Multiobjective Optimization of the Hydrothermal Liquefaction of Microalgae for Cost-Effective Advanced Biofuels Production

Hydrothermal liquefaction (HTL) converts microalgae into bio-oil, a biofuel precursor expected to play a pivotal role in the energy transition. This study investigates the combined influence of temperature and reaction time on the trade-off between bio-oil yield and quality, and its implications for process economics. A multivariable optimization framework integrating experimental design, process simulation, and technoeconomic analysis was developed to identify cost-optimal conditions. Distinct optimal points were obtained depending on production scale, with 287 °C and 30 min for small capacities, and 304 °C and 12 min for upscaled production. These conditions reduced the minimum fuel selling price (MFSP) by 5.16% and 19.27%, respectively, compared to frequent yield-based optimization, while decreasing hydrogen demand for hydrotreatment by up to 4.50%. Despite these improvements, elevated costs (13.04 €/L and 3.77 €/L, respectively) and significant economic uncertainty persist, underscoring the importance of increasing production rates and reducing capital costs.