Community Research and Development Information Service - CORDIS

Solar chemical reactor heated by concentrated solar irradiation

A solar reactor heated by concentrated solar irradiation suited to effect endothermic chemical reactions. The reactor is especially well suited for reactions involving at least one solid partner and resulting in basically gaseous products only. One example is the carbothermic reduction of ZnO. In this case the input is ZnO and a coal or coke and the main products are Zn(g) and CO (if the input mix is nearly stoichiometric, that is one mol of C per mol of ZnO).

- The solar reactor principle:
The innovative "two cavity batch reactor" concept was extensively tested on a laboratory scale of 5-10kW. It consists of two chambers above each other: The upper cavity chamber is heated through a quartz window by the concentrated solar irradiation below the secondary concentrator (CPC). A thin-coated graphite absorber wall separates the upper chamber from the lower reaction chamber and hence protects the window from condensing gases etc. A mixture of ZnO and a carbon material is placed in the reaction chamber prior to the start of the operation. Once the temperature in the lower chamber exceeds some 1000°C the ZnO and C react to gaseous Zn and CO-gas, which leave the reaction chamber through a horizontal offgas pipe to the Zn-dust production and offgas cleaning system (see separate result 4 from ScanArc). The pressure in the reactor is very close to the ambient pressure (approximately: ambient +/- 20 mbar). Since all major products are in gaseous form the height of the ZnO-C-bed is shrinking during the operation, and the reaction chamber is basically empty after an undisturbed operation. After cooling down (typically over night) the lower part of the reactor can be disconnected from the upper one and lowered down for refilling with new ZnO-C-mixture.

- The pilot plant:
The pilot plant has been erected at the Weizmann Institute of Science in Rehovot/Israel, where an optical system to produce concentrated irradiation from the top (beam-down system) for powers up to about 500kW is available.

- Solar reactor:
The solar reactor consists of two parts:
-- The lower reactor part is the lower part of the lower chamber. It can be lifted down to allow for (re-)filling of the reactor. Based on the specific production rates per surface area (a bed diameter of about 1.4m and a bed height of maximal 0.5m were chosen, allowing to load about 450kg ZnO-Carbon mixture for a full day operation with a single batch.

The reactor walls consist of different layers (in both, upper and lower parts of the reactor) with SiC-plates as first wall in contact with the reactants and the produced gases.

The lower (reaction) chamber is heated from above by the radiating separation plates between the two chambers.

-- The stationary upper reactor part includes the whole upper chamber and the upper part of the lower reaction chamber with the offgas pipe and an inlet pipe for carrier gas, which also serves as emergency gas exit. The SiC offgas pipe can be electrically heated from outside by a heating cartridge in order to prevent Zn-condensation.
The aperture diameter of the upper chamber is 48cm. It is closed by a 600mm diameter 12mm quartz window rated for 100 mbar pressure difference. The water cooled copper front holding the window includes 3 inlets for N2 to purge the window from below. This nitrogen-flow can pass the separation plates through small openings.
The window is positioned a few mm below a small water cooled ring for protection of the lowest section of the existing secondary concentrator against re-radiation. This ring has 10 holes allowing optional air cooling of the window from outside.

- The commissioning of the solar reactor including its off gas system started in November 2004. First solar zinc on pilot scale has been produced on December 2, 2004.

- The operation of the pilot plant in 2005 gave very promising results. Only minor hardware adjustments were made to guaranty a smooth operation. The based on the laboratory scale tests expected reaction rates could be demonstrated on pilot scale with Zn production rates of up to 50 kg/h for batches of industrial ZnO mixed with industrial charcoal at a molar ratio C:ZnO of 0.8-0.9. Overall the concept proved to be fully scalable.

- A conceptual design of a 5MW demonstration scale solar reactor based on the same concept has been performed. All 7 hexagonal secondary concentrators foreseen in the optical design of this demonstration plant heat one 7.5m inner diameter solar reactor. Preliminary offers and cost calculations are available for this plant constituting the last development step prior to a commercialisation of the SOLZINC technology. A conceptual design of the respective Zn-quench offgas system has been performed. Depending on the needs the Zn may be recovered from the offgas as Zn-dust (few micrometer size), as Zn-powder (about 100 micrometer size) or as bulk-Zn.

Related information

Reported by

Laboratory for Solar Technology
Paul Scherrer Institute, WSAD
CH-5232 Villigen PSI
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