The first and most important step in any conversion process is to reduce the physical size of LB. In order to obtain the high yields required for commercial success in bioconversion operations, it is vital to pretreat and reduce the biomass into an effective size (Mosier et al.,

2005) . Reducing the LB size from a log to wood chips to even fine powders improves mass and heat transfer as well as increases the surface area of the particle. Increasing the surface area exposes a higher percentage of the glycosidic or ester bonds to the agents in solution

FIGURE 27.4 Schematic flow sheet for biomass conversion to bioproducts. Source: US DOE, 2006. (For color version of this figure, the reader is referred to the online version of this book.)

(Mosier et al., 2005). Catalysts, such as a proton or an enzyme, can only access active chemical bonds when exposed at the solid—liquid interface (Liu, 2003; Yang and Liu, 2005). Smaller particles translate into faster, more uniform reactions and a more complete conversion.

The energy required to reduce the biomass into a treatable size depends on the density of the biomass source. Herbaceous materials do not require as much processing to achieve the needed particle size as it does to reduce wood (Cadoche and Lopez, 1989). Since LB reduction is much more energy intensive, it is imper­ative to adequately define the reduction process. This re­quires an understanding of the quality and condition of the source materials. Qualities such as moisture content, soil particles, foreign matter, and initial cut length will impact efficiency, energy requirements and downstream treatment conditions and requirements. Pretreatment is costly and greatly influences the cost and effectiveness of downstream operations. It affects fermentation toxicity, the rate of enzymatic hydrolysis, enzyme load, powder mix, product concentration, product purity, waste treatment requirements, energy requirements and a host of other process variables (Zhu and Pan, 2010). Thus, it is important to begin the design process with the end in mind. Much effort should be invested to design the whole process up front with specific source materials and conditions defined.