Optimizing ammonia cracking in microwave argon plasma: Temperature control and ammonia delivery

Hydrogen has been conceived as an alternative to hydrocarbons to achieve a carbon–neutral society, with ammonia emerging as the most practical hydrogen carrier for long-distance transportation. Conventional thermo-catalytic cracking of ammonia is an energy-intensive process, leading to the consideration of low-temperature plasmas as potential alternatives. Recently, a surfatron-based Microwave Plasma Jet (MWPJ) was investigated, demonstrating the fundamental physical and chemical mechanisms of NH₃ cracking. In this study, we present an optimization scheme for NH₃ cracking using a surfatron-based argon MWPJ to improve H₂ production rate and efficiency. Our approach included: (i) introducing downstream inlets of NH₃ to decouple NH₃ from plasma generation and (ii) adding additive (N₂ or NH₃) to the main Ar flow to elevate the gas temperature of the MWPJ. We found that both additives to the Ar flow raised the gas temperature from 2700 K up to 4200 K (N₂) or 5400 K (NH₃), showing a trade-off between gas temperature and the height of the MWPJ. The case with NH₃ added to the main Ar flow showed better performance than the case with N₂ due to the higher gas temperature and additional H₂ production from the additive NH₃. The main NH₃ supply in the downstream showed no negative effect on the plasma generation and significantly increased the H₂ production rate and efficiency, primarily due to the sufficient NH₃ supply rate. Increased number of NH₃ inlets also facilitated NH₃ cracking, illustrating saturated performance with four inlets. To scale up this process, the obtained optimal geometry and operating conditions will be further explored using a high-power microwave system.