Comparative study of a vitrinite-rich and an inertinite-rich Witbank coal (South Africa) using pyrolysis-gas chromatography

• Published in: International journal of coal science & technology 2019-12, Vol.6 (4), p.621-632
• Main Authors: Moroeng, Ofentse M., Mhuka, Vimbai, Nindi, Mathew M., Roberts, R. James, Wagner, Nicola J.
• Format: Article
• Language: English
• Subjects: 13C CP-MAS SS NMR; Bituminous coal; Chromatography; Coal; Comparative studies; Energy; Fossil Fuels (incl. Carbon Capture); Gas chromatography; Geological history; Geotechnical Engineering & Applied Earth Sciences; Main Karoo Basin; Mineral Resources; Molecular chemistry; NMR; Nuclear magnetic resonance; Phenolics; Phenols; Polycyclic aromatic hydrocarbons; Py-TD-GC-TOFMS; Pyrolysis; Pyrolysis products; Rankings; Research Article; Xylene
• ISSN: 2095-8293; 2198-7823
• DOI: 10.1007/s40789-019-00274-3

This study aims to compare iso-rank vitrinite-rich and inertinite-rich coal samples to understand the impact of coal-forming processes on pyrolysis chemistry. A medium rank C bituminous coal was density-fractionated to create a vitrinite-rich and an inertinite-rich sub-sample. The vitrinite-rich sample has 83 vol% total vitrinite (mineral-matter-free basis), whereas the inertinite-rich counterpart has 66 vol% total inertinite. The vitrinite-rich sample is dominated by collotelinite and collodetrinite. Fusinite, semifusinite, and inertodetrinite are the main macerals of the inertinite-rich sample. Molecular chemistry was assessed using a pyrolysis gas chromatograph (py-GC) equipped with a thermal desorption unit coupled to a time of flight mass spectrometer (MS) (py-GC/MS) and solid-state nuclear magnetic resonance ( 13 C CP-MAS SS NMR). The pyrolysis products of the coal samples are generally similar, comprised of low and high molecular weight alkanes, alkylbenzenes, alkylphenols, and alkyl-subtituted polycyclic aromatic hydrocarbons, although the vitrinite-rich sample is chemically more diverse. The lack of diversity exhibited by the inertinite-rich sample upon pyrolysis may be interpreted to suggest that major components were heated in their geologic history. Based on the 13 C CP-MAS SS NMR analysis, the inertinite-rich sample has a greater fraction of phenolics, reflected in the py-GC/MS results as substituted and unsubstituted derivatives. The greater abundance of phenolics for the inertinite-rich sample may suggest a fire-related origin for the dominant macerals of this sample. The C 2 -alkylbenzene isomers ( p -xylene and o -xylene) were detected in the pyrolysis products for the vitrinite-rich and inertinite-rich samples, though more abundant in the former. The presence of these in both samples likely reflects common source vegetation for the dominant vitrinite and inertinite macerals.