Zero-Waste Biorefinery: Pyrolysis of Fermentation Residues into Catalytic Biochar for Circular Biohydrogen Systems
This study presents a closed-loop biorefinery strategy that thermochemically upcycles fermentation residues (FRs) from photo-fermentative biohydrogen production (PFHP) into functional biochar catalysts, thereby enhancing the efficiency of the initial PFHP process. Four FRs derived from hydrothermal and ethylene glycol-pretreated corn stover were pyrolyzed at 700{\deg}C. Multi-model kinetic analyses revealed diffusion-controlled mechanisms with activation energies ranging from 157 to 278 kJ/mol, while thermodynamic profiling highlighted the influence of feedstock composition on reaction spontaneity and entropy. Pyrolysis effectively restored porosity compromised during fermentation, yielding biochar with tailored properties: microporous BC3 (185 m2/g) from oxygen-rich precursors and mesoporous BC4 (76.58 m2/g) from graphitized residues. When reintroduced into PFHP, BC3 maximized cumulative hydrogen yield (570 mL) via pH buffering, and BC4 achieved the highest production rate (14.91 mL/h) through electron shuttle mechanisms. The integrated process concurrently generated syngas, bio-oil, and catalytic biochar, enabling waste valorization, renewable energy output, and process enhancement within a circular bioeconomy framework.