Premelting, Melting, and Degradation Properties of Molten Alkali Nitrates: LiNO 3 , NaNO 3 , KNO 3 , and Binary NaNO 3 -
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DUCTION
MOLTEN alkali nitrate salts are used in various metallurgical, chemical, food, nuclear, water desalination, and power generation industries.[1–3] Applications include fertilizer production; gunpowder and explosive compounds; high-temperature electrochemistry; nitriding of steel; bath quenchant; electrolytes for thermal battery; phase change materials (PCMs); heat transfer fluid (HTF); coolant in nuclear reactors; curing of raw red meat; fish processing; caustic soda production; and heat transfer media in metal, chemical, and minerals industries.[1,4–12] In most industries, the nitrate and nitrite salts are used as an isothermal temperature operation, such as in chemical reactors, while in solar thermal power (STP) plants, they are used for dynamic temperature operation (cyclic heating-cooling)[7] in thermal energy storage (TES) systems.[13–21] Hence, MEHEDI BIN MOHAMMAD, GEOFFREY ALAN BROOKS, and MUHAMMAD AKBAR RHAMDHANI are with the Department of Mechanical Engineering and Product Design, Swinburne University of Technology, Hawthorn, Victoria-3122, Australia. Contact email: [email protected] Manuscript submitted September 19, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
understanding of the thermal breakdown and gaseous product evolution behavior of nitrates at the nonisothermal heating condition is important. Due to their versatile use, the alkali nitrates and nitrites gained significant attention compared to other molten salts over the last century.[1,22,23] Various molten nitrate salts have lower melting points and negligible vapor formation compared to other alkali salts.[19–21] They have lower thermal conductivity, viscosity, and heat capacity compared to water steam.[24,25] The nitrates as HTF is used in many STP plants,[13–18] because they have broader operating temperature ranges compared to synthetic oil or hydrocarbons (such as a mixture of biphenyl-diphenyl oxide[26]). A typical working range for various molten nitrate/nitrite salts is from 150 °C to 600 °C.[27] One way to improve the levelized cost of electricity of existing STP plants is to increase the solar-to-electric conversion efficiency (power cycle efficiency). To achieve higher power cycle efficiency, the operating temperature range of an HTF should be broadened by increasing the thermal stability limit and lowering the melting points. Currently, solar salt with a binary 60:40 weight ratio NaNO3-KNO3[28] is frequently used in many parabolic trough and central receiver tower plants. Binary solar salt has an operating temperature ranging from 220 °C to 565 °C (can work up to 600 °C).[1,2,29–34] To improve the stability limit
or upper temperature of the binary salt, in this study, oxygen as blanket gas was used to suppress the degradation point. Some authors define the degradation temperature of nitrate as when the oxygen is first detected or the nitrites are formed in the melt.[35–37] Olivares[38] and Olivares and Edwards[39] used a simultaneous thermal analyzer (STA) to study ternary Hitec salt (a mixture of alkali nitrates and ni
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