Pre-oxidation of natural ilmenite for use as an oxygen carrier in the cyclic methane-steam redox process for hydrogen production

The methane-steam redox process is an alternative methane conversion route to high-purity hydrogen production. Using natural ilmenite as an oxygen carrier, we investigated the effect of three different pre-oxidation temperatures (800, 1000 and 1200 degrees C) on redox performance and stability. We found that the pre-oxidation temperature significantly altered the crystalline properties of the ilmenite sample, which subsequently affected its morphology, reducibility and redox performance. Pre-oxidation above 1000 degrees C caused the formation of pseudo-brookite (Fe2TiO5), while hematite (Fe2O3) was formed at 800 degrees C. The presence of Fe2TiO5 lowered the reduction kinetics, but improved the oxygen-transfer capacity. This resulted in the highest redox activity for the sample pre-oxidised at 1000 degrees C, which had a three to fourfold increase in hydrogen yield compared with samples pre-oxidised at the other temperatures. Redox activity progressively increased during the cyclic redox operation, due to an increase in surface area caused by continual pore and crack development. Hydrogen yield was sustained at double the level of the initial yield, with a purity of more than 98% over 40 redox cycles. (C) 2017 Elsevier B.V. All rights reserved.