Insights into the reaction mechanisms and pathways of shale oil sludge hydrothermal liquefaction

Shale oil sludge (SOS), composed of shale dust, wastewater, and residual shale oil, is an industrial waste that is notoriously difficult to treat. This investigation explores the potential of subcritical water processing as an innovative and sustainable treatment methodology for SOS. HTL experiments of SOS were conducted at various temperatures and reaction times. The results indicate that the reaction temperature increased from 240 °C to 300 °C, and the energy recovery rate (ER) of hydrothermal oil (HTO) increased from 55.52 % to 65.64 % at a sludge-to-water ratio of 1:5 and a reaction time of 60 min. Through characterization of SOS and products, we found that HTL breaks up the emulsification state of SOS and achieves dehydration and oil extraction of SOS. SOS gradually generated HTO, hydrochar, and water through depolymerization and demulsification. A total of 12 types of reactions that may occur in HTO during hydrothermal reaction were identified, such as depolymerization, aromatization, pyrolysis, hydration and others. Finally, the causal relationships between hydrothermal processing parameters and the resulting composition and structure of HTO are explained. The reaction pathways of HTO components and the hydrothermal reaction mechanism of SOS are also reported. This paper provides an important basis for the harmless use of SOS and other oily sludge as resources.