| Coal and gas outburst is a major natural disaster that affects the safety of coal mine production.Studying the adsorption characteristics of coal to methane is helpful to prevent the occurrence of coal and gas outburst accidents.With the increase of mining depth,the gas in coal seam reaches the supercritical state.It is of great significance to study the adsorption characteristics of supercritical gas from high order coal for the treatment of gas in deep coal seam.In this paper,the adsorption characteristics of three kinds of coal samples in supercritical condition are studied based on the pore structure characteristics of coal.The main conclusions are as follows:(1)The experimental results of gravimetric method show that under sup-ercritical condition,the adsorption capacity that measured in the laboratory is excess adsorption capacity,which can not objectively describe the adsorption performance of deep coal seam.Therefore,it is necessary to correct the excess adsorption capacity to obtain the absolute adsorption capacity of coal samples.The absolute adsorption capacity of coal sample under supercritical condition can be calculated by linear fitting of the excess adsorption capacity and the density of free phase gas.(2)The correction results of absolute adsorption capacity show that the increase of temperature inhibits the adsorption characteristics of coal,and the increase of pressure increases the adsorption capacity of coal.However,the adsorption capacity of coal tends to be stable after reaching a certain pressure.With the increase of metamorphism degree,the specific surface area of coal increases,there are more adsorption sites,and the adsorption capacity of coal is improved.(3)The specific surface area of coal can be obtained on the basis of the saturated adsorption capacity of single molecular layer obtained by Langmuir model fitting the adsorption capacity of coal under low pressure.According to the number of adsorption layers of coal at high pressure,it is considered that the adsorption mode of methane in coal under supercritical condition is mainly single-layer adsorption,and there are multi-layer adsorption or filling adsorption in some parts.(4)The surface free energy of coal determines the adsorption capacity of coal.The increase of temperature reduces the surface free energy of coal and inhibits the adsorption.The increase of pressure and metamorphism is helpful to increase the change of surface free energy per unit area of coal matrix and improve the adsorption capacity of coal. |
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