Numerical simulation study on the influence of surface air leakage in shallow thick coal seam on coal spontaneous combustion in goaf

The shallow thick coal seam in the western mine area in China usually adopts the extraction ventilation method. The surface air leakage makes the wind flow disorderly, and the oxygen of the air penetrates the goaf and interacts with the residual coal in goaf to oxidize the coal. Therefore, the coal spontaneous combustion is likely to occur, and the harmful gas such as carbon monoxide exceeds the standard, causing seriously effects on the normal mining of the mine. At present, field measurements, theoretical analysis and experimental research methods are generally used to analyze the gas concentration field and temperature field of coal spontaneous combustion in goaf caused by surface air leakage. However, the spontaneous combustion experiment of surface air leakage is relatively complex. It is difficult to use theoretical analysis and experimental research methods to obtain the influence law of surface air leakage on coal spontaneous combustion in goaf from a three-dimensional perspective.In order to solve the above problems, according to the characteristics of shallow thick coal seam in northwest China, a three-dimensional numerical calculation model is established. Combined numerical simulation and field measurement methods are used to analyze the distribution of "three zones" in the surface air leakage goaf area of shallow thick coal seam. The methods also analyze the distribution law of O2 concentration field, CO concentration field, temperature field and pressure field in goaf under different working conditions. Moreover, the field validation is carried out by the ZD5 coal mine fire multi-parameter monitoring device. The results show that the distribution of the "three zones" and the O2 concentration field in goaf are greatly affected by the surface air leakage. It is found that O2 is easily accumulated at the top of the goaf, which changes the gas flow field distribution in goaf. The concentration range of high volume fraction O2 (volume fraction 18%-23%) in goaf is 0-270 m along the strike direction of the goaf and 3-20 m along the vertical direction of the goaf. In particular, O2 is sufficient in the range of 0-80 m along the strike direction of the goaf and 3-8 m along the vertical direction of the goaf. In this area, there is a certain amount of residual coal and heat is not easily dissipated, raising the risk of coal spontaneous combustion. The pressure at the corner of return air roadway in goaf is the smallest, -10 Pa, the pressure at the return air roadway outlet is the lowest, and the pressure at the air inlet is the highest. The pressure gradually increases along the inclined direction, the vertical direction and the strike direction. The temperature distribution is similar to the CO distribution in goaf. The goaf floor is little affected by the air leakage from the surface, but largely affected by the corner of the intake air roadway of the working face. The heat accumulation and CO accumulation are basically the same as the situation of no air leakage. From the middle of the goaf, the temperature and CO are mainly affected by the surface air leakage, presenting an "O" ring distribution. At the top of the goaf,the temperature and CO reach maximum values at the junction of each fracture zone and the goaf, then decrease along both sides. The maximum value appears at the junction of the deepest fracture zone and the goaf.