Prediction of coal spontaneous combustion in compound gob of shallow seam group mining

The air leakage in the gob caused by the ground fissures seriously threaten the coal mining safety in mining area. To study the air leakage mechanism is useful for the high-yield and efficient development of the coal industry.Through the combination of remote sensing,field measurement and numerical simulation,the authors explored the distribution characteristics,air leakage characteristics,dynamic development and evolution regularity of ground fissures due to mining shallow and near-neighbored coal seams in Sujiagou. The influence of air leakage ground fissures on the distribution of seepage field in composite gob was analyzed,and the spontaneous combustion danger zone in the overlying gob when mining the lower coal seam was determined according to the limit parameters of coal spontaneous com-bustion obtained from the coal spontaneous combustion experiment. The results show that the secondary mining causes new ground fissures on the surface,and the existing fissures are further activated and expanded on the basis of the original development degree. According to the observation,the width and stagger increase about 0.1 m and 0.06 m respectively,which intensify the surface air leakage. In addition,the ground fissure presents regional distribution characteristics in the process of advancing with the working face and follows the dynamic regularity of "increase-decreaseclose". According to the different degree of penetration between the fissure and the gob,the fissure producing area,the fissure development area and the fissure closure area are divided. The through type fissure distributed in the fissure development area is the main air leakage channel on the surface. The velocity of air leakage changes with the change of space position of ground fissure relative to working face. The numerical simulation results show that the surface air leakage enlarges the influence range of gob seepage and complicates the distribution of gob velocity field. Due to the influence of static pressure difference,the air leakage from the working face and the surface is concentrated on the return side of the gob. Based on the analysis of the limit parameters of coal spontaneous combustion and the identification gas of coal natural oxidation,it is found that the spontaneous combustion danger zone in the overlying gob of Sujiagou is distributed at the return air side of the gob within 30-65 m from the working face. The research results reveal the air leakage mechanism of the surface,overlying gob and working face of the lower coal seam,which provides a reference for the accurate control of air leakage and the prevention of fire in the gob when mining near-neighbored coal seams.