Valorization potential of the aqueous products from hydrothermal liquefaction and stepwise slow pyrolysis of wood bark and hemp hurds with yields and product comparison

Abstract

Birch bark, spruce bark, and hemp hurds were processed with hydrothermal liquefaction (HTL) and stepwise slow pyrolysis (SSP) and the compositions of the aqueous liquid products were quantitatively compared. The phenolic profiles of the analyzed products were different, but overall they contained similar concentrations of aromatics and phenolic compounds (4–43 mM and 52–215 mM, respectively). The SSP fractional condensation provided liquids with lower water content (appr. 80 % in SSP, 95 % in HTL), but the HTL process was selective e.g. in producing catechols into the aqueous phase. HTL produced higher amounts (15–4406 mmol kg−1) of the analyzed compound groups per dry matter into the aqueous products than SSP (8–1387 mmol kg−1). This study forms a solid basis for feedstock and process optimization considering the future valorization of these liquid products.