Influence of maceral composition on the structure, properties and behaviour of chars derived from South African coals

Abstract

The generation of coal chars is an important intermediate step in coal conversion processes such as combustion and gasification. The char structure impacts their behaviour in these processes. Detailed investigations on slow-heated coal chars utilizing molecular modelling approaches are limited in comparison to the more extensive work on coal. Also South African (SA) coal is generally not as well studied as the vitrinite-rich coals in the Northern Hemisphere. Both inertinite-rich and vitrinite-rich coals are available in SA, formed with the same inputs but in different depositional environments, thus providing an opportunity to examine maceral influences without the challenges associated with maceral separation. Experimental work was performed to characterize de-ashed chars generated by heating the inertinite- and vitrinite-rich South African Gondwana coals of the Permian Age under inert atmosphere (nitrogen gas) to 1000 °C at a rate of 20 °C/min and maintain this temperature for 60 min before cooling. Experimental work included petrographic analysis, elemental analysis, helium density, 13C NMR, XRD, and HRTEM analyses. The maceral purity was enhanced with density separation prior to pyrolysis. Analytical data was used to construct molecular structures comprising polyaromatic hydrocarbons with oxygen, nitrogen and sulphur functionalities. Aromaticity and skeletal density measurements provided additional constraining data. The inertinite-rich char model comprised 8586 atoms comprising 21 individual molecules with a composition (normalised to 1000 carbon atoms) of C1000H105O14N22S1, while the vitrinite-rich char model was 8863 atoms within 37 molecules with a normalised composition of C1000H125O21N22S3. The two chars were thus chemically similar; however, the size of polyaromatic molecules differed, as observed from HRTEM and XRD data. The inertinite-rich chars contained larger molecules (La (10) = 37.61 ± 2.32 Å) than the vitrinite-rich chars (La (10) = 30.74 ± 0.77 Å). This will influence the fraction of active carbon sites located on the crystallite edges and defects of the basal planes that may contribute to the lower reactivity of inertinite-rich chars in comparison to vitrinite-rich chars. Atomistic representations of these chars was constructed capturing a portion of the structural diversity observed from image analysis of the HRTEM lattice fringe micrographs. While the char generation parameters used in this study produced similar chars derived from the South African inertinite- and vitrinite-rich coals, the crystallite diameter, La, was found to be the property that distinguished the two chars significantly. This may hold promise for exploring structural-reactivity relationships