The fate of Volatile Fatty Acids (VFAs) during the thermodynamic transition from hydrothermal carbonization to hydrothermal liquefaction: HtC-to-HtL

The valorization of biomass via hydrothermal treatment, a green and reliable thermochemical process, produces high-value chemicals. This study focuses on the transformation of Volatile Fatty Acids (VFAs) during the transition from Hydrothermal Carbonization (HTC) to Hydrothermal Liquefaction (HTL) and their impact on product quality. Conducted with anaerobically digested sewage sludge, the hydrothermal treatment varied temperatures from 280 °C to 340 °C. The primary objective was to investigate VFA concentrations and their link to hydrochar yield and gaseous by-product composition. A high-performance Parr 4577A hydrothermal reactor facilitated these experiments. For VFA assessment, a Shimadzu Nexis 2030 plasma GC-BID with an HP-FFAP column was utilized for the first time. The study revealed a U-shaped COD concentration curve (peaking at 10,510 mg/l at 340 °C) and an S-shaped Total Phenols profile (highest at 1314 mg/l at 325 °C). Notably, VFA concentrations dipped significantly at 295 °C and 325 °C but were higher at other temperatures, suggesting a complex underlying mechanism. This variability is partially attributed to the reduction in hydrochar mass yield with increasing temperature (46% at 280 °C to 31% at 340 °C) and a surprising spike in CO2 concentration (90% at 310 °C). These findings emphasize the critical role of carboxylation reactions in the HTC-to-HTL transition, enhancing the quality of liquid hydrothermal products.