Evaluation Method of Coal Spontaneous Combustion Risk Zone in Goaf Based on Multi-Parameters of Fully Mechanized Caving Face: A Case Study on Wangwa No. 2 Coal Mine

Mining technology is the factor affecting coal spontaneous combustion (CSC), and the risk of CSC is especially prominent under fully mechanized caving face (FMCF). Based on the engineering background of 5# coal FMCF of Wangwa No. 2 Mine, a multi-parameter method for determining the dangerous area of CSC, which couples the thickness of floating coal, the limit parameters of CSC, the on-site observation data of the gas tube and numerical simulation, is proposed to accurately demarcate the dangerous area of CSC. The experimental results show that the CSC period of 5# coal in Wangwa No. 2 Mine under the actual conditions of the simulated mine is 22 days. The limit parameters of CSC reach the extreme value at 40°C. With the 115,032 and 215,021 FMCF goaf depth increasing, the O2 concentration decreases slowly at first and then rapidly, while the air leakage intensity decreases rapidly at first and then slowly. Under the FMCF of the same coal seam, the average width of the oxidation zone in the goaf of the 115,032 workface is 74 m, and that of the 215,021 workface is 50 m. The CSC dangerous area of 115,032 goaf is larger than that of 215,021 goaf, and the dangerous area of 115,032 goaf is closer to the workface. It is found that the thickness of floating coal correlates positively with the dangerous zone for spontaneous combustion. At a given floating coal thickness, the greater the upper limit of gas leakage intensity, the smaller the lower limit of oxygen concentration, the wider the oxidation zone, and the closer it is to the workface. The multi-parameter method proposed in this paper comprehensively integrates the thickness of loose coal, oxygen concentration, and air leakage intensity, enabling a more accurate and reliable delineation of CSC dangerous areas in the goaf. After the CSC disaster measures during the recovery period of 115,032 FMCF were guided by the multi-parameter method, the peak CO concentration was reduced from 94 ppm to a safe value, and the maximum temperature of the support was stabilized at 22°C. The research results have theoretical guidance and engineering reference value for the early warning and prevention and control of CSC hazards in goaf areas of similar workface under FMCF.