Toward Reducing the Energy Cost of NO_x Formation in a Spark Discharge Reactor through Pinpointing Its Mechanism

Interest in sustainable nitrogen fertilizer production using plasma technology is rapidly growing, and this method is a promising way to decentralize fertilizer synthesis and achieve carbon-free production. However, improving efficiency remains a major challenge for the industrial application of this technology. Herein, we present comprehensive experiments using spark discharge to pinpoint the key factors affecting the efficiency of this process and provide technical approaches to improve the energy usage. We found that applying a bipolar voltage at high frequencies greatly benefited the NOx yield and its efficiency via the utilization of residual species. Measurements using active cooling for the reactor and electrodes revealed that the chemical loss process was not significant, but the energy loss during the heating of the electrode was the dominant loss process, particularly at high frequencies (>20 kHz) where an anchor state of the spark was reached. However, the beneficial effects of higher frequencies compensated for and mitigated the higher power loss resulting in slowing the increase of the energy cost with the energy density. Moreover, enlarging the plasma zone (reaction channel) by increasing the electrode gap was an effective approach for enhancing the energy usage for the desired reaction. Further, the limitations of enlarging the gap were resolved by inserting a floating electrode in the large gap, yielding higher NOx production (1.8%–3.0%) at a lower energy cost (1.9–4.4 MJ/mol), and further enhancements could be achieved by optimizing the reactor configuration. Therefore, the results provide an important basis for the further development and optimization of plasma reactors for efficient chemical reactions.