Abstract

The transition towards hydrogen-based direct reduction ironmaking represents a major step for carbon-neutral steelmaking. However, the absence of carbon in hydrogen-based direct reduced iron (H₂-DRI) limits its utilization in subsequent steelmaking operations. Controlled carburization, achieved by injecting carburizing gases into the cooling zone of the direct reduction shaft furnace, provides a viable option for reintroducing carbon into H₂-DRI and bridging the carbon gap for its utilization in steelmaking.
In this study, the carburization behavior of H₂-DRI was systematically investigated under CO-rich gas atmospheres. The experiments were designed to simulate the post-reduction stage of a direct reduction process, wherein the reduced pellets are exposed to carbon-bearing gases. Single-pellet experiments were conducted using a thermogravimetric analyzer (TGA) and blast furnace (BF) grade iron ore pellets under controlled gas atmospheres. Each test began with pure hydrogen reduction of the pellets, achieving an average reduction degree of 93%. Following this, isothermal carburization was carried out in CO-based atmospheres at temperatures ranging from 600 °C to 800 °C. Three carburizing atmospheres were examined: (i) 100% CO, (ii) CO + 10% H₂, and (iii) CO + 10% CO₂ mixtures. The real-time mass change of the pellet during carburization was recorded to evaluate carbon uptake and compare carburization rates across different gas compositions and temperatures.
The results show a strong dependence of carburization rate on both gas composition and temperature. In pure CO atmosphere, carbon deposition proceeds primarily via the Boudouard reaction (2CO ⇌ CO₂ + C), with carburization rates decreasing as the temperature increases. The introduction of H₂ enhanced carburization rates across the temperature range. This enhancement can be explained through the coupling of the Boudouard reaction and the reverse water-gas shift reaction (CO₂ + H₂ ⇌ CO + H₂O). H₂ reacts with CO₂, effectively reducing CO₂ partial pressure and generating additional CO, thereby shifting the Boudouard equilibrium forward and increasing the driving force for carbon deposition. Conversely, the addition of CO₂ suppressed the carburization rate, as the elevated CO₂ partial pressure shifts the Boudouard equilibrium backward, lowering the carbon activity at the gas–solid interface.
The hierarchy of carburization rates, with the highest in the CO + H₂ mixture, intermediate in pure CO, and lowest in the CO + CO₂ mixture, was consistent across all studied temperatures. A maximum mass gain rate of 22.0 mg/min was achieved in the case of CO + 10% H₂ at 600 ⁰C, while the lowest rate of 3.2 mg/min was achieved using CO + 10% CO₂ at the same temperature. These findings underline the crucial role of carburizing gas atmospheres and temperature affecting both carbon activity and the kinetics of carburization. From a process metallurgy perspective, such insights provide valuable guidance for tailoring shaft-furnace gas atmospheres to reintroduce carbon into H₂-DRI without compromising the low-carbon objectives of hydrogen steelmaking.