Visual Representation of Carbon Dioxide Adsorption in a Low-Volatile Bituminous Coal Molecular Model

Carbon dioxide can be sequestered in unmineable coal seams to aid in mitigating global climate change, while concurrently CH4 can be desorbed from the coal seam and used as a domestic energy source. In this work, a previously constructed molecular representation was used to simulate several processes that occur during sequestration, such as sorption capacities of CO2 and CH4, CO2-induced swelling, contraction because of CH4 and water loss, and the pore-blocking role of moisture. This is carried out by calculating-the energy minima of the molecular model with different amounts Of CO2, CH4, and H2O. The model used is large ( > 22 000 atoms) and contains a molecular-weight distribution, so that it has the flexibility to be used by other researchers and for other purposes in the future. In the low-level molecular modeling presented here, it was anticipated that CO2 would be adsorbed more readily than CH4, that swelling would be anisotropic, greater perpendicular to the bedding plane because of the rank of this coal, and finally, that, with the addition of moisture, CO2 capacity in the coal would be reduced. As expected with this high-rank coal, there was swelling when CO2 perturbed the structure of approximately 5%. It was found that, on the basis of the interconnected pore structure and molecular sizes, CO2 was able to access 12.4% more of the pore volume (as defined by helium) than CH4, in the rigid molecular representation. With water as stationary molecules, mostly hydrogen bound to the coal oxygen functionality, pore access decreased by 5.1% of the pore volume for CO2 accessibility and 4.7% of the pore volume for CH4 accessibility.