(443a) The Steam Iron Process for Pressurized Hydrogen Production

The commitment to fuel cell powered electric vehicles offer a carbon emission free mobility in the near future. A key requirement to achieve this goal is the supply of low cost hydrogen produced out of renewable resources. Currently more than 90% of the hydrogen production involves the utilization of fossil fuels in large scale centralized plants. The supply of cost-competitive hydrogen by renewable resources requires a different production infrastructure. Transportation distances of resources and of hydrogen itself have to be minimized and the production has to be realized in decentralized small scale production facilities. At Graz University of Technology the reformer steam iron process is developed as a scalable, on-site process, to emphasize on the requirements of natural resources. The process combines the steam reforming with the steam iron process (a chemical looping water splitting process) in one single unit.

In the first step of the process a syngas is generated by catalytic steam reforming of hydrocarbons according to eq.1. C_xH_y + xH₂O -> (x+y/2)H₂+ xCO (1) This syngas is directly used for the reduction of an oxygen carrier (eq. 2) which is a mixture of hematite (Fe₂O₃) and supporting metal oxides, prepared by mechanical mixing. Fe₂O₃ + 3H₂/CO -> 2Fe + 3H₂O/CO₂ (2) The consecutive oxidation of iron with steam leads to the formation of magnetite (Fe₃O₄) and pure hydrogen (eq. 3), an additional oxidation step with air regenerates the magnetite back to its initial hematite state (eq. 4). 3Fe + 4H₂O -> Fe₃O₄ + 4H₂ (3)

2Fe₃O₄ + 1/2O₂ -> 3Fe₂O₃ (4) In the reformer steam iron process the reduction is performed at ambient pressure to utilize a high fuel gas conversion efficiency due to the favorable reaction equilibrium. The oxidation on the other hand can be executed at an increased pressure to yield compressed pure hydrogen without additional gas compressors, which improves the efficiency of the system.

Isothermal cyclic pressure experiments at 750 °C were performed in a lab scale test rig as a proof of concept for pure pressurized hydrogen production. The goal of the experiments was to investigate (i) possible draw-backs of the production of compressed hydrogen by stability constrains of the oxygen carrier or operational limitations, (ii) influences of an increased system pressure on the hydrogen purity regarding carbon contaminations, (iii) the characterization and quantification of the contaminations and (iv) the analysis of the oxygen carrier conversion regarding efficiency and stability.

The reduction reactions were performed at ambient pressure with hydrogen or a synthesis gas mixture of H₂/CO/CO₂. The re-oxidation of the oxygen carrier at an increased pressure was achieved by pumping liquid water into the system which is evaporated in the heated inlet. The steam oxidizes the oxygen carrier by releasing hydrogen. The pressure build-up is realized by closing the out-going valve after the cooling system. In this case the unreacted steam is condensed and the pressure increase can be directly related to the amount of produced hydrogen.

 A successful hydrogen production up to 55 bar was achieved in the experiments with a hydrogen purity to fit the requirements of low temperature fuel cells. The oxygen carrier showed signs of structural degradation with no evident impact of the different pressure conditions in the prior oxidations. The linear pressure increase in the first part of each oxidation maintained at a stable level throughout the entire investigation thus the oxidation rate was not reduced by the solid degradation. Sintering was identified as the source for the loss of cycle stability which is mainly inducted by the high reaction temperatures of 750 °C. The oxidations indicated no visible influence on the purity by the different system pressures. Thus the process is suitable for the direct production of pressurized hydrogen using hydrocarbons as feed.

Acknowledgements

This work is funded by the IEA-Research Cooperation on behalf of the Austrian Federal Ministry of Transport, Innovation and Technology. The authors gratefully acknowledge support from NAWI Graz.