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Industrial applications of coal rely on understanding its macromolecular structure, which is primarily controlled by coal type and rank. The present study assessed five (5) samples from different collieries extracting coal from the No. 4 Seam of the Highveld Coalfield and their float products, produced at relative densities (RD) of 1.7 and 1.9 g/cm3. The aim was to assess changes in maceral composition and coal quality following the density fractionation, and to use Raman Spectroscopy to compare differences in macromolecular structures between the parent samples and the float products. Raman parameters were also determined for specific macerals, i.e., semifusinite and collotelinite. Mean random vitrinite reflectance (%RoV) values for the studied coals range between 0.57 and 0.60% (medium rank D/C bituminous) and the parent coals are inertinite-rich (70.3 to 88.7 vol% mmf), enriched in semifusinite and inertodetrinite. Following density fractionation, reactive macerals (a combination of liptinite, vitrinite, and reactive semifusinite) are enriched in the float products (designated by "F"), specifically in the products obtained at the 1.7 RD. In comparison, the proportion of inert macerals is higher in the F1.9 samples. These differences in maceral composition are reflected in the Raman spectra and parameters. Although the G and D1 bands for the parent coals and F1.9 samples are similar, these bands are narrower than for the F1.7 samples, indicative of greater aromaticity. The G FWHM values for the F1.9 samples are comparable to those for the parent coal samples, and lower than for the F1.7 samples. This reflects larger differences in maceral composition between the parent coals and the F1.7 samples. In contrast, the D1 FWHM values for the float products, particularly the F1.7 samples, are slightly higher than the parent coals, reflecting a disordered aromatic character mainly related to the presence of aliphatic chains. The Raman spectra for the F1.7 samples are more like that for collotelinite. In contrast, the Raman spectra and parameters (G and D1 FWHM) for the F1.9 samples are more comparable to semifusinite. Thus, the increased aliphaticity for the F1.7 samples is attributed to the relative enrichment of reactive macerals, whereas higher aromaticity for the F1.9 samples reflects a larger proportion of inert macerals. Raman spectroscopy expanded on the petrographic data by interrogating the macromolecular structure of the isorank Highveld coals and their float products. This may assist in predicting the behaviour of the coals during industrial applications (i.e., liquefaction, gasification, combustion, and carbon fibre production). [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, carbon-13 cross-polarization magic-angle-spinning solid-state nuclear magnetic resonance ( 13 C CP-MAS SS NMR) was undertaken on a parent coal sample and its density-fractionated derivatives, an inertinite-rich and a vitrinite-rich sample, obtained from the Witbank Coalfield of South Africa (medium rank C bituminous coal). The formation of inertinite macerals has been extensively researched and the conclusions remain largely controversial. However, most research has been confined to northern hemisphere coals, which are typically dominated by vitrinite. South African coals are widely known for their high inertinite content. Earlier workers have ascribed the inertinite macerals, in their diversity, to aerial oxidation. However, this oxidation-only pathway fails to recognize that inertinite macerals can form through other processes such as charring of plant matter. Microscopically, charred matter possesses anatomical structures that closely resembles those observed in inertinite macerals, with lignin and/or cell walls largely preserved, perhaps mechanically fragmented or compressed in some instances. Based on the NMR structural parameters, the average aromatic cluster size in the inertinite-rich sample (petrographically dominated by fusinite, semifusinite of varying reflectance, and inertodetrinite) corresponds to monocyclic, 6-aromatic carbon rings. In the vitrinite-rich sample of the same coal, the cluster sizes are larger, corresponding to multi-ring aromatic hydrocarbons. The 6-carbon rings in the inertinite-rich sample are interpreted to correspond to guaiacol and syringol, the principal products of low-temperature (below 400 °C) lignin pyrolysis. Since charring depletes cellulose and moisture, fire-affected plant matter is interpreted to have a lower compaction potential than vitrinite-forming material, sustaining 6-aromatic carbon ring isolation in the former. Lignin-derived aromatic rings in compactable plant matter merge to form the multi-ring clusters present in the collotelinite- and collodetrinite-rich sample of the same coal. [ABSTRACT FROM AUTHOR]