Pulsed microwave discharge at atmospheric pressure for NOx decomposition

A 3.0 GHz pulsed microwave source operated at atmospheric pressure with a pulse power of 1.4 MW, a maximum repetition rate of 40 Hz, and a pulse length of 3.5 mus is experimentally Studied with respect to the ability to remove NOx from synthetic exhaust gases. Experiments in gas mixtures containing N-2/O-2/NO with typically 500 ppm NO are carried out. The discharge is embedded in a high-Q microwave resonator, which provides a reliable plasma ignition. Vortex flow is applied to the exhaust gas to improve gas treatment. Concentration measurements by Fourier transform infrared spectroscopy confirm an NOx reduction of more than 90% in the case of N-2/NO mixtures. The admixture of oxygen lowers the reductive potential of the reactor, but NOx reduction can still be observed up to 9% O-2 concentration. Coherent anti-Stokes Raman scattering technique is applied to measure the vibrational and rotational temperature of N-2. Gas temperatures of about 400 K are found, whilst the vibrational temperature is 3000-3500 K in pure N-2. The vibrational temperature drops to 1500 K when O-2 and/or NO are present. The randomly distributed relative frequency of occurrence of selected breakdown field intensities is measured by a calibrated, short linear-antenna. The breakdown field strength in pure N-2 amounts to 2.2 x 10(6) V m(-1), a value that is reproducible within 2%. In the case of O-2 and/or NO admixture, the frequency distribution of the breakdown field strength scatters more and extends over a range from 3 to 8 x 10(6) V m(-1).