High-temperature Solar Reactor Research Articles

Experimental Performance of a Nonlinear Control Strategy to Regulate Temperature of a High-Temperature Solar Reactor

Abstract Despite the significant potential of solar thermochemical process technology for storing solar energy as solid-state solar fuel, several challenges have made its industrial application difficult. It is important to note that solar energy has a transient nature that causes instability and reduces process efficiency. Therefore, it is crucial to implement a robust control system to regulate the process temperature and tackle the shortage of incoming solar energy during cloudy weather. In our previous works, different model-based control strategies were developed namely a proportional integral derivative controller (PID) with gain scheduling and adaptive model predictive control (MPC). These methods were tested numerically to regulate the temperature inside a high-temperature tubular solar reactor. In this work, the proposed control strategies were experimentally tested under various operation conditions. The controllers were challenged to track different setpoints (500 °C, 1000 °C, and 1450 °C) with different amounts of gas/particle flowrates. Additionally, the flow controller was tested to regulate the reactor temperature under a cloudy weather scenario. The ultimate goal was to produce 5 kg of reduced solar fuel magnesium manganese oxide (MgMn2O4) successfully, and the controllers were able to track the required process temperature and reject disturbances despite the system's strong nonlinearity. The experimental results showed a maximum error in the temperature setpoint of less than 0.5% (6 °C), and the MPC controller demonstrated superior performance in reducing the control effort and rejecting disturbances.

Design and optimization of solar-dish volumetric reactor for methane dry reforming process with three-dimensional optics-CFD method

Solar thermochemical methane dry reforming process has great potential for long-term energy storage. Most of the current researches focused on the development of efficient catalyst or the operation optimization of solar reactor with certain structure design. However, there is still a lack of research into the comparison study of different reactor structure designs and optimization strategies for chemical conversion process intensification. In this paper, a three-dimensional coupled optical-CFD model is established to explore the effects of different reactor structure designs, catalyst distribution optimization and thermal conductivity improvement on the performance enhancement of methane dry reforming (MDR) reaction. It is found that the cylindrical porous foam reactor has higher heat absorption efficiency and better chemical conversion performance. The distribution of catalyst would obviously affect the overall performance of the reactor and rational adjustment of catalyst distribution has the potential to improve the reactor performance. Improving the heat transfer ability of porous absorber can help to reduce the temperature gradient inside the reactor but has little effect on the reaction conversion. And the relative position of solar reactor can also affect the light propagation path and influencing the reaction process. In general, the obtained results can provide valuable references for the future study of high-temperature solar thermochemical reactors.

Experimental and numerical study on thermal performance of an indirectly irradiated solar reactor with a clapboard-type internally circulating fluidized bed

Fluidized-bed based solar reactors for the gasification of biomass have been widely recognized as a promising approach to produce high-quality syngas due to the merits, e.g. continuous operation and superior heat and mass transport. In this work, a novel high-temperature solar reactor with a clapboard-type internally circulating configuration that can efficiently mitigate the overheating of the bed wall affected by non-uniform solar radiation, is designed, constructed, and experimentally investigated under Singapore’s first 28 kWe high-flux solar simulator. To investigate the flow dynamics in the high-temperature solar reactor, a transient 3D computational fluid dynamics multi-phase model is developed accordingly and validated by the experimental data. The simulated results can well clarify the flow and thermal behaviours in experiments. The effects of the gas flow rate and bed mass on the thermal performances of different Group-B particle materials are studied through experiments and simulation. The results indicate that increasing the bed mass in the solar reactor is capable of mitigating the overheating of the absorber surface caused by hot spots. The gas flow rate is dependent on the particle thermo-physical properties and shape as well as the clapboard geometric parameters. The highest solar-to-thermal conversion efficiency (defined as the ratio of the accumulated sensible heat of the particles to the solar power input) of 10.7 ± 0.4% is achieved by SiC due to its superior thermal conductivity.

Biomimetic hierarchical structure for enhancing concentrated solar energy converting and utilizing efficiency.

Concentrated solar technology has the problems of local overheating, inadequate light absorption, and insufficient diffusion of solar irradiation energy flow. In this study, the idea of using biomimetic hierarchical structure is proposed to solve these problems through radiation regulating and photon managing, thereby enhancing the conversion and utilization efficiency of concentrated solar energy. The performance of uniform porous structure and biomimetic hierarchical structure are comparatively investigated. The geometrical parameters (diameters, ratio, and filling factor) of the biomimetic hierarchical structure are investigated and optimized by finite-difference time-domain (FDTD) method combined with particle swarm optimization algorithm. The results indicate that the biomimetic hierarchical structure can enhance the effective light absorption and energy flow diffusion efficiency of concentrated solar energy, an increasing pore size is better for the biomimetic hierarchical structure than a decreasing pore size, and the feature sizes which serve as the boundary between large and small pores are found. For the effective light absorption, the biomimetic hierarchical structure has an enhancement of 3.4%, and for the energy flow diffusion efficiency, this structure has an enhancement of over 25%, compared with the uniform porous structure. This research can provide general and valuable guidance for enhancing solar energy utilizing efficiency of high-temperature solar thermochemical reactors, solar cells, and photocatalytic carriers, based on the biomimetic hierarchical structure.

Thermal Model of a Solar Thermochemical Reactor for Metal Oxide Reduction

Abstract A transient heat transfer model is developed to study the thermal performance of a high-temperature solar thermochemical reactor for metal oxide reduction. The solar reactor consists of an indirectly irradiated tubular fluidized bed contained in a solar cavity receiver. Radiative heat transfer in the cavity, modeled with the Monte Carlo ray-tracing method, is coupled to conduction in the tube and cavity walls. Incident radiation distributions from a diffuse radiative source and a high-flux solar simulator are implemented separately in the model to study the influence of incident radiation directionality on the performance of the reactor. Maximum temperature, maximum thermal stress, start-up time, energy balance, and particle reduction rate for the proposed reactor concept are calculated to inform the design and optimization of a prototype reactor.

Remarkable performance of microstructured ceria foams for thermochemical splitting of H2O and CO2 in a novel high–temperature solar reactor

Thermochemical splitting of H2O and CO2 applying redox materials constitutes a sustainable option for synthetic fuel production and CO2 valorization. It consists of two-step process based on the creation of oxygen vacancies in non-stoichiometric oxides during solar-driven thermal reduction, followed by the material re-oxidation with H2O and/or CO2 to generate syngas (H2/CO), the building block for a wide variety of synthetic hydrocarbon fuels. In this work, a monolithic solar reactor was designed and tested integrating reticulated porous ceria (open-cell foams) heated by concentrated solar energy. The influence of various operating parameters on the thermochemical reactor performance was investigated. Increasing the temperature or decreasing the pressure in the reduction step was found to enhance the maximum reduction extent reached by the redox material (CeO2-δ), thereby improving the fuel production capacity. In addition, a decrease of the oxidation temperature led to higher fuel production rate, despite an increase of the temperature swing between the reduction and oxidation steps. Increasing the oxidant concentration also sharply enhanced the oxidation rate. Peak CO production rate approaching 10 mL/min/g was achieved with ceria foams (exhibiting micron-sized grains forming an interconnected macroporous network within the struts) during their reoxidation upon free cooling with pure CO2 stream (after reduction at 1400 °C), thus strongly outperforming (by a factor of about x8) the previous maximum values reported to date. This result was attributed to the fine and stable granular microstructure of the reticulated ceria foams. The solar reactor reliability and robustness during high-temperature two-step redox cycling were demonstrated with an average cycle production of 5.1 mL/g of H2 and CO, and peak solar-to-fuel efficiencies above 8%. The highly reactive reticulated foams with 10 and 20 ppi (pore per inch) were cycled for about 69 h (51 cycles) of continuous on-sun operation without any decrease in performance, thus evidencing their noteworthy thermochemical and microstructural stability.

Numerical Modeling of a Novel Solar Driven Sulfuric Acid Decomposition Reactor

An efficient and cost-effective approach to produce hydrogen sees the direct use of thermal power to drive thermochemical water splitting processes. Such cycles break the hydrogen-oxygen bounds through a series of heat-driven chemical reactions, with recirculation of intermediate substances of different type. Among the various water splitting processes, the thermo-electrochemical Hybrid Sulfur (HyS) is one of the most appealing cycles. It has only fluid reactants and is comprised of only two global reaction steps: a low temperature electrochemical exothermic section, operating at temperatures on the order of 100 °C, and a high temperature endothermic thermal section operating at max temperatures of about 800 °C. In the electrochemical section SO2 and H2O are combined to produce electrochemically H2 at the electrolyzer cathode and H2SO4 at the electrolyzer anode. Sulfuric acid is recycled inside the plant, concentrated and decomposed, in the high temperature thermal section, into SO2, O2 and H2O. The endothermic decomposition of sulfuric acid takes place in two main reactions: H2SO4 → SO3 + H2O (1) SO3 → SO2 + 0.5 O2 (2) The required thermal power is provided by external source and by internal heat recovery from the SO2-O2 stream product of the decomposition reactions. Solar driven HyS process is being studied by the Greenway Energy and the University of South Carolina within the DOE HydroGEN program (part of the DOE Energy Material Network initiative) [1], partnering with the Idaho National Laboratory, the Savannah River National Laboratory and the National Renewable Energy Laboratory. The project focuses on the high temperature sulfuric acid decomposition section. One of the main tasks of the project is to develop and fabricate an effective and low cost high temperature solar reactor for the decomposition of the sulfuric acid into sulfur dioxide. Previous work was carried out within the DOE-NHI Initiative and resulted in identifying a reactor concept based on the bayonet heat exchanger as the baseline reactor configuration [2]. The bayonet reactor was tested both from a modeling point of view and from an experimental point of view, demonstrating the potential to achieve almost the reaction equilibrium conditions. At temperatures of about 800 °C and pressures of 14 bar, an SO2 production of approximately 19 mol fraction % was achieved, which is essentially the equilibrium value at the same operating conditions [3,2]. However, a solar driven bayonet concept presents several drawbacks, mainly relative to the need for an additional intermediate heat exchanger to transfer the solar power to the reactive sulfur mixture. The removal of the intermediate solar heat exchanger will result in increased energetic and exergetic efficiencies and decreased capital and operating costs. A novel reactor concept is proposed, described and discussed, based on a direct solar heated cavity receiver system. It realizes the solar driven sulfuric acid decomposition and internal heat recovery in a single unit, directly heated by the incident solar radiation without the presence of any intermediate heat transfer fluid. The reactor includes catalytic structures to decompose H2SO4 and foamed structures in the internal heat recovery channels to increase the overall thermal conductivity. A reactor unit, made of SiC material, and directly heated from solar radiation has preliminarily been demonstrated from a modeling point of view. A detailed transport phenomena model was developed using Finite-Volume approach and run in STAR-CCM+. The kinetics parameters reported in Reference [3] were adopted for the current simulations. Results showed excellent conversion rates and reduced temperature gradients and pressure drops inside the device, achieving better performance than the bayonet configuration. The cavity receiver-reactor also allows: (1) efficient internal heat recovery from the decomposition products and (2) connections with the metallic interfaced HyS equipment (e.g. tubing, valves, etc.) at low temperatures. Results obtained from a first conceptual scale up analysis of the reactor concept also demonstrated the actual feasibility for integration in both centralized and distributed solar tower based plants.

A CFD-DEM study of hydrodynamics with heat transfer in a gas-solid fluidized bed reactor for solar thermal applications

The particles flow and heat transfer characteristics of a high temperature solar thermochemical fluidized bed reactor have been studied for solar beam-down concentrating systems. A numerical model has been developed by the combined approach of computational fluid dynamics (CFD) and discrete element method (DEM) collisional model since it is an effective approach for studying the gas-solid flow. The discrete ordinate model has been used to solve the radiation heat transfer. An experimental visualization of particles circulation pattern and mixing of two-tower fluidized bed system has been presented. A good agreement has been found between the experimental measurements and numerical predictions. The effect of gas superficial velocity, bed mass and inlet gas temperature on the flow pattern and temperature characteristics of the bed have been investigated. The results showed that the maximum and average temperature of the bed, depends on the top layer position and focal point of the concentrated radiation, decreased when increasing the total mass of the bed.

A Novel Iris Mechanism for Solar Thermal Receivers

Variable aperture mechanisms are being used in many fields including medicine, electronics, fluid mechanics, and optics. The main design characteristics of these aperture concepts are the use of multiple blades regulating aperture area and consequently the incoming medium flow. Manufacturing complexities primarily depend on the concept geometry, material, and the process application requirements. Design of a variable aperture demands meticulous methodology and careful consideration of the application field. This paper provides an in-depth methodology on the design of a novel iris mechanism for temperature control in high temperature solar thermal receivers and solar reactors. Such methodology can be used as a guideline for iris mechanisms implemented in other applications as well as in design of different apparatuses exposed to high temperature. Optical simulations in present study have been performed to demonstrate enhanced performance of the iris mechanism over conventional Venetian blind shutter serving as optical attenuators in concentrating solar power systems. Results showed that optical absorption efficiency is improved by 14% while reradiation loss through the aperture is reduced by 2.3% when the iris mechanism is used. Correlation for adaptive control of aperture area was found through computational surface area measurement. Experimental testing with a 7 kW solar simulator at different power levels demonstrated the performance of the mechanism to maintain stable temperature under variable flux.

Optical thermometry for high temperature multiphase environments under high-flux irradiation

A non-intrusive optical method to measure gas phase temperature in strongly scattering multiphase environments under high-flux, broad-band irradiation, relevant to conditions in high temperature solar reactors was developed and demonstrated. The high-flux irradiation with a peak flux of 450kW/m2 was provided by a 6kW metal-halide lamp coupled with a reflector and two concentrators. An ethylene/air diffusion flame, which contains fine soot particles, was employed to provide a high temperature reacting flow (approximately 1800K) with strong optical interference from nano particles having a peak soot volume fractions of ∼16ppm (with irradiation) under conditions of relevance to solar reactors. Under this environment, the proposed laser-based thermometry technique, line-wise two-line atomic fluorescence (TLAF) has been successfully demonstrated to measure flame temperature with good spatial resolution of ∼1mm. It was found that the measurement accuracy in the presence of particle and the high-flux external radiation is 65K at a typical flame temperature of ∼1800K, while the measurement precision is 38K. Results reveal that the presence of high-flux irradiation increases the flame temperature by typically 50–100K. This paper presents a thermometry technique that is suitable for temperature measurement within solar reactors, particularly in hybrid solar-thermal receiver-combustor systems. The experimental setup, measurement methodology and data processing are discussed, followed by the temperature measurements.

Coupled Concentrating Optics, Heat Transfer, and Thermochemical Modeling of a 100-kWth High-Temperature Solar Reactor for the Thermal Dissociation of ZnO

This work reports a numerical investigation of the transient operation of a 100-kWth solar reactor for performing the high-temperature step of the Zn/ZnO thermochemical cycle. This two-step redox cycle comprises (1) the endothermal dissociation of ZnO to Zn and O2 above 2000 K using concentrated solar energy, and (2) the subsequent oxidation of Zn with H2O/CO2 to produce H2/CO. The performance of the 100-kWth solar reactor is investigated using a dynamic numerical model consisting of two coupled submodels. The first is a Monte Carlo (MC) ray-tracing model applied to compute the spatial distribution maps of incident solar flux absorbed on the reactor surfaces when subjected to concentrated solar irradiation delivered by the PROMES-CNRS MegaWatt Solar Furnace (MWSF). The second is a heat transfer and thermochemical model that uses the computed maps of absorbed solar flux as radiation boundary condition to simulate the coupled processes of chemical reaction and heat transfer by radiation, convection, and conduction. Experimental validation of the solar reactor model is accomplished by comparing solar radiative power input, temperatures, and ZnO dissociation rates with measured data acquired with the 100-kWth solar reactor at the MWSF. Experimentally obtained solar-to-chemical energy conversion efficiencies are reported and the various energy flows are quantified. The model shows the prominent influence of reaction kinetics on the attainable energy conversion efficiencies, revealing the potential of achieving ηsolar-to-chemical = 16% provided the mass transport limitations on the ZnO reaction interface were overcome.

A transient heat transfer model for high temperature solar thermochemical reactors

We present a heat transfer model for energy transport in a solar thermochemical reactor for CO2 and/or H2O splitting to produce chemical fuels CO and/or H2. The reactor is comprised of a horizontal cavity-receiver packed with several insulation layers, an array of tubular absorbers in which the reactive metal oxide is loaded, and a windowless aperture that allows the incident concentrated solar energy to enter the receiver. A framework for modeling the complex multimode thermal transport within the reactor system is developed. The concentrated solar radiation from a high flux solar simulator to the cavity-receiver is simulated using a Monte-Carlo ray tracing model. Heat transfer within the tubular absorbers, including conduction, convection, radiation, and chemical reactions, is simulated using a lattice Boltzmann (LB) model. These two models are coupled by taking into account the temperature-dependent radiative emission from the absorber surfaces. Two particular techniques are employed to improve the numerical accuracy and stability: appropriate rescaling of the relaxation coefficients in the LB method and implicit treatment of the heat source/sink terms due to chemical reactions. The numerical method is validated with two test cases for which analytical solutions are available. Simulation results for the heating process and one thermal reduction step under near-vacuum pressures are presented. The predicted solar-to-fuel energy conversion efficiency varies from 5% to 10% and is determined based on the simulated O2 release during the reduction step. The model is able to provide insight into the optimum operating conditions, the reactor design and the scale-up. The results suggest that high temperatures for both reduction and oxidation steps and near-vacuum pressure (∼10−4 atm) for thermal reduction are critical for improved solar-to-fuel conversion efficiency with the ceria-based reactive material.

Heat Transfer Simulation in a High Temperature Solar Reactor

Abstract The fluid flow coupled to radiative, convective and, conductive heat transfer is computed within a solar reactor containing a reticulated porous ceramics (RPC) domain made out of ceria (cerium dioxide, CeO 2 ). A Monte-Carlo radiation model is used in the fluid regions of the reactor with source terms outside the cavity's window to account for the concentrated radiative power input. Darcy's lawfor the viscous regime and theForchheimer's term for the inertial regime are used in the momentum equation to account for the pressure drop within the porous region (RPC). Two separate energy equations for the solid and for the fluid regions of the porous domainare solved in order to capture the non-equilibrium effects in that region. Rosseland's diffusion approximation is used in the solid regions of the RPC domain. The material properties and boundary conditions were taken frompublished experimental measurements. The simulation results are compared to the measurement data collected during the pre-heating and the ceria reduction phases, which sum up to four different radiative power inputs. Results of the comparison are good and constitute the verification that the numerical methods, physical sub-process models and material properties are adequately selected and implemented.

Demonstration of the Entire Production Chain to Renewable Kerosene via Solar Thermochemical Splitting of H2O and CO2

The European consortium SOLARJET has experimentally demonstrated the first ever production of jet fuel via a thermochemical H2O/CO2-splitting cycle using simulated concentrated solar radiation. The key component of the production process of sustainable “solar kerosene” is a high-temperature solar reactor containing a reticulated porous ceramic (RPC) foam structure made of pure CeO2 undergoing a 2-step redox cyclic process. During the first endothermic reduction step at 1450–1600 °C, the RPC was directly exposed to concentrated thermal radiation with power inputs ranging from 2.8 to 3.8 kW and mean solar flux concentration ratios of up to 3000 suns. In the subsequent exothermic oxidation step at 700–1200 °C, the reduced ceria was stoichiometrically reoxidized with CO2 and/or H2O to generate CO and/or H2. The RPC featured dual-scale porosity: millimeter-size pores for volumetric radiation absorption during reduction and micrometer-size pores within its struts for enhanced oxidation rates. For a cycle durati...

Experimental investigation of a reticulated porous alumina heat exchanger for high temperature gas heat recovery

The present study presents an experimental study of a prototype counter-flow heat exchanger designed to recover sensible heat from inert and reactive gases flowing through a high temperature solar reactor for splitting CO2. The tube-in-tube heat exchanger is comprised of two concentric alumina tubes, each filled with reticulated porous alumina with a nominal porosity of 80% and pore density of 5 pores per inch (ppi). The RPC provides high heat transfer surface area per unit volume (917 m−1) with low pressure drop. Measurements include the permeability, inertial coefficient, overall heat transfer coefficient, effectiveness and pressure drop. For laminar flow and an inlet gas temperature of 1240 K, the overall heat transfer coefficients are 36–41 W m−2 K−1. The measured performance is in good agreement with a prior CFD model of the heat exchanger.

Pore-level engineering of macroporous media for increased performance of solar-driven thermochemical fuel processing

The performance of high-temperature solar reactors incorporating porous ceramic materials that serve as radiative absorbers and chemical reaction sites can be improved significantly by tailoring their pore structure. We investigated the changes in their effective heat and mass transport properties with increasing mass loading of porous ceramics fabricated by the replica method. We applied a methodology consisting of the experimental characterization of the structure via 3D tomographic techniques coupled to pore-level direct numerical simulations for the determination of the effective transport properties. This approach was extended by using digital image processing on the structure data to allow for artificial changes in the morphological characteristics – corresponding to actual variations in the fabrication process. We derived transport correlations of porous ceria foam with varying mass loading, i.e. reticulate to dense foams with porosity from 0.85 to 0.45. We observed that the correlations proposed in literature do not accurately describe the behavior of low-porosity foams. The numerical findings of this study provide guidance for pore-level engineering of materials used in solar reactors and other high-temperature heat and mass transfer applications.

Kinetic analysis of high-temperature solid–gas reactions by an inverse method applied to ZnO and SnO2 solar thermal dissociation

This study addresses the kinetic investigation of solid–gas reactions in a high-temperature solar chemical reactor. An inverse method was developed to identify the kinetics of metal oxide thermal dissociation as part of a two-step thermochemical redox cycle for solar splitting of H2O and CO2. This method was applied and further validated by studying ZnO and SnO2 solar thermal dissociation. A solar chemical reactor enabling continuous solid reactant processing was developed in which both the oxide reactant temperature at the front surface and the O2 concentration in the off gas were measured dynamically. The aim of the inverse method was to identify the intrinsic kinetics of the dissociation reaction using only the available experimental data.Different approaches were proposed and compared to investigate the kinetics of solid–gas reactions. The activation energy of the reaction was first estimated roughly using an iso-conversional model-free approach, which can be used as an initialization value for further refinement with the inverse method. The inverse method consists in identifying the reaction kinetics from only the online diagnosis of outlet O2 concentration and using a model enabling parameters fitting via an iterative process. Depending on the considered approach and assumptions for predicting the temperature profile within the reacting oxide rod (succession of stationary states assumption or unsteady state operation), the activation energy of the dissociation reaction was found to be 313±31kJ/mol for ZnO and 353±18kJ/mol for SnO2. Such a method may be implemented for the kinetic analysis of any kind of solid–gas reactions in high-temperature solar reactors.

Review: Photochemical and Thermochemical Production of Solar Fuels from H2O and CO2 Using Metal Oxide Catalysts

Metal oxides are reviewed as catalysts to convert H2O and CO2 to fuels using solar energy. For photochemical conversion, TiO2 has been found to be the most stable and useful oxide material, but it is currently limited by its large bandgap and a mismatch between its conduction band and the redox couples for water splitting and CO2 reduction. A theoretical framework has been utilized to understand the basic thermodynamics and energetics in photochemical energy conversion systems. This is applied to model systems comprised of Ag2O and AgCl to examine why the former reacts thermochemically in air, while the latter reacts photochemically. For thermochemical conversion, zinc-, ceria-, and ferrite-based redox cycles are examined and examples of high-temperature solar reactors driven by concentrated solar radiation are presented. For CO2 splitting, theoretical solar-to-fuel energy conversion efficiencies can be up to 26.8% for photochemical systems, and can exceed 30% for thermochemical systems, provided that sen...

CO2 and H2O reduction by solar thermochemical looping using SnO2/SnO redox reactions: Thermogravimetric analysis

The thermochemical dissociation of CO2 and H2O from reactive SnO nanopowders is studied via thermogravimetry analysis. SnO is first produced by solar thermal dissociation of SnO2 using concentrated solar radiation as the high-temperature energy source. The process targets the production of CO and H2 in separate reactions using SnO as the oxygen carrier and the syngas can be further processed to various synthetic liquid fuels. The global process thus converts and upgrades H2O and captured CO2 feedstock into solar chemical fuels from high-temperature solar heat only, since the intermediate oxide is not consumed but recycled in the overall process. The objective of the study was the kinetic characterization of the H2O and CO2 reduction reactions using reactive SnO nanopowders synthesized in a high-temperature solar chemical reactor. SnO conversion up to 88% was measured during H2O reduction at 973 K and an activation energy of 51 ± 7 kJ/mol was identified in the temperature range of 798-923 K. Regarding CO2 reduction, a higher temperature was required to reach similar SnO conversion (88% at 1073 K) and the activation energy was found to be 88 ± 7 kJ/mol in the range of 973-1173 K with a CO2 reaction order of 0.96. The SnO conversion and the reaction rate were improved when increasing the temperature or the reacting gas mole fraction. Using active SnO nanopowders thus allowed for efficient and rapid fuel production kinetics from H2O and CO2.

Thermogravimetry Analysis of CO2 and H2O Reduction from Solar Nanosized Zn Powder for Thermochemical Fuel Production

This study addresses the thermochemical production of CO and H2 as high-value solar fuels from CO2 and H2O using reactive Zn nanoparticles. A two-step thermochemical cycle was considered: Zn-rich nanopowder was first synthesized from solar thermal ZnO dissociation in a high-temperature solar chemical reactor and the reduced material was then used as an oxygen carrier during the CO2 and H2O reduction reactions. The kinetics of CO2 and H2O reduction was investigated by thermogravimetry to demonstrate that the solar-produced nanoparticles react efficiently with CO2 and H2O. Zn started to react from 513 K and almost complete Zn conversion (reaction extent over 95%) was achieved at 633–773 K in less than 5 min, thus confirming that the active Zn-rich nanopowder exhibits rapid fuel production kinetics during H2O and CO2 dissociation. The reaction mechanism was best described by a nucleation and growth model with an activation energy of 43 kJ/mol and an oxidant order of 0.8. The high reactivity of zinc was attri...