Comparative Study of Aromatic Hydrocarbons in Bitumens and Expelled Oils Generated by Hydrous Pyrolysis of Coal

An immature humic coal sample from the Yaojie Formation of the Muli coal field is pyrolysed in a hydrous closed system. The differences in aromatic hydrocarbon compositon between bitumens and expelled oils are investigated with the simulation temperature ranging from 250 to 500 degrees C. The absolute concentrations of the main aromatic classes in bitumens and expelled oils initially increase and thereafter decrease with increasing maturity. The absolute concentrations of naphthalene are always higher in expelled oils than in the corresponding bitumens; however, the absolute concentrations of alkyl-naphthalenes, phenanthrene, and alkyl-phenanthrenes are higher in bitumens than in expelled oils at the lower-maturity stage, and reversals occur with continuous increase in R-O. Most of the common parameters-MNR, MPI1, MPR, F1, and F2-exhibit regular differences in bitumen-expelled oil pairs. The ratios of different alkylation degrees of naphthalenes and phenanthrenes (such as N/C1-5-N, C-1-N/C1-5-N, C-1-N/C-5-N, P/C1-3-P, and C-1-P/C-3-P) and the ratios of aromatic components (such as naphthalene/phenanthrene, phenanthrene/fluoranthene, and phenanthrene/pyrene) that have different numbers of aromatic rings exhibit regular differences in bitumen-expelled oil pairs. These ratios of expelled oils are always higher than those of corresponding bitumens; reversals may occur at a high maturity stage. The systematic differences between the compositions of the main aromatic hydrocarbons in bitumens and expelled oils could be predominantly controlled by the fractionation effect at a lower-temperature stage and differentiated decomposition at a higher-temperature stage during the hydrous pyrolysis of coal.