Effect of alkali activated potassium persulfate solution immersion on wettability and pore structure of coking coal

To improve the wettability of coking coal and explore wetting mechanism of coal samples, the capillary reverse osmosis experiment was firstly performed to select the composition of alkali-activated potassium persulfate (AAP) solution. By combination with the fitting results of FT-IR spectra for coal samples before and after AAP immersion experiment, the relative content changes of various functional groups on the surface of coal samples were obtained. Based on these results, porous layered structure models of coal molecule were constructed by using Sorption module of Materials Studio software to simulate the adsorption of water molecules on the surface of coal samples. Effects of AAP immersion on contact angle θ, reverse osmosis height, functional group content change, pore structure, gas adsorption constants and adsorption properties of water molecules on coal surface were investigated systematically. The experimental results showed when the mass fraction of potassium persulfate was 1.5%, the mass fraction of sodium hydroxide was 0.5% and the activation temperature was 30 ℃, the wettability of coking coal was most significantly improved by AAP immersion, and the reverse osmosis height of treated coal sample reached 10.3 cm. This was consistent with wettability increasing trend that contact angle θ of coal samples surface decreased from 96.32° to 48.51° (the decline was 49.64%). After AAP immersion, the content of OH-π group and cyclic hydroxyl group on the surface of coking coal samples increased from 9.22% and 13.43% to 17.25% and 24.39%, respectively, and its content change was the most pronounced among all functional groups. As OH-π group and cyclic hydroxyl group were liable to form hydrogen bond with water molecules, the increase of its content facilitated boost the wettability of coal samples. MS simulation results showed that after AAP immersion, the average adsorption heat of water molecules increased from 16.11 kJ/mol to 19.57 kJ/mol, and the adsorption capacity of water molecules increased from 48.6 H₂O/cell to 53 H₂O/cell. The treated coal samples could adsorb a large amount of water molecules even at a lower pressure, which was beneficial to promoting the effects of spray dust suppression and coal seam water infusion. Meanwhile, \mathrmSO_4^\cdot - and ·OH in AAP solution could oxidize and corrode organic small molecule phase in coal, which enabled the pore diameter of primary micropores in coal larger and the number of micropores decreased. After immersing by AAP solution with the optimal composition, average pore diameter of micropores increased by 44.64% and specific surface area decreased by 65.48%. The decrease of gas adsorption constant a value was 12.01%, and the decrease of b value was 19.27%. That is, AAP immersion could remarkably improve the wettability of coal samples and meanwhile effectively reduce methane adsorption amount, providing theoretical reference for developing coal seam water infusion dust suppression and outburst prevention technology.