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

Abstract

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.