The phase diagram prediction and experimental study of ternary same cation systems

doi:10.19799/j.cnki.2095-4239.2021.0320

Abstract

Abstract:

New high-temperature thermal storage/storage materials with higher service temperatures must be developed to improve the efficiency of third-generation concentrated solar thermal power generation technology and reduce the system's operating costs. Based on the same-cation principle, three molten salts, i.e., sodium nitrate (NaNO₃), sodium chloride (NaCl), and sodium carbonate (Na₂CO₃) of the same sodium ion, were selected as base salts, and the phase diagram principle guided the preparation of mixed molten salts. The phase diagram thermodynamic calculation of the NaNO₃-NaCl-Na₂CO₃ ternary system was carried out using the FactSage software program. The thermophysical analysis of several groups of eutectic salts near the lowest eutectic point was carried out using a differential scanning calorimeter. The results showed that when the molar ratio of the molten salts in the ternary system(NaNO₃:NaCl:Na₂CO₃) was 0.92:0.064:0.016, the melting point of the system was the lowest, the lowest eutectic point was 279.9 ℃, and the latent heat was 194.1 J/g. These results were essentially consistent with the theoretical calculation result (291 ℃), verifying the model's accuracy. Other thermophysical properties of the system at the lowest eutectic point were measured. The decomposition temperature of the system at this ratio was 648 ℃ (the mass loss rate was 5%), and the specific heat value was 1.60 J/(g·K) (500 ℃). The measured specific heat value increased by 0.12 compared with Solar Salt and 0.28 compared with Hitec salt. The new ternary system had large latent heat of molten salts and a broad temperature range. It showed significant potential in the preparation of latent heat storage involving composite materials. The system can be used as a reference for developing new thermal storage/storage materials for solar power generation and as a guide for the development of mixed molten salts using phase diagram theory.

Key words: molten salt, phase diagram, concentrating solar power, thermophysical properties

CLC Number:

TK 512

Cong HE, Yuanwei LU, Wenbing SONG, Xiaotong CHEN, Yuting WU, Zhansheng FAN. The phase diagram prediction and experimental study of ternary same cation systems[J]. Energy Storage Science and Technology, 2021, 10(5): 1729-1734.

Figures/Tables 8

Table 1

Table 2

Thermodynamic properties of pure components"

相	状态	H⁰_298.15K/(J·mol^-1)	S⁰_298.15K/(J·mol^-1·K^-1)	温度范围/K	比热容/(J·mol^-1·K^-1)
NaNO₃	S1	-467980.0	116.315	298.1～549.0	$22.62 + 0.222 T$
	S1	-467980.0	116.315	549.0～579.0	139.000
	S2	-464360.0	122.908	298.1～549.0	$22.62 ? + 0.222 T$
	S2	-464360.0	122.908	549.0～579.0	139.000
	L1	-449380.0	148.781	298.1～549.0	$22.62 ? + 0.222 T$
	L1	-449380.0	148.781	549.0～579.0	139.000
NaCl	S1	-411119.8	72.132	298.1～2000.0	$45.940 + 1.63176 × 10 - 2 T$
NaCl	L1	-394956.0	76.076	298.1～1500.0	$77.763 - 7.5312 × 10 - 3 T$
Na₂CO₃	S	-1130768.0	138.797	298～758	$87.121919 + 2.629 × 10 - 2 T + 110904.53 ? T - 21.6668 × 10 - 4 T 2$
Na₂CO₃	L	-1104433.7	161.706	298～723	$87.12 + 2.629 × 10 - 2 T + 110904.53 ? T - 2 + 1.66 × 10 - 4 T 2$

Table 2

Fig. 1

Table 3

Fig. 2

Fig. 3

Fig. 4

Table 4

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熔盐组成(质量分数)/%	熔点/℃	稳定性/℃	比热容/[kJ?(kg?K)^-1]	成本/(USD?kg^-1)
KNO₃/NaNO₃(40/60)	220	530～565	～1.5(400 ℃)	0.50～0.8
KNO₃/NaNO₃/NaNO₂(53/7/40)	142	450～535	1.34(400 ℃)	～0.90
K₂CO₃/Li₂CO₃/Na₂CO₃(32/35/33)	397	>650	1.9(700 ℃)	1.30～2.50
KF/LiF/NaF(59/29/12)	454	>700	1.9(700 ℃)	> 2.00
KCl/MgCl₂/NaCl(17.8/68.2/14.0)	380	>800	～1.0(500 ℃)	< 0.35

NaNO₃:NaCl:Na₂CO₃组成/%		预测值/℃	实验值/℃	潜热/(J·g^-1)	误差/℃
配比1	0.90:0.08:0.02	291.2	280.7	209.0	10.5
配比2	0.92:0.064:0.016		279.9	194.1	11.3
配比3	0.91:0.075:0.015		289.2	195.2	2.0
配比4	0.90:0.085:0.015		288.1	196.3	3.1

熔盐组成(质量分数)/%	熔点/℃	稳定性/℃	比热容/(kJ·kg·K)^-1	比热容提高比例/%
40%KNO₃+60%NaNO₃	240	530～565	～1.5(400 ℃)	8.0
53%KNO₃+7%NaNO₃+40%NaNO₂	142	450～535	1.34(400℃)	20.8
48%Ca(NO₃)₂+7%NaNO₃+45% KNO₃	130.61	554.39	1.272	27.3
20%LiNO₃+52%KNO₃+28% NaNO₃	130.15	600.05	1.091	48.5
30%LiNO₃+10%Ca(NO₃)₂+60% KNO₃	132.15	567.18	1.395	16.1
10%LiNO₃+10%Ca(NO₃)₂+60% KNO₃+20% NaNO₃	132.08	580.36	1.517	6.8
NaNO₃-NaCl-Na₂CO₃(本研究)	279.9	598	1.62	—

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