The Influence of Coal Microstructure on the Thermodynamic Behavior of Water Vapor Adsorption in Coal

The presence of water during coalbed methane development significantly affects gas adsorption and migration. However, the factors governing primary and secondary water vapor adsorption amounts and adsorption heats remain unclear. In this study, the chemical and pore structures of five coals with different ranks were characterized, and water vapor adsorption experiments were conducted at three temperatures. The variation patterns of water vapor adsorption and heat in coal were investigated, along with the factors influencing these parameters. More importantly, the factors affecting the primary and secondary adsorption amounts were examined, as well as the primary and secondary adsorption heats. The results indicate that the hydrophilic/hydrophobic properties of coal surfaces and the micropore fractal dimension are the primary factors influencing the amount of water vapor adsorption in coal. The oxygen-containing functional group structural parameter ‘C’ ranges from 0.22 to 0.55, and the micropore fractal dimension ranges from 2.26695 to 2.41057. The micropore fractal dimension exhibits the strongest influence on secondary adsorption (r = 0.90). The primary and secondary adsorption heats range from 30 to 73 kJ/mol and 34 to 47 kJ/mol, respectively. The initial adsorption heat of water vapor in coal exhibits a strong positive relationship with both the specific surface area (57.24–160.06 m2/g) and volume (0.0184–0.0515 cm3/g) of micropores, with r > 0.90.