ABSTRACT
As a result of requirements of the Energy Independence and Security Act of 2007, by the year 2022, 36 billion gal of biofuels will need to be produced to meet liquid transportation fuel demand, with at least 21 billion gal of “advanced biofuels,” dened as renewable fuels derived from non-cornstarch sources achieving greater than 50% reduction in greenhouse gas (GHG) emission. In meeting this challenge, cellulosic biofuels will likely be a major contributor because of the resource potential of cellulosic feedstocks, which are estimated to be over 1 billion dry tons per year in the United States, sufcient to produce enough biofuels to replace 30% of current demand for transportation fuels (Perlack et al. 2005). The process of breaking down a complex polysaccharide carbohydrate (such as starch, cellulose, or hemicellulose) into monosaccharide components that can be fermented into biofuels is called saccharication. Saccharication of corn starch, alpha-linked glucose polymers, is relatively easy compared with breaking down the beta-linked glucose polymers that make up the structurally aligned and hydrogen-bonded cellulose polymers in cellulose microbrils. In addition, the biological decomposition of cellulosic biomass presents a formidable challenge because of the recalcitrance of cellulose microbrils embedded within the complex, heterogeneous structure of the plant cell wall composed of cellulose, hemicellulose, and lignan (Figure 4.1). This necessitates not only costly, energy-intensive, and environmentally detrimental biomass pretreatment steps (usually involving high temperatures, acids, and/or enzymes) to increase the accessibility and effectiveness of cellulase enzymes, but also high cellulase enzyme loadings (ratio of enzyme mass to biomass), currently approximately 100 times the loadings used for corn starch saccharication.
