WO2020168604A1 - Mixed chloride molten salt having high thermal conductivity, preparation method therefor and application thereof - Google Patents

Abstract

Provided by the present invention is a mixed chloride molten salt having high thermal conductivity. The raw materials of the molten salt comprise a chloride, a metal hydroxide and a metal powder. The molten salt lowers the melting point, increases the upper working temperature limit, and improves the power generation efficiency of solar thermal power generation. At the same time, the molten salt has greater thermal conductivity, better heat transfer and heat storage performance, may avoid the problem of local overheating of an apparatus, may also reduce the amount of media for heat transfer and heat storage, reduce the volumes of related transportation and storage devices, and reduce operating costs.

Description

High thermal conductivity mixed chloride molten salt and preparation method and application thereof Technical field

The invention belongs to the field of energy storage materials, and relates to a molten salt, in particular to a mixed chloride molten salt with high thermal conductivity and a preparation method and application thereof.

Background technique

Renewable energy includes biomass energy, hydropower, wind energy, wave energy, and solar energy. Among green energy sources, solar energy is the best energy source for safety, cleanliness, and permanence. The annual sunshine hours of more than 2/3 of the country’s total area are more than 2200h, especially in western Tibet, southeastern Xinjiang, western Qinghai, southern Yunnan and other regions with abundant solar resources, close to the world’s most famous African desert, ranking The second place in the world, my country is very rich in solar energy resources. If this part of solar energy is used effectively, it will be of great significance to solve energy and environmental problems. At present, there are three main ways to use solar energy, which are converted into electric energy, thermal energy and chemical energy; among them, solar photovoltaic power generation and solar thermal power generation are the most used. Solar photovoltaic power generation has high production costs of polysilicon materials and serious environmental pollution. Solar thermal power generation technology is considered to be one of the most promising technologies for solar power generation. However, due to the shortcomings of intermittent, low energy density, and poor stability of solar energy, it is difficult to meet the demand for continuous energy consumption, and high temperatures will be generated after the sunlight is focused. Therefore, choosing reliable high-temperature heat transfer and heat storage materials will be one of the key technologies to improve the efficiency of solar thermal power generation and solar thermal chemical reaction energy storage.

As an inorganic compound, molten salt has low viscosity, good thermal conductivity, weak corrosiveness, low vapor pressure, wide operating temperature range, and low price, making it the first choice for medium and high temperature heat transfer and heat storage materials. Studies have shown that, compared with high-temperature heat transfer oil, the use of molten salt (nitrate molten salt) can increase the maximum operating temperature of solar power stations to about 500°C and increase the power generation efficiency of steam turbines to 40%.

At present, the most used molten salt for solar thermal power generation is nitrate molten salt as a heat transfer and storage material. It is disclosed that mixed nitrate molten salt (60% KNO ₃ -40wt% NaNO ₃ , Solar Salt) and ternary mixed nitrate molten salt (53 % KNO ₃ -7% NaNO ₃ -40 wt% NaNO ₂ , Hitec). The maximum operating temperature of the mixed nitrate molten salt system is generally 550°C. If tower solar power generation with higher operating temperature and hydrogen production by thermochemical pyrolysis are to be used, problems such as high temperature decomposition and degradation will occur, and molten nitrate salts have very low thermal conductivity, generally 0.4～0.6W/(m· K), under the same thermal power, more heat transfer and heat storage materials are needed, which increases the size of the heat storage system, increases the difficulty of power station operation, and increases the cost.

CN 104109508 A discloses a nitrate system molten salt heat storage material consisting of potassium nitrate and sodium nitrate, the mass ratio of sodium nitrate and potassium nitrate is 30-70:30-70, and the potassium nitrate and sodium nitrate are the ^{Ca 2+, Mg 2+, Cl -} , SO 4 2- ion content of impurities each less than 100ppm, 100ppm, 200ppm and 300ppm. The invention is based on Solar Salt molten salt, and improves the thermal stability of Solar Salt molten salt through purification process and impurity ion control, and extends the service life and operating cost of heat storage materials. However, the molten salt provided by the invention has a low thermal conductivity and a relatively high melting point.

CN 107090273 A discloses a mixed molten salt heat transfer and heat storage working fluid, which is characterized in that it is composed of Ca(NO ₃ ) ₂ , KNO ₃ , NaNO ₃ and LiNO ₃ , and the mass ratio of each component is: Ca( NO ₃ ) ₂ : 8.0wt%; KNO ₃ : 55-60wt%; NaNO ₃ : 7-12wt%; LiNO ₃ : 18-25wt%. The molten salt can be used in the fields of medium and high temperature heat transfer and heat storage, such as solar thermal power generation and energy storage, abandonment of wind and photovoltaic, "coal-to-electricity" heat storage and heating, or industrial waste heat recovery and storage. The thermophysical properties of the mixed molten salt are very stable, there will be no separation of a certain component during use, it has good heat transfer performance in the liquid temperature zone, and the saturated vapor pressure is lower than 2 atmospheres. Since the main raw material of the mixed molten salt is nitrate, its thermal conductivity is low, and there is a risk of decomposition at high temperatures, and the upper limit of use temperature is low.

At present, there is an urgent need for molten salt with the advantages of high operating temperature, lower melting point, and high thermal conductivity to replace the existing nitrate molten salt, so as to improve the power generation efficiency of solar thermal power generation and reduce the operating cost of heat transfer and heat storage.

Summary of the invention

In order to solve the above technical problems, the present invention provides a high thermal conductivity mixed chloride molten salt. The molten salt reduces the melting point, increases the upper limit temperature of the work, and improves the power generation efficiency of solar thermal power generation. At the same time, the molten salt has Larger thermal conductivity, better heat transfer and heat storage performance, can avoid local overheating of the device, reduce the amount of heat transfer and heat storage media, reduce the volume of related transportation and storage equipment, and reduce operating costs.

One of the objectives of the present invention is to provide a mixed chloride molten salt with high thermal conductivity. The raw materials of the molten salt include chloride, metal hydroxide and metal powder.

As a preferred technical solution of the present invention, the chloride includes sodium chloride and/or potassium chloride.

As a preferred technical solution of the present invention, the metal hydroxide is aluminum hydroxide.

Preferably, the metal powder is metal aluminum powder.

In the present invention, the use of aluminum powder can increase the thermal conductivity of the mixed molten salt, thereby improving the heat transfer efficiency and the power generation efficiency.

As a preferred technical solution of the present invention, the raw materials of the molten salt include sodium chloride, potassium chloride, aluminum hydroxide and metal aluminum powder.

The present invention overcomes the problems of low working temperature upper limit and low thermal conductivity of the existing nitrate molten salt. By adding components such as aluminum hydroxide and metal aluminum powder with higher thermal conductivity on the basis of simple sodium chloride and potassium chloride, a new mixed chloride molten salt is formed. Since the melting point of aluminum hydroxide is 300°C, and the melting point of metal aluminum powder is 660°C, both of which are lower than simple inorganic sodium chloride and potassium chloride mono-salts, they can form mixed chloride molten salts with lower melting points and higher working temperature limits. Among them, the thermal conductivity of metal aluminum powder is 270W/(m·K), and a small amount of addition can increase the thermal conductivity of molten salt.

As a preferred technical solution of the present invention, the raw materials of the molten salt in terms of mass fraction include:

Among them, the mass fraction of sodium chloride can be 40%, 45%, 50%, 55%, 60%, 65% or 70%, etc., and the mass fraction of potassium chloride can be 25%, 26%, 28%, 30%. %, 32%, 35%, 38%, 40%, 42% or 45%, etc. The mass fraction of aluminum hydroxide can be 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%, etc. The mass fraction of metal aluminum powder can be 2%, 3%, 4%, 5%, 6% or 7%, etc., but it is not limited to the numerical values listed. The values listed also apply.

In the present invention, the mass fraction of metal aluminum powder in the molten salt is less than 2%, which cannot improve the thermal conductivity of the mixed molten salt, and has little effect on its thermal conductivity; the mass fraction is greater than 7%, and the molten salt causes mixed melting. Salt cannot form a communicative mixture.

As a preferred technical solution of the present invention, the raw materials of the molten salt in terms of mass fraction include:

The second object of the present invention is to provide a method for preparing the above-mentioned molten salt, the preparation method being:

The metal powder is mixed with the chloride and the metal hydroxide to obtain a mixture; the mixture is heated to a molten state for calcination, and the molten salt is obtained after cooling.

As a preferred technical solution of the present invention, the calcination temperature is 780 to 850°C, such as 780°C, 790°C, 800°C, 810°C, 820°C, 830°C, 840°C or 850°C, etc., but is not limited to For the listed values, other unlisted values within the range of values also apply.

In the present invention, the calcination temperature is higher than 850°C, which will cause the molten salt to deteriorate and decompose, and the thermal stability is poor; the calcination temperature is lower than 780°C, which will result in low power generation temperature and low power generation efficiency.

As a preferred technical solution of the present invention, the calcination time is 12-16h, such as 12h, 12.5h, 13h, 13.5h, 14h, 14.5h, 15h, 15.5h or 16h, etc., but not limited to the listed values , Other unlisted values within this value range also apply.

In the preparation method of the present invention, if the particles of the metal powder are too large, the metal powder needs to be ball-milled and refined. The molten salt obtained after calcination and cooling has a relatively large particle size and needs to be crushed and ground into smaller particles for storage and transportation.

The third object of the present invention is to provide an application of the above-mentioned chlorinated modified molten salt, which is used in the field of solar thermal power generation.

Compared with the prior art, the present invention has at least the following beneficial effects:

(1) The mixed chloride molten salt of the present invention can lower the melting point and increase the upper limit operating temperature, improve the power generation efficiency of solar thermal power generation, and reduce the melting point to save operation and insulation costs; at the same time, it also expands the use temperature range of the molten salt and can be applied Solar thermal power generation, solar hydrogen production reaction and industrial waste heat recovery and other fields;

(2) The mixed chloride molten salt of the present invention has higher thermal conductivity, good heat transfer performance, better heat transfer and heat storage effect, and can also avoid local overheating problems of the device.

detailed description

To facilitate the understanding of the present invention, examples of the present invention are listed below. Those skilled in the art should understand that the described embodiments are only to help understand the present invention and should not be regarded as specific limitations to the present invention.

Example 1

This embodiment provides a mixed chloride molten salt with high thermal conductivity. The raw materials of the molten salt in terms of mass fraction include:

According to the above mass fraction, the metal aluminum powder is ball-milled and mixed with sodium chloride, potassium chloride and aluminum hydroxide to obtain a mixture; the mixture is heated to a molten state and calcined at 800°C for 16 hours, and the mixture is obtained after cooling Said molten salt.

Example 2

This embodiment provides a mixed chloride molten salt with high thermal conductivity. The raw materials of the molten salt in terms of mass fraction include:

According to the above mass fraction, the metal aluminum powder is ball-milled and mixed with sodium chloride, potassium chloride and aluminum hydroxide to obtain a mixture; the mixture is heated to a molten state and calcined at 830°C for 12 hours, and the mixture is obtained after cooling Said molten salt.

Example 3

This embodiment provides a mixed chloride molten salt with high thermal conductivity. The raw materials of the molten salt in terms of mass fraction include:

According to the above mass fraction, the metal aluminum powder is ball-milled and mixed with sodium chloride, potassium chloride and aluminum hydroxide to obtain a mixture; the mixture is heated to a molten state and calcined at 800°C for 12 hours, and after cooling, the mixture is obtained Said molten salt.

Example 4

This embodiment provides a mixed chloride molten salt with high thermal conductivity. The raw materials of the molten salt in terms of mass fraction include:

According to the above mass fraction, the metal aluminum powder is ball-milled and mixed with sodium chloride, potassium chloride and aluminum hydroxide to obtain a mixture; the mixture is heated to a molten state and calcined at 820°C for 15 hours, and the mixture is obtained after cooling Said molten salt.

Example 5

This embodiment provides a mixed chloride molten salt with high thermal conductivity. The raw materials of the molten salt in terms of mass fraction include:

According to the above mass fraction, the metal aluminum powder is ball-milled and mixed with sodium chloride, potassium chloride and aluminum hydroxide to obtain a mixture; the mixture is heated to a molten state and calcined at 810°C for 13 hours, and the mixture is obtained after cooling Said molten salt.

Example 6

This embodiment provides a mixed chloride molten salt with high thermal conductivity. The raw materials of the molten salt in terms of mass fraction include:

According to the above mass fraction, the metal aluminum powder is ball-milled and mixed with sodium chloride, potassium chloride and aluminum hydroxide to obtain a mixture; the mixture is heated to a molten state and calcined at 820°C for 14 hours, and the mixture is obtained after cooling Said molten salt.

Example 7

This embodiment provides a mixed chloride molten salt with high thermal conductivity. The raw materials of the molten salt in terms of mass fraction include:

According to the above mass fraction, the metal aluminum powder is ball-milled and mixed with sodium chloride, potassium chloride and aluminum hydroxide to obtain a mixture; the mixture is heated to a molten state and calcined at 820°C for 14 hours, and the mixture is obtained after cooling Said molten salt.

Example 8

This embodiment provides a mixed chloride molten salt with high thermal conductivity. The raw materials of the molten salt in terms of mass fraction include:

According to the above mass fraction, the metal aluminum powder is ball-milled and mixed with sodium chloride, potassium chloride and aluminum hydroxide to obtain a mixture; the mixture is heated to a molten state and calcined at 820°C for 14 hours, and the mixture is obtained after cooling Said molten salt.

Comparative example 1

This embodiment provides a mixed chloride molten salt with high thermal conductivity. The raw materials of the molten salt in terms of mass fraction include:

Sodium chloride 55%;

Potassium chloride 45%;

Sodium chloride, potassium chloride and aluminum hydroxide are mixed to obtain a mixture; the mixture is heated to a molten state and calcined at 820° C. for 14 hours, and the molten salt is obtained after cooling.

Comparative example 2

This embodiment provides a mixed chloride molten salt with high thermal conductivity. The raw materials of the molten salt in terms of mass fraction include:

Sodium chloride 50%;

Potassium chloride 40%;

Metal aluminum powder 10%.

According to the above mass fraction, the metal aluminum powder is ball-milled and mixed with sodium chloride and potassium chloride to obtain a mixture; the mixture is heated to a molten state and calcined at 820° C. for 14 hours, and the molten salt is obtained after cooling.

The melting point, upper working temperature and thermal conductivity of the molten salt provided in Examples 1-8 and Comparative Examples 1 and 2 were tested. The results are shown in Table 1.

Table 1

To	Melting point/℃	Working online temperature/℃	Thermal conductivity/W/(m·K)
Example 1	778	980°C	3.638
Example 2	783	960°C	4.113
Example 3	790	960°C	4.526
Example 4	781	970°C	4.244
Example 5	779	980°C	4.471
Example 6	789	980°C	4.498
Example 7	791	960°C	4.642
Example 8	788	950°C	4.893
Comparative example 1	786	950°C	3.237
Comparative example 2	-(Cannot form a communion)	-	-

It can be seen from the test results in Table 1 that the melting point of the molten salt obtained in Examples 1-8 of the present invention can be as low as 778°C, the upper working temperature can reach 980°C, and the thermal conductivity can reach 4.893W/(m·K) . In Comparative Example 1, no metal hydroxide and metal powder were added, and its thermal conductivity was only 3.237 W/(m·K), while the addition ratio of Comparative Example 2 could not form an eutectic.

The applicant declares that the present invention uses the above embodiments to illustrate the detailed process equipment and process flow of the present invention, but the present invention is not limited to the above detailed process equipment and process flow, which does not mean that the present invention must rely on the above detailed process equipment and process flow. The process can be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent replacement of each raw material of the product of the present invention, the addition of auxiliary components, the selection of specific methods, etc. fall within the scope of protection and disclosure of the present invention.

Claims (10)

1. A high thermal conductivity mixed chloride molten salt, characterized in that the raw materials of the molten salt include chloride, metal hydroxide and metal powder.
2. The molten salt according to claim 1, wherein the chloride includes sodium chloride and or potassium chloride.
3. The molten salt according to claim 1 or 2, wherein the metal hydroxide is aluminum hydroxide;

   Preferably, the metal powder is metal aluminum powder.
4. The molten salt according to claim 2 or 3, wherein the raw materials of the molten salt include sodium chloride, potassium chloride, aluminum hydroxide and metal aluminum powder.
5. The molten salt according to claim 4, wherein the raw material of the molten salt in terms of mass fraction comprises:

   
6. The molten salt according to claim 5, wherein the raw material of the molten salt in terms of mass fraction comprises:

   
7. A method for preparing molten salt according to any one of claims 1 to 6, characterized in that the preparation method is:

   The metal powder is mixed with the chloride and the metal hydroxide to obtain a mixture; the mixture is heated to a molten state for calcination, and the molten salt is obtained after cooling.
8. The preparation method according to claim 7, wherein the calcination temperature is 780-850°C.
9. The preparation method according to claim 7 or 8, wherein the calcination time is 12 to 16 hours.
10. An application of the molten salt according to any one of claims 1-6, wherein the molten salt is used in the field of solar thermal power generation.