Solarthermal dissociation of zinc oxide, Experimental investigation and modelling of the reverse reaction

Erneuerbare Energien gewinnen zunehmend an Bedeutung, da fossile Brennstoffe nicht unbegrenzt verfügbar sind und ihre Nutzung ökologische Risiken in sich birgt. Die solarthermische Dissoziation von Zinkoxid ist Bestandteil eines geschlossenen Stoffkreislaufs zur Speicherung und Transport von Sonnenenergie in Form des solaren Brennstoffs Zink. Der wohl kritischste Punkt im Design eines Solarreaktors ist die Verhinderung der Rekombination von einmal erzeugtem Zink und Sauerstoff. Auf der Basis von experimentellen Ergebnissen wird ein Modell formuliert, das diesen Prozess quantitativ beschreibt. Damit ergeben sich wichtige Hinweise zur Auslegung eines Hochtemperatursolarreaktors zur rein thermischen Darstellung von Zink aus Zinkoxid.Abstract The solar thermal dissociation of zinc oxide is part of a closed material cycle for the storage and transport of solar energy. The efficiency of the solar step and the whole cycle is in part determined by the obtained zinc yield. Whenever the reoxidation to zinc oxide in a high temperature solar reactor occurs the zinc yield decreases. Due to this fact, one of the most critical design aspects is the selection of the quench step in order to prevent the back reaction. This study describes the motivation and performance of high temperature solar processes as a means for emission reduced production of sustainable energy carriers. More specificly it is dealing with the production of zinc. Conventional processes are analysed in order to identify the best quench method for a future solar driven zinc production process. Theoretical aspects of the oxidation of zinc at elevated temperatures are given as well as findings from experimental investigations in the solar furnace and a laboratory bench scale electric furnace. In the electric furnace the reaction is studied mainly at various gas temperatures, various ratios of diluting inert gas to zinc, and with various surface areas exposed to zinc and oxygen gas. Experiments indicate that the oxidation of zinc is of a heterogeneous nature and occurs also above the boiling point of zinc if there is a hot surface available. A two dimensional mass transfer model describes well the rate of oxidation as being limited by the rate of the diffusion of the reactants to the reactor wall when zinc and oxygen are diluted in argon in a laminar flow regime. Experiments in the solar furnace show that an inert dilution gas temperature in a region between the dissociation and the quench zones above the boiling point of zinc leads to higher zinc yields than in the case when the dilution gas is below this temperature. Experimental investigations using a zinc-splash-condenser as a means to quench the system zinc/oxygen are showing that condensor efficencies of 8 % can not be exceeded. The study finishes by showing how the above results can be applied towards design aspects of a high temperature solar reactor for the production of zinc by direct thermal splitting of zinc oxide. One sees how best to set the following design parameters: the reactor material, geometry and flow conditions. Further more one sees how to estimate the best temperature of the inert gas, the entry site of the quench gas, and the heat exchanger design and position