Analysis of the mechanism of temperature influencing methane adsorption in coal from perspective of adsorbed layer thickness theory

Temperature is an important factor influencing gas content in coal seam. The adsorbed layer thickness in coal has not been quantitatively analyzed under different temperatures. To probe the temperature evolvement and its effect on methane adsorbed layer thickness in coal, the adsorbed layer thickness equation is established based on thermodynamic principles. The adsorbed layer thickness increases with decreasing temperature and increasing equilibrium pressure. The low temperature can promote methane adsorption and increase the number of adsorbed layers. When the pore size is larger than the pore radius at which capillary condensation occurs, the adsorbed layer thickness will decrease as the pore radius increases. This phenomenon shows a negative exponential function. If the pore radius increases to a certain extent, the number of adsorbed layers does not change with the increasing pore radius. The number of adsorbed layers in the micropores and mesopores are greater than that in the macropores. The adsorbed methane quantity can be calculated by the pore size distribution function and adsorbed layer thickness function under different temperatures. The adsorbed layer thickness theory not only reveals the mechanism of temperature influencing methane adsorption in coal but also can be used to predict the adsorption isotherms at different temperatures and pressures if the pore size distribution is known.