Solid-Gaseous Biofuels Production E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Written by a team of industry experts and edited by one of the most prolific and well-respected engineering authors in the industry, this exciting new volume covers the latest processes, equipment, and applications for clean biofuel production.
With renewable and alternative energy sources becoming more and more important, and the growth in percentage of the overall energy used, biofuels production is more important than ever and is a huge part of taking up the slack in the transition from fossil fuels. This volume covers many of the newest state-of-the art processes, trends, and changes in the industry, combining information from many disciplines to deliver have-to-have solutions for the engineer or scientist’s daily problems. Whether in the plant or in the classroom, this exciting new volume is a must-have for any engineer, scientist, student, or other industry professional working in biofuel production.
Audience
Engineers, scientists, faculty and students, and industry professionals working in the biofuel industry.
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Veröffentlichungsjahr: 2024
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Table of Contents
Cover
Table of Contents
Series Page
Title Page
Copyright Page
Preface
1 Biofuel Production: Past to Present Technologies
1.1 Introduction
1.2 Types of Biofuels
1.3 Different Generation of Biomass for Biofuel Production
1.4 Conversion Strategies for Biofuel Production
1.5 Roadway to Biofuel Production Technologies
1.6 Conclusions
References
2 Biorefineries for the Sustainable Generation of Algal Biofuels
2.1 Introduction
2.2 Biorefinery Concept
2.3 Algal Biomass
2.4 Biofuels and Processing of Algal Biomass for Biofuel Production
2.5 Other Bioproducts Obtained from Algal Biomass
2.6 Current Situation Regarding Energy Consumption
2.7 Challenges and Future Perspectives
2.8 Conclusion
References
3 Biofuel Production from Waste Materials
3.1 Introduction
3.2 Biofuel From Waste Materials
3.3 Conclusion
Acknowledgments
References
4 Essentials of Liquefied Biomethane Gas (LBG)
4.1 Introduction
4.2 Biogas Upgradation Technologies
4.3 Methods to Produce Liquid Biomethane
4.4 Application
4.5 Challenges and Prospects
References
5 Exploring Cost-Effective Pathways for Future Biofuel Production
5.1 Introduction
5.2 Emerging Technologies for Cost-Effective Biofuel Production
5.3 Conclusion
References
6 Generation of Hydrogen Using Cyanobacteria
6.1 Introduction to Hydrogen Production by Cyanobacteria
6.2 Hydrogen Production Mechanisms by Cyanobacteria
6.3 Economic and Environmental Analysis of Hydrogen Production by Cyanobacteria
6.4 Setbacks of Hydrogen Production by Cyanobacteria
6.5 Hydrogen Patents’ Overview
6.6 Conclusion
References
7 Microstructural Engineering for Bioenergy Production
7.1 Introduction
7.2 Biomass Microstructure and Characterization
7.3 Microbial Engineering for Bioenergy Production
7.4 Plant Cell Wall Engineering for Bioenergy Production
7.5 Nanotechnology for Bioenergy Production
7.6 Microstructural Engineering for Bioreactors and Processing
7.7 Conclusion
References
8 Lignocellulosic Biomass as Feedstock for Biofuels: The State of the Science, Prospects, and Challenges
8.1 Introduction
8.2 Structural Chemistry of Lignocellulosic Biomass
8.3 Sources of Lignocellulosic Biomass
8.4 Energy Content in Lignocellulosic Biomass
8.5 Challenges in Bioconversion of Lignocellulosic Biomass into Biofuels
8.6 Pretreatment of Lignocellulosic Biomass
8.7 Bioconversion of Lignocellulosic Biomass into Biofuels
8.8 Lignocellulosic Biomass–Based Biorefineries
8.9 Conclusion
References
9 Limitations of the Firstand Second- Generation Solid-Gaseous Biofuels in a Time of Climate Emergency
9.1 Introduction: Global Population, Energy Consumption, and Climate Emergency
9.2 Feedstock Diversification of the Firstand Second-Generation Biofuels for Sustainable Bioenergy Production
9.3 Considerations for the Firstand Second-Generation Solid-Gaseous Biofuels Amidst the Climate Emergency
9.4 Conclusions and Future Perspectives
References
10 Advancements in Microbial Fermentation of Agro and Food Processing Wastes for Generation of Biofuel
10.1 Introduction
10.2 Types of Agro and Food Processing Wastes
10.3 Pretreatments and Conditioning
10.4 Supplementation of Wastes
10.5 Fermentation Technologies
10.6 Ethanol Production from Wastes
10.7 Butanol Production from Wastes
10.8 Conclusion and Future Perspective
References
11 Biofuel Prospects by 2030, Based on Existing Production and Future Projections
11.1 Introduction
11.2 Biofuel Generations
11.3 Biofuel Demand: Current Situation and Perspectives
References
12 Microstructural Maneuvering for Bioenergy Production
12.1 Introduction
12.2 Microstructural Maneuvering in Carbon-Based Products for Bioenergy
12.3 Bioenergy from Different Biomasses
12.4 Microstructural Amendments in Coal-Derived Material for Bioenergy Production
12.5 Summary
References
13 Nanotechnology-Based Alternatives for Sustainable Biofuel and Bioenergy Production
13.1 An Overview
13.2 Role of Nanomaterials in Biofuels and Bioenergy Generation from Biomass
13.3 Factors Influencing Nanoparticle Performance in Biofuel Production
13.4 Research on Different Types of Nanomaterial for Biofuels and Bioenergy Production
13.5 Application of Nanomaterial Materials for Biofuels and Bioenergy
13.6 Challenges of Nanomaterial Materials for Biofuels and Bioenergy
13.7 Conclusion and Future Prospects
References
14 New Insights Into Valuable Strategies for Generating Algal Biofuels
14.1 Introduction
14.2 Algal Cultivation Strategies
14.3 Future Prospects
14.4 Conclusions
References
15 Outline of Energy Crop–Based Solid Biofuels: Trends and Opportunities
15.1 Introduction
15.2 Energy Crops
15.3 Pellet Fuel
15.4 Technical Combustion Properties
15.5 Conclusion
References
16 Overview of Gaseous Biofuels Derived from Crops: Progress and Prospects
16.1 Introduction
16.2 Biofuels as an Alternative to Fossil Fuels
16.3 Classification of Biofuels and Generations
16.4 Bio-Hydrogen
16.5 Syngas
16.6 Biogas
16.7 Conclusions
References
17 Recent Advances in Microbial Biodiesel
17.1 Introduction
17.2 Developments in the World’s Biomass-Based Energy Recovery
17.3 Types of Biofuel
17.4 Conventional Methods and Feedstocks for Biofuel Productions
17.4.2 Transesterification
17.5 Recent Advancements in Biofuel Production
17.6 Feedstock Availability for Biofuel Production
17.7 Emerging Technology for the Development of Biofuel Production
17.8 Catalyst for Conversion of Biomass into Biofuels
17.9 Modern Extraction Techniques
17.10 Purification Techniques
17.11 Conclusion
References
18 Thermochemical Conversion Products for Solid Biofuels
18.1 Introduction
18.2 Renewable Raw Materials to Solid Biofuels
18.3 Pretreatments
18.4 Production of Solid Biofuels
18.5 Conclusions
References
19 Coal for Hydrogen Production and Storage
19.1 Introduction
19.2 Sources of Hydrogen Energy
19.3 Coal Makeup Controls on Hydrogen Production
19.4 Effect of Coal Rank on Hydrogen Generation
19.5 Hydrogen Production Techniques From Coal
19.6 Coal for Hydrogen Storage
19.7 Summary
References
20 Fuel Characteristics of Solid and Gaseous Energy Carriers
20.1 Introduction
20.2 Solid and Gaseous Energy Carriers
20.3 Biomass Pretreatments and the Resulting Biofuels
20.4 Description of Solid Biofuel Characteristics
20.5 Description of Gaseous Biofuel Characteristics
20.6 Procedures for Biofuel Property Determination
20.7 Common Solid Biofuel Properties
20.8 Common Gaseous Biofuel Properties
20.9 Concluding Remarks
References
Index
Also of Interest
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Solid biofuels from different wastes.
Table 1.2 Liquid biofuels from different wastes. [2].
Table 1.3 Gaseous biofuels from different wastes.
Table 1.4 The distinctive features of each biofuel generation [14, 15, 58].
Chapter 2
Table 2.1 Use of algae biomass to produce biofuels, bioproducts, or even an al...
Chapter 3
Table 3.1 Classification of biofuels.
Table 3.2 Example of substrates used for bioethanol production.
Table 3.3 Example of various substrates and fermentation processes used for bi...
Table 3.4 Different feedstocks used for biodiesel production.
Table 3.5 Few organic substrates used for biogas production.
Chapter 4
Table 4.1 Permissible limits of biomethane constituents [12].
Table 4.2 Comparison of biogas upgradation technologies [3, 18, 23, 27, 66, 68...
Table 4.3 Advantages and disadvantages of upgradation technologies [12, 15, 65...
Table 4.4 Comparison of liquefied biomethane with conventional fuels [58, 68]....
Chapter 6
Table 6.1 Patents deposited and granted on biological hydrogen production.
Chapter 8
Table 8.1 Significant sources of lignocellulosic biomass and the relative comp...
Table 8.2 The relative percentage of biopolymers, sugar, and biofuel yield of ...
Chapter 10
Table 10.1 Bioethanol production from food and agricultural wastes.
Table 10.2 Biobutanol production from food and agricultural wastes.
Chapter 13
Table 13.1 Importance of nanomaterials in the production of biofuel.
Table 13.2 Nanoparticle effects on biogas production processes and their effec...
Table 13.3 The bioethanol production using different type of NPs.
Table 13.4 The utilization of different NPs in the manufacture of biodiesel.
Table 13.5 Use of nanoparticles in the synthesis of biohydrogen.
Chapter 14
Table 14.1 Some of the popular techniques in algae cultivation.
Chapter 15
Table 15.1 Classifications of energy corps plants.
Table 15.2 Used part of energy crops.
Table 15.3 Conversion factor between different wood logs assortments [33].
Table 15.4 Wood density at absolutely dry soft and hard woods [33].
Table 15.5 Planning figure for evaluation of energy content in a given fuel ba...
Table 15.6 Nanoparticle effects on biogas production processes and their effec...
Table 15.7 Bulk density of different fuel particles (wood fuels and herbaceous...
Table 15.8 Conversion factor between different wood logs [33].
Table 15.9 Particle density of different fuels [33].
Chapter 16
Table 16.1 Source, feedstock, and examples of biofuels produced from the first...
Table 16.2 Different feedstock and pre-treatment methods to increase the biohy...
Table 16.3 Composition of syngas for several feedstock based on the gasificati...
Table 16.4 Gasifiers used for different feedstock.
Chapter 20
Table 20.1 Biomass components affecting fuel characteristics.
Table 20.2 Solid biomass component composition (%) on a dry ash–free basis.
Table 20.3 Procedures and/or references in determining solid biofueal properti...
Table 20.4 Typical properties for selected solid biofuels [11, 13, 17–27].
Table 20.5 Properties for component gases in biofuel gases.
Table 20.6 Biogas properties dependent on source. Adapted from [28, 29].
Table 20.7 Typical syngas composition from biomass gasification. Adapted from ...
List of Illustrations
Chapter 1
Figure 1.1 Overview of solid biofuel production and its application.
Figure 1.2 Feedstock generation and liquid biofuels process.
Figure 1.3 Production of gaseous biofuels.
Figure 1.4 A schematic of the biofuel generation.
Figure 1.5 Thermochemical conversion of biomass to biofuels.
Figure 1.6 Biochemical conversion of biomass to biofuels.
Chapter 2
Figure 2.1 Algal bomass biorefinery. Source: the authors.
Chapter 3
Figure 3.1 Percentage consumption of different energy sources in 2019.
Figure 3.2 Flowchart of bioethanol generation from lignocellulosic biomass.
Figure 3.3 Advantages and disadvantages of bioethanol generation. Green circle...
Figure 3.4 Different biological method for biohydrogen production and their re...
Figure 3.5 Biodiesel production through transesterification process.
Chapter 4
Figure 4.1 Basic layout for biomass to LBM conversion.
Figure 4.2 Biogas upgradation overview.
Figure 4.3 Water scrubbing upgradation.
Figure 4.4 Biogas upgradation through chemical absorption.
Figure 4.5 Pressure swing adsorption biogas upgradation.
Figure 4.6 Biogas upgradation through membrane separation.
Figure 4.7 Biogas upgradation through cryogenic separation.
Figure 4.8
In situ
biological upgradation.
Figure 4.9
Ex situ
biological upgradation.
Figure 4.10 Hybrid biological upgradation.
Figure 4.11 Photoautotrophic biological upgradation.
Figure 4.12 Pure refrigerant cycle.
Figure 4.13 Single mixed refrigerant cycle.
Figure 4.14 Dual mixed refrigerant cycle.
Figure 4.15 Propane precooled mixed refrigerant cycle.
Figure 4.16 Integrated mixed refrigerant cascade cycle.
Figure 4.17 Single N
2
expander.
Figure 4.18 Dual N
2
expander.
Figure 4.19 Cryogenic liquid vaporization.
Chapter 6
Figure 6.1 Different ways of obtaining hydrogen by biological methods.
Chapter 8
Figure 8.1 Overview of the process involved in the biochemical conversion of l...
Figure 8.2 Structural component of lignocellulosic biomass. Cellulose, hemicel...
Chapter 9
Figure 9.1 Environmental impacts of the firstand second-generation biofuels.
Chapter 10
Figure 10.1 The traditional and innovative extraction technologies from food w...
Figure 10.2 Conversion processes for food wastes and their types [8].
Figure 10.3 Different types of agro-processing wastes [11].
Figure 10.4 Different categories of pretreatment methods [32].
Chapter 12
Figure 12.1 Potential coal-derived bioenergy products after different treatmen...
Figure 12.2 Flowchart of physical and chemical activation methods for active c...
Figure 12.3 Arc-discharge setup (after Arora and Sharma [95]); Reuse of this f...
Figure 12.4 Diagrammatic sketch of the arrangement of molten caustic leaching ...
Chapter 13
Figure 13.1 Multidisciplinary application of nanotechnology.
Figure 13.2 Utilization of nanotechnology for bioenergy production.
Figure 13.3 Different types of NPs reported for biofuels production.
Chapter 14
Figure 14.1 Representative model of hybrid cultivation.
Figure 14.2 Schematic representation of the design of
Chlorella
sp. cultivatio...
Figure 14.3 Indoor view of 2,000-L large-scale manufacturing raceway pond [29]...
Figure 14.4 Schematic diagram of enclosed photobioreactor—open race pond hybri...
Figure 14.5 An overview of generating, storing, and adding flue gas to photobi...
Figure 14.6 Graphical representation of chlorophyll component (a1 and a2) in
C
...
Chapter 15
Scheme 15.1 Procedure and process parameters of pellet formation from biomass....
Figure 15.2 Different phases of ash sample during melting (H, height of sample...
Chapter 16
Figure 16.1 Methods of bio-hydrogen extraction.
Figure 16.2 Types of pre-treatment strategies for the anaerobic production of ...
Figure 16.3 Stages of anaerobic digestion (AD).
Chapter 17
Figure 17.1 Types of biofuels and their sources.
Figure 17.2 Gasification process involved in the production of biofuels.
Figure 17.3 Supercritical CO
2
extraction process.
Figure 17.4 Microwave-assisted extraction process.
Chapter 18
Figure 18.1
Chapter 19
Figure 19.1 Hydrogen production routes from different feedstocks after Epelle
Figure 19.2 The diagram shows the single-walled carbon nanotubes (SWCNTs) and ...
Chapter 20
Figure 20.1 Coal classifications based on properties and different systems. Re...
Figure 20.2 Biomasses processed into different solid and gaseous biofuels. Ada...
Guide
Cover Page
Table of Contents
Series Page
Title Page
Copyright Page
Preface
Begin Reading
Index
Also of Interest
WILEY END USER LICENSE AGREEMENT
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e1
Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Solid-Gaseous Biofuels Production
Edited by
Inamuddin
and
Tariq Altalhi
This edition first published 2024 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2024 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-394-20440-3
Front cover images supplied by Pixabay.comCover design by Russell Richardson
Preface
Excessive energy demand in numerous sectors all over the world has resulted in maximal consumption of fossil fuels, which has led to their depletion. Furthermore, the study of different forms of energy is under consideration. In addition, research into diverse forms of energy is being examined for the future world. Biofuels are among the best substitutes for fossil fuels and for combating climate change to attain Net Zero Emissions by 2050. Furthermore, biofuels proved to be the most significant factor in boosting global alternative energies, accounting for approximately fifty percent of renewable energy and six percent of global energy stocks. Developing solid-gaseous-based biofuels requires a significant amount of time, effort, and resources, including the use of wastes and residual flows. Micro/macro-algae are the most diversified category of plants, flourishing in nearly most regions of the globe, and represent the third generation of biofuels. Algae are responsible for more than fifty percent of oxygen sources, and they also generate the feed as well as other essential health foods on a large scale. Although algae constitute the prominent starting substrates for producing biofuels, this issue focuses on advanced technology related to solid/gaseous biofuels. Furthermore, recent research and technological developments in algal biomass for refined biofuel generation, as well as various conversion processes for their prospects, are discussed. This book will help you explore the biofuel advancements of third-generation biomass, which is considered the leading alternative bio reserves business, as well as the related technologies and their many implementations. In the fields of sustainable renewable biofuel industries, it is a great reference tool for students, academics, researchers, professionals, and investors.
Chapter 1 The generation of biofuels has progressed from conventional processes like ethanol fermentation to cutting-edge approaches like enzymatic hydrolysis and algae culture. Modern technology focuses on increasing efficiency, minimizing environmental effects, and maximizing feedstock utilization. The development of biofuel production offers hope for more sustainable energy sources and lower carbon emissions.
Chapter 2 focuses on how algal biofuels can become a viable and sustainable option through bio-refineries. For that, biorefineries, algal, and biofuels are separately explained, then, the production of algal biofuels and other bioproducts are presented, along with data about them and the respective challenges and perspectives for the future.
Chapter 3 discusses the production of biofuel from waste materials like biomass feedstock, livestock waste materials, food processing waste etc. This chapter gives an overview of various biofuels such as bioethanol, biohydrogen, biodiesel, and biogas production. The classification of biofuels based on feedstock is also discussed in this chapter.
Chapter 4 portrays the essential elements required for the upgradation of biogas to biomethane, followed by its conversion to liquid biomethane (LBM). Various biogas to biomethane upgradation technologies through CH4 enrichment and salient features of practical biomethane liquefaction methodologies are also discussed.
Chapter 5 covers recent research and advancements in cost-effective biofuel production. It emphasizes the growing need for sustainable and economically viable biofuels to meet energy demands and reduce greenhouse gas emissions. Topics include utilizing non-edible and waste materials, algae as feedstock, genetic engineering, nanotechnology, optimization, advanced fermentation, and policy support.
Chapter 6 discusses obtaining hydrogen through cyanobacteria. The biophotochemical processes capable of producing it are discussed. In addition, it refers to a small economic and environmental analysis of hydrogen production by cyanobacteria. Production setbacks and an overview of patents are presented.
Chapter 7 has been focused on microstructural engineering used for bioenergy production. Also It has been detailed Microbial Engineering, Plant Cell Wall Engineering and Nanotechnology for Bioenergy Production. On the other hand, It has been covered the subjects of Biomass Microstructure and Characterization and Microstructural Engineering for Bioreactors and Processing.
Chapter 8 explores the potential of lignocellulosic biomass as an affordable, abundant, and environmentally sustainable feedstock for biofuel production through fermentation. It delves into the structural chemistry of lignocellulosic biomass, discusses pre-treatment methods, and critically reviews the contemporary technologies used for the biochemical conversion of lignocellulosic biomass into biofuels.
Chapter 9 focuses on the limitations of the first and second generation of solid-gaseous biofuels amidst the ongoing climate crisis. It deliberates on issues such as land-use changes, biodiversity loss, water security, and greenhouse gas emissions. Additionally, it highlights recent studies emphasizing the importance of diversifying feedstock for sustainable biofuel production.
Chapter 10 emphasizes on types of Agro-Processing wastes. The pre-treatment methods followed by supplementation of these wastes are discussed in detail. Additionally, the techniques used for fermenting these wastes are also covered. The main focus is to understand the substratemicroorganism combinations and pre-treatments performed to manufacture Ethanol and Butanol.
Chapter 11 discusses the current situation of biofuel production worldwide and draws the future perspective on the production and consumption of biofuel by 2030. Countries’ goals for replacing fossil fuels with biofuels were also presented. Moreover, the advantages of green and sustainable feedstock compared to conventional feedstock were discussed.
Chapter 12 portrays various possibilities for generating bioenergy from carbon-based precursors and biomasses through microstructural maneuvering. Renewable energy production from carbon products, industrial wastes, agricultural crops, and municipal wastes offers a carbon-neutral future. The procedures of manipulating physico-chemical properties to augment bioenergy production from these precursors are discussed at length.
Chapter 13 discusses the crucial role of nanotechnology in enhancing the sustainable production of biofuel and bioenergy. It also emphasizes on the key factors that influence the performance of nanomaterials, their various advantages, disadvantages, and challenges for biofuel and bioenergy production.
Chapter 14 outlines the advancements in generating algal based bio-fuels through technological optimization. The ongoing research providing dynamical insight into various parameters that assists in overcoming the existing gaps between laboratory scale and industrial implementation in a cost effective manner remains the focal point of the chapter.
Chapter 15 talks about crop-based solid bio-fuels, the green alternative sources of energy against fossil fuel. There is a discussion on pellets, which are the way to store the solid bio-fuel. They contain the highest calorific values and burned with high combustion efficiency.
Chapter 16 discusses the process involved in the extraction, production, and storage of three vital gaseous biofuels namely bio-hydrogen, syngas, and biogas. The raw materials required for the production of these gaseous biofuels were also discussed, along with their applications and their role in limiting emissions and reducing pollution.
Chapter 17 discusses about the different types of biofuels. And various traditional and modern methodologies adopted to synthesize biofuels. Furthermore, the chapter also addresses the accessibility of feedstock supply and explores different catalysts used in the conversion process of biomass into biofuels. Additionally described in the chapter are contemporary extraction and purification methods along with their functions.
Chapter 18 details the processes to produce solid biofuels (SB). The main feed-stocks to produce SB, the physical and chemical processes, including dewatering and drying, size reduction and shredding, and microwave are also discussed. Also, the processes such as briquetting, pelletizing, pyrolysis, gasification and torrefaction and hydrothermal carbonization are discussed.
Chapter 19 details the application of coal for hydrogen production and storage. Industrial and economic production of hydrogen from coal pyrolysis, gasification, and coal slurry electrolysis are well-demonstrated. Similarly, the coal properties and their influence on hydrogen storage are elaborated to meet the energy demand of the upcoming hydrogen era.
Chapter 20 discusses various characteristics of solid and gaseous biofuels. Emphasis is on identifying standard methods used to establish these characteristics. The importance of understanding biofuel characteristics in energy mix decision making and process or equipment design is also included in the discussions.
Key features:
Provides a broad overview of biofuels
Outlook for the technological advancements in biofuel industries
Discussion across a wide range of scopes and challenges related to biofuels
Future trends of biofuels development
Inamuddin
Department of Applied Chemistry, Zakir Husain College of Engineering and Technology, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh-202002, India
Tariq Altalhi
Department of Chemistry, College of Science, Taif University, 21944Taif, Saudi Arabia
1Biofuel Production: Past to Present Technologies
Manisha Jagadale1, Beula Isabel J.2*, Mahesh Jadhav3, Selvakumar Periyasamy4,5 and Desta Getachew Gizaw4
1ICAR-National Institute of Natural Fibre Engineering and Technology, Kolkata, West Bengal, India
2Department of Energy and Environment, National Institute of Technology, Tiruchirappalli, India
3Dr. Balasaheb Sawant Konkan Krishi Vidyapeeth, Dapoli, Maharashtra, India
4Department of Chemical Engineering, School of Mechanical, Chemical and Materials Engineering, Adama Science and Technology University, Adama, Ethiopia
5Department of Biomaterials, Saveetha Dental College and Hospitals, SIMATS, Saveetha University, Chennai, India
Abstract
Biofuels are renewable energy sources from biological materials such as plants, algae, and animal waste. These fuels are gaining popularity as a sustainable alternative to traditional fossil fuels, which are non-renewable and contribute significantly to climate change. The production of biofuels involves converting biological materials or biomass into usable fuels through various processes such as fermentation, pyrolysis, and transesterification. These processes can be conducted on a small scale, such as in a laboratory, or on a large scale, such as in industrial plants. Different generations of biofuels exist; the first-generation biofuels are typically made from corn, sugarcane, and soybeans, whereas the second-generation biofuels are made from non-food crops such as switchgrass and wood chips. The third-generation biofuels are produced from algae and other aquatic plants. Biofuels have the potential to reduce greenhouse gas emissions, create jobs, and increase energy security. However, there are also concerns about the environmental impact of biofuel production, such as land use changes and water usage. Therefore, it is essential to carefully consider biofuel production’s benefits and drawbacks and to develop sustainable production practices.
Keywords: Biofuel, biomass, conversion routes, sustainability
1.1 Introduction
The world’s population is increasing fast and is projected to reach 9.7 billion by the year 2050, so there is an increase in energy demand [1]. By 2050, global energy demand will increase by 50%, existing at around 900 quadrillion Bt [2]. It is necessary to satisfy the rising energy needs of the globe with resources. Fossil fuels have the largest share (84.3%) of the global energy mix [3]. It is a fretting situation because burning fossil fuel sources creates problems for human health and the environment, atmospheric issues, global warming, and climatic changes due to greenhouse gas emissions [4, 5]. After the 20th century, due to industrialization and globalization, energy demand increased exponentially, resulting in the availability of this fuel, which is also one of the concerns [4]. Awareness has shifted to using sustainable and alternative energy sources to address the above issues [5, 6].
Nowadays, several alternative solutions are available for energy recovery, which are renewable in nature. It includes solar, wind, ocean, geothermal, and bioenergy [7, 8]. Among them, biomass or bioenergy has excellent potential for developing sustainable energy solutions [9]. Green plants absorb the energy from the sun during the photosynthesis process. Biomass is widely accessible, inexpensive, carbon neutral, and low in sulfur and chlorine; has a decentralized supply; and has a short life cycle for non-woody biomasses [10]. Biomass energies are harvested using various conversion processes, i.e., physical, thermochemical, and biochemical [1, 11]. The generated biofuels can be used for transport, heat, and electricity generation. Biofuel production from biomass feedstock gained more attention in the energy sector due to its reliability and positive role in reducing CO2 levels in the atmosphere compared to conventional sources [12, 13].
The first-, second-, third-, and fourth-generation biofuels are the four categories into which biofuels are divided on the basis of the kind of feedstock [14]. Food-vs.-fuel problems may result from the first-generation biofuels from eatable feedstock [15]. Cellulosic waste that cannot be consumed is used to create the second-generation biofuels [16]. Of the four generations of biofuels, only the first and second generations are produced commercially. Algae-based fuels are categorized as the third-generation biofuels, whereas genetically modified (GM) organisms are used to produce the fourth-generation biofuels. Large-scale commercialized production of the thirdand fourth-generation biofuels is less due to high production cost and low biomass production. The feedstock used for biofuel generation can be agricultural waste, forest waste, sewage sludge, wastewater, processed industrial waste, and kitchen food waste [17–19]. So, simultaneously utilizing this waste will solve waste management issues by converting them into value-added products [20]. Thus, generations of biofuels from destruction will help to develop sustainable green cities in the future, as per the Paris Agreement [21].
As per the bibliometric study of Hasan et al. [22] in a study period of 2001 to 2022, after 2006, the publication in the biofuels area increased sub-stantially. This is because the governments have taken initiatives to increase renewable energy areas, climate change issues, and increasing energy demand and consumption—the increased production of biofuel-initiated concept of biofuel economy. The idea of a sustainable biofuel economy is directly correlated with the sustainable development goals (SDGs) of the United States. For example, the use of biofuels ensures good health and well-being of people (SDG3); biofuels provide clean and sustainable energy (SDG7); biofuel generation increases employment opportunities and economic growth of agricultural people and industries (SDG8); biofuel generation assures secure, resilient, and sustainable cities and communities for everyone (SDG11); and biofuel generation ensures taking immediate action to tackle climate change challenge (SDG13) [22, 23].
The many social and environmental advantages have proven the recent headways in biofuel production. Still, its financial sustainability depends on feedstock availability, possible technology, design, project management, and production capacity [22, 24]. Recently, conversion technologies such as hydrothermal carbonization [25, 26], hydrothermal liquefaction [27, 28], pelletizing [29, 30], gasification [31, 32], fast pyrolysis [33, 34], bioethanol production [35, 36], and biodiesel production [37] under research in developed and developing countries to convert waste into biofuels. Thus, this article aims to provide knowledge on the production of biofuels from past to present by using different technologies within a sustainability and economic feasibility framework. This article will overview current research trends for producing solid, liquid, and gaseous biofuels with varying conversion pathways and future roadways for producing biofuels to fulfill global energy security.
1.2 Types of Biofuels
1.2.1 Solid Biofuels
Solid fuels are mainly used for heat and power generation. Conventionally, solid fuels are obtained from coals of lignite, bituminous, and sub-bituminous types [20]. Burning coal for energy has a negative impact as it increases CO2 concentration in the atmosphere, which causes climate and global warming [38]. Several countries are finding it extremely challenging to minimize CO2 emissions in the atmosphere. As a result, both developed and developing nations are working to establish sustainable energy systems to address the problems caused by the use of fossil fuels [9]. In addition, heavy dependency on fossil fuels also leads to low energy sustainability and security [10]. Among the different solutions, solid biofuels play a vital role in circular economy concepts by converting waste to wealth by implementing reduction, reuse, and recycling principles [31]. Solid biofuels can be obtained from agricultural residue, forest residue, food waste, municipal solid waste, and microalgae [20]. The socioeconomic development of so many developing countries relies mainly on agriculture and agro-based industries to meet the growing population’s increasing demand. Consequently, the annual generation of agricultural waste is considerably high [39]. For example, India annually generates 550 Mt of agro residues [40]. The type and quantity of waste vary from ecological zones. The production of solid biofuels depends upon different factors like type of feedstock, feedstock availability, technique used, economic conditions, and energy required.
Solid waste has low bulk density, high moisture content, irregular shape, high volatile matter, heterogeneity, high hygroscopicity, and low calorific value [41, 42]. So, to improve the properties of feedstock, pretreatments such as drying, size reduction, microwave pretreatments, and dewatering are commonly given. To convert the raw feedstock to solid biofuels, different production techniques, i.e., briquetting/pelletizing/ densification, torrefaction, pyrolysis, gasification, and hydrothermal carbonization, can be used as shown in Figure 1.1. Table 1.1 summarizes the common pretreatments given to feedstock to produce solid biofuels by using different technologies, such as microwave torrefaction [32], hydrothermal carbonization [43], vacuum pyrolysis [44], and microwave vacuum pyrolysis [45]. Integrating pretreatment with different technologies brings extra benefits of selective, homogeneous, rapid heating; low energy input; and processing time [43, 44]. The produced solid biofuels can be used as cooking fuel for domestic purposes, industrial boilers, biochar for soil amendment, and wastewater treatment [20]. Therefore, solid biofuels can replace fossil fuel consumption, supporting SDGs and a circular economy [46, 47].
Figure 1.1 Overview of solid biofuel production and its application.
1.2.2 Liquid Biofuels
Liquid fuels are majorly used for transport purposes. According to the International Energy Agency [2], total transport emissions increased by 2.1% (or 137 Mt) in 2022. However, due to the increasing population, the global demand for transport continues expanding (40% by 2035). So, at the world level, efforts are being made to reduce emissions due to transport by replacing fossil fuels with renewable, sustainable, and carbon-neutral liquid biofuels [15]. Biomass may be converted into ethanol, methanol, butanol, biodiesel, and Fischer–Tropsch diesel, among other fuels [4]. With a global yearly production of 200 billion tons, liquid biofuels may be produced from natural, renewable, and sustainable feedstocks already in the environment [48]. These different sources include residue from the agricultural sector, forest sector, food-industrial sector, non-food energy crops, solid waste, and algal biomass.
To convert the raw feedstock to liquid biofuels, different production techniques, i.e., torrefaction, pyrolysis, gasification, hydrothermal carbonization, and aerobic and anaerobic fermentation, can be used, as shown
Table 1.1 Solid biofuels from different wastes.
Raw material
Pretreatment
Technology used
Product
Properties
Scale
HHV, MJ/kg
Application
Ref.
Rice husk
Size reduction
Pelletizing (water as a binder)
Pellets
Y, 65%; BD, 650 kg/m
3
; AC, 18%
Pilot
12–13.5
Thermochemical application for the generation of energy
[
92
]
Banana stalk
Drying and grinding
Hydrothermal carbonization (180°C, 1–3 h)
Hydro char
EY, 57.8% to 75.3%; FC, 16–44; VM, 48–73; AC, 6–10
Lab
18.1–18.9
Potential raw material for energy production
[
93
]
Oil palm trunk
Drying, grinding, and sieving
Pyrolysis (300°C–350°C)
Bio-coal
EY, 27.8%; BD, 87.7 kg/m
3
; FC, 51.8; VM, 39.4; AC, 15.5
Pilot
19.6
Co-combustion in coal-firing power plants; keywords
[
94
]
Jute sticks
Drying, grinding, and sieving
Torrefactions (150°C–350°C)
Solid fuel
EY, 94.03%; EF, 1.18; FR, 0.64
Lab
19.32
Pelletizing and gasification
[
95
]
Cornstalk
Microwave pretreatment
Hydrothermal carbonization
Hydro char
FC, 9.8%– 18%; VM, 74.3–81.3; AC, 3.1–4.0
Lab
22.82
Direct solid fuel or auxiliary fuel
[
96
]
Y, yield; EY, energy yield; BD, pellet density; AC, ash content; FC, fixed carbon; FR, fuel ratio; EF, enhancement factor; VM, volatile matter.
Figure 1.2 Feedstock generation and liquid biofuels process.
in Figure 1.2. However, conversion of waste into liquid biofuels is still at a developing stage as compared to conventional sources, due to bottlenecks of feedstocks itself. Lignocellulosic biomass mainly contains cellulose, hemicellulose, lignin, and extractives, whereas microalgae contain lipids, carbohydrates, and protein. The conversion of this waste into fuels and chemicals is still in the developing stage due to the recalcitrance of feedstock. With the help of pretreatment, undesired functional groups and structures can be modified or removed as per the requirement to improve conversion efficiency [49]. The treatment given to biomass can be classified as physical, chemical, thermal, and biological. Physical pretreatment includes grinding, densification, and pelletization. In contrast, chemical pretreatment includes acid, alkali, hydrothermal, steam explosion, and ammonia fiber expansion pretreatment. Thermal pretreatment methods like drying, torrefaction, and microwave greatly influence biomass and
Table 1.2 Liquid biofuels from different wastes. [2].
Raw material
Pretreatment
Technology used
Product
Scale
Properties
Yield, %
Ref.
Corn and corn stover
NaOH, 121°C for 20 min
Fermentation
Bioethanol
Lab
104.9 g/L
80.47
[
97
]
Rice straw
H
2
SO
4
1%, 15 min, 121°C
Fermentation
Bio-butanol
Lab
13.5 g/L
34
[
98
]
Rice straw
1 M NaOH, 121°C for 1 h and enzymatic
Fermentation
Bioethanol
Lab
30.5 g/L
45
[
99
]
Microalgae
Ionic-liquid extraction
Fermentation
Bio-butanol
Lab
6.6 g/L
23
[
100
]
Soyabean oil
Catalyst: Karanja seed shell ash
Transesterification; Methanol:oil, 10:1; temperature, 65°C; time, 60 min
Biodiesel
Lab
Density, 0.8795 gm/ cm3; kinematic viscosity, 6.52 mm2/S
96%
[
101
]
Banana leaves
Size reduction
Fast pyrolysis
Bio-oil
Lab
HHV (MJ/kg), 5.35; water content, 19%
[
102
]
Spirulina algae
Drying
Hydrothermal liquefaction; temperature (HLT), 300°C; pressure, 10–12 MPa; time, 30 min
Bio-oil
Lab
-
32.6
[
103
]
Water hyacinth
Chopping, pulverization
HLT, 280°C–350°C; pressure, 10–12 MPa; time, 30 min
Bio-oil
Lab
HHV, 23 MJ/kg; H/C ratio, 2.5
37
[
104
]
feedstock behaviour to convert into liquid biofuels [50]. Table 1.2 summarizes the common pretreatments given to feedstock to produce liquid biofuels using different technologies, such as torrefaction, hydrothermal liquefaction, pyrolysis, and fermentation. Few pretreatments have been reported, such as torrefaction [51], steam explosion [52], acid/alkali deep eutectic solvent pretreatment, and biological pretreatment [53]. The liquid biofuels that are produced can be used as transport fuel to replace diesel and petrol. These liquid biofuels will provide a broader outlook with different advanced technologies to use liquid biofuels for road, shipping, and aviation use.
1.2.3 Gaseous Biofuels
Gaseous biofuels are used to generate electricity and heat for cooking purposes and to replace fossil fuels in the transport sector. Gaseous biofuels can be generated from renewable feedstocks of agriculture, forest, microalgae, and sewage waste. Different production techniques, i.e., anaerobic fermentation, gasification, and hydrothermal gasification, can convert the raw feedstock to gaseous biofuels, as shown in Figure 1.3 [54].
Figure 1.3 Production of gaseous biofuels.
Biogas/methane, dimethyl ether (DME), syngas, and hydrogen are the most frequently used gaseous biofuels. Biogas is an excellent alternative to fossil fuel made by anaerobic digestion (AD) from renewable sources of agricultural residue, household waste, municipal waste, and microalgae in an oxygen-free environment [55]. Methane production from feedstock mainly includes hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Biogas rich in methane can produce chemicals as a hydrocarbon source [55]. The gasification process can obtain syngas or producer gas, a mixture of different proportions of CO2, CO, H2, CH4, and N2 [56]. Similarly, from this feedstock, bio-hydrogen can be generated by the dark fermentation process [57]. Moreover, the bio-hydrogen calorific value is ~120 MJ/kg, more significant than any other hydrocarbon fuels like bioethanol, natural gas, and biodiesel, having a calorific value of around 29.9 MJ/kg, 50 MJ/ kg, and 37 MJ/kg, respectively. However, the process of developing bio-H2 from biomass is not economical and is at an under-developing stage. Table summarizes the different technologies used to produce gaseous biofuels by other waste.
Table 1.3 Gaseous biofuels from different wastes.
Raw material
Technology used
Environmental condition
Product
Scale
Yield
Ref.
Potato pulp waste and dairy manure
Anaerobic digestion
Feed: inoculum, 2:1; mesophilic, 37°C; catalyst, apple tree biochar
Biomethane
Lab
87.5%
[
105
]
Corn stalk, rice straw, and peanut shell
Gasification
SBR, 1; temperature, 650°C; catalyst, CaO
Bio-hydrogen
Lab
61.23%
[
106
]
Soyabean straw
Hydrothermal gasification
Fixed bed reactor; temperature, 500°C; pressure, 23–25 MPa; residence time, 45 min; BTW ratio, 1:10
Producer gas
Lab
H
2
, 57.6%; CH
4
, 17.9%; CO
2
, 27%
[
107
]
Sewage sludge
Anaerobic digestion
Feed:inoculum, 1:2; thermophilic, 55°C; catalyst, corn stover biochar
Bio methane
Lab
87.3%
[
108
]
1.3 Different Generation of Biomass for Biofuel Production
Based on the feedstock type, biofuels are classified into four groups, i.e., first-, second-, third-, and fourth-generation biofuels [14]. Of these, the first generations of biofuels derived from edible feedstock can create a food-vs.-fuel crisis [15]. The second-generation biofuels are produced from non-edible cellulosic waste [16]. Only the firstand second-generation biofuels are the only commercially produced out of the four generations. The fuels generated from algae are considered the third-generation biofuels, whereas the fourth-generation biofuels are made from GM algae. The large-scale commercialized production of the thirdand fourth-generation biofuels is less due to a high production cost and a low biomass production. Figure 1.4 shows a schematic of the biofuel generation. The distinctive features of each biofuel generation are presented in Table 1.4.
Figure 1.4 A schematic of the biofuel generation.
Table 1.4 The distinctive features of each biofuel generation [14, 15, 58].
Essential factors
First-generation biofuels
Secondgeneration biofuels
Third-generation biofuels
Fourth-generation biofuels
Food crisis
Adverse effect on food security
No impact on food security
No implications for food security
No impact on food security
Land footprint
Requires arable land for feedstock cultivation
Requires arable land or forest for feedstock cultivation
Non-arable land can be used for cultivation.
Non-arable land can be used for cultivation.
Water footprint
Portable water required
Portable water required
Waste, brackish, and saline water can be used.
Scrap, brackish, and salty water can be used.
Loss of biodiversity
Monoculture of feedstocks results in loss of biodiversity
No impact on biodiversity
No impact on biodiversity
No impact on biodiversity
Conversion to biofuels
Easy process
Need for sophisticated and downstream processing
Easy conversion due to increased hydrolysis and fermentation efficiency
Easy conversion due to increased hydrolysis and fermentation efficiency
Nutrient requirement
Use of fertilizer and pesticide
No need for fertilizer
Large carbon and nitrogen sources are required
Large carbon and nitrogen sources are required
Environmental condition
Temperature and humidity should be maintained.
Temperature and humidity should be maintained.
It can be grown in more polluted water with high pH, salinity, and light intensity.
It can be grown in more polluted water with high pH, salinity, and light intensity.
Regulation
The regulation is reasonably clear.
The regulation is fairly clear.
No regulation
No regulation
Environmental impact
Use of fertilizer and pesticides is the main concern.
No use of chemicals, but deforestation is the foremost concerned.
No expenditure on chemicals, but marine eutrophication is the main concern.
Medium (CO
2
fixation and wastewater treatment are pros, but the release of GM organisms is the main concern.
Harvesting method
Harvesting is done by hand or machine picking.
Harvesting can be done manually or with the help of a machine.
Harvesting is a costly and complicated process.
Harvesting is an expensive and complex process.
Cost economics
Low investment
Low investment
The initial cost for large-scale cultivation is too high.
The initial price for high-scale cultivation is too high.
1.3.1 First Generation
In the first generation, edible biomass, i.e., potato, barley, wheat, soyabean, corn, rye, barley, maize, sugarcane, beetroot, sweet sorghum, rapeseed, soybean, and palm, showed promising results for commercial production as well as from environmental aspect [58]. Countries like the United States and Brazil are the world’s largest producers of the first-generation fuels [59]. The first-generation biofuels include mainly biodiesel, bioethanol, and biogas. Among the first-generation biofuels, ethanol is the major biofuel in the world. The primary feedstock is wheat, sweet sorghum, cassava juice, and corn. However, concerns about this fuel production arose because of the increased production costs, energy spent in cultivation, and food conflict [58]. Commercial production of the first-generation biofuels negatively impacts greenhouse gas emissions, biodiversity, land use, and water usage [59]. All these issues called researchers to explore new avenues for biofuel production.
1.3.2 Second Generation
The second-generation biofuels are mainly generated from non-edible lignocellulosic biomass from agriculture, forest, and municipal waste. The advantages of this biomass are as follows: the net carbon emission is zero; it is abundantly available due to a high yield per area of land; it is inexpensive; it can be produced with less land use; and there is no competition in foodstuff [60]. At present, the production of biofuels from lignocellulosic biomasses is not cost-effective due to the complex structure of biomass. Biomass mainly consists of lignin, hemicellulose, and cellulose, which are recalcitrance in nature. Hence, it requires suitable pretreatment for proper further utilization. The physical, chemical, and biological conversion processes must flow to produce biofuels from lignocellulosic biomasses. Fuels like methanol, ethanol, ether, biogas, syngas, bio-oil, and pellets can be produced from the second-generation crops. At the scientific level, efforts are made to produce commercial biofuels by minimizing production costs [61].
1.3.3 Third Generation
The third-generation biofuels are generated from algae [58]. The thirdgeneration biofuels have attracted attention because of their high yield, lack of arable land, ability to be cultivated in wastewater under open or closed conditions, high carbon sequestration, and easy conversion process [14]. Microalgae are not competing with food, so there is no food-vs.-fuel crisis. So, microalgae-based biofuel tends to be a viable alternative energy source to substitute conventional fossil fuel sources. However, there are some limitations, like the high cost of production, less stability, and more volatile nature at high temperatures [62].
1.3.4 Fourth Generation
The fourth-generation biofuels are obtained from GM microalgae to achieve a high biofuel production. The fourth-generation biofuels are sourced from GM algae biomasses to achieve enhanced biofuel production. Light penetration and reduced photoinhibition modify algae genetic arrangements to improve photosynthetic efficiency [63]. The cultivation of GM algae can be done in open and closed systems. While the impact of each method is significantly different for each other. Many publications studied the environmental benefits of GM algae, including carbon dioxide (CO2) sequestration [64], greenhouse gas emission reduction [62], and wastewater usage with heavy metals [14]. Few articles studied the environmental and health risks due to GM biofuels.
1.4 Conversion Strategies for Biofuel Production
As we discussed in the previous section, there are various generations of biomass, and the nature of every biomass varies. The conversion strategies also vary because of these changes in structure and function. Firstgeneration biomass contains a complex carbohydrate structure, broken down into simple sugars for fermentation to convert to biofuels. Secondgeneration biomass is lignocellulosic in nature and has a rigid form of lignin [11]. This lignin structure must be broken down to make the cellulose and hemicellulose available for fermentation and conversion [65]. Due to the above-discussed first and second-generation constraints, thirdand fourth-generation biomass comes into the picture. Algae is an extraordinary potential candidate for producing biofuels like biodiesel and bioethanol. In addition, the fourth-generation GM biomass can be made into highly potent biofuel biomass. The conversion strategies for all these biomass categories vary, as discussed below.
1.4.1 Thermochemical
Gaseous, solid, and liquid materials must be produced through thermochemical conversion. Each kind is acquired by varying the reaction’s operational circumstances (such as temperature, pressure, and residence time) and divided into different categories. The primary thermochemical process includes pyrolysis, combustion, gasification, hydrothermal liquefaction, and torrefaction, as represented in Figure 1.5 [66]. Biomass pyrolysis is a typical example of a thermochemical conversion process. Pyrolysis has gained interest despite being in its infancy because it immediately transforms biomass into solid (charcoal), liquid (bio-oil), and gaseous (fuel gas) products through the thermal degradation of biomass in the absence of oxygen [67]. The pyrolysis process is grouped as slow, fast, and flash pyrolysis based on the conditions under which it occurs. The common occurrence of slow pyrolysis in traditional charcoal kilns is well recognized. Biomass pyrolysis takes time, and high charcoal continents are related [68]. The operating temperature for slow pyrolysis is typically between 550 K and 950 K. Biomass is thermolyzed at high temperatures (57°C to 977°C) in an inert atmosphere during the rapid pyrolysis process. The flash pyrolysis process operates between 777°C and 1,027°C.
The most common bioenergy pathway worldwide is still the direct combustion of biomass. Combustion is a method for converting the energy held in biomass into heat and power. Depending on the type of biomass, there are
Figure 1.5 Thermochemical conversion of biomass to biofuels.
significant differences in that biomass’ chemical makeup and combustion characteristics [68]. For combustion, a variety of biomass sources can be considered. The chemical makeup of the feedstock might vary depending on the time of year, the region, and the plant parts (such as bark, branches, and leaves) used to produce woody biomass (such as wood chips, wood pellets, and waste wood). The potential of straw as an alternative feedstock is widely recognized [69]. Complete combustion results in the oxidation of carbonand hydrogen-rich biomass to CO2 and H2O, which generates heat. However, the intricate chemical dynamics of the reactions that occur when biomass is burned are complex. Partial combustion releases intermediate, including air pollutants such as particulate matter (PM), CH4, and CO. Sulfure oxides (SOx) and Nitrogen oxides (NOx) emissions are linked to fuel contaminants like sulfur and nitrogen [69]. In essence, straw is a byproduct of the production of crops. This feedstock does not compete with agricultural products for the limited land resources. Besides wood and straw, many waste materials can be used as feedstock, including rice husks, wheat bran, peanut shells, coffee grounds, and bagasse [70]. These are cheap sources of fuel that can be utilized to generate the heat or electricity required for industrial activities. High carbon and hydrogen content and low levels of other elements (oxygen, nitrogen, sulfur, and trace elements) are characteristics of the best fuels [71].
The process of transforming biomass physically and chemically in an aqueous medium at high temperatures (250°C to 374°C) and pressures (4 MPa to 22 MPa) while maintaining pressure-tight conditions is known as hydrothermal liquefaction. By successfully breaking down the cellulose, hemicellulose, and lignin in lignocellulosic biomass into liquid products through hydrothermal processing, the biomass may be made more soluble, and the physical and chemical reactions between the biomass and the aqueous medium can proceed more quickly [72]. The hydrolysis of glycosidic linkages to produce glucose and xylose is the initial step in the cellulose and hemicellulose hydrothermal conversion. By further dehydrating and breaking down glucose and xylose, several furans and other C2–C5 compounds can be produced [73]. The critical advantage of hydrothermal liquefaction over fast pyrolysis may be its ability to deal with high-moisture biomass feedstock without necessitating an energy-intensive drying process. Hydrothermal liquefaction has been developed at a pilot scale. Still, it has a long way to go before it can be used in industrial settings, in contrast to rapid pyrolysis, which has been an industrialized usage for decades [74]. The primary barriers to industrialized uses of hydrothermal liquefaction should be the need for specialized reactor and separator designs and high capital expenditures. Furthermore, feeding biomass fuel into the reactor at high pressures is challenging, which should be a significant issue when operating a large-scale installation. It is difficult to run hydrothermal liquefaction continuously [73].
The process of gasification involves the thermochemical conversion of solid or liquid biomass into a mixture of trace species, the fractions of which are influenced by operational parameters such as the properties of the raw material, the gasifying medium (steam, air, O2, and CO2), the temperature and pressure inside the gasifier, and catalysts [75]. Gasification is a complex combination of many reactions involving overlapping subprocesses like drying, pyrolysis, and partial oxidation. The feedstocks are dried up to a temperature of 120°C, and, below 500°C, volatile species are produced. Char can start to gasify at about 350°C. Exothermic combustion reactions can generate heat inside [76].
1.4.2 Biochemical Conversion
Utilizing yeast and specialized bacteria yeast to transform waste or biomass into usable energy is called a biochemical conversion. The traditional processes include AD, alcoholic fermentation, and photobiological methods, which produce various biofuels, as indicated in Figure 1.6 [77]. The pretreatment process is the most expensive step in turning lignocellulosic biomass into fermentable sugars. This is due to the expense of handling liquids, processing solids after pretreatment, and treating any co-products
Figure 1.6 Biochemical conversion of biomass to biofuels.
or inhibitors that may be present [78]. As a result, an efficient pretreatment should be able to (i) boost sugar yields for subsequent processing, (ii) treat all lignocellulosic feedstock, (iii) assist in recovering lignin for subsequent combustion, (iv) lessen the formation of co-products or inhibitors, (v) lower energy and operating costs, and (v) regenerate valuable lignin co-products. Several pretreatment methods are utilized to produce bioethanol [79].
Potential sources for the manufacture of bioethanol include wheat and rice stalks, sugarcane bagasse, maize cobs, and other agricultural byproducts [80]. Because of the speedy harvest times of these crops, wastes are more accessible all year. Every year, 350 and 450 million tons of crops are harvested, creating enormous agricultural waste [81]. Wastes from the food and pulp sectors and municipal solid waste have all been investigated for ethanol generation. Municipal solid waste is a viable feedstock because it contains carbohydrates that can be fermented to produce bioethanol [82, 83]. It also contains protein and minerals essential for the growth of bacteria that produce ethanol.
Over the past few decades, microalgae have gained recognition as a promising feedstock for the creation of bioenergy. While their carbohydrate content can be used to produce fermentative bioethanol and biobutanol, their lipid content can make biodiesel. After lipid extraction and ethanol fermentation, microalgae can produce gaseous biofuels like biomethane and bio-hydrogen or their byproducts [83]. Microalgae species like Chlamydomonas sp., Chlorella sp., Spirulina sp., Spirogyra sp., and Dunaliella sp. are appropriate for usage as potential feedstocks because they contain up to 64% of starch per dry cell weight. In addition, microalgae have high biomass production rates, photosynthetic activity, and CO2 bio-sequestration [81].
In the presence of yeast or bacteria, fermentable sugars generated from biomass’ cellulose and hemicellulose components can be transformed into bioethanol through the alcoholic fermentation of biomass wastes. For instance, it has been reported that certain microalgae species, like Chlorella, Chlamydomonas, Scenedesmus, Dunaliella, and Spirulina, accumulate significant amounts of starch, glycogen, and cellulose [84]. The basic materials required for the manufacture of bioethanol are these complex polysaccharides. Before feeding, the polysaccharides are hydrolyzed to convert them into simple sugars because the microorganisms have trouble metabolizing them. Enzymes, acids, and alkalis are all used in the most common hydrolysis procedures. However, simple and affordable acid treatment can transform sugars into undesirable forms. Although using enzymes costs more and takes longer, they are practical and do not produce unwanted byproducts. Techniques for cell disruption can be applied before hydrolysis to increase efficiency and shorten processing time. The resulting unfiltered ethanol [85
