CFD simulation of biomass grate furnaces with a comprehensive 3 D packed bed model

A 3D CFD model for biomass packed bed combustion has been developed at BIOENERGY 2020+ in co-operation with BIOS BIOENERGIESYSTEME and KWB in a previous work [1]. It consists of an Euler-Granular model for hydrodynamics of gas-particle multiphase flow and a thermally thin particle model for combustion of biomass particles. In this paper, this model has been improved by the implementation of a layer model for thermally thick particles. The new packed bed model provides the advantages of considering the intra-particle species and temperature gradients and, accordingly, allows for parallel progress of the thermal conversion sub-processes. Moreover, the layer model considers a more realistic shape for biomass particles, e.g. cylinders. Enhanced models for pyrolysis and char oxidation are applied and char gasification reactions are included. Additionally, the products of char oxidation are CO and CO2, whereas the ratio between these species changes depending on the particle temperature. The simulation of a small-scale underfeed stoker furnace has been successfully performed by the application of the new packed bed combustion model. The positions of the drying, pyrolysis and char burnout zones in the fuel bed as well as the temperature distribution among the particles seem to be plausible and could be confirmed by observations. Furthermore, a good qualitative agreement concerning the flue gas temperatures measured by thermocouples at different positions in the combustion chamber and CO emissions measured at boiler outlet could be achieved.

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