Study On The Mechanical-electrical Signature And Ignition Properties Of Coal Mine Goaf Roof

Due to the complexity of the goaf,prevention and control of gas explosions in the goaf and working face has always been the focus and difficulty of coal mine safety.the containment of gas explosion accidents in goaf is of great significance for ensuring China's coal energy security and further fundamental improvement in coal mine safety.The types and signature of ignition sources that cause gas explosions in coal mine goaf have not been well understood due to the complexity and concealment of the goaf as well as the lack of targeted prevention and control work,which results in the goaf becoming the main location of major and serious coal mine gas explosion accidents at home and abroad in recent years.Many fire and explosion cases are attributed to unidentifiable fire sources,and the hidden and unexplained ignition source has become the difficulty of the research.This paper proposes that the electrical effect of roof deformation and fracture under the action of crustal stress is a new disaster generating mechanism of goaf gas explosion,which reveals the ignition characteristics from the perspective of electrical effects.Hence,this paper constructs mechanical–electrical characteristic test system.Study the characteristics and mechanism of electricity generation,discharge,and gas disaster in the process of deformation and fracture of different rocks by physical experiment,numerical simulation,theoretical analysis and combined actual cases.The main results and conclusions achieved are as follows:First,by constructing a mechanical-electrical experiment system,the variation rules of current and voltage of different rock samples under different loading rates and methods were studied,and the electrical characteristics of roof sandstone were revealed.The results show that the higher the loading rate is,the higher the average charge release rate will be,and the loading rate is the decisive factor of the free charge release rate.However,the peak current and voltage formed at the moment of rock sample failure are not proportional to the loading rate,which is related to the compressive strength of rock sample.The piezoelectric effect of rock samples enhances the electrodynamic sensitivity of quartz rocks and shows stronger electrical characteristics.Second,the mineral composition and grain size of different rock samples were analyzed by X-ray diffraction(XRD),and the microscopic pore structure characteristics of rock samples were analyzed by scanning electron microscopy(SEM)and nuclear magnetic resonance(NMR).The results show that both roof sandstone and granite contain a large amount of quartz,while marble does not.The average rate of electricity generation in rocks increases with the increase of quartz grain size.Although the quartz content of granite is lower than that of sandstone,its average rate of electricity generation is higher.The pore size distribution of different types of rocks is different,and the generation of micropore size can determine the charge generation rate of rock samples.The changes in the current and voltage of rocks in the whole process of loading can be summarized as the result of the synergistic effect of piezoelectricity-cracking to produce electricity,or it can be regarded as the synergistic effect of chemical physics to produce electricity,which is the macroscopic manifestation of the two kinds of effects.Again,the discharge characteristics of different rock samples were studied by constructing an experimental system to collect the charge release during the fracture process of rock samples.The results show that the discharge voltage value of the rock sample is small before the main fracture,whether it is loaded at a low speed or at a high speed.When the rock fracture fails,a transient,discontinuous and surging discharge voltage will be generated.Based on the crack propagation,the tip discharge mechanism of roof sandstone is proposed.The field intensity formed by the tip discharge and the sudden disappearance of the piezoelectric effect due to the fracture of quartz crystal produce extremely high field intensity,which is much higher than the breakdown strength of air after calculationThen,a high-speed camera was used to record the morphological characteristics and the law of electric spark generation in the deformation and fracture process of different rock samples under load.The results show that there are dust clouds in the process of rock sample destruction,and the fracture process of rock sample is explosive.The generation of spark is not only related to the quartz content of rock sample,but also to the compressive strength of rock sample.During the experiment,the flash produced by rock fracturing is not the light generated by friction heat,but an electric spark,and the piezoelectric effect of rock becomes the key factor for the intensity of electric spark or whether it can be generated.It is proposed that the generation of electric spark is caused by the ionization of air by the collision of electrons,which is enough to ionize the combustible gas and cause gas explosion,and the addition of water increases the possibility of gas explosion.In this paper,the characteristics of coal seam roof disaster caused by discharge are presented.Finally,the rule of gas disaster caused by roof electric effect is discussed.Based on the accident case and the characteristics of the occurrence and development of the accident site,the author analyzes the generation of the signature gas of coal spontaneous combustion and compares the environmental and historical data of coal spontaneous combustion,so as to exclude the possibility of coal spontaneous combustion as the ignition source.Using FLAC software to simulate the mining process of the space-time evolution of stress field.Based on the mechanical and electrical characteristics of roof rock,the paper puts forward the scenario of roof rupture generating electric spark in goaf,and the premixed gas air gathered in the goaf is broken through and ionized,thus igniting the gas and forming a gas explosion.Finally,the residual coal in the goaf is ignited,and the residual coal becomes a continuous source of fire,resulting in multiple explosions in the goaf.The results in this paper provide new ideas for the correct understanding of the gas explosion in goaf,and confirm its scientificity,which is of great significance for further improving the safety level of coal mines and curbing the occurrence of gas explosion in coal mines.