Changes in chemistry of vitrinite in coal through time: Insights from organic functional group characteristics

Is the signature of the original plant communities preserved in coal, or is that signature overprinted by depositional and diagenetic processes? To address this question, Pennsylvanian coals from the Illinois and Appalachian basins were compared to Cretaceous, Paleocene, and Eocene coals of comparable coalification levels as shown by vitrinite reflectance. Three reflectance intervals—0.52–0.55, 0.58–0.62, and 0.68–0.71%—were considered, and in total 51 samples were analyzed. Although data on proximate and ultimate analyses and petrographic composition were compared on whole coal samples, the maceral vitrinite was the main target of this comparison because’ aside from being the dominant component of all the samples, it offers the best chance to preserve the signature of the original woody material. An in-situ micro-FTIR technique was used to collect data on the functional group distribution of vitrinite, and FTIR-derived parameters were compared between different age groups with a special emphasis on the differences between Pennsylvanian and the Cretaceous vitrinite. The main difference between Cretaceous and Pennsylvanian coals is reflected in their oxygen content, which is higher for the Cretaceous coals. On the functional group level, the only significant difference was detected in the CH₂/CH₃ ratio, which had higher values for vitrinite in the Cretaceous samples. Other parameters both within the coal and the vitrinite showed comparable scatter within age groups as those between the age groups. It is suggested that oxygen content in coal and CH₂/CH₃ ratio in vitrinite reflect differences in the original biomass, and the variations in other parameters such as C, H, N, volatile matter, calorific value, or the majority of FTIR-derived parameters are a function of both depositional and post-depositional factors.