Research On The Law Of Fissure Air Leakage And Coal Spontaneous Combustion In The Mining Of Shallow Buried Close Distance Coal Seam Group

The thick coal seams,shallow buried,and close distance in the western region result in the development of cracks in the surface and overlying strata during the mining of coal seams,the air leakage is serious,which provides continuous oxygen for the spontaneous combustion of coal in the compound goaf.In addition,the spontaneous combustion oxygen consumption and heat release rules of the residual coal in the overlying goaf are not clear,resulting in unclear rules for the spontaneous combustion of coal in the compound goaf and difficult to determine the location of the fire,which brings great difficulties to fire prevention and control.According to the actual conditions of the mining of shallow burial close distance coal seam group in Shendong mining area,the change law of coal spontaneous combustion characteristics in the compound goaf,the law of dynamic development and air leakage of surface fissures,the development of overburden rock fissures and the change of porosity are be studied.Establish a model of coal spontaneous combustion in composite goafs of shallow buried close diatance coal seam group,simulate and study the gas concentration,temperature and flow field distribution characteristics of composite goaf,construct the coal spontaneous combustion prevention system in the composite goaf of shallow buried close distance coal seam group.The following results have been achieved through research:Carry out temperature-programming experiments of coal samples in the compound goaf to study the change law of the spontaneous combustion characteristics of the coal in the compound goaf.The results show that with the increase of temperature,the oxygen consumption rate,gas generation rate and heat release intensity of coal samples increase exponentially.Regression analysis was performed on the experimental results,and the function of the oxygen consumption rate,gas generation rate,heat release intensity with temperature of the coal sample in the compound goaf were obtained,which provides basic parameters for the simulation of coal spontaneous combustion in the compound goaf.Study the variation law of coal spontaneous combustion limit parametersin the compound goaf;when the temperature is the same,the thickness of residual coal increases,the upper limit air leakage intensity linearly increases,and the lower limit oxygen concentration approximately decreases;when the the thickness of residual coal is the same,as the temperature increases,the upper limit air leakage intensity first decreases and then increases rapidly,the lower limit oxygen concentration first increases then increases rapidly,and the extreme value is between 70°C and 80°C.From the perspective of the heat balance of the goaf,it is clarified that the coal spontaneous combustion hazardous zone in the compound goaf is determined by the upper limit air leakage intensity and the lower limit oxygen concentration,and an example is analyzed to give the index parameters of the coal spontaneous combustion hazardous zone with different residual coal thicknesses.Long-term observation of surface fissures,classification of surface fissures according to the morphology and development of surface fissures,and grasping the temporal and spatial distribution and scale characteristics of various fissures.The open type fissures above the two lanes of the working face break and evolve into collapsed fissures after periodic weighting,and then remain stable;the extension direction is consistent with the advancing direction of the working face.The graben type fissures behind the working face abruptly undergo a sudden change when periodic weighting,accompanied by the development and closure of a large number of stretch type fissures,after three mutations,the graben type fissures developed and stabilized.The horizontal fissures account for 2/3 of the various types of fissures on the surface,but the width of the fissures is small;the longitudinal fissures account for1/3,but the width of the fissures is large.Detect the air leakage of various fissures,grasp the change law of the air leakage speed of the surface fissures with the distance from the fissure to the working face,build the fissure air leakage model,verify the correctness of the air leakage model through surface air leakage detection and pressure monitoring,and analyze the factors affecting the fissure air leakage,and provide new ideas and theoretical basis for reducing the surface air leakage.Similar simulation experiments and PFC numerical simulations were carried out to study the dynamic development of air leakage fissures in the goaf during the mining of coal seams.The rock layer is broken and the vertical air leakage fissures rapidly develop upwards when periodic weighting.During the period of pressure,the separation layer air leakage fissures are mainly developed,and the development height is basically unchanged.Under the influence of the mining of the lower coal seam,the vertical air leakage fissures in the overlying goaf developed twice,and the vertical air leakage fissures of each rock layer penetrated each other and the width increased.The evolution law of the number of air leakage fissures,the characteristics of the temporal and spatial distribution of the air leakage fissures are revealed;during the upper coal seam mining process,the number of fissures increases linearly with the advancement of the working face;in the process of the lower coal seam,the number of fissures approximately increases exponentially;after the coal seam is mined,the air leakage fissures are mainly distributed within the first breaking step of the open cut and the stop line,and the fissures in the middle of the goaf are compacted and closed.The change law of the broken bulking coefficient of the goaf was obtained,and the porosity distribution characteristics of the goaf were mastered.After the coal seam is mined,the change patterns of the breaking broken bulking coefficient and the porosity of the goaf are basically similar.The porosity and breaking bulking coefficient on the side of the open-off cut and stop mining line are larger,and the middle part of the goaf is smaller;the closer to the coal seam in the vertical direction,the greater the porosity and breaking bulking coefficient.Under the influence of the mining of the lower coal seam,the porosity and breaking bulking coefficient on both sides of the overlying goaf increased by about 2 times,and the permeability increased by 3.41～4.05 times;the porosity and breaking expansion coefficient in the middle of the goaf increased slightly,and the permeability increased by 1.19～1.55 times,the increase of the permeability of the overlying goaf indicates that the gas flow resistance in the goaf is smaller.Based on the porosity and fissure distribution of the goaf,and the law of rock movement,a discrete fracture-pore model of the composite goaf of a shallow buried close distance coal seam group is established,and the coal and rock mass parameters of the working face are substituted to verify the correctness of the model,which establishes a physical model basis for the simulation of coal spontaneous combustion in the composite goaf.A model of coal spontaneous combustion in the composite goaf of shallow buried close distance coal seam group was established,revealing the characteristics of gas（CO,CO₂）concentration,wind speed and temperature distribution in the composite goaf.The simulation results show that in the goaf of the lower coal seam,from the air inlet side to the return air side,from the working face to the deep part of the goaf,the oxygen concentration gradually decreases,and under the influence of air leakage in the overlying goaf,there is a low-oxygen area at the top of the return air side in the goaf of the lower coal seam.In the overlying goaf,the oxygen concentration near the fissures in the working face is higher,and the oxygen concentration in the deep and surrounding areas of the goaf is lower.The overlying goaf is far from the source of air leakage,and the wind speed near the working face is lower than the wind speed in the goaf of the lower coal seam,but the wind speed inside the goaf decreases slowly,and the wind speed in the deep part of the goaf is higher than the wind speed in the goaf of the lower coal seam.Based on the simulation results,the lower limit oxygen concentration and upper limit air leakage intensity of the compound goaf are used to determine the range of the coal spontaneous combustion dangerous zone.In the lower coal seam goaf,the maximum width of the coal spontaneous combustion dangerous zone is on the inlet side,130.61～421.67m away from the working face;the overlying goaf,the width of the coal spontaneous combustion dangerous azone corresponding to the interlayer fissures on the inlet side is the largest,and the distance from the working face is 0～412.67m.Based on the previous research results and the site situation,the internal and external causes of coal spontaneous combustion in the composite goaf of shallow buried close distance coal seam group were clarified.The technologies of plugging and equalizing pressure,covering oxygen isolation and inerting oxygen reduction were put forward,and a coal spontaneous combustion prevention technology system was constructed in the composite goaf of shallow buried and close coal seams.Field application shows that the technical system can reduce surface air leakage,improve the stability and disaster resistance of the ventilation system of the working face;fill the voids in the overlying goaf,reduce the space for oxygen;cover the residual coal in the goaf of the lower coal seam,absorb heat and cool down;inerting the goaf and reducing the oxygen concentration,effectively ensure the safe and efficient mining of the mine,and has broad application prospects in the western mining area of shallow coal seam group.`The paper has 111 pictures,16 tables,and 220 references.