Molecular beam mass spectrometric and modeling studies of neat and NH 3-doped low-pressure H 2/N 2O/Ar flames: Formation and consumption of NO

Abstract

An experimental and chemical modeling study of neat and NH3-doped H2/N2O/Ar flames (Θ∼1.1) is conducted to understand the fundamental mechanism for NO formation and destruction and to predict the efficacy of NH3 on the rate of conversion of NO to N2. Species concentration and temperature profiles are measured with a molecular beam mass spectrometer and thin-wire thermocouple, respectively. Species profiled include H2, N2O, NH3, N2, NO, and Ar. The experimental mole fractions are compared to both equilibrium and premixed laminar flame code calculations. The flame code employs a chemical mechanism consisting of 87 reactions and 20 species with rate constants obtained from a critical literature review. Equilibrium calculations are in very good agreement with both experimental and flame code calculations for N2O, N2, and H2O in the postflame region of both neat and doped flames. However, they underpredict the H2 and NO mole fraction in both flames, proving that NO is not in equilibrium and prevents full energy release. The flame code profiles of the majority of the species agree well with experiment for the neat flame and reasonably well for the doped flame. A 55% reduction in the NO mole fraction for 4% dopant is predicted in the postflame region, in good agreement with that observed experimentally. The flame calculations overpredict the NH3 mole fractions in the postflame region, however, suggesting that refinements in the model are necessary. Rate and sensitivity analyses reveal that the decrease in NO mole fraction results from less NO formation by the reactions N2O+H=NO+NH and N+OH=NO+H and more of its consumption to N2 by reactions NO+NH2=N2+H2O and NO+NH2=NNH+OH.