Thermodynamic possibilities and constraints for pure hydrogen production by iron based chemical looping process at lower temperatures

Iron offers the possibility of transformation of a syngas or gaseous hydrocarbons into hydrogen by a cycling process of iron oxide reduction (e.g. by hydrocarbons) and release of hydrogen by steam oxidation. From the thermodynamic and chemical equilibrium point of view, the reduction of magnetite by hydrogen, CO, CH4 and a model syngas (mixtures CO + H-2 or H-2 + CO + CO2) and oxidation of iron by steam has been studied. Attention was concentrated not only on convenient conditions for reduction of Fe3O4 to iron at temperatures 400-800 K but also on the possible formation of undesired soot, Fe3C and iron carbonate as precursors for carbon monoxide and carbon dioxide formation in the steam oxidation step. Reduction of magnetite at low temperatures requires a relatively high H-2/H2O ratio, increasing with decreasing temperature. Reduction of iron oxide by CO is complicated by soot and Fe3C formation. At lower temperatures and higher CO2 concentrations in the reducing gas, the possibility of FeCO3 formation must be taken into account. The purity of the hydrogen produced depends on the amount of soot, Fe3C and FeCO3 in the iron after the reduction step. Magnetite reduction is the more difficult stage in the looping process. Pressurized conditions during the reduction step will enhance formation of soot and carbon containing iron compounds. (c) 2007 Elsevier Ltd. All rights reserved.