Chemical looping hydrogen generation using iron oxides as oxygen carriers is a novel technology to convert carbonaceous substances into hydrogen. The reduction characteristics of oxygen carriers greatly impact the process… Click to show full abstract
Chemical looping hydrogen generation using iron oxides as oxygen carriers is a novel technology to convert carbonaceous substances into hydrogen. The reduction characteristics of oxygen carriers greatly impact the process efficiency. In this work, the oxygen carriers of Fe2O3 60wt%/Al2O3 were prepared by physical mixing method. The stepwise reduction kinetics of this material was investigated at 800–1000 °C in a TGA with a thermodynamics-controlled method. For this method, CO/CO2 mixture gases at the volume ratios of 0.11, 1, and 5.67 were selected as fuels to decouple the continuous reduction of Fe2O3→Fe into independent reductions, including Fe2O3→Fe3O4, Fe3O4→FeO and FeO→Fe with little mutual influences. The kinetic mechanisms of Fe2O3→Fe3O4, Fe3O4→FeO and FeO→Fe were well represented by the nucleation and grain-growth function, the diffusion-controlled function and the phase-boundary-controlled function, respectively. Fe3O4→FeO was determined as the rate-limiting step with a lower reaction rate constant (8.89 × 10−6–5.60 × 10−4 s−1) and a higher activation energy (234 kJ/mol). By means of XRD analysis, it was also found that Al2O3, usually regarded as the supported component, became active by forming FeAl2O4 with FeO. At 900–1000 °C, FeAl2O4 impacted on the reduction rate of Fe2+→Fe due to its lower reactivity.
               
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