For particle size reduction, it is preferable to process biomass with low moisture content (after a drying pre-treatment) since its brittleness is increased and higher

shear forces are promoted. Taking that into consideration, the most common size reduction techniques are dry shredding and hammermilling. Dry shredding relies on rotating cutters: A geared roll is mounted with sharp designed metal cutters, which are regularly disposed on its surface. Larger wood pieces can this way be converted into wood chips. As smaller pieces will simply bypass the cutters, there is no need to separate the biomass feed before this step. Dry shredding can easily reduce biomass size down to wood chips-like particulate [10].

If a powder-like feedstock (<500 ц-m) is required for pyrolysis, further size re­duction can be achieved with hammermills [10]. The principle is to grind a material until it reaches a minimal particle size. It is designed to limit particle size by the use of perforated plate outlet whose holes size determines the final average particle diameter. In a small drum, solid metal hammers are mounted on a central shaft. The metal hammers are rotated and the biomass material comes under the action of cen­trifugal force: the biomass is crushed between the hammers and the drum wall. The drum wall has grooves oriented perpendicular to those of the hammers extremities to maximize shear forces. By gravity, the fine particles percolate at the bottom of the drum where the perforated plate controls their exit in the outlet duct.