Stepwise reduction kinetics of iron-based oxygen carriers by CO/CO₂ mixture gases for chemical looping hydrogen generation

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.