Polycyclic Aromatic Compounds in Shale Oils: Influence of Process Conditions

Abstract

Polycyclic aromatic compounds (PAC) are a significant environmental chemical group, with an associated health hazard. They have been shown to be present in oils derived from the pyrolysis of oil shales; however, the influence of process conditions on their concentration is less well known. Oil shales were pyrolysed in a 200 cm³ stainless steel fixed bed reactor in relation to temperature from 400°C to 620°C in both a nitrogen and nitrogen/steam atmosphere. The condensed oils were analysed for their content of PAC, including polycyclic aromatic hydrocarbons (PAH), sulphur-PAH (PASH) and Nitrogen-PAH (PANH). The derived gases were analysed in terms of H₂, CO, CO₂, CH₄ and other hydrocarbons to C₄ and the alkene/alkane ratio determined. The derived shale oils were found to contain significant concentrations of PAH, PASH and PANH some of which have been shown to be biologically active. The main PAH were naphthalene, fluorene and phenanthrene and their alkylated homologues; PASH identified included, benzothiophene, and dibenzothiophene and the PANH included indole, carbazole and quinoline and their alkylated derivatives. The concentration of PAC increased with increasing pyrolysis temperature and their formation was linked to increased secondary reactions as indicated by the gas phase alkene/alkane ratio. The influence of steam was to increase; the yield of oil significantly. The presence of steam also increased the concentration of PAC in the derived shale oil compared to using nitrogen as the sweep gas.

Keywords:

• Pyrolysis
• oil shale
• analysis
• PAC
• PAH
• PANH
• PASH