Abstract
Understanding the macromolecular structure of coal is vital for predicting behavior during industrial deployment. In South Africa, coal is a primary energy source; however, the depletion of older mines necessitates evaluation of the quality and macromolecular structure of the new sources. This study investigates five run-of-mine (ROM) coals from the No. 4 Seam in the Highveld Coalfield, which were density-fractionated at 1.7 and 1.9 g/cm³ to assess the differences in quality and macromolecular structure.
Organic petrography (macerals and microlithotypes), chemical analysis (proximate and ultimate analysis), and X-Ray Diffraction (XRD) for mineralogical analysis, were employed to assess quality and compositional variations following the density fractionation. The coal macromolecular structure was assessed using complementary analyses including Raman Spectroscopy, XRD, Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared (FTIR). The combustion behaviour based on the macromolecular structure was assessed using thermogravimetric analysis (TGA).
The parent coals (ROM) are classified as medium-rank D/C bituminous (%RoV: 0.57–0.60%) and are inertinite-rich, mainly comprising semifusinite and inertodetrinite with varying compositions. The H/C and O/C atomic ratios indicated Type II and III kerogen, suggesting terrestrial origins. Ash yield correlated positively with inertodetrinite, indicating occasional flooding in palaeomire(s). The float products at 1.7 g/cm³ (F1.7) were enriched in reactive macerals (vitrinite + reactive semifusinite + liptinite) (28.6 – 61.2 vol.%), whereas the F1.9 fractions contained more inert macerals, particularly inertodetrinite (12.6 – 23.7 vol.%) which mostly associate with detrital minerals. In terms of microlithotypes, the parent coals and F1.9 samples largely comprised inertite, whereas the F1.7 samples were enriched in vitrite as well as bi- and tri-macerals. Inertite was also high in the sink products, although carbominerite and minerite were relatively higher. Some sink products (S1.7 samples) showed high volatile matter yields and calorific values, meeting the quality specifications for some of Eskom’s coal-fired electricity power-stations.
Samples enriched in inertinite (the parent coals and the F1.9 samples) showed a narrow Raman G-band, indicating that they are highly ordered compared to those enriched in reactive macerals (i.e., the F1.7 samples). Similarly, XRD revealed that the F1.7 samples had a larger interlayer spacing
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(d₀₀₂), whereas the F1.9 samples exhibited higher crystallite height (Lc). Nuclear Magnetic Resonance results indicated that the fraction of aromaticity (fa) was generally higher for the ROM coals and the F1.9 samples. Interestingly, some F1.7 samples enriched in fusinite also exhibit high fa, whereas those dominated by reactive macerals reported lower fa. Despite high semifusinite, fa for the F.17 samples remained lower than the F1.9 samples, suggesting that inertodetrinite contributes more to higher fa values in the latter.
The FTIR analysis revealed that F1.7 samples had longer aliphatic chains, particularly in samples with low vitrinite, while those with higher vitrinite showed more branching (CH₃). Some F1.9 samples, despite being inertinite-rich, occasionally exhibited longer chains due to high durite content (alginite co-occurring with inertodetrinite). Nonetheless, the F1.9 samples and parent coals had shorter aliphatic chains, whereas the fa, degree of condensation (DOC), and index aromaticity (I) were higher. Combustion analysis indicated that the F1.7 samples burned for a longer period and burned out early due to higher CH₂/CH₃ ratios; the latter need to first escape to allow polyaromatization and/or cross-linking to take place. In contrast, the F1.9 and parent samples burned for a shorter period at higher temperatures, suggesting that these samples required less cross-linking due to already high fa, I, and DOC.
Coals from the same seam differ significantly in chemo-structural and petrographic characteristics, resulting in float products of varying qualities at the same or different density cuts. At 1.7 g/cm³, float products suitable for liquefaction or carbonization were produced, whereas at 1.9 g/cm³, the products were appropriate for gasification and carbonization. However, the parent coals, excluding those with higher ash content (sample HCM and HSF), already meet the specifications for gasification and carbonization, suggesting that beneficiation is not necessary unless aiming to meet export-quality standards. These findings highlight the need for revised density cut strategies, as a single-cut approach may not be suitable for the No. 4 Seam across Highveld Coalfield.