Water Vapor's Role in Nitrogen Fixation Using Air Plasma Jets at Atmospheric Pressure

This study systematically investigated the influence of water vapor on the primary nitrogen fixation (NF) products—NO and NO2—produced by DC air plasma jets[1], examining both DC and pulsed discharge modes under varying oxygen-to-nitrogen ratio (VO2: VN2) conditions[2]. The results reveal a complex relationship where the addition of water vapor exerts promotion, equilibrium, or inhibition effects on NF performance, depending on the VO2: VN2 ratio. Under conditions dominated by 100% N2 or low oxygen levels, increasing the concentration of water vapor was found to significantly enhance NO synthesis, highlighting its role as a facilitator under such scenarios. However, as the VO2: VN2 ratio approached a critical value, the concentrations of NO and NO2 initially decreased slightly with rising water vapor levels before stabilizing, suggesting a transitional equilibrium state. In contrast, in environments with high oxygen concentrations, the presence of water vapor had a marked inhibitory effect on NO production, indicating a shift in the plasma chemistry dynamics under these conditions. Additionally, a comparative analysis of the NF performance using dry and humid quenching gases (N2) applied to the jet afterglow plume revealed further insights. Increasing the flow rate of dry quenching gas was shown to improve NF yield, with the NOX production rate (RNOX ) rising by approximately 16% and the energy cost decreasing by around 15%. Interestingly, when the quenching gas flow rate was held constant, the introduction of water vapor had minimal impact on NF performance. This finding underscores the jet afterglow plume's resilience and adaptability to variations in ambient humidity.