Kinetics of rich ammonia flames

Abstract

Abstract We report laser absorption measurements of NH 3 decay within the flame front region of rich, atmospheric pressure ammonia flames. These data are combined with earlier OH, NH, and NH 2 measurements to obtain new estimates for the oscillator strength of NH 2 . This value, f i = 6.4 × 10 −5 for the P Q 1,7 line in the (0,9,0) ← (0,0.0) vibrational band of the A 2 A 1 ← X 2 B 1 transition, suggests Δ H (NH) ≅ 87 kcal/mol. The ammonia profiles were also combined with previous data on NO, NH, NH 2 , and OH to provide an extensive database at fuel equivalence ratios (ø) of 1.28, 1.50, and 1.81 for comparison to our kinetic model predictions. This modeling used a one‐dimensional flame code which explicitly accounts for the diffusional component in our flame experiments. Modeling results using a conventional mechanism predicted concentration profiles which deviated markedly from our observations. It was possible to obtain much more satisfactory fits by postulating reactions between various NH i ( i = 1, 2) species to form N—N bonds. The N 2 H j ( j = 1–3) species could then lose H atoms via dissociation to ultimately form N 2 . Inclusion of these reactions in the mechanism allowed us to predict concentration—distance profiles for five different species at three different equivalence ratios that are in good agreement with experiment. The most important component of this mechanism is the recognition that the NH i + NH i reactions dominate the kinetics in rich flames. A most satisfying aspect of these calculations is that the key rate constants in the NH i + NH i sequence were estimated using simple RRK theory.