A variable-density packed bed system and capture method for low-temperature carbon capture

Abstract

The present invention discloses a variable-density packed bed system and a capture method for low-temperature carbon capture. The variable-density packed bed system includes a flue gas precooler, a cooling device, a flue gas pressurization buffer tank, a variable-density packed bed, a CO2 gaseous product storage tank, a cold energy recovery device, a gas chromatograph analyzer, and a purge gas pressurization buffer tank, and is connected through a plurality of pipelines and valves. The variable-density packed bed is provided with two inlets and two outlets, and a variety of cold storage fillers are filled in the packing area of the variable-density packed bed. The packing density of the cold storage fillers gradually decreases along the flue gas flow direction. The present invention uses variable-density cold storage fillers that match the frost layer migration characteristics during the sublimation process for sublimation carbon capture, alleviates the problems of excessive flow resistance and the risk of freezing blockage during the sublimation process, and the system operates stably, has high heat exchange efficiency, and is structurally compact. At the same time, in the capture method, the capture purity of the CO2 gaseous product is further improved by the residual flue gas purge method, having good technical effects.

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon dioxide capture, and particularly relates to a variable-density packed bed system and a capture method for low-temperature carbon capture. Background Technique

[0002] At present, CO2 capture technologies mainly include: absorption method, adsorption method, membrane separation method and cryogenic separation method. Compared with other methods, the cryogenic separation method uses the phase change temperature difference between gas components to physically remove CO2. The system does not involve chemical reactions during operation, and the captured CO2 product has a high purity and will not cause secondary pollution to the environment. It has the advantages of long operation life and high reliability.

[0003] The cryogenic separation method can be further divided into sublimation separation method and liquefaction separation method according to different phase change processes. The triple point temperature of CO2 is 216.55K, the triple point pressure is 0.518MPa, the critical point temperature is 304.25K, and the critical point pressure is 7.38MPa. The liquefaction separation method uses compression and condensation methods to capture liquid products. Combining with the CO2 phase diagram data, it can be seen that this method usually needs to compress CO2 to a relatively high pressure. And CO2 can undergo sublimation phase change under normal pressure and complete the capture in the form of solid, without the need to introduce compression equipment, and the operation energy consumption is low, so it shows greater application prospects.

[0004] Chinese Patent with Publication No. CN 114087897 A discloses a sublimation heat exchanger for low-temperature carbon capture and its working method. A coolant is introduced into the heat exchanger for precooling. CO2 sublimates in the heat exchanger. The reciprocating motion of the linear slide rail assembly drives the magnet assembly on the heat exchanger body to move, so that the scraper assembly moves back and forth under the action of magnetic force to scrape off CO2 frost crystals. However, the mechanical scraping method is easy to cause equipment damage and reduce the system operation life. When capturing carbon from a multi-component mixed gas, the gas that has not undergone phase change is not purged and replaced, and the residual gas enters the collection area together with the CO2 frost crystals, resulting in a decrease in product purity.

[0005] Chinese Patent with Publication No. CN 213668629 U discloses a boiler low-temperature cooling carbon capture system. A cooling medium circulation pipeline is arranged inside the low-temperature cooler. After the flue gas is desulfurized and denitrified, it cools and sublimates in the cooler. CO2 and the cooling medium conduct indirect heat exchange through the pipeline, increasing the design size requirements of the heat exchanger. The system needs continuous cooling during operation and is difficult to be applied to the industrial cold energy recovery scenario with intermittent cooling.

[0006] Foreign scholars have proposed the design of using a packed bed for low-temperature sublimation carbon capture and carried out analysis and research from the perspectives of operating energy consumption and economy. However, all the packed bed systems disclosed so far use uniformly sized packing materials, and there has been no consideration of the internal packing arrangement in combination with the frost front migration behavior during the sublimation capture process. Therefore, the current packed bed system has a large flow resistance during operation and there is a risk of freezing blockage due to frost layer accumulation.

[0007] In summary, the existing technologies mainly have the following deficiencies: The system using wall heat exchange for sublimation carbon capture increases the design size requirements of the heat exchanger, and the requirement of continuous cooling is difficult to effectively match with intermittent cold sources; The internal packing arrangement of the packed bed system using contact heat exchange does not consider the migration behavior of the frost front and there is a risk of freezing blockage; Mechanical defrosting means are likely to cause equipment damage and reduce the operating life of the system; At the same time, the current sublimation carbon capture system lacks a perfect purging method to displace and purge the residual impurity gases in the heat exchange equipment, resulting in a decrease in the purity of the captured CO2 product. Summary of the Invention

[0008] To solve the deficiencies of the existing technologies, the present invention provides a variable-density packed bed system and a capture method for low-temperature carbon capture, realizing efficient heat exchange, compact design, stable operation, and high-purity capture of the low-temperature sublimation carbon capture system.

[0009] A variable-density packed bed system for low-temperature carbon capture includes a flue gas precooler, a cooling device, a flue gas pressurization buffer tank, a variable-density packed bed, a CO2 gaseous product storage tank, a cold energy recovery device, a gas chromatograph, and a purge gas pressurization buffer tank;

[0010] The hot fluid channel of the flue gas precooler is connected to the first air inlet of the flue gas pressurization buffer tank through a pipeline provided with a first shut-off valve for the incoming flue gas;

[0011] A variety of cold storage packings are filled in the packing area of the variable-density packed bed, and the filling density of the cold storage packings gradually decreases along the flue gas flow direction;

[0012] The variable-density packed bed is provided with two inlets and two outlets; Among them, the first outlet of the variable-density packed bed is divided into two paths after passing through the first outlet shut-off valve of the packed bed. One path is sequentially connected to the purified flue gas shut-off valve, the cold fluid channel of the cold energy recovery device, and the gas chromatograph; The other path is sequentially connected to the second shut-off valve for the precooling working medium, the cold fluid channel of the flue gas precooler, the cooling device, and the first shut-off valve for the precooling working medium to serve as the precooling working medium feed pipeline; The outlet pipeline of the flue gas pressurization buffer tank provided with a second shut-off valve for the incoming flue gas is arranged in parallel with the precooling working medium feed pipeline and is connected to the first inlet of the variable-density packed bed through the first inlet shut-off valve of the packed bed;

[0013] The second outlet of the variable-density packed bed is divided into two paths after passing through the second cut-off valve of the packed bed outlet. One path is connected to the second inlet of the pressurization buffer tank through a pipeline provided with a second cut-off valve for purge gas; the other path is sequentially connected to a CO2 cut-off valve, a CO2 gaseous product storage tank, a first cut-off valve for purge gas, the hot fluid channel of a cold energy recovery device, a purge gas pressurization buffer tank, and after passing through the second inlet cut-off valve of the packed bed, it is connected to the second inlet of the variable-density packed bed.

[0014] Further, the cold storage packing uses three kinds of spherical packings with different particle sizes. Along the flue gas flow direction in the packing area, there are a high-density packing layer, a medium-density packing layer, and a low-density packing layer in sequence. Temperature sensors are respectively arranged in each density packing layer to monitor the precooling effect of the packed bed.

[0015] The incoming flue gas entering from the first inlet of the variable-density packed bed is first cooled by the high-density packing layer, and the temperature rapidly drops below the carbon dioxide frost point temperature. Subsequently, sublimation and frosting occur in the upper regions of the medium-density packing layer and the low-density packing layer; under the heating of the incoming flue gas, the surface temperature of the packing gradually rises, and the frost crystals initially condensed on the packing surface sublimate, and then sublimate again after being cooled by the packing in the lower region, showing an overall downward migration of the frost crystals until the maximum capture limit of the packed bed is reached.

[0016] The packing layers with three packing densities respectively play different functions: rapidly cooling the incoming flue gas, providing carbon dioxide sublimation sites, and providing frosting sites for carbon dioxide sublimation and the downward migration and re-sublimation of frost crystals.

[0017] Optionally, the cold storage packing uses spherical packings with different particle sizes, vertical or corrugated cold storage plates with fins arranged at different intervals.

[0018] Optionally, the material of the cold storage packing is one or more of stainless steel, ceramic, phosphor bronze, and brass.

[0019] Further, the flue gas pressurization buffer tank has a buffering effect on the pressure fluctuations generated during the mixing process of the purge gas and the incoming flue gas, and a diaphragm or airbag is provided in the tank to pressurize the mixed flue gas.

[0020] Further, the flow directions of the logistics at the first inlet and outlet of the variable-density packed bed are opposite to those of the logistics at the second inlet and outlet; the first inlet cut-off valve, the first outlet cut-off valve, the second inlet cut-off valve, and the second outlet cut-off valve of the packed bed are all one-way valves.

[0021] Further, the variable-density packed bed sequentially includes a heat insulation layer, a vacuum interlayer, an electric heating layer, and a packing area from the outside to the inside; when the residual flue gas is purged and replaced, the electric heating layer is started to heat the carbon dioxide frost crystals so that they sublimate to complete the capture in the form of a gaseous product.

[0022] Optionally, the flue gas pre-cooler and the cold energy recovery device are plate-fin heat exchangers or shell-and-tube heat exchangers.

[0023] Optionally, the cooling device includes a refrigeration system driven by external energy or a heat exchange system for recovering industrial waste cold, and the cold energy recovery working medium includes low-temperature fluids such as LNG, liquid nitrogen, and by-products of air separation.

[0024] A low-temperature carbon dioxide capture method using the variable-density packed bed system for low-temperature carbon capture described above specifically includes the following steps:

[0025] Step 1, pre-cooling process: First, open the first cut-off valve for the pre-cooling working medium and the first inlet cut-off valve of the packed bed, and keep the other valves closed. The pre-cooling working medium cools the packing in the variable-density packed bed to a temperature below the CO2 frost point temperature and reaches the sub-cooling temperature at the same time. The temperature sensors in the packing area continuously monitor the temperatures of the packing in different density layers in the packed bed as the basis for judging the pre-cooling process.

[0026] After the temperature of the packing drops to the set working temperature range, open the first outlet cut-off valve of the packed bed and the second cut-off valve of the pre-cooling working medium, and close the other valves. The pre-cooling working medium flows into the flue gas pre-cooler to pre-cool the incoming flue gas, and then flows into the cooling device to restore the pre-cooling capacity.

[0027] Step 2, sublimation separation process: Open the first cut-off valve for the incoming flue gas. The pre-cooled flue gas enters the flue gas pressurization buffer tank and mixes with the purge gas. Open the second cut-off valve for the incoming flue gas, the first inlet cut-off valve of the packed bed, the first outlet cut-off valve of the packed bed, and the purified flue gas cut-off valve, and close the other valves. The flue gas pressurization buffer tank pressurizes the flue gas into the variable-density packed bed for sublimation carbon capture.

[0028] The flue gas flows through the high-density packing layer, the medium-density packing layer, and the low-density packing layer in sequence. In the high-density packing layer, the flue gas quickly cools down to below the CO2 frost point, and then sublimation frosting occurs in the upper regions of the medium-density packing layer and the low-density packing layer. As the capture process continues, under the heating of the incoming flue gas, the surface temperature of the packing gradually increases, and the frost crystals initially condensed on the surface of the packing sublimate, and then sublime again after being cooled by the packing in the lower region, showing an overall downward migration of the frost crystals until the maximum capture limit of the packed bed is reached.

[0029] The CO2 in the flue gas frosts on the surface of the packing to complete sublimation separation. The purified flue gas after removing CO2 enters the cold energy recovery device for rewarming, and then enters the gas chromatograph for continuous monitoring of the CO2 concentration.

[0030] Step 3, purge gas treatment process: A small amount of CO2 product is used as the purge gas. When the purified flue gas flows out of the variable-density packed bed, open the first cut-off valve of the purge gas. After being cooled in the cold energy recovery device, the purge gas enters the purge gas booster buffer tank.

[0031] Step 4, purge process: When the packed bed reaches the maximum capture limit and the CO2 concentration monitored by the gas chromatograph exceeds the allowable working limit, stop feeding flue gas into the variable-density packed bed; open the second inlet cut-off valve of the packed bed, the second outlet cut-off valve of the packed bed, and the second cut-off valve of the purge gas, and close the other valves; the purge gas booster buffer tank presses the gas into the variable-density packed bed, and the residual flue gas is displaced and flows into the flue gas booster buffer tank together with the purge gas.

[0032] Step 5, sublimation collection process: After the residual flue gas in the packed bed is removed, keep the second outlet cut-off valve of the packed bed open, open the CO2 cut-off valve, close the other valves, start the electric heating layer of the variable-density packed bed, and the CO2 frost crystals sublimate and flow into the CO2 gaseous product storage tank to complete the capture in the form of gas.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. The variable-density packing design of the present invention effectively matches the migration characteristics of the CO2 frost layer in the condensation capture process, and the segmented action realizes the rapid cooling of the incoming flue gas and fully utilizes the frosting sites provided by the packing surface for capture.

[0035] 2. The present invention uses a small amount of CO2 product as the purge gas to displace the residual flue gas in the packed bed, improving the capture purity of the product. At the same time, the purge gas is cooled to near the frost point by the purified flue gas, avoiding the premature sublimation of CO2 frost crystals caused by purging; the sublimation method of electric heating overcomes the damage to the system caused by mechanical defrosting and prolongs the service life of the system.

[0036] 3. The present invention directly contacts and exchanges heat between the incoming flue gas and the packing, reducing the heat transfer resistance and making the system more compact; the packing with cold storage function overcomes the deficiency of intermittent cooling in the cold energy recovery scenario, and combined with industrial waste cold recovery, the capture cost can be further reduced. Brief Description of the Drawings

[0037] Figure 1 It is a schematic diagram of the overall structure of a variable-density packed bed system for low-temperature carbon capture according to the present invention;

[0038] Figure 2 It is a schematic diagram of the internal structure of the variable-density packed bed in the embodiment of the present invention;

[0039] Figure 3 It is a flow chart of the purge method for the variable-density packed bed system in the embodiment of the present invention.

[0040] In the figure: 1 - Flue gas precooler; 2 - Cooling device; 3 - First on - flow flue gas shut - off valve; 4 - Flue gas booster buffer tank; 5 - Second on - flow flue gas shut - off valve; 6 - First precooling medium shut - off valve; 7 - First inlet shut - off valve of packed bed; 8 - Variable - density packed bed; 9 - Temperature sensor in high - density packing area; 10 - Temperature sensor in medium - density packing area; 11 - Temperature sensor in low - density packing area; 12 - First outlet shut - off valve of packed bed; 13 - Purified flue gas shut - off valve; 14 - Second precooling medium shut - off valve; 15 - Second inlet shut - off valve of packed bed; 16 - Second outlet shut - off valve of packed bed; 17 - Second purge gas shut - off valve; 18 - CO2 shut - off valve; 19 - CO2 gaseous product storage tank; 20 - First purge gas shut - off valve; 21 - Cold energy recovery device; 22 - Gas chromatograph analyzer; 23 - Purge gas booster buffer tank; 801 - First inlet of packed bed; 802 - Thermal insulation layer; 803 - Vacuum interlayer; 804 - Electric heating layer; 805 - First outlet of packed bed; 806 - Second inlet of packed bed; 807 - Low - density packing layer; 808 - Medium - density packing layer; 809 - High - density packing layer; 810 - Second outlet of packed bed. Specific embodiments

[0041] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the following embodiments are intended to facilitate the understanding of the present invention and do not limit it in any way.

[0042] As Figure 1 and Figure 2 shown, a variable - density packed bed system for low - temperature carbon capture includes a precooling unit, a flue gas booster buffer unit, a carbon dioxide capture unit, and a purge gas treatment unit.

[0043] The precooling unit includes: a flue gas precooler 1, a cooling device 2, a first precooling medium shut - off valve 6, a second precooling medium shut - off valve 14, a temperature sensor 9 in the high - density packing area, a temperature sensor 10 in the medium - density packing area, and a temperature sensor 11 in the low - density packing area. The hot - fluid inlet and outlet of the flue gas precooler 1 are respectively connected to the flue gas and the first on - flow flue gas shut - off valve 3, and the cold - fluid inlet and outlet are respectively connected to the second precooling medium shut - off valve 14 and the cooling device 2; the cooling device 2 cools the heated precooling medium to restore its precooling ability; the first precooling medium shut - off valve 6 connects the cooling device 2 and the first inlet shut - off valve 7 of the packed bed; the temperature sensors 9, 10, and 11 in the high - density, medium - density, and low - density packing areas are placed inside the packing layer to monitor the precooling effect of the packed bed.

[0044] The flue gas pressurization and buffering unit includes: the first incoming flue gas cut-off valve 3, the flue gas pressurization and buffering tank 4, the second incoming flue gas cut-off valve 5, and the second purge gas cut-off valve 17. The flue gas pressurization and buffering tank 4 is provided with two air inlets, which are respectively connected to the first incoming flue gas cut-off valve 3 and the second purge gas cut-off valve 17. The impurity gas purged before the sublimation of the packed bed is mixed and pressurized with the incoming flue gas here; the second incoming flue gas cut-off valve 5 is communicated with the exhaust port of the flue gas pressurization and buffering tank 4.

[0045] The carbon dioxide capture unit includes: the first packed bed inlet cut-off valve 7, the variable density packed bed 8, the first packed bed outlet cut-off valve 12, the second packed bed inlet cut-off valve 15, the second packed bed outlet cut-off valve 16, the CO2 cut-off valve 18, and the CO2 gaseous product storage tank 19. The variable density packed bed 8 is provided with two inlets and two outlets, which are respectively connected to the first packed bed inlet cut-off valve 7, the second packed bed inlet cut-off valve 15 and the first packed bed outlet cut-off valve 12, the second packed bed outlet cut-off valve 16. CO2 sublimates and separates from other gas components in the packed bed. After purging the residual flue gas, the sublimation operation starts, and the capture is carried out in a gaseous form; the CO2 cut-off valve 18 connects the second packed bed outlet cut-off valve 16 and the CO2 gaseous product storage tank 19.

[0046] The purge gas treatment unit includes: the purified flue gas cut-off valve 13, the first purge gas cut-off valve 20, the cold energy recovery device 21, the gas chromatograph 22, and the purge gas pressurization and buffering tank 23. The hot fluid inlet and outlet of the cold energy recovery device 21 are respectively connected to the first purge gas cut-off valve 20 and the purge gas pressurization and buffering tank 23, and the cold fluid inlet and outlet are respectively connected to the purified flue gas cut-off valve 13 and the gas chromatograph 22. The purge gas and the purified flue gas exchange heat here, cooling the purge gas while recovering the cold energy of the purified flue gas.

[0047] The internal structure of the variable density packed bed 8 includes: the first packed bed inlet 801, the heat insulation layer 802, the vacuum interlayer 803, the electric heating layer 804, the first packed bed outlet 805, the second packed bed inlet 806, the low density packing layer 807, the medium density packing layer 808, the high density packing layer 809, and the second packed bed outlet 810.

[0048] The variable density packed bed 8 uses a material with good cold storage performance as the cold storage packing. In the precooling stage and the carbon dioxide capture stage, the precooling working medium, the incoming flue gas and the spherical packing exchange heat in a direct contact manner. The cold storage packing includes any one or more of stainless steel, ceramic, phosphor bronze, brass, etc.

[0049] Refer to Figures 1-2 As shown, the carbon dioxide capture process of the present invention is as follows:

[0050] S01, Pre-cooling process. First, open the first cut-off valve 6 of the pre-cooling working medium and the first inlet cut-off valve 7 of the packed bed, and keep the rest of the valves closed. The pre-cooling working medium cools the packing in the variable-density packed bed 8 to a temperature below the CO2 frost point temperature and reaches a certain sub-cooled temperature at the same time. The temperature sensors 9 in the high-density packing area, 10 in the medium-density packing area, and 11 in the low-density packing area monitor the temperatures of the packing in different density layers in the packed bed in real time, which are used as the basis for judging the pre-cooling process.

[0051] After the temperature of the packing drops to the set working temperature range, open the first outlet cut-off valve 12 of the packed bed and the second cut-off valve 14 of the pre-cooling working medium, and close the rest of the valves; the pre-cooling working medium flows into the flue gas pre-cooler 1 to pre-cool the incoming flue gas, and then flows into the cooling device 2, and the pre-cooling capacity is restored through a refrigeration system driven by external energy or a heat exchange system that recovers industrial waste cold.

[0052] S02, Sublimation separation process. Open the first cut-off valve 3 of the incoming flue gas. The pre-cooled flue gas enters the flue gas pressurization buffer tank 4 and mixes with the purge gas. Open the second cut-off valve 5 of the incoming flue gas, the first inlet cut-off valve 7 of the packed bed, the first outlet cut-off valve 12 of the packed bed, and the purified flue gas cut-off valve 13, and close the rest of the valves; the diaphragm or airbag in the flue gas pressurization buffer tank 4 presses the flue gas into the variable-density packed bed 8 for sublimation carbon capture.

[0053] The flue gas flows through the high-density packing layer 809, the medium-density packing layer 808, and the low-density packing layer 807 in sequence. In the high-density packing layer 809, the flue gas quickly cools down to below the CO2 frost point, and then sublimation frosting occurs in the upper regions of the medium-density packing layer 808 and the low-density packing layer 807; as the capture process continues, under the heating of the incoming flue gas, the surface temperature of the packing gradually increases, and the frost crystals initially condensed on the surface of the packing sublimate, and then sublimate again after being cooled by the packing in the lower region, showing an overall downward migration of the frost crystals until the maximum capture limit of the packed bed is reached; the packing layers with three packing densities play different functions respectively: quickly cooling the incoming flue gas, providing carbon dioxide sublimation sites, and providing frosting sites for carbon dioxide sublimation and the downward migration and re-sublimation of the frost crystals.

[0054] The CO2 in the flue gas frosts on the surface of the packing, completing the sublimation separation. The purified flue gas after removing CO2 enters the cold energy recovery device 21 for rewarming, and then enters the gas chromatograph 22 to continuously monitor the CO2 concentration.

[0055] S03, Purge gas treatment process. Use a small amount of CO2 product as the purge gas. When the purified flue gas flows out of the variable-density packed bed 8, open the first cut-off valve 20 of the purge gas. The purge gas is cooled in the cold energy recovery device 21 and then enters the purge gas pressurization buffer tank 23.

[0056] S04. Purge process: When the packed bed reaches its maximum trapping limit and the CO2 concentration monitored in the gas chromatograph analyzer 22 exceeds the allowable working limit, the flue gas supply to the packed bed is stopped. The second inlet shut-off valve 15 of the packed bed, the second outlet shut-off valve 16 of the packed bed, and the second purge gas shut-off valve 17 are opened, and the other valves are closed. The diaphragm or airbag in the purge gas pressurization buffer tank 23 presses the gas into the variable-density packed bed 8, displacing the residual flue gas, which then flows into the flue gas pressurization buffer tank 4 together with the purge gas.

[0057] S05. Sublimation collection process: After the residual flue gas in the packed bed is removed, the second outlet shut-off valve 16 of the packed bed remains open. The CO2 shut-off valve 18 is opened, the other valves are closed, and the electric heating layer 804 of the variable-density packed bed 8 is started. The CO2 frost crystals sublimate and flow into the CO2 gaseous product storage tank 19, completing the capture in the form of gas.

[0058] Refer to Figure 3 As shown in the figure, the embodiment of the present invention provides a purge method for a variable-density packed bed system for low-temperature carbon capture, including the following steps:

[0059] Step 1: The pre-cooled flue gas is further cooled in the variable-density packed bed 8, and the CO2 in the flue gas sublimates and freezes, completing the low-temperature carbon capture of the incoming flue gas.

[0060] Step 2: The purified flue gas after removing CO2 passes through the first outlet shut-off valve 12 of the packed bed and the purified flue gas shut-off valve 13 in sequence, and exchanges heat with the purge gas in the cold energy recovery device 21.

[0061] A small amount of pure gas is released from the CO2 gaseous product storage tank 19 as the purge gas before the sublimation of the frost crystals in the variable-density packed bed 8. The first purge gas shut-off valve 20 is opened, and the purge gas is cooled by the purified flue gas in the cold energy recovery device 21 and then stored in the purge gas pressurization buffer tank 23.

[0062] Step 3: The heated purified flue gas enters the gas chromatograph analyzer 22, and the CO2 concentration is continuously monitored. As the capture process progresses, the temperature in the packed bed gradually rises, the frost layer on the surface of the packing becomes thicker, and after a period of time, the packed bed loses its sublimation capture ability, and the CO2 concentration in the tail gas rises sharply. The supply of flue gas to the packed bed is stopped.

[0063] Step 4: When the supply of flue gas to the variable-density packed bed 8 is stopped and before the system switches from the sublimation carbon capture mode to the sublimation mode, there is a small amount of uncompleted captured flue gas remaining inside the packed bed. Direct sublimation of the frost crystals will result in a decrease in the capture purity. Therefore, before sublimation, the residual flue gas needs to be purged with pure CO2 gas.

[0064] Open the second inlet stop valve 15 and the second outlet stop valve 16 of the packed bed, close the first inlet stop valve 7, the first outlet stop valve 12 of the packed bed, and the first purge gas stop valve 20. Use the purified CO2 gas after cooling to purge the variable density packed bed 8. The temperature of the cooled CO2 should be as low as possible to avoid premature sublimation of frost crystals caused by heat exchange, resulting in a decrease in the CO2 capture amount.

[0065] In step 5, open the second purge gas stop valve 17 and close the CO2 stop valve 18. The purge gas and the residual flue gas enter the flue gas booster buffer tank 4 together to complete the purging operation of the packed bed. When the system is in the sublimation working mode, the purge gas in the flue gas booster buffer tank 4 enters the variable density packed bed 8 together with the incoming flue gas for low-temperature sublimation carbon capture.

[0066] The above embodiments have detailed the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, supplements, and equivalent replacements made within the scope of the principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A variable-density packed bed system for low-temperature carbon capture, characterized in that, It includes a flue gas precooler (1), a cooling device (2), a flue gas pressurization buffer tank (4), a variable density packed bed (8), a CO2 gaseous product storage tank (19), a cold energy recovery device (21), a gas chromatograph analyzer (22), and a purge gas pressurization buffer tank (23); The hot fluid channel of the flue gas precooler (1) is connected to the first inlet of the flue gas pressurization buffer tank (4) through a pipeline provided with an incoming flue gas first shut-off valve (3); A variety of cold storage fillers are filled in the filler area of the variable density packed bed (8), and the filling density of the cold storage fillers gradually decreases along the flue gas flow direction; The variable density packed bed (8) is provided with two inlets and two outlets; among them, the first outlet of the variable density packed bed (8) is divided into two paths after passing through the packed bed first outlet shut-off valve (12). One path is sequentially connected to the purified flue gas shut-off valve (13), the cold fluid channel of the cold energy recovery device (21), and the gas chromatograph analyzer (22); the other path is sequentially connected to the precooling medium second shut-off valve (14), the cold fluid channel of the flue gas precooler (1), the cooling device (2), and the precooling medium first shut-off valve (6) to form a precooling medium feed pipeline; the outlet pipeline of the flue gas pressurization buffer tank (4) provided with an incoming flue gas second shut-off valve (5) is arranged in parallel with the precooling medium feed pipeline and is connected to the first inlet of the variable density packed bed (8) through the packed bed first inlet shut-off valve (7); The second outlet of the variable density packed bed (8) is divided into two paths after passing through the packed bed second outlet shut-off valve (16). One path is connected to the second inlet of the pressurization buffer tank (4) through a pipeline provided with a purge gas second shut-off valve (17); the other path is sequentially connected to the CO2 shut-off valve (18), the CO2 gaseous product storage tank (19), the purge gas first shut-off valve (20), the hot fluid channel of the cold energy recovery device (21), the purge gas pressurization buffer tank (23), and the packed bed second inlet shut-off valve (15) and then connected to the second inlet of the variable density packed bed (8); The cold storage fillers adopt three kinds of spherical fillers with different particle sizes. Along the flue gas flow direction in the filler area, there are a high-density filler layer (809), a medium-density filler layer (808), and a low-density filler layer (807) in sequence. Temperature sensors are respectively arranged in each density filler layer to monitor the precooling effect of the packed bed; The incoming flue gas entering from the first inlet of the variable density packed bed (8) is first cooled by the high-density filler layer (809), and the temperature rapidly drops below the carbon dioxide frost point temperature. Subsequently, sublimation and frosting occur in the upper regions of the medium-density filler layer (808) and the low-density filler layer (807); under the heating of the incoming flue gas, the surface temperature of the fillers gradually rises, and the frost crystals initially condensed on the filler surface sublimate, and then sublime again after being cooled by the fillers in the lower region. Overall, it shows the downward migration of the frost crystals until the maximum capture limit of the packed bed is reached; The three filler layers with different filling densities perform different functions: rapidly cooling the incoming flue gas, providing carbon dioxide sublimation sites, and providing frosting sites for carbon dioxide sublimation and the downward migration and re-sublimation of the frost crystals after sublimation.

2. The variable density packed bed system for low-temperature carbon capture according to claim 1, wherein The cold storage filler described above uses spherical fillers with different particle sizes, and vertical or wavy cold storage plates with fins arranged at different intervals.

3. The variable-density packed bed system for low-temperature carbon capture according to claim 1, wherein The material of the cold storage filler described above is one or more of stainless steel, ceramics, phosphor bronze, and brass.

4. The variable density packed bed system for low-temperature carbon capture according to claim 1, characterized in that, The flow direction of the first inlet and outlet logistics of the variable density packed bed (8) is opposite to that of the second inlet and outlet logistics; the packed bed first inlet stop valve (7), the packed bed first outlet stop valve (12), the packed bed second inlet stop valve (15), and the packed bed second outlet stop valve (16) are all one-way valves.

5. The variable density packed bed system for low-temperature carbon capture according to claim 1, characterized in that The variable density packed bed (8) described above sequentially includes an adiabatic layer (802), a vacuum interlayer (803), an electric heating layer (804), and a packing area from the outside to the inside; when the residual flue gas is purged and replaced, the electric heating layer (804) is started to heat the carbon dioxide frost crystals so that they sublimate to complete the capture in the form of gaseous products.

6. The variable density packed bed system for low-temperature carbon capture according to claim 1, characterized in that, The flue gas pre-cooler (1) and the cold energy recovery device (21) described above are plate-fin heat exchangers or shell-and-tube heat exchangers.

7. The variable density packed bed system for low-temperature carbon capture according to claim 1, wherein The cooling device (2) described above includes a refrigeration system driven by external energy or a heat exchange system for recovering industrial waste cold.

8. A method for low-temperature carbon dioxide capture, characterized in that, Using the variable density packed bed system for low-temperature carbon capture according to any one of claims 1 to 7, specifically includes the following steps: Step 1, pre-cooling process: First, open the pre-cooling medium first stop valve (6) and the packed bed first inlet stop valve (7), and keep the other valves closed. The pre-cooling medium cools the fillers in the variable density packed bed (8) to a temperature below the CO2 frost point temperature and simultaneously reaches the sub-cooling temperature; the temperature sensors in the packing area continuously monitor the temperatures of the fillers in different density layers in the packed bed as the basis for judging the pre-cooling process. After the filler temperature drops to the set working temperature range, open the packed bed first outlet stop valve (12) and the pre-cooling medium second stop valve (14), and close the other valves; the pre-cooling medium flows into the flue gas pre-cooler (1) to pre-cool the incoming flue gas, and then flows into the cooling device (2) to restore the pre-cooling capacity. Step 2, sublimation separation process: Open the incoming flue gas first stop valve (3), and the pre-cooled flue gas enters the flue gas pressurization buffer tank (4) to mix with the purge gas. Open the incoming flue gas second stop valve (5), the packed bed first inlet stop valve (7), the packed bed first outlet stop valve (12), and the purified flue gas stop valve (13), and close the other valves. The flue gas pressurization buffer tank (4) presses the flue gas into the variable density packed bed (8) for sublimation carbon capture. The flue gas sequentially flows through the high-density packing layer (809), the medium-density packing layer (808), and the low-density packing layer (807). In the high-density packing layer (809), the flue gas rapidly cools to below the CO2 frost point, and then sublimation frosting occurs in the upper regions of the medium-density packing layer (808) and the low-density packing layer (807); as the capture process continues, under the heating of the incoming flue gas, the surface temperature of the fillers gradually increases, and the frost crystals initially condensed on the filler surface sublime, and then sublime again after being cooled by the fillers in the lower region, and the overall performance is the downward migration of the frost crystals until the maximum capture limit of the packed bed is reached. CO2 in the flue gas freezes on the surface of the packing to complete sublimation separation. The purified flue gas after removing CO2 enters the cold energy recovery device (21) for reheating, and then enters the gas chromatograph analyzer (22) to continuously monitor the CO2 concentration; Step 3: Purge gas treatment process. A small amount of CO2 product is used as the purge gas. When the purified flue gas flows out of the variable density packed bed (8), open the first cut-off valve (20) of the purge gas. The purge gas is cooled in the cold energy recovery device (21) and then enters the purge gas booster buffer tank (23); Step 4: Purge process. When the packed bed reaches the maximum capture limit and the CO2 concentration monitored in the gas chromatograph analyzer (22) exceeds the allowable working limit, stop introducing flue gas into the variable density packed bed (8); open the second inlet cut-off valve (15), the second outlet cut-off valve (16) of the packed bed, and the second cut-off valve (17) of the purge gas, and close the other valves; the purge gas booster buffer tank (23) presses the gas into the variable density packed bed (8), and the residual flue gas is displaced and flows into the flue gas booster buffer tank (4) together with the purge gas; Step 5: Sublimation collection process. After the residual flue gas in the packed bed is removed, the second outlet cut-off valve (16) of the packed bed remains open. Open the CO2 cut-off valve (18), close the other valves, and start the electric heating layer (804) of the variable density packed bed (8). The CO2 frost crystals sublimate and flow into the CO2 gaseous product storage tank (19) to complete the capture in the form of gas.

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