Purification of Chloride Salts for Concentrated Solar Applications

Abstract

The increased interest in high temperature molten salt technology, including concentrated solar power (CSP), has resulted in an increased need for high purity alkali and alkaline earth chloride salts. One lingering question remains in the technoeconomic analysis: 'How clean is clean enough?' High-quality corrosion experiments are necessary to design alloys with enhanced corrosion resistance, and ultimately to realize a cost-effective CSP facility. To conduct high-quality, reproducible corrosion experiments, high purity salt with known impurity compositions must be achievable. Commercial high purity salts are qualified on the basis of metal impurity content, with cost increasing significantly as purity increases. However, non-metallic impurities such as oxygen and moisture content are extremely relevant to concentrated solar or nuclear applications are. For this reason, efforts to clean salts at Oak Ridge National Laboratory (ORNL) have centered upon a modified carbochlorination process which addresses oxygen and moisture-based impurities. This report will focus on the modified carbochlorination procedure utilized to purify magnesium chloride-containing salts for concentrated solar power applications.

Molten Chloride Thermophysical Properties, Chemical Optimization, and Purification

“…Very similar behaviors to produce hydroxychloride during salt dehydration had been found in CaCl2 salt as well [16,17,[31][32][33][34][35]. To mitigate the risks of the corrosive hydroxychloride species, multiple methods for salt dehydration and purification have been proposed, such as thermal treatment by controlled heating [26,29,43,31,[36][37][38][39][40][41][42], reduction by elemental Mg [19,22,23,25], dehydration with ammonium chloride [14,29,42,44,45], and carbochlorination with CO, Cl2, and/or CCl4 to remove oxide species [13,14,46,47]. Because the targeting CSP industry needs a significant amount of salt inventory (>10,000 Mt) for each power plant, it is our intent to investigate the most cost-effective and scalable salt purification process.…”

“…1. Gas flowing effect on pre-drying time: A relatively easy method to understand the gas flowing effect is to plot the observed drying time 14 as a function of gas flow rate (i.e., sparging gas, cover gas, or the sum of sparging and cover gas). Figure 5 shows that the water release time is most correlated to the total gas flow rate (i.e., the sum of sparging gas and cover gas) during pre-drying.…”

“…Molten chloride salts have been proposed as an alternative HTF chemistry for the next generation (Gen3) CSP technology operating at higher temperatures in order to maximize the benefit of using the sCO2 power cycle [1,2]. A significant number of research efforts [3,4,[13][14][15][16][17][18][19][20][21][22]5,[23][24][25][26][27][28][6][7][8][9][10][11][12] have been devoted to investigating molten chlorides' chemistry, thermodynamics, corrosion, and properties, especially the ternary system of NaCl-KCl-MgCl2 due to its availability and low cost.…”

Facility to Alleviate Salt Technology Risks (FASTR): Preliminary Design Report with Failure Modes and Effects Analysis

“…At the outset of the FASTR project, the method for chloride salt preparation had not been determined. Oak Ridge National Laboratory (ORNL) was creating highly purified chloride salts through carbochlorination using carbon tetrachloride as the reagent [6,7]. Carbochlorination has been used at the industrial scale to purify MgCl2 through sparging Cl2 gas in the presence of a separate source of solid carbon.…”

Application of In Situ X-ray Absorption Spectroscopy to Study Dilute Chromium Ions in a Molten Chloride Salt

“…In an effort to confirm the chromium coordination environment, we generated structure models which included Cr-O scattering paths. Post-purification titrations of the KCl-MgCl2 eutectic revealed 50-75 ppm of oxygen in the form of MgO 14,15 . Fits obtained following inclusion of Cr-O scattering paths afforded coordination numbers and bond lengths that are close to the CrO2Cl2 literature values.…”