Numerical investigation on hydrogen reduction of iron oxide pellets in a shaft furnace: unveiling the impact of particle size

The reduction process of iron oxide pellets in the hydrogen shaft furnace is investigated numerically with the main purpose of clarifying the extent to which the pellet diameter can be reduced under the particle fluidization constraint, further exploring feasible options for preventing fluidization. A standalone numerical routine is developed to estimate the fluidization factor of the in-furnace burden. The results show that an increase in the gas feed rate yields a higher solid reduction degree, but also causes a larger fluidization factor due to the increase in the pressure drop. Under the conditions where the specific gas feed rate and pellet diameter are 1600 Nm3/t-pellet and 13 mm, respectively, the fluidization factor is smaller than unity, which is the threshold representing the occurrence of particle fluidization. A decrease in the pellet diameter is kinetically beneficial for the reduction reactions but also results in a clear rise of the pressure drop and hence an increase in the fluidization factor, which surpasses unity when the pellet diameter becomes smaller than 11 mm. The fluidization factor can be effectively lowered by adopting a higher top gas pressure, indicating that increasing the operating pressure is a feasible option for preventing fluidization of small particles.