Low-temperature chemistry in plasma-driven ammonia oxidative pyrolysis

Ammonia is gaining increasing attention as a green alternative fuel for achieving large-scale carbon emission reduction. Despite its potential technical prospects, the harsh ignition conditions and slow flame propagation speed of ammonia pose significant challenges to its application in engines. Non-equilibrium plasma has been identified as a promising method, but current research on plasma-enhanced ammonia combustion is limited and primarily focuses on ignition characteristics revealed by kinetic models. In this study, low-temperature and low-pressure chemistry in plasma-assisted ammonia oxidative pyrolysis is investigated by integrated studies of steady-state GC measurements and mathematical simulation. The detailed kinetic mechanism of NH₃ decomposition in plasma-driven Ar/NH₃ and Ar/NH₃/O₂ mixtures has been developed. The numerical model has good agreements with the experimental measurements in NH₃/O₂ consumption and N₂/H₂ generation, which demonstrates the rationality of modelling. Based on the modelling results, species density profiles, path flux and sensitivity analysis for the key plasma-produced species such as NH₂, NH, H₂, OH, H, O, O(¹D), O₂(a¹Δ_g), O₂(b¹Σ_g⁺), Ar^*, H^-, Ar⁺, NH₃⁺, O₂^- in the discharge and afterglow are analyzed in detail to illustrate the effectiveness of the active species on NH₃ excitation and decomposition at low temperature and relatively higher E/N values. The results revealed that NH₂, NH, H as well as H₂ are primarily generated through the electron collision reactions e + NH₃ → e + NH₂ + H, e + NH₃ → e + NH + H₂ and the excited-argon collision reaction Ar^* + NH₃ + H → Ar + NH₂ + 2H, which will then react with highly reactive oxidative species such as O₂^*, O^*, O, OH, and O₂ to produce stable products of NOx and H₂O. NH₃ → NH is found a specific pathway for NH₃ consumption with plasma assistance, which further highlights the enhanced kinetic effects.