Effect of equivalence ratio on the thermal autoignition of aqueous ammonia ammonium nitrate monofuel

The combustion of a carbon-free nitrogen-based monofuel consisting of an aqueous ammonium hydroxide/nitrate (AAN) solution was studied at different equivalence ratios ranging between 0.6 and 12. The main oxidizing and reducing agents in AAN are nitric acid and ammonia, respectively. A combined differential thermal/barometric analysis (DTA/DBA) study was used to investigate the effect of the equivalence ratio on the autoignition temperature (AIT) of this monofuel. Increasing the equivalence ratio was found to increase the AIT value and reduce the energy generated during the ignition. Kinetic gas-phase simulations were used to calculate the theoretical AIT values. Good agreement was found at equivalence ratios close to unity. Rate of production and sensitivity analyses were performed to explore the reaction kinetics leading to the thermal auto-ignition. These indicated that reactions reducing nitrogen dioxide to nitrogen oxide and molecular oxygen inhibit the ignition, while reactions between ammonia and high oxidation state NO_x producing amidogen promote it. The results of this study shed light on the influence of non-stoichiometric conditions on the thermal auto-ignition of AAN.