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About this book

This book provides in-depth coverage on the latest concepts, systems, and technologies that are being utilized in biorefineries for the production of biofuels and value-added commodities. Written by internationally recognized experts, the book provides a comprehensive overview of pretreatment technology for biorefineries and biofuels, enzymatic hydrolysis and fermentation technology for biofuel production, and lignin valorization for developing new products from waste lignin. The book will be a valuable resource for researchers and professionals working in process engineering, product engineering, material science, and systems and synthetic biology in the fields of biorefining, biofuel, biomaterials, environmental waste utilization, and biotechnology.

Table of Contents


Chapter 1. Challenges and Perspectives of Biorefineries

The valorization of lignocellulosic biomass (LCB) to biofuels and value-added commodities in biorefineries shows the promise to address the energy demands, the global climate changes, and the societal needs. However, it is still urgent to develop modern biorefinery scenarios to transform LCB into high-value products for the sustainability and feasibility. This chapter summarizes the challenges and perspectives of biorefineries from the aspects of the LCB properties, the process and product, the conversional and emerging technologies, and the lignin valorization. Due to the complex structures of LCB, a substantial knowledge in understanding of its intrinsic properties is essential to facilitate sustainable conversion. The conversional and emerging technologies related to pretreatment, hydrolysis, and fermentation and the interactions among these units have been described systematically. The development of these technologies would further reduce the process cost of biorefineries. After that, the lignin valorization strategies have been discussed to make a sustainable biorefinery. The requirements of innovative modern biorefineries have been proposed to meet the implication of LCB conversion to biofuels and value-added commodities. The summary of these perspectives of LCB upgrading to a diverse set of products would guide the process design, the technology development, and the implementation of biorefineries by mitigating technical risk for scale-up with the improvement of the profitability of biorefinery. Overall, the improved sustainability of biorefinery holds potential advantages to address the problems facing the energy and the societal needs.
Zhi-Hua Liu

Chapter 2. Deconstruction of Lignocellulose Recalcitrance by Organosolv Fractionating Pretreatment for Enzymatic Hydrolysis

Lignocellulosic biomass is a potential feedstock to produce the second-generation cellulosic ethanol and high-value-added chemicals. In order to increase the enzymatic digestibility of lignocellulosic biomass for efficient bioconversion, various pretreatment approaches have been studied. Among them, organosolv pretreatment is promising owing to its ability to achieve a biomass fractionation in one-pot process with efficient enhancement of the enzymatic digestibility. In this chapter, the research progress in recent years regarding different types of organosolv fractionating pretreatment methods (e.g., alcohol-based, organic acid-based, ketone-based, etc.) has been discussed, in terms of process operation, mechanism for improving enzymatic digestibility, and the lignin reactions during pretreatment. Organosolv fractionating pretreatment not only improves the enzymatic efficiency of the biomass but also shows great potential to achieve a full utilization of the main components. However, most existing organosolv-based biorefineries are just in pilot or demonstration scale mainly due to the high operation cost and energy consumption. To improve the economic feasibility of the organosolv pretreatment, more products with high value added should also be developed in the future.
Ziyuan Zhou, Dehua Liu, Xuebing Zhao

Chapter 3. New Developments on Ionic Liquid-Tolerant Microorganisms Leading Toward a More Sustainable Biorefinery

The growing concerns about climate change and energy security are driving the development of bio-based technologies to produce renewable liquid fuels and chemicals. Ionic liquids (ILs) have demonstrated to be promising solvents to pretreat lignocellulosic residues, promoting efficient enzymatic hydrolysis of lignocellulosic carbohydrates into sugars, which can be further used by microorganisms to produce biofuels and other value-added chemicals. Despite their unique properties to effectively deconstruct plant cell walls, ILs show strong interactions with the pretreated biomass, and their presence is often inhibitory to cellulolytic enzymes and microorganisms. The most advanced biorefinery concepts based on IL pretreatments focus on the development of more biocompatible ILs and more robust microbial strains with higher tolerance to ILs. This chapter provides an overview and a discussion over the main efforts performed on the screening and development of IL-tolerant microbial strains, as well as in more biocompatible IL pretreatment methods. These early research advancements in this field offer a baseline and a platform for future research with the goal of improving the sustainability and economic viability of IL pretreatment-based biorefineries.
André M. da Costa Lopes, Leonardo da Costa Sousa, Rafał M. Łukasik, Ana Rita C. Morais

Chapter 4. Liquid Hot Water Pretreatment for Lignocellulosic Biomass Biorefinery

Liquid hot water (LHW) technology has been thought as a green process which hardly pollutes the environment due to no addition of chemical reagents. It is usually employed as a pretreatment method for producing ethanol, butanol, lactic acid, biogas, and other biochemicals from lignocellulose. It is also used as a technology to directly obtain products from lignocellulose such as xylooligosaccharide, microcrystalline cellulose, hydrochar, and so on. This chapter describes the technological characteristics and development of LHW process, summarizes the way of LHW treatment to influence the physicochemical features of lignocellulose and the biorefinery efficiency, and depicts the application of LHW technology for bioproduct production. It concludes that LHW treatment is a versatile biorefinery technology and outlooks the promising way for the practical application of LHW technology.
Xinshu Zhuang, Wen Wang, Bing Song, Qiang Yu

Chapter 5. Multiproduct Biorefining from Lignocellulosic Biomass Using Steam Explosion Technology

Biorefinery has attracted a great deal of attention in response to energy demand, security, and environmental concerns in recent decades. Multiproduct biorefining of lignocellulosic biomass (LCB) by employing steam explosion technology would facilitate the realization of profitable biorefinery. The purpose of this chapter is to review the multiproduct biorefining from LCB using the integrated process of steam explosion from the perspectives of feedstock, process, and product. First, the intrinsic characteristics of LCB were introduced, while the multiproducts from biorefinery were summarized. Second, the operation mode and mechanism of steam explosion pretreatment had been described. After that, the integrated process employing steam explosion had been further discussed to overcome the intrinsic characteristics of LCB across the diversity of LCB feedstock. Steam explosion pretreatment and its interdependent and synergistic relationships among up- and downstreams were then talked about for the production of multiproducts in biorefinery. Additionally, several pilot- and demonstration-scale operations for the production of multiproducts in biorefinery were described. Finally, several strategies for further development of the multiproduct biorefinery from LCB had been proposed to guide the design of the integrated process of steam explosion. Overall, the development and design of highly effective integrated process employing steam explosion should be economically feasible for biorefinery and could be keys to the implementation of multiproduct biorefinery.
Zhi-Hua Liu

Chapter 6. Fundamentals of Lignin-Carbohydrate Complexes and Its Effect on Biomass Utilization

Lignocellulosic biomass has been admitted as sustainable, renewable, and carbon-neutral energy sources which can be the potential alternative to the fossil fuel. Although the biomass mainly consists of cellulose, hemicellulose, and lignin, each of them does not exist separately. These components are cross-linked to form lignin-carbohydrate complexes (LCCs) which should be considered as the basic structure in cell wall. The existence of LCC is being realized to directly restrict the fractionation and cause the recalcitrance in biomass utilization. Still, there are controversial debates regarding the properties, structure, and composition of LCCs and its effect on biorefinery, which can be attributed to the lack of attention on LCCs which makes progress more sluggish in this field. Discussion in this review critically emphasizes on the fractionation of LCCs, presence and susceptibility of lignin-carbohydrate bonds, and direct/indirect analytical techniques to evoke the attention of researchers toward this, when the world is surrounded by energy and environment issues and realization of biorefinery needs to be addressed.
Usama Shakeel, Saif Ur Rehman Muhammad, Yong Zhao, Hongqiang Li, Xia Xu, Yong Sun, Jian Xu

Chapter 7. Systematic Metabolic Engineering of Saccharomyces cerevisiae for Efficient Utilization of Xylose

Efficient utilization of xylose is indispensable for economic biotransformation of biomass hydrolysates into fuels and other value-added chemicals. In the past two decades, there have been many metabolic engineering efforts in developing fast xylose-fermenting recombinant Saccharomyces cerevisiae strains, which are commonly used in industrial fermentation but are naturally incapable of metabolizing xylose. In this chapter, we review systematic metabolic engineering of S. cerevisiae to improve xylose metabolism via the xylose reductase (XR)-xylitol dehydrogenase (XDH) pathway or the xylose isomerase (XI) pathway. We focus on key regulatory targets that are highly involved in xylose metabolism, including redox imbalance caused by cofactor mismatch between XR and XDH, low activity of upper pathway enzymes (e.g., XR, XDH, XI, and xylulose kinase), inefficient xylose transportation, and incompatible sensing and signaling pathway. Meanwhile, we describe the construction of a new xylose metabolism pathway, the Weimberg pathway, in S. cerevisiae to metabolize xylose in a different manner, providing an option to synthesize more useful chemicals via the tricarboxylic acid cycle.
Jing Han, Guoli Gong, Xia Wu, Jian Zha

Chapter 8. Microbial Lipid Production from Lignocellulosic Biomass Pretreated by Effective Pretreatment

To date, much attention has been paid on developing new strategies for the valorization of abundant, inexpensive, and renewable lignocellulosic biomass into liquid biofuels and chemicals. Lipids are one kind of value-added energy-rich compounds, which can produce by oleaginous microorganisms using biomass and/or biomass-hydrolysates. Recently, the conversion of lignocellulosic biomass into microbial lipid has received significant attention replacing fossil fuels. However, biomass is highly recalcitrant due to its complex structure with cellulose, hemicellulose, and lignin. Pretreatment of biomass is a critical process in the conversion due to the nature and structure of the biomass cell wall that is complex. Although green technologies for microbial production are advancing, the productivity and yield from these techniques are low. Over the past years, various biomass pretreatment techniques have been developed to disrupt the plant cell-wall structure of lignocellulosic biomass, facilitate subsequent enzymatic hydrolysis and microbial lipid fermentation, and successfully employed to improve biomass-to-lipid technology. In this chapter, the progress of pretreatment for enhancing the enzymatic digestion of lignocellulosic material is introduced. In addition, microbial lipid production from lignocellulosic biomass pretreated by effective pretreatment is discussed.
Cui-Luan Ma, Yu-Cai He

Chapter 9. Metabolic Engineering of Yeast for Enhanced Natural and Exotic Fatty Acid Production

Lignocellulose-derived sugars and other biorefinery by-product streams such as glycerol and acetic acid are useful carbon feedstocks for microbes that produce lipids. Lipids have high energy density and are easily converted into versatile biofuels and valuable oleochemicals. Common, robust yeasts such as Saccharomyces cerevisiae and Yarrowia lipolytica have been the most successfully exploited as cell factories for lipid production, and excellent progress has been made in productivity with the implementation of synthetic biology tools and metabolic engineering strategies. Accumulation and storage of standard fatty acids as triacylglycerols or secretion of free fatty acids has been enhanced by modification of metabolic pathways yielding maximal fatty acid titers above 100 g L−1 and productivity of 0.8 g L−1 h−1. Production of higher-value exotic fatty acids that are not native to yeast, such as short chain, hydroxylated, and cyclopropane, has great potential but requires more research into lipid synthesis pathways and new metabolic engineering strategies to achieve similar productivities as achieved for standard fatty acids. In addition, monitoring of cell viability and health, balancing cofactor demands, and minimizing stress are important strategies to avoid or reduce metabolic burden caused by engineering of cells.
Wei Jiang, Huadong Peng, Rodrigo Ledesma Amaro, Victoria S. Haritos

Chapter 10. Advanced Fermentation Strategies to Enhance Lipid Production from Lignocellulosic Biomass

This chapter discusses the related researches on microbial lipid biosynthetic processes, the inhibitor tolerance of lipid-producing microorganisms, and high cell density culture strategies for lipid production from lignocellulosic biomass. The aspects covered here mainly focused on the elucidation of lipid accumulations of oleaginous microorganisms in different fermentation modes including batch, fed-batch, and continuous cultivation coupled with multistage strategies for increasing cell densities of oleaginous microbes thereby improving lipid yield, titer, and productivity. Furthermore, because of the inhibitors generated during hydrolysis processes as by-products that influence the lipid biosynthesis, the strategies to enhance the lipid content through metabolic engineering approach including blocking of competing pathways and multigene methods were discussed in this chapter. It is suggested that the efficiency of the lignocellulosic lipid-based biorefinery process would be greatly improved if the cultivation platform of oleaginous microorganisms could integrate both micro-manipulations for the gene expression and fermentation strategies with the online control-feedback system.
Qiang Fei, Yunyun Liu, Haritha Meruvu, Ziyue Jiao, Rongzhan Fu

Chapter 11. Fractionation, Characterization, and Valorization of Lignin Derived from Engineered Plants

There is an urgent need to address the societal and sustainability challenges in the food, energy, and water (FEW) nexus at the global level. Research on improving the efficiency and sustainability of lignocellulosic conversion technologies and development of biomass feedstocks that are amenable to pretreatments has become a focus of the research community over the past few years. Genetic manipulation of lignin is a promising approach to generate biomass feedstocks with favorable properties. However, it is relatively unexplored in the area of understanding the effects of lignin modification on fractionation, characterization, and upgrading of lignin streams from engineered biomass feedstocks. There is a knowledge gap in linking the chemical complexity of lignin with its biological, spatial, and functional deposition with respect to how to separate and utilize the feedstock in a biorefinery. This knowledge is necessary for development of lignin-engineered plants and downstream processing technologies. This review recapitulates recent progress in lignin genetic modification and the leading technologies to fractionate and characterize lignin streams. Possible lignin valorization pathways including oxidative and reductive catalysis, electrocatalysis, and biological upgrading, in particular for use with engineered biomass feedstocks, are also discussed. Challenges and the outlook for future development are also briefly reviewed.
Enshi Liu, Wenqi Li, Seth DeBolt, Sue E. Nokes, Jian Shi

Chapter 12. The Route of Lignin Biodegradation for Its Valorization

Using lignin to produce high-value-added aromatic fine chemicals and high-grade biofuels such as aromatics, cycloalkanes, and alkanes can reduce the dependence on fossil resources and highly improve the competitiveness of biorefining industry. The biological valorization of lignin includes the biological depolymerization and bioconversion of lignin. With the development of bioprospecting and systems biology technology, more and more lignin-degrading microorganisms have been discovered and separated from the natural habitat of lignin decomposition. The physiological and biochemical characteristics of microorganisms and the molecular- and systematic-level degradation mechanism on lignin and lignin-derived aromatic compounds have also been deeply recognized. All of these have laid a theoretical foundation for precisely controlling the depolymerization and metabolism of lignin and establishing the biological processing pathway of lignin. This chapter will introduce the research progress of lignin valorization from the aspects of lignin-degrading microorganisms and enzymes, lignin degradation metabolic pathways, and the application of biosynthesis in lignin conversion.
Weihua Qiu

Chapter 13. Understanding Fundamental and Applied Aspects of Oxidative Pretreatment for Lignocellulosic Biomass and Lignin Valorization

Oxidative pretreatment of lignocellulosic biomass improves the efficacy of plant cell wall polysaccharides in biochemical conversion processes. In presence of oxidants, recalcitrant lignins are oxidized and dissolved to enhance the access of cellulose and hemicellulose to pretreatment chemicals and hydrolytic enzymes. This chapter focuses on the common types of oxidative pretreatments, their applications in biomass conversion, and the physiochemical changes in biomass structure during the pretreatment.
Younghan J. Lim, Zhenglun Li

Chapter 14. Lignin Valorization in Biorefineries Through Integrated Fractionation, Advanced Characterization, and Fermentation Intensification Strategies

Lignin valorization is essential for achieving profitable and sustainable biorefinery. However, the complex structure of lignin and the presence of lignin–carbohydrate complex (LCC) result in a series of obstacles that contribute to biomass recalcitrance. In addition, the heterogeneity of lignin and the unclear relationship between lignin structure and its activity significantly restrict the lignin valorization process. Therefore, this chapter provides the progress toward lignin valorization in three aspects. First, various fractionation strategies developed in recent years have been summarized to evaluate how uniform lignin fractions could be produced. Second, recent advances in lignin characterization techniques as well as their important roles for understanding lignin structure and providing guidance for lignin processing are systematically investigated and reviewed. Besides these, promising lignin bioconversion approaches through fermentation have been provided in detail. Fermentation intensification strategies are systematically examined from the aspects of microbial strains, substrates, and processes design. With the increase in fundamental understanding of lignin structure–activity relationships, a more directional and controllable lignin valorization path could be developed to contribute to the profitability and sustainability of biorefineries.
Zhi-Min Zhao, Yan Chen, Xianzhi Meng, Siying Zhang, Jingya Wang, Zhi-Hua Liu, Arthur J. Ragauskas

Chapter 15. Novel and Efficient Lignin Fractionation Processes for Tailing Lignin-Based Materials

As the most abundant aromatic biopolymer in nature, lignin has attracted great attention due to the complexity and yet richness of its functional groups for value-added applications. The yield of production of lignin and the reactivity of prepared lignin are very important to guarantee the feasibility of lignin-based material utilization. Various fractionation techniques have been developed to obtain high yield and relative high-purity lignin and carbohydrates (hemicelluloses and celluloses) and to reduce the condensed and degraded nature of conventional biorefinery lignin. In this chapter, novel and efficient lignin fractionation for tailing lignin-based material preparation are summarized and discussed.
Chuanling Si, Jiayun Xu, Lin Dai, Chunlin Xu


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