Physical Pretreatment of Woody Biomass
In the physical pretreatment step, woody biomass is distinctly different from other forms of biomass such as leaf, grass or municipal waste type of non-woody biomass. This is due to major differences between woody and non-woody biomass in their physical properties and chemical compositions. Woody biomass is denser and physically larger and structurally stronger than non-woody biomass. Chemically, woody biomass has higher lignin content than agricultural biomass wastes and grasses and, as a result, woody biomass is more recalcitrant to enzymatic hydrolysis. Consequently, more energy is required to overcome the recalcitrance of woody biomass through pretreatment before enzymatic saccharification, and high energy demand in the pretreatment of woody biomass for cellulosic ethanol production is mentioned in a few review articles [7]. Even though appreciably size-reduced materials were used in all chemical pretreatment experiments using woody biomass materials, and were a critical step in the pretreatment, there are only a small number of literature reports on information and energy evaluations on this step. Indeed, the physical pretreatment step involving size reduction was even not included in process cost analysis in some key literature examples [15, 21, 22]. This is probably because the majority of research on bioethanol has been carried out on non-wood soft biomass forms and energy consumption in the size reduction is not very important in grasses, corn stover, and rice straw type of materials. The energy requirement for mechanical comminution of hardwood materials is very high compared to soft biomass raw materials, as illustrated in Table 5.1.