At the time of harvest, an operation is performed in the field to presize the LB. Herbaceous biomass is pre­pared by shredding or forage cutting. Chipping is the
preferred method for reducing the size of wood. Chip­ping reduces wood to 10—50 mm in two dimensions and 5—15 mm in the third (Zhu and Pan, 2010). This is the minimum treatment necessary to begin conversion. However, additional reduction is often performed. For example, wood chips may subsequently be refined to fi­bers such as that in fiber production, pulverized into wood fibers or wood flour (Zhu and Pan, 2010). Pulver­ization requires much more energy than chipping (Zhu and Pan, 2010).

In addition to chipping and shredding, hammer mill­ing, knife milling, disk or attrition milling, and ball mill­ing are viable alternatives to reduce biomass sizes. Large-scale reduction operations have favored hammer and disk milling (Tienvieri et al., 1999). Chip refining is also an alternative as it can have a large throughput.

Hammer milling is primarily used for making wood flours for composites and pellets. Disk milling is used for wood fiber production at a commercial scale, around 1000 tons per day. Disk milling operations are depen­dent on environmental conditions and the quality of source materials. The energy requirement and the wood particle size and shape depend on these opera­tional parameters (Tienvieri et al., 1999).

Milling operations have a significant impact on downstream energy requirements and the efficiency of enzymatic cellulose saccharification. Since the goal of a biorefinery is to optimize the conversion process, to reduce energy requirement and maximize the enzymatic cellulose saccharification, it is important to attend to the biomass size reduction portion of the process. Failure at this stage amplifies the cost of energy requirements and reduces the effectiveness of subsequent treatments.

Since these mechanical processes can produce a range of particle sizes it is often necessary to control the

particle size used in the biorefinery. Size characterization is accomplished using sieves, screens and imaging analysis. The particle surface area is the most relevant determination of effectiveness, and thus, it is the quality to be controlled. Specific surface area correlates to energy consumption and the efficiencies of a variety of size reduction processes have been compared (Holtzapple et al., 1989).

There is a limit to the effectiveness of size reduction. At this point, additional surface area increases, or parti­cle size reductions will not improve substrate enzymatic digestibility. This critical size is proportional to the pore size in and along the wood cells. Refer to Figures 27.1 and 27.2. A common target size is one that maintains the cell structure while allowing for lignin removal from the middle lamellae.

Size reduction below the cell size will provide a more efficient conversion. To reduce particle size to this smaller level is done by comminution (Vidal et al.,

2011) . Comminution of biomass, especially at the final sizing stage, is energy intensive and the product is of low value. Thus, there is much interest in finding the most efficient milling processes.

To that end, ball milling has been extensively studied. It has been shown to deliver excellent results in terms of the hydrolysis rate and sugar yield. Additionally, this pretreatment method is clean and easy to do. Vibratory ball milling has been shown to be more effective at breaking down the crystallinity of cellulose and improving the digestibility of the biomass over ball mill­ing alone (Millet et al., 1976). Mechanical milling requires long operation times and a large amount of energy (Lynd et al., 1996). The smaller the desired particle size the greater the comminution requirements will be in terms of time and energy (Cadoche and Lopez, 1989).