Highly efficient catalytic pyrolysis of oil shale by CaCl2 in subcritical water

An innovative method combining CaCl2 with subcritical water is proposed for in-situ mining of oil shale. The catalytic hydrothermal pyrolysis of block Huadian oil shale was carried out at 350 °C to evaluate the feasibility and reaction mechanism of this method. After adding CaCl2, the time required for kerogen pyrolysis was shortened from 70 to 40 h, while the maximum expelled oil yield increased from 19.14% to 21.96%. This excellent catalytic activity indicated that CaCl2 was transported into shale matrix by subcritical water to catalyze the pyrolysis of kerogen. It was found that the C–O bonds in kerogen were first broken and then the whole decomposition of kerogen initiated. The compositional analysis of residual kerogen indicated that CaCl2 could facilitate the transportation of hydrogen from subcritical water to kerogen, thereby inhibiting the excessive condensation of kerogen to improve its oil-generating potential. The macromolecular compounds in expelled oil would undergo a secondary cracking reaction. Besides, CaCl2 could also promote the re-cracking and discharge of residual bitumen in oil shale matrix. These findings demonstrated that combining CaCl2 and subcritical water enabled efficient pyrolysis of block oil shale and provided a novel technique for in-situ mining of oil shale.