Luke J. Venstrom

The Effects of Morphology on the Oxidation of Ceria by Water and Carbon Dioxide

The oxidation of three-dimensionally ordered macroporous (3DOM) CeO2 (ceria) by H2 O and CO2 at 1100 K is presented in comparison to the oxidation of nonordered mesoporous and sintered, low porosity ceria. 3DOM ceria, which features interconnected and ordered pores, increases the maximum H2 and CO production rates over the low porosity ceria by 125% and 260%, respectively, and increases the maximum H2 and CO production rates over the nonordered mesoporous cerium oxide by 75% and 175%, respectively. The increase in the kinetics of H2 O and CO2 splitting with 3DOM ceria is attributed to its enhanced specific surface area and to its interconnected pore system that facilitates the transport of reacting species to and from oxidation sites.

Design of a Solar Reactor to Split CO2 Via Isothermal Redox Cycling of Ceria

The design procedure for a 3 kWth prototype solar thermochemical reactor to implement isothermal redox cycling of ceria for CO2 splitting is presented. The reactor uses beds of mm-sized porous ceria particles contained in the annulus of concentric alumina tube assemblies that line the cylindrical wall of a solar cavity receiver. The porous particle beds provide high surface area for the heterogeneous reactions, rapid heat and mass transfer, and low pressure drop. Redox cycling is accomplished by alternating flows of inert sweep gas and CO2 through the bed. The gas flow rates and cycle step durations are selected by scaling the results from small-scale experiments. Thermal and thermo-mechanical models of the reactor and reactive element tubes are developed to predict the steady-state temperature and stress distributions for nominal operating conditions. The simulation results indicate that the target temperature of 1773K will be reached in the prototype reactor and that the Mohr-Coulomb static factor of safety is above two everywhere in the tubes, indicating that thermo-mechanical stresses in the tubes remain acceptably low.

Review of Heat Transfer Research for Solar Thermochemical Applications

This article reviews the progress, challenges and opportunities in heat transfer research as applied to high-temperature thermochemical systems that use high-flux solar irradiation as the source of process heat. Selected pertinent areas such as radiative spectroscopy and tomography-based heat and mass characterization of heterogeneous media, kinetics of high-temperature heterogeneous reactions, heat and mass transfer modeling of solar thermochemical systems, and thermal measurements in high-temperature systems are presented, with brief discussions of their methods and example results from selected applications. [DOI: 10.1115/1.4024088]

Splitting water and carbon dioxide via the heterogeneous oxidation of zinc vapor: Thermodynamic considerations

The heterogeneous oxidation of zinc vapor is proposed as a promising reaction path for the exothermic step in the two-step Zn/ZnO solar thermochemical water and carbon dioxide splitting cycles. This approach circumvents mass transfer limitations encountered in the oxidation of solid or liquid zinc, promising rapid hydrogen and carbon monoxide production rates concurrent with a complete conversion of zinc to zinc oxide. In this paper, a parametric thermodynamic analysis is presented to quantify the benefit of achieving a rapid and complete conversion of zinc via the heterogeneous oxidation of zinc vapor. The conversion of zinc in polydisperse aerosol reactors has been limited to 20% for reaction times on the order of a minute, resulting in a cycle efficiency of ∼6%. The benefit of completely converting zinc via the heterogeneous oxidation of zinc vapor is an increase in efficiency to 27% and 31 % for water and carbon dioxide splitting, respectively. The cycle efficiency could be higher if heat recuperation is implemented.

Study of a Quench Device for the Synthesis and Hydrolysis of Zn Nanoparticles: Modeling and Experiments

The synthesis and hydrolysis of zinc nanoparticles are carried out in a tubular reactor A key component of the reactor is a coaxial jet quench device. Three coaxial and multi-inlet confined jets mix Zn(g), steam, and argon to produce and hydrolyze zinc nanoparticles. The performance of the quench device is assessed with computational fluid dynamics modeling and measurements of hydrogen conversion and particle size and composition. Numerical data elucidate the impact of varying jet flow rates on temperature and velocity distributions within the reactor. Experiments produce hydrogen conversions of 61-79%. Particle deposition on sections of the reactor surface above 650 K favors hydrolysis. Residence time for in-flight particles is less than 1 s and these particles are partially hydroLyzed.

Ceria-based electrospun fibers for renewable fuel production via two-step thermal redox cycles for carbon dioxide splitting

Zirconium-doped ceria (Ce(1-x)Zr(x)O2) was synthesized through a controlled electrospinning process as a promising approach to cost-effective, sinter-resistant material structures for high-temperature, solar-driven thermochemical redox cycles. To approximate a two-step redox cycle for solar fuel production, fibrous Ce(1-x)Zr(x)O2 with relatively low levels of Zr-doping (0 < x < 0.1) were cycled in an infrared-imaging furnace with high-temperature (up to 1500 °C) partial reduction and lower-temperature (∼800 °C) reoxidation via CO2 splitting to produce CO. Increases in Zr content improve reducibility and sintering resistance, and, for x≤ 0.05, do not significantly slow reoxidation kinetics for CO production. Cycle stability of the fibrous Ce(1-x)Zr(x)O2 (with x = 0.025) was assessed for a range of conditions by measuring rates of O2 release during reduction and CO production during reoxidation and by assessing post-cycling fiber crystallite sizes and surface areas. Sintering increases with reduction temperature but occurs primarily along the fiber axes. Even after 108 redox cycles with reduction at 1400 °C and oxidation with CO2 at 800 °C, the fibers maintain their structure with surface areas of ∼0.3 m(2) g(-1), higher than those observed in the literature for other ceria-based structures operating at similarly high temperature conditions. Total CO production and peak production rate stabilize above 3.0 mL g(-1) and 13.0 mL min(-1) g(-1), respectively. The results show the potential for electrospun oxides as sinter-resistant material structures with adequate surface area to support rapid CO2 splitting in solar thermochemical redox cycles.

Solar Thermal Electrolytic Process for the Production of Zn From ZnO: An Ionic Conductivity Study

The ionic conductivities of mixtures of ZnO in Na 3 AlF 6 and in xCaF 2 -yNa 3 AlF 6 mixtures were established with a swept-sine measurement technique. A millivolt sinusoidal voltage at frequencies from 1000 Hz to 25,000 Hz was impressed on a system containing the electrolytes. The systems frequency response was used to establish the conductivities. The influence of these conductivities on the potential of a solar thermal electrolytic process was evaluated using two process performance parameters: the back-work ratio and the fraction of minimum solar thermal energy required to drive the metal production reaction. We found the conductivity of mixtures of ZnO-Na 3 ALF 6 to be independent of the concentration of ZnO for weight percentages of ZnO from 0.5% to 5%. For temperatures 1240-1325 K the conductivity is close to that of pure Na 3 AlF 6, 3 ± 0.5 Ω -1 cm -1 . At temperatures from 1350 K to 1425 K it jumps to 6 ± 0.5 Ω -1 1 cm -1 When CaF 2 is added to the mixture, the electrolytes conductivity drops. We thus expect that calcium cations are not present to any important extent in the electrolyte. When CaF 2 is part of the chemical system, the concentration of ZnO can have a measurable impact on the electrolytes conductivity. Combining the conductivity results with the two solar process performance parameters illustrates the importance of operating the solar process at low current densities when the temperature range is 1200-1 S00 K The results further suggest that one should consider studying the electrolytic process at 1800 K.

The oxidation of macroporous cerium and cerium-zirconium oxide for the solar thermochemical production of fuels

The H2 and CO productivity and reactivity of three-dimensionally ordered macroporous (3DOM) cerium and cerium-zirconium oxide upon H2 O and CO2 oxidation at 1073K is presented in comparison to the productivity and reactivity of non-ordered porous and low porosity cerium oxide. The production of H2 and CO2 constitutes the second step of the two-step solar thermochemical H2 O and CO2 splitting cycles. The 3DOM cerium oxide, with a specific surface area of 25 m2 g−1 , increases the average H2 and CO production rates over the non-ordered porous cerium oxide with a specific surface area of 112 m2 g−1 : the average H2 production rate increases from 5.2 cm3 g−1 min−1 to 7.9 cm3 g−1 min−1 and the average CO production rate increases from 7.7 cm3 g−1 min−1 to 21.9 cm3 g−1 min−1 . The superior reactivity of 3DOM cerium oxide is attributed primarily to the stability of the 3DOM structure and also to the improved transport of reacting species to and from oxidation sites realized with the interconnected and ordered pores of the 3DOM structure. Doping the 3DOM cerium oxide with 20 mol% zirconia further stabilizes the structure and increases the average H2 and CO production rates to 10.2 cm3 g−1 min−1 and 22.1 cm3 g−1 min−1 , respectively.Copyright

Thermal electrolytic production of Mg from MgO: Reflections on commercial viability

We are exploring the commercial viability for producing Mg from MgO for which thermal energy is supplied to the cell as a substitute for some electric energy. The thermal input source may be concentrated sunlight or natural gas. Laboratory-scale electrochemical studies near 1250 K for two cell concepts show that we reached current densities above 0.5 A-cm−2at an overvoltage of 1.0 V. Current efficiency values exceeded 80%. The discussion of the relationship between these bench-top experimental results and the industrial potential of the process has been initiated.

A High-Flux Solar Furnace for Undergraduate Engineering Education and High-Temperature Thermochemistry Research

The optical design and engineering features of a 10 kW solar furnace now operational at Valparaiso University are described. The solar furnace is anticipated to achieve a mean concentration ratio of 3000 suns over a 6 cm diameter focus. It will support high-temperature solar chemistry research and undergraduate engineering pedagogy. Many of the components of the solar furnace were designed and constructed by undergraduate engineering students. Some of these students cite their participation in the solar furnace project as the motivating factor for continuing to work in the area of energy science in industry or graduate school.Copyright