Some feedstocks are characterized by low densities (such as bark and wood residues) and increasing their particle size may be necessary for some reactor technologies. In fluidized bed reactors, for example, particles with low terminal velocities may be rapidly entrained outside of the reaction zone resulting only in partial conversion. If the pyrolysis reactor technology requires larger particles, particle agglomeration techniques can be used. Pelletization, also referred to as densification, is a well — established process, and it is currently used to transform many MSW into denser particulate RDF (Refuse Derived Fuel) feedstocks to be directly used as fuels [10]. These same processes can also be used for biomass pre-treatment for biorefineries.

The most widely used equipment to produce pellets is the extruder [29]. It con­sists of one or two (partially overlapped) cylindrical ducts in which screws force
deformable solids to flow with very high shear forces. At the end of the extruder, a die controls the pellet average size and a binding agent can be introduced with the solids in order to consolidate the agglomerates. Depending on the objective of the extrusion process, the design can consider multiple outlets to remove water.

Since pulp and paper industry wastes contain significant quantities of water, drying can become very expensive. In this instance, some compacting technologies can mechanically remove water from biomass while forming pellets or briquettes. Some extruders and other hydraulic or pneumatic presses perform compaction as well as removal of liquid water. By reaching high pressures using extrusion, Edwards [30] was able to compact a mix of bark and wood residue and lower its moisture content from 56.5 wt% down to 34.8 wt%.