Molecular simulation of relaxation behaviors of coal-aggregated structures

Upper Freeport coal (Argonne Premium Coal Sample, APCS 1) was extracted and fractionated with a carbon disulfide/N-methyl-2-pyrrolidinone mixed solvent, acetone and pyridine at room temperature, and three extract fractions were obtained: acetone-soluble (AS), acetone-insoluble–pyridine-soluble (PS) and pyridine-insoluble (PI) fractions. A molecular dynamics (MD) computer simulation method was used to study the structural relaxation behaviors of the fractions during heating. The calculation was carried out at 373–773 K in order to see changes in various properties by heat. For the PS and PI fractions, the temperature at which the average volume of unit cell discontinuously changed was observed. The DSC thermograms of PS and PI showed a reversible endothermic peak at 610–710 and 650–710 K, respectively, corresponding to the temperature ranges in which the structural relaxation was estimated by the simulation. The simulation suggests that the endothermic peak is the result of physical relaxation of the aggregated structures through dissociation of nonbonded interactions.