Catalytic oxidation of natural gas over supported platinum: Flow reactor experiments and detailed numberical modeling

Noble metal (platinum or palladiuni) combustion catalysts have demonstrated low NO₂ (nitrogen oxides, consisting of both NO and NO₂) emissions in natural-gas-fired turbines. The catalyst permits low temperature combustion below the traditional lean limit. Due to the combined proceses of diffusion and (unknown) surface reaction, the catalyst is typically modeled as a "black box," often described by a global reaction rate expression. While this approach has been useful for proof-of-concept studies, we expect practical applications to emerge from a greater understanding of the details of the catalytic combustion process. We have constructed a detailed numerical model of the catalytic combustion process based on the wellaccepted CHEMKIN chemical kinetics formalism for detailed gas-phase and surface chemistry. Results from an experimental combustor support the model development. We present measured and modeled axial profiles of fuel conversion for natural-gas combustion over platinum catalysts supported on ceramic honeycomb monoliths, NO emissions are below 1 ppm, and CO is observed at ppm levels. The data are taken at several lean equivalence ratios and flow rates, at atmospheric pressure. Fuedl conveersion rates occur in tworegines: a low, constant conversion rate and a higher conversion rate that inereases linearly with equivalence ratio. Both conversion rates are consistent with kinetically limited processes. The jump from kinetic to mass-diffusion limitation, predicted by most accepted theories of catalytic combustion, is not observed. The agreement of the numerical model with the measured data is good at temperatures below 900 K: above this temperature. the predicted fuel conversion is as much as a factor of 2 lower than the measurements. Carbon monoxide is overpredicted by 2-3 ppm for <0.34. and by less than a factor of 2 for >0.34. Results from the numerical model indicate that fuel conversion rate has a linear dependence on the fraction of available surface reaction sites.