This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.cej.2016.03.13

This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.ijhydene.2015.11.06

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.

Chemical looping hydrogen production uses the oxidation and reduction of metal oxides, typically iron, to produce hydrogen. This work focuses on the modification of iron oxide with calcium oxide to form an oxygen carrier containing dicalcium ferrite (Ca₂Fe₂O₅), which presents favorable thermodynamics for achieving higher conversions of steam to hydrogen, compared to chemically unmodified iron oxide. Different methods of synthesis, viz. mechanochemical synthesis and coprecipitation, were used to produce Ca₂Fe₂O₅, and their resulting performances were compared. Consistent with thermodynamic predictions, it was found that CO₂, or steam, was sufficient to fully regenerate the reduced carriers to Ca₂Fe₂O₅. The cyclic stability of the oxygen carriers were studied in fluidized bed reactors and by thermogravimetric analysis (TGA). Good stability of the materials was observed for up to 50 cycles, with no evidence of agglomeration, even up to 950 °C. The rate of deactivation was found to correlate with the purity of Ca₂Fe₂O₅ and the presence of impurity phases such as CaFe₂O₄, which had a tendency to segregate into its constituent elemental oxides. Carbonation of the oxygen carriers was examined by TGA, and it was found to occur appreciably only for the reduced carrier (a mixture of CaO and Fe) between temperatures of 500–700 °C and 0.1–0.5 atm of CO₂, whereas the oxidized carrier (viz. Ca₂Fe₂O₅) did not carbonate. Fresh and cycled materials were characterized by XRD, SEM, and BET analysis. Ca₂Fe₂O₅ is a potentially viable material as an oxygen carrier for hydrogen production; however, because of thermodynamic limitations, it cannot be used for complete fuel oxidation.

Chemical-looping combustion (CLC) has emerged as a promising and low cost carbon capture process which produces a pure stream of CO2. In CLC, oxygen from a solid oxygen carrier (OC) is used to combust fuels. Iron oxide is a promising material, being relatively cheap and environmentally benign. The thermodynamics of the Fe-O system allows reduced iron oxides to be also used for partial oxidation or hydrogen production. However, pure Fe2O3 has the problem of degradation in reactivity over repeated CLC cycles. This work is concerned with inclusion of a suitable CaO support material within the Fe-based OCs to increase its strength and cycle ability. OCs were made from Fe2O3 powder by granulation and followed by impregnation with Ca(NO3)2.4H2O. The reactivity and cyclic stability of the OCs were tested by being subjected to alternating reducing (using H2 or CO) and oxidizing environments (using air and / CO2) for many cycles, in a thermogravimetric analyser and a fluidized bed reactor at 850-950°C. It was found that, for samples with a range of loading of CaO, from 2 to 30 mol%, the one containing 20 mol% CaO showed optimal performances, whereas others showed improvements in oxygen carrying capacity, cyclic stability and reactivity over unmodified iron oxide but to a lesser extent