Condensed-phase pyrolysis mechanism of ammonium nitrate based on detailed kinetic model

Abstract

Ammonium nitrate (AN) is widely used as a fertilizer ingredient and an oxidizer in industrial explosives; however, it can also cause environmental harm through the discharge of nitrogen species such as ammonia and nitrogen oxides to the atmosphere or by catastrophic explosion accidents that injure people and damage surrounding buildings. The purpose of the present work is to fully understand the condensed-phase pyrolysis of AN. Detailed kinetic models for the condensed-phase pyrolysis of AN were developed based on quantum chemistry calculations with the incorporation of solvent effects. Optimized structures for reactants, products, and transition states were obtained at various levels of theory. Rate coefficients were determined to allow the application of transition state theory and variational transition state theory to reactions identified in this study. Models were employed to simulate the pyrolysis of liquid AN under specific heating conditions. The model based on the G4//ωB97XD/SMD method successfully predicted the thermal properties; the shape of the heat flow curve, onset temperature, heat of decomposition, and gases that resulted from condensed-phase pyrolysis. The model also revealed that the condensed-phase pyrolysis mechanism starts with three parallel reactions; NH4+ + HNO3 → NH3NO2+ + H2O, HNO3 + HNO3 → N2O5 + H2O, and HNO3 → NO2 + OH.