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

We report a very low energy cost for 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 energy cost for NO production of about 80 eV/molecule (calculated based on the total direct current power applied to the discharge) is achieved in the spark regime before the transition, whereas the maximum NO yield (of about 10^4 part per million) corresponds to the glow regime. Based on the estimated power absorbed in plasma the energy cost 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 effciency of a single spark may exceed that of the modern Haber-Bosch cycle (with energy cost of about 5 eV/molecule in the case of ammonia, according to Patil et al., Catalysis Today, 256 (2015) 49). The found low energy cost is associated with the discharge non-equilibrium caused by short spark duration.