Morphologies, X-ray parameters, and burnout comparisons of inertinite-rich and vitrinite-rich South African bituminous coal derived chars

Char structure and morphology directly influences combustion behavior as it affects the intrinsic char reactivity and reacting gas transport. In South Africa, the less reactive inertinite-rich bituminous coals are used domestically while the export market is dominated by vitrinite-rich bituminous coals. The degree of thermoplasticity during coal to char transition is strongly dependent on the petrographic composition, rank, and heating rate. These factors affect the transformations to chars of differing structures, morphologies, and hence reactivity. Here the coal to char transformations during rapid-heating pyrolysis of narrow size cuts of two South African coals were compared. These coals had different petrographic compositions but similar carbon content, vitrinite reflectance (mean-max), and organic precursors. Char samples representing different stages of mass loss were generated in a drop-tube reactor under rapid-heating conditions (10⁴-10⁵ °C/s) with a 99%N₂/1%O₂ atmosphere. 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 demineralized chars. The vitrinite-rich (91.8% dmmf) coal reached a high degree of fluidity, producing mostly extensively swollen crassisphere, tenuisphere, and network-type chars. The inertinite-rich (87.7% dmmf) coal had limited fluidity which resulted only in rounding of particle edges and a large fraction of mixed-dense type chars. The final inertinite-rich char had a higher crystallite height (12.6Å) than the final vitrinite-rich char (11.7Å). The vitrinite-rich sample has a higher crystallite width (26.8Å) than the inertinite-rich sample (24.5Å). Even though these stacking parameters were similar, the inertinite-rich parent coal was significantly more ordered than the vitrinite-rich coal prior to heat treatment (11.8Å compared to 4.3Å), indicating a more extensive transformation for the vitrinite-rich coal. Burnout profiles obtained in a thermogravimetrical analyzer at a heating rate of 5°C/min. in air showed the inertinite-rich char had a burnout temperature of 680°C, higher than the vitrinite-rich char's 650°C. The temperature at the maximum rate of weight loss was at 480°C for the vitrinite-rich char and at 530°C for the inertinite-rich char indicating significant reactivity differences.