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## Über dieses Buch

The biorefinery, integration of processes and technologies for biomass conversion, demands efficient utilization of all components. Hydrothermal processing is a potential clean technology to convert raw materials such as lignocellulosic and aquatic biomass into bioenergy and high added-value compounds. This book aims to show fundamental concepts and key technological developments that enabled industrial application of hydrothermal processing. The scope of this book is primarily for scientists working in the biorefinery field as well as engineers from industry and potential investors in biofuels. Therefore, the information in this book will provide an overview of this technology applied to lignocellulosic materials and aquatic biomass, and especially new knowledge. Critically, this book brings together experts in the application of hydrothermal processes on lignocellulosic and aquatic biomass.

## Inhaltsverzeichnis

### Chapter 1. How the Severity Factor in Biomass Hydrolysis Came About

Abstract
The search for a useful index that could provide a simple descriptor of pretreatment operations motivated us, in the 1970s, to explore the concept of severity in chemical reactions. The literature showed that industrially there had been numerous efforts to define such indices. For example, pre-hydrolysis in kraft pulping is an operation which includes steam and aqueous pretreatment methods similar to those that we were pursuing in our labs at the time. This had been modeled by Brasch and Free via the P-factor, as well as the H factor = ∫ (k(T)/k(373)dt, between times 0 and t, with the temperature in °K, with k being the rate constant which traditionally has been considered to follow the Arrhenius temperature dependency. Delignification rates are proportional to the H factor, hence its usefulness in controlling the final pulp consistency. Outside of wood processing, the high temperature cracking of petroleum fractions resulted in similar indexes which had been developed and implemented in plant operations. With increasing analytical insight into petroleum chemistry and processing, the cracking indices have gradually been replaced by a chemical kinetic-based “lumping” approach used for each of the petroleum fractions in a given crude oil.
Esteban Chornet, Ralph P. Overend

### Chapter 2. Effect of Hydrothermal Pretreatment on Lignin and Antioxidant Activity

Abstract
The influence of the hydrothermal processing of lignocellulosic materials on the solubilization of the phenolic fractions is reviewed. Special attention is deserved to autohydrolysis, since this an environmentally friendly technology requiring water as the only reagent. Considerations for the selection of the major variables affecting the performance of this hydrolytic process are presented according to the severity factor and discussed regarding the solubilized phenolic products and their antioxidant properties. Examples on the fractionation of different waste materials are presented.
Andrés Moure, Gil Garrote, Herminia Domínguez

### Chapter 3. Effect of Hydrothermal Processing on Hemicellulose Structure

Abstract
Hydrothermal process is a potential technology to convert lignocellulosic resources into biofuels and value-added chemicals in a green fashion. Fractionation of woody biomass allows the exploration of the concept of integrated forestry biorefinery. The hydrolysis of hemicellulose produces oligosaccharides, pentose (xylose and arabinose), hexose (glucose, mannose, and galactose), acids (acetic acid, formic acid, and levulinic acid), and furans (furfural and 5-hydroxymethylfurfural), as well as resulting insoluble humins as by-products under harsh conditions. This chapter highlights recent results on hydrothermal processing of various lignocellulosic biomasses, including fundamentals of hydrothermal pretreatment and mechanistic and kinetic studies in such process, especially in analysis and characterization of hemicelluloses before and after hydrothermal processing.
Ling-Ping Xiao, Guo-Yong Song, Run-Cang Sun

### Chapter 4. Response of Biomass Species to Hydrothermal Pretreatment

Abstract
Considerable effort has been devoted to the development of biofuels from renewable lignocellulosic biomass. To resolve the challenges associated with the structural barrier of lignocellulosic biomass, hydrothermal pretreatment is applied to alter the structure and improve the accessibility of carbohydrate sugars to microorganisms or chemicals in the subsequent conversion processes. Hydrothermal pretreatment takes advantage of high moisture content of biomass and efficiently converts polysaccharides into monomeric sugars and their corresponding degradation products. To achieve this goal, multiple technologies have been explored using liquid water as the media, with or without addition of chemicals (acids or alkalis). However, there are difficulties in developing an optimized and universal treatment approach due to the heterogeneity of biomass. In this chapter, four major biomass types, wood, bamboo, agricultural residues, and agave, are discussed and compared with respect to feedstock composition and response to the various hydrothermal pretreatments. Moreover, the reaction pathways of individual biomass components (hemicellulose, cellulose, lignin, extractive, and ash) under different treatment conditions (acidic and alkaline) are also comparatively reviewed. Finally, the effects of pH, biomass solid loading, and reactor selection on pretreatment are presented.
Jingqian Chen, Zhaoyang Yuan, Elisa Zanuso, Heather L. Trajano

### Chapter 5. Kinetic Modeling, Operational Conditions, and Biorefinery Products from Hemicellulose: Depolymerization and Solubilization During Hydrothermal Processing

Abstract
Hydrothermal processing is an interesting technology for the conversion of lignocellulosic biomass in biofuels and compounds in terms of a biorefinery. This process is based on the selective solubilization and depolymerization of hemicellulose fraction (xylan), producing a liquid phase of hemicellulose rich in oligomers (xylooligosaccharides), monosaccharides (mainly xylose), and degradation compounds (furfural and formic acid). Therefore, this chapter presents an overview of mathematical modeling based on pseudo-first-order kinetics and the operating conditions as temperature and time (“severity factor”) during the hydrothermal processing in order to predict the conversion and the produced compounds with high value-added in terms of biorefinery.
Elisa Zanuso, Anely A. Lara-Flores, Daniela L. Aguilar, Jesús Velazquez-Lucio, Cristóbal N. Aguilar, Rosa M. Rodríguez-Jasso, Héctor A. Ruiz

### Chapter 6. Combined Severity Factor for Predicting Sugar Recovery in Acid-Catalyzed Pretreatment Followed by Enzymatic Hydrolysis

Abstract
The severity factor developed by Chornet and Overend combines the effects of temperature and time in a single function to allow translation of sugar and oligomer release results from operation at one combination of temperature and time to realize nearly the same release at a different combination of these two variables. This factor has proven very valuable in correlating results from pretreatment of a variety of cellulosic biomass materials with just hot water or steam. The severity factor concept was subsequently extended to facilitate trading off among temperature, time, and acid concentration for pretreatments that employ dilute acid to hydrolyze hemicellulose. The resulting combined severity factor can be derived from simple first-order kinetic models that have been shown to describe sugar release from dilute acid pretreatment. In addition to describing hemicellulose sugar yields, it has been shown that the combined severity factor can provide some insights into expected sugar release yields from subsequent enzymatic hydrolysis of the solids left after dilute acid pretreatment. Furthermore, a simple adjustment in one parameter of the combined severity factor makes it possible to translate from one combination of temperature, time, and acid concentration conditions that maximizes yields from acid-catalyzed breakdown of xylooligomers released in hydrothermal pretreatment of biomass to a different set of conditions for maximum sugar release.
Charles E. Wyman, Bin Yang

### Chapter 7. Hydrothermal Pretreatment of Lignocellulosic Biomass for Bioethanol Production

Abstract
The work of Overend and Chornet on pretreatment severity factors combined with major contributions by others in the field has consistently shown that pretreatment is both an important cost driver of lignocellulose conversion to ethanol and a critical step that enables enzyme hydrolysis. Different lignocellulose pretreatments have a common objective of enhancing hydrolysis by opening up the plant cell wall enabling hydrolytic enzymes to access cellulose and hemicellulose. The work reported in this chapter addresses liquid hot water pretreatment and mechanisms by which it enhances the rates and extents of enzyme hydrolysis of cellulose from different types of lignocellulosic materials. Maintaining pH between about 4 and 7 is an important process variable of liquid hot water pretreatment, since pH can directly influence the formation of aldehydes and other inhibitors from hemicellulose and affect lignin solubilization which in turn also releases molecules that inhibit or deactivate the enzymes. Pretreatment conditions and severities may also change both chemical structure and physical properties of the residual lignin itself, which in turn negatively affects the action of enzymes. This chapter will focus on a detailed review of our work on liquid hot water pretreatment of lignocellulosic materials and its implications for potential use in biorefineries for production of bioethanol and bioproducts with high added value. Correlation of severities to enzyme conversion of different biomass materials and an overview of the potential application of hydrothermally pretreated biomass as a renewable feedstock for enzyme production will also be presented and discussed.
Eduardo Ximenes, Cristiane S. Farinas, Youngmi Kim, Michael R. Ladisch

### Chapter 8. Hydrothermal Pretreatment: Process Modeling and Economic Assessment Within the Framework of Biorefinery Processes

Abstract
Techno-economic evaluation of processes based on hydrothermal pretreatment is needed to set the current status and to identify processing bottlenecks that need to be addressed to make these processes viable. Having suitable models to carry simulations is a prerequisite for conducting such evaluations. The goal of this chapter is to discuss different approaches that can be used to model and simulate hydrothermal pretreatment processes using commercial process simulators. A discussion on possible flowsheets, as well as different ways to model biomass and chemical reactions, is presented. Rather than simply listing the different possibilities, the focus is on the rationale behind the selection of unit operations and models and the consequences each selection has. A brief discussion on possible improvements to be made to the current state-of-the-art models is also presented. Toward the end, the chapter provides a review of different criteria and usual assumptions that are made to calculate costs of biorefinery processes, and a summary of these costs for those flowsheets has analyzed inclusion of hydrothermal pretreatment.
Ana I. Torres, Muhammad T. Ashraf, Tanmay Chaturvedi, Jens Ejbye Schmidt, George Stephanopoulos

### Chapter 9. Bioethanol Production from Pretreated Solids Using Hydrothermal Processing

Abstract
Hydrothermal pretreatment was discussed as an efficient pretreatment method used to process biomass feedstock prior to bioethanol fermentation. Several examples of processes using different reactors and different feedstocks have been presented for comparison. Feedstocks discussed included energy crops (exemplified by prairie grasses), agricultural residues (exemplified by date palm trimmings), and extremophiles (exemplified by Salicornia bigelovii). Optimization studies have been performed to find the most efficient conditions for treating particular biomasses, which in conclusion have been found to produce similar results (200–210 °C and ~10 min). Pretreated solids have been fermented to measure the bioethanol potential following the simultaneous saccharification and fermentation protocol with Saccharomyces cerevisiae. The results revealed a considerable increase (at least 50%) in enzyme digestibility and fermentability of the pretreated pulps compared to the raw biomass, finally reaching 77–100% of theoretical ethanol yield (depending on biomass).
Iwona Cybulska, Mette Hedegaard Thomsen

### Chapter 10. Production and Emerging Applications of Bioactive Oligosaccharides from Biomass Hemicelluloses by Hydrothermal Processing

Abstract
Hemicelluloses are the second most abundant heterogeneous polysaccharides in nature. Among the several treatments that can be used for the solubilization of hemicelluloses to produce oligosaccharides from lignocellulosic biomass, the autohydrolysis reaction is the most widely used. Under suitable conditions, autohydrolysis enables high ligosaccharides yields, however this reaction is not selective and undesired compounds are also present in the reaction media. Because of this, the autohydrolysis media has to be subjected to further processing to improve the purity of oligosaccharide hydrolysate. The chemical and structural characterization of the solubilized products from hemicelluloses is an important aspect as it allows to know the application for which they are more suitable. Oligosaccharides from hemicelluloses can find applications in several fields such as biomedical, food, and biomaterials.
Beatriz Gullón, Izaskun Dávila, María García-Torreiro, Remedios Yáñez, Jalel Labidi, Patricia Gullón

### Chapter 11. Production of Hemicellulases, Xylitol, and Furan from Hemicellulosic Hydrolysates Using Hydrothermal Pretreatment

Abstract
Nowadays, manufacturing products (chemicals, fuels, and other value-added products) from renewable sources (such as lignocellulosic biomass) is one of the main challenges which the society has to face in order to achieve a sustainable growth. Lignocellulosic biomass processing in a biorefinery approach is presented as an alternative and promising solution to achieve that by replacing the oil-based market. Hydrothermal treatment is recognized as the first step of a biorefinery since the hemicellulose fraction is easily solubilized and recovered, remaining a spent treated solid composed by cellulose and lignin. Hemicellulose (ramified polysaccharide composed by pentoses and hexoses) presents a great potential to be used as substrate for value-added product production, such as hemicellulases, xylitol, and furan-derived compounds. Hemicellulases are mainly produced from fungi and have application in several industrial sectors, but the high cost of enzymes is the main bottleneck in enzymatic conversion processes. Xylitol can be obtained by chemical or microbial synthesis from xylose. Currently, commercial xylitol has been obtained by chemical processes which generate pollutant degradation products. In the last years, biotechnological processes have received much interest in order to become more competitive and replace chemical synthesis. Moreover, furan-derived compounds are arising as alternative components of renewable fuel due to interesting features (higher heating values than gasoline) and as building blocks for chemical production (e.g., resins, polymers). This chapter will primarily cover the current scenario regarding hemicellulose-derived products, mainly hemicellulases, xylitol, and furan-derived compounds, as well as production processes, industrial applications, and world market focusing on the potential of hydrothermal treatment for the production of these products.
Michele Michelin, Aloia Romaní, José Manuel Salgado, Lucília Domingues, José A. Teixeira

### Chapter 12. Steam Explosion as a Hydrothermal Pretreatment in the Biorefinery Concept

Abstract
Steam explosion is one of the leading and most promising hydrothermal pretreatment for lignocellulose biorefinery. In the biorefinery concept, this monograph chapter systematically elaborates the basic principles, derived technologies, and process integration of steam explosion biorefinery techniques in development. The theoretical bases of steam explosion technology were elucidated from the transfer rules and hydrothermal mechanism during the multistage pretreatment process. Based on the cognition of steam explosion process and heterogeneity of lignocellulosic feedstock, a series of novel steam explosion-derived biorefinery techniques were explored to achieve the efficient fractionation and conversion of biomass components. Their integrated industrial biorefinery chain was further interpreted by a typical ethanol production demonstration project, in order to boost the development of steam explosion technology from biorefinery engineering. It is anticipated to have some guiding meaning for the better interpretation and application of steam explosion biorefinery techniques in both the experimental research and industrial production.
Hongzhang Chen, Wenjie Sui

### Chapter 13. Adaptation of Severity Factor Model According to the Operating Parameter Variations Which Occur During Steam Explosion Process

Abstract
The severity factor model developed by Chornet and Overend in 1987 has become the standard method to estimate the intensity of steam explosion pretreatment of biomass feedstocks. Continuous records of operating conditions applied during steam explosion show that significant variations occur during the process and have an impact on the treatment severity. The two main parameters that vary significantly during the process are temperature (linked to the pressure) and pH. Temperature variation is especially significant at the beginning of the process when pressure increases to its target value. pH variation is observed throughout the process. pH variation occurs due to the evolution of organic acids during vapocracking.”
This chapter discusses a variation of the severity factor determination, adapted to treat complex dynamics which are not taken into account by the severity factor model currently in use. The first part of the chapter presents the steam explosion technology and its effect on lignocellulosic biomass and describes a typical process installation. The second part is focused on the adaptation of the severity factor model to temperature and pH variation. Finally, a general equation for severity is developed based on the true evolution of temperature and pH during steam explosion.”
Nicolas Jacquet, Aurore Richel

### Chapter 14. Hydrothermal Pretreatment Using Supercritical CO2 in the Biorefinery Context

Abstract
Supercritical fluids have been receiving an increasing attention in biorefinery processes due to guarantying high reaction yields and selectivities. In addition, supercritical fluids, especially CO2, permit to avoid the use of additional catalyst simplifying downstream processing. These benefits are a result of their unique physicochemical proprieties, which are easily adjustable just by slight changes of operational temperature and pressure.
This chapter provides state-of-the-art about the biomass processing in hydrothermal media with supercritical CO2 within biorefinery context. The biomass pretreatment with water is addressed briefly since it is inherently associated. In an effort to demonstrate the employment of supercritical CO2 as a catalyst in hydrothermal pretreatment, the knowledge concerning phase behaviour of CO2 and water mixture is reported. In particular, the role of supercritical CO2 in the perspective of enzymatic valorisation of cellulose is explored thoroughly. This chapter addresses also the importance of supercritical CO2 addition to hydrothermal technologies in the processing of hemicellulose and lignin into C5-sugars and phenolic products, respectively.
Finally, the critical outlook and perspectives of high-pressure biomass pretreatment with supercritical CO2 is given.
Ana Rita C. Morais, Rafal M. Lukasik

### Chapter 15. Scale-Up Hydrothermal Pretreatment of Sugarcane Bagasse and Straw for Second-Generation Ethanol Production

Abstract
The process for producing fuel ethanol from renewable lignocellulosic biomass includes three main steps: pretreatment, to reduce the biomass recalcitrance; hydrolysis, to release biomass polysaccharides into mono- and disaccharide form; and fermentation, to convert these reducing and monomeric sugars into biofuels. The pretreatment processes intend to disorganize the lignocellulosic structure, and the hydrothermal pretreatment has been gaining increasing attention as an environmentally friendly solvent pretreatment since it will undergo hydrolysis reactions in the presence of the hydronium ions generated by water autoionization and acetic acid from hemicelluloses, which acts as catalysts, and an attractive reaction media for a variety of applications. This chapter intends to show a comparative study of hydrothermal pretreatment in laboratory and pilot plant scale, including a complete mass balance approach.
Viviane Marcos Nascimento, Carlos Eduardo Vaz Rossell, George Jackson de Moraes Rocha

### Chapter 16. Pilot Plant Design and Operation Using a Hydrothermal Pretreatment: Bioenercel Experience

Abstract
Over the last century, the world has depended on the oil industry to provide the greatest majority of the raw material necessary for the production of chemical products, textiles, pharmaceuticals, and fuels. Countries with no oil reserves have the necessity to seek new cleaner and sustainable technologies to fulfill its requirements. Lignocellulosic biomass has the potential to replace oil as the raw material to produce chemicals and fuels under a biorefinery concept. To develop this new concept of industry, Bioenercel, a Chilean Consortium, was created to design a strategy to obtain sugars and lignin from wood as raw materials for the production of fuels and high-value products: bioethanol or bio-oil production from sugar fermentation and generation of biomaterials from lignin. This strategy includes a pilot plant design and operation. The plant was designed to handle a variety of feedstocks; the process stages are batched to optimize each step individually and the process as a whole. It also considers a wide range of operation options for process flexibility. The equipment includes a wood digester for biomass pretreatment, a disc refiner, a high consistency horizontal mixer (pre-hydrolyzer) and a hydrolyzer for the enzymatic hydrolysis, a fermenter, and a distillation column. Pre-hydrolyzer, hydrolyzer, and fermenter are combined with a filter press to adjust the operation to a simultaneous saccharification and fermentation process or to a separate hydrolysis and fermentation process. Additionally, all the complimentary equipment is in place for this pilot plant to successfully mimic a biorefinery. This work shows a pilot plant experience using hardwood (eucalyptus wood) to obtain ethanol using autohydrolysis pretreatment and simultaneous saccharification and fermentation with high solid loads.
Preliminary results of the wood-to-ethanol process evaluated in the pilot plant showed 73% yield base on the initial cellulose present in the wood converted into ethanol compared to 79% obtained at bench scale. These promising results were used to assess a preliminary technical-economic evaluation considering the production of ethanol from the sugars and electricity from the solid residue. For a base value of US$74 per dry ton of wood and an estimated selling price of ethanol and electricity of US$ 770/m3 and 100 US$/MWh, respectively, the net present value reaches US$ 112 million, the internal return rate 14%, and the payback time 5.7 years.
Alfred Rossner, Carolina Parra

### Chapter 17. Techno-Economic Aspects in the Evaluation of Biorefineries for Production of Second-Generation Bioethanol

Abstract
During the past decades, biorefineries have been in the research focus, to a large extent driven by the desire to produce biofuels for replacing fossil sources in the transportation sector. The heterogeneous composition of biological feedstocks opens for the possibility to produce a palette of products from any biorefinery, and this is also true for ethanol production from second-generation feedstock. The complexity of the process increases by using different processing paths, which include, e.g., reactions and separation operations. This complexity leads to difficulties in the understanding of the process and establishing a suitable design of the process. In biorefinery operations, relationships also exist between the outcome in terms of yields of the different products, which can be complicated further by the integration of internal and external processes, such as energy and mass flows. Integration can be used as a means to reduce energy utilization in the process; thus, the production cost can be cut. Techno-economic evaluations play an important part in guiding the development toward more economically and environmentally sustainable processes. In this chapter, we discuss topics, which focus on bioethanol biorefineries based on hydrothermal pretreatment. Some suggestions on how to perform techno-economic evaluations are also presented.
Michael Persson, Borbála Erdei, Mats Galbe, Ola Wallberg

### Chapter 18. Minimizing Precipitated Lignin Formation and Maximizing Monosugar Concentration by Formic Acid Reinforced Hydrolysis of Hardwood Chips

Abstract
Prehydrolysis Kraft involves high-temperature steam treatment of wood chips followed by Kraft pulping. In industrial practice, the acidic prehydrolysate is mixed with alkaline spent Kraft black liquor because separate handling of the hydrolysate causes plugging in downstream equipment due to lignin precipitation. However, dilution of black liquor by the hydrolysate significantly increases evaporation cost, and the prehydrolysate sugars are lost as potential feedstock for fuels and chemicals. Thus, there is a strong incentive for reducing or eliminating the formation of the troublesome lignin precipitates in the prehydrolysate. Earlier we reported that addition of 10 g/L formic acid (FA) during hot water (160 °C) treatment of hardwood chips significantly minimized the lignin precipitates in prehydrolysate. Also, FA reinforced prehydrolysates contained more hemicellulose sugars, with more in monomeric form. In the present paper, we show that further increasing the FA concentration and/or addition of a small amount of sulfuric acid leads to almost complete hydrolysis of the oligomers and, more importantly, also further decreases the lignin precipitate concentration. The effect of FA, hydrolysis time, L/W ratio, and H2SO4 concentration on dissolved sugar and lignin precipitate concentrations will be described. The molecular weight and polydispersity of the lignin precipitates decreases with increasing FA concentration. A new mechanism is proposed to explain the effect of FA and other operating conditions on the insoluble lignin concentration. Recovery of FA charged at 10 g/L at L/W ratio of 3.5 L/kg can be achieved by reactive distillation using methanol as reactant. At 95% FA recovery, the net charge of fresh FA to the process is 0.9 g/100 g wood because FA is partially carried forward into the Kraft process with the drained extracted chips. However this chemical requirement is compensated by the production of acetic acid (AA) at 2.3 g/100 g wood, furfural at 1.4 g/100 g wood, and hemicellulose sugars at 10 g/100 g wood, 90% of which are present as monosugars when prehydrolysis is performed at 15 g/L of FA or 10 g/L FA plus 0.25 g/L H2SO4.
Adriaan van Heiningen, Yusuke Yasukawa, Kefyalew Dido, Raymond Francis

### Chapter 19. Microwave-Assisted Hydrothermal Processing of Seaweed Biomass

Abstract
This chapter focuses on the effects of microwave on hydrothermal processing of algal biomass, especially fast-growing green seaweed biomass of Ulva. Microwave enhances the efficiency of hydrothermal treatment of biomass by directly affecting water molecule, catalysts, and biomass substrates. Firstly, the fundamentals of microwave-assisted biorefinery are illustrated. Then, the microwave effects for hydrothermal processing of biomass were summarized for the model sugar reaction and seaweed conversion. Dielectric properties of seaweed biomass relevant to microwave heating are also presented in the last section.
Shuntaro Tsubaki, Ayumu Onda, Tadaharu Ueda, Masanori Hiraoka, Satoshi Fujii, Yuji Wada

### Chapter 20. Hydrothermal Processes for Extraction of Macroalgae High Value-Added Compounds

Abstract
Macroalgae are a complex biomass conformed of unique compounds with high value-added potential for applications in diverse areas such as pharmaceutical, medical, food, material, and biofuel. The pretreatment conditions for macroalgae fractionation into target molecules are essential to remain relevant biological and chemical properties including antiviral, anti-inflammatory, antiangiogenic, antiproliferative, antitumoral, anticoagulant, antioxidant, and gelation activities. Therefore, green technologies such as hydrothermal processes using different heating sources as microwaves, conduction-convection under high-pressure systems, and steam explosion have been recently studied to evaluate the efficiency of this process taking into account the effect over the environment and the economic viability. The present overview describes and discusses the most recent and relevant studies based on hydrothermal processes over the extraction of high value-added and bioactive compounds from macroalgae sources.
Daniela E. Cervantes-Cisneros, Dulce Arguello-Esparza, Alejandra Cabello-Galindo, Brian Picazo, Cristóbal N. Aguilar, Héctor A. Ruiz, Rosa M. Rodríguez-Jasso

### Chapter 21. Hydrothermal Processing of Microalgae

Abstract
As a result of the increasing population and industrial development, there is an enormous energy demand worldwide. For this reason, the research on the potential of microalgae (including also cyanobacteria) as a third-generation feedstock for bioenergy production has markedly increased. Besides biofuels (biogas, bioethanol, biodiesel, etc.), algae biomass is one of the most promising feedstock to produce high-value products in a sustainable way.
The use of microalgae has many advantages over first- and second-generation raw materials. This is due to their fast growth rates due to an efficient solar conversion into biomass; capability of growing under several conditions, including in wastewater; reduced need for water and other resource inputs; and the possibility of not using arable lands for their cultivation.
Independently of the product of interest, most of the production processes from microalgal biomass need some hydrothermal processing step. This implies a pretreatment step at high temperature, even at high pressure, with or without acid addition. Hydrothermal pretreatment may disrupt microalgae cell wall for biogas production or can involve partial biomass hydrolysis as it is the case for bioethanol production.
This chapter includes an updated revision of hydrothermal treatments commonly used to process microalgal biomass mainly for biofuels production and resumes how different temperatures and other treatment parameters affect final product titers and yields.
Cristina González-Fernández, Lara Méndez, Mercedes Ballesteros, Elia Tomás-Pejó

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