Millimeter-scale heat storage particles with a hollow structure and a method for preparing the same

Abstract

The application discloses a kind of millimeter heat storage particles with hollow structure and preparation method thereof, the particle is millimeter size, it has hollow structure in macroscopically, it has micron-nanometer step hole in microcosm, so compared with the millimeter uniform structure particle prepared by traditional extrusion-rounding method has better mass transfer capacity, its reaction rate is obviously promoted.The application is prepared by first preparing core particle that can be completely decomposed in air, then the outer layer of the core particle is coated with a certain thickness of heat storage material, and then the mother core material such as microcrystalline cellulose / polystyrene is removed by calcination to obtain the heat storage particle with hollow shell structure.

Description

Technical Field

[0001] This invention belongs to the field of solar thermochemical storage technology, specifically relating to a millimeter-sized heat storage particle with a hollow structure and its preparation method. Background Technology

[0002] The extensive use of fossil fuels has led to a continuous rise in atmospheric carbon dioxide concentration, exacerbating the greenhouse effect and causing sea-level rise. Vigorously developing renewable energy sources such as solar and wind power can effectively reduce the proportion of fossil fuels and is considered an effective means of achieving carbon neutrality. However, solar energy has inherent drawbacks such as instability and discontinuity. Therefore, in solar energy utilization fields such as concentrated solar thermal power generation, thermal storage systems are usually used. These systems convert solar energy into heat energy and store it when solar irradiance is sufficient, and release the heat for utilization when solar irradiance is insufficient, such as on cloudy days or at night. Among various thermal storage technologies, new thermal storage technologies, represented by calcium carbonate thermochemical thermal storage technology, have advantages such as high thermal density, high operating temperature, low heat loss during long-term energy storage, and large material reserves. They are considered to be a highly promising thermal storage method for next-generation solar thermal power generation technology.

[0003] In recent years, researchers have conducted numerous studies on calcium carbonate-based thermochemical thermal storage. In terms of materials development, research has mainly focused on improving the solar spectral absorption capacity and thermal storage / release cycle stability of calcium carbonate materials through methods such as elemental doping. However, the chemical reaction rate in the thermal storage / release process of calcium carbonate materials is slow, which leads to low efficiency of the thermal storage system. Therefore, it is urgent to achieve a synergistic improvement in the thermal storage density, solar spectral absorption rate, thermal storage / release cycle stability, and fast reaction rate of calcium carbonate thermochemical thermal storage materials. Summary of the Invention

[0004] Objectives of the invention: One objective of the present invention is to provide a millimeter-sized thermal storage particle with a hollow structure; another objective of the present invention is to provide a method for preparing the millimeter-sized thermal storage particle; and a final objective of the present invention is to provide the application of the millimeter-sized thermal storage particle in solar chemical thermal storage.

[0005] Technical solution: The present invention provides a millimeter-sized heat storage particle with a hollow structure, wherein the millimeter-sized particle is a composite calcium carbonate particle, which has a hollow structure on a macroscopic scale and a micron-nano stepped pore structure on a microscopic scale.

[0006] Furthermore, the particle size of the composite calcium carbonate particles is 600~700 μm.

[0007] The method for preparing millimeter-scale thermal storage particles with hollow structures according to the present invention mainly includes processes such as precursor powder preparation, core preparation, centrifugal granulation, and calcination-acidification, and includes the following steps:

[0008] (1) Preparation of composite calcium hydroxide precursor powder and microcrystalline cellulose microspheres;

[0009] (2) Mix microcrystalline cellulose microspheres and composite calcium hydroxide precursor powder, spray binder solution, sprinkle in composite calcium hydroxide precursor powder, and under the action of binder, calcium hydroxide powder will bond and grow until the particle size reaches the target diameter to obtain precursor particles.

[0010] (3) The precursor particles are calcined in air and carbonized in a CO2 atmosphere.

[0011] Further, in step (1), the preparation of composite calcium hydroxide precursor powder includes the following steps: first, mixing micron-sized pore-forming agent with calcium hydroxide powder, adding transition metal element dopant solution and stirring evenly, drying, and ball milling.

[0012] Furthermore, the micron-sized pore-forming agent is selected from microcrystalline cellulose, foxtail grass fibers, straw powder, or charcoal powder with micron-sized rod-like structures, and the transition metal element is one or more of Al, Zr, Mn, Fe, Co, and Cu. The transition metal element dopant is selected to improve the solar spectrum absorption capacity and heat storage / release cycle stability of the calcium carbonate thermochemical heat storage material. The transition metal element dopant can be a transition metal salt or a transition metal oxide, and the transition metal salt can be a nitrate or an organic acid salt. The mass-to-volume ratio of calcium hydroxide powder and micron-sized pore-forming agent is 100:10 to 100:60, and the molar ratio of transition metal element dopant to calcium hydroxide is 100:5 to 100:35. When using the transition metal dopant, it needs to be prepared into a solution with a molar concentration of 2 to 6 mol / L and mixed evenly with the calcium hydroxide powder. After drying and crushing, it is made into calcium hydroxide precursor powder. In the composite calcium hydroxide precursor powder, Al is mainly used to improve stability. 3+ Zr 4 Mn 2+ Total concentration and Ca 2+ A concentration molar ratio of not less than 8:100 is used to improve the solar absorbance of Fe. 3+ Co 2+ and Cu 2+ Total concentration and Ca 2+ The molar ratio of concentrations shall not exceed 5:100.

[0013] Further, in step (1), the preparation of microcrystalline cellulose microspheres includes the following steps:

[0014] A saturated solution of polyvinylpyrrolidone was prepared and mixed with microcrystalline cellulose powder. Microcrystalline cellulose microspheres were prepared by extrusion-spheronization and then sieved.

[0015] Furthermore, the mass ratio of microcrystalline cellulose to polyvinylpyrrolidone powder is 80-100:10-20, preferably 90:10, and the diameter of the microcrystalline cellulose microspheres is 400-450 μm.

[0016] Furthermore, in step (2), the adhesive used is a polyvinylpyrrolidone solution with a mass concentration of 1-5 wt%. The blower temperature is controlled at 30-80°C during spraying. o C, flow rate 1~10 mL / min. Slowly sprinkle in calcium hydroxide precursor powder until the precursor particle diameter increases to 700~800 μm.

[0017] Furthermore, in step (3), the calcination conditions in an air atmosphere are calcination at 600°C ~ 1000°C for more than 2 hours, and the carbonation conditions in a CO2 atmosphere are calcination at 600°C ~ 800°C for more than 1 hour.

[0018] The present invention relates to the application of millimeter-sized thermal storage particles with a multi-level stepped pore structure in solar chemical thermal storage.

[0019] This invention promotes carbon dioxide mass transfer within particles by coupling a macroscopic hollow structure with a micro-nano tiered porous structure, resulting in a significant decrease in the thermal storage reaction temperature and a significant acceleration in the reaction rate. This avoids the problems of decreased thermal storage density and decreased mechanical stability caused by traditional doping modification methods. The preparation method of this invention can realize the construction of the aforementioned novel structure and is expected to achieve large-scale production.

[0020] This invention employs a hollow structure design coupled with pore-forming agent doping, solar spectral absorption-enhancing element doping, and heat storage / release cycle stability element doping modification. Compared to traditional simple doping modification methods, this approach achieves a synergistic improvement in heat storage density, solar spectral absorptivity, heat storage / release cycle stability, mechanical wear resistance, and fast reaction rate. By adding micron-sized pore-forming agents (such as microcrystalline cellulose), micron-sized pores are generated during calcination. Components such as calcium hydroxide and nitrates in the heat storage particle precursor generate nano-sized pores during calcination decomposition, thus constructing a micron-nano ladder-like pore structure. This structure can also effectively promote carbon dioxide mass transfer. The macroscopic hollow structure and the micron-nano ladder-like pore structure together constitute a gas reactant / product mass transfer enhancement structure, synergistically promoting the mass transfer process and accelerating the chemical reaction rate.

[0021] This invention first adds a micron-sized pore-forming agent and a transition metal element dopant solution to calcium hydroxide powder and stirs them evenly. Microcrystalline cellulose is used as the micron-sized pore-forming agent, and the transition metal element dopant is selected to improve the solar spectrum absorption capacity and heat storage and release cycle stability of calcium carbonate thermochemical heat storage materials. The precursor mixture is then dried, and finally crushed into fine powder using a pulverizer and planetary ball mill. The core material for preparing hollow particles is a material that can be completely removed by calcination in air, such as commercial polystyrene microspheres. Alternatively, microcrystalline cellulose powder can be prepared into microspheres of the target size using an extrusion-spheronization method. In preparing hollow structured particles, the centrifugal granulation process fluidizes the core microspheres under centrifugal force. A viscous binder solution, such as polyvinylpyrrolidone, is sprayed onto the core, and composite calcium hydroxide powder is continuously sprinkled into it. Under the action of the binder, the calcium hydroxide powder continuously adheres to the outer layer of the core and grows thicker, gradually increasing the particle size, thus obtaining precursor particles consisting of an inner core layer and a growing outer layer. The calcination process is carried out in air, primarily to remove micron-sized pore-forming agents (such as microcrystalline cellulose), hollow templates from hollow particles, moisture, and nitrate ions from dopants, thereby obtaining composite calcium oxide particles with the corresponding target structure. These particles are then calcined with calcium carbonate under a carbon dioxide atmosphere to obtain composite calcium cuprate particles with the same target structure.

[0022] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: The millimeter-scale thermal storage particles of the present invention can reduce the thermal storage decomposition temperature and increase the chemical reaction rate through hollow structure and stepped pore structure. Compared with the traditional method of accelerating the reaction rate by doping with inert materials, the problem of decreased thermal storage density and deterioration of particle strength of composite materials caused by doping can be avoided. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the process flow for preparing hollow composite calcium carbonate thermal storage particles by centrifugation, wherein (a) is the process flow for preparing microcrystalline fiber microspheres; (b) is the process flow for preparing composite calcium hydroxide precursor powder; (c) is the process flow for preparing composite calcium hydroxide precursor particles; and (d) is the process flow for particle calcination and molding.

[0024] Figure 2 Here are SEM images of the composite calcium carbonate particles obtained in Comparative Example 1 and Example 1, where (a) is a macroscopic SEM image of the particles in Comparative Example 1, (b) is a cross-sectional view of the particles in Comparative Example 1, (c) is a microscopic SEM image of the particles in Comparative Example 1, (d) is a nanostructure SEM image of the particles in Comparative Example 1, (e) is a macroscopic SEM image of the particles in Example 1, (f) is a cross-sectional view of the particles in Example 1, (g) is a microscopic SEM image of the particles in Example 1, and (h) is a nanostructure SEM image of the particles in Example 1.

[0025] Figure 3 The images show the CT morphology of the composite calcium carbonate particles obtained in Comparative Example 1 and Example 1. (a) shows the macroscopic morphology of the particles in Comparative Example 1, (b) shows the cross-sectional morphology of the particles in Comparative Example 1, (c) shows the slice morphology of the particles in Comparative Example 1, (d) shows the top view of the slice morphology of the particles in Comparative Example 1, (e) shows the macroscopic morphology of the particles in Example 1, (f) shows the cross-sectional morphology of the particles in Example 1, (g) shows the slice morphology of the particles in Example 1, and (h) shows the top view of the slice morphology of the particles in Example 1.

[0026] Figure 4 The graphs show the thermogravimetric test results of calcium carbonate particles obtained in Example 1 and Comparative Example 1, where (a) represents the particle dispersion state and (b) represents the particle packing state.

[0027] Figure 5 It is 800 of the calcium carbonate particles obtained in Example 1 and Comparative Example 1. o Figure showing the results of the C isothermal decomposition test;

[0028] Figure 6 This is a comparison diagram of the specific surface area of ​​composite calcium carbonate particles with different structures prepared in Example 1 and Comparative Example 1;

[0029] Figure 7 The graphs show the solar spectral absorption capacity, cycle stability, and mechanical wear resistance test results of the composite calcium carbonate particles prepared in Example 1 and Comparative Example 1. In the graphs, (a) is the solar spectral absorption law diagram of the obtained calcium carbonate particles, (b) is the average solar spectral absorption law diagram of the obtained calcium carbonate particles, (c) is the 25-cycle energy storage density change diagram of the obtained calcium carbonate particles, and (d) is the mechanical wear resistance test result diagram of the obtained calcium carbonate particles. Detailed Implementation

[0030] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0031] Figure 1 This is a schematic diagram of the novel preparation process of the structure described in this invention.

[0032] Example 1: Preparation of hollow structured particles

[0033] (1) First, as Figure 1 As shown in (a), polyvinylpyrrolidone (PVP) solution was poured into microcrystalline cellulose (MCC) powder and thoroughly mixed. The mass ratio of microcrystalline cellulose to PVP powder was 90:10. Subsequently, microcrystalline cellulose microspheres were prepared by extrusion-spheronization method, and microcrystalline cellulose microspheres with a size of 400-450 μm were sieved using a standard sieve for later use.

[0034] (2) with Figure 1 As shown in (b), the composite calcium hydroxide precursor powder was prepared by thoroughly mixing calcium hydroxide and microcrystalline cellulose (MCC) powders at a mass ratio of 100:20. Then, a nitrate aqueous solution was prepared by adding aluminum nitrate nonahydrate, ferric nitrate nonahydrate, and 50 wt% manganese nitrate aqueous solution to deionized water. This nitrate aqueous solution was added to the composite calcium hydroxide and microcrystalline cellulose powder and stirred thoroughly until the color was uniform. 2 + Al 3+ Fe 3+ Mn 2+ The molar ratio is 100:15:10:5. The moist mixture is placed in a drying oven and dried at 80°C for 4-6 hours to completely remove moisture. Because the dried material has high hardness, the large, bonded particles need to be pre-crushed using a crusher to reduce their particle size, making it easier to ball-mill using a planetary ball mill until it is completely reduced to a powder <100 μm, i.e., composite calcium hydroxide precursor powder.

[0035] (3) After preparing the microcrystalline cellulose microspheres and composite calcium hydroxide precursor powder, centrifugal granulation was performed using the centrifugal granulation and coating machine shown in Figure 1(c). The concentration of the polyvinylpyrrolidone aqueous solution as a binder was 1 wt%. 200 g of microcrystalline cellulose microspheres were weighed and placed into the centrifugal chamber of the centrifugal granulation and coating machine, and the blower temperature was set to 30°C. After the microcrystalline cellulose microspheres flowed under the drive of the centrifugal turntable, 1 wt% of the composite calcium hydroxide precursor powder was sprinkled into them. After they were mixed evenly, the core was obtained. The spray system was turned on, and the aforementioned 1 wt% concentration of polyvinylpyrrolidone solution was sprayed onto the core in the chamber. The flow rate of the peristaltic pump of the spray system was 5 mL / min. After the microcrystalline cellulose microspheres were slightly moistened, the composite calcium hydroxide powder was continuously sprinkled into the chamber to coat the surface of the microcrystalline cellulose microspheres. At this time, under the synergistic effect of powdering and spraying, the calcium hydroxide layer continued to thicken and the particle size increased accordingly. The particle size is continuously observed. After the particles grow to a certain size, they are removed and sieved through a standard sieve to retain the particles that reach the target size (700~800 μm). Particles that are too large are removed, and smaller particles are put back into the granulator to continue to grow larger, thus obtaining precursor particles.

[0036] (4) As shown in Figure 1(d), the precursor particles obtained by centrifugal granulation were placed in a muffle furnace and calcined at 900°C in air atmosphere to become composite CaO particles. Then, the composite CaCO3 product was obtained by acidification and calcination in a tube furnace under CO2 atmosphere for 1 hour. The acidification and calcination temperature was 700°C. It was found by standard sieve analysis that the precursor particles with a diameter of 700~800 μm became 600~700 μm after calcination.

[0037] Comparative Example 1

[0038] This comparative example uses a more traditional extrusion-spheronization method to prepare composite calcium carbonate particles with a uniform structure. The specific steps are as follows:

[0039] (1) The preparation of composite calcium hydroxide powder is the same as step (2) in Example 1;

[0040] (2) Add water to the composite calcium hydroxide powder, adjust the composite calcium hydroxide powder to a suitable humidity (the water content is 10 wt% of the composite calcium hydroxide powder), and make the precursor mixture into composite calcium hydroxide granules by extrusion-spheronization granulator;

[0041] (3) The composite calcium hydroxide particles were calcined in air at 900°C for 2 hours to remove nitrate ions, moisture, microcrystalline cellulose, etc., to obtain composite calcium oxide particles with micron-nano ladder pore structure (the ladder pore structure is uniformly distributed and there is no core-shell structure or hollow structure).

[0042] (4) The above-mentioned composite calcium oxide particles were placed in a tube furnace and carbonated and calcined in a pure CO2 atmosphere for 1 hour. The calcination temperature was set to 700°C to obtain composite calcium carbonate particles with a uniform stepped pore structure.

[0043] CT and SEM characterization were performed on the calcium carbonate particles obtained in Example 1 and Comparative Example 1, demonstrating their superior performance. The results are as follows: Figure 2 and 3 As shown. By means of Figure 2 and 3 The SEM and CT morphology characterizations shown reveal that both types of particles exhibit a micron-nano tiered pore structure due to elemental doping and the use of microcrystalline cellulose pore-forming agents. Compared to the uniformly distributed micron-nano pores in the particles prepared by the extrusion-spheronization method in Reference Document 1, the particles prepared in Example 1 based on this invention exhibit a distinct hollow structure.

[0044] Thermogravimetric analysis (TGA) was performed on the calcium carbonate particles obtained in Example 1 and Comparative Example 1. Tests were conducted using small amounts of dispersed particles in both dispersed and packed states. The results are as follows: Figure 4 As shown. Figure 4 Figure (a) shows the results of tests conducted using a small number of dispersed particles. Whether in a dispersed or packed state, the decomposition temperatures corresponding to the maximum decomposition rate of the hollow particles were 20 K and 28 K lower, respectively, than those of the solid particles prepared by the extrusion-spheronization method.

[0045] The isothermal decomposition test of the calcium carbonate particles obtained in Example 1 and Comparative Example 1 at 800°C yielded the following results: Figure 5 As shown. Figure 5The results show that, compared with the uniform particles in Comparative Example 1, the hollow particles prepared in Example 1 have a significantly faster reaction rate, with a peak reaction rate reaching 1.824 K. -1 Compared to the uniform particles in Comparative Example 1, the improvement is 31.79%.

[0046] The specific surface area of ​​the calcium carbonate particles obtained in Example 1 and Comparative Example 1 was measured, and the results are as follows: Figure 6 As shown in the figure. The results indicate that the particles and powder prepared by the extrusion spheronization method in Comparative Example 1 have a larger specific surface area. Furthermore, considering that both types of particles have the same elemental doping ratio and microcrystalline cellulose pore-forming agent ratio, the specific surface area results show that it is indeed the hollow structure of the present invention that gives it a faster decomposition rate, rather than elemental doping or micropores, thus proving the effectiveness of the hollow structure proposed in this invention.

[0047] The calcium carbonate particles obtained in Example 1 and Comparative Example 1 were tested for key properties such as solar spectral absorptivity, heat storage and release cycle stability, and mechanical wear resistance. The results are as follows: Figure 7 As shown. Figure 7 As shown in (a) to (b), due to the use of the same elemental doping ratio, the particles of the two structures have similarly high solar spectral absorbance. Figure 7 As shown in (c), both types of particles exhibit excellent heat storage and release cycle stability, with the heat storage density remaining essentially unchanged after 25 cycles. Furthermore, the hollow-structured particles in Example 1 maintained good resistance to breakage, such as... Figure 7 As shown in (d), after 24 hours of intense flow, collision and friction tests on a planetary ball mill, the hollow structure particles in the agate ball mill jar suffered a mass loss of only 0.24%, and most of the particles maintained their integrity without significant breakage.

[0048] In summary, based on the improvement of solar spectrum absorption capacity and heat storage cycle stability by doping with transition metal elements and the construction of micron-nano ladder pores by doping with pore-forming agents, the hollow structure and its preparation method proposed in this invention achieve a synergistic improvement of fast reaction rate, high heat storage density and the aforementioned performance.

Claims

1. A millimeter-scale thermal storage particle with a hollow structure, characterized in that, The millimeter-sized particles are composite calcium carbonate particles, which have a hollow structure on a macroscopic scale and a micro-nano stepped pore structure on a microscopic scale.

2. The millimeter-sized thermal storage particles with a multi-level stepped pore structure according to claim 1, characterized in that, The particle size of the composite calcium carbonate particles is 600~700 μm.

3. The method for preparing millimeter-scale thermal storage particles with a hollow structure as described in claim 1 or 2, characterized in that, Includes the following steps: (1) Preparation of composite calcium hydroxide precursor powder and microcrystalline cellulose microspheres; (2) Mix microcrystalline cellulose microspheres and composite calcium hydroxide precursor powder, spray binder solution, sprinkle in composite calcium hydroxide precursor powder, and under the action of binder, calcium hydroxide powder will bond and grow until the particle size reaches the target diameter to obtain precursor particles. (3) The precursor particles are calcined in air and carbonized in a CO2 atmosphere.

4. The preparation method according to claim 3, characterized in that, In step (1), the preparation of the composite calcium hydroxide precursor powder includes the following steps: First, mix the micron-sized pore-forming agent with calcium hydroxide powder, add the transition metal element dopant and stir evenly, then dry and ball-mill.

5. The preparation method according to claim 4, characterized in that, The micron-sized pore-forming agent is selected from microcrystalline cellulose, foxtail grass fibers, straw powder or charcoal powder, and one or more of the transition metal elements Al, Fe, Mn, Co, Cu, and Zr. The transition metal element dopant is a transition metal salt or transition metal oxide. The mass ratio of calcium hydroxide powder to micron-sized pore-forming agent is 100:10 to 100:60, and the molar ratio of transition metal element dopant to calcium hydroxide is 100:5 to 100:

35.

6. The preparation method according to claim 3, characterized in that, In step (1), the preparation of microcrystalline cellulose microspheres includes the following steps: After preparing a saturated solution of polyvinylpyrrolidone powder, it was mixed with microcrystalline cellulose powder and microcrystalline cellulose microspheres were prepared by extrusion-spheronization method and then sieved.

7. The preparation method according to claim 6, characterized in that, The mass ratio of microcrystalline cellulose powder to polyvinylpyrrolidone powder is 80-100:10-20, and the diameter of the microcrystalline cellulose microspheres is 400-450 μm.

8. The preparation method according to claim 3, characterized in that, In step (2), the adhesive used is a polyvinylpyrrolidone solution with a mass concentration of 1-5 wt%. The blowing temperature is controlled at 30-80°C during spraying. o C, flow rate 1~10 mL / min. Slowly sprinkle in calcium hydroxide precursor powder until the precursor particle diameter increases to 700~800 μm.

9. The preparation method according to claim 3, characterized in that, In step (3), the calcination conditions in air atmosphere are calcination at 600°C ~ 1000°C for more than 2 hours, and the carbonation conditions in CO2 atmosphere are calcination at 600°C ~ 800°C for more than 1 hour.

10. The application of millimeter-sized thermal storage particles with a multi-level stepped pore structure as described in claim 1 or 2 in solar chemical thermal storage.

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