Effects of HTL Operating Conditions on Fuel-Related Properties and Compositional Evolution Microalgae-Derived Bio-Crude Oil

This study systematically investigated the hydrothermal liquefaction (HTL) of Chlorella vulgaris to understand how operating conditions influence bio-crude oil yield, nitrogen behavior, fuel properties, and compositional changes. A Box–Behnken design (BBD) was used, with temperature (200–300 °C), pressure (50–200 bar), and reaction time (15–45 min) as the process variables. Bio-crude oil yields ranged from 11.10 to 40.79 wt% (dry basis), and nitrogen content varied between 2.34 and 6.22 wt%. Temperature was the primary factor affecting both bio-crude oil yield and nitrogen content. Higher temperatures increasedbio-crude oil production but also led to more nitrogen incorporating into the oil phase, indicating a trade-off between yield and nitrogen retention. Van Krevelen analysis showed that HTL bio-crude had lower O/C ratios than the raw microalgal feedstock, suggesting progressive deoxygenation under more severe HTL conditions. GC–MS analysis revealed that hydrocarbons increased and fatty acids decreased with rising temperature. Nitrogen-containing compounds such as amides and N-heterocyclic compounds, were still detectable at high temperatures. Multi-response optimization identified 300 °C, 200 bar, and 15 min as the optimal conditions, balancing maximized bio-crude oil yield with minimized nitrogen content. These results suggest that HTL bio-crude oil is regarded as an intermediate feedstock requiring further upgrading. Therefore, optimizing the HTL process should simultaneously consider both fuel-related properties and nitrogen behavior.