Combined experimental and kinetic modeling approaches of ammonium nitrate thermal decomposition

• Development of a condensed-phase kinetic mechanism for NH4NO3 decomposition.
• Validation against original TGA-XRD-DSC experiments and literature data.
• Impact of heating rate on reactions pathways and N-selectivity.
• Impact of gas-phase reactions involving nitramide and nitric acid.

This experimental and modeling study aimed at better understanding the impact of the operating conditions on the interplay between ammonium nitrate (AN) decomposition pathways and their coupling with gas-phase chemistry. Experimental analysis and kinetic modeling were combined to elucidate the role of operating conditions on ammonium nitrate thermal decomposition. A new condensed-phase semi-detailed kinetic mechanism was developed and validated against original experiments performed in this study and experiments from other groups. This mechanism was coupled to a gas-phase mechanism, making it possible to quantify the contributions of condensed-phase and gas-phase reaction pathways to AN thermal decomposition at different heating rates. The study revealed that T50 (temperature at 50% conversion) increases strongly with the heating rate, favoring highly activated reactions. Even at typical burning surface temperatures (350 °C), gas-phase reactions involving nitramide and nitric acid play a significant role. The thermal deNOx pathway and hydroxylamine decomposition impact significantly NH3 consumption in the monopropellant flame zone.