Thermochemical Cycling Research Articles

The morphological stability and fuel production of commercial fibrous ceria particles for solar thermochemical redox cycling

Implementation of the solar thermochemical ceria redox cycle to split water and carbon dioxide depends in part on the morphological stability of a porous ceria substrate and the ability to acquire porous substrate in high volume. Here we evaluate the evolution of morphology and fuel production of ceria particles formed of fibers in a commercially relevant manufacturing process. The particles are evaluated over 1000 CO2-splitting cycles (56 h) at 1773K followed by sixteen temperature-swing cycles (5.7h) with oxidation at 1073K. New particles are 78% porous with a specific surface area of 0.14m2g−1 and a grain size of 3.7μm. During isothermal cycling, the morphology stabilized after 500 cycles (28h) to 73% porosity, a surface face 0.08m2g−1 and a grain size of 8μm. The stabilized particles retained 89% of the peak cycle average rate of CO production. During temperature-swing cycling, the specific surface area decreased to 0.06m2g−1. The mass-produced fibrous structures have adequately stable morphologies to produce fuel production performance similar to less scalable (lab-scale) ceria structures of similar pre-cycling surface area.

Study of the performance of Co and Ni ferrites after several cycles involved in water-splitting thermochemical cycles

Water splitting thermochemical cycles based on metal oxide systems and using concentrated solar energy is a promising and sustainable clean process for hydrogen production. In this work, the capability of Ni and Co powder ferrites for hydrogen production via two step redox processes has been tested. In order to avoid decreasing of the hydrogen release with the number of cycles due to powder sintering, a simple handmade pellet preparation has been used. Then, samples in the forms of pellets or powders were tested for the water splitting reaction under the same experimental conditions, and the amount of hydrogen generated in each case has been compared. Results show that pellet samples maintain their shape after the cycling process, although higher hydrogen production rate is obtained for Ni ferrite powder samples. This study also includes a thorough chemical, structural and morphological characterization of the samples after thermochemical cycling.

A solar thermochemical fuel production system integrated with fossil ​fuel heat recuperation

Thermochemical cycling (TC) is a promising means of converting solar energy and storing it in chemical fuels. Isothermal solar TC has the potential of achieving high solar-to-fuel efficiencies contingent on effective gas-phase heat recovery at high temperatures. We propose an approach of simultaneously recovering waste heat and unreacted gas species (e.g. H2O or CO2) downstream the isothermal TC reactor by taking advantage of endothermic reactions of certain fossil fuels (e.g. CH4). Such comprehensive utilization and the syngas produced enable a polygeneration system for simultaneous power and methanol production with the possibility of eliminating the self-generation power plant that is conventionally needed for methanol production. The scheme is proposed based on the type of splitting reaction in isothermal TC, and optimization of the polygeneration system is discussed with solar energy conversion efficiency as the objective. Fossil Fuel Consumption Rate of methanol production is about 22GJ/ton, lower than typical values in current industrial processes. Compared with direct solar reforming or direct combustion of the same amount of fossil fuels, this new approach features lower carbon emission per unit calorific value of fuel obtained due to the incorporation of the isothermal TC upstream.

Implications of Exceptional Material Kinetics on Thermochemical Fuel Production Rates

AbstractProduction of chemical fuels by solar‐driven thermochemical cycling has recently generated significant interest for its potential as a highly efficient method of storing solar energy. Of particular interest is the thermochemical process using non‐stoichiometric oxides, such as ceria. In this process a reactive oxide is cyclically exposed to an inert gas, typically at 1500 °C to induce the partial reduction of the oxide, and then exposed to an oxidizing gas of either H2O or CO2 at a temperature between 800–1500 °C to oxidize the oxide and release H2 or CO. Conventional wisdom has held that material kinetics limit the fuel production rates. Herein we demonstrate that, instead, at 1500 °C the rates of both reduction and oxidation of ceria, and hence also the global fuel production rate, are limited only by thermodynamic considerations for any reasonable set of operating conditions. Thus, in terms of materials design, significant room exists for sacrificing material kinetics in favor of thermodynamic characteristics.

Process concepts to produce syngas for Fischer–Tropsch fuels by solar thermochemical splitting of water and/or CO2

Process concepts for making synthesis gas by solar thermochemical cycling have been screened. The produced gas is delivered at 70°C, 5bar pressure and with a H2/CO ratio of 2.0 which, with moderate adjustments, fit specifications of Fischer–Tropsch synthesis using a cobalt-based catalyst. The cobalt ferrite/alumina assisted hercynite cycle run adiabatically between 1350 and 1330°C was selected for the study. HYSYS process simulations comprise six process combinations; quenching of hydrogen with CO2; separate solar splitting of water and CO2; vacuum depletion of oxygen in cobalt ferrite; flushing out oxygen with nitrogen produced cryogenically; and flushing with nitrogen from high temperature dense membranes. Calculated energy demand eliminates cryogenic concepts whereas the dense membranes require further study. Most promising is the water-only splitting pathway in combination with CO2 quench and vacuum oxygen depletion. The alternative with separate H2O and CO2 solar splitting benefits from half the amount of inert CO2 in the produced syngas and 7% reduction in heliostat size, but suffers from technical challenges with high temperature heat exchangers and severe coking probability; and calculated 7% higher installed cost. The novel chemical quench method limits the use of high temperature heat exchangers to just one for the depleted oxygen.

Solar fuel processing efficiency for ceria redox cycling using alternative oxygen partial pressure reduction methods

Solar-driven non-stoichiometric thermochemical redox cycling of ceria for the conversion of solar energy into fuels shows promise in achieving high solar-to-fuel efficiency. This efficiency is significantly affected by the operating conditions, e.g. redox temperatures, reduction and oxidation pressures, solar irradiation concentration, or heat recovery effectiveness. We present a thermodynamic analysis of five redox cycle designs to investigate the effects of working conditions on the fuel production. We focused on the influence of approaches to reduce the partial pressure of oxygen in the reduction step, namely by mechanical approaches (sweep gassing or vacuum pumping), chemical approaches (chemical scavenger), and combinations thereof. The results indicated that the sweep gas schemes work more efficient at non-isothermal than isothermal conditions, and efficient gas phase heat recovery and sweep gas recycling was important to ensure efficient fuel processing. The vacuum pump scheme achieved best efficiencies at isothermal conditions, and at non-isothermal conditions heat recovery was less essential. The use of oxygen scavengers combined with sweep gas and vacuum pump schemes further increased the system efficiency. The present work can be used to predict the performance of solar-driven non-stoichiometric redox cycles and further offers quantifiable guidelines for system design and operation.

Thermal transport model of a sorbent particle undergoing calcination–carbonation cycling

A numerical model coupling transient radiative, convective, and conductive heat transfer, mass transfer, and chemical kinetics of heterogeneous solid–gas reactions has been developed for a semitransparent, nonuniform, and nonisothermal particle undergoing cyclic thermochemical transformations. The calcination–carbonation reaction pair for calcium oxide looping is selected as the model cycle because of its suitability for solar‐driven carbon dioxide capture. The analyzed system is a single, porous particle undergoing thermochemical cycling in an idealized, reactor‐like environment. The model is used to investigate two cases distinguished by the length of the calcination and carbonation periods. The calcination–carbonation process for a single particle is shown to become periodic after three cycles. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2647–2656, 2015

Thermodynamic study on solar thermochemical fuel production with oxygen permeation membrane reactors

Thermodynamic analysis is performed on a conceptual oxygen permeation membrane reactor driven by concentrated solar energy (heat) for isothermal H2O splitting for the purpose of solar fuel derivation. By way of a plug-flow reactor model, kinetic and thermodynamic factors responsible for conversion rate, reactor dimension, and solar-to-fuel efficiency are analyzed for the case of pump-assisted and methane-assisted scenarios. The pump-assisted case achieves the same solar-to-fuel efficiency (2.9% at 1500 degrees C) as isothermal solar thermochemical cycling, while the methane-assisted case attains much higher efficiencies at much lower temperatures, whose net solar-to-fuel efficiency reaches 63% at around 900 degrees C. The theoretical framework developed in this study can be applied to the solar thermochemical splitting of other gases such as CO2 and can be further extended to the co-splitting of H2O and CO2 for syngas production driven by solar energy only (i.e., without the participation of hydrocarbon fuels). Copyright (c) 2015 John Wiley & Sons, Ltd.

Investigations of nano coated calcium hydroxide cycled in a thermochemical heat storage

Thermochemical heat storage systems are a promising new technology for concentrated solar power plants and can contribute to improve the efficiency of industrial processes Neveu et al. (2013) [21]. However, for example for the reaction system calcium oxide/calcium hydroxide (CaO/Ca(OH)2), the good availability at low cost is accompanied by poor powder properties that demand complex reactor solutions. During thermochemical cycling agglomeration effects occur and originate inhomogeneity resulting in permanent changes of bed characteristics especially related to the heat and mass transport. One approach in order to stabilize the material is to coat the reacting material with nanoparticles in order to minimize attractive forces leading to less agglomeration. But, high temperatures, change of volume and surface configuration, permeance for reaction gas, side reactions and mechanical stresses within the storage represent challenges for nanoparticles. Therefore, in this work, Aerosil® as additive for thermochemical storage is investigated during cycling in an indirect operating pilot-scale thermochemical reactor with regard to side reactions, stability on the surface and various coating configurations. It is shown that the reaction bed properties can be highly improved depending on the modality of the insertion process whereas occurring side reactions lead to a stabilization of the surface structure at the expense of a capacity loss of the thermochemical reactor.

Hydrogen peroxide thermochemical oscillator as driver for primordial RNA replication.

This paper presents and tests a previously unrecognized mechanism for driving a replicating molecular system on the prebiotic earth. It is proposed that cell-free RNA replication in the primordial soup may have been driven by self-sustained oscillatory thermochemical reactions. To test this hypothesis, a well-characterized hydrogen peroxide oscillator was chosen as the driver and complementary RNA strands with known association and melting kinetics were used as the substrate. An open flow system model for the self-consistent, coupled evolution of the temperature and concentrations in a simple autocatalytic schemeis solved numerically, and it is shown that thermochemical cycling drives replication of the RNA strands. For the (justifiably realistic) values of parameters chosen for the simulated example system, the mean amount of replicant produced at steady state is 6.56 times the input amount, given a constant supply of substrate species. The spontaneous onset of sustained thermochemical oscillations via slowly drifting parameters is demonstrated, and a schemeis given for prebiotic production of complementary RNA strands on rock surfaces.

Effect of Morphology on Spectral Radiative Properties of Three-Dimensionally Ordered Macroporous Ceria Packed Bed

In this paper, radiative characterization of a packed bed of novel three-dimensionally ordered macroporous (3DOM) ceria particles is performed in the spectral range relevant to solar thermochemical processes, 0.35–2.2 μm. Normal–hemispherical transmittance and reflectance of three samples of various thicknesses are measured. Monte Carlo ray-tracing (MCRT) and discrete ordinate methods are employed to identify transport scattering albedo and transport extinction coefficient in the spectral range corresponding to weak absorption in the semi-transparency band of ceria. 3DOM ceria particles are characterized by weaker scattering in comparison to sintered ceria ceramics, and increased transparency in the near-infrared spectral range 0.7–2 μm. The ordered pore-morphology of the 3DOM ceria after thermochemical redox cycling between temperatures 1373 K and 1073 K is altered due to sintering of walls of the 3DOM structure. The absorption coefficient of the packed bed is found to be practically independent of morphology. Radiative characterization of 3DOM ceria ceramics before and after thermochemical cycling suggests that preserving the 3DOM structure can lead to scattering characteristics that permit longer attenuation path lengths of incident concentrated solar radiation in the material, as well as be favorable for confinement of the near-infrared radiation during thermochemical cycling leading to favorable thermochemical conditions for fuel production.

Enhanced Oxidation Kinetics in Thermochemical Cycling of CeO2 through Templated Porosity

Two-step thermochemical cycling was achieved using CeO2 with sub-micrometer sized macropores, allowing for substantially improved CO production at fast cycle rates when compared to nonporous CeO2. The effects of porosity, pore order, and packing density were probed by synthesizing ceria materials with different morphologies. Polymeric colloidal spheres were used as templates for the synthesis of three-dimensionally ordered macroporous (3DOM) CeO2 and nonordered macroporous (NOM) CeO2. Aggregated CeO2 nanoparticles with feature sizes similar to those in 3DOM CeO2 were prepared by fragmenting 3DOM CeO2 into its building blocks using ultrasonication. The three templated materials and nonporous, commercial CeO2 were tested in thermochemical cycles using an infrared furnace. CeO2 was reduced at ∼1200 °C, and the reduced CeO2−δ materials were reoxidized under CO2 at ∼850 °C. The high temperatures required for cycling induced changes in the morphology of the porous materials, which were characterized by electron microscopy, X-ray diffraction, and nitrogen sorption measurements. In spite of sintering, the macroporous materials retained an interconnected pore network during 55 cycles, providing a 10-fold enhancement in CO productivity and production rate when compared to nonporous CeO2. Additionally, 3DOM CeO2 provided the fastest rate of CO production of all tested materials and also retained the smallest solid feature sizes. This boost in reaction kinetics allowed for extremely rapid cycling with less than a minute required for complete reduction or oxidation. Characterization of the porous materials also provided some insight into thermal gradients that developed in the sample bed as a result of rapid heating and cooling.

Atomic layer deposited thin film metal oxides for fuel production in a solar cavity reactor

Alumina thin film structures were produced by coating high surface area polymer particles via atomic layer deposition (ALD), using the polymer as a sacrificial template. Burnout of the polymer material left high surface area, high pore volume structures, with 15 nm wall thickness. Further deposition of up to 27 mol% Co and Fe was performed via ALD to produce high surface area CoFe2O4 particles for thermochemical water splitting. The ALD particles were thermally cycled in electrically heated lab reactors and on-sun using a concentrated solar, reflective cavity reactor. Surface area measurements of cycled ALD particles showed improved surface area retention as compared to bulk Fe2O3 nanopowders. Reaction rates as high as 15.2 and 9.8 μmol/s/g were observed, on-sun, for H2O and CO2 splitting respectively. Thermochemical cycling in a concentrated solar cavity reactor showed an order of magnitude increase in solar utilization efficiency between ALD particles and bulk Fe2O3 nanopowders.

Using in-situ techniques to probe high-temperature reactions: thermochemical cycles for the production of synthetic fuels from CO2 and water

Ferrites are promising materials for enabling solar-thermochemical cycles. Such cycles utilize solar-thermal energy to reduce the metal oxide, which is then re-oxidized by H2O or CO2, producing H2 or CO, respectively. Mixing ferrites with zirconia or yttria-stabilized zirconia (YSZ) greatly improves their cyclabilities. In order to understand this system, we have studied the behavior of iron oxide/8YSZ (8 mol-% Y2O3 in ZrO2) using in situ X-ray diffraction and thermogravimetric analyses at temperatures up to 1500 °C and under controlled atmosphere. The solubility of iron oxide in 8YSZ measured by XRD at room temperature was 9.4 mol-% Fe. The solubility increased to at least 10.4 mol-% Fe when heated between 800 and 1000 °C under inert atmosphere. Furthermore iron was found to migrate in and out of the 8YSZ phase as the temperature and oxidation state of the iron changed. In samples containing >9.4 mol-% Fe, stepwise heating to 1400 °C under helium caused reduction of Fe2O3 to Fe3O4 to FeO. Exposure of the FeO-containing material to CO2 at 1100 °C re-oxidized FeO to Fe3O4 with evolution of CO. Thermogravimetric analysis during thermochemical cycling of materials with a range of iron contents showed that samples with mostly dissolved iron utilized a greater proportion of the iron atoms present than did samples possessing a greater fraction of un-dissolved iron oxides.

Ferrite-YSZ composites for solar thermochemical production of synthetic fuels: in operando characterization of CO2 reduction

Ferrites are promising materials for enabling solar-thermochemical cycles. Such cycles utilize solar-thermal energy for the production of hydrogen from water, or carbon monoxide from carbon dioxide. Mixing ferrites with zirconia or yttria-stabilized zirconia (YSZ) greatly improves the cyclability of the ferrites and enables a move away from powder to monolithic systems. This synergistic effect is only partially understood. In order to unravel the underlying mechanisms of the effect and to understand the evolution of thermochemically active phases, we have studied the behaviour of iron oxides co-sintered with 8YSZ (8 mol% Y2O3) using in operando X-ray diffraction and thermogravimetric analysis at temperatures up to 1500 °C and under environments representative of those present in a thermochemical cycle. The solubility of iron oxide in 8YSZ measured by XRD at room temperature, following calcination to 1500 °C in air, was 9.4 mol% Fe. The solubility increased to at least 10.4 mol% Fe when heated between 800 and 1000 °C under inert (He) atmosphere. Furthermore iron was found to migrate in and out of the 8YSZ phase as the temperature and oxidation state of the iron changed. In samples containing insoluble iron (i.e., containing >9.4 mol% Fe) stepwise heating to 1400 °C under helium caused reduction of Fe2O3 (hematite) to Fe3O4 (magnetite) to FeO (wustite). This gradual thermal reduction from hematite to wustite was accompanied by evolution of oxygen. The wustite remained stable upon cooling to room temperature in the helium environment, although after multiple consecutive cycles some of the wustite was observed to disproportionate to Fe metal and magnetite. Exposure of the wustite-containing material to CO2 at 1100 °C enabled re-oxidation of the wustite to magnetite with evolution of CO. Thermogravimetric analysis during thermochemical cycling of materials with iron oxide contents between 1.8 and 27.6 mol% Fe showed that samples with mostly dissolved iron utilized a greater proportion of the iron atoms present than did samples possessing a significant fraction of un-dissolved iron oxides.

Water-splitting via Thermochemical Cycling of Ceria

not Available.

96/01859 Operating procedure for fluidized-bed reactor for gasification of carbonaceous fuels

Solar-driven thermochemical water splitting represents one efficient route to the generation of H2 as a clean and renewable fuel. Due to their outstanding catalytic abilities and promising solar fuel production capacities, perovskite-type redox catalysts have attracted significant attention in this regard. In the present study, the perovskite series La1-xCaxMn1-yAlyO3 (x, y = 0.2, 0.4, 0.6, or 0.8) was fabricated using a modified Pechini method and comprehensively investigated to determine the applicability of these materials to solar H2 production via two-step thermochemical water splitting. The thermochemical redox behaviors of these perovskites were optimized by doping at either the A (Ca) or B (Al) sites over a broad range of substitution values, from 0.2 to 0.8. Through this doping, a highly efficient perovskite (La0.6Ca0.4Mn0.6Al0.4O3) was developed, which yielded a remarkable H2 production rate of 429 μmol/g during two-step thermochemical H2O splitting, going between 1400 and 1000 °C. Moreover, the performance of the optimized perovskite was found to be eight times higher than that of the benchmark catalyst CeO2 under the same experimental conditions. Furthermore, these perovskites also showed impressive catalytic stability during two-step thermochemical cycling tests. These newly developed La1-xCaxMn1-yAlyO3 redox catalysts appear to have great potential for future practical applications in thermochemical solar fuel production.