(113a) Detailed Decomposition Mechanism of Ammonium Nitrate/Chloride Mixtures

Ammonium Nitrate (AN), NH₄NO₃, is widely used as a fertilizer and industrial explosive because of its low cost, high oxygen balance, high nitrogen content, and high gasification rate. On the other hand, AN has a hazardous character and causes numerous accidental explosions, such as in Toulouse, France in 2001, West Texas, USA in 2013, Tianjin, China in 2015, and Beirut, Lebanon in 2020. Some of such explosions are known to be responsible for thermal destabilization and decomposition due to the presence of contaminants. Chloride is well known as a prime example of those contaminants. In previous studies, the mechanism of AN decomposition with chloride has been investigated by several experimental or computational approaches. In the experimental work, kinetics and gas phase products of AN decomposition with chloride were examined and it is known that chloride accelerates the exotherm rate and increases the ratio of N₂ production of AN decomposition. In the computational work, the reaction pathway was surveyed by quantum calculation and the reaction pathway via NO₂Cl and NOCl was proposed. As described, there are a lot of efforts to reveal the mechanism of thermal destabilization by chlorides. However, it has not been still clear what and how the reaction pathway of AN decomposition with chloride affects macroscopic properties such as the onset temperature and the kinetics. Therefore, we focused on the analysis method of the reaction mechanism utilizing a detailed kinetics model, to reveal the relationship between the detailed reaction path and macroscopic thermal properties. We did not only reaction identification, but also examined the method for calculating the kinetic parameters of the detailed kinetics model that simulates molten AN.

In this study, the purpose is to elucidate the decomposition mechanism that is responsible for the destabilization of AN with chlorides. Ammonium chloride (NH₄Cl), which has the same cation species as AN, was chosen as a representative of simple chloride salts. At first, Gas phase and condensed phase products were detected by a mass spectrometer(MS), gas chromatography(GC), and ion chromatography(IC), to identify and formulate elementary reactions of the AN/NH₄Cl pyrolysis. We estimate the elementary reactions based on the results of these experiments and the mechanism proposed by previous studies. Then, a detailed kinetics model of AN/NH₄Cl was constructed, to check that those elementary reactions are responsible for the macroscopic properties. To construct this model, the kinetic parameters for each elementary reaction and thermodynamic data for all chemical species were calculated based on quantum chemical calculations at the CBS-QB3//ωB97X-D/6-311++G(d,p)/SCRF=(SMD,solvent=water) level of theory, transition state theory, statistical thermodynamics. Additionally, in this work, to properly estimate the effect of the diffusion-controlled reaction, we estimated the diffusion rate in molten AN by molecular dynamics and took into account detailed balance about each elementary reaction. And then, it is evaluated how the mechanism proposed in this study affects the macroscopic properties, by comparing the thermal behavior estimated by this model with an experiment DSC. After that, the sensitivity analysis of AN decomposition revealed the rate-determining step, and the effect of initial composition was examined by simulations at various initial compositions. Finally, we discussed the relationship between the decomposition mechanism of AN with chloride and the conditions during storage, such as temperature, pressure, and compositions.