Pore structures and methane sorption characteristics of coal after extraction with tetrahydrofuran

An anthracite and three bituminous coals were extracted with tetrahydrofuran (THF) solvent at 50 °C and atmospheric pressure by microwave-assisted extraction. Low-temperature nitrogen sorption and high-pressure methane sorption isotherms on raw coals and their residues were tested, and their extracts were analyzed via the Gas Chromatography/Mass Spectrometry (GC/MS). The pore structure (specific surface area, pore size distribution, pore volume, etc.) and the gas adsorbance of coals and their residues were comparatively analyzed. The results indicate that the mesopore volumes of all residues were increased compared to the raw coal, whereas the increase or decrease of the specific surface areas and micropore volumes was correlated with their ranks. Meanwhile, the pores of raw coals and their residues were mainly distributed in the range of 3–4 nm. Although the capability of gas adsorption by coal was strengthened after solvent extraction, the ability to adsorb methane was reduced because the amount of space for the dissolution of small organic molecules where methane condensed was reduced. This behavior is caused by two aspects: (1) The enhanced ability of gas sorption due to the enlarged pore structure resulting from the solvent extraction; (2) the weakened ability of gas sorption because of space reduction for dissolving or adsorbing of methane. Therefore, after extraction, the methane sorption characteristics of the coal were jointly dependent on the enlargement of the pore structure and the reduction of small organic molecules.