Effect of water vapor on nitriding of stainless steel walls induced by ammonia flames

Ammonia is a promising candidate fuel for future carbon-free energy systems. However, significant interactions between ammonia flames and metal walls in combustors result in “unwanted” nitriding, compromising safe operation and shortening the lifespan of combustion systems. The substantial water vapor generated during ammonia combustion further influences this flame-wall interaction. This study examines the effect of water vapor on two interconnected processes: the heterogeneous decomposition of ammonia and the nitriding of stainless steel induced by ammonia flames. Ammonia conversion ratios due to heterogeneous decomposition on stainless steel surfaces were measured in a flow reactor under varying water vapor concentrations, and the mechanisms underlying the impact of water vapor on both surface reactivity and surface nitriding were examined. Additionally, the effect of water vapor on nitriding induced by ammonia flames was investigated. The findings confirm that the oxidation effect of water vapor reduces surface reactivity for heterogeneous ammonia decomposition, making it the primary factor behind the hindering effect on nitriding during ammonia combustion.