Molten carbonate salts for advanced solar thermal energy power plants: Cover gas effect on fluid thermal stability

• Published in: Solar energy materials and solar cells 2018-12, Vol.188, p.119-126
• Main Authors: Fereres, Sonia, Prieto, Cristina, Giménez-Gavarrell, Pau, Rodríguez, Alfonso, Sánchez-Jiménez, Pedro Enrique, Pérez-Maqueda, Luis A.
• Format: Article
• Language: English
• Subjects: Carbon dioxide; Carbonates; Decomposition; Gases; Heat transfer; Heat transfer fluid; High temperature; Kinetics; Molten salts; Nucleation; Operating temperature; Potassium carbonate; Power efficiency; Power plants; Reaction kinetics; Reactors; Salts; Sodium carbonate; Solar energy; Solar heating; Solar power; Stability analysis; Temperature effects; Thermal decomposition; Thermal energy; Thermal energy storage; Thermal power; Thermal power plants; Thermal stability; Thermogravimetric analysis; Vaporization
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2018.08.028

The eutectic mixture Li2CO3-Na2CO3-K2CO3 is investigated as a high temperature heat transfer fluid and storage medium alternative for molten salt solar thermal power plants. This salt has an operating temperature range of 400–700 °C, enabling the use of higher temperature/efficiency power cycles. However, this carbonate mixture is known to thermally decompose in air. This study evaluates the thermal stability of the salt mixture under different cover gases: air, nitrogen, carbon dioxide, and an 80/20 carbon dioxide/air mixture. Initial characterization is performed through thermogravimetric analysis (TGA), followed by larger scale testing in a custom-made reactor to simulate conditions closer to its practical use. The results show improved thermal stability with a CO2 atmosphere. The decomposition kinetics under different cover gases are estimated from TGA data. However, larger-scale, longer duration experiments show much slower decomposition rates compared to the classical TGA approach. These findings indicate that the main contribution to mass loss in TGA is due to vaporization rather than thermal decomposition. Thus, a proper evaluation of the molten salt´s thermal stability can only be obtained from reactor experiments where vaporization is inhibited. Very long induction periods (of the order of days) are observed, suggesting that the kinetic decomposition mechanism is a nucleation and growth type. Other considerations for future plants incorporating these high temperature salts are discussed. •The thermal decomposition of LiNaK salt is investigated under different cover gases.•CO2 atmospheres have a stabilizing effect on LiNaK carbonates.•Kinetics confirm increased decomposition rates with CO2/Air mixtures.•Large-scale, long-duration tests are needed to reproduce plant operation behavior.•Decomposition induction periods are 12–42 days depending on cover gas composition.