Geochemical and petrographic alteration of rapidly heated coals from the Herrin (No. 6) Coal Seam, Illinois Basin

Abstract

Coals altered by rapid heating events (i.e., those intruded by dikes and sills) are thought to follow a different geochemical maturation pathway than coals altered through diagenesis. If an igneous intrusion alters the petrographic and geochemical properties of a coal, the effect should also be observable in the coal's molecular structure. In this study, we evaluate whether coals altered by rapid heating follow distinct maturation trends from coals that were altered by slower heating (burial maturation). Petrographic, geochemical, and micro-FTIR analyses were performed on a series of Pennsylvanian Illinois Basin coal samples, collected at various distances from a Permian igneous dike. Standard coal characterization techniques including vitrinite reflectance and proximate and ultimate analyses provide valuable insights on the maturation pathways experienced by rapidly heated coals. These techniques were coupled with reflectance micro-FTIR to provide a better understanding of the molecular changes that occur in the coal structure during relatively short-lived, intensive heating events.With decreasing distance to the intrusion, coals have higher mean random vitrinite reflectance values (Rr) within the dike alteration zone. Coking textures similar to those observed in industrial cokes are observed within 2m of the intrusion. Geochemical data for HCl-treated coals indicate an overall loss of H, O, and N and an increase in C approaching the dike. Intruded coals have higher volatile matter (VM) yields at high rank than coals of similar rank that result from normal burial maturation. When plotted on a van Krevelen diagram or Seyler chart, intruded coals follow different coalification trends than coals matured through normal burial diagenesis.Reflectance micro-FTIR analysis of collotelinite shows increased aromaticity with rank: both the ratio of the aromatic CH stretching band at ~3100–3000cm−1 versus the aliphatic CHx stretching bands between 3000 and 2800cm−1 (AR1), and the ratio of the aromatic out-of-plane deformation bands between ~900–700cm−1 versus the aliphatic CHx band (AR2) increase with increasing Rr. Within the 3000–2800cm−1 region, there is an increase in the area under the asymmetric CH3 peak at ~2960cm−1 relative to the asymmetric CH2 peak at ~2920cm−1 with increased rank. Within the 900–700cm−1 region, the overall intensity of the ~750cm−1 peak (aromatic rings with four adjacent H atoms) relative to the ~870cm−1 peak (aromatic rings with one isolated H atom) increases up to 2.5% Rr, likely reflecting a lower degree of substitution (DOS) of alkyl groups on aromatic ring sites. The prevalence of the 750cm−1 peak at high rank may represent a lower degree of condensation of aromatic rings in the structure of intruded coals compared to normally matured coals.