Effects of organic micromolecules in coal on its wettability

Two types of bituminous coal were extracted with tetrahydrofuran solvent using microwave-assisted method at 50°C and atmospheric pressure. Pore structure parameters and wettability of raw coals and their residues were separately tested with nitrogen adsorption and capillary penetration method. Fourier transform infrared spectrometer was employed for analysis of surface functional groups of raw coals and their residues. Results showed remarkably degraded wettability of coal, especially gas coal, after extraction, whereas wetting height of raw coal reached more than twice that of residual coal within the same wetting time. Given the different storage characteristics of organic micromolecules in coal, pore volume of coal increased after extraction, and solvent extraction exerted different effects on pores. Pores of gas coal expanded with reduced specific surface area and improvement in hydrophobicity. Pores of coking coal increased with increasing specific surface area and degrading hydrophobicity. Organic micromolecules in coal and several side-chain groups in macromolecular skeleton structure were dissolved, reducing contents of functional groups in coal, decreasing quantities of groups, such as methyl and carbonyl, on coal surface, and modifying microcosmic surface characteristics of coal and improving hydrophobicity. Through comprehensive analysis, pore structure, and microcosmic surface characteristics of coal jointly determined its wettability.