Thermochemical Processes: Principles and Models


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To address some of these requirements, we are characterizing the effect of a number of material parameters on performance in a test system for cyclic TR and WO with concurrent quantification of hydrogen and oxygen evolution.


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Although we have tested a number of ferrites, we have decided to focus mainly on a baseline cobalt ferrite to understand the design variables. Processing parameters such as synthesis procedure and particle size, as well as the impact of supporting the ferrite in a zirconia matrix, are of interest. Hydrogen and oxygen yields over successive cycles are the primary metrics. We are also applying characterization techniques such as x-ray diffraction, microscopy, and temperature programed reduction and oxidation to provide insights into the chemical and physical processes occurring in the ferrites during operation We are using robocasting, a Sandia-developed technique for free-form processing of ceramics to manufacture monolithic structures with complex three-dimensional geometries for chemical, physical, and mechanical evaluation Probably because of enhanced mass transport geometries, we have been able to produce more hydrogen and oxygen at faster rates with these monolithic structures than with equivalent amounts of powders.

We have also fabricated reactant fin sections for the CR5 prototype.

Thermochemical Processes: Principles and Models by C.B. Alcock

Design objectives for the fin sections include high geometric surface area, thermal shock resistance, and allowance for light penetration. The design of these monolithic structures is guided by our CFD studies. Figure 5 shows photographs of robocast: a a reactant ring assembly, b prototype reactant fin segments, c the test part as cast, and d the test part after 31 redox cycles. A key result of our material studies is an appreciation of the importance of the zirconia support in enhancing the reduction reaction and avoiding issues associated with melting.


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Studies with iron oxide suspended in stabilized zirconia indicate that a small amount of the iron oxide would be expected to go into a solid solution with the zirconia support. In addition, the high oxygen mobility through zirconia may enhance kinetics. An important consequence is that high porosity may not be needed, and sintering of the porous zirconia may not be an issue. These observations appear to be in good agreement with those obtained by Ishihara et al. They suggest a new type of redox material with high internal oxygen mobility for transport of oxygen to and from dispersed redox sites.

Given the numerous uncertainties, we believe that a proof-of-concept prototype CR5 thermochemical heat engine is needed to establish feasibility and to evaluate many of the unknowns. A proof-of-concept device with the overall objective of demonstrating the key features of the CR5 at a reasonable scale is being developed. The design will be a first attempt to address high-temperature moving parts and other design issues.

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Ancillary hardware such as steam generation equipment and pumps and their integration into the system will not be addressed initially. The prototype reactant rings have an outside diameter of 0. The ring width is 1. Robocast ferrite fin segments will be attached to refractory ceramic fiber board supports and will be assembled into reactant rings.

See Fig. Rotation will be driven by two sets of internal gears. Two sets of sprocket gears will drive the rings. Sunlight will be introduced through a quartz-dome window, and steam will be ducted through ports in the insulation to provide countercurrent sweeping of the reactant fins. Design objectives include experimental flexibility and significant hydrogen production. Solar hydrogen production from water by the use of two-step solar-driven thermochemical cycles is potentially an alternative to fossil fuels. Recognizing that thermochemical cycles are heat engines that convert thermal energy into chemical energy and are, therefore, analogous to mechanical work producing machines, we have conceived a new kind of heat engine.

As in Stirling and Ericsson cycle mechanical work producing counterparts, countercurrent recuperation of sensible heat within the cycle is the key to high efficiency in the CR5. Investigations of the efficiency potential of the CR5 concept suggest that solid-to-solid countercurrent recuperation can be effective and that the cycle can potentially be efficient.

Furthermore, recuperation mitigates the need for complete reaction extent and permits the use of support for the ferrite working material.


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  8. These investigations also suggest that the underlying thermodynamic properties of the iron oxide redox materials are marginal at the temperatures dictated by materials and that a number of schemes will probably be required to compensate. These include adjusting the redox thermodynamics by substituting other metals for iron in the spinel, taking advantage of solid-gas reactions by continuous removal of the product gases, and effectively lowering the product gas partial pressure by countercurrent sweeping. Like other engines, the CR5 involves numerous design issues and tradeoffs.

    It places extraordinary demands on materials and involves high-temperature moving parts.

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    In addition, the CR5 must be designed and operated to avoid a crossover through the recuperator. In the process of evaluating materials for the CR5 heat engine, we have developed a new kind of reactant material in which ferrite particles are dispersed in a monolithic zirconia structure. These materials appear to enhance and maintain reactivity and kinetics, as well as provide the structural support needed in the CR5 heat engine.

    To establish the practicality of the CR5 concept, we are experimentally evaluating materials, exploring the thermodynamic design space, and evaluating fluid flow within the device. Given the potential, uncertainties, and results thus far, we have decided to design, build, and test a prototype device.

    If suitable materials can be developed and the design challenges can be met, the CR5 heat engine concept appears to provide an integrated approach for potentially efficient and low-cost solar hydrogen. This work is supported by the U. Department of Energy under Contract No. A recuperator radiation heat transfer area m 2. Stefan—Boltzmann constant 5.

    Schematic showing heat flows of an ideal iron oxide cycle operating between K and K. In addition, the heat of reaction, Recuperator temperature profile for the case presented in Tables 1 , 2. Recuperation at high temperatures requires less of the recuperator than at low temperatures.

    The as-cast test sample is approximately 15 mm in diameter and 5. Baseline Fe 3 O 4 CR5 heat engine performance parameters.

    Design Principles of Perovskites for Thermochemical Oxygen Separation

    Sign In or Create an Account. Sign In. Advanced Search. Article Navigation. Research Papers. Diver Richard B. Sandia National Laboratories. This Site. Google Scholar.

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    James E. Miller James E. Mark D. Allendorf Mark D. Nathan P. Siegel Nathan P. Roy E. Hogan Roy E. Author and Article Information. Richard B. Nov , 4 : 8 pages. Published Online: September 4, Article history Received:. Standard View Views Icon Views. Issue Section:. Sample design parameters for a CR5 engine using iron oxide as a reactant material are listed in Table 1. In the design, the fin outside diameter is 0. For these calculations the TR and WO reactor sections and the two recuperator sections each used one-quarter of the ring circumference.

    The ring spacing is 6. This is the smallest spacing that we believe to be reasonable in a practical device. HHV H 2.

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    Q solar. T approach. Search ADS. Volume , Issue 4. Next Article. The CR5 Heat Engine. This may take some time to load. Jump to main content. Jump to site search.

    Thermochemical Processes: Principles and Models Thermochemical Processes: Principles and Models
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    Thermochemical Processes: Principles and Models Thermochemical Processes: Principles and Models
    Thermochemical Processes: Principles and Models Thermochemical Processes: Principles and Models
    Thermochemical Processes: Principles and Models Thermochemical Processes: Principles and Models
    Thermochemical Processes: Principles and Models Thermochemical Processes: Principles and Models
    Thermochemical Processes: Principles and Models Thermochemical Processes: Principles and Models
    Thermochemical Processes: Principles and Models Thermochemical Processes: Principles and Models
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