Application of solar thermal energy for the utilisation of CO2 and water for hydrogen and synthesis gas production

Kurzfassung

A two-step thermo-chemical cycle process has been developed. It is based on metal oxide redox pair systems, which are able to reversibly incorporating oxygen atoms them into their lattice. CO2 and water can be reduced to CO and hydrogen by solar-powered thermo-chemical cycles. Concentrated solar radiation and high temperatures are necessary for this. The product synthesis gas is a precursor for a broad variety of chemical commodities and in particular fuels. Straightforward operation is ensured by the combination of a ceramic substrate as absorber structure, which can be heated to high temperatures with concentrated solar radiation, and of a metal oxide coating which is capable of splitting water. This offers advantages over comparable processes, because in this case the entire process can be conducted in a single solar heated converter. In the first step, the steam or CO2 flowing past the metal oxide is split by binding the oxygen to the excited metal oxide lattice, and H2 or CO are produced. In the second step, the oxygen, which has previously been incorporated into the lattice, is released again by applying high temperatures an oxygen-lean atmosphere, and the metal oxide is regenerated. The suitability of different redox materials for this process was screened by thermo-chemical calculations. Some promising candidates have been identified after investigating them as powders and coated on ceramic substrates in dedicated laboratory set-ups. A related process to be run on a solar power tower plant was developed. An operational and control strategy has been derived, validated firstly a mini-plant in DLR solar furnace in Cologne and finally applied for water splitting during the test operation of a pilot-scale plant on a solar tower near Almería in the South of Spain. The present contribution elaborates on the materials development as well as on the design of core components and of the process as a whole.