Physical pretreatment processes employ the combination of mechanical and irradiation processes to change only the physical characteristics of biomass. Physical pretreat­ment is usually carried out before a following processing step which is often needed

Effect of Pretreatment

Cellulose

Ilcmicellulose

Fig. 20.2 Schematic representation on biomass pre-treatment (Mosier et al. 2004) to reduce the particle size, make material handling easier, reduce volume, and increase surface area. This can be done by a combination of chipping, grinding, or milling depending on the final particle size of the lignocellulose biomass (10-30 mm after chipping and 0.2-2 mm after milling or grinding) (Sun and Cheng 2002). Different milling processes (ball milling, two-roll milling, hammer milling, colloid milling, and vibro energy milling) can be employed to improve the enzymatic hydrolysis of ligno- celullosic biomass (Taherzadeh and Karimi 2008). Mechanical pretreatment factors such as operating costs and depreciation of equipment are very important.