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.