Enhanced hydrothermal liquefaction of shale oil sludge: Exploring catalyst combinations and synergistic effects of Fe, Na2CO3, and Ni/Al2O3

Oil shale has garnered significant attention as a supplementary energy source to traditional fossil fuels. In this study, we investigated the catalytic hydrothermal liquefaction (HTL) of shale oil sludge using three catalysts (Fe, Na 2 CO 3 , Ni/Al 2 O 3 ) and three combinations in pairs. The hydrothermal oil (HTO) yield, energy recovery (ER), and conversion rate (CR) of organic matters were comprehensively analyzed under six catalyst conditions. Both Fe + Na 2 CO 3 and Fe + Ni/Al 2 O 3 paired catalyst combinations achieved similar HTO yields of 28.43 % and 28.13 % along with ER values of 56.19 % and 57.94 % respectively. Moreover, these two groups achieved remarkably high CR values of 81.88 % and 81.02 %, respectively. The HTO from the Fe + Ni/Al 2 O 3 combination has the highest H/C ratio and the lowest O/C ratio at 1.682 and 0.032, respectively. Elemental composition, Fourier transform infrared spectroscopy (FT-IR), and gas chromatography-mass spectrometry (GC-MS) analyses of HTO were conducted in this study. The HTO produced by using Fe + Ni/Al 2 O 3 exhibited the most ideal distillation distribution, with fractions below diesel accounting for up to 96.94 %. Based on the catalytic functions of Fe, Ni, and Al 2 O 3 , six potential synergistic mechanisms of Fe + Ni/Al 2 O 3 were summarized. These mechanisms highlighted Fe + Ni/Al 2 O 3 's roles in promoting compound conversion pathways in HTO, ultimately revealing the synergistic catalytic mechanism.