Oxidation and Reduction of Iron-Titanium Oxides in Chemical Looping Combustion: A Phase-Chemical Description

Ilmenite (FeTiO3 ) is being explored as an oxygen carrier in chemical looping processes. Its reduction and oxidation are described by the system Fe-Fe2 O3 -TiO2 -Ti2 O3 . The phase diagram at 1 000°C, presented here, offers a useful tool for predicting reactions and their products. We see that Fe2 TiO5 (pseudobrookite) and TiO2 (rutile) form a stable phase assemblage following the oxidation of FeTiO3 (ilmenite) in air. The subsequent reduction of Fe2 TiO5 at oxygen partial pressures of 10-15.5 atm stabilizes Fe1.02 Ti0.98 O3 , a solid solution of ilmenite. Further reduction will produce metallic iron, which compromises the integrity of the oxygen carrier for chemical looping processes. We speculate that the reduction of Fe-Ti oxides in several practical instances does not reach completion (and equilibrium) under the imposed atmospheres operating in fuel reactors.