The non-isothermal thermal decomposition evolution of the Fushun oil shale kerogen based on ReaxFF molecular dynamics simulation

In this work, reaction force field (ReaxFF) molecular dynamics (MD) simulations were utilized to investigate the reaction mechanism and product distribution during the non-isothermal thermal evolution of the Fushun oil shale kerogen (Chemical formula C240H322O32N7S5, abbreviated as W). The macromolecular scale systems (15 W) were constructed based on the structural model of Fushun kerogen by applying molecular dynamics. Non-isothermal simulations were carried out based on these at different heating rates of 2, 5, 10, 30, 50, 80, and 100 K/ps by applying ReaxFF. The calculation results show that kerogen pyrolysis is mainly an internal molecular change. The initial pyrolysis stage consists of the breakage of the weak C-C and C-O bonds. As the temperature increases, the kerogen reacts intra-molecularly to form large volumes of shale oil (light oil: C5-C13; heavy oil: C14-C40) and gas (organic gas: C1-C4, inorganic gases). There are specific formation patterns of typical pyrolysis products. The tar product decrease with increasing temperature at different heating rates, and the thermal degradation of kerogen tar push backward as the heating rates increase, which is consistent with previous experimental results. At the same time, the species and number of pyrolysis products show a trend of increasing and then decreasing. Shale oil shows different patterns of change at different heating rates, with the lighter oil coming partly from the pyrolysis of the heavier oil when kerogen temperature is reached. The gases produced by the pyrolysis process of kerogen are mainly organic, including CH4, C2H2, C2H4, C2H6, C3H6, C4H8, CH2O, and inorganic, including H2O, CO2, H2, NH3. The above results provide a reliable theoretical basis for the future industrial use of the Fushun oil shale.