Methane adsorption capacity of extracted coals under control of solvent polarity

Based upon isothermal methane adsorption on raw and solvent extracted coals, relationship between solvent polarity and methane adsorption capacity change of the coal was analyzed, and the possible geochemical mechanism was discussed from polarity of the solvents. The results show that methane isothermal adsorption curves on the extracted coals follow the Langmuir equation. Extraction with CS₂ and C₆H₆ enhances methane adsorption capacities on extracted coals, and that with tetrahydrofuran (THF) and acetone is opposite. There is a negative correlation between methane adsorption capacity change of the coal and the solvent polarity, which can be explained by similarity-intermiscibility theory. The polarities of CS₂ and C₆H₆ are weaker, which can extract more alkane and aromatic hydrocarbons with nonpolar molecule structure (-CH₃ and -CH₂-) to increase the adsorbed space of coal surface for methane adsorption. The polarities of THF and acetone are stronger, which can extract more non-hydrocarbon and asphaltene with polar molecule structure (-CHO, -OH and -COOH) to reduce the adsorbed space of coal surface for methane adsorption.