Renewable Hydrogen: Opportunities and Challenges in Commercial Success

By Sudhir Kumar

Renewable Hydrogen: Opportunities and Challenges in Commercial Success presents fundamental principles and the latest research and technological advances in renewable hydrogen commercialization. With commercial scenarios and case studies, the book offers practical guidance for the scale-up of hydrogen production and storage.

Beginning with an introduction to alternative energy resources, Part 1 presents a deep dive into the chemical, biochemical and electrochemical processes of hydrogen production. Part 2 discusses hydrogen storage and transportation, with Part 3 reviewing the applications of hydrogen in the automobile, space and chemical industries. Finally, Part 4 considers future perspectives, including challenges and techno economics.

Renewable Hydrogen: Opportunities and Challenges in Commercial Success is an essential read for those seeking to understand how to successfully apply hydrogen production and storage research to an industrial scale.

• Presents a comprehensive review of hydrogen production and scale-up perspective
• Provides a detailed compilation of commercial scale hydrogen storage, along with opportunities and challenges faced during economical production
• Highlights future trends and government policies that will impact the renewable hydrogen production

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 12, 2024
• ISBN: 9780323953801

Book preview

Renewable Hydrogen

Opportunities and Challenges in Commercial Success

Edited by

Mohit Bibra

Bionova Scientific Inc., Fremont, CA, United States

Rajesh K. Sani

Department of Chemical and Biological Engineering, South Dakota School of Mines and Technology, Rapid City, SD, United States

Sudhir Kumar

Department of Biotechnology and Bioinformatics, Jaypee University of Information Technology, Waknaghat, Himachal Pradesh, India

Table of Contents

Cover image

Title page

Copyright

Contributors

About the editors

Section 1. Hydrogen production

Chapter One. Hydrogen production from biomass by chemical processes

1.1. Introduction

1.2. Methods of hydrogen production

1.3. Recent advancements and research findings

1.4. Challenges and contaminations in hydrogen (H2) production

1.5. Industrial application considerations

1.6. Market insights

1.7. Technoeconomic analysis

1.8. Conclusion

Chapter two. Biohydrogen production by biological methods

2.1. Introduction

2.2. Hydrogen production by biological methods

2.3. Substrates for biohydrogen production

2.4. Reactors

2.5. Commercial-scale production

2.6. Conclusions

Chapter Three. Hydrogen production by electrochemical process

3.1. Introduction

3.2. Hydrogen production: Existing technologies and drawbacks

3.3. Advantages for opting electrochemical process

3.4. Electrochemical process setup

3.5. Process variables and their effect on hydrogen production

3.6. Technical challenges and future prospects

3.7. Conclusion

Chapter Four. Hydrogen production scale-up

4.1. Introduction

4.2. Evolution of hydrogen using a photocatalyst

4.3. Designing effective photocatalysts for solar H2 generation

4.4. Transformation of surface in semiconductor photocatalysts

4.5. Comparison of hydrogen production via photocatalysis

4.6. Conclusion

Section 2. Hydrogen storage and transportation

Chapter Five. Exploring the capabilities of solid-state systems as a means of storing hydrogen

5.1. Introduction

5.2. Various solid hydrogen storage systems

5.3. Applications of hydrogen storage systems and future perspective

Chapter Six. Hydrogen: Empowering sustainable transportation and mitigating greenhouse gas emissions

6.1. Introduction

6.2. Hydrogen—as transportation fuel

6.3. Socioeconomic benefits of hydrogen transportation

6.4. Conclusion

Chapter Seven. Fuel cell: Applications and future prospects

7.1. Introduction

7.2. History of the fuel cell

7.3. Working principle

7.4. Types of fuel cell

7.5. Recent developments

7.6. Application of artificial intelligence in fuel cells

7.7. Challenges and future prospects

Section 3. Applications of hydrogen

Chapter Eight. Hydrogen in transportation

8.1. Introduction

8.2. Fuel cells

8.3. Use of hydrogen in transportation

8.4. Challenges in hydrogen fuel

8.5. Future perspectives

8.6. Conclusions

Chapter Nine. Renewable hydrogen opportunities and challenges

9.1. Introduction

9.2. Hydrogen production technologies

9.3. Renewable energy sources for hydrogen production

9.4. Hydrogen storage and transportation

9.5. Hydrogen utilization and applications

9.6. Economic and environmental aspects

9.7. Technological and research challenges

9.8. Global perspectives and market outlook

9.9. Conclusion and future directions

Chapter Ten. Hydrogen in the chemical industry

10.1. Introduction

10.2. Hydrogen production in the chemical industry

10.3. Gray, blue, and green H2 concepts

10.4. Hydrogen utilization in the chemical industry

10.5. Hydrogen storage and transportation

10.6. Hydrogen-related challenges and solutions

10.7. Current developments and outlook

Section 4. Opportunities and challenges

Chapter Eleven. Technoeconomic analysis of hydrogen production

11.1. Introduction

11.2. Technoeconomic analysis

11.3. Technoeconomic analysis of hydrogen production

11.4. Sensitivity analysis

11.5. Future perspectives

11.6. Conclusions

Chapter Twelve. Commercial-scale hydrogen production

12.1. Introduction

12.2. Commercially hydrogen producers

12.3. Commercial processes

12.4. Hydrogen product specification

12.5. Product storage, distribution, and transportation

12.6. Safety aspects

12.7. Future perspectives

12.8. Conclusions

Chapter Thirteen. Current status of renewable hydrogen production

13.1. Introduction

13.2. Transition to renewable energy

13.3. Policy support for hydrogen

13.4. Challenges

13.5. Future perspective

13.6. Conclusion

Index

Copyright

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Contributors

Isha Agarwal,     Department of Biotechnology and Bioinformatics, Jaypee University of Information Technology, Waknaghat, Himachal Pradesh, India

Upasana Bagri,     School of Chemical Engineering and Physical Science, Lovely Professional University, Phagwara, Punjab, India

Mohit Bibra

Bionova Scientific Inc., Fremont, CA, United States

Department of Chemical and Biological Engineering, South Dakota School of Mines and Technology, Rapid City, SD, United States

Umakant Chaudhari,     Department of Environmental Science and Technology, Central University of Punjab, Bathinda, Punjab, India

Akshay Kumar Chaudhry,     Department of Civil Engineering, Chitkara University Institute of Engineering and Technology, Chitkara University, Rajpura, Punjab, India

Vijay Kumar Garlapati,     Department of Biotechnology and Bioinformatics, Jaypee University of Information Technology, Waknaghat, Himachal Pradesh, India

Rajneesh Jaswal,     Department of Biomedical Engineering, Boston University, Boston, MA, United States

Prasann Kumar,     Department of Agronomy, School of Agriculture, Lovely Professional University, Phagwara, Punjab, India

Sachin Kumar,     Biochemical Conversion Division, Sardar Swaran Singh National Institute of Bio-Energy, Kapurthala, Punjab, India

Sudhir Kumar,     Department of Biotechnology and Bioinformatics, Jaypee University of Information Technology, Waknaghat, Himachal Pradesh, India

Disha Kumari,     Department of Environmental Science and Technology, Central University of Punjab, Bathinda, Punjab, India

Sunil Mittal,     Department of Environmental Science and Technology, Central University of Punjab, Bathinda, Punjab, India

Mamta Pal,     Biochemical Conversion Division, Sardar Swaran Singh National Institute of Bio-Energy, Kapurthala, Punjab, India

Vijaykumar Patel

Department of Chemical and Biological Engineering, South Dakota School of Mines and Technology, Rapid City, SD, United States

School of Chemical Engineering and Physical Science, Lovely Professional University, Phagwara, Punjab, India

Payal Sachdeva,     Department of Civil Engineering, Chitkara University Institute of Engineering and Technology, Chitkara University, Rajpura, Punjab, India

Dipayan Samanta,     Department of Chemical and Biological Engineering, South Dakota School of Mines and Technology, Rapid City, SD, United States

Rajesh K. Sani,     Department of Chemical and Biological Engineering, South Dakota School of Mines and Technology, Rapid City, SD, United States

Ajit Kumar Sharma,     School of Chemical Engineering and Physical Science, Lovely Professional University, Phagwara, Punjab, India

Swati Sharma,     Department of Biotechnology and Bioinformatics, Jaypee University of Information Technology, Waknaghat, Himachal Pradesh, India

Deepak Sharma

Department of Biotechnology and Bioinformatics, Jaypee University of Information Technology, Waknaghat, Himachal Pradesh, India

Department of Biotechnology, Chandigarh College of Technology, Chandigarh Group of Colleges, Mohali, Punjab, India

Joginder Singh,     Department of Botany, Nagaland University, Lumami, Nagaland, India

Gursharan Singh,     School of Allied Medical Sciences, Lovely Professional University, Phagwara, Punjab, India

Harminder Singh,     Department of Chemical Engineering and Physical Sciences, Lovely Professional University, Jalandhar, Punjab, India

Richa Tungal,     Osmose Utilities Services Inc., Peachtree City, GA, United States

Tanishka Tyagi,     Department of Environmental Science and Technology, Central University of Punjab, Bathinda, Punjab, India

About the editors

Dr. Mohit Bibra is working as a Senior Scientist, at Bionova Scientific, Inc., California, USA. With bachelor's and master's (Hons. Science) degree in Microbiology and doctorate in Chemical and Biological Engineering from South Dakota School of Mines and Technology, South Dakota, USA, his research interests center around 6Fs—Future, Fermentation, Feed, Food, Fuel, and Functional molecules for developing sustainable fermentation-based bioprocesses in order to alleviate limitations and problems in the food, animal feed, energy, and functional molecule industries He has supported in business value addition and cost savings worth millions while working with different entities. He has been an author of 10 peer-reviewed articles, 4 book chapters, and 3 invention disclosures. He is also serving as a reviewer for Scientific Reports (Nature), Biotechnology for Biofuels, Bioresource Technology, Frontiers in Bioengineering and Biotechnology, Frontiers in Energy Research, Frontiers Microbiology, ACS, and IEEE Access and served as the member of an organizing committee at the Society of Industrial Microbiology and Biotechnology.

Dr. Rajesh K. Sani is a Distinguished Professor in the Departments of Chemical and Biological Engineering at the South Dakota School of Mines and Technology, South Dakota, USA. He led an NSF-funded consortium called BuG ReMeDEE, with 108 participants who focused on developing a Genome-to-Phenome Infrastructure for Regulating Methane in Deep and Extreme Environments. The consortium team took deliberate steps to ensure effective collaboration and integration among three jurisdiction research groups through active training and exercises. Over the past 18 years, he has been the PI and co-PI on 55 research projects. Sani and his group have published 125 peer-reviewed articles in high-impact factor journals and hold two patents. Dr. Sani has developed new biological engineering courses to actively engage biology, chemistry, environmental, and chemical and biomedical engineering undergraduate and graduate students. He established lab practices of peer mentoring and professional development and actively recruited graduate students traditionally underrepresented in STEM. He developed a set of teaching and lab lessons and broadly disseminated them to middle and high school teachers through hands-on exercises and platform presentations.

Dr. Sudhir Kumar is a Professor in the Department of Biotechnology and Bioinformatics at Jaypee University of Information Technology, Solan, India. He has been the PI and Co-PI of 10 funded research projects sponsored by Indian funding agencies and industries in the areas of biometallurgy, biofuels, and bioremediation. He was also a part of the research team of international projects during his tenure as a Research Scientist at the South Dakota School of Mines, South Dakota, USA. He is passionate for teaching and connecting rural people for developing cost-effective and sustainable technologies in the area of biofuels. He was involved in content creation of courses like genetics, microbiology, genetic counseling, and biofuels. He has 55 peer-reviewed publications in the area of environmental biotechnology. He has also guided 07 PhD students and 15 masters’ students so far. He has 3 patents to his credit on sustainable solutions processes. Beyond academia, his literary power extends to numerous published works, including scholarly articles and book chapters. Dr. Sudhir also shared his viewpoints on sustainable clean stoves in India through Nature’s correspondence section which was published in 2012.

Section 1

Hydrogen production

Outline

Chapter One. Hydrogen production from biomass by chemical processes

Chapter two. Biohydrogen production by biological methods

Chapter Three. Hydrogen production by electrochemical process

Chapter Four. Hydrogen production scale-up

Chapter One: Hydrogen production from biomass by chemical processes

Richa Tungal     Osmose Utilities Services Inc., Peachtree City, GA, United States

Abstract

This chapter delves into industrial-scale hydrogen (H2) production from biomass, emphasizing diverse methods and recent advancements. Crucial considerations for industrial applications, market dynamics, and cost analyses shape the future of large-scale hydrogen production from biomass. Thermochemical processes, including gasification and liquefaction, exemplify human ingenuity, unlocking renewable energy potential. Ongoing research and collaboration are imperative for a sustainable future. Environmental considerations drive cleaner methods, striking a balance for widespread adoption. In conclusion, the journey toward large-scale hydrogen production from biomass signifies continuous exploration, showcasing a collaborative effort to harness biomass' renewable potential for a more sustainable and energy-efficient future.

Keywords

Biomass to hydrogen (H2); Chemical hydrogen production; Hydrogen; Technoeconomic analysis

1.1. Introduction

In the dynamic landscape of industrial-scale hydrogen production, biomass stands out as a versatile and sustainable resource, offering a myriad of methods that span the spectrum from biological processes to advanced thermochemical techniques. These methods collectively represent a crucial stride toward establishing sustainable alternatives for the generation of hydrogen—a quintessential energy carrier and indispensable feedstock for a diverse array of industrial applications. The pursuit of harnessing hydrogen from biomass epitomizes a commitment to environmentally conscious energy practices, aligning with the global imperative for cleaner and renewable fuel sources.

Biological hydrogen production, a standout facet within this spectrum, unfolds a captivating realm where microorganisms become instrumental players in the extraction of hydrogen. The ingenious utilization of bacteria, algae, and other microorganisms brings forth pathways where natural processes yield hydrogen as a by-product. Noteworthy advancements in this realm include the development of enhanced microbial strains and the refinement of fermentation processes, pushing the boundaries of efficiency and scalability in biological hydrogen production.

In parallel, thermochemical processes constitute a powerful arm of biomass-to-hydrogen conversion. Gasification, a prominent member of this category, orchestrates the transformation of biomass into syngas—a precursor laden with hydrogen. The intricate dance of controlled heating and chemical reactions in gasification presents a compelling narrative of technology evolving to unlock the potential of biomass for large-scale hydrogen production. This chapter serves as a gateway to understanding the nuances of gasification and its counterparts, pyrolysis, and liquefaction, where carefully orchestrated processes leverage heat and chemistry to liberate hydrogen from biomass matrices.

As we embark on this comprehensive exploration, this chapter unravels the mechanisms that govern these diverse methods, shedding light on their inner workings, recent advancements, and the intricate web of environmental considerations that envelop their implementation. The symbiosis of technological innovation, sustainability imperatives, and the quest for efficient, clean energy unfolds within these pages, offering a holistic view of biomass-driven hydrogen production in the industrial realm.

1.2. Methods of hydrogen production

1.2.1. Biological hydrogen production

In the realm of biological processes, two standout methods showcase the remarkable potential of harnessing hydrogen through the intricacies of microbial activities.

1.2.1.1. Dark fermentation

Dark fermentation¹ stands as a pioneering method that capitalizes on the metabolic prowess of hydrogen-producing bacteria, notably exemplified by Clostridium species. This process involves the anaerobic fermentation of organic biomass materials such as sugars, starches, and cellulose, orchestrating a biochemical symphony that culminates in the release of hydrogen gas. The dark fermentation pathway illuminates a pathway where microbial alchemy transforms diverse biomass feedstocks into a valuable, clean energy source—hydrogen.

The versatile bacterium Clostridium acetobutylicum takes center stage in dark fermentation, showcasing its ability to metabolize sugars and produce hydrogen along with valuable by-products such as acetone and butanol. This example highlights the potential synergy of biological hydrogen production with the generation of valuable chemical compounds, contributing to the overall efficiency and economic viability of the process.

1.2.1.2. Photofermentation

In a testament to the harmony between biology and light energy, photofermentation emerges as a captivating method for hydrogen production. Harnessing the capabilities of photosynthetic microorganisms such purple nonsulfur bacteria and green algae, this process converts organic compounds into hydrogen gas. The combined activity of microbial photosynthesis and hydrogen evolution paints a picture of sustainable energy generation, driven by the limitless power of sunlight.

The purple nonsulfur bacterium Rhodopseudomonas palustris takes center stage in photofermentation, showcasing its ability to thrive in anaerobic conditions and produce hydrogen by capturing solar energy. The intricate interplay between microbial photosystems and hydrogenase enzymes exemplifies the elegance of harnessing biological processes for renewable hydrogen production.

As we delve into these biological methods, it becomes evident that nature's microbial toolkit offers a rich repertoire of strategies for unlocking hydrogen from diverse biomass sources. The synergy of dark fermentation and photofermentation exemplifies the versatility inherent in biological processes, providing a foundation for sustainable hydrogen production with a touch of microbial ingenuity.²

1.2.2. Thermochemical processes

In the quest for diverse and efficient routes to unlock the energy potential within biomass, thermochemical processes emerge as a formidable alternative, harnessing the transformative power of controlled heating and chemical reactions.

1.2.2.1. Gasification

Gasification stands as a prominent player in the realm of thermochemical processes. This technique coordinates a meticulously regulated interplay of heat and biomass, leading to the generation of syngas, a synthesis gas abundant in hydrogen content. The essence lies in subjecting biomass to elevated temperatures in a controlled environment, where the intricate chemistry unfolds to liberate hydrogen from its organic confines. Gasification, therefore, presents a pathway to tap into the renewable wealth of hydrogen encapsulated within various biomass feedstocks. In other words, gasification is fundamentally a thermochemical procedure that transforms biomass materials into a gaseous form. The outcomes of gasification include producer gas, comprising carbon monoxide, hydrogen, methane, and various other inert gases.

Wood gasification exemplifies the application of this thermochemical process, wherein wood biomass undergoes a sequence of chemical reactions in a low-oxygen environment, leading to the generation of syngas. The hydrogen-rich syngas obtained from wood gasification becomes a versatile precursor for diverse industrial applications, showcasing the adaptability of gasification in harnessing hydrogen from renewable sources.

Gasification reactors for biomass can be categorized into three main types: fixed bed gasifiers, fluidized bed gasifiers, and entrained flow gasifiers. Among these, fixed bed gasifiers are particularly suitable for small-scale power generation plants, typically up to 10 MW. They are further classified as updraft and downdraft gasifiers. In the updraft configuration, biomass is fed from the top while the gasifying agent (GA) is introduced from the bottom in a countercurrent fashion. Conversely, in the downdraft configuration, both biomass and GA are supplied from the top in a cocurrent manner. The operational sequence involves stages of drying, pyrolysis, and reduction, ultimately leading to the combustion zone, with syngas extracted either from the top (updraft) or the bottom (downdraft). It is essential to note that in the downdraft configuration, gaseous products from pyrolysis undergo the reduction process, while in the updraft configuration, they are directly incorporated into syngas.³

Updraft gasifiers exhibit elevated thermal efficiency due to several factors, including effective contact between biomass and the GA, minimal pressure drop, modest slag formation, and a straightforward and sturdy design. However, these gasifiers are not without their limitations. They are prone to higher tar content in the produced syngas and have restricted flexibility in terms of loading and process operation. The operational temperature range spans from a minimum of 650–700°C to a maximum of 950–1150°C.⁴

1.2.2.2. Syngas production via gasification

Gasification plays a pivotal role in the thermochemical transformation of biomass. In the presence of a GA, biomass undergoes conversion, resulting in a versatile gaseous mixture commonly known as syngas or synthesis gas. This syngas holds potential applications in energy production (heat and/or electricity generation), chemical manufacturing (such as ammonia production), and the synthesis of biofuels. Additionally, the process generates a solid residue known as char. Syngas comprises primary components such as CO, H2, CO2, and CH4, along with secondary components such as H2O, H2S, NH3, tar, and other trace species. The specific composition is influenced by factors such as feedstock characteristics, operational conditions (GA, gasifier temperature and pressure, type of bed materials), and the gasification technology employed.

Biomass → CO2 + H2 + CO2 + CH4 → Syngas (SNG)

Based on information from the International Energy Agency (IEA) Bioenergy Task 33E—Thermal Gasification of Biomass database,⁵ the global landscape of biomass gasification reveals 114 operational plants, 14 idle/on-hold facilities, and 13 under construction/planned projects. This totals 141 plants, each serving distinct purposes with the produced syngas. Among them, 106 plants are dedicated to power production, contributing around 356 MW of global electric power and approximately 185 MW of thermal power. Furthermore, 24 plants focus on liquid fuel production, churning out an estimated 750,000 tons/year of liquid fuel from biomass-derived syngas. Gaseous fuel production is handled by eight plants, generating about 3.2 × 108 Nm³/year of SNG and H2. Seven plants are engaged in chemical production, resulting in a global output of around 9000 tons/year of chemicals derived from biomass-derived syngas. Notably, four plants utilize syngas for both power and fuel production simultaneously.⁴,⁶

During gasification, a series of distinct processes within a gasifier, encompassing drying, pyrolysis, combustion, and gasification stages occurs. The dehydration process occurs at approximately 100°C, with resulting steam integrated into the gas flow, potentially participating in subsequent chemical reactions such as the water–gas reaction at elevated temperatures. Pyrolysis unfolds within the range of 200–300°C, releasing volatiles and yielding char, leading to up to a 70% weight loss in biomass. Subsequently, combustion takes place as volatile products and char reacts with oxygen, primarily forming carbon dioxide and minimal carbon monoxide, generating heat for subsequent gasification reactions. The gasification process, initiated as char, reacts with carbon and steam and produces carbon monoxide and hydrogen. Additionally, the reversible gas-phase water–gas shift reaction reaches equilibrium rapidly, balancing concentrations of carbon monoxide, steam, carbon dioxide, and hydrogen. The introduction of a limited amount of oxygen or air allows the burning of organic material to produce carbon monoxide and energy, driving a secondary reaction converting additional organic material to hydrogen and carbon dioxide. Further reactions generate methane and excess carbon dioxide (Fig. 1.1).⁶

C + O2 ↔ CO2 ΔH = −393.5 kJ/mol (1.1)

C + H2O ↔ H2 + COΔH = 131.3 kJ/mol (1.2)

CO + H2O ↔ CO2 + H2   ΔH = −41.1 kJ/mol (1.3)

Figure 1.1  Illustration indicating the distinct phases within the downdraft gasification process.

The basic reaction for gasification is given in Reaction (1.1). Gasification takes place when char engages with carbon and steam, resulting in the production of carbon monoxide and hydrogen through the Reaction (1.2). Furthermore, the reversible gas-phase water–gas shift reaction rapidly attains equilibrium at the temperatures present in a gasifier. This equilibrium ensures a balance in the concentrations of carbon monoxide, steam, carbon dioxide, and hydrogen as shown in Reaction (1.3). Essentially, a controlled quantity of oxygen or air is introduced into the reactor, facilitating the combustion of some organic material to generate carbon monoxide and energy. This process propels a secondary reaction, transforming additional organic material into hydrogen and additional carbon dioxide.

1.2.2.3. Pyrolysis

In the realm of thermochemical alchemy, pyrolysis takes center stage as a transformative process. Here, biomass undergoes controlled decomposition in the absence of oxygen, yielding a triad of valuable products: gases, liquids, and char. Hydrogen, a key constituent of the gaseous fraction, emerges as a sought-after yield from this intricate thermal ballet.

Bioenergy pyrolysis is classified into three main types: slow (conventional) pyrolysis, fast pyrolysis, and flash pyrolysis, each characterized by different heating rates and solid residence times. Traditional slow pyrolysis, known as carbonization, has been historically used for charcoal production, featuring extended residence times (hours to days), relatively low temperatures (300–700°C), and the ability to handle various particle sizes (5–50 mm). This method allows for the gradual thermal decomposition of biomass, particularly lignocellulosic types, with a low heating rate, providing sufficient time for repolymerization reactions to maximize solid yields. Fast pyrolysis involves higher heating rates (410–200°C/second) and shorter residence times (0.5–10 seconds, typically <2 seconds), resulting in biooil yields of up to 50–70 wt%. Flash pyrolysis, characterized by rapid heating rates of 103–104°C/second and short residence times (<0.5 seconds), achieves exceptionally high biooil yields, reaching up to 75–80 wt%.⁷

In the pursuit of advancing biomass pyrolysis for enhanced energy efficiencies and tailored product outcomes, future research emphasizes a comprehensive understanding of the impact of pyrolysis parameters. This includes evaluating factors such as feedstock selection, reaction conditions, reactor configurations, and other variables, aiming to optimize pyrolysis performance, product yields, and properties.

The intricacies of biomass pyrolysis stem from the diverse decomposition behaviors of biomass components, influenced by various reaction mechanisms, rates, and thermal conditions. Previous studies confirm interactions among major woody biomass constituents, such as cellulose, hemicelluloses, and lignin, during pyrolysis, posing challenges in predicting pyrolysis characteristics based solely on the thermal behavior of individual components. Various interactions, such as those between hemicellulose and lignin, significantly impact the distribution of pyrolysis products.

The pyrolysis process involves parallel and series reactions, including dehydration, depolymerization, isomerization, aromatization, decarboxylation, and charring. Biomass pyrolysis generally comprises three stages: initial evaporation of free moisture, primary decomposition, and secondary reactions (oil cracking and repolymerization). These stages are intertwined, observable through thermal analysis. Specific heat, heats of reactions, and degradation pathways of biomass components have been extensively studied, shedding light on the complexity of cellulose, hemicelluloses, and lignin decomposition. Cellulose decomposition, represented by the Waterloo mechanism, involves dehydrogenation, depolymerization, and fragmentation, each dominant at different temperature ranges.⁸

1.2.2.4. Subcritical and supercritical biomass liquefaction

Operating under specific temperature and pressure regimes, subcritical and supercritical biomass liquefaction offers intriguing pathways within thermochemical processes. These methods transform biomass into valuable liquid products—biooil and biocrude in subcritical and supercritical conditions—both potentially carrying hydrogen within their molecular frameworks.

In the quest for sustainable hydrogen production, several researchers⁹–¹¹ have introduced a groundbreaking approach by incorporating water into biomass liquefaction processes. Unlike traditional methods, the inclusion of water serves a dual role as a solvent and a reactant, influencing the transformation of solid biomass into liquid forms. Researchers such as Tungal et al. have meticulously optimize key parameters such as temperature, pressure, and the water-to-biomass ratio, orchestrating conditions that not only enhance overall liquefaction efficiency but, more crucially, maximize the liberation of hydrogen—a pivotal energy carrier from woody biomass and wastepaper.

The experimental results shed light on the intricate dynamics of biomass liquefaction in the presence of water. Through systematic investigations, the researchers decipher the nuanced interactions that govern the liberation of hydrogen. Beyond the increased hydrogen yield, their work reveals the formation of hydrogen-enriched liquid products, potentially opening avenues for versatile applications in the realm of bioenergy. This approach not only contributes to the understanding of biomass transformation but also holds promising implications for scalable and sustainable hydrogen production in the broader context of a clean energy future.

The subcritical approach, employing relatively low temperatures, demonstrates the capability to effectively treat both lignocellulosic and woody biomass.⁹–¹¹ This process utilizes simple catalysts to enhance the production of hydrogen and synthesis gas in the gas phase. In the liquid phase, termed biocrude, various oxygenated hydrocarbons and organic acids, including lactic acid, acetic acid, and formic acid, are present. The biocrude can be processed to extract biooil, which, depending on catalysts and processing parameters, can be upgraded to produce gasoline or jet fuels. The extracted organic acids, rich in lactic acid, acetic acid, and formic acid, hold potential for diverse applications, such as the production of lactic acid polymers. The remaining biocrude undergoes further processing for acid production, utilizing wet oxidation techniques. Remarkably, the liquid water separated from biocrude retains utility for subsequent biofuel processing, as the water-soluble catalyst can be employed in successive biomass batches, ensuring an efficient and sustainable approach to biofuel production. This method generates a minimal amount of char, providing a significant advantage to the overall efficiency of the process.⁹–¹¹

1.3. Recent advancements and research findings

The recent advancements and research findings will be covered in the following categories.

1.3.1. Hydrogen production and environmental sustainability

1.3.1.1. Hydrogen production, storage, and transport for renewable energy and chemicals

Recent research abstracts shed light on advancements and environmental considerations in hydrogen production. Environmental footprint assessments evaluate the eco-benefit and eco-cost of various production methods, with hydrogen production from acid gas showing the highest eco-benefit, while biomass gasification incurs the highest eco-cost.

In one such study, an integrated system is proposed and evaluated for efficiently harvesting energy from the waste of the pulp mill industry, specifically black liquor (BL). The proposed system involves supercritical water gasification (SCWG) of BL, syngas chemical looping, and power generation.¹²

To minimize exergy loss and optimize energy efficiency, the study employs principles of exergy recovery and process integration methods. The main output of the system is hydrogen, with power being generated by utilizing the heat generated throughout the process. The process simulation, conducted using the steady-state process simulator Aspen Plus, defines energy efficiency in three categories: hydrogen production efficiency, power generation efficiency, and total energy efficiency. The results of the simulation indicate very high total energy efficiency, reaching about 73% for both integrated systems.

1.3.1.2. Integrated process for simulation of gasification and chemical looping hydrogen production using artificial neural network and machine learning validation

Integrated processes employing artificial neural networks for simulation and optimization, such as gasification and chemical looping, underscore the importance of predicting syngas composition and optimizing biomass gasification processes.

In the paper titled Thermochemical processes for CO2 hydrogenation to fuels and chemicals: Challenges and opportunities, the authors delve into catalytic thermochemical CO2 conversion technologies. The manuscript explores the potential use of CO2 as a carbon and oxygen carrier, aiming to reintegrate it into the carbon life cycle in the form of fuels or chemicals through thermochemical catalytic pathways. The discussion emphasizes the theoretical versatility of CO2 hydrogenation in forming various fuels and chemicals. However, practical considerations such as selectivity, conversion, operating range, kinetic limitations, and operational costs play pivotal roles in determining the end product.

The authors highlight small olefins (C2H4) and methanol as valuable target species due to their high value and versatility as chemical or fuel products. Nature's primary choices for CO2 conversion, CH4 and CO, are underscored for their thermochemical selectivity and formation pathways over a wide range of operating conditions. While less valuable as end products, CH4 and CO can serve as intermediate species, contributing to increased overall CO2 hydrogenation yield through secondary processes. The paper addresses the challenge of hydrogen requirement for CO2 hydrogenation, considering both the environmental impact and cost implications. The authors discuss alternatives to pure hydrogen, proposing low-value, small-chain paraffins as potential hydrogen carriers, emphasizing the direct pathway to olefin formation through carbon dioxide–assisted oxidative dehydrogenation of paraffins. The comprehensive analysis draws on theoretical considerations and recent advancements in both academia and industry, providing valuable insights into the challenges and opportunities in thermochemical CO2 hydrogenation processes.¹⁴

Researchers¹³,¹⁴ have spearheaded groundbreaking research in the realm of integrated processes for biomass conversion, offering a multifaceted perspective on optimizing the utilization of renewable resources. Their comprehensive approach combines various methodologies, seamlessly integrating biological and thermochemical processes to achieve a synergistic effect in biomass conversion. The research explores the intricate interplay between biological hydrogen production, thermochemical gasification, and subsequent power generation, envisioning a holistic system that maximizes energy efficiency and minimizes environmental impact.

In this work, a key highlight is the integration of supercritical water gasification (SCWG) of black liquor, a by-product of the pulp mill industry, with syngas chemical looping and power generation. The synergistic combination of these processes not only facilitates efficient energy recovery from waste biomass but also positions hydrogen as a central output. The authors employ advanced process simulation tools and integration principles to optimize exergy recovery, resulting in remarkably high total energy efficiency. The integrated processes exemplify a pioneering paradigm