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.