Article ID: ISIJINT-2025-089
Reducing the greenhouse gas emissions from steel production can be done through direct reduction inside a shaft furnace using hydrogen gas as a reductant, generating water as an off gas. The temperature varies along the height of the shaft furnace, and studying the non-isothermal reduction is therefore necessary. In this work, industrial hematite pellets were non-isothermally reduced in a vertical tube furnace. Different gas mixtures containing water and hydrogen were used for reduction. The reduction gas used contained water vapor contents of 5%, 10%, and 20%, respectively, and the remaining gas was hydrogen. The experimental setup was carefully designed for the reductions to be carried out under well-controlled experimental conditions. It was clear that the water present in the reduction gas significantly decreased the reduction rate, especially at the lower temperatures. Moreover, the onset temperature of reduction was increased to around 525 °C when water was present, compared to 450 °C when pure hydrogen was used. Water contents above 5% lead to a low-rate stage at reduction degrees between 0.11 to 0.15. The low-rate stage ended when the wüstite phase became stable, changing the mechanism of reduction, which altered the chemical reaction rate. The reduction rate was less affected by water when the heating rate increased, since an increasing heating rate led to the reduction occurring at a higher temperature. Finally, the present study showed that the kinetics of non-isothermal reduction, using different water vapor contents, are very different from isothermal reduction.
