Identifying the Transition from Hydrothermal Carbonization to Liquefaction of Biomass in a Batch System

The transition from mild to harsh hydrothermal conditions involves the passage from progressive hydrothermal carbonization (HTC) to liquefaction (HTL) of biomass, shifting products from solid to oily phases. Understanding the reaction pathways biomass undergoes during hydrothermal processing can help researchers tune operating conditions according to the desired products. This work investigates the transition from HTC to HTL of two model and one heterogeneous biomass: cellulose, lignin, and wood chips. The experimental method approaches a “real-time” study by sampling the reaction slurry during hydrothermal operation. Analyses were performed on the solid and liquid (aqueous and oily) products obtained from processing over a range of temperatures (220–320 °C) and residence times (0–2 h). Hydrochars’ thermal stability and carbon content increase with process severity (time and temperature). For all the substrates, thermogravimetric analyses show that volatile, thermally unstable compounds form at moderate temperatures. Samples drawn during the reaction with immediate liquid analysis enabled us to follow the evolution of organics, mainly carboxylic acids, furans, and phenol derivatives, providing an insight into hydrothermal reaction pathways. Overall, as temperature increases, organic acids and furans progressively transform to ketones and phenol-derivatives, while 5-hydroxymethylfurfural yield reaches a maximum at 30 min at 250 and 280 °C.