Calcium-based composite heat storage material based on calcium zirconate doping and preparation method thereof

Abstract

The invention discloses a calcium-based composite heat storage material based on calcium zirconate doping, Ca (OH) 2 / CaO is used as a matrix, and CaZrO3 is doped in the matrix; the invention further discloses a preparation method of the calcium-based composite heat storage material based on calcium zirconate doping, a three-dimensional interpenetrating network structure is constructed by regulating and controlling the molar ratio of calcium to zirconium, and full contact with the outside in the reaction process is guaranteed by arranging an organic ligand; calcium zirconate is adopted as an inert skeleton to improve the local sintering agglomeration problem of the calcium hydroxide / calcium oxide reaction system heat storage material in the repeated heat charging and discharging cycle process, and the synergistic effect of calcium zirconate, calcium zirconate and calcium oxide improves the high-temperature degradation resistance and the cycle service life of the material. A segmented pyrolysis process is also arranged, so that the hydration reaction fraction loss is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of heat storage materials, and particularly relates to a calcium-based composite heat storage material doped with calcium zirconate and a preparation method thereof. Background Art

[0002] The energy problem is the main source of carbon emissions. Therefore, on the basis of ensuring energy security, renewable energy should be vigorously developed, and a low-carbon, clean, safe and efficient new energy system mainly composed of wind energy, water energy, geothermal energy, tidal energy and solar energy should be accelerated. Solar thermal power generation has become one of the key technologies that can be expected. Its characteristics of large-scale energy storage, independent coordination and local peak shaving can fill the gap for volatile renewable energy, and the solar thermal power generation coupled with large-scale heat storage has stable power output.

[0003] Thermal energy storage is an advanced technology that can reduce the environmental impact brought by fossil energy and promote the development of a more efficient and clean energy system. Compared with traditional energy storage systems, thermochemical energy storage systems have the advantages of large energy storage density, high heat output temperature, and can store heat without loss at ambient temperature for a long time. Therefore, chemical energy storage, as a new form of energy storage, has received more and more attention and exploration. Among them, due to the advantages of wide availability, low cost and good temperature range of Ca(OH)2, the thermochemical energy storage method based on the reversible decomposition reaction of Ca(OH)2 has attracted extensive attention and research. A large number of studies have found that fast and effective reaction kinetics, high reaction enthalpy (99.5 kJ / mol) and adjustable temperature range (410 - 600 °C) make the Ca(OH)2 / CaO system have great application prospects in CSP power plants.

[0004] However, in its practical application, repeated charge and discharge cycles cause sintering of calcium-based materials. Particle sintering or aggregation will pose a physical obstacle to the contact between gas and solid, resulting in slower reactions, and ultimately the loss of chemical activity after long-term cycling, causing the decline of its heat storage performance. Moreover, the dilution effect of inert additives on active components will also lead to a decrease in the initial energy storage density. Summary of the Invention

[0005] In order to overcome the above deficiencies of the prior art, the purpose of the present invention is to provide a calcium-based composite heat storage material doped with calcium zirconate to improve the stability and cycle service life of the material.

[0006] Another object of the present invention is to provide a preparation method of a calcium-based composite heat storage material doped with calcium zirconate.

[0007] The object of the present invention is achieved by the following technical solutions: A calcium-based composite heat storage material doped with calcium zirconate, with Ca(OH)2 / CaO as the matrix and CaZrO3 doped in the matrix.

[0008] Preferably, by weight percentage, it includes: Ca(OH)2 / CaO matrix 70 - 92; CaZrO3 doped phase 8 - 30.

[0009] A preparation method of a calcium-based composite heat storage material doped with calcium zirconate, comprising the following steps: (1), Dissolve the calcium source and zirconium source in deionized water, stir until completely dissolved to form a precursor solution; (2), Add an organic ligand to carry out a complexation reaction to form a stable sol; (3), Carry out the first-stage low-temperature calcination on the sol, with a heating rate of 3 - 8 °C / min, a calcination temperature of 270 - 400 °C, and a heat preservation time of 1 - 3 hours, and pyrolyze to generate the CaZrO3 doped phase and a porous structure; (4), Then carry out high-temperature calcination on the pre-calcined product, with a heating rate of 10 - 20 °C / min, a calcination temperature of 900 - 1100 °C to form a calcium zirconium oxide solid solution, and a heat preservation time of 1 - 3 hours; (5), Finally, activate with water vapor to construct a Ca(OH)2 / CaO biphasic structure.

[0010] Preferably, the calcium source is calcium acetate, the zirconium source is zirconium acetate, and the organic ligand is a hydroxycarboxylic acid compound.

[0011] Preferably, the hydroxycarboxylic acid compound includes one of D-gluconic acid solution or citric acid monohydrate.

[0012] Preferably, the molar ratio of calcium acetate to zirconium acetate is 100:3 - 12.

[0013] Preferably, the molar ratio of the hydroxycarboxylic acid compound to metal ions is 1 - 3:2.

[0014] Preferably, in step (1), the complexation reaction temperature is controlled at 40 - 80 °C, and the reaction time is 3 - 6 hours.

[0015] Preferably, the specific steps of the water vapor activation in step (3) are as follows: Place the obtained medicine in a crucible as a fixed bed, preheat it to 120 °C - 150 °C in a reaction kettle, and introduce water vapor with a relative humidity ≥ 75% into it, and the medicine is hydrated and activated to form a CaO / Ca(OH)2 biphasic matrix.

[0016] The present invention has the following advantages and beneficial effects compared with the prior art: (1). The calcium-based composite heat storage material prepared by the present invention adjusts the thermal decomposition kinetics of the precursor by setting the molar ratio of calcium zirconate, balances the relationship between its heat storage density and cycle stability, and constructs a three-dimensional interpenetrating network structure by using its phase change expansion effect (volume expansion rate > 200%); the gas generated during the decomposition of the organic ligand punches holes in the composite material, ensuring that the composite material has a fluffy structure and a large specific surface area, ensuring sufficient contact with the outside world during the reaction. The specific surface area of the prepared porous structure is 15 - 25 m 2 ·g -1 、The pore volume is 0.050 - 0.110 cm 3 ·g -1 。The dopant acts as an inert skeleton to ensure that the porous structure is not easily collapsed, effectively reducing the grain migration rate of the calcium oxide matrix, thereby improving the local sintering and agglomeration problems generated in the heat storage material of the calcium hydroxide / calcium oxide reaction system during repeated charge and discharge cycles. The synergistic effect of the three enhances the high-temperature deterioration resistance and cycle service life of the material.

[0017] (2). The in-situ generated calcium zirconate phase exhibits chemical inertness characteristics in a high-temperature and high-pressure system by virtue of its high Tamman temperature characteristics. As a structurally stable phase, it can not only maintain the integrity of the calcium-based skeleton but also avoid the dilution effect of traditional inert additives on the active components. The dual protection mechanism enables the composite material to obtain significantly improved anti-sintering performance while maintaining a high heat storage density and a long cycle life, and can play the role of a structurally stable phase under high-temperature cycling conditions. The calcium zirconate nanocrystals are uniformly distributed in the CaO matrix, inhibiting the abnormal growth of Ca(OH)2 grains. This microscopic structure regulation enables the material to still maintain a pore volume decay rate < 20% after 20 thermal cycles, effectively solving the problem of pore channel collapse during high-temperature calcination.

[0018] (3). The present invention's segmented pyrolysis process causes the material to expand at low temperature in the first stage of pyrolysis, and then randomly removes carbon at high temperature. Through secondary pore formation, the material forms a hierarchical porous network with micro-mesopore synergy, enabling the CaZrO3 doped phase to be uniformly distributed in the CaO / Ca(OH)2 matrix, thereby reducing the loss of the hydration reaction fraction.

[0019] (4). The present invention adopts a metal carboxylate system, and by optimizing the proportion of the organic ligand and the doping proportion of the inert substance, a porous calcium-based composite heat storage material with high heat storage density and high cycle stability is prepared. Using a hydroxycarboxylic acid compound as a ligand, a stable sol is formed at a low temperature of 40 - 80 °C, with low energy consumption and easy industrial scale-up. By adjusting the molar ratio of calcium acetate and gluconic acid, the best initial adsorption performance can be obtained. Too little gluconic acid cannot protect the expansion characteristics of the raw material calcium acetate from being easily affected by acetate ions, while too much gluconic acid leads to precipitation during constant-temperature stirring, resulting in uneven mixing solutions.

[0020] (5) The calcium-based composite heat storage material prepared by the present invention is applicable to high-temperature scenarios such as solar thermal storage and industrial waste heat recovery. It has better reaction stability, reaction uniformity and faster reaction rate. At the same time, it is convenient to prepare, the reaction process is green and pollution-free, and it has stronger applicability. When the charging and discharging temperature difference exceeds 300 °C, it still maintains high-efficiency heat storage / discharge capacity, meeting the intermittent energy supply requirements of fluctuating renewable energy. Description of the Drawings

[0021] Figure 1 It is the X-ray diffraction analysis diagram of the heat storage materials prepared in each example.

[0022] Figure 2 It is the curve diagram of the hydration reaction fraction in 20 cycles of the heat storage materials prepared in each example.

[0023] Figure 3 It is the scanning electron microscope surface morphology diagram after 20 repeated cycles of charging and discharging of CaCG-100-6.

[0024] Figure 4 It is the scanning electron microscope surface morphology diagram after 20 repeated cycles of charging and discharging of Ca100. Detailed Embodiments

[0025] The invention object of the present invention will be further described in detail below in conjunction with the drawings and specific embodiments. The embodiments cannot be elaborated one by one here, but the embodiments of the present invention are not limited to the following embodiments.

[0026] Example 1 The calcium-based composite heat storage material doped with calcium zirconate provided in this embodiment is obtained by the sol-gel method Step S1: Using calcium acetate (analytical pure) as the calcium source and zirconium acetate (analytical pure) as the zirconium source, the two are accurately weighed according to the calcium-zirconium molar ratio of 100:6 (0.04 mol: 0.0024 mol).

[0027] Step S2: Dissolve the above raw materials in 50 ml of deionized water, and stir at a constant temperature of 60 °C until completely dissolved to form a homogeneous precursor solution.

[0028] Step S3: Dropwise add 0.02 mol of D-gluconic acid solution (low cost, water-based system) to the mixed solution, and continuously stir in a water bath at a constant temperature of 80 °C for 8 hours to form a stable sol. Control the addition rate of gluconic acid by constant-rate titration to ensure sufficient complexation of metal ions and ligands, and avoid agglomeration caused by local over-concentration.

[0029] Step S4: Primary drying, transfer the gel to a porcelain boat, and dry it in an oven at 80 °C for 12 hours to remove free moisture.

[0030] Step S5: Low-temperature calcination. Heat up to 270 °C (at a rate of 5 °C / min), hold for 2 hours, to promote the decomposition of acetate and organic ligands and release gases (CO2, H2O), causing the material to expand violently to form a primary porous framework.

[0031] Step S6: High-temperature carbon removal. Transfer the pre-calcined product to a muffle furnace, heat up to 1000 °C at a rate of 15 °C / min and calcine for 1 hour. The residual carbon particles are oxidized to form CO2, further etching the pores and increasing the specific surface area (23.9 m² / g) and porosity (0.109 cm³ / g).

[0032] Step S7: After natural cooling, sieve (30 mesh) to obtain a composite heat storage material with a calcium / zirconium molar ratio of 100:6.

[0033] Step S8: Place the obtained medicine in a 50 ml crucible as a fixed bed, preheat it to 120 °C in a 1000 ml reaction kettle, and introduce water vapor with a relative humidity ≥ 75% for 1 hour. The medicine is hydrated and activated to form a CaO / Ca(OH)2 biphasic matrix, which is named CaCG-100-6.

[0034] Example 2 In this example, except for the following different technical features, other technical features are the same as those in Example 1: The difference is that in step S1, calcium acetate and zirconium acetate are accurately weighed according to a molar ratio of 100 / 3 (0.04 mol:0.0012 mol); And in step S3, the hydroxycarboxylic acid is replaced with citric acid monohydrate. Add 0.04 mol of citric acid solution (pre-prepared, 0.04 mol of citric acid dissolved in 50 ml of deionized water) dropwise to the mixed solution, and continuously stir in a constant temperature water bath at 80 °C for 8 hours to form a stable sol.

[0035] The CaO / Ca(OH)2 biphasic matrix prepared in this example is named CaCCi-100-3.

[0036] Comparative Example 1 Step S1: Provide calcium hydroxide as the precursor of the calcium-based composite heat storage material; Step S2: Directly put 0.04 mol of calcium hydroxide into a muffle furnace and calcine at 1000 °C to obtain pure calcium oxide as Comparative Example 1, named Ca100.

[0037] Table 1

[0038] Table 1 shows the specific surface area (BET) of the fresh and cycled thermal storage materials prepared in each embodiment. It can be seen from Table 1 that the specific surface area of ​​the composite material decreases with the increase of the doping amount of calcium zirconate. In the initial cycle, Ca100 has a higher reaction fraction, and after 20 cycles, the specific surface area of ​​Ca100 decays to 4.4992 m 2 •g -1 Although the specific surface area of ​​the composite materials doped with calcium zirconate decayed to varying degrees after cycling, CaCG-100-6 retained good water absorption performance, and its specific surface area was still as high as 21.3628 m2·g -1 .

[0039] Figure 1 The X-ray diffraction analysis diagram of the heat storage material prepared in each embodiment is shown in FIG. Figure 2 It can be seen that the prepared material has a high-purity phase. In addition to the characteristic peak of the main component calcium oxide in the spectrum, the characteristic diffraction peak of calcium zirconate shows significant intensity. The in-situ generated calcium zirconate phase can play a role as a structurally stable phase under high-temperature cycling conditions due to its high Tammann temperature characteristics, effectively reducing the grain migration rate of the calcium oxide matrix, thereby inhibiting the sintering of the material. By comparing the XRD spectra of different doping systems, it was found that the characteristic peak positions of each sample were highly consistent with the standard calcium oxide spectrum, and no out-of-phase diffraction signals were detected, which not only verified the parameter controllability of the preparation process, but also showed that the method has excellent inter-group repeatability and chemical regulation accuracy.

[0040] Figure 2 is a curve diagram of the hydration reaction fraction in 20 cycles of the heat storage material prepared in each embodiment, Figure 2 It can be seen that as the number of cycles increases, the reaction fraction of the material gradually decreases, and the greater the slope, the greater the loss of the reaction fraction. After 20 cycles, the reaction fraction loss of CaCCi-100-3 and CaCG-100-6 (Example 1-2) is smaller than that of Ca100 (Comparative Example 1), and the composite material with the smallest loss is CaCCi-100-3, whose hydration reaction fraction loss is only 5.14%. The largest loss of hydration reaction fraction is Ca100. As the number of cycles increases, Ca100 gradually sinters, causing the reaction fraction of Ca100 to drop rapidly. The composite material prepared in Example 1-2 is subjected to a secondary pore-forming process (low-temperature expansion + high-temperature decarbonization), and the material forms a hierarchical porous network (micropore-mesopore synergy), and the CaZrO3 doped phase (≈12wt%) is evenly distributed in the CaO / Ca(OH)2 matrix, thereby reducing the loss of hydration reaction fraction.

[0041] Figure 3 This is the SEM surface morphology of CaCG-100-6 after 20 cycles of charging and discharging. Figure 3It can be seen that the composite material after cycling presents a relatively porous microstructure with abundant nanoscale grains distributed on the surface. It can be clearly observed that there are a large number of nanoscale zirconium-based stabilizer grains between the large-sized calcium oxide grains, which, as spacers, effectively inhibit the growth of calcium oxide grains. Thus, it can be seen that in the calcium-based composite heat storage material doped with calcium zirconate, the cyclic stability of the calcium-based composite heat storage material is effectively improved.

[0042] Figure 4 Figure 4 shows the SEM surface morphology of Ca100 after 20 repeated cycles of charging and discharging. Figure 4 It can be seen that after 20 cycles, the surface of Ca100 is severely sintered, and the agglomeration of calcium oxide particles blocks the pores.

[0043] The above specific embodiments are the preferred embodiments of the present invention and cannot limit the present invention. Any other changes or other equivalent replacement methods made without departing from the technical solution of the present invention are included in the protection scope of the present invention.

Claims

1. A calcium-based composite heat storage material doped with calcium zirconate, characterized in that: Ca(OH)2 / CaO is used as the matrix, and CaZrO3 is doped into the matrix.

2. The calcium-based composite heat storage material doped with calcium zirconate according to claim 1, characterized in that: Included by weight percentage: Ca(OH)2 / CaO matrix 70-92; CaZrO3 doped phase 8-30.

3. The method for preparing the calcium-based composite heat storage material doped with calcium zirconate according to claim 1, characterized in that: The following steps are involved: (1) Dissolve the calcium source and zirconium source in deionized water and stir until completely dissolved to form a precursor solution; (2) Adding organic ligands to carry out complexation reaction to form a stable sol; (3) subjecting the sol to a first stage of low-temperature calcination, with a heating rate of 3-8°C / min, a calcination temperature of 270-400°C, and a holding time of 1-3 hours, to pyrolyze and generate a CaZrO3 doped phase and a porous structure; (4) calcining the pre-calcined product at a high temperature, with a heating rate of 10-20°C / min, a calcination temperature of 900-1100°C to form a calcium zirconium oxide solid solution, and a holding time of 1-3 hours; (5) Finally, the Ca(OH)2 / CaO dual-phase structure is constructed by water vapor activation.

4. The method for preparing a calcium-based composite heat storage material based on calcium zirconate doping according to claim 3, characterized in that: The calcium source is calcium acetate, the zirconium source is zirconium acetate, and the organic ligand is a hydroxyl-containing carboxylic acid compound.

5. The method for preparing a calcium-based composite heat storage material doped with calcium zirconate according to claim 3, characterized in that: The hydroxyl carboxylic acid compound includes one of D-gluconic acid solution or citric acid monohydrate.

6. The method for preparing a calcium-based composite heat storage material based on calcium zirconate doping according to claim 4, characterized in that: The molar ratio of the calcium acetate to the zirconium acetate is 100:3-12.

7. The method for preparing a calcium-based composite heat storage material based on calcium zirconate doping according to claim 4, characterized in that: The molar ratio of the hydroxyl carboxylic acid compound to the metal ion is 1-3:

2.

8. The method for preparing a calcium-based composite heat storage material doped with calcium zirconate according to claim 3, characterized in that: In step (1), the complexation reaction temperature is controlled at 40-80°C, and the reaction time is 3-6 hours.

9. The method for preparing calcium-based composite heat storage material based on calcium zirconate doping according to claim 3, characterized in that: The specific steps of water vapor activation in step (3) are as follows: The obtained drug is placed in a crucible as a fixed bed, preheated to 120°C-150°C in a reactor, and water vapor with a relative humidity of ≥75% is introduced therein to hydrate and activate the drug to form a CaO / Ca(OH)2 dual-phase matrix.

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