Downstream oxygen quenching of microwave nitrogen plasma: A pathway to NO_x production

Plasma-assisted nitrogen fixation can decarbonize and decentralize fertilizer production if its performance can be enhanced [1,2]. One innovative approach involves separating the plasma-induced nitrogen activation step from subsequent reactions with O₂, allowing a higher degree of activated nitrogen to be transported downstream, thereby enhancing NO_x production. This method offers a direct route from molecular nitrogen to nitric oxide, which bypasses the need for the conventional ammonia synthesis stage.

In this work, we investigate the process performance and temperature evolution during oxygen injection into the afterglow of a nitrogen plasma, using FTIR and Raman spectroscopies, respectively. Different quenching nozzles are employed to improve the mixing of N-radicals generated in the discharge with O₂ quench gas. Systematic parameter-space measurements are conducted, including scans of plasma power, flow ratio and discharge pressure, to boost NO_x production. The highest NO_x yield is observed with O₂ flow fractions exceeding 50% at atmospheric pressure. Temperature imaging indicates a range of 1400-2000 K for an O₂ flow fraction of 23% and 1200-1400 K for a flow fraction of 50%. Overall, these findings indicate that higher O₂ flow fractions result in better mixing and more rapid quenching, and suggest that N-radical transport likely plays a key role in downstream NO_x production.