Modelling of particle pyrolysis in a packed bed combustor

Abstract

The combustion process of a dry wood particle within a packed bed combustor starts with the heat up and devolatilization of the wood to form char which then travels down through the reduction and oxidation zones to complete the combustion process. This thesis describes the development of a computational model for overfeed packed bed combustion of wood and other biomass fuels, including the modelling and tracking of individual particles within the devolatilization region of the bed. It builds on an earlier continuum bed model, with no description of processes within individual particles. A numerical model for the heat-up and pyrolysis of an individual particle is developed and numerically tested to determine the preferred grid sizing, time step (Deltat) and node number. The impact of particle density, size, the heat of pyrolysis and kinetics on devolatilization are studied. The particle model is then incorporated into the overall bed model by developing a mechanism for particle tracking in the devolatilization region of the bed, coupled with interpolation processes to transfer information between the bed and the particles. The resultant model shows that the model not only produces more detail about particle processes in the combustor but also produces profiles that compare well to the Girgis (2004) experimental data.