RU2654044C2 - Heat-storing salt composition - Google Patents

Abstract

FIELD: heat-and-power engineering.

SUBSTANCE: invention relates to heat power engineering, in particular to a heat-storing salt composition. Heat-storing salt composition (HS) is a system of LiF-LiCl-SrFCl-SrCO₃. Composition provides the serviceability of the mixture at the temperature range from 395 to 1200°C at the following ratio of components, mol%: lithium fluoride - 7.0-7.1; lithium chloride - 66.9-67.0; strontium chloride fluoride - 0.99-1.1; strontium carbonate - 24.95-25.1. Total amount of accumulated energy while heating of the composition up to 800°C is 619.497 kJ/g.

EFFECT: invention provides a heat-storing composition with a serviceability at a wide temperature range, an increased specific heat of fusion and a density with low corrosive activity with respect to structural materials of thermal accumulators.

1 cl, 6 dwg, 5 tbl, 2 ex

Description

APPLICATION AREA

The invention relates to a power system, in particular to the development of heat storage compositions and mixtures, including halides and carbonates of alkali and alkaline earth metals, which are used in thermal batteries.

When designing a phase-transition heat accumulator, the primary task is to search for heat-storage materials, the properties of which have certain requirements. The choice of such materials is dictated, first of all, by the level of the working temperature of thermal accumulation and the amount of thermal energy that must be accumulated. Therefore, such properties of heat storage materials as the melting temperature and the specific enthalpy of the phase transition are of particular importance. The melting temperature of the material should correspond to the operating temperature of the battery, at which there is an accumulation and release of thermal energy. The volume of the material used is associated with the value of the specific enthalpy of the phase transition, and, consequently, the compactness and cost of the heat accumulator. In addition to the set melting temperature and high specific enthalpy of the phase transition, the main criteria for choosing heat storage materials are: high specific heat, high thermal conductivity, high density, low viscosity in the liquid phase, thermochemical stability in the operating temperature range, low coefficient of thermal expansion, low vapor pressure, prevalence in nature, minimal cost, safety of use, ease of absorption and release of heat, stability of thermophysical x properties under repeated cycles, no toxicity and no aggressiveness with respect to the construction materials [2].

Known heat storage mixture of lithium chloride and strontium carbonate [3], proposed by us.

To improve its thermophysical and thermodynamic characteristics, we continued the study of the ISS, since the salt composition used in heat accumulators operates at high temperatures (from 412 ° C), has relatively low density, heat capacity and, as a result, less total accumulated energy.

Prototype. Closest to the proposed, in technical essence and the achieved result is a heat-accumulating salt mixture, including lithium chloride and strontium carbonate in mol. %:

lithium chloride - 72.5-73.1;

strontium carbonate - 26.9-27.5.

The melting point of the mixture is 412 ° C, the specific heat of melting is 326.45 J / g

To improve the technological, chemical and thermophysical characteristics of this heat-accumulating mixture, in particular, decrease the melting temperature of the composition, increase the density and specific heat of fusion, we continued the study of the ISS.

The purpose of the invention is an increase in specific heat, a decrease in corrosion activity, an increase in density and ensuring the operability of a heat storage composition in the range of 395-1200 ° C.

This goal is achieved by introducing energy-intensive components of lithium fluoride and the SrFCl compound, which has reduced corrosion activity and a stabilizing effect.

The heat storage mixture includes lithium fluoride and chloride, strontium chloride fluoride and strontium carbonate in the following ratio, mol. %:

lithium fluoride 7.0-7.1; lithium chloride 66.9-67.0; strontium chloride fluoride 0.99-1.1; strontium carbonate 24.95-25.1.

The proposed salt mixture was investigated on the installation of a synchronous thermal analyzer, modification STA 409RS (thermal analyzer).

During an experimental study in this four-component system, one invariant point of the eutectic nature of melting was revealed, containing, mol. %: 7 LiF, 67 LiCl, 1 SrFCl, 25 SrCO ₃ and melting at a temperature of 395 ° C (Fig. 1).

Our salt composition is characterized in that, this heat storage mixture in comparison with the prototype has a higher specific heat of fusion, high density and has a reduced operating temperature of 15-17 ° C, compared with the analogue, which leads to lower energy costs and increase the efficiency of TA .

To assess the total amount of heat accumulated, we studied the density of the heat storage mixture we are offering, since an increase in the density of the material entails an increase in the specific heat, therefore, and the amount of heat accumulated in the volume of the material. The density of the material in the liquid phase changes stepwise during melting and decreases linearly with increasing melt temperature, which is accompanied by an increase in the volume of the melt by 10-30%. Therefore, when designing a phase-transition heat accumulator, a certain free volume is usually provided in it, based on the experimental values of the density of the heat-accumulating material at the maximum working temperature.

The temperature dependence of the density of molten mixtures was measured by hydrostatic weighing of a platinum ball on a VLR balance with an accuracy of 0.01 g. A float was a platinum ball weighing 37.44, which was suspended on a thin platinum thread to one arm of an analytical balance beam and weighed first in air (weight M ₁ ). Then the float was immersed in a crucible with molten salt and re-weighed (weight M ₂ ). The volume of the ball depending on the temperature was found by weighing it in the melt of the eutectic mixture (from 405 to 620 ° C) and pure lithium chloride (from 620 to 800 ° C) in the range of 422-800 ° C. In the calculations, the melt densities of this salt from [4] were used. Knowing the volume of the float (V _t ) at the appropriate temperature, the density of the molten mixture was determined, namely: D _t = M ₁ -M ₂ / V _t g / cm ³ .

The temperature of the molten mixture was measured using calibrated platinum-platinum-rhodium (10% rhodium) thermocouples and maintained with an accuracy of ± 1K. Cold junctions of thermocouples were thermostated at 0 ° C in a Dewar vessel filled with a mixture of water and ice. Millivolts were converted to degrees on a scale of translation taken from [5]. The test samples, weighing 20 g, were placed in corundum crucibles.

To exclude the effect of oxygen and air moisture on melts containing hygroscopic lithium chloride, measurements were carried out in an argon atmosphere. For research used salt brand "h.ch." The reliability of the method was verified by direct determinations of the densities of the starting salts. A schematic diagram of an apparatus for measuring density is given in FIG. 2.

The components of the installation are indicated as follows:

1. Measuring thermocouple 7. Cylinder with argon 2. Platinum ball 8. The device combined digital Shch-300 3. The melt 4. Heating furnace 9. Dewar vessel 5. Microscrews 10. Scales of VLR 6. Tripod 11. Autotransformer

For an experimental study of density polytherms, we took the composition of our heat storage mixture. For the density measurement compounds started at T _m + 10 K and was completed at temperatures of 1073K.

To study this heat storage mixture, we used the installation of a synchronous thermal analyzer, modification STA 409RS (thermal analyzer) (Fig. 3), manufactured by the German company NETZSCH and designed to measure the thermodynamic characteristics (temperature and enthalpy of phase transitions, heat capacity) and detect changes in the mass of solid and powder materials in a wide temperature range from + 25 ° C to + 1500 ° C.

The studies were carried out with a heating and cooling rate of 5 ° C per minute, in a helium atmosphere and in platinum crucibles.

With increasing temperature from 683 to 1073K, the density of the molten salt composition LiF-LiCl-SrFCl-SrCO ₃ decreases by 6.1382% (Fig. 4). The volume of the composition increases by 0.4035 cm ³ . When recalculated to one ton, this heat-accumulating salt composition at a melting point (395 ° C) will occupy a volume equal to 308450 cm ³ or 3.0845 m ³ . When the operating temperature rises to 800 ° C, this salt composition will occupy a volume of 328625 cm ³ or 3.28625 m ³ (Fig. 4).

On the graph of dependence (Fig. 4) for clarity, the dependence of density on temperature and for the prototype is given, the equations of linear dependence and approximation of reliability are also shown there. Analysis of experimental data (tab. 1 and Fig. 4) show that this salt composition has a higher density and slight volume expansion compared to the prototype.

In addition, to determine the corrosion activity of this salt composition on structural materials of heat accumulators, we determined the average corrosion rate of various steel grades from which TA tanks are made in a temporary mode (the experiment is as close as possible to the real conditions of TA operation).

,The corrosion rate was studied by the gravimetric method, the essence of which is to weigh the test sample before and after the experiment. The test sample at an appropriate temperature was kept for a long time from 50 to 200 hours in a eutectic molten mixture. To create the temperature regime, a laboratory MPL-6 mini-electric furnace was used with a TP-400 temperature regulator, which is intended for automatic temperature control according to the proportional-integral-differential (PID) law or for on-off regulation. The step of setting the temperature is 1 ° C, the dead zone is 0.5 ° C. After the set time, the alloy was quenched, quickly cooled. Hardened samples with different quantitative ratios of the starting components were subjected to etching. Etching was carried out with acid to clean samples from corrosion products. Salt melt was analyzed chemically for the content of the main alloying components of steel. The corrosion rate was determined by the formula:

,

where: m is the mass difference of the samples before and after the experiment, g; S is the surface area of the samples, cm ² ; t - time, h

Stainless steels under conditions of increased corrosion activity in contact with oxidizing media are highly prone to passivation, and this attracts attention for use as structural materials and in halide-carbonate melts of alkali and alkaline earth metals. Steels containing alloying elements such as chromium, nickel, titanium are widely used as structural materials of containers for molten salts. Therefore, we selected the following grades of steel for corrosion studies: 14Х17Н2Т and 12Х18Н10Т, the chemical composition of which is presented in table 2.

As samples, plates of sheet steel of various grades were used, which were weighed on an analytical balance before the experiment. For the preparation of salt mixtures, reagents of the "hch" qualification were used.

The purpose of this study is to identify the corrosive behavior of steels of grades 14X14H2T, 12X18H10T in a melt of a chloride - carbonate system of alkali and alkaline earth metals, depending on the exposure time in the atmosphere. We carried out kinetic tests in the melt of a four-component eutectic of the composition (Table 3) in platinum crucibles.

The exposure time was 50-100-150-200 hours, at a temperature of 450 ° C (i.e., at the melting temperature of the mixture plus 37-38 ° C). According to experimental data, the dependence of corrosion losses on the exposure time of samples in the melt was calculated (Table 4), which well reflects their behavior, depending on time.

An analysis of the results suggests that 14Kh17N2T steel is characterized by an increase in the corrosion rate at a holding time of 50-100 h - one maximum is observed.

Further, the corrosion rate decreases (Fig. 5).

For the corrosion rate of 12X18H10T steel, a maximum at 100 hours and a minimum at 150 hours exposure were noted (Fig. 6).

Composing the physicochemical properties of the metals of the iron family, it can be assumed that the increase in corrosion resistance in the Fe-Cr-Ni series in the melt (Table 5) is obviously due to a decrease in the diffusion coefficient D of the cations of these metals, their atomic volume V, and the solubility of the oxides of these metals - products of the corrosion process, as well as raising the value of the ionization potential of the second order of I ₂ atomic elements.

One of the causes of corrosion of 12Kh18N10T steel in the melt is the presence of oxidizing agents. The components of the atmosphere, impurity cations contained in the electrolyte, and also structural materials in contact with the melt can act as oxidizing agents. In the presence of oxygen on the surface of chromium-containing steels, chemical interaction of chromium oxide with sodium chloride is possible: Cr ₂ O ₃ + 4NaCl + 5 / 2O ₂ = 2Na ₂ CrO ₄ + 2Cl ₂ ; Cr ₂ O ₃ + 2NaCl + 2O ₂ = Na ₂ Cr ₂ O ₇ + Cl ₂ . In the future, the formed chlorine and chromate ions themselves act as oxidizing agents, intensifying the corrosion of steel. Chlorine in molten chlorides of alkali and alkaline earth metals can also be formed by the exchange reaction:

1 / 2O _{2 (gas)} + Cl ^- _(dec) = 1 / 2Cl _{2 (dec)} + O ^2- _(dec)

However, the presence in the melt of cations of polyvalent metals, such as Fe ²⁺ , Cr ²⁺ , Ti ³⁺ , which form sparingly soluble oxides, shifts the equilibrium towards the formation of free chlorine, which can subsequently act as a direct oxidizing agent [6].

In this case, intergranular corrosion (MCC) can be neglected, since austenitic steels become prone to MSC after prolonged heating in the range of 500-850 ° C.

An experimental study of the corrosion of steel grades (14X14H2T and 12X18H10T) in the eutectic melt of the salt composition LiF-LiCl-SrFCl-SrCO ₃ and their analysis.

An analysis of the results suggests that 14Kh17N2T steel is characterized by an increase in the corrosion rate at a holding time of 50-100 hours - one maximum is observed (Table 4). Further, the corrosion rate decreases (Fig. 5).

For the corrosion rate of steel 12X18H10T, a maximum at 100 hours exposure and a minimum at 150 hours exposure were noted (Fig. 6).

The analysis of experimental data shows that the corrosion rate (corrosion activity) in our salt composition is insignificant and after 150 hours of operation is practically reduced to zero values.

To determine the cost of 1 g of the heat storage mixture of the prototype and the heat storage mixture we offer, we carried out the calculation taking into account the prices of the reagents of Chemist LLC (St. Petersburg, updated July 14, 2015). One gram of the heat storage mixture we offer costs 0.5396 rubles, while a gram of the prototype costs 0.5434 rubles. Thus, compared with the prototype, we offer a heat storage mixture, has a reduced cost of 1.01 times.

Example. 1. To obtain 1 g of the heat-accumulating mixture, anhydrous salts of the grade of chemical grade are melted in a shaft-type electric furnace. in amount, g: lithium fluoride 0.0268 (7.0 mol%), lithium chloride 0.4086 (67.0 mol%), strontium chloride fluoride 0.0209 (1.05 mol%), strontium carbonate 0 5437 (24.95 mol%). The melting point of the mixture is 395 ° C, and the specific heat of fusion is 356.58 J / g.

Example. 2. Anhydrous salts of qualification x are melted in an electric furnace. including in an amount of g: lithium fluoride 0.0273 (7.01 mol%), lithium chloride 0.4080 (66.9 mol%), strontium chloride fluoride 0.0189 (0.99 mol%), strontium carbonate 0.5458 (25.1 mol%). The melting point of the mixture is 398 ° C, and the specific heat of fusion is 352.23 J / g.

An increase or decrease in the concentration of the components does not allow to obtain the desired melting point.

The results obtained and the calculation of economic efficiency show that the proposed heat storage mixture has a reduced operating temperature of 15 °; value 1.01 times; the cost of generated energy is 1.23 times.

Claim

The heat-accumulating salt composition, including lithium chloride and fluoride, strontium chloride fluoride, and strontium carbonate, has insignificant corrosion activity, a higher density, a high specific heat of fusion and ensures the operability of the mixture in the temperature range from 395 to 1200 ° C with the following ratio of components in mol. %:

lithium fluoride 7.0-7.1; lithium chloride 66.9-67.0; strontium chloride fluoride 0.99-1.1; strontium carbonate 24.95-25.1.

Literature

1. Gamataeva B.Yu. Physico-chemical interaction in multicomponent systems containing alkali and alkaline earth metal salts. Development of heat storage materials. Diss. ... doctor of chemical sciences - M .: IONH, 2002 .-- 317 p.

2. Hasanaliev A.M., Gamataeva B.Yu. Heat storage properties of melts. / Advances in Chemistry, 2000, T. 69, No. 2, S. 192-200.

3. A.M. Hasanaliev, B.Yu. Gamataeva A.I. Rasulov, M.T. Tagzirov. Low melting heat storage salt mixture, No. 2524959 Russia, IPC S09K 5/06 (2006.01); declared 03/20/2012; publ. 08/10/2014, Bull. Number 22

4. Handbook of molten salts // Ed. Morachevsky A.G., - L .: Chemistry, 1971, T. 1, 357 p.

5. Bunin P.P., Dzhannet X.A. Workshop on solid state physics. - DSC, Makhachkala, 1969 .-- 260 p.

6. A.V. Abramov, I.B. Polovov, V.A. Volkovich, O.I. Rebrin, D.G. Lisienko. Spectroelectrochemical study of corrosion processes of stainless steels in the melt of an equimolar mixture of NaCl-KCl // Rasplavy, 2011, P. 71-80.

TABLES

Density Polytherms for Melt System

LiF-LiCl-SrFCl-SrCO ₃

Claims (2)

The heat-accumulating salt composition, including lithium chloride and lithium fluoride, strontium chloride and strontium carbonate, ensuring the efficiency of the mixture in the temperature range from 395 to 1200 ° C in the following ratio of components, mol. %: lithium fluoride                        7.0-7.1 lithium chloride            66.9-67.0 strontium chloride fluoride 0.99-1.1 strontium carbonate             24.95-25.1

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