A pyrolysis mechanism for ammonia

Abstract

The mechanism of NH 3 pyrolysis was investigated over a wide range of conditions behind reflected shock waves. Quantitative time‐history measurements of the species NH and NH 2 were made using narrow‐linewidth laser absorption. These records were used to establish an improved model mechanism for ammonia pyrolysis. The risetime and peak concentrations of NH and NH 2 in this experimental database have also been summarized graphically. Rate coefficients for several reactions which influence the NH and NH 2 profiles were fitted in the temperature range 2200 K to 2800 K. The reaction and the corresponding best fit rate coefficients are as follows:with a rate coefficient of 4.0 × 10 13 exp(−3650/RT) cm 3 mol−1 s−1,with a rate coefficient of 1.5 × 10 15T−0.5 cm 3 mol−1 s−1 and with a rate coefficient of 5.0 × 10 13 exp(−10000/RT) cm 3 mol−1 s−1. The uncertainty in rate coefficient magnitude in each case is estimated to be ±50%. The temperature dependences of these rate coefficients are based on previous estimates. The experimental data from four earlier measurements of the dissociation reaction were reanalyzed in light of recent data for the rate of NH 3+ H → NH 2 1 + H 2, and an improved rate coefficient of 2.2 × 10 16 exp(−93470/RT) cm 3 mol−1 s−1 in the temperature range 1740 to 3300 K was obtained. The uncertainty in the rate coefficient magnitude is estimated to be ± 15%.