A carbon dioxide low-temperature capture system using circulating nitrogen combined with cold energy cascade utilization
The low-temperature carbon dioxide capture system, which combines circulating nitrogen with cascaded utilization of cold energy, solves the problems of low-temperature CO2 sublimation in low-concentration flue gas, such as small processing capacity and high energy consumption. It achieves continuous capture and efficient utilization, especially with near-zero energy consumption low-temperature CO2 separation when LNG cold energy is available.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- ZHEJIANG UNIV
- Filing Date
- 2023-12-15
- Publication Date
- 2026-06-19
AI Technical Summary
Existing low-temperature CO2 sublimation methods suffer from problems such as small processing capacity, high energy consumption, complex systems, and inability to continuously capture low-concentration carbon dioxide. In particular, there is a lack of efficient cold energy utilization solutions in the low-carbon dioxide concentration flue gas scenario of LNG power plants.
A carbon dioxide cryogenic capture system that combines circulating nitrogen with cascaded utilization of cold energy utilizes N2 circulation loop and CO2 collection and storage flow path. It uses N2 as a cold source and cold energy carrier to achieve cascaded utilization of cold energy. By changing the N2 circulation direction through a combination of switching valves, it achieves continuous, low-energy consumption, and high-capacity CO2 capture.
It significantly reduces the power consumption of multi-stage compression, improves the purity and separation rate of CO2 separation, and realizes continuous capture and efficient utilization of low-temperature CO2. Especially when LNG cold energy is available, it has near-zero energy consumption and is suitable for CO2 separation of low-concentration flue gas.
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Figure CN117732191B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of CO2 separation, and in particular relates to a low-temperature carbon dioxide capture system that uses circulating nitrogen combined with the cascade utilization of cold energy. Background Technology
[0002] Current industrial-grade traditional carbon capture technologies—absorption, adsorption, membrane separation, and cryogenic liquefaction—each have their own shortcomings in low-concentration carbon dioxide capture scenarios, making it difficult to simultaneously achieve system reliability, durability, economy, and high capture rates. Cryogenic sublimation, however, can be combined with waste cooling recovery to separate low-concentration carbon dioxide flue gas at atmospheric pressure, achieving high capture rates and high purity with low energy consumption. Compared to cryogenic liquefaction, cryogenic sublimation can capture carbon dioxide with higher purity without pressurization. However, current research on cryogenic sublimation has several limitations, such as the lack of switching devices (only localized sublimation) and high energy consumption in industrial applications.
[0003] Although gases with carbon dioxide content above their triple point pressure are generally suitable for gas-liquid separation by cooling and partial condensation, the increased compression power consumption reduces the competitiveness of cryogenic gas-liquid separation when using flue gas with carbon dioxide content below its triple point pressure for cryogenic capture. Therefore, gas-solid separation by cooling and carbon dioxide sublimation becomes the preferred method.
[0004] Research progress on cryogenic carbon dioxide sublimation separation technology is slower than that of the aforementioned absorption, adsorption, and membrane separation methods. This is mainly because cryogenic methods require additional refrigeration systems, increasing system complexity. As of 2021, my country's LNG power plants consumed 44 million tons of LNG annually, generating 10 billion kWh of vaporization cooling energy. This far exceeds the cooling energy required for capturing the 120 million tons of carbon dioxide emitted annually through sublimation. Therefore, utilizing LNG cooling energy can solve the cooling energy source problem for sublimation methods, eliminating the need for additional refrigeration systems.
[0005] Liquefying carbon dioxide at 300K requires 70 atmospheres of pressure, while sublimation at around 200K requires only 1 atmosphere, significantly reducing compression energy consumption. LNG vaporization temperature is approximately 110K, which perfectly meets the temperature required for CO2 sublimation in flue gas with low carbon dioxide concentrations (5%-20%). Compared to liquefaction, which is suitable for high-concentration pre-combustion capture and ultra-high operating pressures, sublimation, performed at low temperature and under normal pressure, is more suitable for post-combustion flue gas carbon capture in LNG power plants where there is potential for cold energy.
[0006] Chinese patent CN113975938A discloses a device and method for capturing carbon dioxide from flue gas by rotating low-temperature adsorption. CO2 separation is achieved by continuous rotation of an integral adsorption bed during the adsorption, desorption and cooling stages. However, its structure is relatively complex, has high requirements for waste heat, and the energy consumption required for continuous rotation of the adsorption bed is high.
[0007] Chinese patent CN115025512A discloses a switching CO2 sublimation separation system. The system uses a cold source in a heat exchanger to directly exchange heat with the flue gas, causing the CO2 in the flue gas to sublimate. However, the temperature of the cold source may be lower than the sublimation point of the flue gas with low CO2 concentration, resulting in a large temperature difference in heat exchange and a lot of energy waste.
[0008] Chinese patent CN107677044A discloses an oxygen-enriched combustion exhaust gas treatment system employing a low-temperature sublimation method. This system treats the high CO2 concentration exhaust gas after oxygen-enriched combustion by using a cold source to sublimate the pre-cooled high CO2 concentration exhaust gas. The resulting CO2 frost is then used to pre-cool the relatively high-temperature, high-CO2-concentration exhaust gas after oxygen-enriched combustion, turning the CO2 frost into liquid CO2 for storage. This patent relates to equipment such as air separation units; the CO2 capture design utilizes gaseous, liquid, and solid states, making the capture process relatively complex.
[0009] In light of the above background, the main technical challenges currently faced by LNG power plants in dealing with low-carbon dioxide concentration flue gas include small processing capacity, high energy consumption, inability to continuously capture CO2, and system complexity. Therefore, it is necessary to explore a new type of carbon dioxide sublimation capture system. Summary of the Invention
[0010] To address the shortcomings of existing low-temperature carbon dioxide capture technologies in flue gas with low carbon dioxide concentrations, this invention provides a low-temperature carbon dioxide capture system that utilizes circulating nitrogen combined with cascaded cold energy utilization, enabling continuous, low-energy-consumption, and high-capacity carbon dioxide capture.
[0011] A low-temperature carbon dioxide capture system employing circulating nitrogen combined with cascaded utilization of cold energy is characterized by comprising a flue gas flow path, an N2 circulation loop, and a CO2 collection and storage flow path.
[0012] The N2 circulation loop includes the second branch of the precooling heat exchanger, the seventh switching valve, the second bypass valve, the second branch of the second heat exchanger, the second mixing chamber, the eighth switching valve, the cold source heat exchanger, the fifth switching valve, the first mixing chamber, the second branch of the first heat exchanger, the first bypass valve, and the sixth switching valve connected in sequence.
[0013] The flue gas flow path includes two paths. The first path sequentially connects to the dehumidifier, compressor No. 1, first switching valve, second switching valve, third switching valve, first branch of heat exchanger No. 1, fourth switching valve, and fifth switching valve before converging into the N2 circulation loop. It then connects to the external environment via a pipeline from the seventh switching valve to the eleventh switching valve and the first branch of the CO2 heat exchanger. The second path sequentially connects to the dehumidifier, compressor No. 1, first switching valve, third switching valve, second switching valve, first branch of heat exchanger No. 2, ninth switching valve, and eighth switching valve before converging into the N2 circulation loop. It then connects to the external environment via a pipeline from the sixth switching valve to the eleventh switching valve and the first branch of the CO2 heat exchanger.
[0014] The CO2 collection and storage flow path includes two paths. The first path sequentially connects the first branch of the first heat exchanger, the fourth switching valve, the tenth switching valve, the second compressor, the second branch of the CO2 heat exchanger, and the CO2 storage tank. The second path sequentially connects the first branch of the second heat exchanger, the ninth switching valve, the tenth switching valve, the second compressor, the second branch of the CO2 heat exchanger, and the CO2 storage tank.
[0015] In this invention, N2 in the N2 circulation loop serves as a cold energy carrier for both the cold source and the solid CO2 energy. The cold energy is gradually released in three heat exchangers: the precooling heat exchanger, the first heat exchanger, and the second heat exchanger, thus achieving cascaded utilization of the cold source energy. By changing the circulation direction of N2 in the N2 circulation loop, the condensation and sublimation states of the first and second heat exchangers are altered, thereby achieving continuous, low-energy-consumption, and high-capacity CO2 capture.
[0016] Furthermore, the precooling heat exchanger, heat exchanger No. 1, heat exchanger No. 2, and CO2 heat exchanger in the flue gas flow path operate in coordination in the temperature range to achieve cascade utilization of the cold source's cooling capacity.
[0017] Furthermore, the N2 in the N2 circulation loop is used as a cold source carrier for the cold energy and the CO2 solid cold energy;
[0018] In the N2 circulation loop, N2 passes through a cold source heat exchanger, where the cooling energy from the cold source is transferred to the N2 in the N2 circulation loop, thus transferring cooling energy from the cold source to N2. When the first heat exchanger is in a sublimation state, CO2 in the flue gas is sublimated by the N2 in the N2 circulation loop, and the cooling energy of the N2 in the N2 circulation loop is transferred to the CO2 and nitrogen-rich flue gas in the flue gas. The nitrogen-rich flue gas flowing out of the first heat exchanger enters the N2 circulation loop, mixes evenly with the N2 in the first mixing chamber, and then enters the precooling heat exchanger. The flue gas is precooled by the N2 in the N2 circulation loop, thus transferring cooling energy from N2 to the flue gas. At this time, the second heat exchanger is in a sublimation state, and the solid CO2 in the second heat exchanger is sublimated by the N2 in the N2 circulation loop flowing out of the precooling heat exchanger, thus transferring cooling energy from CO2 to N2.
[0019] Furthermore, by connecting the pipeline between the No. 1 bypass valve and the No. 2 bypass valve, and by connecting the N2 circulation loop with the No. 1 mixing chamber and the No. 2 mixing chamber through the fifth switching valve, the N2 circulation loop becomes a variable flow circulation loop.
[0020] When heat exchanger 1 is in the condensation state and heat exchanger 2 is in the sublimation state, the nitrogen-rich flue gas with a flow rate of 'a' from heat exchanger 1 and the circulating N2 with a flow rate of 'b' from cold source heat exchanger 1 are mixed in mixing chamber 1, and the flow rate of the mixed circulating N2 is a+b. By calculating the precooling requirements of the precooling heat exchanger, the required N2 flow rate is a'. Therefore, the circulating N2 with a flow rate of a+b is divided into two streams in bypass valve 1. One stream with a flow rate of a' is introduced into the second branch of the precooling heat exchanger, and the other stream with a flow rate of a+b-a' passes through bypass valve 2.
[0021] Furthermore, in the CO2 collection and storage flow path, the sublimated CO2 gas in the No. 1 heat exchanger and the No. 2 heat exchanger is compressed and then cooled by the CO2 heat exchanger before finally entering the high-pressure CO2 storage tank for storage.
[0022] During system operation, N2 in the N2 circulation loop periodically changes the flow direction by using the opening and closing combinations of multiple switching valves (fourth switching valve, fifth switching valve, sixth switching valve, seventh switching valve, eighth switching valve, ninth switching valve, etc.), forming flow direction A and flow direction B. Flow direction A is: cold source heat exchanger → No. 1 heat exchanger → precooling heat exchanger → No. 2 heat exchanger → cold source heat exchanger; flow direction B is: cold source heat exchanger → No. 2 heat exchanger → precooling heat exchanger → No. 1 heat exchanger → cold source heat exchanger.
[0023] When the flow direction is A, it is the first mode, with heat exchanger No. 1 in the condensation state and heat exchanger No. 2 in the sublimation state. At this time, the CO2 cryogenic capture process is as follows:
[0024] Step 1: In the first branch of the No. 1 heat exchanger, CO2 in the flue gas is sublimated by N2 in the N2 circulation loop. The remaining nitrogen-rich flue gas enters the No. 1 mixing chamber and mixes with N2 from the N2 circulation loop of the cold source heat exchanger, and then re-enters the second branch of the No. 1 heat exchanger; thus completing the first stage of utilization of the remaining flue gas cold energy.
[0025] Step 2: The N2 in the N2 circulation loop is diverted by the No. 1 bypass valve, and part of it enters the second branch of the precooling heat exchanger to precool the flue gas, completing the second stage of utilization of the remaining flue gas cold energy; the remaining part enters the CO2 heat exchanger to cool the CO2 that has passed through the No. 2 compressor at high temperature and high pressure, completing the third stage of utilization of the remaining flue gas cold energy before being discharged.
[0026] Step 3: In the No. 2 heat exchanger, the CO2 solid condensed in the previous cycle is sublimated by the N2 in the other part of the N2 circulation loop that is diverted through the No. 1 bypass valve. The cold energy in the CO2 solid is recovered and sublimated into CO2 gas.
[0027] Step 4: The sublimated CO2 gas is finally stored in the CO2 storage tank through the No. 2 compressor.
[0028] In the cold source heat exchanger, the cold energy in the cold source is transferred to the N2 in the N2 circulation loop. In the No. 1 heat exchanger, which is in a sublimation state, a portion of the cold energy in the cold source is transferred to the flue gas through the N2 in the N2 circulation loop, causing the CO2 in the flue gas to sublimate. In the No. 2 heat exchanger, which is in a sublimation state, the cold energy in the already sublimated CO2 solid is transferred to the N2 in the N2 circulation loop, causing the CO2 solid to sublimate. The cold energy in the cold source is transferred and utilized through the N2 in the N2 circulation loop.
[0029] When the flow direction is B, it is the second mode, with heat exchanger 1 in the sublimation state and heat exchanger 2 in the condensation state. In this case, the CO2 cryogenic capture process is as follows:
[0030] Step 1: In the first branch of the second heat exchanger, CO2 in the flue gas is sublimated by N2 in the N2 circulation loop. The remaining nitrogen-rich flue gas enters the second mixing chamber and mixes with N2 from the N2 circulation loop of the cold source heat exchanger, and then re-enters the second branch of the second heat exchanger; thus completing the first stage of utilization of the remaining flue gas cold energy.
[0031] Step 2: The N2 in the N2 circulation loop is diverted by the No. 2 bypass valve, and part of it enters the second branch of the precooling heat exchanger to precool the flue gas, completing the second stage of utilization of the remaining flue gas cold energy; the remaining part enters the CO2 heat exchanger to cool the CO2 that has passed through the No. 2 compressor at high temperature and high pressure, completing the third stage of utilization of the remaining flue gas cold energy before being discharged.
[0032] Step 3: In heat exchanger No. 1, the CO2 solid condensed in the previous cycle is sublimated by the N2 in another part of the N2 circulation loop that is diverted through bypass valve No. 2. The cold energy in the CO2 solid is recovered and sublimated into CO2 gas.
[0033] Step 4: The sublimated CO2 gas is finally stored in the CO2 storage tank through the No. 2 compressor.
[0034] In the cold source heat exchanger, the cold energy in the cold source is transferred to the N2 in the N2 circulation loop. In the second heat exchanger, which is in a sublimation state, a portion of the cold energy in the cold source is transferred to the flue gas through the N2 in the N2 circulation loop, causing the CO2 in the flue gas to sublimate. In the first heat exchanger, which is in a sublimation state, the cold energy in the already sublimated CO2 solid is transferred to the N2 in the N2 circulation loop, causing the CO2 solid to sublimate. The cold energy in the cold source is transferred and utilized through the N2 in the N2 circulation loop.
[0035] Alternatively, the precooling heat exchanger, heat exchanger No. 1, heat exchanger No. 2 and cold source heat exchanger may be indirect heat exchangers such as wall-mounted, shell-and-tube heat exchangers or any other form of indirect heat exchanger.
[0036] Alternatively, the switching valve is not limited to a solenoid valve and can be any valve capable of changing the multi-pass path.
[0037] Alternatively, the cold source may be an external cold source such as LNG vaporization cold energy or refrigeration mechanism cold energy.
[0038] Compared with the prior art, the present invention has the following beneficial effects:
[0039] 1. This invention adopts the cascade utilization of cold energy. The temperature of the flue gas gradually decreases in heat exchangers in different temperature zones until CO2 in the flue gas sublimates. This can better utilize cold energy of different grades and has the effect of cascade recovery and utilization of low-temperature cold energy.
[0040] 2. This invention utilizes N2 in the N2 circulation loop as a cold source and a carrier for the solid CO2 cooling capacity. By using a variable flow rate, the system process is simplified while also achieving better matching of materials and energy. The periodic change in the N2 flow direction in the N2 circulation loop enables changes in the state combination between heat exchangers, achieving continuous CO2 capture by the system.
[0041] 3. This invention significantly reduces the power consumption caused by multi-stage compression while improving CO2 separation purity and separation rate. Pre-cooling the incoming flue gas with clean flue gas further reduces energy consumption. If a recyclable external cold source (such as LNG cold energy) is available, the cryogenic sublimation CO2 separation method can achieve almost "zero energy consumption," offering advantages unmatched by liquefaction separation methods.
[0042] 4. This invention uses two heat exchangers to perform switching separation of CO2 in flue gas. The low-temperature N2 flow rate can be flexibly adjusted in the early stage according to the different flue gas flow rate and CO2 concentration, thereby meeting the CO2 separation requirements of large flow rate, low pressure and low concentration flue gas. Attached Figure Description
[0043] Figure 1This is a schematic diagram of the overall structure of a carbon dioxide cryogenic capture system that utilizes circulating nitrogen combined with cascaded cold energy according to the present invention.
[0044] Figure 2 This is a schematic diagram of the first operating mode of the system of the present invention (i.e., flow direction A);
[0045] Figure 3 This is a schematic diagram of the second operating mode of the system of the present invention (i.e., flow direction B).
[0046] In the diagram: 1. Dehumidification device; 2. Compressor No. 1; 3. First switching valve; 4. Second switching valve; 5. Precooling heat exchanger; 6. Third switching valve; 7. Heat exchanger No. 1; 8. Fourth switching valve; 9. Fifth switching valve; 10. Mixing chamber No. 1; 11. Bypass valve No. 1; 12. Sixth switching valve; 13. Seventh switching valve; 14. Bypass valve No. 2; 15. Heat exchanger No. 2; 16. Mixing chamber No. 2; 17. Eighth switching valve; 18. Cold source heat exchanger; 19. Ninth switching valve; 20. Tenth switching valve; 21. Compressor No. 2; 22. CO2 heat exchanger; 23. CO2 storage tank; 24. Eleventh switching valve. Detailed Implementation
[0047] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the embodiments described below are intended to facilitate the understanding of the present invention and do not constitute any limitation thereof.
[0048] like Figure 1 As shown, a carbon dioxide cryogenic capture system employing circulating nitrogen combined with cascaded utilization of cold energy includes a flue gas flow path, an N2 circulation loop, and a CO2 collection and storage flow path.
[0049] The N2 circulation loop includes the second branch of the precooling heat exchanger 5, the seventh switching valve 13, the second bypass valve 14, the second branch of the second heat exchanger 15, the second mixing chamber 16, the eighth switching valve 17, the cold source heat exchanger 18, the fifth switching valve 9, the first mixing chamber 10, the second branch of the first heat exchanger 7, the first bypass valve 11, and the sixth switching valve 12, which are connected in sequence.
[0050] The flue gas flow path includes two paths. The first path sequentially connects to dehumidifier 1, compressor 2, first switching valve 3, second switching valve 4, third switching valve 6, the first branch of heat exchanger 7, fourth switching valve 8, and fifth switching valve 9 before flowing into the N2 circulation loop. It then connects to the external environment via pipelines from seventh switching valve 13 to eleventh switching valve 24 and the first branch of CO2 heat exchanger 22. The second path sequentially connects to dehumidifier 1, compressor 2, first switching valve 3, third switching valve 6, second switching valve 4, the first branch of heat exchanger 15, ninth switching valve 19, and eighth switching valve 17 before flowing into the N2 circulation loop. It then connects to the external environment via pipelines from sixth switching valve 12 to eleventh switching valve 24 and the first branch of CO2 heat exchanger 22.
[0051] The CO2 collection and storage flow path includes two paths. The first path sequentially connects the first branch of heat exchanger 7, the fourth switching valve 8, the tenth switching valve 20, compressor 21, the second branch of CO2 heat exchanger 22, and CO2 storage tank 23. The second path sequentially connects the first branch of heat exchanger 15, the ninth switching valve 19, the tenth switching valve 20, compressor 21, the second branch of CO2 heat exchanger 22, and CO2 storage tank 23.
[0052] Reference Figure 2 This is the first operating mode of the system of the present invention, with LNG as the cold source.
[0053] The N2 flow direction in the N2 circulation loop is: cold source heat exchanger 18 → No. 2 heat exchanger 15 → precooling heat exchanger 5 → No. 1 heat exchanger 7 → cold source heat exchanger 18. No. 1 heat exchanger 7 is in the sublimation state, and No. 2 heat exchanger 15 is in the condensation state.
[0054] The first switching valve 3 opens its channel to the second switching valve 4 and closes its channel to the third switching valve 6; the second switching valve 4 opens its channel to the first branch of the precooling heat exchanger and closes its channel to the first branch of the second heat exchanger 15; the third switching valve 6 opens its channel to the first branch of the first heat exchanger 7 and closes its channel to the first switching valve 3; the fourth switching valve 8 opens its channel to the fifth switching valve 9 and closes its channel to the tenth switching valve 20; the fifth switching valve 9 simultaneously opens its channels to the first mixing chamber 10 and the second branch of the cold source heat exchanger 18. The sixth switching valve 12 opens to the second branch of the precooling heat exchanger 5 and closes to the eleventh switching valve 24; the seventh switching valve 13 opens to both the second bypass valve 14 and the eleventh switching valve 24; the eighth switching valve 17 opens to the cold source heat exchanger 18 and closes to the ninth switching valve 19; the ninth switching valve 19 opens to the tenth switching valve 20; the tenth switching valve 20 opens to the ninth switching valve 19, closes to the fourth switching valve 8, and closes to the eighth switching valve 17.
[0055] The CO2 in the flue gas is sublimated by the N2 in the N2 circulation loop. The remaining nitrogen-rich flue gas enters the first mixing chamber 10 and mixes with the circulating N2 from the cold source heat exchanger 18. It then re-enters the first heat exchanger 7, completing the first stage of utilization of the remaining flue gas cold energy.
[0056] The circulating N2 in the N2 loop is diverted by the first bypass valve 11, and part of it re-enters the pre-cooling heat exchanger 5 to pre-cool the flue gas, completing the second stage of utilization of the remaining flue gas's cooling energy.
[0057] Finally, the gas enters the CO2 heat exchanger 22, where the high-temperature and high-pressure CO2 from the second compressor 21 is cooled, completing the third stage of utilization of the remaining flue gas's cold energy before it is discharged.
[0058] At this time, heat exchanger 15 is in a sublimation state. The CO2 solid condensed in the previous cycle is sublimated by another part of the flow diverted through bypass valve 11. The cold energy in the CO2 solid is recovered and sublimated into CO2 gas.
[0059] As the CO2 capture process continues, the amount of solid CO2 on heat exchanger 7 increases, and the thermal resistance of heat exchanger 7 increases. Heat exchanger 15 retains less and less solid CO2 from the previous cycle, and cannot further sublimate to release cooling energy and maintain the continuous operation of the system.
[0060] At this point, by changing the connection method of the fifth switching valve 9, the sixth switching valve 12, the seventh switching valve 13, the eighth switching valve 17, the ninth switching valve 19, and the tenth switching valve 20, the N2 circulation direction in the N2 circulation loop is changed to the flow direction B, as shown in the reference. Figure 3 The specific connection method is as follows:
[0061] The first switching valve 3 opens its channel to the third switching valve 6 and closes its channel to the second switching valve 4; the third switching valve 6 opens its channel to the first branch of the precooling heat exchanger 5 and closes its channel to the first branch of the first heat exchanger 7; the second switching valve 4 opens its channel to the first branch of the second heat exchanger 15 and closes its channel to the first switching valve 3; the ninth switching valve 19 opens its channel to the eighth switching valve 17 and closes its channel to the tenth switching valve 20; the eighth switching valve 17 simultaneously opens its channels to the second mixing chamber 16 and the second branch of the cold source heat exchanger 18; the seventh switching valve 13 opens its channel to the second branch of the precooling heat exchanger 5 and closes its channel to the eleventh switching valve 24; the sixth switching valve 12 simultaneously opens its channels to the first bypass valve 11 and the eleventh switching valve 24; the fifth switching valve 9 opens its channel to the cold source heat exchanger 18 and closes its channel to the fourth switching valve 8; the fourth switching valve 8 opens its channel to the tenth switching valve 20 and closes its channel to the fifth switching valve 9.
[0062] At this point, the CO2 in the flue gas is sublimated by the N2 in the N2 circulation loop. The remaining nitrogen-rich flue gas enters the first mixing chamber 10 and mixes with the circulating N2 from the cold source heat exchanger 18. It then re-enters the second heat exchanger 15, which no longer contains CO2 solids, thus completing the first stage of utilization of the remaining flue gas cold energy.
[0063] The circulating N2 in the N2 loop is diverted by the second bypass valve 14, and part of it re-enters the pre-cooling heat exchanger 5 to pre-cool the flue gas, completing the second stage of utilization of the remaining flue gas's cooling energy.
[0064] Finally, the gas enters the CO2 heat exchanger 22, where the high-temperature and high-pressure CO2 from the second compressor 21 is cooled, completing the third stage of utilization of the remaining flue gas's cold energy before it is discharged.
[0065] At this time, heat exchanger 7, which contains a large amount of CO2 solids, is in a sublimation state. The CO2 solids condensed in the previous cycle are sublimated by another portion diverted through bypass valve 11, and the cold energy in the CO2 solids is recovered and sublimated into CO2 gas. This achieves continuous capture.
[0066] In the implementation case, the parameters of the heat exchanger and material state in each state are shown in Tables 1 to 3 below.
[0067] Table 1: Parameters of Precooling Heat Exchanger
[0068]
[0069] The ratio of flue gas mass flow rate to the diverted circulating nitrogen mass flow rate is 9:13.
[0070] Table 2: Parameters of heat exchangers in the condensation state
[0071]
[0072] The ratio of circulating nitrogen mass flow rate to liquid nitrogen mass flow rate is 2:9:7.
[0073] Table 3: Parameters of heat exchangers in sublimation state
[0074]
[0075] The ratio of circulating nitrogen mass flow rate, split circulating nitrogen mass flow rate, and supplementary nitrogen mass flow rate is 7:6:1.
[0076] The embodiments described above provide a detailed explanation of the technical solutions and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.
Claims
1. A carbon dioxide cryogenic capture system employing cyclic nitrogen gas combined with cold energy cascade utilization, characterized by: This includes the flue gas flow path, the N2 circulation loop, and the CO2 collection and storage flow path; The N2 circulation loop includes the second branch of the precooling heat exchanger (5), the seventh switching valve (13), the second bypass valve (14), the second branch of the second heat exchanger (15), the second mixing chamber (16), the eighth switching valve (17), the cold source heat exchanger (18), the fifth switching valve (9), the first mixing chamber (10), the second branch of the first heat exchanger (7), the first bypass valve (11), and the sixth switching valve (12) connected in sequence. The flue gas flow path includes two paths. The first path connects sequentially to the dehumidifier (1), compressor (2), first switching valve (3), second switching valve (4), third switching valve (6), first branch of heat exchanger (7), fourth switching valve (8), and fifth switching valve (9) before flowing into the N2 circulation loop. It then connects to the external environment via the seventh switching valve (13) through a pipeline to the eleventh switching valve (24) and the first branch of CO2 heat exchanger (22). The second path connects sequentially to the dehumidifier (1), compressor (2), first switching valve (3), third switching valve (6), second switching valve (4), first branch of heat exchanger (15), ninth switching valve (19), and eighth switching valve (17) before flowing into the N2 circulation loop. It then connects to the external environment via the sixth switching valve (12) through a pipeline to the eleventh switching valve (24) and the first branch of CO2 heat exchanger (22). The CO2 collection and storage flow path includes two paths. The first path is sequentially connected to the first branch of the first heat exchanger (7), the fourth switching valve (8), the tenth switching valve (20), the second compressor (21), the second branch of the CO2 heat exchanger (22), and the CO2 storage tank (23). The second path is sequentially connected to the first branch of the second heat exchanger (15), the ninth switching valve (19), the tenth switching valve (20), the second compressor (21), the second branch of the CO2 heat exchanger (22), and the CO2 storage tank (23).
2. The system for low-temperature carbon dioxide capture using cyclic nitrogen gas combined with cold energy cascade utilization according to claim 1, characterized in that: The precooling heat exchanger (5), No. 1 heat exchanger (7), No. 2 heat exchanger (15), and CO2 heat exchanger (22) in the flue gas flow path operate in coordination under different temperature zones to realize the cascade utilization of the cold source's cooling capacity.
3. The carbon dioxide cryogenic capture system using circulating nitrogen combined with cascaded utilization of cold energy as described in claim 1, characterized in that: The N2 in the N2 circulation loop is used as a cold source and a carrier for the CO2 solid cold energy. The N2 in the N2 circulation loop passes through the cold source heat exchanger (18), and the cold energy in the cold source is transferred to the N2 in the N2 circulation loop, so that the cold energy is transferred from the cold source to the N2; when the first heat exchanger (7) is in the sublimation state, the CO2 in the flue gas is sublimated by the N2 in the N2 circulation loop, and the cold energy of the N2 in the N2 circulation loop is transferred to the CO2 and nitrogen-rich flue gas in the flue gas; the nitrogen-rich flue gas flowing out of the first heat exchanger (7) enters the N2 circulation loop, mixes evenly with the N2 in the first mixing chamber (10), and enters the precooling heat exchanger (5) together. The flue gas is precooled by the N2 in the N2 circulation loop, so that the cold energy is transferred from the N2 to the flue gas; at this time, the second heat exchanger (15) is in the sublimation state, and the CO2 solid in the second heat exchanger (15) is sublimated by the N2 in the N2 circulation loop flowing out of the precooling heat exchanger (5), so that the cold energy is transferred from the CO2 to the N2.
4. The carbon dioxide cryogenic capture system using circulating nitrogen combined with cascaded cold energy utilization as described in claim 1, characterized in that: By connecting the pipeline between the No. 1 bypass valve (11) and the No. 2 bypass valve (14), and by connecting the No. 5 switching valve (9) to the No. 1 mixing chamber (10) and the No. 8 switching valve (17) to the No. 2 mixing chamber (16), the N2 circulation loop is connected to the flue gas loop, making the N2 circulation loop a variable flow circulation loop. When heat exchanger 1 (7) is in the condensation state and heat exchanger 2 (15) is in the sublimation state, the nitrogen-rich flue gas with a flow rate of a from heat exchanger 1 (7) and the circulating N2 with a flow rate of b from cold source heat exchanger (18) are mixed in mixing chamber 1 (10), and the flow rate of the mixed circulating N2 is a+b. By calculating the precooling requirements of precooling heat exchanger (5), the required N2 flow rate is a'. Therefore, the circulating N2 with a flow rate of a+b is divided into two streams in bypass valve 1 (11). One stream with a flow rate of a' is fed into the second branch of precooling heat exchanger (5), and the other stream with a flow rate of a+b-a' passes through bypass valve 2 (14).
5. The carbon dioxide cryogenic capture system using circulating nitrogen combined with cascaded utilization of cold energy as described in claim 1, characterized in that: In the CO2 collection and storage flow path, the sublimated CO2 gas in the No. 1 heat exchanger (7) and the No. 2 heat exchanger (15) is compressed and then cooled by the CO2 heat exchanger (22), and finally enters the high-pressure CO2 storage tank (23) for storage.
6. The carbon dioxide cryogenic capture system using circulating nitrogen combined with cascaded cold energy utilization as described in claim 1, characterized in that: When the system is running, N2 in the N2 loop uses the opening and closing combination of multiple switching valves to periodically change the flow direction, forming flow direction A and flow direction B; flow direction A is cold source heat exchanger (18) → No. 1 heat exchanger (7) → precooling heat exchanger (5) → No. 2 heat exchanger (15) → cold source heat exchanger (18); flow direction B is cold source heat exchanger (18) → No. 2 heat exchanger (15) → precooling heat exchanger (5) → No. 1 heat exchanger (7) → cold source heat exchanger (18).
7. The carbon dioxide cryogenic capture system using circulating nitrogen combined with cascaded utilization of cold energy as described in claim 6, characterized in that: When the flow direction is A, heat exchanger 1 (7) is in a condensation state and heat exchanger 2 (15) is in a sublimation state. At this time, the CO2 low-temperature capture process is as follows: Step 1: In the first branch of the No. 1 heat exchanger (7), CO2 in the flue gas is sublimated by N2 in the N2 circulation loop. The remaining nitrogen-rich flue gas enters the No. 1 mixing chamber (10) and mixes with N2 in the N2 circulation loop from the cold source heat exchanger (18), and then re-enters the second branch of the No. 1 heat exchanger (7). Step 2: The N2 in the N2 circulation loop is diverted by the No. 1 bypass valve (11), and part of it enters the second branch of the precooling heat exchanger (5) to precool the flue gas. The remaining part enters the CO2 heat exchanger (22) to cool the high temperature and high pressure CO2 that has passed through the No. 2 compressor (21). Step 3: In the second heat exchanger (15), the CO2 solid condensed in the previous cycle is sublimated by the N2 in the other part of the N2 circulation loop that is diverted through the first bypass valve (11). The cold energy in the CO2 solid is recovered and sublimated into CO2 gas. Step 4: The sublimated CO2 gas is finally stored in CO2 storage tank (23) through compressor No. 2 (21).
8. The carbon dioxide cryogenic capture system using circulating nitrogen combined with cascaded cold energy utilization according to claim 6, characterized in that: When the flow direction is B, heat exchanger 1 (7) is in the sublimation state and heat exchanger 2 (15) is in the condensation state. At this time, the CO2 low-temperature capture process is as follows: Step 1: In the first branch of the second heat exchanger (15), CO2 in the flue gas is sublimated by N2 in the N2 circulation loop. The remaining nitrogen-rich flue gas enters the second mixing chamber (16) and mixes with N2 in the N2 circulation loop from the cold source heat exchanger (18), and then re-enters the second branch of the second heat exchanger (15). Step 2: The N2 in the N2 circulation loop is diverted by the No. 2 bypass valve (14), and part of it enters the second branch of the precooling heat exchanger (5) to precool the flue gas. The remaining part enters the CO2 heat exchanger (22) to cool the high temperature and high pressure CO2 that has passed through the No. 2 compressor (21). Step 3: In the No. 1 heat exchanger (7), the CO2 solid condensed in the previous cycle is sublimated by the N2 in another part of the N2 circulation loop that is diverted through the No. 2 bypass valve (14). The cold energy in the CO2 solid is recovered and sublimated into CO2 gas. Step 4: The sublimated CO2 gas is finally stored in CO2 storage tank (23) through compressor No. 2 (21).