Hybrid Poly-generation Energy Systems: Thermal Design and Exergy Analysis [1 ed.] 9780323983662, 0323983662

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Hybrid Poly-generation Energy Systems

Hybrid Energy Systems Series

Hybrid Poly-generation Energy Systems Thermal Design and Exergy Analysis

Mehdi Mehrpooya Faculty of New Sciences and Technologies, the University of Tehran, Tehran, Iran

Majid Asadnia Mechanical Engineering Department, Faculty of Engineering at the Kar Higher Education Institute, Tehran, Iran

Amir Hossein Karimi Faculty of New Sciences and Technologies, the University of Tehran, Tehran, Iran

Ali Allahyarzadeh-Bidgoli Department of Mechanical Engineering (PME), Escola Polite´cnica-University of Sa˜o Paulo (EP-USP), Sa˜o Paulo, Brazil

Academic Press is an imprint of Elsevier 125 London Wall, London EC2Y 5AS, United Kingdom 525 B Street, Suite 1650, San Diego, CA 92101, United States 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom Copyright © 2024 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). MATLABs is a trademark of The MathWorks, Inc. and is used with permission. The MathWorks does not warrant the accuracy of the text or exercises in this book. This book’s use or discussion of MATLABs software or related products does not constitute endorsement or sponsorship by The MathWorks of a particular pedagogical approach or particular use of the MATLABs software. Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-98366-2 For Information on all Academic Press publications visit our website at https://www.elsevier.com/books-and-journals

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Contents

1.

What does it mean? Hybrid polygeneration systems 1.1 Background 1.2 Definition and classification of hybrid polygeneration systems 1.3 Objectives and necessity of using hybrid polygeneration systems References

2.

How to use renewable energy sources in polygeneration systems? 2.1 Introduction 2.2 Proposed process frameworks for solar polygeneration systems 2.3 Solar power plant 2.4 Solar thermochemical reactors 2.5 Wind energy-based polygeneration systems 2.6 Hydrogen production by a multipurpose cycle consisting of wind turbine and heliostats 2.7 Conceptual configuration of using wind energy and geothermal energy to produce hydrogen chloride 2.8 Multipurpose combinations of wind and solar energy for power and refrigeration generation, energy storage, water desalination, food drying, and water electrolysis 2.9 CO2 capturing using wind energy in a multiproduction energy system 2.10 Geothermal energy and polygeneration systems 2.11 Biomass energy used in polygeneration systems 2.12 How to combine hydroenergy systems and polygeneration systems? 2.13 Hybrid power generation of hydropower 2.14 Polygeneration systems that use wave energy resources References

3.

Energy storage type and size in PGSs 3.1 Introduction 3.2 Operational possible ways for thermal energy storage in PGSs 3.3 Benefits and limitations of mechanical energy storage in PGSs 3.4 Proposed process configurations for electrochemical energy storage in PGSs 3.5 How to store electrical energy in PGSs? References

1 1 2 5 9 11 11 11 21 33 49 50 51

52 53 54 68 102 104 113 115 125 125 126 136 137 144 145

vi

4.

Contents

Exergy, energy, environmental and economic analysis of hybrid poly-generation systems: methods and approaches 4.1 Introduction 4.2 The concept of exergy 4.3 Preliminary and advanced environmental analysis of PGs 4.4 Preliminary and advanced economic analysis of PGSs References

147 147 149 156 160 164

5.

Solar-based hybrid energy systems 5.1 Introduction 5.2 Power production by solar PGSs 5.3 Heating production by solar PGSs 5.4 Cooling production by solar PGSs 5.5 Hydrogen production by solar PGSs References

167 167 174 182 184 185 190

6.

Technical and economic prospects of fuel cells combination with polygeneration systems? 6.1 Fuel cell 6.2 Electrolyze 6.3 SOFCs in polygeneration systems References

193 193 213 237 305

7.

Biomass-based hybrid energy systems 7.1 Introduction 7.2 Thermochemical biomass gasification combined processes References

313 313 315 368

8.

Chemical looping combustion in polygeneration systems 8.1 Introduction 8.2 Fuel cell 8.3 Solid oxide fuel cell 8.4 Proton exchange membrane fuel cells 8.5 Expander power process 8.6 Vapor (or steam) power cycle 8.7 Gas and combined power cycles 8.8 Heat recuperation 8.9 Two reactor conversion process configurations References

373 373 374 374 375 375 375 375 376 378 391

9.

A framework for sustainable hydrogen production by polygeneration systems 9.1 Introduction

393 393

Contents

9.2 9.3

High-temperature hybrid electrolyzers Biomass and photobiological processes to produce hydrogen 9.4 GS reactor temperature effect on hydrogen production rate 9.5 Sulfuric acid system References 10. Integration of oxyfuel power plants in polygeneration systems 10.1 Integration of oxyfuel power plants 10.2 Energy and exergy analysis of integrated oxyfuel hybrid power plants 10.3 Environmental and economic analysis of oxyfuel hybrid power plants References 11. Basic power and cooling production systems in combination with polygeneration systems to trigeneration of cold, heat, and power 11.1 Basic power and cooling production systems 11.2 Thermoelectric/thermionic generators in polygeneration systems 11.3 Stirling engines and polygeneration systems 11.4 ORCs in polygeneration systems 11.5 Joule Brayton refrigeration processes in combination with polygeneration systems 11.6 Cryogenic air separation 11.7 Absorption refrigeration-based polygeneration systems References 12. Integration of carbon dioxide capturing processes in hybrid energy systems 12.1 Integration of carbon dioxide capturing processes 12.2 Absorption-based postcombustion capture of carbon systems and polygeneration systems 12.3 Exergy and energy analysis of hybrid CCSs 12.4 Environmental and economic analysis of hybrid CCSs References 13. Why advanced analyses? 13.1 Introduction 13.2 Advanced economic, environmental, and exergy analyses of polygeneration systems 13.3 Advanced method procedure 13.4 Accessible and inaccessible sector variables and assessment

vii

419 460 507 517 546 553 553 561 562 571

573 573 608 609 611 612 614 615 616

619 619 629 633 638 641 643 643 643 646 652

viii

Contents

13.5 13.6

Avoidable and unavoidable sector variables and assessment Benefits and disadvantageous of advanced analysis methodology for energy systems References Index

653 659 660 661

What does it mean? Hybrid polygeneration systems

1.1

1

Background

The process of producing both electricity and useable heat simultaneously using a thermal turbine or power plant is referred to as combined heat and power or co-generation plant. Co-generation has been in use for quite some time, being the first application of polygeneration system. It originated in Europe during the 19th century and in the United States during the early 1900s, when coal boilers and steam turbine generators were the primary sources of energy for most industries. The steam generated from these facilities was utilized for commercial operations [1,2]. One or more primary energy sources are combined in a process called polygeneration to produce two or more energy products. Applications for polygeneration systems are numerous in the power plant, energy, industrial, and utility sectors. Using polygeneration systems for concurrent heating, cooling, and power generation is an efficient way to improve energy efficiency. Such systems combined and produces three or more outcomes, including energy outcomes, through one or more natural resources. By doing so, it is possible to improve overall performance significantly and, in turn, reduce pollutants and emitting greenhouse gases. Small-scale polygeneration systems combine old and new technology to produce heating, cooling, power, and additional products such as energy storage, CO2 (in liquid or gas phases), biofuels, and other things. The elements of a polygeneration system can be combined to form its subsystems. Hybrid systems can generate electricity by combining two or more technologies. Various configurations can create hybrid systems, such as hydrogen, renewable energy, gas, and steam cycles. Examples of this technology include integrating the fuel cell system with renewable sources such as wind turbines and PVs, cogeneration and tri-generation processes integrated with fuel cell technology, etc. Simultaneous electricity and heating, cooling, or both generation is an excellent solution to improve energy efficiency. For co-generation processes, the required fuel to generate power and heating energy is significantly smaller than when power and heat are generated distinctly in conventional systems. Therefore these systems are very suitable in terms of efficiency. Triple-generation systems also include processes that use a single fuel source to generate and consume electricity and heat simultaneously. Simultaneous energy use results in high energy efficiency, fewer emissions, storage safety, less waste, and lower costs.

Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00005-0 © 2024 Elsevier Inc. All rights reserved.

2

1.2

Hybrid Poly-generation Energy Systems

Definition and classification of hybrid polygeneration systems

Providing a clean, eco-friendly option is increasingly crucial than ever in light of the rise in energy use and the issues of fossil resource shortage. Even though numerous renewable energy sources have already been implemented recently, they have constantly been constrained by problems with the economy, the level of technology, and other factors. As a large portion of fossil fuels are utilized in the current inefficient electricity generation, greenhouse gases (GHG) have been emitted into the environment more frequently in recent years. Co-generation systems that simultaneously produce two different products have been created to increase efficiency, lower costs, and reduce pollutant emissions. By integrating several processes, polygeneration systems enable the production of multiple energy sectors (more than two), such as heat, cold, electricity, and energy carriers like hydrogen [3,4]. High energy efficiency, declining fossil fuel usage, financial impact, combination ability with sustainable power, and minimal pollutant production are only a few benefits of these systems [5,6]. Combining heat and power (CHP), one type of polygeneration technology, is crucial for maximizing energy efficiency and utilizing available energy resources [7]. In order to meet the demand for cooling, integrated cooling, heating, and power (CCHP) processes, which generate heating, refrigerating, and electricity simultaneously, were developed. In this manner, waste heat from conventional systems is utilized to create valuable goods. In essence, it uses the waste heat from primary energy sources, including internal combustion engines, steam turbines, micro gas expanders, Stirling engines, and fuel cells, to create useful products [8]. The polygeneration system’s core components are a prime mover as well as an energy conversion device. The appropriate prime mover and energy conversion device are selected according to the power production, usage, and cost associated. Fig. 1.1 illustrates the potential for manufacturing fuels, energy, and other goods using a polygeneration system. Fig. 1.2 illustrates the various prime movers and energy conversion devices employed. Renewable energy sources, as sustainable, easily accessible, and reproducible, are the best choice among other energy sources for a polygeneration system. These energy sources can be converted to other forms of energy conventionally, resulting in heat, cooling, power, and other energy source generations. Wind, solar, biomass, and geothermal are the primary and most used renewable energy sources, including for hybrid polygeneration systems. Wind energy is applicable in wind turbines for power production. As shown in Fig. 1.3, solar energy can be used by thermal applications such as solar collectors, photovoltaics, and thermoelectrics—for instance, the solar organic Rankine cycle. Biomass is used by two utilization: thermochemical and biochemical. Gasification and pyrolysis are the solutions for using biomass’s thermochemical cycle in polygeneration systems. Biodiesel and biogases are the products of the biochemical process as the energy sources of energy generation systems. Geothermal renewable energy sources can be implemented by geothermal

What does it mean? Hybrid polygeneration systems

3

Ventilation Waste heat for domestic hot water Cooling

Preheating of air

Heat

Renewable energy/low grade heat/ fossil fuel

Polygeneration

Production of alcohols, hydrogen and chemicals Drying

Prime mover /Energy conversion device

Electrical power/ mechanical output

Figure 1.1 Polygeneration systems’ potential routes [3].

Polygeneration system Prime movers

Internal combustion engines

Energy conversion device

External combustion engines

Fuel cells

Thermoelectric converters

Solar, wind, geothermal

Figure 1.2 An illustration of the various prime movers and energy conversion devices [3].

Renewable energy sources

Wind

Thermal

Biomass

Solar

Photovoltaic

Thermochemical

Gasification

Pyrolysis

Biochemical

Biodiesel

Geothermal

Geothermal well

Borehole Heat exchanger

Biogas

Thermoelectric

Figure 1.3 Most used renewable energy sources for polygeneration energy systems.

4

Hybrid Poly-generation Energy Systems

wells and boreholes (heat exchangers) in a hybrid polygeneration system. However, each energy source can be integrated with another to complete a production process or generate a second production, such as heating, cooling, and power in a hybrid polygeneration system presented in this book’s following chapters. The utilization of renewable resources presents a significant challenge due to their intermittent nature, particularly in power delivery systems such as the electrical grid. This issue is especially evident in solar energy, which is not accessible during nighttime hours. Consequently, implementing an energy storage system is imperative for preserving surplus energy and utilizing it during periods of unavailability. Energy storage is a highly viable solution, as it enables energy production units to store their excess output and retrieve it when necessary, thereby ensuring a consistent and stable energy supply [9]. Fig. 1.4 shows the critical energy storage used in hybrid polygeneration energy systems. Energy storage can be categorized into various types such as biological, thermal, chemical, mechanical, electrochemical, and electrical. Among these categories, some of the most well-known energy storage methods include phase change materials of thermal and methanol dimethyl ether ammonia from the chemical category. However, there are also more conventional energy storage methods such as flywheel, hydraulic accumulator, compressed air, solid mass gravitational, and pumped hydro from mechanical and supercapacitors from electrical. These energy storage methods play a critical role in ensuring that energy is efficiently stored and utilized in hybrid polygeneration energy systems. The system depicted in Fig. 1.5 is designed to incorporate the sweet process of flue gases, utilizing two distinct mechanical energy storage mechanisms, namely turbines and compressors, in conjunction with amine. The heating generated by the system is harnessed to prepare the amine for its removal duties. Chapter 3 of this book delves into the subject of energy storage methods and offers an in-depth explanation of their applications in hybrid polygeneration energy systems.

Energy storage

Biological

Thermal

Latent

Sensible

Chemical

Mechanical

Thermochemical

Electrical Supercapacitors

Compressed air Liquid Fuels

Phase change Materials (PCM) Cryogenic liquids Liquefied natural gas Liquid nitrogen Liquid oxygen

Electrochemical

Flywheel Pumped-Hydro

Methanol Dimethyl ether (DME) Ammonia

Hydraulic accumulator

Solid mass gravitational

Figure 1.4 Most used energy storage used for polygeneration energy systems.

What does it mean? Hybrid polygeneration systems

5

Lean Amine (85 °C, 500 kPa, 687.8 kgmole/h)

Sweet gas (84.06 °C, 6860 kPa, 2.631 kgmol/h)

Flue gas (85 °C, 500 kPa, 60 kgmole/h) Absorber column (discharge mode) Rich Amine (123.2 °C, 6895 kPa, 728.3 kgmole/h) Heat duty 1610.4 kW

Acid gas (54.41 °C, 100 kPa, 41 kgmole/h)

Auxiliary Heat 708.5 kW

Air (25 °C, 2433 kPa, 1773 kgmole/h)

Desorption column (charging mode)

Heat duty 891.5 kW Mechanical energy storage: turbines (discharge mode)

Mechanical energy storage: compressors Air (charging mode) (100 °C, 25000 kPa, 1773 kgmole/h)

Figure 1.5 Application of mechanical energy storage methods for preparing amine for the separation process [10].

1.3

Objectives and necessity of using hybrid polygeneration systems

The hybrid polygeneration objectives can be categorized into the following item: (1) maximizing the application of sustainable energy systems; (2) utilizing a variety of energy inputs to operate fuel cells applications; (3) optimizing the polygeneration system; and (4) utilizing various polygeneration system applications in residences, business, and industrial usages [11]. To achieve the primarily mentioned objectives, the following criteria are typically used to decentralize the hybrid polygeneration systems: (1) heat engines (rotary internal combustion, micro gas expanders, fuel cells); (2) low-turn equipment (absorption or electric chillers); (3) supplementary equipment (heating systems, fuel burner absorption cooling systems or heat pumps, engine-based chiller); (4) potential source of renewable energy (solar, organic matter, wind, hydroelectric); and (5) ethanol, hydrogen, etc. As a result, numerous possible designs for polygeneration systems can be found. Given the wide range of hybrid topologies that can be used, such as hydrogen, renewable energy, gas cycles, etc., the designs, including fuel cells, are especially interesting among these options [3,12]. Therefore the main objectives of hybrid polygeneration systems are power production, providing heating or auxiliary heating demands, cooling and refrigeration systems, preparing energy storage devices, and generating energy resources. Moreover, natural or wasting energy resources, heat exchangers, separators, boilers, burners, compressors, expanders, and pressure valves are the most common and necessary equipment for conventional hybrid polygeneration systems. Fig. 1.6 shows the diagram of a polygeneration process. The combustible, air, and GT parameters of this system’s Brayton power-generating cycle are in a power plant.

6

Hybrid Poly-generation Energy Systems

WNET 133.3 MW

MEA

Flue gas

T=40.2°C P=1.7 atm m=1500 kg/s

T=231.5°C P=1. atm m=508.7 kg/s

Natural gas

CO2

T=8.6°C P=21.1 atm m=8.7 kg/s

T=60.8°C P=1. atm m=4 kg/s Power generation cycle (Brayton)

Air

CO2 capture from flue gas

Heat of combustion chamber outlet stream

Heat of flue gas

T=6°C P=0.86 atm m=500 kg/s

H 2O T=20°C P=1 atm m=3.4 kg/s

Air

T=42.8°C P=1. atm m=498.6 kg/s

Qcooling 9.9 MW

Sorption-enhanced chemical looping reforming

T=20°C P=1 atm m=4.3 kg/s

Flue gas

Heat of flue gas

High pressure steam generation

Heat of flue gas

Absorption chiller

CH4 T=25°C P=1 atm m=1 kg/s

H2

CO2

N2+O2

T=103.1°C T=45°C T=40°C P=1 atm P=1 atm P=1 atm m=0.38 kg/s m=2.6kg/s m=3.4 kg/s

H2 O

HP steam

0.3NH3 + 0.7H2O

T=20°C P=3.9 atm m=50 kg/s

T=180°C P=9.9 atm m=50 kg/s

T=44.6°C P=2.6 atm m=60 kg/s

Figure 1.6 Conceptual diagram of a hybrid polygeneration system [12].

This hybrid polygeneration system provides a solvent process for carbon dioxide sequestration from exhaust gases, a natural gas-based generating process, a SECLR for hydrogen production, an high-pressure steam generator, and an absorption cooler for temperature reduction of requirement. Concurrent hydrogen liquefaction and carbon dioxide capture are advantages of using SECLR rather than traditional SMR. Furthermore, this unit’s CLC eliminates the need for an outside heat source during the hydrogen production process. The system is sustainable and dramatically reduces CO2 emissions thanks to MEA-based CO2 sequestration for the combustion gases. One of the most used energy carriers and the fuel of the future is hydrogen. Hydrogen production was considered the objective of several systems such as energy, cryogenic, chemical, poly, and hybrid polygeneration systems [13 25]. Instead of being an energy source, hydrogen is an energy carrier. By combining different technologies, including electrolysis, steam methane reformation, or gasification in combination with either direct combustion of fossil fuels or power produced from renewable, fossil, or nuclear energy sources, hydrogen may be created from an energy source. Climate effects from different hydrogen-generating processes vary. There are several classification schemes to identify hydrogens produced from various fuels and electric suppliers. The color categorization of hydrogen is based on the original energy source and manufacturing method, as shown in Fig. 1.7 [26]. Fig. 1.8 presents the hydrogen production methods that are categorized into biomass, water splitting, reforming, and chemical looping methods. This book discusses various methods for producing hydrogen, including biomass, water splitting,

What does it mean? Hybrid polygeneration systems

7

THE COLORS OF HYDROGEN GREEN Hydrogen produced by electrolysis of water, using electricity from renewable sources like wind or solar. Zero CO2 emissions are produced.

BLUE

GREY

Hydrogen produced from fossil fuels (i.e., grey, black, or brown hydrogen) where CO2 is captured and either stored or repurposed.

Hydrogen extracted from natural gas using steam-methane reforming. This is the most common form of hydrogen producon in the world today.

PURPLE/PINK

TURQUOISE

Hydrogen produced by electrolysis using nuclear power.

Hydrogen produced by thermal spling of methane (methane pyrolysis). Instead of CO2, solid carbon is produced.

BROWN/BLACK Hydrogen extracted from coal using gasificaon.

YELLOW

WHITE

Hydrogen produced by electrolysis using grid electricity from various sources (i.e., renewables and fossil fuels).

Hydrogen produced as a byproduct of industrial processes. Also refers to hydrogen occurring in its (rare) natural form.

Figure 1.7 Hydrogen production categorization by color classification [26].

Hydrogen production

Biomass

Gasification

Water splitting

Thermochemical cycles

Reforming

Elecrolyzer

Chemical looping

Two reactor system

Three reactor system

Syngas Pyrolysis

Zn-SI S-I Mg-CI Cu-CI

Low temperature High temperature SOEC

Figure 1.8 Hydrogen production methods applicable in hybrid polygeneration systems.

reforming, and chemical looping. To achieve a diverse range of options for hydrogen production, it is crucial to integrate these methods into a hybrid-polygeneration cycle. Throughout the book, the production of hydrogen as a key objective is thoroughly explored in several chapters. It is crucial to utilize renewable energy sources to establish a sustainable energy system. However, reducing carbon emissions is still necessary to address environmental and climate change concerns. Carbon capture and storage remain practical approaches

8

Hybrid Poly-generation Energy Systems

CO2 capturing, separation, and liquefaction

Post combustion

Oxyfuel combustion

Absorption separation

Cryogenic Separation

Chemical solvents

Physical solvents

Amine scrubbing

Water scrubbing

Reverse water gas shift reaction

Electroreduction

CO2 + H2

CO + H2O

Ionic liquids

Figure 1.9 CO2 capturing, separation, and liquefaction methods. O2 (g) 25 °C, 1.4 bar, 99297 kg/h

456580 kg/h

Oxy Fuel Power Plant Electrical Power 122183 kW

Exhaust Gas (H2O+CO2) 67.9 °C, 1.4 bar, 1468321 kg/h

NG 35 °C, 40 bar 480898 kg/h

Mixed refrigerant cycle

24048 kg/h

LNG -164 °C, 1 bar, 436441 kg/h

Propane refrigeration cycle LNG production and CO2 capturing and liquefaction sub-system H2O

925 kg/h

CO2 (L) -1 °C, 35 bar, 84776 kg/h

Figure 1.10 Oxyfuel power plant integrated with LNG liquefaction, CO2 capture, and liquefaction [27].

to sequester carbon, but they require significant energy consumption for heating and power. To enhance this system’s energy efficiency, it can be integrated into a hybridpolygeneration energy system. Fig. 1.9 showcases CO2 capture, separation, and liquefaction techniques, including postcombustion, oxy-fuel combustion, electroreduction, and reverse water gas shift reaction. Meanwhile, in Fig. 1.9, the CO2 sequestration and liquefaction methods are discussed in this book’s chapters. In Fig. 1.10, a hybrid polygeneration energy system is presented to use a part of a natural gas flow for power generation by an oxy-fuel power plant. The produced

What does it mean? Hybrid polygeneration systems

9

power and flue gases are forwarded to the refrigeration system (mixed and propane refrigeration). LNG production and CO2 capturing and liquefaction as subsystem is integrated into an oxyfuel power plant. In other words, this hybrid polygeneration system receives natural gas at 35 C and 40 bar, oxygen and delivers LNG at -164 C and 1 bar, liquid CO2 at -1 C and 35 bar, and water. The required power of this liquefaction system is more than 122 MW, which is supplied through a mass flow of the receiving gas. The main subject of this publication is the study of Hybrid Polygeneration Energy Systems: Thermal Design and Exergy Analysis. This book delves into the latest developments and concepts in hybrid energy systems, with a specific emphasis on thermal applications. The authors begin by offering an overview of hybrid polygeneration and storage possibilities before exploring six distinct types of hybrid systems: solar, fuel cells, combustion, heating, and cooling. Furthermore, the publication examines the economic and environmental impacts of each system and provides techniques and approaches for analyzing energy and efficiency.

References [1] W.D. Sawyer, A History of Industrial Power in the United States, 1780 1930. Vol. 2: Steam Power. 1988, JSTOR. [2] Babcock and W. Company, Steam: Its Generation and Use with Catalogue of the Manufactures of the Babcock and Wilcox Company, Vol. 36, The Company, 1923. [3] S. Murugan, B. Hora´k, Tri and polygeneration systems-a review, Renewable and Sustainable Energy Reviews 60 (2016) 1032 1051. [4] M. Sheykhi, et al., Investigation of the effects of operating parameters of an internal combustion engine on the performance and fuel consumption of a CCHP system, Energy 211 (2020) 119041. [5] J. Yan, Handbook of Clean Energy Systems, 6 Volume Set, Vol. 5, John Wiley & Sons, 2015. [6] I. Dincer, Y. Bicer, Integration of renewable energy systems for multigeneration. Integrated Energy Systems for Multigeneration, 2020: pp. 287 402. [7] K.K. Roman, M. Hasan, H. Azam, CCHP System Performance Based on Economic Analysis, Energy Conservation, and Emission Analysis, Energy Systems and Environment, IntechOpen, London, UK, 2018. [8] R. Segurado, et al., Techno-economic analysis of a trigeneration system based on biomass gasification, Renewable and Sustainable Energy Reviews 103 (2019) 501 514. [9] A.S. Alsagri, A. Chiasson, M. Gadalla, Viability assessment of a concentrated solar power tower with a supercritical CO2 Brayton cycle power plant, Journal of Solar Energy Engineering 141 (5) (2019). [10] M. Mehrpooya, P. Pakzad, Introducing a hybrid mechanical Chemical energy storage system: Process development and energy/exergy analysis, Energy Conversion and Management 211 (2022) 112784. Available from: https://doi.org/ 10.1016/j.enconman.2020.112784. [11] F. Calise, Design of a hybrid polygeneration system with solar collectors and a solid oxide fuel cell: dynamic simulation and economic assessment, International Journal of Hydrogen Energy 36 (10) (2011) 6128 6150.

10

Hybrid Poly-generation Energy Systems

[12] R. Habibi, et al., A natural gas-based eco-friendly polygeneration system including gas turbine, sorption-enhanced steam methane reforming, absorption chiller and flue gas CO2 capture unit, Sustainable Energy Technologies and Assessments 52 (2022) 101984. [13] H. Quack, The key role of the cryotechnology in the hydrogen energy industry. Die Schluesselrolle der Kryotechnik in der Wasserstoff-Energiewirtschaft. Wissenschaftliche Zeitschrift der Technischen Universit¨at Dresden, 2001. 50. [14] H. Quack, Conceptual design of a high efficiency large capacity hydrogen liquefier, AIP Conference Proceedings, American Institute of Physics, 2002. [15] A. Steinfeld, Solar thermochemical production of hydrogen—a review, Solar energy 78 (5) (2005) 603 615. [16] J. Stang, P. Neksa, E. Brendeng, On the Design of an Efficient Hydrogen Liquefaction Process. 2006. [17] A. Z’Graggen, et al., Hydrogen production by steam-gasification of petroleum coke using concentrated solar power—II Reactor design, testing, and modeling, International Journal of Hydrogen Energy 31 (6) (2006) 797 811. [18] A. Z’Graggen, A. Steinfeld, Hydrogen production by steam-gasification of carbonaceous materials using concentrated solar energy V. Reactor modeling, optimization, and scale-up, International Journal of Hydrogen Energy 33 (20) (2008) 5484 5492. [19] Walnum, H.T., et al., Principles for the liquefaction of hydrogen with emphasis on precooling processes, in: 12th Cryogenics, 2012: pp. 8. [20] F. Khalid, I. Dincer, M.A. Rosen, Analysis and assessment of an integrated hydrogen energy system, International Journal of Hydrogen Energy 41 (19) (2016) 7960 7967. [21] D. Cai, et al., Integration of wind turbine with heliostat based CSP/CPVT system for hydrogen production and polygeneration: a thermodynamic comparison, International Journal of Hydrogen Energy 47 (2020). [22] M. Luqman, Y. Bicer, T. Al-Ansari, Thermodynamic analysis of an oxy-hydrogen combustor supported solar and wind energy-based sustainable polygeneration system for remote locations, International Journal of Hydrogen Energy 45 (5) (2020) 3470 3483. [23] R. Zhuang, et al., Waste-to-hydrogen: Recycling HCl to produce H2 and Cl2, Applied Energy 259 (2020) 114184. [24] M. Mehrpooya, B. Ghorbani, M. Khalili, A new developed integrated process configuration for production of hydrogen chloride using geothermal and wind energy resources, Sustainable Energy Technologies and Assessments 45 (2021) 101173. [25] M. Mehrpooya, et al., Conceptual design and performance evaluation of a novel cryogenic integrated process for extraction of neon and production of liquid hydrogen, Process Safety and Environmental Protection 164 (2022) 228 246. [26] Tanya Stasio, J.C. The “Colors” of Hydrogen. 2021; Available from: https://aeclinic. org/aec-blog/2021/6/24/the-colors-of-hydrogen. [27] T.M. Mehrpooya, B. Ghorbani, Introducing a hybrid oxy-fuel power generation and natural gas/carbon dioxide liquefaction process with thermodynamic and economic analysis, Journal of Cleaner Production 204 (2018) 1016 1033. Available from: https://www.sciencedirect.com/science/article/pii/S0959652618327197#fig1.

How to use renewable energy sources in polygeneration systems?

2.1

2

Introduction

Renewable energy sources lead to clean, green energy generation and possibly sustainable and polygeneration systems (PGSs). These sources can be divided into renewable fuels such as biomass and hydrogen, solar power and heating, and wind and wave energy. These energy sources are usually available at a low cost. The cost is only for exploiting these resources. Therefore using these resources in the context of energy crises and environmental problems related to fossil fuels is precious. These energy sources are used in different ways discussed in the following section. As we observed in the previous chapter, renewable energy sources play as both prime mover and conversion energy devices in a typical polygeneration system. A renewable energy source can be converted to electrical power or mechanical output. Heating directly by a solar collector and cooling indirectly, for instance, in an absorption refrigeration system, are achievable in PGSs. In the following section, these applications are theoretically introduced for each renewable energy source.

2.2

Proposed process frameworks for solar polygeneration systems

Today, using solar energy as a primary source of power generation is unavoidable. The use of solar energy technology has many positive effects on the environment, such as reducing greenhouse gases and toxic emissions, reducing the requirements for transmission lines in the electricity network, rehabilitating barren lands, and improving the quality of water resources. Socio-economic benefits of solar technologies include increased national and regional energy independence, increased job opportunities, increased acceleration of rural electricity supply in remote areas, diversity, and security of energy supply. In general, since solar energy is an inexhaustible, safe, and clean energy source, it has attracted much attention as an alternative to conventional energy sources. Solar energy is used in two ways: photovoltaic and solar thermal technology. In the first method, sunlight is converted directly into electricity, which is done by photovoltaics. In the second method, first, the heat of the sun is absorbed by the Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00004-9 © 2024 Elsevier Inc. All rights reserved.

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Hybrid Poly-generation Energy Systems

fluid inside the collector, then this heat is converted to power or its heat is used to provide the heat load of another system. This type of solar application is called a solar power plant.

2.2.1 Photovoltaic In photovoltaic cells, there is an electric field with the positive side behind the cell and the negative side with the cell facing the radiation. As a result of the collision of photons scattered on the cell surface, electrons are separated from atoms in semiconductor materials, creating an electron-cavity pair. An electrical circuit is formed if the electrical conductors are connected positively and negatively. The electric current from electrons is called photocurrent (Iph). As can be seen from this description, a photovoltaic cell cannot function in the dark; thus it acts as a diode. In other words, the p-n bond in the dark produces no current or voltage. If the connector is connected to a large external voltage source, it passes through what is called a diode current or dark current (ID). As such, a photovoltaic cell is typically electrically modeled with a diode. Fig. 2.1 shows the equivalent electrical circuit of a photovoltaic cell. The circuit in Fig. 2.1 is used for a single cell, a module consisting of several cells, and an array consisting of several modules. Fig. 2.1 shows that the equivalent electrical circuit consists of a current source (Iph), a diode, and a series resistor (Rs), indicating intracellular resistance. The diode also has an internal resistor indicated by the circuit’s parallel resistance (RSH). The net current equals the difference between the photocurrent and diode current. So in Eq. (2.1), we have [1,2]: 
  q0 ðV 1 IRs Þ V 1 IRs I 5 Iph 2 ID 5 Iph 2 I0 exp 21 2 ΓkTc RSH

(2.1)

2.2.1.1 Γ : diode ideal factor It should be noted that the parallel resistor is usually much larger than the load resistor, while the series resistor is much smaller than the load resistor, thus wasting a tiny amount of power generated inside the cell. Given these two resistances, the net current is equal to the difference between the photocurrent and the cell current obtained from Eq. (2.2) [1,2].

Figure 2.1 Equivalent electrical circuit of a cell.

How to use renewable energy sources in polygeneration systems?


   q0 V I 5 Iph 2 ID 5 Iph 2 I0 exp 21 ΓkTc

13

(2.2)

where k is the Boltzmann constant (1.381 3 10223j/k), TC is the Cell temperature (k), q0 is the electron charge (1.602 3 10219j/v), V is the voltage applied to the two ends of the cell (v), I0 is the Dark saturation current, which strongly depends on temperature (A) If the two ends of the junction cell are shortened, the current [short circuit current (Isc)] is maximized, and the voltage is zero. If the two ends of the photovoltaic cell circuit are open, the voltage of the two circuits [open-circuit voltage (Voc)] is maximized, and the current is zero. In both cases (open circuit and short circuit), the power is zero but between these two states has a nonzero value. As a result, the maximum power density (Pel) can be obtained from the Eq. (2.3) [3]: Pel 5 Voc 3 FF 3 jsc

(2.3)

where Voc is the ppen circuit voltage, FF is the density coefficient, jsc is the short circuit current density.

2.2.1.2 Thermophotovoltaic The thermophotovoltaic system increases combustion chamber heat recovery, power generation, and overall efficiency in the following process. It is worth noting that power generation in a thermovoltage system requires emitting radiation. For this purpose, we assume the combustion chamber’s temperature and the emitter surface’s temperature [4], assuming that the heat transfer between the combustion chamber and the emitter surface is 100% efficient. The assumption is valid and reasonable. As mentioned earlier, a thermophotovoltaic system for heat recovery consists of four main parts: 1. 2. 3. 4.

Emitter Optical filter Cells Cooling system

In the meantime, another essential component is not part of the system but affects the system’s performance. The conceptual model structure of a thermophotovoltaic system is shown in Fig. 2.2. Fig. 2.2 shows that current (1) represents the radiation power emitted from the transmitter to the optical filter system. The current (2) reflects the reflected radiation from the filter to the transmitter because, in the general case, the collision of electromagnetic waves with any surface may occur in three states: pass, reflection, and absorption. Each of these three situations is likely to depend on the type of filter. Therefore current (2) represents the general relationship between the emitter and the optical filter. Also, in general, the viewing angle should be taken into account. The current (3) represents the radiation power that passes through the filter; since it contains two convertible and nonconvertible parts by the cells, part of it

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Hybrid Poly-generation Energy Systems

Primary

1

Useful Energy

4

2 Emitter

connversion

Processing

Optical filter

3

5

Load

6

Cooler

TPV cell

8

7

Figure 2.2 Conceptual model of the thermophotovoltaic system [5].

is converted into electrical current, and the remainder is divided into two parts: reflection and absorption. The reflection part is determined by the current (4), and the absorbed and nonconvertible part causes the cells to warm. If the optical filter performs poorly due to limitations in fabrication or cost, the cell temperature rises in such a way as to affect the overall performance of the system, forcing the use of cooling. The cooling system may consist of a combination of blades and fans for proper operation. Flow (7) reflects the effect of the cooling system on cell function. The current (6) reflects the power required to operate the cooling system. The task of the cooling system is to transfer heat from the cells to the environment, the power generated by the system is transferred to the load, but the type and amount of consumption in this part, depending on the current required, partially affects the cell’s performance. Therefore the current shows a dominant relationship between this part and the cells (5). Since both the coolant and the filter are responsible for preventing the cell temperature from rising, their performance is affected by the relationship reflected by the current (8). Given the flow and relationships governing the entire system, two essential factors determine the system’s overall performance. These two factors are: 1. Emitter temperature 2. Cell temperature

Other factors affect system performance, such as cell type (by energy bandwidth), filter type (by irreversible radiation reflectance), and emitter type, including wavelength selector and constant wavelength selector. They are important and somehow subject to temperature (especially emitter temperature), so the influence of temperature will be of greater importance, which is considered in the computational model. The short-circuit current density is obtained from Eq. (2.4) [4]: jsc 5

ð λg λ0

F:εðλÞ:eλb ðλÞ:SPλ :τðλÞdλ

(2.4)

where εðλÞ denotes the wavelength emission factor λ, eλb ðλÞ denotes the Planck spectral relation emission power distribution, SPλ denotes the spectral response

How to use renewable energy sources in polygeneration systems?

15

proportional to the type of thermophotovoltaic cell, τðλÞ denotes the coefficient of the passage of the optical filter at wavelengths λ, λ denotes the wavelength, λ0 denotes the low wavelength limit, λg denoted the cell cutoff wavelength, and F denotes the visibility coefficient between cell and emitter. For the emitters used in thermophotovoltaic systems, silicon carbide emitters with a Gaussian coefficient of about 0.9 is typically considered. This emitter type is most commonly used in thermophotovoltaic systems that use the combustion chamber as a heating source. The German physicist Max Planck proposed the spectral dependence of the power emitted by the unit of surface emission at different temperatures for a black body in the form of Eq. (2.5) [2]. eb ðλ; TÞ 5

2πhc2 1 3 hc 5 λ eλkT 2 1

(2.5)

Using Eq. (2.6), one can obtain the wavelength intensity for the black object. The temperature dependence of the wavelength distribution of radiation intensity is shown in Fig. 2.2 [2]. As shown in Fig. 2.3, as the emitter temperature increases, the wavelength with the highest radiation reaches the smallest wavelength. It should be noted that only a few objects behave like the ideal black body. Many objects at the same temperature as the black body have less radiation. The emission rate (ε) is the amount that determines the relationship between actual radiation and black body radiation.

Figure 2.3 Distribution of radiation intensity in terms of wavelength at different temperatures [6].

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Hybrid Poly-generation Energy Systems

The spectral response determines the amount of radiation that the cell converts to electric current and is obtained from Eq. (2.6) [3]. SPλ 5

QEext q0 λ ; λ # λg hc

(2.6)

External Quantum Efficiency [-]

QEext is the external quantum efficiency, q0 is the charge of an electron [q0 5 1.602 3 10219 (C)], h is the Planck’s constant [h 5 6.626 3 10234 (j.s)], c is the speed of light ðc 5 3 3 108 ðmsÞÞ. The spectral response depends on the cell type. The quantity that reflects celltype dependence in Fig. 2.4 is the external quantum efficiency (QEext ). To estimate the conversion efficiency of a cell, one of the basic parameters is the external quantum efficiency, which is the probability that a photon with λ wavelength is absorbed by the cell and produces an electron that is absorbed by the output port (positive terminal). This quantum efficiency considers the probability of reflection and absorption of the incoming photons and the probability of producing and collecting minority carriers. Thus the external quantum efficiency describes the behavior of the p-n bond very precisely. External quantum yields for different semiconductors selected as thermophotovoltaic have been calculated and their behavior is shown in Fig. 2.4 [7]. Fig. 2.5 shows that most materials used to make thermophotovoltaic cells have high external quantum efficiency over a wide range of wavelengths near their energy band to smaller wavelengths. The amount of external quantum efficiency

100

InGaAs

90

InAsSbP

80 70 60

GaSb

50 40 30 20

Ge

10 0 0

500

1000

1500

2000

2500

3000 3500 wavelength [nm]

Figure 2.4 External quantum efficiency of thermophotovoltaic cells [3].

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17

EQE For GaSb

1

EQE

0.8 0.6 0.4 0.2 0 0

0.5

1

1.5

2

λ(μm) Figure 2.5 External quantum efficiency of GaSb cells [3].

Table 2.1 External quantum efficiency values corresponding to cell energy gap [3]. Eg(ev)

QEext

1.1 0.72 0.55

0.75 0.75 0.65

for photons with wavelengths drops sharply to about 1000 (nm), but it should be noted that this region has wavelengths in standard thermophotovoltaic systems, and the system also has an optical filter with negligible emission. For this reason, thermophotovoltaic cells can convert a considerable portion of the radiation of a black body to electricity [7]. In all the papers reviewed, the external quantum efficiency for all the transverse wavelengths to a fixed cell and an average value assumed to correspond to the cell’s energy bandgap is presented in Table 2.1 [3]. Since external quantum efficiency has a direct impact on the cell’s electrical output density, it has a significant influence on the cell’s output power density. As shown in Fig. 2.5, as the wavelength of the input spectrum to the cell increases, its external quantum efficiency decreases rapidly. Therefore it is suggested that the external quantum efficiency be entered into the model as a function of the wavelength of the input spectrum. The quantum efficiency for thermophotovoltaic cells has been measured experimentally, so the relation obtained is also experimental. According to Fig. 2.5, one can fit a function to any of the curves in Fig. 2.4 to obtain the quantum efficiency relation in terms of the wavelength of the input-cell spectrum. In this study, two GaSb and InGaAsSb cells were examined. Because these cells have the highest output and utilization, GaSb’s external quantum efficiency graph, Fig. 2.5, and InGaAsSb cell, Fig. 2.6 have been used to fit the quantum efficiency function in terms of wavelength [8]. As shown in Fig. 2.5, the GaSb quantum efficiency diagram has two local maximums relative to the wavelength of the input spectrum. The first peak in this graph

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Hybrid Poly-generation Energy Systems

EQE

EQE for InGaAsSb 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.5

1

1.5

2

2.5

λ(μm) Figure 2.6 InGaAsSb external quantum efficiency [8].

Table 2.2 Sum of least squares for both curves. Curve type

R2

First local maximum Second local maximum

0.9924 0.9582

corresponds to a wavelength of 0.5μm, which is the threshold of the visible blue spectrum, and the second peak is to the wavelength of 1.5μm, that is, the beginning of the infrared region (near the visible spectrum). Therefore if the wavelength of the emitted spectrum decreases either too much (below the blue spectrum) or too much (beyond the infrared spectrum), the quantum efficiency rapidly tends to zero, in other words, disrupting the system’s performance. The reason for the decrease in quantum efficiency at shorter wavelengths than the blue spectrum is that they are likely to pass through the semiconductor potential barrier. So the best way to solve this problem is to convert Fig. 2.5 into two curves so that each local maximum is in one curve. So we have two curves, each with an absolute maximum, and as a result, the external quantum efficiency function becomes a binomial function. The sum of the least squares is given in Table 2.2. Thus the quantum yield relationship of the GaSb cell is given by Eq. (2.7).  EQEðλÞ 5 

1187:6λ4 2 2473:2λ3 1 1903:8λ2 2 640:95λ 1 79:957 2 1:8338λ4 1 6:3487λ3 2 8:18λ2 1 5:2902λ 2 0:9398 0:4 # λ # 0:627449 0:62745 # λ # 1:8

(2.7)

Since λmax is the wavelength with the highest radiation at the corresponding temperature and λ0 is the smallest wavelength at which the temperature exists

How to use renewable energy sources in polygeneration systems?

19

max ðλ0 5 λ2:5 Þ, it is recommended to put the mean value in the EQE(λ) relation. In other words in Eq. (2.8):

λmax 1 λ5 2

λ max

2:5

5 0:7λmax

(2.8)

According to the Vienna Displacement Law [9]: λmax 5

b ; b 5 2:898 3 1023 ½m:k T

(2.9)

The model’s operating temperature range is based on quantum yields in the range of 852 C4727 C: T 5 47273 C ! λ 5 0:4ðμmÞ T 5 8523 C ! λ 5 1:8ðμmÞ Thermophotovoltaic systems use optical filters to reflect part of the radiation into the emitter with wavelengths larger than the cell’s energy bandgap. Therefore the filter results in two effects: 1. Prevent excessive warming of the cell 2. Reduce emitter cooling rate

The filter operation mechanism is schematically illustrated in Fig. 2.7 [10]. Fig. 2.7 shows that the optical filter reflects the irreversible part of the radiation (less energy than the cell’s energy band gap). This specificity is not possible for normal solar cells, which causes thermophotovoltaic cells to reach potential. They achieve a high conversion rate of the incoming photon to the

Figure 2.7 Concept of optical filter [10,11].

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Hybrid Poly-generation Energy Systems

Figure 2.8 How the radiation spectrum of the emitter is distributed at different temperatures [11].

electric current because the incoming radiation corresponds to the area where the external quantum efficiency is maximal; how the spectrum of radiation corresponds to the external quantum efficiency for GaSb and InGaAsSb cells is shown in Fig. 2.8 [11]. As shown in Fig. 2.8, the maximum external quantum efficiency for the GaSb cell occurs at 1500K for the emitter and decreases with the emitter temperature decreasing very rapidly and tilting to zero. By lowering the energy band gap of a thermophotovoltaic cell, such as InGaAsSb, at lower temperatures, the emitter can be expected to have higher external quantum efficiency [11]. The optical filter system is not limited to one object or one type, but there are several mechanisms for filtering the spectrum inappropriately: 1. 2. 3. 4. 5.

Interference filters Plasma filters Combination of interference and plasma filters Resonant array filters Spectral control using the back surface reflector (BSR)

How to use renewable energy sources in polygeneration systems?

21

In an isotropic, homogeneous, linear, and stable environment, without an electric charge, the electric field follows the wave Eq. (2.10). !

r2 E 2 εμ

@2 E @E 50 2 μσ @t2 @t

(2.10)

The results of the solution of Eq. (3.32) are flat wave harmonics, which are shown in Eq. (2.11). !

!

! !

E 5 E0 exp½ jðωt 2 k : x Þ

(2.11)

!

where k is the wave vector that specifies the direction of wave propagation. One feature of the flat wave is that interacting can increase or decrease the electromagnetic fields. This process is called interference. An optical filter can produce interference with high throughput for a particular spectral region and high reflectivity outside the region. Interference can be achieved by controlling

the path  of the flat !

waves, the wave vector (k ) or the ambient refractive index n 5

λ! 2π k

. An interfer-

ence filter consists of several thin layers with different refractive indexes. The thickness of the layers is an adjustable parameter for reaching the desired path length. Interfering filters are generally composed of dielectric materials whose refractive indexes are true and constant, so their absorption rate can be neglected [2].

2.3

Solar power plant

The technologies for converting solar energy to thermal energy are thought to have great potential and the majority of the applications refer to medium to high temperatures. In other words, solar collectors are a particular type of heat exchanger that converts solar radiant energy into thermal energy. By absorbing the incoming solar radiation and converting it to heat, this device transfers heat to the operating fluid through the collectors. Solar collectors are classified into two types depending on whether they are stationary or mobile.

2.3.1 Stationary collectors The primary differences among solar energy collectors are their motion—stationary, single-axis tracking, and two-axis tracking—as well as their working temperature. These types of collectors do not follow the sun since they are set in place all the time. This category includes three different kinds of collectors [12]: 1. Flat-plate collector (FPC). 2. Stationary compound parabolic collector (CPC). 3. Evacuated tube collector (ETC).

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Hybrid Poly-generation Energy Systems

2.3.2 Concentrating collectors Concentrating collectors are typically known as parabolic trough solar collectors (PTC) concentrating solar power plants. This is primarily because PTC is a proven technology with a competitive price (about US$ 200 per square meter) and a respectable thermal performance [13]. In a typical PTC, the mirrors focus the inbound solar radiation onto the collector, which typically has a tiny contact area. The receiver comprises an absorber tube that distributes the absorbed heat to the working fluid and a concentric transparent cover, which is often made of glass. Stainless steel is commonly used for the absorber tube that is covered with a coating to improve thermal and optical efficiency (higher absorptivity and lower emissivity). The concentric translucent cover reduces heat leaks to the surrounding air, guards against absorber deterioration, and preserves the pressure differential between the cover and absorber tube. The vacuum cage is primarily utilized to reduce the losses in heat transfer by convection and to protect the selected surface from oxidation brought on by the absorber’s high-temperature operation. In order to maintain the vacuum pressure at the specified design ranges and prevent heat transfer between the tube and cover, sealing and metal bellows are placed at either end of the receiver. To increase the thermal efficiency of the PTC module and provide the necessary safety standards, chemical getters are applied in the vacuum zone to absorb the hydrogen that is produced by the heat transfer fluid (HTF). Fig. 2.9 shows the most common collector that is used for solar PGSs. In collectors with a concentration ratio greater than one, reflective surfaces are used to increase radiation over a small area. Concentrating the sun’s rays on a small surface is done in order to achieve high temperatures. Use flat concentrators to increase the number of direct beams emitted to the collector surface. Table 2.3 shows the characteristics of each collector, and Fig. 2.9 shows the types of static and concentrating collectors [12].

2.3.2.1 Governing equations of geometry It is considering that the concentration coefficient of the flat mirror is one and is obtained to calculate the concentration coefficient composed of the multiplication of this variable, and considering the assumption that the flat mirror is larger than the parabolic dish collector. The pattern of solar radiation affects how much energy from the sun is absorbed in the reception cavity in parabolic solar collectors. It is difficult to quickly acquire the radiation pattern on the inside surface of the receiving using the empirical system for a cavity with inversion symmetry and a parabolic collector. A mechanism like this ensures that the dish’s surface is perfectly parabolic (with no contour defects) and that the sun’s core beam strikes it parallel to the concentration axis. The realistic parabolic system is referred to as a “real” system even though it only approximates a genuine system since these two requirements cannot be satisfied in practice. A half-length cross-section of an ideal parabolic collector (Fig. 2.10A) and its actual counterpart (Fig. 2.10B), as well as its design, are shown in Fig. 2.10. D and f,

Figure 2.9 Types of solar collectors (A) flat plate collector, (A-a) illustration of a flat plate collector, (A-b) photo of cutting the upper part and riser of flat plate collector; (B) vacuum tube collector; (C) Linear parabolic collector; (D) Fresnel collector, (D-a) Fresnel lens collector, (D-b) Fresnel type linear parabolic collector; (E) parabolic dish collector; (F) Heliostat collector [12,14].

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Hybrid Poly-generation Energy Systems

Figure 2.9 (Continued).

respectively, are the collector diameter and focal lengths. The collector is described by an x, y, and z Cartesian coordinate system. The reference is at the top of the container, and the optical axis’ z-axis is also positioned there. On the surface of the real collector, at point P, with radius R, are the coordinates x, y, and z given in Eq. (2.12). These coordinates follow the equation of conic cut, which is: z5

x2 1 y2 4f

(2.12)

How to use renewable energy sources in polygeneration systems?

Figure 2.9 (Continued).

25

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Hybrid Poly-generation Energy Systems

Table 2.3 Specifications of collectors [12]. Parameter type

Collector

Attractor

Focus ratio

Temperature range index ( C)

Static

Flat plate collector Evacuated tube collector Compound parabolic collector Linear fresnel reflector Cylindrical trough collector Parabolic trough collector Parabolic dish reflector Heliostat field collector

Flat Flat Cylindrical Cylindrical Cylindrical

1 1 15 515 1040 1550

3080 50200 60240 60300 60250 60300

Cylindrical Pointwise Pointwise

1085 6002000 3001500

60400 1001500 1502000

Uniaxial tracking

Biaxial tracking

f

z

∆z

y

P’(y, z’)

P(y, z)

b

∆zmax

a

y

R

x

D/2

P, P’ (x, y)

Figure 2.10 Half-length collector: (A) Ideal parabolic collector. (B) Real collector [15].

How to use renewable energy sources in polygeneration systems?

27

The following analysis assumes relatively modest deviations from the ideal x and y coordinates of the locations on the collector surface in front of the z component. Furthermore, it is presumable that the divergence from the z component is more considerable the further they are from the focal axis. As a result, the true collector’s surface point P’ has the coordinates x, y, and z. The circumference of two circles with the same radius passes through these two places (Fig. 2.10). The ideal collector is represented by the first, and the level of the real collector by the second. If the maximum deviation on the component z is denoted by Δzmax then Δz for the point P’ is obtained as follows: Δz 5 Δzmax 3

2R D

(2.13)

Given that in Eq. (2.13), R is obtained as follows: R5

pffiffiffiffiffiffiffiffiffiffiffiffiffiffi x2 1 y2

(2.14)

From Eq. (2.3), it follows that for the component z the point P’ is calculated as follows: pffiffiffiffiffiffiffiffiffiffiffiffiffiffi x2 1 y2 z 5 z 1 Δzmax 3 D 0

2

(2.15)

Substituting the equation z from Eq. (2.1) into Eq. (2.3), the real collector equation is obtained as follows: x2 1 y2 z 5 1 2 3 Δzmax 3 4f 0

pffiffiffiffiffiffiffiffiffiffiffiffiffiffi x2 1 y2 D

(2.16)

It should be noted that Eq. (2.17) is used to calculate the collector diameter as follows: D 5 4 3 f 3 ðcscðrimÞ 2 cotðrimÞÞ

(2.17)

To investigate the radiation distribution within the receiving walls, we first examine Fig. 2.11, which shows the design of a real parabolic dish collector with diameter D and focal length f. The collector’s head is the source of the Cartesian coordinates system x, y, z where the z-axis is the same as the optical axis, b. A plate ξ is in a random position in the space above the collector; it can be, for example, in the line of one of the internal surfaces of the receiver. The typical unit vector nξ specifies its orientation relative to the Cartesian coordinate system. Point C is located on plane ξ. Its coordinates, which are removed from the figure for clarity, are xc, yc, and zc. When viewed from the ground to a parabolic dish, the solar disk is inside a radiant cone with an angle of 32 degrees. A radiant cone of this type has

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Hybrid Poly-generation Energy Systems

Ac

M

Rc [- plane

n[ n[ M C [

δ

’

nP

C

nK

n PM

32’

ne

16

f b

a

z

P dACol y

x D

Figure 2.11 Parabolic dish collector [16].

its central beam at an angle of 8 degrees (solar tracking error) to the optical axis of the collector, as shown in Fig. 2.11. The solar radiation is also reflected at point P on the collector, whose coordinates are xp, yp, and zp (again removed from the figure), and produces an elliptical image of the solar reflection on the ξ plane. A section perpendicular to the central beam through point C of the reflected beam cone has a circular cross-section with center M, radius Rc, and surface Ac. To continue the solution, it is appropriate to define the orientation of this cross-section by a unit vector nPM that goes from point P to point M and is perpendicular to the plane Ac. The line PM is between the points P and M shown in Fig. 2.11. Moreover, it is obtained as follows: PM 5 PC 3 cosζ

(2.18)

where ζ is the angle between the center beam and the line PC. Since the C points are considered to be within the elliptical region, the following criterion for the angle ζ is considered: 2160 # ζ # 160

How to use renewable energy sources in polygeneration systems?

29

The radius Rc of this circular section is obtained as follows: Rc 5 PM 3 tan160

(2.19)

According to Eqs. (2.18) and (2.19), it results that for the area Ac, this part is calculated as follows:  2 Ac 5 π 3 PC 3 cosζ 3 tan160

(2.20)

A small differential area of the dACol collector is assumed to exist on the xy surface and receives radiant energy as intense as s: _ 5 s 3 cosδ 3 dACol dQ

(2.21)

where s only involves direct contact of sunlight reaching the earth’s surface. That part of the energy that is emitted by the suspended particles as they pass through the atmosphere cannot be used and reduces the total radiation [17]. If the collector reflection coefficient for short-wavelength solar radiation is indicated by ρCol, Eq. (2.22) shows the amount of radiant energy considering the reflection coefficient: _ 5 ρCol 3 s 3 cosδ 3 dACol dQ

(2.22)

while we know that the sun is slightly brighter in its center than its surroundings [17], this effect is small and is not considered here. Therefore the intensity of distribution in the solar plane is homogeneous. The surface Ac, which is oriented perpendicular to the central ray of the reflected cone, is irradiated evenly by the point P of the collector with the following energy flux: q_ AC ;P 5

ρCol 3 s 3 cosδ dACol AC

(2.23)

The plane ξ and the area Ac are generally not parallel to each other. For example, their normal unit vectors nPM and nξ collide with an angle γ (Fig. 2.11). The intensity of radiation reflected at point P on the collector and radiation at point C perpendicular to the plate ξ is obtained from the following equation: q_ C;P 5 q_ AC ;P 3 cosγ

(2.24)

According to Eqs. (2.20) and (2.23), it follows that: q_ C;P 5

ρCol 3 s 3 cosδ 3 cosγ  2 dACol π 3 PC 3 cosζ 3 tan160

(2.25)

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Hybrid Poly-generation Energy Systems

From the size PC we get from Fig. 2.11 according to Eq. (2.26): PC 5

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðxP 2xC Þ2 1 ðyP 2yC Þ2 1 ðzP 2zC Þ2

(2.26)

where (xC, yC, zC) and (xP, yP, zP) are the coordinates of points P and C. To determine the angles γ and ζ in Eq. (2.24), the normal vectors nPM and nPC must first be obtained. The normal vector nPC in the direction of the line PC in the form of Eq. (2.26): nPC 5 2

1 ðxP 2 xC ; yP 2 yC ; zP 2 zC Þ PC

(2.27)

To obtain the normal vector nPM, following the radiation cones and the reflected radiation is necessary. By selecting the x and j axes from the coordinate system shown in Fig. 2.11. In that, the angle between the center beam of the incident radiation cone and the x-axis is equal to the angle between this ray and the y-axis, the unit vector (ne) in the direction of the central radiation descending through the following equation is obtained:  ne 5 nejx jnejy jnejz 5 ð 2 0:707 3 sinδj 2 0:707 3 sinδj 2 cosδÞ

(2.28)

The following formula can be used for the nPM unit vector in the direction of centrally reflected radiation [18]: 0

1 0 10 1 nPM;x ne;x 1 2 2n2k;x 2 2nk;x nk;y 2 2nk;x nk;z @ nPM;y A 5 @ ne;y A@ 2 2nk;x nk;y 1 2 2n2k;y 2 2nk;y nk;z A nPM;z ne;z 2 2nk;x nk;z 2 2nk;y nk;z 1 2 2n2k;z

(2.29)

where the components nk,x, nk,y, and nk,z of the normal vector nk are perpendicular to the actual surface of the collector at point P. These components can be defined for a level in the desired pattern, which by a system of components (x, y, z) are: 1

0 δz 2 δy

C  B 1 C nk 5 nk:x ; nk:y ; nk:z 5 B ; rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi @rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi δz 2 δz2 δz 2 δz2 δz 2 δz2 A 1 1 δx 1 δy 1 1 δx 1 δy 1 1 δx 1 δy δz 2 δx

(2.30) If Eq. (2.30), that is, the real collector equation, is added to Eq. (2.16) and z’ is used instead of z, the unit vector equation nk is obtained as follows: 0  B xP yP nk 5 nk;x ; nk;y ; nk;z 5 B @2 c ; 2 c ;

1  2c

1 4f

1 Δzffiffiffiffiffiffiffiffiffiffi max ffi 1 p 2 2 D

xP 1 yP

C C A

(2.31)

How to use renewable energy sources in polygeneration systems?

31

where x P and y P are the components x and y of point P and Δz max is the maximum collector surface error on the z-axis. The following equation obtains the value of c: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 1 u   1 x2P 1 y2p c5u t Δz 1 max ffi 4 3 4f 1 pffiffiffiffiffiffiffiffiffiffi 2 2

(2.32)

xP 1 yP

D

The components (nPM,x, nPM,y, nPM,z) of the unit vector nPM reflected in the direction of the central ray can be from Eq. (2.17) and with the help of Eqs. (2.28), (2.30) and Eq. (2.32) is obtained. The angle ζ can be obtained through Eq. (2.33): cosζ 5 nPM  nPC 5

1  nPM;x :ðxC 2 xP Þ 1 nPM;y :ðyC 2 yP Þ 1 nPM;z :ðzC 2 zP Þ PC (2.33)

Assume the normal vector of the unit nξ from the plane ξ as follows:  nξ 5 nξjx jnξjy jnξjz

(2.34)

By comparing Eq. (2.32), the angle γ is obtained as follows:  cosγ 5 nPM :nξ 5 nPM;x :nξ;x 1 nPM;y :nξ;y 1 nPM;z :nξ;z

(2.35)

Therefore the intensity of solar radiation q_ C:P is reflected from point P on the collector surface at point C on the plane ξ can be obtained with the help of Eq. (2.25). Of course, if the normal vector of a plate is known at a random point above the collector. Depending on the receiver, the vector of the unit receiving the radiation within the walls must be known. Determining the total intensity of solar radiation qc at point C requires integration on the collector surface. It should be noted that the receiver casts a shadow on the part of the collector surface. The diameter of this shadow is approximately equal to the outer diameter D0 of the receiver. If D is considered as the collector diameter, it is irradiated. Thus the effective surface is equal to: ACol:e 5

π 2 D 2 D20 4

(2.36)

From which the intensity of solar radiation q_ C:P at point c is obtained by considering Eq. (2.25) as follows: q_ C 5

ρCol 3 s 3 cosδ π 3 tan2 160

ð ACol:e



cosγ PC:cosζ

2 dACol

(2.37)

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Hybrid Poly-generation Energy Systems

The application in Eq. (2.37) is made numerically and the radiation on the parabolic plate can be calculated for each point on the inner surface of the receiver. The reflection coefficient ρCol of the entire collector surface is assumed to be uniform. Thus ρCol can be assumed to be constant for integration in Eq. (2.37). It should be noted that the Monte Carlo irradiation method was used to calculate the values provided. Also, one of the parameters for evaluating the characteristics of the solar receiver system is the concentration ratio, which is the ratio of the flux that occurs to the solar flux of the receiver. A high concentration ratio usually means a strong concentrator for concentrating solar radiation. When the focus ratio is calculated, the radiation input flux on the focal plane or receiver surface can be calculated. The results of the Monte Carlo radiation drawing code to calculate the concentration ratio of different cavity structures are compared with various existing computational methods models for calculating the concentration ratio [19]. Examining the elements of the different concentrator levels at the rc point and the receiver at the focal plane at the r point is shown in the figure below [20] (Fig. 2.12). It is the surface normal in the concentrator and focal plane rc nc and rn and O is the focal point. Therefore the angles are defined in the following order: δ1 5 +Orc r; θc 5 +rrc nc ; θ 5 +rc rn The focus ratio in r is defined as follows: ð 1 fcosðθÞcosðθc Þ CðrÞ 5 dAc I0 jr2rc j2

(2.38)

Ω



 2 f ðδÞ 5 I0 =ðπsin ψm δ # ψm 0 δ . ψm

Figure 2.12 Paragliding dish solar concentrator diagram.

(2.39)

How to use renewable energy sources in polygeneration systems?

33

In these equations, 1. 2. 3. 4. 5.

f (W/m2) denotes radiation intensity Ω denotes surface correlation clearly at the collector level I0 (W/m2) denotes solar flux ψm denotes the maximum solar angle of the screen dAc (m2) denotes collector surface element differential

Eq. (2.39) assumes that the flux of solar radiation as a spatial function on the solar panel is not different, meaning that no darkness is observed. Therefore it is possible to extend Eq. (2.39). Considering the relations presented regarding the calculation of the reflected heat in the receiver and the concentrator, it should be noted that the energy conservation equation has been used to calculate the heat, which is as follows: ρCp

@T 1 ρCp u:rT 1 r:q 5 Q:q 5 2 krT @t

(2.40)

2.3.2.2 Governing equations of heat transfer Regarding the initial conditions, it should be noted that to calculate the heat distribution at the collector level according to the continuity equation as well as the equations related to energy conservation, the initial value for ambient temperature is temperature. The walls are also assumed to be insulated. It should be noted that the outer walls also have a convective heat transfer medium.

@T k

5 hðTwall 2 TÞ (2.41) @x wall Permeable cavity walls are also assumed to calculate the amount of radiation in the receiver cavity.

2.4

Solar thermochemical reactors

Solar thermochemistry is a technology that has great potential for development. Optimizing the appropriate reactors for each chemical process is necessary to perform efficient and effective solar thermochemical reactions. Researchers working on solar chemistry have created and used many different types, sizes, and forms of solar reactors during the past 30 years. Solar thermochemistry’s goal is to employ radiation from the sun to produce hydrogen and solar fuels, which convert solar energy into long-term energy carriers and transportable energy, demonstrating that it is advantageous to make a significant contribution to addressing the requirements of high-energy systems. Solar reactors are specialized solar receivers used as heat sinks to conduct thermally activated chemical processes. The basic

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Hybrid Poly-generation Energy Systems

categorization of sun detectors, which may be separated into two categories of direct or indirect solar reactors, can thus be used to relate a preliminary categorization of solar reactors.

2.4.1 Indirect reactors Solar energy is used indirectly by reactor types for heating the reaction chamber’s opaque (nontransparent) exterior wall. The heat transfer rate delivered to the reactors by the wall current drives the solar thermochemical reaction. Indirect radiation reactors are often advancements over catalytic tubes, tubes with catalysts set in the interior that induce gas to stream through them [21]. Along the tube’s outside wall, the solar heat transfer is dispersed. For thermochemical implementations, another idea for an indirect reactor has indeed been advanced, including two-cavity reactors with a reaction chamber that is practically distant from the radiation receiver [22,23].

2.4.2 Direct reactors The direct reactants are irradiated and heated directly by the concentrating input radiation of the sun. Open or closed reactors employ a transparent window through which the sun’s rays enter the reaction chamber, and the reaction takes place. Because of the solar irradiation at the reactants’ surfaces, a higher operating temperature is expected in direct radiation reactors. The majority of sun reactors examined, particularly on a lab scale, outside of volumetric reactors, where processes have been thoroughly explored in terms of volumetric adsorption, are solar particle reactors [24]. Therefore the collection and design of experiments, structure, and experimental studies that have been performed on solar particle reactors so far.

2.4.3 Reactor particles classification Another way to classify solar thermochemical reactors is to classify them based on the reactor particles. Based on how long they must remain within the reactor if a transporter stream is present, and how they interact with other particles, the particles may be organized in various forms. Within the reaction zone, these indications directly impact mass and heat transmission. Therefore solar reactor particles can divide the types of reactors into fluidized bed reactors, stacked reactors, and entrained reactors and distinguish between the transport of gas and solid particles by the fluid flow [25], as shown in Table 2.4. Since the beginning of research on solar reactor particles, many authors have designed various specimens with specific properties and characteristics, and Villermaux’s [25] selection criteria have not been followed. However, the combination between the fluidized bed reactor and the accumulator and adsorption is what distinguishes these reactors. For instance, Puig-Arnavat et al. [26] worked on the effect of solar reactors used for the gasification of carbonated raw materials. Steinfeld [27] has also studied hydrogen from different thermochemical paths for different solar reactors.

How to use renewable energy sources in polygeneration systems?

35

Table 2.4 Classification of gas-solid reactors by Villermaux [25]. Stacked beds

Fixed Mobile Brewed oven

Fluidized or suspended beds

Entrained beds

Conveyor belt Blast-furnace Multistage Rotary kiln

Vibrated or pulsated Fluidized beds Circulating fluidized bed Blown bed Drooping bed Pneumatic transport Cyclone Falling particles

2.4.4 Entrained reactors While the Sandia National Laboratories (SNL) had begun to develop the initial sense of a solar solid central receiver (SPCR), chemical engineers from CARSENSIC, France, introduced the new concept of solar cyclonic reactor particles [28]. The continuous pyrolysis of sawdust and wood is in 1143K. The results show that such a reaction can be carried out on a large scale in a new type of reactor using concentrated solar energy. In 1991, Imhof et al. [29] proposed a new solar silicon reactor to develop gas-to-solid heat processes. In this regard, they looked for a simple method to combine some volumetric sensors, such as high-radiation absorbers, with a constant provision of reactants and removal of outputs. The silicon reactor shown in Fig. 2.13 consists of a short 30 cm long conical cavity that opens to the atmosphere, with a ceramic insulating layer covering the inside walls. Two circular cylinders forming a conical slit for gas passage make up the remaining portion of the hollow. The projected reactor for use in a solar furnace (surface) (17 KW) at the Paul Scherrer Institute (PSI). The authors studied the thermal decomposition of calcium carbonate as an example of a gassolid reaction. The solar reactor is a separate silicon to collect solid products and a projected heat exchanger to decrease the exhaust gas stream temperature. It is noted that at temperatures around 1300K, the reactants showed a higher degree of universal performance for the calcination process. This performance is regarding the ratio of the total absorbed energy and the thermal system to the energy occurring in the diaphragm. Most prototypes with the same configuration and shape but larger dimensions were built and tested on the McDonnell Douglas 55 kW dish [31]. A new solar chemical reactor was projected by Stanfield et al. [30], which was restricting the flow of fluid to the solar cavity receiver. Initially, a

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Hybrid Poly-generation Energy Systems

Figure 2.13 Schematic of a solar cyclonic reactor and the laboratory complex of Steinfeld [30].

prototype size of 5 kW, called Synmet, was fabricated and examined in a PSI solar furnace, thought to generate zinc metal simultaneously and start synthesizing gas from zinc oxide (ZnO) and natural gas. This includes stone cavities supplied with a quartz window. The reactants and cavity walls are heated by directed radiation that enters through a window. A flow of natural gas is used to carry ZnO particles, which are continually delivered into the cavity by a tangent inlet (NG). Reactants flow through the reactor from the back to the front. The temperature obtained exceeded 1600K and the thermochemical conversion from ZnO to Zn reached 90%. In the new design of the entire reactor, the window is continuously cooled, and an additional stream of gas is injected into the window tangentially and radially to keep the particles cool and clear. This reactor previously generated CaO and syngas by combining CaCO3 and CH4 modification processes [32]. In the SYNPET project, after several attempts, the main changes related to the reactive feeding system were finally a calcareous slurry generator [33,34]. The reactor tested in the high-flux solar furnace range of 1800K1300K had an efficiency of over 78% air conversion. The conversion efficiency of solar energy to chemical, without calculating the perceptible heat of 9%, and when the sensible heat is restored to produce steam and preheating, 20% was obtained. The reactor current then increased from 5 to 300 kW [35]. This increase in productivity raised solar to chemical energy to 24% due to the economic surface-to-volume ratio. The extensive conversion dependence on the particle size of the residence time inside the reactor was also realized. Fig. 2.14 shows both designs of the 5 kW reactors, which were proposed in 1998 and 2006.

How to use renewable energy sources in polygeneration systems?

37

Figure 2.14 Schematic of two 5 kW vortex reactors. Left: ZnO reaction prototype design with NG; Right: Steam-gas prototype design [31].

2.4.5 Fluidized bed reactors Unlike entrained reactors, fluidized bed reactors improve solid or gas contact and increase particle life. Such a feature may indicate a particular advantage for those chemical reactions associated with a slower kinetic mechanism. Primary fluid solar beds were introduced by Flament [36] in 1980, which consisted of a transparent silica tube (34 mm in diameter) between two metal brackets, which is directly exposed to concentrated sunlight (Fig. 2.15). The device was tested in a 2 kW solar furnace to heat refractories at 873K1573K and calcite calcification at 1123K. Graphite particles attach to calcite to increase the number of chemicals. Also, the thermochemical conversion to graphite had risen above 14%. Flament et al. [36] later found an increase of 50 kW in this reactor [37], then other concepts such as a fluidized bed with a transparent window at the top or a fluid annular reactor with opaque outer walls, which is also irradiated from above were suggested. It was straightforward and they suggested improving the prototype. Preliminary experiments on solar and nonsolar thermochemistry are often performed on a laboratory scale because of the ease of construction and their composition and function in a fluidized bed. In Fig. 2.16, a fluidized bed reactor shown in 2009 was used in successive applications of calcium oxide carbonation (CaO) and lime (CaO3) to capture carbon dioxide from the atmosphere [38]. The solar reactor also includes a quartz tube with an outer diameter of 25 mm and a height of 25 cm, which has a fluidized bed of reactive particles. A solar flux simulator with a power of more than 75 kW was used to irradiate the top of the tube, with the simulator focal design in the middle of the fluidized bed. Successful results of CO2 mass equilibrium were obtained after five consecutive cycles of nearly 99%, and the maximum temperature for the liming stage reached 1150K.

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Hybrid Poly-generation Energy Systems

Figure 2.15 Schematic of Flament et al. [30] reactor on the laboratory scale.

Gokan et al. [39] presented a solar fluid bed based on the idea that concentrated solar radiation passes through a transparent window above a reactor and directly heats the inner mobile bed of the reacting particles as can be seen in Fig. 2.16.

2.4.6 Stacked reactors The categorization described before states that stacked reactors should be separated into three categories: stationary, mobile, and rotational. Rotating reactors have often

How to use renewable energy sources in polygeneration systems?

39

Figure 2.16 Solar fluidized bed reactor test in high flux solar simulator and process set-up [38].

been seen to facilitate the transfer of mass and heat. However, designing, constructing, and running stationary reactors is simpler and more cost-effective. Particularly, prototypes used in laboratories to start chemical processes frequently have a fixed bed.

2.4.6.1 Fixed reactors Tremper is a stationary reactor made [40] for chemical kinetics of second-scale reactions at temperatures above 2100K (Fig. 2.17). The trimmer is made up of a number of components that are all contained in a quartz tube. A small hole in a copper base that has been water-cooled is used to hold the sample. These settings stop the sample from interacting with the support. The sample receives concentrated sunlight from the horizontal source through the 45 degree mirror. To stop the evaporated material from further evaporating, the carrier gas is angled toward the quartz tube, collected in a silica tube, and the gas flow is directed to the mirror. There is a defrost unit present. Flow pattern analysis determines the device’s overall performance as an active reservoir; therefore it is easy to derive the amount of gas emission from the gas analysis at the outlet line [41]. The greatest chemical transformation for iron oxide was 5%, however, for manganese oxide, it was 85%. Manganese was decreased in a trampoline. A fixed reactor to separate the precipitated zinc oxide can be used [42,43]. The outline of the reactor used is shown in Fig. 2.18. The reactor chamber is filled with stabilized ZrO2 insulation particles. In the center of the chamber, at the focal point of the solar furnace, the ZrO2 tank receives a ZnO pellet. The concentrated solar

40

Hybrid Poly-generation Energy Systems

Figure 2.17 Schematic of a thermometer reactor [40].

energy passes through the window and illuminates the opposite surface of ZnO. Several inert gas streams prevent product vapors from condensing on the window. The products of this inert gas and gas reaction flow continuously through the reactor to a flow and the heat exchanger in which the products are deposited. The temperature obtained in the above sample is 2100K. Some studies show the solar thermochemical cycle of H2OCO2 decomposition [44], which also uses cerium oxides. A new fixed solar reactor was used to perform the two steps involved in this cycle. The first is at high temperature and the second at low temperature. The general design of this solar device is shown in Fig. 2.19. It consists of an insulated thermal chamber receiver containing a porous series cylinder. The focused solar radiation enters through the window and spreads on the inner walls of the series. Radial reaction gases are poured into the cavity throughout the porous series, while the exhaust

How to use renewable energy sources in polygeneration systems?

41

Figure 2.18 Fixed bed reactor for ZnO decomposition [42].

gases of the products are discharged through the outlet duct at the end. Complete cycle feasibility and rapid and stable generation of H2 and CO were achieved by separating CO2 and H2O. The material proof was also shown over 50 thermal cycles. However, solar energy conversion efficiency to fuel was only 0.7 to 0.8%.

2.4.6.2 Mobile reactor One example of a mobile reactor is a reactor designed by [45], abbreviated as GRAFSTRR, designed, built, and tested to reduce zinc (Zn). The reactor with a quartz window and a schematic of a conical inverted surface on the reaction surface and a continuous decrease with the movement of the bed, in the direction of a thermochemical reaction at high temperature on the surface of the cavity, is exposed to the solar concentrator. The inner wall of the decomposition reaction chamber is made of fifteen pure aluminum trapezoidal tiles supported by three layers of porous insulating ceramic. They are located inside an inverted cone with a slope angle of 40 degrees. The zinc oxide solution feeds the surface of each tile individually and vibrates to form a moving bed reaction layer. After the decomposition reaction, the vapor of the zinc product is directed downwards into the outlet located in the center. This is because the fluid flow originates from the top of the source valve and connects to the outlet pipe through the reaction chamber. The reactor that was expected to operate in the range of 10 to 20 kW is shown in Fig. 2.20. Preliminary

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Hybrid Poly-generation Energy Systems

Concentrated Solar Radiation

Quartz Window

CPC Inlet Inlet

s Purge Ga H O, CO2 2

Alumina Insulation Porous Ceria Outlet Oxygen Evolution Half-Cycle Fuel Production Half-Cycle

Purge Gas, O2 H2, CO

Figure 2.19 Schematic of a proposed solar reactor for studying the H2O-CO2 solar thermochemical decomposition cycle [44].

experiments using a successful high-flux solar simulator showed the mechanical stability of the reactor and the initial system, that is, the stability of the moving particles in the fluid flow, the connection of the moving bed to the reaction surface, and the solid particle transfer and exit system. Temperatures between 1100K and 1900K appeared on the reaction surface. Rotating reactors are widely used in industrial processes such as liming, cement and gypsum production, the food industry, etc. The phenomenon of heat and mass that occurs in rotating reactors leads to chemical conversion. Other advantages include versatility, long service life, and low maintenance costs. However, an essential weakness of these reactors is the high energy demand of these reactors, which is mainly supplied by fossil fuels. If concentrated solar energy is used as the energy source, such a weakness can be overcome by integrating solar radiation with the heating reaction in the reactor.

How to use renewable energy sources in polygeneration systems?

43

Figure 2.20 GRAFSTRR reactor [45].

A type of 10 kW rotating solar reactor is designed by [46] to reduce the heat of zinc oxide and oxygen. Fig. 2.11 shows a design of a prototype called a ROCA. They came up with the idea of designing a reactor that would take into account the chemical reaction and use materials with low thermal capacity that could withstand thermal shocks. The main component of the reactor was a conical rotary receiver chamber made of internal steel consisting of a small valve that allowed concentrated solar energy to enter through the window. The window had water cooling and was included in the concentric conical cover. The reactants, the zinc oxide particles, are continuously fed into a rotating chamber with a screw-like solution located at the rear of the reactor. The centrifugal acceleration of the zinc oxide solution causes the wall to form a thick layer of zinc oxide that insulates and reduces the thermal load in the inner wall chamber. The gaseous products are expelled by a continuous stream of inlet gas, which enters the chamber tangential receiver from the front and exits the extinguisher with an outlet. This cleaning keeps the window cool and free of particles and dense gases. Thus zinc oxide is used simultaneously as an absorber of solar radiation, thermal insulation, and chemical reactants. Experimental tests have concluded that the reactor is able to operate at temperatures close to 2000K with a uniform distribution along the chamber. They also proposed low thermal inertia and high resistance to thermal shocks. In addition, some defects in the design of the ROCA were detected and an operation was attempted to propose a new reactor with equal power and force called ZIRRUS [47]. A rotating chamber also forced the centrifugal acceleration of the reactive zinc oxide to cover the chamber. Unlike ROCA, where zinc oxide was both a reactant and an insulator, ZIRRUS separated the two functions. The ZIRRUS chamber is therefore made of an inner wall that is ineffective against gas emission but is able to operate at the decomposition temperature of zinc oxide in a corrosive medium containing zinc oxide, gaseous zinc, and oxygen. A layer of zinc oxide of the desired thickness is placed inside the feeder and connects to the outside of the chamber. The back of the chamber was mainly

44

Hybrid Poly-generation Energy Systems

Figure 2.21 Rotating reactor designed to reduce ZnO [48].

a high-temperature resistance insulator, and neutral (inert) gas flowed through the reactor window, extinguishing the products toward the device. However, the neutral gas in ZIRRUS was heated to a temperature above the zinc condensate before entering the chamber. Gas emission through ROCA walls [46] was prevented by using a concentrated gas cavity of HFO2. The gaseous products flowed between the feeder and the rotating wall of the water-cooling cylinder. Experiments performed on the PSI solar panel resulted in successful results. The conversion of zinc oxide to zinc oxide was more than 90% compared to the 35% result obtained in ROCA. A chamber temperature of 1900K was measured. More than 80% of the products were modified after each test, and about 20% were regenerated in ROCA. Successful reduction of zinc oxide was demonstrated in subsequent experiments. In addition, the reactor was on a scale of more than 100 kW and was tested in the Odeillo solar orbit (one million watts) [49]. For large-scale prototypes, mechanical and thermal resistance was shown in [48]. ZIRRUS is depicted in Fig. 2.21. The cell receiving solar radiation and the reduced materials are designed with a rotating cold water chamber 30 mm in diameter and a hemispherical glass surrounding the window [48]. The cell is filled with a ceramic cylinder (i.e., zirconia) surrounded by an insulating layer. First, the insulation chamber is heated until it reaches a thermally stable state. Therefore the reacting particles are continuously fed from inside the chamber by a screw feeder located at the rear of the reactor along the horizontal axis. Solid particles are mainly heated by radiation from the hot walls of the chamber by direct solar radiation entering the valve. Many studies have been done on the quality of materials, as well as zinc oxide reduction tests. Zinc was generated and recycled significantly with high purity, and the reactor showed a satisfactory thermal response equal to that of concentrated solar energy. Separation performance was reported to be 78%. The chamber temperature reached above 1500oC.

2.4.7 Solar reactors governing equations The solar reactor is one of the main components of a solar fuel production system in which the thermochemical cycling reaction occurs. As explained in detail in the previous chapter, the thermochemical cycle is the design heart, the basis of heat calculations, and other system parts. According to the information presented in the previous chapter,

How to use renewable energy sources in polygeneration systems?

45

the thermochemical cycle selected for the system is the zinc-oxide/zinc (Zn/ZnO) cycle. As mentioned before, the ZnO/Zn cycle is two-stage and its thermochemical reaction is performed according to the following equation [50]: ZnOðsÞ

solar thermal energy

!

ZnðgÞ 1

1 kJ O2 ðgÞ :ΔH 5 456 :T 5 170022000o C 2 mol (2.42)

ZnðsÞ 1 H2 OðgÞ ! H2ðgÞ 1 ZnOðsÞ :ΔH 5 2 104

kJ :T 5 3502400o C mol

(2.43)

As shown in (2.42), the solar reactor generates a reaction to generate Zn and oxygen (O2), which is an endothermic reaction. Therefore solar radiation generates the heat required for the reaction in a solar reactor. It should be noted that, according to the purpose of investigating the amount of Zn generated to investigate in the hydrogen generation reactor, this model is considered reliable. All transfer phenomena are assumed, which has doubled the importance of the model. According to the above, the geometry, the assumptions and equations of the reactor model, the boundary conditions and the initial value for the heat distribution, and the amount of Zn and O2 generated are described in the next sections. A typically applied geometry for the solar reactor is shown in Fig. 2.22. The heat transfer relations of solar reactor components for mentioned materials in Fig. 2.23 are presented in Tables 2.5 and 2.6. In the solar reactor section, a three-dimensional model includes three sections of heat transfer, mass transfer, and fluid flow can be used. To investigate the heat transfer of the system, the energy conservation equation for the porous substrate can be written as a convection-diffusion equation considering the source [51]:     @T  1 ε ρCp f u:rT 5 r:ðkcond rTÞ 1 r:qrad 1 q’’chem ε ρCp f 1 ð1 2 εÞ ρCp s @t (2.44) Here the subtitles f and s show the liquid and solid phases, respectively, and also ε is porosity, ρ is material density, Cp is heat capacity, t is time, T is temperature, u is

Figure 2.22 Schematic of a solar reactor modeled on a comsol [51].

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Hybrid Poly-generation Energy Systems

A

C-1

D-1

B-1 B-2

C-2

D-2 D-1

E

C-1

C-2

D-2

C-1 D-1 C-1

E B-2 B-1

D-1 C-2

D-2

A

Figure 2.23 Solar reactor on x-r plane; (A) Microprose insulation layer; (B) Alumina buster insulation layer M-15, (B-1) Internal insulation layer, (B-2) Central insulation layer; (C) Alumina buster insulation layer M-35, (C-1) Inner front surface, (C-2) rear surface; (D) insulating layer of alumina buster blanket, (D-1) insulating layer of absorbent tubes, (D-2) outer surface; (E) porous bed [5,51].

fluid velocity, kcond is effective thermal conductivity of the porous structure, qbed is flux Radiant heat, and q”chem is the rate of heat during the reaction. It should be noted that the porosity in the pipes is assumed to be ε 5 0.776 [51]. Eq. (2.45) also assumes the equation of energy in mean volume with local heat equilibrium. In addition, it is presented for the solar thermal reaction chemical reaction to generate Zn and oxygen [52]. q’’chem 5 2 r’’ΔHr ðTÞ

(2.45)

In Eq. (3.33), the negative sign indicates that the reaction is superheated, r” is the reaction rate, and ΔHr(T) is the enthalpy of the reaction, which is a function of temperature. It is used to calculate the enthalpy of the reaction [52].   ΔHr ðTÞ 5 5:96 3 106 2 161:3T 2 2:66 3 1022 T2 J:kg21

(2.46)

Regarding the mass transfer equation, the mass conservation equation and the concentration changes in the system are calculated using Fick’s law [43]:
 @cj @ @ εcj 2 Deff;j 2 vg cj 5 Rj @t @x @x

(2.47)

Table 2.5 Heat conduction of solar reactor components. Value (W/mK)

Symbol

Material

kcond =kf 5 ðks =kf Þ0:28020:757logε20:057logðks =kf Þ 4:2 3 1023 1 5:6 3 1025 T 2 2:6 3 1028 T2 1 1:1 3 10211 T3 2 1:6 3 10215 T4 6 23 5:5 1 34:5e23:3 3 10 ðT2273:15Þ 22 2:2024 3 10 1 1:3924 3 1024 T 1 5:3512 3 1029 T2 1:1685 3 1021 2 1:7636 3 1024 T 1 1:5032 3 1027 T2  0:083; T # 800K 2 2:435 3 1021 1 3:607 3 1024 T 1 1:25 3 1028 T2 ; T . 800K 9:1640 3 1024 1 9:0320 3 1025 T 2 1:1810 3 1027 T2 1 6:1469 3 10211 T3

kcond

Porous substrate

kf kS ktube kins،1 kins،2 kins،3

Argon (reference gas during vacuum) Zn Aluminum pipes [53] Insulation material 1 [54] Insulation material 2 [54] Insulation material 3 [54]

kins،4

Insulated material 4 [52]

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Hybrid Poly-generation Energy Systems

Table 2.6 Heat capacity of solar reactor components. Value (J/kgK)

Symbol

Material

299:86957 1 2:69766 3 1021 T 2 1:271 3 1024 T3 1:04 3 103 1 1:74 3 1021 T 2 2:80 3 107 =T2

cp:bed cp:tube

447:6996 1 1:5987T 2 1:3797 3 1023 T2 1 4:0 3 1027 T3

cp:ins123

800.0

cp:ins4

Porous substrate Aluminum pipes [53] Insulation material 13 [54] Insulated material 4 [52]

In Eq. (3.47), Cj represents the concentration, and j represents the gases Zn and O2. Deff, j indicates the molecular diffusion coefficient of components j and ε indicates porosity, Rj also indicates the material transfer as well as the material production equation. To calculate the reaction rate, r”, Eq. (3.36) is also presented [52].   2 Ea r’’ 5 k0 exp (2.48) RT In this equation, Ea is the reaction activation energy, k0 is the reaction factor, T is the temperature, and R is the constant of the gases. Regarding the boundary conditions for the energy equation, it should be noted that due to the high temperature of the system, surface-to-surface radiant heat transfer is also considered. Regarding the boundary condition in the outer walls to realize the problem with the outside environment, it performs heat transfer, so for the outer walls reactor Eq. (2.49) will be valid: hðT 2 Twall Þ 5 k

@T

@r r5rwall

(2.49)

It should be noted that since the heat transfer from the surfaces is in the form of natural convection, Eq. (2.38) is used to calculate h. 8 k 1=4 > > 0:5RaL ; if T . Twall ; and 104 # RaL 107 > > L > > > >

> > > > k 1=4 > 5 10 > > : L 0:2RaL ; if T # Twall ; and 10 # RaL 10

(2.50)

In this equation, RaL, Rayleigh number, and L is the length of the medium for heat transfer and k is the coefficient of thermal conductivity. It should be noted that the initial value of the heat transfer values of the walls is the ambient temperature.

How to use renewable energy sources in polygeneration systems?

2.5

49

Wind energy-based polygeneration systems

The pollution caused by fossil resources and the eventual degradation of their supplies makes it challenging to minimize emissions by using alternative renewable resources. Wind energy has attracted much consideration in industrialized nations compared to other renewable sources because of its superior performance. Wind energy is expanding the quickest globally because of benefits including being green, having little safety concerns, and using sustainable resources. The amount of wind power being installed globally has rapidly increased as a trustworthy and sustainable source of electricity. The wind turbine is the popular method of harvesting wind energy to generate power. These machines may typically be divided into vertical-axis turbines and horizontal-axis turbines. With the emergence of technologies for converting fossil fuel energy into mechanical and electrical energy, it seems that the era of converting wind energy into useful energy is over. However, in the late 1960s, interest in wind energy gradually increased, so the capacity to generate energy from wind increased five times, and it was a sign of a successful resurgence. The 1990s are remarkable for the construction of megawatts-size wind turbines and the stabilization of wind turbine manufacturing, and the development of offshore wind power plants. This process continued at the beginning of the 21st century. The developed countries strengthened and supported this approach to achieve domestic sustainable energy sources and reduce the emission of pollutants [55]. Five main drivers for this return can be listed. The most important factor was the need for an inexhaustible energy source and the need to preserve the environment. The scarcity of fossil fuel resources and the destructive effects of their use on the environment provided the necessary motivation to find an alternative energy source. The second factor was the potential of using wind energy. The wind is everywhere, and it has the right conditions to convert it into useful energy in many places. The wind had a long history of being used for windmills, driving sailing ships, and producing mechanical energy. Thus it could be considered again. The third factor was technological advancements in various fields, which made wind energy affordable and attractive. These three factors were the drivers of the revival of wind energy, but two other factors strengthened this stream; the approach of new methods of using wind and sovereign supports. The founders of the new approach were a group of scientists such as Albert Betz, Poul la Cour, Percy Thomas, and Palmer Putnam, followed by E. W. Golding, Johannes Juul, Ulrich Hutter, and William Heronemus. Moreover, many others soon followed suit. In the beginning, wind energy was more expensive than fossil fuel energy; therefore the support of governments was necessary to allocate costs for developing wind technologies. Support was also needed to connect the wind power grid to the fossil fuel power grid and provide motivations to accelerate the expansion of wind energy use. This type of support was first formed in the United States, Denmark, and Germany, and then many countries made policies in this field. This section aims to introduce PGSs that utilize wind energy and answer questions such as: How can wind energy, combined with geothermal energy, be used to

50

Hybrid Poly-generation Energy Systems

produce hydrogen chloride? By combining solar photovoltaic and wind energy, how can you provide the necessary energy for a water desalination unit? How is the energy and exergy analysis of a polygeneration system in which the wind turbine is part of its cycle configuration done? How can oxygen be produced by combining heliostat and wind turbines?

2.6

Hydrogen production by a multipurpose cycle consisting of wind turbine and heliostats

Fig. 2.24 shows the process flow diagram of a multiproduct plant that simultaneously generates electricity, produces useful products such as oxygen and hydrogen, desalinizes water, and provides cooling. This plant has seven main parts: solar area, wind area, desalination section, two low-temperature Rankine cycles, water electrolysis section, and cooling absorption cycle. Streams 8 and 9 exchange solar thermal energy with the Rankine cycle boiler, and superheat steam 11 enters turbine 1. The temperature of extraction stream 12 increases in the boiler and enters turbine two along with stream 13 which leaves

Sea Water

Fresh Water

26 PEM Electrolyzer 2b

Water

2

O2

23 28

SC 2

HEX 1

HEX 2

Electricity

29

27

3

CPWH 2

35

36 11

Turb 2

Turb 1

13 12

9

SC 1

18

8

16 7

34

ST 2

Qcond

22

37

20 ST 1

P1

38

19

21

Water in

V1

47

Evaporator

S. EX 45

48

P4

42

17

39

Gen 46

Absorption Cycle

P2

33

40

Hot Water Out

32

OFWH

41

15

CFWH 1

30

Condenser

Hot Water Chamber

10

P3

14

Bioler

31

Turb 3

5

H2

1

Rejected Brine Water

24

4

Electricity Wind Turbine

25 Thermal Desalinaon

43

44

49 Qabs

Absorber

6 Qevap

Heliostats

Figure 2.24 Process flow diagram of wind polygeneration plant [56].

How to use renewable energy sources in polygeneration systems?

51

turbine 1. The generated electricity three is integrated with the electricity generated by wind turbines. The other extraction stream 14 enters the heat exchanger HEX 1 and preheats the feed water of the steam boiler by stream 15 after passing through the heat exchanger 1. The Rankine cycle condenser is replaced by a hot water chamber that provides the hot-water stream of 22 and the feed-water stream of 19. Wind turbines produce electricity that can be used in various applications. The current of electricity 2 enters a PEM electrolyzer that splits water into useful products of oxygen and hydrogen. Produced hydrogen can be used to fuel hydrogen vehicles. The thermal desalination unit needs heat energy which is provided by heat exchanger HEX1 through the heat exchanging of streams 23 and 24. The noteworthy point is that the boiler of the second Rankine cycle has been replaced by the HEX 1. Stream 28 is used as the driver of turbine 3, and stream 31 enters the heat exchanger 2. HEX 2 is typically the second Rankine steam cycle condenser in the SC2 area of Fig. 2.24. Streams 29 and 30 are the extraction streams of turbine 3 that preheat HEX 1 feed water. Moreover, stream 31 enters HEX 2 to heat stream 38. The absorption cycle generator needs heat and the necessary heat is provided by stream 38.

2.7

Conceptual configuration of using wind energy and geothermal energy to produce hydrogen chloride

Fig. 2.25 shows the block flow diagram of the hybridization of wind turbines and a geothermal energy source. Accordingly, three main systems can be seen: (1) a thermal cracking of ethylene dichloride (EDC) unit to produce HCL, (2) An organic

Figure 2.25 Block flow diagram of the integrated EDC cracking with the wind turbine, geothermal HTHP, and ORC [57].

52

Hybrid Poly-generation Energy Systems

Rankine cycle (ORC) to use heat duty and produce electricity, and (3) a renewablebased power plant that consists of wind turbines system and a geothermal hightemperature heat pump (HTHP) system to consume electricity and provide heat duty. In the configuration shown, the HTHP cycle consumes all the heat from the geothermal reservoir and provides the necessary thermal energy for the EDC cracking unit. Also, the heat energy for the ORC system is provided by the HTHP cycle. Wind turbines and the ORC system provide the necessary electrical energy for the HTHP cycle and the EDC cracking unit. A furnace usually provides the heat for the EDC cracking units; however, the geothermal power plant provides the heat in this configuration. The HCL can produce high-purity hydrogen that may be used as fuel for fuel cell-based power plants [58].

2.8

Multipurpose combinations of wind and solar energy for power and refrigeration generation, energy storage, water desalination, food drying, and water electrolysis

Global energy consumption is on the rise and fossil energy sources are decreasing. More importantly, fossil fuel consumption causes many environmental problems; therefore more legal restrictions are imposed yearly to reduce fossil energy sources’ use [59]. Therefore developing energy systems based on renewable energy sources for the domestic section has been the focus of attention. Fig. 2.26 shows the schematic of an integrated energy system that combines wind and solar energy to supply electricity for a domestic area. As can be seen, wind turbines and solar photovoltaic cells provide electricity for a PEM electrolyzer that splits water into oxygen and hydrogen. Produced hydrogen and oxygen feed a fuel cell system that produces fresh water, electricity, and heat needed for the residential section. The three main characteristics of a multiproduction system are resources, configuration, and uses of its outputs [60]. A system with renewable resources must have stable production in different weather conditions. Moreover, it must have a variety of outputs to meet the needs of the residential area. Fig. 2.27 shows another novel configuration using wind and solar energy to meet the domestic area’s cold and heat duty needs during day and night and cloudy weather. Accordingly, seawater stream 11 is desalinated in the desalination unit, and on its way, it heats the airflow 18 of the food dryer in the HX3 heat exchanger. Then, in the flow divider, a part of it is consumed as fresh water in Stream 21, and another part enters the electrolysis unit with Stream 26. The electrolyzer receives the necessary energy from the electrical current 29 from wind turbines. The produced oxygen and hydrogen in the electrolysis unit are stored in H2 and O2 storage tanks and will be burned in the H2-oxy combustor under unfavorable atmospheric conditions to provide fresh water stream 44 and the necessary heat for the Rankine cycle in Stream 42. The heat energy required by the Rankine cycle in normal

How to use renewable energy sources in polygeneration systems?

53

Figure 2.26 Block flow diagram of the integrated wind-solar system for domestic uses [61].

conditions is provided by the solar energy stored in thermal energy storage by Stream 38. The Rankine cycle condenser for cooling Stream 7 is a heat exchanger that exchanges heat with the water desalination unit. A refrigeration cycle that takes its energy from the electrical current 30 of wind turbines cools the space of 37 and its condenser is used to heat Stream 35 of the residential unit (Fig. 2.27).

2.9

CO2 capturing using wind energy in a multiproduction energy system

CO2 has a 60% share of global warming and is, therefore one of the main culprits [62]. Systems that can absorb carbon dioxide and not produce it themselves will greatly mitigate global warming. Fig. 2.28 depicts an integrated configuration for CO2 capturing and liquefying it. This configuration has five main components that

54

Hybrid Poly-generation Energy Systems

Figure 2.27 A multitask polygeneration systems can produce in various weather conditions, day and night [63].

together purify a factory flue gas. Accordingly, the sun shines on the parabolic trough collectors, and the resulting heat duty is transferred to the carbon dioxide removal system and absorption refrigeration cycle (ARC). Wind turbines and a low-temperature Kalina cycle also provide the necessary electrical energy for the carbon removal system and ARC. The heat duty of the Kalina power cycle boiler is also provided by the heat resulting from the chemical reactions of the carbon removal system. As can be seen, a significant amount of carbon dioxide is absorbed and liquefied, and no more carbon dioxide is produced (Fig. 2.28). Wind energy has come a long way historically, but there is still a long list of prospects [64]. Restrictions on using carbon-based fossil fuels and Ukraine’s postcrisis energy policy have provided many opportunities for research [65]. There are many creative combinations of wind energy-based systems with other energy systems and the list of hybrid systems is added daily. The stability of supply in different weather conditions and the production of different products should focus on developing wind-energy PGSs. Moreover, adding CO2-capturing capability is a significant competitive advantage.

2.10

Geothermal energy and polygeneration systems

Renewable energy sources are increasingly recognized as the best option in current years, with a bright outlook predicted for these energy sources. Geothermal energy has become one of the most dependable green energy sources that might reduce the

How to use renewable energy sources in polygeneration systems?

55

Figure 2.28 A CO2-capturing wind/solar-based polygeneration systems [66].

quantity of GES emissions in power generation. The International Energy Agency estimates that if the resources are fully used, geothermal energy might provide 3.5% of global energy consumption by the year 2050. This renewable energy source is suitable for accomplishing thermal demands such as solar or cryogenic absorption cycles and/or providing the required power demands of a hyper polygeneration system (HPGS). In the following items, we can see the implemented geothermal system as a part of the HPGS.

2.10.1 Application of HPGSs ORC combination with geothermal The fact that geothermal energy is conserved at levels of up to 3000 m is noteworthy. The geothermal heat sources’ temperature is between 50 C and 350 C, and 70% of this enormous natural energy is low-enthalpy, water-dominated resources with temperatures under 150 C. Geothermal wells are bored into a geothermal resource, and geothermal liquid is transported to the surface to use hydrothermal resources to generate electricity. Using turbines, geothermal power plants transform the heat into energy. This energy source is becoming appealing for forgoing investigations due to its benefits, including the absence of environmental contaminants, sustainable energy sources, nontoxic (steam or hot water) operational streams, and

56

Hybrid Poly-generation Energy Systems

Figure 2.29 The simple ORC [67].

the capacity to be employed as a heat-free source. Geothermal energy’s accessible temperature serves as a heat sink for some developed Rankine cycles, for example, ORC. The ORC were created as a result to generate power at low temperatures as well (about 150 C160 C). Fig. 2.29 shows the fundamental nonregenerative ORC. The cycle displayed consists of a heat recovery unit (HRS), a vapor turbine, a condenser, and a pump. The HRS is a heat exchanger that transfers heat from a geothermal fluid to an organic flow, the operational stream in the ORC. The organic working fluid enters the HRS as a subcooled liquid (p2) and exits as a superheated vapor (p3). The economizer, evaporator, and superheater are the three compartments that make up the HRS. The heat exchanger portion, known as the economizer, converts the subcooled liquid into a saturated liquid (p21). The heat exchanger component, the economizer, transforms the subcooled stream into a saturated fluid (p21). The superheater creates a superheated vapor, whereas the economizer is in charge of a vaporization enthalpy exchange to generate the saturated vapor (p22). The quantity of superheating that is typically carried out is minimal; nevertheless, this variable relies on the operating environment and the working stream. More particularly, the positive sign of the T-s saturation curve of the dry operational stream indicates that superheating is neither useful nor only in modest quantities (5K to 10K) as can be seen in Fig. 2.30 [68]. The superheated vapor is released to a reduced pressure (state 4) in the vapor turbine well after HRS to generate productive work. The electric generator transforms the work into power based on the First Law of Thermodynamics. The condenser is to reject the heat from the surrounding air. The organic flow is a saturated fluid at reduced pressure near the condenser’s exit (state 1). The pump receives the saturated liquid and boosts the pressure while just requiring a minimal quantity of

How to use renewable energy sources in polygeneration systems?

57

Figure 2.30 Temperature-specific entropy (T-s) of ORC [69].

Figure 2.31 The process of the heat recovery system as a function of temperature and heat flux (Q-T) [69].

work. This action completes the process. Fig. 2.31 describes the heat transfer in the HRS. The pinch-point, which refers to the lowest temperature differential between the hot side and the organic operational stream, is typically set to range from 5 K and 20 K [70].

58

Hybrid Poly-generation Energy Systems

The fundamental equations for the mathematical explanation of the described simple ORC are shown below. The 1st Law for ORC for a control volume is used to estimate the input heating in the HRS (Qin): Qin 5 morcUðh3 2 h2 Þ

(2.51)

2.10.1.1 Solar collector-geothermal heat pump Heat pumps effectively and sustainably transfer thermal energy between two sources of heat. A kind of heat pump called a geothermal HTHP delivers heat from the geothermal source to a heat sink. The vapor-compression cryogenic cycle serves as the structural foundation of a heat pump system. Typically, the heat pump cycle operational stream works as a refrigerant. In addition, a heat pump cycle with internal heat exchangers can supply the necessary internal thermal energy for the system, such as that needed by the evaporator in an ORC. The main parts of the heat pump cycle are the evaporator, compressor, internal heat exchanger, condenser, and expansion valve. Utilizing geothermal heat pumps has several drawbacks, such as the subsurface heat exchangers losing heat, which might result in a drop in the heat pump’s coefficient of performance (COP) over time and when a heavier thermal demand than usual is needed. Geothermal heat pumps’ high initial price and reduced dependability when used alone are further drawbacks. Combining geothermal heat pumps with thermal solar collectors can address several of these issues. For instance, as shown in Fig. 2.32, a heated flow from the tank will be processed into the diverter of 15. The flow can then be utilized to heat the greenhouse

1

Green house

11 16 9

21

2

22

18 8

19 14

13

16

On /off On /off

10

12

17

Ground level

20

3

6

5

7

23

1: solar collectors 2: storage tank 3: ground heat exchanger 4: evaporator 5: condensor 6: compressor

7: expansion value 8: internal exchanger 9,10,11,12: pump 13,14,15,16: diverting value 17,18,19,20: mixing value 21,22,23: temperature controller

Figure 2.32 A conceptual design of a combined geothermal heat pump with a solar collector [71].

How to use renewable energy sources in polygeneration systems?

59

or enter the diverter of 13 on a combined basis. In the diverter of 13, the flow is utilized to either recuperate heat from the ground around the heat exchanger during nonheating times and/or to preheat the flow approaching the heat pump’s evaporator up to 12 degrees. Controller 13 determines the efficiency of diverter 15 by taking into account the temperature of the fluid exiting mixer number 17, which is a critical aspect to remember because thermal controllers generally specify how valves should operate. However, the function of diverters 14 and 16 is identical to that of diverter 15, and the overall efficiency of pumps 10 and 12 and diverter 13 may be determined by an on/off switch. The COP, which is the proportion between the heat energy obtained in the condensation and the energy utilized by the compression, is a crucial determinant of how efficient a heat pump cycle is [72]: COP 5

Capheating P_ heating

(2.52)

2.10.1.2 LNG heat sink: carbon dioxide power cycle driven by geothermal energy with liquefied natural gas as its heat sink The schematic diagram of the transcritical CO2 geothermal system for power generation using Liquified natural gas cold energy is shown in Fig. 2.33. The evaporator CO2 Turbine 3

4 Vapor Generator

c2

c1 LNG Tank

CO2 Cycle Condenser b2 c3 Chilled Water

g1 1 Geothermal Heat Source g2

2 c4

Pump

Ambient Water b1

NG Turbine c5

Figure 2.33 Conceptual design of carbon dioxide power cycle driven by geothermal energy with liquefied natural gas as its heat sink [73].

60

Hybrid Poly-generation Energy Systems

received its water supply from the profound geothermal well. The geothermal water is first recirculated into the earth, releasing significant heat. The supporting pump I was utilized to increase the operating pressure of the liquid CO2 from the condensation to the supercritical stage. This liquid CO2 then flows in the evaporator to absorb heat and create high-temperature and high-pressure supercritical CO2. Utilization of a CO2 expander, the supercritical CO2 expanded, driving a power supply to generate electricity. Pump II first inflated the LNG in the storage tank to raise the LNG pressure to a supercritical pressure. LNG functions as a heat sink in the condenser to decrease the operating temperature of the CO2 expander exhaust gas. It continuously enters the heater and absorbs heat from the ambient water to raise the temperature of the heated natural gas. A natural gas expander uses high-pressure, heated natural gas that expands to generate electricity. Finally, the natural gas drained from the natural gas expander is forwarded to the natural gas distribution center to supply natural gas to the users. Enthalpy versus temperature for the geothermal power generating system is plotted in Fig. 2.34. The important benefit of this procedure is using LNG’s cold energy to lower the back-pressure of the CO2 expander since doing so increases the CO2 expander’s electrical output. Additionally, the output power of the entire system was increased by the natural gas heated by the local water expanding by the NG expander. The thermodynamic characterization of the flows changes the heat transfer procedure, particularly close to the critical point, which may alter the heat transfer coefficient calculation. As a result, the complete heat transfer process has to be

T

g1 3 g2 2

b1

2s b2 4 4s

1

c4

c3 c2s

c2

c5s

c5

c1

S Figure 2.34 Temperatureentropy graphs of Carbon dioxide power cycle driven by geothermal energy with liquefied natural gas as its heat sink [73].

How to use renewable energy sources in polygeneration systems?

61

discretized into several parts. It is important to note that it is possible to assume that each subsection’s stream characteristics are constant. The heat exchangers are built using the mean temperature difference method and are of the tube-and-shell variety. The following is an expression for the heat transfer formula for each subdivision: Qi 5 Ui Ai Δti

(2.53)

Δti is mean temperature and the logarithmic mean temperature difference as follows: Δti 5

ðTf 2;i11 2 Tf 1;i11 Þ 2 ðTf 2;i 2 Tf 1;i Þ Tf 2;i11 2 Tf 1;i11 Tf 2;i 2 Tf 1;i Þ

lnð

(2.54)

The total heat transfer coefficient can be expressed as below: 1 1 δ 1 5 1 1 Ui Hf 1 λ Hf 2

(2.55)

The influence of the thermal resistance of pipe walls is neglected to make them more accessible in the computation. Consequently, the Eq. (2.56) can be written as follows: 1 1 1 5 1 Ui Hf 1 Hf 2

(2.56)

Water comes through the shell of the evaporator while supercritical CO2 flows through the tube. The following uses the semiempirical definition of the coefficient of heat transfer of the geothermal water side presented by [74]: Uw 5 0:023Re0:8 Pr0:3

kw D

(2.57)

The semiempirical formulation of heat transfer coefficient of the CO2 side, which was proposed by [75], is employed as below: " #
0:35

 fbulk kCO2 Cp Kbulk 20:33 μbulk 20:11 8 Rebulk Pr UCO2 5 D 12:7ðfbulk Þ0:5 ðPr23 2 1Þ 1 1:07 Cpbulk Kwall μwall 8 (2.58) It ought to be noted that the heater’s primary premise is that ambient water runs in the shell and supercritical natural gas streams in the tube. The following are the two semi-empirical formulas for the heat transfer coefficient. On the waterside,

62

Hybrid Poly-generation Energy Systems

Eq. (2.55) is utilized once more, and on the natural gas side, Wang’s semiempirical equation is used as follows [76]: 0:5 ULNG 5 0:0156Re0:82 bulk Prbulk ð

ρw 0:3 Cp 0:4 Kbulk Þ ð Þ ρbulk Cpbulk D

(2.59)

The operating stream condition at the expander output determines whether the process of heat transfer in the condensation consists of simply a two-phase heat transfer, a two-phase and a single-phase heat transfer process, or both. The primary premise is that CO2 flows in the shell and higher-pressured LNG flows in the tube [77]: 0:33 UCO22TP 5 0:05Re0:8 eq Prliq

Kliq D

(2.60)

In which the equivalent Reynolds Number can be defined as the following: Reeq 5 Revap ð

μvap ρliq 0:5 Þð Þ 1 Reliq μliq ρvap

(2.61)

Reliq 5

Dw m_ f ð1 2 xÞ Af μsat:liq

(2.62)

Revap 5

Dw m_ f x Af μsat:vap

(2.63)

Eq. (2.54) is also employed for the single-phase area. The total heat exchange area of the system can be determined as follows: Atot 5 Aevp 1 Acnd 1 Aht

(2.64)

2.10.1.3 Combined desalination and CCHP system driven by geothermal energy The projected plant’s conceptual diagram is shown in Fig. 2.35. The well is used to obtain hot, highly pressurized geothermal water, which is then supplied to the flashing apparatus. When the pressure in this component is reduced, some water evaporates and produces saturated vapor, while the remaining water leaves the flashing equipment as a liquid. The steam turbine produces electricity as the created vapor expands. For the purpose of heating, the saturated liquid from the flashing process is mixed once more with the expander turbine exit stream after being condensed and pushed to a higher pressure. Before merging, the saturated liquid water’s (Stream 3) energy is utilized to power a Kalina cycle that may also create electricity

How to use renewable energy sources in polygeneration systems?

2

Steam Turbine

27 28

11

4 1

63

14

Kalina Turbine

16

Valve 1

Cold water out 34

17

Flash 29 30

Cond 2 10

Cond 1

13

Sep 1

Sep 2 Evaporator

5 3 Pump 1 6

15

18

Valve 2

12

33 19

Vapor Generator

20 Valve 3

25

26 8

22

21

7

32

Heat exchanger 1

23

24 Cond 3

Pump 2

Hot water out

29 28

Heat exchanger 2 Permeate water 36 9

31 Water in

35 Feed water

RO Unit Brine water

37

Figure 2.35 Schematic view of a combined desalination and CCHP system driven by geothermal energy [78].

and cooling concurrently. To do this, the operating stream of the Kalina cycle is pumped to the steam generator, which is an ammonia and water solution. Throughout this part, the ammonia-water mixture is heated by hot water, which causes it to evaporate. One benefit of the Kalina cycle over other low-temperature cycles (such as the ORC cycle, TLC cycle, etc.) is that it experiences temperature variations during the vaporization process. This is because ammonia vaporizes more readily at cooler temperatures than water due to its volatility. As a result, the mixture’s concentration drops, and its temperature rises. Better heat transmission occurs in the vapor generator due to this procedure. A turbine that produces electricity expands an ammonia-rich flow. The turbine outflow is again fed to a splitter to enrich the solution. Rich-solution goes by an expansion valve after condensing into saturated liquid in the condenser. This valve’s outlet stream has a very low temperature and low pressure. The evaporator uses this low temperature to create the necessary cooling effect. The primary solution combines the rich ammonia solution and weak solution outflows from the separators. The mass flow rate of the produced ammonia-water in the vapor generator is equal to: m_ 3 ðh3 2 h7 Þ 5 m_ 10 ðh10 2 h26 Þ

(2.65)

64

Hybrid Poly-generation Energy Systems

It should be noted that the following relation exists between hot water and ammonia-water mixture due to the second law of thermodynamics: Ta 5 Tb 2 ΔTVG

(2.66)

This relation could be better understood using the picture shown in Fig. 2.36. It should be noted that ΔTVG is the minimum temperature difference of the vapor generator. Since the operating stream of the Kalina cycle is a mixture, both concentration and mass balances should be applied to the separators. These equations for separators 1 and 2 are written as equations Eqs. (2.67)(2.70): m_ 10 5 m_ 11 1 m_ 12

(2.67)

m_ 10 x10 5 m_ 11 x11 1 m_ 12 x12

(2.68)

m_ 13 5 m_ 14 1 m_ 15

(2.69)

m_ 13 x13 5 m_ 14 x14 1 m_ 15 x15

(2.70)

Power generated by the Kalina turbine is computed as Eq. (2.71): W_ Kalina 5 m_ 11 ðh11 2 h13 Þ

(2.71)

Also, the energy balance for the second condenser is expressed as: m_ 14 ðh14 2 h16 Þ 5 m_ 27 ðh28 2 h27 Þ

(2.72)

Temperature

3

b

a

10

7

26

Entropy

Figure 2.36 T-s diagram of ammonia water and the point where minimum temperature difference occurs [78].

How to use renewable energy sources in polygeneration systems?

65

The produced cooling effect is calculated by Eq. (2.73): Q_ eva 5 m_ 17 ðh18 2 h17 Þ

(2.73)

The energy balance for the third condenser, pump, and heat exchanger is written as: m_ 23 ðh23 2 h24 Þ 5 m_ 28 ðh29 2 h28 Þ

(2.74)

W_ pump;Kalina 5 m_ 24 ðh25 2 h24 Þ

(2.75)

m_ 12 ðh12 2 h21 Þ 5 m_ 25 ðh26 2 h25 Þ

(2.76)

The integral equation to model the RO system is developed by Dessouky [79]. Mass and salt balances for the system are expressed as: m_ 35 5 m_ 36 1 m_ 37

(2.77)

m_ 35 xf 5 m_ 36 xp 1 m_ 37 xb

(2.78)

A parameter named recovery ratio (RR) defines the ratio between feed mass flow rate and permeate mass flow rate, and it is expressed as Eq. (2.79): RR 5

m_ 36 m_ 35

(2.79)

The mass flow rate of the permeate water is a function of water permeability, area of reserve osmosis membrane, and permeate hydraulic and osmotic pressures as Eq. (2.80): m_ 36 5 ðΔp 2 ΔπÞKw Am

(2.80)

Permeate hydraulic and osmotic pressures are defined as Eqs. (2.81) and (2.82): Δp 5 p 2 pp

(2.81)

Δπ 5 π 2 πp

(2.82)

In Eqs. 2.81 and 2.82, p and π are the average feed water pressure and osmotic pressures on the feed side and brine side, which can be calculated as:  p 5 0:5 pf 1 pb

(2.83)

 π 5 0:5 πf 1 πb

(2.84)

66

Hybrid Poly-generation Energy Systems

Also, osmotic pressures can be expressed by: πf 5 75:84xf

(2.85)

πb 5 75:84xb

(2.86)

πp 5 75:84xp

(2.87)

Finally, the power consumed by the RO system is calculated by Eq. (2.88): ΔPm_ 35 W_ RO 5 ρf ηp

(2.88)

where ρf is the density of feed water, ΔP is the net pressure difference across the high-pressure pump, and ηp is the driving pump mechanical efficiency.

2.10.1.4 Integrated process configuration for production of hydrogen chloride using geothermal and wind energy resources According to [80], ethylene dichloride (EDC) thermal decomposition produces hydrogen chloride (HCl) and vinyl chloride monomer (VCM): C2 H4 Cl2 ! C2 H3 Cl 1 HClΔH 5 71kj=mole

(Reaction 2.2 -1)

The gaseous phase’s residence duration (1030 s) and temperature (480 C540 C) must be carefully controlled during the EDC thermal cracking reaction [80]. EDC decomposition is typically performed up to 55%63% to prevent a rise in side products. A hybrid renewable energies approach is proposed for the parametric analysis of a hybrid system comprising EDC reactions, powered by a geothermal HTHP and an ORC. To demonstrate the provision of the required thermal energy for the EDC thermochemical conversion process, it is vital to highlight the importance of sustainable sources of energy. The diagram (BFD) of the mentioned hybrid system is depicted in Fig. 2.37. It consists of three primary units: an ORC, a hybrid renewable energy power plant, and an EDC unit that produces HCl (wind turbines system and geothermal HTHP). According to the suggested process idea, an HTHP cycle absorbs all the heat output produced by the geothermal reservoir and uses the EDC cracking unit to supply the necessary thermal energy. The ORC unit receives the excess heat responsibility to generate electricity. In this unit, the EDC is transformed to HCl and VCM by the cracking reaction, and the internal power usage is provided by an ORC unit that operates in conjunction with the wind turbine system. C2 H4 Cl2 1 2HCl ! H2 1 C2 Cl4

(Reaction 2.2 -2)

How to use renewable energy sources in polygeneration systems?

Ethylene dichloride cracking process

67

High temperature heat pump

Organic Rankine cycle (ORC)

B6

B4 Generator

B17 B14

Wind turbine rotor

B15

B16

HX7

T101

T100

B19 HX5 B21

B18

B20

B11 P103 B7

VLV5

P104

Injection Well B12 B10

B3

B9

B8

A3

HX6

HX9

HX8

B1

A2

A1

Production Well

VLV6

R100

C100

P100

A28 A4

Heat Duty A5 A15 A14 HX1

A34

A33

A12 A11

Column1

A13

HX3 A31

A30

VLV1

A32

A10 A9

A8

FD

Flash drum

A35

VLV2

P101

Heat exchanger

A16

Column

A18 A17

A6

A37

A25

Valve HX4

HX2

A29

A28

A23

Pump

A22 A36

A24

Column2

A7

VLV3 A27

Reactor

A21 A20

A19

A26 VLV4

P102

Figure 2.37 Conceptual view of an integrated process configuration for the production of hydrogen chloride using geothermal and wind energy resources [57].

C2 H4 Cl2 1 H2 ! ClC2 1 HCl

(Reaction 2.2 -3)

Since EDC pyrolysis is a complex radical and chemical process driven by Cl. The primary reaction [reaction (21)] does not accurately reflect the reaction occurring in the cracker. Hence, the aforementioned chemical processes [reactions (21), (22), and (23)] are regarded as cracking processes. The flows exiting the cracker are split into streams A6 and A7, which go into columns 1 and 2, respectively, of the distillation apparatus. The image shows that just one distillation stage is required to create HCl. The unit to create a VCM product must distill in two steps. In a flash drum (D100), HCl is split with a mole fraction of 81.71% in stream A34, the top columns’ outputs are combined before entering.

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Hybrid Poly-generation Energy Systems

Additionally, a hybrid renewable energy system at the beginning of the process supplies the necessary thermal load for the EDC cracking procedure. A wind turbine and an ORC system powered by geothermal energy compensate for the hybrid renewable energy unit. Flow fills the geothermal resource at ambient temperatures and warms to 350 C (B25). Then it transfers heat generated with the working stream (C1tream2H10) of the HTHP process in heat exchanger HX9. The ORC evaporator (HX7) is supplied with a heat source in the HTHP cycle output flow (B4). On the other side, the net output power of the ORC and wind turbines provides the electricity needed by the HTHP cycle.

2.11

Biomass energy used in polygeneration systems

The world’s population is growing daily; on the other hand, countries are striving for economic growth and industrial development. Therefore today we are witnessing an increase in energy consumption in the world, and consequently, its effects on the environment have become a global concern. These factors significantly increase energy demand and consumption. It is predicted that energy demand in 2030 will increase by about 65% compared to energy demand in 2004 [81]. Therefore given the limited fossil energy resources, the energy crisis, pollution, and environmental damage, finding an alternative energy source is clear and undeniable. Climate change due to the use of fossil fuels is a 21st-century human concern that could seriously threaten his health and other living things on Earth. Increased mortality due to some diseases, increased occurrence of storms and floods, intensification of drought and submergence of parts of the land, etc., are only a small part of the adverse consequences of global climate change [82]. Crude oil and other fossil fuels, such as natural gas and coal, account for 85% of the world’s energy demands. Features such as limited fossil fuels, greenhouse gas emissions, and global warming, as well as unexpected increases and fluctuations in the price of these fuels, have led studies and research to find a suitable alternative to fossil fuels, invest in renewable energy, and make biofuels affordable and economical [83]. Biomass is one of the largest sources of energy in the world (fourth after coal, oil, and natural gas), which provides about 14% of the world’s energy needs [84]. Biomass is currently accepted as a renewable energy source that has the potential to meet a large part of future energy demand. Due to the increasing demand for energy in the world, it is estimated that biomass can provide 25% of this demand [85]. Biomass is converted into biofuels such as biodiesel, bioethanol, etc., through various thermal, biological, and physical methods. Thermal energy production processes from biomass sources include direct combustion of biomass, gasification, and pyrolysis. Direct combustion of biomass exposes its oxidation to air. This process is one of the old processes with low efficiency of about 10%, which generates

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heat, mechanical energy, and electricity at high temperatures (about 900 C). It should be noted that this method is associated with the production of a significant amount of pollution [86]. Direct combustion of all types of biomass with a moisture content of less than 50% is possible. Gasification is the process by which biomass is converted to a mixture of combustible gases. This process takes place at a temperature of about 850 C, the main product of which is syngas with a calorific value of 46 MJ/Nm3, which can be used to produce chemical products [87]. Gasification is a process during which partial oxidation occurs, while pyrolysis is an endothermic process and a combination of direct combustion and gasification [88]. Pyrolysis is known as a promising and effective process to produce a liquid product from biomass. Comparing the products of pyrolysis, gasification, and direct combustion processes, it can be noted that the storage and transfer of liquids are much easier than solids and gases. Therefore biooil extracted from pyrolysis is one of the types of biological fuels that has been considered by many researchers and can be mentioned as a suitable alternative to fuels used in transportation. Besides, bio-oil can be used in turbines, boilers, and engines and also as feed for refineries. Also, during the fast pyrolysis process, by-products such as char and incompressible gases are produced that can be used in the process itself or can be sold. This process is currently under development to be able to compete with conventional fossil fuels. It is necessary to overcome the technical and economic challenges ahead [89]. Fast pyrolysis is an interesting process for converting biomass to energy. Fast pyrolysis is a thermochemical process that takes place at medium temperatures (approximately 500 C) during which the biomass is rapidly heated and decomposed under oxygen-free conditions. Thermal decomposition of biomass yields products such as compressible and incompressible gases (CO, CO2, H2, and CH4) and char. A dark brown viscous liquid is extracted by cooling the compressible gases called bio-oil. The amount of bio-oil produced by this method is about 80% by weight [90]. The calorific value of this liquid product is about 1418 MJ/kg [91]. Biomass can be produced using a variety of agricultural and forest lignocellulose wastes. Pyrolysis of various types of biomass to produce bio-oils and by-products has been studied in detail in many articles. For example, the studied biomass includes beechwood [92], Bagasse [93], wood biomass [94,95], straw [96], municipal solid waste [97,98], and so on. Production of bio-oil through pyrolysis is a complex process and it is affected by various factors such as feed type, reaction temperature, and pressure, the moisture content in the feed, residence time, heat rate, feed particle size, catalyst, etc.

2.11.1 Biomass Renewable and inexhaustible sources of energy are called renewable energy sources, such as solar energy, wind energy, biomass energy, geothermal energy, and so on. Renewable energy is always referred to as a clean source of energy. Using these God-given energy sources minimizes pollution and environmental damage reduces waste production and relieves and reduces the effects of greenhouse

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gas emissions and global warming. Biomass is the only renewable energy source that has a unique feature, and this special feature is that it is possible to produce solid, liquid, and gas products from biomass using various processes. The amount of each, according to the process conditions, will differ from these products [99,100]. Biomass was one of the first sources of energy used by humanity. This divine source is known as one of the renewable energy sources for energy supply that is found in abundance around the world and is easily accessible. Biomass generally includes agricultural and forest wastes, as well as industrial and municipal wastes that are degradable and can be used to produce energy. The types of biomass that can be used as process feed are classified into three categories: starches, which include sugars, triglycerides, and lignocellulosic substances [87]. Sugars and starches are used as human and animal feed. There are various sources of triglycerides including vegetables, plants, fats, and animal oils. However, these biomass sources have limitations in their usage. The most suitable biomass source for triglyceride development is lignocellulosic, which includes agricultural and livestock wastes, urban wastes, forest wastes, marine plants, and algae. Many studies have been conducted on this source and continue to be developed [101]. Despite the complexities of using biomass, its use is increasing. This energy source can be used in agriculture, petrochemical industries, and private companies. The evolution and progress in the field of biomass use are generally known in the form of two generations [102,103]: The first generation of biomass use includes the production of biofuels such as biodiesel from vegetable oils and bioethanol from sugars and starches. The scale of products and energy produced from first-generation fuels (less than 100 MW) was much lower than that produced from oil refineries (several gigawatts). Despite some advantages of using first-generation biofuels, their biggest problem was the competitiveness of these fuels with fossil fuels. Subsequently, the use of biomass grew and evolved, leading to the creation of a new generation in this field. The second generation of biomass use involves the use of lignocellulosic resources. This generation is divided into several groups: 1. Hydrolysis and fermentation of cellulosic materials to produce ethanol 2. Gasification of lignocelluloses to produce synthetic gases and then convert them to methanol, petrol, and gasoline 3. Liquidation and pyrolysis of lignocelluloses

Initially, starches and sugars were used as raw materials for the production of bioethanol, but because these materials were used as food for humans and animals, in the second generation of bioethanol production, lignocellulosic materials were used. These materials are cheap and renewable and are found in abundance. Lignocelluloses are mainly composed of cellulose, hemicellulose, and lignin. There are different types of lignocellulosic materials, each of which is composed of different proportions of these constituent materials. Their general composition is shown in Fig. 2.38.

How to use renewable energy sources in polygeneration systems?

Other % 20-15 Lignin % 20-15

71

Cellulose 35-50%

Hemicellulose 35-20%

Figure 2.38 General composition of lignocelluloses [104].

2.11.1.1 Cellulose Cellulose is a linear polymer that forms plants’ cellular network and promotes plants’ vertical growth. Cellulose is the main and most abundant organic polymer that forms lignocellulosic biomass, which makes up about 40%50% of dry wood. It has a crystalline structure made up of thousands of long chains of glucose molecules linked together by hydrogen bonds. In other words, cellulose is a polysaccharide consisting of linear D-glucose chains. These chains are linked together by β-1,4-glycosidic. Therefore these materials have high strength. These compounds have the chemical formula (C6H10O5)x, which x is in the range of 500 to 4000 and mainly contains glucose [105,106]. The shape of these materials in lignocelluloses is in the form of fibers and thin filaments. Celluloses are insoluble in water and most organic solvents [107]. Cellulose is reported to be stable up to a temperature of about 310 C and then slowly converted to incompressible gases and compressible organic vapors at a temperature of 410 C310 C [108].

2.11.1.2 Hemicellulose Hemicelluloses have the chemical formula (C5H8O4)x, in which x is in the range of 50 to 200 and mainly contains Xylans [109]. These substances are heterogeneous compounds of biopolymers and include five-carbon monosaccharides (β-D-xylose and α-L-arabinose), six-carbon sugars (β-D-mannose, β-D-glucose, and α-D-galactose), and acids. They are inorganic (α-D-glucuronic, α-D-4-Omethyl-galacturonic, and α-D-galacturonic acids) and have low molecular weight. The structure of these compounds is branched and irregular, resulting from the repetition of nearly 150 saccharide monomers. These materials wrap around cellulose fibers and must be separated to obtain cellulose. These materials are susceptible to operating conditions such as temperature and residence time. Therefore we must carefully consider the operating conditions to prevent producing undesirable products [103]. Unlike cellulose, hemicelluloses are hydrolyzed in contact with dilute acids and acid-based materials such as sodium hydroxide. Hemicellulose is the first substance of lignocellulose that decomposes due to temperature increase and its

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decomposition starts from 220 C and continues up to 400 C. At a temperature of 260 C, we see the highest rate of decomposition of these materials due to the decomposition of xylans [110].

2.11.1.3 Lignin Lignin has the chemical formula [C9H10O3(OCH3)0.91.7]n. These materials are aromatic polymers that are formed from the combination of phenylpropanoid and are in the form of complex matrix networks [111]. Lignin generally consists of three-dimensional branched compounds and aromatic polymers bonded together by ether and carbon-carbon bonds. These bonds’ presence causes lignin’s thermal stability at high temperatures. Although lignin decomposition begins slowly at 160 C and continues through the temperature range of 900 C800 C, during fast pyrolysis, when the temperature reaches about 500 C, approximately 40% of lignins decompose [112,113]. Tables 2.7 to 2.9 list some sources of lignocellulose along with the composition of the percentage of constituents, and their physical and chemical properties. Also, the higher heating value (HHV) of raw and coal biomass obtained from the pyrolysis process can be calculated as follows [114].  HHV kJ=kg 5 354:68C 1 1376:29H 2 15:92Ash 2 124:69ðO 1 N Þ 1 71:26 (2.89) In the above formula, the number of constituent elements (C, H2, O2, N2, and Ash) is expressed based on the mass percentage. In the biofuel production process, the more cellulose the lignocellulosic material we use as feed contains, the better the product quality. Also, according to some studies that have examined different types of biomass feeds, the higher the amount of lignin in the feed, the more coal is produced at medium temperatures (about 500 C) [114]. Also, the ratio of volatiles, fixed carbon, ash, and water content in the biomass directly affects the distribution of products obtained from fast pyrolysis. Usually, the higher the amount of volatiles in the biomass, the higher the production of biooils and syngas. While increasing the amount of fixed carbon, we see an increase in the amount of coal produced. Moisture content in biomass directly affects the heat transfer process and product distribution [86]. The lower the number of minerals and nitrogen content in the biomass, the higher the production of bio-oils and syngas [115]. The most suitable method for isolation and decomposition of lignocelluloses consists of three steps. An example of this method is given in Fig. 2.39, which includes the following steps: 1. Disruption of hydrogen chains in cellulose crystals 2. The disintegration of lignin and hemicellulose networks 3. Increased porosity and cross-section of cellulose

Table 2.7 Biomass lignocellulosic resources composition percentage (based on the dry matter). Lignocellulosic biomass

Barley hull Barley straw Com cobs Com stover Cotton stalks Wheat straw Rise straw Rye straw Oat straw Soya stalks Sunflower stalks Switchgrass Sugarcane bagasse Sweet sorghum bagasse Forage sorghum Olive tree pruning Poplar Spruce Oak

Celluloseglucan

33.6 33.8 33.7 38.3 14.4 30.2 31.1 30.9 39.4 34.5 42.1 39.5 43.1 27.3 35.6 25.0 43.8 43.8 45.2

Hemicellulose

Lignin

Xylan

Arabinan

Galactan

Mannan

Acid-insoluble lignin

Acid-soluble lignin

30.5 21.9 31.9 21.0

6.1

0.6

Trace

ND

2.7 14.4 2.8 3.6 ND

2.1

ND

0.8 ND ND

ND ND ND

2.1

2.6

ND

1.4 1.8 2.4 ND ND ND

ND ND 1.5 ND ND ND

ND ND 0.8 ND 14.5 4.2

19.3 13.8 6.1 17.4 21.5 17 13.3 22.1 17.5 9.8 13.4 17.8 11.4 14.3 18.2 16.2 29.1 28.3 21.0

18.7 18.7 21.5 27.5 24.8 29.7 20.3 31.1 13.1 18.4 11.1 14.8 6.3 20.3

3.2

4.0

2.2 0.53 3.3

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Table 2.8 Physical properties of some biomass resources [105]. Feedstock

Density (kg/m3)

Moisture content (%)

Ash content (%)

Volatile matter (%)

Fixed carbon (%)

Poplar Pine Birch Grateloupia filicina Wood Bituminous coal Switchgrass Hybrid polar Wheat straw Miscanthus Barley strew Sugarcane baggage Rice straw Firewood Danish pine

120 124 125  1186  108 150 1233 70100 210 1198 200  

16.8 17 18.9 4.93 20 11 1315 45 16 11.5 30  6 7.74 8

0.007 0.03 0.004 22.37 0.41 811 4.55.8 0.52 4 1.54.5 6 3.25.5 4.3 1.98 1.6

   55.93 82 35   59 66.8 46  79 80.86 71.6

 16 20 17.01 17 45   21 45.9 18  10.7 17.16 19

Table 2.9 Chemical properties of some biomass resources [104,106]. Feedstock

Carbon (%)

Hydrogen (%)

Oxygen (%)

Nitrogen (%)

Ash (%)

Wheat straw Bituminous coal Wood Barley straw Cypress Scots Birch Olive baggage Polar Switchgrass Pine Willow Lolium perenne Switchgrass Reed canary grass

48.5 73.1 51.6 45.7 55 56.4 44 66.9 48.1 44.77 45.7 47.78 43.12 44.77 45.36

5.5 5.5 6.3 6.1 6.5 6.3 6.9 9.2 5.3 5.79 7 5.9 5.8 5.79 5.81

3.9 8.7 41.5 38.3 38.1  49 21.9 46.1 49.13 47 46.1 49.8 49.13 48.49

0.3 1.4 0.1 0.4  0.1 0.1 2 0.14 0.31 0.1 0.31 1.28 0.31 0.34

4 9 1 6 0.4 0.09 0.004  0.007 4.3 0.03 1.3 6.2 4.3 5.1

There are different methods for decomposing lignocellulosic materials, as follows: 1. Physical methods (slicing, grinding, etc.) 2. Chemical methods (alkaline and acidic methods, ozone reduction, and ionic methods)

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Figure 2.39 Isolation and decomposition of lignocelluloses [118]. 3. Physiochemical methods (steam combustion, carbon dioxide combustion, and use of wet oxidants) 4. Biological methods

2.11.2 Biomass advantages and disadvantages to produce biofuels The main advantages of using biomass to produce biofuels are as follows [119]: 1. 2. 3. 4. 5. 6. 7. 8.

Renewable source of energy supply Greenhouse gas emissions reduction Preservation and protection of fossil fuels Storage and trapping of toxic compounds in the form of ash Diversify the energy basket and increase energy security Revitalization of villages, along with income generation and job creation Less ash, C, FC, S, N, Si content, and other elements Less dependence on energy imports

The main disadvantages of using biomass to produce biofuels are as follows [119,120]: 1. Diversity of biomass resources and their characteristics 2. Limitation of some types of biomass

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3. 4. 5. 6. 7. 8.

High investment cost Low energy density compared to fossil fuels Acidic and corrosive properties High water content Relevant infrastructure and culture-building requirements Low volume of products produced during a year (usually less than 10 tons, which depends on geographical conditions)

2.11.3 Conversion of biomass to energy Biomass can be converted into useful products with the help of thermochemical and biochemical processes [121]. The choice of each of these processes depends on the type and amount of biomass feed.

2.11.3.1 Thermochemical processes These processes themselves are divided into two main categories. The first category includes the direct use of biomass as a fuel for combustion and subsequent heat generation and electricity generation. The second category includes the conversion of biomass into other useful energy carriers previously used as energy sources. These processes include direct combustion, gasification, liquefaction, hydrogenation, and pyrolysis [121]. One of the benefits of these processes is the conversion of all biomass (even lignin) into a product. However, its disadvantages include the need for high energy to provide high heat rates and operating temperatures.

2.11.3.1.1 Direct combustion In this process, the biomass in the combustion chamber is used as a fuel for combustion in boilers in the presence of sufficient air to produce steam (heat source). Then, with the help of turbines, heat and electricity can be generated from steam simultaneously. Biomass direct combustion is generally divided into fixed bed systems and fluidized bed systems [122].

2.11.3.1.2 Gasification Biomass gasification is more efficient than direct combustion. During this process, the feed is converted to combustible gases. These combustible gases are known as syngas, which include hydrogen (H2), carbon dioxide (CO2), methane (CH4), carbon monoxide (CO), water vapor (H2O), nitrogen (N2), and hydrocarbons. Other lights and some impurities are such as ammonia (NH3), hydrogen sulfide (H2S), and hydrogen chloride (HCl). The process occurs when a controlled amount of oxygen carrier (pure oxygen, air, steam) reacts at high temperatures with the carbon in the feed [123]. In the process of gasification, biomass is converted directly into syngas in the presence of air, which can then be converted to diesel with the help of the FischerTropsch process or hydrogen can be produced by purification [114]. Syngas can be used in internal combustion engines or refrigeration systems to generate heat or electricity. Studies indicate that the construction of a large gasification

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unit depends on the location of the unit and the availability of resources and feed, which plays a significant role in the cost of production [120].

2.11.3.1.3 Pyrolysis The process of thermal decomposition of biomass under anaerobic conditions and medium temperature is called pyrolysis. Pyrolysis occurs at operating temperatures of 350 C350 C. The final products produced by this process are in the form of gas, liquid, and solid rich in carbon (coal). The distribution of these final products depends on the feed and operating conditions. This process is divided into two main groups: slow pyrolysis and fast pyrolysis, which are based on operating conditions such as temperature and residence time, etc. Also, to accelerate this process, various catalysts can be used [124].

2.11.3.2 Biochemical processes These processes convert biomass into valuable additives such as ethanol, hydrogen, and methane. However, many biochemical processes have been reported to produce ethanol and hydrogen, including anaerobic digestion, fermented ethanol production, and fermented hydrogen production [109].

2.11.3.3 Anaerobic digestion During this process and in the absence of oxygen, microorganisms convert biomass into biogas, which is a mixture of methane and carbon dioxide. Biogas is used as fuel to generate heat and energy. One of the essential factors in this process is the pH of the reaction medium, the manner and amount of heating, the size of the biomass components, the type of biomass, etc. Among these factors, the pH of the environment is more important and influential than the others [109].

2.11.3.4 Fermented ethanol production This process consists of three steps: The first step involves preparing the food and slicing it. The second stage involves the conversion of cellulose and hemicellulose into monosaccharides and six-carbon sugars. In the next step, glucose is converted to ethanol by fermentation by microorganisms. Some studies suggest that the ethanol mixture produced by this method can compete with some fuels, such as diesel [103]. It is economical to use this method in places where many sources of carbohydrates (especially sugar) are available, such as Brazil. Using this method, a small amount of ethanol is obtained, which can be considered about 10%12% by feed volume. It also takes a long time (612 h) to extract ethanol by fermentation. This process is relatively simple and consumes low energy. By-products of this process include carbon dioxide [103].

2.11.3.5 Fermented hydrogen production Hydrogen is the fuel for the future, which is a clean fuel with a high energy density. One of the methods of hydrogen production is the fermentation of organic waste.

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Municipal wastewater treatment plants produce a significant amount of activated sludge as a by-product that can be used for fermentation and hydrogen gas production, but usually, the amount of hydrogen produced by this method is low [125]. Studies on fermented hydrogen production of agricultural wastes are limited. During fermentation, anaerobic bacteria convert carbohydrates into hydrogen, volatile fatty acids, and carbon dioxide. This process can be divided into photofermentation and dark-fermentation, in which the type of active bacteria and operating conditions are different [126].

2.11.4 Types of pyrolysis Pyrolysis is an effective thermochemical process for converting biomass to bio-oil in a reactor in oxygen-free conditions at temperatures of 300 C300 C [97]. The word pyrolysis has Greek roots, which is a combination of the two words “Pyro” meaning fire and “Lysis” meaning decomposition into constituent elements. More than 500 years ago pyrolysis technology was used to produce coal in southern Europe and the Middle East. Pyrolysis was also used to produce bitumen to prevent water from seeping into boats and being embalmed in ancient Egypt [94]. Using the pyrolysis process under certain conditions, higher proportions of biooil can be produced from biomass. In several studies, the maximum production of bio-oil from the fast pyrolysis process has been reported up to 80% [94,114,117,127,128]. Therefore in recent decades, pyrolysis has been considered as an effective way to convert biomass into biofuels [98]. Depending on the operating conditions of the pyrolysis process, it is classified into various types: slow pyrolysis, fast pyrolysis, flash pyrolysis, ultra-fast pyrolysis, hydropyrolysis, vacuum pyrolysis, and methane-pyrolysis, of which fast pyrolysis and slow pyrolysis are the most common. It is better known to briefly describe these processes and the types of reactors used in pyrolysis. Table 2.10 also compares the types of pyrolysis.

2.11.4.1 Slow pyrolysis This process has been used for thousands of years to produce coal at low temperatures and low heat rates. Solid product (coal) is the goal and main product of slow Table 2.10 Pyrolysis types comparison [104,119]. Pyrolysis process

Residence time

heating rate

Final temperature (K)

Particle size(mm)

Product yield (%) Oil Char Gas

Slow Fast Flash Vacuum Hydropyrolysis

530 min ,2s ,1s 230 s , 10 s

Low Very high High Medium High

873 B773 , 923 673 , 773

550 ,1 ,0.2  

30 50 75

35 20 12

35 30 13

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pyrolysis process, while in fast pyrolysis, the goal is to convert biomass to the maximum amount of liquid product (bio-oil) [98]. In this process, the residence time of steam is very long, which causes the gaseous compounds to react with each other [88]. Due to some technical limitations in the slow pyrolysis process, this process is not suitable for producing quality bio-oils. Due to the long residence time, primary product cracking occurs, which adversely affects the amount and quality of bio-oil produced. In addition, long residence time and low heat transfer rate require a large amount of energy [127,128].

2.11.4.2 Fast pyrolysis Fast pyrolysis is the most well-known process of cellulose biomass conversion in which the amount of liquid product produced is significant and is produced at a relatively low cost. Pyrolysis products are in the form of gas, liquid, and solid, of which the amount of liquid production is significant. The resulting liquid product can be used in transportation as fuel, direct combustion in thermal power plants, and combustion in gas turbines [127]. The resulting charcoal can be sold or used to provide the heat required by the process. The gas obtained from this process has a moderate calorific value that can be used as a heat supplier of the process, or it can be used in the gasification process. Studies have shown that fast pyrolysis is a relatively mature and well-developed technology that is very close to the commercialization stage, but the improvement and promotion of bio-oil still remain in the laboratory and Pilot scales, which is one of the main challenges in fast pyrolysis [127,128,130]. Pyrolysis has been used to produce coal for thousands of years, but in the last 30 years, the fast pyrolysis at temperatures of about 500 C during a short stay of about 2 seconds has attracted much attention. One of the reasons for this is the possibility of producing 75% by weight of bio-oil. Essential features of the fast pyrolysis process for further extraction of bio-oil are as follows [131133]: 1. High heat rate and high heat transfer rate (1000 C10000 C/s) 2. Requiring dry feed (moisture content less than 10%) 3. Small biomass components (usually, the size of the components should be less than 3 mm). 4. Pyrolysis reaction precise temperature control; because the amount of bio-oil produced depends on the reaction temperature. (Approx. 650 C450 C) 5. Atmospheric pressures (0.511.5 MPa) 6. Residence time shorter than two seconds to avoid side effects (0.25 s) 7. Fast cooling of pyrolysis vapors to extract bio-oil

The fast pyrolysis process involves drying the feed, and this mechanism will continue until the moisture content of the feed reaches less than 10% so that there is less water in the final products and the calorific value of the products is higher. The feed is cut into small pieces for a better reaction and enters the fast pyrolysis reactor. In the next step, the produced solid particles (coal) are separated and the

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Figure 2.40 Fast pyrolysis process [134].

resulting vapors are cooled and the liquid product is collected. An overview of the fast pyrolysis process is given in Fig. 2.40.

2.11.4.3 Flash pyrolysis With the help of this technology, solid, liquid, and gas products can be produced from biomass, and the production of bio-oil is possible up to more than 70%. In this process, rapid biomass decomposition occurs at high temperatures between 450 C and 1000 C, high heating rates, and very short residence times for gas (less than 1 s) [135]. Technical limitations of this method include poor thermal stability, the high corrosiveness of bio-oil, high amount of solid particles in bio-oil, high viscosity, high content of alkaline compounds in coal, high amount of insoluble compounds in biooil, etc. [131].

2.11.5 Reactors used in fast pyrolysis The fast pyrolysis process includes various parts such as a condenser, filter, gas collection unit, gas preheater, reactor, etc. Among these parts, the reactor is like the beating heart of the fast pyrolysis process, which has been the subject of many articles and studies [97]. Initially, it was assumed that the smaller the residence time and the size of the biomass components (less than 1 mm), the more bio-oil was extracted, but later with increasing studies and research, different results were obtained. Biomass particle size and residence time had little effect on the amount of bio-oil produced, while these parameters had a major effect on bio-oil constituents [114]. With the growth and development in the field of pyrolysis, we see various models of reactors, each of which aimed to optimize pyrolysis performance and produce as much bio-oil as possible. However, each type of reactor has its own characteristics, limitations, and advantages. For example, it should be noted that Bubbling

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fluidized bed reactors and Circulating fluidized bed reactors exist on an industrial scale, while other types of reactors exist on a laboratory scale. In the following, we will briefly describe the most important of them.

2.11.5.1 Bubbling fluidized bed The construction and operation of this type of reactor is relatively simple. Using these reactors results in better temperature control, better contact between gas and solid, better heat transfer, and higher capacity. This reactor has been used in the chemical and petrochemical industries for over 50 years. With these reactors’ help, a high heat transfer rate can be provided while the bed temperature is uniform. Both of these factors are essential in the fast pyrolysis process. With the help of this type of reactor, 70%75% of dry biomass can be converted into bio-oil. That is why the use of these reactors is common. Based on research, this reactor seems to be ideal for biomass from agricultural waste [98]. Generally, these reactors can produce the highest amount of bio-oil under operating temperatures of 500 C550 C and a residence time of 0.5 s, although larger systems with lower temperatures and higher residence times can produce the maximum amount of bio-oil. Of course, the temperature can vary depending on the type of biomass. Proper heat supply for the substrate, heat transfer, good mass transfer, and the need for feed with small particles in the size of 23 mm, are the features of this reactor. Another feature of this reactor is “self cleaning,” in which the coal produced is transported outwards by the gases and vapors produced from the reactor. This requires the size of the feed components to be appropriate, and if the size of the biomass components is large, the produced coal will remain. Therefore the density of coal produced must be less than the density of feed particles so that the coal produced can float to the top of the reactor. One of the advantages of keeping coal floating on top of the reactor is that catalysts perform better in reaction and create a better contact surface. One of the disadvantages of these reactors is inadequate heat transfer at scale-up. A view of these reactors is shown in Fig. 2.41. Flexibility against different types of feed, proper mixing of solid particles and gas, good temperature control, and high mass transfer are other advantages of these reactors.

2.11.5.2 Circulating fluidized bed These types of reactors are very similar to Bubbling fluidized bed reactors. Distinctive features of this type of reactor include a high heat transfer rate and low steam residence time. Both of these conditions are essential for the fast pyrolysis of biomass. One of the special and complex features of this type of reactor is the use of hot sand (or other intermediate fluid). This hot sand constantly rotates between the combustion chamber and the pyrolysis unit. For many years, refineries used this type of solid to transfer heat to the catalytic cracking unit, so these reactors have reached the stage of industrial and commercial applications. In addition, various systems have been designed, the main

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Gas, Char and Oil vapours and areosol

Freeboard

Biomass Fluid bed Heat: Feeder Distributor plate Fluidizing gas

Figure 2.41 Bubbling fluidized bed reactor [136].

Gas and oil vapours and aerosol

Pyrolyzer

Combustor

Biomass

Flue gas

Sand and char Distributor plate

Hot sand

Feeder

Air Fluidizing gas

Figure 2.42 Circulating fluidized bed reactor [137].

difference being the reactor heat supply mechanism. A view of these reactors is shown in Fig. 2.42. The feed parts size of these reactors is even smaller than the feed parts size of bubbling fluidized bed reactors. The range designed for the feed parts size of these reactors is 12 mm. Also, the residence time in this type of reactor is about 15 s. If the feed portion size is large, there will not be enough time to transfer heat. This is exacerbated when the coal produced on the outer surface of the feed increases and acts as an insulator against heat transfer.

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The movement of sand in the system causes scratches on the coal produced, but generally, the interaction between the sand and the coal at the knee points and bending occurs more [136].

2.11.5.3 Rotating cone pyrolyser Mixing inert biomass and inert hot particles is one of the most effective ways to improve heat transfer in the fast pyrolysis process. In these reactors, instead of the biomass being mixed with hot gases, it is mixed with hot sand particles. This type of reactor was built at the University of Twente in the Netherlands in 1990 and its development process continued. Recent achievements indicate an increase in the scale of this reactor for a feed capacity of 200 kg/h. The reactor technology is similar to a circulated fluidized bed in which hot feed and sand are mixed. The feed and hot sand are close to each other at the beginning of the inlet to the cone until the rotating cone rotates and creates a centrifugal force that causes solids to move toward the top edge of the cone. The hot feed and sand are thoroughly mixed and at the same time, the produced vapors are directed to the condenser. The coal and sand are then directed to the combustion chamber, where the sand is reheated and exposed to fresh feed. A view of this reactor is shown in Fig. 2.43. Using this reactor and suitable conditions, fuel oil production is about 70%. The advantages of this method include ease of product recovery and reduction of abrasion effects. Disadvantages of this reactor include scaling problems due to the complexity of Biomass

Vapours and aerosol

Hot sand

Rotation

Figure 2.43 Rotating cone pyrolyzer reactor [137].

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equipment such as rotating cones, bubble bed for coal combustion and pneumatic transfer (with the help of compressed air) of sand and then return it to the reactor [136].

2.11.5.4 Vacuum pyrolysis This reactor is mainly used in the slow pyrolysis process because in this type of reactor, the heat transfer rate is low and the amount of bio-oil produced is about 30%45% wt., which is a very small amount compared to the product obtained from other reactors. One of the most important advantages of these reactors is the ability to use feed with larger parts (25 cm). In these reactors, biomass movement is based on gravity and through rotating blades. As the biomass moves, its temperature rises from 200 C to 400 C. It then transfers the feed to the combustion chamber with the help of a metal belt until it reaches a temperature of 500 C, which is a complex mechanical process. A view of this reactor is shown in Fig. 2.44. Due to this type of reactor’s special design and operation and vacuum operating conditions, its maintenance and investment costs are high. Its advantages include the high quality of the product, the large size of feed parts, and ease of product extraction [136].

2.11.5.5 Auger reactor According to some research, using this type of reactor has great potential in reducing the price of bio-oil produced. But these reactors are more suitable for smallscale pyrolysis. Inside these reactors, feed and hot sand (as heat carriers) that act as reaction heat providers, mix. The heat carrier can be hot sand or hot steel. These materials are

Scrapper driver Biomass

Bio-oil Char Multiple hearth vacuum pyrolysis reactor

Figure 2.44 Vacuum pyrolysis reactor [137].

Condenser

Vacuum pump

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Biomass

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Hot sand

Vapours and aerosol to condenser

Char and sand Auger driver

Auger reactor

Figure 2.45 Auger reactor [137].

heated independently before entering the reactor. Volatile vapors and aerosols (liquid particles in the gas) escape from various parts of the reactor, and coal has an axial motion along the reactor that is collected with the heat carrier in a predetermined chamber. The phases are separated based on density and, in effect of gravity. A view of this reactor is given in Fig. 2.45 [136]. Studies and results indicate that these reactors have a good potential to increase the scale. These reactors can operate continuously despite the small amount (and even lack) of gas carriers. Inside the reactor are blades such as a meat grinder, which increase the contact area of the phases and improve heat transfer, and the coal-generated automatically moves forward. Due to the complexity of the shape and movement of the blades, we see less numerical analysis of rapid biomass pyrolysis in such reactors [138].

2.11.6 Affecting factors in the fast pyrolysis reaction Several factors affect the fast pyrolysis and the number of products produced from it, which we will briefly address:

2.11.6.1 Heat rate and temperature These two operational parameters are the most important factors that affect the amount of bio-oil produced. If the temperature is low, more coal is produced. If the operating temperature increases, the amount of monoxide produced increases. In the study of the effect of temperature on bio-oil production, it has been observed that first, with increasing temperature, the production of bio-oil increases sharply (450 C500 C), and then with the continuation of increasing temperature, the production of bio-oil decreases [136].

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Peng Pang and Petya investigated the effect of temperature (400 C500 C) on the number of products produced by the pyrolysis process in a fluidized bed reactor fed by cassava stems. As shown in Fig. 2.46, as the temperature rises from 400 C to 500 C, bio-oil production increases, coal production decreases significantly, and the production of gas increases. As the temperature rises from 500 C to 550 C, we see a decrease in the amount of bio-oil produced, a decrease in the amount of coal produced, and a significant increase in the number of gaseous products. Therefore in this experiment, the maximum conversion rate of bio-oil at a temperature of 500 C has been determined.

2.11.6.2 Biomass type Different types of biomass have a significant role in the products due to having different compounds. Different types of biomass lead to producing different amounts of bio-oil under the same conditions. The more cellulose in the biomass, the more bio-oil is produced. In other words, the feed type is the biggest factor in the fast pyrolysis process. Each type of biomass, due to its different constituents, produces a certain amount of bio-oil, which is discussed in Table 2.11.

Figure 2.46 Temperature effect on the number of pyrolysis products [94].

Table 2.11 Biomass type effect on the amount of bio-oil produced [84]. Type of feedstock

Yield of bio-oil (wt.%)

Palm oil (palm shell) Sunflower-oil cake Cassava stalk Cassava rhizome Rice husk

47.3 48.69 62 65 41.7

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2.11.6.3 Feed preheat As the preheater temperature increases, the amount of gas production increases slightly and the amount of bio-oil production decreases slightly [94]. The effect of this parameter on other factors is insignificant. For this reason, few studies have been conducted in this field.

2.11.6.4 Biomass moisture content The presence of water and moisture in the biomass entering the pyrolysis reactor causes some adverse effects on the stability, viscosity, pH, corrosion, and other properties of the liquid obtained from pyrolysis. Therefore biomass drying is essential. If the water content of biomass is very high, the bio-oil produced will also have a high water content, and its calorific value will decrease. Generally, the moisture content in the pyrolysis process feed should be around 5%15% wt. [94]. According to studies on the drying temperature, duration of biomass, and their effect on the properties and chemical composition of products, increasing the drying temperature affects the distribution of pyrolysis products, while increasing the drying time has no sensible effect on pyrolysis products distribution. In addition, with increasing the drying temperature, the quality of the produced bio-oil improves due to the reduction of water content and acidic compounds in the liquid product produced from pyrolysis [94].

2.11.6.5 Biomass particle size The size of biomass components has a major effect on the applied heat rate so that it is one of the most important parameters in controlling the drying rate and pyrolysis operation. In fact, the larger the feed portion size, the smaller the amount of biooil extracted. For example, changing the feed size from 5363 mm to 270500 mm in a sample reduced the production of the desired product from 53% to 38% (percentage by weight of the desired product produced from the feed). However, reducing the size of biomass components is costly and directly affects the process economy, so according to studies, if the feed size changes from 2.5 mm to 250 μm, it can add up from $ 1.8/ton to $ 6.5/ton to costs [94]. The size of feed portions directly impacts the distribution of products from the fast pyrolysis process. As the feed portions’ size increases, the mass waste increases. Therefore the percentage of conversion of feed into products decreases. If we consider the heat rate as constant, the larger the size of the feed parts, to provide the heat rate, we must increase the feed’s residence time, which reduces the production of compressible gases by at least 10%. It also causes a significant increase in coal production.

2.11.6.6 Biomass particle size Residence time is an important parameter in the pyrolysis process so if the residence time is too short, not all biomass components will become the product. On

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the other hand, if the residence time is longer, the amount of gas production will increase and the amount of bio-oil production will decrease. As a suitable approximation in the calculations, the residence time can be obtained by dividing the height of the reactor by the velocity of the exhaust gases [139].

2.11.6.7 Catalyst In the pyrolysis process, catalysts are used in two ways. The catalysts can be mixed directly with the feed (biomass) and then introduced into the fast pyrolysis reactor (in situ), or the catalysts can only be exposed to the resulting hot vapors after the fast pyrolysis reactor (ex situ). Catalysts include acidic solids such as zeolites, silica-alumina, silica, alumina and molecular sieves, or metal oxides such as zinc oxide, copper chromate, etc. In addition to solid catalysts, there are also inorganic catalysts such as metal chlorides, phosphates, alkaline catalysts, etc. The use of appropriate catalysts in bio-oil production increases the product type’s selectivity. For example, with the help of a catalyst, the oxygen in the biomass can be extracted in the form of coke or gas so that the produced bio-oil has a higher viscosity [138]. Catalysts used in fast pyrolysis are generally divided into several groups: inorganic solutions, metal oxides, microporous materials, mesoporous materials, and metal-based materials.

2.11.6.7.1 Inorganic solutions These types of catalysts include KCl, MgCl2, NaCl, FeSO4, ZnCl2, etc. The presence of these catalysts, especially K and Ca in the process, increases the production of coal and reduces the production of gases and vapors, changes in the decomposition temperature, and reduces the weight of the products. Potassium increases the production of CO2 and CO. For example, the presence of 2% wt. of KCl significantly affects the amount of coal produced. The use of MgCl2 has no effect on the products’ decomposition temperature and weight loss but increases the production of aldehydes and ketones [138].

2.11.6.7.2 Metal oxides In many reactions, these catalysts are used heterogeneously. These catalysts include MgO, NiO, Al2O3, ZrO2, TiO2, MnO2, CeO2, Fe2O3, etc., which are divided into three categories [140]. The use of acidic metal oxide catalysts increases gas and coal production, reduces the production of liquid products, changes the composition of the bio-oils produced, and increases the production of CO. Al2O3, SiO2, etc., are suitable catalysts in this group. Metal oxide-based catalysts are known as active catalysts for the ketonization and condensation of carboxylic acid and carbonyl compounds. Oxides of alkaline earth compounds such as MgO and CaO fall into this group. Other metal oxide catalysts include Nio, ZrO2, ZnO, etc. Using these catalysts reduces the production of liquid and organic compounds and increases the production of gas, water, and coal. The use of NiO also increases the production of CO2 and H2.

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2.11.6.7.3 Microporous materials These types of catalysts are easily used in the cracking process in fluidized catalytic cracking (FCC) in oil refineries. Zeolites fall into this category [140]. In many cases, zeolite with different porous structures is used in the catalytic pyrolysis of biomass and to improve the quality of bio-oil produced. ZSM-5 zeolite is the most well-known catalyst for catalytic cracking and this type of catalyst is more used in biomass cracking than other types. The choice of a suitable catalyst is related to several factors, such as acidity, shape structure, selectivity, rate of inactivation by the coke produced, high thermal stability, etc. In experiments, it has been reported that in comparison with the performance of 4 catalysts—ZSM-5, Y-, -β, and ordinary zeolite catalysts (SiO2/Al2O3 5 1225)—more bio-oil can be produced using ZSM-5. Also, the amount of acid production was lower, but the amount of ketone production was higher [94].

2.11.6.7.4 Mesoporous materials In porous materials, the porosity size affects the activity, selectivity, and price of the catalyst. Primary products from pyrolysis and products with a diameter equivalent to glucose cannot penetrate through the cavities of mesoporous materials and only materials with a size of 215 nm can pass through it. If these catalysts are used, they are mixed with feed and then introduced into the pyrolysis reactor [94].

2.11.6.7.5 Metal-based materials Hydrodeoxygenation (HDO) is an effective way to improve the quality of bio-oils. HDO involves the production of a liquid product at a temperature of 250 C450 C with the application of hydrogen pressure in the range of 57.30 MPa in the presence of CoMo or NiMo sulfide catalysts. This process requires high operating pressure, investment cost, and high operating costs. Studies on carbon-based metal catalysts such as Cu/C, Fe/C, Pd/C, etc. in the HDO process show that they have good performance and less acidic product, and more selectivity, but their costs are very high [141].

2.11.7 Bio-oil In many sources, the product of the liquid produced is pyrolysis, referred to as biooil, but other titles have been used for this liquid, such as bio-crude oil, biofuel, pyrolysis oil, etc. The quality and amount of bio-oil obtained from pyrolysis depend on several factors, such as temperature, humidity, biomass size, air-to-feed ratio, etc. Bio-oil smells like smoke and irritates the eyes. Water content and the number of light compounds are among the factors affecting its viscosity. Bio-oil usually contains lower amounts of nitrogen than petroleum derivatives and is mainly free of sulfur and metals [33]. The resulting liquid is usually dark brown, but depending on the type of feed and operating conditions, the resulting liquid can be black, dark red, or even dark green (if nitrogen is high). The density of this product is very high and is about l1.2 kg, while the density of light fuels is 0.85 kg/l. This means that based

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on weight comparison, this liquid has about 42% of the energy content of light fuels, and this parameter is 61% by volume. Therefore the use of this liquid in pure form (without mixing with other fuels or without refining it) requires a new design in combustion engines and pumps. Other factors influencing the use of fuels include their viscosity [141]. During the pyrolysis process, some impurities such as minerals and metals are separated from the biomass and produce coal, so there is no trace of these substances in the bio-oil produced. This is one of the benefits of producing fuel oil from biomass, from an environmental point of view and the point of view of preparation and refining stages of the product. Compared to oil, bio-oils have lower sulfur levels but higher nitrogen and chlorine levels. Table 1.6 compares fuel oil with gasoline and diesel. Bio-oils contain hundreds of species of organic matter in the form of alkenes, aromatic hydrocarbons, phenol derivatives, and small amounts of ketones, esters, ethers, sugars, amines, and alcohols with H/C molar ratio higher than 1.5. The composition of biomass (C, H, O, N, and S), ash content, moisture content and HHV are the factors that determine the quality of bio-oil produced. To extract higher quality bio-oil, the biomass is refined before the process. For example, with acid purification, part of the oxygen content can be removed from the biomass. The higher the amount of ash and potassium in the biomass, the lower the bio-oil produced [91]. Bio-oil is made up of different substances with different molecular weights, the lightest of which is water with a molecular weight of 18 g/mol and the heaviest of which can be even more than 5000 g/mol. The average molecular weight of bio-oil ingredients can be considered in the range of 703000 g/mol [142]. Based on the information in Table 2.12, bio-oils contain amounts of heavy aromatic compounds that are opposed to gasoline and diesel. Conventional transport fuels (gasoline and diesel) are a mixture of short-chain hydrocarbons (520). The H/C ratio in bio-oils is about 1.21.4, which indicates that bio-oils are more prone to aromatics than alkanes. The O/C ratio in bio-oil is high due to this product’s high water and oxygen content. The high oxygen content of bio-oils results in instability, corrosion, and incompatibility with petroleum fuels. Therefore there is a serious need to improve the quality of bio-oil, one of the most important goals in improving the

Table 2.12 Comparison of some characteristics of bio-oil with gasoline and diesel [131,132].

Carbon chain Length H/C ratio O/C ratio Density (kg/m3) Viscosity (mm2/s) LHV (MJ/kg) Water (wt.%)

Bio-oil

Gasoline

Diesel

Up to 100

510 branched Alkanes, aromatics 12 0

1220 Linear alkanes B2 0 820845 (15 C) 24.5 (40 C) 42.5 0.02

1.21.4 0.5 11001260 (20 C) 13297 (40 C) 14.3420.9 18.232

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quality is the removal of oxygen and cracking of large aromatics into smaller molecules. Finally, the improved product must contain small amounts of water and oxygen, which reduces the acidity and viscosity and increases the calorific value.

2.11.7.1 Properties of bio-oils 2.11.7.1.1 Homogeneity Bio-oil generally has a uniform and homogeneous appearance, but the situation may be different for some types of biomass, such as forest and agricultural wastes. According to product reports, the liquid produced from the extractive-rich biomass can be separated into two phases: the lighter phase of the liquid, which is rich in the extract, and the heavier liquid phase, which is similar to ordinary bio-oils. The lighter phase’s ratio depends on the feed type and can be up to 20% wt. of the total liquid product produced. All types of biomass contain small amounts of extracts, the same small amount of which is well dispersed in the bio-oil and eventually produces a homogeneous liquid product [142].

2.11.7.1.2 Multiphase microscopic structure Bio-oil ingredients come in a variety of sizes and nature, including high-polarity materials (water, acids, and alcohols), low-polarity materials (esters, ethers, and phenolics), and nonpolar materials (hexane and other hydrocarbons). Not all of these substances are completely soluble. According to reports, bio-oils structure can be considered a microscopic multiphase, which includes water and water-soluble molecules as a continuous phase. Some multipolar materials play a role in maintaining the stability of the bio-oil structure. Some reports even mention coal particles and waxy substances in the bio-oil. It is further stated that with the increase of temperature to approximately 60 C, there will be no trace of these particles anymore, which is due to the melting of the waxy materials [142].

2.11.7.1.3 Oxygen The amount of oxygen in the feed varies and can be around 35% wt. (based on wet). Oxygen is present in many organic constituents of bio-oils, which is the most significant difference between bio-oils and petroleum fuels, causing polarization and consequent insolubility of bio-oils in petroleum fuels. Large amounts of oxygen affect the calorific value, corrosion, and instability.

2.11.7.1.4 Water The water content of the fuel oil is in the range of 15%30% wt., which depends on the feed’s initial moisture content and the pyrolysis operating conditions. If the amount of water in the bio-oil reaches more than 30%35% wt., we will have a biphasic liquid that is no longer homogeneous and causes many problems in its use. Removing water from bio-oils is a complicated process. The presence of water in bio-oil has two positive and negative effects on its storage and use. On the one hand, it reduces the calorific value and even causes the fuzzy separation of bio-oils; on the other hand, it delays combustion, reduces the adiabatic temperature of the flame during combustion, viscosity, combustion contaminants emission, and

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increases sprayability. OH radicals in water also prevent the formation of soot. Low combustion temperature and water content reduce NOX production [105]. The water content of bio-oil is higher than that of gasoline, which has an adverse effect on combustion.

2.11.7.1.5 Calorific value The bio-oil produced from the fast pyrolysis process has an LHV of 1418 MJ/kg, which is a small amount compared to petroleum fuels (4143 MJ/kg), and a small energy density relative to petroleum fuels (about 60% by volume). One reason for this is that bio-oil contains 25% by weight of water (some of this water is produced by the reaction and the other part is moisture that is not removed from the feed during the drying phase). There are different methods for measuring HHV, but for measuring LHV, relationships and formulas can be used if HHV is available [105].

2.11.7.1.6 Solid particles Bio-oil contains coal particles, more or less. Cyclones are commonly used to separate solids from gases, and because of their price, they are used to separate particles larger than 10 μm. Particles smaller than this amount cool with steam and form as bio-oil. In most cases, the amount of these substances reach 3% wt. with the size of 1200 μm. Solutions such as ethanol and the combination of methanol and methane dichromate can be used to separate these particles. The presence of solid particles disrupts storage and combustion. These particles tend to settle, and this happens if there is no stirrer. Of course, these solids can be separated with the help of filters. The presence of solid particles increases the viscosity and causes problems in pumping and spraying bio-oils. Also, the presence of solid particles causes erosion and wear, clogging fuel injection pores and affecting the activity of the catalyst [105].

2.11.7.1.7 Ash Some biomass sources, such as rice straw, contain significant amounts of ash (15% wt.), which may result in coal production containing large amounts of ash (50% wt.) and, of course, the number of solid particles in bio-oil may increase. The presence of metals in the ash increases the acidity and corrosion properties. Also, ash separation through filtration is almost not possible [95]. According to studies, the most important factors affecting the viscosity of bio-oil are temperature and water content. Also, the viscosity of bio-oils does not depend on pH.

2.11.7.1.8 Viscosity Viscosity is a property that expresses the resistance of a fluid to flow due to the application of force, which plays an important role in the design and use of fuel in the fuel injection system. The viscosity of bio-oil can be 12511000 m2s (at 40 C), depending on the feed material and water content. Therefore sometimes to improve the properties of bio-oil, it is combined with different proportions of alcohols such as methanol and ethanol [94]. The longer a bio-oil lives, the higher its viscosity. To make better use of bio-oil and have a better viscosity, it can be preheated and then used in the process [91].

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2.11.7.1.9 Flashpoint, pour point, and cloud point Flashpoint is taken into account to control fuel against fire and explosion hazards. The lighter and more volatile the compounds in the bio-oil, the higher the flash temperature which can be in the range of 40 C100 C. The pour point is the lowest temperature at which a liquid flows by itself. Decrease in this parameter usually results from increased viscosity or crystallization of vaccine materials. The pour point of most bio-oils is around -33 C to -12 C. The cloud point is the maximum temperature at which the waxy material begins to form crystals and is visible. There are various laboratory methods such as ASTM D 2500 for this measurement of cloud points [103].

2.11.7.1.10 Thermal conductivity and specific heat capacity Knowing these two parameters is essential for designing heat transfer equipment such as heat exchangers. According to the reported results, the thermal conductivity of the biomass is about 0.350.45 W/m.K and its specific heat capacity is in the range of 2.53.5 kJ/kg K at a temperature of 20 C60 C. Both of these parameters are higher for bio-oil than Gasoline [103].

2.11.7.1.11 Cetane number It is a parameter with which the combustion quality of liquid fuel is measured. In fact, it indicates the ignition of the fuel when entering the combustion engine. A high cetane number means that the combustion delay period is short. The cetane number for diesel is about 48, but measuring the cetane number for bio-oil is troublesome because the combustion of pure bio-oil in conventional combustion engines is very difficult, according to studies that measure the cetane number. The bio-oil mixture has been studied with different proportions of diesel and then with the help of linear interpolation method, it has been concluded that the bio-oil cetane number is about 5.6 [103]. It should be noted that bio-oil is combustible but not flammable and we need a lot of energy to ignite it [144].

2.11.7.1.12 Acidic and corrosive properties The pH of this product is between 2 and 4 and for its storage, acid-resistant materials such as stainless iron and polyolefins should be used [144]. Bio-oil is corrosive due to its acetic acid and formic acid and causes severe damage to carbon steel, aluminum, and some other materials [144]. In this dissertation, previous research in the field of bio-oil production through fast pyrolysis and its improvement processes are reviewed. According to previous studies, it was necessary to study different operating conditions and different types of biomass. Therefore in order to achieve the stated goals, the following items have been investigated: (1) Design and simulation of bio-oil production process through fast pyrolysis mechanism and then upgrade it in a biorefinery using Aspen Plus and Aspen Energy Analyzer simulation software (2) Evaluation of bio-oil production efficiency of four different types of biomass (3) Sensitivity analysis of the modeled process and study of the effect of operational parameters (4) Study of thermal integration effect, evaluation of the results and process optimization for bio-oil production from biomass.

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Using unlimited biomass resources, bio-oil can be produced, which is an ideal alternative to fossil fuels. The use of bio-oil increases energy security, diversifies the energy basket, generates employment, reduces energy imports and, most importantly, reduces pollution. The most suitable process for the production of bio-oil from biomass is the pyrolysis process. The produced bio-oil has some undesirable properties that it is not possible to use directly in diesel and petrol engines, so it seems necessary to improve its properties before using it as a fuel source. In the following, we will study the applications and research done on bio-oils.

2.11.8 Properties of bio-oils Biomass is one of the renewable energy sources that are abundantly available on the planet and has a very high potential for use in transportation. Today, the transportation sector plays an important role in the economic and social growth of human societies [131]. To use bio-oil as an alternative fuel in the transportation sector, it must be refined and its quality improved, but its use as a combustion fuel in burners and static combustion chambers is appropriate. The use of bio-oil for combustion in engines has more advantages than direct biomass combustion. For example, bio-oil has a good energy density, which with the advancement of technology can be considered competitive with fuels used in transportation. During the pyrolysis process, a significant amount of ash is removed along with the coal, so bio-oil (relative to the direct use of biomass in combustion) has small amounts of ash, the use of which in combustion reduces ash production and emissions, soot and particulate matter (PM) [131]. Biofuels can be used as fuel in generators to generate electrical energy, as alternative fuels to gasoline in boilers, burners, engines, electric and thermal energy generating turbines, as fuel in transportation, and direct combustion in thermal power plants and combustion in gas turbines [91]. For the use of bio-oils in boilers and burners, they are classified into eight groups based on ASTM D7544, which is shown in Table 2.1. According to this classification, G-to-D-grade bio-oils can be used in industrial boilers, but they are not suitable for use in residential heaters, small commercial boilers, and engines. Dgrade bio-oil is not suitable for commercial/industrial use, but it is suitable for residential heaters and engines, and to use them, care must be taken in selecting the desired equipment [91]. Using bio-oil as fuel for diesel engines is more useful in low and mediumacceleration engines. In diesel engines with higher efficiency and higher acceleration, a mixture of 72% bio-oil with 24% methanol and 4% additives can be used to improve the octane number, although the use of petroleum fuel is necessary at the beginning to ignite [91]. The use of bio-oil in boilers is associated with changes in the boilers, because if these materials are used in the combustion chamber, compared to the use of conventional fuels, there are differences in combustion efficiency, soot formation, spraying, and the amount of gases produced. The production of carbon monoxide and particulate matter increases and the production of NOX and SO2 decreases (Table 2.13).

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Table 2.13 Classification of bio-oils based on ASTM [134]. Compounds

Peak temperature (TGA)

Mass average molecular weight Mw (GCMS)

Boiling point Tb

A Volatile nonpolar compounds B Volatile polar compounds C Monolignols D Polar compounds with moderate volatility E Sugars F Extractive-derived compounds G Heavy nonpolar compounds H Heavy polar compounds

116 C153 C

106112 g/mol

136 C162 C

120 C130 C 176 C192 C 170 C215 C B260 C

72112 g/mol

123 C187 C

94182 g/mol 110210 g/mol

182 C266 C 245 C277 C

178 C198 C 235 C300 C

390486 g/mol (GPC) 256414 g/mol

267 C450 C

255 C296 C

6521927 g/mol (GPC)

B355 C

561772 g/mol (GPC)

One of the main challenges in the field of bio-oils is the high water and oxygen content and the low calorific value. High water content affects local temperature, evaporation rate, and specific heat. High oxygen content is related to NOX emissions and production. Various methods are used to improve these cases. For example: combining bio-oil with other fuels, using organic solvents, emulsification, deoxygenation, spraying it in the combustion chamber, etc. Despite the challenges in terms of properties and use of bio-oil, several studies are underway to use it in various fields; for example, it has recently been reported that carbon emissions in the aviation industry account for about 12% of total carbon dioxide emissions in the transportation sector, and this has become an incentive to use bio-oil to produce bio-jet fuel. Biofuel jets are generally produced from nonedible biomass sources based on previously commercialized processes that include hydrodeoxygenation and hydroisomerization/cracking with hydrotreating catalysts and then distillation. Although this requires a large amount of hydrogen to remove the oxygen molecule from the bio-oil (420300 m3H2 per cubic meter of bio-oil), to reduce the amount of hydrogen consumption, decarboxylation is under study as a way to remove the oxygen molecules [145].

2.11.9 Bio-oils quality improvement Generally, four methods are used to improve the properties and quality of bio-oils [131]:

2.11.9.1 Hydrogen process to reduce oxygen content The first studies on hydrogenation of bio-oils with the aim of reducing oxygen were conducted by Elliott et al. in 1996. The reaction was performed on a laboratory

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scale and in the dimensions of a table consisting of two fixed-bed reactors at a pressure of 21 MPa. During that, they used Ni-Mo and Co-Mo as catalysts for the hydrogen curing process. Zheng and Wei studied the oxygen content of distilled bio-oils in 2011 and found that the oxygen content of distilled bio-oils was 9.2% lower than that of bio-pyrolysis. The product obtained from distillation has better stability, lower calorific value, and less corrosion.

2.11.9.2 Superheated water method to increase the calorific value In 2011, Duan and Savage [146] improved the bio-oil derived from algae (as a process feed) using the superheated water method, and with this method, they were able to produce products with an approximate calorific value of 42 MJ/kg with a lower pH than the pure oil. They also concluded that the use of zirconium/titanium and ZSM-5 catalysts with high cross-sections increases the production of liquid product in the pyrolysis process. This liquid product contains less oxygen and more aromatics.

2.11.9.3 Bio-oils esterification in atmospheric conditions The process of bio-oil stress using light molecular weight alcohols seemed to be a good solution to overcome the undesirable properties of bio-oils, which attracted a lot of attention. In recent years, many efforts have been made in this field. There are many efforts to stress bio-oils under atmospheric pressure using solid acids [147149]. The results of their work showed that the pH, density, calorific value, and stability of the final bio-oil were improved.

2.11.9.4 Bio-oils catalytic cracking Bio-oil contains oxygen and heavy hydrocarbons that can be converted to lighter materials with less oxygen and less oxygen by catalytic cracking. Reports indicate that zeolite catalysts (such as HZSM-5, HY, etc.) are very suitable and efficient in converting bio-oils to light aromatic hydrocarbons (benzene, toluene, xylene, and naphthalene). Liquid product production from the pyrolysis process began in earnest in the first experimental experiments in the 1970s. Then, various types of pyrolysis processes and pyrolysis reactors were studied and it went so far that today fast pyrolysis is known as an acceptable process for producing renewable fuel to be replaced by fossil fuels. Therefore many research institutes and universities have studied in this field, so since 1990, we have seen numerous articles and reports on large-scale fast pyrolysis units. Palm and Peacocke [150] have studied the key features of fast pyrolysis, its reaction mechanism and the results obtained, and the progress of this process over 20 years during the years before 2000 AD. The amount of bio-oil production and its properties depend on several factors. With increasing temperatures from 400 C to 500 C, the amount of bio-oil and gas production increases, which is due to the thermal decomposition of coal into

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incompressible gases and water vapor. Then, with increasing temperature, we see the thermal decomposition of bio-oil, which increases the produced gases and consequently decreases the amount of bio-oil. As mentioned in the previous sections, bio-oil has some undesirable properties that make its direct use as a fuel problematic. In the following, to overcome the undesirable properties of bio-oil, we introduced methods to improve bio-oil. It should be noted that with the help of these methods, the nature of bio-oil does not change much and we only try to improve some of its properties. Unfortunately, we see limited studies and articles in this field, which we will discuss below. The amount of greenhouse gases emitted from the biohydrocarbon product produced by fast pyrolysis. The bio-oil produced from the fast pyrolysis unit reacted with hydrogen and then entered the zeolite cracking unit to improve its properties. The final product produces less greenhouse gases than fossil fuels, and studies indicate that the amount of soil organic carbon in the feed has a direct effect on the amount of greenhouse gases emitted from the crop, and the higher the amount of biomass organic carbon, the higher the amount of greenhouse gas emissions. In addition, the amount of nitrogen injected into the pyrolysis reactor affects the emission of these gases. Also, the maximum storage potential of greenhouse gas emissions of products from hydroprocessing and zeolite cracking units is equal to 87% and 77%. A mixed bio-oil was produced from the fast pyrolysis of pine wood with gas oil and then introduced the mixture into the catalytic cracking unit of the fluidized bed to finally obtain gasoline and diesel. As a result of this process, most of the oxygen content in the bio-oil is removed and converted to water vapor, carbon monoxide, and carbon dioxide (to a very small extent) [151]. However, we see an increase in oxygenated compounds, especially alkyl phenols, in the final products. In this study, bio-oil and gas oil were tested with two ratios (ratio of bio-oil to gas-oil) 1:19 and 1:9. Due to the high polarity of bio-oil, the mixture of these two compounds is incompatible in two phases, so bio-oil and gas oil were injected into the reactor in two separate places. The results showed that increasing the temperature increased polymerization and coke formation and intensified the degradation of biooils. Therefore the bio-oil was not preheated before being injected into the reactor and entered the reactor at a temperature of 30 C, while the gas oil reacted at a temperature between 220 C and 320 C. Preheating the gas oil reduces the viscosity of the products. The catalytic cracking reaction took place at a temperature of 540 C and a pressure of 2.7 bar. Catalyst reduction was also performed at 690 C. A view of this reactor is shown in Fig. 2.47. In this experiment, it was found that using older biomass (approximately 9 months) does not cause operational problems, but using older biomass (approximately 21 months) may affect the operating conditions of the reaction. In laboratory work, the reaction of yellow poplar wood pyrolysis was performed in a fluidized bed reactor with a capacity of 200 g/h, at a temperature of 500 C and a vapor residence time of 2 s. The products were passed through the reaction through a cyclone, then into a cooler and cooled to 0 C. During cooling, compressible gases were converted to crude bio-oil, which converted about 63% wt. of dry biomass to crude bio-oil.

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Flare Flue Gas

Freon cooler Cold Water Regenerator

Reactor Riser

Stripper Steam Torch oil

Water condensate Liquid product Feed

Spent Catalyst Regen. Catyst

Feed Regen. Air Lift Steam

Figure 2.47 Schematic of a fluidized catalytic cracking reactor [132].

The crude bio-oil extracted from pyrolysis is then introduced into the HDO unit to reduce its water content and increase its calorific value. The HDO process took place in an autoclave reactor with a capacity of 40 g. In this system, temperature control was done with the help of electric heater and water coil. Hydrogen gas was also injected into the reactor at a pressure of 40 bar. This reaction was investigated under the following conditions. A schematic of the pyrolysis unit is given in Fig. 2.48, which is divided into nine sections: feeding, crushing, insulation, main reactor, cyclone, coal collector, cooler, electrostatic settling, and filter. The final products of the reaction were gas, coal, and two liquid phases (aqueous and organic) called light oil and heavy oil. The results of the experiment showed that the temperature, reaction time, and the amount of catalyst in the HDO process have a direct effect on the distribution of products. Increasing the temperature and reaction time leads to a decrease in the amount of heavy oil (organic phase) because with increasing temperature and increasing the reaction time, thermal cracking was intensified and more gas and coal were produced. The resulting liquid products had a lower water and acid content than crude bio-oils. The acidity of the produced heavy oil was less than that of

How to use renewable energy sources in polygeneration systems?

P

V2

99

P

V3

V1

P 7 1

N2

4

5

9

2 M

6

TC

8 gas

TC 3

oil

Figure 2.48 Schematic of the process mentioned above [132].

the crude bio-oil so its pH was about 3.7 to 4.5. Water content and calorific value of heavy oil were in the range of 0.19 wt.% and 7.428.37 MJ/kg. The calorific value of the produced heavy oil was almost twice the calorific value of the crude bio-oil. The biorefinery included a hydrogen processing unit, a hydrogen cracking and distillation unit, and a steam reforming unit. In this process, in a biorefinery, bio-oil is converted into a combined biofuel, which is a mixture of gasoline and petrol. To simulate each type of biomass, they defined unique kinetics for the pyrolysis reaction that increased the accuracy of their work. One of the by-products of pyrolysis is coal. In this process, part of the coal produced in the pyrolysis unit was used to produce steam in the steam reforming unit of the biorefinery and the other part of the coal was used to provide the heat required in the fast pyrolysis process. The flow chart of this process is shown in Fig. 2.49. According to their studies, the amount of greenhouse gas emissions from the final product of this method is 54.5% less than gasoline and fossil diesel, and it is predicted that with increasing thermal efficiency, this number will improve and even the limit reached 60%88%. Therefore this process can be considered a lot. The flowchart includes the following [132]: 1. Biomass transport: The capacity of the pyrolysis unit was 11 t /h of feed (based on wet) at 7000 h/y, so 38,500 tons of feed (based on dry) was needed annually, which led to the consideration of the biomass transmission unit in the flow block diagram. Therefore a circle with a diameter of 44 km was considered in the center which was the pyrolysis unit, and the average distance between biomass resources and the pyrolysis unit was considered to be 15.5 km. 2. Pyrolysis unit: At first, the biomass entered the dryer (using hot exhaust gases from the combustion of coal and gas produced in the pyrolysis unit) and its humidity reached 7%. It was then cut into 3 mm pieces and then fed into the pyrolysis reactor, whose output

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Figure 2.49 BFD of a bio-oil process [132]. consisted of bio-oil, coal, and gas. The products were then removed from the reactor, and coal was obtained by cyclones and bio-oil by the cooling process. The gas and coal produced were burned in the combustion chamber to provide heat for the reaction. Then, with the help of a cyclone, the ash was separated from the exhaust gases of the combustion chamber, and these hot gases were used in the drying section. 3. Bio-oil transfer unit: The average distance of bio-oil transfer from the pyrolysis unit to the bio-refinery was considered to be 200 km.

In a refinery, bio-oil is converted into petrol and gasoline, which is consisted of 3 parts: hydrogen processing, product separation (including hydrocracking), and steam reforming. The refinery was operating at 7000 h per year [132]. In the hydrogen processing unit, the goal was to remove oxygen from the biooil, which was done in two steps. Finally, the oxygen content of bio-oils was less than 2%. In the distillation and hydrocracking unit, the water is separated from the bio-oil and its heavy compounds are converted into lighter compounds, and biofuel and biogasoline are extracted. In the steam reforming unit, part of the hydrogen required by the hydrogen processing and hydrocracking units is produced with the help of light gaseous hydrocarbons produced in the distillation section. Another part of hydrogen was produced through the consumption of natural gas. The pyrolysis unit produced 68.8% bio-oil, 14.3% gas, and 16.9% coal. 61% of the coal produced was burned to provide heat to the process. In the biorefinery, 0.96 MJ of bio-oil was used to produce 1 MJ of biofuels, in addition to which 0.27 MJ of natural gas energy and 0.04 MJ of electric energy were used. Finally, liquid products produce about 54% petrol, 35% gasoline, and 11% heavy waste. 70% of the electrical energy was used in the pyrolysis section, most of which was related to air compressors to supply the air needed for combustion. Fig. 2.50 compares the amount of greenhouse gases released from fossil fuels and the fuels produced through this process. According to their studies, the amount of greenhouse gas emissions from the final product of this method is 54.5% less than gasoline and fossil fuel.

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Figure 2.50 Greenhouse gas emissions comparison from produced fuels [95].

Figure 2.51 BFD of the biorefinery [132].

The exergy of biorefinery performance and the pyrolysis process, in which biooil produced from fast pyrolysis is converted into fuels such as gasoline and biodiesel. In this process, the biorefinery consists of three main parts: the hydrogen processing unit, the distillation and hydrocracking unit, and the steam reforming unit. The flow chart of this biorefinery is shown in Fig. 2.51. With the help of exergy analysis, the useful part of energy can be identified and the dysfunctional parts of thermodynamics can be accurately identified. Current exergy consists of four parts: potential exergy, kinetic exergy, physical exergy, and

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chemical exergy. The calculation of the exergy balance and exergy efficiency for all compounds in the units shows that the total exergy efficiency for this process was 60.1%, while the exergy efficiency of the biorefinery section was 77.7%. In the biorefinery section, the steam reforming reactor was the most inefficient part, the main reason being the high operating pressure in this unit, and in the pyrolysis unit, the most inefficient part was related to coal and gas combustion. Compared to other biomassrelated methods such as gasification by FischerTropsch process or ethanol production process from biomass, the mentioned process had higher energy efficiency.

2.12

How to combine hydroenergy systems and polygeneration systems?

Impoundment, diversion, and pump storage installations are the three types of hydroelectricity plants. Hydropower facilities vary in their utilization of dams. Even though not all dams were constructed for hydropower, these structures have successfully supplied vast amounts of renewable energy to the grid. As of 2020, less than 2300 of the more than 90,000 dams in the United States were producing electricity. The other dams are utilized for water supply, irrigation, flood control, stock/ farm ponds, and recreation. Hydropower facilities come in various sizes, from modest initiatives providing energy for a single house or community to massive undertakings. Learn more about hydropower plant sizes [152,153].

2.12.1 Impoundment An impoundment structure is the kind of hydroelectric power plant that is most prevalent. A dam is used in an impoundment plant, usually a big hydroelectric plant, to hold river water in a dam. Freed water from the dam spins an expander such as a turbine as it passes across, starting a generator that generates power. The water may be released to address shifting power and other requirements, including flood control, leisure, fish migration, and other aquatic and environmental quality requirements [152,153] (Fig. 2.52).

2.12.2 Diversion A diversion, often known as a “run-of-river” plant, directs a valley segment via a waterway and a penstock to harness the energy-producing potential of the river’s natural elevation decrease. Water flow is controlled by gates, valves, and turbines in a penstock, a controlled pipe that directs water to turbines. A diversion may not need a dam [153,154] (Fig. 2.53).

2.12.3 Pump storage Pumps storage hydroelectric, often known as PSH, is another form of hydropower that functions similarly to a large battery. The power generated by alternative

How to use renewable energy sources in polygeneration systems?

Figure 2.52 A conceptual design of impoundment [153].

Figure 2.53 A view of a diversion hydrosystem [153].

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energy sources, such as solar, wind, and nuclear, can be stored at a PSH plant for future usage. Water pumped from a resource at a lower altitude to storage at a higher level is how these systems store energy. A PSH plant stores energy by pumping water from a lower reservoir to a higher pool when the requirement for power is low. When there is a significant requirement for energy, the water is discharged back into the lower reservoir, where it drives a turbine and produces electricity [155].

2.13

Hybrid power generation of hydropower

Recently, there has been much interest in the idea of employing hydrosources to integrate with other power generation and balance out the unpredictable power supply and increase plant reliability [155,156]. For example, in terms of integration with other renewable resources such as solar energy, the key argument in favor of statements that hydropower can incorporate variable solar energy sources is the compatibility of their temporal characteristics, particularly the hydropower’s adaptability owing to its storage capability. Several strategies and alternatives may be utilized to assist and smoothly integrate variable renewable energy sources (VRES), particularly photovoltaic systems and wind generation. Going to connect scattered generators, utilizing supplementary, nonvariable energy sources to supply requirements, implement real-time responses and demand-side management, batteries where it is generated, sizing up VRES to supply demand and hydrogen production, storing electricity where it is consumed, and in electric cars, as well as using weather predictions to estimate the power generation of photovoltaic or wind turbines are all examples of ways to meet the requirement [157].

2.13.1 Hydrosolar hybrid power generation systems Considering all the factors mentioned earlier, it can be argued that solar and water resources show tremendous promise for combining into a single hybrid energy source. Most of the time, hydropower is recognized as a backup asset utilized to balance the network when other VREs are overcapacity or unable to meet the requirement for energy. Nevertheless, modest unregulated run-of-the-river (RoR) power facilities with power capacities under 10 MW may contribute significantly to localized hydropower production [158]. The level of complementarity across PV and solar RoR provides decision-makers on the assessment’s temporal and hydrological scales. It is critical to examine various hydrological forecast techniques’ propensity to anticipate the synergy involving solar PV power and tiny RoR power. By utilizing the standard error of the variations between power production and load, the complementarity between RoR and solar PV should be evaluated. The larger the standard error of the energy output, the fewer the complements and the higher the balancing costs are; this indicator is a good proxy of the balancing costs owing to energy transit and storage activities necessary for rebalancing generation-

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load variations. The energy balance time series are often estimated for different energy balance possibilities, including RoR and solar PV power [158]. The potential of flowing water is used to generate power hydroelectrically. Runof-river electricity is produced by power facilities situated across a river channel; in other words, no upstream dam regulates the water flows into the plant. The production of RoR power during time is determined via: PROR ðtÞ 5 ηHgh ρ qðtÞ

(2.90)

where g represents the acceleration of gravity (59.81 m3/s2), PRoR is the supplied power by the plant generator (kW), ηH shows the generator’s efficiency, the water flow going through the turbines (m3/s),ρ the water-density (51000 kg/m3), and h is the failure height (m). The output of an RoR power plant is constrained through technical or ecological restrictions, as shown in Fig. 2.3. Flow of the design Qd is related to the instrument’s features (e.g., turbine design, penstock capacity). It reflects the greatest amount of river water that has been extracted. The environment’s movement Qmin represents the minimum river flow necessary to meet specific ecological goals. The production must cease in order to avoid causing damage to the structure when the river flow surpasses a safety threshold or Qmax. The hydropower plant’s intended use determines the values of Qmin, Qd, and Qmax. The following sets Qmin, Qd, and Qmax to the 95th, 25th, and 2nd percentiles of the water discharge (Fig. 2.54). In Fig. 2.55, a gray shaded region denotes the design volume Vd or the water volume used for electricity production. At a particular time t, the horizontal global sun irradiation Leff (W/m2) and Ta, ambient temperature determine how much energy a horizontal photovoltaic generator (PPV) can produce [159].    PPV ðtÞ 5 BIeff ðtÞ 1 2 μ Ta ðtÞ 2 Tc;STC 2 μCIeff ðtÞ (2.91)

Discharge (m3s-1)

Qmax

Qd

Qmin t(Qmax) t(Qd) t(Qmin) Percent of me indicated flow was equaled or exceeded

Figure 2.54 A long-term total flow graph in schematic form. The lowest and highest discharges are Qmin and Qmax. The design flow is Qd. The region of light gray shows the design volume employed for power generation. A gray shadow region serves as a representation of the design volume [160].

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Figure 2.55 Normalized recorded and predicted mean periods of flowing water for the recipient basins over a sliding period of thirty days, the cycles are flattened [158].

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Using T and C, which stand for the temperature- and radiation-dependent performance decrease indices, as well as B, the PV element’s surface area multiplication by the generator and inverter performances. Tc, STC is the cell temperature compared to the usual test scenario. The local demand is determined by averaging the domestic consuming timeseries data in accordance with the Gross Domestic Product of the administrative units included in the transect, which is approximately equivalent to 8% of the Italian GDP. The modeling approach by [160] implies that the temperature and a few other sociological and economic factors affect how much power is consumed (such as the distinction between weekdays, weekends, and vacations.). However, the efficiency is reasonably strong, with NSE values for the validation and calibration phases of 0.85 and 0.79, accordingly (200609 and 201012, respectively). The assumptions behind the energy supply predictions assume that the power demand may only be met by power by PV and RoR energy plants. The sequence for the generation of RoR and PV power is adjusted enough so, on average, the generation equals the load for the whole period. pðtÞ 5

PðtÞ hLt i; hP t i

(2.92)

where P is the power produced through one source of power, L is the demand, and p is the time series period of proportional energy output. The typical operator is , .. This presumption relates to a 100% sustainable condition for the territory. Mixed power generating scenarios are generated by combining PV and RoR power generation with a distribution coefficient known as SPV. PMix 5 SPVpPV 1 ð1 2 SPV ÞpH;

(2.93)

where PMix is the integrated system’s power output. When solely RoR power plants generate electricity, SPV 5 0 and, conversely, 1 once all electricity is produced by the photovoltaic system. The energy balance for a given time step t is defined as the deviations Δ between the generation PMix and the electricity consumption L.

2.13.2 Hydrowind hybrid power generation systems It is preferable to rectify the fluctuating production of wind power in actual time utilizing hydrowind power than to use the maximum regulation capability of hydropower facilities. Essentially, water initially utilized to create electricity may be stored in dams, and hydroelectric lower production when the wind energy output rises. In order to make up for the loss of wind power whenever it occurs, the hydropower supply is immediately boosted. The inconsistent and erratic wind power production is now a reasonably consistent quantity when superposed by the hydropower’s compensating output. The production of consistent and dependable hydrowind energy is then achieved [162,163]. In particular, hydropower plants

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regulate the peak load of the electrical system and make up for the unpredictable and intermittent nature of wind energy production. The ability of hydroelectric plants to regulate, nevertheless, has been greatly limited by the extensive needs of several water-using agencies. In order to minimize hydropower restriction, it is imperative to utilize the hydrowind compensation capability fully. The real power production of hydropower plants must be higher than the forced energy output of hydroelectric in order to satisfy the requirements for water and stream for various dam usage activities and prevent hydroelectric curtailment [163,164]. One of the primary proposed hybrid applications of a hydrowind hybrid power generation system is presented for Greek islands [165]. In that, a reversiblehydropower plant, a complementary water-pump plant, and a wind generator are all operationally explained. The goal of this project is to significantly lessen the islands’ reliance on energy derived from petroleum and its byproducts. Fig. 2.56 displays the schematic design and energy flow of the implemented hybrid system. The following stages provide the basis for how this system operates: When there is extra power, it is transferred to the water-pump plant, which transports from a lower reservoir to a superior stage and stores it in the form of hydrodynamic energy. The energy generated by the wind generators is supplied to the customers. The hydroelectric system generates energy utilizing the power generated by the water held in the higher storage when the wind turbine cannot adequately meet user power needs. The system employs the APS for as long as this severe circumstance lasts when the shortage of energy (lower energy output from the wind generators and low-level water in the reservoir) is projected to be long-term. When the energy requirement is too great for either wind or hydrosystems to provide, the usage of internal combustion engines is also required. It is advised to add an energyconsuming device (like a desalination plant) to use the excess energy generated by the wind farm [165]. Another possibility of developing a hybrid wind-hydropower generation plant is contingent upon achieving the highest degree of energy system independence from petroleum products and having a low initial installation expense. As a result, a methodology for estimating the best wind-hydrosolution is created and then used in several typical island situations in the Aegean Sea in order to determine the best layout for this renewable facility. Fig. 2.57 shows a combined wind-hydrohybrid system [166]. The total electrical balance for a sample 10-year span for all islands under investigation is also shown to clarify the process and ensuring of this system. Based on these observations and findings, a significant portion of energy demand is supplied by wind farms, and any energy shortfall is primarily filled by hydroelectric turbines, reducing the need for the local autonomous power station (APS). In every example examined, the saturation of renewable energy sources may be greater than 85%, which would minimize the associated exchange loss caused by the decline in imported oil as well as the majority of the harmful ecological impact associated with the use of internal combustion engines. Fig. 2.57 shows a diagram of a hydrowind hybrid power generation system with an upper and lower reservoir. Windhydropower system (WHPS) in a hybrid form can be further developed in many parts of the world and can now be developed in places where fossil fuels

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>

(A)

Hydro Pumps

>

Wind Turbine

>

Diesel Generator

>

>

>

Parallel Operation

Hydro Turbine

Dump

Dump

>

>

>

Local Grid

(B)

Dump at WTG

Wind Turbine Generator

Dump at WTG Local Grid

Hydro Turbine Generator

Figure 2.56 Conceptual diagram of a hydrowind hybrid system [165].

are typically used, primarily due to new technological developments and economic conditions. This PGS can be used as a storage technology and thus become an optimum complement to the rapidly increasing harnessing of intermittent sun and wind power. Today, pumped hydroelectric storage is one of the most economically viable energy storage methods. At times of low electrical demand, excess electrical capacity is used to pump water into an upper reservoir [163]. Whenever we have a greater demand, an expander such as a turbine releases water back into the lower reservoir, producing hydropower. Reversible turbine/generator combinations can perform pump and expander functions. Nevertheless, the main technological,

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Figure 2.57 A schematic diagram of a hydrowind hybrid power generation system [166].

economic, and environmental obstacles prevent the expansion of hydrowind power generation systems: The year-round accessibility and fluctuation of water and wind sources affect production. It is vital to put in place advanced control methods that raise operating expenses in the event of undesirable weather conditions (such as cold temperatures). It is essential to create policies to safeguard the environment when a WHPS is being built, operated, and dismantled.

2.13.3 Hydro, solar, and wind hybrid power generation systems The previous method of hybridization is shown in Fig. 2.58, in which a hybrid charge controller (HCC) is used to combine the output of the PV and wind turbines, and the electricity from the HCC is delivered into the large battery to charge the batteries. Moreover, a battery charger with the given ability is used to charge the batteries using the MHP’s power. The DC electricity in the battery bank is subsequently transformed into AC power by an inverter and sent into the grid. This arrangement is straightforward and inexpensive, but if the producing systems are far apart, it can be less affordable because two distinct lines are needed to send electricity to the customer and recharge the batteries [167]. The latter hybridization method employing GTI is shown in Fig. 2.59. Similar to the earlier method, an HCC combines PV and wind turbines to charge the battery bank. The new methodology was utilized to hybridize the three RE systems, but instead of utilizing a battery charger, this technique basically synchronizes the hybrid PV-wind system with the mini-grid to provide a dependable electricity supply for the towns. Using a stage process transformer, medium voltage (MV)

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Figure 2.58 A hybrid charge controller and a battery charger, the PV, wind, and hydroelectric generators are hybridized [167].

Battery Bank

Grid Interactive Inverter

PV Generator

Wind

Solar

Hybrid Charge Controller ~ Load

Wind Generator

Generator Water Turbine

ELC

Heater

Figure 2.59 Employing a grid tie inverter to synchronize the hybrid PV-wind and microhydropower systems [167].

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transmission lines (11,000 V) were built to connect the two communities. Two stepdown transformers were erected in various places to reduce the power to 380 V (phase-to-phase voltage) and deliver it to the homes [167]. In places where the public power grid is costly, an HRES is a comparatively cost-effective alternative, making it appropriate for electrical applications in distant locations. The dependability and reliability of the power are improved when scattered producing units are integrated into a mini-grid. In order to combine PV, wind, and hydroelectricity into a single small grid, the article presented two hybridization strategies. The findings showed that these systems might be made hybrid using a HCC, a GTI, and an appropriate electronic load controller (ELC). Other off-grid systems might benefit from the use of such a strategy [167]. The HY-5 kW wind turbine, PV module, hydrogenerator, converter, battery, diesel generator, primary and deferrable loads make up the Taba (B) hybrid system’s components. 15% daily and hourly noise level is added to the principal load to account for random volatility. Fig. 2.60 depicts a hybrid arrangement produced. The power curve for the chosen HY-5 kW wind turbine is shown in Fig. 2.60. Figs. 2.8 and 2.9 exhibit the combined wind speed PDF and sun radiation data [168]. The society’s total hourly power demand, which includes lights, a TV, radio, oven, medical center, hospital, pumping stations, and grain mills, is calculated. The main peak usage for the entire neighborhood (about 10,500 homes) is predicted to

Figure 2.60 An example of a hydro, solar, and wind hybrid power generation systems [168].

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be 1424 kW, the non -dispatchable peak consumption to be 52.25 kW, and the storage capacity to be 1248 kWh. According to its resource, Taba (B) is assigned a 3.45% part of the overall load for analysis [168].

2.14

Polygeneration systems that use wave energy resources

In the electrical power industry, it is predicted that the percentage of renewable power sources will rise from 25% in 2017 to 85% in 2050, with marine renewable resources such as wave, tidal, geothermal, and salinity gradient accounting for 4% of the total power output. To achieve that objective, new methods for designing the electricity system, operating the system and the market, and formulating rules and policymaking will be needed. The percentage of power used by users will have to rise from about 20% in 2015 to 40% in 2050 as the share of low-carbon electricity increases, and it emerges as the promising energy carrier [169]. Due to the frequently dramatic damaging impacts of waves, ocean energy from waves is the most obvious form of energy. Since the waves are created by wind, they constitute a secondary clean power source. The primary drawback of wave energy is its (primarily stochastic) unpredictability across various periods, including from wave to wave, with ocean conditions, and from month to month. The evaluation of the wave energy resources is a fundamental requirement for the development of wave energy devices as well as for the strategic plan of its use. Although the behavior of the wave conditions has been performed in the past for other uses, such as route planning and marinas, coastal, and offshore technology (in which wave is viewed as an annoyance), the data requested does not match that required for planning and designing wave energy usage [170]. Wave energy utilization is now the subject of several proposals and concepts. The transformation for wave energy may be broken down into three basic steps, as shown in Fig. 2.61: the primary conversion stage, the secondary conversion stage, and the tertiary conversion stage. By exchanges between the wave bodies, the wave exchanger first converts the waves’ kinetic energy (e.g., buoy oscillation, airflow, or water flow). Through the power take-off (PTO) system, the body movement energy is converted into power in the secondary conversion stage. The tertiary stage uses power electronic linkages to tailor the generated power’s characteristics to the grid’s demands [171].

Figure 2.61 Principal concepts of wavy energy system [171].

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Figure 2.62 A schematic concept (A) and real view (B) of hanging wavy energy system [171].

A hanging component coupled to the pumping units, hydraulic storage, a hyperbaric tank, and a generator make up this system, as shown in Fig. 2.62. The pump actuator, which moves the water inside the closed loop to a hydropneumatic collector, is propelled vertically by the moving object because of the interactions between the wave and the body. A previously compressed hyperbaric chamber is linked to the accumulator. The pressurized water is then used to power an electrical generator and a hydraulic turbine. The hyperbaric chamber functions as an energy storage mechanism, reducing the power variations brought on by the sea waves’ cyclical nature. 250400 m of the water column is the range of the applied pressure (m.wc) [171]. Energy prices must be compatible with solar and onshore wind power generation through large-scale applications, training curves, and development. In the next 3 years, implementation in the open waters is anticipated for around 27% of the current plans, which are now in the predeployment stage. Most research, development, and testing initiatives have been undertaken by universities and entrepreneurs, except for the tidal range technology, which is already near the commercialization stage [171].

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Energy storage type and size in PGSs

3.1

3

Introduction

Renewable energy sources can be unpredictable, causing issues for power delivery systems like the electrical grid. Energy storage systems are a practical solution, allowing excess energy to be saved when unavailable. This ensures a consistent and reliable energy supply [1]. Effective energy storage systems, especially those for power storage, play an essential role in meeting the immediate demand for renewable energy sources. By doing so, we can eliminate the need for flexible fossil fuel auxiliary systems and promote a more sustainable approach to energy development [2]. Reliable and sustainable production of renewable energy can be achieved through the development of better energy storage system [3]. Methods for storing energy may be divided into electric power and thermal energy storage (TES). Electrical storage specifies five categories: (1) mechanical, (2) chemical, (3) electrochemical, (4) magnetic superconductivity, and (5) cryogenic. Thermal storage can be described in the paragraphs below. The three primary forms of mechanical energy storage are flywheel, compressed air energy storage (CAES), and pumped hydro storage (PHS). Mechanical energy storages, such as CAES and PHS, are used more on a grid scale than other energy storage technologies. However, PHS is often constrained by geological and topographical requirements, as well as some commitments on water rights, whereas CAES merely requires an appropriate cave for grid sizes. Moreover, CAES provides advantages such as a vast storage capacity, cheap capital cost, extended lifetime, and practical cycling that constantly draw scientific studies instead of PHS, which appears to have developed. The traditional gas turbine idea serves as the basis of the CAES technology. The extra electricity drives a compressor, which allows airflow to gather in large amounts. Next, at peak times, the collected air is forced by an expander to produce electricity. The primary CAES project, a 290 MW facility, started to operate in 1978 with a 43% productivity. It was constructed close to Huntorf in Germany. The subsequent one, which was put into use in Alabama, United States, 13 years later, was 12% more effective, thanks to the recovery of combustion gases. Both of these plants fall within the classic system category of diabatic-CAES (D-CAES), which loses heating in the process of compression, necessitates extra supplementary energy, and produces greenhouse emissions. This problem attracted study interest before adiabatic compression energy storage (A-CAES) was invented as a new tool for expansion, replacing TES. Compressed air storage (CAS) was where the initial versions of A-CAES stored high-temperature air after compression. However, it has not yet been commercialized due to its poor energy density, low round-trip efficiency (RTE), and other Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00013-X © 2024 Elsevier Inc. All rights reserved.

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constraints on the CAS element. Therefore a novel sort of A-CAES was considered to store thermal energy during compression and release it during expansion. Reusing thermal energy in A-CAES through TES results in a more environmentally friendly system and helps compress cooled air, resulting in a more effective process. The three components of TES are practical heat storage (SHS), latent heat storage (LHS), and thermochemical energy storage (TCES), which is more analogous to the relevant theoretical processes. In the past, compacted beds and double tanks of fluid heat storage were used in solar power systems to store sensible heat. Contrastingly, latent heat storage (often called PCM) is gaining interest because of its high energy density. Highly contextualized modeling of packed bed thermal energy storage (PBTES), but no definitive study. In several manufacturing cycles, PBTES is a TES device that collects heat inside its porous solid components. It was anticipated that PBTES would be more than 70% in RTE.

3.2

Operational possible ways for thermal energy storage in PGSs

3.2.1 Thermochemical energy storage in PGSs TCES may be seen as an energy-efficient strategy providing several opportunities for energy resource conservation and decarbonization [4]. In order to store energy as chemical potential, TCES uses a reversible chemical process and benefits from strong chemical reactions. Owing to regardless of the temperature method of storing, this sort of energy storage potentially delivers increased energy efficiency with little loss of efficiency over prolonged storage [5]. Both heat and mass transport processes could be enhanced by utilizing high porosity designed carrier materials distributed with reactive material within [6]. The fundamental workings of a conventional TES device are shown in Fig. 3.1. Thermochemical storage systems are receiving much attention nowadays due to their advantages over other option technologies, such as high working temperature and energy density. Solar energy is converted into chemical energy and preserved in thermochemical storage devices through a reversible reaction. When sunlight cannot be accessed, the heat produced by the endothermic reversible reaction is utilized for the heat transfer fluid (HTF) in the power process. This is the way these processes function. Concentrated solar energy is used to advance the reaction. As shown in Fig. 3.1, several substances, such as hydroxides, metal oxides, carbonates, organic matter, hydrides, and ammonia, have currently been studied for their potential to storage thermochemical energy. Regardless of the fact that there are several possible candidates for TCES, exceptions are made for substances like PbCO3 (Fig. 3.1), which may be dangerous to humans and are typically not taken into account seriously for more research. Comparable to sulfates sulfurates’ usage might be forbidden because of sulfur oxides, a gas-phase reaction product that is poisonous and destructive [8]. The associated

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127

Figure 3.1 Thermochemical TES system at medium and high temperatures [7].

reaction temperatures between 50
C and 300
C make several hydroxides and carbonates, such as Mg(OH)2, Mn(OH)2, ZnCO3, and MgCO3, unsuitable for elevatedtemperature applications in energy storage [9]. The necessity to store reactants such as CO2 or H2O and the potential have to distinct or evaporation reaction products while dealing with carbonates, hydroxides, and biological processes creates significant challenges for the energy storage industry [10].

3.2.2 Thermochemical energy storage using CaO/CaCO3-CaCl2 The CaO/CaCO3 cycle has garnered interest among the numerous TCES kinds currently being developed owing to superior energy density, competitive prices, and accessibility of raw resources. Reaction (3.1) of the calcination reaction serves as the starting point. CaCO3 breaks down into CaO and CO2 in this highly exothermic process. CaCO3 ðsÞ2CaOðsÞ 1 CO2 ðgÞ calcination=carbonation

(Reaction 3.1)

Moreover, it is important to note that this process results in energy storage. In the reverse exothermic reaction, CaCO3 is made from CaO and CO2, and heat is released during the discharge phase. The deterioration of CaO over numerous reactions is one of the challenges with this process. A simple and effective way to solve this issue is to dissolve CaO in CaCl2. Additionally, this approach enhances the procedure and boosts energy storage density. The charge and discharge reactor supplies are CaCO3-CaCl2 and CaO-CaCl2, accordingly. Each of these compounds forms eutectic between 650
C and 850
C. Along with storing chemical energy through the appropriate reaction, the CaCO3/CaO-CaCl2 cycle also stores sensible and latent heat. In this manner, the

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Hybrid Poly-generation Energy Systems

solid composition of CaCO3-CaCl2 results in energy storage in the charge mode through the calcination process and melting. By the carbonation process and solidification, the CaO-CaCl2 also releases energy in the discharge mode. As a result, the phase transition process and TCES work together to address the degradation issue and enhance the energy storage process. The mentioned operating condition is suitable for using the Mg-Cl hydrogen production process. This production cycle works in a thermochemical low temperature. In the following, we observe how a combination of CaO/CaCO3-CaCl2 TCES with Mg-Cl hydrogen production process using solar energy as a renewable resource leads to a hybrid poly-generation. The Mg-Cl cycle is a water-splitting thermochemical process for producing hydrogen. T, heat dissociation of water produces hydrogen and oxygen through several processes (net reaction: H2O H2 1 O2). Due to its low temperature, energy and exergy efficiencies, and versatility in combining with other energy sources, particularly renewable ones such as solar, the Mg-Cl cycle is among the most promising. MgCl2 ðsÞ 1 H2 OðgÞ ! MgOHCl 1 HClðaqÞ MgOHCl ! MgO 1 HClðgÞ MgO 1 Cl2 ! MgCl2 1 0:5O2

hydrolysis

(Reaction 3.2)

decomposition

(Reaction 3.3)

chlorination

(Reaction 3.4)

2HClðgÞ ! H2 ðgÞ 1 Cl2 ðgÞ ð1:4 VÞ dry electrolysis

(Reaction 3.5)

2HClðaqÞ ! H2 ðgÞ 1 Cl2 ðgÞ ð1:8 VÞ

(Reaction 3.6)

aqueous electrolysis

Reactions (3.2)(3.5) occur in hydrogen production (Reaction 3.6). MgCl2 and water H2O undergo a first reaction known as hydrolysis, in which they combine to form MgOHCl and HCl. This exothermic reaction happens at a temperature of 280
C. The generated MgOHCl is broken down into MgO and HCl at 450
C via endothermic dissolution. The MgO produced during the breakdown process interacts with chlorine in the following step, known as chlorination, to produce MgCl2 and oxygen. The MgCl2 result of this endothermic reaction, which frequently occurs at 450
C500
C, serves as the reactant for the hydrolysis process. The creation of hydrogen from HCl is the last stage. The first and the third processes’ outputs of HCl would result in hydrogen. It is due to the hydrolysis reaction’s high water-to-hydrochloric-acid ratio, which makes the HCl aqueous, increasing the work and power required during the electrolysis phase. Therefore it seems illogical to execute electrolytic processes in a single step. By participating in aquatic and dry electrolysis, HCl (aq) from the hydrolysis process and HCl (g) from the chlorination reaction create hydrogen. At 70
C, both electrolysis processes take place. Fig. 3.2 depicts the schematic design of the CaO/CaCO3-CaCl2 TCES and Mg-Cl hydrogen production systems combined with a solar collector. A solar

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129

Q=4.298 MW W=11 MW

Auxiliary Heater QDischarge=9.53 MW

QCharge=11.01 MW

Mg-Cl Hydrogen Production

Central Receiver m=30.2 kg/s

H2 production = 35.6 mol/s

60%wt NaNO3/40%wt KNO3

Heliostat Field = 30000 m2

Figure 3.2 Schematic of an Mg-Cl hydrogen production process and CaO/CaCO3-CaCl2 thermochemical energy storage [11].

tower structure is used as the heat source in two cycles due to the increased temperature. Nevertheless, the cycle has a backup heater in case there is little or no irradiation. The solar unit supplies the heat load for such processes, which produce hydrogen and store energy in the TCES and Mg-Cl parts. The fluid of the solar system, molten salt, is heated up in the primary solar collector during charging. It penetrates the TCES if the output temperature of the primary receiver is high enough (905
C). If not, it avoided the TCES. The supplementary burner elevates the temperature to the requested value whenever the output temperature of the TCES or bypass flow is below the operating temperature of 515
C. It then goes into the hydrogen production system and returns to the primary receiver. The hydrogen-producing unit’s output stream reaches the TCES during the discharging process. The TCES heats the thermochemical process that was stored in the recharging state. It must be observed that if the stored energy runs out, the flow leaves TCES unchanged. Here is a breakdown of each of the element functions. Hence, energy storage and hydrogen generation are accomplished using the energy of the unique solar collector. In case of limited sun irradiation, it results in low heat-gain, which causes the energy stored in the TCES to be used to produce H2. An additional heat exchanger is running parallel, if needed, to elevate the operating stream temperature of the solar unit to the desired temperature.

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Hybrid Poly-generation Energy Systems

3.2.3 Thermochemical energy storage uses CaO/Ca(OH)2 In another example, the TCES can be a heat exchanger that is roughly comparable to an air-cooled heat exchanger. In charging mode, the air drawn in by the fan warms up by flowing over the banks of tubes, as depicted in Fig. 3.3A. The energy from the heated air is subsequently used to start the endothermic reaction, whose byproducts are calcium oxide and steam. It was believed that the storage of the reaction products would not vary. The disintegrated components will be repurposed in discharging mode (Fig. 3.3B). The air passes over the substance undergoing the exothermic reverse reaction, which generates heat, and absorbs it. The banks of the tubes receive the heat after that. Next, the mentioned TCES system is linked with a solar thermal power generating process using the Rankine cycle. The TCSE in Fig. 3.4 employs molten salt (60 wt.% NaNO340 wt.% KNO3) as the HTF and a parabolic dish collector to supply Inlet HTF (T1)

High-Temperature

Low-Temperature

H2O Ca(OH)2 CaO Outlet HTF (T2)

(A) Inlet HTF (T9)

High-Temperature

Low-Temperature

H2O Ca(OH)2 CaO Outlet HTF (T2)

(B) Figure 3.3 A schematic view of CaO/CaCO3-CaCl2 thermochemical energy storage and discharging: (A) charging and (B) discharging mode [12].

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131

Figure 3.4 Solar Rankine cycle with thermochemical energy storage is shown schematically [12].

the system’s necessary thermal energy. The thermochemical energy storage, which uses Ca(OH)2 as the chemical coupled, receives the outflow flow from the collectors (point 1) and stores it. The flow reaches the thermochemical storage if the temperature is high enough to initiate the process; it reaches the supplementary heater (p2) devoid of transferring additional heating. Supplementary heating has been considered to compensate for the system’s insufficient energy needs. Additionally, there might be occasions when the TCES system is unable to provide the system with the energy it requires. The supplementary heating turns on if the TCES’s temperature difference (p2) is below the appropriate level. The steam Rankine cycle is conducted when the stream reaches the heat exchanger (p16). Fig. 3.4 shows that the heat-superheated exchanger’s saturated combination and compressed fluid components are divided. The output stream from the heat exchanger (p5) then passes via the solenoid valve, which the controller operates to check the flow direction. The flow feeds pump 1 (p6) if, indeed, the radiation from the sun is adequate (charging mode); if not (discharge mode), it reaches pump 2 (p8). The evaporator controls the mass flow rate of the Rankine cycle, which is influenced by the backflow and pressure of the steam turbine, and the temperature and rate of flow of the hot side stream (p16) of the heat exchanger, among other factors (p10). Cooled by a water cooling tower, the condenser is subsequently entered by the turbine outflow stream (p11). The liquid fluid then enters the pump to once again raise its pressure.

3.2.4 CaCO3/Cao 1 CO2 thermochemical energy storage Sensible energy storage (SES), TCES, and Latent Heat Storage are the three methods used to store thermal energy (LES) [13,14]. One of these approaches, thermochemical

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Hybrid Poly-generation Energy Systems

energy storage, promises to resolve issues with renewable energy technologies, particularly those centered on solar power plants [14]. Contrary to latent and SES, which has received much attention, TCES is still in its infancy; however, it has received attention due to its greater energy storage intensity and limitless storage period. One TCES technology is the oxidative energy storage of metal oxides [15]. This process offers numerous benefits, including the ability to operate at high temperatures, using air as a reactant and a heat transmission medium, and the distinction of the products due to gas-solid interactions [16]. The primary objective of incorporating the solar system, storage, and hydrogen production processes is to use solar collectors to achieve some of the energy needed for the hydrogen production process and store it for hours throughout the day when the radioactivity is slight, thereby lowering the heat the system’s load and increasing thermal performance. Solar energy is transient; thus, using storage devices is quite effective. The energy required to generate hydrogen can be provided during the hours when radiation is at its strongest. When there is insufficient irradiation to generate hydrogen, the surplus energy is stored and utilized later. The energy absorbed or delivered when molecular bonds are shattered or distorted in an entirely reversible chemical process drives thermochemical systems. The quantity of storage material, reaction heat, and reaction volume affect how much heat can be stored. Chemical reactions are used in this process. The heat enthalpy of the reaction is the variation between the enthalpies of the reactants at the initiation and the conclusion of the reaction when a chemical reaction takes place. Heat is emitted in an exothermic process, whereas heat is absorbed in an endothermic reaction. The reaction concerning the enthalpy alters from the given formulas yields this quantity of heat that has been stored: Q 5 ΔH 5 mΔh

(3.1)

Material A undergoes an elevated-temperature transformation into materials B and C by the charging process. The reaction products may be conveniently divided and stored in reserve as long as the evacuation procedure is necessary. After proper temperature-controlled mixing of components B and C, pressure and energy are liberated. Compared to the heat of reaction, heat-loss from storage units is often restricted to thermal impacts that may be felt. A selective reaction must be carried out at a temperature where the hydrogen production system’s output current can start, and the storage system’s outlet heat can help with some of the energy needs. The specific reaction should be carried within the equal temperature variation of the ZnSI system, as its operating temperature is 1123K. The reverse reaction of calcium carbonate, which decomposes to produce calcium oxide and CO2. CaCO3 1 H ! CaO 1 CO2

(Reaction 3.7)

The stoichiometric constants predict that a mole of calcium carbonate during this first-order chemical reaction will break down into 1 mol of gaseous phase of

Energy storage type and size in PGSs

133

carbon dioxide and 1 mol of calcium oxide. The response time may alternatively be calculated by using this formula:
 E r 5 Aexp 2 RT

(3.2)

The Arrhenius coefficient (s21), activation energy (kJ/mol), and ideal gas constant (R), which is equal to 8.314 J/kmol, are all used in the equation above. The reaction enthalpy is used to calculate the amount of heat that is stored. Qstorage 5 nUΔH

(3.3)

In the above relation, n is the mole value of calcium carbonate, and Qstorage is the amount of heat stored in the tank. m c 5 n 3 Mc

(3.4)

The dimensions of the storage tank for CaCO3 and CaO are calculated according to the following equation [16]: Vc 5

mc ρc

(3.5)

For gaseous carbon dioxide storage, the dimensions of the tank for storage can be obtained according to the following formula [16]: V5

n:RT P

(3.6)

In the above relations, mc is the molecular mass of the substance, ρc is the density of the substance, and P and T are the pressure and temperature of the substance in the storage tank. It needs to be remembered, nevertheless, that it is not feasible to execute a reverse reaction using the materials in the storage tank every hour. This is because there are periods when the energy needed for the ZnSI system is supplied: therefore, storage is no longer necessary. This makes it possible to determine the volume needed for storage using the cumulative frequency. Calculating the quality of material concentration requires knowing the quantity of production, consumption, intake, and output to the tank. The following equation may be used to do this: Accumulation 5 Input 2 output 1 production 2 consumption

(3.7)

The system described here consists of a chemical energy storage reservoir, heliostat field collectors (HFC), and a zinc-sulfur-iodine (ZnSI) production of hydrogen process. With the help of solar power, this system will produce hydrogen. The system’s

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Hybrid Poly-generation Energy Systems

Molten salt

T > 1123 K

Heliostat collector

Molten salt

Auxillary heater

T < 1123 K

CO2 1138 K 1 bar

1248 K

CO2 HEATER PUMP

ZnSI hydrogen production system

Molten salt

319.80 K 75 bar

HX

867 K 75 bar 343.86 K 1 bar CO2

CALCINER

HX

318.95 K 1 bar 0.186 kg/s

CaCO3 CaO

745.80 K 45 bar

457.913 K VALVE 1 bar

CO2 STORAGE

HEATER

TURBIN

condenser

1138 K 1 bar

comp

457.913 K 3 bar

1053 K 45 bar

1053 K 1 bar VALVE

1053 K 1 bar

1053 K

CaO

CaO

cooler

CARBONATOR

CaO STORAGE CaCO3 CaO

1053 K 1 bar 1053 K CaCO3 STORAGE

Figure 3.5 General schematic of the process [17].

thermal load may be lowered due to the thermal combination, and the thermal and round-trip performance can be determined. HFC, a TCES reservoir, a supplementary burner, and a Rankine cycle make up the process. The Rankine cycle comprises an expander, condenser, pump, pressure valve, and evaporator. An overview of the procedure is shown in Fig. 3.5. In the solar concentrator, molten salts of lithium fluoride and beryllium fluoride are combined in a two-to-one composition ratio as the HTF. At 290
C and 1 atm, this flow reaches the collectors. Due to its large heat flux, molten salt is an excellent choice for high-temperature operations. The flow reaches the hydrogen production process to provide the necessary heat for the synthesis of hydrogen if the output temperature of the collector is sufficient. The flow, in contrast, absorbs heat from the supplementary burner to attain the necessary temperature when the output temperature of the collectors is lower than the needed amount. Additionally, in instances where the energy generated by the collector is excess, after providing the energy needed for hydrogen generation, the surplus energy is stored so that it may be released back into the system when needed.

3.2.5 Co3O4/CoO redox pair thermochemical energy storage system Co3O4 quality is its capacity to operate in several processes without lowering storage efficiency. Inside the TCES systems, the Co3O4/CoO redox pair is used.

Energy storage type and size in PGSs

135

The series of chemical reactions illustrate the decrease and oxidation stages: Co3 O4 ! 3CoO 1 1=2O2

ΔH298:15K 5 844 kJ=kg

(Reaction 3.8)

3CoO 1 1=2O2 ! Co3 O4

ΔH298:15K 5 2 844 kJ=kg

(Reaction 3.9)

Each reactor design includes benefits and drawbacks. Rolling bed reactors benefit from a wide surface area for heat transmission owing to material displacing; however, one of the issues is the movement of reactor parts at elevated temperatures. Even though they have issues with efficient heat distribution and have a more significant pressure penalty, fixed-bed reactors seem to be more common because of their straightforward form and simplicity of building. Honeycomb-shaped structures are used in fixed-bed reactors to improve heat transmission and better distribute heat. High surface area and efficient heat transmission are properties of honeycomb structures. This method utilizes a honeycomb-shaped thermochemical reactor covered with Co3O4. The intermittent nature and night-time inaccessibility of solar energy are critical limitations of its growth. TES throughout the day and later recovery, if solar energy is not accessible, becomes a way to accomplish ongoing CSP functioning. The yearly solar energy accessibility could be increased by 25%65% by using a solar energy tower with thermal storage. Fig. 3.6 presents a novel concept for a triple (sensible, latent, and chemical) storage system coupled with a high-temperature solar collector to address this problem. The initial section discusses methods for boosting heat transfer to raise the solar collector’s output temperature and then integrate it with the thermochemical reactor. Instead of using an inner tube, this design incorporates three new pipe models (sinusoidal, triangular, and trapezoidal) to achieve its objectives. Corrugated tubes outperform straight tubes in terms of heat transmission and pumping power, while

Thermal energy absorbing

C0304

Endothermic reactor

3C00 + 1/2 02

Thermal energy releasing Exothermic reactor

Figure 3.6 Schematic diagram of the mentioned system above [7].

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Hybrid Poly-generation Energy Systems

the sinusoidal variant leads to high thermal-hydraulic performance. The Co3O4/CoO redox pair-using thermochemical reactor with a honeycomb construction is considered in the second section. With a solar receiving performance of 68% and an energy storage capacity of 137 MJ, this specially-built solar collector can maintain the turbine’s needed input temperature (1073K) for approximately 100 minutes when the sun is not shining. Additionally, this system can store up to three times more energy than the storage of latent heat alone by satisfying the primary limits imposed by the micro-gas expander.

3.3

Benefits and limitations of mechanical energy storage in PGSs

Techniques for mechanical energy storage employ a force of gravity or kinematics to preserve applied energy. While the mechanism of mechanical systems—such as spinning a flywheel or weightlifting up a hill—are sometimes very straightforward, the methods that make it possible to utilize these forces effectively and efficiently are especially sophisticated. These methods are practical for use because of advanced materials, advanced computer controllers, and creative projection [18]. The most known methods for mechanical energy storage are flywheel energy storage systems (FESS), CAES, and Isothermal CAES. A flywheel is a mechanism that rotates and is employed to store immediately accessible rotary motion. From the most known fundamental mechanism, a flywheel has a rotating weight in the middle that is powered by an engine. Once energy is required, the rotating force powers turbine-like equipment to generate electricity while lowering the rotational speed. By utilizing the engine to accelerate the flywheel’s rotating velocity once more, a flywheel gets reloaded. With the help of the flywheel technique, we can enhance the capabilities of the present electrical infrastructure. During times, a flywheel may absorb energy from sporadic power sources and provide a steady stream of constant electricity to the network. Additionally, flywheels can rapidly react to network signals, providing harmonic management and improving the reliability of the electricity [18]. Flywheels are typically built of metal (normally steels) and revolve on standard axles, often only capable of a few thousand RPM of rotation. Modern flywheels can rotate at rates of up to 60,000 RPM since they are composed of carbon-fiber materials, are vacuum-stored to decrease drag, and use permanent magnets in place of traditional bearings.

3.3.1 Flywheel energy storage systems FESS has several important benefits, including minor regular maintenance, long lifespans (several flywheels can withstand well over 100,000 complete depth discharge cycles, and the most recent structures can withstand even over 17,000 complete thorough discharge cycles), and low impact on the environment. Having

Energy storage type and size in PGSs

137

outstanding cyclic and load-following properties, flywheels may fill the gap between short-term ride-through power and long-term energy storage. By substantial group, essential qualities, highly sophisticated FESS can achieve appealing energy capacity, high performance, and minimal idle losses (over intervals of several minutes to many hours): (1) A spinning volume with a max tensile proportion produced of polymer or fiber-glass coatings; (2) one that spins in a vacuum to reduce aerodynamic drag; (3) one that spins at a high rate, and (4) one that uses air or ferromagnetism bearing devices to adapt high rotor velocity. Improved FESS work at tip velocities of more than 1000 m/s and rotary frequencies greater than 100,000 RPM.

3.3.2 Compressed air energy storage An approach to storing energy produced for one moment to be used later is CAES. At the end-user level, the energy produced in off-peak hours can be released to meet load demand hours when energy demands are greater. CAES systems have been used since the 1870s to deliver efficient, on-demand electricity to towns and businesses. Although there are several minor implementations, the first utility-scale CAES system with a capacity factor of over 290 MW was installed in the 1970s. Small-scale, on-site energy storage options are possible with CAES, as bigger facilities may supply significant network energy stores. Regarding its usage, CAES facilities are comparable to pumping system energy plants. However, in a CAES plant, atmospheric air or other gases can be compressed and kept pressurized in a dank cave or vessel rather than moving water from a deeper to an upper pool throughout times of extra electricity. When energy is needed, heated and expanded pressure air is used to generate power via a gas turbine. Pumped-hydro power plants may be effectively replaced with CAES power plants. The CAPEX and OPEX for the diabatic facilities that are already in operation are competitive. A developing technique called isothermal CAES aims to get beyond some of the drawbacks of conventional (diabatic or adiabatic) CAES. Traditional CAES employs turbomachinery to compress air to around 70 bar before storing. Without intercooling, the air would heat up to around 900K, rendering the processing and storage of the gas impractical (or prohibitively costly).

3.4

Proposed process configurations for electrochemical energy storage in PGSs

Batteries or other highly energetic or high-power density technologies are the foundation of electrochemical energy storage. Uses that call for high energy and power densities in the same material are becoming more and more necessary in the present and the coming years [19]. The term “secondary batteries” refers to all kinds of electrochemical energy storage. Rechargeable batteries use an oxidation reaction reverse process to transform

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Hybrid Poly-generation Energy Systems

the chemical energy found in their active components into electric energy. Batteries are currently made in various dimensions for a broad range of purposes. The provided powers range from Watt to dozens of kilowatt (Compare to the batteries used in big vehicles, power plants, and pacemaker power supplies) [20].

Using a fuel cell electrochemical for a power plant system The expanders convert chemical energy to mechanical energy to provide the necessary power for condensation operations. The performance of the fuel-conversion process is a crucial factor that significantly impacts condensation process prices. Fuel cells are a novel class of energy-conversion technologies that use electrochemical processes to transform the potential of fuel as chemical energy into electrical power. Although fuel cell conversion performance is greater than traditional power generation, these techniques remain in the developing stage for industrial implementations due to economic and technical issues. Power facilities that use solid oxide fuel cells can be coupled with hybrid gas turbines (GT-SOFC). Additionally, other potential integration techniques include linking the different thermodynamic cycle processes, such as Brayton and Rankin, and combining SOFC in the form of poly-generation, trigeneration type power plant with an absorption refrigeration system [21]. In contrast, one of the most crucial factors to consider when planning and operating refrigeration is providing the necessary cryogenic conditions. There are several techniques commonly used to achieve cryogenic temperatures, with the majority of them operating at temperatures below 150
C. This section introduces a novel concept for a hybrid electrochemical power generation and an integrated natural gas liquefaction system. In the liquefaction process, the absorption refrigeration system’s evaporator temperature, around 30
C, replaces the precooling cycle. In order to produce the necessary refrigeration during the precooling cycle of the cryogenic system, an H2O-NH3 absorption refrigeration system is created. The dual mixed refrigerant (DMR) process is contrasted with the method’s parameters and results. As was mentioned, the SOFC hybrid system has a more significant electrical and overall performance than traditional gas turbine power plants. An incorporated SOFCCCHP system can supply the necessary power and heat load in the condensation cycles that utilize ARSs as the precooling refrigeration cycle. Conceptual diagrams of a hybrid poly-generation utilizing the two and three processes of cryogenic cycles are shown in Figs. 3.1 and 3.2. Natural gas from pipelines is split into two portions. After undergoing a treatment process, the following section starts the liquefaction operation. Additionally, the power plant supplies the necessary heat load in the ARS and the power in the vapor compression cryogenic system. Therefore the necessary power and heat load can be computed for a specific LNG mass flow rate. The following hybrid power plant can be developed according to the necessary power and heat requirements. The SOFC and power process provides the necessary power, and the combustion products from the hybrid system provide the necessary heat duty. The liquefaction process requirements may be used to establish the intakes for hybrid SOFC power plants. The liquefaction process’s power and heat load

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139

requirements depend on how the operation is set up and used. Fig. 3.7 shows a conceptual view of the hybrid power plant and two-cycle NG condensation cycle, and Fig. 3.8 presents a block flow diagram of a hybrid SOFC power plant and a three-cycle natural gas liquefaction process.

Pipeline natural gas Exhaust gases

Fuel Cell-Steam Turbine hybrid (CCHP) power plant

Waste Heat

Treatment

Power

Absorpon Refrigeraon system

Compression Refrigeraon system

First Cycle (Precooling)

Second Cycle (Liquefacon)

LNG Figure 3.7 A conceptual view of the hybrid power plant and two-cycle NG condensation cycle [22].

Pipeline natural gas

Exhaust gases

Fuel Cell-Steam Turbine hybrid (CCHP) power plant

Waste Heat

Treatment

Power

Absorpon Refrigeraon system

Compression Refrigeraon system

First Cycle (Precooling)

Second Cycle (Subcooling)

Power Compression Refrigeraon system Third Cycle (Liquefacon)

LNG

Figure 3.8 A conceptual view of the hybrid power plant and three-cycle NG condensation cycle [22].

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Hybrid Poly-generation Energy Systems

The SOFC electrodes additionally perform some additional fuel synthesis. The following reaction equations govern the transformation of methane [23]:
 82000 Rr 5 4274 3 PCH4 3 exp 2 3 As RT

(3.8)

where methane vapor compression in the gas bulk is represented by the formula PCH4, average cell temperature by T. The vapor alteration reaction rate is measured in mol/S/m3. The watergas shift reaction equation reads as below [24]: !
 2103191 PH2 PCO2   Rs 5 0:0171 exp PH2 O PCO 2 RT 0:019 exp 4276 T

(3.9)

The following electrochemical processes occur in the anode and cathode: 2H2 1 2O22 .2H2 O 1 4e2 O2 1 4e2 .2O22

ðanodeÞ

ðcathodeÞ

(Reaction 3.10) (Reaction 3.11)

Following the equation, electrochemical production levels for H2 are determined: RH2 5 2

i 2F

(3.10)

where F is the Faraday constant, and i is the current density. According to the steam-to-carbon ratio (S/C), anode gas recycling (AGR) is achieved as follows: S=C 5

ðH2 OÞAGR ðCOÞAGR 1 ðCOÞfuel 1 CH4 1 2C2 H6 1 . . .

(3.11)

Eq. (3.12), which is merely the Nernst voltage mines the voltage wastage, is used to compute the actual voltage, V.   (3.12) V 5 VI 2 ηohm 1 ηact 1 ηconc The process flow schematic for the SOFC system is shown in Fig. 3.9. As operational sources, methane, air, and vapor are utilized. The polarization profile of the solid oxide fuel cell is shown in Fig. 3.10. The efficiency of the fuel cell is affected by the temperature, as shown in Fig. 3.11. One factor that has a considerable impact on power density is the working temperature. Three temperatures within the system’s range of practicable operating temperatures are examined. Due to irreversible power losses, voltage drops off with increasing current density. Until a maximum value, power density rises with the current density and then falls.

Energy storage type and size in PGSs

141

123

Water

101-SC

114-SC

Boiler

103 EXHAUST GAS

NATURAL GAS

125

124 Air pre heater1

Heat recovery

102

AC-Power

112

B-2

Inverter

120

ANODE DC-power 108 109 107 49 110 121 Fuel-pre Tee-101 heater 50 PREREFORMER

104

Heat loss

COMBUSTOR

106

M-1 9

E-101 111

AIR

Air Blower1

Mix-103

113

119 Mix-102 115

Tee-102

114

122 Air-pre heater2

116

117

118 Heat Loss

Air Blower2 CATHODE

Figure 3.9 SOFC system process flow diagram [22].

Voltage (V), Power density (W cm-2)

Voltage

Power density

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

0.5

1

1.5

2

2.5

3

3.5

4

Current density (A cm-2) Figure 3.10 Polarization profile of the SOFC [22].

Power density rises with the operating temperature at 927
C, reaching a high of 1.51 W cm22 at 3.3 ( A cm2). Voltage and power densities are 0.737 V and 0.86 W cm22, correspondingly when the current density is 1.17 (A cm2). The impact of the fuel utilization number on the fuel cell voltage is shown in Fig. 3.8. The fuel usage number causes a drop in the cell voltage (Fig. 3.12). The conception and evaluation of a new solid oxide fuel cell-combined cooling, heating, and power (SOFC-CCHP) hybrid power generation are developed based on

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Hybrid Poly-generation Energy Systems

Voltage (V), Power density (W cm-2)

1.6 727 C

1.4

827 C 927 C

1.2

727 C

1

827 C 927 C

0.8 0.6 0.4 0.2 0 0

1

2

3

4

5

Current density (A cm-2) Figure 3.11 Impact of the temperature of the efficiency of the fuel cell [22].

Figure 3.12 Impact of the fuel utilization number on the fuel cell voltage [22].

the preceding section. The block process flow of the hybrid system is shown in Fig. 3.13. The procedure is divided into four parts: the SOFC system, the steam process, the absorption refrigeration system, and the cycle for condensation and subcooling. The process’s intake supplies are air, water, and NG; its output is LNG. In reality, CCHP plants are built so that their production cooling, heating, and electricity will correspond to the needs of the LNG operation. The combination plan’s flow charts are shown in Fig. 3.10. There are four subsystems in this operation, each with a unique operating fluid. They are identified as solid

Energy storage type and size in PGSs

Water 151.8 °C, 5.0 bar, 0.0378 kgmole sec-1

143

SOFC Cyde Produced Electrical Power 21677 kW Natural Gas 25 °C, 60 bar, 2.015 kgmoles sec-1

Air 629.5 °C, 1.27 bar, 0.923 kgmole sec-1

SOFC Cycle Heat Loss from SOFC Cycle 30528 kW

Water 25° C, 1 bar, 2.03256 kgmole sec-1

Air 25 C, 1 bar, 0.396 kgmole sec-1

Steam Turbine Cycle

Liquefied Natural Gas Process N2 853.9 °C, 1.27 bar, 0.527 kgmole sec-1

Absorption Chiller Cycle

Heat Loss from ST Cycle 60275 kW

ST Cycle Produced Electrical Power 13458 kW

Cold Energy to DMR Process 324021 kW

Liquefaction and Sub-cooling Cycle

LNG -163.8 °C, 1bar, 1.846 kgmole sec-1

Figure 3.13 Block flow diagram of the integrated system [22].

oxide fuel cell cycle (3738394041424344454647484950 515253545556575859606162636465), ammoniawater absorption refrigeration system (8910111213141516171819 202122), steam turbine (Rankin) cycle (66676869707172 7374757677787980), and the liquefaction and subcooling cycles (12 345678/252627282930313233343536) as can be seen in Fig. 3.14. In the fuel compression, the pressure of NG rises. After that, the stream (37) reaches the prereformer with some fuel cell anode reflux and water flow. Using the established convergence ratio, some methane is transformed into hydrogen in the prereformer following the reforming reactions. An energy balancing is performed using the H-1 heat exchanger to determine the preadiabatic reformer’s efficiency. Additionally, the natural gas’s ethane component enters the equilibrium reactor and undergoes reforming processes. Two distinct reactors working under two different operating conditions are designed to transform methane and ethane in two distinct proportions. Once heating up to the proper temperature, the prereformer outlet temperature is combined with a flow of pure oxygen (52). Stream (53) enters the fuel cell’s anode, where the electrochemical and reforming processes occur concurrently. Reforming uses some of the heat produced by the electrochemical processes since it is an endothermic process. To regulate the temperature of the fuel cell, the cathode input air stream must absorb the remainder of the heat emitted by the electrochemical processes. Before approaching the cathode, the air stream (44) is additionally compressed to the required pressure in B-1 and C-4 compressors. The HE-8 and HE-9 heat exchangers then preheat it to cathode temperature. The anode output stream (54) and a part of the cathode output stream (58) are mixed before entering the combustion chamber. A portion of the fuel not transformed in the anode is burnt in the combustion chamber. Because the gases coming out of the chamber are very hot, they may be used to warm up the fuel and air streams before they enter the anode

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Hybrid Poly-generation Energy Systems

77

Steam cycle NATURAL GAS

SOFC

76

WATER

37

79

78 M-7

P-2

1 TEE-1

H-2 80

P-4

63

67 TEE-4

75

71

M-8

ST-2

57 WASTE HEAT

B-2 COMBUSTOR

Natural Gas

NET POWER

65

H-1 39 M-3

38

EXHAUST GAS

E-1

V-4 HE-3

42

41

43

53

62 HE-6

M-5

Equitrium WGS-2

56 54 TEE-2 55

48 M-4 52

B-1

44

19

ANODE

40

PREREFORVER

45

V-1 6

61

64

7 5

Inverter

HE-8 47

HE-9 46

FLASH GAS

31

TEE-5

ST-1

Mixed refrigerant cycle

S-1

72

70

69

HE-7

Absorpon cycle

LNG TO STORAGE

68 66

M-6

59

60

58 TEE-3

49

AR

51 CS-1

V-2

50

C-4 30 4

2 27

20

CATHODE

HE-5 22

32

8 CO-4

33 M-2

M-1

13 V-5

36 V-3 35

14

9 12

29 3

HE-4

P-1

16 21

10 HE-1 Con-1

25 34 28

17

15 F-3

S-2

18

HE-2

CO-5 23

AC-1 25-a 25-b

AC-2 25-c 25-d

25-e

AC-3

26 74 T-1

C-1

C-2

C-3

73 11

24

Re-1 P-3

Figure 3.14 A schematic view of the system [22].

and cathode. In Rankin’s steam cycle, the hot gases’ removed heat is also utilized to create the necessary steam (HE-7). After being pumped to the desired pressure in P-4, stream (80), composed entirely of pure water, enters the (HE-2) as a boiler and, through heat exchange with hot gases, transforms into superheated steam. From the passage of two steam turbines (HP (ST-1) and LP (ST-2) turbines), steam is converted into electricity in two phases. By combining with flow (68), a portion of the high-pressure turbine’s output raises the boiler’s average operating temperature, improving the steam cycle’s efficiency (74). A predetermined temperature is changed in the low-pressure turbine output stream (70). As a result, it can supply the ammoniawater absorption cycle generator with the necessary temperature. The absorption chiller system and the steam turbine cycle are thermally integrated.

3.5

How to store electrical energy in PGSs?

First, it was raised that electricity cannot be stored in any meaningful way; it can be transformed into various kinds of energy that can be stored and instantly transformed into electricity. Batteries, flywheels, compressed air, and pumped hydro storage are

Energy storage type and size in PGSs

145

all forms of electrical energy storage [25]. Lately, capacitors, supercapacitors, and superconducting magnetic energy storage (SMES, superconducting storage coil) have been categorized as electrical energy storage. Making an affordable energy storage system that can store electricity at times of peak demand and delivering it in the future is the challenging part of electrical energy storage. Among the quickest energy storage and delivery technologies, supercapacitors can deliver high currents quickly while storing and supplying energy. Over the past several years, there has been a hasty development and study of supercapacitor technology [26,27]. Although both definitions of electrical energy storage are applicable to hybrid poly-generation energy systems due to their nature conversion and storage of energy.

References [1] A.S. Alsagri, A. Chiasson, M. Gadalla, Viability assessment of a concentrated solar power tower with a supercritical CO2 Brayton cycle power plant, Journal of Solar Energy Engineering 141 (5) (2019) 051006. [2] A.S. Alsagri, A. Arabkoohsar, A.A. Alrobaian, Combination of subcooled compressed air energy storage system with an organic Rankine cycle for better electricity efficiency, a thermodynamic analysis, Journal of Cleaner Production 239 (2019) 118119. [3] J. Ren, X. Ren, Sustainability ranking of energy storage technologies under uncertainties, Journal of Cleaner Production 170 (2018) 13871398. ˙ ˙ [4] P. Zuk, P. Zuk, National energy security or acceleration of transition? Energy policy after the war in Ukraine, Joule 6 (4) (2022) 709712. [5] L. Irwin, et al., Thermochemical energy storage for concentrating solar thermal (CST) systems, in: M. Blanco (Ed.), Advances in Concentrating Solar Thermal Research and Technology, Elsevier, 2017, pp. 247267. [6] H. Bao, Z. Ma, Thermochemical energy storage, in: T. Letcher (Ed.), Storing Energy, Elsevier, 2022, pp. 651683. [7] P. Pardo, et al., A review on high temperature thermochemical heat energy storage, Renewable and Sustainable Energy Reviews 32 (2014) 591610. [8] L. Andre´, S. Abanades, L. Cassayre, High-temperature thermochemical energy storage based on redox reactions using Co-Fe and Mn-Fe mixed metal oxides, Journal of Solid State Chemistry 253 (2017) 614. [9] S. Tescari, et al., Thermochemical solar energy storage via redox oxides: materials and reactor/heat exchanger concepts, Energy Procedia 49 (2014) 10341043. [10] S.A. Kalogirou, Solar thermal collectors and applications, Progress in Energy and Combustion Science 30 (3) (2004) 231295. [11] R. Habibi, M. Mehrpooya, P. Pourmoghadam, Integrated Mg-Cl hydrogen production process and CaO/CaCO3-CaCl2 thermochemical energy storage phase change system using solar tower system, Energy Conversion and Management 245 (2021) 114555. Available from: https://doi.org/10.1016/j.enconman.2021.114555. [12] P. Pourmoghadam, M. Mehrpooya, Dynamic modeling and analysis of transient behavior of an integrated parabolic solar dish collector and thermochemical energy storage power plant, Journal of Energy Storage 42 (2021) 103121. Available from: https://doi. org/10.1016/j.est.2021.103121.

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[13] A. Gil, et al., State of the art on high temperature thermal energy storage for power generation. Part 1—Concepts, materials and modellization, Renewable and Sustainable Energy Reviews 14 (1) (2010) 3155. [14] S. Kuravi, et al., Thermal energy storage technologies and systems for concentrating solar power plants, Progress in Energy and Combustion Science 39 (4) (2013) 285319. [15] C. Prieto, et al., Review of technology: thermochemical energy storage for concentrated solar power plants, Renewable and Sustainable Energy Reviews 60 (2016) 909929. [16] S. Wu, et al., A review on high-temperature thermochemical energy storage based on metal oxides redox cycle, Energy Conversion and Management 168 (2018) 421453. [17] A. Sami, M. Mehrpooya, A. Noorpoor, Investigation of an integrated thermochemical hydrogen production and high temperature solar thermochemical energy storage and CO2 capture process, Applied Thermal Engineering 214 (2022) 118820. Available from: https://doi.org/10.1016/j.applthermaleng.2022.118820. [18] Energy Storage Association, Mechanical energy storage. ,https://energystorage.org/whyenergy-storage/technologies/mechanical-energy-storage/., 2020 (accessed 09.04.22). [19] S. Sagadevan, et al., Fundamental electrochemical energy storage systems, in: N. Arshid, M. Khalid, A.N. Grace (Eds.), Advances in Supercapacitor and Supercapattery, Elsevier, 2021, pp. 2743. [20] A.F. Zobaa, Energy Storage: Technologies and Applications, BoDBooks on Demand, 2013. [21] X. Zhang, et al., A review of integration strategies for solid oxide fuel cells, Journal of Power Sources 195 (3) (2010) 685702. [22] M. Mehrpooya, Conceptual design and energy analysis of novel integrated liquefied natural gas and fuel cell electrochemical power plant processes, Energy 111 (2016) 468483. Available from: https://doi.org/10.1016/j.energy.2016.05.126. [23] E. Achenbach, E. Riensche, Methane/steam reforming kinetics for solid oxide fuel cells, Journal of Power Sources 52 (2) (1994) 283288. [24] B. Haberman, J. Young, Three-dimensional simulation of chemically reacting gas flows in the porous support structure of an integrated-planar solid oxide fuel cell, International Journal of Heat and Mass Transfer 47 (1718) (2004) 36173629. [25] P. Komarnicki, P. Lombardi, Z. Styczynski, Electric Energy Storage Systems, Springer, 2017, pp. 3795. [26] S. Devasahayam, C.M. Hussain, Nano Tools and Devices for Enhanced Renewable Energy, Elsevier, 2021. [27] K. Mensah-Darkwa, et al., Supercapacitor energy storage device using biowastes: a sustainable approach to green energy, Sustainability 11 (2) (2019) 414.

Exergy, energy, environmental and economic analysis of hybrid poly-generation systems: methods and approaches

4.1

4

Introduction

4.1.1 Energy analysis procedure in PGSs The first Law of thermodynamic analysis or energy assessment is the essential and primary step of a poly-generation system (PGS) investigation. This evaluation is a statement of how the energy was converted. For every conversion system of energy and utilization of the source of energy, this analysis creates an energy equilibrium while allowing for the determination of losses. Also, energy analysis is a wellknown method for determining how well a system is performing and may be used to assess energy consumption and performance. Energy equilibrium for a control volume of a thermodynamic system may be expressed as: Q_ 2 W_ 5

X

m_ out hout 2

X

m_ in hin

(4.1)

where Q_ and W_ are the heat rate and power, respectively; m_ is the mass flow rate of a material stream and h is the specific enthalpy. Coefficient of Performance (COP) is very common, which is a criterion used to evaluate the efficiency of the cryogenic system and is given by: COP 5

QHex WComp

(4.2)

where QHex is the total heat transferred by refrigerant fluids (refrigerant duty in MW) and WComp is the power required by compressors (in MW). The First Law of Thermodynamics serves as the basis for the energy equilibrium of each unit, and the formulae for compressors and heat exchangers may be found in Eqs. (4.3) and (4.4), respectively [1].

WComp 5

  3 m _ inlet h0i;outlet 2 hi;inlet X i51

ηi

Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00007-4 © 2024 Elsevier Inc. All rights reserved.

(4.3)

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Hybrid Poly-generation Energy Systems

and QHex 5

3 X

m_ j;inlet cpj ΔTlnj ;

(4.4)

j51

where ΔTln 5

ðTW 2 T inhÞ ln

Ð Ð UTdA ðTWi2 T out Þ , with T 5 ÐA Ð

TW 2 T in TW 2 T out

A

UdA

Finally, the pressure drop is calculated as λ: ΔP 5 p in 2 p out

(4.5)

Ð Ð pdA where p 5 ÐA Ð A

dA

Several variables can impact a compressor’s efficiency, including inlet temperature, secondary chilling, outlet pressure, and the molar fraction of the suction flow. The compressor driver’s required braking horsepower (BHP) may be determined using the formulas below.  n k 2 1 o3 2 CR k 2 1 qin 3 Ps ZZout k21 in k 5 3 ð1% mechanical lossesÞ W_ BHP 5 4 ð229 3 ηi Þ

(4.6)

where qin, Ps, Cr, k, Z, and ηi stand for inlet volumetric flow rate, inlet pressure, pressure ratio, specific heat ratio, compressibility coefficient, and isentropic compressor efficiency, respectively. Pumps are employed to raise the fluids’ pressure. They require much less energy than compressors. The amount of power needed by a pump to raise the mass flow rate from one level to another may be described as follows if it is assumed to be an ideal machine.

4.1.2 Classification of energy forms One aspect of evaluating the quality of an energy form depends on how it is stored, which can be regular or irregular; in the latter case, there are different degrees of irregularity.

4.1.2.1 Symmetric energy (regular) Two types of energy fall into this category of energy forms: 1. Potential energy can be stored in the field of magnetic force, electric force, or gravity. An utterly flexible spring’s stored energy takes the form as well. 2. Regular kinetic energy: for instance, for spinning or the fountain of an ideal fluid. The particle path of a moving system in which energy is stored in parallel to each other.

Exergy, energy, environmental and economic analysis of hybrid poly-generation systems

149

Regular energy has the following characteristics: G

G

G

In the reversible state, one form of regular energy is completely transformed into another. Regular energy transfer between two systems manifests itself in the form of work (not heat) at the interface between the two. In other words, regular energy work is in transition. Regular energy transfer is performed without changing the systems’ entropy and can only be analyzed with the help of the first Law.

4.1.2.2 Asymmetric energy (irregular) Various sorts of irregular energy include chemical, thermal, and internal energy. A similar category of energies likewise includes the energy which characterizes a turbulent flow, which ultimately transforms regular energy into energy brought on by the erratic movement of molecules (irregular energy). Below are some implications of this topic on mechanisms that aim to transform as much irregular energy as possible into regular energy; G

G

G

The process must be completely reversible. The second law should do process analysis. Typically, conversion is done by changing the entropy of other systems.

4.2

The concept of exergy

The quality (or ability to change) of irregular forms of energy, determined by entropy, varies and depends on the form of energy (chemical, thermal, etc.) and the parameters of the energy carrier and the environment. On the other hand, regular forms of energy, which are not characterized by entropy, have an unchanging quality and are able to be completely transformed into other forms of energy through the interaction of work. In calculating the variable quality of different forms of irregular energy in the analysis of chemical and thermal units, a comprehensive measure of quality is needed. The primary and most common criteria are the maximum work that can be obtained from a particular form of energy as a reference state and using environmental parameters. This measure of energy quality is called exergy. Exergy balance in the thermal system analysis is one of this concept’s main uses. Exergy balance is similar to energy balance, but the two fundamentally differ. While energy balance is an expression of the energy conservation law, exergy balance can be considered an expression of the Law of decreased energy quality. Since all actual processes are irreversible, a drop in energy quality is equivalent to the irreversible degradation of exergy.

4.2.1 Control volume Before considering the form of exergy in the control volume, it will be useful to discuss the two basic concepts used to define exergy sentences. Initially, the

150

Hybrid Poly-generation Energy Systems

environment is described as the first fundamental concept. As a particular concept of the exergy method, the environment is an inclusive body or environment in a state of complete thermodynamic equilibrium. Therefore, this hypothetical environment is free of gradients or changes in pressure, temperature, chemical potential, potential, or kinetic energy; therefore, there is no possibility of producing work due to the interaction between the components of the environment. Any system outside the environment that has one or more parameters, such as pressure, temperature, or chemical potential, that differs from its corresponding parameter in the environment has the potential to work relative to the environment. Thus, the environment is a comprehensive basis for assessing the work potential of different types of systems. The environment can interact with systems in three different ways: G

G

G

As a source (heat or well) of thermal energy at temperature T0, through thermal interaction: the high heat capacity of the environment enables it to exchange heat with any system without significant temperature change. As an unusable source of work through mechanical interaction: This form of interaction occurs only in systems that are subjected to volume change under a certain pressure, such as a closed system that undergoes an expansion process. This mode does not include continuous flow processes. As a source of material with low chemical potential in chemical equilibrium through chemical interaction: This interaction occurs when an open system pushes matter into the environment or removes material with low chemical potential from the environment. It is assumed that the environment consisting of these substances is in chemical equilibrium with each other.

In all of these interactions, the environment is defined as the zero base level for pressure, temperature, and chemical potential to evaluate the exergy. The second basic concept is equilibrium, which itself is divided into the following two groups. 1. Environmental state: Restricted equilibrium is a condition in which a mechanical and thermal balance is established between the system and the environment while equalizing the pressure and temperature of the system and the environment. The limited attribute indicates that under these conditions, the system’s material is restrained by a physical barrier that prevents material exchange between the system and the environment. Therefore, there is no general chemical equilibrium in the limited equilibrium between the system and the environment. The state of equilibrium limited by the environment is also called the state of the environment. 2. Dead state: In the unrestricted balance between the environment and the system, the mechanical, thermal, and chemical balance conditions are established. Thus, in addition to pressure and temperature, the chemical potentials of the materials of the system and the environment must be equal. In this situation, when there is a complete thermodynamic balance between the system and the environment, the system cannot tolerate a change of state through any interaction with the environment, and these conditions are called dead state. The dead state is, in fact, the same environmental conditions that are usually considered to be the temperature of 25 C and the pressure of one atmosphere [2].

Exergy, like enthalpy and entropy, is a state function; therefore, its change during a process is equal to the difference in exergy between the initial and final states.

Exergy, energy, environmental and economic analysis of hybrid poly-generation systems

151

Heat Source QH

Exin (Inflow of Matter)

ExQin

Control Volume I≥0

W

QL

Exout (Outflow of Matter)

ExQout

To Environment (P0, T0, x0) Figure 4.1 Exergy balance of a control volume [3].

In the analysis of the control region shown in Fig. 4.1, the exergy balance is described as [3]: Exin 1 ExQin 5 Exout 1 ExQout 1 W 1 I

(4.7)

In the above relation, Exin, Exout, ExQin, ExQout, W, and I are, respectively, the exergy of input material flow, exergy of output material flow, the exergy of input energy flow, exergy of output energy flow, axial work performed on or by the system, and irreversibility (destroyed exergy), each of which will be described later.

4.2.2 Stable flow A flowing substance contains energy and can therefore be defined as an exergy. Regardless of the kinetic energies, potential, electrical, magnetic, and surface tension effects, the exergy rate of a whole system can be considered as follows [2]: e 5 eph 1 ech

(4.8)

The following equation can define the physical exergy of a flow of matter: eph 5 ðh 2 h0 Þ 2 T0 ðS 2 S0 Þ

(4.9)

where h0 and s0 are the enthalpy and entropy of the flow at ambient temperature and pressure, respectively. The following equation obtains chemical exergy for the

152

Hybrid Poly-generation Energy Systems

flow of matter; ech 5

X

xi e0i 1 RT0

X

xi lnxi γ i

(4.10)

In the above relation, ei0, xi, and γ i are the standard chemical exergies at ambient temperature and pressure, respectively. The ith represents the molar composition of the components in a flow. This equation is completely true for calculating the chemical exergy of non-ideal solutions. Since the coefficient of activity can be more or less than one, calculating the chemical exergy of a non-ideal solution does not seem easy [4]. Because each current in the process has its temperature and pressure, and the value of the activity coefficient of each component in each current will be different from the other current. To solve this problem, it can be shown that the second sentence to the right of the Eq. (4.11) is equal to the change in Gibbs free energy due to the mixing of different compounds and the formation of a solution at ambient temperature and pressure [5]. Therefore, the following equation can be used to calculate the chemical exergy of a non-ideal solution; ech 5

X

xi e0i 2

X

xi G i

(4.11)

Now, with the help of the above relation, it is possible to quickly calculate the amount of chemical exergy of the flow by having common exergy values in the references and the Gibbs free energy of each component and mixture at ambient temperature and pressure in the simulation software.

4.2.3 Solid fuels Szargut and Styrylska [6] considered the ratio of chemical exergy to the net calorific value of LHV equal to a parameter called ϕ. After calculating the values of ϕ for several organic matters, it consists of C, H, O, N, and S, correction expressions that indicate the dependence of ϕ on the atomic ratios of H/C, O/C, N/C, and in some cases, S/C has been extracted. It is assumed that the resulting expressions can also be extended to cover industrial fossil fuels. For dry organic matter in solid fossil fuels containing C, H2, O2, and N2 with a weight ratio of oxygen to carbon less than 0.667, the following expression is obtained in terms of mass ratios; ϕdry 5 1:0437 1 0:1882

  o  n h 1 0:0610 1 0:0404 c c c

(4.12)

where c, h, o, and n are the mass fractions C, H2, O2, and N2, respectively. Due to the limitation of the high weight ratio of oxygen to carbon, the above relationship applies to a wide range of solid industrial fuels except for wood. The accuracy of this relationship is estimated to be more than 6 1.

Exergy, energy, environmental and economic analysis of hybrid poly-generation systems

153

For fossil fuels whose weight ratio of oxygen to carbon is in the range of 0.667 to 2.67 (in some cases, including wood), the following equation is used; ϕdry 5

1:0437 1 0:1882

h c

    1 0:2509 1 1 0:7256 hc 1 0:0383 nc o 1 2 0:3035 c

(4.13)

The accuracy of this relationship is also estimated at 6 1. The following experimental equation is used to calculate the chemical exergy of a fuel with moisture; h  i ech 5 ðLHV 1 2442wÞ 3 ϕdry 1 9417s

(4.14)

In the above relation, w and s are solid fuel mass fractions of water and sulfur, respectively. LHV is the net calorific value of the fuel, which is in kJ/kg. As a result, it must be noted that the amount of chemical exergy of the fuel is obtained in kJ/kg. The exergy of ash in the fuel is also considered minor. The accuracy of this relationship is estimated at 6 0.38.

4.2.4 Efficiency criteria Exergy is the amount of work that results from changing the state of a system from a specific state to environmental conditions in a reversible process. Therefore, it can be said that exergy is equivalent to reversible work. When the process works between the initial and final conditions, reversible work is the highest amount of productive work or the lowest work consumption in equipment. Thus, when the final conditions, environmental conditions, are considered, the concept of reversible work and exergy can be considered the same for a system [7,8]. The difference between reversible and valuable work is irreversible (wasted work). The concepts of irreversibility and the rate of exergy destruction can be equated. Friction, flow mixing, chemical reaction, heat transfer, condensation, and expansion in valves produce entropy and exergy degradation. Therefore, the rate of exergy degradation can be considered equivalent to the entropy generation (T0Sgen), which is always greater than zero for real systems and equal to zero for reversible systems. If the rate of exergy destruction is negative, the process is impossible. Of course, it should be noted that the number of exergy changes in a process can be positive, negative, or zero. In addition to the rate of exergy degradation, another criterion called exergy efficiency can be used to analyze the process. Exergy efficiency is defined as the ratio of product exergy to feed exergy. Feed exergy refers to the input exergy of each process equipment, and product exergy expresses the difference between feed exergy and exergy degradation rate. Exergy degradation rates and exergy efficiencies can be achieved by exergy balancing around each piece of equipment for each device separately [9,10]. Table 4.1 shows a summary of energy, mass balances, and exergy destruction of the most used equipment in hybrid-ploy generation systems.

Table 4.1 Summary of energy and mass balances and exergy destruction of most used equipment in hybrid ploy-generation systems [10]. Components

Schematic figure

Heat exchanger

m_ 1 5 m_ 2 m_ 3 5 m_ 4

. m1 . m4 . m2

. m3

.

Separator

Mass balance

m2

B_d

HE

5

i P ðB_i

in

2 B_i

out Þ 1

P

P

valve

5 ðB_i

in

2 B_i

out Þ 1

m_ 1 5 m_ 2

2 W_ in 1 m_ 1 h1 5 m_ 2 h2

B_d

Comp

5 ðB_i

in

2 B_i

out Þ 2 W in

.

Q B_

1

B_d

m3

. m2 . Win

 m_ 1 ðh2 2 h1 Þ 2 Q_ leak   5 m_ 3 ðh3 2h4 Þ2 Q_ loss

hfeedð1Þ 6 Q_ Duty 5 hvapourð2Þ 1 hliquidð3Þ or hfeedð1Þ 6 Q_ Duty 5 hvapourð2Þ 1 hheavy 1 hlightð3Þ

.

. m1

Exergy destructing

m_ 1 5 m_ 2 1 m_ 3

m1

Compressor

Energy balance

_

Q B_

Mixer

. m1 . m2

Splitter . m3

. m3

. m1 . m2

Valve

Multi stream HE

C2_OUT C2-HOT_IN

C2_IN C2-HOT_OUT

C1_IN

C1_OUT

NG_IN

NG_OUT

The Qleak and Qloss are assumed to be zero in this research.

P 5 ð B_i

m_ 3 5 m_ 1 1 m_ 2

m_ 1 h1 1 m_ 2 h2 5 m_ 3 h3

B_d

Mixer

m_ 3 5 m_ 1 1 m_ 2

m_ 3 h3 5 m_ 1 h1 1 m_ 2 h2

B_d

Splitter

m_ 1 5 m_ 2

m_ 1 h1 5 m_ 2 h2 h1 5 h2

B_d

valve

m_ in 5 m_ out

Eqs. (4.13) and (4.14)

B_d

HE

5 ðB_i

5 ðB_i

5ð

P

in

in

B_i

in

2 B_i

2

P

2 B_i

in

2

out Þ

B_i

out Þ

out Þ

P

B_i

out Þ

156

Hybrid Poly-generation Energy Systems

4.3

Preliminary and advanced environmental analysis of PGs

For the mathematics relations of the second Law indicators, exergoeconomic and exergoenvironmental analysis are comparable [11]. Comparatively, the exergoenvironmental study identifies the source and scope of the ecological implications of the system’s thermodynamics irreversibility [12]. Additionally, the kth element’s impact on the environment balancing calculation is described as follows: bF;k E_ F;k 1 Y_ k 5 bP;k E_ P;k

(4.15)

B_ 5 bE_

(4.16)

Where Y_ k : environmental impact of the kth component bF;k : sepecific environmental impact of the kth component fuel bP;k : sepecific environmental impact of the kth component product _ the environmental impacts that blongs to fuel or product B:

The environmental impact of the kth items Y_ k are taken into account by the system’s life. On the other side, it also takes into account the following environmental implications that arise during varying phases of the element: CO OM DI Y_ k 5 Y_ k 1 Y_ k 1 Y_ k

(4.17)

Where CO Y_ k : Environmental impacts of manufacturing, transport and installation stages OM Y_ k : Environmental impacts of operarion and maintenance stages

DI Y_ k : Environmental impacts of disposal stage

Supplementary environmental effect calculations are also required, similar to exergoeconomic analysis, to resolve the linear system pertaining to fuel and the end product of exergy flows [13]. Table 4.1 displays the environmental effect balance’s primary and supplementary calculations for each element (Table 4.2). Exergoenvironmental parameters might be built accordingly to assess the environmental efficiency of parts. Initially, the comparative variation in the particular environmental effects of the kth component’s fuels and products, bF;k and bP;k , can be a diagnostic tool for identifying opportunities to lessen environmental consequences. It is a comparable level of the exergoenvironmental effect connected to a part taking into account the exergoenvironmental effects of its fuels and product. The definition of this exergoenvironmental parameter is as [12]: rb;k 5

bP;k 2 bf ;k bf ;k

(4.18)

Table 4.2 Environmental impact balance equations of the components [14]. Component

Main equations

Auxiliary equations

Component

Main equations

Auxiliary equations

HX-1

B_ H4 1 B_ R3 1 Y_ HX21 5 B_ H5 1 B_ R4

b304 5 b305

C-1

B_ H3 1 B_ W2C21 1 Y_ C21 5 B_ H4

None

HX-2

B_ H6 1 B_ R5 1 Y_ HX22 5 B_ H7 1 B_ R6

b306 5 b307

C-2

B_ H5 1 B_ W2C22 1 Y_ C22 5 B_ H6

None

HX-3

B_ H8 1 B_ R7 1 Y_ HX23 5 B_ H9 1 B_ R8

b308 5 b309

C-3

B_ H7 1 B_ W2C23 1 Y_ C23 5 B_ H8

None

HX-4

B_ H9 1 B_ H21 1 Y_ HX24 5 B_ H10 1 B_ H22

b321 5 b322

AC-101

B_ 100 1 B_ W2AC2101 1 Y_ AC2101 5 B_ 101

None

HX-5

B_ H11 1 B_ H20 1 Y_ HX25 5 B_ H12 1 B_ H21

b320 5 b321

AC-102

B_ 107 1 B_ W2AC2102 1 Y_ AC2102 5 B_ 108

None

HX-6

B_ H129 1 B_ H16 1 Y_ HX26 5 B_ H13 1 B_ H17

b316 5 b317

AC-201

B_ 200 1 B_ W2AC2201 1 Y_ AC2201 5 B_ 201

None

HX-101

B_ 102 1 B_ 104 1 Y_ HX2101 5 B_ 103 1 B_ 105

b104 5 b105

AC-202

B_ 207 1 B_ W2AC2202 1 Y_ AC2202 5 B_ 208

None

HX-102

B_ 108 1 B_ 111 1 Y_ HX2102 5 B_ 109 1 B_ 112

b111 5 b112

S-1

B_ H14 1 Z_ S21 5 B_ H15 1 B_ H16

bH15 5 bH16

HX-103

B_ 110 1 B_ H1 1 Y_ HX2103 5 B_ 111 1 B_ H2

b110 5 b111 , bH1 5 0

Mix-1

B_ H2 1 B_ H22 5 B_ H3

None

HX-201

B_ 202 1 B_ 204 1 Y_ HX2201 5 B_ 203 1 B_ 205

b204 5 b205

Mix-2

B_ H17 1 B_ H19 5 B_ H20

None

HX-202

B_ 208 1 B_ 211 1 Y_ HX2202 5 B_ 209 1 B_ 212

b211 5 b212

Mix-101

B_ 106 1 B_ 112 5 B_ 100

None

HX-203

B_ 210 1 B_ 401 1 Y_ HX2203 5 B_ 211 1 B_ R2

b210 5 b211 , bR1 5 0

Mix-201

B_ 206 1 B_ 212 5 B_ 200

None

P-101

B_ 101 1 B_ W2P2101 1 Y_ P2101 5 B_ 102

None

Tee-1

B_ R2 5 B_ R3 1 B_ R5 1 B_ R7

bR3 5 bR5 5 bR7 (Continued)

Table 4.2 (Continued) Component

Main equations

Auxiliary equations

Component

Main equations

Auxiliary equations

P-102

B_ 201 1 B_ W2P2201 1 Y_ P2201 5 B_ 202

None

Tee-2

B_ H10 5 B_ H11 1 B_ H18

bH11 5 bH18

EXP-1

B_ H18 1 Y_ EXP21 5 B_ H19 1 B_ W2EXP21

None

V-101

B_ 105 5 B_ 106

None

EXP-2

B_ H13 1 Y_ EXP22 5 B_ H14 1 B_ W2EXP22

None

V-102

B_ 109 5 B_ 110

None

T-101

B_ 103 1 B_ Qi2T2101 1 Y_ T2101 5 B_ 104 1 B_ 107 1 B_ Qo2T2101

b104 5 b107

V-201

B_ 205 5 B_ 206

None

T-102

B_ 203 1 B_ Qi2T2201 1 Y_ T2201 5 B_ 204 1 B_ 207 1 B_ Qo2T2201

..1. b204 5 b207

V-202

B_ 209 5 B_ 210

None

Exergy, energy, environmental and economic analysis of hybrid poly-generation systems

159

The next factor assessed to show how to lessen environmental effects is the exergy destruction’s influence on the environment. The kth element of this indicator is described as follows: B_ D;k 5 bF;k E_ D;k

(4.19)

The final element is the exergoenvironmental aspect. It indicates that the origin of an element’s environmental effect creation is equally specified. This criterion contrasts the component’s environmental effect with the exergy destruction’s environmental impact. The following is a definition of the exergoenvironmental component: fb;k 5

Y_ k Y_ k 5 Y_ k 1 B_ D;k B_ t;k

(4.20)

Y_ k is the primary cause of environmental influence if the exergoenvironmental element is larger than 0.7, whereas exergy destruction is prominent when its value is less than 0.3 [12]. We observe an environmental analysis applied in a cryogenic system with the possibility of integrating it into a hybrid poly-generation system in the following, and this analysis obtained valuable results. Fig. 4.2 shows a hydrogen liquefaction system by a flow diagram. The cryogenic process is divided into three stages, as shown in Fig. 4.2. One major portion contains a hydrogen flow, H1, while the other two supplementary sections are meant to refrigerate the principal flows and serve as a cooling system. Heat exchangers (HX), compressors (C), expanders (TE), air coolers (AC), pumps (P), reactors (R), and separators are all components of the hydrogen refrigeration process (S). The environmental analysis showed that With 474.59 and 359.50 mPts/h, respectively, expander TE-5 and heat exchanger HX-6 had the most significant environmental 200

201 C-3

202

203

AC-3

C-4

204

205 C-5

AC-4 117

106

105

AC-2

C-2

102

S-1

Mix-1

103

104

P-1 101

AC-1 H1

107

112

108

109

110 124

C-1

211

TE-2 115

210

111 123 H2

122

HX-2

121 H3

HX-3

120 H4 R-1

HX-8

208

215

213

TE-5

209

HX-4 H5

217

212

TE-4

Mix-3

221

216 207

HX-7

116

Mix-2 HX-1

119

Mix-4 Tee-1

TE-3

114 TE-1

100

118

113 S-3

S-2

206 AC-5

214 H6

H7 HX-5 R-2

Figure 4.2 Conceptual design of a cryogenic hydrogen process [14,15].

HX-9 220

218

TE-6 H11

219

HX-6 H8

H9

H10

S-4

TE-7

H12

160

Hybrid Poly-generation Energy Systems

effect on the rate of energy destruction. To reduce the negative influence on the environment, one must take into account this significant environmental impact. Conversely, the pump P-1 and the expander TE-1, with their respective environmental impacts of 0.13 and 3.16 mPts/h, had the least amount of energy destruction. According to the element study, the heat exchanger HX-6 (2.4%) and the pump P-1 (96%) have the greatest and lowest exergoenvironmental factors, respectively. In order to lessen the process’s environmental impact, the pump P-1 constructionrelated environmental impact must also be reduced. Moreover, heat exchanger HX6 efficiency and performance must be improved to reduce the environmental effect of the process. The expanders TE-5 and TE-6 and the heat exchanger HX-6 must be targeted for improvement to make the process more environmentally sustainable, according to the exergoenvironmental analysis [14].

4.4

Preliminary and advanced economic analysis of PGSs

The initial investment of every element is estimated following the calculation of each flow’s exergy. The suggested formulae to assess the initial element investment are shown in Table 4.3. As observed, functionalities of acquired equipment are chosen from sources [16,17]. Fig. 4.3 presents the mentioned items below in a schematic view (Table 4.3). The price equilibrium calculation for the kth component is therefore expressed as below: cF;k E_ F;k 1 Z_ k 5 cP;k E_ P;k

(4.21)

Table 4.3 The cost equations of the process components [14]. Component

Purchased equipment cost (PEC) functions

Heat exchanger

PECHX 5 8500 1 409(A)0.8 [18]

Compressor

PECC 5 7900(HP)0.62 [16]

Expander

PECE 5 378(HP)0.81 [16]

Air cooler Pump

PECAC 5 30(A)0.4 [16]  0:26 1 2 η  P PECP 5 800 W10P [17] η

Drum

PECD 5 1.218(42 1 163 W) [16]

Tower

PECT 5 1.218[Cb 1 NCt 1 Cp] [16] Ct 5 457.7 exp (0.1739D), N 5 number of trays Cb 5 1.218 exp[6.629 1 0.1826(ln W) 1 0.02297(ln W)2], Cp 5 300D0.7396 L0.7068

P

Exergy, energy, environmental and economic analysis of hybrid poly-generation systems

HX-102

200

100

112

108

107

106 Mix-101 AC-101

AC-102

V-101

Rectifier

109 T-101

V-102 110

111

Mix-1

C-1

203

HX-201

P-201

R6

HX-1

H6 C-2

H7

HX-2

211

Reboiler

210

R1

R7

H8 C-3

V-202

T-201

R2

R5 R8

H5

209

204

H2 H4

5 4 3 2 1

HX-202

AC-202

Rectifier

202

HX-101 P-101

R3

208

205

104

R4

H3

V-201

201

102

103

Reboiler

H1

HX-103

5 4 3 2 1

207

AC-201 Mix-201 206

101

105

212

161

H9

Tee-1

H10

HX-3

H18 H11 Tee-2

H22

H21 HX-4

HX-203

H19 EXP-1 H20

HX-5

H13

H12

Mix-2

H14

EXP-2

S-1 H15

H16

H17 HX-6

Figure 4.3 PFD of the proposed hybrid LHL with the SEC of 12.7 kWh/kgLH2 [14].

C_ 5 cE_

(4.22)

Where

  c: unit exergy cost $=GJ _ exergy cost rate $=h C:   Z_ : capital investment cost flow rate $=h

To arrive at the appropriate values, a linear function describing the financial equilibrium of equipment must be calculated. Nevertheless, many calculations include several uncertainties. Supplementary formulas would be required in such circumstances. Following the rules of P and F, supplementary equations are constructed for the fuel and the end product of the exergy flows [19]. Table 4.4 demonstrates the supplementary calculations and materials cost balancing formulas. The outcomes are used to construct three criteria that exergoeconomically assess individual elements. Firstly, the proportion of the variation between the median price of its goods and fuels to the estimated cost of its fuels is known as the comparative difference in the cost of the kth element. This exergoeconomic parameter measures the proportionate quantity of energy lost by an element while taking into account the expenses of its fuels and byproducts. These definitions apply to these criteria: rk 5

CP;k 2 Cf ;k Cf ;k

(4.23)

Second, the cost of the exergy destruction is another exergoeconomic indicator that is defined for the kth component as follows: C_ D;k 5 cF;k E_ D;k

(4.24)

Table 4.4 Exergoeconomic cost balance equations of the components [14]. Component

Main equations

Auxiliary equations

Component

Main equations

Auxiliary equations

HX-1

C˙H4 1 C˙R3 1 Z˙HX-1 5 C˙H5 1 C˙R4

c304 5 c305

C-1

C˙H3 1 C˙W˙-C-1 1 Z˙C-1 5 C˙H4

None

HX-2

C˙H6 1 C˙R5 1 Z˙HX-2 5 C˙H7 1 C˙R6

c306 5 c307

C-2

C˙H5 1 C˙W˙-C-2 1 Z˙C-2 5 C˙H6

None

HX-3

C˙H8 1 C˙R7 1 Z˙HX-3 5 C˙H9 1 C˙R8

c308 5 c309

C-3

C˙H7 1 C˙W˙-C-3 1 Z˙C-3 5 C˙H8

None

HX-4

C˙H9 1 C˙H21 1 Z˙HX-4 5 C˙ H10 1 C˙H22

c321 5 c322

AC-101

C˙100 1 C˙W˙-AC-101 1 Z˙AC-101 5 C˙101

None

HX-5

C˙H11 1 C˙ H20 1 Z˙HX-5 5 C˙H12 1 C˙ H21

c320 5 c321

AC-102

C˙107 1 C˙W˙-AC-102 1 Z˙AC-102 5 C˙108

None

HX-6

C˙H12 1 C˙ H16 1 Z˙HX-6 5 C˙H13 1 C˙ H17

c316 5 c317

AC-201

C˙200 1 C˙W˙-AC-201 1 Z˙AC-201 5 C˙201

None

HX-101

C˙102 1 C˙ 104 1 Z˙HX-101 5 C˙103 1 C˙105

c104 5 c105

AC-202

C˙207 1 C˙W˙-AC-202 1 Z˙AC-202 5 C˙208

None

HX-102

C˙108 1 C˙ 111 1 Z˙HX-102 5 C˙109 1 C˙112

c111 5 c112

S-1

C˙H14 1 Z˙S-1 5 C˙H15 1 C˙ H16

C˙H15 5 C˙ H16

HX-103

C˙110 1 C˙ H1 1 Z˙HX-103 5 C˙111 1 C˙H2

c110 5 c111, cH1 5 0

Mix-1

C˙H2 1 C˙H22 5 C˙ H3

None

HX-201

C˙202 1 C˙ 204 1 Z˙HX-201 5 C˙203 1 C˙205

c204 5 c205

Mix-2

C˙H17 1 C˙H19 5 C˙H20

None

HX-202

C˙208 1 C˙ 211 1 Z˙HX-202 5 C˙209 1 C˙212

c211 5 c212

Mix-101

C˙106 1 C˙112 5 C˙100

None

HX-203

C˙210 1 C˙ 401 1 Z˙HX-203 5 C˙211 1 C˙R2

c210 5 c211, cR1 5 0

Mix-201

C˙206 1 C˙212 5 C˙200

None

P-101

C˙101 1 C˙ W˙-P-101 1 Z˙P-101 5 C˙102

None

Tee-1

C˙R2 5 C˙ R3 1 C˙ R5 1 C˙ R7

C˙R3 5 C˙R5 5 C˙R7

P-201

C˙201 1 C˙ W˙-P-201 1 Z˙P-201 5 C˙202

None

Tee-2

C˙H10 5 C˙H11 1 C˙H18

C˙H11 5 C˙ H18

EXP-1

C˙H18 1 Z˙EXP-1 5 C˙ H19 1 C˙W˙-EXP-1

None

V-101

C˙105 5 C˙106

None

EXP-2

C˙H13 1 Z˙EXP-2 5 C˙ H14 1 C˙W˙-EXP-2

None

V-102

C˙109 5 C˙110

None

T-101

C˙103 1 C˙ Qi-T-101 1 Z˙T-101 5 C˙104 1 C˙107 1 C˙ Qo-T-101

c104 5 c107

V-201

C˙205 5 C˙206

None

T-201

C˙203 1 C˙ Qi-T-201 1 Z˙T-201 5 C˙204 1 C˙207 1 C˙ Qo-T-201

c204 5 c207

V-202

C˙209 5 C˙210

None

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Another exergoeconomic criterion, the expense of energy destruction, is calculated for the kth element as below. The final indication is an exergoeconomic component that weighs the price of energy destruction against the expense of purchasing and maintaining the equipment. The significant exergoeconomic component demonstrates that the equipment’s contribution to the investment costs and maintenance is more significant than the expense of its exergy destruction; as a result, the equipment’s expense must be decreased in order to lower the cost of the system. The tiny exergoeconomic component states that system element performance should be increased to lower the system’s cost [13]. The exergoeconomic factor is calculated as below: fk 5

Z_ k Z_ k 5 C_ t;k Z_ k 1 C_ D;k

(4.25)

Next, we discuss an economic analysis of a hybrid poly-generation system. A hybrid CCHP system that includes SOFC, a concentrated solar dish, an absorption refrigeration system, and an ORC is built to deliver the necessary demand in a tower, as seen in Fig. 4.4. As previously mentioned, the SOFC integrated system is the most significant emerging technology that may be readily included in polygeneration systems. Two identical SOFC units are created here. The combined cycle system’s flowchart schematic is shown in Fig. 4.1. Air, water, and natural gas are used as the system’s intake supplies. The solar collector and the fire tube burner supply a portion of the necessary thermal energy for the SOFC system. In a doubleeffect LiBr-H2O absorption chiller system, thermal energy is provided by using the exhaust from one of the SOFCs. It is blended with another SOFC exhaust gas and utilized to provide ORC with the necessary thermal energy. For the needs of the tower, ORC generates electricity and hot water [20].

Water 25°C, 2 bar, 6.692 kgmole h-1

Boiler

Solar system A=602.88m2

Power SOFC:570.3 kW

858.6°C, 52.88 kgmole h-1

950°C, 52.88 kgmole h-1

Duty: 602.2 kW

Refrigeraon: 7033.7 kW,6.7°C Absorpon SOFC Cycle

refrigeraon system

Power 64.41 kW

Air 25°C, 1.013 bar, 72 kgmole h-1 Fuel

ORC

Exhaust gas 85°C, 1.4 bar, 80.33 kgmole h-1

SOFC Cycle

25°C, 1.2 bar, 9.2 kgmole h-1 Hot water 41.45°C, 5 bar, 1145 kgmole h-1 Power SOFC:570.3 kW

Figure 4.4 Block flow diagram of the process [20].

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From economic analysis, we can observe that the SOFC unit is the component with the highest cost in this hybrid poly-generation energy system. The fuel-cell system’s price seems to influence the system’s overall cost significantly, and the process is substantially more expensive than traditional methods (approximately double as much of this expense is attributable to the expensive SOFC process). Furthermore, compared to fuel cell-generated power, the dimension of the solar equipment has less of an influence on the overall cost.

References [1] A. Allahyarzadeh-Bidgoli, D.J. Dezan, J.I. Yanagihara, COP optimization of propane pre-cooling cycle by optimal Fin design of heat exchangers: Efficiency and sustainability improvement, Journal of Cleaner Production 271 (2020) 122585. [2] T.J. Kotas, The Exergy Method of Thermal Plant Analysis, Paragon Publishing, 2012. [3] B. Tirandazi, et al., Exergy analysis of C2 1 recovery plants refrigeration cycles, Chemical Engineering Research and Design 89 (6) (2011) 676689. [4] O. Govin, et al., Evaluation of the chemical exergy of fuels and petroleum fractions, Journal of Thermal Analysis and Calorimetry 62 (1) (2000) 123133. [5] A. Vatani, M. Mehrpooya, A. Palizdar, Advanced exergetic analysis of five natural gas liquefaction processes, Energy Conversion and Management 78 (2014) 720737. [6] J. Szargut, T. Styrylska, Approximate evaluation of the exergy of fuels, Brennst. W¨arme Kraft 16 (12) (1964) 589596. [7] M. Mehrpooya, M. Khalili, M.M.M. Sharifzadeh, Model development and energy and exergy analysis of the biomass gasification process (Based on the various biomass sources), Renewable and Sustainable Energy Reviews 91 (2018) 869887. [8] Y.A. Cengel, M.A. Boles, M. Kano˘glu, Thermodynamics: An Engineering Approach, 5, McGraw-hill, New York, 2011. [9] D. Marmolejo-Correa, T. Gundersen, A comparison of exergy efficiency definitions with focus on low temperature processes, Energy 44 (1) (2012) 477489. [10] A. Allahyarzadeh-Bidgoli, M. Mehrpooya, J.I. Yanagihara, Geometric optimization of thermo-hydraulic performance of multistream plate fin heat exchangers in two-stage condensation cycle: Thermodynamic and operating cost analyses, Process Safety and Environmental Protection 162 (2022) 631648. [11] Y. Lara, et al., An exergy-based study on the relationship between costs and environmental impacts in power plants, Energy 138 (2017) 920928. [12] L. Meyer, et al., Exergoenvironmental analysis for evaluation of the environmental impact of energy conversion systems, Energy 34 (1) (2009) 7589. [13] A. Lazzaretto, G. Tsatsaronis, SPECO: A systematic and general methodology for calculating efficiencies and costs in thermal systems, Energy 31 (89) (2006) 12571289. [14] H. Ansarinasab, M. Mehrpooya, M. Sadeghzadeh, An exergy-based investigation on hydrogen liquefaction plant-exergy, exergoeconomic, and exergoenvironmental analyses, Journal of Cleaner Production 210 (2019) 530541. [15] M. Aasadnia, M. Mehrpooya, H. Ansarinasab, A 3E evaluation on the interaction between environmental impacts and costs in a hydrogen liquefier combined with absorption refrigeration systems, Applied Thermal Engineering 159 (2019) 113798. Available from: https://doi.org/10.1016/j.applthermaleng.2019.113798.

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[16] J.R. Couper, et al., Chemical Process Equipment: Selection and Design, Gulf Professional Publishing, 2005. [17] L. Khani, et al., Energy and exergoeconomic evaluation of a new power/cooling cogeneration system based on a solid oxide fuel cell, Energy 94 (2016) 6477. [18] V. Eveloy, et al., Energy, exergy and economic analysis of an integrated solid oxide fuel cellgas turbineorganic Rankine power generation system, International Journal of Hydrogen Energy 41 (31) (2016) 1384313858. [19] A. Lazzaretto, G. Tsatsaronis, On the quest for objective equations in exergy costing, ASME International Mechanical Engineering Congress and Exposition, American Society of Mechanical Engineers, 1997. [20] M. Moradi, M. Mehrpooya, Optimal design and economic analysis of a hybrid solid oxide fuel cell and parabolic solar dish collector, combined cooling, heating and power (CCHP) system used for a large commercial tower, Energy 130 (2017) 530543.

Solar-based hybrid energy systems

5.1

5

Introduction

5.1.1 Concentrated and photovoltaic solar systems in polygeneration systems The concentrated solar and photovoltaic systems details and theories are explained in Chapter 2. The applications of concentrated and photovoltaic solar systems in PGSs are presented and discussed in this Chapter.

5.1.1.1 Hybrid photovoltaic-thermal collector and ejector refrigeration cycle This system, which uses solar PV-thermal collectors, a cryogenic absorption cycle, and a phase-change material storage system in a refrigerator in Bushehr and close to the Persian Gulf, is an interconnected tri-generation cycle for electricity production, heating, and cooling. The absorption cryogenic cycle’s generator receives heat from the PV-thermal collectors, which are also used for storing energy using the phase-change material storage method throughout the day. At night, the heat that has been accumulated in the storage unit is discharged into the cryogenic absorption process. Fig. 5.1A shows conceptual diagrams of a photovoltaic thermal concentrated collector with combined cooling, heating, and power (CCHP) system. This application of solar photovoltaic is an integrated tri-generation system of CCHP by vapor absorption system and phase-change material storage system. This hybrid process generates 6666 kW of electricity, 5395 kW of cold duty for a refrigerator throughout the day, and 73.95 C of heating water demand as can be seen in Fig. 5.1B. The flowchart diagram for the created combined structure is displayed in Fig. 5.1A and B. The cooling stream is produced in the refrigerator and stored in PCM using the same combination structure of the cryogenic absorption process and the PVthermal equipment. Two-power systems plus a refrigeration process make up the cryogenic process. Below, the water cycle for thermal PV energy transfer between the solar system and the cryogenic absorption process is depicted in red. The energy from the solar circuit reaches the E100 heat exchanger during the operation of the cryogenic absorption process and the R-141b refrigerant vapors under elevated pressure. The first stream into the ejector is elevated pressure flow produced in the boiler (Stream 100) at 90.08 C and 290 kPa pressure.

Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00003-7 © 2024 Elsevier Inc. All rights reserved.

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Figure 5.1 A process flow (A) and schematic diagram (B) for the combined system utilizing reduced-temperature PCM, absorption refrigeration process, and solar thermal collectors [1].

The evaporator exit flow (flow 101), which is 7.968 C and 39.93 kPa as the secondary flow enters the nozzle of the ejector, is created by raising the velocity and lowering the starting stream pressure in the nozzle of the ejector. The two streams are mixed and injected into the diffuser to change their respective velocities and pressures.

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5.1.1.2 Hybrid hydrogen purification and LNG, organic Rankine cycle (ORC), and photovoltaic panels Installation of the H2 generation process is expensive, and occasionally it is impossible to build them where needed. Hence, it is acceptable for hydrogen to be transferred from other locations (H2 production process) to systems that require hydrogen. The enormous size and delivery issue will be resolved by holding hydrogen as a liquid phase. The complexity of accurately predicting multi-component cryogenic thermodynamics characteristics and elevated specific energy ought to be seen as the principal cause for the lack of an integrated system structure of hydrogen purifying and condensation. The combination of the cryogenic system with the central aspect in the form of hot and cold compound schematics is taken into consideration when designing the hybrid construction. The hybrid framework purifies and liquefies hydrogen using the natural gas steam reforming (NGSR) method and an ORC, as shown in Fig. 5.2. The NGSR system produces 82031 kg/h of syngas from 68317 kg/h of NG, 12643 kg/h of water, 15.79 MW of electricity, and 33.89 MW of heating. The ORC’s input heat is provided by the heated output from either the reforming reactor at a rate of 18.67 MW. The gas flow joins the precooling and cryogenic cycles after cooling and removing some contaminants. Solar panels deliver the needed electricity for compressors and the NGSR procedures.

Figure 5.2 A schematic view illustrates a novel integrated framework for the liquefaction and purification of hydrogen using the organic Rankine cycle, NGSR, and solar cells [2].

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Hybrid Poly-generation Energy Systems

5.1.1.3 Combined solar collector power generation and solid oxide electrolyzer for hydrogen production The necessary power for an electrolyzer is generated using a solar dish collector incorporated into a compressed air unit for energy storage. The cost of power would be quite expensive if it could solely be generated via solar energy. Hence, a novel method is created to lower the price of hydrogen production. The fact that this unit utilizes the grid electricity at non-peak periods and is not entirely dependent on solar energy, which is its most significant advantage (at night). Using the other solar dish collector, the necessary thermal energy is provided. Solar dish-collector might perform effectively with solid oxide electrolyzer because it can reach greater temperatures than parabolic trough collector (PTC). This system’s conceptual view is shown in Fig. 5.3. It comprises four parts: a power process to supply the electrical energy, a thermal energy process, an elevated-temperature electrolyzer, and an ORC to recuperate wasting energy and generate more electricity. Compression and expansion equipment trains are the two distinct parts of a compressed air energy storage system. The two divisions function independently. The compression train works to create pressurized air at nighttime when electricity prices are quite low. In the compression train, there are air compressors. The intercooler and after-cooler heat exchangers are incorporated into the unit to lower air Intercooler

1

Aftercooler

3

2 Comp 1

Comp 2

5

Air

7

6

Flue gas Liquid sodium

Compressed Air Storage

4

9

Oxygen

8

H2O+H2

Fuel

12

14

Organic fluid

Preheater

13

11

Water

10

Fuel

Turbine

15

Combustion Chamber

27 Economizer

Superheater

Evaporator

24 30

29

28

18

19

37

26

Turbine

Electric Heater 1

20 38

Heat exchanger

22

21

32

17 Water Tank

Electric Heater 2

H2+H2O

39

36

25

Condenser

16 23

Cathode

SOEC O2 Anode

31

34 35

33

Figure 5.3 A schematic diagram combined solar collector power generation and solid oxide electrolyzer for hydrogen production [3].

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171

temperature and, as a result, lower the power required by these compressors. In a cavern, pressurized air is kept. Once there is sufficient solar irradiation throughout the day, the expansion train starts to operate. The temperature of the compressed air rises as it passes by a solar dish collector and is heated by sunlight. The temperature of the dish output varies throughout the day due to variations in solar radiation. A burner is positioned after the dish collector to regulate the air temperature by consuming a small quantity of fuel, preventing thermal stresses in the gas turbine. It lowers the amount of electricity used from the grid by enabling the gas turbine to provide consistent power during the discharge period. The preheater accomplishes energy recovery from the gas turbine exit stream. Another dish collector is utilized to supply thermal energy. Dish collectors’ ability to attain high temperatures is their most significant benefit. In order to raise the temperature of the water as much as possible, liquid sodium is employed in this study as a heat transfer fluid. The boiling point of sodium is 873 C, allowing superheated water creation at high temperatures. A portion of the water from the water tank escapes the dish collector cycle and goes via an electric heater if the dish collector cannot, for any reason, provide enough energy to superheat the water. Utilizing energy from solid-oxide electrolysis cell (SOEC) output streams, a heat exchanger is utilized to raise the temperature of the water further. Another electric heater is used to modify water temperature since it needs to be at the electrolyzer cell’s operational temperature. In order to recover energy from electrolyzer output streams, an ORC is employed. These two streams still contain high-quality energy after the heat exchange with the water in the heat exchanger, which might be utilized to generate more power and improve plant efficiency.

5.1.2 Proposed solar PGSs for water desalination 5.1.2.1 A solar dish collector power plant with ARS and desalination unit A concentrated solar power (CSP) system that included the multi-effect desalination (MED) operation with the cryogenic absorption cycle or absorption refrigeration system (ARS) is presented in Fig. 5.4, where a conceptual schematic diagram can be seen. The unit consists of a cryogenic absorption system, a MED process, and a CSP with parabolic collectors. The primary process streams in the system are broken down into parts and the streams in the integrated system. Fig. 5.5 shows the system’s process flowchart in more clarity. The diphenyl ether (C12H10O) used as the heat transfer fluid (HTF) is heated by the light that hits the concentrated area of the solar dish, which is located above the dish. By the heat exchanger E102, the heated HTF transfers the solar energy from the solar dish to the operating water. When hot water enters the steam turbine at 570 C and 13 bar, 4632 kW of electrical power is generated. Following Fig. 5.5, the heat exchanger E110 is subsequently employed to warm the HTF output through the turbine outlet (flow 304). After that, the water beneath flow 305, which is at 1 bar and 333 C, reaches E101 (which serves as a

Figure 5.4 The integrated solar thermal power plant’s conceptual diagram, the MED cycle, the absorption refrigeration system, and other components [4].

Figure 5.5 Process view of the combined CSP plant, ARS, and MED system [5].

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reboiler for the distillation tower in cryogenic absorption system) and transfers 2029 kW of heating to the base of the column. The steam power plant’s water is heated by the reboiler output (stream 306), which is supplied to heat exchanger E109 at 162 C and one pressure and exits at 105.8 C and 1 bar (stream 307). The Flow 307 is directed to E111, which functions as the HRSG and supplies 14,390 kW of heating to the ejector intake to complete the desalination cycle. The cooling water of the utility uses the heat exchanger E103 to chill flow 308, which has a temperature of 89.28 C. After passing by pump P101, the cooled water’s pressure rises to the turbine input pressure of 3000 kPa. It is worth noting that all heat exchangers were thought to have minimal pressure drops. When designing pumps and expanders, the adiabatic efficiency can be considered between 0.7 and 0.9. Additionally, 298.16 C and 1 atm were assumed to represent the atmospheric temperature and pressure. Due to pumping a sizable volume of water for cooling water utility, pump P103 had the greatest power consumption (37.66 kW) among the pumps. The steam power plant pump, P101, has the secondhighest power need due to the steam power plant cycle’s high mass flow rate (1200 kgmol/h) and existing high-pressure differential (2900 kPa). A single-stage ammonia-water (NH3-H2O) absorption refrigeration system is considered here for the cryogenic unit. The use of sustainable and environmental refrigerants and the ability to run on reduced waste thermal energy rather than electricity are two prospective benefits of absorption refrigeration systems over vaporcompression refrigeration. In absorption refrigeration systems, the NH3-H2O operating couple is among the most well-liked and frequently employed due to its particular advantages, including the ability to provide extremely low refrigeration temperatures (down to 60 C), uses for chilling, ease of use, compactness, easier maintenance, and no crystallization. Initially, the high NH3-H2O solution (flow 200) with 73.93% water and 26.07% ammonia reaches P100 to boost its pressure up to the generator operating pressure, which is 1300 kPa. In order to heat recovery and raise the refrigeration performance, considering the heat exchangers’ duty and the required power consumption, the highly pressurized flow 201 is warmed by the hot and weak solution from the generator outflow (stream 204) before entering the generator (T100). This is done in a counterflow heat exchanger (E105). Ammonia evaporates and exits the generator at flow 215 after being heated by the reboiler E101 utilizing the waste heat from the steam turbine output (stream 305). The vapor leaving the generator still contains a small amount of water vapor in addition to ammonia vapor (flow 215). The excess water of flow 215 must be reduced, though, since too much water might build up in the evaporator and reduce system performance. The concentrated solar, which directs and focuses sunlight in the concentrated area of the dish, is part of the analyzed CSP plant. At the concentrated point, a receiver (which contains HTF) absorbs the solar energy of radiation and transforms it into thermal energy at a temperature of 600 C. The concept of the parabolic dish collector is shown in Fig. 5.5. The concentrator focuses the beam radiations onto the absorber plate, and the other arriving irradiation will be dispersed in the air because the receiver size is considerably less than the area of the concentrator.

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Hybrid Poly-generation Energy Systems

A MED procedure with simultaneous water supply is taken into consideration for MED. For every effect, there is a flash pool to detach the evaporated vapor from the brine, a J-T valve to reduce pressure, and a heat exchanger to dry up the provided fresh water while also condensing the evaporated vapor from the previous phase. Fig. 5.5 displays the MED system’s process flow diagram. The prepared freshwater (flow 529) passes via the heat exchanger E112 and V102 at the initial effect, where it experiences a pressure decrease (from 0.9732 to 0.2593 bar) and temperature increase (from 56.25 C to 67.96 C). Following the evaporation of the provided salt water, it is put into the flash separator D100, where the flashed gaseous phase (only water) is divided underflow 502, and the residual liquid phase (known as brine) inside the flash pool is delivered to the flash pool of the following effect. The initial effect (flow 502) evaporating vapor is then cooled by E113 before entering the desalination process. Repeating this procedure gives us the desired result. The saltwater (flow 528) is preheated in E115 before reaching the subsequent effects, and the heat is recovered from the condensed desalinated water. Each effect’s heat exchanger simultaneously fulfilling two different functions. Seawater is first evaporated, and then the vapor from the prior action is condensed. The driving vapor is flow 522, which travels by the vapor ejector’s convergent part and draws uncondensed vapor from the final effect (flow 519). Instantaneously combined, the two streams emerge from the steam ejector at 83.95 C and 0.3489 bar (stream 500). The vapor line from the ejector output provides the first effect’s (E112) evaporator with the thermal energy it requires (stream 500). To optimize water and energy recovery, the working pressure during the desalination process is kept below atmospheric pressure (1.0132 bar). The relevant data was gleaned from the literature in order to recreate the MED cycle.

5.2

Power production by solar PGSs

Power production is one of the main objectives of the solar PGSs cycle in order to provide the requested demands of the cycles and outside demands. Conventional solar power production is divided into photovoltaics and transforms thermal energy into electrical energy. The nature of a PGS is not only power production but also other applications supplying electrical energy. In this subsection, some conventional and novel power productions of a PGS are presented and discussed.

5.2.1 Micro gas expander and solar dish collector A unique system that combines a micro expander, a compressed air energy storage (CAES) mechanism, and a parabolic solar collector is created specifically for household use. The power and the hot water might be supplied via the suggested method. The most typical method of generating power on a household scale is by using a micro expander. We may also generate power during peak hours by using the CAES system. The sun’s strongest irradiation coincides with peak electrical demand. The suggested system can take advantage of this advantage by adding a parabolic solar collector, which lowers the system’s dependency on fossil fuels and,

Solar-based hybrid energy systems

175

Intercooler 1

1

Intercooler 2

3

2

7

6

Comp 1

Comp 2

5

Compressed Air Energy Storage 9

4

8

10 12 14

Combustion Chamber 13 Gas Turbine

Preheater 11

15

Figure 5.6 An example of a micro expander for power production by solar dish collector [5].

consequently, resolves environmental issues as can be seen in Fig. 5.6. The power plant starts using the compressed air stored in the cavern at midday since a strong demand for energy and enough solar irradiance are available. The temperature of the air rises as it passes by the dish collector. The dish is designed to ensure that the most significant temperature it may reach is below the input temperature of an expander. An engine block is positioned after the dish collector to raise the air temperature using enough fuel. Fig. 5.6 shows that the air expands in the turbine to generate electricity, and before it reaches the dish collector, it is heated by the expander’s hot output exhaust gases. At peak hours, the expander can produce all the necessary electricity.

5.2.2 Solar organic Rankine There are two ways to convert solar energy into electricity: through photovoltaic technology and by using stationary or concentrating collectors to produce hot water and vapor. The process involves capturing solar energy in the collectors and converting it into thermal energy, which can then be used as a source of power. The operating temperature of HTF can be used for an ORC cycle in order to create power generation. Therefore, the idea of a solar ORC can be created for power generation. A solar concentrator with a temperature operating with the variation of 60 C to 250 C is called the linear Fresnel solar reflector (LFR). An innovative concept is to use thermal energy from concentrated solar receivers like LFRs to provide the ORC with the required heating. This process may be used for various organic

176

Hybrid Poly-generation Energy Systems

fluids at various temperatures, with little expense to install, and as decentralized generation facilities. This cycle has the ability to produce heat, electricity, and store energy. Solar energy is captured by the LFR, which converts it into thermal electricity. The system produces electricity by heating the ORC’s evaporator with the heat produced in this manner. The suggested system configuration is shown in Fig. 5.7. Solar energy is captured by the LFR and converted to thermal energy. The process produces electricity by heating the ORC’s evaporator with the heat produced in this manner. The pumped-hydro-compressed air (PHCA) storage system pump then receives this electricity. The storage system comprises a pump, an expander, a water reservoir, a storage container, and a compressor. The storing process includes two stages: the initial state is the charging phase, in which the pump transports water from the tank to the container using electricity produced by the ORC system. Energy is stored as the air is compressed. Next, in discharging process, water is pumped from the container to the PHCA-expander, which turns the expander and produces electricity. When the air is compressed during charging, the temperature rises, and heat is transmitted from the air to the water. The properties of the water have an impact on

Figure 5.7 Process schematic diagram [6].

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177

the compression or expansion of air. The air condition changes as the water’s density and heat capacity rise, enhancing the heat transfer rate. Two mechanisms for compressing air that is directly connected to the water flow rate are covered: the first is isothermal compression, which happens when water flow rates are extremely high (low heat transfer rate), and the second is quasi-isothermal compression, which develops when water flow rates are low. The thermodynamic model of the PHCA pump, turbine, and vessel must be enlarged based on energy and mass balance to study the functioning of PHCA.

5.2.3 Kalina cycle driven by a parabolic trough solar collector In contrast to conventional pure operating stream, binary mixtures like NH3 and H2O show a range in vaporization temperature that is well matched with the heat source. Due to this thermal feature, Kalina developed a bottoming cycle using an NH3-H2O combination as the operating stream in 1984. In contrast to the Rankine cycle, the Kalina cycle uses an absorption condenser instead of a conventional condenser [7]. The solar Kalina cycle’s schematic is shown in Fig. 5.8. It comprises a Kalina power process, a thermal storing tank, and several parabolic trough collectors. Due to the Kalina cycle’s reliability and the need to provide steady power throughout the day, the storage tank’s presence is essential. The PTC directs sunlight onto a glass tube with thermal oil as a working fluid, which is used as a heat transfer fluid. The heated oil then fills the storage tank, providing the tank with usable heat. After that, it goes into the steam generator to heat the NH3-H2O combination and power the steam expander. A supplementary heater supplies heating to working fluid if the tank’s operating temperature dips under a specified temperature point or needs the energy to achieve the minimum temperature difference in a steam generator. The Kalina cycle consists of two separators: a vapor expander and a water-cooling condenser, and consists of two recuperators for enhancing thermal performance: a pressure valve and a pump. The NH3-H2O combination is substantially vaporized before entering separator 1, where

Figure 5.8 A Kalina cycle driven by a parabolic trough solar collector [8].

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Hybrid Poly-generation Energy Systems

nearly pure H3 emerges from the top and powers an expander. The clean water is subsequently removed from the bottom of Separator 1, and its energy is obtained using Recuperator 1. A pressure valve is used to lower the flow’s operating pressure so that it may be combined with the expander’s exit stream. Following that, the combined stream reaches recuperator 2 to recuperate its energy. The two phases of the lowpressure mixture are separated by separator 2. The two phases are therefore combined to form one phase in the water-cooling condenser. The cycle is finished by pumping the primary way via the vapor generator.

5.2.4 Low-temperature solar energy and novel oxy-fuel power generation cycle Solar oxy-fuel energy production combines the subcritical Rankine cycle and the CO2 oxy-fuel transcritical cycle into one power production cycle. Both processes utilize LNG’s cold energy, but only the oxy-fuel transcritical cycle uses NG’s chemical and solar heat energies, as can be seen in Fig. 5.9. A high-temperature solar cycle with a thermal storage tank (TST) is employed to prepare the process’s heat source. Designing, simulating, and analyzing a high-efficiency new CO2-based power cycle are the primary goals of this research. The following items are the process’s innovative elements: G

G

Internal heat recovery combined with LNG evaporation and electrical energy production. High performance and reasonably priced transformation of solar heat; sustainable usage of fossil fuels.

O2(g) 7.00 °C, 110 kPa 1.44×104 kg.h-1

NG chemical energy 8.00 °C, 7000 kPa 3.6×103 kg.h-1

LNG -161.1 °C, 101.3 kPa 3.89×105 kg.h-1

Oxy-fuel power Generation with CO2 capturing Solar Energy 7.13×104 kW

Power Generation sub-cycle Outlet NG 25.0 °C, 7000 kPa 3.85×105 kg.h-1

Electrical Power Wnet=791 kW

CO2(L) -53.0 °C, 2605 kPa 1.03×104 kg.h-1

Electrical Power Wnet=4.27×104 kW

Figure 5.9 A schematic view of low-temperature solar energy and novel oxy-fuel power generation cycle [9].

Solar-based hybrid energy systems

G

G

G

G

G

179

Employing CO2 as a working fluid (WF) improves heat flux temperature distribution adherence. CO2 capture with low energy consumption coupled with energy conversion in the most practical and workable form. The cold recovery idea is ensured using LNG as a heat sink at the lowest temperature possible. Rankine and CO2 oxy-fuel transcritical cycles are used in an integrated framework to increase overall net electrical power output. High-temperature solar energy cycles may function in any climate.

The overall process schematics are shown in Fig. 5.10. The system involves CO2 oxy-fuel transcritical and subcritical Rankine cycle energy production parts. The low temperature of 2 156 C of liquid CO2 as the WF is compressed in P-1 to approximately 2.8 MPa (20 to 1) in the CO2 oxy-fuel transcritical cycle. Stream (1) is split into (2) and (3), each having the specified value. Heat exchangers (HE-1), (HE-2), and (HE-3) are used, in that order, to heat stream (2). By exchanging heat with the thermal heating fluid (THF) in the heat exchanger (HE-4), (6 to 7), WF (6) is heated to 30 C. In a heat recuperation heat exchanger (HE-5), streams (7) are heated to the necessary temperature (1000 C) (7 to 8). The air separation unit (ASU) creates stream (46) of pure atmospheric O2, which is subsequently pressured in the compressor (CR-1) (46 47). In the combustion chamber (CC), which is powered by natural gas (NG) stream (32), streams (8) and (48) are burnt up to their maximum value, or the turbine inlet temperature (TIT) (8, 48, and 32 9). To

TST

35

37

CPC

M-1 HE-4

P-5

47

46

28 LNG -161 °C, 101 kPa

44 43

45

36 H-2

P-4

O2 (g) 7 °C, 110 kPa

41 40 T-3 7

38

CO2 THF LNG H2O O2

39

48 8

CR-1

10

11

GT-1

9 CC

P-2

29

4 20

6 16

19

HE-6

15

HE-1 CR-2 18

1

S-1 17

5 13

HE-3

12 HE-2

S-2

14

42 34

P-1

22

21

2 3

25

T-1

H2O (L) 1 °C, 110 kPa

24

HE-9

GT-2

P-3

H2O (L) 1 °C, 110 kPa

G-2

23

30 HE-8

G-1

HE-5

27

H-1

26 CO2 (L) -53.0 °C, 2605 kPa

Figure 5.10 Flow diagram of the indicated process [9].

HE-7 31

33 32 T-2

NG 25 °C, 7000 kPa

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Hybrid Poly-generation Energy Systems

produce electricity, the WF temperature in the (GT-1) rises to 101 kPa (9 to 10) before cooling in the (HE-5), (10 to 11). The CO2 component of the gaseous exhaust stream (11) may be separated by lowering its temperature. After separation, heat exchangers (HE-2) and (HE-3).

5.2.5 Simultaneous generation of power and oxy-fuel power plant by solar dish collection and ORC This unitarily includes a power plant, an oxy-fuel generating station with CO2 collection, and an air separator system for delivering oxygen. Two axes of the solar parabolic dish follow the sun. Additionally, the materials utilized in the construction must be carefully chosen to achieve the solar collector segment’s requisite temperature of 1200K. Niobium, molybdenum, and tantalum are resistant metals that must be employed at higher temperatures up to 1473K, along with toughened alloys [10]. The ORC power-generating process is depicted in Fig. 5.11A (in orange). There are two primary sources of heat used in the steam power plant. The elevated pressure boiler in the heat exchanger HX8 receives some of its heat from the exhaust of the expander unit that produces oxy-fuel electricity and captures CO2. The solar dish collectors contribute the other portion of the heat into the steam generating cycle. Stream 57 enters the heat exchanger HX9 at a temperature of 198 C and a pressure of 186 bar. When the temperature reaches 542 C, the stream passes into turbine T2, where it produces 26.53 MW of electricity. A portion of the expander T2 output flows into the turbine T3 for preheating, and another portion of that stream 60 generates 19.53 MW of electricity. A portion of the T3 turbine’s exit stream reaches the HX18 for preheating, while the remaining portion (stream 63) is warmed up to 536 C by the solar cycle’s heated flow of oil. Flow 64 enters the T4 turbine and produces 23.23 MW of electricity at a temperature and pressure of 536 C and 3100 kPa, respectively. A portion of the T4 turbine’s outlet stream passes through the T5 turbine for preheating, while the remaining portion produces 32.90 MW of power. Similar to this, the flow reaches the steam expanders T6, T7, and T8 passes via temperature reduction in condensers HX19 and HX20 using utility water and then increases pressure in pumps P103 and P102, becoming the HX22, HX23, and HX24, respectively as can be seen in Fig. 5.11B.

5.2.6 Solar thermionic generator and thermoelectric device This solar-powered hybrid unit is depicted schematically in Fig. 5.12 and includes a concentrated solar power plant (PDC-type), thermionic generator (TIG), thermoelectric generator (TEG), and thermoelectric cooler (TEC) to generate the power and refrigeration load. The PDC captures sunlight from the beam as a temperaturecontrolled heat source for TIG before transforming it into thermal energy. Two cathode and anode plates linked to the hot end of the TEG and the solar collector, respectively, make up the TIG. In order to transform part of the heat into electrical

Solar-based hybrid energy systems

181

(A)

Heat Duty 107590 kW

Heat Duty 13857 kW

Absorption Refrigerant System

Organic Rankine Cycle

Liquid CO2 -9.014 °C, 3000 kPa, 2151 kgmol/h

Natural gas Power 65.17 kW

Power 11593 kW

Power 53453 kW Air Separation Unit (ASU)

Solar Dish Collectors

35 °C, 4000 kPa, 2003 kgmol/h

Oxygen Heat Duty 4344 kW

Power Plant and Heat Recovery

Heat Duty 329900 kW Power 148265 kW

Nitrogen 27 °C, 101 kPa, 20215 kgmol/h

Power 29972 kW

Steam Oxy Fuel Power Plant and CO2 Capture

0.2367 °C, 470 kPa, 4200 kgmol/h

Heat Duty 46217 kW

Organic Rankine Cycle

Power 847 kW

(B)

Figure 5.11 A simultaneous generation of power and oxy-fuel power plant by solar dish collection and ORC. (A) A schematic view and (B) processes [11].

energy, the produced electrons must flow across the gap between the electrodes, condense in the anode, and return to the cathode through an external load [12]. The waste from the TIG anode plate is transferred to the TEG heating element in the following phase, which produces more electricity. The TEC module then absorbs the heat from the chilled region to supply the cooling load. The TEG provides the necessary electrical power to carry out this task. As a result, the suggested hybrid system creates refrigeration and electrical load. It needs to be emphasized that the PDC is employed as a specific temperature of thermal power generation, converting the sun’s light energy into heat.

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Hybrid Poly-generation Energy Systems

Figure 5.12 A schematic view of a solar thermionic generator and a thermoelectric device [13].

5.3

Heating production by solar PGSs

5.3.1 Heating greenhouses: combined solar collector-geothermal heat pump A combined process is created so that solar collectors and a geothermal heat pump can work together or separately. Solar heat can be used to either warm the heat pump’s evaporator to a predetermined temperature or recover heat from the ground. To raise COP and inhibit its decline when heat transfer from the earth is greater than usual, the system is designed to restore ground heat and pre-heat the fluid reaching the heat pump’s evaporator up to a set temperature throughout heating periods. In order to prevent the ground’s temperature from dropping over years of operation due to heat extraction from the soil, recovery is also carried out during non-heating seasons. In Fig. 5.13, the process flow diagram is displayed. As shown in Fig. 5.13, warm fluid from the tank will be converted to diverter number 15. The fluid can then be utilized to heat the greenhouse or to enter diverter number 13 on a combined basis. In diverter number 13, the fluid is utilized to

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1

Green house

11 18

18 9

21

2

22

8

19 14

13

16

On /off On /off

10

12

17

Ground level

20

3

6

5

7 23

1: solar collectors 2: storage tank 3: ground heat exchanger 4: evaporator 5: condensor 6: compressor

7: expansion valve 8: internal exchanger 9,10,11,12: pump 13,14,15,16: diverting valve 17,18,19,20: mixing valve 21,22,23: temprature controller

Figure 5.13 A flow diagram of a combined solar collector-geothermal heat pump system [14].

either recover heat from the soil around the heat exchanger during non-heating seasons or to pre-heat the fluid entering the heat pump’s evaporator up to 12 C. The performance of diverter number 15 will be determined by controller number 13 by taking into account the temperature of the fluid exiting mixer number 17, which is an important aspect to remember because thermal controllers generally specify how valves should operate. However, the overall performance of pumps numbers 10 and 12 and diverter number 13 is identical. They may be identified by an on/off switch, and diverter numbers 14 and 16 operate similarly to diverter number 15.

5.3.2 A hybrid photovoltaic solar, proton exchange membrane fuel cell, and thermoelectric device system The hybrid model presented and examined in this work consists of thermoelectric, water electrolyzer, PEM fuel cell, and photovoltaic solar panels. The hybrid system conceptual design is shown in Fig. 5.14. Solar PV energy is supplied into the electrolyzer, which creates H2. In order to generate electricity and heating, the electrolyzer’s oxygen and hydrogen are supplied to the cathode side and anode side of the fuel cell, respectively. Keep in mind that synthesizing hydrogen and fuel cells does not necessarily result in power production. The inverter converts the DC electrical power produced from the PEMFC into AC electrical power in order to meet electrical demands.

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Hybrid Poly-generation Energy Systems

Q2 Heating demand

PEM electrolyzer

H2

Solar PV

Electrical demand

O2

DC/DC Converter

PEMFC T

Q1

Water Tank TEG q1

H2O

Ambience (T0)

q2

Power

TEC Qc Cooled space (Tc)

Figure 5.14 A conceptual design of a hybrid photovoltaic solar, proton exchange membrane fuel cell and thermoelectric device system [15].

5.4

Cooling production by solar PGSs

5.4.1 Cryogenic biogas process using solar energy A process applicable for cryogenic biogas uses a hybrid solar ORC power generating system to supply the necessary electricity. To ensure sustainable development, the usage of renewable resources is prioritized. Planning and research are done on parabolic solar collectors for a certain geographic situation at various times. A mixed refrigerant (MR) process, ORC, absorption system, and parabolic trough solar collector are all employed in a hybrid process. The temperatures required in the ORC and absorption stages in the cogeneration are provided via the parabolic trough solar collector system, which considers environmental sustainability. The block diagram for the system is shown in Fig. 5.15. The four distinct parts of this operation are the heat supply, power production, cryogenic cycle, and biogas process. The ORC and refrigeration processes are heated using a parabolic trough solar collector setup. The supplementary heater makes up for the heat loss if there is insufficient sun radiation due to the climate. Water is the operating medium of the solar system. The produced vapor is sent to the ORC and absorption cycles by the pipelines. According to Fig. 5.15, the solar collector generates around 35 MW of heat, 91% of which goes to the ORC and 9% to the absorption cycle.

Solar-based hybrid energy systems

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Biogas 25 °C, 200 kPa, 320 kmol/h Acid gas -55 °C, 200 kpa, 12.88 kgmole/h Biogas Upgrading Process (Cryogenic separation method)

Power 1145 kW

Power 3078 kW

Upgraded Biogas 23.7 °C, 500 kPa, 172.8 kmol/h

Power 17.33 kW Organic Rankin Cycle

Heat 31810 kW

Mixed Refrigerant Cycle

Absorption Cycle

Heat 3032 kW

Power 2426 kW

Porabolic trough collector & auxiliary boiler

Figure 5.15 A schematic diagram Cryogenic biogas process using a parabolic trough collector [16].

5.5

Hydrogen production by solar PGSs

5.5.1 Hydrogen liquefaction by solar PTC and ORC In this plant for hydrogen liquefaction, the structure integrates parabolic solar dish collectors, an ARS, and an ORC with ideal energy efficiency. Pre-pressurizing, precooling, deep or cryogenic chilling, and expanding are the four essential components of a hydrogen liquefier’s equipment. Fig. 5.16 provides a comprehensive overview of the entire system. Since the feed gas temperature is 25 C and the mass flow rate is expected to be 290 tons daily, the hydrogen feed conditions are comparable to those of the primary system. This mass flow rate may deliver the LH2 necessary for 270,000 540,000 hydrogen cars to operate in a significant urban area. As observed, the first MR process occurs by precooling regular hydrogen at a temperature of 25 C to 50% para-hydrogen at a temperature of 2 195 C, and the second MR phase deep-cools it at a temperature of 2 254.5 C. These two refrigeration systems need 13,234 and 32,166 kW of electricity to liquefy hydrogen, respectively, as shown in Fig. 5.16. A solar-assisted ORC supplies the necessary electricity [17,18]. The improved structure’s flowcharts diagram is shown in Fig. 5.17. As a result, three additional renewable-based systems help hydrogen refrigeration create. Therefore, the fundamental hydrogen liquefaction process and the ARS meet at three heat exchangers, HX13, HX14, and HX15. The ARS’s external chilling flow

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Hybrid Poly-generation Energy Systems

1.103 kWh/kgLH2

95% Para - H2(L)

50% Para - H2(g)

Hydrogen (g) 25 °C, 21 bar

2.919 kWh/kgLH2

Mixed refrigerant cycle (First)

-195 °C, 21 bar

Mixed refrigerant cycle (Second)

-254.5 °C, 1.3 bar

Power 32166 kW

Power 13234 kW

Power plant

Heat Duty 237617 kW

Cold Duty 10806 kW

Power 74 kW

Solar Dish Collectors & Auxiliary Boiler Absorption refrigerant system

Heat Duty 22518 kW

Figure 5.16 A conceptual of hydrogen liquefaction by solar PTC and ORC [19].

Figure 5.17 A solar-based hybrid system for hydrogen liquefaction by solar PTC and ORC [19].

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187

cools the second MR cycle’s streams. The ARS’s re-boiler and the HX23 heat exchanger also serve as the corresponding connecting points for the solar dish collecting system and the ARS. As a result, the solar system’s K103 outgoing heating stream enters the re-boiler and provides the necessary heat for the ARS. The HX23 heat exchanger receives the red-hot boiler’s stream exit, which heats the ORC as needed. In the operation of liquefying hydrogen, the ORC contains two expanders, Ex108 and Ex109, which are connected to compressors, C101 to 106 C, and pump 102 in the ARS. They provide the compressors with the necessary power.

5.5.2 Hydrogen generation from seawater via a desalination unit and low-temperature electrolysis by solar-based setup Therminol-66 oil, the operating stream of the solar subsystem, receives solar energy via PTSCs and transfers it to the solar subsystem as shown in Fig. 5.18. Energy storage is not taken into account, and only the ORC is set up, generating electricity using n-octane working liquid at moments whenever the energy of the renewable radiation is poor, notably the solar mechanism. It commonly happens between 6 a.m. to 8 a.m. and 4 p.m. to 6 p.m.; when radiation from the sun power surges, the solar and storage mode takes control, which normally occurs between 8 a.m. and 4 p.m. throughout the day. The storage component of the solar subsystem is employed in this mode in parallel to ORC beginning, allowing it to be utilized for ORC function at periods during which there is no availability of solar irradiance or the storing phase, typically between 4 p.m. and 6 a. m. in the following day. Therminol-66, on the other hand, transmits its energy to the organic operating fluid (n-octane) used in the ORC employing evaporation in the ORC. When this working fluid (n-octane) meets the essential thermodynamic criteria for reaching the ORC turbine, it produces mechanical energy in the expander, which is then used to power an external power supply, which creates electricity. This system generates electricity as a semi-product, which is then utilized by the electrolyzer to generate hydrogen. Fig. 5.18 shows that the stream that reaches the expander must be modified in a specific process to make it appropriate for returning the expander since it does not have the necessary pressure to generate power. Thus to be acceptable to approach the pump and produce electricity, it must remove heat and change into saturated water. Condensers and cooling towers are frequently used in conventional systems to disperse this heat, which results in the loss of this vital energy source. Nevertheless, in modern systems, this stream movement serves several functions. This stream is appropriate for accessing the ORC pump because of its optimal temperature capacity while employing the multi-heat recovery approach. It is important to note that heat exchangers 1 and 2—which play a crucial role in the HDH and hydrogen generation subsystems—implement this multi-heat recovery.

188

Hybrid Poly-generation Energy Systems

Figure 5.18 A flow diagram of Hydrogen generation from seawater via a desalination unit and low-temperature electrolysis by a solar-based system [20].

Solar-based hybrid energy systems

189

Figure 5.19 Schematic diagram of a combined solar thermophotovoltaic power generation and solid oxide electrolyzer for hydrogen production [21].

5.5.3 A combined solar thermophotovoltaic power generation and solid oxide electrolyzer for hydrogen production The system is described schematically in Fig. 5.19 as having two primary subsystems: the solar thermophotovoltaic (STPV) instrument and the SOEC process. A solar collector collects the sun’s rays, which radiate to the absorber’s surface area. The absorber serves as a light-harvesting layer for the STPV system, converting solar light into thermal energy. This surface decreases heat emission in the infrared spectral domain while maximizing solar energy uptake in the sight and nearinfrared range. The absorber area must function at high temperatures with significant energy to guarantee thermostability. Another selective surface that emits heat energy in the direction of the PV cell is the emitter. In light of the fact that photons with energies below the minimum necessary to produce electron-hole pairs are essentially useless and photons with energies above the bandgap result in extra energy at the PV surface and poor efficiency, single-junction solar PV has a substantial limitation in utilizing solar energy [22]. Emitters provide the most effective spectra for the PV panel by adjusting the bandgap. The quaternary InGaAsSb diode with 0.53 eV is the thermophotovoltaic (TPV) diode used for simulation. The innovative STPV system, on the other hand, offers electrical energy that passes by the SOEC to produce hydrogen. A diagram of the STPV mechanism and its energy flow is shown in Fig. 5.19.

190

Hybrid Poly-generation Energy Systems

References [1] B. Ghorbani, M. Mehrpooya, M.M.M. Sharifzadeh, Introducing a hybrid photovoltaicthermal collector, ejector refrigeration cycle and phase change material storage energy system (Energy, exergy and economic analysis), International Journal of Refrigeration 103 (2019) 61 76. [2] B. Ghorbani, M. Mehrpooya, M. Amidpour, A novel integrated structure of hydrogen purification and liquefaction using natural gas steam reforming, organic Rankine cycle and photovoltaic panels, Cryogenics 119 (2021) 103352. [3] A. Mohammadi, M. Mehrpooya, Techno-economic analysis of hydrogen production by solid oxide electrolyzer coupled with dish collector, Energy Conversion and Management 17 (2018) 167 178. [4] M. Mehrpooya, B. Ghorbani, S.S. Hosseini, Thermodynamic and economic evaluation of a novel concentrated solar power system integrated with absorption refrigeration and desalination cycles, Energy Conversion and Management 175 (2018) 337 356. [5] A. Mohammadi, M. Mehrpooya, Exergy analysis and optimization of an integrated micro gas turbine, compressed air energy storage and solar dish collector process, Journal of Cleaner Production 139 (2016) 372 383. 2016. [6] M. Marefati, M. Mehrpooya, F. Pourfayaz, Performance analysis of an integrated pumped-hydro and compressed-air energy storage system and solar organic Rankine cycle, Journal of Energy Storage 44 (2021) 103488. [7] H. Junye, C. Yaping, W. Jiafeng, Thermal performance of a modified ammonia water power cycle for reclaiming mid/low-grade waste heat, Energy Conversion and Management 85 (2014) 453 459. [8] M. Ashouri, A.M.K. Vandani, M. Mehrpooya, M.H. Ahmadi, A. Abdollahpour, Techno-economic assessment of a Kalina cycle driven by a parabolic Trough solar collector, Energy conversion and management 105 (2015) 1328 1339. [9] M. Mehrpooya, M.M.M. Sharifzadeh, A novel integration of oxy-fuel cycle, high temperature solar cycle and LNG cold recovery energy and exergy analysis, Applied Thermal Engineering 114 (2017) 1090 1104. [10] M. Moradi, M. Mehrpooya, Optimal design and economic analysis of a hybrid solid oxide fuel cell and parabolic solar dish collector, combined cooling, heating and power (CCHP) system used for a large commercial tower, Energy 130 (2017) 530 543. [11] F. Piadehrouhi, B. Ghorbani, M. Miansari, M. Mehrpooya, Development of a new integrated structure for simultaneous generation of power and liquid carbon dioxide using solar dish collectors, Energy 179 (2019) 938 959. [12] J.-H. Lee, et al., Optimal emitter-collector gap for thermionic energy converters, Applied Physics Letters 100 (17) (2012) 173904. [13] M. Mehrpooya, M. Marefati, Parametric design and performance evaluation of a novel solar assisted thermionic generator and thermoelectric device hybrid system, Renewable Energy 164 (2021) 194 210. [14] M. Mehrpooya, H. Hemmatabady, M.H. Ahmadi, Optimization of performance of combined solar collector-geothermal heat pump systems to supply thermal load needed for heating greenhouses, Energy Conversion and Management 97 (2015) 382 392. [15] M. Marefati, M. Mehrpooya, Introducing a hybrid photovoltaic solar, proton exchange membrane fuel cell and thermoelectric device system, Sustainable Energy Technologies and Assessments 36 (2019) 100550.

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[16] M. Mehrpooya, B. Ghorbani, A. Manizadeh, Cryogenic biogas upgrading process using solar energy (process integration, development, and energy analysis), Energy 203 (2020) 117834. ¨ . Kizilkan, S. Niˇzeti´c, G. Yildirim, Solar assisted organic rankine cycle for power [17] O generation: a comparative analysis for natural working fluids, Energy, Transportation and Global Warming, Springer, 2016, pp. 175 192. [18] Z. Fergani, D. Touil, T. Morosuk, Multi-criteria exergy based optimization of an Organic Rankine Cycle for waste heat recovery in the cement industry, Energy Conversion and Management 112 (2016) 81 90. [19] B. Ghorbani, M. Mehrpooya, M. Aasadnia, M.S. Niasar, Hydrogen liquefaction process using solar energy and organic Rankine cycle power system, Journal of Cleaner Production 235 (2019) 1465 1482. [20] M. Delpisheh, M.A. Haghghi, H. Athari, M. Mehrpooya, Desalinated water and hydrogen generation from seawater via a desalination unit and a low temperature electrolysis using a novel solar-based setup, International Journal of Hydrogen Energy 46 (10) (2021) 7211 7229. [21] R. Daneshpour, M. Mehrpooya, Design and optimization of a combined solar thermophotovoltaic power generation and solid oxide electrolyser for hydrogen production, Energy Conversion and Management 176 (2018) 274 286. [22] W. Shockley, H.J. Queisser, Detailed balance limit of efficiency of p-n junction solar cells, Journal of Applied Physics 32 (3) (1961) 510 519.

Technical and economic prospects of fuel cells combination with polygeneration systems?

6.1

6

Fuel cell

A fuel cell is an electrochemical device that converts a chemical reaction’s energy directly into electrical energy. This conversion of energy takes place as long as the fuel is injected into the cell. A fuel cell increases energy conversion efficiency by eliminating intermediate stages of energy conversion. The steps of energy conversion in thermal power plants are as follows, each of which reduces efficiency. Chemical energy ! Thermal energy ! Mechanical energy ! Electrical energy The function of a fuel cell is not like an energy-storing battery. In a fuel cell, a state of energy is converted to another state to consume no materials. Also, the battery’s energy density is lower than that of the fuel cell, and the process of charging the battery is much more complicated than filling the fuel tank of the fuel cell. In batteries, the power of electrochemical conversions decreases after several charges, while in fuel cells, there is no such limit [1,2]. The benefits of fuel cells include the following: 1. High efficiency, depending on the type of fuel cell, design, and operating conditions; the efficiency varies between 40%60%, which reaches over 80% using the heat of the exhaust gases. 2. Due to high efficiency for the same power produced, reducing pollutant emissions is less than other pollution emission processes. 3. Variety of fuel consumption, the ability to produce hydrogen from different types of fossil fuels and use it in fuel cells, or direct use of fossil fuels in batteries depending on the type of cell. 4. Ability to combine with the thermal cycle and energy production using the heat of the cell’s exhaust gases. 5. The need for minor repairs is due to the lack of moving parts that increase the useful life of system components.

A fuel cell has the following general arrangement [3]: 1. Oxidizer (oxidizer): Oxygen is converted to oxygen ions by accepting electrons from the cathode. Performing this reaction requires the presence of a catalyst in the electrode, especially at low temperatures. The choice of the type of catalyst used is very effective in the efficiency of the cell. 2. Cathode electrode: Through this electrode, electrons are transferred to oxygen, and a halfreduction reaction occurs. The electrode’s surface is porous to allow oxygen to penetrate, Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00012-8 © 2024 Elsevier Inc. All rights reserved.

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and the electrode uses a catalyst to accelerate the reduction reaction half. The electrode components must be catalytic, conductive, and porous to speed up the reactions. 3. Electrolyte: It is responsible for transferring ions from the cathode to the anode or vice versa. Electrodes and external circuits conduct electrons, and electrolytes carry ions. 4. Anode electrode: In this electrode, a half-oxidation reaction occurs, and hydrogen is converted to an ion by losing an electron. 5. Oxidizing: It is the same fuel consumption that can be in the form of pure hydrogen, or fossil fuels can be used directly in the cell. These fuels can be converted to hydrogen in the cell and then used, or they can be directly oxidized. The consumed fuel penetrates through the electrode holes and then reaches the catalyst’s surface and is oxidized.

There are three general classifications of electrolytes: aqueous, polymeric, and ceramic. Regardless of the classification, an electrolyte must have high ionic conductivity, low electron conductivity, high stability (at both the anode and cathode), no fuel transfer, and high mechanical strength (if solid) [4]. As mentioned, a suitable catalyst must be used at the electrode to accelerate the reduction and oxidation reactions. The electrode structure used in fuel cells is a function of various factors; one of the most important is its operating temperature. Essential properties that a catalyst in an electrode should include high mechanical strength, high electron conductivity, good porosity, easy fabrication, corrosion resistance, and high catalytic activity. Platinum is one of the best catalysts used in polymer fuel cell electrodes. Nickel is often used for high-temperature fuel cells [4]. Regardless of the type of catalyst, the catalyst’s thickness is also a hidden factor that must be considered. The gaps in the fuel cell electrodes are essential for the following reasons: 1. They create areas where oxidation-reduction of gas and liquid occurs. 2. The conductors of the ions are inside a three-phase retainer (gas-electrolyte-electrode surface), so the electrode must be made of materials that conduct electrons well. 3. Provide a suitable environment for gas and electrolyte phase separation.

As the reaction rate of ions increases at high temperatures, the catalyst’s role at low temperatures becomes more critical.

6.1.1 Types of fuel cells Fuel cells are divided according to the type of electrolyte used and the operating temperature. The following are the top five fuel cell classifications based on the type of electrolyte [5]: 1. 2. 3. 4. 5.

Alkaline Fuel Cell (AFC) Polymer Solid Fuel Cell (PSFC) or Proton Exchange Membrane Fuel Cell (PEMFC) Phosphoric Acid Fuel Cell (PAFC) Molten Carbonate Fuel Cell (MCFC) Solid Oxide Fuel Cell (SOFC)

The study of the different types of fuel cells mentioned is omitted separately and an attempt is made to provide a brief and useful comparison of the fuel cells by providing the requirements. Table 6.1 shows a comparison of fuel cell types [6,7].

Table 6.1 Comparison of fuel cell types [6,7]. Prospects for achieving price reduction

Process stage

Application

Output power (kW)

Efficiency

Operating temperature ( C)

Conductor ion

Reactions

Electrolyte

Type of cell

Medium to good

Laboratory

Power plant

53000

40%60%

7001000

O22

2H2 1 2O22 ! H2 O 1 4e2

Zr0:92 Y0:08 O1:96

SOFC

Li2 CO3 =K2 CO3

MCFC

H3 PO4

PAFC

2H2 1 4OH2 ! 4H2 O 1 4e2 O2 1 2H2 O 1 4e2 ! 4OH2

KOH/NaOH

AFC

Anode: 2H2 ! 4H1 1 4e2

Nafion

PEMFC

O2 1 4e2 ! 2O22

Medium to high energy density uses carbon monoxide as fuel Medium

Practical

Power plant

, 11000

50%60%

650

CO22 3

CO3 22 1H2 ! H2 O 1 CO2 1 2e2 1 2 O2

1 CO2 1 2e2 ! CO3 22

Low energy density, nickel catalyst instability, need for carbon dioxide cycle Medium

Commercial

Power plant

501000

40%50%

160210

H1

H2 ! 2H1 1 2e2 1 2 O2

1 2H1 1 2e2 ! H2 O

Medium energy density, use of platinum as a catalyst, sensitive to carbon monoxide Good

Space

Submarine and space

10100

50%55%

6080

OH2

, 1250

40%50%

5085


 H1 H3 O1

High energy density, carbon dioxide intolerance Good

Prototypes

Small vehicles and power plants

High energy density, Pt catalyst, sensitive to carbon monoxide, need to keep the membrane moist permanently.

O2 1 4H1 1 4e2 ! 2H2 O

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Hybrid Poly-generation Energy Systems

6.1.1.1 SOFC SOFC electrolyte combines yttria and zirconia, containing 8% yttria and 92% zirconia ((ZrO2)0.92(Y2O3)0.08)). Because yttria causes the crystalline structure of zirconia to remain cubic, it is called yttria-stabilized zirconia (YSZ). The material of the anode and cathode in this cell is different. The anode electrode is a mixture of metal-ceramic. The most common of which is a combination of nickel and stabilized zirconia with yttria. The metal used in the anode electrode is used as a catalyst and electron conductor. Its ceramic is used to conduct ionic, stable, mechanical, and high porosity, as well as increase the effective surface and thermal expansion compatibility. The cathode electrode is also made of ceramic, which includes ionic conductivity and electron conductivity. The material of the cathode electrode also includes the following [4]: 1. 2. 3. 4.

Strontium-doped Lanthanum Manganite (SLM), LaxSr1-xMnO3 Lanthanum-Strontium Ferrite (LSF), LaxSr1-xFeO3 Lanthanum-Strontium Cobaltite (LSC), LaxSr1-xCoO3 Lanthanum-Strontium Cobaltite Ferrite (LSCF), LaxSr1-xCoyFe1-yO3

These materials have been seen in the cathode environment as a suitable catalyst and have shown good oxidation resistance. The SOFC temperature is in the range of 600 C1000 C, which has its advantages and disadvantages. Among the benefits that can be mentioned: 1. Flexibility in terms of fuel, despite the high temperature, is possible to use direct fossil fuels. For example, natural gas can be introduced directly into the cell and used during the steam reforming reaction to hydrogen production and then consumed, or natural gas that can be oxidized directly [8]. 2. It has high electrical efficiency (40%60%), and by combining it with suitable thermal cycles, an efficiency of 85% can be achieved. 3. It is less sensitive to sulfur compounds such as hydrogen sulfide. 4. Despite carbon monoxide, electrodes, and electrolytes are not destroyed, but carbon monoxide is used as fuel. 5. The SOFC size can be changed according to the type of application and the amount of power required. 6. There is no need to use precious metals as a catalyst

Among its disadvantages, we can mention the intensification of corrosion and damage of components due to high temperature and high cost of some components. At high operating temperatures in a SOFC, oxygen ions pass through the electrolyte. At the cathode, oxygen molecules in the air combine with four electrons. When a hydrogen-containing gas fuel passes through the anode, a negatively charged stream of oxygen ions passes through the electrolyte to oxidize the fuel. The electrons created at the anode pass through an external circuit and go to the cathode, which completes the electrical circuit and generates electricity. Fig. 6.1 shows the overall performance of a SOFC. SOFC reactions are in the form of Reactions (6.1) and (6.2). 2H2 1 2O22 ! 2H2 O 1 4e2

(Reaction 6.1)

Technical and economic prospects of fuel cells combination with polygeneration systems?

197

Figure 6.1 Solid oxide fuel cell performance (with internal reforming) [8].

O2 1 4e2 ! 2O22

(Reaction 6.2)

As mentioned, the electrolyte of this cell is a mixture of yttria and zirconia. Y and Zr are intermediate elements with atomic numbers 39 and 40, respectively. The chemical component of zirconia is ZrO2, and the chemical composition of yttria is Y2O3. The presence of yttria particles in the structure of zirconia creates a space for O22 ions in the structure of zirconia. This space created for the movement of these ions increases the ionic conductivity in the electrolyte structure. Pure zirconia consists of an ionic network consisting of Zr41 and O22 as shown in Fig. 6.2. By adding Y31 ions to this structure, a kind of turbulence in the charge balance is created. As shown in the figure, for every two Y31 ions that replace one Zr41 ion, there is a space for oxygen to balance the charge. Adding 8% molar yttrium creates 4% of space for oxygen. At high temperatures, these voids allow oxygen ions to move inside the electrolyte. Voids are a kind of carrier of oxygen ions inside the crystal structure. As the amount of yttrium inside the electrolyte increases, the amount of voids increases, increasing ionic conductivity. Unfortunately, the increase in yttria is limited, and increasing this amount from one value onward reduces the ionic conductivity. This increase in concentration for YSZ reaches 8% mol, after which the crystal structure interacts and reduces the ionic conductivity within the crystal structure. Therefore the best compound for SOFC electrolytes is a concentration of 8% molar yttria within the zirconia structure. Fig. 6.3 shows the changes in yttria concentration in terms of ionic conductivity [4].

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Hybrid Poly-generation Energy Systems

Figure 6.2 (A) Pure ZrO2 and (B) yttria-stabilized zirconia [4].

Figure 6.3 YSZ conductivity in the percentage of Y2O3 (mol) [4].

6.1.1.1.1 Required SOFC fuel The low density of hydrogen energy in the gaseous state makes it challenging to use as an energy carrier, meaning that it has less volumetric energy than liquid fuels such as gasoline or methanol. Liquid hydrogen has a good energy density but must be stored at very low temperatures and high pressures, making it difficult to store and transport. On the other hand, although hydrogen is the second most abundant element in nature, it does not exist in pure form in nature [3]. It must be derived from water or fuels such as coal, oil, natural gas, methanol, ethanol, and ammonia in molecular structure. Hydrogen itself is prepared. Therefore the fuel cell’s hydrogen fuel is provided by the electrolysis of water or conventional fuels conversion. In the latter case, the presence of fuel exchangers is mandatory, so it can be said that the operation of a fuel exchanger involves the conversion of hydrogen-rich

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fossil fuel into hydrogen and other by-products such as CO2. Although there are different types of fuel exchangers, often derived from a combination of different technologies, the main types of exchangers common in this field are: (1) Exchangers with steam reforming, (2) partial oxidation exchangers, (3) autothermal exchangers, (4) pyrolysis or cracking. The presence of sulfur impurities in the fuel is one of the significant problems of exchangers whose input fuel is natural gas or oil derivatives. These sulfurcontaining organic compounds must be removed from the fuel because sulfur acts as a poison to the converter catalysts. In the case of natural gas, sulfur compounds are odorous compounds that are added to the gas for safety. The desulfurization system must be carefully designed to ensure the absence of sulfur derivatives in the gas passing through the converter catalysts and the fuel cell mass. A desulfurization reactor is used for this purpose. In this reactor, hydrogenation reactions take place in the presence of molybdenum oxide-nickel or molybdenum oxide-cobalt catalysts. During a series of chemical reactions, the sulfur-containing organic compounds are converted to hydrogen sulfide gas, and the hydrogen sulfide gas produced by water is converted to zinc oxide [9]. There are two ways to supply the fuel needed for a SOFC from natural gas or other hydrocarbon fuels: 1. Convert hydrocarbon fuel to hydrogen outside the fuel cell and then send hydrogen into the fuel cell for consumption. 2. Conversion of fossil fuel to hydrogen inside the fuel cell and its simultaneous consumption inside the cell.

Several methods and reactions are for converting fossil fuels to hydrogen, which use steam-fuel reforming reactions. This method has high efficiency and low production and operating costs. The reaction for fuel reforming is in the form of Reaction (6.3). Following this reaction, carbon monoxide is converted to hydrogen and carbon dioxide during Reaction (6.4). The overall reaction is presented in Reactions (6.2)(6.5), and the heat from the electrochemical reaction is presented in Reaction (6.6). CH4 1 H2 O2CO 1 3H2

ΔH298 5 206 Kj=mol

(Reaction 6.3)

CO 1 H2 O2CO2 1 H2

ΔH298 5 2 41 Kj=mol

(Reaction 6.4)

CH4 1 2H2 O2CO2 1 4H2

ΔH298 5 165 Kj=mol

(Reaction 6.5)

H2 1 1=2O2 ! H2 O

ΔH298 5 2 243 Kj=mol

(Reaction 6.6)

In the external reforming mode, all reforming reactions occur outside the fuel cell, and the fuel in the cell enters the hydrogen. The reaction with water vapor conversion is highly endothermic and must occur at high temperatures and low pressures to produce more hydrogen and carbon monoxide. The pressure (0.23.5 MPa) is usually constant, so to achieve the required high temperature (500 C900 C), the reactor in which the reaction takes place is placed in the radiant part of a furnace. In this

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Hybrid Poly-generation Energy Systems

process, the ratio of vapor to carbon in the feed is 3.5. Under these conditions, 98% of methane is converted. The produced carbon monoxide then enters another reactor to increase the hydrogen conversion rate, but at a lower temperature (about 360 C500 C) due to the exothermic reaction. This reaction rapidly approaches equilibrium at higher temperatures [9]. One method of supplying the heat required for endothermic reactions in converting low molecular weight hydrocarbons to hydrogen is to use the heat generated by the electrochemical reaction inside the fuel cell. Internal reforming method is possible in fuel cells with high operating temperatures such as SOFC. In the fuel cell with internal reforming, various hydrocarbon fuels such as natural gas, gasoline, and kerosene can be used. Internal reforming can be used in both direct and indirect contact. In the indirect state, fuels such as methane are converted to hydrogen and other by-products using exchangers in direct heat contact with the fuel cell mass. The reforming reaction and the electrochemical reaction occur separately, and only the heat generated by the electrochemical reaction is transferred through the exchanger to the reforming reaction. In indirect internal reforming, methane is first converted to hydrogen using the cell’s heat. Then the hydrogen produced enters the cell through another path. Ni has been observed to be a suitable catalyst for the reforming reaction. Among the problems in this method are the different speeds of electrochemical reactions and reforming. The speed of the reforming reaction is faster than the speed of the electrochemical reaction, which causes a sudden drop in temperature at the beginning of the cell, resulting in mechanical problems with the cell. One way to solve this problem is to reduce the speed of the reforming reaction, which can be achieved by making changes in the catalyst [10]. In internal reforming, by direct contact, conversion reactions take place in the anode cell. In a SOFC, the high operating temperature and nickel presence as a catalyst cause direct conversion reactions on the anode electrode. The advantage of the direct method is that in addition to excellent heat transfer, the steam from the electrochemical reaction at the anode can be used in the fuel conversion reaction. Also, the fuel conversion of heat transfer reaction can be used to control the mass temperature. However, the fuel conversion reaction cannot consume all the heat from the electrochemical reactions, and there must be a heat management and control system. As to solve the sudden drop in temperature at the beginning of the fuel cell problem, a part of the reforming operation (20%30%) can be done before entering the fuel cell to present hydrogen when the fuel enters the fuel cell. Another problem is the direct reforming of carbon formation on the anode, which leads to reduced cell performance. High temperature (T . 700 C) and a decrease in steam-to-carbon ratio (S/C) are among the factors that cause carbon formation [11]. Fig. 6.4 shows the difference between internal and external reforming, and the technology is moving towards the use of direct internal reforming. Internal reforming has some advantages over external reforming: 1. System cost is reduced due to the lack of need for an external model. 2. Since water vapor is one of the products of electrochemical reaction, the need for water vapor for reforming is reduced.

Technical and economic prospects of fuel cells combination with polygeneration systems?

Indirect Internal Reforming

External Reforming H2, CO

H2, CO

H2O, CO2

800°C

CH4, H2O

700°C H2, CO

Anode

Direct Internal Reforming

CH4 H2O

H2, H2O CO, CO2

H2, CO

Electrolyte Cathode

CH4, H2O

CO, CO2

H2, H2O CO, CO2

H2O, CO2

Anode

H2, H2O CO, CO2

Electrolyte Cathode

Anode Electrolyte Cathode Air (O2)

Air (O2)

CH4, H2O

201

Air (O2)

Direction of Technology Advancement

Figure 6.4 Comparison of internal and external reforming and the movement of technology towards direct internal reforming [12].

3. Using internal conversion increases cell efficiency. Because in this case, the fuel cools the fuel cell mass, and the need for additional air to control the system temperature is reduced. This, in turn, leads to a reduction in energy consumption in the air compressor [9].

6.1.1.1.2 SOFC structure For practical applications, SOFCs have different structures, each of which has advantages and disadvantages. However, most of the materials and components used in these different designs are the same. There are currently two major types of structural design, including tubular and plate design. In the plates’ design, the battery components are stacked on top of each other in flat plates, which can be rectangular or circular. In the design of a tube, one electrode (anode or cathode) is made in the form of a long tube with a porous surface, then a layer of electrolyte is placed on the outer surface of the electrode, and then another electrode. Many cells are placed in series with connecting plates in tube and plate designs to achieve the desired voltage. Most early studies focused on the design of the tube type. Since the late 1990s, plate-type development has received much attention due to its high current density and easy fabrication [13]. In pipe design, due to longer electrical conduction paths, the failures are more, and their maximum power density is limited to 250300 mW/cm2. The type of plate has a higher potency due to its shorter navigation paths. High power density and lower manufacturing costs have led to exceptional attention to this type of fuel cell. However, the main problem with this type of fuel cell is sealing. The low power density of tubular SOFCs has led to their use only in static power generation applications. There is a slight tendency to use them in mobile applications. This type of plate can be used in the mobile system due to its high power density [14]. A comparison between a tube and a plate design is given in Table 6.2. Figs. 6.5 and 6.6 also show a schematic of each tube and plate design.

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Hybrid Poly-generation Energy Systems

Table 6.2 Comparison of plate and tube structure of solid oxide fuel cell [13]. Parameter

Plate structure

Power per unit level Power per unit volume Ease of construction ease of fabrication Simple sealing Long-term resistance Thermocycling stability

X X X

Tube structure

X X X X

Interconnection

Ni felt Anode Interconnect

Electrolyte Cathode

Electrolyte

Fuel Flow

Buffer Cathode Air Flow

Figure 6.5 Tube solid oxide fuel cell [15,16].

Figure 6.6 Plate solid oxide fuel cell [17].

Anode

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203

On the other hand, SOFCs can be divided into two categories: self-supporting and external support. In the base type itself, one of the fuel cell components is responsible for its structural support. Thus a single cell can be anode-enhanced (anode-enhanced), cathode-supported (cathode-enhanced), or electrolyte-dependent (electrolyte-supported). The whole cell is placed as a thin layer on the interconnectors or a porous substrate [14]. The types of these structures are shown in Fig. 6.7. One of the base electrodes’ crucial advantages is that it reduces the electrolyte thickness and reduces ohmic resistance losses. As a result, it is possible to work with a lower operating temperature, which in turn reduces costs [13].

6.1.1.1.3 Development of SOFC systems Siemens-Westinghouse built the first 100 kW SOFC system. Each cell’s diameter is 12 mm, and its active length is 1500 mm. A total of 1152 cells are arranged vertically in 12 rows and controlled by five other subsystems. These systems include: 1. Temperature control system that is used to control blowers, air pipes and air control valves, and cell outlets. 2. An electrical regulator system that regulates the electricity distributed throughout the system and the electronic hardware that controls the entire assembly. 3. Fuel system which consists of a small steam generator to start the work of the cell mass (the generator is connected to a water tank). This system also has pure (desulfurized) fuel control valves. 4. Electric heating system that is used to start the work of the cell mass by heating the air. 5. Direct to alternating current conversion system.

The above systems are placed in a chamber with dimensions of 8.42 meters in length, 2.75 meters in width, and a maximum height of 3.85 meters. Westinghouse has tried to design a 250 kW system of SOFCs that operate at a pressure of 3.5

Figure 6.7 Types of flat-type fuel cell structures in terms of the type of support [18].

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Hybrid Poly-generation Energy Systems

atmospheres. This system is combined with a generator with a capacity of 50 kW. The output efficiency of such a system is 60%. The second plan for developing solid mass units of an oxide fuel cell is the construction of 1 MW, in which four SOFC masses and a 250 kW turbine are used. An efficiency of more than 60%. Due to turbines and the addition of 4 battery masses to the above unit, it is possible to increase efficiency up to 70% [19]. Ztek was founded in 1984 to develop SOFC systems. The company has done extensive work to develop and build SOFC hybrid systems and develop and construct a 200 kW SOFC hybrid system with a gas turbine. Materials and Systems Research Incorporated (MSRI) has also focused on developing anode-base plate SOFCs (for medium temperatures). The company has successfully built 1 and 3 kW stacks and is currently planning to build 10100 kW systems with different fuels. Sulzer-Hexis is another active and leading company in the development of platetype SOFCs. The company successfully tested its 1 kW stack in 1999. In addition to these companies, many other companies worldwide have been formed to develop the position of SOFCs [14]. Many applications such as transportation, portable applications, and stationary applications (large and small) can be imagined for SOFCs, considering their advantages. However, now more than SOFCs for medium and large power plants and combined cycles that are static applications are used. Due to its high power density and lack of harmful effects on the environment, SOFC is the best type of battery for use in cars. Westinghouse built the tubular SOFC in the late 1950s. Currently, this type of battery with a power of 25.4 and 100 kW is in operation. The flat type of SOFC is in the early stages of development. In the future, for the optimal use of SOFCs, more work needs to be done in the following areas: 1. What materials should the fuel cell be made of to be more resistant to thermal stresses? Also, the cell mass becomes more straightforward and more compact to reduce manufacturing costs. 2. To optimize the cell’s performance, in the long run, it is necessary to conduct primary research on the choice of materials, the penetration of the internal materials of the cell components, and their ability to be synthesized. 3. The thickness of the cell should be less, and its other dimensions should be more extensive. 4. Carbon dioxide separation and technology related to SOFC recycling should be examined [20].

The lower cost and higher power density of SOFCs have led to using this fuel cell type to simulate and design the combined cycle process, as well as to increase the fuel cell’s efficiency and reduce the operating temperature of the fuel cell. In the following, the thermodynamic performance of SOFCs is investigated.

6.1.1.1.4 Voltage-current graph (i-v curve) The flow diagram can summarize the performance of a fuel cell in terms of voltage. This diagram shows the output voltage of the cell in terms of the output current of the cell. For comparison between large-area fuel cells and small-area fuel cells, the resulting current is divided by the cell area, which indicates the current density

Technical and economic prospects of fuel cells combination with polygeneration systems?

205

(A/cm2). For this reason, the desired graph is drawn in terms of current density. For each fuel cell, the open-circuit voltage (OCV) or its thermodynamic voltage can be calculated. Nevertheless, this voltage is only relevant to the ideal condition, and this is the case if, in actual operating conditions, the output voltage is less than the theoretical value. This discrepancy is due to failures in operating conditions. The fuel cell, like other systems, has a series of leaks that vary depending on the conditions; also, according to this diagram, the higher the output current from the actual fuel cell, the lower the output voltage. The fuel cell’s output power is also obtained by multiplying the current at the output voltage by Eq. (6.7). P5i3V

(6.7)

The power density graph representing the power density changes according to the current density changes is also extracted from the graph (i-v). The current density graph is obtained by multiplying the graph’s voltage (i-v) by the corresponding current density. As in Fig. 6.8, as the current density increases, the power density also increases until it reaches a maximum value and decreases at higher current densities due to a sharp drop in mass transfer inside the cell. It happens. In the following, this issue will be examined. In the actual case, the fuel cell is trying to operate at or below this point. At subpoint current densities, the maximum voltage efficiency improves, but the power density decreases. At current densities above this point, both the power density and the voltage decrease [21]. As mentioned, the output voltage of the actual fuel cell is lower than the voltage obtained from the ideal thermodynamic conditions, which is due to the following three drops:

Fuel cell voltage (V)

1.2

Power density curve

0.7

1.0

0.6

0.8

0.5 0.4

0.6

0.3

i–V curve

0.4

0.2 0.2 0

0.1

Fuel cell power density (W/cm2)

1. Activation losses of oxidation and reduction half-reactions: At the cathode electrode, a half-reduction reaction occurs, and at the anode electrode, a half-oxidation reaction

0 0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Current density (A/cm2) Figure 6.8 Diagram of current density and power density in terms of output voltage [22].

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Hybrid Poly-generation Energy Systems

occurs. Each reaction, depending on the type of reactants and products it has, will require initial activation energy to initiate the reaction. Therefore at the anode and cathode electrodes, part of the energy is spent on activation. 2. Ohmic losses due to ionic and electron conductivity: To perform a continuous reaction, electrons travel from the anode to the cathode via an external circuit. The electrolyte also transports ions. According to Ohm’s law, the fuel cell components resist ions and electrons’ path; there is a voltage drop in the path of ions and electrons, which increases with increasing current. 3. Concentration Losses: As mentioned, for the reaction to take place, the fuel must pass through a porous medium to reach the catalyst surface, and then the reaction takes place. Passing through this porous medium reduces the fuel concentration at the catalyst surface relative to the fluid bulk. Now suppose that the current density increases, which increases the intensity of the reaction performed on the catalyst surface. The reactants need to approach the catalyst surface more rapidly. However, due to the layers’ porosity, the transfer of reactants to the catalyst surface is limited, which increases the voltage drop. The high current density exacerbates this decrease.

Therefore by calculating the ideal thermodynamic voltage and then subtracting all the drops from it, the actual cell voltage is obtained according to Eq. (6.8). V 5 Ethermo 2 ηact 2 ηohmic 2 ηconc

(6.8)

Fig. 6.9 also shows Eq. (6.8) in the image view. As shown in this figure, the activation drops with the current density changes as an exponential function. The

Ohmic loss

Cell voltage (V)

Cell voltage (V)

Cell voltage (V)

Activation loss

Reversible voltage

Current density (A/cm2)

Current density (A/cm2)

Net fuel cell performance

Cell voltage (V)

Cell voltage (V)

Concentration loss

Current density (A/cm2)

Current Density (A/cm2)

Current Density (A/cm2)

Figure 6.9 Calculation of the actual voltage by reducing the drop from the ideal thermodynamic voltage [22].

Technical and economic prospects of fuel cells combination with polygeneration systems?

207

decrease due to ionic and electron conduction also changes linearly, and the decrease due to mass transfer also changes exponentially.

6.1.1.1.5 Thermodynamics of fuel cells Gibbs free energy from Eqs. (6.9) and (6.10) is used to obtain the maximum amount of energy obtained from the fuel cell. Reversible processes, steady flow, and work have done assumed to be electrical energy. G 5 U 2 TS 1 PV

(6.9)

dG 5 dU 2 TdS 2 SdT 1 VdP 1 PdV

(6.10)

Given that the process is reversible, relations of (6.11) and (6.12) are established. dQ 5 TdS

(6.11)

dU 5 dQ 2 dW.dU 5 TdS 2 ðPdV 1 dWelec Þ

(6.12)

According to the presented relations, Eq. (6.13) is obtained. dG 5 2 SdT 1 VdP 2 dWelec

(6.13)

Since the fuel cell’s temperature and pressure do not change much, it can be concluded that the maximum energy available for conversion to electrical energy is equal to the Gibbs free energy shown in Eq. (6.14). dG 5 2 dWelec .ΔG 5 2 Welec

(6.14)

The electrical energy obtained is equal to the current multiplied product by the fuel cell’s ideal voltage. Given that electrons are flowing in an external circuit, the relationship between electrical energy and the number of electrons is (6.15). Welec 5 E 3 ne 3 F

(6.15)

where ne is the number of electrons participating in the redox reactions. F is the Faraday constant that is equal and is calculated from Eq. (6.16). F 5 NA 3 ðeÞ 5 6:022 3 1023

electrons C C 3 1:68 3 10219  96400 mol electrons mol (6.16)

According to the obtained results, the equation is (6.17). E52

ΔG ne 3 F

(6.17)

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Hybrid Poly-generation Energy Systems

Therefore the cell voltage can be calculated according to the Gibbs free energy obtained from the electrochemical reaction. Consider, for example, the electrochemical reaction performed on a SOFC under standard conditions in which relations (6.18) to (6.21) are valid. H2 1 1=2O2 ! H2 O ΔS298 5 2 0:04435

ΔH298 5 2 243 Kj=mol

Kj 3K mol

ΔG 5 ΔH  TΔS

ΔG298 5 2 243 2 ð298 3 ð2 :04435ÞÞ 5 2 229:78 Kj=mol E 5 2

2 229:78 3 103 5 1:19v 2 3 96400

(6.18) (6.19) (6.20) (6.21)

The enthalpy and entropy relations concerning temperature and pressure must be calculated according to Eq. (6.22) to obtain the voltage in conditions other than standard conditions. Assuming that gas is ideal, there are also Eqs. (6.23)(6.25) [2]. ΔGðT; PÞ 5 ΔH ðT; PÞ 2 TΔSðT; PÞ

(6.22)

ΔGðT; PÞ 5 ΔH ðT Þ 2 TΔSðT; PÞ

(6.23)

ΔS 5 R

ðT

CP ig dT P 2 ln P0 T0 R T

ΔH 5 ΔH0 1 R

(6.24)

ðT

CP ig dT T0 R

(6.25)

According to the above relations, the entropy and enthalpy values can be calculated in actual cell conditions. Then the cell voltage can be obtained by calculating the Gibbs free energy. For example, suppose a SOFC operates at a temperature of 1000 C and a pressure of 1 atmosphere. The calculated values will be as follows. ΔH 5 2 294

kJ mole

ΔS 5 2 0:058

kJ mole:K

ΔG 5 2 249 2 ð1273 3 0:058Þ 5 2 175 E5

2 175 3 1000 5 0:91V 2 3 96400

kJ mole

Technical and economic prospects of fuel cells combination with polygeneration systems?

209

The issue of chemical potential must be addressed to study voltage changes in terms of concentration changes. Here it is enough to write the general result of Eq. (6.26) (refer to the reference [21] for information on proving the equation). 

E5E 2

RT Laproducts vi ln nF areactants vi

(6.26)

ν i is the stoichiometric coefficient of the reaction, and α is the coefficient of activity, which, assuming the ideal gas state, is equal to the ratio of system pressure to standard pressure. Consider, for example, the electrochemical reaction in a SOFC, which is Eq. (6.27), and the cell voltage is derived from Eq. (6.28). H2 1 1=2 O2 ! H2 O 

E5E 2

(6.27)

RT PH2 O RT xH 2 O RT  ln ln lnðPÞ 5E 2 1 0:5 0:5 nF PO2 3 PH2 nF xO2 3 xH2 2nF

(6.28)

Finally, considering all the conditions, Eqs. (6.29) and (6.30) are obtained to calculate the ideal thermodynamic voltage. Values A, B, C, and D are also available in J. M. Smith’s book. Eq. (6.31) is also written after extracting the constants.      ΔS , ΔCP . s T T T 3 T0 3 ðT 2 T0 Þ 2 ln E 5 1:18 2 2 T0 T0 T0 nF nF 2

RT xH 2 O RT ln lnðPÞ 1 nF xO2 0:5 3 xH2 2nF 



(6.29)

     D τ 11 τ 21 T 3 τ5 , ΔCP . s 5 R 3 A 1 BT0 1 CT0 1 2 2 3 2 lnðτ Þ T0 τ T0 

2

(6.30) E 5 1:18 2 0:06855 3 ðτ 2 1Þ 2 0:0165 3 ðτlnðτ Þ 2 τ Þ 2

RT xH2 O RT ln lnðPÞ 1 nF xO2 0:5  xH2 2nF

(6.31)

6.1.1.1.6 Ideal and real fuel cell efficiency As shown in the above equation, the maximum amount of energy available to do the work is equal to Gibbs free energy. The total energy produced is equal to the heat of the electrochemical reaction. Therefore the ideal fuel cell efficiency is calculated from Eq. (6.32). εthermo;fc 5

ΔG ΔH 2 TΔS TΔS 5 512 ΔH ΔH ΔH

(6.32)

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Hybrid Poly-generation Energy Systems

The actual efficiency of the fuel cell is less than the ideal thermodynamic efficiency, which is due to two main reasons: 1. Voltage drop: There are multiple drops inside the fuel cell; the actual voltage is less than the ideal thermodynamic voltage. Thus the voltage efficiency is equal to the actual cell voltage ratio to the ideal thermodynamic voltage, shown in ([23]). εvoltage 5

V E

(6.33)

2. Fuel consumption drop: Due to the non-participation of all fuels in the electrochemical reaction, which may have participated in adverse reactions or left the cell without reacting. Eq. (6.34) indicates fuel efficiency.

εfuel 5

j=nF nfuel

(6.34)

The overall efficiency of the cell is also obtained from Eq. (6.35) [21]: εreal 5 εthermo;fc 3 εvoltage 3 εfuel 5

ΔG V j=nF 3 3 ΔH E nfuel

(6.35)

6.1.1.1.7 Voltage loss in the cell Activation losses: Reduction and oxidation half-reactions performed at the cathode and anode require activation energy to initiate the reaction. Therefore the total voltage drop includes the voltage drop across the anode and cathode, denoted by ηact. This voltage drop is shown as the BalterVolmer equation in relation to (6.36) [21].      ne F ne F ηact 2 exp 2 ð1 2 αÞ ηact i 5 i0 exp α RT RT

(6.36)

α: The transfer coefficient for most electrochemical reactions is between 0.20.5 and i0: The current density is for the OCV state, where the reciprocating reactions at each electrode are in equilibrium. The relationship obtained depends on hydrogen and oxygen uptake and desorption, and which stages determine the velocity. Depending on the circumstances, different relationships have been obtained, discussed in reference [24]. For example, Eqs. (6.37) and (6.38) are presented for the cathode and anode, respectively [25].  i0;c 5 γc

PO2 Pref

 i0;a 5 γa

PH2 Pref

0:25 

  Eact;c exp 2 RT

   PH2 O Eact;a exp 2 Pref RT

(6.37)

(6.38)

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211

Assuming that α is equal to 0.5 for the cathode and anode electrodes, then the BalterVolmer relationship is written as Eq. (6.39).     2RT 21 i 2RT 21 i ηact 5 sin sin 1 (6.39) ne F 2i0;c ne F 2i0;a Of course, the modified BalterVolmer equation is also presented [26]. One of the advantages of SOFCs is the high operating temperature, which requires less activation energy, so it also experiences less voltage drop.

Ohmic losses Because the fuel cell components are resistant to the load movement, some of the load path’s cell voltage is reduced. According to the law, the most important relationship is Eq. (6.40) as follows: R5

L σ3A

i 5 j=A

V 5i

L σ

V 5j3R

(6.40)

This voltage represents the voltage that must be used to move the load. As mentioned, there are two types of conduction inside the fuel cell: ionic conduction inside the electrolyte and electron conduction at the electrodes and external circuit. The result is an ohmic drop from Eq. (6.41). ηOhmic 5 j 3 ROhmic 5 j 3 ðRelec 3 Rionic Þ

(6.41)

Ionic conductivity is typically much lower than electron conduction, so the significant share of voltage drop is ionic conductivity. Therefore care must be taken in choosing the type of fuel cell electrolyte and its thickness. For SOFCs, the appropriate electrolyte is (ZrO2)0.92(Y2O3)0.08, which has the highest ionic conductivity. Various relationships have been proposed to obtain a SOFC’s electrolyte conductivity, each of which is subject to operating conditions. The general relationship presented for the conductivity of SOFC electrolytes is 42 [21]:
 act C 3 ðZ 3 F Þ2 3 D0 3 exp 2 ΔG RT σ5 RT

(6.42)

In Eq. (6.42), σ is the ionic conductivity (1/Ω. cm), ΔGact is the activation Gibbs free energy and is usually between 50 and 120 kj/mol (at temperatures between 800 C and 1000 C, this value is between 80 and 90 kj/mol). C Indicates the number of moles carrying the charge per unit volume (1001000 mol/m3), and Z is the number related to the ion charge (for example, for o22, this number is equal to Z 5 2 2). Eq. (6.43) has been proposed for the YSZ electrolyte [2]. YSZ electrolyte

  3:584 3 107 10300 exp 2 σ5 T T

ðΩ:mÞ21

(6.43)

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Hybrid Poly-generation Energy Systems

Concentration losses Since the fuel cell’s voltage is a function of the concentration of the components participating in the electrochemical reaction, reducing these components’ concentration leads to a decrease in the cell’s voltage. Electrodes have a porous structure that the fuel and air must pass through to take place the electrochemical reaction. Passing through this porous medium leads to a concentration gradient in the electrode structure. The concentration of fuel and air in the fluid mass is greater than the concentration at the catalyst surface, which leads to a decrease in cell performance (Fig. 6.10). Reducing the concentration of reactants also reduces cell performance in two ways [21]: 1. Reduces the ideal thermodynamic voltage (Nernst equation). 2. Since the reactions performed at the anode and cathode electrodes are a function of the reactants’ concentration, a decrease in concentration leads to increased activation losses.

The drop due to mass transfer is equal to the sum of each of the above drops. After calculating the effect of each of the above, the overall result for calculating the voltage drop due to concentration is ηconc 5 C ln

iL iL 2 1

Figure 6.10 Schematic of mass transfer in a fuel cell electrode [21,27].

(6.44)

Technical and economic prospects of fuel cells combination with polygeneration systems?

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iL Indicates the current density when the reactant concentration at the catalyst surface is assumed to be zero. Therefore the actual SOFC voltage is calculated from Eq. (6.45). V 5 1:18 2 0:06855 3 ðτ 2 1Þ 2 0:0165 3 ðτlnðτÞ 2 τÞ 2

RT xH2 O ln 0:5 nF xO2  xH2

     RT 2RT 21 i 2RT 21 i lnðPÞ 2 sin sin 1 1 2nF ne F 2i0;c ne F 2i0;a 2 j 3 ðRelec 3 Rionic Þ 2 ηconc

(6.45)

In the continuation of this chapter, the principles of electrolysis and its types will be examined. The solid oxide electrolyzer cell (SOEC) is also examined in general, and at the end, the combined cycle for the simultaneous production of electricity and hydrogen is examined.

6.2

Electrolyze

Water electrolysis is a process that was discovered before the discovery of fuel cells. However, its use was abandoned due to the emergence of fossil fuels (such as oil). The electrolysis process has recently received much attention again, as fuel prices and environmental problems have raised concerns. Nevertheless, this process still accounts for a minimal share of hydrogen production worldwide (about 4%). Compared to other available technologies, this method’s advantage is the production of very high-purity hydrogen ( . 99.9%). This process is very convenient on a small scale and will be much more efficient if the required electricity is supplied from nonrenewable sources. Although hydrogen has advantages such as availability, flexibility, and high purity, the use of hydrogen produced by water electrolysis requires improved energy efficiency, safety, durability, portability, and reduced installation and operating costs. These specifications have created many opportunities for the research and development of water electrolysis technology.

6.2.1 History of water electrolysis About two hundred years have passed since discovering the phenomenon of electrolytic decomposition of water into hydrogen and oxygen to modern electrolyzers’ development. After discovering electricity in 1798, J.R. Deiman and A.P. Van Troostwijk used an electrostatic generator with two gold systems embedded in a tube full of water to discharge electricity, which led to the production of gases. Alessandro Volta invented the first battery in 1800. William Nicholson and Anthony Corlisle later used the cell to electrolyze water electrolytes. The gases

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produced by the electrolysis of water from their experiments were later known as hydrogen and oxygen. With the development of electrochemistry, a proportional relationship between the consumption of electrical energy and the number of gases produced was published, and the law of electrolysis was called Faraday. Finally, the concept of water electrolysis was scientifically defined. With the invention of the Gramme machine in 1769 by Zenobe Gramme, water electrolysis became an economical method for producing hydrogen. Another technique for the industrial synthesis of hydrogen and oxygen from water electrolysis was developed in 1888 by Dmitry Lachinov. In 1902, more than 400 industrial water electrolyzers entered the operational phase. The period from 1920 to 1970 was the golden age of flourishing and the development of water electrolysis technology. In 1939, the first significant water electrolysis plant with a capacity (10,000 Nm3H2/h) entered the operational phase, and in 1948, the first industrial pressure electrolyzer was built by Zanski/Lonza. Later, following the industrial development of electrolyzers, the cell components changed. In 1966, the first solid polymer electrolyte system was developed by General Electric. The first membranes to enter the operational phase were made of asbestos. Due to the asbestos’s resistance to corrosion, the membrane was gradually replaced with other materials. From 1970 onwards, polymers based on Perfluoro Sulfonic acid or Poly-tetra fluoro ethylene were used as the gas separator. The cell configuration was also rebuilt several times over time. In 1972, the first solid-state electrolysis unit was developed. The first advances in alkaline systems began in 1978. The material of choice for the electrode must have high corrosion resistance. Conductivity, high catalytic effect, and low cost are also very important. Stainless steel was a cheap material with low over-potential, but it did not last in an alkaline environment. Noble metals were costly as electrode materials. Nickel was then identified as the electroactive (active electrode) material of the cathode with high corrosion resistance in an alkaline solution and quickly became popular during water electrolyzers’ development. Subsequently, nickel-based alloys were the subject of extensive research. In the first half of the twentieth century, the demand for hydrogen production increased. Due to the discovery of oil and the widespread use of hydrocarbons in industry and the achievement of the importance that large-scale hydrogen production through coal gasification and natural gas reforming is less costly than water electrolysis, the economic benefits of water electrolysis gradually diminished, and all research and progress in this area was stopped. In the 1970s, due to the oil crisis that arose worldwide, many researchers again considered water electrolysis. The new ideology of the hydrogen economy saw it as the carrier of future energy and the key to solving the energy supply problem. Therefore improving the efficiency of water electrolysis became an important goal, and incredible progress was made. In 1966, General Electric was the first to use Nafion membranes to power energy space projects. The proton exchange membrane discovery led to electrolyzers’ development using this membrane, called solid polymer electrolyzers (SPE), whose operating principles are essentially the opposite of a proton exchange membrane fuel cell. Extensive studies have also been performed to reduce membrane

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costs. In the early 1970s, small-scale proton exchange membrane electrolyzers were used for military and space applications. Efforts are currently being made to integrate electrolyzer technologies with different energy sources. In this case, in the next chapter, some examples of research have been mentioned.

6.2.2 Principles of electrolysis Electrolysis is an electrochemical process that uses electrical energy as a driving force for chemical reactions. Fig. 6.11 shows the most straightforward water electrolysis unit, which consists of an anode and a cathode connected through an external source of electricity and immersed in a conductive electrolyte solution. As mentioned earlier, water decomposes into hydrogen and oxygen in a nonspontaneous reaction by passing an electric current through it in the presence of a suitable substance called the electrolyte. The electric current creates ions with a positive charge H1, and these ions migrate to the cathode with a negative charge. At the cathode, ions are reduced to form hydrogen atoms. The formed atoms then combine to form gaseous hydrogen molecules. Oxygen is also formed in the other electrode. As shown in Reaction (6.46), the stoichiometry of the total water decomposition reaction consists of two volumes of hydrogen and one volume of oxygen. 2H2 O ! 2H2 1 O2

Figure 6.11 Water electrolysis unit [28].

(6.46)

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Hybrid Poly-generation Energy Systems

The most crucial part of building an electrolyzer unit is using suitable electrodes to prevent unwanted reactions that cause impurities in the hydrogen gas. Another vital component of an electrolyzer unit is the selection of the separation membrane, which allows the passage of ions or electrons but prevents the passage of oxygen or hydrogen atoms. These membranes are used to keep gases separate to prevent the gas mixture explosion (H2-O2). The following is the theory of water electrolysis.

6.2.3 Water electrolysis theory Water electrolysis is a known principle for the production of hydrogen gas and oxygen. Fig. 6.12 shows a schematic of an electrochemical cell [29]. An electrolyzer unit’s core is its electrochemical cell, filled with pure water, and has two electrodes connected to an external power supply. At a specific voltage between the electrodes, called the critical cell, hydrogen gas is produced on the negative electrode side, and oxygen gas is produced on the positive electrode side. The amount of gas produced per time unit depends directly on the current flowing through the electrochemical cell. A certain percentage of ionic species in water is represented by the equilibrium Eq. (6.47). H2 OðaqÞ ! H1 ðaqÞ 1 OH2 ðaqÞ

(6.47)

Hydrogen and oxygen gases can be produced at noble metal electrodes by electrolysis of water in Eqs. (6.48) and (6.49). Anodeðpositive electrodeÞ: 4OH2 ! 2H2 O 1 O2 1 4e2

(6.48)

Cathodeðnegative electrodeÞ: 2H1 1 2e2 ! H2

(6.49)

In the case of acidic water, the reaction that takes place on the electrodes’ surface is slightly different. Water electrolysis has no side effects, so the net balance is in Eq. (6.50). 2H2 O 1 4e2 ! 2H2 1 O2

Figure 6.12 Schematic of an electrochemical cell [29,30].

(6.50)

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The minimum voltage required to start the electrolysis under standard conditions (constant temperature and pressure) is given in Eq. (6.51). 



Ecell 5 ΔG =nF

(6.51)

where ΔG is the Gibbs free energy changes that are transmitted under standard conditions, and n is the number of electrons. In the case of a closed electrochemical cell, the condition slowly deviates from the standard condition. In the open cell, the pressure and temperature are constant, and in the closed electrochemical cell, the temperature and volume are constant. Because the volume change is less than the pressure change. So instead of ΔG , free energy Helmholtz ΔA is used. The voltage required to overcome the Helmholtz energy is given in Eqs. (6.52) and (6.53). 



Ecell 5 ΔA =nF 

(6.52)



ΔA 5 ΔH 2 RTΔn 2 TΔS



(6.53)

The following values are given for water electrolysis: 

ΔH 5 285:8kJ=mole Δn 5 1:5 

ΔS ðO2 Þ 5 130:6kJ=mole:K 

ΔS ðH2 Þ 5 205:1kJ=mole:K 

ΔS ðH2 OÞðLÞ 5 70kJ=mole:K as a result: 1  ΔStotal 5 130:6 1 205:1 2 70 5 163:14kJ=mole:K 2 

ΔA 5



ΔG 5 1:23V nF

Therefore the minimum cell voltage is calculated with Eq. (6.54). The standard Gibbs free energy is obtained from Eq. (6.55), and the minimum cell voltage in open circuit mode and standard conditions of 1 bar and 25 C is obtained from Eq. (6.56). 



Ecell 5 1:21V 5 ΔA =nF

(6.54)

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Hybrid Poly-generation Energy Systems







ΔG 5 ΔH 2 TΔS 5 237:2kJ=mole 



Ecell 5 1:23V 5 ΔG =nF

(6.55) (6.56)

For the reaction to begin, it is necessary to overcome the energy barrier, the activation energy (Eact). The number of molecules that can overcome this energy is a reaction rate controlling factor, r, obtained using the static relationship with the exponential MaxwellBoltzmann behavior, presented in Eq. (6.57). Thus the activation energy determines the reaction rate. rBr exp

  Eact RT

(6.57)

The maximum efficiency for a closed electrochemical cell and the electrochemical cell’s actual efficiency is obtained from Eqs. (6.58) and (6.59), respectively. εmax 5

ΔH 2 ΔH 5 ΔA nFEcell

(6.58)

εreal 5

2 ΔH nEelec

(6.59)

ΔEelec relation is also obtained using I in the form of relation 6.60 ΔEelec 5 ΔA 1 IR 1

X

η

(6.60)

In relation (6.59), R is the total ohmic series resistance in the cell, including the circuit’s external resistance, electrolyte, electrodes, and membrane materials. Also, Ση is the sum of the total polarizations (including the two electrodes’ activation polarization and the concentration polarization). Equilibrium energy during water electrolysis is shown in Fig. 6.13. The additional activation potential increases with increasing current density and can be reduced using electrodes with catalytic action such as Pt. In water electrolysis under ideal reversible conditions, the maximum theoretical efficiency concerning the electrical energy source will be εmax 5 120%. So heat flows from the inside into the cell. When the denominator’s value in Eq. (6.58) becomes 1.48 nF (polarization 0.25 V), the electrochemical cell operates at 100% efficiency. Under these conditions (Ση 5 0, ΔS 5 0, therefore ΔG 5 ΔH) the cell does not exchange heat (or cold), and the value Etn 5 ΔH nF 5 1.48 is shown as thermo-neutral. The electrochemical cell generates heat at potentials higher than 1.48 volts. At lower potentials, it absorbs this amount of heat, and at a potential of 1.48 volts, the cell temperature is kept constant. In practice, the IR drop can be 0.25 volts. Polarization ri must also be kept low to maximize efficiency and minimize heat generation. In other words,

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6K ∆Eelec- IR

Eoct

∆A

Figure 6.13 Energies involved in the reaction [29,30].

the lower the polarization, the calmer the reaction. One of the best ways to increase the current without increasing the polarization is to increase the contact surface between the electrodes and the electrolyte [30]. In the next section, the types of electrolyzers and SOECs are described in detail.

6.2.4 Types of electrolyzers Electrolyzers are divided into two categories based on the operating temperature and the nature of the electrolyte. The hydrogen separator membrane options are listed in Table 6.3. Here only the main processes will be examined: 1. Low-temperature electrolysis cell a. Alkaline Electrolyzer Cell (AEC): Its electrolyte is a hydroxide salt (mainly potassium hydroxide) with an operating temperature of about 100 C. b. Proton Exchange Membrane Electrolyzer Cell (PEMEC): Its electrolyte is a proton exchange membrane (mainly Nafion) with an operating temperature range of 100 C. 2. High-temperature electrolysis cell: SOEC: Its electrolyte is a solid oxide with an operating temperature of about 800 C.

In the following, each of the mentioned types of electrolyzers is examined. Since the SOEC is one of the main components of the hybrid system, it will be examined in detail.

6.2.4.1 Alkaline electrolyzer cell Alkaline electrolyzers have been used in industrial applications since the 1920s [2]. Today, this technology is the most mature electrolyzer available. In fact, the oldest and only commercially available technology is alkaline electrolyte-based water electrolysis. The alkaline electrolyzer cell generally consists of two electrodes, a microporous separator and an alkaline aqueous phase electrolyte with approximately 30 wt.% of potassium hydroxide or sodium hydroxide. In other words, the system of this electrolyzer is based on the alkaline fuel cell. The use of potassium hydroxide is preferable to sodium hydroxide due to its better conductivity. Nickel is also a good choice for electrodes due to its low price, good performance, and availability. Nickel with a catalytic coating in the cathode electrode, such as platinum, is usually used in the cathode electrode. In the anode,

Table 6.3 Membrane options for hydrogen separation [31]. Membrane type

Dense polymer

Microporous ceramic

Dense metal

Porous carbon

Dense ceramics

Temperature range (K) H2 selectivity Hydrogen flux at 1 bar (mol per square meter seconds 1023) Stability issues

373 .

873473

873575

1173773

1173873

Low Low

5139 60300

1000 , 60300

420 10200

1000 , 680

Swelling compaction SOxHCl Polymers

Stable in water

Phase transition

Brittle

Stable is carbon dioxide

Silica, alumina, zirconia, titania, zeolites On a laboratory scale— requires proof of durability and stability

H2S HCl CO Palladium or its alloys

Strong adsorbing vapors Carbon

H2S Proton-conducting ceramics

Pilot-scale testing requires proof of long-term stability and practical applications

Air-scale test for hydrogen penetration requires proof of hydrogen operational recovery

Laboratory scale, need to increase hydrogen flux by combination with hydrogen permeable metals

Poisoning issues Material

Progress stage

Commercially available

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nickel or copper with a coating of metal oxides such as manganese, tungsten, or ruthenium are used [31]. Since the electrolyte of this electrolyzer is liquid, an aperture is needed to separate the gases produced to prevent them from mixing. The two chambers of the electrolyzer are also separated by an aperture (such as NiO). The liquid electrolyte in this electrolyzer does not react but must be emptied and refilled each time due to system losses. In an alkaline electrolyzer, water enters the cathode and decomposes into hydrogen and hydroxide ions, according to Eq. (6.61). OH2 migrates from electrolyte to anode, and oxygen is formed during the anode reaction of (6.62). Cathode: 2H2 O 1 2e2 ! H2 1 2OH2

(6.61)

1 Anode: 2OH2 ! O2 1 H2 O 1 2e2 2

(6.62)

H2 O ! H2 1

1 O2 2

ΔH 5 2 288kJ=mole

The typical current density in this 100300 mA/cm2 electrolyzer depends on the Lower Heating Value (LHV) value of the efficiency in the range of 50%60% [32]. The alkaline electrolyzer is suitable for refueling applications and has an operating pressure of 25 bar [33]. Although this technology has reached the commercial stage, there are drawbacks to the vector, such as the lower purity of the gas than other technologies and the relatively limited ability to respond to input power fluctuations. This technology is also the most energy-dependent. Besides, carbon dioxide can be absorbed into the membrane, and carbonation occurs, limiting the electrolysis function. These limitations allow for the research and development of this technology by simply improving its performance. Alkaline electrolyzers usually include the main components in Fig. 6.14. In [34], a proper study has been done on this electrolyzer.

O2

KOH Electricity Transformer/ rectifier

Water

Deionizer/ reverse osmosis

O2/KOH Gas separator

Electrolytic cell block

KOH

H2/KOH Gas separator

Figure 6.14 Alkali electrolysis process diagram [33].

Deoxidizer

Dryer

H2

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Hybrid Poly-generation Energy Systems

6.2.4.2 Proton exchange membrane electrolyzer cell Proton-exchange membrane electrolyzers have emerged following recent advances in PEMFC technology. This technology is also known as SPE and operates based on a fuel cell inverted proton exchange membrane. This electrolyzer does not require any liquid electrolytes, which dramatically simplifies its design. Its electrolyte is a poppy-like polymer membrane that separates the electrodes and acts as a gas separator [32]. The use of Nafion is a great advantage for this electrolyzer because it makes the system more cohesive and reduces corrosion problems. However, the opium membrane is expensive and requires noble metal electrocatalysts (e.g., Platinum). Pt Black, Iridium, Ruthenium, and Rhodium are also used for electrode catalysts [32,35]. A good study on this technology is given in the reference [36]. For any electrolyzer, the membrane must be mechanically strong to withstand the operating pressure of the system. It must also be chemically stable due to the reduction and oxidation of chemicals (hydrogen and oxygen) to prevent gases combustion. The target membrane must be a suitable proton conductor and a weak electron conductor. Besides, it must withstand operating temperatures (up to 100 C120 C) and operating pressures (up to 30 bar or more). As mentioned, the most widely used membrane in this electrolyzer is the Nafion (as used in PEMFC) [3]. One of the most critical parameters in the membrane is to achieve the best hydration performance. If too little hydration occurs, the membrane dries out, and the migration of protons is reduced. As a result, the membrane is severely damaged. However, if the hydration is too high, the membrane becomes too wet, the electrodes drowned, and the chemical reactions are quickly blocked [6]. The water electrolysis principles based on proton exchange membrane electrolyzer are stated in Eqs. (6.63) and (6.64). In this electrolyzer, water enters the anode and is converted to protons and oxygen according to Eq. (6.63). Protons penetrate through the membrane to the cathode, where hydrogen is formed. Oxygen gas does not react with water and remains, and the general reaction is the same as that which occurs in alkaline electrolysis. A schematic of a single proton and alkaline exchange membrane electrolyzer cell is shown in Fig. 6.15. In this water electrolysis method, depending on the required purity, a dryer is used to remove the remaining water after the gas/liquid separation unit. Cathode: 2H1 1 2e2 ! H2

(6.63)

1 Anode: H2 O ! O2 1 2H1 1 2e2 2

(6.64)

1 H2 O ! O2 1 H2 2

ΔH 5 2 288kJ=mole

These electrolyzers have been commercially available for several years but are not suitable for energy storage due to their inability to generate hydrogen on a large

Technical and economic prospects of fuel cells combination with polygeneration systems?

Alkaline electrolysis 40 - 90 °C Cathode -

PEM electrolysis 20 - 100 °C

+ Anode

OH-

H2

223

1

/2

O2

Cathode -

H2

+ Anode

H+

1

/2O2

H2O

H 2O Anode Ni/CoFe

Cathode Ni/C Diaphragm

2OH- o1/2 O2 + H2O + 2e- Anode 2H2O + 2e- oH2 + 2OH- Cathode Total reaction H2O oH2 + 1/2O2

Anode Ir

Cathode Pt Membrane

Anode H2O o2H+ + 1/2 O2 + 2e2H + 2e- oH2 Cathode Totalreaction H2O oH2 +1/2 O2 +

Figure 6.15 Schematic of how proton and alkaline exchange membrane electrolyzer operation [36].

scale. Since this electrolyzer can be designed with operating pressures several hundred times, it is suitable for refueling and portable applications. PEM electrolyzers have low ionic resistance, so currents higher than 21,600 mA/cm2 can be achieved, and efficiencies of about 55%70% can be produced [37,38]. This technology operates at an operating temperature similar to an alkaline electrolyzer or higher and is more efficient. This technology’s advantages include a higher turn-down ratio, higher safety due to lack of liquid electrolyte, more coherent design for higher densities, higher operating pressure, faster response to input electrical fluctuations, and higher purity hydrogen production. Relatively high cost, low capacity, low efficiency, short life, uncertainty, and inability to be used on a large industrial scale are some of the disadvantages of this technology that are the basis for future research on this electrolyzer.

6.2.4.3 SOEC Solid oxide water electrolysis cells have great potential for economic production and hydrogen production. This technology, also called high-temperature electrolysis, was developed to develop a high-temperature SOFC. The electrical energy required to split water at 1000 C is less than the energy required at 100 C. This means that hightemperature electrolyzers operate dramatically, with a higher total process interval than conventional low-temperature electrolyzers. High temperatures reduce cathode and anode losses, which wastes power in electrolysis, thereby increasing electrolysis efficiency. For example, increasing the temperature from 375 K to 1050 K reduces the required thermal energy composition by about 35% [23].

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Hybrid Poly-generation Energy Systems

A solid oxide electrolysis cell is a SOFC that works in the opposite direction and is shown in Fig. 6.16. Because SOFCs typically operate at 700 C1000 C, the electrode reactions are more reversible, and the fuel cell reactions are easier to reverse. These systems replace some of the electrical energy needed to split down water with heat [39]. Solid oxide electrolyzers are the least commercially advanced. The SOEC system has not entered the commercial phase and has not been tested for comparison with other electrolyzers. Table 6.4 provides a comparison of water electrolysis methods. Because they operate at much higher temperatures than other water electrolysis technologies, they consume much less electricity. However, high operating temperatures require unique materials that can withstand process conditions. The components of this electrolyzer will be described in this section.

electricity H2O

Steam Electrolysis

H2O Electrolysis

H2, H2O

FT Reactor/ FT Liquid Shift Reactor

CO2

electricity H2O

Coelectrolysis

Syngas H2O/CO2 Coelectrolysis with RSR

CO2

FT Reactor

FT Liquid

Figure 6.16 Comparison of steam electrolysis and simultaneous high temperature [31]. Table 6.4 Comparison of types of electrolyzers [2,40]. Type of electrolyzer

Reactants

Ion carrier

Electrolyte

Electrodes

Operating temperature ( C)

Alkaline electrolyzer

Water

OH 2

Nickel (Nibased)

50100

Proton exchange membrane electrolyzer

Water

H1

Sodium or potassium hydroxide Polymer (Nafion)

20100

Solid oxide electrolyzer

Water and carbon dioxide

O22

Graphite with Platinum, Platinum/ Iridium, Polymer Nickel, Ceramics, Ni-cermet

ceramic

6001000

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In addition to providing the highest Faradic efficiency, this technology also allows the direct electrolysis of carbon dioxide. Also, this technology can be used for the simultaneous electrolysis of water and carbon dioxide, as shown in Fig. 6.16. The product of this simultaneous electrolysis is synthesis gas [41,42]. In this way, solid oxide electrolyzers can store/carry chemical energy while converting renewable energy or excess energy from power plants into hydrogen or synthesized gas. On the contrary, it shows a SOFC and a cross-sectional view. First, the water vapor’s temperature entering the solid oxide electrolyzer increases to 800 C1000 C. Then, as shown in Fig. 6.17, water vapor enters the porous cathode. By applying an electric current, water molecules penetrate the reaction sites and at the junction of the cathode and the electrolyte, are converted to hydrogen gas in oxygen ions. The hydrogen produced penetrates the cathode surface and collects (Fig. 6.18).

e-

CATHODE H2O+2e-=>H2+O2-

POWER

ELECTROLYTE

H2O

e-

ANODE O2-=>1/2 O2 + 2e-

H2

H2

O2-

ELECTROLYTE

O2

O2

e-

ANODE H2+O2-=> H2O+2e-

O2-

CATHODE 1/2 O2 + 2e- => O2-

LOAD POWER

e-

Figure 6.17 The basic basis of the work of a solid oxide electrolysis cell and a solid oxide fuel cell [43].

Figure 6.18 Cross-sectional view of a solid oxide electrolyzer cell [31].

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Hybrid Poly-generation Energy Systems

Oxygen ions migrate from the dense electrolyte to the anode. At the anode, oxygen ions are oxidized to oxygen gas and penetrate the surface from inside the porous anode. A solid oxide electrolysis cell’s reactions are in Eqs. (6.65) and (6.66). Cathode: H2 OðgÞ 1 2e2 ! H2 ðgÞ 1 O22

(6.65)

1 Anode: O22 ! O2 ðgÞ 1 2e2 2

(6.66)

1 H 2 O ð gÞ ! O 2 ð gÞ 1 H 2 ð gÞ 2 The total energy used in a SOEC (ΔH) to produce hydrogen is 67. ΔH 5 ΔG 1 TΔS

(6.67)

In Eq. (6.67), free Gibbs energy, electrical energy, and TΔS thermal energy are required. As can be seen from this relation, an increase in temperature leads to a decrease in the electrical energy required, while the total amount of energy required remains constant. Therefore SOEC is advantageous at high temperatures because of the opportunity to use the industry’s waste heat energy. The equations for the electrolyzer model is already presented in Chapter 4. The main components of SOEC, such as SOFC, include the dense electrolyte ion conductor, anode, and porous cathode. The following points should be considered in the production and use of these components [44]: 1. Electrodes must have good chemical stability in strong oxide and reduction environments and also be electron conductors. 2. Electrodes must have good porosity for two reasons: a. Easily transfer gas between the electrode surface and the joint between the electrode and the electrolyte. b. Provide a three-phase gas-electrode-electrolyte boundary. 3. The coefficient of thermal expansion of the electrodes should be close to the electrolyte. At high temperatures, material failure may occur due to mechanical stresses due to differences in thermal expansion coefficient. 4. To produce hydrogen on an industrial scale, the cells must be joined together by interconnects, which must also have good chemical stability in severe oxidation and reduction environments.

High-temperature electrolysis based on solid oxide electrolyzers does not have the problems of liquid electrolytes and current distribution in the electrolyzer due to the use of solid electrolytes, which are noncorrosive. A suitable electrolyte must be dense and have high ionic conductivity, high chemical stability, and poor electrical conductivity. The presence of electrical conductivity will reduce ionic conductivity. In addition to the above, the electrolyte must be impermeable to gas to prevent any reaction between hydrogen and oxygen. Choosing the suitable material for the electrolyte depends on operating temperature,

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price, and compatibility with other components. Various materials have been proposed for SOEC electrolytes, the most important of which are discussed below.

Stabilized zirconia The most common material used for SOEC electrolytes is YSZ, which has good ionic conductivity and mechanical strength. The melting temperature of ZrO2 is about 2973 K and at 1373 K changes phase from monoclinic to tetragonal structure and in 2673 Kelvin from tetragonal to cubic [45]. This phase change is associated with an increase in volume and can be destructive. The addition of dopants to zirconia also stabilizes the cubic and tetragonal phases and increases ionic conductivity by creating an oxygen void. For this purpose, various oxides such as Y2O3, Yb2O3, Sc2O3, La2O3, and CaO are used [46]. Among the oxides mentioned, Sc2O3-doped zirconia has the highest ionic conductivity due to the proximity of the radius of Sc31 to Zr41. However, due to its high price, this material is not widely used in high-temperature systems [47]. Hence YSZ is the best option. The ionic conductivity of the electrolyte is also affected by the dopant concentration [48]. As shown in Fig. 6.19, ionic conductance increases and decreases again with increasing dopant concentration. For Y2O3, this is 8% of the molar fraction. At low dopant values, oxygen vacancies increase the conductivity, and at high dopant values, these dopant cation vacancies form complex defects and reduce ionic conductivity. Other factors affecting ionic conductivity include temperature [45], grain boundary [49], and electrolyte thickness [50].

Figure 6.19 Dependence of electrolyte ion conductivity on dopant concentration [48].

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Hybrid Poly-generation Energy Systems

Doped LaGaO3 Despite the increase in reaction rate and ionic conductivity at high temperatures, this temperature makes SOEC longevity problems and interconnectors’ choice difficult for the cells. Thus finding a material that can have good ionic conductivity at lower temperatures was investigated, and LaGaO3 was introduced as a suitable option. It has good ionic conductivity in the temperature range of 673K to 1073K. In different experiments, the effects of Sr and Mg dopants on LaGaO3 ionic conductivity were investigated, and the results showed that La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) had better ionic conductivity than YSZ and ScSZ [51]. It has higher ionic conductivity than YSZ at average temperatures as well as at high temperatures. Of course, the electronic conductivity of this material is very low [52]. The main problem in using LSGM is its reactivity with the nickel electrode, which forms Nickalates Lanthanum [53]. Zhang et al. found that LSGM forms LaNiO3 with NiO at 1423 K, which dramatically reduces ionic conductivity [54]. However, this problem can be solved by placing a thin layer of SDC (SamariaDoped Ceria) between the electrolyte and the electrode [55].

Ceria-based oxides (CeO2) Like LSGM, doped ceria has ionic conductivity at medium temperatures (773 K to 1073 K). The ion series conductivity can be increased by introducing 2 and 3 valent cationic dopants [19]. Experiments have shown that Sm31 and Gd41 produce the highest ionic conductivity and are known as SDC and GDC [2,20]. The diagram shown in Fig. 6.3 (which shows the effect of concentration in the pent on ionic conductivity) also applies here. The simultaneous introduction of two elements (such as La, Y) has also been shown to increase ion ceria conductivity [19]. The main problem with using ceria is the partial conversion of Ce41 to Ce31 in the reduction environment. This conversion leads to mechanical failure of the part [25]. The ceria is combined with YSZ and LSEM electrolytes [26] to solve this problem.

Other oxygen conductor ceramics Bismuth is another option for use at medium temperatures that has a higher ionic conductivity than YSZ. However, this material decomposes at low partial pressures of oxygen and has limited application. This problem is also solved by combining bismuth with other electrolytes. For example, the combination Bi2O3-Y2O3-CeO2 has good ionic conductivity [26]. It is necessary to research its interaction with other materials and its long-term stability to use bismuth. From the introduction of the electrolyte in the previous paragraphs, it can be seen that YSZ is the most widely used electrolyte in high-temperature applications. Although ScSZ has higher conductivity than YSZ, it is not economical to use. LSGM is also a better option for working at lower temperatures, which only needs to solve the problem of reacting with nickel. Materials such as SDC and GDC are other options for use at moderate temperatures, for which there is a problem with reducing Ce41 to Ce31, which reduces their properties. So far, the materials used in solid oxide cell electrolyzer electrolytes have been studied. This electrolysis also has two other main components, namely anode, and

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cathode. Here the materials used in the anode valley and then the cathode are examined; Only two groups of materials are used for solid oxide electrolyzer anode under an intense oxidation medium: 1. Noble metals such as platinum and gold 2. Composition of conductive oxides

Due to the high cost of precious metals, only a series of conductive oxides of electricity is suitable for this purpose. So far, the most suitable material for the anode is LSM (Sr doped in LaMnO3) with a perovskite structure. This material has good electrical conductivity, catalytic properties for oxygen ion oxidation, and good chemical and mechanical compatibility with YSZ. Also, its coefficient of thermal expansion is close to the YSZ electrolyte. However, this material’s ionic conductivity is weak, limiting its efficiency because the oxygen ion oxidation reaction occurs in a small area between the electrolyte and the electrode. An excellent solution to this problem is to prepare an LSM-YSZ composite. However, this composite is still not ideal because, during the process, the MnOX in the LSM may penetrate into the YSZ, reducing conductivity or susceptibility [56]. Recently, LSC and LSF have also been used, and their effectiveness has been reported as follows [57]: LSM-YSZ . LSF-YSZ . LSC-YSZ Research has also shown that the LSCYSZ composite’s stability decreases after about 100 hours due to the reaction between LSC and YSZ. Hydrogen production by SOECs usually requires about 1 to 1.5 volts for practical applications. Since generating electricity from renewable sources is still a costly process, many efforts have been made to reduce electricity consumption at SOEC. A practical solution for this purpose is anode depolarization. To do this, they use the reaction of carbon and hydrocarbons with oxygen in the anode. This reaction leads to a decrease in the chemical potential between the two electrodes [58]. Martinez et al. [59] proposed a SOEC system with natural gas to reduce electricity consumption. Fig. 6.20 shows a simple SOEC block, and Fig. 6.21 shows the SOEC system with natural gas. In this system, natural gas reacts with the oxygen produced in the anode electrode and reduces its electrical potential. This reaction can be done with all or part of the oxygen, both shown in Fig. 6.21. Their research

Hydrogen, steam

Water

Natural Gas

Natural Gas Heat

Oxygen

Electricity SOEC

Figure 6.20 Schematic of solid oxide electrolyzer cell (with natural gas feed) [60].

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(A)

2e-

(B)

2eH2O H2

H2O H2 Porous cathode

Porous cathode

H2O+2e- → H2 +O2-

H2O+2e- → H2 +O2Electrolyte

Electrolyte

O2O2- → 0.5O2+2eCH4 +2O2 →CO2 +2H2O

Porous anode

O2O2- → 0.5O2 +2eCH4 +0.5O2 → CO +2H2

Porous anode CH4

CH4 CO2; H2O 2e-

CO; H2 2e-

Figure 6.21 SOEC system with natural gas. (A) Complete reaction with oxygen. (b) Incomplete reaction with oxygen [59,61].

showed that with the above system’s help, the voltage reaches about 0.5 volts at a current density of 2100 A/m2. Water vapor and hydrogen penetration must be done well at the cathode of a SOEC. The cathode must also provide active locations for water vapor reduction. In general, the use of metal electrode materials is possible when the partial pressure of oxygen at the cathode is in the range of 10212 to 1026. Platinum, as well as nickel and cobalt, can be used for the cathode. However, the use of noble metals is not recommended due to their high cost and the formation of volatile oxides at high temperatures. Nickel is the most common material for the SOEC cathode. Although nickel can regenerate hydrogen, it only conducts electrons. As a result, the electrochemical reaction can occur at the three-phase cathode-electrolyte-gas boundary. Ni particles are combined with ion-conducting particles (which is the electrolyte, YSZ) to increase the electrochemical reaction region. Usually, Ni/YSZ composite can be fabricated by sintering NiO and YSZ particles and reducing hydrogen under the atmosphere [62]. The proposed methods for the synthesis of the above electrode include colloidal acid deposition (which is a cheap method for thin-film fabrication) [59] and mechanical alloying method [63]. In addition to Ni/YSZ, Ni/SDC can also be used for the cathode [64]. One of the things to consider when using a SOEC is the cells’ geometry. So far, two plate modes [65] and a tube [39] have been introduced to make this electrolyzer (such as an SOFC), as shown in Fig. 6.22. Tube geometry has better mechanical strength and insulation than the plate. Nevertheless, the plate model has better performance due to the uniform distribution of gas components and has received more attention. Another point to consider when using SOEC is the use of exhaust gas heat to preheat the incoming water vapor. This reduces energy losses and the cost of hydrogen production. The electrolysis efficiency of water at high temperatures depends on the temperature and the thermal coherence. If the efficiency is only a function of the input electricity, about 85%90% has been reported [66]. Although the efficiency decreases significantly when using a heat source (such as geothermal,

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Figure 6.22 Types of solid oxide electrolyzer cell geometry. (A) Tubes. (B) Page [61].

nuclear, conventional combustion, and so on.), the integration of SOEC with a high-temperature nuclear reactor has resulted in an efficiency of about 60% [6]. Of course, operating at such a temperature, in addition to using a heat source, requires the use of more expensive materials and manufacturing methods. In connection with the combination of SOEC with other technologies, we can refer to research that uses the simultaneous electrolysis of water and carbon dioxide to produce synthetic gas (H2 1 CO). The resulting synthetic gas is also used as a gas feed in the processes of synthesis of hydrocarbons (ethanol, methanol, dimethyl ether, and else) and Fischer-Tropsch reactions. In addition to combining SOFC technologies with SOECs, it can be mentioned that the heat and electricity generated in the SOFC section are used to supply electrolyzer energy to produce hydrogen. In some cases, hydrogen from the SOEC can also be used for SOFC feed to generate electrical power. Sometimes the combination of SOEC with the methane production process (methane refining photo) is used to produce natural gas and use it in the natural gas distribution network.

6.2.4.4 Reversible fuel cells Fuel cells can theoretically work in the opposite direction to act as an electrolyzer. Much research has been done to reduce costs by using one device to perform two

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operations, but they still face problems. Because proprietary fuel cells or single electrolyzer systems are designed to achieve optimal efficiency, reversible systems in both cases cannot operate at maximum efficiency, which affects the operating cost of the system.

6.2.4.5 Integration of electrolyzers with other systems All water electrolyzer systems can be integrated with other technologies to reduce costs. It is essential to find cheap energy sources to provide the heat and electrical power needed to run electrolyzers. For example, if SOEC is integrated with nuclear power plants, the extra heat available can generate some of the energy needed to produce hydrogen. In the future, many parts of the world will use more renewable energy. However, the problem with such energy sources (such as solar or wind) is that they cannot produce energy 24 hours a day, seven days a week. In this case, when coupling these sources with electrolysis power devices, part of the generated energy can feed the electrolyzer in the absence of sunlight or wind. When an electrolyzer, for example, is integrated with a wind energy source, the electrolyzer’s job is to produce hydrogen and store it after absorbing the electrical oscillations generated by the wind turbine.

6.2.5 Combined systems Hybrid systems are systems that can generate electricity by combining two or more technologies. The purpose of this work is to optimize the efficiency of the processes involved in these systems. In addition to optimizing electrical and thermal efficiency, reducing emissions is also very important. Various configurations can create hybrid systems, such as hydrogen, renewable energy, gas, steam cycles, etc. Examples of this technology are hybridization processes of fuel cells with wind and photovoltaic turbines, co-generation and trigeneration processes integrated with fuel cell technology, and so on. Simultaneous generation of electricity and heat (hot, cold, or both) is an excellent solution to improve energy efficiency. In cogeneration systems, the fuel required to generate electrical and thermal energy is much less than when electricity and heat are generated separately in conventional systems. Therefore these systems are very suitable in terms of efficiency. Triple-generation systems also include processes that use a single fuel source to generate and consume electricity and heat at the same time. One of the technologies that have the best performance in this field is the fuel cell. Simultaneous energy use results in high energy efficiency, fewer emissions, storage safety, less waste, and lower costs. SOFC is the most promising technology for generating high-efficiency electricity from natural gas in either SOFC mode alone or integrated with other technologies. This fuel cell has a high operating temperature (600 C1000 C). Therefore this fuel cell’s exhaust gases have an excellent potential for use in different heating cycles. The high temperature of the exhaust gases from the SOFC can be used in other cycles such as Rankin and Brighton to generate additional power or provide

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heating/cooling (simultaneous production/triple production). Temperature analysis shows that SOFC has an electrical efficiency of 40%60% in normal conditions, which increases to 85% by combining it with simultaneous power and heating systems. The heat generated by the cell’s exhaust gases can be used both directly and indirectly in thermal cycles. In direct thermal combinations, the fluid used in the thermal cycle and the fuel cell are almost identical and interdependent. However, in the indirect case, the fluid used in the thermal cycle is different from the fluid in the fuel cell, and only the heat of the exhaust gases is used through a heat exchanger. Since the fluid exiting the cell is a gaseous fluid, it is based on the Brighton cycle. In direct thermal composition, the fuel cell’s exhaust gases act as the Brighton cycle’s operating fluid. This type of combination is divided into two modes of the system under pressure and atmospheric pressure. In a pressurized system, the air pressure increases by passing through the compressor. By passing through the fuel cell and performing an electrochemical reaction, it combines with the gases from the anode and enters the turbine, and the work is produced. In the atmospheric pressure mode, the air pressure increases as it passes through the compressor, and after exchanging heat with the exhaust cell fuel gases, it enters the turbine and is produced, and the turbine outlet is used as the inlet air to the cell. According to the Nernst equation, increasing the pressure increases the output voltage and increases cell efficiency. Disadvantages of this method can be faster wear of the battery and the need to increase the battery’s walls and components’ thickness, which increases prices. In general, the system has better performance and efficiency under pressure. Also, the temperature of the exhaust gases in the atmospheric pressure state is usually higher than in the pressurized state, resulting in lower heat recovery. Another mode of combining is that the heat of the cell’s exhaust gases is used separately. This is called indirect thermal composition. In addition to the SOFC cycle, other cycles can include the Brighton cycle, the Rankin cycle, the CHP, the refrigeration cycle, etc. As mentioned earlier, hydrogen has been proposed to meet energy needs. Because of the benefits of hydrogen, examines the simultaneous production of hydrogen and power. As mentioned, the SOFC is one of the most suitable cogeneration technologies due to its high operating temperature. In the next part, the SOFCSOEC combination process will be studied.

6.2.6 SOFCSOEC combined cycle As mentioned earlier, hydrogen is used as clean energy to replace hydrocarbon fuels, preventing greenhouse gas release. The global market has considered that hydrogen should be used as a driving force for other technologies, such as fuel cells, whose performance is entirely dependent on the presence of hydrogen. Hence, they seek to generalize a high-efficiency process to produce hydrogen. Hydrogen is produced in many ways, and on a small scale, the electrolysis of water produces pure hydrogen and oxygen gases. Alkaline electrolyzers produce 406403 kWh/ Nm3 of hydrogen. Assuming that electricity is generated by natural gas in an

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advanced combined cycle with an efficiency of 53% and a loss of 6%, the total conversion rate of natural gas to hydrogen is barely 34%. While proton exchange membrane electrolyzers are an excellent solution for reducing electricity consumption compared to alkaline electrolyzers, they are not a good proposition for hydrogen production because they have low hydrogen output. The most suitable method is steam electrolysis, which achieves good efficiency by using SOECs. Fig. 6.23 shows a schematic of a fuel coupling with various hydrogen generation technologies based on a SOFC. Due to the high operating temperature, the total losses inside the cell are reduced. There is a chance to replace part of the energy required by SOEC with the thermal energy available in other processes. Of course, high operating temperatures (around 1000 C) require relatively expensive materials, which leads to higher operating costs. For this reason, researchers have focused on developing lowertemperature SOECs (in the range of 850%550%), called medium-temperature SOECs (IT-SOEC). Therefore the temperature range is considered in this range. Hydrogen can also be produced through fossil fuels through several different processes. Steam reforming is one of the best and least expensive processes available. According to the data reported in [67], the conversion efficiency of natural gas to hydrogen varies from 74% to 85%. The above-combined cycle was also used to produce pure oxygen. In fact, the hydrogen produced in SOEC was considered as SOFC input fuel in this system [67]. A schematic of such systems is shown in Fig. 6.24. The combination of SOFCs and SOECs has been used to produce pure hydrogen from natural gas. The same operating temperature of the two cells means no additional load for the other system, which is a significant advantage. Due to the possibility of internal reforming in SOFC and providing its hydrogen, the steps and cost of natural gas reforming are reduced. However, 25% of natural gas is converted to prevent temperature fluctuations in the cell before entering the fuel cell. In this

1.Steam reforming 2.Thermo water-splitting 3.Coal gasification 4.Diesel reformer 5.Gasoline reformer 6.PV electrolyzer 7.Biomass gasification

Fuel

HRU

SOFC

Hydrogen Production Exhaust HE

Air

Figure 6.23 Integration of a SOFC based on fuel coupling schematic [8].

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Air

Electricity

Depleted Air

H2O-H2 mixture

ANODE

Heat

SOEC CATHODE

ANODE

CATHODE

SOFC

O2

Figure 6.24 Conceptual arrangement of SOFCSOEC integration system for pure oxygen production [67].

combined cycle, SOFC is fed with natural gas and supplies electricity to the SOEC. The heat generated by the irreversible SOFC processes is used for thermal recovery of SOEC reactions, and hydrogen is produced in SOEC. Here are two possible solutions for heat transfer from SOFC to SOEC: (1) physical separation of SOFC and SOEC in two separate stacks (can provide advantages in the design and management of inlet and outlet flows). In this case, the heat can be transferred from the SOFC output to the SOEC input currents through a heat transfer circuit or through heat recovery. (2) Using the sandwich configuration (where the organization and current density inside the fuel cell and electrolyzer are constant and the same). According to the above, SOEC must supply all or part of the energy generated by SOFC electricity to supply electrical (and often thermal) energy. Fig. 6.25 shows a schematic of the SOFC/SOEC integration system’s process design, and Fig. 6.26 shows the overall triple-generation system (hydrogen, power, and heat). The compressed natural gas enters the reformer after preheating, and the output from the reformer enters the SOFC anode after heating. The anode valley completes the reforming process, and hydrogen is oxidized. The electricity and heat required by the SOEC are also generated. The air trapped in the exchangers is also preheated and enters the SOEC cathode. Water vapor also enters the SOEC cathode at high temperatures and produces hydrogen and oxygen. To improve the results in such a system, various methods can be used, for example, the oxygen produced in the SOEC anode valley can be removed from the system as a by-product or used in the SOFC cathode to increase the oxygen concentration, and the air discharged from the SOFC cathode. The combustion section can be sent to the furnace to supply the required oxygen (to ignite the SOFC anode output current), or part of the SOFC anode output (water-rich) can be returned to the pre-reformer to provide the proper vapor-to-carbon ratio, and so on.

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Hydrogen, steam

Water

Heat

SOEC Heat

Oxygen Electricity

Natural Gas

Flue gases SOFC Electricity

Air

Figure 6.25 Schematic of the SOFC/SOEC integration system [60]. H2O

H2/H2O

H2O

H2 H2 USERS

H2O TANK

SOEC Q

H2O

Q USERS

O2/H2O H2O

COMBUSTOR POWER POWER H2/H2O/CO2 CH4/H2O

N.G SOURCE

SOFC CO2 STORAGE

O2 depleted air

AIR Flue gases CO2

CO2 RECOVERY

H2O

Figure 6.26 Schematic of SOFC/SOEC integration system for simultaneous production of hydrogen, power, and heat from natural gas feed [68].

Several SOFCs and SOECs were used in a stack with the same fuel (natural gas) [69]. Then hydrogen was produced in the SOEC section and electricity in the SOFC section, and the efficiency was higher than 69%. The fuel consumption factor was low, about 40%, and coke deposition problems were still present. At the end of the next chapter, a complete overview of the SOEC and SOFC systems will be provided.

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Hydrogen production is one of the most critical research areas to provide energy for the future. The SOFC and the systems integrated with it are the SOEC and then the technologies and resources that make it possible to combine with this electrolyzer. Moreover, hydrogen production and power using the combined cycle of SOFCSOEC is a high-efficiency process. Therefore the process, performed on the SOFCSOEC hybrid system, is discussed in this chapter.

6.3.1 SOFC The fuel cell is an alternative technology to traditional power generation systems with higher electrical efficiencies and fewer disadvantages. The high operating temperature of a SOFC allows it to be integrated with traditional thermal cycles to improve thermal efficiency. These cells have more accessible reliability and maintenance and lower repair costs due to the absence of vibration, noise, movement of internal components during operation. The high operating temperature makes it possible to use a variety of fuels to produce hydrogen. The device’s size is also flexible so that it can be used to generate power in any range from watts to megawatts [70]. A direct carbon SOFC can be utilized to generate power with solid carbon fuel (DC-SOFC). Then, it was used a two-dimensional model to simulate a tube to generate electricity and carbon monoxide simultaneously. Activated carbon was injected into the anode and air was into the cathode. First, oxygen in the anode reacts with activated carbon to produce carbon dioxide, and then carbon dioxide reacts with carbon during the Boudouard reaction to convert to carbon monoxide. This model investigated the effects of operating temperature, current density changes in terms of voltage, the longitudinal distance of the carbon chamber to the anode section of the electrolyte-support configuration, and the anode support. It was shown that the system performance decreases by increasing the distance between the carbon chamber and the anode. Increasing the operating temperature increased the molar fraction of carbon monoxide and the system output’s current flow. Next, the current density increased slowly due to electrochemical fuel production due to the odor reaction compared to hydrogen-powered fuel cells. It used supported electrolyte and supported configurations to compare the effects of cell configuration type on electrical output power and carbon monoxide production and found that supported anode configurations had higher electrical power than supported electrolyte configurations. It is not suitable for producing carbon monoxide, especially at longer distances from the carbon chamber to the anode [71]. The mass transfer in the anode section of a SOFC was compared to the Fickian, Stefan-Manwell, and dusty-gas models. Graham’s law was used to calculate the flux ratio of components and validate the isotherm’s assumption (low-pressure gradient). The prediction of the SOFC anode section’s additional concentration potential is accomplished with a supported anode configuration. Two types of gas mixtures, including carbon dioxide and carbon monoxide, and a mixture of water,

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hydrogen, and argon, were used as feed. The electrochemical reactions were related to the oxidation of hydrogen and carbon dioxide. The assumption of constant temperature and stable conditions and the one-dimensional Cartesian model was implemented to express the concentration of components along with the anode thickness. The results compared this model with empirical data from the work of Suwanwarangkul. The dusty-gas model, with an approximate stoichiometric flux ratio for the components, was found and it is the best option for modeling multicomponent mass transfer in a SOFC system. This method presented a good performance as compared with Fickian in mass transfer expression at low current density [71]. The effect of current density and sulfur concentration on a SOFC’s electrochemical performance with methane fuel (experimental) is important. The methane feed consisted of 0100 ppm of hydrogen sulfide gas, and the system used was a SOFC with a reinforced anode configuration. With increasing the concentration of hydrogen sulfide in the fuel entering the anode and increasing the current density, the cell’s voltage drop increased. By apparent resistance spectroscopy, the presence of hydrogen sulfide directly affects mass transfer and charge transfer at the anode. In the OCV mode, hydrogen sulfide’s main adverse effect at the anode was the increase in charge transferpolarization resistance of the three-phase boundary (TPB). In contrast, its effect on the fuel conversion-mass transfer process was less significant. However, at high current densities, the sensitivity of this process to the presence of hydrogen sulfide increased. Finally, they showed that mass transfer was process-limiting due to high fuel consumption and hydrogen production at high current densities [72]. Using an electrochemical model needed using the ButlerVolmer equation, Fick’s model, and Ohm’s law to determine the additional activation potentials, concentration, and ohmicity, respectively. The results should also analyze three configurations of the supported anode, supported cathode, and electrolyte support, operating conditions, pore size, and electrode porosity. Cell performance became maximized by optimizing the porosity and size of the electrode cavities. Moreover, increasing the temperature significantly reduced the additional activation and ohmic potential and increased excess concentration potential due to decreasing gas density and increasing component penetration coefficients in rising temperatures. The effect analysis of increasing the concentration of hydrogen in the feed and increasing the operating pressure showed that these two parameters positively affect the cell’s output power [73]. Two-dimensional computational fluid dynamics (2D CFD) model can be used to calculate the fuel cell’s performance with a supported anode configuration at medium operating temperatures. The area-specific resistance and potential increase due to temperature increase were significant at high current density. Moreover, the cathode excess potential’s contribution was very significant at low current densities. The additional concentration potential at the anode and the additional activation potential at the cathode was considerably high. One of the most important is that the electrochemical reaction was more active at the electrolyte-electrode contact surface, where the temperature was high. Since the model was two-dimensional, the effects of contact resistance, ohmic loss, and nonuniform flow generation were not considered in the model equations [74].

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To analyze a cross-flow plate fuel cell with methane-free biogas-boosted electrolyte configuration is necessary to consider the input feed consisting of hydrogen and carbon dioxide, the oxidation reactions of hydrogen and carbon monoxide at the electrolyteanode contact surface, and the reduction of oxygen at the electrolyte-cathode contact surface as the gas-water reaction at the anode electrode. The high molar fraction of carbon dioxide in the inlet feed activates the reverse gas-water heater reaction and reduces the airflow’s need to cool. Hence, the use of methane-free biogas feed with a high concentration of carbon dioxide is activated by the reverse gas-water reaction, resulting in a more uniform flow distribution [75].

6.3.1.1 Integrated systems with SOFCs User needs generally determine SOFC hybrid systems integration strategies. In most cases, pollution and efficiency are considered in the design of these systems. In some cases (often aerospace and marine), reliability and noise level are more important. The integration of SOFC with the Brighton cycle is a direct thermal coupling process in which SOFC inlet air is supplied in two ways. In the first case, which is called pressure, the SOFC inlet air is supplied from the Brayton cycle compressor outlet, and in the second case, which is called non-pressure, the SOFCrequired air is supplied from the Brighton cycle gas turbine outlet. Also, SOFC can be combined with Rankin flood and is considered an indirect thermal composition. The above hybrid systems were thermal couplings. Combined fuel coupling systems are also available that use various technologies to produce the hydrogen required by SOFC. These systems use hydrocarbon reforming, water electrolysis, biomass gasification, coal gasification, etc [8]. It is important to note that increasing the hard consumption factor from 0.6 to 0.95 decreases the fuel cell voltage due to further fuel reduction and polarization loss at the anode. In addition, to achieve the highest efficiency, the system had to work with a fuel consumption factor of 0.85. According to the fuel consumption factor of 0.85, with increasing current density from 1600 amps per square meter to 2400, system efficiency and potential decreased due to ohmic, and concentration loss, but the output power increased with increasing density. Achieving an electrical efficiency of about 68% is significant compared to the combined cycles of power plants (about 50%) [76]. In a combined plate SOFC system, a gas turbine power plant and the model of coherent and heterogeneous flow pattern for the fuel cell, increasing temperature and pressure increased cell voltage and output power. However, their values in the heterogeneous current are more significant than the alignment. The optimal current density value of 8000 amps/m2 is obtained for both current patterns, and increasing the fuel consumption factor reduces the cell voltage in both current patterns. The voltage in the co-current was always greater than the counter-current in any fuel consumption factor. In an optimal combination of a SOFC system, ammonia-water absorption cooling cycle, and Rankin steam cycle, the output power can achieve 500 kW. The flow density, boiler exhaust gas temperature, steam turbine pressure, generator

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temperature, and cooling water consumption are key parameters to optimize the cycle. In addition, the combined system efficiency can reach to be 62.4% under optimal conditions and 22.1 kW heat recovery. In that, regardless of the absorption cycle, the combined system’s investment cost is about US $2917 per kW. The suitable range of current and voltage densities are reported to be 0.035.5 A/cm2 and 0.073.8 V, respectively. The highest oxygen loss is related to the boiler, air preheater, and combustion chamber.

6.3.1.2 SOEC Hydrogen is a potential alternative fuel and energy carrier for future energy supply. In addition to traditional fossil fuel methods, hydrogen can also be produced by the thermochemical decomposition of water, photocatalytic decomposition of water, and electrolysis. The first two methods have very low operating efficiencies and are therefore not competitive with the electrolysis process. High-temperature solid oxide electrolysis cells have the advantage of a higher chemical reaction rate with lower electrical energy compared to low-temperature ion exchange membrane electrolysis cells and alkaline electrolyzers [61]. The evaluation of the effect of pressure on solid oxide electrolysis cells theoretically and experimentally in the pressure range of 110 bar and a temperature of 800 C can be performed by plotting potential graphs in terms of flow, additional potential in terms of flow, and flow in terms of cermet porosity. The OCV increased with increasing pressure. Besides, the limiting current density increased due to the increase in pressure, which improved hydrogen production. Also, the limiting current density’s improvement is related to reducing the cathode’s excess concentration potential due to the pressure increase. Furthermore, an optimal point for pressure in these operating conditions so that pressures above the optimum pressure lead to a slight decrease in the rate of hydrogen production, which is mainly due to the increase in OCV [77]. The investigation of the theory of producing synthetic and hydrogen fuels using solid oxide electrolysis cells with the input feed consisting of water and carbon monoxide needs to consider heterogeneous catalytic reactions, including methanation reaction (reverse methane vapor reforming) and gas-water inverse reaction. In addition, the extra concentration, ohmic, and activation potential are a pre-step of the parametric analysis, such as thermodynamic voltage, fuel utilization factor, carbon dioxide consumption factor, and hydrogen-carbon dioxide and hydrogen-carbon ratio, to determine the system performance. It is important to note that increasing the concentration of carbon dioxide in the input feed led to an increase in its consumption factor and fuel consumption factor due to the high progression of the gas-water reverse reaction. Also, the higher electrochemical losses (especially activation) in the anode section are because of the input feed’s high carbon dioxide concentration. Therefore the composition of the exhaust gas from the electrolysis process, in case the water-carbon ratio in the feed is high and is suitable for the methane production process and in the case where the water-carbon ratio is close to one for the FischerTropsch process [78].

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Analyzing the hydrogen production system using a solid oxide electrolysis cell to investigate the effects of current density, operating temperature and pressure, molar fraction, and input flow rate on cell performance are as the following: the increase in cell potential is achieved by increasing the current density and decreasing the operating temperature. Also, increasing the input hydrogen flow rate increased the output hydrogen flow rate but decreased the hydrogen production rate. Finally, a high-temperature coupling reactor with a high-temperature electrolysis system leads to an efficiency of nearly 36% [79].

6.3.1.3 Integrated systems with SOECs Carbon dioxide from fossil fuels or carbon dioxide absorbed in chemical processes can produce valuable fuels. In a similar process to water electrolysis, the simultaneous electrolysis of water and carbon dioxide can produce synthetic gas, producing liquid hydrocarbons during the FischerTropsch process [80]. In a high-temperature electrolysis process of water and carbon dioxide using a solid oxide electrolysis cell to produce synthetic gas and, subsequently, convert it to liquid fuels during the FischerTropsch process, the synthetic gas emitted from the solid oxide electrolysis cell in a slurry bubble column can be converted to liquid fuels during the FischerTropsch process, considering the once-through conversion of CO and the Anderson-Schulz-Flory model determined the distribution of hydrocarbon products. Therefore the system’s overall efficiency for producing liquid hydrocarbon fuels from electrical energy reaches about 58.8% HHV and 51% LHV. Also, the electrolyzer’s performance at low pressure (1.6 bar) compared to a higher pressure (5 bar) led to a 2.6% increase in process efficiency. In terms of cost analysis, the cost of producing liquid fuels ranged from 4.4 $/GGE to 15 $/GGE for the cost of electricity 0.02 $/kWh to 0.14 $/kWh and a plant capacity factor of 90% 40% changes [80]. In a combined SOEC system and a catalytic reactor to produce hydrocarbon fuels, including location and dimethyl ether, the fuel entering the SOEC included water, hydrogen, and carbon dioxide, and the electrolysis reactions took place simultaneously at the electrolyzer anode. Also, gas-water reactions and methanation (reverse methane vapor reforming) can be considered in the SOEC and introduced the exhaust gas into a catalytic reactor to produce dimethyl ether. Moreover, in the electrolyzer section, odd reactions (solid carbon formation reactions) from the decomposition of carbon monoxide and hydrogen can be incorporated with the reaction between carbon monoxide and hydrogen. The positive effect of decreasing temperature and increasing pressure on methane production shows the need for high temperature and operating pressure to produce dimethyl ether and prevent the formation of carbon in the high fuel consumption factor [81]. An economic analysis of methanol production by electrolysis of CO2 and H2O in a solid oxide electrolysis cell is essential for its operating cost. The electrolysis reaction of water and carbon dioxide and the equilibrium gas-water reaction are considered in the solid oxide electrolyzer section. The synthetic gas emitted from the electrolyzer synthesizes ethanol during a catalytic reaction directly, and the

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conversion reactions of ethanol, methanol, and methane are considered. The energy consumption is 4.383 MJ/kg ethanol, electricity consumption is 36.9 MJ/kg ethanol, and the cost of ethanol production is $ 1.1 per kg ethanol [82]. In terms of a natural gas synthesis process from a combination of SOEC and the catalytic methanation process, two different ways of producing natural gas can be created. In the first method, water vapor electrolysis is used, and simultaneous electrolysis of water vapor and carbon dioxide occurs in the second method. The high operating pressure in simultaneous electrolysis reduces the electrical energy required. However, most of the heat required to produce steam and preheat the electrolyzer inlet water came from the methane-generating exotherm. With this technique, the efficiency of the process increased to over 80%. In the simultaneous electrolysis process, the LHV efficiency reaches 81.4% [83]. A hybrid system including photovoltaic and SOEC hybrid power plants with biomass fuels in four different structures can decrease the irreversibilities of sources. For example, the combined hybrid units as a reference plant and comparing its performance with a reference unit’s combined system and a SOEC. In the combined system, due to the electrolyzer’s inherent irreversibility and the decrease in the efficiency of the reference unit due to the outflow of water vapor for evaporation of the electrolyzer inlet water, operational problems arose compared to the reference unit. The severity of these problems depended on the power consumption of the SOEC, the heat demand, and the pressure dissipation within the plant components. The hybrid system’s maximum efficiency can be obtained when the electrolysis system is operating with a return sweep gas stream. However, a decrease of 6.3% 7.5% in the combined system is achievable compared to the reference unit. Moreover, exergy analysis shows that removing electric heaters reduces the negative exergy points of the process [84]. On the other hand, the combination of a SOEC and the FischerTropsch process can produce synthetic fuels. The synthetic gas produced by the simultaneous electrolysis of water and carbon dioxide is considered the feed of the FischerTropsch process. Three different configurations (structures) are typically used to combine the SOEC and the FischerTropsch process. The first is a simple combination of SOEC and the FischerTropsch process. In the second case, the FischerTropsch process’s lighter exhaust gases are first introduced into the reformer section and then added to the SOEC input. In the third case, the FischerTropsch process’s lighter exhaust gases enter the electrolyzer directly. In the first case, the LHV electrolyzer efficiency is 79%, and the stack size achieves 10.43 square meters to supply FischerTropsch feeds at a rate of 42 gallons per day. The highest efficiency of the first and second laws was obtained in the third case. The third mode’s energy efficiency was slightly higher than the second mode (about 1.5%). From the exergy point, the third state has a higher electricity consumption than the second [85].

6.3.1.4 Combined system of SOFCSOEC In some cases, a combination of SOEC and SOFC can be used to optimize operations and increase efficiency. The hydrogen required in SOFC is supplied from the

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SOEC section, and the heat and electricity required in SOEC are supplied from the SOFC section. An essential point in integrating these two technologies is the transfer of heat from the fuel cell to the electrolyzer, for which two solutions have been proposed. The first way is that SOFC and SOEC are designed in two separate stacks, in which case the management of input and output currents and the design of stacks is a kind of advantage. In this design, heat can be transferred from the fuel cell to the electrolyzer through a heat transfer circuit, or with a suitable recovery design, heat can be transferred from the SOFC outlets to the electrolyzer input stream. The second design method allows heat transfer by placing SOFC and SOEC as overlapping layers. This method, known as the sandwich configuration, is preferable in terms of heat and electricity transfer between the SOFC and the SOEC. SOFCSOEC hybrid technology is used to produce high-efficiency pure oxygen. The heat, electricity, and water vapor generated by SOFC enter the SOEC section and are consumed there. Hydrogen produced in SOEC is considered as SOFC feed and oxygen as output. The different operating conditions show that when the integrated system operates at low current density (less than 1000 amps per square meter), pure oxygen with electricity consumption is comparable to medium-sized units of air separation and high power consumption. Fewer small-sized units can be produced based on oscillating pressure absorption technology. At a current density of 500 amps per square meter, they reported 0.3 kWh of electricity consumption per kilogram of oxygen output [86]. In a close configuration to the mentioned cycle above, SOFCSOEC hybrid technology can be integrated to produce pure hydrogen from natural gas fuels. Thus the heat and electricity generated in the SOFC section are considered as the required heat and electricity in the SOEC section. While providing the electrolyzer’s required electricity deficit during the grid connection and returning O2 to the SOFC anode inlet current to improve its performance, it achieved twice the average water electrolysis efficiency. The most complex operating mode increased system efficiency by up to 87% [67]. In terms of exergy assessment of a designed system for the simultaneous production of hydrogen, heat, and electricity from natural gas based on the SOFCSOEC combined system: The exergetic efficiency is about 45%. The three options used to sweep oxygen gas at the SOEC anode are water vapor, air, and synthetic gas. The required potential is not much different from each other. Also, the exergic efficiency in terms of the water vapor consumption factor of SOEC cathode shows an upward trend of efficiency for all three sweep gas types [68]. The SOFCSOEC hybrid system is a low-scale unit of oxygen production. The technologies for low-scale oxygen production units can by varying current density and operating temperature. Under optimal conditions, electricity consumption ranged from 0.35 to 0.5 kWh/kg oxygen, which is lower than PSA technology and higher than centralized modeling [67]. A double-sided cell design for SOFC can operate in both SOFC and SOEC modes and medium temperatures. The cell consisted of a series of five layers with different compositions. This cell’s existence and its reversibility are proved by alternating current spectroscopy [87].

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Hybrid Poly-generation Energy Systems

Using an electrochemical model for the operating mode of the fuel cell and solid oxide electrolyzer can be accomplished to predict the performance and efficiency of this process by considering two-dimensional and flat SOFC models together and at a voltage of 0.8 Volts for each cell with efficiencies above 80% and total electrochemical efficiencies of 68% [88]. A high-temperature reversible solids (rSOCs) cells can be combined with the SOFC and SOEC models to form a SOC unit. The cell’s behavior under different operating conditions presents that the voltage consumption is reduced if the operation is performed at high pressure due to the advancement of the heater reaction. Also, increasing the pressure significantly affects the cell’s thermal behavior [89]. The physical state equation is one of the most critical simulation parameters. The equation of state estimates the fluid’s properties using constants, simulator databases, and operating conditions. The appropriate state equation must be selected depending on the type of process, devices, materials, and the system’s temperature and pressure range. Records show the state equations’ accuracy in the system’s software. For gas processes, the PengRobinson (PR) equation is proposed. This equation can be used in many operational variables and provides relevant and accurate results. The Soave Redlich Kwong (SRK) state equation works similarly to PR in this system. The allowable temperature and pressure ranges for these two equations are shown in Table 6.5. Depending on the fuel cell and electrolyzer system’s operating conditions, the choice of PR or SRK equation for this system is no different and has the same accuracy. Here the PR mode equation is selected.

6.3.1.5 Fuel cell system The most crucial fuel cell power plant components include the fuel converter, fuel cell mass, AC to AC equipment, heat exchangers, compressors, and blowers. The amount of heat lost for recovery cycles to generate power, heat, or cold varies depending on the cell’s operating temperature. The rate of waste heat recovery has essential effects on economic and environmental parameters and system efficiency. Fig. 6.27 shows the general components of a fuel cell system. As mentioned, most fuel cells’ fuel is hydrogen, which must be cleared of sulfur compounds before entering the fuel cell. If the fuel used is hydrocarbon compounds, it must be converted to hydrogen during the reforming reactions inside or outside the fuel cell. After its operation, the fuel enters the stack of fuel cells, and electrochemical reactions are performed on the fuel. The efficiency of the cell depends on the type of fuel cell. The fuel cell generates direct electricity, which is converted to AC power by a converter with an efficiency of 98%90%. Then, using conventional heat recovery techniques, the whole system’s efficiency is increased (Fig. 6.28). Table 6.5 Temperature and pressure range allowed for the state equation. Equation of state

Temperature ( F)

Temperature ( F)

Pressure (psia)

Pressure (KPa)

Peng-Robinson SRK

. -457 . -225

. -271 . -143

,1500 ,5000

,100,000 ,35,000

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Depleted Anode Gas

AC-POWER

DC-POWER

H2 rich gas

Fuel

Fuel Processor

Inverter

Fuel Cell

Air POWER or Process Heat Heat and Power Recovery subsystem Low Grade Heat

Air

Figure 6.27 Overview of solid oxide fuel cell system [90]. 7 Hydrogen Fuel pre Heater

Fuel Compressor 1

2

Anode

3

Net Power AC Power Air

Inver ter

8

DC-Power

4 Air Blower

5

SOFC Stack

6

Cathode Combustor

Air pre Heater 9 Heat Recovery Exhaust

Figure 6.28 Overview of solid oxide fuel cell with hydrogen fuel [90].

Fuel cell system design and knowledge of chemistry, electrical, electrochemical, thermal, and materials require the correct choice of conditions such as fuel consumption ratio, operating temperature, and fuel cell voltage. Fig. 6.29 shows the components of a fuel cell system with hydrogen fuel, in which the required hydrogen is compressed depending on the compressive conditions of the system and then enters a preheater with a temperature of about 4060 C less than the nominal temperature of the mass. The fuel cell is preheated [90]. The fuel then enters the fuel cell. The air is compressed and preheated according to the system’s compression and temperature conditions and finally enters the fuel cell mass. After electrochemical reactions are performed in the cell, the direct current generated is converted to alternating current. It provides the power required by compressors, blowers, and pumps, with additional power as net power is taken out of the system. Since not all of the hydrogen entering the cell is consumed, the rest is burned in a combustion chamber. It enters the existing exchangers

246

Hybrid Poly-generation Energy Systems 9 External Reformer 3 Natural Gas

2

Fuel Compressor 1

4

Net Power

Anode

Desulfurizer

5

10

W-C AC Power

SOFC Stack

DC-Power

Inver ter

8

W-B Air

6

Cathode

7

Combustor Air pre Heater

Air Blower

W-P

11 Steam

Water

Boiler

Pump

12

Heat Recovery Exhaust

Figure 6.29 Overview of solid oxide fuel cell with natural gas [90].

from the combustion chamber’s outlet to provide the heat needed to preheat the fuel and air, and retain residual heat. Since the hydrogen required by fuel cells does not exist independently and in pure form, to supply it, one must go to hydrogen-rich sources and produce the required hydrogen with the necessary conversions on them. Hydrogen-rich sources include water and hydrocarbon fuels, which can be converted to hydrogen (as mentioned in Chapter 1). About 90% of the hydrogen produced is obtained by reforming steam-fired hydrocarbon fuels. This method has high efficiency and low production and operating costs [91]. As mentioned in Chapter 2, natural gas is one of the most economical sources of hydrogen production. Fig. 6.30 shows a natural gas fuel cell system in which the fuel enters a desulfurization reactor after compression and then undergoes a steam reforming process to convert it to hydrogen.

6.3.1.5.1 Necessary operations on fuel Desulfurization Sulfur compounds are harmful to all fuel cells, so they should be prevented from entering the system. The permissible sulfur for SOFCs is reported to be around 110 ppm. Different desulfurization methods exist, such as (6.68) and (6.69) reactions. The operating temperature for these reactions is 300 C400 C. The choice of desulfurization method depends on the economic costs and the type of desulfurization composition in the fuel. Sulfur compounds can also be removed from the environment using activated carbon, but fixed and operating costs increase [90]. ðC2 H5 Þ2 S 1 2H2 ! 2C2 H6 1 H2 S

(6.68)

H2 S 1 ZnO ! ZnS 1 H2 O

(6.69)

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247

1.2

T=1073 K model data

1

T=973 K T=873 K

experimental data

Cell Voltage, (V)

0.8

T=1073 K T=973 K T=873 K

0.6

0.4

0.2

0 0

5000

10000

15000

20000

25000

30000

35000

Current density, (A/mΛ2) Figure 6.30 Comparison of model results with experimental data [92].

The process is based on natural gas. Table 6.6 shows the components of natural gas and their molar fraction. According to this table, the desulfurization section is omitted due to the absence of sulfur compounds in natural gas. Steam natural gas reforming The natural gas conditions are given in Table 6.7. In a preheater reactor, which is a plug reactor, the vapor-to-carbon ratio is assumed to be S/C 5 2, and 25% of the inlet gas in the preheater is converted to hydrogen. To prevent hydrocarbons heavier than methane from entering the SOFC (due to the increased possibility of solid carbon formation), they are converted to hydrogen before entering the cell by an equilibrium reactor. The steam required in the reactor can be calculated according to the assumed steam-to-carbon ratio. The equation expressing the ratio of steam to carbon is given in Eq. (6.70). S H2 O 5 C CO 1 CH4 1 2C2 H6 1 . . .

(6.70)

The operating temperature of this reactor depends on the system’s available heat, and its pressure depends on all the pressure drops inside the system. Reforming

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Hybrid Poly-generation Energy Systems

Table 6.6 Except molar natural gas. Composition (mole %)

Component

0.927 0.0343 0.00 0.0081 0.0095 0.0151 0.0053

Methane Ethane H2 O CO CO2 N2 O2

Table 6.7 Conditions and properties of natural gas. Parameter

Value

Unit

Temperature Pressure Molar flow

25 101.3 10

 C kPa kmole=h

0.6985 4.675e4 5.147e4

kg=m3 kJ=kg kJ=kg

Real properties at 14.7 psia and 60 F Density Lower heating value (LHV) Higher heating value (HHV)

reactions can be performed externally or internally in a SOFC stack. Here, 25% of the reforming is done externally (in a plug reactor), and the rest of the methane is done in the fuel cell anode along with an electrochemical reaction (in a plug reactor). To remove heavier hydrocarbons after the first reactor, ethane is reformed in an equilibrium reactor. The following sections present the reactions, kinetics, and simulation hypotheses of reforming and electrochemical reactions. Note that ρ is required to convert the rate unit to a catalytic reaction. External reforming Various kinetics have been proposed for steam methane reforming and water gas shift reaction. These kinetics depend on the operating conditions and the type of catalyst used. The investigation of the kinetics of two steammethane reforming reactions and gas-water displacement on a commercial Ni/ α-Al2O3 catalyst is carried out with a test pressure range of 600120 kPa [93]. The obtained kinetics are based on the Langmuir-Hinshelwood Hougen-Wastone (LHHW) model and Freundlich adsorption theory. This section’s reactions are given in Eqs. (6.71)(6.73). This reaction’s kinetics are given in Eqs. (6.74)(6.77). The parameters related to the kinetics of these reactions are shown in Table 6.8. CH4 1 H2 O ! CO 1 3H2 ΔH298 5 206kJ=moler1

(6.71)

CO 1 H2 O ! CO2 1 H2 ΔH298 5 2 41kJ=moler2

(6.72)

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Table 6.8 Parameters related to the kinetics of external reforming reactions [93]. Unit

Value

kmol kgcat 3 kpa0:25 3 s kmol kgcat 3 kpa 3 s kmol kgcat 3 kpa0:25 3 s 2

kpa

kpa2 1 kpa 1 kpa0:5

2
 Ki or ki 5 Ai exp Ei =RT

Parameter

E

A

209.2e3

5.922e8

15,000

6.028e 2 4

k2

109.4e3

1.093e3

k3

26830 2 4400 22430 2 1.4e5

1.198e17 1.767e 2 2 2.117e15 5.127e 2 13

KP1 KP2 KP3 KCO

2 93,400

5.68e 2 10

KH2

15,900

9.251

KH2O Catalytic density 5 1780 kgcatalyst =m3

k1

CH4 1 2H2 O ! CO2 1 4H2 ΔH298 5 165kJ=moler3 0 r1 5

k1 B 3@ PH2 1:25 0

r2 5

k2 PH2 0:5

3@

3 PH2 3

P1

k3 3@ PH2 1:75

H2 O

0:5

ðDENÞ2 P

3 PH2

PCO 3 PH2 O 0:5 2 K CO32 P

0 r3 5

P

PCH4 3 PH2 O 0:5 2 K CO3 P

P2

H2 O

ðDENÞ2 PCH4 3 PH2 O 2 ðDENÞ

0:5

1 C A

(6.74)

1 A

(6.75)

1

PCO2 3 PH2 4 KP3 3 PH2 O A 2

DEN 5 1 1 KCO PCO 1 KH2 PH2 0:5 1 KH2 O

(6.73)

PH2 O PH2

(6.76)

(6.77)

Reforming of heavier hydrocarbons (ethane) Reforming of heavier hydrocarbons (here ethane) is done due to the increase in input H2 concentration as well as the lack of coke formation in the SOFC anode. The equilibrium reactions of ethane reforming are as follows. 1. Equilibrium reaction of ethane conversion [Eq. (6.78)] 2. Equilibrium reaction of water-gas [Eq. (6.79)]

C2 H6 1 2H2 O22CO 1 5H2

(6.78)

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Hybrid Poly-generation Energy Systems

CO 1 H2 O2CO2 1 H2

(6.79)

External reforming reactions Because the SOFC allows internal fuel reforming, as a result, reactions performed inside the fuel cell include reforming reactions and electrochemical reactions. Electrochemical reactions are examined in the next section. Three different types of methane reforming kinetics have been reported in Eqs. (6.80)(6.82). Depending on the problem, the Achenbach-Riensche and Lein Felder model is used. The kinetic model proposed for Aachen Bach-Ring and Linfelder is Arnius type and for Drescher model is Langmuir type. The laboratory conditions according to which the following kinetics are obtained are reported in the reference [94]. Units related to reaction rate parameters are listed in Table 6.9.   82e3 Rr;AchRie 5 4274 3 PCH4 3 exp 2 3 As RT

(6.80)

  205e3 Rr;Lei 5 30:8e10 3 PCH4 3 PH2 O 3 exp 2 3 As RT

(6.81)


 288:52 3 PCH4 3 PH2 O 3 exp 2 11e3 RT 
Rr;Dre 5 3 As 1 1 16PCH4 1 14:3PH2 O 3 exp 39e3 RT

(6.82)

In these equations, P is the partial pressure; based on the load, T is the fuel cell temperature, in Kelvin, as is the active surface area by volume. The kinetics of carbon monoxide reaction with water vapor is also presented in Eq. (6.83). !   103191 PCO2 PH2
 Rs 5 0:0171 exp 3 PCO PH2 O 2 RT 0:019exp 4276 T

(6.83)

A plug reactor has been used to simulate this part, which is the SOFC fuel cell anode part. Electrochemical reaction The SOFC’s electrochemical reaction and its related kinetics are presented in Eqs. (6.84) and (6.85). The kinetics of this reaction is Table 6.9 Unit of parameters in the kinetics of internal reforming reactions [94]. Unit

Parameter

mole m3 :s

Ri Pi T R As

Bar K j mol:K active area volume ,

(cm21)

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251

proportional to the cell current density. In the following sections, the current density will be discussed [95]. H2 O 1 2e2 ! H2 1 RH2 O 5

1 O2 2

i i i RO 5 2 RH 2 5 2 2F 2 4F 2F

(6.84)

(6.85)

6.3.1.5.2 Fuel cell mass To increase the output voltage of the fuel cell, several fuel cells are connected in series, which is called the fuel cell mass set. Each mass consists of units, each with an anode, cathode, electrolyte, two fuel, air transfer plates, and two current collecting plates. Because the output voltage per cell is less than 1 volt, most applications require more voltage than this (for example, commercial electric motors often operate at voltages of 200300 volts). The required voltage is obtained by connecting the fuel cells in series and forming a network of fuel cells. The area of a single cell can vary from a few square centimeters to a thousand square centimeters. The area of each cell is considered to be 225 square centimeters. A network can also consist of several to several thousand cells connected in series using bipolar plates. Many networks are used in series or parallel for applications requiring high power. The production capacity of a fuel cell mass depends on various parameters defined by Eq. (6.86): Pstack 5 Ncell 3 Acell 3 icell 3 Vcell

(6.86)

where N is the number of cells used, A is the surface area of each cell, i is the current density, and V is the cell’s output voltage. The amount of fuel Utilization (Uf) in the fuel cell can be defined as the fuel consumption ratio to the input fuel per cell. The amount of fuel used in the fuel cell can be defined as the ratio of fuel consumed in the cell to the input fuel per cell. If the fuel consumed is only hydrogen, this number is defined as the ratio of hydrogen used to the hydrogen input to the cell. Now, if the fuel consumed is hydrocarbon fuel, this ratio is defined as follows: nH2 ;equivalent 5 nH2 1 nCO 1 4nCH4 1 7nC2 H6 1 . . . Ufuel 5 1 2

nH2 ;anode exhaust nH2 1 nCO 1 4nCH4 1 7nC2 H6 1 . . .

(6.87) (6.88)

This definition is based on the fact that each component is equivalent to several hydrogens. For example, methane and ethane gas produce 4 and 7 hydrogen molecules in steam reforming reactions, respectively. The value of the fuel utilization factor for SOFC in commercial applications ranges from 0.750.85. The fuel

252

Hybrid Poly-generation Energy Systems

required for the fuel cell system depends on the fuel cell mass flow rate, the relationship of which is given in Eq. (6.89). The important thing to consider is getting the fossil fuel entering the system. In this case, after obtaining hydrogen-equivalent fuel, we convert it to real fuel according to the relationship between hydrogenequivalent fuel and the molar component of hydrocarbon fuel. Some of the heat generated in SOFCs is removed by the excess air entering the cathode, which is defined as the ratio of excess air as follows: λO2 5


 nO2 cathode inlet moles of oxygen supplied with air
 5 moles of oxygen needed for stoichiometry 0:5 3 nH2 ;equivalent (6.89)

The amount of excess air entering the cell regulates the operating temperature of the cell. The allowable temperature change inside the fuel cell (outlet temperatureinlet temperature) is 100 C, which is adjusted by adding extra air to the cathode. The number related to fuel consumption is also defined as the ratio of oxygen consumed in the cell to the oxygen input to the cell. Assumptions used in fuel cell simulation include: 1. The one-dimensional model of the fuel cell is considered, based on which the distribution of component concentration and output voltage of the cell is obtained, and we have omitted the temperature distribution along the cell. 2. The cell’s performance is based on the desired nominal temperature for the fuel cell, and we exclude the temperature distribution throughout the cell. 3. The fuel cell is simulated based on stable conditions. 4. The fuel cell anode is simulated with a plug reactor in which the electrochemical reaction, the methane reforming reaction with water vapor, and the WGS reaction occur simultaneously. 5. The cathode is simulated by a Gibbs reactor. 6. The molar composition of the air entering the cathode is 79% N2 and 21% O2. 7. The total heat lost in the fuel cell and combustion chamber is 3% of the system input energy.

6.3.1.5.3 Voltage calculation According to the previous chapters’ contents, it is necessary to calculate the fuel cell voltage, calculate the ideal thermodynamic voltage, and triple the voltage drop. Using the Gibbs free energy thermodynamic relationships and entropy, the ideal thermodynamic potential is obtained from Eq. (6.90). E 5 1:18 2 0:06855ðτ 2 1Þ 2 0:0165 3 ðτlnðτ Þ 2 τ Þ 2

RT PH 2 O ln nF PO2 0:5 3 PH2 (6.90)

T τ 5 298:15 In Eq. 4.23, T is the temperature of the fuel cell, PH2 O, PH2 , and PO2 are the partial pressure of water vapor in the anode gas mass, the partial pressure of hydrogen

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in the anode gas mass, and the partial pressure of oxygen in the cathode gas mass, respectively. Also, R is the gas constant equal to 8.314 J/mole/K, and F is the Faraday constant equal to 96485 C/mol. Losses due to ionic and electron conduction are also obtained from Eq. (6.91). ηOhmic 5 ic 3

X
21 σj 3 δj j

(6.91)

In Eq. 4.24, δ and σ are, respectively, the thickness and ionic conductivity (ionic conductivity unit 1/Ω.m) of each component (cathode, anode, electrolyte, and connector). Activation losses from Eq. 4.25 and the exchanged currents’ density values at the cathode and anode, Eqs. (6.92) and (6.93), respectively, are used.     2RT ia 2RT ic 21 21 3 sinh 3 sinh ηact 5 1 nF nF 2i0;an 2i0;cat i0;cat 5 γ cat 3

  RT Eact;cat 3 exp 2 nF RT

  RT Eact;an 3 exp 2 i0;an 5 γan 3 nF RT

(6.92)

(6.93)

(6.94)

In the above relations, Eact,cat, and Eact,an are the cathode and anode activation energies, respectively, and γcat and γan are the cathode and anode prediction coefficients. Information and parameters related to SOFC are reported in Table 6.10. The concentrations of hydrogen, water, and oxygen at the anode-electrolyte and cathode-electrolyte interface must be calculated to calculate the voltage drop associated with mass transfer losses. The relationship between the voltage drop due to mass transfer is ! ! PH2 O P0H2 P0O2 RT RT 3 ln  0 3 ln  ηcon 5 1 2F 4F PO2 PH2 PH2 O

(6.95)

In Eq. 4.28, P H2 , P H2 O, and P O2 , respectively, are the partial pressures of hydrogen, water, and oxygen at the reaction site, which through Eqs. (6.96)(6.98) are calculated. PH2 5 P0H2 2

i3δ3R3T n 3 F 3 Deff H2

PH2 O 5 P0H2 O 1

i3δ3R3T n 3 F 3 Deff H2 O

(6.96)

(6.97)

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Hybrid Poly-generation Energy Systems

Table 6.10 Fuel cell requirements parameters [96]. Parameter for SOFC Anode Thickness (δ A ) Average pore radius (rp) Average particle diameter (dp) Specific area (As) Porosity (ε) Tortuosity (τ) Charge-transfer coefficient (α) Electrolyte Thickness ðδE ) Cathode Thickness (δ C ) Average pore radius (rp) Average particle diameter (dp) Porosity (ε) Tortuosity (τ) Charge-transfer coefficient (α) Parameters for exchange current density Pre-exponential factor for anode (γ an) Activation energy for anode (Eact,an) Pre-exponential factor for cathode (γcat) Activation energy for cathode (Eact,cat) Condition Fuel cell temperature Operating pressure Fuel utilization Active cell area Number of cells

PO2 5 P0O2 2

i3δ3R3T 2 3 n 3 F 3 Deff O2

Value

Unit

0.5 0.5 2.5 1025 0.35 3.8 0.5

mm μm μm -

25

μm

30 0.5 2.5 0.48 5.4 0.5

μm μm μm -

6.54e11 140 2.35e11 137

A=m2 kJ=mol A=m2 kJ=mol

11001200 120130 0.85 250 5965

K kPa cm2 -

(6.98)

In the above relations, Dieff is the effective penetration coefficient of substance i at the anode and cathode obtained from Eq. (6.99). Deff i 5

  ε 1 1 21 1 τ Di;M Di;K

(6.99)

In Eq. (6.99), ε represents the electrodes’ porosity, and τ represents the curvature or curvature of the endo-cathode. As in this connection, diffusion into porous materials includes molecular diffusion and Knudsen diffusion. Molecular penetration

Technical and economic prospects of fuel cells combination with polygeneration systems?

255

becomes more critical when the pores are larger than the average free distance between the molecules. Suppose the diameter of the cavities is less than the average free distance of the molecules. In that case, the Knudson penetration becomes more critical and must be considered in the calculations. In fact, in the Knudsen penetration, the molecules collide more with the cavity walls. The Knudsen penetration coefficient of each component is obtained from Eq. (6.100) [97]. pffiffiffiffiffiffiffiffiffiffiffiffi Di;K 5 4850 3 dpore 3 T=MA (6.100) In Eq. (6.100), the dpore of the hole diameter in the anode or cathode structure in centimeters, T represents the cell temperature in Kelvin and MA represents the molecular mass of each component. The Wilke equation calculates the diffusion coefficient values in a multi-component gas mixture [97]. Wilke’s relation is given in Eq. (6.101). Di;M 5

1 2 xi x Σnj6¼i Djij

(6.101)

In Eq. (6.101), Dij is a double diffusion coefficient. Using EnskogChapman’s theory, the two-component diffusion coefficient inside the gas mass is obtained through the relation (6.102). 3

DAB 5 0:0018583

T2 0:5 P 3 MAB 3 σ2AB 3 ΩD

(6.102)

In Eq. (6.102), P is the total cell pressure in terms of atmosphere. Also, MAB is the average molecular mass, εAB is the intermolecular force, σAB is the collision diameter in angstroms, and Ω D is the collision diffusion integral presented in the relations (6.103) to (6.107), respectively. 

1 1 MAB 5 2 1 MA MB

21 (6.103)

εAB 5

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi εA 3 εB

(6.104)

σAB 5

σA 1 σB 2

(6.105)

2B

ΩD 5 AT  1 C 3 expð 2 DT  Þ 1 E 3 expð 2 FT  Þ 1 G 3 expð 2 HT  Þ (6.106) The equations of Eq. (6.106) (Ω D) are presented in Table 6.11, and the value of T is calculated from Eq. (6.107) (Table 6.12). T 5

k T εAB

(6.107)

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Hybrid Poly-generation Energy Systems

Table 6.11 Ω D [97]. Coefficient

Value

A B C D E F G H

1.06036 0.1561 0.193 0.47635 1.0.3587 1.52996 1.76474 3.89411

Table 6.12 Voltage drop parameters [96,97]. Cell component

A1

A2

Electrolyte Anode Cathode Interconnect

3340 95e4 42e4 9.3e4

10300 1150 1200 1100

P ηohmic 5 ic 3 j σj 21 3 δj
 σj 5 A1 exp 2A2 =T

where K is the Boltzmann constant. Therefore the fuel cell voltage is calculated using Eq. (6.108). V 5 E 2 ðηOhmic 1 ηact 1 ηcon Þ

(6.108)

The dimensions and conditions of the SOFC used [92] are summarized in Table 6.13. According to Fig. 6.30, it can be seen that the results of the model are consistent with the laboratory results. Fig. 6.30 shows that this consistency is higher at high temperatures.

6.3.1.5.4 Increase the efficiency of SOFC system One way to increase the efficiency of the Anode Gas Recycle (AGR) is the fuel cell for pre-forming the gas. With this operation, the required steam is provided in the prereformer, and there is no need to produce steam for the prereformer reactor. Advantages of AGR include: (1) no need to produce steam for the prereformer; (2) reduction of the number of fuel cells due to reduction of fuel consumption number; (3) reduction of steam in the exhaust gases of the fuel cell system, which leads to increased efficiency. A blower, fan, or injector also performs AGR. The return flow ratio also depends on the steam-to-carbon ratio (S/C) required in the preheater, which is obtained from the following equation. S ðH2 OÞAGR 5 C ðCOÞAGR 1 ðCOÞfuel 1 CH4 1 2C2 H6 1 . . .

(6.109)

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Table 6.13 Parameters required to validate the electrochemical [92]. Parameter

Value

Unit

Anode thickness Cathode thickness Electrolyte thickness Fuel/Airstream inlet pressure Cell mean temperature Porosity Tortuosity Pore diameter Fuel Air

1000 20 8 1 8731073 0.48 5.4 1 0.97% H2, 0.03% H2O 0.21% O2, 0.79% N2

μm μm μm bar  C

μm

One way to increase the system’s efficiency is to return some of the Cathode Gas Recycle (CGR) to preheat the inlet air to the cell. This reduces the size of the air preheater and blower. One of the disadvantages of this work is the reduction of oxygen concentration in the fuel cell cathode, which leads to a decrease in fuel cell performance. How heat recovery is also essential and how and where the heat treatment is performed must be achieved by balancing system efficiency with unit cost.

6.3.1.5.5 Parameters of other components of SOFC system Other components of a SOFC system include compressors, blowers, pumps, and heat exchangers. The parameters for these components are given in Table 6.14. It should be noted that these parameters have been corrected in the final simulation, and an attempt has been made to be closer to the experimental results. One of the parameters that indicate the effectiveness of the heat exchanger is the effective number of the exchanger, which is defined as ε5

qactual qmax

(6.110)

In Eq. (6.110), qactual represents the heat exchanged in the heat exchanger and qmax represents the maximum heat exchanged when the temperature of the hot inlet gases reaches the temperature of the inlet cold fluid. Assumptions about other components of the fuel cell system include the following: 1. The heat losses of the fuel cell system components have been ignored, and the total heat losses have been concentrated in the combustion chamber and the fuel cell. 2. The prereformer reactor is simulated by a plug reactor that contains information about the kinetics of methane reforming reactions with steam and gas-water conversion. 3. The combustion chamber is simulated by a conversion reactor, which includes combustion reactions of hydrogen, carbon monoxide, and methane. 4. The compressor simulates the required blowers to compress the air and return the anode and cathode current.

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Hybrid Poly-generation Energy Systems

Table 6.14 Fuel cell parameters required for simulation [90,98,99]. Other parameters in PFD

Value

Unit

Temperature for all input stream Ambient pressure Adiabatic compressors efficiency Pump efficiency Blower efficiency DCAC inverter efficiency Pre-reformer conversion Cathode gas recycle ratio S/C in prereformer Anode gas recycle ratio Inlet anode gas temperature Temperature increment in fuel cell Pressure drop in fuel cell Pressure drop in prereformer Conversion in combustor

25 101.3 0.7 0.75 0.60.65 0.92 25%30% 0.50.7 2 0.50.65 870 90110 3 3 100(%)



C kpa

ðmol=molÞ 

C C kPa kPa 

6.3.1.6 SOEC Fig. 6.31 shows the process of hydrogen production using electrolysis. The electrolyzer model includes a conversion reactor for the electrolysis reaction, a component splitter for hydrogen flow separation (cathode electrode), oxygen (anode electrode), material and energy flow lines, and four actuators to regulate temperature, pressure, and reaction rate. The conversion reactor in the simulation uses a stoichiometric equation to show the reaction of decomposition of water (in liquid or vapor state), and the flow of each product after the reaction is determined using the conversion percentage. Fig. 6.32 shows the concept of high-temperature steam electrolysis using SOEC. In this section, the information required for modeling a solid oxide electrolyzer is first discussed, and then the hypotheses and parameters related to the system components are presented.

6.3.1.6.1 High-temperature electrolyzer model Thermodynamically, a SOEC involves the reaction of a SOFC. SOEC reactions, as mentioned earlier, are in the form of Eqs. (6.111) and (6.112), and the general equation of water refraction is in the form of (6.113). Cathode: H2 OðgÞ 1 2e2 ! H2 ðgÞ 1 O22

(6.111)

Anode: O22 ! 1=2O2 ðgÞ 1 2e2

(6.112)

H2 OðgÞ ! H2 ðgÞ 1 1=2O2 ðgÞ

(6.113)

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259

H2 producon

Cathode (H2,H2O) H2O,H2 input

H2O/H2 Separator

Split energy

T

Sweep gas input

H2O Ext

Electrolysis Process heat

Anode N2,O2)

Electrolysis power

Figure 6.31 Overview of the electrolysis process for hydrogen production [79].

Nitrogen (N2) Hydrogen (H2)

Electric Furnace Pump (HPLC)

Sand bath (Heating)

Cathode

Heating band

Hydrogen (H2)

SOEC Stack Anode

MFC

Condenser (Separator)

Heating band

Water (H2O)

MFC

Nitrogen (N2) Water (H2O)

Sweep Gas (N2+O2)

Sweep Gas +O2

MFC

+ DAS & Control

-

DC Power Supply

Figure 6.32 Concept of high-temperature steam electrolysis using SOEC [79].

Because the operating temperature of the solid oxide electrolyzer system is high and in the range of 700 C1000 C, water enters the system in a vapor state. As can be seen in Fig. 6.33, the total energy required to electrolyze water vapor at high temperatures can be considered a function of Gibbs free energy and heat energy. Eq. (6.112) shows this relationship: ΔH 5 ΔG 1 TΔS

(6.114)

Table 6.15 shows the standard thermodynamic parameters for high-temperature electrolysis at atmospheric pressure and temperature of 298 K [79,100]. Of course,

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Hybrid Poly-generation Energy Systems

Figure 6.33 The total energy required to produce hydrogen [79].

other equations for specific heat are presented as a function of temperature [101]. The energy required for electrolysis is shown in Fig. 6.33, showing the total energy required by ΔH, ΔG for electrical energy, and TΔS for thermal energy. The minimum electrical energy for electrolysis at 850 C is approximately 25% less than at ambient temperature. High-temperature electrolysis would be theoretically very efficient if TΔS could be sourced from other energy sources. According to this diagram, the required electrical energy decreases as the temperature increases. On the other hand, providing thermal energy is much easier and less expensive than electricity. The electrical and thermal energy required by the electrolyzer is supplied from the fuel cell. The chemical equilibrium of the electrolysis reaction is written as Eq. (6.114). Gibbs free energy in the decomposition reaction of water is a function of temperature and pressure, which equates (6.115). 1

fH2 fO 2 2 ΔGðT; PÞ 5 ΔGf ðT; PÞ 1 RTln fH2 O

! (6.115)

In relation (6.115), f is the molar fraction of the components, R is the constant of gases (8.314 J/mole.kg), T is the operating temperature of the system (K), and ΔGF(T,P) is Gibbs free energy difference of products and reactants It is known in temperature and pressure as shown in Eq. (6.116). ΔGf ðT; PÞ 5 Gf 2H2 ðT; PÞ 1 1=2Gf 2O2 ðT; PÞ 2 Gf 2H2 O ðT; PÞ

(6.116)

Table 6.15 Thermodynamic data for high-temperature steam electrolysis (atm 1 pressure and 298.15 K temperature).     Cp0
Substance (state) ΔH ðkJ=moleÞ ΔG ðkJ=moleÞ S ðkJ=mole:KÞ R J=mole:K a0

a1

a2

a3

a4 2 2.3e-12 1.79e-12  Cp0
R H2Oliquid 5 75:35 6.32e-12

H2 (gas) O2 (gas)

0 0

0 0

0.131 0.205

2.883 3.63 H2 O (liq)

3.68e-3 2 1.794e-3 237.2

2 7.72e-6 6.58e-6 285.8

6.92e-9 2 6.01e-12 0.0699

H2 O (gas)

228.9

241.8

0.188

4.395

2 4.186e-3

1.405e-5

2 1.564e-8

Cp0 R

5 a0 1 a1 T 1 a2 T 2 1 a3 T 3 1 a4 T 4

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Hybrid Poly-generation Energy Systems 

ΔGF(T,P) is written in terms of ΔG F(T,P)-GF(T,Pstd) and the standard pressure is considered to be 0.101 MPa. The Gibbs free energy variation with the electrical voltage required for electrolysis is calculated from Eq. (6.117). 21  ΔGf ðT; PÞ 2 RTln Vnernst 5 2F

!

fH2 O 1

fH2 fO 2 2

P

212 !!

PSTD

(6.117)

In the above relation, the standard pressure of 0.101 MPa is considered. P is the cell or mass pressure and F is the Faraday constant (96485 C/mole). The cell voltage is obtained from Eq. (6.118). Vcell 5 Vnernst 1 i:ASR

(6.118)

In Eq. 4.60, i is the current density (A/cm2) and ASR is the Area Specific Resistance (Ω.cm2). The electrical work done in the cell is also calculated from Eq. 4.60. WEL 5 VOP :I 5 VOP :i:A

(6.119)

In Eq. 4.61, I is the current in terms of amps and A is the area of the cell (or mass) (cm2). The energy balance in the electrolyzer model is written as Eq. (6.120). X i

n_ P2i HP2i ðTP ; PÞ 5

X

n_ R2i HR2i ðTR ; PÞ 1 QHeat 1 WEL

(6.120)

i

In this relation n is the molar flow rate of the components (kgmole/s), H is the enthalpy per mole (kJ/kmole), QHeat is the heat energy transferred from outside the cell to the electrolyzer (kW), T is the temperature ( C), and P is the pressure (kPa). R is a subtitle for reactants and P is for products. The required energy of the electrolyzer model is obtained according to Eq. (6.119). The sum of external and internal energy values must be in balance. This is done through operator and spreadsheet calculations.

QHeat 5 n_ H2 O HH2 O ðT; PÞ1 n_ H2 HH2 ðT; PÞ1 n_ O2 HO2 ðT; PÞ outlet

2 n_ H2 O HH2 O ðT; PÞ1 n_ H2 HH2 ðT; PÞ1 n_ O2 HO2 ðT; PÞ inlet 2 WEL

(6.121)

The synthetic reaction is the opposite of the relationships presented in the electrochemical reaction section of the SOFC [73]. Table 6.16 presents the required parameters of the SOEC, Table 6.17 presents the parameters required to validate the electrochemical model, and Fig. 6.34 compares the model and reference result [73].

Technical and economic prospects of fuel cells combination with polygeneration systems?

263

Table 6.16 Requirements for electrolyzer [73,79]. Parameter for SOEC Anode Thickness (δ A ) Average pore radius (rp) Porosity (ε) Tortuosity (τ) Charge-transfer coefficient (α) Electrolyte Thickness ðδ E ) Cathode Thickness (δ C ) Average pore radius (rp) Average particle diameter (dp) Porosity (ε) Tortuosity (τ) Charge-transfer coefficient (α) Parameters for exchange current density Pre-exponential factor for anode (γ an) Activation energy for anode (Eact,an) Pre-exponential factor for cathode (γcat) Activation energy for cathode (Eact,cat) Condition Electrolyze cell temperature Operating pressure Active cell area Current density ASR Number of cell

Value

Unit

100 1.07 0.48 5.4 0.5

μm μm   

100

μm

500 1.07 1.5 0.48 5.4 0.5

μm μm μm   

2.051e9 120 1.334e10 100

A=m2 kJ=mol A=m2 kJ=mol

11001200 120130 225 0.8 0.25 895

K kPa cm2 A:cm2 Ω:cm2 

Table 6.17 Parameters required for validation [73]. Parameter

Value

Unit

Anode thickness Cathode thickness Electrolyte thickness Fuel/Airstream inlet pressure Cell mean temperature Porosity Tortuosity Pore diameter Fuel Air

100 100 1000 1 8731073 0.48 5.4 1.07 0.4H2, 0.6H2O 0.21O2, 0.79N2

μm μm μm bar  C

μm

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Hybrid Poly-generation Energy Systems

2.2

2

Cell voltage, (V)

1.8

1.6

1.4

T=1273 K model datas

1.2

T=1223 K T=1173 K T=1273 K

experimental datas

1

T=1223 K T=1173 K

0.8 0

1000

2000

3000

4000

5000

6000

7000

Current density, (A/m2) Figure 6.34 Electrolyzer model validation [73].

6.3.1.6.2 Solid oxide electrolyzer mass As in the SOFC section, the cell mass is used to increase the electricity produced, in this section, to improve the amount of hydrogen produced, a number of electrolyzers can be transferred to each other in series. Eq. (6.85) is also true in this section. The area of a single electrolyzer can vary. The area of each electro-laser cell is 225 square centimeters. Input parameters to the electrolyzer mass are output temperature, pressure, and molar component of input compounds, current density, and active surface. The output of the cell is the voltage, the required input power, and the molar percentage of the output components.

6.3.1.6.3 Assumptions and information used SOECs can also be used for the simultaneous electrolysis of carbon monoxide and water. The air is also used as a sweeping gas and to regulate the temperature of the exhaust gases from the electrolyzer. Table 6.18 shows the parameters and values used in the electrolyzer simulation. The simulations of solid oxide electrolyzer simulations are as follows: 1. A one-dimensional model of a solid oxide electrolyzer is considered and the temperature distribution along the cell is omitted. 2. Simulation conditions are isothermal and isobar electrolyzer systems. 3. The electrolyzer cell is shielded in stable conditions.

Technical and economic prospects of fuel cells combination with polygeneration systems?

265

Table 6.18 Solid oxide electrolyzer parameters for simulation [79,98]. Other parameters in PFD

Value

Unit

Temperature for all input stream Ambient pressure Adiabatic compressors efficiency Blower efficiency Inlet cathode gas temperature Outlet cathode & anode gas temperature Pressure drop in electrolyzer

25 101.3 0.7 0.60.65 800 800 0



C kPa 

C C kPa 

4. Reaction and conversion reactor is used to perform the electrochemical reaction of this section. 5. The input current to the electrolyte is pure water. 6. The molar component of the air entering the electrolyzer is considered as sweep gas, N2 79% and O2 21%.

Overall efficiency of a hybrid system with fuel cell. This subsection presents different efficiency equations for evaluating a combined or hybrid system. The overall efficiency of the system is an essential indicator and is typically defined as Eqs. (6.122) and (6.123). ηOveral;LHV 5


Wnet 1 QRecover heat  mfuel;in 3 LHVfuel system inlet

(6.122)

Wnet 1 QRecover heat  mfuel;in 3 HHVfuel system inlet

(6.123)

ηOveral;HHV 5


If we affect the recovered heat in the efficiency equations, we will have Eqs. (6.124) and (6.125). W  net ηNet Overal;LHV 5
mfuel;in 3 LHVfuel system inlet 2 QRecover heat ηNet Overal;HHV 5


mfuel;in 3 HHVfuel

W  net system inlet

2 QRecover heat

(6.124)

(6.125)

Electrical efficiency is also defined by Eqs. (6.126) and (6.127). ηelec;LHV 5


Wnet  mfuel;in 3 LHVfuel system inlet

(6.126)

Wnet  mfuel;in 3 HHVfuel system inlet

(6.127)

ηelec;HHV 5


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Hybrid Poly-generation Energy Systems

The efficiency of the fuel cell based on the calorific value of the fuel entering the anode is ηSOFC;HHV 5


WDC  mfuel;in 3 HHVfuel anode inlet

(6.128)

The performance of the electrolyzer system is also obtained from Eq. (6.129). ηSOEC;LHV 5

LHVH2 3 nH2;Out Qelectric 1 Qheat;SOEC 1 Qheat;H2O

(6.129)

Concerning 129 nH2 ,Out flow rates of hydrogen and Qelectric and Qheat,SOEC, respectively, the electrical energy and thermal energy input to the solid oxide electrolyzer and Qheat,H2 O the ratio of the input thermal energy to the heat required to heat the incoming water stream. According to the second law of thermodynamics, there are other efficiencies, which we will discuss in the exergy section. Another suitable parameter to evaluate the system’s performance is the ratio of heat recovered to the electricity generated in the system, which is called the Thermal to Electrical Ratio (TER) and is obtained from Eq. (6.130). TER 5

QREcover heat Wnet

(6.130)

6.3.1.7 Investigation of SOFC performance and its general system The designed SOFC system is presented in Fig. 6.35. 25%30% of the natural gas is converted to a prereformer before entering the fuel cell to prevent a sudden drop in temperature at the beginning of the fuel cell, and the rest of the natural gas is sent to the fuel cell for internal reforming. The steam required for the process is supplied using the heat of the exhaust gases from the system. To improve system efficiency, some of the exhaust air from the cathode is returned and mixed with the inlet air to the cathode. The values for the system inputs are given in Table 6.19. The effect of different parameters on the performance of SOFCs is investigated and its results are presented in the combined cycle. According to the inputs of Table 6.19, the results obtained from the fuel cell performance are presented in Table 6.20. One of the critical parameters in a hybrid system is the type of energy requirement. For example, the type of energy requirement for using the system in the home or industrial sector is different. Therefore fuel cell examination should be considered based on the type of energy required and economic conditions. In addition, the type of system and its operations significantly impact system performance. As mentioned, steam methane reforming is one of the best energy efficiency and economic cost methods. The choice of this reaction, due to the possibility of doing part of it inside the fuel cell, has a positive effect on increasing the efficiency of the

Technical and economic prospects of fuel cells combination with polygeneration systems?

267

Figure 6.35 Solid oxide fuel cell system [102].

Table 6.19 Input data to fuel cell for simulation. Input parameter

Value

Cell properties Fuel inlet temperature Air inlet temperature Amount of methane conversion Steam to carbon ratio Fuel utilization Cathode gas recycle Nominal cell temperature Air gas inlet composition Cell current density

Tables 6.8, 6.10, 6.11 and 6.12 827 827 2530 2 0.85 0.65 1100 0.21 O2, 0.79 N2 0.8

Unit 

C C % 

K A=cm2

cell. Details of the designed system, such as the AGR, CGR, and how to recover heat, etc., are discussed in this section. Fig. 6.35 shows the details of a natural gas fuel cell system, which includes an anode-reinforced fuel cell mass, a prereformer, and a steam generator for fuel reforming. Fuel and air are compressed to a pressure of 1.2 times. About 25% 30% of the fuel in the preheater is converted to hydrogen before entering the cell. The air entering the fuel cell is preheated to a temperature of 780 C and enters the

268

Hybrid Poly-generation Energy Systems

Table 6.20 Fuel cell output results. Output parameter

Value

Unit

Fuel feed Air feed Anode inlet composition Cell Power Cell voltage Cell efficiency Exhaust gas temperature

1.845 20.73 0.08C1,0.22H2,0.27H2O 0.21CO2,0.03CO,0.184O2,0.004N2 860 0.716 0.4728 580

mol=s mol=s

kW V 

C

battery to some extent to regulate the temperature of the fuel cell. The rest of the fuel is converted to hydrogen inside the fuel cell anode (during internal reforming). Inlet water is also used to initial run the system. The direct power generated is 863 kW, produced at an average voltage of 0.7192 volts and an average current density of 0.8 amps per square centimeter. Direct current is converted to alternating current with an efficiency of 0.92, and then after supplying the required power to the unit, the net power with a value of 618 kW is removed from the system. Fuel cell efficiency is 47.49%. In a system that uses hydrocarbon fuels. Internal fuel reforming reduces blower consumption. Because the reforming reaction is endothermic, part of the heat generated by the fuel cell is consumed by this reaction, and as a result, less air is needed to cool the cell, which reduces the power consumption of the air blower. As can be seen in Fig. 6.35, AGR is also used. The return of part of the anode current provides the steam needed by the prereformer and there is no need to produce steam through the boiler. This both increases the thermal efficiency and eliminates the cost of the boiler. AGR reduces the air entering the system and its components’ size. Another advantage is the reduction of water vapor leaving the fuel cell system, which increases system efficiency by increasing the available thermal energy in the exhaust gases. Reducing the size of the thermal gradient of the cell (due to uniform current density) and reducing the formation of solid carbon (due to the presence of carbon dioxide) are other advantages of this method. However, one of the disadvantages of this method is the reduction of catalyst activity used in the preheater, which increases the amount of catalyst in the preheater. It has also been observed that this method reduces methane conversion rate at the fuel cell anode. The AGR value is considered 0.65. The amount of air entering the system also plays an essential role in regulating the temperature of the fuel cell mass and providing the required heat to the process. For this reason, the amount of incoming air has been determined using Adjust. The amount of internal reforming and the allowable increase in cell temperature depends on the required thermal conditions and temperature limits. The heat load and the size of the air preheater, the amount of heat recovered, the efficiency of the

Technical and economic prospects of fuel cells combination with polygeneration systems?

269

whole system, the outlet temperature of the combustion chamber, etc., all depend on the amount of air entering the system. The use of GCR is also suitable for improving system performance. The purpose of GCR is to reduce excess air in the system and reduce the size of the air preheater. Reversing the cathode current significantly increases the system’s electrical efficiency and overall efficiency. Disadvantages of this method include reduced cell performance due to reduced oxygen concentration at the fuel cell cathode. Of course, this performance loss is compensated by adjusting (reducing) the inlet air to the system. Another important parameter is the allowable temperature increase along the fuel cell channel. As this value increases, it significantly reduces the air entering the cell and the power consumption of the air blower. Therefore along with increasing the efficiency of the system, the size of the exchanger and air blower decreases.

6.3.1.7.1 Various SOFC parameters Fig. 6.36 shows the performance of a SOFC in terms of average current density. This diagram is based on the constant level and temperature of the cell. As the current density increases, the cell voltage decreases due to irreversible losses. Graph of power changes with increasing current density, due to inverse voltage changes with current density, has a maximum point. In fact, as the current density increases, the power reaches a maximum and then decreases. The maximum power value is 1.3001 MW at a current density of 2.1 A/cm2. For a current density of 0.8 A/cm2, power, and voltage equal 863 kW and 0.719 V, respectively. Fig. 6.37 shows the changes in fuel consumption and inlet air of the cell in terms of current density, and assuming the surface is constant, the nominal temperature and number of cells are plotted. As can be seen in the figure, increasing the current 1

1.4

0.9

1.2 1

Voltage, (V)

0.7 0.6

0.8

0.5 0.6

0.4 0.3

0.4

Power, (W*10^-6)

0.8

0.2 Voltage 0.1

0.2

Power

0

0 0

0.5

1

1.5

2

2.5

3

Current density, (A/cm^2) Figure 6.36 Graph of fuel cell voltage and power changes in terms of current density [103].

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Hybrid Poly-generation Energy Systems

Figure 6.37 Graph of changes in fuel consumption and inlet air of fuel cell in terms of flow density [103].

density increases the air required by the cell. As the current density increases, the voltage decreases due to irreversible drops, and as a result, the cell efficiency decreases and a smaller part of the generated heat are converted to electricity by an electrochemical reaction, and the rest as free heat. Therefore the air required to remove heat from the cell and cool it increases. Input fuel also increases with increasing current density due to the increase in hydrogen equivalent to input fuel. Fig. 6.38 shows the changes in cell output power in terms of voltage and is plotted with the assumptions of Fig. 6.37. As the current density increases, the dissipation increases, and the voltage decreases. Because power is a function of current density and voltage, the output power decreases after reaching the maximum value. At this point, the voltage is 0.45 volts. Figure 5.5 shows the amount of hydrogen consumed in the cell with voltage changes and is plotted to assume a constant level. As shown in this diagram, with increasing voltage, the hydrogen consumption in the cell decreases due to the decrease in current density and the rate of progress of the electrochemical reaction (Fig. 6.39). Fig. 6.40 shows the effect of the fuel consumption factor on the current density of a SOFC at 1100 K. Fig. 6.40 shows that the voltage decreases with increasing fuel consumption at a given current density. As fuel consumption increases, the losses due to activation and concentration drop increase, which leads to a decrease in actual voltage and cell performance, thus increasing current density. In fact, temperature and fuel consumption are fundamental parameters for SOFC systems. The higher the cell temperature, the higher the cell efficiency and, consequently, the economic costs. As for the fuel consumption factor, the higher the value, the higher the cell efficiency. However, in consumption factors higher than 0.85, the concentration drop is more severe, and the efficiency decreases. In addition, an excessive increase in the number of fuel uses leads to more heat production in the cell, which increases the need for cooling air. As mentioned earlier, increasing the air consumption increases the power consumption of the blowers and reduces the efficiency of the combined cycle.

Technical and economic prospects of fuel cells combination with polygeneration systems?

271

1.4 1.2

Power, (MW)

1 0.8 0.6 0.4 0.2 0 0.2

0.4

0.6

0.8

1

Voltage, (V) Figure 6.38 Graph of fuel cell output power in terms of voltage [103].

H2 consumed Molar Flow Rate, (mol/s)

25

20

15

10

5

0 0.2

0.4

0.6

0.8

1

Voltage, (V) Figure 6.39 Diagram of changes in hydrogen consumption by voltage [103].

The changes in production power in terms of current density in different fuel consumption factors are investigated in Fig. 6.41. As mentioned, with increasing the amount of fuel consumption factor at a certain current density, the voltage decreases and the cell performance decreases due to the increase of activation losses and voltage concentration. In fact, a SOFC requires a higher current density by increasing the fuel consumption factor to achieve the same performance. Fig. 6.42 shows the changes in the required molar flow rate of natural gas and cell air consumption in fuel consumption factors of 0.75, 0.85, and 0.95 with

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Hybrid Poly-generation Energy Systems

0.9 Uf=0.75 Uf=0.85

0.8

Uf=0.95

Voltage, (V)

0.7

0.6

0.5

0.4

0.3 0

0.5

1

1.5

2

2.5

3

Current Density, (A/cm^2) Figure 6.40 Diagram of the effect of fuel consumption factor on voltage in terms of current density [103].

1.6 1.4

Power. (MW)

1.2 1 0.8 0.6 0.4 Uf=0.75 Uf=0.85 Uf=0.95

0.2 0 0

0.5

1

1.5

2

2.5

3

Current Density, (A/cm^2) Figure 6.41 Graph of changes in production power in terms of current density in different fuel consumption factors [103].

3.6

34

3.4

33

3.2

32

3 31 2.8 30 2.6

Uf=0.75 Uf=0.85 Uf=0.95 Uf=0.75 Uf=0.85 Uf=0.95

2.4 2.2

NG 29

Air

273

Air Molar Flow Rate, (mole/s)

NG molar Flow Rate, (mol/s)

Technical and economic prospects of fuel cells combination with polygeneration systems?

28

2

27 0.2

0.4

0.6

0.8

1

Voltage, (V)

Figure 6.42 Diagram of changes in fuel requirements and air consumption with voltage in different fuel consumption factors [103].

voltage. This diagram is drawn assuming constant temperature, production power, surface, and number of cells. As mentioned, increasing the voltage reduces the required fuel and air due to a decrease in current density. What is remarkable in this figure is the decrease in the molar flow rate of fuel and air by increasing the fuel consumption factor. In fact, by increasing the fuel consumption factor and the potential loss of the cell, a larger portion of the fuel energy is converted into electricity (compared to heat) and thus the cell’s performance is improved. But since this diagram is drawn assuming that the output power is constant, the need for input fuel is reduced. Also, as the fuel consumption factor increases at a constant voltage, the amount of inlet air required decreases slightly. The increase in fuel conversion to electricity and the decrease in heat output at a constant total output power justifies the reduction in required air. In fact, less heat at the fuel cell outlet reduces the need for air to cool the fuel cell. It is clear from the diagram that this is somewhat more pronounced at higher voltages, but still, this change is very small, probably due to the low air consumption factor. The air consumption factor is 0.15, which indicates the low effect of the fuel consumption factor on the incoming air. The effect of temperature on fuel cell performance is shown in Fig. 6.43. This figure is drawn assuming a fuel consumption factor of 0.85 and a constant level. As shown in this figure, the cell performance decreases sharply at low temperatures. In fact, at low temperatures, the resistance due to the passage of ions through the electrolyte increases and decreases the cell’s performance. Fig. 6.44 shows the temperature changes of the exhaust gases from the fuel cell mass and the combustion furnace section with the flow density in different fuel

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Hybrid Poly-generation Energy Systems

1.2

1600 1400

1

Voltage

T=900K T=1000K

Power

T=900K T=1000K

0.6

1000 800

T=1100K 600 0.4 400 0.2 200 0 0

5000

10000

15000

20000

25000

30000

0 35000

Current Density, (A/m^2) Figure 6.43 Diagram of the effect of temperature on fuel cell performance [103].

2100 1900

Temperature, ( ͦ C)

1700 1500 1300 1100

Uf=0.75

900

Uf=0.85

700

Uf=0.95

Exhust Cell gas Combuson outlet Exhust Cell Gas Combuson outlet Exhust Cell Gas Combuson outlet

500 0

0.5

1

1.5

2

2.5

3

Current Density, (A/cm^2) Figure 6.44 Diagram of temperature changes of exhaust gases from fuel cell mass and combustion chamber with flow density in different fuel consumption factors [103].

Power, (kW)

T=1100K

0.8

Voltage, (V)

1200

Technical and economic prospects of fuel cells combination with polygeneration systems?

275

0.7

1.4

0.6

1.2

0.5

1

0.4

0.8

0.3

0.6

0.2

0.4 Efficiency

0.1

Power, (MW)

Cell Efficiency,

consumption factors. As expected, as the current density increases due to reduced cell performance, the heat output of the cell increases and the combustion temperature increases. Since this figure is plotted on a constant amount of inlet air, the proposed temperature change makes sense. On the other hand, the temperature of the exhaust gas mass of the fuel cell depends on the temperature of the furnace, which is calculated based on the amount of fuel available in the combustion chamber. In the low fuel consumption factor, the access to fuel for the combustion chamber is higher, so the temperature rises. As the fuel consumption factor increases, more of the heat generated is converted to electricity (relative to heat) by an electrochemical reaction. As a result, less heat is applied to the exhaust gases and the outlet temperature of the combustion chamber decreases; consequently, the fuel cell mass output decreases (increases less). As mentioned, a SOFC should operate at a high fuel consumption factor, but this factor should not be so high that the concentration loss values increase. The appropriate values of fuel consumption factor reported in the references are 0.750.85. Fig. 6.45 shows the efficiency and production capacity of the cell in terms of current density. This figure is obtained by assuming the input fuel’s nominal temperature of the cell and its area to be constant. It has already been said that as the current density increases, the cell efficiency decreases due to the increase in irreversible losses, and as a result, less of the heat generated is converted to electricity. Also, with increasing current density, the voltage decreases and the output power decreases after reaching the maximum value. Fig. 6.46 shows the effect of SOFC voltage change on the total power of the whole system, direct power generation and power consumption in the fuel cell

0.2

Power 0

0 0

0.5

1

1.5

2

2.5

3

Current Density, (A/cm^2) Figure 6.45 Diagram of efficiency and cell production capacity in terms of current density [103].

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Hybrid Poly-generation Energy Systems

1400

94 W-net

1300

92 W-total;

W-total & W-net , (kW)

W-consumed

90

1100 88 1000 86 900 84 800

W-consumed, (kW)

1200

82

700

80

600

78

500 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Voltage, (V)

Figure 6.46 Graph of total power, net output power, and fuel cell power consumption in terms of voltage [103].

system. As the voltage decreases, the direct current produced increases and at a voltage of 0.419 V reaches a maximum of 1340.57 kW and then decreases. The maximum net power output of the system reaches the maximum value of 1251.35 kW by maintaining the trend of direct power changes at a voltage of 0.419 volts. At the maximum net power output of the system, the required power of the system is 89.22 kW. According to the system’s need for electricity, the system voltage is selected in the range of 0.60.8 volts. It is noteworthy that the diagrams in this section are drawn without considering the power consumption in the electrolyzer and the performance of the SOFC system is examined here alone. The effect of voltage on changes in electrical efficiency-LHV and the efficiency of the whole system can be seen in Fig. 6.47. It should be noted that this figure is drawn in variable power, so it has a maximum like the power diagram. As shown before, as the voltage decreases, the efficiency increases due to the increase in net power. At a voltage of about 0.43 volts, both efficiencies are maximal, and then the efficiency decreases with further voltage reduction. Low voltage indicates the operation of the cell at a high current density that will damage the cell. The point to be noted in system efficiency is that the economic value of the efficiency generated at high voltage is much higher than that obtained at low voltage. As can be seen in this figure, the maximum electrical efficiency and the whole systems are 72.5% and 54%, respectively.

Technical and economic prospects of fuel cells combination with polygeneration systems?

0.75

0.6

0.7

0.55

277

0.5 0.45 0.6 0.4 0.55 0.35

Electrocal efficiency,

Total efficiency

0.65

0.5 0.3 Total efficiency

0.45

0.25

Electrical efficiency

0.2

0.4 0.2

0.4

0.6

0.8

Voltage, (V)

Figure 6.47 Diagram of the effect of voltage on electrical efficiency and total LHV efficiency [103].

SOEC performance assessment. Fig. 6.48 shows a typical SOEC process. As can be seen, the electrolyzer model includes a conversion reactor, a material separator and actuators to regulate temperature, pressure, water vapor conversion rate, etc. The designed electrolyzer system consists of a cathode-supported electrolyzer mass that considers the air as a sweeping gas. At the cathode, pure water vapor enters, and at the anode, air enters at a temperature of 800 C and a single pressure, which contains 79% nitrogen and 21% oxygen, and removes the oxygen produced by the electrolysis of water vapor slowly. The amount of air entering the designed electrolyzer system plays an important role in regulating the temperature of the electrolyzer mass. The above settings are made in the software for the constant temperature operation of the electrolyzer. The input and output values of the electrolyzer system are presented in Table 6.21, respectively.

6.3.1.7.2 SOEC parameters Fig. 6.49 shows the changes in the voltages and the number of solid oxides electrolyzer mass cells with current density. The diagram is drawn in temperature, input power, and fixed cell surface. As can be seen in this diagram, as the current density increases, the cell voltage increases. In fact, the cell’s potential is a function of temperature, current density, and system losses. When the current density is zero, the cell voltage is equal to the Nernst voltage, which is not shown in this figure. The

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Hybrid Poly-generation Energy Systems

Figure 6.48 Electrolyzer cell [103].

Table 6.21 Table of input and output values of the electrolyzer cell. Parameter

Value

Cell properties Fuel inlet temperature Air inlet temperature Cell current density Water utilization Nominal cell temperature Feed gas composition Air gas inlet composition Cell voltage Cell power Cell number

Tables 6.136.15 800 800 0.8 0.85 1073 Pure water 0.21 O2, 0.79 N2 1.135 183 807

Unit 

C C A=cm2



K

V kW

voltage of Nernst solid oxide electrolyzer with a temperature of 1073 K is calculated as 0.9401 volts. On the other hand, a solid oxide electrolyzer works in the opposite direction of a SOFC, and as a result, the loss statements are formally summed to obtain the true voltage with the Nernst voltage. As a result, increasing the current density, which increases the voltage dissipation, increases the potential of the electrolyzer cell. On the other hand, due to the constant input power of the system, with increasing current density and voltage, the number of batteries required by the electrolyzer mass decreases. Carefully in this diagram, it is clear that at a current density of less than 0.4 A/cm2, the number of electrolyzer cells is significantly high, which is not economically justifiable. Also, in the current density higher than 0.83 A/cm2, no significant change in the number of cells is observed. Therefore the appropriate range of current density is 0.04.8 A/cm2.

1.2

12000

1

10000

0.8

8000

0.6

6000

0.4

4000 Voltage

0.2

279

Number of Cells in a stack

Voltage, (V)

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2000

N-cell 0

0 0

0.2

0.4

0.6

0.8

1

1.2

Current Density, (A/cm^2)

Figure 6.49 Diagram of voltage changes and number of electrolyzer mass cells in terms of current density [103].

Changes in the molar flow rate of hydrogen produced and the percentage of water vapor conversion entering the voltage in the electrolyzer are reported in Fig. 6.50. This figure is drawn with the assumptions of Fig. 6.49 and the number of cells in the electrolyzer mass varies. As the current density increases due to system losses, the voltage increases. Increasing losses means reducing the electrolyzer system’s performance and the conversion rate of the water electrolysis reaction. The lower the electrolysis of water, the lower the amount of hydrogen production. Fig. 6.51 shows the changes in voltage and number of cells relative to temperature. The input power, current density, and cell surface are considered constant to draw this diagram. As mentioned, the potential of a cell depends on its temperature. As the cell temperature increases, the system losses decrease, the cell performance improves, and the voltage decreases. Therefore the number of cells must be increased to keep the input power constant. In fact, at low temperatures, the resistance of the ion passing through the electrolyte increases and reduces the cell performance. As a result, with constant power and current density, decreasing voltage increases the number of electrolyzer cells. The effect of temperature on hydrogen production and water vapor conversion rate during the electrochemical reaction is investigated by the assumptions of Fig. 6.51 in Fig. 6.52. As the temperature of the cell improves with increasing temperature, the electrolysis reaction of water takes place more, increasing hydrogen production. The effect of voltage on the power and number of cells required for steam electrolysis is given in Fig. 6.53, assuming that the current density, cell area, and steam flow rate to the electrolyzer are constant. This diagram shows that with increasing voltage due to increased losses and conversion rate in the electrolysis reaction, the required power and the number of cells increase. As shown in this figure, increasing

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Hybrid Poly-generation Energy Systems

Figure 6.50 Diagram of the amount of hydrogen produced and the percentage of water vapor conversion in the electrolyzer in terms of voltage [103].

1.08

1260

1.06

1240

1.04

1220

Voltage, (v)

1180

1 Voltage

0.98

N-cell 0.96

1160 1140 1120

0.94

number of cell

1200

1.02

1100

0.92

1080

0.9

1060

0.88 700

800

900

1000

1100

1200

1040 1300

Temperature, ( ͦ C)

Figure 6.51 Diagram of voltage changes and number of cells in terms of temperature [103].

the potential after a voltage of 1.2 V has little effect on the power and number of cells required. Because the input steam flow rate is constant and the conversion rate is high. Therefore the amount of electricity required to convert water vapor to hydrogen and oxygen increases with a lower slope. Fig. 6.54 shows the effect of input power on water vapor conversion rate and hydrogen production. This diagram is drawn with the assumptions of Fig. 6.53 and shows the increase in the progress

1.16

0.58

1.14

0.57

281

0.56

1.12

0.55

1.1

0.54 1.08 0.53 1.06 0.52 1.04

H2O Conversion,

H2 Producon rate, (mol/s)

Technical and economic prospects of fuel cells combination with polygeneration systems?

0.51

1.02

0.5 H2 producon

1

H2O conversion 0.98 750

850

950

1050

0.49

0.48 1250

1150

Temperature, ( ͦC)

Figure 6.52 Diagram of the effect of temperature on hydrogen production and water vapor conversion percentage [103].

600

2500

500

2000

1500 300 1000

Number of Cell

Power (kW)

400

200 500

100

Power N-cell 0

0 0.9

1

1.1

1.2

1.3

1.4

Voltage, (V) Figure 6.53 Diagram of the effect of voltage on the power and number of cells required for electrolysis [103].

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Hybrid Poly-generation Energy Systems

2.5

1 0.9 0.8 0.7

1.5

0.6 0.5

1

0.4

H2O Conversion

H2 producon Rate, (mol/s)

2

0.3 0.5

0.2 H2 producon 0.1

H2O conversion 0

0 0

100

200

300

400

500

600

Power, (kW)

Figure 6.54 Diagram of the effect of input power on water vapor conversion rate and hydrogen production rate [103].

of the electrolysis reaction in proportion to the input power. The higher the rate of reaction, the higher the rate of hydrogen production, which is well understood in this diagram. Fig. 6.55 shows the changes in the water conversion rate and the hydrogen produces molar flow rate relative to the pure water vapor entering the electrolyzer. This diagram assumes the power, current density, and cell surface are constant. The diagram shows the percentage of water vapor conversion in the electrolysis reaction. As can be seen, due to the constant amount of input power, with increasing the molar flow rate of hydrogen, the rate of refraction of water to hydrogen and oxygen during the electrolysis process decreases. This is because, in this case, the voltage must be reduced to keep the power constant. However, the percentage of water conversion at higher flow rates decreases with a lower slope. According to Figure 521, the hydrogen produced increases with the increasing molar flow rate of incoming water vapor. The percentage of water-to-hydrogen conversion is decreasing, but the molar flow rate of hydrogen production is increasing. For example, according to the diagram, at 1 mol/s, 90 mol/s of electrolysis water is electrolyzed and produces 0.9 mol/s of hydrogen; at 2 mol/s, 50% of the incoming water is electrolyzed, which is equal to 1 mol/s. As can be seen, although the percentage of water conversion decreases, the molar flow rate of hydrogen produced increases. The effect of the molar flow rate of pure inlet water vapor on the potential and number of piles is shown in Fig. 6.56. This diagram is drawn with the assumptions of Fig. 6.55. The previous diagram shows that the conversion percentage decreases

Technical and economic prospects of fuel cells combination with polygeneration systems?

1

1.06

0.9

1.04

0.8

1

0.6 0.98 0.5 0.96 0.4 0.94

0.3

H2 Producon, (mol/s)

1.02

0.7

H2O Conversion

283

0.92

0.2 H2O conversion

0.1

0.9

H2 producon 0.88

0 0

1

2

3

4

5

6

Inlet H2O molar flow, (mol/s)

1.18

1140

1.16

1120

1.14

1100

1.12

1080

1.1 1.08

Voltage

1060

N-Cell

1040

1.06

1020

1.04

1000

1.02

980

1

960

0.98

Number of Cell

Voltage, (V)

Figure 6.55 Graph of changes in the rate of water conversion and the amount of hydrogen produced relative to the molar flow rate of pure water vapor entering the electrolyzer [103].

940 0

1

2

3

4

5

6

Inlet H2O molar flow, (mol/s) Figure 6.56 Diagram of voltage changes and number of electrolyzer cells in terms of the molar flow rate of pure input water vapor [103].

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Hybrid Poly-generation Energy Systems

with increasing molar flow rate. As the conversion rate decreases, the potential is negligible, consequently decreasing the actual cell potential. On the other hand, the number of cells in the electrolyzer mass increases due to the constant input power. Fig. 6.57 shows the changes in the rate of water vapor conversion during the electrolysis reaction and the molar flow rate of water leaving the electrolyzer mass by changing the molar fraction of the input hydrogen. This diagram is assumed to show constant input power, current density, and cell surface, as well as shows an increase in water conversion and a decrease in the molar flow rate of water in the electrolyzer output by increasing the molar fraction of input hydrogen. As the molar fraction of hydrogen inlet to the electrolyzer increases (or the molar fraction of inlet water decreases), the Nernst potential increases with a slight slope and the reaction does not usually occur. Because the amount of water available for electrolysis is very small. Adding a small amount of hydrogen to a high-temperature electrolyzer indeed prevents oxidation of the cathode, which is usually nickel-based, but its large amount reduces the concentration of incoming water and reduces the water available for electrolysis. On the other hand, the rate of reaction conversion increases due to the decrease in the amount of water and the constant input power. In fact, the high rate of water conversion causes the molar flow rate of water at the outlet to decrease as the molar fraction of the hydrogen inlet increases. The effect of the molar fraction of hydrogen input on the rate of hydrogen production is shown in Fig. 6.58. This diagram is drawn with the assumptions of Fig. 6.57. As the amount of hydrogen entering the system increases, the rate of hydrogen production by the electrochemical reaction decreases due to less access to water. Of course, the molar flow of hydrogen increases at the output, but the hydrogen produced during the 1.2

3

1

H2O Conversion

2.5 0.8 2 0.6 1.5 0.4 1 H2O Conversion

0.2

Outlet H2O Molar Flow, (mol/s)

3.5

0.5

H2O Molar Flow 0

0 0

0.2

0.4

0.6

0.8

Inlet H2 Mole Fracon,(%) Figure 6.57 Diagram of the effect of molar fraction of input hydrogen on the rate of conversion of water vapor and its molar flow rate from the electrolyzer [103].

Technical and economic prospects of fuel cells combination with polygeneration systems?

285

0.98

H2 producon Rate, (mol/s)

0.96 0.94 0.92 0.9 0.88 0.86 0.84 0.82 0

0.2

0.4

0.6

0.8

Inlet H2 Mole Fracon

Figure 6.58 Diagram of the effect of the molar fraction of input hydrogen on hydrogen produced in the electrolyzer [103].

electrolysis reaction decreases. For example, suppose the input fuel of an electrolyzer is one mole per second and consists of water vapor and hydrogen. According to Fig. 6.58, if the molar fraction of the input hydrogen is 0.1, about 0.97 mol/s of hydrogen is produced and the molar flow rate of the output hydrogen will be 1.07 mol/s. However, if 50% of the input current of the electrolyzer is hydrogen, about 0.885 mol/s of hydrogen is produced in the electrolyzer, and 1.335 mol/s of hydrogen is released from the electrolyzer system, which is less than 1.7 mol/s. Fig. 6.59 shows the effect of changing the molar fraction of the input hydrogen on the potential and number of cells. This diagram is presented with the assumptions of Fig. 6.57. As mentioned, the higher the amount of hydrogen entering the electrolyzer, the lower the concentration of inlet water, resulting in reduced cell performance and increased system losses, hence the higher the voltage. At a molar fraction above 50% hydrogen, the voltage diagram increases with a greater slope. At a fraction of less than 10% of water vapor at the electrolyzer inlet, the slope of the diagram increases so much that water electrolysis (due to the very small amount of water available) requires more steps and more accurate calculations. Of course, Figs. 6.56.25 do not show this range. On the other hand, due to the constant input power, current density and area, the number of cells required to compensate for the increase in voltage is reduced.

6.3.1.8 Combined system of SOFCSOEC The SOFCSOEC hybrid system is shown in Fig. 6.60. In this hybrid system, natural gas is compressed, and after mixing with the anode backflow, it enters a

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Hybrid Poly-generation Energy Systems

1.35

1050

1.3 1000

950

1.2 1.15

900

Number of Cell

Voltage, (V)

1.25

1.1 850 1.05

Voltage N-Cell

1

800 0

0.2

0.4

0.6

0.8

H2 Mole Fracon Figure 6.59 Diagram of the effect of the molar fraction of input hydrogen on the voltage and number of cells in the electrolyzer mass [103].

111 AR Blower-AR Combustor exhaust gas Pre-reformer Fuel Compressor P-NG Natural gas

Anode-FC

E-110

Pre-reformer outlet

Pre-reformer inlet

To separator

heat

V-sep

VLV-100 Air-1

103

Anode inlet

Anode outlet

MIX-101

109

110-2 VLV-Out

102

Fuel preheater

Q-PR E-100

Heat Loss

combustor

To pre-reformer MIX-108

Water in

TEE-103

104

105

MIX-102

103

Air Compressor 108

114-out

Air-111

103

Air-112

Air-EX2

Air-EX1 110

MIX-103 110-1

Cathode outlet

E-101 Phase separator

TEE-101

Ambient-Air L-Sep E-102

Cathode inlet

Cathode-FC

110-1

Air-Blower

DC-Power

CGR Cathode-EC

Feed-EC

Cooling And separang Pure H2 TO Tank Vapor-EC CRV-SOEC

AC-Power

X-SOEC

DC-AC inverter

Liquid-EC

Power to SOEC

O2 recycle to SOFC

Anode-EC

Air to EC

MIX-EC

TO Tank for O2 by-product Anode-final

Figure 6.60 Combined system of SOFCSOEC in the second case [103].

preheater. The return of the anode flow is significant because of the reforming process’s proper vapor-to-carbon ratio. After the performing stage, the fuel is reheated and enters the fuel cell anode. The reforming process is completed at the anode,

Technical and economic prospects of fuel cells combination with polygeneration systems?

287

and hydrogen is oxidized. It provides electricity, heat, and sometimes water vapor the electrolyzer needs. The air is preheated and sent to the fuel cell cathode and the electrolyzer anode. The air mixes with the oxygen produced in the SOEC and removes it from the electrolyzer system. Part of the air rich in the cathode can be consumed in the fuel cell cathode, or the oxygen in it can be considered a byproduct. The evacuated air from the fuel cell cathode, which has lost 15% of its oxygen, is sent to a combustion chamber to provide the oxygen required for the combustion process of the non-returned part of the exhausted cell exhaust gases. Water vapor is also supplied from the exhaust gases of the fuel cell anode, in which case it is necessary to reduce the flow temperature to 50  C to remove other components, separate the liquid water from them, and then lower the water temperature before entering the electrolyzer, which raised the temperature to 800 degrees Celsius. At the output of the electrolyzer cathode, water can be stored in condensate and, finally, pure hydrogen, not shown in Fig. 6.60. In this section, the parameters of the hybrid system are examined and the system’s energy efficiency is obtained. Table 6.22 shows the required values of the hybrid system.

6.3.1.8.1 Check the hybrid system In this hybrid system, the water required for the electrolyzer is supplied from inside the fuel cell system, and as a result, changes in the parameters of the two parts of the system have a greater effect on each other. The graph of changes in total power and net power output and power consumption of the combined system in the second case with current density is presented in Fig. 6.61. To draw this graph, the current density of the two parts of the combined system is changed by the same value, and all values of this graph are reported at the same current density of the two cells. It should be noted that the fuel cell’s fuel consumption factor and the water conversion rate in the electrolyzer are 0.85. This diagram shows that increasing the current density increases the total and net production capacity and their values in the current density 2.25 A/cm2 with maximum values of 1361 and 952.6, respectively. It is in kilowatt. As expected, the total power and net power diagrams decrease due to the increase in concentration loss at current densities above 2.25 A/cm2. It is Table 6.22 Combined system values [79,104]. Parameter

Value

Unit

SOEC and SOFC Current density SOEC steam utilization SOFC fuel utilization SOFC anode recycle to natural gas molar flow ratio SOFC stack inlet temperature SOEC stack inlet temperature Air blower isentropic efficiency Air blower organic/electric efficiency Pressure losses within the SOFC anode/cathode Pressure losses in SOEC cathode

0.8 8085 85.01 6 827 800 72 85 1/2 0

A=cm2 % %  C  C % % % %

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Hybrid Poly-generation Energy Systems

1400

500 450 400

1000

350 300

800

250 600

200 150

400 W-total W-net W-consumed

200

W consumed, (kW)

W Total and Net, (kW)

1200

100 50 0

0 0

1

2

3

Current density, (A/cm^2)

Figure 6.61 Graph of changes in total net power and consumption in terms of current density in a hybrid system [103].

noteworthy that the amount of power increases with an increasing current density of the solid oxide electrolyzer. As a result, the power consumption of the entire system is increasing. Fig. 6.62 shows the changes in fuel cell output power and electrolyzer power consumption in a hybrid system relative to current density. This diagram is obtained with the conditions of Fig. 6.61. As mentioned, a SOFC’s production capacity increases and decreases with increasing current density. The power required by the electrolyzer increases as the current density increases due to more significant loss and reduced cell performance. Note that this diagram is drawn at 85% of one mole per second of water vapor entering the system. Fig. 6.63 shows the effect of current density on total, electrical efficiencies and the ratio of heat recovery to net power output. This diagram is drawn with the assumptions of Fig. 6.61. As can be seen, the total efficiency and the electrical efficiency of this combined system increase at low values of the current density and decrease after reaching a maximum value. The amount of net generating power of a hybrid system determines the electrical efficiency. As can be seen in Figs. 6.61 and 6.62, the power output of the fuel cell and the power consumption of the electrolyzer is increasing with the current density. On the other hand, as the current density increases, the amount of fuel consumed by the hybrid system also increases. It seems that increasing the fuel consumption of the system at current densities higher than 0.85 A/cm2 overcomes the increase in the net power of the system and reduces the electrical efficiency of the system. The overall efficiency is also higher than the electrical efficiency due to heat recovery, but despite the increase in heat recovery in the hybrid system in proportion to the increase in current density, this graph also decreases due to excessive fuel consumption. The TER efficiency also decreases in

Technical and economic prospects of fuel cells combination with polygeneration systems?

280

1600

260

1400

240 220 1000

200

800

180 160

SOEC power, (kW)

SOFCPower, (kW)

1200

600 140 FC

400

120

EC 200

100 0

1

2

3

Current density, (A/cm^2)

Figure 6.62 Diagram of the effect of fuel cell output power and electrolyzer power consumption in terms of current density [103].

2.5

0.9

2

0.7 0.6

1.5 0.5

TER,

Overal & Electrical Efficiency,

0.8

0.4 1 0.3 0.2

0.5

Electrical Total

0.1

TER 0

0 0

0.5

1

1.5

2

2.5

3

Current density, (A/cm^2)

Figure 6.63 Diagram of the effect of current density on the overall electrical and TER efficiencies of a combined system [103].

289

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Hybrid Poly-generation Energy Systems

low current density, indicating that the total net power prevails over the heat recovery rate. As the current density increases, the TER graph increases after reaching its lowest value. Figs. 6.64 and 6.65 show the power and efficiency changes of the hybrid system of the second mode with the molar flow of water entering the electrolyzer, respectively. These two diagrams are drawn assuming the conversion rate of incoming water vapor to hydrogen and oxygen. This amount equals 85% of the incoming water, water vapor, and power required in all stages is fully supplied by the SOFC system. Figs. 530 shows the effect of the molar flow rate of water vapor entering the electrolyzer on the net output power, power consumption of the hybrid system, and power consumption of the electrolyzer. By increasing the molar flow rate of water and adjusting the system to the conversion rate, the power required by the electrolyzer to convert 85% of the electrolyzer inlet water increases. As a result, the net production capacity is reduced. The power consumption of the whole system, in addition to being affected by the power consumption of the electrolyzer, also depends on other components of the system, all of which increase in the conditions examined in this diagram. In fact, with increasing the molar flow of water entering the electrolyzer, due to the increase in the volume of water that must be condensed, the heat load of the cooler increases and releases a lot of heat. Fig. 6.66 is also obtained with the conditions of the previous diagram and shows the general, electrical, and TER efficiencies in the combined system by changing the molar flow rate of the inlet water to the electrolyzer. By reducing the production 450

800

400 350

600 300 500

250

400

200 150

SOEC Power, (kW)

Net & Consumed Power. (kW)

700

300 100

W-net 200

W-consumed

50

SOEC-power 0

100 0

0.25 0.5 0.75

1

1.25 1.5 1.75

2

2.25 2.5

H2O Molar Flow. (mol/s)

Figure 6.64 Graph of changes in total, net, consumption, and consumption power in the electrolyzer concerning the molar flow rate of water entering the electrolyzer in the combined system [103].

Technical and economic prospects of fuel cells combination with polygeneration systems?

291

2

1 0.9

Overall

0.8

TER

1.8 1.6

0.7 0.6

1.4

TER

Overall & Electrical Efficiency

Electrical

0.5 1.2

0.4 0.3

1

0.2 0.8 0.1 0

0.6 0

0.25 0.5 0.75

1

1.25 1.5 1.75

2

2.25 2.5

H2O Molar Flow, (mol/s)

Figure 6.65 Graph of changes in total efficiency, electrical, and TER in terms of the molar flow of water entering the electrolyzer in the combined system [103].

Input streams

Qin

Output streams

Practical Process

Win

Qout Wout

Figure 6.66 Exergy balance around a system.

capacity while increasing the molar flow of water entering the electrolyzer, the electrical efficiency of the hybrid system decreases. As described in the previous diagram, the system’s heat recovery increases slightly, but not enough to overcome the reduction in net output. For this reason, the overall efficiency of the hybrid system decreases with a lower slope than the electrical efficiency. Also, the TER efficiency is increasing due to the increase in thermal efficiency and the decrease in net production capacity. In the combined system of SOFC and SOEC, the water vapor required by the electrolyzer section is provided from inside the process and from the exhaust gases of the SOFC anode. This current is used to preheat the air currents required by the hybrid system due to the high temperature before entering the electrolyzer section. The current 110, which is in Fig. 6.65, has a temperature of about 800 C and a single pressure. It should be noted that due to the presence of carbon dioxide, nitrogen,

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hydrogen, and oxygen in this current, which are not suitable for entering the electrolyzer cell, liquid water is separated from other gases by condensing the current and passing it through a fuzzy separator. The liquid water is then heated to a temperature of 800 C and prepared to enter the electrolyzer. This system has a higher electrical and overall efficiency than the first mode system and their values are 42.32% LHV and 78.58% LHV, respectively. The results of the mentioned system are given in Table 6.23. As can be seen from the results, the combined system of SOFCSOEC with a supply of required water vapor from inside the system, despite the phase separator and cooling of the anode exhaust gas flow with high heat loss, offers 42% electrical efficiency-LHV. If the output power of the fuel cell is not considered and we want to separate the total output power and total water in the output line of the fuel cell anode output and enter it as power and feed of the electrolyzer, in the current density 0.8 A/m2 the values obtained for the electrolyzer are summarized in Table 6.24. The laws of nature are based on the principle of non-creation or destruction of energy (the first law of thermodynamics). Energy comes in many forms that can be converted to each other with limitations. The most valuable energies are electrical and mechanical energies that can be wholly and quickly converted into thermal energy. In contrast, converting thermal energy into mechanical or electrical energy is a significant waste. Therefore it can be concluded that different forms of energy have different qualities and the rate of conversion of different energies into mechanical work is different. This limitation indicates that the quality of energy decreases Table 6.23 Results of two combined systems. Parameter

Value

Feed molar flow to SOEC Feed substances to SOEC Exhaust gas Temperature ( C) SOEC power (kW) SOFC power (kW) Total consumed power (kW) Net power Heat recovery (kW)

1 Water 49 183 863 240.4 622.6 533.5

Efficiencies Electrical-LHV (%) Electrical-HHV (%) Overall-LHV (%) Overall-HHV (%) Actual-LHV (%) Actual-HHV (%) TER (%) H2 generation rate (mol=s) LHV production efficiency of pure H2 (%)

42.32 38.44 78.58 71.37 66.39 57.31 85.68 0.85 76

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Table 6.24 Combined system values by removing the power generated by the combined system. Parameter

Value

Unit

SOEC voltage Number of cells in SOEC stack H2O conversion H2O mass flow H2 production rate

1.236 3256 98 12.4658 12.14676

V % kmol=h kmol=h

at each stage of the transformation (the second law of thermodynamics). The concept of entropy explains energy quality. High entropy equals low energy quality. The second law of thermodynamics considers conversion to be practical only if the total entropy increases. Entropy and energy can be considered simultaneously by introducing the concept of exergy. Exergy knowledge helps to define these qualities and control the optimization of energy expression. Electrical and mechanical energy can be called pure exergy. While heat energy, while converting to another type of energy, taking into account the initial heat of the system and its surroundings, causes losses. In recent decades, with the introduction of process integration issues, the idea that by optimizing each process of a system, the best-operating conditions of the system can be found has been discarded. Therefore exergy analysis tools and pinch application tools are used to study the interaction of processes and their interaction with each other and on the whole system. In this chapter, after the general definition of exergy, the combined exergy system of the SOFC and SOEC is analyzed [68]. Exergy equations Energy analysis is based on the first law of thermodynamics and is defined as the ratio of power output to energy input to the system. Various relationships to express the efficiency of the first law of thermodynamics have been presented in previous chapters. The most critical parameters affecting the fuel cell system are temperature and current density, which were thoroughly discussed in the previous chapter on the energy efficiency of the two parts of the system and the combined system. The word exergy is derived from the Greek words ex (foreign) and ergon (work or force) and was coined in 1824 by Carnot in the equation of work and heat. Exergy is based on the second law of thermodynamics and Gibbs free energy. In fact, exergy is the axial work obtained from a current, if that current is brought to thermodynamic equilibrium and the same components with the environment in a reversible process. Exergy analysis to evaluate industrial processes has been around since the 1980s. Exergy analysis refers to the determination of exergy losses in a process that result from the irreversibility of that process. This identification leads to the improvement of process technology to reduce energy consumption or the amount of waste discharged into the environment, whereas a process, if only energy analysis, does not give such knowledge to the designer.

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To more accurately study the thermodynamic performance of the system, it is necessary to examine the degree of irreversibility of each component of the system according to the second law of thermodynamics. The lower the irreversibility of a given state change, the greater the amount of work done. Therefore the greater the irreversibility, the greater the reduction of available resources in all processes. Another reason to do something with the least return is the cost factor. Often, doing something with less irreversibility costs less. Nevertheless, to achieve the most economical plan, one must pay attention to many factors that affect the overall cost. Exergy analysis is used to determine irreversibility. As mentioned, exergy is defined as the maximum work achievable from the system if it reaches standard conditions. Standard conditions also refer to the temperature of 25 C and the pressure of one atmosphere [105]. The maximum achievable performance of a system is achieved when it reaches the standard state in an irreversible process. According to this definition, exergy can be called reversible work. Reversible labor is the most productive labor or the least labor consumed in equipment when it goes from one state to another. In fact, the difference between reversible work and irreversible work is known as exergy losses. The expression of the second law is that in each process, the total entropy change is positive, while the value of zero is obtained only in a reversible process. In no process can the total entropy be reduced. According to this statement, it can be concluded that the amount of exergy losses is proportional to the total entropy produced in the process. In fact, the higher the total entropy production, the higher the irreversibility and the higher the exergy losses [106]. The purpose of process exergy analysis is to obtain the amount of axial work wasted and exergetic efficiency in process units. With the help of these tools, these tools can be the most suitable unit for modification and optimization in terms of energy consumption. Exergy loss or irreversibility is a small measure of process inefficiency. The analysis of a multicomponent unit indicates the irreversible distribution of the whole unit among its components and determines the largest contributions to the inefficiency of the whole unit. Two flow and single methods are recommended for oxygen analysis. The flow method is based on the oxygen balance between a process unit’s input and output currents. In the steady state, the amount of entropy production in an open system is obtained from Eq. (6.131). ΔðSm_ Þfs 2

X Qj 5 S_G $ 0 Tσ;j j

(6.131)

By combining the first and second laws of thermodynamics, the amount of work lost (exergy loss), which is equivalent to the ideal (reversible) work differently from the actual work, is obtained by Eq. (2), where T is the source temperature and SG(Stot) is the production of the total entropy. I 5 W_ lost 5 T0 S_G

(6.132)

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Therefore the larger the irreversibility of the process, the greater the intensity of entropy production and the amount of energy available to do the work but not available. As a result, any irreversibility comes with a cost. ðTotal entropy production 3 Ambient temperature 5 exergy lossesÞ Exergy is transmitted in three ways, exergy transmission with work, heat effects, and mass flow, which are discussed in the following sections. Exergy caused by work Exergy is work material. Therefore the work transferred in the process is fully known as exergy. In other words, the exergy of power is equal to power itself. Exergy caused by heat transfer Exergy due to heat transfer in the system is equivalent to the maximum work that can be achieved from a certain amount of heat to environmental conditions. This amount of work is proportional to the efficiency of the Carnot cycle, which is defined as Eq. (6.133).   T0 W_ max 5 EQ 5 Qr 1 2 (6.133) Tr In Eq. (6.3), Q is heat transfer and Tr is the temperature at which heat is transferred [105]. Exergy due to material flow Exercise mass flow consists of physical, chemical, mixing, kinetic, and potential exergy. Eq. (6.134) expresses the exergy relation of mass flow, which generally ignores the last two, kinetic and potential. E 5 Ek 1 EP 1 Eph 1 Ech

(6.134)

Therefore the most important components of material flow exergy include physical and chemical exergy. Physical exergy is the maximum work that can be received from a stream by bringing its temperature and pressure to ambient temperature and pressure (without chemical change). Therefore physical exergy includes exergy due to temperature and pressure, which is shown in Eq. (6.135). Eph 5 EΔT 1 EΔP

(6.135)

In total, physical exergy can be calculated from Eq. (6.136). Eph 5 ðh 2 h0 Þ 2 T0 ðS 2 S0 Þ

(6.136)

In relation (6.134), h0 and s0 are the standard enthalpy and entropy, respectively. Since exergy is a state function, a flow’s transmitted exergy can be calculated according to its input and output conditions through Eq. (6.139). Eph 5 ðh1 2 h2 Þ 2 T0 ðS1 2 S2 Þ

(6.137)

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Chemical exergy is the chemical energy stored in matter molecules. The difference in extractable energy from the conversion of matter molecules to the free conversion of their constituent elements is called chemical exergy. For an ideal mixture, chemical exergy is obtained from Eq. (6.138). Ech;m 5

X

xi E0;i 1 RT0

X

i

xi lnðxi Þ

(6.138)

i

In the above relation, E0,i is the molar exergy of each component and xi is the molar component. In real mixtures, the activity coefficient (γ) is also used to calculate the chemical exergy, which is in the form of Eq. (6.139). Ech;m 5

X

xi E0;i 1 RT0

X

i


 xi ln γ i xi

(6.139)

i

According to the presented thermodynamic relations, it can be concluded that chemical exergy is calculated from Eq. (6.140). The values of E0,i are given in Table 6.25. Ech;m 5

X

xi E0;i 1 ΔG

(6.140)

i

By writing the exergy balance around a system (Fig. 6.66), the lost work, the system efficiency, and the data needed to analyze the exergy are obtained. The exergy efficiency of each device is defined according to the task for which it is intended. In the steady state, the exergy equilibrium is written as Eq. (6.141). X

Ein 1

stream

X

Q Ein 1

X

Win 5

X

Eout 1

X

Q Eout 1

X

Wout 1 I

(6.141)

stream

Table 6.25 Standard exergy values [105]. Substance

State


 E0 kJ=kmole

Methane Ethane H2O H2O CO CO2 O2 N2 N2(air) H2

Gas Gas Gas Liquid Gas Gas Gas Gas Gas Gas

836,510 1,504,360 11,710 3120 275,430 20,140 3970 720 690 238,490

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In Eq. (6.141), I is irreversible in the system. Using this relationship, the amount of loss in each device is obtained. In the following, we define the efficiency of the second law for each device and by which the distance from the ideal state is measured. According to the exergy balance for the whole system, the equation is X X ΔEin 5 ΔEout 1 I (6.142) The ratio of output exergy to input exergy is less than one. The value of this ratio depends on the degree of irreversibility of the whole system. According to this ratio, the percentage of process completion is known in terms of the second law, which is defined as the efficiency of the second law for the whole system. Eqs. (6.143) and (6.144) represent the efficiency of the second law. P ΔEout Ψ5 P ΔEin Ψ512 P

I ΔEin

(6.143)

(6.144)

The role of each component of the system in reducing the efficiency of the second law is also calculated from Eq. (6.145) [105]. δi 5 P

Ii ΔEin

(6.145)

Equilibrium and exergy efficiency of combined cycle components Turbine and compressor The turbine is used in the steam cycle by which the heat generated by the steam is converted into electrical energy [107]. Exergy balance and efficiency for this device are calculated with Eqs. (6.146) and (6.147), respectively. Ein 2 Eout 5 W 1 I

(6.146)

W Ein 2 Eout

(6.147)

Ψ5

A compressor increases the fuel and air pressure required in the process. The relationships between exergy balance and compressor efficiency are given in Eqs. (6.148) and (6.149). W 5 Eout 2 Ein 1 I

(6.148)

Eout 2 Ein W

(6.149)

Ψ5

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Hybrid Poly-generation Energy Systems

Valves Suppression valves are devices for reducing pressure and in some cases producing lower temperatures. Assuming this device is very good, it can be assumed without heat transfer. The loss rate from Eq. (6.150) and the exergy efficiency of this device can be obtained as the output ratio to input exergy from Eq. (6.151). Ein 2 Eout 5 I

(6.150)

Eout Ein

(6.151)

Ψ5

The relationships given above are not suitable for valves that operate at low temperatures or produce cold. In such systems, where the main purpose is to reduce the temperature by reducing the pressure, the exergy of the current is divided into two parts. The part is due to the pressure difference between the system and the environment (EΔP) and the rest is due to the temperature difference from the standard temperature (25 C) (EΔT). Exergy due to temperature difference is easily calculated from Eq. (6.152) [108], and exergy due to pressure difference at standard temperature is also easily calculated from Eq. (6.153). EΔT 5

ð T

T 2T0 dh T T0

 (6.152) p

EΔP 5 T0 ðS0 2 Si Þ 2 ðh0 2 hi Þ

(6.153)

It is clear that physical exergy is the sum of the total exergy caused by temperature and pressure. This relationship is expressed in Eq. (6.152). In addition, due to the performance of the throttle valves, their efficiency is in the form of Eq. (6.153). Eph 5 EΔP 1 EΔT Ψ5

ΔT ΔT Eout 2 Ein ΔP ΔP Ein 2 Eout

(6.154) (6.155)

Heat exchanger Many heat exchangers have been used in this system, which plays an important role in process optimization [107]. The balance and exergy efficiency of the heat exchanger are calculated from Eqs. (6.156) and (6.157), respectively. Eh;in 2 Eh;out 5 Ec;out 2 Ec;in 1 I Ψ5

Ec;out 2 Ec;in Eh;in 2 Eh;out

(6.156) (6.157)

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Mixers It has been used in many parts of mixers. The relationships between exergy balance and exergy efficiency are given in Eqs. (6.158) and (6.159), respectively. Imix 5 Ein;1 1 Ein;2 2 Eout;3 Ψ512

(6.158)

Imix Ein;1 1 Ein;2

(6.159)

SOFC One of the most important parts of the combined cycle is the SOFC in which electrochemical reactions and methane reforming occur. The exergy balance is written as Eq. (6.160) according to Fig. 6.67. ISOFC 5 Ea;in 1 Ec;in 2 Ea;out 2 Ec;out 2 WFC

(6.160)

For the fuel cell, various definitions of the efficiency of the second law are given [109115], which are mentioned in relations (6.161) to (6.164). The third definition is used. Ψ5


Ψ5

Ea;in 1 Ec;in



WFC
 2 Ea;out 1 Ec;out

(6.161)

WFC Ea;in 1 Ec;in

(6.162)


 WFC 1 Ec;out 2 Ec;in
 Ψ5 Ea;in 1 Ea;out Ψ512

(6.163)

ISOFC Ea;in 1 Ec;in

(6.164)

SOEC Another major component of this hybrid system is the solid oxide electrolyzer. The relationships in this section are similar to SOFCs. With the difference that

W 3

1 Anode SOFC Stack

2

Cathode

Figure 6.67 Exergy balance around the fuel cell.

4

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Hybrid Poly-generation Energy Systems

the work is in the input. The efficiency of solid oxide electrolyzer is calculated from Eq. (6.165) [115]. Ψ5

EH2 3 nH2O;out WSOEC 1 Exheat;SOEC 1 Exheat;H2O

(6.165)

In relation (6.165), the amount of electrical energy input to the electrolyzer and the input energy exergy E are the denominators of this fraction. It also contains hydrogen exergy. In order to evaluate the efficiency of the second law for the whole combined cycle, in addition to the defined exergy efficiency, the exergy performance coefficient can be used, which is defined as the ratio of net production labor to total system exergy losses [109]. Wnet EPC 5 P Ii

(6.166)

Also, the ratio of exergy degradation for each of the devices in the combined cycle is defined as the ratio of exergy degradation in the device to the total fuelinduced input exergy, which is presented in Eq. (6.167) [116]. YD;i 5

Ii Efuel

(6.167)

Exergy analysis Table 6.26 shows the amount of exergy degradation and the efficiency of each component of the hybrid system, taking into account the vapor-tocarbon two ratios and the temperature of the fuel cell and electrolyzer mass of 1100 K. Most exergy damage occurs in the following components: 1. Compressors: The higher the total pressure drop of the combined cycle, the higher the exergy losses. The pressure drops are placed according to the practical results so that the obtained results are closer to reality. Of course, another point that has increased exergy losses in compressors is the low isentropic efficiency of compressors. This value is considered 0.65 for each compressor. 2. Combustion chamber: One of the most important reasons for the increase in exergy losses in the furnace is the increase in excess air entering it. In fact, the more excess air entering the combustion chamber, the less the amount of exergy released by the reaction in this chamber becomes less of the exergy caused by the reaction products (resulting from the temperature of the products). Of course, combining fuel with excess air also leads to increased losses due to chemical exergy. Therefore the excess air entering the cathode must be reduced to reduce the exergy losses in the furnace. Heat loss also plays an important role in increasing exergy losses. Also, increasing the temperature of the combustion chamber increases the exergy losses in the related heat exchangers. 3. SOFC: As the current density in the cell increases, its efficiency decreases. Therefore reducing the current density is one of the factors in reducing exergy losses in this area. However, an excessive increase in current density greatly increases the required cell

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Table 6.26 Exergy losses of each system component. Component

Destruction Rate (kW)

Efficiency (%)

YD (%)

Fuel-comp Air-comp Blower-air Blower-AR Fuel preheater Air-EX-1 Air-EX-2 E-100 E-101 E-102 E-110 VLV-100 Mix-101 Mix-102 Mix-103 Mix-108 Mix-EC Combustor Prereformer Fuel cell Electrolyzer DC-AC inverter Gas exhaust

0.386 5.824 2.892 1.122 16.274 57.057 30.848 76.5 8.767 3.634 100.4 4.8 21.980 41.34 10.37 27.462 0.927 42.181 12.691 90.631 17.533 66 12

0.776 0.727 0.884 0.879 0.850 0.354 0.778 0.843 0.802 0.993 0.596 0.999 0.991 0.940 0.988 0.988 0.931 0.933 0.995 0.773 0.831 0.92 -

0.0253 0.38 0.189 0.0733 1.06 3.72 2.01 5.00 0.572 0.237 1.06 0.0318 1.43 2.7 0.677 1.79 0.0605 2.75 0.828 5.91 1.14 4.31 0.783

4.

5.

6.

7.

surface. So a balance must be struck between exergy and economic issues. Excess air entering the fuel cell increases the exergy losses, which can be compensated for by increasing the cathode backflow. However, this value is adjusted according to the allowable temperature in the furnace and the oxygen concentration at the cathode. SOEC: Increasing the current density in the electrolyzer also reduces cell efficiency. As a result, the current density in this cell must be reduced to improve the exergy loss. Of course, in the electrolyzer, the amount of water entering the cell also changes the exergy losses. In other words, as the efficiency of hydrogen production increases, the system losses decrease. Current density: With increasing current density, exergy losses increase, and in high currents, these losses are more. Therefore from the exergy point of view, the lower the current density, the lower the irreversibility. These results are in Tables 6.27 and 6.28. Air Preheater Exchanger: Due to the high heat potential of the exhaust gases from the combustion chamber, improving the losses of an exchanger does not affect the total losses. Because these losses are automatically transferred to another exchanger. Here we have tried to establish the highest efficiency by observing the temperature rules in the exchangers. One way to improve exergy losses in this system is to use gas turbines and high-pressure operations, which have advantages and disadvantages. Exhaust gases: The flow of exhaust gases with a temperature of 49 degrees Celsius. The whole exergy from these gases is completely discharged into the environment. Reducing the amount of vapor in these gases is one way to reduce their exergy losses.

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Table 6.27 Exergy values in current density of 0.8 A /cm2. Parameter

Value

Total destruction (kW) Total efficiency EPC SOFC destruction (kW) SOEC destruction (kW)

651 0.41 0.955 90.5 17.5

Table 6.28 Effect of current density on exergy values. Parameter

i 5 0:5A=cm2

i 5 1A=cm2

i 5 1:5A=cm2

i 5 2A=cm2

Total destruction (kW) Total exergy efficiency EPC

487 0.43 1.24

729 0.399 0.99

1009 0.334 1.09

1339 0.255 1.37

8. DC to AC Converter: Since most devices operate on AC power, converting the direct power generated in the fuel cell system to AC power is necessary. Converting directly to alternating current is another factor in increasing exergy losses. Conversion efficiency has a great impact on the first and second laws. Due to this converter’s choice of 92% efficiency, its exergy losses are significant. For 662 kW in the combined system, 66 kW of exergy losses occur in this converter.

Finally, it can be concluded that by improving the following parameters, exergy losses can be reduced: Flow density Pressure drop DC to AC efficiency Allowable increase in temperature along the cathode The allowable outlet temperature of the combustion chamber Reducing the amount of water vapor output with the exhaust gases (of course, affects the amount of power consumed by the electrolyzer.) 7. Water vapor conversion efficiency in an electrolyzer 1. 2. 3. 4. 5. 6.

Considering the aim to design and evaluate the combined system of SOFC with SOEC for simultaneous production of power and hydrogen, the process of reviewing and implementing the system and its results were presented in several sections. The different parts of the process are as follows: 1. Structural knowledge of fuel cell and electrolyzer: As examined, the plate structure was selected as a suitable option for fuel cell and solid oxide electrolyzer due to higher power density and lower cost. Anode-reinforced fuel cell and cathode-reinforced electrolyzer allow the cell to operate at a lower temperature while operating high. The system used at medium temperature is powered by an electrolyzer fuel cell. Since the electricity generated in the fuel cell is used in the electrolyzer, the combined system is a good option for producing hydrogen.

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2. SOEC performance: This system operates at a constant temperature and pressure with a current density of 0.8 amps per square centimeter. As the cell temperature rises, its performance improves. Although increasing the temperature of the electrolyzer cell improves performance, it also increases system depreciation and has a negative effect on the exergy. The appropriate temperature was selected for this 1073 K electrolyzer. The appropriate voltage range of this section is 0.22.9 volts. Air was used for the cooling of the electrolyzer system. 3. Fuel cell performance is also strongly influenced by temperature and current density. Increasing the current density, increasing system losses, and reducing voltage will also reduce the system’s exergy.

A summary of the results can be summarized as follows: 1. Because the inlet fuel is converted to hydrogen inside the fuel cell, it reduces the amount of air entering the system, as internal reforming absorbs some of the heat of the fuel cell electrochemical reaction. Of course, as mentioned, 25% of the fuel in a preheater is converted to hydrogen to prevent a sharp drop in temperature at the beginning of the fuel cell and damage to the battery. 2. Using the reverse part of the anode current to supply the moraine vapor required to convert the fuel to hydrogen has increased system efficiency and reduced the level required by the converter. 3. Using the reverse part of the cathode current also reduces the amount of air required by the system and the size of the downstream components of this part. 4. The use of oxygen-rich airflow in the anode cell of the electrolyzer due to the increase in oxygen concentration in the fuel cell has reduced the amount of air required by the fuel cell and the size of its downstream components. Of course, it should be noted that the amount of molar flow of this flow is small, but it has had its effect. 5. The flow of exhaust gases from the anode, in addition to water vapor, includes other components that, if used directly, will produce hydrogen impurities. Therefore part of this current is separated to supply the pure water vapor required by the electrolysis reaction. 6. Naturally, in order to increase the production of hydrogen in the electrolyzer, the power and steam of the water entering it must be increased. On the other hand, as the input power to the electrolyzer increases, the energy efficiency of the hybrid system decreases due to the decrease in net output power. Also, with increasing water vapor, the electrolyzer section’s heat load and the condenser section’s exergy losses increase. 7. The combined system includes a SOFC and a SOEC. The same temperature of the two cells used is one of the most important advantages of this system. In fact, neither part of the system creates an additional thermal load on the other. The fuel cell also supplies the power and water vapor the electrolyzer needs. This system in the same current density of two cells of 0.8 A/cm2, S/C 5 2, CGR 5 0.65 and temperature 1100 Kelvin, has the electrical efficiency of HHV 5 38% of combined system and the total efficiency of HHV 5 71%. 8. Using exergy analysis, the most important points of loss of the hybrid system were identified and attempts were made to improve them. Exergy analysis revealed that current density is one of the most important parameters of exergy loss. In fact, exergy losses decreased with decreasing current density. Of course, reducing current density increases system costs. The exergy efficiency of the combined system at 1100K, and S/C 5 2 with a current density of 0.8 A/cm2, was equal to 41%. Also, the exergy efficiency of the electrolyzer cell was 83%.

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6.3.2 Molten carbonate fuel cells in polygeneration systems Electricity is provided using molten carbonate fuel cells (MCFC) using CO2 and oxygen as fuel. Additionally, the anode electrode’s outflow stream has a high proportion (40% of volume) of carbon dioxide, which may cause environmental issues. Due to air pollution and rising global temperatures, lowering the quantity of CO2 in the atmosphere is currently one of the most significant issues facing many nations. So it appears to be beneficial to build an MCFC process with minimal CO2 emission. A novel method integrated into MCFC is shown to separate and liquefy carbon dioxide from the anode exit [117]. This technique reduces greenhouse gas emissions while simultaneously producing a byproduct. Additionally, it was found that postcombustion removal provides better accessibility in energy production in this method. The MCFC have been the subject of numerous studies. For nine examples of particulate carbon generated from fuel oil, details of an MCFC, including cell polarization, surface, primary particle density, and crystallization index, are provided [118,119]. In the following, we can observe MCFC with a carbon removal system in the form of a poly-generation energy system. The four primary steps of the system are burning the NG in the combustion process, fuel cell electrochemical processes, high-temperature output streams cooled using a water cycle, and removal and condensation of the anode’s outlet CO2 gas (Fig. 6.68).

Figure 6.68 Conceptual design of the cycle [120].

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The gas expander receives the combustion process’s output, which continues toward the fuel cell cathode. An electrochemical reaction takes place in the cathode as follows [121]: CO2 1 0:5O2 1 2e2 ! CO22 3

(6.168)

Stream with the fuel cell anode after being combined with the steam produced in the cooling section of the process and being adjusted in temperature in the E-1 heat exchanger. In this section, steam and hydrocarbons interact to produce CO and hydrogen. Then, through Reactions (6.169) and (6.170), hydrogen and CO are transformed into CO2 and water [121]. 2 H2 1 CO22 3 ! CO2 1 H2 O 1 2e

(Reaction 6.169)

2 CO 1 CO22 3 ! 2CO2 1 2e

(Reaction 6.170)

References [1] B. Ghorbani, Development of an integrated structure for the tri-generation of power, liquid carbon dioxide, and medium pressure steam using a molten carbonate fuel cell, a dual pressure linde-hampson liquefaction plant, and a heat recovery steam generator, Sustainability 13 (15) (2021) 8347. [2] J. Larminie, A. Dicks, M.S. McDonald, Fuel Cell Systems Explained, 2, J. Wiley Chichester, UK, 2003. [3] M. Mehrpooya, M.R. Ganjali, S.A. Mousavi, N. Hedayat, A. Allahyarzadeh, Comprehensive review of fuel-cell-type sensors for gas detection, Industrial & Engineering Chemistry Research 62 (6) (2023) 23872409. Available from: https://doi. org/10.1021/acs.iecr.2c03790. [4] R. O’hayre, et al., Fuel Cell Fundamentals, John Wiley & Sons, 2016. [5] K.-D. Kreuer, Fuel Cells: Selected Entries From the Encyclopedia of Sustainability Science And Technology, Springer Science & Business Media, 2012. [6] J.D. Holladay, et al., An overview of hydrogen production technologies, Catalysis today 139 (4) (2009) 244260. [7] J.I. San Martı´n, et al., Hybrid fuel cells technologies for electrical microgrids, Electric Power Systems Research 80 (9) (2010) 9931005. [8] X. Zhang, et al., A review of integration strategies for solid oxide fuel cells, Journal of Power Sources 195 (3) (2010) 685702. [9] R. Taherian, M.J. Hadianfard, A.N. Golikand, Manufacture of a polymer-based carbon nanocomposite as bipolar plate of proton exchange membrane fuel cells, Materials & Design 49 (2013) 242251. [10] P. Dokmaingam, et al., Modeling of IT-SOFC with indirect internal reforming operation fueled by methane: Effect of oxygen adding as autothermal reforming, International Journal of Hydrogen Energy 35 (24) (2010) 1327113279.

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Biomass-based hybrid energy systems

7

Biomass is a reliable source of energy that can help solve the energy crisis and reduce environmental damage. Along with rising petroleum and gas prices, as well as the growing energy crisis, an alternative type of energy that is more environmentally friendly and considerably inexpensive is ever so demanding. Biomass is one of the choices between these types of energy sources. This source, the oldest known source of energy for humans, does not accumulate much to the carbon dioxide level on Earth. The reason behind this case is that most biomass is produced by photosynthesis by absorbing carbon dioxide from the atmosphere, and the process only releases the absorbed carbon dioxide after its conversion to energy. Biomass can be reproduced, and it does not take millions of years.

7.1

Introduction

Biomass is classified as a renewable energy source. Also, we can utilize a wide variety of biomass as raw materials for energy production, such as wood chip residues, agricultural residues, animal excreta, waste, and more. Biochemical or thermochemical pathways can convert this biomass. For thermochemical conversion, thermal energy generation is the primary stimulus for this conversion. The biomass is converted to gas and then synthesized to the desired chemical or used directly. Direct combustion, pyrolysis, and gasification classify as thermochemical processes. Traditional biomass combustion shows low efficiency in energy use and, therefore cannot compete with fossil fuels. Biomass gasification for combined heat and power (CHP) products offers higher energy efficiency. This technology has been commercialized successfully in some countries [1,2]. Gasification is the process of converting carbon solids into synthetic gases under conditions of a wide range of temperatures and the absence of oxygen [3].

7.1.1 Types of biomass There are many ways to categorize biomass. In general, they classify into two groups: Virgin biomass and residual biomass. A detailed classification of biomass [1,4] is shown in Table 7.1. Untouched biomass is obtained directly from plants. Wooden biomass includes trees, shrubs, shrubs, and foliage. Plant biomass is plants that die annually after the growing season. Energy products are plants that are used exclusively for energy production. These products have a high energy density and a short growth period. Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00002-5 © 2024 Elsevier Inc. All rights reserved.

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Table 7.1 Types of biomass. Virgin biomass

Waste biomass

Woody biomass Herbaceous biomass Energy crops Agricultural waste Municipal waste Industrial waste Forestry waste

Energy products such as willow, spruce, and a type of switchgrass consume for energy production [1]. Waste biomass or secondary biomass is a type of intact biomass obtained during various production stages or from industrial and urban waste. Agricultural waste mainly includes straw, sugar beet leaves, and animal excreta. Forest waste includes tree bark, wood chips, leaves, and other similar items, and industrial waste reaps from sawdust during wood production and destruction of wood products while also including excess oil and fat. Municipal solid waste (MSW), municipal wastewater, and gas are in landfills.

7.1.2 Biomass composition Biomass consists of a complex combination of organic compounds, moisture (M), and an ineffective solid called ash (ASH). Organic compounds include carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and small amounts of chlorine (Cl) and sulfur (S). Biomass composition is one of the most critical pieces of information that requires determination for designing a gas generator or combustion chamber. Final analysis and comparative analysis are the two most widely used combination analysis methods. For the final analysis, we analyze the essential components of hydrocarbon fuel. In addition to moisture (M) and fuel ash (ASH), a final analysis describes as follows: C 1 H 1 O 1 N 1 S 1 Cl 1 M 1 ASH 5 100%

(7.1)

The components of the above equation express the mass percentage of the corresponding components in the fuel. For an approximate analysis, the composition of the hydrocarbon fuel is given as the volatile substance (VM). A volatile substance is heat that is released by heating fuel. Fixed carbon is a solid carbon that remains after pyrolysis. The approximate analysis can be expressed as follows. FC 1 VM 1 M 1 ASH 5 100%

(7.2)

7.1.3 Thermodynamic properties of biomass Biomass gasification involves a series of thermochemical reactions. Therefore to achieve appropriate reactions at different stages and to optimize the process, it is

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necessary to assess the thermodynamic properties of biomass [1]. Specific heat capacity, or a summary of specific heat, includes the thermal capacity of a material that is strongly affected by temperature. Humidity and the type of biomass also affect it. Thermal value is the maximum heat released during the complete combustion of a specific amount of fuel in the presence of air under standard conditions. The calorific value depends on the phase of water produced after combustion. If water is in the gas phase, the amount of heat released is called the lower thermal value (LHV). LHV and higher thermal value (HHV) differ in latent evaporation heat [5]. Biomass ignition heat is a valuable thermodynamic property when designing a gas unit. During the gasification process, combustion is required to provide energy for drying and pyrolysis. Gas-reactive reactions are mainly endothermic; therefore combustion can provide energy for endothermic reactions. The biomass is first preheated and then dried to dissolve in pyrolysis in the absence of oxygen and broken down into volatile substances, liquids including tar and heavy hydrocarbons, and carbon solids called char. Gasification is then processed in a gaseous environment.

7.2

Thermochemical biomass gasification combined processes

7.2.1 Gasification There are two basic types of conversions for biomass: Biochemistry and thermochemistry. The thermochemical conversion studied in this section involves processes performed with thermal energy. Combustion involves the complete conversion of biomass to CO2 and H2O at high temperatures with an additional oxidizer (usually air). In contrast, Gasification involves the conversion of biomass into an environment deficient in O2. Pyrolysis occurs in an environment with a relatively low temperature without O2 [6] (Fig. 7.1). Gasification is a thermochemical process in which a carbon-based fuel is converted to a combustible gas known as synthesis gas, which includes H2, CO, CH4, H2O, N2, heavier hydrocarbons, and impurities (i.e., tar, NH3, H2S, and HCl). The process occurs when a controlled amount of a gaseous agent (pure O2, air, or steam) reacts at high temperatures with the carbon in the fuel in a gas gasifier. Gasification converts biomass into a gas that can be used in advanced power generation systems such as fuel cells and provides higher electrical efficiency than combustion-based technologies [7]. Therefore as shown in Fig. 7.2, it offers higher flexibility in applications (electricity, heat, transport fuels, and chemicals).

7.2.1.1 Gasification theory The following initial reactions occur during pyrolysis and Gasification of biomass or coal [8]. Heat

Biomass
! Char 1 Liquids 1 Gases

(7.3)

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Figure 7.1 Biomass conversion paths and processes [6].

Figure 7.2 The flexibility of the gasification process [7]. Heat

Char 1 Gasification Medium
! Gases 1 Ash

(7.4)

The gasification process can be divided into four stages: drying (100 C
200 C), pyrolysis (200 C
500 C), Gasification, and combustion. When biomass enters the gas generator, it dries first. Pyrolysis then occurs, which produces char, H2, CO, CH4, CO2, H2O, tar, and hydrocarbons. Char is a solid residue rich in carbon [9] (Fig. 7.3). The main gasification reactions are given in Table 7.2. Gasification is highly dependent on the environment for its use (pure O2, air, or steam). Gasification with air produces synthetic gas with low thermal energy values, HHV about 4
7 MJ/m3, while O2 and steam produce synthetic gas with a thermal value of about (HHV) 10
18 MJ/m3 [10]. Biomass gasification with pure oxygen is practically inaccessible due to the high cost of O2 production using commercial technology (air refrigeration separation). Supplying O2 to the gasifier is one of the most costly parts of any gasification project [11]. New technologies such as membranes can reduce O2 production costs and thus make it attractive for

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Figure 7.3 Biomass gasification steps and reaction paths [9].

Table 7.2 The main reactions of gasification. Reaction

Heat of reaction

Reaction name

(2111 MJ/kmol) (1172 MJ/kmol) (1131 MJ/kmol) (275 MJ/kmol)

Char partial combustion Boundouard Water-gas Methanation

(2283 MJ/kmol) (2242 MJ/kmol) (241 MJ/kmol) (1206 MJ/kmol)

CO combustion H2 combustion CO-shift Steam-methane reforming

Heterogeneous reactions: C 1 0:5O2 ! CO C 1 CO2 #2CO C 1 H2 O ! CO 1 H2 C 1 2H2 ! CH4 Homogeneous reactions: CO 1 0:5O2 ! CO2 H2 1 0:5O2 ! H2 O CO 1 H2 O ! CO2 1 H2 CH4 1 H2 O ! CO 1 3H2

NH3, H2S and HCl formation reactions: 5N2 1 1:5H2 #NH3 H2 1 S#H2 S Cl2 1 H2 #2HCl

nrb nr nr

NH3 formation H2 S formation HCl formation

biomass gasification. Cold gas efficiency (CGE) can show the performance of a gas generator and is defined as follows: CGE 5

m_ gas :HVgas 3 100 m_ fuel :HVfuel

(7.5)

where m_ gas and m_ fuel are mass flow rates of synthesis gas and biomass with units of Kg/s, and HVgas and HVfuel are thermal values of synthesis gas and biomass (HHV or LHV) with units of KJ/Kg. Higher CGE provides a higher level of synthesis gas energy, which results in higher system efficiency.

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Table 7.3 Common main reactions that occur depending on the shape and configuration of the gasification in the gasifier reactor. C 1 O2 ! CO2 C 1 0:5O2 ! CO H2 1 0:5O2 ! H2 O C 1 H2 O#CO 1 H2 C 1 2H2 O#CO2 1 H2 C 1 CO2 #2CO C 1 2H2 #CH4 CO 1 H2 O#CO2 1 H2 CO 1 3H2 #CH4 1 H2 O C 1 H2 O#0:5CH4 1 0:5CO2

Δh298 5 2 394 kJ=mol Δh298 5 2 268 kJ=mol Δh298 5 2 241:8 kJ=mol Δh298 5 131:3 kJ=mol Δh298 5 89:7 kJ=mol Δh298 5 172:1 kJ=mol Δh298 5 52:47 kJ=mol Δh298 5 2 41:2 kJ=mol Δh298 5 2 88 kJ=mol Δh298 5 7:7 kJ=mol

Depending on the configuration of the gasification, the primary reactions in a gas generator are as follows (Table 7.3): The second to fourth reactions are exothermic combustion reactions. Most of the oxygen injected into the gas reactor is consumed in these reactions. The purpose of these reactions is to provide the heat needed to dry the biomass and to initiate exothermic reactions. Gasifiers that provide the heat they need on their own are called autothermal, and those that require external input heat are called allo-thermal; most common gasifiers are called autothermal [12]. The composition and thermal value of the gas product depend on what gasification agent (air, oxygen, or steam) is used, as well as the fuel, the type of gasifier, and the operating conditions. Steam is produced in a separate production system and is superheated before entering the gasifier. Steam alone cannot be used for gasification, and because the steam reaction with fuel is endothermic, it requires more heat than the amount of steam provided. Gasification heating sources can be supplied in two ways: 1. Combusting part of the feed with air or oxygen 2. External heating system.

Gasification with steam produces a moderate thermal value and increases the gas product’s hydrogen component. Steam gasification is appropriate when synthesis gas with high hydrogen content is required; for example, for the production of methanol or usage in fuel cells. Different types of gasifiers are classified in detail in Fig. 7.4.

7.2.2 Biogas upgrading combined processes 7.2.2.1 Chemical looping Chemical looping combustion (CLC) involves using a metal oxide as an oxygen carrier, and two interconnected reactors accompany this process. The gas fuel enters the fuel reactor (FR), which is oxidized by the oxygen the metal oxide provides.

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Figure 7.4 Partial classification of gasifiers based on gas-solid contact mode.

Complete combustion in this reactor produces CO2 and water vapor, which is easily separated by condensing water vapor without the need for additional energy consumption. Waterless CO2 can be used for other applications [13]. This technology, which may be used in a single unit of power to generate electricity, achieves a higher thermal efficiency of the unit than other technologies for capturing carbon. The highest importance of these technologies is the amount of solid (carrying oxygen) required to achieve complete gas conversion, which is believed to be accountable for the highest cost of the CLC power unit [14]. The advantage of producing hydrogen with chemical looping is that the heat required to convert CH4 to H2 and CO is supplied without the need for costly oxygen production and without mixing air with gaseous fuels, which include carbon [15]. Chemical looping involves using solid oxygen carriers (OC) to provide the oxygen needed for the oxidation reaction in processes. After the oxidation reaction takes place in the FR, the OC enter the air reactor to be re-oxidized by air or pure oxygen. CLC is a promising technology for the combustion of fossil fuels without diluting CO2 with combustion gases, especially nitrogen. In the CLC, the solid oxygen

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carrier provides the stoichiometric oxygen needed to form CO2 and water, creating a nitrogen-free mixture and reducing NOX production. As a result, the need to separate CO2 from combustion gases and the high cost of capturing CO2 is eliminated. A suitable oxygen carrier for CLC can react with the gas fuel and oxidize again in contact with oxygen. OC generally consist of a metal oxide and a neutral and inert substrate, providing fluidity and mechanical stability. The reactivity and stability of OC are fundamental challenges for CLC. According to studies conducted in this field on fluidized bed reactors, while the oxides of Ni, Fe, Cu, Mn, and Co have the potential to carry oxygen, recent research has shown that Ni is the best option for CLC. Ni-based OC have shown acceptable activity and stability during repeated oxidation and reduction cycles [13]. The general outline of the chemical cycle process is shown in Fig. 7.5. The following reaction takes place in the FR: ð2n 1 mÞMy Ox 1 Cn H2m ! ð2n 1 mÞMy Ox21 1 mH2 O 1 nCO2

(7.6)

After the fuel oxidation is complete, the metal oxide MyOx-1 (or metal) is transferred to the air reactor to oxidize according to the following reaction: 1=2My Ox21 1 1=2O2 ðairÞ ! My Ox 1 ðair : N2 1 unreacted O2 Þ

(7.7)

A reduction reaction is endothermic and an oxidation reaction is exothermic. The reactions’ heat depends on the fuel type and the metal oxide carrying the oxygen. The heat of the reaction, the rate of fuel flow, and the oxygen-carrying

Figure 7.5 Schematic of the combustion process with chemical cycles [13].

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capacity of the oxygen carrier determine the specific rate of solid rotation between the reactors [13]. Depending on the type of fuel used, the FR may be fluidized by steam, CO2, or the fuel itself (when the fuel is gaseous) [16].

7.2.2.1.1 CO2 capturing with chemical cycles process As CO2 is one of the leading causes of air pollution and the primary source of greenhouse gases, efforts to prevent it from entering the environment are among the biggest concerns of the current century. There are three methods to capture CO2 [17]: 1. Precombustion processes separate carbon before combustion. 2. Combustion of oxide fuels (oxy-fuels) that uses pure oxygen obtained by removing nitrogen from the air. 3. Separation after combustion, which separates CO2 from flue gas by different methods [18].

All these processes are associated with overall combustion efficiency and generated electricity price increase. Considering all of these factors, CLC provides a low-cost and high-performance technology [13]. As mentioned, CLC is an indirect combustion process in which carbon fuels ignite without direct contact with air. The oxygen transfer between the fuel and air reactors by OC results in high-purity CO2 flow. This feature is excellent for CO2 capture applications because it does not require additional energy to separate CO2 from N2, while processes after CO2 separation combustion require energy consumption [19].

7.2.2.1.2 Hydrogen generation with a chemical looping process Chemical looping processes are an excellent way to generate hydrogen and synthesize gas. The main advantage of generating hydrogen with chemical cycles is that the heat required to convert CH4 to H2 and CO is provided without producing oxygen at a high cost and without mixing air with the carbon in the gas fuel. Hydrogen generation with the chemical loop is a promising process for supplying hydrogen that competes with methane vapor reforming with higher revenues. Using chemical cycles in hydrogen production has significant effects on the process economy. Hydrogen is generated from various sources, including fossil fuels such as natural gas and coal, nuclear energy, and the use of renewable energy sources such as biomass, wind energy, solar energy, geothermal energy, and hydraulic power over a wide range of processes [15]. Today, catalytic methane reforming (MR), natural gas, or heavy hydrocarbons, is an economical process for the generation of hydrogen or synthesis gas, which is the primary source for the production of ammonia, methanol, and many other compounds. In the partial oxidation of methane to produce synthetic gas, the source of oxygen is obtained from pure oxygen, which is obtained by refrigeration processes. In chemical looping, for partial oxidation of methane, oxygen with OC source is used instead of pure oxygen, which avoids the initial and current costs of building and operating an oxygen unit. Proper OC must have sufficiently high conversion rates in oxidation/reduction reactions, high-density stability, complete fuel conversion to

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CO and H2, low carbon deposition and hardening, and economic and environmental benefits [20]. Methane vapor reforming Chemical looping for hydrogen generation follows the same basic CLC principles. The main difference is that the products are synthesized instead of heat, hydrogen, and, finally, gas. The reaction also takes place in conditions lower than stoichiometry. Hydrogen generation with chemical looping can be achieved in different ways: CH4 1 H2 O ! CO 1 3H2 O

(7.8)

The above reaction is the sum of the following reactions: Mx Oy 1 δCH4 ! Mx Oy2δ 1 δð2H2 1 COÞ ðmetal oxide reductionÞ Mx Oy2δ 1 δH2 O ! Mx Oy 1 δH2 O ðmetal oxide oxidationÞ

(7.9) (7.10)

Since methane and steam do not enter the process simultaneously, the problem of gas purification, which is present in different steam reforming units, can be eliminated in a chemical looping-steam methane reforming (CL-SMR) [15]. Methane carbon dioxide reforming (dry reforming): CH4 1 CO2 ! 2CO 1 2H2

(7.11)

Partial oxidation of methane with oxygen or air CH4 1 O2 ! 2CO 1 4H2

(7.12)

Chemical looping hydrogen generation (CLHG): Fuel 1 H2 O 1 O2 ! H2 1 CO2

(7.13)

In the chemical looping reforming (CLR) process, a suitable oxygen carrier between two reactors is passed using methane. Methane is partially oxidized to synthesis gas. In the CLR process, the air-to-fuel ratio is kept low to prevent complete oxidation of the fuel in CO2 and water generation [21]. As mentioned earlier, the main advantage of the CLR process is that the heat required to convert CH4 to H2 and CO occurs without the need for costly oxygen generation and without mixing air with carbon-containing gaseous fuels, which is deemed attractive in both CO2 separations in combustion processes as well as in the H2 generation in reforming processes [15]. In the case of CLHG systems, the system consists of 3 reactors: the FR, the steam reactor (SR), and the air reactor (AR), as shown in Fig. 7.6 Gas fuel (CnH2m) enters FR and reacts with OC (MyOx):

m m n1 My Ox 1 Cn H2m ! n 1 My Ox22 1 mH2 O 1 mH2 1 nCO2 2 2

(7.14)

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Figure 7.6 Chemical looping hydrogen generation with oxygen carrier based on Fe [15].

Once the reactive gas is fully converted, pure CO2 can be obtained after condensing steam. Reduced OC (MyOx-2) then enters the SR and the following reaction occurs:


n1


m m My Ox22 1 mH2 O ! n 1 My Ox21 1 mH2 2 2

(7.15)

OC, which is partially oxidized (MyOx-1), then enters the AR, where it is oxidized to its original state:


n1


m 1 m My Ox21 1 O2 ! n 1 My Ox 2 2 2

(7.16)

The completely oxidized OC (MyOx) returns to FR, and a new cycle begins. This process has several advantages (ideally): there is no need for purification steps, hydrogen has high purity, and the overall reaction is exothermic [22]. Most OCs suitable for CLC are not suitable for use in CLR or direct hydrogen generation. The most suitable OC for CLR are Ni-based OC because of their strong catalytic properties. Cerium oxide (CeO2) is better environmentally friendly for CLR, while OC based on Mn3O4 and CuO are not suitable due to their weakness in selective synthesis gas and low methane conversion. Fe2O3 and CeO2 have been identified as suitable OCs for direct CLHG [23]. CL processes have typically been studied for hydrogen generation in fixed bed reactors, moving beds, and circulating fluidized beds. However, the optimal choice for reactor design depends on the reaction kinetics and the required process flow dynamic [15].

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7.2.2.1.3 Fuel used in chemical process The CLC process is divided into three categories based on the primary fuel used: 1. CLC gas fuel 2. CLC solid fuels 3. CLC liquid fuel.

CLC gas fuel As the name implies, CLC for gas fuels means that combustion is done with a primary gas fuel such as natural gas, refinery gas, methane, coal gas, synthesis gas, and so on. The oxygen carrier reacts directly with the primary fuel. CLC solid fuels In CLC solid fuels, the primary fuel is solid, such as coal, pet coke, solid waste, or biomass. CLC Solid fuels are classified based on whether the fuel reacts directly or indirectly with the oxygen carrier. In the indirect process, the fuel injected into the CLC system is gaseous, although the primary fuel is solid. Therefore solid fuel gasification is first performed in a gasifying system, and subsequently, the generated synthesis gas enters the CLC system. This process is known as the CLC with synthesis gas fuel or Syngas-CLC for short. The gasification process before combustion is endothermic by nature and requires an energy supply. To provide this energy, one of two methods is considered. Either pure oxygen is used as the gasifying agent, or energy is supplied directly from the CLC system, for example, by installing a gasifier inside the reactor. In contrast, in the direct process, solid fuel enters the FR directly into the CLC system. This process is called Solid fulled-CLC. Solid fulled-CLC is classified into two types: In-situ gasification chemical looping combustion (iG-CLC) In iG-CLC, there is no trace of the gasifier. So there will be no need for the corresponding oxygen. In this case, the solid fuel is mixed with the oxygen carrier in the FR. The oxygen carrier reacts with the gasification products of the solid fuel generated inside the FR. The FR is fluidized by H2O and/or CO2 as the gasification agent. The iG-CLC reactions are as follows: CoalðCÞ ! Volatile matter 1 CharðCÞ

(7.17)

CharðCÞ 1 H2 O ! H2 1 CO

(7.18)

CharðCÞ 1 CO2 ! 2CO

(7.19)

H2 1 CO 1 Volatile matter 1 nMex Oy ! CO2 1 H2 O 1 nMex Oy21

(7.20)

H2 O 1 CO ! H2 1 CO2

(7.21)

Mex Oy21 1 0:5O2 ! Mex Oy

(7.22)

Reactions (7.17) to (7.19) show solid fuel gasification inside the FR. The main combustion reactions with oxygen-carrying gaseous products under oxidation, reduction, and reproduction stages are shown in reactions (7.20) to (7.22).

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Chemical looping with oxygen uncoupling In CLOU, the oxygen carrier is separated to release the gaseous oxygen for combustion. This process has recently been used to overcome the slow phase of coal gasification in the iG-CLC process. The main reactions of the CLOU process are as follows: 2Mex Oy ! 2Mex Oy21 1 O2

(7.23)

CoalðSolid FuelÞ ! Volatiles 1 CharðCÞ 1 H2 O

(7.24)

CharðCÞ 1 O2 ! CO2

(7.25)

Volatiles 1 O2 ! CO2 1 H2 O

(7.26)

The distinction and separation of oxygen from the oxygen carrier are shown in Reaction (7.23). This gaseous oxygen then reacts with volatile materials and generates coal, according to reactions (7.24) to (7.26). Eventually, the oxygen carrier in the air reactor is oxidized again. Numerous studies have been conducted on solid fuels to apply in CLC processes. German lignite and coal, for example, have been identified as suitable for the CLC process due to their high reactivity [24], and the CLC process has been used for the dual purpose of waste management and power generation for plastic waste [25]. Adanez-Rubio used the first biomass fuel CLOU unit to investigate the effect of FR temperature on CO2 capturing and combustion efficiency [16]. CLC liquid fuel This is one of the newest aspects of CLC technology. Here, liquid fuels are used instead of gaseous or solid fuels for combustion. The fuels that have been used so far are sulfur and non-sulfur croissants. However, the use of heavy oil and heavy vacuum distillation residues, which are generally produced in crude oil refining, is one of the main goals for future work. In bitumen and asphalt, the two materials are first gasified in a unit separate from pyrolysis, and the resulting synthesis gas enters the batch fluid bed reactor. In another laboratory, a native oil fuel and a heavy oil fuel were injected into a batch fluid bed reactor. The use of cooking waste oil in a packed bed and batch reactor to obtain synthesis gas was reported. Liquid fuel CLC research aims to develop technology to utilize and process heavy waste oils with intrinsic CO2 capturing.

7.2.2.1.4 Chemical looping system reactors The performance of the CLC system depends on the contact between the oxygen carrier and the fuel. Therefore choosing a specific type of reactor is an important parameter for CLC processes. The essentials for designing an optimal CLC system are as follows: 1. There should be sufficient movement of OC particles between FR and AR to achieve complete fuel combustion.

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2. It should have enough time of contact between fuel/air and solid OC to reach the maximum conversion rate. 3. High temperature and pressure should be used to achieve higher overall efficiency and high pressure is also considered for better separation of CO2. 4. It should limit CO2 leakage from FR to AR.

Based on the mentioned parameters, the CLC process can be efficient in different types and forms of reactors, including two interconnected reactors of movable or substrate bed, alternating bed or recessed bed reactors, and rotating reactors. Two or more interconnected reactors of a fluid bed or a movable bed To perform the CLC process, two interconnected substrate reactors (FR and air reactor) are usually considered, with OC moving between units. When designing a largescale CLC fluid bed reactor, the shape and configuration of the two interconnected reactor beds (air and FRs) are important options and parameters. Although critical issues need to be investigated, this arrangement and the order in which the reactors are properly placed cause the high displacement of solids and the high degree of flexibility of the process with minimal leakage and air and fuel pollution [13]. Two moving bed reactors or a fluid bed consisting of a high-speed bed reactor as an air reactor and a low-speed bubble bed reactor as a FR are the most common forms of all reactors. And so far, it has been most used in existing CLC units. Fig. 7.7A shows a schematic of two interconnected fluid bed reactors. In this type of reactor, fuel is converted to FR, and the oxygen carrier inside the AR is oxidized. In addition, two loops have been used to prevent gas leaks between AR and FR. Most OC need more time for their particles to react better. The choice of such

(A)

(B) N2/O2

CO2+H2O N2/O2

AR/FR

AR/FR

CO2/H2O

Gaseous Fuel Air

Air

Gaseous Fuel

Figure 7.7 Reactors used in the process of chemical cycles. (A) Two interconnected reactors of a fluidized bed or a mixed bed. (B) Periodic reactors of a fluidized bed or a packed bed) [16].

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shapes of reactors is based on the unique reaction properties of the carriers. Some authors have also suggested bubble fluid beds for both AR and FR. A 25kw CLC system with interconnected fluidized bed reactors with circulating fluidized bed AR and a two-stage bubble fluidized bed FR has been reported [26]. Carbon capture has been reported to be about 90% efficient [27]. CLC unit with two interconnected fluidized bed reactors is developed with 88% carbon capturing at 991 C with 470 Kg/MWth solid material in the FR [16]. Periodic reactors of packed or fluidized bed An example of this type of reactor is shown in Fig. 7.7B. This type of reactor consists of at least two reactors used in parallel and periodically to continuously ensure a high-temperature gas flow to the downstream gas turbine. The process consists of cycles of oxidation and intermittent reduction in two separate reactors, which alternate with short periods of light bed fluidization in each cycle to lower the temperature and the concentration profile. The main advantages of this technology are avoidance of the possibility of working under high pressure and separating gas and particles. The disadvantage of this technology is the need for high-temperature and high-flow gas switching systems. The way the CLC works was displayed on a dynamically packed bed reactor. The pressure rose to 7 bar and the highest temperature in the reactor was 340 C, which shows that the pressure system could be used with a CLC-packed bed [28]. Rotating reactors In these reactors, OC are placed radially in the reactor. Fuel, the air needed to revive OC, and inert gases are also located in sectors of the circle. Fig. 7.8 shows an example of this type of reactor.

Figure 7.8 Types of reactors used in the process of chemical cycles (rotary reactors) [16].

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7.2.2.1.5 OC use a chemical looping process Choosing a suitable oxygen carrier is one of the most important parameters for the proper CLC processes’ performance. The effect of the manufacturing method, like the nature of the oxygen-carrying metal on the activity of the carriers, has been studied in the microfluidic bed reactor. Two methods of saturation and sedimentation have been investigated. It has been reported that some of the oxygen-carrying particles produced by sedimentation were too small and impossible to fluidize. At the same time, saturated particles are easily fluidized. Saturated carriers performed better than those prepared by sedimentation [29]. Optimal characteristics of oxygen carriers The oxygen carrier used in chemical cycle processes must have the following characteristics: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

It should have an acceptable environmental performance. It has satisfactory thermodynamic properties to convert fuel to CO2 and H2O. It should be able to carry enough oxygen. It has high reactivity for both oxidation and reduction steps. It can be used in many reduction reactions in multiple cycles. It has a high melting point. It is easy to fluidize. It has good resistance to abrasion to minimize the loss of metal particles. It has high mechanical stability and resistance to pressure and abrasion in particle displacement along the cycles. Carbon depletion on carriers should be minimized. It satisfies the size of the appropriate particles. It should be economically viable. It should be resilient to density [30].

Optimal characteristics of oxygen carriers 1. 2. 3. 4. 5. 6. 7.

Ni-based Cu-based Fe-based Mn-based Co-based Perovskites Other types of OC.

Ni-based oxygen carriers After conducting a comparison of OC with Fe, Ni, Cu, Ce, and Mn bases, along with coating on inert materials such as Al2O3, SiO2, TiO2, and Mg-ZrO2, as well as preparation by various methods, it was found that Ni-based carriers have the most robust catalytic properties. Therefore, they are considered to be the most attractive carriers [31,32]. In fact, the metal Ni is used as a catalyst in most commercial steam reforming processes [21]. OC particles containing Ni are prepared by different methods (granulation by freezing, saturation, and spray drying) and using different supports (such as Al2O3, MgAl2O4, and NiAl2O4); they have shown acceptable results in CLR in small laboratory units like pilot units [32]. A distance before Ni-based oxygen carriers is needed.

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Ni-free oxygen carriers Ni-free metal oxides are considered energy carriers for hydrogen generation by chemical looping processes. The well-known advantages of iron oxide are low cost, chemical stability, nontoxicity, the low temperature requirement for sintering, and low fluidity [33]. In fact, the principle of using CLHG from iron oxide as an oxygen carrier for the production of hydrogen is the vapor-iron process. CLHG focuses not only on hydrogen production but also on CO2 uptake. The CLHG basis is slightly different from CLC. In CLC and CLR, two interconnected reactors are commonly used. While CLHG with iron oxide as an oxygen carrier, it consists of three reactors: FR, steam reactor, and air reactor. In the FR, the initial OC (Fe2O3) is reduced. Reduced OC (Fe or FeO) is directed to the vapor reactor and oxidized by the vapor to the intermediate state (Fe3O4), and hydrogen is generated. Finally, in an air reactor, Fe3O4 is recovered with oxygen or air to Fe2O3 [33]. Iron oxides have high strength and stability in CLC processes but low reactivity. However, the iron carriers of oxygen have been upgraded to high activity in the reaction of hydrophilicity to produce hydrogen [20,22]. However, when Fe2O3 is coated without CLR, its reactivity declines rapidly after several oxidation/reduction cycles, making it unsuitable for long-term operations and cycles [34]. Higher temperatures increase hydrogen efficiency and improve iron oxides’ activity. Lower temperatures are more suitable for carbon hardening [26,32]. Although the duration of the reduction reaction affects the production of hydrogen, and the longer the reduction reaction, the deeper the OC and the greater the increase in hydrogen production (in the vapor oxidation phase), the conversion of CO at the end of the reaction time is much shorter. Another critical factor is particle size, which affects hydrogen generation in the vapor oxidation phase. Hydrogen generation with smaller particles (125
200 mμ) increases compared to 200
300 μm and 300
450 μm [33]. It has been reported that the preparation of Fe2O3/Al2O3 OC using ultrasonic waves improves the surface properties of iron-based OC and particle uniformity [15]. Pure metal oxides do not satisfy all the desirable properties of OC. Reaction rates drop rapidly after several cycles and reactions of oxidation and reduction of metal oxides. Therefore different metal oxides are associated with coatings or sometimes combined with other metal oxides. This increases the reaction’s surface area, mechanical stability and abrasion resistance, and ionic conductivity of the solids. So far, thousands of energy carriers have been tested for CLC [16]. Cu-based oxygen carriers Cu-based OC have advantages over stability during cycle processes, high oxygen displacement capacity, and mechanical stability. Reactions between CuO and gaseous products are exothermic and will be helpful for biomass reforming. In addition, Cu-based OC have higher activity than Fe-based OC, and they are cheaper than Ni-based OC and more environmentally friendly [14]. It should be noted that the reduction of CuO and the oxidation of Cu are endothermic reactions. Therefore Cu-based OC do not need heat to maintain the operating temperature to reduce the reaction in the CLR process. The most important disadvantage of Cu-based OC is their hardening and deposition, and dehydration due to the relatively low melting point of the metal Cu.

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The use of copper can play a role in heat transfer between two reaction zones in the CLR process. Since CuO releases heat with methane during its reduction, it must react with another metal that reacts exothermically (such as Fe2O3). Therefore using CuO can limit the temperature drop in the FR. In addition, the combination of copper and iron should improve carrier stability by maintaining high reactivity in oxidation and reduction reactions. Ce-based oxygen carriers CeO2 is one of the most appealing oxides in the field of catalysts and materials because the lack of oxygen can be eliminated quickly and formed in a wide range of operating temperatures, which causes a high capacity to store oxygen. The three-capacitance ions in Ce reduce the activation energy required to transport oxygen. In contrast, adding smaller ions increases the oxygen storage capacity by delaying the reduction of oxygen storage capacity due to the lack of oxygen at high temperatures. Pure CeO2 loses its ability after ten successful reduction cycles [15]. Table 7.4 shows the oxygen carriers tested in the continuous CLC units. Table 7.5 also shows the storage capacity of oxygen in oxygen carriers. Perovskite carriers Complex perovskite metal oxides are a series of combined oxides with the general formula of ABO3, A is usually a lanthanide ion and/or alkali metal and B is an intermediate metal ion [15]. The ideal perovskite structure can be octagonal BO6, with A in the center. Sometimes several defects in the structure are usually caused by a lack of oxygen. Therefore the structure of perovskite is expressed as ABO3-δ, which indicates the amount of oxygen deficiency. In terms of thermal stability, good mechanical properties and good reactivity of perovskite oxides are considered catalysts for oxidation/reduction reactions and OC for CL processes, and their certain types have shown a good ability to regenerate themselves. There are two different types of oxygen in perovskite oxides. In one type, oxygen is absorbed at the surface, and in another type, it is located in the mass structure, which plays different roles in CL processes. In the first type, oxygen participates in the complete combustion of methane, while in the second type, oxygen causes the partial oxidation of methane to CO and H2 [15]. As shown in Table 7.6, the nine perovskite oxide structures synthesized by different methods were examined in a series of experiments as mass carriers of oxygen and coating for iron oxides. Other types of oxygen carriers Thermodynamic studies of possible oxidation/ reduction pairs have shown that tungsten contains substances that have the potential as an alternative to iron oxide due to the high melting point of the system, which offers higher stability for precipitation. One of the problems with the CLR process is the hardening and precipitation of OC particles, which reduces the activity during repetitive cycles with the use of various additives (Ce and Zr) to carry the oxygen of tungsten oxide to improve and stabilize the carrier, this problem is less common. Iron-based OC have been reported to have the highest resistance to deposition and hardening and the highest capacity to absorb oxygen surfaces with other materials.

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Table 7.4 Various oxygen carriers were tested in continuous CLC units [15]. Oxygen carrier(s)

Support

Application

Operational time (h)

NiO 40% by weight NiO 40% by weight Ni, Fe, and Mn-based NiO 60% by weight CuO NiO Ni-based Ni-based Ni-based Low Ni content NiO CuO 15% by weight Cu-based Cu-based Fe2O3 Fe2O3 α-Fe2O3 Fe2O3 Fe2O3 Fe-ESF from bauxite Tierga iron ore Mn-based Mn3O4 Co-based Fe2O3 1 NiO Ilmenite Ilmenite NiO 1 CuO

NiAl2O4 NiAl2O4

10 KW continuous CLC 10 KWth 300 Wth CLC 50 KWth CLC 10 KWth CLC 10 KWth CLC 120 KWth CLC 10 KWth CLC 500 Wth CLC 500 Wth CLC 300 Wth CLC 10 KWth CLC 1.5 KWth CLOU 300 Wth CLC 300 Wth CLC 500 Wth CLC 1 KWth CLC 500 Wth CLC 10 KWth CLC 500 Wth CLC 500 Wth CLC 300 Wth CLC 300 Wth CLC 50 KWth CLC 1 KWth CLC 100 Wth CLC 300 Wth CLC 500 Wth CLC

100 . 1000

Bentonite γ-Al2O3 MgO or MgAl2O4 α-Al2O3 CaO/ Al2O3 γ-Al2O3

Al2O3 Al2O3 γ-Al2O3

ZrO2/MgO CoAl2O4 Bentonite

Al2O3

28 120 . 50 160 100 90 54 120 45 40 50 20 75 78 40 50 17 25 12 80 67

Table 7.5 Oxygen transport capacity for different metal/metal pair oxides [16]. Metal oxide

Moles of O2/ mole matal

NiO/Ni CuO/Cu Cu2O/ Cu Fe2O3/ Fe3O43 Mn2O3/MnO Mn3O4/MnO Co3O4/Co CoO/Co

0.5 0.5 0.25 0.083 0.25 0.17 0.67 0.5

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Table 7.6 Summary of mixed and combined oxides [35]. Sample no.

Composition (wt.%)

Preparation method

LSF-0 BSF-0 CTF-SG-0 LSF-SG-0 LSF-1 BCF-1 CTF-1 CTF-2 CTF-SG-1 YSZ

100% La0.7Sr0.3FeO3 100% BaCe0.7Fe0.3O3 100% CaTi0.85Fe0.15O3 100% CaTi0.85Fe0.15O3 50% Fe2O3, 50% La0.7Sr0.3FeO3 50% Fe2O3, 50% BaCe0.7Fe0.3O3 50% Fe2O3, 50% CaTi0.85Fe0.15O3 50% Fe2O3, 50% CaTi0.85Fe0.15O3 50% Fe2O3, 50% CaTi0.85Fe0.15O3 60% Fe2O3, 40% YSZ

Solid state reaction Solid state reaction Solid state reaction Sol-gel Solid state reaction Solid state reaction Solid state reaction Solid state reaction Sol-gel Solid state reaction

Table 7.7 Oxygen carriers in terms of CL processes [15]. OC type

CLpartial oxidation of methane with air/ oxygen

CL-steam reforming of methane

CLwater splitting

Perovskites Ni-based

11 1

1 11

Fe-based

1

11

1

Cu-based Ce-based Other OCs

1 1

1 1 1

1 1

CLoxidation or reforming of other fuels 1(CO) 1(Kerisen, glycerol) 11(CO, CO2) 1(biomass)

CLHG

CLCSR

1 1

1 Have been reviewed. 11 has been extensively studied and has yielded acceptable results.

Although copper has been shown to have a high oxygen transport capacity, its resistance to hardening, precipitation, accumulation, and compaction is relatively low. Oxygen consumption and hydrogen production capacity increase with increasing temperature to an optimal value. Further rise in temperature, due to the phenomenon of hardening and deposition, results in less hydrogen production. Of course, reducing operating costs to build OCs in CL processes is desirable, so the use of cheaper natural minerals such as ilmenite (ilmenite-FeTiO-Fe2TiO5) has attracted much attention in recent years [21,36]. Table 7.7 shows the CL processes for hydrogen production based on studied OC and more acceptable ones. Most OCs are Ni-based OC for SMR processes.

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However, due to their price and toxicity, they are intended to be replaced by Febased materials. With Fe-based OC, acceptable results were obtained. In addition, they are actively investigated for processes using proposed fuels (CO, CO2, and biomass) along with Cu-based OCs and perovskites. Fe-based OCs are promising for use in CLHG. Although several types of improved and advanced OC have been identified, a full evaluation of their performance and further optimization studies is needed. OC thermal stability still needs further improvement, for example, with more concentration and purification, introducing a permeable liquid provided by another oxide, and/or adding pure oxides with higher oxygen storage capacity [15]. There are a limited number of studies that have examined the lifespan of OC [20].

7.2.2.1.6 Operational conditions in the chemical looping process In chemical looping processes, several operating conditions affect the amount of CO2 captured, the amount of hydrogen generated, the combustion efficiency, and any target defined in the process. Solid flow rate, oxygen hold-up time, fuel feed rate, air feed flow rate, steam feed flow rate, oxygen carrier to fuel ratio, FR temperature, purification of OC from carbon, etc., are known as factors affecting the process efficiency. The starting temperature of the reaction, the preheating of the feed, and the time elapsed since the oxidation and reduction reactions also affect the conversion rate.

7.2.3

Hybrid biomass energy systems to produce power

Over the last three decades, the fast pyrolysis process has been considered by many researchers as a promising technology to meet energy demand and reduce greenhouse gas emissions. Fast pyrolysis is the process by which organic matter, such as biomass, is converted to liquid fuel by thermochemical decomposition in the absence of oxygen. In this process, the biomass heats up quickly to intermediate temperatures of about 500 C, and the residence time of the vapors is short, about 2 s. In fast pyrolysis, just as the heating rate is high and fast, the cooling operation is fast to convert the compressible vapors into the main product, which is a liquid called biooil. One of the characteristics of bio-oils extracted from fast pyrolysis is the ability to store and transfer quickly to solid biomass. Incompatible gases and coal are pyrolysis by-products that can heat the process or sell coal with good energy content. Therefore in a sense, almost no waste and disposal are produced during this process. It should be noted that the enthalpy and density of biomass are calculated with the help of biomass properties. The approximate and final analysis of biomass and the chemical compounds used in this process are listed in Tables 7.8 and 7.9.

7.2.3.1 Process description and simulation In this section, the fast pyrolysis unit was first investigated and simulated. Based on the results of pyrolysis of four types of feed, which are discussed in the next

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Table 7.8 Analysis and chemical composition of biomass used [37
40]. Feedstock

Hybrid poplar

Eucalyptus

Pine

Wheat straw

82.7 16.9 0.4

84 15.9 0.1

74.8 18.1 7.1

50.1 6 44 0.1 0.01

50.5 6.4 43 0.1 0.01

45.9 5.67 40.5 0.7 0.16

45.3 25 20.9

42 23 24

43.2 24.1 22

Proximate analysis (wt.% dry basis) Volatiles matter Fixed carbon Ash

85.6 12.3 2.1

Ultimate analysis (wt.%) C H O N S

50.5 5.97 40.8 0.6 0.02

Chemical composition (wt.% dry basis) Cellulose Hemicellulose Lignin

43.8 20.4 29.1

Table 7.9 The main compounds defined in the process. Component name

Alias

Component name

Alias

Water Nitrogen Oxygen Carbon-dioxide Carbon-monoxide Methane Ethane Ethylene Propylene Hydrogen Hydrogen-sulfide Ammonia Hydrogen-cyanide Acetic-acid Formic-acid Propionic-acid Phenol Acetol Acetaldehyde Glycol-aldehyde

H2 O N2 O2 CO2 CO CH4 C2H6 C2H4 C3H6
2 H2 H2 S H3 N CHN C2H4O2
1 CH2O2 C3H6O2
1 C6H6O C3H6O2-D1 C2H4O-1 C2H4O2-D1

Glyoxal Ketene Acetone Naphthalene Furfuryl-alcohol Methanol Ethanol Ethylene-glycol Acrylic-acid Guaiacol 2-pyrrolidone Pyrrole Hydrogen-chloride L-glutamic-acid Dpg-monomethyl-ether Propane N-butane M-cresol 2,3-xylenol 2-methyl-pentane

C2H2O2 C2H2O C3H6O-1 C10H8 C5H6O2 CH4O C2H6O2 C2H6O2 C3H4O2
1 C7H8O2-E1 C4H7NO-D2 C4H5N-2 HCL C5H9NO4 C7H16O3 C3H8 C4H10
1 C7H8O-4 C8H10O-5 C6H14
2 (Continued)

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335

Table 7.9 (Continued) Component name

Alias

Component name

Alias

Formaldehyde Methylcyclohexane N-propylcyclohexane Bicyclohexyl Chrysene Benzene Toluene M-xylene Ethylbenzene N-propylbenzene N-pentane N-hexane N-heptane N-octadecane Cyclopentane Cyclohexane Cyclohexene Carbon-graphite

CH2O C7H14
6 C9H18
1 C12H22 C18H12 C6H6 C7H8 C8H10
2 C8H10
4 C9H12
1 C5H12
1 C6H14
1 C7H16
1 C18H38 C5H10
1 C6H12
1 C6H10
2 C

Anethole Furfural Furan 1-propanol N-butanol N-octane N-nonane N-decane N-undecane N-dodecane N-tridecane N-tetradecane N-pentadecane Methylcyclopentane 1-hexanol 1-pentanol Cyclohexanol

C10H12O C5H4O2 C4H4O C3H8O1 C4H10O1 C8H18
1 C9H20
1 C10H22
1 C11H24 C12H26 C13H28 C14H30 C15H32 C6H12
2 C6H14O1 C5H12O1 C6H12O1

chapter, due to the production of more bio-oil by the hybrid poplar biomass, this biomass was considered the most suitable feed for the process. The biorefinery is intended as a downstream pyrolysis unit for refining and improving the properties of bio-oil obtained from fast pyrolysis and its conversion to petrol and gasoline. The biorefinery includes hydrogen processing, distillation, hydrocracking, and steam reforming units. Also, as by-products of the pyrolysis reactor, incompressible coal and gases enter the gasification unit to be converted into syngas. The syngas then enters the steam reforming unit to produce the hydrogen required. It should be noted that considering this unit, the process does not require the consumption of natural gas and its conversion to hydrogen. In addition to the mentioned units, the power generation unit is also considered to generate electricity from the thermal energy recovery of the process. The flow block diagram is shown in Fig. 7.9. This will describe the details and steps of different process units.

7.2.3.1.1 Fast pyrolysis The process of producing bio-oil from fast pyrolysis includes four stages: biomass drying, fast pyrolysis, solid particle separation, and bio-oil recovery. The process flow diagram of the fast pyrolysis unit is shown in Fig. 7.10. First, the biomass is crushed, has a moisture content of 26 wt.% (flow P1), and enters the drying stage. As mentioned earlier, biomass drying is essential due to increased process efficiency and production of better-quality bio-oils. Biomass

336

Hybrid Poly-generation Energy Systems Gas 920°C , 1 bar 8.6 kg/s

Inert gas 25°C , 1 bar 7.3 kg/s Biomass 25°C , 1 bar 8.3 kg/s

Bio-char 500°C , 1 bar 1.2 kg/s

Fast Pyrolysis

Gasification

Feed water 25°C , 1 bar 12.5 kg/s Bio Oil 15 °C , 1 bar 5.7 kg/s

Syn Gas 350°C , 47 bar 9.5 kg/s

H2O Steam

Electricity Power 5.6 MW

Power Generation H2O

Biorefinery

H2

Distillation & Hydrocracking

Hydrotreating

Steam Reforming Gases Diesel 62°C , 0.01 bar 1 kg/s Gasoline 32°C , 2.3 bar 1.1 kg/s

Figure 7.9 Block diagram of the studied process flow [41].

S6 (To Steam Reforming)

P7

S21 (To Steam Reforming)

P9 C-1

P11 (To Power Generation)

P6

H5 (From Hydrotreating)

EX-2

DRYER

P10 (To gasification)

S5 (From Steam Reforming)

H6 (To Hydrotreating)

EX-1

P2

EX-3 FL-1

S20 (From Steam Reforming)

P1

P8 (To hydrotreating) P5 P3 PYROLYSIS S18 (From Steam Reforming)

CYC-1 P4 (To gasification)

Figure 7.10 Process diagram of fast pyrolysis unit flow [41].

drying takes place in the R-Stoic stoichiometric reactor. This reactor is used when the reaction rate is unknown or does not matter, and the reaction’s stoichiometry with the conversion rate is known. To dry the biomass, partial heat recovery of the exhaust from the direct combustion reactor (S18 flow) is used.

Biomass-based hybrid energy systems

337

The heat required for biomass drying is supplied to the direct combustion reactor by controlling the amount of incoming air (flow S13). The amount of biomass moisture output from the drying reactor is reduced to 8 wt.% and then enters the next stage, the pyrolysis reactor. To simulate the pyrolysis reactor in the software, the CSTR reactor is used, whose temperature and operating pressure are equal to 520 C and 1 bar. This reactor is used to simulate equilibrium and kinetic reactions. With the help of this reactor, the amount and composition of the obtained products can be well predicted. In this reactor, nitrogen gas (P2 flow) is used as a fluidity agent in the fluidized bed reactor (520 C and 1 bar). In a pyrolysis reactor, the biomass decomposes under high heat into compressible gases, incompressible gases, and coal. The kinetic model is shown in Table 7.10. In the next step, the products of the pyrolysis reactor enter the cyclone. In cyclones, unreacted coal and biomass (flow P4) are separated from incompressible vapors and gases. The high-temperature incompressible vapors and exhaust gases from the cyclone then enter the bio-oil recovery phase. For this purpose, we cool them in three steps up to 15 C. To cool these gases, we put them in two heat exchangers EX-1 and EX-2 to use their heat energy to heat other currents. The cooling process under operating conditions must be performed quickly, as this prevents the cracking of organic compounds in the gas phase. The cooled gases and condensate then enter the separator. In this separator, incompressible gases (flow P9) are separated from condensate (flow P8). Compressible gases enter the gasification reactor after heating in the EX-3 exchanger and reaching the gasification reaction temperature. In addition, unreacted coal and biomass are sent to the gas reactor, while the recovered condensate (P8 flow), or in other words, the recovered bio-oil, which is a combination of about 30 different substances, leaves the pyrolysis unit and enters the biorefinery to refine and convert it into gasoline and biodiesel. It should be noted that in operating conditions, as the separation of solid particles in cyclones does not occur ideally, we usually see a small number of solid particles in bio-oil.

7.2.3.1.2 Gasification By-products of the fast pyrolysis process, coal (P4 flow) and hot gases (P10 flow), enter the gasification reactor. In addition, water vapor (flow E11) enters the reactor under 927 C and 1 bar as a gassing agent produced in the power generation unit. Figure 3
3 shows the process flow diagram of this unit. The RGibbs reactor at 927 C and 1 bar are used to simulate the gasification reactor. In this reactor, there is no need to specify the reaction’s stoichiometry. This reactor predicts equilibrium constants and equilibrium products by minimizing Gibbs’s free energy. It should be noted that because the coal flow in Aspen Plus software is defined as nonconventional flow, the Fortran calculator block is used for its calculations. Biomass gasification involves several reactions, as shown in Table 7.11 (Fig. 7.11). The products are then transferred from gasification to cyclone to separate solid particles (G9 flow) from gases (G2 flow), and then these gases lose their heat and

Table 7.10 The applied relations for the kinetic model [41]. Reaction

Rate

Unit

K 5 8 3 1013 expð 2 46000=RTÞ K 5 1 3 109 expð 2 30000=RTÞ

½s21  ½s21 

K 5 4 3 T 3 expð 2 10000=RTÞ K 5 8 3 107 expð 2 32000=RTÞ

½s21  ½s21 

K 5 1 3 1010 expð 2 31000=RTÞ K 5 3 3 109 expð 2 27000=RTÞ K 5 3 3 T 3 expð 2 11000=RTÞ K 5 1 3 1010 expð 2 33000=RTÞ

½s21  ½s21  ½s21  ½s21 

K 5 4 3 1015 expð 2 48500=RTÞ K 5 2 3 1013 expð 2 37500=RTÞ K 5 1 3 109 expð 2 22500=RTÞ K 5 5 3 106 expð 2 31500=RTÞ

½s21  ½s21  ½s21  ½s21 

K 5 1 3 105 expð 2 20500=RTÞ K 5 8 3 T 3 expð 2 12000=RTÞ K 5 1:2 3 109 expð 2 30000=RTÞ

½s21  ½s21  ½s21 

Lumped kinetic scheme of Cellulose pyrolysis CELL ! CELLA CELLA ! 0:95HAA 1 0:25Glyoxal 1 0:20C3 CHO 1 0:20C3 H6 O 1 0:25HMFU 1 0:02CO2 1 0:15CO 1 0:1CH4 1 0:9H2 O 1 0:65Char CELLA ! LVG CELL ! 5H2 O 1 6Char Lumped kinetic scheme of Hemicellulose pyrolysis HCE ! 0:4HCEI 1 0:6HCE2 HCEI ! 2:5H2 1 0:125H2 O 1 CO 1 CO2 1 0:5CH2 O 1 0:25CH3 OH 1 0:125C2 H5 OH 1 2Char HCE1 ! XYLOSE HCE2 ! 1:5H2 1 0:125H2 O 1 0:2CO2 1 0:7CH2 O 1 0:25CH3 OH 1 0:125C2 H5 OH 1 0:8GfCO2 g 1 0:8GfCOH2 g 1 2Char Lumped kinetic scheme of Lignin pyrolysis LIG 2 C ! 0:35LIGCC 1 0:1pCoumary1 1 0:08Phenol 1 1:49H2 1 H2 O 1 1:32GfCOH2 g 1 7:05Char LIG 2 H ! LIGOH 1 C3 H6 O LIG 2 O ! LIGOH 1 CO2 LIGCC ! 0:3pCoumary1 1 0:2Phetol 1 0:35C3 H4 O2 1 1:2H2 1 0:7H2 O 1 0:25CH4 1 0:25C2 H4 1 1:3GfCOH2 g 1 0:5GfCOg 1 7:5Char LIGOH ! LIG 1 0:5H2 1 H2 O 1 CH3 OH 1 GfCOg 1 1:5GfCOH2 g 1 5Char LIG ! C11 H12 O4 LIG ! 0:7H2 1 H2 O 1 0:2CH2 O 1 0:5CO 1 0:2CH2 O 1 0:4CH3 OH 1 0:2CH3 CHO 1 0:2C3 H6 O2 1 0:4CH4 1 0:5C2 H4 1 GfCOg 1 0:5GfCOH2 g 1 6Char

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339

Table 7.11 Gasification reactions [41]. Reaction name

Chemical equation

Heat of reaction ΔH (KJ/mol)

Carbon combustion Carbon partial oxidation Boudouard reaction Water gas reaction Water gas shift reaction Methanation of carbon Hydrogen partial combustion Steam reforming of methane

C 1 O2 ! CO2 C 1 0:5O2 ! CO C 1 CO2 ! 2CO C 1 H2 O ! CO 1 H2 CO 1 H2 O ! CO2 1 H2 C 1 2H2 ! CH4 H2 1 0:5O2 ! H2 O CH4 1 H2 O ! CO 1 3H2

2393.0 2112.0 1172.0 1131.0 241.0 274.0 2242.0 1206.0

C-3

G4

EX-5

H11 (From Hydrocracking) G7

S3 (To Steam Reforming)

G8 (To Steam reforming)

C-2 S2 (From Steam Reforming)

G3

CMP-1

S15 (From Steam Reforming)

G2

E11 (From Power Generation)

P4 (From pyrolysis)

G1

H10 (From Hydrocracking)

FL-2

EX-4

P10 (From pyrolysis)

EX-6 G6

S16 (To Steam Reforming)

G5 (To Power Generation)

CYC-2

GASIFIRE

G9

Figure 7.11 Process diagram of gasification unit flow [41].

cool to 25 C in three stages of EX-4, EX-5, and C-2 heat exchangers. The use of its energy heats other process flows. These gases then enter the separator to separate the condensed liquids (G5 flow) from the gas and do not damage the compressor. This liquid flow is used in the power generation unit to produce water vapor. The gas flow from the separator enters the compressor and is compressed to 44.5 bar, increasing the temperature (G6 flow). This compressor has a polytropic efficiency of 79% and a mechanical efficiency of 95%. To use this current in the steam reforming unit, its temperature must be reduced to 350 C. This is done in EX-6 and C-3 exchangers.

7.2.3.1.3 Hydrogen process unit In this unit, crude bio-oil is converted to an almost oxygen-free product, which is operated in the industry in two catalytic reactors, and the catalyst used in the operating conditions is usually Co-Mo [42]. A view of this unit is shown in Fig. 7.12.

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Hybrid Poly-generation Energy Systems

H10

C-4

H1

P8 (From Pyrolysis)

H11 (To Distillation)

EX-7 P-1 S22 (From Stem Reforming)

H2

FL-3

HT REACT-1

M-1

H12 (To Steam Reforming) H9

H4

COMP-2 P6 (To Pyrolysis) H3

HT REACT-2

H5 H6

H7

EX-8 EX-1 P5 (From Pyrolysis) H8

Figure 7.12 Process diagram of hydrogen processing unit flow [41].

Crude bio-oil (flow P8) is pumped from the pyrolysis unit, and its pressure is increased to 170 bar (flow H1). This stream is then heated to 164 C and mixed with condensed hydrogen from the reforming unit (stream H3). This temperature is heated to about 280 C in the EX-8 exchanger (H4 current) and enters the first hydrogen processing reactor. This reactor operates under conditions of 270 C and 17 bar and stabilizes the products. Hydrogen curing reactions are exothermic, the heat energy of which is used to supply the heat required by distillation tower boilers. The output of this reactor enters the EX-1 exchanger to increase the temperature and passes through the pressure relief valve before entering the second reactor. The second reactor operates at higher temperatures and lower pressures so that its operating temperature and pressure are 350 C and 150 bar. The output products of the second reactor (H8 current) are deoxygenated and stabilized. Both reactors are simulated by the RYield reactor. This reactor is used when the distribution of products is known, but the stoichiometry and reaction speed is unknown. The information in the articles has been used to predict the output of these reactors [43
46]. The product of hydrogen-treated reactors is then cooled to EX-8, EX-7, and C-4 in three stages at a temperature of 22 C and then fed into a separator. In this equipment, organic and aqueous compounds are separated from each other. The product with the aqueous phase (current H12) enters the steam reforming unit, and the product with the organic phase (H11) is sent to the distillation unit.

7.2.3.1.4 Distillation and hydrocracking unit Fig. 7.13 shows the process flow diagram of this unit. In this unit, organic compounds from the hydrogen processing unit are distilled in two stages and converted to biogasoline and diesel. Organic compounds (flow H11) are mixed with the cool flow output from the hydrocracking reactor (flow D1) and enter the flash tower.

Biomass-based hybrid energy systems

341 D2 (To Steam Reforming)

H11 (From Hyrotreating)

M-2 D5 (To Steam Reforming) D6 (Gasoline) D1

ATM

FL-4

D8 (Diesel)

VAC D7 D4 D9

D13

M-3 C-5

D3

P-2

HC REACT

G6 (From Gasification)

EX-9 S23 (From Steam Reforming)

D11

D10

D12

EX-6

G7 (To Gasification)

COMP-3

Figure 7.13 Process flow diagram of distillation and hydrocracking unit [41].

In this tower, unreacted hydrogen (current D2) is separated from the organic compounds and sent to the steam reforming unit. The liquid leaving the flash tower then enters the pressure relief valve and heats up to about 160 C in the EX-9 exchanger (current D4) and enters the atmospheric distillation tower. The tower operates under atmospheric conditions and consists of eight trays, while the vacuum distillation tower operates in near-vacuum conditions with nine trays. RadFrac distillation tower has been used to simulate both distillation towers from the tower in Aspen Plus software. This tower is used to simulate multistage separators accurately. The atmospheric distillation tower produces biogasoline (stream D6) and a small amount of gas (stream D5) which enters the steam reforming unit. The heavy residue of this tower (flow D7) enters the vacuum distillation tower to be converted to biogasoline (flow D8). Finally, the heavy residue of the vacuum distillation tower (flow D9) is pumped, and its pressure is increased to 89 bar. The flow is then mixed with the concentrated hydrogen produced by the steam reforming unit and heated to an EX-6 exchanger temperature of 652 C before entering the hydrocracking reactor. Due to the lack of sufficient information on the bio-oil hydrocracking reactions, the RYield reactor has been used to simulate the hydrocracking reactor. Heavy hydrocarbons are converted to lighter hydrocarbons with smaller carbon chains during the hydrocracking reaction. In industrial conditions, these operations usually occur under 675 C and 89 C and in the presence of a commercial nickel catalyst. The information in the articles [43,46] has been used to simulate this reactor.

7.2.3.1.5 Steam reforming unit Mainly hydrogen requirement for hydrocracking and hydrogen processing units is one of the limitations of these processes. To meet this need, natural gas is usually

342

Hybrid Poly-generation Energy Systems

used as feed in the reforming unit. In the proposed process, there is no need to use natural gas, and instead of using natural gas, a small gasification unit is considered. The syngas produced from it enters the steam reforming unit to meet the hydrogen demand of the various units. As shown in Fig. 7.14, this unit supplies hydrogen for hydrocracking and hydrogen processing reactors. Some of this hydrogen is supplied by light hydrocarbons from hydrogen distillation and processing units (streams H12 and D2). The gas flow of light inlet hydrocarbons from the distillation unit (flow D2) is divided into two parts. One of these currents (S1) enters the compressor and condenses to a pressure of 50 bar, and the other current enters the pressure relief valve (S12) and enters the direct combustion reactor as fuel. After condensing in the compressor, current S1 is mixed with water vapor (flow S21), heated in two stages in EX-5 and H-1 exchangers to 500 C, and enters the steam reforming reactor. The RGibbs reactor was used to simulate the steam reforming reactor. In industry and under operating conditions, this reactor operates in the presence of a commercial nickel catalyst, and its operating temperature and pressure are equal to 950 C and 48 bar [47
49].

D2 (From Distillation)

S12

DT COMB

S-1 S1

G4 (To Gasification)

ST REACT

M-4 COMP-4

S2

S3

S18 (To Pyrolysis)

H-2 S17

G2 (From Gasification)

EX-5

H-1

M-6

EX-10 S4

S16

G3 (From Gasification)

S15 S5

P10 (To Pyrolysis)

P9 (From Pyrolysis)

EX-4 G3 (To Gasification)

D5 (From Distillation)

EX-3

S14 S11

S13

S6

M-7

S21 COMP-5

C-6 M-5 G8 (From Gasification)

WGS REACT

S8

FL-5

P6 (From Pyrolysis) H12 (From hydrotreating)

C-7 S10

S20

M-7

P-3

S-3 PSA S23 (To Hydrocracking) S19 (To Power Generation)

EX-2 P7 (To Pyrolysis)

S22 (To Hydrotreating)

S9

S7

S-2

Figure 7.14 Steam reforming unit flow process diagram [41].

Biomass-based hybrid energy systems

343

The steam reforming reaction of hydrocarbon gases is as follows [50]: Cn Hm Ok 1 ðn 2 kÞH2 O ! nCO 1 ðn 1 m=2 2 kÞH2

(7.27)

Also, a water-gas shift reaction occurs during the reforming, which is given as follows: nCO 1 nH2 O2nCO2 1 nH2

(7.28)

Finally, the general reaction can be summarized as follows: Cn Hm Ok 1 ð2n 2 kÞH2 O ! nCO2 1 ð2n 1 m=2 2 kÞH2

(7.29)

The steam reforming reaction is an endothermic reaction in which burning waste gases in a direct combustion reactor supplies the required heat. The products of steam reforming (flow S4) are cooled in three stages in exchangers EX-10, EX-3, and C-6 to a temperature of 350 C and mixed with the flow of synthetic gases obtained from the gasification unit (flow G8). It then enters in the water-gas shift reactor. The water-gas shift reaction takes place in a REquil reactor under 350 C and 48 bar conditions. This reactor is used when the stoichiometry of the reaction is known. The output of this reactor is cooled to a temperature of 77 C (current S8) and then enters the flash tower. In this tower, unreacted water (flow S10) is separated from gas flow (flow S9). The gas stream then enters the pressure swing adsorption (PSA) stage, and the unreacted water stream is divided into two parts. Part of it (current S19) is sent to the power generation unit, and the rest is mixed with the aqueous phase of the processing unit with hydrogen (current H12) and enters the pump, and its pressure increases to 50 bar (Current S20). The EX-2 is then heated to 137 C, and the water in this stream is converted to steam. The separation equipment uses the PSA unit, which separates around 90% of the hydrogen from the inlet gas (flow S9). The separated hydrogen is then directed toward the hydrogen processing units (flow S22) and hydrocracking (flow S23). Meanwhile, the remaining gas (flow S11) is mixed with light gaseous hydrocarbons from the distillation unit (flow D5) and the necessary airflow for combustion (flow S13). It is heated in three stages in EX-4, EX-10, and H-2 exchangers and burned in a direct combustion reactor. The direct combustion reactor is simulated by the RGibbs reactor operating at a temperature of 1090 C and a pressure of 2 bar.

7.2.3.1.6 Power generation Fig. 7.15 shows that this unit comprises a steam turbine through which electricity is generated. In this unit, first, the water flow pressure inside the cycle (flow E1) is reached 50 bar by the pump. The high-pressure water is then split into two streams to convert to superheated water vapor at 450 C and 50 bar. The E2 current is converted to steam in the EX-11 exchanger by recovering the hot exhaust current from the dryer. In contrast, the flow of E4 is converted to steam through the flow of

344

Hybrid Poly-generation Energy Systems E3

EX-11 E12

P11 (From Pyrolysis) E2

S-4 M-8 E4

E5

P-4

Power

H-3 Heat Flow From C-2,4,5,6 E1

E6

G5 (From Gasification)

ST-1

C-8 E7

S19 (From Steam Reforming)

M-9 FL-6

E8

M-10

E9

H-4

E11 (To Gasification)

E10 Heat Flow From C-1,3,7

Figure 7.15 Power unit flow process diagram [41].

energy received from coolers 2, 4, 5, and 6 in the H-3 exchanger. Then two superheated steam streams mix and enter the turbine. In a superheated steam turbine, energy is lost, causing the blades and shafts of the turbine to rotate, generating electricity. Finally, the output current from the turbine (current E6) enters the cooler, and the cycle is repeated. Also, in this unit, two currents of the water phase of gasification units (current G5) and steam reforming (current S19), and compensating water (current E9) are used to supply the steam required for gasification. The incoming water streams from the gasification and reforming units are mixed and enter the flash tower to separate the small incompressible gases that accompany it from the water. The remaining water flow is then mixed with the compensatory water flow (flow E10) and then enters the H-3 exchanger to be converted to the steam required in gasification. It should be noted that the energy flow of coolers 1, 3, and 7 has been used to supply the energy required by the H-3 exchanger. Providing the correct operating conditions for any process can improve performance and efficiency. Therefore the study of process input parameters such as the

Biomass-based hybrid energy systems

345

type of biomass and its constituents, operating temperature and pressure and feed-tosteam ratio, etc., leads to the identification of sensitive parameters in the process. In this process, the biomass’s fast pyrolysis stage is considered the main and most important unit, and we have tried to investigate this unit using kinetic reactions. In the following, the process efficiency and the results obtained from pinch technology and thermal integration are examined to determine the potential for reducing energy consumption and optimizing it. Finally, the economic analysis of the proposed process is studied.

7.2.3.2 Process sensitivity analysis In the studied process, the fast pyrolysis unit is the heart of the system, and the main product and feed of the biorefinery are produced in this unit. Therefore this unit is upstream of other units, the results of which directly impact the performance of other units.

7.2.3.2.1 Biomass type effect on bio-oil compounds Due to their different physical and chemical properties, the different biomasses have a direct impact on the chemical composition and properties of the bio-oils obtained from the fast pyrolysis process. Fig. 7.16 shows the distribution of products obtained from rapid pyrolysis of four biomass types under 500 C and atmospheric pressure. The results indicate that the higher the amount of cellulose and hemicellulose compounds in the biomass (similar to Eucalyptus), the lower the amount of lignin obtained in the bio-oil and 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Eucalyptus

Wheat straw

Pine

Hybrid poplar

Gas

Lignin derived

Alcohols

Sugar derived

Aldekydes & Kenones

Acids

Water

Char

Figure 7.16 Biomass-type effect on obtained bio-oil compounds [41].

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Hybrid Poly-generation Energy Systems

the amount of organic acids in the bio-oil obtained from fast pyrolysis is higher. Generally, the higher the amount of volatiles in the feed (such as Pine and Hybrid poplar), the higher the production of bio-oils and incompressible gases. Also, the higher the amount of fixed carbon in the biomass (such as wheat straw), the higher the coal produced. It is a bit difficult to compare the results with other papers and experimental laboratory work, as few papers have studied the constituents of bio-oil constituents in detail. Figs. 7.17 to 7.18 compare the simulation results with the laboratory results of other papers. As can be seen from the comparison of the figures, there is good coordination between the results and similar tasks. Based on the results and a similar paper, the amount of sugar-derived is very close to each other, especially in the bio-oil obtained from Eucalyptus biomass. Also, comparing the number of ketones, aldehydes, and lignins obtained from the simulation and a similar article, we see a good match and coordination. It should be noted that there is some water in acidic compounds, the amount of which has not been studied in the same article. Hence, the amount of water content in bio-oil obtained is slightly different from the same article. Therefore it can be considered a suitable model for predicting the properties of bio-oils produced through fast pyrolysis of lignocellulose biomass (Fig. 7.18).

7.2.3.2.2 Pyrolysis type effect on bio-oil compounds Four types of biomass were subjected to fast and slow pyrolysis to investigate the operating conditions of pyrolysis on the obtained products and bio-oil compounds. 0.4 0.9 4.6 6.3 31 14

19 21 3.6

Sugar-derived

Water

Ketones

Aldehydes

Lignin-derived

Organicacids

Alcohols

Nitrogen content

Others

Figure 7.17 Eucalyptus fast pyrolysis bio-oil compounds.

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347

2 17 30

25

25

Sugar-derived

Water

Aldehydes & Ketones

Lignin-derived

Others

Figure 7.18 Eucalyptus fast pyrolysis bio-oil compounds. 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Eucalyptus

Wheat straw

Pine

Hybrid poplar

Gas

Lignin derived

Alcohols

Sugar derived

Aldekydes & Kenones

Acids

Water

Char

Figure 7.19 Bio-oil compounds obtained from types of rapid biomass pyrolysis [41].

Fast pyrolysis was performed under the conditions of 500 C and residence time S 1 and slow pyrolysis was performed under the conditions of 425 C and residence time of 30 min, the results of which are shown in Figs. 7.19 and 7.20. As mentioned earlier, the amount of bio-oil produced by the fast pyrolysis process is significantly higher than the slow pyrolysis. In slow pyrolysis, the long

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Hybrid Poly-generation Energy Systems

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Eucalyptus

Wheat straw

Pine

Gas

Lignin derived

Sugar derived

Aldekydes & Kenones Acids

Water

Char

Hybrid poplar

Alcohols

Figure 7.20 Bio-oil compounds obtained from types of slow biomass pyrolysis [41].

residence time causes the gaseous compounds to decompose, react with each other, and produce more incompressible gases. Also, due to the low operating temperature, the coal produced does not decompose well in this process. Its amount is higher in the obtained products. Therefore fast pyrolysis is not a good option for producing as much bio-oil as possible. A comparison of the results showed that using hybrid poplar biomass as a single pyrolysis feed would produce more bio-oil. The conversion rate of this biomass into bio-oil is equal to 68.9%, and the conversion rate to gaseous and coal products is 16.43% and 14.7%, respectively.

7.2.3.2.3 Temperature effect on production rate by fast pyrolysis One of the most important factors affecting the distribution of fast pyrolysis products is the effect of reaction temperature. Fig. 7.21 examines the effect of reaction temperature on the distribution of products obtained from poplar fast pyrolysis and compares it with laboratory work, which shows good coordination between the results obtained from the simulation and laboratory work. In the pyrolysis of poplar biomass, the conversion rate of this biomass to bio-oil is 71%, 6% water, 12% gas, and 11% coal [51]. The biomass conversion rate under the temperature of 500 C to the following products has been reported: 62% bio-oil, 15% water, 10% gaseous product, and 12% coal [52]. In addition to the above, we see good coordination and agreement in comparing the results obtained from the simulation and the experimental sample. Especially in the temperature range of 500 C
550 C is the highest amount of bio-oil that can be extracted in this period [53].

Product yield (wt%)

Biomass-based hybrid energy systems

80 70 60 50 40 30 20 10 0 380

349

Bio-oil (Literature) Gas (Literature) Bio-char (Literature)

Bio-oil (Simulation) Gas (Simulation) Bio-char (Simulation)

430

530

480 Temperature (°C)

580

Figure 7.21 Comparison of simulation results with experimental sample [41].

7.2.3.2.4 Simulation achieved process information The results obtained from fast pyrolysis of hybrid poplar biomass show that the conversion rate of biomass to bio-oil is equal to 68.9% and the conversion rate of biomass to gaseous and coal products is equal to 16.43% and 14.7%, respectively. The bio-oil obtained from the rapid pyrolysis unit enters the biorefinery to be converted to biofuel and biogasoline, and the by-products of the fast pyrolysis process (gases and coal produced) quickly enter the gasification unit. Then the syngas produced by the gasification unit to produce hydrogen enters the reforming unit in the biorefinery. Finally, in the power generation unit, electrical energy is generated from the thermal energy in the exhaust gases and coolers. A summary of the thermodynamic and mass results of the process flows is shown in Tables 7.12 and 7.13. Fig. 7.22 also overviews the integrated process (except for the power generation unit).

7.2.3.2.5 Steam to carbon ratio effect on gasification Fig. 7.23 shows the rate of change of steam to carbon over the amount of H2, CO, and CO2 produced in the gas stream produced in gasification. As shown in the figure, the ratio of steam to carbon significantly affects the amount of hydrogen production. The reaction for hydrogen production is endothermic, and initially, as the vapor-to-carbon ratio increases, the amount of hydrogen production increases due to the heat supply required for the reaction. By increasing this ratio, hydrogen production decreases and incomplete combustion of hydrogen occurs. We also continue to see the dilution of syngas by steam, due to the cessation of the reaction of synthetic gases with steam. The process of hydrogen and carbon monoxide produced in this unit is then sent to the water-steam shift unit. Therefore the optimal amount of steam to carbon is equal to 1.2.

Table 7.12 Main process flow compositions. Name

P1 (Biomass)

P8 (Bio-oil)

P4 (Bio-char)

P9 (Pyrol gas)

G2 (Gasifi gas)

D6 (Gasoline)

D8 (Diesel)

Temperature ( C) Pressure (bar) Mass flow rate (kg/s)

25 1 8.3

16 1 5.7

520 0.9 1.2

16 1 8.6

927 1 17.9

32 2.3 1.1

62 0.01 1

























1.116 0.003 0 0.003 0 0 0 0 0.004 0 0 0 0.011 0.203 0.021 0.045 0.014 0.054 0.074 0.665 0.013 0.095

























0.05 7.214 0 0.371 0.387 0.054 0.006 0.027 0.086 0.014 0 0.002 0.019 0.006 0.002 0 0 0 0.158 0.001 0.124 0.06

5.303 7.244 0 3.065 1.882 0 0 0 0 0.422 0 0 0 0 0 0 0 0 0 0 0 0

0.003 0 0 0.01 0 0 0.001 0 0.001 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Component (kg/s) H2O N2 O2 CO2 CO CH4 C2H6 C2H4 C3H6 H2 H2S H3N CHN C2H4O2 CH2O2 C3H6O2 C6H6O C3H6O2 C2H4O C2H4O2 CH2O C2H2O2

C2H2O C3H6O C10H8 C5H6O2 CH4O C2H6O C2H6O2 C3H4O2 C7H8O2 C4H7NO C4H5N HCL C5H9NO4 C7H16O3 C3H8 C4H10 C7H8O C8H10O C6H14
2 C10H12O C6H6 C7H8 C8H10 C8H10 C9H12 C5H12 C6H14 C7H16 C8H18































0 0.11 0.045 0.366 0.058 0.049 0.074 0.723 0.014 0 0.116 0 1.292 0.377 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0































0.003 0.058 0 0 0.015 0.006 0 0.006 0 0 0.002 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0.003 0 0 0 0 0 0 0 0 0.004 0.017 0 0 0.002 0 0.059 0.019 0.019 0.011 0.023 0.036 0.044 0.056 0.068

0 0 0.034 0 0 0 0 0 0 0 0 0 0 0 0 0 0.001 0.001 0 0.008 0 0 0.001 0 0.032 0 0 0 0.001 (Continued)

Table 7.12 (Continued) Name

P1 (Biomass)

P8 (Bio-oil)

P4 (Bio-char)

P9 (Pyrol gas)

G2 (Gasifi gas)

D6 (Gasoline)

D8 (Diesel)

C9H20 C10H22 C11H24 C12H26 C13H28 C14H30 C15H32 C18H38 C5H10 C6H12 C6H10 C6H12 C7H14 C9H18 C12H22 C18H12 C5H4O2 C4H4O C3H8O C4H10O C6H14O C5H12O C6H12O Biomass Bio-char
























8.3 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0


























0 1.2

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0



0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0



0.057 0.004 0 0 0 0 0 0 0.057 0.149 0.105 0.1 0.113 0.073 0 0 0 0.049 0.004 0.004 0.003 0.004 0.008



0.019 0.061 0.076 0.08 0.074 0.077 0.086 0.083 0 0 0 0 0 0.084 0.188 0.137 0 0 0 0 0.001 0 0.009



Table 7.13 Process flow information. Stream name

Temperature ( C)

Pressure (bar)

Mass flow (kg/s)

Streamname

Temperature ( C)

Pressure (bar)

Mass flow (kg/s)

P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 G1 G2 G3 G4 G5 G6 G7 G8 G9 H1 H2 H3 H4 H5

25.5 520.1 520.1 520.1 520.1 327.5 230 16.3 16.3 924 840 927 927 504.4 130 25.1 714.6 684.8 350 927 24.6 163.9 145.3 282.2 270

1 1 1 0.99 0.99 0.99 0.99 1.06 1.06 1.06 1.9 1.06 1.06 1.06 1.06 1.05 44.4 44.4 47.7 1.06 170 170 170 170 170

8.3 7.3 15.6 1.22 14.38 14.38 14.38 5.71 8.66 8.66 26.27 18.09 17.92 17.92 17.92 4.99 12.92 12.92 12.92 0.17 5.5 5.5 6 6 6

H8 H9 H10 H11 H12 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 S1 S2 S3 S4 S5 S6 S7

372 246.5 208 22 22 22 22.2 22.2 157.8 32 32 259.8 62 277.4 277.9 652 675 106.6 22.2 125.7 231 950.3 927 375 350

139 139 139 35 35 35 20 3.9 3.9 2.3 2.3 2.8 0.01 0.02 89.8 89.8 35 35 20 50 50 48.5 48.5 48.5 47.7

6 6 6 3.41 2.59 3.67 1.31 2.35 2.35 0.02 1.1 1.234 1.05 0.182 0.26 0.26 0.26 0.26 1.21 6.11 6.11 6.11 6.11 6.11 19.03 (Continued)

Table 7.13 (Continued) Stream name

Temperature ( C)

Pressure (bar)

Mass flow (kg/s)

Streamname

Temperature ( C)

Pressure (bar)

Mass flow (kg/s)

H6 H7 S10 S11 S12 S16 S17 S18 S19 S20 S21 S22 S23 E1 E2

372 371.9 77 77 22.2 594 610 1090 77 48 137 77 77 24.6 26

170 130 35 35 2.4 2.4 2.4 1.9 35 50 50 35 35 1 50

6 6 3.57 14.57 0.11 24 24 24 1.26 4.99 4.99 0.5 0.1 13.37 6.59

S8 S9 S13 S14 S15 E3 E4 E5 E6 E7 E8 E9 E10 E11 E12

77 77 22 136.1 89.2 450 26 450 222 35 35 25 33 927 136

35 35 1.01 2.4 2.4 50 50 50 5 1 1 1 1 1 1.9

19.03 15.47 9.3 9.32 24 6.59 6.78 6.78 13.37 0.04 6.22 1.88 8.09 8.09 26.27

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S6 P2

DRYER

PYROLYSIS

EX-3

P9

C-6

P10 S5

P1

P7

G4

FL-1 C-1

EX-2

FL-2

EX-5

S3

S20

H-1

P-3

G7

EX-6

G8

M-5

C-3

G5

S2 P6

G6

COMP-1

C-2

G3

H6

EX-1

EX-4 M-7

P5 P3

HC REACT

P8

CYC-1

D11

G2

E11

GASIFIRE

P4

CYC-2 G1

H11

G9 H-10 H1

D10

M-2

FL-3 C-4

H2

EX-7

M-1

D12

C-5

H12

D13

P-1

COMP-2

D6 (Gasoline)

FL-4

D1

H9

ATM

H7 H3

D3

HT REACT-1

D8 (Diesel)

D4

EX-9

VAC

EX-8

S22

D7

ST REACT

H4

D2

H8

S-3

S23

HT REACT-2

H5

D9

P-2 S-1 S4

S13

S1

COMP-3

M-3

M-7 S12

S18

S17

COMP-5

S16

EX-10

H-2

COMP-4

S14

DT COMB S11

M-4

M-6

PSA

S21 S15

S7 FL-5

S10 S19

S8

S-2

WGS REACT

C-7

Figure 7.22 Integrated process of fast pyrolysis, gasification, and biorefining [41].

Mass fraction of CO2

Mass fraction of CO

0.0265

0.3

0.026

0.25

0.0255

0.2

0.025

0.15

0.0245

0.1

0.024

0.05

0.0235

0

Mass fraction of H2 (%)

Mass fraction of CO & CO2 (%)

Mass fraction of H2 0.35

0.023 0.5

1

1.5

2 S/C ratio

2.5

3

3 .5

Figure 7.23 Steam-to-carbon ratio effect on gasification [41].

7.2.3.2.6 Steam reforming temperature effect The feed of this unit consists of unreacted hydrogen. The distillation unit and the aqueous phase flow are the hydrogen processing unit. Fig. 7.24 shows the effect of

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Hybrid Poly-generation Energy Systems

Mole fraction of H2

Mole fraction of CH4

Mole fraction of CO2

Mole fraction of CO

Products mole fraction (%)

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 650

750

850

950

1050

1150

1250

Temperature (°C) Figure 7.24 Steam reforming temperature effect on the product’s molar percentage composition [41].

steam reforming temperature on the composition of the molar percentage of products. As can be seen, the amount of hydrogen produced increases rapidly with increasing temperature and the consumption of available methane. We see the highest amount of hydrogen production with total methane consumption, which occurs at a temperature of 950 C. Also, the amount of carbon monoxide increases with the reduction of hydrogen production and the water-gas shift reaction.

7.2.3.2.7 Steam to carbon ratio effect on steam reforming The ratio of vapor to carbon in the inlet stream directly affects the distribution of products. As this ratio increases, the reaction to produce more hydrogen and carbon dioxide progress. We also see a decrease in carbon monoxide levels by increasing the vapor-to-carbon ratio shifts the reaction equilibrium toward the products. On the other hand, increasing the amount of steam in the inlet stream requires much energy to produce steam. Spending too much energy to produce steam reduces process efficiency. Therefore choosing the correct ratio of steam to carbon, optimizing energy consumption for steam production, and the amount of steam consumption in steam reforming are essential. According to Table 7.14, at steam-to-carbon ratios above 3.5, slight changes in hydrogen production are seen. Therefore, the optimal amount of steam to carbon is 3.5.

7.2.3.3 Energy efficiency To more accurately evaluate the energy efficiency of the proposed process, two subsystems are considered: Fast pyrolysis system and gasification and biorefinery system.

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357

Table 7.14 Steam-to-carbon ratio effect on steam reforming [41]. S/C ratio

2.5 3 3.5 4 4.5 5

Mass flow rate of H2 (kg/min)

23.90 24.45 24.65 24.95 25.06 25.25

Fraction of by-products CO2

CO

CH4

0.47 0.54 0.56 0.61 0.62 0.66

0.48 0.44 0.42 0.38 0.37 0.34

0.04 0.03 0.02 0.01 0.01 0.01

7.2.3.3.1 Energy efficiency of fast pyrolysis-gasification system The total biomass energy content is calculated based on its mass flow rate and biomass calorific value equal to 99.35 MW [54]. The energy content of bio-oil is estimated to be 137.2 MW. The amount of electricity consumed by Comp-1 is equal to 13.85 MW. The energy efficiency of this system is equal to 65.8%, calculated from the following equation. ηe 5

m_ bo  qbo _ _ ðm bm  qbm Þ 1 W

(7.30)

7.2.3.3.2 Energy efficiency of biorefinery system The total energy content of biofuels produced is estimated at 90.58 MW. The energy content of bio-oil that enters the biorefinery is equal to 99.35 MW. Also, the amount of electrical energy consumed by pumps and compressors equals 4.55 MW. The energy efficiency of the biorefinery is 86.7%. The energy efficiency of the whole process (considering the power generation unit) is calculated to be 61.82%, obtained from the following equation. The power generation unit’s share in the process’s total energy efficiency was equal to 3.6%. ηe 5

  ðm_ d  qd Þ 1 m_ g  qg 1 W_P _  qbm Þ 1 W_tot ðm

(7.31)

bm

7.2.3.4 Greenhouse gas emission rate Table 7.15 shows the amount of emitted gases per 1 MJ of biofuels produced (gasoline and diesel) according to the European Commission’s Renewable Energy

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Hybrid Poly-generation Energy Systems

Table 7.15 Emission rate of greenhouse gases from biofuels obtained [41]. Fuel

Conventional gasoline

biogasoline

Conventional diesel

Biodiesel

Emission amount (gCO2,eq) Saving amount (gCO2,eq) Saving rate (%)

89.35

37.46

83.25

36.55




51.46




46.7




57.6




56.1

Hot streams

Cold streams ΔTmin

1000 900

Temperature (°C)

800 700

Cooling Utility

600

Maximum heat recovery

500

Heating Utility

400 300 200 100 0 0

20000

40000

60000

80000

100000

Entalpy (kJ/s) Figure 7.25 Studied process composite diagram [41].

Directive (RED). Comparing biofuel and biogasoline with conventional fossil fuels, we see a decrease in greenhouse gas emissions of 57.6% and 56.1%, respectively.

7.2.3.5 Thermal integration In the proposed process, the heat energy of the process currents is recovered by eleven heat exchangers (EX-1 to EX-11).

7.2.3.5.1 Composite curve Fig. 7.25 shows a composite curve of cold and hot process flows, with a blue flow diagram in blue and a hot flow diagram in red. This diagram shows the temperatureenthalpy of the process flow network. The minimum temperature difference selected

Biomass-based hybrid energy systems

359

for the process must be greater than the minimum vertical distance of the cold and hot composite curves. The temperature at which the minimum vertical distance between hot and cold currents occurs is called the pinch temperature. As shown in this figure, the minimum fluid temperature difference is 8 C, which is at 927 C. The common area between the hot and cold flow diagrams shows the maximum heat recovery that the heat exchangers can use. The parts of the diagram in which there is no commonality between hot and cold flows indicate the minimum amount of heat and cold energy required to regulate the temperature of the process fluids. In this figure, the minimum amount of energy required to cool hot streams is specified in blue and the minimum amount of energy required to heat cold streams is specified in red. As shown in Fig. 7.25, the maximum rate of heat recovery from hot streams and their transfer to cold streams is 63.84 MW.

7.2.3.5.2 Minimum temperature difference effect on the total investment cost When two hot and cold flow diagrams come into contact in a composite curve diagram, there will be no driving force for heat transfer, and in this case, we need an infinite heat transfer surface to transfer heat, which affects the process economy. Also, heat can only be transferred from a hot source to a cold source, and vice versa is not possible. Therefore the amount of heat energy recovery from process flows is limited. With increasing ΔTmin, the amount of heat transfer required decreases and, consequently, the amount of fixed investment. Nevertheless, on the other hand, the amount of energy consumption and operating costs of the process increase. Therefore a balance must be struck between the fixed investment cost and the operating cost of the process. Fig. 7.26 shows the effect of ΔTmin on process costs. Based on the results, the optimal ΔTmin is 15 C. Also, in these conditions, using 29 heat exchangers, maximum heat energy recovery can be achieved. Under these conditions, the minimum required cold utility energy is equal to 40.89 MW, and the minimum hot utility energy is equal to 1.97 MW. ΔTmin is assumed to be equal to 10 C. Under these conditions, the energy consumption for cooling is 54.56 MW, and the energy consumption for heating is 16.18 MW. Also, using EX-1 to EX-10 converters, 49.75 MW of thermal energy has been recovered.

7.2.3.5.3 Grand composite curve This diagram shows the temperature-enthalpy of the output currents of the process, which can be used to achieve the minimum amount of energy required for cooling and heating the process, as well as the pinch temperature. Unlike the composite curve diagram, this diagram is based on the result of hot and cold process flows. In this diagram, at a temperature whose enthalpy value is zero, that temperature is considered the pinch temperature. In this diagram, there is a lack of heat in the upper part of the pinch point, and excess heat is observed in the lower part of the pinch temperature.

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Hybrid Poly-generation Energy Systems

Total Cost index

Operating Cost index

Capapital Cost index

0.89 0.87 0.85

Cost indexes

0.83 0.81 0.79 0.77 0.75 0.73 0.71 0.69 3

6

9

12

15

18

21

24

27

30

33

36

39

ΔT min (°C) Figure 7.26 ΔTmin effect on process costs [41].

Fig. 7.27 shows the grand composite curve for the process. According to this figure, the minimum cooling and heating required for the process are 40.69 MW and 1.77 MW, respectively, as shown in Fig. 7.27.

7.2.3.5.4 Demand curve These diagrams are used to evaluate the effect of ΔTmin on the energy consumption required for cooling and heating the process in pinch analysis. If we see a breaking point in this diagram, this point is known as a large change in the minimum energy required for cooling and heating the process. If we see a breaking point in the chart, we see a complex challenge. In these circumstances, accurate and detailed economic evaluation and determination of optimal ΔTmin are essential. Fig. 7.28 shows the demand curve for the process. As shown in the figure, this diagram for the process is shown as two parallel lines with no breakpoints. Under these conditions, decreasing or increasing ΔTmin does not significantly affect the amount of energy required for cooling and heating the process.

7.2.3.5.5 Heat exchangers network Fig. 7.29 shows the network of heat exchangers at maximum energy recovery (MER). Maximum thermal energy recovery and minimum energy consumption for heating and cooling are obtained from the composite curve and grand composite curve diagrams available using this converter network. This usually requires many heat exchangers (29 heat exchangers for the process), leading to high costs. For this reason, this is rarely seen in the industry.

Biomass-based hybrid energy systems

361

1200

Min heating duty 1000

Temperature (°C)

Pinch temperature: 927°C 800

600

400

200

Min cooling duty

0 0

5000

10000

15000

20000

25000

30000

35000

40000

45000

Enthalpy (kW) Figure 7.27 Grand composite curve diagram of the process [41].

Heating utility

Cooling utility

45000

Minimum utility load (kw)

40000 35000 30000 25000 20000 15000 10000 5000 0 0

5

10

15

20

25

30

35

ΔTmin (°C)

Figure 7.28 Demand curve diagram of the process [41].

40

45

50

55

60

65

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Hybrid Poly-generation Energy Systems

Hot Utility

421kW 148kW 2797kW

S4

808kW

13716kW

2322kW

G2

3438kW 7858kW

G6

906kW

D12

2756kW

3807kW

779kW

3203kW

P5

668kW 10229kW

H8 2024kW

12747kW

S15

5919kW 842kW

4683kW

P9

1128kW

7316kW

D10

517kW

S2

5787kW

H5 3921kW

H3 4889kW

H1

2008kW

D3 868kW

S20

Cold Utility

Figure 7.29 MER network of heat exchangers of the studied process [41].

Hot Utility

H-2

C-6 C-2

S4

10

-3

(G4) EX

(G3) EX

-5

-4

G2

(G7) EX

C-3

G6

-6

D12

(D13) EX

C-5 C-1

H-1

(S5) EX-

(S6) EX

-9

(P6) EX

(P7) EX-

P5

-1

2 C-4

(H10) EX

(H9) EX

-7

-8

(S8)

C-7 EX4

S15 EX (P10)

P9

H8

(S16)

EX -10

(S17)

H-2

-3 EX -6

D10 EX -5

S2

(D11)

(S3)

H-1 EX -1

H5 EX -8

H3 EX -7

H1

EX -9 EX -2 C-1

(H4)

(H2)

D3 S20

(H6)

(D4)

(S21) C-2

C-3

C-4

C-5

C-6

C-7

Cold Utility

Figure 7.30 The network of heat exchangers proposed by the studied process [41].

Fig. 7.30 shows the network of heat exchangers proposed for the process. In addition, ten heat exchangers (EX-1 to EX-10) are provided in this network to recover the heat energy of process currents. Also, in this process, the inlet feed temperature to the steam reforming reactor and direct combustion is used using two heaters (H-1 and H-2). Also, seven coolers (C-1 to C-7) regulate hot currents. In addition, the heat energy of these coolers and the heat energy of the hot exhaust

Biomass-based hybrid energy systems

363

gases are used to produce the steam required for the gasification unit and the power generation unit (not shown in the figure).

7.2.3.5.6 Thermal integration effect investigation Pinch technology is used to minimize the number of ancillary services and energy consumption for cooling and heating the process. The best heat exchanger network layout can be selected by balancing the investment costs with the operating costs of the process. By comparing the conventional and integrated processes, we see a 75.02% and 47.80% reduction in energy consumption in air conditioners and heaters, significantly impacting operating costs and the cost of manufactured products. Also, the amount of heat energy recovered by EX-1 to EX-11 converters in the process equals 69.82 MW, as shown in Fig. 7.31. In addition, in the studied process, a small power generation unit can produce 5.62 MW of electrical energy. In Fig. 7.31, a value less than zero is related to the energy produced per power generation unit.

7.2.3.6 Economic analysis According to estimates, the complete investment expense for the fast pyrolysis of hybrid poplar biomass, bio-oil conversion to biofuels in a biorefinery, and electricity production are approximately US $53.7 million. This cost includes both direct Pyrolysis

Hyrotreating

distillation

reforming

power generation

100

Energy Consumption (MW)

80

60

40

20

0

Conventional Process

Proposed Process

-20

Figure 7.31 Energy consumption comparison of integrated process and conventional process [41].

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Hybrid Poly-generation Energy Systems

and indirect expenses. Direct and operating costs of the process are obtained. Table 7.16 shows the share of each fast pyrolysis unit, hydrogen processing, distillation and hydrocracking, steam reforming, gasification, and power generation at the total investment cost. The results show that the steam reforming unit, with a total investment cost of US $16 million, accounts for about one-third of the total investment. Many units are used in this unit, which is often in the gas phase and under high pressure, which increases the volume and thickness of the equipment, affecting the process costs. The results of the economic analysis are shown in Table 7.17. According to this table, the annual operating cost is estimated at US $19 million, including maintenance costs, energy costs, etc. The revenue of this process is from the sale of biofuel and biogasoline as well as electricity produced. The amount of biofuels and electricity produced equals 181 T/day and 5.62 MW, respectively. The concept of net present value (NPV) is used to calculate the net profit comprehensively. When the NPV was set to zero, the biofuel value was 7.21 USD/G. For NPV calculations, the useful life of the process was 20 years, and the interest rate was 10%. NPV is calculated based on the following formula. NPV 5

N X ðRt 2 Tt Þ t51

ð11iÞt

2 I0

(7.32)

Table 7.16 The share of each process unit in the total investment cost [41]. Process units

Share of contribution (%)

Capital cost (MUSD)

Pyrolysis Hydrotreating Distillation Steam reforming Gasification Power generation Total

9.20 15.37 12.38 29.85 14.84 18.36 100

4.94 8.25 6.65 16.03 7.97 9.86 53.7

Table 7.17 Economic analysis results [41]. Parameter

Value

Plant feed (T/day) Plant biofuels production (T/day) Plant electricity production (MW) Total capital cost (MUSD) Total operating cost (MUSD/year) Product value (USD/G)

717 181 5.62 53.7 19 7.21

Biomass-based hybrid energy systems

365

In the above formula, I0 is the initial investment cost, Tt is the annual operating cost, Rt is the annual income, i is the interest rate, and t is the tenth year of the process life. The use of fossil fuels pollutes the environment and exacerbates the greenhouse effect. By using renewable biomass sources and using them in the process of rapid pyrolysis, it is possible to produce a liquid product called bio-oil, which has much less pollution and greenhouse gas emissions than petrol and gasoline. Therefore this bio-oil obtained through this technology has an excellent potential to be replaced with conventional fossil fuels. The analysis of bio-oil properties indicates that it is essential to refine this liquid to improve its properties and the following items can be mentioned: 1. The fast pyrolysis of four biomasses of hybrid poplar, Eucalyptus, Wheat straw, and Pine was investigated. According to the results of fast pyrolysis and its comparison with experimental and laboratory work, we see good coordination and compliance for bio-oil production. The results showed that the use of hybrid poplar biomass produced more bio-oil. Fast pyrolysis of this biomass produced 68.9% bio-oil, as well as 16.43% and 14.7% of it were converted into gas and coal products. Operating conditions such as biomass type, temperature, and residence time of the reaction directly affected the amount of bio-oil produced from the biomass. 2. The analysis of the process’s overall efficiency indicates that the process’s overall efficiency, considering the power generation unit, is equal to 61.82%. The energy efficiency of the process without considering the power generation unit is equal to 58.2%. Therefore it is clear that adding this unit to the process increases the process energy efficiency by 3.6%. Also, the result obtained from the economic analysis indicates that this unit has about 10% of the total investment cost. Therefore it seems that the existence of this unit has no economic justification. Also, more than 70% of electricity consumption is in the gasification sector, which is related to the compressor of the gasification unit. One of the reasons for this is the low mechanical efficiency of this compressor (75%) and the high input flow rate with it. 3. The assessment of thermal integration of the process shows that using pinch technology, the consumption of utility by coolers and heaters is 57.02% and 47.80%, respectively, compared to the conventional process. Also, the maximum rate of thermal energy recovery in the process is equal to 69.82 MW. It should be noted that the excess thermal energy of hot exhaust gases and thermal energy of coolers in the power generation unit has been used to provide the required steam to the gasification reactor and then 5.62 MW of electrical energy was produced by the steam turbine.

Development and progress in this field to achieve an efficient and appropriate example on a commercial scale is one of the challenges in pyrolysis, which requires using catalysts in other stages of the process. Usually, with the increased unit capacity, we see a decrease in the cost price of manufactured products. Therefore increasing the capacity of the process to a suitable capacity can reduce the price of biofuels produced by this process to some extent and make it competitive with conventional fossil fuels. It is also suggested that a system consisting of several medium-capacity pyrolysis units located in different locations be collected, from which the produced bio-oils be collected and sent to a high-capacity biorefinery for refining. Examining such a system can lead to meaningful results.

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Hybrid Poly-generation Energy Systems

One of the methods that we see in some laboratories conducted is mixing fossil fuels with biofuels obtained from fast pyrolysis, which can lead to positive results, and by evaluating the possibility of determining the best mixture and percentage, an important step is taken in this direction.

7.2.4

Tri-generation and integration of cold, heat, and power by biomass-based hybrid systems

This subsection describes the application of biomass-based in the hybrid polygeneration system. Therefore a hybrid energy system integrating cold, heat, and power by biomass is shown in Fig. 7.32. This system, which includes a refrigeration air separation section, biomass co-gasification, CO2 power unit, LNG regasification, and refrigerated CO2 collection, is schematically depicted in Fig. 7.32. Two distillation stages are utilized in the air-separation system, and blue lines show this unit in the graphical view of the hybrid process in Fig. 7.33. Stream 1 contains N2 and O2, with 79% and 21% mole fractions, respectively. The hybrid system uses no supplementary heating or cooling; instead, a self-heat recovery technique is employed. Additionally, no additional chilling procedure is required due to the system’s usage of LNG gasification recovery. Besides producing cleaner energy and a sufficient syngas product, fractional burning of biomass provides the heat needed for the gasifier. Ultrapure N2 and O2 flow rates, m_ N2 and m_ O2 , are shown in this chapter. In addition, C-1, C-2, and E-1 specify the total amount of electricity utilized by ASU, and formula (7.33) showed its calculation [55]. WTot;ASU 5

n X

WC2i 2

i51

m X

WE2i 5 WC21 1 WC22 2 WE21

(7.33)

i51 LNG

Pure N2 Air

Lean N2

Cryogenic Air Separation

Air

Coal Water Biomass Pure O2

Biomass/Coal Gasification

Combustion Chamber

CO + H2 Water Gas Shift

CO2 Power Generation Power

CO2 + H2

NG

Steam Cycle NG

Cryogenic CO2 Capture NG

H2

Water CO2

Figure 7.32 A schematic view of the hybris energy system by biomass [55].

Power

Biomass-based hybrid energy systems

367

16

Air 25 °C, 101.3 kPa 50000 Sm3/h

4

2

1

14

6 T-1

3

C-1

5

27

N2, 22 °C, 101.3 kPa

10 DT-1

20 DT-2 V-1 12 13

24 33

E-1

18 SR-1

22

25

23 LNG -162 °C, 140kPa 63.97 Kg/s

30

29

34

31

32

T-3

36

P-2

19

28

P-6

SW-Out 2

F-1 68

RC-2

C-3

B-1

63

74 RC-3 70

69

66

64

72

77 HX-8

71

55

45

75 84

F-4

60

58

80 78

E-6 86

87 88 89 90

SR-4

49

P-5

HX-5

E-2 43

42

C-5

82

C-6 85

HX-4

SW-ln 1

83

57

56

HX-6

HX-7

Steam

73 59

37

SW-Out 1

RC-4

67

M-1 38

SR-3 81

C-4 46

CC 44 47

NG, 5 °C, 7000kPa Exhaust Gas

HX-9

HX-3

54 F-3

51

H2O, 10 °C, 2000kPa CO2, 30 °C, 11000kPa H2, 710 °C, 200kPa NG, 5 °C, 7000kPa LN2, 25 °C, 101.3kPa

T-4 48

53

52

39 40

41 50

SW-In 2

76

E-5

65

Air 25 °C, 101.3 kPa 40530.5 Sm3/h

21

V-2

9

P-3

RC-1

NG, 5 °C, 7000kPa

15

62

61

Coal , Biomass 25 °C, 101.3kPa 11125.63 Kg/h

T-2

C-2

HX-2

11

8

7

HX-1

35 Water 25 °C, 101.3kPa 2750 Kg/h

17 SR-2

26

P-1

P-4

79

Figure 7.33 The process diagram of the integrated systems [55].

7.2.5

Proposed systems for hybrid solar and biomass power plants

Fig. 7.34 describes a hybrid energy system that draws energy from clean, sustainable sources. Additionally, this mechanism stops contaminants from entering the atmosphere. In order to be more effectively utilized in the process’ following phases, biomass is initially gasified to create syngas as the feedstock. A solar source supplies the heating duty of this unit. In order to generate electricity and hydrogen, gasification products are used in SOFC and Ca-looping systems, respectively. After generating power and hydrogen, the extra energy would reach the storage to be utilized on overcast days or during low-radiation times. The naturally occurring chemical looping portion naturally captures the generated CO2. In this way, the zero-emission objective is achieved, as shown in Fig. 7.35. In order to evaluate the thermal functionality, the following concepts are used to calculate the thermal conversion efficiency [56]:   Thermal energy content ðHHVÞ of gas kg=kg   TECE 5 3 100 Chemical energy content of feedstock kg=kg

(7.34)

The amount of H2, CO2, and H2O generated by the gasification process rises with each S/C or S/B molar or mass ratio increase. Up to an S/C ratio of roughly

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Hybrid Poly-generation Energy Systems

Pure H2 H2 rich gas

Syngas Biomass Steam

Gasification

Hot gases

WGS

Heat

HRSG

CLHG

H2

Recovered heat and generated steam return to process

Heat Water

CSP

SOFC

Solar heat

Air

Electricity

Figure 7.34 A schematic view of the hybris energy system by biomass and solar source [56]. Pump 1 298.15 K, 1bar 1500 kg/h Water (H2O) S32 B6 873.15K, 1bar 1448 kg/h 100% H20

S2 298.15 K, 1bar 6456.73 kg/h Steam

Carbonator

Collector

R1

MX1

H1 S6 Calcinator

WGS

Gasifier Elements3

C1

9

S9

Recycle

Compressed-Air Splier

Syngas

Pump 2

Pure H2

Syngas-B

Compressor

Air

11

S1

Sungas-A

S5

S29

10

10

S20

423.15 K, 1bar 1980.4 kg/h (H2O)

S26

R4 HX3

Air-preheater

Steam2

R7

S16 S30

S4

R2 Elements3

Anode

2

S22

C2

HX2

O2

O2

S23 Pyrolysis Elements

532.55 K, 1bar 500 kg/h (H2O)

Cathode

B4

B3 Biomass

303.15 K, 1bar 3929.6 kg/h (CO2)

S33

R3

HX4

Elements2

R5

4

2300 kg/h

HX5

S3 Impurity

AT

473.15 K, 1 bar 9.38 kg/h (10.4% H2S, 50.6% HCl, 38.9% NH3)

531.42 K, 1bar 4952.78 kg/h (N2)

ST1

S25

ST2

551.93 K, 1bar 1000 kg/h (H2O) R6

HX6

HEATER-1 CA(OH)2

EVAPORATOR

PUMP

H2O-A HPSTEAM

HPWATER H2O-1

TURBIN

H2O CALONER

LPWATER

H2O-STORAGE

HEATER-2 H2O(L)

LPSTEAM-2

CONDENSE

VALVE

LPSTEAM-1

HEATER-3 H2O-B

CAO CARBONATOR CAO-A CAO-STORAGE

COOLER

CA(OH)2-A

CA(OH)2-P CA(OH)2-STORAGE

Figure 7.35 The process diagram of the integrated systems using solar and biomass [56].

0.5, the CO molar flow rate increases before declining. One may say that at 0.5 in the S/C ratio, CH4 generation approaches zero. The generation of H2 rises with temperature and falls with pressure in the WGS portion, where the goal is to transform additional CO and CH4 to H2. Additionally, CO transformation alters negatively with temperature and positively with pressure. Both CO conversion and H2 molar flow rate rise with improved S/C.

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Chemical looping combustion in polygeneration systems

8

One approach that may be utilized for ignition to stop carbon dioxide emission in the environment is chemical looping combustion (CLC), an oxyfuel burn process with no interaction of O2 (air) and fuel mixture. Chemical looping hydrogen generation (CLHG) and chemical looping reforming (CLR) are the processes used for producing H2 with built-in CO2 removal. Similar techniques that utilize CLC technique include gasification and airflow sequestration, sometimes known as chemical looping gasification (CLG) and chemical looping air segregation CLAS, respectively.

8.1

Introduction

Fast social and commercial growth, particularly in emerging nations, has increased the demand for energy supplies. Fossil fuels account for 85% of the world’s energy production, compared to green sources’ 7% share. However, after 2050, the amount of sustainable energy will have doubled, yet fossil fuels will continue to be the primary source of energy for everyone. As a result, in the coming half-century, concern about carbon dioxide emissions will grow into a global problem. Climate change is primarily caused by emissions of greenhouse gases (GHG) such as carbon oxide, oxides of nitrogen, Sulfur, and methane, of which CO2 is in the vast majority. One-third of the CO2 produced via burning during electricity production comes from burning fossil fuels [1]. Consequently, CO2 collection is a crucial choice. A variety of approaches to decrease GHG emissions, including decreasing energy usage, promoting sustainability, switching to zero-carbon and sustainable energy sources, improving plant growth, and carbon dioxide collection [2]. The major cause of global climate change was already attributed to the higher atmospheric carbon dioxide, 400 ppm presently. The Intergovernmental Panel on Climate Change (IPCC) predicts that a rise in the global temperature further than 2
C would significantly impact the environment [3]. The nations announced their plans to reduce global warming to 2
C approximately by the end of the 21st century. According to estimates, in order to achieve this goal, GHG emissions must be reduced by 40%70%, and zero-emission must be adopted by the end of 2100. Experts say that the goal of limiting global temperatures up to 2
C could be more complicated and costlier than imagined if early choices and actions had not been taken. Chemical looping (CL) is a potential technique that primarily consists of CLC, CLR, and CLHG with the objectives of separating carbon dioxide generated by Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00009-8 © 2024 Elsevier Inc. All rights reserved.

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burning, fuel reformation, and producing H2, in that order. Reactor projection, fuel, and O2-carrier types employed, as well as working circumstances, are key factors in the economics and performance of this technique.

8.2

Fuel cell

Fuel cells are a kind of equipment that efficiently transfer chemical energy from combustible to power electricity via electrochemical processes. There are several varieties of fuel cells, each with unique benefits and drawbacks. Focusing on electrolyte components and spent combustibles is common in categorizing fuel cells. Alkaline fuel cells (AFC), Proton-exchange membrane fuel cells (PEMFC), direct methanol fuel cell (DMFC), Phosphoric acid fuel cells (PAFC), molten-carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC) are six primary groupings into which they are divided. Fuel cells are being used increasingly often because they are quiet, highly efficient, have a simple structure, and emit little pollution. Hydrocarbon reforming and partial oxidation might be combined with the primary method for producing H2 or synthesis gas for use as combustible in fuel cells. The SOFCs and PEMFCs are the best forms of fuel cells for combining the large capacity electrical generation installation because of their solid-phase electrolytes and other benefits [4].

8.3

Solid oxide fuel cell

Due to the emphasized features over other power generation technologies, SOFC has proven to be appealing. These fuel cells can operate in high-temperature environments and are fuel-friendly. The electrodes’ conditions, such as the porosity, also substantially influence the results, resulting in significant power densities and considerable energy-conversion efficiencies. Additionally, due to the high working temperature of this fuel cell, internal-reforming of hydrocarbons may be possible. This temperature ranges from 600
C to 1000
C, based on the kind of electrolyte used. As shown by the working temperatures and the kind of cell support, SOFCs are categorized. SOFCs may function with hydrogen, carbon monoxide, and methane. The highest output current and electrical performance among fuel cells, regular generating electricity, and combinations of them are found in the hybrid SOFC/gas turbine (GT) application in a cogeneration plant. Methane and carbon dioxide may be fed directly into this apparatus, where they can be converted to synthesis gas and used to generate energy. The coupling with the gasification process is a novel SOFC hybrid energy system idea. Appropriate synthesis gas mixture characteristics have an impact on cell efficiency. The efficiency of combining a gasifier, SOFC, and GT may improve productivity to between 50% and 60%.

Chemical looping combustion in polygeneration systems

8.4

375

Proton exchange membrane fuel cells

Hydrogen and O2 are the output flows for the proton exchange membrane and PEMFC, respectively. PEMFCs present a prolonged lifetime and operate at low temperatures. PEMFCs have a high power density due to their excellent efficiency. PEMFC with high temperatures offer various advantages over those with low temperatures. However, in order to provide high temperatures, the cell must be warmed up with exterior heat sources. Cogeneration of electricity and heating energy will be conceivable once PEMFC operates at high operating temperatures of 100
C to 200
C.

8.5

Expander power process

Thermodynamic energy processes are split into two categories: gaseous and steam cycles, according to the phase state of the operating stream. As opposed to vapor cycles, in which the operating stream alternates between the liquid and vapor phasestates, gas cycles have an operating stream that is always in the gas state. In addition to being divided into closed and open cycling, the thermodynamic process may also be classified from a different angles opposed to closed cycles, which regenerate the operating stream and restore it to the starting condition after each process, open cycles include the renewal of the operating stream at the final of each cycle.

8.6

Vapor (or steam) power cycle

Vapor is the most commonly used operating stream for steam power cycles for a variety of reasons, including the fact that it is readily available, relatively inexpensive, and has a high enthalpy for vaporization. Energy is supplied to the operating stream in vapor power cycles, which are classified as external combustion engines. Examples of such sources include burners, geothermal, nuclear plants, and energy from the sun’s heat, amongst other types of energy resources. The analysis of thermodynamics, as well as the implementation of optimization strategies for these plants, appears to be required given their poor performance, the high levels of pollution they create, and the restricted sources available for combustible.

8.7

Gas and combined power cycles

GTs are flexible machines that can utilize several combustibles. The performance of the GT cycle may be improved by raising the expander input temperature, improving mechanical components, and altering the basic cycle by including reheating, intercooling, and other changes. In a nutshell, GT equipment raises the temperature and pressure of the air via a compressor. When pressurized air enters the combustor, combustion product is then delivered to the expander to produce electricity.

376

Hybrid Poly-generation Energy Systems

Cooling methods are implemented because the GT’s temperature can rise over the deforming point of the building materials it utilizes. There are three categories of GTs: open-cycle, closed-cycle, and semiclosed-cycle. Unlike small-size internalcombustion engines, which are cheaper and therefore more prevalent, micro GTs emit less pollution. Because the temperature of the GT’s flue gases is much greater than the vapor expander’s working temperature, including a lowest-point vapor expander electrical generation unit may result in the heat recovery of the GT heap to produce steam for the vapor expander. Combined cycle electricity generation plants are more efficient than GT and vapor expander power processes operating separately. The combination of the Bryton and Rankine cycles yields greater than 60% in combined heat and power (CHP) cycles. In a CHP, the essential variable for achieving improved efficiency is the efficiency of the GT.

8.8

Heat recuperation

In a typical power generation unit, approximately one-third of the combustible energy is converted to power owing to thermodynamics limitations, with the remainder being dissipated through (waste) heat into the environment. Waste heat recuperation (known as waste heat recovery) is a cost-effective solution. Internal combustion by organic Rankine cycle (ORC) systems may achieve a 10% up to 25% in terms of thermal performance and a total system efficiency of approximately 60% up to 90%. The hot flows are used in heat recovery steam generators (HRSG) units to generate vapor from the heat transmission of waste heat to added fresh water. Utilizing generated steam vapor improves plant productivity. This vapor might be utilized to operate a vapor electrical generation unit, as a gasification agent, or for other purposes. The majority of heat recovery systems operate at high temperatures of 350
C500
C and medium of 200
C350
C operating temperatures. The Rankine cycle, including ORC, steam Rankine cycle (SRC), supercritical Rankine cycle (SCRC), Kalina cycle (KC), flue GT processes, and thermoelectric generators are the principal heat recuperation techniques. The fundamental equilibrium of a basic CL-based process is shown in Fig. 8.1. As can be anticipated, a process includes various components, each having a unique design influenced by inlets, resources, the procedure developers’ preferences, the intended outcomes, and other factors. According to the top-mentioned valuable and productive results, operations are divided into several categories. Processes need to be classified into three major groups to be classified, but also because electricity and cooling are nearly typically provided by heat production and recuperation equipment as specialized products. Power generating systems were initially categorized with non-fuel engine power generators having precedence (with one fuel cell). The following groups belong to the second category: CPC, GT cycles, and STC. The remaining three subsystems would have been part of systems that used fuel cell power generators. However, in these setups, non-fuel engines are a source of

Chemical looping combustion in polygeneration systems

377

Air or pure oxygen

Heating Fuel

Process with chemical looping technology

Feed water or steam

Heat

Power

Cooling

Oxygen-carrier make-up Gasification agent

Figure 8.1 Inputs and outcomes required for or potentially produced by a regular polygenerational chemical looping process [5]. Solid-oxide fuel cells (SOFC) Thermo-electrical power generation

Proton-exchange-membrane fuel cells (PEMFC) power

Main product

Steam turbine power plants

Thermo-mechanical power generation

cooling

Gas turbine power plants

Combined cycle power plants

Figure 8.2 Chemical looping processes are categorized from the perspective of the products [5].

unsuitability even though processes use expander e power cycles. Similar to the category for cooling production, even though other products are outside cooling, the focus on cooling production is considered to be put in this category. Additionally, this classification is shown in Fig. 8.2.

378

Hybrid Poly-generation Energy Systems

8.9

Two reactor conversion process configurations

Typically, CLC involves utilizing O2-carrier with a metal-based to provide the necessary O2 for an oxidation process. O2-carriers are transported to the air reactor following the oxidation event in the combustible reactor. With the help of airflow or pure O2, these decrease inside the air reactor. The combustible reactor’s reactions are similar to: ð2n 1 mÞMy Ox 1 Cn H2m ! ð2n 1 mÞMy Ox21 1 mH2 O 1 nCO2

(8.1)

The MyOx-1 (or metal) is delivered to AR in order to occur the reactions of the bottoming once the combustible oxidation in FR is complete as regards: /2My Ox21 1 1/2O2 ðairÞ ! My Ox 1 ðair: N2 1 unreacted O2 Þ

1

(8.2)

Generally, endothermic and exothermic processes are used to describe redox reactions accordingly. The heat of the reaction is influenced by the combustible and O2 carrier. Fig. 8.3 shows a schematic view of a CL process with two-reactor.

8.9.1 Electrochemical electrical production/CL-based processes Electricity production uses fuel cell-based electricity generation. Such procedures can also be used in expander-based such as turbine power plants, although, as was already noted, the categorization priority begins in non-fuel power plants.

8.9.1.1 Solid oxide fuel cell/CL-based processes Coal gasification, CLHG, SOFCs, and GT technologies may be used to construct a system that generates electricity,H2, and captures CO2. This plant shows that rising MyOx-1 CO2 + H2O

Oxygen depleted air

Fuel reactor

Air reactor

Fuel

Air/Pure oxygen MyOx

Figure 8.3 Two-reactor chemical looping process [6].

Chemical looping combustion in polygeneration systems

379

operating pressure and cell temperature may increase to a maximum of 45% net power efficiency. Additionally, increased combustible usage leads to improved net power efficiency. Reduced energy losses resulting from coupling CL gasification with SOFCs. Additionally, it is substantially more efficient than both the CDCLcombined cycle and the traditional integrated gasification combined cycle (IGCC). On the other hand, CL technology for burning may be used to construct a CHC power plant linked with SOFCs. The carbonator, calcinator, and hydrator are the three reactors used in this CLC system, which is structured on Ca-looping. It is possible to reach 54.2%, 55.8%, 87%, and 100% for SOFCs performance, plant net power efficiency, H2 power efficiency, and CO2 separation performance. Thermodynamic evaluations of an ORC/Heat Recovery Steam Generation combined SOFC/CLC technology process have been conducted in compact and large sizes. Electric efficiencies (lower heating value (LHV)) greater than 66% were attained in both sizes. It has been shown that SOFC voltage has the greatest impact on the facility’s performance and fuel-cell dimensions. The system in Fig. 8.4 shows how the gasification output passes the SOFC and joins two CLC reactors. The outgoing gases from the CLC and heat with the heat recovery steam generation cycle power a hybrid power facility. Upon pressurization and inter-cooling, a portion of the incoming air to the operation proceeds to the air separation unit (ASU) in a hybrid trigeneration system. Air

Cathode Fuel

Fuel reactor

SOFC

O2 Gasifier

Anode

CO2 Pure Oxygen

Solid oxide fuel cell

Gasificaion

Power

Air reactor

chemical looping system with two reactors

Steam turbine

HRSG

Heat Recovery Steam Generation

Power Gas turbine

Combined cycle power plant

Figure 8.4 Solid oxide fuel cell, combined cycle power plant, fuel gasification, two-reactor chemical looping process, and heat recovery steam generation [5,7].

380

Hybrid Poly-generation Energy Systems

Steam and O2 are introduced to the gasification area together with coal input. In the combustible reactor is synthesis gas. Lowered particles move into the air reactor, completely oxidize there, and then back to the combustible reactor. The carbon dioxide and H2O-containing combustible reactor gas output are refrigerated in the heat recovery steam generator before being compressed and split. A portion of the steam reactor’s hydrogen-water flow that has been passed through the hydrogen turbine reaches the negative electrode, while the remaining portion goes to a separator to purify the hydrogen after chilling in the heat recovery steam generator. The positive electrode also receives decreased air from the air reactor. After combustion in the GT, the SOFC results and the nitrogen gas portion of the air separation unit output are sent to the heat recovery steam generator. A heat recovery steam generator provides the necessary load in four-stage SRC and ORC power plants. A power-generating system employing a steam turbine and SOFC is shown in Fig. 8.5. Two different two-reactor setups enable CL. Some of the necessary steam is provided by the heat recovery steam generator cycle.

8.9.1.2 PEMFC/CL processes The efficiency of a PEMFC and CLR combined process analysis showed that the needed electrical has an impact on the PEMFC stack’s overall efficiency. Larger current numbers cause efficiencies of roughly 25%, with small loads producing efficiencies of approximately 45%. The H2 generated by CLR from underground coal Reformer

Fuel reactor

Steam

Fuel

Steam

Gasifier Steam Calciner

Gasificaion

chemical looping system with two reactors Air

Steam reactor

chemical looping system with two reactors

Cathode

SOFC Anode

HRSG

Steam turbine

Power

Heat Recovery Steam Generation Solid oxide fuel cell

Steam cycle power plant

Power

Figure 8.5 SOFC, steam cycle power plant, gasification, two chemical looping systems, HRSG [7].

Chemical looping combustion in polygeneration systems

381

gasification (UCG) is utilized in operation to fuel the production of electricity in a PEMFC system. In contrast to traditional reforming, which results in a net power conversion efficiency with CO2 sequestration of more than 37%, the process’s net efficiency with carbon sequestration is 43.6%. A high-temperature PEMFC and a combined process (biogas sorption-enhanced CLR or SECLR) have also been studied. The findings demonstrated that the exegetic efficiency rose from 16.6% to 26.72% with the addition of heat. Additionally, it is established that increasing cell temperature positively impacts total exergy destruction, whereas increasing current density has the opposite effect. Fig. 8.6 shows a PEMFC power-generating system using a water gas shift system and two CLC reactors.

8.9.1.3 Steam turbine power plants/chemical looping-based systems The capability of a system for a synthesis gas based on CL application for coproduct with power and H2 is up to 450 MW net electricity and generation of 0200 MW of H2 (using LHV). A deeper look at possibilities shows the application of gasifier selection, gasifier supply systems, heat and electricity combination processes, and other possible efficiency-boosting measures. Moving bed reactors are used in a 25 KWth subpilot unit with a three-reactor CL-H2 production using synthesis gas to improve fluidized bed reactors. A 100 MWth CLC plant assessment that integrates CLC reactors, a vapor cycle, and a heat recuperation system shows a net process performance of up to 42.8% and a 2% reduction in combustible conversion. As a result, the reactor configuration is crucial to obtaining the desired efficiency. It is essential to note that this process reduces the net system efficiency by around 1%. Additionally, the type of combustible determines how the reaction will occur in the combustible reactor (exothermic or endothermic). To establish whether the combustible oxidation process is exothermic or endothermic, the temperatures of the combustible and air reactors must be carefully changed. It is determined that Air

Fuel reactor

Fuel Cathode WGS

PEMFC

steam Anode

Power Air or Pure Oxygen Air reactor

Water-gas-shift

Proton exchange membrane fuel cell

chemical looping system with two reactors

Figure 8.6 PEMFC, WGS, and chemical looping system with two reactors [7].

382

Hybrid Poly-generation Energy Systems

shifting between synthesis gas and nitrogen gas is possible after considering a few system-related factors. With CO2 sequestration, a chemical looping with oxygen uncoupling (CLOU) power plant’s 42% net electric performance and optimization of efficient components may achieve more than 90%. In addition, using the inverted Brayton cycle with CLC and integrating it with other power production systems demonstrates its advantages. In contrast, a combination procedure known as CL-oxy combustor, which consists of a CLAS facility with inter-connection of the oxyfuel combustor, does have many benefits over traditional processes. As a result, a more 6% effectiveness in an integrated boiler system (IBS) with a vapor cycle can be obtained. Compared to cryogenic air separation unit (CASU) and CLOU methods, the energy loss of the chemical looping oxy- combustor (CLOC) approach is nearly 3%4%, 50%, and around 45 times lower, respectively. A straightforward vapor turbine power generating structure relies on a CLC process with two-reactor, as shown in Fig. 8.7. Combustible enters a combustible reactor on a semicommercial size, and the flue gas from the reactor is utilized to preheat the feed-water and air that approach the air reactor. Three more reactors operate on steam continuously from the air reactor. Combustible is introduced into the combustible reactor of a gas-fired power plant. Following pre-heating the expander outputs water and creates vapor; its exhaust gases are used to condense water and store carbon dioxide. In an air reactor, warmed vapor and air are introduced, and the vapor that exits runs an expender. In a hybrid CLOU system, coal reaches the combustible reactor and is returned to it after being stripped of its CO2 and oxidized in the air reactor. The CO2 stripper and combustible reactor both employ CO2 streams. The air reactor and GT, which run at three different levels of pressure to produce electricity, receive superheated and reheated water. Upon flowing via the economizer, GT flue water reaches the combustible reactor. CO2 and Nitrogen gas are introduced into the combustible reactor

Fuel reactor

Fuel

Steam turbine

Air or Pure Oxygen Air reactor

Steam cycle power plant

chemical looping system with two reactors Figure 8.7 Steam turbine, chemical looping process with two-reactor [7].

Power

Chemical looping combustion in polygeneration systems

383

via a process that utilizes the inverted Bryton cycle for the CLC cycle. Once decreased particles and air react in the air reactor, some of the released gas vapor is used to heat the first steam process, while the remainder enters an inverted Brayton and subsequently a subsequent vapor process. Furthermore, recognizing the recommended technique involves adding limestone and water to the coal while using an oxycirculating fluidized bed combustor. The exhaust gas enters the reduction reactor with CH4 as the feed after being stripped of its ash and sulfur in a cyclone and running through a three-step vapor process. The returning exhaust gases introduced to the reducing reactor are heated by the hot outlet vapor, which contains oxygen gas, carbon dioxide, and water. The oxidation reactor is followed by air and water. The resulting vapor is heated by the generated gas to power an steam turbine (ST). A vapor cycle-based power-generating system using two CLC reactors and an oxyfiring combustor is shown in Fig. 8.8. A coal flow with 26 vol% oxygen enters an oxy-PF furnace during the CLAS operation. Following the warming of the vapor, ash, and sulfur-containing intake of oxygen, the exhaust gases are divided into two portions. The carbon dioxide processing system receives one portion, while the decreased reactor receives the unused amount. The methane stream also reaches this reactor. The reduction reactor uses air to operate. An ST is powered by hot water from the oxidation reactor. After passing the oxy-PF, another water vapor operates a three-stage vapor power plant. Another procedure that has been presented has a similar overall structure to the one that was previously mentioned. However, an additional component is used. A reduction reactor’s additional heat process utilizes solar heating to raise particle temperatures between the oxidation and reduction reactors by 90
C. Coal or natural

Oxy-PF

Oxy-pulverized fuel firing

Steam turbine

Power Steam cycle power plant

Fuel reactor Fuel

Air or Pure Oxygen Air reactor

chemical looping system with two reactors

Figure 8.8 ST electrical unit, oxy-PF, chemical looping process with two reactors [7].

384

Hybrid Poly-generation Energy Systems

gas is introduced into an IGS or IBS during an integrated chemical looping air separation (ICLAS) operation. The reduction reactor receives carbon dioxide 1 water with vapor at more than 550
C. An oxyfurnace receives its exhaust flow and coal input from the boiler. Condenser water is transferred to a steam drum before being fed to the ST. Exhaust gas is introduced to a purification and carbon dioxide process plant, after which CO2 is deposited. An air blower provides the necessary air for the oxidation reactor. A two-reactor CLC with a steam turbine for power production and an HRSG system for heat recovery is shown in Fig. 8.9.

8.9.2 Three reactor conversion process configurations Three reactors make up the majority of the loop in the H2 generating process or CLHG. The combustible is fed into the combustible reactor, where it interacts with the O2 carrier:

n1m=2 y Ox 1 Cn H2m ! n 1 m=2 My Ox22 1 mH2 O 1 mH2 1 nCO2

(8.3)

Fuel reactor Fuel

Steam turbine

Air or Pure Oxygen

Power Steam cycle power plant

Air reactor

chemical looping system with two reactors

HRSG

Heat Recovery Steam Generation Figure 8.9 Steam turbine power plant, chemical looping system with two reactors, HRSG [7].

Chemical looping combustion in polygeneration systems

385

Following the vapor condenses, pure carbon dioxide may be produced just after the reactant has been entirely transformed. The bottoming reaction subsequently takes place when the decreased OC (MyOx-2) reaches the steam reactor:

n 1 m=2 My Ox22 1 mH2 O ! n 1 m=2 My Ox21 1 mH2

(8.4)

Inside the air reaction chamber, fractionally MyOx-1 is further oxidized to reach the main state:

n 1 m=2 My Ox21 1 1/2O2 ! n 1 m=2 My Ox

(8.5)

Completely MyOx switches back to the fuel reactor, and a subsequent process begins. Fig. 8.10 presents a conceptual diagram of a CL process with three-reactor. In certain circumstances, a loop may necessitate more than a reductive or oxidation. Such conditions call for more intricate arrangements of reactors for redox. Reactor designs may be placed in a series, parallel, or a mix of these modes. A particular kind is carried out during each reduction or oxidation phase of a loop in order to finish the loop perfectly. The placement of O2 carriers is radially in spinning reactors. Sectors of the circle are filled with combustible, inert gases and the air needed for OC reduction. Rotary motions improve interaction in these reactors. A trigeneration power plant that can produce electricity, heat, and H2 may employ a three-reactor CL process. A waste heat recovery unit composed of a Rankine cycle, an ORC, and space heating may be combined in combination with a Solid oxide fuel/GT power generation unit production. Calculations provide 56.9% and 45.05% of total energy and exergy efficiency, respectively. The power efficiency is determined to be 23.47% and 37.3%, respectively, with and without H2 generation, taken into account. Additionally, increased SOFCs pressure leads to increased SOFCs and general efficiency. Once the operating pressure is increased to 1800 kPa, the efficiency rises MyOx-2

CO2 + H2O

Fuel reactor

Fuel

H2 + H2O

MyOx-1

Steam reactor

Steam

MyOx

Figure 8.10 Three-reactor chemical looping process [5].

Oxygen depleted air

Air reactor

Air/Pure oxygen

386

Hybrid Poly-generation Energy Systems

by 5%. Pressure is more effective than raising the temperature. One method of integrating CL with SOFCs systems is the ZECA procedure. It has never been attempted practically. The efficiency of this process for a 600 MW plant is calculated by modeling to be 68.8% (HHV) with a SOFCs efficiency of 59% (LHV). The coal supply contains the gasification of carbon dioxide and oxygen that enters the gasifier in the procedure described below. Synthesis gas is followed to the anode following desulfurization and preheating, and the electrode output subsequently reaches the combustible reactor. On the contrary, pressurized air reacts in the cathode before being combined with air with more oxygen and entering the air reactor. After leaving the GT, the flue gas from the combustible and air reactors are directed to the carbon dioxide and air HRSG, both of which receive heating from the combustible reactor and heat burner. After becoming steam, the intake H2O flows to the aforementioned heat recovery steam generations and then on to the steam expander. After leaving streams, carbon dioxide and water are subsequently pressurized and condensed, respectively. A structure of gasification-based CCPP and HRSG systems is shown in Fig. 8.11. It uses a three-reactor chemical loop cycle to generate power from a SOFCs stack. The gasifier receives steam, oxygen, and coal inside a combined cycle. Since it has been cleaned, generated synthesis gas is sent to the combustible reactor. Steam

Power Air

Cathode Steam turbine

SOFC Anode

Gas turbine Fuel

Solid oxide fuel cell Gasifier

Combined cycle power plant Gasification agent (O2/CO2/Steam)

Steam Carbonator or Hydrator or Fuel reactor Steam reactor

Gasificaion

Air or Pure Oxygen HRSG

Calcinator or Air reactor

Heat Recovery Steam Generation

chemical looping system with three reactors

Figure 8.11 Solid oxide fuel cell, combined cycle power plant, fuel gasification, three-reactor chemical looping process, and heat recovery steam generation [5].

Chemical looping combustion in polygeneration systems

387

and the reactor’s output are introduced into the steam reactor. Partially oxidized carriers reach the air reactor well after the reaction, and the generated hydrogen reaches the negative electrode. Utilizing the gas expander’s output, the air reactor thoroughly reacts with oxygen to the fragments before sending them to the combustible reactor. The positive electrode receives the outgoing air. The discharge stream from a fuel cell is burned in a boiler before entering a GT. Combustion air from the exhaust drives the three-stage steam turbine and the heat recovery steam generation. In the same manner as its use in carbon dioxide HRSG, the release of carbon dioxide from the combustion reactor differs from that of water in three distinct phases. In a different procedure, biomass is separated into two components after being gasified in a steam gasifier by pre-heating the intake air to the positive electrode. The initial portion following reformation reaches the negative electrode, and the following subsection reaches the carbonator by mixing the negative electrode response. After reacting with the steam that has been admitted to the hydrator, the solid carbonation output exits the calcinator and joins the carbonator to conclude the cycle. After the temperature decreases in the heat recovery steam generator, hydrogen is separated from the carbonator product gas and sent to the storage system, while carbon dioxide (the carbonator gas product) passes through the heat recovery steam generator after pressuring. The outcome of heating in the heat recovery steam generator is hot steam approaching the gasifier and reformer. The negative electrode product that exits the GT also passes the HRSG. HRGS manages a steam turbine with three different levels. Fig. 8.12 depicts a SOFC/Chemical Loop Cycle/GT-based power plant that uses an air separation unit to deliver O2 to the gasifier and AR. In a CLC system with three reactors, combustion takes place. The combustible reactor receives biogas heated via vapor from the steam generator. The air, after heating, is forwarded to the air reactor. Following passing via the heat exchanger, the combustible reactor’s product reaches the intermediate temperature H2O-gas-shift reactor with vapor from the steam generator. The flue stream enters CO preferential oxidation reactor (COPROX) along with an oxygen gas flow. Therefore, the negative electrode is reached upon passing via a heat exchanger. The positive electrode additionally receives air. A three-reactor CL cycle arrangement with polymer electrolyte membrane fuel cells-based energy production is shown in Fig. 8.13. The combustible reactor may be supplied with synthesis gas following heating up. To renew O2-carrier, the solid product is circulated through three combustible, calcination, and air reactors. The reactors’ gas emissions heat the hydrogen of the polymer electrolyte membrane fuel cells’ anode output from the combustible reactor, while its positive electrode receives air. The power generation using polymer electrolyte membrane fuel cells and a CPC design is shown in Fig. 8.14. ASU supplies the O2 and combustion occurs in a three-reactor CL hydrogen due to the incorporation of a heat recovery steam generation. Upon compression, air reaches the air reactor in a hybrid UCG, polymer electrolyte membrane fuel cell, and integrated electricity generation unit. The air reactor’s output, together with GT exhaust, then travels to the combustible reactor.

388

Hybrid Poly-generation Energy Systems

Air Fuel

Cathode

steam

SOFC

Gasifier

Anode

Power Solid oxide fuel cell

Gasificaion

Water

Fuel reactor

Air

Steam reactor

Pure Oxygen

ASU

Gas turbine

Power Gas cycle power plant

Air reactor

Air Separation Unit

chemical looping system with three reactors PRC plant

Power SRC plant

HRSG

Power

Figure 8.12 Solid oxide fuel cell, gas cycle power plant, air separation unit, gasification, three reactor chemical looping system, heat recovery steam generation, organic Rankine cycle, SRC [7].

H2

Air

Fuel reactor Calcination reactor Fuel Cathode

PEMFC Air or Pure oxygen

Anode Air reactor

chemical looping system with three reactors

Proton exchange membrane fuel cell

Figure 8.13 PEMFC, chemical looping system with three reactors [7].

Power

Chemical looping combustion in polygeneration systems

389

Steam

Fuel reactor

Steam reactor

Fuel

H2

Steam turbine

Power Air reactor

Gas turbine

chemical looping system with three reactors Combined cycle power plant

Air

Pure oxygen ASU

Air Separation Unit

Cathode

HRSG

PEMFC Heat Recovery Steam Generation Anode

Power Proton exchange membrane fuel cell

Figure 8.14 PEMFC, CCPP, ASU, chemical looping system with three reactors, HRSG [7].

Following preheating and energy generation in the GT, generated synthesis gas flows via a heat recovery steam generator before returning to the combustible reactor. Reduced particles in the combustible reactor pass to the vapor reactor. Following a heat recovery steam generator, completely oxidized particles returned to the air reactor, and the gas exits into the negative electrode. Pure oxygen from the air removal unit reaches the positive electrode, and surplus oxygen is reused. A heat recovery steam generator is crossed after air from an air reactor has been released. The vapor that is generated turns a vapor expander. An elaborate polymer electrolyte membrane fuel cell-based power production unit utilizing the CL process is shown in Fig. 8.15. A Ca-looping and chemical-based solvent cleansing method for carbon dioxide collection, in which pre-heated exhaust gases from a coal-fired power generation (CFPP) reach a carbonator. Pure exhaust gases exit the operation after the product gas has passed through the economizer. The calciner is filled with fuel, oxygen from ASU, recovered carbon dioxide, and limestone. The resulting vapor powers a threestage vapor ST power unit while the outgoing carbon dioxide reaches the CCU. First and sequence super-heaters are used as HRSG by the carbonator and the calciner. Three-reactor CL process with gasification and steam turbine energy production is shown in Fig. 8.16.

390

Hybrid Poly-generation Energy Systems

Figure 8.15 An oxygen and carbon dioxide blown combined typical synthesis gas reformer PEMFC process electricity plant’s full flow sheet [8]. Steam

Fuel reactor

Steam reactor

Fuel Gasifier

Gasification agent (O2/CO2/Steam)

Steam turbine

Air or Pure oxygen

Power Air reactor

Gasificaion

Steam cycle power plant

chemical looping system with three reactors

Figure 8.16 Steam turbine electricity plant, gasification, chemical looping-process with three reactors [7].

Synthesis gas from the gasifier island, vapors, and air flows approach combustible, vapors, and air reactors, correspondingly, in the combined cycle system described below. Solid particles move back and forth between these reactors to regenerate O2-carriers. The power-generating segment receives the vapor portion of the exports. Following dryness and compression, carbon dioxide from the combustible reactor is sent to the storage facility. Hydrogen from the vapor reactor is delivered to the compression and filtration stages, with just a portion utilized for power production.

Chemical looping combustion in polygeneration systems

391

Steam

Fuel reactor(reducer)

Steam reactor(oxidizer)

Fuel

Gasifier

Gasification agent (O2/CO2/Steam)

Steam turbine

Power

Air or Pure oxygen

Gasificaion

Air reactor(combustor)

Steam cycle power plant

chemical looping system with three reactors

HRSG

Heat Recovery Steam Generation

Figure 8.17 Steam turbine unit, gasification, chemical looping-process with three reactors and heat recovery steam generator [7].

Fig. 8.17 describes a combined system of gasification, three-reactor CL-process, ST electricity generation unit, and heat recuperation cycle.

References [1] A. Allahyarzadeh-Bidgoli, P.E.B. de Mello, D.J. Dezan, et al., Thermodynamic analysis and optimization of a multi-stage compression system for CO2 injection unit: NSGA-II and gradient-based methods, J Braz. Soc. Mech. Sci. Eng. 43 (2021) 458. Available from: https://doi.org/10.1007/s40430-021-03164-5. [2] A. Allahyarzadeh Bidgoli, N. Hamidishad, J.I. Yanagihara, The impact of carbon capture storage and utilization on energy efficiency, sustainability, and production of an offshore platform: thermodynamic and sensitivity analyses, ASME, J. Energy Resour. Technol. 144 (11) (2022) 112102. Available from: https://doi.org/10.1115/1.4053980. [3] A. Allahyarzadeh-Bidgoli, N. Hamidishad, J.I. Yanagihara, Carbon capture and storage energy consumption and performance optimization using metamodels and response surface methodology, J. Energy Resour. Technol. 144 (2022) 050901. Available from: https://doi.org/10.1115/1.4051679. [4] M. Mehrpooya, M.R. Ganjali, S.A. Mousavi, N. Hedayat, A. Allahyarzadeh, Comprehensive review of fuel-cell-type sensors for gas detection, Ind. Eng. Chem. Res. 62 (2023) 23872409. Available from: https://doi.org/10.1021/acs.iecr.2c03790.

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Hybrid Poly-generation Energy Systems

[5] M. Rajabi, et al., Chemical looping technology in CHP (combined heat and power) and CCHP (combined cooling heating and power) systems: A critical review, Applied Energy 253 (2019) 113544. [6] M.M. Hossain, H.I. de Lasa, Chemical-looping combustion (CLC) for inherent CO2 separations—a review, Chemical Engineering Science 63 (18) (2008) 44334451. [7] M. Rajabi, M. Mehrpooya, Z. Haibo, Z. Huang, Chemical looping technology in CHP (combined heat and power) and CCHP (combined cooling heating and power) systems: a critical review, Appl. Energy 253 (2019) 113544. Available from: https://doi.org/ 10.1016/j.apenergy.2019.113544. [8] L. Yan, G. Yue, B. He, Exergy analysis of a coal/biomass co-hydrogasification based chemical looping power generation system, Energy 93 (2015) 17781787.

A framework for sustainable hydrogen production by polygeneration systems

9.1

9

Introduction

Energy is used in all aspects of life and allows the existence of ecosystems to make human civilization possible. World energy demand increases with population growth. According to the International Energy Agency, the energy demand from fossil fuels is 81.3%, while nuclear and renewable energies are 5.8% and 10.7%, respectively [1]. In this regard, energy has been essential in building environmental policies, sustainable issues, social dimensions, etc. [2]. Based on fossil fuels, today’s energy systems cannot be considered stable [3]. Worldwide, the use of fossil fuels also leads to environmental and energy problems. Emissions of greenhouse gases (GHGs) and CO2 and declining fossil fuels, leading to rising fossil fuel prices, are examples of these problems. These reasons have led people to look for more efficient and cheaper energy conversion options [4]. Hence, an alternative to fossil fuels has been considered. Increasing energy demand will lead to the growth of nuclear and renewable energy in order to solve sustainable solutions. Therefore it is expected that there will be a change in the energy supply from fossil fuels to nuclear and renewable energy sources due to increased energy demand as well as environmental issues such as global warming [5,6]. Renewable energy sources, such as water heaters, wind, biomass, geothermal and solar photovoltaics, and solar heat that do not emit GHGs, can be a good solution. Therefore more research and funding are devoted to renewable energy technologies worldwide. However, renewable energy sources have challenges [7]. Therefore alternatives to renewable energy sources, such as nuclear energy, should be considered. The growing number of countries adopting a nuclear energy program reflects global efforts to develop nuclear power plants. However, in the future, all energy systems are expected to be hybrid systems that combine different energy sources and energy conversion methods to maximize efficiency and minimize environmental impact and wasted energy [8]. As an essential energy carrier and fuel, hydrogen will help solve several energy challenges we face today. Oxidation does not emit GHGs or cause climate change if clean energy sources are used [9]. H2 has been widely proposed as an absorber energy carrier due to its high calorific weight and ability to propagate the effective CO2 line to a source point ready for removal, transport, and storage [10]. Numerous researchers predict that hydrogen will replace petroleum products for Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00011-6 © 2024 Elsevier Inc. All rights reserved.

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vehicle fuels, reducing dependence on oil. Industrial sectors such as petrochemicals, agriculture, food, plastics, and manufacturing use hydrogen as a commodity [10,11]. Hydrogen is abundant in nature in the form of water. However, pure hydrogen has to be produced. There are several methods to achieve this goal, including steam reforming of natural gas, water electrolysis, coal gasification, and thermochemical water decomposition. Hydrogen production using thermochemical water splitting has a cleaner and more cost-effective capability than other hydrogen production methods. Studies have shown that although hydrogen generation systems using the thermochemical cycle have not yet been commercialized, such systems can compete with conventional H2 generation methods, including methane vapor reforming [12,13]. Today, using solar energy as a primary source of power generation is essential. The use of solar energy technology has many positive effects on the environment, such as reducing GHGs and toxic emissions, reducing the requirements for transmission lines in the electricity network, rehabilitating barren lands, and improving the quality of water resources. Socio-economic benefits of solar technologies include increased national and regional energy independence, job opportunities, increased acceleration of rural electricity supply in remote areas, and diversity and security. In general, since solar energy is an inexhaustible, safe, and clean energy source, it has attracted much attention as an alternative to conventional energy sources. Solar energy is used in two ways: photovoltaic (PV) solar technology and solar thermal technology. In the first method, sunlight is converted directly into electricity, which is done by photovoltaics. In the second method, first, the heat of the sun is absorbed by the fluid inside the collector, then this heat is converted to power, or its heat is used to provide the heat load of another system.

9.1.1 Hydrogen production technologies Fossil fuels such as oil, gas, and coal are the primary energy sources for electricity generation, transportation, and residential services. These fuels have been composed of organic matter for millions of years and have contributed significantly to the last century’s global development. Simultaneously, due to the non-renewability of these resources, increasing levels of carbon dioxide in the atmosphere, increasing energy demand in the world, environmental risks, and forecasts of the energy crisis, many efforts to replace clean fuel are in process. Since energy supply is one of the biggest challenges of the age ahead, hydrogen has been proposed as an alternative and desirable fuel for the future. Hydrogen has many advantages as a clean energy carrier because it can be used in all energy consumption areas and has better flexibility and overall efficiency. In addition to the features that distinguish hydrogen from other fuel options, we can mention the abundance, very low emission of pollutants, and the reduction of greenhouse effects. The technical specifications of hydrogen are such that it can be used for transportation, power generation, and heat, as well as for the replacement of today’s fuels in all existing applications. In addition to all these benefits, the challenge of hydrogen storage and production methods is an obstacle to creating a

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hydrogen economy. Hydrogen can be produced from various foods, including fossil fuels (including natural gas and coal) and renewable sources (such as biomass and water), using renewable energy sources such as sunlight. They become wind and waves or hydropower. Various technologies can produce hydrogen, including chemical, biological, electrolytic, photolytic, and thermochemical technologies. Each of these technologies is at different development stages, and each has opportunities, challenges, and advantages. Local access to feedstock, the maturity of the technology, market and demand applications, policy issues, and costs can influence the appropriate hydrogen production method. Fig. 9.1 shows the types of hydrogen production technologies. Various technologies are available in the industrial market for hydrogen production. The first commercial technology dating back to the 1920s was the electrolysis of water to produce pure hydrogen. In the 1960s, industrial production of hydrogen shifted to fossil fuels, which today are the primary source of hydrogen production. More than 98% of hydrogen is produced through renewable fossil sources [14]. The most common method of producing hydrogen is to reform methane fuel or natural gas. Most hydrocarbon fuels contain small amounts of sulfur, which can cause catalyst poisoning. Catalyst poisoning is the biggest challenge of hydrocarbon fuels to produce hydrogen, which can be eliminated by techniques such as adsorption and chemical reactions (such as hydrodesulfurization and alkalinity) [14]. Large-scale hydrogen production is possible in the not-too-distant future, and water electrolysis and natural gas and coal reforming methods are good options for distributing distributed hydrogen. Water electrolysis and natural gas reforming are the technologies of choice today and future. These two technologies are suitable for implementing an infrastructure for producing hydrogen used in transportation. Small natural gas reformers that are commercially limited have been tested and proven in various projects. Other hydrogen production methods are still far from commercialization and require further research and development. The following section briefly discusses the various methods of hydrogen production using different feedstocks.

9.1.1.1 Fuel-based hydrogen production methods In recent years, hydrogen has been proposed as a clean and desirable fuel for the future. Several methods are used to produce hydrogen. High-temperature electrolysis produces remarkably high-efficiency and pure hydrogen, and when combined with new solid oxide fuel cell technology, it provides intensifying effects [15]. Today, fuel cells are a viable alternative to internal combustion engines. This technology produces energy without causing pollution; its only by-product is water. This device does not follow the Carnot cycle and directly converts chemical energy into electricity during an electrochemical reaction. The solid oxide fuel cell operates at 600
C1000
C due to its solid structure. This cell’s efficiency is approximately 4555%, which, when combined with a turbine, increases the overall efficiency by about 70% [16]. Advanced thermodynamic systems, fuel cells, and

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Hydrogen Producon

Physical-Chemical

Hydrocarbon

Biologic

Water-based

Water biophotolysis

Steam reforming

Eletrolysis

Organic compound photodegradaon

Natural gas thermo cracking

Photolysis

Hybrid systems

Heavy hydrocarbon paral oxidizaon

Thermolysis

Coal Gasificaon

Thermochemical processes

Pyrolysis

Ammonia reforming

Figure 9.1 Hydrogen production methods [17].

hybrid systems are the best hydrogen and power generation choices. Due to the importance of the cost factor of hydrogen production from different fuels and fuel resources in our beloved country, the proposed input fuel is natural gas. It should be noted that natural gas is a suitable fuel for the solid oxide fuel cell and solid oxide electrolyzer cell cycles due to the high operating temperature of the battery. The efficiency of conventional water electrolysis by a solid oxide electrolyzer is about 40% [18]. In the above hybrid process, the process efficiency will reach nearly twice the normal electrolysis of water and about 83% [18]. The solid oxide

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fuel cell can be fed with natural gas as fuel and provides electricity and heat for use in the solid oxide electrolyzer cell. In an electrolyzer, water vapor decomposes into hydrogen and oxygen during the electrolysis process. Thus in this hybrid process, the same operating temperature of the two cells in the system is one of the configuration benefits. Hydrogen is one of the most abundant elements on the earth. This element does not exist in nature in its pure form and must be produced from various other compounds in different ways. Hydrogen is the primary option as an energy carrier. Fig. 9.2 shows the amount of hydrogen in different fuels. The following reviews hydrogen production from significant sources such as fossil fuels, biomass, and water. It should be noted that hydrogen can be produced from other fuels, such as methanol, ethanol, ammonia, and other substances, which are not discussed here.

9.1.1.1.1 Hydrogen production from fossil fuels Hydrogen can be produced from most fossil fuels. Hydrogen production from natural gas and coal is briefly described here. Carbon dioxide, a by-product of these fuels, must be removed through processes.

Figure 9.2 The ratio of hydrogen to carbon in different fuels [19].

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Hydrogen production from natural gas Three different chemical processes are used to produce hydrogen from natural gas: 1. Steam methane reforming (SMR) 2. Partial oxidation (POX) 3. Autothermal reforming (ATR)

Steam reforming involves converting methane and water vapor heat exchangers to hydrogen and carbon monoxide during Eq. (9.1). The heat required for the process is often obtained from the combustion of part of the methane feed. This process typically occurs at a temperature of 700800
C and a 325 bar pressure. The product gas also contains approximately 12% of carbon monoxide, converted to carbon dioxide and hydrogen in Eq. (9.2). This equation is known as the Water Gas Shift (WGS) reaction. CH4 1 H2 O 1 heat ! CO 1 3H2

(9.1)

CO 1 H2 O ! CO2 1 H2 1 heat

(9.2)

Partial oxidation of natural gas is when hydrogen is produced Eq. (9.3). During this exothermic process, the partial oxidation of methane with oxygen gas leads to carbon monoxide and hydrogen production. Due to the absence of an external thermal reactor in this process, it is possible to design a coherent system. Also, carbon monoxide can be converted to hydrogen during the gaswater reaction: CH4 1

1 O2 ! CO 1 2H2 1 heat 2

(9.3)

Auto-thermal reforming combines steam reforming processes [Eq. (9.1)] and partial oxidation [Eq. (9.3)]. The overall reaction is exothermic. The reactor’s outlet temperature is 1100950oC, and the gas pressure is high and about 100 times. The carbon monoxide produced during the gaswater reaction is also converted to hydrogen. Exhaust gas purification from this process significantly increases costs and reduces overall efficiency. Each process has its advantages and challenges, as summarized in Table 9.1. Hydrogen production from coal Various gasification processes include the fixed bed, fluid bed, and entrained flow. In practice, high-temperature train flow processes are considered because of the maximum conversion of carbon to gas. The typical reaction of the gasification process is according to Eq. (9.4), in which carbon is converted to carbon monoxide and hydrogen. Carbon monoxide is converted to carbon dioxide and hydrogen if the gaswater reaction’s operating conditions are provided: CðsÞ 1 H2 O 1 heat ! CO 1 H2

(9.4)

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Table 9.1 Reforming technologies comparison [14]. Disadvantages G

G

More air pollution System complexity

Advantages G

G

G

G

G

G

G

G

Trade restrictions Air or oxygen required Less purity of produced hydrogen

G

G

G

G

G

G

G

G

Low hydrogen-to-carbon monoxide ratio High operation temperature Process complexity due to soot formation Lower efficiency and purity of hydrogen produced

G

G

G

G

G

Technology

Most extensive industrial technology Better efficiency No oxygen required Lower operating temperature The best hydrogen-to-carbon monoxide ratio for hydrogen production

Steam reforming

Lower operating temperature than partial oxidation Low methane slip Small size compared to steam reforming Simpler system Reduced need for the desulfurization process No catalyst needed Low methane slip Small size compared to steam reforming Simpler system

Autothermal reforming

Partial oxidation (POX)

Although coal gasification is commercially mature, it is more complex than hydrogen production from natural gas. The costs of producing hydrogen are often high, but because coal is abundant in many parts of the world, finding clean technology to use coal is invaluable.

9.1.1.1.2 Hydrogen production from biomass In the not-too-distant future, biomass will be an organic and renewable resource. It will be a possible alternative to oil. Biomass can be obtained from various sources, such as animal waste, municipal solid waste, crop waste, wood materials, agricultural waste, waste paper, grain, and other useless materials. Biomass-to-hydrogen conversion technologies include gasification, pyrolysis, supercritical extraction, liquefaction, hydrolysis, and more. In biomass conversion processes, hydrogen is produced like coal gasification. Coal gasification technology is a type of pyrolysis. The partial oxidation of the material is converted into a mixture of hydrogen, methane, carbon dioxide, carbon monoxide, and nitrogen. The gasification process has low thermal efficiency due to the evaporation of moisture in the biomass. This process can be performed as a catalytic or noncatalytic reaction in a fixed or mobile bed reactor. The gaswater reaction produces more hydrogen, followed by a purification process to increase

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Recycle

Biomass

Drying and chipping

Gasificaon and gas cleaning

Reforming, shiing, CO2 separaon

Catalysis, separaon

Methanol

Separaon

Hydrogen

Catalysis, separaon

Refining

Gas turbine or boiler

FT diesel

Electricity

Steam turbine

Figure 9.3 Hydrogen production from biomass (biomass gasification) [20].

hydrogen’s purity. Fig. 9.3 shows a typical flowsheet producing hydrogen from biomass. Sometimes, it is used to convert biomass to hydrogen, reforming and producing biological hydrogen. Due to the increasing importance of waste minimization, much research has been done on biohydrogen in recent years. The leading technologies for bio-H2 production include photolytic hydrogen production from water by green algae or cyanobacteria, fermentative hydrogen production, the photofermentation process, and hydrogen production through the gaswater reaction. In the production of biological hydrogen, water is used in the photolysis process, and biomass is used in the fermentation process. Fig. 9.3 shows hydrogen production from biomass (biomass gasification).

9.1.1.1.3 Hydrogen production from water splitting Water splitting produces hydrogen through a variety of processes. The first commercial use of this method dates back to the 1980s. This section briefly studies various water refraction technologies such as electrolysis, photoelectrolysis, photobiological production, and high-temperature water decomposition. Water electrolysis The simplest water-splitting method uses an electric current passing through two electrodes to decompose water into hydrogen and oxygen through Eq. (9.5). According to this equation, the required energy increases slowly with temperature, while the required electrical energy decreases: H2 O 1 electricity ! H2 1

1 O2 2

(9.5)

This reaction is done through electrolyzer cells. Commercial low-temperature electrolyzers have good systematic efficiency. The most common electrolysis technology is based on alkaline cells, and proton exchange membrane fuel cell (PEMFC) is being developed. Although minor development has taken place in solid oxide fuel cells (SOEC), these electrolyzers are the most electrically efficient. This

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technology challenges corrosion, seals, and thermal cycles. Proton exchange membrane electrolyzers that do not have the corrosion effect and solid oxide electrolyzer seals are more efficient than alkaline electrolyzers. Nevertheless, the cost of making them is higher. The most researched alkaline systems have lower investment costs and electrical returns. As a result, they consume the most electrical energy. Photoelectrolysis Photoelectrolysis of water is a process in which light is used to decompose water into hydrogen and oxygen directly, and semiconductor materials similar to PV cells are used. PV cells are combined with electrolyzers to perform photoelectrolysis. These systems are flexible, meaning that their output can be electricity produced from PV cells or hydrogen from electrolyzers. The principles of direct photoelectrolysis are shown in Fig. 9.4. A more comprehensive explanation is given in reference [21]. Thermochemical split of water Thermochemical decomposition, the conversion of water into hydrogen and oxygen during a series of chemical reactions with a thermal driving force, is usually called thermolysis [14]. The overall efficiency of such a process is about 50% [22]. During this process, water decomposes at 2500
C. However, items that are resistant to this temperature and environmentally friendly heat sources are not readily available. Research in this area began in the late 1970s and 1980s and ceased around the mid-1980s. However, recently, especially in the last ten years, it has attracted the attention of scientists. Due to the lack of resistance of the material at 2500
C in most thermochemical processes, the operating temperature is significantly reduced, and the operating pressure is increased. An example of a thermochemical process is the iodine/sulfur cycle. Eqs. (9.6)(9.8), and Fig. 9.5 illustrate this process. More than three hundred water failure cycles have been studied in [97].

Oxygen

Hydrogen Incident solar radiaon

Sensized glass surfaces Electrolyte Water

Figure 9.4 Principles of direct photoelectrolysis [20].

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Hydrogen

Oxygen Nuclear or solar heat ½O2

H2 900°C

400°C

H2 + I2

2HI

Rejected heat 100°C

I2

H2SO4

+

2HI

I (Iodine) circulaon

+ H2O

I2

H2SO4

+ SO2+H2O H2O

½O2 + SO2+H2O

S (Sulfur) circulaon SO2 + H2O

Water

Figure 9.5 Principles of thermochemical process iodine/sulfur [20].

850
C: 1 H2 SO4 ! SO2 1 H2 O 1 O2 2

(9.6)

120
C: I2 1 SO2 1 2H2 O ! H2 SO4 1 2HI

(9.7)

650
C: 2HI ! I2 1 H2

(9.8)

SUM: H2 O ! H2 1

1 O2 2

In addition to the methods mentioned, photobiological hydrogen production and high-temperature decomposition also use water to produce hydrogen. Photobiological hydrogen production consists of photosynthesis [Eq. (9.9)] and catalytic hydrogen production [Eq. (9.10)]. This method requires long-term research, but a long-term solution to hydrogen production is created if this research is successful. Figs. 16 present the principles of photobiological hydrogen production: 2H2 O ! 4H1 1 4e2 1 O2 4H1 1 4e2 ! 2H2

(9.9) (9.10)

A framework for sustainable hydrogen production by polygeneration systems

Algae producon bioreactor (light-aerobic) Sunlight

403

Algae concentrator H2 and adaptaon chamber Hydrogen photobioreactor (light-anaerobic) (dark-anaerobic)

CO2 O2

Sunlight

Algae

Algae

Nutrient recycle

Algae recycle

Figure 9.6 Principles of biological production of hydrogen [20].

Decomposition of water at high temperatures also occurs at 3000
C. In this process, 10% of the water is decomposed at this temperature, and the rest is returned. Of course, thermochemical decomposition of water is a type of this technology. The most critical challenges in using such a process are developing corrosionresistant materials, high-temperature membranes, separation processes, heat exchangers, and equipment. The type of design and safety is also essential for these processes can be seen in Fig. 9.6. As mentioned, hydrogen production is one of the essential goals considered. Additionally, the simultaneous production of hydrogen electricity, and heat is an excellent way to provide energy. Integration of SOFC with other systems such as absorption refrigerators, gas turbines, combined heat and power systems, biomass gasification, and other systems can produce decent electrical power with efficiencies of at least 20% and up to 80%. On the other hand, researchers have found that hydrogen can be produced from various fuels using SOFC. Thus by creating various hybrid processes, an attempt has been made to produce pure hydrogen. If pure hydrogen production is in line with the desired power, the combined cycle of SOFC and solid oxide electrolyzer cell will be a good option. This combined cycle includes a fuel cell and a high-temperature electrolyzer, which can withstand high operating temperatures due to their solid components. Natural gas as the fuel of this system is also proportional to this process’s high operating temperature. According to Fig. 9.3 (showing the ratio of hydrogen to carbon in different fuels) presented in the previous section, it is clear that the amount of hydrogen in natural gas is appropriate and valuable. Table 9.2 shows the cost of hydrogen production from feed types. Table 9.2 shows that the most economical sources of hydrogen production are natural gas and coal.

9.1.1.2 A brief look at process units Population rates are rising, and the need for energy is growing. Energy production processes, such as electricity generation, heating, cooling, and other applications,

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Table 9.2 Cost of hydrogen production from feed types [13]. Feedstock

Cost ($)

Natural gas Nuclear energy Solar energy and natural gas Solar energy Wind energy Coal

2.333.17 2.745.36 7.53 2.238.27 2.627.77 0.136.83

emit pollutants harmful to the ecosystem [23]. Carbon dioxide is an important GHG; the continuous increase of CO2 concentration in the atmosphere and rapid growth in the use of fossil fuels such as coal, oil, and natural gas seriously impact the global greenhouse [24]. One strategy is to reduce CO2 emissions and use H2 as a clean energy source. Studies on CO2 removal methods and H2 production show that the thermochemical cycle is suitable due to its ability to bind to alternative energies with suitable temperatures, such as solar and geothermal [25]. To this end, the Zn-S-I thermochemical cycle reduces CO2 emissions through thermal energy and produces a promising energy carrier (H2). Due to the cycle’s ability to couple with renewable energies, thermal integration is considered to reduce the system’s thermal load. In this section, to produce hydrogen and reduce CO2 emissions, known as one of the most important GHGs, the Zn-S-I (ZincSulfurIodine) thermochemical cycle is presented.

9.1.1.3 Introduction to conceptual process design Hydrogen is produced in various ways, which will be briefly reviewed in this section. We briefly mentioned the thermochemical cycle types and their process methods and subsystems in the following sections. Since various energy sources provide hydrogen production, the introduction of the solar system has been considered in this research.

9.1.1.3.1 Thermochemical cycles As stated in the topic of hydrogen production methods, large amounts of hydrogen come from fossil fuels, and although hydrogen is a carrier of clean energy, it can have adverse environmental effects during its production. For example, steammethane reforming, which is the most widely used method, produces large amounts of GHGs during H2 production. Thermochemical cycles, especially Cu-Cl, S-I, and Zn-S-I, are considered promising options for hydrogen production with less environmental impact.

9.1.1.3.2 Mg-Cl The Mg-Cl thermochemical decomposition cycle combines heat and electricity to split water into hydrogen at a maximum temperature of 550
C. The Mg-Cl cycle

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consists of three main phases (two thermochemical and one electrochemical), which are as follows: Step 1 (hydrolysis step at 723K823K): MgCl2 ðsÞ 1 H2 OðgÞ ! MgOðsÞ 1 2HClðgÞ

(9.11)

Step 2 (Chlorination step at 673K773K): MgOðsÞ 1 Cl2 ðgÞ ! MgCl2 ðsÞ 1

1 O2 ðgÞ 2

(9.12)

Step 3 (Hydrogen production step at 343K363K): 2HClðgÞ ! H2 ðgÞ 1 Cl2 ðgÞ

(9.13)

In the hydrolysis step, a solid-gas reaction occurs, which is produced during the hydrolysis of magnesium chloride MgCl2, hydrogen chloride (HCl), and magnesium oxide (MgO). The reactants for this step are H2O and MgCl2. This is an endothermic reaction with a temperature range of 450
C550
C, the highest temperature required for the Mg-Cl cycle. In the chlorination phase of the Mg-Cl cycle, MgO(s) and Cl2(g) are introduced as reactants to form MgCl2 and O2 at a reaction temperature of about 400
C500
C. In order to recycle all the magnesium compounds, MgCl2 is returned to the hydrolysis stage to form a closed inner ring. Hydrogen is produced in a thermochemical cycle or electrochemical reaction in the third stage of the cycle.

9.1.1.3.3 Fe-Cl The Fe-Cl thermochemical cycle is a three-step cycle consisting of iron chloride (FeCl3) and its steps are as follows [26]: Step 1 (Decomposition of FeCl3 at 700K): 6FeCl3 ! 3Cl2 1 6FeCl2

(9.14)

Step 2 (hydrolysis at 920K): 3FeCl2 1 4H2 O ! Fe3 O4 1 6HCl 1 H2

(9.15)

Step 3 (Chlorination at 420K): 3Cl2 1 2Fe3 O4 1 12HCl ! 6FeCl3 1 6H2 O 1 O2

(9.16)

The hydrolysis reaction occurs at 920K, the highest cycle temperature. Among the cycle characteristics are the low number of reactions, the use of abundant Fe and Cl elements, and the low reaction temperature of approximately 200K, which is lower than the S-I cycle described in a section below [27].

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9.1.1.3.4 Cu-Cl The Cu-Cl thermochemical cycle uses a series of copper and chlorine compounds. Chemical reactions consist of a closed inner ring that continuously recycles all chemicals without GHGs or other substances. Hence, the primary inputs to the cycle are water, heat, and electricity. According to the initial chemical reactions of the cycle, there are three, four, and five stages. The five main stages of a chemical reaction are: Step 1 (hydrolysis at 723K): 2CuCl2 ðsÞ 1 H2 OðgÞ ! CuOCuCl2 ðsÞ 1 2HClðgÞ

(9.17)

Step 2 (Copper oxychloride of decomposition at 773K): 1 CuOCuCl2 ðsÞ ! 2CuClðlÞ 1 O2 ðgÞ 2

(9.18)

Step 3 (Copper production at 298K): 4CuClðaqÞ ! 2CuCl2 ðaqÞ 1 2CuðsÞ

(9.19)

Step 4 (Drying CuCl2 at 363K): 2CuCl2 ðaqÞ ! 2CuCl2 ðsÞ

(9.20)

Step 5 (Hydrogen production at 723K): 2CuðsÞ 1 2HClðgÞ ! 2CuClðlÞ 1 H2 ðgÞ

(9.21)

In the first stage, hydrogen chloride is produced using equipment such as a fluidized bed. In the second stage, copper oxychloride is produced, decomposed, and oxygen is produced. In step three, copper is produced to recover all the materials. In step four, CuCl2 is dried, and hydrogen is produced. The reaction temperature varies for each step.

9.1.1.3.5 S-I Among the S-I thermochemical cycles, the most common is the three-phase cycle, which includes the Bunsen reaction, the decomposition of sulfuric acid, and the decomposition of hydrogen iodide. This cycle is based on three main reactions at three different temperature levels. The main stages of the S-I cycle are defined as follows: Step 1 (Bunsen-hydrolysis reaction at 393K): I2 ðl 1 gÞ 1 SO2 ðgÞ 1 2H2 OðgÞ ! 2HIðgÞ 1 H2 SO4 ðlÞ Step 2 (Sulfuric acid decomposition—oxygen production at 1123K):

(9.22)

A framework for sustainable hydrogen production by polygeneration systems

H2 SO4 ðgÞ ! SO2 ðgÞ 1

1 O2 ðgÞ 1 H2 OðgÞ 2

407

(9.23)

Step 3 (Hydrogen decomposition of iodide - hydrogen production at 723K): 2HIðgÞ ! I2 ðgÞ 1 H2 ðgÞ

(9.24)

The first stage is exothermic hydrolysis, which occurs at 393K. The endothermic oxygen production stage is the second stage, which occurs at temperatures above 1100K, and the final stage is the hydrogen production stage, which is an endothermic reaction and occurs at around 700K.

9.1.1.3.6 Zn-S-I A new thermochemical cycle has been developed that uses Zn/ZnO/ZnI2 recycling factors and the S-I thermochemical cycle. The most common Zn-S-I cycle consists of six stages, which are as follows: Step 1 (Bunsen reaction in 293K393K): I2 1 SO2 ðgÞ 1 2H2 O ! H2 SO4 ðaqÞ 1 2HIðaqÞ

(9.25)

Step 2 (Sulfuric acid decomposition at 1073K1273K): 2H2 SO4 ! 2SO2 ðgÞ 1 2H2 O 1 O2 ðgÞ

(9.26)

Step 3 (Hydrogen iodide decomposition at 573K773K): 2HI ! I2 1 H2 ðgÞ

(9.27)

Step 4 (CO2 reduction reaction at 1050K): Zn 1 CO2 ðgÞ ! ZnO 1 COðgÞ

(9.28)

Step 5 (Zinc-iodide production at 313K): 2HIðaqÞ 1 ZnO ! ZnI2 1 H2 O

(9.29)

Step 6 (ZnI2 decomposition at 1073K): ZnI2 ! Zn 1 I2

(9.30)

The first three steps are the steps of the S-I thermochemical water splitting cycle. The Zn-S-I thermochemical cycle consists of four subsystems called the Bunsen system, the sulfuric acid system, the HIx system, and the zinc system, each of which describes the process mechanism. Fig. 9.7 shows the general diagram of the hydrogen production process in the thermochemical cycle.

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Bunsen The Bunsen system includes a Bunsen reactor, a liquid-liquid separator, two refining reactors, and several auxiliary equipment. The input of the Bunsen system is water, and the return stream from the rest of the system’s cycle and output subsystems is HI from the upper phase and H2SO4 from the lower phase of the system. The reaction temperature range of the system is between 25
C and 408
C. The Bunsen reaction reacts with the reported excess water and iodine in the Bunsen reactor at the reported temperature range to produce two immiscible aqueous solutions of sulfuric acid and hydrogen iodide [29]. The aqueous solutions are separated in the next two steps and sent to the H2SO4 and HIx phases, producing hydrogen and oxygen during the reactions [30]. The output stream has two immiscible liquid phases with additional water and iodine. In both phases, there are small but undeniable impurities. Therefore to purify the streams, the Bunsen reverse reaction was

Figure 9.7 General diagram of the six-step hydrogen production process [28].

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used in the second reactor, and a large amount of energy load is spent on separating impurities and water evaporation [31]. Sulfuric acid The equipment in the H2SO4 system is a flash tank for sulfuric acid concentration and a decomposition reactor. The operating temperature range of the system is 85
C850
C. Due to the greater volatility of water than sulfuric acid, a simple flash can increase the concentration of sulfuric acid. Concentrated H2SO4 is heated before entering the decomposition reactor and decomposed into a mixture of O2, SO2, and water at a specified conversion rate and operating temperature range and returns to the Bunsen reaction. Hydrogen iodide The HIx system includes the HI decomposition reactor, distillation column, and other equipment. The temperature range of the system reactions is between 40
C and 450
C. Because the solution is meta-azeotropic, the pure HI solution remains at the bubble point temperature (43
C) without increasing its concentration, and the pressure increases to 11.7 bar [32]. The products of the distillation column are pure HI vapor from the top of the column, which is at the same temperature as the boiling point of HI, and the liquid solution from the end of the column at the poso-azeotropic saturation temperature [32,33]. The downstream column returns to the Bunsen system. Based on the third step reaction performed in the HI decomposition reactor, H2 is collected as the final product and I2 is recycled to the distillation column together with the decomposed HI [32,34]. Zinc The zinc system consists of three reactors: the Zn-CO2 reactor, ZnI2 production reactor, and ZnI2 decomposition reactor, as well as a ZnI2 crystallizer and desiccator, a filter and solvent, and other auxiliary equipment. The operating temperature range of the reactions is 25
C800
C. Through exothermic oxidation, Zn reacts with CO2 to produce CO and ZnO according to the reaction of the fourth step in the first reactor. After separating the carbon monoxide vapor, the solid ZnO produced is combined with part of the HI vapor extracted from the distillation column in the HIx system with excess water in the ZnI2 reactor. The aqueous solution of ZnI2 is heated, and solid ZnI2 is obtained by water evaporation. Due to the thermal decomposition of solid ZnI2 through endothermic reaction in the third reactor, the produced I2 was returned to the Bunsen reaction and separated due to the different solubility of Zn and ZnI2 produced. The filtered solid Zn was sent to the first reactor, the refined ZnI2 was sent to the drying process, and the evaporated water was returned to the Bunsen reaction. Thus the separation of carbon dioxide and water into carbon monoxide, hydrogen, and oxygen is achieved only with the input of carbon dioxide and water and heat of the appropriate high-temperature process. In order to simplify and improve the six-stage thermochemical cycle, Zn-S-I thermochemical cycles called open-loop system, five-stage cycle, and electrochemical cycle have been studied, which are briefly described. Open-loop thermochemical cycle Concentrated sulfuric acid is removed from the system as a product instead of splitting and decomposition in the H2SO4 subsystem reactor, and the SO2 required for the Bunsen reaction is obtained by burning sulfur.

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The primary sulfuric acid system is divided into the concentrated H2SO4 system and the SO2 production system, while the Bunsen, HIx, and Zn systems remain unchanged. This part of the cycle includes a sulfur melting tank, a sulfur combustion chamber, a back-pressure steam turbine, and some utilities called the SO2 generation system. After melting, liquid sulfur enters the combustion chamber and reacts with air to produce high-temperature flue gas combined with SO2, SO3, O2, and N2. The high-temperature flue gas must then be cooled to 358K, which converts the water into superheated steam by heating the waste heat recovery boiler. Therefore a stream of SO2 is sent to the Bunsen system along with the impurities to participate in the Bunsen reaction. At the same time, the superheated steam goes to the back pressure steam turbine to generate power and is recycled to the waste boiler after a series of heat exchanges. The flow paths of the Bunsen system, the HIx system, and the Zn system are similar to the primary closed-loop cycle. Fig. 9.8 shows the ZnS-I open-loop thermochemical cycle diagram.

Figure 9.8 Zn-S-I open-loop thermochemical cycle diagram [35].

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Five-stage cycle In the sixth cycle reaction, the ZnI2 decomposition ratio can be increased, breaking the reaction equilibrium and sending the reaction to the right, producing more products. According to the reaction of the fourth cycle stage, in exchange for increasing Zn, the amount of CO2 consumption also increases, so it replaces Eq. (9.29) with two reactions, which makes it a short and flexible system for work. The Zn-S-I cycle is divided into four parts: the Bunsen system, the H2SO4 system, the HIx system, and the Zn system, where Zn system is simplified and the other three parts are unchanged. The five-stage thermochemical cycle consists of three main reactions: the S-I cycle, the fifth-stage reaction, and the reaction (9.29): ZnI2 1 CO2 ! ZnO 1 CO 1 I2

(9.31)

The simplified Zn system includes a ZnI2 generator, a ZnI2-CO2 reactor, and several auxiliary equipment. Excess ZnI2 reacts with CO2 to produce ZnO, CO, I2 vapor, and slightly unreacted ZnI2. After the separation of solids, gas products are separated by cooling to melting point and converted to liquid iodide and carbon monoxide vapor. The liquid iodide is then returned to the Bunsen reactor while CO is removed from the system as the final product. Mixtures of ZnI2 and ZnO are fed as solid products to the ZnI2 generator, where half of the HI vapor from the HIx system is dissolved in excess water. Therefore the aqueous solution of ZnI2 is generated and heated, and then solidification is performed to return to the combined reaction of this cycle (223), and at the same time, the steam produced is sent to the Bunsen reaction. Fig. 9.9 shows the Zn-S-I thermochemical cycle diagram in a five-step cycle. Electrochemical cycle In the electrochemical cycle, only the Bunsen system is changed, and the rest remain the same. Due to the transmission characteristics of proton exchange membranes, the H2SO4 and HIx phases are considered by two separate battery poles. The H2SO4 phase at the anode and the HIx phase at the cathode is produced based on the reactions (9.32) and (9.33). Excess water and I2 are used to improve the reactions, making spontaneous classification of the mixed solution necessary, leading to the removal of the treated units. Meanwhile, the H2SO4 and HI concentrations in the electrochemical Bunsen reactor outputs are designed to reduce the heat load required for sulfuric acid enrichment and to remove the equipment for HI concentrations. Therefore the Bunsen system consists of only one Bunsen electrochemical reactor and some utilities, thus simplifying the whole system. When the Bunsen reaction is complete, the H2SO4 and HIx phases enter the H2SO4 and HIx systems, respectively, which are similar in method to the primary system. At the same time, the Zn system remains unchanged. The ZnSI electrochemical cycle diagram is shown in Fig. 9.10. Anode reaction: SO2 1 2H2 O ! H2 SO4 1 2H1 1 2e2

(9.32)

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Figure 9.9 Five-stage Zn-S-I thermochemical cycle diagram [35].

Cathode reaction: I2 1 2H1 1 2e2 ! 2HI

(9.33)

9.1.1.4 Solar energy Solar power plants can be supplied in two ways, one is solar PV, which includes photovoltaic thermal (PVT) and concentrating photovoltaic thermal (CPVT), and the other is solar thermal power with centralized and decentralized collector system.

9.1.1.4.1 Photovoltaic, photovoltaic thermal, and concentrating photovoltaic thermal systems PV solar technology is more focused on supplying electricity and heat through solar PVT systems. The PVT solar system consists of a PV panel that generates heat by the panel during operation by water, air, or cooling extracted. Different solar energy technologies for heat pumps and PV systems, along with other technologies, are used to supply electricity and heat. CPVT-based systems can generate heat and

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Figure 9.10 Zn-S-I electrochemical cycle diagram [35].

electricity by concentrating direct sunlight on PV cells while removing heat from the cell surface by means of a cooler.

9.1.1.4.2 Solar thermal Solar thermal collectors have been studied over the years, including analyzing two centralized and decentralized collectors and different heating, cooling, and electricity generation applications. Solar collector The main component of any solar system is the solar collector. Solar collectors are a particular type of heat exchanger that converts the sun’s radiant energy into the internal energy of a heat transfer fluid. Solar collectors can be distinguished based on the type of heat transfer fluid (water, nonfreezing liquid, air, or heat transfer oil). There are generally two types of solar collectors: decentralized or stationary collectors and sun-tracking concentrating collectors The decentralized collector has the same area for receiving and absorbing solar radiation. In contrast, the sun-tracking concentrating solar collector has a concave

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reflecting surface for receiving and concentrating the intensity of sunlight on a small receiving surface, whereby the flux radiation increases. There are three types of decentralized collectors: flat plate collectors, parabolic collectors, and vacuum tube collectors. Concentrating collectors are suitable for high-temperature applications. We have four types of concentrator collectors: parabolic trough collector (PTC), linear Fresnel reflector (LFR), parabolic dish reflector (PDR), heliostat field collector (HFC), as shown in Fig. 9.11 [36]. The parabolic trough system consists of a PTC and uses a solar tube receiver along the focal length to concentrate sunlight. The solar parabolic trough is usually aligned with the axis from north to south, and the liquid absorbs the solar energy inside the tubes along the channel line. The maximum temperature of the heat transfer fluid is not more than 450
C, which is not enough to heat the whole process of the thermochemical cycle. The Heliostat solar tower uses arrays of two-axis tracking mirrors to reflect directly to the receiver, and the reactor is mounted on top of the center tower. The Heliostat solar tower is an essential advantage in achieving large production capacities in one unit, which is obtained from the focused reflection of thousands of mirrors. The temperature of the heat transfer fluid in this system can go up to 1000
C. In the parabolic dish system, the point collector follows the sun on two axes and concentrates the isolation of the sun’s radiation on the (A)

(B) Concentrator Receiver

Receiver

Heliostats

(C)

Reflective Tower

(D)

Receiver

Concentrator

CPC Heliostat Receiver

Figure 9.11 Schematic of solar concentrating systems: (A) parabolic through; (B) heliostat power tower; (C) parabolic dish; (D) dual concentrator [37].

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receiver located at the dish’s focal point, which can reach a temperature of more than 1500
C. The dual concentrating system consists of a heliostat field, a reflecting tower, and a ground receiver equipped with a secondary focusing device. The receiver or reactor on the ground achieves a temperature of more than 1300
C [36].

9.1.1.5 Thermal integration The main stage of processing systems, including reactors, separators, heat exchangers, etc., after mass and energy balance, is the heat duty stage of heating and cooling, which is known as the heat recovery system. In pinch analysis, any current that needs to be heated or cooled without mixing its components is considered a current. Hot and cold currents, known as process currents, decrease during the process and increase the temperature of the hot stream and increase the temperature of the cold stream, respectively. In order to implement and exploit this process, steam heater is needed as a cold stream and water heater as a hot stream. To make the heat exchanger between hot and cold streams possible, the temperature of the hot stream must be higher than the cold stream in all parts. In addition, by splitting the streams, a large number of utilities can be covered. By covering some cold streams through the heat of the hot stream, energy consumption can be reduced and cause steam and water reduction to meet the remaining heat duty. Hot and cold streams exchange heat by adjacent streams in the heat exchanger, which results in the exchangers’ thermal integration, reducing the heat load and increasing the heat efficiency. The adjacency of hot and cold streams in the heat exchanger is selected through the heat capacity of the currents, which can be calculated by enthalpy. In order to obtain the optimal heat load, the process of hot and cold streams must be specified, and the hot and cold flow characteristics, including inlet and outlet temperature, inlet and outlet enthalpy, and molar flow, must be provided. Utility stream heat load is applied when the heat exchanger’s hot or cold process streams are not practical or economical. The minimum temperature difference (ΔTmin) is determined for the amount of heat recovery. The process pinch point is calculated by equating the net heat flow to zero in the cascade diagram. The heat added to the distance from the beginning to the pinch point is defined as the minimum hot utility (QHmin). Above the pinch point behaves like a heat well and takes the heat from the utility, and no heat is returned. The heat removed from the distance between the pinch point to the end is called the minimum cold utility (QCmin). At the bottom, the pinch behaves similarly to a heat source. It does not absorb heat but provides heat for cold utility. Because hydrogen is an energy-efficient and low-emission fuel, it is believed to be one of the most promising energy sources for the future. When hydrogen is burned in the air or used in a fuel cell to generate electricity, only its products become water and a small amount of NOX. In addition, it is renewable and many compounds, including water, fossil fuels, and biomass, contain hydrogen. The most straightforward approach to water separation is the thermal decomposition of water itself. To facilitate this process, the minimum temperature should be 2500K, but since the amount of Gibbs free energy for the decomposition of water to H2 and O2

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does not reach zero until 4310K, a higher temperature for the process is desirable. The unfortunate problem for direct solar hydrogen production is the low resistance of materials required at high temperatures. Even if the process is successful, H2 and O2 need to be separated. Hydrogen can be produced by several methods, such as water electrolysis, SMR, biochemical and electrochemical catalysts [38], photocatalytic systems [39], solar thermochemical cycles [40], and thermochemical water splitting. Hydrogen can be associated with various energy sources, including nuclear, fossil fuels, and renewable energy [41]. Currently, 96% of the world’s hydrogen is produced using fossil fuels. Natural gas is the raw material, among which SMR is the most widely used method. Although hydrogen is known as a clean source, because SMR is most widely used, it has adverse environmental effects during hydrogen production and emits large amounts of GHGs. Therefore new methods are tested.

9.1.1.5.1 Thermochemical cycle Because the operating temperature required to split water into hydrogen and oxygen is very high, one of the most promising routes for hydrogen production is the thermochemical splitting of water, which, using multiple reactions, can achieve water decomposition in temperatures less than what is required. Many inevitable losses in efficiency are associated with multiple reactions and separation. However, because thermochemical cycles produce H2 and O2 in separate phases, they prevent new compounds, which is an important advantage of the cycle [42]. Due to economic problems, very few thermochemical cycles went beyond theoretical calculations to prove experimental work. After examining several factors such as availability and abundance of materials, simplicity, chemical capability, thermodynamic feasibility, and safety issues, eight important cycles for commercials are: sulfur-iodine (S-I), copper-chlorine (Cu-Cl), cerium-chlorine (Ce-Cl), iron-chlorine (Fe-Cl), magnesium-iodine (Mg-I) [43]. Among them, the S-I, Cu-Cl, and Zn-SI cycles can be considered promising options. The comparison of SI and CuCl cycles from the perspectives of heat quantity, temperature, thermal efficiency, related engineering challenges, and hydrogen production methods. The heat required for the two cycles is similar, although the Cu-Cl cycle has a lower maximum temperature than the S-I cycle. Depending on the heat recovery rate of the systems, the overall energy efficiency is between 37% and 54%. The cost estimation showed that both cycles have the same cost for hydrogen production [44]. Among the various hydrogen production methods, the S-I sulfur iodide thermochemical water separation cycle is one of the most efficient, promising, and CO2free methods. Many institutions, including the Japan Atomic Energy Agency [45], the General Atomics [46], the Atomic Energy Commission (France) [47], and the Sandia National Laboratory [48], are actively researching the S-I cycle. This process has been studied in many other countries, including the United States, Japan, France, Italy, Korea, and China [33]. General Atomics first proposed the S-I process in the 1970s [49], which is capable of producing hydrogen at lower temperatures (less than 1000
C). Research and development on the water-splitting cycle have been done using large-scale hydrogen production methods in S-I thermochemical

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cycle. The thermal decomposition of H-I and H2SO4 water splitting is possible by coupling the Bunsen reaction. The production of hydrogen on a large scale [50] is possible by water-splitting. The solar thermochemical cycle consists of two steps to separate water and carbon dioxide from the metal oxide of the redox pair, which have been obtained as significant values for achieving the artificial carbon cycle. The chemical stages of hydrogen production, operating conditions, inlet, and outlet of the two-stage cerium oxide thermochemical cycle system are performed. This cycle produces hydrogen without fossil fuels, and the results show that the two-stage thermal cycle is a promising process for hydrogen production.

9.1.1.5.2 Thermal efficiency Thermochemical cycles have presented a theoretical thermal efficiency range of 50% [49]. One factor affecting the high efficiency of the S-I thermochemical process is the decomposition of sulfuric acid, which significantly affects the process’s thermal efficiency due to its high thermal requirement. The stability temperature considered for sulfuric acid decomposition is 650
C, which has the highest thermal efficiency. Then, with the change in thermodynamic conditions, other influential factors on thermal efficiency are investigated and calculated, and the concentration of HI in the system is known as the compelling factor. To support hydrogen production and improve the performance of the S-I thermochemical process, experiments were conducted, including constructing a laboratory to produce hydrogen and using helium gas as a process heat supply, and testing components. In order to increase the energy conversion efficiency, low reaction temperatures and a more complex system than the S-I cycle system and the two-phase thermochemical metal/metal oxide cycle are required. As a result, a six-step thermochemical cycle is proposed to separate CO2 and H2O. The thermal efficiency of the ZnSI cycle is lower than that of the S-I cycle. The product of the S-I cycle is hydrogen, while the products of the Zn-S-I cycle are hydrogen and CO2 simultaneously. The Zn-S-I thermochemical cycle has proposed alternative energies such as solar, geothermal, and other energy to supplement its energy sources. The separation of water and carbon dioxide through the Zn-S-I thermochemical cycle, coupled with a renewable energy source such as a high-temperature heat source, is a promising solution to produce hydrogen and carbon monoxide and reduce CO2 emissions from fossil fuel combustion. The upper limit of thermal efficiency is calculated according to the first law of thermodynamics, which according to the results, the maximum efficiency is 43.5% [25]. The Zn-S-I thermochemical cycle is based on the S-I cycle and the proposed two-phase thermochemical metal/metal oxide cycle and calculates the thermal efficiency by changing the operating conditions. This cycle consists of four subsystems. The results of these studies are as follows: the different compositions of Bunsen products have little effect on all units, and the factors affecting the thermal efficiency of the system, HI concentration in HIX phase, and ZnI2 decomposition ratio are true in all units [51]. In order to simplify and improve the Zn-S-I cycle, the design of three open-loop, five-phase, and electrochemical simulation designs

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and thermal efficiency concerning a constant amount of hydrogen production were studied. The open-loop thermal efficiency is improved, but the electrochemical cycle is almost the same, and the reason for the loss of efficiency is the lack of access to all the experimental data of the Bunsen system. The thermal efficiency of the five-stage cycle is slightly lower than the initial value due to a change in the Zn system and a reduction in heat dissipation. Therefore the Zn-S-I thermochemical cycle becomes more flexible for work by simplifying the chemical process and reducing the number of zinc system units [35].

9.1.1.5.3 Heat source for integration During research and development, the heat required for the process can be provided in various ways to improve thermal efficiency in thermochemical hydrogen production using water separation. The use of natural gas as a heat source for SMR integration and thermochemical hydrogen production technology has been investigated. Hydrogen generation methods include SMR, S-I thermochemical cycle, Cu-Cl thermochemical cycle, Zn-S-I thermochemical cycle, and SMR/Cu-Cl power plant integration. The results show that integrating the Cu-Cl cycle with SMR is promising because hydrogen production costs are reduced, and the process’s overall performance is improved [52]. Geothermal energy has also been investigated as a heat source in connection with the thermochemical cycle of hydrogen production. Six thermochemical potentials and hybrid cycles are compared, including the Cu-Cl cycle under operating conditions, temperature range, cycle phenomena, and performance aspects. The results show that the Cu-Cl cycle is the most promising lowtemperature for hydrogen production [53]. Nuclear reactors have been introduced as a suitable option for generating high-quality process heat for water separation [54]. Simultaneous production of thermochemical hydrogen using the heat of nuclearmolten salt reactors (MSRs) has been proposed, and various conceptual designs of heat exchangers for heat transfer from SMR to S-I and Cu-Cl have been proposed [55]. In the next decade, high-temperature gas-cooled reactors (HTGR) made significant progress [56]. HTGR provide the required heat for the process, and helium gas is used for cooling [56]. The system is coupled to a high-temperature reactor to estimate the upper limit and the best efficiency for the thermochemical separation cycle of sulfur iodide water. Separating water and sulfuric acid, and separating water from HI, seems to consume much energy. According to the considered heat efficiency, a pinch temperature is considered for the design of heat exchangers and is calculated after the actual evaluation [57]. Due to the hot and cold utility of the Zn-S-I system, it needs significant utilities; Thermal integration can be used to reduce the utility of this type of process. The Japan Atomic Energy Agency is exploring technology to produce hydrogen integrated with a high-temperature reactor with a cooling gas. The technology of this research maintains the performance of the reactor against thermal disturbances caused by the hydrogen production system, which prevents the penetration of tritium from the explosion of combustible gas [58]. The concentration of sunlight can also be used to produce the high temperature required to produce thermochemical hydrogen. This is achieved using a heliostat

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that directs sunlight into a receptor [59]. It is assumed that the solar complex is made with a linear parabolic composition, and the reactor capacity designed to produce hydrogen per day is 3.6 kg [60]. The solar power plant coupled to the Cu-Cl thermochemical plant for hydrogen production across Canada is being analyzed. The proposed linear parabolic solar technology provides the heat required in the oxygen generation phase. Molten salt is considered a medium of heat transfer. The dimensions of the storage unit are estimated based on the energy of the oxygen reactor and the energy collected by the solar power plant [61]. The operating temperature range of different solar thermal energy technology is analyzed by Wang according to their compatibility with solar hydrogen production through thermal cycles [62]. A new and promising way is solar thermochemical hydrogen, in which solar energy is integrated with steam methanol-reforming to produce hydrogen from solar energy. Experimental studies show that producing solar thermochemical hydrogen at medium temperatures has led to the development of hydrogen production technology and effectively combines the solar thermal energy of synthetic fuels [63]. During the analysis of the copper sulfate hydroxide hybrid cycle for separating thermochemical water, the thermal integration of hydrogen production using pinch technology was considered. Sensitivity analysis (SA) for low and high temperatures shows that a low driving force can further improve the process [64]. Energy evaluation for an initial model of a hydrogen power plant was designed by the S-I thermochemical cycle, assuming the process is powered by solar energy. Due to the minimum and maximum temperature range, two solar units are required. The efficiency of the thermochemical cycle without power generation is 34% [65]. The most critical disadvantage of solar energy compared to nuclear energy is the change of sunlight with the weather, time of day, and year. This can be overcome quickly by storing additional thermal energy [66]. The method of hydrogen production through the thermochemical cycle is a recent method that has been tested and has a lower greenhouse gas emission rate than other methods. Studies on thermochemical cycles have shown that the S-I cycle has lower thermal efficiencies than Zn-S-I but can produce higher efficiencies under suitable conditions, especially favorable Bunsen reaction conditions. The ZnS-I cycle requires a lower operating temperature than other thermochemical cycles, which is one of the essential advantages of this cycle, and in addition to producing hydrogen, it also produces CO as the final product. Due to the high temperature, the solar dish collector is considered the most suitable heat source for coupling with the hydrogen production cycle.

9.2

High-temperature hybrid electrolyzers

Given the world’s energy limitations, one of the favorite concepts of researchers worldwide is the design of high-potential renewable systems. Thermochemical water splitting solar reactors, with the generation of hydrogen and solar fuels, is one of the high-potential solar thermal systems.

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9.2.1 Thermochemical reactions Due to day and night changes, solar energy is not always available; therefore storing and using this renewable resource is considered an essential issue. Conventional energy storage methods do not have the possibility of long-term storage and the possibility of moving it to the required location, which requires many costs. As a result, thermochemical reactions are one of the leading methods for storing energy for a long time. Thermochemical reactions use solar energy to produce solar fuel, which can also be transported and stored. Hydrogen generation from water is one of the most important targets of solar fuel production. Hydrogen can be very cost-effective when combined with a fuel cell. Hydrogen is mainly produced using fossil fuels, especially natural gas. When hydrogen is generated from fossil fuels by reforming gas, it has no particular economic advantage, especially if CO2 is not separated. In addition, the generation of hydrogen from fossil fuels contains small amounts of carbon monoxide (CO), which contaminates the catalyst used in the fuel cell. Under these circumstances, using renewable energy sources such as solar thermal energy to generate hydrogen can be an alternative to fossil fuels. This is because the hydrogen generated in this way does not contain the toxic substances of the fuel cell catalyst. Information on the direct decomposition of water is one of the most important issues to justify the use of interfaces to perform solar thermochemical reactions. Fig. 9.12 shows the effect of temperature changes on ΔH
, TΔS
, and ΔG
for the

350 H 2O

300

p

H2 1/2O2 1 bar

250 200 150 100 50 4330K 0 -50 300

1000

2000

3000

4000

5000

Temperature / K

Figure 9.12 Temperature changes on ΔH
, TΔS
, and ΔG
for direct water splitting at 1 bar pressure [67].

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direct reaction of water splitting. Direct decomposition of water is performed according to the following equation: Δ

H2 O ! H2 1

1 O2 2

(9.34)

According to Fig. 9.12, Gibbs free energy changes for direct thermochemical decomposition of water occur at a pressure of 1 bar and zero point, at a temperature of about 4300K. Much research has investigated the possibility of producing hydrogen directly at high temperatures [67]. The possibility of direct thermochemical decomposition of water is by solar heat at high temperatures [68]. The report shows that it is thermodynamically challenging to establish the temperature required for direct thermal decomposition and that no model for the separation process is provided (the significant decomposition depends on the higher operating temperature of 2500K). In addition, the direct solar thermal decomposition of water requires extremely hard technology. A diffused gas separates the hydrogen generated by the decomposition process of water through a porous ceramic membrane. Fig. 9.13 shows the equilibrium of the hydrogen generation ratio as an equation of temperature and pressure [37]. At a pressure of 0.01 bar, water splitting is challenging to perceive at 2000K, and with increasing temperature up to 2500K, the hydrogen ratio reaches more than 15% at the same pressure. The 0.3 2500 K 0.25

H2 yield

0.2

0.15 2200 K 0.1

0.05

2000 K

0 0.001

0.01

0.1

1

10

Pressure / bar Figure 9.13 Equilibrium of H2 generation ratio for the direct splitting of water as an equation of temperature and pressure [37].

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problems encountered during the development of the solar thermal decomposition process of water are as follows: 1. A very high temperature of the chemical reactor can be achieved by secondary concentration. 2. The problem of materials in the construction of solar reactors is faced. 3. A special porous ceramic membrane should be developed which has high resistance to clotting at very high temperatures.

Given the above discussion, the problems of direct decomposition are probably complicated to solve, so other methods should be sought to use the decomposition of water into hydrogen. Therefore generating temperature and eliminating the need for high-temperature gas separation will be proposed for hydrogen generation by thermochemical decomposition of multi-stage water. Hydrogen can be generated by the thermochemical splitting of water, which is a multistep chemical reaction to change water composition. Water and heat are system inputs, and hydrogen and oxygen are the only outputs of the system. The rest of the chemicals and detectors are recycled in a closed cycle. For example, a thermochemical cycle with more than two stages, such as UT-3 and IS, can decompose water to H2 and O2 at temperatures below 1273K. It should be noted that some processes are very complex for solar applications, and some highly corrosive gaseous materials react or are generated at high temperatures in the current, which results in reactive fabrication problems. Regarding the history of the use of thermochemical reactions, it is noteworthy that the interest in the thermochemistry of water splitting developed in the late 1970s and early 1980s with the oil crisis [69,70]. Subsequently, global research declined (the last review was presented in 1989 [70]), and most works continued in Japan, where reliance on national energy resources was considered. Although several hundred cycles were proposed, primary research was conducted to prove the technique’s feasibility and the potential for high efficiency for only a few. In the following section, the current thermochemical cycles in process temperature, process efficiency, safety, and environmental compatibility, process complexity, and process economy are examined, and finally, a list of suitable processes for further research in this field is presented.

9.2.1.1 Cycle process temperature The survival of the cycles depends on the amount of thermal energy activated by the reactions, which depends on the temperature of the reactor supplied by the concentrators. Therefore mainly, two- and three-stage thermochemical cycles are performed at a suitable temperature level to concentrate solar energy. This temperature is limited by the heat efficiency of the receiver-concentrator system as well as the heat transfer properties of the system. If too high a temperature is required for the process, the issue of ingredients and significant re-irradiation becomes apparent. In

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Table 9.3 Deleted cycles due to high process temperature. Temperature (
C)

Chemical reaction

Cycle

3713 1543 2977 2656 3910 884

MoO2 ðsÞ ! Mo 1 O2 Mo 1 2H2 O ! MoO2 ðsÞ 1 2H2 SiO2 ! SiOðgÞ 1 12 O2 SiOðgÞ 1 H2 O ! SiO2 1 H2 WO3 ðsÞ ! W 1 32 O2 W 1 3H2 O ! WO3 ðsÞ 1 3H2

Mo/MoO2 SiO2/SiO W/WO3

Table 9.4 Deleted cycles due to low efficiency. Temperature (
C)

Chemical reaction

Cycle

700 200 Electrochemical, 300 430 Electrochemical, 2575 550 .

H2 O 1 Cl2 ! 2HCl 1 12 O2 2HCl 1 2CuClðsÞ ! 2CuCl2 ðsÞ 1 H2 2CuCl2 ðsÞ ! 2CuClðsÞ 1 Cl2

US-Chlorine [71]

2Cu 1 2HCl ! H2 ðgÞ 1 2CuCl 4CuCl ! 2Cu 1 2CuCl2

Argonne Cu, CI

2CuCl2 1 H2 O ! 12 O2 ðgÞ 1 2HCl 1 2CuCl

addition, direct decomposition of water occurs for temperatures above 2500K3000K. Therefore the three high-temperature cycles presented in Table 9.3 are omitted for these reasons.

9.2.1.2 Cycle process efficiency If the solar concentrating processes of heat generation are hybridized with cycles involving the electrochemical phase, the overall efficiency is eliminated due to the low electricity efficiency to thermal energy. These reactions are shown in Table 9.4.

9.2.1.3 Safety and environmental compatibility One of the reasons for the development of solar thermochemical cycles is environmental compatibility. Therefore due to the production of toxic or corrosive substances, environmental issues, and process safety, cycles containing Cd, Hg, or bromide compounds are eliminated. Also, the problem of chemical corrosion in the process, due to the presence of substances such as potassium hydroxide, is usually one factor that eliminates the cycle. Therefore cycles containing relatively corrosive compounds, such as sulfuric acid or hydrochloric acid, are not considered. The cycles that fall into this group are presented in Table 9.5.

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Table 9.5 Deleted cycles due to safety and environmental issues. Temperature (
C)

Chemical reaction

Cycle

360 600 Electrochemical, 25 1400 850 250 650 700 1500 150 400 1600 150 725 825 125

HgðgÞ 1 H2 O ! HgOðsÞ 1 H2 HgOðsÞ ! Hg 1 12 O2 CdðsÞ 1 H2 O ! CdOðsÞ 1 H2

HgO/Hg

CdOðsÞ ! CdðgÞ 1 12 O2 3FeBr2 ðlÞ 1 4H2 O ! Fe3 O4 ðsÞ 1 6HBrðgÞ 1 H2 Fe3 O4 1 8HBr ! 3FeBr2 1 4H2 O 1 Br2 H2 O 1 Br2 ! 2HBr 1 12 O2 2Al2 O3 1 6Br2 ðlÞ ! 4AlBr3 1 3O2 4AlBr3 1 6WO3 ðsÞ ! 2Al2 O3 1 6Br2 1 6WO2 ðsÞ 6WO2 ðsÞ 1 6H2 O ! 6WO3 1 6H2 Sc2 O3 1 3Br2 ðlÞ ! 2ScBr3 1 32 O2 2ScBr3 1 3WO3 ðsÞ ! Sc2 O3 1 3Br2 ðgÞ 1 3WO2 ðsÞ 3WO2 ðsÞ 1 3H2 O ! 3WO3 ðsÞ 1 3H2 2KOH 1 2K ! 2K2 O 1 H2 2K2 O ! K2 O 1 H2 K2 O2 1 H2 O ! 2KOH 1 12 O2

CdO/Cd

FeBr2 GIRIO Al2O3

ScBr3

KOH

9.2.1.4 Cycle process complexity First, the number of chemical reactions can help simplify the process as a factor. Multi-step processes are associated with heat loss, and it is not easy to obtain products and reactants during the reaction in each work cycle. Two- and three-stage cycles are easier to implement. Second, the complexity of the process usually depends on the number of steps of chemical separation of the cycle. Separation steps increase the cost of the process. Therefore cycles that require gas separation techniques using advanced technology, such as membranes, fall into this category. Carbon base cycles are also eliminated due to the low efficiency of the gas phase separator to generate pure hydrogen contaminated with carbon oxides, as the polymer fuel cell electrode becomes contaminated at low concentrations of carbon oxides. It should be noted that the increase in HI decomposition problems (HI/H2O separation) and total energy efficiency in the I-S cycle eliminate all cycles that have the same reaction. These omitted reactions are presented in (Table 9.69.8). The following are cycles that have a solid part separation and are eliminated; it has been shown:

9.2.1.5 Cycle economical evaluation The cycle of elements that are not commonly found in the earth’s crust, ocean, or atmosphere undergo cycles where they are extracted and utilized, eventually

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Table 9.6 Deleted cycles due to process complexity. Temperature (
C)

Chemical reaction

Cycle

700 1700 550 500 900 700 250 1400 500 25 700 500 800 50 500 850 200 8001000 120 450 200 900 450 20 550 1000 25 425 475 20 450 100

CO 1 H2 O ! CO2 1 H2 CO2 ! CO 1 12 O2 CO 1 H2 O ! CO2 1 H2 CO2 1 SO2 1 H2 O ! H2 SO4 1 CO H2 SO4 ðgÞ ! H2 OðgÞ 1 SO2 ðgÞ 1 12 O2 C 1 H2 O ! CO 1 H2 CO 1 2Fe3 O4 ! CðsÞ 1 3Fe2 O3 ðsÞ 3Fe2 O3 ! 2Fe3 O4 1 12 O2 2NH4 I ! 2NH3 1 I2 1 H2 2NaI 1 2NH3 1 CO2 1 H2 O ! Na2 CO3 ðsÞ 1 2NH4 I Na2 CO3 1 I2 ! 2NaI 1 CO2 1 12 O2 2NH4 I ! 2NH3 1 I2 1 H2 BaCO3 1 I2 ! BaI2 1 CO2 1 12 O2 BaI2 1 2NH3 1 CO2 1 H2 O ! BaCO3 1 2NH4 I 2NH4 I ! 2NH3 1 I2 1 H2 2CuO 1 I2 ! 2CuI 1 12 O2 2CuI 1 NH3 1 H2 O ! 2CuO 1 2NH4 I H2 SO4 ðgÞ ! H2 OðgÞ 1 SO2 ðgÞ 1 12 O2 ðgÞ I2 ðlÞ 1 SO2 ðaqÞ 1 2H2 OðlÞ ! H2 SO4 ðaqÞ 1 2HIðlÞ 2HIðlÞ ! I2 1 H2 ðgÞ SO2 1 H2 O 1 I2 ! SO3 1 2HI SO3 ! SO2 1 12 O2 2HI ! H2 1 I2 1 1 2 Sb2 O3 1 H2 O 1 I2 ! 2 Sb2 O5 1 2HI 2HI ! H2 1 I2 1 1 1 2 Sb2 O5 ! 2 Sb2 O3 1 2 O2 LiNO2 1 I2 1 H2 O ! LiNO3 1 2HI 2HI ! I2 1 H2 LiNO3 ! LiNO2 1 12 O2 2FeSO4 1 I2 1 2H2 O ! 2FeðOHÞSO4 1 2HI 2HI ! I2 1 H2 2FeðOHÞSO4 ! 2FeSO4 1 H2 O 1 12 O2

CO/CO2 Schulten C,S

Carbon-iron

Hitachi

Osaka75 GIRIO 4 Cu, I, N

IS Cycle (Ispra Mark 16) THEME S-3

Miura [72]

Argonne

Yokohama Mark 3

removed due to the high cost of access. These elements are usually heavy elements that block the solid flow, and the mass ratio required to generate H2 is not optimal. These cycles are as follows: Cycles involving the ferrite oxide compound are also not considered due to the very high mass ratio of solids required per mole of H2 generated (about 1020 moles of solid per 1 mol of H2 ). These cycles are presented in Table 9.9.

Table 9.7 Deleted cycles due to separation complexity. Temperature (
C)

Chemical reaction

Cycle

800 850 550 700 900 100 900 100 700 650 100 850

Cr2 O3 ðsÞ 1 4SrðOHÞ2 ! 2Sr2 CrO4 ðsÞ 1 3H2 O 1 H2 2SrCrO4 ðsÞ 1 43 SrðOHÞ2 ðlÞ ! 32 Sr5 CrO4 ðsÞ 1 3H2 OðgÞ 1 12 O2 2SrCrO4 ðsÞ 1 32 Sr5 ðCrO4 Þ3 OHðsÞ 1 5H2 O ! Cr2 O3 ðsÞ 1 2SrCrO4 ðsÞ 1 Cr2 O3 1 4BaðOHÞ2 ðlÞ ! 2Ba2 CrO4 ðsÞ 1 3H2 O 1 H2 2BaCrO4 ðsÞ 1 BaðOHÞ2 ðlÞ ! Ba3 ðCrO4 Þ2 1 H2 OðgÞ 1 12 O2 2Ba2 CrO4 ðsÞ 1 Ba3 ðCrO4 Þ2 ðsÞ ! Cr2 O3 ðsÞ 1 2BaCrO4 ðsÞ 1 5BaðOHÞ2 1 1 2 Cr2 O3 1 3KOH ! K3 CrO4 1 2 H2 O 1 H2 5 1 3K3 CrO4 1 2 H2 O ! 2 Cr2 O3 ðsÞ 1 2K2 CrO4 ðsÞ 1 5KOH 2K2 CrO4 1 2KOH ! 2K3 CrO4 1 H2 O 1 12 O2 3FeCl2 ðsÞ 1 4H2 OðgÞ ! Fe3 O4 ðsÞ 1 6HClðgÞ 1 H2 ðgÞ Fe3 O4 ðsÞ 1 Fe2 O3 ðsÞ 1 6HCl 1 2SO2 ! 3FeCl2 1 2FeSO4 1 3H2 O 2FeSO4 ! Fe2 O3 ðsÞ 1 2SO2 ðgÞ 1 12 O2

ORNL Cr, Sr 16 3 SrðOHÞ2

ORNL Cr, Ba [73]

ORNL Cr, K

S, Fe, Cl Tokyo

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Table 9.8 Deleted cycles for economic reasons. Temperature (
C)

Chemical reaction

Cycle

390 323 1000 20 227 700 600 90 600

2EuO 1 H2 O ! Eu2 O3 ðsÞ 1 H2 I2 ðgÞ 1 SrOðsÞ ! SrI2 ðsÞ 1 12 O2 Eu2 O3 ðsÞ 1 SrI2 ðlÞ ! 2EuO 1 I2 ðgÞ 1 SrOðsÞ I2 ðsÞ 1 MgðOHÞ2 ðsÞ 1 U3 O8 ðsÞ ! MgI2 1 3UO3 ðsÞ 1 H2 OðlÞ MgI2 1 H2 OðgÞ ! I2 ðgÞ 1 MGðOHÞ2 ðsÞ 1 H2 ðgÞ 3UO3 ðsÞ ! U3 O8 ðsÞ 1 12 O2 Sr3 U2 O8 1 3SrðOHÞ2 ! 2Sr3 UO6 1 2H2 O 1 H2 2SrUO6 1 3H2 O ! Sr3 UO6 1 2H2 O 1 H2 Sr3 U2 O9 ! Sr3 U2 O8 1 12 O2

Eu, Sr

Mg, I, U

Sr, U

Table 9.9 Deleted cycles involving the ferrite oxide compound. Temperature (
C)

Chemical reaction

Cycle

1100 600

Ni0:5 Mn0:5 Fe2 O4 ! Ni0:5 Mn0:5 Fe2 O42δ 1 2δ O2 Ni0:5 Mn0:5 Fe2 O42δ 1 δH2 O ! Ni0:5 Mn0:5 Fe2 O4 1 δH2 MnFe2 O4 1 3CaO 1 ð1 2 yÞH2 O ! Ca3 ðFe; MnÞ3 O82y 1 ð1 2 yÞH2 Ca3 ðFe; MnÞ3 O82y ! MnFe2 O4 1 3CaO 1 ð1 22 yÞ O2

Ni-Mn ferrite: Tamaura et al. [74] Ca/Mn ferrite: Tamaura et al. [75]

1000 600

9.2.1.6 Suitable cycles for research Of the cycles studied, about 30 cycles are presented in Table 9.10, which met the specified criteria and are suitable for further research. Among the proposed cycles, ZnO/Zn, Fe3O4/FeO cycles, and CeO2/Ce2O3 cycles are among the most important ones being studied today. The energy conversion efficiency of ZnO/Zn has been obtained by more than 50%, with the highest efficiency among other cycles. In addition, the hydrolysis reaction with zinc is fast and has higher repeatability than other cycles. Therefore according to the mentioned advantages, the zinc cycle will be more suitable for use on an industrial scale than other cycles offered.

9.2.2 Solar concentration As discussed in the previous chapter, the solar concentrator, as a heat exchanger in the system, generates the energy needed to carry out the thermochemical reaction. It should be noted that the modeling objectives in this section are to obtain the diameter of the cavity opening, the amount of heat received in the receiver, the distribution of radiation and temperature on the concentrating surface, and also the amount of radiation distribution in the cavity. In this regard, the model geometry,

Table 9.10 Appropriate cycles for further research [76]. Chemical reaction

Highest temperature (
C)

Chemical steps

Elements

Cycle

ZnO ! Zn 1 12 O2 Zn 1 H2 O ! ZnO 1 H2 Fe3 O4 ! 3FeO 1 12 O2 3FeO 1 H2 O ! Fe3 O4 1 H2 In2 O3 ! In2 O 1 O2 In2 O 1 2H2 O ! In2 O3 1 2H2 SnO2 ! Sn 1 O2 Sn 1 2H2 O ! SnO2 1 2H2 MnSO4 ! MnO 1 SO2 1 12 O2 MnO 1 H2 O 1 SO2 ! MnSO4 1 H2 FeSO4 ! FeO 1 SO2 1 12 O2 FeO 1 H2 O 1 SO2 ! FeSO4 1 H2 CoSO4 ! CoO 1 SO2 1 12 O2 CoO 1 H2 O 1 SO2 ! CoSO4 1 H2 Fe3 O4 1 6HCl ! 3FeCl2 1 3H2 O 1 12 O2 3FeCl2 1 4H2 O ! Fe3 O4 1 6HCl 1 H2 3FeOðsÞ 1 H2 O ! Fe3 O4 ðsÞ 1 H2 Fe3 O4 ðsÞ 1 FeSO4 ! 3Fe2 O3 ðsÞ 1 3SO2 ðgÞ 1 12 O2 3Fe2 O3 ðsÞ 1 3SO2 ! 3FeSO4 1 3FeOðsÞ 3FeOðsÞ 1 H2 O ! Fe3 O4 ðsÞ 1 H2 Fe3 O4 ðsÞ 1 3SO3 ðgÞ ! 3FeSO4 1 12 O2 FeSO4 ! FeO 1 SO3 Fe2 O3 ðsÞ 1 2SO2 ðgÞ 1 H2 O ! 2FeSO4 ðsÞ 1 H2 2FeSO4 ðsÞ ! Fe2 O3 ðsÞ 1 SO2 ðgÞ 1 SO3 ðgÞ SO3 ðgÞ ! SO2 ðgÞ 1 12 O2 6CuðsÞ 1 3H2 O ! 3Cu2 OðsÞ 1 3H2 Cu2 OðsÞ 1 2SO2 1 32 O2 ! 2CuSO4 2Cu2 OðsÞ 1 2CuSO4 ! 6Cu 1 2SO2 1 3O2

2000 1100 2200 400 2200 800 2650 600 1100 250 1100 250 1100 200 1500 700 200 800 1800 200 300 2300 125 700 1000 500 300 1750

2

Zn

ZnO/Zn [77]

2

Fe

Fe3O4/FeO [78]

2

In

In2O3/In2O

2

Sn

SnO2/Sn

2

Mn, S

MnO/MnSO4 [79]

2

Fe, S

FeO/FeSO4 [79]

2

Co, S

CoO/CoSO4 [79]

2

Fe, Cl

Fe3O4/FeCl2 [80]

3

Fe, S

FeSO4 Julich [70]

3

Fe, S

FeSO4_4 [79]

3

Fe, S

C7 IGT

3

Cu, S

Shell process [70]

Cu2 OðsÞ 1 H2 OðgÞ ! CuðsÞ 1 CuðOHÞ2 CuðOHÞ2 1 SO2 ðgÞ ! CuSO4 1 H2 CuSO4 1 CuðsÞ ! Cu2 OðsÞ 1 SO2 1 12 O2 SO2 1 H2 O 1 BaMoO4 ! BaSO3 1 MoO3 1 H2 O BaSO3 1 H2 O ! BaSO4 1 H2 BaSO4 ðsÞ 1 MoO3 ðsÞ ! BaMoO4 ðsÞ 1 SO2 ðgÞ 1 12 O2 3FeCl2 1 4H2 O ! Fe3 O4 1 6HCl 1 H2 Fe3 O4 1 32 Cl2 1 6HCl ! 3FeCl3 1 3H2 O 1 12 O2 3FeCl3 ! 3FeCl2 1 32 Cl2 H2 O 1 Cl2 ! 2HCl 1 12 O2 2HCl 1 2FeCl2 ! 2FeCl3 1 H2 2FeCl3 ! 2FeCl2 1 Cl2 2CrCl2 ðs; Tf 5 815
cÞ 1 2HCl ! 2CrCl3 ðsÞ 1 H2 2CrCl3 ðs; Tf 5 1150
cÞ ! 2CrCl2 ðsÞ 1 Cl2 H2 O 1 Cl2 ! 2HCl 1 12 O2 6MnCl2 ðlÞ 1 8H2 O ! 2Mn3 O4 1 12HCl 1 2H2 3Mn3 O4 ðsÞ 1 12HCl ! 6MnCl2 ðsÞ 1 3MnO2 ðsÞ 1 6H2 O 3MnO2 ðsÞ ! Mn3 O4 ðsÞ 1 O2 H2 O 1 Cl2 ! 2HCl 1 12 O2 2TaCl2 1 2HCl ! 2TaCl3 1 H2 2TaCl3 ! 2TaCl2 1 Cl2 Cl2 ðgÞ 1 H2 OðgÞ ! 2HClðgÞ 1 12 O2 ðgÞ

1500 100 1500 300

2VOCl2 ðsÞ 1 2HClðgÞ ! 2VOCl3 ðgÞ 1 H2 ðgÞ 2VOCl3 ðgÞ ! Cl2 ðgÞ 1 2VOCl2 ðsÞ H2 O 1 Cl2 ! 2HCl 1 12 O2 2BiCl2 1 2HCl ! 2BiCl3 1 H2
 2BiCl3 Tf 5 233O c; TEB 5 441
c ! 2BiCl2 1 Cl2 3FeðsÞ 1 4H2 O ! Fe3 O4 ðsÞ 1 4H2 Fe3 O4 1 6HCl ! 3FeCl2 ðgÞ 1 3H2 O 1 12 O2 3FeCl2 1 3H2 ! 3FeðsÞ 1 6HCl

170 200 1000 300 1700 700 1800 1300

1300 680 900 420 1000 600 350 200 1600 1000 700 100 1000 1000 100 2200 1000

3

Cu, S

CuSO4 [81]

3

Ba, Mo, S

LASL BaSO4

3

Fe, Cl

Mark 9

3

Fe, Cl

Euratom 1972 [70]

3

Cr, Cl

Cr, Cl Julich [70]

3

Mn, Cl

Mark 8

3

Ta, Cl

Ta Funk

3

V, Cl

Mark 3 Euratom JRC Ispra (Italy)

3

Bi, Cl

Bi, Cl

3

Fe, Cl

Fe, Cl Julich

(Continued)

Table 9.10 (Continued) Chemical reaction

Highest temperature (
C)

Chemical steps

Elements

Cycle

3 3 5 2 FeOðsÞ 1 2 FeðsÞ 1 2 H2 O

1000 1800 700 900 100 600 700 80 1000 800 100 1500 500 100 1300 400 1700 700 850 100 1000

3

Fe, Cl

Fe, Cl Cologne

3

Mn, Na

Mark 2 [41]

3

Mn, Li

Li, Mn LASL [41]

3

Mn, Na

Mn PSI

3

Fe, M ORNL

3

Fe, (M 5 Li, K, Na) Sn

Sn Souriau

3

Co, Ba

Co ORNL

3

Ce, Ti, Na

Ce, Ti ORNL [82]

3

Ce, Cl

Ce, Cl GA [83]

! Fe3 O4 ðsÞ 1 52 H2 Fe3 O4 1 6HCl ! 3FeCl2 ðgÞ 1 3H2 O 1 12 O2 3FeCl2 1 H2 O 1 32 H2 ! 32 FeOðsÞ 1 32 FeðsÞ 1 6HCl Mn2 O3 ðsÞ 1 4NaOH ! 2Na2 O:MnO2 1 H2 O 1 H2 2Na2 O:MnO2 1 2H2 O ! 4NaOH 1 2MnO2 ðsÞ 2MnO2 ðsÞ ! Mn2 O3 ðsÞ 1 12 O2 6LiOH 1 2Mn3 O4 ! 3Li2 O:Mn2 O3 1 H2 3Li2 O:Mn2 O3 1 3H2 O ! 6LiOH 1 3Mn2 O3 3Mn2 O3 ! 2Mn2 O4 1 12 O2 2MnO 1 2NaOH ! 2NaMnO2 1 H2 2NaMnO2 1 H2 O ! Mn2 O3 1 2NaOH Mn2 O3 ðlÞ ! 2MnOðsÞ 1 12 O2 2Fe3 O4 1 6MOH ! 3MFeO2 1 2H2 O 1 H2 3MFeO2 1 3H2 O ! 6MOH 1 3Fe2 O3 3Fe2 O3 ðsÞ ! 2Fe3 O4 ðsÞ 1 12 O2 SnðlÞ 1 2H2 O ! SnO2 1 2H2 2SnO2 ðsÞ ! 2SnO 1 O2 2SnOðsÞ ! SnO2 1 SnðlÞ CoOðsÞ 1 xBaðOHÞ2 ðsÞ ! Bax CoOy ðsÞ 1 ðy 2 x 2 1ÞH2 1 ð1 1 2x 2 yÞH2 O Bax CoOy ðsÞ 1 xH2 O ! xBaðOHÞ2 ðsÞ 1 CoOðy2xÞ ðsÞ CoOðy2xÞ ðsÞ ! CoOðsÞ 1 ðy 2 x2 2 1Þ O2 2CeO2 ðsÞ 1 3TiO2 ðsÞ ! Ce2 O3 :3TiO2 1 12 O2 Ce2 O3 :3TiO2 1 6NaOH ! 2CeO2 1 3Na2 TiO3 1 2H2 O 1 H2 CeO2 1 3NaTiO3 1 3H2 O ! CeO2 ðsÞ 1 3TiO2 ðsÞ 1 6NaOH H2 O 1 Cl2 ! 2HCl 1 12 O2 2CeO2 1 8HCl ! 2CeCl3 1 4H2 O 1 Cl2 2CeCl3 1 4H2 O ! 2CeO2 1 6HCl 1 H2

8001300 800 150 1000 250 800

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Sun

Receiver Parabolic Concentrator

(A)

Heliostats Field

(B)

Figure 9.14 Solar concentrator; (A) Schematic of the system; (B) Schematic of the threedimensional model of the parabolic solar concentrator [76].

the model hypotheses, governing equations, and finally, the boundary conditions and the initial value are presented. It should be noted that the purpose of modeling in this section is to investigate the heat generated by the solar concentrator and the concentrating power of the dish collector.

9.2.3 Model geometry The general schematic for a solar concentrator combines a flat collector and a parabolic dish collector (Fig. 9.14). According to the relationships that will be presented in the next section, only the parabolic dish-focusing model is needed to simulate the system; Therefore to simulate the solar concentrator, a three-dimensional model consisting of a parabolic solar concentrator and a receiver has been used, which is shown in Fig. 9.14. It should be noted that the size of the design variables is presented in Table 9.11.

9.2.3.1 Material properties As described in the previous section, the concentrator system consists of two parts: collectors and cavity-receiver. It should be noted that the collectors are made of silica glass and their specifications are presented in Table 9.12.

9.2.3.2 Model hypothesis The assumptions of the solar concentrator are as follows: The amount of solar radiation intensity is 21 kW/m; Collector surface slope error from the ideal mrad is 1.75; The maximum deviation of the concentration of solar radiation from the ideal state is 4.65 mrad; Wind speed is 16 m/s and the value of convection heat transfer coefficient to air is 20 W/m2.K. In addition, the air temperature in a day is considered according to Fig. 9.15.

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Table 9.11 Information on the structure of the centralization model [76]. Cavity-recipient

Parabolic dish concentrator

Value

Symbol

Variable

Value

Symbol

Variable

35 cm 43.276 cm

h D

Height Diameter

4 cm 45


t Rim

Collector thickness Edge angle

Rear cone 5 cm 5 cm 060 degrees

h2 r2 Semiangle

Front cone Height Lower radius Angle

7.5 cm 25 cm 60 degrees

h1 r1 Semiangel

Height Lower radius Angle

Table 9.12 Information on materials [76]. Value

Symbol

Properties

2203 1.38 703

ρsilica glass ksilica glass CPsilica glass

Density (kg/m3) Heat conductivity (W/mK) Heat capacity (J/kgK)

Figure 9.15 Ambient air temperature [76].

It should be noted that information about the geometry of the materials is provided in Table 9.13. It should be noted that the material of 14 absorbent tubes is alumina.

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Table 9.13 Specifications related to the geometry of the solar reactor [76]. Value (cm)

Symbol

Variable

5 43.276 30.48 5.08 0.3175 60.96 30.48 5.283 60 5.283 4.283 20.921 3.048 51.842 5.08 27.432 62.408 22.352

Daper Dcav Lcav Dtube ttube Ltube Lbed tA LA tB21 tB22 tC21 LC21 DC22 LC22 LD21 DD22 LD22

Aperture diameter Cavity inner diameter Cavity length Absorbent tube inner diameter Absorbent tube thickness Absorbent tube lenght Porous bed lenght Insulation layer thickness A Insulation layer length A Insulation layer thickness B-1 Insulation layer thickness B-2 Insulation layer thickness C-1 Insulation layer length C-1 Insulation layer diameter C-2 Insulation layer length C-2 Insulation layer length D-1 Insulation layer diameter D-2 Insulation layer length D-2

Table 9.14 The density of solar reactor components [76]. Value (kg/m3)

Symbol

Material

5000.4 3950.0 560.7 240.3 150.0 230.0

ρbed ρtube ρins;1 ρins;2 ρins;3 ρins;4

Filled substrate Zn Aluminum pipe [84] Insulation material 1 (buster M-35) [85] Insulation material 2 (buster M-15) [85] Insulation material 3 (cover buster) [85] Insulation material 4 (microporous insulation) [86]

9.2.3.3 Material properties As described in the geometry section, the solar thermochemical reactor comprises four types of insulation materials, micro process insulation layer, M-15 alumina buster insulation layer, M-35 alumina insulation buster, and coating alumina buster insulation layer. The substrate pipes are also made of alumina, and the substrate is filled with ZnO particles. It should also be noted that argon is an inert gas for transporting process products. According to the mentioned contents, the specifications related to each of the materials are presented in Tables 9.149.16.

Table 9.15 Heat conduction of solar reactor components [76]. Value (W/mK) kcond =kf 5 ðks =kf Þ0:28020:757logε20:057logðks =kf Þ 4:2 3 1023 1 5:6 3 1025 T 2 2:6 3 1028 T2 1 1:1 3 10211 T3 2 1:6 3 10215 T4 6 23 5:5 1 34:5e23:3 3 10 ðT2273:15Þ 22 2:2024 3 10 1 1:3924 3 1024 T 1 5:3512 3 1029 T2 1:1685 3 1021 2 1:7636 3 1024 T 1 1:5032 3 1027 T2  0:083; T # 800K 2 2:435 3 1021 1 3:607 3 1024 T 1 1:25 3 1028 T2 ; T . 800K 9:1640 3 1024 1 9:0320 3 1025 T 2 1:1810 3 1027 T2 1 6:1469 3 10211 T3

Symbol

Material

kcond kf kS ktube kins;1 kins;2 kins;3

Porous substrate Argon (reference gas during vacuum) Zn Aluminum pipes [84] Insulation material 1 [85] Insulation material 2 [85] Insulation material 3 [85]

kins;4

Insulated material 4 [86]

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Table 9.16 Heat capacity of solar reactor components [76]. Value (J/kgK)

Symbol

Material

299:86957 1 2:69766 3 10 T 2 1:271 3 10 T 1:04 3 103 1 1:74 3 1021 T 2 2:80 3 107 =T2

cp:bed cp:tube

447:6996 1 1:5987T 2 1:3797 3 1023 T2 1 4:0 3 1027 T3

cp:ins123

800.0

cp:ins4

Porous substrate Aluminum pipes [84] Insulation material 13 [85] Insulated material 4 [86]

21

24 3

9.2.3.4 Model hypothesis The solar thermochemical reactor has the following characteristics: it is a stable and three-dimensional mode model; the inlet heat to the system is assumed to be a constant flux from the inlet plate of the 10 kW cavity; volumetric reaction rate was considered; inert argon (Ar) gas was used as the carrier; the activation energy of the Ea reaction is equal to 361kJ/mol; the reaction factor, k0, is considered 1.35 3 108 kg/m3/s.

9.2.3.5 Governing equations In the solar reactor section, the proposed three-dimensional model includes three sections of heat transfer, mass transfer, and fluid flow, which will be discussed in each of the following sections. To investigate the heat transfer of the system, the energy conservation equation for the porous substrate can be written as a convection-diffusion equation considering the source [87]: h

 i @T
 1 ε ρCp f u:rT 5 r:ðkcond rT Þ 1 r:qrad 1 q00chem ε ρCp f 1 ð1 2 εÞ ρCp s @t (9.35) Here the subtitles f and s show the liquid and solid phases, respectively. In addition, ε is the porosity, ρ is the material density, Cp is the heat capacity, t is the time, T is the temperature, u is the fluid velocity, kcond is the effective thermal conductivity of the porous structure, qbed is the flux Radiant heat, and qvchem is the rate of heat during the reaction. It should be noted that the porosity in the pipes is assumed to be ε 5 0.776 [87]. Eq. (9.35) also assumes the energy equation in mean volume with local heat equilibrium. In addition, it is presented for the solar thermal reaction chemical reaction to generate Zn and oxygen [88]. q00chem 5 2 r 00 ΔHr ðT Þ

(9.36)

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Table 9.17 Penetration value for materials in the system [76]. Value (cm2/s)

Symbol

Variable

1.96 2.34 2.45

DZn2Ar DAr2Ar DO2 2Ar

Zn in Ar diffusion coefficient Ar in Ar diffusion coefficient Ar in O2 diffusion coefficient

In Eq. (9.36), the negative sign indicates that the reaction is superheated, rv is the reaction rate, and ΔHr(T) is the enthalpy of the reaction, which is a function of temperature. It is used to calculate the enthalpy of the reaction [88].   ΔHr ðT Þ 5 5:96 3 106 2 161:3T 2 2:66 3 1022 T2 J:kg21 (9.37) Regarding the mass transfer equation, the mass conservation equation and the concentration changes in the system are calculated using Fick’s law:   @
 @ @cj εcj 2 Deff ;j 2 v g c j 5 Rj @t @x @x

(9.38)

In Eq. (9.38), Cj represents the concentration, j represents the gases Zn and O2. Deff, j indicates the molecular diffusion coefficient of components j and ε indicates porosity, Rj also indicates the material transfer as well as the material production equation. To calculate the reaction rate, rv Eq. (9.39) is also presented [88].

2 Ea 00 r 5 k0 exp (9.39) RT In this equation, Ea is the reaction activation energy, k0 is the reaction factor, T is the temperature, and R is the constant of the gases. These calculations show the value of the diffusion coefficient of materials in the argon phase (neutral material to carry the product and create the appropriate pressure for the reaction) [25] (Table 9.17).

9.2.3.6 Boundary conditions and initial value Regarding the boundary conditions for the energy equation, it should be noted that due to the high temperature of the system, surface-to-surface radiant heat transfer is also considered. Regarding the boundary condition in the outer walls to realize the problem with the outside environment, it performs heat transfer, so for the outer walls, in reactor [Eq. (9.40)]: hðT 2 Twall Þ 5 k

 @T  @r r5rwall

(9.40)

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It should be noted that since the heat transfer from the surfaces is in the form of natural convection, Eq. (9.41) is used to calculate h. 8 k 1=4 > > 0:5RaL ; if T . Twall ; and 104 # RaL 107 > > L > > > >

> > > > k 1=4 > 5 10 > > : L 0:2RaL ; if T # Twall ; and 10 # RaL 10

(9.41)

In this equation, RaL is the Rayleigh number, L is the length of the medium for heat transfer, and k is the coefficient of thermal conductivity. It should be noted that the initial value of the heat transfer values of the walls is the ambient temperature. Also, the boundary condition for absorbent pipes is in the form of Eq. (9.41). It should also be noted that the initial value of temperature for the fluid inside the pipe and the substrate at the inlet is assumed to be the same. Regarding mass transfer, it should be noted that the input concentration for Zn and O2 is zero. Also, in the case of momentum transmission, the fluid is assumed to be slow due to the low velocity in the tubes, and the value of fluid input in each tube is assumed to be 100 nm/s.

9.2.4 Separation of zinc from oxygen with nanofluid coolant After the generation of Zn in the gas phase in the solar reactor, O2 and Zn must be separated immediately. Because with the decrease of temperature to ambient temperature and conversion of Zn from a gas phase to a solid phase, in the presence of O2, ZnO is generated. In this cycle phase, the quenching process is used to separate the products of the solar reactor stage. In this way, the output current of the solar reactor at high temperature is mixed with the cooling flow of argon and oxygen, and by changing the pressure and temperature, Zn is brought from the gas phase to the solid phase. It should be noted that this step is performed at a high distance after the fluid leaves the reactor. One of the innovations in this design is the use of nanosized cooling. Therefore for this modeling part, only heat transfer and momentum transfer modeling are required. The following topics are related to modeling.

9.2.4.1 Model geometry The process model in this section consists of a tube in which the quenching fluid containing cold argon enters the fluid, and also slightly cold argon is injected from the side of the wall to prevent cooling and deposition in the wall of the tubes; In addition to the cold inlet fluid, a nanosized coolant enters the tube adjacent to the quench chamber. It should be noted that the amount of nanocurrent should not be such that the walls of the quench tube cool down quickly and is intended only to aid the cold argon fluid that enters from the side of the tube. After the arrival of the

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Hybrid Poly-generation Energy Systems Quench Flow

Nano Fluid Flow Annual Flow Product of Solar Reactor Flow

Annual Flow Nano Fluid Flow

Figure 9.16 Schematic of separation phase by quenching process and nanofluid cooling [76]. Table 9.18 Specifications related to Quinch process geometry. Value (cm)

Symbol

Variable

1.3 0.5 1.5 1 40

DQF DAF DRF DNF LQ

Quinch flow inlet diameter AF current inlet diameter Reactor current inlet diameter Nano oil flow thickness Quenching process pipe length

quenching fluid, the volume of the pipe reaches a uniform temperature faster. The simulated geometry in this model is shown in Fig. 9.16. It should also be noted that the dimensions of the model geometry are according to Table 9.18 [89].

9.2.4.2 Material properties In this model, nanofluid flow is used for cooling. The nanofluid used in cooling combines aluminum oxide particles and VPI oil. All thermophysical properties of the base fluid (composite oil) and nanoparticles (aluminum oxide), such as density viscosity and thermal conductivity for different temperatures, have been obtained experimentally. The relations related to the density of the base fluid (b), nanofluid (n), and effective fluid (eff) can be obtained through the following equations [90]:

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ρb 5 2 0:90797 3 T 1 0:00078116 3 T 2 2 2:367 3 1026 3 T 3 1 1083:25 (9.42) ρn 5 3850

(9.43)

ρeff 5 ð1 2 φÞρb 1 φρn

(9.44)

It should be noted that the density unit in high relations kg/m3 and T indicates the fluid’s temperature. The relations related to the viscosity of the base fluid as well as the operating fluid are as follows [91]:
 ν b 5 exp 544:149=ðT 1 114:43Þ 2 2:59578

(9.45)

μb 5 ρb 3 ν b

(9.46)

μeff 5 ð1 1 2:5φÞμb

(9.47)

In the mentioned relations, φ is the concentration of nanoparticles inside the base fluid. Also, the unit of viscosity for the mentioned relations is μPa.s. The following equations are used to calculate thermal conductivity [84]: Kb 5 2 8:19477 3 1025 3 T 2 1:92257 3 1027 3 T 2 1 2:5034 3 10211 3 T 3 2 7:2974 3 10215 3 T 4 1 0:137743

(9.48)

Kn 5 5:5 1 34:5 3 e20:0033 3 ðT2273Þ

(9.49)

Keff

  Kn 1 2 3 Kb 2 2 3 ð11β Þ3 3 φ 3 ðKb 2 Kn Þ 5 Kb 3 Kn 1 2 3 Kb 1 ð11β Þ3 3 φ 3 ðKb 2 Kn Þ

(9.50)

Concerning thermal conductivity, β is the ratio of the thickness of the nanolayer to the diameter of the nanoparticles, which is considered 0.1 [85]. It should also be noted that the unit of thermal conductivity in the above equations is W/m.K. In addition, the heat capacity is calculated from the following equations [63]: CPb 5 0:002414 3 T 1 5:9591 3 1026 3 T 2 2 2:9879 3 1028 3 T 3 1 4:4172 3 10211 3 T 4 1 1:498

(9.51)

CPn 5 1:046 1 1:74 3 1024 3 T 1 2:79 3 1024 3 T 2

(9.52)

CPeff 5 ð1 2 φÞ 3 CPf 1 φ 3 CPn

(9.53)

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Considering that pressure changes in terms of temperature are important in this section, the equations of heat transfer and momentum transfer have been studied. Given this, the continuity equation is as follows. @ρ 1 @ @ 1 ðρrϑr Þ 1 ðρϑZ Þ @t r @r @z

(9.54)

The energy equation is also as follows:



@ @ @ 1@ @T @ @T @Sφ rk k ðcP ρT Þ 1 ðcP ρϑr T Þ ðcP ρϑz T Þ 5 1 1 @t @r @r r @r @r @z @z @t (9.55) It should be noted that the momentum equations are in the radial direction of Eq. (9.56) and the longitudinal direction of Eq. (9.57).

 

@ @ϑr @ϑr @p @ 1@ @ 2 ϑz 1μ 1 ϑz ðρϑr Þ 1 ρ ϑr ðrϑr Þ 1 2 52 @t @r @r r @r @r @z @z

(9.56)

 



@ @ϑz @ϑz @p 1@ @ϑz @2 ϑz 1μ r 1 ϑz ðρϑz Þ 1 ρ ϑr 52 1 2 @t @z r @r @r @z @r @z

(9.57)

9.2.4.3 Boundary conditions and initial value In this modeling, the initial values for the inlet temperature of the quench current, nano current, reactor current, and AF current are 298K, 298K, 1800K, and 773K, respectively; also, the tubular walls of the nano current according to Eq. (9.58) isolates are assumed.  @T  50 @x nano wall

(9.58)

9.2.5 Hydrogen generation reactor The last step in the desired thermochemical cycle is hydrogen generation in terms of use in the polymer fuel cell. Therefore the Zn separated from oxygen at this stage is converted to hydrogen in water vapor and neutral argon gas. At the end of the process, the Zn particles are converted to ZnO, making the cycle complete. It should be noted that the optimal temperature for the reaction at this stage is 603K633K [86]. The geometry of the model as well as the governing hypotheses and equations are presented as follows.

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Nano Fluid Zn(g) Bed

H2O (+Ar)

Nano Fluid

Figure 9.17 Schematic of a hydrogen generation reactor [76].

Table 9.19 Specifications related to the hydrogen process. Value (cm)

Symbol

Variable

3 0.5 35 24.5

DR tN LR Lbed

Reactor diameter Nano current thickness Reactor length Bed length

9.2.5.1 Model geometry At this stage, a substrate of Zn generated is considered to model the hydrogen generation reactor, which generates ZnO and H2 by passing water vapor through the substrate. Nanofluid flow has also been used to keep the bed temperature constant at the desired temperature. The geometry of the desired model is displayed in Fig. 9.17. It should be noted that the specifications of the displayed schematic size are available in Table 9.19.

9.2.5.2 Model hypothesis This model is also considered in three dimensions, and the three mass and heat transmissions and momentum are solved simultaneously. To keep the temperature constant in this system and increase the efficiency of hydrogen generation, the cooling nanofluid of the separation stage has been used. It should be noted that the specifications of nanofluids are described in the previous section. Three transfers of heat, mass, and momentum are not presented in this section as the transfer relations are already presented in the previous sections. Of course, it should be noted that the equation of thermochemical reaction to convert Zn to ZnO is essential in this section, which will be discussed in the following part. The reaction of Zn with water vapor is a surface reaction whose products are ZnO and H2. The specific surface area SSA, of Zn particles, is 5.11 6 0.35m2/g. It should be noted that the particle diameter is 164 6 10 nm. The reaction rate of Zn to ZnO conversion is also obtained [86]:

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rZn 5 k0 y0:5 exp

2 EZn RT

(9.59)

In this regard, 1. k0 is equal to 2 3 1025 mol/cm2.s, 2. EZn is equal to 42.8 6 7.4 kJ/mol, 3. R, T, and y are the universal constants of gases, fluid temperature, and the composition of the percentage of H2O in the fluid, respectively.

Regarding the initial value, it should be noted that the inlet temperature of the nanofluid is 623K, and the outer walls of the reactor are isolated.  @T  50 @x nanowall

(9.60)

All the equations governing the solar thermochemical cycle were explained in detail, and the method of solving the dynamics of computational fluids was introduced as the solution to the equations. The schematic of the ZnO/Zn cycle is shown in Fig. 9.18.

Figure 9.18 Schematic of the ZnO/Zn cycle [76].

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Figure 9.19 The average amount of heat produced by a solar concentrator in a year in terms of the focal length of the concentrator [76].

9.2.6 Solar concentrator As mentioned earlier, a solar concentrator acts as a heat exchanger to generate enough heat to perform a thermochemical reaction. For this purpose, a threedimensional dynamic model is investigated to examine the heat generated by the parabolic dish concentrator.

9.2.6.1 Collector-generated heat evaluation The thermal energy production factor in the solar concentrator is the main factor in the solar thermochemical cycle. Because the solar concentrator is considered a heat generation system in the cycle process. Therefore according to the dimensions of the solar reactor and thermal calculations, the minimum value for creating a solar thermochemical reaction in the dimensions of the reactor is 10 kW. Therefore initially, according to the amount of heat required, an analysis was performed on the dimensions of the solar concentrator. Fig. 9.19 shows the average amount of heat generated by the concentrator in one year in terms of the focal length of the concentrator at the edge angle of 45 degrees. According to the calculations, to achieve a temperature of 10 kW, the focal length should be 5.168 m. The importance of heat distribution shows the average heat generated in different months. It should be noted that temperatures below 10 kW cannot produce solar fuel in the solar reactor and with increasing heat, the reaction speed increases, which will be discussed in the next section (Fig. 9.20).

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Figure 9.20 The average amount of production temperature in different months of the year by the solar concentrator [76].

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Figure 9.20 Continued.

445

446

Figure 9.20 Continued.

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447

Figure 9.21 Distribution of solar radiation on the solar concentrator in the times of (A) 7.5 in the morning, (B) 9.5 in the morning, (C) 11.5 in the afternoon, (D) 1.5 in the afternoon, (E) 5/3 pm, (F) 4 pm [76].

9.2.6.2 Temperature distribution at the specified ratio Solar radiation is actually a variable in calculations to obtain the amount of heat reflected by the sun. First, according to the sun’s position and the trend of changing

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Figure 9.22 Distribution of solar temperature on the solar concentrator in the seasons (A) 7.5 in the morning, (B) 9.5 in the morning, (C) 11.5 in the afternoon, (D) 1.5 in the afternoon, (E) 5/3 pm, (F) 4 pm [76].

solar radiation during the day, the amount of solar radiation and the temperature distribution on the plate concentrator plate in winter is displayed in order, as shown in Figs. 9.21 and 9.22.

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Figure 9.23 Solar radiation path (A) ideal state, (B) true [76].

As per the results from Figs. 9.21 and 9.22, most of the solar radiation is related to the time of 10a.m. to 2p.m., which is comparable to the diagrams obtained in Fig. 9.20. It should also be noted that, as mentioned in the previous chapter, the MonteCarlo model was used to calculate the reflectance of solar radiation from the concentrator to the receiver. In the calculations performed in the current model, the reflected radiation path is considered in both ideal and real states. Solar reflection radiation is the basis for calculating the flux distribution at the focal point and the reactor walls. It can also calculate the optimal amount of aperture in a solar reactor. Fig. 9.23A is the solar radiation in the ideal state and Fig. 9.23B is the solar radiation in the real state. As shown in Fig. 9.23A and B, most of the reflected rays hit the receiving walls, increasing the temperature of the receiving walls in the system in question, and increasing the heat in the bed tubes.

9.2.6.3 Distribution of reflected radiation flux at the focal point One of the important variables in the subject of concentrator design is the distribution of the reflected flux from the concentrator. In fact, by calculating the reflected flux distribution, the amount of heat distribution generated by the solar concentrator will be calculated. Therefore the distribution of solar flux reflected at the focal point in both ideal and accurate states is shown in Fig. 9.24. It should be noted that the beam’s arrival time from the dish concentrator to the focal point plate is 6.0042e28s. As shown in Fig. 9.24, the reflection flux distribution is in the ideal state and according to the reflection from the ideal surface, all the reflections are displayed at one point. Also, in the proper mode, the reflection is distributed at the receiver level according to the Monte-Carlo model. One of the results’ applications is obtaining the input of the cavity and the power of concentration. According to the results, the hole for this research is 5 cm.

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Figure 9.24 Distribution of reflected radiation flux at the focal point (A) ideal and (B) real [76].

Figure 9.25 Distribution of reflected radiation flux at a focal point, (A) ideal and (B) real [76].

9.2.6.4 Distribution of reflected radiation flux in the reactor walls Since the porous substrate is used in the design, the amount of radiation flux reflected in the concentrating wall is very important. Therefore this section shows the distribution of radiation flux in both ideal and real states. It should be noted that, as mentioned in the previous chapter, the amount of radiation in this section is considered to be 1 kW/m2. It should be noted that the time of collision of the sun’s rays from the concentrator to the walls of the cavity is 6.3377e28s. The results are shown in Fig. 9.25. As can be seen in the results, most of the reflective flux reaches the cavity walls, indicating this point’s high potential.

9.2.7 Solar reactor The solar reactor is modeled to produce a solar thermochemical thermoset reaction to generate Zn and O2 from the ZnO substrate. It should be noted that in this model,

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451

Figure 9.26 Heat distribution in the reactor, (A) YZ 5 0, (B) YZ 5 9, (C) YZ 5 13, (D) YZ 5 16 [76].

three heat transfers, mass, and momentum, have been observed. In addition to presenting the reactor’s mass, heat, and fluid distribution, the validation is also examined.

9.2.7.1 Heat distribution Heat distribution is the main factor in performing a thermochemical reaction. Because the thermochemical reaction requires heat due to the reaction’s heat reaction, the reaction interface with temperature was presented in the previous chapter. In this section, due to the high temperature of the reactor, heat transfer is investigated by considering all three types of heat transfer methods, including conductive, conductive, and radiant heat transfer, as well as heat caused by the reaction. Fig. 9.26 shows the heat distribution in the solar thermochemical reactor at different locations on the YZ plane. As can be seen in the results, the maximum amount of heat is observed in the tube wall, which was not unexpected according to the results obtained in the concentrator section. For further investigation into the importance of heat distribution in pipes, Fig. 9.27 is presented, which focuses on the temperature distribution in pipes. As shown in Fig. 9.27, the highest temperature is created in the center of the porous substrate, which is expected to advance the thermochemical reaction further.

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Figure 9.27 Heat distribution in absorber tubes [76].

Figure 9.28 Zn distribution in the bed of absorbent tubes [76].

9.2.7.2 Mass distribution in pipe profile One of the most critical parts of the present modeling is the presentation of mass distribution in the bed of flux-absorbing pipes. This is because a thermochemical reaction takes place in these tubes. As mentioned in the previous chapter, the solar thermochemical reaction depends on the bed temperature, and, as shown in Fig. 9.27, the amount of mass distribution on the surface facing the cavity axis that receives the most radiation will be more. Keeping this in mind, Figs. 9.28 and 9.29 show the amount of Zn and O2 generated in the bed tubes, respectively. It should be noted that to show the effect of heat on the progress of the reaction, Fig. 9.30 shows the mass distribution at different distances from the center of the tube. As expected, the reaction takes place faster on the surface of the pipe.

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Figure 9.29 O2 distribution in the bed of absorbent tubes [76].

Figure 9.30 Mass distribution at different distances from the center of the pipe, (A) distance from the center 0, (B) distance from the center 10.9 mm, (C) distance from the center 22.2 mm [76].

9.2.8 Separation of zinc from oxygen with nanofluid coolant First of all, it should be noted that separation using nanofluid coolant and hydrogen generation is essential. The separation process considered in this research is

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Figure 9.31 Zn fuzzy diagram in terms of temperature and pressure [76].

quenching process. By cooling the outlet stream from the cavity, the mixture Zn in O2 is converted from a gaseous state to a solid state and separated from the flow by a filter. Fig. 9.31 shows the Zn fuzzy diagram regarding temperature and pressure. The main point to be considered in this process is the rapid and volumetric cooling of the flow in order to prevent Zn particles from changing too much, in addition to preventing Zn deposition in the walls. Therefore according to this issue, the cooling flow is considered in the opposite direction to the flow of the reactor products, because the rapid cooling of the walls causes sediment in the walls. However, it should be noted that to prevent sediment in the walls of the separator chamber, a cold Ar current enters the separator next to the wall, as discussed in the previous chapter. According to the description of the process, the validation and presentation of the results are discussed in the next section.

9.2.8.1 Temperature profile comparison in different cooling flow ratios As mentioned, the temperature profile is one of the essential factors in performing the quenching process. Because in this process, by reducing the volume of the quench tube temperature, Zn particles are converted from the gas phase to solid. Therefore steady-state modeling was first proposed to calculate the amount of nanofluid cooling flow. It should be noted that the annual fluid flow entering the pipe from the sides must be in the same order as the cooling flow; Because nucleation occurs with an excessive increase of the cooling current in the walls. According to

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Figure 9.32 Temperature profile in different ratios of nanofluid cooling flow [76].

the given explanations, it shows the temperature profile in different ratios of nanofluid cooling flow in Fig. 9.32. According to the calculations performed, the best ratio, according to the test points of Gstoehl and his colleague [89], is presented in the test points as a ratio of AF to nanofluid is equal to 1. Other data have also been calculated accordingly.

9.2.8.2 Temperature distribution at the specified ratio The temperature distribution is the main parameter; this is because by changing the temperature of Zn particles from reactor temperature to ambient temperature, they become solid. The temperature profile changes along the tube as shown in Fig. 9.33. As shown in the figure, most of the temperature stresses occur at the beginning of the pipe and there is no specific stress afterward, and during the temperature pipe, the temperature reaches the inlet temperature of the cooling fluid. In addition, Fig. 9.34 shows the temperature changes over time.

9.2.8.3 Pressure distribution at a specified ratio In addition, the representation of the streamline in the tube is important, as it represents the path of the particle; because in this part, Zn particles should not reach the pipes. Fig. 9.35 shows this value in the quenching process.

9.2.9 Hydrogen generation reactor The hydrogen generation reactor is the last part of the thermochemical cycle in which solid Zn is converted to ZnO, and also, at this stage, H2O is converted to H2. At this stage of the model, the goal is to determine the amount of H2

Figure 9.33 Temperature change profile in quinch tube, (A) external temperature of the tube. (B) temperature of the tube center, (C) isotherm profile of the tube [76].

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457

Figure 9.34 Temperature profile in the pipe in terms of time [76].

Figure 9.35 Flow line in quenching process [76].

generated in the system. Due to this, the present model has been investigated in both steady state and time functions.

9.2.9.1 Mass distribution in the reactor As mentioned in the previous sections, the amount of hydrogen generation depends on the square of the molar percentage of H2O. According to this issue, for squares of different percentages from 0.1 to 0.9, the steady state is shown in Fig. 9.36. It should be noted that the reason for the non-zero concentration of H2 at the beginning of the process due to the lack of substrate is due to the low velocity of the fluid and the diffusion coefficient. According to this information, the composition of 0.5% (the square of this value is 0.7), considering that a large amount of water vapor interferes with the reaction, was investigated for the time-dependent mode. Fig. 9.37 also shows the distribution of H2 over time. Fig. 9.38 also shows the amount of hydrogen generation at different times in three dimensions

Figure 9.36 Concentration distribution in steady state [76].

Figure 9.37 Distribution of concentration at different times [76].

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Figure 9.38 Profile of concentration change at different times, (A) t 5 25, (B) t 5 50, (C) t 5 75, (D) t 5 100 [76].

Due to the limited nature of renewable energy and the need to achieve sustainable development, research on renewable energy is fundamental. Among renewable energies, research on solar energy is critical due to its high potential. Therefore development in the solar energy field is one of the researchers’ essential goals worldwide. In this research, a solar reactor has been designed and modeled to generate hydrogen using metal interfaces and nanofluid coolers. To receive the heat of about 10 kW, the appropriate collector size was reported to be around a focal length of 5.168 m. At this size, the average solar power per year is about 10 kW. In addition, to investigate the heat received by the receiver from optical analysis based on the Monte-Carlo model, the distribution of reflected beam power on the collector plate was performed to calculate the receiver cavity opening. Based on the calculations, the best diameter was 5 cm. With the modeling of the best fluid flow mode for this part, the ratio of nanofluid flow to AF fluid is equal to 1, and finally, by examining the hydrogen generation reactor, the amount of hydrogen generation in the system was reported to be about 34 mol/m. The distribution of mass at different times was also shown.

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Hybrid Poly-generation Energy Systems

Biomass and photobiological processes to produce hydrogen

Simulation software is widely used in process design today. The applications of these applications in the field of work range from the simple calculation of thermophysical properties of flows or even pure materials to the design of complete plants with auxiliary facilities, feed pipelines, or product transfer and control system review. Because this method is simpler, faster, and more accurate than manual computation, it can be easily replicated under different conditions and with much less time to predict a complete set of process functions in different modes and in this way. At the same time, reducing fixed investment costs (additional devices) and operating costs (water, energy, etc.) provides more flexibility in process design and offers optimization in terms of cost, psychological operation, safety, environment, and so on. In addition, since the process design is not separated from the design of mechanical devices, equipment, piping, instrumentation, electrical systems, structures, and buildings, the information obtained from the simulation in different modes can be used.

9.3.1 Multiple production systems 9.3.1.1 Plasma gas A plasma gasification unit with a gasification capacity of 7.2 tonnes of waste per day. This device was built by the Environmental Resources Agency in 2007. The plasma reactor core is a conventional fixed-bed, upstream-flow gasification reactor. The design of the reactor is shown in Fig. 9.39. Generally, a reactor is a fixed bed gas cylinder with a plasma melting chamber beneath it. Plasma burners are embedded in the melting chamber ceiling. Initial air flows through the burners into the melting chamber; it is ionized and causes the plasma jets to exit the tip of the burners. The plasma jet temperature may reach 6000 K [92]. Plasma jets provide the heat needed to melt the mineral components of the raw material (which has reached the bottom of the reactor). Secondary air nozzles are placed around the plasma nozzle. Secondary air (at room temperature) is injected through the secondary air nozzles. The secondary air flow rate is adjustable so the air feeding rate can be controlled in total. The steam enters the reactor at a temperature of 1000
C from the steam nozzles located in the sidewall of the melting chamber. The feed tube is located above the reactor. The municipal solid waste (MSW) is pumped into the reactor every half hour from the top of the cylinder. The overall height of the reaction cylinder is 7.02 m, and the fixed bed height is 6.11 m [92]. Table 9.20 shows the characteristics of municipal solid waste [92].

9.3.1.1.1 Drying The first process in a gas reactor is drying the inlet feed. In this process, the residual moisture entering the reactor, in contact with the hot exhaust gas at the top of the reactor, loses its moisture and dries. Waste drying consists of three stages: (1) evaporation of free moisture; (2) evaporation of moisture absorbed in the waste; (3)

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461

Syngas outlet

Reaction shaft

Secondary air Plasma torches

10° Concretionary slag Melting chamber

Steam Inlet

Figure 9.39 Schematic of a plasma reactor [93].

Table 9.20 Biomass characteristics [93]. Proximate analysis, wt.% Moisture Fixed carbon Volatile matter Ash

20 7/10 6/77 7/11 Ultimate analysis, wt.%, dry basis

Carbon Hydrogen Nitrogen Chlorine Sulfur Oxygen Ash

9/47 0/6 2/1 1/0 . 3/0 9/32 7/11 Heating value (kJ/kg)

LHV

12,890

Slag outlets

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chemical separation and evaporation of water. According to the temperature gradient within the incoming residual particles, it is assumed to evaporate at the pressure of an atmospheric residual moisture temperature at 120
C. The balance of waste drying energy is defined as follows: P i

m_ i

Ð TsyngasðinÞ TsyngasðoutÞ

1 m_ steam

Ð

Cp;i dT 5 m_ MSWðdryÞ

TMSW ðoutÞ TMSW ðinÞ

Ð TMSWðoutÞ TMSWðinÞ

Cp;steam dT 1 Lsteam

Cp;MSWðdryÞ dT (9.61)

In this equation, m_ steam is the evaporated water mass, m_ MSWðdryÞ is the dry mass of waste material, m_ i is the mass of gaseous elements in the synthesis gas, Lsteam is the latent heat of evaporation, Cp,steam is the water specific heat, Cp,i is the specific heat capacity of element i, Tsyngas(out) is the gas temperature of the reactor outlet, and Tsyngas (in) is the temperature of the synthesized gas before contact with the wet residue and leaving the reactor. In the assumed relation, the input residue to the gasifier is isothermal and is dried at 120
C. A heater, cooler, and an RStoic reactor were used to simulate the drying process shown in Fig. 9.40.

SYNGAS

WET-MSW

Q-SYN

HT-FEED

DRYER

HOT-MSW

MSW-MOIS

Figure 9.40 Drying section of incoming wet feed [94].

COOL-SYN

SYNGAS-3

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463

MSW-MOIS

HOT-MSW

SEPARAT

Q-DEC

DRY-MSW

COMB-OUT

DEC-OUT

DECOMP

COMBUST

Figure 9.41 MSW analysis and pyrolysis section of dried MSW [94].

At this stage, by defining a chemical reaction in the DRYER block and a relationship in the calculator, the output residual moisture content is determined to be 10%.

9.3.1.1.2 Pyrolysis The process is then carried out using an SSplit block to remove the moisture in the feed from the dried MSW. Before starting the pyrolysis process, it should be noted that since MSW is a non-conventional Bohemian solid, its Gibbs-free energy cannot be calculated. For this reason, the DECOMP block, which is an RYield reactor, was used before the RGibbs reactor (Fig. 9.41). In this reactor, the reaction products are constituents of MSW, which are C, N2, O2, H2, S, H2, and ASH. The yields of the products of this reaction are calculated from approximate analysis and final analysis. Product returns are calculated as follows:
wmoisture  3 mMSW 1 H2 O yH2 O 5 100 (9.62) mMSW 1 H2 O

wN  2 1 2 wmoisture 3 mMSW 3 100 100 (9.63) yN 2 5 mMSW 1 H2 O

w O  2 1 2 wmoisture 3 mMSW 3 100 100 (9.64) yO 2 5 mMSW 1 H2 O

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wS  3 mMSW 3 100 yS 5 mMSW 1 H2 O

wH  wmoisture 2 1 2 100 3 mMSW 3 100 yH 2 5 mMSW 1 H2 O

wC  wmoisture 1 2 100 3 mMSW 3 100 yC 5 mMSW 1 H2 O

ASH  1 2 wmoisture 3 mMSW 3 w100 100 yASH 5 mMSW 1 H2 O 12

wmoisture 100



(9.65) (9.66) (9.67) (9.68)

The biomass pyrolysis process begins at 200
C and is completed at 500
C. Increasing the temperature to over 500
C, the primary tar changes formation and becomes gas and secondary tar. The gas produced in pyrolysis contains some tar. In gasification, the process must be at a higher temperature to convert the primary tar to a smaller amount of secondary tar and produce more gas. The gas usually contains 15% of the secondary tar. The overall MSW pyrolysis reaction can be illustrated as follows: MSW ! αGas 1 βTar 1 γChar

(9.69)

The weight distribution of the pyrolysis reaction products is simulated using Gibbs free energy minimization.

9.3.1.1.3 Gasification Coal-fired by thermal decomposition will react with gaseous agents (H2O and O2). Many homogeneous and heterogeneous reactions are involved in this process. Given the high temperature in the unburned coal gasification and combustion sector, chemical equilibrium is assumed, and Gibb’s free energy theory applies. In this way, the equilibrium composition of the products is obtained by directly minimizing Gibbs free energy without entering any specification of the reactions that may occur within the system. This model can also be used to calculate the phase equilibrium of solid solutions and liquidvaporsolid systems. One must enter the temperature, pressure or enthalpy, and output pressure to use this model. The model calculates the minimum amount of Gibbs free energy given the atomic equilibrium (equilibrium constraints) and the positives of the output materials model (inequality constraints). Therefore there is no need to define the chemical reaction in this model. Depending on the temperature profile inside the gaseous plasma reactor, the reactor is divided into two regions: high temperature (HTZ) and low temperature (LTR) for modeling. As a result, the organic gasification process of the MSW section is performed in two RGibbs reactors. Using the HTZ reactor operating at an average temperature of about 250
C, the central area of the plasma reactor is simulated, in which the plasma airflow directly affects the waste. In the LTR region, operating at about 8001200
C, the gasification process is complete, and the organic portion is converted to the synthesis gas.

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Fig. 9.42 shows that the ash is first separated from the gas stream by the ASHSEP block. Subsequently, the unburned coal is separated from the organic portion of the gas stream by the CHAR-SEP block. Unburned coal enters the HTZ reactor to perform the gasification reaction. The gaseous reaction products and the organic fraction of the gas stream are mixed in the MIXER block and sent to the LTR reactor for synthesis gas. The output from the LTR reactor is the synthesis gas which is then discharged from the top of the reactor as a final product after mixing with the moisture contained in the MSW in the first section. The temperature of the synthesized gas produced before contact with the wet feed is about 990
C. Plasma airflow and steam flow are injected directly into the HTZ reactor as a gaseous agent. The flow chart for this section is shown in Fig. 9.42.

9.3.1.1.4 Plasma melting The inorganic residue enters the plasma melting section after pyrolysis and gasification reactions and exits the reactor after melting. In this section, no chemical reaction occurs. The melting process is highly endothermic and high solids temperatures should be avoided. Experimental results show that the ash melts at 1650
C and is carried out of the reactor as slag. Four blocks of HEATER were used to simulate this section. The PLAS-GEN block is used to simulate the plasma burner, the HT-LOSS block is used to simulate the process loss, and the PLS-EX and ASH-MELT blocks are used to

LTZ-GASF

SYNGAS-3

MIX SYNGAS-2

SYNGAS-1

MOISTURE

MIXER

PYRO-GAS ASH-SEP COMB-OUT

CHAR-SEP

GAS

PRODUCT CHAR

ASH

HTZ-GASF STEAM1

PLASMA

Figure 9.42 High-temperature and low-temperature gasification section [94].

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Hybrid Poly-generation Energy Systems

ASH

ASH-MELT SLAG

Q-MELT

PLAS-AIR

PLS-AIR2

QPLASMA Q

MIXER-1

HT-LOSS

PLAS-GEN

PLS-AIR3

PLS-AIR4

PLASMA

PLS-EX Q-LOSS AIR-INJ Q

Figure 9.43 Ash melting section [94].

simulate the ash melting process (Fig. 9.43). The process energy efficiency of the plasma burner is assumed to be 10%. Plasma burner power, in this case, is 260 kW and the ash melting temperature is 1650
C. The heat capacity (CP, Ash) and latent heat of melting ash (LAsh) can be calculated in the following equations [(70) and (71)]. At low equations (ωi), the mass fraction of each inorganic element is in the ash: CP;Ash 5

X

ωi CP;i

(9.70)

i

LAsh 5

X

ωi Li

(9.71)

i

According to the results of the empirical analyzes of other papers, it is assumed that the inlet to the plasma smelter has a weight distribution of 50% silicon oxide, 30% aluminum oxide, and 20% calcium oxide [95]. Finally, the plasma gasification reactor is shown in Fig. 9.44.

9.3.1.2 Synthesis of refrigerant gas The heater and cooler are used to transfer heat between currents to avoid the problem of temperature crossing in the heat exchangers. Outputs from the plasma gas reactor go to the synthesized gas cooling section. The cooling process of the synthesized gas is divided into two parts: hightemperature and low-temperature cooling. In the high-temperature cooling section, the complete cooling method is used. In this section, we use a mixer (shown in

A framework for sustainable hydrogen production by polygeneration systems

SYNGAS

WET-MSW

Q-SYN

HT-FEED

DRYER

467

COOL-SYN

SYNGAS-3

HOT-MSW

LTZ-GASF MIX SYNGAS-2

SYNGAS-1

MSW-MOIS MIXER

HOT-MSW

SEPARAT

Q-DEC

PYRO-GAS ASH-SEP

DRY-MSW

COMB-OUT

DEC-OUT

PRODUCT

COMBUST

DECOMP

CHAR-SEP

GAS

CHAR

ASH ASH-MELT

HTZ-GASF

SLAG

STEAM1

Q-MELT

MIXER-1

HT-LOSS

PLAS-GEN PLS-AIR2

PLAS-AIR

PLS-AIR3

PLS-AIR4

PLASMA

PLS-EX Q-PLASMA

Q-LOSS AIR-INJ

Q

Q

Figure 9.44 Plasma gas reactor [94].

Fig. 9.45) to simulate hot gas passage from the water spray housing. Hot gases leave the gas reactor at 900
C and enter the heat exchanger directly. The hot TOQENCH stream and the ALL-COND stream mix in the QENCH block. In this section, using a Design-Spec, the mass flow rate of ALL-COND is determined to reach the outlet flow temperature of Quench to 220
C. The flow of NH3FREE then enters the low-temperature cooling section. After this step, the gas flow enters the low-temperature cooling section (Fig. 9.46). The gas flow is initially cooled in the COOL1 heat exchanger to a temperature of 211
C. COLL2, COOL3, and COOL4 blocks are exchangers that cool the gas stream to a temperature of 166
C. The gas flow is cooled by passing COOL5 and COOL6 to 54
C and 38
C, respectively. All components of the condensed mixture in the above exchanger are blended in COND-MIX. The heat recycled in the exchanger is used to generate steam for the heat recovery steam generator (HRSG) section. The steam produced in the HRSG section is used as the gaseous agent in the plasma reactor.

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NH3

QENCH TO-QENCH

NH3TOWER COOL-GAS

NH3FREE

ALL-COND

Figure 9.45 High-temperature cooling and ammonia separation [94].

Figure 9.46 Complete hot gas cooling and steam generation in the HRSG unit [94].

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9.3.1.3 The low-temperature cleaning section The output synthesized gas enters the acidic gas removal section from the previous section. Several solvents are available for the physical absorption of H2S from the gas stream. This section describes the Selexol process that uses DMPEG solvent to remove H2S. The Selexol process has been patented by Allied and has been in use since 1960. In this process, dimethyl ether polyethylene glycol solvent (DMPEG) is used to adsorb H2S from the gas stream. The chemical formula for this solvent is CH3(CH2CH2O)nCH3 where n can be 39 [96]. The operating temperature of this solvent is usually between 0
C and 40
C. The ability of this solvent to operate in this temperature range reduces the cooling cost. Because H2S solubility is much higher than COS solubility, there is usually a hydrolysis unit before this unit, which converts H2S to COS. However, COS is not considered a gaseous product. This process is illustrated in Fig. 9.47.

9.3.1.4 The hydrogen production sector 9.3.1.4.1 Gaswater displacement reactors The gaswater displacement reaction system consists of two sequentially filled catalytic reactors. The following reaction occurs in these reactors: H2 O 1 CO2CO2 1H2

Figure 9.47 Selexol process [98].

(9.72)

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Hybrid Poly-generation Energy Systems

Figure 9.48 Gaswater displacement reaction system [100].

Figure 9.49 The gaswater displacement reactor [94].

The product of the gas reactor is combined with the current and enters a series of exchangers. The exchangers cool the compound to the desired reactor inlet temperature before entering the first displacement reactor. Because much heat is released in the first reactor, iron oxide, which is effective at higher temperatures, has been used as a catalyst, as can be seen in Fig. 9.48. The product flow from the first displacement reactor is sent to another set of exchangers before entering the second displacement reactor. Less heat is released into the second reactor, so copper oxide, which is effective at lower temperatures, is used as a catalyst. The product flow from the second displacement reactor is sent to the third set of exchangers to separate the water from the gas product and condense it. The separated water is combined with freshwater and sent to a set of

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471

Figure 9.50 Oscillator-pressure absorber [100].

reciprocating exchangers to form a heater for gaswater displacement reactors. Outflow from gaswater displacement reactors mainly involves carbon dioxide and even more hydrogen with little methane, water, and carbon monoxide [97]. Reactors and reciprocating exchangers are shown in Figs. 9.49 and 9.50. The gaswater discharge reaction system is simulated as two successive reactors with intermediate coolers and reciprocating exchangers. Two reactors have been used sequentially to achieve higher overall CO conversion. Displacement reactions occur in catalytically packaged tubular reactors. Because of the large amount of heat released in the first reactor, the iron oxide catalyst that works at high temperatures is used. Also, less heat is released into the second reactor, so a copper oxide catalyst that is effective at lower temperatures is used. Each reactor is simulated with a Gibbs reactor. All components are considered reaction products. Each reactor is adiabatic and the inlet temperature is 457.4
F. The ratio of steam to CO in the first reactor is considered 3:1. This steam is a mixture of reclaimed water and water added to the process that is generated from the heat of the intermediate exchangers. Additional water coolers (COOL1 and COOL2) are needed to provide additional cooling for the inlet flows of the reactor that is necessary to achieve gaswater discharge inlet temperatures. The product from the second gaswater displacement reactor is cooled to a temperature to separate and condense the excess water from the gas product. The condensed water is reused in the process again.

9.3.1.4.2 Pressure swing adsorption blocks and purging The pressure oscillation adsorption unit is a set of substrates that are filled with the molecular sieve, where all compounds are adsorbed except hydrogen. This results in increased purity of H2 (more than 99%). Part of this pure H2 is used to reproduce surface adsorbent by purging the substrates [97]. Figs. 9.50 and 9.51 show the mentioned units.

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Hybrid Poly-generation Energy Systems

Figure 9.51 PSA purification [94]. PSA, Pressure swing adsorption.

9.3.1.5 Methanol production An RGibbs reactor is used for methanol production. The output stream of synthesis gas from the Selexol unit enters the three-stage compressor and is compressed to 70 bar pressure. The reactor’s operating temperature is 200
C, and its pressure is 70 bar. The outflow from the reactor cools to 10
C and enters the high-pressure separator. The pressure of this separator is 25 bar. The gas stream is separated from the liquid stream and sent to the low-pressure separator in this separator. At this stage, the outflow pressure of the outlet from the high-pressure separator is lowered to separate the remaining reactive gases in the liquid stream. The pressure of this separator is 2 bar. The outflow fluid from the low-pressure separator consists of methanol, water, and many other substances. We need a distillation tower to separate water from methanol, as shown in Fig. 9.52.

9.3.1.6 Combined cycle sections The combined cycle of a gas turbine consists of a gas turbine and a steam cycle, which comprises the HRSG and steam turbines.

9.3.1.6.1 Gas fuel saturation Cleansed and acid-free gases from the Selexol unit go to the saturated portion of the gas fuel to moisten and saturate. A mixer called MIXSAT is used to simulate this part. The mass flow rate of the H2O-SAT must be such that the moisture content in the SATGAS stream reaches 28.2%. Calculations of the amount of water added can be done as follows: mH2 O 5

yH2 O;wt 3 mH2 O 1 2 yH2 O;wt

(9.73)

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473

Figure 9.52 Methanol production unit [94].

The saturated gas fuel in HEATER is heated to 570
F and flows into the combustion chamber.

9.3.1.6.2 Gas fuel saturation model The gas turbine consists of a multistage compressor, a combustion chamber, and a multi-stage expander (turbine) [99]. The compressor compresses atmospheric air. In the last stage, some compressed air is removed and sent to the turbine for cooling. The synthesis gas burns with the compressed air in the combustion chamber. The gases from the combustion exhaust expand in the turbines. The gases from combustion expand, and the shaft work is generated and converted to power through generators. Finally, hot exhaust gases are sent to the steam cycle to increase system efficiency and generate more power, as shown in Fig. 9.53. The flow diagram of this process is shown in Fig. 9.54. The environment is assumed to have a temperature of 15
C, and atmospheric pressure and relative humidity of 60%. The conventional compression ratio for a Class 7F gas turbine is 15.5 and its combustion temperature is 1288
C. The three-stage compressor is simulated using three GT-COMP1, GT-COMP2, and GT-COMP3 blocks. As mentioned, the compressor has three stages. The pressure ratio is assumed to be constant at each step. Therefore the pressure ratio at each stage can be calculated as Eq. (9.74), where the ratio of the pressure of each step is expressed by rC,i, the compressor total pressure ratio is expressed by PR, and the number of compressor steps is expressed by i. rC;i 5 PR1=3

(9.74)

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Hybrid Poly-generation Energy Systems

Figure 9.53 Gas turbine cycle flowchart [99].

Part of the exhaust air from each compressor stage is separated from the mainstream and sent to the turbine section to cool the turbine blades. The synthesis gas is mixed with compressed air (AIR6) in a GT-MIXER mixer and then burned in the combustion chamber called the GT-BURN. A stoichiometric block reactor simulates the combustion

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475

Figure 9.54 The gas turbine cycle section.

chamber. The chemical reactions inside the combustion chamber mainly include the combustion reactions of CO, H2, and CH4. O2 and N2 reactions also simulate the formation of NOX compounds. The following reactions occur in the combustion chamber: 2CO 1 O2 ! 2CO2

(9.75)

CH4 1 1:5O2 ! CO 1 2H2

(9.76)

2H2 1 O2 ! 2H2 O

(9.77)

2H2 S 1 3O2 ! 2H2 O 1 2SO2

(9.78)

COS 1 1:5O2 ! CO2 1 SO2

(9.79)

2NH3 1 2:445O2 ! 0:1N2 1 1:71NO 1 0:09NO2 1 3H2 O

(9.80)

The above reactions indicate the oxidation of gaseous fuel. The combustion chamber pressure drop equals 4% of the inlet pressure of the combustion chamber. Hot combustion gases are expanded in a three-stage expansion turbine simulated by the GT-TURB1, GT-TURB2, and GT-TURB3 blocks. The pressure ratio at each

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Hybrid Poly-generation Energy Systems

stage can be calculated as Eq. (9.81), where the pressure ratio of each step is expressed by rT,i, the total pressure ratio of the turbine is expressed by PR, and the number of turbine steps is expressed by i. The output pressure of each step can be calculated using the ratio of pressure and inlet pressure. rT;i 5 PR1=3

(9.81)

Before each gas turbine stage, a mixer block is simulated by the GT-MIX1, GTMIX2, and GT-MIX3 blocks. In these blocks, the cooling air sent from the compressor is mixed with the inlet gas flow to each of the turbine stages. Hot exhaust gases from the last stage of the turbine are sent to the HRSG section to produce steam at high temperatures and pressures, as shown in Fig. 9.54.

9.3.1.6.3 Steam cycle The steam cycle includes the HRSG section, steam turbine, and other auxiliary equipment. Hot combustion gases from the turbine are sent to the steam generating unit using thermal recycling (HRSG). The heat of the hot gases emitted from the turbine produces high-temperature steam. The steam produced in the steam turbine expands; as a result, the shaft work is produced and eventually converted to power. Heat generation using thermal recovery The HRSG section comprises gas-gas heat exchangers, heaters, evaporators, and superheaters, which recover the perceptible heat of the hot gas from the gas turbine and generate water vapor. HRSG is used to preheat boiler feedwater, reheat medium steam water, generate highpressure steam and 7 bar pressure steam, and superheating high-pressure steam. The flow chart of the HRSG section is shown in Fig. 9.55. The HRSG consists of a superheater with a working pressure of 100 bar, a working temperature heater of 535
C, two economizers, a high-pressure boiler, and a low-pressure boiler. The low-pressure boiler generates steam for the flue gas vent, leaving the economizer at 185
C. The heat dissipation of the HRSG process is simulated through the QSPLIT block. The cooling process of the hot gas outlet from the gas turbine is simulated through a series of heat exchangers called SH-HRSG, HP-HRSG, E2-HRSG, LP-HRSG, and E1-HRSG. Fig. 9.56 shows that the heat recovered from the E1-HRSG, E2-HRSG, and HP-HRSG heat exchangers is aggregated by a mixer called QMIX. Also, the residual heat recovered by the SH-HRSG heat exchanger is divided into three heat flows by the QSPLIT block, one representing heat loss. The other heat current is sent to the steam turbine section, and the last heat current is combined with other heat flows in the QMIX block. The heat accumulated in the QMIX block is sent to an economizer simulated by a heat exchanger called ECONOMZR. ECONOMZR heats water to 290
C. Subsequently, the heated water in the ECONOMZR is sent to the HPBOILER boiler to produce highpressure steam. The steam produced in the high-pressure boiler is sent to the SUPERHTR superheater, and eventually, the superheater is produced at a temperature of 535
C, which is sent to the high-pressure steam turbine. Also, using a low-pressure boiler simulated by the BOIL100 block, water vapor is produced at a pressure of 7 bar.

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477

Figure 9.55 HRSG unit flowchart [99].

Plasma filters Plasma filters are made of several conductors and semiconductors whose refractive indexes have both real and imaginary portions and as a result, have high absorbency. These filters use semiconductors such as silicon and InGaAs

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Hybrid Poly-generation Energy Systems

HP-HRSG

SH-HRSG

GTPROD

HP-IN

QSH-HRSG

E2-HRSG E2-IN

QHP-HRSG

Q-SPLIT

E1-HRSG

LP-HRSG LP-IN

QE2-HRSG

E1-IN

QLP-HRSG

STACKGAS

QE1-HRSG

Q-MIX

Q-SUPER

Q-LOST

Q-REHEAT

QTOTHRSG

SUPERHRT SH-STEAM

HP-STEAM HP-BOILER

WATER-1

ECONOMIZ

PUMP-1

H2OSPLIT

TO-ECONO

TO-ECONO

Q-HPBOIL

HP-BFW

HP-BLOWDN LP-BFW

TO-BOIL

LP-STEAM LP-BOILER

PUMP-2

LP-BLOWDN

Figure 9.56 HRSG unit [94].

and transparent conductive oxides such as indium oxide, tin oxide, and zinc oxide. Plasma filters are described by the Drude model. In this model, electrons behave like free gas particles, like plasma where the gas is ionized [101]. Combination of interference and plasma filters Interference filters have a high permeability range and do not have high reflectivity at long wavelengths, which are unsuitable for thermophotovoltaic systems. Instead, plasma filters have a broad reflection for long wavelengths but have high absorption for those wavelengths that require high pass-through. To achieve proper performance, the two filters can be combined and a suitable filter can be obtained. By placing a dielectric interference filter in front of a plasma filter, a suitable coefficient of passage for short wavelengths and a large reflectance coefficient for long wavelengths can be achieved [101]. Resonant array filters Periodic distribution of small fragments of highly reflective materials such as gold is a simple description of resonant array filters. The dimensions of these small fragments are smaller than the wavelength of the radiation. In these filters, the transmission range is near the infrared spectrum and they have been developed for thermophotovoltaic applications [101]. Spectral control using the back surface reflector The simplest method for spectral control is to use a high-reflectivity surface such as gold on the back of the cell array. This level (BSR) reflects photons that are not absorbed by the cell array toward the emitter. For this method to be successful, materials used for cells must have low absorption for photons whose energy is less than the cell’s energy bandgap [101].

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According to the explanations earlier, if any filter is used, the mechanism is such that at the wavelengths below the cut-off wavelength, it has the highest transmission coefficient (minimum absorption and reflection coefficient), and it has the lowest transmittance coefficient at wavelengths greater than the cutoff wavelength. According to the literature, an optical crystalline photonic filter for InGaAsSb cells and nine dielectric metal filters were selected for GaSb cells. Fig. 9.57 shows the reflectance coefficient of a 9-layer dielectric filter used in the GaSb cell thermoforming system [102]. As can be seen, the reflectance coefficient near the cutoff wavelength of this cell, which is about 1.7μm, is close to zero (the crossing coefficient is about one), which means that this filter is an almost ideal filter for this cell (Fig. 9.57). The pass-through coefficient of the experimental optical filter [3,4] and the fit for the InGaAsSb cell are shown in Fig. 9.58. The fitted equation of the crystalline photon filter coefficient of passage with the sum of the least-squares error is 0.9553 in the form of a relation, that is, Eq. (9.82):

% Reflectance

εðλÞ 5 2 1:2133λ4 1 5:9581λ3 2 9:7247λ2 1 6:092λ 2 0:5092

(9.82)

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1

2

3 microns

4

5

Figure 9.57 Reflection coefficient of a dielectric filter [102]. Experimental

Fied

1.2

Transmission

1 0.8 0.6 0.4 0.2 0 0.5

1

1.5

2

2.5

λ(μm) Figure 9.58 Crystal photonic filter passage diagram for InGaAsSb cell in wavelength [103].

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Hybrid Poly-generation Energy Systems

An experimental optical filter pass-through [104] and a fitted metal dielectric for GaSb cells are shown in Fig. 9.59. As can be seen, a good match between the original curve and the best-fit function curve cannot be seen in Fig. 9.59. Figs. 9.60 and 9.61 show two fit graphs. Thus the fit of the two-parameter equation for the dielectric filter coefficient with the sum of least squares is 0.949 and 0.996 respectively, and it is represented as Eq. (9.35): 

τðλÞ 5

2 23:149λ4 1 61:847λ3 2 61:953λ2 1 27:551λ 2 3:7201 2 0:128λ4 1 3:0922λ3 2 12:509λ2 1 17:095λ 2 6:6655



0:38 # λ # 0:9095 0:9096 # λ # 2

(9.83)

Experimental

Fied

1

Transmission

0.8 0.6 0.4 0.2 0 0

0.5

1

1.5

2

λ(μm)

Figure 9.59 Metal dielectric filter pass rate diagram for GaSb cell by wavelength [103].

Experimental

First fied

1

Transmission

0.8 0.6 0.4 0.2 0 0

0.5

1 λ(μm)

1.5

Figure 9.60 First fit curve for the dielectric crossing coefficient [109].

2

A framework for sustainable hydrogen production by polygeneration systems

Experimental

481

Second fied

1

Transmission

0.8 0.6 0.4 0.2 0 0

0.5

1

1.5

2

2.5

λ(μm)

Figure 9.61 Second fit curve for dielectric crossing coefficient [109].

9.3.1.6.4 Upper and lower Integral limit (cell and cutoff wavelength) Cell wavelength The cell wavelength (λg) is obtained from the relation (9.86) [105]: λg 5

hc Eg

(9.84)

It should be noted that since the energy gap is in units of electron volts, the Planck constant must be divided by the electron charge so that its unit becomes electron volts instead of joules.

9.3.1.6.5 Cutoff wavelength The cutoff wavelength (λ0) can be approximated by the relation (9.85) [105]: λ0 5

λg 2:5

(9.85)

λ0 is considered such that if λg also corresponds to the highest energy band (the energy band corresponding to normal solar cells), then all wavelengths are smaller. In other words, 2.5 is the smallest number for which all wavelengths smaller than the cell energy band are considered in the model. If any larger number is included in the model, it will not affect the model performance, as the Planck spectral relation tends to approach zero rapidly for very small wavelengths, and the model error is zero at this point.

9.3.1.7 Open circuit voltage (Voc) The open-circuit voltage (Voc) is obtained from Eq. (9.86) [105]:

nkTc jsc ln 11 Voc 5 e j0

(9.86)

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Hybrid Poly-generation Energy Systems

n: Factor is the ideal degree of the
diode,  which is usually
 considered 1. k: Fixed Boltzmann ð8:625 3 1025 evk 5 1:38 3 10223 kj Tc: Cell temperature jsc: Short circuit current density j0: Saturation of current density

9.3.1.8 Saturation current density (j0) The saturation current density (j0) is obtained from Eq. (9.87) [105]: j0 5 1:5 3 105 exp

2 Eg e kTc

(9.87)

In the two above relationships, the Boltzmann constant unit is assigned to Joule.

9.3.1.8.1 Cell energy gap (Eg) Because thermophotovoltaic cells deal with close to the cell’s energy bandwidth, a small changes in the output electrical current. The material depends on its temperature and Eq. (9.88) [106]: Eg ðTÞ 5 Eg ð0Þ 2

the bulk of photons whose energy is change in the bandgap results in large energy bandgap of the semiconductor follows the relationship shown in

αT 2 T 1β

(9.88)

where α and β are constant values depending on the cell type and material. The energy bandgap changes to the cell temperature for the two GaSb and InGaAsSb cells are shown in Fig. 9.62 [106]. As shown in Fig. 9.62, the energy bandwidth changes with temperature are linear. Therefore the relationship of (9.88) is estimated as the ratio of (9.89) and (9.90) for GaSb and InGaAsSb cells [106,107]: Eg ðTÞ 5 0:7276 2 ð3:990 3 1024 ÞðT 2 300Þ

(9.89)

Eg ðTÞ 5 0:5548 2 ð1:952 3 1024 ÞðT 2 300Þ

(9.90)

9.3.1.8.2 Fill factor Open circuit voltage and short circuit current are the maximum voltage and current obtained from a PV cell, respectively. But as shown in Fig. 9.63, at both these points, the output power of the cell is zero. The fill factor (FF) is a parameter that relates the open-circuit voltage and short-circuit current of the cell to its maximum output power. Thus FF is defined as the relation (9.91) [108]. FF 5

Pmax Vmax Imax 5 Voc Jsc Voc Jsc

(9.91)

A framework for sustainable hydrogen production by polygeneration systems

483

Figure 9.62 Energy band gap changes with cell temperature [106].

I

1/Ropt A

Isc

Imax

B C

D

1/R

E 0

Vmax

Figure 9.63 Current curve—photovoltaic cell voltage [108].

Voc

V

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Hybrid Poly-generation Energy Systems

For a good cell, the FF is greater than 0.7 and decreases with increasing cell temperature [108]. To determine the FF independent of the maximum current and voltage (Vmax ; Imax ), the relation (92) is used. FF 5 β

v 2 lnðv 1 0:72Þ eVoc ;v5 v11 kTc

(9.92)

whereβ: Frequency coefficient β shows the effect of parasitic dissipation as a result of resistive aberrations on FF. The value of β is usually assumed to be 0.96.

9.3.1.8.3 Cell temperature determination One of the main determinants of the output power of the thermophotovoltaic system and therefore its efficiency is the cell temperature. Due to the emitting radiation, the cell becomes warm after a while. For the following reasons, cell warming can only be modeled by a radiant heat transfer mechanism: 1. Existence of filter and convection heat transfer mechanism Inhibition 2. The predominance of radiative mode over convection

Therefore assuming that the transmitter and the cell have only heat exchange together, the equation governing this heat transfer can be considered as the relation (9.93) [110]. 4 εemitter σAðTemitter 2 T04 Þ 5 mcell Ccell

@Tcell @t

(9.93)

εemitter : Emission factor of emitter, mcell : Cell mass Temitter : Emitting temperature, Ccell : Heat capacity of the cell Tcell : Cell temperature, σ: StephenBoltzmann constantt: time A: Emitter surface The energy emitted by the transmitter and having a wavelength less than the cell energy band gap is converted by the cell to electric current, so this spectrum of energy emitted by the transmitter does not increase the cell temperature. Thus Eq. (9.93) has to be modified to consider this section. To determine the amount of radiation energy converted to electrical power, we use the ratio shown in Eq. (9.92). We can now write the relation of the cell temperature rise to the relation (9.94): σcell 3 Pemitter 2 Pel 3 A 5 mcell Ccell

@Tcell @t

(9.94)

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485

Pemitter in Eq. 3.46 is obtained from Eq. (9.95): Tcell ðtÞ 5 T0;cell 1

1 mcell Ccell

ð t

ðσcell 3 Pemitter 2 Pel 3 AÞdt

(9.95)

0

Increasing the temperature of the cell reduces its output power. The relation (9.96) indicates the effect of increasing temperature on the output power of the cell [108]. pmax ðTÞ 5 pmax ðT0 Þ 3 ½1 2 γðT 2 T0 Þ

(9.96)

where γ: The temperature drop coefficient of the cell due to its temperature rise

9.3.1.8.4 Influence of distance between cell array and emitter (F) The electromagnetic waves propagate in a spherical plane, the emitter and the cell being parallel to each other, so the amount of radiation emitted by the transmitter does not always reach the cell array. To consider this loss and the effect of the distance between the cell array and the emitter, a visibility factor should be used. The visibility factor for the two plates, in general, is obtained from Fig. 9.64 and Eq. (9.97) [101]: Fij 5

1 πAi

ð ð Ai

cosθi cosθj dAj dAi s2 Aj

(9.97)

where Fij : Visibility factor (determines radiation reaching from level i to level j) S: The distance between two pages Θ: the angle between the normal of element and line drawn between two plates

dAj θj Aj θi

S

ioi(xi,θi,Ti,λ) dAi Ai

Figure 9.64 Geometric visualization for radiation reaching from surface i to surface j [101].

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Hybrid Poly-generation Energy Systems

b

A1

A2

X=

a h

Y=

b h

a

h

Figure 9.65 Arrangement of cells and emitters [101].

The arrangement of the array of cells and the emitter, as shown in Fig. 9.65, is parallel, two-axis, and quite similar. For the geometrical state shown in Fig. 9.65, the result of the relationship shown in Eq. (9.97) give us the relation (9.98) [101]: 2 F12 5 πXY

(  1

pffiffiffiffiffiffiffiffiffiffiffiffiffi ð11X 2 Þð11Y 2 Þ 2 X 21 2 ffiffiffiffiffiffiffiffiffiffiffiffiffi p ln 1 X 1 1 Y tan 11X 2 1Y 2 1 1 Y2



pffiffiffiffiffiffiffiffiffiffiffiffiffiffi Y 1 Y 1 1 X 2 tan21 pffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 Xtan21 X 2 Ytan21 Y 1 1 X2

) (9.98)

If the two pages are not identical as shown in Fig. 9.66, the result of the relation (9.97) is the Eq. (9.99) [101]: ( !  2 2 X ð11Y 2 Þ12 2 F12 5 ln ½X 2 ð1 2 Y 2 Þ 1 2½X 2 ð1 1 Y 2 Þ 1 2 πX 2 " !  2 12 Xð1 2 YÞ 2 21 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 X ð12Y Þ14 Xð1 2 YÞtan X 2 ð1 2 Y 2 Þ 1 4 !#) Xð1 1 YÞ 21 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 Xð1 2 YÞtan (9.99) X 2 ð1 2 Y 2 Þ 1 4 We can now modify the relationship of (9.73) and (9.95) as (9.100) and (9.101), respectively, and consider the effect of geometrical losses in the model. e0λb 5 eλb 3 F

(9.100)

P0emitter 5 Pemitter 3 F

(9.101)

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b a A2

b

A1 a c X=

a c

y=

b a

Figure 9.66 Two parallel plates, both axial and nonaxial [101].

9.3.1.9 Thermophotovoltaic system efficiency The optical filter efficiency is defined as the ratio of the input convertible power (passing through the optical filter) to the input power to the filter (emitting radiation), as represented in Eq. (9.102). Ð λg ηoptical 5

εðλÞ:eλb ðλÞ:τðλÞdλ ÐN 0 εðλÞ:eλb ðλÞdλ

λ0

(9.102)

The thermophotovoltaic cell efficiency can be defined as the ratio of the output electrical power (PEL) to the input convertible power (bypassing the optical filter) as represented in Eq. (9.103): ηcell 5 Ð λg λ0

Pel εðλÞeλb ðλÞτðλÞdλ

(9.103)

Finally, the efficiency of the thermophotovoltaic system is obtained by multiplying the efficiency of the cell by the optical filter efficiency, which can be defined as the ratio of the output electrical power (pel) to the input radiation power, as shown in Eq. 9.104: ηTPV 5 ηoptical 3 ηcell 5 Ð N 0

pel εðλÞeλb ðλÞdλ

(9.104)

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9.3.1.10 Model solving algorithm Finally, taking into account the issues raised in the thermophotovoltaic section, a computational model was developed to calculate the output power of the thermophotovoltaic system in MATLABs . The general procedure for calculating the electrical power of a thermophotovoltaic system is as follows: Choosing basic terms and assumptions Defining emitter temperature mode (combustion chamber temperature) Vision constant calculation based on size and distance of emitter from cell Initial energy bandwidth and cutoff wavelength of the two cells calculation Cell spectral response and equating optical filter relationships calculation Preparation of relevant equations and simultaneous solving of open-circuit voltage equations, short circuit current density and density coefficient with cell energy balance equation 7. Power calculation, cell efficiency, and thermophotovoltaic efficiency 1. 2. 3. 4. 5. 6.

9.3.2 Calculating efficiency 9.3.2.1 Cold gas efficiency According to the research conducted in recent years, different criteria have been proposed to evaluate the efficiency of gasification processes. In isothermal processes, the process energy efficiency, or cold gas efficiency (CGE), is calculated as the ratio of the low thermal value (LHV) of the cold gas to the total LHV of solid waste and the added energy (fuel or electricity) per kilogram of solid waste. This parameter is calculated by the following equations:
 LHV of cold gas kJ Nm23 3 Fuel gas productionðNm3 kg21 Þ
 η5 LHV of waste treated kJ kg21 1 Allothermal PowerðkWÞ=Waste flow rateðkg s21 Þ

(9.105) η5

m_ Syngas 3 LHVSyngas 3 100 ðm_ Feedstock 3 LHVFeedstock Þ 1 PSteam 1 PPlasma

(9.106)

where m_ Syngas and m_ Feedstock represent the mass flow rates of the synthesized gas and raw materials, respectively, while LHV Syngas and LHV Feedstock are mass-based thermal values. PSteam is the power used to heat steam and PPlasma is the plasma power. To calculate the energy efficiency of the waste gasification process by thermal plasma, the source of energy used to create the plasma must be taken into account. If the electrical energy consumed is generated in another part of the process, the thermal energy will be equal to the energy consumed for making the plasma.

9.3.2.2 The overall efficiency of the methanol, hydrogen, and electricity cogeneration process Efficiency (or productivity) indicates the avoidance of wasting materials, energy, capital, and time in getting things done. This concept is quantifiable in many branches of physics, engineering, mathematics, and economics. Returns are usually

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defined as the ratio between "useful output" to "total input". The overall efficiency of the multiple production system processes is defined as follows: ηt ð % Þ 5

Pnet 1 ðm_ METH 3 LHV METH Þ 1 ðm_ H2 3 LHV H2 Þ ðm_ MSW 3 LHV MSW Þ

(9.107)

Pnet 5 PGT 1 PST 1 PTPV 2 PPlasma

(9.108)

where m_ METH is the output methanol flow rate, m_ H2 is the output hydrogen flow rate, PGT is the gas turbine cycle power generation, PST is the steam cycle power generation, PTPV is the thermophotovoltaic system power generation, and PPlasma is the plasma burner power consumption.

9.3.3 Economic evaluation of the process Economic discussion of any process is very important because any process that is not economically profitable will not be sustainable. Success in solving engineering problems often depends on the ability to evaluate both economic and technical factors. Engineers should be responsible for the economic interpretation of their work. To understand the relationship between the technical and economic aspects of engineering work, it is necessary to master the basic concepts of economic analysis. This section describes the metrics used to accurately evaluate the costs and benefits of the process. In this section, we obtain the relationships to calculate the costs of each simulated segment and then form the profit target function.

9.3.3.1 Cost of direct investment The direct cost of each unit can be calculated for other years by the unit cost index (PCI), as shown in Table 9.21. DC2017 5 DC2001 3

553 394:3

(9.109)

The direct cost of each process segment is then calculated. Table 9.21 Device Cost Index for 19902017 [112]. Year

Device cost index

Year

Device cost index

Year

Device cost index

1991 1992 1993 1994 1995 1996 1997 1998 1999

3/361 2/358 2/359 1/368 1/381 7/381 5/386 5/389 6/390

2000 2001 2002 2003 2004 2005 2006 2007 2008

1/394 3/394 6/395 0/402 2/444 2/468 6/499 4/525 4/575

2009 2010 2011 2012 2013 2014 2015 2016 2017

9/521 8/550 7/585 6/584 3/567 7/575 4/578 7/541 553

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9.3.3.1.1 Thermal plasma gasification unit The main drawback of the thermal plasma gasification process that has been mentioned by many researchers and engineers is the consumption of one of the most expensive sources of energy, namely electricity. Economic evaluation of this process involves many variable parameters including regional characteristics, type of solid waste material, capacity, etc. Currently, the average cost of building a thermal plasma unit is estimated at $0.13 to $ 0.39 million per tonne of solid waste material (million US$/TPD). Fig. 9.67 shows the construction cost based on the unit capacity of the plasma [111].

9.3.3.2 Plasma gasification Four sets of experiments were performed using a plasma reactor. In every four cases, the rate of MSW entering the reactor was set to 300 kg/h. Other process variables such as plasma power, plasma air rate, secondary air feed rate, rate, and temperature of the injected steam are shown in Table 9.22. The results of this series of experiments, including percent composition, LHV, and gaseous efficiency, are as follows. The four stages of the experiment were simulated using the developed model, and the simulated results were compared with the experimental results.

9.3.4 Thermophotovoltaic system model results The model is developed so that it is possible to evaluate the cell and thermophotovoltaic system performance in different modes. The assumptions and parameters considered in the model and which can be examined are as follows: 1. Emitter temperature The emitter temperature in the model is regarded in such a way that it can be investigated at different temperatures. According to the earlier explanations, the emitter temperature is considered equal to the temperature of the combustion chamber.

Figure 9.67 Construction cost of thermal plasma unit based on unit operating capacity [111].

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Table 9.22 Operating parameters for plasma gasification reactor experiment. Row

Steam temperature (
C)

Injected steam rate (kg/h)

Secondary air rate (kg/h)

Plasma air rate (kg/h)

Plasma power (kW)

Flow rate MSW (kg/h)

1 2 3 4

1000 1000 1000 1000

70 100 70 70

60 60 35 13

120 120 120 130

240 240 240 260

300 300 300 300

2. Cells array temperature The array of cells’ temperature increases with time. Moreover, this increase in temperature affects its performance. 3. Distance between cells array and emitter The array of cells and emitter are placed parallel to each other at a distance of a few centimeters. The distance between the cells array and the emitter is also regarded in the model to consider the radiative losses of the emitted spectrum at its lateral edges. 4. External quantum efficiency This efficiency determines the amount of radiation spectrum a cell can convert regarding wavelength. In most references, this variable is considered to be average. However, given the landing wavelength’s sensitivity, the quantum efficiency in this model is regarded in such a manner that it can be investigated in quantum efficiency dependent on the emitter wavelength for every cell. 5. Optical filter transmissivity coefficient The emission coefficient is assumed to depend on the wavelength of the input photon corresponding to each cell. The results are presented in two cases. In the first case, a metal-dielectric optical filter depends on input photon wavelength, and in the second case, a crystal photonics optical filter at a temperature equivalent to the combustion chamber. The distance between the cell array and emitter is 1 cm, and the quantum efficiency changeable with wavelength and the silicon carbide emission coefficient is 0.9, are to be reviewed. 6. First case GaSb cell with an initial energy bandgap of 0.725 electron-volts varies with cell temperature. 7. Second case

InGaAsSb cell with an initial energy bandgap of 0.55 eV varies with cell temperature.

9.3.4.1 Short-circuit current density of each cell According to Eq. (9.75), the short-circuit current density is a function of the transmitter temperature. According to the relations (9.77) and (9.73), the short-circuit current density is also a function of the EQE. According to this model, it is also capable of considering changes in EQE in terms of the wavelength emitted. Fig. 9.68 shows the variations of the short-circuit current density in terms of the emitter temperature for both cells at variable wavelength quantum efficiency.

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Hybrid Poly-generation Energy Systems

As shown in Fig. 9.68, the short-circuit current density is directly related to the temperature of the emitter and increases rapidly with the temperature of the emitter. It is also observed that the short-circuit current density for the InGaAsSb cell is higher than that of GaSb, due to the cell’s lower energy band gap, which increases the wavelength cutoff. As a result, it uses the bulk of the selective emitter radiation in the temperature range. Therefore the short-circuit current density decreases as the cell energy band gap increases. According to the previous chapters, the short-circuit current density depends on the cell’s temperature. Fig. 9.69 shows the variations of the short-circuit current GaSb

InGaAsSb

8 7

Jsc (A/cm2)

6 5 4 3 2 1 0 973

1073

1173

1273

1373

1473

1573

1673

T emitter (K) Figure 9.68 Short-circuit current density vs. emitter temperature [113]. GaSb-1273 K

InGaAsSb-1273 K

GaSb-1173 K

InGaAsSb-1173 K

1.7

JSC (A/cm2)

1.4

1.1

0.8

0.5

0.2 27

37

47

57

67

77

87

T cell(c) Figure 9.69 Short circuit current density changes versus cell temperature [113].

97

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density in terms of cell temperature. As can be seen, as the temperature of the cell increases, the short-circuit current density slightly increases, which is due to the low energy band gap resulting from the increase in cell temperature. Given the linearity of the relationship of the energy bandwidth to the cell temperature, the short circuit current density diagram changes linearly with the cell temperature. According to Figs. 9.69 and 9.70, increasing the cell’s temperature increases the short-circuit current density slightly, and increasing the transmitter temperature significantly increases the short-circuit current density. Thus the effect of the transmitter temperature on the short-circuit current density is far greater than the effect of the cell temperature.

9.3.4.2 Open circuit voltage of each cell The open-circuit voltage variations according to the cell temperature are shown in Fig. 9.71. As can be seen, open-circuit voltage variations are inversely related to cell temperature. Moreover, according to Eq. (9.87), the saturation current density of the cell is an exponential relationship with temperature. This indicates explicit independence of open-circuit voltage on cell temperature. Given the simultaneous variations in cell temperature and transmitter temperature in the open circuit’s voltage amount, it is observed that the effect of cell temperature is more significant than others. Figs. 9.70 and 9.71 show that the opencircuit voltage for GaSb cells is higher than that of InGaAsSb cells due to the greater bandgap of GaSb cell energy. Although in short circuit current density, this issue is the opposite of the latter.

GaSb-1173 K

InGaAsSb-1173 K

GaSb-1273 K

InGaAsSb-1273 K

GaSb-1373 K

InGaAsSb-1373 K

0.44 0.42

Voc (v)

0.4 0.38 0.36 0.34 0.32 0.3 0.28 27

37

47

57

67

77

T cell(c)

Figure 9.70 Open circuit voltage changes versus cell temperature [113].

87

97

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Hybrid Poly-generation Energy Systems

GaSb

InGaAsSb

0.6

Voc (v)

0.5 0.4 0.3 0.2 0.1 0 973

1073

1173

1273

1373

1473

1573

1673

T emitter (k) Figure 9.71 Open circuit voltage variations versus transmitter temperature [113].

9.3.4.3 Compression factor The compression coefficient is one of the effective parameters in the output power of the thermophotovoltaic system. As it is known, the cell temperature has more influence than the emitter temperature, and the density coefficient decreases with increasing cell temperature.

9.3.4.4 Influence of cells array and transmitter Because of the bursts from the lateral edges of the emitter mentioned at the beginning of this section, not all emitter’s emitted power reaches the cell array. The visibility parameter determines the amount of power loss and the power that arrives at the cell array. According to Fig. 9.72, the visibility factor is 0.84 within 1 cm between the array of cells and the transmitter if they are identical and parallel. The visibility factor at 2 cm between the array of cells and the transmitter is 0.7 and 0.5 at 4 cm if they are entirely similar and parallel. Therefore the amount of power lost to the sides increases with increasing distance between the transmitter and the cells. This indicates the importance of the distance between the transmitter and the array of cells in the thermophotovoltaic systems. The effect of cell distance from the emitter versus visibility factor is shown in Fig. 9.72.

9.3.4.5 The output power density of each cell As mentioned in this section, the output power density of the cell is both a function of the emitter temperature and the cell temperature. The power density variations of the output of each cell concerning cell temperature are shown in Fig. 9.73.

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Figure 9.72 Visual factor value in terms of the emitter and cell distance [113].

GaSb-1173 K

InGaAsSb-1173 K

GaSb-1273 K

InGaAsSb-1273 K

GaSb-1373 K

InGaAsSb-1373 K

6 5.5 5

Pel(kw/m2)

4.5 4 3.5 3 2.5 2 1.5 1 0.5 27

37

47

57

67

77

87

T cell(c) Figure 9.73 Output power density changes of each cell versus cell temperature [113].

97

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Hybrid Poly-generation Energy Systems

GaSb

InGaAsSb

25

Pel(kw/m2)

20 15 10 5 0 973

1073

1173

1273

1373

1473

1573

1673

T emitter(K) Figure 9.74 Output power density variations of each cell versus emitter temperature [113].

As can be seen, increasing cell temperature reduces the output power density. The reason behind this is stated earlier that as the cell temperature increases, the shortcircuit current density increases. On the other hand, the open-circuit voltage and compression coefficient decrease, and the effect of reducing these two is more than increasing the short-circuit current density. It is also observed that the output power density for the InGaAsSb cell is higher than that of GaSb, due to the lower cell energy band gap, which increases the wavelength cutoff. As a result, the input power to the array of cells is increased, which increases the power output density of the cell with less energy bandgap. However, it is also worth noting that the InGaAsSb cell is more sensitive to the cell temperature than the GaSb cell, and its power decreases with increasing cell temperature compared to another cell. Output power density variations according to the emitter temperature are shown in Fig. 9.74. As can be seen, as the emitter’s temperature increases, the cell’s output power density also increases rapidly.

9.3.4.6 Efficiency As stated previously, the efficiency of the thermophotovoltaic system is affected by the radiation efficiency, the optical filter efficiency, and the efficiency of the thermophotovoltaic cell. In all this, the cell’s efficiency holds more significance.

9.3.4.6.1 Thermophotovoltaic cell efficiency The efficiency of each cell is determined by Eq. (9.103). This relationship represents the amount of electrical power density produced by each cell relative to the input convertible power (passing through the optical filter). As shown in Fig. 9.75, which shows the variation of the efficiency of each cell in terms of the emitter temperature, increasing the emitter temperature increases the efficiency of the cell. When the temperature of the emitter and the cell are 900
C and 27
C, the efficiency of GaSb and InGaAsSb cells is 20% and 16%, respectively. It can

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be seen that the GaSb cell efficiency is higher than InGaAsSb, which is contrary to the output power density mentioned in the previous section. This is due to the higher input power of the InGaAsSb cell, despite its higher output power density, which results in lower efficiency of the InGaAsSb cell compared to the GaSb cell.

9.3.4.6.2 Total efficiency of the thermophotovoltaic system The overall efficiency of the thermophotovoltaic system is obtained from Eq. (9.104). This relation represents the amount of electrical power density of each cell relative to the incoming radiation power of the transmitter. According to Fig. 9.76, due to the Cell eff. GaSb

Cell eff. InGaAsSb

0.3

Cell Efficiency

0.25 0.2 0.15 0.1 0.05 0 973

1073

1173

1273

1373

1473

1573

1673

T emitter(k) Figure 9.75 Changes in the efficiency of each cell vs. emitter temperature [113]. TPV eff. GaSb

TPV eff. InGaAsSb

0.1 0.09

TPV Efficiency

0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 973

1073

1173

1273

1373

1473

1573

1673

T emitter(k) Figure 9.76 Changes in the total efficiency of the thermophotovoltaic system versus emitter temperature [109].

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Hybrid Poly-generation Energy Systems

similar behavior of each cell’s efficiency with the total system efficiency, the total system efficiency also increases with increasing emitter temperature. As can be seen, the efficiency of the whole system at the emitter and cell temperatures at 900
C and 27
C for InGaAsSb and GaSb cells is 5% and 3%, respectively. As a result, InGaAsSb cells are used to achieve higher efficiency in the thermophotovoltaic system.

9.3.5 Total efficiency of the proposed hybrid system Assuming that the cell temperature is 27
C and the distance from the emitter is 1 cm, given that the combustion chamber temperature is about 1288
C and also taking Fig. 9.78 into account, the electrical power density of this system is 12.96 kW/m2. The proposed thermophotovoltaic system consists of 200,000 proposed InGaAsSb cells, each with a surface area of 2 cm2. Therefore this system, with an area of more than 40 m2, generates 0.5 MW. As a result, using Eq. (9.107), the overall efficiency of the proposed system is calculated to be 68.17%. In fact, with the use of a thermophotovoltaic system for the thermal recovery of combustion protection radiation, the system’s overall efficiency has been increased by about 3.5%. For the two processes, we calculate (A) methanol and hydrogen coproduction and (B) multiple productions (Table 9.23). As shown in Table 9.23, the efficiency of the multiple production processes will be higher than that of methanol and hydrogen production. The main reasons for the higher efficiency of the multiple-generation system include: 1. Use of the unreacted part of the methanol synthesis unit gas in the combined cycle for power generation 2. Provide power required for plasma burners through the combined cycle 3. Generating power in the TPV system through the thermal recovery of combined cycle combustion protection

Therefore due to the higher efficiency of the multiple production processes, this process was selected for economic evaluation and optimization, which will be examined in the continuation of this research.

9.3.6 Economic evaluation of the process Using the results, we can calculate the total investment cost, operating cost, revenue, profit amount, and rate of return. The results are summarized in Table 9.24. As can be seen, the methanol and hydrogen production process has a higher return on capital due to its lower overall cost and higher revenue. So the total investment cost is returned to the investor over two years and nine months. In contrast, in the process of multiple productions, we are experiencing a marked increase in overall costs. It is important to note that since the thermo-voltage system is one of the new energy systems, it still does not have high economic competitiveness. One of the reasons for this is the high cost of the initial investment of this system. As such, each TPV cell should cost around US $ 50. Also, as maintenance and

Table 9.23 Comparison of the efficiency of cogeneration and multiple production processes. Process type

Municipal solid waste (kg/h)

Methanol (kg/h)

Hydrogen (kg/h)

Combined cycle generation power (MW)

System production capacity TPV (MW)

Efficiency (%)

Cogeneration Multiple production

4166.667 4166.667

1056.854 528.427

227.031 113.515

0.0 6.770

0.0 0.518

62.22% 68.17%

Table 9.24 Evaluation of economic indicators of processes. Process type

Total cost (TPC) (M $)

Overall cost per unit year (TPC0) (M $/Year)

Operation and maintenance cost (O&M) (M$/Year)

Income (M $/Year)

Profit (M $/Year)

Return of investment (ROI%)

Co-production of methanol and hydrogen Multiple production

50.43

3.39

3.69

24.97

17.89

35.47

63.41

4.26

4.34

19.63

11.03

17.40

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operation costs are a function of overall costs, we will experience an increase in operating and maintenance costs. All of these factors lead to a significant reduction in the rate of return on capital for the multiple production processes. However, considering the design of a power plant that incorporates a thermophotovoltaic system was part of the objectives, and the power required for plasma gas reactor burners to be provided locally. Given the higher efficiency of the multiple production processes, this process is justified. Thus optimization will be done in this process. Before optimization, it is necessary to identify and evaluate the parameters affecting the process. For this purpose, it is necessary to investigate the parameters affecting the process.

9.3.7 Model thermodynamic sensitivity analysis Sensitivity analysis (SA) studies the impact of output variables on the input variables of a model. In other words, it is a way of systematically modifying the inputs of a model to predict the effects of these changes on the output of the model. We first examine the thermodynamic sensitivity of the model and analyze the total productivity sensitivity after defining the thermodynamic behavior of the system.

9.3.7.1 Influence of steam rate on synthesized gas composition

Mole Fraction (%)

High-temperature steam injection affects the performance of the plasma reactor in two ways. First, water vapor is involved in chemical reactions such as water-gas (WG) and water-gas transport (WGS) reactions and thus affects the chemical equilibrium of the gas system. Second, high-temperature water vapor changes the overall balance of mass and energy within the reactor and will affect the system’s energy balance. Fig. 9.77 illustrates the effect of increasing water vapor rate on the percentage of synthesized gas. As shown in the figure, an increase in the high-temperature steam feed rate will impact the values of H2, CO2, and CO. As the amount of high-temperature 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0

500

1000

1500

2000

2500

3000

Steam Flow Rate (kg/h) H2

CO

CO2

CH4

Figure 9.77 Influence of vapor rate on synthetic gas composition percentage [109].

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Hybrid Poly-generation Energy Systems

water vapor increases, the molar percentage of H2 and CO2 in the final synthesis gas increases, while the molar percentage of CO gas is reduced. This phenomenon can be justified that as the water vapor rate increases, the WG displacement reaction (CO 1 H2O!H2 1 CO2) is promoted, resulting in CO being consumed and more H2 and CO2 gases being produced. The molar percentage of CH4 gas in the final product also decreases with increasing vapor. Since by increasing the amount of water vapor, the vapor reforming reaction (CH4 1 H2O!H2 1 CO) is affected significantly, the molar percentage reduction of CH4 is expected.

9.3.7.2 Influence of inlet air rate on synthesis gas composition In conventional gasification reactors, the energy required for drying, pyrolysis, and unburned coal gasification is supplied by incomplete combustion of unburned coal. In plasma gasification reactors, heat from combustion is supplied by plasma air and high-temperature water vapor. Therefore the amount of air required for gasification in plasma reactors is lower than for other gas reactors. Increasing the air intake rate means it is near complete combustion in the gas reactor, which results in some combustible components of the gas mixture being consumed and reduced during the combustion process. In addition, increasing the amount of nitrogen entering the reactor will dilute the gas produced and reduce its thermal value. Therefore increasing the rate of inlet air will not have a favorable effect on the synthesis gas produced. Fig. 9.78 illustrates the effect of increasing water vapor rate on the percentage of synthesized gas. As it is known, the values of H2 and CO have initially increased, indicating incomplete combustion in this area. As the inlet air rate and the combustion rate in the gaseous reactor increase, combustible products are consumed, and their amount decreases. In contrast, complete combustion process products, including CO2, have increased. The N2 value has also increased as the inlet air rate increases with a relatively steep slope 0.45 0.4 Mole Fraction (%)

0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 1000

1500

2000

2500

3000

3500

4000

4500

5000

Plasma Air Flow Rate (kg/h) H2

CO

CO2

CH4

N2

Figure 9.78 Influence of inlet air rate on synthesis gas composition percentage [109].

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9.3.7.3 Influence of inlet air rate on thermal value of synthesized gas Fig. 9.79 shows the effect of the inlet air rate on the low thermal value of the synthetic gas (LHV). As can be seen, initially, with the increase in inlet air rate, despite the increase in H2 and CO, we see a decrease in the thermal value of the synthesized gas. The LHV reduction in this range can be related to the reduction of CH4 gas. Further, as the rate of inlet air increases, LHV decreases with a relatively steady slope, which is reasonably expected due to the dramatic increase in N2 in the output and dilution gas product as well as the decrease in H2 and CO.

9.3.7.4 Influence of plasma power on gasification process temperature High-temperature plasma air injection is the most critical factor affecting the plasma gasification process. Plasma air provides the required degree for the melting of inorganic MSW compounds. In addition, the residual heat provides the heat needed for gasification. Plasma power affects the energy balance of the entire gas system and directly affects the thermal profile, the gas composition, the amount of Tar produced, and the stability of the gasification process. Fig. 9.80 shows the effect of increasing plasma power on the temperature of the high-temperature gasification region, the low-temperature gasification region, and the average gasification temperature of the gasification process. Increasing the plasma power will increase the temperature of the inlet air to the reactor and thus provide more heat for the gasification process, increasing the gasification temperature.

9.3.7.5 Influence of plasma power on the composition of synthesis gas Fig. 9.81 shows the changes in the synthesized gas composition percentage produced by increasing plasma power. As can be seen, the molar percentage of H2 and CO gases 12000 Syngas LHV (KJ/m3)

11000 10000 9000 8000 7000 6000 5000 4000 0

1000

2000

3000

4000

5000

6000

Plasma Air Flow Rate (kg/h)

Figure 9.79 Influence of inlet air rate on the thermal value of synthesis gas [109].

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Hybrid Poly-generation Energy Systems

3000

Temperature (°C)

2500 2000 1500 1000 500 0 0

1

2

3

4

5

6

7

Plasma Power (MW) High Temp.

Low Temp.

Gasification Temp.

Figure 9.80 Influence of plasma power on the temperature of the gasification process [109]. 0.45

Mole Fraction (%)

0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0

1

2

3

4

Plasma Power (MW) H2

CO

CO2

CH4

Figure 9.81 Influence of plasma power on synthesis gas composition percentage [109].

increases with increasing plasma power. This phenomenon can be attributed to the increase in the thermal breakdown of unburned coal due to the increase in gasification temperature. It is also observed that the amount of CH4 decreases markedly with increasing plasma power. Since the CH4 reforming reaction with water vapor is an endothermic reaction, increasing the plasma power and gasification temperature makes the CH4 reforming reaction more intense, and consequently, CH4 is reduced.

9.3.7.6 The effect of plasma power on gasification efficiency Fig. 9.82 shows the gasification efficiency changes with increasing plasma power. As shown in Fig. 9.70, increasing the plasma power will decrease the percentage of methane in the synthesized gas. Consequently, with the decrease in methane gas content, the thermal value of the synthesized gas (LHV) also decreases; Therefore concerning Eq. (9.104), the CGE reduction is entirely justified.

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505

0.92 0.9

CGE (%)

0.88 0.86 0.84 0.82 0.8 0.78 0

1

2

3

4

5

6

7

Plasma Power (MW)

Figure 9.82 Influence of plasma power on gasification efficiency [109].

350

Tar Content (ppb)

300 250 200 150 100 50 0 1

2

3

4

5

6

Plasma Power (MW)

Figure 9.83 Influence of plasma power on tar production [109].

9.3.7.7 The effect of plasma power on tar production rate As mentioned earlier, one of the main benefits of plasma gasification is the elimination of tar from the synthesis process. Fig. 9.83 illustrates the effect of plasma power on the amount of tar in the synthesized gas produced. Obviously, with increasing plasma power and gasification temperature, the thermal breakdown of the tar increases, and its amount in the final synthesis gas decreases dramatically. The tar is eliminated by increasing the plasma power to more than 4 MW. Plasma gasification is called a tar-free process.

9.3.7.8 Effect of reaction conditions on methanol production This section presents the effect of temperature and pressure of a methanol synthesis reactor on CO conversion and methanol production. For the sensitivity analysis of

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Hybrid Poly-generation Energy Systems

Methanol Production (kmol/hr)

the methanol production process, 100,000 kmol/h of feedstock with the composition of 45% H2, 35% CO, 15% CO2, and 5% CH4 was used. As can be seen in Fig. 9.84, the conversion rate of CO and methanol production increases significantly with increasing reactor pressure. A decrease in mol accompanies the stoichiometric reaction of methanol synthesis. As a result, it can be said that the increase in pressure improves the reaction toward more methanol production, thereby increasing CO conversion and increasing methanol production. Although increasing pressure will favor methanol synthesis performance, 70 bar pressure is recommended for the methanol synthesis reactor due to economic constraints. The effect of reactor temperature changes on CO conversion and methanol production is shown in Fig. 9.85. As shown above, methanol production decreases exponentially as the reactor temperature increases. Therefore increasing the reactor temperature can be considered an undesirable change in methanol production. It should also be noted that due to the inactivation of the catalysts at high temperatures, the reactor temperature should be set below 250
C. 26500 26400 26300 26200 26100 26000 25900 25800 57.5

60

62.5

65

67.5

70

72.5

75

77.5

80

82.5

Reaction Press. (bar)

Methanol Production (kmol/hr)

Figure 9.84 Methanol synthesis reactor pressure versus methanol production rate [109]. 28000 27000 26000 25000 24000 23000 22000 21000 20000 140

150

160

170

180

190 200 210 220 Reaction Temp. (°C)

230

240

250

260

Figure 9.85 Methanol synthesis reactor temperature versus methanol production rate [109].

A framework for sustainable hydrogen production by polygeneration systems

9.4

507

GS reactor temperature effect on hydrogen production rate

Since the reaction in the WGS reactor is reversible and exothermic, Thus as the reactor temperature rises, the reaction shifts to the left, producing less hydrogen. This is clearly shown in Fig. 9.86.

9.4.1 Sensitivity analysis of total system efficiency 9.4.1.1 Impact of inlet water steam rate The influence of inlet steam rate on overall system efficiency was investigated. The result of this review is shown in Fig. 9.87. As shown previously, increasing the vapor input rate increases the H2/CO ratio in the synthesized gas. Increasing this ratio has a favorable effect on the production of methanol and hydrogen. Increasing the H2/CO

Hydrogen Production(Kg/hr)

113 112.5 112 111.5 111 110.5 110 190

200

210

220

230

240

250

260

270

280

290

300

310

Reaction Temp. (°C) Figure 9.86 WGS reactor temperature versus hydrogen production rate [109]. 0.7 0.69

Efficiency

0.68 0.67 0.66 0.65 0.64 0.63 0.62 300

400

500

600

700

800

900

1000

1100

Steam Flow Rate (kg/hr) Figure 9.87 Effect of water vapor rate on total system efficiency [109].

1200

1300

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Hybrid Poly-generation Energy Systems

0.7

Efficiency

0.65 0.6 0.55 0.5 0.45 0.4 1600

1800

2000

2200

2400

2600

2800

3000

3200

Plasma Air Flow Rate (kg/hr)

Figure 9.88 Influence of plasma air rate on overall system efficiency [109].

ratio will also increase the percentage of combustion components of the synthesized gas and thereby increase the generated power of the combined cycle. As a result, we will see overall system productivity increase as steam rates increase.

9.4.1.2 Influence of plasma air rate The impact of the plasma air rate on the overall system productivity is then investigated, which is shown in Fig. 9.88. One of the main effects of increasing the rate of inlet plasma air is the decrease in the H2/CO ratio of the synthesized gas with the increase in inlet air rate, which in turn reduces methanol production. However, with the decrease in methanol production, the amount of reactive synthesis gas returned to the combined cycle increases, which increases the power of the combined cycle. However, increasing the rate of inlet air will reduce the combustible components of the synthetic gas and thus reduce the cycle of power generation. In addition, plasma power also increases with increasing inlet air rate. All of these factors reduce overall system productivity.

9.4.1.3 Impact of mainstream split ratio Another vital process parameter to be considered is the mainstream split block, which sends the two streams to methanol and hydrogen cogeneration units and the combined cycle. Fig. 9.89 shows the effect of this parameter on the total system productivity. As it is known, the total system productivity increases with the increase of the split ratio. This is because methanol and hydrogen production will increase and positively impact overall system efficiency with the increase in the split ratio. Although increasing this ratio decreases the power generation capacity of the combined cycle, overall, the effects of the combined cycle power reduction and the increase in methanol and hydrogen production rates on overall system productivity are evaluated positively.

9.4.1.4 Impact of side stream split ratio Another important process parameter to be considered is the subcurrent split block that sends the two currents to the methanol and hydrogen production units.

A framework for sustainable hydrogen production by polygeneration systems

509

0.76

Efficiency

0.74 0.72 0.7 0.68 0.66 0.64 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Primary Split Fraction

Figure 9.89 Influence of mainstream split ratio on overall system productivity [109].

0.695

Efficiency

0.69 0.685 0.68 0.675 0.67 0.1

0.3

0.5

0.7

0.9

1.1

Secondary Split Fraction

Figure 9.90 Influence of substream split ratio on overall system efficiency [109].

Fig. 9.90 shows the effect of this parameter on the total system productivity. As it is known, the total system productivity increases with the increase of the split ratio. As hydrogen production increases as the split ratio increases, it will have an adverse effect on methanol production. Moreover, the amount of unreacted gas will decrease in the combined cycle, which reduces the power output of the combined cycle, but overall, given that the thermal value of hydrogen gas is about 6 times the thermal value of methanol, the effect of increased hydrogen on methanol reduction and The reduction in combined cycle power is overcome and overall system efficiency is increased.

9.4.1.5 Influence of return flow The next parameter to be considered is the effect of the return current. The impact of the return current on the overall system productivity is shown in Fig. 9.91. As mentioned earlier, part of the unreacted synthesis gas in the methanol production

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Hybrid Poly-generation Energy Systems

0.74

Efficiency

0.72 0.7 0.68 0.66 0.64 0.62 0.6 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Recycle Ratio

Figure 9.91 Influence of return flow on overall system efficiency [109].

process can be sent to the combined cycle to increase the overall system productivity through a reversible flow. This increases the production capacity of the gas turbine. Increasing the current return ratio as much as it would favor the combined cycle power generation would also have a detrimental effect on methanol production. Thus in general, the additive effects of power generation and the reduction of methanol production counteract each other to maintain overall system productivity in a certain amount.

9.4.1.6 Impact of WGS reactor temperature Since the reaction in the WGS reactor is reversible and exothermic, as the reactor temperature rises, the reaction shifts to the left, producing less hydrogen. As a result, increasing the temperature of the WGS reactor has a negative impact on the overall productivity of the system. After all, the reduction in hydrogen production is low and has little effect on the overall efficiency function. More information is shown in Fig. 9.92.

9.4.1.7 Impact of methanol synthesis reactor pressure The impact of methanol reactor pressure on overall system efficiency is shown in Fig. 9.93. As described in the previous section, increasing the reactor pressure increases the methanol production rate. On the other hand, the generated power is reduced due to the reduction of the unreacted synthesis gas and the reduction of the volume of combustion gases sent to the combined cycle.

9.4.1.8 Influence of temperature on methanol synthesis reactor The results of the evaluation of the effect of temperature on the system’s overall productivity are shown in Fig. 9.94. Increasing the reactor temperature reduces methanol production. However, due to the increase in the synthesis gas, the reaction volume will not increase, and the volume of gases sent to the combined cycle will

A framework for sustainable hydrogen production by polygeneration systems

511

0.69

Efficiency

0.685

0.68

0.675

0.67 220

230

240

250

260

270

280

290

WGS Reaction Temp. (°C)

Efficiency

Figure 9.92 Influence of WGS reactor temperature on overall system efficiency [109]. 0.7 0.695 0.69 0.685 0.68 0.675 0.67 0.665 0.66 0.655 0.65 55

60

65

70

75

80

85

Methanol Reaction Press. (bar)

Figure 9.93 Methanol synthesis reactor pressure vs. overall system efficiency [109].

increase the production capacity of the combined cycle. Thus overall, the cumulative effects of the combined cycle power generation and reduction in methanol production rate is that the system’s overall productivity is almost independent of the methanol synthesis reactor temperature.

9.4.1.9 Impact of combustion chamber temperature In addition, the effect of combustion protection temperature on overall system efficiency was evaluated. The result is shown in Fig. 9.95. As we know, by increasing the inlet temperature of the turbine, the output power of the turbine increases. As a result, the overall productivity of the system increases as the methanol and hydrogen production rates are constant. However, in selecting the combustion chamber temperature, it

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0.7

Efficiency

0.695 0.69 0.685 0.68 0.675 0.67 160

180

200

220

240

260

Methanol Reaction Temp. (°C)

Efficiency

Figure 9.94 Methanol synthesis reactor temperature vs. overall system efficiency [109].

0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 900

1000

1100

1200

1300

1400

1500

1600

Combustor Temp. (°C)

Figure 9.95 Combustion cycle combustion temperature versus overall system efficiency [109].

should be noted that chemical reactions in which the oxygen and nitrogen in the combustion air form thermal NOx. At a high temperature, a gas turbine burner occurs, in which, with increasing temperature, NOx production increases. Finally, we can optimize the process by examining the main variables affecting the process. These variables are: 800 # m_ Steam # 1200Kg=h 1650 # m_ PlasmaAir # 2000Kg=h 0:3 # SPPrimary # 0:7 0:3 # SPSecondary # 0:7

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513

0:3 # SPRecycle # 0:7 240 # TWGS2Reaction # 285
C 150 # TMETH2Reaction # 250
C 60 # PMETH2Reaction # 80 bar 1100 # TCombustor # 1500
C We also consider the following constraints for optimization: 250 # m_ METH # 1000 50 # m_ H2 # 200

Kg h

Kg h

2000 # Wnet # 7000KWh

9.4.2 Overall process efficiency optimization

Objective Function (Efficiency)

The function that must be optimized is energy efficiency. Using the optimization tool, we define the optimization variables and, using the model sensitivity analysis results, define the range of variations of each variable. The process constraints mentioned in the previous section are also considered by the constraint tool in optimization. The SQP algorithm worked out after seven iterations. The graph of the energy efficiency objective function value in terms of the SQP algorithm iteration is shown in Fig. 9.96. The optimal values of the operational variables with the energy efficiency objective function are shown in Table 9.25. 0.7 0.695 0.69 0.685 0.68 0.675 0.67 1

2

3

4

5

6

7

Iteration Number Figure 9.96 The value of the energy efficiency objective function in terms of the iterative SQP algorithm [109].

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Table 9.25 Optimal values of operational variables with the objective function of overall system efficiency. Unit

Final value

Variable type

Kg/h Kg/h   
C
C bar
C

1200 194/1656 515/0 676/0 312/0 042/246 649/217 66 53/1274

Inlet steam rate Plasma air rate Mainstream split ratio Substream split ratio Return flow ratio WGS reactor temperature Methanol reactor temperature Methanol reactor pressure Combustion chamber temperature

Table 9.26 Results of single-objective optimization of overall system efficiency. Unit

Optimal value

baseline base value

Objective functions

% Kg/h Kg/h MWh MWh

42/69 06/345 07/152 29/6 49/0

17/68 43/528 52/113 77/6 52/0

KWh

75/3242

19/3535

Overall System Productivity Methanol production rate Hydrogen production rate Combined cycle generating power Thermophotovoltaic power generation Plasma power consumption

As a result of this optimization, energy efficiency will increase by more than 1%, from 69.17% to 68.42%. More information about the result of this optimization is shown in Table 9.26. This section explores the process of multiple methanol, hydrogen, and electricity generation in terms of energy efficiency. Then, before optimization, model sensitivity analysis was performed to evaluate system behavior. After that, the optimization variables and constraints were defined using an optimization tool. Using the model sensitivity analysis results, each variable’s range of variations was determined, and finally, the optimization program with The SQP algorithm was implemented. As a result of this optimization, energy efficiency will increase by more than 1%, from 69.17% to 69.42%.

9.4.3 Hydrogen from solar-wind-based polygeneration systems The five-stage Zn-S-I thermochemical cycle is a complete and flexible system in which the number of units of Zn unit is reduced. In order to reduce the heat load of the system using an energy analyzer, the system is integrated, and half of the heat

A framework for sustainable hydrogen production by polygeneration systems

515

SEP7 407A

408

407 E407 406 SEP6 405

403

E405

405A

SEP8 R401

CO2

CO2-A

R402 401

404A

402A

402 E402

E-CO2

SEP5

R201

CO

CO

E-CO

I2-A

205A

206

404

E203

I2

205

E-I2 208

208A H2

E204 106A

207

106

SEP4

E104

204

H101

SEP1

R102

R101

SEP2

H201 201

102

101

H2O

102*

103

104

103A

105

105A E103

E102

E101

202 203

PUMP

203A

203B E202 O2 SEP3

R103

110A 107A

107

108

H301

E301

E105

303A

303

301A

301

R301

E302

109 302 110

300

304

E107

Figure 9.97 Zn-S-I process general schematic [114].

load required by the process is reduced. In order to improve the thermal efficiency, we have modeled the solar collector dish with MATLAB, and the equations are summarized.

9.4.3.1 Zn-S-I thermochemical cycle Thermochemical cycles have less greenhouse gas emissions during hydrogen production, and among all thermochemical cycles, Cu-Cl, S-I, and Zn-S-I have promising cycles with environmental effects. The Zn-S-I cycle increases the system’s thermal efficiency due to its low operating temperature and is newer than other cycles. Due to the mentioned advantages, we have chosen this cycle to simulate and perform thermal integration. A five-step cycle is chosen among the types of Zn-S-I thermochemical cycles because it is short and flexible to operate. As a result, the Zn-S-I thermochemical cycle was simulated with ELECNRTL and SRK equation of state, which describes the cycle’s process and expresses the system’s flow characteristics. Fig. 9.97 shows a block diagram and Fig. 9.98 shows a general schematic of the hydrogen production process in the Zn-S-I thermochemical cycle.

9.4.3.1.1 Bunsen system The first reaction in the Bunsen reactor (9.110) is the Bunsen reaction, a solution of water, SO2, and iodine, which occurs at a conversion ratio of SO2 to a value of 1 and a temperature of 85
C. The products are separated into two phases: upper H2SO4 and lower HI. The output stream has two phases of immiscible liquid, which

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Figure 9.98 Zn-S-I process general block diagram [114].

have low but negligible impurities in both phases. The lower phase of HI in the treatment reactor returns SO2 to the Bunsen reactor during the reaction with a conversion ratio of H2SO4 to a value of 1, which facilitates the reaction to return the material to the Bunsen reactor (9.111) and the remaining reaction products are sent to the HIX system with some unreacted HI. In the third reactor, the Bunsen system reacts (9.111) with a conversion ratio of HI to a value of 1, and the reaction products are reacted with some H2SO4 and sent to the sulfuric acid system. The block diagram of the Bunsen system is shown in (Fig. 9.99). Table 9.25 shows the flow characteristics, Table 9.26 shows the equipment settings, and Table 9.27 shows the process reaction of the Bunsen system. I2 1 2H2 O 1 SO2 ! 2HI 1 H2 SO4

(9.110)

2HI 1 H2 SO4 ! I2 1 2H2 O 1 SO2

(9.111)

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517

358 K, 1 Bar From-Zinc System Mol% H2O: 100 8.568 Kmol/hr

H2 O

Bunsen Reactor

298 K, 1 Bar Mol% H2O: 100 1.8 Kmol/hr

O2 358 K, 1 Bar Mol% O2:100 5.184 Kmol/hr

From-HIx System

358 K, 1 Bar Mol% H2O: 42.53 Mol% HI: 78.63 Mol% I2: 49.6 391.464 Kmol/hr

HI, H2SO4 358 K, 1 Bar Mol% H2O: 46.2 Mol% HI: 12.9 Mol% I2: 37 Mol% H2SO4: 39.1 503.676 Kmol/hr

From-H2SO4 System 358K, 1 Bar Mol% H2O: 82.37 Mol% I2: 0.62 Mol% SO2: 9.87 Mol% O2: 4.75

Mol% H2SO4: 2.39

L-L Separator

358 K, 1 Bar From-Zinc System Mol% I2: 100 8.568 Kmol/hr

HI 358 K, 1 Bar Mol% H2O: 37.5 Mol% HI: 15.62 Mol% I2: 45.34 Mol% H2SO4: 1.53 410.4 Kmol/hr

H2SO4 358 K, 1 Bar Mol% H2O: 84.47 Mol% HI: 0.89 Mol% I2: 0.29 Mol% H2SO4: 14.35 93.276 Kmol/hr

Purificaon Reactor

Purificaon Reactor

To-HI System 373 K, 1 Bar Mol% H2O: 40.57 Mol% HI:12.55 Mol% I2: 46.87 410.4 Kmol/hr

To-H2SO4 System 403 K, 1 Bar Mol% H2O: 85.99 Mol% H2SO4: 14.01 92.592 Kmol/hr

109.245 Kmol/hr

Figure 9.99 Bunsen system diagram block [114]. Table 9.27 Bonsen unit process reaction [114]. Process

Description

No.

I2 1 2H2 O 1 SO2 ! 2HI 1 H2 SO4 2HI 1 H2 SO4 ! I2 1 2H2 O 1 SO2

Bunson reactor Purification reactors

R101 R102, R103

9.5

Sulfuric acid system

According to the block diagram of the sulfuric acid system shown in Fig. 9.100, the system consists of a flash tank and a decomposition reactor. The sulfuric acid concentration entering the reactor must be increased to ensure good decomposition performance. Due to the greater volatility of water than H2SO4, the concentration of H2SO4 increases in a flash tank by evaporation at 137/152K and under atmospheric pressure. The concentrated sulfuric acid is heated before entering the decomposition reactor, and the solution is assumed to evaporate and decompose to SO3 and steam (E302). The reactor decomposes at 850
C and a pressure of 1 bar with a conversion ratio of H2SO4 of 0.8, according to the reaction (9.110) to sulfur dioxide and oxygen. The remaining SO3 is converted to sulfuric acid with water during cooling until it reaches the Bunsen system. 2H2 SO4 ! 2SO2 1 O2 1 2H2 O

(9.112)

Hybrid Poly-generation Energy Systems

H2O, H2SO4

H2 O Decomposion reactor Concentrated H2SO4 410.152 K, 1 Bar Mol% H2O: 30.60 1123 K, 1 Bar Mol% H2SO4: 69.40 410.152 K, 1 Bar Mol% H2O: 46.99 18.67 Kmol/hr Mol% H2O: 99.97 Mol% SO2: 30.29 Mol% H2SO4: 0.03 Mol% O2: 15.15 73.92 Kmol/hr Mol% H2SO4: 7.57 To-Bunsen reacon 34.23 Kmol/hr

410.152 K, 1 Bar Mol% H2O: 85.99 Mol% H2SO4: 14.01 92.592 Kmol/hr

681 K, 1 Bar Mol% H2O: 82.37 Mol% I2: 0.63 Mol% SO2: 9.87 Mol% O2: 4.75 Mol% H2SO4: 2.39 109.245 Kmol/hr

Flash Tank

518

Figure 9.100 Block diagram of sulfuric acid system [114].

Table 9.28 HIX system process response [114]. Process

Description

No.

2HI ! H2 1 I2

Decomposition reactor HI Distillation tower HI

R201 H201

9.5.1 HIX system In the hydrogen iodide system, we have a pressure increase of up to 11.7 bar, which is supplied by the pump at the beginning of the system, and the temperature of the inlet feed to the distillation tower remains at the bubble point. It is assumed that the distillation tower operates under ideal conditions and has no pressure drop during the process. According to this assumption, the distillation parameters are calculated. The top product of the distillation tower is pure HI steam, which has the same temperature as the inlet feed of the distillation tower, and the lower product of the tower is the liquid solution which is returned to the Bunsen system. Half of the HI vapor at the top of the tower is transferred to the system and the other half is converted to hydrogen by reaction (9.113) in a decomposition reactor with an HI conversion ratio of 0.21 and a temperature of 450
C. The reactor’s output is 0.5 mol/s H2, which is taken as the output of the system, and the generated I2 is returned to the system. The HIx system process schemas are shown in Figs. 9103 and Table 9.28 system equipment settings and unit reactions, respectively (Fig. 9.101). 2HI ! H2 1 I2

(9.113)

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519

H2 313 K, 1 Bar Mol% H2: 100 1.8 Kmol/hr

HI decomposion reactor

To-Zinc System

313 K, 11.7 Bar Mol% HI: 100 34.272 Kmol/hr

316 K, 11.7 Bar Mol% H2O: 39.11 Mol% HI: 15.28 Mol% I2: 45.61 425.736 Kmol/hr

300 K, 1 Bar Mol% H2O: 40.57 Mol% HI: 12.55 Mol% I2: 46.87 410.4 Kmol/hr

disllaon column

723 K, 11.7 Bar Mol% HI: 88.26 Mol% I2: 11.74 15.336 Kmol/hr

313 K, 1 Bar Mol% HI:100 17.136 Kmol/hr

.

To-Bunsen System

471 K, 11.7 Bar Mol% H2O: 42.53 Mol% HI: 7.86 Mol% I2: 49.6 391.464 Kmol/hr

Figure 9.101 HIX system diagram block [114].

9.5.2 Zinc system The mentioned advantages of the five-stage thermochemical cycle, in addition to the improvement in thermal efficiency in case of suitable operating conditions, are the reduction of the number of units of the zinc system unit. As shown in system block diagram of Fig. 9.105, the system consists of two reactors and some utilities. The CO2 introduced into the ZnI2-CO2 reactor, reacted through reaction (9.114) with a conversion ratio of 0.42 ZnI2 and CO2 to a value of 1, and CO as the output product and I2 after cooling to the Bunsen system and ZnO and some unreacted ZnI2, are sent to the ZnI2 production reactor (Fig. 9.102). The reaction (9.115) that takes place in the ZnI2 production reactor with HI inputs from the HIX and ZnO units and ZnI2 from the reactor (R401) and the conversion ratio of HI to a value of 1, produces an aqueous solution of ZnI2. The final stage of the drying system is the ZnI2 aqueous solution, where the separated water is returned to the Bunsen system, and ZnI2 is returned to the reactor (R401). In order to understand the details of flow and settings of equipment and process reactions, we can refer Table 9.29. ZnI2 1 CO2 ! ZnO 1 CO 1 I2

(9.114)

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Hybrid Poly-generation Energy Systems

To-Bunsen System

313 K, 1 Bar Mol% H2O: 100 25.704 Kmol/hr

H2 O

ZnI2

H2O, ZnI2 313 K, 1 Bar Mol% H2O: 58.33 Mol% ZnI2: 41.67 58.752 Kmol/hr

313 K, 1 Bar Mol% ZnI2: 100 24.48 Kmol/hr

CO2 298 K, 1 Bar Mol% CO2: 100 8.568 Kmol/hr CO 313 k, 1 Bar Mol% CO: 100 8.568 Kmol/hr 1073 K, 1 Bar Mol% I2: 100 8.568 Kmol/hr

I2

To-Bunsen System

ZnI2-CO2 reactor

ZnO, ZnI2 1073 K, 1 Bar Mol% ZnO: 0.35 Mol% ZnI2: 0.65 24.48 Kmol/hr

ZnI2 producing reactor

From-HI System

313 K, 1 Bar Mol% H2O: 100 8.568 Kmol/hr

313 K, 1 Bar Mol% HI: 100 17.136 Kmol/hr

Figure 9.102 Zinc system block diagram [114].

Table 9.29 Zinc system output reaction [114]. Process

Description

No.

ZnI2 1 CO2 ! ZnO 1 CO 1 I2 ZnO 1 2HI ! ZnI2 1 H2 O

Reactor ZnI2-CO2 Reactor ZnI2

R401 R402

ZnO 1 2HI ! ZnI2 1 H2 O

(9.115)

9.5.3 Solar system A solar power plant consists of a parabolic concentrator dish to direct and focus sunlight on the dish’s focal point and a receiver containing heat transfer fluid at the focal point to convert sunlight into heat energy. Modeling of solar dish collectors has been done using MATLAB. Because the receiver aperture area is much smaller than the focus area, only radiation beams will be effective. As a result, the concentrator focuses only on the radiant beams on the absorber surface, and the rest of the beams are scattered in the air. Fig. 9.103 shows an overview of parabolic dish collector beams.

A framework for sustainable hydrogen production by polygeneration systems

Sun rays Qs

521

QL

Receiver Qr

Qu

Dish paraboa

Figure 9.103 Overview of parabolic dish collector beams [114].

A parabolic concentrator with a length of (L) of 0.254 m and a rim angle of 48.16 has been selected, and the collector area can be calculated using Eqs. (9.116)(9.118). f 1 5 d 4 3 tan ψ2rim p5

23f 1 1 cosψrim

Aa 5

2 π
2 3 p 3 sinψrim 4

(9.116)

(9.117)

(9.118)

where d, F, ψrim, and p are the aperture diameter, focal length, rim angle, and parabolic radius, respectively. Their values are shown in Table 9.30. There are many relationships to calculate the area of the collector opening, the relation (9.118) has been used to calculate the area. Power dissipation at the focusing level occurs due to light losses throughout the focusing surface. Therefore optical efficiency is defined as the ratio of the power generated by the collector to the force from the sun, which depends on the reflection of the reflective material. Focusing light efficiency can be calculated by the following equation: η0 5 λρατγ

(9.119)

where λ, ρ, α, τ, and γ are shadow absence factor, mirror reflection coefficient, receiver absorption coefficient, glass coating transfer coefficient, and interception factor, respectively. Other power losses occur in the receiver and collector. Hence, the receiver efficiency (ηr) and the collector efficiency (ηc) can be defined as follows:

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Hybrid Poly-generation Energy Systems

Table 9.30 Heat exchanger settings of the integration system [114]. Number of sides

LMTD (K)

Heat duty (kW)

Heat exchanger no.

2 2 2 2 2 2 2 2 2 2 2

37/26 12/26 114/26 12/02 101/47 85/02 25/06 685/86 37/50 178/69 39/72

2/26 0/37 416/53 122/16 5/85 56/41 12/97 9/3 935/26 33/29 14/94

HX1 HX2 HX3 HX4 HX5 HX6 HX7 HX8 HX9 HX10 HX11

LMTD, Logarithmic mean temperature difference.

ηr 5

Qu Qr

(9.120)

ηc 5

Qu Qs

(9.121)

where Qu is the useful thermal power that reaches the receiver. In addition, the practical power can be obtained by reducing the power dissipation in the receiver (Ql) from the power obtained from the receiver (Qr) according to Eq. (9.122), and the amount of power from the sun’s rays reaching the surface of the dish can be calculated using Eq. (9.123), where Qs is the power obtained from the dish’s surface (W), Is is the amount of solar radiation reaching the concentrating surface (W/m2), and Aa is the area of the parabolic concentrating surface of 12.82 m2. Qu 5 Qr 2 Ql

(9.122)

Qs 5 Is Aa

(9.123)

The power obtained from the receiver (Qr) is the product of the light output multiplied by the power obtained from the surface of the dish, which is as follows: Q r 5 Q s η0

(9.124)

Waste power in the receiver is caused by heat transfer through conduction, transmission, and radiation and can be described as follows: Ql 5 Qlk 1 Qlc 1 Qlr

(9.125)

In the equation, Qlk, Qlc, and Qlr mean the power dissipated from the receiver through conductive heat transfer, convection, and radiation, respectively. A transparent window is provided in the receiver opening to prevent airborne dust, which

A framework for sustainable hydrogen production by polygeneration systems

523

can prevent forced movement to some extent. In addition, conductive heat loss has been neglected due to its small contribution compared to convection and radiation. Qlk 5 0 It is noteworthy that the effect of wind is neglected in calculations, and the dissipative power of the receiver through convection heat transfer is calculated as follows: Qlc 5 hc Aw ðTr 2 Ta Þ

(9.126)

where hc is the convection heat transfer coefficient (W/m2K) whose relationship is described in Eq. (9.127), Aw is the area of the internal cavity of the receiver (m2), Tr is the temperature of the receiver (
C), and Ta is the ambient temperature (
C). hc 5 Nul K=L

(9.127)

where Nul is the Nusselt number, whose relation is calculated as follows, K is the thermal conductivity of the ambient air (W/mK), and L is the length of the parabolic concentrator.
0:18
s
 Nul 5 0:10Gr l 1=3 Tr =Ta 4:256Ac =Aw h ϕl

(9.128)

In the Nusselt relation, Grl, Ac, h(ϕl), and ϕl, are, respectively, Grashof number, receiver aperture surface area (m2), convection heat transfer coefficient between the absorber and ambient air (W/m2K), and cavity collision angle in which their equations are as follows:
 Gr l 5 gβ Tr =Ta L3 =ν 2

(9.129)


0:5 s 5 0:56 2 1:01 Ac =Aw

(9.130)

Ac 5 Aa =C


 h ϕl 5 1:1677 2 1:0762sin ϕl 0:8324

(9.131) (9.132)

where ν, β, and C are the kinematic viscosity of ambient air (m2/s), the volume expansion coefficient of ambient air (1/K), and the concentrating geometric ratio, respectively. In addition, the receiver power dissipation due to radiant heat transfer (Qlr) is calculated using the following equations.
 Qlr 5 Ac εeff σ Tr 4 2 Ta 4 εeff



  1 Ac 5 1= 1 1 21 εc Aw

(9.133) (9.134)

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Hybrid Poly-generation Energy Systems

where εeff is the effective infrared emission from the cavity and εc is the surface emission of the cavity. In order to calculate the power received from the dish, the total solar radiation is required in the horizontal plane, which is obtained according to the following equation: πH Is 5 ð0:409 1 0:5016sinðωs 2 60Þ 1 ð0:6609:0:476sinðωs 2 60ÞcosωÞÞ 24 ! cosω 2 cosωs 3 (9.135) s sinωs 2 2πω 360 cosωs In this relation, H is the total daily radiation, which is defined by the daily cleaning index KT, which defines the ratio of radiation for a given day to extraterrestrial radiation (H0) for the same day: H (9.136) KT 5 H0 where H0 is the outdoor radiation (W/m2) which is calculated based on the following equation, and φ is the latitude specified for a city. H0 5

24 3 3600 3 Gon πωs cos [ cos δ sin ωs 1 sin [ sin δ π 180

(9.137)

where Gon is a function of extraterrestrial radiation that is irradiated to a normal level on the nth day of the year, which the following equation can calculate:

360n Gon 5 Gsc 1 1 0:033 cos (9.138) 365 where Gsc is a solar constant whose value is 1367 (W/m2) and δ is the declination. The angle between the earth-sun and the equatorial axis varies throughout the year and is obtained by the following equation. In both relations, n represents the day of the year.

284 1 n δ 5 23:45 sin 360 (9.139) 365 ωs is the angle of the clock at sunset, the following equation is used to calculate it: ωs 5 cos21 ð 2 tan [ tan δÞ

(9.140)

where ω is the angle of the clock to degrees for a given time that the following equation can calculate: 


 ω 5 15 60h 1 9:87 sin 2 3 360ðn 2 81Þ=364 2 7:53 cos 360ðn 2 81Þ=364

 2 1:5 sin ðn 2 81Þ=364 2 4 120 2 Llog =60Þ 2 12

(9.141)

A framework for sustainable hydrogen production by polygeneration systems

525

The values considered for the parameters of the collector dish to calculate the outlet temperature of the fluid and to evaluate the amount of heat supply are given previously. The outlet temperature of the collector is calculated based on the following equation:
 Tout 5 Tin 1 Qu =m Cp

(9.142)

Solar fraction is the amount of heat load provided by the solar system to the total heating load required by the process, which is calculated according to the following equation: SF 5

Qu Qh

(9.143)

Thermal efficiency is the ratio of optimal energy to the energy consumed according to Eq. (9.144). This parameter is selected to evaluate the whole system and electricity generation is not considered: Efficiency 5

mH2 3 EH2 :HHV 1 mCO 3 ECO:HHV 3 100 Hheat

(9.144)

where mH2 and mCO are hydrogen and carbon monoxide mass flow rates in kg/s, respectively. EH2,HHV and ECO. HHV are the higher heating values of hydrogen and CO, respectively, in kJ/kg, and Hheat is the denominator of the thermal load of the whole system (kW).

9.5.4 Process integration The process integration takes place in the energy medium of the analyzer according to the specific heat capacity, which can also be calculated through enthalpy. In order to reduce the thermal load of the process, the integration takes place in two stages.

9.5.4.1 Thermochemical cycle integration In order to achieve the desired thermal load, the hot and cold streams of the cycle are determined. The characteristics of hot and cold streams include inlet and outlet temperatures, enthalpy of inlet and outlet, and molar flow rate, which are identified. Adjacent streams provide the required thermal load of hot and cold streams in the heat exchanger. Utility streams are applied to hot and cold streams if the heat exchanger is not economical or practical. The utility shows the maximum possible amount of heat recovery during the process for the minimum temperature difference specified. The total energy input to the system is 8023 kW, of which 3685 kW is in bunsen, 737 kW in sulfuric acid, 998 kW in hydrogen iodide, and 2422 kW in zinc. The hot and cold utility of each subsystem before integration is shown in Fig. 9.104, and

526

Hybrid Poly-generation Energy Systems

Figure 9.104 Thermal charge of the ZnSI thermochemical cycle diagram [114].

Figure 9.105 Zn-S-I thermochemical cycle thermal load integration diagram [114].

Fig. 9.105 shows the hot and cold utility after two separate and general integration stages. Due to the hot and cold streams in the cycle, there is a need for heating and cooling. The heating load is the amount of heat energy required to obtain the process temperature to be added to the system, and the cooling load is the amount of energy required to be removed from the system to keep the temperature within acceptable limits. To

A framework for sustainable hydrogen production by polygeneration systems

527

SEP7

408

406

407

SEP6

HX9 405

403

E405

405A

SEP8

R402

402 402A

R401 CO2-C

404A

402B

401 E402

E-CO2 CO2

HX6 CO

CO

I2-B E-I2-A

HX7

CO2-A

CO2-B I2-A

I2-C

SEP5

HX8

R201

E203-A

407A

205A

206 I2

106B 106 R102

102

102*

HX5

104

201

105

202

107A

203

PUMP

203A E202

107B

110A

110C

HX3

H201

105B E103-A

203B

103 103A

H2

204 105A

107

E204-A

205 SEP4

SEP2

HX2

O2

208B

H2O

101 106A

H101

SEP1

208A

207

HX1

E104-A

R101

404

205A 208

SEP3

R103

H301

HX4 107C

108

E301

E105-A

303A

303

301A

301

R301 E302

109 302

110B E107-A

110

300

304

Figure 9.106 Schematic of separate system integration [114].

calculate the heat load of process heat exchangers, including heaters and coolers, all inlet and outlet temperatures, pressures, and flow rates must be checked. The thermal integration process is performed in such a way that we first examine each subsystem separately and then consider all the subsystems together. By performing different heat integration, nine heat exchangers have been added, which has removed three heaters and one cooler from the process. This reduces the heat load of the remaining air conditioners and heaters. The exchangers added to each subsystem to improve the process are as follows: Bunsen system four heat exchangers, HI system one heat exchanger, system on four heat exchangers. Fig. 9.106 shows a schematic of a separate integration system, and Table 9.30 presents the settings of the integration system of applied heat exchangers.

9.5.4.2 Thermochemical cycle integration with a solar unit The temperature of the inlet fluid to the collector is considered to be 300
C, and the outlet temperature is converted to heat after receiving solar energy and enters a

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Hybrid Poly-generation Energy Systems

H2, CO, CO2

Thermochemical cycle Solar Dish Collector Thermal Integraon

Auxiliary Heater

Figure 9.107 General schematic of thermochemical cycle integration with solar collector [114].

utility exchanger to enter the Zn-S-I cycle at a constant temperature of 810
C. The temperature of the collector fluid decreases after returning the heat load of the cycle heaters and returns to the collector path. Fig. 9.107 shows a general schematic of the thermochemical cycle integration with the solar collector. Collectors are used in a parallel formation. The flow path of the fluid with a specific flow rate and temperature is from the high-temperature heater to the lowtemperature heater, respectively. Due to the intersection of the streams inside the heat exchanger, the heat load of the Zn-S-I cycle heaters is reduced. The integration of this stage has been done in two ways, which have almost the same results but differ in terms of economic estimation. Figs. 9.108 and 9.109 show the first and second integration state of the system. As mentioned in previous chapters, the system is the ZnSI thermochemical cycle, which has undergone thermal integration to improve the process and reduce the required thermal load. Following the reduction of the heat load of the remaining heaters, we have coupled the cycle with a solar collector. This section analyzes the results and analyses performed. The amount of heat load due to thermal integration and the energy consumption of each unit have been investigated, and the profiles of output temperature and heating load have been analyzed.

9.5.5 Energy consumption per each unit Fig. 9.110 shows the amount of energy consumption of each unit in which the hot and cold heat load of each subsystem is presented separately. Further process heat

A framework for sustainable hydrogen production by polygeneration systems 405 T= 1033.4 K

From Dish T=1083 K

CO2 T= 920.865 K

1

405A T= 1073 K

205 T= 448 K

2

CO2-A T= 1073 K

303 T= 410.152 K

107 T= 375.628 K

3

529

107A T= 403 K

205A T= 723 K

To Dish T= 573 K

5

4

303A T= 466 K

From Dish T=1083 K

To Dish T= 774.4 K

6

303B T= 1082 K 303C T= 1123 K

E302

Figure 9.108 First state integration between collector and cycle currents [114]. 405 T= 1033.4 K

From Dish T=1083 K

1

405A T= 1073 K

CO2 T= 920.865 K

2

CO2-A T= 1073 K

303 T= 410.152 K

205 T= 448 K

3

4

303A T= 1062 K

From Dish T=1083 K

6

303B T= 1082 K 303C T= 1123 K

107 T= 375.628 K

To Dish T= 573 K

5

205A T= 707 K

107A T= 378 K

To Dish T= 774.56 K

8

7

205A T= 723 K

107A T= 403 K

E302

Figure 9.109 Second state integration between collector and cycle currents [114].

is required in the decomposition of SO3 in the H2SO4 system, reboilers in HI distillation, purification in the Bunsen system, and zinc system reactors.

9.5.6 Thermochemical cycle integration As explained earlier, QHmin and QCmin are the least heat taken or given by hot and cold utilities obtained at the lowest specified temperature (ΔTmin). Cold utilities are responsible for the system’s maximum heat duty, and with integration, 1.56 MW of each hot and cold utilities is reduced. In addition, the integration is calculated depending on the enthalpy and temperature range of hot and cold flow. In order to investigate the heat load of each subsystem, the heat load of each step is given in Fig. 9.111. In this stage, the maximum heat load required by the process is in the zinc subsystem, which is due to bringing the streams to the operating temperature of the process reactors. In the Bunsen system, the treatment

530

Hybrid Poly-generation Energy Systems

Consumpon Load 3000

Thermal load (kW)

2500

2000

1500

1000

500

0 Bunsen

H2SO4 Qh

HI

Zn

Qc

Figure 9.110 Energy consumption of each subsystem [114].

4500 4000

Thermal load KW

3500 3000 2500 2000 1500 1000 500 0 bunsen HI

simulaon

single-integraon

2422.1456

394.2727

overall-integraon 348.9267

998.208

985.6596

971.5662

H2SO4

737.2579

737.2579

704.0134

Zn

3865.0735

2782.9417

2782.9417

Figure 9.111 Heat load of subsystems at each stage [109].

A framework for sustainable hydrogen production by polygeneration systems

531

Table 9.31 Heat duty of each step [114]. Heat duty (MW) 10/18 16/67 17

Main system Separate integration General integration

operation requires a high heat duty, so this unit also includes a high heat duty. In the second stage, which is the separate integration stage of the process, we have the most significant reduction in thermal load. At this stage, each subsystem has heat exchange with its single streams and depending on the temperature range and the enthalpy of the currents of each subsystem, the degree of integration is calculated. The greatest reduction in thermal load occurs in the Bunsen system. As shown in the diagram, during the overall integration stage of the process, the system’s thermal load remains constant and a slight change in the thermal duty of other systems occurs. Thermal efficiency is expressed as a criterion for the overall evaluation of the system. According to the relationship expressed in the previous chapter, thermal efficiency is related to the thermal duty of the system. Reducing the thermal duty of the system increases thermal efficiency. The calculated values of the thermal efficiency of each step are reported in Table 9.31.

9.5.7 Thermochemical cycle sensitivity analysis Sensitivity analysis performed in other papers focuses on simulated cycle analysis, which examines three effective performance parameters to evaluate thermal efficiency, including ZnI2 decomposition ratio, bunsen product composition, and HI concentration in the ZnO-HI reaction. We have now examined these three parameters in the integrated system. Due to the elimination of the ZnI2 decomposition reactor in the simplified zinc system, the ZnI2 decomposition ratio parameter is not considered in the sensitivity analysis. Only by changing the conversion ratio of the Bunsen reactor can the composition of the Bunsen products, which is considered one of the effective parameters, be changed, and the effect of which changes on the heat duty of the system can be investigated. The SO2 conversion ratio is considered 100 in the Bunsen simulated reactor, reducing the heat duty and increasing the thermal efficiency. Changes in heat duty and thermal efficiency due to conversion ratio changes are shown in Figs. (9.112) and (9.113), respectively. The change in the HI conversion ratio in the ZnO-HI reactor increases the concentration of hydrogen iodide, which destroys the mass balance of the reactor, thus not being included in the sensitivity analysis. Sensitivity analysis has been performed to determine the thermal efficiency in the thermal integration system. The goal is to find parameters that affect the amount of heat duty that can affect the system’s efficiency. The effect of changing the temperature

532

Hybrid Poly-generation Energy Systems

4910 heat-Fc

4900

Heat duty (kW)

4890 4880 4870 4860 4850 4840 4830 88

90

92

94

96

98

100

102

Conversion rao Figure 9.112 Heat duty due to change in the conversion ratio of Bunsen reactor [114]. 16.9

Thermal efficiency

16.85 16.8 16.75 16.7 16.65 16.6 88

90

92

94

96

98

100

102

Conversion rao Figure 9.113 Thermal efficiency due to change in reactor conversion ratio [114].

of the heat exchanger and the flow rate has been studied. Changing the temperature parameter only affects the system’s heat duty and does not change the flow rate. In the sensitivity analysis of heat exchanger temperature change and its effect on heat duty due to the temperature range of the exchanger, the only reportable sensitivity analysis flows 402 temperature change. In other cases, temperature change has little effect on heat duty. The process of temperature analysis in a heat exchanger is such that by changing the output temperature of the exchanger, the secondary flow temperature of the exchanger changes and causes a change in the heat duty of hot and cold streams.

A framework for sustainable hydrogen production by polygeneration systems

T-Qc

T-Qh

6000

533

T-Duty

5500

Heat duty (kW)

5000 4500 4000 3500 3000 2500 2000 1500 1000 200

300

400

500

600

700

800

900

1000

1100

(K)402 flow temperature Figure 9.114 Heat load due to change in flow temperature 402 [114].

0.17 0.165

Efficiency

0.16 0.155 0.15 0.145 T-Effi. 0.14 0

200

400

600

800

1000

1200

(K)402 flow temperature Figure 9.115 Thermal efficiency by changing the flow 402 temperature [114].

Temperature changes in heat exchanger nine have been investigated. With an increasing temperature of flow 402, the heat load of heater E402 and cooler E405 increases, thus increasing the heat load of the whole system. Heat exchanger analysis nine is shown in Fig. 9.114. The result of temperature changes is a reduction in the thermal efficiency of the entire system. Fig. 9.115 shows the changes in thermal efficiency due to temperature changes in flow 402.

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Hybrid Poly-generation Energy Systems

720000

Auxiliary heater heat duty (W)

710000 700000 690000 680000 670000 660000 650000 640000 630000 620000 610000 0

5

10

15

20

25

30

Time (hour)

Figure 9.116 Auxiliary heater heat duty overnight [109].

9.5.8 Parabolic solar collector A parabolic dish collector is a subset of a solar heat collector that receives the sun’s radiant energy to convert it into heat energy with the help of the operating fluid. Fluctuation of the auxiliary heater is due to measurement during the day and night. In order to better understand the heat duty profile, we present the auxiliary heater load overnight in Fig. 9.116.

9.5.9 Solar collector sensitivity analysis The parameters studied in collector sensitivity analysis are operating fluid flow rate and collector area. As the flow rate changed, the solar fraction factor, which is a function of solar radiation, remained unchanged. Also, the change in fluid flow does not affect the useful heat duty received from the receiver, which is according to the relationship between the difference in power dissipation from the power obtained from the receiver. Fig. 9.117 shows the sensitivity analysis based on the flow rate change and its effect on the solar fraction and the useful heat duty received from the receiver. Since the useful heat duty of the system remains constant, changing the flow rate has an inverse effect on the collector outlet temperature. Since the system was inspected at 8760 h per year, the sensitivity analysis was performed on the maximum outlet temperature of the collector and the average outlet temperatures in one year, which is presented in Fig. 9.118. Finally, due to the output temperature effect on the heat duty of the auxiliary exchanger, the analysis has been done. The lower the outlet temperature, the more

A framework for sustainable hydrogen production by polygeneration systems

535

6.00E+02

1.50E+05

4.00E+02

1.00E+05

3.00E+02

2.00E+02

5.00E+04 mass flow- Quseful

Solar fracon

Useful heat duty (w)

5.00E+02

1.00E+02

mass flow- SF 0.00E+00

0.00E+00 0

0.2

0.4

0.6

0.8

1

1.2

Flow rate (kg/s)

7.00E+02

3.70E+02

6.00E+02

3.65E+02

5.00E+02

3.60E+02

4.00E+02 3.55E+02 3.00E+02 3.50E+02

2.00E+02 mass flow-Tmax 1.00E+02

3.45E+02

mas flow- T out

Average outlet temperature (°C)

Maximum output temperature (°C)

Figure 9.117 The effect of the flow rate change on heat duty and solar fraction [114].

3.40E+02

0.00E+00 0.55

0.65

0.75

0.85

0.95

1.05

Flow rate (kg/s) Figure 9.118 Flow rate change effect on the collector maximum and average output temperature [114].

heat exchanger the auxiliary exchanger must provide. Fig. 9.119 shows the flow effects on the heat load of the auxiliary exchanger. In general, according to the analysis, with increasing the flow rate of the operating fluid, the outlet temperature of the collector decreases, which causes a higher heat duty than the auxiliary exchanger, which is not desirable for the process. Also,

Hybrid Poly-generation Energy Systems

1.20E+06

3.70E+02

1.00E+06

3.65E+02

8.00E+05

3.60E+02

6.00E+05

3.55E+02

4.00E+05

3.50E+02 mas flow-Q Auxiliary

2.00E+05

3.45E+02

mas flow- T out 0.00E+00

Average outlet temperature (°C)

Auxiliary exchanger heat duty (w)

536

3.40E+02 0

0.2

0.4

0.6

0.8

1

1.2

Flow rate (kg/s) Figure 9.119 Flow rate change effect on the auxiliary exchanger heat duty [114].

the lower the flow rate, the lower the temperature effect due to the integration with the cycle streams and the lower the temperature it can cover. The selected flow rate is considered according to the desired output temperature and proper coverage in the integration. The second parameter is the collector area, for which we have examined the effect of changing the collector area on the useful heat duty received. As the collector area increases, the solar energy received increases and produces more heat duty. Since the solar fraction has a direct relationship with the useful heat duty, increasing the heat duty directly affects the solar fraction. Fig. 9.120 shows the growing solar fraction and heat duty trend due to increasing collector area. On the other hand, the flow rate of the fluid is assumed to be constant, and according to the formula, the outlet temperature of the collector is directly related to the useful heat duty received and the collector area. As the collector area increases, the average and maximum outlet temperature increase, as shown in Fig. 9.121. By increasing the outlet temperature of the collector, the auxiliary exchanger produces less power and the heat load of the auxiliary exchanger is reduced. As a result of increasing the collector area and reducing the heat duty of the auxiliary exchanger, the profile of which is reported in Fig. 9.122. Finally, with increasing the collector area, the useful heat load received from the collector increases, which increases the system’s renewability and decreases the amount of heat load that the auxiliary converter must provide. The need for energy has increased because of the vision for the future to burn and reduce fossil fuels. CO2, as a greenhouse gas, causes global warming. In this sense, hydrogen is an energy carrier of alternative fuels. Hydrogen reduces the problems caused by fossil fuels. In order to develop an environmentally friendly and economical process, the raw materials for the process are considered stable and non-fossil to create a clean

537

6.50E+02

1.30E+05

6.00E+02

1.20E+05

Solar fracon

1.10E+05 5.50E+02 1.00E+05 5.00E+02 9.00E+04 4.50E+02 8.00E+04

Useful heat duty (w)

A framework for sustainable hydrogen production by polygeneration systems

area-SF 4.00E+02

7.00E+04

Area-Q useful 3.50E+02

6.00E+04 9

10

11

12

13

14

15

16

Area (m2)

3.70E+02

700

3.65E+02

600 500

3.60E+02

400 3.55E+02 300 3.50E+02 200 area- Tout 3.45E+02

100

area-Tout max 3.40E+02

Maximum output temperature (°C)

Average outlet temperature (°C)

Figure 9.120 Area change effect on solar fraction and applicable heat duty received [114].

0 9

10

11

12

13

14

15

16

Area (m2) Figure 9.121 Area change effect on the average and maximum output temperature [114].

by-product for the environment. The advantage of H2 is that it can be generated using thermal energy from water, and such a production line has a strong potential for realizing economical hydrogen. As mentioned in previous chapters, there are several ways to generate H2 using the thermal energy of water. The Zn-S-I thermochemical cycle has been considered as a promising candidate due to its novelty and greater flexibility than the S-I cycle. Zn-S-I cycle subsystems have a high heat duty due to the temperature of the process

538

Hybrid Poly-generation Energy Systems

3.70E+02

8.70E+05

3.65E+02

8.60E+05 8.55E+05

3.60E+02

8.50E+05 area-Qaux 8.45E+05

3.55E+02 Area-T out

8.40E+05

3.50E+02

8.35E+05 8.30E+05

3.45E+02

8.25E+05 8.20E+05

Average outlet temperature (°C)

Auxiliary exchanger heat duty (w)

8.65E+05

3.40E+02 9

10

11

12

13

14

15

16

Area (m2) Figure 9.122 Collector area effect on the heat duty of the auxiliary converter [114].

reactions. In order to make the system economical, we have reduced the amount of energy consumed through integration and reduced the heat duty of the remaining heaters by coupling it with a solar collector that is considered clean and renewable energy. Different integration in each subsystem has reduced the heat duty of the heaters and coolers of the cycle by half. Coupling of the thermochemical cycle with the solar dish system has resulted in the removal of process heaters. Based on the sensitivity analysis performed in the integrated cycle, the only factor affecting the heat duty and thermal efficiency is the change in the conversion ratio of the Bunsen reactor. Based on the collector sensitivity analysis, the change in current flow does not affect the solar fraction and the useful heat duty received from the receiver. However, due to the constant Qu, the output temperature of the collector decreases according to the relation with increasing flow rate. Based on the collector sensitivity analysis, the change in the collector area directly affects the solar fraction, the valuable heat duty received, and the outlet temperature.

9.5.10 Economic aspects and environmental impacts of hydrogen production by poly generation systems The cost of building a thermal plasma unit with a capacity of 750 tonnes of waste per day will be $150 million. The cost of constructing a TPD 300 unit of thermal plasma in Utah, Japan, was estimated at $50 million. It also cost about $100 million to build a TPD 600 thermal plasma unit in St. Lucie, Canada. Generally speaking, for units with a capacity of between 250 and 750 tonnes of waste per day,

A framework for sustainable hydrogen production by polygeneration systems

539

Construction Cost (M$)

400 350 300 250 200 150 100 50 0 0

500

1000

1500

2000

2500

3000

MSW Treatment (TPD)

Figure 9.123 Power model for estimating cost of manufacturing a thermal plasma unit [111].

0.170.22 million US$/TPD will be spent on construction costs. At capacities above TPD 2000, this cost is reduced to 0.13 million US$/TPD. These results show that if the thermal capacity of the process increases, the process of thermal plasma gasification will be more economically justified. Based on the above data, a linear or power model can be presented to estimate the cost of building thermal plasma units, which is considered a power model for this section. This model will estimate unit construction costs with acceptable accuracy of 10 tonnes/day to 2700 tonnes/ day. This model is shown in Fig. 9.123. Given the curve fitting, the power model cost estimate of the thermal plasma unit will be as follows: DCPG 5 0:5858 3 ðm_ MSW Þ0:8115

(9.145)

10 TPD # m_ MSW # 2700 TPD

9.5.10.1 Complete cooling unit The following model is presented for the complete cooling section. DCTQ 5 3:5818 3 M0:79 SYN;TQ;O ð$1000Þ

(9.146)

9.5.10.1.1 Complete gas clearing unit The direct cost of this unit is Selexol. The direct costs of this part of the relationship come from:

MSyn;s;i 0:98 NT;S 3 DCS 5 0:2136 3 ð$1000Þ (9.147) ð1 2 βH2 S Þ NO;S 0:835 # β HS # 0:997

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Hybrid Poly-generation Energy Systems

Figure 9.124 Cost of constructing a combined cycle with extended power [115].

In the above relation β H2 S is the sulfur removal efficiency. Assuming β H2 S 5 0:98 and NT;S 5 NO;S 5 1: DCS 5 0:3247 3 M0:98 Syn;s;i ð$1000Þ

(9.148)

Combined cycle section In this section, the cost of building a combined cycle unit was calculated using a power model [115]. Fig. 9.124 presents the cost of constructing a combined cycle with an extended power model. This powerful model calculates unit construction costs using the unit cost index (PCI), where the WCC is the combined cycle output power. More information is shown in Fig. 9.124. DC CC 5 1:6945 3 ðWCC Þ0:2009 ð$1000Þ

(9.149)

Methanol and hydrogen production unit Using an exponential model, the direct investment costs of these units are calculated [116]. This exponential model is defined as follows: k S C 5 C0 3 (9.150) S0 where S0 is the reference production capacity, S is the simulated unit capacity, C0 is the direct investment cost of the reference unit, k is the exponential coefficient of conversion cost, and C is the estimated investment cost for methanol and hydrogen production units. More information is shown in Table 9.32. It should be noted that the cost of the units is stated in million euros [116].

Table 9.32 Cost of direct investment of methanol and hydrogen generating units according to unit capacity [116]. Cost of installation per year2017 (M$)

Cost of installation per year 2010 (Mh)

Exponential conversion factor

Capacity

Cost scale

Process type

66/6

0/5

67/0

10.000

Synthesis of compressed gas

63/42

0/32

67/0

30.540

13/2

6/1

7/0

3331

79/16

6/12

67/0

1377

Compressor work (kW) Methanol production rate (Kg/hr) H2 flow rate (m3/hr) Inlet synthesis gas MWth

Methanol synthesis, return compressor, distillation, and purification Hydrogen production PSA section WGS reactor

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Hybrid Poly-generation Energy Systems

Consequently, the calculation of the initial investment cost per unit capacity for methanol and hydrogen production units will be as follows. Where mMETH is the methanol production rate in Kg/hr, and WCOMP is the consumed methanol unit compressor used for compressing the synthesis gas in KW, VH2 volumetric hydrogen flow rate, and PH2 heat input value of the hydrogen synthesis unit. DC METH:SYN 5 42161:1 3 ðm_ METH Þ0:67

(9.151)

DC METH;COMP 5 13918:1 3 ðWCOMP Þ0:67

(9.152)

DC H2:WGS 5 1293:9 3 ðPH2 Þ0:67

(9.153)

DC H2;PSA 5 7293:6 3 ðVH2 Þ0:7

(9.154)

Thermophotovoltaic system The proposed thermophotovoltaic system consists of 200,000 proposed InGaAsSb cells. The total cost of investing in a thermophotovoltaic system segment is calculated according to the relationship (9.155): DC TPV 5 NumCell 3 PriceCell

(9.155)

It should be noted that the price of each cell is US $50.

9.5.10.1.2 Process facility cost These costs include two parts: the direct costs resulting from the previous section’s relationships and the indirect costs for water cooling systems, air tools, operating water, and electrical systems. Indirect costs are calculated: Indirect costs 5 fIC 3

X

DCI

(9.156)

1

fIC 5 Indirect cost factor 5 0:2 The total cost of this section includes direct costs related to each of the sectors and indirect costs: PFC 5 ð1 1 fIC Þ 3

X

DCI

(9.157)

1

9.5.10.1.3 General facility cost Costs include road construction costs, office buildings, laboratories and costs of sales, transportation, and more. These costs account for 15%17% of the process costs. The costs related to this part are obtained from the following relation: GFC 5 fGF 3 PFC

(9.158)

A framework for sustainable hydrogen production by polygeneration systems

543

fGF 5 General facility cost factor 5 0:15

9.5.10.1.4 Engineering and home office fees The costs of this segment are usually 7%15% of the process costs. These costs can be derived from the following relationship: EHOF 5 fEHOF 3 PFC

(9.159)

fEHOF 5 0:1

9.5.10.1.5 Project contingency The costs for this part of the relationship are calculated as follows: PC 5 fPC 3 PFC

(9.160)

fPC 5 0:15

9.5.10.1.6 Total plant class Overall costs include direct costs, indirect costs of construction and construction, engineers and administrative staff salaries, sales tax, costs during construction, project event cost, and probable process cost, so the total costs are the overhead costs, given as follows: TPC 5 PFC 1 GFC 1 EHOF 1 PC

(9.161)

9.5.10.1.7 Operating and maintenance costs (variable and fixed) Fixed operating costs include operational workers, maintenance costs, maintenance personnel, costs related to executive and support staff, etc. Moreover, variable operating and maintenance costs include water consumption, catalyst consumption, replacement, electricity supply, etc. For synthetic gas cooling and purification units, hydrogen and methanol production, and the combined cycle, annual operating and maintenance costs (fixed and variable) can equal 5% of the total process costs. Since the thermal plasma unit is one of the new technologies, the existing models for calculating its operating costs may be very flawed. For this purpose, a detailed economic evaluation was carried out for the TPD 100 thermal plasma unit, including the total operating costs. The details of this evaluation are shown in Table 9.33. Operating costs include fixed costs, variable costs, and insurance costs. In fixed costs, labor costs, depreciation costs, and overhead costs such as employee perks, safe maintenance costs, training costs, daily expenses, and mission costs are included. Total fixed costs will be US $2.39 million/year. Variable costs include the cost of maintenance, the cost of electricity, the cost of chemicals, and the cost

Table 9.33 Economic evaluation of TPD 100 thermal plasma gasification unit. Cost of M $ (year 2012)

Cost of M $ (year 2012)

Type of costs

23/46

24/8

Construction cost

0/54 0/16

0/57 0/17

1/56

1/65

Workers 14 Advantages of workers Cost of safe maintenance The cost of education Daily expenses Mission costs etc Depreciation period 5 15 years

2/26

2/39

Total fixed costs

0/77

0/82

0/11

0/12

Cost of electricity supply Maintenance cost Chemical cost Environmental cost etc Construction 5% cost

3/14

3/34

Total operating expenses in one year plus insurance

111 US$ 101 US$

Including 9% tax Excluding tax

Labor cost Overhead costs

Fixed costs

Depreciation expense

Variable costs

Insurance

Operating cost per ton of MSW, including 330 working days per year

Operating costs in one year

A framework for sustainable hydrogen production by polygeneration systems

545

Table 9.34 Average operating costs of a thermal plasma unit. Amount

Cost type

Construction cost 15% Construction cost 5% Construction cos 5%

Total fixed costs Total variable costs The cost of insurance

Table 9.35 Feed prices and products. Price (US $) $ 0/0 0.39 $/Kg 10 $/Kg 0.112 $/KWh

Municipal Solid Waste (MSW) Methanol Hydrogen Electricity

Feed Products

of water, which will cost up to $0.82 million/year. All variable costs plus insurance costs will be US $0.94 million/year. The operating cost per tonne of municipal solid waste, accounting for 9% of VAT, will be US $111. The average fixed and variable operating costs of a thermal plasma unit are shown in Table 9.34.

9.5.10.1.8 Revenue Revenue comes from the sale of products. Products include methanol, hydrogen, and electricity, the prices of which are listed in Table 9.35. One of the main benefits of using municipal solid waste as feed is the elimination of feed costs from process costs. Feed costs would have to be considered if coal or other types of biomass were used. Earnings are calculated from the following relationship: X Income 5 PriceðpÞ 3 Product rate 3 OperatingTime (9.162) P

The capacity factor is defined as follows: CF 5

Operating time ðhÞ ð365 3 24ÞðhÞ

(9.163)

A capacity factor of 1 is considered. The revenue from the sale of methanol, hydrogen, and electricity in millions of US dollars (MUS $) is also derived from the following relationship: Income 5 1026 3 ½ð3416:4 3 MMETH Þ 1 ð87600 3 MHYD Þ 1 ð981:12 3 Wnet Þ (9.164)

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Hybrid Poly-generation Energy Systems

In the above relation, MMETH is methanol’s mass flow rate, MHYD is hydrogen’s mass flow rate in kg/h, and Wnet is the total energy produced in KW.

9.5.10.1.9 Profit function The annual profit function is defined as follows: Profit 5 Income 2 O&M 2 ðTPC 3 CRFÞ

(9.165)

Factor in the recovery of capital (CRF) is a factor in the recovery of capital, which is the annual income of a monetary investment unit that is needed to pay the principal and the yield of the capital (at a specific interest rate). This factor can convert total investment cost (TPC) to total cost per unit of the year. Eq. (9.166) calculates the CRF where i is the interest rate and n is the system’s lifetime, which is 3% and 20 years, respectively. Therefore the CRF value is calculated as 0.0672. CRF 5

ið11iÞn ð11iÞn 2 1

(9.166)

Equilibrium of investments and costs is always possible under a certain rate, which we call the return on investment (ROI). The rate of ROI is the most common profitability ratio, calculated by dividing the profit by the cost of investment and expressed as a percentage or ratio. This is the benchmark that measures the profitability of a company, the higher the index or rate, the better the company has used its capital and the better the profitability. ROI is an economic indicator for the economic evaluation of the process, which is defined as follows: ROI ð%Þ 5

Profit 3 100 TPC

(9.167)

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[98] B. Miller, 8 - Greenhouse gas  carbon dioxide emissions reduction technologies, in: B. Miller (Ed.), Fossil Fuel Emissions Control Technologies, Butterworth-Heinemann, 2015, pp. 367438. Available from https://doi.org/10.1016/B978-0-12-801566-7.00008-7 [99] H.C. Frey, N. Akunuri, Probabilistic Modeling and Evaluation of the Performance, Emissions, and Cost of Texaco Gasifier-Based Integrated Gasification Combined Cycle Systems Using ASPEN. Prepared by North Carolina State University for Carnegie Mellon University and US Department of Energy, Pittsburgh, PA, 2001. [100] Z. Hoffman. Simulation and Economic Evaluation of Coal Gasification with Sets Reforming Process for Power Production. Louisiana State University and Agricultural & Mechanical College, 2005. Available at: https://digitalcommons.lsu.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=3268&context=gradschool_theses. [101] D. Chubb, Fundamentals of Thermophotovoltaic Energy Conversion, Elsevier, 2007. [102] W. Yang, et al., Development of a prototype micro-thermophotovoltaic power generator, Journal of Physics D: Applied Physics 37 (7) (2004) 1017. [103] Y. Nam, et al., Solar thermophotovoltaic energy conversion systems with twodimensional tantalum photonic crystal absorbers and emitters, Solar Energy Materials and Solar Cells 122 (2014) 287296. [104] A. Ghanekar, et al., Optimal design of wavelength selective thermal emitter for thermophotovoltaic applications, Journal of Thermal Science and Engineering Applications 10 (1) (2018). [105] K. Qiu, et al., Generation of electricity using InGaAsSb and GaSb TPV cells in combustiondriven radiant sources, Solar Energy Materials and Solar Cells 90 (1) (2006) 6881. [106] C. Walker, Towards a High-Efficiency Micro-Thermophotovoltaic Generator, Massachusetts Institute of Technology, 2010. [107] W. Chan, et al., Modeling low-bandgap thermophotovoltaic diodes for high-efficiency portable power generators, Solar Energy Materials and Solar Cells 94 (3) (2010) 509514. [108] S.A. Kalogirou, Solar Energy Engineering: Processes and Systems, Academic press, 2013. [109] A. Karimi, Optimum design of combined thermo-photovoltaic power plant for biogas gasification and hydrogen production, Master thesis of University of Tehran, 2018. [110] T. Butcher, et al., Heat transfer and thermophotovoltaic power generation in oil-fired heating systems, Applied Energy 88 (5) (2011) 15431548. [111] Y. Byun, et al., Thermal plasma gasification of municipal solid waste (MSW), Gasification for Practical Applications 1 (2012) 183210. [112] W.M. Vatavuk, Updating the CE plant cost index, Chemical Engineering 109 (1) (2002) 6270. [113] M. Mehrpooya, R. Khodayari, S.M. Ali Moosavian, A. Dadak, Optimal design of molten carbonate fuel cell combined cycle power plant and thermophotovoltaic system, Energy Conversion and Management 221 (2020) 113177. Available from: https://doi. org/10.1016/j.enconman.2020.113177. [114] N. Ahangar, M. Mehrpooya, Thermaly integrated five-step ZnSI thermochemical cycle hydrogen production process using solar energy, Energy Conversion and Management 222 (2020) 113243. Available from: https://doi.org/10.1016/j. enconman.2020.113243. [115] D. Pauschert, Study of Equipment Prices in the Power Sector. 2009. [116] I. Hannula, E. Kurkela, Liquid Transportation Fuels Via Large-Scale Fluidised-Bed Gasification of Lignocellulosic Biomass. 2013: VTT Technical Research Centre of Finland.

Integration of oxyfuel power plants in polygeneration systems

10.1

10

Integration of oxyfuel power plants

A type of electricity plant called an oxygen-fuel power generation substitutes pure O2 for the oxidizer. High-combustion temperatures are attainable, and fuel usage is decreased because the N2 component of air does not burn. As oxyfuel permits higher combustion temperatures than are possible with an air-fuel burn, the welding, and machining of metallic materials, particularly steel, has traditionally been the principal purpose of oxyfuel burning [1]. Since a significant portion of the air is made up of nitrogen, this is accomplished by separating the two. A combination of CO2 and H2O is the burning reaction’s end product, water. CO2 and H2O are simple to separate since additional contaminants are rarely present. A fraction of the output vapor is forwarded into the input feed because the burn temperature increases significantly when pure oxygen is used.

10.1.1 Integration of oxyfuel cycle, high-temperature solar process, and LNG cold recovery Among the most effective methods of carbon dioxide collecting in electricity generation is oxygen fuel utilization [2]. To improve CO2 sequestration, a combined operation of natural gas and oxygen fuel burning can be used [3]. Substantial carbon dioxide quantities are produced while burning oxygen-based fuels, which facilitates CO2 collection following water condensation. Regarding low-sulfur coals, postcombustion carbon dioxide collection devices may be more affordable than oxyfuel combustion from a cost perspective. Additionally, this technique may be enhanced by adding other systems, such as using membranes or multistage combustion [4]. The concept of the liquefied natural gas (LNG) cooling sink and hightemperature solar source-benefiting oxyfuel carbon dioxide transcritical and subcritical Rankine process is explored (see Fig. 10.1A). The procedure integrates two electricity-generating stages: the subcritical Rankine process and the carbon dioxide oxyfuel transcritical process. Both processes employ LNG’s cold energy, but only the oxyfuel transcritical process uses NG’s chemical and solar collector heat energies. A high-temperature solar collector cycle with thermal storage (TST) provides the application’s heat source. In order to achieve this goal, a TST is employed to preserve high-temperature solar energy when there is insufficient solar irradiation. Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00001-3 © 2024 Elsevier Inc. All rights reserved.

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Figure 10.1 (A) Oxyfuel Carbon dioxide trans-critical and Rankine electricity generating process schematic representation. (B) Graph of the projected electricity generating cycles’ production line [2].

Fig. 10.1B demonstrates how the internal energy is recovered via the vaporization of LNG as well as the production of electrical energy. Additionally, environmentally friendly and efficient fossil fuel utilization may be achieved using solar heat transformation, which is relatively inexpensive. A higher conformity with the heat source temperature profile is also provided by employing CO2 as a working fluid. Although employing LNG at the coldest temperature possible serves as a heat sink to ensure the cool recuperation idea, low-energy consumption carbon dioxide

Integration of oxyfuel power plants in polygeneration systems

555

collection is combined with the energy transformation stage in the most practical and operable way. As a result, using Rankine cycles, a carbon dioxide oxyfuel transcritical system, and a high-temperature solar energy process in an integrative approach improves net electrical power output overall. The TS and PH graphs for the carbon dioxide oxyfuel transcritical and Rankine electricity generating processes are shown in Figs. 10.3A and B. The pressure increment in the liquid state is shown by paths (20)!(1) and (3)!(21); heat absorbing from the THF of the high-temperature solar collector is shown by line (6)!(7). The heat dissipation procedure (25)!(26) presents CO2 condensation and withdrawal from the cycle. Phase shift happens during the heat dissipation stage in the carbon dioxide oxyfuel transcritical process. In this instance, the triple-point pressure is more significant than to flow (20). Additionally, the temperature of the stream (6) is less than the surrounding air’s temperature, which lowers evaporating pressure.

10.1.2 A hybrid oxyfuel power generation, CO2 liquefaction process, and water desalination An approach that may be utilized to reduce energy usage is a thermal combination. Integration reduces the amount of infrastructure necessary and lowers energy usage. The environment is negatively impacted by a rise in CO2 in the atmosphere, raising concerns worldwide. Consequently, various methods for absorbing and storing CO2 have been suggested. The desalination process, arguably the most fundamental method for producing drinkable water, depends on fossil fuels to provide the necessary energy. A compact industry may create 1869 kW of power, 65194 kW of thermal demand, and 83.22 kg/s of potable water using a trigeneration system of solar flat-plate collectors and a multistage desalination machine. The multi-effect desalination (MED) technique may generate distilled water utilizing the solar Rankine cycle (SRC’s) thermal energy. A hybrid system that combines the production of LNG, Carbon dioxide liquefaction, oxyfuel electric plants, and MED is described. Oxyfuel power plants provide the necessary heating and electricity for the LNG cycle. In order to supply the necessary cooling via the employed cryogenic process in this cycle, Carbon dioxide liquefaction is linked with LNG cycle. The MED procedure takes advantage of the carbon sequestration approach’s heating duty [1]. Choosing the finest and most appropriate LNG process is a complicated and delicate task that relies on several performance characteristics. The attractive configuration of other variables, including evaluation of the heat exchanger’s temperature, rotary equipment efficiency, choice of the suitable compositions for refrigerants of cryogenic process, and so forth, varies depending on the operating conditions. Several other specifications, for example, the composition of NG feed and the temperature of the section, are usual for all methods. The operation’s methodology and the devices are chosen depending on operational and economic factors.

556

Hybrid Poly-generation Energy Systems

The interaction between the different parts of the combined cycle for producing LNG, separating and liquefying CO2, and producing electricity using only O2 is shown in Fig. 10.3. When designing low-temperature operations, the heat exchanger network and cryogenic unit are often designed after the reactor or scrubber columns, which serve as the system’s central components. As a result, any adjustment to either the process or the heat exchanger network alters the cryogenic system’s variables, such as its electricity requirements, needed refrigeration capacity, and needed temperature level. In low-temperature operations, the construction of the refrigeration cycle is complicated by existent connections between the parts. The flowchart for the procedure is shown in Fig. 10.2. Heat exchanger HX1 receives the input flow of natural gas1 at 35 C and 4000 kPa. The flow is chilled to a temperature of more than 26 C by HX1, and subsequently, flow one is chilled to about 2128 C and 2163 C, via HX2 and HX3. Two phases of liquid and gas make up V4 outflow flow, a two-phase flow. LNG at 2164 C and 100 kPa from the scrubber to stream four as it reaches separator D3 and goes into the storage. Two cryogenic processes are used to supply the necessary cooling. NH3-H2O water absorption refrigeration system (ARS) is the precooling stage of cryogenic. In the liquefaction step, a mixed refrigeration (MR) vapor-compression process is employed (Fig. 10.3). The thermal desalination technique with more than one effect is employed here (at MED). This approach is employed when excess heat from the motor or energy installation can provide the necessary heat. There are multiple continuous

(A)

BubblePt

7000 6000

4

5000

20 2

5

3

19

18

BubblePt

1000

DewPt

17 1413 12

1110

16

9

1000

7 17

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16 13 20 200 1 2 0 400

15

0 -425000 -405000 -385000 -365000 -345000 -325000 -305000

8 26

Cycle

800

8

7

6

3000 2000

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T (˚C)

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DewPt

-200 25

6

10

9

11 19

15

75

125

h (kJ/kgmole)

12

14 175

225

S (kJ/kgmole)˚C)

(B)

(B) 20000

24 25

DewPt

T (˚C)

10000

BubblePt

26

15000

P (kPa)

5 18

3 4

Cycle

21 22

640

BubblePt

540

DewPt

440

Cycle

340 240 140

23

5000

23 22

40

26

25 24

21

-60 0 -420000

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-400000

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h (kJ/kgmole)

-370000

-360000

30

50

70

90

110

130

150

170

S (kJ/kgmole)˚C)

Figure 10.2 TS and PH graph of CO2 oxy-fuel transcritical and subcritical for Rankin system (A and B) [2].

190

Integration of oxyfuel power plants in polygeneration systems

557

Seawater 56.25˚C,0.3 bar, 3967 kgmole/h

Oxygen 25˚C, 1.4 bar, 2967 kgmole/h

Oxy Fuel Power Plant and CO2 Capture

Desalinated Water

Heat Duty 15MW

50˚C,0.15 bar, 4141 kgmole/h

Desalination System (MED)

Brine 60.9 ˚C,0.18 bar, 4825 kgmole/h

Duty Power CO2 Heat 105 MW 7.7MW Natural gas

LNG +CO2 Liquefaction

26.8˚C, 40 bar, 36346 kgmole/h

Absorption Refrigerant System

Mixed Refrigerant System

LNG -164˚C,1 bar, 34017 kgmole/h

Liquid CO2 -1˚C,35 bar, 1926 kgmole/h

Figure 10.3 Schematic of the hybrid system in blocks [1].

processes; some salty water evaporates, condenses, and becomes potable water at each step. The process works according to multiple phases of freshwater generation. Another method to boost freshwater output in MED desalination is composed of the feed of water in steam condition. Typically, thermal and mechanical methods are used to prepare steam. A steam ejector is utilized in the thermal compression shown in Fig. 10.4 to condense it. In contrast, steam from an external source and the vapor created in the preheater enters the ejector. These two streams are combined and discharged using the correct pressure and temperature. The evaporator’s design determines the kind of steam condensation that occurs. The ejector also controls the system’s internal pressure and aids in the removal of noncondensable gases. When analyzing the effectiveness of thermal desalination, the gain-output ratio (GOR) is a crucial consideration. GOR measures how much fresh water the desalination process produces using vapor [1]. GOR 5

Md Mm

(10.1)

Upon reaching the desalination condenser, the temperature of the salty water rises, and some of it exits the process as chilling water. The feed water is then poured into the evaporative pipes through each level, proceeding to the initial step, and a part of it evaporates when it reaches boiling point. The salty water spraying water on the evaporator coils is now partially vaporized. Additionally, a small quantity of steam is produced due to the abrupt pressure decrease. Vapor is created in a flash box as a result of an abrupt reduction in water pressure, and it moves to the next stage with the vapor created in the previous phase.

558

Hybrid Poly-generation Energy Systems

Figure 10.4 A hybrid oxyfuel power generation, CO2 liquefaction process, and water desalination [1].

The CC of the operational heat exchangers is shown in Fig. 10.5. Since the lowest temperature gets closer, the heat exchanger’s total volume and needs heat transfer area both drop. As the lowest temperature approaches, the application’s energy use and exergy destruction rise. HX1 has an extra capacity between the hot and cold CCs. The temperature differential between the hot and cold composites is reduced in heat exchangers. This quantity, typically between 1 C and 3 C, cannot be lowered in any way.

10.1.3 A hybrid oxyfuel power generation and NG/ CO2 liquefaction process An energy system with almost negligible emissions uses oxygen-fueled power production with CO2 collection, similar to a downstream operation. Although it is possible to liquefy the collected CO2, doing so requires a significant amount of energy. NG transportation benefits from the use of liquid gas. Its manufacturing is considered to be a high-consumption energy process, nevertheless. The greatest fossil-based resource, natural gas, is still in the growing market worldwide. LNG is among the technologies being promoted for NG transport to make use of this energy carrier across large distances. LNG is transported to the

Integration of oxyfuel power plants in polygeneration systems

559

40

HX1 Hot Composite Curve

-135

HX3

30

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Cold Composite Curve

-30 -40

-165 0

14000

7000

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40000

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HX2

160000

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-5

Hot Composite Curve

-65 -85 Cold Composite Curve

-105

120000

15

Temperature (°C)

Temperature (°C)

-45

80000

Heat Flow (kW)

Heat Flow (kW)

-25

Hot Composite Curve

-45 -65 -85 -105

Cold Composite Curve

-125

-125

-145 -145

-165 0

50000

100000

150000

Heat Flow (kW)

200000

250000

0

50000

100000 150000 200000 250000 300000 350000 400000

Heat Flow (kW)

Figure 10.5 Composite curves of the applied multistream heat exchangers [1].

required place for this reason. The capacity of NG is significantly reduced (by around 600 times) via liquefaction, which enables transport to distant locations cost-effective. LNG manufacturing, however, requires much energy. The efficiency of the LNG manufacturing system may be improved by identifying the stages of excess energy. Depending on the difficulties highlighted, it is a strong chance to build an LNG cycle in which its cryogenic processes provide the necessary cooling for refrigeration CO2 collecting. Additionally, CO2 is extracted from the flue gas of an oxyfuel power installation, generating the power needed for the LNG. A refrigeration CO2 capture, oxyfuel electricity production, and the C3MR cryogenic cycle’s design are shown in Fig. 10.6. The power electrical plant and LNG process are the two critical components of the primary system (see Fig. 10.6). The oxyfuel process uses 460 Mg/h of natural gas at 35 C and 44 bar and 10^6 g/h of pure oxygen at 25 C and 1.4 bar to generate 430 MW of electricity. Moreover, 24 MW of electrical power may be generated by using 190 Mg/h of water as the operating stream in the steam-generating process at 25 C and 1 atm. The two processes create 110 MW of electrical power in total. Two mixed and one single cryogenic cycles make up the LNG process. In the single and mixed refrigeration cycles, respectively, the operating stream is C3 and a combination of C1, C2, and C3 [8]. The combined cryogenic and natural gas may be cooled using the pure C3 stage to a temperature of about 34.0 C. A mixed

560

Hybrid Poly-generation Energy Systems

Figure 10.6 A flow diagram of the integrated process [5].

refrigerant cycle supplies the remaining cold energy needed for LNG. The LNG process provides refrigeration (9 MW) for the liquid CO2. The energy consumption parts should have an isothermal efficiency of 0.75. The burner is working as a Gibbs reactor. Additionally, because the input air is roughly 35.0 C, the air conditioning system works in actual atmospheric conditions. The specified heat exchangers are plate fin and shell, and tube. Fig. 10.7 displays the composite curves (CC) graph for the multistream heat exchangers as well as the process’s overall CC. As shown in Fig. 10.7, the heat exchangers’ thermodynamic configuration has been executed with acceptable performance. The CCs of HX4 and HX5 present nearly consistent inclination over the full operational temperature variation. In a portion of the temperature changes, the cold CCs in HE1, HE2, and HE3 are flat. The pure refrigerant dissipates at a constant temperature, which is the cause. The cold and hot needs of the cycle are thoroughly addressed according to the system’s overall CCs. The pinch analysis presents a temperature of around 2.0 C.

Integration of oxyfuel power plants in polygeneration systems

50

561

5

HX1

HX2

0 Hot Composite Curve

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Temperature (°C)

Hot Composite Curve

Cold Composite Curve

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-10 Cold Composite Curve

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Temperature (°C)

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Hot Composite Curve

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150000

Heat Flow (kW)

Heat Flow (kW) 60

HX5

Overall Composite Curve

-130 20 Hot Composite Curve

Temperature (°C)

Temperature (°C)

-135 -140 -145 -150 -155

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Hot Composite Curve

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Figure 10.7 Composite curve of plate-fin heat exchangers [5].

10.2

Energy and exergy analysis of integrated oxyfuel hybrid power plants

The thermodynamic analysis relations are shown in order to calculate energy and exergy analyses and evaluate the efficiency of the system elements. Exergy analysis helps define projection efficiency enhancement strategies and understand the irreversibility sources. It offers logical ideas to enhance the system with electrical power.

562

Hybrid Poly-generation Energy Systems

Table 10.1 Thermal and energy efficiency of the equipment [6]. Equipment

Energetic efficiency

Gas turbine

ηth 5

Solar collector

ηen;FPC 5

Transferred heat to the THF Solar energy input

Thermal storage tank

ηen;FPC 5

Heat stored in the TST Heat load from heating elements

Combustion chamber

ηCC 5

W_ W_ S

5

Tin 2 Tout Tin 2 Tout;s

5

Q_ u Aaperture IT

CP;cc 3 ðT o 2 Ti Þ LHV 3

mfuel mAir

Compressor

ηth 5

W_ S W_

5

Tout;s 2 Tin Tout 2 Tin

Pump

ηth 5

W_ S W_

5

Tout;s 2 Tin Tout 2 Tin

Table 10.1 shows the calculated energetic efficiency of different equipment used in this hybrid energy system. The exergy balance of the integrated system is calculated as the following [7]: 05

 X X X T0 _ D m_ i 3 exi 2 m_ i 3 exi 2 EX 12 3 Q_ 2 W_ 1 T out in

(10.2)

Table 10.2 shows the exergy efficiency of each equipment of the integrated system. Table 10.3 shows the system’s efficiency based on the different evaluations, and Table 10.4 presents the exergy destruction equation of each equipment. Fig. 10.8 demonstrates that shell and tube presents the largest exergy destruction in operating plant equipment. Shell & tube account for more than 38% of all exergy destruction, whereas air conditioners cause the least amount—less than 1%—of all exergy destruction. According to Fig. 10.9, the shell and tube are responsible for around 90% of the energy destruction in the water-desalination process. The reactor (Gibbs type) is responsible for 24.8% of the exergy destruction throughout the electricity and heating generating process due to using the combustion chambers.

10.3

Environmental and economic analysis of oxyfuel hybrid power plants

This part does a thorough economic analysis of the Oxyfuel hybrid systems. Due to their high instrument expenses and energy demands, low-temperature operations are

Integration of oxyfuel power plants in polygeneration systems

563

Table 10.2 Exergy efficiency of the equipment. Equipment Heat exchangers

Exergetic efficiency Pn  Pm  _ Þ _ Þ ðmΔe ðmΔe 2 Pm1 ηex 5 1 2 Pn1 P

1

_ Þi 2 ðm:e W

Compressors

ηex 5

Turbines

ηex 5 P ðm:e _ Þ

Pumps Air coolers

P ηex 5

i

_ Þ ðmΔh

P

h

1

_ Þ ðmΔh

c

_ Þo ðm:e

WP 2 ðm:e _ Þo

P

_ Þi 2 ðm:e W

_ Þo ðm:e

P ðm:e _ Þo ηex 5 P ðm:e _ Þ 1W i

Expansion valves Gibbs reactor

eΔT 5

Ð T0 T 2 T0 T

T

dh ; ePh 5 eΔT 1 eΔp ; ηex 5

P ðm:e _ Þo ηex 5 P ðm:e _ Þ i

Column

ηex 5

Wmin Wmin 1 LW

X

; Wmin 5

Separator=Tee=mixer

nb

in to stream

Total irreversibility in cycle ηex 5 1 2 Total consumed power in cycle P ðm:eÞ _ o ηex 5 P ðm:eÞ _

h

Solar collector ηex 5 Combustion chamber

X

nb 2

Out of stream

Process/cycle or overall

ΔT eΔT o 2 ei Δp eΔp i 2 eo

ηex 5

i



 

m_ THF Cp;THF To 2 Ti 2 Ta ln



To Ti

IT ACPC NCPC 1 2 TTas



i 2ρΔP

THF

h E_o  i

E_i 1 Q_ L 1 2

Ta Tbs

h

Thermal storage tank ηex;TST 5

E_ s E_ t

5

T Q_ t 2 Ta ðρν_ Cp ÞTHF ln T THF;i 2 Q_ L 1 2 T Ta THF;o

i

THF

T Q_ t 2 Ta ðρν_ Cp ÞTHF ln T THF;i

THF;o

among those that need much energy. Thus the process’ economic assessment is done using the ACS (annualized cost of the system) technique. Annualized capital cost (acap), annualized replacement cost (arepc), annualized maintenance cost (amain), and annualized operational cost are expenses that are taken into account (aope). The plant’s life span is expected to be 20 years. Hence, the expense of replacing the devices is disregarded. Marshal and Swift Cost Index updates the relationships employed for economic research. One of the most crucial critical elements in the economic assessment of the projects is the period of return (POR), rate of return (ROR), and incremental worth or additional value (AV). These formulas are used to compute them (Table 10.5).

Table 10.3 The efficiency of each system. Parameter

Equation

Final electrical efficiency ðHHV BaseÞ

ηOxy;HHV 5

W_ Turs 2 W_ Comps 2 W_ Pumps m_ fuel 3 HHVfuel

Final electrical efficiency ðLHV BaseÞ

ηOxy;LHV 5

W_ Turs 2 W_ Comps 2 W_ Pumps m_ fuel 3 LHVfuel

Overall thermal efficiency ðHHV BaseÞ

ηOverall;HHV 5

m_ LNG 3 HHVLNG 1 m_ CO2 3 HHVCO2 1 Q_ HRSG 1 W_ Turbines 2 W_ Compressors 2 W_ Pumps ðm_ fuel 3 HHVfuel ÞTotall

Overall thermal efficiency ðLHV BaseÞ

ηOverall;LHV 5

m_ LNG 3 LHVLNG 1 m_ CO2 3 LHVCO2 1 Q_ HRSG 1 W_ Turbines 2 W_ Compressors 2 W_ Pumps ðm_ fuel 3 LHVfuel ÞTotall

Table 10.4 Exergy destruction of equipment. Equipment

Thermal storage tank

Exergy destruction P P _ i 2 EX _ o5 I_ 5 EX ðm:eÞ _ i2 ðm:eÞ _ o P P _I 5 EX _ i 2 EX _ o5 _ _ i1W 2 _ o ðm:eÞ ðm:eÞ P P _ i 2 EX _ o5 _ i2 _ o I_ 5 EX ðm:eÞ ðm:eÞ   P P ΔP 1 _ i2 _ o 2 ACPC NCPC ηO IT Ta TCPC I_ 5 ðm:eÞ ðm:eÞ 2 T1s 2 m_ THF ρTHF 3 h  i P P _ o 2 Q_ L 1 2 TTbsa _ i2 ðm:eÞ I_ 5 ðm:eÞ h i P P a _ o 2 Q_ L 1 2 TTTHF _ i2 ðm:eÞ I_ 5 ðm:eÞ

Process/cycle or overall

I_ 5 Summation of irreversibility of all devices

Heat exchangers Compressorsand Pumps Separator=Tee=mixer Solar collector Combustion chamber

Ta lnðTToa Þ ðTo 2 Ti Þ

    2 Ti Þ _ p ÞTHF Ta ln TToi 2 ðTTo CPC 2 ðmC

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Hybrid Poly-generation Energy Systems

10.2%

12.6% Cold Boxes

5.3%

Heat Exchangers Turbines

11%

Compressors & Pumps Reactor

38.8%

11.2%

Valves and Drums Air Coolers Towers

9.9%

Distribution of exergy destruction

Figure 10.8 The exergy destruction of each equipment per percentage [1].

100% 90% 80%

Ejector

70%

Towers

60%

Air Coolers

50%

Valves and Drums Reactor

40%

Compressors & Pumps

30%

Turbines

20%

Heat Exchangers

10%

Cold Boxes

0% LNG Product Cycle

Power Cycle

Desalination Cycle

Figure 10.9 The distribution of exergy destruction of the main processes [1].

POR 5

CC NAB

ROR 5

NAB CC

AV 5 COP 2 PC

Table 10.5 Illustrates the relationships and processes involved in economic analysis. Definition

Parameter

Annualized cost of system

ACS 5 CacapðComponentsÞ 1 CarepðComponentsÞ 1 CamainðComponentsÞ 1 CaopeðLabor Cost 1 Fuel Cost 1 Insurance CostÞ

Annualized capital cost

Ccap 5 1:1 of Total capital cost Cacap 5 i  ð11iÞYproj j2f ;i5 Ccap:CRF ði; YprojÞ 5 Ccap Yproj 1 2f ð11iÞ 21 j 5 17; f 5 20%

Annualized replacement cost

Crap 5 CcapðIn BaseÞ:ð11iÞYproj Carep 5 Crap:FSF a ðI; YprojÞ 5 Crap:

j ð11iÞYproj 2 1

Annualized maintenance cost

For Yproj 5 20; Camain 5 0:05 of capital cost

Annualized operating cost

OFC 5 ðLabor Cost 1 Fuel Cost 1 Insurance Cost 1 UtilityÞ Number of labor 5 30; Labor Cost 5 400 US$ per Month Fuel CostðNatural Gas PriceÞ 5 2ðUS$ per Million BtuÞ Fuel CostðOxygen priceÞ 5 0:04 US$ per kg Fuel CostðElectrical Energy PriceÞ 5 0:15ðUS$ per kWhÞ Insurance Cost 5 0:02 of Capital Cost

Net present value

NPV 5 ACS=CRFði; YprojÞ

C1 5 Electricity produced price ðUS$ per YearÞ C2 5 liquid carbon dioxide Price ðUS$ per YearÞ NEW ACS 5 ACS 2 C1 2 C2

ðElectrical energy priceÞ 5 0:15ðUS$ per kWhÞ ðLiquid carbon dioxide priceÞ 5 100ðUS$ per tonÞ

(Continued)

Table 10.5 (Continued) Definition

Parameter

Levelized cost of product Total product in one yearðkg LNGÞ Prime cost

LCOP 5 NEW ACS=total product in one year VOP 5 Volume of Product; PC 5 OFC=VOP

Summary of product cost

COP 5 Cost Of Product; SOPC 5 VOP: COP COP 5 6ðUS$perMillion BtuÞ

Annual benefit

AB 5 SOPC single bond OFC

Net annual benefit

NAB 5 AB:ð1-Tax percentÞ; Tax 5 0:1ðABÞ

Period of return

POR 5 Ccap=NAB

Rate of return

ROR 5 NAB=Ccap

Additive value

AV 5 COP 2 PC

Integration of oxyfuel power plants in polygeneration systems

569

It is possible to conclude that this technique is economically viable in light of the findings and taking ROI and the cost of the finished items into consideration. The final cost is 0.29 US dollars per kilogram of Liquefied natural gas. LNG costs from 0.24 to 0.32 USD per kilogram. Figs. 10.9 and 10.10 show how the return time and price of the product vary concerning the expense of the O2 and energy generated during production. Fig. 10.9 depicts the impact of the expense of the LNG market upon that time of return for various liquefied CO2 pricing. The cost of LNG ought to be greater than 6.5 US dollars per MMBTU again until the system is commercially viable at a CO2 price of 80 US dollars per ton (period return less than four years). (Figs. 10.11 and 10.12) It is important to note that the price of LNG is increased significantly in 2022, and it is predicted that the price increments will continue in 2023. Therefore the return time is decreased. For the integration of oxyfuel cycle, high-temperature solar process, and LNG cold recovery system, The significant amount of energy and exergy efficiency is due to the whole heat combined framework and the best use of the LNG cold energy. The operation’s energy performance rises due to utilizing solar thermal radiation as the source of heat. TST usage in the high-temperature solar subsystem also results in a reduction in the territory of the collector. In hybrid water desalination, oxyfuel power generation, and CO2 liquefaction process, in order to lower the needed energy usage, LNG procedure with ARS is employed. In order to use oxyfuel energy output in the LNG and CO2 cryogenic process, a process that can generate more than 105 MW of power and more than 7 MW of thermal load is designed. This system additionally transfers 15 MW for 3/2

Prime Cost of Product (US$ per kg LNG) Period of Return (Year)

0/34

3

0/33

2/8

0/32

2/6

0/31

2/4

0/3

2/2

0/29 0/02

2 0/04

0/06

0/08

0/1

0/12

0/14

0/16

0/18

0/2

Oxygen Price (US$ per kg) Figure 10.10 Comparison between the system’s basic expense and the cost of the application’s input O2 [5].

Period of Return (Year)

Prime Cost of Product (US$ per kg LNG)

0/35

Hybrid Poly-generation Energy Systems

2/6

0/5

Period of Return (Year) Prime Cost of Product (US$ per kg LNG)

2/55

Period of Return (Year)

0/4 2/5 0/3

2/45 2/4

0/2

2/35 0/1 2/3 2/25 0/03

0 0/06

0/09

0/12

0/15

0/18

0/21

0/24

0/27

Prime Cost of Product (US$ per kg LNG)

570

0/3

Electricity Price (USS per kWh)

Figure 10.11 Comparison between the return time and the system’s basic expense and the cost of the power generated throughout the operation [5].

Figure 10.12 Variation of the return time vs. the stock’s expense of liquefied natural gas at various prices for the generated liquefied CO2 [5].

use in heat recovery steam generators, resulting in about 30 tons of vapor per hour at 72 C and 0.33 degrees for water-desalination, which provides about 75 tons per hour of potable water. The pressure valves’ energy efficiency is poorer when compared to that of other equipment, and their irreversibility is also lower.

Integration of oxyfuel power plants in polygeneration systems

571

According to the presented findings, it is possible to conclude that the C3 precooling LNG method is capable of being successfully combined with the CO2 collecting and cryogenic cycle. The suggested technique uses roughly 0.28 kWh of electricity per kilogram of Liquefied natural gas. In this scenario, the oxyfuel cycle’s electrical effectiveness and power output may approach more than 40% and 170,000 kW, correspondingly. The power density of the oxyfuel process and the amount of electricity needed to liquefy CO2 are inversely related to the molar concentration of CO2. Energy efficiency and oxygen-fuel stage energy production rise with the O2 mass flow rate. The CO2 may be liquefied at fewer pressures by raising the C3 mass flow. Since the SPC for LNG generation rises. According to the economic assessment findings, the production’s prime value and the intervals rise by 25.6% and 14% when the price of O2 is raised to 90%. The time of return and the prime expense of the product both rise by 10.6% and 64.4%, respectively, including a rise in energy cost to approximately 90%.

References [1] B. Ghorbani, M. Mehrpooya, H. Ghasemzadeh, Investigation of a hybrid water desalination, oxy-fuel power generation and CO2 liquefaction process, Energy 158 (2018) 11051119. [2] A.A. Bidgoli, N. Hamidishad, J.I. Yanagihara, The impact of carbon capture storage and utilization on energy efficiency, sustainability, and production of an offshore platform: Thermodynamic and sensitivity analyses, Journal of Energy Resources Technology 144 (11) (2022) 112102. Available from: https://doi.org/10.1115/1.4053980. [3] A. Allahyarzadeh-Bidgoli, P.E.B. de Mello, D.J. Dezan, F. Saltara, L.O. Salviano, J.I. Yanagihara, Thermodynamic analysis and optimization of a multi-stage compression system for CO 2 injection unit: NSGA-II and gradient-based methods, Journal of the Brazilian Society of Mechanical Sciences and Engineering 43 (2021) 119. Available from: https://doi.org/10.1007/s40430-021-03164-5. [4] M. Mehrpooya, M.M.M. Sharifzadeh, A novel integration of oxy-fuel cycle, high temperature solar cycle and LNG cold recoveryenergy and exergy analysis, Applied Thermal Engineering 114 (2017) 10901104. [5] M. Mehrpooya, B. Ghorbani, Introducing a hybrid oxy-fuel power generation and natural gas/carbon dioxide liquefaction process with thermodynamic and economic analysis, Journal of Cleaner Production 204 (2018) 10161033. [6] M. Mehrpooya, M. Saedi, A. Allahyarzadeh, S.A. Mousavi, A. Jarrahian, Conceptual design and performance evaluation of a novel cryogenic integrated process for extraction of neon and production of liquid hydrogen, Process Safety and Environmental Protection 164 (2022) 228246. Available from: https://doi.org/10.1016/j.psep.2022.05.076. [7] A. Allahyarzadeh-Bidgoli, M. Mehrpooya, J.I. Yanagihara, Geometric optimization of thermo-hydraulic performance of multistream plate fin heat exchangers in two-stage condensation cycle: Thermodynamic and operating cost analyses, Process Safety and Environmental Protection 162 (2022) 631648. Available from: https://doi.org/10.1016/j.psep.2022.03.088. [8] A. Allahyarzadeh-Bidgoli, D.J. Dezan, J.I. Yanagihara, COP optimization of propane pre-cooling cycle by optimal Fin design of heat exchangers: Efficiency and sustainability improvement, Journal of Cleaner Production 271 (2020) 122585. Available from: https:// doi.org/10.1016/j.jclepro.2020.122585.

Basic power and cooling production systems in combination with polygeneration systems to trigeneration of cold, heat, and power

11

High-efficiency combined heat and power systems can produce both electrical and thermal energy. Hybrid energy systems near the point of use can reduce energy waste during generation and transmission. Common CHP technologies are gas turbines, fuel cells, ORCs, and RCs. This chapter focuses on ORC and gas expander systems for electricity conversion, often used for water heating.

11.1

Basic power and cooling production systems

11.1.1 Cogeneration cycle This chapter presents the novel cogeneration cycle, as the previous chapter presented the basic power and cooling production systems in combination with PGSs using conventional operating fluids and conditions. Therefore as the first step, the application of nanofluid as a working fluid will be discussed. This subsection presents and discusses a cogeneration cycle consisting of three main parts: air compression, gas turbine, and heat recovery. In the heat recovery sector, they typically generate electricity by producing steam and using a steam turbine. Fig. 11.1 shows a general schematic of a cogeneration process. In the heat recovery sector, they typically generate electrical power by producing steam and using a steam turbine. The general outline of the process is shown in Fig. 11.2. A typical natural gas composition is presented in Table 11.1 to be used in gas turbines. The analysis of the inlet gas to the system is given in Table 11.2. As can be seen, gas enters at 1950 kPa pressure. Due to the system’s high pressure, injecting inlet gas into a turbine and lowering the pressure to the required amount generates electrical power. The exhaust gas pressure from the turbine is 8 bar. Fig. 11.3 shows this part of the process. The gas enters the gas turbine after passing the turbine and reducing pressure. In this part, the gas enters the combustion chamber after being mixed with compressed air. Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00006-2 © 2024 Elsevier Inc. All rights reserved.

574

Hybrid Poly-generation Energy Systems

Figure 11.1 A general schematic of the cogeneration process [1].

POWERNET POWER1

W POWEROUT

POWER2 POWER3X

WATER14 WATER1

HP-STEAM PREGEN

GASTURB

HRSG

NATGAS2

NATGAS HIERARCHY

HIERARCHY

HOTGAS1

AR

MP-STEAM

HIERARCHY

LP-STEAM

WATER24 HOTGAS9

NOXSTEAM

Figure 11.2 A general outline of the process [2].

Table 11.1 Flowchart of inlet gas flow composition. Material

Mass fraction

Argon Methane Ethane Propane

0.018 0.8362 0.0733 0.0725

Basic power and cooling production systems

575

Table 11.2 Properties of inlet gas stream. Temperature (
C) Pressure (bar) Flowrate (kg/h)

52 19.5 25,000

Figure 11.3 Initial turbine to reduce gas pressure and generate power [2].

Table 11.3 Analysis of inlet air to gas turbine. Material

Mass fraction

N2 O2 CO2

0.7555 0.2316 0.0005

Table 11.4 Properties of the inlet air stream. Temperature (
C) Pressure (bar) Flowrate (kg/h)

15 1.013 1,324,000

Tables 11.3 and 11.4 show the analysis of the constituents of inlet airflow and properties of the inlet air stream, respectively. The inlet air pressure reaches 8 bar after passing the compressor, and the compressor outlet gas temperature reaches 269.15
C with 90% efficiency and enters the combustion chamber after mixing with the inlet fuel. Also, to prevent the production of NOx gases in the combustion chamber, steam enters the chamber at 45,000 kg/h and has a pressure of 8 bar along with the inlet gas.

576

Hybrid Poly-generation Energy Systems

11.1.1.1 Turbine gas A gas turbine is an internal combustion engine from the rotary engine type that works based on combustion gas energy [3]. Each gas turbine comprises a compressor for compressing air, a combustion chamber to mix air with fuel to burn it, and a turbine to convert the internal energy of hot and compresed gases into mechanical energy [4]. Part of the mechanical energy produced in the turbine is spent by rotating the compressor itself. Depending on the gas turbine application, the rest of the energy may rotate the electric generator and accelerate the air directly or in the same way after velocity variation by the gearbox. In Fig. 11.4, the outline of the gas turbine is shown. The R-Gibs block was used to simulate the combustion chamber. In this section, the equilibrium condition is used to simulate. The pressure drop and heat loss in the combustion chamber are also ignored. The characteristics of the gas flow out of the combustion chamber are shown in Table 11.5. As can be seen, the fuel is completely transformed. Table 11.5 shows the exhaust product of the combustion chamber. As it is known in the process, the exhaust gas enters the turbine at a temperature of 977
C and 8 bar pressure. The turbine is one of the backpressure types, and the hot gas inlet pressure of the turbine is reduced to 1/1 bar pressure. The turbine’s efficiency is 90%, and the exhaust gas temperature also reaches 551
C. The turbine output power is 200504.63 kW, and 97108.6975 kW is used to generate the power the compressor requires to compress the air. The positive output of the gas turbine is 103395.93 kW. NDC1

NATGAS2(IN)

BURN1

NATGAS2 NOXSTEAM(N)

MIXGAS

NOXSTEAM

HOTGAS EXP2

AIR(IN)

AR1

AR2 HOTGAS1

POWER2A

AIRCOMP

ACPOWER

WORKMX HOTGAS1(OUT)

POWER2

POWER2(OUT)

Figure 11.4 Overview of gas turbine [2]. Table 11.5 Analysis of exhaust gas from the combustion chamber. Material

Mass fraction

Water N2 O2 CO CO2 Ar

0.0704 0.72 0.151 1.67 e-9 0.049 0.012

Basic power and cooling production systems

577

11.1.1.2 Heat recovery Since the gas turbine gas has a large amount of energy due to burning in the gas turbine, it is necessary to prevent energy loss and its loss to increase the system’s efficiency. Therefore These usually recover the gas from the gas turbine differently. These methods include steam generation, hot water production methods, and the use of these gases as a hot source elsewhere. In the following, the mentioned cogeneration system is used to recover heat from the steam production system. Thus two different streams of water enter the system with the following specifications (Table 11.6). Inlet gas to thermal recovery system during several steps of temperature reduction is from 551
C to 125.1
C. In such a way that gas enters the E-100 exchanger. In this exchanger, steam enters at 288
C and exits at 500
C. This steam then enters the steam turbine. The output gas temperature of the E-100 exchanger is then reduced to 480.5
C. Then the gas enters the E-101 converter, reducing its temperature to 313
C. The water is recycled at 288
C to achieve 50% steam quality. The gas then enters the E-102 exchanger, reducing the gas temperature to 241
C. In this exchanger, the water rises from 160
C to 291
C. The gas then enters the E-103 exchanger and cools to 237
C. Water vapor is also heated from 155.5
C to 217.1
C. The exhaust gas enters the E-104 converter, and its temperature drops to 179.9
C, where the water reaches an output quality of 50% at 155.5
C. At this point, the gas is divided into two parts with ratios of 0.725 and 0.227 and the current with a lower ratio flows to the E-105 exchanger. The temperature is reduced to 140
C, and the current with a higher ratio flows to the E-106 exchanger, where the temperature is reduced to 119.4
C and is mixed with the exhaust gas from exchanger E-105 and it temperature reaches 125
C. Using the recovered heat from the exchangers in this system, three steam levels are extracted from the turbine and their generated power. The output stream from the E-100 exchanger enters the K-100 turbine at 80% efficiency, at a temperature of 500
C and a pressure of 72.5 bar, which is reduced to 24 bar. The power output of this turbine is 12595.7 kW, and 15% of the output stream is extracted from the K-100 turbine as high-pressure steam, and the rest is sent to the K-101 turbine with 80% efficiency. The power produced in this turbine is 11910.7 kW. The steam outlet pressure in this turbine is 5 bar. The output of this turbine is STM11. The output steam streamflow is divided by the E-103 exchanger into two ratios (0.15 STM21 and 0.85 STM22). The STM21 flow is used as the intermediate pressure steam flow. The temperature and pressure of this steam stage are 217.6
C and

Table 11.6 Characteristics of inlet water to thermal recovery system.

Temperature (
C) Pressure (bar) Flowrate (kg/h)

W14

W1

70.8 6.9 42,600

73.16 76.5 180,800

578

Hybrid Poly-generation Energy Systems

5 bar, respectively. The STM22 steam stream is combined with the STM11 stream from the K-101 turbine and sent into the K-102 turbine with 80% efficiency. The output pressure is 1.01 bar, and the generated power of this turbine is 11657.4 kW. The steam output from the turbine enters the two-phase region to separate the liquid and the steam into a flash system to separate the steam and liquid. STM23 as flow with low pressure is used, and the outlet current will be used as hot water. The steam output from the turbine enters a two-phase region. To separate the liquid and vapor, it flows into a flash system. The STM23 separated steam is used as a lowpressure current, and the output current is used as hot water. The system is to generate electrical power, steam, and hot water. Moreover, since the system aims to employ nanofluids, it is necessary to execute the process in the liquid phase and prevent the nanofluid from being removed. Therefore it is necessary to review the heat recovery of the hot exhaust gases in this section. In the following, another cycle is simulated. This cycle (Fig. 11.5) is more straightforward and does not have the complexities of the previous system [5]. Fig. 11.6 illustrates the outline of the exhaust gas heat recovery process from the gas turbine seen in this section. As it is known, this system is similar to other systems in other parts of gas turbine and steam production and heat recovery in gas exhaust gas turbine.

Figure 11.5 Schematic of the simulated exhaust gas recovery of the gas turbine [2].

Basic power and cooling production systems

579

Figure 11.6 An overview of a cogeneration system [2].

Table 11.7 Characteristics of air inlet to the compressor. Material

Mass fraction

N2 O2 Ar

0.7809 0.2095 0.096

In this system, 262.8 t/h of air (current 1) is compressed into a compressor with 5.6 t/h of natural gas (fuel 1) and then sent into the combustion chamber. The pressure drop in the combustion chamber is approximately 0.5 bar and is assumed adiabatic. After the combustion of natural gas in the combustion chamber, its temperature reaches 1103.5
C and then enters the turbine, and its temperature is reduced to about 524
C. Given that the system is a type of backpressure turbine, the pressure at the gas turbine outlet reaches about 1.1 bar. The turbine’s power is 21.1 MW, with 10 MW used to operate the compressor. The hot gas is then driven to the thermal recovery system from the gas turbine. Besides, due to the need to produce steam at high temperatures, it is necessary to reuse some natural gas as fuel. Therefore 5.9 t/h of natural gas (fuel 2) with the exhaust gas from the gas turbine is burned again in a combustion chamber. The exhaust gases are then fed into the superheated converter. After passing the gas through the converter, the outlet temperature reaches 125
C. The superheated steam generated by the system enters the steam turbine at 443
C and 61 bar pressure, reducing its pressure to atmospheric pressure and generating power of 13.9 MW. The inlet gas flow to the system consists of methane and inlet air at 25
C and a pressure of 1.01 bar. The compressor’s inlet composition is shown in Table 11.3. The air inlet and gas flow from the combustion chamber is also shown in Tables 11.7 and 11.8, respectively.

580

Hybrid Poly-generation Energy Systems

Table 11.8 Analysis of exhaust gases from the combustion chamber. Material

Mass fraction

N2 O2 Ar H2O

0.757 0.143 0.0296 0.0593

11.1.1.3 Combined cooling and heating systems with nanofluid A specific case is presented here to investigate the effect of adding nanofluid. Some of the most essential and cost-effective natural energy sources are wind, solar, geothermal, etc. These energy sources are usually available at a low cost. The cost is only for exploiting these resources. Therefore using these resources in the context of energy crises and environmental problems related to fossil fuels is very valuable. This energy is used in different ways. One of these ways is to use the ORC. The operating fluid circulates through the four main cycle stages and generates electricity during this cycle. This cycle consists of four main stages: operating fluid evaporation, steam expansion in the turbine, condensation of the steam output from the turbine, and increasing the operating fluid pressure to the inlet pressure. The fluid in this cycle is the normal pentane. Normal pentane enters the E-100 heat exchanger at a temperature of 314K and 410 kPa, and its temperature reaches 354K, and the normal pentane is completely vaporized. The steam then enters the turbine with 85% adiabatic efficiency. The output pressure in the turbine is reduced to 130 kPa. The exhaust gas from the K-100 turbine then enters the heat exchanger, cools to a temperature of 314K, and becomes completely liquid. The fluid then enters pump 56 and pumps up to 410 kPa pressure and then enters the liquid and again into the heat exchanger of the evaporator. The rate of heat input to the evaporator and the output heat rate in the condenser and the turbine generating power are reported in Table 11.9 (Fig. 11.7). Also, the ORC yields are as follows: Efficiency 5 ðPower generated in a turbine  Power consumed in pumpÞ 3 100=ðInlet heat to evaporatorÞ Efficiency 5 8:07% Natural sources such as geothermal and solar energy can supply the heat required by the evaporator. This research uses solar energy to supply a hot energy source. According to available sources, solar energy was used as a heat source. Thus water in this system first enters a solar system and then enters the evaporator. Heat exchangers are the most important equipment in the heat transfer debate. Therefore to investigate the effect of adding nanoparticles to the base fluid, it is necessary to model the heat exchangers and thereby observe the effect of the nanoparticles on

Basic power and cooling production systems

581

Table 11.9 Thermal profile of the Organic Rankine cycle. Gas turbine (kW) Evaporator (kW) Condenser (kW) Pump (kW)

175.8 2137 1964 3.2

Figure 11.7 Organic Rankine cycle with nanofluid [2].

the reduced surface area of the designed heat exchanger. Various heat exchangers are available in the industry, and shell and tube heat exchangers are one of the most commonly used heat exchangers. In this research, shell and tube converters have also been presented. Therefore the most important equations used for modeling are discussed further. The first step from the start of modeling is to measure the heat transfer and energy balance in the shell and tube to obtain the heat exchanged and the output temperature of the converter: Q 5 u 3 A 3 F 3 LMTD

(11.1)

Eq. (11.2) is the energy balance equation to calculate the amount of heat exchanged, where U is the overall heat transfer coefficient, A is the effective surface area of the heat exchanger, LMTD is the logarithmic mean temperature, and F is the temperature correction coefficient. Q 5 ms 3 CPS 3 ðTis 2 Tos Þ 5 mt 3 CPt 3 ðTit 2 Tot Þ

(11.2)

The rate of heat transfer is obtained from the following equation, where ms, Cps, Tis, and Tos are, respectively, the fluid inflow into the shell, the thermal capacity, the fluid inlet and outlet temperatures within the shell. Also, mt, Cpt, Tit, and Tot are the inlet fluid discharge, heat capacity, and fluid inlet and outlet temperatures.

582

Hybrid Poly-generation Energy Systems

Eq. (11.3) also shows the overall heat transfer coefficient in the converter. In this equation, hs is the heat transfer coefficient of the shell side, and ht is the coefficient of heat transfer of the flux side of the pipes. U5

1 hS 1 RFS 1

do di




RFT 1

1 hT



(11.3)

Also, do is the outer diameter of the tubes and di is the inner diameter of the tubes, which is assumed. di 5 0=8 3 do

(11.4)

Rft and Rfs are the sediment resistance in pipes and shells. The value of the heat transfer coefficient of the shell is also obtained from the following equation. In this respect, ks is the conductivity heat transfer coefficient, De of the shell equivalent diameter, Res is the fluid Reynolds number in the shell, and Prs is the fluid Prandtl number in the shell. t is the viscosity of the fluid side of the shell, and μw is the viscosity of the water. The relation expressed for the computation of the heat transfer coefficient is on the side of μ in single-phase mode. When we change the phase, the calculations are as follows. As water usually enters the evaporators supercooled, the heat transfer coefficient is divided into two parts for calculations. Therefore they perform as follows for calculations. h5

qs hs

Q 1

qv hv

(11.5)

In this respect qs and qv are heat exchanged for the preheating and evaporating segments, respectively. Then Δtw is calculated as follows: Δtw 5

hio ðTc 2 tc Þ hio 1 ho

(11.6)

Δth 5 Thi 2 Tco

(11.7)

Δtc 5 Tho 2 Tci

(11.8)

Tc 5 Tho 1 ðThi 2 Tho Þ

(11.9)

tc 5 tci 1 ðTco 2 Tci Þ

(11.10)

After calculating Δtw from Fig. 11.8, the values of the heat transfer coefficients for the vapor-liquid phase are calculated. And then, after examining the heat transfer coefficients obtained, we continue with the calculation. Therefore according to the following equation, the heat transfer coefficient of displacement is calculated.

Basic power and cooling production systems

1000

583

Maximum for water

800

400

Maximum for organics

300

t tra

nsf

er

200

hea

100

ent

80

Lat

Heat transfer coefficient , h ,Btu/(hr)(ft2)(°F)

600

60

at by

le he

50

sib Sen

40

tion

nvec

co free

30 20 4

5

6 7 8 910

20

30

50

100

200

300

Temperature difference (∆t)w between tube wall and liquid

Figure 11.8 The diagram to calculate the evaporation convection heat transfer coefficient [8].

The equivalent hydraulic diameter is obtained from the following equation [6,7]. In this equation, Pt is the square tube step. Also, the triangular tube step is obtained from the following relation. The Res is obtained from the following relation: Res 5

ρs 3 vs 3 De μsj

In this respect, vs is the velocity of the fluid in the shell: ms vs 5 as 3 ρ s

(11.11)

(11.12)

In this respect, as is the specific level and is calculated from the following equation [8]: as 5

Ds 3 B 3 Cl Pt

(11.13)

584

Hybrid Poly-generation Energy Systems

In this respect, Ds is the shell, B is the baffle’s cross-sectional area, and Cl is the cross-sectional area of the baffle and is derived from the following relation [8]: Cl 5 Pt 2 do

(11.14)

The following equations calculate the heat transfer coefficient at the pipe side. The friction coefficient is also obtained from the following equations.

11.1.1.4 Pressure drop We also use the equations to calculate the pressure drop in the shell and pipe. These equations are calculated for the shell and pipe sections separately. Typically, the pressure drop in the pipe section results from the fluid pressure drop along the pipe and the fluid pressure drop in the bending and joint sections. The constant value of p varies in different references and is assumed to be 4 [8]. The pressure drop calculations in the shell are also calculated as follows [9]. The following is a flowchart of the exchanger design information (Tables 11.10 and 11.11). Modeling was done using the information provided by the converter. The modeling results are shown in Table 11.12. As shown in Table 11.10, the results of the modeling error have an acceptable error with the values in the reference. As mentioned earlier, the addition of nanoparticles to the base fluid causes changes in the properties of the base fluid. These properties include viscosity, heat capacity, conductive heat transfer coefficient, and density. Many relationships are provided for each of these properties. Table 11.10 K1 and n1 values for Pt 5 1.25 3 d0 [5]. No. of passes

Triangular pitch

1 2 4 6 8

Square pitch

K1

n1

K1

n1

0.319 0.249 0.175 0.0743 0.0365

2.142 2.207 2.285 2.2499 2.675

0.215 0.156 0.158 0.402 0.0331

2.207 2.291 2.263 2.617 2.643

Table 11.11 Exchanger fluid specifications.

Shell side methanol Tube side water

m (kg/s)

Ti (
C)

To (
C)

ρ (kg/m3)

Cp (kj/kg)

μ (Pa.s)

27.8

95

40

750

2.84

0.00034 0.19

0.00033

68.9

25

40

995

4.2

0.0008

0.0002

k (w/m.k)

0.59

Rf (m2.k/w)

Basic power and cooling production systems

585

Table 11.12 Results of heat exchanger modeling comparison. Area

A(m2)

Available exchanger reference Modeling

278.6 261.5

Therefore it is useful to model heat transfer in nanofluid to provide a model for each of the mentioned properties. Different sources need to be analyzed to model the conductivity heat transfer coefficient. Many relationships and information are provided for each nanoparticle. One of the appropriate relationships for modeling titanium nanoparticle conductivity heat transfer coefficient (TiO2) is the Maxwell model described in the equations below: knf kp 1 2Kbf 1 2φðkp 2 kbf Þ 5 kp 1 2Kbf 2 φðkp 2 kbf Þ kbf

(11.15)

The following semiempirical relation is also suggested for titania nanoparticles [10]: knf 5 ð1 1 2:92φ 2 11:99φ2 Þ kbf

(11.16)

In this respect, knf, kbf, and kp are the conductivity coefficients of nanofluid, base fluid, and nanoparticles, respectively. Also, Ø is the volume fractions of nanoparticles per nanofluid. This valid relationship for modeling this property is based on many available sources. We also use the following semiempirical relation to model alumina nanoparticle conductivity heat transfer coefficient: knf 5 ð1 1 7:47φÞ kbf

(11.17)

The following semiempirical relation is also used for silver nanoparticles [11]: knf 5 ð0:9692φ 1 0:9508Þ kbf

(11.18)

The copper conductivity heat transfer coefficient was calculated from the values in the papers as illustrated in Fig. 11.9 [12]. Viscosity modeling has also provided many relationships. Among the most important is the following relationship [13,14]: μnf 1 5 μbf ð12φÞ2:5

(11.19)

μnf 5 ð1 1 2:5φÞ μbf

(11.20)

586

Hybrid Poly-generation Energy Systems

Figure 11.9 Increased rate of conductive heat transfer coefficient of copper nanoparticles in water [12].

In this respect, μnf is the nanofluid viscosity μbf is the viscosity of the base fluid. This μ relation was used to model the viscosity. The thermal capacity of nanofluids is usually lower than the base fluid due to the low thermal capacity of nanoparticles. Different relationships have been proposed to model nanofluids. The relation used in this research is an equation [15]: Cpnf 5

ρp φCp 1 ð1 2 φÞρbf Cbf ρp φ 1 ð1 2 φÞρbf

(11.21)

In this respect, the symbol p is related to the nanoparticles, and the symbol bf is to the base fluid, which has a high affinity for different nanoparticles. The density of nanofluids increases concerning the base fluid due to the high density of the nanoparticles used. This relation is used to model the density [16]: ρpnf 5 ρp φ 1 ð1 2 φÞρbf

(11.22)

This relationship can also be attributed to different nanoparticles due to different sources. Other evolved parameters include evaporation enthalpy. The following equation for the evaporation enthalpy usually decreases the evaporation enthalpy for the water-based fluid by adding nanoparticles. ðρhfg Þnf 5 ð1 2 φÞρbf hfg;bf

  Tb;bf 1 φρ hfg;p Tb;bf p

(11.23)

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Table 11.13 Characteristics of nanoparticles applied [9].

Ag Al2O3 Cu TiO2

ρ(kg/m3)

Cp(kj/kg)

k(w/m.k)

10525 3970 8945 4175

0.234 0.76 0.3831 0.692

428 40 401 8.4

In this respect, Tb is the boiling point and hfg is the vapor enthalpy and Ø is the nanofluid volume fraction. bf and p also represent base fluids and nanoparticles, respectively [9]. Since the addition of nanoparticles to water reduces the evaporation enthalpy, high evaporation enthalpy is one of the essential parameters in heat transfer so as not to be added to situations where the nanoparticles evaporate (Table 11.13). The table shows the properties of silver, copper, alumina, and titania nanoparticles.

11.1.1.4.1 Heat transfer coefficient in a heat exchanger Different equations have been proposed to calculate the heat transfer coefficient of the fluids. These relationships are due to the conditions and physics of the problem that may change. One of the most important effects of increasing nanoparticles in the base fluid is increasing the fluid’s heat transfer coefficient. This requires increasing the power consumption of the pump without adding nanofluids. Therefore appropriate equations for modeling the heat transfer coefficient are presented. Two meaningful relationships have been used for the application of nanoparticles.

11.1.1.4.2 Copper The following relationships for copper nanoparticles are for copper. These relationships are one of the most important and applicable in terms of performance [1]: Nunf 5 0:4328ð1 1 11:28φ0:754 Penf 0:218 ÞRenf 0:333 Prnf 0:4 Laminar flow

(11.24)

Nunf 5 0:0059ð1 1 7:63φ0:6886 Penf 0:001 ÞRenf 0:9238 Prnf 0:4 Turbulent flow

(11.25)

These relationships are usually applicable in different cases. This relationship has also been applied to silver nanoparticles.

11.1.1.4.3 Silver Besides, Asirvatham et al. proposed a relation for the silver displacement heat transfer coefficient which cannot be used in many cases due to limitations [17]. It is valid to Reynolds of 12,000: Nunf 5 0:023Renf 0:8 Prnf 0:3 ð0:61φ 2 0:135ÞRenf ð0:445φ20:37Þ Prnf ð1:081φ21:305Þ

(11.26)

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11.1.1.4.4 Alumina and titania Different relationships have been expressed for the Nusselt number modeling of alumina and titania nanoparticles. Pak et al. proposed the following equation for calculating the Nusselt number of titania and alumina nanoparticles, and given the experimental results, this equation had little error for the experiment performed by this group [18]. Nunf 5 0:021Renf 0:8 Prnf 0:5

(11.27)

The Nusselt number is used to calculate the heat transfer coefficient. After studying and selecting the appropriate relationships, heat exchangers are sized according to the conditions. Nunf 5

f 8 ðRenf

2 1000ÞPrnf qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 12:7 8f ðPr2=3 2 1Þ

(11.28)

After extracting the appropriate equations for the thermal transfer properties, we first examine the performance of the nanofluid in the thermal equation modeled in the reference. The important parameters such as Reynolds, Nusselt, Prandtl, and overall heat transfer coefficient are investigated. Thus the results for two copper and silver nanoparticles are analyzed (Figs. 11.10 and 11.11). The results showed that the modeling error reaches 12% in most cases, and in general, the modeling results have good compatibility with the experimental results. Modeling for copper nanoparticles has also shown promising results and comparisons with experimental results from Xuan et al.

Figure 11.10 Comparison of modeling and laboratory results for copper heat transfer coefficient [12].

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Figure 11.11 Comparison of modeling and laboratory results for copper heat transfer coefficient [12].

Figure 11.12 Changes of convection heat transfer coefficient with increasing concentration of nanoparticles [1].

As shown in Fig. 11.12, increasing the concentration of silver and copper nanoparticles increased the base fluid heat transfer coefficient. In this case, the heat transfer coefficient is relatively high as the water flows through the pipes. The addition of silver nanoparticles also increased it. We also see (in Fig. 11.13) the effect of increasing silver nanoparticles on the overall heat transfer coefficient of the exchanger. As can be seen, the nanofluid convection heat transfer coefficient on the shell side controls the heat transfer. Because of the low heat transfer coefficient on

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Figure 11.13 Overall heat transfer coefficient of the converter [1].

the shell side, as the nanoparticles increase, the coefficient of transfer on the nanofluid side increases, but it has little effect on the overall heat transfer coefficient. As shown in Fig. 11.12, increasing the volume fraction of silver nanoparticles increased the Reynolds number. What is known is that increasing the volume fraction of silver nanoparticles increases the heat transfer coefficient and subsequently reduces the surface area of the converter. Due to the constant flow and diameter of the pipes, this reduces the pipe temperature and increases the speed. So the Reynolds number increases. Also, as can be seen in Fig. 11.13, the Prandtl number decreases with increasing the volume fraction of nanoparticles. This is due to the reduction of the thermal capacity of the nanofluid relative to the base fluid and the increase of the thermal conductivity of the nanofluid. Also, adding silver nanoparticles increases the conductivity of the fluid by increasing the conductivity of the fluid more than the copper. Fig. 11.14 shows the Nusselt number changes. As is known, the Nusselt number also increased significantly, as did the nanofluid heat transfer coefficient. Also, due to copper’s lower conductivity and heat conductivity, the Prandtl number was higher but resulted in a higher Nusselt number than silver. However, this coefficient is increased for silver compared to copper nanoparticles due to the direct effect of the conductive heat transfer coefficient on the heat transfer coefficient (Figs. 11.15 and 11.16). In the following, we investigate the reduced surface area in the heat exchanger. It was found that increasing silver nanoparticles increases the overall heat transfer coefficient and decreases the surface area of the exchanger due to the constant heat transfer rate. This is visible in Fig. 11.17. As seen previously, a greater decrease in the initial concentration of silver nanoparticles occurred. Because a further increase in silver nanoparticles increases the nanofluid heat transfer coefficient, but due to very low variations of the shell fluid

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Figure 11.14 Nanofluidic Reynolds number changes with nanoparticle volume fraction [1].

Figure 11.15 Prandtl number changes in nanoparticles with the volume fraction of nanoparticles [1].

heat transfer coefficient, the overall coefficient of heat transfer coefficient is not significant and the increase in concentration has no major effect on increasing the overall coefficient of heat transfer as well as on the deduction on exchanger’s surface area. Following previous contents, increasing the nanoparticle volume fraction increases the conductivity heat transfer coefficient. Usually, the low heat capacity

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Figure 11.16 Changes of Nusslet versus volume for silver and copper [1].

Figure 11.17 Changes in the surface area of the exchanger with increasing the volume fraction of silver nanoparticles [1].

of the nanoparticles relative to the base fluid, reduces the heat capacity of the base fluid. Therefore decreasing the heat capacity reduces the logarithmic average temperature, and considering the energy balance and the overall interface of the heat exchanged in the converter for heat exchanger design, increasing the nanoparticles increases the heat transfer coefficient on one hand, and decreases the logarithmic average temperature on the other. This causes an increase in nanoparticle volume fraction to face a downward trend at some point.

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The results were also evaluated for copper nanoparticles. The results show that the results are close to one another due to the close properties of the copper and silver nanoparticles. One of the important parameters in the heat exchanger design is heat. One of the disadvantages usually associated with adding nanoparticles is the decrease in heat capacity. The results show that adding nanoparticles reduces the practical level of heat transfer. Next, we will review and design the converters in the cogeneration process. As previously mentioned, the purpose of using nanoparticles is to increase the amount of heat recovered or reduce the heat exchangers’ area. Therefore with the introduction of nanofluids, the properties of adding nanoparticles are virtually ineffective. Therefore it is necessary to prevent evaporation and two-phase nanofluid. Therefore it is necessary to use liquid nanoparticles to recover the heat in the exhaust gas from the gas turbine. Therefore in most cogeneration systems, the purpose is to produce steam. Therefore this steam is used in various units. Moreover, if the nanofluid evaporates, the nanoparticles will eventually be lost. For this reason, it is aimed to recover the heat in the exhaust gas of the gas turbine and generate hot water to circulate the system. Therefore the heat recovery rate is the same, and the same amount reduces the gas temperature as before. The difference is that the other output is the hot water system. In the initial system, the percentages of the turbine are combined with the following percentage added by the exhaust gas from the turbine (Table 11.14). The output gas from the gas turbine has a pressure of 1.1 bar, a temperature of 524
C, and a flow rate of 872 kg/s. This gas exchanges heat with water with the following characteristics. Water enters the heat exchanger at a temperature of 25.4
C and a flow of 870,287 kg/s and exits at a temperature of 132.4
C. In this case, the aim is to investigate the rate of decrease of the designed heat exchanger surface by increasing the concentration level of nanoparticles. The following results were obtained after design. In this situation, by increasing the nanofluid flow rate, the exchanger is redesigned, and the effect of increasing the nanoparticles to the base fluid is investigated. In this case, only the flow rate of the nanofluid increases to 4.287. Increasing the constant-diameter flow rate increases the Reynolds number and turbulence and consequently increases the heat transfer coefficient slope, and the surface area of the converter decreases. These observations can be seen in Figs. 11.18 and 11.19. Table 11.14 Analysis of exhaust gas from the combustion chamber. Materials

Mass fraction

H2O N2 O2 CO CO2 Ar

0.0704 0.72 0.151 1.67 e-9 0.049 0.012

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Figure 11.18 Changes in convection heat transfer coefficient with the volume fraction of nanoparticles [1].

Figure 11.19 Total heat transfer coefficient with nanofluid volume fraction [1].

In the following, we examine the variations of the Reynolds number, Prandtl, and Nusselt. As can be seen, the Prandtl number decreases with increasing nanoparticle volume fraction. As can be seen, the percentage decrease in the surface area decreased compared to the first case due to the increased Reynolds number and increased turbulence inside the tubes, and the viscosity of the nanofluid heat transfer coefficient increased. In contrast,

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Figure 11.20 Reynolds number variations with nanofluid volume fraction [1].

Figure 11.21 Prandtl number variations with the volume fraction of nanoparticles [1].

the heat transfer coefficient in the shell is not affected significantly, and this issue increases the heat transfer resistance (Figs. 11.2011.23). In this case, again, we reduce the turbulence on the side of the pipe by reducing the length of the pipe. As the overall heat transfer rate and the overall heat transfer equation increase, while the exchanger’s length decreases, the number of tubes used in the converter increases, thus reducing the Reynolds velocity and number within the tubes. In the former case, the exchanger’s length was assumed to be 4 m; in this

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Figure 11.22 Nanofluid Nusselt number changes with increasing nanoparticle volume fraction and all of the above parameters affect the designed exchanger area [1].

Figure 11.23 Changes in the area of the exchanger with nanofluid volume fraction [1].

part, its length was reduced to 3 m. Moreover, the flow rate is 4.0287 kg/s, just as the former. The heat transfer coefficient increased with increasing volume fraction, as shown in Figs. 11.24 and 11.25. It can be seen that the heat transfer coefficient of nanofluid increased as before, but the overall heat transfer coefficient increased more than in the former case. This is due to the decrease in heat transfer resistance. In this case, the fluid side of the shell is the heat transfer controller. In each case, the coefficient of heat transfer of the shell is about 106 W/m2K, but in the first case, due to the more significant current turbulence, the coefficient of heat transfer coefficient on the pipe side is higher than in the second case. As a result, the heat transfer resistance is higher

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Figure 11.24 Changes of convection heat transfer coefficient with nanofluidic volume fraction [1].

Figure 11.25 Changes in the overall heat transfer coefficient with the volume fraction of nanoparticles [1].

than the latter. The numbers of Nusselt and Prandtl, and Reynolds have changed, as can be seen in Figs. 11.2611.29. Finally, the effect of heat exchanger reduction is investigated. As can be expected, the decrease in surface area is observed due to the increase in the rate of nanofluid heat transfer coefficient and the slight increase in heat transfer coefficient on the shell side.

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Figure 11.26 Reynolds number changes with nanofluid volume fraction [1].

Figure 11.27 Prandtl number changes with the volume fraction of nanoparticles [1].

11.1.1.4.5 Second cogeneration cycle In the second cogeneration cycle, according to the above content, it is necessary to use water to recover the heat from the turbine so that the water output does not convert to steam. In this case, the exhaust gas from the gas turbine cools from 524
C to 164
C. On the other hand, water with 30
C and pressure of 1.1 bar is used for cooling. The analysis of the exhaust gas from the gas turbine is presented in Table 11.15.

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Figure 11.28 Nusselt number estimation with nanofluid volume fraction [1].

Figure 11.29 Heat exchanger surface changes with the volume fraction of nanofluid [1].

Table 11.15 Analysis of exhaust gas from the combustion chamber. Materials

Mass fraction

N2 O2 CO2 H2O

0.757 0.143 0.0296 0.0593

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Hybrid Poly-generation Energy Systems

Figure 11.30 Changes in heat transfer coefficient with the volume fraction of nanoparticles [1].

Figure 11.31 Changes in the overall heat transfer coefficient with the volume fraction of nanoparticles [1].

The results of the modeling are presented below in Figs. 11.30 and 11.31. As can be seen, the heat transfer coefficient of the nanofluid and the overall heat transfer coefficient increase with the increasing volume fraction of the nanoparticles (Figs. 11.32 and 11.33). As can be seen, the Nusselt number and, consequently, the heat transfer coefficient increase with increasing volume fraction. On the other hand, increasing the volume fraction reduces the heat capacity of the base fluid and decreases with

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Figure 11.32 Reynolds number changes with the volume fraction of nanoparticles [1].

Figure 11.33 Prandtl number variations with the volume fraction of nanoparticles [1].

increasing volume fraction. Reducing the heat capacity increases the output temperature and decreases the logarithmic temperature mangle, thereby increasing the designed exchanger area. Therefore increasing the nanoparticles, on the one hand, reduces the logarithmic mean temperature and, on the other hand, increases the heat transfer coefficient. As a result of these two effects, we can see the changes observed in Fig. 11.34. Economic feasibility is one of the most important steps in any project. This section examines the economic impact of adding nanofluids to the converter current.

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Figure 11.34 Nusselt number changes with nanofluid volume fraction [1].

Figure 11.35 Changes in the area of exchanger with nanofluid volume fraction [1].

We use the following equation for the economic estimation of heat exchangers (Fig. 11.35). Ct 5 Ci 1 CoD

(11.29)

In this respect, Ct is the total cost, which includes initial investment cost (Ci) and energy cost (Ce) and annual operating cost (Co), and annual total operating cost (Cod). Ci 5 a1 1 a2 Aa3

(11.30)

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The initial investment cost is calculated using the Hall relation, a function of the area of the converter. In this respect, a1 5 8000, a2 5 259.2 and a3 5 0.9 for stainless steel. Also, A is the area of the converter. The total annual operating cost relationship is also calculated from the following equation: CoD 5

ny X Co k k51 ð11iÞ

(11.31)

In this respect, Co is obtained from the following relation: Co 5 P 3 CE 3 H

(11.32)

  1 mt ms ΔPt 1 ΔPs P5 η ρt ρs

(11.33)

where t represents the pipe section and s represents the shell section. ɳ is the pump efficiency and is assumed to be 0.7. Calculations are also made for the ten years ny 5 10, i 5 10% Ce 5 0.12, and H 5 7000 h/yr. The following are the results of economic validation from various studies. Consider equal sizing modeled for the exchanger at the beginning of the chapter. As can be seen in the figure, changes in the heat exchanger cost estimation are observed by changing the volume fraction of silver nanoparticles, which decreases the heat exchanger surface area, decreasing the initial cost and, on the other hand, increasing the pressure drop in the exchanger, which this increase in pressure drops causes the increase of initial expenses related to the exchanger. Fig. 11.36 also shows the percentage change in the total cost of the converter (Figs. 11.3611.41).

Figure 11.36 Exchanger cost estimation variations with the volume fraction of silver nanoparticles [1].

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Figure 11.37 Changes in estimated cost of exchanger with volume fractionation of copper nanoparticles [1].

Figure 11.38 Changes in estimated cost of exchanger with volume fractionation of alumina nanoparticles [1].

These values are also estimated for copper nanoparticles. This is visible in Figs. 11.3711.40. We also discuss the effect of nanofluid enhancement on the designed Rankin cycle. The results were investigated for three copper and silver nanoparticles and alumina and titania. As seen in the previous figures, as the volume fraction of the nanoparticles increases, the area of the exchanger is reduced and optimized at the specified point.

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Figure 11.39 Estimated cost exchanger variations with the volume fraction of titania nanoparticles [1].

Figure 11.40 Percent change in the total cost of exchanger with nanofluid volume fraction [1].

Adding nanoparticles, on the one hand, reduces the exchanger area and, on the other hand, increases the pressure drop of the exchanger. Lowering the exchanger level reduces initial investment costs, and increases in pressure drop also increase annual operating costs. It is well known that the addition of nanoparticles is optimized at some point. As shown in Fig. 11.42, silver nanoparticles have the most significant cost savings due to their high thermal conductivity. This process is followed by copper, alumina, and titania nanoparticles, respectively (Figs. 11.43 and 11.44).

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Figure 11.41 Changes in the economic estimation of the exchanger by adding silver nanoparticles [1].

Figure 11.42 Economic estimation changes of exchanger by addition of copper nanoparticles [1].

All in all, increasing the nanofluid volume fraction increases the conductivity, density, viscosity, and also heat transfer coefficient. Adding nanoparticles to the fluid also reduces the thermal capacity of the nanofluid. Considering the relationships between the exchanger size, increasing the conductive heat transfer coefficient and displacement can increase the heat transfer rate or decrease the designed exchanger surface. On the other hand, increasing the viscosity parameters and reducing the nanofluid’s heat capacity increases the surface area and thus decreases

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Figure 11.43 Economic estimation changes of the converter by addition of alumina nanoparticles [1].

Figure 11.44 Percent reduction in heat exchanger costs by adding nanoparticles [1].

the heat transfer rate. Therefore adding nanoparticles to the base fluid increases to a certain point. Adding nanoparticles to the base fluid also causes more fluid pressure to drop during the exchanger’s length, which also has a cost. Therefore adding nanoparticles to the base fluid reduces the exchanger surface and increases the pressure drop in the converter. The overall cost of the exchanger also shows that adding nanoparticles to a certain point is economically essential.

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Hybrid Poly-generation Energy Systems

Thermoelectric/thermionic generators in polygeneration systems

An integration combined power and cooling (CCP) that utilizes molten carbonate fuel cell (MCFC), thermoelectric generator (TEG), linear fresnel solar collector (LFR), and a powering expander is shown. MCFC generates thermal and electrical power. The MCFC flue gas is extremely hot, and this heat may be used for electricity. A portion of this heat loss is delivered to the TEG to generate further electricity, while the remainder is discharged into the atmosphere. The solar field’s heat exchanger creates some of the needed thermal power for the process. In addition, two gas and steam expanders generate electricity in this setup. Fig. 11.45 depicts a graphical view of the CHP process. This method includes an MCFC to supply thermal and electrical power, an LFR solar thermal field to supply some of the needed thermal energy, a TEG to generate extra electricity from the MCFC’s heat losses, and two steam and gas expanders to provide the electrical power. The intake streams in this operation include water, NG, and air. Initially, the NG and water pass through a series at 20
C and 101 kPa, correspondingly, and their pressure is raised to 110 kPa by C-1 and pump P-1. Following that, water heated to 165
C by the heat exchanger HE-1 is coupled with NG. The combination (stream 6) is transferred to some other heat exchanger. The temperature of the combination rises to 298.5
C in this heat

Figure 11.45 Conceptual diagram of the CCP system with TEG [19].

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exchanger, whose thermal energy is provided by the LFR solar unit. The heat load of LFR required to raise the temperatures of the mixture by 166
C is 1728 kW. Because LFR, while comparable to other solar units, does not produce uniform production at all times and all times of the year, an auxiliary unit is needed to supply the requisite thermal energy at night, on overcast days, and at times with lower radiation from the sun. The additional equipment is an NG-powered boiler (Fig. 11.45). Thermoelectric generator modeling generates additional power by using a portion of the MCFC’s heat losses (QH) at temperature T, while the remainder of the heat losses (q) are released into the atmosphere at temperature T0. QL in Fig. 11.45 indicates the value of heat transmission from the TEG cold side to the ambient. A TEG creates an electrical current by moving a high-temperature duty (QH) from the hot side to the cold end (QL). Each TEG component comprises parallel (thermal) and series (electrical) connections between P-type and N-type semiconductor legs. 1. The temperature of the hot junction equals that of the MCFC output; 2. The temperature of the cold connection is the same as the ambient temperature. 3. The temperature does not affect the TEG’s electrical resistance, Setbeck factor, heat capacity, and FOM. 4. To solve the problem of the big temperature difference, the Thomas effect in the TEG is not taken into account. 5. The irreversibility of outside heat flux between the heat reservoir and TEG is disregarded.

The equation below shows how to calculate the efficiency of TEG [19]: ηTEG 5

PTEG Z 3 i 3 ðT 2 T0 Þ 2 i2 5 2 QH Z 3 ðT 2 T0 Þ 1 ðZ 3 T 3 iÞ 2 i2

(11.34)

It is essential to notice that when the current density rises, the voltage and MCFC performance amount decline while the nondimensional electrical current of the TEG grows. However, the performance of the TEG and the combination rise first before falling. Additionally, the power generated by the integrated unit grows as the working pressure and temperature rise. In addition, raising the fuel mass flow rate seems to have no appreciable influence on the power conversion efficiency of MCFCs. Furthermore, as the pressure drop in T-1 increases, so does the overall current, and as the pressure change in T-2 improves, the new hybrid power system’s electrical and total efficiencies decrease.

11.3

Stirling engines and polygeneration systems

A hybrid CCHP process is described with a dual-effect LiBr/H2O absorption refrigeration, solar field, steam expander, and Stirling engine. The electricity is generated in this procedure using steam turbines and SOFC. The Stirling engine used excess fuel cell heating to generate extra electrical energy. Additionally, the Stirling engine’s excess heating powers the absorption refrigeration system, which creates a

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647 kW

878 kW Turbine 2

Steam for heating

Turbine 1

Combustion Chamber Electrical power

413 kW

5446 kW

Steam

water

Heat

Fuel

Reformer

SOFC 2169 kW

Stirling engine

NG

656 kW

Heat recovery

Oxidant Duty=961.7 kW

Heat

1513 kW

Air

Cooling

Solar Field (LFR)

Chiller

2119 kW

Figure 11.46 Conceptual design of a Stirling engine with polygeneration systems [20].

cold duty. A part of the application’s necessary heating energy is generated in this hybrid process’s solar field as a heat exchanger. Fig. 11.46 depicts the operations’ process flow. At 25
C and 1.32 bar, natural gas reaches the system as fuel for SOFC. Since being heated to 135
C, it mixes with steam at 190
C and reaches a reformer to provide the hydrogen for the SOFC [20]. Compared to internal-combustion engines, the SE is an external-combustionengine with excellent performance and minimal emissions. This motor can operate with an external energy source similar to heat from fuel cells. The Stirling engine used operates with SOFC flue gases at 850
C. Two processes with equal size and two procedures with isothermal temperature put the SE in its optimum condition. 1. Isentropic procedures: able to absorb energy from the SOFC and then extend; 2. Constant-volume procedures: lowering the operating stream’s temperature by dismissing heat to the regenerator; 3. Isentropic systems: dismissing the operating stream’s temperature to 240
C after which pressurizing; and 4. Constant-volume operations: raising the operating stream’s temperature by absorbing heat from the regenerator.

The thermal efficiency of Stirling engines is calculated as follows: ηthermo 5

12τ 1 1 ð1 2 τ Þ 3 ð1 2 eÞ=ðk 2 1Þ 3 ln

V1 V2

τ5

TC Cp ;k5 TH Cv

(11.35)

Basic power and cooling production systems

11.4

611

ORCs in polygeneration systems

This hybrid energy system with ORC configuration includes five subsystems: ORC, parabolic trough solar collector (PTC), diffusion absorption refrigeration (DAR), phase-change material (PCM) and a battery bank. The heat transmission flow is first heated in the PTC area and then sent to the DAR, wherein its energy is utilized in the burner. After exchanging heat with the ORC working fluid in the E-105 heat exchanger, the fluid is pumped to the PTC fie by the P-101. The detailed process flow diagram (PFD) of the solar-based cogeneration configuration is depicted in Fig. 11.2. The DAR subsystem consists of ammonia, water, and hydrogen (3.63% NH3, 0.35% H2O, and 96.06% H2). The DAR’s generator, reservoir, and bubble pump are housed within the T-102 column. First, a rich-liquid solution of ammonia water (Stream 1) enters the solution heat exchanger, which is preheated by a weak liquid solution of ammonia water at the output of the bubble pump. In this instance, the heat load causes it to vaporize, producing an ammonia-water vapor mixture. Precooling the water increases its capacity to absorb more ammonia through the absorber, thereby increasing the system’s effectiveness. The T-102 rectifier condenses the ammonia vapor flowing out of the generator into liquid, resulting in pure ammonia vapor (Stream 3), and rejects heat to the ambient environment via an exothermic reaction. Gravity returns the water condensate to the generator. The absorber collects the preheated rich ammonia-hydrogen vapor mixture (stream 10) from the E-104 and showers it with the pre-cooled weak liquid solution of ammonia-water (stream 12) from the E-102 in order to absorb the ammonia into the water, thereby producing a rich ammonia-water liquid solution (stream 13) at the absorber’s exit. This process is exothermic and releases heat into the surrounding atmosphere. Herein, the hydrogen is extracted from the top of the column as stream 14 and from drum where the residue hydrogen is extracted from the richliquid solution of ammonia-water as stream 15 (Fig. 11.47). In the ORC subsystem, the working fluid i-Butane (stream 22) is warmed up in the E-105 heat exchanger with the heat transfer fluid in the PTC field. Afterward, it (stream 23) enters the Ex-101 expander, where its thermal energy is converted into mechanical energy and electricity. The Ex-101 meets the electrical demand of the pumps and other devices of the cogeneration configuration, and the excess are saved in the battery bank. By discharging its energy, the battery bank satisfies the nighttime electrical demand. E-107 is cooled after passing through E-106, where it exchanges heat with the return streams (2722). Finally, it is pumped by P-102, and this cycle continues [21]. The energy balance of the presented system in a volume control is calculated as follows, considering an absorption refrigeration system and ORC. Q_ k 2 W_ k 5

X

m_ out;k hout;k 2

X

m_ in;k hin;k

(11.36)

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Figure 11.47 Flow diagram of SORC poly-generation unit [21].

11.5

JouleBrayton refrigeration processes in combination with polygeneration systems

In order to accomplish the simultaneous generation of heat and electricity, this strategy is an effective means of enhancing the effectiveness of the utilization of energy. However, the optimization of the JouleBrayton power cycle produced by earlier researchers, which primarily concentrated on the power-output characteristic of the cycle, may not be optimum for cogeneration purposes. The JouleBrayton cogeneration plant comprises a JouleBrayton heat engine and a heat recovery device. The JouleBrayton heat-engine cycle comprises two constant pressure and two isentropic processes. Because any irreversibilities caused by flow, heat transfer, and combustion are not considered, the JouleBrayton power cycle is considered an entirely reversible model. The JouleBrayton heat engine generates usable work for the user while simultaneously releasing waste heat into the surrounding environment. Recovery of the JouleBrayton heat engine’s wasted heat from the exhaust gas reduces the fuel required. Therefore power and heat can be generated at the same time. This irreversibility is caused by heat transfer occurring across a finite temperature difference. The endoreversible JouleBrayton cogeneration cycle is the name given to this particular cycle [22]. In Fig. 11.48, combined heat and power - Pumped thermal electricity storage (CHPPTES) framework comprised four circles: the warm pump circle, the warm motor circle, the warming circle, and the cooling circle. The framework gadgets included one

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Figure 11.48 A sample of JouleBrayton refrigeration processes in polygeneration systems [23].

packed-bed HR and one packed-bed CR, four warm exchangers, two buffer vessels (BV), two compressor/expander sets, and adornments counting circulating fans, pumps, vacuum pumps, and valves. Amid the charging preparation, the ordinary temperature and low-pressure helium medium were compressed by the compressor to a tall temperature and middle-pressure state of around 515
C and 1.05 MPa, separately, and after that, streamed through the hole of the HR. The warm exchange handle between the gas and the strong warm capacity fabric lowered the gas temperature. At that point, the helium at ordinary temperature and high pressure was extended to 143
C and 0.105 MPa separately and then streamed through the CR. Cold vitality was put away within the strong fabric of the CR as sensible warmth amid this handle. Finally, the gas at an ordinary temperature returns [23].

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The cooling efficiency considering the JouleBryton registration system is calculated as [23]: ηcold 5

11.6

Ecold 1 Erec;cold Einput 1 Epump

(11.37)

Cryogenic air separation

The most common method of air separation is cryogenic (refrigeration) distillation. High-purity oxygen, nitrogen, and argon can be produced cryogenically as byproducts used in constructing semiconductor devices. In this method, very low temperatures produce gas and liquid products; Thus air components are separated from each other, and a product with a certain degree of purity is obtained. Also, economically, this separation method is the most economical and efficient production method in the industry. Today, cryogenic is used in almost all industries to liquefy large volumes of gases. Refrigeration separation processes require special heat exchangers and high-efficiency separation columns. Air compressors supply all the energy required for refrigeration at the inlet, so this process is a system with significant energy consumption.

11.6.1 Cryogenic air separation steps Step 1: The first step in refrigerating air is removing and compressing contaminants (filtration). After filtering and compressing, the air temperature (hot fluid) approaches ambient temperature with the help of water or air as a cold fluid in heat exchangers. The reason for this cooling is the release of compressed air with a very high temperature after passing through the compressor. Mechanical refrigeration systems can also cool the air to a lower temperature. This will improve the process of removing pollutants and reduce energy consumption [24]. Step 2: The remaining carbon dioxide and water vapor in the air are separated in the second step. Water vapor and carbon dioxide have properties that distinguish them from the air. These compounds evaporate at 0
C and 2 79
C at atmospheric pressure, respectively. If these gases are present in air separation processes, the thin tubes in the heat exchanger and the holes in the mesh trays in the distillation column close. Therefore before the air enters the distillation stage, all the contaminants in it must be treated because at low distillation temperatures, water, carbon dioxide, and other contaminants freeze and cause damage to the process equipment. Step 3: The air is cooled to near the dew point temperature, usually done using heat exchangers. The output currents of the separation process are usually used as cold fluids in exchangers. It is also possible to use wasting energy, such as the energy contained in liquefied natural gas, when it re-evaporates to form a gas. It should be noted that in 1979, for the first time, a combined method for the liquefaction of air was introduced, in which the cold energy of liquefied natural gas was used in the process, significantly reducing energy consumption.

Basic power and cooling production systems

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Step 4: In the fourth step, the compressed and purified air, which has cooled to a shallow temperature, enters the distillation columns to separate the desired products. The number of columns depends on the number of products and their purity.

11.7

Absorption refrigeration-based polygeneration systems

The process flow of the combined LNG and helium separation process is displayed in Fig. 11.49. This form of cryogenic employs MFC technology. Three cryogenic processes are used to supply the system’s refrigeration needs. An ARS could fill the role of the initial pre-cooling phase. Once at a mass flow rate of 1179 kg/s, the system’s input feed comprises 87.87% C1, 0.5% N2, 0.05% He, and 7.1% C2-C5. Helium at 5
C and 12 pressure, LNG (999.8 kg/s) at 160.5
C and 1.3 bar, and the fuel gas are the three by-products of this operation. Helium extraction unit (HEU)

Figure 11.49 (A) Schematic and (B) flowchart representation of the combined system for extracting helium and liquefying natural gas [25].

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Hybrid Poly-generation Energy Systems

is produced via a hybrid flash and distillation method [25]. The combined helium and Liquefaction procedure diagram is shown in Fig. 11.2. 100 streams of natural gas enter the operation at 20
C and 60 bar. There are two streams inside it: 106 and 101. The first component, 101, is heated to 145
C after passing via HX1, HX3, and HX4. Stream 105 gets combined with some other component, 106, which has been chilled in HX5 to around 146
C. A flash is entered via a mixing output, 108, via a crossing throttle valve, V3. This drum’s gaseous output runs through C7, C8, and C9, at an operating pressure of 40 bar. Stream 113 travels from the HX6 multi-stream heat exchanger to the T100 distillation tower. The cryogenic absorption units stream 500 to 521 have taken the role of the pre-cooling cryogenic unit in this procedure. An NH3/Water ARS with a flow temperature of 513 and an evaporating temperature of 29.9
C is taken into account in this process. Low-grade heat duty at 173
C is necessary to provide the necessary heat load in the reboiler of section T200 in the ARS generator (Fig. 11.49).

References [1] M. Mehrpooya, et al., Heat transfer and economic analyses of using various nanofluids in shell and tube heat exchangers for the cogeneration and solar-driven organic Rankine cycle systems, International Journal of Low-Carbon Technologies 17 (2022) 1122. [2] M. Dehghani, Modeling, Simulation and Optimization of Cogeneration Production Cycle Using Nanofluid, Master thesis in University of Tehran, 2018. [3] A. Allahyarzadeh Bidgoli, N. Hamidishad, J.I. Yanagihara, The impact of carbon capture storage and utilization on energy efficiency, sustainability, and production of an offshore platform: thermodynamic and sensitivity analyses, Journal of Energy Resources Technology 144 (11) (2022) 112102. [4] A. Allahyarzadeh-Bidgoli, N. Hamidishad, J.I. Yanagihara, Carbon capture and storage energy consumption and performance optimization using metamodels and response surface methodology, Journal of Energy Resources Technology 144 (5) (2022) 050901. [5] B. Jabbari, et al., Design and optimization of CCHP system incorporated into kraft process, using Pinch Analysis with pressure drop consideration, Applied Thermal Engineering 61 (1) (2013) 8897. [6] B. Farajollahi, S.G. Etemad, M. Hojjat, Heat transfer of nanofluids in a shell and tube heat exchanger, International Journal of Heat and Mass Transfer 53 (13) (2010) 1217. [7] G. Roy, C.T. Nguyen, P.-R. Lajoie, Numerical investigation of laminar flow and heat transfer in a radial flow cooling system with the use of nanofluids, Superlattices and Microstructures 35 (36) (2004) 497511. [8] D.Q. Kern, D.Q. Kern, Process Heat Transfer, Vol. 5, McGraw-Hill, New York, 1950. ¨ . Kızılkan, M. Reppich, A new design approach for shell-and-tube heat [9] R. Selba¸s, O exchangers using genetic algorithms from economic point of view, Chemical Engineering and Processing: Process Intensification 45 (4) (2006) 268275. [10] A.A. Abd, S.Z. Naji, Analysis study of shell and tube heat exchanger for clough company with reselect different parameters to improve the design, Case Studies in Thermal Engineering 10 (2017) 455467.

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[11] A. Hadidi, A. Nazari, Design and economic optimization of shell-and-tube heat exchangers using biogeography-based (BBO) algorithm, Applied Thermal Engineering 51 (12) (2013) 12631272. [12] M.S. Peters, K.D. Timmerhaus, R.E. West, Plant Design and Economics for chemical Engineers, Vol. 4, McGraw-hill, New York, 2003. [13] J.C. Maxwell, A Treatise on Electricity and Magnetism, Vol. 1, Clarendon press, 1873. [14] W. Yu, S. Choi, The role of interfacial layers in the enhanced thermal conductivity of nanofluids: a renovated Maxwell model, Journal of Nanoparticle Research 5 (1) (2003) 167171. [15] J. Buongiorno, Convective Transport in Nanofluids, 2006. [16] L. Godson, et al., Experimental investigation on the thermal conductivity and viscosity of silver-deionized water nanofluid, Experimental Heat Transfer 23 (4) (2010) 317332. [17] L. Godson, et al., Heat transfer characteristics of silver/water nanofluids in a shell and tube heat exchanger, Archives of Civil and Mechanical Engineering 14 (3) (2014) 489496. [18] B.C. Pak, Y.I. Cho, Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles, Experimental Heat Transfer an International Journal 11 (2) (1998) 151170. [19] M. Marefati, M. Mehrpooya, Introducing and investigation of a combined molten carbonate fuel cell, thermoelectric generator, linear fresnel solar reflector and power turbine combined heating and power process, Journal of Cleaner Production 240 (2019) 118247. [20] M. Marefati, M. Mehrpooya, S.A. Mousavi, Introducing an integrated SOFC, linear fresnel solar field, Stirling engine and steam turbine combined cooling, heating and power process, International Journal of Hydrogen Energy 44 (57) (2019) 3025630279. [21] S.A. Mousavi, M. Mehrpooya, M. Delpisheh, Development and life cycle assessment of a novel solar-based cogeneration configuration comprised of diffusion-absorption refrigeration and organic Rankine cycle in remote areas, Process Safety and Environmental Protection 159 (2022) 10191038. [22] X. Hao, G. Zhang, Maximum useful energy-rate analysis of an endoreversible JouleBrayton cogeneration cycle, Applied Energy 84 (11) (2007) 10921101. [23] H. Zhang, et al., Combined cooling, heating, and power generation performance of pumped thermal electricity storage system based on Brayton cycle, Applied Energy 278 (2020) 115607. [24] M. Mehrpooya, M. Saedi, A. Allahyarzadeh, S.A. Mousavi, A. Jarrahian, Conceptual design and performance evaluation of a novel cryogenic integrated process for extraction of neon and production of liquid hydrogen, Process Safety and Environmental Protection 164 (2022) 228246. [25] A. Zaitsev, et al., Novel integrated helium extraction and natural gas liquefaction process configurations using absorption refrigeration and waste heat, International Journal of Energy Research 44 (8) (2020) 64306451.

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Several economies are highly dependent on and are projected to use fossil fuels to develop a gross domestic product (GDP) for decades. The most cleaned fossil fuel is natural gas, and its production after combustion produces carbon dioxide [1]. On the other hand, carbon dioxide emission is a global environmental issue for many businesses, mainly for industries that use high CO2 emission operations, such as power plants. Carbon capture and storage (CCS) is a remarkable method for mitigating emissions through fossil fuel-based industries. Hence, integrating carbon dioxide capturing processes in a polygeneration system to make the hybrid energy systems is engaging to present the solutions for energy demands and environmental concerns.

12.1

Integration of carbon dioxide capturing processes

To store carbon dioxide first, it must be separated from other combustion gases or industrial processes. Three possible methods are postcombustion, precombustion, and oxyfuel [2,3]. Finally, the adsorbed carbon dioxide must be prepared for transport and storage during the purification and compression processes.

12.1.1 Postcombustion In this process, carbon dioxide is absorbed from the exhaust gases after the combustion of fossil fuels and carbonaceous materials (such as biomass). Carbon dioxide can be adsorbed using liquid solutions or other separation methods [4]. Today, combustion-based power plants provide much of the world’s electricity. In an advanced coal-fired power plant, chopped coal is integrated with air and burned in a furnace or boiler. Combustion heat generates the required water vapor in the turbine generator (see Fig. 12.1). An effective method for the adsorption of carbon dioxide from the exhaust gases of this power plant is chemical adsorption with the help of organic solvents such as monoethanolamine (MEA) [2]. Usually, the absorption of carbon dioxide in this method is about 85%
90%. Also, the chemical adsorption method with the help of amine can be used in the natural gas combined cycle (NGCC), which has a better adsorption efficiency than coal-fired power plants. Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00010-4 © 2024 Elsevier Inc. All rights reserved.

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Figure 12.1 Coal fuel power plant with the postcombustion process of carbon dioxide by amine [2].

12.1.1.1 Precombustion The precombustion process converts the fuel to a gaseous mixture of hydrogen and carbon dioxide. Hydrogen is separated and can be burned without producing carbon dioxide. Carbon dioxide can also be compressed for transport and storage. Removing carbon from the fuel before combustion must first be converted to an absorbable form. In a coal-fired power plant, this process is carried out by reacting coal with water vapor and oxygen at high pressure and temperature, called partial oxidation or gasification. Most of the exhaust gas from this method contains hydrogen and carbon monoxide, which is known as syngas. Electricity can be generated by burning syngas in a combined cycle power plant, such as an NGCC. After removing the impurities in the syngas, the carbon monoxide is converted to carbon dioxide in the shift reactors and separated from the hydrogen. Fig. 12.2 presents a type of precombustion unit.

12.1.1.2 Oxyfuel In oxy-fuel systems, pure oxygen is used for combustion, which removes a large amount of nitrogen in the output stream. After removing the ash, the final gas flow consists only of steam and carbon dioxide. However, a small amount of sulfur dioxide (SO2) and nitrogen oxides (NOX) must be removed. Steam is also easily separated by cooling and compression. A coal-fired power plant with an oxy-fuel process as presented in Fig. 12.3. The oxyfuel process requires less cost than the postcombustion process. Of course, it is essential to note that the amount of pure oxygen required in this method is approximately three times that of other methods, which can add to its cost. Oxygen purity in the oxyfuel process should be about 95%
99%. This section introduces and describes the proposed process for energy recovery in liquefied natural gas (LNG) by cryogenic air liquefaction, coal gasification, and electric power generation. Moreover, the energy and exergy analysis governing the processes is also expressed.

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Figure 12.2 Coal gasification combined cycle with the pre-combustion process of carbon dioxide [5].

Figure 12.3 Coal-fired power plant with oxyfuel adsorption process of carbon dioxide [5].

According to the contents of the previous chapter, in the process of air separation, in addition to producing high-purity oxygen and nitrogen, the use of latent heat in the distillation towers and the energy of the currents in the heat exchangers has somewhat reduced energy loss. In the coal gasification process, flow energy and LNG energy recovery have reduced energy consumption. The Rankine cycle with carbon dioxide as the working fluid has been practiced in the power generation cycle. The proposed process shows air separation, coal gasification, and carbon dioxide adsorption along with the power generation cycle and re-evaporation of LNG, each of which is described separately.

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Hybrid Poly-generation Energy Systems

12.1.1.3 Cryogenic air separation First air, flow (1), which contains 78% nitrogen, 21% oxygen, and 1% argon, is cooled in the HE-1 exchanger during this operation at a temperature of 25 C and a pressure of 101.51 kPa, as shown in Fig. 12.4. Two streams are formed from the cooled air (2) and (3). After passing through the multistream heat exchanger and the primary HE-1 heat exchanger, the flow temperature (2) drops to a temperature of about
181.8 C, and its pressure rises to 566 kPa in the CR-1 compressor. The lowest high-pressure column, D-1, which has 48 steps and an operating pressure of 460 kPa, is where the high-pressure and cold airflow (6) enters. The two-phase current (5) is split into two currents (8) after being squeezed in CR-2 (9). Column D-1 receives flow (8) from the 20-column stage. Pure nitrogen gas is extracted from the top of the column in D-1 after being separated by distillation. After passing through the EX-1 expander, the nitrogen (10) net stream travels back to the primary HE-1 heat exchanger. The remaining compressed air flow, flow (9), and the lower product of the high-pressure distillation tower, flow (11), go through the HE-2 heat exchanger before being lowered to the ideal pressure of 144 kPa in valves V-2 and V-1 before entering the D-2 column. Before being employed as low-pressure column feed, the liquid and gas phases of currents (14) and (15) are separated in the S1 and S-2 phase separators. Phase separators are used to lower the D-2 distillation

Figure 12.4 Proposed process of air separation, coal gasification, carbon dioxide adsorption along with power generation cycle and natural gas liquefaction re-evaporation.

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623

column’s energy usage. Column D-2 top is where currents (18) and (19) and additional feed currents enter step 12. There are 50 stages in the D-2 distillation column, which operates at a pressure of roughly 101 kPa. The bottom and top of the tower discharge high-purity liquid oxygen flow and relatively low-purity nitrogen gas flow following distillation in this tower. The HE-2 heat exchanger may utilize currents (20) and (21) as cold currents. In order to lower the temperature of the exchanger’s incoming air, currents (22) and (23) likewise return to the HE-1 heat exchanger. The HE-7 exchanger uses its low temperature by receiving a poor-purity nitrogen stream at a temperature of 2155 C. The coal gasification unit receives the final stream of 99.99% pure oxygen, with a temperature, pressure, and purity of 23.5 C and 101 kPa. Gas turbine units may employ pure nitrogen flow, with the following specifications: 22 C, 101 kPa, and 100% purity. A high-pressure distillation tower condenser and a low-pressure distillation tower reboiler work together to decrease heat loss and energy in the distillation columns.

12.1.1.4 Liquefied natural gas re-evaporation Fig. 12.4 shows a proposed process of air separation, coal gasification, carbon dioxide adsorption along with power generation cycle and natural gas liquefaction reevaporation. The process involves a LNG storage tank, two pumps, and four heat exchangers. LNG is stored at 2162 C and 140 kPa [6]. The assumed compounds of LNG can be seen in Table 12.1. LNG evaporation is a frequent example of an effective procedure whose chilly energy may be used for other procedures [7]. The easiest way to get LNG into the gas phase for long-distance transmission is to pump the pressurize to the necessary level (7000 kPa). Because, as mentioned, the amount of energy consumed to pump in this manner is lower, and if a pressure increase occurs after the natural gas 2 liquid phase change, power consumption will increase more than 21 times. As a result, the P-1 and P-2 pumps boost the pressure of LNG for the distribution lines by up to 70 times following the T-3 splitter. The primary heat exchanger HE-1 of the air separation process receives the current (32) as a cold fluid. Additionally, this design incorporates LNG’s cold energy into producing electricity. Therefore current (34) is employed as a cold current in the HE-4 heat exchanger, which serves as a carbon dioxide power cycle condenser, to avoid the loss of cold energy. Finally, heat up to 5 C is applied to the current (36) in the HE-6 heat exchanger. Hot air or saltwater may provide the heat needed for the HE-6 heat exchanger. If air is used, the cool air from the heat exchanger can be used for air conditioning. The outflow seawater can also be used for cryogenic desalination to separate the treated water from the seawater after the pretreatment processes. Table 12.1 Natural gas liquefied compounds. Components

CH4

C2H6

C3H8

N2

Total

Mole fraction (%)

90.38

5.37

4.04

0.21

100

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Hybrid Poly-generation Energy Systems

12.1.1.5 Carbon dioxide power generation cycle The power generation unit is a process in which a pump, turbine, heater, and two heat exchangers are used. In this cycle, carbon dioxide is used to generate power. Liquid carbon dioxide flow (37) enters the P-3 pump at 211.8 C and a pressure of 26.05 bar and exits with a pressure equal to 240 bar. The current (40) is preheated in the HE-3 heat exchanger and then enters the H-1 heater of the power generation cycle. At constant pressure, the temperature of the working fluid increases to 700 C. The heat required in this section can be provided by solar energy or heat lost in other processes. The working fluid in the gas phase enters the EX-2 expander to produce power at a pressure of 240 bar and a temperature of 700 C. The output current (41) is cooled after passing through the HE-3 and HE-4 heat exchangers to reach its initial conditions in the current (37).

12.1.1.6 Coal gasification process Fig. 12.4 depicts the water-gas transition and coal gasification processes as brown dashed lines. As a coal gasifier, the gasifier is related to the coal gas regulator. The coal gasifier model includes the gasification process and two phases of coal degradation. Coal breaks down into its structural components, such as C, H, N, O, S, and moisture, in the R-1 breakdown reactor at a temperature and operating pressure of 25 C and 1 atmosphere. The yield of the elements in the decomposition reactor has been calculated using a calculator block. Coal’s chemical makeup is identified using an approximation analysis and an elemental or latent analysis. The table lists the variables pertaining to the elemental analysis of coal (carbon, hydrogen, oxygen, nitrogen, and sulfur) as well as the proximate analysis of coal (moisture, volatile matter, ash, and fixed carbon) (12.2). Additionally, the return of elements depending on element analysis is decided as follows in the calculator block: Fact 5 ð100 2 MoistureÞ=100

(12.1)

YieldðH2 OÞ 5 Moisture=100

(12.2)

YieldðiÞ 5 UltimateParameterðiÞ 3 Fact=100

(12.3)

i 5 O2 ; H2 ; N2 ; S; C; Ash It should also be noted that the amount of moisture content in the above relationships is obtained from the proximate analysis of coal. After decomposition, the flow of ground coal (45) is combined with the flow of water (44) before entering the gasifier to form a slurry. Additionally, the flow (28) is transferred to the gasification unit as the pressure in the oxygen gas product unit of the air separation process rises to that of the gasifier’s working pressure. The R-2 reactor uses Gibbs free energy minimization to perform its calculations as a gasifier for the partial oxidation of elements in coal. Thirty-two atmospheres are comparable to the gasifier’s

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operating pressure. The R-1 decomposition reactor and the R-2 gasification reactor are regarded as entrained flow gasifiers, which should be emphasized. The HE-7 heat exchanger cools the exhaust gas stream (48) using low-purity nitrogen streams (25) and LNG (33). In F-1, a solid waste by-product called coal ash is separated from gasification products. Ash transforms into low-viscosity slag at the high temperature of the R-2 reactor (around 1300K), which makes removal from the gasifier simple. A minor quantity of water, carbon dioxide, nitrogen, ammonia, methane, and hydrogen sulfide are also present in the syngas stream (50), which is mainly composed of carbon monoxide (50 mol%) and hydrogen (30 mol%). The current (50) enters the hydrogen sulfide and ammonia removal unit B-1, which in this design is considered a preprepared and simple unit with the help of a separator. The water-gas shift unit consists of two pre-filled catalytic reactors, first the high-temperature R-3 reactor and the other low-temperature R-4 reactor. The amount of hydrogen produced increases during the water-gas shift reaction. Flow (52) and steam enter above reactor R-3 at a pressure of 550 kPa and a temperature of 310 C. The HE-7 heat exchanger reduces the output current temperature from the first reactor to 200 C. Flow (58) contains more carbon dioxide and hydrogen. Iron oxide and copper oxide can also be used as catalysts in high-temperature and low-temperature reactors, respectively [8] (Table 12.2).

12.1.1.7 Carbon dioxide adsorption process The output current from the coal gasification unit is sent to the cryogenic carbon dioxide adsorption unit to separate the carbon dioxide and hydrogen from each other. CR-4 high-pressure and CR-5 low-pressure compressors are the essential components of this unit. The current (58) enters the CR-4 compressor after cooling in the HE-6 heat exchanger. The current (60) is then cooled in the AC-1 cooler, and Table 12.2 Coal characteristics and approximate and elemental analysis. Parameter

Quantity

Proximate analysis (wt.%) Volatile matter (VM) Fixed carbon (FC) Moisture content Ash

36.97 55.44 9.07 7.59

Ultimate analysis (wt.%) C H O N S HHV (Btu/lb)

78.03 5.06 5.66 1.69 1.97 12775

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the output current enters an S-3 dryer to remove water. A stream of water-free gas (62) is transferred to HE-7. The two-phase output current in the S-4 separator is divided into two streams of liquid and gas at a temperature of 2150 C and a pressure of 200 kPa. Finally, a gas stream (66) with a molar percentage of 98% hydrogen is used as the cold fluid in the HE-7 heat exchanger. In addition, the pressure of liquid carbon dioxide flow (67) is increased by the CR-5 compressor to 11,000 kPa and then passes through the HE-7 to a temperature of 30 C to be prepared for storage. Absorbed high-pressure carbon dioxide is often transported through pipelines or injected underground for storage. The Equilibrium model and the Kinetic categories are used to categorize system gas reactors. A mixture of these two parts is also sometimes employed. In equilibrium models, heat dissipation is not taken into account since it is assumed that the gas reactor is insulated. However, it should be remembered that this assumption is never accurate, and there is some heat loss from the reactor to the surrounding environment. Additionally, the gasifier’s mixing is complete, and the reactor’s temperature is thought to be consistent throughout. The equilibrium model’s underlying assumptions state that it may sometimes fail to address the demand for simulation adequately. The conversion during the gasification process is investigated using the kinetic model using data from the tests [9]. The combustion of volatile materials and coal with complete mixing is presumed, and assuming the gasifier’s equilibrium, the Gibbs minimization reactor is used in the simulation.

12.1.1.8 Cryogenic air separation The total power consumed (WTot,ASU) in this unit is calculated based on two twostage compressors (WCR-1, WCR-2) and one expander (WEX-1); WTot;ASU 5 WCR21 1 WCR22 1 WEX21

(12.4)

Two parameters are defined for specific energy consumption in the air separation process. One is based on the amount of power consumed relative to the amount of pure oxygen produced (γO2,ASU), and the other is based on the amount of power consumed relative to the total amount of oxygen and nitrogen produced (γO2 1 N2, ASU). WO2 ;ASU 5

WTot;ASU MO2

γO2 1N2 ;ASU 5

WTot;ASU MO2 1 MN2

(12.5)

(12.6)

In the above relations, MO2 and MN2, the mass flow rates of oxygen and nitrogen are pure, respectively. Also, it refers only to high-purity nitrogen flow. Pay attention to the fact that the lower the value of this index, the more efficient the process.

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12.1.1.9 Coal gasification The performance of a gas gasifier is typically evaluated through two efficiencies: carbon conversion efficiency (CCE) and cold gas efficiency (CGE). CCE (ηCC) is defined as the ratio of reacted carbon (MCre) to feed carbon in the gasifier (MCin); ηCC ð%Þ 5

MCre 3 100 MCin

(12.7)

Cold gas efficiency (ηCG) is also expressed as the ratio of the chemical energy of the gaseous products leaving the gas generator at ambient temperature to the chemical energy of the incoming coal; ηCG ð%Þ 5

HHVsyn 3 Msyn 3 100 HHVcoal 3 Mcoal

(12.8)

ηCG ð%Þ 5

LHVsyn 3 Msyn 3 100 LHVcoal 3 Mcoal

(12.9)

In the above relations, HHVsyn is the higher heating value, Msyn is the mass flow rate, and LHVsyn is the lower heating value of the syngas. The calorific value of coal and syngas can be defined as HHV or LHV. In LHV calculations, the heat required to condense the water in the syngas after gasification is not considered. However, to calculate HHV, this value must be considered [10]. HHVsyn 5

4 X

yi 3 HHVi 5yH2 3 HHVH2 1 yCO 3 HHVCO 1 yCH4

i51

3 HHVCH4 1 yH2 O 3 HHVH2 O LHVsyn 5

4 X

(12.10)

yi 3 LHVi 5 yH2 3 LHVH2 1 yCO 3 LHVCO 1 yCH4 3 LHVCH4

i51

(12.11) In the above equations, yH2, yCO, yCH4, and yH2O, are the molar fractions of hydrogen, carbon monoxide, methane, and water, respectively.

12.1.1.10 Cryogenic adsorption of carbon dioxide Compressors and expanders determine total power consumption (WTot,CCC) in this process; WTot;CCC 5 WCR24 1 WCR25 1 WEX24

(12.12)

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The following equation also obtains the process efficiency; ηCCC 5

MTot;CCC MSC

(12.13)

In the above relation, ηCCC is the cryogenic adsorption efficiency of carbon dioxide, and MSC is the mass flow rate of the separated carbon dioxide.

12.1.1.11 Carbon dioxide power generation cycle According to the HE-3 heat exchanger, the amount of hot and cold energy transferred is precisely the same. Therefore the total amount of the calculated heat in the power generation cycle is expressed in the following equation;     QTot;PG 5 Q38;39;40 1 Q41;42;37 5 Q38;39 1 Q39;40 2 Q41;42 1 Q42;37 5 Q39;40 2 Q42;37

(12.14) LNG provides the cold energy required in the power generation cycle condenser (HE-4 heat exchanger). As a result, the amount of heat received can be obtained with the help of the following equation; Qnet 5 Q39;40 5 ðh40 2 h39 Þ 3 MLNG

(12.15)

In the above relation, MLNG is the mass flow of LNG and h39 and h40 are the enthalpy of currents (39) and (40), respectively. The following equation is used to determine the total power of the cycle: Wnet 5 WEX22 1 WP22 5 W40;41 2 W37;38 5 ððh40 2 h41 Þ 2 ðh38 2 h37 ÞÞ 3 MLNG (12.16) In the mentioned relation, h37, h38, and h41 are the enthalpy of currents (37), (38), and (41), respectively. The method of calculating the thermal efficiency for the power generation cycle is Eq. (12.17): ηth 5

ððh40 2 h41 Þ 2 ðh38 2 h37 ÞÞ 3 MLNG ðh40 2 h41 Þ 2 ðh38 2 h37 Þ 5 ðh40 2 h39 Þ 3 MLNG ðh40 2 h39 Þ

(12.17)

12.1.1.12 Re-evaporation of liquefied natural gas The recovered cold energy from the LNG evaporation process is obtained as follows [11]: QLNG 5 ΔhLNG 3 MLNG

(12.18)

Integration of carbon dioxide capturing processes in hybrid energy systems

12.2

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Absorption-based postcombustion capture of carbon systems and polygeneration systems

In the past, fossil fuels were used to address the fundamental energy issue. However, today, due to dwindling supplies and rising carbon dioxide concentrations in the global society, alternative sustainable sources are the only option. Owing to the day-night cycle, solar radiation is not always constantly accessible. Consequently, the storage and exploitation of this sustainable energy source are among the most pressing issues. Using the stream of gases from solid oxide fuel cells and solar PTC collectors, a carbon dioxide power production process, postcombustion carbon dioxide removal system, and NH3-water absorption cryogenic process, a combined design for power production, heat, and liquefied carbon dioxide is suggested. This hybrid energy system is designed to enable effective long-term energy production and storage. Hence, one of the significant ways of long-term energy storage that would be delivered to distant places effectively, the generation of liquefied carbon dioxide, is created as one of the primary inventions of this system. Fig. 12.5 shows that this hybrid system produces 346.5 kW of electricity, 82.2 kW of heating demand, and 158.7 kg/h of liquefied carbon dioxide by receiving 203.7 kW of thermal energy from solar PTC and 60.79 kg/h of NG in a fuel cell. In that, the hot flue gas out of the fuel cell is employed to give intake heating to the CO2 power production process; thus preheating the feed flows to the fuel cell and supplying intake heating to the absorption generator of the cryogenic process. The flue flow reaches the postcombustion carbon dioxide removal system, and the extracted CO2 subsequently reaches the cryogenic absorption process and condenses. In the postcombustion carbon dioxide removal system, solar PTCs supply

Figure 12.5 Schematic diagram of the integrated system with CCS and PTC [12].

630

Hybrid Poly-generation Energy Systems

heating for the vapor reaching the reformer and the intake warmth to the tower burner, as shown in Fig. 12.5. Fig. 12.6 depicts the operational flowchart for the concurrent production of liquefied carbon dioxide and electricity. This hybrid system comprises a solid oxide fuel cell, a carbon dioxide electricity generating process, a CO2 capture-liquefaction unit, and an NH3-H2O absorption cryogenic cycle [12]. Presently, the majority of the world’s largest enterprises employ fossil fuel burning, particularly coal in natural gas, to create and deliver energy. In most businesses, 80% of the air required for burning is N2, and the resulting exhaust gases have a carbon dioxide concentration of 3
15 vol% with 1 atm. The carbon dioxide removal from the exhaust gases of burning fossil fuels—including coke, gas, and petroleum—and biofuels, is the postcombustion capturing technique that has achieved the greatest economically and technologically developed in comparison to other approaches. A CO2 removal unit makes up the whole of the molecular adsorption. The absorber is a typical separation tower that does not necessitate a separate energy supply, but the remover incorporates a fractionation column with a condensing unit (HX21) and reboiler (HX5) for improved stripping efficiency characteristics. The discharge stream P101 is introduced into the HX19 and raised to 100 C. The recuperation of low-returns heat gives the needed heat at the stripper, which raises the stream’s temperature. The CO2 capture procedure takes place in the absorber upon the amine solutions of the subsequent processes; therefore MEA (A47 flow) is introduced. In order to adjust the tower temperature and complete the removal of remaining particles in the gases, A48 flow is introduced at low discharge from the top of the absorption tower. Throughout this t, tower, the consumed solvent (A39 flow) arrives from the top and is recycled to the carbon dioxide capture cycle after being recuperated by a reboiler utilizing incoming heat. The heat exchanger of the

Figure 12.6 The instantaneous production of liquefied carbon dioxide and electricity [12].

Integration of carbon dioxide capturing processes in hybrid energy systems

631

stripper receives its needed energy from solar PTC, while the condenser receives its necessary coolness from flowing H2O (Fig. 12.7). The fuel cell conversion occurs prior to the injection of fuel into the cell. By removing the intermediate energy transfer phases, a fuel cell boosts energy conversion performance. In power plants, each of these conversions influences the system’s productivity. NG is utilized to deliver the essential fuel for solid oxide fuel cells. Solid oxide fuel cells can operate with high-carbon hydrocarbons, CO, and CH4. Prior to entering the fuel cell anode [13], CO and CH4 are typically transformed into carbon dioxide and hydrogen outside of reforming. CO is manufactured via the water
gas reaction process shown in Eq. (12.19). CH4 1 H2 O ! 3H2 1 CO CO 1 H2 O ! H2 1 CO2

(12.19)

The negative electrode turns the hydrogen from Eq. (12.20) into oxygen. Solid oxide fuel cells may also utilize multicomponent fuel streams with neutral or ideal gases such as N2, CO2, and vapors. However, the efficiency goes down as the accumulation of dispersant gas goes up. Moreover, eventually, here is what the general reaction was: CH4 12H2 O ! 4H2 1CO2 175

(12.20)

Outlet Temperature

45

Ambient temperature

170

165 35

160

155

30

150

Outlet Temperature (°C)

Ambient temperature (°C)

40

25 145

140

8

10

12

14

16

18

20

Time (h) Figure 12.7 Variations in the temperature of the environment and the HTF temperature [12].

632

Hybrid Poly-generation Energy Systems

The kinetics relations for the synthesis gas production reactor and water-gas shift processes are distinct. These kinetics rely upon that working circumstance and supplied catalyst. The trial compression limit is 120
600 kPa, and the kinetics of the synthesis gas production reactor and water-gas shift in catalyst Ni/-Al2O3 are presented. T the resultant kinetics were determined. The kinetics variables are shown here. 0 11 0 3 P 3 ð P Þ H2 AC B ðPCH4 Þ 3 ðPH2O Þ0:5 2 @ CO 0:5 C B K 3 ð P Þ P1 H2O C B K1 B r1 5 3 B 2 C C 1:25 ðPH2 Þ C B A @ 11KjCO PCO 1KH2 ðPH2 Þ0:5 1KH2O PPH2O H2 22:09 3 105 RT

K1 5 5:92 3 10 e 8

KCO 5 5:12 3 10213 e

KH2 1:4 3 105 RT

210

5 5:68 3 10

e

(12.21)

9:34 3 104 RT

21:59 3 104 RT

KH2 O 5 9:25 3 e 22:683 3 104 T

KP1 5 1:198 3 1017 e

0

11 P 3 ð P Þ CO2 H2 AC B ðPCO Þ 3 ðPH2O Þ0:5 2 @ B KP2 3 ðPH2O Þ0:5 C C B 3B 2 C C B C B A @ 11KjCO PCO 1KH2 ðPH2 Þ0:5 1KH2O PPH2O H2 0

r2 5

K2 ðPH2 Þ0:25

22

KP2 5 1:767 3 10 e 0 r3 5

K3 ðPH2 Þ1:75

4:4 3 103 T

24

21:5 3 104 RT

K2 5 6:028 3 10 e 0 1 1 4 P 3 ðPH2 Þ A C B ðPCH4 Þ 3 ðPH2O Þ 2 @ CO2 C B K 3 ðPH2O Þ P3 C B B 3 B 2 C C C B A @ 11KCO PCO 1KH2 ðPH2 Þ0:5 1KH2O PPH2O H2 21:094 3 10 RT

K3 5 1:093 3 10 e

5

(12.22)

2:243 3 10 T

(12.23)

4

KP3 5 2:117 3 10 e   PH O DEN 5 1 1 KCO PCO 1 KH2 PH2 0:5 1 KH2 O 2 PH 2 3

15

The equations regarding the efficiencies of the subsystem in terms of energy are presented below: ηCO2 power cycle 5

W_ net Efficacy of the CO2 power generating process _ QInlet to power cycle (12.24)

Integration of carbon dioxide capturing processes in hybrid energy systems

ηSOFC; elec 5

W_ DC SOFC electrical 2 efficiency m_ fuel 3 LHVfuel

633

(12.25)

ηSOFC;th 5

W_ DC 2 W_ Compressors 2 W_ Pumps 1 Q_ Inlet to power cycle SOFC thermal 2 efficiency m_ fuel 3 LHVfuel

ηOverall;th 5

W_ Turbines 2 W_ Compressors 2 W_ Pumps 1 W_ DC 1 Q_ heat 1 m_ product 3 LHVproduct Overall 2 efficiency Q_ Solar 1 m_ fuel 3 LHVfuel

(12.26)

(12.27)

12.3

Exergy and energy analysis of hybrid CCSs

The thermodynamic analysis of a hybrid CCS is calculated based on the equations presented in Chapter 6. The variations in the air temperature and the solar PTC’s oil HTF temperature are shown in Fig. 12.8. These variations demonstrate that the ambient and PTC output stream temperatures are at their highest during noon. The impact of modifications in the fuel consumption factor on the electrical and overall efficiencies of the SOFC employed in this study is shown in Fig. 12.8. The quantity of H2 used by the fuel cell grows as the fuel consumption factors rise while the fuel intake output remains constant. Consequently, the fuel cell’s ability to generate power likewise rises. In contrast, the fuel cell’s output gases are discharged less often, yet considering their high temperature. SOFC typically increases as the fuel consumption factors of the outflow gases rise. As a result, the cycle’s overall thermal-energy efficiency also improves. Fig. 12.9 illustrates a rise in the fuel consumption coefficient related to the quantity of fuel cells and thermal energy efficiency while keeping the fuel intake flow constant. The quantity of H2 used, the amount of electric current, and the amount of energy generated by the fuel cell all rise as the fuel consumption factor rises while the fuel intake outflow remains constant. The exergy analysis of the primary process flows are shown individually in Table 12.3. Table 12.4, Fig. 12.10, and the results of the exergy assessment of process equipment are also presented in Fig. 12.11. As can be seen, the R-1 reactor, followed by the AC-1 air conditioner, the R-2 reactor, and the HE-7 heat exchanger, has the highest degree of irreversibility. Expanders have a poorer energy efficiency when compared to other components, but they also have a lesser irreversibility. This leads to the conclusion that a device’s energy consumption performance should be assessed concurrently in terms of irreversibility and energy efficiency. The gadget may have a high rate of lost effort, yet it is still quite effective. For instance, the HE-7 heat exchanger’s energy

0.6

SOFC electrical efficiency 0.46

Overall thermal efficiency 0.54

0.42 0.48

0.38

0.42

0.34

0.36

0.3

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.3

Figure 12.8 Changes in the fuel consumption coefficient’s impact on the effectiveness of the employed SOFC’s electrical performance and overall energy efficiency [12].

450

0.75 SOFC thermal efficiency

Cell number of SOFC

0.7

410

0.65

370

0.6

330

0.55

290

0.5

0.6

0.65

0.7

0.75

0.8

0.85

0.9

250

Figure 12.9 Impact of the variety of cells and SOFC thermal energy performance employed on the fuel consumption factor [12].

Table 12.3 Process stream exergy values. Stream

Physical exergy (kW)

Chemical exergy (kW)

Total exergy (kW)

Stream

Physical exergy (kW)

Chemical exergy (kW)

Total exergy (kW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

0 0 0 1330.16 798.39 3562.11 2492.23 670.45 1817.15 491.04 8030.35 8376.17 2574.02 8340.25 2541.98 164.11 105.05 8171.83 2433.58 2439.59 1672.84 1845.95 1028.63 226.07

50.68 35.48 15.20 35.48 15.20 35.48 15.20 4.11 11.10 41.56 38.02 38.02 11.10 38.02 11.1 6.12 4.31 37.87 10.07 90.58 330.77 90.58 330.77 41.56

50.68 35.48 15.20 1365.64 813.59 3597.59 2507.43 674.56 1828.25 532.6 8068.37 8414.19 2585.12 8378.27 2553.08 170.23 109.36 8209.7 2443.65 2530.17 2003.61 1936.53 1359.4 267.63

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10512.83 11506.99 17339.34 32805.47 18093.87 8648.12 0 3.06 0 0 678.02 8854.51 3032.21 3025.62 6.59 3009.07 16.65 1340.5 2137.69 5118.13 4075.34 4037.73 3481.96 6865.19

22560.93 22560.93 22560.93 22560.93 22560.93 22560.93 132.29 132.29 91349.44 92842.17 92969.15 67958.14 67958.14 67958.14 0 66593.23 1389.39 66593.23 66593.23 64435.52 64435.52 64377.73 64377.73 64377.73

33073.76 34067.92 39900.27 55366.4 40654.8 31209.05 132.29 135.35 91349.44 92842.17 93647.17 76812.65 70990.35 70983.76 6.59 69602.3 1406.04 67933.73 68730.92 69553.65 68510.86 68415.46 67859.69 71242.92 (Continued)

Table 12.3 (Continued) Stream

Physical exergy (kW)

Chemical exergy (kW)

Total exergy (kW)

Stream

Physical exergy (kW)

Chemical exergy (kW)

Total exergy (kW)

25 26 27 28 29 30 31 32 33 34 35 36

1338.5 0 0 1451.35 32482.64 23167.41 9315.23 23233.81 9305.04 20218.94 13562.15 13478.4

90.58 330.77 41.56 330.77 1730473.92 1234114.25 496359.68 1234114.25 496359.68 1234114.25 1234114.25 1234114.25

1429.08 330.77 41.56 1782.12 1762956.56 1257281.66 505674.91 1257348.06 505664.72 1254333.19 1247676.4 1247592.65

61 62 63 64 65 66 67 68 69 70 71 72

3196.85 3160.38 12.92 1153.05 4513.53 1044.63 2755.87 2785.98 1709.89 423.43 5419.44 0

64377.73 63842.88 556.69 63842.88 63842.88 60913.05 3643.94 3643.94 3643.94 60913.05 496359.68 90.58

67574.58 67003.26 569.61 64995.93 68356.41 61957.68 6399.81 6429.92 5353.83 61336.48 501779.12 90.58

Integration of carbon dioxide capturing processes in hybrid energy systems

637

Table 12.4 Process equipment exergy efficiency and degradation. Exergy destruction (kW)

Exergy efficiency (%)

Component

Exergy destruction (kW)

Exergy efficiency (%)

AC-1 CR-1 CR-2 CR-3 CR-4 CR-5 D-1 D-2 EX-1 EX-2 EX-3 EX-4 H-1 HE-1 HE-2 HE-3 HE-4

13700.47 503.38 251.40 142.51 529.95 29.62 2232.40 834.41 75.91 1806.70 401.56 710.83 2364.02 2160.92 135.16 3613.40 4792.08

86.74 41.41 40.97 91.07 86.46 50.41 91.37 92.36 66.70 87.72 75.93 64.59 86.74 99.83 99.06 95.16 99.62

HE-5 HE-6 HE-7 P-1 P-2 P-3 P-4 R-1 R-2 R-3 R-4 S-1 S-2 S-3 S-4 V-1 V-2

0.02 0.05 9659.29 411.79 202.52 425.98 0.16 20198.47 11591.59 4389.95 16.3 0.04 0.07 1.71 0.02 35.92 32.04

99.99 99.99 98.76 98.26 97.87 96.43 95.07 82.13 87.85 94.47 99.98 99.99 99.99 99.99 99.99 99.57 98.75

91.37 92.36

91.07 86.46

67.26

86.74

100

50.41

60

41.41 40.97

80

40 20 0

AC-1 CR-1 CR-2 CR-3 CR-4 CR-5 D-1 D-2 EX-1 EX-2 EX-3 EX-4 H-1 HE-1 HE-2 HE-3 HE-4 HE-5 HE-6 HE-7 P-1 P-2 P-3 P-4 R-1 R-2 R-3 R-4 S-1 S-2 S-3 S-4 V-1 V-2

Exergy Efficiency (%)

120

87.72 75.93 64.59 86.74 99.83 99.06 95.16 99.62 99.99 99.99 98.76 98.26 97.87 96.43 95.07 82.13 87.85 94.47 99.98 99.99 99.99 99.99 99.99 99.57 98.75

Component

Component Figure 12.10 Process equipment exergy efficiency.

efficiency is 98.76%. However, it has a heat loss of 9644.05. Of course, irreversibility and energy efficiency can have an inverse connection. For instance, the irreversibility and energy efficiency of the S-1 mixer are 0.04 kW and 99.99%, respectively.

Hybrid Poly-generation Energy Systems

5000 0

4389.95

10000

16.3 0.04 0.07 1.71 0.02 35.92 32.04

15000

13700.47 503.38 900 142.51 529.95 29.62 2232.4 834.41 75.91 1806.7 401.56 710.83 2364.02 2160.92 135.16 3613.4 4792.08 0.02 0.05 9659.29 411.79 202.52 425.98 0.16

20000

AC-1 CR-1 CR-2 CR-3 CR-4 CR-5 D-1 D-2 EX-1 EX-2 EX-3 EX-4 H-1 HE-1 HE-2 HE-3 HE-4 HE-5 HE-6 HE-7 P-1 P-2 P-3 P-4 R-1 R-2 R-3 R-4 S-1 S-2 S-3 S-4 V-1 V-2

Exergy Destruction (kW)

25000

11591.59

20198.47

638

Component Figure 12.11 Process equipment exergy degradation.

12.4

Environmental and economic analysis of hybrid CCSs

The expense of constructing a carbon dioxide-supplying process is significant, and it is rarely viable to construct one in the desired area. The transport of CO2 from some other site (the generation of CO2 in the hybrid process) to equipment needing CO2 is justifiable for this purpose. Although CO2 gas storage is big and challenging to carry, this issue will be overcome for industrial facilities if CO2 is kept in a liquid phase. This combined plant’s viability assessment might include an economic analysis as one of its primary considerations. This evaluation will use the annualized cost of system (ACS) technique. This approach calculates the total expense of the process throughout its anticipated technical lifetime. These expenses include the yearly capital cost (Cacap), annualized replacement cost (Carep), annualized maintenance cost (Camain), and annual operational expenses (operating cost) (Caope) [12]: costreference year 5 Costoriginal year

cost indexreference cost year cost indexoriginal cost year

(12.28)

The equation used to calculate ACS for the hybrid system is as follows: ACS 5 Cacap ðComponentsÞ 1 Carep ðComponentsÞ 1 Camain ðComponentsÞ 1 Caope ðLabor Cost 1 Fuel Cost 1 Insurance CostÞ

(12.29)

The annualized cost of capital, the rate of return, and the prime cost are crucial characteristics for selecting the suitable hybrid process. The formulae utilized to compute the equipment cost of the created hybrid process are shown in Table 12.5. The economic analysis of the created hybrid process using the annualized costs of system (ACS) technique is shown in Table 12.6.

Table 12.5 Equipment costs [12]. Equipment

Purchased equipment cost functions

Compressor Turbine General heat exchanger (Reboiler) Condenser Pump

Cc 5 7:90ðHPÞ0:62 ; Original year: 2013Cc 5 Cost of compressorðk$Þ CT 5 0:378ðHPÞ0:81 ; Original year:2013CT 5 Cost of expanderðk$Þ CRiboler 5 8500 1 409 3 A0:85 Reboler ; Original year:2013 Ccondenser 5 516:621 3 ACondenser 1 268:45; Orginal year:2013 CP 5 fM fT Cb CP 5 Cost of pumpð$Þ Cb 5 1:39exp½8:833
0:6019ðlnQðH Þ0:5 Þ 1 0:0519ðlnQðH Þ0:5 Þ2 ; Q in gpm; H in ft headfM 5 Material factor fT 5 exp½b1 1 b2 ðlnQðH Þ0:5 Þ 1 b3 ðlnQðH Þ0:5 Þ2  b1 5 5:1029; b2 5 2 1:2217; b3 5 0:0771; Original year:2013 Cb 5 1:128expð6:629 1 0:1826ðlogW Þ 1 0:02297  ðlogW Þ2 Þ Cp1 5 300ðD0:7395 ÞðL0:7068 ÞC1 5 1:218½ð1:7Cb 1 23:9V1 1 Cp1Þ C2 5 Cost of installed manholes; trays and nozzles C3 5 Cost of condenser C4 5 Cost of reboiler CAb 5 C1 1 C2 1 C3 1 C4 CAb 5 Cost of drumð$Þ; Original year:2013 CPTC 5 150 Að$Þ; A 5 areaðm2 Þ; Original year:2013 CSOFC 5 2000W ð$Þ; W 5 workðkW Þ; Original year:2013  0:7   CSOFC 5 204 M=1125 ðk$Þ; m:H2 mass flow rate kg=d ; Original year: 2013

Absorber

Parabolic trough collector Solid oxide fuel cell Prereformer

Table 12.6 Economic analysis of the integrated system by the ACS method: [12]. Definition

Parameter

Annualized cost of system

ACS 5 CacapðComponentsÞ 1 CarepðComponentsÞ 1 CamainðComponentsÞ 1 CaopeðLabor cost 1 fuel cost 1 insurance costÞ Ccap 5 1:1 of Total capital cost 5 1:1 of ðdirect cost 1 Indirect cost 1 Other outlaysÞCacap 5 Ccap:CRFði; YprojÞ 5 Ccap: i:ð11iÞYproj j2f i5 j 5 17; f 5 20% Yproj 1 2f ð11iÞ 2 1

Annualized capital cost

Annualized replacement cost

Crap 5 CcapðIn BaseÞ:ð11iÞYproj Carep 5 Crap:FSFðI; YprojÞ 5 Crap: j ð11iÞYproj 2 1

Annualized maintenance cost Annualized operating cost operating flow cost

For Yproj 5 20; Camain 5 0:01 of Capital Cost OFC 5 ðLabor Cost 1 Fuel Cost 1 Insurance CostÞ Number of labor 5 30; Labor cost 5 400 US$ per Month Fuel costðNatural Gas PriceÞ 5 2ðUS$ per Million BtuÞ Fuel costðElectrical energy priceÞ 5 0:15ðUS$ per kWhÞ Insurance cost 5 0:02 of Capital cost NPV 5 ACS=CRFði; YprojÞ LCOP 5 NEW ACS=Total product in one year

Net present value Levelized cost of product total product in one year ðkWh electrical energyÞ Prime cost Summary of product cost

Annual benefit Net annual benefit Period of return Rate of return Additive value

VOP 5 Volume of product; PC 5 OFC=VOP COP 5 Cost of product; SOPC 5 VOP:COP COP 5 0:15ðUS$ per kWhÞ AB 5 SOPC-OFC NAB 5 AB:ð1-Tax percentÞ; Tax 5 0:1ðABÞ POR 5 Ccap=NAB ROR 5 NAB=Ccap AV 5 COP 2 PC

Integration of carbon dioxide capturing processes in hybrid energy systems

641

References [1] K. Dyl, Annual energy outlook 2018 with projections to 2050, Annual Energy Outlook 44 (8) (2018) 1
64. [2] A. Allahyarzadeh Bidgoli, N. Hamidishad, J.I. Yanagihara, The impact of carbon capture storage and utilization on energy efficiency, sustainability, and production of an offshore platform: thermodynamic and sensitivity analyses, Journal of Energy Resources Technology 144 (11) (2022) 112102. Available from: https://doi.org/ 10.1115/1.4053980. [3] A. Allahyarzadeh-Bidgoli, N. Hamidishad, J.I. Yanagihara, Carbon capture and storage energy consumption and performance optimization using metamodels and response surface methodology, Journal of Energy Resources Technology 144 (5) (2022) 050901. Available from: https://doi.org/10.1115/1.4051679. [4] A. Allahyarzadeh-Bidgoli, P.E.B. de Mello, D.J. Dezan, et al., Thermodynamic analysis and optimization of a multi-stage compression system for CO2 injection unit: NSGA-II and gradient-based methods, Journal of the Brazilian Society of Mechanical Sciences and Engineering 43 (2021) 458. Available from: https://doi.org/10.1007/s40430-02103164-5. [5] E.S. Rubin, CO2 capture and transport, Elements 4 (5) (2008) 311
317. Available from: https://doi.org/10.2113/gselements.4.5.311. [6] E. Roszak, M. Chorowski, Exergy analysis of combined simultaneous Liquid Natural Gas vaporization and Adsorbed Natural Gas cooling, Fuel 111 (2013) 755
762. [7] T. Morosuk, G. Tsatsaronis, LNG
based cogeneration systems: evaluation using exergy-based analyses, Natural Gas—Extraction to End Use, InTech, 2012, pp. 235
266. [8] R. Doctor, J. Molburg, P. Thimmapuram, Oxygen-blown gasification combined cycle: carbon dioxide recovery, transport, and disposal, Energy Conversion and Management 38 (1997) S575
S580. [9] M. Puig-Arnavat, J.C. Bruno, A. Coronas, Review and analysis of biomass gasification models, Renewable and Sustainable Energy Reviews 14 (9) (2010) 2841
2851. [10] J. Yazdanfar, et al., Energy and exergy analysis and optimal design of the hybrid molten carbonate fuel cell power plant and carbon dioxide capturing process, Energy Conversion and Management 98 (2015) 15
27. [11] H. Dong, et al., Using cryogenic exergy of liquefied natural gas for electricity production with the Stirling cycle, Energy 63 (2013) 10
18. [12] B. Ghorbani, M. Mehrpooya, K. Shokri, Developing an integrated structure for simultaneous generation of power and liquid CO2 using parabolic solar collectors, solid oxide fuel cell, and post-combustion CO2 separation unit, Applied Thermal Engineering 179 (2020) 115687. [13] M. Mehrpooya, M.R. Ganjali, S.A. Mousavi, N. Hedayat, A. Allahyarzadeh, Comprehensive review of fuel-cell-type sensors for gas detection, Industrial & Engineering Chemistry Research 62 (6) (2023) 2387
2409. Available from: https://doi. org/10.1021/acs.iecr.2c03790.

Why advanced analyses?

13.1

13

Introduction

A fraction of inadequacies that may be modified or eliminated is unable to be identified using typical exergy analysis. Additionally, these assessments cannot solely determine the percentage of exergy destruction brought on by an element’s efficiency. Recognizing the system better and much more practically increases with the determination of the portion of irreversibility which may be averted. The irreversibility of an element is separated into two categories in developed exergetic analysis. Exergy destruction is split into two categories from the start. The first half is influenced by the inefficiency of the element under consideration, whereas the second section is influenced by the plan’s design and the inefficiency of the process’s remaining elements. From a different angle, exergy destruction may be classified into two categories: preventable, which can be stopped, and second is inevitable, which is impossible to stop due to technological and financial constraints. It can further divide the exogenous and endogenous components into preventable and inevitable elements. This division makes it easier to comprehend how the components are connected and to calculate the reason for optimizing. These systems’ capacity for strengthening is discovered by evaluating them, and improvement plans may be created. Numerous energy conversion technologies have recently undergone sophisticated exergetic investigation. A rather more in-depth investigation of the economic performance of the process’s constituent parts, as well as their interactions with other equipment, is conducted in advanced exergoeconomic analysis.

13.2

Advanced economic, environmental, and exergy analyses of polygeneration systems

The source and removal capacity of the element exergy destructions cannot be determined by typical exergy analysis. The classification of the exergy destruction of the system elements is the fundamental concept of advanced exergy analysis. Exergy destruction has two components: endogenous and external, according to the theory of irreversibility: EN EX E_ D;k 5 E_ D;k 1 E_ D;k

Hybrid Poly-generation Energy Systems. DOI: https://doi.org/10.1016/B978-0-323-98366-2.00008-6 © 2024 Elsevier Inc. All rights reserved.

(13.1)

644

Hybrid Poly-generation Energy Systems

Since other elements function optimally, endogenous exergy destruction is computed. This aspect of exergy annihilation is connected to the element’s inherent irreversibilities. The procedure’s remaining elements bring about exogenous exergy loss. It is calculated by taking the proper exergy destruction and reducing the endogenous exergy destruction. EX EN E_ D;k 5 E_ D;k 2 E_ D;k

(13.2)

Based on the capacity to remove irreversibility, there are two types of energy destruction: preventable and unavoidable. UN AV E_ D;k 5 E_ D;k 1 E_ D;k

(13.3)

Amounts of inevitable exergy destruction are not able to be reduced due to financial and technical constraints. However, advancements may eliminate unnecessary exergy loss in technology in the application’s parts. The analyzer’s assumption of unavoidable circumstances determines the unavoidable component. The unavoidable and preventable exergy destructions may be determined through the use of the calculations.
UN E_ D;k UN _ _ ED;k 5 EP;k E_ P;k AV UN E_ D;k 5 E_ D;k 2 E_ D;k

(13.4)

(13.5)

The following formulas are used to compute both unavoidable exogenous and endogenous exergy degradation.
UN UN;EN EN E_ D;k E_ D;k 5 E_ P;k E_ P;k UN;EX UN UN;EN E_ D;k 5 E_ D;k 2 E_ D;k

(13.6)

(13.7)

The following relations are used to determine unnecessary exogenous exergy loss and preventable endogenous exergy loss. AV;EN EN UN;EN E_ D;k 5 E_ D;k 2 E_ D;k

(13.8)

AV;EX AV AV;EN E_ D;k 5 E_ D;k 2 E_ D;k

(13.9)

Why advanced analyses?

645

Exergy destruction and capital costs are broken down into four categories in advanced exergoeconomic analyses: unavoidable, avoidable, endogenous, and exogenous portions. The following benefits result from the irreversibility of the kth element: endogenous/exogenous; inevitable/avoidable expense. EN EN C_ D;k 5 cF:k E_ D;k

(13.10)

EX EX C_ D;k 5 cF:k E_ D;k

(13.11)

UN UN C_ D;k 5 cF:k E_ D;k

(13.12)

AV AV C_ D;k 5 cF:k E_ D;k

(13.13)

The following benefits result from the kth component’s exergy destruction that cannot be avoided: UN;EN UN;EN C_ D;k 5 cF:k E_ D;k

(13.14)

UN;EX UN;EX C_ D;k 5 cF:k E_ D;k

(13.15)

The following benefits result from the exergy degradation of the kth component that may be avoided: AV;EN AV;EN C_ D;k 5 cF:k E_ D;k

(13.16)

AV;EX AV;EX C_ D;k 5 cF:k E_ D;k

(13.17)

By assessing the values of E_ P;k , one may derive endogenous capital costs. EN

EN EN Z_k 5 E_ P;k




Z_ E_ P

real (13.18) k

By deducting the endogenous capital costs from the total capital cost, the exogenous capital cost is derived. EX EN Z_k 5 Z_k 2 Z_k



_ E_ P Z=

UN k

(13.19)

concept is used to determine capital costs that cannot be avoided.

UN Z_k 5 E_ P;k




Z_ E_ P

UN (13.20) k

646

Hybrid Poly-generation Energy Systems

Comparable to how the unavoidable capital cost is subtracted from the total investment cost to get the avoidable capital costs. AV UN Z_k 5 Z_k 2 Z_k

(13.21)

These definitions apply to unavoidable endogenous and exogenous capital costs. UN;EN EN 5 E_ P;k Z_k




Z_ E_ P

UN (13.22) k

UN;EX UN UN;EN Z_k 5 Z_k 2 Z_k

(13.23)

The following is a definition of avoidable endogenous and exogenous capital costs. AV;EN EN UN;EN 5 Z_k 2 Z_k Z_k

(13.24)

AV;EX EX UN;EX Z_k 5 Z_k 2 Z_k

(13.25)

The revised energy efficiency (εmodified), the adapted exergeoeconomic factors AV;EN ðfkAV;EN Þ, and the overall operational expenses ðC_ tot Þ are three of the most crucial performance indicators in advanced exergoeconomic assessments. According to the avoidable endogenous component, these metrics are written in terms of: εmodified 5

fkAV;EN 5

E_ P;k UN AV;EX E_ F;k 2 E_ D;k 2 E_ D;k

AV;EN Z_k AV;EN AV;EN C_ D;k 1 Z_k

(13.26)

(13.27)

AV;EN AV;EN AV;EN C_ tot 5 C_ D;k 1 Z_k

Approaches used to divide the overall expense of each piece of equipment are shown in Fig. 13.1. Tables 13.1 and 13.2 provide current cost-balancing, auxiliary, and cost formulae for the application’s constituent parts.

13.3

Advanced method procedure

An exergoeconomic assessment makes use of data on fuel costs, capital costs, effectiveness costs, and maintenance expenses. A system element’s cost is partially

Why advanced analyses?

Figure 13.1 Dividing the exergy costs related to capital expenditure and exergy loss [1].

647

648

Hybrid Poly-generation Energy Systems

Table 13.1 Cost calculation for the system’s individual parts. Component

Purchased equipment cost function

Pumps

0:8 Z 5 1120 3 W_ 0:7 Z 5 4405 3 W_   Z 5 c1 3 Q_ in ; c1 5 283 $=kWth

Turbine Heater Separator Heat exchanger

Z 5 280:3 3 m_ 0:67 in Z 5 2143 3 A0:514

Table 13.2 Hypotheses of the advanced exergoeconomic assessment. Component

Ideal and unavoidable conditions

Pumps Pumps Recuperators Absorber and condenser

ηideal 5 99:99%; ηUN 5 95% ηideal 5 100%; ηUN 5 95% ðΔTmin Þideal 5 0K; ðΔTmin ÞUN 5 0:25K ðΔTmin Þideal 5 0K; ðΔTmin ÞUN 5 0:25K

Table 13.3 The hypotheses and coefficients of exergoeconomic analysis. Economic parameter

Value

Average annual money cost rates (ieff) Average nominal escalation operating and maintenance cost rates (rOMC) Average nominal escalation fuel rates (rFC) Economic lifetime of plant (book life) Process total annual operating time (based on full load operation)

10% 5% 5% 25 years 7300 h

determined by magnitude [2]. These variables may alter throughout the system’s financial lifetime. Thus, the expenditures are calculated using yearly levelized amounts [3]. For exergoeconomic assessment, the Electricity Supply Research Institute’s Total Revenue Requirement (TRR) technique can be used as the economic model [4]. The strategy mentioned above takes into account the overall yearly cost of the system outputs. Carrying charges (CC) and costs are the two primary components of spending expenses [5]. Taxation, insurance, and total capital recovery are included in the investment expenses. Costs incurred throughout the process, such as fuel costs (in equations, FC) and operating and maintenance expenses, are what lead to expenditures (OMC) [6]. Table 13.3 lists the hypotheses and coefficients of exergoeconomic analysis. The capital recovery factor (CRF) and deviation parameter are used to calculate the levelized amounts for the overall yearly revenue need (TRRL) because of the nonuniform sequence of carrying charges (CCi) and operating expenditures (FCi, OMCi) for the ith year. It is due to the fact that the combustible and capital expenses show opposing tendencies as the system’s operating year increases [7].

Why advanced analyses?

649

The capital expense is falling, whereas the expense of gasoline is rising. The levelized yearly income need is determined using the following equation [8]. TRRL 5 CRF

BL X TRRi i 1 ð11ieff Þ

(13.28)

wherein TRRi , BL, and ieff represent the yearly level of efficient devaluing, the economic life of the system, and the income required in the ith annual operating, correspondingly. The CRF provides why all accounting entries are completed after each operational year of the operation [9]. CRF 5

ieff ð11ieff ÞBL ð11ieff ÞBL 2 1

(13.29)

Total capital requirement (TCR), return on investment (ROI), F.C., and OMC are the four yearly expenses for the proposed process (OMC). Hence, it may be determined using Eq. (13.30). TRRi 5 TCRi 1 ROIi 1 FCi 1 OMCi

(13.30)

The combustible expense, which includes the expense of the electricity supply used in it, was calculated from the following equation throughout the first annum of the process: FC0 5 cw 3 W_ 3 τ

(13.31)

whetherτ and W_ stand for the planned application’s estimated 7300 running hours and the electricity (kW) used by its various parts, accordingly. There is a cw factor (equivalent to up to nearly 0.07 $/kWh) related to the expense of electricity as well. After adding up the combustible expense at the start of the first period of activity by a reference amount, known as an inflation levelization parameter, as shown below, one may get the power costs for the ith system operating year and the levelized worth of fuel cost (FCL ). FCj 5 FC0 ð11rFC Þi FCL 5 FC0 3 CELF 5 FC0

(13.32)   BL kFC 1 2 kFC CRF ð1 2 kFC Þ

(13.33)

The continuous exponential levelization factor (CELF) and kFC are produced in this situation as below: kFC 5

1 1 rFC 1 1 ieff

(13.34)

650

Hybrid Poly-generation Energy Systems

where rFC represents the yearly percentage increase in gasoline prices and remains constant. A similar process as described below may be used to calculate the levelized yearly operation and maintenance cost (OMCL ): OMCL 5OMC0 3 CELF5OMC0 kOMC 5

  BL kOMC 1 2 kOMC CRF ð1 2 kOMC Þ

1 1 rOMC 1 1 ieff

(13.35)

(13.36)

rOMC is a constant-valued nominal expansion rate of operating and maintenance costs. The levelized carrying charges (CCL) may therefore be calculated as: CCL 5 TRRL 2 FCL 2 OMCL

(13.37)

The connection across CCL and OMCL by PECk may be used to write the expense of the kth element of the system for yearly capital investment (CI) and yearly operation and maintenance expenses (OM). CCL PECk CI P Z_k 5 τ k PECk

(13.38)

OMCL PECk OM P Z_k 5 τ k PECk

(13.39)

CI OM where (Z_k ) and (Z_k ) are yearly investment and service and operating expenditures, respectively. The item, τ, indicates the amount of time the system operated at peak capability for an annum, and PECk is the retail price of the kth element. The formulas for calculating expense interaction for the system elements are shown in Table 13.4. The exergy expense level (Z_k ) for the kth system element relies on the CI and OMC and is described by:

CCL 1 OMCL PECk OM CI P Z_k 5 Z_k 1 Z_k 5 τ k PECk

(13.40)

For the entire system, the levelized expenses index for provided expenses may be calculated as: FCl C_ F 5 τ

(13.41)

Why advanced analyses?

651

Table 13.4 Formulas relating to system element expenses. Component Heat exchanger Compressor Turboexpander Pump

Equipment cost equations  A 0:78 PECHE 5 130 0:093 PECE 5 8500 1 409ðAÞ0:8 PECC 5 7900ðHPÞ0:62 PECTE 5 378ðHPÞ0:81  0:26   1 2 ηis;p _ PECP 5 800 W10P η is;p

Flash drum Tower

PECF 5 1:218ð42 1 163W Þ PECT 5 1:218½Cb 1 NCt 1 Cp  Ct 5 457:7 expð0:1739DÞ; N 5 number of trays Cb 5 1:218 exp½6:629 1 0:1826ðlnW Þ 1 0:02297ðlnW Þ2 ; Cp 5 300D0:7396 L0:7068

OM CI Through the exergoeconomic assessment, levelized expenses including, Z_k ,Z_k _ and C F are employed as parameters. The exergy rate of flow and mean price per unit of exergy may be multiplied to get the expense rate C_ i of the ith flow. The expense level linked with the energy and mass flows for the kth system element may be written, including employing expense equilibrium and associated secondary calculations [10].

X

OM CI ðci E_ i Þk 1 Z_k 1 Z_k 5

i

X

ðc0 E_ 0 Þk

(13.42)

0

Typically, there are several outlet flows from each system element. There are more undetermined expense variables in this instance than in the expense equilibrium approach. Therefore it is required to use the thermodynamic auxiliary equations [11] as the foundation for the fuel and product (P and F) regulations. An array of linear functions is generated using the expense equilibrium and associated auxiliary equations [12]: ½E_ k  3 ½ck  5 ½Z_k 

(13.43)

To determine each element exergoeconomic parameter in the system, the unit exergetic expenses, such as the expense rates corresponding with each mass and energy flow, can be used [10]. In order to determine the unit exergy expenses involved with each system flow, the exergy assessment’s findings of the exergy destruction rate (E_ D ), exergy destruction ratio (yk ), and exergy performance (ε) are computed and used to calculate the expense equilibrium and associated auxiliary equations. The expense rate of the combustible or fuel (CF) and product (CP) may be calculated by taking into account the F and P parameters for the system element. The expense rate of the combustible and product, which become correlated to the fuel exergy (EF,k) and product exergy (Ep,k), respectively, are (EF,k) and (Ep,k) for the kth element of the system. The expense

652

Hybrid Poly-generation Energy Systems

rate for exergy destruction, which has to be calculated by exergoeconomic evaluation, is connected to the kth element and is known as the invisible expense rate. Therefore an additional combustible cost is taken into account to maintain the product’s mean price at a fixed amount and make up for the exergy destruction. An intriguing feature of the exergoeconomic assessment is comparing the system element’s exergy destruction expense rate (C_ D;k ) and capital expense (Z CI k ). This comparison compels developers to concentrate on changes needed to increase the system’s overall economic performance. The rise in the mean expense of unit of energy between the combustible and product of the kth system element is measured by the proportional expense difference (of the kth system element). The ratio of the capital expense rate to the total expenditure and energy destruction price rate may be used to compute the exergoeconomic element of the kth system element (fk). The exergoeconomic element determines the relative relevance of expenses that are connected to exergy (expense of exergy destruction rate and losses of exergy) and expenditures that are unrelated to exergy (investment expenditure in addition to maintenance and operation expense). To reduce the overall cost of the operation unless the exergoeconomic worth of a single element is low, the element efficiency must be enhanced by boosting the first law of the thermodynamic energy assessment parameter, even if it means raising the system’s capital investment [13]. However, if the worth is high, the investment expenditure must be reduced for a lower overall cost by investing in more energy-efficient equipment [13]. The comparative cost difference (rk) and exergoeconomic component (fk) supply metrics for examining the economic efficiency of the system elements, whereas exergoeconomic parameters such as capital cost rate (Z CI k ) and exergy destruction cost rate (CD,k) serve as instruction for the choice of a system element. The function fk depends on economic factors such as interest rate, yearly inflation rate, amortization component, and system-building operation. For analyzing costs depending on elements, the relative cost difference (s) is helpful [14]. Table 13.5 below presents the equation for exergoeconomic assessment.

13.4

Accessible and inaccessible sector variables and assessment

In order to understand an application of the mentioned methodologies above, an integrated process is presented in this subsection for natural gas refrigeration and N2 Table 13.5 Equations used for exergoeconomic analysis. Parameter

Equation

Exergy destruction cost rate Relative cost difference

C_ D;k 5 cF;k E_ D;k rk 5 cP;kc2F;kcF;k 5

Exergoeconomic factor

fk 5

Z_k Z_k 1 C_ D;k

1 2 εk εk

1

Z_k cF;k E_ P;k

Why advanced analyses?

653

317

318 221

Mix-203

222

AC-201

219

Mix-202

220

C-202

AC-302

Mix-201

218

C-203

316

217

AC-301

C-302

C-301

315

C-201

223 209

203

208 211

Tcc-201

NG

301 101

HE-101

212

C-101 304

128

305

114

113

302 102

303 103

111 313

112 314

210

HE-102

D-102

124

125 214

129

LNG

216

Tcc-202

204 115

130

306

V-203

V-202 206

202

D-203 D-301

213

207

V-201

205

215

D-202

D-201 201

Mix-301 312

308

HE-103

V-302 311 310

307 V-301

104

HE-104

127

126 309

V-102 HE-105

110

119

107 108

Pure propane refrigeration cycle Mixed refrigerant cycle Natural gas Nitrogen Stream for converting to LNG

Tee-102

120

118

Tee-101 121 109

D-101

116

122 117

V-101 105 123

T-101

106

Figure 13.2 A refrigeration of natural gas and N2 separation [15].

separation processes. Parts of the system for the refrigeration of natural gas and N2 separation make up the two primary components of the combined cycles. The production workflow is shown in Fig. 13.2. The fluid flow 106 from the lowest part of the tower (T-101) reaches the heat exchangers in the N2 separation subsystem at a temperature of 119 C and contains a nominal quantity of nitrogen (approximately 1.2%). A cost equilibrium based on linear equations and supporting formulae for the elements is shown in Tables 13.6 and 13.7. Exergy study outcomes showed that the system’s exergetic efficiency and exergy destruction rates are more than 41% and approximately 90 MW, respectively. The outcomes of the application’s exergy and exergoeconomic analyses are shown in Table 13.8. Exergoeconomic evaluation yields the exergy destruction expense which is a crucial metric. The system’s heat exchanger-105 and compressor-201 present the highest and lowest hourly exergy destruction expense rates, respectively, of 3210 MW and more than 40%. The exergy efficiency estimates are lowest (about 44%) and highest (approximately 98%) for tower-101 and air-cooler-301. Additionally, the irreversibility rates of compressors-301 and 201 are the maximum and lowest, correspondingly.

13.5

Avoidable and unavoidable sector variables and assessment

The advanced exergy investigation of the system is shown in Fig. 13.3. All system elements’ exergy destructions are endogenous. It is preventable for compressor

Table 13.6 Cost equilibrium for each component [15]. Equipment

Main equation

Equipment

Main equation

Equipment

Main equation

Equipment

Main equation

C 2 101

C_ 124 1 C_ W 1ZC-101 5 C_ 125

AC 2 302

C_ 318 1 C_ W 1ZAC-302 5 C_ 301

Tee 2 101

C_ 118 5 C_ 119 1 C_ 120

D 2 101

C_ 116 1 ZD2101 5 C_ 117 1 C_ 118

C 2 201

C_ 127 1 C_ W 1ZC-201 5 C_ 218

V 2 101

C_ 121 5 C_ 122

Tee 2 102

C_ 107 5 C_ 108 1 C_ 110

D 2 102

C_ 128 1 ZD2102 5 C_ 129 1 C_ 130

C 2 202

C_ 219 1 C_ W 1ZC-202 5 C_ 220

V 2 102

C_ 127 5 C_ 128

Tee 2 201

C_ 202 5 C_ 204 1 C_ 206

D 2 201

C_ 201 1 ZD2201 5 C_ 202 1 C_ 203

C 2 203

C_ 221 1 C_ W 1ZC-203 5 C_ 222

V 2 201

C_ 223 5 C_ 201

Tee 2 202

C_ 208 5 C_ 210 1 C_ 212

D 2 202

C_ 207 1 ZD2202 5 C_ 208 1 C_ 209

C 2 301

C_ 315 1 C_ W 1ZC-301 5 C_ 316

V 2 202

C_ 206 5 C_ 207

Mix 2 201

C_ 215 5 C_ 216 1 C_ 217

D 2 203

C_ 213 1 ZD2203 5 C_ 214 1 C_ 215

C 2 302

C_ 317 1 C_ W 1ZC-302 5 C_ 318

V 2 203

C_ 212 5 C_ 213

Mix 2 202

C_ 209 1 C_ 211 1 C_ 218 5 C_ 219

D 2 301

C_ 304 1 ZD2301 5 C_ 305 1 C_ 306

AC 2 201

C_ 222 1 C_ W 1ZC-201 5 C_ 223

V 2 301

C_ 307 5 C_ 208

Mix 2 203

C_ 203 1 C_ 205 1 C_ 220 5 C_ 221

T 2 101

C_ 105 1 C_ 109 1 C_ 122 1 C_ 123 1 ZT2101 5 C_ 106 1 C_ 107

AC 2 301

C_ 316 1 C_ W 1ZC-301 5 C_ 317

V 2 302

C_ 310 5 C_ 311

Mix 2 301

C_ 308 1 C_ 312 5 C_ 313

HE 2 101

C_ 101 1 C_ 301 1 C_ 114 1 C_ 204 1 Z˙HE2101 5 C_ 102 1 C_ 302 1 C_ 115 1 C_ 205

HE 2 102

C_ 102 1 C_ 302 1 C_ 113 1 C_ 210 1 Z˙HE102 5 C_ 103 1 C_ 303 1 C_ 114 1 C_ 211

HE 2 103

C_ 103 1 C_ 303 1 C_ 112 1 C_ 214 1 C_ 314 1 Z˙HE103 5 C_ 104 1 C_ 304 1 C_ 113 1 C_ 216 1 C_ 315

HE 2 105

C_ 309 1 C_ 126 1 C_ 120 1 C_ 108 1 C_ 110 1 C_ 311 1 Z˙HE105 5 C_ 310 1 C_ 127 1 C_ 121 1 C_ 109 1 C_ 111 1 C_ 312

HE 2 104

C_ 306 1 C_ 305 1 C_ 125 1 C_ 104 1 C_ 117 1 C_ 106 1 C_ 119 1 C_ 111 1 C_ 313 1 Z˙HE104 5 C_ 309 1 C_ 307 1 C_ 126 1 C_ 105 1 C_ 124 1 C_ 116 1 C_ 123 1 C_ 112 1 C_ 314

Table 13.7 Auxiliary equations for each component. Equipment

Auxiliary equation

Equipment

Auxiliary equation

T 2 101

C_ 106 E_ 106 C_ 119 E_ 119 C_ 108 E_ 108 C_ 206 E_ 206 C_ 210 E_ 210 C_ 204 E_ 204 C_ 210 E_ 210

D 2 101

C_ 117 E_ 117 C_ 129 E_ 129 C_ 202 E_ 202 C_ 208 E_ 208 C_ 214 E_ 214 C_ 305 E_ 305

Tee 2 101 Tee 2 102 Tee 2 201 Tee 2 202 HE 2 101 HE 2 102

5 5 5 5 5 5 5

C_ 107 E_107 C_ 120 E_120 C_ 110 E_110 C_ 204 E_204 C_ 212 E_212 C_ 205 E_205 C_ 211 E_211

D 2 102 D 2 201 D 2 202 D 2 203 _ ; CE_ 114 114 _ ; CE_ 113 113

5 5

C_ 115 E_ 115 C_ 114 E_ 114

_ C_ 101 ; CE_102 2 _ 102 2 E 101 _ C_ 102 ; CE_103 2 _ 103 2 E 102

5 5

C_ 302 2 C_ 301 E_ 302 2 E_ 301 C_ 302 2 C_ 301 E_ 303 2 E_ 302

D 2 301 HE 2 104

5 5 5 5 5

C_ 118 E_118 C_ 130 E_130 C_ 203 E_203 C_ 209 E_209 C_ 215 E_215 C_ 306 E_306

5 C_ 313 5 E_ 313

C_ 314 C_ 111 C_ 112 C_ 117 C_ 124 C_ 106 C_ 116 C_ 119 C_ 123 ; 5 ; 5 ; 5 ; 5 _ _ _ _ _ _ _ _ E314 E111 E112 E117 E124 E106 E116 E119 E_ 123

C_ 105 2 C_ 104 C_ 309 2 C_ 306 C_ 307 2 C_ 305 C_ 126 2 C_ 125 5 5 5 E_ 105 2 E_ 104 E_ 309 2 E_ 306 E_ 307 2 E_ 305 E_ 126 2 E_ 125 HE 2 103

C_ 214 E_ 214

5

C_ 216 C_ 314 ; E_216 E_ 314

5

C_ 315 C_ 112 ; E_ 315 E_ 112

5

C_ 113 C_ 104 2 C_ 103 ; E_ 113 E_ 104 2 E_103

5

C_ 304 2 C_ 303 E_ 304 2 E_ 303

HE 2 105

C_ 311 E_ 311

5

C_ 312 C_ 110 ; E_ 312 E_ 110

5

C_ 111 C_ 127 2 C_ 126 ; E_ 111 E_ 127 2 E_ 126

5

C_ 121 2 C_ 120 E_ 121 2 E_ 120

5

C_ 109 2 C_ 108 E_ 109 2 E_ 108

5

C_ 310 2 C_ 309 E_ 310 2 E_ 309

Table 13.8 Results of exergy and exergoeconomic analysis of the process components. Component

E˙ F (kW)

E˙ P (kW)

E˙ D (kW)

cF ($/Gj)

cP ($/Gj)

C˙ D ($/hr)

˙ Z($/hr)

ε (%)

yD (%)

r (%)

f (%)

C-101 C-201 C-202 C-203 C-301 C-302 AC-201 AC-301 AC-302 HE-101 HE-102 HE-103 HE-104 HE-105 T-101

2967.26 2207.14 7290.34 40145.49 71657.47 27949.3 45740.43 81054.31 99011.76 3935.54 6731.42 8299.72 73150.62 64802.66 13356.03

1841.67 1634.72 5489.33 32119.2 56782.42 22312.78 43443.91 79268.98 96554.04 624.9 3223.44 4869.68 71148.06 52337.23 5872.02

1125.59 572.42 1801.01 8026.29 14875.05 5636.52 2296.51 1785.32 2457.72 3310.63 3507.98 3430.05 2002.56 12465.43 7484.01

19.72 19.72 19.72 19.72 19.72 19.72 19.72 19.72 19.72 117.27 117.64 95.69 60.02 71.55 4.77

45.44 39.44 34.2 28.59 28.08 29.23 21.17 20.37 20.4 744.6 246.9 164 61.78 88.68 12.64

79.91 40.64 127.86 569.8 1056.01 400.15 163.03 126.74 174.48 1397.69 1485.62 1181.56 432.66 3210.75 128.43

90.62 75.43 158.22 455.62 652.55 364 63.36 58.55 60.7 13.57 14.37 16.03 19.38 17.2 38.03

62.07 74.07 75.3 80.01 79.24 79.83 94.98 97.8 97.52 87.35 88.67 89.29 90.73 90.43 43.97

0.74 0.37 1.18 5.24 9.72 3.68 1.5 1.17 1.61 2.16 2.29 2.24 1.31 8.14 4.89

130.43 100.01 73.41 44.97 42.38 48.24 7.34 3.29 3.43 534.93 109.88 71.39 2.94 23.95 165.19

53.14 64.99 55.31 44.43 38.19 47.63 27.99 31.6 25.81 0.96 0.96 1.34 4.29 0.53 22.85

Why advanced analyses?

657

Figure 13.3 Advanced exergy destruction rates of the process components [15].

exergy destructions. Therefore enhancing their efficiency may raise the system’s efficiency. However, exergy destruction in the heat exchangers and air coolers is unavoidable. According to Fig. 13.3, the compressors-301 and -203 and heat exchanger-105 have greater endogenous exergy degradation rates than the other components. Per preventable endogenous exergy degradation rates, system efficiency may be enhanced by increasing the compressors’ thermodynamic efficiency. The splitting of the exergy destruction costs of the system elements is shown in Fig. 13.4. The application’s whole exergy destruction cost is endogenous. As a result, the interactions between process components cannot considerably alter the cost of exergy destruction. Exergy destruction rates in the compressors are avoidable, much as exergy destruction rates. Heat exchangers and coolers must be used when in the air. Compressors offer the possibility for development in this situation, whereas air coolers and heat exchangers are constrained by cost and technology. The heat exchangers have a greater rate of unavoidable exergy destruction compared to the remaining system elements, as shown in Fig. 13.4. The high-value of preventable endogenous exergy destruction expense rates suggests that modifications must be made to each element in order to reduce the element’s expense. The compressors used in operation are relevant in this scenario. The findings of partitioning the elements’ capital cost rates are shown in Fig. 13.5. All process element parts’ capital cost rates are endogenous. Compressor investment cost rates cannot be avoided, although they may be in air coolers and heat exchangers. According to Fig. 13.5, compressor-301 has the greatest unavoidable and unavoidable endogenous capital cost rates among the system elements. The heat exchangers’ unavoidable endogenous investment cost rates are more

658

Hybrid Poly-generation Energy Systems

2400

Advanced exergy destruction cost ($/hr)

2200 2000 1800 1600 1400 1200 1000 800 600 400 200 0 •

CD(EN) • CD(EX) • CD(UN) • CD(AV) • CD(AVEN) • CD(AVEX) • CD(UNEN) • CD(UNEX)

C-101 71.28 8.63 26.87 53.04 47.31 5.73 23.97 2.90

C-201 35.53 5.10 13.68 26.95 23.57 3.38 11.97 1.72

C-202 117.14 10.72 43.02 84.84 77.72 7.11 39.41 3.61

C-203 436.22 133.58 192.74 377.07 288.67 88.40 147.55 45.19

C-301 815.92 240.09 363.04 692.97 535.42 157.55 280.50 82.54

C-302 AC-201 AC-301 AC-302 HE-101 HE-102 HE-103 321.94 128.05 105.00 140.48 978.13 1138.89 929.06 21.75 34.99 34.00 419.56 346.73 252.50 78.21 69.45 114.12 88.72 122.14 1215.99 1292.49 945.24 52.34 181.70 193.13 236.31 330.70 48.91 38.02 266.06 38.41 42.14 127.16 148.06 185.81 31.50 50.50 6.52 45.08 64.64 10.20 54.54 10.50 55.88 98.34 850.97 990.84 743.25 73.50 89.63 24.49 23.80 365.02 301.66 202.00 13.57 15.22

HE-104 271.68 160.98 303 129.80 81.50 48.29 190.18 112.69

HE-105 2203.94 1006.81 2247.52 963.22 661.18 302.04 1542.76 704.77

Figure 13.4 Advanced exergy destruction cost rates of the process components [15].

600

Advanced investment cost ($/hr)

550 500 450 400 350 300 250 200 150 100 50 0 •

Z(EN) • Z(EX) • Z(UN) • Z(AV) • Z(AVEN) • Z(AVEX) • Z(UNEN) • Z(UNEX)

C-101 80.83 9.78 81.56 9.06 8.08 0.98 72.75 8.81

C-201 65.95 9.47 67.88 7.54 6.60 0.95 59.36 8.53

C-202 144.95 13.27 142.40 15.82 14.50 1.33 130.46 11.94

C-203 348.81 106.82 410.06 45.56 34.88 10.68 313.92 96.13

C-301 504.19 148.36 587.29 65.25 50.42 14.84 453.77 133.52

C-302 292.85 71.15 327.60 36.40 29.29 7.11 263.57 64.03

AC-201 49.76 13.60 28.51 34.85 27.37 7.48 22.39 6.12

AC-301 48.50 10.05 26.35 32.20 26.68 5.53 21.83 4.52

AC-302 48.87 11.83 27.31 33.38 26.88 6.50 21.99 5.32

HE-101 9.50 4.07 6.11 7.47 5.22 2.24 4.27 1.83

HE-102 11.02 3.35 6.47 7.91 6.06 1.85 4.96 1.51

HE-103 12.61 3.43 7.22 8.82 6.93 1.88 5.67 1.54

HE-104 12.17 7.21 8.72 10.66 6.69 3.97 5.48 3.24

HE-105 11.80 5.39 7.74 9.46 6.49 2.97 5.31 2.43

Figure 13.5 Advanced investment cost rates of the process components [15].

significant than those of the remaining system elements. The process may enhance the compressor’s efficiency because of the avoidable endogenous investment cost rates.

Why advanced analyses?

659

Table 13.9 Comparison of the evaluated results between the exergy and exergy and exergoeconomic analyses for the conventional and advanced methods. Component

C-101 C-201 C-202 C-203 C-301 C-302 AC-201 AC-301 AC-302 HE-101 HE-102 HE-103 HE-104 HE-105

Conventional

Advanced

ε (%)

f (%)

˙ CTOT ($/h)

ε modified (%)

f AV,EN (%)

˙ CTOTAV,EN ($/h)

62.07 74.07 75.3 80.01 79.24 79.83 94.98 97.8 97.52 87.35 88.67 89.29 90.73 90.43

53.14 64.99 55.31 44.43 38.19 47.63 27.99 31.6 25.81 0.96 0.96 1.34 4.29 0.53

170.53 116.06 286.07 1025.42 1708.56 764.15 226.39 185.29 235.17 1411.26 1500 1197.59 452.04 3227.94

73.43 83.12 83.37 88.76 88.28 85.62 98.77 99.44 99.39 97.48 90.22 90.03 99.47 95.32

14.59 21.87 15.72 10.78 8.61 9.92 41.6 45.86 38.94 3.95 3.93 3.6 7.59 0.97

55.4 30.16 92.22 323.55 585.84 295.34 65.78 58.18 69.02 132.38 154.12 192.75 88.2 667.67

The comparison of the conventional and advanced exergoeconomic analyses is shown in Table 13.9. The major performance indicators are provided, including exergy efficiency, exergoeconomic factors, and overall expenses. The adjusted exergy efficiency is greater than the exergy efficiency for each system’s elements. The heat exchanger-105 must initially be adjusted based on overall expense. In the traditional exergoeconomic study, the compressor-201 has the greatest exergoeconomic component, but the air cooler-301 has the greatest adjusted exergoeconomic indicator. In both basic and advanced exergoeconomic analyses, the heat exchanger-105 has the lowest exergoeconomic component.

13.6

Benefits and disadvantageous of advanced analysis methodology for energy systems

According to advanced exergy evaluation, advanced exergoeconomic assessment divides capital costs and exergy destruction percentages of the system element into exogenous/endogenous and avoidable/unavoidable portions. As expressed in a different sense, expense percentages are exogenously produced through other system elements while endogenously induced by each element [13]. Between preventable and unavoidable expenditures, there is a distinction. The unnecessary expense may be disregarded throughout system revamping and servicing intervals and is based on the system operating action. Analyzing such expenses is beneficial since it helps identify charges that do not increase earnings [16,17]. Costs cannot be avoided because of both technical and

660

Hybrid Poly-generation Energy Systems

financial constraints. This indicates that such charges are necessary since there is a discrepancy between a device’s actual and planned functioning.

References [1] M. Mehrpooya, S.A. Mousavi, Advanced exergoeconomic assessment of a solar-driven Kalina cycle, Energy Conversion and Management 178 (2018) 7891. [2] L. Khani, et al., Energy and exergoeconomic evaluation of a new power/cooling cogeneration system based on a solid oxide fuel cell, Energy 94 (2016) 6477. [3] G. Tsatsaronis, Design optimization using exergoeconomics, Thermodynamic Optimization of Complex Energy Systems, Springer, 1999, pp. 101115. [4] Pietsch, J., et al., TAG [trademark] Technical Assessment Guide: Volume 2, Electricity end use. 1992, Electric Power Research Inst., Palo Alto, CA (United States); Pietsch . . .. [5] U. Yildirim, A. Gungor, An application of exergoeconomic analysis for a CHP system, International Journal of Electrical Power & Energy Systems 42 (1) (2012) 250256. [6] A. Kazim, Exergoeconomic analysis of a PEM fuel cell at various operating conditions, Energy Conversion and Management 46 (78) (2005) 10731081. [7] A. Bejan, G. Tsatsaronis, M.J. Moran, Thermal Design and Optimization, John Wiley & Sons, 1995. [8] S. Mert, I. Dincer, Z. Ozcelik, Exergoeconomic analysis of a vehicular PEM fuel cell system, Journal of Power Sources 165 (1) (2007) 244252. [9] M. Mehrpooya, F. Gharagheizi, A. Vatani, Thermoeconomic analysis of a large industrial propane refrigeration cycle used in NGL recovery plant, International Journal of Energy Research 33 (11) (2009) 960977. [10] A. Abusoglu, M. Kanoglu, Exergoeconomic analysis and optimization of combined heat and power production: a review, Renewable and Sustainable Energy Reviews 13 (9) (2009) 22952308. [11] A. Lazzaretto, G. Tsatsaronis, On the quest for objective equations in exergy costing, ASME International Mechanical Engineering Congress and Exposition, American Society of Mechanical Engineers, 1997. [12] A. Ozbilen, I. Dincer, M.A. Rosen, Development of a four-step CuCl cycle for hydrogen productionPart I: exergoeconomic and exergoenvironmental analyses, International Journal of Hydrogen Energy 41 (19) (2016) 78147825. [13] A. Gungor, et al., Advanced exergoeconomic analysis of a gas engine heat pump (GEHP) for food drying processes, Energy Conversion and Management 91 (2015) 132139. [14] H. Ozcan, I. Dincer, Exergoeconomic optimization of a new four-step magnesiumchlorine cycle, International Journal of Hydrogen Energy 42 (4) (2017) 24352445. [15] M. Mehrpooya, M.M.M. Sharifzadeh, H. Ansarinasab, Investigation of a novel integrated process configuration for natural gas liquefaction and nitrogen removal by advanced exergoeconomic analysis, Applied Thermal Engineering 128 (2018) 12491262. [16] F. Cziesla, G. Tsatsaronis, Z. Gao, Avoidable thermodynamic inefficiencies and costs in an externally fired combined cycle power plant, Energy 31 (1011) (2006) 14721489. [17] A. Lazzaretto, G. Tsatsaronis, SPECO: A systematic and general methodology for calculating efficiencies and costs in thermal systems, Energy 31 (89) (2006) 12571289.

Index

Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively. A Absorption refrigeration-based polygeneration systems, 615 616 Acidic property, 93 94 Activation losses, 205 206, 210 211 Additional value (AV), 562 569 Adiabatic compression energy storage (A-CAES), 11 Advanced analyses benefits and disadvantageous of, 659 660 description, 643 Air Preheater Exchanger, 301 Alkaline Fuel Cell (AFC), 194, 374 Alumina and titania, 588 597 Ammonia (NH3), 76 Anode electrode, 194 Anode Gas Recycle (AGR), 256 257 Ash, 92 Autothermal reforming (ATR), 398

Biomass to energy, conversion of, 76 78 anaerobic digestion, 77 biochemical processes, 77 fermented ethanol production, 77 fermented hydrogen production, 77 78 thermochemical processes, 76 77 Biomass transport, 99 Bio-oil, 89 94 BFD of, 100f characteristics of, 90t classification of, 95t properties of, 91 94 quality of, 95 102 Bio-oil recovery, 335 Bio-oil transfer unit, 100 Biorefinery, 99 100, 335 BFD of, 101f Bismuth, 228 Braking horsepower (BHP), 148 Bunsen system, 408 409, 515 516, 517f

B Back surface reflector, spectral control using, 478 480 Biochemistry, 315 Biomass drying, 335 Biomass energy, 68 75 advantages, 75 characteristics, 461t complexity of, 70 composition, 314 conversions for, 315, 316f disadvantages, 75 76 first generation of, 70 and photobiological processes, 460 506 second generation of, 70 thermodynamic properties of, 314 315 types of, 70, 313 314, 314t

C CAES. See Compressed air energy storage Calcium oxide carbonation (CaO), 37 Calorific value, 92 Capital cost rate, 652 Capital recovery factor (CRF), 648 650 Carbon capture and storage (CCS), 619 Carbon conversion efficiency (CCE), 627 Carbon dioxide (CO2), 76 adsorption process, 625 626 capturing, separation, and liquefaction methods, 7 8, 8f cryogenic adsorption of, 627 628 liquefaction, 555 power generation cycle, 624, 628 Carbon monoxide (CO), 76 Catalyst, 88 89

662

Catalyst (Continued) inorganic solutions, 88 mesoporous materials, 89 metal-based materials, 89 metal oxides, 88 microporous materials, 89 Cathode electrode, 193 194 Cathode Gas Recycle (CGR), 257 CCHP. See Combined cooling, heating, and power Cell energy gap (Eg), 17t, 482 Cells array and emitter, 491 and transmitter, 494 Cells array temperature, 491 Cell temperature determination, 484 485 Cellulose, 71 Cell wavelength (λg), 481 Ceria-based oxides (CeO2), 228 Cetane number, 93 Chemical energy storage reservoir, 133 134 Chemical exergy, 296 Chemical looping (CL), 318 333, 373 374 Chemical looping processes, 321 operational conditions in, 333 two reactor, 378 391, 378f Chemical looping combustion (CLC), 318 319 gas fuel, 324 liquid fuel, 325 solid fuels, 324 solid fulled, 324 Chemical looping system reactors, 325 327 optimal, 325 326 oxygen carrier use, 328 333 periodic reactors of packed or fluidized bed, 327 rotating reactors, 327 two or more interconnected reactors of a fluid bed or a movable bed, 326 327 Chemical looping with oxygen uncoupling (CLOU), 325 reactions of, 325 CHP. See Combined heat and power Climate change, 7 8 CLC. See Chemical looping combustion Cloud point, 93 CO2 capture, 5 6, 8f with chemical cycle process, 321

Index

using wind energy in a multiproduction energy system, 53 54 Coal gasification, 627 process, 624 625 Coefficient of Performance (COP), 127 Cogeneration cycle, 573 607 Cold gas efficiency (CGE), 488, 627 Combined cooling, heating, and power (CCHP), 167 Combined cycle sections, 472 481, 540 gas fuel saturation, 472 473 gas fuel saturation model, 473 476 Combustion chamber, 300 Complete cooling unit, 539 546 Complete gas clearing unit, 539 542 Compressed air energy storage (CAES), 125, 136 137 Compressed air storage (CAS), 125 126 Compression factor, 494 Compressor, 297, 300 Concentrated solar power (CSP) system, 171 Concentrating collectors, 414 Concentrating photovoltaic thermal (CPVT) system, 412 413 Concentration losses, 206, 212 213 Constant-volume procedures, 610 Cooling effect, 65 Copper, 587 Corrosive property, 93 94 Cost of direct investment, 489 490 Cryogenic air separation, 614 615, 622 623, 626 Cryogenic biogas, 184, 185f Cu-Cl thermochemical cycle, 406 Current density, 301 Cutoff wavelength (λ0), 481 D DC to AC Converter, 302 Dead state, 150 Decentralized collector, 413 414 Desulfurization, 246 247 Deviation parameter, 648 650 Diabatic-CAES (D-CAES), 11 Diesel, 77 Direct combustion, 76 Direct methanol fuel cell (DMFC), 374 Distillation and hydrocracking unit, 340 341, 341f

Index

Diversion, 102 Doped LaGaO3, 228 Drying, 460 463, 462f E Economic analysis, 363 366, 364t Economic evaluation, 498 501 ECONOMZR, 476 Efficiency, 496 498 of proposed hybrid system, 498 Electrical efficiency, 265 266 Electrical energy, 144 145 Electrical storage, 125 Electric power, 125 Electrochemical cycle, 411 412, 413f Electrochemical energy storage, proposed process configurations for, 137 144 fuel cell electrochemical for power plant system, 138 144 Electrochemical reaction, 250 251 Electrolyte, 194 Electrolyze(r), 213 236 alkaline electrolyzer cell, 219 221 combined systems, 232 233 history of water, 213 215 integration of electrolyzers with other systems, 232 principles of, 215 216 proton exchange membrane electrolyzer cell, 222 223 requirements for, 263t reversible fuel cells, 231 232 SOEC, 223 231 SOFC SOEC combined cycle, 233 236 types of, 219 232 water electrolysis theory, 216 219 Electronic load controller (ELC), 112 Emitter temperature, 490 Endogenous exergy destruction, 125 Energy analysis, 147 Energy consumption, 528 529 Energy efficiency, 356 357 of biorefinery system, 357 of fast pyrolysis-gasification system, 357 of equipment, 562t Energy storage, 4, 4f Engineering and home office fees, 543

663

Entropy, 149 Environment, defined, 150 Environmental effects, 159 Environmental state, 150 Exergoeconomic analysis, 646 648 equations used for, 652t hypotheses and coefficients of, 648t hypotheses of advanced, 648t Exergoeconomic factor, 163 Exergy analysis, 300 303 asymmetric energy (irregular), 149 classification of, 148 149 concept of, 149 155 control volume, 149 151 due to heat transfer, 295 due to material flow, 295 297 due to work, 295 efficiency criteria, 153 155 equations, 293 295 solid fuels, 152 153 stable flow, 151 152 symmetric energy (regular), 148 149 Exergy destruction cost rate, 652 distribution of, 566f of equipment, 565t Exergy efficiency, 153 and equilibrium, combined cycle components, 297 300 of equipment, 563t Exhaust gases, 303 Expander power process, 375 External exergy destruction, 643 645 External quantum efficiency, 491 External reforming, 248 249 reactions, 250 F Fast pyrolysis, 69, 80, 335 337 auger reactor, 84 85, 85f bubbling fluidized bed, 81 circulating fluidized bed, 81 83 flow diagram of, 336f reactors used in, 80 85 rotating cone pyrolyser, 83 84 vacuum pyrolysis, 84 Fast pyrolysis reaction, affecting factors in, 85 89 biomass moisture content, 87

664

Fast pyrolysis reaction, affecting factors in (Continued) biomass particle size, 87 biomass type, 86 catalyst, 88 89 feed preheat, 87 heat rate and temperature, 85 86 FESS. See Flywheel energy storage systems Fill factor, 482 484 Five-stage cycle, 411, 412f Flashpoint, 93 Flywheel, 125, 136 Flywheel energy storage systems (FESS), 136 137 Focus ratio, 32 33 Fuel cell, 193, 374, 573 comparison of, 195t mass, 251 252 necessary operations on, 246 251 output results, 268t requirements parameters, 254t system, 244 257 types of, 194 213 Fuel cell mass, 251 Fuel reactor (FR), 318 319 G Gain-output ratio (GOR), 557 Gas and combined power cycles, 375 376 GaSb cells, 17, 17f Gasification, 69, 76 77, 315 318, 337 339, 464 465 heating sources, 318 high-temperature and low-temperature, 465f reactions of, 317t, 339t theory, 315 318 Gasifiers, types of, 319f Gas turbines, 573, 576, 576f General facility cost, 542 543 Geothermal energy, 54 68 combined desalination and CCHP system by, 62 66 Global energy consumption, 52 GRAFSTRR reactor, 41 42, 43f Graham’s law, 237 238 Greenhouse gas emissions, 101f, 373 rate, 357 358, 358t Gross domestic product (GDP), 619

Index

H Half-length collector, 22 27, 26f HCC. See Hybrid charge controller Heat exchanger, 298 Heat recovery, 415, 577 579 Heat recovery unit (HRS), 56 Heat recuperation, 376 377 Heat source for integration, 418 419 Heat transfer coefficient, 587 Heat transfer fluid (HTF), 22 Heliostat field collectors (HFC), 133 134, 414 Hemicellulose, 71 72 HIX system, 518 diagram block, 519f process response, 518t Higher thermal value (HHV), 315 High-temperature decomposition, 402 403 High-temperature electrolysis cell, 219 High-temperature heat pump (HTHP), 51 52 High-temperature hybrid electrolyzers, 419 459 cycle economical evaluation, 424 426, 427t cycle process complexity, 424, 425t, 426t cycle process efficiency, 423, 423t cycle process temperature, 422 423, 423t safety and environmental compatibility, 423, 424t suitable cycles for research, 427, 428t thermochemical reactions, 420 427 Homogeneity, 91 HPGS. See Hyper polygeneration system HRS. See Heat recovery unit Hybrid biomass energy systems, 333 366 Hybrid CCS environmental and economic analysis of, 638 640 exergy and energy analysis of, 633 637 Hybrid charge controller (HCC), 110 Hybrid energy systems, 9 Hybrid polygeneration systems, 5 9 Hybrid systems, 232, 557f Hydrocarbons, 76, 321 Hydrodeoxygenation (HDO), 89 Hydrogen (H2), 77 78, 240, 397

Index

Hydrogen chloride (HCl), 66, 76 integrated process configuration for production of, 66 68, 67f Hydrogen generation reactor, 440 442, 441f mass distribution in the reactor, 457 459 model geometry, 441 model hypothesis, 441 442 Hydrogen iodide, 409 Hydrogen process unit, 339 340, 340f Hydrogen production, 5 7, 7f from biomass, 399 400, 400f from coal, 398 399 from fossil fuels, 397 399 from natural gas, 398 from water splitting, 400 403 fuel-based, 395 403 by a multipurpose cycle consisting of wind turbine and heliostats, 50 51 principles of, 403f Hydrogen production sector, 469 471 gas water displacement reaction system, 469 471, 470f pressure swing adsorption blocks and purging, 471, 472f Hydrogen sulfide (H2S), 76 Hydropower, hybrid power generation of, 104 113 hydro, solar, and wind hybrid power generation systems, 110 113, 112f hydrosolar hybrid power generation systems, 104 107 hydrowind hybrid power generation systems, 107 110, 109f, 110f Hyper polygeneration system (HPGS), 55 application of, 55 68 Hypothetical environment, 149 150 I Ideal parabolic collector, 22 27, 26f Impoundment structure, 102 Incremental worth, 562 569 InGaAsSb, 17, 18f, 20 21 In-situ gasification chemical looping combustion (iG-CLC), 324 Interference, 21 Intergovernmental Panel on Climate Change (IPCC), 373 Iodine/sulfur cycle, 401, 402f Ionic conductivity, 211

665

Isentropic procedures, 610 Isentropic systems, 610 Isothermal CAES, 136 137 J Joule Brayton refrigeration processes, 612 614, 613f K Kalina cycle, 177 178, 177f Kalina turbine, 64 L Latent heat storage (LHS), 125 126 Lignin, 72 75 Lignocellulose isolation and decomposition of, 72 74, 75f Lime (CaO3), 37 Linear Fresnel reflector (LFR), 414 Liquified natural gas (LNG), 559 560 compounds, 623t cooling sink, 553 heat sink, 59 62, 59f re-evaporation of, 623, 628 Lower thermal value (LHV), 315 Low-temperature cleaning section, 469 Low-temperature electrolysis cell, 219 M Mechanical energy storage methods, 5f, 125 benefits and limitations of, 136 137 Methane reforming (MR), 321 Methane vapor reforming, 322 323 Methanol and hydrogen production unit, 540 542 Methanol production, 472, 473f Mg-Cl thermochemical decomposition cycle, 404 405 Minimum cold utility (QCmin), 415 Minimum hot utility (QHmin), 415 Mixers, 299 Model geometry, 431 437 boundary conditions and initial value, 436 437 governing equation, 435 436 material properties, 431, 432t model hypothesis, 431 432 Model solving algorithm, 488

666

Model thermodynamic sensitivity analysis, 501 506 Molten Carbonate Fuel Cell (MCFC), 194, 304 305, 374 Multi-effect desalination (MED)operation, 171 Multiphase microscopic structure, 91 N Natural gas, 35 36, 248t, 321 Net present value (NPV), 364 365 Nickel, 219 221 Nitrogen (N2), 76 Nonconventional flow, 337 NPV. See Net present value Nusselt number, 588, 594 O Ohmic losses, 206, 211 Open circuit voltage (Voc), 481 482, 493 vs. cell temperature, 493f vs. transmitter temperature, 494f Open-loop thermochemical cycle, 409 410, 410f Operating and maintenance costs, 543 545, 545t Optical filter, 19f Optical filter efficiency, 487 Optical filter transmissivity coefficient, 491 ORC. See Organic Rankine cycle Organic Rankine cycle (ORC), 51 52, 573 with nanofluid, 581f simple, 56f temperature-specific entropy (T-s) of, 57f thermal profile of, 581t Osmotic pressure, 65 Output power density, 494 496 vs. cell temperature, 495f vs. emitter temperature, 496f Oxidizer (oxidizer), 193 Oxidizing, 194 Oxyfuel, 620 621, 621f Oxyfuel electric plants, 555 Oxy-fuel power generation, 8 9 Oxygen, 91, 330 Oxygen carriers (OC), 319 Ce-based, 330 Cu-based, 329 330 iron-based, 330 332

Index

Ni-based, 328 Ni-free, 329 optimal characteristics of, 328 perovskite, 330 reactivity and stability of, 319 320 types of, 330 333 Oxygen conductor ceramics, 228 231 Oxygen-fuel power generation, 553 P Packed bed thermal energy storage (PBTES), 126 Parabolic dish collector (PDR), 27 28, 28f Parabolic dish reflector (PDR), 414 Parabolic solar collector, 534 Parabolic trough solar collectors (PTC), 22, 414 Partial oxidation (POX), 398 Peng-Robinson (PR) equation, 244 Period of return (POR), 562 569 Permeate hydraulic pressure, 65 Permeate water, 65 Phase change materials (PCM), 4 Phosphoric Acid Fuel Cell (PAFC), 194, 374 Photobiological hydrogen production, 402 403 Photoelectrolysis, 401 principles of, 401f Photovoltaic cells, 12 21, 12f Photovoltaic thermal system, 412 413 PHS. See Pumped hydro storage Physical exergy, 298 Pinch technology, 363 Plasma filters, 477 478 combination of interference and, 478 Plasma gas, 460 466, 461f, 467f Plasma gasification, 490 operating parameters for, 491t Plasma melting, 465 466, 466f Polygeneration, 1 Polygeneration systems (PGSs), 11, 54 68 biomass energy used in, 68 102 concentrated and photovoltaic solar systems in, 167 171 energy analysis procedure in, 147 148 hydroenergy systems and, 102 104 ORCs in, 611 preliminary and advanced economic analysis of, 160 164

Index

SOFCs in, 237 305 Stirling engines and, 609 610, 610f thermoelectric/thermionic generators in, 608 609 use wave energy resources, 113 114 Polymer Solid Fuel Cell (PSFC), 194 Postcombustion, 619 628, 620f Potassium hydroxide, 219 221 Pour point, 93 Power electrical plant, 559 560 Power generation, 343 345 Practical heat storage (SHS), 125 126 Prandtl number, 594 Pressure drop calculation, 584 607 Preventable energy destruction, 619 Process facility cost, 542 Process integration, 525 528 thermochemical cycle integration, 525 527 thermochemical cycle integration with a solar unit, 527 528 Process sensitivity analysis, 345 356 biomass type effect on bio-oil compounds, 345 346, 345f Eucalyptus biomass, 346, 346f, 347f pyrolysis type effect on bio-oil compounds, 346 348 simulation achieved process information, 349 steam reforming temperature effect, 355 356, 356f steam to carbon ratio effect on gasification, 349 354, 355f steam to carbon ratio effect on steam reforming, 356, 357t temperature effect on production rate by fast pyrolysis, 348 Profit function, 546 Project contingency, 543 Proton Exchange Membrane Fuel Cell (PEMFC), 194, 374 375 CL processes, 380 381 PTC. See Parabolic trough solar collectors Pumped hydro storage (PHS), 125 Pump storage hydroelectric (PSH), 102 104 Pyrolysis, 77, 463 464 fast, 79 80 flash, 80

667

slow, 78 79 types of, 78 80 Pyrolysis unit, 99 100 R Radiation intensity, distribution of, 15f Rankine cycles (RCs), 573 Rate of return (ROR), 562 569 Real collector, 22 27, 26f Recovery ratio (RR), 65 Reduction reaction, 320 321 Reforming of heavier hydrocarbons (ethane), 249 250 Refrigerant gas, synthesis of, 466 468 Renewable energy sources, 2 4, 69 75, 393 Residual biomass, 313 Resonant array filters, 478 Restricted equilibrium, 150 Revenue, 545 546 Reversible labor, 294 Reynolds Number, 62 ROCA, 43 “Run-of-river” plant, 102 S Sandia National Laboratories (SNL), 35 Saturation current density (j0), 482 486 Secondary batteries, 137 138 Second cogeneration cycle, 598 607 Selexol process, 469, 469f Sensible energy storage (SES), 131 132 Short-circuit current density, 491 493 vs. cell temperature, 492f vs. emitter temperature, 492f Silicon reactor, 35, 36f Silver, 587 S-I thermochemical cycles, 406 407 Soave Redlich Kwong (SRK), 244 SOEC, 223 231, 240 241, 258 266, 299 300 assumptions and information used, 264 266 combination of SOFC and, 242 244, 285 303 current density and efficiency, 301 high-temperature electrolyzer model, 258 263 integrated systems with, 241 242

668

SOEC (Continued) parameters, 277 285 performance, 277 solid oxide eletrolyzer mass, 264 SOFC electrolyte, 196 213 development of, 203 204 ideal and real fuel cell efficiency, 209 210 integrated systems with, 239 240 required SOFC fuel, 198 201 structure, 201 203 thermodynamics of fuel cells, 207 209 voltage-current graph, 204 207 voltage loss in the cell, 210 211 Solar-based hybrid energy systems combined solar thermophotovoltaic power generation and solid oxide electrolyzer for hydrogen production, 189 concentrated and photovoltaic solar systems in poly-generation systems, 167 171 cooling production by solar PGSs, 184 cryogenic biogas process using solar energy, 184 heating production by solar PGSs, 182 183 heating greenhouses: combined solar collector-geothermal heat pump, 182 183 hybrid photovoltaic solar, proton exchange membrane fuel cell, and thermoelectric device system, 183 hybrid hydrogen purification and LNG, organic Rankine cycle (ORC), and photovoltaic panels, 169 hybrid photovoltaic-thermal collector and ejector refrigeration cycle, 167 168 hydrogen generation from seawater via a desalination unit and lowtemperature electrolysis by solar-based setup, 187 188 hydrogen liquefaction by solar PTC and ORC, 185 187 hydrogen production by solar PGSs, 185 189 Kalina cycle driven by a parabolic trough solar collector, 177 178

Index

low-temperature solar energy and novel oxy-fuel power generation cycle, 178 180 micro gas expander and solar dish collector, 174 175 power production by solar PGSs, 174 181 proposed solar PGSs for water desalination, 171 174 simultaneous generation of power and oxy-fuel powerplant by solar dish collection and ORC, 180 solar dish collector power plant with ARS and desalination unit, 171 174 solar organic Rankine, 175 177 solar thermionic generator and thermoelectric device, 180 181 Solar collector, 413 415 sensitivity analysis, 534 538 types of, 25f Solar collector-geothermal heat pump, 58 59 Solar concentration, 427 431 Solar concentrator, 443 450 collector-generated heat evaluation, 443 446 distribution of reflected radiation flux at the focal point, 449, 500t distribution of reflected radiation flux in the reactor walls, 450, 450f temperature distribution at the specified ratio, 447 449 Solar dish collectors, 180 Solar energy, 11 12, 175 176, 412 415 cryogenic biogas process using, 184 Solar organic Rankine Cycle, 2 4 Solar oxy-fuel energy production, 178 Solar polygeneration systems (PGSs), 11 21 cooling production by, 184 heating production by, 182 183 hydrogen production by, 185 189 power production by, 174 181 for water desalination, 171 174 Solar power plant, 21 33 collectors, specifications of, 26t concentrating collectors, 22 33 governing equations of geometry, 22 33 governing equations of heat transfer, 33

Index

half-length collector, 26f solar collectors, 25f stationary collectors, 21 Solar reactor, 450 452 heat distribution, 451, 451f, 452f mass distribution in pipe profile, 452 Solar system, 520 525 Solar thermal collectors, 413 415 Solar thermochemical reactors, 33 48 direct reactors, 34 entrained reactors, 35 36 fixed reactors, 39 41 fluidized bed reactors, 37 38, 38f, 39f governing equations, 44 48 indirect reactors, 34 mobile reactor, 41 44 reactor particles classification, 34 stacked reactors, 38 44 Solid oxide electrolyzer cell (SOEC), 213 schematic of, 229f types of, 231f Solid oxide electrolyzer mass, 264 Solid Oxide Fuel Cell (SOFC), 194, 237 303, 374 CL-based processes, 378 380 current density and efficiency, 301 efficiency of, 256 257 investigation of, 266 285 parameters of other components of, 257, 269 277 Solid particles, 92 Solid particle separation, 335 Specific heat capacity, 93 Stabilized zirconia, 227 Steam cycle, 476 480 Steam methane reforming (SMR), 398 Steam natural gas reforming, 247 250 Steam reforming unit, 341 343, 342f Steam turbine power plants/CL based systems, 381 384 Storage, 4, 7 8 Subsurface heat exchangers, 58 Sulfuric acid, 409, 517 546, 518f Sulfur impurities, 199 Syngas, 76 T Tangent inlet (NG), 35 36 Γ: diode ideal factor, 12 13

669

Thermal conductivity, 93 Thermal efficiency, 417 418, 562t Thermal integration, 358 363, 415 419 composite curve, 358 359, 358f demand curve, 360, 361f effect investigation, 363 grand composite curve, 359 360, 361f heat exchangers network, 360 363, 362f minimum temperature difference effect on the total investment cost, 359, 360f Thermal plasma gasification unit, 490, 490f Thermal recycling (HRSG), 476 unit flowchart, 477f, 478f Thermal to Electrical Ratio (TER), 266 Thermal value, 315 Thermochemical cycles, 404, 416 417 integration, 529 531 sensitivity analysis, 531 533 Thermochemical energy storage (TCES), 125 126, 131 132 CaCO3/Cao 1 CO2, 131 134 CaO/Ca(OH)2, 130 131, 130f Co3O4/CoO redox pair, 134 136 in PGSs, 126 127 using CaO/CaCO3-CaCl2, 127 129 Thermochemical split of water, 401 403, 402f Thermochemistry, 315 Thermolysis, 401 Thermophotovoltaic cell efficiency, 487, 496 497 Thermophotovoltaic system, 13 21, 542 conceptual model of, 14f efficiency, 487 vs. emitter temperature, 497f external quantum efficiency of, 16f model results, 490 498 total efficiency of, 497 498 Total plant class, 543 Total system efficiency, sensitivity analysis of, 507 513 combustion chamber temperature, 511 513, 512f inlet water steam rate, 507 508 mainstream split ratio, 508, 509f methanol synthesis reactor pressure, 510, 511f methanol synthesis reactor temperature, 510 511, 512f

670

Total system efficiency, sensitivity analysis of (Continued) plasma air rate, 508, 508f return flow, 509 510, 510f side stream split ratio, 508 509, 509f WGS reactor temperature, 510, 511f Tremper, 39 Trigeneration power plant, 385 Turbine, 297 U Unavoidable energy destruction, 643 645 Untouched biomass, 313 314 Upper and lower Integral limit, 481 V Vapor (or steam) power cycle, 375 Variable renewable energy sources (VRES), 104 Vinyl chloride monomer (VCM), 66 Virgin biomass, 313 Viscosity, 92 Visibility factor, 485 Voltage calculation, 252 256 VRES. See variable renewable energy sources W Water, 91 92 Water electrolysis, 216, 400 401 Water-gas (WG), 501

Index

Water Gas Shift (WGS) reaction, 398 Water-gas transport (WGS), 501 Water vapor (H2O), 76 Wave energy system hanging, 114f principal concepts of, 113f Wave energy utilization, 113, 113f, 114f Wind energy, 54 Wind energy-based polygeneration systems, 49 50 Z Zinc from oxygen with nanofluid coolant, separation of, 437 440 boundary conditions and initial value, 440 material properties, 438 440 model geometry, 437 438 pressure distribution at a specified ratio, 455 temperature distribution at the specified ratio, 455 temperature profile comparison in different cooling flow ratios, 454 455, 455f Zinc oxide (ZnO), 35 36 Zinc-sulfur-iodine (ZnSI), 133 134 Zinc system, 409 412, 519 520 block diagram, 520f output reaction, 520t Zn-S-I thermochemical cycle, 407 412, 515 516