Experimental study on physicochemical structure of different rank coals under acid solvent treatments and its effects on heat of gas adsorption

Physicochemical structure characteristics and its effects on the heat of gas adsorption are the foundation for coalbed methane (CBM) development. Herein, this paper selected different rank coals before and after acid treatments to analyze the relationship among nanopore structure, chemical functional groups and the heat of gas adsorption through low-temperature N2 adsorption, Fourier transform infrared spectroscopy (FTIR) and calorimetric tests. This enabled us to explore the effect of acid solvents on nanopore structure and functional groups, and finally reveal a coupling mechanism between physicochemical structure and gas adsorption. Results showed that the specific surface area of Brunauer-Emmett-Teller model (BET-SSA) and pore volume of Barret-Joyner-Halenda model (BJH-PV) decrease first and subsequently increase with increasing coal evolution before and after acid treatments. However, both pore structure parameters and fractal dimension show a decrease or an increase at different degrees under pore expansion and increase effects. On the other hand, after acid treatments, aliphatic structures evidently decrease and aromatic structures relatively increase. For oxygen-containing functional groups affected by ester hydrolysis and Fourier reaction, more carboxyl and hydroxyl groups appear but ether groups reduce. This suggests that acid treatment is actually accompanied by more physicochemical changes. Then, according to its physicochemical response, less aliphatic structures and more aromatic structures significantly contribute to appear more micropores. However, a non-significant effect of aliphatic structure on gas adsorption is observed, while micropores, aromatic structure and oxygen-containing groups give a positive effect on gas adsorption.