A mechanism for ignition of high-temperature gaseous nitromethane—The key role of the nitro group in chemical explosives

Abstract

A detailed chemical mechanism describing ignition of high-temperature pure gaseous nitromethane was compiled and tested using shock tube experiments. The temperatures and pressures behind the reflected shock were in the range 1000–1600K and 1–10 atm. Measurements were made of the time evolution of the pressure at the end wall, as well as of the simultaneous pressure and NO absorption at a short, fixed distance from the end wall. Mass and infrared spectroscopy were used to identify the final products. In the reaction mechanism proposed, initiation starts with the CN bond breaking which yields CH3 and NO2. Methoxy and CH2NO2 radicals then propagate the reaction through two major parallel pathways, both producing CH2O. Formaldehyde is then reduced to HCO and carries the reaction toward completion. The radical reactions do not release enough energy to compensate for the energy consumed in breaking the CN bond. Although most of the radicals reach their maximum concentration early in the reaction process, ignition does not occur until virtually all of the nitromethane is consumed. The calculations show that the nitro group is the key to explosion: NO2 produces OH through its reaction with H radicals. Hydroxyl reactions, which are fast and exothermic, lead to an accelerated consumption of the explosive with heat release. Comparison with the experiments shows that the mechanism predicts correct induction times for the pressure and temperature range of the experiments.