Preprocessing of biomass is an important step in preparing and supplying biomass to a bio­refinery. Literature indicates that the particles used for hydrolysis and subsequent fermenta­tion should be in the range of 2 mm. For pulping applications size ranges should not be less than 20 mm and not more than 40 mm. For pyrolysis, particle size influences the speed of the pyrolysis. For fast pyrolysis where bio-oil are produced, the smaller the particle is the more efficient the process becomes because of high rate of heat transfer. For slow pyrolysis like gasification and charcoal making, the size of particle can be as big as 50 mm where the process of heat treatment is very slow. For pelletization of biomass to small diameter pellets, a particle size in the range of 1-2 mm is preferred. For larger cube sizes (10-30 mm) the size of particles can increase.

Loose-cut biomass has a low bulk density ranging from 50 to 120kg/m( depending on the particle size (Table 7.3). In case of chopped and ground biomass, the bulk density can be increased substantially (~25%) by vibrating the biomass holder (e. g., truck box, con­tainer). To increase density, the biomass must be mechanically compacted (Sokhansanj et al. 1999( . When biomass is densified to briquettes, cubes, or pellets, densities in the range of 300-700kg/m3 can be obtained.

Pellets are usually in the form of a hardened biomass cylinder, 4.8-19.1 mm in diameter, with a length of 12.7-25.4 mm. Pellets are made by extruding ground biomass through round or square cross — sectional dies. The unit density of pellets (density of a single pellet) is 960-1120kg/m3. Bulk density of pellets may be as high as 750kg/m-. Cubes have a lower density than pellets. Typical bulk density of cubes range from 450 to 550kg/m- depending upon the size of cubes.