Combustion of methane in catalytic honeycomb monolith burners

The heat transfer efficiency, stability, and pollutant emissions characteristics of ultra-lean methane–air combustion in some precious metal-based honeycomb monoliths were investigated. The interpretation of the experimental results was assisted using numerical modelling of the gas-phase combustion process. The thermal radiation output of the monoliths varied between 27 and 30 per cent of the thermal input, and this compared favourably with equivalent porous inert media burners. The minimum fuel concentrations for very-low emission stable combustion were found to be significantly lower than for conventional gas-phase combustion and were shown to vary with the nature and loading of the catalyst, as well as with flow rates. The palladium catalyst was found to have a larger window of mixture strengths and flow rates for stable operation than the platinum one. During all the runs under stable combustion conditions, only extremely small amounts of CO, NOx and unburnt hydrocarbons were detected. Thus, the operating conditions verified ‘near-zero’ pollutant emissions that only a catalytic combustion process can achieve at present. Temperature profiles inside the monoliths channels proved that the catalyst's role was not only to enable the ignition of fuel mixtures below flammability limits, but also to ensure the complete oxidation of the fuel to CO2 via surface reactions in the steady state. The reaction zone inside the catalysts was found to end at about 10 mm from the monolith's entrance. The effect of monolith length was investigated and a reduction of 70 per cent in the original length was found possible. Copyright © 2000 John Wiley & Sons, Ltd.