Continuous-flow production of petroleum-replacing fuels from highly viscous Kraft lignin pyrolysis oil using its hydrocracked oil as a solvent

The development of solvent-free lignin pyrolysis processes is highly desirable, because these processes would allow the depolymerized product to be used directly as a renewable energy source and chemical feedstock, without removal of solvnets. However, this product is typically highly viscous and cannot be used in continuous-flow reaction systems. In this study, lignin pyrolysis oil was prepared from Kraft lignin using bench-scale fixed-bed batch pyrolysis and then hydrocracked to produce less-viscous liquid products. Oligomers were degraded into smaller molecules via reactions involving hydrogen (hydrocracking) using CoMo/Hβ and CoMo/Al₂O₃ as catalysts. A low viscosity of 21 cP, a liquid yield of 76.6%, and a low coke yield of 1.6% were successfully attained using a reaction temperature of 400 °C, a reaction time of 60 min, and the CoMo/Hβ catalyst. Thus, these conditions were selected to achieve the highest liquid yield with sufficient fluidity, although the lowest viscosity of 3.2 cP was achieved after 240 min. When 30 wt% of the resulting hydrocracked oil was used, it dissolved sticky raw lignin pyrolysis oil, significantly reducing its viscosity from 751 cP to 111 cP, which is sufficient to ensure flow in a typical petroleum pipeline. Using the hydrocracked/raw lignin pyrolysis oil mixture, the proposed continuous-flow hydrodeoxygenation successfully produced petroleum-replacing deoxygenated fuels.