Study On Prevention And Control Technology Of Toxic And Harmful Gases In Overlying Goaf Of Close Coal Seam Group

In the early mining process of some small coal kilns,there are some problems,such as low efficiency,more waste coal and serious air leakage,which are easy to cause spontaneous combustion of waste coal.When spontaneous combustion occurs in the mined-out area of small coal kilns in close distance coal seam group,harmful gas in the fire area is very likely to pour into the coal-face through cracks under the action of air pressure.Aiming at the possible leakage of toxic and harmful gases in the mined-out area of overlying goaf on the 39105 working face of Yuanbaowan Coal Mine,this thesis adopts the research methods of investigation,theoretical analysis,experimental testing and numerical simulation to study the spontaneous combustion characteristics of overlying coal seam,the law of crack development in the mining process of 39105 working face and the law of gas migration in composite goaf.It is proposed to adopt pressure equalization ventilation to prevent toxic and harmful gases from leaking down,determine the reasonable pressure boost value of working face and optimize the pressure equalization parameters,and finally verify the implementation effect.The main conclusions are as follows:（1）The characteristic parameters of spontaneous combustion of No.4 coal seam in Yuanbaowan Mine,such as spontaneous combustion tendency,ignition period and indicator gas,are expounded.The test results show that coal No.4 is a type II spontaneous combustion coal seam.The shortest spontaneous ignition period was 76days.In the process of spontaneous combustion,CO can be used as the main index and C₂H₄,C₃H₈,C₂H₂ and C₂H₄/C₂H₆ as the auxiliary indexes to judge the degree of spontaneous combustion of coal.（2）Based on the actual situation of Yuanbaowan Mine,FLAC^3D software is used to simulate the variation law of plasticity,stress and displacement of overlying rock in the mining process of 39105 working face.The results show that the height of caving zone and fracture zone in goaf is 22.3m and 51m,respectively.In the process of working face mining,the goaf of 39105 working face is connected with the goaf of 4#coal seam overlying,forming a composite goaf.When the working face is drawn up to 200m,the central area of the goaf is compacted again,and the air leakage channel between the two goaf areas is mainly concentrated in the loose accumulation area.Based on the variation law of subsidence of overlying strata in goaf,the distribution law of porosity and permeability in composite goaf is obtained.（3）Fluent software was used to simulate the migration law of harmful gas before pressure equalization and under different pressure boost conditions.The simulation results show that the pressure in the upper corner of the working surface is lower than that in the overlying goaf before the implementation of pressure equalization ventilation,and CO intrusions into the upper corner through cracks,with a peak concentration of 94ppm.After the implementation of pressure equalization ventilation,the pressure in the upper corner gradually increases.When the pressure of the working face increases by 101Pa,the CO concentration of the working face can be guaranteed to be in the safe range.However,with the increase of working face pressure,the amount of oxygen leaking into the overlying goaf increases,which increases the risk of spontaneous combustion in the overlying goaf.When the pressure boost value is 265Pa,it can ensure that the overlying goaf will not occur spontaneous combustion.Therefore,according to the simulation results,the pressure boost interval of the working face is determined to be 101Pa～265Pa.（4）A reasonable pressure equalization scheme is developed and the pressure equalization parameters are optimized.The combined pressure equalizing mode of"fan and window"is selected.The fan model is FBDNo8.0（2×75k W）.It is calculated that the required air volume of the working face is 1058m³/min,and the pressure boost interval of the working face is 98.72Pa～297.48Pa.Considering the factors of prevention and control of toxic and harmful gases and fire prevention,the pressure boost range of the working face is finally determined to be 101Pa～265Pa,and the adjustable area of the wind window is 0.99m²～1.54m².The safety technical measures during the implementation of pressure equalization ventilation are formulated.（5）The effect of the pressure equalization scheme is verified.The results show that the pressure equalization measures can effectively solve the problem of toxic and harmful gas emission in the overlying goaf.The concentration of CO in the upper corner was always lower than 7ppm during production,and the concentration of CH₄,CO₂ and other gases are within the safe range.This thesis has a total of 48 drawings,20 tables,75 references.