Structural evolution and reaction pathways of hematite reduction by biomass-lignin derived bio-coke

In this study, bio-cokes with different mass ratios were prepared by co-pyrolysis of pine sawdust and lignin, and their hematite reduction behavior, gas release characteristics, and carbon structural evolution at 600-1000 °C were systematically investigated. The results show that hematite reduction follows a stepwise pathway of Fe₂O₃ → Fe₃O₄ → FeO → Fe, with the FeO-to-Fe transition identified as the key rate-controlling step because it becomes thermodynamically favorable only above 800 °C. Increasing the lignin proportion improved the fixed carbon content of bio-coke, leading to higher reduction efficiency at all tested temperatures, with a maximum reduction rate of 96.8% at 1000 °C. TG-FTIR and kinetic analyses revealed that the P2-L1 sample released CO and CO₂ more intensely and concentratively in the high-temperature region, thereby enhancing gas-phase reduction through the Boudouard reaction. Raman spectroscopy and microscopic observations further confirmed that higher lignin content promoted ordered carbon structural reorganization during high-temperature treatment, forming a more stable and graphitized carbon structure. This facilitated deeper deoxygenation and higher metallic iron yield. This work investigates the influence of lignin content variation in the blended feedstock on bio-coke structure and reduction performance, providing guidance for the development of biomass-derived reducing agents in low-carbon metallurgy.