A pyrolysis mechanism for ammonia

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:\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm NH}_{\rm 2} + {\rm H} \to {\rm NH + H}_{\rm 2} $$\end{document}with a rate coefficient of 4.0 × 10 13 exp(−3650/ RT ) cm 3 mol −1 s −1 ,\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm NH}_{\rm 2} + {\rm NH} \to {\rm N}_{\rm 2} {\rm H}_{\rm 2} + {\rm H}$$\end{document}with a rate coefficient of 1.5 × 10 15 T −0.5 cm 3 mol −1 s −1 and\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm NH}_{\rm 2} + {\rm NH}_{\rm 2} \to {\rm NH + NH}_{\rm 3} $$\end{document}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\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm NH}_{\rm 3} + {\rm M} \to {\rm NH}_{\rm 2} {\rm + H + M} $$\end{document}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%.