| Spontaneous combustion of goaf in coal mines is a typical disaster which seriously affects some normal mining of workface.In a variety of fire fighting methods,the effect of CO2 heavy gas inserting on goaf is the best,but there are also problems.In order to reduce the threat of CO2 leaking into the working face and reduce the cost,the method of CO2 and N2 joint injection is adopted.How to optimize the gas injection parameters to achieve a reasonable ratio and ensure the safety of CO2 application is an urgent problem to be solved.In this paper,a theoretical system of CH4-CO2-N2 multi-element inertia in stope is proposed,and the "oxygen consumption criterion number" weighted by position is introduced,which comprehensively reflects the change of oxygen concentration and coal oxidation intensity in goaf.Combining with field data,experimental research,engineering measurement and numerical simulation,the general quantitative law of gas storage inertia,CO2 or N2 inertia gas injection and inertia effect in goaf and the safety of working face are systematically studied,so as to scientifically guide the spontaneous combustion prevention and extinguishing work in goaf.Oxygen concentration in goaf is directly related to coal consumption.A comprehensive experimental platform of closed oxygen consumption and natural temperature rise was set up.Several coal samples from Hongqingliang and Xiaonan Coal Mines were tested respectively.The relationship between oxygen concentration and oxygen consumption time was measured,and the oxygen consumption rate,critical oxygen concentration for asphyxiation,and the shortest spontaneous combustion period of coal samples were calculated.The 3D CFD simulation model of goaf is established,including the spatial distribution function of the coefficient of collapse and expansion of"O" ring,the model of non-uniform gas emission source,and the boundary conditions of the model are set according to the measurement of face-airing,air leakage and the gas extraction-injection inert gas flow rate.In order to verify the correctness and applicability of the model,CFD theoretical results are consistent with the actual measurement of nitrogen injection goaf gas beam tube in Fuxin Wulong Coal Mine 3123 working face after comparison.Oxygen concentration in goaf is closely related to gas concentration in goaf and extraction parameters.Simulation results show that the relationship between oxygen consumption criterion number and gas emission intensity change is negative index,and that and gas extraction intensity is quadratic curve of "increase first and then decrease";On the premise of considering gas,various inserting schemes such as single injection of N2,CO2,N2-CO2 mixed injection,double-port single injection at the same position(hereinafter referred to as co-injection),and N2 and CO2 injection at different positions(hereinafter referred to as no co-injection)are simulated and analyzed.The results show that the oxygen consumption criterion number of goaf decreases firstly and then increases with the inert gas injection position,and decreases exponentially with the N2 and CO2 injection flow rate.The optimum position of inert gas injection is 50-80 m behind the working face,in which the inserting effect of CO2 at the same high positionN2 injection at the same low position is the best.The best ratio of N2 to CO2 flow for mixed injection is 6:4.With the increase of CO2 mixing ratio,the oxygen consumption criterion number for evaluating the spontaneous combustion risk in goaf firstly decreased sharply and then increased slowly.In order to investigate the safety of CO2 injection,the simulation study of CO2 leakage to the working face after gas injection shows that the highest CO2 concentration in the working face is not always in the upper corner of the working face;the shallower the gas injection position is,the location of the leakage danger zone moves down.When the gas injection position is less than 30 m,the dangerous area of the middle part of the working face is more than 5%(the value of the uncomfortable CO2 concentration of the human body).In the scheme of CO2 injection 10 m away from the working face,CO2 concentration higher than 5%is found in the working face,and the CO2 injection flow rate has a greater impact on the safety of the working face.When the flow rate is lower than 6 m3/min,the maximum concentration of CO2 is less than 5%.According to the research,the three factors affecting the safety of the working face by CO2 injection in the goaf are:initial concentration of CO2 injection,initial flow rate and initial position.Combining with the engineering example of 11301 working face in Hongqingliang Coal Mine,the digital model of 11301 working face is established according to the field data.The simulated oxygen concentration in goaf is close to the field measurement.The shortest spontaneous combustion period is 27 days,and the lowest safe advance of 11301 working face is 4.48 m/d.The theory of multiple inertia in goaf should be applied.When there is no gas in the goaf,the simulation results show that the minimum safety advance of working face can be reduced to 3.8 m/d when nitrogen injection is 10 m3/min,and CO2 injection is not needed at this time.The inerting scheme of the S5903 goaf in Daxing Mine with high gas emission is optimized.From the perspective of inerting effect,inerting cost and safety of working face,the injection of N2-CO2 in the gob area of Daxing Mine S5903 is finally determined.The flow rate is 1200m3/h,which is the optimal solution. |
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