Preparation method and application of molten salt modified magnesium oxide adsorbent particles for carbon dioxide capture

Abstract

The present invention discloses a method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture. The method involves reacting magnesium chloride as a magnesium source with sodium bicarbonate to produce light basic magnesium carbonate, which is then calcined in stages to produce a microporous magnesium oxide precursor. The microporous magnesium oxide precursor is then impregnated with a mixed salt consisting of nitrate and carbonate, and dried to produce an alkali metal molten salt-modified magnesium oxide powder. A binder is added to the powder, and the powder is extruded to form a granular adsorbent. Applications of the adsorbent are also disclosed. The prepared adsorbent particles have good structural stability and are recyclable. When adsorbing and capturing CO2 at approximately 300°C, the loaded mixed salt forms a molten salt layer on the microporous surface of the magnesium oxide, enhancing CO2 adsorption. The CO2 adsorption capacity reaches over 10 mmol / g (adsorbent), which is more than 10 times higher than that of unmodified magnesium oxide adsorbent. The CO2 desorption rate is also significantly improved, reaching 100% at 400°C.

Description

Technical Field

[0001] The present invention relates to the technical field of greenhouse gas carbon dioxide capture, and in particular to a method for preparing and forming an adsorption material for medium-temperature carbon dioxide adsorption capture. Background Art

[0002] Carbon dioxide capture technology can ensure the large-scale, low-carbon utilization of fossil fuels. Among the various CO2 capture technologies, chemical absorption (solution) and chemical adsorption (solid) offer high capture efficiency and good flue gas adaptability, making them the primary approaches for large-scale application and attracting widespread attention and research and development. The development of high-efficiency CO2 capture materials, novel carbon capture equipment, and innovative carbon capture processes will promote the large-scale application of next-generation CO2 capture technologies in my country's high-carbon emission industries, such as thermal power, steel, and cement.

[0003] At present, the main difficulty in using adsorption method to capture CO2 is to develop adsorbent materials with good performance. The adsorbent materials used for CO2 capture need to have the following important characteristics: ① high CO2 adsorption capacity; ② good CO2 selectivity; ③ efficient adsorption / desorption kinetics; ④ good adsorption / desorption cycle stability, etc. According to the applicable operating temperature of the adsorbent, it can be classified into room temperature and low temperature adsorbents (<200℃), medium temperature adsorbents (200℃-400℃), and high temperature adsorbents (>400℃). Room temperature and low temperature adsorbents include activated carbon, zeolite, metal organic framework materials, and amine solid adsorbents. Among them, metal organic framework materials and amine solid adsorbents have better performance in adsorbing CO2 in flue gas, but they are still in the research and development stage, and industrialization still requires breakthroughs in key technologies. Magnesium oxide has a high CO2 theoretical adsorption capacity (22.3mmol·g -1 ), has always been considered a promising medium-temperature CO2 adsorbent. However, since magnesium oxide adsorbs CO2 to form a MgCO3 shell, it hinders the subsequent CO2 diffusion and adsorption process, resulting in a low actual CO2 adsorption capacity and unable to achieve the adsorption effect required by industrial production. Calcium oxide is a commonly used high-temperature adsorbent. The temperature for capturing CO2 is usually above 600°C. The adsorption process is achieved through a carbonation reaction, which is exothermic. Therefore, it can effectively recover a large amount of energy released during the carbon dioxide capture process and reduce the energy consumption caused by external heat. At the same time, the raw material cost of CaO adsorbent is low. This type of adsorbent and adsorption process is very economical, but its adsorption capacity will decrease with the increase in the number of cycles, and the conversion rate of CaO will continue to decrease and eventually stabilize at a very small value. Therefore, preventing the sintering problem of CaO adsorbent during use is the key to solving the problem of reduced CO2 adsorption capacity.

[0004] In recent years, many researchers have found that alkali metal molten salt modified magnesium oxide adsorbent has a high CO2 adsorption capacity. At around 300 °C, the adsorption capacity exceeds 10 mmol·g -1 (Adsorbent), compared with pure magnesium oxide, the CO2 adsorption capacity has increased by an order of magnitude, and these adsorbents can be quickly regenerated at 400 ° C, which has good industrial application prospects. The high CO2 adsorption capacity of alkali metal molten salt modified magnesium oxide adsorbent is due to the alkali metal salt loaded on the surface of magnesium oxide is in a molten state at around 300 ° C, CO2 can quickly diffuse through the molten salt layer and react with the Mg dissolved in the molten salt. 2+ With O 2- Ion-pair interactions generate MgCO₃, allowing CO₂ to bind to active sites within the adsorbent pores, enhancing the adsorption capacity of magnesium oxide and preventing the formation of a magnesium carbonate shell on the surface of the magnesium oxide. Currently, research on alkali metal molten salt-modified magnesium oxide adsorbents is mostly focused on the development of powdered adsorption materials, which is limited to laboratory research. Powdered adsorbents are not suitable for large-scale capture of low-concentration carbon dioxide in flue gas, requiring granulation technology for alkali metal molten salt-modified magnesium oxide powder. Given that powder packing is no longer suitable for industrial applications in fixed beds or circulating fluidized beds, millimeter-sized granular adsorbents are needed to reduce the pressure drop in the packed column, and further research is needed. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to provide a method for preparing low-cost, high-CO2 adsorption-performance alkali metal molten salt-modified magnesium oxide adsorbent particles, thereby providing an advanced adsorbent material for large-scale capture of low-concentration carbon dioxide from flue gas. Using abundant and inexpensive magnesium chloride as a magnesium source, the microporous magnesium oxide precursor is prepared, reducing the adsorbent material preparation cost and making the molten salt-modified magnesium oxide adsorbent and the CO2 adsorption capture process highly economical.

[0006] The technical solution of the present invention is a method for preparing molten salt modified magnesium oxide adsorbent particles for carbon dioxide capture, comprising the following steps:

[0007] (1) Preparation of microporous magnesium oxide precursor:

[0008] Magnesium chloride is used as a magnesium source, reacted with sodium bicarbonate at a reaction temperature of above 80°C to synthesize light basic magnesium carbonate, and then the light basic magnesium carbonate is calcined in two steps to prepare a microporous magnesium oxide precursor; the two-step calcination method comprises a first calcination at 270-330°C for 1-3 hours, and then the temperature is raised to 420-480°C for a second calcination for 1-3 hours;

[0009] (2) Modified microporous magnesium oxide precursor by mixed salt impregnation composed of nitrate / carbonate:

[0010] A mixed salt composed of nitrate and carbonate is dispersed in an organic substance and impregnated with a microporous magnesium oxide precursor. After impregnation, the organic solvent is removed by vacuum drying to obtain a mixed salt modified magnesium oxide adsorbent powder.

[0011] (3) Mixed salt modified microporous magnesium oxide powder is formed into granular adsorbent material:

[0012] The mixed salt modified magnesium oxide adsorbent powder is mixed with a binder and extruded into a certain shape, calcined at 400-500°C to remove moisture, and then crushed and sieved to obtain a granular adsorbent material.

[0013] According to the method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture of the present invention, preferably, in step (2), the nitrate is selected from potassium nitrate and lithium nitrate; and the carbonate is selected from sodium carbonate and potassium carbonate.

[0014] According to the method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture of the present invention, preferably, in step (2), the mixed salt components composed of nitrates and carbonates are potassium nitrate, lithium nitrate, sodium carbonate, and potassium carbonate.

[0015] A mixed salt composed of nitrates and carbonates is loaded into the micropores of magnesium oxide at low or room temperature. The adsorbent adsorbs CO2 at a temperature of approximately 300°C. At this temperature, the mixed salt loaded into the micropores of magnesium oxide can form a molten salt, forming a molten salt layer on the surface of the micropores of magnesium oxide, thereby enhancing carbon dioxide adsorption.

[0016] Preferably, the nitrate is selected from potassium nitrate and lithium nitrate, and the carbonate is selected from sodium carbonate and potassium carbonate. A molten salt mixture composed of potassium nitrate / lithium nitrate / sodium carbonate / potassium carbonate has a stable structure and is best for enhancing CO2 adsorption.

[0017] According to a method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture of the present invention, further, in step (2), the molar ratio of potassium nitrate, lithium nitrate, sodium carbonate, and potassium carbonate is 2-3:1-2:1-2:1-2.

[0018] According to the method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture of the present invention, preferably, in step (2), the organic solvent used to impregnate the magnesium oxide powder with the mixed salt is selected from one of the following: methanol, ethanol, and acetone. After impregnation, vacuum drying is required to remove organic matter and avoid carbon deposition in the adsorbent micropores during subsequent calcination.

[0019] According to the present invention, a method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture, preferably, the binder in step (3) is selected from one of water, aluminum sol, silica sol, and cellulose. Magnesium oxide itself is a commonly used binder and is easy to shape. It can be formed by high-pressure extrusion after adding a small amount of water. Adding aluminum sol, silica sol, and cellulose can increase the hardness of the formed adsorbent.

[0020] Preferably, the binder accounts for 0.1% to 5% by mass of the adsorbent. Further, the binder accounts for 1% to 5% by mass of the adsorbent.

[0021] In step (3), preferably, high pressure extrusion is used to control the amount of adhesive used to be as small as possible to avoid negative impact on the distribution of the molten salt layer formed on the surface of the magnesium oxide micropores. High pressure refers to 10 to 30 atmospheres.

[0022] According to the method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture of the present invention, preferably, the calcination time in step (3) is 1-4 hours. The calcination removes moisture brought by the binder.

[0023] The total molar ratio of nitrate to carbonate is 2:1-2. Within this range, appropriately increasing the carbonate ratio in the mixed salt composed of nitrate / carbonate and increasing the alkalinity of the molten salt layer can improve the capture efficiency of low-concentration carbon dioxide.

[0024] The present invention also provides the use of the molten salt modified magnesium oxide adsorbent particles prepared by the above-mentioned method for preparing molten salt modified magnesium oxide adsorbent particles for carbon dioxide capture in capturing carbon dioxide from medium-temperature flue gas in the range of 200°C-400°C.

[0025] Preferably, the molten salt modified magnesium oxide adsorbent particles are placed in an adsorption tower, and the operating pressure in the adsorption tower is from normal pressure to 40 atmospheres.

[0026] Furthermore, the operating pressure in the adsorption tower is from atmospheric pressure to 30 atmospheres. Still further, the operating pressure in the adsorption tower is from atmospheric pressure to 20 atmospheres.

[0027] When low-concentration carbon dioxide is adsorbed and captured (the carbon dioxide content in the feed gas is 1%-50% by mole fraction), pressurizing to increase the carbon dioxide partial pressure is beneficial to the adsorption and capture of CO2 in the tower. It is preferred to pressurize to high pressure to increase the low-concentration carbon dioxide capture rate.

[0028] The feed gas contains 1%-50% carbon dioxide by mole, while common coal-fired flue gas contains 10%-30% CO2 at a pressure of approximately 1 atmosphere. The adsorption tower operates at pressures between atmospheric and 30 atmospheres. Raising the operating pressure within the adsorption tower to within the 1-40 atmosphere range increases the carbon dioxide partial pressure in the gas phase. This improves the recovery rate and adsorption rate of low-concentration carbon dioxide in the flue gas.

[0029] In step (1), magnesium chloride, which is abundant in resources and cheap, is used as a magnesium source to react with sodium bicarbonate to synthesize light basic magnesium carbonate, which is then calcined in two steps to prepare a magnesium oxide precursor with high specific area and rich micropores.

[0030] The present invention relates to three key steps in the preparation process of alkali metal molten salt modified magnesium oxide adsorbent particles, including: preparation of low-cost microporous magnesium oxide precursor, impregnation of modified magnesium oxide with alkali metal mixed salt, and molding of powdered adsorbent into granular adsorbent. Specifically, the present invention uses abundant and low-cost magnesium chloride as a magnesium source, synthesizes light basic magnesium carbonate by reacting with sodium bicarbonate, and then prepares a microporous magnesium oxide precursor by two-step calcination; impregnates microporous magnesium oxide with a mixed salt composed of nitrate and carbonate to obtain molten salt modified magnesium oxide powder; and molds the prepared mixed salt modified magnesium oxide powder into granules to reduce the pressure drop of the adsorbent layer during the application of fixed bed or circulating fluidized bed, thereby providing advanced adsorption materials for the capture of low-concentration carbon dioxide in large-scale flue gas. Figure 1 Schematic diagram showing the preparation process of mixed salt-modified microporous magnesium oxide particles composed of nitrate / carbonate.

[0031] Preparation of low-cost microporous magnesium oxide precursor: This invention uses abundant and low-cost magnesium chloride as a magnesium source, which reacts with sodium bicarbonate to synthesize light basic magnesium carbonate. Carbon dioxide gas is generated during the synthesis process, and there is an optimal reaction temperature of approximately 80°C or above. The specific reaction equation is:

[0032] 2MgCl2+4NaHCO3=Mg2(OH)2CO3+4NaCl+3CO2+H2O

[0033] The synthesized light basic magnesium carbonate is calcined in steps, calcined at about 300°C to decompose Mg(OH)2, and calcined at about 450°C to decompose MgCO3, to obtain a magnesium oxide precursor rich in micropores.

[0034] Mg2(OH)2CO3=MgCO3+H2O+MgO

[0035] MgCO3=MgO+CO2

[0036] Beneficial effects:

[0037] One of the advantages of the present invention is that magnesium chloride, which is abundant in resources and low in cost, is used as a magnesium source to synthesize light basic magnesium carbonate by reacting with sodium bicarbonate, and then the light basic magnesium carbonate is calcined in two steps to prepare a microporous magnesium oxide precursor. The prepared magnesium oxide precursor has a rich microporous structure and a high specific surface area, and the preparation cost is low.

[0038] The second advantage of the present invention is that a mixed salt composed of nitrate and carbonate is used to modify the magnesium oxide adsorbent, and the ratio of the mixed salt is adjusted so that at about 300°C, the loaded mixed salt forms a molten salt layer on the surface of the magnesium oxide micropores. The nitrate in the molten salt layer provides oxygen anions to enhance CO2 adsorption, and the addition of carbonate increases the alkalinity of the molten salt layer to improve the adsorption of low-concentration CO2. Under the synergistic effect of nitrate and carbonate, the CO2 adsorption amount can reach more than 10 mmol / g (adsorbent), which is more than 10 times higher than that of the unmodified magnesium oxide adsorbent.

[0039] The CO2 desorption rate is also significantly improved, reaching 100% at 400°C.

[0040] The third advantage of the present invention is that the adsorbent powder is formed into a granular adsorption material by extrusion using a binder such as water. The formed adsorbent particles have good strength, the CO2 adsorption capacity of the adsorbent remains basically unchanged after forming, the adsorption forming is easy, and the formed granular adsorbent can be used for carbon dioxide adsorption capture in fixed beds and fluidized beds.

[0041] The fixed bed refers to the "adsorption tower" mentioned above.

[0042] The fourth advantage of the present invention is that the fixed bed filled with molten salt modified magnesium oxide adsorbent particles can capture low-concentration CO2 by adsorption, and its CO2 capture capacity can reach more than 10 mmol / g (adsorbent). Further increasing the operating pressure in the adsorption tower to increase the CO2 gas partial pressure, and optimizing the carbonate content in the mixed salt to increase the alkalinity of the molten salt layer will significantly improve the low-concentration CO2 adsorption capture efficiency, so that the fixed bed medium-temperature carbon dioxide adsorption carbon capture technology has good industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of the preparation process of mixed salt modified microporous magnesium oxide particles composed of nitrate / carbonate.

[0044] Figure 2a It is the microscopic morphology of magnesium oxide powder. Figure 2b It is the microscopic morphology of molten salt modified magnesium oxide particles.

[0045] Figure 3a and Figure 3b The cyclic CO2 adsorption and desorption performance of molten salt modified magnesium oxide particles.

[0046] Figure 4 This is the outflow curve of low-concentration CO2 adsorption capture in a fixed bed of molten salt-modified magnesium oxide adsorbent particles. DETAILED DESCRIPTION

[0047] In order to better understand the present invention, the content of the present invention is further illustrated below in conjunction with the embodiments, but the content of the present invention is not limited to the following embodiments.

[0048] Preparation of microporous magnesium oxide precursor: The present invention uses abundant and low-cost magnesium chloride as a magnesium source, reacts with sodium bicarbonate to synthesize light basic magnesium carbonate, and then uses a two-step calcination method to prepare a microporous magnesium oxide precursor. The prepared magnesium oxide precursor has a rich microporous structure and a high specific surface area, and the preparation cost is low.

[0049] Specific operations:

[0050] (1) Add magnesium chloride aqueous solution to a reactor, heat the reactor to a set reaction temperature, and then gradually add sodium bicarbonate aqueous solution under stirring. After the reaction is completed, vacuum filtration, centrifugal dehydration, drying, crushing, and sieving are performed to obtain basic magnesium carbonate.

[0051] (2) Basic magnesium carbonate is placed in a high-temperature furnace for heating and decomposition. The decomposition temperature is controlled in two steps: the temperature is raised to about 300°C, and the temperature is kept constant for 2 hours to decompose. The temperature is then raised to about 450°C and the temperature is kept constant for 2 hours to decompose. During the decomposition process, a certain amount of air is constantly introduced into the furnace.

[0052] Alkali metal mixed salt impregnation modified magnesium oxide: The present invention is to configure a mixed salt of nitrate and carbonate with a molar ratio of potassium nitrate / lithium nitrate / sodium carbonate / potassium carbonate of 2.28mol / 1.77mol / 1mol / 1mo. The mixed salt formed according to this ratio is a molten salt at about 300°C. The mixed salt according to this ratio is impregnated into the porous magnesium oxide powder prepared above. The mixed salt is loaded in the micropores of magnesium oxide. When adsorbed at about 300°C, the loaded mixed salt forms a molten salt layer on the surface of the magnesium oxide micropores. CO2 can quickly diffuse through the molten salt layer and react with the Mg dissolved in the molten salt. 2+ With O 2- The ion pairs interact to generate MgCO3, which enables CO2 to bind to the active sites in the pores of the adsorbent, enhancing the adsorption capacity of the adsorbent and preventing the formation of a magnesium carbonate shell on the surface of magnesium oxide.

[0053] Specific operations:

[0054] (1) A mixed salt having a molar ratio of potassium nitrate / lithium nitrate / sodium carbonate / potassium carbonate of 2.28 mol / 1.77 mol / 1 mol / 1 mol is dispersed in an organic solvent, and then a measured amount of microporous magnesium oxide powder is added and stirred for impregnation;

[0055] (2) Vacuum-evaporating the organic solvent, vacuum-drying, crushing, and sieving to obtain mixed salt-modified magnesium oxide adsorbent powder.

[0056] The powder adsorbent is formed into a granular adsorbent: The present invention forms the mixed salt modified magnesium oxide powder into granules to reduce the pressure drop of the adsorbent layer filled during the application of the fixed bed or circulating fluidized bed, providing an advanced adsorption material for the capture of low-concentration carbon dioxide in large-scale flue gas.

[0057] Specific operations:

[0058] (1) Take a small amount of water or other adhesive and add it dropwise to the mixed salt modified magnesium oxide powder, add the adhesive while stirring until the powder has a certain adhesion and agglomeration phenomenon, then stop adding the adhesive, place the adhered adsorbent on the tablet press, adjust the set pressure and press the adsorbent into a set shape such as a thin sheet.

[0059] (2) The flakes are placed in a vacuum drying oven to dry out moisture to obtain adsorbent flakes with a certain hardness, which are then calcined in a muffle furnace, cooled, crushed, sieved, etc. to obtain granular adsorbent.

[0060] The preparation of microporous magnesium oxide precursor is as follows:

[0061] (1) Ingredients: Weigh 50.40 g of magnesium chloride hexahydrate and 41.66 g of sodium bicarbonate into two 500 mL beakers, then add 248 mL and 496 mL of water, respectively, to prepare 0.5 mmol / g magnesium chloride solution and sodium bicarbonate solution;

[0062] (2) Preparation of light basic magnesium carbonate: The prepared magnesium chloride solution is first added to a reactor, and the reactor temperature is controlled at 80°C;

[0063] The stirring speed of the reactor was 300 r / min; the sodium bicarbonate solution in the flask was slowly added to the reactor using a peristaltic pump, the feeding rate was controlled to be 6 mL / min, and the reaction was carried out for 83 minutes; after the reaction was completed, the reaction was aged for 30 minutes, and then the obtained white suspension was filtered, washed, and dried to obtain a light basic magnesium carbonate product.

[0064] (3) Calcination of light basic magnesium carbonate to obtain microporous magnesium oxide precursor: The prepared light basic magnesium carbonate is placed in a muffle furnace and calcined in stages. The temperature is set at a heating rate of 5°C / min and raised to 300°C for calcination for 2 hours. Then, the temperature is raised to 450°C at a heating rate of 5°C / min and calcined for a second time for 2 hours. After the two calcinations, microporous magnesium oxide powder is obtained.

[0065] The modified magnesium oxide powder is impregnated with mixed salt. The specific steps are as follows:

[0066] (1) Ingredients: Weigh 30 g of microporous magnesium oxide powder and place it in a 500 mL beaker, marked as beaker a; weigh 4.21 g of potassium nitrate, 2.26 g of lithium nitrate, 1.97 g of sodium carbonate, and 2.57 g of potassium carbonate, place them in a mortar and grind them into powder, and then place them in beaker b; use a measuring cylinder to measure 400 mL of anhydrous ethanol, add it to beaker b, and mix the mixed salt in beaker b with the magnesium oxide in beaker a, so that the molar ratio of nitrate and carbonate to magnesium oxide is 10% and 5%, respectively.

[0067] (2) Impregnation with a mixed salt consisting of potassium nitrate / lithium nitrate / sodium carbonate / potassium carbonate: Place the beaker containing the magnesium oxide powder and the mixed salt on a constant temperature magnetic stirring table, set the temperature to 75°C, the speed to 300 r / min, and heat and stir for 10 h;

[0068] (3) Vacuum rotary evaporation: After the stirring is completed, the mixed salt impregnation sample is transferred to a rotary evaporator for rotary evaporation. The rotary evaporation temperature is set to 55°C, the vacuum degree is 0.08 MPa, and the speed is 70 r / min. The rotary evaporation is carried out until a powder sample is obtained;

[0069] (4) Vacuum drying: After the rotary evaporation is completed, the obtained powder sample is placed in a vacuum drying oven, the vacuum drying temperature is set to 100°C, and the drying time is 24 hours. After the drying is completed, the obtained sample is ground to obtain a mixed salt modified magnesium oxide adsorbent powder;

[0070] The mixed salt modified microporous magnesium oxide powder is formed into a granular adsorbent material. The specific operation is as follows:

[0071] (1) Molding: The obtained mixed salt modified microporous magnesium oxide powder is placed in a beaker, and water is slowly added thereto in a metered ratio of 0.5 mL of water per 1 g of adsorbent, while the adsorbent powder is stirred until a cohesive sample is formed; and the cohesive sample is pressed into tablets using a tablet press.

[0072] (2) Calcination: The pressed sample was placed in a vacuum drying oven at 100°C for 24 hours. After drying, the sample was placed in a muffle furnace and calcined at 450°C for 2 hours to obtain a sheet-like adsorbent with a certain mechanical strength.

[0073] Then, the flake sample is taken out and crushed to obtain a granular adsorbent with a particle size of 1-2 mm.

[0074] Evaluation of CO2 adsorption performance of prepared molten salt modified magnesium oxide particles

[0075] According to the above preparation method, magnesium chloride is used as a magnesium source and reacted with sodium bicarbonate to prepare light basic magnesium carbonate, which is then calcined in two steps to obtain a microporous magnesium oxide precursor; the microporous magnesium oxide precursor is impregnated with a mixed salt consisting of potassium nitrate / lithium nitrate / sodium carbonate / potassium carbonate to obtain a mixed salt-modified magnesium oxide powder; the powder adsorbent and a binder are mixed and extruded to form a 1mm-2mm granular adsorbent, named (Li / KNO3) 0.1 (Na2 / K2CO3) 0.05 -MgO. Then, the prepared granular adsorbent was evaluated using thermogravimetric testing and fixed bed experiments. The evaluation results are as follows:

[0076] (1) Micromorphology of prepared molten salt modified magnesium oxide particles

[0077] The surface morphology of the prepared magnesium oxide powder and molten salt modified magnesium oxide particles was characterized using field emission scanning electron microscopy, as shown in Figure 2. The magnesium oxide sample obtained by step-by-step calcination of basic magnesium carbonate has a petal-shaped surface with rich pore structure and a high specific surface area, which can reach 200m 2 / g or more, as shown in Figure 2(a). After being impregnated with a mixed salt consisting of potassium nitrate, lithium nitrate, sodium carbonate and potassium carbonate, part of the pores on the surface of magnesium oxide are occupied, the microporous structure is reduced, and its specific surface area is greatly reduced after the modification and molding steps, which is about 20m 2 / g, this is because the alkali metal salt loaded on the adsorbent is in the solid phase at room temperature and will occupy part of the pores of the adsorbent, resulting in a decrease in its specific surface area and pore volume, as shown in Figure 2(b).

[0078] (2) Cyclic CO2 adsorption and desorption performance and stability evaluation of molten salt modified magnesium oxide particles

[0079] Thermogravimetric experiments were conducted to test the cyclic CO adsorption and desorption stability of molten salt-modified magnesium oxide particles. The cyclic adsorption conditions were 300°C, CO2 atmosphere (50 mL / min) for 60 min, and 400°C, N2 atmosphere (50 mL / min) for 60 min. Figure 3(a) shows a decreasing trend in CO2 adsorption over the first 25 cycles. This is due to a certain degree of agglomeration of the adsorbent's microcrystal structure during the adsorption and desorption processes, which reduces the number of available adsorption sites and lowers the adsorption capacity. After 25 cycles, the adsorbent's microcrystal structure aggregates to form stable microcrystal clusters, with virtually no change in adsorption sites. Consequently, the adsorption capacity stabilizes. After stabilization, the CO2 adsorption capacity of the granular adsorbent reaches 6.0 mmol / g, as shown in Figure 3(b). The adsorbed CO2 can be rapidly desorbed, with a desorption rate exceeding 99%. Furthermore, Figure 3(b) shows that the unmodified magnesium oxide has a very low adsorption capacity, while the modified magnesium oxide increases CO2 adsorption by more than an order of magnitude. The prepared adsorbent particles have a high cyclic adsorption capacity, which is significantly higher than the cyclic CO2 adsorption capacity of alkali metal salt modified magnesium oxide adsorbent reported in the current literature, and have certain competitiveness in the cyclic adsorption and desorption process.

[0080] (3) Evaluation of low-concentration carbon dioxide adsorption and capture performance in a fixed bed of molten salt-modified magnesium oxide particles

[0081] Figure 4 The breakthrough curve of low-concentration CO2 adsorption in a fixed bed of molten salt modified adsorbent particles is shown. The experimental test conditions are 300°C, 130kPa, and 30% CO2 feed gas is passed into the adsorption tower at a flow rate of 100mL / min. The adsorption tower is filled with about 23g of prepared adsorbent particles. Figure 4 It can be seen that the first-stage breakthrough time of CO2 in the fixed bed is 6 minutes. After the CO2 breakthrough, the CO2 outflow rate enters a stable state (the second stage), constant at about 10kPa CO2 partial pressure (39kPa CO2 partial pressure of the feed gas). After a stable time of about 200 minutes, it quickly breaks through again (enters the third stage), and its CO2 concentration quickly reaches or approaches the CO2 concentration of the feed gas. After 600 minutes, the CO2 adsorption capacity reaches 12.95mmol / g. Even for the adsorption and capture of low-concentration CO2 in flue gas, the fixed bed can achieve a high CO2 adsorption capacity, so the prepared granular adsorbent has good industrial application prospects.

Claims

1. A method for preparing molten salt modified magnesium oxide adsorbent particles for carbon dioxide capture, characterized in that: The steps include: (1) Preparation of microporous magnesium oxide precursor: Magnesium chloride is used as a magnesium source, reacted with sodium bicarbonate at a reaction temperature of above 80°C to synthesize light basic magnesium carbonate, and then the light basic magnesium carbonate is calcined in two steps to prepare a microporous magnesium oxide precursor; the two-step calcination method comprises a first calcination at 270-330°C for 1-3 hours, and then the temperature is raised to 420-480°C for a second calcination for 1-3 hours; (2) Modified microporous magnesium oxide precursor by mixed salt impregnation composed of nitrate / carbonate: A mixed salt composed of nitrates and carbonates is dispersed in an organic substance, and a microporous magnesium oxide precursor is impregnated. After impregnation, the organic solvent is removed by vacuum drying to obtain a mixed salt modified magnesium oxide adsorbent powder. The mixed salt composed of nitrates and carbonates comprises potassium nitrate, lithium nitrate, sodium carbonate, and potassium carbonate, and the molar ratio of potassium nitrate, lithium nitrate, sodium carbonate, and potassium carbonate is 2-3:1-2:1-2:1-2. (3) Mixed salt modified microporous magnesium oxide powder is formed into granular adsorbent material: The mixed salt modified magnesium oxide adsorbent powder is mixed with a binder and extruded into a certain shape, calcined at 400-500°C to remove moisture, and then crushed and sieved to obtain a granular adsorbent material.

2. The method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture according to claim 1, wherein: In step (2), the nitrate is selected from potassium nitrate and lithium nitrate; and the carbonate is selected from sodium carbonate and potassium carbonate.

3. The method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture according to claim 1, characterized in that: In step (2), the organic solvent used for impregnating the magnesium oxide powder with the mixed salt is selected from one of the following: methanol, ethanol and acetone.

4. The method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture according to claim 1, wherein: The adhesive in step (3) is selected from one of water, aluminum sol, silica sol and cellulose.

5. The method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture according to claim 1, characterized in that: The binder accounts for 0.1% to 5% by mass of the adsorbent.

6. The method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture according to claim 1, characterized in that: The calcination time in step (3) is 1-4 hours.

7. Use of the molten salt modified magnesium oxide adsorbent particles prepared by the method for preparing molten salt modified magnesium oxide adsorbent particles for carbon dioxide capture according to claim 1 in capturing carbon dioxide from medium temperature flue gas in the range of 200°C-400°C.

8. The use according to claim 7, characterized in that: The molten salt modified magnesium oxide adsorbent particles are placed in an adsorption tower, and the operating pressure in the adsorption tower is from normal pressure to 40 atmospheres.

Images