Circular valorization of oil palm empty fruit bunch residue: biochar-assisted SSF for enhanced bioethanol production

The conversion of oil palm empty fruit bunches (EFB) into bioethanol via simultaneous saccharification and fermentation (SSF) is often limited by low conversion efficiency and prolonged fermentation duration. To overcome these limitations and achieve circular utilization of biomass residues, this study proposes a residue-to-biochar recycling strategy, where EFB fermentation residue is pyrolyzed to generate biochar and subsequently repurposed as an additive in the SSF process. The effects of residue-derived biochar on ethanol yield were evaluated by varying biochar dosage and pH, and the optimal SSF conditions were systematically identified. Among the tested residue-derived biochar, EBC-500 showed the most pronounced enhancement effect. Under optimized conditions (biochar dosage of 0.2 g, corresponding to 10% (w/w) relative to dry biomass loading，and pH 5), the ethanol concentration reached 7.57 g/L after 24 h, corresponding to a maximum ethanol yield of 0.44 g/g, representing a 72% increase compared with the biochar-free control. Biochar enhanced ethanol productivity within an existing 24 h kinetic window, and the superior performance of EBC-500 was linked to surface chemical features rather than surface area alone. This study provides a practical and sustainable approach for increasing bioethanol production via the valorization of fermentation residues, while offering mechanistic insights into biochar-assisted SSF under cost-effective enzyme system conditions for circular bioresource utilization in the oil palm industry.