Pressurized hydrogen production by fixed-bed chemical looping

Abstract

Fuel cell cars powered by hydrogen enable CO2-emission free mobility. A main requirement for the success of this technology is the availability of an area-wide and affordable hydrogen supply. A significant cost factor in the hydrogen supply chain is the multi-stage gas compression to provide the mandatory filling pressure for the pressurized tanks. One way to address this issue is to perform the hydrogen production process at elevated pressure. In this paper the feasibility of compressed hydrogen production without additional gas compression based on the steam iron process is discussed. Experiments were performed in a lab-scale test rig using fixed bed reactor technology. The focus was to evaluate the influence of pressurized hydrogen production on the cycle stability, on the conversion efficiency and on the structural integrity of a Fe2O3–Al2O3 (90+10wt%) oxygen carrier. The oxidations were performed at different pressure levels of 7–22bar (g) at a temperature of 750°C with steam. The steady steam supply was ensured by a HPLC pump which delivered 0.03gmin−1 (at room temperature) of water, which was evaporated in the heated inlet section. The water was introduced for approximately 100min until the oxygen carrier was fully oxidized. The iron oxide was reduced in the subsequent reaction steps at 750°C and ambient pressure with 25Nmlmin−1 H2 as reducing agent. The reduction reactions were analyzed to evaluate possible influences of its prior oxidations. The results revealed no signs of negative repercussions. The oxygen carrier conversion of initially 84% remained at a steady behavior between the 15 performed cycles. Only small losses of 0.8% per cycle caused by thermal sintering of the contact mass were observed, which was independent from the different pressure levels of the prior oxidations. The evaluation of the pressurized oxidations did not reveal any performance decrease as well. The rise of pressure in each oxidation showed a consistent characteristic throughout the complete test series. Scanning electron microscopy analysis of the oxygen carrier sample after the experiment revealed some structural changes, which are related to thermal sintering but the structural integrity of the sample stayed intact. The oxidations yielded an average of 18mmolgFe−1 hydrogen with a maximum hydrogen pressure of 22bar (g). The conducted experiments showed that the steam iron process is very suitable for the production of compressed hydrogen and that the process is not negatively influenced by the elevated system pressure.