Effect of Particle Shape on the Reverse Combustion Process of a Packed Bed

The effect of fuel particle shape on the combustion front propagation rate of the reverse combustion in a packed bed was studied experimentally. We used shredded rectangular papers having different aspect ratio and size as model particles. Our discussion focuses on the combustion in which the surface reaction is dominant. Experimental results showed that the combustion front propagation rates under constant gas velocity depended on the particle shape, but had no correlation with geometric parameters such as length, area and aspect ratio. We presumed that the difference in propagation rate depending on the particle shape was attributable to the difference in effective surface area in the surface reaction. Then, we estimated the effective specific surface area through the permeability method. It was found that the combustion front propagation rates at consrtant gas velocity increased with the effective specific surface area. This fact implies that the effective specific surface are are presents the difference in surface reaction rates dominating the combustion propagation rate under our experimental conditions. Furthermore, we estimated the mass transfer coefficient by means of analogy of j-factor. In the estimation, we defined the effective equivalent diameter calculated from the effective specific surface area as the representative length of the fuel particle. As a result, it was found that the propagation rate correlated with the mass transfer coefficient estimated by the effective equivalent diameter, independently of the particle size, shape and gas velocity. This fact shows that the effect of the particle shape on the combustion front propagation rate in a reverse combustion process of a packed bed can be evaluated by the permeability method.