Low Molecular Weight Compounds Contained in Coal and the Behaviors of Methane Adsorbed by Coal at Isothermal High-Pressure Condition

Abstract

Coal seam mehtane deposit at depth has been paid attention to. With the theory of coal structure, techniques of isothermal high-pressure adsorption and the method of Soxhlet solvent extraction, the behaviors of methane adsorbed by coal sample and its residuals were studied. At the condition of the isothermal and high methane pressure, the results of adsorption experiment show that when the methane pressure is continuously increased, the capability of methane absorbed by Pingdingshan fat coal sample gradually increases, reach to a maximum, then decrease, this phenomenon do not accord with the Langmuir’s model, but the capability of methane absorbed by Jiaozuo anthracite sample and Pingdingshan fat coal’s residuals with tetrahydrofuran gradually increase to a fixed value, and is not changed any longer. Solvent extraction of coal indicate that low molecular weight compounds contained in coal have three modes of occurrence, including free, absorbent and co-soluble mode. We can infer to conclusion: (1) low molecular weight compounds contained in coal affect greatly on the behaviors of methane adsorbed by coal at isothermal high-pressure condition, (2) low molecular weight compounds contained in fat coal is high in content, and the free mode of them are forced to remove and displace the potential space of methane molecule occupancy, the capability of methane absorbed by coal is reduced at condition of methane pressure above certain value. And it is concluded that at fat coal rank, methane content gradually increases, reach to a maximum, then decrease, but at rank of the anthracite, it gradually increases to a fixed value, and is not changed any longer with the increase of buried deepness and confining pressure in coal seam.