A comparison of the thermal conversion behaviour of marine kerogens isolated from oil shales by NaOH-HCl and HCl-HF methods

Abstract

The inherent reactivity of kerogens isolated from marine oil shales has not been extensively studied. Determining the conversion and products of demineralized oil shales simplifies modelling of oil shale pyrolysis because the complications introduced by the mineral matter are removed. Accordingly, the reactive behaviour of demineralized products (kerogens) isolated from two marine oil shales (type II kerogen) by NaOH-HCl and HCl-HF methods have been compared with each other and with those of untreated shales. Pyrolysis experiments have been carried out under N2 and H2 in a sealed system, and also in a flow through autoclave system, which is closer to practical pyrolysis conditions, at temperatures in the range 355 °C to 425 °C. The two isolation methods gave a similar conversion in sealed autoclaves and the only major difference was the higher asphaltol yield for the NaOH-HCl kerogens, so that asphaltol yield cannot be used to characterize the intrinsic reactivity of the shale organic matter. The El-Lajjun shale gave higher conversion to organic materials than the kerogens but the opposite was noted for Julia Creek, especially for pyrolysis under N2.Pyrolysis experiments in a flow through system gave similar overall conversion for the kerogens and their untreated shales but the yields of condensates for the Julia Creek shale were significantly lower for the oil shales than for the kerogens. The chemical complexity of the condensates from the flow through experiments was much greater than that for the CH2Cl2 soluble samples from the sealed autoclave pyrolysis experiments.The product compositions for the NaOH-HCl and HCl-HF kerogens were, as for the total yields, very similar, suggesting that the NaOH-HCl method is a valuable alternative in countries where the use of HF is severely restricted. Further, determining the reactive behaviour of kerogens isolated by two different methods makes it likely that features of the reactivity common to both types of kerogens are features of organic matter reactivity rather than of the isolation method and thus provides a firm foundation for future work. The isolated kerogen could also be considered as a source material for the production of fine chemicals and other applications.