Effect of low-temperature oxidation of coal with different metamorphic degrees on coal quality characteristics and outburst comprehensive index

Outburst prevention and control measures such as protective seam mining and coal seam gas pre-drainage can cause low-temperature oxidation of coal seam, and there is a lack of research on the changes in coal quality characteristics and outburst comprehensive index during low-temperature oxidation. In this paper, the changes of coal quality characteristic indexes during oxidation of coals with different metamorphic degrees, the initial gas emission velocity and the firmness coefficient were measured, the influence of coal quality characteristics on outburst comprehensive index is studied, and the change of outburst comprehensive index K=Δpf in the low-temperature oxidation process was analyzed. The results show that for coal with different metamorphic degrees, the coal with a lower metamorphic degree enters the stage of inherent moisture evaporation later and is more likely to undergo decomposition and volatilization of volatile matters. In the midterm of low-temperature oxidation (120–160°C), the growth rates of △p values and the decline rates of f values of coal samples with higher metamorphic degrees tend to decrease. It can be known that the K value of bituminous coal and anthracite are more than 25 during the whole oxidation heating stage, which shows a serious outburst risk. For these coals, effective outburst prevention measures must be taken to prevent outburst risk and inhibit coal spontaneous combustion. In contrast, for lignite coal, the growth rate of △p value and the decline rate of f value keep increasing in the midterm of low-temperature oxidation, due to the joint action of moisture evaporation and volatile decomposition and volatilization. The K value of Lignite is more than 15 when oxidation temperature rises to 150 °C, which shows a general outburst danger. According to the analysis results in the midterm, appropriate measures must be taken to inhibit coal oxidation when local or overall temperature of coal seam reaches 130 °C.