Experimental study on microwave plasma discharge and combustion of premixed methane and air at atmospheric pressure

Abstract

In this work, we carry out the experiments on an atmospheric-pressure premixed methane and air microwave plasma discharge combustion with premixed methane and air to study the morphology, the spatial distribution of species, and the temperature characteristics for various microwave power values and methane-to-air equivalent ratios (<i>Φ</i>) at a series of measurement positions. The experimental results show that the equivalent ratio of 0.4 corresponds to the limit value for lean-combustion of premixed methane and air. And for <i>Φ</i> < 0.4, the discharge flame is mainly characterized by the combustion induced by premixed methane and air microwave plasma discharge, while, for <i>Φ</i> ≥ 0.4, the discharge flame is constricted into filaments radially, and the natural combustion occurs in the region of low reduced electric strength and the combustion induced by plasma discharge in the region of high reduced electric strength, which affect each other. The variations of emission intensity of OH (A-X) band, NH (A-X) band and CN (B-X) band with the measuring position and the equivalent ratio <i>Φ</i> are measured by using optical emission spectrometry (OES). It is found that the discharge combustion occurs in near afterglow and the combustion in far afterglow. And the vibrational temperature and the rotational temperature of the plasma discharge combustion are determined by analyzing the emission bands of CN(B-X) and the variations of both vibrational and rotational temperatures with equivalent ratio, exhibiting very different varying tendencies for cases of <i>Φ</i> < 0.4 and <i>Φ</i> ≥ 0.4, respectively. Finally, the reaction pathway and mechanism are discussed on the basis of the comparative analyses of the emission spectra of CH<sub>4</sub> diffusion combustion flame, premixed methane and nitrogen microwave plasma discharge, and premixed methane and air microwave plasma discharge combustion.