Abstract:
Coal contains a significant concentration of free radicals as a result of the coalification process. One of the
experimental methods sensitive to the presence of radicals is electron spin resonance (ESR), and differences in ESR spectra
for different macerals may provide insight into coal-forming processes. In this study, ESR data along with the H/C atomic
ratio (to infer the aromatic fraction) are used to characterize coal samples with the aim of assessing a fire-origin for
dominant inertinite macerals. A medium rank C bituminous Witbank No. 4 Seam Upper coal (the parent) was densityfractionated
to create vitrinite-rich and inertinite-rich samples. The parent sample consists of 42 vol% vitrinite and 49 vol%
inertinite. The density-fractionated samples comprise of 81 vol% total vitrinite (dominated by collotelinite and collodetrinite),
and 63 vol% total inertinite (dominated by fusinite, semifusinite, and inertodetrinite). The H/C ratio is 0.74 for the
inertinite-rich sample, and 0.85 for the vitrinite-rich counterpart, suggesting the former sample is more aromatic. The ESR
spectra obtained for the three samples were found to fit best using a Lorentzian distribution. The fit is noticeably better for
the aromatic inertinite-rich sample, for which the spectrum is symmetric. This is attributed to pronounced electron mobility
and exchange interactions. The higher radical content of the inertinite-rich and parent samples is attributed to the presence
of specific inertinite macerals, namely: fusinite, semifusinite, and inertodetrinite. And, owing to the greater radical content
of the inertinite-rich sample, the dominant inertinite macerals are interpreted to have formed through charring of plant
matter.
