Biochemical Conversion

Biochemical conversion of switchgrass straw, and other lignocellulosic biomass, to biofuel typically has the following three major steps: pretreatment, saccharification, and fuel synthesis. Biochemical conversion is also known as direct microbial conversion and biological conversion. First, the harvested and chopped biomass is pretreated to breakdown its microstructure and improve accessibility of the polysaccharides. Conventional pretreatments include a combination of heat, pressure, acid, and/or base treatment (Agbor et al. 2011). Use and recycling of ionic liquids is an example of a new and highly effective pretreatment (Li et al. 2010). In the second step, enzymes are added to the neutralized slurry to breakdown the cellulose and other polysaccharides into monosaccharides, a process known as saccharification. Finally, in the fuel synthesis step, microbial metabolism, typically fermentation, is enlisted to convert the sugars into fuel. The prototype fuel is ethanol, though recent research has demonstrated synthesis from sugars of higher energy-content fuels, including butanol, alkanes, and fatty acid esters, also known as biodiesel (Peralta-Yahya et al. 2010). In this chapter, we will provide essential information about each of these steps with a focus on how they relate in particular to the use of switchgrass as a bioenergy crop.

While higher and more consistent biomass yields for switchgrass and other bioenergy crops are essential, currently the cost and inefficiency of saccharification represent the greatest barriers to wide-spread commercial realization of lignocellulosic biofuel production (Lynd et al. 2008). This chapter will review the major research efforts dedicated toward optimizing lignocellulose composition and the enzymes that degrade it toward improving sugar yields from grasses. Research on plants has focused on understanding the synthesis and regulation of plant lignocellulose toward developing plant biomass that results in the highest yields of monosaccharides per unit mass (Carpita 2012; Youngs et al. 2012). Of course, stature and plant health must be maintained in the quality-optimized genotypes. Research on enzymes that hydrolyze lignocellulose has delved into understanding their basic mechanisms and the biophysics of their interactions with biomass. Moreover, researchers continue to use advanced methodologies to identify and generate additional hydrolase diversity. These two fields intersect with the overexpression of lignocellulolytic enzymes by plants themselves. This and other approaches to consolidate the steps of biofuel production are thought of as being important ways to improve biochemical biofuel production efficiency (Lynd et al. 2008). We note that the production of co-products, i. e., valuable uses for biomass components that do not become fuel, is extremely important in the life cycle analysis of the economic and environmental feasibility of biofuel production (Farrell et al. 2006; Lynd et al. 2008), especially via biological conversion, but will not be covered here.