Investigation of chemical kinetics in plasma-assisted ammonia conversion processes

Ammonia, as a green hydrogen carrier, has significantly accelerated the widespread application and sustainable development of hydrogen energy. This work presents a theoretical analysis of the decomposition kinetics and key reaction mechanisms in plasma-assisted ammonia decomposition for hydrogen production, based on a model of the Ar/NH₃ mixed gas plasma DC discharge fluid. It focuses on uncovering the mechanisms that regulate ammonia decomposition pathways during discharge mode transitions and analyzes the effects of NH₃ concentration and current density on NH₃ decomposition yield and kinetics. The results indicate that in the normal glow discharge regime, ammonia is mainly converted through energy transfer reaction (NH₃ + Ar^* → NH + 2H + Ar) and charge transfer reaction (NH₃ + Ar+ → NH₃⁺ + Ar). In the abnormal glow discharge regime, the conversion of ammonia and the generation of hydrogen are primarily achieved through electron collision reactions. The main consumption pathway of NH₃ is through the vibrationally excited states NH₃(v) and electronic excitation state NH₃(e) generated by electron collision excitation, with the remaining NH₃ decomposition reactions primarily driven by electron collision reactions. It is noteworthy that the concentration of ammonia decomposition products shows a non-monotonic dependence on discharge power. While the H₂ concentration continuously decreases with increasing discharge power, the peak concentrations of H and other species occur in the abnormal glow regime. The theoretical model established in this work provides an important theoretical basis for optimizing the design of plasma hydrogen production reactors.