A promising new technology for sustainable fuel production is the splitting of water and carbon dioxide by the non-stoichiometric two-step metal oxide redox cycle. Development of oxide materials and reactors to realize the cycle is currently in infancy, with significant room for improvement over previous demonstrations. Research efforts have gone into developing and characterizing reactive metal oxide materials for the cycle, while less literature is devoted to the design and understanding of non-stoichiometric redox reactors. The work presented attempts to close the gap by exploring multiple levels of modeling analysis to determine the important considerations for designing a reactor to perform high efficiency non-stoichiometric redox cycling. Cerium oxide (ceria) is considered as the reactive material for reactors in this work. A reactor for non-stoichiometric redox cycling should allow for continuous use of the solar input and should implement heat recovery. In the first stage of the research, thermodynamic analysis is carried out on a model reactor system to quantify the potential efficiency benefits of heat recovery and to determine the effects of the reduction temperature and sweep gas flow rate. Heat recovery is found to improve reactor efficiency from 4% to 16%. The selection of reduction temperature is important to high efficiency. For many cases the heating of gases is a major source of heat loss, indicating that heat recovery should be applied to the gas flows as well as the solid metal oxide. In the second stage of the research, a reactor is presented which incorporates continuous redox cycling of ceria and heat recovery from the solid ceria by using counter-rotating hollow cylinders of ceria and inert material. Heat transfer modeling is applied to this concept to explore its performance potential and identify the important design factors for effective heat recovery. Energy conservation is applied using a finite volume method with detailed modeling of radiative heat transfer by the Monte Carlo method and the Rosseland diffusion approximation. A simplified model of the rotating cylinders and a more complete model of the full reactor geometry are applied. It is determined that the proposed design can recover over 50% of the heat from the ceria, and provide a temperature differential of 400 K between the reaction steps. Geometric and material parameters are varied in a parametric study to determine which are important forheat recovery. The important parameters for heat recovery and chemical utilization of the material are those which define the heat transport across the ceria cylinder wall. Temperatures, heat transfer rates, heat fluxes, and the chemical state of the material are predicted. Using the heat transfer model results and other analysis, values of thermal design parameters for a prototype reactor are selected as part of an effort leading to a prototype reactor to be built and tested at the University of Minnesota. Heat recovery is found to be a path with great potential for improving the efficiency of solar-driven non-stoichiometric redox cycles. The prototype reactor described has the potential to demonstrate high levels of heat recovery and unprecedented efficiency. However, a careful understanding of the properties of the reactive material and the geometric parameters of the reactor is needed to ensure that heat which is input or removed is effectively transported across the cylinder wall for heat recovery. The models described here account for the important effects and explore the complexity needed to investigate the problem. Primary future improvements to the modeling work will include coupling of heat transfer and fluid mechanics, implementation of chemical rate expressions, and the addition of high-temperature and spectral material properties as they become available.
University of Minnesota Ph.D. dissertation. August 2013. Major: Mechanical Engineering. Advisor: Wojciech Lipinski. 1 computer file (PDF); xvi, 160 pages + 1 zip file of computational code.
Lapp, Justin L..
Thermal modeling and design of a solar non-stoichiometric redox reactor with heat recovery.
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