The effect of CO2 and increased initial pressure on hydrothermal liquefaction for the production of high value fatty acids

• Study on the effect of CO 2 , N 2 and pressure on HtL. • CO 2 creates a reactive medium that drives carboxylation reactions in HTL. • CO 2 enhanced VFA production, yielding 2 g/ L acetic acid. • Hydrochar HHV peaked at 35 MJ/ kg at 350 °C. • Pressure significantly affected VFA/FAME yields at ≤ 350 °C. Hydrothermal liquefaction (HTL) of waste biomasses has been widely studied, yet the influence of the reaction atmosphere on carboxylation chemistry is often overlooked. In this study, HTL of anise ( Pimpinella anisum L.) waste was performed at 310, 350 and 390 °C solely with air (no added pressure), under N 2 with and without increased initial pressure, solely with CO 2 , and with combined N 2 /CO 2 and increased pressure. The aim was to assess how total pressure and gas/ atmosphere composition affect product distribution and high‑value compound formation. Dissolved CO 2 was found to acidify the aqueous phase and participate in carboxylation reactions that convert hydrolysis intermediates into volatile fatty acids (VFAs), as measured via a GC-BID in the form of fatty‑acid methyl esters (FAMEs). Chemical oxygen demand (COD) in the aqueous phase increased from 10,000 to 30000 mg/ L and was consistently higher under CO 2 . Hydrochar higher heating value peaked at approximately 35 MJ/ kg at 350 °C and declined at 390 °C. CO 2 ‑rich runs exhibited an order‑of‑magnitude increase in VFA production, with acetic‑acid concentrations up to 2 g/ L. FAMEs formation increased with temperature; at 390 °C the combined N 2 / CO 2 system produced methyl hexanoate concentrations of approx. 13 g/ L and methyl octanoate 11.9 g/ L, more than threefold higher than in single‑gas or non‑pressurized runs. One‑way ANOVA revealed that pressure significantly influenced VFA and FAME yields at 310–350 °C but this effect diminished at 390 °C. These results highlight the importance of optimizing both gas atmosphere and pressure in HTL processes and demonstrate that reactive CO 2 atmospheres can drive carboxylation pathways leading to higher yields of valuable fatty acids.