Inhibiting the interaction between FeO and Al2O3 during chemical looping production of hydrogen

Abstract

Hydrogen of high purity can be produced by chemical looping using iron oxide as an oxygen carrier and making use of the reaction between steam and either iron or FeO. However, this process is viable only if the iron oxide can be cycled between the fully-oxidised and fully-reduced states many times. This can be achieved if the iron oxide is supported on refractory oxides such as alumina. Unfortunately, the interaction between alumina and oxides of iron to form FeAl₂O₄ hinders the kinetics of the reactions essential to the production of hydrogen, viz. the reduction of Fe(II) to metallic iron by a mixture of CO and CO₂ prior to the oxidation by steam. Here, oxygen carriers containing Fe₂O₃ and Al₂O₃ were doped with Na₂O and, or, MgO, in order to inhibit the formation of FeAl₂O₄ by forming NaAlO₂ or MgAl₂O₄, respectively. The performance of the modified oxygen carriers for producing hydrogen, i.e. cyclic transitions between Fe₂O₃ (or Fe₃O₄) and metallic Fe at 1123 K were investigated. It was found that the interaction between FeO and Al₂O₃ was successfully mitigated in an oxygen carrier containing Mg, with an Al: Mg ratio of 2, resulting in consistently stable and high capacity for producing hydrogen by chemical looping, whether or not the material was oxidised fully in air in each cycle. However, the oxygen carrier without Mg only remained active when a step to oxidise the sample in air was included in each cycle. Otherwise it progressively deactivated with cycling, showing substantial interaction between Al₂O₃ and oxides of Fe.