The combustion of methane on platinum—alumina fibre catalysts—II design and testing of a convective-diffusive type catalytic combustor.

Abstract

A convective—diffusive catalytic combustor has been used to study the total oxidation of methane over a platinum on alumina fibre catalyst. Temperature profiles across and through the catalyst pad have been measured as a function of methane feed rate, as has the radiant energy and the gas composition near the pad. Carbon dioxide and water were the only detectable products of reaction. The results have been compared with the predictions of a one dimensional—one phase model. Good agreement was obtained with experimental results, except near to the edge of the combustor: this is probably due to small inaccuracies in the thermal conductivity values used. The model has been used to predict the behaviour of a combustor as a function of variables in the system. The combustor performance is found to depend upon a balance between different effects. Heat generation depends on the rate of chemical reaction, and an increased feed rate of methane may (a) increase the rate by increasing the concentration of methane (b) decrease the rate by limiting the diffusion of air from the other side of the pad or (c) result in increased methane slippage by reducing the contact time in the pad. Varying packing density has a similar effect, with the added problem that radiation (heat recovery) is also affected by the density of fibres. It is shown that only ca. 30% of the fibre pad radiates significantly. As a result, although the thickness of the bed increases the internal bed temperature, it has little effect on heat recovery.