The structural transformations of two Permian-aged South African coals, one vitrinite-rich [91.8% dry mineral matter free (dmmf)] and one inertinite-rich (87.7% dmmf), and their resultant char morphologies were compared in this study. With these two maceral-rich coals, the opportunity presented itself to compare the degree of thermoplasticity during coal-to-char formations for the macerals without the need to use maceral separation techniques. The thermoplasticity is affected by its petrographic composition and, consequently, influences the combustion behavior. Pyrolysis chars were generated from wet-screened coal (200 × 400 mesh), under rapid-heating conditions (104-105 °C/s) in a drop-tube reactor, to closely resemble chars generated in pulverized combustion conditions. The chemical and physical structures of the chars were characterized through a range of different analytical techniques, including scanning electron microscopy, X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), nitrogen adsorption, and optical microscopy, to quantify the factors contributing to reactivity differences. Results indicated that the inertinite-rich coal experienced limited fluidity during heat treatment, resulting in slower devolatilization, limited growth in crystallite height (11.8-12.6 Å), and only rounding of particle edges and producing >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 (4.3-11.7 Å). Nitrogen adsorption revealed that both chars were mostly mesoporous but that the inertinite-rich char had double the average pore size, which also resulted in a larger nitrogen surface area (3.9 m2/g compared to 2.7 m2/g). SAXS data showed that the vitrinite-rich char had 60% higher frequencies of pores in the micropore range. Helium porosimetry indicated that the inertinite-rich coal and resultant char had higher densities than the vitrinite coal and char, 1.6 and 2.0 g/cm3 compared to 1.3 and 1.9 g/cm3 (on a dry basis). To evaluate combustion reactivity, non-isothermal burnout profiles were obtained through thermogravimetrical analysis in air. The burnout profiles showed that the inertinite-rich char had a burnout temperature of 680 °C, slightly higher than that of the vitrinite-rich char, of 650 °C. This along with the peak shape and position in the burnout profiles indicates that the vitrinite-rich char has a higher reactivity. The higher reactivity is due to a combination of factors, likely including less organization, greater porosity and access to the reactive site, less ash blocking, and char morphology differences. The char samples were de-ashed, which resulted in an increase in combustion reactivity, because the ash acted as a barrier to the reactive surface area. The maximum reaction rate of the high-ash (36% ash yield) inertinite-rich char increased 80% after de-ashing, while the vitrinite-rich char, with an ash yield of 15%, had a 20% increase in reactivity after de-ashing.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology