Chlorella vulgaris thermochemical valorization: experimental and life cycle assessment of hydrothermal liquefaction versus pyrolysis

This study provides a comprehensive comparison of the thermochemical conversion of Chlorella vulgaris via hydrothermal liquefaction (HTL) and pyrolysis (PYR), integrating detailed characterization of all product streams and life cycle assessment (LCA). Although HTL and PYR provide comparable yields of liquid and solid products, their composition, quality and valorization potential differ substantially. PYR oils contain lighter components and a higher proportion of valuable nitrogenated compounds than bio-crudes. Likewise, pyrochars possess higher carbon content and stability than hydrochars, whose elevated ash content and lower carbonization degree limit their suitability for soil amendment. Clear differences are also observed in the aqueous fractions. PYR aqueous phase is more concentrated and contains potentially valuable compounds (acetic acid and 1,4-diaminobutane). In contrast, the HTL aqueous phase is highly diluted and contains residual dichloromethane from the bio-crude recovery step, limiting its direct downstream utilization. These compositional differences are reflected in the LCA results, which show substantially higher environmental impacts for HTL, driven mainly by its fossil energy demand, CO 2 emissions and solvent-intensive bio-crude recovery step. Although PYR requires additional energy for microalgae drying, this is outweighed by its lower carbon and fossil energy footprints and by the greater economic potential linked to its higher yield of valuable products. In summary, despite the technical and economic barriers that both routes still face for commercial deployment, pyrolysis appears, under the conditions studied here, as a more sustainable and feasible pathway for microalgae valorization.