Thermodynamic properties of high-rank tectonically deformed coals during isothermal adsorption

The adsorption capacity and thermodynamic properties of high-rank tectonically deformed coal (TDC) samples from the Yangquan coalfield in North China were analyzed by combining isothermal adsorption and Polanyi potential theory. The adsorption capacities of mylonitic- (10.7015–17.2065 cm³/g) and scaly (9.8237–11.5386 cm³/g) coals are higher than those of cataclastic- (8.3496–9.5238 cm³/g) and schistose (7.7990–8.0467 cm³/g) coals. The primary- and wrinkle coals have the lowest adsorption capacities. The adsorption potential (the change of surface free energy per mole during physical adsorption) decreases with the increasing adsorption space in a parabola characteristic. For different TDCs, the interval length of the adsorption potential distribution is in the same order with the adsorption capacity. The interval lengths of primary-, schistose-, and mylonitic coals are 0–0.014, 0–0.020, and 0–0.025 cm³/g, respectively. The reduction amounts of surface free energy for mylonitic- (6.040–10.082 kJ/cm²) and scaly (2.075–6.047 kJ/cm²) coals are higher than those of the cataclastic- (3.069–4.249 kJ/cm²) and wrinkle (2.222–3.434 kJ/cm²) coals. The initial and saturated adsorption surface free energies of mylonitic- and scaly coals are higher than those of the primary- and schistose coals. Based on the difference in the surface free energy reduction values, the TDCs, especially the mylonitic- and scaly coals, have a greater adsorption potential than primary- and other TDCs.