The characterisation of slow-heated inertinite- and vitrinite-rich coals from the South African coalfields

Abstract The development of coal char structures with pyrolysis has been extensively investigated but has traditionally been focused on the carboniferous coals in the northern Hemisphere. In this investigation, the properties of pyrolysis chars generated from inertinite- and vitrinite-rich coals (81% and 91% mmb by volume, respectively) collected from selected South African coalfields were determined. Chars were generated at 450, 700 and 1000°C. The properties of coals and chars were examined using the chemical, physical, petrographic, solid-state ¹³C NMR and X-ray diffraction analytical techniques. The objective was to generate results for further studies including molecular modelling and atomistic reaction kinetics. A good correlation was found between the total maceral scan (rank) and aromaticity as the pyrolysis temperature increased, as well as between the aromaticity measurements with XRD and NMR techniques. The chemical structure of the intertinite-rich and vitrinite-rich chars at 700-1000°C was remarkably similar in terms of the proximate, ultimate, total maceral scan and aromaticity values. Greater transition occurred in the vitrinite-rich coal, implying a thermally more activated coal. Differences in the physical structure of the chars at these temperatures were observed in terms of the surface area using the Dubinin-Radushkevich (D-R), the Brunauer-Emmet-Teller (BET) and Langmuir methods as well the microporosity from the CO₂ adsorption method. The macerals were not distinguishable at 700-1000°C. However, the differences in maceral composition of the coals resulted in substantially different char forms during thermal conversion. The intertinite-rich coal formed more denser chars and higher proportions of thicker-walled networks (60-65% by volume). The vitrinite- rich coal showed higher proportions of isotropic "coke" (91-95% by volume), which contributes to a high distribution of surface area and micropores. Therefore, on a chemical level, the high temperature chars were similar. Differences existed in the physical structure at high temperatures. The physical structures, char forms and crystallite diameter (L_a) significantly distinguished the chars at high temperatures, where L_a for inertinite-rich chars was 37.6 Å compared with 30.7 Å for the vitrinite-rich chars. The L_a property, in particular, played a significant role in investigations for the molecular structural properties of the inertinite- and vitrinite-rich chars, including their reactivity behaviour with carbon dioxide gas.