Mechanisms of gas temperature variation of the atmospheric microwave plasma torch

Abstract

The gas temperature is a key parameter that affects the process of microwave plasma chemistry in industrial applications. Based on the molecular emission spectrometry of the A2Σ+→X2Πr electronic system of OH radicals, the gas temperature of the atmospheric air microwave plasma core at different absorbed microwave power levels, gas flow rates, gas humidities, and volume fractions of CO2 in air was analyzed. In the experiment, the absorbed microwave power, gas flow rate, gas humidity, and volume ratio of CO2 in air was varied from 560 to 1750 W, 10 to 24 l min−1, 30% to 95%, and 0% to 40%, respectively. Moreover, the axial gas temperature distribution of the plasma torch was measured. The experimental results showed that (i) the plasma gas temperature mainly ranged from 4000 to 7000 K, (ii) the plasma gas temperature rose with increasing absorbed microwave power but was hardly affected by the feeding gas flow rate, (iii) the plasma gas temperature decreased by ∼400 K for every 20% increase in the fraction of CO2 in air and decreased with increasing gas humidity, and (iv) the plasma torch gas temperature decreased along the axial direction. Due to the lack of a prevailing microwave discharge theory, an in-depth analysis of the mechanisms of gas temperature variation was performed based on the heat balance equation.