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 degrees 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 degrees C and increased when in the 460-520 degrees 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 degrees 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 degrees 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 degrees C, the pyrrolic N in the char was seen to change slightly, while when the temperature increased from 340 degrees C to 520 degrees 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 degrees 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. (c) 2021 Elsevier Ltd. All rights reserved.