Flow reactor study of the effect of pressure on the thermal de-NOx process

Abstract

The effect of pressure on the thermal de-NOx reaction was studied in a flow reactor capable of operating at pressures up to 50 bar. Hot combustion products were supplied to the reactor by a burner operating with a lean, premixed CH4-air mixture. Nitric oxide, ammonia and nitrogen were injected into the combustion products at the throat of a converging-diverging nozzle through radially opposed jets oriented perpendicular to the flow to provide rapid mixing. Nearly isothermal conditions exist downstream of the mixing region. Typical axial temperature variations are ±5 K. Axial profiles of NO, NH3, NO2, and N2O mole fractions were measured in the reactor at pressures of 1, 2, 5, and 10 bar for temperatures of 1200 K and 1240 K using extractive sampling techniques. Initial injected mole fractions of NO and NH3 corresponded to 250 ppm and 500 ppm, respectively, in the combustion products, and the O2 mole fraction was near 3%. At a given temperature, increasing pressure resulted in a modest decrease in the rate of NO removal, with little effect on the amount of NO2 and N2O formed. Measured NO2 and N2O mole fractions were less than 20 ppm for all conditions of this study. To allow comparison of the experimental data with predictions, a model was developed that couples the entrainment and mixing processes in the flow reactor with a detailed thermal de-NOx reaction mechanism. To improve agreement between measured and calculated species profiles, particularly at elevated pressures, the reaction mechanism was optimized by systematic variation of key kinetic parameters within reported uncertainties. This optimization reduced the overall rate and branching fraction of the NH2 + NO → products reaction to values consistent with recent experimental studies of this reaction and provided a modest improvement in agreement between the calculated and measured species profiles.