The use of ilmenite as an oxygen carrier in chemical-looping combustion

The feasibility of using ilmenite as oxygen carrier in chemical-looping combustion has been investigated. It was found that ilmenite is an attractive and inexpensive oxygen carrier for chemical-looping combustion. A laboratory fluidized-bed reactor system, simulating chemical-looping combustion by exposing the sample to alternating reducing and oxidizing conditions, was used to investigate the reactivity. During the reducing phase, 15 g of ilmenite with a particle size of 125–180 μm was exposed to a flow of 450 mL_n/min of either methane or syngas (50% CO, 50% H₂) and during the oxidizing phase to a flow of 1000 mL_n/min of 5% O₂ in nitrogen. The ilmenite particles showed no decrease in reactivity in the laboratory experiments after 37 cycles of oxidation and reduction. Equilibrium calculations indicate that the reduced ilmenite is in the form FeTiO₃ and the oxidized carrier is in the form Fe₂TiO₅ + TiO₂. The theoretical oxygen transfer capacity between these oxidation states is 5%. The same oxygen transfer capacity was obtained in the laboratory experiments with syngas. Equilibrium calculations indicate that ilmenite should be able to give high conversion of the gases with the equilibrium ratios CO/(CO₂ + CO) and H₂/(H₂O + H₂) of 0.0006 and 0.0004, respectively. Laboratory experiments suggest a similar ratio for CO. The equilibrium calculations give a reaction enthalpy of the overall oxidation that is 11% higher than for the oxidation of methane per kmol of oxygen. Thus, the reduction from Fe₂TiO₅ + TiO₂ to FeTiO₃ with methane is endothermic, but less endothermic compared to NiO/Ni and Fe₂O₃/Fe₃O₄, and almost similar to Mn₃O₄/MnO.