Enzymatic Hydrolysis of Lignocellulosic Biomass

Abstract

Lignocellulosic biomasses are non-food energy resources, the worldwide terrestrial availability of which is estimated to be around 200 x 10 kg (220 billion ton) annually (Foust et al., 2008). A USDA and USDOE report estimates that the United States has the potential of producing 1.3 billion dry tons of biomass annually after meeting food, feed and fiber demands, and exports (USDA and USDOE joint report, 2005), which could theoretically substitute more than 30% of the nation’s petroleum consumption. Thus, biomass may play an important role in the domestic bio-based economy through production of a variety of biofuel and biomolecules. Major lignocellulosic biomass sources include forest and woody products along with agricultural residues, agricultural processing byproducts, and energy crops. Broadly, there are two major pathways for producing biofuels from lignocellulosic feedstocks: biochemical conversion and thermochemical conversion. The biological route to obtain ethanol from lignocellulosic biomass is based on microbial fermentation of sugars derived from saccharification of cellulose biomass. A simplified process overview of ethanol production from lignocellulosic biomass via the biochemical route is shown in Figure 10.1. As cellulose is protected by lignin and intertwining hemicellulose and pectin, it is not easily accessible to the enzymes for saccharification, thus necessitating pretreatment. Pretreatment is the first operation in lignocellulosic ethanol production, which essentially prepares biomass to enzyme hydrolysis. Various forms of pretreatment are: physical, e.g. mechanical comminution, extrusion; physical-chemical, e.g. steam explosion, ammonia fiber explosion, CO2 explosion; chemical, e.g. ozonolysis, high temperature acid treatment, alkali hydrolysis, organosolv; and biological pretreatment, e.g. fungal treatments of biomass. They are well reviewed by several researchers (See Chapter 9 on Biomass Pretreatment). Depending on the type of pretreatment, the following can be accomplished: lignin breakup and crystallinity reduction in cellulose (mechanical), hemicellulose removal (acid treatment), delignification (alkaline, oxidative delignification, biological), breakup of internal lignin and hemicelluloses bonds (organosolv), size reduction (comminution), or the generation of high shear, high pressure (extrusion) conditions. In all cases, pretreatment prepares the biomass for