Lignocelluloses are often a major or sometimes the sole components of different waste streams from various industries, forestry, agriculture and municipalities. Hydrolysis of these materials is the first step for either digestion to biogas (methane) or fermentation to ethanol. However, enzymatic hydrolysis of lignocelluloses with no pretreatment is usually not so effective because of high stability of the materials to enzymatic or bacterial attacks. The present work is dedicated to reviewing the methods that have been studied for pretreatment of lignocellulosic wastes for conversion to ethanol or biogas. Effective parameters in pretreatment of lignocelluloses, such as crystallinity, accessible surface area, and protection by lignin and hemicellulose are described first. Then, several pretreatment methods are discussed and their effects on improvement in ethanol and/or biogas production are described. They include milling, irradiation, microwave, steam explosion, ammonia fiber explosion (AFEX), supercritical CO2 and its explosion, alkaline hydrolysis, liquid hot-water pretreatment, organosolv processes, wet oxidation, ozonolysis, dilute- and concentrated-acid hydrolyses, and biological pretreatments.

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Pretreatment of lignocellulosic wastes to improve ethanol and biogas production: a review.

Production of ethanol by bioconversion of lignocellulosic biomass has attracted much interest in recent years. However, the pretreatment process for increasing the enzymatic digestibility of cellulose has become a key step in commercialized production of cellulosic ethanol. During the last decades, many pretreatment processes have been developed for decreasing the biomass recalcitrance, but only a few of them seem to be promising. From the point of view for integrated utilization of lignocellulosic biomass, organosolv pretreatment provides a pathway for biorefining of biomass. This review presents the progress of organosolv pretreatment of lignocellulosic biomass in recent decades, especially on alcohol, organic acid, organic peracid and acetone pretreatments, and corresponding action mechanisms. Evaluation and prospect of organosolv pretreatment were performed. Finally, some recommendations for future investigation of this pretreatment method were given.

Organosolv pretreatment of lignocellulosic biomass for enzymatic hydrolysis

Lignocellulosic biomass is recalcitrant to biodegradation due to the rigid and compact structure of plant cell wall The recalcitrance of biomass is mainly constructed by its chemical compositions that build a spatial network as a protective bulwark Generally, the factors affecting the accessibility of biomass cellulose can be divided into direct and indirect factors The direct factors refer to the accessible surface area, and the indirect factors include biomass structure-relevant factors (pore size and volume, particle size, and specific surface area), chemical compositions (lignin, hemicelluloses, and acetyl group), and cellulose structure-relevant factors (cellulose crystallinity and degree of polymerization) Pre-treatment is actually the process to alter indirect factors and improve direct factors thus enhancing the accessibility of cellulose In this review, we summarize the effects of chemical compositions and physical structures on the enzymatic digestibility of lignocellulosic biomass We suggest that future work should be focused on but not limited to the molecular mechanisms of biomass recalcitrance by investigating the microscale and nanoscale features as well as hydrogen bonds network of lignocellulosic biomass © 2012 Society of Chemical Industry and John Wiley & Sons, Ltd

Biomass recalcitrance. Part I: the chemical compositions and physical structures affecting the enzymatic hydrolysis of lignocellulose

Lignocelluloses are often a major or sometimes the sole components in different waste streams from various sources such as industries, forestry, agriculture and municipalities. It represents an as-of-yet untapped source of fermentable sugars for significant industrial use. Many physico-chemical, structural and compositional factors hinder the hydrolysis of components present in the biomass to sugars and other organic compounds that can later be converted into fuels. During the past few years, a large number of chemical pretreatment methods including lime, acid, steam explosion, sulfur dioxide explosion, ammonia fiber explosion, ionic liquid and others have been developed for efficient pretreatment of biomass. Many pretreatment methods have shown high sugar yields i.e. more than 90% of the theoretical yield from lignocelluloses. In this review, we discuss various chemical pretreatment processes, feasibility of the processes at industrial scale in terms of the mechanisms involved, advantages, disadvantages and economic assessment. It is not possible to define the best pretreatment method as it depends on many factors such as type of lignocellulosic biomass, process parameters, environmental impact, economical feasibility, etc. However, some of these chemical pretreatments have disadvantages such as formation of inhibitory compounds especially furfural and 5-hydroxyl methyl furfural (HMF).

Importance of chemical pretreatment for bioconversion of lignocellulosic biomass

Bioconversion of lignocellulosic biomass to ethanol is significantly hindered by the structural and chemical complexity of biomass, which makes these materials a challenge to be used as feedstocks for cellulosic ethanol production. Cellulose and hemicellulose, when hydrolyzed into their component sugars, can be converted into ethanol through well established fermentation technologies. However, sugars necessary for fermentation are trapped inside the crosslinking structure of the lignocellulose. Hence, pretreatment of biomass is always necessary to remove and/or modify the surrounding matrix of lignin and hemicellulose prior to the enzymatic hydrolysis of the polysaccharides (cellulose and hemicellulose) in the biomass. Pretreatment refers to a process that converts lignocellulosic biomass from its nativ form, in which it is recalcitrant to cellulase enzyme systems, into a form for which cellulose hydrolysis is much more effective. In general, pretreatment methods can be classified into three categories, including physical, chemical, and biological pretreatment. The subject of this paper emphasizes the biomass pretreatment in preparation for enzymatic hydrolysis and microbial fermentation for cellulosic ethanol production. It primarily covers the impact of biomass structural and compositional features on the pretreatment, the characteristics of different pretreatment methods, the pretreatment study status, challenges, and future research targets. Keywords: lignocellulosic biomass, pretreatment, cellulosic ethanol, enzymatic hydrolysis, fermentation, cellulose, hemicellulose, lignin DOI: 10.3965/j.issn.1934-6344.2009.03.051-068 Citation: Yi Zheng, Zhongli Pan, Ruihong Zhang. Overview of biomass pretreatment for cellulosic ethanol production. Int J Agric & Biol Eng, 2009; 2(3): 51

https://www.ijabe.org/index.php/ijabe/article/viewFile/168/83

Overview of biomass pretreatment for cellulosic ethanol production.

A number of pre-treatment methods have been developed during the last few decades to overcome bio- mass recalcitrance. Although the mechanisms of these pre-treatments are different, the fi nal objective is the same - increasing cellulose accessibility to cellulase enzymes. Generally, pre-treatment is the process to disrupt the compact structure of lignocellulosic biomass and expose cellulose fi bers, which can be achieved by mechanical comminu- tion, chemical modifi cations of biomass compositions, biological degradation, or a combination of these methods. After pre-treatment, the accessible surface area is increased resulting in the enhancement of cellulose digestibility. In this paper, we review the recent progress of the fundamental researches of various biomass pre-treatment proc- esses, especially on how these pre-treatments alter chemical composition and physically change cell wall struc- ture. Understanding these changes would be helpful to further optimize existing pre-treatments, and would aid in developing novel pre-treatment methods.© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd

Biomass recalcitrance. Part II: Fundamentals of different pre-treatments to increase the enzymatic digestibility of lignocellulose

Comparative study on chemical pretreatment methods for improving enzymatic digestibility of crofton weed stem.

Siam weed [Chromolaena odorata (L.) King & Robinson], an invasive exotic weed in China, was proposed as a feedstock for bioethanol production. This would be a promising way of using for an invasive weed that needs management and control. It was found that the glucan content of the weed stem was similar to that of sugarcane bagasse, but higher than those of corn stover and wheat straw. Several chemical pretreatment methods were applied to the weed stem to increase its enzymatic digestibility. Mild sulfuric acid (<120 degrees C) or alkali pretreatment did not markedly increase the enzymatic digestibility. However, peracetic acid (PAA) pretreatment dramatically enhanced the enzymatic hydrolysis of the weed stem. Compared to some other common agricultural residues, the weed stem was more difficult to pretreat and digest by cellulase. Fourier transform infrared (FTIR) spectra analysis indicated that the cellulose-related bands became more intensive after pretreatment, especially for PAA-pretreated samples. According to X-ray diffraction spectra, the biomass solids had higher crystallinity indices after pretreatment, although these indices were similar for all of the pretreated samples.

Lihua Zhang

Papers

Biomass recalcitrance. Part I: the chemical compositions and physical structures affecting the enzymatic hydrolysis of lignocellulose

Biomass recalcitrance. Part II: Fundamentals of different pre-treatments to increase the enzymatic digestibility of lignocellulose

Comparative study on chemical pretreatment methods for improving enzymatic digestibility of crofton weed stem.

Pretreatment of Siam weed stem by several chemical methods for increasing the enzymatic digestibility.