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

Abstract

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, CH 4 and a model syngas (mixtures CO + H 2 or H 2 + CO + CO 2) and oxidation of iron by steam has been studied. Attention was concentrated not only on convenient conditions for reduction of Fe 3O 4 to iron at temperatures 400–800 K but also on the possible formation of undesired soot, Fe 3C 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/H 2O ratio, increasing with decreasing temperature. Reduction of iron oxide by CO is complicated by soot and Fe 3C formation. At lower temperatures and higher CO 2 concentrations in the reducing gas, the possibility of FeCO 3 formation must be taken into account. The purity of the hydrogen produced depends on the amount of soot, Fe 3C and FeCO 3 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.