Advances in Synthesis Gas: Methods, Technologies and Applications, Volume 3: Syngas Products and Usages 0323918786, 9780323918787

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Advances in Synthesis Gas: Methods, Technologies and Applications

Advances in Synthesis Gas: Methods, Technologies and Applications Volume 3: Syngas Products and Usages Edited by Mohammad Reza Rahimpour Department of Chemical Engineering, Shiraz University, Shiraz, Iran

Mohammad Amin Makarem Methanol Institute, Shiraz University, Shiraz, Iran

Maryam Meshksar Department of Chemical Engineering, Shiraz University, Shiraz, Iran

Contributors Mitra Abbaspour Department of Chemical Engineering, Shiraz University, Shiraz, Iran Amr Abdalla Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada Waqar Ahmad Department of Chemical and Biological Engineering, Monash University, Clayton, VIC, Australia Chayene Gonc¸ alves Anchieta Paul Scherrer Institut, Villigen, Switzerland Prakash Aryal Department of Chemical and Biological Engineering, Monash University, Clayton, VIC, Australia Nooshin Asadi Department of Chemical and Biological Engineering, Monash University, Clayton, VIC, Australia Koroosh Asghari Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada Elisabete Moreira Assaf University of Sa˜o Paulo, Sa˜o Carlos Institute of Chemistry, Sa˜o Carlos, Sa˜o Paulo, Brazil Jose Mansur Assaf Federal University of Sa˜o Carlos, Chemical Engineering Department, Sa˜o Carlos, Sa˜o Paulo, Brazil Manuel Bailera Department of Mechanical Engineering, Universidad de Zaragoza, Campus Rı´o Ebro, Bldg. Betancourt, Zaragoza, Spain; Graduate School of Creative Science and Engineering, Waseda University, Tokyo, Japan Carlos Gilberto Temoltzin Caballero Monterrey Institute of Technology and Higher Education, Puebla, Mexico Joa˜o Sousa Cardoso Polytechnic Institute of Portalegre, Portalegre; Instituto Superior Tecnico, University of Lisbon, Lisbon, Portugal Jose Antonio Mayoral Chavando Polytechnic Institute of Portalegre, Portalegre, Portugal Silvio de Oliveira Junior Department of Mechanical Engineering, Polytechnic School of University of Sa˜o Paulo, Sa˜o Paulo, Brazil Meire Ellen Gorete Ribeiro Domingos Department of Chemical Engineering, Polytechnic School of University of Sa˜o Paulo, Sa˜o Paulo, Brazil Moises Teles dos Santos Department of Chemical Engineering, Polytechnic School of University of Sa˜o Paulo, Sa˜o Paulo, Brazil Swarit Dwivedi Department of Chemical and Biological Engineering, Monash University, Clayton, VIC, Australia Daniela Eusebio Polytechnic Institute of Portalegre, Portalegre, Portugal Azharuddin Farooqui Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada

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Contributors Carla Ferna´ndez-Blanco Chemical Engineering Laboratory, Faculty of Sciences and Centre for Advanced Scientific Research (CICA), University of La Corun˜a, La Corun˜a, Spain Daniel A. Flo´rez-Orrego Department of Mechanical Engineering, Polytechnic School of University of Sa˜o Paulo, Sa˜o Paulo, Brazil; Faculty of Minas, National University of Colombia, School of Processes  and Energy, Medellin, Colombia; Department of Mechanical Engineering, Ecole Polytechnique Federale de Lausanne, Switzerland Foroogh Mohseni Ghaleh Ghazi Department of Chemical Engineering, Shiraz University, Shiraz, Iran Ashwin Hatwar Department of Chemical and Biological Engineering, Monash University, Clayton, VIC, Australia Fatemeh Khodaparast Kazeroonian Department of Chemical Engineering, Shiraz University, Shiraz, Iran Christian Kennes Chemical Engineering Laboratory, Faculty of Sciences and Centre for Advanced Scientific Research (CICA), University of La Corun˜a, La Corun˜a, Spain David M. Kennes-Veiga CRETUS, Department of Chemical Engineering, University of Santiago de Compostela, Santiago de Compostela, Spain Hadiseh Khosravani Department of Chemical Engineering, Shiraz University, Shiraz, Iran Ananda Vallezi Paladino Lino Federal University of Sa˜o Carlos, Chemical Engineering Department, Sa˜o Carlos, Sa˜o Paulo, Brazil Pilar Lisbona Department of Mechanical Engineering, Universidad de Zaragoza, Campus Rı´o Ebro, Bldg. Betancourt, Zaragoza, Spain Nader Mahinpey Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada Mohammad Amin Makarem Methanol Institute, Shiraz University, Shiraz, Iran Tayebeh Marzoughi Department of Chemical Engineering, Shiraz University, Shiraz, Iran Maryam Meshksar Department of Chemical Engineering; Methanol Institute, Shiraz University, Shiraz, Iran Rafael Nogueira Nakashima Department of Mechanical Engineering, Polytechnic School of University of Sa˜o Paulo, Sa˜o Paulo, Brazil Virginia Perez Centre for the Development of Renewable Energy - Centre for Energy, Environment and Technology Research (CEDER-CIEMAT), Soria, Spain Elham Rahimpour Shiraz University of Medical Sciences, Shiraz, Iran Hamid Reza Rahimpour Department of Chemical Engineering, Shiraz University, Shiraz, Iran Mohammad Reza Rahimpour Department of Chemical Engineering, Shiraz University, Shiraz, Iran Tayebe Roostaie Department of Chemical Engineering; Methanol Institute, Shiraz University, Shiraz, Iran Sonia Sepahi Department of Chemical Engineering, Shiraz University, Shiraz, Iran Mohammad Javad Shahbazi Department of Chemical Engineering, Shiraz University, Shiraz, Iran Nazanin Abrishami Shirazi Graduate Faculty of Environment, University of Tehran, Tehran, Iran Valter Silva Polytechnic Institute of Portalegre, Portalegre; Centre for Environmental and Marine Studies (CESAM), Department of Environment and Planning, University of Aveiro, Aveiro, Portugal

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Contributors Anahita Soleimani Department of Nano-Chemical Engineering, Faculty of Advanced Technologies, Shiraz University, Shiraz, Iran Akshat Tanksale Department of Chemical and Biological Engineering, Monash University, Clayton, VIC, Australia Luı´s A.C. Tarelho Centre for Environmental and Marine Studies (CESAM), Department of Environment and Planning, University of Aveiro, Aveiro, Portugal Marı´a C. Veiga Chemical Engineering Laboratory, Faculty of Sciences and Centre for Advanced Scientific Research (CICA), University of La Corun˜a, La Corun˜a, Spain Shabnam Yousefi Department of Chemical Engineering, Shiraz University, Shiraz, Iran

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Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2023 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). 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: 978-0-323-91878-7

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Preface Vol. 3: Syngas products and usages Synthesis gas (syngas) and its products such as hydrogen are indispensable in chemical, oil, and energy industries. They are important building blocks and serve as feedstock for the production of many chemical compounds such as ammonia and methanol. Hydrogen is expected to become a common energy carrier no later than the middle of the 21st century since it offers considerable energy density and releases negligible pollutants. It is also utilized in petroleum refineries for producing clean transportation fuels, and its consumption is expected to increase dramatically in the near future as refineries need to intensify production capacities. Many publications have hitherto focused on syngas production and purification methods, as well as its applications in industrial production units. Despite the fact that extended studies have been undertaken, there is still room for improvement. The four volumes of this book series explain the conventional and state-of-the-art technologies for the production, purification, and conversion of syngas meticulously. The development of different technologies for transforming syngas or the major ingredients into value-added products such as hydrogen, ammonia, ethanol, and methanol has attracted the attention of researchers in both academic and industrial communities. Thanks to their undeniable significance, many studies have been hitherto devoted to the development of these processes and there is a wealth of information on the conventional technologies, which should be collected in a comprehensive contribution. However, the progress that is being made at a breakneck pace should not be overlooked. For this goal to be achieved, this engaging text touches on the details of different products of syngas, as well as their operating conditions and challenges, and would thus serve as a connection between the scientists in research laboratories and the operators in industrial plants. To do so, the book is divided into three sections of chemical and energy productions from syngas with related challenges. The first section includes applications and utilization of syngas for producing a vast variety of chemical materials such as hydrogen, methanol, ethanol, methane, ammonia, acetic acid, fuel gas, and biofuel, while the second addresses power generation in fuel cells either solely or power and heat generation in tandem, as well as iron reduction in detail. In the third section, the environmental challenges of syngas and its future prospects and industrial outlook are presented.

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Preface The editors feel obliged to sincerely appreciate the authors of the chapters for their contributions, hard work, and great assistance in this project. Furthermore, the authors, as well as the editors, are grateful to all the Elsevier staff for their invaluable and irreplaceable step-by-step assistance in preparing this book. Mohammad Reza Rahimpour Mohammad Amin Makarem Maryam Meshksar

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Reviewer Acknowledgments The editors feel obliged to appreciate the dedicated reviewers (listed below) who were involved in reviewing and commenting on the submitted chapters and whose cooperation and insightful comments were very helpful in improving the quality of the chapters and books in this series. Dr. Mohammad Hadi Sedaghat School of Mechanical Engineering, Shiraz University, Shiraz, Iran Dr. Ali Bakhtyari Chemical Engineering Department, Shiraz University, Shiraz, Iran Dr. Javad Hekayati Department of Chemical Engineering, Shiraz University, Shiraz, Iran Ms. Parvin Kiani Department of Chemical Engineering, Shiraz University, Shiraz, Iran Ms. Samira Zafarnak Department of Chemical Engineering, Shiraz University, Shiraz, Iran

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About the Editors Prof. Mohammad Reza Rahimpour is a professor in Chemical Engineering at Shiraz University, Iran. He received his PhD in Chemical Engineering from Shiraz University in cooperation with the University of Sydney, Australia, in 1988. He started his independent career as assistant professor at Shiraz University in September 1998. Prof. Rahimpour was a research associate at the University of California, Davis, from 2012 to 2017. During his stint at the University of California, he developed different reaction networks and catalytic processes such as thermal and plasma reactors for upgrading lignin bio-oil to biofuel with the collaboration of UCDAVIS. He has been a chair of the Department of Chemical Engineering at Shiraz University from 2005 to 2009 and from 2015 to 2020. Prof. Rahimpour leads a research group in fuel processing technology focused on the catalytic conversion of fossil fuels such as natural gas and renewable fuels such as bio-oils derived from lignin to valuable energy sources. He provides young distinguished scholars from developing countries with perfect educational opportunities in both experimental methods and theoretical tools to undertake in-depth research in the various fields of chemical engineering including carbon capture, chemical looping, membrane separation, storage and utilization technologies, novel technologies for natural gas conversion, and improving the energy efficiency in the production and use of natural gas in industries.

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About the Editors Dr. Mohammad Amin Makarem is a research associate at Methanol Institute, Shiraz University. His research interests are focused on gas separation and purification, nanofluids, microfluidics, catalyst synthesis, reactor design, and green energy. In the gas separation field, his focus is on experimental and theoretical investigation and optimization of the pressure swing adsorption process, and in the gas purification field, he is working on novel technologies such as microchannels. Recently, he has investigated methods of synthesizing bio-template nanomaterials and catalysts. He has collaborated in writing and editing various books and book chapters for famous publishers such as Elsevier, Springer, and Wiley, in addition to guest editing journal special issues.

Maryam Meshksar is a research associate at Shiraz University. Her research is focused on gas separation, clean energy, and catalyst synthesis. In the gas separation field, she is working on membrane separation processes, and in the clean energy field, she has worked on different reforming-based processes for syngas production from methane experimentally. She has also synthesized novel catalysts for these processes, which have been tested for the first time. She has reviewed novel technologies like microchannels for energy production. Recently, she has written various book chapters for famous publishers such as Elsevier, Springer, and Wiley.

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CHAPTER 1

Introduction to syngas products and applications Hadiseh Khosravania, Maryam Meshksara,b, Hamid Reza Rahimpoura, and Mohammad Reza Rahimpoura Department of Chemical Engineering, Shiraz University, Shiraz, Iran bMethanol Institute, Shiraz University, Shiraz, Iran

a

1. Introduction The extreme use of fossil fuels due to the fast industrial development, as well as population increment, has created many environmental problems like increased global warming gases concentration into the atmosphere (CO2, CH4, etc.) [1]. Recently, the increased rate of CO2 concentration (1.5 ppm/year) forced the international society to decrease the concentration of greenhouse gases by lowering their productions or consuming them in other plants [2–4]. For this purpose, many approaches have been reviewed by biological and chemical researchers [5,6]. Synthesis gas as an important intermediate or feedstock in many manufactures or processes can be made from a diversity of sources containing natural gas (NG), coal, oxygen, carbon dioxide, or nearly each hydrocarbon feedstock [7]. From such mentioned raw materials, NG is the most common and the lowest priced one for syngas production [8–10]. Methane is the most important constituent of NG, which main source is reserves of oil and gas, as well as landfill gas. As a result of about 20% of global warming caused by CH4 emission into the atmosphere, the development of technologies for converting methane into valuable products is essential [11,12]. Different convectional processes exist for producing syngas from methane, listed as steam reforming, dry reforming, partial oxidation, in addition to the new technologies like the plasma process [13–16]. As synthetic gas generation is very endothermic, it needs high temperatures and is, therefore, costly [17]. In Table 1, the summaries of different syngas producing processes used in ammonia plants regarding their energy consumption and costs [18]. By lowering the activation energy of the desired reactions in the syngas production processes, the catalyst plays a key role in increasing reaction kinetics and achieving a maximum yield of syngas production. Several efforts have been done to develop catalytic systems that have enhanced resilience for coke formation and cheaper precursors. These attempts include Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91878-7.00014-9 Copyright # 2023 Elsevier Inc. All rights reserved.

3

4

Chapter 1

Table 1 Relative plant investment cost and energy consumption for various feedstock used for syngas production for ammonia synthesis [18]. Technology Relative Investment Feedstock (GJ/tNH3) Fuel (GJ/tNH3) Total (GJ/tNH3) a

Steam methane reforming

Coil gasification

Water electrolysis

Authothermal reforming

Partial oxidation of fuel oil

1.0

1.8–2.1

2.2–2.5

1.2

1.5

22.1

–

–

24.8

28.8

7.9–9.0

–

–

3.6–7.2

5.4–9.0

30.0–31.1

49.0–59.0

37.0a

28.4-32.0

34.2-37.8

Depends on the efficiency of the electricity generation.

Fuel gas Synthetic natural gas

Town gas

Synthesis gas

Waxes

Fischer-Tropsch liquids Kerosene

Naphtha Acetic acid

Methanol Dimethyl ether

Urea

Hydrogen Ammonia

Fig. 1 Examples of chemicals made from synthesis gas [7].

the alteration of synthesis conditions and methods in addition to applying mixed support approaches and bimetallic catalysts [11,19]. Synthetic gas may produce a broad range of environmentally friendly chemicals and fuels, which conventional uses have been steadily increasing. Fig. 1 lists the most important chemicals produced from syngas. Synthetic gas is a great source of hydrogen that can be used to make almost all chemicals from hydrogen like ammonia [3,4]. In 1910, Haber and Bosch developed a method for producing ammonia from N2 and H2, and the first industrial ammonia

Introduction to syngas products and applications 5 synthesis plant was built in 1913 [20]. Besides, the methanol—another product of syngas—has exhibited tremendous development for synthesizing methyl ethers used as octane boosters in vehicle fuels, despite remaining the second-largest consumer of synthesis gas. Fisher-Tropish synthesis is still the third-largest consumer of synthetic gas in which kerosene, naphtha, waxes, etc., are the main products [7]. In the following, each application of syngas is introduced and discussed in more detail.

2. Chemicals from synthesis gas As mentioned, different chemicals are produced from synthetic gas, which are introduced in the following sections with their production methods in detail.

2.1 Hydrogen Hydrogen is largely used in the production of ammonia and methanol, as well as in petroleum refining. It can be also be utilized to generate energy or be applied as a transportation fuel. In addition to various hydrogen applications listed in Table 2, its different producing methods are also mentioned. Among different processes listed previously for hydrogen production, 80%–85% of hydrogen needed in different factories comes from SMR (steam methane reforming), which is shown in Fig. 2 [21]. Different separating methods can be used for producing high-purity H2 from syngas, such as pressure swing adsorption (PSA) and membrane-based technologies, each of which is discussed in more detail in the following. 2.1.1 Swing adsorption technologies PSA, the most mature cyclic adsorption technology, has been widely applied for gas separation and purification. The adsorption-desorption cycle is achieved by the drop in solid adsorption capacity with decreasing pressure. PSA processes have been successfully applied in several processes to separate various gas mixtures, such as CO2 capture, olefin-paraffin separation, methane recovery, NG upgrading, and air separation. PSA has become the most widespread technology used to produce high-purity H2 from a gas mixture. Currently, PSA techniques are used in over 85% of global H2 plants for high-purity H2 production [22]. This technology can be arranged into single or multicolumn arrangements that depend on the number of adsorbers. Multicolumn PSA units having 4–12 parallel columns are frequently used for industrial applications, as for large-scale plants, continuous operation is desired. Fig. 3 shows a typical double-bed PSA setup: one adsorbing while the other regenerates [20]. For separating H2 from syngas, high-pressure syngas is fed into the PSA absorber, and its impurities mainly consist of CO, CO2, CH4, and N2 are selectively adsorbed on the surfaces of

6

Chapter 1 Table 2 Different applications and producing methods of hydrogen [7]. Hydrogen applications

Naphtha hydrotreater: • Hydrogen is used to desulfurize naphtha from atmospheric distillation; the naphtha must be hydrotreated before being sent to a catalytic reformer unit. Distillate hydrotreater: • After atmospheric or vacuum distillation, desulfurizes the distillates; in some units, aromatics are hydrogenated to cycloparaffins or alkanes. Hydrodesulfurization: • As a feed for Claus plants, sulfur compounds are hydrogenated to hydrogen sulphide H2S. Hydroisomerization: • To improve the product’s characteristics, normal (straight-chain) paraffin is transformed into isoparaffins (e.g., octane number). Hydrocracker: • Hydrogen is used to convert heavy fractions into higher value, lower boiling products. Hydrogen production Catalytic reformer: • Hydrogen is produced as a by-product of the conversion of naphtha-boiling range molecules into higher octane reformate. Steam-methane reformer: • Hydrogen is produced for the hydrocracker or the hydrotreaters. Steam reforming of hydrocarbon compounds with a greater molecular weight: • Other than methane, it produces hydrogen from low-boiling hydrocarbon derivatives. Off-gases from refineries can be recovered: • Hydrogen in the range of 50% (v/v) is common in process gas. Gasification of petroleum residua: • Gasification units produce synthesis gas, which can be recovered. Gasification of petroleum coke: • In gasification units, synthesis gas is recovered. Partial oxidation processes: • Produce synthesis gas from which hydrogen can be extracted, similar to the gasification process.

the adsorbing bed. Therefore, the product of the first column is high purity H2. Adsorption typically occurs at relatively high pressure (20–50bar) and up to 50–60°C temperature [23]. In continuous adsorption, the syngas feed stream is switched to another bed by reaching the adsorbents capacity to its saturation amount. Then, the saturated adsorbents are recovered via depressurization aided through a purge-gas supply for releasing the trapped impurities into

Introduction to syngas products and applications 7 Pre-heater Natural gas

Syngas (H2, CO, CO2)

Heat recovery

Steam reforming

Shift reactors (high and low T)

Compressor Sulfur removal Oxygen Steam

Pressure swing absorption

Hydrogen

PSA purge gas

Fig. 2 Schematic of SMR for H2 production [20]

adsorbents. These released gases are called off-gas. Part of the H2 feed-stream (usually up to 20%) may be lost in the off-gas, depending on the operating condition. Off-gas is commonly partially burnt in the reformer or applied as a heat source, avoiding unnecessary emissions. The CO content in the tail-gas improves the reformer flame stability and enhances the reforming equilibrium [14]. Despite good advantages of PSA technology, including relatively simple operation, notable performance at ambient temperatures, high regeneration efficiency, equipment compactness, and low energy intensity, it has some drawbacks listed as high syngas losses in the bed after purging in the depressurization step, low H2 recoveries, and limited impurities sorption into the adsorbents [2]. Therefore, a complete recovery of H2 by coupling PSA with other separation technologies is highly demanded. Membrane technologies offer significant advantages for H2 purification with their typically high recovery rates, facile operation, and energy efficiency [4]. Therefore, by using membrane reactors in the syngas producing unit, the number of stages will be reduced as both purification and production steps occur simultaneously at the same time. 2.1.2 Membrane-based technologies Membranes act as a physical barrier that allow mass species selective transport, which are widely applied for purification and separation in many industries. Membranes are divided into three categories: inorganic, organic, and hybrids of organic/inorganic membranes. Inorganic membranes can be classified into ceramic and metallic constituents, while organic ones are

8

Chapter 1

H2Product

Pure Gas

Valve close

Column Adsorbing

Column Regenaration

Valve Open Syngas Off-Gas

Fig. 3 Typical PSA double-bed arrangement for H2 recovery [22].

divided into biological and polymeric membranes. Membrane performance in the separation process is shown in Fig. 4, in which the driving force is the concentration of feedstock species or pressure gradient [24]. The criterion for selecting a membrane is based on its application. Important properties of membranes including mechanical integrity and durability, as well as separation selectivity and productivity, must be balanced against their cost [25]. However, permeance (permeation rate) and selectivity are the main properties of membranes. Pt-based membranes, dense ceramic membranes, and microporous membranes are three major types of H2 separating membranes in which the researchers have primarily focused on solving

Introduction to syngas products and applications 9

Fig. 4 Membrane performance for separating gases components [24].

their major challenges, including maximizing hydrogen flux and selectivity, as well as minimizing membrane cost and membrane failure created by the chemical interactions or thermal cycling [26]. The characteristics and operating conditions of these three membranes are summarized in Table 3. The generic drawback of these membranes is recovery of hydrogen at low pressure, which is appropriate for fuel cell applications, for processes where high-pressure hydrogen is required, H2 needs to be compressed before being applied. However, the pressurization of hydrogen is difficult and needs high costs as it has small molecular sizes. More complete CO to H2 shift is required in a single reactor for removing hydrogen in a membrane reactor. This reaction is exothermic, so H2-membranes need heat removal to prevent catalyst deactivation caused by coke formation or sintering at high temperatures. This is a significant challenge of fixed-bed reactors as in these systems, hot spots are formed because of gases mixing less than ideal and natural heat flow into the reactor [20].

2.2 Methanol production Methanol, also known as methyl alcohol or methyl hydrate with the formula CH3OH, is the simplest alcohol. It is a colorless, light, volatile, flammable, and toxic material. Methanol consists of a methyl group linked to a polar hydroxyl group and is a polar chemical, thus is completely soluble in water and organic solvents and only slightly soluble in fat and oil [27]. Methanol has a molecular weight of 32.04, an octane number of 113, and its density is about half that of gasoline. It can be used as a transportation fuel in the role of additive to gasoline. Combining 10% methanol with 90% gasoline can cause an octane number up to 130. Any additional percentage of methanol to fuel advances the performance of internal combustion engines (ICE) and reduces the pollutants such as hydrocarbons, NOx, SOx, and particulates [23]. Methanol, as the second most common synthesis gas product after hydrogen, can be consumed as a solvent, chemical fuel, as well as being applied as a raw material in different industries like formaldehyde, methyl tert-butyl ether (MTBE), acetic acid, dimethyl ether (DME), methyl,

10

Chapter 1 Table 3 Characteristics of H2 membranes [20]. Dense ceramic membrane

Ceramic membrane

Microporous membrane

Metallic membrane

Zeolites, silica on ceramic

Pd, Pd-Ag, or Pd-Cu on ceramic or stainless steel

300–600

300–750

Materials

Silica, perovskite

Operating tempeature (°C) H2 flux 3 (ftH2/ 2 ftmembrane × 100 psi) H2 purity (%) Thermal stability

900

Mixed of Pd with dense ceramic 400–600

2

36

100

36

100

100

>90

100

High

Medium, phase change inhibited Medium tolerant to feedstream impurities

High

Low, reduction in H2 flux at high temperatures, Pd alloy is more efiicient

High tolerant to sulfur, carbonates at high temperatures are formed by the reaction of some membranes with CO2 Low

Low, CO and sulfur are two major poisons of Pd, side reactions and defections are occurred by alloys High (for pure Pd) to medium (for alloys)

Chemical stability

High

Cost

Low

Medium

vinyl acetates, methyl methacrylate (MMA), methylamines, melamine resin, ethylene, and propylene production. As methanol has an important effect on energy-related conversion and the rate of energy requirement has been increased, the development of methanol-synthesizing industries with lower costs is an urgent necessity in the future. Methanol synthesis procedure can be summarized in three steps as follows, which details are illustrated in Fig. 5: • • •

Synthesis gas production Methanol synthesis Product separation/purification

As previously mentioned, synthesis gas can be obtained by NG or liquid hydrocarbons reforming or coal or biomass gasification. The most widely used route for syngas production is steam reforming of NG, commonly referred to methane steam reforming (MSR), by which more than 75% of syngas are produced [29]. In the following, methanol synthesis plants are discussed in more detail.

Introduction to syngas products and applications 11

Natural Gas

Refined Methanol

Distillation

Crud Methanol

Methanol synthesis

Steam

Steam System

Reformed Gas

Heat Recovery

Make up Gas

Feed Gas

Steam Reforming

Purge Gas

Natural Gas

Feed Gas Purification

Steam

Flue Gas

Process Steam

Heat Recovery

Syngas

Compression

Fig. 5 Steps in the conventional methanol production process [28].

2.2.1 Methanol synthesis plant Methanol synthesis is originally a high-pressure process, initially worked under temperature 320–450°C and pressure 250–350bar, which was first developed in 1923 [30]. In the process, ZnO/Cr2O3 catalysts were widely used due to their unique characteristics, including high temperature and mechanical resistance. The high-operating pressure balances out the decrement in moles. As high-pressure processes require high investment in operating costs and factory design, low-pressure processes received much attention. The low-pressure process works at temperature of 200–300°C and pressure of 50–100 bar. Despite the advantages of working at equilibrium conditions which operate at low temperatures, the activity of the catalysts decreases intensively. By increasing the reaction temperature, the catalyst activity is enhanced, resulting in a catalyst deactivation by coke deposition [31]. Therefore, the optimization of reaction temperature and also its pressure is an important issue in this process. The low-pressure process was the only process employed in the market by 1999. According to this technology employment, syngas should be washed, compressed, and heated before being used as a raw material of the methanol synthesis process. This fresh feed is mixed with recycled unreacted syngas and sent to a methanol converter where the following reactions are taken place [32]:

12

Chapter 1 2H2 + CO $ CH3 OH ΔH 0 ¼ 90:77 kJ=mol

(1)

3H2 + CO2 $ H2 O + CH3 OH ΔH 0 ¼ 49:58 kJ=mol

(2)

H2 + CO2 $ H2 O + CO

ΔH 0 ¼ 41:19 kJ=mol

(3)

CO reduction (Eq. 1) is an exothermal process limited by equilibrium and favored at low temperatures in the gas phase. To achieve a reasonable industrial conversion rate, the use of an appropriate catalyst and operating condition is essential [33]. The applied catalyst in the low-pressure methanol synthesis process is based on copper oxide, zinc oxide, and alumina (CuO/ZnO/Al2O3). The selectivity of this catalyst toward methanol is quite high. However, the selectivity per pass conversion is low for commercial plants, necessitating product recycling. Fig. 6 shows a typical methanol synthesis section. 2.2.2 Separation and purification section The produced methanol is not pure, and the impurities, including dissolved gases, hydrogen, and water, must be removed before the produced methanol is applied. However, the amount of lateral current present in the synthesized methanol is low under normal operating conditions. Therefore, several distillation columns are needed to separate low-boiling impurities from methanol. The physically dissolved gases are flashed off in a flash vessel [34]. After the flash vessel is distilled in a two-stage system, the stabilized methanol is first under pressure and second at atmospheric pressure to obtain a specific purification. Afterward, the higher boiling point components are removed in a distillation column. The process flow sheet of such a methanol purification plant is shown in Fig. 7.

Pre-heater Natural gas

Syngas (H2, CO, CO2)

Gas cooling

Cooling and distillation

Steam reforming Compressor Compressor Desulfurization

Methanol converter

Oxygen

Syngas recycle

Steam Purge

Fig. 6 A typical methanol production unit [20].

Methanol

Introduction to syngas products and applications 13

Light Ends Fule Gas Light Ends Column

Pure Methanol Column (Elve,Pressure)

Pure Methanol Column (Atmospheric)

Aire Cooler Final Cooler

Expansion Gas Vessel

Steam/Reformed Gas Reboiler Steam/Reformed Gas Reboiler

Process Water Pure Methanol

Crude Methanol

Fig. 7 A two-stage methanol distillation process flow diagram [35].

2.2.3 Methanol to gasoline Although methanol can be used as a transportation fuel, current needs (without engine modifications) dictate converting the methanol to immediately usable gasoline [36]. The conversion of methanol to gasoline (MTG) is based on appropriate catalysts, including zeolite-based ones. The technology of producing gasoline from coal via methanol interstage production involves coal gasification by the reaction of oxygen and steam under pressure in which the composition of the synthesis gas produced is adjusted by a shift reaction (which converts carbon monoxide and steam to hydrogen and carbon dioxide) (Fig. 8). Afterward, the produced syngas is purified and then converted to methanol. The crude intermediate methanol, which contains about 15% water, is then fed to the M-Gasoline unit and converted to the gasoline end-product in a two-stage fixed-bed conversion process. In order to convert methanol directly to high-octane gasoline, the Mobil MTG process has been developed in which both fluidized-bed and fixed-bed reactors have been used (Fig. 9). In this process, methanol is first dewatered to produce dimethyl ether, and then further dewatering with a ZSM-5 catalyst ends to a series of C+5 hydrocarbon production. Zeolite catalysts have an important role in controlling the distribution of hydrocarbon products due to their specific geometry and size pores. Therefore, molecules with larger sizes than zeolite pores cannot be made in this process. After vaporization, the methanol feedstock is fed to the first converter for

Chapter 1 Sulfure Plant

coal

Shift Convertion

Gas Purification

Methanation

SNG

Ash

Gasification

Sulfur

CO2

Stream

Oxygen

Asid Gas

Methanol Senthesis

Synthesis Gas

Methanol Convertion

Gasoline Blending

Fractionation

Hydrogen Recovery

Gasoline

Gases

14

H2

Naphta Hydrotreating

High-octan gasoline

C4

Naphta HF alkylation

LPG

Fig. 8 M-Gasoline process [37].

Condenser

Catalyst Separator Catalyst Regenerator

Gasoline

Reactor Catalyst Water

Methanol Vapor

Fig. 9 The Mobil MTG process [36].

Introduction to syngas products and applications 15 converting to dimethyl ether, H2O, and unreacted methanol mixture at operating conditions of 300 psi and 300°C. This reaction can be seen in Eq. (4) [37]. 2CH3 OH ! CH3 OCH3 + H2 O

(4)

After that, the first-stage product is mixed with a recycled stream and sent to a second-stage converter. While the temperature at the entrance is around 345°C, the outflow temperature is around 400°C, which is filled with a catalyst for gasoline components production. Several chain steps occur here fractionation of a raw product, alkylation of light ends, hydration of a recovered, and blending products for producing gasoline. Fig. 9 illustrates the methanol to gasoline process in which vaporized methanol first moves upward from a catalytic bed at a pressure of 25psi and a temperature of 415°C, and then converted to water and hydrocarbons. After the reaction, the catalyst is removed from the products at the top of the reactor, then the overhead product is condensed and separated from water and small quantities of coke (Fig. 10). The catalyst’s selectivity is an essential factor in the M-gasoline process for having high-octane gasoline. This selectivity is higher than that of the traditional FT catalysts as the selectivity of Mobil catalysts is about 85%, whereas FT catalysts have a 50% selectivity [37].

Gasoline(CS+)(60)

products of MTG process %W/W

Propylene(5)

Methanol + ether (0.2) n-Butane(1.7)

Water(56) Light Gas (5.6)

Hydrocarbons (43.5)

Isobutane(14.5)

CO,CO2 (0.1)

Propane (5.9)

Butenes(7.3)

Coke (0.2)

Fig. 10 Products of the MTG process (T ¼ 410°C, P ¼ 25 psi) [36].

16

Chapter 1

2.3 Ammonia production For ammonia production, the first hydrogen should be separated from produced syngas, and then it is combined with nitrogen according to the high-temperature Haber–Bosch catalytic process for obtaining ammonia in industries. This process was developed it in the first decade of the 20th century, which reaction was according to Eq. (5) [7]. 3H2 + N2 ! 2NH3

(5)

Low amounts of oxygen-containing compounds like CO, H2O, and CO2 should be used in syngas as they act as a poison of the ammonia synthesis catalysts especially ones with multipromoted magnetite. While the needed H2 for ammonia synthesis is widely comes from SMR, approximately pure N2 can be achieved using an air separation instrument with much lower costs. However, additional O2 removal is required. The operating cost of the ammonia synthesis step is much higher than SMR as the absolute pressures of about 375–525psi is needed for H2 production, while it ranges between 900 and 2700psi for ammonia production. At these operating conditions, conversions rates below 30% can be obtained using an iron-based catalyst. Other catalysts relying on ruthenium are allegedly 10 times more active whereas running at milder operating conditions [38], such as those found in the Kellogg advanced ammonia process (KAAP). For increasing the pressure of syngas up to the operating conditions of the ammonia loop (>150 bar), an intercooled multistage compression train is required [39]. It is a very energy-intensive process representing almost one-third of the total power demand. A circulator is also required to recycle the unreacted mixture and overcome the pressure drop in the ammonia loop (Fig. 11). Together, the makeup syngas compressor and circulator represent about 45% of the overall power consumption in the ammonia plant [40]. Fig. 11 also shows other equipment of the ammonia loop, such as the chemical converter beds, the ammonia separation process, and the waste heat recovery units, which are described in more detail next. Syngas Compr Heat Recovery

Purified Syngas

Makeup Syngas Cond

Heat Recovery

Circulator Reactor Feed

Recycle Separator

Separator 1

Bed 2

Bed 3

Heat Recovery

Bed 1

Heat Recovery

Preheater

Heat Recovery

Heat Recovery

Refrigeneration Ammonia Purge Ammonia

Fig. 11 Ammonia compression, synthesis, condensation, and separation units [18].

Introduction to syngas products and applications 17

2.4 Fischer-Tropsch chemicals The end chemical products of synthesis gas conversion to higher molecular weight liquid fuels and other chemicals are FT chemicals. FT is an only commercial method in which coal is directly converted to liquids at a pressure of 75–4000psi and temperature of 200–350°C. In this process, as can be seen in Fig. 12, syngas is converted to waxes, ketones, alcohols, and liquid hydrocarbons with the aim of a fixed-bed and entrained-bed reactor filled with an iron-cobalt catalyst. Table 4 summarizes the composition of these two reactors’ products. MgO and K2O are the two additives of the FT catalysts that act as a promoter and chemical enhancer, respectively [41]. The ratio of hydrogen to carbon monoxide is normally around 2.2:1 or 2.5:1. Because the next stage of liquids production may require up to three volumes of hydrogen, the synthesis gas must

Low T FTS Slurry phase or tubular reactors

Wax (>C20) Hydroprocessing

Diesel

Cobalt or iron Air Oxygen Steam

catalysts Particulate removal Tar conversion Wet scrubbing

Gas cleanup Syngas production

and

Clean syngas (H2 and CO)

conditioning

Sulfur removal Water gas shift Natural gas Coal Biomass

High T FTS

Olefins (C3-C11)

Circulating or fast

Oligomerization

fluid bed reactors

Isomerization

Iron catalysts

Hydrogenation

Fig. 12 FT chemicals production via syngas [20].

Gasoline

18

Chapter 1 Table 4 Composition of FT products [41,42]. Composition (% v/v) Product

Carbon number

Fixed-bed reactor

Entrained-bed reactor

Hard wax Gasoline Liquefied petroleum gas Soft wax Middle distillate

C31-C60 C5-C11 C3-C4 C21-C30 C11-C20

18.0 33.4 5.6 11.8 16.6

– 72.3 7.7 – 3.4

be converted to the desired level of hydrogen using the water-gas shift reaction (Eq. 6), after which the gaseous mix must be purified and converted to a variety of hydrocarbons: CO + H2 O ! CO2 + H2

(6)

CO + ð2n + 1ÞH2 ! Cn H2n+2 + H2 O

(7)

These reactions generally produce low- and medium-boiling aliphatic compounds; current commercial targets are centered on the circumstances that lead to the creation of n-hydrocarbons, olefins, and oxygenated materials [43].

2.5 Ethanol and mixed alcohols Various processes can be used to make mixed alcohols from H2 and CO, such as variants of FT, isosynthesis, and oxosynthesis involving homologation of methanol, hydroformylation of olefins, and lower molecular weight alcohols to make higher alcohols. When several gas-to-liquid methods such as methanol synthesis and FT were developed, it was discovered that higher alcohols were the by-products of these processes when catalysts or conditions were not optimized. Therefore, for shifting the products of FT or methanol synthesis processes to higher alcohols, alkali metals can be added to their used catalysts as a promoter. Higher alcohol synthesis (HAS) is also optimized at lower space velocities and higher temperatures than methanol synthesis, with an H2 to CO ratio of roughly 1:1 instead of 2:1 or greater. While other syngas-to-liquids technologies have found commercial success, HAS has struggled due to low product yields and poor selectivity. HAS yields are on the order of 10% syngas conversion to alcohols in a single pass, with methanol is usually the most abundant alcohol generated [20,44]. In 1923, Fisher and Tropsch developed the “Synthol” alcohol manufacturing technique. At >10 MPa and 400–450°C, they used an alkalized iron catalyst to convert syngas to alcohols. Commercial mixed alcohol production was carried out with alkalized ZnO/Cr2O3 catalysts between 1935 and 1945. With the increased availability of petroleum and the requirement for pure alcohols for manufacturing chemicals, the demand for mixed alcohol production from syngas has been increased after 1945 [45].

Introduction to syngas products and applications 19 Two types of catalysts are currently promising for the direct-ethanol synthesis from syngas: rhodium-based and copper-based catalysts. At high pressures, rhodium-based catalysts have a selectivity of more than 50%. However, this selectivity is high only at very low conversions [46]. Based on the used catalyst, both the direct and indirect syntheses of ethanol from methanol are ended to the production of CH4, C2-C5 olefins and alkanes, esters, ketones, acetic acid, and aldehydes as a by-product. Therefore, both the catalyst and reaction conditions must be better tuned to limit methanation activity to increase ethanol production selectivity. Higher selectivity can be attained with homogeneous catalysts, according to a study of the literature on the conversion of syngas into ethanol and higher alcohols [47], but commercial systems based on these catalysts require extremely high-operating pressures. Heterogeneous catalysts based on rhodium preferentially create ethanol over other alcohols. However, because of the low ethanol yield and high cost of rhodium, such catalysts are less appealing for commercial use, especially when significant metal loadings are required. Compared to methanol synthesis, alcohol production rates are much lower, necessitating at least a two- to threefold increase in alcohol production rate for commercial viability. Standard fixed-bed reactor technology has been combined with customized cooling designs for methanol synthesis or hydrocarbon FT synthesis in HAS catalyst R&D. By executing the reactions in slurry reactors with temperature control and efficient heat removal, the product yield and selectivity may be improved [47].

2.6 Dimethyl ether DME synthesis from syngas can be performed at moderate pressure (1000psi) and temperature (250°C) in the liquid phase, which is applicable as diesel, cooking fuel, refrigerant, or as a chemical feedstock [20]. The bifunctional catalyst comprises of a mixture of methanol synthesis catalyst (Cu/ZnO/Al2O3) and methanol dehydration catalyst (γ-Al2O3) slurried in a liquid oil medium in this single-stage process. The process of DME production consists of a two-step process; first, methanol synthesized, and after that the produced methanol is dehydrated over an acidic catalyst like γ-Al2O3, which reactions are listed as Eqs. (8)–(11) [48]. 2H2 + CO ! CH3 OH

ðMethanol synthesisÞ ΔH ¼ 21=6 kcal=mol

2CH3 OH ! H2 O + CH3 OCH3 H2 O + CO ! H2 + CO2

ðMethanol dehydrationÞ ΔH ¼ 5=6 kcal=mol

ðWater gas shift reactionÞ

ΔH ¼ 9=8 kcal=mol

(8) (9) (10)

Overall response: 3CO + 3H2 ! CO2 + CH3 OCH3

ΔH ¼ 58=6 kcal=mol

(11)

It is worth noting that one of each reaction’s products is devoured by another. Syngas conversion to DME yields higher conversions than syngas conversion to methanol due to the

20

Chapter 1

synergy between both reactions [49]. The per-pass and total conversions for the synthesis of DME are 50% and 95%, for methanol are 14% and 77%, and for MeOH/DME are 18% and 85%, respectively [50]. Like methanol, DMR can also be used as a fuel in diesel engines that causes improved fuel efficiency and fewer emissions. Besides, it can be utilized with liquefied petroleum gas (LPG) in Otto engines or fuel cell vehicles due to its easy regeneration from methanol. Dimethyl ether possesses physical properties similar to LPG and may be handled as a liquid using the same infrastructure [7].

2.7 Methane Within a methanation unit, several exothermic processes may occur at the same time (Eqs. 12–16). A variety of metals have been used as catalysts for the methanation reaction; nickel and ruthenium appear to be the most common and most effective. Therefore, methanation catalysts with nickel are the most widely used ones in the industrial scale [43]. Because sulfur molecules quickly deactivate (poison) the catalysts, the synthesis gas must be desulfurized before the methanation step. When the concentration of carbon monoxide in the methanation stream is too high, a problem might occur because substantial amounts of heat must be evacuated from the system to avoid high-temperature deactivation of catalyst by sintering and carbon deposition [43]. Therefore, reaction temperature should be kept below 400°C to avoid this problem. 3H2 + CO ! H2 O + CH4

ðMethanation reactionÞ

(12)

4H2 + CO2 ! 2H2 O + CH4

(13)

2H2 O + C ! CO2 + 2H2

(14)

H2 + 2C ! C2 H2

(15)

2H2 O + CH4 ! 4H2 + CO2

(16)

3. Energy production from synthesis gas Because fossil fuel resources are limited and their use rates are still high due to population expansion, the energy issue has become a major challenge for human life in the future [51]. As fuel usage rises and reserves fall, energy carrier prices will rise. As a result, the world needs to create new technologies that convert energy more efficiently and/or replace fossil fuels with renewable energy sources [52]. It should be mentioned that, due to their high capital expenditures and intermittent nature, the usage of some renewable energy sources for power generation (such as solar and wind energy) remains a severe concern. Using an energy carrier or a storage media to solve the intermittency problem is a viable option. The present focus of activities worldwide is on different technologies, mostly for composing heat and potency

Introduction to syngas products and applications 21 (CHP) generation, to demonstrate economic viability and address different issues. These technologies come with technological dangers. Biomass-integrated gasification is a method of converting biomass into energy. Gaseous fuels with greater calorific values can reach better efficiency [53–55]. Paisley and Welch [56] discussed how biomass gasification could be used as a key component in a high-efficiency combined cycle system with a gas turbine. Savola and Keppo [57] used the optimization technique to simulate the potential of increasing authority generation and the authority-to-heat ratio of 1–20 MW CHP facilities utilizing biomass fuels.

3.1 Cogeneration Cogeneration is defined as the continuous creation of needed heat and authority (electrical or mechanical) or the improvement of low-level energy for authority generation. Compared to segregated energy generation facilities, this serial energy generation saves fuel because both the heat and authority stipulations are met from a popular/sole fuel source. The heat from the authority generation method is recovered and used where heat is required (which would otherwise necessitate a separate fuel source), resulting in significant fuel savings. By recent rises in gas and petroleum prices, advancements in gas turbine and gas mutual motor fuel flexibility, along with a global aim to minimize GHG (green house gas) emissions, increase authority safety (via localization of authority production), and memorable production system efficiency levels, cogeneration applications have sparked renewed fondness. In topping or bottoming cycles, power can be cogenerated. In a topping cycle, authority is generated prior to delivering thermal energy to the process. The following are some instances of typical topping cycles: (i) Cycles of noncondensing vapor turbines. (ii) Heat recovery and composed cycles utilize thermal energy generated by using exhaust energy from a gas turbine or heat from a gas reciprocating motor to suit method stipulations. (iii) Local heating/cooling applications are found in urban areas where electric authority plants also provide thermal energy. Authority is generated in bottoming cycles by recovering thermal energy that would otherwise be lost to the heat sink. Examples of typical bottoming cycles include exothermic process interactions that provide electricity, and heat is recovered from kilns, process heaters, and furnaces [58].

3.2 Trigeneration For generating air conditioning and/or refrigeration, a combination of gas engines and absorption chillers is the best option. For this purpose, lost heat from the combination intercooler, the motor petroleum, the motor cooling water, and the exhaust gas van be powered

22

Chapter 1

to the chillers. When an absorption refrigeration system combines a cogeneration facility, seasonal surplus heat can be used for cooling. Total efficiencies (authority, air cooling, and/or refrigeration) of up to 75% can be achieved with trigeneration, improving annual capacity and overall plant efficiency [58].

4. Conclusion Growing worldwide energy demand while limiting the future focus on renewable energy technology is maintained due to the impact of carbon dioxide emissions from fossil fuel consumption on universal climate change. Biofuels developed from first-production technologies that are sustainable, as well as the development of sophisticated conversion technologies, provide a way to reduce transportation’s carbon intensity. A thermochemical conversion method integrates biomass gasification and catalytic fuel synthesis to produce syngas from a broad range of biomass input. That syngas, once purified, may be transformed to forwarding fuels using a variety of catalytic fuel synthesis techniques [20]. The original Fischer-Tropsch (FT) process has been adjusted and enhanced throughout time to improve efficiency and acceptability. Synthetic crude oil is the name given to the hydrocarbon production admixture that exits the FT reactor [7]. Besides the application of syngas for fuel synthesis, it has a wide range of applications for DME, MeOH, ethanol, or alcohols production. Cogeneration is being discovered to give great efficiency and maybe environmental benefits as rather industrials, developers, commercial/educational establishments, and utilities throughout the universe hunt for little expense electric energy and heat applications. One of the most significant aspects of success is the industrial steam host, which provides thermic energy needs that may be met by very effective cogeneration systems also earth for developers and utilities to build new power plants [58].

Abbreviations and symbols CHP DME FT GHG HAS ICE KAAP LPG MMA MSR MTBE MTG NG PSA SMR

composing heat and potency dimethyl ether Fischer-Tropsch green house gas higher alcohol synthesis internal combustion engines Kellogg advanced ammonia process liquefied petroleum gas methyl methacrylate methane steam reforming methyl tert-butyl ether methanol to gasoline natural gas pressure swing adsorption steam methane reforming

Introduction to syngas products and applications 23

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Chapter 1

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CHAPTER 2

Hydrogen production from syngas Tayebe Roostaiea,b, Mitra Abbaspoura, Mohammad Amin Makaremb, and Mohammad Reza Rahimpoura Department of Chemical Engineering, Shiraz University, Shiraz, Iran bMethanol Institute, Shiraz University, Shiraz, Iran

a

1. Introduction Fossil fuels supply more than 80% of the current worldwide primary energy demand [1,2]. However, in recent decades, the world has witnessed a significant reduction in oil supplies and a growing shortage of cheap oil [3]. In addition to the problems associated with the availability and cost of fossil fuels, concern about greenhouse gas (GHG) emissions originating from burning fossil fuels has issued a warning about the future of the energy sector [4,5]. Along with unstable political conditions and the ambiguous future of oil-based fuels [6], All of this has led to more research to find alternative fuels for our near and far future. In this regard, hydrogen is considered as the most promising alternative energy carrier that effectively reduces GHG emissions [7]. The popularity of hydrogen stems from the fact that it is the most abundant chemical element in the universe and makes up 75% of the material [8,9]. However, it is found in the form of components of raw materials and cannot be used directly [6,10]. More importantly, it has a high energy content compared to conventional fuels [11]. Using proper storage techniques, hydrogen can be used as a domestic energy carrier with easy and safe transportation [12,13]. H2 can be stored as pressurized gas, cryogenic liquid, or solid hydride for further use [14,15]. Hydrogen is considered as a secondary energy source, meaning that primary sources are needed to produce it [16]. However, the hydrogen manufactured is not necessarily environmentally friendly unless produced from clean energy sources through a green path, without emitting GHGs [6]. Hydrogen could be produced from various primary energy sources, which are divided into two main categories: conventional and renewable sources. Conventional sources of H2 include fossil fuels that employ the common reforming and hydrolysis methods. The latter renewable sources include biomasand water [17]. Today, more than 96% of hydrogen is produced from fossil fuels. As a clean and renewable method, only a tiny portion of H2 is generated by the electrolysis of water [18]. To be more precise, H2 can be produced from natural gas, coal, Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91878-7.00019-8 Copyright # 2023 Elsevier Inc. All rights reserved.

27

28

Chapter 2 4% 18% 48%

30%

Natural gas

Oil

Coal

Electrolysis

Fig. 1 Feedstock used for hydrogen production [1].

light hydrocarbons, petroleum fractions, biomass, and water. Fig. 1 presents the main raw materials for hydrogen production and their current contribution. Hydrogen is used as the raw material in petrochemical companies to produce strategic products such as ammonia and methanol or to manufacture metals from their oxides [3,9]. It is also described as a clean energy carrier. Furthermore, high-quality hydrogen is utilized as feedstock for fuel cells [19]. A high portion of hydrogen is consumed in refineries for desulphurization, hydrocracking, and other processes. A reliable supply of purified hydrogen helps downstream processes to perform better and cleaner while converting raw materials into higher value products, such as low-sulfur diesel and gasoline. The primary consumers of hydrogen are represented in Fig. 2. From the increased global capacities of ammonia and methanol plants, it is evident that the worldwide demand for hydrogen is growing rapidly. The heavier feedstock of refineries and 6%

51%

35%

8% Ammonia

Methanol

Refining

Other

Fig. 2 The main uses of hydrogen [9].

Hydrogen production from syngas 29

2010

43

2025

7

50

0

10

20

8

30

40

50

60

70

Fig. 3 Global hydrogen demand in 2010 and its prediction in 2025 (million of tons) [21].

the requirement for hydro-desulfurization with hydrogen are other proofs of the increased demand for hydrogen [20]. More importantly, the growing interest in using hydrogen as a new energy carrier results in higher hydrogen demand in the near future. Fig. 3 represents an outlook of hydrogen demand in 2010 and its development in 2025. As the figure reveals, the hydrogen demand will witness an increase of 17% till 2025 to fulfill the required H2 for manufacturing methanol, ammonia, refinery operations, and so forth as predicted. Several approaches for producing hydrogen are mainly classified into four groups:electrolysis, photolysis, biolysis, and thermolysis [22]. Choosing the hydrogen production method depends on several factors, including the feedstock from which H2 will be produced, the source of the required energy for this production, the utilized catalyst, and the H2 plant scale. The photonic, electric, chemical, bioenergy, and heat could be utilized as the energy source [23]. As mentioned previously, the main source of manufacturing H2 is fossil fuels, including natural gas, heavy oil, naphtha, and coal, participate in manufacturing more than 96% of worldwide hydrogen supply [1,24]. The processes used for hydrogen production using fossil fuels, including steam reforming, partial oxidation (POX), autothermal, dry reforming, and plasma reforming [25], belong to the thermolysis class, in which heat is the primary source of energy. The fossil fuel-based methods’ maturity and efficiency are presented in Table 1.

Table 1 The fossil fuels used for hydrogen production [23]. Feedstock

Technology

Maturity (1–10)

Efficiency (%)

Fossil fuel Fossil fuel Fossil fuel Fossil fuel Fossil fuel Fossil fuel

Steam reforming Partial oxidation Autothermal Dry reforming Plasma reforming Membrane reactor

10 7–9 6–8 6–8 1–3 7–9

60–85 60–75 60–75 – 9–85 64–90

30

Chapter 2

As proved previously, there is a growing demand for hydrogen, which is likely to be met by fossil fuels. Almost all hydrocarbon feedstock used to produce hydrogen-rich stream contains at least a small amount of sulfur, which is the most challenging issue encountered in reforming technologies. Hence, the first step in the reforming methods is desulfurization, which is described in the following section. The subsequence sections introduce various reforming techniques to manufacture syngas from fossil fuels. In fact, syngas production is an intermediate stage for generating hydrogen. The synthesis gas obtained through these technologies is a mixture of hydrogen and carbon oxides. Hence, several purification technologies are employed to generate hydrogen-rich streams, which are described in this chapter. Fig. 4 depicts the flow chart of hydrogen generation technologies from different forms of feedstock, which are explained in this chapter. It is worthwhile mentioning that the discussion presented in this chapter focuses on the technologies and pathways to produce hydrogen only from synthetic gas.

2. Desulfurization Depending on the type and source of fossil fuels, they most probably contain at least a minute amount of sulfur, which needs to be removed due to the subsequent problems associated with sulfur, such as reforming and water-gas shift (WGS) catalyst poisoning [3,26]. Practical techniques for removing sulfur-containing compounds are either chemical reactions or adsorptive approaches. Hydrodesulfurization (HDS) reaction is one of the chemical methods that omits sulfur in large-scale plants via partially or completely hydrogenating

Solid Fuel

Gasification & Desulfurization

Liquid Fuel

Desulfurization

Gas Fuel

Desulfurization

Reforming

WGS Reaction

Purification

PSA

Fig. 4 Producing hydrogen from different technologies [3,20].

Membrane

Hydrogen production from syngas 31 sulfur-containing compounds to release sulfur in the form of hydrogen sulfide [27,28]. Alkylation is another chemical method to remove sulfur contents in fossil fuels, which is mainly applied in pilot-scale plants [29]. There is no report on employing this method for large-scale units. Alkylation operates by enhancing the weight of organo-sulfur compounds. Thus, the boiling point of sulfur compounds is increased, which provides the possibility of the distillation process, through which sulfur is omitted. The HDS requires high-pressure hydrogen, while there is no limitation in the alkylation approach. In adsorptive methods, sulfur is removed using suitable adsorbents. The sulfur content can be completely omitted by adsorbents such as activated carbon and zeolite. The method operates at ambient temperature and pressure using a conventional fixed bed reactor that makes it is easy to use. Sulfur can also be adsorbed on the surface of metals like nickel. In these cases, adsorbents are more complex due to the requirement of operating at high temperatures and pressures in the fluidized bed reactor. However, the adsorptive methods generally suffer from capacity constraints. Choosing the adsorbent based on the sulfur-containing material type is crucial to ensure that all sulfur has been omitted since they have to be highly selective for that kind of sulfur. For instance, active carbon is preferred in the natural gas case [29].

3. Syngas production from hydrocarbons reforming Reforming is the process of converting hydrocarbon fuels into hydrogen under optimum conditions through various techniques. In addition to the hydrocarbon, other reactants are also involved in the reforming reaction, including steam, oxygen, or a combination of these materials. Depending on which reactants participate in the reaction, there are different reforming methods, including steam reforming, PO, and autothermal [25]. In addition to these reforming techniques, dry reforming of methane (DRM), also called CO2 reforming, has been introduced [30–33]. Before getting into details, several benefits and drawbacks of each reforming method are presented in Table 2. Table 2 Advantages and disadvantages of various reforming methods [34,35]. Method

Advantages

Disadvantages

Steam reforming

Produce syngas with high H2/CO

Partial oxidation

The possibility of a noncatalytic process Omitting desulfurization step Obtain low methane slip

Require high amount of energy A high amount of emission Produce syngas with low H2/CO Require high temperature Require oxygen or air Noncommercial

Autothermal

32

Chapter 2

3.1 Steam reforming Steam methane reforming (SMR) is the most profitable method for producing syngas from fossil fuels on large scales, mainly due to its competitive resource price, low production cost, and reliable technology [36,37]. Light to heavy hydrocarbons up to naphtha could be used in steam reforming (SR) instead of methane. If the organic-sulfur compounds are present in the feedstock, the desulfurization process is the first step in SMR to avoid catalyst poisoning, which has been explained in the previous section. Subsequently, in the reforming step, hydrocarbon reacts with steam in the presence of a reforming catalyst to produce hydrogen and carbon oxides, which are called syngas [8]. Syngas is manufactured in SMR at moderate to high temperatures and pressures up to 3.5MPa [38], according to Eq. (1). In a more general path, if any hydrocarbon is used, the SR reaction is according to Eq. (2). CH4 + H2 O>CO + 3H2

ΔH ¼ +206:1

kJ mol

(1)

Cm Hn + mH2 O>mCO + ðm + n=2Þ H2

(2)

SMR is capable of coke formation due to moderate to high temperatures. Over Ni-based catalyst, H2O/C >2.5 is required to gasify the formed coke. However, less coke is formed when using the noble metals of group VIII. Coke formation also could be minimized using promoters such as K or MgO. Regarding the pros and cons of SMR, on the one hand, it is a highly endothermic reaction and requires a high amount of external energy to occur, which is considered the most challenging issue in this method. SMR also suffers from high gas emissions. On the other hand, it benefits from a high H2/CO ratio (about 3), resulting in more hydrogen produced than other methods. In SMR, the limiting steps are heat and mass transfer, and the reaction is not limited by the catalyst activity, providing the opportunity to use economic Ni-based catalysts. The final catalyst effectiveness factor of SMR is 5%, which could be enhanced using microchannel reactors. After syngas production in the SMR, the syngas is involved in the WGS reaction to produce additional H2 [39]. The exit gas is then passed through pressure swing adsorption (PSA) to produce high-purity hydrogen [39]. Membrane may also be used for increasing hydrogen purification. WGS, PSA, and membrane methods are explained in the following sections. Fig. 5 shows a simplified flowsheet in SMR.

Steam H2

CH4

Desulfurization

Steam reforming

Water-gas shift

Fig. 5 SMR simplified flowsheet.

PSA

Hydrogen production from syngas 33

3.2 Partial oxidation method Partial oxidation (POX) technology is another promising method to convert methane into syngas, by the advantage of less energy consumption compared with SMR due to its exothermic nature. Another main advantage of POX is that a wide range of hydrocarbon feedstock could be used, ranging from natural gas to liquid fuels and heavy oil to residual refinery oil. As the name reveals, in the POX, hydrocarbon is partially combusted using an oxidant, which could be oxygen, air, or enriched air. In large-scale plants, oxygen is preferred due to smaller POX reactors and reduced subsequence cleanup units. A small portion of steam may also be fed in some cases, which enhances the flame temperature controllability and reduces the carbon content of syngas. Nevertheless, hydrogen efficiency is reduced due to higher fuel consumption. This method has the lowest methane slip compared to other reforming methods. Based on the selected feedstock and process, POX could occur either in the presence or absence of catalyst according to Eqs. (3) and (4), respectively. Higher temperatures are required for noncatalytic reactions. The catalytic reaction occurs at 950°C using feedstock ranging from methane to naphtha. In contrast, the noncatalytic reaction is carried out at 1150–1500°C and pressure 25–80bar, using methane, naphtha, and heavy oil as feedstock [39]. The efficiency of hydrogen production is found to be 70%–80% in the case of noncatalytic POX. In the noncatalytic process removing feed’s impurities is not required; therefore, heavier feedstock could be utilized. In the catalytic POX, oxygen is completely converted and is relatively completely converted in the catalytic bed. CH4 +

1 O >CO + 2H2 2 2

CH4 + H2 O>CO + 3H2

ΔH ¼ 36

kJ mol

ΔH ¼ 216:15

kJ mol

(3) (4)

In a more general way, if other hydrocarbons are used, POX is according to Eq. (5): Cm Hn + m=2 O2 >mCO + n=2H2

(5)

Before POX, the desulfurization step is employed to ensure removing sulfur content in the feedstock. A pure oxygen stream is then used to partially oxidize the hydrocarbon for producing syngas. The obtained synthesis gas subsequently undergoes purifications stages as mentioned in the SMR and will be described in the following sections. Pollutants such as NOx and SOx are not produced during the POX process due to the reduced flame temperature compared with the combustion step in SMR, which is another main advantage of this method. In POX, the H2/CO ratio of obtained syngas is 2, less than SMR, which indicates that more purification processes are needed to obtain high-yield hydrogen. However, from an economic point of view and by considering all costs, including the oxygen supply and the desulfurization cost, POX provides a financially viable path to produce hydrogen [8]. If heavier

34

Chapter 2

feedstock such as coal and heavy oil residue is used, POX is the most promising method to generate H2 [25]. The reason is that heavier feedstock contains lower hydrogen to carbon, and hence more H2 is manufactured.

3.3 Autothermal reforming method The autothermal reforming (ATR) method combines the SMR and POX technique in a single reactor. It uses the heat generated in the exothermic POX reaction to provide the required energy for highly endothermic SMR, by exposing natural gas or other hydrocarbons to oxygen and steam simultaneously. Hence, this thermally neutral method eliminates the requirement of heat supply from outside, omitting the heat exchanger cost as well and providing more compact and simpler reformers, leading to more economical technology. In comparison with POX, higher efficiency is available via ATR. Unless POX, ATR is only carried out in the presence of catalysts. Using catalyst helps to boost hydrogen yield, and its selection plays a crucial role in achieving the desired conversion and selectivity. The operation pressure is lower in ATR compared with POX. Nevertheless, a pure oxygen supply is required, which is costly. The ATR technique usually incorporates a higher steam/carbon compared to POX, providing an opportunity to generate more hydrogen due to more facile SMR and WGS reactions. In ATR, the temperature varies from 900°C to 1150°C, and pressure ranges from 1 to 80 bar [20]. If methane is used as feedstock: CH4 +

1 1 H O + O2 >CO + 5=2H2 2 2 2

(6)

Otherwise, if heavier hydrocarbons are used: Cm Hn +

m m H2 O + O2 >mCO + ðm=2 + n=2ÞH2 2 4

(7)

4. Water-gas shift technology A significant amount of carbon oxides is produced in the reforming methods, which comes from burning fossil fuel either as a feedstock or an energy source. Large quantities of CO produced in reforming methods reduce hydrogen yield as well as poison downstream catalysts. To enhance the hydrogen yield by producing extra hydrogen moles, CO is engaged in a reversible, slightly exothermic reaction with water, known as a WGS reaction. The WGS reaction occurs according to Eq. (8), which is carried out at a low temperature, while pressure does not affect it: CO + H2 O>H2 + CO2

ΔH ¼ 41:15 kJ=mol

(8)

Hydrogen production from syngas 35 The reverse WGS reaction is according to Eq. (9). CH2 + CO2 >O + H2 O ΔH ¼ +41:15 kJ=mol

(9)

WGS could lower the CO content of the product by consuming 1mol of CO to generate 1mol H2. Considering that the WSR equilibrium constant declines with increasing temperature, it is favorable to operate at lower temperatures to enhance the conversion. To facile the reaction, a shift catalyst is utilized. Based on the aim of the WGS process, one or more fixed bed reactors may be required. To produce high yield H2, two to three stages at the gradually lower inlet and outlet temperature are adequate. Fast kinetics is achieved at high temperatures, but high temperatures shift the reaction to produce CO and decrease the H2 yield. To overcome this problem, WGS is carried out via a two-stage process, known as high-temperature shift (HTS) and low-temperature shift (LTS). In the first stage, the CO concentration is declined from about 10% to 2% using HTS catalysts, commonly iron catalyst, adiabatically with the inlet temperature of 350–550°C and pressure of 20–30 bar. Although the reaction is fast in HTS, the thermodynamics limits it to the amount of carbon monoxide that can be shifted. Therefore, the CO concentration is declined to 0.2%–0.4% in the LTS stage, with an adiabatic temperature of 50°C, commonly using copper catalysts. It is necessary to use a cooling system before the LTS step [20]. HTS and LTS reactions could be combined in a single microreactor. A preferential oxidation reactor has been proposed for further H2 purification. In this method, a fuel cell, such as a proton-exchange membrane fuel cell (PEMFC), is employed to reduce the content of CO. To use a preferential oxidation reactor, the air concentration needs to be measured; hence, air measurement tools should be added to the system, which makes it more complex. CO selective methanation reactor is another method to enhance the purification of hydrogen. This seems a more straightforward operation due to not requiring air. However, it needs a careful operation to avoid unnecessary hydrogen consumption. Both mentioned methods use noble metals on alumina catalysts. The PSA or membrane may be used after these processes to exceed the hydrogen purity, which is considered in the following sections. CO + O2 >CO2 H2 +

ΔH ¼ +283 kj=mol

1 O >H2 O ΔH ¼ 242 kj=mol 2 2

(10) (11)

5. Hydrogen purification methods Hydrogen is produced through various processes in an impure form and needs to be purified before being consumed [40]. The reality is that selection of the syngas method is of little importance; what is more critical is how to purify the obtained hydrogen from the steam, which consists of H2, CO, CO2, N2, and other components present in the gas mixture. In this regard,

36

Chapter 2

PSA, membrane-assisted separation, and syngas redox process (SGR) are among the most common hydrogen separation and purification methods, which will be discussed in the following sections.

5.1 Hydrogen recovery and purification by pressure swing adsorption process The PSA process has captured considerable attention during the past decades, with the main use in the recovery and purification of hydrogen, methane, carbon dioxide, and so on for a wide variety of industrial applications. PSA process is also utilized in refining and petrochemical units, ammonia production, as well as processes like manufacturing steel [41]. Today, more than 85% of hydrogen generation plants use the PSA process to purify hydrogen. This notice is not only because of its cost-effectiveness but also for its simplicity. Using PSA technology, pure hydrogen could be manufactured from a 60 to 90 mol% hydrogen-containing gas [42]. The PSA step usually comes after the SMR and WGS steps. The hydrogen-containing gas comes into contact with a solid adsorbent in a packed column that operates at high pressure in the PSA process. The non-hydrogen species are adsorbed on the adsorbent pores, which could be silica gel, active carbon, and zeolite. Subsequently, the pressure is declined to desorb the adsorbed species and regenerate the adsorbent, and CO and CO2 are led off into burner to generate heat for new hydrogen production [43]. The desorption step could be done by either depressurization or purging free-hydrogen steam. Despite the simple concept of PSA, it contains a multistage adsorption process that operates under a cyclic system, which makes it a relatively complex technology [20]. During the PSA process temperature is constant, while pressure is changed in adsorption and desorption steps. At constant temperature, the adsorption isotherm shows the relationship between the partial pressure and the equilibrium loading on the adsorbent material [44].

5.2 Hydrogen purification using membrane technology The membrane has played a significant role in hydrogen production from conventional resources, as in many fields of the chemical industry. By definition, a membrane is a permeable structure that allows passing elements based on a driving force that could be pressure, temperature, or concentration gradient [45,46]. Various continuous flow layouts are possible in membrane-assisted separation consisting of cocurrent, countercurrent, mixed, cross, and dead-end flow [47]. Membrane technology is highly demanding due to the lack of moving parts and having steady conditions, consuming less energy, and reducing the unit’s volume [48]. The hydrogen membrane system offers a simple and facile method for separating hydrogen from syngas. The pressure gradient is the main driving force in membrane purification technology, the same as in the PSA method. The size of the plant in which hydrogen is going to be purified is the most crucial factor that determines either membrane-based technique or PSA technology is employed. PSA is the preferred technology in the larger plants, while in the smaller scales, membrane technology is superior. Hydrogen separation using a membrane is

Hydrogen production from syngas 37 also desired in the moving platforms or in cases where a shock may affect the PSA adsorbent bed. However, the membrane separation method is continuous, while PSA is a cyclic process. Membrane-assisted technology benefits from more controllable operations, which are only limited to the controllability of membrane temperature, feed, and permeate pressure. Foy hydrogen production aim, based on the separation regime, four categories of membranes have been either used or proposed; polymeric, porous, dense, and ion-exchange membranes. A suitable membrane for hydrogen production needs to have high selectivity for hydrogen, great stability, and high permeability to perform at high streams and restricted surfaces [19,49]. Polymeric membranes are the most suitable for hydrogen production, while ion-conductive membranes are the less favorable ones in this area. Regarding polymeric membranes, they are classified into two groups of asymmetric and composite membranes. The former type consists of a single polymer composition in which the thin, dense permselective layer, and the latter one is a thick, porous layer covered by a thin, dense permselective layer composed of a different polymer composition. Porous membranes including ceramic, metal, and carbon are under development in this aim. A schematic of the hydrogen-selective membrane process is presented in Fig. 6. High purity of hydrogen, 85–95 vol%, is achievable via the membrane-assisted method. This presents a facile, flexible, reliable, and easy-to-operate method for separating hydrogen from syngas. Besides all mentioned benefits, the long life of the membrane is another positive point of this method. Membrane technology could easily be integrated into the exciting plants without requiring a considerable change in the unit. The membrane method has minimal utility usage.

5.3 Syngas redox process to produce hydrogen from syngas Syngas redox (SGR) is a cyclic process that includes a reduction step followed by a regeneration process, meanwhile yielding high purity hydrogen [50]. To be more precise, a metal oxide is reduced to the metal via the syngas stream, which has been derived from coal.

Fig. 6 A simple schematic of a hydrogen separation membrane [20].

38

Chapter 2

It is then regenerated via steam while producing hydrogen. The reduction and generation reactions are presented in a general form as Eqs. (12) and (13). Additional reduction reaction using oxygen-rich steam should be used if the metal oxide could not entirely be regenerated via Eq. (13) [51]. Reduction: Metal oxide + syngas>Metal +

CO2 H2 O

(12)

Regeneration: Metal + steam>Metal oxide + H2

(13)

If the syngas is reduced almost completely in the redox reaction, the product is a mixture of CO2 and H2O. In a situation that H2O is condensed, the remaining gas only consists of CO2, which could further be used in other processes like enhanced oil recovery (EOR), cutting the CO2 removal costs. However, this is not an advantage for the cases that contain incondensable nitrogen, such as producer gas. The metals such as Ni, Fe, Mn, Cd, Cu, Co, and so forth proved to be proper in this process because of their thermodynamic equilibrium limitations. A schematic of SGR is presented in Fig. 7. As mentioned in the previous sections, fossil fuels are the main source of producing hydrogen. Furthermore, steam reforming is the most common method for fabricating hydrogen from

Fig. 7 A schematic of the SGR process [51].

Hydrogen production from syngas 39 hydrocarbons on large scales [52]. However, H2 is produced in centralized plants and then transferred to long distances, which increases the final price of hydrogen. Instead, hydrogen could be produced in situ using a small-scale chemical looping water splitting (CLWS) unit to cut the transportation and storage costs and meanwhile manufacture hydrogen via renewable hydrocarbons and an environmentally friendly path. In this process, pure hydrogen is produced by combining SMR and steam-iron process in a fixed-bed reactor. In more detail, hydrogen-rich syngas is initially produced by the SMR method. The obtained syngas is directly employed to reduce the iron-based oxygen carrier. Fe2O3 and Fe3O4 are reduced to FeO and finally to Fe in the reduction step. The metal is further oxidized with steam, and hydrogen is produced. These reactions occur in a cyclic process, and the initial metal oxide is reproduced [16]. In similar research, calcium oxide has been used as a reacting solid in the cyclic process. In this method, the syngas obtained from coal has been used as the feed gas to produce H2 in high temperatures and high pressures using a single-step fluidized system. Fe2O3/CaO has been used as sorbents, in which Fe2O3 acts as an oxygen carrier and CaO absorbs CO2. The feed gas, synthesis gas, is a mixture of CO, H2O, and H2. The WGS reaction initially occurs to convert a fraction of CO to CO2 and H2. The other portion of CO reacts with Fe2O3 to be oxidized and reduces Fe2O3. CO is then adsorbed by CaO [53]. A series of reactions occurs in the two-step regenerative system, which is presented as Eqs. (14)–(26). In the H2 enrichment step, hydrogen and carbon dioxide are initially produced in the WGS reaction (Eq. 14). Subsequently, H2 and CO reduce ferric oxide to iron in two steps while producing H2O and CO2 (Eqs. 15–18). Fe2O3 is considered as the oxygen transfer agent here. After reducing to Fe, iron becomes a catalyst in the Boudouard reaction (Eq. 19). CO2 capture using calcium oxide through Eq. (20) reduces the amount of CO2 and therefore shifts the WGS reaction forward; hence, more H2 is produced. The DRM reaction occurs (Eq. 21), and the carbon which has resulted from CO2 capture participates in the steam reforming reaction and generates further hydrogen. After finishing the H2 enrichment step, the regeneration step began via replacing syngas with air. In this step, iron is oxidized to its original form of Fe2O3 in two steps (Eqs. 23 and 24), and CaCO3 decomposes to its previous state (Eq. 26). Hydrogen enrichment step: CO + H2 O ! CO2 + H2

(14)

H2 + Fe2 O3 ! H2 O + 2FeO

(15)

H2 + FeO ! H2 O + Fe

(16)

CO + Fe2 O3 ! CO2 + 2FeO

(17)

CO + FeO ! Fe + CO2

(18)

2CO ! CO2 + C

(19)

40

Chapter 2 CaO + CO2 ! CaCO3

(20)

CO2 + CH4 ! 2CO + 2H2

(21)

C + H2 O ! CO + H2

(22)

2Fe + O2 ! 2FeO

(23)

4FeO + O2 ! 2Fe2 O3

(24)

C + O2 ! CO2

(25)

CaCO3 ! CaO + CO2

(26)

Regeneration step:

6. Conclusion The present chapter is devoted to the hydrogen production methods from synthesis gas. The syngas itself has been produced from fossil fuels such as natural gas and coal. Other sources of producing hydrogen, such as biomass and water, are not in the scope of this study. Syngas production technologies are first presented, including SMR, POX, and ATR methods. Among various reforming methods, SMR has been the most common and widely used one in the industry up to now. However, the H2 obtained with these methods is not pure enough to be used in different processes. Hence, various possible routes for the purification and recovery of hydrogen have been discussed and compared. WGS reaction is also used to increase the hydrogen yield. Hydrogen is separated from other components of syngas by techniques such as PSA, membrane-assisted methods, and SGR, which are presented in this work.

Abbreviations and symbols ATR CLWS DRM EOR GHG HDS HTS LTS PEMFC POX PSA RWGS SGR SMR SR WGS

autothermal reforming chemical looping water splitting dry reforming of methane enhanced oil recovery greenhouse gas hydrodesulfurization high-temperature shift low temperature shift proton-exchange membrane fuel cell partial oxidation pressure swing adsorption reverse water-gas shift reaction syngas Redox steam methane reforming steam reforming water-gas shift reaction

Hydrogen production from syngas 41

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42

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[25] H.L. Chen, H.M. Lee, S.H. Chen, Y. Chao, M.B. Chang, Review of plasma catalysis on hydrocarbon reforming for hydrogen production—interaction, integration, and prospects, Appl. Catal. B Environ. 85 (2008) 1–9, https://doi.org/10.1016/j.apcatb.2008.06.021. [26] C. Song, Fuel processing for low-temperature and high-temperature fuel cells: challenges, and opportunities for sustainable development in the 21st century, Catal. Today 77 (2002) 17–49, https://doi.org/10.1016/S09205861(02)00231-6. [27] I.V. Babich, J.A. Moulijn, Science and technology of novel processes for deep desulfurization of oil refinery streams: a review, Fuel 82 (2003) 607–631. [28] C. Song, X. Ma, New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization, Appl. Catal. B Environ. 41 (2003) 207–238. [29] C. Song, An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel, Catal. Today 86 (2003) 211–263, https://doi.org/10.1016/S0920-5861(03)00412-7. [30] A. Abdulrasheed, A.A. Jalil, Y. Gambo, M. Ibrahim, H.U. Hambali, M.Y. Shahul Hamid, A review on catalyst development for dry reforming of methane to syngas: recent advances, Renew. Sust. Energ. Rev. 108 (2019) 175–193, https://doi.org/10.1016/j.rser.2019.03.054. [31] W.J. Jang, J.O. Shim, H.M. Kim, S.Y. Yoo, H.S. Roh, A review on dry reforming of methane in aspect of catalytic properties, Catal. Today 324 (2019) 15–26, https://doi.org/10.1016/j.cattod.2018.07.032. [32] K. Herrera Delgado, L. Maier, S. Tischer, A. Zellner, H. Stotz, O. Deutschmann, Surface reaction kinetics of steam- and CO2-reforming as well as oxidation of methane over nickel-based catalysts, Catalysts 5 (2015) 871–904, https://doi.org/10.3390/catal5020871. [33] M. Meshksar, M.R. Kiani, A. Mozafari, M.A. Makarem, M.R. Rahimpour, Promoted nickel–cobalt bimetallic catalysts for biogas reforming, Top. Catal. (2021), https://doi.org/10.1007/s11244-021-01532-y. [34] D.J. Wilhelm, D.R. Simbeck, A.D. Karp, R.L. Dickenson, Syngas production for gas-to-liquids applications: technologies, issues and outlook, Fuel Process. Technol. 71 (2001) 1–3. [35] J.L.G. Fierro, Hydrogen production reactions from carbon feedstocks: fossil fuels and biomass, Chem. Rev. (2007) 3952–3991, https://doi.org/10.1021/cr0501994. [36] S. Koumi, D. Njomo, An overview of hydrogen gas production from solar energy, Renew. Sust. Energ. Rev. 16 (2012) 6782–6792, https://doi.org/10.1016/j.rser.2012.07.027. [37] W. Chul, D. Yeon, M. Won, S. Done, K. Kang, K. Kwang, C. Hee, S. Jeong, C. Sik, Continuous operation characteristics of chemical looping hydrogen production system, Appl. Energy 113 (2014) 1667–1674, https:// doi.org/10.1016/j.apenergy.2013.08.078. [38] A. Erso, Investigation of hydrocarbon reforming processes for micro-cogeneration systems, Int. J. Hydrog. Energy 33 (2008) 7084–7094, https://doi.org/10.1016/j.ijhydene.2008.07.062. [39] M. Steinberg, H.C. Cheng, Modern and prospective technologies for hydrogen production from fossil fuels, Int. J. Hydrog. Energy 14 (1989) 797–820, https://doi.org/10.1016/0360-3199(89)90018-9. [40] M.A. Makarem, M. Farsi, M.R. Rahimpour, CFD simulation of CO2 removal from hydrogen rich stream in a microchannel, Int. J. Hydrog. Energy 46 (2021), https://doi.org/10.1016/j.ijhydene.2020.07.221. [41] A. Abdeljaoued, F. Relvas, A. Mendes, M.H. Chahbani, Simulation and experimental results of a PSA process for production of hydrogen used in fuel cells, J. Environ. Chem. Eng. 6 (2018) 338–355, https://doi.org/ 10.1016/j.jece.2017.12.010. [42] S. Sircar, T.C. Golden, Purification of hydrogen by pressure swing adsorption, Sep. Sci. Technol. (2011) 37–41. [43] S. Sircar, W.E. Waldron, M.B. Rao, M. Anand, Hydrogen production by hybrid SMR-PSA-SSF membrane system, Sep. Purif. Technol. 17 (1999) 11–20, https://doi.org/10.1016/S1383-5866(99)00021-0. [44] S. Sircar, Pressure swing adsorption, Ind. Eng. Chem. 41 (2002) 1389–1392. [45] T. Roostaie, M. Meshksar, M.R. Rahimpour, Biofuel Reforming in Membrane Reactors, Elsevier Inc., 2020, https://doi.org/10.1016/b978-0-12-817384-8.00015-7. [46] M. Meshksar, T. Roostaee, M.R. Rahimpour, Membrane Technology for Brewery Wastewater Treatment, Elsevier Inc., 2020, https://doi.org/10.1016/B978-0-12-816823-3.00010-1.

Hydrogen production from syngas 43 [47] P. Nikolaidis, A. Poullikkas, A comparative overview of hydrogen production processes, Renew. Sust. Energ. Rev. 67 (2017) 597–611, https://doi.org/10.1016/j.rser.2016.09.044. [48] E. Drioli, A. Brunetti, D. Pro, G. Barbieri, Green Chemistry Process Intensification Strategies and Membrane Engineering, 2012, https://doi.org/10.1039/c2gc16668b. [49] Y. Shirasaki, I. Yasuda, Membrane Reactor for Hydrogen Production from Natural Gas at the Tokyo Gas Company: A Case Study, Woodhead Publishing Limited, 2013. https://doi.org/10.1533/9780857097347.2. 487. [50] A. Singh, J. Klausner, J. Petrasch, Hydrogen Production via the Iron/Iron Oxide Looping Cycle, ASME, 2016, pp. 1–10. [51] P. Gupta, L.G. Velazquez-vargas, L. Fan, Syngas Redox (SGR) process to produce hydrogen from coal derived syngas, Energy Fuels (2007) 2900–2908. [52] Hydrogen and Fuel Cells Emerging Technologies and Applications, 2012. [53] P. Taylor, L.S. Denton, E. Hippo, T. Wiltowski, Optimization of the production of hydrogen from syngas using a 1∶7 mass ratio of hematite to calcium oxide in a pressurized fluidized bed, Sep. Sci. Technol. (2006) 37–41, https://doi.org/10.1080/01496390500527076.

CHAPTER 3

Ammonia production from syngas ´rez-Orregoa,b,c, Rafael Nogueira Nakashimaa, Meire Ellen Gorete Daniel A. Flo Ribeiro Domingosd, Mois
es Teles dos Santosd, and Silvio de Oliveira Juniora a

Department of Mechanical Engineering, Polytechnic School of University of Sa˜o Paulo, Sa˜o Paulo, Brazil Faculty of Minas, National University of Colombia, School of Processes and Energy, Medellin, Colombia c Department of Mechanical Engineering, E
cole Polytechnique F
ed
erale de Lausanne, Switzerland d Department of Chemical Engineering, Polytechnic School of University of Sa˜o Paulo, Sa˜o Paulo, Brazil b

1. Introduction Nitrogen is one of the most important elements needed for crops growth. Despite its abundance in the atmosphere, this essential nutrient must be firstly transformed (or fixed) into a more accessible form before plants can absorb it, due to its strong dinitrogen triple bond [1]. Currently, nitrogen fixation is possible on an industrial scale thanks to the invention of the synthetic nitrogen fertilizers (SNFs), which are reportedly responsible for at least fifty percent of the global yield of food crops [2]. The basic feedstock for all SNFs is ammonia (NH3), a concentrated source of nitrogen (85% wt.) that can be further processed to produce urea or nitrates [3]. The era of nitrogen fertilizers was born when the ammonia synthesis was perfected by Fritz Haber, who devised a catalytic reactor to convert the nitrogen in the air into liquid ammonia. Next, Carl Bosch applied this process to the industrial production scale [4]. The first ammonia plant built in 1913 had a production of 30 tons of ammonia per day [5], and by 1940, the original version of the current Haber–Bosch process consumed 100 GJ/tNH3 by using coal and coke [6]. As of 1960, natural gas replaced coal as main feedstock, and between 1955 and 1966, the energy requirements of the best plants dropped from 55 to 40 GJ/tNH3. A major advance in the technology of ammonia production occurred in early 1960s, when the M.W. Kellogg Company introduced the jumbo-size single train ammonia plants equipped with centrifugal compressors, where ammonia could be produced at half of the costs of smaller plants equipped with the conventional reciprocal compressors [7]. The decline in the energy expenditure since 1960 to the present days has been more gradual, achieving about 30GJ/tNH3 [4]. During the last years, ammonia plants have undergone various developments aimed to reduce the energy consumption of the chemical processes [8–12], the enhancement of the waste heat recovery [13–19], the drop of energy consumption in the CO2 removal process [20–22], Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91878-7.00016-2 Copyright # 2023 Elsevier Inc. All rights reserved.

45

46

Chapter 3

and the design of catalysts with higher activity [6,8,23–25]. However, the ideally achievable energy demand (18–21 GJ/tNH3) [4] is still much lower compared to the most efficient configurations built so far (28–31 GJ/tNH3) [8,12]. As a result, the production of fertilizers is responsible for a large proportion of energy consumption in agriculture, accounting for two percent of the global energy demand [26]. The use of nitrogen fertilizers has steadily increased at almost a rate of 2 million of tons (N) per year, achieving a worldwide production of 235 Mt in 2019 [27]. The fact that 90% of the ammonia production relies on nonrenewable energy resources suggests the need for further processes intensification and emissions cutdown in the existing and new ammonia plants [28,29].

2. Typical ammonia plant layout The conventional ammonia production plant can be generally arranged into four main subprocesses (see Fig. 1), namely the syngas production unit (reforming and shift reactions), the syngas purification unit (CO2 removal and methanation), the ammonia synthesis loop (conversion, refrigeration, separation, and purge), and the utility systems (combined steam and power generation and heat exchanger network) [30]. The operating condition of one subprocess is strongly related to each other, thus, any variation in one system simultaneously affects the Syngas production unit

CO2

CO2 Waste Heat Boiler

MP Steam Feed (NG)

Steam Methane Reforming Unit

Shift Reaction Unit

Heat Duty Fuel (NG)

BFW Heat Recovery

Syngas

Proc. Air

Carbon Capture and Methanation

Fresh Feed Gas

Ammonia Synthesis and Condensation Unit

Comb. Gases

Comb. Air

Heat Recovery Convection Train

Stack Gases

CO2 captured

Purge Gas Treatment Unit

Input Output (Cold) (Hot) HP Steam Power Utilities Plant Make up Water

Ammonia

Purge Gas

CO2

Combustion Furnace

Ammonia synthesis unit

Syngas purification unit

LP / MP Steam Cond.

Steam network and Utility systems

Fig. 1 Layout of an integrated syngas and ammonia production plant [31].

Fuel Gas LP / HP H2 & N2

Ammonia production from syngas 47 different sections of the plant concept. In practice, various configurations of processes and technologies may be licensed depending on site-specific conditions and commercial vendors include Kellogg, Brown and Root (KBR), Linde, Haldor Topsoe, Casale, and ThyssenKrupp. In order to produce ammonia from water, natural gas, and air based on the reforming of methane, a stoichiometric ratio of 3:1 H2:N2 is required, as it is shown in Eq. (1) [32]: CH4 + 0:303O2 + 1:131N2 + 1:395H2 O ! CO2 + 1:131N2 + 3:395H2 ! CO2 + 2:262NH3 (1) Accordingly, up to 2.4 kg of ammonia and 2.75 kg of carbon dioxide (fuel emissions not included) can be produced per kg of methane, whereas up to 1.6 kg of steam is required as feedstock.

3. Syngas and hydrogen production technologies For many years, the predominant feedstock for ammonia synthesis was coke, but now it is only used under special economic and geographical circumstances [4]. At this moment, below 10% of the ammonia produced comes from solid feedstock, such as coal, coke, or lignite. In general, the process is similar to the original Haber process, but involves gasification of carbonaceous materials and a stringent syngas purification process [6]. Electrolysis of water is also a relatively minor source of hydrogen for ammonia production. Its competitiveness is not as dependent on the plant size as on its localization, as it strongly relies on the cost of the electricity. Hence, this technology is mostly implemented where inexpensive electricity from hydro or other renewable power plants is available [33]. Typical power consumption of water electrolysis is 4.3 kWh per m3 of H2 produced (approx. 190 GJ/tH2) [4]. Considering that, for ammonia synthesis, additional energy is required to separate the nitrogen from the air and to compress the reactants; the total power consumption may achieve 10,200 kWh/tNH3 (or 37GJ/ tNH3). In addition, depending on the technology employed for generating the electricity, the overall energy consumption may double [34]. A threefold specific investment cost is also expected, compared to steam methane reforming (SMR) [4]. Thus, this process is often economically unsustainable, except for few specific scenarios. In practice, other energy resources and technologies dominate the syngas production for ammonia manufacturing, including partial oxidation of hydrocarbons (POX), and, more prominently, steam (SMR) and autothermal (ATR) reforming of natural gas [35]. Each process produces different H2:CO ratios (SMR 3.0–5.0; ATR 1.5–2.5; POX 1.5–1.8) [36,37] and offers specific operational advantages and technological challenges [38]. For instance, SMR does not require oxygen separation, operates at the lowest process temperatures, and yields the major H2: CO ratios. Notwithstanding, it also produces the largest emissions due to the external fuel consumption for heating purposes. The ATR runs on a lower temperature than POX, as well as presents a lower methane slip (i.e., unconverted methane); but its commercial experience is

48

Chapter 3

more limited. Meanwhile, POX does not need catalyst, produces the lowest H2:CO ratios, and operates at more severe conditions (>50 bar and >1400°C). Partial oxidation offers the advantage of a broader choice of feedstock and a better tolerance to fuel impurities, although it adds complexities to the purification process. The most common feedstock in POX is heavy residual oil from petroleum refining [39]. However, due to its higher H2:CO ratio, its relatively low energy consumption, and its reduced capital cost, SMR is nowadays considered as the most cost-efficient technology for this application (see Fig. 2), accounting for about 80% of the feedstock for ammonia synthesis [40]. Table 1 compares the relative plant investment cost and the relative energy consumption for various feedstock used in the ammonia production. In the following sections, the various unit operations used to produce hydrogen via the SMR are briefly described. Air

Syngas Production Unit

Process Air Compr.

Preheater

Process Air Primary Reformer

Water Steam Boiler

Sat. Steam Heat Recovery

Heat Recovery

Heat Recovery

Natural Gas Preheater Desulfurizer

HT Shift

Prereformer

LT Shift

Cond.

Secondary Reformer Reformer Duty

Heat Duty

Air

Combustion Gases Flue Gas Heat Recovery Convection Train

NG Fired Furnace

Natural Gas (Fuel)

Combustion Air

Fig. 2 Syngas production via steam methane reforming and water gas shift processes [31].

Table 1 Relative plant investment cost and energy consumption for various feedstock used in the ammonia production [31]. Technology Fuel Relative investment Feedstock (GJ/ tNH3) Fuel (GJ/tNH3) Total (GJ/tNH3) a

SMR Natural gas

ATR Natural gas

POX Fuel oil

Gasification coal

Electrolysis water

1.0

1.2

1.5

1.8–2.1

2.2-2.5

22.1

24.8

28.8

–

–

7.9–9.0 30.0–31.1

3.6–7.2 28.4–32.0

5.4–9.0 34.2–37.8

– 49.0–59.0

– 37.0a

Depends on the electricity generation efficiency.

Ammonia production from syngas 49

3.1 Desulfurizer In order to avoid the poisoning of catalysts in the downstream processes, the sulfurous compounds in the natural gas must be removed by using a two-step hydrogenation-absorption process called hydrodesulphurization. First, a small amount of hydrogen is added to the feedstock, which is heated up to 400°C and passed through a cobalt molybdenum or nickel molybdenum catalyst. There, sulfur is converted into hydrogen sulfide (H2S), according to Eqs. (2) and (3) [41]: CH3 SH + H2 ! CH4 + H2 S

(2)

C2 H5 SH + H2 ! C2 H6 + H2 S

(3)

Next, hydrogen sulfide is removed from the feedstock gas through an adsorption process producing zinc sulfide, Eq. (4), by involving zinc oxide pellets (ZnO) that can be later regenerated [24]. ZnO + H2 S ! ZnS + H2 O

(4)

3.2 Prereformer A prereformer is an adiabatic reactor filled with a highly active catalyst at low temperature (18% molar) and partial pressure (>6bar). The maximum amine strength (kgDEA/kgSln) is limited by its corrosivity, which ends up affecting the absorption capacity and the recirculation rate of the solvent. In addition, the degradation susceptibility limits the temperature of the chemical solvent below 120°C [4]. The highest and lowest CO2 loadings of the amine are typically set as 0.45 and 0.04 kmolCO2/kmolDEA, respectively [52]. Inhibitors may be used to increase the amine stability and reduce the corrosion at higher strengths and acid gas loads [44]. In the absorber, most of the CO2 in the syngas (99.7%) reacts with the chemical solvent producing a CO2-rich bottoms effluent (35 bar). Next, the rich solution is expanded to 2 bar and

Ammonia production from syngas 55 Heat Recovery

Preheater

Purified Syngas

Methanator

Dryer

Cond. CO2 captured

Makeup

Cond. Duty

Lean Amine

Reboiler Duty Syngas

Absorber

Desorber

Fig. 7 Syngas purification unit based on chemical absorption solvents [53].

preheated up to 93°C [30] before it enters the desorption column, wherein the reaction is reversed and the CO2 is stripped out at higher temperature and lower pressure. The desorption process demands a substantial amount of steam at the reboiler (3.4–4.0 MJ/kgCO2 or 0.1 kg sat steam @ 4bar/LDEA Solution). It must supply (i) the energy to raise the temperature of the rich solution; (ii) the enthalpy of reaction to break the chemical bonds between the CO2 gas and the amine; and (iii) the enthalpy of vaporization of the water to produce the stripping effect. Meanwhile, the overhead CO2-rich gas leaving the desorber is cooled at the top of the desorber column to provide the reflux of water and small amounts of vaporized amine, whereas the separated CO2 gas can be conditioned and exported. The partially stripped lean amine solution is withdrawn through the desorber bottoms and cooled by using a lean-rich (L/R) amine heat exchanger. Since some amount of water is carried over with the overhead stream, makeup water is required to maintain the amine solution strength. Finally, the lean amine stream is pumped and recycled back to the absorber, so that the loop can be restarted. Physical absorption systems are commonly favored over the chemical units when streams with very high partial pressures and elevated CO2 concentrations are involved. Fig. 8 shows a typical syngas purification unit based on the physical absorption principle. In the high pressure absorber (30bar), a mixture of dimethyl ethers of polyethylene glycol (DEPG) contacts the raw syngas, also producing a CO2-rich bottoms solution. But, unlike the heat-consuming chemical desorption process, the rich DEPG solution can be fairly regenerated by a gradual pressure letdown, whose expansion energy may be recovered [54]. The most notorious advantage of using DEPGs is the lower energy requirements for the solvent regeneration, due to the lack of a

56

Chapter 3 Heat Recovery

Preheater Purified Syngas

Dryer

Cond.

Methanator Makeup Lean Selexol

Cooler

Cooler

CO2 Captured

CO2 Flash Syngas

Lean Selexol

Absorber

Fig. 8 Syngas purification unit based on physical absorption solvents [53].

reboiler component. Furthermore, DEPG performs a partial gas dehydration during the absorption processes and the CO2 separation temperatures are close to the ambient conditions. Finally, DEPG presents more stable and noncorrosive operating conditions, thus, special metallurgy is seldom required, which compensates for the larger equipment and circulation rates required, compared to the chemical absorption plants [55].

3.7 Syngas purification via methanation process After the bulk CO2 removal, the syngas may still contain up to 0.32% mol CO and 600 ppm of CO2 [30]. Thus, a methanator removes the residual oxygen compounds at expense of a fraction of the hydrogen produced. Poisonous components are converted into inert methane over a nickel catalyst. Three highly exothermic reactions are involved in the methanation unit, namely the WGS reaction (7) and the reverse reforming reactions (i.e., reverse of Eqs. 5 and 6), all of them carried out at temperatures above 290°C [56,57]. Downstream the methanator, the purified syngas is cooled and the excess moisture is removed [58].

4. Ammonia synthesis and purge gas recovery An intercooled, multistage compression train is responsible for increasing the pressure of the syngas up to the operating conditions of the ammonia loop (>150 bar) [22]. It is a very energy-intensive process that represents almost one third of the total power demand. A circulator compressor is also required to recycle the unreacted mixture and overcome the pressure drop in the ammonia loop (Fig. 9). Together, the makeup syngas compressor and circulator represent about 45% of the overall power consumption in the ammonia plant [59].

Ammonia production from syngas 57

Fig. 9 Ammonia compression, synthesis, condensation and separation units [31].

Fig. 9 also shows other equipment of the ammonia loop, such as the chemical converter beds, the ammonia separation process, and the waste heat recovery units, which are described in more detail next. Contrary to the reforming process, ammonia synthesis is a volume-decreasing, highly exothermic reaction, Eq. (10). Consequently, higher equilibrium conversions could be theoretically achieved if the reaction temperature is reduced and the reactor pressure is increased [25]. However, at very low temperatures, the reaction rate becomes extremely slow and the self-sustainability of the reaction is not guaranteed. Moreover, very elevated pressures may bring about more power consumption and increased equipment cost. Accordingly, there exists a compromise between high yields, reasonable residence times and affordable setups [60]. Those operating conditions can be determined based on process synthesis and optimization techniques. See literature related to ammonia production modeling and simulation for more information [60,61].
(10) N2 + 3H2 $ 2NH3 ΔHo298K ¼ 92 kJ=kmol In practice, reactor temperatures ranging from 350°C to 550°C and pressures between 150 and 300 bar are commonly adopted as a compromise between high equilibrium conversion and acceptable reaction rates. At these operating conditions, conversions rates below 30% can be obtained using an iron-based catalyst. Other catalysts relying on ruthenium are allegedly 10 times more active, whereas running at milder operating conditions [62], such as those found in the Kellogg Advanced Ammonia Process (KAAP). Other factors that strongly influence the extent of buildup and recirculation of inerts and noncondensables in the loop are the design of the waste heat removal and the localization of the points of ammonia and purge gas withdrawal. The ammonia condensation starts with the recovery of the high-grade waste heat from the converter effluent. Considering that the enthalpy

58

Chapter 3

Fig. 10 Multiple-stages refrigeration system with a separator used (a) for intercooling or (b) vapor separation only [59].

of reaction of the ammonia synthesis is roughly equivalent to 9% of the overall energy consumption in a typical ammonia plant (2.72 GJ/tNH3), it is desirable to recover the largest amount of this energy. To this end, various proposals, ranging from steam generation to direct mechanical energy recovery using expanders, have been suggested as interesting solutions from a thermodynamic point of view [63–65]. After the major waste heat recovery is performed, the bulk of the ammonia produced is condensed by using cooling water and removed in a first separation vessel. Thereafter, the noncondensed stream joins the fresh makeup syngas before they are cooled down to 20°C with the aid of a vapor compression refrigeration unit (Fig. 10) [66]. As for the best location of purge gas withdrawal, an obvious choice is where inerts concentration is higher. This normally occurs after ammonia bulk removal and before the makeup syngas addition. Moreover, since the makeup syngas entering the loop contains some impurities, they can be suitably dissolved in the liquid ammonia, avoiding their access to the catalytic beds. Fig. 11 compares different configurations of ammonia loops that can be devised depending on the level of impurities.

Ammonia production from syngas 59 Makeup gas

Preheater Converter Circulator Bed 1

Intercooler

(a) Only suitable for make-up gas with low moisture and impurities (e.g. after thorough nitrogen washing). The impurities are not dissolved and removed with the ammonia liquid product.

Purge gas Bed 2

Intercooler

Bed 3

Separator

Water cooler

Ammonia

Refrigerator

Preheater Converter Circulator Intercooler

Bed 1

(b) Only recycled and makeup gases are compressed. The condensing ammonia is diluted by mixing it with the makeup syngas. Impurities are dissolved and removed with the ammonia liquid product.

Purge gas Intercooler

Bed 2 Makeup gas Bed 3 Separator

Refrigerator

Water cooler

Ammonia

Preheater Converter

Intercooler

(c) Impurities are dissolved and removed with the condensed ammonia. Both the ammonia produced and the recycled gases are recompressed, increasing the circulator duty. A two-step separation process using cooling water and refrigeration increases the ammonia recovery.

Bed 1 Purge gas

Intercooler

Bed 2 Makeup gas Bed 3

Separator Separator

Circulator Refrigerator

Water cooler

Ammonia

Preheater Converter Circulator Intercooler

Bed 1

(d) Only recycle and makeup gases are compressed. Impurities are dissolved and removed with the ammonia liquid product. The two-step separation process using cooling water and refrigeration increases the ammonia recovery.

Purge gas Intercooler

Bed 2 Makeup gas Bed 3

Separator Separator

Water cooler

Refrigerator

Ammonia

Fig. 11 Flow diagrams of various ammonia loop arrangements [31].

60

Chapter 3

Even if the small fractions of methane and argon entering the ammonia loop behave as inerts, their excessive built-up hinders the overall conversion and increases the circulation rate. Thus, a fraction of the recirculated stream must be continuously purged, so as to maintain inerts levels (e.g., ammonia, argon and methane) at acceptable values (1100°C). It evidences the need for a high degree of energy integration; otherwise, large driving forces may

Ammonia production from syngas 67 trigger higher energy consumption and destruction rates in those facilities. In the following sections, various proposals aimed to enhance the ammonia plant performance are discussed.

7.1 Chemically recuperated gas expansion systems for syngas production and enhanced carbon capture Fig. 18 is an upgraded version of the conventional syngas production unit, in which air is enriched up to 40% in oxygen before it enters the secondary reformer [12]. The combustion gases leaving the externally fired reformer are at atmospheric pressure, which renders them unsuitable for direct power generation. Thus, they are used instead to produce superheated steam that will be expanded in an extraction-condensing steam turbine, as well as to preheat other process streams, including the combustion air. Due to the large driving forces arisen from the marked temperature differences in the heat exchange network of the existing syngas production units, the avoidable inefficiencies increase along with the associated environmental impact. Thus, better energy integration approaches must be worked out in order to reduce the process irreversibilities and the fuel consumption. More radical approaches proposed a chemically recuperated gas turbine concept (Fig. 19) that supplies the heat duty to the reforming process, whereas generates electricity and process steam [51]. This configuration profits from the occurrence of both endo- and exothermic chemical reactions in the same production unit. In this way, a gas heated (GHR) and an ATR could be coupled so that the gases leaving the latter reformer can be previously expanded before exchanging heat with the endothermic one. In the ATR, the water produced from the partial combustion process acts as excess feedstock, which reduces the need for steam generation in recovery boilers. In this way, the losses associated to the exergy in the flue gases of an externally fired primary reformer can be avoided. The dilution of the carbon dioxide in the process gas is also minimized, which renders the capture process easier and less consuming. Thus, the introduction of the chemically recuperated gas turbine concept for energy integration purposes in existing ammonia plants may radically reduce the energy consumption and the atmospheric emissions [73]. A combination of the previous cogeneration approaches, i.e., a syngas production unit with a steam network and a chemically recuperated gas turbine system, may reduce even more the fuel consumption and the environmental impact (Fig. 20). In fact, since the heat exchange occurs between streams at elevated pressure, the heat transfer rate is enhanced in the GHR and the waste heat steam generators. The reliability of the reformer components also improves, as the externally heated tubes of the GHR are no longer subject to excessive pressure differences. Table 4 summarizes the energy consumption and environmental performance of the configurations shown in Figs. 18–20. Notably, the emissions associated to the conventional layout (2.2 tCO2/tH2) are up to 52% higher than those related to the setups that implement a chemically recuperated gas turbine system. The water consumption in the latter configurations

Natural Gas (Fuel) Air Separation Unit

Preheater

Air

Enriched Air Compressor

Cooler

Air

Syngas Purification Process

Methanator

Air (12 bar)

Air Compressor

Syngas

Cond.

N2 rich O2 rich

Molecular Sieves

Air (6 bar)

Makeup Water

Syngas to Methanator

CO2

Secondary Reformer

Cryogenic pump

Cond. Duty

Lean Amine

CO2 Capture Waste Heat Boiler

Chemical Absorption

SMR Reformed Low Pressure Column

Desorber

HT Shift High Pressure Column

HP Cond./ LP Reb. Duty

Primary Reformer

Syngas to Purification

Absorber Cond.

LT Shift

Reboiler Duty Makeup Water

Boiler Feedwater

Reforming Section

Preheater Condesate Pump Natural Gas (Feed)

Saturator

LP Steam MP Steam

Heat Duty

Overall power generation (steam turbine)

Heat Duty Saturated Steam

Backpressure Turbine

Heatt Duty

Air

Flue Gas NG Fired Furnace Natural Gas (Fuel)

Reformer Duty

Superheated Steam

Feed Streams Preheating

Combustion Airr

Fig. 18 Conventional configuration based on a steam network as utility system [51].

Heat Recovery Steam Generation

Natural Gas (Fuel) Air Enriched Air Compressor

Air Separation Unit

Cooler

Air

Enriched Air Compressor

Air (12 bar)

N2 rich O2 rich

Air Molecular Compressor Sieves

Expanded Syngas

Air (6 bar)

Cryogenic pump

SMR Reformed

MP Secondary Reformer

Methanator

MP ATR Product

HP ATR Product

Cond. HP Secondary Reformer

Low Pressure Column

Chemically Recuperated Gas Expansion

HP Primary Reformer

Steam Boiler

Natural Gas (Feed)

HP Saturated Mixture

Cond. Duty CO2 Capture Chemical Absorption

Heat Recovery

Cooled Syngas

Cond.

Reb. Duty

Syngas to Purification

Desorber

Fuel (CCS) Aux. Boiler

Heat Recovery

MP Primary Reformer Steam Boiler

Process Water

Lean Amine

Absorber

Heat Duty

Natural Gas (Feed)

HP / MP Reformer Duty

Preheater

LT Shift

Saturator

Makeup Water CO2

Expanded Syngas

HP - Feed

Process Water

Syngas

Overall power generation (gas expander)

Syngas to Methanator

SMR Reformed

High Pressure Column

HP Cond./ LP Reb. Duty

Gas Expander

Gas Expander

Recompressor

Heat Recovery MP - Feed

Steam to WGS

Preheater Saturator

Aux. Boiler

MP Saturated Mixture

Fuel (WGS) Boiler Feedwater

HT Shift BFW Pump

Heat Duty Reforming Section

Fig. 19 Syngas production with a chemically recuperated gas turbine concept equipped with reheat [51].

Syngas Purification Process

Fig. 20 Syngas production with a dual pressure, chemically recuperated gas turbine concept equipped with a steam network [51].

Ammonia production from syngas 71 Table 4 Energy consumption and environmental performance of the syngas production facilities shown in Figs. 18–20 [51]. Process parameter Feedstock (kmol/h) Feedstock fraction to ATR (kmol/h) Fuel consumed in the reformer (kmol/h) Fuel consumed in the auxiliary boiler (kmol/h) Methane slip (kmol/h)a Hydrogen production rate (kmol/h) Specific fuel consumption (kmolCH4/kmolH2) Air flow rate (kmol/h) Oxygen-rich flow rate (kmol/h) Air enrichment (%) Nitrogen-rich flow rate (kmol/h)b S/C ratio at HP inlet S/C ratio at MP inlet Pressure at HP inlet (kPa) Pressure at MP inlet (kPa) Pressure at LP inlet (kPa) Cooling demand (MW) Specific cooling demand (MJ/kmolH2) Methane conversion (%) 1° reformer outlet T (°C) at HP 2° reformer outlet T (°C) at HP 1° reformer outlet T (°C) at MP 2° reformer outlet T (°C) at MP Split fraction of enriched air to HP line (%) Split fraction of feedstock to HP line (%) Split fraction of fuel to HP line (%) Carbon dioxide sequestration rate (kmol/h) Spec. carbon dioxide sequestration (tCO2/tH2) Spec. carbon dioxide emission (tCO2/tH2) Water consumption (m3/h) Condensate production (m3/h) Syngas inerts content (%) Hydrogen to nitrogen molar ratio a

Unreacted methane in syngas. 97.75% N2, 25°C 1 atm.

b

Upgraded conventional

CRGT + reheat

Dual pressure CRGT+ steam network

1800 280 610

1090 1250 –

850 1370

–

400

440

87 6170 0.4210

150 6200 0.4180

120 6190 0.4100

2520 440 30 1440 3.000 – 3500 – – 128 75

2480 910 37 2980 3.000 3.000 3000 1340 600 125 73

2450 720 35 2350 3.490 4.150 3000 1400 900 117 68

96.76 700 960 – – –

96.68 730 1090 330 900 85

98.67 520 1050 770 960 74

–

95

5

– 1980

77 2180

93 2100

7.1

7.8

7.5

2.183

1.432

1.563

97 58 1.54 3.036

69 35 2.44 2.999

66 29 2.02 3.075

72

Chapter 3

is also reduced due to the lower steam to carbon ratios required in the ATR and the additional water production via partial oxidation. Additional technologies, such as unmixed reforming process [74], microchannels [75], and chemical looping reactors [76] have been also reported as alternatives for increasing the efficiency of the steam reforming process. The performance of the chemical absorption process can be improved by adopting two individual lean solvent streams with different CO2 concentrations instead of a single lean stream fed to the top. This strategy increases the absorption and may help cutting down the reboiler consumption [43]. Better syngas purification techniques and solvents, such as those based on additivated tertiary amines or on physical solvents, also influence the energy integration and the fuel consumption [53].

7.2 Dual pressure ammonia production systems Due to the equilibrium-limited conversion at high temperatures, single bed ammonia reactors have been superseded by designs with multiple beds, in order to increase the per-pass conversion. Fig. 21 depicts a typical ammonia loop equipped with three intercooled catalytic beds running at 200 bar (SP). The ammonia loop is also equipped with a cryogenic purge gas recovery unit. The thermodynamic properties of the numbered streams are summarized in Table 5. Since the ammonia synthesis is a decreasing volume reaction, some authors have proposed a gradual increase of the pressure of the reactants in order to increase the yield and reduce the power input per unit of ammonia produced [43]. For instance, in the dual pressure concept (DP) [77,78], about 18% of the total amount of ammonia is firstly produced at 83 bar in a series of once-through reactor beds located upstream of the main ammonia loop [61]. After 60% of ammonia is refrigerated and separated, the unreacted mixture is further compressed up to 200 bar in order to complete the ammonia conversion in the usual synthesis loop (Fig. 22). By using this strategy, a reduction of 11% in ammonia production costs has been reported for a 3300 tNH3/day plant, compared to a conventional 2000 tNH3/day plant with a single pressure ammonia loop [77]. By removing the bulk of ammonia after the once-through reactor beds, the volumetric flow of the gas is reduced, thus decreasing the power consumption in the high-pressure compression train and in the circulator of the loop. Table 6 summarizes the properties of selected streams of a 1000 t per day ammonia plant operating under the dual pressure design (DP, 83 and 200 bar). Table 7 shows a comparison between the two studied ammonia loops. Notably, an important volume of inerts and ammonia produced in the once-through section of the DP layout is withdrawn ahead of reaching the main loop. In this way, the fresh syngas compression duty is significantly lowered, along with the circulation rate. Also, although an additional refrigeration stage is required in the low-pressure section, the use of a DP loop reduces the overall

Fig. 21 Flowsheet of a typical ammonia loop equipped with multiple catalytic beds operating at a single pressure (SP) of 200 bar; cf. Table 5 for the properties of numbered streams [61].

Table 5 Selected process data of a 1,000 tNH3/day plant operating at a single pressure (SP) of 200 bar, cf. Fig. 21 for stream numbers [61]. No.

Stream

n (kmol/h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Makeup syngas W compr. total Fresh syngas To 2° separator W loop refrig Recycled syngas W circulator To ATHE Bed 1 feed Bed 1 outlet Bed 2 feed Bed 2 outlet Bed 3 feed Bed 3 outlet BFW inlet Steam outlet To gas-gas HEX Purge gas NH3 product Flashed NH3 Aquammonia Purge to refrig. W cryo. refrig To cold box To cryo. sep Expanded liquid HP H2-N2 mixture W expander LP H2-N2 mixture Fuel gas

5282 – 5421 16,206 – 15,412 – 15,412 15,412 13,731 13,731 13,305 13,305 12,873 3997 3997 12,873 312 2570 11.5 118.9 281.5 – 282 282 89 146 – 46 89

T (°C)

P (bar)

BCH (kW)

BPH (kW)

N2 (%)

H2 (%)

NH3 (%)

CH4 (%)

Ar (%)

35.1 – 36.2 20 – 11.6 – 241.9 310.0 496.9 429.3 477.7 380.0 430.0 110.1 310.0 300.0 30.0 15.0 2.6 52.5 10.0 – 30.0 195.0 199.1 83.2 – 34.0 12.5

34.7 – 200.0 198.7 – 198.7 – 200.0 200.0 199.6 199.6 199.2 199.2 198.7 100.0 100.0 198.7 198.7 198.7 80.0 80.0 79.6 – 79.6 79.6 1.0 225.1 – 79.6 1.0

264,574 – 273,712 982,612 – 911,877 – 911,877 911,877 897,816 897,816 894,570 894,570 891,367 999 999 891,367 20,527 235,915 1057 1739 17,786 – 17,787 17,786 5867 9121 – 2880 5867

12,927 9954 20,206 60,927 3090 57,281 101 64,309 68,414 74,705 69,333 71,120 63,748 65,364 1124 13,466 56,547 1151 4637 19.2 10 855.6 40 869 1527 500 564 144 140 0.1

25.2 – 24.7 21.1 – 22.2 – 22.2 22.2 18.8 18.8 17.8 17.8 16.8 – – 16.8 19.4 0.4 0.1 0.0 21.5 – 21.45 21.45 57.6 4.8 – 4.8 57.6

73.4 – 74.1 65.2 – 68.5 – 68.5 68.5 58.6 58.6 55.7 55.7 52.4 – – 52.4 60.7 0.7 0.2 0.0 67.3 – 67.3 67.3 7.7 94.9 – 94.9 7.7

0.0 – 0.0 6.5 – 1.8 – 1.8 1.8 14.3 14.3 17.9 17.9 21.9 – – 21.9 9.8 97.7 99.1 16.2 0.0 – 0.0 0.0 0.0 0.0 – 0.0 0.0

1.0 – 1.0 5.5 – 5.7 – 5.7 5.7 6.3 6.3 6.6 6.6 6.8 – – 6.8 7.7 1.2 0.6 0.0 8.5 – 8.5 8.5 26.6 0.1 – 0.1 26.6

0.41 – 0.2 1.7 – 1.8 – 1.8 1.8 2.0 2.0 2.0 2.0 2.0 – – 2.0 2.4 0.0 0.0 0.0 2.7 – 2.7 2.7 8.1 0.2 – 0.2 8.1

Ammonia production from syngas 75

Fig. 22 Flowsheet of an ammonia loop equipped with multiple catalytic beds operating under a dual pressure design (DP); cf. Table 6 for the properties of numbered streams [61].

refrigeration duty by 34.6%, compared to the SP configuration. It is worthy to notice that the fresh syngas compression consumes almost 80% of the power demand in the SP loop, followed by the refrigeration unit (24%) and the circulator (Ir>Rh>Ni>Co>Os>Pt>Fe> Mo>Pd>Ag [16]. When the surface area of the active metal is also accounted and a specific activity is calculated, the activity trend slightly varies as follows Ru>Fe>Ni>Co>Rh>Pd>Pt >Ir [17]. Ruthenium appears as the most active metal under methanation conditions but is less selective and more expensive than other nonnoble metals that focus the main effort for commercial applications [18]. With regard to sulfur tolerance, noble metals and molybdenum show the best performances. However, the activity of Mo is low and its selectivity to C2 + hydrocarbons makes it a less favorable alternative for methanation [19]. As well as molybdenum, iron-based catalysts are used for larger hydrocarbons production mostly through the Fischer–Tropsch process [20]. Nickel and cobalt present similar selectivity and activity, but the lower price of Ni makes it the most widely spread active metal in commercial catalysts for methanation. Different promoters can improve the activity of the catalyst. Among them, MgO enhances the thermal stability, carbon deposition resistance, and catalytic activity of Ni-based catalysts [21]. Also CeO2 increases the thermal stability and catalytic activity of Ni/Al2O3 [22]. Table 1 summarizes the main catalyst formulation and the operation parameters in catalytic methanation reaction. Table 1 Catalysts and operation parameters of catalytic methanation reaction [23]. Catalysts

Reaction conditions

CO2 conversion

CH4 selectivity

2.5 wt%Ru/TiO2 5 wt%Ru/r-TiO2 Ce0.95Ru0.05O2 3 wt%Ru/Al2O3 3.7 wt%Ru/CeO2/r 5 wt%Ru/CeO2 Mesoporous Rh 1 wt%Rh/γAl2O3 10 wt%Ni/CeO2 5 wt%Ni/ZrO2 10 wt%Ni/CeO2-ZrO2 2 wt%Ni@CeO2-ZrO2 5 wt%Ni/MSN 20 wt%Ni-20 wt%CeO2/MCM-41 10 wt%Ni-1 wt%MgO/SiO2 10 wt%Ni-La2O3/SBA-15(C) 10 wt%Co/ZrO2 2 wt%Co/ZrO2 Co3O4 nanorods 20 wt%Co/KIT-6

325°C—1 bar 300°C—1 bar 450°C—1 bar 350°C—1 bar 350°C—1 bar 300°C—1 bar 550°C—1 bar 25°C—1 bar 350°C—1 bar 350°C—1 bar 275°C—1 bar 350°C—1 bar 300°C—1 bar 380°C—1 bar 350°C—1 bar 320°C—1 bar 400°C—1 bar 400°C—30 bar 230°C—10 bar 280°C—1 bar

>80% 65% 55% 85% >75% 83% 98.9% 25% 93% 79.1% 55% 58% 64.1% 85.6% >67% 90.7% 92.5% 85% 70% 48.9%

100% 100% 99% 85% 99% 99% 81.2% 100% 100% 76.5% 99.8% > 97.5% 99.9% 99.8% > 98% 99.5% 99.9% 99% 99% 100%

Methane production from syngas 97 Coke deposition is one of the deactivation mechanisms of metal catalysts taking place during CO hydrogenation processes. This can be prevented through the addition of steam controlling an appropriate H2/CO ratio. Sulfur poisoning is another major challenge for methanation catalysts that strongly depends on the composition of reactants streams. Deactivation of the catalyst by attrition can be induced in fluidized beds, while thermal stress might sinterize the particles. Future research in this field should focus on different designs and mechanisms for temperature control, the improvement of coke and sulfur resilience, and the improvement of catalyst deactivation caused by fouling and mechanical stress while keeping high activity and selectivity. The optimal operating conditions for catalytic methanation will be obtained after techno-economic analysis, which accounts for the capital investment of high operating pressures and the cost of catalysts, accounting for the capital investment of high operating pressures and the cost of the catalysts that lead to significant activity at low temperatures. Nowadays, Ni-based catalysts still present the most competitive prices that make them the most attractive active metal selection.

2.2 Catalytic methanation concepts: design and facilities The main catalytic methanation technologies are divided into (i) two-phase (gas–solid) methanation reactors that include adiabatic and cooled fixed-bed and fluidized-bed and (ii) three-phase (gas–liquid–solid) methanation reactors. Some of them are already in a commercial-stage, while others are still under demonstration or research [11]. Current research mainly focuses on improving temperature control, cost efficiency of the process, and methanation flexibility regarding the availability of renewable syngas. Among the adiabatic fixed-bed concepts, Air Liquide (two reactors—intermediate cooling) [24,25] and Haldor Topsøe (TREMP process: 3–4 reactors—high-temperature methanation) [26,27] are the two most well-known commercial technologies developed in the 70s but still implemented in new projects. British Gas and Conoco also developed a similar process in the 70s (HICOM process: 3 reactors, intermediate cooling, and gas recycling) [28], which is currently commercially exploited by Johnson Matthey. Imperial Chemicals Industries and Koppers developed an integrated methanation concept for Koppers–Totzek gasifier syngas (several reactors–intermediate cooling and steam addition) [29]. Another commercial technology is the Vesta methanation process (3 reactors–steam addition) developed by Clariant and Foster Wheeler based on the Imperial Chemicals Industries concept [30]. Noncommercially available concepts include the reactor developed by Ralph M. Parsons company (4–7 reactors-intermediate cooling & staged gas feed) [28]. The cooled fixed-bed methanation concept developed by the Linde company included a first cooled reactor with integrated heat exchanger and a second adiabatic reactor [31]. Finally, this concept did not derive into commercial methanation reactors although these isothermal concepts were applied

98

Chapter 4

to methanol synthesis. Further alternatives to the concept of cooled methanation reactors are molten salts [32] and heat pipes. The main limitations of adiabatic fixed-bed reactors are the temperature hot spots and high-pressure drops. Investigations to facilitate temperature control through new cascades of adiabatic or cooled fixed-bed reactors [33–35] or the design of structured reactors are under development [36–38]. The internal structure of these reactors enhances heat transferability in 2–3 orders of magnitude and reduces the pressure drop [39,40]. These reactor designs are characterized by a high surface-to-volume ratio but present difficulties in the placement and replacement of the catalyst in the metallic structure [41]. Investigations with structured reactors are also oriented to improve the flexibility of biomass to methane production. Engler-Bunte-Institute coordinated the commissioning of a methanation pilot plant using structured isothermal honeycomb reactors in combination with a WoodRoll gasifier in Sweden [42]. Sorption-enhanced methanation reactors are other types of structured fixed-bed reactors in which water formed during methanation is adsorbed on catalyst support reducing the thermodynamic limitation of CO2 conversion [43,44]. This advantage can be exploited to increase methane production yield, and further research is done in this kind of reactor [45,46]. Besides the commercially available fixed-bed reactors, several concepts of fluidized-bed methanation reactors have been developed. The Bi-Gas-Process was one of the first fluidized-bed methanation industrial reactors developed by Bituminous Coal Research Inc. and included two internal heat exchangers [47,48]. Thyssengas GmbH, Didier Engineering GmbH, and the DVGW-Forschungsstelle developed the COMFLUX process including fluidized-bed reactor and internal heat exchanger [49]. Several groups are working on the demonstration of methanation plants using fluidized-bed reactors. The Paul Scherrer Institute commissioned a facility with an output of 1 MWSNG to demonstrate the CO methanation from biomass gasification [32] and a pilot plant with an output power of 160 kWSNG [50]. A similar concept was chosen for the biomethane pilot plant commissioned by Engie in Lyon to demonstrate technological feasibility under semiindustrial conditions (GAYA project) [51]. Nevertheless, fluidized-bed reactors also present drawbacks such as poor reaction kinetics and attrition of catalyst. Specific catalysts designed for fluidized-bed operating conditions are under investigation [52]. Another technology option for effective cooling implies three-phase methanation reactors. Chem Systems Inc. developed in the 70s a concept (LPM process: 3-phase reactor, 70 bar) in which catalyst was suspended in mineral oil, enabling a proper temperature control [53]. The limitations of three-phase methanation reactor concepts are associated with the additional resistance related to gas–liquid mass transfer and the decomposition and evaporation of the heat transfer fluid. The investigation is focused on hydrodynamics and heat transfer [54] as well as

Methane production from syngas 99 Table 2 Syngas catalytic methanation projects at large scale [11]. Project and location

Capacity

Methanation

Feedstock

GAYA (Engie)—Saint Fons (France) GOBIGAS (G€ otegorg Engie)—Gothenburg (Sweden) BioSNG (EU project)—G€ ussing (Austria) Great Plains Synfuels Plant—Beulah (USA) DemoSNG (EU project)—K€ oping (Sweden) CPI (CPI Xingjiang Energy)—Yili City (China) Keqi project (Datang)—Chifeng (China) Fuxin project (Datang)—Fuxin (China) Huineng project—Ordos (China) Xinwen (Xinwen Mining)—Yili City (China) Qinghua (Qinghua Group)—Yili City (China) POSCO (POSCO)—Gwangyang (South Korea)

400 kWSNG 20 kWSNG

PSI TREMP

Biomass Biomass

1 MWSNG 1500 MWfuel 50 kWSNG 6 billion m3 SNG/a 4 billion m3 SNG/a 4 billion m3 SNG/a 1.6 billion m3 SNG/a 4 billion m3 SNG/a 5.5 billion m3 SNG/a 0.7 billion m3 SNG/a

PSI Lurgi KIT TREMP HICOM HICOM TREMP HICOM TREMP TREMP

Biomass Coal Biomass Coal Coal Coal Coal Coal Coal Coal

on the development of optimized catalysts [55]. The isothermal operation of this type of reactors has been proven at high load variations [56,57]. A summary of the main worldwide projects that combine coal or biomass gasification with catalytic methanation stage is presented in Table 2.

2.3 Catalytic methanation kinetic models Mathematical modeling methanation processes are commonly applied to simulate and predict the outcome of different process configuration, as well as to optimize the process yield and productivity of the desired products. Different research groups have studied the CO and CO2 catalytic methanation kinetics. Some authors make use of basic power rate laws [58–60], while more detailed models consider the reaction mechanism following the Hougen Watson formalism [61]. A large number of different kinetic approaches of catalytic methanation reaction are found in literature depending on the catalyst used [11,17,29,62,63]. Langmuir–Hinshelwood is commonly applied in simulation models [29,64], but the validity of this kinetics models will depend on the catalyst and operation parameters. Other authors adapted these models to their experimental results obtained for novel catalysts [65,66]. A review of CO and CO2 methanation rate equations was published together with their validity [67]. This study recommends for dynamic simulations the use of rates given by equations from [62,66] including a term for the reverse methanation reaction. While CO methanation rate can be complemented with a rate for the reverse water–gas shift reaction [67].

100 Chapter 4

3. Biological conversion of syngas into methane Besides the chemical conversion route, a novel trend proposes the biological methanation of syngas (syngas biomethanation process). Methanogenic archaea are microorganisms that can be exploited as natural catalysts to produce CH4 from a source of carbon and H2 with a lower energy demand than the thermochemical route [68]. The integration of syngas from the gasification process and methanogenic fermentation presents some interesting synergies. The upgrading of syngas from biomass gasification through biological methanation could significantly limit the requirement for gas cleaning compared to catalytic chemical methanation [69]. Methanogenic microorganisms tolerate and need sulfur compounds and may even facilitate the decomposition of tars through fermentation. Ash wastes from the gasifier can supply nutrients to the microorganisms, which implies reducing costs as additives. The major challenge in the biological methanation of syngas is the mass transfer between the gas phase and the microbial cell to achieve high conversion of the syngas components. These technologies present a significant potential cost-efficient and environmentally friendly alternatives [70].

3.1 Biological mechanism and process conditions Syngas methanation via methanogenic archaea transform CO2 and CO with H2 into CH4 through hydrogenotrophic methanogenesis (Eq. 6) and carbon monoxide conversion (Eq. 7). CO2 + 4 H2 $ CH4 + 2 H2 O

(6)

4CO + 2 H2 O $ CH4 + 3 CO2

(7)

Biological methanation reactions can be carried out under anaerobic conditions at mild temperatures between 20°C and 70°C (mesophilic and thermophilic) and atmospheric pressure in trickle-bed or stirred tank reactors [71]. The technical implementation of biological methanation is still a challenge. Methane formation rate (MFR), the gas hourly space velocity (GHSV), and the methane content in the final gas are the most significant parameters to assess the performance of the reactor. The methanation efficiency will depend on the selected microorganisms, microorganism concentration, type of reactor, pressure, pH, and temperature (Table 3). The main advantage of biological methanation is the higher efficiency related to lower operating temperatures and pressure. Reactions (Eq. 6) and (Eq. 7) are catalyzed by microorganisms that tolerate impurities and loading changes of the substrate gas to a higher degree than metallic catalysts required in the thermochemical route [76]. The conversion of

Methane production from syngas 101 Table 3 Operation parameters of biological methanation reactors under continuous operation [70].

a

T (ºC)

P (bar)

Reactor

MFR (h21)

CH4 (%)

GHSV (h21)

Ref.

60 65 65 37 65

1–1.22 3 1 1 1

CSTR CSTR CSTR Trickle-bed Fixed-bed

1.4–2 28.7 5.6–21.3 0.05 4.6

81–82.7 13.4 60–85 98 34

8.6–12 300 29–120 0.3 30

[72]a [71]a [73]a [74]b [75]b

Based on liquid volume. Based on bed volume.

b

Table 4 Growth temperature and pH of syngas biomethanation microorganisms (methanogenic archaea, acetogenic, and hydrogengenic microorganisms). Microbial culture

T (ºC)

Topt (ºC)

pH

Ref.

Methanothermobacter archaea thermautotrophicus Ruminococcus productus Morella thermoacetica Rhodospirillum rubrum Carboxydothermus hydrogenoformans

40–70 35–40 55–60 22–35 40–78

65–70 37 60 30 70–72

7.2–7.6 7 5.7–7.7 6.8–7.2 6.8–7.0

[72] [77] [77] [78] [79]

syngas is higher achieving methane contents near 98% for suitable syngas composition [74]. Table 4 summarizes the operation temperature of the main microbial cultures that are used in the biomethanation process of syngas. The limitation of biological methanation is mainly related to the slower kinetics compared to two-phase catalytic conversion since microorganisms are present in a fermentation broth (aqueous solution). Thus, an additional gas–liquid mass transfer resistance must be overcome. The effective methanation rate can be increased by improving the mass transfer coefficient or by increasing the solubility. A good contact surface obtained through stirring enhances gas microorganisms mass transfer and favors kinetics. But the application of liquid-phase stirred tank reactors still represents a challenge given the limited solubility of hydrogen in the liquid phase [80]. Applying Henry’s Law, carbon dioxide is over 20 times more soluble than hydrogen in the aqueous media. Thus, increasing the hydrogen supply to the microorganisms by improving the mass transfer represents the most significant challenge. Continuous stirred-tank reactor (CSTR) is the most spread concept to study biological methanation. The increase of the mass transfer coefficient (higher presence of hydrogen into the liquid) can be achieved in these reactors through a more intense stirring rate. However, more intense agitation of the tank also leads to higher energy consumption. Elevated pressures in the

102 Chapter 4 CSTR also increase the mass transfer of carbon dioxide and hydrogen to the broth and the MFR [71,72]. A higher supply of CO2 to the broth generates a reduction of pH, which must be controlled to ensure suitable conditions for the microorganisms. Further research to develop an effective pH control method in pressurized reactors must be carried out. On the other hand, trickle-bed reactors in which microorganisms are immobilized on a solid carrier can be used. Carrier materials include polyethylene rings and polyvinylidene fluoride sachets [74,81].

3.2 Biological methanation concepts Different concepts are under study to integrate biomass gasification and methanogenic fermentation. In a first proposal, the fermenter directly connects with the outlet of the gasifier acting as a methanation reactor and gas scrubber, which eliminates ashes and unburned coke, tars, and H2S. The ashes from the gasifier supply nutrients to the methanogenic bacteria, while syngas sensible heat provides heat if tank’s design requires an additional supply. A second proposed concept is the integration of the syngas from gasifier into a conventional anaerobic digestion plant capable of digesting partially or totally tars. The addition of syngas to the anaerobic digester may lead to the heat of the fermenter through the condensation of steam content. These two concepts must deal with the issues derived from the impact of carbon monoxide and heavy tars presence on the microorganisms. Another promising concept integrates the syngas from gasifier into a biological power to gas, using additional hydrogen from electrolyzers in the biological methanation stage. The electrolysis requires a continuous power supply or the availability of complex and expensive hydrogen storage tanks. Thus, providing the baseload of hydrogen with the syngas from the gasifier makes possible a more flexible consumption of electricity, the avoidance of hydrogen storage, and the optimization plants operation [82].

3.3 Biological methanation kinetic models Unstructured kinetic models are the most commonly used given their simplicity since they only makes use of a limited number of variables to define the evolution of the concentration of microbial, substrates, and products. Grimalt-Alemany et al. summarized a complete selection of well-known unstructured kinetic models that determines the properties of syngas-converting cultures under several conditions and process configurations [6]. These models successfully simulate the effects of these main variables on the microbial growth and the productivity in batch and continuous processes representing a relevant tool for optimization of bioprocesses design and operation [83].

Methane production from syngas 103 The growth kinetics of methanogenic archaea has been assessed through kinetic models based on Monod, Andrew and modified Gompertz model [84–87]. However, these existing models do not account for the influence of the CO concentration on the kinetics on methanogenic archea growth since most of them were developed for anaerobic digestion processes. Further research to investigate the effect of CO partial pressure on methanogenic archaea growth must be carried out. Besides methanogenic archaea, acetogenic, and hydrogengenic microbial cultures can grow on syngas to produce methane through alternative metabolic pathways. The acetogen Ruminococcus productus growth follows a modified Monod equation, which is able to account for the inhibition effects of high partial pressures of CO [88]. CO conversion rate is calculated in this model as a function of the gas loading rate and the volumetric mass transfer coefficient in both a CSTR and a bubble column. The growth of acetogen Morella thermoacetica on CO may be simulated through (i) a dual-substrate kinetic models able to account for the initial pressure of syngas on the consumption of H2 and CO in which the growth behavior was simulated with the combination of Luong/Monod kinetic models [89] or (ii) an Andrew equation for kinetic parameters estimation and a novel expression for microbial growth, cell decay, and CO inhibition [90]. The kinetic parameters of hydrogenogen Rhodospirillum rubrum growth on CO and syngas were determined through a Monod model using CSTR experimental data with dual impellers [91]. The dilution rate for the process configuration can be optimized to obtain the maximum possible hydrogen productivity for this reactor. The characterization of the kinetics of hydrogenogen Carboxydothermus hydrogenoformans used the Han & Levenspiel model to include the effects of the partial pressure of CO and to account for the influence of the substrate to biomass ratio on the behavior of the microbial culture [92]. All these kinetic models have been developed for pure microbial groups in syngas biomethanation processes, but no model has been developed to simulate the simultaneous growth of several microbial groups. The simulation of kinetic competition among microbial cultures or the effects of biomethanation operating parameters on the growth of each group will facilitate the selection of the optimal parameters for syngas biomethanation and the control of the metabolic pathways that are more significant in the microbial consortium. Structured models that include the metabolic interactions of mixed cultures have been mainly proposed for anaerobic digestion processes and fermentations [93,94]. A preliminary metabolic network model for syngas biomethanation processes based on a flux balance analysis was proposed but presented accuracy limitations related to the low number of components considered in the metabolic network [95]. Recent studies have developed kinetic models for syngas biomethanation process carried out by a combination of mesophilic and thermophilic microbial consortia to find control strategies of the catabolic routes through the regulation of operating parameters. Thermophilic microbial consortium presented much higher apparent specific methane productivity that was also increased with initial CO partial pressure. The control of CO2 partial pressure is highlighted as a promising operational strategy to enhance the selectivity toward methane [96]. Further investigation to develop more complex models is still

104 Chapter 4 required given the potential of these structured kinetic models for the optimization of syngas biomethanation processes [6].

4. Conclusion The methanation process presents the potential to store large amounts of renewable energy as chemical energy, which also allows diverting electricity into the transportation sector. The environmental analysis highlights the significance of the selected CO2 and H2 to assess the benefits of the methanation process. Renewable sources of both chemicals such as syngas from biomass or waste gasification reduce the concept’s global warming impact. Methane production from syngas via the Sabatier reaction represents an efficient and well-established technology for utilizing large amounts of carbon dioxide (95% conversion). The composition of the obtained gas after water removal is suitable for injection in the gas network. The major challenges are energy input, heat management, syngas composition (SOx and NOx could poison catalysts), and availability since renewable sources often fluctuate. Future research on dynamic systems should focus on developing flexible concepts able to adapt to the fluctuating supply of energy and hydrogen. Regarding the suitable metals used as catalysts, nickel’s low cost, high activity, and selectivity make it the most predominant catalysts in commercial methanation processes. Future research should focus on improving thermal stability and sulfur and coke resilience of these materials. The management of heat and the resulting temperature control represent issues for both CO and CO2 methanation. Many concepts have been proposed, studied, and brought to pilot or commercial stage, but only a few fixed-bed concepts are commercially available. They predominantly include gas recycling and intermediate cooling. To reduce capital investments and operational maintenance, the gas recycling compressor tends to be removed in future applications. Further research is required to better understand the methanation mechanism for novel catalysts and different inlet syngas compositions, as well as to develop new reactors and process concepts with enhanced performances under dynamic operating conditions. Thus, the main aspects for future investigations are temperature control and improvement of cost efficiency and flexibility. The current state of the art, GoBi gas plant in Gothenburg, Sweden, still requires very large facility given the complexity of the syngas conditioning stage prior to the methanation. The improvement of economic competitiveness through the economy of scale finds other limitations related to biomass supply and logistics. The concepts of biological methanation have gained special interest. microorganisms are potentially more tolerant to syngas pollutants such as sulfur and tars, leading to less complex process chains. Other attractive options are the pressurized gasifiers together with hot gas cleaning systems or the integration of green hydrogen to enhance the methanation process. The technology readiness level of these options is low, and further investigations are still required.

Methane production from syngas 105

Abbreviations and symbols CCUS CSTR GHG GHSV LHV MFR SNG

carbon capture, utilization, and storage continuous stirred-tank reactor greenhouse gases gas hourly space velocity low heating value methane formation rate syngas

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CHAPTER 5

Methanol production from syngas Sonia Sepahi and Mohammad Reza Rahimpour Department of Chemical Engineering, Shiraz University, Shiraz, Iran

1. Introduction To satisfy the daily needs of humanity, large amounts of energy are devoured. Coal, natural gas, and oil comprise our principal energy sources that tender the raw materials to generate the secondary products. Unfortunately, these assets are restricted and are not capable of being renewed on the human time scale; thus, we will probably deplete fossil fuels, coal, natural gas, and oil within three centuries [1]. In addition to the previous disadvantages, fossil fuels are liable for the emission of greenhouse gases such as CO2, CH4, and N2O, which have a role in global warming. Therefore, all practical substitutes must be considered to find feasible and long-term solutions. Besides the power generating problem, a significant dilemma is using and storing energy efficiently. A convenient alternative could be hydrogen and liquid methanol. From an energy point of view, using hydrogen appears to be the most promising way. As indeed, hydrogen, compared to any known fuel, has the highest energy level per unit weight (142 kJ/g), and it is environmentally safer than other natural gases. The problems of using hydrogen are related to the expenses of purification processes and substructural difficulties of storage and transport. Methanol can function as an easily transportable fuel, feasible energy storage medium, solvent, and raw material for producing intermediates and hydrocarbons like polymers. Thus, it might be a key material in the global economy. Methanol can benefit the substitution of fossil fuels for renewable energies [1–3]. Rather than the traditional production along with synthesis gas, many other different ways can be applied in methanol production. The carbon source for manufacturing methanol could be natural gas, coal, biomass, or CO2 that can be recovered from industrial units or the atmosphere. Regarding the existence of greenhouse gases in the atmosphere, these new ways would relieve global warming [4].

Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91878-7.00012-5 Copyright # 2023 Elsevier Inc. All rights reserved.

111

112 Chapter 5

1.1 History Before the modern industrial age, methanol production was produced by heating wood in an anaerobic condition. The wood distillation products were a mixture of methanol (also called wood spirit) and many impurities. Sir Robert Boyle improved the process by purifying the extract (so-called wood vinegar) with the help of a reaction with milk of lime (an aqueous suspension of calcium hydroxide) in the 1660s. Nevertheless, methanol was not commercialized for about two centuries [5]. Methanol’s chemical composition was first discovered by J.V. Liebig and J.B.A. Dumas in the 1800s. In 1835 based on their work, the term methyl was officially introduced into chemistry. Also, production volumes at that period were small, e.g., about 10–20 L per ton of wood. Methanol started being marketed in the 1900s due to the increase of requirements by the chemical industry. In 1905, Sabatier suggested a new synthetic pathway for producing methanol. He tried CO hydrogenation on a metal-based catalyst and among studying the numerous compounds, he found out that methyl alcohol can be obtained on a nickel-based catalyst [6]. In 1923, the first commercialized methanol plant was proposed by a German company called Badische Anilin und Soda Fabrik (BASF). This patent was based on syngas supplied via coal gasification. This syngas conversion required a ZnO-Cr2O3 catalyst and high temperature and pressure (300–400°C and 250–350 atm) called the BASF process [7,8]. However, the BASF process had main limitations such as difficult operating conditions, low syngas conversion, and high production costs. Still, the BASF process remained as the dominant technology for around 45 years. To improve the economic process, new technologies have been developed in which pressure and temperature levels are reduced. In the 1940s, the Swiss Lonza Company began using the electrolytic hydrogen and carbon dioxide in order to synthesis methanol in industrial volumes. The carbon dioxide was derived from Ca (NO3)2 synthesis. For the first time, through purifying nitrous vapors, the reactant gases had been formed, and the ZnO-based catalyst was used for carrying out the reaction. The process was developed in Italy by Prof. Giulio Natta for the methanol synthesis from CO and H2 [9,10]. According to the invention of the methane steam reforming, purer syngas (a mixture of H2, CO, and CO2) and a more active Cu/ZnO catalyst have been obtained. The process pressure and temperature were reduced to about 100 atm and 300°C, respectively. This significant improvement was presented by Imperial Chemical Industries (ICI) in 1966 [11]. Later, Lurgi proposed a process with even lower operating temperature and pressure (230–250°C and 40–50 atm). Nowadays, the methanol synthesis process is a well-known technology and therefore most methanol plants function at low pressure conditions based on syngas conversion. The commercial methanol plants consist of two major steps: feedstock reforming to syngas and catalytic synthesis of methanol from syngas. Today, more than 90% of

Methanol production from syngas 113 methanol in the world is produced from natural gas, and producing methanol from coal and other hydrocarbons is less common [12].

2. Methanol applications Methanol, also known as methyl alcohol or methyl hydrate with the formula CH3OH, is the most simple alcohol. It is a colorless, light, volatile, flammable, and toxic material. Methanol consists of a methyl group linked to a polar hydroxyl group and is a polar chemical, thus it is completely soluble in water and organic solvents and slightly soluble in oil and fat [2]. Methanol has a molecular weight of 32.04 and an octane number of 113 and the density of about half of gasoline. It can be used as a transportation fuel in the role of additive to gasoline. The combination of 10% methanol to 90% gasoline can cause an octane number up to 130. Any additional percentage of methanol to fuel, advances the implementation of internal combustion engines (ICE) and reduces the pollutants such as NOx, SOx, hydrocarbons, and particulates [13]. Methanol is used for producing different chemicals such as formaldehyde, acetic acid, methyl tert-butyl ether (MTBE), dimethyl ether (DME), methyl methacrylate (MMA), methyl and vinyl acetates, methylamines, melamine resin, ethylene, and propylene. Also, many artifacts are manufactured from methanol products: antifreezes, adhesives, paints, resins, silicones, plastics, textiles, and so on. Recently, after ammonia production, methanol synthesis has become the second source of hydrogen consumption [14–16]. As shown in Fig. 1, about 29% of methanol is expended to produce formaldehyde. Formaldehyde, also called methanal, is the simplest aldehyde and is a colorless, strong-smelling, flammable chemical. Which is stored as formalin that is the aqueous solution of it. It is commonly used as a germicide, fungicide, and disinfectant and as a preservative in mortuaries and medical laboratories. In 1910, the first generation of formaldehyde began. Methanol and air react at 600°C on a silver catalyst as Eq. 1 that is an endothermic reaction [17–20]: CH3 OH ! CH2 O + H2

ΔH ¼ 84 kJ mol1

(1)

Due to the high temperature level of this equation, finding an alternative process to increase selectivity and stability at lower temperature conditions was an important task. In 1931, Adkin et al. offered an iron-base catalyst for direct oxidation of methanol to formaldehyde. This process needs a lower temperature and the yield was 90% [21]: CH3 OH +

1 O ! CH2 O + H2 O ΔH ¼ 159 kJ mol1 2 2

(2)

114 Chapter 5 THE USES OF METHANOL

other uses

Chemical Precursors

%11 fuels & additives Blending

Methyl methacrylate

10%

Chloromethane 2%

Dimethyl ether 10% Methyl tert-butyl ether

Biodiesel 4%

2%

Methylamine

10%

%34

%55

3%

Acetic acid 9% Methanol to Olefin/ Methanol to Paraffin 10% Formaldehyde 29%

Fig. 1 Resulting products of methanol. Rearranged from The Uses of Methanol. http://www.mefco2.eu/news/theuses-of-methanol.php (Accessed 13 September 2021).

In the last few years, methanol has become one of the C1 building blocks in the petrochemical industry and a broad part of the produced methanol is used in manufacturing DME. DME has an autoignition temperature and cetane number near diesel fuel. With DME addition to diesel fuels, the NOx emission, smoke, and engine noise are reduced. Also, it is used as feedstock for producing numerous chemicals such as acetic acid, short olefins (ethylene and propylene), hydrogen, and dimethyl sulfate. The manufacturing process of DME from methanol is [22]: 2CH3 OH ! CH3 OCH3 + H2 O

(3)

Hydrogen is known as a clean energy source that plays an important role in the chemical and electronic industry. Hydrogen is difficult to store and transport, thus a liquid feedstock like methanol with an excellent H2 source is a good alternative for easy transporting. Many technologies are developing for the conversion of methanol to hydrogen. Some of these technologies are direct decomposition, MSR reaction, and POM. The summary of methanol application for hydrogen manufacturing is represented in Table 1. Nowadays, as the use of chargeable systems (mobile devices, computers, etc.) grows, researchers try to find a feasible alternative for external charging forces. Proton exchange membrane (PEM) fuel cells are electrochemical devices that can produce electrical energy from

Methanol production from syngas 115 Table 1 Summary of methanol application for hydrogen manufacturing. Method Methanol decomposition (MD) Methanol steam reforming (MSR) Partial oxidation of methanol (POM) Autothermal reforming (ATR) (combination of MSR & POM) Methanol-water solution electrolysis

Reaction CH3OH > CO + 2H2

ΔH ¼ 90.1 kJ mol

Ref. 1

(4)

CH3OH + H2O > CO2 +3H2 Δ H ¼ 49.7 kJ mol1 CH3OH +0.5O2 > 2H2 + CO2

ΔH ¼ 192.3 kJ mol1

[23] (5) (6)

4CH3OH + 3H2O + 0.5O2 > 11H2 + 4CO2 ΔH¼ 44 kJ mol1 (7) CH3OH + H2O ! CO2 + 6H+ + 6e anode (8) 6H+ + 6e- ! 3H2 cathode (9) CH3OH + H2O ! CO2 +3H2 overall (10) Hydrogen produced is very pure (95.5–97.2 mol%), and the voltage of the system is significantly lower (0.03 V compared to 1.23 V in water electrolysis)

[24] [25] [26]

[27,28]

chemical reactions. Also, methanol solution can act as a fuel cell for converting chemical energy to electrical power. This system works at ambient temperature and is like methanol-water solution electrolysis [29]: CH3 OH + 1:5O2 ! CO2 + 2H2 O

(11)

3. Methanol production Methanol can be produced from any feedstock that contains carbon such as natural gas, coal, biomass, and CO2.

3.1 Methanol production from syngas About 90% of methanol production is currently from natural gas, and other technologies cannot be substituted on an industrial scale according to undesirable efficiency. Production of methanol from syngas can be summarized into three basic steps: a. Production of synthesis gas (syngas) b. Syngas conversion to methanol c. Methanol distillation to reach to desired purity.

116 Chapter 5 3.1.1 Syngas production Syngas (mixture of H2, CO, and CO2) is mainly derived from natural gas by steam reforming (SR) and auto thermal reforming (ATR). It can also generate by partial oxidation (PO) of methane, petroleum coke, heavy oils, coal, or biogas [30,31]. The purification of syngas is a pivotal step in methanol production. When natural gas is the feedstock, more than half of the total expenditure is required and up to 70%–80% of total spending when coal is the raw material [11]. Methane steam reforming is a high endothermic process in which the operating temperature is in the range of 800–1000°C, the pressure is about 20–30 atm, and the reaction occurs on a Ni-based catalyst. Also, CO and water react, and CO2 is produced in the water gas shift (WGS) reaction [32–34]. CH4 + H2 O>CO + 3H2 CH4 + 2H2 O>CO2 + 4H2 CO + H2 O>CO2 + H2 CH4 + CO2 >2CO + 2H2

ΔH298K ¼ 206 kJ mol1 ΔH 298K ¼ 165 kJ mol1 ΔH 298K ¼ 41 kJ mol1 ΔH298K ¼ 247 kJ mol1

(12) (13) (14) (15)

High-temperature and low-pressure benefit the production of syngas. In the WGS reaction, with no mole change, the temperature increase will reduce the production of CO2 and promotes the generation of CO. In auto thermal reforming (ATR), methane transforms to syngas in a reaction with oxygen, CO2, or steam. The reaction is exothermic due to methane oxidation. The main difference between SR and ATR is that in SR, no use of oxygen happens. The advantage of ATR is that the ratio of H2 to CO can be varied, which helps control the conversion of products. In the case of methanol synthesis, the process should operate with a very low H2/CO (thus very low steam to carbon ratio) in the feed to produce CO-rich syngas. 3 O ! CO + 2H2 O 2 2

(16)

1 O + CO2 ! H2 + 2CO + H2 O 2 2

(17)

CH4 + CH4 +

CH4 + O2 + 2H2 O ! 10H2 + 4CO

(18)

Syngas production from partial oxidation of methane is exothermic and ideal for the production of methanol because of its favorable H2/CO ratio. Furthermore, CO and H2 can be oxidized to CO2 and H2O, thus make the H2/CO Undesirable: CH4 + 0:5O2 ! CO + 2H2

(19)

Methanol production from syngas 117 Coal gasification is another route for producing syngas and raw material in countries where natural gas is not available. In this method with partial oxidation and steam treatment, syngas is produced. C+

1 O >CO 2 2

(20)

C + H2 O>CO + H2

(21)

CO + H2 O>CO2 + H2

(22)

CO2 + C>2CO

(23)

The design of the gasifier depends on factors including the type of coal (lignite, subbituminous, graphite, etc.) and its water, ash, and impurity content. Although the produced syngas has hydrogen deficiency, the WGS reaction can increase the hydrogen content. An innovative pathway can be generating the syngas from acid gases [35–37]. The overall reaction is. 2H2 S + CO2 >H2 + CO + S2 + H2 O

(24)

The composition of syngas is specified by the stoichiometry number S: S¼

½H2   ½CO2  ½CO + ½CO2 

(25)

S is the ratio of mole difference between hydrogen and CO2 to mole summation of CO2 and CO. For the synthesis of syngas, the stoichiometry number should be at least 2. Values above 2 indicate a hydrogen excess, whereas values below 2 show the deficiency of hydrogen. An ideal value for producing methanol for S is slightly above 2 because values below 2 (hydrogen deficiency) strongly reduce methanol selectivity, and excess hydrogen will increase the synthesis loop duo to hydrogen accumulation. 3.1.2 Methanol synthesis from syngas Methanol production from syngas can be generated from both CO and CO2. The reactions are [38,39]: CO + 2H2 >CH3 OH

ΔH 298K ¼ 90:77 kJ mol1

CO2 + 3H2 >CH3 OH + H2 O ΔH 298K ¼ 49:16 kJ mol1

(26) (27)

Both reactions are exothermic and have a reduction in mole and volume thus decreasing temperature and increasing pressure will benefit the reaction. Also, the reverse reaction of WGS (RWGS) happens that is an endothermic reaction: CO2 + H2 >CO + H2 O ΔH298K ¼ 41:21 kJ mol1

(28)

118 Chapter 5 The first productions of methanol were under high pressure. In 1923, Mittasch and Pier used the waste gas from ammonia synthesis and the same iron-based catalyst to produce methanol in 500°C and 100 bars. Due to impurities of the reactant gas and undesired by-products, the yield was very low [40]. After that, two catalysts, ZnO/Cr2O3 and ZnO/CuO, were presented for methanol synthesis, which reactions were carried out at 250–300bar (still high pressure) and 320–450°C (still high temperature). The difference was a slight reduction in pressure and temperature and also the reaction between CO and the catalyst was prevented. Currently, methanol production is in low-pressure conditions. According to the ICI method, the reactions are performed in 30–60 bar and 200–300°C. This procedure requires an active and highly selective catalyst like a copper-based catalyst (Cu/ZnO/Al2O3). Catalysts can have different compositions. Cu ranges between 20% and 80%, ZnO percentage is between 15% and 50%, and Al2O3 amount is from 4% to 30% [41]. Copper is vulnerable to poisoning, and if the concentration of sulfur and chlorine compounds exceeds a certain amount, the catalyst may poison. The allowable amount of sulfur in the stream is limited to 0.5 ppmv, whereas if the sulfur concentration goes above 0.8 wt%, the catalyst completely deactivates. Any form of chlorine content up to a 1 ppb limit is allowed. Chlorine strongly reacts with copper and deactivates the catalyst. Liquid water can also accelerate deactivation by speeding up the growths of copper crystals and destroying the catalyst matrix. The addition of ZnO helps reducing the susceptibility of catalyst to sulfur. MgO is also an additive that can stabilize catalyst activity, improve selectivity, and has a part in slowing down the thermal sintering. Therefore, purification of the synthesis gas before entering the reactor is required [12,42,43]. Additionally, the sintering of copper causes the crystal formation and therefore deactivation. To prevent sintering, an acceptable maximum operating temperature is 300°C. Following the tips together, the catalyst owns an average lifetime of 2–5 years [12,42]. We can categorize methanol production in low-pressure conditions into adiabatic reactors and isothermal reactors [8]. The main reactions for methanol synthesis are exothermic and cause a temperature increase. Due to the exothermic nature of these reactions, a temperature increase within the reactor may affect the conversion, production of unwanted byproducts, and recycling rate. Recycling the unreacted syngas can improve the methanol conversion to 80%–100% although one pass of reactant gas has about 25% conversion. Fig. 2 shows a simple methanol synthesis flow diagram in low-pressure conditions including an isothermal reactor: Adiabatic reactors

The reaction zone of adiabatic reactors does not include an external cooling system. In quench reactors, when multiple catalyst beds are in series in a single shell, the reaction is quenched by adding cooling gas at several points. In adiabatic reactors, syngas flows through

Methanol production from syngas 119

Fig. 2 Methanol synthesis flow diagram. (A) Reactor; (B) heat exchanger; (C) cooler; (D) separator; (E) recycle compressor; (F) fresh gas compressor [44].

several reactor beds with a series arrangement, and the heat generated from reactions will be removed within intermediate heat exchangers [39,45]. The problem of adiabatic reactors is the development of high temperatures within the reactor, which is not favorable for methanol production and may decrease the catalyst lifetime due to sintering. Adiabatic reactor with axial flow has a simple design in spite of a relatively large pressure drop and large vessel diameter that results in high material costs. Also, although reactors with the radial flow have lower pressure drops, their designs are more complicated and more expensive [11,12]. Isothermal reactors

In isothermal reactors, an attempt is to make the temperature remain constant at a favorable low level by cooling the reactor, catalyst, and syngas that are called quasi-isothermal reactors. Indirect cooling is carried out to prevent reactant gas diluting. The advantages of these reactors are the longer lifetime of catalysts, higher conversion of methanol synthesis due to optimum temperature kept, and energy recovery in the coolant. These types of reactors are divided into two most common categories: gas cooling (gas-cooled reactor (GCR)) and cooling through the water to steam generation (boiling water reactor (BWR)). The common construction of these reactors is tubular with cooling. When the catalyst is located in tubes, the surrounding boiling water is in the shell and removes the heat of generation with the aim of constant temperature. In this case, the synthesis gas has an axial flow through the tubes [12]. In some shell and tube reactors called variobar reactors, the cooling tubes are located in the catalyst packing on the shell side. In such reactors, the tubes are double walled, and the syngas

120 Chapter 5

a: Synthesis gas compressor b: Recycle gas compressor c: Trim Heater d: Trim Heater e: Combined converter system f: Final cooler g: Methanol separator h: Expansion vessel i: Light ends column j: Pure methanol pressure column k: Atmospheric methanol column

e

d

c

b

Syngas

Gas Reformed

Pure Methanol

Convertor

f

k Process Water to Saturator

Boiler Feed Water

a

MP Steam

g i Purge Gas

Hydrogen

Pressure Swing Adsorption

j h

Fuel Gas to Fire Heater

Fig. 3 Lurgi Mega process for methanol synthesis with energy saving distillation. Rearranged from E. Fiedler, G. Grossmann, D.B. Kersebohm, G. Weiss, C. Witte, Methanol, in: Ullmann’s Encyclopedia of Industrial Chemistry, 2000.

feed is first preheated by flowing through the inner tube and then indirectly flows along the catalyst. As the syngas moves within the catalyst, the reaction occurs, and the heat generated is transferred to cold feed gas and boiling water in the outer tube. In this case, the synthesis gas can flow either radially, axially, or axially radially through the catalyst bed [44]. Here in Fig. 3, you can see the Lurgi Mega methanol synthesis process. In this method, the reaction happened in two different conversion parts. The feed (syngas) is pressurized and then preheated to the required temperature for the first methanol synthesis. In this reactor, the catalyst is in tubes, and the reactor is an isothermal boiling water reactor. The reason for placing the catalyst and thus the reaction in the tube is that the reactant gas that enters this reactor is very active and ensures that most of the generated heat will be removed. The second conversion reactor is a gas-cooled reactor where the preconverted gas enters the shell side. The heat of the reaction removes continuously and boosts the methanol conversion. Also, heat generated is used for preheating the first reactor inlet gas (which is in the tubes). The outlet gas flow from the second reactor is cooled and then goes to a separator where the unreacted gas and crude methanol are separated. The unreacted gas is pressurized and recycled,

Methanol production from syngas 121 and a part of it is purged out to prevent inert accumulation. The crude methanol goes to the distillation section for more purification. More information and details about the methanol reactors are given in Section 3.5. 3.1.3 Methanol distillation The outlet gas from the reactor contains methanol, unreacted syngas, natural gas, water, inerts such as N2, and unwanted by-products. In order to divide methanol and water from gases, the flow is cooled down to near 40 °C. Unreacted gases are separated in the methanol separator. This separated gas includes unreacted syngas that is a reactor feed itself; therefore, the main part of the gas is recycled via recycle gas compressor to improve overall conversion. A small amount of the gas is purged to avoid accumulation of inserts in the recycle loop. Apart from water, condensed crude methanol includes impurities (byproducts) and dissolved gases. Thus, raw methanol should be purified to achieve the desired purity. The impurities can have either higher or lower boiling points than the methanol from which they are removed in the distillation section [10,12]. Methanol distillation is done in three steps. At first, dissolved gases are removed in an expansion vessel by flashing at a low pressure. Ethers, ketones, aldehydes, and formiates are light ends that are driven out simultaneously with the remaining dissolved gases in a prerun column. In this column, the components with lower boiling point than methanol are separated. Thus, these components are discharged from the top of the column and the flow consists of methanol which exits from the end (item i in Fig. 3 and b in Fig. 4) [12]. At the final step, the heavy ends (water, ethanol, and higher alcohols) in a pure methanol column are separated from the stabilized methanol. The overhead stream of the column is pure methanol. Depending on what Lurgi offers, this section can consist of either one or two columns. The investment cost saving generally consists of two columns, which is used for plain steam reforming or steam importing of column reboilers if possible (Fig. 4), and another energy saving design with three column distillation is used if large plant capacities are required (Fig. 3). In Fig. 3, the overhead stream of the pressure column is the heat source for the ambient pressure column and is condensed simultaneously. This coupling of energy reduces the heat demand almost 35%. The methanol purification is the same in these two methods [10,12].

3.2 Direct methanol synthesis from CO2 CO2 is the main source of carbon for methanol production in this method. CO2 is available in high concentrations alongside petroleum, natural gas, and geothermal hot water and can be directly drawn out from the atmosphere via air capture. The CO2 taken from different sources should be purified for further reaction. This method is useful for CO2 capture and utilization. Converting excess CO2 to a material such as methanol will benefit the storage and

122 Chapter 5

Fig. 4 Lurgi investment cost saving methanol distillation [12].

transportation of energy in a green way. Direct conversion of CO2 to methanol is exothermic (Eq. 27), but a traditional route represents a reverse WGS reaction (RWGS) (Eq. 28) and further methanol manufacturing of CO (Eq. 26) [46]. It can be seen that as the overall equation of direct methanol synthesis from CO2 is exothermic and there is a reduction in the number of moles, thus lower reaction temperature and higher pressure help methanol production. Higher reaction temperature is helpful for CO2 activation, and also under the reaction condition, there are some competing reactions that make many side products same as methane, formaldehyde, and formic acid. Vapor water and other side products may cause catalyst deactivation and reaction inhibition [47]. When CO2 and H2 are exclusively presented in the feed, the H2/CO2 ratio of 3 certifies an S value of 2. But if the H2/CO2 ratio is 4, a competitive methane reaction occurs [48]. CO2 is nonflammable, nontoxic, noncorrosive, inexpensive, and abundant in which natural gas-fired, flue gas from coal-fired and gas streams in several industries such as ammonia and aluminum production, cement factories, WGS, and coal gasification units, and fermentation plants are good resources for CO2. For direct conversion of CO2, H2 is also needed. The most common and commercial production of H2 is methane steam reforming, partial oxidation of light oil residues, and coal gasification.

Methanol production from syngas 123 Table 2 Thermochemical routes for H2 production [46]. Thermochemical route

Reactions 850°C

Sulfur-iodine process

H2SO4 ƒƒƒ! SO2 + H2O + 12O2 (29) SO2 + I2 + 2H2O ! 2HI + H2SO4 (30) 2HI ! H2 + I2 (31) Overall: H2O ! H2 + 12O2 (32)

Copper-chlorine process

2Cu + 2HCl ƒƒƒ! 2CuCl + H2 (33) 400°C 2CuCL2 + H2O ƒƒƒ! Cu2OCl2 + 2HCl (34)

450°C

500°C

2Cu2OCl2 ƒƒƒ! 4CuCl + O2 (35) 2CuCl ! CuCl2 + Cu (36) Overall: H2O ! H2 + 12O2 (37)

From a green chemical point of view, H2 can be produced from the electrolysis of water using renewable electrical energy (wind, biomass, solar, tides, waves, etc.). Other routes are still under research and practice. One of these routes refers to using sunlight energy to activate microorganisms that produce H2 biologically while another is applying a water-splitting process synergy by photo semiconductor catalysts. Also, another method for H2 production is the thermochemical route in which the energy for water splitting comes from atomic energy or solar energy. The sulfur-iodine process and copper-chlorine process are instance reactions for this method that is representative in Table 2 [13,49]. The carbon-oxygen bond in CO2 is very strong; thus, it requires high energy to break. Due to the high thermochemical stability of CO2 and the difficulty of its conversion, selecting a good catalyst is significant. Also, the hydrogenation of CO2 to methanol competes with the methanation reaction. Thus, a high selectivity is required in which the catalyst adsorbs and actives CO2 but breaks only one of the CdO bonds [50]. At first, the catalyst of this process was Cu/ZnO/Al2O3, but it was concluded that CO2 conversion needs a more selective and active catalyst. Many investigations are developing on metal-oxide and metal-carbide catalysts and their combination with other chemicals. It has obtained the catalysts with more than one metal and intermetallic chemicals such as Ni5Ga5 and GaPd2 and hybrid oxide catalysts such as MOx/TiO2 (M can be Ce, Mg, Co, or Ru) could have a high performance. Also, it was perceived that Pd has an essential role in controlling the direct reaction of CO2; thus, Pd supported on VeO2, Ga2O3, SiO2, and carbon nanotubes (CNTs) is a good offer for methanol synthesis. By considering the positive effect reported of Zr in CO2 hydrogenation, ZrO2 can replace Al2O3 in the future [47,48,50]. Also, for photochemical CO2 reduction, there are many nano photocatalytics in which the reaction may take place in gas and aqueous phase. Table 3 is a summary of these photo catalysts [51].

124 Chapter 5 Table 3 Categorization of various materials for photocatalytic CO2 reduction [51]. Type

Material

Advantages

Crystalline (oxide and nonoxide)

TiO2, ZnO, GaP, Ga2O3, Cu2O, WO3, Bi2WO4, BiVO4, MoS2, SnS2, WS2, etc.

• Electronic and absorp-

Cocatalyst (modified oxide and nonoxide)

Cu, Pt, Au, Ag-TiO2, etc.

tion behavior

• Improve photo absorption

• Increase charge carrier lifetime

Nanostructure and nanoporous support

Pt/TiO2, CdS/TiO2, Bi2S3/TiO2, n-ZnO, AgCl:Ag, AgBr:Ag, ZNGaNO, FeTiO3/TiO2, ZIF-8/Zn2GeO4, etc.

• Efficient electron transport

• High surface area • Active sites • Charge separation and transfer

• Increase photon absorption Carbonaceous hybrid materials

g-C3N4, GO, Cu-GOs, TiO2-rGO, Pt-rGO, SnS2/ C3N4, MoS2/TiO2/rGO, Cu-GO, TiO2/g-C3N4, etc.

• • • • •

High surface area High chemical stability High porosity High electron conductivity Low cost

3.3 Biomass as energy-feedstock for methanol synthesis Biomass is a good renewable source for generating energy and as a building block for synthesizing chemicals. Biomass is any substance produced by living creatures. In other words, biomass can be defined as the carbonous material produced by organism activity through the photosynthesis process. Not using biomass and its accumulation may cause socioeconomic problems. Methanol production from biomass is quite the same as using the origin method production from wood but more efficient. For using biomass as feedstock for methanol production there are four steps that consist of syngas production, syngas treatment and purification, methanol synthesis, and product separation, respectively. The main way for producing biogas and biofuel is biomass gasification through a thermochemical process at about 600–1000°C. 3.3.1 Biomass gasification The process of biomass gasification has four main steps including drying, pyrolysis, oxidation, and reduction, respectively. The biomass used for gasification should be dry and solid and the moisture usually should be less than 10%–15%. Thus, the first step in biomass gasification is

Methanol production from syngas 125 drying that removes the humidity in the temperature range 100–200°C. The higher the amount of moisture the higher the heat required for the process [52–54]. Reducing the biomass size leads to a higher reaction rate and lower residence time. For this reason, the biomass is crushed to chips less than 20 to 80mm. In the pyrolysis step, thermochemical decomposition of dry fuel happens in oxygen absence and the temperature range of 250–700°C. In this decomposition, biomass converts to gases, liquids, and solids as follows: Biomass ðdryÞ>H2 + CO + CO2 + H2 OðgÞ + light hydrocarbons ðgÞ + tar ðlÞ + char ðsÞ (38) During the oxidation step, the fuel can come from the converted biomass. This exothermic step supplies the required heat for other steps that are all endothermic. Mainly the products of the oxidation step are CO2, CO, and H2O. Also, nitrogen can be present in products due to the use of air for oxidation. In the reduction stage, the gaseous and solid products of the oxidation and pyrolysis step react together by Boudouard reaction, water-gas reforming, WGS, and methanation. These reactions occur in the oxygen absence and the temperature range of 800–1000°C. Eqs. 39–42 list the most common reactions: C + CO2 >2CO Boudouard reaction, endothermic C + H2 O>CO + H2 CO + H2 O>CO2 + H2 C + 2H2 >CH4

(39)

Water-gas reforming, endothermic

(40)

Water-gas shift reaction, exothermic

(41)

Methanation, exothermic

(42)

These reactions together result in an endothermic process. In addition, the composition of the products is determined by the temperature of the process. Increase in the formation of char and reduction in the conversion of tars take place in higher temperatures, and a lower temperature leads to an increase in the energy level of the syngas and decreases the risk of ash sintering in the process. The current gasification processes are planned for temperature ranges between 800°C and 1100°C; however, if pure oxygen is used instead of air for the oxidation step, temperature ranges between 500°C and 1600°C are also possible [55]. For the aim of increasing the carbon conversion using catalysts is a good approach. Selected catalyst ought to be inexpensive, effective, chemically and thermally stable, strong, persistent to coke formation, and regenerable [56]. In the thermochemical conversion of biomass, some preferable catalysts are dolomite, alkali and alkaline earth metal, and nickel-based catalysts. Dolomite is a cheap and abundant chemical and is useful to decrease tar production and CO2 adsorption, but it has problems regarding thermal stability. Alkali and alkaline earth metals are good catalysts for tar reduction, enhanced produced gas, and are favorable in resistance of coke deposition, but are expensive and have the problem of high ash production, agglomeration at high temperature, and are not regenerable. Nickel-based catalysts are

126 Chapter 5 common and commercial catalysts and are good in decreasing tar production, but their active life is short and is not regenerable either [57,58]. Bio syngas synthesis from biomass contains 5–6vol% methane, 35–40vol% hydrogen, 16–18 vol% carbon dioxide, 18–21 vol% carbon monoxide, and 17–21 vol% nitrogen [59]. The produced syngas has a far S ratio amount from the demand ratio due to the elemental composition of biomass. In this bio syngas, there is a deficiency in hydrogen. Hydrogen is only 6%–7% and carbon is about 47%–49% in dry, ash-free biomass. Adding pure H2 and applying WGS reactions are two strategies for regulating the S ratio. Adding pure hydrogen will have a high CO2 emission due to traditional ways of producing hydrogen based on fossil sources. More expensively, from renewable electricity sources hydrogen can produce via water electrolysis with a low carbon footprint. In the second strategy, the syngas goes to a WGS converter. In the WGS reaction, CO and steam convert to H2. The catalyst at high temperature is iron-chrome oxide and at low temperature is copper-zinc oxide. This reaction achieves a considerable amount of CO2. To adjust the carbon dioxide concentration for methanol synthesis, alkanol amines are used as chemical absorbents. If the partial pressure of CO2 is high in the bio syngas stream, applying a series of physical and chemical absorption columns is more noteworthy [59–61]. Frequently, the favor and desire syngas, convert to methanol as said in Eqs. (26)–(28). The product stream should be purified to produce pure methanol due to including impurities and unwanted by-products. The amount of the above impurities depends on reaction condition, quality of feed gas, catalyst type, and its lifetime. At first, the unconverted syngas is divided from the flow and recycled to the methanol production reactor. To prevent inert gases accumulation, a part of the recycled stream will be purged. Further, methanol and water separate in a distillation column [39,60]. 3.3.2 Biological process Biogas can be manufactured from biomass in anaerobic biological digestion. In this process, microorganisms grow and produce biogas by metabolizing. This produced biogas contains 50%–60% methane, 40%–50% carbon dioxide, and further gases such as ammonia, hydrogen, and hydrogen sulfide and could be used in different applications specially in methanol production. Syngas can be produced by steam reforming due to the high amount of methane in biogas. Before any change, the biogas should be clean and purified. The biogas quality depends on the digestion process, operating condition, type of organic waste, and the existence of toxic compounds [60]. Two common digestion methods for biomass converting are mesophilic and thermophilic. Mesophilic is the most common process for anaerobic digestion and occurrence in atemperature range between 30°C and 40°C and a time period of 15–40 days. In thermophilic digestion, the temperature is maintained at 55°C and a retention time of 12–14 days. Thermophilic

Methanol production from syngas 127 digestion leads to higher biogas and lower pathogen and virus production and is more efficient but due to high temperature controlling is more expensive and complex [60]. Commonly hydrolysis, acidogenesis, acetogenesis, and methanogenesis are the four main steps in anaerobic digestion. The hydrolysis stage is the first step and provides chemical bonds cleavage by water addition. This stage is relatively slow and can limit the digestion rate especially when substrates are solid. The related reaction is [62]: ðC6 H10 O5 Þn + nH2 O ! nC6 H12 O6 + nH2

(43)

Eq. (43) is the formation of glucose from hydrolysis of cellulose via the addition of water. Homogeneous and heterogeneous acids can catalyze the reaction. In acidogenesis, cellulose converts to fatty acid, carbon dioxide, and hydrogen. This stage is a fermentation stage and the process happens by the bacteria well known as acidogenic bacteria (fermentative microorganisms) and the important acid in this step is acetic acid. Eqs. (44)–(46) summarizes these reactions [63]: C6 H12 O6 >2CH3 CH2 OH + 2CO2

(44)

C6 H12 O6 + 2H2 >2CH3 CH2 COOH + 2H2 O

(45)

C6 H12 O6 ! 3CH3 COOH

(46)

Acetogenesis stage approximately provides the most of acetic acids that is a major key in digestion and is consumed in the methanogenesis step to produce methane. Also, volatile fatty acids from the previous stage are broken down to acetic acid and hydrogen [64]. These reactions are as follows: CH3 CH2 COO + 3H2 O>CH3 COO + H+ HCO3  + 3H2

(47)

C6 H12 O6 + 2H2 O>2CH3 COOH + 2CO2 + 4H2

(48)

CH3 CH2 OH + H2 O>CH3 COO + 2H2 + H+

(49)

Methanogenesis is the fourth and final step of the anaerobic digestion process. This process occurs by methanogen bacteria that are very sensitive to only small values of oxygen. Methanogens have an important role in digestion due to their slow growth and being extremely vulnerable to changes in conditions [62]. CH3 COOH ! CH4 + CO2

(50)

CO2 + 4H2 ! CH4 + 2H2 O

(51)

2CH3 CH2 OH + CO2 ! CH4 + 2CH3 COOH

(52)

Also, produced biomethane could convert to biomethanol by methanotrophs. These microorganisms belong to the proteobacteria family subset category of prokaryotes that are

128 Chapter 5 found in soils, lakes, wetlands, and water. These bacteria convert methane to methanol via oxygen and an enzyme known as methane monooxygenase (MMO). Methanotrophic bacteria are classified into two categories of α-proteobacteria and γ-proteobacteria. The methanotrophs belonging to α-proteobacteria are put into four classes: Methylocystis, Methylosinus, Methylocapsa, and Methylocella. The methanotrophs in γ-proteobacteria are more manifold and have 12 genera, including Clonothrix, Crenothrix, Methylobacter, Methylocaldum, Methylomicrobium, Methylomonas, Methylohalobius, Methylosarcina, Methylosphaera, Methylosoma, Methylococcus, and Methylothermus [65]. Biogas has impurities such as tar, char, dust, ash, and alkali compounds that can further poison or deactivate the catalyst. Biogas streams can be purified by filtration, condensation, scrubbing, electrostatic precipitation, adsorption, and absorption processes. Tar can be completely removed from biogas by using electrostatic precipitation and ash also can be separated with cyclones, bags, and electrostatic precipitation filters. Additionally, unwanted by-products such as NH3 and acid gases like H2S and HCl can affect the downstream processes. They can be removed by continuous wet scrubbers pursued by chemical adsorption on adsorbent beds and absorption via chemical solvents [60,66].

3.4 Methanol production from a feedstock other than syngas About a century ago, Christiansen expressed the methanol synthesis from methyl formate. In this two-step route, the first reaction is methanol carbonylation and the latter forms two molecules of methanol. These reactions need potassium or sodium methoxide and copper-based catalysts, respectively. These reactions are [67]: CH3 OH + CO>HCOOCH3

(53)

HCOOCH3 + 2H2 >2CH3 OH

(54)

CO + 2H2 >2CH3 OH : overall

(55)

If there is water and carbon dioxide in the syngas stream, it would react with sodium methoxide and make impurities. Thus, extra separation is needed. Methanol can be directly produced from methane oxidation without the requirement of syngas. The reaction path is CH4 +

1 O ! CH3 OH 2 2

(56)

CH4 + O2 ! CH2 O + H2 O

(57)

CH4 + 1:5O2 ! CO + 2H2 O

(58)

CH4 + 2O2 ! CO2 + 2H2 O

(59)

The main disadvantage of this route is very active side products. Methane oxidation can occur either in a homogeneous gas phase or in a heterogeneous catalytic setup. In the

Methanol production from syngas 129 homogeneous gas phase, methane oxidation takes place at high pressure (30200atm) and in the temperature range 200–500°C. It observed that at optimal conditions at 450°C, 65atm, and 5% O2 content the conversion is 8%–10%. Other studies suggested a lower pressure (about 3–60 atm), in which the conversion is reduced to 5%–10%. since at high pressure, radical reactions occur and are dominant and limit the catalyst activity [11,68]. For improving the selectivity, a lower temperature is beneficial (less than 250°C). The problem is that current catalysts are not active in this range of temperature and new investigations are required for a desirable catalyst. To solve this problem, liquid phase reactions are suggested that can operate at moderate temperatures. These reactions need a high acidity chemical named super acids, whereas its acidic strength is a million times more than concentrated sulfuric acid. In the presence of superacids, methane can react with H2O2 and produce high selective methanol in the liquid phase and ambient temperature. In this reaction, a hydrogen peroxide (H3O+2 ) molecule that is protonated is placed into the CdH bond of methane. Due to this acidity condition and methanol protonation (CH3OH+2 ), methanol oxidation is prevented. According to its benefits, H2O2 cannot be used in large-scale manufacturing and superacids are very costly. For that reason, looking for more economic and applicable peroxides and superacids is still desired [1]. A two-step process with methyl-halide intermediates can also produce methanol. In methyl-halide, the halogen acts as a catalyst and does not consume in the reaction. Methyl-chloride and methyl-bromide are produced catalytically from methane and are hydrolyzed to synthesize methanol and HBr or HCl. HBr or HCl could also be reoxidized as follows [69]: X2

H2 O

CH4 ƒƒ! HX + CH3 X ƒƒ! CH3 OH + 2HX 1 2O2

2HX ƒƒ! X2 + H2 O

(60) (61)

X ¼ Cl, Br From an economic point of view, it is important to recycle hydrogen halides from Eq. 60. Hydrogen bromide oxidizes readily to bromine by air, but reoxidation of hydrogen chloride requires a difficult technology. A summary of the current available technologies for methanol production and technologies that are still under development is shown in Fig. 5:

3.5 Existing technologies One of the most important industrial equipment is the reactor. Since methanol synthesis consists of exothermic reactions, providing an appropriate temperature control structure has a great deal of importance. The produced heat should be eliminated and used in another process for process economical optimization. Another factor that affects operational cost is downstream separation cost that can be reduced by increasing the per pass conversion [8]. To sum up, the

130 Chapter 5 biological digestion

Biomass

gasification

• Natural gas • Coal • Petroleum

reforming

Biogas (Bio methane)

methanotrophic

Applications: • Chemical precursor • Additive, etc.

reforming

Syngas

Methanol

oxidation

Methane

reduction

Carbon dioxide

Fig. 5 Summary of the current available technologies for producing methanol [12].

reactor design technologies are divided into two major groups, gas- and liquid-phase technologies. 3.5.1 Gas-phase technologies There are two types of gas-phase reactors, adiabatic or isothermal. The adiabatic unit has a multibed quench design with direct cooling or multibed design with heat exchange between stages (indirect cooling). In Fig. 6, the visual illustration for methanol reactors is shown. Adiabatic reactors

Multiple adiabatic beds (up to five) that are placed in series inside a pressurized shell form a quench converter (Fig. 7). The most usual type of low-pressure quench convertor in the industry is The ICI (Johnson Matthey) that works at 270°C and 50–100bar. The catalyst is loaded and supported in a single bed and is a combination of Cu/ZnO/Al2O3. To reduce the reactor temperature, a cooling gas is distributed in beds using lozenges. The fresh and recycled syngas maintains the reactor temperature and optimizes the reactor design. The cold gas is delivered to lozenges by a central pipe that runs horizontal pattern across the converter from side to side [70]. The production capacity of this simple and reliable design is up to 3000 t/day. Additionally, the produced heat is recovered and used in other parts. The different void fractions along the reactor bed make the flux distribution irregular, then each catalyst pellet receives different gas flows. As a result, hot and cold zones (catalyst deactivation or by-products formation and low reaction rate, respectively) might exist in the reactor [11].

Methanol production from syngas 131

Fig. 6 (A) Adiabatic reactor with direct cooling; (B) adiabatic reactor with indirect heat exchange; (C) reactor with external cooling [11].

Fig. 7 Quench reactor representing quench gas inlet [11].

132 Chapter 5

Fig. 8 Spherical reactor [11].

The tubular reactor design makes the vessel manufacturing and catalyst loading and unloading easier than other designs. In contrast, some of the disadvantages are the complexity of reactor performance tuning, less effective heat recovery, and lower conversion. To compete with these drawbacks, separating each adiabatic reactor in a single shell can increase the capacity up to 10,000 t/day [11]. Kellogg, Brown, and Root come up with a new reactor design that consists of a series of fixed beds, adiabatic reactors. Their designed reactor is spherical, and catalysts are placed between inner and outer spherical shells (Fig. 8). The lower wall thickness of the reactor (half of the conventional reactors) reduces costs of manufacturing, transportation, and installation [71]. The flow direction in these reactors is from the exterior wall, then to the catalyst layer, and finally the inner spherical shell. This radial flow reduces the pressure drops and consequently investment and operational costs. One major issue is the reduction of catalyst beds during start-up. As mentioned in the previous design, the catalyst voids may lead to an irregular flow regime and should be avoided. To remove this limitation, 10vol% of the total catalyst will be added after the start-up that loads the empty areas. For removing the reaction heat, some intercoolers were considered. In addition, the fresh syngas enters the first reactor that enhances

Methanol production from syngas 133

Fig. 9 ARC technology (courtesy of Casale SA) [11].

reaction rate and reduces necessary catalyst volume in comparison with other quench-type units [11]. The heat exchangers and gas distributors are simple. Although Haldor Topsoe and Krupp Uhde suggested multivessel adiabatic reactor systems, the Kellogg, Brown, and Root design assures higher yields and a smaller recycle stream with respect to others [11]. Casale’s solution was a quench-cooled converter named ARC (Advanced Reactor Concept) (Fig. 9). This design was in collaboration with ICI and since it was used for rebuilding several old plants which use quench-lozenge technology, its reliability and efficiency were tested and increased by 20%. Moreover, the effectiveness of quench gas redistribution is more than the

134 Chapter 5 Catalyst Loading Gas Inlet

Catalyst Centeral Pipe Cooling Tube

Steam Outlet

Boiler water inlet Gas Outlet & Catalyst Unloading

Fig. 10 Toyo MRF-Z reactor [71].

others. Unlike the adiabatic bed reactors, in this design, some distribution plates separate the bed into different parts. The next design was proposed by Haldor Topsoe and was called Collect-Mix-Distribute reactor (CMD). The catalyst beds, same as Halliburton reactors, are separated through vertical support beams. The fresh feed enters from the bottom of the reactor and is mixed with the cold quench gas after passing through the first catalyst bed and then flows to the next bed. This design enhances the reagent’s conversion and controls to temperature to increase the life of the catalyst [11]. The MRF-Z reactor is a multistage radial flow reactor with intermediate cooling that is designed by the Toyo Engineering Corporation (TEC) (Fig. 10). Blade boiler tubes are considered to remove heat, and the catalyst is loaded in concentric beds with a reasonably simple replacement method. By keeping the temperature near to the maximum allowed value, the per pass conversion becomes optimum and in comparison with quench converters with the same production capacity, the volume of required catalyst beds is reduced by almost 30%. The produced heat by this high temperature can be used to generate steam to synthesize the syngas feedstock. This method reduces about 50% of the required steam for syngas feedstock production and increases the catalyst life by providing mild temperature conditions. One of

Methanol production from syngas 135 Steam out

Gas inlet

Circulating water

Gas outlet

Fig. 11 Linde isothermal reactor [11].

these reactors has a production capacity of about 5000–6000 t/day. The radial flow configuration in these reactors compares with the axial flow, decreasing the pressure drop across the catalyst bed by almost 90%. The lower pressure drop makes the recompression of unreacted syngas more economical and also makes the scale-up (increase height in constant diameter) easier. For instance, in radial flow, the pressure drop across the bed is independent of reactor height [71]. Isothermal reactors

The first type of isothermal reactor is called Linde, which has helically tubes located in the catalyst bed that makes the heat exchange indirect (Fig. 11). This design is appropriate for exothermic, endothermic, liquid/liquid, gas/liquid, and gas/gas catalytic reactions. Inserting the heating or cooling tubes in this design maximizes the reaction rate and provides an optimum temperature profile. Therefore, this type of reactor has the following advantage: higher productivity and catalyst lifetime, lower by-products, efficient heat recovery, and less operating costs [71]. The helical design for the tubes lessens the chance of stress generation because of axial temperature differences. This problem usually exists in straight fixed-bed reactors.

136 Chapter 5 Saturated Steam

Gas-Cooled Reactor (second reactor)

Boiler Feed Water

Preheated Syngas

Gas inlet

Catalyst Catalyst

Product Gas Outlet

Caralyst Discharge

Water-Cooled Reactor (first reactor)

Start-up steam

Caralyst Discharge

Syngas Inlet

Fig. 12 Lurgi combined methanol converters [71].

Furthermore, by placing the catalyst outside the tubes, the heat transfer on the catalyst section will become significantly efficient, and the required reaction volume will be reduced. As a result, this design reduces the necessary cooling area and material costs and can guarantee up to 4000 t/day production capacity. One of the most common reactors mainly used in large-scale plants is the Lurgi Converter (now known as a company of Air Liquide). A reactor with a design of fixed bed and shell and tube, is also used by this company in the Fischer-Tropsch (FT) reaction. On the shell side, there is cooling water, and the catalyst is placed in the reactor tubes. The reasons for choosing this configuration are tight temperature control and high-pressure steam production. The steam pressure is the manipulating variable for controlling the reaction temperature. The typical operating pressure is 50–100 bar, and the reaction temperature should be around 230–265°C. The required steam pressure is about 40–50bar and can be compressed and reused or sent to the distillation unit. The plant production rate is about 1200–1400t/day, which results from higher yield and lower recycle ratios compared to the other methods [66,72,73]. Lurgi came up with another design: a two-stage converter with a higher production capacity (Fig. 12). This new design is the association of two Lurgi methanol converters. The first stage

Methanol production from syngas 137

Second pathway (Reaction) Steam Outlet

Outer Tube Catalyst Water Inlet Gas Out Diaphragm

Feed Gas Inlet

Flexible Hose

Fig. 13 Mitsubishi superconverter [1].

works at higher temperature and space velocity than a single-stage converter, so it partially converts syngas to methanol and is smaller. The higher temperature results in higher steam pressure. The feed of the second reactor is the discharged gas of the first reactor, which flows in the shell side, and cold fresh gas enters the tubes with countercurrent direction. This design reduces production costs by preserving the driving force and heat recovery. If the production capacity is lower than 3000t/day, the two stages could be combined in a single vessel, but two separate converters are more efficient for a larger capacity [71]. The next isothermal reactor is designed by two companies, Mitsubishi Gas Chemical (MGC) and Mitsubishi Heavy Industry (MHI), and named MGC/MHI Superconverter [74]. This reactor has tubes with two walls, and the space between these walls is filled with a catalyst (Fig. 13). The gas-phase feed enters from the bottom of the vessel by flexible tubes and then flows in the walls of the tubes. When the gas reaches the end of the tube, it turns downward and enters the annulate space in which the catalyst is loaded. Outside the tubes cooling water flows and removes the heat of reactions. Finally, the product, methanol, and unconverted syngas exit from the bottom of the vessel. From the inlet to the outlet section, the temperature in the catalyst bed decreases due to the exchange of heat with fresh gas. This method provides a conversion per pass of almost 14% because it maximizes the reaction rate. Furthermore, mechanical stability and safe operation are the other advantages of this design [44].

138 Chapter 5 Gas-phase fluidized bed converter

The cooperation of two companies, the Petroleum Endowment Center (PEC) and New Energy and Industrial Technology Development Organization (NEDO), in 1993 led to the design of large-scale facilities for producing methanol. This project aimed to reduce production costs by using novel technologies. The converter is a fluidized bed with 50–60 μm catalyst particles that is fluidized by fresh gas feeded from the bottom. The cooling pipes are responsible for temperature control and allow the heat of reaction recovery at high pressure. One of the most crucial factors in this design is the possibility of catalyst replacement during the operation for stable and constant operating conditions [11].

3.5.2 Liquid phase technologies As a result of efficient heat removal and tight temperature control, the liquid phase methanol production process is the future of the methanol industry. The industrial methanol production methods have used three-phase (gas-solid-liquid) systems so far. More specifically, when the synthesis gas is coal-derived, the low H/C ratio can cause cocking, making it impossible to use gas-phase fixed-bed reactors (GPFBR). Slurry converters and trickle bed reactors (TBRs) are the solutions to this limitation [75]. TBRs have advantages of both fixed bed reactors and slurry, and have higher efficiency. To clarify, in a TBR, the catalyst is loaded in a fixed bed, and a cocurrent gas-liquid stream flows through the bed. Compared with the fixed bed reactors, this type provides better heat removal, the possibility of using the coal-derived feed, and higher gas space velocity. In TBR and slurry reactors, the absorption of methanol in the liquid phase and its elimination from the reaction system cause higher conversion due to the mass action law. This characterization reduces recycling in comparison with conventional fixed bed reactors. The recycle ratio in the gas phase process is about 5:1, while the liquid phase reduces to 1:1 or 2:1 [11,75]. Two essential issues of trickle bed and slurry reactors are modeling and scale-up problems, which prevent industrial use. Even scale-up from lab-scale is impossible, so these technologies have not been progressed and are still under research. LPMEOH (slurry liquid phase reactor developed by Air Products) is one of the examples of this type (Fig. 14). The suspension of catalyst particles in an inert mineral oil allows faster mass transfer, better temperature control, lower operational, and capital costs compared to older versions. The catalyst replacement is straightforward, and mass and heat transfer are very efficient. The history of this technology goes back to 1997 and Eastman Chemical’s coal gasification complex (Kingsport, Tennessee, USA), a commercial methanol facility with 300,000 lit/day production capacity. An internal heat exchanger has the duty of taking the heat off the mineral oil for temperature control. The conversion rate is higher, and the recycling is

Methanol production from syngas 139 Gas & vapor outlet

Disengagement Zone

Steam

Catalyst powder slurred in oil

Boiler feed water

Gas Inlet

Fig. 14 LPMEOH slurry reactor (Air Products) [11].

significantly reduced. Moreover, this process can operate both continuously and discontinuously [76]. Another process was developed by Brookhaven National Laboratory (BNL) in N.Y. (USA) for methanol production by the pathway of methyl formate reaction at lower pressure (CH3 OCH3 + H2 O

(62)

In the presence of bifunctional catalysts like CuO/ZnO/Al2O3 for methanol production and γ-Al2O3 for the dehydration of methanol, the above reaction will occur, and methanol production and WGS reaction take place simultaneously. There is a thermodynamic equilibrium limitation for the above reaction, but this effect is not a barrier as it was for the methanol synthesis reaction. Since the DME synthesis reaction is exothermic, the lower temperature is more favorable. Moreover, the total number of moles is constant, so the pressure change does not affect the extent of the reaction.

4. Conclusion Methanol synthesis from many sources and feedstocks with many possible routes have been briefly reviewed. According to the carbon, hydrogen, and energy storage of methanol, the demand amount of this chemical is increasing. Nowadays, most of the methanol is produced from syngas. Syngas can be generated from different sources including natural gas, coal, and biomass. From an economic point of view, the optimal way to produce methanol in high quantities is by converting the syngas that is produced from natural gas to methanol. In which this process is more common, efficient and the catalysts are cheaper and easier to dispose of. Due to global warming and the increase in CO2 emission, efforts for avoiding CO2 accumulation are charming. Thus, biomass can be used as a feedstock in green technology, and by producing bio-methanol and its applications, the green CO2 cycle can be closed. By using biomass as the source, the gasification process is more desirable. Currently, the emission of greenhouse gas and environmental pollutants is a significant concern. Therefore, producing energy from a renewable feedstock such as biomass is a good approach to follow.

Abbreviations and symbols ARC ATR BASF

advanced reactor concept autothermal reforming Badische Anilin und Soda Fabrik

142 Chapter 5 BNL BWR CMD CNT DME FT GCR GPFBR ICE ICI LPMEOH MD MGC MHI MMA MMO MSR MTBE MtO MtP NEDO PEC PEM PO POM RWGS S SR TBR TEC WGS

Brookhaven national laboratory boiling water reactor collect-mix-distribute carbon nanotube dimethyl ether Fischer Tropsch gas-cooled reactor gas-phase fixed-bed reactor internal combustion engines imperial chemical industries liquid phase methanol process methanol decomposition Mitsubishi gas chemical Mitsubishi heavy industry methyl methacrylate methane monooxygenase methanol steam reforming methyl tert-butyl ether methanol to olefin methanol to paraffin new energy and industrial technology development organization petroleum endowment center proton exchange membrane partial oxidation partial oxidation of methanol reverse water gas shift stoichiometry number steam reforming trickle bed reactor Toyo engineering corporation water gas shift

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CHAPTER 6

Ethanol production from syngas David M. Kennes-Veigaa, Carla Ferna´ndez-Blancob, Marı´a C. Veigab, and Christian Kennesb a

CRETUS, Department of Chemical Engineering, University of Santiago de Compostela, Santiago de Compostela, Spain bChemical Engineering Laboratory, Faculty of Sciences and Centre for Advanced Scientific Research (CICA), University of La Corun˜a, La Corun˜a, Spain

1. Introduction Many countries are currently implementing initiatives to reduce environmental pollution and to ensure the transition of societies to a circular economy. The European Union (EU) recently adopted a new circular economy action plan, updating its circular economy actions implemented since 2015. The new action aims at valorizing waste and other pollutants and at achieving the EU’s 2050 climate neutrality target [22]. Still, the annual waste generation, in the EU, is projected to increase by 70% by 2050 [38], and the overall consumption of biomass, fossil fuels, and other similar materials is expected to double between 2020 and 2060 [58]. In order to reduce pollution and ensure a more sustainable economic development, it is necessary to look for environmentally friendly production processes, and search for alternative energy sources and transport biofuels, generating less waste and less emissions. In that respect, bioethanol, as an alternative fuel and chemical, can contribute to the circular economy of zero waste discharges, as it is more eco-friendly than fossil fuels. One of the main applications of ethanol, or ethyl alcohol, is indeed as a fuel, mainly mixed with gasoline or in replacement of gasoline, but it can also be used as a solvent or for other purposes. According to current EU regulations, in terms of biofuel use, a maximum of 10% (vol:vol) ethanol can be mixed with gasoline, which does not require any modification of spark-ignition engines under such circumstances. Ethanol is also used in the manufacture of drugs, plastics, lacquers, polishes, plasticizers, and it is a suitable starting compound to prepare some chemicals, such as ethyl ether or ethyl amines, among others. It has also applications in medicine and drugs. Additionally, products such as varnishes, cosmetics and perfumes do also often contain ethanol [73]. In terms of toxicological aspects, high ethanol emissions or ingestion of large amounts are noxious to humans. It may result in coma and even be lethal above a given threshold concentration in blood; typically above 0.4% (wt:vol) or 400 mg/dL-blood in adults [78]. Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91878-7.00013-7 Copyright # 2023 Elsevier Inc. All rights reserved.

147

148 Chapter 6 Syngas or synthesis gas is a gas mixture generally rich in carbon monoxide and hydrogen, and eventually other gases such as carbon dioxide, nitrogen, methane, or other minor compounds, depending on the gasifier and the gasification conditions, e.g., temperature, oxygen level, and gas residence time. Gasification is the thermal treatment of carbonaceous materials, of many different possible natures such as biomass, waste, wood, or coal, among others. It does generally take place at temperatures in the range of about 600–900°C. The presence of an oxidant, at concentrations below the amount required for the stoichiometric combustion, results in the partial oxidation of the original carbonaceous material. Air, pure oxygen, or air-oxygen mixtures are commonly used to ensure the oxidation process. Additionally, steam is also another suitable gasification agent. Using air will lead to the presence of significant amounts of nitrogen in the syngas mixture, typically 50% (vol) or more, compared to gasification with pure oxygen. Besides syngas, a solid by-product containing char and ashes will also be formed. As will be addressed later in this chapter, the optimal chemical conversion of syngas to desired products needs to look for specific CO:H2 ratios, while this ratio is less important for optimal bioconversion processes. Depending on the chemical reaction of interest, different gas mixtures will be required. The desired ratios may be obtained through adequate selection of the gasifier and fine-tuning of the gasification conditions.

2. Characteristics of ethanol vs gasoline and fossil fuels As a fuel, ethanol offers some advantages over gasoline, though it has also some drawbacks. Ethanol is a pure compound, made of C, H, and O atoms, and it burns cleaner than gasoline or gasoline-ethanol blends, which are of more complex composition. Gasoline is composed of linear and branched alkanes, cycloalkanes, alkenes, cycloalkenes, olefins, and aromatic compounds (e.g., benzene derivatives), with typical average hydrocarbon chain length ranging from C4 to C12 [57]. Some minor compounds found in crude oil, e.g., sulfur-related ones, may also end up in the final fuel, though measures are generally adopted to remove them, which is also often mandatory in many environmental regulations. In that sense, the sulfur content in most fuels is generally limited to 10ppm in the European Union as well as in the United States. Besides compounds typical of crude oil, gasoline contains also some additives and blending agents, e.g., octane enhancers. Ethanol, classified as an oxygenate compound, has a higher oxygen content than gasoline, and it allows for a cleaner combustion, with less air needed for its combustion than for gasoline. Apart from gasoline, assays have also been done on blending ethanol with diesel. However, the cetane number of ethanol (50) [48]. In terms of environmental benefits, ethanol in the form of bioethanol is produced from more environmentally friendly feedstocks and through processes generally complying better with the requirements of a circular economy. It can be produced from syngas, obtained from the gasification of biomass, waste, or other similar sources, rather than from oil-related raw

Ethanol production from syngas 149 Table 1 Characteristics of gasoline and ethanol. Parameter

Gasoline

Ethanol

Starting raw material Formula Molecular weight (g/mol) C:H:O mass ratio Octane number Cetane number Energy content (MJ/L)

Crude oil C4–C12 (e.g., C8H16) 100
20 86:14:0 95 – 33.5

Syngasa C2H5OH 46.07 52:13:35 108 5–10b 21.3

a

As the main raw material considered in this chapter. Quite low compared to the cetane number required for diesel.

b

materials. When trying to compare gasoline and ethanol or ethanol-gasoline blends in terms of their respective emissions, some apparently contradictory information may be found in the scientific literature. However, this can generally largely be attributed to the different conditions applied by different researchers to estimate such emissions. In that sense, NOx emissions released to the atmosphere have been claimed to be lower with ethanol than with gasoline in some studies, while other reports conclude the opposite. This apparent inconsistency can be due, among others, to the fact that higher emissions will occur if the combustion process is closer to stoichiometric with a correspondingly increased flame temperature [81]. In that respect, forced induction will increase NOx concentrations, especially at high engine loads with increases in compression ratios. Other parameters may also play a role on emissions of that air pollutant. On the other side, there is rather common agreement on decreases of CO emissions when using ethanol or its blends [20], which can find its explanation in the lower amount carbon in the alcohol compared to the fossil fuel (Table 1). The higher oxygen content also favors complete oxidation to CO2, rather than partial combustion to CO. Therefore, significant CO2 reduction with ethanol, compared to gasoline, is generally not observed; and, with both fuels, the values are thus generally rather similar, though this will again also depend on the test conditions. In terms of hydrocarbons, there is good agreement between most authors who report reduced emissions when using ethanol because of its higher oxygen content and improved combustion [17,35]. Some additional characteristics of ethanol, compared to gasoline, are provided in Table 1. The energy content of ethanol (i.e., MJoules/liter) is somewhat lower than in gasoline. However, ethanol and ethanol-gasoline blends have higher octane numbers than pure gasoline, which increases the engine’s combustion performance and efficiency, as it reduces its risk of knocking [8]. On the other side, ethanol is more corrosive in conventional engines, which may be a drawback compared to gasoline. In any case, the gradual depletion of fossil fuels requires searching for alternative fuels that can be obtained from renewable sources, waste, or even greenhouse gases, among others, and, in that sense, ethanol is a suitable candidate for this.

150 Chapter 6 The following sections provide an overview of the major bioprocesses and catalytic processes available, at lab-scale, pilot-scale, or commercial scale, for the production of bioethanol. Some of those processes are being used at commercial scale, for several years now, while others are still at the level of laboratory research.

3. Catalytic conversion of syngas to ethanol Methanol is among the most common alcohols produced catalytically from syngas, which is often obtained from natural gas or otherwise other carbon-rich gasifiable feedstocks, e.g., coal and biomass, while thermocatalytic ethanol production from syngas is rather less developed. Nonbiological ethanol production is typically achieved through ethene hydration at temperatures of around 300°C and pressures close to 60–70 atm, according to the following overall reaction, in the presence of steam: CH2 ¼ CH2 + H2 O! C2 H5 OH

(1)

Another option consists in using syngas. Nowadays, mainly two ways are being considered for the catalytic production of ethanol from syngas, namely the direct conversion of syngas into ethanol and the indirect conversion via intermediates such as methanol and dimethylether. Both alternatives are illustrated in Fig. 1 and described in the next sections. The figure also shows the biological route, which will be described later as well.

3.1 Direct catalytic conversion of syngas to ethanol Similarly, as for ethene conversion to ethanol, the syngas-based reactions require high temperatures and pressures and generally expensive catalysts. Direct CO hydrogenation to ethanol, with a single catalyst, can be summarized in the following reaction, taking place at typical temperatures of 300–500°C and pressures of 10–50 bar:

Fig. 1 Overview of direct and indirect catalytic ethanol production, as well as its biological production.

Ethanol production from syngas 151 2CO + 4H2 ! C2 H5 OH + H2 O

(2)

A similar reaction would be observed with CO2 as starting reactant: 2CO2 + 6H2 ! C2 H5 OH + 3H2 O

(3)

It is an energy-intensive process, with low selectivity, that suffers from multiple reactions, though research is currently ongoing aiming at improving those aspects [37]. Several different products, including methane, higher hydrocarbons, and oxygenates, may be formed from CO and/or CO2 hydrogenation; while trying to reach higher reaction selectivities will often result in lower conversions [72]. This poor selectivity will also lead to cost-intensive downstream operations of separation. Methanation includes several side reactions (Reactions 4 and 5) that should be minimized in this process, as they are competing with the ethanol production technology from syngas. CO + 3H2 ! CH4 + H2 O

(4)

CO2 + 4H2 ! CH4 + 2H2 O

(5)

This direct syngas to ethanol production process is also characterized by rather low yields. Although selectivities close to 40% ethanol can occasionally be reached, this will frequently lead to very low conversions of hardly 5%–10% in most cases. Besides, though some catalysts such as the rhodium-based ones exhibit higher selectivity for C2 oxygenates, their cost may be prohibitive at large scale and may not be suitable for industrial applications, compared to other, cheaper, catalysts, such as copper-based ones. Though research and optimization of direct syngas conversion to higher alcohols is somewhat more advanced than for ethanol, it does usually not exceed TRL6 (Technology Readiness Level). Despite several decades of intensive research, the direct syngas-to-ethanol technology is still mainly being developed and optimized at laboratory scale [80]. The larger development for higher alcohols (e.g., C3, C4) results, among others, from their higher selling prices. Among the issues to be solved, one major research target in this field is still focusing on finding catalysts with higher activity, stability and selectivity. In terms of temperature and pressure, CO and CO2 hydrogenation are exothermic reactions. Lower temperatures would favor the production of ethanol, e.g., below 350°C, while increasing temperature will direct the reaction toward the accumulation of more unconverted reactants. Conversely, higher pressures stimulate the conversion of syngas to ethanol, although both parameters, i.e., temperature and pressure, are related and need to be optimized together.

3.2 Indirect catalytic conversion of syngas to ethanol Since the direct conversion of syngas to ethanol is still far from being sufficiently optimized for possible commercialization, indirect conversion processes have meanwhile been considered as viable alternative options. The indirect conversions of syngas to ethanol, which are at a more

152 Chapter 6 advanced stage than the direct route, consist of producing first another compound, e.g., methanol, to transform the latter subsequently into ethanol, with possible other intermediate compounds. Syngas is thus not directly converted to ethanol in this case. 3.2.1 Methanol route Different indirect routes have been proposed, and one such process has recently been scaled-up for commercialization, transforming waste into syngas, used for methanol production in a first stage, with its further conversion to ethanol. The potential and suitability for commercialization of this indirect ethanol process has recently been proven, though the technology started already being studied and developed more than a decade ago, when the company Enerkem (Alberta, Canada) first focused on the sorting of municipal solid waste for its use as heterogeneous material to be gasified in order to produce ultra clean syngas [9]. Syngas is then converted into methanol, which is subsequently used in catalytic reactions to produce ethanol. The latter technology has been scaled-up and recently commercialized by Enerkem in Canada, using municipal solid waste and heterogeneous residues as feedstock. A few years ago, it was already proven, before scaling-up, that the economics of the process should be favorable, with reasonable operational costs and a tipping fee provided by the municipality for the conversion of the ultimate residue [53]. The company’s first commercial plant, in the city of Edmonton (Qu
ebec, Canada), can process 100,000 t of dry municipal solid waste per year to produce 38,000 m3 ethanol at full capacity. The first step of this type of waste-to-energy technology consists of gasifying the feedstock, i.e., waste, in order to obtain syngas with an optimal CO/H2 ratio (Fig. 2). This ratio is important in the catalytic conversion, contrary to the bioconversion process where more flexibility in the syngas composition is allowed, as will also be explained in further sections. Besides, metal catalysts are very sensitive to the syngas composition and to the possible presence of impurities that may affect the catalyst’s activity and therefore the long-term stability of the process [13]. Some typical impurities found in syngas may include volatile tars, ethane (C2H6), benzene (C6H6) hydrogen sulfide (H2S), sulfur dioxide (SO2), carbonyl sulfide (COS), ammonia (NH3),

Fig. 2 Major steps of feedstock conversion to ethanol, in which methanol from syngas is converted to methyl acetate and subsequently to ethanol, avoiding acetic acid as intermediate compound from methanol to methyl acetate (MA).

Ethanol production from syngas 153 nitrogen (N2), hydrogen cyanide (HCN), nitrous oxide (N2O), methane (CH4), acetylene (C2H2), and ethylene (C2H4) [34]. The effect of such minor compounds on chemical catalysts depends on the nature of the impurity. For example, the common presence of nitrogen in waste materials may generate compounds such as HCN and NH3. NH3 in turn may be converted to NH4Cl, causing fouling problems, while HCN will generally deactivate the catalyst. Similar problems may occur with sulfur compounds. They are found in many wastes and appear as volatile compounds in syngas (e.g., H2S, CS2, and COS), which may also deactivate hydrogenation catalysts and affect the process. Although some ongoing research is focusing on reducing such problems, through improved gasification technologies, significant further improvements are still needed [54]. Once pure syngas is obtained, the next step is its catalytic conversion to methanol, through CO hydrogenation. Methods to produce methanol from syngas are addressed in another chapter and will not be described in full details here. It involves the following three highly exothermic reactions: CO + 2H2 ! CH3 OH

(6)

2CO2 + 3H2 ! CH3 OH + H2 O

(7)

CO + H2 O ! CO2 + H2

(8)

Besides CO and H2, the presence of some CO2 (generally less than 10%) in the syngas mixture would be beneficial for the synthesis of methanol and for maintaining an optimal catalyst activity. A stoichiometric H2/CO ratio of 2 is generally preferred in order to reach near full conversion to methanol; or alternatively an (H2 + CO2)/(CO + CO2) ratio of 2 can also be considered. For the catalytic conversion, different metal catalysts and mixtures would be suitable, including copper, zinc oxide, alumina, and magnesia. Besides its conversion to ethanol, methanol could, actually, directly be used as a fuel as well, or for producing other compounds, such as dimethylether. Contrary to ethanol production, the commercial syngas to methanol process is an established state of the art technology. Finally, methanol is catalytically converted to ethanol. Temperatures and pressures typically up to 350°C and 15–80bar, respectively, are commonly used. The process is based on three main reactions, in which methanol and CO yield methylacetate, which is in turn converted to ethanol, according to the following reactions, and avoiding the accumulation of undesired acetic acid as possible intermediate compound from methanol conversion to methylacetate: CH3 OH ! CO + 2H2

(9)

2CH3 OH + CO ! CH3 COOCH3 + H2 O

(10)

CH3 COOCH3 + 2H2 ! C2 H5 OH + CH3 OH

(11)

Carbonylation of carbonylatable reactants, such as methanol, with carbon monoxide in the gas phase, in the presence of a catalyst is a well-developed commercial technology, and variations

154 Chapter 6 of continuous carbonylation processes have been patented over the years [63] as well as several other recent aspects and improvements [19,67]. Methanol and CO gases are mixed in a packed bed reactor, in the presence of a Rh-based catalyst and methyl iodide as co-catalyst and mediator. Subsequently, the generated methylacetate is mixed with H2 to produce ethanol, with methanol as a co-product, according to the above reactions. 3.2.2 DME route An alternative to the methanol route described above, consists of producing dimethylether (DME, CH3OCH3), from syngas, for subsequent DME conversion to methylacetate, according to the following reactions for the direct, one step, synthesis of DME from syngas [30]: 3CO + 3H2 ! CH3 OCH3 + CO2

(12)

CH3 OCH3 + CO ! CH3 COOCH3

(13)

Methanol is a possible intermediate in the production of DME. Thus, DME may either be produced directly from syngas as indicated above or, otherwise, syngas is converted first to methanol and subsequently to DME. The latter multistep process is more common and corresponds to the following reactions: CO + 2H2 ! CH3 OH

(6)

2CH3 OH ! CH3 COCH3 + H2 O

(14)

CH3 COCH3 + CO ! CH3 OOCH3

(15)

Methylacetate is then formed though a reaction between CO from syngas and DME, using H-Mordenite as catalyst (Fig. 3). However, that catalyst and the production of methylacetate are not efficient enough yet for cost-effective industrial application. The latter reaction is usually run at moderate temperatures and pressures, of around 200°C and 15bar. Afterward, ethanol and also some methanol are obtained from the hydrogenation of methylacetate with H2, through a well-established reaction, similarly as in the methanol route, using a Cu/ZnO catalyst in most

Fig. 3 Feedstock conversion to ethanol through the DME route.

Ethanol production from syngas 155 cases [65], and according to Reaction (11). Some other catalysts, such as Cu/ZnO/Al2O3 or Rh/ SiO2, among others, have been tested as well. Methanol, which is produced in the reaction, besides ethanol, and excess methylacetate are recycled to the corresponding reactors. Produced methanol can easily be converted to DME and thus recycled to the process. It is worth highlighting that, contrary to the direct route, in the above indirect catalytic conversions, there is no substantial accumulation of water in the end products, and thus no need to separate water from the final mixture in this case.

4. Bioconversion of syngas to ethanol Another alternative to the catalytic conversion of syngas to ethanol is its bioconversion. Syngas obtained from the gasification of different feedstocks, e.g., biomass, waste, coal, can be fermented, in bioreactors, in a similar way as in the catalytic routes described above. In case of considering biomass and lignocellulosic wastes as feedstocks, the latter can yield either different sugars obtained from their extraction from the cellulosic and hemicellulosic fractions or they can yield syngas from their full (i.e., cellulose, hemicellulose, but also lignin) gasification. Although sugar fermentation to ethanol is still leading the market, syngas and CO2 fermentation to ethanol is gaining increased interest and commercial acceptance [43]. It allows to produce a valuable product, i.e., ethanol, while simultaneously solving environmental problems, among others through waste gasification and valorization or the use of greenhouse gases such as CO2. Syngas fermentation is thus a recent, viable, alternative to the more common sugar fermentation technology [43]. In sugar fermentation, yeasts metabolize starch and simple sugars, generally obtained from corn, sugarbeet, or sugarcane. The choice of the feedstock depends on the country, their abundance and (low) cost; with corn being most common in the United States while sugarcane is more popular in countries such as Brazil. These feedstocks can also be used as food or feed and, in order to avoid a food-fuel dilemma, cellulosic ethanol has therefore gained much interest in recent years, though further efforts still need to be made in order to reduce its production costs compared to fossil fuels [51]. Cellulosic ethanol uses biomass, agricultural residues, or similar materials from which sugars are extracted and then fermented into ethanol. Besides their bioconversion to ethanol, sugars can also be fermented to a broad range of other different metabolites of interest, providing a high flexibility to this technological platform [6]. Similarly, to sugar fermentation, syngas bioconversion also requires the use of biological rather than chemical catalysts. However, instead of yeasts, as used for sugar fermentation, specific anaerobic bacteria will catalyze those reactions when considering gases, as described hereafter. Several acetogens can metabolize gases, i.e., syngas, but also sugars, though the end products may sometimes be slightly different, as different metabolic steps may be involved [11,26].

156 Chapter 6

4.1 Biocatalysts and syngas bioconversion pathway Contrary to the expensive metal catalysts needed in catalytic syngas conversion, cheap microbial catalysts, i.e., microorganisms, drive the reactions in biological processes. Besides, most bioreactors are operated at near ambient temperature and pressure, thus resulting in much cheaper running conditions, in that respect, than with catalytic reactors. Though the direct catalytic conversion of syngas into ethanol is still being optimized at laboratory scale, the direct biological conversion of syngas related mixtures, including gases such as CO and CO2, relying on one single bacterial species, i.e., biocatalyst, is already a reality at large scale [76]. In that sense, the company LanzaTech built a commercial demonstration plant in Ghent, Belgium, in 2017, to treat emissions from the steel mill plant of ArcelorMittal. Those emissions contain CO and H2 mainly, similarly as in syngas obtained from the gasification of different feedstocks. Steel mill emissions may also include variable concentrations of other gases, such as CO2. Their actual composition depends on the specific process being considered. At full capacity, 380.000m3 ethanol per year could be produced from ArcelorMittal’s waste gas plant. Besides, the first full commercial syngas fermentation plant producing ethanol (Beijing Shougang LanzaTech New Energy Science & Technology Co., Ltd) was started in May 2018 [44]. It can treat unsorted, nonrecyclable, gasified municipal solid waste, to produce ethanol from gas fermentation. The bacterial catalysts involved in syngas fermentation are acetogenic bacteria, also called acetogens. The main end product of any acetogen, metabolizing syngas, is acetic acid, and this conversion process is called acetogenesis [50]. Besides, a limited number of strains can also produce ethanol as an end product, in which case the reaction is referred to as solventogenesis. Although more than 100 acetogens are known and produce acetic acid; at the time of writing, hardly less than 10 had been identified as native solventogenic strains. Ethanol can either directly be obtained from the fermentation of gases such as CO2, CO, H2 (e.g., syngas) or, more frequently, acetic acid will be produced first, through gas fermentation, and then that acid is further metabolized, by the same biocatalyst, to generate ethanol as the end product. The key steps of the metabolic pathway are shown in Fig. 4. In that pathway, called Wood-Ljungdahl pathway (WLP), the gaseous substrates, CO2, CO, H2, are first converted into the intermediate compound, acetyl-CoA. Acetyl-CoA could then directly be converted into acetaldehyde and, subsequently, ethanol [2]. However, the most common and energetically favorable pathway for the bacteria consists in converting acetyl-CoA first to acetic acid, which generates energy and ATP (adenosine-tri-phosphate) in that specific step and stimulates biomass growth [49] (Fig. 4). Once produced, acetic acid can then further be metabolized to yield acetaldehyde and finally ethanol. The ethanol producing pathway, via acetic acid as intermediate compound, is much more favorable for the biocatalyst, from a bioenergetic point of view, and is thus the highly prevailing bioconversion route. It could even roughly be the sole pathway followed by many acetogens in practice. Indeed, thermodynamic and stoichiometric analyses estimated that

Ethanol production from syngas 157

Fig. 4 Metabolic Wood-Ljungdahl pathway for the bioconversion of syngas (CO2, CO, H2) to acetic acid and ethanol.

158 Chapter 6 during autotrophic growth of C. autoethanogenum on H2 +CO2, the overall ATP yield is only 0.5 ATP/mol ethanol in the route converting acetyl-CoA directly to acetaldehyde and then ethanol. Conversely, in the route in which acetic acid is produced first, then converted into acetaldehyde and, later, into ethanol, 1.2 ATP/mol ethanol would be produced via acetic acid reduction to ethanol. Similarly, with pure CO as carbon and energy source, the indirect route, via acetic acid, would yield 1.875 ATP/mol ethanol, while the direct route would only yield 1.375 ATP/mol ethanol, according to the same analysis [46,55]. It is worth highlighting that, irrespective of the route followed by the organism, all the reactions can be carried out by one same, single, biocatalyst, according to the same WLP; simply the pathway and specific intermediates leading to ethanol as final product, would be different. The main overall reactions of syngas bioconversion to ethanol can be summarized as follows and can use different mixtures of CO, CO2, H2 gases, or even pure gases (e.g., pure CO): 6CO + 3H2 O ! C2 H5 OH + 4CO2

(16)

2CO2 + 6H2 ! C2 H5 OH + 3H2 O

(17)

2CO + 4H2 ! C2 H5 OH + H2 O

(18)

3CO + 3H2 ! C2 H5 OH + CO2

(19)

Pure carbon monoxide can be used as carbon and energy source by acetogens, and it does not need the presence of any other gas. Conversely, carbon dioxide is a suitable carbon source, but it requires, additionally, the presence of an energy source, such as hydrogen. Therefore, many acetogens are able to metabolize pure CO, mixtures of CO2/H2, or any other related mixtures similar to syngas, e.g., CO/H2, CO/CO2/H2. Carbon monoxide and hydrogen can both act as electron donors, and carbon dioxide plays the role of electron acceptor. Therefore, CO2 will be reduced in the bioconversion process, while CO and H2 will be oxidized. The latter two gases are, in most cases, proportionally more abundant in syngas compared to carbon dioxide and, in such case, the amount available hydrogen may be enough to metabolize all the carbon dioxide present in the gas mixture. It is worth highlighting that the conversion of CO will generate some CO2, as can also be concluded from the above overall reactions. Thus, even when dealing with syngas or waste gases very rich in CO, it is still desirable to have sufficient hydrogen in order to remove any produced CO2 and to fully convert all the gases into ethanol. Common acetogenic bacteria of the Clostridium genus that have been proven to produce ethanol are listed in Table 2. Occasionally, some solventogenic strains not belonging to the Clostridium genus have been reported to accumulate some ethanol as well, i.e., Butyribacterium methylotrophicum [68] and Alkalibaculum bacchi [7], though they are rather scarce, produce only low alcohol concentrations, and have hardly been studied and described in the scientific literature [27]. As can be observed in Table 2, and as indicated earlier, the number of so far known solventogenic acetogens is rather limited. It is also worth to mention that some

Ethanol production from syngas 159 Table 2 Main end products and optimal growth conditions of common solventogenic bacteria. Possible alcohols produced

Typical pH for alcohol production

Typical pH for growth

Optimal Temperature (°C)

Reference

Ethanol; 2,3Butanediol Ethanol; 2,3Butanediol Ethanol; 2,3Butanediol Ethanol; Butanol

4.5

6.0

37

[74]

4.8

5.8–6.0

37

[1]

5.5

6.3

37

[33]

–

5.5–7.5

30–37

[47]

Ethanol

6.6–6.9

8.3

30

[10]

Ethanol; Butanol; Hexanol

4.8–5.0

6.0–7.0

37–40

[47]

Bacterial species Clostridium ljungdahlii Clostridium autoethanogenum Clostridium ragsdalei Clostridium drakei Clostridium aceticum Clostridium carboxidivorans

´ ., Veiga, M. C., & Kennes, C. (2017b). H-B-E (hexanol-butanol-ethanol) fermentation for the production of Adapted from Ferna´ndez-Naveira, A higher alcohols from syngas/waste gas. J. Chem. Technol. Biotechnol., 92(4), 712–731. https://doi.org/10.1002/jctb.5194.

acetogens can produce higher alcohols as well (e.g., butanol or even hexanol), but usually at lower concentrations than ethanol and in mixture with it [16,25,59]. Most solventogenic clostridia are mesophilic organisms, with optimal activity near 30–37°C, while a pressure of 1 atm, or otherwise a slight overpressure, is generally applied in most research studies and even at large scale. Though some thermophilic (50–70°C) acetogens are known, they are mainly acetate rather than ethanol producers. It is worth to keep in mind that applying higher temperatures will reduce the aqueous solubility of the gases [43]. Typical solubilities at 30°C and 60°C are given in Table 3 [27]. This is an important point, as the ethanol producing biocatalyst is generally growing in suspension in a liquid nutrient broth [56] or, alternatively, it could grow in the form of a wet biofilm whenever working with a packed bed bioreactor [21]. The low gas solubility will limit the availability and mass transfer of the gases to the biocatalyst. In terms of conversion optimization, the bioreactor operating conditions, the reaction medium, as well as other aspects can be selected to produce mainly ethanol, with no or minor concentrations of acetic acid as side-product [4], as will be described later in the corresponding section dealing with parameters affecting ethanol production.

Table 3 Typical water solubilities of CO, H2, CO2 (g/L) at a pressure of 1 atm [27]. Temperature (°C)

CO

H2

CO2

30 60

0.026 0.015

0.0015 0.0012

1.32 0.59

160 Chapter 6

4.2 Parameters affecting ethanol production from syngas 4.2.1 pH and redox potential As highlighted above, acetic acid is the main natural end product of acetogenic bacteria grown on syngas. That acid can then be converted into ethanol, and this will occur mainly when stress conditions are exerted on the biocatalysts (e.g., pH below optimal for growth), as can also be deduced from the pH data reported in Table 2. Indeed, several acetogenic bacteria have an optimal pH for growth and acetogenesis roughly around 6.0–6.5, while solventogenesis will take place at lower pH values, often around 4.5–5.0, for those same strains. At such low pH values, growth is hardly observed while ethanol production (i.e., solventogenesis) is highly stimulated. Few strains, such as C. aceticum, have a higher optimal pH for growth, around 8.3 [15], and the stress pH conditions will then correspond to a near neutral pH, close to 6.6–6.9, at which ethanol production will take place with limited or no biomass growth [10]. A lower, i.e., more negative, redox potential is also favorable to solventogenesis and allows reaching better and faster bioconversion of acetic acid into ethanol. Therefore, adding some reducing agent, such as cysteine-HCl, or increasing its concentration has been proven to stimulate alcohol production [3]. Reducing agents are also involved in the conversion of NAD(P) to NAD(P)H, which favors the production of alcohols as well. 4.2.2 Trace metals in the fermentation medium Specific metals, added in trace concentrations, in the fermentation broth, may stimulate specific enzymes involved in solventogenesis, and thus the bioconversion of acetic acid into ethanol. This occurs because several enzymes of the WLP are metalloenzymes. It has been observed, for example, that tungsten stimulates the activity of the aldehyde:ferredoxin oxidoreductase enzyme involved in acetic acid conversion to ethanol (Fig. 4). In that respect, some studies have shown that the addition of trace amounts of tungsten (W) in the fermentation broth significantly increase the final ethanol/acetic acid ratio [42]; acetic acid may even end-up being completely converted into ethanol under optimized conditions [4]. The same positive effect of tungsten on solventogenesis observed with pure bacterial cultures has been reported in the case of complex mixed microbial communities [18]. Molybdate (Mo) is considered to be an analogue of tungsten and binds on the active sites of some enzymes, and also the acetaldehyde: ferredoxin oxidoreductase, which is either a molybdenum- or a tungsten-iron-sulfur protein, depending on the biocatalyst. Interestingly, trace metals such as tungsten are also involved in the enzyme formate dehydrogenase catalyzing the initial reduction of CO2, and therefore their effect on solventogenesis may sound somehow striking, though clearly proven experimentally [27,29]. Some other authors reported that adding or increasing the concentration of trace metals, such as 2 Ni2+, Zn2+, SeO2 4 , had a similar effect as WO4 , and that they stimulate ethanol production in the species C. ragsdalei [66]. However, a similar clear positive effect of trace metals such as

Ethanol production from syngas 161 selenium on ethanol production could not be confirmed with other biocatalysts, e.g., C. autoethanogenum, C. carboxidivorans [4,29]. In the case of zinc, some researchers observed that increasing the Zn2+ content, from 7 to 280 μM, improved alcohol production through an increase in the gene expression of the carbon fixation and alcohol dehydrogenase in C. carboxidivorans [45]. Additional studies on the influence of trace metals would be useful and would allow to shade light on their specific effect, either individually or in mixtures, on different biocatalysts and their enzymes. 4.2.3 Other compounds of the fermentation medium The culture medium used for optimal growth and activity of acetogens is sometimes enriched with supplements such as yeast extract (YE), beef extract, peptone, or similar additives, mainly in lab-scale research. However, the use of high concentrations of the latter could be prohibitive at commercial scale. Growth of acetogens may be improved in the presence of some B-vitamins, such as vitamin B5 (pantothenate) and vitamin B12 (cobalamin), among others, besides the positive effect of trace metals explained earlier. The advantage of supplements such as YE is that they provide a broad range of minerals, vitamins, amino acids, and even macronutrients such as nitrogen, that stimulate growth and bacterial activity. However, research has also demonstrated that although the addition of YE will stimulate and improve bacterial growth, it will most often reduce the efficiency of some processes not linked to growth, such as solventogenesis, and high concentrations of such supplements may then negatively affect the final ethanol/acetic acid ratio [3]. Few lab-scale studies have shown that additives such as YE can efficiently be replaced by cheaper ones such as corn steep liquor, with a rather similar effect, while allowing to reduce costs [52]. 4.2.4 Substrate and product toxicity Acetogenic bacteria are living organisms and may get inhibited if the end product’s concentration exceeds a given threshold value in the fermentation broth. At ethanol concentrations exceeding 20–25 g/L, some inhibitory effect on the activity of C. carboxidivorans, fermenting syngas, started being observed [24]. This inhibitory effect can be alleviated by regularly removing the end product from the bioreactor during the bioconversion process. Different downstream processes have been optimized over the years for the separation of bioethanol from fermentation media [77]. In bioreactors with continuous feed of both the gas phase, i.e., syngas, as well as the liquid phase, the ethanol concentration can be maintained below inhibitory levels, while adjusting the liquid flow rate and optimizing productivity in order to maintain the latter as high as possible. The IC50, which is the concentration corresponding to 50% inhibition of the biocatalyst’s growth rate and activity, was reported to be as high as about 35g/L ethanol in C. carboxidivorans, which is already a highly acceptable concentration for process optimization [24].

162 Chapter 6 On the other side, not all acetogenic bacteria can effectively metabolize all three major gases of interest in syngas fermentation, i.e., CO, CO2, and H2. In most cases, carbon monoxide is the problematic compound. Indeed, taking Acetobacterium woodii as an example, and although literature data are sometimes contradictory, it has generally been considered that that organism can grow on CO2 and H2, but that it is not able to metabolize CO nor grow on that gas [14]. Actually, it is probable that A. woodii can metabolize CO, but under certain specific conditions only. In presence of CO2 and H2, it was observed, in some study, that higher concentrations of biomass were produced in the presence of low amounts of CO as cosubstrate, suggesting that carbon monoxide was indeed metabolized in such mixture [14]. However, at high CO concentrations or pressures, that species was found to be inhibited. In some acetogens, carbon monoxide may inhibit hydrogenases as well as the hydrogen-dependent CO2 reductase, an enzyme involved in H2 consumption. In such case, H2/CO2 consumption would then be reduced. Hydrogenase enzymes of many other acetogens may, however, not be sensitive to carbon monoxide; and several acetogens are able to grow on pure CO. Since some strains may face inhibition only at high CO concentrations, when they are cultivated in suspended-growth bioreactors (e.g., stirred tanks), the actual gas concentration potentially able to affect and inhibit the bacteria will always be low, because of the low aqueous solubility of the syngas components. Conversely, CO inhibitory effects could be different and more significant in packed bed bioreactors, in which the bacteria grow in attached form, on a packing material; though this has not been studied in detail, to the best of our knowledge. Additionally, impurities may be found in syngas, at different concentrations and of different nature, depending on the gasified feedstock, among others. Though some impurities may exhibit inhibitory effects; others do apparently not [79]. In any case, if needed, such impurities can then be removed. Some other factors may also need to be taken into account, such as the salinity of the fermentation broth, as it could also have some inhibitory effect [28]. The salinity may vary and increase, among others during pH regulation, when adding acids or bases for pH adjustment in the bioreactor. However, experimental evidence suggests that, in most cases, the salinity of the medium remains generally below threshold inhibitory levels. 4.2.5 Bioreactors Bioreactors have been used for decades for the treatment (biodegradation) of volatile organic or inorganic pollutants and odors emitted from wastewater treatment plants, waste treatment facilities or industries, among others. The most common bioreactor configurations that have been used at commercial scale for such air pollution control are the biofilter, the biotrickling filter, the bioscrubber, and activated sludge or suspended growth bioreactors with polluted air diffusion [40,41]. They all have largely been optimized for such purpose. Other bioreactor configurations have been considered almost only in lab-scale or pilot-scale research, e.g., the membrane bioreactors, rotating biological contactors, and monolith bioreactors [40,41]. Only recently has the bioconversion and valorization of gas emissions, and also syngas fermentation,

Ethanol production from syngas 163 Table 4 Examples of continuous bioreactor configurations used for syngas fermentation into acids and/or ethanol and other alcohols with acetogenic bacteria. Bioreactor configuration Bubble column bioreactor Stirred tank bioreactor Gas-lift bioreactor Biotrickling filter Hollow fiber membrane bioreactor Moving bed biofilm reactor Monolith bioreactor Rotating biological contactor

Biocatalyst

Gas composition

Reference

Clostridium carboxidivorans

CO:CO2:H2, 25:15:60 CO:CO2:H2:Ar, 55:10:20:15

[61]

CO:CO2, 4:1 CO:CO2:H2:N2, 38:28.5:28.5:5 CO

[62] [21]

CO:CO2:H2, 40:30:30 CO:CO2:H2:N2, 20:15:5:60 CO:CO2:H2:N2, 20:15:5:60

[32]

Clostridium ljungdahlii Clostridium carboxidivorans Clostridium ragsdalei C. autoethanogenum DSM10061

Clostridium ragsdalei, Butyribacterium methylotrophicum, or Clostridium ljungdahlii Clostridium carboxidivorans Clostridium carboxidivorans

[56]

[36]

[70] [69]

´ ., Veiga, M. C., & Kennes, C. (2017b). H-B-E (hexanol-butanol-ethanol) fermentation for the production of Adapted from Ferna´ndez-Naveira, A higher alcohols from syngas/waste gas. J. Chem. Technol. Biotechnol., 92(4), 712–731. https://doi.org/10.1002/jctb.5194.

been considered as innovative alternatives, using similar or adapted bioreactor configurations as for conventional air pollution control [27]. The different bioreactor configurations tested for syngas fermentation and its anaerobic bioconversion to ethanol, higher alcohols, or other bioproducts are listed in Table 4. Roughly, two types of bioreactors are available; those in which the biocatalyst growth in suspension and others in which it grows in the form of a biofilm attached on a solid material. Although syngas components are poorly soluble in aqueous phase, suspended-growth bioreactors still represent the most popular configuration. Stirred tank bioreactors are often used in lab-scale research studies, also because fully automated set-ups can directly be purchased from different providers. However, reactors similar to gas-lift systems are closer to configurations most commonly used at larger scale. Since optimizing mass transfer of the gaseous substrate to the biocatalyst is a key issue, pressurized gas can occasionally be fed to the reactor as well, though typically used pressures are quite lower than in catalytic processes. Although suspended growth reactors are most popular, attached growth bioreactors (e.g., biotrickling filters, to mention one example) would theoretically result in a better mass transfer, among others because the gas needs only to transfer from the gas phase to the wet biofilm through a thin liquid layer rather than through a liquid suspension [39].

164 Chapter 6 4.2.6 Biocatalytic vs catalytic processes As explained in the above sections, syngas can be converted either with biological catalysts or with metal-based chemical catalysts. Each one has its own characteristics with specific advantages and drawbacks. Direct gas conversion to ethanol with one single biocatalyst is possible, while the direct catalytic conversion needs further research and optimization studies and is still largely being explored at lab-scale. At larger scale, indirect catalytic processes are much more developed, with the conversion of syngas to a product such as, for example, methanol, followed by methanol conversion to ethanol, in different steps or with different intermediate compounds. Both biological syngas fermentation and indirect catalytic syngas to ethanol conversion via methanol have reached commercial scale in recent years (around 2017–2018), after intensive research and optimization, and with gradual scaling-up from lab-scale to pilot-scale and, later, demonstration scale. In terms of syngas composition, many acetogenic bacteria can metabolize a broad range of gas mixtures, from pure CO to different ratios of CO/H2 or CO/CO2/H2 [43]. Instead, catalytic reactions are highly sensitive to the syngas composition and require rather well-defined CO/H2 ratios, as variations of such ratio will affect the selectivity and conversion efficiency. Besides, the presence of other, contaminant gases needs to be considered as well. Some studies have shown that bioprocesses can be affected by some of the contaminants possibly present in syngas, and some inhibitory effects have sometimes been reported, although they are generally less problematic than when dealing with chemical catalysts [5]. On the other side, some syngas impurities could have a positive effect on bioprocesses, such as some sulfur species. One major advantage of biocatalysts is that they are living organisms (bacteria), which may decay in the presence of specific impurities found in syngas; though other cells will remain active and grow or regenerate in continuous cultures. Instead, poisoning of chemical metal catalysts or their deactivation by thermal sintering has frequently been reported, as a result of the presence of syngas impurities, such as sulfur compounds, chlorine, ammonia, or other impurities [75]. Relying on any possible self-regeneration of inert chemical catalysts is generally not an option. Gas cleaning may then be required or, otherwise, the use of methods that alleviate the poisoning effect. In terms of conversion efficiency and productivity, if one compares selectivity and productivity in either biological or catalytic reactors, selectivity is generally rather higher in bioreactors, as already partly explained above. In syngas fermentation, in the best case, ethanol could basically be the only dominant end metabolite. However, in catalytic processes, syngas will often yield a range of different products. Conversely, the concentration of end product(s) is usually higher, and thus more favorable, in catalytic reactors than in bioreactors. Besides, high ethanol concentrations, exceeding 30–35 g/L, may have inhibitory effects on biocatalysts. However, there is potential for improving the productivity of syngas fermentation processes through the development of strains with superior characteristics, which is already the focus of ongoing

Ethanol production from syngas 165 Table 5 Ethanol and some possible products that can be obtained from syngas fermentation. Biocatalyst(s)

Product(s)

Reference

Acetogenic bacteria Acetogenic bacteria Acetogenic bacteria (e.g., C. carboxidivorans) Acetogenic bacteria +chain elongating bacteria (e.g., C. kluyveri) Acetogenic bacteria (e.g., C. carboxidivorans) Acetogenic bacteria +chain elongating bacteria (e.g., C. kluyveri) Acetogenic bacteria +PHA-accumulating aerobic bacteria Acetogenic bacteria +Microbial oil accumulating yeasts or bacteria Anaerobic bacteria Recombinant acetogenic bacteria Recombinant acetogenic bacteria Acetogenic bacteria or acetogenic bacteria +other biological/chemical conversion

Acetic acid Ethanol Medium chain fatty acids (e.g., butyric, hexanoic acids)

[71] [76] [29]

Medium chain fatty acids (e.g., butyric, hexanoic acids) Higher alcohols (e.g., butanol, hexanol)

[23]

Higher alcohols (e.g., butanol, hexanol)

[23]

Biopolymers (e.g., poly-hydroxy-alkanoates, PHA)

[60])

Microbial oils (fatty acids methyl esters, FAME)

[64]

Methane Acetone Iso-propanol Propanol, 2,3-butane-diol, butylene, isoprene, succinic acid, lactic acid, 1,3-butadiene, etc.

[31] [12] [12] –

[27]

research. In that sense, progress in metabolic engineering and genetic engineering tools needs also further studies. An interesting aspect of the biological syngas-based process is that the same bioreactor set-up can be used to produce ethanol as well as different other products, with only slight adaptations, among others concerning the biocatalyst and the downstream processing stage [44]. This is quite more difficult with catalytic reactors that are highly product-specific adapted processes. Some other products that have been studied in lab-scale research are listed in Table 5. Those products are obtained with acetogenic biocatalysts or combining such acetogenic bacteria with other microorganisms, e.g., aerobic or anaerobic bacteria, yeasts, or fungi. Acetic acid and ethanol, which are the most extensively studied products of syngas fermentation obtained with acetogens, can be converted to a broad range of other platform chemicals or fuels, through subsequent chemical or biological reactions. Regarding temperature and pressure, bioprocesses take place optimally under near ambient conditions, i.e., room temperature and pressure, though higher pressures would increase the gas solubility and thus improve the mass transfer of the gaseous substrates to the biocatalyst, as the latter is growing in liquid suspension or as an attached wet biofilm. Instead, catalytic processes always need high temperatures and pressures, and both parameters will significantly affect gas conversion and selectivity. Even in case bioreactors are operated above atmospheric pressure,

166 Chapter 6 such pressures would still be lower than those used in most catalytic reactions. Less stringent conditions are thus required for the construction of bioreactors, which also results in lower investment and operating costs compared to catalytic reactors. Besides, space velocity of the gas feed is another aspect that affects the conversion and selectivity of catalytic reactions. In bioreactors, increasing the gas flow rate may reduce the conversion efficiency as a result of the shorter residence time of the gaseous substrate in the system, while it is expected to increase the gas-liquid mass transfer and thus the final product concentration. In case of bioprocesses, considering the low gas solubility and since the active biocatalyst (i.e., acetogenic bacteria) generally grows in liquid suspension or in the form of a biofilm, optimizing kLa is a key issue.

5. Conclusion Ethanol is considered as an interesting fuel and chemical. Several processes, either catalytic or biological ones, are suitable for its production from syngas. Though several indirect catalytic routes have been developed and even recently commercialized, there is room for significant improvements in terms of conversion efficiency, cost-effectiveness, and optimization of the catalysts used. The direct catalytic conversion of syngas into ethanol is also possible and requires less reaction steps. Thus, the latter sounds more attractive, though it still needs significant research in order to become technically and economically effective. Conversely, indirect routes, through some intermediate compounds, have reached a quite higher development stage. Most recent progress on direct catalytic conversion is still largely occurring at laboratory scale. A common feature of catalytic reactions is that they require high temperatures and pressures, and they commonly need expensive metal-based catalysts. On the other side, bioprocesses allow the direct bioconversion of syngas and even waste gases containing CO or CO2 into ethanol, and they take place at near room temperature and pressure. In the biological alternative, one same biocatalyst, i.e., microorganism, is able to convert syngas directly into ethanol or it can also be converted into acetic acid, as metabolic intermediate, and then further into ethanol by one same strain. Bioprocesses for ethanol production through syngas fermentation have recently reached commercial scale, though additional research is here also possible for further improvement of the technology. In contrast to the chemical processes, bioprocesses are effective over a wider range of gas ratios, e.g., CO, H2.

Acknowledgments Part of our ongoing research on gas fermentation at University of La Corun˜a is funded by the Ministry of Science and Innovation (project PID2020-117805RB-I00). DKV (ED481A-2018/113) and CFB (ED481A-2020/028) thank Xunta de Galicia for financing their doctoral contracts. The BIOENGIN group also thanks Xunta de Galicia for financial support to Competitive Reference Research Groups (ED431C 2021/55).

Ethanol production from syngas 167

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CHAPTER 7

Mixed higher alcohols production from syngas Tayebeh Marzoughi and Mohammad Reza Rahimpour Department of Chemical Engineering, Shiraz University, Shiraz, Iran

1. Introduction Global studies on alternative technologies for the production of synthetic fuels and chemicals have been prompted by the inevitable discharge of crude oil resources and growing environmental concerns. Conversion of synthesis gas (syngas) to value-added compounds is a substantial topic of study [1–5]. Syngas, a blend of hydrogen and carbon monoxide with trace amounts of carbon dioxide in some instances, can be produced from a variety of carbon-based resources, including natural gas, coal, biomass, and even organic waste. Numerous products, such as methanol, ultra-clean gasoline and diesel, high-quality waxes, alcohols, olefins, and other compounds, may be derived by catalytic conversion of syngas [6,7]. One prominent pathway to convert syngas to value-added chemicals is the higher alcohol synthesis (HAS) [8–13]. Higher alcohols (HA), which are usually described as alcohols with two or more carbon atoms (alcohols with a molecular weight greater than methanol), have got tremendous attention due to their diverse range of uses. They are extensively used as feedstocks and intermediates in the chemical and polymer industries for the production of both commodity and specialized chemicals [14–17]. Furthermore, it has been demonstrated that adding HA to methanol raises the water’s phase separation tolerance, decreases the fuel’s volatility as well as vapor lock propensity, and results in higher volumetric heating values [12,18,19]. Currently, HA such as ethanol and isobutanol are primarily synthesized by fermenting sugars achieved from sugarcane and starch [20–23], whereas heavier alcohols are most often synthesized through the hydration process of the relevant petroleum-derived alkene over acid catalysts [24]. The abundant energy request of the former techniques owing to the distillation stages needed to the products isolation [23], besides the slight single-pass conversion of the latter processes, which is about 5%, constitute disadvantages that limit the widespread industrialization of these techniques. Two more HA synthesis technologies based on the syngas Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91878-7.00020-4 Copyright # 2023 Elsevier Inc. All rights reserved.

173

174 Chapter 7 conversion have achieved commercialization, although their portion in total market volume is quite small. Particularly, fermentation of syngas using the microorganism Clostridium ljungdahlii is carried out on limited scale in companies such as Lanzatech or INEOS Bio in order to manufacture ethanol and isobutanol [25]. Additionally, alcohol with two carbon atoms is synthesized by carbonylation of methanol to acetic acid followed by reduction [26]. To fulfill future HA demand, broader use of syngas-based methods is more enticing, since unconventional natural gas, biomass, or even CO2 would be used to produce this feedstock resulting in noticeable sustainability benefits in the future. These processes are shown in blue color in Fig. 1. Apart from the two processes outlined before, three more processes have been studied in the literature, including methanol homologation to HA [26], methanol coupling with CO to generatedimethyl oxalate (DMO) and subsequent hydrogenation to ethanol [28–30], and the direct conversion of syngas to HA. Herein, the latter is a very attractive alternative in terms of environmental friendliness as well as capital and operational expenses, since it involves fewer unit activities and is performed in a single stage. These processes are shown in red color in Fig. 1. HAS is a complicated series of processes that necessitates an integrated investigation strategy that takes into account catalytic activity and physical characterization of catalysts, thermodynamic, kinetic, as well as process considerations. HAS is comparable to methanol synthesis process, Fischer-Tropsch (FT) synthesis, and also ammonia synthesis in which the reaction’s exothermic nature and stoichiometry require that the catalyst be operated at high pressures and low temperatures [31]. The primary reaction in this process is the formation of

Biomass

Fossil fuels Syngas Methanol synthesis Cu/Zn/Al

Methanol carbonylation

Methanol and Co coupling

Pd

Pd

Sugar fermentation

Oxygenates hydrogenation

Methanol homologation

Direct conversion

DMO hydrogenation

Fermentation

Alkene hydration

Invertase or Eschrichia coli

Cu

[Co(CO) 4]2

Rh,Cu,Co, Fe or Mo

Cu

Clostridium ljungdahlii

H3PO4

Higher alcohols

Fig. 1 Commercialized (gray) and contemplated (dark gray) pathways for HAS. The review’s primary topic, direct syngas conversion, is highlighted in black [27].

Mixed higher alcohols production from syngas 175 alcohol, whereas the secondary reactions are the generation of hydrocarbons and the water-gas-shift (WGS) reaction: Alcohol formation : nCO + 2nH2 ¼ Cn H2n+1 OH + ðn  1ÞH2 O

(1)

Hydrocarbon formation : nCO + ð2n + 1ÞH2 ¼ Cn H2n+1 OH + nH2 O

(2)

Water  gas  shift reaction equilibrium : CO + H2 O ¼ CO2 + H2

(3)

Typically, the aforementioned HAS reactions occur in the presence of catalysts capable of hydrogenation, CdO bond breaking, and CO insertion. Alcohols are formed by inserting CO into a metal-alkyl bond, forming an acyl intermediate which may be hydrogenated to generate an alcohol molecule [8]. The intermediates are mostly affected by the catalyst’s shape, structure, composition, and also reaction conditions. The active metals, preparation formula, and process conditions all influence the creation and subsequent transformation of the intermediate, which enables the selective control of the low-carbon mixed alcohols. Moreover, the design of the process is critical for the effective heat removal and supreme productivity of the alcohols produced [9]. The goal of this study is to look at how catalysts and reactors have evolved over time. We will first examine the types of catalysts that are effective for HAS, and then we will explore reactor and process technology innovation as a critical factor for increasing selectivity and yield toward HA while minimizing the proportion of by-products in the synthesis of mixed alcohols.

2. Catalyst design for higher alcohol synthesis The heterogeneous catalysts employed in the synthesis of HA may be roughly categorized into four groups: Rh-based, Mo-based, modified FT, and modified methanol catalytic systems (MS). Before discussing developments in catalyst synthesis, it is appropriate to explain the basic mechanism of HAS with respect to FT and MS, since it is critical for the understanding of the catalyst structure and composition. [32,33]. HAS is a very convoluted catalytic reaction process that involves a large number of simultaneous and sequential reactions. The following are some examples of reactions: nCO + 2nH2 ¼ Cn H2n+1 OH + ðn  1ÞH2 O

(4)

nCO + 2nH2 ¼ Cn H2n + nH2 O

(5)

nCO + ð2n + 1ÞH2 ¼ Cn H2n+1 OH + nH2 O

(6)

CO + 2H2 ¼ CH3 OH

(7)

CO + H2 O ¼ CO2 + H2

(8)

2CO ¼ C + CO2

(9)

The reactions (5) and (6) are representative of FT reactions, whereas the reaction (7) is representative of the methanol synthesis process. HAS has a few surface reactions in common

176 Chapter 7 with FT and methanol synthesis. As regards, HAS, FT, and methanol synthesis, all have distinctive features. FT may be propounded as a surface polymerization process that involves the incorporation of methylene into alkyl groups on the surface during the chain propagation stage. Dehydrogenation to produce alkenes or hydrogenation to generate alkanes is the last step for the surface alkyls [34,35], as depicted in Fig. 2. There are ongoing disagreements over the method by which syngas is converted to methanol. CO is converted to CO2 via the WGS reaction, and in the subsequent step CO2 is hydrogenated to generate methanol via the formation of surface formate and subsequently a methoxy species, as depicted in Fig. 3 [37]. FT necessitates chain propagation, which requires the dissociation of CdO bonds but does not need oxygenation, which requires the introduction of oxygen into the carbon chain through an undissociated CdO bond. While oxygenation is required for methanol synthesis, chain propagation is not required. Both chain propagation and oxygenation are required for HAS. Since chain propagation and oxygenation occur at distinct locations, HAS requires a dual site for bifunctional catalysis (see Fig. 4). On the dual site, one active site type catalyzes chain propagation while the other site catalyzes oxygenation. According to the synergy between these two sites, HA are formed. Due to the need for a dual site in HAS, growing the surface concentration of the dual site, enhances the higher alcohol formation, and any structural alteration that reduces the surface concentration of the dual site results in reduced higher Olefin

Paraffin

H2O CH2 O

C

H

CH2

CH2

R

H2C

R CH2

+ Propagation

Fig. 2 Schematic of carbon chain growth and termination in FT [36].

O

O

O C

O

H

C O

O

H

CH3 H

C O

O

H O CH3OH

Fig. 3 Schematic of methanol synthesis surface reaction route [36].

Mixed higher alcohols production from syngas 177 H2O

O

H

C

CH2

ROH R

*C=O

H2 C M1

*C=O M1

M2

Fig. 4 Schematic of the dual site in HAS. M1: chain growth center; M2: oxygenation center. (The interaction of M1 and M2 results in the production of HA. The hydrogenation of dissociatively adsorbed CO to generate CH2 is seen on the left.) [36]

alcohol selectivity. Furthermore, a balance between these two kinds of active sites should always be maintained. When one active site surpasses the other one, the catalyst’s performance is diminished. Therefore, the design strategy for HAS catalysts should be the construct of a dual site at the atomic level to optimize the synergy between the oxygenation and propagation sites [36]. Hereon, we represent the current advancements obtained in the use of carriers and promoters as well as usual conditions of the reaction will be detailed in order to illustrate the ranges of the operating and possible consequences with regard to design of the catalyst for each catalyst group.

2.1 Rh-based catalysts Since 1978, Rh has been well identified as the catalyst for converting syngas to ethanol and HA. Despite some slight differences, the synthesis of HA by hydrogenation of CO over Rh monometallic catalyst is possible to deduce that it follows the CO insertion method [27,33,38]. The production of the atomically adjacent Rh0-Rhn+ species is claimed to catalyze both CO dissociation and CO insertion, resulting in the synthesis of C2 oxygenates such as ethanol, acetaldehyde, and acetic acid during the conversion of syngas [39]. First, CO molecules are adsorbed linearly on Rh0 sites and in the germinal form on Rhn+ sites [40]. The adsorbed CO on Rh0 sites then disintegrates to produce the CHx* intermediate, which can be hydrogenated to form hydrocarbons or inserted into the CHx* on ionic Rhn+ species to form the CHxCO* intermediate, which can then be hydrogenated to form ethanol or reacted with adsorbed CO to generate higher oxygenates [41]. An assessment of the surface oxidation states of the Rh species showed that the ethanol selectivity is influenced by the relative number of atomically adjacent Rh+/Rh0 species [42]. However, this type of catalyst is unfavorable due to rhodium’s scarcity and expensive cost. To reduce the amount of Rh metal in the catalysts to a minimum, various types of supports are often utilized in Rh monometallic catalysts. Additionally, it has been shown that the type of support plays a key role in obtaining a high C2+ OH yield. The

178 Chapter 7 neutral and acidic supports (SiO2 and ZrO2) are typically more active in the generation of C2-oxygenates such as acetaldehyde and ethanol, whereas the basic supports such as CeO2 and MgO are more active in the mixed alcohol’s production like ethanol and methanol [43]. Table 1 summarizes the state-of-the-art for this catalyst family by listing the top 10 Rh-based catalysts published in the recent decade, ranked by the yield of their C2+ oxygenates. Here, it is obvious that the upgrade caused by binary or tertiary combinations of Li, Mn, and Fe is essential to attain higher efficiency by enhancing CO insertion and CO dissociation activity as well as decreasing the capacity of alkyl intermediates hydrogentation (Table 1, entries 10, 7, 2, and 3) [44–47]. A major exception seems to be the first catalyst (Table 1, entry 1), which has a CO conversion of about 39% and a selectivity for C2+ oxygenates of about 54% utilizing simply Mn as a promoter. Here this exception might be justified by the relatively high sample’s Rh loading which is about 6 wt%, the application of a oxidation-reduction cycle (redox cycle) as an activation technique, and also the use of high pressure and low GHSV [48]. As seen by the top two and top three catalysts, which share identical metal loading and formulation, the sequence in which the promoters are added is significant. Additionally, high yields of C2+ oxygenates have been produced over solids including conventional supports, but using unusual techniques like the colloidal pathway and also firm electrostatic adsorption (Table 1, entries 4 and 8). The scalability of such protocols is definitely less than that of conventional methods. Nonetheless, the recent commercialization of BASF’s colloidally manufactured alkyne hydrogenation catalyst, NanoSelect [53], demonstrates that such technologies may be utilized on a wide scale provided low-cost surfactants and reducing agents are utilized, as well as eco-friendly solvents (aqueous mediums). It is worth mentioning that the high loading of the pricey Rh (3–5wt%), as well as the unfavorable HA distribution, is this catalyst family’s primary limitations and preclude its widespread application. In fact, even under optimal reaction conditions (P ¼0.1–3 (MpPa) (typically 2 (MPa)), T ¼ 523–593 (K), GHSV ¼ 8000–10,000 (h1), and H2/CO ¼ 2), usually ethanol is the predominant alcoholic product (ethanol:propanol:butanol¼22:6:2 (molar basis)), owing to the difficult improve of the inherent low CdC coupling capability of Rh [54]. Recently, different types of the single site catalysts have been presented as the most effective system in selective hydrogenation process [55]. However, although this technique efficiently reduces the metal concentration to acceptable levels, it prevents the adjacent Rh0 and Rhn+ formation and functionalities essential for the process. Moreover, the stability of catalysts based on Rh has been tested infrequently. A recent catalytic run, for example, demonstrated steady performance for 60 h on stream [56].

2.2 Mo-based catalysts Molybdenum sulfide doped with transition metals and alkali was patented for HAS for the first time in 1980 by Dow chemical (now Dow DuPont) [57]. The synthesis gas conversion activity of Mo is low, and the majority of its products are hydrocarbons. Due to this, most of the research on Mo catalysts has focused on compounds containing the ligands C, O, S, or P.

Table 1 Top 10 supported and promoted Rh-based catalysts [43–52].

Entry

Catalysta

1 2

Rh-Mn/SiO2 Rh-Mn-Li/Fe/ SiO2 Rh-Mn-Li-Fe/ SiO2 Rh-Mn/SiO2 Rh/Ce0.8Zr0.2O2 Rh/Mn-FeOMC Rh-Mn-Li/SiO2 Rh-Mn/MSN Rh-Mn-Li-Fe/ CMK-9 Rh-Mn/SiO2

3 4 5 6 7 8 9 10 a

T (K)

P (MPa)

GHSVb (h21)

H2/CO (2)

XCO (%)

SMeOHc (%)

S C2+ Oxyd (%)

SHC (%)

SCO2 (%)

Y C2+ Oxy (%)

Ref.

553 573

5.4 3

1700 10,000b

2 2

39 28

3 1

54 (54) 64 (33)

42 34

1 1

21 18

[48] [47]

573

3

10,000b

2

28

1

58 (23)

40

1

16

[44]

558 548 573

2 2.4 5

– 2400b 12,000

2 2 2

36 27 26

0 8 4

43 (14) 44 (40) 46 (35)

52 38 38

0 10 12

16 12 12

[49] [43] [50]

573 543 593

3 3 3

10,000b 6600b 12,000

2 2 2

19 20 14

2 4 4

54 (27) 47 65 (55)

42 47 22

1 2 9

10 10 9

[45] [51] [52]

543

3

4000

2

17

1

46 (20)

52

1

8

[46]

CMK, cubic-ordered mesoporous carbon; MSN, mesoporous silica nanoparticles; OMC, ordered mesoporous carbon. 1 Space velocity in cm3 g1 cat h . c MeOH: methanol. d Oxy: oxygenates. Selectivity of HA is shown in brackets. b

180 Chapter 7 Other metals, especially alkali metals (AM) and Group VIII metals, are usually added to the Mo-based catalyst to enhance its activity. These catalysts operate under a wide variety of conditions. MoP- and MoS2-based systems are typically utilized between P ¼ 5–10 (MPa) (typically 8.3 MPa), T ¼ 533 and 603 (K), molar H2/CO ¼ 1, and GHSV ¼ 3600–5000 (h1), whereas Mo2C-based catalysts operate between P ¼3–10 (MPa), T ¼493 and 633 (K), molar H2/CO¼1, and GHSV¼2000–5000 (h1). (more typically 8MPa, 573 (K), and 2000 (h1)). Based on two lately published works on MoOx, the following parameters are appropriate: P ¼2–8 (MPa), T ¼ 488–593 (K), molar H2 /CO ¼ 2, and GHSV ¼ 1800–10,000 (h1). Zaman et al. [58] examined HAS on Mo-based compounds, focusing on the role of the specific promoters, specifically metals of Group VIII. Many new researches, however, have been published since then, focusing on the effect of the carrier’s inherent features, like CNT, AC, GMC, mixed metal oxides (MMO), and also Al2O3, on the selectivity and activity, and the use of new synthetic methods, particularly microemulsion, to achieve the best performances. The aforementioned points will be covered in detail below for each of the four families. As a concise summary of this subgroup, Table 2 ranks the top 10 Mo-based catalysts based on their C2+ alcohols yield. According to this ranking, materials based on MoS2 seem to be the greatest compounds (Table 2, entries 1 and 2). Here, the formulation of these catalysts confirms that although the alkali promotion is a necessary prerequisite, but it is insufficient for the catalyst to be effective. In fact, AM increase the selectivity for whole alcohols due to act as a CO insertion site for alkyl groups formed through the CO dissociation process over MoS2, however, do not dramatically change the distribution of Anderson-Schulz-Flory (ASF), i.e., the selectivity is greater in the order methanol > ethanol > propanol, as recently demonstrated by supported K-MoS2 studies [68,69]. Additionally, they reduce the activity of the catalyst by clogging catalytic centers and are unable of suppressing CO2 production (440% selectivity). Group VIII metals were shown to be capable of increasing the selectivity of HA by altering the distribution of ASF in alcohols and compensating the activity loss owing to AM promotion by promoting the growth of the carbon chain. Extensive studies have been conducted on these compounds. Hereon, Ni and Co being the favored additives because of their high hydrogenation capabilities. In both instances, the appropriate promoter/Mo ratio was found to be 0.5, as previously expressed. Despite the previous Dow Chemical patents [70], which suggested that Co was superior than Ni in terms of the increase in selectivity and conversion over K-MoS2, new studies on different supported materials indicate the contrary tendency (Table 2, entries 1 and 7). Here, Ni was superior in terms of increasing the conversion of CO and maintaining the selectivity of CO2 in the range of 20%–25%. Nevertheless, the coexistence of an additional metal (Rh) and carbon nanotubes (CNT) as the carrier allowed K-CoMoS2 to achieve an exceptionally high selectivity of HA which is about 64% and a low CO2 selectivity which is about 8% (Table 2, entry 2). Recent studies have shown that tiny crystallites of MoS2 with few oxide layers are necessary for the synthesis of HA, while highly organized or single layer MoS2 crystallites are not efficient [71]. Furthermore, it has been mentioned that a mixed phase containing MoS2 and also dispersed

Table 2 Top 10 supported and promoted Mo-based catalysts [59–67].

a

Entry

Catalyst

T (K)

1 2 3 4 5 6 7 8 9 10

K-NiMoS2/Al2O3-mont K-CoRhMoS2/CNT K-FeMo2C NiMoS2 K-NiMoOx-Ni/CNT-h K-CNT-NiMoS2 K-CoMoS2/Al2O3 K-MoP/SiO2 K-RhMoP/SiO2 K-NiMoOx/CNT

553 603 573 583 558 593 573 548 548 538

a

P (Mpa)

GHSVb (h21)

H2/CO (2)

XCO (%)

SMeOHc (%)

S C2+ Oxyd (%)

SHC (%)

SCO2 (%)

YHA (%)

Ref.

10 8.3 8 6 8 8 5 8.2 8.2 5

2400 3600b 2000 1044 10000b 4000b 5000 3960 3960 5000b

2 1.25 1 2 1 1 1 1 2 1

62 21 73 33 15 13 9 8 18 14

16 22 4 28 17 9 4 2 0 17

34 (34) 64 (64) 16 (16) 33 (31) 46 (31) 33 (32) 31 (31) 40 (21) 37 (11) 32 (15)

30 6 33 14 27 20 32 13 18 34

20 8 48 25 10 37 34 45 44 16

21 13 12 10 5 4 3 2 2 2

[59] [60] [66] [67] [61] [62] [63] [64] [64] [65]

CNT, carbon nanotubes; CNT-h, herringbone-type carbon nanotubes; Mont, montmorillonite. 1 Space velocity in cm3 g1 cat h . c MeOH: methanol. d Oxy: oxygenates. Selectivity of HA is shown in brackets. b

182 Chapter 7 atoms are responsible for increasing the selectivity of HA in Co- and Ni-promoted systems, respectively, and this incredibly high promoter/Mo ratios favor the initiation of segregated CoSx and NiSx species, which enhance the production of HC [72–74]. In terms of synthesis approaches, hydrothermal method, microemulsion (Table 2, entry 4), and ball milling have all been shown to generate solids with more metal dispersion and smaller MoS2 particle sizes than materials made by thermal decomposition, coprecipitation, and impregnation. Whereas various supports that have been investigated over the last decade like CNT, AC, Al2O3, and also MgAl MMO, as summarized earlier, future work should focus on developing a better comprehension of the impact of CNT, since its use as a carrier (or additive) has resulted in catalysts with the greatest HA selectivities and the lowest CO2 generation (Table 2, entry 2). Nonetheless, its impact as a promoter was limited, owing to the considerable selectivity of CO2 (Table 2, entry 6). Eventually, it should be emphasized that, unlike with Rh-based catalysts, several research groups have conducted stability experiments spanning hundreds of hours. For example, Hensley et al. [75] performed an experiment with a time span of 4000-h over K-CoMoSx and discovered that the process can be proceed successfully with H2S concentration about 100ppm in the feed stream, here the catalyst deactivation was caused by surface oxidation, carbon deposition, and carbide production. This is a favorable discovery, but evokes the basic drawbacks of MoS2-based catalysts to need a detrimental acid additive in the gaseous feed. The second best form of Mo-based catalyst is promoted Mo2C (Table 2, entry 3). Alkali promoters have a similar impact on Mo2C as they do on MoS2, particularly, they shift selectivity away from HC to HA by favoring the CO insertion stage. Besides, AM was also observed to favor the conversion of methanol to ethanol and possibly HA [76]. Even if they are mild, acid centers diminish the HA selectivity by facilitating the side reactions like coke production and alcohol dehydration. Therefore, alkali cations have been added to the catalyst surface to neutralize it and prevent these competitive reactions. In terms of maximum HA yield, MoOx-based systems are comparable to Mo2C-based catalysts. Generally, these materials have received little notice. Indeed, despite the fact that the partial reduction of the oxidic phase to metallic Mo provides a considerable CO conversion level under the reaction circumstances, HA are not the major products even with the normal addition of K. The current research has focused on the use of supports and promoters to raise the amount of Mo species on the surface and in a higher oxidation state in order to improve the catalytic selectivity. It is demonstrated that Ni deposited on the oxide and decorated CNTs added to the catalyst formulation as a carrier or an addition was particularly successful in this regard (Table 2, entries 5 and 10). Furthermore, these samples are equivalent in terms of low selectivity of CO2 attained by the best MoS2-based system (10 to 16 vs 8%) while operating without the use of H2S. Here, MoP-based catalysts have surpassed Mo-based compounds in terms of efficiency (Table 2, entries 8 and 9). In comparison to the other groups, these solids have a substantially distinct product distribution. They clearly benefit by reducing the production of methanol and HC, but have an unfavorable high WGS activity and nonalcoholic oxygenates selectivity.

Mixed higher alcohols production from syngas 183

2.3 Modified FT catalysts Because of their strong selectivity for simple chains and HA (namely, C2+ OH), between all sorts of materials for HAS, study on improved FT catalysts has been the most comprehensive. The evaluation of Co-based materials for this reaction began in 1978, with the publication of several patents by the Institut Franc¸ais du Petrole (IFP). The studies on Fe-based catalysts started at around the same period. In comparison to catalysts based on Co, those based on Fe exhibit increased activity and selectivity for C2+ OH. Moreover, catalysts based on Fe have a larger WGS activity, rendering them suited for hydrogen-lean syngas containing CO2 and produced from biomass or coal [77–83]. The catalysts based on Co and Fe are commonly run at temperature range of 473–623 (K), a pressure range of 2–7 (MPa), a GHSV range equal to 3000–10,000 (h1), and a H2 to CO molar of 1–2, although the highest performance was obtained at a temperature and pressure range of 493–573 (K) and 3–5 (MPa), respectively. Except in kinetic trials, the GHSV seldom surpassed 8000 (h1) [9]. The majority of research on solids based on Co has been conducted using mixtures including Cu. Consistent with prior studies, Co and Cu were identified as the two necessary sites for HAS. Besides, the CO implementation technique for increased alcohol production described by Xu et al. [8] is generally recognized for enhanced FT catalysts. Also, HAS is considered to be a hybrid of methanol synthesis and FTS in the CO insertion procedure, or as an intermediate process between methanol synthesis and FTS. Propagation (kp) and chain initiation (kd, k1) continue in a manner similar to that of the FT reaction to create surface alkyl species (CnHz*), and also the reaction governing the termination of alkyl species controls the products generated. From acyl species on the surface ((CnHzCO*) coupled by hydrogenation (kH0 ), the termination effected by CO insertion (kCO) produces alcohols, while termination resulting from dehydrogenation or hydrogenation (kH) produces olefins and paraffins, in the same order. It is self-evident that the creation of alcohols competes with the synthesis of hydrocarbons. The CO insertion process explains why the improved FT catalysts mostly generated linear primary alcohols with a chain growth factor comparable to that of hydrocarbons. The CO inclusion technique elucidates in detail the reaction pathways that the surface species take. The precise architecture on modified FT catalysts of the dual site, on the other hand, remains unknown. Xu et al. [8] postulated that the Cu-M center (with M¼Co, Fe, and similar elements) serves as a dual location for increased alcohol generation, a hypothesis that has been accepted by a large number of studies [9,84]. CO is then dissociated over the FT element (including Fe, Co, and the like) and subsequently hydrogenated to create methylene compounds that trigger chain growth to generate alkyl compounds on the surface, whereas CO adsorbs over Cu and enters into the alkylmetal bond, oxygenating the carbon chain. In this regard, Fig. 5 demonstrates that the CO adsorbed onto Cu relocates to the FT element’s surface that follows by CO placement onto the CdM bond, and the FT element’s surface alkyl group moves to the Cu site that is followed by the inclusion of CO to produce an acyl over the surface. Alcohols are formed when the acyl groups are hydrogenated. The two approaches require the interaction of the FT and Cu components. The

184 Chapter 7 RCH2OH

Hydrocarbons H

CO

R

COR

COR C

R CO CO

Cu

H2O

O

CHx

CO

H

M

Fig. 5 Formation of alcohol on Cu-M dual sites [36].

absence of this synergistic effect would result in a reduction in the selectivity toward alcohols. As seen in Fig. 5, the synthesis of HA necessitates the transfer of species on the surface from one place to another without causing side reactions (for instance, desorption and hydrogenation). To improve the selectivity of alcohols, the FT and Cu elements ought to be in close atomic proximity to minimize side reactions. Clearly, separating the Cu and FT parts would break the CuM dual site’s synergy and result in decreased alcohol selectivity [36]. Following the discussion of these mechanistic aspects, we will shift our focus to the promoters (such as La2O3, Rh, CNT, AM, and so on) that have extensively been studied over the course of the previous decade. The principal characteristics of ternary mixtures will then be discussed. Among these, CuCoMn [85,86] outperformed (Table 3, entry 7) CuCoCe [95,96], CuCoNb [97], and CuCoMo in terms of performance [98]. The present review on supports will consider the vast variety of materials examined (i.e., CNT, AC, ZrO2, Al2O3, La2O3, MgO, OMC, SiO2, ZnO, MMO, and also carbon fibers (CF)). The most effective CoCu-based materials are Co3Cu/CNT-h, (Cu1Co2)2Al/CNT, LaFeO3 mixtures, and CoCu dispersed on graphene sheets, which yield 22%–28% of HA (Table 3, entries 2, 5, and 8). Interestingly, the CNT significantly reduced the formation of HC in the bulk solid (Cu1Co2)2Al while maintaining the high activity and low selectivities for CO2 and methanol (Table 3, entry 6). To achieve additional improvements in this sort of material, it will be necessary to further reduce HC production. Carbonaceous supports have been proven to be effective in the production of outstanding K-CoMo-based catalysts that also exhibit remarkable selectivity for either CO2 or methanol (Table 3, entries 3 and 4). K-FuCu supported onto bimodal silica and CuFeMg-MMO was selected as the best candidates after the screening of promoters (Mn, [83,99,100], Zn [83,100], K [101,102], Zr [103] and Ce [103]) and carriers (SiO2 [87,104–106], MMO [94,103], and attapulgite (ATP) [107]) (Table 3, entries 1 and 10). It should be noted that the first catalyst corresponds to the best CoCu base material in terms of HA, compensating the lower selectivity (as a result of considerable methanol and HC formation) with a higher activity [96–98].

Table 3 Top 10 promoted and supported modified FT catalysts [86–93]. Entry 1 2 3 4 5 6 7 8 9 10 a

a

Catalyst

a

K-CuFe/SB (Cu1Co2)2Al/CNT CoMo/AC K-CoMo/(Co/CNT-h) CoCu/GE-LFOa (Cu1Co2)2Al CoCuMn/AC Co3Cu/CNT-h CoCu/LaFeO3 CuFeMg-MMO

T (K)

P (Mpa)

GHSVb (h21)

H2/CO (2)

XCO (%)

SMeOHc (%)

SHA (%)

SHC (%)

SCO2 (%)

YHA (%)

Ref.

593 503 523 593 573 523 493 573 573 573

5 3 7.5 5 3 3 3 5 3 4

6000 3900b 2000 10,000b 3900b 3900b 500 7200b 3900b 2000

2 2 1 2 2 2 2 1 2 2

56 45 42 57 50 52 58 39 76 57

14 3 18 1 6 1 1 16 6 12d

49 62 65 46 51 45 39 58 29 37d

9 34 16 13 35 51 55 21 39 39

28 1 2 40 8 3 5 5 26 12

28 28 26 26 23 23 23 22 22 21

[87] [88] [94] [93] [92] [89] [86] [90] [91] [87]

CF, carbon fibers; GE-LFO, a composite of graphene sheet and LaFeO3; SB, Bi-modal silica made of silica sol and silica gel. 1 Space velocity with the units cm3 g1 cat h . c MeOH: methanol. d MeOH selectivities based on carbon and also HA yields were calculated using the assumed distribution of total alcohols. b

186 Chapter 7

2.4 Modified methanol synthesis catalysts It was observed in the 1930s that while the methanol synthesis takes place, the alkali residues in MS catalysts result in the production of oxygenates [108]. Significant research has been undertaken since then to modify the basicity of such solids in order to alter the CO conversion to shift toward HA. The suggested improved MS systems may be roughly categorized into two categories: (1) low-T catalysts based on Cu (for instance Cu/CeO2, Cu/ZrO2, Cu–ZnO–Al2O3, CudMn core-shell NPs) and (2), high-Tcatalysts based on Cr (for example CrdZn mixed oxides). The previous elements are recognized to be one of the best set-ups with the largest activity to generate branched alcohols. Nevertheless, enhancement of the catalyst by a Co-promoter permits an increase in primary alcohols, most notably ethanol [109]. In comparison to Cr-based catalysts, alcoholic products have a lower average carbon number and a relatively high concentration of ethanol and propanol. AM’s (alkali metal) effects on this catalyst class, particularly the Cu-ZnO-Al2O3 combination, have been extensively explored. To avoid sintering, low-T catalysts based on Cu have usually been run at a GHSV range of 1200–9600 (h1), a temperature range of 523–573 (K) (more specifically, 523–623 (K) for materials containing CeO2 and La2O3), a pressure range of 1–5 (MPa), and a H2 to CO molar ratio of 1–2.5. Due to the less intense coke deposition, high-T catalysts based on Cr are famous for their relatively extended catalytic lifetime. They exhibit large activities (on average 10%– 30% CO conversion) and selectivity for alcohols, particularly branched ones, with methanol and isobutanol accounting for up to 94% of the alcoholic mixture [110]. Nevertheless, it cannot be forgotten that they also create a significant quantity of HC (for example, selectivity of 35% over 5% Cs-Cr/ZnO, on a CO2-free basis) and that frequently, no detailed info regarding the selectivity toward CO2 was provided [111]. Some publications recommended Zn to be present in an excess quantity in the creation of Cr-Zn spinels and generating ZnO to serve as the catalytic center [111] for isobutanol synthesis, whereas others stated that this is a nonstoichiometric spinel phase that results in HA [92,93]. Typically, these composites are evaluated at GHSV¼3000 (h1), T ¼623 or 673 (K), P ¼7–10 (MPa), and a H2/CO molar ratio of 2.3–2.6. The focus of investigations on catalysts based on Cu has been on the integration of promoters, with a focus on the evaluation of K and Cs, the influence of reaction settings, and the impact of the AM content on the carrier’s characteristics, as outlined hereon [112,113]. With regard to high-T materials, current studies have continued to focus on fundamental issues, evaluating the catalytic behavior of Zn-Cr spinels to structural changes caused by various treatment conditions and formulations. The top 10 structures in this catalyst category were rated based on their total alcohols yield (Table 4). It is worth noting that all of the catalysts on this table are made entirely of low-T compounds based on Cu. The optimal catalyst is a Cs-Cu hybrid supported by Ce0.8Zr0.2O2 that displays high selectivity for total alcohols and relatively low selectivity for CO2 (Table 4, entry 1). Increased CO conversion was observed with the addition of Zr to ceria, but the addition of Cs dramatically reduced the methanol proportion of total alcohols (Table 4, entries 5 and 6).

Table 4 Top 10 promoted and supported modified MS catalysts [82,114–117]. Entry 1 2 3 4 5 6 7 8 9 10 a

Catalyst Cs-Cu/Ce0.8Zr0.2O2 CuMnZrO2 K-La-Cu/ZrO2 K-Cu45Zn45Al10 Cu/CeO2 Cs-Cu/CeO2 Fe0.3CuMnZnO Cu/ZnO Cs-Cu/ZnO K-Cu45Zn45Mn10

T (K) 573 573 633 573 573 573 533 573 573 593

P (Mpa) 3 8 10 4 3 3 4 3 3 4

GHSVb (h21) 2400 8000 3000 4860b 2400 2400 6000 2400 2400 4860b

H2/CO (2) 2 2 2.5 2 2 2 2 2 2 2

XCO (%) 35 37 63 22 24 22 45 17 17 12

S

Total alc.

(%) a

84 (46) 64 (5)a 34 (20)a 86 (11) 83 (13)a 81 (40)a 38 (27) 78 (5)a 78 (33)a 82 (12)

HA yields and selectivities based on carbon were computed based on the estimated distribution in the total alcohols. 1 Space velocity with the units of cm3 g1 cat h . c Both the yield and selectivity of C2+ oxygenates are listed in parentheses. b

c

SHC (%) 8 18 31 3 8 9 36 10 11 3

SCO2 (%) 6 18 27 11 8 8 25 10 9 16

Y

Total alc.

c

(%)

Ref. a

29 (16) 24 (2)a 21 (12)a 19 (2) 19 (3)a 18 (9)a 17 (12) 14 (1)a 13 (5)a 10 (1)

[114] [82] [117] [116] [114] [114] [115] [114] [114] [116]

188 Chapter 7 Besides, this latter point could be made in comparison to ZnO-supported Cu catalysts that are less active (Table 4, entries 8 and 9). The system in entry two, namely, CuMnZrO2, had comparable activity to the standout catalyst, nonetheless a worse selectivity for total alcohols (Table 4, entry 2). Additionally, its HA selectivity was pretty poor, most likely owing to the acidity imparted by Zr. The utilization of La as a promoter significantly increased the catalytic activity and HA percentage in total. Nevertheless, the selectivity for total alcohols was not particularly encouraging (Table 4, entry 3). Fe was found to be more beneficial compared to Co or Ni. Its inclusion into a CuMnZnO solid resulted in selectivity for total alcohols comparable to that of the system promoted by La (34% vs 38%) and a greater HA proportion (71%). Yet, the overall alcohols yield dropped because of the decreased catalyst activity (Table 4, item 7). K-Cu-ZnO-Al2O3 (designated as K-CuZnAl in the table of interest) is one of the earliest known, nevertheless very successful systems. The total alcohols selectivity achieved the greatest value among all of the systems under study by tweaking the Cu/Zn/Al ratio (45/45/10), while methanol was still the major product (Table 4, item 4). By substituting Mn for Al, the selectivity was little affected, but the CO conversion was decreased (Table 4, entry 10). Inhibition of HC formation was overcome in some situations, although restricting methanol production continues to be an unanswered question. The inclusion of additional promoters and an increase in the oxygen storage capability of ceria-based supports may be attractive avenues to pursue low-T materials. With regard to the improved Zn-Cr oxides, the primary objective would be to enhance the nonstoichiometric Zn-Cr spinel percentage, which may be accomplished by the use of alternate synthesis techniques, reaction settings, or promoters. In terms of the first feature, the newly evaluated preparation techniques include a significant number of factors that may be modified to achieve more control over the catalyst’s characteristics. They are, however, highly sophisticated and need the use of costly accessories or reaction precursors. This necessitates the development of more simple, economical, but efficient methods.

3. Process configurations The findings of the various thermodynamic and kinetic analyses serve as the foundation for general considerations in the development of commercial HA manufacturing systems. Low reaction temperatures have been proven to be more suitable for selectivity toward methanol generation as opposed to HA. In general, weight ratios of 30:70 to 50:50 for HA/methanol reflect the industrial goal. Thus, when utilizing low-T catalysts based on Cu for the production of methanol, it may be required to consider not just operating with substoichiometric H2/CO ratios but also processes that incorporate methanol recycling. This prevents the creation of methanol because of the reverse methanol synthesis process. By employing alkali-enhanced high-T catalysts, recycling of methanol is unnecessary, and a product in the liquid phase with a larger HA to methanol ratios is achieved [118].

Mixed higher alcohols production from syngas 189 According to the data and analysis presented in many studies, the input feed to the reactor ought to be free from CO2 and have an H2/CO ratio less than 1:1 to maximize the formation of C2+ oxygenates. However, determining an economically optimal feed ratio necessitates an integrated study of the process, which includes both the syngas generation and the liquid product separation phases. The existing technology of natural gas-based syngas generation limits the viability of extremely substoichiometric H2/CO ratios. Indeed, it appears that syngas with an H2 to CO ratio of less than 1:1 cannot be manufactured on a large scale from this raw material. Coal and heavy residue gasifiers, on the other hand, may provide such ratios. In fact, because of the low H2/CO syngas required, HAS is well tailored to other sources different from natural gas, such as coal. Because large productions of HA are always coupled with large productions of CO2, removing CO2 from the synthesis loop necessitates the installation of a separation unit directly in the synthesis loop, which may raise the dilemma of how to efficiently make use of the CO2 extracted. CO2 sequestration and subsequent recycling up to the point of syngas manufacture are included in many of the procedures proposed thus far. This approach provides for the partial reuse of CO2 extracted from the reactor input stream, as well as the adjustment of the ratio of H2 to CO of syngas to the required low-value levels. It should be remembered that input CO2 content directly impacts the H2O present in the process output through the WGS reaction, with effects on the product splitting, and that the exclusion of CO renders the reaction more exothermic overally, since it speeds up the rate of creation of HA and also since the direct WGS reaction produces heat. A block schematic of a representative design for the integrated process of interest for the HAS is shown in Fig. 6. Syngas is produced by partial oxidation of natural gas at high temperatures, and the removal unit for CO2 is placed further into the product stream of the reactor. To maximize the overall carbon output of the process, the CO2 expelled from the CO2 removal unit alongside much of the H2-rich purge gases of the reaction section are returned to the partial oxidation stage to favorably modify the H2/CO ratio. However, a large quantity of CO2 may be released. To avoid this, the Snamprogetti method features CO2 recirculation to the reactor intake, allowing the HAS to run basically at conditions resulting in negligible net CO2 generation due to the WGS reaction’s equilibrium [76,120]. While this solution significantly increases the process’s C consumption, it also significantly reduces the rate of synthesis of HA, resulting in a larger H2O concentration in the final mixture. The separation section’s design is mostly determined by the water present in the unpurified liquid product stream. When the alcohol level is high, it may be dehydrated using the usual azeotropic distillation procedure. Methanol is first distilled in a column from the water/C2+ alcohol combination. This combination is then dehydrated in two azeotropic columns, with benzene serving as a water entrainer while the water itself and the heavier alcohols serving as the column’s bottom products. DOW has introduced an alternate separation process that is ideal

190 Chapter 7

Natural Gas Oxygen

OPT. CO2 Purge

O2 Plant

Syngas Production (Partial Oxidation)

Recycled CO2

Recycle Compressor Recycled Reactor Purge

Alcohols Synthesis

CO2 Removal Unit

Recycled Syngas

Vapor/Liquid Separation

Reactor Purge

Purge (fuel-gas)

Raw Alcohol

Alcohol Fractionation

Alcohol Mixture

Fig. 6 Integrated IFP process for producing alcohol mixtures [119].

for drying final liquid products with low water content. The DOW process utilizes system of zeolite units with a low-energy requirement to remove the majority of the residual water, resulting in a finished product stream with as little as 0.2% of water [121].

4. Reactor configurations Apart from the search for high-performance catalysts, reactor and process technology improvement is a major factor in achieving greater yields and improved selectivity for HA while minimizing the fraction of unfavorable by-products in the synthesis of mixed alcohols.

Mixed higher alcohols production from syngas 191 It should be kept in mind that the classic gas phase and fixed-bed tubular reactor’s greater alcohol production and selectivity are generally poor. The double-bed reactor was then developed to optimize methanol production at a lower reaction temperature over the first catalyst bed and from syngas, as well as enhance the CdC bond formation and the further synthesis of HA. When comparing the dual bed Cs/Cu/ZnO/Cr2O3jjCs/ZnO/Cr2O3 catalyst to the single-bed Cs/Cu/ZnO/Cr2O3 catalyst in the synthesis of isobutanol, it has been observed that the efficiency was more than doubled [122,123]. Because of the superior stability of branched alcohols compared to linear alcohols, double-bed reactors are now mostly employed for their manufacture. Mixed alcohol synthesis is a hydrogenation reaction that converts carbon monoxide to products that mostly consist of mixed C1–C6 alcohols and often paraffin hydrocarbons with a varying length of the carbon chain. The reaction of interest is highly exothermic, resulting in the formation of hotspots that facilitate the deposition of carbon and catalyst sintering. As a result, for the development of appropriate reactors, quick removal of reaction heat is also a crucial concern. Because of its improved heat transmission, mass transfer, and the ability to replace the catalyst online, the slurry-bed reactor has been adopted for alcohol synthesis. The use of a slurry-bed reactor for alcohol synthesis can reduce CO2 output and byproduct generation. For the synthesis of both methanol and HA, the supercritical synthesis procedure appeared to overcome most of the difficulties usually encountered. The equilibrium condition for methanol synthesis may move beyond its thermodynamic limit if a suitable supercritical solvent is used, and selectivity for HA during the synthesis is likely to increase by employing this method [124].

5. Conclusion and future outlook The synthesis of mixed alcohols, being one of the alternate ways to use petroleum resources, is gaining traction as a result of the rising need for liquid fuels and dwindling oil sources. This section provided an overview of the methods and procedures used to create the four major types of catalysts, namely Mo-based, Rh-based, modified FT, and enhanced methanol synthesis catalysts to maximize the yield and selectivity of HA while minimizing the ratio of by-products in the synthesis of mixed alcohols. Among the different routes mentioned for HAS, direct synthesis of HA from syngas is especially intriguing as it is a one-step process that may be carried out using gas feedstocks either from renewable sources or uncommonly from natural gas. Extensive theoretical and experimental research have also been carried out in the last 10 years on enhanced Rh-based catalysts for methanol synthesis, but they have not recognized the correct strategy to fix the innate inefficiencies of these mixtures. Particularly, the old solids retained an abnormally high selectivity for acetaldehyde and ethanol. Moreover, the Rh content could not be reduced further to compensate for the high metal costs without jeopardizing efficiency. With regard to the second catalyst class, despite some notable breakthroughs in

192 Chapter 7 decreasing the selectivity to methanol and CO2, further progress toward industrial scale is nearly unattainable. Thus, the future scientific endeavor should be focused on the two remaining catalyst types. Besides, Mo-based catalysts revealed a strong affinity for HA, and many studies indicated the potential of significantly suppressing their generally high CO2 selectivity. Certain performance characteristics have been established in MoS2-based materials, but the actual nature of the active spots remains unknown. Numerous theoretical investigations have detailed the CO and H2 dissociation and adsorption phenomena in detail, but only a few insights into the reaction mechanism have been acquired thus far. Closure of these gaps is required for the development of improved Mo-based materials. In this regard, future research should take into account the dynamic character of MoS2 and Mo2C under reaction settings. All in all, in the previous decade, great progress has been made in the field of modified FT synthesis catalysts. Effectively, the best members of this catalyst group today represent the most viable elements. They exhibit remarkable selectivity and activity, significantly reducing the generation of all C1 species and regulating the production of hydrocarbons, which was recognized to be their primary obstacle thus far. Molecular-level investigations have focused on CoCu-based systems, revealing information on the functions of these two metals. Acquiring additional understanding about their mechanism will aid in further reducing their potential to generate hydrocarbons. When feasible, the most significant mechanistic findings from methanol synthesis and FT have been related to the synthesis of higher alcohol. The study on these two advanced technologies has shown the critical need of combining theoretical and experimental techniques when attempting to gain a basic understanding. Even though many attempts have been undertaken to speed up the development of mixed alcohol synthesis technologies, there are still certain unknowns and risks in this practice. First, there is a tradeoff between selectivity and conversion in most HAS catalysts, resulting in poor HA yields. Second, owing to the deactivation issue caused by metal sintering or phase separation, HAS catalysts have a limited lifespan. Thirdly, producing HA is energy intensive and requires high temperatures and pressures, resulting in high production costs. Finally, despite the intricacy of the reaction pathways and the constraints of characterization approaches, the nature of the active centers remains unclear. However, in the commercial manufacture of mixed alcohols, the most important goal is to attain better C2+ alcohol selectivity with equivalent alcohol output compared to the current levels. Aside from a thorough insight into the mechanisms underlying the synthesis of mixed alcohols, future research must continue to focus on the development of high-performance catalysts and integrated process technologies in order to meet the economic feasibility and market needs for oxygenated fuel sources.

Mixed higher alcohols production from syngas 193

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CHAPTER 8

Acetic acid and co-chemicals production from syngas Waqar Ahmad, Nooshin Asadi, Prakash Aryal, Swarit Dwivedi, Ashwin Hatwar, and Akshat Tanksale Department of Chemical and Biological Engineering, Monash University, Clayton, VIC, Australia

1. Introduction Acetic acid (AA), also called ethanoic acid (CH3COOH), is an important colorless and corrosive organic acid that has a strong pungent odor and burning sour taste [1]. It is also known glacial AA that contains Fe. Nickel also presents CO hydrogenation activity, despite the H2 chemisorption on Ni is much stronger than CO dissociation, leading to a great methane formation due to the high H/C ratio created on the catalyst surface. Oppositely, Ru leads to the lowest selectivity toward CH4 and the greatest production of long-chain hydrocarbons. On the other side, Ru is not used as the metallic phase for industrial catalysts because of its expensive cost, which makes both Fe- and Co-based catalysts the materials applied to the F-T units [7,37]. The Co is more selective toward long-chain paraffins than Fe, making it suitable for the production of both diesel and aviation fuel, and it also deactivates less than Fe. The Fe-based catalysts are less expensive than Co, despite being less active for the hydrogenation. Besides that, they show greater operation flexibility than Co, being possible to operate with Fe-based

Fuel gas from syngas 249 catalysts in HTFT with no increment of CH4 selectivity rather than C+2 products, as observed with Co-based catalysts [7,37]. Hence, Fe is useful to produce light olefins and naphtha under HTFT operation condition [58]. Moreover, Fe is also active toward the water-gas shift (WGS) reaction (Eq. 13), which make these types of catalysts less dependent on the H2/CO ratio in the feed stream and useful to convert less H2-enriched streams, as commented in Section 2.1 [7]. The F-T synthesis can be interpreted as a polymerization reaction, and its first step is the formation of the monomers, that are polymerized to hydrocarbons with a wide range of carbons chain distribution [7,56]. The reactions that take place in a F-T reactor are the alkanes or paraffins and alkenes or olefins formation (represented by Eq. 20 and Eq. 21, respectively), and in a less extent, the formation of alcohols and carbonyls compounds (represented by Eq. 22 and Eq. 23, in this order) [56]. ð2n + 1ÞH2 + nCO ! Cn H2n+2 + nH2 O

(20)

2nH2 + nCO ! Cn H2n + nH2 O

(21)

2nH2 + nCO ! Cn H2n+1 OH + ðn  1ÞH2 O

(22)

2nH2 + ð2n  1ÞCO ! Cn H2n O + ðn  1ÞH2 O

(23)

The carbide mechanism is the most accepted to explain the F-T mechanism, despite it does not explain the formation of some oxygenates products, as pointed by Eqs. (22) and (23). In this case, a CO insertion mechanism in the chain growth step might explain the production of these oxygen-containing molecules [56]. The initiation step in carbide mechanism consists of the CO dissociation, producing the M]C (carbide) and M]O species (Eq. 24), where M corresponds to the active site; the H2 dissociation on the metallic sites generates atomic H adsorbed species (Eq. 25), that will react with both M]C and M]O species, producing CHx (x ¼0–3) intermediates (Eq. 26) and H2O molecules (Eq. 27), respectively [7,56]. CO + 2M ! M〓C + M〓O

(24)

H2 + 2M ! 2M▬H

(25)

xM▬H + M〓C ! CHx ðMÞ▬+M

(26)

2M▬H + M〓O ! H2 O + 3M

(27)

The CO dissociation assisted by the adsorbed atomic hydrogen is another initiation step mechanism that is also accepted (Eq.28) [56]: M▬H + M▬CO ! CHðMÞ▬+M〓O

(28)

250 Chapter 10 The chain growth or propagation proceeds throughout the CdC coupling. Fischer and Tropsch original mechanism proposed that the CdC coupling occurs by the reaction between the growing hydrocarbon chain with the adsorbed CH2 (methylene) species (Eq. 29). Nowadays, it is accepted that several CHx species (x ¼ 0–3) contribute to the CdC coupling [7,56]: CH2 ðMÞ▬+CH2 ðMÞ▬ ! CH3 CHðMÞ▬+M…ðnÞ ! CH3 ðCH2 Þn2 CHðMÞ▬+M

(29)

Finally, the termination steps determine whether paraffins or olefins are produced. This depends on the catalyst and/or the reaction temperature [58,59]. The former products are obtained through the reaction between the adsorbed alkyl group with the adsorbed atomic hydrogen (Eq. 30) or with the adsorbed methyl species (Eq. 31). Olefins are produced by the elimination of a hydrogen atom linked to a carbon in the beta position (Eq. 32) [56]: CH3 ðCH2 Þn2 CHðMÞ▬  M▬CH3 ! Cn H2n+2 + 2M

(30)

CH3 ðCH2 Þn2 CH2 ðMÞ▬  M▬H ! Cn H2n+2 + 2M

(31)

CH3 ðCH2 Þn2 CH2 ðMÞ▬+M ! Cn H2n + M▬H + M

(32)

The product distribution of the F-T reaction can be predicted by statistical models since it consists of a polymerization based on the CdC coupling, which governs the chain growth. The Anderson-Schulz-Flory (ASF) model is commonly used to predict the hydrocarbons distribution. In this case, the molar fraction of a hydrocarbon with “n” atoms of carbon (Mn) is related to the chain growth probability (α) according to the following expression (Eq. 33) [7]: Mn ¼ ð1  αÞαn1

(33)

As α expresses the ratio between the rates of the chain growth and of the chain termination, it. assumes values from 0 to 1. When α value tends to zero, only CH4 is expected to be produced. As α approaches to unity, more long-chain hydrocarbons are expected [37]. The values of α also depends on the metal active phase and on the operating conditions [60]. Methane is the main product obtained when the CO hydrogenation is carried out with the Ni-based catalysts, which feature low α values. As for Co, which presents high values of α, hydrocarbons with high molecular weight, as waxes, are obtained. Intermediate values of α are ascribed to Fe-based catalysts, giving a wider product distribution among the three metals [37]. The effects of the chain growth probability (α) on the products distribution in F-T process are represented in Fig. 6. Mousavi et al. [61] proposed a generalized equation describing the kinetic rate of the F-T synthesis, considering that the chain propagation occurs due to the participation of the CH+H monomers rather than the CHx, as suggests the carbide mechanism. According to them, this mechanism should describe better a generalized equation that is valid for both Fe- and Co-based catalysts.

Fuel gas from syngas 251 1.0 0.9

CH4

0.8

Weight fraction

0.7

Wax (C35-C120)

0.6

Gasoline (C5-C11)

0.5 0.4 0.3

Diesel (C9-C25)

C2

0.2

C3 0.1 0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

a 100%

Weight distribution

C31-C60 Hardwax

80%

C11 -C12 C8-C11 Heavy pet

C13-C20 Diesel

C21-C30 Softwax

60% C5-C7 Light pet

40% C3-C4

20% C1-C2 SNG

0.7

0.75

0.8

a

0.85

0.9

0.95

Fig. 6 ASF products distribution as a function of α. Reproduced with permission from M. Martı´n, I.E. Grossmann, Process optimization of FT-diesel production from lignocellulosic switchgrass, Ind. Eng. Chem. Res. 50 (2011) 13485–13499. All rights reserved to ACS Publications.

As the rate of the CO consumption corresponds to the hydrocarbons formation added to the production of CO2 by means of WGS reaction, the former can be represented according to Eq. (34). Moreover, the rate of F-T synthesis corresponds to the rate of hydrocarbon production, which is equivalent to the rate of the CO consumption in both initiation and propagation steps (Eq. 35). The termination is not considered because CO does not participate in this step. Once it

252 Chapter 10 was considered that both chain initiation and growth depend on the CH+H insertion, the rate of F-T synthesis is equivalent to the rate of CH + H insertion, as represented by Eq. (36). rCO ¼ rHC + rCO2

(34)

r FT ¼ rHC ¼ r initiation + rpropagation

(35)

r FT ¼ rCHH

(36)

Thus, a generalized equation rate was derived (Eq. 37) [61]: ! 3=4 PCO PH2 rFT ¼ A  ð1 + kb PCO Þ2

(37)

Previous works of Shiva et al. [62] presented kinetic models for methane, paraffins, and olefins formation during F-T synthesis, developed according to Langmuir-HinshelwoodHougen-Watson (LHHW) approach. In this case, a H-assisted CO dissociation mechanism, according to Eq. (28), was considered due its potential role in the F-T mechanism for both Fe and Co catalysts. In this model, rate-determining steps are represented by Eqs. (38)–(40): CH2 ∗+CH2 ∗ ! C2 H4 ∗…Cn1 H2n2 + CH2 ∗ ! Cn H2n ∗…Cn H2n ∗+CH2 ∗ ! Cn H2n CH2 ∗+∗ (38) CH3 ∗+H∗ ! CH4 + ∗…Cn H2n+1 ∗+HS∗ ! Cn H2n+2 + 2∗

(39)

Cn H2n ∗ ! Cn H2n + ∗

(40)

Thus, the discriminate hydrocarbons formation rates are presented, according to LLHW mechanism [62]:   k01 eE1=RT PCO P3H2 =PH2O (41) rCH4 ¼ ð1 + KPCO Þ2   E2 P P2 k02 eRT PCOH2OH2 αð1 + αÞ rolefin ¼ (42) 1 + KPCO   E3 P P3 k03 eRT PCOH2OH2 αð1 + αÞ rparaffin ¼ (43) ð1 + KPCO Þ2 where α is the chain growth probability, described by Eq. (44) [62]: α¼

1+

1 aP1 H2 PCO PH2O

1 1 2 + bP2 H2 PCO PH2O + cPH2 PH2O

(44)

Fuel gas from syngas 253 The determination of a kinetic model that describes the F-T synthesis assumes a great complexity because the development of a model that precisely describes the wide range distribution of the hydrocarbon products along the polymerization is a very difficult task. According to M
endez and Ancheyta [63], a model that fully describes the kinetics of the products distribution considering all the parameters regarding to the formation of intermediary species and initiators that contribute for the chain growth of hydrocarbon is still required. In general, increasing the temperature of the F-T reaction favors the desorption of the alkyl species, leading to the formation of short-chain hydrocarbons, as consequence of the reduction of the contact time between the growing hydrocarbon chain and the catalytic sites; in other words, increasing the reaction temperature decreases the chain growth probability (α). Similar effect is observed as the H2/CO ratio in the feed stream increases: the H/C ratios on the surface of the catalyst increases, accelerating the chain termination reaction, as represented by Eq. (30). As for the total pressure of the reagents, it does not influence on the α values, despite of the fact that high pressures (>10 bar) are required to increase the hydrocarbons yields [60]. Usually, the liquid hydrocarbons (i.e., the C5+ fraction) are the desired products in the F-T process. The presence of methane and light compounds (C2–C4) in the syncrude implicates a loss of selectivity toward the C5+ fraction. van Steen et al. [64] compared the performances of both Co/ TiO2 and Fe/Cu/K/SiO2 catalysts. The former presented high selectivity toward the C5+ products (90%), with CO conversions ranging from 30% to 70%, and then decreased to 83%, when the CO conversion achieved 93%, followed by an increment of the selectivity toward methane (6%, with CO conversions lower than 60%, to 10% with CO conversion values superior to 90%). Similar trend was observed for the Fe-based catalyst; however, the selectivity toward methane was lower and that toward C2–C4 products was higher than the Co/TiO2 catalyst along the all the CO conversions ranges. In fact, Fe-based catalysts show greater flexibility than Co-based catalysts, despite its lowest activity and lifetime, as discussed previously. These last properties can be improved for the iron catalysts with the addition of alkali metals, as K [59]. The addition of such alkali metal increases the strength of the CO adsorption on the iron sites, creating conditions in the catalysts surface to produce olefins rather than paraffins, making this type of catalyst widely employed to the Sasol-Synthol process (340°C) [59,65]. Moreover, K also increments the formation of iron carbides, which are responsible for the hydrocarbon chain propagation [65]. At lower temperatures (LTFT: 200–240°C), Fe-based catalysts promoted with potassium are also useful to obtain liquid paraffinic hydrocarbons, as the Co-based materials [58]. As for the paraffins production in the C2–C4 pool to increment the quality of both the SN and SNG, the situation is the opposite, and the C5+ yields must be minimized. Besides, the olefins concentration in the light fraction must also be minimized in order to achieve high paraffin/ olefin ratios, since olefins feature lower heat power than the paraffins, as commented previously [5].

254 Chapter 10 The next section will provide a more detailed discussion regarding the catalysts used for the F-T reaction aiming to increase the production of the fuel gas.

3.2 Production of fuel gases through F-T reactions: Catalysts and operating conditions The evaluation of the role of each Co-Mn-Ru/Al2O3 catalyst component on the production of SNG with high calorific power was proposed by Lee et al. [66]. In this case, the MnO acted as a Lewis acid site, allowing the hydrocarbon chain growth, as the hydrogen spillover character ascribed to Ru ensured the increment of CO conversion even at temperature as lower as 250°C, once more Co0 could be formed during the catalyst activation step. Ru also enhanced the H2 dissociative adsorption, which created a hydrogenation environment that promoted the desorption of short-chain hydrocarbon. As a consequence, the yields of paraffins in the C2–C4 range could be incremented. Iron-based catalysts can also be considered as good candidates to produce light hydrocarbons fraction, especially in the HTFT condition, as previously discussed. However, as iron hydrogenation ability is lower than cobalt, more olefins are produced than paraffins, which is an undesired picture in the case of the F-T process focusing in obtaining C2–C4 paraffins. To make the Fe-based catalysts suitable for such purpose, the promotion with metals like Zn or Cu, that increase H2 adsorption property of the catalyst, is considered as a solution: Zn is responsible for both increasing the iron activity and selectivity toward paraffins due to its H2-spillover; as for Cu, it increases the iron oxides reducibility, promotes the formation of paraffins and diminishes the selectivity toward C5+ products, which presence hinders the fuel gas transportations, as they are easily condensed [67,68]. Lee et al. [67] also reported that the pretreatment conditions during the activation step influence the paraffin/olefin (P/O) ratios: the magnetite (Fe3O4) was the main phase that constituted the Fe-Zn catalysts after they were activated with H2 (400°C, 1h), while the activation with syngas (H2:CO¼3:1, 500°C, 1h), known as carburization, led to the formation of more iron carbides (FexCy) species. The CO adsorption is stronger on FexCy compared to the adsorption on the magnetite, and the H2 adsorption occurs much easier on the partially reduced iron oxides. Hence, the P/O ratios in the C2–C4 fraction were higher using the catalyst activated under H2 flow compared to the P/O ratios with the catalysts that were carburized. The temperatures of Fe-Cu catalysts carburization also affect the composition of the activated catalyst: as the temperature was increased from 300°C to 500°C, the catalysts composition changed from Fe3O4 to FexCy, which caused the P/O ratio to diminish from 2.0 to 1.6, contributing to increase the selectivity toward the C5+ liquid products [68]. The combination of both Co and Fe in a bimetallic Al2O3-supported catalyst (5% Co-15% Fe, % wt) showed some positive effects regarding the production of the light paraffins, as the combination of these metals diminished slightly the selectivity toward methane and increased

Fuel gas from syngas 255 the selectivity toward C2–C4 products compared to the monometallic 20% Co/Al2O3 catalyst. Besides, the P/O ratio in the C2–C4 fraction was higher with the bimetallic catalyst compared to the monometallic 20% Fe/Al2O3 catalyst because cobalt increased the reducibility of iron, and it also increased the hydrogenation ability by promoting the H2 dissociation on the catalyst surface, allowing higher paraffins production [48]. The operating conditions, as temperature, H2/CO ratios in the feed stream, pressure, and space velocity, also influence the production of fuel gases through F-T reaction mechanism, besides the catalysts and their pretreatment (activation) protocols, as previous discussed. An optimum condition was found by Jo et al. [48] to produce light hydrocarbons with a high volumetric concentration of paraffins (98%) in the C2–C4 fraction, using H2/CO ¼ 3, 10 bar, 1 300°C, and space velocity of 6000mLg1 cat h , with a 5%Co-15%Fe/γ-Al2O3 catalyst. Besides the high selectivity toward paraffins (27.6%) with these reaction conditions, they also obtained great conversions of CO (91.5%), and acceptable selectivity toward the undesired products, as CO2 (22.5%) and C5+ liquids (26%). Increasing the temperature (250–400°C) and H2/CO ratios in the feed stream (1–3), have increased the CO conversions, as well as the production of both CH4 and light paraffins, while the selectivity toward C5+ molecules decreased because both higher temperatures and H2/CO ratios accelerate the desorption rates of the short-chain hydrocarbons, as commented previously. Because of the iron activity toward the WGS reaction (Eq. 13), which allows an in situ adjustment of both C and H concentrations on the catalyst surface, the concentration of paraffins in the C2–C4 products was little affected by changing the H2/CO ratio from 2 to 3 at temperature superior to 300°C. Posterior studies performed at 10bar 1 and H2/CO¼3 also showed that increasing the space velocity (from 4000 to 8000mLg1 cat h ) increased the methane production at the expense of the C2–C4 formation, which was not a positive result considering as target the production of a fuel gas with high calorific power [5]. Higher selectivity toward the methane is in fact expected as the space velocity increases in the F-T reaction, as the contact time between the reagents and active sites is shortened, hindering the chain growth. Increasing the pressure from 10 to 20 bar allowed higher CH4 production, probably because the direct CO2 (produced through WGS reaction) hydrogenation to methane was favored under higher pressures. As it was noticed that the pressure does not change the selectivity toward the C2–C4 and C5+ products; it is interesting to select a condition in which both C5+ and CO2 production are minimized, as CO2 also contributes to diminish the calorific power of the fuel gas mixture, associated to great CO conversions (close to 99%) (Fig. 7). Thus, 1 lower space velocity (4000 mL g1 cat h ), higher pressure (20 bar) and higher temperature (350°C) were the parameters considered for a suitable production of a fuel gas with high paraffins content (90%) in the C2–C4 fraction, whose selectivity was about 36% [5]. Double-layered catalytic beds are also considered as alternative pathways to increase the production of the light hydrocarbons from the syngas. In this case, a cracking catalyst that is commonly used for the hydroprocessing of the syncrude can be located right below the F-T

256 Chapter 10

Fig. 7 Yields of CH4 (A), C2–C4 (B), C5+ (C), and CO2 (D) with 5%Co-15%Fe/γ-Al2O3 catalyst, under different space velocities as function of temperature and pressure. Reproduced with permission from S.B. Jo, T.Y. Kim, C.H. Lee, J.H. Woo, H.J. Chae, S.-H. Kang, J.W. Kim, S.C. Lee, J.C. Kim, Selective CO hydrogenation over bimetallic Co-Fe catalysts for the production of light paraffin hydrocarbons (C2–C4): effect of space velocity, reaction pressure and temperature, Catalysts 9 (2019) 779. This figure is licensed under a Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/legalcode).

catalyst. This cracking material is responsible to transform the C5+ produced through the F-T mechanism to light molecules (C2–C4) and CH4, thus increasing the fuel gas yields, as the hydrocarbon formation is limited by a statical distribution (ASF model). In a typical F-T unit, the F-T reaction, and the hydroprocessing steps occur in separate reactors. The employment of such configuration aims to combine these stages in a single reactor, and it also allows a selective production of the desired specific fraction of the hydrocarbons, i.e., the C2–C4 fractions, avoiding the limitation of the ASF distribution [69,70]. Jo et al. [51] worked with such

Fuel gas from syngas 257 F-T Catalyst:5%Co-15%Fe/Al2O3 Quartz wool Cracking catalyst 1: SAPO34 Cracking catalyst 2: 10%Ni/Al2O3

H2/CO 3/1

XCO = 91.0% H2/CO

SCH4 = 27.5%

3/1

SC2-C4 = 31.2%

XCO = 91.5%

SC5+ = 22.7%

SCH4 = 23.5%

SCO2 = 19.3%

SC2-C4 = 28.5% SC5+ = 26.0% SCO2 = 22.3%

XCO = 100% H2/CO

SCH4 = 59.1%

3/1

SC2-C4 = 25.6% SC5+ = 8.4%

Fig. 8 Catalytic performances for fuel gas production from syngas using a F-T catalyst and double-layered catalytic beds (hybrid system), according to the results shown in the work of Jo et al. [51]. From Made by the authors.

configuration, using a SAPO34 zeolite or a Ni/Al2O3 as the cracking catalyst and the 5% Co-15%Fe/Al2O3 as the F-T catalyst (Fig. 8); the acid sites of both zeolite and alumina are responsible to convert the high weight hydrocarbons to low weigh products, as nickel methanation activity converts the unreacted CO and the CO2 produced through WGS to CH4, contributing to diminish the selectivity toward CO2. The SAPO34 alone did not convert the syngas, as the Ni/Al2O3 alone converted the CO (98% of conversion) to CH4 and CO2 (90.2% and 9.8% of selectivity, respectively). Double catalytic beds were also studied by Corsaro et al. [69], using a HZSM-5 zeolite right below a commercial F-T catalyst. They verified that changing the temperature of the catalytic bed ascribed to the zeolite from 250°C to 350°C and keeping the temperature of the F-T catalytic bed in 190°C increased the selectivity toward the LPG production from values below 10% to near 20%, as the selectivity of the liquid products diminished from 65% to 47% due to the cracking of the C6+ hydrocarbons as the temperature increased.

3.3 Fuel gas production from syngas through non-Fischer-Tropsch mechanisms To control the selectivity toward the production of the light paraffins and even break the ASF hydrocarbons distribution limitations as a result of the polymerization mechanism of the F-T synthesis, hybrid catalysts that comprise a methanol synthesis catalyst and a zeolite have been studied [6,53]. Besides the possibility of obtaining a narrower hydrocarbon distribution in the desired range of light hydrocarbons, a syngas with H2/CO ratios near 2 can be used,

258 Chapter 10 and the H2 + CO mixture can be produced by the combination of the methane reforming processes, as discussed in the Section 2.1 [53]. With this type of hybrid catalyst, the syngas is transformed to hydrocarbons through the follow steps: (1) syngas transformation to methanol (Eq. 13); (2) methanol dehydration to DME (Eq. 14); (3) DME dehydration to light olefins (DME ➔ olefins + H2O); and (4) light olefins hydrogenation to the light paraffins (olefins + H2 ➔ paraffins). The step (1) occurs on the methanol catalyst; steps (2) and (3), on the cracking material/zeolite, and step (4), on the metallic phase supported on the zeolite. WGS reaction (Eq. 13) is also a parallel equilibrium reaction that also take place [52,53]. The construction of a multifunctional catalyst that is capable to perform all the steps previously described in a single reactor is not simple, and the main difficulty is to control the active sites distribution and the contact between the constituents of the catalyst that are responsible for each reaction step [52]. Besides, coke deposition on the cracking catalyst is one of the main causes of deactivation [53]. The presence of H2O, from the dehydration reactions, and CO2, formed through WGS, also contributes for the deactivation of the catalyst responsible for the methanol synthesis [71]. One of the simplest ways to produce these hybrid catalysts is through physical mixture, employing the methods of powder or granule mixture. For the former method, the methanol catalyst and the zeolite with the required weights are mixed, pelletized, and then crushed and sieved to a desired particle size range. As for the granule mixture, each catalyst (methanol catalyst and zeolite) is shaped separately into the same particle size range and then mixed with the required weights to constitute the hybrid catalyst [53,71]. The synergetic effect between the methanol synthesis catalyst and the zeolite promotes the CO conversion to methanol, as it is dehydrated to DME by the zeolite material. Thus, the thermodynamic barriers ascribed to the methanol production can even be surpassed in the presence of the hybrid catalyst [71]. Besides the method employed to the physical mixture for the preparation of the hybrid catalyst, the operating conditions, as temperature and pressure, also influences on the performance, as shown by Xiangang et al. [53]: CO conversion reaches a maximum at 265°C (55%) when the temperature increased from 200°C to 370°C at 21bar; at 300°C, the CO conversion decreased to 46%, and no DME was observed, due to its complete dehydration to the hydrocarbons. Further increments in the temperature (>300°C) compromise the hydrocarbons production, once both methanol and DME dehydration are exothermic reactions, being suppressed at higher temperatures. As for the pressure, moderate values (30–40bar) are recommended in order to keep a high CO conversion and to avoid the production of long-chain hydrocarbons (C5+). One of the challenges ascribed to the hybrid catalysts is to control the interfaces of the active sites, which are responsible for all the reactions, as commented previously. The preparation of a

Fuel gas from syngas 259 core-shell hybrid catalyst (Fig. 9), where the core material is the methanol catalyst Cu-Zn/ Al2O3, and the shell is a beta-zeolite, was evaluated by Lu et al. [6], and the results are shown in Table 2. In this case, a previous synthesized Cu-Zn/Al2O3 catalyst was added to the mother liquid during the preparation of the beta in order to obtain a ratio of methanol catalyst to zeolite weight of 10:1 in the final catalyst, and then the hydrothermal step was carried out. This synthesis method creates a confinement effect of the methanol catalyst (core), which is encapsulated by the zeolite (shell). Compared to the physical mixture catalyst, this core-shell material was less selective toward the CO2 because the well-organized structure allows an efficient H2O separation, produced from the methanol/DME dehydration over the acidic sites of the zeolites, by avoiding its contact with the CO molecules, thus minimizing the occurrence of WGS reaction. Moreover, the light olefins produced from the DME dehydration can be hydrogenated to the light paraffins in the Cu sites at the interface of the core containing catalyst and the zeolite (Fig. 10), which eliminates the need of noble metals, as Pd, supported on the cracking materials. Thus, the selectivity toward C3d-C4d paraffins increased from 19.2% for the physical mixture catalyst to 37.3% with the core-shell material, as the CO conversion also increased.

3.4 Production of fuel gas through F-T or non-F-T mechanism From the reported in the previous sections, it is not possible to choose one path over the other, as both the mechanisms feature advantages and disadvantages. In general, the production of the fuel gas from F-T mechanism can be performed with the two most used active metals that are employed in the F-T units, that is, Fe and Co, that are of low cost, especially the former. Once Fe features lower chain growth probabilities than Co, it can even be more convenient to obtain the short-chain hydrocarbons. The employment of Fe-Co as bimetallic catalysts also seems to be a successful pathway to produce C2–C4 fraction with high P/O ratios, which is

Fig. 9 Scheme of a core-shell catalysts. Reproduced with permission from P. Lu, J. Sun, D. Shen, R. Yang, C. Xing, C. Lu, N. Tsubaki, S. Shan, Direct syngas conversion to liquefied petroleum gas: importance of a multifunctional metalzeolite interface, Appl. Energy 209 (2018) 5. License number: 5217010558094.

Table 2 Catalysts used for fuel gas production through non-F-T pathway commonly reported in literature. Catalyst

Synthesis method

Catalytic performance

Reaction conditions 1

Cu-Zn-Al2O3/Pd-YZeolite 1/1 in weight

Physical mixture

T ¼ 300°C; P ¼ 40 bar; GHSV ¼ 1500 h . Feed composition: 36.7% H2, 19.1% CO; 7.4% CO2; 0.5% CH4; 36.3% N2

Pd-SiO2/SilicateHZSM-5 16.7%wt zeolite

Core-double shell

T ¼ 350°C; P ¼ 50 bar; W/F ¼ 10 g h mol1. Feed composition: 62.78% H2, 26.3% CO; 4.95% CO2; 2.97% Ar

Cu-Zn-Al2O3/βZeolite 10/1 in weight

Physical mixture

T ¼ 350°C; P ¼ 30 bar; W/F ¼ 10 g h mol1. Feed composition: H2/CO/Ar ¼ 2/1/1

Cu-Zn-Al2O3/βZeolite 10/1 in weight

Core-shell

T ¼ 350°C; P ¼ 30 bar; W/F ¼ 10 g h mol1. Feed composition: H2/CO/Ar ¼ 2/1/1

XCO ¼ 70.6%; SCO2 ¼ 34.4%; Smethanol ¼ 0%; SDME ¼ 0%; SCH4 ¼ 5.8%; SC2 ¼ 6.3%; SC3-C4 ¼ 50.1%; SC5 ¼ 3.1%; SC6+ ¼ 0.3% XCO ¼ 14.1%; XCO2 ¼ 10.7%; Smethanol ¼ 2.2%; SDME ¼ 2.0%; SCH4 ¼ 24.3%; SC2 ¼ 6.3%; SC3-C4 ¼ 34.4%; SC5 ¼ 2.5%; SC6+ ¼ 1.6% XCO ¼ 34.0%; SCO2 ¼ 55.9%; Smethanol ¼ 0%; SDME ¼ 7.0%; SCH4 ¼ 10.8%; SC2 ¼ 4.1%; SC3-C4 ¼ 19.2%; SC5+ ¼ 3.0% XCO ¼ 50.2%; SCO2 ¼ 48.4%; Smethanol ¼ 0%; SDME ¼ 0%; SCH4 ¼ 6.6%; SC2 ¼ 3.9%; SC3-C4 ¼ 37.3%; SC5+ ¼ 3.7%

Reference [53]

[47]

[6]

[6]

Fuel gas from syngas 261

Fig. 10 Distribution of the elements on the interface of the Cu-Zn/Al2O3 core-shell catalyst. Reproduced with permission from P. Lu, J. Sun, D. Shen, R. Yang, C. Xing, C. Lu, N. Tsubaki, S. Shan, Direct syngas conversion to liquefied petroleum gas: importance of a multifunctional metal-zeolite interface, Appl. Energy 209 (2018) 4. License number: 5217010558094.

more convenient to produce a fuel gas with suitable quality. Relatively low pressures (10–20 bar) can be used to produce these light hydrocarbons. One of the negative points is the hydrocarbon distribution limitation, as the F-T synthesis is nonselective reaction that leads to a wide hydrocarbon distribution, as predicted by the ASF statistical model. Additionally, a H2/CO ratio superior to 2 in the feed stream is more convenient to produce light paraffins, once it ensures a better hydrogenation environment. This can, however, make difficult the use of biomass sources to produce the syngas. The choice of hybrid catalysts through the non-F-T pathway is an intelligent alternative to obtain a narrower hydrocarbon distribution in the desired range. In this case, the syngas is converted to methanol and dehydrated to DME that is further transformed to olefins, and finally, they are hydrogenated to the paraffins. The most challenging fact is to design a catalyst capable to perform all these steps in a single reactor. The Cu-Zn-Al2O3/zeolite hybrid catalyst prepared by means of core-shell technique, where the core is the methanol synthesis catalyst and the zeolite as the shell seems to be a successful material to selectively produce the desired light paraffins. The presence of copper sites in the core-shell interface eliminates the need of impregnating the cracking material (i.e., the zeolite) with a noble metal, as Pd, which is responsible to hydrogenate the olefins produced through the DME dehydration. Less H2-enriched syngas streams can be used with these catalysts; thus, the syngas can be produced from biomass. Relatively higher pressures are required (30–40 bar) to achieve high CO conversions to methanol and high hydrocarbons yields, which may be a negative point. Another

262 Chapter 10 disadvantage is the CO2 production through WGS reaction, as the CO reacts with the H2O produced from the dehydration steps. Besides, the copper-based catalysts responsible for the methanol synthesis are prone to the deactivation by the presence of water and the zeolite suffers with the coke accumulation. The above-mentioned considerations show that both methods have positive and negative points. In general, the source of the syngas and the processes/reactions that will be selected to produce H2 + CO mixture seem to be of great importance to determine whether a F-T or a non-F-T pathway must be employed to produce the fuel gas from the syngas.

3.5 DME production DME is one of the chemicals that can be formed from syngas and is considered as an alternative to substitute diesel in compression-ignition engines [72]. DME is also applied as intermediary of different synthesis process of fuels. According to the composition of syngas formed by dry or steam reforming and partial oxidation of methane [73–75] or by these combined reactions with water-gas shift on the tri-reforming reaction [76], DME can be produced by a direct route. Two chemical routes are described in the literature to directly produce DME using syngas, resulting in products rich in CO2 or H2O; these technologies are licensed by Japan Future Enterprise (JFE), Korea Gas Corporation (KOGAS), Air Products & Chemicals from the United States (Eq. 45), and from Denmark the Haldor Topsoe (Eq. 46) [77]. The reactions are showed as following (Eqs. 45 and 46): 3CO + 3H2 $ CH3 OCH3 + CO2

ΔH ¼ 246:0 kJ=mol

(45)

3CO + 4H2 $ CH3 OCH3 + H2 O

ΔH ¼ 205:0 kJ=mol

(46)

With H2/CO ¼ 1, CO2 is produced, and with a higher syngas ratio, H2/CO ¼ 2, H2O is a co-product of the reaction. As both direct syntheses of DME are highly exothermic, these reactions are favored in the low range of temperature of 200–300°C and pressures between 30 and 70 bar [8]. These one step technologies avoid intermediates, like methanol formation. Moreover, DME could be synthesized using a CO2-based method according to Eq. (15) in which an ideal ratio of H2/CO2 ¼3 is expected. However, in this case, methanol is formed as intermediate, as the hydrogenation of CO2 is the predominate reaction, even with the reverse water-gas shift (reverse of Eq. 12) occurring [78]. In the same way, an indirect formation of methanol is possible using syngas due through the CO hydrogenation to methanol (Eq. 13), after that the methanol is dehydrated to form DME (Eq. 14). Methanol is formed by both CO and/or CO2, under exothermic reaction conditions; however, the syngas conversion of methanol is limited by the equilibrium [8]. Feeding the

Fuel gas from syngas 263 reactor with syngas containing CO2 is a possible alternative to synthesize DME and methanol, which contributes for greenhouse gases reduction and is considered an environmentally friendly technology [75]. Different catalysts have been applied to both, one and two-step reactions. Bifunctional catalysts, with metal and acid sites, are needed to convert syngas into DME [79]. Methanol contamination needs to be considered as this chemical is produced using syngas. Acid active sites are responsible for the methanol dehydration and the rate of formation of DME. To increase DME yields most part of authors uses highly active catalysts for methanol synthesis. Alumina and zeolites are widely used as support due to its acid character [80]. γ-Al2O3 is the ideal structure due to the DME selectivity [81], and HZSM-5 has high stability, hydrophobic properties, and strong Bronsted acid site, which reduce hydrocarbon formation [82,83]. ZrO2 is used as support due to Br€onsted and Lewis acid sites that can dehydrate methanol [84]. Cu-based catalysts combined with Zn, Zr, Ce, Al, Co, Cr, and Fe by a physical mixture are attractive. Zr-promoted Cu-based catalysts can improve the specific metallic area, which allows higher directly CO conversions [85]. Also, it was reported that the content (%wt) of ZrO2 into CuO/ZnO/ZrO2/HZSM-5 catalysts had influence in the CO conversion and DME selectivity and yields. 8%wt of ZrO2 achieved 72% of CO conversion, 83% of DME selectivity, and 60% of DME yields [86]. CuFe and CuCo over V-AlPILC (pillared clays) were evaluated as CO2 hydrogenation catalysts; the authors also showed a relationship between CO2 conversion the Cu metallic area and iron addition improves the selectivity related to methanol and DME using a molar ratio of H2/CO2 ¼3 [87]. The CuCe over V-AlPILC is promising for DME synthesis as the catalysts has a balance between basic, acid, and Cu+ species [88].

3.6 DME mechanisms The involved mechanisms in the DME production are not completely elucidated and are considered controversial [89]. It is well known that the strength of Bronsted acid sites is the key for the methanol dissociation and DME production. The mechanism over HZSM-5 zeolites proposed by Wen-Zhi Lu et al. [90] for DME production is the nucleophilic displacement because of the CH3OH adsorption on a carbocation (Fig. 11). The catalysts configuration could affect the direct reactional mechanisms of DME synthesis. Kaoru Takeishi [91] proposed that sol-gel Cu/Al2O3 catalysts are better for the direct DME synthesis than the commercial catalysts obtained via impregnation method. The commercial catalysts showed three different reaction steps, including the methanol synthesis and it dehydration followed by the occurrence of water-gas shift reaction. On the other hand, with the sol-gel catalyst directly DME formation was observed. The proposed mechanism is shown in Fig. 12.

264 Chapter 10 CH3OH (ads) + HX

↔

HXCH3OH

↔

HXCH3OH

CH3+X– + H2O (ads)

CH3+ CH3+X– + HXCH3OH

↔

HXCH3

O

H + X–

CH3+ CH3+X– + HXCH3OH ↔ HXCH3

CH3+ H

↔

H+

↔

O

CH3+X– + HXCH3OH ↔ HXCH3

O

H+

CH3+ CH3+X– + HXCH3OH ↔ HXCH3

HXCH3

O

O

CH3

↔

HXCH3

O

CH3 + H+

CH3OCH3 + HX

Fig. 11 Mechanism over HZSM-5 zeolites proposed by Wen-Zhi Lu et al. [90] for DME production.

Fig. 12 Mechanism proposed by Kaoru Takeishi [91] for: (A) Commercial catalysts and (B) the sol-gel synthesized Cu/Al2O3.

4. Conclusion This chapter introduced the routes of syngas production, including coal and methane as raw materials, in the industrial scale, and the applications and usages of the synthesis gas, mainly for production of synthetic gaseous fuels.

Fuel gas from syngas 265 The main process addressed for the fuels production involves the F-T synthesis, with the syncrude produced constituted by gaseous products, LPG, naphtha with low and high boiling temperature, middle distillate, and waxes of low and high melting temperature. In this case, is very important to choose the adequate active metals and the operational conditions of reaction to obtain a selective process. Many metallic sites were studied, as Fe, Co, Ni, Ru, and others, the reaction mechanism was presented, and some kinetics models proposed in literature were discussed. An extensive description of the fuel gas production by means of F-T and non-F-T routes were discussed, presenting their respective positive and negative points. The source of the syngas, as well as the processes that are selected to produce H2 +CO mixture, appears to be of great importance to determine whether an F-T or a non-F-T pathway must be employed to produce the fuel gas from the syngas. Two possible routes were considered for DME production. In the first, syngas is converted to methanol, which is dehydrated to produce DME. In the second route, DME is synthesized from syngas by one-step technology, which avoids the formation of methanol as an intermediate. In this case, bifunctional catalysts containing metal (Cu and additives) and acid (alumina, zeolites) sites are required. The mechanisms involved in DME synthesis, so far, have not been fully elucidated.

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CHAPTER 11

Biofuel production from syngas Foroogh Mohseni Ghaleh Ghazi, Mitra Abbaspour, and Mohammad Reza Rahimpour Department of Chemical Engineering, Shiraz University, Shiraz, Iran

1. Introduction The increasing rise of the human population, the global economic downturn, global energy usage, and the eventual fossil fuel decline need developing alternate and renewable fuel resources [1]. Coal, oil, natural gas, and other energy resources are being utilized rapidly, and their depletion in the coming decades is a significant issue for the world. According to reports, some oil-rich countries would be unable to supply global energy demand in the following decades [2]. Meanwhile, worldwide CO2 emissions, the primary factor of global warming, grew by 3% in 2011, reaching an all-time high of 34 billion tons. [3]. As a result, biomass, a plentiful renewable energy feedstock resource, has considerable environmental and economic potential for producing syngas high-value fuel through a conversion technique while addressing emerging concerns such as global energy demands and carbon emissions [4]. Thermochemical conversion methods are promising alternatives for using biomass as an energy source. Many academics are interested in the pyrolysis process because it has been shown to be a viable way of converting biomass to products with high value. Flash pyrolysis, a well-known method for the conversion of biomass to heavy and light hydrocarbons, is particularly noteworthy. In the recent decade, many pyrolysis reactors and techniques have been applied to extract gas, bio-oil, and char from biomass [5,6]. Gasification, in addition to the pyrolysis process, is a viable option for producing syngas. Several research studies have investigated the impact of different factors on biomass gasification and usage as a renewable energy source [7–11]. Traditionally, synthetic gas has been made from fossil fuels. However, recent interest in alternate manufacturing approaches, such as biomass gasification, electrocatalytic reduction of CO2, or water electrolysis, has grown significantly in recent years. In addition to presenting substantial price variations, the composition of the produced syngas might vary dramatically [12]. Syngas has a density of about half that of natural gas and is mainly utilized as a transportation fuel and chemical manufacturing [13]. Syngas is mainly made up of CO and H2, Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91878-7.00011-3 Copyright # 2023 Elsevier Inc. All rights reserved.

271

272 Chapter 11 thoroughly cleaned to eliminate particulates and other contaminants (H2S, NH3, and metals). Other chemicals produced by high-temperature gasification (750–800°C) include CO2, CH4, C2H2, C2H4, C2H6, ash, tar, char NOx, and SOx based on the raw materials [14,15]. First-generation biofuels, particularly biodiesel and bioethanol, should no longer be considered technological limitations; they have already been integrated into the sociotechnical system, and their dispersion is extensive and well-established worldwide. Approximately 2% of the world’s farmland is now cultivated for biofuel feedstock utilizing a well-established method. However, because of the more outstanding production and processing costs associated with first-generation biofuels, their commercial competitiveness remains low compared to fossil fuels (except Brazil’s sugarcane ethanol). Because the biomass used in the manufacture of biofuels is considered carbon-neutral, they have the potential to reduce transportation-related carbon dioxide emissions. Biofuels are becoming increasingly popular due to this exact reason. Nonetheless, some elements such as increased land use and food production rivalry may make them unsustainable, particularly for first-generation biofuels. Considering all essential economic, social, and environmental factors, biofuels’ long-term viability should be carefully assessed. To prevent the consequences throughout the production process, the complete life cycle of biofuels should be investigated for sustainability, including the cultivation of feedstock and biofuel manufacturing methods. Additionally, the competitiveness of biofuels is closely tied to changes in the price of crude oil, creating uncertainty for both producers and investors. It is possible that demand for biofuels would soar if oil prices remain high that the market will be unable to meet the demand. On the other hand, the biofuels business faces crippling financial losses if oil prices remain low for an extended period. We will now go more into the economic and technical characteristics of the various forms of biofuel.

2. Syngas conversion into biofuels The majority of syngas generated is used in ammonia production (50%) and hydrogen synthesis (25%) worldwide, with the remains utilized for Fischer-Tropsch (FT) synthesis, methanol production, and other applications. For the manufacturing of second-generation biofuels, syngas generated from biomass serves as a raw material in a variety of thermochemical mechanisms, including the production of liquids (such as ethanol, methanol, Fischer-Tropsch diesel, and ethanol, dimethyl ether (DME)) as well as gases (e.g., synthetic natural gas (SNG), and hydrogen). The type and manufacturing technique of biomass significantly impact the composition and heating value of biomass fuels. As an energy transporter, liquid biofuel production might be very cost effective due to utilizing the storage system, infrastructure, and transportation as liquefied petroleum gas [16]. Fig. 1 depicts several syngas transformation routes and applications.

Biofuel production from syngas 273 Olefins Gasoline

Fe, Co, Ru

Ag

d pe do

Isosynthesis

Cu/ZnO

Syngas CO + H2

ThO2 or ZrO2

h

se

DME

Olefins Gasoline

tU

Ethanol Aldehydes Alcohols

zeolites MTO MTG rec

,R

H2

Methanol

Di

Co

c sis the syn

N2 over Fe/FeO (K2O, Al2O3, CaO)

) o Ox (CO) 4 P(Bu 3 )3 )3 o HC o(CO (PPh 3 HC (CO) Rh

H2O WGS Purify NH3

Formaldehyde

acidic ion exchange ca CH rbo n 3 O yla H tio Co ,R +C n O h, Ni

Fischer-Tropsch

isobutylene

O3 liAl 2 ka O/ Al O 3 /Zn Cr 2 Cu O 3 O/ O; /Al 2 Zn /Zn oO Cu O/C Cu oS 2 M

I - C4

Acetic Acid

MTBE

Al2O3

Waxes Diesel

hom olo gat Co ion

Mixed Alcohols

M100 M85 DMFC

Fig. 1 Pathways of syngas conversion to biofuels and chemicals [17]. From an open-access article distributed under the terms of the Creative Commons CC-BY license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Table 1 Conditions for syngas conversion to different types of biofuels [18]. Pressure (bar)

Temperature (°C)

250–300 50–100 55–65 70–105 30–70

350–450 200–300 230–300 200–300

FTS

10–40 7–12

Hydrogen SNG

1–30 1–25

Biofuel Methanol Ethanol DME

Catalyst

H2/CO (mol/mol)

ZnO/Cr2O3 Cu/ZnO/Al2O3 Rh MoS2 CuO-ZnO-MnO and zeolite

3 2 2 1–1.2 2; 3

300–350 200–240

Fe Co

0.6–1.7; 2 2.0–2.15

200–1100 200–450

Ni, Fe, Mo Ni, Co, Fe, Ru

2 3

CO2 4–8% v/v 85

83–87

N.A.

2–6.5

N.A.

[45]

N.A.

3000

900–930

N.A.

N.A.

N.A.

N.A.

15–20

N.A.

[46]

N.A.

5

900–1000

417–1250

N.A.

40–80

N.A.

3.7–26.5

N.A.

[47]

16–55 1

25 1000

900–940 930

4505 156

92 N.A.

96–98 N.A.

N.A. N.A.

11–25 12–17

10–24 N.A.

[48] [49]

1.6 1.-4.3

1000 1.5

900 900–960

100 235–1150

50 N.A.

90 94.7–99.3

N.A. N.A.

20 0

N.A. 0

[50] [51]

1.1

1.5

900–945

950

96

96–98.4

N.A.

0

0

[52]

N.A.

10

930–960

770–1660

N.A.

N.A.

75.2–88.4

7.7–11.2

N.A.

[53]

CuO (iG-CLC) Fe2O3 (iG-CLC) Fe2O3 (iG-CLC) CaSO4 (iG-CLC) Hematite (iG-CLC) Ilmenite (iG-CLC) Ilmenite (iG-CLC) Ilmenite (iG-CLC) CuO/MgAl2O4 (CLOU) CuO-Fe2O3/ MgAl2O4 (CLOU) CaMnO0.9Mg0.1O3δ (CLOU)

Petcoke

Power generation from syngas 313 Syngas-CLC can be used in power plants with GTs and heat generation system, in which the outlet stream from air reactor drives a GT to provide power, after which the stream out of the GT is directed toward heat recovery system through steam generation, and the generated steam is then used in a steam turbine to provide additional power. While the outlet stream of the fuel reactor is directed to run a CO2 turbine, after which the flue gas stream is cooled to separate existing water and consequently CO2 is concentrated and can be used for either sequestration or other CO2 conversion processes. Alternatively, other options such as CLC with steam turbine system or CLC with combined cycle system have also been suggested [40]. Gaseous CLC of syngas and natural gas in a 300 W system were investigated using Mn3O4/ Mg-ZrO2 oxygen carrier [42]. Results for the natural gas case revealed that CH4 was not completely combusted, and some traces of CH4 together with low concentrations of CO and H2 were found in the flue gas. On the other hand, syngas was completely converted in CO2 and H2O with no traces of CO and H2 in the exiting flue gas stream. The effects of pressure on the syngas-CLC performance of NiO(40%)/Al2O3, CuO(10%)/Al2O3, and Fe2O3(45%)/Al2O3 oxygen carriers were studied in the thermogravimetric analyzer for up to 30atm. It was shown that increasing pressure would result in negative impacts on rates of reactions for the three oxygen carriers. In a different study by Tian et al. [54], CuO(60%)/bentonite and CuO(45%)/ barium-hexaaluminate using simulated syngas without impurities at different temperatures (700–900°C), different particle sizes, and different pressures (up to 100psi). It was found that particle size and temperature have a minor impact on the performance of CuO/bentonite. Moreover, and CuO/barium-hexaaluminate displayed better activity and stability than CuO/ bentonite. However, increasing pressure was also found to adversely affect the reaction rates because of slow diffusion through bulk materials at high pressures. The effect of syngas composition on NiO(18%)/Al2O3 performance was studied at different temperatures (800–880°C) [55]. High combustion efficiencies were obtained throughout the temperature range, and the composition of syngas fed to fuel reactor had a minor effect on combustion parameters since high and comparable combustion efficiencies were obtained for a range of syngas compositions even with high fractions of CO. Furthermore, Shen et al. investigated the effects of the presence of sulfur contamination (H2S) in syngas feedstock over NiO(35%)/ Al2O3 [56]. Sulfur contamination led to Ni3S2 formation on the oxygen carrier after reduction without the presence of NiS or NiS2. Ni3S2 formation is reversible during syngas-CLC redox reactions, however, its low Tamman temperature induced sintering on the oxygen carrier’s surface and consequently constrained the extent of H2S conversion into SO2 with the cycles progress. CLC process was first developed for gaseous fuels, and most of the initial progress made was on gaseous CLC. However, recent developments and research interests on the process have been more focused on solid and liquid fuels. The idea of integrating the chemical looping cycles with GTs has been investigated in the literature [57–61] and compared with the conventional natural gas-fired combined cycles. It was argued that the NGCC power plant without carbon capture

314 Chapter 12 may reach an efficiency of 58%, but when the postcombustion capture unit is integrated, a substantial drop in overall efficiency was reported with large increased capital costs and operating costs in the postcombustion capture unit. The integration of natural gas-fired CLC with the combined cycle has achieved an efficiency close to 50% with 100% capture rate [57]. The penalty of 8% can be improved by optimization of the air and fuel reactors temperatures and developing high oxygen-carrying capacity oxygen carriers. The network done by the syngas-CLC would be much lesser due to the lower LHV of the input fuel and efficiency would be also much lower. To improve the efficiency, the process integration of CLC with an additional combustor that is fired either by only H2 or syngas is suggested [57,62]. CLC has a TRL of 3–5 [24] and with recent efforts in large-scale pilot deployment, it could be considered in the range of 5–6. With oxyfuel combustion technology and CLC producing CO2 and H2O as the emissions, there has been an effort to consider converting the CO2/H2O to syngas through redox cycles similar to the chemical looping process. The metal oxides such as CeO2, Co3O4, Nb2O5, WO3, SiO2, Ir2O3, CdO, perovskites, and mixed metal oxides have been investigated as potential oxygen carriers for CO2/H2O splitting in an oxidation reactor at a temperature (800–1100°C) [9,63]. The reduction of oxygen carrier is carried out by providing solar heat to a very high temperature of the order of 1400–1600°C in very low partial oxidation conditions as shown in Fig. 11. Conventionally syngas is produced from reforming processes as discussed in Section 2 and the issues of lower LHV of such fuels will always result in lower efficiency when integrated with combined cycles through chemical looping processes. This issue would be challenged by converting the emissions CO2/H2O through thermochemical redox cycles and producing syngas eliminating or limiting the use of natural gas. Therefore, solar thermochemical redox cycles can be integrated into power plants (such as CLC power plants or oxy-fired power plants) that produce only CO2/H2O as an emission. The reformed

Fig. 11 Solar thermochemical redox cycles for power production.

Power generation from syngas 315 emissions to syngas can be supplied back to the power plant to compensate for the energy penalty from carbon capture integration [64,65]. The investigation of this integration of chemical looping thermochemical redox CO2/H2O splitting has been extended to polygeneration plants for producing methanol, dimethyl ether, jet fuels along with electricity [25].

6. Conclusion The current drive on carbon capture and utilization to curb the global warming rates have led to the use of low-carbon fuels for power generations. Syngas has been considered as a potential low-carbon fuel that could easily replace natural gas with little or no modification and able to use the existing infrastructure. Many gas turbines manufacturers have started their research in using syngas or hydrogen as fuel for power production. Since syngas is considered as low calorific value fuel, in order to maintain similar generation efficiency, the flowrate required increases and the pressure conditions pushes the compressor close to the surge limits. Issues of flammability and flame temperature also require close consideration in operation. These issues were suggested to be tackled by using air bleeding and integration degree method in IGCC. Materials for GT components should be selected with care for syngas and hydrogen-fired power plants, as these turbines undergo different conditions than those using natural gas because of different calorific values and required firing temperatures, and the potential presence of impurities. Development and optimization of single-crystal superalloys are suggested for first stages blades and vanes to tolerate syngas and hydrogen firing temperatures. Improvement of bond coating and thermal barrier coating techniques are favored to endure the negative effects of persistent impurities from syngas and hydrogen feedstocks. Modifications of Ni-based alloys are suggested for syngas turbine discs, whereas ceramic matrix composites are proposed to be used for combustor liners. Testing materials suggested for syngas or hydrogen turbines would require the use of experimental and analytical techniques to assess their performance in real operational power plants. New research directions to tackle material and operational challenges of the syngas-fired gas turbines have been evolved. Chemical looping combustion has evolved as a promising pathway for producing power that uses a two-reactor concept with metal oxides circulating between the reactors as an oxidizer. Syngas as fuel has been explored in CLC that produces flue gas streams consisting mainly of CO2 and H2O. With higher H2/CO ratios, syngas will reflect to have lesser CO2 emissions. Even though chemical looping technology has lesser technology readiness level, however, it has the potential in using syngas as fuel for combustion and integration with the gas turbines. More research would be required in this domain to see the clear potential of using chemical looping combustion for syngas combustion in large-scale power generation units.

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CHAPTER 13

Combined heat and power application of syngas Foroogh Mohseni Ghaleh Ghazi, Mitra Abbaspour, and Mohammad Reza Rahimpour Department of Chemical Engineering, Shiraz University, Shiraz, Iran

1. Introduction In recent years, nonrenewable energy sources like crude oil, coal, and natural gas have supported technological progress and advancements in a wide range of productive sectors [1]. Furthermore, nonrenewable energy has caused a variety of problems such as global warming [2], water pollution, and smog production. A biomass-based renewable energy source [3] has been proposed for producing energy carriers capable of covering this demand. Syngas, specific energy vector, is used in cogeneration systems to generate heat and power [4]. It would be possible to use syngas from biomass gasification as a chemical platform for generating value-added chemicals. The vast majority of produced electricity is generated by heat engines, which transform heat into electricity. This energy is generated from various sources. A majority of it is provided by the combustion of fossil fuels, such as steam plants, gas turbine plants, and reciprocating engines [5]. A nuclear power plant produces heat that is used to drive a heat engine, usually a steam turbine, while biomass is burned [6] to generate heat. The majority of thermal power plants’ energy is wasted. Between 40% and more than 80% of it is wasted. Heat is a waste product that is discarded in one way or another. In some cases, it is dispersed into cooling waters that pass through power plants and are then returned back to rivers or the sea, but most of the time it is dispersed into the atmosphere via heat exchangers The heat produced by this process is considered pollution. [7]. Though these energies are not used to generate electricity, they can still be used for other purposes. Steam can be generated from high-grade heat and used in some industrial processes, while low-grade heat can be used to produce hot water or to heat rooms. By utilizing waste heat from power generation, we can replace high-value energy sources such as gas, oil, or even electricity. Consequently, overall energy efficiency is greatly improved. This type of system is known as CHP system [8].

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322 Chapter 13

2. CHP systems A CHP is simply a way of generating electricity and heat simultaneously. A CHP system can utilize many different energy sources, such as coal or natural gas, nuclear fuel, or renewable energy sources such as syngas generated from renewable sources, geothermal energy, biomass, and solar thermal power. Similar technology is used in district heating and cooling (DHC), which is referred to as cogeneration. As illustrated in Fig. 1, the combustion process considers various CHP technologies. Using an external combustion process or no combustion process shows better fuel adaptability [10]. According to the maximum capacity of the CHP system, three categories are distinguished: microcogeneration (less than 50 kWe), small-scale/mini cogeneration (50 kWe–1 MWe), and cogeneration (more than 1 MWe). The investigations show that the proposed technologies for the production of CHP on a small scale (size 100 kW–1 MW) consist of two main paths: (i) biomass preprocessing linked to the internal combustion engine (ICE) and micro gas turbine (MGT), (it should be noted that biomass preprocessing occurs through gasification or thermal decomposition) and (ii) direct combustion in mesh boilers or fluidized bed to propel MGT, Stirling engines, steam turbines, or organic Rankine cycles (ORCs) [11].

2.1 Advantages and disadvantages of CHP technologies Considering heat consumption, CHP systems can operate at up to 90% energy efficiency. This results in significant savings in fuel costs and overall environmental degradation. However, while its benefits are widely known, CHP implementation remains low [7]. Additionally, because existing heat and electricity generation will be used more efficiently, less investment will be required in the electricity sector. Due to the production of local heat and electricity by CHP systems, the electricity required for central production is reduced and as a result, less investment will be required in the transportation and distribution systems. CHP power plants are therefore a cost-saving alternative to centralized power plants. [8]. Some of the problems are related to the historical and general issues of large central power plants. In order to easily feed electricity into the grid, large power plants are usually built nearby the main transmission line. They may also be located near a fuel source. Most of the time, this means that their waste heat cannot be used by consumers near them. Having central power plants in or near urban areas will be able to provide heat as well as power through district heating systems using waste heat from the plant. By using renewable energy sources in combined heat power (CHP) generation, greenhouse gas emissions can be reduced with social, economic, and environmental advantages [12]. Utilizing CHP technology to exploit this energy offers benefits around the world. Economic and environmental benefits are both involved. As a result of CHP, fuel resources can be utilized more efficiently, resulting in more cost-effective energy. An analysis by the International Energy Agency (IEA) suggests that indirect electricity costs can be

Combined heat and power application of syngas 323

Fig. 1 CHP technologies and the applications [9] (with permission).

reduced slightly by the growth in CHP markets [13] if the savings are allocated for this purpose. Many other reports indicate that using CHP increases end-user electricity costs, as the IEA has pointed out. In addition, if heat is managed better during energy generation, those receiving it can benefit economically. Increasing the efficiency of existing heat and electricity generating capacities could reduce financial investments in the power sector as a result of increased heat

324 Chapter 13 and electricity generating capacity. As CHP provides local power and heat, it reduces the amount of power used that must be produced at a central location, thus reducing the need for new transmission and distribution systems, and the investment in central power stations can be minimized. Furthermore, CHP has environmental benefits, such as reducing emissions of greenhouse gases and halting global warming.

2.2 CHP principles Most CHP systems generate electricity using thermal motors. Except for the fuel cell, which produces electrochemical power, steam turbines, gas turbines, and reciprocating motors are heat engines used to generate electricity. In the 19th century, all of these engines were designed using thermodynamic principles that governed their development. Over the past century, their use has increased, and today they rank among the top suppliers of electricity in the world. When considering the technology of CHP, it is helpful to recognize two types of CHP systems. The first cycle, also known as a topping cycle, uses fuel primarily for electricity production, while heat generated by the electric force is retained and can be used for ancillary purposes. According to the type of electricity-generating system, it may be low-grade heat, and it is suitable for heating the space and producing hot water, or a higher degree heat used to raise the steam are used directly in industrial processes. As Fig. 2A illustrates the typical topping cycle of an electrical generation system: first, an electrical generator, such as a gas turbine, reciprocating engine (or fuel cell) is used to generate electricity, and then the waste heat produced is captured and used for space heating, steam generation or process heat. Generally, in this kind of system, the heat generated by electricity generation will be relatively low. However, systems can be designed that are capable of generating high-grade heat or steam by adjusting the overall balance between heat and power. The second type of CHP system is known as the bottoming cycle. In this type of system, the electricity is generated at the bottom of the list and must make do with the remaining energy after the primary heat user has consumed it, and also heat production is done by fuel, which is commonly used in process with very high temperatures or large amounts of heat, for example, iron smelting. Electricity is produced from the energy left over after this process. The bottom-cycle CHP method is also known as waste heat recovery or waste energy recovery. In addition to heat, combustible gases produced during iron production can also be used to generate electricity. High-pressure exhaust gases from a process can also be utilized for electricity production when the pressure drop is used as an engine drive. Considering that bottoming cycle CHP can provide electricity without any additional fuel consumption, they are highly attractive. The schematic of this process is shown in Fig. 2B. For a bottoming cycle to be economically viable, a thermodynamic fluid must be heated, and an industrial process must generate enough heat energy to propel a turbine. The heat energy inside the steam turbine is at a relatively low temperature, so an organic Rankin cycle turbine can be used to change low-temperature heat into electricity. In some bottoming cycle systems, energy can be recovered in the form of combustible gases, such as from blast furnaces, or from high-pressure gas streams that could drive gas turbines. In addition, CHP

Combined heat and power application of syngas 325 Water

Heat recovery unit

Steam or hot water

Cooling/heating

Hot exhaust gas Engine or turbine

Fuel

Electricity Generator

Building or facility

Grid

(A) Thermal energy Heat exchanger

Building or facility Turbine generator

Waste heat

Fuel

Electricity

Grid

Fuel combusted as part of an industrial process

(B) Fig. 2 CHP topping (A) and bottoming (B) cycle schematic [14] (with permission).

systems provide a bridge between the two. Steam turbine-based CHP would be an example of this, the steam generated at the input or partway through the steam turbine not only drives the steam turbine but also generates industrial process heat. This type of system is used in some industries. A CHP system can integrate all types of heat engine-based power generation. As a result, this technology can be applied to both fossil fuel combustion power plants as well as biomass power plants [15].

3. Syngas potential of producing energy Coal, petcoke, lignocellulosic biomass, and municipal solid waste gasification yield syngas of varying compositions, including those of low quality and low energy content, depending on factors like gasifier design, gasifying medium employed, raw material composition, and operating pressure. A number of authors have examined these variables to find the best conditions for gasification. There is evidence that different kinds of syngas can have different heating values [16]. The syngas that is obtained from coal and pet coke has a higher high

326 Chapter 13 heating value (HHV) than gasification from solid wastes or lignocellulosic biomass. Due to this reason, these raw materials have been found to be most appropriate for industrial use. Optimum operating conditions could, however, lead to significant variations in composition. The use of biomass has the potential to reduce the exploitation of coal in different regions of the world by substituting it for the primary raw material. To perform the gasification process, the three most common gasifying mediums are air, oxygen, and steam. Based on differences in H2 and CO concentrations in each medium, each provides a different level of heat transfer to syngas [17]. As well as producing heat and power, syngas can also be used in the production of products with added value such as biofuels (methanol, ethanol), ammonia, hydrogen, synthetic natural gas, and dimethyl ether. The composition of syngas influences the production of the composites mentioned above.

3.1 Applying syngas in CHP systems The main alternatives for sustainable development are heat and electricity generation from biomass [18,19]. One of the most attractive sources of energy is lignocellulose biomass, which is both widely available and renewable in nature. By replacing fossil fuels with biomass in efficient thermochemical conversion processes, Greenhouse gas emissions can be reduced. An advanced processing path for converting biomass to a valuable fuel is gasification In the process of gassing, the biomass at a temperature of more than 700 degrees Celsius turns into a gas mixture rich in carbon monoxide (CO) and hydrogen (H2), which is called synthetic gas. This gas also contains carbon dioxide (CO2), methane (CH4), water (H2O), hydrocarbons, including tar, and small amounts of mineral impurities. In gas engines, turbines, or fuel cells, synthetic gas is used to generate electricity. The overall efficiency range of CHP generation in integrated systems is between 60% and 90% [20,21]. For small- and medium-scale power plants from 1 to 10MW, internal combustion gas (IC) engines are suitable [22], which are more convenient and less expensive than turbines. Although laboratory facilities have shown the high potential of CHP gas-fired power plants, the technology has not yet reached the commercial stage. The Gussing Experimental CHP plant demonstrates one of the best technologies available. An autothermal steam gasifier with an IC motor is capable of generating 2MW of electricity and 4.5MW of heat (for regional heating) from 8MW of biomass fuel [23]. There is a major obstacle in the development of the biomass gas conversion unit in reducing the tar content in the synthesized gas. Tar causes clogging and damage to process equipment and engines [24,25] and also reduces the lifetime of the equipment. Heat and power generation from synthesized gas in CHP systems cannot be classified as a conversion technology. Rather, thermodynamic cycles are regarded as a method of obtaining useful energy from synthesized gases. Cogeneration usually involves using turbines, compressors, generators, boilers, and heat exchangers, and it is adjusted to be highly efficient [26]. Fig. 3 shows the overview of the wood gasification CHP plant.

Combined heat and power application of syngas 327 exhaust gas flue gas

oil

electricity

IC Engine

air

Bag filter

fresh water

flue filter dust

electricity

olivine Tar cracker

Cyclone

ELECTRICITY

Syngas cooling

Bag filter

clean syngas

Water scrubber

used oil

Comp

flue gas

electricity

syngasR

sand WET WOOD

Dryer

syn. filter dust

raw syngas

flue gas

Gasifier

char

electricity

Combustor

Wastewater treatment

waste water

tar sludge tar sludge flue gas

water cooling

electricity

electricity

air

air

stripped air

air heating district heating

Fig. 3 Overview of the wood gasification CHP plant [26] (with permission).

4. SOFC-based CHP application Recently, the production of electricity by renewable and sustainable sources has received much attention. These sources include the solid oxide fuel cell (SOFC) which is a suitable choice for green energy production and has benefits such as fuel types, quiet performance, low or zero emissions, and powerful effectiveness. SOFC is mostly used at medium operating temperatures (400–700°C) [18,21]. When this temperature is reached, proton-conducting electrolytes exhibit good electrochemical performance. Oxygen and hydrogen are commonly used as fuels and oxidizers in fuel cells. Using biomass gasification as a feedstock for SOFCs shows a positive trend in the use of biological products. In the process of gassing, carbonaceous materials are converted to gaseous products by carbonated agents such as air, oxygen, and steam alone or in a mixture [27]. When SOFC operates at high pressures, it can work with gas turbines to achieve high efficiency for generating electricity [20]. SOFC is a green energy source that produces few or no emissions and can be used as an alternative energy source to fossil fuels. By adding a CHP system, the fuel effectiveness of SOFC can be increased from about 60% to 90% [24,25,28].

328 Chapter 13 Natural gas is used by the CHP system to generate electricity and heat. SOFC-based CHP is still a relatively new technology that uses biomass gasification for fuel. Fig. 4 shows the simple diagram of biomass gasification-SOFC-based CHP, and biomass gasification-integrated fuel cell-based CHP systems are shown in Fig. 5.

Biomass Gasifier Air

Particulate Filter

Gas Cleaning

Char/Ash

Heat losses Water

Solid Oxide Fuel Cell (SOFC) Combined heat and power generation (CHP)

Power Useful Heat Flue gas

Fig. 4 The simple diagram of biomass gasification-SOFC-based CHP [9] (with permission).

A/C: Air Conditioning CB: Control Box DHW: Domestic Hot Water EAs: Electrical Appliances

A/C System

Space Heating System

Hot Water

Grid

EAs

CB

Fuel Cell CHP System

Biomass Syngas gasification

DHW

Lighting Heat

Heat Storage

Heat

Auxiliary Boiler

Plug loads

Natural Gas Network District Heating Network

Fig. 5 Biomass gasification-integrated fuel cell-based CHP system [9] (with permission).

Combined heat and power application of syngas 329

4.1 Development of SOFC SOFCs are typically capable of operating at high temperatures (more than 700°C), allowing the use of a wide range of fuels. The development of anodes, electrolytes, and cathodes for SOFCs has been conducted. Chemical stability and electrochemical performance are targeted for improvement [18,21,29–32]. SOFC electrolytes are commonly made from Yttria-stabilized zirconia (YSZ). Several studies have been conducted on its electrochemical properties [25]. Taking into consideration the heat-consumption effect of fuel cells, specifically SOFCs, and their environmental effect, the intermediate temperature solid oxide fuel cell (IT-SOFC), which runs at 400–700°C, has been developed. Protons conduct most efficiently on BaCeO3 and BaZrO3 at intermediate temperatures. It is possible to partially dope, co-dope, or combine conventional materials [33,34]. Anode, electrolyte, and cathode are the components of fuel cells [35]. Fueling a machine with natural gas has many advantages, including efficiency, availability, and environmental friendliness. Hydrocarbons of heavier density (such as gasoline and jet fuel) may also be used but must be reformatted externally. Alternative hydrogen-rich fuels like methanol may be used to supplement gasoline. High temperatures require more heat energy, which increases the costs. This development in intermediate temperatures can lead to a reduction in energy costs. Optimum fabrication of IT-SOFC is enabled by proton conductive electrolytes operating at intermediate temperatures [18,21,30,36]. There are several perovskite electrolytes with excellent electrochemical properties in this temperature range (general formula, ABO3) [37,38]. Although oxide ion conductors are common in SOFCs, problems still persist with mismatched materials, hard tolerable operating conditions, and low fuel efficiency. Ion conducting materials with proton conductors are shown in this issue to offer the potential for improving the performance of operations [18,39,40]. Additionally, the quality of the gas used in the producer part of a SOFC will affect its performance, in particular the impurities. It is chosen a high-temperature SOFC (HT-SOFC) due to the extremely high temperature of the producer gas.

4.2 Advantage of SOFC As a form of green energy, SOFCs have become increasingly popular due to the fact that they can burn a wide range of fuels and produce a low level of emissions [41,42]. Gas engines, gas turbines, gas motors, and diesel engines have higher temperatures and efficiency than combustion engine-based power generation systems. During normal operation, the SOFC has an electrical efficiency of about 60%, and among combined heat and power (CHP) systems, it can provide about 90%. Polymer electrolyte membrane fuel cells (PEMFCs) are less affected by impurities than SOFCs because they can use a variety of fuels, like natural gas and hydrocarbons, as hydrogen-rich sources. Natural gas may also be used in cogeneration if biomass and other renewable resources are used as fuel [10]. A conventional SOFC generates electrical energy with few or no emissions by feeding hydrogen/hydrogen-rich fuel to the anode and air to the cathode [43]. By operating at high temperatures (700–1000°C) fuel reactions are

330 Chapter 13 accelerated and materials degradation is prevented [44]. In response to growing demand for small-scale, low-carbon energy generation, SOFCs have been miniaturized as micro-SOFCs for use in auxiliary power units (APUs), uninterrupted power sources (UPS), and combined heat and power (CHP) systems [45].

5. Gasification process In the process of gasification, carbonaceous materials such as biomass are converted to fuel gas through partial thermochemical oxidation in the presence of carbon dioxide, usually air. This type of gas is known as synthetic gas, and it contains the following components: CH4, H2, CO, CO2, N2, small amounts of coal, ash, tar, and oil. Usually, the gasification process operates at a temperature between 500°C and 1400°C under a pressure of 33bar. Gasification consists of four typical processes, (i) drying: in this process, the moisture is removed from the raw material; (ii) pyrolysis: in this process in order to obtain 3 different fraction including solid, liquid, and gas fractions, the carbonaceous materials are decomposed thermo-chemically; and (iii) oxidation: The heat released in one exothermic reaction is used as thermal energy in the next reaction, which is endothermic, thus maintaining the operating temperature without oxygen; (iv) reduction: In this process, the pyrolysis and oxidation steps are repeated. If the necessary energy is produced inside the same gas generator by partial combustion of the biomass, gasification is called an autothermal process. Sufficient heat is required to supply the gas generator through hotbed materials in an efficient autothermal gasification process, which generally results from exhaust fuel and SOFC air currents. Air, oxygen, or steam is used as an oxidizer in the gasification process. The heat required for the endothermic reactions in the gas generator is provided by oxidation reactions and no external energy source of gasification is required. Biomass gasification combined with SOFCs is a useful technology in gasification processes. Air and oxygen are used as oxidants in the gasification process. Gasification by air oxidizer is more common because it is more cost effective than oxygen. Biosyngas produced in the gasification process contains a significant amount of nitrogen. As a result of high nitrogen content in the air, nitrogen can reduce cooling air required at the cathode of SOFCs in gasifier-SOFC-GT systems. Additionally, nitrogen dilutes the product gas due to its presence in the air. This leads to low calorific values of 4–6 MJ/m3n in the gasification of air. Biosyngas produced in the process of gasification by steam and oxygen has an average calorific value of 10–20MJ/m3n. If hydrogen is used as a gasifying agent, the calorific value will be 40MJ/Nm3 [46].

5.1 Gasifier Basically, gasifiers can be categorized as a fluidized bed and fixed bed [47]. The three main types of fixed bed gasifiers are updrafts, downdrafts, and cross-drafts. It is highly recommended to use a fixed bed downdraft gasifier due to its moderate cost, low impurity

Combined heat and power application of syngas 331 levels, and ability to produced power more than 250 kW [48]. In comparison to a downdraft gasifier, which is sensitive to moisture content and fuel size, the updraft gasifier or counter-current gasifier generates cleaner gas, which is ideal for use in engine applications. During the combustion/oxidation phase of a downdraft gasifier, biomass feedstock is fed from the top while air is fed from the bottom. The combustion zone provides the heat needed for the drying, pyrolysis, and reduction zones. During a reduction zone, where syngas is produced and carbon dioxide and water vapor are reacting with hot char, ash is what remains. Drying and pyrolysis zones produce biosyngas, which is subjected to oxidation (at high temperatures) resulting in secondary cracking of tar. The gas produced in the gas generator is then transferred to the gas cleaning unit. The gas is still much cleaner than updraft gasifiers, despite the high proportion of particulates in the gas. In particular, small-scale installations can benefit from a fixed bed for biomass gasification. A fixed bed downdraft gasifier was found to work well for biomass gasification because there was little tar produced, reducing emissions and gas cleaning costs.

5.2 Gasification process and the role of operating parameters It is also important to keep in mind that different operating parameters can also affect a gasification process. The composition of a producer gas can be altered by this factor. Gasifying agents such as air possess a lower heating value (LHV) than other media, ranging from 4 to 6 MJ/Nm3. It is the most common yet economical source of LHV gas. Although oxygen can also be used, its price makes it an unattractive option. Consider the equivalent ratio (ER) as a way to counter the disadvantages of air. For stoichiometric combustion, ER is the ratio of air to fuel weight. ER should be between 0.2 and 0.4 in order to achieve an appropriate calorific value while also controlling tar content [49,50]. The low ER (less than 0.18) is not appropriate because high tar levels are in parallel with the low ER (less than 0.18), and gas production is unproductive at the low ER (less than 0.18). During gasification at lower temperatures, less char is converted to fuel and the amount of tar in the producer gas is relatively high. Although higher temperature in the gasification process produces more H2 and CO, the gasification temperature is limited to 1000°C due to the melting point of the ash [51]. In order to obtain the maximum amount of hydrogen and the least amount of tar, the gasifying agents and biomass ratio must be adjusted [51,52].

5.3 Gas cleaning Before the gasifier can be used, a catalytic reforming unit must be installed. In the process of producing gas, we expect the gas compositions to be nearly equal with the least tar. As the gasification temperature is kept low as possible [20], the producer gas will be cleaned up until 600°C. To remove HCl and H2S, gas is cooled down to 600°C in the high temperature gas cleaning process A software called “Facstage” is used to calculate the thermodynamic equilibrium. This software can estimate the carbon deposition tendency of the gas before

332 Chapter 13 cooling it down. In the gas cleaning unit, impurities can be removed without the loss of heat to the atmosphere and the gas composition remains unchanged. As a result of combustion, carbonaceous materials can retain hydrocarbon materials, such as tar and other impurities. Synthesis gas must be cleaned in the gas cleaning process after conversion to gas so that it can be used as fuel for SOFCs. In the process of gasification, various pollutants may be produced depending on the type of raw material, type of gasifier, carbonated agent, and operating parameters. To eliminate contaminants, several cleaning methods are used. Physical processes are used to clean impurities in solids and liquids, while chemical processes are used to clean impurities in gases [53,54].

6. The agnion heatpipe-reformer Agnion heatpipe-reformer uses heatpipes to address the problem of heat transfer inside an autothermal reformer [55]. Heat pipe-based processes are able to operate under high pressure with smaller reactors because the gasification and combustion processes are decoupled by heat pipes. Thus, the integration of reactors with their systems produces economic and energetic efficiencies competitive with large systems. Syngas is produced from the heatpipe-reformer using a broad range of solid fuel inputs and applications. As a result of high conversion ratios and suitable gas composition, the SNG and BTL syngas transformation processes utilize pressured autothermal steam gasification. The technology is therefore suitable for small- or medium-scale polygeneration appeals. Pressurized gasification is also beneficial to the CHP process, as it permits direct port injection of hydrogen-rich fuel gases together with cylinder-selective combustion. This combination improves the gas engine’s operating stability and combustion efficiency. As the fluidized bed of the combustor warms up to 900°C, high-temperature heat is generated. Through internal evaporation, immersed heat pipes absorb heat at high temperatures. The fluidized beds of the reformers are heated between 800°C and 860°C at the working fluid condensator end of the heat pipes, depending on the amount of heat necessary to fuel the autothermal gasification process. Steam generation is used to cool flue gas and syngas before mid-temperature particle filtration is applied in order to remove ash, char, and bed material from both gases. It is necessary to cool the syngas to 60–75°C before it can be burned in a gas engine. The gas is heated to 80°C upstream of the organic solvent scrubber unit in order to prevent tar and water residues from condensing in it. This control of tars and water at the dew point of the engine prevents fouling and condensate accumulation. With less fuel input, hydrogen concentration increases and methane concentration decreases, which indicates greater activity in reforming and gasification of fixed carbon and lengthy residence times for char particles. As a result of the pressurized fuel feeding system, nitrogen is added to the syngas. As reformer fuel input increases, the flow rate decreases because it is a constant volume flow. Fig. 6 shows the heatpipe-reformer CHP flow chart.

Combined heat and power application of syngas 333 RME

Water

Biomass Reformer Heat pipe Heat flux

El. Power

Air

Water

Heat

Syngas Syngas cooler

Syngas filter

RME scrubber

Steam

Gas engine

Flue gas Steam/ Heat

ID fan Flue gas

Combustor Air Biomass

Flue gas cooler

Particle filter Cyclone

Stack

Water

Char, Ash, RME

Ash

Fig. 6 Heatpipe-reformer CHP flow chart [56].

7. Conclusion and future outlook There are a number of problems caused by nonrenewable energy, including global warming, water pollution, and smog. Our planet generates most of its electricity through heat engines, which convert heat into electricity. Approximately 40%–80% of the energy produced by thermal power plants is wasted. Gas, oil, or even electricity can be replaced with waste heat from power generation. In general, energy efficiency is greatly enhanced. In addition, by reducing fuel consumption and overall environmental impact, fuel costs can be drastically reduced. Heat engines are used to generate electricity using steam turbines, gas turbines, and reciprocating motors. Thermal motors are commonly used in CHP systems. However, fuel cells generate electricity using electrochemistry. Combined heat and power systems are also known as CHP systems. When analyzing CHP systems, two types of configurations should be analyzed: a topping cycle and a bottoming cycle. An energy source is a primary, or top, user of a topping cycle because it is the system that generates the electricity. During the electric force generation, heat remains and is used as an auxiliary energy source. CHP systems with bottoming cycles are the second type. A typical industrial process involves the production of high temperatures or large amounts of heat, like iron smelting, which leads to the electricity generating system, which must operate with the remaining energy after the primary heat users receive it. In addition, heat is produced by fuels, which are used at very high temperatures or large amounts of heat. A gasification process is composed of four typical processes: (i) drying; (ii) pyrolysis; (iii) oxidation; and (iv) reduction. The following components are found in synthetic gas: CH4, H2, CO, CO2, N2, and small amounts of coal, ash, tar, and oil. To provide a

334 Chapter 13 guide for future development, all the anode materials have been discussed. Also, one of the most important and fundamental technologies in the field of sustainable energy sources in the future is the conversion of gas into a bed of solid fuels.

Abbreviations and symbols APU BTL CHP DHC ER HHV ICE IEA LHV MGT ORC PEMFC SNG SOFC UPS

auxiliary power unit biomass to liquid combined heat and power district heating and cooling equivalent ratio high heating value internal combustion engine International energy agency lower heating value micro gas turbine organic Rankine cycles polymer electrolyte membrane fuel cell substitute natural gas solid oxide fuel cell uninterrupted power sources

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CHAPTER 14

Application of syngas in fuel cell Fatemeh Khodaparast Kazeroonian and Mohammad Reza Rahimpour Department of Chemical Engineering, Shiraz University, Shiraz, Iran

1. Introduction Nowadays, there is a growing demand for energy due to the increase in the world population and development in the global economy. Fossil fuels are still the primary energy sources worldwide; however, there are serious issues associated with burning them [1,2]. Fossil fuel resources are limited, and their price goes upward as supplies go down [3,4]. In addition, hydrocarbon fuel combustion causes detrimental impacts on the environment, such as global warming, ozone layer depletion, and air pollution by CO2, SOX, and NOX emission. In response to the mentioned challenges, researchers concentrate on adopting alternative renewable resources of energy such as solar energy, wind energy, wave energy, and fuel cell [4–6]. Compared to other green energy sources like solar energy, fuel cells are known as promising renewable energy sources because of their high efficiencies, reduction in the emission of greenhouse gases, and eco-friendly process [7]. Furthermore, they do not cause sound pollution during operation and occupy little space [8,9]. Having no mechanical part in their structure, the cost of maintenance of fuel cells is considerably low [3]. However, there are several limitations in fuel cell commercialization and industrial applications, for instance, low hydrogen production quality and high cost of hydrogen production, storage, and transportation [8]. The fuel cell history goes back to 1839 when William Grove discovered that reverse electrolysis of water could generate power. Grove immersed a platinum electrode in sulfuric acid solution and generated only a voltage of 1 V. He named his invention “gas batteries” [10,11]. In 1889, Ludwing Mond and Carl Langer developed the grove device by increasing the surface of the cell using porous electrodes and powered electrocatalysts and coined the term “fuel cell” for the first time [10–12]. In 1893, the functions of fuel cell elements and the relation between them were expressed by Friedrich Wilhelm Ostwald. [5,11]. During 1933–1959, Francis Thomas Bacon developed a fully operational alkaline fuel cell (AFC) for the first time. NASA used the AFC fuel cell (based on the Bacon fuel cell) in the Apollo space program in the1960s. In the same time, a proton exchange membrane fuel cell was developed by the General Electric Company and was applied in NASA’s Gemini space program [5,11]. In 1968, Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91878-7.00021-6 Copyright # 2023 Elsevier Inc. All rights reserved.

337

338 Chapter 14 Dupont Company introduced Nafion as a membrane in the proton exchange membrane fuel cell (PEMFC). It was found that Nafion could increase generated power in PEMFC by four times [11]. From the early twentieth century, extensive researches on fuel cells were undertaken to develop reliable fuel cell technology appropriated for commercial applications, including stationaries and transportations [5].

2. Fuel cell operation principle Fuel cell is an electrochemical device that directly converts the chemical energy of fuel into electrical energy based on the simple combustion reaction [5,13]. It involves two electrodes, anode and cathode as the positive and negative terminals, and an electrolyte in contact with electrodes, as shown in Fig. 1. The anode and cathode are electronic conductors, while ionic species are conducted through the electrolyte. The anodic and cathodic reactions are accomplished by anode and cathode at the anode/electrolyte and cathode/electrolyte interfaces. The reactions that take place in the fuel cell are as the following equations [3,14]: H2 ! 2H+ + 2e

Reaction in the anode : Reaction in the cathode : Overall reaction :

1 O + 2H+ + 2e ! H2 O 2 2

1 O ðgÞ + H2 ðgÞ ! H2 O + Electricity + Heat 2 2

(1) (2) (3)

In fuel cells, hydrogen fuel and oxidant (oxygen from the air) are supplied continuously into the anode and cathode electrodes, respectively. Hydrogen fuel decomposes into the positive

e−

e−

e−

e−

Water and Heat Out

Excess Fuel H+ H2 → 2H++2e−

H2

O2

2H++2e−+1/2O2→H2O

H+ Air in

Fuel in Anode Electrolyte Cathode

Fig. 1 Fuel cell schematic [3].

Application of syngas in fuel cell

339

and negative ions (H+ and e) at the anode section according to Eq. (1). An intermediate electrolyte between anode and cathode is electron barrier and only allows positive ions to pass through the electrolyte. On the other side of the fuel cell, to achieve stability, electrons tend to recombine. As a result, free electrons pass through an external electrical circuit, then in the cathode section, positive and negative ions recombine and form pure water and depleted oxidant (Eq. 2) [3,14,15].

3. Fuel cell types Fuel cells are developed in different types based on electrolyte material and utilized fuel [3,15]. Each type works in different operating temperatures and has various power outputs and efficiencies [5]. The main classification of fuel cells is mentioned as the following [15]: • • • • • •

Proton exchange membrane fuel cell (PEMFC) Solid oxide fuel cell (SOFC) Alkaline fuel cell (AFC) Phosphoric acid fuel cell (PAFC) Direct methanol fuel cell (DMFC) Molten carbonate fuel cell (MCFC)

Fuel cells are also classified according to the operating temperature as the following [10]: ▪ ▪ ▪

Low-temperature fuel cells operate in the temperature range of 20–100°C. Medium-temperature fuel cells operate in the temperature range of 200–300°C. High-temperature fuel cells operate in the temperature range of 600–1500°C.

Each of these fuel cells types is introduced in the following sections. Comprehensive comparison among different types of fuel cells based on material, working principle, fuels, applications, advantages, and disadvantages is mentioned in Table 1.

3.1 Proton exchange membrane fuel cells As mentioned in the introduction section, polymer fuel cells were first used in NASA’s Gemini space program [13]. Proton exchange membrane fuel cells are the most popular kind of fuel cells that are broadly used in domestic and transportation applications [3]. In the PEMFCs, a solid polymeric membrane is applied as the electrolyte for the purpose of ion exchange. The polymeric membrane is a great conductor for producing proton ions in the anode, i.e. H+, and an excellent barrier to electrons. The electrochemical reactions in PEMFC are as the Eqs. (1)–(3) [15]. The fuel of PEMFC is mainly pure hydrogen because PEMFC cannot tolerate impurities in the fuel especially carbon monoxide [15]. PEMFCs operate at 1–2 bar pressure and below

Table 1 A comprehensive comparison among different types of fuel cell based on material, working principle, fuels, applications, advantages, and disadvantages. Fuel cell type

SOFC

MCFC

PEMFC

AFC

PAFC

DMFC

Electrolyte [21]

Porous ceramic material

Molten carbonate salt

Polymeric ion exchange membrane

Potassium hydroxide (KOH)

Phosphoric acid

Solid polymer membrane

Operating temperature °C [1,11]

700–1000

550–700

Around 80

Around 70°C

Around 200

< 100°C

Efficiency [15,20]

50%–60%

50%

40%–50%

60%

40%

40%

Anode [21]

Nickel alloy

Nickel alloy

Platinum

Nickel

Platinum-carbon

Platinum-ruthenium

Cathode [21]

Lanthanum strontium manganite

Nickel oxide-lithium

Platinum

Silver

Platinum-carbon

Platinum-ruthenium

Charge carrier [15]

O

CO3

H+

OH

H+

H+

Catalyst [1,11]

Perovskites

Nickel

Platinum

Platinum or Ni alloys

Platinum

Pt/Ru (1:1)

Fuel [11,21]

H2, CO, methanol, and hydrocarbons

H2, Methanol, and, hydrocarbons

Pure H2

Pure H2

H2, LNG, and Methanol

Methanol

Oxidant [11,15]

O2 in Air

Air/CO2

O2 in Air

O2 in Air

O2 in Air

O2 in Air

Reaction

2

!H2O+2e



Anode

H2 +O

Cathode

1 2 O2

+ 2e ! O2

Overall

1 2 O2

+ H2 ! H2 O

-

 CO2 3 +H2!H2O+2e

+ CO2 CO2 + 2e ! CO2 3 1 2 O2 + H2 ! 2H2 O



H2 ! 2H + 2e +

1 + 2 O2 2H 

1 2 O2 + 

+ 2e ! H2 O + H2 ! H2 O

1 2 O2



H2 ! 2H + 2e



H2 +2OH !2H2O+ 2e H2 O + 2e + 12 O2 ! 2OH 1 2 O2 + H 2 ! H 2 O

2H+ + 2e ! H2 O 1 O 2 2 + H2 ! H2 O

+

1 2 O2 + 

CH3OH+H2O!6H+ + 6e + CO2 3 + 2 O2 + 6H + 6e ! 3H2 O 3 2 O2 + CH3 OH ! 2H2 O + CO2

Energy conversion efficiency [22]

Up to 90%

85%

85%–90%

85%

85%–90%

85%

Operating life cycle (h) [22]

Up to 40,000

Up to 15,000

40,000–50,000 (stationary) Up to 5000 (mobile)

Up to 5000

Up to 40,000

10,000–20,000

Major contaminants [5] Applications [1,15,20]

Sulfides

• • • •

Auxiliary power Distributed generation Electric utility Residential

Sulfides Halides

• • •

Distributed generation Transportations Industries

CO, H2S

• • • •

Backup power Transportation, Residential Emergency services

CO2

• • • • •

Spacecraft Transportation Portable power Submarines Backup power

CO, H2S

• • •

Distributed generation Transportation portable power

CO

•

Power generation in cell phones; laptops and portable devices

• • •

Power density (kW/m3) [15]

• • •

• • • •

•

• • • • • •

• • •

Long-life expectancy High efficiencies High-quality waste heat Good resistance to contaminants Internal reforming Fuel flexibility Inexpensive catalyst

• • •

Appropriate for CCS systems High efficiencies High-quality waste heat Good resistance to

contaminants

• • • • • • • • • •

Industry Portable power distributed generation in small scale

3.8–6.5

0.7–1.0

•

Low flexibility to employ in different applications Low electrolyte conductivity High cost Slow start-up Sensitive to sulfur Low-power density Extreme thermal stresses

Utility power plants Auxiliary power Electric utility distributed generation in large scale 1.5–2.6

0.8–1.0

• • •

Disadvantages [1,5,11,15]

•

0.1–1.5

Cell voltage [15] Advantages [1,5,11,15]

Utility power plants Commercial cogeneration; Portable power

1.1

• • • • • •

Simple scale-up High-power density Fast start-up Vast power range Modular Compact

Internal reforming fuel flexibility Inexpensive catalyst High activity Low-power density Cathode and catalyst dissolution High cost Corrosive electrolyte sensitive to sulfur Slow start-up Air crossover Need CO2 in the cathode

1

1.1

1

• • • • • • • •

High efficiency Low cost Fast kinetics High-power density Fast start-up High activity Easy heat management Inexpensive catalyst

0.8–1.9

• • • • • • •

• • • • • •

Intolerant to CO, H2S Heat and water Management issues Pure H2 requirement Low-quality waste heat Expensive catalyst

• • • • •

Sensitive to CO2 Expensive catalyst Pure H2 and O2 requirement Low-power density Corrosive electrolyte

0.6

• • • • • • •

High stability Moderate operating temperature Inexpensive Applicable for CHP systems Low vapor pressure Higher tolerance to CO2 High-quality waste heat Low efficiency and power density Intolerant to CO, H2S Low electrolyte conductivity Corrosive electrolyte Slow start-up Expensive catalyst High cost

0.2–0.4

• • • • •

•

• • • • • •

Low cost Without CO2 emissions Fast start-up Good resistance to CO Methanol is cheap with simple storage condition Compact

Lower efficiency and power density Fuel crossover Expensive catalyst Cathode poisoning Methanol is toxic and flammable Complex water management

342 Chapter 14 100°C temperature and is categorized in low-temperature fuel cells [1,3,10]. Working at such a low temperature makes them applicable for transportation, battery recharging, and technologies like cell phones and laptops [15,16]. There are several benefits associated with PEM fuel cells includes high-power density, fast start-up especially for vehicles, great mechanical structure, large range power output, and simple scale-up [1,15]. However, their operating efficiency is relatively low, around 40%–45%. For overcoming this issue, Pt catalyst is commonly used in the PEMFCs to accelerate the electrochemical reaction and increase its efficiency. Also, there are other negative points regarding PEMFCs such as weak management of heat and water, expensive catalyst, pure hydrogen requirement, high cost for pure hydrogen transportation, and high risk of CO poisoning of Pt catalysts [1,15,16]. The most common proton exchange membrane is Nafion by DuPont Company because of its good proton conductivity. However, other kinds of membranes like nanocomposites, phosphoric, and sulfuric acid-based membranes can work at higher temperatures. Working at higher temperature results in considerable increase in the resistance of PEMFC to carbon monoxide, therefore hydrogen with lower purity can be produced from natural gas as PEMFC fuel [13,16].

3.2 Solid oxide fuel cell Solid oxide fuel cells are categorized as the high-temperature fuel cells. The operation temperature is between 600°C and 1100°C [1]. Yttrium-stabilized zirconia (YSZ), a solid ceramic, is a common material applied as the SOFC electrolyte [1,15]. SOFC electrolyte is an oxygen-ion conductor, and only oxygen ions are allowed to pass. In the anode, O2 reacts with hydrogen and carbon monoxide in the fuel. Reaction products include H2, CO, CO2, and H2O are generated in the outlet of the anode [17]. Schematic of SOFC process is shown in Fig. 2. The electrochemical reactions in SOFC are as following: Reaction in the anode :

H2 + O2 ! H2 O + 2e

Reaction in the cathode : Overall reaction :

1 O + 2e ! O2 2 2

1 O ðgÞ + H2 ðgÞ ! H2 O + Electricity + Heat 2 2

(4) (5) (6)

The SOFC cathode materials must meet several criteria like catalytic activity, thermal stability, and stable ionic conductivity. A promising candidate with above-mentioned feathers is lanthanum strontium manganite (La, Sr) MnO3. Nickel-based YSZ (Ni/YSZ) also can be used as the anode to accelerate the oxidation of hydrogen [1]. SOFC stacks are categorized into different types includes planar and tubular [18]. SOFCs have several advantages includes (1) high efficiency about 50%–60% in the normal operating condition and 90% in combined heat and power (CHP) operation, (2) low emission of

Application of syngas in fuel cell

e− H2O+H2 +CO+CO2

H2O

e−

CO2 2−

H2+O

→H2O+2

CO+O2−→CO2+2e−

e− O2− O2−

e−

H2

e−

Depleted air with reduced fraction of O2 O2 O2

CO

343

1/2O2+2e− →O2−

O2− Air in

Fuel in

Anode

Electrolyte

Cathode

Fig. 2 SOFC operation principle schematic [20].

greenhouse gases, (3) using affordable catalysts as SOFCs can operate efficiently even without noble metals, (4) highly tolerant toward fuel impurities, and (5) ability to use a variety range of fuels such as natural gas, hydrocarbons, biogas, syngas, and methanol because of the ability to reform fuels internally at a high operating temperature [1,6,9,15]. However, due to the high operating temperature, material selection is a challenging issue. Moreover, the start-up of this process is slow, and it has a low tolerance to sulfur poisoning [1,2]. As a result of their outstanding benefits, SOFCs are employed in a wide range of applications, especially in cooling, heating, and power generation systems with medium and large capacities [2,15,19].

3.3 Alkaline fuel cells Alkaline fuel cells are the oldest type of fuel cells. They are classified as the low-temperature fuel cells with an operating temperature of around 70°C [1,3,15]. Due to the low operating temperature and low resistance to fuel impurities, the AFC is only fed by pure hydrogen [3]. AFC has high efficiency of 60%–70%. The applied electrolyte in this type of fuel cell is an alkaline-based solution like NaOH or KOH [1,15]. Schematic of AFC process is shown in Fig. 3. The AFC electrochemical reactions are as following [1]: Reaction in the anode : Reaction in the cathode : Overall reaction :

H2 + 2OH ! 2H2 O + 2e H2 O + 2e +

1 O ! 2OH 2 2

1 O ðgÞ + H2 ðgÞ ! H2 O + Electricity + Heat 2 2

(7) (8) (9)

344 Chapter 14

e−

e−

e− H2O+H2

e−

Depleted air with reduced fraction of O2

−

OH H2O

H2+OH−→2H2O+2e−

OH− H2

O2

1/2O2+2e− +H2O→2OH−

OH− Fuel in

Air in

Anode

Electrolyte

Cathode

Fig. 3 AFC operation principle schematic [20].

Fast start-up, high efficiency, simplest heat management, high activity, and inexpensive material are some of the benefits of AFCs. However, AFCs are intolerant to CO2 because CO2 consumes electrolyte, resulting in reduction of the hydroxide concentration and system efficiency. Therefore, it is necessary to use an external purification system to separate CO2 from air [1,15]. Another drawback is the short lifetime of AFCs because the electrolyte is corrosive that makes them not applicable for industrial applications. The main applications of AFCs are transportation and space shuttle [13,15].

3.4 Phosphoric acid fuel cell PAFCs are the first commercial fuel cells in which the mechanism of electrolyte is proton-driven same as PEMFCs. PAFCs use phosphoric acid (H3PO4) as the electrolyte, pure hydrogen as fuel, and usually platinum as the electrode. The PAFC electrochemical reactions are as the Eqs. (1)–(3) [1,3,13]. It is considered as a medium-temperature fuel cell and works at a temperature range of about 150–220°C. Because of medium operating temperature, waste heat can be exploited in PAFCs. Compared to PEMFCs, PAFCs have higher activity, resulting in higher resistance to CO poisoning and impurities in the fuel and lower need of Pt catalyst. However, they have slower start-up and less ionic conductivity. One of the main applications of PAFCs is stationary power generation, and it is a promising candidate for CHP systems [1,3,15].

Application of syngas in fuel cell

345

3.5 Direct methanol fuel cell DMFCs, similar to PEMFC, uses polymer membrane electrolyte. DMFC is fed by alcohol, commonly methanol (MeOH), as fuel and converts it to hydrogen internally. It works at a temperature below 100°C and is classified as low-temperature fuel cells [1,3,13]. The DMFC electrochemical reactions are as following: Reaction in the anode :

CH3 OH + H2 O ! 6H+ + 6e + CO2

Reaction in the cathode : Overall reaction :

3 O + 6H+ + 6e ! 3H2 O 2 2

3 O + CH3 OH ! 2H2 O + CO2 + Electricity + Heat 2 2

(10) (11) (12)

DMFCs have the ability to supply a maximum voltage of 0.3–0.5 V, which make them appropriate for portable power applications below 260 W such as cameras and computers [1,3,15]. A layer of Pt and Ru is used as the catalyst in DMFC. PtCOH and PtCO are produced as undesirable products by the reaction between platinum and methanol are toxic agents of catalyst in case of using Pt solely. Therefore, Ru can protect the Pt from CO poisoning [1,13]. Waste resources can be used for methanol and ethanol production as fuel, which is one of the great benefits of DMFCs. Other advantages of DMFCs include fast start-up, less footprint, cost-effectiveness, and no need for external fuel reformer. In addition, storage, utilization, and transportation of methanol are comfortable because methanol is liquid at the standard condition [1,13,15].

3.6 Molten carbonate fuel cell In molten carbonate fuel cells (MCFCs), the electrolyte is a mixture of molten carbonate salts, typically lithium and potassium (or sodium) carbonates, stored in a porous ceramic matrix or tile [13,17]. Porous electrodes with high conductivity are used as anode and cathode [15]. Molten carbonate salts show good conductivity at the temperature as high as 600°C, and MCFC is considered a high-temperature fuel cell [13]. This kind of fuel cell, because of its high operating temperature, has the ability for internal reforming of fuels. So a wide range of fuels such as hydrocarbons and natural gas can be applied in the MCFCs [1,3,15]. The schematic of MCFC process is shown in Fig. 4. The MCFC electrochemical reactions are as the following [13]: Reaction in the anode :

 CO2 3 + H2 ! H2 O + 2e + CO2

Reaction in the cathode : Overall reaction :

1 O + CO2 + 2e ! CO2 3 2 2

1 O ðgÞ + H2 ðgÞ ! H2 O + Electricity + Heat 2 2

(13) (14) (15)

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e− e−

H2O+CO2 +H2 CO2

e− e−

H2O CO32− H2+CO32− →H2O+CO2+2e−

Unconverted O2 and CO2

CO32−

H2

CO2 O2

1/2O2+CO2+2e− →CO32−

CO32− Fuel in

O2+CO2 in

Anode

Electrolyte

Cathode

Fig. 4 MCFC operation principle schematic [20].

According to the above equations, produced CO2 in the anode can be used in the cathode. CO2 also can be supplied from an external resource. So, MCFC can be employed as a carbon capture and storage system (CCS) [11,13].

4. Fuel cell applications Fuel cells have a wide range of applications. Based on the type of fuel cell, generated electricity varies from 1 W for compact portable power units such as cell phones to 10 MW for massive stationary power plants. Operating characteristics and conditions, cost, efficiency, fuel availability, and fuel processing are several factors that affect fuel cell application [5,13,14]. Low-temperature fuel cells include PEMFC and AFC are appropriate for emergency and portable applications due to the fast warm-up. In contrast, utilization of waste heat in the medium- and high-temperature fuel cells (SOFC, MCFC, and PAFC) make them applicable for heating in domestic heating applications and CHP systems [5]. The classification of number fuel cell units based on fuel cell type and region that were shipped over five years between 2014 and 2018 are indicated in Figs. 5 and 6, respectively [10]. Fuel cells are mainly used for stationary power plants, private, and public transportation, maritime applications, building heating systems, and portable power generation [14]. The number of shipped fuel cell units from 2014 to 2018 for portable applications, stationary power plants, and transportation as main applications of fuel cells are reported in Fig. 7 [10]. Some of the applications are described in the following sections.

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80

Thousands of Units Shipped

70 60 50 40 30 20 10 0

2014 PEMFC

2015 SOFC

2016 DMFC

PAFC

2017 MCFC

2018 AFC

Fig. 5 Fuel cell shipments by fuel cell type [10].

Thousands of Units Shipped

80 70 60 50 40 30 20 10 0

2014 Asia

2015 North America

2016 Europe

2017

2018

Rest Of World

Fig. 6 Fuel cell shipments by region [10].

4.1 Transportation applications In recent years, a significant application of fuel cells is private and public transportation. Based on the environmental protection agency reports, about a quarter of greenhouse gas emissions result from transportation devices fueled by fossil fuels. So, the first and foremost reason for fuel cell development in the transportation sector is using hydrogen as a clean and eco-friendly fuel instead of fossil fuels for decreasing the emission of toxic and greenhouse

348 Chapter 14 80

Thousands of Units Shipped

70 60 50 40 30 20 10 0

2014

2015

Portable application

2016

Stationary application

2017

2018

Transport application

Fig. 7 Fuel cell shipments by application [10].

gases into the environment. Second, by replacing the combustion engines with fuel cells system in vehicles, make them simple by removing complicated mechanical parts [3,11,13]. Mentioned advantages attract big car manufacturers’ attention to shift their market to using fuel cells, especially PEMFCs [11]. Toyota, Honda, and Hyundai were the first hydrogen car manufacturers [12]. Currently, General Electric Company is one of the pioneer manufacturers who put fuel cell technology in their future design. They have a plan to replace mechanical parts of the vehicles with wires and electrical components to reduce the risk of failure [13]. It is anticipated that fuel cell vehicles will replace combustion engine vehicles by 2030 [3]. Despite all the mentioned fuel cell benefits in automotive applications, the main difficulty associated with applying fuel cells, is controlling and converting electrical energy to mechanical energy. Another challenge is the storage system for hydrogen, which is heavy, large, and expensive, and also has safety hazards. Therefore, car manufacturers are focused on solving problems facing mass production [3,11].

4.2 Stationary power plant Stationary applications of fuel cells include supply of emergency backup power, distributed generation or CHP systems, stand-alone power stations, and power supply for remote places [10,11]. A number of stationary power plants exploiting fuel cell technology have been constructed during recent years. At the moment, Ballard Generation Power Plant is the largest plant that uses PEMFC fueled by natural gas with a capacity of 250kW [12]. However, due to the low efficiency of PEMFC resulting from external reforming requirements,

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high-temperature fuel cells such as SOFCs and MCFCs with high efficiency can be employed [11]. In cogeneration systems and CHP generation, large quantities of waste heat and hot water that are produced in stationary power plants, can be exploited in other processes such as space heating that results in increasing the efficiency of the plant. As a result, high-temperature fuel cells and, in particular SOFCs, are more appropriate for CHP generation application [12,23]. According to the literature, the stationary power application of fuel cells has been increased from 2014 to 2018, and it has an specific place in the commercial market of fuel cells (Fig. 7) [10]. Currently, Japan is the pioneer in using fuel cells for remote residential places for the purpose of power generation and water heating [11].

4.3 Portable applications Fuel cells are promising candidates for power generation in compact portable systems and can efficiently replace batteries [11,12]. Portable fuel cells are commonly used to generate power from 5W to 500W for various applications. The applications can be mainly categorized into two groups: (1) portable power generation for emergency relief efforts and light commercial and personal applications such as camping and portable signage in the remote areas where there is no access to grid connection and (2) portable power generation for consumer electronic devices such cellphones, radio, laptops, and devices use batteries. Other rapidly growing markets are educational remote control vehicles, portable battery chargers, kits, and toys [5,23]. The energy density of fuel cells is 5–10 times higher than batteries that makes devices more compact and lighter for the same amount of generated power. Furthermore, by employing fuel cell technology, the device can be recharged by refilling or replacement of fuel cartridge without any need for electricity sources. Due to the fact that operation for a long time, being compact and low weight are critical requirements for portable electronic devices like cell phones and laptops, fuel cell technology has a great potential for future portable electronic devices. However, fuel cells are expensive and fueling them with hydrogen is followed by storage and safety problems. Hence, liquid fuels and methanol are attractive fuels in portable applications [5,11,23].

4.4 Maritime application In recent years, due to some features of fuel cells such as making almost no noise, utilizing hot water as by-product in onboard reformers, and infrared signals are attractive for submarine applications and ships [3,12]. In the ship application, fuel cells are mainly used as advanced electric ship demonstrators (AESD). However, it needs more space as well as more care about people and vulnerable equipment around it, because of high operating temperature of fuel cell. The most suitable type for ship application is PEMFC due to its low operating temperature, fast start-up, and long lifetime [3]. MCFC and SOFC are the other most used fuel cell types for

350 Chapter 14 maritime applications. Maritime applications of fuel cells are widely viewed in literature [21,24].

5. Fuels for fuel cells Pure hydrogen and oxygen are the typical fuel and oxidant in the fuel cells, respectively. However, there are some fuel cells that can be fueled by other fuel resources, including methanol, ethanol, natural gas, syngas, hydrocarbons, and ammonia [3,6,7,21]. Generally, low-temperature fuel cells use only pure hydrogen as fuel, while high-temperature fuel cells can apply hydrocarbons and methane, and reform them internally to hydrogen gas [24].

5.1 Hydrogen fuel for fuel cell Hydrogen is the main fuel for fuel cells due to fast electrochemical oxidation [24]. It is considered a clean and sustainable energy source that can be a promising solution to the world concerns about the use of fossil fuels [25,26]. Compared with internal combustion engines, the efficiency of hydrogen conversion to electricity in fuel cells is higher. However, the low storage density of hydrogen makes hydrogen storage and transport difficult and costly, which is the main drawback of using hydrogen as fuel [21,24]. Furthermore, there is a need for a great space for storage even by liquefication of hydrogen through a temperature of 53°C or pressure of 350 bar [21]. To address this issue, onsite hydrogen production has been extensively investigated [27]. Although hydrogen is one of the most plentiful elements on the earth, molecular hydrogen is hardly seen in nature [24,25,28,29]. Hydrogen gas is produced from fossil fuels (mainly naturalgas), coal, biomass, and water. Currently, fossil fuel reforming and water electrolysis are the two main hydrogen production technologies [21]. About 96% of hydrogen is produced from sources that are not renewable such as natural gas and coal, only 4% is produced by water electrolysis [5,26]. It is anticipated that other methods like photoelectrochemical and biological conversion also could be used in the future [21].

5.2 Methanol fuel for fuel cell Methanol or methyl alcohol is liquid in the atmosphere condition. So methanol’s storage, transportation, and safety conditions are without any complexities and can be applied for traditional liquid fuel foundation without any needs to make significant adjustments [24,30]. Moreover, these characteristics make methanol appropriate for portable applications. Methanol oxidation is as the following equation [31]: CH3 OH + H2 O ! CO2 + 6H+ + 6e

(16)

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Natural gas is the main source of MeOH production; however, it can be made from synthesis gas and biogas. MeOH can be directly used as fuel for DMFCs. An alternative application is reforming of MeOH to hydrogen in both separated and combined system with the fuel cell units at the moderate temperature. A good example can be integrating a methanol-reforming system with a high-temperature PEMFC that shows a favorable result [24,30]. Methanol crossover is one of the most significant challenges associated with employing MeOH as a fuel in the fuel cell [31].

5.3 Dimethyl ether fuel for fuel cell Due to the great features of dimethyl ether (DME), it can be considered a promising source for both hydrogen production and fuel for fuel cells. DME can be converted to hydrogen-rich gas through fuel processing methods or used directly as a fuel in the direct DME fuel cell. It is produced from synthesis gas or dehydration of methanol. The energy density of DME is greater than the methanol (8.2 kWh/kg and 6.1 kWh/kg for DME and MeOH, respectively). Furthermore, fuel crossover for DME is less than methanol as the lower dipole moment in DME (1.3 D and 1.69 D for DME and MeOH, respectively). DME oxidation occurs as the following equation [24,32]: CH3 OCH3 + 3H2 O ! 2CO2 + 12H+ + 12e

(17)

Since there is no CdC bond in the structure of DME, complete oxidation of DME is possible, and it can help reduce coke formation in the fuel cells, a common issue in fuel cells fed by fuels containing carbon–carbon bonds like ethanol [24]. DME is less toxic and can be liquefied easily at ambient conditions. Therefore, DME storage and transportation are simple and inexpensive and consequently could solve the storage challenges of hydrogen utilization as a fuel [32,33].

5.4 Ammonia fuel for fuel cell Due to the hydrogen storage and transportation matters, ammonia can be used as hydrogen carrier for fuel cells. Ammonia is in the liquid phase at 33°C and atmospheric pressure. Therefore, the storage and transportation of ammonia are easy and low cost. For the purpose of hydrogen provision for fuel cells, ammonia decomposes to H2 and N2 gas at the temperature of 300–520°C. It can decompose internally in SOFC at high operation temperature in the presence of catalyst. Due to the absence of sulfur and carbon elements, fuel cells do not face CO and sulfur poisoning as well as coke formation. However, ammonia energy density is slightly lower than MeOH, and ammonia can cause several issues for human health and the environment [21,24].

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5.5 Natural gas fuel for fuel cell Natural gas is one of the main sources of energy all over the world. Natural gas normally comprises mainly methane more than 85 Vol%, 2–3 Vol% ethane (C2H6), 1–2 Vol% propane (C3H8), less than 1 Vol% butane (C4H10), less than 1 Vol% pentane (C5H12), less than 1 Vol% H2S, 1–5 Vol% N2, and slightly higher hydrocarbons. It can be come from fossil feedstock as the main source and biogas [24,34]. Liquefied natural gas (LNG) has a lower effective volumetric energy density than diesel fuels. However, because of its environmental benefits compared to diesel fuels, natural gas is more attractive. Fewer greenhouse gases, SO2 and NOx, are emitted to the environment by natural gas burning rather than coal and petroleum. For instance, carbon emission is half compared to burning coal. The H/C ratio for natural gas is 4, while this ratio is 2 and 1 for oil and coal, respectively. Therefore, a greater proportion of energy per emitted CO2 is obtained for natural gas. At present, natural gas is the most significant source of hydrogen and methane. About 75% of hydrogen is produced by natural gas reforming [24,34]. Using LNG to produce on-board hydrogen production is more effective, denser, and less expensive than applying pure hydrogen. Natural gas can also be used as fuel directly in the fuel cells, especially it is one of the appropriate fuels for SOFCs. Most high-temperature fuel cells are currently constructed to be fed by natural gas because of their high efficiency [24,35].

5.6 Syngas fuel for fuel cells Although syngas is a gas mixture that mainly contains H2 and CO, based on the type of process and fuel source employed for syngas production, other components like CH4, CO2, N2, H2O, and impurities also can be found in the syngas [6,21,36–38]. For instance, steam can be found in produced syngas by steam reforming; however, in partial oxidation reforming due to oxidation of fuel by air, there is a significant amount of N2 in produced syngas [37]. Syngas is an eco-friendly alternative to fossil fuels that can be produced from the gasification of biomass and coal and the reforming of methane, natural gas, and biogas [6,36]. One of the main applications of syngas is fuels for fuel cells [39]. Based on fuel cell type, syngas can be fueled to the fuel cell, directly or after several purification processes. Syngas requirements to be fed into the different types of fuel cell are reported in Table 2. Due to the fuel flexibility and low sensitivity of high-temperature fuel cells to impurities in comparison to low-temperature fuel cells, syngas is a potential alternative fuel instead of pure hydrogen for SOFC and MCFC [6,14,37,40,41]. Among high-temperature fuel cells, MCFC because of corrosive electrolyte is not very applicable, while due to the solid electrolyte in SOFC that results in high mechanical resistance, SOFCs are more appropriate to be fueled by syngas or hydrocarbons [6,41,42]. With regard to operating temperature as high as 800°C in SOFC, it not only can tolerate CO in the syngas but also can be fueled by pure CO [37]. SOFC can convert

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Table 2 Syngas requirements to be fed to the different types of fuel cell [40]. Impurity Fuel cell types

Particulate

Total sulfur

Total chlorine

NH3

Tar