Introduction Zinc and synthesis gas (syngas), besides being important material commodities, are attractive as energy carriers. Zinc finds applications in Zn/air fuel cells and batteries, and it can also be reacted with water to form hydrogen. Syngas is the building block of a wide variety of synthetic liquid fuels, including methanol - a promising substitute of gasoline for fuelling cars. However, the current industrial production techniques of both zinc and syngas carry severe environmental consequences, especially CO 2 emissions. These emissions can be reduced substantially, by combining both the production of Zn and syngas and by replacing fossil fuels with concentrated solar energy as the source of high-temperature process heat. The proposed solar combined process, called "SynMet", can be represented by the overall reaction: ZnO + CH 4 --> Zn + 2H 2 + CO The use of solar energy for supplying the enthalpy of the reaction upgrades the calorific value of the initial reactants by 39%. Thus, using the SynMet process, solar energy is converted into storable and transportable chemical fuels.

Scheme of the process In the first, endothermic step, concentrated solar radiation is used for co-producing zinc and syngas by the combined ZnO-reduction and Natural Gas reforming processes in a solar chemical reactor called SynMet; syngas could further be processed to methanol. In the second step, zinc is either used to split water and form H 2 in a water-splitting reactor, or, alternatively, zinc is used in a Zn/air fuel cell or battery to generate electrical work. In either case, the chemical product of the second step is ZnO which, in turn, is recycled to the first step.

The SynMet reactor The SynMet-reactor consists of a cavity-receiver (1) with an aperture (2) to let in concentrated solar energy. Particles of ZnO and natural gas are fed at the back of the reactor (3). The gas-particle stream forms a vortex flow that progresses towards the front following a helical path. The chemical products exciting the reactor are zinc an syngas (4). The window (5) is actively cooled and kept clear of particles by means of an auxilary flow (6). With this arrangement, the particles of zinc oxide are directly exposed to the high-flux solar irradiation. Such a concept provides efficient radiation heat transfer directly to the reaction site, where the energy is needed. Energy absorbed by the reactants is used to raise their temperature and to drive the simultaneous reduction of zinc oxide and reforming of natural gas.

related publications

• Kräupl, S., and Steinfeld, A., "Monte Carlo radiative transfer modeling of a solar chemical reactor for the co-production of zinc and syngas", Journal of Solar Energy Engineering, in press, 2004
• Kräupl, "Chemische Speicherung von Sonnenenergie durch solares Reformieren von CH ₄ mit ZnO", Swiss Federal Institute of Technology Zurich 2002, Diss. ETH Nr. 14744
• Kräupl, S., and Steinfeld, A., "Operational Performance of a 5 kW Solar Chemical Reactor for the Co-Production of Zinc and Syngas", Proceedings of Solar 2002: Sunrise on the Reliable Energy Economy, June 15-20, 2002, Reno, Nevada and Journal of Solar Energy Engineering, Vol. 125, 124-126, 2003.
• Kräupl S. and Steinfeld A., "Operational Performance of a 5 kW Solar Chemical Reactor for the Co-Production of Zinc and Syngas", PSI Scientific Report 2001 - Volume V, pp. 33-34, 2001. pdf file 319 kB
• Kräupl, S., and Steinfeld, A., "Experimental Investigation of a Vortex-Flow Solar Chemical Reactor for the Combined ZnO-Reduction and CH ₄ -Reforming", Journal of Solar Energy Engineering, Vol. 123, 237-243, 2001 and Proceedings of Solar Forum 2001: Solar Energy: The Power to Choose, April 21-25, 2001, Washington, DC
• Kräupl, S., and Steinfeld, A., "Pulsed gas feeding for stoichiometric operation of a GAS-SOLID VORTEX FLOW solar chemical reactor", Journal of Solar Energy Engineering, Vol. 123, 133-137, 2001.
• Kräupl S. and Steinfeld A., "Stoichiometric Operation of The SynMet Reactor", PSI Scientific Report 2000 - Volume V, pp. 31-32, 2000. pdf file 220 kB
• Werder M., Steinfeld A., "Life Cycle Assessment of the Conventional and Solarthermal Production of Zinc and Synthesis Gas", Energy - The International Journal, in press.
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• Steinfeld A., Larson C., Palumbo R., Foley M., "Thermodynamic Analysis of the Co-Production of Zinc and Synthesis Gas Using Solar Process Heat", Energy - The International Journal, Vol. 21, No. 3, pp. 205-222, 1996.
• Steinfeld A., Frei A., Kuhn P., Wuillemin D., "Solarthermal Production of Zinc and Syngas Via Combined ZnO-Reduction and CH ₄ -Reforming Processes", International Journal of Hydrogen Energy, Vol. 20, No. 10, pp. 793-804, 1995.
• Steinfeld A., Brack M., Meier T., Weidenkaff A., Wuillemin D., "A Solar Chemical Reactor for the Co-Production of Zinc and Syngas", PSI Annual Report 1997 - Annex V, pp. 6-7, 1997. pdf file 326 kB