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Solar carbothermic production of Zn from ZnO (SOLZINC)

Deliverables

The achievement is the monitoring of the behaviour of different custom complex systems with parts operating at high temperature (the parts of a 500kg-capacity chemical reactor and a 50kg/h metallic powder zinc quenching system): - To be able to run the plant by monitoring the main operating parameters, - To be able to compare the real behaviour of the systems to the predicted modelled behaviour, - To be able to enhance and optimise the whole plant. This monitoring includes low to high temperature parameters (ambiant to 1600 K) of solids, liquids and gases, pressure of gases, flow rates of liquids and gases, gas composition real time analysis, for a total of about 150 parameters directly for the plant, without including all the auxiliary systems. A complete instrumentation system has been designed to achieve this. First, the relevant parameters to be monitored have been identified together with our partners. Then, adapted instrumentation devices have been selected and purchased to fulfill the mission. Together with WIS, we have installed and checked in place these instruments. Exploitation of the data collected is being conducted for different purposes: - Control of the process, in real time or for after operation analysis - Correspondence of the reality to the theoretical models - Basis for further development and cost studies To achieve this, PROMES-CNRS has further developed its existing expertise in high temperature solar based chemical processes instrumentation and monitoring. This knowledge can be applied or adapted to other high temperature processes.
The achievement is determining the thermal efficiency of a solar reactor whose input thermal power is about 300 kW, the working temperature is 1600 K. Its operation time is a complete day: daytime for heating thanks to concentrated solar energy, nighttime for cooling down to be able to prepare the plant for the next operating day. The determination of the thermal losses through the walls of reactor is realized with a combinaison of a set of 17 thermocouples and a fluxmeter (sensor delivering a voltage directly proportional to the thermal flux through it) placed at different choosen points of the reactor, both inside and outside. The combinaison of a pair of thermocouples and the fluxmeter allows the direct measurement of the thermal resistance of the walls of the reactor. This parameter, combined with the thermocouples-based temperature map of the reactor, thereafter allows the determination of the thermal losses of the reactor with a good accuracy only degraded by local geometrical complexities of the reactor. Exploitation of the resulting data is a key improvement to realize more reliable energetic assessment of the plant, as the thermal losses are an important part of the balance of such systems. This instrumentation configuration and its associated methodology can be used to enhance the thermal efficiency of a reactor, as we developed here for a chemical solar-based application, leading to substantial energy and money savings. This knowledge can be applied further to any system working at high temperature (several hundred of Kelvins and higher).
Zinc/air fuel cells, which use commercial zinc powders and those powders produced by the carbothermic reduction inside a solar reactor. One main product during discharge of the the cell is zinc oxide (ZnO). After a removal of the discharged anode the ZnO can be led back to the solar reactor. Herewith the circular flow of the Zn is closed.
ScanArc's part in the Solzinc project is to develop a process to convert the zinc metal vapor produced in the solar furnace into a zinc powder suitable for electrolytic cell electrode production. The conventional way to produce zinc powder is to first condense the zinc vapor into zinc metal and then produce the powder according to one of the three alternative routes: - Atomisation of liquid zinc. - Condensation of evaporated zinc. - Electrolytic precipitation. Since the zinc vapor from the solar furnace is diluted by CO and H2, and if operated at higher carbon utilization also CO2 and H2O, the diluting gas will be oxidizing to the zinc when the temperature drops and the condensation product will be a mixture of zinc metal and zinc oxide. To avoid the zinc oxide formation the zinc from the vapor has to be dissolved into liquid lead and two alternatives were studied: the ISP splash condenser and the ScanArc spray condenser. Both alternatives are possible to use with some advantage for the ScanArc condenser since it is a true counter-current reactor giving lower sensitivity to oxidizing gases. The condensation into zinc metal followed by powder production is a quite possible and safe route, but rather investment and cost intensive. We have therefor worked along another concept to produce powder direct from the zinc vapor containing gas from the solar reactor. After a series of introductory tests we managed to produce a zinc powder with a good yield but the content of ultrafine particles made the powder too reactive for electrodes. The powder production was discussed with a commercial powder producer (Harzer Zinc) and the test equipment has been modified with good success. This design was upscaled for the pilot scale offgas system. It performed well and produced a Zn-dust with a typical size of a few microns and with a purity of around 95%, as targeted.
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.
The achievement is measuring the gradient of temperature inside a reacting porous media exposed to high temperature (up to 1300K) during the operation of the plant which last one day. This material is a mix of ZnO and beech charcoal in neutral sealed atmosphere, whose height is shrinking due to the carboreduction of the material in gaseous Zn, CO and CO2 thanks to concentrated solar indirect heating. A 650 mm ceramic lance made of 5 sheathed thermocouples of different height each protected by an alumina outer sheath was realised and operated for some of the operations of the Solzinc plant. Among its main characteristics is its resistance to the gaseous zinc inside the reactor and the high temperature. The complete exploitation of the results is being conducted. The main purpose is to check the thermo-chemical dynamic model of the carboreduction inside the reactor. This design can be used to determine temperature gradients in powder beds that can change of height in the time, leaving place to aggressive atmosphere. The spatial resolution of the instrument can be modified by adding or removing sensing elements. The operating temperature range can be modified by changing the sensing element type, allowing continuous measurements up to about 1800K.