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The hydrolysis reaction of the two-step ZnO/Zn and SnO 2 /SnO thermochemical cycles was kinetically investigated for solar hydrogen production. Nanoparticles of Zn and SnO were synthesized by solar thermal reduction of the oxides and neutral gas quenching of the vapors. They were then hydrolyzed to quantify and compare the H 2 yields and the kinetic rate laws in fixed-bed. The hydrolysis of Zn nanoparticles reached only up to 55% of H 2 yield, whereas SnO hydrolysis was almost complete. In contrast, Zn hydrolysis was much faster than SnO hydrolysis, but Zn deactivation occurred suddenly. Models of solid–gas reactions were applied to identify the controlling mechanisms and the associated kinetic parameters. The kinetic models were fitted to both isothermal and non-isothermal (temperature ramp) hydrolysis experimental data. Activation energies and reaction orders were found to be 122 ± 13 kJ/mol and 2.0 ± 0.3 for SnO, and 87 ± 7 kJ/mol and 3.5 ± 0.5 for Zn, respectively. Finally, a shrinking core approach was applied to the case of SnO to account for the reaction-controlling mechanisms. more...

This study deals with the production of hydrogen from water-splitting thermochemical cycles based on mixed metal oxides. The solar synthesis of mixed oxides based on ceria was achieved by melting mixed powders of component oxides at high temperature. The reduced oxides (Ce2Ti2O7, Ce2Si2O7, CeFeO3, CeVO4, and CeNbO4) produced hydrogen in a three-step thermochemical cycle except CeVO4. The three-step cycle uses an alkali hydroxide (NaOH or KOH) in an activation reaction to improve oxidation and to facilitate water-splitting during the third step. Experimental investigations were performed to demonstrate the concept and to quantify the reactions performance for hydrogen production. The high temperature synthesis of reduced cerium-based mixed oxides was performed in a laboratory-scale solar reactor. The mixed oxides were analysed by X-ray diffraction and characterized by XPS, which identified Ce(III) species in the structure. The activation reaction with NaOH or KOH producing hydrogen was studied to determine the effect of temperature and particle diameter, and to quantify the chemical conversion for each cerium-based mixed oxide. more...

The kinetic rate laws of heavy metal (HM) vaporization from municipal solid waste during its incineration were studied. Realistic artificial waste (RAW) samples spiked with Pb, Zn, and Cd were injected into a fluidized bed reactor. Metal vaporization wastracked by continuous measure ofthe above metals in exhaust gases. An inverse model of the reactor was used to calculate the metal vaporization rates from the concentration vs time profiles in the outlet gas. For each metal, experiments were carried out at several temperatures in order to determine the kinetic parameters and to obtain specific rate laws as functions of temperature. Temperature has a strong influence on the HM vaporization dynamics, especially on the vaporization kinetics profile. This phenomenon was attributed to internal diffusion control of the HM release. Two types of kinetic rate laws were established based on temperature: a fourth- or fifth-order polynomial rate law (r(x) = k0e(-E(A)/RT)p(x)) for temperatures lower than 740 degrees C and a first-order polynomial (r(x) = k0e(-E(A)/ RT(q-q(f) for temperatures higher than 740 degrees C. more...

A converter of focused optical radiation into electric current is considered on the basis of the photovoltaic effect in plasmas. The converter model is based on analysis of asymmetric spatial distributions of charge particle number density and ambipolar potential in the photoplasma produced by external optical radiation focused in a heat pipe filled with a mixture of alkali vapor and a heavy inert gas. Energy balance in the plasma photoelectric converter is analyzed. The conditions in which the external radiation energy is effectively absorbed in the converter are indicated. The plasma parameters for which the energy of absorbed optical radiation is mainly spent on sustaining the ambipolar field in the plasma are determined. It is shown that the plasma photoelectric converter makes it possible to attain a high conversion efficiency for focused solar radiation. more...

The production of hydrogen from a novel two-step thermochemical cycle based on SnO2/SnO redox reactions is presented. This process targets CO2-free hydrogen production by using renewable solar energy and water in a high-temperature water-splitting cycle. The cycle consists of a solar endothermic reduction of SnO2 into SnO(g) and O2 followed by a non-solar exothermic hydrolysis of SnO(s) to form H2 and SnO2(s). The objective of this study was to demonstrate this innovative concept for H2 production and to establish the potential of cycle implementation in an integrated solar chemical process. The reduction and hydrolysis reactions were experimentally tested in order to define optimal operating conditions, chemical conversion and hydrogen yield. The thermal reduction occurs under atmospheric pressure at about 1600 °C and over. The solar step encompasses the formation of SnO nanoparticles that can be hydrolysed efficiently in the temperature range 500–600 °C with a H2 yield over 90%. A preliminary process design is also proposed for cycle integration in solar chemical plants. more...

The study deals with a dynamic modeling of a solar thermochemical reactor operating continuously to simulate its behavior during transient periods. This reactor is devoted to the thermal reduction of volatile metal oxides which are involved in water-splitting cycles for hydrogen production. Unsteady mass and energy balances are solved to determine the evolution of the reactor temperature and of the outlet gas composition versus time. The kinetics of the chemical reaction is considered in the specific case of zinc oxide dissociation for which reliable data are available. For the chosen reactor design, the thermal inertia of the reactor materials has a weak influence on zinc production during short solar flux interruptions. Energy losses by conduction through reactor walls are the highest at small scale (ranging between 30% and 40% at 1 kW scale), whereas radiative losses through the aperture become predominant at large scale (50 MW scale) and greatly depend on the solar concentration ratio. Then, simulations show that a minimum concentration ratio of 2500 is necessary to reach a sufficient temperature (above 2000 K) allowing efficient ZnO dissociation. more...

A novel two-step thermochemical water-splitting cycle based on SnO2/SnO is proposed from the detailed study of the whole tin oxide systems involving three redox pairs. The thermal reduction of tin(IV) oxide occurs in the temperature range 1400-1600°C following a zero order kinetic law of Arrhenius with an activation energy of 394.8 kJ mol -1 and a pre-exponential factor of 8.32 X 10 8 g s -1 at atmospheric pressure. The operating conditions that prevent gaseous stannous oxide (SnO) from recombining with O 2 are defined. The effect of a quenching device (water-cooled finger) is negligible whereas operation at low total pressure or low O 2 and SnO partial pressures leads to nearly pure SnO product. The comparison of SnO and metallic tin hydrolysis in a fixed bed reactor reveals a higher reaction rate in the case of SnO. Hydrolysis of these reduced compounds shows nearly complete conversion producing hydrogen by a solid/gas reaction proceeding at moderate temperature, thus easy to implement in a common reactor technology. more...

This study deals with the thermal cracking of natural gas for the coproduction of hydrogen and carbon black from concentrated solar energy without CO2 emission. A laboratory-scale solar reactor (1 kW) was tested and modeled successfully. It consists of a tubular graphite receiver directly absorbing solar radiation, in which a mixture of Ar and CH4 flows. A temperature increase or a gas flow rate decrease results in chemical conversion increase. Methane conversion higher than 75% was obtained. Reaction occurred near the wall where temperature is maximal and gas velocity is minimal due to the laminar flow profile. The work focused also on the design of a medium-scale tubular solar reactor (10 kW) based on the indirect heating concept. A reactor model including gas hydrodynamics and heat and mass transfers coupled to the chemical reaction was developed in order to predict the reactor performances. Temperature and species concentration profiles and final chemical conversion were quantified. According to the r... more...

The sewage sludge from San Juan, Argentina, was characterized in an attempt to improve the thermal treatment efficiency and to reduce the environmental impact of incineration. The ash content is about 50% dry basis, and its weight loss at 105 °C was high. Taking into account the moisture content, it required drying for an autothermal combustion, thus imposing an additional combustible for incineration. The sludge contains high concentrations of several trace elements, and all but Hg are two to fivefold more concentrated in its ash. Thermogravimetric analyses were carried out on dry samples of sludge in inert and oxidative atmosphere. Three peaks can be observed in all differential thermogravimetric curves during the organic matter decomposition. The pyrolysis curve is over the combustion curve and parallel to it up to high temperatures. A kinetic model of the sludge weight loss during its incineration, based on these studies and combining the multi-step pyrolysis and combustion processes, is proposed and discussed. more...

This study addresses the exploration of an unconventional route for potentially cost effective hydrogen production with solar thermal energy. The process co-produces hydrogen-rich gas and high-grade Carbon Black (CB) from concentrated solar energy and methane. The approach is based on a single-step thermal decomposition (pyrolysis) of methane to hydrogen and CB, without catalysts and without emitting carbon dioxide since solid carbon is sequestered as carbon black. A high temperature nozzle-type solar chemical reactor has been developed in which the solar radiation absorbed by a graphite nozzle is transferred to the flow of reactant (fluid-wall reactor). The conversion of methane dissociation depends strongly on the solar power input, on the geometry of the graphite nozzle, and on the inert and reactive gas flow rates, which determine both the residence time and the inlet CH 4 mole fraction. Maximum chemical conversion of methane was up to 99% (yield of hydrogen up to 90%). It was obtained with a vertical-axis solar furnace (2 m-diameter concentrator), with a given geometry of the nozzle determining the fluid-wall reaction surface area, and with the lowest flow rate of methane investigated (0.1 L n /min). For each target tested, the higher the inlet flow rate of methane or argon, the lower the gas residence time, thus the lower the chemical conversion. more...

This article presents an XPS study of Ce 3d emission spectra dominated by atomic multiplet effects in core level spectroscopy of rare earth compounds (Ce oxides). Core level spectroscopy has been used to study the electronic states of Ce 3d5/2 and Ce 3d3/2 levels in Ce4+ and Ce3+ states. The well-resolved components of Ce 3d5/2 and Ce 3d3/2 spin-orbit components, due to various final states (4f0, 4f1, 4f2 configurations), were determined on 3d XPS spectra from commercial powders (CeO2, CePO4). These results were used to study the 3d spin-orbit component of mixed cerium-titanium oxide. This compound was prepared by co-melting commercial powders of CeO2 and TiO2 at 1800 K under air using a solar furnace with a flux density of 16 MW.m−2 at the focal point of the parabolic concentrator. The mixed oxide Ce2Ti2O7 was produced and contained Ce(III) species which may be reactive with water to give back the initial metal oxides and generate hydrogen, a valuable product considered as a promising energy carrier in the future in replacement of oil. The 3d photoemission spectra revealed the presence of mixed components attributed to mainly Ce(III) and Ce(IV) species. Copyright © 2008 John Wiley & Sons, Ltd. more...

Direct thermocline thermal energy storage is the cheapest sensible thermal energy storage configuration. Indeed, a thermocline tank consists in one tank instead of two and reduces costs. Thermocline thermal energy storages are often filled with cheap solid materials which could react with the heat transfer fluid in the case of incompatibility. PROMES laboratory is building a pilot-scale parabolic trough solar loop including a direct thermocline thermal energy storage system. The working fluid will be a synthetic oil, the Jarytherm® DBT, and the thermal energy storage tank will be filled with stabilized solid materials elaborated from vitrified wastes. Compatibility tests have been conducted in order to check on one hand if the thermo-mechanical properties and life time of the energy storage medium are not affected by the contact with oil and, on the other hand, if the thermal oil performances are not degraded by the solid filler. These experiments consisted in putting in contact the oil and the solid materials in small tanks. In order to discriminate the solid materials tested in the shortest time, accelerating aging conditions at 330 °C for 500 hours were used. The measurements consisted in X-Ray Diffraction and Scanning Electron Microscopy for the solids, and thermo-physical and chemical properties measurements for the oil. Regarding the solid samples, their crystalline structure did not change during the test, but it is difficult to conclude about their elementary composition and they seem to absorb oil. While thermal properties still makes Jarytherm® DBT a good heat transfer fluid after the accelerated aging tests, this study results in differentiating most compatible materials. Thus according to our study, Jarytherm® DBT can be used in direct thermocline thermal energy storage applications when compatibility of the solid material has been demonstrated. more...

International audience; A novel application of powders relies on their use as heat transfer medium for heat capture, conveying and storage. The use of powders as heat transfer fluid in concentrated solar systems is discussed with respect to current technologies. The specific application reported upon is the use of powder loops in Solar Power Tower plants. In the proposed receiver technology, SiC powder is conveyed as a dense particle suspension through a multi-tube solar receiver in a bubbling fluidization mode, the upwards flow being established by pressurizing the powder feed. Tests were conducted with a single-tube receiver unit at the 1 MW solar furnace of CNRS (Odeillo Font-Romeu, F). The measured wall-to-suspension heat transfer coefficient is a function of operating temperature, applied air velocity and imposed solid circulation flux: values increased with increasing solids flux from ∼430 to 1120 W/m2 K. Empirical approaches and a heat transfer model were applied to compare experimental and predicted values of the heat transfer coefficient, with a fair agreement obtained. The research moreover provides initial data concerning the overall economy of the system. The high temperature of the circulating powder leads to an increased power cycle efficiency, an increased storage density, reduced thermal power requirements, reduced heliostat field size, reduced parasitic power consumption and increased plant capacity factor. more...

A 50-150 kWth pilot solar rig comprising the key equipments of a real plant and that uses silicon carbide Dense Particles Suspension as the heat transfer fluid has been tested at the 1 MW solar furnace at Odeillo-Font Romeu, France. The tests were carried out under large ranges of operating parameters and controlling the mass flow rate when higher temperature was required and when changes on DNI (direct normal irradiation) occurred. This paper presents experimental results on particle outlet temperature, dynamic response of the system to solid mass flow rate and solar power variations, and receiver thermal efficiency (η). Mean and maximum particles’ temperature up to 585°C and 720°C respectively was reached. The receiver thermal efficiency was measured in the range 50-90%. more...

Dense Particle Suspension (DPS) can be used as high temperature heat transfer fluid in solar receiver. Tests conducted with a one-tube experimental setup in real conditions of concentrated solar irradiation resulted in determining heat transfer coefficients for the DPS flowing upward in a vertical tube. They have been obtained for solid fluxes in the range 10-45 kg/m2.s and outlet temperatures up to 1020 K. The influence of solid flux, aeration and temperature is outlined in this paper. Heat transfer coefficient variations are correlated as a function of the solid flux and the temperature for given aeration conditions. more...

A laboratory-scale solar reactor was designed and simulated for the thermal reduction of metal oxides involved in water-splitting thermochemical cycles for hydrogen production. This reactor features a cavity-receiver directly heated by concentrated solar energy, in which solid particles are continuously injected. A computational model was developed by coupling the fluid flow, heat and mass transfer, and the chemical reaction. The reactive particle-laden flow was simulated, accounting for a multiphase model (solid–gas flow). A discrete phase model based on a Lagrangian approach was developed. The kinetics of the chemical reaction was considered in the specific case of zinc oxide dissociation for which reliable data are available. The complete model predicts temperature and gas velocity distributions, species concentration profiles inside the reactor, particle trajectories and fates, and conversion rate assessing the reaction degree of completion. The reaction extent is highly dependent on temperature of the radiation-absorbing particles. Initial diameter of injected particles is also a key parameter because it determines the available surface area for a given particle mass feed rate. The higher the particle surface area, the higher the conversion rate. As a result, reaction completion can be achieved when particle temperature exceeds 2200 K for a 1 μ m initial particle diameter. more...

International audience; The aim of this work is to develop an experimental device for the measurement of porous media textural properties during physical or chemical transformation. The method is based on gas tracer experiments. The system investigated consists of a spherical reactor containing a spherical porous media. To avoid surface stresses, there is no contact between the sample and the reactor. We called the cell a particle in box, or ``PIB''. The porosity of the solid and the gas effective diffusivity were determined using a parametric identification method. The model is based on the perfectly stirred reactor approach. The PIB methodology was first checked at room temperature to determine the solid porosity and the effective diffusion coefficient. Experiments were then performed during transformation of a sample composed of a mixture of hydroxylapatite particles, glycerol, and water. In situ determination of the porosity and diffusivity from room temperature to 573 K (300 degrees C) was demonstrated. more...

This paper presents a thermodynamic and experimental study of three-step thermochemical cycles for hydrogen production involving hydroxides (NaOH and KOH). Solar concentrated energy was successfull... more...

This study deals with solar hydrogen production from the two-step iron oxide thermochemical cycle (Fe 3 O 4 /FeO). This cycle involves the endothermic solar-driven reduction of the metal oxide (magnetite) at high temperature followed by the exothermic steam hydrolysis of the reduced metal oxide (wustite) for hydrogen generation. Thermodynamic and experimental investigations have been performed to quantify the performances of this cycle for hydrogen production. High-temperature decomposition reaction (metal oxide reduction) was performed in a solar reactor set at the focus of a laboratory-scale solar furnace. The operating conditions for obtaining the complete reduction of magnetite into wustite were defined. An inert atmosphere is required to prevent re-oxidation of Fe(II) oxide during quenching. The water-splitting reaction with iron(II) oxide producing hydrogen was studied to determine the chemical kinetics, and the influence of temperature and particles size on the chemical conversion. A conversion of 83% was obtained for the hydrolysis reaction of non-stoichiometric solar wustite Fe (1− y ) O at 575 °C. more...

Both single‐walled and multi‐walled carbon nanotubes (SWNTs and MWNTs) are produced using a solar reactor working under nitrogen atmosphere. Samples were studied by transmission electron microscopy (TEM), Raman spectroscopy and electron energy loss spectroscopy (EELS). The produced MWNTs exhibit the typical morphologies observed for nitrogen‐doped MWNTs synthesized with other methods and EELS measurements point out the presence of substituted nitrogen within the tubes. The doping level is not uniform and the maximum value reaches 11 at.%. more...

A high-temperature fluid-wall solar reactor was developed for the production of hydrogen from methane cracking. This laboratory-scale reactor features a graphite tubular cavity directly heated by concentrated solar energy, in which the reactive flowing gas dissociates to form hydrogen and carbon black. The solar reactor characterization was achieved with: (a) a thorough experimental study on the reactor performance versus operating conditions and (b) solar reactor modeling. The results showed that the conversion of CH 4 and yield of H 2 can exceed 97% and 90%, respectively, and these depend strongly on temperature and on fluid-wall heat transfer and reaction surface area. In addition to the experimental study, a 2D computational model coupling transport phenomena was developed to predict the mapping of reactor temperature and of species concentration, and the reaction extent at the outlet. The model was validated and kinetics of methane decomposition were identified from simulations and comparison to experimental results. more...

Samples of single wall carbon nanotubes (SWNTs) were prepared using a solar reactor. Graphite targets containing different catalysts (Ni/Co, Ni/Y, Ni/Ce) allowed the synthesis of SWNTs soot in which nanotubes had different diameter distributions. Several consecutive stages of HCl treatment and thermal oxidation in air (HCl protocol) purified the samples. Another protocol involving HNO3 treatment and H2O2 oxidation (HNO3 protocol) was also used. Isotherms of hydrogen adsorption were volumetrically measured at 253 K under pressures below 6 MPa on raw and treated samples. The highest adsorption capacity (0.7 wt%) was measured on raw soot. HCl protocol clearly increases the BET surface area ( S BET ) and the microporous volume ( W 0 ( N 2 ) ) measured by N2 at 77 K of the treated samples with respect to the as-produced materials, whereas HNO3 protocol decreases them. A correlation between textural properties and hydrogen storage capacities is discussed. more...

The project BESTFACADE, sponsored by the Energy Intelligent Europe programme of the European Union, and led by MCE-Anlagenbau, Austria, accumulated the state of the art of double skin facades (DSFs) in seven European countries (Austria, Belgium, France, Germany, Greece, Portugal and Sweden). Twenty-eight facades of different buildings in all partner countries of BESTFACADE have been analysed for the aspects, types of facade in different countries, DSFs in different climatic regions of Europe, existing simulations and measurements, thermal behaviour, indoor air quality, comfort, user acceptance, energy demand and consumptions, control strategies, integrated building technology, cost (investment, maintenance and operation), resource conservation, environmental impact, comparison to conventional glass facades (CGFs), integration of renewable energy sources into DSFs, as well as non-energy related issues, such as, acoustics, aesthetics, fire protection, moisture, corrosion, durability, maintenance an... more...

Among new carbonaceous materials, herringbone carbon nanofibers (HCNFs) are one of the most promising precursors for the production of original adsorbents. Nevertheless, little information is available in the open literature on this subject and is mainly restricted to KOH activation. In the present study, the effects of the main activating agents currently used in chemical activation of conventional precursors, namely KOH, NaOH, ZnCl2 and H3PO4, are compared for the same HCNFs. The adsorptive properties of the obtained activated fibers are described through the corresponding nitrogen isotherms and conventional textural parameters. All chemical activations led to a common microporous texture but different efficiencies in terms of developed porosity per activating agent weight. At low activation yield (first activation regime), a microporosity is initiated resulting in a sharp decrease of the apparent mean pore size (from 20 to 15 A) and Lc002 and simultaneously to a small increase in d002. At higher activation yield (second activation regime), the mean pore size is found to be almost constant while the specific microporous volume is greatly increased (from 0.1 to 0.32 cm3 × g−1) and the d002 and Lc002 parameters remain nearly constant. It was observed that the KOH activation is much more efficient than the other ones. more...

High temperature solar receiver is a core component of solar thermal power plants. However, non-uniform solar irradiation on the receiver walls and flow maldistribution of heat transfer fluid inside the tubes may cause the excessive peak temperature, consequently leading to the reduced lifetime. This paper presents an original CFD (computational fluid dynamics)-based evolutionary algorithm to determine the optimal fluid distribution in a tubular solar receiver for the minimization of its peak temperature. A pressurized-air solar receiver comprising of 45 parallel tubes subjected to a Gaussian-shape net heat flux absorbed by the receiver is used for study. Two optimality criteria are used for the algorithm: identical outlet fluid temperatures and identical temperatures on the centerline of the heated surface. The influences of different filling materials and thermal contact resistances on the optimal fluid distribution and on the peak temperature reduction are also evaluated and discussed. Results show that the fluid distribution optimization using the algorithm could minimize the peak temperature of the receiver under the optimality criterion of identical temperatures on the centerline. Different shapes of optimal fluid distribution are determined for various filling materials. Cheap material with low thermal conductivity can also meet the peak temperature threshold through optimizing the fluid distribution. more...

International audience; Char samples were produced from pyrolysis in a lab-scale solar reactor. The pyrolysis of beech wood was carried out at temperatures ranging from 600 to 2000 °C, with heating rates from 5 to 450 °C/s. CHNS, scanning electron microscopy analysis, X-ray diffractometry, Brunauer–Emmett–Teller adsorption were employed to investigate the effect of temperature and heating rate on char composition and structure. The results indicated that char structure was more and more ordered with temperature increase and heating rate decrease (higher than 50 °C/s). The surface area and pore volume firstly increased with temperature and reached maximum at 1200 °C then reduced significantly at 2000 °C. Besides, they firstly increased with heating rate and then decreased slightly at heating rate of 450 °C/s when final temperature was no lower than 1200 °C. Char reactivity measured by TGA analysis was found to correlate with the evolution of char surface area and pore volume with temperature and heating rate. more...

International audience; The design of solar concentrating systems and receivers requires the spatial distribution of the solar flux on the receiver. This article presents an integral formulation of the optical model for the multiple reflections involved in solar concentrating facilities, which is solved by a Monte Carlo ray-tracing (MCRT) algorithm that handles complex geometries. An experimental validation of this model is obtained with published results for a dish configuration. The convergence of the proposed algorithm is studied and found faster than collision-based algorithms. In addition, an example of the use of the sensitivity of the power on a target to the mirror rms-slope is given by treating an inverse-problem consisting in finding the equivalent rms-slope of mirrors that best match the flux map measurements. more...

A new thermochemical cycle for H2 production based on CeO2/Ce2O3 oxides has been successfully demonstrated. It consists of two chemical steps: (1) reduction, 2CeO2 → Ce2O3 + 0.5O2; (2) hydrolysis, Ce2O3 + H2O → 2CeO2 + H2. The thermal reduction of Ce(IV) to Ce(III) (endothermic step) is performed in a solar reactor featuring a controlled inert atmosphere. The feasibility of this first step has been demonstrated and the operating conditions have been defined (T = 2000 °C, P = 100–200 mbar). The hydrogen generation step (water-splitting with Ce(III) oxide) is studied in a fixed bed reactor and the reaction is complete with a fast kinetic in the studied temperature range 400–600 °C. The recovered Ce(IV) oxide is then recycled in first step. In this process, water is the only material input and heat is the only energy input. The only outputs are hydrogen and oxygen, and these two gases are obtained in different steps avoiding a high temperature energy consuming gas-phase separation. Furthermore, pure hydrogen is produced (it is not contaminated by carbon products like CO, CO2), thus it can be used directly in fuel cells. The results have shown that the cerium oxide two-step thermochemical cycle is a promising process for hydrogen production. more...

Hydrogen, a promising and clean energy carrier, could potentially replace the use of fossil fuels in the transportation sector. Currently, no environmentally attractive, large-scale, low-cost and high-efficiency hydrogen production process is available for commercialization. Solar-driven water-splitting thermochemical cycles may constitute one of the ultimate options for CO2-free production of hydrogen. The method is environmentally friendly since it uses only water and solar energy. First, the potentially attractive thermochemical cycles must be identified based on a set of criteria. To reach this goal, a database that contains 280 referenced cycles was established. Then, the selection and evaluation of the promising cycles was performed in the temperature range of 900–2000 °C, suitable to the use of concentrated solar energy. About 30 cycles selected for further investigations are presented in this paper. The principles and basis for a thermodynamic evaluation of the cycles are also given. more...

This study addresses the single-step thermal decomposition (pyrolysis) of methane without catalysts. The process co-produces hydrogen-rich gas and high-grade carbon black (CB) from concentrated solar energy and methane. It is an unconventional route for potentially cost effective hydrogen production from solar energy without emitting carbon dioxide since solid carbon is sequestered. A high temperature solar chemical reactor has been designed to study the thermal splitting of methane for hydrogen generation. It features a nozzle-type graphite receiver which absorbs the solar power and transfers the heat to the flow of reactant at a temperature that allows dissociation. Theoretical and experimental investigations have been performed to study the performances of the solar reactor. The experimental set-up and effect of operating conditions are described in this paper. In addition, simulation results are presented to interpret the experimental results and to improve the solar reactor concept. The temperature, geometry of the graphite nozzle, gas flow rates, and CH4 mole fraction have a strong effect on the final chemical conversion of methane. Numerical simulations have shown that a simple tubular receiver is not enough efficient to heat the bulk gas in the central zone, thus limiting the chemical conversion. In that case, the reaction takes place only within a thin region located near the hot graphite wall. The maximum CH4 conversion (98%) was obtained with an improved nozzle, which allows a more efficient gas heating due to its higher heat exchange area. more...

The temperature required for carbothermal reduction of silica—in the range 2100–2300 °C—is past the upper limit for combustion process heat. It is therefore an interesting candidate for solar–thermal processing. The production of solar-grade silicon from carbothermally produced silicon requires an energy-intensive long-duree high-temperature purification process. We propose here a two-step solar process for the production of silicon from silica: first, a carbothermal reduction in the presence of nitrogen to yield silicon nitride and second, the solar dissociation of the nitride to yield silicon. This last step could be combined with purification of the silicon if solar-grade silicon is the desired end-product. In this paper, we report on experimental results that indicate that silicon nitride can be dissociated to yield silicon with no detectable nitride content. more...

The purification of upgraded metallurgical silicon by extraction of boron and phosphorus was experimentally demonstrated using concentrated solar radiation in the temperature range 1550–1700 °C. The process operated with a flow of Ar at reduced pressure (0.05 atm) for elimination of P, and with a flow of H 2 O for elimination of B. Impurity content decreased by a factor of 3 after a 50-min solar treatment, yielding Si samples with final average content of 2.1 ppm w B and 3.2 ppm w P. more...

Single-walled carbon nanotubes were synthesized in gram quantities with a 50 kW solar reactor. Vaporization rate was in the range 6–15 g/h and two samples were collected for each run. The structure of the products was studied by SEM, TEM and Raman spectroscopy as a function of the buffer gas (helium or argon), the target length and the collected location. Good quality samples containing 1.2–1.6 nm diameter SWNTs were produced with helium at 450 hPa, 8 Nm 3 /h and a 15 cm target length using 2 at.% Ni and 2 at.% Co as catalyst. The material quality increased with the target length in helium. On the contrary, poor quality product was obtained whatever the target length in argon. TGA analysis showed that the best product purity was at least 35 wt%. more...

A high-temperature solar reactor has been developed for co-producing hydrogen-rich gas and high-grade carbon black (CB) from concentrated solar energy and methane. The approach is based on a single-step thermal decomposition (pyrolysis) of methane without catalysts and without emitting carbon dioxide since solid carbon is sequestered. In the tested reactor, a graphite nozzle absorbs concentrated solar radiation provided by a solar furnace. The heat is then transferred to the reactive flow. The experimental setup, first test results, and effect of operating conditions are described in this paper. The conversion of methane was strongly dependant on the solar furnace power input, on the geometry of the graphite nozzle, on gas flow rates, and on the ratio of inert gas-to-reactive gas. CB was recovered in the carbon trap, and maximum chemical conversion of methane-to-hydrogen and CB was 95%, but typical conversion was in the range 30–90%. more...

Experimental results with a 50kW solar reactor producing 10-15g∕h single-wall carbon nanotube’s rich soot have shown that good quality product was obtained with helium and rather bad product with argon. This result is explained using a computational fluid dynamics model of the reactor accounting for fluid flow, heat transfer, and mass transfer. The bad results in argon were linked to the quenching of carbon vapor in the vaporization zone that resulted in the growth of carbon nanoparticles in spite of carbon-metal clusters (single-wall carbon nanotube precursor). By contrast, the vaporized materials (C, Ni, and Co species) at the target surface were well mixed in helium, and single-wall carbon nanotubes (SWNTs) might grow in the annealing and cooling zone at the backside of the reactor. The same numerical approach may be used to design modifications of the reactor in order to favor the growth of SWNTs. more...

International audience; Different families of carbon nanostructures produced by a continuous plasma process are presented. Due to the flexibility of this original technology, properties of classical carbon black products can be adjusted more freely during synthesis and an even wider range of parameters is accessible. Novel products with distinctive characteristics are observed when treating nanostructured material in the high-temperature plasma. Evaluation of the application properties of selected materials indicates in certain cases an excellent performance. Plasma-synthesised or plasma-treated carbon nanomaterials have an important potential to improve the performance of carbon-black-containing materials. more...

A three-dimensional model is developed to predict the bed-to-wall radiative heat transfer coefficient in the upper dilute zone of circulating fluidized bed (CFB) combustors. The radiative transfer equation is solved by the discrete ordinates method and Mie scattering theory is applied to calculate the absorption and scattering efficiency factors of particles existing in CFB combustors. Empirical correlations calculate both spacial variation of solid volume fraction and temperature distribution at the wall. The model considers the influences of the particle properties (including particle size distribution, particle optical constants and solid composition) on the radiative heat transfer coefficient. Simulation results show that the particle properties have significant influences on the bed-to-wall radiative heat transfer coefficient in CFB combustors. A very good agreement of predicted results is shown with experimental data. more...

The total extraction of sulfur by vaporization from iron sulfide ores leads to a residue that is constituted in great part of iron. The aim of this article is to follow up the secondary metals composition in the residue during the sulfur extraction. When the pyrrhotite is treated alone, copper is completely evaporated after Cu2S decomposition. While in the presence of carbon, iron is present as the core form in the iron sulfide. In this case, the kinetic of vaporization is fast and no secondary metals trace is detected in the iron core. more...

Heavy metal (HM) vaporization dynamics in a circulating fluidized bed (CFB) riser was numerically analysed based on a Lagrangian approach. Simulation was performed by combining an HM vaporization model and a Lagrangian model developed for CFB, coupled by including the HM transport equation in gas flow. CdCl2/Al2O3 was chosen as typical HM/mineral matrix system in the model. The simulation results address the HM vaporization dynamics of a particle, the heterogenous particle flow structures, and the HM distribution in the CFB riser. The particle heterogeneous distribution affects significantly the HM concentration distribution in the riser. more...

Since solid exhibits a wide particle size distribution (PSD) in a circulating fluidized bed (CFB) boiler, it is difficult to predict the hydrodynamic behavior. Therefore, a model based on the semi-empirical approach is developed to approximate the local particle size distribution in CFB. The core/annulus flow structure is applied to this model and the particles in the bed are discretized into several size groups. The model accounts for the disintegration and shrinking of coal particles during the combustion process of each group of particles. It shows that coarser particles are gathered near the walls and the average particle diameter decreases along the boiler height, and this trend is more significant in the splash region. Moreover, coal combustion behavior affects both axial and radial segregations in a CFB boiler. All these phenomena are discussed against experimental data reported in literature. more...

Selenium (Se) is one of the most volatile trace metals emitted from coal-fired combustors and will most probably be considered for regulation in the electric power industry due to its significant environmental impact. Novel technologies are needed to remove Se vapor. This requires first a clear understanding of the behavior of selenium in combustion. In this work, coal and coke samples spiked with Se were prepared and the effect of Se on combustion was evaluated in a thermobalance. Impregnation with Se had a negligible impact on the combustion of coke; this allowed a systematic experimental study of the vaporization, condensation, and association of Se through burning cokes spiked with Se. Capturing the aerosol in the combustion flue gas on a membrane filter, coupled with identification of the species present using X-ray photoelectron spectroscopy, was established as a useful approach for coal combustion. The high volatility of Se and its affinity for oxygen, as well as the effect of chlorine on its vaporization, supported our previous calculations. The dominant Se species were identified as atomic Se and SeO 2 , which also confirmed the thermodynamic predictions. more...

The turbulent gas–particle flow in a bubbling fluidized bed is numerically studied using large eddy simulation and discrete particle method (LES-DPM). The gas-phase model is based on locally averaged two-dimensional Navier–Stokes equations for two-phase flow with fluid turbulence calculated by LES, in which the particle effect on subgrid-scale (SGS) gas flow is taken into account. The particle motion is treated by a DPM, in which the particles are assumed to interact through binary, instantaneous and non-elastic collisions. The gas–particle turbulent flow structures are predicted. The distributions of the simulated particle anisotropic velocity show that the particles in the fluidized bed do not have any local equilibrium. The vertical components of both the gas and particle turbulent intensities are higher near the walls than in the centre, whereas these horizontal components are always low. The prediction results indicate that the profiles of the mean gas and particle velocities and of the turbulent intensities are rather insensitive to C k value, whereas the instantaneous particle structure and individual trajectories are sensitive to it in this turbulent model. more...

This paper aims at the optimum design of a solar reactor for carbon product processing by vaporization. The chosen reference case is the CNRS 1 MW solar furnace and graphite vaporization for fullerene synthesis. The method, which accounts for heat transfer and chemical reaction kinetics, is based on the combination of both experimental data obtained at laboratory scale and numerical simulation. For this very high temperature process (carbon vaporization is only significant at temperatures higher than 3200 K), the optimum diameter of the target is 22 cm and the soot production ranges between 80 and 150 g/h for an effective power of 325 kW ( P =240 h Pa). The model is validated by experiments using a 6 cm o.d. graphite target. more...

International audience; Thermal plasma processes show certain advantages over conventional gas phase synthesis process for the production of carbon nanomaterials. Due to the extreme temperature conditions and the unique flexibility, very different structures can be produced at high selectivity. An overview is given of the three principal families of nanostructures, carbon blacks, fullerenes and nanotubes, produced so far by the original three-phase AC plasma technology presented. Various examples of individual structural types are used to illustrate and explain the multitude of different particle formation processes adjustable in plasma systems. Numerical simulation of arc region, kinetics and fluid dynamics appears as a central tool for the control of the three main process parameters: temperature profile, particle residence time and species concentrations. more...

Fullerene mixture of C 60 and C 70 was produced in gram quantities in a solar reactor using partially (50 kW) the power of the 1 MW CNRS solar furnace at Odeillo. Fullerene yield was studied as a function of buffer gas (helium and argon), pressure (in the range 80–500 hPa) and gas flow rate. Mean Fullerene yield of 13.5% was measured with helium at 450 hPa and 10 sm 3 /h, at a carbon vaporization rate of 21 g/h. This mean value refers to three samples collected inside the experimental set up; the maximum yield for one single sample (inside the reactor) was 16%. The dramatic effect of pressure and gas flow rate on the process selectivity is correlated to the dilution number (number of carbon atoms versus number of buffer gas atoms) and to the results of a numerical simulation related to the temperature distribution in the annealing zone of the reactor. more...

Gas-dynamical expansion and radiation transfer of Al vapour breakdown plasma induced by nanosecond laser action with an intensity of 109?2 ? 1010?W?cm?2 at the wavelengths of 1.06, 0.512, and 0.248??m are modelled. Plasma evolution is described in the approximation of non-stationary radiative gas dynamics in the two-dimensional axially symmetrical formulation. Radiation transfer is found to exert a considerable effect on the evolution of the laser plasma. As the intensity increases, the radiative energy losses also increase, reaching 60% at 2 ? 1010?W?cm?2, and cause the plasma temperature to be reduced proportionally. The energy escapes mainly through the side and, to a smaller extent, the frontal boundaries, the amount of energy escaping in the direction of the target being negligible. The dependence of plasma processes on the laser wavelength is due to specific features of the absorption mechanisms and the photo-absorption contribution under the action of visible and ultraviolet ranges of radiation. The spectral composition of the escaping radiation differs considerably from that of the equilibrium spectrum and is typical of plasma with a variable optical density. It is possible to take into account the influence of radiation on the plasma characteristics by solving the equation of radiation transfer in the multi-group approximation with several tens of spectral intervals. If the maximum available number of groups is used for selected spectrum intervals, the computational results can be compared to the experimental data of plasma emission. more...

The distinct element method (DEM) is used to model combustion of coal particles in a bubbling fluidized bed. The gas phase is modeled as a continuum and the particle phase is modeled by DEM. The chemical reactions consist of the heterogeneous reactions of char with O 2 , CO, CO 2 , NO and N 2 O and in the homogeneous reactions involving CO, O 2 , NO and N 2 O. The colliding particle-particle heat transfer is based on the analysis of the elastic deformation of the spheres during their contact. The model predicts the particle heterogeneous flow structure, the thermal characteristics of burning coal particles, and the gaseous emissions from a fluidized binary mixture. Results show that the instantaneous contribution of the collision heat transfer ranges from 0 to about 1.0% of the total heat transfer during the coal combustion. The mean excess of coal temperature is approximately 140K, and the maximum excess of coal particle temperature is approximately 270 K after 2 s. more...

This study addresses the emission of heavy metals during the incineration of municipal solid waste. A global method was developed to determine the vaporization rate of the metal from the on-line analysis of exhaust gas. This method differs from direct models, which predict the time course of the metal concentration in the gas from the vaporization rate profile, but which are not practicable because this vaporization rate cannot be measured in real incinerators burning real wastes. The method is based on the determination of the global rate of release of heavy metal from the combustion of model wastes in a fluidized bed. It is an inverse method, which involves only the measured concentration of heavy metal in the exhaust gases and a model developed at the reactor scale. The thermal treatment of model wastes spiked with a metal was performed in a laboratory- scale fluidized bed. In fact, a solid matrix derived from real waste was dosed with Cd, Pb, or Zn and burned to simulate the metal’s release during the incineration of municipal solid waste. An on-line analysis system was linked to the gas outlet of the reactor, and the metal’s vaporization was tracked successfully by continuously measuring by inductively coupled plasma optical emission spectroscopy (ICP-OES) the relative concentration of the metal in exhaust gases. On the theoretical front, a bubbling bed model was developed and validated to calculate the metal’s vaporization rate from its concentration-time profile in the outlet gas. The inverse method consists in identifying these vaporization rates at the particle level from only the on-line diagnostic results and using the model, whatever the waste considered. The data obtained may be used in any process, in which wastes are heated rapidly (several hundreds of degrees per second), as in fluidized beds. more...