Thermal analysis of molten ternary lithium-sodium-potassium nitrates

The pre-melting, melting and degradation events of a ternary LiNO3-NaNO3-KNO3 (29.63-1323-57.14 wt %) salt have been investigated using simultaneous thermal analyzer (STA) for potential use as a heat transfer and storage fluid (HTF). This composition was selected based on the thermochemical calculation using the FactSage 7.0 to find the lowest solid-liquid equilibrium temperature. The melting point obtained from STA (122.8 degrees C) indicated a good agreement with the theoretical value determined by FactSage (120.8 degrees C), which is 100 degrees C less compared to binary NaNO3:KNO3 solar salt. The salt was heated from 50 to 800 degrees C in 3 atm (argon, air, and O-2) at three scanning rates (2.5, 5 and 10 degrees C/min) to investigate its decomposition limits. At 10 degrees C/min scanning rate the salt started to degrade rapidly at 545 degrees C in argon, 571 degrees C in air and 600 degrees C in oxygen, respectively. In argon at a higher heating rate (10 degrees C/min), the salt degradation was delayed by 42 degrees C. The main gaseous products of decomposition were found to be O-2, N-2, NO, N2O and NO2. After melting, at higher temperature the evolution of O-2 and NO indicated that primary and secondary decomposition reactions are concurrent and overlapping. The heating-cooling rates have low effect on the onset of melting and offset freezing temperatures but, have an effect on peaks height, peaks width, and transition enthalpies. The alpha/beta transition (75 degrees C peak temperature) during cooling only appeared at the 1 degrees C/min cooling rate. The ternary salt offers 70 degrees C greater operating temperature range compared to binary solar salt in argon. At a higher partial pressure of O-2, decomposition was delayed by 55 degrees C (at 10 degrees C/min), which could be used to increase the power cycle efficiency of solar electricity generation. (C) 2016 Elsevier Ltd. All rights reserved.