Mechanistic study on the plastic phenomena of coal

Highly coking coal, Goonyella coal, and slightly coking coal, Witbank coal, were submitted to ruthenium ion catalyzed oxidation (RICO) reaction to clarify the structural features of the two coals. Expected structural differences between the two coking coals were meager but interesting: Witbank coal has a little bit larger amount of longer methylene bridges (C15-C25) and longer alkyl chains (C15-C30) than Goonyella coal. Combined use of ¹³C NMR and ¹H NMR spectra gave valuable information of the average aromatic ring size: Goonyella coal has larger aromatic rings than Witbank coal on average. To obtain the information about the evolution of volatile materials during heating which are supposed to be significant for the appearance of coal plasticity, two steps of heat treatment of the coals were performed: the first step is heating to the softening temperature, and the second step is heating the resulting sample to resolidification temperature. Witbank coal gave a relatively larger amount of tar in the first step of the heating than in the case of Goonyella coal, while Goonyella coal gave a larger amount of tar in the second step of the heating compared with the corresponding fraction of Witbank coal. These strongly suggest that a larger amount of metaplast, which is indispensable for the appearance of fluidity, could be produced during the plastic range (the temperature range from the softening temperature to the resolidification temperature) in the case of Goonyella coal, this leading to its higher Gieseler maximum fluidity. We previously reported that these two coking coals have almost the same amount of transferable hydrogen. Therefore, Witbank coal is supposed to consume relatively large amounts of transferable hydrogen for the formation of tar during the heating to softening temperature, probably via oxygen functional group related reactions and carbon-carbon bond breaking reactions. Due to a lower amount of transferable hydrogen in the char, subsequent bond cleavage reactions at the plastic range lead to recombination to show a very low value of its Gieseler maximum fluidity, although the three-dimensional structure framework of Witbank coal collapses to a small extent.