The characterization of free radical reaction in coal low-temperature oxidation with different oxygen concentration

Abstract

To explore the mechanism of low-temperature oxidation of residual coal at different depths in goaf, the oxygen concentration of airflow was dynamically adjusted to simulate the natural oxidation of coal at different depths in goaf with the aid of a self-built simulation experiment system of low-temperature coal oxidation. Based on electron spin resonance (ESR), analyses were made on the variation of free radicals during coal oxidation at low temperatures in different concentrations (volume fraction) of O2. Besides, an industrial analyzer and a gas chromatograph were also applied to the determination of the change of volatile matter and the emission of gaseous products. The results show that line height, the free radical concentration, volatile matter, and CO concentration change in stages with the increase of oxidation temperature. At about 130 °C, they shift from slow-varying to rapid-changing, reflecting that the coal steps out of a slow oxidation stage and enters a rapid one. The variation trend of line height and free radical concentration is basically the same, and g values are always in the range of 2.00141–2.002034, with little change. As O2 concentration rises, the tendency of each parameter with the oxidation temperature remains unchanged, but the overall variation gradually increases. When O2 concentration is lower than or equal to 9%, free radicals undergo reaction at a low speed and CO generation delays in the low-temperature stage. When it is higher than 9%, there is a growing possibility of coal-oxygen reaction. The increase in temperature improves the excitation of free radical reaction, so that CO concentration rises exponentially. Since different O2 concentrations have varying effects on coal oxidation, it is of great importance to adopt prediction and prevention technology in different-depth areas of goaf to ensure the safe mining of coal seam.