An experimental, theoretical and kinetic modeling study of ammonia borane combustion

Ammonia borane (NH₃BH₃) is a promising high-energy solid fuel candidate for ramjet propulsion systems. However, the detailed chemical kinetic mechanisms remain insufficiently understood. In this study, the ignition delay times (IDTs) of NH₃BH₃/air mixtures were experimentally investigated using a high-pressure shock tube under conditions of 5.0 and 10.0 bar over 1000 to 2300 K. A comprehensive kinetic mechanism comprising 58 species and 292 reactions was developed to describe the combustion of ammonia borane. Ab initio calculations were conducted to study hydrogen abstraction, unimolecular, and chemically activated reactions on the potential energy surfaces of NBH₆ and NBH₅. Geometry optimizations, vibrational frequency calculations, and dihedral angle scans were performed at the M06-2X/6-311++G(d,p) level of theory. Single-point energies (SPEs) for all species were determined using the CCSD/CBS method. The kinetic and thermochemical parameters obtained from high-level calculations were incorporated into the C3MechV3.3 framework to construct a revised mechanism. The reliability of the developed mechanism was evaluated through IDT simulations, and sensitivity and flux analyses were performed to identify the key reactions controlling the reactivity. The results indicate that the phase transition reaction NH₃BH₃(s) → NH₃BH₃ and the unimolecular decomposition reaction NH₃BH₃ → NH₃ + BH₃ play dominant roles during ammonia borane combustion.