Formation of nitrogen oxides from atmospheric electrodeless microwave plasmas in nitrogen-oxygen mixtures

Electrodeless microwave plasmas were produced in nitrogen-oxygen mixtures at atmospheric pressure to investigate the formation of nitrogen oxides (NO_x) from the plasma. The oxygen content in the mixtures is varied in the range of 1%-3%, and the total flowrate is varied in the range of 25-45 slpm while the microwave power is fixed at 2 kW. The rotational and vibrational temperatures of the plasma are measured based on plasma optical emission spectroscopy, and the amount of NO_x is measured using a NO_x analyzer far downstream from the plasma. The temperatures at the plasma region reach ∼6700 K, and little difference is observed between the rotational and vibrational temperatures as a result of fast vibrational-translational relaxation. Moreover, these temperatures are found to be independent of the flowrate. As the flowrate decreases and the oxygen content in the mixture increases, the level of NO_x is increased from 1612 ppm to 9380 ppm. For detailed investigation, plasma kinetic simulations considering trans-rotational, vibrational, and electron temperatures separately are developed and conducted for the plasma region. The level of NO_x from the kinetic simulations is found to be considerably smaller than that measured. As the equilibrium mole fraction of NO_x is the highest at a temperature of 3120 ± 100 K, with the variation attributable to the composition of species, significant production of NO_x is expected to occur at the post-plasma region when the plasma stream is quenched by mixing with the surrounding flow.