Kinetic analysis of the thermal decomposition of liquid ammonium nitrate based on thermal analysis and detailed reaction simulations

A detailed reaction mechanism for the liquid-phase decomposition of ammonium nitrate (AN) was modeled based on ab initio calculations, and this model was employed to simulate heat flow curves associated with the decomposition of AN at various heating rates (1, 2, 4, 5, and 8 K min(-1)) over the temperature range of 443-623 K. A kinetic analysis using the model-free Friedman method determined the kinetic triplet for AN decomposition. The predicted activation energy for the exothermic decomposition had a range of approximately 162-168 kJ mol(-1) with alpha values between 0.1 and 0.5. The activation energy value is good agreement with the experimental one of 170 kJ mol(-1). The Friedman kinetic model was used to predict the time to maximum rate under adiabatic conditions (TMRad) at various initial temperatures, and these values were compared to the TMRad values obtained directly using the detailed reaction mechanism. Accurate predictions for TMRad were obtained at initial temperatures below 623 K. The difference between the TMRad values obtained from the thermal analysis and detailed reaction models at 823 K was larger than one order of magnitude. It was also found that the dominant decomposition mechanism changes from ionic to radical with increasing temperature, and this explains the difference in the TMRad values at 823 K.