Consideration on coal plasticity by referring to the distribution and structural properties of skeletal and volatile fractions from two different coals in both open and closed heat-treating systems

Coal plasticity was investigated using two coals, a strongly coking coal (Goonyella; GNY) and slightly coking coal (Witbank; WIT) by the heat treatment followed by solvent extraction and the analyses of the resulting fractions. The two different methods of heat treatment were employed: one was heat treatment under nitrogen flow and the other one was heat treatment in a batch container. The heat-treatment followed by pyridine/CS₂ extraction gave volatile, solvent-soluble fraction and solvent-insoluble fraction, the last one corresponding to the skeletal component during development of plasticity while the volatile + soluble fraction is believed to act as a lubricant against the skeletal component. The elemental analyses, the measurements of FT-IR and solid-state 13_C NMR spectra, the measurement of swelling index, and the estimation of the amount of transferable hydrogen were conducted for the samples obtained to elucidate how coal plasticity appears and develops. In the open and closed systems, WIT tended to be paralysed at the early stage of plasticity; however, GNY was paralysed little in the open system and a little in the closed system, respectively. The amount of transferable hydrogen and swelling index of solvent-insoluble fractions (PI) strongly depended on the heat-treatment temperature in the open and closed heat-treating systems, respectively. In the closed system, WIT showed the relatively higher swelling index of PI along with fairly small amount of transferable hydrogen while GNY coal showed lower swelling index and could keep a lot of transferable hydrogen and low-molecular-weight component, which significantly released at the latter stage of plasticity.