Study On Generation,Migration And Accumulation Of CO And Over-limit Prevention Technology In Working Face Of Shallow Thick Coal Seam

Most of the coal seams mined in the western mining areas of China are shallow buried and the bedrock is thin,mostly thick and extra-thick coal seams.In the process of high-intensity mining of coal seams,the mining fissures formed by the fracture and instability of the overlying rock connect the goaf and the ground surface,resulting in surface air leakage.The air leakage leads to disordered ventilation in the working face,CO in the return corner of the working face exceeds the safety limits,interferes with the prediction and early warning of residual coal spontaneous combustion in the goaf,and seriously threatens the underground safety production.Taking the Geological Occurrence and production technical conditions of Lijiahao Coal Mine as the engineering background,this study analyses the distribution law of overburden fractures,the characteristics of air leakage in the goaf,the law and source of CO generation in the goaf and the migration and accumulation law of CO in the goaf under multi-source air leakage in the mining of shallow thick coal seam through numerical simulations,experiment tests,observations and theoretical analyses.Ultimately,the prevention and control technology of CO overrun in the goaf and return corner of the shallow thick coal seam is put forward.The research results have important theoretical significance and guiding value for the prevention of CO overrun in working face and the treatment of spontaneous combustion in the goaf,which are generally faced by coal seam mining in Western mining areas.Based on the geological occurrence and production technical conditions of shallow buried and thick coal seams in the Lijiahao Coal Mine,the distribution and evolution law of mining fissures in the overlying rock in the goaf is studied by using FLAC^3D software.The results show that within 110 m of the goaf on the inlet and return sides of the 31115 working face,the mining fractures can conduct gas.Through the tracer gas air leakage test technology,the air leakage characteristics of the goaf are obtained.The surface air converges to the return corner through the mining fissure of overburden and 2-2 coal goaf,and the air leakage of the working face flows out from the return roadway through the goaf.Based on coal samples collected from Lijiahao Coal Mine,the basic parameters of coal spontaneous combustion are measured,and the spontaneous combustion tendency of coal samples is analyzed.Through the temperature-programmed experiment,the law of CO generation by oxidation of residual coal in the goaf is studied.The results show that there is an exponential growth relationship between the CO generation rate and the coal temperature,and the critical temperature for rapid oxidation of coal samples is between 70°C and 80°C.The source of CO in the working face and the proportion of CO in the return corner are analyzed.It is determined that the source of CO in the working face is mainly produced by the oxidation of residual coal in the goaf,the gas discharge in the overlying goaf,the coal cutting and crushing of shearer and the operation of trackless rubber tyred vehicle.The gas discharge in the overlying goaf and the oxidation of residual coal in the goaf account for the largest proportion of CO in the return corner,about 83%.The model of CO migration and accumulation in the goaf with multi-source air leakage is established,and the distribution law of air leakage flow field and migration and accumulation of CO in the goaf under the condition of multi-source air leakage are revealed.The research results show that under the effect of multi-source air leakage,CO from the overlying goaf and the residual coal in the goaf of this coal seam are migrated to the air return side of the goaf,resulting in the accumulation of CO in the return corner.According to the previous research results and the actual conditions of the working face,the prevention measures of CO overrun in the goaf and return corner of the working face of shallow thick coal seam are put forward.Including suppressing the oxidation of residual coal in the goaf to generate CO by proper air distribution in the working face,nitrogen injection in the goaf and spraying of the chemical suppresses,preventing gas leakage from the overlying goaf by pressure ventilation,preventing CO accumulation in return corner by hanging wind curtain,air guide curtain and extraction non-sparking fan in the working face.Field application results show that the implementation of the above technical measures can effectively prevent the CO excess in the goaf,reduce the surface air leakage,and reduce the risk of spontaneous combustion of residual coal in the goaf.