Propagation of glowing combustion front in a packed bed of activated carbon particles and the role of CO oxidation

The propagation of a glowing combustion front in a packed bed of activated carbon (AC) particles was investigated with particular attention to the role of gas phase oxidation of carbon monoxide (CO). The AC particles were loosely packed in a cylindrical quartz column. A N₂/O₂ mixture of varying O₂ concentration flowed through the bed from the bottom. Following ignition at the top of the bed, a glowing combustion front was formed and propagated downwards. Experiments were conducted at different oxygen mass fluxes with varying oxygen concentration and total flow rate. The bed mass loss rate, propagation velocity and temperature of the glowing front, were measured. A transient two-dimensional axisymmetric numerical model considering surface and gas phase reactions and transport properties were developed to describe the combustion phenomena. By comparing with the experiments, the validity of the proposed numerical model was confirmed. The results demonstrated that both the C-O₂ and C-CO₂ surface reactions contribute to carbon consumption and a constant mass loss rate is attained at a given oxygen flux. The gas phase oxidation of CO acted as a major heat source to sustain the propagation of combustion front through the bed. Suppressing CO oxidation would reduce the propagation velocity but may increase the combustion front temperature because of the prolonged residence time per unit of bed mass.