Defined purge gas composition stabilizes molten nitrate salt - Experimental prove and thermodynamic calculations

Abstract

Thermal energy storage systems are integrated in concentrating solar power plants to improve 8 the flexibility of the electricity generation. Commonly, the liquid storage material Solar Salt, a 9 nitrate salt mixture, is applied to transport and store solar thermal energy. The lifetime and the 10 temperature range of Solar Salt in the storage units are restricted by decomposition reactions 11 of the material. Oxide ion formation is one of the fundamental issues. So far, it has not been 12 proven if oxide ion formation can be prevented by addition of gaseous reaction products to the 13 gas atmosphere. Also, a reliable reaction equation for the oxide ion formation is missing. In 14 the presented experiments, molten salt at 600 and 620 °C is purged with a gas mixture of 15 nitrogen, oxygen, and nitrous gases. Post-analysis of salt samples reveals stabilizing effects, 16 depending on the purge gas compositions. Chemical equilibrium of the oxide ion forming 17 reaction is demonstrated. It is proven that oxide ion formation can be controlled and 18 suppressed. Reaction equations are evaluated and selected in order to quantify the reaction 19 thermodynamics. The results contribute to recommendations for operating conditions and gas 20 handling in storage systems of solar thermal power plants, which finally ensure reliable and 21 constant material properties for extended lifetime and high temperatures.

Investigation of Regeneration Mechanisms of Aged Solar Salt

“…Similarly, the nitrite concentration equilibrated at 10.4, 9.8, 9.7 (±0.1) mol% respectively. In general, these results agree with previously reported Solar Salt equilibrium data [ 7 , 14 , 15 , 16 ].…”

“…In the reference experiment (Ref/80_0) with pure Solar Salt ( Figure 3 , orange) the nitrite content increased before it reached a steady state at 6.0 (±0.1) mol%, while the nitrate content stabilized at 93.9 (±0.1) mol%. This result is consistent with thermodynamic expectations (93.3(±0.3):6.7(±0.1) mol% NO 3 − :NO 2 − [ 7 ]) and in agreement with Equation (1), where increasing the oxygen partial pressure will push equilibrium composition to the nitrate side, thereby lowering the nitrite content. The detected oxide ion concentration increased continuously, with a final value of 0.14 (+0.01 − 0.02) mol% at 1150 h. This behavior is again explained by the fact that all gases evolving from the molten salt are purged out of the test rig, allowing no equilibration of Equation (2).…”

“…The summarized data of experiments performed under p(O 2 ) = 0.2 atm expressed as anion concentrations over time, is illustrated in Figure 2 . Reference experiments of pure Solar Salt already published are marked orange and named according to the applied purge gas (Ref/20_0, Ref/20_200) [ 7 ]. Within 200 h, all experiments exhibited a steady concentration of nitrates and nitrites.…”

“…Beside these ions, the evolution of several reactive gas species, namely oxygen, nitrogen and nitrous oxide (N 2 O) has been discovered [ 7 ]. It has to be emphasized that the exact stoichiometry of Equation (2) has not been determined experimentally and may also depend on atmospheric composition.…”

Molten Salt Storage for Power Generation

“…The interest is optimization and identification of molten salts mixtures with higher operation temperature . One research line is the optimized operation of existing Solar Salt 18, 19. Other work focuses on the identification and qualification of new salt classes.…”