Solar Technology Laboratory
Characterisation of the Reactivity of the System Zn / O |
| Contact: Ivo Alxneit |
| Running Time: 01. January 2000 - 31. December 2003 |
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Funding: Swiss Federal Office of Energy |
IntroductionPSI's Laboratory for Solar Technology is dedicated to the goal of converting highly concentrated solar radiation into transportable and storable energy carriers, so called 'solar fuels'. Thermochemical cycles are a path to reach this goal. Currently our research in this field is focussed on a cycle based on Zn/ZnO. In this cycle zinc oxide is thermally dissociated in a solar reactor at temperatures above 2000 K. The resulting gaseous mixture of zinc vapor and oxygen has to be separated, presumably by a fast quench. As a result, metallic zinc is obtained in which solar energy is intermediately stored. Zinc is then either reacted with steam to produce hydrogen as the final energy carrier or is used in a zinc air battery for direct electricity generation. In both cases, ZnO is again obtained and the cycle is closed. It is obvious that the efficiency of the process is directly proportional to the zinc yield. Thus reoxidation of zinc vapor before and during the quench has to be avoided. To design an optimized quench process information on the kinetics and on the reaction mechanism) of the following two competing reactions, schematized below, is needed:
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Starting from zinc vapor homogeneous nucleation and subsequent growth of small zinc clusters eventually leads to bulk zinc (solid or liquid). Reoxidation of zinc vapor is a much more complicated process. It is not known at present whether reoxidation allready starts on a atomic scale by the formation of mixed zinc / oxygen clusters, as suggested in the above scheme, or only later by the oxidation of larger zinc droplets / crystallites. |
Homogeneous Nucleation of Zinc VaporAn experiment to study the homogeneous nucleation is evaluated. In the current concept, diluted zinc vapor is adiabatically expanded in a Laval nozzle. In a Laval nozzle supersonic flows with very stable temperature gradients can be obtained (see figure below). The flow as well a the temperature-, pressure-, and density fields inside the nozzle only depend on the stagnation conditions and on the exact shape of the nozzle. |
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Nucleation can be monitored by two different approaches: (i) the appearance of large particles can be observed by light scattering techniques (ii) the disappearing of atomic zinc vapor can be detected by laser induced fluorescence. Both techniques allow for spatially resolved measurements. To analyze the experimantal data extensive modelling of the flow and the nucleation has to be performed. |
Reoxidation of Zinc VaporRekin, an experiment to study the kinetics of the reoxidation of zinc vapor is currently being built. The experimental setup is shown schematically in the following figure. |
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Zinc vapor optionally diluted with inert gas leaves the central nozzle and is confined in a stream of oxygen feeded by the ring-nozzle. The oxygen stream can also be optionally diluted with inert gas. A gas heating system will allow tp preheat the gases to temperatures up to 1000 K. Therefore, it can be controlled whether only zinc atoms or also droplets will participate in the reaction with oxygen. Depending on the exact geometry of the nozzle system, a laminar flow or a turbulent mixing can be realized after the nozzle system. Thus the situation corresponding to a diffusion flame as well as the one approaching a premixed flame can be realized. By choosing an appropriate geometry of the nozzle system the case of a diffusion limited reaction can be excluded to a large extent and measurements of the reaction rates become possible. The concentration of various species (zinc atoms, oxygen molecules or intermediates) in the reaction zone will be probed spatially resolved by planar laser-induced fluorescence (PLIF). |
References
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