Study On The Law Of Air Leakage In Cracks And Spontaneous Combustion In Mined-out Areas In Shallow And Short-distance Coal Seam Group Mining

Located in pu county in Shanxi Province of China-coal mine minable seam buried shallow,spacing,rock strength is low,in the process of mining with caving of the rock,fracture makes up and down seam well versed in goaf,form a composite crack continue up to the surface,made continued flow of external fresh air composite mined-out area,provide oxygen for legacy of coal spontaneous combustion in goaf.In addition,the complex and irregular development of cracks in the overlying rock under the surface,and the unstudied law of spontaneous combustion of abandoned coal in the goaf make it difficult to determine the risk area of spontaneous combustion of coal in the goaf,which brings great difficulties to the prevention and control of spontaneous combustion of coal.-coal mine based on the China#2 and#3 close distance coal seam mining regulations and actual conditions as the background,research complex goaf coal spontaneous combustion characteristic parameters and limit parameters,the surface rock migration under the law,laws of evolution of fracture,stress distribution,temporal variation and distribution of porosity,establish a composite model of goaf coal spontaneous combustion,The distribution characteristics of oxygen concentration in composite goaf were simulated.According to the above research,prevention and control measures for spontaneous combustion in shallow and close-distance coal seam mining are proposed to effectively avoid the occurrence of coal spontaneous combustion disasters.The following research results are obtained:Temperature-programmed experiments were carried out to study the characteristic parameters and limit parameters of spontaneous combustion of coal in composite goaf.The experimental results show that with the increase of temperature,the oxygen consumption rate and heat release intensity of coal samples increase continuously,and the gas generation rate increases at the same time,and the growth trend accelerates obviously when the characteristic temperature reaches about 70℃.By regression analysis of the experimental results,the fitting formulas of oxygen consumption rate,CO,CO₂production rate,heat release intensity and temperature of coal samples were obtained,which provided basic parameters for the subsequent establishment of coal spontaneous combustion model.The limit parameters of coal spontaneous combustion in the compound goaf were studied and it was found that the overlying coal seam had stronger spontaneous combustion characteristics due to the reason of air drying after soaking water.The geological conditions,meteorological characteristics and coal seam characteristics were analyzed,and the surface cracks were observed on site.Because the mining area where Huasheng coal industry is located is arid all the year round and has less precipitation,the surface is dry and cracked,and it is difficult to close and annihilate the cracks under natural conditions after the formation of cracks.According to the geological conditions and coal seam characteristics,the causes of cracks are analyzed and the height of the caving zone and the water-conducting fracture zone is calculated.The surface fissures are classified according to the causes,positions,widths and trends of surface fissures.The proportion of different types of fissures is mastered by field measurement.The graben and collapse fissures have large width and depth and are easy to be connected with the lower strata to form air leakage channels.The transverse fractures parallel to the direction of the working face are less in number than the longitudinal fractures parallel to the trend direction,but the width is generally larger,which is the key object of fracture sealing measures.PFC numerical simulation was carried out to study the law of rock strata movement,crack development,stress distribution and porosity space-time distribution in the mining process of upper and lower coal seams.In the process of coal mining,as the immediate roof and main roof caving,delamination crack formation,vertical crack up continuously,gradually caving goaf central overburden compaction inside the mined-out area,the coal mining process,the key strata is not broken,fracture rock near the minable seam,did not develop to the surface;After the mining lower coal,influenced by second mining,the movement of overburden rock caving and fracture rate significantly faster,with the key layer breaking caving,crack up continuously,large number is formed within the rock,complex crisscross in a crack in the channel,until the development to the surface,surface subsidence and cracks,formed through the earth’s surface and complex goaf air leakage passage.Coal seam mining process,the formation stress near the gob clusters,clusters with increasing of coal seam mining,the stress peaks rising,rising trend in the late mining slow until stable within a certain value,the seam roof has obvious phenomenon of pressure relief,in the upper formation stress release area,as the central goaf compaction gradually development.Porosity is larger in the area before cutting and near the rear of stop-mining line,and smaller in the middle of mined-out area.According to the regular distribution of fracture development area and fracture compaction area,porosity in each area increases to varying degrees under the influence of secondary mining after mining of lower coal seam.Establish a shallow buried close distance coal seam group of composite model of goaf coal spontaneous combustion,simulate the oxygen concentration distribution characteristics and spontaneous combustion"three zones"area,through comparing without air leakage and oxygen concentration and mined-out area under the influence of the surface air leakage was spontaneous combustion"three zones",researching the air leakage,with the result of the influence of mining coal to determine spontaneous combustion dangerous area,The simulation results were verified by field gas production and laboratory gas measurement.Based on the above research results and combined with the on-site situation of the mining area,the causes of ignition in the short-distance goaf of shallow buried coal seam were analyzed,and the combined spontaneous combustion prevention measures of fracture plugging and nitrogen injection in goaf were proposed to ensure the safe mining of coal mine.