high temperature materials and solar fuels - promes · 40: axis 1 materials and extreme conditions...

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AXIS 1 : MATERIALS AND EXTREME CONDITIONS | High Temperature Materials and Solar Fuels

High Temperature Materials and Solar Fuels

Identity Composition of the team (or participants) Team leader: M. Balat-Pichelin (DR) Permanent personnel: S. Abanades (DR), L. Charpentier (CR) Not permanent personnel: Post Doc: A. Demont (oct. 2013 - dec. 2014, EADS), J. Puig (since dec. 2014, Labex SOLSTICE 1st year, Labex SOLSTICE+PSA 2nd year), M. Nair (june 2015 – may 2016, IHI) PhD students: (1) achieved Ph’D: J. Iacono (2013/12/12, CNES+CNRS), Y. Prévereaud (2014/06/24, MESR, price Aerospace Valley 2015, Midi-Pyrénées), G. Lévêque (2014/10/16, MESR), E. Brodu (2014/10/27, CNES+SAO Harvard USA), M. Sanchez-Brusset (2015/06/17, CEA+EDF); (2) Ph’D in progress: Q. Bellouard (since 2014/10, ADEME+CEA), L. André (since 2014/11, FP7_STAGE-STE), L. Barka (since 2015/10, Labex SOLSTICE+CNES), C. Piriou (since 2015/10, Limousin Area+SPCTS Limoges+PROMES) Z. Zhang, invited Prof. (2015/04 – 2016/03) Keywords Ceramics, High temperature, Thermochemistry, Hydrogen, CO2 valorization Topics Chemical reactivity and thermal radiative properties of materials (ceramic and metal) at high temperature Solar metallurgy: synthesis and recycling of materials (nano-structured for propulsion; rare earths and Nb, Ta, Mg...) Synthesis of hydrogen by solar thermochemical cycles and from hydrocarbon resources (cracking, reforming, gasification) and CO2 valorization for the production of synthetic fuels Collaborations National - P. Boubert (CORIA, Rouen), J.L. Vérant (ONERA, Toulouse), H. Combes, J. Annaloro et P. Omaly (CNES, Toulouse),

A. Maître, S. Foucaud et R. Lucas (SPCTS, Limoges), T. Cutard et G. Bernhart (ICA, EMAC, Albi), D. De Sousa Meneses (CEMTHI, Orléans), P. Piluso (CEA Cadarache), A. Michaud-Soret (Herakles, Le Haillan), M. Le Flem et C. Sauder (CEA Saclay), M. Pons (SIMAP, Grenoble), A. Julbe et M. Drobek (IEM, Montpellier), J.C. Jumas et J. Olivier Fourcade (ICG, Montpellier), S. Rodat et N. Dupassieux (CEA-INES, Le Bourget du Lac).

International - J.C. Kasper et M. Freeman (SAO Harvard, USA), O. Chazot et T. Magin (VKI, Rhode St Genese, Belgique), D. Sciti et

L. Silvestroni (ISTEC-CNR, Faenza, Italie), D. Alfano (CIRA, Caserta, Italie), M. Rutigliano (IMIP-CNR, Bari, Italie), A. Vesel et M. Mozetic (JSI, Ljubljana, Slovénie), K. Nickel (Univ. Tuebingen, Allemagne), H. Kimura et H. Otsuka (IHI Corporation, Japon), H.I. Villafàn-Vidales (UNAM/IER, Mexique), N. Calvet (Masdar Inst. Sci. Technol., Abu Dhabi, Emirats Arabes Unis), Z. Zhang (Univ. Technol., Hangzhou, Chine)

Contracts - ESA n°4000103145/NL/CP (CNRS n°066016), Passive/active oxidation of CMC structural materials, coordinator - Amendment to ESA n°4000103145/NL/CP, CCN 1 (CNRS n°093327), Passive/active oxidation of CMC structural

materials, coordinator - ESA via AAC n°4000105487/12/NL/AF-PROMES (CNRS n°080395), Near-sun mission heat shield material

assessment NESSMA, partner - CNES, R&T for 3 years (CNRS n°081628), Simulation of materials behavior for the Solar Probe Plus mission – heat

shield materials and materials for the instrumentation SWEAP and FIELDS, coordinator - CNES (CNRS n°096503), Oxidation and measurement of the total hemispherical emissivity at high temperature of

metallic materials from satellites, coordinator - CNES R&T for 3 years (CNRS n°111989), Oxidation study and measurement of thermo-radiative properties at high

temperature of metals and alloys in vacuum and air plasma conditions (Programme Design for Demise D4D), coordinator

- Herakles-Safran (CNRS n°122586), Measurement of the first decomposition gases from elastomer (thermal protection), coordinator

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- CNES (CNRS n°128069), Measurement of thermo-radiative properties of Elgiloy and abraded Niobium for the Solar Orbiter mission (antennas RPW), coordinator

- NEEDS (CNRS n°124791), Oxidation of SiC-based composite materials with metallic liner in accidental conditions for the GFR, coordinator

- EADS Foundation (CNRS n°084210), Renewable aviation fuel production from H2O and CO2 using concentrated solar thermal energy, coordinator

- IHI Corporation (Japan), (CNRS n° 095753 et CNRS n°109421), Development of a solar reactor for thermo-catalytic decomposition of natural gas, coordinator

- Cellule Energie CNRS (exploratory project 2012), "Sn-O Mossbauer" project: Characterization of the SnO2/SnO system for the syngas production from CO2 and H2O using a solar thermochemical cycle, coordinator

- MI-CNRS, ENRS challenge (Exploratory projects 2014: "CO2 Emergence"), Project VALTHER-CO2, Thermochemical valorization of CO2 by solar route from new perovskite structures, coordinator

- References 1, 16, 30, 32, 35, 43, 49, 50, 61, 63, 64, 76, 77, 78, 82, 86, 87, 88, 93, 100, 101, 102, 103, 104, 110, 122, 131, 134, 143, 147, 152, 153, 160, 167, 168, 169, 172, 173, 182, 183, 184, 198, 207, 209, 226, 263, 269. Large equipment Leica DCM 3D profilometer, purchased in march 2013, contract funding by M. Balat-Pichelin, cost 117 k€

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Scientific report INTRODUCTION

The team "High Temperature Materials and Solar Fuels" develops research under the three themes defined above. All these themes is mainly using the solar furnaces of Odeillo to simulate extreme conditions of temperature, in controlled atmosphere environments encountered during the use of some materials in fields such as space and energy. The researches thus allow to qualify these materials and, in particular, their surface properties and their surface interactions in highly reactive environments such as non-equilibrium plasmas or highly ionized media. The target applications concern the materials for the cladding of future nuclear fission reactors (Generation IV), those of future solar receivers for concentrated solar power plants (tower) and the protective materials (heat shields) of solar probes (Solar Probe Plus, Phoibos) and of reusable space vehicles (IXV) or also for the study of space debris within the LOS (Space Operations Act). These researches require the development of original test facilities and efficient methods of temperature measurement (pyro-reflectometry) and surface properties of the materials studied. Regarding the production of synthetic fuels without emitting greenhouse gases, the main objective of the work concerns the development of solar thermochemical processes at high temperature using different ways. The goal is to use concentrated solar energy as a high temperature energy source in thermochemical transformations. The solar fuels referred are hydrogen, and synthetic gas (CO and H2 as main components), key precursors for the synthesis of various derivative fuels (methanol, DME, synthetic or other liquid fuels). The processes developed are based on the use of hydrocarbon resources (fossil or biomass) for production of hydrogen or synthetic gas, and on the decomposition of H2O and CO2 by means of redox cycles in two stages. The thermochemical energy storage using similar systems is also a research axis in progress. The new theme relative to solar metallurgy and recycling of materials presented at the previous laboratory evaluation has begun. Promising results were obtained for the production of metal nanopowders from the carbo-reduction of MgO and Al2O3. All of the research topics of the team MHTCS is oriented towards the development and the optimization of materials for their use properties, in very severe conditions, their characterization, modeling and prediction at different scales (material, reactor, process). Technological breakthroughs concern the control of in situ measurements in high temperature reactors and with coupled constraints, the reactivity control in harsh environments, the evaluation of the materials life (space probes, solar receivers...) and the optimization of processes for the large scale production of high-value energy carriers and materials. Finally, a highlight on the works of our team is related to the atmospheric reentry demonstration flight of the first european spacecraft IXV (Intermediate eXperimental Vehicle) which was held on february 11, 2015. Indeed, as part of a contract with the European Space Agency (ESA) and the two industrial Herakles (France) and MT Aerospace (Germany), we have demonstrated the good behavior and the possible reusability of the thermal protection system (nose and flaps). The studies were carried out on the MESOX facility (device for solar oxidation) placed at the focus of the 6 kW Odeillo solar furnace of and on the Plasmatron facility in collaboration with the Von Karman Institute in Rhode St Genèse, Belgium. These studies led to the determination of the transition between passive and active oxidation regimes for the ßSiC-C/SiC material, the determination of a maximum loss of mass rate in active oxidation conditions and the evaluation of its catalycity in conditions of pressure, temperature and ‘plasma' simulating the atmospheric re-entry phase.

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Summary

1. Chemical reactivity and thermal radiative properties of materials (ceramic and metal) at high temperature

1.1. In extreme environment (aerospace, atmospheric re-entry)

1.1.1. Chemical reactivity in air plasma - Application to the study of space debris

1.1.2. Textured materials for the SWEAP instrumentation and the heat shield of the Solar Probe Plus (SPP) mission

1.2. In standard atmosphere (solar energy, CSP, nuclear)

1.2.1 Materials for concentrated solar power

1.2.2. Cladding materials for the 4th generation of nuclear system (NEEDS)

1.2.3. Ageing protocol for ceramics materials

2. Solar metallurgy: synthesis and recycling of nano-structured materials for propulsion

3. Production of synthetic fuels by solar thermochemistry at high temperature

3.1. Production of hydrogen and syngas from hydrocarbons

3.2. Solar fuels from thermochemical H2O/CO2-splitting cycles

3.3. Thermochemical energy storage via reversible solid/gas reactions

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1. CHEMICAL REACTIVITY AND THERMAL RADIATIVE PROPERTIES OF MATERIALS (CERAMIC AND METAL) AT HIGH TEMPERATURE

1.1. In extreme environment (aerospace, atmospheric re-entry) 1.1.1. Chemical reactivity in air plasma - Application to the study of space debris The application of the LOS (Space Operations Act) requires from 2021 that all the satellites launched by France have no more fuel at the end of the mission (passivation) and they will be destroyed on re-entering the Earth's atmosphere to clear the Low Earh Orbit LEO currently crowded of debris that can cause collisions with active satellites and the international space station. For this, in collaboration with CNES, a study is conducted on the most difficult materials to destroy than are metallic alloys such as TA6V, Inconel 718 and 316L (PhD L. Barka). For the certification of the satellite launch, the DEBRISK code developed by CNES is used. This code requires high temperature input data and measured when possible in air plasma conditions such as the heat of oxidation, the thermophysical properties and in particular the total hemispherical emissivity. The first experiments were carried out on Inconel 718 for which it has been shown that around 1600 K, under air plasma, the mass gain versus time follows a parabolic law of the type, with ∆m/S expressed in mg/cm2 and t in s: - at 300 Pa ∆m/S = 0.6312 t1/2 with a correlation coefficient of 0.90823 - at 2000 Pa ∆m/S = 0.6508 t1/2 with a correlation coefficient of 0.94846. The temperature of 1600 K was chosen because it is close to the melting temperature and, according to the literature, under standard air, the weight gain is important for this temperature. The duration of the tests was in between 70 and 480 s. Since the oxides formed on Inconel 718 (Figure 1) are adherent and regardless of the temperature or the oxidation duration, the parabolic kinetics is confirmed by the presence of a passivating oxide layer. As a first approximation, in a model as DEBRISK, only one law can be used between 300 and 2000 Pa, for a temperature of 1600 K close to the melting temperature, of the type ∆m/S = 0.64 t1/2.

ref. 1320 K 1574 K 1643 K 1718 K

Figure 1 : Pictures of the Inconel 718 samples before and after oxidation in air plasma conditions XRD characterization performed on different samples indicates the presence of mixed oxides on Inconel 718 such as for example NiNb2O6, FeNbO4, CrNbO4 and Fe2O3. It may be noted that for the experiments conducted at about 1600 K and at 300 Pa of air, a change in the composition of the oxide layer occurs over time. For the shortest time (70 s), the mixed oxides such as NiNb2O6, FeNbO4 are visible and when increasing time up to 455 s, the hematite Fe2O3 content increases more than the one of mixed oxides. These results will be complemented by XPS analysis of extreme surface. The mass change as a function of temperature was also studied for a constant duration of oxidation of 300 s at 300 and 2000 Pa air plasma. It is found that the mass gain as a function of inverse temperature follows an Arrhenius law. Both laws determined for the evolution of the mass in the temperature range 1080-1720 K for each total air pressure are: - at 300 Pa Δm/S.t’’ = 0,7203 exp (- 4795/T) with a correlation coefficient of 0.6094 - at 2000 Pa Δm/S.t’’ = 0.9139 exp (- 4436/T) with a correlation coefficient of 0.9076.

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The data discrepancy for the pressure of 300 Pa and for the lowest temperatures leads to a poor correlation coefficient. The activation energy of the oxidation process can be calculated from the Arrhenius law and the values obtained are very close and equal respectively to 39.8 and 36.9 kJ/mol at 300 and 2000 Pa. This confirms the low influence of pressure on the oxidation process between 300 and 2000 Pa. Measurements of the total (0.6-40 µm) directional (0-80°) emissivity and the calculation of total hemispherical emissivity - obtained by integration of the directional values - were performed on Inconel 718 in high vacuum (10-4 Pa) in the MEDIASE reactor. Then measurements were made on samples previously oxidized in air plasma in atmospheric reentry conditions (MESOX) and on samples oxidized in situ for low total air pressure in MEDIASE. Figure 2 presents these results that can be compared to values obtained in high vacuum for non-oxidized Inconel 718 and for which the total hemispherical emissivity is constant and equal to 0.20 over the temperature range from 1050 to 1450 K. Taking into account the emissivity of oxidized surfaces - 4 times higher - causes strong changes in DEBRISK calculations.

(a)

(b) Figure 2 : Total emissivity (a) normal for pre-oxidized Inconel 718 samples in air plasma conditions and (b) hemispherical for samples oxidized in situ in standard air (the red arrow corresponds to the temperature increase and the blue one to the decrease after the high temperature oxidation) versus temperature 1.1.2. Textured materials for the SWEAP instrumentation and the heat shield of the Solar Probe Plus (SPP) mission The Solar Probe Plus mission (NASA, launched in July 2018) will approach the solar corona at about 6.6 million km (9.5 solar radii) from the surface of the sun in a region no other spacecraft has never met, to understand the heating of the solar corona and the solar wind acceleration. A thermal protective shield protects the payload from the thermal radiation and the solar wind (protons). However, some metal parts of the on-board instrumentation will directly face the sun to perform in-situ measurements in the solar corona: the Faraday cup SWEAP (Solar Wind Electrons, Alphas and Protons Investigation, SAO, Harvard University, USA) and antennas FIELDS (Electromagnetic Fields Investigation, SSL from the University of Berkeley, USA). Among the important materials properties for these two instruments exposed to the sun radiation, the thermo-radiative properties are critical because they influence the thermal equilibrium (Ph’D E. Brodu). Two solutions were chosen to optimize the thermal design of the Faraday cup of the SWEAP instrumentation for lowering its equilibrium temperature: 1- using textured high-emissivity coatings on the metal parts of the cup non-directly exposed to the solar flux and 2- covering the bottom of the cup, directly exposed to the sun with metal having a low / ratio: solar absorptivity/ total hemispherical emissivity. The high emissivity coatings studied are textured refractory metals: W of W, Mo and Re on TZM alloy. These coatings are heated up to 1900 K in high vacuum (10-5 Pa) using concentrated solar energy in the reactor MEDIASE placed at the focus of the 1 MW solar furnace. The emissivity in the different spectral ranges is obtained by using filters, as for example from 0.6 to 2.8 µm approaching the solar absorptivity. The total emissivity (0.6 to 40 µm) directional (measured) and hemispherical (obtained by integrating directional data) of the samples were obtained in the temperature range from 1100 to 1900 K. This study has focused on the evolution of the thermo-radiative properties and the microstructure of the coatings as a function of temperature and time. The results obtained for two of the high-emissivity coatings are shown in Figure 3 and were compared to those of non-textured samples (W, Re, Mo, W-25Re) obtained previously.

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(a)

(b)

Figure 3 a et b. Total hemispherical emissivity and SEM images after emissivity measurement of (a) textured Re coating deposited on TZM alloy and (b) textured W on W as a function of temperature and time. The scale bar for SEM images is 10 µm for Re/TZM and 100 µm for W/W. For the two solar missions planned (SPP, NASA and PHOIBOS, ESA), a thermal protection system (TPS) will protect the payload from the thermal radiation and the solar wind (protons). For this purpose, a film (130 µm) of pyrolytic boron nitride (pBN) deposited by CVD on C/C composites provided by Herakles (France) has been studied as a potential candidate. The material used as the outer layer of such TPS must have the following three characteristics: (1) a low ratio of solar absorptivity

to total hemispherical emissivity to maintain the equilibrium temperature as low as possible for a given distance from the Sun; (2) it must also be able to simultaneously withstand the high temperatures and the two characteristics of the solar environment: ion bombardment of the solar wind and VUV (Vacuum Ultra Violet) radiation and (3) the material must have a low mass loss rate to avoid the pollution of the measurement performed by the scientific on-board instrumentation. To simulate the conditions experienced by the materials near the Sun, the MEDIASE reactor, developed in collaboration with CNES and installed at the focus of the 1 MW solar furnace of Odeillo, was used. Samples can be heated up to 2500 K using concentrated solar energy, in high vacuum (10-5 Pa), and with or without the addition of proton bombardment and VUV radiation. This reactor is also instrumented to perform several in situ measurements: pressure, temperature via a pyro-reflectometer developed at PROMES, mass loss rate via QCM, qualitative species ejected with open source mass spectrometer (neutral and ions), and the measurement of total and directional spectral emissivity, or within narrow ranges via a radiometer. Figure 4 shows the advantage of the pBN coating against a shield in bare C/C composite in the face of the species emitted during processing at 2100 K with or without ions, and the evolution of the normal spectral emissivity. It is clear that in the part of the solar spectrum beyond 5000 cm-1 (collaboration with CEMTHI Orleans), the massive pBN (2 mm thick) presents a very low emissivity and this is partly retained for the 130 µm coating (red curve) compared to the bare C/C composite (black curve). After the high temperature treatment (2300 K) with or without ions, the surface morphology was not modified, nor in the thickness (Figure 5), the pBN layer being perfectly adherent to the C/C composite thus showing the interest of the pyrolytic boron nitride as TPS material.

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(a)

(b) Figure 4. (a) Mass spectra of the emitted species for the bare C/C composite, coated with the pBN and heat treated at 2100 K with or without ions and (b) comparison of the normal spectral emissivity of a massive pBN sample (2 mm thickness), of the bare C/C composite and of the assemblage pBN+C/C around1300 K, and correlation with the directional-hemispherical reflectivity measured on a massive pBN sample (0.6 mm thickness) at room temperature by Manara et al. (2002)

(a)

(b) Figure 5 : SEM images of (a) the surface and (b) the cross-section of the pBN (130 µm thickness) coating on the C/C composite after treatment at 2300 K 1.2. In standard atmosphere (solar energy, CSP, nuclear) 1.2.1. Materials for concentrated solar power Ultra High Temperature Ceramics materials (UHTC) such as carbides (ZrC, HfC…) owing excellent mechanical properties at high temperature could be candidates for applications as receivers of concentrated solar power plants (tower). The ISTEC-CNR laboratory from Faenza, Italy elaborates grades of HfC and ZrC materials with additives like MoSi2, TaSi2 or ZrSi2. Grades HfC/10% vol. ZrSi2 (HCZ) and HfC/10% vol. TaSi2 (HCT) have been treated in air at atmospheric pressure using the REHPTS reactor (High pressure and Solar Temperature Reactor) during 20 min. at temperatures of 1800, 2000, 2200 and 2500 K. On another side, the SPCTS laboratory from Limoges has developed a process to coat ZrC powders with SiC from polymeric precursors. These powders were sintered using SPS technique to obtain ZrC/SiC grades with a SiC content from 10 to 30 %wt. These samples have been oxidized using the REHPTS reactor in air during 20 min. at temperatures from 1400 to 2400

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Figure 6 shows images after the oxidation of the samples HCZ, HCT and ZrC/SiC. We observe a slight degradation of the materials HCZ and HCT while ZrC/SiC material supports an important damage due to the peeling off of the formed oxide layer. SEM analyses of the oxide layers have shown that that is due to a difference in the composition of the produced oxides. HCT can oxidize into the mixed compound Hf6Ta2O17, whereas mixed phases (Hf, Zr)O2 can be found in the oxide layers formed at the surface of HCZ. We cannot find mixed phases in the oxide layers formed at the surface of the ZrC/SiC: the layer involves grains of zirconia surrounded with silica, the layer is therefore more fragile and less prevents the diffusion of oxidizing species.

HCT

HCZ

ZrC/20%SiC

Figure 6 : Pictures of three samples exposed in air during 20 min. at 2200 K. For the ZrC/SiC sample, the oxide layer was completely peeled off from the substrate during the cooling phase.

1.2.2. Cladding materials for the 4th generation of nuclear system (NEEDS) This investigation, supported by the transdisciplinary mission NEEDS, in collaboration with Herakles (France) and CEA, Saclay (France) aimed at studying the behavior of the sandwich assemblage SiC/SiC composite + metallic liner (Nb or Ta), this association being envisaged as cladding material for the nuclear fuel in the 4th generation nuclear system using gaseous coolant (helium). The investigation of the behavior of the metallic material, treated during 10 min. in helium flow (with the residual presence of oxygen impurity, pO2 = 2 Pa) evidenced the recrystallization of tantalum according to a preferential direction (211) and the amorphization of niobium above 1550 K (figure 7a). The experimentations carried out on the superposition of SiC/SiC+Ta or Nb evidenced the carburation of the metallic liner (figure 7b). If these materials are weakly damaged in nominal operating conditions (temperature below 1200 K), they can be fragilized by the active oxidation of the composite and the carburation of the metallic liner in case of an accident with temperature increase above 2000 K.

(a)

(b) Figure 7 :XRD patterns of (a) reference Nb and after 10 min. oxidation at various temperatures in helium flow and (b) the metallic liner of the sandwich Ta/SiC/SiC oxidized during 10 min. at 2200 K in helium flow 1.2.3. Ageing protocol for ceramics materials In the frame of the European collaborative program STAGE-STE, an ageing protocol for the wall materials of the solar thermochemical reactors has to be developed, the REHPTS reactor being used for this study. Damaging tests have been performed on AlN elaborated using HT-CVD (High Temperature Chemical Vapor Deposition) at SIMAP laboratory (Grenoble, France). Experiments of cyclic oxidation (20 min. heating at 1600 K, followed by 5 min. cooling)

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have been carried out, validating the development of an ageing procedure (figure 8). This procedure will be used on AlN and SiC/SiC materials, and compared with other ageing procedures developed in parallel at CIEMAT (Spain).

Figure 8 : Cycles thermiques effectués sur AlN dans le cadre du développement d’une procédure de vieillissement accéléré

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2. SOLAR METALLURGY: SYNTHESIS AND RECYCLING OF NANO-STRUCTURED MATERIALS FOR PROPULSION

In the transportation sector, there are several alternatives to the use of fossil fuels: Li-ion batteries, bio-fuels, H2 ... However, these new fuels have, for some of them, resource limitations (Li, bio-fuels), social acceptability (bio-fuels, H2) or are still under consideration (H2 production/storage...). The combustion of metallic powders finely divided is a new energy process envisaged for the propulsion of future vehicles (long-term). Terrestrial reserves of the most abundant metals such as Si, Al, Mg or Fe are important and the unique products of their combustion are metal oxides. Metals regenerated from their oxides with a good efficiency can enable sustainable use of these resources. In this work, the concentrated solar energy route is employed to properly reduce metal oxides formed after a combustion process. Possible metal fuels in the transportation sector include Si, Al and Mg thanks to their high energetic densities. Thermodynamic calculation using the software GEMINI (Thermodata) has shown that the reduction of MgO and Al2O3 in the presence of carbon (reducing agent) is achieved at 1800 K at 100 and 0.1 mbar respectively. SiO2 reduction is achievable above 2200 K under 0.1 mbar, which makes it difficult to recycle from an industrial point of view (too large energy and thermal stresses). The first experiments of the carbothermal reduction of MgO and Al2O3 were performed between 10 and 16 mbar. During these experiments, MgO:C or Al2O3:C pellets (9 mm in diameter and 2-3 mm thick) were positioned close to the focus of a 2 kW solar furnace on a sample-holder in a batch reactor under controlled atmosphere. Condensates from the carbothermal reduction were recovered on a water-cooled metal finger and on a microporous ceramic filter (Figure 9a). Optimization of the reaction parameters in the presence of argon was conducted with different stoichiometries of reagents, various grain sizes and some varieties of carbon. The influence of a pre-grinding, experiment duration, solar flux,... were analyzed. Magnesium and aluminum contents from 60 to 80%wt. were obtained by XRD semi-quantitative analysis in the collected powders. Magnesium and aluminum global conversion efficiencies near 50% were obtained. Microscopic observations (SEM) have shown that the powders formed are composed of agglomerates of nanoscale grains (Figure 9b), which appears to be an advantage for their future combustion. The compounds identified in the collected powder after the experiments are Mg, MgO (after a carbothermal reduction of MgO; example in Fig. 9c), Al, Al2O3, Al2OC and Al4C3 (after a carbothermal reduction of Al2O3).

Figure 9 (a) “Heliotron” reactor during an experiment: the sample is placed just below the focus of the solar concentrator, the green and black squares are respectively located on the ceramic filter and on the water-cooled finger areas (b) SEM micrograph of the powder collected on the filter and (c) XRD patterns of the powders collected on the two deposit areas after a carbothermal reduction of MgO A new reactor named Sol@rmet was recently designed for optimizing the carbothermal reduction process. The reaction temperature will be measured using an optical pyrometer and an analysis of the outlet gases (CO and CO2) from the reactor will allow a better control/monitoring of the reaction. The formation mechanisms of the by-products (carbides, oxides) will be better described and the reaction kinetics will be optimized using these new equipments that should allow the metal yield increase.

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3. PRODUCTION OF SYNTHETIC FUELS BY SOLAR THERMOCHEMISTRY AT HIGH TEMPERATURE

The considered processes are based on the one hand on the use of hydrocarbon resources (fossil or biomass) for the production of hydrogen or synthetic gas. The works from 2013-2015 carried out on these processes are focused on the thermo-catalytic methane decomposition and biomass gasification. These hybrid solar/hydrocarbon paths offer short/mid term development prospects. On the other hand, the production of H2 and CO from H2O and CO2 splitting is also studied by considering thermochemical cycles based on oxygen-exchange redox materials. The CO2 conversion using solar energy is a novel method for the valorization and the recycling of CO2. Finally, the thermochemical energy storage via reversible solid/gas redox reactions is also a research field currently developed because it involves systems closely similar to thermochemical cycles. Thermochemical storage exploits the heat effects of reversible endo/exothermic chemical reactions at high temperature (in the range 500-1200°C). The stored energy is then transferred to the heat transfer fluid for continuous electricity generation via the thermodynamic cycle. For example, the utilization of metal oxide redox pairs or mixed oxides is investigated using air as both the heat transfer fluid and the reactant in open loop. The research conducted in these different fields are related to the synthesis and characterization of novel active materials, the investigation of the reactivity of the solid/gas systems, and the development of appropriate solar receivers/reactors (design, experimental testing, modelling). 3.1. Production of hydrogen and syngas from hydrocarbons The considered routes for H2 synthesis from hydrocarbons are natural gas cracking, reforming and pyrolysis/gasification. The thermal treatment of hydrocarbons or biomass consists in substituting fossil fuel combustion (heat supply for endothermic reactions) by solar concentrated energy. The thermal dissociation of methane co-produces hydrogen and carbon black as a high value nanomaterial without CO2 emission (CH4 Æ Csolid + 2 H2, ∆H° = 75 kJ.mol-1). The thermo-catalytic methane decomposition has been studied in a joint research project with IHI Corporation (Japan). The objective is the decrease of the reaction temperature in the range of 900-1200°C by using carbonaceous catalyst (e.g., carbon black). The joint research has been devoted to the study of the catalytic activity of different types of carbons, the design/modelling/experimentation of different reactor concepts, and their performance assessment under concentrated solar irradiation. A tubular solar reactor prototype (1 kWth) has been developed for methane dissociation with continuous catalyst feeding (Carbon Black particles) in the reactive gas stream (Figure 10). The research has been focused on the design and simulation of the reactor (3D CFD model), the construction of the experimental set-up with solid particles injection, and the experimental evaluation of the reactor performances for direct thermal decomposition without catalyst up to 1400°C and with carbon particles injection. Another reactor configuration featuring an indirectly-irradiated packed-bed of catalyst has also been investigated. This reactor configuration enables almost complete CH4 conversion, 100% selectivity to H2 with no side-products, and thermochemical reactor efficiency up to 6% in the range of 1050-1250°C. The catalytic activity of different types of carbon catalysts has been further studied between 900 and 1200°C using a solar thermogravimetric device (Figure 11). The current research work related to hybrid systems focuses on the solar pyrolysis/gasification processes of carbonaceous feedstock (lignocellulosic biomass) for the production of syngas. New solar thermochemical reactors are currently developed and experimentally tested, in collaboration with CEA/LITEN (LSHT-INES, Chambéry and LTB, Grenoble, France) and with ADEME support.

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Figure 10 : Solar reactor prototype for thermocatalytic methane decomposition (collab. IHI Japan)

Figure 11 : Evolution of the relative carbon gain with time for Carbon Black (SB905, SB285) and activated carbon (AC) powders

3.2. Solar fuels from thermochemical H2O/CO2-splitting cycles This research aims to study the conversion of H2O and CO2 into fuels from sunlight using concentrated solar energy as the source of high-temperature process heat. The investigated processes are based on two-step thermochemical metal oxide redox cycles: Solar endothermic step (T>1200°C) : MOx Æ MOx-y + y/2 O2 Exothermic step (T<1200°C) : MOx-y + y H2O/CO2 Æ MOx + y H2/CO The produced syngas (H2/CO mixture) can be further processed to synthetic liquid hydrocarbon fuels via conventional Fischer-Tropsch or other catalytic processes. This topic was notably supported by Airbus Group foundation (2012-2015) through a research project on jet fuel production from CO2, H2O and concentrated solar energy. The performed studies have provided a detailed understanding of the cycles based on SnO2/C/Sn and SnO2/SnO, and have also led to the development of relevant methods and tools for the investigation of other candidate cycles for the production of synthetic fuels (Ph’D G. Levêque). The carbothermal reduction of SnO2 into Sn was investigated using activated carbon or solar-produced carbon black as reducing agents (amorphous carbons with different structures and morphologies), in order to identify the prevailing reaction mechanisms and to propose an application for the carbon obtained from CH4 splitting. In addition, the production of CO/H2 from SnO and Sn as solar nanopowders, and the associated reaction mechanisms have been characterized using different techniques (kinetics via TGA, tin speciation via XRD and Mössbauer spectrometry of 119Sn). Concerning the high-temperature solar step, the kinetics of ZnO and SnO2 thermal dissociation have been identified from an inverse method by coupling experimentally measured data (temperature and outlet oxygen concentration) with a reactor model (transient heat transfer coupled with chemical reaction). Besides, a new solar-driven thermogravimetric reactor has been developed to directly determine the kinetics of high-temperature solid-gas reactions in controlled atmosphere.

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The customized system was applied to the case of ZnO and SnO2 volatile oxides to determine the kinetics of thermal dissociation at different operating pressures up to 1600°C (1st step of the cycles) and also to the carbothermal reduction of ZnO, SnO2 and MgO (Figure 12).

Figure 12 : Kinetic study of the high-temperature ZnO decomposition reaction as a function of pressure in a solar thermogravimeter

Another specific research field developed is related to the non-stoichiometric redox systems for splitting water and CO2. The use of mixed oxides such as doped ceria (MxCe1-xO2- ) has been shown to enhance the oxygen exchange and mobility in the crystal lattice, which favors solid-state thermal reduction below 1400°C (formation of oxygen vacancies) and rapid fuel production kinetics. A project with RHODIA-SOLVAY was focused on the evaluation of doped ceria compounds for H2 production. Besides, perovskite structures (ABO3- ) have been identified as promising materials for the solar thermochemical fuel production (demonstration of their potential during exploratory project VALTHER-CO2, MI CNRS 2014) and have been examined (Post-doc A. Demont and M. Nair).

Figure 13 : Reactivity of perovskites with A and B-site doping for thermochemical CO2 splitting (thermogravimetric analysis during two successive cycles)

The attractive properties of such non-stoichiometric structures can be attributed to their high oxygen storage capacity and solid-state ionic conductivity thanks to the presence of vacancies and defects, and the related mechanisms in ionic solids. Various substituted perovskites of Ln1-xAexBO3- or (Ln1-xAex)2BO4- series have been first synthesized and tested for the splitting of water. A screening of doped lanthanum manganites has been conducted (within the composition space La1 xSrxMnO3), and optimal redox performances are obtained for 50% Sr while Al/Mg B-site doping results in more active and thermally stable materials. (Figure. 13).

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3.3. Thermochemical energy storage via reversible solid/gas reactions Thermochemical cycles are developed for application to thermochemical storage of solar energy at high temperature based on reversible solid/gas reactions. This storage can be associated to solar thermal power plants in order to match a variable electricity demand with an intermittent energy source supply, enhancing energy generation dispatchability. The research on this topic has been initiated since fall 2014 (Ph’D L. André). Thermochemical storage exploits the heat effects of reversible chemical reactions: MO2x+1 + ∆H MO + xO2. The utilization of a redox pair involving solid oxides of multivalent metals (Fe2O3/Fe3O4, Mn2O3/Mn3O4, Co3O4/CoO…) or mixed oxides is most attractive because air can be used as both the heat transfer fluid and the reactant in open loop. The research objectives are the selection of the most suitable thermochemical systems (equilibrium temperature, heat storage capacity, reversibility…), the investigation of the reaction kinetics during heat charge/discharge, and the design of solar thermochemical reactors/heat exchangers. A list of potential candidate systems for thermochemical energy storage has been proposed based on defined criteria such as suitable temperature range and non-toxicity, and a thermodynamic equilibrium analysis has been first conducted to determine the theoretical transition temperatures. An experimental study using TGA-DSC and XRD analysis mainly aims to determine the temperature and enthalpy of each reaction, and to demonstrate the complete reversibility during successive cycles. Different cycling experiments of Co oxide (Figure 14) have been carried out without any reactivity losses (TGA, fixed bed, and porous foam). The influence of the synthesis method on the reactivity of Mn oxide was also observed. The improvement of the system redox properties via doping or mixed oxide synthesis is currently investigated (binary systems of Mn-Fe, Mn-Cu, Mn-Co, Co-Fe). The influence of the materials shaping and structure is also examined (mesoporous structure, core-shell…).

Figure 14 : Experimental study of the Co3O4/CoO redox system during several cycles for thermochemical heat storage in fixed bed

high temperature materials and solar fuels - promes · 40: axis 1 materials and extreme conditions...

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