Fuel particle shape effects in the packed bed combustion of wood

Abstract

Experiments on the overfeed packed bed combustion and gasification of seven different shapes of parallelepipedal wood particles are presented. Attention is focussed on the part of the bed in which char conversion occurs; results for the pyrolysis zone at the top of the bed are not included. It is shown that fuel particle shape can affect conversion through the sphericity of the particle, through the orientation of the wood grain in the particle, and through the overlap of particles in the bed. These effects were incorporated into an existing numerical model of packed bed combustion and gasification. Particle sphericities for input to the model were determined directly from particle geometry, and particle overlap factors were estimated photographically. Comparison of predicted gas analyses and temperatures in the bed with experimental values then allowed the effect of the orientation of the original wood grain on char conversion to be estimated, with the conclusion that the rate of carbon conversion by the CO2 reduction reaction is faster by a factor of about 5 on surfaces normal to the wood fibres compared to the rate on surfaces parallel to the fibres. The carbon oxidation reaction at the bottom of the bed, on the other hand, is controlled by external gas phase diffusion and is not affected by the fibre orientation.