Molecular characterization of flash pyrolysates of two Carboniferous coals and their constituting maceral fractions

Abstract

Two high-volatile bituminous Upper Carboniferous coals from the Argonne Premium Coal Set and their constituting liptinite, vitrinite, and inertinite fractions, obtained by density gradient centrifugation (DGC), were characterized by Curie point pyrolysis-gas chromatography-mass spectrometry. A detailed analysis of the pyrolysis products was performed to relate individual macerals to their (plant-derived) precursors. Liptinite fractions were found to be mainly composed of sporinite, the diagenetic product of sporopollenin. Its aliphatic character was reflected in its pyrolysate by the abundance and characteristic distribution of n-alk-1-enes and n-alkanes showing maxima at C₁₁ and C₂₅. Moreover, a selective enrichment of aromatic components derived from linear carbon skeletons via cyclization and aromatization of functionalized, linear precursors was observed. In the sulfur-rich Illinois No. 6 coal, the addition of abiotic inorganic sulfur species to functional groups was reflected by a selective enrichment of organic sulfur compounds in the liptinite maceral. Diagenetically altered lignin derived from gymnospermous wood is the most likely precursor for vitrinite, the major constituent of both coals. Flash pyrolysates of vitrinite fractions were dominated by alkylbenzenes and alkylphenols. Inertinite fractions exhibited relatively high amounts of (po1y)aromatic pyrolysis products. Thermal extraction experiments revealed that these components were mainly present as such adsorbed to or entrapped in the pore system of the maceral matrix.