Modelling of NOx removal by a pulsed microwave discharge

A kinetic model is developed to study NOx removal in N-2/O-2/NO gas mixtures treated by a pulsed microwave discharge at atmospheric pressure. The model considers the coupling between electronic, vibrational and chemical kinetics, taking into account a large set of neutral, excited and charged species. The time behaviour of the species densities is calculated for the given input parameters, namely the reduced electric field, the initial gas composition (N-2, 500 ppm NO and 0%, 2%, 5% 02), the effective pulse duration (1.5 mus) and repetition rate (40 Hz), and the gas temperature. The rate coefficients of the processes considered are either calculated from the electron energy distribution function or taken from published data. The vibrational temperature of N-2 is derived from the calculated population densities of the first two vibrational levels of N-2, and the results obtained are compared with those observed by coherent anti-Stokes Raman scattering (CARS) technique. The experimental data from the FTIR measurements of the NOx densities after reduction agree well with the calculations. The kinetic study shows the dominant role of the N-4(+) ions during the discharge phase. The calculated N density is about an order of magnitude lower than the O density. Nevertheless, it is the N radical that plays the major role in the reduction of the NO concentration even in O-2-containing gas mixtures. Although there is oxidation of NO to NO2, an overall NOx reduction can be achieved, provided the microwave pulses are short enough (similar to1 mus) and the gas temperature low (similar to500 K).