Change Characteristics of Radicals in Different Stages of Coal Spontaneous Combustion

Residual coal in mine goafs will be oxidized in the presence of air leakage, which can be well tracked through the radical reaction. This study aimed to explore the changes in radical parameters and the reactivity and persistence of newly produced radicals of coal at different temperature stages. To this end, an in situ electron spin resonance (ESR) spectrometer was adopted to measure changes in radical parameters at the heating, maintaining and cooling stages in real time. The following conclusions were drawn. At the heating stage, the radical concentration first rises slowly and then surges; the g factor remains small initially and increases rapidly later; the linewidth falls continuously. At the maintaining stage (230 degrees C), the radical concentration keeps increasing, while the g factor and the linewidth remain stable. At the cooling stage, the radical concentration first decreases and then stabilizes at a relatively high level, and both the g factor and the linewidth nearly return to the initial values. Based on the changes in radical parameters at different oxidation stages, the radicals in coal are divided into transient radicals and persistent radicals according to their survival times. Transient radicals, which are strongly reactive, are quickly consumed, while persistent radicals, which is of weak reactivity, can exist in the system for a long time. That is, the active radicals in coal are transient, while the stable ones are persistent. In addition, this paper gives insight into the changes in contents of active radicals and stable radicals at these stages. Finally, the changes in indicator gas and oxygen concentrations at these stages were analyzed. The research finding further explains the mechanism of radical reactions during coal spontaneous combustion.