Advancing hydrothermal liquefaction of Canadian forestry biomass for sustainable biocrude production: co-solvent integration, co-liquefaction, and process optimization†

Canadian hardwood and softwood species were screened for hydrothermal liquefaction to produce sustainable biocrude. Based on the availability of the feedstock, their biocrude yield, and oxygen content, spruce (softwood) and poplar (hardwood) species were found to be promising and selected for the optimization of process parameters to maximize biocrude yield while minimizing the oxygen content. Solvent (ethanol) assisted hydrothermal liquefaction was performed to evaluate the effect of process parameters such as temperature, retention time, catalyst loading, and different ethanol concentrations. The highest yield of biocrude obtained from spruce and poplar was ∼36 wt% with an HHV of ∼27 MJ kg⁻¹ under the optimized HTL conditions. HTL experiments were conducted to study the effect of recycling the hydrothermal liquefaction aqueous phase and co-liquefaction of hardwood and softwood species. The HTL aqueous phase recycling improved the quantity (47 wt%) and quality (HHV of 29.9 MJ kg⁻¹) of the biocrude obtained from spruce liquefaction. The co-liquefaction of spruce and poplar (50 : 50 wt%) showed a potential synergistic effect on biocrude yield and quality at a lower reaction temperature (260 °C). The GC-MS analysis of spruce and poplar wood biocrude indicated that the majority of the compounds were phenolic in nature. BET results confirmed the high surface area of spruce and poplar wood-derived hydrochar. The gaseous products formed during HTL were mainly composed of CO₂, CO, H₂, O₂, CH₄, and C₂H₂.