Browsing by Subject "Thermochemical"

Now showing 1 - 5 of 5
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    Improving the efficiency of a ceria reduction-oxidation cycle through the choice of operating conditions and ceria morphology
    (2014-05) De Smith, Robert Michael
    Pathways for improving the efficiency of ceria-based thermochemical cycling for solar-driven fuel production are investigated. First, the operating conditions of an isothermal CO2 splitting cycle are optimized to improve process efficiency. The optimum conditions are a sweep gas flow rate of 150 mL min-1 g-1, a CO2 flow rate of 50 mL min-1 g-1, a reduction time of 100 s, and an oxidation time of 155 s. A quasi-equilibrium model is developed to predict the rates of ceria reduction and oxidation. Finally, a new ceria morphology, wood templated ceria, is used to improve the heterogeneous oxidation reaction rates by maintaining a high surface area when exposed to the extreme temperatures required for ceria reduction. Wood templated ceria performs well at reduction temperatures up to 1400 °C, reaching peak CO production rates of 9 mL min-1 g-1, but rates decrease due to sintering when it is reduced at 1500 °C.
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    Numerical modeling of transport phenomena in reactive porous structures for solar fuel applications
    (2013-08) Keene, Daniel Joseph
    The focus of this work is the development and use of a numerical model to study solar thermochemical fuel production using nonstoichiometric ceria in the form of a reactive porous medium. Volume averaging theorems are applied at the pore-level to obtain a set of governing equations that use effective transport properties to describe the heat and mass transfer processes in the reactive porous medium on the macroscale. Reaction rate expressions are formulated for both the thermal reduction and oxidation steps as an interphase mass flux whose dependence on the partial pressures, solid temperature, and nonstoichiometry is derived from the ionization state of the oxygen vacancies.The porosity and pore-level solid feature size (as represented by a Sauter mean diameter) of a porous monolith are varied from 0.6 to 0.9 and 10 to 1000 μm, respectively, to determine their impact on the rate of oxygen release and the solar-to-chemical energy conversion efficiency during thermal reduction while the macroscale geometry and operating conditions are held fixed. The process is carried out in a batch mode by placing the reactive medium inside a cavity where it is directly irradiated with a concentrated solar flux as a sweep gas passes through its pore network. The best performance is obtained with a non-uniform heating of the solid, which is attributed to the nonlinear temperature dependence of the equilibrium nonstoichiometry. The solar-to-chemical energy conversion efficiency attains its highest value of 10.9 % with a porosity of 0.9 and a Sauter mean diameter of 10 μm. These results imply that high porosities and small feature sizes are preferred, but it is recognized and discussed that these findings are strongly tied to the particular set of operating conditions considered.The mathematical model is also used in conjunction with available experimental data to develop a method for determining a reaction rate expression that characterizes the oxidation of ceria by carbon dioxide. The volume-averaged conservation equations for the porous medium are used to simulate a packed bed of porous particles and the axially-dispersed plug flow model is used to compare numerical predictions of the effluent composition with experimental measurements. Reaction rate expressions are developed and their kinetic parameters are determined by minimizing the difference between the numerical predictions and the experimental measurements as quantified by a root mean square error for a temperature of 1203 K. At this temperature, numerical results obtained using a reaction rate expression based on singly ionized oxygen vacancies provide excellent agreement with the experimental data.
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    Solar gasification of biomass: design and characterization of a molten salt gasification reactor
    (2013-12) Hathaway, Brandon Jay
    The design and implementation of a prototype molten salt solar reactor for gasification of biomass is a significant milestone in the development of a solar gasification process. The reactor developed in this work allows for 3 kWth operation with an average aperture flux of 1530 suns at salt temperatures of 1200 K with pneumatic injection of ground or powdered dry biomass feedstocks directly into the salt melt.Laboratory scale experiments in an electrically heated reactor demonstrate the benefits of molten salt and the data was evaluated to determine the kinetics of pyrolysis and gasification of biomass or carbon in molten salt. In the presence of molten salt overall gas yields are increased by up to 22%; pyrolysis rates double due to improved heat transfer, while carbon gasification rates increase by an order of magnitude. Existing kinetic models for cellulose pyrolysis fit the data well, while carbon gasification in molten salt follows kinetics modeled with a 2/3 order shrinking-grain model with a pre-exponential factor of 1.5*106 min-1 and activation energy of 158 kJ/mol.A reactor concept is developed based around a concentric cylinder geometry with a cavity-style solar receiver immersed within a volume of molten carbonate salt. Concentrated radiation delivered to the cavity is absorbed in the cavity walls and transferred via convection to the salt volume. Feedstock is delivered into the molten salt volume where biomass gasification reactions will be carried out producing the desired product gas. The features of the cavity receiver/reactor concept are optimized based on modeling of the key physical processes. The cavity absorber geometry is optimized according to a parametric survey of radiative exchange using a Monte Carlo ray tracing model, resulting in a cavity design that achieves absorption efficiencies of 80%-90%. A parametric survey coupling the radiative exchange simulations to a CFD model of molten salt natural convection is used to size the annulus containing the molten salt to maximize utilization of absorbed solar energy, resulting in a predicted utilization efficiency of 70%. Finite element analysis was used to finalize the design to achieve acceptable thermal stresses less than 34.5 MPa to avoid material creep.
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    Thermal modeling and design of a solar non-stoichiometric redox reactor with heat recovery
    (2013-08) Lapp, Justin L.
    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.
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    Zinc aerosol hydrolysis in a transverse jet reactor.
    (2011-12) Haltiwanger, Julia Frances
    Some of the major challenges---both technical and economic---of the Zn/ZnO two-step thermochemical hydrogen production cycle are investigated in this study. Technically, complete hydrolysis of Zn in the hydrogen production step remains a major barrier to implementation, and much attention has been given to Zn nano-scale reacting aerosols as a solution. Smaller particles favor faster reaction kinetics, and because they can be entrained and reacted in a gas flow, a continuous controllable process is possible. However, success of this continuous process depends on achieving high particle yields and high conversions in the aerosol, neither of which have yet been achieved in laboratory reactors. The ability of a new reactor concept based on transverse jet fluid dynamics to control the flow field and rapidly cool the Zn vapor is investigated. In the transverse jet reactor, evaporated Zn entrained in an Ar carrier gas issues vertically into the horizontal tubular reactor through which cooler H2O and Ar flow. Particles are formed in the presence of steam at ~450 K. The objective of controlling the flow field is to keep Zn away from the walls, thereby reducing particle deposition in the reactor and increasing particle yields on the filter. A computational fluid dynamics (CFD) model indicates that the trajectory of the jet can be controlled so that the majority of the Zn mass is directed down the center of the reactor, not near the reactor walls. Furthermore, the model shows that quench rates of 2x10^4 K/s are achieved and reactants are well mixed. Experimentally, maximum particle yields of 93% of the mass entering the reactor are obtained. Hydrolysis experiments are conducted in the transverse jet reactor at 418 K, 573 K, 603 K, and 713 K to assess the mechanisms of particle growth and hydrolysis. Experiments are conducted with and without steam to assess the effect of the reacting gas on particle morphology. SEM images of particles collected on a filter downstream from the reaction zone indicate that particle growth is dominated by condensation, resulting in hexagonal particles generally with lengths across their hexagonal face of 300 nm to 1micron in experiments with stream, and 1 to 3 micron in experiments without steam. Furthermore, the SEM images indicate that in hydrolysis experiments, ZnO forms on the surface of particles early on, protecting them from re-evaporation. Particle yield on the filter, Y, is defined as the fraction of the total mass entering the reactor that is collected on a filter placed downstream of the reaction zone. Overall conversion, X, is measured by monitoring the H2 content of the effluent gas throughout experiments with a gas chromatograph. Conversion of aerosol particles, Z, is the ZnO content (by mole) of particles collected from the downstream filter; it is measured by x-ray diffractometry with the internal standard calibration method. At all temperatures, particle yield remains high---generally 70 to 80% in hydrolysis experiments---and particle deposition on the walls of the reaction zone is eliminated for temperatures of 573 K and above. However, the conversion in the aerosol is <7% and decreases with reaction zone temperature. The overall conversion ranges from 11% at 418 K to 49% at 713 K. The higher overall conversion than conversion in the aerosol is attributed to heterogeneous Zn vapor hydrolysis. Visual observation proves heterogeneous hydrolysis occurs on the reactor walls; it is inferred that the heterogeneous Zn vapor reaction also occurs on the surface of aerosol particles. In this study, high particle yields are achieved for the first time---an important step forward for the continuous aerosol process. However, complete conversion of the aerosol particles remains a major challenge. In an economic and policy study of the Zn/ZnO cycle, the time frame for economic viability is assessed through the use of experience curves under minimal input, mid-range, and aggressive incentive policy scenarios. For the technology to become cost competitive, incentive policies that lead to early implementation of solar hydrogen plants will be necessary to allow the experience effect to draw down the price. Under such policies, a learning curve analysis suggests that hydrogen produced via the Zn/ZnO cycle could become economically viable between 2032 and 2069, depending on how aggressively the policies encourage the emerging technology. Thus, if the technical challenges are resolved, the Zn/ZnO cycle has the potential to be economically viable by mid-century if incentive policies--such as direct financial support, purchase guarantees, low interest rate loans, and tax breaks--are used to support initial projects.