CO, H2 or C3H8 assisted catalytic combustion of methane over supported LaMnO3 monoliths

The effect of the addition of a second fuel such as CO, C3H8 or H-2 on the catalytic combustion of methane was investigated over ceramic monoliths coated with LaMnO3/La-gammaAl(2)O(3) catalyst. Results of autothermal ignition of different binary fuel mixtures characterised by the same overall heating value show that the presence of a more reactive compound reduces the minimum pre-heating temperature necessary to burn methane. The effect is more pronounced for the addition of CO and very similar for C3H8 and H-2. Order of reactivity of the different fuels established in isothermal activity measurements was: CO > H-2 greater than or equal to C3H8 > CH4. Under autothermal conditions, nearly complete methane conversion is obtained with catalyst temperatures around 800 degreesC mainly through heterogeneous reactions, with about 60-70 ppm of unburned CH4 when pure methane or CO/CH4 mixtures are used. For H-2/CH4 and C3H8/CH4 mixtures, emissions of unburned methane are lower, probably due to the proceeding of CH4 homogeneous oxidation promoted by H and OH radicals generated by propane and hydrogen pyrolysis at such relatively high temperatures. Finally, a steady state multiplicity is found by decreasing the pre-heating temperature from the ignited state. This occurrence can be successfully employed to pilot the catalytic ignition of methane at temperatures close to compressor discharge or easily achieved in regenerative burners. (C) 2003 Elsevier Science B.V. All rights reserved.