Ilmenite Activation during Consecutive Redox Cycles in Chemical-Looping Combustion

Ilmenite, a natural mineral composed of FeTiO3, is a low-cost and promising oxygen carrier (OC) for solid fuels combustion in a chemical-looping combustion (CLC) system. The aim of this study is to analyze the behavior of ilmenite as an OC in CLC and the changes in its properties through redox cycles. Experiments consisting of reduction−oxidation cycles in a thermogravimetric analyzer were carried out using the main products of coal pyrolysis and gasification, that is, CH4, H2, and CO, as reducing gases. Characterizations of ilmenite through scanning electron microscopy−energy-dispersive X-ray (SEM−EDX), X-ray diffraction (XRD), Hg porosimetry, N2 fisisorption, He pycnometry, and hardness measures have been performed. Both fresh and previously calcined at 1223 K ilmenite have been used as initial OCs. Fresh ilmenite reacts slowly; nevertheless, there is a gain in reactivity in reduction as well as in oxidation with the number of cycles. This activation occurs for all tested fuel gases and is faster if ilmenite has been previously calcined. The initial oxygen transport capacity was measured to be 4%, and it decreases with the number of cycles up to 2.1% after 100 redox cycles. Nevertheless, ilmenite shows adequate values of reactivity and oxygen transport capacity for its use in CLC technology with solid fuels. The trade-off between the increase in reactivity and decrease in oxygen transport capacity on ilmenite performance in a CLC system has been evaluated through the estimation of the solids inventory needed in the fuel reactor. If fresh or calcined ilmenite is fed into the CLC system, the activation process could happen in the CLC itself. Also, a previous step for activation can be designed.