Combined effects of pore structure and surface chemistry on water vapor adsorption characteristics of coal: Equilibrium, thermodynamic and kinetic studies

The combined effects of surface chemistry and pore structure on water vapor adsorption characteristics of coal were studied by evaluating the equilibrium, thermodynamic and kinetic properties. Four coal samples of different rank were fully characterized with gas (N2 and CO2) sorption and Fourier Transform infrared spectroscopy (FTIR) methods. Then measurements of water vapor adsorption equilibrium and kinetics were undertaken at elevated temperatures. Equilibrium and kinetic data were fitted by the modified BET model and the unipore model, respectively. The thermodynamic parameters, as well as diffusion activation energy were estimated based on the adsorption data. The results demonstrate that water vapor adsorption depends on the surface chemistry but unrelated to the pore structure, because the pore space is not completely filled by water molecules even at the saturation pressure. The monolayer adsorption capacity decreases with increasing coal rank. Moreover, water vapor needs lower work to attain equilibrium on high-ranked coal. Also, the binding affinity of water molecule with primary sites is stronger than secondary sites. Furthermore, the diffusion coefficient decreases with coal rank, and the diffusion activation energy for high-volatile bituminous is higher than the other coals, as a result of the complexity and poor connectivity of its pore network.