Thermochemical regeneration of iron-based materials using syngas derived from carbon dioxide gasification of digested...

Conventional regeneration of iron-based materials heavily relies on fossil-derived reducing gases, necessitating sustainable thermochemical alternatives. This study investigates a CO₂-assisted thermochemical regeneration strategy that integrates CO₂ gasification with iron redox cycling, simultaneously enabling sludge valorization. Commercial activated carbon and digested sludge biochar were evaluated under CO₂/steam atmospheres to characterize syngas generation as a function of the steam-to-CO₂ ratio and the gas flow rate. The generated CO-rich syngas was utilized for the thermochemical reduction of oxidized iron species (Fe₂O₃ and oxidized zero-valent iron, ZVI), followed by functionality assessment via H₂ production performance. Both carbon sources drove a stepwise reduction pathway (Fe₂O₃ → Fe₃O₄ → FeO → Fe), and biochar-derived syngas promoted more rapid progression toward metallic Fe under the specific conditions of the in-situ XRD experiment. Extensive Fe₂O₃ reduction toward metallic Fe was achieved under specific conditions, producing submicron metallic Fe particles that exhibited higher cumulative H₂ production than did virgin ZVI (<150 μm). Pre-oxidized ZVI samples were also functionally restored; both the FeCO₃ formed via reactions of ZVI with H₂O and CO₂ and that generated in the presence of sludge residues exhibited viable H₂ production capacities after regeneration. These results indicate that effective utilization of CO₂-derived syngas for iron-material regeneration provides a potential pathway for sludge-derived carbon valorization and integrated resource recovery.