The use of a noble-metal combustion catalyst such as platinum or palladium in a natural-gas fired turbine can lower NOx (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 NOx is low or nonexistent. Second, the catalyst permits low temperature combustion below the traditional lean limit, thus inhibiting NOx 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 combustion based on the well-accepted CHEMKIN chemical kinetics formalism for gas-phase and surface chemistry. To support the model development, we built a research combustor. We present measured and modeled axial profiles of temperature, fuel conversion, and pollutant emissions for natural-gas combustion over platinum catalysts supported on ceramic honeycomb monoliths. NOx 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 increases 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.