Construction And Performance Study Of Multi-Chloride Molten Salts System

Facing to the escalating energy and environmental crises, the energy transformation is urgent. High-efficient heat transfer and storage technology provide a better solution for energy conservation and low-cost large-scale use of clean energy. With the quickly development of efficient molten salt heat storage technology and its promotion to other applications like high-temperature waste heat recovery and nuclear power plants, higher requirements were proposed for improving the energy conversion efficiency, reducing the cost of electricity and designing high-temperature efficient heat transfer and storage materials. Molten nitrate salts, which was used to the high-temperature heat transfer and storage have been widely used in foreign solar thermal power stations and gained much success. But the instability and NOx emissions of molten nitrate salt restricted its high temperature applications. Therefore, the development of inexpensive, high temperature stability, excellent heat transfer and storage performance material was the key point of heat storage technology.In this paper, two lower-cost multi-chloride molten salts with excellent performance were built. According to regular solution model and conformal ion solution model, the phase diagram of ternary chloride molten salt system consisting of Na Cl, Ca Cl2 and Mg Cl2 was calculated and its eutectic point was predicted. Differential scanning calorimeter(DSC) was used to validate the eutectic point and the best ratio of ternary molten salt. The eutectic point was tested 424 oC, which was in excellent agreement with that value 424.05 oC predicted by phase diagram. Then the best composition of molten salt system was constituted. The quaternary eutectic salt system consisting of Na Cl, KCl, Ca Cl2 and Mg Cl2 was also designed according to the phase diagram. The eutectic point and the best ratio of quaternary molten salt were also confirmed by DSC.The molten salts were prepared by static melting method according to the optimum ratio. Then, the thermo-physical properties of molten chloride salts at high temperature were also measured including melting point, heat of fusion, heat capacity, density and viscosity, which were respectively determined by TG-DSC, Archimedes and vibration-rotation methods. For ternary molten salt system(Na Cl-Mg Cl2-Ca Cl2), the melting point was 424 oC, heat of fusion was up to 191.7J?g-1, the heat capacity was average 0.83 and 1.19J?g-1?K-1 for solid and liquid phase. For quaternary molten salt system(Na Cl-KCl-Mg Cl2-Ca Cl2), the melting point was 385 oC, heat of fusion was 151J?g-1, the heat capacity was average 0.91 and 1.41J?g-1?K-1 for solid and liquid phase. Both density and viscosity decreased linearly with increasing temperature.Mass loss experiments under isothermal conditions and thermal cycle experiments were used to test the thermal stability of multi-chloride molten salts. The mass loss of molten salts all accelerated with the augment of temperature. The optimal operating temperature range of ternary molten salt was 480~700oC and the best operating temperature range of quaternary molten salt was 430~650oC. The steady thermal cycling curves and the negligible change of melting and freezing point all indicated that these two multi-chloride molten salts had excellent thermal cycling stability.Finally, the corrosion of stainless steel in high temperature molten chloride salts was researched. The short-term corrosion results found that the corrosive rate of the molten salt can be inhibited by blocking the air. 316-L stainless steel showed the best corrosion resistance due to containing the molybdenum element. Few iron entered the molten salt after corrosion. Long-term corrosion experiment indicated that the corrosive rate of stainless steel reduced with increasing days and tended to be steady. XRD(X-ray diffraction) and SEM(scanning electron microscope) were used to analyze the morphology of corrosion products. The results showed that the Mg O Obtained by thermal decomposition of Mg Cl2 could attach to the stainless steel surface and fill the corrosive hole, which could prevent further corrosion of stainless steel.