Physical pre-treatments are the key to control feedstock properties, which sig­nificantly influence gas/solid hydrodynamics as well as heat/mass transfer in the pyrolysis reactor. Recommended feedstock pre-treatments depend on the initial biomass characteristics, the pyrolysis conditions as well as the reactor type. Pre­treatments also allow the homogenization of the feedstock characteristics with time.

Intrinsic feedstock properties such as specific heat, thermal conductivity and density (dry and true) cannot be easily modified and constitute limitations for ther­mal processes. On the other hand, feedstock moisture and particle size are the main physical parameters that can be adjusted to optimize the pyrolysis process performance.

Particle fluidizability has been correlated to its average size and density. Geldart classified particles into four groups (Geldart classes A, B, C and D) based on their fluidization behaviour at ambient conditions [27]. Figure 11.1 illustrates the Geldart classification of powders and indicates the properties of common feedstocks for biorefineries.

In the Geldart classification, class C (cohesiveparticles) and D (large particles) can be detrimental to gas/solid mixing as well as heat/mass transfer. For example, these particles cannot be easily and uniformly fluidized in a reactor: class C particles lead to channelling [28]. Furthermore, large particles (class D) are more subject to internal temperature gradients and species diffusion effects, which affect the final pyrolysis product distribution and composition. Species intra-particle diffusion increase the species exposure time to the pyrolysis conditions (additional time for reactions) while temperature gradients lead to uncertainties related to the characterization of the pyrolysis conditions.

Based on typical wood densities (various species) and Fig. 11.1 sawdust particles (50-500 ^m) are classified as Geldart A. On the other hand, coarser bark or wood residue particles are classified as Geldart B or Geldart D.