| The present study was initiated and designed to assess experimentally the feasibility of methane combustion emissions control with catalytic oxidation of methane at relative low temperatures to avoid NOx formation using inexpensive metal oxides as catalysts. The oxidation reactions of heated low velocity streams of homogeneous lean fuel-air mixtures were investigated at atmospheric pressure using a specially designed packed bed tubular reactor. The main fuel considered was methane, however, other common gaseous-fuels, e.g. propane, carbon monoxide, hydrogen and ethylene as well as their mixtures were also examined for comparative purposes. It was shown that binary cobalt oxide/chromium oxide catalysts can be effective in the oxidation of very lean fuel-air mixtures. Furthermore, there is an optimum catalyst composition, corresponding to a cobalt oxide/chromium oxide mass ratio that could produce a significant improvement to the low temperature oxidation of the lean mixtures examined, and the corresponding resulting emissions. Overall, the mixed oxides catalysts prepared in the laboratory exhibit somewhat higher ignition temperatures and lower activities than Pt catalyst, but are superior to noble catalysts for use at higher temperatures, i.e. above 823K, when sintering process of platinum commences.; In general, the increased reactivity of fuel mixtures when approaching stoichiometric proportions increases the quantity of fuel converted in homogeneous gas-phase reactions. Therefore at higher temperatures more reactive fuels exhibit improved conversion with increased equivalence ratio. However, at lower temperatures, where surface reactions are dominant, increasing equivalence ratio has a detrimental effect on the overall fuel conversion. The conversion of all fuels decreased with an increase in the mixture approach velocity. The severity of the decrease was influenced by the type of fuel, in particular its reactivity, and the reactor temperature.; The addition of other common gaseous fuels to methane-air mixtures, did not improve significantly the methane conversion characteristics, where there was a large disparity between methane and the fuel added in their reactivity toward oxidation. This was the case of methane/ethylene-air and methane/hydrogen-air mixtures. However, when the conversion temperatures of methane and its additive coincided, e.g. methane/carbon monoxide-air mixture and methane/propane-air mixture, methane reacted more readily than it did as a single fuel.; For all mixtures, a more reactive fuel when added to methane it was oxidized at much lower temperatures than those needed for pure methane-air mixtures. The heat generated could then increase the temperature of the catalytic bed and as a result decrease the amount of external heating required to initiate the catalytic oxidation of methane. |
