A detailed chemical kinetic model for the combustion of gas phase ammonium nitrate

In this work, a mechanism for gas phase combustion of ammonium nitrate (AN) was identified and investigated. The optimized structures of reactants, products, and transition states were generated at the omega B97XD/6-311++G (d,p) level of theory and the total electron energies of such structures were calculated at the CBS-QB3 level of theory. The new kinetic model was subsequently used to predict low-pressure AN decomposition products, and the results were compared with the experimental data in the literature. Good agreement was found in terms of the concentrations of decomposition products, although the simulation predicted lower amounts of some products than were determined experimentally, suggesting that surface catalytic decomposition on the reactor walls may affect the AN decomposition process. A modified model including surface catalytic reactions provided better predictions. Detailed chemical reaction calculations were used to determine the AN ignition mechanism. During an induction period, the homolytic cleavage of HNO3, with a high energy barrier, initiates a chain reaction by generating OH center dot and NO2 center dot, after which OH center dot attacks NH3 to yield NH2 center dot. This NH2 center dot reacts with NO2 center dot to yield HONO via NH2O center dot. Finally, HONO attacks HNO3 to yield t-ONONO2 and this compound decomposes to start a chain-branching reaction : t-ONONO2 -> NO2 center dot + H2O. It was determined that, due to the stability of NH3, this species is not attacked by NO2 center dot but solely by OH center dot. The production of OH center dot was therefore determined to be the rate-determining step for AN decomposition in the gas phase. The results of this work also demonstrate that, following sufficient accumulation of radicals, the mixture of gas phase HNO3 and NH3 ignites and the temperature rises sharply.