A large number of pretreatment approaches have been investigated across a variety of biomass feedstock types. Published studies are widely available and there are several re­view articles available that provide a general overview of the field (10-14). Unfortunately, standard experimental and analytical methodologies have not been utilized across much of the published pretreatment literature, making it difficult to conduct comparative eval­uations based on published findings. Recently, several pretreatment research teams across North America have undertaken the first broad-ranging coordinated effort to develop com­parative process performance and economic evaluation data for several leading pretreatment options. While this collaboration, known as the Biomass Refining Consortium for Applied

Fundamentals and Innovation (CAFI), does not fully encompass all possible pretreatment technologies or potential biomass feedstocks, it does serve as a model for how comparative data can be developed and made available for various stakeholders and potential commer­cializes of biomass conversion technologies. A series of papers that cover the comparative findings from a recently completed CAFI project on a common corn stover feedstock have been published (15-22).

14.5.1 Physical pretreatments

14.5.1.1 Comminution

Most pretreatment approaches require that collected biomass undergo some degree of me­chanical size reduction prior to introduction into a pretreatment reactor. Woody biomass can be chipped in a manner similar to that commonly practiced in the pulp and paper industry. Depending on the pretreatment process and associated heat and mass transfer considerations, woody biomass is commonly comminuted to particle sizes smaller than typical wood pulping chips, as pretreatment processes are often practiced at much shorter residence times than wood pulping processes. Other biomass feedstocks, such as agricul­tural residues and herbaceous energy crops, can be coarsely chopped during or after the feedstock harvesting operation. Again, further comminution may be employed on these feedstock types as well, depending on the pretreatment process and associated heat and mass transfer considerations.

Intensive comminution of various biomass types has been practiced as an actual pretreat­ment process, without any further pretreatment prior to enzymatic hydrolysis. Methods include various types of ball milling (dry, wet, and vibratory processes), other types of attrition milling, compression milling, and wet or dry disk refining (23-27). While these methods can increase the enzymatic digestibility of the comminuted biomass by increasing the available surface area and by decrystallizing cellulose, most studies have concluded that the high mechanical power requirements cause comminution to be cost-prohibitive for use as a stand-alone pretreatment in a biomass to ethanol conversion process.

14.5.1.2 Irradiation

The use of high-energy electron beam and microwave energy sources as a biomass pretreat­ment approach has been investigated. These methods are believed to mechanically disrupt plant cell wall structure and decrease the crystallinity of cellulose, resulting in an increased enzymatic digestibility of cellulose. Issues with cost, energy intensity, and the practical­ity of applying such approaches in commercial processes have limited the development of irradiation as a viable pretreatment approach (12, 28).