| Gas control in goaf is the top priority of coal safety work,and the law of gas migration determines the method of gas extraction control.In dealing with the problem of over-limit gas concentration in working face,a combination of various extraction methods is usually adopted to ensure the safety of working face.However,gas extraction has a great influence on the internal flow field of goaf.Different extraction methods will have different changes in the internal flow field of goaf,and the flow field can affect the distribution law of gas.Therefore,mastering the law of gas migration in goaf under the condition of joint extraction can put forward more reasonable suggestions on gas control.On the basis of previous research results,by analyzing the failure law of overlying strata in the goaf,the goaf is divided into regional types,and the characteristics of porosity in each region of the goaf are analyzed,the boundary conditions and parameters of the goaf are determined,the amount of gas gusher in the goaf is determined,and the occurrence state and movement form of gas in the coal seam are analyzed according to the characteristics of porous media.The mathematical model of goaf is established by seepage control equation.Based on the actual situation of Zhu Xianzhuang Ⅱ1055 working face,the influence of different extraction methods on goaf gas was studied by using Fluent software.(1)In the numerical simulation of goaf without pumping conditions,vortex will be generated in the lower corner of air inlet diversion.The gas enrichment area of goaf is in the fracture zone and the compacting stability zone,among which the gas concentration at the upper corner and that at the return air inlet are 5% and 3.7% respectively,seriously exceeding the limit.Therefore,corresponding pumping measures should be taken to reduce the gas concentration and ensure operation safety.(2)Numerical simulation of goaf under the conditions of surface drilling and extraction,gas concentration of working face can be effectively reduced due to the interception of pressure relief gas by surface drilling.With the increase of distance from the working face,the gas concentration at the return air side of the goaf presents the distribution form of decreasing--increasing--decreasing--increasing.The gas concentration in the goaf gas rich area is more than 52% and 85% in the middle of two surface drilling positions and after200 m deep in the goaf.At this time,the gas concentration in the upper corner and the gas concentration in the return air lane are 1.2% and 0.56% respectively,and the gas concentration in the upper corner is still over the limit.With the increase of drilling spacing,the gas concentration between drilling holes will gradually increase with the increase of drilling spacing,and the optimal spacing is about 100 m between drilling holes.(3)Numerical simulation of goaf under extraction condition of high directional long borehole shows that pressure relief gas in goaf is intercepted by negative pressure of extraction of directional long borehole.A low-concentration gas area will appear under the directional drilling hole near the extraction port.With the increase of the vertical distance of the goaf,the low-concentration gas area becomes smaller and smaller.At this time,the gas enrichment in the goaf is mainly 150 m away from the air inlet side to the working face,and the average gas concentration is 75%.At this time,the gas concentration in the upper corner and the gas concentration in the return air lane are 1.52% and 0.62% respectively,and the gas concentration in the upper corner is still over the limit.With the increase of high directional drilling horizon,the concentration of gas in goaf decreases first and then increases.The optimal extraction height of borehole is 25 m.(4)The numerical simulation of goaf under the condition of joint extraction by surface drilling,directional drilling and buried pipe in goaf shows that the gas rich area in the fractured zone will not change significantly.Through surface drilling,directional drilling and goaf buried pipe combined extraction,the gas concentration in the upper corner and the return air roadway is 0.72%,0.35% and 0.83%,0.49%,respectively.However,due to the influence of coal seam mining,the amount of gas emission will be abnormal,so it is still possible to make the gas concentration in the upper corner exceed the limit.(5)In the numerical simulation of goaf under the condition of the combined extraction of the three methods,the flow field on the return air side changes dramatically under the influence of the combined extraction of the three methods.The highest concentration of gas in the goaf is only 50%,the area of low concentration gas at the extraction opening of directional drilling is larger,and the area of high concentration gas between surface drilling disappears.The gas enrichment area in the goaf under the combined extraction of the three methods is also 150 m away from the working face from the inlet side,but the average gas concentration is only 40%,and the gas concentration in the upper corner and the return air roadway is reduced to 0.3% and 0.17% respectively,which can ensure the safe mining of the working face.Through the field investigation of 1055 working face of Zhu Xianzhuang Ⅱ,through the analysis of the extraction effect of surface drilling,directional drilling and goaf buried pipe,it is concluded that the surface drilling and directional drilling are used as high concentration gas extraction area,the extraction concentration is more than 70% and 50%respectively,and the buried pipe in goaf is used as low concentration gas extraction area,the extraction concentration is 2%.The average gas concentration in the upper corner and the return air roadway is reduced to 0.25% and 0.15% respectively,which is little different from the simulation results of 0.3% and 0.17%,which can well solve the problem of gas overlimit and provide guarantee for the safe mining of the working face.Figure [72] table [9] reference [58]... |
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