Hydrogen reduction of combusted iron powder: Role of the fluidization regime on the conversion

Fluidized bed systems play a crucial role in industrial processes such as combustion and gasification. In the Iron Power Cycle, fluidized bed systems are essential for enabling the reduction of combusted iron back to iron, making them a critical component in the regeneration step of the cycle. This study investigates the impact of operating gas velocity on conversion by performing reduction experiments at three distinct fluidization numbers ( u s / u mf ): 16 (bubbling regime), 55 (transition region), and 100 (fully turbulent regime). Experiments were conducted to determine the appropriate velocities for each regime, ensuring optimal fluidization conditions across reduction temperatures ranging from 500 to 700 ∘ C. The results reveal that conversion rates increase significantly with gas velocities. At 500 ∘ C, operating at approximately six times higher velocity leads to a sixfold improvement in conversion when using iron-oxide particles with a Sauter mean diameter of 61 µm. However, while enhanced velocities improve reaction efficiency, challenges remain at elevated temperatures (T ≥ 500 ∘ C), where iron undergoes defluidization when exposed to hydrogen. Once defluidization occurs, refluidization proves impossible with either hydrogen or nitrogen, raising concerns about process stability. These insights highlight the potential for optimizing fluidized bed reduction through velocity control, while also underscoring the need for additional measures to mitigate unstable fluidization during high-temperature iron oxide reduction. • At 500 ∘ C, a conversion of 11 wt% is achieved after one hour of reduction in bubbly regime (fluidization number of 16). This increases to 66 wt% under identical conditions when operating in a turbulent regime (fluidization number of 100). • Particle agglomeration occurs in all experiments, becoming more pronounced at T > 500 ∘ C. • Defluidization at high temperature ( T ≥ 500 ∘ C) with iron is irreversible, as the bed remains defluidized once it occurs.