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Energies 2020, 13, 5859 7 of 23 3. TCES Systems Based on Carbonates Metal carbonates present the advantage of being cheap and largely available materials. Several of them have demonstrated attractive performances for TCES application, such as CaO/CaCO3, SrO/SrCO3 or BaO/BaCO3 [33]. Calcium-Looping (CaL) technology in particular, coupled with CSP, is being thoroughly studied for adaptation to TCES power plant (Equation (2)) [3,12,34–38]. This reversible cycle requires operating temperatures between 850 and 950 ◦C. CaO(s) + CO2(g) CaCO3(s) (∆H◦ = −178.2 kJ/mol) (2) However, due to sintering, the material gradually loses its porosity and facility for the reactive gas to access the active sites within the material [39,40]. To address this issue, the addition of an inert material for structure stabilization and sintering inhibition is a proven approach (Figure 4). For example, CaO/SiO2 composites were synthesized using rice husk as support [41]. The composites containing 70 and 90% CaO retained the morphology of rice husk and showed enhanced conversion, as compared to limestone, and decreased pore-plugging effect. Pure CaO and nano silica doped systems (molar ratio 1:1) were compared, and a shift in reaction temperature was observed [42]. The pure material performed better between 750 and 925 ◦C, while for the silica-doped samples the decarbonation happened at lower temperatures, between 700 and 800 ◦C. Al2O3 has been demonstrated to efficiently stabilize CaO/CaCO3 [43]. Thanks to using a space-confined chemical vapor deposition (CVD) method, Han et al. [43] presented a new way to synthesize a Al2O3 (5 mol%)-CaO composite which demonstrated high stability over 50 calcination/carbonation cycles as compared to samples using SiO2 or TiO2 as inert additives. The space-confined CVD method allowed CaO crystalline grains to be coated with inert oxide nanoparticles, as high contribution to thermal stability of the composite material. Further work was carried out by the same team on CaO-based materials, using the same method, and resulted in the synthesis of dense CaO grains using calcium formate as precursor, with Al2O3 deposited on the surface [44]. The resultant composite, optimized with 10 mol% Al, presented high volumetric energy storage density (2.07 GJ/m3) after 20 cycles. The properties of the Al2O3-doped CaO system were compared to CeO2-doped CaO and to a novel Al2O3/CeO2 co-doping [45]. The Al2O3/CeO2 co-doped CaO-based material was synthesized via a wet-mixing method and comprised a mixture of CaO, Ca12Al14O33 and CeO2. Co-doping of CaO using 5 wt% of Al2O3 and 5 wt% of CeO2 gave the best results in terms of energy storage capacity, and the material proved to retain a good stability over 30 cycles with 7% conversion rate loss. The Al2O3/CeO2 co-doping of CaO/CaCO3 then also showed the benefit of enhancing the carbonation reactivity of the material, attributed to the presence of Ce3+ ions on its surface. ZrO2 was also considered as a stabilizing agent and compared to Al2O3 [46]. CaCO3 doped, via ball-milling, with ZrO2 (40 wt%) or Al2O3 (20 wt%) both presented excellent cyclic stability. The CaCO3-Al2O3 system managed to retain more than 80% of its cyclic stability over the course of 500 calcination/carbonation cycles. SiO2 also proved to be an interesting dopant to stabilize CaCO3 as it improved the system’s energy storage capacity, enhanced the calcination kinetics and stabilized CaCO3 over TCES cycles [47]. The stabilization of CaO/CaCO3 was also attempted via the synthesis of composites composed of CaCO3 nanoparticles and antioxidative graphite nanosheets [48]. Graphite nanosheets impregnated in H3BO3 showed higher antioxidant property. The porous structure of the composite helped to enhance CO2 transportation within the material and to obtain a higher thermal conductivity. With only a 3 wt% graphite nanosheet, the composite was capable of cycling for 50 cycles under CO2 and of maintaining a high heat storage capacity (1333 kJ/kgcomposite), while pure CaCO3 was deactivated after 50 cycles and the released heat decreased (down to 452 kJ/kgCaCO3 ). The stabilization of CaO was also studied through the use of sodium sulphate covering the surface of CaO particles at high temperatures. The molten salt was used to form a performant screen on the surface of reactive particles to prevent sintering during TCES cycles [49]. Another approach to hinder the agglomeration of CaO powder was proposed by Raganati et al. (2020) [50] who presented aPDF Image | Hi Temp Thermochemical Energy Storage via Solid Gas Reactions
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