Geochemical and petrographic alteration of rapidly heated coals from the herrin (no. 6) coal seam, Illinois Basin.

摘要

A petrographic, geochemical, and molecular assessment was performed on a series of Illinois Basin coal samples that were collected at various distances from a Permian-age igneous dike. Standard coal characterization techniques such as vitrinite reflectance, proximate, and ultimate analyses provide valuable insights to maturation pathways experienced by rapidly heated coals. These techniques were coupled with reflectance micro-FTIR and KBr-FTIR analysis to provide a deeper understanding of the molecular changes that occur in the coal structure during relatively short-lived, intensive heating events. With increasing proximity to the contact with the dike, coals and coals macerals exhibit increased maturation on both petrographic and molecular scales. KBr-FTIR spectra were collected to distinguish bulk trends expressed in coals with increasing maturity, whereas reflectance micro-FTIR spectra were collected to analyze characteristics of vitrinite macerals in-situ..;With increasing proximity to the intrusion, coals have higher mean random vitrinite reflectance values (Rr) within the dike alteration zone. Liptinite macerals are not distinguishable at reflectances equal to or higher than 1.36%, and coking textures are developed within 2 m of the intrusion. Geochemical data indicate an overall loss of H, O, and N and an increase in C approaching the dike. Carbonate minerals contribute significantly to geochemical data, specifically volatile matter (VM), C content, and O content. Even after carbonate removal with HCl, coals have higher VM contents at high rank than expected compared to normal coalification trends. When plotted on a van Krevelen diagram or Seyler chart, intruded coals follow different coalification trends than coals matured through normal burial metamorphism.;FTIR analysis shows increased aromaticity with rank, with the ratio of the aromatic CHx band at 3100-3000 cm-1 versus the aliphatic CHx band at 3000-2800 cm-1 (AR1), and the ratio of the aromatic out-of-plane deformation band at 900-700 cm-1 versus the aliphatic CHx band (AR2) becoming larger with increasing Rr. Within the 3000-2800 cm-1 bandwidth, there is an increase in the area under the asymmetric CH3 peak at ~2960 cm -1 relative to the asymmetric CH2 peak at ~2920 cm-1 with increased rank. The overall intensity of the aromatic out-of-plane deformation modes at 900-700 cm-1 increase relative to aliphatic CHx bandwidth response at high rank as well. This trend is much more traceable in micro-FTIR spectra, which avoids the influence of mineral matter bands in the 900-700 cm-1 range. Within the 900-700 cm-1 band, the overall intensity of the ~750 cm-1 peak relative to the ~870 cm-1 peak increases with rank, likely reflecting a lower degree of substitution (DOS) of alkyl groups on aromatic ring sites in high rank coals. This trend likely also represents the building of large aromatic clusters at high rank. General FTIR spectral trends for whole-coals and individual vitrinite macerals are similar. Semi-quantitative measurements become less reliable at high coal ranks, mostly due to very small aliphatic bandwidth areas.;FTIR analysis of coals and coal macerals provides insight to the molecular changes that occur in coals during rapid heating. Research of this nature can offer deeper insights into coalbed methane development in response to localized heating (Gurba and Weber, 2001).