A Model for the Transient Behavior of Catalytic Combustors

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Abstract
-A transient tridimensional model has been developed which accounts for both the heterogeneous and homogeneous reactions occurring in the combustor, as well as for the axial heat conduction in the solid and radial heat losses. The heterogeneous global kinetics used here has been obtained earlier, experimentally, for the catalytic combustion of propane on a mixed transition metal oxide catalyst (3.38 percent CraOa, 1.13 percent CoaOj by weight deposited on alumina washcoated monolith). At short times, the transient behavior depends on the reactions occurring on the surface, on the heat capacity of the solid and on the advection of heat downstream by the flowing gas. The temperature of the solid near the entrance exceeds that of the gas, the latter being heated by the heat convected from the solid. At locations further downstream, the fuel concentration is low and the solid temperature falls below that of the gas, the former being heated by the latter. As the combustor heats up, the peaks in the solid and gas temperatures propagate in the flow direction and the exit fuel concentration continuously decreases. Once the bed temperature becomes high enough, homogeneous reactions are induced causing the conversion to go to completion and the gas temperature in the bed rises sharply. The large temperature gradients thus produced in the zone of homogeneous reactions cause significant heat transfer through the substrate upstream. This results in a combustion front that propagates in the upstream direction. The computations show that the combustor length required to achieve complete conversion under steady state conditions may be inadequate to attain complete conversion during the start-up from cold conditions. In addition, the time required to reach steady state is computed for a variety of conditions.

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