Ternary molten salt as well as preparation method and application thereof

Abstract

The invention relates to ternary molten salt as well as a preparation method and application thereof. The ternary molten salt comprises the following components in percentage by weight: 51-60% of potassium nitrate, 34-40% of lithium nitrate and the balance of sodium chloride. The preparation method comprises the following steps: mixing potassium nitrate, lithium nitrate and sodium chloride, grinding into powder, melting, cooling and crystallizing, and grinding into powder. Compared with the prior art, sodium chloride is introduced into a potassium nitrate and lithium nitrate system, the eutectic effect is triggered, a nitrate-sodium chloride mixed system (KNO3-LiNO3-NaCl) is constructed, and the ternary molten salt has the characteristics of low melting point and high latent heat (the melting point is 112.1 DEG C, and the phase change enthalpy is 189.2 J / g) by optimizing the proportion, and can be suitable for low-temperature waste heat recovery (100-200 DEG C) and medium-high temperature heat storage (lt; 580 DEG C).

Description

Technical Field

[0001] The present invention belongs to the technical field of thermal storage systems and relates to a ternary molten salt and a preparation method and application thereof. Background Art

[0002] As the global energy landscape undergoes profound transformation, efficient energy utilization and sustainable development have become core issues of our time. With the vigorous development of renewable energy sources such as solar and wind power, their inherent intermittent and unstable nature has become increasingly prominent, posing a serious challenge to energy storage technology. Energy storage technology has become a key technology for improving energy efficiency and optimizing the energy mix. Traditional molten salt thermal storage materials suffer from high melting points and low phase change enthalpy, limiting their application in medium- and high-temperature heat storage. By adding modified salts, the thermal properties of molten salt can be significantly improved, enhancing its heat storage performance and heat transfer efficiency.

[0003] Prior art CN107312500A discloses a ternary molten salt and its preparation method, the ternary molten salt comprising the following components in weight percentage: 35-45% lithium nitrate, 10-20% sodium nitrate, and 35-55% potassium nitrate; the preparation method of the ternary molten salt comprises the following steps: adding 35-45% lithium nitrate, 10-20% sodium nitrate, and 35-55% potassium nitrate, respectively, to a crucible and mixing them evenly, heating at 200-400°C for 3-7 hours, and then cooling to room temperature to obtain the ternary molten salt, which has the advantages of low melting point, high stability, and high latent heat. The process for preparing the ternary molten salt is simple and easy to operate, can achieve standardization of the preparation of the ternary molten salt, and is environmentally friendly. However, the melting point of the ternary mixed molten salt obtained by this method reaches 165°C, and the phase change enthalpy reaches about 170 J / g, which still cannot meet the heat storage performance and heat exchange efficiency requirements in the heat storage field.

[0004] Prior art CN108285777A discloses a method for preparing a ternary mixed molten salt medium-temperature phase change energy storage material. In the proportioning process, a mixture of lithium nitrate, sodium nitrate, and potassium chloride solid particles with a total weight of 40g is calculated and weighed, wherein the mass ratio of lithium nitrate, sodium nitrate, and potassium chloride is 9:10:1; in the grinding process, a ball mill is used to grind the mixed salt at a speed of 500r / min for 90 minutes until the mixed solid particles become a solid powder with a fineness of 0.1μm. In the melting process, the ground mixed salt is placed in a muffle furnace and heated from room temperature to 400℃ at a heating rate of 5℃ / min. The temperature is kept at 400℃ for 120 minutes. After it is completely melted, it is removed from the furnace and air-cooled when it cools to about 170℃ until it reaches room temperature; the blocky mixed molten salt is removed from the corundum crucible, ground, and dried to obtain a sample of the ternary mixed molten salt. The addition of potassium chloride improves the thermophysical properties of the mixed molten salt, making it more suitable for application. However, the melting point of the ternary mixed molten salt obtained by this method is 162.5°C, which is still relatively high. When used in solar thermal power stations, problems such as frozen pipes are still prone to occur. Summary of the Invention

[0005] The purpose of the present invention is to provide a ternary molten salt and its preparation method and application, which are used to solve the problems of high melting point and low phase change enthalpy of existing ternary mixed molten salt, so that it is suitable for medium and low temperature phase change heat storage and recovery of industrial waste heat with low temperature; at the same time, it has a low melting point and a high decomposition temperature, and is suitable for sensible heat storage, and is used in fields such as solar thermal power generation.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A first aspect of the present invention provides a ternary molten salt comprising the following components and their weight percentages: 51%-60% potassium nitrate, 34%-40% lithium nitrate, and the balance being sodium chloride.

[0008] In some preferred embodiments, the ternary molten salt includes the following components and their weight percentages: 55%-60% potassium nitrate, 36%-40% lithium nitrate, and the balance sodium chloride.

[0009] In some more preferred embodiments, the ternary molten salt includes the following components and their weight percentages: 57.6% potassium nitrate, 38.4% lithium nitrate, and the balance sodium chloride.

[0010] The second aspect of the present invention provides a method for preparing the ternary molten salt as described above, comprising: mixing potassium nitrate, lithium nitrate, and sodium chloride, grinding them into powder, melting them, cooling and crystallizing them, and grinding them into powder again to obtain.

[0011] In some specific embodiments, the potassium nitrate, lithium nitrate, and sodium chloride are dried before mixing to effectively eliminate the weighing error caused by moisture absorption of the molten salt, thereby making the ratio more accurate.

[0012] In some specific embodiments, during the melting, the melting temperature is 350-500° C., and the melting time is preferably 1.5-2.5 h.

[0013] In some preferred embodiments, during the melting, the melting temperature is 400° C. and the melting time is preferably 2 h.

[0014] In some specific embodiments, during the melting, the heating rate is 4-6°C / min.

[0015] In some preferred embodiments, during the melting, the heating rate is 5°C / min.

[0016] In some specific embodiments, the secondary grinding is used to refine the product to ensure mixing uniformity and material particle uniformity, thereby improving the stability of heat storage performance. Grinding in a mortar is simpler to operate.

[0017] In some specific embodiments, the cooling crystallization includes natural cooling to room temperature. The process of natural cooling to room temperature in a furnace can minimize the contact of the molten salt with ambient moisture during the cooling process, thereby avoiding the impact of secondary moisture absorption on material properties.

[0018] A second aspect of the present invention provides an application of the ternary molten salt as described above, including using the ternary molten salt as a heat storage material or as a solar thermal power generation heat storage and heat transfer material.

[0019] Compared with the prior art, the present invention has the following characteristics:

[0020] The present invention introduces sodium chloride into the potassium nitrate and lithium nitrate system to induce the eutectic effect, constructing a nitrate-sodium chloride mixed system (KNO3-LiNO3-NaCl), and optimizing the ratio so that the ternary molten salt has both low melting point and high latent heat characteristics. Compared with the potassium nitrate and lithium nitrate system (melting point 136.5°C, phase change enthalpy 170.8J / g), the melting point is reduced by 17.9% to 112.1°C, and the phase change enthalpy is increased by 10.8% to 189.2J / g. It has the characteristics of low melting point, high latent heat and wide temperature range, and can be suitable for low-temperature waste heat recovery (100-200°C) and medium and high temperature heat storage (<580°C) scenarios.

[0021] The components of the ternary molten salt prepared by the present invention are only a physical mixture of KNO3, LiNO3 and NaCl, and no new chemical phase is generated, indicating that the preparation process does not trigger a chemical reaction and the material stability is reliable. This proves the process controllability and material reliability at the microscopic level. Therefore, it has excellent high-temperature stability. In hundreds of cycles at 50-500°C, the heat storage performance (melting point, phase change enthalpy) basically does not change. It has good application potential in the field of long-life thermal energy storage devices.

[0022] The preparation process of the invention is simple, environmentally friendly, and the raw materials are cheap and readily available, and has good industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Thermogravimetric curve of a ternary molten salt prepared in Example 4;

[0024] Figure 2 XRD pattern of a ternary molten salt prepared in Example 4;

[0025] Figure 3 Cyclic stability diagram of a ternary molten salt prepared in Example 4 during 100 cycles;

[0026] Figure 4 A process flow chart of a method for preparing ternary molten salt in an embodiment. DETAILED DESCRIPTION

[0027] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0028] The following are more detailed implementation cases, which further illustrate the technical solutions of the present invention and the technical effects that can be obtained.

[0029] In the following examples, unless otherwise specified, raw materials, reagents or processing techniques are all conventional commercially available products or conventional processing techniques in the art.

[0030] Comparative Example

[0031] This comparative example provides a binary molten salt and a preparation method thereof. The binary molten salt includes the following masses of molten salt raw materials: 60 g of potassium nitrate, 40 g of lithium nitrate, and 0 g of sodium chloride.

[0032] The preparation method of the above binary molten salt is as follows: Figure 4 As shown, the following steps are included:

[0033] S1: Drying: Weigh sufficient amount of molten salt raw materials and place them in a constant temperature drying oven at 120°C for 24 hours;

[0034] S2: Weighing: Weigh the dried molten salt raw materials according to the above ratio using an electronic balance for later use;

[0035] S3: Grinding: Grind the weighed molten salt raw materials into powder using a mortar to obtain a mixed molten salt raw material;

[0036] S4: Melting: Place the ground mixed molten salt raw materials into a muffle furnace and set the program to increase the temperature to 400°C at a rate of 5°C / min, and then keep the temperature constant for two hours to fully mix the eutectic;

[0037] S5: Crystallization and grinding: After the constant temperature is completed, the temperature is naturally cooled to room temperature, and then the mixture is taken out and ground again to obtain a uniformly mixed binary molten salt.

[0038] Example 1

[0039] This embodiment provides a ternary molten salt and a preparation method thereof. The ternary molten salt includes the following masses of molten salt raw materials: 59.4 g potassium nitrate, 39.6 g lithium nitrate, and 1 g sodium chloride.

[0040] The preparation method of the above-mentioned ternary molten salt comprises the following steps:

[0041] S1: Drying: Weigh sufficient amount of molten salt raw materials and place them in a constant temperature drying oven at 120°C for 24 hours;

[0042] S2: Weighing: Weigh the dried molten salt raw materials according to the above ratio using an electronic balance for later use;

[0043] S3: Grinding: Grind the weighed molten salt raw materials into powder using a mortar to obtain a mixed molten salt raw material;

[0044] S4: Melting: Place the ground mixed molten salt raw materials into a muffle furnace and set the program to increase the temperature to 400°C at a rate of 5°C / min, and then keep the temperature constant for two hours to fully mix the eutectic;

[0045] S5: Crystallization and grinding: After the constant temperature is completed, the temperature is naturally cooled to room temperature, and then the mixture is taken out and ground again to obtain a uniformly mixed binary molten salt.

[0046] Example 2

[0047] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following molten salt raw materials: 58.8 g potassium nitrate, 39.2 g lithium nitrate, and 2 g sodium chloride. The rest is the same as in Example 1.

[0048] Example 3

[0049] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following molten salt raw materials: 58.2 g potassium nitrate, 38.8 g lithium nitrate, and 3 g sodium chloride. The rest is the same as in Example 1.

[0050] Example 4

[0051] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following molten salt raw materials: 57.6 g potassium nitrate, 38.4 g lithium nitrate, and 4 g sodium chloride. The rest is the same as in Example 1.

[0052] Example 5

[0053] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following molten salt raw materials: 57 g potassium nitrate, 38 g lithium nitrate, and 5 g sodium chloride. The rest is the same as in Example 1.

[0054] Example 6

[0055] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following molten salt raw materials: 56.4 g potassium nitrate, 37.6 g lithium nitrate, and 6 g sodium chloride. The rest is the same as in Example 1.

[0056] Example 7

[0057] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following mass of molten salt raw materials: 55.8 g potassium nitrate, 37.2 g lithium nitrate, and 7 g sodium chloride. The rest is the same as in Example 1.

[0058] Example 8

[0059] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following molten salt raw materials: 55.2 g potassium nitrate, 36.8 g lithium nitrate, and 8 g sodium chloride. The rest is the same as in Example 1.

[0060] Example 9

[0061] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following molten salt raw materials: 54.6 g potassium nitrate, 36.4 g lithium nitrate, and 9 g sodium chloride. The rest is the same as in Example 1.

[0062] Example 10

[0063] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following mass of molten salt raw materials: 54g potassium nitrate, 36g lithium nitrate, and 10g sodium chloride. The rest is the same as in Example 1.

[0064] Example 11

[0065] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following molten salt raw materials: 53.4 g potassium nitrate, 35.6 g lithium nitrate, and 11 g sodium chloride. The rest is the same as in Example 1.

[0066] Example 12

[0067] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following molten salt raw materials: 52.8 g potassium nitrate, 35.2 g lithium nitrate, and 12 g sodium chloride. The rest is the same as in Example 1.

[0068] Example 13

[0069] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following molten salt raw materials: 52.2 g potassium nitrate, 34.8 g lithium nitrate, and 13 g sodium chloride. The rest is the same as in Example 1.

[0070] Example 14

[0071] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following molten salt raw materials: 51.6 g potassium nitrate, 34.4 g lithium nitrate, and 14 g sodium chloride. The rest is the same as in Example 1.

[0072] Example 15

[0073] This embodiment provides a ternary molten salt and a preparation method thereof, wherein the ternary molten salt comprises the following mass of molten salt raw materials: 51 g potassium nitrate, 34 g lithium nitrate, and 15 g sodium chloride. The rest is the same as in Example 1.

[0074] Application Examples

[0075] This example illustrates the performance of the ternary molten salt by performing melting performance tests, thermal stability analysis, XRD analysis, and cyclic stability tests on the molten salts prepared in the comparative example and Examples 1-15.

[0076] Melting point and phase change enthalpy test of ternary molten salt

[0077] The phase change characteristics of the ternary molten salt were tested using a differential scanning calorimeter (DSC). The principle of the test is that when the sample undergoes a phase change during heating and cooling, the sample will absorb or release a certain amount of heat. Based on the heat flow fluctuation, the heat flow curve of the material phase change process was obtained. The melting / solidification starting point, melting / solidification end point and melting / solidification latent heat value of the sample can be obtained through the analysis software. Nitrogen was used as the protective gas during the test. The flow rate of nitrogen during the heating process was set to 40ml / min, and the flow rate of nitrogen during the cooling process was set to 60ml / min. The heating rate was set to 10℃ / min, the temperature measurement accuracy was less than ±0.1℃, and the calorimetric measurement accuracy was less than 1%.

[0078] Table 1 Melting point and phase change enthalpy of ternary molten salt

[0079]

[0080]

[0081] From the data in the above table, it can be seen that the binary molten salt in the comparative example and the ternary molten salt in the example both have relatively low melting points. Compared to the binary molten salt in the comparative example, the melting point of the ternary molten salt is significantly lower after adding sodium chloride. Storing heat requires the molten salt material to have a large phase change enthalpy, so Example 4 has both a low melting point and a high phase change enthalpy.

[0082] 2. Thermal stability analysis

[0083] The ternary molten salt in Example 4 was selected as the experimental object, and the temperature at which the mass loss of the ternary molten salt was 3% was selected as the thermal decomposition point. The thermal decomposition point can be obtained from the TG curve. Figure 1 It can be seen that the mass loss rate of the ternary molten salt in Example 4 is essentially zero before 550°C, and the thermal decomposition point is 582°C. At 582°C, the mass loss is 3%, and the mass of the sample drops sharply after 582°C. The maximum operating temperature of the ternary molten salt in Example 4 is 582°C.

[0084] 3.XRD test

[0085] The ternary molten salt in Example 4 was selected as the experimental object. Figure 2The XRD pattern is shown. It can be seen from the figure that the main phases in the prepared ternary molten salt are LiNO3, NaNO3 and NaCl. No characteristic diffraction peaks of other substances were found through XRD diffraction testing. It can be clearly seen that LiNO3 has characteristic diffraction peaks at 23.8°, 28.1°, 33.1°, and 40.6°, 23.8°, 28.1°, 33.1°, and 40.6° are characteristic diffraction peaks of KNO3, and 31.6°, 45.6°, and 53.9° are characteristic diffraction peaks of NaCl. This shows that there are no other impurities in the prepared KNO3-LiNO3-NaCl ternary molten salt, and also shows that no chemical reaction occurs during the preparation process of the mixed molten salt.

[0086] 4. Cyclic stability test

[0087] The ternary molten salt in Example 4 was selected as the experimental subject for cyclic stability testing: one cycle consisted of heating from 50°C to 500°C at a heating rate of 10°C / min and then cooling naturally to room temperature. After 100 cycles, the phase change enthalpy was tested and found to remain unchanged. Repeated experiments confirmed that the melting point and phase change enthalpy of the ternary molten salt remained essentially unchanged within the test range, meeting the heat storage requirements.

[0088] Table 2 Data of 100 thermal cycles of ternary molten salt

[0089]

[0090] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention, without departing from the scope of the present invention, should be within the scope of protection of the present invention.

Claims

1. A ternary molten salt, characterized in that: The invention comprises the following components and their weight percentages: 51%-60% potassium nitrate, 34%-40% lithium nitrate, and the balance sodium chloride.

2. The ternary molten salt according to claim 1, characterized in that The invention comprises the following components and their weight percentages: 55%-60% potassium nitrate, 36%-40% lithium nitrate, and the balance sodium chloride.

3. The ternary molten salt according to claim 2, characterized in that The invention comprises the following components and their weight percentages: 57.6% potassium nitrate, 38.4% lithium nitrate, and the balance sodium chloride.

4. A method for preparing a ternary molten salt according to any one of claims 1 to 3, characterized in that: The method comprises the following steps: mixing potassium nitrate, lithium nitrate and sodium chloride, grinding the mixture into powder, melting the mixture, cooling and crystallizing the mixture, and grinding the mixture into powder again to obtain the product.

5. The method for preparing the ternary molten salt according to claim 4, wherein: In the melting, the melting temperature is 350 to 500°C.

6. The method for preparing the ternary molten salt according to claim 5, characterized in that: In the melting, the melting temperature is 400°C.

7. The method for preparing the ternary molten salt according to claim 5, characterized in that: During the melting, the heating rate is 4-6°C / min.

8. The method for preparing the ternary molten salt according to claim 7, characterized in that: During the melting, the heating rate was 5°C / min.

9. The method for preparing the ternary molten salt according to claim 4, characterized in that: The cooling crystallization includes naturally cooling to room temperature.

10. A use of the ternary molten salt according to any one of claims 1 to 3, characterized in that: The ternary molten salt is used as a heat storage material or as a solar thermal power generation heat storage and heat transfer material.

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