Irmene Ortiz*, Rodolfo Quintero

Departamento de Procesos y Tecnologia, Universidad Autonoma Metropolitana — Cuajimalpa, Mexico D. F., Mexico

*Corresponding author email: irmene@correo. cua. uam. mx

OUTLINE

Trends in Pretreatments 62

Other Pretreatments 62

Pretreatment Modeling 65

Environmental and Economical Aspects 65

Concluding Remarks 66

References 66

LIGNOCELULLOSIC BIOMASS

Lignocellulosic biomass is composed primarily of cel­lulose, hemicelluloses (mainly xylan), lignin and smaller amounts of other compounds. Typically, the composi­tion of lignocellulosic biomass by weight is 40—50% cel­lulose, 20—40% hemicellulose, 10—30% lignin and other components such as minerals, oils, soluble sugars, pec­tins, proteins, and ashes (Jorgensen et al., 2007; Lewis et al., 2005; Wyman et al., 2005).

Cellulose, hemicelluloses and lignin are present in varying amounts in the different parts of the plant and they are intimately associated to form the structural framework of the plant cell wall; also, the content of the different sugars of the hemicelluloses varies signifi­cantly between different plants depending on plant spe­cies, age and growth conditions (Jorgensen et al., 2007).

Cellulose is the most abundant constituent of the plant cell wall; its linear structure enables the formation of both intra — and intermolecular hydrogen bonds
resulting in the aggregation of chains into elementary crystalline fibrils of 36 cellulose chains, while hemicellu — loses are complex branched heterogeneous polysaccha­rides composed of monomeric residues: D-glucose, D-galactose, D-mannose, D-xylose, L-arabinose, D-glucur — onic acid and 4-O-methyl-D-glucuronic acid; and lignin is a complex amorphous network formed by polymeri­zation of phenyl propane units and constitutes the most abundant nonpolysaccharide fraction in lignocel — lulose (Jorgensen et al., 2007; Lewis et al., 2005).

Biofuels produced from native lignocellulose are known as second-generation biofuels. In this process the cellulose is converted into glucose, which is easily fermented to ethanol, while the hemicellulosic fraction is converted into monomeric sugars (mainly pentoses), a fermentation that is considerably harder to accomplish (Dias et al., 2011). The physicochemical and structural compositions of native lignocellulose are, however, recalcitrant to direct enzymatic hydrolysis of cellulose (Mosier et al., 2005). Therefore, a pretreatment step is

Bioenergy Research: Advances and Applications http://dx. doi. org/10.1016/B978-0-444-59561-4.00004’8

invariably required to render the cellulose amenable to enzymatic hydrolysis (Zheng et al., 2009).

The total estimated availability of usable biomass in the world is about 2 billion dry tons per year (Lewis et al.,

2005) . Therefore, the enormous potential of second- generation fuels and the increasing interest toward devel­oping effective, low-cost and environmentally friendly pretreatments for breaking down the close association of the structures of the biomass.