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

Abstract

Carbon dioxide can be sequestered in unmineable coal seams to aid in mitigating global climate change, while concurrently CH 4 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 CO 2 and CH 4 , CO 2 -induced swelling, contraction because of CH 4 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 CO 2 , CH 4 , and H 2 O. 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 CO 2 would be adsorbed more readily than CH 4 , 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, CO 2 capacity in the coal would be reduced. As expected with this high-rank coal, there was swelling when CO 2 perturbed the structure of approximately 5%. It was found that, on the basis of the interconnected pore structure and molecular sizes, CO 2 was able to access 12.4% more of the pore volume (as defined by helium) than CH 4 , 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 CO 2 accessibility and 4.7% of the pore volume for CH 4 accessibility.

Simulation of Adsorption–Desorption Behavior in Coal Seam Gas Reservoirs at the Molecular Level: A Comprehensive Review

“…The simulation results matched well with the experimental results. The values for CH 4 varied as 0.04–0.91 mmol/g, whereas 0.68–1.92 mmol/g held for CO 2 . , Narkiewicz and Mathews employed a macromolecular model (C 13781 H 8022 O 140 N 185 S 23 ) to look into the swelling effect induced by CO 2 adsorption and observed that the direction perpendicular to the bedding plane exhibited the highest swelling effect, which was also in line with the experimental results . Van Niekerk and Mathews used the macromolecular model C 1000 H 846 O 69 N 16 S 2 to represent bituminous coal in order to probe the interaction between coal and various adsorbates, and the authors found that the nonbonding interaction dominated the coal swelling .…”

“…As C usually occupies 60–95% of the coal matrix depending on the coal rank, O and H always comprise approximately 5% of the total mass of dry and ash-free coal. , The majority of the remainding elements include S and N. To obtain a more realistic coal model, these heteroatoms are doped on the graphene surface. ,,− According to previous reviews, the minor elements in coal are Si, Al, Ca, Mg, K, Na, Fe, Mn, and Ti, as well as halogens including F, Cl, Br, and I, and the trace elements are B, As, Cd, Hg, Mo, and P. In some models, these elements are included for a more accurate prediction . Liu et al.…”

“…Wu et al described CO 2 as a simple one-center sphere to reduce the computation . Rao and Jenkins, however, believed that a molecule with a linear structure and polarity could not be treated as a one-center LJ expression, as the kinetic diameter was insufficient to describe the interaction potential without rotation in the slit pores. , In the study of Murthy et al, two and three LJ n –6 centers with quadrupole moments were explored, and the author observed that the three-center model was more accurate than the two-center CO 2 model . In addition, a 12–6 LJ potential was in good line with a wide range of the properties of CO 2 ; however the computed result was slightly lower than the experimental result with respect to the quadrupole moment .…”

A Review on the Application of Molecular Dynamics to the Study of Coalbed Methane Geology

“…MD was another efficient method to reproduce the swelling behaviors of gaseous adsorbates into coal. Narkiewicz and Mathews (2009) calculated the CO 2 -induced swelling of coal via MD and proposed that the anisotropic swelling was greater perpendicular to the bedding plane. The deformation of coal induced by CH 4 adsorption was also quantified via the quenched solid density functional theory (QSDFT) and clarified the adsorption-induced deformation effect (Yang et al, 2010).…”

“…MD was another efficient method to reproduce the swelling behaviors of gaseous adsorbates into coal. Narkiewicz and Mathews (2009) calculated the CO 2 -induced swelling of coal via MD and proposed that anisotropic swelling was greater perpendicular to the bedding plane. Based on the poromechanical model, Brochard et al (2011) conducted effective MD work for swelling deformation behaviors of the CS1000 model induced from the adsorption of CO 2 and CH 4 in micropores, clarifying that the differential swelling was independent of the geological temperatures and pressures.…”