Catalytic combustion of natural gas over supported platinum: Flow reactor experiments and detailed numerical modeling

The use of a noble-met al combustion catalyst such as platinum or palladium in a natural-gas fired turbine can lower NO_x, (nitrogen oxides, consisting of both NO and NO₂) emissions for two reasons. First, most of the combustion occurs on the catalyst surface; surface production of NO_x, is low or nonexistent. Second, the catalyst permits low temperature combustion below the traditional lean limit, thus inhibiting NO_x, formation routes in the gas phase. Due to the complexity of the catalytic combustion process, the catalyst has traditionally been modeled as a "black box" that produces a desired amount of fuel conversion. 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 numerical model of catalytic aorabustion based on the well-accepted CHEMKDI chemical kinetics formalism for gas-phase and surface chemistry. To support the model development, we built a research combustor. We present mea:sured and modeled axial profiles of temperature, fuel conversion, and pollutant emissions for natural-gas combtistion over platinum catalysts supported on ceramic honeycomb monoliths. NO_x, emissions are below 1 ppm, and CO is observed at ppm levels. The data are taken at several lean equivalence ratios and flow rates. Fuel conversion rates occur in two regimes: a low, constant conversion rate and a higher conversion rate that increasei linearly with equivalence ratio. The agreement of the numerical model with the measured data is good at temperatures below 900 K; above this temperature, fuel conversion is underpredicted by as much as a factor of two. The predicted surface ignition temperatures agree well with the measured values. Results from the numerical model indicate that the fractional conversion rate of fuel has a linear dependence on the fraction of available surface reaction sites.