Elucidation of the structural and molecular properties of typical South African coals

Abstract

Advanced analytical techniques were performed on four South African coals, three noncoking and one coking, containing different maceral contributions (vitrinite contents ranging between 24 and 66 vol %, m.m.f.b.). Inertinite-rich (INY, UMZ) and vitrinite-rich (G#5 and TSH) coals were investigated to quantify differences and similarities in structural properties. Coals from the Witbank region (INY, UMZ, and G#5) have similar carbon contents (79.2−83.8 wt % d.a.f.), in contrast to a carbon content of 90.8 wt % (d.a.b.) for coal TSH. The free swelling index (FSI) indicated that the Witbank coals were noncoking while TSH was strongly coking. Solid state nuclear magnetic resonance indicated that the vitrinite-rich (65.9 vol % m.m.f.b.), higher rank (1.23 RoV %) coal TSH was more aromatic (81%) and more polycondensed than the other three coals. Coal G#5 was the least aromatic (66%) and was characterized by its larger proportion of protonated aliphatics as compared to the other coals. Coals UMZ, INY, and G#5 had similar average aromatic cluster sizes (ranging between 19 and 21 aromatic carbons) and number of cluster attachments (5 to 6) as estimated from NMR data. Furthermore, the cluster attachments of coal TSH were concentrated more in the side-chains, whereas the attachments of the other three coals were more prominent in bridge-and/or loop structures. XRD carbon crystallite analyses showed that coal G#5 contained the largest amount of amorphous carbon (67%), consistent with a higher volatile matter yield in comparison to the other coals. Laser-desorption ionization mass time-of-flight spectroscopy indicated that all four coals displayed similar molecular weight distributions ranging up to 1800 m/z. Coal TSH showed a maximum abundance at a higher molecular mass (608 m/z) in comparison to the other three coals. HRTEM analyses confirmed the presence of slightly more aromatic fringes in the higher molecular mass range for coal TSH in comparison to the other coals.