Response of Methane Diffusion in Varying Degrees of Deformed Coals to Different Solvent Treatments

In the communication, we analysed four semianthracite coals with different degrees of deformation from the Huoerxinhe colliery in China that were extracted by tetrahydrofuran (THF) and carbon disulphide (CS₂), and treated with hydrochloric acid (HCl) solution. Low-temperature nitrogen adsorption, water contact angle measurement and methane diffusion of untreated coals and their residues were carried out. As well, mineral composition of untreated coals and their residues treated with HCl are performed. Overall, compared with untreated coals, specific surface area has increased tendency after THF and CS₂ extraction due to the removal of the soluble organic components in coal, depends jointly on mineral types and their respective content after HCl treatment. Regardless of coal un- or treated by solvents, diffusion coefficient grows with increasing coal deformation. Further study shows that the diffusion coefficient of coal with same deformation increases as follows: HCl treated coal residues, THF extracted coal residues, untreated coals, CS₂ extracted coal residues. The aforementioned phenomena result from two aspects: on one hand, the enlargement of pore after solvent treatment reduces the collision between methane molecule and pore walls, thus improving the efficient of methane diffusion; on the other hand, pore is narrowed or even blocked completely due to the retention of foreign matters, which increases the resistance of methane diffusion. Therefore, methane diffusion after solvent treatment is jointly dependent on pore change because of the removal of soluble components and the retention of foreign matters.