Dewatering and drying should be considered, since heating the biomass may only vaporize the water while delaying the biomass pyrolysis reactions and increasing the operation costs. Also, studies suggest that steam explosion of biomass could influence the quality of pyrolysis products [55].

Particle size reduction is also important to minimize heat transfer limitations and internal temperature gradients in biomass particles. The Biot number criterion of

0. 1 suggests that the maximum wood chips size that should be fed to a BFB to avoid heat transfer limitations (and product yields issues) is ~2 cm (by estimation with typical wood properties). This calculation assumes that the wood chips have a thickness five times lower than their diameter (parallelepiped shape). Particle size reduction is therefore recommended for biomass particles that are larger than that value. Heterogeneous feedstocks such as MSW would not be recommended with this technology since pyrolysis operation temperatures do not promote slagging, in opposition to gasification. Undesired particulate would then need to be removed from the bed, which is a difficult operation in BFBs, considering the wide particle size and densities distribution of the inorganics.