Mechanisms of sulfur and nitrogen transformation during Longkou oil shale pyrolysis

Recently, shrinking reserves of crude oil have led to increased interest in the utilisation of various unconventional resources. Oil shale is one of these promising alternatives to crude oil. However, the transformation of S/N that takes place during the oil shale pyrolysis cannot be ignored since they contribute to the release of harmful gases and production of poor-quality shale oil. In order to study the mechanism of S/N migration during pyrolysis, S/N species in the pyrolysates produced from the Longkou oil shale were qualitatively and quantitatively examined over a temperature range of 340–520 °C. The results revealed that a majority of the S retained in char during pyrolysis was primarily present as aromatic S, sulfone and sulfate. Aromatic S in the char decreased when at a temperature below 460 °C and increased when in the 460–520 °C range; sulfone in the char increased during the initial stage and decreased in the last stage; and sulfate in the char showed a fluctuating pattern. At lower temperatures (≤400 °C), more gases (H₂S, CH₃SH and COS) were produced via the decomposition of pyrite and cleavage reactions of aliphatic and aromatic S, when compared with the liquid S species. At higher temperatures (≥400 °C), an increasing amount of S migrated to shale oil, with the most abundant form being thiophenes. Concerning the transformation of N during pyrolysis, abundant N was present in the char, with a predominant occurrence of pyrrolic N. At temperatures below 340 °C, the pyrrolic N in the char was seen to change slightly, while when the temperature increased from 340 °C to 520 °C, it decreased significantly. By raising the temperature in the tested range, the N compounds were promoted to migrate into both shale oil and gas, and the production of liquid N species was recorded as higher than that of gaseous N (NH₃ and HCN), especially after surpassing 400 °C. More non-basic or weak basic N species, such as indoles, carbazoles and aliphatic nitriles, were present in shale oil than basic N species (anilines and quinolines). These findings suggest that regulating and controlling S/N transformation during oil shale pyrolysis would improve the quality of shale oil. In addition, it would provide basic data to be used in the subsequent processing for the efficient and clean utilisation of oil shale.