Micro-Nanostructure of Coal and Adsorption-Diffusion Characteristics of Methane

The adsorption and diffusion characteristics of coal are important parameters for coalbed methane (CBM) extraction and mine gas control. However, the adsorption test can only obtain the apparent adsorption amount, and it cannot obtain the actual adsorption amount, which leads to a large error during the calculation of the coal diffusion coefficient. Taking the anthracite coal in the Jiulishan Mine as the research object, the micro-nanostructure and instantaneous apparent methane adsorption isotherms of the primary structure coal and tectonic coal were determined by low-temperature CO₂ adsorption, mercury intrusion and methane diffusion kinetics tests, and the instantaneous apparent adsorption isotherms of methane were corrected to the instantaneous actual adsorption isotherm by the Langmuir model. The results demonstrate that the micro-nanopore, Density Function Theory (DFT) pore volume and specific surface area values below 1-2 nm in tectonic coal are larger than those in the primary structure coal, which is the fundamental reason why the ultimate adsorption capacity of tectonic coal is larger than that of the primary structure coal. The apparent adsorption amounts of the tectonic coal and the primary structure coal reach the maximum at 8 MPa and 10 MPa, respectively. Thereafter, the instantaneous isotherms of the apparent adsorption amount decrease with increasing of gas pressure. However, the instantaneous isotherms of the actual adsorption amount tend to be stable. The diffusion coefficient undergoes a rapid decay with time under low gas pressure, and undergoes a slow decay with under the high gas pressure.