Upgrading Distillation Residue from Hydrothermal Liquefaction Biocrude by Slurry Hydrocracking

Biocrudes are a promising feedstock for producing biofuels used in transportation subsectors that are difficult to electrify. However, upgrading and refining them into liquid fuels are challenging, largely due to the presence of high-boiling material that concentrate the most problematic oxygen compounds for processing. This study investigates the upgrading of the hardest-to-process residue fraction of a hydrothermal liquefaction (HTL) biocrude from forest biomass by slurry hydrocracking, a petroleum refining technology designed for the heaviest and most difficult-to-convert petroleum feedstocks combined with a two-stage temperature approach. Experiments were conducted in a batch reactor unit with a commercial hydrotreating catalyst at variable temperatures and catalyst loadings, with and without the use of a solvent. The results demonstrated that the residual fraction of the HTL biocrude can be converted with a 70.3% gross yield of distillate-range liquid fractions, while increasing the overall H/C ratio. The initial stage at low temperature (320 °C) was shown to help reduce residue gross yield. The lowest second-stage temperature tested, 380 °C, gave the highest increase in distillate yield and H/C ratio, whereas higher temperatures (400 and 420 °C) led to a growth in high-boiling byproducts at the expense of distillate yield, suggesting that the catalyst was unable to suppress the excessive condensation reactions occurring at such temperatures. Increasing the catalyst loading helped address this issue but only to a certain extent. The use of solvent appeared to reduce CO2 formation by improving heat transfer, but did not improve distillate generation. Advanced characterization revealed that the liquid products were largely composed of alkanes, cycloparaffins, and phenols, whereas the heavier products were rich in condensed aromatic compounds.