Hydrogen of very high purity can be produced via the steam-iron process, in which steam oxidises metallic Fe in 3/4Fe+H2O→1/4Fe3O4+H2. It is then advantageous to oxidise Fe3O4 in air to Fe2O3, an oxygen-carrier. This higher oxide of Fe is then reduced to regenerate metallic iron by reacting with synthesis gas, producing metallic Fe and possibly some wüstite (FexO, 0<x<1). In this three-stage process, the reduction of FexO to Fe is the slowest reaction. This paper is concerned with the kinetics of the reduction of wüstite (FexO) by reaction with CO, and, or H2. Starting with pure (99wt%) wüstite, the intrinsic kinetics of its reduction to metallic iron were measured in fluidised beds at different temperatures. The reaction was found to have 3 distinct stages, (i) the removal of lattice oxygen in wüstite, (ii) rate increasing with conversion of solid and (iii) rate decreasing with conversion of solid. A random pore model was used to simulate the latter stages of the reduction of wüstite by either H2 or CO or a mixture of the two. It was found that the intrinsic rate of reduction in H2 is substantially faster than with CO, whereas the resistances to diffusion of H2 and CO through the product layer of Fe are comparable; these factors account for differences in the overall rates observed with these gases.
Liu, W., Lim, J. Y., Saucedo, M. A., Hayhurst, A. N., Scott, S. A., & Dennis, J. S. (2014). Kinetics of the reduction of wüstite by hydrogen and carbon monoxide for the chemical looping production of hydrogen. Chemical Engineering Science, 120, 149–166. https://doi.org/10.1016/j.ces.2014.08.010