Enhancing the Yield and Performance of Bio-Oil Produced from the Hydrothermal Liquefaction of Microalgae with High Solid Contents: Optimization and Modeling

The hydrothermal liquefaction reaction was carried out with Chlorella vulgaris as the feedstock, which has a high solid content (20–100 wt %). The effects of the reaction temperature (250–350 °C) and solid content on the characteristics of bio-oil (including yield, higher heating value (HHV), and elemental composition) were studied via central composite design (CCD) and response surface methodology (RSM) to model the relationships among these variables. This study reliably established a reaction model via response surface analysis, and the optimum reaction conditions calculated from the model were 350 °C and 59.04 wt % feedstock, resulting in a 37.94 wt % bio-oil yield, 37.36 MJ/kg HHV, 8.07 wt % N content, 0.375 wt % S content, and 6.69 wt % O content, respectively. Increasing the solid content under high-temperature conditions can effectively reduce the content of heteroatoms in bio-oil. GC–MS analysis of the bio-oil revealed a substantial increase in the concentrations of saturated hydrocarbons, unsaturated hydrocarbons, aliphatic alcohols, and aromatic compounds with increasing solid content. The most notable increase was observed for aromatic compounds, with a corresponding increase in content of up to 27.63%. The main condensation reaction occurs during the gradual increase in solid content, resulting in a hexadecanamide content of up to 10.84% at a solid content of 60 wt %. As the solid content increases beyond this point, the components in the bio-oil undergo a noticeable dehydration reaction, leading to a gradual increase in the hexadecanenitrile content to 4.33%, while the fatty acid content decreases. This change facilitates the subsequent hydrodesorption of heteroatoms.