Carburisation of hydrogen-reduced iron ore pellets by CH₄–H₂ gas mixtures for sustainable steelmaking

Decarbonisation is critical for transitioning the steel industry toward sustainable, low-carbon production. As a major greenhouse gas emitter, the industry is exploring greener alternatives, such as producing Direct Reduced Iron (DRI) using hydrogen as a reductant to replace carbon. However, retaining a specific carbon content in DRI is essential for efficient steelmaking. This study investigates the carburisation of DRI pellets that were obtained by hydrogen reduction through using hydrogen-methane gas mixtures with varying CH₄ concentrations (10%–50%). Weight changes during reduction and carburisation steps were monitored using thermogravimetric analysis. The mineralogy, microstructure and chemical properties of the oxide reduced and carburised pellets were analysed using XRD, SEM, XRF and optical microscopy. Thermodynamic calculations with HSC Chemistry 9 were performed to evaluate experimental results. The findings show a complete reduction of iron oxide to metallic iron during the reduction step by hydrogen. During carburisation, increasing CH₄ concentration enhanced cementite formation, with a notable transition occurring above 20 vol% CH₄. Initially, cementite is formed via a direct reaction of methane with metallic iron, and at higher CH₄ levels, methane cracking precedes the cementite formation. This study identifies optimal carburising gas compositions for carburising DRI and elucidates mechanisms involved in CH₄–H₂ carburisation.