Experimental Study On Oxidative Spontaneous Combustion Characteristics Of Loaded Crushed Coal

At present,the shallow coal resources in coal mining are nearly exhausted,and have begun to enter the status of deep mining.With the increase of coal mining depth,the degree of coal and rock fragmentation increases.The more obvious the influence of geotemperature and geostress is,the more prominent the problem of coal spontaneous combustion will be.In order to compensate for the lack of considering the axial stress loading in the research results of coal spontaneous combustion at present,a testing device for the permeability evolution and spontaneous combustion characteristics of confined crushed coal body was designed and developed.The device was combined with GC-4000A series gas chromatograph to simulate the change characteristics and response mechanism of voidage and permeability of coal body under different stress states and surrounding rock temperature,and under the coupling effect of stress and temperature.The research shows that the voidage and permeability of coal body decrease during loading,which results in the decrease of air leakage and oxygen supply capacity,thus inhibiting the spontaneous combustion of coal;and the decrease of voidage will lead to the increase of thermal conductivity and promote the spontaneous combustion of coal.The combination of the two trends leads to the most severe coal-oxygen complex reaction at the same temperature when the axial stress is 9MPa.The increase of the axial stress promotes the spontaneous combustion of coal first and then inhibits it.The characteristics of spontaneous combustion and oxidation of coal reveal the cause of formation of differential oxidation of crushed coal under confined pressure.Through the above research,the following main conclusions are obtained:1.At the beginning of the experiment,with the increase of temperature,the flow rate of outlet gas decreases linearly.When the temperature reaches 120?,small water droplets with condensation are found in the outlet,resulting in a sharp drop in the flow rate of outlet gas.When the temperature reaches a certain value,the experimental temperature continues to rise,and the flow rate of outlet gas basically does not change.With the increase of temperature,the oxygen concentration did not change before the critical temperature?80??,and began to decrease continuously after reaching the critical temperature?80??.When the axial pressure is 9MPa,the oxygen concentration at the outlet is the smallest,which indicates that the oxidation degree is the greatest at9MPa.With the increase of temperature,the oxidation temperature rises to the oxygen-poor state,and the oxygen concentration at the outlet basically does not change.2 During the spontaneous combustion of coal sample under pressure,the production of CO and CO₂ increases with the increase of temperature,and increases first and then decreases with the increase of axial stress.At the beginning of the experiment,before the coal sample temperature reaches the critical temperature,the production of CO and CO₂ increases approximately linearly;after reaching the critical temperature,the production of CO and CO₂ increases almost exponentially.With the increase of the axial stress,the production of CO and CO₂ gas increases first and then decreases.When the axial stress is 9MPa,the production of CO and CO₂ gas is the largest.And the temperature of CO₂ gas is earlier than that of CO gas entering exponential growth stage.3 CH₄ gas is produced during the whole experiment.Before the critical temperature,the production of CH₄ gas increases linearly under different axial stress conditions.With the increase of axial stress,the production of CH₄ gas changes in a parabolic form.The maximum amount of CH₄ gas is generated when the axial stress is9MPa.The temperature of C₂H₆ gas in non-pressurized state is 60?,and that of C₂H₄gas is 100?.With the increase of axial stress,the temperature point of C₂H₆ gas appears ahead of time,and the temperature of C₂H₄ gas appears at 100?without pressure.However,with the increase of axial stress,the temperature point of C2H6 gas appears ahead of time and then lags behind.4.The difference between the oxidation state and the non-oxidation state permeability was found.During the axial stress loading process,the permeability under non-oxidation conditions showed a negative exponential change,and the permeability during the temperature rise oxidation process showed a linear decrease.The experimental results show that under the action of compaction,the coal body is broken twice to form smaller particles to plug the effective voids,which leads to the decrease of the voids and permeability of the coal body.The decrease of voidage leads to the increase of thermal conductivity of coal,and the formation of free radicals in coal is easier,which is conducive to the occurrence of coal-oxygen complex.The decrease of permeability makes the heat generated by coal oxidation more easily accumulated,the oxygen consumption and exothermic intensity increase in a certain range,and the oxidation process of coal accelerates,which promotes the occurrence of coal spontaneous combustion.With further compaction,the porosity and permeability continue to decrease,and the increase of thermal conductivity leads to the increase of heat release intensity and oxygen consumption rate,which requires more oxygen consumption.Coal oxidation process enters the oxygen-poor state,thus inhibiting the occurrence of spontaneous combustion of coal.This paper reveals the cause of formation of oxidation difference of pressure crushed coal,and provides a reference basis for guiding fire prevention and extinguishing in goaf.