Carburization in direct reduction: Effects of various CO-H2 gas mixtures

Hydrogen and carbon monoxide govern distinct pathways in direct reduction of iron ores, influencing process efficiency and product quality. H 2 enables rapid, nearly CO 2 -free reduction, but is strongly endothermic and yields carbon-free direct reduced iron. In contrast, CO enables reduction and supplies in-situ carbon for secondary steelmaking. Balancing these gases, and establishing renewable CO sources such as biomass-derived syngas, is central for decarbonizing steelmaking. Systematic datasets linking carburization with reduction kinetics and product characteristics under industrially relevant conditions remain scarce. Single DR-grade pellets were reduced isothermally (973–1273 K) in H 2 /CO atmospheres under moderate flow conditions. Weight evolution was tracked by thermogravimetry, and characterization (SEM–EDS, XRD, nanoindentation) resolved carbon distribution, phase composition, and mechanical properties. Complementary trials with simulated biogenic gas showed slower early reduction than syngas but higher final metallization; only at 973 K did a slight late-stage mass gain indicate controlled carburization. Gas composition and temperature governed the balance between Fe 3 C formation and free-carbon deposition. The resulting dataset enabled calibration of a porous–solid model that couples oxygen removal, carbon deposition, and carbide formation, providing a mechanistic basis for optimizing metallization and controlled carburization in hydrogen-rich and renewable CO-containing DR processes. • Pure H 2 gives fast reduction, full metallization, and a dense uniform α-Fe structure. • Pure CO exhibits slow reduction and forms iron whiskers at 1273 K with minimal carbon. • H 2 /CO forms carburized structures, which potentially hinder reduction. • Simulated biogenic gas attains high metallization with controlled carburization. • The 1D porous-solid model links oxygen removal, carbon uptake, and carbide formation.