SYNTHESIS GAS PRODUCTION BY SOLAR THERMOCHEMICAL REDUCTION OF CO2 AND WATER

Kurzfassung

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. A two-step cycle based on a metal oxide redox pair system, which can split the mentioned gaseous oxides by abstracting oxygen atoms and reversibly incorporating them into their lattice, coated on ceramic honeycombs that can absorb concentrated solar radiation has been developed. An easy way of operation is gained 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, at temperatures of 1150-1200 °C, the oxygen, which has previously been incorporated into the lattice, is released again, and the metal oxide is regenerated. Because of the immobilization of the redox pair material on the substrate, not only no solids need to be circulated, but H2/CO production and oxygen release take place at different steps, eliminating thus the need for high-temperature gas separation processes. The synthesis gas produced represents a potential precursor for the production of liquid fuels and other organic chemicals. The suitability of different redox materials for this process was screened by thermo-chemical calculations. Some promising candidates were screened as powders and coated on ceramic substrates in dedicated laboratory set-ups. The most promising material families are based on doped ferrites, perovskites and ceria. The second part of the investigation concerned the set-up of scalable hardware to run such a process on a solar power tower. An operational and control strategy has been developed, 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 core part of the pilot plant is the receiver-reactor with two identical chambers, in which the two steps of the process are carried out in a cyclic mode. The present contribution describes the realisation and successful test operation of this 100kW pilot plant. In parallel to this, a system and control model of the plant has been developed and validated. It is used to simulate the process to enable an optimisation of the operational strategy and the daily output.