Hydrochar fractionation and composition in batch and continuous hydrothermal liquefaction

Understanding differences in hydrochar characteristics in batch and continuous hydrothermal liquefaction (HTL) is crucial in determining suitable valorisation routes for this byproduct, given its high nutrient load and carbon sequestration potential. This study thoroughly characterised hydrochar from batch and continuous HTL at 300, 325 and 350 ◦C, elucidating their main differences and shedding light on the operational parameters causing them. For this purpose, a bench-scale continuous HTL unit with in-line solids separation was commissioned. It was possible to differentiate primary and secondary char and infer their formation mechanism. The results showed that batch hydrochar yields were higher (24–26%) than continuous (primary and secondary) char yields (15–19%). In both reactions, higher temperatures led to chars with lower carbon and nitrogen contents. The ash content of batch hydrochars was lower than that of continuous primary chars, revealing that the in-line char separator effectively removed inorganic impurities at reaction conditions and produced a cleaner biocrude. The nutrient distribution in the HTL products showed batch biocrudes were more contaminated by Na, K and Fe, while in the continuous biocrudes, only Fe was detected. Moreover, less carbon migrated to the solids from continuous HTL, indicating that removing inorganics may reduce secondary char formation. Batch hydrochars showed a higher presence of oxygenated and nitrogenated compounds, while the continuous primary chars had a higher share of alkanes and alkenes. These differences may imply that batch reactions may not serve as indicators, in terms of hydrochar characteristics, for HTL upscaling to industrial plants.