Distribution of Polycyclic Aromatic Compounds in Coals of High Rank

The concentration of 131 polycyclic aromatic compounds (PAC) has been determined by capillary gas chromatography in the solvent extracts of eight bituminous coals with vitrinite reflectance (Rr) in the range of 0.9–1.9%. The distributions of PAC that eluted between naphthalene and coronene are dominated by naphthalenes, biphenyls, fluorenes, phenanthrenes, fluoranthenes, pyrenes and their C1-C3 derivatives, oxygen-containing compounds (dibenzofurans), and sulphur heterocycles (dibenzothiophenes, benzo[b]-naphthothiophenes) which were selectively measured by an electrolytic conductivity detector. Discrepancies in concentration profiles among PAC can generally be related to differences in kinetic rather than thermodynamic stabilities. However, thermodynamic equilibration is indicated in certain instances, e.g., with methylphenanthrenes and methyldibenzothiophenes extracted from coals having reflectance values between 0.9 and 1.3% Rr where the total concentration of each group of isomers remains fairly constant. A late phase of intense PAC generation in coals of high rank is inferred from the concentration profiles of a number of parent compounds and their β-methyl derivatives maximizing in the range of 1.2–1.7% Rr. In contrast, concentrations of α-methyl derivatives generally decrease drastically at this stage. Owing to the lower kinetic stability of those derivatives, generation is overridden in this instance by a conversion to inextractable material.