Science and Technology of Energetic Materials

Vol.78, No.6 (2017)

Research paper

A detailed chemical kinetic model for the combustion of gas phase ammonium nitrate
Yu-ichiro Izato, and Atsumi Miyake


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 ωB97XD/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· and NO2·, after which OH· attacks NH3 to yield NH2·. This NH2· reacts with NO2· to yield HONO via NH2O·. Finally, HONO attacks HNO3 to yield t-ONONO2 and this compound decomposes to start a chain-branching reaction : t-ONONO2 →NO2· + NO2· + H2O. It was determined that, due to the stability of NH3, this species is not attacked by NO2· but solely by OH·. The production of OH· 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.

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ammonium nitrate, kinetic model, combustion, ignition mechanism, DFT

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