Minimum fluidization velocity and reduction behavior of combusted iron powder in a fluidized bed

The fluidization and reduction behavior of micron-sized iron oxide powder, produced by iron combustion, is studied in a lab-scale cylindrical fluidized bed. The minimum fluidization velocity umf is found to stabilize above normalized static bed heights of 0.5 H/D (static bed height divided by the bed diameter). umf is measured as a function of temperature between 280 and 860 K for both H2 and N2 as fluidizing gas. The experimental results start to deviate from the Ergun correlation at temperatures above 560 K, both for N2 and H2. A new correlation, taking the cohesive inter-particle solid bridge force into account, is proposed in this work to predict the minimum fluidization velocity at high temperature. Reduction experiments are carried out for a total time of 5 h at constant excess velocity with 50, 75 and 100 vol% of H2 and temperatures between 623 and 823 K. Gradual defluidization occurs when the operating temperature exceeds 800 K. A maximum reduction degree of 61% is obtained at 807 K and 100 vol% H2. [Display omitted] •Minimum fluidization velocity is studied as function of temperature.•A new model is proposed for predicting the minimum fluidization velocity.•The reduction behavior of combusted iron is studied at constant excess velocity.•Gradual defluidization occurred at experiments above 800 K.