Multistage mechanism of thermal decomposition of hydrogen azide

A kinetic mechanism for combustion of hydrogen azide (HN$_3$) comprising 61 reactions and 14 flame species (H$_2$, H, N, NH, NH$_2$, NNH, NH$_3$, HN$_3$, N$_3$, N$_2$H$_2$, N$_2$H$_3$, N$_2$H$_4$, N$_2$, Ar) was developed and tested. The CHEMKIN software was used to calculate the flame speed at a pressure of 50 torr in mixtures of HN$_3$ with various diluents (N$_2$ and Ar), as well as the self-ignition parameters of HN$_3$ (temperature and pressure) at a fixed ignition delay. The modeling results of the flame structure of HN$_3$/N$_2$ mixtures show that at a 25–100% concentration of HN$_3$ in the mixture, the maximum temperature in the flame front is 25–940 K higher than the adiabatic temperature of the combustible mixture. Analysis of the mechanism shows that burning velocity of a HN$_3$/N$_2$ mixture at a pressure of 50 torr is described by the Zel'dovich–Frank-Kamenetskii theory under the assumption that the burn rate controlling reaction is HN$_3$ + M = N$_2$ + NH + M (M = HN$_3$) provided that its rate constant is determined at a superadiabatic flame temperature. The developed mechanism can be used to describe the combustion and thermal decomposition of systems containing HN$_3$.