Effect of coal addition on reduction behavior of hematite in a H₂ atmosphere

Gas maldistribution in hydrogen-based shaft furnaces can result in low local reduction rates during the gas-solid reduction process. This study examines the incorporation of lignite into hematite to enable partial solid-solid reduction, enhancing reduction rates in a H₂ atmosphere. Between 350 and 700 °C, the hydropyrolysis of lignite consumes H₂ and emits substantial volatiles, which hinder the diffusion of external H₂ to hematite surfaces, reducing the overall reduction rate. Beyond 700 °C, hydropyrolysis shifts to the condensation stage, with H₂ becoming the dominant volatile, increasing H₂ concentration at the reaction interface. Together with carbon, this accelerates the reduction. Above 850 °C, the reduction is further expedited by CO and H₂ produced via the carbon gasification reaction. Unlike the Boudouard reaction, which starts at 900 °C in the coal-only reduction, the primary carbon gasification reaction in the synergistic reduction may be the steam‑carbon reaction, commencing at 750 °C. At heating rates of 30, 40, and 50 °C/min, coal addition lowers the reduction termination temperature by 49, 58, and 107 °C, respectively, while increasing the average reduction rate by 18.35 %, 19.97 %, and 27.14 %. The three reduction steps of hematite follow the Avrami-Erofeev nucleation model with n = 1.5, 2, and 2, respectively. The corresponding activation energies for these steps are 117.76, 84.68, and 69.51 kJ/mol. Moreover, SEM images confirm that adding lignite mitigates adhesion and sintering of iron whiskers.