Multi-physics modelling of packed bed biomass combustion

•A transient 3D model was developed to simulate fuel bed and freeboard of biomass grate furnaces.•The intra-particle gradients, the bed shrinkage and variations of the bed porosity are considered.•Detailed kinetic mechanisms are used for the homogeneous gas phase reactions.•The results are in a good agreement with experimental data.

A transient 3D model for two main zones, namely the fuel bed and the freeboard, of biomass packed bed combustion systems was developed. It integrates the models for the biomass conversion sub-processes and solves the governing equations for the gas and solid phase and their interactions. The intra-particle gradients are included by considering the biomass particles as thermally thick particles. The shrinkage of the packed bed and the variations of the bed porosity due to the uneven consumption of the fuel are taken into account. Detailed kinetic mechanisms are used for the simulation of homogeneous gas phase reactions. To verify the model and to increase the understanding of packed bed combustion, laboratory-scale fixed-bed batch experiments have been performed in a reactor with 9.5 cm diameter and 10 cm length. The model performance was extensively validated with gas phase measurements (CO, CO2, CH4, H2, H2O and O2) above the fuel bed, temperatures at different heights in the bed and in the freeboard, and the propagation rate of reaction front. The simulation results are in a good agreement with the measured values.