Study on spontaneous combustion law of residual coal in goaf under the condition of gas extraction in the low-level roadway

The gas extraction in the low-level roadway in the coal seam roof is an important technical measure to solve the problem of gas concentration exceeding the limit in the upper corner of the working face. But the high-flow mixed extraction in the low-level roadway causes serious air leakage in goaf and increases the risk of spontaneous combustion of residual coal. At present, there are few studies on the synergistic effect of low-level roadway layout and extraction flow on the spontaneous combustion of residual coal in goaf. According to the actual situation of gas extraction in existing goaf with the low-level roadway in 10106 working face of Jiajiagou Coal Mine, a three-dimensional fluid-solid-thermal multi-field coupling numerical model of heterogeneous goaf is established by COMSOL software. The spontaneous combustion law of residual coal in goaf induced by gas extraction in the low-level roadway is analyzed by numerical simulation. The results show that gas extraction in the low-level roadway can reduce the gas concentration in the upper corner of the working face. The gas extraction flow rate is proportional to the maximum width of the spontaneous combustion oxidation zone and the maximum temperature of goaf. With the increase of gas extraction flow rate, the maximum width of the spontaneous combustion oxidation zone and the maximum temperature of goaf will increase. But too high gas extraction pressure will cause the air near the upper corner to "flow back" to the goaf. This will increase the risk of spontaneous combustion of residual coal in goaf. When the gas extraction flow rate of low-level roadway is constant, the smaller the dislocation distance is, the larger the maximum width and the maximum temperature of the spontaneous combustion oxidation zone in goaf are. Combined with the numerical simulation results and engineering practice, it is determined that the dislocation distance in the low-level roadway of Jiajiagou Coal Mine is 15 m. The gas extraction flow rate is 45 m³/min. The gas volume fraction in the upper corner is 0.875% and the maximum width of the spontaneous combustion oxidation zone in goaf is 59.14 m. The scheme effectively solves the problem of gas concentration exceeding the limit in the upper corner. The risk area of spontaneous combustion of residual coal in goaf is not significantly increased.