Effects of Organic Micromolecules in coal on its Pore Structure and Gas Diffusion Characteristics

Abstract

Coal has complex pore structures including micro-, meso-, and macropores and cracks, and contains organic micromolecules. To in-depth study the effects of organic micromolecules in coal on the pore structure and gas diffusion characteristics, organic micromolecules were extracted by tetrahydrofuran using microwave-assisted method from anthracite and bituminous coal samples to obtain the residual coal samples at 50 \(^{\circ }\)C and atmospheric pressure. Changes in raw and residual coal samples were explored using a series of methane desorption and low-temperature nitrogen adsorption experiments and their pore structure parameters were compared using samples with same granule size (0.180–0.250 mm). The results showed that (1) although the micropores of both raw and residual coal granules are characterized by fractal, residual coal has lower fractal dimension than raw coal; (2) residual coal samples have higher gas emission amount and rate than the raw coal samples and lower gas diffusion resistance, indicating that gas in residual coal is easier to flow from micropores through mesopores into macropores and cracks. Based on the fractal theory and the diffusion model, extraction of organic micromolecules increased the number and diameter of pores and cracks and dredged the rawly clogged pores and cracks, all of which decreased the resistance of gas diffusion in the coal matrix. In addition, extraction of organic micromolecules has the most obvious effect on diffusion pores. After extraction, the proportion of meso-/macropores increased, while that of micropores reduced, thereby reducing the resistance of gas flowing from micropores through mesopores into macropores and cracks, and subsequently increasing the amount and rate of gas emission. The study is of great significance for pushing forward the boundary of our recognition of the influences exerted by micromolecules on gas diffusion.