Stabilization of Pyrolysis Oil from Woody Biomass Using Ni/Al₂O₃ and Sulfided NiMo/Al₂O₃ Catalysts

In the action against climate change, it is vital to find alternatives to conventional fossil fuels. One such alternative is biomass-derived pyrolysis oils. However, these pyrolysis oils contain high levels of oxygen and water and therefore need to be upgraded to be suitable for further processing in a refinery. It has been proposed to upgrade pyrolysis oils in two steps to minimize operational issues, particularly coking and reactor plugging: a stabilization step at lower temperatures followed by hydrodeoxygenation at elevated temperatures. Two different catalysts (metallic Ni/Al₂O₃ and sulfided NiMo/Al₂O₃) were used in this study to stabilize wood fast pyrolysis bio-oil in a batch reactor for 2 h at 90 bar initial H₂ pressure and 120, 180, and 220 °C. The impact of the catalyst to oil ratio as well as reaction time were also investigated. The stabilized pyrolysis oil was characterized for elemental composition, carbonyl number, and micro carbon residue (MCR). The best results toward stabilizing the pyrolysis oil, in terms of lowering the carbonyl number and MCR, were achieved with the Ni/Al₂O₃ catalyst at 15 wt % catalyst to oil ratio. However, leaching of Ni was observed for all experiments with the Ni/Al₂O₃ catalyst, which may prove to be a significant issue for continuous operation. The sulfided NiMo/Al₂O₃ catalyst only reduced the carbonyl number marginally, and increasing the reaction time did not improve this.