Numerical Simulation Of Gas Distribution In Overlying Goaf In Close Coal Seam

Coal plays an important role in today's world energy structure and the frequent occurrence of energy crisis makes people pay more and more attention to rational coal mining.In the process of close coal seam mining,the overburden strata stress is destroyed,the adjacent coal seam affects each other,and the fracture becomes the main channel of oxygen infiltration.In the overlying goaf,the coal has undergone an oxidation reaction during the initial mining,accumulating a certain amount of heat,and the oxygen leaking into the overlying goaf easily increases the possibility of spontaneous combustion of the coal.Therefore,it is very meaningful to study the law of gas distribution in the overlying goaf in close coal seams.In this paper,the characteristics of coal seam as porous medium,gas migration theory of overburden goaf,fracture distribution in horizontal and vertical directions,and the characteristics and influencing factors of spontaneous combustion in close coal seam are analyzed by using fluid mechanics and molecular diffusion and mass transfer.The upper and lower goaf of the coal seam are close to each other,and influence each other.And the upper goaf has low wind speed,good heat storage condition and easy spontaneous combustion.Based on the law of conservation of mass,the law of conservation of component and the law of conservation of energy,the governing equations of seepage field and concentration field in overlying goaf are derived,and the relations between the two physical fields are analyzed.By introducing the features of the numerical simulation software and the calculation steps,the physical model of the mined-out area in the near coal seam is established,and the calculation model is reasonably simplified,and the boundary conditions and physical parameters are set up.COMSOL Multiphysics software is used to simulate the seepage field,the oxygen concentration field and the hazardous area in the goaf of the coal seam in close distance,and the gas distribution law of overlying goaf of the coal seam was obtained by setting different porosity and ventilation rate.The simulation results show that porosity of porous media and ventilation rate of air inlet in goaf will affect the gas distribution in the goaf of coal seam.When the porosity of porous media in the goaf is constant,the ventilation rate of the inlet increases,the gas flow rate around the working face and the shallow part of the goaf becomes faster,the area of high oxygen concentration in goaf becomes larger,and the risk of upper goaf increases.When the porosity is small,the gas infiltrated into the goaf is less,the gas change inside the goaf is not obvious,the oxygen concentration in the shallow part of the goaf is low,and the dangerous area is mainly distributed in the shallow part and the middle part of the goaf.When the porosity is large,there are more gas seeping into the goaf,and the gas flow around the working face changes obviously.The oxygen concentration in the shallow part of the goaf is high,the dangerous area moves towards the deep part gradually,and the risk of upper goaf is higher than that of lower goaf.In the practical engineering application,the gas distribution in the goaf overlying the coal seam should be monitored at all times,and the spontaneous combustion hazard protection of residual coal should be done well to improve the safety of mining.