Study On The Mechanism Of Coal Spontaneous Combustion And The Disturbance Effects Induced By The Upper Tunnel Gas Extraction

A numeber of mines are symbiotic disasters with high gas and spontaneous combustion in China,in order to solve the problem of the gas emission and overrun in upper corner,the upper tunnel gas extraction as one of the pressure relief gas extraction technologies is widely used.But because of the upper tunnel gas concentration low,there is a high concentration of oxygen in the upper tunnel,which may cause the air leakage expanding,and introduce the hidden danger of coal spontaneous combustion.Thus,although the upper tunnel gas extraction can solve the problem of gas disasters,but it also can bring out the problem of air leakage and spontaneous combustion of coal in gob.Therefore,in view of the problem of spontaneous combustion of coal under the upper tunnel gas extraction,this paper starting from the coal depositional environment in gob,and combining with theoretical analysis,in-situ measurement,experimental analysis and numerical simulation,the environment of spontaneous combustion of coal,the overlying strata caving and the fracture characteristics of upper tunel,the characteristics of stress recovery in gob,the permeability parameter evolution of compacted broken coal,the disaster mechanism of spontaneous combustion of coal,the disturbance effects of the air leakage and the oxidation zone with gas extraction amount and layout parameters of the upper tunnel,and the reasonable gas extraction amount were studied.Through the above researches,corresponding progress have been made in the following aspects:1)Combinating with the characteristics of stress recovery and gas-liquid-solid environment in gob,the relationship between stress field and spontaneous combustion of coal was analyzed,and the mechanism of action of the the stress field to the coal self-heating was identified.According to the spatial location of upper tunnel and the mode of ventilation,the air leakage flow pattern,passage existence mode and the air leakage power structure were analyzed.Basing on the internal vertical air leakage and temperature gradient in gob,the endogenous hot wind pressure theory and the mathematical model were established,and the air leakage environment was identifiedy.2)Using FLAC3 D and rock mass mechanics parameters inversion,according with the verification of the reliability of simulation,the overlying strata caving state,fracture height,the sectional area shrinkage rate of the upper tunnel were studied,and the breaking distance and breaking caving characteristics of the upper tunnel were determined.The air leakage formation mechanism induced by upper tunnel gas extraction was identified.According to the caving evolution characteristics of the upper tunnel,finding that the upper tunnel linked up the working face by the caved roof,and developed a built-in air leakage passage.The air leakage was segmented into lower air leakage and upper air leakage,which stereoscopicly flow in gob and had the characteristics of "a source two sink".3)Using FLAC3 D and double yield constitutive relation,the characteristics of the subsidence strata stress and stress state were analysised under different excavation distance along the working face advanced,combining with the advance support stress and the displacement of roadway roof monitoring,the stress recovery distance and the floor stress recovery curve were determined,and the pressure state of broken coal in gob was identificatied.4)Using independent development seepage test device,the deformation and seepage experiments of the compacted broken coal were carried out.The evolution characteristics of the broken coal's bulking coefficient,the porosity and permeability were obtained under loading.On the basis of the function relation between the bulking coefficient and vertical stress,and combinating with the stress recovery curve in gob,the bulking coefficient,porosity and permeability distribution model was established,which can provide a new setting method of the permeability parameters in gob.5)Combined with the air leakage formation mechanism of upper tunnel and the multiple field coupling theory of coal spontaneous combustion in gob,the mechanism of coal spontaneous combustion induced by the upper tunnel gas extraction was established.The high and low positon air leakage were coexisted,once coal-oxygen compounded in gob,then the low temperature of the coal oxidation occurrence and development were further prompted because of the the heat storaged and temperature rised,eventually,which lead to the occurrence of coal spontaneous combustion.6)The effects of air leakage and the spontaneous combustion area with different gas extraction volume,the vertical height and the inner distance were comprehensively analyzed.The function relationship between the leakage air volume and the gas extraction volume was determined,and the characteristics of the influences of inducting air leakage was made sure.The disturbance effects of the oxidation zone induced by the volume of gas extraction were mastered.The disturbance effects of the gas emission and the oxidation zone under different vertical height and inner distance were been clear.7)Through the characteristics of ventilation air methane drainage,and the change law of the air leakage and the oxidation zone with the increaseing of the gas extraction volume,the relation between gas extraction volume and the spontaneous combustion of coal was analysised,and the contradiction between gas extraction and spontaneous combustion of coal was determined.The theory model of the reasonable gas extraction volume was built with the safety permission concentration of the ventilation air methane and the acceptable condition of oxidation area in gob.Through the above studies,the mechanism of spontaneous combustion of coal and the disturbance effects which induces by the upper tunnel gas extraction are identified.Meanwhile,basing on the collaborative symbiosis disaster of gas and spontaneous combustion of prevention and controlling,the theory model of reasonable gas extraction volume is established,which provides a reference basis for setting up the reasonable extraction amoun and coordinating the contradiction between gas extraction and spontaneous combustion of coal.