Structure and combustion reactivity of inertinite-rich and vitrinite-rich South African coal chars: Quantification of the structural factors contributing to reactivity differences

ABSTRACT South Africa has large reserves of Permian-aged, high-ash yielding bituminous coals. Although these coals are of great economic importance, their behavior has not been well studied. This study compares the devolatilization and subsequent combustion behavior of an inertinite-rich (87.7% dmmf) and a vitrinite-rich (91.8% dmmf) South African coal, wet-screened to a narrow particle size distribution of 200 x 400 mesh. Pyrolysis chars were generated under rapid-heating conditions (104−105 °C/s) in a drop-tube reactor to closely resemble chars generated in pulverized combustion conditions. The inertinite-rich coal took 400ms to devolatilize in the drop-tube, compared to only 240ms for the vitrinite-rich sample. The combustion reactivities of the chars were correlated to a range of chemical, physical, and optical characteristics including the maceral differences and high ash yields. To evaluate the combustion reactivity, isothermal and non-isothermal thermogravimetrical analyses (TGA) were utilized. The vitrinite-rich char had on average 20% higher reaction rates than the inertinite-rich char under the various combustion conditions. To verify these results, temperature programmed oxidation was used and confirmed the higher reactivity of the vitrinite-rich char, where the vitrinite-rich char reached a higher maximum carbon dioxide signal at 590oC compared to 650oC for the inertinite-rich char. The char samples were de-ashed with HCl and HF acid which resulted in an increase in combustion reactivity. The maximum reaction rate of the high-ash (36%) inertinite-rich char increased with 80% after de-ashing. This suggested that the ash acted as a barrier, and the removal of ash most likely increased the access to reactive surface area. While the vitrinite-rich char was slightly more reactive than the inertinite-rich char the similar char reactivities once de-ashed are consistent with the emerging recognition that these interitite-chars are far more reactive than N. American inertinite chars The chemical and physical structures of the chars were characterized through a range of different analytical techniques to quantify the factors contributing to reactivity differences. The morphologies of the chars were characterized with SEM and optical microscopy, while quantitative information on the ordered nature of chars was obtained through XRD on de-ashed chars. The inertinite-rich coal experienced limited fluidity during heat-treatment, resulting in slower devolatilization, limited growth in crystallite height (11.8 to 12.6A), only rounding of particle edges, and over 40% of mixed-dense type chars. The vitrinite-char showed more significant structural transformations; producing mostly (80%) extensively swollen crassisphere, tenuisphere, and network-type chars, and XRD showed a large increase in crystallite height (4.3 to 11.7A). Nitrogen adsorption and small-angle X-ray scattering (SAXS) were utilized to compare the nitrogen surface areas and pore size distributions of the two chars. Both chars were mostly mesoporous but the…