Evidence of near-the-limit energy cost NO formation in atmospheric spark discharge

Abstract

We report a very low energy cost (EC) for nitric monoxide (NO) radical formation found in atmospheric spark discharge. For this purpose non-equilibrium air discharges working in spark and glow regimes were studied showing the optimum performance in the vicinity of the spark-to-glow transition. The minimum EC for NO production of about 80 eV/molecule (calculated based on the total direct current power applied to the discharge generator) is achieved in the spark regime before the transition, whereas the maximum NO yield (of about 104 part per million) corresponds to the glow regime. Based on the estimated power absorbed in plasma the EC below 4 eV/molecule is achievable in the spark regime, which is close to the Zeldovich reaction enthalpy of NO formation (about 3 eV/molecule). The result implies that the energetic efficiency of a single spark likely exceeds that of the modern Haber–Bosch cycle (with EC of about 5 eV/molecule in the case of ammonia, according to Patil et al (2015 Catal. Today 256 49). The found low EC is associated with the discharge non-equilibrium caused by short spark duration.