The Thermal Stability of Molten Lithium–Sodium–Potassium Carbonate and the Influence of Additives on the Melting Point

Abstract

The thermal stability of a molten LiNaK carbonate salt, potentially suitable for thermal energy storage, was studied up to a temperature of 1000 °C. The salt investigated was the eutectic Li2CO3–Na2CO3–K2CO3 in the proportions 32.1–33.4–34.5 wt. % and the study was done by simultaneous differential scanning calorimetry (DSC)/thermogravimetric–mass spectrometric (TG–MS) analysis in gas atmospheres of argon, air, and CO2. It was found that (i) under a blanket gas atmosphere of CO2 the LiNaK carbonate salt is stable up to at least 1000 °C. (ii) In an inert atmosphere of argon, the salt evolves gaseous CO2 soon after melting and begins to decompose at between 710 °C and 715 °C with acceleration in the CO2 evolution rate from the melt. An increase in the rate of weight loss is also observed after 707 °C. (iii) Under a blanket atmosphere of air, the gaseous CO2 evolution from the salt is observed to commence at 530 °C, the onset of decomposition detected by DSC analysis at 601 °C and the rapid rate of weight loss determined by TG analysis at 673 °C. The melting point of the LiNaK carbonate studied was between 400 °C and 405 °C. Thermodynamic modeling with Multi-Phase-Equilibrium (MPE) software developed in CSIRO Process Science and Engineering indicated that additives such as NaNO3, KCl, and NaOH lower the melting point of the LiNaK carbonate eutectic, and this was experimentally verified.

Thermal Investigation of a Doped Alkali-Metal Carbonate Ternary Eutectic for Direct Carbon Fuel Cell Applications

“…Interestingly, the addition of NaNO 3 , KCl, and NaOH lowered the melting point of the alkali-metal carbonate eutectic even further. 16 These literature results were confirmed by the use of thermodynamic modeling software, as well as experimentally; however, the reasons for this complex thermal behavior were left to further study.…”

“…A high activation energy, as in the case of TiO 2 , literally implies that kinetics of melting are sluggish, meaning that it has the ability to store substantial energy. 16 It can also be seen from Figure 10 that the profile of the plot of E A vs α varies in accordance with the type of contaminant used. Some mixtures start with a relatively high activation energy, which decreases as the conversion or reaction approaches completion.…”

“…16 It was found that the unmodified eutectic gave rise to a single melting peak in a differential thermal analysis (DTA) experiment, signifying the melting of a single phase material. Interestingly, the addition of NaNO 3 , KCl, and NaOH lowered the melting point of the alkali-metal carbonate eutectic even further.…”

A One‐Pot Synthesis of Hydrogen and Carbon Fuels from Water and Carbon Dioxide

“…[ 27 ] These electrolytes melt at 346 °C to 420 °C, and facilitate dissolution of both water and carbon dioxide. The eutectic carbonates/hydroxides had not been considered for electrolyses, but are solar thermal storage molten salt candidates.…”

“…For example, the eutectic lithium mix studied had 84.3:15.7 mol% LiOH:Li 2 CO 3 with a melting point of 434 °C, and the 87.5:12.5 mol% NaOH:Na 2 CO 3 sodium mix studied had a melting point of 330 °C. [ 27 ] ). Electrolysis was measured at 500, 600, and 700 °C in the lithium hydroxide/carbonate electrolyte and at 400 °C in the sodium hydroxide/carbonate electrolyte.…”