Bulk permeability characteristics in a biomass moving bed and their effects on reactor design and scaling

Moving or fixed bed reactors are commonly used for continuous processing of biomass in a wide range of applications, such as torrefaction, slow pyrolysis, gasification, and incineration. Many of these moving bed reactors for biomass applications behave like a quasi-fixed bed reactor because of longer solid residence times in comparison to fluid. In order to ensure effective heat and mass transfer through such beds, a good understanding of the bulk hydrodynamic characteristics in the bed is of critical importance. There lacks sufficient data in the literature documenting these characteristics with respect to different types of biomass of various particle sizes. In this paper, we describe both theoretical and experimental frameworks to determine and quantify the bulk bed hydrodynamic characteristics. We assumed—and verified—that the gas flow inside a biomass bed moving at a slow speed can be roughly approximated via Darcy’s law. The bulk porosities of different types of biomass are obtained and are observed to often diverge from predictions derived from common empirical correlations. The newfound data is then used for a specific application of designing a torrefaction reactor for scale-up, which showed that under certain conditions, the reactor can be powered entirely by the stack effect generated by a hot column of rising gas without any external pressure drive. It is shown that the developed framework can be useful in informing some fundamental scaling questions regarding biomass reactor operation and performance.