Kinetics of the reduction of wüstite by hydrogen and carbon monoxide for the chemical looping production of hydrogen

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 (Fe_xO, 0<x<1). In this three-stage process, the reduction of Fe_xO to Fe is the slowest reaction. This paper is concerned with the kinetics of the reduction of wüstite (Fe_xO) 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.