NOx production in spark and corona discharges

This paper presents an experimental study on the production of NO and NO2 as a function of electrical energy in laboratory sparks and corona. The results show that both the NO and NOx yield increase almost linearly with the energy per unit length of the discharge. From the data, it is estimated that the spark discharges with energy in the range of 0.5–2.5 J generate (2.76±0.2)×1016 NO molecules/J and (2.97±0.2)×1016 NOx molecules/J. In order to test the effect of humidity on the NOx production, experiments were conducted both at 37% and 87% relative humidity. However, any significant difference in the NO or NOx yield with humidity was not detected. Experiments were also conducted to evaluate NOx production in corona discharges. In the experiment, a DC corona discharge is maintained by applying a high voltage to the central wire of a coaxial metal cylinder. The applied voltage and the current were measured as a function of time and from this the energy dissipation in the corona is estimated. The results show that the efficiency of corona (with an average current of about 300 μA) in producing NOx is 5.7×1014 and 3.6×1014 molecules/J, respectively, for positive and negative polarity. The NOx yield was slightly higher for positive corona while the NOx production in corona is in general two orders of magnitude smaller than that of spark discharges.