The reaction of free radicals and functional groups during coal oxidation at low temperature under different oxygen concentrations

Abstract Safety and environmental problems caused by coal mining and utilization are becoming increasingly severe. The air leakage and uneven oxygen distribution in goaf seriously affect coal’s natural oxidation. Therefore, an in-depth understanding of the changes of free radicals and functional groups in coal under different oxygen concentrations during low-temperature oxidation is essential for dynamically preventing and controlling the spontaneous combustion of residual coal in goaf, which is of great significance for mine safety and environmental protection. Based on Electron spin resonance (ESR) and Fourier transform infrared (FTIR) spectroscopy, an analysis was made on the influence of different oxygen concentrations on the reaction activity of micro-groups during low-temperature coal oxidation. The results show that with increasing oxidation temperature in a range of 30℃–230℃, the concentration of free radicals Ng improves. When the oxygen concentration exceeds 9 %, the production of free radicals Ng significantly accelerates. The content of −CH2 and −CH3 and −OH, major reaction groups in coal oxidation, declined to various degrees with the gradual increase of temperature. The reaction rate of C O and −COOH was positive in the whole heating range (30℃–230℃), indicating that C O and −COOH are the products. During the entire oxidation heating process, when the oxygen concentration reaches 3% and 21 %, the C O content increases by 38.65 % and 59.71 %, respectively, and the −COOH content increases by 32.74 % and 51.96 %. At low oxygen concentrations (O2≤9 %), the range of each active group’s change is relatively small, which implies that low-oxygen conditions can inhabit the coal-oxygen reaction. At high oxygen concentrations (O2 > 9 %), active groups are more likely to react. The rise of temperature ushers in a rapid oxidation stage of coal. Besides, it is necessary to strengthen the fire extinguishing and cooling efforts in goaf deep low-oxygen area.