Comparative preparation of waste-derived K-modified activated biochars and proof-of-concept application in sewage-sludge hydrothermal liquefaction

Hydrothermal liquefaction (HTL) of sewage sludge offers a route for converting wet waste into biocrude but developing low-cost catalysts that enhance conversion while influencing product distribution remains challenging. This study comparatively evaluated biochars derived from sewage sludge, agricultural residues, and banana peel under N 2 and CO 2 pyrolysis, followed by K 2 CO 3 activation and proof-of-concept application in sewage-sludge HTL. Activation effects were strongly feedstock-dependent, with K 2 CO 3 activation of CO 2 -derived banana-peel biochar (BPBK-CO 2 ) increasing its BET surface area from 3.17 to 311.9 m 2 g -1 . However, HTL at 310 °C with 5 wt.% biochar showed that increased surface area did not translate directly into higher biocrude production. Biocrude yield increased from 25.75 wt.% in the catalyst-free baseline to 37.88 wt.% with non-activated BPB-CO 2 and 31.35 wt.% with BPBK-CO 2 . Biochar addition altered the relative biocrude composition by decreasing the contribution of oxygen-containing compounds and shifting the detected products toward lower carbon numbers. The non-activated biochar produced the strongest shift toward lighter compounds, whereas the activated biochar retained a comparatively greater contribution from the intermediate carbon-number range. ICP-OES showed that BPB-CO 2 contained substantially more potassium than BPBK-CO 2 and produced greater aqueous-phase potassium enrichment during HTL. The correspondence between potassium availability and biocrude yield is consistent with a proposed alkali-mediated contribution involving dissolved and surface-associated potassium species, which may facilitate hydrolysis, depolymerization, and oxygen-removal reactions while limiting repolymerization toward solid products. Overall, the findings suggest that HTL performance reflects the combined influence of potassium availability, carbon structure, and surface properties rather than surface area alone.