Effect of C-Bearing Industrial Off-Gases on Carburization and Melting Behavior of H$$_2$$-DRI: Combined Thermodynamic and Experimental Study

Abstract Hydrogen-based ironmaking is one of the promising strategies for achieving a carbon-neutral ironmaking process. However, Hydrogen-Direct Reduced Iron (H $$_2$$ 2 -DRI) contains almost no C, resulting in higher energy consumption during the smelting process. Introducing additional C into H $$_2$$ 2 -DRI may lower its melting temperature, thereby reducing the energy consumption. In the present study, the feasibility of industrial off-gas as carburizing agents of H $$_2$$ 2 -DRI was investigated through CALPHAD thermodynamic calculations and a series of laboratory-scale experiments. Carburizing potential, represented in terms of the extent of Fe $$_3$$ 3 C formation, was evaluated for carbon-containing gases. After carburization, the sequential melting behavior of carburized H $$_2$$ 2 -DRI was observed between 1100 °C and 1550 °C using horizontal and drop quench furnaces. It was found that CO and CH $$_4$$ 4 were effective in carburization at low and high temperature, respectively. Among the off-gases, COG–LDG mixtures exhibited higher carburizing potentials for H $$_2$$ 2 -DRI than COG–BFG, as the former includes relatively low CO $$_2$$ 2 content. Carburization at high temperature was less favorable because the formation of Fe $$_3$$ 3 C was kinetically hindered while free C deposition was promoted. Carburization Ratio (CR) and Carbon Utilization Ratio (CUR) were defined to assess the extent of carburization and the effectiveness of carburization. Observation of H $$_2$$ 2 -DRI melting behavior revealed that carburization alone is insufficient to induce complete melting of H $$_2$$ 2 -DRI. Thus, fluxing and carburization should be considered at the same time to accelerate melting of H $$_2$$ 2 -DRI in the smelting process. These findings provide fundamental knowledge on gaseous carburization based on comparisons between thermodynamics and practice, emphasizing the importance of considering complex reactions in carburization and fluxing to promote complete melting of H $$_2$$ 2 -DRI.