Study On Characteristics Of Low Temperature Oxidation Of Air-dried Coal Soaked In Water

The coal seams in western mining areas,such as Shendong mining area,have the characteristics of easy self-ignition,shallow burial depth,close distance and thin bedrock.The upper coal seams of many mines have been mined or soon,and have begun to mine the lower coal seams.It put the residual coal of goaf in a constant state of water soaking as the surface water and groundwater leaks into the goaf of upper coal seams along the mining-induced fracture.The physical and chemical properties of the coal soaked and swelled in water are bound to change after air-dried and dehydration processes,with significant difference of low temperature oxidation characteristics compared with the untreated coal.The low temperature oxidation characteristics of soaked coal have not yet been systematic studied after air-dried and dehydration,and it restricts the recognition to the early warning indicator gases and self-ignition time of coal spontaneous combustion.It is of great scientific significance to sudy on characteristics of low temperature oxidation of air-dried coal soaked in water for the coal spontaneous combustion prevention of shallow close distance seams.The paper select different air-dried coals soaked in water as the study object,including different coal ranks、different particle sizes and different soaking time.The surface pore structure features of air-dried coal soaked in water are studied by the scanning electron microscopy(SEM),the study result shows that the air-dried coal has more developed surface pore structures than untreated coal.The change characteristics of free radicals of air-dried coal soaked in water are studied by the electron spin resonance( ESR) during low temperature oxidation process,the study result shows that the longer the soaking time, the higher the free radicals concentration of air-dried coal.The free radicals concentration of air-dried coal soaked in water has an increasing trend over temperature.Besides the same soaking time,the bigger the particle size,the greater the concentration of the free radicals concentration and g-value of air-dried coal.With the rise of the coal rank,the free radicals concentration of air-dried coal increases and g-value decreases.The gaseous product change of air-dried coal soaked in water are studied by the test system of coal spontaneous combustion characteristic during low temperature oxidation,the study result shows that the CO production quantity of air-dried coal is greater than the untreated coal,as well as the greater oxygen consumption rate and the lower crossing point temperature.In a certain range of soaking time,the longer the soaking time of air-dried coal,the greater the CO production quantity and oxygen consumption rate,as well as the lower crossing point temperature.In a certain range of particle size,the bigger the particle size,the greater the change of CO production quantity and crossing point temperature between the air-dried coal and untreated coal.The change rules of apparent activation energy of air-dried coal soaked in water are studied by the test method of critical temperature of coal spontaneous combustion based on the temperature programmed experiment during the rapid temperature-rising period,the study result shows that the apparent activation energy of air-dried coal is lower than the untreated coal.The longer the soaking time,the lower the apparent activation energy of air-dried coal.The bigger the particle size,the greater the change of apparent activation energy between the air-dried coal and untreated coal.The spontaneous combustion tendency of air-dried coal soaked in water are studied by the test method of oxidation kinetics for the propensity of coal to spontaneous combustion,the study result shows that the spontaneous combustion tendency is higher compared with the untreated coal.The longer the soaking time of air-dried coal,the higher the spontaneous combustion tendency.The bigger the particle size,the greater the change of spontaneous combustion tendency between the air-dried coal and untreated coal.