Advances in Synthesis Gas: Methods, Technologies and Applications, Volume 1: Syngas Production and Preparation 0323918719, 9780323918718

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Advances in Synthesis Gas: Methods, Technologies and Applications

Advances in Synthesis Gas: Methods, Technologies and Applications Volume 1: Syngas Production and Preparation 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 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 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 Manuel Bailera Graduate School of Creative Science and Engineering, Waseda University, Tokyo, Japan; Department of Mechanical Engineering, Universidad de Zaragoza, Campus Rı´o Ebro, Bldg. Betancourt, Zaragoza, Spain Idris Bakare Interdisciplinary Research Center for Hydrogen and Energy Storage, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia Ali Bakhtyari Chemical Engineering Department, Shiraz University, Shiraz, Iran Mohammad Bonyadi Department of Chemical Engineering, Faculty of Engineering, Yasouj University, Yasouj, Iran Felipe Gomes Camacho Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, Calgary, AB, Canada Guoxing Chen Fraunhofer Research Institution for Materials Recycling and Resource Strategies IWKS, Alzenau, Germany Camilla Fernandes de Oliveira Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, Calgary, AB, Canada Silvio de Oliveira Junior Polytechnic School, University of Sao Paulo, Department of Mechanical Engineering, Sao Paulo, Brazil Maryam Delshah Department of Chemical Engineering, Shiraz University, Shiraz, Iran Meire Ellen Gorete Ribeiro Domingos Polytechnic School, University of Sao Paulo, Department of Chemical Engineering, Sao Paulo, Brazil Moises Teles dos Santos Polytechnic School, University of Sao Paulo, Department of Chemical Engineering, Sao Paulo, Brazil Swarit Dwivedi Department of Chemical and Biological Engineering, Monash University, Clayton, VIC, Australia Razieh Etezadi Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA, United States

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Contributors Azharuddin Farooqui Department of Chemical and Petroleum Engineering, University of Calgary; Vorsana Environmental Inc, Calgary, AB, Canada Daniel Flo´rez-Orrego Polytechnic School, University of Sao Paulo, Department of Mechanical Engineering, Sao Paulo, Brazil; Faculty of Minas, National University of Colombia, School of Processes and Energy, Medellin, Colombia Andrea Galeazzi Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”, Piazza Leonardo da Vinci, Milan, Italy Xingyuan Gao Department of Chemistry and Material Science, Guangdong University of Education, Engineering Technology Development Center of Advanced Materials & Energy Saving and Emission Reduction in Guangdong Colleges and Universities, Guangzhou, People’s Republic of China; Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore, Singapore Jonathan Harding Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, United Kingdom Baishali Kanjilal Bioengineering, University of California, Riverside, CA, United States Milanpreet Kaur Department of Chemistry, Faculty of Science, University of Calgary, Calgary, AB, Canada Sibudjing Kawi Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore, Singapore Mohammad Hasan Khademi Department of Chemical Engineering, College of Engineering, University of Isfahan, Isfahan, Iran Parvin Kiani Department of Chemical Engineering, Shiraz University, Shiraz, Iran Soheila Zandi Lak Department of Chemical Engineering, Faculty of Engineering, Yasouj University, Yasouj, Iran Pilar Lisbona Department of Mechanical Engineering, Universidad de Zaragoza, Campus Rı´o Ebro, Bldg. Betancourt, Zaragoza, Spain Mohammad Lotfi-Varnoosfaderani Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran Nader Mahinpey Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, Calgary, AB, Canada Mohammad Amin Makarem Methanol Institute, Shiraz University, Shiraz, Iran Zuhair Omar Malaibari Chemical Engineering Department; Interdisciplinary Research Center for Refining and Advanced Chemicals, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia Flavio Manenti Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”, Piazza Leonardo da Vinci, Milan, Italy Arameh Masoumi Bioengineering, University of California, Riverside, CA, United States Maryam Meshksar Department of Chemical Engineering; Methanol Institute, Shiraz University, Shiraz, Iran Masoumeh Mohandesi Department of Chemical Engineering, Shiraz University, Shiraz, Iran Galal Nasser Interdisciplinary Research Center for Hydrogen and Energy Storage, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia

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Contributors Francesco Negri Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”, Piazza Leonardo da Vinci, Milan, Italy Rafael Nogueira Nakashima Polytechnic School, University of Sao Paulo, Department of Mechanical Engineering, Sao Paulo, Brazil Iman Noshadi Bioengineering, University of California, Riverside, CA, United States Alireza Palizvan Department of Chemical Engineering, College of Engineering, University of Isfahan, Isfahan, Iran Song Won Park Polytechnic School, University of Sao Paulo, Department of Chemical Engineering, Sao Paulo, Brazil Virginia Perez Centre for the Development of Renewable Energy - Centre for Energy, Environment and Technology Research (CEDER-CIEMAT), Soria, Spain Kristiano Prifti Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”, Piazza Leonardo da Vinci, Milan, Italy Shuxian Qiu Department of Chemistry and Material Science, Guangdong University of Education, Engineering Technology Development Center of Advanced Materials & Energy Saving and Emission Reduction in Guangdong Colleges and Universities, Guangzhou, People’s Republic of China Hamid Reza Rahimpour Department of Chemical Engineering, Shiraz University, Shiraz, Iran Mohammad Reza Rahimpour Department of Chemical Engineering, Shiraz University, Shiraz, Iran Fatemeh Salahi Department of Chemical Engineering, Shiraz University, Shiraz, Iran Mohammed Sanhoob Interdisciplinary Research Center for Hydrogen and Energy Storage, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia Akshat Tanksale Department of Chemical and Biological Engineering, Monash University, Clayton, VIC, Australia Theodore Tsotsis Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA, United States Xin Tu Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, United Kingdom Luis D. Virla Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, Calgary, AB, Canada Ni Wang Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, United Kingdom Yaolin Wang Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, United Kingdom Shabnam Yousefi Department of Chemical Engineering, Shiraz University, Shiraz, Iran Samira Zafarnak Department of Chemical Engineering; Methanol Institute, Shiraz University, Shiraz, Iran Zahra Zarei Chemical Engineering Department, University of Sistan and Baluchestan, Zahedan, Iran Fatemeh Zarei-Jelyani Department of Chemical Engineering, Shiraz University, Shiraz, Iran Linghao Zhao Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA, United States Sara Zolghadri Department of Chemical Engineering, Marvdasht Branch, Islamic Azad University, Marvdasht, 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: 978-0-323-91871-8

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Preface Vol. 1: Syngas production and preparation 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 producing syngas has attracted the attention of many academics, researchers, and industries owing to the innate benefits. In the current scenario, a number of conventional processes exist for producing syngas including, but not limited to, steam reforming, dry reforming, partial oxidation, and gasification. Selecting the syngas production method is mainly based on feedstock type and the desired composition for downstream utilization. Following the necessity to address the challenges of producing syngas such as deactivation of catalysts with coke deposition and sintering, this engaging text touches on all the cutting-edge strategies serving as a link between the scientists in the research laboratories and the operators in industrial plants to solve these problems. The book has two sections, the first of which introduces common syngas production methods and technologies including steam reforming, gasification, partial oxidation, and pyrolysis, as well as novel technologies such as electrolysis, plasma, and microchannels in separate chapters. In the second section, syngas production sources such as coal, black liquor, food and plastic wastes, lignin, and microalgae are described in detail. These two sections together provide insights into the current status and the prospect of syngas generation on a large scale.

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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 such as 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

Syngas characteristics: Physical and chemical properties Masoumeh Mohandesi, Hamid Reza Rahimpour, and Mohammad Reza Rahimpour Department of Chemical Engineering, Shiraz University, Shiraz, Iran

1. Introduction The usage of fuels has evolved since the discovery of fire, with man transitioning from timber to coal, then to oil, and eventually to natural gas, while simultaneously increasing the worldwide use of fuels by an order of magnitude [1]. The ensuing CO2 emissions increased, greater than 400 ppm concentration in the atmosphere (ppm), which may be the primary driver of climate change as a result of global warming [2]. As a result, the demand for alternative and clean energy is a major subject of interest for academics around the world working to develop sustainable energy sources. When it comes to producing sustainable energy, synthetic gas has enormous promise. It can meet the rising demand for liquid and gaseous fuels and power while also reducing greenhouse gas emissions. Synthesis gas is a promising technology for improving energy efficiency, fuel production, electricity generation, and pollutant elimination, while serving as a transitional step from carbon-based to hydrogen-based fuels. Carbon monoxide, hydrogen, and other gases in small quantities are mixed together to form synthesis gas, which is also known as syngas. Gas derived from natural gas and other hydrocarbons is used to create this gas. The production of syngas is accomplished through the reforming process that involves either an endothermic or an exothermic reaction, depending on the chemical technique that is used to implement the methane catalytic reforming process. Biomass can also be used to generate synthetic gas [3]. This can be accomplished through a variety of processes including biomass gasification [4,5] and bio-oil reforming using steam [6–9]. Every year, approximately 6 EJ of syngas is produced around the world, about 2% of the global total primary energy consumption at present. Many applications require the generation of synthesis gas, such as the production of hydrogen from natural gas for use in fuel cell operations. Fischer–Tropsch is used to generate light synthetic crude oil or heavy

Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91871-8.00003-9 Copyright # 2023 Elsevier Inc. All rights reserved.

3

4

Chapter 1

Fig. 1 Current global market for syngas [3].

waxy hydrocarbons, as well as the synthesis of methanol and ammonia [3,10,11]. On the worldwide syngas market (mostly derived from fossil fuels such as natural gas, coal, and oil/ residues), the ammonia industry holds a dominant position [12]. Other important applications include the creation of hydrogen intended for use in refineries, as well as hydrogen synthesis and the production of methanol. Fig. 1 depicts syngas present market usage at the time of writing [13]. A more ecologically friendly, adaptable, and cost-effective alternative would be to synthesize higher alcohols directly from syngas (CO + H2), rather than relying on fermentation of sugars (ethanol and isobutanol) or hydration of alkenes produced from petroleum (heavier alcohols). Since its inception in the 1930s, this reaction’s research effort has fluctuated with the price of oil, it has increased significantly in the last decade due to an increased focus on shale gas and other renewable resources as supplies of gaseous feedstocks [14]. To further boost the performance of the internal combustion engine, syngas has lately begun to be used in the transportation sector.

2. Production of syngas A syngas process can be broken down into the following fundamental components (Fig. 2):

Fig. 2 Process phase in syngas power plants [15].

Syngas characteristics: Physical and chemical properties 5 Where: • • •

Feed preparation includes unit operations such as heating, purifying the feedstock, and adding additives such as steam, CO2, etc. Reactions include steam reforming, gasification, pyrolysis, etc. Purifications are unit operations that change the composition of the product to meet downstream needs and remove things that aren’t good for the product, such as water if the product needs to be dry, H2S, CO2, etc.

Syngas processes are often defined by the following: • • • • • •

Huge manufacturing volumes Catalytic reactions in the gaseous phase High pressure (up to 100 bar) Temperatures ranging from 0°C to 2500°C during condensation and combustion Generation of steam, superheating of steam, and condensation of steam as water Extremely energy-efficient processes with feed consumptions of several hundred MW [15].

2.1 The reaction of methane reforming When it comes to producing syngas, reforming is the most common method used in both industry and laboratory research. This includes a steam reforming of methane (SRM), dry reforming of methane (DRM), and partial oxidation of methane (POM), as well as, bireforming of methane (BRM) and autothermal reforming of methane (ARM) [16–18]. The advantage of SRM is that it generates syngas with a large concentration of hydrogen, which is advantageous because methane is distinguished to have the highest H/C ratio of all of the hydrocarbons in use. Additionally, the method is based on inexpensive nickel-based catalysts. Heat is essential for the creation of both steam and the progression of the endothermic reaction. The heat from cooling syngas can be used to make heat without needing to burn methane, which is often used as a heat source. At around 900°C, a full conversion of methane can be achieved. The use of hydrogen separation during the process, such as membrane utilization, allows to achieve 100% methane conversion at a lower temperature, and focused solar heat might be used to substitute methane burning, thereby lowering the process’s carbon output. High pressure is detrimental to the progression of the reaction due to the increased mole number. But when compressed hydrogen is required, pressures up to 30bar are being used to reduce the capacity of the reactor, as well as the amount of electricity required for the compression [18]. In the DRM, the interaction between CH4 and CO2 is the most important one among a number of other reactions. Apart from the feed ratio and pressure, thermodynamic properties have an effect on the optimal temperature for the reaction and the number of occurred side reactions. Additionally, with regard to temperature and CO/H2O ratio, the reaction system adjusts [19]. In most cases, gasification (coal, heavy hydrocarbons) and reforming are the primary sources of

6

Chapter 1

this H2/CO combination (light hydrocarbons) [20]. Carbon sources are reacted at high temperatures by steam, oxygen, or carbon dioxide mixes. The reaction conditions and the content of the generated syngas might substantially vary depending on the agent used in different technologies. A different mixture is needed for each application. For methanol and Fischer–Tropsch synthesis, the most frequent H2/CO ratio is 2, but higher ratios are needed for the hydrogen production [18]. Lower H2/CO ratios of roughly 1 are used in the production of ethanol and other alcohols, as well as dimethyl ether and oxo-alcohols. Further lowering of the ratio results in pure carbon monoxide, which is a toxic gas when breathed in. Formic acid and acetic acid are two examples of tiny organic molecules that can be synthesized using this gas alone, without the assistance of hydrogen [21]. Table 1 depicts various types of reforming procedures, as well as some information about each method. To illustrate how different fuels react in different processing steps, Table 2 includes equations for four representative fuels: natural gas (CH4) and liquefied propane gas (LPG) for stationary use, liquid hydrocarbon fuels (CmHn), methanol (MeOH) for mobile use, and coal gasification for large-scale industrial use to produce syngas and hydrogen [23].

2.2 Gasification of biomass/coal for the production of fuels and chemicals When using a solid material such as biomass or coal as a starting point, the feedstock must be gasified before it can be used. The particle size distribution is determined by the kind of gasification [24]. After gasification, the end result is a gas referred to as producer gas, which contains impurities and must be cleaned up before it can be utilized. Prior to synthesis, the gas producer typically requires a ratio of H2 to CO [13]. 2.2.1 Gasification As previously mentioned, synthesis gas can be produced from any hydrocarbon feedstock, such as coal, heavy oil, or combustible biomass, via gasification. Low pressure (1–20bar) and high temperature (800–1000°C) are required for a successful reaction, and according to the technologies, the H2/CO ratio ranges between 0.5 and 1.8. O2, H2O, air, or a combination of these agents is used to partly oxidize the solid carbon. CxHyOz is primarily composed of methane with trace amounts of hydrocarbons such as ethane and ethylene (Table 3). In the majority of gasifiers, gas may also contain heavier hydrocarbons such as toluene, naphthalene, and benzene according to the feedstock and operational conditions. Hydrocarbons that are greater in weight than benzene are frequently referred to as “tars” [3]. In most cases, the gasification efficiency is calculated on the basis of the lower heating value. In order to compute the efficiency, the total amount of energy contained in the producer gas (practical as well as chemical) is expressed as a percentage of the total energy contained in the

Table 1 Compare and contrast the conventional mainstream methane reforming procedures[22]. Reforming type

Reaction conditions

SRM

H2/CO ratio

ΔH (KJ/mol)

Advantages

P ¼ 3–25 atm T ¼ 250–1000°C CH4/H2O ¼ 1.5

3

228

1. A greater concentration of the active ingredient in the product 2. High operational efficiency 3. Maturity of industry

1. Relatively inefficient energy use 2. Relatively high reaction unit requirement 3. Inappropriate products for F-T synthesis

POM

P ¼ 100 atm T ¼ 950–1100°C CH4/O2 ¼ 2

2

22.6

1. Enhanced susceptibility to sulfides 2. Short duration of contact 3. More energy efficient 4. High conversion/ selection efficiency

1. Expensive pure O2 feedstock 2. Possibility of uncontrolled combustion

ARM

CH4/H2O/ O2 ¼ 1/1/0.5

1 or 2 based On feed Composition

Negative value Depended on feed Composition

1. More energy efficient 2. Reduced contact duration 3. Decreased coke deposition 4. Adjustable and adaptable composition of the feed

1. Decreased H2 yield 2. A maximum H2O/O2 ratio 3. Possibility of uncontrolled combustion

BRM

CH4/H2O/ CO2 ¼3/2/1 T ¼ 650–900°C

2

220

1. Lower levels of coke deposition 2. Flexible and customizable composition of the feed

1. Relatively high reaction unit requirement 2. Expensive separating of CO2 from target goods

Disadvantages

Continued

Table 1

Compare and contrast the conventional mainstream methane reforming procedures—cont’d

Reforming type

Reaction conditions

DRM

P ¼ 1 atm T ¼ 650–900°C CH4/CO2 ¼ 1

1. The process of converting two greenhouse gases simultaneously 2. Generation of clean power and fuel 3. Optimum H2/CO for the synthesis of F-T

1. Difficulties associated with catalyst deactivation as a result of coke and metal sintering 2. Extremely stringent requirements for CO2 activation

Plasmaassisted

T ¼ 400 ° C

1. High ionization activation energy efficiency 2. Mild thermodynamic circumstances relative to others

1. Overpriced glow discharge systems 2. Ambiguous mechanism of reaction

Photoassisted

T ¼ 400 ° C

1. Breaking beyond the thermodynamics limit and decreasing energy usage

1. Available catalysts in photocatalytic materials 2. Few comparative research

Microwaveassisted

T ¼ 800 ° C

1. Instantaneous heat generation 2. Relatively straightforward experimental design

1. Inadequate management of a scalding hot zone 2. Difficulty in monitoring the temperature of the catalyst

H2/CO ratio

ΔH (KJ/mol)

Advantages

Disadvantages

Syngas characteristics: Physical and chemical properties 9 Table 2 Equations for the probable reactions that occur during the various processing steps [23]. Name of reactions

Possible reactions CH4 + H2O $ CO + 3H2 CmHn + mH2O $ mCO + (m + n/2)H2 CH3OH + H2O $ CO2 + 3H2 CH4 + O2 $ CO + 2H2 CmHn + m/2O2 $ mCO + n/2H2 CH3OH $ CO + 2H2 CH4 + 1/2H2 + 1/2O2 $ CO + 5/2H2 CmHn + m/2H2O + m/4O2 $ mCO + (m/2 + n/2)H2 CH3OH + 1/2H2O + 1/4O2 $ CO2 + 2.5H2 C + H2O $ CO + H2 C + O2 $ CO2 C + 0.5O2 $ CO C + CO2 $ 2CO CO + H2O $ CO2 + H2 CO2 + H2 $ CO + H2O (reverse water gas shift) CO + O2 $ CO2 H2 + O2 $ H2O

SRM

POM

ARM

Gasification of carbon (coal, coke)

Carbon formation Selective co-oxidation

Table 3 Syngas fuel composition by coal and gasifier type [25]. Composition of syngas fuel (in mole fraction %) Type of coal Brown coal Bituminous Lignite Coke Sub bituminous

H2

CO

CH4

N2

CO2

Low calorific value (MJ/kg)

25 24.8 12 15 17.3

16 17.2 22 29 14.7

5 4.1 1 3 3.3

40 42.7 55 50 51.6

14 11 10 3 12.4

6.28 6.19 3.76 5.86 4.40

Gas composition (vol%, dry) Gasifier type

H2

CO

CH4

N2

CO2

Low calorific value (MJ/kg)

Bubbling fluidized bed Updraft Downdraft

9

14

7

20

50

3.57

11 17

24 21

3 1

9 13

53 48

3.56 3.47

feedstock (the heating value). Mechanical gasifier efficiency can range from 60% to 75%, dependent on the gasifier’s kind and construction and also the fuel quality. The gasification efficiency (percent) utilized in engine applications can be defined as follows: ηm ¼

H g  Qg  100 H s  Ms

(1)

10

Chapter 1

where ηm_m denotes the efficacy of gasification (percentage) (mechanical), Hg indicates the gas’s heating value of the gas (kJ/m3), Qg represents the gas’s volume flow (m3/s), Hs is the gasifier fuel’s lower heating value (kJkg1), and Ms defines the solid fuel consumption of the gasifier (kg/s) [13,26].

3. The properties of a synthesis gas 3.1 A brief description of synthesis gas physical and chemical properties Syngas is a blend of CO and H2. Syngas may also have CO2 plus other constituents such as H2O. Due to the fact that syngas is typically utilized at greater pressures to synthesize chemicals and fuels, the N2 level of syngas must typically be kept to a minimum [27]. Hydrogen is a chemical element that can be used in a variety of applications. However, despite the fact that it is nearly exclusively found in a mixed form on earth, it might be created from any main resource by separating it from H2O or other compounds, which includes the element and converted into electricity and other low-pollution energy forms [24,28]. There have been a variety of technologies used to create hydrogen (Table 4), including SRM, POM, water electrolysis, and biomass gasification, among others. When it comes to producing hydrogen from water, electrolysis and photocatalytic water splitting are two of the most prevalent methods. However, in order to produce both hydrogen and oxygen at the same time, further separation and purification of hydrogen from oxygen are required. Additionally, electrolysis of water is [30] expensive, and photocatalytic water splitting is limited by photo conversion efficiency. SRM and POM of hydrocarbons and carbon-based feedstocks such as natural gas, petroleum, and coal are the main ways to make hydrogen for commercial use [31–33].

Table 4 The most common methods of hydrogen synthesis [29]. Process

Energy source

Hydrogen donor

Coal gasification Hydrocarbons partial oxidation Hydrocarbons reforming Kvaerner process Electrolysis Thermolysis Photoelectrolysis Photosynthesis Biomass gasification Biomass anaerobic digestion Reaction water/hydride

Coal Hydrocarbons

Water/coal Water/hydrocarbons

Fossil/nuclear/renewable

Methane Water

Nuclear/renewable Sun Water/biomass

Chemical hydride

Water/hydride

Syngas characteristics: Physical and chemical properties 11 Hydrogen is colorless, odorless, and tasteless; it is almost insoluble in water; it is extremely combustible; and it is harmless (simple asphyxiate). Hydrogen is a diatomic molecule containing covalent bonds, with protons having either parallel (ortho-hydrogen) or antiparallel (para-hydrogen) spins. The molecular weight of this compound is 2.016amu. At or above room temperature, H2 seems to be mix (75% ortho-hydrogen, 25% para-hydrogen), which is referred to as normal-hydrogen [29]. Several other properties of hydrogen are listed in Table 5. Carbon monoxide is a colorless, odorless, combustible gas that is lighter than air. While increasing amounts of CO are entering the atmosphere as civilization advances, it has always been present, mostly as a result of volcanic activity. Unfortunately, because there are so many natural sources of CO in the atmosphere, it is impossible to accurately figure out how much CO is released into the air. In metropolitan locations, high levels of breathed CO can have a detrimental effect on individuals health. It is frequently generated by incomplete combustion of carbon-containing substances, most notably in internal combustion engines. Additionally, it can be kept and transported compacted, and close association with fire or high temperatures can result in container explosions [35]. CO is a very poisonous molecule, and inhalation of significant amounts frequently results in central and peripheral nervous system and cardiovascular system diseases. The severity of symptoms is dose-dependent and ranges from mild headaches to quick death: CO concentrations greater than 1000 ppm are deemed life-threatening. Table 5 contains a summary of some features of carbon monoxide [34]. Table 5 also describes the properties of other gases in synthesis gas, such as carbon dioxide and nitrogen.

Table 5 Hydrogen, carbon monoxide, carbon dioxide, and nitrogen characteristics [29,34]. Properties Molecular weight Critical temperature (K) Critical pressure (bar) Density at 20°C, 1 atm (kg m3) Fusion point (K) Boiling point (K) Gas constant (kJ kg1 K1) Specific heat at p constant (kJ kg1 K1) Speed of sound (m s1) Lower heating value (MJ kg1) Higher heating value (MJ kg1) CAS number

H2

CO

CO2

N2

1.00784 33.25 13.20 0.0892 14.0 20.30 4.1200 14.890

28.01000 133.16 34.98 0.0400 68.0 81.50 0.2968 1.040

44.01000 304.13 73.96 0.0187 216.1 194.67 0.1889 0.840

28.01300 126.20 33.95 0.0160 63.0 77.20 0.2968 1.087

1270 119.930 141.860 1333-74-0

337 10.112 10.112 630-08-0

258 Noncombustible 32.912 124-38-9

354.4 Noncombustible Noncombustible 7727-37-9

12

Chapter 1

3.2 Syngas basic combustion properties It is critical to understand the basic combustion efficiency of syngas fuel in order to use it effectively in combustion equipment. The most effective combustion devices generate significant turbulence, which results in improved blend of gaseous air and fuel prior to ignition, so the combustion process is categorized as premixed in many applications. Different syngas properties can have an effect on the combustion procedure in engines with internal combustion (IC). The limit of flammability of syngas is a critical property for IC engine safety and fuel efficiency [36]. Additionally, burning velocity is a vital factor that governs the blended combustion process. Depending on the nature of the burning velocity of the flow field, which might be laminar or turbulent, it may be used to manage the premixed combustion process. The following sections discuss the syngas’s flammability limit, procedures for measuring its burning velocity, the syngas’s composition, and its calorific value [3]. 3.2.1 Limits on the flammability of syngas The flammability restriction is frequently used as a proxy for the flammability of the gas. When applied to a given temperature and pressure, this phrase refers to a variety of fuel contents in the fuel and air mixture that allow the flame to spread and maintain the flammability limitations. These are commonly referred to as the fuel–air areas that allow flame propagation and those that do not allow fire propagation. The fuel, the direction in which it spreads, the size and shape of combustion chamber, as well as the temperatures and pressures therein are all significantly affected. Besides, there are two unique flammability boundaries for the fuel–air mixture; the smallest possible fuel limit across which flames can spread is referred to as the lower limit of flammability, while it is regarded as the upper flammability limit for the richest. Due to the fact that hydrogen and carbon monoxide are the primary components of syngas that are heat-resistant, these gases’ properties are inherited. Inert gases such as nitrogen and carbon dioxide can be added to gas mixtures to make them less flammable [37]. 3.2.2 The velocities of syngas laminar flames The frequency of laminar flashes is the rate at which a flame spreads in the direction it is pointed of the expansion wave surfaces under a laminar flow scenario using unbranded silent fuel– oxidant mixtures [38]. Because laminar flame velocity is extremely sensitive to the operation of the combustion chamber and the performance of the emission system, it is critical to conduct combustion chamber operation investigations. It is affected by the fuel concentration, the equivalency ratio of mixtures, the pressure, and the temperature. 3.2.3 Syngas ignition delay times Syngas ignition delay times are important combustion variables that affect both the performance and operation of burners and gas turbines. Using preignition while the supply is low could cause damage to the equipment, making it critical to accurately characterize ignition

Syngas characteristics: Physical and chemical properties 13 delay durations. Additionally, ignition delay times are frequently used to validate reaction mechanisms. Generally, machines for fast compression and facility for shock tube are used to determine ignition delay times under a variety of thermodynamic circumstances [3,39].

3.3 Composition of syngas and its calorific value The manufacturer’s gas composition is determined by the gas flow rate, feedstock, flow of feedstock, particle diameter, designs of reactors, catalyst, operating circumstances or gasification process, gasificator, and duration of gas residence. However, it is mostly influenced by the reactor’s temperature, which is affected by the energy rating value. Additionally, the quantities of hydrogen, carbon monoxide, and methane in producer gas are regulated by chemical processes occurring throughout the gasification process [37]. As a result, the type of chemical agent that oxidizes used in gasification has a big impact on the gas’s calorific value that comes out of it.

4. Conclusion and future outlook In order to produce sustainable energy, experts from around the world are increasingly focused on further innovative and clean energy sources. Synthesis gas has significant potential for use in the production of sustainable energy, which can help to meet the growing need for liquid and gaseous fuels, as well as for electricity. The synthesis gas is described as a gas containing primarily hydrogen and carbon monoxide as fuel components, with the limitation of flammability and the velocity of laminar flame serving as the primary syngas parameters. Syngas can be created in a variety of ways, including biomass or coal gasification, as well as natural gas reforming. The future use of syngas and hydrogen will necessitate the development of more efficient hydrogen plants, lower-cost large-scale syngas plants for gas-to-liquids plants, and small-scale applications of syngas technologies for fuel cells, whether for stationary or automotive applications in the future [18].

Abbreviations and symbols ARM BRM CO2 CO CH4 DRM H2 IC POM SRM

autothermal reforming of methane bireforming of methane carbon dioxide carbon monoxide methane dry reforming of methane hydrogen internal combustion partial oxidation of methane steam reforming of methane

14

Chapter 1

References [1] D. Bessarabov, P. Millet, Brief Historical Background of Water Electrolysis. PEM Water Electrolysis, Academic Press, Cambridge, MA, 2018, pp. 17–42. [2] C. Wang, et al., Recent advances during CH4 dry reforming for syngas production: a mini review, Int. J. Hydrog. Energy 46 (7) (2021) 5852–5874. [3] A. Arman, et al., Syngas production through steam and CO2 reforming of methane over Ni-based catalyst—a review, in: IOP Conference Series: Materials Science and Engineering, IOP Publishing, 2020. [4] M.P. Aznar, et al., Hydrogen production by biomass gasification with steam—O2 mixtures followed by a catalytic steam reformer and a CO-shift system, Energy Fuel 20 (3) (2006) 1305–1309. [5] R. Radmanesh, J. Chaouki, C. Guy, Biomass gasification in a bubbling fluidized bed reactor: experiments and modeling, AICHE J. 52 (12) (2006) 4258–4272. [6] P.N. Kechagiopoulos, et al., Hydrogen production via steam reforming of the aqueous phase of bio-oil in a fixed bed reactor, Energy Fuel 20 (5) (2006) 2155–2163. [7] L.A. Garcia, et al., Catalytic steam reforming of bio-oils for the production of hydrogen: effects of catalyst composition, Appl. Catal. A Gen. 201 (2) (2000) 225–239. [8] C. Rioche, et al., Steam reforming of model compounds and fast pyrolysis bio-oil on supported noble metal catalysts, Appl. Catal. B Environ. 61 (1–2) (2005) 130–139. [9] S. Czernik, et al., Hydrogen by catalytic steam reforming of liquid byproducts from biomass thermoconversion processes, Ind. Eng. Chem. Res. 41 (17) (2002) 4209–4215. [10] K. Asimakopoulos, H.N. Gavala, I.V. Skiadas, Reactor systems for syngas fermentation processes: a review, Chem. Eng. J. 348 (2018) 732–744. [11] H. Ebrahimi, M. Rahmani, Modeling chemical looping syngas production in a microreactor using perovskite oxygen carriers, Int. J. Hydrog. Energy 43 (10) (2018) 5231–5248. [12] J.A. Dı´az, et al., Cobalt and iron supported on carbon nanofibers as catalysts for Fischer–Tropsch synthesis, Fuel Process. Technol. 128 (2014) 417–424. [13] R.A. El-Nagar, A.A. Ghanem, Syngas production, properties, and its importance, in: A. Inayat, C. Ghenai (Eds.), Sustainable Alternative Syngas Fuel, IntechOpen, 2019, pp. 1–408. [14] J.R. Rostrup-Nielsen, Syngas in perspective, Catal. Today 71 (3–4) (2002) 243–247. [15] J. Rostrup-Nielsen, L.J. Christiansen, Concepts in Syngas Manufacture, Vol. 10, World Scientific, 2011. [16] C. Chong, et al., Robust Ni/dendritic fibrous SBA-15 (Ni/DFSBA-15) for methane dry reforming: effect of Ni loadings, Appl. Catal. A Gen. 584 (2019) 117174. [17] N.A.K. Aramouni, et al., Catalyst design for dry reforming of methane: analysis review, Renew. Sust. Energ. Rev. 82 (2018) 2570–2585. [18] J.R. Rostrup-Nielsen, New aspects of syngas production and use, Catal. Today 63 (2–4) (2000) 159–164. [19] B. Abdullah, N.A. Abd Ghani, D.-V.N. Vo, Recent advances in dry reforming of methane over Ni-based catalysts, J. Clean. Prod. 162 (2017) 170–185. [20] D.M. Alonso, et al., Production of liquid hydrocarbon transportation fuels by oligomerization of biomassderived C 9 alkenes, Green Chem. 12 (6) (2010) 992–999. [21] J. Falbe, Carbon Monoxide in Organic Synthesis, Vol. 10, Springer Science & Business Media, 2013. [22] A. Abdulrasheed, et al., A review on catalyst development for dry reforming of methane to syngas: recent advances, Renew. Sust. Energ. Rev. 108 (2019) 175–193. [23] K. Liu, C. Song, V. Subramani, Hydrogen and Syngas Production and Purification Technologies, John Wiley & Sons, 2010. [24] X. Yu, et al., Fischer-Tropsch synthesis over methyl modified Fe2O3@ SiO2 catalysts with low CO2 selectivity, Appl. Catal. B Environ. 232 (2018) 420–428. [25] E. Jithin, et al., A review on fundamental combustion characteristics of syngas mixtures and feasibility in combustion devices, Renew. Sust. Energ. Rev. 146 (2021) 111178. [26] C.G. Visconti, et al., CO2 hydrogenation to lower olefins on a high surface area K-promoted bulk Fe-catalyst, Appl. Catal. B Environ. 200 (2017) 530–542.

Syngas characteristics: Physical and chemical properties 15 [27] V.N. Nguyen, L. Blum, Syngas and synfuels from H2O and CO2: current status, Chem. Ing. Tech. 87 (4) (2015) 354–375. [28] S. Bahri, A.M. Venezia, S. Upadhyayula, Utilization of greenhouse gas carbon dioxide for cleaner FischerTropsch diesel production, J. Clean. Prod. 228 (2019) 1013–1024. [29] D. Candelaresi, G. Spazzafumo, Introduction: the power-to-fuel concept, in: Power to Fuel, Elsevier, 2021, pp. 1–15. [30] H.F. Abbas, W.W. Daud, Hydrogen production by methane decomposition: a review, Int. J. Hydrog. Energy 35 (3) (2010) 1160–1190. [31] Y. Li, D. Li, G. Wang, Methane decomposition to COx-free hydrogen and nano-carbon material on group 8–10 base metal catalysts: a review, Catal. Today 162 (1) (2011) 1–48. [32] J.N. Armor, The multiple roles for catalysis in the production of H2, Appl. Catal. A Gen. 176 (2) (1999) 159–176. [33] M. Pen, J. Gomez, J.G. Fierro, New catalytic routes for syngas and hydrogen production, Appl. Catal. A Gen. 144 (1–2) (1996) 7–57. [34] W. Adach, M. Błaszczyk, B. Olas, Carbon monoxide and its donors-chemical and biological properties, Chem. Biol. Interact. 318 (2020) 108973. [35] V.D. H€ahnke, S. Kim, E.E. Bolton, PubChem chemical structure standardization, J. Cheminform. 10 (1) (2018) 1–40. [36] E. Esmaili, E. Mostafavi, N. Mahinpey, Economic assessment of integrated coal gasification combined cycle with sorbent CO2 capture, Appl. Energy 169 (2016) 341–352. [37] F. Jiang, et al., Insights into the influence of support and potassium or sulfur promoter on iron-based Fischer– Tropsch synthesis: understanding the control of catalytic activity, selectivity to lower olefins, and catalyst deactivation, Catal. Sci. Technol. 7 (5) (2017) 1245–1265. [38] S. Mandal, et al., Synthesis of middle distillate through low temperature Fischer-Tropsch (LTFT) reaction over mesoporous SDA supported cobalt catalysts using syngas equivalent to coal gasification, Appl. Catal. A Gen. 557 (2018) 55–63. [39] S. Wang, G. Lu, G.J. Millar, Carbon dioxide reforming of methane to produce synthesis gas over metalsupported catalysts: state of the art, Energy Fuel 10 (4) (1996) 896–904.

CHAPTER 2

Syngas production by gasification processes Virginia P
ereza, Manuel Bailerab,c, and Pilar Lisbonac a

Centre for the Development of Renewable Energy - Centre for Energy, Environment and Technology Research (CEDER-CIEMAT), Soria, Spain bGraduate School of Creative Science and Engineering, Waseda University, Tokyo, Japan cDepartment of Mechanical Engineering, Universidad de Zaragoza, Campus Rı´o Ebro, Bldg. Betancourt, Zaragoza, Spain

1. Introduction Syngas is a very important intermediate product, which is mainly composed of hydrogen (H2) and carbon monoxide (CO). It is used for the manufacture of chemicals such as ammonia and methanol and fuels such as hydrogen and synthetic natural gas (SNG). It can also be used for the production of thermal or electrical energy. In addition, syngas is a fuel gas, and therefore, it is used in internal combustion engines. Syngas can be produced by different methods such as gasification of coal, biomass, or waste; steam reforming of natural gas; reforming of liquid hydrocarbons; or alternative technologies such as pyrolysis or catalytic partial oxidation. The reforming processes are based on the chemical reactions of steam reforming (CnHm +nH2O!nCO + (n+m/2)H2 and CH4 +H2O! CO + 3H2) and the water gas shift reaction (CO + H2O ! CO2 + H2). In addition, many other chemical reactions take place during gasification, including partial oxidation reactions. The presence of catalysts and the operating conditions influence the above reactions [1]. Gasification is a thermochemical process by which fuel is broken down into a product gas and ash. It occurs at high temperatures using a gasifying agent under substoichiometric conditions. This product gas, or gasification gas, generated by gasification processes differs chemically from syngas. The product gas is like a raw syngas containing H2 and CO and significant amounts of hydrocarbons, such as methane (CH4). This gas may also contain carbon dioxide (CO2), water (H2O), and nitrogen (N2), as well as some undesirable compounds. Gasification process can accept a multitude of fuels, such as coal, biomass, or waste; and a variety of gasifying agents, such as air, oxygen, and water vapor. Different technologies can be used to carry out this process. In addition, in order to obtain a quality syngas, the product gas must undergo gas cleaning and upgrading operations. Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91871-8.00020-9 Copyright # 2023 Elsevier Inc. All rights reserved.

17

18

Chapter 2

2. State of the art of gasification Gasification of solid and liquid fuels is a proven process. The first studies on gasification date to the year 1659, on methane produced in a coal mine; and 1739, on coal distillation. But it was at the beginning of the 19th century when the first light generators appeared, using the so-called city gas, which is produced from coal. Afterward, gasification was used for the steel industry, small furnaces, and internal combustion engines. During World War II, it was widely used in gasogene devices to obtain a gas from wood; that gas was used in standard petrol engines for vehicles. Subsequently, during Apartheid in South Africa, gasification was used as a process for the production of hydrocarbons from coal, using the Fischer–Tropsch (FT) process. After the oil crisis in 1973, there was an interest to develop new forms of energy that could offer an alternative to the use of oil; mainly focused on the production of gas from coal [2,3]. The highly efficient and low-polluting integrated gasification combined cycle plants were among the best performing coal-fired plants of their time when they were commissioned at the end of the 20th century. After that, new advances in coal and synthesis gasification technologies have been developed; notable examples include gasification for transport and hot syngas cleaning. However, units based on coal synthesis gas for power production and other uses have difficulties in the current market situation [4]. The increase in both the cost and demand for energy and raw materials and the need to meet environmental requirements to limit global warming to a maximum of 2°C, ideally 1.5°C, above preindustrial level, established by the Paris Agreement [5], have grown the interest in the gasification of other fuels, such as biomass and waste, which are positioning themselves in different applications at industrial level. According to the data from the Global Syngas Technology Council, the gasification was a technology presented in almost 30 countries around the world in December 2020; 686 gasifiers operating in 272 large-capacity plants have a synthesis gas generation capacity close to 200 GWth [6]. A further 238 gasifiers in 74 new plants are under construction, adding another 83 GWth of synthesis gas capacity [7]. This situation can be compared with a Gasification Database made in 2010 by the National Energy Technology Department of the US Department of Energy. At that moment, there were nearly 412 gasifiers operating in 150 plants with a synthesis gas generation capacity of over 71 GWth [4]. These data are presented in Fig. 1 Fig. 2 shows the use of synthesis gas in the worldwide gasification plants in 2020. The production of (liquid) chemicals and fuels amounts around 160 GWth; gaseous fuels, mainly natural gas, 30 GWth; and power generation 10 GWth [7]. The main fuel for the gasification plants is coal, followed by oil and petroleum coke. The use of nonfossil feedstocks such as biomass and wastes is more limited and is generally consigned to units with a smaller capacity, with around 100 installations and a total capacity of some few GWth [7].

Syngas production by gasification processes 19 250

Synthesis Gas (GWth)

200 150 100 50 0 2010

2020

Under construction in 2020

Fig. 1 Worldwide synthesis gas production and planned growth in 2020 (own elaboration using data from refs. [3, 6]). 180

Synthesis Gas (GWth)

160 140 120 100 80 60 40 20 0 Chemical and liquid fuels

Gaseous fuels

Power

Fig. 2 Worldwide use of synthesis gas in 2020 (own elaboration using data from ref. [7]).

3. Gasification process Gasification is a thermochemical conversion process whereby a solid or liquid fuel is turned into a combustible gas, using a net reducing atmosphere at high temperatures. The product gas or raw syngas is the main product of the gasification process, containing H2 and CO. Additionally, other compounds, such as CO2, H2O, N2, CH4, other hydrocarbons and impurities, may be present in the gas, depending on the fuel, the reducing atmosphere, the gasification technology, and the operating conditions, such as temperature, pressure, and heating rate.

20

Chapter 2

The net reducing atmosphere is achieved by using an oxidizing agent, also called gasifying agent, under substoichiometric conditions. The gasifying agent may be air, oxygen, steam, or a mixture of these or with carbon dioxide. The amount of which is less than the theoretical amount to burn the fuel completely. Gasification is a complex process between pyrolysis and combustion, in which the fuel and the gasifying agent are the raw materials. This process produces product gas, or raw syngas, which is the product of interest, and a solid, mainly composed of ash. It has the advantage of accepting flexible feedstock and products, which gives versatility of applications. Gasification is a four-step endothermic process of converting solid fuel into a combustible gas [8,9]: (a) Heating and drying: The free water content in the fuel evaporates, leaving dry material. The water steam produced can be reacted in the subsequent stages. This is an endothermic process. (b) Pyrolysis or devolatilization: When the dry fuel is exposed to increasing temperature, the weakest chemical bonds are broken and the large and complex fuel molecules are thermally degraded into smaller volatile gas molecules such as hydrocarbon gases, H2, CO, CO2, H2O vapor, and tar, mainly composed of heavy polycyclic aromatic hydrocarbons (PAHs). Char is a carbonaceous solid resulting from the incomplete reaction of the combustible material, does not vaporize, and is a by-product of this process that will undergo gasification reactions. Oxygen is not required at this stage or at the previous one. The temperatures are up to about 600°C. This process is also endothermic and is described by the first reaction (Eq. 1) in Table 1. (c) Partial oxidation: The volatile products and some of the char from the previous stage react with limited oxygen to produce gases CO2, CO, H2O vapor, and hydrocarbons with more than two carbon atoms. This stage is exothermic and releases the heat needed for subsequent gasification reactions. Looking at the enthalpy of carbon monoxide oxidation, carbon oxidation, and carbon partial oxidation (Eqs. 3–5) in Table 1, it can be deduced that in autothermal gasification processes, about from 19% to 23% of the heating value of carbon is invested in the gasification stage to ensure that the remaining heating value is conserved in the gas for heating, drying, pyrolysis, and decomposition reactions [8]. (d) Reduction or gasification: This stage involves homogeneous and heterogeneous gasification reactions and determines the final gas mixture in the syngas. The remaining char reacts with water steam and CO2 to produce H2 and CO (Eqs. 8 and 17 in Table 1). Additionally, water–gas shift reaction (WGSR) (Eq. 9 in Table 1), methanation reaction (Eq. 14 in Table 1), and other reactions take place. The overall process is endothermic. Ash and fly ash are produced at this stage and the previous one. In these last two stages, the decomposition of tar and hydrocarbons also takes place.

Syngas production by gasification processes 21 Table 1 Main reactions during gasification process [heterogeneous reactions (underlined) and homogeneous reactions (italic)] [8–11]. Main reactions

ΔH0 (kJ/mol)

Equation

Endothermic

(1)

286 239 394 123 803 0 206 75

(8) (9) (10) (11) (12)

88 227

(13) (14) (15) (16)

160 247 >0

(17) (18) (19)

>0 >0

(20) (21) (22) (23) (24) (25)

Primary devolatilization Fuel ! light gas (CO, CO2, CH4, CXHY, H2O) + Char + Primary tar Oxidation reactions H2 oxidation: CO oxidation: Carbon oxidation: Carbon partial oxidation: CH4 oxidation: CnHm partial oxidation:

H2 + ½ O2 ! H2O CO + ½ O2 ! CO2 C + O2 ! CO2 C + ½ O2 ! CO CH4 + 2 O2 ! CO2 + 2 H2O CnHm + (n/2) O2 ! n CO + (m/2) H2 Gasification reactions involving steam

Water gas: Water gas shift (WGSR): Steam reforming: Steam methane reforming: Other reaction:

C + H2O ! CO + H2 CO + H2O ! CO2 + H2 CnHm + n H2O !n CO + (n +m/2) H2 CH4 + H2O ! CO + 3 H2 C + 2 H2O ! CO2 + 2 H2 Gasification reactions involving hydrogen

Hydrogasification: Methanation: Hydrogenation: Other reaction:

C + 2 H2 ! CH4 CO + 3 H2 ! CH4 + H2O CnHm + (2n (m/2)) H2 ! n CH4 CO2 + 2 H2 ! H2O + C Gasification reactions involving carbon dioxide

Boudouard: Dry reforming of CH4: Dry reforming:

C + CO2 ! 2 CO CH4 + CO2 ! 2 CO + 2 H2 CnHm + n CO2 !(2n) CO + (m/2) H2 Decomposition reactions of tar and hydrocarbonsa

Dehydrogenation: Carbonization: Thermal cracking: Thermal cracking: Steam alkylation: H2 alkylation:

pCnHm ! qCxHy + H2 CxHy ! n C + (m/2) H2 CnHm ! CxHy + CnxHmy CnHm ! CxHy + C CnHm + n H2O! CxHy +q CO + p H2 CnHm + n H2 ! CxHy + q CH4

a In this table, generally, CnHm are hydrocarbons with more than two carbon atoms. Nevertheless, in decomposition reactions of tar and hydrocarbons, note that CnHm represents tars and, in general, the heavier fuel fragments are produced by thermal cracking; and CxHy represents hydrocarbons with smaller number of carbon atoms and/or larger degree of unsaturation than CnHm.

22

Chapter 2

Depending on the gasification technology used, these stages can be produced in separate areas or together. This already complex picture is further complicated by parallel comminution phenomena that can significantly change the particle size of the fuel and strongly affect its conversion process [12]. The main reactions during gasification process are presented in Table 1, where the heterogeneous reactions are underlined and the homogeneous ones are in italic. Values of the enthalpy of each reaction (ΔH0) refer to standard conditions (25°C and 1 atm). The rate of reactions depends mainly on the reactivity of the fuel, the type of gasifying agent, temperature, and pressure. Additionally, combustion reactions are faster than gasification reactions, and therefore, in an autothermal process, combustion reactions occur first, rapidly consuming the available oxygen and providing the heat needed for other reactions. The equilibrium constants (K) of the chemical reactions involved in the gasification process are favored or hindered by the effect of temperature. Fig. 3 represents the evolution of the logarithm 50 Reactions: (Eq. 2) H2 + ½O2 = H2O (Eq. 3) CO + ½O2 = CO2 (Eq. 4) C + O2 = CO2 (Eq. 5) C + ½O2 = CO (Eq. 8) C + H2O = CO + H2 (Eq. 9) CO + H2O = CO2 + H2 (Eq. 11) CH4 + H2O = CO + 3H2 (Eq. 13) C + 2H2 = CH4 (Eq. 17) C + CO2 = 2CO

40

30 (2) 20

(3)

log K

(5)

10

(4)

(13) (11)

(9) 0

(8)

–10

(17) –20

0

500

1000

1500

2000

2500

3000

Temperature (K)

Fig. 3 Calculation equilibrium constant for some gasification reactions (own adaptation using data from ref. [13].

Syngas production by gasification processes 23 of K as a function of temperature for some of the reactions in Table 1. It can be seen that all reactions show the greatest variation of the equilibrium constant at temperatures below 800°C. In most of the reactions, K has the highest values at temperatures below 1000°C and decreases with temperature. On the opposite side, a few reactions have the lowest values of K at temperatures below 1000°C, and they increase with temperature. The last behavior can be observed in Steam reforming of CH4, Boudouard, and water–gas reactions. Furthermore, it is important to mention that endothermic reactions will only occur if sufficient thermal energy is available. As a result of the gasification process, gas and a solid are obtained. On the one hand, the product gas or raw syngas is the product of interest and is composed of CO and H2, and depending on the reactions, the gas also contains CO2, CH4, N2, H2O vapor, other light hydrocarbons, and several undesirable compounds such as organic impurities like tar, inorganic substances like H2S, HCl, NH3, HCN, and alkali metals, and solid particles. On the other hand, the solid may consist of a carbonaceous residue or char, ash, and the inorganic matter of the fuel.

3.1 Gasification fuel Gasification has the advantage of being able to use a wide range of fuels as feedstock, such as coal, petroleum coke and refinery streams, other fossil fuels, or biomass and waste; and turning them into synthesis gas. The flexibility comes from gasification’s ability to take any feedstock containing hydrocarbons and then thermochemically break it down into a gas containing simple compounds. At this moment, the most commonly used fuel for gasification is coal. The four main types of coal are [4] as follows: - Lignite: This is the youngest of the coal types and has the lowest energy content, containing between 25% and 35% carbon. - Subbituminous: It has higher energy content than lignite coal, containing between 35% and 45% carbon. - Bituminous: It has higher energy content than subbituminous carbon, containing between 45% and 86% carbon. - Anthracite: It has a slightly lower heating value than bituminous coal, containing between 86% and 97% carbon. In recent years, the global consumption of coal has decreased, whereas the use of other fossil fuels, such as oil continue, is increasing worldwide [14]. Moreover, the use of other fuels for gasification, such as biomass and waste, is increasing in importance [2,9]. The reasons for this increase are, on the one hand, the rising cost and demand for energy and raw materials and, on

24

Chapter 2

the other hand, the need to comply with environmental requirements to control CO2 emissions, such as The European Green Deal [15], which leads to the search for new energy sources to prevent global warming. According to article 2(c) of the Renewable Energy Directive in Europe, “biomass” means the biodegradable fraction of products, waste, and residues from biological origin from agriculture including vegetal and animal substances, forestry, and related industries such as fisheries and aquaculture, as well as the biodegradable fraction of waste, including industrial and municipal waste of biological origin [16]. Some examples of waste include municipal solid waste, sewage sludge, plastic type waste, or waste from paper industry, wood residues, agricultural waste, crops residues, and others. Gasification can be considered a waste-to-energy technology option [17,18]. It has the unique characteristic of being a technology that can even convert waste, from municipal solid waste to agricultural or crop residues, into a useful and high-quality energy source [9]. Nowadays, the disposal of any kind of waste is complicated due to environmental regulations and legislations. Therefore, waste gasification is an increasingly important waste management option to comply with the hierarchy established by the waste directives, where it can even be considered as a recycling method with priority over other waste management options, such as recovery or disposal, when it is used to obtain raw materials, also contributing to the circular economy. When syngas is burned, gasification offers the advantage of separating harmful substances from the fuel gas before combustion. Therefore, a wide variety of fuels can be used in the gasification process. Table 2 presents data on the chemical characterization and low heating value (LHV) of three different examples of fuels used in gasification: coal, wood chips, and sewage sludge. The calorific value of coal is higher than that of biomass and waste. In addition, the fixed carbon of coal is also higher, while the volatile content is lower. Comparing the data from the ultimate analysis, it can be seen that carbon contains more C and less O than biomass and waste. In addition, there are some studies focused on the relationship between elements and heating value of biomass. In fact, there is a connection between the H/C atomic ratio and the O/C atomic ratio for the different fuels, which was firstly studied by Van Krevelen. Fig. 4 shows this relationship for coal, biomass, and waste. Generically, the higher-quality fuels, e.g., anthracite, are in the lower left corner of the diagram, while waste is in the upper right corner of the diagram. Therefore, the nature and the physical and chemical properties of the fuel used in the gasification process influence the operating conditions and the quality of the gas obtained. For instance, while air gasification of coal can generate a syngas with a CO content ranging from 30% to 60%, from 25% to 30% for H2, from 5% to 15% for CO2, and from 0% to 5% for CH4 [4]; sewage sludge gasification can generate a syngas with ranges from 5% to 8% of CO, from

Syngas production by gasification processes 25 Table 2 Gasification process reactions. Parameter

Unit

Bituminous coal [19]

Moisture Ash Volatile Fixed carbon

% w.b. % d.b. % d.b. % d.b.

8.7 21.9 36.1 42.0

Wood chips [20]

Sewage sludge [21]

8.8 0.5 83.1 7.6

9.4 41.2 53.5 5.3

49.55 6.5 0.16 43.27 0.02 0.02

31.5 4.7 4.8 16.8 1.0 0.06

18.51

13.1

Proximate analysis

Ultimate analysis C H N O S Cl

% % % % % %

d.b. d.b. d.b. d.b. d.b. d.b.

61.5 4.2 1.2 6.0 5.1 – Heating value

LHV

1

MJ kg

27.0

w.b., wet basis; d.b., dry basis.

2 1,75 WASTE

1,5 BIOMASS

10 H/C

1,25 1 0,75 COAL

0,5 0,25 0 0

0,25 0,5 0,75

1 1,25 1,5 1,75 O/C

2

Fig. 4 Relationship between the H/C atomic ratio and the O/C atomic ratio for different fuels (own elaboration using data from ref. [22]).

3% to 9% of H2, from 12% to 16% of CO2 and from 2% to 4% of CH4 [21]. This difference in gas composition means that gas from sludge gasification has a much lower calorific value, from 2 to 5 MJ/Nm3 as reported in ref. [21], than gas from coal, from 10 to 15 MJ/Nm3 [23]. In addition, another factor to consider is the amount of volatiles: the higher volatile content of biomass and waste, compared with coal, intensifies the pyrolysis stage, favoring the release of

26

Chapter 2

primary tars and forcing operation at lower temperatures. This results in biomass or waste gasification gas generally having a higher tar content than coal. Another alternative is based on the use of two fuels at the same time in the gasification process, so-called cogasification, which makes it possible to compensate for the negative properties of some fuels. The fuel used in a gasification process is a key variable. Its reactivity depends not only on its chemical composition and nature, but also on other parameters such as its moisture content, size, or particle density. Therefore, in most cases it is necessary to pretreat the fuel with different operations such as drying to suitable low moisture content, broken down to a suitable size or fuel densification, e.g., by pelletization. It should also be free from undesirable ingredients, such as stones or metals, which could cause operational problems.

3.2 Gasifying agent The gasification process requires a net reducing atmosphere, under substoichiometric conditions. Stoichiometric condition is that theoretical condition at which the proportion of the air-to-fuel is such that all combustible products will be burned with no oxygen remaining in the combustion air. The gasifying agent may be air, oxygen, steam, or a mixture of these or with carbon dioxide. The nature of the gasifying agent and its quantity influence the gas composition, the tar formation [2], the economy of the process [24], and consequently, the final application of the gas. Table 3 shows typical ranges of syngas composition using different gasifying agents and the low heating value. They are obtained from biomass gasification using different types of gasification technologies [25].

Table 3 Syngas compositions using different gasifying agents (own elaboration using data from ref. [25]). Gasifying agent

Air

Oxygen

Steam

Gasification technology CO (%) CO2 (%) H2 (%) CH4 (%) N2 (%) LHV (MJ/Nm3)

Fixed bed 13–18 12–16 2–6 45–60

Entrained flow 45–55 10–15 23–28 0–1 0–1 10–12

Fluidized bed 25–30 20–25 35–40 9–11 0–5 12–14

4–6

Syngas production by gasification processes 27 Air gasification produces a gas containing about 50% of N2, and therefore, it has a low calorific value, in the case of using biomass as fuel. It is the easiest and most economical process, because the air comes from the environment, usually supplied by an air blower. Oxygen-based gasifiers produce a gas containing a relatively high concentration of H2 and CO and with a heating value higher than in the case of using air. During oxygen gasification, the combustion products (CO2, H2O) are in the product gas and promote the WGSR. Oxygen gasification has the advantage of minimizing the size of the gasification reactor and its auxiliary process system. However, the oxygen for the process must be separated from the atmosphere using a technology based on cryogenic distillation [4]. Steam gasification produces a gas containing a higher amount of H2. The hydrogen found in the syngas does not originate only from the fuel, but also from the steam. Steam favors steam reforming reactions, promoting the formation of light gases such as H2, CO, and CH4. However, these reactions are very endothermic, which can lead to a decrease in temperature in the reactor, which sometimes needs to be compensated by using the mixture steam–air or steam–oxygen.

3.3 Syngas composition The syngas composition depends not only on the fuel and gasifying agent used as feedstock, but also on the technology used and the operating conditions (temperature, pressure, equivalence ratio, etc.). Therefore, it is very difficult to predict the exact composition of the gasification gas [13]. H2 and CO are the most interesting products of the syngas. However, this gas also contains other compounds such as CO2, H2O vapor, CH4, and other hydrocarbons. When the gasifying agent is air, it also contains N2. In addition, small amounts of undesirable compounds may also occur: particles, halides, alkali compounds, other trace metals as Hg, nitrogen compounds, sulfur compounds, and tars. Table 4 summarizes the main pollutants and their associated problems associated. Table 4 Pollutants of syngas and associated effects. Contaminant

Example

Problem [26,27]

Particles Halides Alkali metals

Ash, char Cl, Br, F Na, K

Nitrogen compounds Sulfur compounds Tar

NH3, HCN H2S, COS Aromatic compounds

Erosion, corrosion, clogging Clogging, catalyst poisoning Corrosion, catalyst poisoning, agglomeration of bed particles Emissions Emissions, corrosion, catalyst poisoning Clogging, catalyst poisoning

28

Chapter 2

3.3.1 Tar content The tar content in syngas from biomass and waste is much higher than that from coal, due to their different chemical composition. The higher the volatile content, the more the volatilization of primary tars is favored during pyrolysis stage. In fact, the high tar content when gasifying biomass or waste, compared with coal gasification, is the main obstacle to further industrial-scale implementation. Tars are a mixture of compounds present in the syngas that condense easily at temperatures below 400°C. They can cause a lot of problems such as clogging pipes, filters or engine valves; causing deposition on turbine blades; or interfering with synthesis of fuels and chemicals [28]. According to the Technical Specification (TS) of the European Committee for Standarization (CEN), CEN/TS 15439:2006 Biomass gasification—Tar and particles in product gases— Sampling and analysis, tars are any organic compound present in the syngas, excluding gaseous hydrocarbons of 1–6 carbon atoms [29]. They are a complex mixture of hundreds of compounds with a wide variety of physical and chemical properties [30]. Their amount and composition depend on the fuel, the gasifying agent, the technology used, the operating conditions, etc. In some studies, tar production is mainly focused on the total amount of tar, expressed in g/Nm3 or mg/Nm3. However, tar composition is the dominant factor, when examining the impact on downstream equipment. Numerous tar classifications exist based on their reaction process, solubility, condensability, boiling point, molecular weight, etc. A tar classification system based on the physical tar properties (water solubility and tar condensation) was developed by The Netherlands Organization for Applied Scientific Research (TNO) and is widely used. This system categorized tars into five different classes [31]: - Class 1. Gas chromatography (GC) undetectable tars: This class includes the heaviest tar compounds that condense at high temperatures even at very low concentrations. They are gravimetric tars and contain compounds heavier than coronene. - Class 2. Heterocyclic components: These are components that generally exhibit high water solubility due to their polarity such as pyridine, phenol, quinoline or o-cresol, m-cresol or p-cresol, whose structures are shown in Fig. 5. Class 2 tars are GC detectable tars, as Class 3, Class 4, and Class 5 tars. - Class 3. Aromatic components: Light hydrocarbons that are not important regarding their condensation and water solubility issues. They are tar compounds formed by only one benzene ring as styrene, toluene, ethylbenzene, o-xylene, m-xylene, or p-xylene, whose structures can be seen in Fig. 6. - Class 4. Light polyaromatic hydocarbons (PAHs): They are composed of 2–3 benzene rings and condense at relative high concentrations and intermediate temperatures. Fig. 7 shows the

Syngas production by gasification processes 29

N

N

OH

Pyridine

Phenol

OH

Quinoline OH

OH CH3

CH3

o-Cresol

CH3

m-Cresol

p-Cresol

Fig. 5 Class 2 tar components (own elaboration). CH3

CH3

CH2 Styrene

Toluene

CH3

CH3

Ethylbenzene

H3C

CH3

CH3

CH3 o-Xylene

m-Xylene

p-Xylene

Fig. 6 Class 3 tar components (own elaboration).

structure of naphthalene, 1-methylnaphthalene, biphenyl, ethenylnaphtalene, acenaphthylene, acenaphthene, fluorine, phenanthrene, and anthracene. - Class 5. Heavy PAHs formed by 4–6 benzene rings. These components condense at relatively high temperatures and low concentration. Fig. 8 shows the structure of some

30

Chapter 2

CH3

Naphthalene

1-Methylnaphtalene

Biphenyl

Ethenylnaphtalene

Acenaphthylene

Acenaphthene

Fluorene

Phenanthrene

Anthracene

H2C

Fig. 7 Class 4 tar components (own elaboration).

examples: fluoranthene, pyrene, benzo(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, perylene, dibenzo(a,h)anthracene, benzo(g,h,i) perylene, indeno(1,2,3-c,d)pyrene, and coronene. TNO has developed a model for calculating the dew-point temperature from the composition of tar. The tar dew-point temperature or condensation temperature is an important thermodynamic property that determines the quality of the syngas [31].

3.4 Operating and performance process parameters In addition to the fuel and the gasifying agent, there are many other factors that influence the gasification process. On the other hand, the performance of the gasification process can be evaluated on the basis of a number of performance parameters.

Syngas production by gasification processes 31

Fluoranthene

Pyrene

Benz(a)anthracene

Chrysene

Benzo(b)fluoranthene

Benzo(k)fluoranthene

Benzo(a)pyrene

Perylene

Dibenzo(a,h)anthracene

Benzo(g,h,i)perylene

Indeno(1,2,3-c,d)pyrene

Coronene

Fig. 8 Class 4 tar components (own elaboration).

3.4.1 Operating parameters The operating conditions play an important role during the gasification process in aspects such as gas composition, production of tars and other undesirable compounds, carbon conversion, and the efficiency of the gasification process. Some of these variables are specific to the gasification technology used. It must be taken into account that all these variables are interrelated and the optimal value of one variable depends on others. Some of these variables are as follows: -

Feedstock: Fuel and gasifying agent Equivalence ratio Temperature Pressure

32 -

Chapter 2 Throughput Inlet temperature of the gasifying agent Additives and bed materials Others

The influence of the fuel and the gasifying agent on the gasification process has been explained in Sections 3.1 and 3.2. Equivalence ratio and temperature

The equivalence ratio (ER) is the ratio between the oxygen content in the oxidant supply and that required for complete stoichiometric combustion. It is likely the most important operating parameter for allothermal processes, since it strongly affects the gas composition (including tar content) and its heating value [8]. In an autothermal process, the ER determines the temperature in the reactor. A higher ER implies a higher amount of gasifying agent, which favors the exothermic oxidation to take place, and, therefore, increases the temperature of the system. Fig. 9 shows the influence of the temperature in the gasification process and the quality of the final product. As the temperature increases, the calorific value of the gas and the tar content decrease, while the carbon conversion and the sintering risk increase. Gasification temperature ranges for some fuels are also included in the figure. In general, it is noted that coal can operated at higher temperatures than biomass [32]. An increase in the operating temperature may be favorable for higher carbon conversion and lower tar content, but, on the negative side, it decreases the calorific value of the gas and favors sintering. Therefore, it is necessary to optimize the process according to the final application of the gas. Other ratios are used when the gassing agent is steam or oxygen. In the case of steam, the steam to biomass ratio (SB) is used, defined as the amount of steam divided by the amount of biomass.

Fig. 9 The influence of the gasification temperature (own elaboration).

Syngas production by gasification processes 33 For oxygen, the parameter used is the oxygen purity (OP), which is defined as the percentage of oxygen in the enriched air. Pressure

The working pressure is one of the variables that most influences the operation of the gasifier, since it is very different to work under pressure than over pressure. In the latter case, the equipment must be optimally sealed to avoid gas leaks. The influence of pressure on the quality of the gas is also important; in general, it favors the reduction of the total tar content. Throughput

The throughput or the feed rate to the gasifier is defined as the ratio between the fuel flow rate, as received including moisture and ash, and the gasifier cross section. It influences the gas composition and the tar content. Increasing the throughput leads to a decrease in residence time, which disfavors the cracking and reforming reactions, resulting in a decrease in H2 content and an increase in tar content. In addition, an increase in the calorific value of the gas has also been observed, in autothermal reactors [21,33]. Inlet temperature of the gasifying agent

The gasification process is influenced by the inlet temperature of the gasifying agent. Generally speaking, an increase in this temperature improves gas quality and gas production [34]. Additives and bed materials

The use of additives or bedding materials containing catalytic properties is quite common in the gasification process [24]. In-bed catalysts are based on transition metals such as calcined rocks (lime, magnesite, dolomite, etc.), olivine, clay materials, iron oxides, or alumina. However, their effectiveness is low mainly due to the tendency to attrition or creep, moderate catalytic activity, and deactivation by coking, which increases the costs of the process [24]. Some of the most frequently used bed materials are lime, dolomite, olivine, and alumina, which give high tar conversion [24,35]. In addition, such materials help to prevent agglomeration [36]. Others

The gasification process can be also influenced by the space–time of the gas in the bed [37,38], the design of the gasifier or the bed height [37]. 3.4.2 Performance parameters The performance of the gasification process can be assessed by different parameters. The most important are cold gas efficiency, hot gas efficiency, carbon conversion efficiency, etc.

34

Chapter 2

Cold gas efficiency

Cold gas efficiency (CGE) is the ratio between the chemical energy of the produced syngas (obtained as syngas flow rate (Q) multiplied by its net heating value) and the chemical energy of the fuel fed to the gasifier (obtained as fuel flow rate multiplied by its net heating value).
(26) CGE ¼ Qsyngas  LHVgas =ðQfuel  LHVfuel Þ This parameter does not take into account the sensible heat of the gas but only its chemical energy. It is useful when the gas is going to be used at low temperatures, for instance, in an engine. It is important to notice that a syngas with an elevated CGE can be adequate for an energy production process but less for an application focused on conversion to liquid fuels or to chemicals [8]. Hot gas efficiency

Hot gas efficiency (HGE) is the ratio between the sum of chemical energy and the sensible heat of the produced syngas (Hout) and the sum of chemical energy and sensible heat of the fuel fed to the plant (Hin).
HGE ¼ Hout + Qsyngas  LHVgas =ðHin + Qfuel  LHVfuel Þ (27)

Carbon conversion efficiency

Carbon conversion efficiency (CCE) is the ratio between the carbon flow rate converted to gaseous products and that fed to the reactor with the solid fuel.

CCE ¼ Qsyngas  Qcarbon_syngas = Qfuel  Qcarbon_fuel

(28)

Qcarbon_fuel is the carbon fraction in the fuel, and Ccarbon_syngas is that of carbon in gaseous component of the syngas (i.e., dust and tar excluded). It gives an indication of the amount of unconverted material that must be treated using other techniques or sent to disposal and therefore provides a measure of chemical efficiency of the process. Other process performance parameters

Gas composition and tar content are the most important parameters because they define the applications of the gas and the problems that tars can cause in the equipment [8]. Other important parameter is the syngas heating value, which is generally expressed as MJ/Nm3.

Syngas production by gasification processes 35

4. Gasification technologies The gasification technology and design influence the kinetics of the reaction, the product’s performance, and the gas quality [39]. There are several gasifier classifications attending different criteria. For instance, attending on heat supply method, they can be categorized in autothermal and allothermal; according to the type of contact between the fuel and the gasifying agent, the most common types of gasifiers are fluidized bed, fixed bed, entrained flow, and other types of gasifiers such as rotary gasifiers or plasma gasifiers; attending the operating pressure, atmospheric or pressurized can be distinguish; depending on the gasifying agent used, air, oxygen, or steam gasification can be identified; according to the number of fuels fed to the reactor, gasification and cogasification can be carried out; according to the work temperature, low-temperature and high-temperature processes can be performed; and other criteria. Attending on the heat supply method, there are two main types of gasifiers: - Direct or autothermal gasification, when the combustion of part of the fuel occurs to generate the thermal energy required to perform the gasification process. Their great advantage is the direct internal heating of the reagents, which provides greater efficiency in energy utilization and low operating costs. On the other hand, partial oxidation with air generates a producer gas diluted by the atmospheric nitrogen (up to about 60%) and a gas with low calorific value is obtained [8]. They are widely used at industrial level. - Allothermal or indirect gasification is characterized by the separation of the processes of heat production and heat consumption. The external heat can be supplied by electric power, solar energy, or plasma torch, but, at industrial level, the allothermal facility almost always consists of two reactors, connected by an energy flow. Fuel is gasified in the first reactor and the char or product gas is combusted in the second reactor, to produce the heat for the first process. The transport of the heat can be done either by heat exchanger or by circulating a bed material [40]. Different types of gasifiers can be recognized according to the type of contact between the fuel and the gasifying agent. Some of the most common types of gasifiers are fluidized bed, fixed bed, and other gasifiers such as entrained flow and other types of gasifiers such as rotary gasifiers or plasma gasifiers.

4.1 Fluidized bed gasifier Fluidized bed reactors have a hot bed of particles and fuel that is kept in constant motion by a fluidizing agent flowing at an ascending velocity to produce a mixture that favors the transfer of matter and energy between the fuel and the gas. Generally, the fluidizing agent is the gasifying agent that can be air, oxygen, steam, or mixtures of them or with carbon dioxide.

36

Chapter 2

Fluidization mechanism consist of solid particles, which are initially static, acquiring a fluid behavior when an ascending flow or fluid, either gas or liquid, affects them upon reaching a certain velocity under specific conditions. The behavior of the bed depends on the gas flow introduced and on the diameter of the particles. The different stages of the gasification process; drying, pyrolysis, oxidation, and reduction; take place simultaneously across the entire bed, thanks to the intense mixing of the fuel and the gasifying agent. Fluidized beds have high levels of matter and energy transfer, providing a good mixture in the solid phase, resulting in reacting at high levels, short residence times of small particles, and constant temperatures in the bed. In Fig. 10, two types of gasifiers can be distinguished according to the fluidization velocity: bubbling fluidized bed (BFB) and circulating fluidized bed (CFB). In a BFB, the fluidization velocity can be from 1 to 3 m/s, this is between the minimum fluidization velocity and the terminal velocity. In a CFB, the fluidization velocity can be greater than 4m/s, this occurs when it is higher than the terminal velocity so that there is particulate entrainment. Afterward, these particles are separated from the gas stream and returned to the gasifier using a solid valve. Fluidized bed gasifiers are best suited to relatively reactive coals, low-rank coals, and other fuels such as biomass and waste [4], and it is very common in large facilities.

Fig. 10 Fluidized bed gasifiers: BFB and CFB (own elaboration).

Syngas production by gasification processes 37 Although fluidized bed gasification has a complex operation compared with other types of gasifiers, it has several advantages, which include [4,28,41]: - Fuel and load flexibility. It can gasify a wide range of feedstocks, regarding the particle size or moisture and ash contents. - Good control of bed temperatures and high transfer rates. - High efficiency: CCE and CGE. (CCE is higher than in fixed bed but lower than in entrained bed gasifiers.) - High capacity. - Easy to scale up to larger-size plants. This technology also has some disadvantages such as the high temperature of the exhaust gas, high concentration of solid particulates in the exhaust gas, medium tar content, and high economic investment. There are several configurations of fluidized bed reactors that can be also dual systems or can be single-stage or multistage [8]. Examples of coal fluidized bed gasifiers include the catalytic gasifier technology being commercialized by Great Point Energy, the Winkler gasifier, and the KBR transport gasifiers [4]. In the case of biomass and waste, the RadGas Technology is an interesting example of technology. There are also some tens of stationary and circulating fluidized bed gasifiers at a scale of 10–140 MWth that use various types of biomass residues and wastes to generate LCV product gas for use as a fuel to lime and cement kilns and for cofiring with coal in power plants. In Vaasa in Finland, about a third of the fuel capacity in a 500 MWth peat-fired power plant comes from solid biomass, and in Lahti, a waste and solid biomass plant produces 50 MWe and 100 MWth district heat for the city [7].

4.2 Fixed bed gasifiers In a fixed bed gasifier, also known as moving bed, the gasifying agent flows through a fixed bed of fuel solid particles. It consists of one combustion zone and other zone where the gas and the solid are mixed. Attending on the flow direction of the gasifying agent through the bed of fuel, three types of gasifiers can be distinguished (see Fig. 11): - Countercurrent or updraft. The solid enters at the top and the gasifying agent enters at the bottom, circulating in opposite directions. Some of its advantages are: high thermal efficiency and low output temperature. Some of its weak points are: high tar content, fuel size requirements, and low scale-up potential. - Cocurrent or downdraft. Fuel and gasifying agent streams flow in the same direction. A relatively clean gas of tars and achieving high carbon conversion are some of its strong points. Some of its drawbacks include high output temperature, strict fuel size requirements, and low scaling potential. This configuration is optimal for internal combustion engines and gas turbines.

38

Chapter 2

Fig. 11 Fixed bed gasifiers: Updraft, downdraft, and cross-draft gasifiers (own elaboration).

- Cross-current or cross-draft. The fuel and gasifying agent streams are perpendicular to each other. It generates a gas of similar composition to updraft or downdraft, but it has few applications. Some of the main attractions of fixed bed gasifiers are the configuration and operation simplicity, that they can be built in small sizes, and that can be suitable for biomass and waste. Some examples of fixed bed gasifier are Lurgi Dry-Ash gasifier and British gas/Lurgi gasifier that are widely used for coal gasification.

4.3 Other types of gasifiers Other types of gasifiers are as follows: - Entrained flow gasifiers where solids are entrained along with the gas flow, operating in isocurrent, with very fine particle size and low residence times at high temperature. High carbon conversion and low tar content are some of the strong points of this technology. The disadvantages are related to the high working pressure and low experience for biomass gasification. - Rotary gasifiers. This technology has a mechanical part that facilitates the movement of the solids. It has short processing times, fuel without may specifications, and good thermal performance; however, it produces high content of tars and oils. - Plasma gasifiers. This technology decomposed the fuel into very simple molecules. Its main advantage is no tar, char, dioxin, furan, or soot formation. It has several disadvantages due to its high cost, high temperature, and long residence time.

Syngas production by gasification processes 39

5. Cleaning and upgrading of syngas The gasification gas has several applications, but, generally, the gas leaving the gasifier is not suitable for direct use. Therefore, the gasification gas may be subjected to a purification and conditioning process to improve its properties. The removal of pollutants is necessary to reach the tolerance levels of the downstream conversion system and to comply with environmental regulations. Conditioning is necessary to adjust the CO/H2 ratio and improve the quality of the gas prior to energy recovery or conversion process [42].

5.1 Gas cleaning Gas cleaning and gas quality improvement processes can be carried out in two ways: As a primary method, carried out during the gasification process, and, as a secondary method performed after the gasification process. Primary methods are all measures that are carried out in the gasification process itself to minimize the content of impurities in the gas. They are cheaper than secondary methods. Some of the primary methods are the selection of the operating conditions, use of different gasifying agents, use of different additives or catlysts in bed or the design of the gasifier [43]. The secondary gas cleaning methods has been carried out using wet scrubbing, traditionally. This involves gas cooling to ambient temperature or below temperature, making some uses as gas turbine steam generation less efficient. In addition, it involves high costs and large volume of effluents to be treated by expensive methods [42]. However, it is used in some plants, especially for low-temperature gas applications, as it has proven efficiency, it is a mature technology, and it is easy to design [27]. However, the hot gas cleaning, at temperatures above 300°C, allows for greater integration with catalytic systems, a better energy balance, and improving the economy of the process, while eliminating the problems of the effluent management [42]. 5.1.1 Low-temperature gas cleaning Typically, the gas leaving the gasifier undergoes a coarse particle separation stage in a cyclone. This can be followed by a series of stages, which constitute the low-temperature cleaning, which includes a water wash and the removal of acid gases. During the water scrubbing stage, particulates, tars, NH3, HCN, and halides are removed from the gas stream. Some trace compounds are also removed by condensation [42]. There are some commercial processes that use a gas washing method to remove tars, such as OLGA technology at TNO or the technology used at G€ussing Renewable Energy, which uses a patented oil as washing liquid, in a multistage washing system [44–46]. The OLGA process is based on the fact that tar condensation starts before water condensation, due to the gas dew

40

Chapter 2

point is lowered but without reaching the temperature of the water. Afterward, tar is absorbed, reducing the gas’ dew point temperature, but not the water’s. Therefore, the gas dew point temperature is lower than the water dew point temperature. Additionally, there are studies addressing the use of different cleaning agents such as water, diesel, biodiesel, vegetable oil, motor oil, and waste cooking oil reaching important efficiencies in the elimination of tar [47,48]. 5.1.2 High-temperature gas cleaning In the first stage of particle removal, the tendency is to increase the operating temperature and to reduce the particle concentration and particle size, through the development of different filtration systems: ceramic candles, ceramic cross-flow filters, electrostatic cross-flow ceramic filters, high-temperature and high-pressure electrostatic high-temperature and pressure electrostatic precipitators, granular bed filters, and fabric/fiber filters [42]. The removal of sulfur compounds (H2S and COS) is carried out by adsorption. Among the technologies for the removal of NH3 and HCN, there are catalytic decomposition processes. The simultaneous removal of NH3 and H2S is also being investigated by means of dual adsorbents [27]. For halides, the current trend is to use alkali or alkaline metal adsorbents. With regard to alkalis, they are removed with multifunctional adsorbents. The combined use of ceramic filters and adsorbents or catalysts allows the joint removal of particulates, alkalis, and halides. Technologies are also used for hot removal of noncondensed heavy metals, mainly by adsorption. In the specific case of Hg, selective active carbons are being developed [27]. Finally, the removal of tars at high temperature is based on cracking processes, breaking large molecules into other smaller molecules, thus converting tar into permanent gases, basically, H2 and CO. The energy content of the tar is recovered with the molecules formed. This process may be thermal cracking, if heat is used to break up the molecules; or catalytic cracking, when lower temperatures are used but the presence of a catalyst is required [2].

5.2 Gas upgrading In addition to the removal of impurities from the gasification gas, it is necessary to undergo a series of stages to adjust its composition according to the end use for which it is intended [27]: - The first stage is reforming with water vapor or dry (with CO2) or a combination of both or partial oxidation of CH4 or other light hydrocarbons to reduce their content in the gas and increase the H2 content. - The second step is the adjustment of the CO/H2/CO2 ratio by means of the WGSR.

Syngas production by gasification processes 41 - The next stage is the production of H2 from the output stream of the shift stage. It is carried out by separation of the H2, or by separation of CO2 and subsequent purification of the H2-rich stream. - Finally, the chemical synthesis comprises a set of processes that allow the conversion into fuels (diesel and gasoline via FT-gasoline, methane, methanol, ethanol, dimethyl ether, dimethyl ether, methane, methanol, methanol, ethanol, dimethylether, etc.) of the CO and H2 contained in the synthesis gas in the synthesis gas. The desirable H2/CO ratio, operating conditions, and catalysts used depend on the final product.

6. Syngas applications The versatility and efficiency of gasification make it an attractive technology with a wide variety of markets. Gasification gas can be used in a multitude of applications in order to obtain different products, as shown in Fig. 12, for instance: chemical synthesis of methanol and other compounds, hydrogen, synthetic advanced fuels via FT, heat generation in a boiler, power generation in an engine, heat and power generation in a gas turbine and in a steam turbine [integrated gasification combined cycle (IGCC)], and power generation in a fuel cell, bioproducts, etc. [19]. Some of the syngas applications are as follows: - Thermal energy production by the combustion of the syngas directly in furnaces or boilers. It is the least restrictive method, as it requires hardly any gas cleaning and upgrading phase, admitting high contents of tars and impurities. - Electricity production by cogeneration systems (engines and turbines), in IGCC or fuel cells. It is an optimal solution for isolated areas. - Chemical synthesis of gaseous fuels such as hydrogen, synthetic natural gas, or methane. - Chemical synthesis of advanced liquid fuels, such as diesel or gasoline. There are two basic paths to obtain it: The first one is using the FT reaction, and the second one is obtaining

Fig. 12 Gasification gas applications (own elaboration).

42

Chapter 2

Table 5 Syngas requirements as a function of possible uses (using some data from refs. [19,27]).

H2/CO CO2 Hydrocarbons Pollutant Calorific power Pressure, bar Temperature, °C

Synthetic fuels

Methanol

Hydrogen

Fuel gas

Fuel gas

Fuel gas

FT-Gasoline 0.6 # # 823 K

+247

973 K

+247 +206  880

1.0

>823 K

+247 +206  36

1.5–2.0

for long-chain hydrocarbons synthesis in the Fischer–Tropsch process [19]. In common SMR, energy is supplied via fossil fuels combustion results in generating CO2 greenhouse gas emissions, whereas in DMR, CO2 is consumed for synthesis gas production in contact with an appropriate catalyst [20]. While partial methane oxidation (PMO) as another convectional process of syngas production does not need such large amount of energy inputs due to its exothermic nature, it requires a pure oxygen stream. Furthermore, the partial oxidation reaction with high exothermicity causes hot spots in the applied catalysts. Thus, the mentioned operational difficulties of this process limit its application for syngas production. For reducing high energy requirements in reforming processes, autothermal reforming as a combination of PMO and SMR or PMO and DMR was developed in 1950. Despite such advantages of autothermal reforming, oxygen separation from syngas is costly [7]. Another catalytic technique for methane conversion to syngas is methane trireforming (MTR), which is represented as a combination of SMR, DMR, and PMO in a single reformer. This process can be considered as a promising technique for curbing CO2 emissions due to utilizing flue gas CO2 as a carbon source. In spite of the advantages of MTR, it is not accomplished in the industries as it suffers from the lack of stable and highly active catalysts [17]. DMR reaction (Eq. 1) is highly endothermic, which requires high temperatures for achieving equilibrium conversion for producing syngas.
(1) CH4 + CO2 ! 2CO + 2H2 ΔH298K ¼ +247 kJ mol1

Dry reforming for syngas production 99 Although as previously mentioned, DMR reaction could produce synthesis gas with H2/CO ¼ 1:1 molar ratio, the simultaneous incidence of reverse water shift gas (RWSG) reaction (Eq. 2) results in H2/CO ratio of lower than unity.
(2) ΔH298K ¼ +41 kJ mol1 CO2 + H2 ! CO + H2 O In addition to the RWGS, sidelong reactions such as methane decomposition in which CH4 completely separates on the catalyst to produce carbon and form H2 (Eq. 3), as well as Boudouard reaction, for CO disproportionation to form surface carbon and CO2 (Eq. 4) may occur simultaneously. The side reactions are strongly influenced by partial pressure and operating temperature [21].
(3) CH4 $ C + 2H2 ΔH298K ¼ +75 kJ mol1
2CO $ 2C + O2 ΔH298K ¼ 171 kJ mol1 (4) By lowering the activation energy of the desired reactions in the process, the catalyst plays a key role in the DMR reaction, increasing reaction kinetics and achieving a maximum yield of syngas production (Fig. 1). Moreover, because of the endothermic nature of the DMR process, the presence of an appropriate catalyst helps the reaction to decrease the operating temperature significantly for obtaining the desired products. Several efforts have been done for developing catalytic systems that have enhanced resilience for coke formation and cheaper precursors. These attempts include the alteration of synthesis conditions and methods in addition to applying mixed support approaches and bimetallic catalysts [8,22]. The DMR catalysts are categorized into two main classes including the noble and non-noble metal-based catalysts [23]. Despite of high reactivity, stability as well as anti-coking effects of noble metals such as Pd, Rh, Pt, and Ru, their high undercosts restrict their applications for industrial scales [24]. Therefore, non-noble metals including Fe, Co, and Ni are widely

DMR

CH4

Promoter

CO

Active metal (Monometal-Bimetal)

CO2

Support

Fig. 1 Schematic of catalysts for DMR reaction.

H2

100 Chapter 5 used in the large-scale DMR process [25]. Among these metals, Ni-based catalysts have shown good performance in the DMR process due to their high activity and low costs [26].

2. Ni-based catalyst Nickel has been identified as the most acceptable non-noble catalyst in DMR; researchers are currently concentrating their efforts on improving the stability of nickel-based catalysts. The main problems with these catalysts are sintering of Ni active sites at high reaction temperatures, as well as activity loss due to carbon deposition [7]. Fast carbon deposition on the surface of catalyst can deactivate catalysts by destroying active site particles and blocking their pores. The balance toward the gasification and formation of carbon results in the creation of coke. Carbon gasification on Ni surfaces is frequently slower than the formation of coke, resulting in carbon deposits accumulating. The dissolving of carbon in Ni particles is an important step in the growth of carbon whiskers, which began with the production of Ni carbide. It was reported that by decreasing particle size of nickel active sites, Ni carbide formation has been reduced, and therefore, lower deposited coke was formed [27].

2.1 The role of Ni content Jose-Alonso and coworkers have recently studied the effect of Ni content of 1.3–7.7wt% (1.3, 2.5, 4.4, and 7.7) on a pelletized g-alumina for DMR at 973 K. They investigated the effect of metal loading on the catalyst stability toward coke formation and catalytic activity. The catalyst with 1.3 wt% Ni displayed the lowest deposited carbon amount and the highest stability. However, the highest methane conversion was related to 7.7wt% Ni (54% conversion) after 6h of testing at 973K. The CH4 conversion of other Ni contents, such as 1.3wt%, 2.5wt%, and 4.4 wt%, was reported to be 29%, 38%, and 40%, respectively. On the other side, by increasing the reaction temperature to 1023 and 1073 K, increased methane conversion was observed using a catalyst with the lowest Ni content, and a very small amount of carbon deposited on its surface according to characterization tests. The action of this catalyst at 1073K showed that the high conversion of 74% was achieved, which was considerably higher than that related to 7.7 wt% Ni content at 973 K. Therefore, the catalyst with a metal content of 1.3 wt% Ni was the most efficient one at 1073 K [28].

2.2 The role of supporting material As is well known, support is a key part of the catalyst because it influences the catalytic performance of the catalyst by dictating the dispersion of the metal active sites. The metal–support interaction can have a significant impact on numerous aspects of the catalyst, such as size of particle and active sites dispersion, in addition to metal–support interface and so on. All of these parameters have an impact on the sintering resistance, coke resistance, and

Dry reforming for syngas production 101 catalytic activity. Aside from that, the support’s instinctive qualities such as redox, acid–base, and so on are important factors. Different materials listed as zeolites, metal oxides, composite metal oxides, silica, carbon-based materials, ordered mesoporous materials, etc., have been examined as supports of Ni-based catalysts in DMR, the results of which are summarized in Table 2 [40]. Despite the considerable properties of Ni/Al2O3 catalyst such as high dispersion of active phase, high availability of reactant molecules, and good particle size distribution, small coking on catalyst surface reduces CO2 and CH4 conversions also H2 and CO yields in DMR [36]. Mei et al. suggested that by using plasma reactor for dry reforming of biogas, the amount of coke can be reduced to as low as 3.9% [41]. Talkhoncheh et al. [36] synthesized Ni-based nanocatalysts over CeO2, clinoptilolite, and Al2O3 supports by impregnation method for DMR in temperature ranging from 550 to 850°C. Ni/Al2O3 nanocatalyst indicates much higher conversions (96% and 93% for CO2 and CH4, respectively) and yields (93% and 90% for CO and H2, respectively) at 850°C because of the desired physicochemical properties of Al2O3. Additionally, the Ni/Al2O3 activity was steady after 1440 min. While the reaction conversion of Ni/clinoptilolite is not as much as Ni/Al2O3, it showed good performance in DMR and its lower costs than other synthesized catalysts were mentioned as its superiority than others. CeO2 is so useful supporting material for Ni-based catalysts in DMR reactions because it has high oxygen mobility and storage capacity in the nickel oxidation state [8]. In terms of thermodynamic, reduction of ceria by CO2 to 1000°C is suitable procedure for increasing its mobility property [19]. It was found that the metal–ceria catalyst may directly separate CO2 into CO and active oxygen species and thus reduces the coke deposition and improves the redox reversibility of Ni species [36]. The consequences have displayed that adding cerium decreases the size of particles and crystallites [42]. The good thermally stable feature of SiO2 causes wide applications of this supporting material for Ni-based catalysts. Furthermore, the low surface acidity of SiO2, reduces the deposition of coke on the catalyst surface [43]. However, the difficult creation of strong interaction between metal and support over the catalysts with SiO2 support, due to heat aggregation of the Ni active site, these catalysts deactivate quickly [44]. Wang et al. [33] synthesized core– shell structure of Ni-SiO2 via water-in-oil microemulsion method. The Ni@SiO2 core–shell catalysts had outperformed stability and catalytic performance in the DMR process than traditional Ni/SiO2 catalysts, which was reported to be the reason of inhibition effect of core– shell structure for Ni nanoparticle sintering due to the strong interaction between metal and support in the Ni@SiO2 core–shell catalysts. Also, tiny Ni nanoparticles were observed in their synthesized catalysts, which had lower coke deposited. Mesoporous silica materials such as MCM-41, KIT-6, and SBA-15 have been extensively explored in the field of catalysis due to their small pore size distribution and organized pore channels, which are beneficial for active metal dispersion and approachability [31]. The comparison between the activity of such

Table 2 Ni-based catalysts with different supports in DMR. Metal

Support

Temperature (K)

Pressure (atm)

CH4 conversion (%)

H2/CO molar ratio

Ref.

Ni Ni Ni Ni Ni

CeO2 Al2O3 Al2O4 SBA-15 SiO2 (core-shell) SiO2 (impregnation) ZrO2 ZSM-5 Clinoptilolite Al2O3 CeO2 SiO2 TiO2 Al2O3 ZrO2 MgO MgO-AL2O3 CeO2-SiO2 SBA-15 (solid-state) KIT-6 MCM-41 SBA-15 (impregnation)

1073 1023 1073 1023 1023 1023 1123 1073 1123 1123 1123 1023

1 1 1 1 1

2.00 0.89 0.81 0.98 0.98 0.68 1.00 0.95 0.94 0.97 0.93 –

[29] [30] [31] [32] [33]

1073 973

1 1

95 55 90 92 86 74 97 66 88 93 75 88 3 78 88 90 78 98 76 75 59 71

1.20 1.10 1.00 1.00 0.92

[38] [39]

Ni Ni Ni

Ni

Ni Ni

1 1 1

1

[34] [35] [36]

[37]

Dry reforming for syngas production 103 mentioned mesoporous silicas as a support of Ni active sites in DMR at 973K during 100h was in the order of Ni-SBA-15, Ni-KIT-6, and Ni-MCM-41 from more to less, respectively. Also, the carbon formation was decreased in the following sequence: Ni-MCM-41 > Ni-KIT-6 > Ni-SBA-15. Metal sintering and deposition of carbon on the Ni-SBA-15 were the lowest because of the strong interaction between the Ni species and support in addition to high dispersion of Ni particles [39]. SBA-15 as a silica-based support has a superior properties for dispersing and hosting nickel nanoparticles such as high specific surface area, regular mesoporous channels with great hydrothermal stability and thick pore walls. The effect of operating conditions such as reaction temperature on the catalytic performance of 10% Ni/SBA-15 catalyst for DMR reaction was investigated by Omoregbe et al. [32]. The DMR reaction was performed at different CH4 to CO2 volume ratios (3:1 to 1:3) and temperatures ranging from 923 to 1023K. Catalytic activity was found to be stable for 4h at 973–1023K, with slight decline in activity at 923 K. The results of the impact of Al2O3, SiO2, ZrO2, MgO and TiO2 supports on Ni active sites performance in DMR were reported as the following [37]: The NiO/SiO2 initial activity was very high, but it gradually decreased by time on stream, implying that NiO/SiO2 has low catalytic stability. This is perhaps due to the poor NiO-SiO2 interaction that makes it hard to prevent active site of nickel from sintering over the surface of SiO2 [45]. The NiO/TiO2 catalyst has the lowest initial activity of all the studied catalysts with CO2 and methane conversion dropping to less than 5% in just 20 h. The TiO2 support with low surface area and poor NiO-TiO2 interaction caused the formation of large Ni grains and therefore ended weak performance of NiO/TiO2 catalyst. The difficulty in reduction of NiO/Al2O3 was reported as a result of the NiAl2O4 spinel formation due to the intense NiO/Al2O3 interaction. As a consequence, NiO/Al2O3 had a very low catalytic activity. Furthermore, NiO/Al2O3 catalyst deactivated quickly, possibly due to the acidic sites on the its surfaces [46]. During DMR reaction at high temperatures, poor interaction between NiO and ZrO2 in Ni/ZrO2 catalyst caused accumulation and sintering of Ni active sites, and thus, rapid deactivation was observed with cycling time.

2.3 The role of promoters An appropriate promoter can not only control the metal atoms’ electronic density, but also can form smaller metal active site crystals, change the basicity and acidity of catalyst surfaces, improve the interaction between support and active sites, in addition to enhancing the deposited carbon resistance or improving the reaction performance. In the DMR reaction, different kinds of promoters have been used mainly to improve the stability rather than the catalyst activity. Generally, three types of promoters have been used in this process: (i) alkali and alkaline earth metals such as Ca, Mg, Ba, and Li; (ii) rare earth metals such as La, Ce, Pr, Zr, and Sm; and (iii) other metals including Bi, Sn, Au, Pb, Ag, As, and Cu. These promoters and their effective performance in DMR are listed in Table 3.

Table 3 Ni-based catalysts with different promoters in DMR. Metal

Support

Promoter

Temperature (K)

Pressure (atm)

CH4 conversion (%)

H2/CO molar ratio

Ref.

14.0 wt% Ni

Al2O3

1023

1

Al2O3

973

1

Ni

Al2O3

1123

1

Ni Ni Ni

Al2O3 ZrO2 SiO2

973 1073 1073

1 1 1

5.0 wt% Ni

Al2O3

973

1

10.0 mol% Ni

CeO2

1033

1

2.5 wt% Ni

MgO-Al2O3

1023

1

10.0 wt% Ni

SBA-15

1123

1

84.7 71.0 81.3 19.0 25.0 63.0 63.4 71.3 55.2 60.5 70.0 90.0 74.0 95.2 96.6 94.3 97.0 97.4 92.8 90.7 78.0 77.0 72.5 70.0 65.2 21.0 12.8 11.8 50.0 60.0 85.0 97.8

0.97 0.40 0.88 0.28 0.44 0.90 0.87 0.90 0.92 1.00 1.00 1.00 0.94 0.97 1.22 1.09 1.16 1.14 1.07 1.02 0.88 0.92 0.93 0.92 0.82 0.42 0.36 0.34 0.78 0.72 – –

[47]

15.0 wt% Ni

0 0.5 wt% Ca 0.5 wt% K 0.5 wt% Sn 0.5 wt% Mn 0 5.0 wt% ZrO2 10.0 wt% ZrO2 15.0 wt% ZrO2 10.0 wt% ZrO2 0 CeO2 MgO La2O3 0 (3.0Si/Ce) CeO2 (9.0Si/Ce) CeO2 (18.0Si/Ce) CeO2 (30.0Si/Ce) CeO2 CeO2 0 3.0% MgO 3.0% CaO 3.0% BaO 0 0.3 mol% Ag 0.6 mol% Ag 2.4 mol% Ag 0 La 0 3.0%wt MgO

[48]

[49] [50] [51] [38]

[52]

[53]

[54] [55]

Dry reforming for syngas production 105 The research on the performance of Ni-MgO-Al2O3 in DMR showed that the small Ni particles have been created in contact with MgO promoters with loadings up to 15wt% having the highest catalytic stability and activity. The CO2 and CH4 conversions have been enhanced with the increment in temperature from 823 to 973K due to the endothermic nature of the DMR reaction. Temperature programmed reduction (TPR) results demonstrated that by increasing the Ni content higher than 15wt%, the reducibility of the catalysts became enhanced due to the weaker interaction of nickel with MgO-promoted Al2O3 support, which therefore made larger nickel crystallites. The obtained results demonstrated that one of the important parameters of the catalytic activity during the DMR reaction is feed ratio (CO2/CH4) in such a way that increased CO2/CH4 ratio has decreased both H2/CO molar ratio and CO2 conversion and increased the CH4 conversion [50]. Akbari et al. [56] studied the influence of CeO2 addition as a promoter for Ni-MgO-Al2O3 catalysts with different ceria contents and preparation methods such as coprecipitation method and impregnation methods in DMR. The obtained results displayed that the addition of Ce to Ni-MgO-Al2O3 has a significant effect on the coke deposition, reduction properties, and metal crystallite size. CeO2 as a promoter has improved the physicochemical catalysts properties. H2-TPR profiles demonstrated that Ce has improved the catalytic reducibility because of the reduced cerium oxidation potential. In addition, CeO2-Ni-MgOAl2O3 had higher activity than Ni-MgO-Al2O3 catalyst. While increasing the CO2/CH4 feed ratio from 1/6 to 6 has boosted CH4 conversion, it showed a negative effect on carbon dioxide conversion and H2/CO molar ratio. Also, by increasing the gas hourly space velocity (GHSV) value, the reactant conversions were reduced. The binary CeO2-SiO2 (CS) with different cerium loading (3, 9, 18, 30 wt%) was synthesized via deposition precipitation (DP) and tested as support of Ni catalyst, the results of which displayed that the performance of CeO2-SiO2 (Ni/xCS) catalysts was better than that of Ni/CeO2 and Ni/SiO2 ones in the field of Ni particles dispersion, size, basicity, reducibility, and deactivation by coke formation. In this study, 9wt% ceria was the optimum loading, which exhibited the highest stability, catalytic performance, and lowest carbon deposition [38]. Different metal oxide promoters including CeO2, MgO, and La2O3 were added to Ni/Al2O3 catalyst for improving its performance in DMR at the temperature range of 823–1123 K [49]. Among these promoters, CeO2 showed an excellent mechanical and thermal resistance and because of its unique property of releasing and storing oxygen, it caused better Ni active metals dispersion and, as a result, has decreased the deposited coke on the catalyst surface [57]. Adding 10% cerium as a promoter resulted in a lower overall surface area but a more uniform distribution of tiny nickel species, as well as higher catalytic stability and activity than the Ni/Al2O3 catalyst were observed over 10 h DMR at 850°C. Alkaline earth metal oxides such as BaO, CaO, and MgO to Ni/Al2O3 were also added by impregnation method, the results of which in DMR showed that although by addition of these promoters, reduction in catalyst surface area has been occurred, their activity in the process has been increased. Among these promoters, MgO had the best activity. TPR analysis displayed that the addition of MgO

106 Chapter 5 into the Ni/Al2O3 catalyst has increased the reducibility of Ni catalyst and reduced the NiO species reduction temperature. In spite of the reduction in methane conversion using BaO and CaO promoters in Ni/Al2O3 catalyst, the deposited coke was decreased in DMR after 30min with these promoters [52]. The catalytic performance of Ni/SBA-15 catalysts with a range of Ni contents (5–15wt%) and 1–7wt% MgO was examined in a continuous flow setup for DMR. In spite of approximately the same methane conversion and CO selectivity over 10%Ni/ SBA-15 and 10%Ni-3%MgO/SBA-15 at 1123 K, the CO2 conversion was 77% and 86%, respectively, after 120 h of reaction [55]. It was reported that citric acid complex, as well as La addition to Ni catalysts, not only limited Ni reoxidation but also enhanced Ni particles dispersion and reduced the rate of cocking during the reaction. The results illustrated that the low nickel content has prevented rapid growth and carbon deposition of particles [54]. A series of Ni/La2O3-ZrO2 catalysts were synthesized by sol–gel method for DMR at 1073K and atmospheric pressure. The purpose of this study was said to investigate the effect of calcination temperature and Ni loadings on the catalytic activity, coke deposition, and long-time stability. The 10wt% Ni content catalyst had the lowest coke deposition and superior catalytic performance, of which CO2 and CH4 conversions were 95.8%, and 95.2%, respectively, and CO and H2 selectivities were 97.5% and 94.3%, respectively. These high conversions and selectivities were expressed to uniformly disperse small particles of Ni. When calcination temperature was increased to 1073 K, the catalytic performance was enhanced. However, more increment in calcination temperature to 1173 K destroyed the catalyst structure in terms of BET surface area and nickel particles sintering, which then decreased the catalytic activity and increased the quantity of carbon deposited to the maximum value of 8.4 wt% [51]. The effect of different metals addition including Mn, K, Ca, and Sn with 0.5wt% loading on the of a Ni-Al2O3 catalyst in DMR showed that K-modified catalyst had the highest stability and lowest carbon deposition among others, which had less than 4% reduction in catalytic activity after 30 h. It was also mentioned that potassium has a prevention effect on carbon accumulation on catalyst surface, promotes reducibility by likely altering the metal–support interaction, and has no effect on the size or structure of the Ni particles [47]. Therdthianwong et al. [48] investigated the improvement of Ni/Al2O3 performance in DMR by addition of ZrO2. The deactivation of this catalyst was mostly caused by coke deposition, which was prevented by the addition of ZrO2. They claimed that the method of catalyst preparation has influenced the catalyst texture and surface area because of the formation of ZrO2-Al2O3 composites with plugged pore by ZrO2 particles. As a consequence, co-impregnated 15Ni/10% ZrO2/Al2O3 catalyst had a larger BET surface area and activity than the one sequentially impregnated. Munoz et al. [58] inserted Ni active sites into Ce0.15Zr0.85O2 and CeO2-YSZ (yttria-stabilized zirconia) for the DMR reaction at 1023 K. The high catalytic stability and activity of these

Dry reforming for syngas production 107 two catalysts are due to the good oxidation properties of their preservatives, which cause oxidation of carbonaceous materials over DMR reaction. In contrast to commercial Ni-based catalysts, which lose 70% of their initial activity after less than 2 h, Ni/CeO2-YSZ and Ni/CeO2-ZrO2 catalysts demonstrated stable CO2 and CH4 conversions with less than 4% deactivation occurrence after 24 h. Noble metals can also be employed to prevent the nucleation and coke growth into graphite or whiskers carbon, in addition to stimulating the coke gasification that has been deposited on the catalyst surface. For instance, Ag metal has been used to improve the Ni/CeO2 stability in the DMR reaction under various temperatures from 833 to 1033 K using different raw materials as reaction feedstock [27]. As a result of surface reconstruction and coke deposition, the pristine Ni/CeO2 was not stable during the process [53]. Ag significantly has decreased the intrinsic activity of nickel particles as a consequence of increased activation energy as well as long-time stability under varied raw material conditions. While 3 mol% Ag addition was sufficient to cover every uncoordinated step location on the nickel surface, extra Ag has been bounded and migrated with CeO2 support. Ag promoter has not only reduced coke content but also changed the type of coke formed from graphitic or whisker carbon to amorphous one [53].

3. Co-based catalyst In addition to Ni-based catalysts, cobalt-based catalysts also showed good activity in the DMR reaction. The main problem of these catalysts is deactivation by carbon deposition [59]. Supports such as SiO2 and Al2O3 have shown good stability over cobalt catalysts. Other support materials such as Nd2O3, La2O3, MgO, ZrO2, and CeO2 are also usable for Co catalysts [8]. The addition of an appropriate promoter has demonstrated good ability for reducing deposited coke on the catalyst surface and improving the catalyst activity in DMR. Among the applied promoters in the DMR process such as potassium, lanthanum, strontium, and cerium, potassium has shown the highest performance for reducing carbon deposited in addition to enhancing catalytic activity [59]. The effect of different parameters such as Co loading, supporting material and different promoters on the catalyst performance in DMR is reviewed in the following.

3.1 The role of Co content It was reported that high Co loadings (among 12 and 20wt%) are exposure for deactivation as a result of coke deposition, whereas for low values (about 2 wt%), it is due to the cobalt oxidation [60]. Jose-Alonso and coworkers have recently studied the effect of different Co loadings (1, 2.5, 4.3 and 7.6wt%) on a pelletized g-alumina for the DMR reaction at 973K. By reducing the Co metal content from 7.6 to 4.3 wt%, methane conversion was decreased,

108 Chapter 5 along with a slight decrease in the stability after 6 h of reaction. The relatively close CH4 conversion was given by 4.3 wt% and 2 wt% Co, which illustrated that conversion does not directly depend on cobalt content at low Co loadings. Catalyst with 1wt% Co was initially very active, but it was deactivated very fast due to coke formation [28].

3.2 The role of support The efficient effect of CeO2 support for dispersion of cobalt showed 80% methane conversion and H2/CO ratio close to 1 at 45 kPa reactants partial pressure in DMR [61]. Abasaeed and coworkers [62] tested impregnated cobalt active sites on nanosized ZrO2 and CeO2 supports in DMR. It was concluded that a higher H2/CO molar ratio was produced for Co/ZrO2 than Co/CeO2 with similar Co contents. Furthermore, the increment in calcination temperature from 773 to 1173 K caused a reduction in the BET surface area of both series of synthesized catalysts, which then reduced their activity in the process. The reports of TPR analysis demonstrated that for Co catalysts with ZrO2 support, a reduction peak has shifted into the lower temperatures when calcination temperature was increased displaying a more reducible Co phase formed at lower calcination temperatures. According to TPO and TEM images, the type of carbonaceous residue formed on ZrO2 and CeO2 supports was disparate. Carbon nanotubes (CNTs) with different sizes were viewed on Co/ZrO2, whereas a reactive carbon was deposited on Co/CeO2, which then oxidized because of the high oxygen storage capacity of CeO2 support. The wet impregnation method was used for the synthesis of 20%Co/La2O3 catalyst for DMR in Ayodele et al.’s experiment [63]. At 1023 K reaction temperature and 1.0 feed ratio, Co/La2O3 catalyst evaluated in a fixed-bed stainless steel reactor has shown maximum methane and CO2 conversion of 50% and 60%, respectively. Nd2O3 as another supporting material hasahigh capacity for storing and releasing oxygen as a rare earth metal oxide. During the DMR reaction, valence oxygen of Nd2O3 can be released for gasification of coke deposited on the catalyst surface. Therefore, in another experiment, Ayodele et al. [64] used a 20% Co/Nd2O3 catalyst for catalytic dry reforming of methane for the first time. The obtained results in reaction conditions of the same as their previous work showed 82% and 62.7% CO2 and CH4 conversions, respectively, which are both higher than that of 20%Co/La2O3 catalyst. As it was previously mentioned, MgO is commonly used as a catalyst supporting material because of its inexpensive cost and great thermal stability. The higher melting point of MgO (3123K) compared with other oxide supports allows it to maintain a comparatively wide surface area [65]. Substantial Lewis basicity of MgO has induced strong CO2 adsorption and decreased carbon deposition on active sites posited on MgO support. The high stability and activity of Co/MgO catalyst have been attributed to the organization of a CoO-MgO solid solution [66]. Compared with other supports, the interactions in Co/MgO catalyst were the greatest, making it difficult to be reduced. Mirzaei et al. [67] used the coprecipitation approach

Dry reforming for syngas production 109 Table 4 Co-based catalysts with different supports in DMR. Metal

Support

Temperature (K)

Pressure (atm)

CH4 conversion (%)

20wt% Co 20wt% Co 20wt% Co 5 wt% Co 10wt% Co 15wt% Co 20wt% Co 30wt% Co

CeO2 La2O3 Nd2O3 MgO

1023 1023 1023 973

1 1 1 1

78.0 50.0 62.7 25.0 69.0 21.0 25.0 24.0

H2/CO molar ratio Pt-Ag, and the yield of H2 for these catalysts was in the order of Rh > Co > Fe > Ni ≫ Ru > Pt-Ag. The most active catalyst for syngas production was Ni/LaGaO3.

3.4 Biofuels Renard et al. [45] studied the catalytic partial oxidation of glycerol over various catalysts. They used a nebulizer to mix glycerol droplets with air at room temperature for reactive flash volatilization. Then, the reaction occurred at 600°C during 30–90 ms. They investigated the partial oxidation of glycerol to equilibrium products (synthesis gas) and nonequilibrium products (other chemicals). Results showed that Rh catalyst improves equilibrium selectivity to syngas, and Pt catalyst produces nonequilibrium products such as olefins and acetone. Adding steam to the reactants leads to more H2 production due to the water–gas shift reaction. They also tested the partial oxidation of glycerol over the RhdCe catalyst. The synthesis gas was produced with a good yield of over 75 h, and after that, the catalyst began to deactivate. Nguyen and Leclerc [46] studied the partial oxidation of methyl acetate over Rh and RhdCe catalysts. They found that the primary products are H2O and CO, rather than H2O and CO2. Higher methyl acetate conversion and more hydrogen production were achieved at high GHSV over the RhdCe catalyst compared with the Rh catalyst due to more water–gas shift reaction. Cuba-Torres et al. [47] evaluated the partial oxidation of methyl oleate over MoO2 catalyst. The reaction was started at 750°C, and no coke deposition was observed during 6 h under feed conditions with an O2/C ratio in the range of 0.6–0.7 at 850°C. They showed that the MoO2 catalyst has higher catalytic performance than Ni. Also, a change in WHSV from 1 to 10h1 had no significant impact on the partial oxidation of methyl oleate.

4. Catalyst development history In the 1940s, Prettre et al. [48] investigated catalytic partial oxidation of CH4 over a 10% by weight nickel catalyst in the temperature range of 973–1173 K and a pressure of 1 atm. They showed that the thermodynamic equilibrium is achieved at the reactor outlet temperature with methane to oxygen feed ratio of 2. According to the results and temperature profile, the process was split into two parts: exothermic and endothermic steps. The first step involves methane

Partial oxidation process for syngas production 205 oxidation to carbon dioxide and water; then methane reforming produces carbon monoxide and hydrogen. Because these reactions are equilibrium reactions and do not have the required efficiency without adequate oxygen; the reaction must be carried out at relatively high temperatures to obtain proper efficiency and a larger yield. Huszar et al. [49] published research on the nickel catalyst with mullite grain in 1970 with limitations due to the penetration of oxygen into the gaseous layer around the catalyst. They investigated the effect of oxygen penetration by examining 25% CH4–air mixtures at 760–1000°C. It was found that the catalyst was deactivated at temperatures ranging from 1030°C to 1773°C. The production of NiAl2O4 may explain this inactivation and can be removed using hydrogen-based techniques. The ignition and extinction properties of the catalyst were studied. It was observed that in an oxidative environment, the nickel catalyst is degraded but can be reactivated in a reduction environment. Gavalas et al. [50,51] examined the effects of calcination temperature on the activity of NiO/α-Al2O3 and NiO/ZrO2. The specific activity of supported nickel oxide drops significantly with calcination temperature, but it may be restored under reaction circumstances by hydrogen reduction followed by reoxidation. The activity of calcined catalysts remains stable throughout time, while the activity of non-calcined catalysts or reduced and reoxidized catalysts gradually diminishes over time. In 1984, they also studied the effect of prereduction temperatures on the activity of NiO/ɑ-Al2O3 catalyst at 843–1033 K. In 1990, Ashcroft et al. [52] investigated ruthenium lanthanide oxide catalysts such as Pr2Ru2O7, Eu2Ru2O7, Gd2Ru2O7, Dy2Ru2O7, or Lu2Ru2O7 and Ru catalyst on pure Al2O3 and 1 to produce RuO2 support at 1043 K, atmospheric pressure, and GHSV of 4  104 mL1 cat h syngas from methane partial oxidation. Methane conversion rates above 90% and CO and H2 selectivity in the range of 94%–99% have been reported. Vernon et al. [52,53] noted the equilibrium efficiency of the reaction, using catalysts made of a combination of Rh, Pt, Ni, Pd, Ru, and Ir oxides or other comparable metals on Al2O3 support. The impact of pressure (1–20 atm) on the catalyst performance was studied, and it was found that conversion and selectivity decline with increasing pressure, as predicted from the thermodynamic viewpoint. They also indicated that partial oxidation is more selective than methane reforming when air oxidation is not feasible, but requires pure oxygen. The low-temperature partial oxidation route had lower costs than the high-temperature partial oxidation procedure, but it could not be used efficiently at high pressures. Lunsford et al. [54] investigated the syngas production over a 25% by weight Ni/Al2O3 catalyst at temperatures ranging from 723 to 1173 K. The selectivity of carbon monoxide at temperatures above 973 K has been reported to be 95%, with a methane conversion rate of approximately 100%. They also concluded that the synthesis gas production from CH4/O2 mixtures may be feasible at higher pressures and is preferred for commercial operation.

206 Chapter 9 Under the above operating conditions, they revealed that the calcined catalytic bed includes three regions: (1) NiAl2O4, (2) NiO+Al2O3, and (3) Ni/Al2O3. The feed CH4/O2/He came into contact with NiAl2O4 at the first section of the reactor, where it only had mild activity for complete methane oxidation to CO2 and H2O. The next section of the reactor contained NiO + Al2O3 phase that all methane was converted to CO2 through complete oxidation and consumed all of the O2. The third section of the catalyst bed was a reduced Ni/Al2O3 phase. In this section, H2 and CO, according to the thermodynamic equilibrium at the bed temperature, were produced through methane reforming with CO2 and water produced during the complete oxidation by the NiO + Al2O3 phase. Vermeiren et al. [55] studied Ni-based catalysts on HdY zeolite support and compared it with 1% and 5% by weight nickel catalysts on the support of SiO2, Al2O3, SiO2-ZrO2, and SiO2-Al2O3. Ni-based catalyst on the HdY zeolite support exhibited less selectivity of CO than other 5% by weight nickel catalysts. When Al2O3 and SiO2-ZrO2 supports were combined with 1% nickel, the results were similar to equilibrium circumstances. Catalysts built of acidic supports with low metal concentrations produced a mixture of gases with a considerable deviation compared with thermodynamic predictions. Hickman and Schmidt [56] demonstrated the behavior of the methane partial oxidation at short contact periods (0.1–10 ms) in an adiabatic reactor employing a 12–20 wt% PtdRh or Pt catalyst on an α-Al2O3 integrated metal support. By adding NH3 to the intake gas, they could reduce the combustion temperature from around 873 to 473K. They showed that preheating the feed improved the selectivity of the products while at the same time raising the reaction temperature. Due to the short residence time employed in these experiments, they were able to investigate direct oxidation independently of the reforming and water–gas shift reactions. Choudhary et al. [57,58], who pioneered research on cobalt catalyst, tested this catalyst on various oxide supports such as CeO2, Nd2O2, Sm2O3, Yb2O3, and compared that with CoO-MgO solid solution. The catalyst’s activation and reaction initiation, which is accompanied by the reduction of NiO and the formation of active sites, occurred at about 535–560°C depending on the type of the catalyst. During the process, carbon deposition on the catalysts, especially NiO-Gd2O3, NiO-Tb4O7, and NiO-Dy2O3, was very fast. Although the carbon deposition on the catalyst did not effect the activity or selectivity, it caused a pressure drop across the catalyst bed due to the carbon’s blockage of the void space between the catalyst particles. Based on the experimental results, CO and H2 production was controlled kinetically. Subsequent studies revealed that data collected from this reaction temperature were probably not sufficient to prove this [59]. Claridge et al. [60] observed carbon deposition on the transition metals. The comparative rates were in the following order: Ni > Pd > Rh > Ru > Pt, Ir. Based on the findings of 28% by weight Co/MgO catalyst, Chang and Heinemann [59] proposed a two-step combustion modification method that was consistent with the study of Prettre et al. [48]. Their experiments were conducted under circumstances that were extremely

Partial oxidation process for syngas production 207 near to adiabatic operation. Their findings showed that high-temperature homogeneous reactions, in which methane reacts with oxygen in an exothermic combustion reaction, are the predominant reactions, resulting in extremely high temperatures, and the remaining methane reacts with steam and carbon dioxide to form carbon monoxide and hydrogen. They found that reactions may be initiated at 773K under catalysts with high cobalt loading. Using an infrared radiation thermometer, they also observed that the catalyst temperature was very high (1473–1573 K) during methane partial oxidation to synthesis gas, despite the furnace temperature being set at 773 K. Thorniainen et al. [61] investigated the behavior of Pd, Ir, Ni, Pt, Rh, Pd-La2O3, Fe, Co, Re, and Ru catalysts during the brief contact time (10ms) in an autothermal NH3 ignition-assisted flow reactor. High selectivity and conversion rates for synthesis gas were reported over Rh, Pt, and Ir catalysts, while Pd has produced considerable coke. Output product distribution based on thermodynamic equilibrium was not found in any of the catalysts, and Ni and Co catalysts were inactivated due to alumina formation (NiAl2O4, CoAl2O4). Iron seemed to oxidize quickly and was unable to convert CH4 to synthesis gas. Finally, experimental studies performed between 1993 and 1994 revealed significant aspects of the catalyticmethane partialoxidation. These included nitrogen dilution and the general effects of pressure, thermodynamic limitations, and temperature. Water–gas shift equilibrium was typically achieved at high temperatures on noble metals and nickel, and the potential of multistage reaction pathways was suggested. Noble metal-based catalysts, such as Ni and Co, produced synthesis gas, while Fe seemed ineffectual [62].

4.1 Noble catalysts Noble metal catalysts have the most significant proportion of research regarding the number of studies and articles accessible for the catalytic conversion of methane to synthesis gas. Scientists have conducted extensive studies on various noble metal catalysts and their kinetics [16,17,63]. A summary of noble metal catalysts, supports, and operating conditions is listed in Table 3. Most metal catalysts in this class are far more reactive than nonnoble catalysts such as nickel [111]. Despite their high activity, the exceptionally high price of noble metals is a significant concern, which is why much research has been performed to minimize the amount of metal in the catalyst structure and prevent deactivation. Platinum-based catalysts, for example, are more expensive than Ni catalysts. Noble metals such as Ir, Pd, and Ru are less costly but not yet industrially viable. Synthesis gas production using various noble metal catalysts on Al2O3 support revealed that Rh and Ni are the only metals with good efficiency. Rh-based catalysts have the lowest deactivation rate, and Ni-based catalysts must operate at low temperatures to reduce the catalyst deactivation rate. Other metals appear to have significant deactivation issues, while Rh seems

208 Chapter 9 Table 3 Noble metal catalysts for partial oxidation of methane. Catalyst

Support

Temperature (K)

Pressure (atm)

References

Rh

Al2O3

723–1023 773 650–1050 943–1130 1060–1400 673–1123 623–1193 733–1273 973 623–873 673–813 1123 773–1023 923–1173 573–1073 923–1123 573–1073 873–1173 473–1173 923–1323 773–1173 923–1123 1173–1373 923–1123 873–1073 723–1073 773 1023–1073 673–973 673–1073 1023–1073 673–1073 1073 773–1073 847–1050 673–973 1073 1023–1073 1073 730–1270 723–1173 773–1073 1123 573–1073 1273

1.0 1.0 1.0 1 7.89 0.98 1.0 1 1.77 1.0 0.34 1.0 1.0 1.0 1.0 1.0 1.0 – 1.0 1.0 1.0 1.0 1.0 1.0 – 1.0 1.0 0.49 – 1.0 0.49 1.0 – 1.0 1.77 – 1.0 0.49 1.0 1.0 1.0 1.0 1.0 1.0 1.0

[14,64] [64] [60,65,66] [67] [65]

ɑ-Al2O3 monolith CeO2/Al2O3 MgO/Al2O3 Calcium aluminate Al2O3–MgO

CeO2 La2O3/CeO2 La2O3 MgO

Nb2O5 Si3N4 SiO2

Pt

Al2O3

CeO2/Al2O3

MgO CeO2

ɑ-Al2O3 monolith ZrO2/Al2O3 Calcium aluminate Al2O3–MgO Barium hexaaluminate

[66] [68] [69] [70] [71] [72] [73] [74] [75] [74] [76] [77,78] [79] [75] [80] [75] [77] [78] [64] [81] [82] [60] [81] [83] [84] [85] [86] [82] [87] [81] [88] [61] [89] [90] [71] [91] [92]

Partial oxidation process for syngas production 209 Table 3 Catalyst

Pd

Noble metal catalysts for partial oxidation of methane—cont’d Support

Temperature (K)

Pressure (atm)

References

CeO2/ZrO2/Al2O3

1073 1073 1073 1173–1373 673–873 1073 773 650–1050 730–1270 773–1073 573–1073 973–1073 973–1073 373–1023 973–1073 773–1173 953–1073 375–875 373–1023 373–1023 373–1023 973–1073 673–973 673–1123 1173–1373 673 823–1023 823–1023 650–1050 1073 823–1023 823–1023 773–1073 730–1270 1050 1050 1050 1050 1050 1050 1050 1050 1050

1.0 – 1.0 1.0 – 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 9.8  10–4 1.0 1.0 1.0 1.0 1.0 1.28 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

[93] [84] [87] [80] [94] [87] [64] [60] [61] [90] [91] [95] [96,97]

CaO Si3N4 TiO2 Y2O3 Al2O3 ɑ-Al2O3 monolith Calcium aluminate Barium hexaaluminate CeO2 La2O3 MgO

SiO2 Y2O3 ZrO2

Ru

Oxidized diamond Al2O3/cordierite monolith Metal gauze SiO2 Al2O3

YSZ TiO2 ɑ-Al2O3 monolith Pr2O3 Sm2O3 Eu2O3 Gd2O3 Tb2O3 Dy2O3 Tm2O3 Yb2O3 Lu2O3

[82,96] [98] [99] [100] [100] [100] [95] [96] [67] [97] [101] [102] [102] [60,103] [102] [104,105] [106] [61] [52] [52] [52] [52] [52] [52] [52] [52] [52] Continued

210 Chapter 9 Table 3 Catalyst

Noble metal catalysts for partial oxidation of methane—cont’d Support

Temperature (K)

Pressure (atm)

References

MgO/Al2O3 Calcium aluminate Al2O3–MgO

623–873 673–813 923–1173 853–873 573–1073 873–1173 473–1173 923–1323 1173–1373 673–873 730–1270 650–1050 650–1050 673–873 773–1173 673–873 673–973 573–1473 1073–1473

1.0 0.34 1.0 1.0 1.0 – 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.28 1.0

[69] [70] [73] [107] [74] [76] [108] [109] [80] [104] [61] [60] [60] [110] [79] [104] [96] [103] [29]

La2O3/CeO2 La2O3 MgO

Ir

Si3N4 TiO2 ɑ-Al2O3 monolith Al2O3 Eu2O3 La2O3 MgO TiO2 Oxidized diamond Al2O3 foam monolith

to be the most effective catalyst for the production of synthesis gas. Although Pt and Ir have lower selectivity than other catalysts, they are stable metals [61]. The impact of adding CeO2 to the Pt/Al2O3 catalyst was described by Yan et al. [81], and the findings showed that the presence of CeO2 increases the Pt dispersion and improves the selectivity for gas synthesis. Adding 1%–2% by weight of CeO2 was adequate, and further ceria addition had no significant influence on Pt particle size, methane conversion, or selectivity. It has been proposed that lattice oxygen from CeO2 contributes to this process, such as the duty of reducing ceria in an oxidation cycle. Although Otsuka et al. [112] showed output equilibrium efficiencies over CeO2 support, the reported activity of the catalyst was far lower than that of typical transition metal catalysts. Silva et al. [84,93] discovered that when employing Pt catalysts on an alumina support, a Ce/Zr promoter had a substantial impact on decreasing catalyst deactivation and the coking rate. The author’s results revealed that Pt/Ce0.75Zr0.25O2 catalyst had the highest activity and stability on partial methane oxidation. This is owing to the strong oxygen exchange capacity of the catalyst support, which keeps the metal surface free of carbon. The Pt/Ce0.75Zr0.25O2/Al2O3 catalyst has suffered severe inactivation due to the low-level alumina coating, which hinders the redox mechanism for carbon removal. As a result, the balance between a high degree of alumina coatings and more oxygen storage capabilities might explain their catalytic activity.

Partial oxidation process for syngas production 211 Furthermore, the presence of metal (>1.5%) is needed for catalyst stability. However, at high temperatures, the loss of Pt crystal due to catalyst sintering becomes a significant problem. Changing the γ-Al2O3 support to the stable α-Al2O3 was responsible for sintering, which resulted in larger Pt crystallites [24,25,113]. Tang et al. [114] investigated the influence of the deuterium isotope on the partial oxidation of methane over a Pt/α-Al2O3 catalyst. CO formation reflected the impact of changing CH4/O2 to CD4/O2 and was considerably reduced in the presence of deuterium, but CO2 selectivity was raised. Another well-known noble metal is Rh, which is utilized in the methane catalytic partial oxidation processes. However, due to the high expense of employing Rh, most research has focused on decreasing its presence in the catalyst structure [27,28,115]. Fathi et al. [68] proposed that adding 0.5% Rh as a promoter could enhance the conversion of methane, affect the reduction of temperature, and improve the reducibility of RhdO species on CeO2/Al2O3 supports. The extent of reduction of the CeO2 determines the selectivity of synthesis gas. These findings point to an extremely selective path for producing syngas from methane and oxygen in a reaction with CeO2. In this case, CeO2 acts as a transitional oxygen carrier. The required temperature in such a process, which is about 700°C, is considerably lower than the temperatures used in the production of synthesis gas by traditional methods, which is about 900°C. Ru-based catalysts have attracted more attention than other noble metals due to their low cost. Metallic Ru has the potential to achieve high selectivity and conversion rate by accelerating methane conversion through the direct partial oxidation mechanism [116]. However, only a few experimental studies fully confirm this concept, which is probably due to the low stability of Ru when interacting with the catalyst support. Ru catalysts may be inactivated owing to irreversible oxidation, reducing their ability to switch between oxidation and reduction states indefinitely [13,30]. Changes in the oxidation states of the catalyst can modify the combustion and reforming reactions process. On silica support, Ru has been found to deactivate quickly, while alumina support is a feasible alternative to improve selectivity. Many efforts have been undertaken to enhance and improve Ru performance using various supports. Lanza et al. [31,32,117,118] worked hard on the microemulsion technique to produce the Ru catalysts, which could improve the conversion of CH4 to equilibrium conversion and increase the selectivity of H2/CO. Matsui et al. [76] stated that using the Ru catalyst on La2O3 support increases CH4 conversion. Over the last decades, many researchers have tried to reduce the amount of noble metal in catalyst manufacturing to 0.01% or less [62]. However, at large-capacity processes, the complicated production of this catalyst has not made significant headway in reducing prices. Other techniques, such as enhancing nonnoble catalysts and mixing noble and nonnoble catalysts, have been explored to boost activity and lower costs.

212 Chapter 9

4.2 Nonnoble catalysts Noble metal catalysts have high conversion rates, high catalytic activity, and excellent selectivity in the methane conversion process, but their high cost has prevented their widespread use. As a consequence, many efforts have been undertaken to discover nonnoble alternatives. Fe, Co, Ni, and Cu have been demonstrated to be acceptable alternatives [62,105,119–121]. However, they have some issues, such as thermal stability and toxicity. A summary of nonnoble metal catalysts, supports, and operating conditions is given in Table 4. Nickel-based catalysts are among the most active catalysts that may be used to decompose CH4 and produce hydrogen. High CH4 conversion and high-temperature levels in the region of 900–1100°C [119] demonstrate the intense activity of Ni-based catalysts. Because of these advantages, the performance of Ni-based catalysts has been extensively researched in this field [54,139–141]. Ni catalysts are much more costly than other metals. Aside from the economic issue, the main issue with this kind of catalyst is the rapid deactivation rate due to the carbon deposition on the surfaces and the degradation of the catalyst [62].

Table 4 Nonnoble metal catalysts for partial oxidation of methane. Catalyst

Support

Temperature (K)

Pressure (atm)

References

Co

Al2O3

Ni

MgO–Al2O3 La2O3/Al2O3 CaO MgO SiO2 TiO2 Al2O3

823–1023 808–1298 723–1073 673–1273 1123–1198 473–973 773–1173 823–1023 625–1019 973–1273 998–1173 773–1073 1023 473–923 773–1173 1073 973–1173 873–1023 850–1123 773–1223 573–1073 1023 973–1123

1.0 – 2.96–49.34 16.77 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.98–6.90 1.0 1.0 1.0 – 1.0 1.0 1.0 1.0 1.0 1.0

[122] [123] [124] [125] [126] [58] [127] [122] [128] [129] [54] [130] [131] [132] [133] [134] [55] [135] [136] [137] [91] [138] [20]

CaO/Al2O3 CeO2/Al2O3 La2O3/Al2O3 MgO/Al2O3 Al2O3–SiO2 Al2O3–ZrO2 Mo2C/Al2O3 Barium hexaaluminate Calcium aluminate NiAl2O4

Partial oxidation process for syngas production 213 Although deactivation of nickel-based catalysts on alumina supports is possible due to the NiAl2O4 formation, the output species are generally close to thermodynamic equilibrium [49,54]. Carbon deposition, which can limit the number of active sites or destroy the catalyst by producing whiskers or layered structures or removing metal particles from the surface through metal corrosion, is another critical issue for nickel-based catalysts. As a result, a significant portion of the research on nickel catalysts has focused on catalytic modification to prevent carbon deposition. Because carbon formation on nickel catalysts is structurally sensitive [142,143], changes are often made to reduce the size of the crystals and stabilize them for sintering. The use of different precursors and pretreatments, as well as the inclusion of metal or oxide promoters, are examples of these changes. Choosing the appropriate materials to employ as a catalyst support or promoter may help to enhance catalyst performance. Alumina is typically used to support catalysts, which may contribute to catalyst sintering and carbon deposition while also having a high nickel reactivity [60]. Deactivation of Ni-based catalysts, on the other hand, is a common occurrence owing to the production of NiAl2O4, which decreases reactivity [144,145]. NiAl2O4 formation is an obvious result of the strong interaction of nickel with alumina. As a consequence, various thermal techniques to reduce NiAl2O4 production have been explored [20,50,54]. Metal oxides, such as La2O3 [36,146,147] and CeO2 [148], have garnered much interest as catalytic supports. Pantaleo et al. [149] investigated the activity of Ni on La2O3, CeO2, and a combination of La2O3 and CeO2. Nickel binds well to the La2O3 support, forming NidLa oxides. Organic nickel sources, such as Ni-acetylacetonate, may aid in the reduction of carbon deposition on the catalyst [150]. Catalyst preparation techniques are also effective in minimizing carbon deposition, which enhances catalyst stability. Cu-based catalysts are a low-cost option for oxygen carriers. Most early investigations on copper-based catalysts demonstrated that catalysts are active at low temperatures due to their low melting point (1080°C) [151]. Metal oxides such as ZrO2 (or CaZrO3 and SrZrO3) [152,153], TiO2 [154], Al2O3 [155,156], SiO2 [154] have been investigated in order to enhance the thermal stability of copper. With Cu-based catalysts, Al2O3 has the greatest propensity to form CuAl2O4 oxide in the same manner as Ni [157]. CuAl2O4, on the other hand, has been demonstrated to enhance the impact of oxygen transport rather than decrease activity. The activity of copper-based catalysts is still exceedingly low and cannot be compared with that of nickel. The partial oxidation of methane on the alumina-supported reduced cobalt catalyst often produces chemical species in conditions close to equilibrium [125,151], except that it is inactivated at higher temperatures compared with the nickel catalyst due to oxidation [125]. Co and Fe-based catalysts, which perform similarly to nickel-based catalysts, have received more attention. Several current processes, such as the Haldor-Topsoe, use Fe and Co catalysts

214 Chapter 9 for catalytic methane reforming. These metals are excellent for catalytic partial oxidation processes due to their ability to produce metal oxides with varying oxidation states. FeO, Fe2O3, Fe3O4, CoO, and Co3O4 may improve methane oxidation and reduce carbon deposition [158–160]. The interaction of Co or Fe with the catalyst support may result in oxidation states, thus reducing the tendency to produce carbon. Fe is a considerably less expensive alternative to Co; however, Fe2O3 and Fe3O4 have more stable oxidation states. Due to thermodynamic constraints, only the redox between the Fe and FeO pairs can catalyze the partial oxidation of methane, whereas the Fe2O3-Fe3O4 pair can complete the oxidation of CH4 [160]. Wang and Rokenstein [151] examined the impact of calcination temperature on the characteristics of the 24wt% cobalt catalyst on γ-Al2O3 support and observed the capability to reduce species in the order of Co3O4 >Co2AlO4 >CoAl2O4. Due to the less reducible Co2AlO4 and CoAl2O4, the reducibility of cobalt catalyst on γ-Al2O3 support decreased with calcination temperature. Sokolovskii et al. [123] investigated cobalt catalysts on γ-Al2O3 support modified by butadiene pyrolysis-induced surface carbon. They found that the active catalyst contained cobalt in the cube form and was deactivated with the loss of metal cobalt and CoAl2O4 formation. Co-based catalysts are more costly, but have a greater oxygen capacity. However, high temperatures (e.g., 900°C) deactivate Co3O4 catalysts faster than Ni and Fe catalysts owing to the breakdown of Co3O4 to CoO [119]. Strong interactions between metal oxide supports (e.g., TiO2, MgO, and Al2O3) and Co-based catalysts might produce inactive species [161–163]. CoO-MgO was combined to form a solid solution that may be detected by XRD. Reduced CoO-MgO demonstrated high gas selectivity and conversion rate, whereas pure MgO was inactive and pure CoO was unstable, resulting in more production of carbon dioxide and water. CoO-MgO catalysts with various molar ratios of Co/Mg were produced by a complete combination of finely powdered pure magnesium carbonate and cobalt nitrate, so that the Co/Mg ratio of 0.5–1.0 was appropriate for partial oxidation [58]. Sokolovskii et al. [124] reported that the CoO-MgO-Al2O3 catalyst has strong performance and excellent stability. This catalyst has shown high selectivity at nearly 100% methane conversion, as well as exceptional stability under reaction conditions. Rockenstein and Wang [151] investigated the combination of methane partial oxidation and methane dry reforming on reduced Co/MgO catalysts and compared the results with Co/CaO and Co/SiO2 catalysts. For the combined reaction, the reduced Co/MgO catalyst had high and stable activity. However, the reactions on the Co/CaO and Co/SiO2 catalysts had no activity to produce the synthesis gas. No catalyst deactivation was observed over the Co/MgO for a period of 110 h, and a selectivity of 94%–95% relative to H2 and CO was obtained at high space velocities. During the methane partial oxidation reaction, catalysts support had a significant effect on CH4 conversion and carbon deposition on nickel-loaded catalysts. Besides alumina, other supports

Partial oxidation process for syngas production 215 have been explored to prevent deactivation owing to CoAl2O4 production. In addition to cobalt on alumina and CoO-MgO supports, considerable research has been conducted on cobalt-based catalysts with various supports, including ZrO2 [164,165], TiO2, and SiO2 [122,151]. Oxidized diamond has been proven to be one of the best supports [96,166]. Choudhary et al. [57] were the first to employ cobalt-based catalysts in the partial oxidation of methane associated with rare earth metal oxides. Unreduced catalysts induce combustion at temperatures over 773K; however, the catalysts can operate at lower temperatures following by a reduction of reactants and products. Cobalt-based catalysts usually have lower methane conversion and selectivity than nickel catalysts. Nishimoto et al. [96] investigated the behavior of cobalt and nickel catalysts on oxidized diamond supports. No deactivation or carbon deposition was observed on the Ni catalyst with diamond supports during the reaction at 973 K. The Co catalyst on diamond supports exhibited similar but somewhat lower activity than Ni catalyst in the partial oxidation of methane. Carbon deposition was not detected on cobalt catalysts; however, it was observed on nickel catalysts containing 3wt% nickel at temperatures below 923 K. These findings imply that oxidized diamond may be utilized as a novel support material even under oxidative conditions. Even though a large number of transition metal-based catalysts have been investigated, the lack of appropriate catalysts for industrial processes remained a source of concern. Catalyst performance evaluation criteria include high reactivity, high mechanical strength, low carbon deposition, high thermal stability, and low cost. Preparing a mixed metal oxide catalyst is a straightforward procedure [167–169]. Complex metal oxides outperform individual metal oxides by creating synergistic effects that result in a better catalytic partial oxidation of methane. For example, CuO-Fe2O3 on MgAl2O4 support can improve the thermal stability of CuO at higher temperatures [170]. On the other hand, improving catalyst performance does not always lead to better results; however, various tests of a catalyst with a specific metal using different preparation and fabrication methods have shown that even the catalyst fabrication method is effective in catalyst performance. By combining Ni and Fe catalysts, Pans et al. [171] found that physical mixing improved catalyst performance while chemical mixing decreased its performance. Some research has focused on the design and fabrication of mixed metal oxide catalysts for methane partial oxidation [119,172]. ConocoPhillips and Dason Technology have introduced an innovation to increase efficiency and reduce production costs by coating precious metals (e.g., Pt and Rh) on the surface of low-cost, well-structured metals (e.g., Al, Cu, Ni, and Co). Mesh, honeycomb, integrated, fabric, wire, and pellet structures resulted in significant cost savings in catalyst fabrication [173,174]. These new designs can significantly improve the catalytic partial oxidation of C1–C5 hydrocarbons, mainly methane and natural gas. The catalysts coated with the precious metal have a much higher synergistic effect than mixed metal oxide designs.

216 Chapter 9 Another potential method to fabricate a suitable catalyst for synthetic gas production is to synthesize oxygen carriers of the perovskite structure (ABO3) [175–178]. When using ABO3 oxygen carriers, lattice oxygen replaces molecular oxygen and simplifies the process by reducing the number of gas separation steps. Cations A and B are routinely substituted by other metal ions with various valences to generate defects, enhance catalytic characteristics, and increase the mobility of lattice oxygen. Typically, the A-site affects the oxygen mobility to control the stability of the catalyst [34,73,179]. Lanthanum is a well-known A-site candidate that can help to develop perovskite structures containing active B elements [180,181].

5. Reaction mechanisms and kinetic 5.1 Reaction mechanisms Due to the highly complex mechanisms of catalytic partial oxidation of hydrocarbons, this section only discusses methane partial oxidation, which is the same for all catalysts. The main challenges for determining mechanisms are: (1) more than one mechanism may occur for a reaction system, (2) the dominant mechanism can be affected by the nature of the catalyst that changes over time, and (3) the reaction mechanism can change by varying operating conditions including space velocity, temperature, O/C ratio, etc. [1]. Two main mechanisms have been suggested for catalytic partial oxidation of methane in literature: direct partial oxidation (DPO) mechanism and combustion and reforming reactions (CRR) mechanism [182]. In the DPO mechanism, methane is first decomposed to hydrogen and carbon; oxygen adsorbed on the catalyst’s surface reacts with carbon to produce CO, and then H2 and CO are desorbed from the catalyst surface. On the other hand, the CRR mechanism happens in two separate zones of the reactor. At first, methane is decomposed to C and H and reacts with oxygen to form CO2 and H2O, and then the remaining methane reacts with steam and carbon dioxide through steam reforming and CO2 reforming to produce syngas. Fig. 1 shows a schematic comparison between CRR and DPO mechanisms. It is concluded that the mechanisms are corresponded to oxidized and reduced catalyst surfaces, so that oxidized sites promote the CRR mechanism and reduced sites promote the DPO mechanism. In addition, the CRR mechanism is more strongly supported for working catalysts, while the DPO mechanism presented by Hickman and Schmidt [56,66,183,184] is reasonable at low surface coverage. Hickman and Schmidt [37,38] reported the first modeling of methane partial oxidation based on Pt and Rh catalysts. The model consisted of 19 elementary reactions corresponding to adsorption, desorption, and surface reaction steps. Because the reactions were flux-limited rather than mass transfer-limited, a plug flow tubular reactor model could well explain the experimental findings for catalysts with high interphase mass-transfer rates, such as foam monoliths. This simulation was used to generate the energy diagrams displayed in Fig. 2. The observed selectivity variations could be

Partial oxidation process for syngas production 217 CH4(g) +

CH4

(a)

H

H C

(b)

H

H

(c)

+nO2(g)

Direct mechanism C

Indirect mechanism H2O

O (f)

+2H2(g)

(d)

Steam reforming

CO

CO2

CH4

(e)

+nCH4(g)

H2O

(g)

CO2 reforming CH4

CO2

2CO

2H2

(i)

CO(g) CO

3H2

(h)

(j) CO(g) + 3H2(g)

2CO(g) + 2H2(g)

Fig. 1 Schematic comparison between CRR and DPO mechanisms [1].

explained by reaction barriers, especially OH formation, which occurs much more on Pt catalysts than Rh. CO2 absorption through the dry reforming and water–gas shift reactions on Rh catalyst were easier than that of Pt [32]. A summary of studies on DPO and CRR mechanisms for methane partial oxidation over various catalysts is presented in Table 5. 5.1.1 Direct partial oxidation mechanism A complete set of reactions (including 24 reactions) for the DPO mechanism was presented by Wang et al. (1996) using the Rh/Al2O3 catalyst, in which the mechanism was separated into five steps: (1) adsorption step and dissociation of CH4, (2) surface reactions step, (3) desorption step, (4) events on Al2O3 sites, and (5) inverse spillover from the support to the metal surface (see Table 6). It is noticeable that reaction (2) is dissociative adsorption of oxygen, which is not indicated in Fig. 2. Reactions (3)–(7) explain the adsorption and dissociation of methane. In reaction (9), the oxygen and surface carbon react to produce CO. The formation of hydroxyl groups and the

218 Chapter 9 10.3

0

CH4(g) +

Pt

2O2(g)

–80

18.1 + 2x52

2H2(g)

–100

2x18

Potential Energy –120 (kcal/mol)

15 CO(g)

–140

42 –160

–180

24

2x2.5

44 C(a) + 2O(a) + 4H(a) + 2O(a) 30

2OH(a) + 2H(a)

2x37

2H2O(g)

CO(a) + O(a)

CO2(g) 6 CO2(a)

2x10.8

a)

2H2O(a)

5

0

CH4(g) +

Rh –100

2O2(g)

18.1 + 2x70 2x8

–120 Potential Energy –140 (kcal/mol)

2x18 CO(g)

–160

40

2H2(g)

2x5

2x20

2OH(a) + 2H(a)

2x37

15 C(a) + 2O(a) + 4H(a) + 2O(a)

2H2O(g)

31.6 –180

2x15 2x5

32

CO2(g) 6 CO2(a) –200

25 CO(a) + O(a)

b)

2x10.8 2H2O(a)

Fig. 2 Potential energy diagram for the CH, oxidation reaction system at low surface coverage over (A) Pt and (B) Rh [38].

oxidation of surface carbon species to CO are presented in reactions (8), (10), (11), and (14). Although CO2 and H2O were produced in reactions (12), (13), (17), and (18), it was not considered as CRR mechanism because they did not involve steam- and CO2-reforming. Reactions (23)–(25) illustrate the effect of support and its role in water formation. Reducing the space velocity leads to a lower methane conversion, followed by less formation of combustion products, which promotes the DPO mechanism. Also, the difference between the

Partial oxidation process for syngas production 219 Table 5 Summary of studies on DPO and CRR mechanisms for methane partial oxidation over various catalysts. Author

Catalyst Direct partial oxidation mechanism

Hickmann and Schmidt [56,66,183,184] Walter et al. [185] Buyevskaya et al. [186,187] Mallens et al. [188] Hu and Ruckenstein [189,190] Au et al. [191] Wang et al. [192] Qin et al. [79] Passos et al. [87] Cihlar et al. [193]

Pt/Al2O3, Rh/Al2O3, Pt-Rh/Al2O3 Rh/Al2O3 Rh/Al2O3 Pt Ni/La2O3 Ni/SiO2, Cu/SiO2 Rh/Al2O3 Ru/MgO Pt/Y2O3 La-Ca-M-(Al)-O (M ¼ Co, Cr, Fe, and Mn)

Combustion and reforming reactions mechanism Oh et al. [194] Dissanayake et al. [54] Hochmuth [195] Vermeiren et al. [55] Boucouvalas et al. [196] Heitnes et al. [197,198] Wang et al. [192] Heitnes et al. [199] Van Looij et al. [99] Horn et al. [14] Passos et al. [87] Lanza et al. [200] Nguyen et al. [201] Scarabello et al. [202] Urasaki et al. [203] Costa et al. [204]

Rh/Al2O3, Pd/Al2O3, Pt/Al2O3, Rh-Ce/Al2O3, Pd-Ce/Al2O3, Pt-Ce/Al2O3 Ni/Al2O3 Pt-Pd/CeO2-Al2O3 Ni/Al2O3, Ni/SiO2, Ni/ZrO2, Ni/zeolites Rh/Al2O3 Pt/Al2O3, Ni/Al2O3, Pd/Al2O3 Rh/Al2O3 Pt metal gauze Pd/SiO2 Rh/Al2O3 Pt/Al2O3, Pt/ZrO2, Pt/CeO2 Ru-Pt/Al2O3-ZrO2-CeO2 Ni/La2O3 Rh/ZrO2, Rh/ZrO2-CeO2 Rh/MgO LaNi0.5Nb0.5O3, LaNiO3, NiO/Nb2O5

catalyst surface and bulk gas temperature was reported as 300°C [54]. Therefore, lowering the temperature leads to a high probability of DPO mechanism and fewer combustion products. 5.1.2 Combustion and reforming reactions mechanism During the CRR mechanism, all oxygen is consumed at the first part of the reactor through the total oxidation of methane (reaction (26)), which is an exothermic reaction. This section is controlled by the O/C ratio and is 10% of the total bed [1]. The remaining methane then reacts with the water produced in the first part of the reactor through secondary reforming

220 Chapter 9 Table 6 Direct partial oxidation mechanism reported by Wang et al. [192]. Reaction

No.

Adsorption steps and dissociation of methane O2 + 2∗ $ 2O∗ CH4 + ∗ $ CH4∗ CH∗4 + ∗ $ CH∗3 + H∗ CH3∗ + ∗ $ CH2∗ + H∗ CH2∗ + ∗ $ CH∗ + H∗ CH∗ + ∗ $ C∗ + H∗

(2) (3) (4) (5) (6) (7)

Surface reaction steps O∗ + H∗ $ OH∗ + ∗ C∗ + O∗ $ CO∗ + ∗ C∗ + OH∗ $ COH∗ + ∗ COH∗ + ∗ $ CO∗ + H∗ CO∗ + O∗ $ CO∗2 + ∗ CO∗ + OH∗ $ CO2∗ + H∗ CHx∗ + OH∗ $ CHx+1O∗ + ∗ CHx∗ + ∗ $ CHxO∗ + ∗ CHxO∗ + ∗ $ CO∗ + xH∗ OH∗ + H∗ $ H2O∗ + ∗ 2OH∗ $ H2O∗ + O∗

(8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18)

Desorption steps CO∗ $ CO + ∗ 2H∗ $ H2 + 2∗ CO2∗ $ CO2 + ∗ H2O∗ $ H2O + ∗

(19) (20) (21) (22)

Events on Al2O3 sites H2O + # O # $ OH # + H  O# OH # + H  O # $ O # + H2O#

(23) (24)

Inverse spillover from the support to the metal surface H2O # + ∗ $ H2O∗ + #

(25)

*Metal sites and # Al2O3 sites.

reactions, including steam reforming (27) and CO2 reforming (28), which are endothermic. The overall reaction is still exothermic, and experiments showed that higher temperatures at the first part of the bed are caused by the combustion reaction and the consumption of all oxygen. CH4 + 2xO2 ! xCO2 + 2xH2 O + ð1  xÞCH4

(26)

CH4 + H2 O ! CO + 3H2

(27)

CH4 + CO2 ! 2CO + 2H2

(28)

Partial oxidation process for syngas production 221

5.2 Reaction kinetic 5.2.1 Kinetic model for catalytic partial oxidation of methane Catalytic partial oxidation of methane is a very fast reaction and can cause some problems in kinetic measurements due to mass transfer limitations. The reaction kinetics based on CRR and DPO mechanisms have been studied by several researchers. Boucouvalas et al. [205] studied the mechanism of DPO over the Ru/TiO2 catalyst. They evaluated the mass and heat transfer limitations for the Rh catalyst because it was the most active catalyst for the partial oxidation of methane. They faced the mass transfer limitation and the temperature difference between the gas phase and the catalyst surface as two main challenges for experiments. Increasing the space velocity solved the mass transfer limitation problem; on the other hand, it raised the temperature in front of the catalyst bed, where reactant mixture and catalyst bed were diluted to overcome this challenge. Hu and Ruckenstein [190] investigated the DPO mechanism over Ni/La2O3 catalyst at low temperatures (450–700°C) and atmospheric pressure. They found that the formation of CO through surface carbon reaction with oxygen was the rate-limiting step. Tsipouriari and Verykios [198] studied the kinetics of methane partial oxidation over Ni/La2O3 catalyst. They suggested that both methane decomposition and CO formation were rate-controlling steps for the CRR mechanism. It was concluded that low partial pressure of oxygen triggers the DPO mechanism, and the CRR mechanism can occur under typical pressure. The following rate expression for the CRR mechanism was presented, which was in good agreement with the experimental results: R¼

k1 K 2 K 3 PCH4 PO2 k1 K 2 PCH4 PO2 + k1 PCH4 + K 2 k3 PO2

(29)

Gubanova et al. [8] used a 1.4-wt% Pt/Pr0.3Ce0.35Zr0.35Ox/α-Al2O3 catalyst and the same mechanism presented by de Smet et al. [206] to account for the reversible oxygen adsorption, carbon species oxidation, and CO2 formation. Table 7 shows these reaction steps (Eqs. 30–38) and the corresponding rate equations. Instead of being produced via water–gas shift, reforming reactions, or methane decomposition, hydrogen was formed through water dissociation. Although this mechanism was not precise for hydrogen formation and not associated with the methane partial oxidation, this kinetic model was consistent with experimental data. They also presented another reaction scheme (Eqs. 39–51) and rate expressions for methane partial oxidation and showed that H2 forms through the surface reaction of hydrogen (Eq. 48) and the water dissociation (Eq. 51). In this model, oxygen was not involved in methane activation. The data over the Pt/PrCeZrO/Al2O3 catalyst have been compared with literature data over a Pt gauze. In this way, the role of ceria can be unraveled. Ceria provides a parallel reaction pathway for the oxidation of CO with a much lower energy barrier than over Pt. The kinetic model takes into account the cofeeding of carbon dioxide, thus the methane dry reforming. This occurs

222 Chapter 9 Table 7 Reactions scheme and corresponding rate expressions reported by Gubanova et al. [8]. Reaction

Rate expression

No.

O2 + 2∗ ! 2O∗ 2O∗ ! O2 + 2∗ CH4 + 2O∗ ! C∗ + 2H2O + ∗ C∗ + O∗ ! CO∗ + ∗ CO∗ + ∗ ! C∗ + O∗ CO∗ + O∗ ! CO2 + 2∗ CO2 + 2∗ ! CO∗ + O∗ CO∗ ! CO + ∗ CO + ∗ ! CO∗ O2 + 2∗ ! 2O∗ 2O∗ ! O2 + 2∗ CH4 + 5∗ ! C∗ + 4H∗ C∗ + O∗ ! CO∗ + ∗ CO∗ + ∗ ! C∗ + O∗ CO∗ + O∗ ! CO2 + 2∗ CO2 + 2∗ ! CO∗ + O∗ CO∗ ! CO + ∗ CO + ∗ ! CO∗ 2H∗ ! H2 + 2∗ H2 + 2∗ ! 2H∗ 2H∗ + O∗ ! H2O + 3∗ H2O + 3∗ ! 2H∗ + O∗

0.11PO2θ ∗ 1.7  1013 exp(200/RT)θ2O 2.4  105 exp(48.2/RT)PCH4θ2O 1.0  1013 exp(62.8/RT)θCθO 1.0  1011 exp(184/RT)θCOθ∗ 1.9  109 exp(30/RT)θCOθO 6.3  102 exp(28/RT)PCO2θ ∗ 2 1.0  1013 exp(126/RT)θCO 0.84PCOθ∗ 0.68PO2θ ∗ 2 1.0  1014 exp(200/RT)θ2O 9.35  103 exp(125.2/RT)PCH4θ5O 1.0  1013 exp(62.8/RT)θCθO 1.0  1011 exp(184/RT)θCOθ∗ 3.2  1013 exp(37.4/RT)θCOθO 9.9  103 exp(10/RT)PCO2θ ∗ 2 1.0  1013 exp(126/RT)θCO 0.71PCOθ∗ 1.0  1014 exp(159.7/RT)θ2H 0.02PH2θ ∗ 2 1.7  1014 exp(20/RT)θ2HθO 1.0  106 exp(69/RT)PH2Oθ ∗ 3

(30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47) (48) (49) (50) (51)

2

through the dissociation of carbon dioxide catalyzed by ceria. A reaction path analysis has revealed the main reaction steps for the production of carbon monoxide. Groote and Froment [207] developed a kinetic model for the CRR mechanism of catalytic partial oxidation over Ni catalyst. They used two models to simulate the Ni catalyst, called varying degrees of reduction and bivalent models. The results of the presented models were in good agreement with the data of industrial autothermal reformers. It was also found that the main difference between the two models was in predicting the catalyst temperature, which was lower for the bivalent model catalyst. The reaction steps and kinetic equations are listed in Table 8. 5.2.2 Kinetic model for catalytic partial oxidation of other hydrocarbons Higher hydrocarbon reactions usually occur through CRR mechanisms, which are not presented in this section due to the complexity of the reactions. The catalytic partial oxidation mechanism of methanol is similar to the CRR mechanism of methane except that the first step after the adsorption is the production of methoxy species, followed by the formation of formate

Partial oxidation process for syngas production 223 Table 8 Reaction steps and kinetic expressions reported by Groote and Froment [207]. Reaction CH4 + 2O2 $ CO2 + 2H2O CH4 + H2O $ CO + 3H2 CH4 + 2H2O $ CO2 + 4H2 CO + H2O $ CO2 + H2

Rate expression r1 ¼ r2 ¼ r3 ¼ r4 ¼

k1 C CH4 C O2

ð1 + K 1 C CH4

+ K 01 C O2 Þ 2

ð1 + K CO PCO + K H2 PH2

k01 C CH4 C 0:5 O2 1 + K 1 C CH4 + K 01 C O2

2

ð1 + K CO PCO + K H2 PH2




2

+ K CH4 PCH4 + K H2 O PH2 O =PH2 Þ

k3 =P2:5 PCH4 P2H H

2O

2




P4H PCO2 =K 3 2

+ K CH4 PCH4 + K H2 O PH2 O =PH2 Þ

2

k4 =PH2 ðPCO PH2 O PH2 PCO2 =K 4 Þ 2 ð1 + K CO PCO + K H2 PH2 + K CH4 PCH4 + K H2 O PH2 O =PH2 Þ

r5 ¼

2CO $ C + CO2

r6 ¼

C + H2O $ CO + H2

+

k2 =P2:5 PCH4 PH2 O P3H PCO =K 2 H

CH4 + CO2 $ 2CO + 2H2

CH4 $ C + 2H2

2

r7 ¼

k5 PCO k05 PCO2 =PCO 1 + K CO2 PCO2 =PCO 0 0:5 k6 PCH4 =P1:5 H k6 PH 2

ð1 + K H2 PH2 Þ k7 PH2 O =PH2

ð1 + K H2 PH2

2

2

+ K H2 O PH2 O =PH2 Þ

2

r8 ¼ f(PO2)

species and finally surface O, H, and C. Then, these surface species are combined to form synthesis gas. One of the mechanisms is presented in Eq. (52) [34]. CH3 O∗ !CH2 O∗ + H∗ !CHO∗ + 2H∗ !CO∗ + 3H∗

(52)

Hognon et al. [208] studied the catalytic partial oxidation of propane over CeO2 catalyst in an isothermal jet stirred reactor. They presented a precise homogeneous and heterogeneous mechanism consisting of 71 direct elementary steps. Pagani et al. [209] investigated the partial oxidation of propylene over Rh/Al2O3 catalyst. They proposed a kinetic model for the indirect mechanism of the reaction, which involves total oxidation followed by steam reforming. A summary of rate expressions for the catalytic partial oxidation of synthetic diesel, propane, isooctane, and gasoline is given in Table 9. Khan et al. [210] proposed a kinetic model (Eq. 53) for the catalytic partial oxidation of synthetic diesel over 5 wt% Ni/ Ce0.5Zr0.33Ca0.085Y0.085O2-δ catalyst. The kinetic rate expressions (Eqs. 54–62) for the partial oxidation of propane as well as side reactions have been reported by Pagani et al. [211] over a 2% Rh/Al2O3 catalyst in an annular microreactor. A kinetic study of partial oxidation of isooctane over a Ni/γ-Al2O3 catalyst was performed by Ibrahim and Idem [212] in a microreactor. They developed 18 different mechanistic and empirical rate models based on the irreversible power-law model, Langmuir–Hinshelwood–Haugen–Watson (LHHW), and

224 Chapter 9 Table 9 A summary of rate expressions for the catalytic partial oxidation of higher hydrocarbons. Reaction

Rate expression

Ref.

No.

[210]

(53)

[211]

(54)

Catalytic partial oxidation of synthetic diesel C12.87H24.81 + 6.435O2 ! 12.87CO + 12.41H2

rA ¼ 3.22  1015e(16000/RT)N2SDNO20.5 Catalytic partial oxidation of propane

C3H8 + 5O2 ! 3CO2 + 4H2O C3H8 + 3H2O ! 3CO + 7H2

k

r Ox r SR

C 3 H8

C 3 H8

CH4 + 2O2 ! CO2 + 2H2O

r Ox

CH4 + H2O ! CO + 3H2

r SR

CH4

¼

P

C 3 H8 C 3 H8 ¼ 1 +Oxkads σ H2 O H O PH O 2

2

kSR C3 H8 PC3 H8 ð1ηSR C 3 H8 Þ 1 + kads CO PCO + kO2 PO2 σ H2 O k

CH4

¼

(55)

P

(56)

¼ 1 +OxkadsCHH4 OCHPH4 O σ H2 O 2

2

kSR CH4 PCH4 ð1ηSR CH4 Þ 1 + kads CO PCO + kO2 PO2 σ H2 O

(57)

CO + H2O ! CO2 + H2

rWGS ¼ KWGSPH2O(1  ηWGS)σ CO

(58)

CO + H2O ! CO2 + H2

rRWGS ¼ KRWGSPCO2(1  ηRWGS)σ H2

(59)

CO + 3H2 ! H2O + CH4

rMet ¼ KMetPH2(1  ηMet)σ CO

(60)

0.5O2 + H2 ! H2O

rOx_H2 ¼ KOx_H2PH2(1  ηMet)σ CO

(61)

0.5O2 + CO ! CO2

rOx_CO ¼ KOx_COPCOσ O2

(62)

Catalytic partial oxidation of isooctane C8H18 + 4O2 ! 8CO + 9H2

C8H18 + 4O2 $ 8CO + 9H2

rA ¼ 

1:251023 eð2:8110 1 + ð5:8510

4

CA Þ

1=2

5 =RT Þ

3=2

CA

+ 2:51107 C A

Catalytic partial oxidation of gasoline   4 rA ¼  1+

1:910 =RT Þ 1011 eð C4 D 2 68:7C B 9 9:210 C A + CD





C A C 4B 

CD CC KP

2

I2:4

[212]

(63)

[213]

(64)

2

+ 1:3109 C C + 1:7108 C D

Eley–Rideal (ER) mechanism. Among them, an LHHW model requiring the molecular adsorption of oxygen and the dissociative adsorption of i-C8H18 on a single site as well as the assumption of the surface reaction, as the rate-determining step, was the most probable pathway for this reaction. The kinetic rate formula is presented by Eq. (63). One year later, this researchers group investigated a kinetic model for the partial oxidation of gasoline over a Ni–CeO2 catalyst in a fixed-bed reactor [213]. They showed that among 13 different mechanisms, including the reversible power law, LHHW, and ER model, the most appropriate model (Eq. 64) to describe the partial oxidation reaction was a dual-site LHHW mechanism, involving the dissociative adsorption of C8.27H15.10 and O2 followed by bimolecular surface reaction as the rate-determining step.

Partial oxidation process for syngas production 225

6. Conclusion Due to the importance of methane as a common fuel for catalytic partial oxidation, this chapter focused on the partial oxidation of methane and its mechanisms. Two reaction pathways, namely direct partial oxidation mechanism and combustion and reforming reactions mechanism, were introduced as the main mechanisms of this catalytic reaction. In addition, several kinetic models reported in the literature for partial oxidation of methane were mentioned. A brief history of the development of catalysts and an extensive review of two categories of catalysts, including noble and nonnoble metals, were performed. The literature showed that although noble metal catalysts have high methane conversion rates, high catalytic activity, and excellent selectivity in the methane partial oxidation process, their expensive cost has prevented their widespread use. Nonnoble catalysts were a suitable alternative to noble catalysts; however, they had some drawbacks such as low thermal stability and toxicity. Besides, due to the complexity of the partial oxidation mechanism of other hydrocarbons such as light hydrocarbons, high hydrocarbons, oxygenated hydrocarbons, and biofuels and the lack of corresponding kinetic models, only a review of the partial oxidation of these hydrocarbons and the rate expressions of some hydrocarbons were presented.

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CHAPTER 10

Co-electrolysis process for syngas production Sara Zolghadria, Hamid Reza Rahimpourb, and Mohammad Reza Rahimpourb a

Department of Chemical Engineering, Marvdasht Branch, Islamic Azad University, Marvdasht, Iran Department of Chemical Engineering, Shiraz University, Shiraz, Iran

b

1. Introduction Global energy consumption has been rising since the advent of industrialization. This rise may be ascribed to the world’s rapid economic expansion as well as to the world’s growing population. Based on International Energy Agency (IEA) 2019 and others, fossil fuel use increased by 2.3% in 2018, almost double the rate of growth recorded since 2010 [1,2]. The negative consequences of increased energy use include total CO2 emissions of 33.1 Gt, which is 1.7% more than its amount in 2017. According to the IEA and others, this equates to a 2.4 ppm increase in total CO2 content in the atmosphere [3–5]. According to NASA research and a recent United Nations report, by 2052, the continuous acceleration of carbon dioxide levels would raise the earth’s temperature by 1.5°C, causing the melting of the North Pole ice and dangerous sea-level rise. These factors, together with the steady depletion of fossil fuels, have created a strong argument for the expansion and deployment of renewable energy (RE) sources, including wind energy, solar energy, geothermal, and tidal energy [6]. As a result, there is an opportunity to create environmentally acceptable replacements for fossil fuels, as well as carbon-neutral energy technology, in order to tackle the energy and environmental challenges [7–9]. Because of its clean burning, hydrogen is undoubtedly most appropriate energy carriers for future fuel. Moreover, the well-known challenges of hydrogen storage, which require intensive technology innovation, as well as extra hydrogen infrastructural development, impede the development of hydrogen fuel applications [10–12]. Because synthetic liquid fuels are so close to prevalent hydrocarbon fossil fuels, they can be stored and transported using extant infrastructure [13–15]. Furthermore, liquid fuels have a upper energy density than H2, implying smaller fuel storage capacity and hence lower capital investment costs [14]. As a result, liquid fuels offer viable energy carriers for the long term. Clean energy sources, such as wind energy, hydra, and tidal energy, have recently seen a surge in investment to generate power electricity [16].

Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91871-8.00009-X Copyright # 2023 Elsevier Inc. All rights reserved.

237

238 Chapter 10 Unfortunately, as most renewable energy sources are intermittent, grid control has become extremely difficult. As a result, huge energy conversion as well as retention is unavoidable in a futuristic energy system that is both sustainable and resilient [10,17,18]. The solid oxide co-electrolyzer (SOCE) provides a promising method for decomposing steam and carbon dioxide into synthesis gas (syngas is a blend of carbon monoxide and hydrogen), converting electrical energy into chemical energy, which can then be converted into fuel. It is used as a raw material in the Fischer–Tropsch (FT) process, which is one of the most widely used methods for producing synthetic fuel [11,12]. Syngas may be made by splitting steam and carbon dioxide separately and afterward mixing them together [19]. However, splitting steam and carbon dioxide concurrently by co-electrolysis has some major advantages. To begin with, due to the rapid total electrochemical kinetics, conducting co-electrolysis is more cost-efficient and fuel-efficient [12,20]. The electrochemical behavior of the co-electrolysis process is comparable with that of single steam electrolysis. However, according to the low carbon dioxide separation kinetics, dry electrolysis has a significantly greater activation energy requirement. As a result, as compared with single CO2 electrolysis, co-electrolysis has reduced chemical stability and overpotential [11,20–22]. Fig. 1 shows the different complete syngas manufacturing processes [23–26]. In this study, operating modes and thermodynamics of the co-electrolysis methods are described to have a comprehensive summary of the SOCE method. Models for determining the Syngas production methods

Renewable Source

Fossil Fuels

Water Splitting Hydrocarbon Reforming

Electrolysis Steam Reforming

Dry Reforming

Partial Oxidation

Thermolysis

Autothermal Reforming

Solid Oxide

PEM

Fig. 1 Various syngas production methods [18–21].

Alkaline

Co-electrolysis process for syngas production 239 Nernst potential are emphasized to demonstrate the systematic link between operating circumstances, electrochemical performance, and synthesis gas generation. Electrochemical cell performance, materials, and degradation problems for SOCEs are discussed. The goal of this chapter is to offer a quick overview of current advances in SOCE at high-temperature technology.

2. Water electrolysis technologies Water electrolysis is the most capable techniques for manufacturing hydrogen since it employs renewable water and produces only pure oxygen as a by-product. Furthermore, the electrolysis process makes use of direct current (DC) electricity from renewable energy sources including biomass, wind, and solar energy. However, owing to cost constraints, only 4% of H2 can be produced by electrolysis of water at the moment [27,28]. Furthermore, water electrolysis has several advantages, including high efficiency and a higher rate of H2 generation with high purity, which is advantageous for conversion into electrical energy via room-temperature fuel gas [29]. The water molecule is the reactant in the electrolysis process, and under the effect of electricity, it is split from hydrogen and oxygen. Water electrolysis is categorized into four classes depending on the electrolyte, operating circumstances, and ionic molecules, although the working principles can be similar in both situations. There are four types of electrolysis methods: (i) alkaline water electrolysis (AWE) [30–32], (ii) solid oxide electrolysis (SOE) [33,34], (iii) microbial electrolysis cells (MEC) [35,36], and (iv) PEM water electrolysis [37,38].

2.1 Alkaline water electrolysis (AWE) At the cathode surface of the alkaline water electrolysis method, two substances of alkaline solution are decreased toward one particle of hydrogen as well as hydroxyl group ions are generated. The generated H2 evaporates from the cathodic sites to intermix in the gas phase, while the hydroxyl ions move to the anode under the effect of the electronic circuit between cathode and anode via the pore diaphragm, where it is separated to 1/2 O2 and one particle of water (H2O) [30,39–41]. The oxygen is remerged at the electrode surface and exits as hydrogen, as illustrated in Fig. 2. Finally, anion exchange membranes (AEMs) composed of polymers having anionic conductivity, rather than an asbestos diaphragm, are being developed as a novel method in alkaline electrolysis. In the realm of AWE, this novel method looks to be fascinating [43–47]. Some of the problems with alkaline water electrolysis include that the product gas tends to mix, which ultimately reduces the efficiency of the electrolyzer. Because these electrolyzers use a

240 Chapter 10 Alkaline Eletrolysis Cathode

+

– Anode

H2

½ O2 OH –

H2O

Diaphragm Anode: 2 OH – Cathode: 2H2O + 2 e– Overall: H2O

H2O + ½ O2 + e– H2 + 2 OH – H2 + ½ O2

Fig. 2 Schematic representation of AWE [42].

liquid electrolyte, they cannot function at high pressure. Finally, they cannot produce high due to the presence of high resistance current density [48].

2.2 Microbial electrolysis cell (MEC) Microbial electrolysis cell (MEC) method may produce hydrogen from organic matter, such as biomass resources and sewage. MEC process is closely linked to microbial fuel cells (MFCs), although the operational concept is the opposite of MFCs [35]. Electric energy is transformed to chemical energy in MECs. Under the influence of an electric flow, MECs generated H2 from organic molecules. Microbes oxidize the material in the anode surface of the microbial electrolysis method, resulting in carbon dioxide, electrons, and protons. The electrons transfer to such cathode via the electrical current, and the protons move to the cathode through a proton carrying membrane (electrolyte), where the electrons and protons mix to form hydrogen [49].

Co-electrolysis process for syngas production 241

2.3 PEM water electrolysis Proton exchange membrane (PEM) electrolysis method is comparable to PEM fuel cell technology in that solid polysulfonated membranes are employed as a proton conductor (electrolyte) [50–53]. The proton exchange membranes provide a number of benefits, including decreased gas permeability, lower thinness, high proton conductivity, and high-pressure performances. PEM water electrolysis is one of the most environmentally friendly technologies for converting renewable sources to high-purity hydrogen in terms of energy and environmental effect. Other potential PEM water electrolysis features a compact structure, large current density (over 2 A cm2), high performance, rapid response, small space, performs at room temperature (20–80°C), and produces pure hydrogen [52–55].

2.4 Solid oxide electrolysis (SOE) The SOCE is one of the methods for syngas production from steam and carbon dioxide and converts electrical energy into chemical energy [11,12]. Furthermore, when the energy source is insufficient to meet the demand, the SOCs may be driven in reverse to transform chemical energy to electricity energy in the solid oxide fuel cell (SOFC) state [12,14]. As a result, SOCs may be used as a unique power source to establish the right balance between both the creation of fuel and the generation of electricity. Given that the quantity of carbon dioxide required in the co-electrolysis method is like that created in the oxidation of the synthetic gas, mixing the SOCEs using carbon dioxide collection technologies might result in an overall carbon dioxide neutral cycle [56,57]. Next, in the co-electrolysis process, a significant percentage of CO2 conversion to CO occurs in a reversible water gas shift (RWGS) chemical process (Eq. 1), resulting in a significant reduction in overall electricity usage to create syngas [58]. H2 ðgÞ + CO2 ðgÞ ! CO ðgÞ + H2 O

(1)

In the process of co-electrolysis, steam is converted to CO2, and in the case of electrolytic dry, CO2 will be deeply decomposed into carbon, resulting in huge coking and degradation of cell performance [59,60]. As a result, the co-electrolysis of steam/CO2 at high temperatures is a cost-effective and long-lasting method for syngas generation. However, the co-electrolysis reactivity, which includes different gas species (CO, CO2, O2, H2, and H2O), is far more complex than only steam or carbon dioxide electrolysis, in which the RWGS process happens all at the same time as the electrolysis response, and any of these should be correctly quantified to assess the overall performance of the synthesis gas [11,61–64]. Moreover, unlike with the linear relationship between the stated charge density and the H2/CO rate of production defined in Faraday’s law, the electrolysis of H2O or CO2, respectively, electrochemical behavior and product mixture are related directly to cell performance parameters such as inlet gas mixture, working temperature, and current voltage. [34,65,66]. As a result, evaluating electrocatalytic activity and the composition of the product only on the basis

242 Chapter 10 of current density is insufficient. As a consequence, in order to truly comprehend the production of syngas through the SOCEs, this same relationship between both operating point and catalytic activity should be thoroughly investigated, and traditional research methods for separating the electrolysis of water and carbon dioxide should be adapted [16]. In SOEC, a Ni–YSZ cathode is commonly employed as a cathode catalyst [67]. Moreover, as previously noted, Ni–YSZ poses challenges for long-term, steady electrolysis. With this said, solid oxide electrolyzers used during water electrolysis exhibit excellent efficiency and the potential to employ nonnoble catalyst materials, prompting significant study in this area [48]. Eventually, water electrolysis to create hydrogen alone helps to reduce dependency on fossil fuels, but it does not inherently help to reduce CO2 levels in the atmosphere [30]. High-temperature electrolysis of CO2 in an SOEC is one of the methods for reducing CO2 concentrations in the environment. CO2 can be effectively transformed into high-value compounds or used to make liquid fuels. CO2 is decreased at the cathode to generate CO in this system, while oxygen ions flow via the semipermeable membrane to make oxygen at the anode. The CO generated is subsequently converted to CO2, which is then returned to SOEC. In this scenario, a Ni–YSZ cathode is typically employed, although it has numerous drawbacks, including deterioration over time, coke production, and variations in thermal coefficients of expansion, making it inappropriate. As a result, a lot of work is being done right now to offer special cathode materials such as perovskite oxide [68]. Finally, the co-electrolysis mixing carbon dioxide and water, which is the subject of this research, is another way to reduce our dependency on fossil fuels while also helping reduce carbon dioxide levels in the atmosphere. Carbon dioxide and water are co-electrolyzed in this system to produce synthesis, which is a CO/H2 gas combination [58,69]. Unlike in previously mentioned electrolysis techniques, co-electrolysis mixing water and carbon dioxide offers an effective electrolysis pathway that needs less energy and enables the use of less expensive, earth-pleasing catalysts. Because liquid hydrocarbons are simpler to transport and store than pure hydrogen, they have an advantage over H2 production [69]. Overall, co-electrolysis mixing water and carbon dioxide has been proven to be a more viable pathway for reducing dependency on fossil fuels and lowering CO2 levels in the atmosphere. Table 1 outlines the benefits and drawbacks of each option.

3. Process description for SOCE 3.1 Operating principle The operating concept of SOCEs for synthesis gas generation is shown in Fig. 3. A steam and carbon dioxide mixture is sent into the cathode, where the reactants absorb electron energy sources to generate oxygen ions and synthesis gas [12]. CO2 + 2e ! CO + O2

(2)

Co-electrolysis process for syngas production 243 Table 1 Benefits and drawbacks of various water electrolysis methods. Electrolysis process SOE

PEM

AWE MEC

Benefits

Drawbacks

Refs.

Conversion of electrical energy to chemical energy. Higher efficiency (90%–100%) in high temperature. High densities Rapid reaction High-pressure operations lower gas permeability High efficiency High proton conductive low cost Energy efficiency ranges between 70% and 80% Produced hydrogen by organic matter

Lack of stability and degradation on a large scale.

[50,67,70–72]

High production cost Unstable in high efficiency Expensive established components

[50,52–55,73]

Limited current densities Low operating pressure Low energy efficiency Low rate of hydrogen generation Low purity of H2

[30,74]

CO + H2

O2

CO O2

Anode

H2O

Electrolyte

CO2

Cathode

H2

CO2 + H2O

[35,49]

Air Optional

Fig. 3 The principles of operation for the SOCE of water and carbon dioxide process [12].

H2 O + 2e ! H2 + O2

(3)

As a result, Eq. (4) may be used to represent the total co-electrolysis at high-temperature process that happened in the SOCs: Electricity + Heat

nCO2 ðgÞ + mH2 OðgÞ







! nCOðgÞ + mH2 ðgÞ +

m+n O2 ðgÞ 2

(4)

The synthesis gas is generated in conjunction with water and carbon dioxide on the cathode side, while oxygen gas is produced as a by-product on the anode side. Moreover, the

244 Chapter 10 electrochemical process, the inverse water gas shift (RWGS) chemical reaction, as described in Eq. (5), happens on the cathode side at a high temperature with a rapid kinetic rate. CO2 ðgÞ + H2 ðgÞ ! COðgÞ + H2 OðgÞ

(5)

RWGS reaction activity was detected by area-specific resistance (ASR) analysis of electrolysis cells operating in three distinct modes. According to published data, single cells made up of Ni–YSZ/YSZ/LSM electrolysis cells had an ASR of 1.38 Ω cm2 for steam/carbon dioxide co-electrolysis, which was similar to 1.36 Ω cm2 for alone steam electrolysis, and just 36% of 3.84 Ω cm2 for solo carbon dioxide electrolysis [11]. Similar findings were reported during high-temperature co-electrolysis, with ASRs of 0.70–0.79 Ω cm2, equal to 0.75 Ω cm2 for steam electrolysis, while significantly lower than 1.06 Ω cm2 for carbon dioxide electrolysis [22]. These findings suggest that carbon dioxide electrolysis converts almost little carbon dioxide to CO throughout the high-temperature SOCE mechanism. The ratio of the RWGS reactivity to CO generation has often been quantified using an electrochemical system that is suitable for the present temperature, intake gas composition, and implemented voltage gain. The RWGS reaction was discovered to contribute considerably to CO generation at a low applied voltage level despite potentially consuming CO at high running cell voltage [58]. As a result of the RWGS reaction, the co-electrolysis processes at high temperatures on SOCs, which are, in theory, oxygenation cells, have become more complicated.

3.2 Thermodynamics Using SOCEs, energy is required to divide steam and carbon dioxide for synthesis gas generation. The overall energy change, ΔH, is made up of two components: the Gibbs free energy change, ΔG, and the reaction entropy modification, Q, in which ΔG becomes an electrical request based on the transient voltage between both the anode and the cathode, and Q is heat. The temperature influence on the co-electrolysis thermodynamics process to create synthesis gas through the use of a hydrogen/CO proportion of 1 is depicted in Fig. 4. [61]. The overall energy requirement stays practically the same throughout the range of temperature between 200°C and 1000°C, at 528 kJ, whereas ΔG has dropped from 462 to 349 kJ mol1, as well as Q has risen from 65 to 181 kJ mol1 because of entropy change, ΔS. Also, with operating temperatures expanding between 200°C and 1000°C, the proportion of ΔG to ΔH has fallen by 88%, showing that the electrical requirements have been significantly reduced while running the SOCEs at extreme temperatures. As a result, performing the electrolysis method at greater operating temperatures is more advantageous, not only because of the increased electrochemical rate of reaction, but mostly because of the lower electrical energy usage. Unless waste heat by other industrial operations is combined with electricity produced by renewable energy, significant economic expansion can be obtained. Wind, tidal,

Co-electrolysis process for syngas production 245

Fig. 4 The thermodynamics from the H2O/CO2 separating reaction used to generate syngas are affected by temperature [75].

solar, and hydra energy are examples of renewable energy sources [76]. Furthermore, by replacing oxygen in the anode with certain reducing chemicals (Fig. 5A and B) [75], including CH4, C, and CO [75,77–81], significant reductions in electricity consumption may be accomplished.

3.3 Nernst potential The Nernst potential is an important property of the SOCE system because it reflects the minimal amount of electrical energy necessary to start the steam/carbon dioxide separation process [75]. Despite the fact that the RWGS process has increased cathode reactivity, the Nernst potential may still be calculated utilizing Nernst potential methods since the SOCEs are essentially oxygenation cells. In this scenario, it is necessary to determine the equilibrium composition of the partial pressure of oxygen or H2O (g)- H2 or CO2-CO at the cathode and anode to use the Nernst, which is as follows [6]: ! ΔG f ,H2O ðT Þ RT Y 1,H2O (6) Ln  VN ¼ 0:5 2F 2F Y 1,H2 Y O2 VN ¼

ΔG

f ,H2O

2F

ðT Þ

  Y 1,CO2 RT Ln  0:5 2F Y 1,O2 Y O2

(7)

where ΔGf, H2O(T) and ΔGf, CO2(T) are the Gibbs free energy changes for the production of carbon dioxide and water carbon dioxide, respectively, during which the anode stands exposed to the air, yO2 is the mole percentage of molecular oxygen on the anode surface is 0.21.

246 Chapter 10

Fig. 5 The operation of SOCEs in two modes: (A) conventional and (B) POM-aided [70].

The equilibrium ratios of H2, CO, H2O, and CO2 are y1, H2, y1, CO, y1, H2O, and y1, CO2, respectively, at high temperature. Certain reducing gases, including hydrogen, are incorporated into the input gas such as the gas phase for the cutting-edge SOCEs using a Ni-based ceramic cathode. To begin, a reducing environment must be maintained to prevent Ni oxidation, which might result in the Ni material losing its catalytic and directing functions in the SOEC procedure. Second, addition of hydrogen helps to enhance the chemical equilibrium reaction (Eq. 8), which shifts the path of formation of CO and leads to an enhanced carbon dioxide conversion efficiency. H2 + CO2 ! CO + H2 O

(8)

Third, because hydrogen is a component of synthesis gas products, it eliminates the need for an additional product separation procedure. Although the introduction of a large volume of

Co-electrolysis process for syngas production 247 hydrogen has resulted in a reduced oxygen pressure, a large value for OCV and, as a result, low energy/cost yield, it is required and frequently employed among the intake gases. As a result, the intake water and carbon dioxide will invariably react together through the RWGS process, resulting in a completely different balance gas composition than the beginning amount of the cold input. The chemical stability co-electrolysis method was created to assess the development of gas composition via combining mass balance rules through the equilibrium reaction of RWGS [11]. There was no mass change of C, H, or O components before the co-electrolysis reaction throughout the re-equilibrium phase of H2, CO, H2O, and CO2 heat transfer of the cold intake toward the maximum temperatures (500°C). As a result, Eqs. (9)–(11) could be constructed for the carbon, hydrogen, and oxygen components, which were connected to the original gas composition y0, x (x ¼ H2, CO, H2O, and CO2) as well as four unknown factors y1, x (x ¼ H2, CO, H2O, and CO2). y0,CO + y0,CO2 ¼ y1,CO + y1,CO2 (9) y0,H2O + y0,H2 ¼ y1,H2O + y1,H2

(10)

y0,CO + 2y0,CO2 + 2y0,H2O ¼ y1,CO + 2y1,CO2 + 2y1,H2O

(11)

The reaction equilibrium constant of RWG, that is, a factor of the operating temperature, was used to construct the other equations necessary to solve composition y1, x (y ¼H2, CO, H2O, and CO2). K¼

y1,H2O y1,CO y1,H2 y1,CO2

(12)

Eqs. (9)–(12) described a system including four equations and four unknown factors, and the equilibrium mole proportion of CO, CO2, H2, and H2O was determined by solving the system simultaneously. After determining the stable gas mixtures, then Nernst potential might be calculated via Eqs. (2) and (3). The method was successfully verified on co-electrolysis cell types using Ni–YSZ/YSZ/ LSM-YSZ and SFM-SDC/LSGM/SFM-SDC cell configurations, with excellent accordance between anticipated mathematical Nernst potential and empirical OCV [61,63]. Furthermore, when the working temperature was increased, the Nernst potential fell, indicating a lower minimum electrical energy consumption for synthesis gas generation through the solid oxide steam/carbon dioxide co-electrolysis procedure.

4. Materials for SOCEs SOCEs employ materials that are comparable to those used in SOFCs [60,82]. The most popular electrolyte for SOCs employing oxygen ion carrying electrolytes contains 8% mol Y2O3 stabilized ZrO2, which has strong ionic conductivity as well as thermal stability. However, when the working temperature is reduced, the oxygen ion conductance drops rapidly,

248 Chapter 10 resulting in a considerable increasing trend in voltage drop. To achieve better cell electrocatalytic activity, slender electrolyte membranes made by sample or physical deposition, and structural components with greater ionic conductivity, including ScSZ, gadolinium-doped ceria (GDC), samarium-doped ceria (SDC), and crystalline substances, including strontium and magnesium-doped lanthanum gallate (LSGM), can be used [58,59,83,84]. The most traditional materials for oxygen electrodes are lanthanum magnetite doped with strontium (LSM) and strontium and iron-doped lanthanum cobalt (LSCF) [66,85,86]. By enhancing ionic conductivity while expanding the triple phase boundaries (TPBs) so at electrode/electrolyte surfaces, the electrolyte phases are often integrated into the electrode to produce a composite electrode that improves unit electrocatalytic activity. Because the intake gas stream mixture of the cathode side is the primary distinguishing characteristic between co-electrolysis and electrolysis of carbon dioxide or steam, the cathode efficiency has a significant impact on the entire cell’s electrochemical activity and synthesis gas generation. As a result, the cathode substance options for solid oxide co-electrolysis cells have received special attention. The prospective SOEC cathodes must fulfill the following criteria: (i) excellent phase stability, (ii) proper temperature compatibility, (iii) acceptable catalytic activity, (iv) appropriate conductivity, and (v) sufficient porosity and minimal tortuosity to enable reactant and product diffusion [87]. The performance of co-electrolysis at high temperature on various SOCs exposed to a range of H2O-CO2-H2-CO atmospheres is summarized in Table 2.

Table 2 The SOE electrochemical performance for steam and CO2 co-electrolysis. Elements of the test

Results performance

Cell structure

%H2O

%H2

%CO2

%Ar

Temp.

i @ 1.3 V (A cm22)

R @ OCV (Ω cm2)

Ref.

Ni–YSZ/YSZ/ LSM-YSZ

45 54.9 45

10 22.5 10

45 22.6 45

– – –

850°C 800°C 850°C

1.56 0.32 2

– 1.38 –

[57] [11] [88]

10 Cell stack Ni–YSZ/ScSZ/ LSM-ScSZ Ni–YSZ/YSZ/ LSCF-GDC,LSCF Ni–YSZ/YSZ/ GDC/LSCF

28.6

14.3

28.6

28.6

750°C

0.36

–

[83]

50

25

25

–

800°C

1.37 0.13

0.13

[14]

20

20

60

800°C

0.58 0.45

0.45

[22]

6 Cell stack LSCM-GDC/ LSGM/LSM-ScSZ SFM-SDC/LSGM/ SFM-SDC

1

5

1

1

850°C

0.1

–

[89]

16

20

16

–

850°C

0.73

0.48

[61]

Co-electrolysis process for syngas production 249

4.1 Ni-based cathode Due to its excellent electrical conductivity, strong catalytic activity during the high-temperature steam/carbon dioxide process, and reduced cost, the Ni-based cermet material is one of the most popular cathode substance for SOCEs units [33,60]. To improve ionic conductivity, correlate the coefficient of thermal expansion through the electrolyte, and lengthen the TPBs, an ionic conductor electrolyte method using GDC or YSZ is included inside the cathode. SOCE systems using Ni-based cermet cathodes have exhibited outstanding electrocatalytic activity, with low resistance and excellent electrolysis flow density. The DTU-Ris investigators revealed flat Ni–YSZ-based SOCs with a hydrogen electrode made of a 300 m thick base of Ni–YSZ porous, a 10 m Ni–YSZ surface functional layer, a 12 m YSZ electrolyte, and a (La0.75 Sr0.25)0.95 MnO3-YSZ metal matrix oxygen electrode for high thermal co-electrolysis of CO2, H2O, and H2 at 45, 45, and 10%, respectively. The cell released electrolysis current densities of 2, 1.25, and 0.69A at 850°C, 800°C, and 750°C with a voltage source of 1.3 V, respectively [88]. An electrolyte-based 10-cell stack was reported by Idaho National Laboratory of approximately 150 μm of ScSZ electrolyte manufactured by ceramatec Inc., nicermet cathode and doped manganate anode. At 800°C and a voltage of 13 V, the packet performed on 54.9% water–22.5% hydrogen– 22.6% carbon dioxide. The electrolysis flow was 20.4 A. Simultaneously, synthesis gas with an H2/CO 2:1 ratio and a conversion rate of CO2/CO equal to 100% was obtained [90]. Zhan et al. identified SOECs composed of a Ni–YSZ hydrogen electrolytic aid, a slim YSZ electrolyte, as well as a La0.6Sr0.4Co0.2Fe0.8O3(LSCF)-Ce0.9Gd0.1O1.95(GDC) air electrolytic capacitor that achieved an electrolysis flow of 0.66, 1.04, and 1.37A cm2 at 700°C, 750°C, and 800°C, respectively, while operating with an inlet gas current of 25% H2, 50% H2O, and 25% CO2 under an applied voltage of at 1.3 V [14]. The Ni–YSZ cathode is generally made by physically combining NiO and YSZ particles and afterward shaping them into cathode surfaces utilizing ceramic manufacturing methods, including tape casting, freeze tape casting, and dry pressing [14,66,85,91]. It is desirable to have enough porous and interconnected gas channels to have low tortuosity for successfully transporting water and carbon dioxide, which have lower diffusion coefficients than H2 [14].

4.2 Ceramic cathode Though nickel-based cermet is the most commonly used cathode for SOEs, nickel is less resistant to impurity toxicity and is prone to changes in microstructure caused by heating or redox cycles, which lead to active reaction sites. Battery loss and performance decline during overtime operation [92–94]. As a result, novel ceramic materials based on LaVO3, LaCrO3, Sr2Fe1.5Mo0.5O6, and SrTiO3 have been investigated as stable cathodes using SOCEs.

250 Chapter 10 4.2.1 Sr-doped LaVO3 (LSV) At 800°C, perovskite Sr-doped LaVO3 (LSV) has a high electrical conductivity of around 120S cm1, is extremely stable at higher of oxygen partial pressures of up to 1022 atm [95], and has good sulfur tolerance and coking resistance [96,97]. Yoon et al. have revealed the co-electrolysis achievement of LSV-YSZ matrix composites as cathode for electrolyte-supported cells using the functionality LSV-YSZ/YSZ/La0.8Sr0.2FeO3 (LSF)-YSZ, which demonstrated a cell electrical resistance of 0.89 at 800°C in a 30% H2O, 80% CO2, and 20% CO atmosphere [98] When cerium oxide (CeO2) nanostructures were put to the LSV-YSZ cathode, the electrolysis density was 1.0 A at a voltages of 1.5 V. Simultaneously operating conditions, while Pd and CeO2 nanomaterials were cointroduced to the LSV-YSZ cathode, the electrical resistance was reduced to 0.53 and the electrolysis current density was raised to 1.6 A. In addition, the LSV-based cathode substance demonstrated adequate long-dated stability, indicating that LSV is a potentially viable cathode substance for synthesis gas generation during co-electrolysis. 4.2.2 LaCrO3-based materials In both reducing and oxidizing environments, perovskite LaCrO3-based ceramics remain stable as well as conductive. They’re often utilized as connections and also have been used as an H2 electrode alternative in SOCs [99,100]. The electrical and ionic conductivity of LaCrO3 may be tuned by co-doping divalent ions including Mg2+, Sr2+, and Ca2+ to the A-sites and transformation metal ions such as Mn2+, Fe2+, and Co2+ to the B-sites [90,101]. When the LSCM-GDC material was confronted with a combination of 10mLmin1 (20%H2O–5% H2/Ar) and 20mL min1(70%CO2-Ar), Yue and Irvine measured the electrolysis density of 0.1A cm2 underneath the voltage source of 1.3 V, including the CO manufacturing rate of 0.2 μmol s1 whenever the LSCM-GDC material was revealed to be mixed at 20 mL min1 [89]. The relatively minimal co-electrolysis activity may be reduced by the distress of steam transfer and the low conductivity of the LSCM-GDC cathode. Because of the doping in LSCM, charge compensation was obtained primarily through the creation of oxygen vacancies in the decreasing atmosphere, resulting in a rise in ionic conductivity, however, a lower electronic conductivity [21]. As a result, electronic conducting phases including Cu were infiltrated into the LSCM electrode to enhance electrode conductivity and TPBs. SOCEs to Cu-infiltrated LSCM cathodes have reduced conductive and electrode resistance than those with infiltrated LSCM cathodes. Under a 1.3 V supplied voltage, the LSCM-Cu/LSGM/LSCF sole cells achieved an electrolysis density of 0.83, 1.3, and 2 A cm2 at 750°C, 800°C, and 850°C, respectively [21]. Other catalyst phases, such as Ru, were also implemented into the LSCM-based electrode to enhance catalytic performance, and 0.5 wt% Ru infiltrated LSCM-BaCe0.5Zr0.3Y0.16Zn0.04O3 (BCZYZ) composite electrodes were investigated for symmetrical proton conducting SOEs, which showed improved electrocatalytic activity for steam/carbon dioxide converting to H2/CO [82].

Co-electrolysis process for syngas production 251 4.2.3 Sr2Fe1.5Mo0.5O6 (SFM) Sr2Fe1.5Mo0.5O6 (SFM) is a conductor that is both electronic and ionic. It has been studied as an anode and cathode side for SOFCs [102–105]. SFM’s thermal conduction and catalytic performance are highly dependent on the blended valent couplings, Mo5+/Mo6+ and Fe3+/Fe4+, which are characterized by the overlap of Mo 4d, Fe 3d and contribute to good electrochemical efficiency [106–108]. SFM had an electrochemical performance of around 32 S cm1 in wet hydrogen at 800°C as well as 15S cm1 in air at 750°C [109], which was significantly greater than that of other ceramic materials investigated, for example, SOFC H2 electrodes, including 1.5 S cm1 for La0.75 Sr0.25Cr0.5Mn0.5O3 and 16 S cm1 for recorded at 900°C in a 5% H2/Ar environment [110]. 16 S cm1 for La0.4Sr0.6TiO3 measured in H2 at 1000°C [111] and 9.3 S cm1 for Sr2MgMoO6 measured in H2 at 800°C [112]. The formation of oxygen vacancies in SFM occurs preferentially along the comparatively weak Fe-O-Fe links instead of the Mo-O-Fe or Mo-O-Mo connections, as shown by initial principle calculations [113], resulting in an increased oxygen ionic conductivity of 0.13 S cm1 at 800°C in air. The ionic conductivity coefficient in SFM was indeed significantly greater than 5.93107 S cm1 for La0.8Sr0.2MnO3 substances [104], and even similar to cobalt-based oxygen electrodes, including 0.008S cm1 for La0.6Sr0.4Co0.2Fe0.8O3 and 0.22 S cm1 for La0.6Sr0.4CoO3 [114]. 4.2.4 LaxSr1 xTiO3+δ (LST) Another possible ceramic cathode substance for co-electrolysis is perovskite oxide LaxSr1 x TiO3 + δ (LST). It’s an n-type semiconductor in its most basic form. When moist H2 is reduced, it can attain an electrical conductivity of up to 30Scm1 at intermediate temperatures [115–117]. The electrical conductor La0.8Sr0.2TiO3+ was used by Xie et al. (LST) LST-CGO mixed cathode for co-electrolysis application made of Ce0.9Cd0.1O2-δ (CGO) and oxygen-ion conductor Ce0.9Cd0.1O2 (CGO). LSTCGO/YSZ/(La0.8Sr0.2) 0.95MnO2-δ (LSM) CGO electrolyzer setup. The solid oxide electrolyzer works well under electrolysis conditions, even if hydrogen is not used as a typical requirement. The reducing gas is in an electrolytic cell that uses nickel-based hydrogen electrodes. However, at 650°C, the LST-CGO composite cathode had a high polarization resistance of 8.5 Ω cm2 under 66.7% CO2–33.3% H2O. The progressive transition from Ti4+ to Ti3+ in the reducing environment resulted in improved cathode activity at increased electrolysis voltage [118].

5. Long-term performance degradation of SOECs The major concern with high-temperature solids is long-term deterioration. As a viable synthesis gas generation method, an oxide co-electrolysis cell was developed. Table 3 summarizes a lot of long electrochemical research studies, the majority of which were conducted on electrolysis cells using Ni-based cathodes. The cell structure and cell components, fuel gas components, and operating circumstances including temperature and

Table 3 Literature on the lengthy performance of the SOCEs. Fuel gas components %H2O %CO2 % H2

Cell structure

Temp. (°C)

Duration (h)

Constant current (A cm22)

Ni–YSZ/YSZ/ LSM-YSZ

850 800

785 920

0.5 1

45 45

45 45

10 10

1.5/785 21/920

850

11.2

2.25

45

45

10

3/11.2

Degradation Electrochemical performance (%/h) Microstructural/composition – (a) Ni grains Ni grain development (b) Fractures and oxygen bubbles inside electrolyte (c) Interface delamination Carbon deposition

Ref. [119] [88]

[120]

Co-electrolysis process for syngas production 253 electrolysis current all have a role in the durability of SOCEs for synthesis gas generation [92–94]. Because of the same cell architecture, some identified basic degradation processes in SOFCs, such as impurity toxicity and microstructural harm caused by grain coarsening and heat stress, also might be relevant to cells operating in a co-electrolysis method. Furthermore, new forms of degradation unique to SOCEs may arise from profound electrolysis under rising current densities and rising humidities in fed gas. Under a constant electrolysis flow of less than 1 A cm2, as evidenced by the durability findings of SOCEs, a destruction rate of roughly 5%/1000h is often found. Although, more extensive degradation processes occur, as evidenced by microstructural damage and a faster growing rate of cell voltage with increasing operating temperatures and current densities, which are connected to distinct decomposition mechanisms. For instance, Graves et al. described the development of resistance spectra using NiYSZ/YSZ/ YSZ-LSM at SOCEs evaluated at various electrolysis current densities [57]. The theory of distribution of relaxation times (DRT) was used to analyze the impedance spectra, which revealed in low current densities of 0.25A cm2, decomposition at the Ni/YSZ electrode was influential, though at greater current densities of 0.5 A cm2 and 1.0 A cm2, the Ni/YSZ electrode proceeded to degrade; however, decomposition at the LSM electrode starts to play a significant role in the overall cell performance loss. Sun et al. investigated the endurance and prestructure of a Ni–YSZ/YSZ/YSZ-LSM electrolyzer at high temperatures and current densities over 1 A cm2 [88].

6. Conclusion There is an urgent need to produce liquid fuels for sustainable energy carriers in order to alleviate energy consumption and environmental concerns. SOECs provide a viable alternative for converting electrical energy into chemical energy, which may then be saved in liquid fuels, by concurrently separating steam and carbon dioxide into synthesis gas, which is used to generate synthetic fuels through the Fischer–Tropsch process. The operating principle of SOCEs demonstrates that it can be basically assumed as an oxygen concentration cell,; however, the inclusion of the RWGS reaction has added a great deal of complexity to the high-temperature co-electrolysis procedure, which includes many reactants as well as reactions. A thermodynamic study of the co-electrolysis process reveals that the increment in the working temperatures significantly reduces electrical utilization, making the process more energy-efficient. A chemical equilibrium of the co-electrolysis process is established to assess the fuel component evolution and Nernst potential as a factor of electrolysis current density, feed gas composition, and operating temperature. It also allows the synthesis gas production process to be designed and controlled to analyze the relationship between product gas composition and electrochemical performance. The most frequent materials utilized as SOEC cathodes are Ni-based cermets that have high electrochemical performance and a low cost. Besides, their electrolytic flow is high, and their polarization resistance is low. Despite their high synthesis gas generation rate, they have a low tolerance for impurities and contaminants.

254 Chapter 10 However, sensitivity to impurities and structural changes due to the heating cycle lead to significant loss of active catalyst surface area and degradation of electrochemical properties over time. Novel perovskite-type-based ceramic materials, including SFM, LSV, LSCM, and LST, have been thoroughly investigated and assessed as replacement cathodes for SOECs. They have adequate electrical conductivity, high phase stability and catalytic characteristics. Despite the significant potential for huge industrial synthesis gas generation by high-temperature co-electrolysis of steam and carbon dioxide, further research is needed before this method can be commercialized.

Abbreviations and symbols AEM ASR ASR ASR AWE BCZYZ BCZYZ CeO2 CGO CO/H2 CO2 DC DRT DRT FT GDC GDC GDC H2 H2O IEA k KOH LSCF LSCF LSCF LSF LSGM LSM LST LSV LSV) MEC MFCs NaOH NASA O2

anion exchange membranes area-specific resistance area-specific resistance area-specific resistance alkaline water electrolysis BaCe0.5Zr0.3Y0.16Zn0.04O3 BaCe0.5Zr0.3Y0.16Zn0.04O3 cerium oxide Ce0.9Cd0.1O2 synthesis gas carbon dioxide direct current distribution of relaxation times distribution of relaxation times the Fischer–Tropsch process gadolinium-doped ceria Ce0.9Gd0.1O1.95 Ce0.9Gd0.1O1.95 hydrogen water international energy agency reaction equilibrium constant Potassium hydroxide strontium and iron-doped lanthanum cobalt La0.6Sr0.4Co0.2Fe 0.8O3 La0.6Sr0.4Co0.2Fe 0.8O3 La0.8Sr0.2FeO3 strontium and magnesium-doped lanthanum gallate lanthanum magnetite doped with strontium LaxSr1  xTiO3+δ LaVO3 Sr-doped LaVO3 microbial electrolysis cells microbial fuel cells sodium hydroxide national aeronautics and space administration oxygen

Co-electrolysis process for syngas production 255 OH PEM Q RE RWGS RWGS SDC SFM SOCs SOCEs SOE SOFC TPBs y0 , x y1 ΔG Δ Gf, CO2 Δ Gf, H2O ΔH ΔS

two hydroxyl ions proton exchange membrane heat renewable energy reversible water gas shift water gas shift samarium-doped ceria Sr2Fe1.5Mo0.5O6 solid oxide cells solid oxide co-electrolysis cells solid oxide electrolysis solid oxide fuel cell triple phase boundaries gas composition equilibrium compositions Gibbs free energy Gibbs free energy changes for the production of carbon dioxide Gibbs free energy changes for the production of water overall energy variation energy entropy

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CHAPTER 11

Photocatalytic process for syngas production Baishali Kanjilal, Arameh Masoumi, and Iman Noshadi Bioengineering, University of California, Riverside, CA, United States

1. Introduction A decline in global fossil fuels has prompted the need for alternative and sustainable energy development [1]. Syngas production from H2O and CO2 provides to address these challenges by supplying clean fuels with enhanced efficiency in energy utilization and eliminates known pollutants to decouple energy utilization and greenhouse gas emissions [2]. Synthesizing fuel from CO2 and water was first proposed by Steinberg [3]. Sunlight is an ideal energy source, which may be utilized for converting CO2 and water vapor to hydrocarbon fuels. While it can be used to produce hydrogen through photocatalytic water splitting, this study will concentrate only on syngas production techniques and catalysts. Syngas, also known as synthesis gas, consists of hydrogen and carbon monoxide, produced as an intermediary in ammonia and methanol production. It is often produced by one of several ways, one of which is by reacting hydrocarbons, from natural gas with water [1–3] CH4 + H2 O Ð CO + 3H2

(1)

An alternative process, dry reforming, or carbon dioxide reforming produces syngas by reacting carbon dioxide with hydrocarbons such as methane, catalyzed by Ni or Ni alloy metal catalysts [1,2]. CO2 + CH4 ! 2H2 + 2CO

(2)

Syngas is also made by steam reforming reaction or coal gasification. Its importance has increased significantly owing to concerns of climate change and global warming, in that it uses two greenhouse gases to produce chemical building blocks [1–3]. Yet another method of producing syngas is by partial oxidation that reacts with hydrocarbons with a limited amount of oxygen. Among these, partial oxidation, as a process, has achieved greater technical maturity. The exothermic process entails mixing natural gas, heavy hydrocarbon fuels, or heating oil with oxygen in limiting amounts.

Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91871-8.00002-7 Copyright # 2023 Elsevier Inc. All rights reserved.

261

262 Chapter 11 C24 H12 + 12O2 ! 24CO + 6H2

(3)

The concept of carbon capture, utilization, and storage has gained momentum in recent times. CO2 can be converted to useful chemicals catalytically, with processes being well studied owing to their potential market and benefits [4,5]. Herein, catalytic utilization, particularly photocatalytic processes have gained tremendous impetus owing to the natural interest in harnessing the infinite power of the sun [6], and it entails hydrogenation [7], carboxylation [8], solar thermochemical CO2 splitting [9], and dry reforming [10]. The research works have been depicted in Fig. 1. Syngas production via CO2 reforming of alkane has gained traction owing it its utilization of atCO2 and light hydrocarbons, with a net emission of greenhouse gases to environment. Dry reforming of methane (DRM) is the most widely used process owing to the easy availability of the gases in biogas in a 1:1 ratio [11,12]. The reforming of alcohol, on the other hand, known as dry reforming of ethanol (DRE), is promising owing to utilization of CO2 and renewable ethanol as raw materials. Thermodynamically, DRE was favorable above 318°C compared with DRM, which requires a temperature of 600°C [13,14]. Yet another possible raw material is glycerol, as by-product of biodiesel [15]. While doped Ni, La, Al, Ce-based catalysts have been studied in DRM, owing to their enhanced oxygen storage capacity of the catalyst through redox, the process is highly dependent on the stoichiometric ratios at partial pressure and leads to significant secondary reactions on the side [16]. The field of syngas production by way of utilization of solar energy depends primarily on solar efficiency, syngas yields, and selectivity. These are incumbent on the catalyst efficacy much more than process maneuverings. The approach of this chapter hence shall place an emphasis on the chronological development of ever more effective catalytic structures, with

Fig. 1 (A) Methods of CO2 conversion (% of volume of CO2 converted by respective method). (B) Number of publications from 2000 to 2020 on CO2 syngas catalysts and dry reforming catalyst [3–5].

Photocatalytic process for syngas production 263 increasing innovation in chemistries employed to achieve ever greater efficiencies. This study will go through the decade of metallic structure use, diving into the creation of their processes. This will be followed by sections with specific emphasis on families of metal compounds employed with innovative support chemistries. The ensuing section will focus on catalysis based on phosphorous, triazines, and other complex organic structures. The next section will delve into polymers, followed by a section on plasma-based processes for synergistic advantage of solar conversion. This will be followed by a conclusion. The development of novel materials has been followed chronologically. The field has slowly grown from the generic use of nickel to more complex molecular structures, harnessing organic frameworks or selectively functionalized nano/meso porous, multilayered materials. A small section on plasma processes and photosynthetic cells of novel designs for highly tunable and efficient photocatalytic syngas production has been included toward the end.

2. Shrinking the carbon cycle The idea of photocatalytic syngas production encompasses, essentially replicating and significantly shortening the carbon cycle, as shown in Fig. 2A [17]. CO2 reduction process is highly endothermic making its sustainability feasible only if combined with renewable energy, such as solar. Solar photocatalytic syngas production proffers a great way to recycle CO2 and of store intermittent solar energy as syngas. The process, however, is challenging [18]. Inoue et al. (1979) first reported photocatalytic CO2 reduction from a mix of methane, methanol, methanoic acid, as well as formaldehyde. The wide-band-gap semiconductors used

Fig. 2 (A) Schematic representation of the natural photosynthetic CO2 cycle and triforming sustainable conversions. (B) Schematic of CO2 photoreduction on a platinum semiconductor surface [17,23].

264 Chapter 11 started off a slew of research in photochemical production of syn-fuels from CO2 [18–20]. The process entails light photon absorption in a photocatalyst material, converting them into electron–hole pairs, spatially separated for chemical redox, and the electrolyte–semiconductor interface. In CO2 photoreduction entailing water as the reducing species, the processes of CO2 reduction and water oxidation have to occur simultaneously. These are multielectron transfer reactions. An energy schematic of this is shown in Fig. 2B. A minimum energy threshold for photo-excited electrons is needed to reduce CO2 as well as breach conduction–valence energy gap of the photocatalyst. Only UV and near-UV spectrum can be used with TiO2-based photocatalysts causing low absorption and conversion efficiency [20]. Very few semiconductors satisfy the requisites of the band gaps and catalytic activity for efficient carbon dioxide reduction. Approaches from enhancing catalyst efficiency have entailed doping, co-catalyst usage on surface as nanoparticles and using the photo and co-catalysts as nanocrystalline form [21,22]. There is superior charge separation with heterojunction structures with disparate band gaps as well as matching band potentials. This also leads to better lifetime as well as better interfacial charge-transfer efficiency [23–28]. Nanoclusters have increased photocatalytic activity with increased electronic transfer to absorbed metal/semiconductor clusters and better possibility of electron–hole recombination [26,27] Water has been dissociated at photoanode with concomitant CO2 reduction at the photocathode. Here the two electrodes were separated by a proton-conducting membrane, making it a superior photoelectron-catalytic device [28]. The key issues remain in optimizing and scaling up device efficiency for photo-induced CO2 reduction. The following section will discuss the development of photocatalysts for CO2 photoreduction. The section chronologically follows the development of photocatalytic structures, which typically started out with nickel, titanium, and cobalt. The catalysts then began using silver and gold as well as composite structures such as cobalt with carbon nitride. Further development was then sought with supported structures with co-catalysts and the usage of iron and phosphorous with a view toward process and cost efficiency. The development of nitrogenated structures, metal organic frameworks (MOFs), and conjugated polymers has opened the doors for tailored structure suited to process requirements. These are explored in the ensuing sections.

3. Catalysts for photocatalytic process 3.1 Nickel Heterogeneous photocatalysis is considered as one of better approaches in producing hydrogen rich [23,29,30]. The semiconductor catalyst is photoexcited. The photons that have an energy higher than the band gap are absorbed leading to holes and electrons and concomitant oxidation and reduction. Valance and conduction band potentials are important at the surface of the semiconductor photocatalyst. Here the solvent, such as water, can suppress recombination of

Photocatalytic process for syngas production 265 electrons and holes through electron capture. An alternative to water is methanol. Its nonpolar nature makes it incapable of suppressing this recombination. This makes methanol suitable for photocatalytic hydrogen production, employed in designing efficient processes for hydrogen-rich syngas obtained from methanol. The process uses a laser pulsed light (355nm). The light photolytically splits methanol in the vicinity of semiconducting NiO as a photocatalyst. The products yielded are molecular hydrogen (H2) and carbon monoxide (CO). A high H2/CO ratio is obtained in addition to a high yield rate even at room temperature. NiO proffers the advantages of good chemical stability, opto-electronic properties [31]. It’s a p-type semiconductor with a band gap of 3.5eV. The potential of valance bands and conduction bands is +3.0 and 0.5V, respectively, suitable for water oxidation [32]. Band edges shift to negative potentials in aprotic solvents. The hydrogen yield confirms that the conduction band has shifted to a negative value. The 355 nm photons once absorbed by NiO generate conduction band electrons in addition to valance band holes. These holes provide the oxidation centers for methanol. Meanwhile, photoexcited conduction band electrons reduce H+ ions. The general mechanism is as follows: NiO ! NiOðhvb , ecb Þ

(4)

CHOH + h+ ! CHO+ H CH O+ H ! CH O + H + H+

(5)

3

2

(6)

H+ + e ! H_ H_+H ! H

(7)

HCO + h+ ! CO + H+ + H_

(9)

H2 CO + 2e ! CO + H2

(10)

2

(8)

The formation of methane occurs via free radicals as does the production of hydrogen proceeding via the photocatalytic dissociation of methanol (11) CH3 OH + H ! CH4 + HO NiO shows semiconductivity with either defect sites or with oxygen vacancies. This happens when these are exposed to laser photons [33]. This is due to hopping of electrons between the Ni2+ and Ni3+ species with oxygen vacancies giving it better catalytic activity [31,32] via reduction of Ni2+ to Ni+. Photons with energy higher than that of the band gap causes electronic transfer between oxygen (2p) valence band to the Ni(3d) conduction band. This reduces it from Ni2+ to Ni+. This also weakens the Ni+2 O bond and causes pushing out of the lattice oxygen forming a valence band hole. This oxidizes the methanol, which then produces H+. The H+ thus formed inserts itself into defect sites to form HxNiO1 x. These species are then are then converted to hydrogen. Given the potential of fuel cell technology, it can be retrofitted to solar production of syngas. Methane steam reforming and partial oxidation are the conventional

266 Chapter 11 sources of hydrogen. These produce hydrogen with a remnant level of carbon monoxide that cannot be handled by proton exchange membrane–based fuel cells [34]. As alternatives, ZnO-based CH4 reduction has been explored by [35–37]. In other works, Fe3O4/FeO and Fe3O4/a-Fe redox pairs are used for CH 4 reforming. The dark radiant absorber particles can be irradiated by solar high-flux beams. It was shown that Fe3O4 + 4CH4 attains chemical equilibrium at 2/3rd H2 and 1/3rd CO. The reactions happen in the gaseous phase at 1 atm pressure and above 1300K. The reactions were carried out in fluidized bed solar furnace with a solar receiver. The solar flux densities were 900kW/m2 [36]. Other studies have encompassed Fe3O4, ZnO, SnO2, In2O3, WO3,or bivalent and trivalent metal doping. In studies with metal-doped magnetites (ferrites of Ni(II), Zn(II), and Co(II)), Ni(II) ferrite supported on ZrO2 (Ni0.39Fe2.61O4/ZrO2) was seen to be good for cyclic steam reforming process, while reforming of methane using reduction of metal oxide was achieved successfully using Ni0.39Fe2.61O4/ ZrO2 under direct irradiation by the simulated solar-simulated light at flux densities of 180–500 kW/m2 [37,38], underscoring that these particles were simultaneous radiant absorbers and chemical reactants.

3.2 Titanium Akhter et al. [39] studied high surface area 11nm nano-TiO2 nanoparticles and mesoporous titanium dioxide structured on KIT-6 silica template/have been used for photocatalytic CO2 reduction the company of H2O vapor for making syngas. The nanoparticles had a surface area of 151m2/g with high adsorption capacity. The meso TiO2 particles showed better syngas production together and superior stability. The reaction mechanism showed a competition between CO2 and H2O adsorption on catalyst surface. The primary influences were UV source and intensity, stoichiometric ratio, and catalyst shapes. Various reaction mechanisms for syngas production have been reported based on TiO2 photocatalysts [40–43]. It is speculated that there are significant branching pathways that produce several side products. Thus, an optimization of process parameters is needed in order to scale up such processes. CO2 is considered stable and inert making it difficult to reduce or oxidize. Upon reduction to the CO 2 anion, it professes a strongly negative electrochemical potential (1.9V) [40]. The following general half equations represent the conversions, also shown schematically in Fig. 3. CO2 + e ! CO2 

(12)

2H + +2e ! H2

(13)

CO2 + 2H+ + 2e ! CO + H2 O

(14)

CO2 + 8H+ + 8e ! CH4 + 2H2 O

(15)

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

(16)

Photocatalytic process for syngas production 267

Fig. 3 Pathways of photolytic conversion of CO2 and H2O to various products [39].

Semiconductors cannot thermodynamically provide a single photo electron to CO2, but several electrons may be transferred. But conversion of water to hydrogen straddles a much lower standard potential and hence is easier. Thus H2 and CO formation may be favored with a need for lesser electrons or protons [44]. That said, CH4 and CH3OH needing more electrons may be more difficult to make. Hydrogen is generated by two electrons and two protons (0.41 V reduction potential) while carbon monoxide needs two electrons and two protons (0.53 V reduction potential). This compared with eight and six protons and eight and six electrons required for methane and methanol formation, respectively. The catalysts are also seen to be easily regenerated via simple evaporation making them promising for scale-up and process applications. Ternary hybrid catalysts prepared by grafting a dye with CoIII catalyst (capable of producing hydrogen) and ReI catalyst on TiO2 (for producing CO) have been used for producing syngas with high yields from CO2-saturated dimethylformamide water solution with a 0.1m sacrificial electron donor. Process control employing stoichiometry has been employed to maneuver the H2/CO ratios. The catalyst stoichiometry ratio was also seen to have profound effect on H2/CO ratios. Syngas yield is controlled by competitive electron flow from TiO2 to CO2-reduction as well as H2 producing sites. The efficacy was seen to depend on catalyst cycles under given conditions. The electron transfer schematic for heterogeneous ternary dual molecular hybrid photocatalytic system is shown in Fig. 4. [45]. The formation of H2 and CO has been seen using photochemical and electrochemical reduction of CO2 and water over Ni, Re, and Ru catalysts [46,47], but they offered little control over the composition. In 2017, Angew. Chem. Int. Ed. reported a hybrid catalyst system consisting of immobilized transition-metal complex and light absorbing TiO2. The ReI complex was considered as CO producing and the Co(III) complex was considered as a hydrogen generator [48,49]. The structure and the scheme are illustrated in the figure above. The components and

268 Chapter 11

hv (1)

Dye•+

E(Dye)ox*

0% H2O (v/v) 3% H2O (v/v)

Dye

= –1.6 V

10% H2O (v/v) 20% H2O (v/v)

(4) ED

CO

Cl

CO

Re N

Ph

N

H

N

N S

Dye

O

H O

CO

N

NC

Cl

N

Co

N N

N

O

O H

H2O3P

S

ED

N

ED•+

COOH

H2O3P

Catalyst 1 (ReP)

H2O3P

Catalyst 2 (CoP)

Fig. 4 Heterogeneous ternary dual molecular hybrid photocatalytic system, an electron transfer schematic [45].

reaction pathway of photocatalytic production of syngas are based on a TiO2 hybrid antenna with two complexed transition-metal catalysts. The antenna here comprises a cyano-thiophen-acrylic acid–based dye structure ((E)-2-cyano-3-(50 -(500 -(p-(diphenylamino) phenyl)thiophen-200 -yl)thiophen-20 -yl)-acrylic acid), The CO producing catalyst 1 is fac[Re(4,40 -bis(diethoxyphosphor-ylmethyl)-2,20 -bipyridine)(CO)3Cl] [ReP]. Here catalyst 2 is [CoIIICl(dimethylglyoximato)2(pyridyl-4-phosphonic acid) [CoP]. Also here, the sacrificial electron donor is 1,3-dimethyl-2- phenyl-1,3-dihydrobenzimidazole [SED]. The hypothetical mechanistic photochemical pathway entails an injection of electrons from excited dye structure into TiO2 as the initiation procedure [50–54]. This is followed by competitive electron transfer to ReP and CoP catalyst through TiO2. This is followed by the chemical processes on the Re and Co sites for CO2 reduction and hydrogen production, respectively. This proceeds due to supply of electrons from TiO2. The dye can be recovered by dyeC+ reduction with SED. The CO and H2 ratio depends on electron supply rates from TiO2 to the two sites of catalysis, ascertained mainly by reduction potentials of Re and Co with respect to conduction [45].

3.3 Cobalt Yet another visible-light-driven H2 evolution system was studied by Lakadamyali et al. [55]. The system comprised a RuII dye and CoIII proton reduction catalysts, respectively denoted as RuP and CoP. These were immobilized on TiO2 nanoparticles and mesoporous films.

Photocatalytic process for syngas production 269 This was seen to photo evolve hydrogen with good efficiency in pH neutral aqueous solutions. It was speculated that a molecular Co species, and not a metallic Co or an oxide precipitate, was responsible for H2 generation on TiO2. Quantitative electron injection is thought to have occurred within 180 ps from RuP into TiO2. The sacrificial donor generates the dye. Then it leads to the formation of an electron at the TiO2 conduction band. Electron transfer occurs from this conduction band to CoP catalysts. This is 100 faster than charge recombination with oxidized RuP. This work has been used to study the transfer mechanism of electrons in multicomponent systems, especially for photo-CO2 reduction, the energetics of which are depicted in Fig. 5 as a plethora of electron transfer and relaxation processes between RuP, TiO2, and CoP in nano and meso catalytic structures. A study by Dewang Li et al. [56] showed a photocatalyst made from Rh, Au, and SrTiO3 for low-cost syngas photo production from CO2/H2O using visible light. Synergy between Rh and Au on SrTiO3 caused a 22–153 fold enhanced activity for syngas yield. This is contrary to Au-SrTiO3 or Rh-SrTiO3 catalysts. Till before this work, syngas production by artificial photosynthesis had been carried out mostly using UV light. This work embarks upon usage of high-efficiency photoreduction of CO2 and H2O by visible photocatalysis. This is also important given earlier works ended up with methane as their main product in the carbon dioxide/water mixture photo reduction. This was due to the fact that CH4 reduced from CO2 occurs over a lower thermodynamic energy requirement than CO [57–59]. Visible-light-mediated syngas production is a challenge. SrTiO3 has a high conduction band potential. When it combines with Au and the Rh co-catalyst, Au is a visible light sensitizer, while the Rh is a photo-electron accumulator. It also applicable for CH4 reforming. The synergy

Fig. 5 Schematic of electron transfer and relaxation processes between RuP, TiO2, and CoP in nano and meso catalytic structures [55].

270 Chapter 11 thus accumulated over SrTiO3 achieves selectivity and enhancement for yielding syngas from CO2 and H2O mixture in the presence of visible light.

3.4 Silver and gold Photocatalysis has been slowly proving to be a promising approach for reforming under normal temperature conditions [60–62]. While DRM has been investigated using Cu/CdS–TiO2/SiO2 [63], photocatalytic reforming of CH4 with CO2 has caught on over ZrO2 catalyst with CO and H2 products under UV-light irradiation [64]. DRMs for syngas production have been studied with Au-Rh/TNTs, Pt/TiO2 photocatalyst under UV light, La/TiO2, as well as Ni-MMT/ TiO2, with H2 and CO production using UV light [65,66]. Recently, graphitic carbon nitride (g-C3N4) has been used in lieu of TiO2 for photocatalysis applications owing to low cost, abundance, stability, and medium band gap energy and its allowance for metal doping [67,68]. Recent works have reported CO2 reduction over Cu or Ag/g-C3N4 [28,29]. Thus works lately have begun to look at photo-BRM as a feasible process, scalable for hydrogen-rich syngas production. In 2019, Tahir et al. [60] studied silver nanoparticles (Ag-NPs) for CO2 reduction. These NPs were supported over protonated g-C3N4 and prepared using ultrasonication and photo-deposition. Photocatalytic reduction of CO2 from methane steam reforming process (SRM) and methane bireforming (BRM), as shown in the equation below, was carried out using these hybrid catalysts with differential results. 3CH4 + CO2 + 2H2 O ! 4CO + 8H2

(17)

Enthalpy for the reaction in + 220 kJ/mol, while the free energy change is +151 kJ/mol [60]. While pure graphitic-C3N4 was seen to favor evolution of CO, while protonated g-C3N4 promoted H2 and methanol production. CO yield over Ag/pg-C3N4 was nearly 2.5 times more than CO over C3N4, primarily owing to more rapid charge separation because of presence of Ag. Photocatalyst behaved differently with reforming systems with enhancement in the BRM process as compared with SRM and DRM, underscoring a better feasibility of photocataytic BRM process for making CH3OH, CO, and H2. The photocatalytic production of syngas via different reaction systems using Ag/pg C3N4 catalyst and the process schematic for photocatalytic reforming over the same catalyst using a fixed bed reactor are shown below in Fig. 6A and B. The selective CO production is explained via reduction potentials of CO2/CO (0.48V) and the g-C3N4 conductance band (1.12 V). Reduction potential for making CO is lower than the g-C3N4 conductance band. This ensures favorable CO production. The potential for H+/H2 (0.41 V) reduction significantly lower than g-C3N4 conductance band, causing enhanced H2, and CO by way of protonated g-C3N4 and modifying with Ag-NPs. In 2020, Renones et al. [65] studied a bimetallic Au  Ag/TiO2 for CO2 photocatalytic conversion with water as reductant. The metals were deposited on TiO2 such that it resulted in silver NPs

(A)

(B) H2

-

+

(i)

e +2

2H

e-

e- e- eCB -1.23

CH4, CO, CH3OH

(ii) -

CO2, 2H+ +

e +2

2H

e-

Ag

H2

e-

Ag

2

+

(iii) -

2H

e-

H+ ,O

2

CO, CH3OH CO2, 2H+, CH3

-

e e eCB -1.23

eAg

Ag

-

+

2H

e +2

-

e

CH4

2

N2/CH4/ CO2

3 CH + +H

F e e d

Quartz window

CO, CH3OH CO2, 2H+, CH3

-

e e e CB -1.23

e-

Water saturator

Ag VB +1.44 h+ h+ h+

H+ ,O

H3

C

Ag

h+ h+ h+ 3

-

-

OH

OH

CH4

(iv)

H2

VB +1.44 CH + +H

Ag

H2O + h+

hu

+

,H CH 3

e-

VB +1.44 h+ h+ h+

H2O + h+

H2

-

e +2

e- e- eCB -1.23

Ag

VB +1.44 h+ h+ h+ H+ ,O

CO, CH3OH

OH

CH + +H

CH

OH

Ag/g-C3N4

Detector

3

3

H2O + h+

CH4

Syngas and hydrocarbons

Fig. 6 (A) Photocatalytic production of syngas via different reaction systems using Ag/Pg C3N4 catalyst. (B) Process schematic for photocatalytic reforming over the same catalyst using a fixed-bed reactor [61, 66].

272 Chapter 11 exposure on the surface. The metal NPs -TiO2 contact allows superior interphase charge transfer under ultraviolet (UV) and visible light. These bimetallic catalysts, under UV, were able to achieve greater methane selectivity, rather than just syngas production over bare TiO2. The metal NPs are thought to cause extraction of photoexcited electrons. However, this selectivity is not seen under visible light with preferential syngas production. Additional interfacial interaction between TiO2 and metallic NPs causes electronic photoexcitation band gap on the surface, leading to near-surface electron–hole pairs.

3.5 Cobalt with carbon nitride Despite g-C3N4 being promising photocatalyst for CO2 reduction, the easy electron–hole recombination coupled with lack of adequate active catalyst sites lowers its overall activity. Thus in 2021, Jianga et al. [69] bettered its performance by coordinating single Co sites with reduced graphene oxide (rGO) integrated using Van der Waals forces with g-C3N4 to form heterostructures. These offered excellent charge separation and transfer for photocatalytic CO2 reduction. These Van der Waals structures allowed interfacial π-π interaction to speed up expedite transfer of photo electrons from g-C3N4 to isolated Co catalyst sites. Hence these 2D Van der Waals structures (Co-rGO/C3N4) showed a better photocatalytic efficiency toward CO2 reduction. They were also able to change the CO/H2 ratio in the syngas produced with promising strategy for better charge transfer dynamics. Fig. 7A and B, respectively, show the schematic for the synthesis of Co-rGO/C3N4 and the energetics of the processes for g-C3N4 and Co-rGO/C3N4 [69].

(a)

(b) –1.5

CBM –1.46V, e–

–1.39V,

e–

GO-Co2+

Freeze dry 550 °C Ar

C3N4

GO rGO

Potential Vs. NHE (V)

+ Eg = 2.71eV

CO-rGO/C3N4

Coordinated Co site

CO2/CO H2O/H2

0.0

h+

C3N4 Co2+

Eg = 2.67eV

1.25V

h+

1.28V

VBM

1.5 C3N4

Co-rGO/C3N4

Fig. 7 (A) Diagram for synthetic procedure: Co-rGO/C3N4 catalyst. (B) Energy-band diagrams: g-C3N4 and Co-rGO/C3N4 [69].

Photocatalytic process for syngas production 273

4. Usage of catalyst support and structural co-catalysts In 2019, Hu et al. [70] showed a facile fabrication of active CoN4 structure supported on SiO2, via self-coordinated Co2+ ions with aminated SiO2 NPs. The CoN4-SiO2 catalyst showed excellent catalytic activity of photo-syngas production using g-C3N4 as a photosensitizer. Syngas achieved had a Co/H2 ratio of nearly 1:2 and high catalytic activity (5053mmolg1 and 36 mmol g1 h1 based on the catalyst) as well as >120h stability. The mechanism was shown to ascertain Co(I) as the active species generated via electron movement induced by light, from g-C3N4 to CoN4-SiO2. The Co2+ is dissociated from aminated SiO2 support, while g-C3N4 is decomposed under light leading to systemic inactivation post after long-term photocatalysis. The schematic showcases the mechanism. The CoN4-SiO2 shows bifunctional activity. It mediates CO2-reduction and conversion of 2H+to H2 photochemically. The CoN4-SiO2 here is a catalyst, g-C3N4 is the photosensitizer, and triethylamine (TEA) is the electron donor, while the solution is H2O/CH3CN. Plausibly, the CoIIN4 on the SiO2 substrate begets an electron, under visible light, from an excited g-C3N4. It generates the CoI-species, which is the main catalyst. It binds to CO2 or to protons in order to realize the CO2 to CO or 2H+ to H2 conversions [71]. TEA, here is the electron donor, oxidized by the g-C3N4 holes. It then releases protons for hydrogen production [72]. Additionally, g-C3N4 and NH2-SiO2 mediate a small amount of CO2 and proton reduction [73]. Fig. 8A shows a composite CoN4-SiO2 nanoparticle preparation procedure, using the SiO2 as a support. Fig. 8B shows the synthesis of C-ZIFs with Co and their photocatalytic function [70,74]. MOFs have been modified in various ways for enhanced CO2 capture and photocatalytic catalytic [74,76]. For example, in the zeolitic imidazolate framework Co-ZIF-9 and Co-ZIF- 67 with nano morphologies have been shown to promote CO2 photoreduction by integrating functions of cobalt such as electron mediation, by confining it using imidazolate structure in heterogeneous structures [77]. Leaf morphology of the 2D Co-ZIF-67 is seen to enhance reduction of CO2. It achieves this through facilitating co-catalyst to photosensitizer [Ru(bpy)3]2+ electron transfer. Nonetheless, the balancing of catalyst ability and MOF stability remains difficult especially in the presence of water [78]. As an alternative, MOF carbonization has made heteroatom-doped complexes of metal and carbon with rich active reduction sites. Iron precursors with Zn-ZIF-8 have been ball milled to make Fe–N–C electrocatalysts with single Fe atom centers and NPs. These reduced CO2 and evolved hydrogen, respectively [74,76,77]. Ionic exchange of Zn-ZIF-8 with Ni caused these Ni sites to exhibit superior selectivity toward CO [79]. Unsaturated NidN sites doped in porous pyrolysis carbon, from bimetallic ZIF-8, showed that the produced syngas could be tuned by Ni–Nx coordination [80]. In 2018, Mu et al. utilized these concepts in photocatalysis syngas production using carbonized composites of cobalt. These were made from bimetallic Zn/Co zeolitic imidazole frameworks. The C-BMZIFs were used as co-catalysts for [Ru(bpy)3]2+ photocatalyzed CO2 reduction with visible light. Ratio of Zn/Co could be used to

274 Chapter 11

Fig. 8 (A) CoN4-SiO2 composite nanoparticle preparation. (B) Synthesis of C-ZIFs with Co and their photocatalytic function. (C) Syngas photocatalytic production with an MOF structure—(Co/Ru)n UiO-67 (bpydc) [70,74,75].

regulate the active Co site size and activity with tunable syngas compositions being produced. A 3:1 Zn/Co ratio showed a high CO yield (1.1104 mmol g1 h1). This work was significant in designing stable MOF structure-based co-catalysts for tunable and efficient photo syngas production [74]. Further to the tetradentate complex structures described above, MOFs have been used for photocatalytic water splitting and CO2 reduction systems [75,81]. Lin et al. [75] introduced Pt NPs as well as transition-metal-based sandwich POMs into MOFs. This enhanced photocatalyzed hydrogen evolution reaction (HER) efficacy [82,83]. Photocatalytic CO2 reduction was explored by Liu et al. in 2019 with a two-step self-assembly to functionalize phosphorescent MOFs with a single site catalyst of (Co/Ru)n-UiO- 67(bpydc). The syngas yield was 13,600 μmol g1 with H2:CO ¼ 2:1, in 16 h, nearly 29 times higher than a homogeneous catalyst system. The mechanism is delineated in Fig. 8. Fig. 8C shows syngas photocatalytic production with an MOF support structure—(Co/Ru)n UiO-67 (bpydc). MOFs are discussed separately in a section below. The main issues with syngas photo production can be summed up as unsuitable CO/H2 ratio and charge recombination. Li et al. [84] mitigates this by maneuvering CuPt alloys and by way of constructing a hollow TiO2 mesoporous sphere. This spatially separates co-catalysts to promote

Photocatalytic process for syngas production 275 charge separation with a concomitant increase in evolution rate (84.2mmol/hg1 of CO, CO/H2 ratio of 1:2). Fig. 9A shows the schematic and mechanism of an MTCP-MS structure where electrons and holes are induced to move in different directions by spatially segregated co-catalysts. Fig. 9B is the stepwise energy diagram of CO2 reduction showing intermediate species. The yield for solar energy conversion is 0.108%, higher compared with oxide or sulfide catalysts. Spatially separated hole and electron traps have been studied for facilitated charge segregation [85–87]. Such catalysts have their overall photocataytic activity determined by surface properties, which are often loaded with oxidation co-catalysts making them only suitable for oxidation [85]. Here in this study, TiO2 hollow spheres, MnOx and CuPt alloys act like the main redox co-catalysts, respectively. The spatial separation of the MnOx particles and CuPt alloys causes electrons and holes to flow in opposing directions and enhances charge separation efficacy. A novel SiO2 NPs supported catalyst SiO2-TPA, coanchoring a [Co(TPA)Cl]Cl, was shown by Guia et al. [88], as a robust dual activity catalyst for syngas photo production. This noble-metal free system saw a 120 h syngas production with a total of 10.18 mmol syngas (CO/H2 ¼ 1.0:1.6–1.0:1.0), along with high stability and efficiency (10.18μmol mg1 cat ). A mechanism of

(a)

(c)

Cu-Pt Alloy

MnO2 TiO 2

N

N

2–

SO3

N

CO H2

h+ e– +

h e

Si

NH2

Co N N N

N N

N N

SiO2

–

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TEA/EtOH

SiO2

Si

CoCl2×6H2O

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Cl CoN N N

N

CO2 H2O

SO42–

SiO2

SiO2 SiO2-NH2

(b)

N N

N

h+ e– h+ e–

SiO2

SiO2-TPA

SiO2-TPACo

(d)

+0.6

Free Energy (eV)

N Cl

N

CO +H2O COOH* +

–

+H +e CO2+ CuPt Cu7Pt 2H++ CuPt3 2e– Cu3Pt –0.6 Step 1

Step 2

CO + H2 e– –1.54 V +

CO2 + 2H

CO*+ H2O

Et3N◊+

CO + H2 –1.25 V

hv Et3N

e–

SiO2-TPACoII/I CO2 + 2H+

h+ g-C3N4

Step 3

Fig. 9 (A) Schematic and mechanism of an MTCP-MS structure where electrons and holes are induced to move in different directions by spatially segregated co-catalysts. (B) Stepwise energy diagram of CO2 reduction showing intermediate species. (C) Schematic representation of a grafted [Co(TPA)Cl]Cl molecular catalyst onto SiO2 nanoparticles. (D) Proposed mechanism of the [Co(TPA)Cl]Cl catalyst [84,88].

276 Chapter 11 photoinduced electron transfer from g-C3N4 to [Co(TPA)Cl]Cl moiety onto SiO2 NP was speculated to occur during photocatalysis. Fig. 9C is a schematic representation of a grafted [Co(TPA)Cl]Cl molecular catalyst onto SiO2 nanoparticles, while Fig. 9D shows the proposed mechanism of the [Co(TPA)Cl]Cl catalyst.

5. Iron and phosphorous: A development driven by cost efficacy and process scaling feasibility 5.1 Iron-based structures The high cost of traditional metal catalysts for syngas production has prompted research on alternative and innovative structures such as in the work by Irfan et al. in 2020. A homogeneous molecular iron catalyst was combined with CdS nanorods (NRs) semiconductor to form highly efficient photocatalysts to make syngas from formic acid at room temperature and atmospheric pressure. A high catalytic activity (150 mmol gcatalyst1 h1 of H2) was achieved. Fig. 10A shows the schematic of photocatalyzed conversion of formic acid into syngas using a CsS-Fe catalyst. The quantum yield was 16.8% for visible light. The mechanism studied by cyclic voltammetry suggests a FeI specie formation as a key step. There was a detectable rapid transfer of electrons from CdS nanorods to the Fe catalyst. This suppresses the photo charge carriers causing better photocatalytic activity through monometallic pathways [89].

5.2 Phosphorous-based structures In recent times, phosphorous-based catalysts have been explored. For example, Pollak et al. [90] showed the efficacy of carbon nitride loaded with single Co2+ sites in combination with black phosphorus to make a hybrid catalyst. Photocatalytic H2 and CO evolution with ratios ranging from 0.30 to 5.16 could be obtained by just changing the % of phosphorous. A max gas evolution of 70 μmol g1 h1 was obtained at a H2/CO ratio of 2. A schematic representing a Co-C3N4 reduction catalyst structure is shown in Fig. 10B [90]. Despite the semiconductor properties of black phosphorous, it stability and activity in syngas production are still suspect. In 2021, Chen et al. [91] studied a 2D heterostructure of black phosphorous/bismuth tungstate (Bi2WO6) for photo syngas production via CO2 reduction. The BP nanosheets were anchored to very small platinum (Pt) nanoparticles of dimensions 2nm. This increased charge transfer efficiency through making an heterostructure of S-Scheme 2D/2D with oxygen defects. CO and H2 generation were achieved, respectively, of 20.5 and 16.8 μmol g1 h1. This is higher than with BP catalysts at CO/H2 ratios (1:1–2:1). The ambient-stable 2D/2D heterostructure of Pt/BP-Bi2WO6 with oxygen vacancies signified as (Pt/BP-OvMBWO), with photocatalytic ability for CO2 reduction to syngas, can be scaled up for industrial production, The Pt nanoparticles (2 nm) act as protecting agents and co-catalysts. Heterojunctions of BP/monolayer Bi2WO6 (MBWO) with some oxygen defects

Photocatalytic process for syngas production 277

Fig. 10 (A) Photocatalyzed conversion of formic acid into syngas using a CsS-Fe catalyst [89]. (B) Schematic representing of a Co-C3N4 reduction catalyst structure [90]. (C) Photocatalytic reduction using a Pt/BP-Ov MBWO heterojunction [91].

cause enhanced charge separation and electronic transfer [92,93]. The photocatalytic reduction using a Pt/BP-Ov MBWO heterojunction is shown in Fig. 10C [91]. The heterojunction is made by electrostatic assembly of Pt/BP nanosheets with positive OvMBWO nanosheets.

6. Tailorable structures 6.1 Nitrogenated structures He et al. [94] demonstrated an efficient system integrating Co single active sites, in covalent triazine frameworks (CTFs), in the form of photoabsorber, for syngas production from CO2 in aqueous solution. Triazines with their intramolecular heterojunctions, with the Co complex,

278 Chapter 11 caused high catalytic efficiency and a yield of 3303 μmol g–1 syngas (CO:H2 ¼ 1.4:1) in 10 h. The heterojunction enhances the yield by three times. The coordinated single Co centers are spectated to accept photo electrons from CTF. They then act as CO2 conversion sites via adsorption-activation reaction. Fig. 11A shows a schematic of synthesis and photocatalytic application of encapsulated Co sites in covalent triazine frameworks. In 2021, a nitrogen vacancy-based polymeric carbon nitride photocatalyst for syngas production was reported by Yang et al. [95]. Fig. 11B shows the formation of selective nitrogen vacancies in carbon nitrides for efficient visible light syngas production. DFT calculations

Fig. 11 (A) Schematic of synthesis and photocatalytic application of encapsulated Co sites in covalent triazine frameworks. (B) Formation of selective nitrogen vacancies in carbon nitrides for highly efficient visible-light-induced syngas production [94,95].

Photocatalytic process for syngas production 279 speculated that the nitrogen vacancies would improve activation and reduction of CO2, in addition to accelerating separation and transfer charge carriers of P-doped- g-C3N4, (PCN), photogenerated, with concomitant excellent catalysis activity for syngas production. The rate was 10 X higher than with pristine PCN and a tunable H2/CO ratio (0.24:1 and 6.8:1) obtained by adjusting NV concentrations, even under low-energy red light (610 nm). Research has proven PCN efficiencies in visible light CO2 photo reduction [96]. PCNs have been oxygen doped with five times higher photocatalytic CO2 reduction, than pristine PCN [97]. They have been made into TiO2/PCN/Ti3C2 composites with better transfer processes of charges and hence better CO2 reduction [98]. Nitrogen defects within PCN have been shown to serve as active sites for CO2 photo reduction [98].

6.2 Metal organic frameworks In 2020, Fu et al. [99] showed that tethered molecular catalysts—a rhenium complex, [Re(bpy) (CO)3Cl], together as a crystalline covalent organic framework leads to a heterogeneous photocatalyst with high visible light absorption. It also showed excellent CO2 binding affinity and catalytic performance. The bipyridine sites inside the framework ligated the Re complex to a p-conjugated backbone. This is a robust light harvesting structure. CO production rates of 1040 mmol g1 h1 with 81% selectivity were measured. The presence of platinum produced syngas, the ratio of the components being tunable to ratio of COF to platinum. The same catalytic efficacy was not realized in an amorphous structure.

6.3 Conjugated polymers CO2 electroreduction to syngas needs additional energetic bias for reduction, which is provided by sunlight and semiconductor catalysts [100]. While the field of photo catalysis has been dominated by inorganic or metal-based structures, recent progress has been made in conjugated photocatalysts materials since they proffer tunability, synthetic, and design freedom [101]. These structures have spanned conjugated microporous polymers- or triazine-based structures [102]. Some others have looked into covalent organic frameworks or other conjugated polymers [103]. The challenges faced by these materials are no different, straddling the CO-hydrogen selectivity ratio in addition to yield a catalyst usability [104]. As an example, in 2021, Fu et al. [105] studied conjugated polymer as photocatalysts for carbon dioxide reduction in the presence of water with a sacrificial hole scavenger. These polymers were based on dibenzo thiophene sulfones, and its copolymer with phenylene with high levels of CO production. The homopolymer however showed better hydrogen production rates perhaps due to favored proton reduction. Hydrogen evolves due to palladium in the catalyst, and it can be used to control the ratios of the evolved CO and hydrogen. Fig. 12A shows the conjugated organic polymer structures used for photocatalytic syngas production. Fig. 12B shows the predicted redox potentials of photocatalysts [105].

280 Chapter 11

H N n

4

p-sexiphenylene

n

P1

O O S

P4

O O S

N n

n

P7

P10

N

P29 N

S

N

N N

n

P30

N

P31

n

n

P74

n

Adiabatic Potential (V Vs SHE)

(b)

(a)

–3 IP EA

–2 –1 0

EA* IP*

CO2/CO (pH 8.3) H*/H2 (pH 8.3) TEOA/TEOA* (pH 8.3)

1 SX 1

4 7

10 29 30 31 74

Fig. 12 (A) Monomeric repeat units of conjugated organic polymers for photocatalytic syngas production. (B) Predicted redox potentials of photocatalysts [105].

7. Novel processes While a greater part of getting photocatalytic syngas production has been focused on developments in the efficacy and scalability of catalytic materials in order to make the process thermodynamically feasible, process engineering also plays an important role in designating optimal performance. In recent times, with an impetus on solar power harnessing, solar photo voltaic devices or photoelectrochemical cells are being explored for fuel generation [106]. Their applicability continues to be marred by low catalytic yields. However, an alternative approach is via process, entailing PEC as an example, exploiting energetic electrons to drive chemical conversion, allowing for easier process scalability [106]. The process needs stable photocathodes, for example, at interfacial metal/semiconductor/p-Si photocathodes with Si, Cu, InP, or NiO structure, unfortunately still riddled by low syngas conversion efficiency of 0.87% [107]. In 2018, to mitigate the low solar yields, two compartment artificial photosynthetic (APS) cells were designed using natural photosystem reactivity entailing the first step of water oxidation [106]. Fig. 13A shows a representation of double-compartment artificial photosynthetic cells for unable syngas production. This is accompanied by multielectron carbon dioxide reduction [109]. Here the anode is a semiconductor and the cathode is the CO2 reduction site. The compartment has a quartz window with NaOH and argon gas, while there exists a dark compartment with NaHCO3 solution with CO2 gas at a pH of 7.2. This structure avoids backflows leading to low efficiencies. It achieves, under solar lighting, a CO2-reduction rate of 154.9 mmol g1 h1 with a syngas CO/H2 between 1:2 and 5:1 and a solar efficiency of nearly 13.6%. The equilibrium

Photocatalytic process for syngas production 281

Fig. 13 (A) Double-compartment artificial photosynthetic cells to unable syngas production. (B) Schematic plasma-light-induced catalysis process for syngas by dry reforming [106,108].

between the water oxidation and CO2 reduction, occurring in separate chambers, determines the choice of electrodes. Cathodes with a low CO production overpotential give high selectivity. The strong hydrogen evolution of noble metal excludes them from consideration here, in addition to a possibility of CO conversion to methane [106]. Graphene electrodes can be used in half of the cell in the form of S/N codoped structures with atomically dispersed nickel, with concomitant high solar efficiency for CO production [106]. The photoanode harvests photons and oxidized water to protons and oxygen, releasing the electrons, which are transferred to the doped graphene electrodes, and used to reduce carbon dioxide to CO with an

282 Chapter 11 additional voltage application. Alternative photo semiconductor structures could entail N-doped TiO2 or Sn-doped BiVO4 or Sn-doped Fe2O3 [106,107]. A further development in process efficiency could be CO2 and CH4 being converted simultaneously into syngas. In 2018, a plasma-process-based photocatalysis technique was successfully studied, using a spark discharge reactor with a perovskite photocatalyst for this hybrid conversion. Syngas production rates of 13.0–18.5mol per kilowatt-hour were achieved, with a 42% improvement over the non-plasma process [108]. Fig. 13B shows the schematic of plasma photocatalytic process for syngas production by dry reforming [108]. In fact, hybrid syngas production has been studied using various techniques wherein the product of DRM, syngas, is used as a feedstock for hydrocarbon generation by Fischer–Tropsch process [110]. In the nonthermal plasma process, electrons are accelerated to collide with molecules for excitation via vibration, electronic changes dissociation ionization, and hence leading to reaction. Nonthermal plasma processes have been studied using discharges such as corona [111], dielectric barrier arc or spark discharge, or microwave [108,112]. These processes, compared with catalysis, exhibit lower greenhouse gas conversion and syngas selectivity [112]. These can be circumnavigated by combining into a hybrid process photocatalysis and plasma process, by either packing the catalyst in the discharge region or behind it [113]. Li et al. studied TiO2 photocatalyst into a dielectric barrier discharge reactor with enhanced hydroxyl radical formation [108]. In solar syngas generation, perovskites have shown several advantages such as ease of substitution into their structure, thermal stability, resistivity, tailorability of valence as well as electromagnetic properties [108]. Electromagnetic character influences photocatalysis ability of perovskites as well as their interactions with plasma. Within the ambit of a plasma reactor perovskite catalysts such as NiTiO3, LaFeO3, and AgNbO3 have been studied owing to their bandgap 100°C, CH4:CO2 ¼ 1:1 Pyrex glass reactor, 300 W, Xe arc lamp Pyrex glass reactor, Solar Xe arc lamp

Product yield

Ref.

CO yield ¼ 237.5 micromol/h/g cat, selectivity (CO) ¼ 18.9%, Selectivity (H2) ¼ 80.5% CO yield ¼ 750 micromol/h/g cat H2 ¼ 1126 micromol/h/g cat Higher H2/CO ratio obtained with MMT support. CO ¼ 237.5 micromol/h/gcat

[114]

Methyl formate: 60.5 micromol/h/ gcat, (Selectivity 41.1%) Methyl acetate: 57.5 micromol/h/gcat, (Selectivity 45.7%) H2 ¼ 71 mmol/h/gcat, CO ¼ 189 mmol/h/gcat

[115]

CH4 ¼ 1.78 micromol/h/gcat, CH3OH ¼ 0.58 micromol/h/gcat CH4 ¼ 0.22 micromol/h/gcat

[66]

[115]

[116]

[117] [118]

Fixed Bed reactor, 200 W Hg Lamp, T ¼100°C, CH4:CO2 ¼ 1:1 Quartz Fixed Bed reactor, 500 W Hg lamp, T ¼ 130°C, CH4: CO2 ¼ 1:1 Quartz Fixed Bed reactor, 125 W Hg lamp, T ¼ 200°C, CH4: CO2 ¼ 1:1 Xe lamp

CO¼780micromol/h/gcat, H2 ¼253 micromol/h/gcat

[60]

CO ¼ 11.9 micromol/h/gcat, H2 ¼ 104 micromol/h/gcat

[114]

CH4 conversion ¼ 0.54%, CO conversion ¼ 0.79%

[63]

CO ¼ 40.2 micromol/m2/h

[119]

300 W Xe Lamp

CH4 ¼ 2.17 micromol/g/h

[120]

5 W LED Lamp

CO ¼ 12.49 micromol/h/g cat H2 ¼ 2.82 micromol/h/gcat

[121]

284 Chapter 11 the ability to reduce the net energy requirement for syngas production and its efficient scale-up. While process efficiency can be improved by intelligent engineering, the crux of the success still largely lies upon the excellence of catalytic engineering, by way of novel and tailorable material platforms. It is also expected that the methane dry reforming way will gain greater importance because it handles methane and carbon dioxide simultaneously two greenhouse gases converting them to useful products and mitigating their effects on the environment. With ever greater impetus on climate considerations, the development of these technologies to aid in fruitful transition to cleaner methods is imperative.

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CHAPTER 12

Membrane technology for syngas production Samira Zafarnaka,b, 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 Energy consumption is increasing as the global population has been enhanced. More than 90% of the world’s energy consumption comes from fossil fuels, which have taken millions of years to be formed. Besides their long formation time, the increasing rate of consumption in addition to their environmental pollution impact has hunted humanity for centuries to replace fossil fuels with renewable and more environmentally friendly energy sources [1–7]. For this purpose, a lot of approaches have been reviewed by biological and chemical researchers [8,9]. Hydrogen is considered as an essential raw material in chemical industries due to its great properties. Synthesis gas, which is mainly composed of H2 and CO, is an important intermediate or feedstock in many manufactures processes [10]. Fig. 1 lists the most important chemicals produced from syngas. Syngas is a great source of hydrogen, which therefore 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 after that, the first industrial ammonia synthesis plant was built in 1913 [12]. Besides, 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. Fischer–Tropsch synthesis is still the third largest consumer of synthetic gas in which kerosene, naphtha, waxes, etc., are the main products [11]. Syngas can be made from a diversity of sources containing natural gas, coal, oxygen, carbon dioxide, or nearly each hydrocarbon feedstock [11]. From such mentioned raw materials, natural gas is the most common, as well as the lowest priced one, for syngas production [13–15]. Methane is the most important constituent of natural gas, the main source of which is reserves of oil and gas, as well as landfill gas. As a result of about 20% global warming caused by CH4 emission into the atmosphere, the development of technologies for converting methane into the valuable products is basically an essential [16,17]. Different convectional Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91871-8.00007-6 Copyright # 2023 Elsevier Inc. All rights reserved.

291

292 Chapter 12

Fig. 1 Examples of chemicals made from synthesis gas [11].

processes exist for producing syngas from methane listed as steam reforming (Eq. 1), dry reforming (Eq. 2), partial oxidation (Eq. 3), in addition to the new technologies such as plasma process [18–23]. As the synthetic gas generation is very endothermic, it needs high temperatures, and therefore, it is costly [24]. By lowering the activation energy of the desired reactions in the syngas production processes, the catalyst plays a key role for increasing reaction kinetics and achieving a maximum yield of syngas production. Several efforts have been done for developing catalytic systems that have enhanced resilience for coke formation and cheaper precursors. These attempts include the alteration of synthesis conditions and methods in addition to applying mixed support approaches and bimetallic catalysts [16,25]. CH4 + H2 O $ CO + 3H2

ΔH ð298°Þ ¼ 206:3 kJ=mol

(1)

CH4 + CO2 $ 2CO + 2H2

ΔH ð298°Þ ¼ 247:3 kJ=mol

(2)

CH4 + ½O2 $ 2H2 + CO

ΔH ð298°Þ ¼ 36:6 kJ=mol

(3)

Depending on the feedstock type, as well as applied process, for generating syngas, it has lots of impurities including hydrogen sulfide, carbon dioxide, water, halogenated chemicals, etc., which should be purified before syngas being used in chemical production processes. The composition of produced syngas from the gasification process in different references is summarized in Table 1.

Membrane technology for syngas production 293 Table 1 Composition of produced syngas in different reference. Content

Vol% [26]

Mole fraction [27]

Vol% [28]

Mole fraction [29]

H2 CO2 CH4 N2 CO

40.00 21.00 10.00 1.50 25.00

0.29 0.12 – 0.03 0.29

80.67 1.571 4.53 – 1.18

0.11 0.12 0.05 0.51 0.22

For producing high-purity H2 from syngas, established methods, such as pressure swing adsorption (PSA) and chemical amine absorption, need high capital investments, bring high operational and maintenance costs. PSA is particularly successful at high-purity H2 production. However, PSA has a drawback of low H2 recovery ( 50 nm)

The gas transport mechanisms are commonly divided into four different mechanisms in the porous materials as shown in Fig. 3: • • • •

Knudsen diffusion Surface diffusion Capillary condensation Molecular sieving

As in macroporous membranes, gas transport takes place via rather nonselective mechanisms such as viscous flow (i.e., convective flow), intermolecular collisions become dominant, resulting in a significant pressure drop in overall, requiring the application of a positive pressure gradient between the two sides of the membrane [9]. If a chemical potential (difference) across the membrane exists for various components, molecular diffusion could occur even in the absence of applied pressure gradient, as governed by Fick’s law, again, without providing much

Membrane technology for syngas production 295 Dense membranes

(a)

porous membranes

(b)

(c)

(d)

(e)

Fig. 3 Major selective gas transport mechanisms: (A) solution diffusion, (B) Knudsen diffusion, (C) surface diffusion, (D) capillary condensation, and (E) molecular sieving [35].

selectivity. If the mean free path of gas molecules is comparable to pore dimensions and the pressure is low enough, which typically occurs in the mesoporous region, the collisions between gas molecules and pore walls happen more frequently than their intermolecular collisions. Therefore, in this case, which is known as Knudsen diffusion, the diffusion rates of the gas molecules become proportional to the square root of their molecular weights (Graham’s law), providing a low degree of selectivity [35]. On the other hand in the microporous regime, the gas molecules tend to adsorb on the pore walls or surfaces and hop between the adsorption sites across the pore network (the surface diffusion phenomenon). Therefore, the gas molecules with the higher affinity between the walls would transfer much faster through membrane structure than those with lower surface adsorption capability, providing substantially higher selectivity. Besides, if one of the components in a gas mixture partially condenses in the membrane channels, it will block the passage of other components and thus gains an advantage in transport [8]. If the pore size of the membranes is in a similar range or close to the diameter of gas molecules, the molecular sieving effect can occur. As a result, smaller gas molecules could pass through the pores, while larger ones get blocked, leading to high selectivity between gas pairs. Typically, the membranes constructed from zeolite, carbon molecular sieves (CMSs), metal–organic frameworks (MOFs), covalent– organic frameworks (COFs) follow molecular sieving effect [12], which are typically good candidates for H2 separation performances. An important factor for increasing the selectivity of the sieving mechanism is the size of gas molecules. The empirically determined kinetic diameters of gas molecules serve as the basis for molecular sieving. Additionally, the quantum mechanically determined physical diameters of

296 Chapter 12 molecules, which is the largest contour value of the 3D molecular shape, including the electron clouds, should also be considered because it may differ from the kinetic diameter for calculations with different isoelectron density values, which represents the boundary of the electron cloud [11]. For a molecule with nonspherical and nonuniform electron density, the electron cloud reshapes based on the interaction with the surface. While the kinetic diameter may be highly representative for low-polarizability materials including polymers and porous carbons, the physical size of the gas may distort for materials with inhomogeneous electric fields such as zeolites. As a conclusion, it should be mentioned that to maximize the molecular separation efficiency in all cases, the pore size of the molecular sieving membrane should ideally be between the molecular sizes of H2 and other components of syngas [35].

3. Membrane material selection The selection of an appropriate material for membranes is dependent on several key factors including the feed gas composition and flow rate, the membrane material qualities, as well as the process operating conditions [33]. Each of these mentioned factors is described in detail in the following sections.

3.1 Feed gas composition and flow rate The effect of feed composition and flow rate is variable according to the membrane type. To have a better understanding of this subject, a review on the literature works on it has been conducted. Darowna and colleagues [36] studied the influence of feed composition and flow rate on the stability and fouling of a UE10 polyether sulfone ultrafiltration membrane in a photocatalytic membrane reactor. They discovered that whereas a high concentration of inorganic anion mixture had a detrimental influence on photocatalytic membrane reactors (PMRs) permeate flux, a high concentration of each inorganic anion alone has a beneficial effect. In the other study by Pishnamazi et al. [37], two promising solutions including mono ethanol amine (MEA) and MEA +[Bmim][BF4] addition for removing CO2 molecules inside the polytetrafluoroethylene-co-perfluorinated alkyl vinyl ether membrane contactor (PFAMC) were studied. The increment of the hollow-fiber density, membrane packing density, and inner fiber radius increased the possibility of CO2 molecules reacting with the utilized solution. This resulted in an increase in CO2 molecular mass transfer inside PFAMC and therefore, in its molecular removal. The results of their investigations showed that compared with utilizing MEA absorbent alone, adding [Bmim][BF4] ionic liquid to MEA (MEA+[Bmim] [BF4]) greatly enhanced CO2 molecular mass transfer and therefore its removal. Chen et al. [38] noticed that when the energy price is higher, a higher feed flowrate, a greater number of effects, and a larger membrane area are preferred. In continuity, they understand that when low-cost thermal energy is available, lower input flowrates and smaller membrane surfaces are preferred. According to Olatunji and Camacho [39], as the feed flow rate increases, the mass

Membrane technology for syngas production 297 transfer coefficient at the feed boundary layer increases, reducing the concentration polarization impact and potentially leading to membrane wetness. Several tests by Alkhudhiri et al. [40] looked into the effect of feed flow rate on heavy metal removal rejection. At constant feed and cooling temperatures, they adjusted the starting feed concentration and feed flow rate from 1 to 5 L/min. Regardless of concentration, the heavy metal rejection increased as the feed flow rate enhanced.

3.2 Membrane material properties A significant element that requires more exploration is the selection of suitable materials for certain membrane application. The nature and amplitude of interactions between permeants and membranes can be controlled by choosing the right membrane material. It determines the polymer chains that make up the solid sections of the membrane’s packing density and segment mobility. Although both material selection and membrane preparation processes have a significant effect on the mechanism of transport, membrane stability, and membrane performance, the latter impacts membrane shape, which is influenced by physical qualities such as steric hindrance on the rate of permeation [41]. Generally, membranes are classified into organic and inorganic materials depending on their morphology, material, chemical structure, nature, form, average pore size, and shape. Natural or man-made polymers are used in almost all industrial membrane processes. Wool, rubber (polyisoprene), and cellulose are the examples of natural polymers. Polysulfone (PSf ), polyethersulfone (PES), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), polyamide-imide (PAI), polyamide, polystyrene, and polytetrafluoroethylene (Teflon) are the examples of polymeric membranes for lab and industrial applications [42]. Organic membranes provide several advantages, including ease of installation, a wide range of diameters, and a low cost. They are not, however, appropriate for use under harsh situations as weak storage and membrane conservation, short lifetime, low bursting resistance, and inability to steam sterilize are all disadvantages of polymeric membranes [41,43]. Inorganic membranes, such as metals or ceramics, have superior mechanical, chemical, and thermal qualities to polymeric membranes, but they are more expensive to manufacture and hence are not preferred in large scale. Metallic, ceramic, and zeolite membranes are the most prevalent inorganic membranes. When compared with organic membranes, inorganic membranes have higher thermal and chemical stability, are resistant to microbiological degradation, and are easier to clean after fouling. Inorganic membranes, on the other hand, have greater initial capital expenditures due to the precise thickness requirements needed to sustain pressure drop variations compaction. The disadvantages of inorganic membranes are being more brittle, having lower selectivity, and being more expensive [41,43]. On the other hand, due to the difficulty of adjusting the material characteristics of membranes, it is difficult to gain a good knowledge of how membrane material properties may affect the performance of artificial photosynthesis devices [44]. The representatives of membrane material properties are permeabilities, separation factors, and selectivities [45]. According to Lin and colleagues [46], the material properties of the

298 Chapter 12 membrane affect the transport of HCO3  from the catholyte to the anolyte in artificial photosynthesis devices employing anion exchange membranes, and membranes with high ionic conductivities would reasonably be expected to contribute to low CO2 utilization efficiencies. In anion exchange membrane (AEM)-based CO2 removal reactors, the movement of HCO3  over the AEM severely reduced the device’s CO2 consumption efficiency.

3.3 Process operating conditions The operating conditions comprising flux and membrane rejection (pressure, temperature, and recirculation rate) have an effect on membrane material selection. To better understand this issue, the influence of flux and membrane rejection is explained. The influence of flux as a function of recirculation (turbulence) rate depends on different factors such as applied pressure, the porosity of the membrane and nature, the concentration, and mass of the macromolecular solute in the feed. There is no substantial difference in rejection when the solute concentration is increased. Low pressure, high recirculation rate (turbulence), and low solute content are ideal operating conditions [47]. Shahabadi and Reyhani [48] investigated the effect of membrane operating conditions on total organic carbons rejection, cross-flow velocity, temperature, transmembrane pressure, and permeation flux and resistance to fouling via using a commercial PAN350 ultrafiltration membrane. According to the study, transmembrane pressure had a substantial effect on fouling resistance and permeation flow, whereas cross-flow velocity had a considerable effect on total organic carbons removal. These findings imply that data tuning is critical to attain the optimum results depending on produced water quality and membrane operation conditions. Shen et al. [49] evaluated the impact of operating parameters for dispersing hollow-fiber membrane bioreactors. They concluded that as the permeate flux raised, the packing density of hollow-fiber decreased. The average permeate flux values were 31.3 and 20.7 L/m2.h, respectively, for packing densities of 7.52  30.08 and 45.1  160.15 threads/cm2. At somewhat low pressures, a relatively high flux was produced, and with membrane dispersion, the increase rate of pressure decreased. When the packing density is reduced, the permeate flux increases, leading in a drop in pressure.

4. Application of membrane-based processes for syngas separation/production Syngas, a mixture of hydrogen and carbon monoxide, has numerous industrial applications, including the manufacture of liquid fuels via Fischer–Tropsch, methanol, heat and power, and other chemical products [50]. The membranes can be immersed in the reactor’s liquid medium or externally linked in series with the reactor, depending on the application [51–53]. Thomas Graham was the first to propose the use of membranes in gas separation procedures [54]. The membrane separation principle is based on the differential in permeability between materials and substances. These procedures are classified according to their driving force, membrane

Membrane technology for syngas production 299 Table 2 A number of commercial uses for membrane gas separation [58]. Separation/production H2/N2 H2/CO H2/hydrocarbons CO2/CH4 H2O/CH4 H2S/hydrocarbons He/hydrocarbons or He/N2 Hydrocarbons/air Volatile organic compunds (such as C2H4 or C3H6)/light gases (such as N2)

Process NH3 synthesis Adjusting syngas ratio Recovering H2 for refineries Sweetening NGa, upgrading LGb Dehydrating NG Treating sour gas Separating He Recovering hydrocarbons, controlling air pollution Purifying polyolefin PGc

a

Natural gas. Landfill gas. c Purge gas. b

type and design, or their removal capabilities and mechanisms. Besides, it combines high evaporation efficiency, low energy consumption, simplicity of operation, and zero pollution. Membrane performance is quantified by the membrane’s capacity to prevent, regulate, or promote penetration. Numerous factors comprising the penetrating molecule’s driving force and size influence the rate of penetration and the mode of transport [55,56]. Recent studies are provided to better understand this issue. As an example, Qiu et al. [57] investigated the production of syngas using a membrane reactor fitted with a fluorine-doped barium cobaltite perovskite membrane at 900°C, achieving 99% methane conversion and 88% carbon monoxide selectivity. Table 2 summarizes the major industrial applications developed for membrane gas separation. Some of the industrial processes will be discussed in greater detail in the following paragraphs.

4.1 Hydrogen recovery Hydrogen is mostly produced by steam reforming natural gas, which produces a gaseous mixture of hydrogen, carbon dioxide, and carbon monoxide. One of the first large-scale commercial applications of membrane gas separation/production technology was hydrogen recovery. Membrane technology was first used in large-scale manufacturing in the mid-1970s when a hollow-fiber prism system for recycling hydrogen from ammonia purge gases was commercially successful, setting the stage for its widespread use. Indeed, the ammonia reactor operates at elevated pressures (approximately 130 bar), providing the necessary driving force for separation; the H2/N2 membrane has a high selectivity, and the feed gas is contaminant-free. Increasing demand for hydrogen (for hydrotreating, hydrocracking, or hydrodesulfurization processes) due to new environmental regulations is driving the development of membrane gas

300 Chapter 12 separation/production in the petrochemical industry. Prism membranes are polysulfone hollow fibers coated with a thin silicone layer. Because of their stability and unique separation factors, polyimide membranes are currently used in refineries to recover H2. Hydrogen pressure would be decreased by the use of standard membranes that let the passage of H2 while trapping other gases. Even if a CO2 compression step is added, solubility-controlled membranes that allow larger gas molecules such as CO2 and polar molecules to pass through allow the bulk gas to be kept at feed pressure. For H2 purification, hydrogen membrane rejection and contaminate permeation are currently being investigated, primarily using carbon-based membranes [58]. Also, utilizing a hollow-fiber membrane as an external gas– liquid contactor coupled to a bioreactor as the reservoir significantly increases the CO mass transfer rate owning to the fact that syngas flowing through the membrane’s lumen diffuses over the microporous membrane without forming bubbles [59]. This system’s high gas–liquid mass transfer allows for low gas flow rates and high gas conversion. The membrane could potentially sustain biofilms as they pass by the liquid stream [49,60].

4.2 CO2 separation Today, polymeric membranes for CO2 separation/production are more developed and commercially available. Due to their high flux rates and selectivity, rubbery polymers have garnered increased interest for CO2 separation/production from H2. Glassy polymers (primarily cellulose acetate and polyimides) are used extensively in industrial CO2 separation/production applications, such as the separation of CO2 from CH4, CO, N2, and other hydrocarbons [58,61]. For CO2 and H2S removal, membranes are attractive because they are permeable to these species, and treatment can be accomplished via using the high wellhead gas pressure as a driving force for separation/production. Because of the high CO2 concentrations (often >50%) and high pressures (up to 140bar) involved in enhanced oil recovery methods, membranes are an attractive option for removing CO2 from natural gas. Polymeric membranes are utilized to separate the natural gas stream from the CO2-enriched stream. The CO2-enriched stream is recompressed and fed into the wells after being compressed. Plasticization occurs as a result of high CO2 and H2S solubility in cellulose acetate, causing the polymer to swell, disturbing the polymer matrix, increasing chain mobility, and modifying membrane permeability parameters. Polyimides have grown in popularity as plasticizers due to their outstanding thermal, chemical, and mechanical stability, as well as their high CO2 selectivity and permeability [62]. As detailed in the paper by Fedotov et al. [63], the original hybrid membrane-catalytic reactor for coproduction of syngas and ultrapure hydrogen from renewable natural sources (methane and ethanol) and synthetic feedstock (dimethyl ether) is discussed. They found that raising the pressure in the hybrid membrane-catalytic reactor (HMCR) membrane mode from 2 to 5 atm increased hydrogen extraction from 20% to 80% during dry reforming of methane (DRM) in HMCR. In HMCR membrane mode, the increase in methane conversion in DRM in comparison with SRM is significant owing to the former, which is attributed to the quicker adsorption of

Membrane technology for syngas production 301 hydrogen on the palladium-containing membrane surface. At temperatures higher than 450°C, the generated methane concentration is lower than in the flow-through mode, showing that methane conversion to syngas is accelerated parallel to the flow-through mode. Thus, the yield of ultrapure hydrogen separation was boosted using a palladium-containing membrane.

5. Conclusion The importance, as well as application, of syngas in lots of industries, such as ammonia, dimethyl ether (DME), acetic acid, ethanol, and methanol, has forced industries to produce this gas via different methods such as steam reforming, dry reforming, autothermal reforming, partial oxidation of methane, gasification, biomass pyrolysis, etc., which are a few of many explored research methods. Based on the syngas production method and its feedstock type, the composition of produced syngas is different, therefore needing further purification to be used in upstream industries. Membrane technologies offer significant advantages for syngas production/purification with their typically high recovery rates, facile operation, and energy efficiency. As a result, by the use of 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. For this purpose, different factors affect the selection of used membrane such as feed gas composition and flow rate, the membrane material qualities, as well as the process operating conditions. Generally, membranes are classified into organic and inorganic materials depending on their morphology, material, chemical structure, nature, form, average pore size, and shape. While organic membranes provide several advantages including ease of installation, a wide range of diameters, and a low cost, they suffers from being used under harsh situations as weak storage and membrane conservation, short lifetime, low bursting resistance, and inability to steam sterilize. Inorganic membranes, such as metals or ceramics, have superior mechanical, chemical, and thermal qualities to polymeric membranes. However, they are more costly to manufacture and hence are not preferred for large-scale applications. It should be noted that the selection of membrane type and material is widely dependent on the produced syngas applications in which what components should be separated from. As an example, for the production of H2 end product from produced syngas hollow-fiber prism system or carbon-based membranes are widely applicable. In addition to membrane material, the operating temperature and pressure also influence the membrane efficiency. It was reported that increment in reaction pressure from 2 to 5atm has increased hydrogen extraction from 20% to 80% during membrane-based DRM.

Abbreviations and symbols AEM CMS COF DRM HMCR

anion exchange membrane carbon molecular sieve covalent–organic framework dry reforming of methane hybrid membrane-catalytic reactor

302 Chapter 12 MEA MOF PAI PAN PES PFAMC PIM PMR PSA PSf PTMSP PTFE PVDF

monoethanolamine metal–organic framework polyamide-imide polyacrylonitrile polyethersulfone polytetrafluoroethylene-co-perfluorinated alkyl vinyl ether membrane contactor polymers of intrinsic microporosity photocatalytic membrane reactor pressure swing adsorption polysulfone poly(1-trimethylsilyl-1-propyne) polytetrafluoroethylene polyvinylidene fluoride

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CHAPTER 13

Micro-channel reactor technology for syngas production Shabnam Yousefia, Mohammad Amin Makaremb, Maryam Delshaha, Hamid Reza Rahimpoura, and Mohammad Reza Rahimpoura Department of Chemical Engineering, Shiraz University, Shiraz, Iran bMethanol Institute, Shiraz University, Shiraz, Iran

a

1. Introduction Synthesis gas is a critical chemical feedstock that has been used as a source of fuels and chemicals for decades. The term “synthesis gas” refers to a mixture of carbon monoxide, carbon dioxide, and hydrogen. CO2 emissions are particularly harmful due to their high energy absorption efficiency, which is a result of their widespread release and prolonged residence in the atmosphere [1,2]. CO2 capture, sequestration, and utilization have proven to be the most efficient reduction techniques for reversing this trend [3]. Despite the success of carbon capture and sequestration (CCS), the usage of massive amounts of CO2 that have been stored has been constrained. CO2 and natural gas (NG) comprising primarily CH4 are transformed into synthetic gas. Syngas is a crucial intermediary chemical in the synthesis of valuable compounds and synthetic fuels, via a variety of processes including steam reforming (SR), autothermal reforming (ATR), partial oxidation (POX), oxidative coupling (OC), and dry reforming (DR), in order to reduce gas flaring underutilization, waste, and greenhouse gas emissions [4,5]. Syngas can be created in a variety of ways, including through the reaction of natural gas, coal, biomass, or almost any hydrocarbon feedstock with steam or oxygen [6–9]. Syngas is a critical intermediate material in the manufacture of hydrogen, ammonia, methanol, and synthetic hydrocarbon fuels [10]. The specific gravity of produced syngas varies owing to the changing molar ratios of hydrogen and carbon monoxide. Syngas can be utilized as a hydrogen source for the manufacture of ammonia, methanol, and hydrogenation in refining activities as well as a source of energy. Syngas production can be classified into two distinct processes depending on the input feedstock: one based on natural gas or hydrocarbons and the other predominantly on coal. While large coal resources are more affordable than hydrocarbon resources, the financial investment required to build coal-based syngas production facilities is three times that required for natural-gas-based plants [11]. Natural gas appears to be an Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91871-8.00019-2 Copyright # 2023 Elsevier Inc. All rights reserved.

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306 Chapter 13 excellent candidate for syngas production due to its lower fuel use. As a result, many businesses rely heavily on synthesis gas (syngas) production. This H2/CO combination is generated mostly via gasification (coal, heavy hydrocarbons) or reforming (light hydrocarbons). At elevated temperatures, steam, oxygen, carbon dioxide, or combinations of them operate as reforming agents and react with the carbon source. The conditions of the reaction and the content of the produced syngas might vary significantly between technologies, depending on the agent used [12].

2. Procedure of producing syngas In industries and laboratory investigations, reforming is regarded the most rational technique of manufacturing syngas. Numerous reforming techniques, including steam reforming of methane (SRM), methane partial oxidation (SRM), autothermal reforming of methane (ARM), and dry reforming of methane, are utilized to accomplish this (DRM) [13,14]. Table 1 compares the distinctive reforming process and the POX. Each procedure will be explained in detail in following sections.

2.1 Partial oxidation (POX) This is a non-catalytic process that produces a negligible amount of energy. Due to the presence of a variety of elements in coal, the syngas produced using this method requires reactions and purification in order to produce syngas of extremely high purity [17]. In general, it operates without a catalyst, has a low methane slip, and is more sulfur-tolerant than the other processes. The process occurs at elevated temperatures with some soot formation, and the preferred H2/CO Table 1 Comparison between different reforming processes [15,16]. Technology

Advantages

Disadvantages

SR

Most considerable experience in terms of industry There is no need for oxygen Process temperature is at its lowest point For H2 generation, the best H2/CO ratio Process temperature is lower than POX Low methane leakage Desulfurization requirements are reduced There is no need for a catalyst Low methane leakage Use of two greenhouse gases with a high global warming potential Renewable energy and other important chemicals are produced

Highest levels of air pollution

ATR POX

DR

Commercial experience is limited Air or oxygen is required H2/CO ratio is low Temperature of processing is really high The development and treatment of soot adds to the complexity of the operation Noble metals are exorbitantly costly Formation and deposition of carbon Catalysts sintering at elevated reaction temperatures

Micro-channel reactor technology for syngas production 307 ratio for inputs to hydrocarbon synthesis reactors such as Fischer–Tropsch is 1:1 to 2:1 [16]. The benefits of POX include a higher sulfur tolerance, the absence of a heat source, a compact system, and speedy startup. The disadvantages of this process include a low H2/CO ratio, the highest temperature, the generation of coke, the need for oxygen or air, and an excessive amount of heat output [16].

2.2 Hydrocarbon reforming This is a high-temperature catalytic reaction between hydrocarbons and a reforming agent. Water vapor, carbon dioxide, oxygen, or a mixture of these can be used as the reforming agent. How the syngas is made depends on what kind of hydrocarbon is used, how much reforming agent is used, how the process is run, and what kind of catalyst is used [18]. 2.2.1 Steam reforming Catalytic steam reforming of natural gas or naphtha is now used to make synthesis gas [19]. In a primary reformer, natural gas is partially reacted with steam over a Ni/a-A12O3 catalyst to create syngas with a 3:1 H2:CO ratio, as illustrated in Fig. 1. However, steam reforming of natural gas in massive multi-tubular fixed-bed reactors is currently used to produce the majority of hydrogen on an industrial scale (more than 80%) [16,20]. Hydrogen, carbon monoxide, and carbon dioxide are produced during the reforming process. Hydrocarbons’ endothermic SR requires an external heat source. While they are endothermic processes, the amount of heat generated is proportional to the amount of hydrocarbon raw materials used. SR reformate contains no oxygen and has a high hydrogen-to-carbon ratio (3:1), which is advantageous for hydrogen generation [16]. The process is highly endothermic and takes place in a fired tube furnace at 900°C and pressures of 15–30atm (Fig. 2). Because carbon can build up on the catalyst, extra steam is added to keep

CO2 and other emissions

Pure hydrogen

HT Shift Reformer LT Shift PSA

Fuel

Fig. 1 Scheme of conventional SR reaction [21].

308 Chapter 13 CH4

900ºC 15-30 atm Ni/Al2O3

H2O

1000ºC Ni/Al2O3

O2+H2O Primary reformer

200ºC Cu

400ºC Fe2O3

H2O

Secondary reformer

CO H2

H2O Water gas shift-1

Water gas shift-2

Fig. 2 Schematic representation of typical steam reforming: At 900°C, CH4 and surplus water are reacted in a primary reformer over Ni/Al2O3. In a secondary reformer, the unconverted CH4 (8%–10%) is reacted with O2 and additional H2O to produce equilibrium CO and H2. The product H2/CO ratio is then altered in two stages of water gas shift [19,23].

it clean. The H2O/CH4 mol ratios in the feed water are usually between 2 and 6, depending on how the syngas will be used [22]. 2.2.2 Dry reforming Dry reforming, the most promising chemical technique, transformed CO2 and CH4 in equal amounts and produced syngas with an optimal H2/CO ratio of around 1 [24–26]. Additionally, the DRM process is more practical than other processes since it eliminates the need for sophisticated product separation from the outflow and incorporates the creation of biogas (CO, CO2, and CH4) for the production of clean and sustainable energy [25]. As a result, if the process is fully developed and matured, it has the potential to significantly lower the entire environmental impact of syngas and the fuels and chemicals it produces. Although the DRM is characterized by the reaction between CH4 and CO2, it is also determined by a variety of secondary reactions [27]. Along with feed ratio and pressure, thermodynamic parameters impact the ideal reaction temperature and number of side reactions [28].

2.3 Autothermal reforming ATR is a method that incorporates both POX and SR. The steam-to-carbon (S/C) ratio is the amount of H2O added to the process in relation to the amount of carbon in the fuel on a molar scale [29]. ATR processes are usually thermally self-sustaining, which means they do not need to get or use heat from outside sources [30]. Each method has specific benefits and drawbacks that are crucial during the research and design process. Table 1 compares reforming technologies. Syngas obtained by the Fischer–Tropsch method can be used as a precursor in the production of synthetic petroleum for use as a fuel or lubricant. Thus, natural gas is typically the most cost-effective raw material for the production of synthetic gas, which generates excess hydrogen-rich gas in conjunction with steam and also must be in a stoichiometric ratio per unit CO or reverse hydrogen conversion unit, as 2H2/CO is compatible with synthetic gases [31].

Micro-channel reactor technology for syngas production 309 Historically, syngas has been generated using the endothermic process of steam reforming natural gas on nickel-containing oxide catalysts [32–34]. Catalytic partial oxidation of methane (POM) or natural gas to syngas has become a popular approach in recent years due to its ease of use and low cost [35]. Syngas is chemically distinct from the gases that are commonly created using low-temperature gasification processes, such as fluidized bed reactors [36]. In the meantime, one method of producing syngas is through the use of micro-channel reactors. A micro-channel reactor is one that consists of numerous small channels with a width of less than a few millimeters. The micro-channel structure has several advantages, including the following: • • • • •

Better chemical operation performance Homogeneous blending Specific temporal dominance Continuous operation Quick phase separation

Micro-reactors have garnered attention due to their superior thermal and mass transfer characteristics. However, micro-reactors have been used mostly for limited purposes, such as the small-scale manufacture of medications and other high-priced commodities. Micro-reactors’ limited capacity is one of the difficulties restricting their use [27]. While increasing the number of channels (numbering up) is one way to increase capacity, it is difficult to efficiently deploy multiple channels (1000–10,000 or more) inside the device and continually supply fluids to the channels. In order for micro-channel techniques to be used commercially, they need to be well designed and have good catalyst performance. They also need to be made in a way that is both practical and cost-effective [37]. In this chapter, the production of syngas in micro-reactors will be discussed in detail.

3. Micro-channels as efficient reaction media The phenomena of mixing are so pervasive that we encounter them on a regular basis. Mixing is used in a wide variety of applications, from chemical processes in industry to environmental research involving pollution. The critical nature of mixing in micro-scale applications cannot be overstated [38]. In comparison to macro-scale applications, micro-channels introduce unique problems for proper mixing, the primary reason being the micro-channel flow’s low Reynolds number value. Furthermore, the small and complicated dimensions of the flow-conduit in micro-channels rule out any mechanical stirrer that could help mix [38]. The development of microfluidic devices begins with the fabrication of microfluidic channels. Over time, a variety of manufacturing techniques for a variety of applications and materials have been developed and accepted. These operations make use of traditional photolithography and other time-consuming methods [39],

310 Chapter 13 etching [40], and ultrafast femtosecond laser processing [41]. Different materials are preferred for micro-channel-based devices depending on their intended use. Due to the growing need for micro-channel-based devices, a range of new hybrid ways for generating cost-effective and efficient micro-channels have been developed [42].

3.1 Micro-channel reactors fabrication Fabrication of micro-channels is a critical challenge in the context of microfluidic device development. A lot of different manufacturing methods have been tried out over the years for a lot of different reasons and with a lot of different types of materials [42]. Micro-channels can be created via a variety of substrates, including polymeric, glass, silicon, and metal. While polymeric and glass substrates are used in biological and chemical devices, electronic and mechanical engineering utilizes silicon-based and metallic substrates [43]. Throughout the years, micro-channels with unique cross sections have been created. Micro-channels are divided into two types: rectangular micro-channels and square micro-channels. This is a common misunderstanding [44], circular micro-channels [45], half circular micro-channels [44], U-shape micro-channels, and Gaussian beam micro-channels [46] are the most frequently used cross sections. Except for circular cross-sectional micro-channels, on the surface of the base material, various types of micro-channels are produced. Typically, circular micro-channels are produced inside the bulk of the material. For optical and viewing clarity, a few microns of thickness are normally left on top of the circular cross-sectional micro-channels along the length [45].

3.2 Materials Numerous materials have been employed to fabricate various micro-channel-based devices. These compounds can be categorized into three broad sections: 3.2.1 Polymeric and glass substrates Due to the low cost of polymeric substrates in recent years, many microfluidic instruments have been made using polymeric substrates instead of silicon or glass [47]. Additionally, their low cost enables them to be manufactured as single-use devices. The primary polymeric substance used in microfluidic devices is polymethyl methacrylate (PMMA) [48] and poly dimethyl siloxane (PDMS) [49]. Due to the inexpensive initial investment and associated simpler manufacturing procedures, PMMA has developed as a technology for a wide variety of microfluidic instruments in recent years. PMMA is a translucent material that is frequently used in place of glass. It can be employed as a drug delivery system or an implant, and due to a number of its properties, it has a significant amount of biocompatibility [50]. Fabricating micro-channels on glass is a significant challenge for conventional fabrication techniques. The process entails a succession of steps, which increases the total cost of the

Micro-channel reactor technology for syngas production 311 procedure significantly. Nonetheless, creating micro-channels on glass substrates might be a lengthy process. Due to its high degree of transparency, inertness to most substances, and ability to maintain a more stable temperature, it is the preferred material for the majority of microfluidic instruments [51].

3.2.2 Metallic substrates Micro-technology-based energy and chemical systems (MECS) that use micro-scale characteristics to increase their surface area to volume ratio may aid in increasing the heat and mass transfer rates in heat exchangers. Numerous endothermic and exothermic processes are also permissible on such metallic substrates, and temperatures can reach as high as 650°C [52]. Metals such as stainless steel are becoming less popular for MECS devices because they are not as durable as they used to be [53].

3.2.3 Semiconductors, ceramics, and composites Silicon constituted the foundation for the majority of early discoveries in micro-channel applications [54] and was originally developed for the integrated circuit (IC) manufacturing industry [55]. Due to their increased performance, high-speed digital circuits with a large number of logic gates should be cooled more quickly. These devices are cooled using either forced air convection or liquid-based micro-channel technology. Because forced air convection cooling is not very efficient, liquid cooling has been shown to be better [56]. Silicon is used in more than just microelectronics; it is also used in fuel cells [57]. In addition to silicon, quartz is another material that can also be utilized as a substrate for a variety of micro-channel-based devices. Quartz is sometimes a better choice for certain applications than silicon since it is (i) chemically inert and stable, (ii) optically clear, (iii) less expensive, and (iv) a good electrical insulator [58]. Gallium nitride is another significant semiconductor material that has risen in popularity in recent years due to its wideband crevice properties (GaN). Other optical devices, such as light-emitting diodes (LEDs) and laser diodes, can also be made from this material. Gallium nitride is another critical semiconductor material that has been resurrected in recent years as a result of wideband crevice (GaN). This material may also be utilized in other applications such as optical devices (LEDs and laser diodes) [59]. Due to the fact that metal materials are commonly used in the fabrication of micro-channel reactors [60], ceramics will outperform their metallic counterparts in terms of performance [61]. As ceramic reactors can be manufactured by utilizing low-cost materials and high-throughput fabrication technologies, they are more economic materials for large-scale industries. Without additional material preparation, the high-surface-area ceramic materials that typically support catalysts adhere effectively to the micro-channels of the ceramic reactor [62].

312 Chapter 13

3.3 Arrayed micro-channel manufacturing Micro-channel-based systems have the potential to revolutionize the electrical and chemical processing sectors. Parallel rows of micro-channels in chemical reactors make them easy to tell apart. Micro-channels have typical dimensions of 0.1–5.0mm. By shortening the heat and mass transfer distances, the process is expedited 10–1000 times, lowering the resistance of the process fluid to the channel wall. When compared with conventional reaction unit hardware, the volume of the reactor can be reduced by a factor of 10 or more. These micro-reactors can be used to cut down on the cost of preparation equipment, to keep the structure of the product in line, and to improve the performance of the structure [37]. Chemical and energy industries gain from micro-channel innovation because it increases production and potency. As demonstrated in Fig. 3, the expanding scale numbering technique decreases the danger, while keeping the characteristics of reaction physics and channel flow hydrodynamics. This may vary based on the scale at which routinely applied modifications alter the reaction mechanics and hydrodynamics. Nevertheless, because each of the hundreds of micro-channels in a reactor must be the same size so that current can flow and the reactor can work properly, the numbering method has its own set of problems [63]. Various micro-channel manufacturing methods could be used to create small, identical entries, depending on the material used. Most large-scale programs, on the other hand, need the use of ferrous alloys or similar compounds. Micro-channel lamination, also known as miniaturized scale lamination, is one of the technologies being pursued for industrial-scale micro-channel reactors. Micro lamination is the process of interlacing (stacking) and joining (connecting) thin sheets of material (laminae or shims) with micro-channel designs to create multiple parallel micro-channels. Fig. 4 depicts the phases of a miniature scale cover. This strategy is appropriate Micro-channel

Conventional

Increasing Scale

Increasing Scale

Critical dimensions remain the same

Critical dimensions change along with process physics

Number up

Scale up

Fig. 3 Critical parameters stay constant in micro-channel numbering-up [63].

Micro-channel reactor technology for syngas production 313

Plating

Shim Pattering

Stacking

Bonding

Machining

Fig. 4 Laminate construction technique to produce micro-channel reactors [64].

for cost-effectiveness, adaptability design tailored to a single component’s complex collection of chemical unit processes, and the resistances required to shape adequately uniform inner routes. In addition to developing more cost-effective processes, other advantages of micro-channel fabrication advancement include [63] the following: i. ii. iii. iv. v. vi. vii.

Membranes and other inhomogeneous layers are merged. Utilize retentive coatings to reduce corrosion and coking. New methodologies for the degradation and repair of catalysts. Fluidic sensors and actuators are being consolidated. Predict whether or not a production performance issue will fail. Tools for laying out manufacturing capabilities have evolved over time. New methods of machine equipment development, such as novel forming, deposition, and furnace equipment. viii. Strategies for building more complicated pathways—in addition to rectangular and circular. ix. Plants and pipelines are integrated with micro-channel components.

3.4 Micro-channels development program While methods for manufacturing micro-channel arrays are now available, each stage of the manufacturing process can be improved to lower manufacturing costs and enhance the quality of micro-channel reactors, consequently expanding renewable energy markets and increasing their productivity. The process begins with the sourcing of materials and continues through area setup, which includes the connection of micro-channel components to industrial-scale plant piping. Indeed, despite the fact that additional contemplation is necessary to develop a precise strategy, the parts beneath highlight the primary difficulties associated with micro-channel fabrication [63]. Material sourcing: Micro lamination necessitates the stacking of hundreds of shims stacked on top of the other. This means that a tiny change in thickness can lead an asymmetric reactor.

314 Chapter 13 Additional material resource issues associated with grain size estimates, such as grain size estimation, compound similarity, and surface conditions, can have a significant impact on downstream design and holding. Current alloys created specifically for micro-channel devices, which simplify stacking and joining and allow high-temperature activities, could be investigated as part of assisted development efforts [63]. Corrosion protection and passivation: Reactors on a smaller scale accelerate chemical reactions. While this acceleration normally favors desired products over by-products, it can accelerate corrosion and initiate side reactions on occasion. There are a few treatments such as surface treatment, which have been shown to be effective for controlling erosion and suppressing adverse effects (such as coking) via aluminization passivation [65]. However, owing to the complication of a micro-channel device’s internal parts, it is difficult to apply coatings completely and uniformly. Numerous more surface treatments are required to modify the chemical and thermal properties of surfaces. These surfaces can be employed in bundling processes that need a high temperature to modify the surface properties. Bundling these surfaces requires modern methods [63]. Catalyst application: Numerous catalyst configurations are compatible with micro-channel design including packed beds, engineered supports, and wash coatings. In any event, several specific challenges arise in terms of accurately and consistently employing the catalyst within the reactor, along with restoring and replacing the catalyst. Enhancement is necessary to overhaul catalyst integration, extend the life of the catalyst, and advance ways for refurbishing or replacing the catalyst [63].

4. Micro-channel applications Micro-channel innovation is a versatile stage that has the potential to make substantial strides in a variety of applications. Table 2 depicts some applications of micro-channels and their associated benefits.

5. Syngas production in micro-channels For a few really quick reactions, the system is mass-transfer limited. In other words, the pace at which the product is formed is determined by the movement of the reagents and not by the reaction’s velocity. On the other hand, certain reactions are slower and are kinetically constrained. Micro-channels appear to have the best performance for mass-transfer regulated reactions between gases [43]. Assuming that the high surface areas allow for exact energy exchange during reactions, a splitting of the channel diameter across quadruples the reactor intensity in terms of the number of reactions that can be carried out inside a given volume of reactor. This makes a strong case for advanced energy generation frameworks, as well as fuel cells [67].

Micro-channel reactor technology for syngas production 315 Table 2 Micro-channel applications, their related advantages, and their innovation state [66]. Application

Advantages

Conditions

Biofuels (Fischer–Tropsch)

Assist in the manufacture of biofuels at a low cost

Hydrogen (steam methane reforming) Ethylene (oxidative dehydrogenation) Vinyl acetate monomer and others (selective oxidation) CO2 concentrator and other separations (thermal swing absorption) Ocean thermal energy conversion and others (heat exchangers) Hydro processing biofuels and heavy hydrocarbons

Increased energy productivity and the prospect of offshore installation Allows for the creation of a novel pathway and a 20% increase in energy Enhancement of selectivity and reduction in energy usage Separations proceeded more efficiently due to lower energy waste

A small commercial exhibit will open in mid-2011, followed by a demonstration Extensive laboratory experiments validated the benefits Benefits confirmed by laboratory tests Benefits have been established through laboratory experiments Individual components operated separately

C2 splitter and others (distillation) Personal care and others (emulsions)

Reduced size and pressure drop have taken a toll on the whole system Selectivity has improved, and the requirement for abundant hydrogen has decreased Significantly reduced height enables the addition of additional stages and increased separation Emulsions that are extremely stable without over-sheering

Modeling enables key enhancements to be demonstrated Benefits have been established through laboratory experiments Benefits established through labscale experiments Illustrations set in a commercial environment

Due to their uncomplicated and efficient design, fixed-bed reactor systems were chosen as the typical solution for synthetic natural gas generation. Nonetheless, honeycomb and micro-channel reactors were deemed more ideal in terms of catalyst exposure within the reactor [68]. As mentioned before, there are various methods to produce syngas. In the following, the studies on producing syngas by reforming and gasification processes in micro-reactors will be reviewed.

5.1 Reforming A study was conducted on the use of chemical looping modifications to the production of gas in a novel micro-reactor comprising two reactors, one exothermic (oxidation) and one exothermic (reduction) [29]. First, two adiabatic reactors were modeled. However, on the reduction side, more heat was lost within the solid support compared with the oxidation side. It may also reduce hotspot risk by no longer conveying all of the heat required to generate synthetic gas. As seen by the temperature distribution inside the reactors, the same temperature was reached across the basin, including the oxygen-carrying layers in the reduction and oxidation channels.

316 Chapter 13 Less recent reactants and ungenerated oxygen carriers were available near the reactor’s input start and during the overall reaction time. Contrary to conductivity, convection is the predominant mechanism of heat movement. A low H2/CO ratio of less than 3 can be produced by introducing CO2 into the feed, for example, via posttreatment synthesis gas. A 38% decrease in H2/CO led to a 100% increase in CO2/CH4. Increasing CO2 had no discernible effect. More oxygen carriers were transformed on both sides of the channel wall, increasing oxidation and hydrogen production. The effect of reducing the channel size or effective reaction surface on the overall reactor volume was studied. This shows that as the channel size rises, the reactor’s effective surface area decreases, reducing heat and mass transfer. Another study looked into the mathematical modeling of synthesis gas production. First, two distinct adiabatic reactors were modeled. As a result of this, while the oxidation heat was liberated, more heat was distributed within the solid support on the reduction side. As a result, hotspot danger was reduced, and the heat required to produce the synthesized gas was supplied. The temperature distribution inside the reactors showed that the same temperature was attained in all sections of the basin, including the reduction and oxidation channels, where the oxygen-carrying layers were present. The results depicted that the highest rate of reactions occurred at the start of the reactor input and throughout the reaction period, where large volumes of new reactants and ungenerous oxygen carriers were available. Thermal analysis showed that convection outperforms conductivity in heat transfer. Using posttreatment synthesis gas, a low H2/CO ratio below 3 can be achieved, which is used for the gasoline to liquid (GTL) operation. A 38% decrease in H2/CO led to a 100% increase in CO2/CH4. Adding CO2 has little influence. The higher the thermal conductivity of the channel wall, the more oxygen carrier conversion, oxidation, and hydrogen production. The effect of lowering the channel size or the effective reaction surface to the full synthetic gas reactor volume was studied. This means that as the channel size grows, the reactor’s effective surface area shrinks, reducing heat and mass transfer. Another study focused on synthesis gas generation mathematical modeling [69], using the perovskite oxygen carrier La0.7Sr0.3FeO3, a chemical ring method was developed. A dynamic approach was used to construct a fixed-bed micro-reactor. Throughout the solid particle reduction process, mass transfer and catalytic and non-catalytic reactions were taken into account. The solid-state reaction mechanism for each of the physical principles studied in their research is shown in Table 3 [69]. Table 3 Integral forms for gas–solid kinetics models. Model Order-zero (Polanye Winger equation) Controlled reaction using phase boundaries (2D) Reactions governed by phase boundaries (3D)

Model abbreviation R1 R2 R3

f ðx Þ5 1k

dx dt

1 2(1  x)1/2 3(1  x)3/2

g(x) 5 kt x 1  (1  x)1/2 1  (1  x)1/3

Micro-channel reactor technology for syngas production 317 Among 21 gas–solid kinetic models that were congruent with experimental data, the most effective model with adequate statistical criteria was chosen. It was established that one-parameter models of R1 and R3 behaved favorably. However, due to its performance across all kinetic parameter assessments, R3 was picked. The reduction of perovskite under isothermal conditions has been proposed using a solid–gas system with a catalytic process. The model’s kinetic parameters were determined using data from a fixed-bed reactor. As a result, whereas a complete reduction of La0.7Sr0.3FeO3 took around 2500s, only 50s was sufficient to completely eliminate the NiO particles. Numerous essential parameters have been tested in the reactor, including reactant conversion and product compounds. Additionally, the H2/CO ratio and carbon composition were studied. The results indicated that in order to avoid the creation of carbon in the reactor, the reduction should be halted shortly after 500 s. It has become recognized that a range of approximately 50–300 s may be beneficial for maintaining a low H2/CO ratio. For the current kinetic investigation, isothermal conditions were established. However, thorough kinetic analysis, as well as determination of the temperature dependence of reaction coefficients and activation energy, can be carried out via experiment at various temperatures. Additionally, the stability of the oxygen carriers, particularly at some point during the scale-up process, can be evaluated using a comprehensive model. The following research examined ethylene glycol (EG) as a feedstock for synthetic gas production by SR and oxidative vapor reforming in micro-channel test reactors [70]. At a steam-to-carbon ratio (S/C) of 4.0, a reaction temperature between 625°C and 725°C, and an atmospheric pressure at volume hourly space velocities (VHSV), the product composition should be between 100 and 300NL/h (gcat.h). Catalysts with varying rhodium loadings were assembled and characterized using two independent fabrication processes (conventional saturation and synthesis of individual nanoparticles). These catalysts were evaluated in micro-structured reactors under EG gas vapor phase correction conditions. It has been widely documented that while dealing with micro-reactors, the restrictions of heat and mass transmission are removed, transforming them into the most impactful system for investigating rapid and extremely exothermic or exothermic reactions [71]. Additionally, operating micro-reactors allows for volume hourly space velocities (VHSVs) than conventional fixed-bed reactors, intensifying the process [70]. Additionally, heat provision is far more efficient in a micro-channel heat exchanger than with conventional technology. Catalysts containing Rh have been obtained using a variety of different preparation procedures, and their activity in steam modification and oxidative vapor modification of ethylene glycol has been evaluated. While samples prepared using the usual saturation process often exhibit greater activity than catalysts derived from Rh nanoparticles, all samples, independent of oxygen content, yielded by-products of species such as acetaldehyde, ethane, and ethylene. Additionally, considerations were made to [36] determine that small methane stack micro-reactors for synthetic gas production are unstable due to external heat losses, which

318 Chapter 13 contribute to combustion channel failure [72]. Parallel plate micro-reactors with alternating combustion channels and steam correction with platinum and rhodium catalytic surfaces were simulated using computational fluid dynamics (CFD). Using a previously reported micro-kinetic model, the single-stage rate expression for hydrogen combustion on platinum is determined. Stability is increased by burning hydrogen or charging the platinum catalyst in the outer combustion channels. These findings should apply to other nonoptimal systems on a small scale. Finally, many stabilization measures are inefficient and unproductive. Stability, efficiency, and placement of small micro-reactor depots that convert natural gas to synthetic gas have all been evaluated. In most channels, increasing catalyst load or using a more reactive fuel such as hydrogen or synthetic gas improves stability. However, increasing the input temperature (via heat recirculation) dramatically increases throughput through placement. Last but not least, hydrogen combustion in the outside channels improves maximum stability but reduces efficiency. A micro-reactor approach based on relative n-butane and propane oxidation was also investigated. In this work, CFD simulation of parallel plate micro-reactors with alternating combustion channels and steam correction with platinum and rhodium catalytic surfaces was used to investigate stability improvement strategies. The formulation of the single-stage rate for hydrogen on platinum combustion is obtained from a previously published micro-kinetic model. Stabilization is best achieved by burning hydrogen or charging the platinum catalyst in the outer combustion channels. Not all systems operate well on a small scale. Finally, several strategies for strengthening stability are inefficient and ineffective. Stability, efficiency, and placement of small micro-reactor depots that convert natural gas to synthetic gas have been investigated. In most channels, increasing catalyst load or using a more reactive fuel such as hydrogen or synthetic gas improves stability. Increasing catalyst load has no influence on placement efficiency, while increasing inlet temperature (through heat recirculation) greatly boosts placement efficiency. In the outer channels, hydrogen combustion increases maximum stability but reduces efficiency. n-butane and propane were also examined in a micro-reactor design for high gas efficiency [73]. A doped rhodium catalyst placed in a foam-like ceramic substrate inside a micro-reactor was used to execute the CPOX reaction. Under continuous heating settings with tspace ¼ 40–44 ms, the micro-reactor was capable of producing up to 60% n-butane and 50% propane at ideal C/O ratios of 0.7 and 0.8. Sufficient gases (7.2 and 6.8 mmol/s, corresponding to 1.85 and 1.76 watts for n-butane and propane, respectively) were generated to power a micro-SOFC device. These experimental data indicate that it behaves similarly to thermodynamic equilibrium, particularly for gas mixtures near stoichiometric partial oxidation conditions. Additionally, it was demonstrated that the micro-reactor is capable of reaching a steady-state temperature and producing thermally stable synthetic gas. However, the synthesis gas efficiency for n-butane was lowered by 39 drops in this scenario due to poor heat distribution across the reactor, increased heat loss by the reactants, and shorter residence time. Despite its higher heat loss, the self-sufficient reaction is the most efficient heating mode when the entire reactor’s heat loss is considered. In all of the trials, the micro-reactor substrate demonstrated excellent thermal and catalytic stability. By and

Micro-channel reactor technology for syngas production 319 large, the catalytic results of this study indicate that this novel micro-reactor is well suited for high fuel conversion and high synthesis gas efficiency. Apart from its intrinsic usefulness, the current micro-reactor has been created and tested for integration with a micro-SOFC membrane and a combustion after the integration of the first portable micro-SOFC power plant.

5.2 Gasification Hu et al. [74] looked into the potential of a micro-channel reactor for producing alcohols and C+2 oxygenates from biomass-derived syngas. They chose supported Rh catalysts to connect with a micro-channel reactor for this project. Temperature, pressure, H2/CO ratio, and activation procedures were all studied to see how they affected the activity and product selectivity of supported Rh catalysts. It was observed that the creation of the unwanted product, methane, can be inhibited under low temperature, high pressure, and a low H2/CO ratio. Interestingly, different catalysts reacted differently to the reduction–oxidation cycle therapy (RedOx). A hybrid catalyst system containing CuZnAl and Rh Mn/SiO2 continued to expand in activity after being treated with RedOx. A significant increase in specific activity was seen when the Rh-Mn/SiO2 catalyst was coated on a FeCrAlY metallic felt substrate and then inserted into a micro-channel reactor, showing a high potential for commercial process intensification. Chaudhari et al. [75] examined steam gasification of two biomass-derived chars in a fixed-bed micro-reactor at temperatures of 700°C, 750°C, and 800°C with steam flow rates ranging from 1.25 to 10g/h/g of char. Following their investigation, they came to the following conclusions: At temperatures between 700°C and 800°C, steam gasification of biomass-derived chars can convert them to hydrogen and/or synthesis gas, with steam flow rates ranging from 1.25 to 10 g/h/g of char. Steam gasification at a temperature of 700°C and a steam flow rate of 1.25–10 g/h/g of biomass-derived char can create hydrogen with a molecular weight of up to 75%. Steam gasification of bagasse and commercial char produced synthesis gas (H2 + CO) in the 80–88mol% and 77–84mol% ranges, respectively. Steam gasification at temperatures between 700°C and 800°C with steam flow rates ranging from 1.25 to 10 g/h/g of char can yield the synthesis gas, which has a very high H2/CO molar ratio of 4–7 for bagasse char and 4–15 for commercial char. The findings suggest that a simple steam gasification process for producing hydrogen and syngas from biomass generated chars has a lot of potential. In a new micro-reactor intended for exploring the inherent chemical kinetics of extremely exothermic processes, Federici et al. [76] investigated the oxidation of synthesis gas over a supported Pt/Al2O3 catalyst. In the absence of CO, the H2 reaction occurred at room temperature. However, when both CO and H2 were available, H2 conversion took place only after significant CO conversion had taken place. CO was inhibiting H2 catalytic oxidation sites, as evidenced by this. Hydrogen, on the other hand, had a more subtle impact on CO oxidation kinetics. Adding hydrogen to a CO input boosted CO oxidation at low hydrogen-to-carbon ratios (H2:CO¼1). At higher mole fractions relevant for natural gas reforming, H2 had a minor enhancing effect on CO oxidation that is independent of the hydrogen mole reaction. As the mole percentage of

320 Chapter 13 H2 increased, the reaction sequence of CO oxidation with regard to H2 changes. At low mole fractions, positive order kinetics had been seen (up to a H2:CO ratio of 1:1). The CO reaction rate remained constant as the hydrogen mole percentage in the mixture grows at greater hydrogen mole fractions. When the H2 mole fraction was changed, hysteresis was also seen. Piermartini et al. [77] used a laboratory-scale micro-structured reactor to examine how to improve the H2/CO ratio in synthesis gas produced from biomass feedstock through the use of the water gas shift reaction at high pressure and temperature. The usual synthesis gas composition from dry biomass gasification was recreated using a model combination of carbon monoxide, carbon dioxide, water, and hydrogen. The performance and properties of previously examined catalyst layers created utilizing a mix of incipient wetness impregnation and sol–gel technology were compared using an incipient wetness impregnation of commercial ceria to help scale-up. At temperatures ranging from 400°C to 600°C and pressures up to 45 bar, the catalytic activity of these Pt/CeO2 powders was compared with that of a commercial high-temperature iron water gas shift catalyst. As the pressure grew, CO conversion increased, resulting in the creation of hydrocarbons (such as CH4 and C2H6) and coke. Due to their larger surface area, catalyst coatings had better catalytic activity than powders. Tavasoli et al. [78] examined the production of hydrogen and syngas in a continuous down flow fixed-bed micro-reactor by gasifying corn and wheat dry distiller grains (DDGS) with oxygen. The effect of reaction length (15–45min), reactor temperature (700–900°C), and oxygen-to-nitrogen ratio (0.08–0.2 vol./vol.) on the composition of the generated gas, yield, low heating value (LHV), and carbon conversion efficiency was investigated in a series of tests. The results appeared to indicate that the operating conditions for both biomasses were optimized for a gasification temperature of around 900°C, an oxygen-to-nitrogen ratio of 0.08, and a reaction time of 30 min, resulting in some gas rich in hydrogen and carbon monoxide, but deficient in carbon dioxide and hydrocarbons over the range of experimental conditions used. The results showed that maize DDGS gasification produces much more H2 and CO (11% and 56.5%, respectively) than wheat DDGS gasification (10.5% and 51.5%). Furthermore, gasification of corn DDGS boosted gas output (0.42 m3/kg), LHV (10.65 MJ/m3), and carbon conversion efficiency (44.2%).

6. Conclusion Historically, analytical chemistry has made by the use of micro-reactors. Micro-reactors have demonstrated exceptional processability in a variety of applications, including the synthesis of inorganic, metal nanoparticles, and organic molecules, which have already been applied in pharmaceutical and chemical engineering in a flexible and controllable manner due to their unique physical and chemical properties. Until now, these micro-structured reactors have been extensively researched as micro-mixers and micro-separators. As a result, tiny structures with variable degrees of porosity, such as channels, have the potential to significantly assist species diffusion. More research is needed to develop and build micro-fabricated reactors with the

Micro-channel reactor technology for syngas production 321 optimal channel structure and connections, channel physical properties, and channel volume for increased yield and selectivity in diverse systems. Multifunctional nanomaterials may be used into micro-reactors to enhance their functionality and performance in high-efficiency hydrogenation applications. Inspired by the direct fabrication of micro-reactors from a variety of materials, it would be ideal to invest much effort in the wall-coated micro-reactor by selecting suitable surfactants and optimizing polymerization conditions. Increased surface reaction would enable us to reconnect functional groups to newly made micro-reactors contained within nanocomposites or micro-channels. Significant progress has been achieved in the investigation of composite-based micro-reactors. It has been demonstrated that micro-reactors with additional functions can be manufactured immediately utilizing defined pore zeolite or ceramic foams. Pilot-scale micro-reactors are especially advantageous when rapid reactions are required, the outcome is highly dependent on mixing quality, and interactions at elevated temperatures and pressures are examined. While the studies suggest that micro-channel conversion of synthesis gas to methanol, DME, or Fischer–Tropsch products is a viable small-scale process, synthesis gas generation technology is still a challenge. The data supplied sheds light on why this is the case. In terms of temperature profile, product selectivity, carbon generation, and cycling stability, micro-channel monoliths have shown promise in partial oxidation and oxidative steam reforming of lower hydrocarbons. This needed the employment of a high-temperature alloy and a method for creating the alumina layer. Furthermore, the data suggest that gas phase reactions at high temperatures could be slowed. Metal dusting corrosion is a major concern that could harm micro-channel reactors for methane steam reforming with better heat exchange geometry. This is owing to the increased surface area of the catalytic reaction volume, as well as interactions between the surface structure and composition and the (local) reaction environment that may exist within or near the catalytic reaction volume. This is demonstrated by research on carbon formation on alloy 800, which appears to be a suitable material choice. Finally, in Fischer–Tropsch synthesis, micro-channel reactors allow for the use of highly active catalysts and process conversions of up to 90% without thermal runaway or considerable catalyst degradation. Furthermore, our findings reveal that pressure has an effect on C5+ selectivity in a micro-channel reactor over 40%Co-1%Re-γ-Al2O3 under particular conditions. Scaling up the micro-channel technique for Fischer–Tropsch synthesis has been demonstrated to be promising by changing the channel length or adding more channels. Velocys has also demonstrated the ability to rebuild cobalt-based catalysts in situ following a lengthy process. At the end, it should be considered that when utilizing micro-reaction technology, numerous technical concerns must be resolved, including how to connect the equipment, how to manage fluid and process parameters, how to integrate and supervise, and how much the technology costs. Micro-reaction technology is certain to advance and develop into a new environmentally friendly processing method in the near future.

322 Chapter 13

Abbreviations and symbols CPOX DDGS EG GaN GTL IC LED LHV MECS OSR PDMS PMMA POM SNG SOFC SR VHSV

catalytic partial oxidation dry distiller grains ethylene glycol gallium nitride gas to liquid integrated circuit light-emitting diode low heating value micro-technology-based energy and chemical systems oxidative steam reforming poly dimethyl siloxane poly methyl methacrylate partial oxidation of methane synthetic natural gas solid oxide fuel cells steam reforming volume hourly space velocities

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CHAPTER 14

Plasma technology for syngas production Yaolin Wanga, Ni Wanga, Jonathan Hardinga, Guoxing Chenb, and Xin Tua a

Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, United Kingdom Fraunhofer Research Institution for Materials Recycling and Resource Strategies IWKS, Alzenau, Germany b

1. Introduction Over the past century, the global utilization of fossil fuels has increased significantly due to the growing global population. As a result, energy-related CO2 and CH4 emissions, referred to as greenhouse gases, are soaring, they have destructive effects on the environment by inducing climate change and global warming. Thus, it is imperative to develop efficient technologies to address the problems associated with increasing CO2 and CH4 emissions. Recently, plasma technology is attracting increasing interest in this regard because of the highly reactive feature of plasma, which is an ionized gas and consists of various reactive species such as electrons, ions, excited species, radicals, and photons. These energetic and reactive species enable reactions that are thermodynamically unfavorable to proceed at low temperatures and atmospheric pressure. In addition, since plasma is typically driven by electrical power, it can be easily switched on and off, offering great flexibility to integrate with renewable electricity, particularly intermittent renewable electricity for decentralized chemical production and storage. Among the various strategies for tackling the global energy challenges, dry reforming of methane (DRM) and CO2 to syngas (a mixture of H2 and CO) has attracted significant interest (R1) as it can utilize both CO2 and CH4 from various sources (e.g., shale gas, biogas, and landfill gas). Syngas is an essential chemical feedstock for producing various platform chemicals and synthetic fuels, such as liquid hydrocarbons via the Fischer-Tropsch process. Nevertheless, due to the highly stable nature of CO2 and CH4 molecules, high temperatures (>700°C) are always desired for the activation of CO2 and CH4 in the conventional thermal process. Consequently, the sluggish thermodynamic kinetics of this process limits the conversion of CO2 and CH4 and the yield of syngas at low temperatures. In addition to high energy consumption in the thermal process, catalyst sintering and coke deposition cause catalyst deactivation at high temperatures, thus limiting the implementation of this process on a commercial scale. Therefore, emerging technologies that can be operated efficiently under mild Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91871-8.00014-3 Copyright # 2023 Elsevier Inc. All rights reserved.

327

328 Chapter 14 conditions have attracted increasing attention. In this endeavor, nonthermal plasma (NTP) stands out as a promising and potential alternative to thermal catalysis for DRM to syngas as this process can be operated at atmospheric pressure and low temperatures [1]. CO2 ðgÞ + CH4 ðgÞ ! 2CO ðgÞ + 2H2 ðgÞ, ΔH° ¼ 247 kJ mol1

(R1)

To date, significant progress has been achieved in the investigation of plasma-based DRM using a variety of NTP systems with different discharge properties and operating conditions (e.g., temperature and pressure) [2–6]. The type of the plasma system can influence the distribution of the products, which we will discuss in the following sections. Meanwhile, the effects of different processing parameters (e.g., dilution gas, input power, flow rate, gas ratio, packing configurations, etc.) on the plasma DRM processes have been investigated in terms of the gas conversion, the selectivity, and yield of reaction products, as well as the energy efficiency. To further tailor the DRM reaction to boost the reaction performance, the coupling of plasma and catalysts, known as plasma catalysis, has been broadly investigated for the DRM reaction. Generally, adding an appropriate catalyst into the discharge region reduces the activation barrier for certain reaction routes and tailors the plasma properties at the same time, this can lead to multiple impacts that affect reaction performances. In addition, the plasma-catalyst coupling has great potential to generate a synergistic effect resulting from the physicochemical interactions between the plasma and catalysts, which can enhance the performance of the hybrid plasma-catalytic process [7,8]. This chapter will review the recent advances in DRM for syngas production using the emerging NTP plasma technologies. The commonly used NTP systems for chemical processes will be introduced briefly. We will discuss the influence of different processing parameters on the performances of the plasma DRM process. The recent development of supported metal catalysts for plasma-catalytic DRM will be reviewed and highlighted. We will also outline the challenges and future perspectives for further development in this emerging area to establish NTP technology as a viable solution for achieving the net-zero economy.

2. Different NTP reactors for DRM Although thermal plasmas have been used for DRM, this chapter will only focus on the dry reforming of CH4 using NTPs. Rapid progress has been achieved in NTP technology over the last two decades, especially for atmospheric pressure NTPs, showing great advantages in a variety of chemical processes. Therefore, in the following sections, a brief introduction of NTPs used for the dry reforming of CH4 to syngas will be presented, such as corona discharge [9–12], glow discharge [13–19], microwave discharge [20–23], gliding arc discharge [4,24,25], and dielectric barrier discharge [26,27].

Plasma technology for syngas production 329

2.1 Corona discharge Corona discharges can be produced via alternating current (AC) or continuous/pulsed direct current (DC) at atmospheric pressure. Fig. 1A shows the geometry of a typical point-to-plane pulsed corona discharge reactor [28]. Corona discharges comprise many streamers that derive from the sharp points or edges of the electrode when a sufficiently high electrical field is applied. Corona discharges are always inhomogeneous, and their characteristics can be changed when varying the electrode geometries and parameters of the pulsed generators (Fig. 1B) [29]. Corona discharge can be divided into the positive corona, negative corona, bipolar corona, or AC corona. Previous studies demonstrated that positive corona has higher gas conversions and CO selectivity for the plasma DRM reaction compared with negative corona due to the large active volume and high electron energy [9]. While the negative corona is favorable for enhancing the H2 selectivity, resulting in a higher H2/CO ratio than that using the positive corona. Li et al. [10] found that the DRM performances for syngas production in different corona reactors follow the order of positive corona > AC corona > negative corona. However, they also reported that the effect of different corona discharges on the H2/CO ratio in the products exhibited an opposite trend: negative corona>AC corona>positive corona. They attained remarkable CH4 (90%) and CO2 (74%) conversions using a positive corona discharge [10]. Dai et al. [12] reported a high CO yield of 33% in DRM using a pulsed corona discharge reactor. They found that higher pulse voltage and repeated frequency gave higher gas conversions. Corona discharge is always nonuniform, with a high electron density but a low

Fig. 1 (A) A typical point-to-plane pulsed corona discharge reactor; and (B) corona discharge images of different electrode geometries with different pulsed power excitations. (A) From H. Taghvaei, M.R. Rahimpour, Upgrading of anisole using: in situ generated hydrogen in pin to plate pulsed corona discharge, RSC Adv. 6 (2016) 98369–98380, https://doi.org/10.1039/c6ra21417g and (B) From T. Shao, V.F. Tarasenko, C. Zhang, et al., Runaway electrons and x-rays from a corona discharge in atmospheric pressure air, New J. Phys. 13 (2011) 113035, https://doi.org/10.1088/1367-2630/13/11/113035.

330 Chapter 14 current density. In general, a high voltage is required to increase the electric field at the corona site for generating a stable corona discharge. The design of asymmetrical electrodes efficiently improves the stability of corona discharges but compromises treatment capacity for the DRM reaction.

2.2 Atmospheric pressure glow discharge (APGD) APGDs have been used for syngas generation because of their advantages such as high electron energy, large discharge volume, and high electron density for the reaction. APGD can be operated over a wide range of gas flow rates from tens to thousands of milliliters per minute, thus offering a flexible process capacity [17]. Fig. 2 shows four discharge stages during the activation process via manipulating the current-voltage of the system in the APGD [30]. Chen et al. [15] investigated the plasma DRM reaction using a pulsed glow discharge reactor under different experimental conditions. The highest conversion of CH4 (92%) and CO2 (83%) was achieved alongside the highest selectivity of CO (81%) and H2 (62%) at a discharge power of 495W. Ghorbanzadeh et al. [19] reported a maximum energy efficiency of 40% in the plasma DRM reaction using an APGD reactor. Yang [11] suggested that the pseudo-glow discharge is more suitable for syngas production compared with the corona discharge due to the higher energy efficiency of the former than the latter. Currently, the scalability of APGD reactors is still challenging [18].

2.3 Microwave (MW) plasma For DRM in the atmospheric microwave discharge, the energy dissipation is larger than other plasma reactors due to the high frequency (from 300 MHz to 10 GHz) used for MW [20]. The properties of MW plasmas are comparable with those of thermal plasmas [20]. An MW plasma has a high ion density of more than 1011 cm3 due to the concentrated energy in a localized volume, and thus a plasma with a high density can be created in MW. An MW discharge is an electrode-free discharge, which can avoid electrode contamination. Fig. 3 shows a schematic diagram of a typical MW plasma system [21]. The electrons in the bulk gas absorb the MW energy to initiate the discharge when the gas intersects with a waveguide. The MWs propagate along the interface between the quartz tube and the plasma column. MW discharges were proven to be a highly potential plasma system for DRM because of their high conversion, product selectivity, and large treatment capacity. Zhang et al. [22] reported that syngas was the major product in a pulsed MW plasma reactor. The CO2 and CH4 conversions were as high as 68% and 72%, respectively, with a CO selectivity of 74% in the products. Jasinski et al. [23] investigated DRM for syngas production in a coaxial-line-based MW plasma reactor and a metal-cylinder-based MW reactor, The authors reported that the metal-cylinder-based MW

Plasma technology for syngas production 331

Fig. 2 (A) Schematic diagram of the APGD; (B) optical photos of the discharge patterns; and (C) currentvoltage dependence of the system with optical photographs of the self-organized stratified interface patterns. From Z. Chen, S. Zhang, I. Levchenko, et al., In vitro demonstration of cancer inhibiting properties from stratified self-organized plasma-liquid interface, Sci. Rep. 7 (2017) 1–11, https://doi.org/10.1038/s41598-01712454-9.

plasma reactor had a higher hydrogen yield and a better energy efficiency than the coaxial-line-based MW plasma reactor. Moreover, the energy efficiency of MW plasmas for syngas production is comparable to atmospheric pressure plasma jet (APPJ), spark discharge, and APGD. However, the complicated MW plasma configuration poses a big challenge for industry scaling-up [22].

332 Chapter 14 Gas Outlet Plasma Quartz Tube

Magnetron Head

Moveable plunger

Waveguide Circulator Three Stub Tuner

Microwave plasma source (MPS) Igniter

CH4 CO2

Fig. 3 Schematic diagram of a microwave plasma system. Reproduced from A.A. Zamri, M.Y. Ong, S. Nomanbhay, P.L. Show, Microwave plasma technology for sustainable energy production and the electromagnetic interaction within the plasma syste a review, Environ. Res. 197 (2021) 111204. https://doi.org/10.1016/j. envres.2021.111204 with permission.

2.4 Gliding arc (GA) GA discharges are a very common type of NTP for chemical reactions including DRM. In its basic form, a GA can be generated by applying a potential difference between a pair of diverging electrodes with a gas flow at their base. As shown in Fig. 4, the arc is initiated at the narrowest interelectrode gap between two diverging electrodes and then glides along the electrodes with the gas flow before extinguishing and reforming at the shortest electrode gap. Therefore, the GA discharge generally needs a high gas flow rate to sustain the arc motion and evolution on the electrodes. The design of a rotating GA reactor with a tangential gas flow could significantly extend the reaction time (see Fig. 4). The electron temperature of GA discharges is lower than that of DBDs at 1–2eV [28]. However, the electron density of GAs is significantly higher than that of other types of NTP such as DBD and corona [4]. Therefore, GA can be far more energy-efficient for the activation of inert molecules such as CO2 due to the use of the lower-energy vibrational dissociation pathway. Conventional GA reactors have significant limitations in that they do not process the entire gas flow with only a limited amount passing through the discharge region between the electrodes, this is a particular problem with large reactor vessels. Great efforts are being done to overcome the limitations of the gliding arc discharge with multielectrode systems, using an external

Plasma technology for syngas production 333

Fig. 4 Schematic representation of a conventional GA with the knife-shaped electrodes (left) and the rotating GA configuration (right). From R. Snoeckx, A. Bogaerts, Plasma technology—a novel solution for CO2 conversion? Chem. Soc. Rev. 46 (2017) 5805–5863. https://doi.org/10.1039/c6cs00066e.

magnetic field or vortex flow to create a rotating gliding arc reactor (RGA), a gliding arc plasmatron (GAP), a fluidized-bed GAD, as well as many others [31–33]. For example, Dinh et al. [25] developed a novel 3D nozzle-type RGA reactor for the DRM reaction. This RGA reactor showed a higher gas conversion and energy efficiency compared with the conventional RGA reactor, which could be attributed to the reduced heat dissipation with the use of thermal insulation [25].

2.5 Dielectric barrier discharge A dielectric barrier discharge (DBD) is formed between two electrodes, one high voltage and one ground. At least one of these electrodes must also be covered with a dielectric layer. Materials such as quartz, glass, and ceramics can be used as dielectric barriers. A DBD can be generated using an AC or pulsed voltage and has a wide working frequency (typically 50Hz–1 MHz) [34]. It has been widely used in plasma-based chemical reactions for decades due to its high operating flexibility (e.g., coupled with catalysis) and easy scalability [35]. However, the partial dissipation of thermal energy to the dielectric and the electrodes in a DBD lowers the energy efficiency of DBD-based plasma chemical processes. Fig. 5 shows the configurations of typical planar and cylindrical DBD reactors [36,37]. Cylindrical DBD reactors have been commonly used in plasma chemical processes. DBDs typically have two different discharge modes: the homogeneous glow discharge, which avoids the formation of streamers, and the filamentary mode, which is considered to have more industrial relevance for breaking chemical bonds. The lifetime of the filamentary discharge is extremely short (10–100 ns), but the current density can be as high as 0.1–1 kA cm2. Each filament is a micro-discharge and could propagate across the surface of the dielectric to create

334 Chapter 14

Fig. 5 Common (A) planar and (B) cylindrical dielectric-barrier discharge configurations. Reproduced from F. Judee, S. Simon, C. Bailly, T. Dufour, Plasma-activation of tap water using DBD for agronomy applications: identification and quantification of long lifetime chemical species and production/consumption mechanisms, Water Res. 133 (2018) 47–59. https://doi.org/10.1016/j.watres.2017.12.035; N.D. Tran, N. Harada, T. Sasaki, T. Kikuchi, Effect of dielectric in a plasma annealing system at atmospheric pressure, In: Dielectric Material, 2012, p. 181 with permission.

surface discharge. The macroscopic characteristics of DBDs depend on a range of operating parameters such as working gas, plasma power, frequency, and dielectric barrier.

3. Effect of processing parameters The reaction performance of the plasma DRM process for syngas production is governed by a variety of processing parameters (e.g., discharge power, gas flow rate, CH4/CO2 molar ratio, reaction temperature, and packing material). Any change in the discharge power leads to a change in the density of energetic electrons and reactive species that contribute to the plasma chemical reactions. The variation in the total flow rate affects the residence time of the reactants in the plasma reaction zone. Since the specific energy input (SEI) combines the influence of both gas flow rate and discharge power, it is a key processing parameter to determine the performance of the plasma-based DRM reaction. Varying the CO2/CH4 molar ratio in the feed gas at a constant total flow rate results in changes in the discharge properties and affects the concentration of reactants, thus influencing the reaction pathways and product distribution. More details on how different processing parameters influence the performance of the plasma DRM reaction will be discussed in the following sections.

3.1 Effect of discharge power Discharge power has been proven to be one of the determining factors that govern the reaction performance of plasma-based DRM. Discharge power can be changed by altering the applied voltage and/or frequency [16,19,38]. In general, raising the discharge power increases the conversions of CO2 and CH4 in all types of plasma systems (e.g., GA, MW, DBD, and corona)

Plasma technology for syngas production 335 [4,9,26,38,39]. Based on the reported results, increasing the discharge power at a constant frequency generates more energetic electrons and reactive species, such as CO, O, and CHx, all of which facilitate the plasma DRM reaction [40]. In a DBD reactor, raising the discharge power typically increases the number of micro-discharge filaments, providing more reaction channels for chemical reactions [5,41]. Moreover, a higher discharge power leads to higher reaction temperatures, which also contributes to the enhanced gas conversions. Yan et al. [42] reported that the conversion of CO2 increased by 12% when increasing the discharge power from 150 W to 220 W in a pulsed DC arc plasma reactor. Interestingly, varying the discharge power has a minor effect on the selectivity of syngas and the H2/CO molar ratio [3]. Zhang et al. [2] reported that increasing the discharge power from 25W to 75W in a DBD reactor enhanced the CO2 and CH4 conversion by 14% and 23%, respectively, but had almost no change on the H2/CO molar ratio (0.8). However, the enhancement in the gas conversion and syngas is at the expense of energy efficiency when the discharge power rises to a certain high value. Li et al. [9] found that the energy efficiency dropped by around 6% when changing the discharge power from 27 W to 63 W using a positive corona discharge for DRM. Choosing an appropriate discharge power is important to balance the conversion and energy efficiency in the plasma DRM process.

3.2 Effect of gas flow rate Lowering gas flow rate is beneficial for enhancing the syngas yield due to the increased residence time of the reactants in the plasma field. Consequently, more interactions occur between the reactant molecules and the reactive species in the plasma. In this case, the subsequent reactions involving the dissociation of higher hydrocarbons (C2–C4) via electron impact reactions are more likely to produce more CO and H2. When the flow rate is increased, the changes in the conversions of CO2 and CH4 are more pronounced than those seen in the selectivities of CO and H2. As the flow rate increases, the gas conversions drop significantly, whereas the selectivities of CO and H2 only slightly decrease before they reach a constant value [29]. Interestingly, varying the feed flow rate has a very limited influence on the H2/CO molar ratio in the product [2]. The SEI can be manipulated via a change in discharge power and/or gas flow rate. Varying the gas flow rate has a more profound influence on the gas conversions in comparison to discharge power [41]. Therefore, a suitable gas flow rate and discharge power should be considered to achieve higher syngas production and energy efficiency simultaneously.

3.3 Effect of CO2/CH4 molar ratio The CO2/CH4 molar ratio in the feed gas significantly influences both gas conversion and syngas selectivity in the plasma DRM. A higher CO2/CH4 molar ratio favors the CH4 conversion due to the increased amount of active oxygen from CO2 decomposition. However,

336 Chapter 14 the increase in CO2 concentration in the mixture gas is typically accompanied by a weak drop in CO2 conversion. In addition, the variation of the CO2/CH4 molar ratio in the feed gas remarkably changes the selectivity of H2 and CO in the produced syngas. In general, the increase of CO2 content in the reactants enhances the selectivity of CO and H2, which can be attributed to the increased number of active O species produced via electron impact dissociation of CO2. Nevertheless, the increment in the CO selectivity is much more pronounced compared with that in the H2 selectivity when increasing the CO2/CH4 molar ratio. For instance, Zeng et al. reported that the H2/CO molar ratio significantly dropped from 9.1 to 0.3 when varying the CO2/CH4 ratio from 1:9 to 9:1 in the plasma DRM reaction using a DBD reactor [29]. Zhang et al. [2] reported that increasing the CO2/CH4 molar ratio from 2:3 to 5:1 decreased the H2/CO ratio from 2.2 to 0.4. Thus, manipulating the CO2/CH4 molar ratio in the gas mixture can be utilized to obtain a desired H2/CO molar ratio in the production of syngas for a specific application.

3.4 Effect of reaction temperature As mentioned above, DRM for syngas production is a strongly endothermic reaction. Increasing the reaction temperature could promote the gas conversion thermodynamically. It is important to note that a change in reaction temperature has a more complicated effect on the plasma DRM reaction due to the changes in plasma properties such as gas density and the reduced field (E/N). As shown in Fig. 6, increasing the reaction temperature enhances the conversions of CO2 and CH4 but lowers the H2/CO ratio [38]. Li et al. [43] reported a similar

Fig. 6 Influence of the reaction temperature on the conversions of CO2 and CH4 (voltage¼18kV, frequency ¼300Hz, total flow rate¼22.5mL/min). Reproduced from V. Goujard, J.M. Tatiboue¨t, C. Batiot-Dupeyrat, Use of a non-thermal plasma for the production of synthesis gas from biogas, Appl. Catal. A Gen. 353 (2009) 228–235, https://doi.org/10.1016/j.apcata.2008.10.050 with permission.

Plasma technology for syngas production 337 trend for the conversion of CO2 and CH4 when the temperature was increased from 350°C to 550°C in a DBD plasma reactor. However, Rahmati and Ghorbanzadeh [44] found that increasing temperature negatively affected on the performance of DRM using a GA plasma reactor, which can be ascribed to the increased relaxation rates of vibrational levels of the molecules at high temperature.

3.5 Effect of packing materials In general, packing materials into the plasma area affects the discharge volume and discharge properties. Wang et al. [45] studied the effect of dielectric constants of different materials on the plasma characteristics in a packed-bed DBD reactor using modeling and intensified charge-coupled device (ICCD) imaging. As shown in Fig. 7, the dominant discharge pattern changed from surface discharges to local filamentary discharges when the dielectric constant of the packing materials increased from 5 to 1000 [45]. They reported that packing materials with

Fig. 7 The influence of packing materials on the plasma properties: (A) The images of the packing beads without discharge; (B) the images of plasma discharge in a packed bed reactor using an ICCD camera; and (C) the electron density distribution around the packing beads. Reproduced from W. Wang, H.H. Kim, K. Van Laer, A. Bogaerts, Streamer propagation in a packed bed plasma reactor for plasma catalysis applications, Chem. Eng. J. 334 (2018) 2467–2479, https://doi.org/10.1016/j.cej.2017.11.139.

338 Chapter 14 a higher dielectric constant showed a faster streamer propagation and discharge development, which facilitated the production of reactive species. Kim et al. [46] investigated the influence of packing materials on the discharge properties in a positive pulsed corona discharge using a nanosecond time-resolved ICCD camera. The propagation of streamers on the surface of γ-Al2O3 and Ag/TiO2 could be observed in a packed-bed reactor, while only filamentary discharges were found in the plasma reactor without packing. Gallon and coauthors [47] studied the effect of the various packing materials (quartz wool, alumina, and zeolite 3A) on the plasma DRM reaction using a coaxial DBD reactor. They found that placing quartz wool into the discharge gap promoted the CH4 conversion and H2 yield because of the increased intensity of microdischarge filaments in the discharge gap compared with a DBD without any packing material. Ray et al. [48] discovered that packing glass beads into a DBD reactor enhanced the DRM performance regardless of the operating conditions, which could be attributed to the enlarged discharge strength, decreased breakdown voltage, and increased transferred charge compared with the plasma-alone system. Recently, Bouchoul et al. [49] investigated the influence of four alumina materials on the plasma DRM reaction in a packed-bed plasma reactor. The different reaction performances can be ascribed to the different discharge properties when using different alumina materials.

3.6 Other process parameters As previously reported, other process parameters also affect the reaction performance of the plasma DRM process, such as the reactor geometry. For instance, the discharge gap and discharge length greatly affect the reaction performance in a DBD plasma. Manipulating the discharge gap and/or discharge length leads to changes in the residence time of gas reactants and power density at a constant discharge power and gas flow rate. Specifically, enlarging the gap distance and/or discharge length can extend the retention time of the reactants in the plasma field facilitating gas conversion. Conversely, increasing the discharge gap and/or discharge length at a constant discharge power also lowers the energy density. Khoja et al. investigated the influence of discharge gap and discharge length on the plasma DRM in a cylindrical DBD reactor [50]. The highest syngas yield was achieved at an optimized discharge gap (3mm) and discharge length (30cm) when packing quartz into the discharge gap [50]. In addition, using different electrode materials resulted in different performances of plasma DRM reactions. Li et al. found that the Ti electrode exhibited the highest CO2 and CH4 conversions compared with other electrodes using Al, Fe, and Cu in a DBD reactor [51]. Wang et al. [27] developed a multistage DBD reactor for plasma DRM reaction and found that this reactor design enhanced the CO2 conversion and syngas selectivity compared with the conventional one-stage DBD reactor. The authors attributed the improved syngas production to the restraint of the reverse reactions in this multistage DBD reactor.

Plasma technology for syngas production 339 Besides, optimizing other experimental conditions (i.e., reaction pressure, the addition of dilution gas, and the feed gas combination system) could also enhance the production of syngas. Zhu et al. [52] reported that increasing the pressure in a spark discharge plasma significantly enhanced the production of CO and H2. Zhang and coauthors [2] investigated the influence of Ar addition in the feed gas on the plasma DRM reaction. They found that adding 50vol% Ar to the reactants increased the CH4 conversion by 15% due to the presence of additional reaction pathways induced by excited Ar: CH4 +Ar∗!CH3 +H+Ar [2]. More recently, Li et al. [43] studied the effect of dilution gas (N2, H2, CO, and H2O) on the gas conversions and the selectivity of H2 and CO in DRM using a DBD reactor. They found that the excited N2* species were beneficial for the breakdown of chemical bonds in CH4 and CO2 molecules, whereas the addition of H2 or CO lowered the CH4 conversion [43]. Moreover, adding water vapor (>5%) increased the conversion of CH4 and the selectivity of H2 and CO [43]. As previously reported, an incompatible trade-off exists in the plasma DRM reaction. Each plasma system has different processing parameters that need to be optimized for viable gas conversions and energy efficiencies, showing a need for adding suitable catalysts into the plasma system to reach high energy efficiencies and conversions simultaneously.

4. Plasma catalysis for DRM 4.1 Plasma catalysis: Configurations and plasma-catalyst interactions Introducing a catalyst into the plasma chemical reactions, known as plasma catalysis, can overcome the kinetic and thermodynamic limitations in chemical processes and control the reaction system to enable the selective generation of target products. Typically, there are two different configurations for the coupling of plasma and catalysts, namely single-stage plasma catalysis and two-stage plasma catalysis. In a single-stage configuration, catalysts can be placed in the plasma region (also called in-plasma catalysis, IPC) in different ways such as packing catalysts into discharge gap or coating catalysts on the surfaces of electrodes/ dielectric materials. In the single-stage plasma catalysis system, there are complex interactions between the plasma and catalysts as catalysts come into direct contact with the plasma. In a two-stage plasma catalysis system, a catalyst bed can be placed before or after the plasma reaction zone. However, it is more common to place catalysts downstream of the plasma reaction zone, also known as postplasma catalysis (PPC). In some cases, the catalyst bed can be heated using extra heating apparatus. In the two-stage system, short-lived species (e.g., radicals and excited species) generated by the plasma generally cannot make it to the catalyst surface, with only long-lived species (e.g., ozone) and reaction intermediates able to reach the catalyst surface.

340 Chapter 14 Plasma-catalytic chemical processes typically involve both plasma-driven gas-phase reactions and plasma-assisted surface reactions. The physicochemical interactions between plasma and catalysts are of primary importance to drive both the gas-phase and surface reactions in the hybrid plasma catalysis system [53–57]. Packing catalysts inside the plasma zone can affect the characteristics of the plasma. For example, the presence of a catalyst in the plasma can induce a packed-bed effect and change the discharge properties such as the local and average electric field, breakdown voltage, electron energy, electron density, and electron energy distribution function, as well as changing the discharge mode and discharge area/volume. In the hybrid plasma catalysis system, the retention time of reactants in the discharge area can also be changed due to the adsorption of plasma species on the catalyst surface. On the other hand, the plasma can also affect the properties and performance of the catalysts in different ways. For example, placing a catalyst in the plasma might change the physicochemical properties of the catalyst such as morphologies, specific surface area, porosity and pore structure, number of defects and oxygen vacancies, metal valence, metal particle size and dispersion on the surfaces of supported metal catalysts, and work function. In addition, the presence of plasma discharges might generate localized hot spots on the surfaces or reduce carbon deposition on the catalyst surfaces due to the changes in the reaction pathways [58]. In some cases, the plasma discharge might be formed in the pores of catalysts, particularly porous catalytic materials. Fig. 8 illustrates the key reaction mechanisms and species at the interfaces between the plasma and catalyst surfaces [59]. In the plasma-assisted surface reactions, both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms could contribute to the generation of target products.

Fig. 8 Key mechanisms and species at the interface between the plasma and catalyst surface. From A. Bogaerts, X. Tu, J.C. Whitehead, et al., The 2020 plasma catalysis roadmap, J. Phys. D. Appl. Phys. 53 (2020) 443001, https://doi.org/10.1088/1361-6463/ab9048.

Plasma technology for syngas production 341

4.2 Plasma-catalytic DRM reaction The combination of NTP and heterogeneous catalysis is a promising solution to enhance the conversions of CO2 and CH4 in plasma-assisted DRM. Meanwhile, the selectivities of targeted products (e.g., syngas) can be improved by using suitable catalytic materials. In this hybrid system, the synergistic benefits obtained by combining plasma with catalyst can further improve the inherent activity and stability of the catalyst by reducing the coking, poisoning, and sintering of the catalyst under plasma conditions. So far, great efforts have been devoted to achieving the DRM reaction through plasma catalysis to produce syngas in different plasma systems, such as DBD [7,50,60–64], GA [65–69], pulsed plasma [31–33,70], MW discharge [71–74], corona discharge [75], and glow discharge [11,17,76]. Meanwhile, the screening and optimization of catalysts under specific plasma conditions have become an important and interesting topic in this field. They have been studied continuously by different groups in recent decades [7,47,63,77–82]. Since the reactor structure of DBD is relatively easy to combine with catalyst materials, most of the plasma-catalytic studies have been conducted using DBD systems compared with other plasma discharges. So far, the related research in this field is still in the “trial and error” stage; the catalysts proved to be active in thermal-catalytic dry reforming have been chosen for the initial stages of the plasma-catalytic DRM reaction. The following section will mainly focus on the catalyst design and optimization for syngas production and the synergistic effects in the plasma-catalytic process. As one of the mainstream catalyst types in thermal catalysis, supported metal catalysts have been widely studied in plasma-catalytic DRM, we will discuss the effect of supported metal catalysts on the plasma-catalytic DRM reaction from three different aspects including metal phases, supports, and promoters. 4.2.1 Effect of active metals Until now, different transition metals (e.g., Ni, Cu, Co, Fe) and noble metals (e.g., Pt, Pd) supported by metal oxides have been tested, such as Ni/Al2O3 [76,82–85], Cu/Al2O3 [29,86], Co/Al2O3 [29], Mn/Al2O3 [29], Fe/Al2O3 [87], Ni/MgAl2O4 [60], Pt/CeO2 [72], Pt/Al2O3 [72], Pd/Al2O3 [29,88], and Ag/Al2O3 [88], etc. Ni/Al2O3 is the most popular catalyst for the plasma-catalytic DRM reaction (Table 1). Song et al. [83] investigated plasma-catalytic DRM over a Ni/γ-Al2O3 catalyst using a DBD system and highlighted the presence of Ni/γ-Al2O3 enhancing the conversion of both CH4 and CO2, and the yield of syngas. However, they found that varying the Ni loading did not substantially change the gas conversion and product selectivity. By contrast, Mahammadunnisa et al. [95] reported that changing Ni loading (10–30wt%) of Ni/Al2O3 changed the catalytic performance of the same reaction. The Ni/Al2O3 with 20 wt% Ni loading exhibited the highest syngas selectivity, gas conversion, and H2/CO molar ratio (Fig. 9). Interestingly, Wang et al. [82] developed a fluidized bed DBD reactor for plasma-catalytic DRM over Ni/γ-Al2O3. This

Table 1 Comparison of performances of various metals loaded on Al2O3 in plasma-catalytic DRM. Reaction conditions

Conversion (%)

Selectivity (%) H2

Flowrate (mL/min)

CH4/ CO2

Temperature (°C)

Power (W)

Frequency

CH4

CO2

DBD, packed bed DBD DBD DBD

250

1

525

50

–

45.5

40

30 30 50

1 1 1

– 400 –

130 38.4 7.5

20 kHz – –

RF-DBD (100 mBar) DBD (1.2 Bar)

100 (95% He)

3/2

–

50

13.6 MHz

16.7

1

240

20

6 kHz

Microwave

60

1

450

55.7 30 19.5 14 15 17.5 35 36 45.9 45.2 49.0 38

33.5 25 9.2 8 8.1 9.5 25.5 23 20.4 28.3 29.6 50

Metal

Loading

Type

Ni

10%

Ni Ni Ni Cu Co Mn Cu Pd Fe Pd Ag Pt

5% >10% 10% 10% 10% 10% 3% 3% 3% 3% 3% 8%

2.45 GHz

CO

H2/CO

Ref. [89]

51.9 96 33 44.5 43.5 34 84 82 20.8 20.7 21.5

60.9 97 37.5 44 42.5 35 65 70 13.6 23.3 19.3

1.04 1.21 1.30 1.35 1.25 1.75 1.65

[90] [91] [56]

[92] [93] [94]

0.75

[81]

Plasma technology for syngas production 343 CO

H 2+

+CO 2 CH 4

(a) Plasma

50

30Ni/Al 20Ni/Al 10Ni/Al Plasma

% of conversion

40

Catalyst

(b)

30

20

10

0 2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

SIE (kJ/L)

Fig. 9 (A) Schematic presentation of the DBD reactor packing with catalysts for DRM. (B) Effect of various Ni loadings on the conversions of CO2 and CH4 at different specific input energies (SIE) (CH4/CO2 ¼ 2:1). Reproduced from S. Mahammadunnisa, P. Manoj Kumar Reddy, B. Ramaraju, C. Subrahmanyam, Catalytic nonthermal plasma reactor for dry reforming of methane, Energy Fuels 27 (2013) 4441–4447, https://doi.org/10.1021/ef302193e with permission.

reactor design enables a unique contact mode between the plasma and catalyst, which demonstrated better reaction performance than the conventional fixed-bed DBD reactor at high temperatures (650–800°C). Furthermore, the fluidized catalyst particles inside the discharge zone can effectively inhibit carbon deposition compared with the fixed bed DBD system. This work has shown that the reactor design and plasma-catalyst coupling mode are critical for plasma-catalytic chemical reactions. Meanwhile, plasma-catalytic DRM over Ni/Al2O3 has also been investigated using the other types of NTP. Zhu et al. [69] placed Ni/γ-Al2O3 downstream of an RGA reactor for the plasma-catalytic conversion of CO2-rich biogas over Ni/γ-Al2O3 (Fig. 10). The conversion of CH4 increased with the increase of Ni loading from 6wt% to 10wt% and reached its maximum at 58.5% when using the 10 wt% Ni loading. Long et al. [89,96] combined an NTP jet with

344 Chapter 14

Resistance Inlet MFC

CO2

CH4

MFC

Rotating gliding are reactor

Desiccant Plasma area Catalyst

10kv DC

Thermocouple

Power source

Quartz cover Vent PTFE Base A Magnet Soap-film flowmeter

Value

A

Inner electrode

Tangential inlet Inlet

Are Catalyst

Outer electrode

GC

A-A

Cylinder Cavity

Vacuum pump Outlet

Fig. 10 Schematic diagram of rotating gliding arc reactor for plasma catalysis. Reproduced from F. Zhu, H. Zhang, X. Yan, et al., Plasma-catalytic reforming of CO2-rich biogas over Ni/γ-Al2O3 catalysts in a rotating gliding arc reactor, Fuel 199 (2017)430–437, https://doi.org/10.1016/j.fuel.2017.02.082 with permission.

Ni/γ-Al2O3 for the plasma-catalytic DRM reaction. Compared with DRM using plasma only, the presence of 12wt% Ni/γ-Al2O3 in the plasma jet enhanced the yield of CO and H2 by 11% and 18%, respectively, at a discharge power of 700 W. Both transition and noble metals have been evaluated for the plasma-catalytic DRM reaction under different conditions. Zeng et al. [29] compared the activities of different Al2O3 supported metal catalysts (Ni, Co, Cu, and Mn) for plasma-catalytic biogas reforming. Compared with Co- and Cu-based catalysts, using Ni- or Mn-based catalysts (Ni/Al2O3 and Mn/Al2O3) increased the CH4 conversion but limited the selectivity of CO and H2. Among the four catalysts under investigation, Ni was found to be the most active metal for biogas reforming to syngas. Plasma-catalytic DRM on Fe/Al2O3 was also carried out at a temperature range of 130–340°C [87]. Compared with the plasma-only process, the coupling of plasma with Fe/Al2O3 slightly lowered the conversions of CH4 and CO2 but substantially reduced the production of CO and H2, indicating that iron was unfavorable for syngas production in this reaction. Furthermore, Sentek et al. [88] investigated plasma-catalytic DRM over Ag/Al2O3 and Pd/Al2O3 in a DBD reactor. The results showed that the coupling of DBD with Pd/Al2O3 slightly decreased the conversion of CH4 and CO2 compared with the plasma-only system but formed more C2 hydrocarbons instead of C3–C4.

Plasma technology for syngas production 345 70

(a)

With 50% Ar Without Ar

Cu-Ni/Al2O3a

80

Plasma+Cu-Ni/Al2O3

70 Conversion (%)

CH4 conversion (%)

60

90

50

40

(b)

Plasma Only

60 50 40 30

30

20 10

20 0

100

400 500 200 300 Reaction time (min)

600

700

0

CH4

CO2

Fig. 11 (A) Effect of Ar added into the feed gases on the conversion of CH4 and the reaction stability (60 mL/min, CH4/CO2 ¼1, 60W, 120°C). (B) Comparison of results between pure plasma, pure catalyst, and the hybrid plasma catalysis in the DRM reaction. (For the thermal conditions: CH4/CO2 ¼1, 50% Ar in the feed gas, GHSV¼ 1800h1, 450°C, 0.1g; For the plasma conditions: 60mL/min (30mL/min for Ar), CH4/CO2 ¼1, 60W, GHSV¼ 1800h1, 450°C.) Reproduced from A.J. Zhang, A.M. Zhu, J. Guo, et al., Conversion of greenhouse gases into syngas via combined effects of discharge activation and catalysis, Chem. Eng. J. 156 (2010) 601–606, https://doi.org/10.1016/j.cej.2009.04.069 with permission.

Supported bimetallic catalysts have also been investigated in plasma-catalytic DRM reactions. Zhang et al. [2] evaluated the effect of the Ni/Cu ratio of bimetallic Ni-Cu/Al2O3 catalysts on plasma-catalytic DRM for syngas production at high temperatures. They found that the addition of argon as the feed gas could significantly enhance the CH4 conversion and the reaction stability (Fig. 11A). 12 wt% Ni-12 wt% Cu/Al2O3 showed the highest conversion of CH4 (69%) and CO2 (75%) alongside with the maximum CO selectivity of 75% at 450°C and 60W (Fig. 11B). Zheng et al. [90] synthesized Ni-Fe bimetallic nanoparticles with a spinel structure embedded in a silica support (denoted as NiFe2O4#SiO2) for plasma-catalytic DRM (Fig. 12). Compared with Ni-based catalysts (Ni-Fe/SiO2, Ni/Al2O3, NiFe2O4, and Ni-Fe/Al2O3), NiFe2O4#SiO2 showed the best performance with high resistance to carbon deposition during the plasma DRM process under ambient conditions. The special structure of NiFe2O4#SiO2 effectively restrains the aggregation of Ni-Fe metallic sites and thus inhibits the generation of solid carbon over a 60-h time-on-stream testing. Kado et al. [91] reported that packing Ni0.03Mg0.97O in a DC pulsed discharge gave a much higher CO2 conversion of 79.5% in the DRM reaction compared with the plasma only system. Meanwhile, the presence of NiMgO in the plasma dramatically decreased the selectivity of C2 from 33.6% to 1% and increased the CO selectivity from 65.4% to 99%, which demonstrates a promising way for use of a catalyst to tune the selectivity of gas products.

346 Chapter 14

Fig. 12 (A, B) TEM images and (C) SAED profiles of synthesized NiFe2O4 nanoparticles; (D, E) TEM images and (F) EDAX spectrum of NiFe2O4#SiO2. Reproduced from X. Zheng, S. Tan, L. Dong, et al., Plasma-assisted catalytic dry reforming of methane: highly catalytic performance of nickel ferrite nanoparticles embedded in silica, J. Power Sources 274 (2015) 286–294, https://doi.org/10.1016/j.jpowsour.2014.10.065 with permission.

Core-shell bimetallic catalysts with uniformly distributed metal sites have also been studied in plasma-catalytic DRM reactions. Even at high temperatures, the strong interaction between core and shell can effectively prevent carbon deposition and sintering of metallic nanoparticles (NPs). As a result, this type of catalyst usually exhibits better stability. Zheng et al. [97] evaluated the activities of LaNiO3@SiO2 core-shell NPs in the plasma DRM reaction. The transmission electron microscope (TEM) images of this catalyst (Fig. 13) show different structures of LaNiO3 NPs and core-shell LaNiO3@SiO2 [92]. Compared with other Ni-based catalysts (LaNiO3/SiO2, Ni/SiO2, and LaNiO3), LaNiO3@SiO2 showed higher syngas selectivity and gas conversion. For instance, the highest conversion of CH4 and CO2 reached 88.3% and 77.8%, respectively, alongside the highest selectivity of CO and H2 being 92.4% and 83.7%, respectively [92]. In addition, LaNiO3@SiO2 also exhibited excellent catalytic stability (Fig. 14) since the SiO2 shell protects the Ni sites in the core from sintering during the DRM reaction [97].

Plasma technology for syngas production 347

Fig. 13 TEM images of (A, B) LaNiO3 NPs and (C, D) core-shell LaNiO3@SiO2. Reproduced from X. Zheng, S. Tan, L. Dong, et al., Silica-coated LaNiO3 nanoparticles for non-thermal plasma assisted dry reforming of methane: experimental and kinetic studies, Chem. Eng. J. 265 (2015) 147–156, https://doi.org/10.1016/j.cej.2014.12. 035 with permission.

4.2.2 Effect of support materials Catalyst supports strongly affect the activities of catalysts, thus determining the reaction performance of catalytic processes. The morphologies, pore properties, hydrothermal stability, and surface chemical properties (e.g., acidity or basicity, surface functional groups, surface electron distribution, and defects) of catalyst supports can affect the catalytic reaction pathways. The metal-support interaction (MSI) is also crucial in catalytic processes. Various types of catalyst support, such as metal oxides, zeolites, metal-organic frameworks (MOFs), have been investigated in plasma-catalytic DRM for syngas production. For metal oxide supports (Table 2), γ-Al2O3 has been widely used in plasma-catalytic DRM reactions because of its low cost, availability, high surface area (typically around 150 m2 g1), and high hydrothermal stability [93]. Mei et al. [94] evaluated the effect of different support materials (i.e., SiO2, γ-Al2O3, MgO, and TiO2) on plasma-catalytic DRM over Ni-based catalysts in a coaxial DBD reactor. Compared with the other three supports, the γ-Al2O3 supported Ni catalyst achieved the highest conversion of CO2 and CH4 (26.2% and 44.1%, respectively) at an

348 Chapter 14 90

80

85

75 only plasma plamsa + Ni/SiO2 plamsa + LaNiO3/SiO2

75

CO2 conversion (%)

CH4 conversion (%)

80 plamsa + LaNiO3 plamsa + LaNiO3@SiO2

70 65

70 only plasma plamsa + Ni/SiO2 plamsa + LaNiO3/SiO2

65 60

plamsa + LaNiO3 plamsa + LaNiO3@SiO2

55 50

60 45 55

(a)

40

50 0

5

10 15 20 time on stream (h)

25

30

95

(b) 0

5

100 only plasma plamsa + Ni/SiO2 plamsa + LaNiO3/SiO2

90

95 CO Selectivity (%)

80 75

10 15 20 time on stream (h) only plasma plamsa + Ni/SiO2 plamsa + LaNiO3/SiO2

plamsa + LaNiO3 plamsa + LaNiO3@SiO2

85 H2 selectivty (%)

35

25

30

plamsa + LaNiO3 plamsa + LaNiO3@SiO2

90

85

70 85

65

(c) 60

0

(d) 5

10 15 20 time on stream (h)

25

30

75 0

5

10 15 20 time on stream (h)

25

30

Fig. 14 The conversions of (A) CH4 and (B) CO2 and the selectivities of (C) H2 and (D) CO with respect to the time on stream over Ni-based catalysts with different structures. Reproduced from X. Zheng, S. Tan, L. Dong, et al., LaNiO3@SiO2 core-shell nano-particles for the dry reforming of CH4 in the dielectric barrier discharge plasma, Int. J. Hydrog. Energy 39 (2014) 11360–11367, https://doi.org/10.1016/j.ijhydene.2014.05.083 with permission.

SEI of 72kJ/L. In addition, packing Ni/γ-Al2O3 into the DBD reactor showed the highest yield of CO and H2. This excellent performance of Ni/γ-Al2O3 could be ascribed to the higher reducibility of NiO NPs on γ-Al2O3, the larger surface area, and the stronger basic sites provided by γ-Al2O3. The CO2-TPD analysis of Ni/γ-Al2O3 confirmed the enhanced chemisorption and activation of CO2 compared with other Ni-based catalysts, thereby reducing the carbon deposition on Ni/γ-Al2O3. Tu and Whitehead [3] investigated the influence of different calcination temperatures (300, 500, and 800°C) of Ni/γ-Al2O3 on plasma-catalytic DRM reaction. All these catalysts were reduced at low temperatures (300°C) using an Ar/H2 plasma instead of conventional thermal reduction at high temperatures. Typically, a lower

Table 2 Comparison of performances of various supports loaded by Ni in plasma-catalytic DRM. Conversion (%)

Reaction conditions Support

Type

Flowrate (mL/min)

CH4/CO2

Temperature (°C)

Power (W)

Al2O3 Al2O3 MgO SiO2 TiO2 MgAl2O4 La2O3-MgAl2O4

DBD DBD

50 50

1 1

– –

7.5 30

DBD

20

–

350

100

Selectivity (%)

CH4

CO2

H2

CO

H2/CO

Ref.

19.5 26 26 25 24.8 76 86

9.2 16.3 16.2 15.7 15.5 74 84.5

33 34.6 34.3 33.9 33.5 43.5 50

37.5 48.8 48.4 48.2 47.5 45 49.5

1.21

[56] [94]

0.86 1.01

[98]

350 Chapter 14 calcination temperature weakens the MSI of Ni/γ-Al2O3, forming NiO as the major Ni species on γ-Al2O3, while higher calcination temperatures (e.g., 800°C) induce a stronger MSI with the formation of a NiAl2O4 spinel. They found that Ni/γ-Al2O3 prepared at a lower calcination temperature of 300°C showed higher catalytic activities in plasma DRM reaction, which can be ascribed to the availability and accessibility of Ni NPs from the low-temperature reduction of NiO by using the Ar/H2 plasma. By contrast, the Ni/γ-Al2O3 calcined at 800°C exhibited poorer activities in the plasma DRM reaction as the formed NiAl2O4 spinel from the high-temperature calcination cannot be reduced by the plasma at low temperatures (300°C), resulting in the absence of Ni active sites for the activation of CO2 and CH4. Zeolites are widely used in heterogeneous catalysis because of their controllable pore structures, large specific surface areas, adjustable acidities, and high thermal stabilities [99]. Using zeolites in plasma-catalytic DRM reaction has also been explored. Plasma-catalytic DRM over different zeolites was firstly investigated by Eliasson et al. [62] in 2000. They found that higher hydrocarbons can be produced in the plasma DRM reaction using zeolite NaX. Compared with the plasma DRM reaction without a catalyst, the presence of zeolite NaX in the plasma lowered the gas conversion and carbon deposition but enhanced the selectivity of C2–C4 hydrocarbons. Zhang et al. [100,101] evaluated the activities of zeolite NaY, zeolite NaX, and zeolite HY in the plasma-catalytic DRM reaction using a DBD reactor. Their results demonstrated that zeolite HaY gave the highest production rate of syngas with CO and H2 selectivity being 42.7% and 25.7%, respectively. Interestingly, all these three zeolites were favorable to produce C4–C5 hydrocarbons. Gallon et al. [47] studied the influence of different packing supports on the production of H2 via DRM in a DBD reactor. Compared with the plasma reaction using quartz wool or alumina, packing zeolite 3A in the DBD reactor increased the H2 selectivity by >30% but lowered the selectivity of CO. Meanwhile, the coupling of DBD with zeolite 3A enhanced the formation of unsaturated C2 hydrocarbons. Jiang et al. [102] noted that combining plasma with zeolite A provided a lower selectivity of CO and H2 and a higher selectivity of C2–C4 compared with the plasma-only system. Notably, most reported studies have confirmed that zeolites are favorable for producing hydrocarbons particularly higher hydrocarbons rather than syngas in the plasma-catalytic DRM reaction. However, packing zeolites into a DBD reactor can reduce the gas conversion. Recently, MOF materials have attracted increasing interest for a range of catalytic processes. MOFs have an extremely high specific surface area, structural diversity, and modifiability, together with tailorable porous structure, and straight microenvironment, enabling rational catalyst design by anchoring additive species into their pores, cages, and channels [103]. The use of MOF materials in plasma catalysis has also been reported. Vakili et al. [7] explored the performance of Pt NPs@UiO-67 in the plasma-assisted DRM process using a DBD system. They concluded that the UiO-67 MOF could be stable in the discharge region with no significant character changes under different operating conditions, such as discharge time, power, and feed gas. Their results showed that the coupling of DBD with UiO-67 enhanced the

Plasma technology for syngas production 351 conversion of CO2 and CH4 by 10% and 18%, respectively, and led to higher selectivity of C2H2 and C2H4 rather than C2H6. Interestingly, the Pt NPs@UiO-67 catalyst exhibited good stability and recyclability under the plasma conditions. This work has demonstrated the potential of MOF-based catalysts in tuning the selectivity of reaction products in plasma-catalytic chemical processes. 4.2.3 Effect of promoters and dopants The addition of promoters and dopants to supported metal catalysts has been considered an effective solution to tune the catalytic performance such as the dispersion and reducibility of metals on catalyst surfaces, the amount of basic/acidic surface sites, and physical morphologies of catalysts. Table 3 summarizes the promoters used for Ni/Al2O3 in the plasma-catalytic DRM process. Zeng et al. [79] investigated the effect of different promoters (K, Ce, and Mg) of the Ni-based catalyst on syngas production via the plasma DRM using a DBD system. Compared with Ni/Al2O3, the addition of promoters improved the conversion of CH4, the yield of H2, and the energy efficiency of the hybrid process. Among the catalysts, the K-promoted Ni catalyst exhibited the highest conversion of CH4 (31.6%) and CO2 (22.8%). Meanwhile, adding Mg to Ni/Al2O3 increased the H2/CO molar ratio to 2.2 due to its lower CO2 conversion. Furthermore, compared with Ni/Al2O3, the presence of promoters increases the carbon deposition by 22%–26% during the reaction. They found that over 80% of formed carbonaceous species on the catalyst surface were reactive carbon species, which can be easily removed by in situ oxidation during the plasma-assisted DRM reaction. Khoja et al. [98] performed plasma-catalytic DRM over La2O3-promoted Ni-based MgAl2O4 catalysts. Compared with Ni/ MgAl2O4, the presence of La2O3 promoter increased the conversion of CO2 and CH4 to 84.5% and 86%, respectively, alongside the increased syngas production. The H2/CO molar ratio was greatly increased from 0.86 (without La2O3) to 1.01 over Ni/La2O3-MgAl2O4.

5. Conclusion and future outlook In this chapter, we reviewed the research progress for the plasma-assisted dry reforming of methane to syngas. Unlike thermal catalytic reactions at high temperatures, NTP technology, as a fast-evolving field, exhibits a great advantage of overcoming the high energy barrier for the DRM reaction at low temperatures. For the plasma-driven chemical reactions, both plasma types and reactor geometries play decisive roles in determining the reaction performances including gas conversion and energy efficiency due to the different plasma properties. Energy efficiency is a key indicator for assessing the potential of emerging technologies. As reported, in the plasma-assisted DRM process, the energy efficiency using plasma-based technology needs to be over 60% to be competitive with the conventional and other alternative developing technologies [53]. Among various types of plasma, MW and GA plasmas enable the energy efficiencies to meet such criteria, which means they have certain prospects for practical applications. For the DRM reaction, there are very limited reported results using MW plasmas.

Table 3 Comparison of performances of various promoters for Ni/Al2O3 in plasma-catalytic DRM. Conversion (%)

Reaction conditions Promoter

Type

Flowrate (mL/min)

CH4/CO2

Temperature (°C)

Power (W)

– 2% Mg 2% Ce 2% K – MgO CeO2

DBD

50

1.5

160

16

DBD

30

1

–

2.7

Selectivity (%)

CH4

CO2

H2

CO

H2/CO

27 31.5 32 31.8 21 34.7 30

19.5 15.2 21.9 22.2 10 13 12.1

41.9 41.4 41.5 43.3 23 35 27

29.9 29.4 31 31.1 50 54 68

1.96 2.2 1.9 1.92 0.64 0.94

Ref. [88]

[104]

Plasma technology for syngas production 353 Many interesting and exciting results have been reported using GA discharge, although ns-pulsed discharge, corona discharge, plasma jet, and APGDs are also promising. Furthermore, DBDs are the most popular plasma type investigated for the DRM process, but typically with a lower energy efficiency of 5%–15%, suggesting that research breakthroughs of DBDs are still needed to make DBD plasma more competitive with other NTP technologies. In addition, the plasma properties are also sensitive to the processing parameters, including CH4/ CO2 ratio, gas flow rate, discharge power, packing materials, etc., which can greatly affect the discharge characteristics, thus influencing the reaction pathways and product distribution positively or negatively. Different plasma systems have their own set of process parameters that need to be optimized to achieve better reaction performances. Clearly, due to the high complexity of plasma systems, process parameters are always coupled when conducting plasma chemical reactions. Thus, synthesis and decoupling analysis are necessary to understand and optimize the plasma-driven chemical reactions. Although plasma can generate a variety of reactive species, the process cannot be chemically selective in producing the expected products. One of the major challenges is how to directionally drive the chemical reactions and produce the target products, and it is possible to enable the selective chemical reactions by developing the hybrid system combining catalysts with the plasma. Theoretically, catalysts can effectively tune the product distribution by creating a certain chemical environment through the surface reactions, whereas the discharge properties can also be changed by packing with the catalytic materials in this hybrid system. Thus, when considering a hybrid plasma-catalytic process, the chemical and physical interactions between the catalyst and plasma should be considered. As reported, there already are various plasma systems that have been combined with the catalyst beds successfully, such as DBD, GA, rotating GA, pulsed corona plasma, plasma jet, etc. Compared with other different plasma systems, DBDs are considered to be the most suitable reactors due to their simple structure, easy amplification, and direct implementation of catalysts. Therefore, in this chapter, we mainly focused on the catalyst design for the syngas production through plasma-assisted DRM reaction in the DBD systems, and the effects of active metals, supports, and promoters on the reactant conversion and product selectivity have been discussed in detail. Obviously, compared with conventional heterogeneous catalysis, the catalyst designs in the plasma-catalytic system with different plasmas still have immense room for studies and development, especially regarding the plasma environment. In addition to the supported metal catalysts, related reports on other kinds of catalysts, such as single-atom catalysts, zeolites, and MOFs, are still very limited. With the assistance of plasma, the syngas ratio (CO/H2) can be tuned at a wide range by changing the initial feed gas CH4/CO2 ratio. However, considering the catalytic performances under plasma conditions, the conversion should be further improved, as well as the selectivity of syngas. As proven by many studies, the synergistic effect of plasma catalysis has a strong influence on improving reaction performance. Until now, various combinations of plasma and catalyst,

354 Chapter 14 thermodynamic and kinetic analysis, and the catalyst surface reactions have been investigated to explain the synergies from different aspects. In situ spectroscopic diagnostics including in situ FTIR and plasma diagnostics should be further developed to gain new insights into the plasma-catalyst interactions, particularly plasma-assisted surface reactions, to elucidate the reaction mechanisms and pathways in the hybrid plasma-catalytic system. Furthermore, as coking is a common issue during DRM reactions, the rational design of the catalysts to alleviate the deactivation caused by coke formation and metal sintering needs to be emphasized for industrial applications. Additionally, most of the research work reported has focused on the investigations of plasma chemical processes (e.g., DRM) using a lab-scale plasma system. Scaling up plasma systems is a key step to validate the implementation of this process on a large scale. The integration of scale-up plasma reactors with upstream and downstream processes should also be considered. Techno-economic assessment of the integrated process should also be carried out to prove the economic and commercial feasibility of the emerging plasma technology for DRM to syngas.

Acknowledgments We acknowledge the funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 823745. N. Wang and X. Tu acknowledge the support of the British Council Newton Fund Institutional Links Grant (No. 623389161). Y. Wang and X. Tu acknowledge the support of the Engineering and Physical Sciences Research Council (No. EP/V036696/1).

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CHAPTER 15

Syngas from coal Linghao Zhao, Razieh Etezadi, and Theodore Tsotsis Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA, United States

1. Introduction Coal has been used to produce syngas for close to 100years. The gasification process used to covert coal or other carbonaceous feedstock into syngas is technically mature with a number of commercial systems presently in operation [1-3,5]. Table 1 lists several mature gasification processes that are practiced commercially today. Some of these gasifiers use specific types of coal (coal is classified into four main ranks based on the carbon content, see Table 2), while others can process any coal rank. Table 3 shows the various components of syngas derived from coal via gasification. Major components include CO, CO2, H2, and N2 (if air is used as the gasifying agent) with smaller quantities of CH4 and other hydrocarbons also being present [3,6,7]. Coal-derived syngas also contains numerous other compounds at lower concentrations, such as H2S, COS, NH3, and various metal-containing (e.g., Hg) volatile compounds. These trace compounds make further utilization of syngas for production of fuels and chemicals rather challenging, as they can damage the catalysts utilized and the processing equipment. This, then, requires additional extensive purification steps downstream of the gasifier prior to any further beneficial use of the syngas [1,8–11]. Such uses include the synthesis of ammonia, methanol, and various liquid hydrocarbons [12,13]. The main function of the gasifier is to convert solid coal into syngas. However, coal contains water in its pore structure, the amounts varying depending on the type of coal utilized. Typically, in the gasifier, the first step is to vaporize the water from the coal structure, which consumes energy (
44 kJ/mol of H2O vaporized). The key process through which coal is converted into syngas is via its reaction with oxygen and steam inside the gasifier, which entails the following three primary reactions [12,14]: 2CðsÞ + O2 ðgÞ ! 2COðgÞ

ΔH ¼ 221:31 kJ=mol

Advances in Synthesis Gas: Methods, Technologies and Applications. https://doi.org/10.1016/B978-0-323-91871-8.00013-1 Copyright # 2023 Elsevier Inc. All rights reserved.

363

(1)

364 Chapter 15 Table 1 Different types of gasifiers and their key characteristics. Ash conditions

Gasifier type Lurgi

Dry ash

BGL

Slagging

Winkler, HTW, CFB, Transport gasifier KRW, U-Gas

Dry ash Agglomerating

Shell, Siemens

Slagging

GE, E-Gas, MCSG

Slagging

Category

Oxidant

Preferred coal rank

Reference

Moving bed Moving bed Fluidized bed Fluidized bed Entrained flow Entrained flow

O2

Any

[3,4]

O2

Bituminous

[5]

O2

[2,3]

O2

Subbituminous, Lignite Any

O2

Any (dry feed)

[1]

O2

Bituminous (slurry feed)

[2,3]

[3]

Table 2 Ranks of coal (US Energy Information Administration). Rank of coal

Carbon%

Anthracite Bituminous Subbituminous Lignite

86–97 45–86 35–45 25–35

Table 3 Syngas composition from various kinds of gasifiers. Shell (128°C) %

Shell (160°C) %

GE %

CO H2 CO2 H2O CH4 Ar N2 H2S

56.4 29.7 1.4 7 – 0.7 4.53 0.24

49.6 26.3 1.3 18.1 – 0.6 3.86 0.21

15.6 15.1 7.3 6.1 – 0.8 0.8 0.12

COS

0.02

2

0.08

Others Reference

0.01 [6]

0.01 [6]

– [3]

Gasifier type

E-Gas % 42.2–46.7 32.31–34.40 14.89–17.13 – 1.04–2.29 – – 17.28–107.2 ppm 9.03–162.13 ppm – [6]

GE (airblown) %

GE (oxygenblown) %

13.89–20.14 10.14–14.73 6.09–8.85 8.18–12.00 0.02–0.05 0.60–0.66 49.50–54.11 0.26–0.32

39.09–43.44 28.94–32.91 9.32–13.53 13.11–19.91 0.02–0.03 0.60 0.41–0.45 0.58–0.64

0.02

0.03–0.04

– [3]

– [6]

Syngas from coal 365 CðsÞ + O2 ðgÞ ! CO2 ðgÞ

ΔH ¼ 393:98 kJ=mol

(2)

CðsÞ + H2 OðgÞ . CO ðgÞ + H2 ðgÞ

ΔH ¼ +131:46 kJ=mol

(3)

The gasification mechanism is, however, still not well understood presently and is thought to include numerous other reactions taking place including the following [12,14]: 2COðgÞ + O2 ðgÞ ! 2CO2 ðgÞ

ΔH ¼ 566:65 kJ=mol

(5)

2H2 ðgÞ + O2 ðgÞ ! 2H2 OðgÞ

ΔH ¼ 484:23 kJ=mol

(6)

CðsÞ + 2H2 ðgÞ ! CH4 ðgÞ

ΔH ¼ 74:94 kJ=mol

(7)

CðsÞ + CO2 ðgÞ ! 2COðgÞ

ΔH ¼ 172:67 kJ=mol

(8)

ΔH ¼ 71:44 kJ=mol

(9)

2CH4 ðgÞ + O2 ðgÞ ! 2COðgÞ + 4H2 ðgÞ COðgÞ + H2 OðgÞ . CO2 ðgÞ + H2 ðgÞ

ΔH ¼ 41:21 kJ=mol

(10)

CH4 ðgÞ + H2 OðgÞ . COðgÞ + 3H2 ðgÞ

ΔH ¼ 206:20 kJ=mol

(11)

Steps (10) and (11) above are the well-known water gas shift (WGS) and steam reforming reactions and play a key role in determining the (H2/CO) molar ratio of the syngas, which in turn is important for the choice of its downstream application, for example, methanol or ammonia synthesis, Fischer-Tropsch synthesis for the production of liquid fuels, or power generation. In many instances, depending on the chosen application, the H2/CO molar ratio will need to be adjusted via an added separate WGS reactor.

2. Conventional coal gasifiers Coal has been used to produce synthesis gas for close to a century. Several of the key gasification processes still in use today were first developed and commercially established in the period between 1926 and 1950. A key driver for technology growth at that time was the commercialization of the air separation process during the 1920s that made relatively pure oxygen available for use in continuous gasification processes to produce synthesis gas. For example, the Winkler fluidized-bed process was developed in 1926 by Rheinbraun AG in Germany for gasifying lignite coal, the Lurgi dry-ash gasification technology was developed by Lurgi GmbH in the 1930s, and the Koppers-Totzek entrained-flow process was developed in the 1940s. In the 1950s, with the discovery of plentiful natural gas and petroleum resources that can also be used to produce syngas, the importance of (and interest in) coal gasification processes waned. Since the beginning of this century, the relatively high and fluctuating prices of oil and natural gas have resulted in renewed interest in coal gasification technologies. The huge demand for energy, particularly in developing countries such as India and China, has greatly increased the use of coal for power generation, but it is also raising great concerns today about the impacts such increased use has on climate change. Concerns about CO2 emissions

366 Chapter 15 from coal-based power generation are, thus, spurring new interest in the gasification technology, as it is the first stage of the so-called Integrated Gas Combined Cycle (IGGC) power generation process from coal with reduced CO2 emissions [1]. According to a report by the Office of Fossil Energy of the US Department of Energy (DOE) [15], synthesis gas production from coal gasification is currently growing at a rate of approximately 10% per year, which means that the technology still remains of significant industrial importance. There are different types of gasifiers that are available today for producing syngas from coal [12], but due to space limitations, here we only discuss three of the most frequently used ones, specifically the Sasol-Lurgi dry-ash, the Winkler, and the Koppers-Totzek gasifiers, in order to describe the key features of the gasification technology. Other gasifiers in use share many of the same features as the three gasifiers described here, such as the GE Energy gasifier, the Shell gasifier, the E-Gas gasifier, the Tsinghua OSEF gasifier, the Kellogg, Brown, & Root (KBR) gasifier, the transport gasifier (TRIG), the U-GAS gasifier, and so on.

2.1 Sasol-Lurgi dry-ash gasifier The Lurgi gasifier technology first came in commercial use in 1927 and has been utilized for the gasification of lignite since 1931. The Sasol Technology Company acquired the rights to the Lurgi gasifier technology in 1955 [16] and has significantly further expanded and developed the technology since, with the systems offered now known as the Sasol-Lurgi gasifiers. Today, the Sasol-Lurgi coal gasification process is the world’s most broadly utilized commercial technology and is the largest producer of syngas worldwide [17–20]. Presently, the Sasol-Lurgi gasifiers can process various types of coals ranging from anthracite to lignite with widely varying properties and carbon content (see Table 2). Fig. 1 shows a schematic of a typical Sasol-Lurgi gasifier. Sasol synfuel plants in South Africa and the Great Plains Synfuels Plant in North Dakota are two representative examples of using this technology to produce syngas. During operation of the gasifier, the coal is first crushed down to a size around 6–50 mm [12,21,22] and is loaded in the so-called coal bunker section at the top of the gasifier (see Fig. 1) under atmospheric pressure and temperature conditions. The valve in between the coal bunker and the next section of the gasifier underneath (known as the coal lock) opens when the coal bunker is full, so that the coal can drop into the coal lock section below. In the coal lock section, the coal is pressurized to around 25–30bar [12,23]. When the desired pressure is reached, the valve at the bottom of the lock section will open, and the coal is allowed to enter the main gasifier section through a distributor mechanism to react with the gasifying media (typically oxygen and steam) coming from the grate section at the bottom of the gasifier. The highest temperature in the gasifier is near the bottom of the bed where temperatures can reach up to 600–760°C to gasify the coal char that is more concentrated at the bottom of the gasifier. The hot syngas that is generated at the bottom continues to rise inside the gasifier where it contacts the cold coal that is dropping down from the top. The unreacted part of the coal, known as coal

Syngas from coal 367 COAL COAL

COAL LOCK

COAL LOCK

TAR RECYCLE JACKET STEAM

COAL DISTRIBUTOR

WASH COOLER

TAR RECYCLE JACKET STEAM

COAL DISTRIBUTOR

WASH COOLER RAW GAS STEAM AND OXYGEN

GRATE WATER JACKET STEAM & OXYGEN

COOLING WATER

ASH LOCK SLAG LOCK

ASH

SLAG

Fig. 1 Sasol-Lurgi gasifier system (left) and BGL gasifier system (right) [12].

ash, which includes the inorganic fraction of the coal, but often also high-molecular-weight organic compounds known as tars, collects at the bottom of the gasifier section, and is then collected in a different section of the gasifier known as ash lock for further treatment and potential disposal. The hot syngas produced and leaving the gasifier at temperatures in the range 370–600°C, typically, contains tars devolatilized from the incoming coal feed, which must be removed prior to further processing of the syngas (e.g., for hydrogen production). To avoid overheating and for temperature control, the gasifier is surrounded by a jacket employing cooling water [3,12,14]. There exists a modification of the Lurgi gasification process, the so-named British Gas/Lurgi (BGL) gasifier, which was developed specifically to deal with the slagging ash and is also shown in Fig. 1 [14]. In the BGL gasifier, the lower grate in the Sasol-Lurgi gasifier was removed so that the tars can be recycled and enter the lower part of gasifier.

2.2 The Winkler gasifier The Winkler gasifier was first commercialized in 1926 by Rheinbraun AG in Germany for use with lignite coal and was also among the first known industrial applications of fluidized bed

368 Chapter 15 FEED GASIFIER

FEED BIN

WASTE HEAT STEAM BOILER RAW SYNGAS

LOCKHOPPER SYSTEM

SYNGAS HTW GASIFIER COOLER

LOCK-HOPPER CYCLONE CHARGE BIN

CYCLONE

FREEBOARD

BFW FEED SCREW AIR/OXYGEN/ STEAM COOLING SCREW

SCREW FEEDER AIR OR OXYGEN

FLUID BED COLLECTION BIN

SOLIDS TO AUXILIARY BOILER

LOCK-HOPPER DISCHARGE BIN

Fig. 2 Winkler gasifier system (left) and high-pressure Winkler gasifier system (right) [12].

reactor technology. In the original Winkler gasifier process (for a schematic, see Fig. 2), coal is first crushed into small size particles 700°C) with the controlled ratio of steam or oxygen [15,146]. The resulting gas is called synthesis gas (syngas), which can further be fermented via anaerobic microorganisms to produce fuels and chemicals. Gasification is performed using oxygen, air, steam, or a mixture of these as a gasifying agent [108,147]. Air gasification leads to the generation of low calorific value products (4–7Nm3) [148,149], whereas steam gasification or a combination of steam and oxygen produces syngas with medium heating value (10–18MJNm3) [148,150,151]. When pure O2 is used, the heating value can be up to 28 MJ Nm3 [152].

Syngas from agricultural waste

393

Gasification reactors typically operate at high temperatures (800–900°C or even higher), regardless of type or shape. Gasification can be described in four basic stages: drying, pyrolysis/devolatilization, partial oxidation or combustion, and reduction [6,108]. In the gasifier dryer, the moisture in the biomass is released as steam, and the biomass does not disintegrate because there is not adequate temperature (up to 120°C) to cause a chemical reaction [6,153,154]. In the devolatilization or pyrolysis step, the steam produced by the drying process can lead to the thermal decomposition of biomass into bio-syngas in the absence of oxidizing agents of the biomass constituents [6,155]. This reaction occurs in the absence of oxygen/air at a temperature of about 350–400°C [6,15]. The oxidation process occurs with air or oxygen and involves a series of exothermic reactions in which volatiles are produced mainly as CO, CO2, and H2O. The gases produced by oxidation, the residual carbon, and volatile gases from the pyrolysis process enter the reduction region [108,156–158]. In the reduction region, tar particles are reduced at a temperature of about 1000°C in the absence of oxygen by chemical reactions, namely the Boudouard reaction, primary and secondary water–gas reactions, methanation reactions, and water–gas shift reaction. These are endothermic reactions that need energy to occur, except methanation [155,157,159]. The main biomass gasification reactions are as follows, which are adapted from ref. [158]. (1) Drying (2) Pyrolysis reaction (3) Oxidation reaction (4) Boudouard reaction (5) Steam gasification reaction (6) Water gas shift reaction (7) Steam reforming of CH4

H2O (l)! H2O (g) CHxOyNz ! char+ volatiles C + O2 ! CO2 2C + O2 ! 2CO C + CO2 $ 2CO C + H2O $ CO + H2

ΔH ¼ 393.4 KJmol1 ΔH ¼ 221.3 KJmol1 ΔH ¼ +167.9 KJmol1 ΔH ¼ +131.7 KJmol1

CO + H2O $ CO2 + H2 C + 2H2 $ CH4 CH4 + H2O $ CO + H2

ΔH ¼ 41.1 KJ mol1 ΔH ¼ 87.5 KJ mol1 ΔH ¼ +206 KJ mol1

The reactions mentioned above are the main gasification reactions. One reaction overrides another depending on the operating circumstances, and the product composition varies as a result [153].

6. Factors affecting performance of biomass gasification Feedstock properties (such as particle size and structure, chemical composition, moisture content, and heating value) and gasification conditions (such as reactor type, temperature,

394 Chapter 16 pressure, residence time, type and flow of oxidizing agent, biomass flow, and catalyst used) have considerable influence on syngas composition and performance of the gasification [157,160].

6.1 Feedstock size The small particle size of biomass can result in a higher carbon conversion efficiency (CCE) and syngas yield due to larger surface area, which accelerates heat transfer rates and gasification reactions. Also, the particle size of the feedstock affects the pressure drop across the gasifier bed. Small particles lead to an increase in pressure drop through the gasifier because they need less time for complete gasification. Furthermore, feedstock flow hindrance can occur if large feedstock particles are used in a moving-bed gasifier. Larger particles cause less-uniform heating distribution because larger particles heat slower and therefore have lower average temperatures [153,160].

6.2 Biomass moisture content Several researchers have investigated the effect of moisture content in the feedstocks on the gasification process. Quaak et al. [161] investigated the effect of heating value and moisture content on biomass residues and found that heating value for feedstocks with higher humidity needs more energy to evaporate moisture content before the gasification stage. Theoretically, nearly all types of biomass with a moisture content of 5%–30% can be converted to syngas; however, not all biomass fuels lead to successful gasification [162]. According to Quaak et al.’s research, the moisture content of the feedstock for downdraft gasification must be less than 25% [161]. Kaushal et al. [162] and Gungor et al. [163] conducted research in fluidized-bed reactors and indicated that high-moisture-content feedstock reduces the average temperature of the gasifier and leads to slower kinetics, which in turn produces low-quality syngas. Moisture content can also be decreased by pretreatment of the feedstock via drying [164]. However, this work increases the overall syngas generation costs [106]. Monteiro et al. [165] described that more moisture-content increases H2 and decreases CO, mainly because of the consumption of this last compound in the water–gas shift reaction, which increases CO2 and hydrogen. La Villetta et al. [166] also demonstrated that for high moisture content inside the gasifier, reducing CO yield has a more significant effect than increasing H2 production because the heating value of CO is higher than that of H2. Low-moisture-content fuel is favorable since higher moisture content requires more energy to evaporate liquid forms of moisture. In other words, for a given heat input, high-moisture fuel leads to a lower temperature, which affects the composition of gas produced and thus produces a gas with a lower heating value [153].

Syngas from agricultural waste

395

6.3 Reactor type Gasifiers are classified according to the types of bed and the availability of heat sources to drive the gasification reactions (direct or indirect) [160,167]. Heat can be provided directly or indirectly to the gasifier. In direct gasification, air or oxygen as a gasifying agent reacts with biomass inside the gasifier and supplies the heat needed for the endothermic gasification reactions. In indirect gasification, biomass is combusted in a discrete enclosed space, and heat exchanger tubes bring the heat from the combustion chamber to the gasification chamber [168]. Direct gasification using air (air gasification) leads to the production of N2 diluted syngas (50%–65%) with low concentrations of H2 and CO, while indirect gasification, which typically uses steam or oxygen as an oxidizing agent, yields syngas with higher H2 and CO concentrations [167]. There are three main groups of gasifier beds, namely fixed beds (updraft or downdraft fixed beds), fluidized beds (bubbling and circulating fluidized beds), and entrained flow reactors [156,167]. A fluidized-bed gasifier can be utilized for a broad range of fuels because it can manage a wide variety of fuels over a great range of particle sizes, followed by entrained beds, which are mainly used for coal gasification. However, if the fuel has a low bulk density and a high moisture content, it may be utilized on a limited scale, in which case, a fixed-bed gasifier would be the best option [153]. In general, entrained flow or fluidized-bed gasifiers are favored for large-scale enterprises, while fixed-bed gasifiers are recommended for small-scale gasification [108]. The syngas composition highly depends on the used feedstocks, as well as the operating conditions of the gasifier. Table 1 represents syngas composition produced from agricultural residues and operating conditions of the gasifier. The capacity of the unit determines the kind of gasifier or reactor to use, and the specification must be compatible with the unit’s ultimate application or downstream operating system [169].

6.4 Reaction temperature and steam-to-biomass ratio (S/B) Gasification temperature and steam-to-biomass ratio are the most important and easily controllable operation factors that affect the composition and characteristics of syngas [167,178]. Temperature and steam-to-biomass ratio (SBR) both positively affect tar cracking and synthesis gas production [179,180]. Both parameters show the same tendency. Steam-to-biomass ratio (SBR) determines the proportion between the flow rate of the incoming steam and the biomass feeding to the gasifier [157,181]. With a higher S/B ratio, more steam is injected to the feed, and the water gas and steam methane reforming processes speed up, raising H2 and CO concentrations. Nonetheless, when the S/B ratio is increased, the CO concentration falls. This is due to the water–gas shift reaction, which increases H2 and CO2 concentrations while decreasing CO concentrations when interacting with steam [182]. Higher SBR and gasification temperatures increase yield due to higher

Table 1 Synthesis gas composition generated from agricultural wastes and operating conditions of used gasifiers. Syngas composition Feed stock

Gasifier type

Catalyst

H2

CO

CO2

CH4

Rice husk

Fluidized bed

NanoNiO/γ-Al2O3

49

25

21

5

Rice straw

Fluidized bed

CaO/C

60.28

16.56

12.63

8.75

Wheat straw

Fluidized bed

MoS catalyst

25.4

27.5

22

16.3

Corn stover

Fluidized bed

MoS catalyst

26.9

24.7

23.7

15.3

Coffee husk

Downdraft fixed bed

–

6.6

13.8

12.1

14.8

Citrus peels

Fixed bed

Dolomit, MgO & Ni/Al2O3

60–65

15–25

15–23

225g/m2 and 225g/m2, respectively. Cardboard is mostly used in commercial industries for packaging and

Syngas production from wood and cardboard waste

413

safe delivery of goods and materials to the customer’s doorstep. Due to COVID-19 pandemic, a dramatic surge in e-commerce has increased the production, supply, and demand of cardboard globally. This not only increased the price of the cardboard but also increased the quantity of waste cardboard collected back from the customer’s end. The cardboard and packaging paper-based materials also come from natural origin, where cellulose is the prime constituent of it, which is mainly obtained from plants [15]. In recent times, the global supply chain and accessibility of woody biomass sources, including wood and cardboards, received an enormous focus due to its growing demand and consumption in the renewable energy generation sector. However, there are certain criteria for the biomass usage, i.e., native forest protection, biodiversity enhancement, sustainability, forest products competitiveness, and appropriate development of policies, etc., which must be fulfilled during bioenergy production from woody biomass [16]. The proceeding sections will elaborate briefly the availability and supply chain of woody biomass sources in different regions of the world.

2.1 Australia Australia’s potential bioenergy resources are large with several underutilized resources in plantation forest residues and waste streams. Potential feedstocks of the future include plantation of new type of trees. Several studies have been conducted to assess Australian bioenergy resource potential (spatial and temporal) estimates for several different categories [17,18]. The proportion of biomass potentially available will depend on the value of biomass relative to competing uses, impact of their removal (retention of biomass in situ returns nutrients to soil, improves soil structure and moisture retention), and global oil prices. The right economic conditions may result in some of the biomass potentially being used for bioenergy production. Depending on the price point, biomass may be diverted to biofuels or electricity generation; sawmill residues otherwise sold for garden products (mulch and potting mix), pulpwood chipped and exported to overseas or used for paper production. Surplus of any of these may be diverted to bioenergy if it is a higher-value energy products or value added chemical products such as hydrogen. Australia’s total plantation area in 2010 was nearly 2 Mha, distributed within 15 NPI regions (Fig. 1 and Table 1). Assuming little change in future plantation area when forecasting wood supply, estimated amount of biomass potentially available from plantations ranges from about 11 to 14 Mt/y over the period 2010–2050 (Table 1). For 2010, the potential availability of biomass ranges from 40 kt to over 2.3 Mt between the NPI regions. Four NPI regions have estimated potentially available biomass between 1.2 and 2.3 Mt/y [20].

2.2 Asia (India and China) In 2015, Zhang et al. reported the resources for forest biomass energy in China, which estimated that 169 Mt of forest biomass resources were available in 2010. These sources were bamboo residue (6%), fire wood (15%), fuel wood (4%), residue of wood felling and bucking (38%),

414 Chapter 17

Fig. 1 Geospatial map of Australia showing forest, agriculture and potential bioenergy/biomass plant location. Reprinted from A. Lang et al., Australia’s under-utilised bioenergy resources, Waste Biomass Valoriz. 5 (2) (2014) 235–243.

and residue of wood processing (37%) [21]. Zhao also described the biomass resources in China, which included the industrial wood waste and forestry residue [22]. The authors separated the wood species into three groups—broadleaved, pine, and spruce. The contribution of each group was based on seventh forest inventory, which presented the percent of broadleaved group (49.2%), pine group (17.8%), and spruce group (33.0%). Furthermore, seventh forest inventory also showed the annual productivity of forest as 3.85m3/ha. In 2030, 77 Mha land will be designated for reforestation, which will not be harvested until 2060. Overall, 223.04Mha land will be allocated for total forest, and 124.09Mha land will be occupied for special purpose and protected forest. Commercial forest will be used for remaining timber land. The purpose of protected forest is linked with the maintenance and protection of natural sources, biodiversity as well as cultural resources [22].

Syngas production from wood and cardboard waste

415

Table 1 Plantation forest biomass (kt/y) potentially available for bioenergy production in 15 National Plantation Inventory Regions in Australia [19]. SN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

NPI region

2010

2030

2050

Western Australia Northern Territory Mt Lofty Ranges Green Triangle North Queensland South-East QLD Northern Tablelands North Coast of NSW Central Tablelands Southern Tablelands Murray Valley Central Victoria Central Gippsland East Gippsland Tasmania Total

2347 367 133 2334 86 652 104 181 501 44 1211 382 570 336 1683 10,931

2676 288 208 2968 189 921 120 845 482 81 1199 424 785 375 2659 14,220

2007 430 115 2610 171 873 162 332 506 78 1207 354 864 319 2816 12,844

In 2017, Brack revealed the supply and demand of woody biomass for generation of power and heat in different countries. It showed that wood and other natural sources of biomass are extensively used for cooking and heating purposes in India due to its massive rural population, which is resulting in forest degradation [23]. Large quantities of forest and agricultural waste are produced in India, where bulk portion of this waste is consumed in domestic, industrial, and commercial activities such as domestic fuel, cattle fodder, boiler fuel, cardboard manufacturing, rural housing construction material, and other relevant applications. In 2005, 85% consumption of countryside energy fuel was dependent on biomass, while majority of it was used for cooking purpose [24,25]. Therefore, the production of biomass energy in India is highly focused on the bagasse waste and agriculture residue [23]. Overall, current biomass availability in India is expected to be over 500 Mt/y, with 120–150 Mt/y being utilized for energy production.

2.3 Europe Wood waste is produced in all the major European countries such as Germany, France, Finland, France, Italy, and the United Kingdom. Among these, Germany is considered as the largest wood waste producer. Factors contributing to the growth of wood waste in Germany include construction and demolition, wood packing, waste from municipal sources, industries of wood processing, import of wood, and other sources (construction of railways and personal households). Overall, 11.9 million tonnes of wood waste was produced during 2015 in Germany [26].

416 Chapter 17 According to bioenergy Europe statistical report 2019, seven EU28 member states had at least 50% forest and other woodland coverage in 2015, with almost three-quarters of Finland and Sweden covered in forest. The report also revealed that 42% of EU28 land area (182 Mha) was covered with forest and other woodland in 2015. Among this land, 161 Mha consisted of forest area in which 134Mha of land was used for wood supply. Moreover, the area for forest and wood land was also showing an increasing trend from 1990 to 2015. Along these lines, an average 0.2% yearly increase was observed for forest and other types of wood land area in EU28 [27]. Most popular species are Populus, Eucalyptus, Paulownia, Robinia, and Salix with their average yield (tDMha1) of 10–33, 10–20, 3–45, 2–10, and 8–12, respectively [28].

2.4 North and South America The United States is utilizing majority of its wood for the production of bioenergy to meet the domestic and international demands, which makes it the largest wood producer [23]. Short rotation woody crops (SRWC), harvest residue, municipal and construction/demolition waste, and mills residue are most widely used [29]. Moreover, almost 170 million acres of cropland was suitable for plantation of SRWC during 2000 in the United States [29,30]. Biomass yield of 37–55 dry tons per acre was achieved from 900 trees per acre under a 6-year managed rotation [29,31]. However, production of SRWC for energy use in Latin America is limited [32], even though most of the South American countries have tropical conditions, which are suitable high biomass yield from SRWC. Biomass yield of 30–50 tons/ha can be projected in 3–4 year rotation with 10–20 tons/ha/year increment. Moreover, fertilization of soil can be adopted to avoid its degradation for continuous production of biomass [32]. Globally, Canada is the biggest biomass producer on the basis of per capita [33], with 347Mha of its land being covered with forest or 9% of the total forest in the world [34]. Canada is also the principal producer of harvesting wood products (HWP) [34] and third largest timber exporter [33]. Additionally, 8.4 million odmt (oven dry metric tonnes) of woody biomass is available in British Columbia, Canada, which includes sawmill wood off-cuts and residue of forest harvesting. This woody biomass can be used as feedstock for production of bioenergy [35,36]. However, the production of bioenergy via woody biomass is not mature in Canada [33]. The production of plantation wood in Brazil was 15.5 million m3 in 2009, of which 93% was utilized for domestic purpose [37]. In Brazil, the planted forest occupied 7.84 Mha of land of which 5.67 Mha area was allocated to Eucalyptus [38]. The total industrial production of Eucalyptus was recorded as 138 million m3 in 2013 [37]. In addition, the southern region of Brazil (state of Rio Grande do Sul) is the biggest firewood producer, which provided 24.2% of overall country production during 2013 [38]. Brazil also remained at top in the world for production of renewable energy resources with a contribution of 43.5% in 2016 [38].

Syngas production from wood and cardboard waste

417

2.5 Rest of the world Woody biomass is also produced in many other countries, where the major part of it is exported due to unavailability of biomass-based energy sector. However, it is domestically utilized for cooking and heating purposes in these countries [23]. Forestry is one of the leading sectors of the Russian economy, where 82.1 billion m3 of wood stock is available, containing 77% of value-added species, and 635 million m3 is the allowable volume of cutting area. Russia fulfills its domestic wood goods and timber requirements. It also generates revenue through export of these products in several countries of the world [39]. In Northwest Russia, almost 52% of land area is covered with forest, of which 10 billion m3 stock includes aspen (6%), birch (21%), pine (29%), spruce (42%), and other species of trees (1%), and 6 billion m3 of this forest stock was overmature or mature. Among 106.2 million m3 u.b. (under bark) defined allowable cut, approximately 50 million m3 u.b. of this was actually cut in 2006. Therefore, the contributions of mature stand, thinning, and felling were counted as 40.2, 5.1, and 4.7 million m3, respectively [40]. Malaysia is the largest palm oil exporter in the world due to the tropical climate of the country. Sabah and Sarawak are the main states for plantation of palm oil. It covers 1.2Mha area, which comprises 30% of total country palm oil cultivated area [41]. The overall area for palm oil plantation is around 4 million ha, where 70% of this land area is situated in Johor, Pahang, Sabah, and Sarawak. Furthermore, the production of empty fruit bunches and fresh fruit bunches accounted for 17 and 75 million tonnes, respectively [41,42]. More than 20Mha of Turkey’s land area is covered in forest, which is approximately >25% of its land area. In addition, 50% of this area is productive, where broad-leaved trees and conifers are the major wood tree species with a contribution of >45% and 55%, respectively. Several types of woody plants are found in Turkey forest, which also includes hardwood plants such as alder, beech, chestnut hornbeam, oak, etc. In Turkish Forest, 30 million m3 of wood is produced, among which around 60% is consumed to meet the firewood requirement [41]. According to Food and Agriculture Organization (FAO) report, 1633 Mt of total woody biomass was generated from productive forest area in Turkey, which was about 160.5 tons/ha [16,43]. Subsequently, 935 million m3 of total forest potential is predicted in Turkey with an increment of 25 million m3/year. This new plantation provides around 90% of productive forest. It was also expected that the demand of wood will climb up to 43 million m3/year in 2020. On the other hand, 5 Mha of forest land area is allotted to the energy sector in Turkey [41,44].

2.6 Cardboard supply and demand growth Cardboard was originally invented by Scottish born Robert Gair in 1890 [45], which made it possible to move goods and materials from heavy and expensive wooden box, casks, and crates

418 Chapter 17 to lightweight, strong, and easy-to-store and -assemble cardboard boxes. Today, the boom in e-commerce and restaurant/grocery home delivery and the huge spike in the number of businesses involved in e-commerce, demand for cardboard has been increasing rapidly. Christmas 2020 was a record-breaking delivery of about 1.5 billion packages worldwide over the festive period and in the United Kingdom only, around 200 million extra home deliveries were placed. Although accurate data can’t be found in the literature, an analysis estimated that only Amazon shipped 415 million packages in July 2020 alone. According to WIRED 2021, cardboard production company DS Smith alone produced 17 billion boxes worldwide in the year 2020, 84% were sent to customers and remaining for Industrial packaging, and this volume demand is 30% up on the year 2019 cardboard demand. Due to the growing trend of e-commerce either because of increasing trend as global e-commerce retailer giants such as Amazon, Alibaba, ebay or uber have increased marketing and system reliability, or it is due to Covid-19 pandemic (lockdown and other travel/shopping restrictions), global consumption of cardboard has been increasing dramatically from 2020. According to Confederation of Paper Industries (CPI) UK, around 80% of the corrugated cardboard packaging is recycled, which contributes about 75% of the material for newly made cardboard, and remaining 25% material is added up from virgin fiber cellulose. Cardboard can be recycled unless it is contaminated due to the following reasons [46]: • • • • •

Pizza boxes, takeout containers, or other boxes with food residue on them Cardboard that’s gotten wet Frozen food containers Waxed cardboard items such as milk and juice containers Plastic-lined cardboard items such as produce containers

Even if only the unrecycled (20%of the global) cardboard supply can be potentially used for syngas production, it will be a significant contribution to substitute fossil-fuel-based energy production and contribute in reducing GHG emission.

3. Structure and composition of wood and cardboard The energy content and the composition of the produced gas from biomass gasification depend mainly upon used biomass structure and composition [47]. Also, the type and design of the gasifier, gasification operating conditions, and pretreatment of the biomass depend upon the structure and composition of the biomass [48]. For instance, biomass containing a significant percentage of ash and moisture needs to be treated and gasified under different operating conditions than the biomass having lower ash and moisture concentration. The methods for estimating biomass composition can be broadly divided into three different analyses, namely proximate, ultimate, and structural analysis. Over the years, several empirical correlations have been developed that also estimate the density of biomass from the proximate analysis. In the

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ultimate analysis, the percentage of carbon, hydrogen, nitrogen, sulfur, and oxygen is estimated [49].

3.1 Structural analysis Lignocellulosic biomass, such as wood, is mainly composed of cellulose, hemicellulose, and lignin, whose relative composition is determined by structural analysis. Cellulose is crystalline, whereas hemicellulose and lignin are relatively more amorphous and more volatile. Hardwoods are rich in cellulose content, whereas herbaceous biomass such as agriculture and forest residues is richer in hemicellulose and lignin content. Cellulose content requires additional energy to break their crystalline lattice structure. Although hemicellulose requires lower energy to decompose, biomass with more hemicellulose content (herbaceous biomass) contains more moisture, which may not be desirable during thermal gasification. However, herbaceous biomass with more moisture content is desirable for biological gasification. Lignocellulosic biomass contains more than 10% (w/w) of lignin [50]. Cardboard and waste paper mainly consist of cellulose, hemicellulose, lignin, and ash. During the manufacturing of paper and cardboards, inorganic additives are added as per the specifications of the product. Therefore, the cardboard waste consists of a greater amount of ash than woody biomass. Total lignin content in cardboard waste is calculated by the acetyl bromide method and is estimated to range from 8% to 12% [51,52]. Sotoudehnia et al. [51] reported that corrugated cardboard municipal waste contains acid-soluble lignin of approximately 0.02%, and Klason lignin was approximately 11.3%.

3.2 Proximate analysis Proximate analysis estimates moisture content, volatile matter, ash content, and fixed carbon. European standards (EN) EN14774, EN14775, and EN15148:2009 can be used to analyze moisture, proximate, and ultimate content of the biomass, respectively [53]. Generally, moisture content of 200°C), some hydrocarbons and sulfur decompose from the biomass to release vapor along with permanent gases such as CO, CO2, and H2, which constitutes the volatile matter [54]. Volatile matter content is desirable for syngas production and higher volatile matter results in more energy-efficient gasification. Fixed carbon and ash content are the solid content remaining in the biomass when the biomass is heated in the absence of oxygen. The fixed carbon is converted into CO and CO2 by gasification in the presence of oxygen as presented in the reaction 1–6 in Section 5.1 of this chapter. However, the ash is composed of solid metals that remain in the gasifier bed after complete gasification. Generally, Si, Al, Fe, Ca, S, and a small amount of alkali metal and alkaline earth metals such as Mg, K, Ti, Na, and P are present in the biomass [55]. The ash content is not desirable as it can result in slag formation in the gasifier [53]; however, it may

420 Chapter 17 help in enhancing catalytic activity due to the presence of alkali metal and alkaline earth metals [55]. Table 2 shows the proximate analysis of woody biomass and cardboard reported in the literature. In woody biomass and cardboard waste, the moisture content is generally in the range of 10%–15%. The volatile matter in woody biomass ranges from 76% to 86% on a dry basis, which on average is higher in softwood biomass. In cardboard waste, more than 80% (dry basis) volatile matter content is reported. Fixed carbon content in woody biomass ranges from 10% to 25% as presented in Table 2. However, for cardboard waste, the fixed carbon content is less than 10%. The ash content in most of the woody biomass is