Small molecules from multistep extraction of coal and their effects on coal adsorption of CH4

Abstract The influence mechanism of small molecules on methane adsorbed in coal has always been one of the research hotspots in coal mine gas control and coalbed methane extraction. In this paper, three solvents, n-hexane, benzene, and tetrahydrofuran (THF), were used to make multistep extractions from three different metamorphic coals in mining areas, including Buertai (B), Huaibei (H), and Yangquan (Y). The composition of the extracts had been tested by the Gas Chromatography-Mass Spectrometer (GC/MS). The pore structures of residues were characterized by low-temperature N2 and high-pressure isothermal adsorption. According to the research results, it can be shown that the types and contents of small molecules in the extracts decreased with increasing metamorphism. The ability of adsorbing methane, and changes in specific surface area and pore volume are closely related to the degree of coal metamorphism. By adding different solvents, different organic small molecules can be extracted. The multistep solvent extractions of coal will lead to the increase of porosity and the removal of small organic molecules in coal. On the one hand, the increase of porosity by extractions can increase the amount of adsorption methane on coal. On the other hand, less methane was adsorbed on coal when less small organic molecules present. The influences of extractions have the double effects in methane adsorbing on coal.