Influence of separation mechanism and position on char formation and characteristics in continuous hydrothermal liquefaction

Hydrothermal liquefaction (HTL) can convert wet biomass into biocrude, a renewable alternative to fossil fuels, under high-temperature and −pressure. HTL solid by-product, hydrochar, poses challenges such as reactor fouling and clogging. Despite advancements in HTL technology, solids formation and separation remain a critical area of development. To address these challenges, this study evaluated in-line solids separation (i) before the reaction zone at different reaction temperatures (300, 325 and 350 °C), (ii) at reaction conditions via passive settling and filtration using different filter pore sizes (5, 20, and 40 µm) and (iii) in the cooling zone, while solids from biocrude were extracted using DCM. Cow manure was used as feedstock and hydrochar fractions were characterised using TGA, ICP-OES, py-GC–MS, SEM-EDX, NMR, laser diffraction, and elemental analysis. Results showed that solids collected before the reaction zone exhibited lower ash content and a more aliphatic structure, while those separated at reaction conditions contained up to 78 % ash with more aromatic structures. Reaction temperature affected yields and carbon content, while filter pore size had minimal impact. Settling was less effective for inorganics separation, increasing solids downstream. The findings provide valuable insights for optimising hydrochar management and valorisation in continuous HTL systems.