Study on the pyrolysis behavior and product characteristics of Balikun oil shale with different water pressures in sub- and supercritical states

•Using newly developed equipment studied the effect of water pressure on oil shale pyrolysis.•Studied changes in product yield and composition over different time periods in pyrolysis process.•Only below 23 MPa, increasing water pressure improves pyrolysis efficiency in this experiment conditions.•Oil shale pyrolysis product yield for supercritical water is higher than subcritical water.•Quality of oil products for supercritical water pyrolysis of oil shale is poor. To study the influence of water pressure on sub/supercritical water pyrolysis of oil shale, a self-developed organic rock pyrolysis experiment system was used to conduct pyrolysis experiments of Balikun oil shale under different water pressures in an isothermal environment set at 400 °C, and the products were characterized by GC, GC-MSD, column chromatography and other methods. The results show that there is an optimal water pressure value of 23 MPa. When the water pressure is lower than 23 MPa, the yields of oil and gas products and the conversion rate of organic matter increase with the rise of water pressure, while when the water pressure is higher than 23 MPa, they decrease. The yields and contents of gas components exhibit non-monotonic variations with water pressure, but the yield of CO2 decreases with increasing water pressure under supercritical water conditions. The content of saturated hydrocarbons decreases with increasing water pressure, while the content of resins and asphaltenes decreases with increasing water pressure. Changes in water pressure result in variations in water solubility, permeability, fluid density of the pyrolysis environment, and chemical properties of the supercritical water. These variations influence the pyrolysis characteristics of the organic matter, leading to differences in both the yields and components of pyrolysis products. At optimal water pressure, the highest returns can be obtained. The selection of optimal water pressure is related to the composition and mineral structure of the oil shale. The conclusions of this study provide theoretical support and references for selecting optimal water pressures for the extraction of organic matter from deep oil shale using sub/supercritical water.