Experimental investigation on microstructure evolution and spontaneous combustion properties of aerobic heated coal

• The index gas release concentration of aerobic heated coal exhibits obvious segmentation characteristics. • The aerobic heated coal has a greater risk of self-heating in the slow oxidation stage. • Compared to raw coal, the aerobic heated coal has larger specific surface area and pore size. • The determination of the oxidation zone provides a basis for targeted fire prevention measures. Since coal oxidation and heat accumulation are prerequisites for coal spontaneous combustion, it is of great significance to study the characteristics of coal oxidation and heat accumulation to grasp characteristics of spontaneous combustion of residual coal and to locate the area with coal spontaneous combustion in goaf. In this study, three types of lignite and bituminous coal with a strong spontaneous combustion tendency were selected to prepare raw coal samples and aerobic heated coal samples. Then ultimate and proximate test, core pore and surface area test, SEM test, TG test, FTIR test and temperature-programmed test were undertaken, respectively. After coal undergoes aerobic heated progress, its surface micro-cracks, internal pore volume and specific surface area surge, and the active functional groups vary significantly; the contents of C and H fall, and that of O rises. These changes are conducive to coal oxidation. The temperatures corresponding to the first peak points in the DTG curves for the aerobic heated coal samples are lower than those for the raw coal samples, which indicates that the aerobic heated coal samples enter the self-oxidation acceleration stage earlier. Besides, according to the results of the temperature-programmed experiment, the cross-point temperatures of CO generation for the aerobic heated coal samples and the raw coal samples are 91 °C, 82 °C and 76 °C, respectively. A comprehensive analysis discloses that the aerobic heated coal samples are more likely to combust spontaneously in the initial stage of oxidation and to generate more CO. The heat dissipation zone, oxidation zone and suffocation zone were divided by monitoring the oxygen concentration changes in goaf at different stages on 5–20101 working face. It is determined that the oxidation zone of the goaf is 71–152 m on the inlet side and 59–128 m on the return side.