Construction of Macromolecular Structural Models of Coal and Simulation of Adsorption under Acidification

We explored the influence of acidification on the structure, wettability, and adsorption capacity of coal based on X-ray diffraction, water adsorption tests, elemental analysis, 13C nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, isothermal adsorption tests, and molecular simulations. The results indicated that the silicate content in the experimental coal samples was high and the optimal acidification condition for improving wettability was 4% hydrofluoric acid with a 6 h reaction time. Under acidification, the aromatic structure in coal changed slightly, while the aliphatic structure and functional groups were significantly altered. In addition, the aliphatic chains were broken to form shorter chains, and the carbon structures changed from methylene and hypomethylene to methyl. Moreover, acidification removed nitrogen atoms and generated a greater number and variety of oxygen-containing groups. The macromolecular structural models of the coal samples and the acidified samples were constructed according to the above tests, and adsorption simulations were subsequently performed by using the models. In the CH4 adsorption simulation, the adsorption capacity decreased after acidification, and the changing trend was highly consistent with the isothermal adsorption tests. In the simulation of the adsorption of H2O molecules and gas–water binary components, the macromolecular model of acidified coal exhibited better performance in absorbing H2O, and the improved wettability further inhibited its adsorption capacity for CH4. Our findings are crucial for exploring the mechanisms underlying the acid modification of wettability and adsorption capacity of coal at the microscopic level.