Molecular dynamic simulation of coal-solvent interactions in Permian-aged South African coals

Molecular dynamic simulations were used to examine the initial stages of solvent-coal interactions during solvent swelling. Large-scale (>10,000 atoms) vitrinite-rich Waterberg and inertinite-rich Highveld coal models used in this study were previously constructed. Isothermal-isobaric molecular dynamics simulated the experimental conditions used for the solvent swelling of these coals. Partially solvent swollen structures were constructed by the addition of solvent molecules to the original coal molecules using an amorphous building approach. The various solvated coal models were simulated using pyridine, N-methylpyrrolidone (NMP) and CS(2)/NMP solvents. The changes in bonding and nonbonding energies due to solvent swelling were determined by comparing original coal models to corresponding swollen models. Simulation studies showed that coal-coal nonbonding interactions changed due to disruption of the van der Waals interaction energies. The distributions of hydrogen bonds were calculated and provided a method to evaluate solvent-coal hydrogen interactions. It was found that hydroxyl groups associated with the bituminous coal structure are the dominating hydrogen bond donor in solvent interaction. Therefore, the contributions of nonbonding interactions in coal play an important role during coal-solvent swelling. Molecular modeling and simulation is a useful tool to probe these changes in energies and nonbonding interactions in coal with various solvents. (C) 2010 Elsevier B.V. All rights reserved.