Lignocellulosic materials can be comminuted by a combination of chipping, grinding, and milling to reduce cellulose crystallinity. This reduction allows the cellulases to access the biomass surface in an easier way increasing the conversion of cellulose into glucose. The energy requirements of mechanical comminution of agricultural materials depend on the final particle size and waste biomass charac­teristics (Sanchez and Cardona, 2008). In some specific cases, it has been demon­strated that wet or dry crushing used as a sole pretreatment method provokes the conversion of biomass into glucose in the following process steps of 56 to 60% for rice straw. For cane bagasse, a 25% conversion using wet crushing and 49.2% conversion for dry crushing have been observed (Rivers and Emert, 1988). In these cases, particle size plays a crucial role in the efficiency of the method. An important amount of data has been presented on the yields during the hydrolysis of timothy grass and alfalfa for different fractions of milled material with dif­ferent particle sizes (Alvo and Belkacemi, 1997). Thus, 56.4% yield for timothy grass (53 to 106 pm) and 62.5% for alfalfa (53 to 106 pm) during 24 h of pretreat­ment using roller mills have been achieved. In contrast, the hydrolysis yields for nonmilled materials were 51.4% for timothy grass and 38% for alfalfa.

The energy requirements of the mechanical comminution of agricultural mate­rials depend on the final particle size and biomass properties (Sun and Cheng,

2002) and are usually very elevated. Cadoche and Lopez (1989) reported that power supply for mechanical comminution in a plant processing different ligno — cellulosic residues should be maintained below 30 kW/ton wastes by ensuring a final particle size in the range of 3 to 6 mm. On the other hand, it was demon­strated that milling and sieving of such residues as wheat straw could lead to an increased efficiency of the pretreatment using dilute acid due to the removal of noncarbohydrate biomass components before the physical-chemical pretreatment (Papatheofanous et al., 1998). Milling with vibratory balls is an effective method of transferring the energy input into size reduction and altering the crystalline cellulose structure. Although mechanical pretreatment methods increase cellu­lose reactivity toward enzymatic hydrolysis, they are unattractive due to their high energy and capital costs (Ghosh and Ghose, 2003).

Besides comminution, pyrolysis has been tested as a physical method for pre­treatment of lignocellulosic biomass since cellulose rapidly decomposes when treated at high temperature. In particular, it has been reported that this method can be improved in the presence of oxygen. When zinc chloride or calcium car­bonate is added as a catalyst, the decomposition of pure cellulose can occur at a lower temperature (Sun and Cheng, 2002). For instance, the pyrolysis of 100 g of waste cotton produces 80 g of a highly viscous pyrolyzate with a high content of levoglucosan (43% by weight), an intramolecular glucoside whose hydrolysis using dilute acid forms significant concentrations of glucose (Yu and Zhang,

2003) . Main physical methods employed for pretreatment of lignocellulosic mate­rials are presented in Table 4.2.