Law Of Gas Exchange Between Sealed Goaf And Outside Space,and Application Of Gas Explosion Prevention Theory

Recently,coal mines in China mainly adopt high-strength mining methods such as fully mechanized mining and fully mechanized caving.The advancing speed of working face is fast,and the forming speed of large-area sealed goaf is also fast.As time goes by,the gas reserves in the sealed goaf will become larger and larger.Because persistently influenced by the in-situ stress and mining operation,the sealing quality of the protective pillars and sealing facilities will be reduced.When the external environment conditions of the goaf change,there will occur different degrees of "breathing" phenomenon at near the sealing facilities.When the breathing phenomenon is serious,it will affect the safety production of adjacent mining space.Therefore,it is of great guiding significance and engineering value to grasp the characteristics and mechanism of "breathing" phenomenon in sealed goaf under different surface atmospheric parameters and to design a safe and feasible control technical scheme for preventing abnormal gas emission and gas exceeding at ouside of sealed goaf.The mechanism of "breathing" phenomenon is theoretically analyzed based on the ventilation energy equation and state equation of ideal-gas,and the expression of relationship between differential pressure and gas concentration is obtained with Darcy law.The results show that,"breathing" phenomenon results from the combining action of ground atmospheric parameters,airflow parameters,roadway characteristics and gas state parameters of sealed goaf.Under the continuous action of the pressure difference between seal's inside and outside,gas exchange will occur between sealed goaf and outside space.The basic characteristics of "breathing" phenomenon of sealed goaf are studied in situ.The gas pressure in sealed goaf is gradually decreasing or increasing as the differential pressure is positive or negative.The gas pressure in sealed goaf has small change,the fluctuation of static pressure of the seal's outside is the dominant factor that results in positive and negative alternation of differential pressure.The variation trend of the static pressure of the seal's outside is similar to that of the ground atmospheric pressure and opposite to that of the ground atmospheric temperature when roadway characteristics and air quantity are stable.On the premise that the seal's permeability and the air quantity of seal's outside are relatively stable,the gas concentration is positively correlated with the differential pressure when the positive differential pressure appears,so that,the change trend of gas concentration is similar to that of differential pressure.The "breathing" phenomenon of sealed goaf was reproduced based on the self-built experimental system.The effects of atmospheric parameters at shaft head,air resistance of roadway,static pressure of fan and ambient temperature around sealed space on the pressure difference between seal's inside and outside are studied.Experimental results show that:(1)on the premise that ventilation condition kept unchanged,the static pressure of seal's outside changes synchronously with atmospheric pressure at shaft head,and the change trend of gas pressure in sealed volume is similar to that of ambient temperature.In the period when the gas pressure in sealed volume is relatively stable,the change trend of pressure difference between seal's inside and outside is opposite to that of the static pressure of seal's outside.In the period when the static pressure of seal's outside is relatively stable,the pressure difference is sensitive to the gas pressure of seal's inside,and the change trend of both is basically similar.(2)The change of ventilation state will cause sudden change of pressure difference between seal's inside and outside in a short time.In the ventilation system with 1 inlet shaft and 1 return shaft,reducing the air resistance of main shaft,increasing the air resistance of return shaft and increasing the static pressure of the fan will cause a large increase of static pressure at seal's outside,while the gas pressure of sealed volume is basically stable in a short time,so that,the pressure difference between seal's inside and outside will decrease significantly.In the ventilation system with 2 inlets shaft and 1 return shaft,reducing the air resistance of auxiliary shaft will cause a sudden decrease of static pressure at seal's outside,while the pressure in sealed volume is basically stable in a short time,so that,the pressure difference between seal's inside and outside will increase suddenly.(3)Reducing or increasing the amount of gas in the model sealed volume will cause the air pressure of seal's volume to decrease or increase,and the pressure difference between seal's inside and outside will decrease or increase accordingly.The influence of leakage coefficient on the concentration of methane and oxygen in sealed goaf is analyzed by using the mathematical model of the concentration change of sealed gas when the sealed goaf is in "exhalation" and the "inhalation" state respectively.The results show that,the methane concentration increases gradually and oxygen concentration decreases gradually when the leakage coefficient remains unchanged under the condition of "exhalation".Under the same positive pressure difference,the rising speed of methane concentration and the decreasing speed of oxygen concentration increases with the increase of leakage coefficient.Under the "inspiration" condition,when the leakage coefficient remains unchanged,the methane concentration decreases gradually and the oxygen concentration increases gradually.Under the same negative pressure difference,the decreasing speed of methane concentration and the rising speed of oxygen concentration increase with the increase of leakage coefficient.Based on the analysis results of gas composition in sealed goaf,the variation law of explosion risk coefficient under different leakage coefficients in the process of"breathing" is studied based on the principle of Coward method and explosion risk coefficient.The results show that the moving speed of state point and the change speed of explosion risk coefficient increase with the increase of the leakage coefficient.The change of the explosion risk level in sealed goaf is constrained by the pressure difference between seal's inside and outside,the leakage coefficient and the initial position of state point.The numerical model of gas release zone is established by CFD,and the gas diffusion law in release zone is analyzed under different air flow rate of roadway and gas flow rate of pipeline.The results show that gas diffuses to the left side and roof of air return roadway under the influence of roadway air flow.When the gas flow rate in pipeline or the air volume in roadway increases,the gas diffusion range gradually enlarges along the direction of air flow.From the distribution of gas concentration on each cross section along the downwind direction of the release vent,the further away from the release pipe mouth,the smaller the gas concentration.The variation law of pressure difference between pipe's inside and outside and that of gas concentration in the releasing zone during the gas releasing period are observed on the spot.The joint control criteria is determined based on the threshold values of differential pressure and gas concentration,and the grading regulation scheme of valve opening is put forward.An automatic control system for gas release of sealed goaf is developed,it mainly consists of electronic valve,micro pressure difference sensor,low concentration methane sensor and automatic control box.The automatic powder spraying and explosion suppression system is installed on gas release pipeline,which ensures the safety and reliability of the release system.