Evaluation of Bio-Oil Quality from Hydrothermal Liquefaction of Chlorella vulgaris Using Entropy–TOPSIS Method

Bio-oil production from microalgae presents a promising approach to address both energy crisis and environmental pollution. This study investigates the hydrothermal liquefaction (HTL) of Chlorella vulgaris for bio-oil production under varying solvents (methanol, ethanol, and propanol), catalyst types (zeolite, activate carbon, and graphene oxide) and catalyst loadings (0, 5, and 10 wt % of relative to dry algae). HTL reactions were conducted at temperatures ranging from 250 to 280 °C for 30 min, with biomass cake concentrations of 42.8–60.0 wt %. The objective was to determine optimal conditions for producing high-quality bio-oil using the Entropy–TOPSIS multicriteria decision-making method. Catalyst properties were characterized using SEM–EDS and nitrogen sorption analysis. Bio-oils were analyzed for higher heating value (HHV) and chemical composition via elemental analysis and gas chromatography-simulated distillation. Results showed bio-oil yields ranging from 43.65 to 71.51 wt %, with HHVs between 23.92 and 40.36 MJ/kg, indicating their suitability as transportation fuels. Among the solvents, propanol produced the highest amount of solid residue (biochar), while methanol promoted higher oil yields. The Entropy–TOPSIS analysis identified the most favorable conditions as HTL using methanol with 5 wt % graphene oxide catalyst, followed by methanol with 5 wt % activated carbon, and ethanol with 10 wt % zeolite. Energy recovery ranged from 63.69% to 96.18%, outperforming comparable biomass conversion processes. These findings support the potential for scalable, catalyst-enhanced HTL systems in commercial microalgae-based biofuel production.