Liquefied natural gas cold energy cascade utilization system

Abstract

The application discloses a liquefied natural gas cold energy cascade utilization system, which comprises an LNG dry circuit, an air separation subsystem, a seawater purification subsystem, a low-temperature power generation subsystem and a cold chain refrigeration subsystem; the cold energy released in the vaporization process of LNG is sequentially used in the air separation, seawater purification, low-temperature power generation and cold chain refrigeration subsystems, the LNG cold energy is cascade utilized, the energy utilization rate is improved, meanwhile, the system is embedded in various subsystems in a nested mode, the energy utilization rate is improved, high-quality industrial products are generated, and the operation cost is saved.

Description

Technical Field

[0001] This invention belongs to the field of liquefied natural gas (LNG) cold energy utilization technology, specifically relating to a cascaded LNG cold energy utilization system. Background Technology

[0002] With the rapid economic development of various countries, energy consumption is also increasing. However, compared to traditional coal and oil, natural gas, as a clean energy source with a higher calorific value, has long been a prominent player in global energy utilization. Under standard atmospheric pressure, the liquefaction temperature of gaseous natural gas is approximately -162°C. To meet the transportation requirements of domestic natural gas pipeline networks, liquefied natural gas (LNG) typically needs to be vaporized into ambient temperature gas before being integrated into the pipeline network. When the temperature of LNG rises from -162°C to 25°C, it releases approximately 830 kJ / kg of cold energy. However, most LNG receiving terminals currently release this cold energy into seawater or the air, resulting in a significant waste of cold energy.

[0003] Freezing and cryogenic storage are widely used to preserve the quality and nutritional components of seafood and its products. Approximately 50% of processed seafood and 21% of seafood products are supplied to the market in frozen form. Freezing requires a significant amount of cooling energy; however, the cooling energy released during the vaporization of natural gas at LNG receiving terminals far exceeds this requirement. Therefore, cooling energy cannot be solely used for preserving seafood.

[0004] Existing public literature indicates that most LNG cold energy utilization systems are currently too simplistic and cannot fully utilize the large amount of LNG cold energy. However, different methods of LNG cold energy utilization have different temperature ranges, which makes the graded utilization of LNG possible. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a cascade utilization system for liquefied natural gas cold energy.

[0006] Technical solution: The liquefied natural gas cold energy cascade utilization system of the present invention includes an LNG trunk line, an air separation subsystem, a seawater purification subsystem, a cryogenic power generation system, and a cold chain cooling subsystem; The LNG inlet supplies fuel to the gas-fired boiler via the LNG trunk line, which is equipped with a primary LNG heat exchanger, a secondary LNG heat exchanger, a tertiary LNG heat exchanger, and a quaternary LNG heat exchanger in sequence along the fuel supply direction. The air separation subsystem is connected to the LNG primary heat exchanger, which is used to exchange heat between the cryogenic fuel in the LNG trunk and the compressed air in the air separation subsystem. The seawater purification subsystem is connected to the LNG secondary heat exchanger, which is used to exchange heat between the cryogenic fuel in the LNG trunk and the ice maker in the seawater purification subsystem. The cold chain cooling subsystem is connected to the LNG three-stage heat exchanger, which is used to exchange heat between the cryogenic fuel in the LNG trunk line and the cold storage air in the cold chain cooling subsystem. The cryogenic power generation system is connected to the LNG fourth-stage heat exchanger, which is used to exchange heat between the cryogenic fuel in the LNG trunk line and the exhaust steam output from the turbine.

[0007] Preferably, the air separation subsystem includes a molecular sieve purifier, an expander, and a distillation column. Compressed air flows through the LNG primary heat exchanger for cooling, then passes through the molecular sieve purifier for purification and the expander for further cooling before the product is fed into the distillation column. The top outlet of the distillation column is connected to a liquid nitrogen tank, and the bottom outlet is connected to a liquid oxygen tank.

[0008] Preferably, the seawater purification subsystem includes a seawater pool, an ice maker, and an evaporation chamber. The seawater in the seawater pool is fed into the ice maker to form ice slurry, which is then washed and flows into a purified water pool. The seawater in the seawater pool is fed into the evaporation chamber to form high-temperature steam, which is then cooled and flows into the purified water pool.

[0009] Preferably, the evaporation chamber is heated by the combustion exhaust gas of the gas boiler and the compressed air of the air separation subsystem. The high-temperature steam generated in the evaporation chamber is used to heat the treated water transported from the purification tank to the gas boiler.

[0010] Preferably, the cold chain cooling subsystem includes a refrigeration unit, which is a cold-side cooling unit consisting of a cold air primary heat exchanger and a waste steam primary heat exchanger. The waste steam generated by the turbine flows into the waste steam primary heat exchanger for primary cooling and then is sent to the LNG fourth-stage heat exchanger for secondary cooling. The condensate formed after the secondary cooling of the waste steam flows into the purification water pool. The cold storage air supplied from the cold storage air inlet flows into the cold air primary heat exchanger for primary cooling and then is sent to the LNG third-stage heat exchanger for further cooling. The cold storage air that has been cooled again is sent back to the cold storage.

[0011] Preferably, after the cold air is cooled once by the primary heat exchanger, it is fed into the ice slurry heat exchanger for a secondary cooling before being sent to the LNG tertiary heat exchanger. The cold source of the ice slurry heat exchanger is the washed ice slurry.

[0012] Preferably, the liquid oxygen tank outlet is connected to the cold side channel inlet of the liquid oxygen heat exchanger, the liquid oxygen heat exchanger cold side channel outlet is connected to the cold side channel inlet of the ice storage tank, the ice storage tank cold side channel outlet is connected to the gas boiler, and the purified water tank is used to supply purified water to the ice storage tank.

[0013] Preferably, the gas-fired boiler is used to supply superheated steam to the steam turbine and drive the steam turbine to rotate, wherein the rotating shaft of the steam turbine is coaxial with the rotor of the generator.

[0014] Furthermore, in the cryogenic power generation system, the fuel in the LNG trunk is heated in stages by a four-stage LNG heat exchanger before being transported to the gas-fired boiler. The oxygen obtained from the air separation subsystem is transported to the gas-fired boiler as a combustion aid. The purified water in the purification water tank is drawn by the boiler feedwater pump, passes through the feedwater processor and feedwater preheater, and is then transported to the gas-fired boiler. Natural gas is fully combusted in the gas-fired boiler with oxygen enrichment to heat the purified water and generate high-temperature steam. The high-temperature steam is used to drive the turbine to rotate, and the turbine drives the generator rotor to rotate and supply power to the grid. The exhaust steam generated after driving the turbine is cooled twice by the exhaust steam primary heat exchanger and the LNG fourth-stage heat exchanger and then liquefied to form purified water, which flows back to the purification water tank. The high-temperature tail gas generated by the combustion of natural gas in the gas-fired boiler is transported to the high-temperature side of the evaporation chamber. The seawater in the seawater tank is transported to the evaporation chamber by the second seawater pump. The high-temperature steam generated by the high-temperature evaporation is transported by pipeline to the feedwater preheater to preheat the purified water. The high-temperature steam releases heat in the feedwater preheater and forms purified water, which flows to the purification water tank.

[0015] Furthermore, in the cold chain cooling subsystem, peak-valley electricity is introduced from the power grid via a peak-valley interface to provide power to the refrigeration units and output cold energy. The cold energy output from the refrigeration units is driven by a second refrigerant circulation pump, flowing sequentially through the cold side of the primary cold air heat exchanger and the cold side of the primary exhaust steam heat exchanger before returning to the refrigeration units. Cold storage air in the cold storage is driven by a fan and transported from the cold storage air inlet through a cold storage air duct to the primary cold air heat exchanger for primary cooling. A cold storage air valve is installed in the cold storage air duct. After primary cooling, the cold storage air is input into the ice slurry heat exchanger for secondary cooling before being transported to the LNG tertiary heat exchanger for further cooling. The second-cooled cold storage air is then transported back to the cold storage. Purified water in the purified water tank is transported to the ice storage tank by an ice bucket makeup pump for ice making. Liquid oxygen obtained from air separation by the air separation subsystem flows through the ice storage tank before being transported to the gas boiler, serving as the cold source for ice making. The cold chain cooling subsystem provides cooling through both cold air and ice making.

[0016] Furthermore, the ice maker in the seawater purification subsystem is a scraper-type ice maker. A first seawater pump draws seawater from the seawater pool through a pipeline and delivers it to the top inlet of the scraper-type ice maker. The bottom drain of the scraper-type ice maker is connected to a waste liquid pool through a pipeline. High-concentration seawater accumulates at the bottom of the ice maker and is discharged from the drain, maintaining a low seawater concentration within the ice maker. The upper layer of ice slurry from the scraper-type ice maker is drawn out by a first ice slurry transport pump and sent to the washing chamber for washing. The washed ice slurry is then transported by a second ice slurry transport pump to the cold end of the ice slurry heat exchanger to cool the air in the cold storage. After passing through the ice slurry heat exchanger, the ice slurry flows into the purification water pool. The purified water in the purification water pool is driven by a washing water centrifugal pump and delivered to the top spray nozzle of the washing chamber to wash away the salt adhering to the surface of the ice slurry. The washing water outlet at the bottom of the washing chamber is connected to the inlet of the evaporation chamber, allowing the washing water to flow into the evaporation chamber for secondary use. The evaporator in the seawater purification subsystem uses both the high-temperature flue gas from the gas-fired boiler and the compressed air heat exchanger in the air separation subsystem as heat sources to evaporate the saline content. The saline content in the evaporator comes from seawater pumped from the seawater pool by a second seawater pump and washing water flowing from the washing chamber. The wastewater outlet of the evaporator is connected to the waste liquid pool via a pipe to maintain a low raw material concentration within the evaporator. The high-temperature steam generated by the evaporator flows through the hot-side channel of the feedwater preheater in the low-temperature power generation system to preheat the feedwater for the gas-fired boiler. After releasing heat in the feedwater preheater, the high-temperature steam forms cooling water that flows to the purification water pool. The seawater purification subsystem purifies seawater through two methods: ice making and evaporation. The resulting purified water is stored in the purification water pool and used for the water needs of each subsystem.

[0017] Furthermore, the heat transfer oil on the cold side of the compressed air heat exchanger is driven by a heat transfer oil circulation pump to achieve heat exchange on the high-temperature side of the evaporator; the LNG secondary heat exchanger and the secondary heat exchanger bypass valve are connected in parallel. By adjusting the flow rate of the LNG trunk pump and the opening of the secondary heat exchanger bypass valve, the cooling capacity exchanged between the LNG secondary heat exchanger and the seawater purification subsystem meets the conditions for low-temperature seawater purification. The outlet of the cold side channel of the LNG secondary heat exchanger and the two ends of the secondary heat exchanger bypass valve merge into the LNG trunk pipeline to flow to the next process.

[0018] Furthermore, in the air separation subsystem, the air feedstock enters through the air inlet, flows through the air valve into the air dust collector for dust removal, and is then delivered to the air compressor. The compressed air temperature rises, and the compressed air passes through the hot side of the compressed air heat exchanger, transferring heat to the evaporation chamber in the seawater purification subsystem for seawater evaporation. The compressed air, after undergoing a first cooling in the compressed air heat exchanger, is delivered to the hot side channel of the LNG primary heat exchanger for a second cooling before being delivered to the molecular sieve purifier. The molecular sieve purifier has two outlets. One outlet is connected to the waste gas tank via a pipeline, separating water, carbon dioxide, nitrogen oxides, and hydrocarbons from the products to improve product purity and ensure production safety. The other outlet of the molecular sieve purifier is connected to the expander via a pipeline. After isentropic expansion, the temperature is reduced to the target temperature of approximately -200°C. The expander is connected to the distillation column via a pipeline. The top outlet of the distillation column is connected to the liquid nitrogen tank, and the bottom outlet of the distillation column is connected to the liquid oxygen tank.

[0019] Furthermore, the pure oxygen in the liquid oxygen tank is used for natural gas combustion. The process is as follows: After a portion of the liquid oxygen is extracted by a liquid oxygen pump, its cooling capacity is first exchanged through the cold side channel of a liquid oxygen heat exchanger. This cooling capacity is then used for cryogenic utilization in the cold chain cooling system. The outlet of the cold side channel of the liquid oxygen heat exchanger is connected to the inlet of the cold side channel of the ice storage tank via a pipeline, allowing the cooling capacity to be used to chill purified water. After the purified water freezes, it is used for cold chain cooling. Finally, after two heat exchanges, the liquid oxygen is maintained at a safe pressure by a pressure relief valve before entering the gas-fired boiler for oxygen-enriched combustion.

[0020] Furthermore, the liquid oxygen heat exchanger is connected in parallel with the liquid oxygen bypass valve to regulate the liquid oxygen flow rate within the liquid oxygen heat exchanger. The heat transfer oil in the hot side channel of the liquid oxygen heat exchanger is driven by a centrifugal pump. The LNG primary heat exchanger is connected in parallel with the primary heat exchanger bypass valve. By adjusting the flow rate of the LNG trunk pump and the opening of the primary heat exchanger bypass valve, the cooling capacity entering the air separation subsystem is dynamically controlled to meet the cooling capacity requirements for air separation. The outlet of the cold side channel of the LNG primary heat exchanger and the two ends of the primary heat exchanger bypass valve converge to the LNG trunk pipeline to flow to the next process.

[0021] Furthermore, after flowing in through the LNG inlet, the cryogenic liquefied natural gas is driven by the LNG trunk pump and flows sequentially through the cold side channel of the four-stage LNG heat exchanger before being transported to the gas-fired boiler.

[0022] Beneficial effects: This system combines the cold energy requirements of the seaside plant operation with the sequential use of the cold energy released during LNG vaporization in subsystems such as air separation, seawater purification, cryogenic power generation, and cold chain refrigeration, based on the different required cooling temperatures. This cascade utilization of LNG cold energy improves energy efficiency. In addition, the system generates a variety of industrial products, including liquid oxygen, liquid nitrogen, ice, purified water, and electricity. The coupling and synergy of the various subsystems also improves the utilization rate of by-products. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the cascade utilization system for liquefied natural gas cold energy in this invention.

[0024] Figure 2 This is a schematic diagram of the air separation subsystem in this invention.

[0025] Figure 3 This is a schematic diagram of the seawater purification subsystem in this invention.

[0026] Figure 4 This is a schematic diagram of the low-temperature electron generation system in this invention.

[0027] Figure 5 This is a schematic diagram of the cold chain cooling subsystem in this invention. Detailed Implementation

[0028] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0029] A liquefied natural gas (LNG) cold energy cascade utilization system includes an LNG trunk line, an air separation subsystem, a seawater purification subsystem, a cryogenic power generation system, and a cold chain cooling subsystem, such as... Figure 1 As shown, the LNG trunk line includes an LNG inlet 1, which is connected in sequence to an LNG trunk line valve 2 and an LNG trunk line pump 3 via pipelines. The outlet of the LNG trunk line pump 3 is connected in parallel to the inlet of the cold side channel of the LNG primary heat exchanger 4 and one end of the primary heat exchanger bypass valve 5 via pipelines. The outlet of the cold side channel of the LNG primary heat exchanger 4 is connected to the trunk line via pipelines to both ends of the primary heat exchanger bypass valve 5. The trunk line LNG pipeline is connected in parallel to the cold side channel of the LNG secondary heat exchanger 6 and one end of the secondary heat exchanger bypass valve 7 via pipelines. The outlet of the cold side channel of the LNG secondary heat exchanger 6 is connected to the trunk line via pipelines to both ends of the secondary heat exchanger bypass valve 7. The trunk line LNG pipeline is connected to the inlet of the cold side channel of the LNG tertiary heat exchanger 8 via pipelines. The outlet of the cold side channel of the tertiary heat exchanger 8 is connected to the inlet of the cold side channel of the LNG quaternary heat exchanger 9 via pipelines. The outlet of the cold side channel of the LNG quaternary heat exchanger 9 is connected to the gas boiler 40 via pipelines.

[0030] In this embodiment, the LNG primary heat exchanger 4, LNG secondary heat exchanger 6, LNG tertiary heat exchanger 8, LNG quaternary heat exchanger 9, compressed air heat exchanger 14, liquid oxygen heat exchanger 23, ice slurry heat exchanger 35, feedwater preheater 39, cold air primary heat exchanger 50, and exhaust steam primary heat exchanger 51 all include a cold side channel and a hot side channel; the heat exchange medium includes two types, namely heat transfer oil and refrigerant. The distillation column 18 includes a bottom inlet, a top liquid nitrogen outlet, and a bottom liquid oxygen outlet; the ice storage tank 26 includes a cold side channel, a water inlet, and an ice outlet; the scraper-type ice maker 30 includes a refrigerant shell inlet, a refrigerant shell outlet, a bottom drain outlet, a seawater inlet, and an ice slurry outlet; the washing chamber 33 includes an ice slurry inlet, an ice slurry outlet, a washing water spray outlet, and a washing water drain outlet; the purified water tank 43 includes three water outlets and three water inlets; and the evaporation chamber 45 includes two hot side channels, two water inlets, and one steam outlet.

[0031] In this embodiment, the air separation subsystem is as follows: Figure 2 As shown, the air inlet 10 is connected in sequence to the air valve 11, the air dust collector 12, the air compressor 13, the hot side channel of the compressed air heat exchanger 14, the hot side channel of the LNG primary heat exchanger 4, and the inlet of the molecular sieve purifier 15 via pipes. The molecular sieve purifier 15 has two outlets. One outlet is connected to the waste gas tank 16 via a pipe, and the other outlet is connected to the expander 17 via a pipe. The expander 17 is connected to the distillation column 18 via a pipe. The top outlet of the distillation column 18 is connected to the liquid nitrogen tank 20, and the bottom outlet of the distillation column 18 is connected to the liquid oxygen tank 19. The liquid oxygen tank 19 is connected to the liquid oxygen valve 21 and the inlet of the liquid oxygen pump 22 via pipes. The outlet of the liquid oxygen pump 22 is connected in parallel to the inlet of the cold side channel of the liquid oxygen heat exchanger 23 and one end of the liquid oxygen bypass valve 24 via a pipe. The outlet of the cold side channel of the liquid oxygen heat exchanger 23 and the two ends of the liquid oxygen bypass valve 24 converge and are connected to the inlet of the cold side channel of the ice storage tank 26 via a pipe. The outlet of the cold side channel of the ice storage tank 26 is connected to the pressure relief valve 27 and the gas boiler 40 via pipes. The outlet of the centrifugal pump 25 is connected to the inlet of the hot side of the liquid oxygen heat exchanger 23 via a pipe.

[0032] In this embodiment, the seawater purification subsystem is as follows: Figure 3As shown, the shell outlet of the scraper-type ice maker 30 is connected in sequence by pipes to the refrigerant circulation valve 28 and the inlet of the first refrigerant circulation pump 29; the outlet of the first refrigerant circulation pump 29 is connected by pipes to the inlet of the hot side channel of the LNG secondary heat exchanger 6; the outlet of the hot side channel of the LNG secondary heat exchanger 6 is connected by pipes to the shell of the scraper-type ice maker 30; the bottom drain outlet of the scraper-type ice maker 30 is connected by pipes to the waste liquid pool 31; the seawater pool 46 is connected by pipes to the inlet of the first seawater pump 56; the outlet of the first seawater pump 56 is connected by pipes to the top water inlet of the scraper-type ice maker 30; the upper floating ice outlet of the scraper-type ice maker 30 is connected by pipes. The inlet of the first ice slurry transport pump 32 is connected; the outlet of the first ice slurry transport pump 32 is connected to the ice slurry inlet of the washing chamber 33 via a pipe; the ice slurry outlet of the washing chamber 33 is connected to the inlet of the second ice slurry transport pump 34 via a pipe; the outlet of the second ice slurry transport pump 34 is connected to the cold side channel inlet of the ice slurry heat exchanger 35 via a pipe; the cold side channel outlet of the ice slurry heat exchanger 35 is connected to the purified water tank 43 via a pipe; the purified water tank 43 is connected to the inlet of the washing water centrifugal pump 44 via a pipe; the outlet of the washing water centrifugal pump 44 is connected to the top spray nozzle of the washing chamber 33 via a pipe; the washing water outlet at the bottom of the washing chamber 33 is connected to the water inlet of the evaporation chamber 45 via a pipe. Seawater tank 46 is connected to the inlet of second seawater pump 57 via a pipe; outlet of second seawater pump 57 is connected to the inlet of evaporation chamber 45 via a pipe; outlet of cold side channel of compressed air heat exchanger 14 is connected to inlet of heat transfer oil circulation pump 55 via a pipe; outlet of heat transfer oil circulation pump 55 is connected to hot side channel of evaporation chamber 45 and inlet of cold side channel of compressed air heat exchanger 14 via pipes in sequence; wastewater outlet of evaporation chamber 45 is connected to waste liquid tank 31 via a pipe; steam outlet of evaporation chamber 45 is connected to inlet of hot side channel of feedwater preheater 39 via a pipe; outlet of hot side channel of feedwater preheater 39 is connected to purified water tank 43 via a pipe.

[0033] In this embodiment, the cryogenic electron-generating system is as follows: Figure 4As shown, the cold-side outlet of the LNG fourth-stage heat exchanger 9 is connected to the fuel inlet of the gas-fired boiler 40 via a pipeline; the purified water tank 43 is connected to the inlet of the boiler feedwater pump 37 via a pipeline; the outlet of the boiler feedwater pump 37 is connected to the feedwater processor 38 and the cold-side channel inlet of the feedwater preheater 39 via pipelines in sequence; the cold-side channel outlet of the feedwater preheater 39 is connected to the gas-fired boiler 40 via a pipeline; the tail flue gas of the gas-fired boiler 40 is connected to the hot-side channel of the evaporation chamber 45 via a pipeline; and the superheated steam outlet of the steam boiler 40 is connected to the steam turbine 41 via a pipeline. The exhaust steam outlet of turbine 41 is connected to the inlet of the hot side channel of exhaust steam primary heat exchanger 51 via a pipeline; the outlet of the hot side channel of exhaust steam primary heat exchanger 51 is connected to the inlet of the hot side channel of LNG fourth-stage heat exchanger 9 via a pipeline; the outlet of the hot side channel of LNG fourth-stage heat exchanger 9 is connected to the purified water tank 43 via a pipeline; the rotating shaft of turbine 41 is coaxial with the rotor of generator 42; the steam outlet of evaporator 45 is connected to the inlet of the hot side channel of feedwater preheater 39 via a pipeline; the outlet of the hot side channel of feedwater preheater 39 is connected to the purified water tank 43 via a pipeline.

[0034] In this embodiment, the cold chain cooling subsystem is as follows: Figure 5 As shown, the cold-side outlet of the LNG stage 3 heat exchanger is connected to the cold-side inlet of the LNG stage 4 heat exchanger via a pipeline; the exhaust steam outlet of turbine 41 is connected to the hot-side inlet of exhaust steam stage 1 heat exchanger 51 via a pipeline; the hot-side outlet of LNG stage 4 heat exchanger 9 is connected to the purified water tank 43 via a pipeline; the purified water tank 43 is connected to the inlet of ice bucket makeup water pump 36 via a pipeline; the outlet of ice bucket makeup water pump 36 is connected to the inlet of ice storage tank 26 via a pipeline; the refrigeration unit 48 is powered by off-peak electricity, and the outlet of the refrigeration unit 48 is connected to the second refrigerant circulation pump via a pipeline. 49 is connected to the inlet; the outlet of the second refrigerant circulation pump 49 is connected to the inlet of the cold side channel of the cold air primary heat exchanger 50 via a pipeline; the outlet of the cold side channel of the cold air primary heat exchanger 50 is connected to the inlet of the cold side channel of the exhaust steam primary heat exchanger 51 via a pipeline; the inlet of the cold side channel of the exhaust steam primary heat exchanger is connected to the inlet of the refrigeration unit 48 via a pipeline; the cold storage air inlet 52 is connected to the cold storage air valve 53, the fan 54, the hot side channel of the cold air primary heat exchanger 50, the hot side channel of the ice slurry heat exchanger 35, and the hot side channel of the LNG tertiary heat exchanger 8 via pipelines in sequence.

[0035] The specific working principle of a liquefied natural gas (LNG) cold energy cascade utilization system is as follows: In this embodiment, in the air separation subsystem, liquid natural gas flows sequentially through LNG inlet 1, LNG trunk valve 2, and LNG trunk pump 3; at the outlet of LNG trunk pump 3, liquid natural gas flows into the cold side channel of LNG primary heat exchanger 4 and the primary heat exchanger bypass valve 5, respectively. After heat exchange, the liquid natural gas vaporizes into gaseous natural gas, but the temperature is still low. Air flows sequentially through air inlet 10, air valve 11, and air dust collector 12, where large particulate impurities are removed. The air is then compressed in air compressor 13, with an outlet air pressure of approximately 1 MPa and a temperature of approximately 130°C. The compressed air flows through the hot-side channel of the compressed air heat exchanger 14, exchanging heat with the heat transfer oil in the cold-side channel. The heat-carrying oil is then transferred to the evaporation chamber 45 via the heat transfer oil circulation pump 55, where the heat is used for distillation purification of seawater in the air purification subsystem. The cooled compressed air is further cooled in the LNG primary heat exchanger 4. The low-temperature, high-pressure air from the hot-side channel outlet of the LNG primary heat exchanger 4 flows into the molecular sieve purifier 15 to separate carbon dioxide, water, hydrocarbons, and nitrogen oxides from the air, preventing these substances from entering the distillation column 18 and causing a safety accident. After the molecular sieve purifier 15 sieves the air, carbon dioxide, water, hydrocarbons and nitrogen oxides are discharged into the waste gas tank 16. The purified air enters the expander 17 for expansion. After the air enters the expander 17, the temperature needs to be maintained at about -200℃. If the temperature is higher or lower than -200℃, the opening of the bypass valve 5 of the first-stage heat exchanger or the flow rate of the LNG trunk pump 3 can be adjusted to achieve the target temperature. The expanded air flows into the distillation column 18. Due to the difference in boiling points, liquid nitrogen with a lower boiling point flows into the liquid nitrogen tank 20 at the top and is stored. Liquid oxygen with a higher boiling point is stored in the liquid oxygen tank 19. Since the content of liquid argon in the air is about 1%, and the purity requirement of oxygen in this invention is not high, it is stored together with liquid oxygen in the liquid oxygen tank 19. Liquid oxygen in liquid oxygen tank 19 is pumped to liquid oxygen heat exchanger 23 through the cooperation of liquid oxygen valve 21 and liquid oxygen pump 22. Since the boiling point of liquid oxygen is -183℃, it meets the conditions for cryogenic utilization. The liquid oxygen is then heat-exchanged through the cold side channel of liquid oxygen heat exchanger 23. Oxygen from the outlet of liquid oxygen heat exchanger 23 flows into the cold side channel of ice storage tank 26 for heat exchange, freezing the water in the ice storage tank into ice. By adjusting the flow rate of liquid oxygen pump 22 or the opening of liquid oxygen bypass valve 24, the temperature of the ice in the tank is maintained at -25℃ to -30℃. This part of the ice can be used for freezing and preserving seafood. Oxygen from the outlet of the cold side channel of ice storage tank 26 passes through pressure relief valve 27 to relieve excessive pressure, keeping the oxygen pressure entering gas boiler 40 within a safe range. Because the oxygen content is very high, it is an oxygen-enriched combustion, which improves the boiler combustion efficiency.

[0036] In this embodiment, natural gas in the seawater purification subsystem flows into the LNG secondary heat exchanger 6 and the secondary heat exchanger bypass valve 7 respectively before converging into the main line; the refrigerant in the shell layer of the scraper-type ice maker 30 flows through the hot side channel of the LNG secondary heat exchanger 6 for heat exchange, and the temperature is maintained at -54℃ to -44℃ by adjusting the opening of the secondary heat exchanger bypass valve 7, the refrigerant circulation valve 28, and controlling the flow of the first refrigerant circulation pump 29; the seawater in the purification pool 43 is pumped into the inner layer of the scraper-type ice maker 30 by the first seawater pump 56, and the refrigerant with a lower temperature inside the shell layer transfers the cold energy to the inner layer, lowering the temperature of the seawater in the inner layer and forming ice crystals on the inner layer wall. The scraper in the inner layer peels off the ice layer in time, reducing the heat exchange resistance and improving the ice-making efficiency; the scraped ice crystals float on the upper water surface due to density and are pumped into the washing chamber 33 by the first ice slurry transport pump 32; and the concentrated brine in the lower layer of the inner seawater also accumulates in the lower part of the inner layer due to density, and this part of the concentrated brine is... The seawater is discharged into wastewater pool 31 to maintain the seawater concentration in the scraper-type ice maker 30 within a suitable range. Part of the purified water in the purification water pool 43 is pumped to the washing chamber by the washing water centrifugal pump 44 for rinsing the ice slurry, removing the salt from the surface of the ice slurry. The first ice slurry transport pump 32 transports the ice slurry produced in the scraper-type ice maker 30 to the washing chamber 33. Since the salt is removed from the ice crystals when seawater freezes, fresh water can be obtained simply by washing, separating, and melting the ice crystals. The ice slurry transported to the washing chamber 33 is placed on a conveyor belt. The top of the conveyor belt has a purified water spray nozzle that sprays the ice slurry evenly on the surface to wash away the salt. The ice slurry is then conveyed by the belt to the second ice slurry transport pump 34. The liquid after rinsing the ice slurry cannot flow back to the purified water tank 43 because it carries the salt from the surface of the ice slurry. Instead, it flows to the evaporation chamber 45 for evaporation. The ice slurry that has undergone heat exchange in the cold side channel of the ice slurry heat exchanger 35 melts and flows into the purified water tank 43.

[0037] The heat from the compressed air in the compressed air heat exchanger 14 is transferred to the evaporation chamber 45 by the heat transfer oil circulation pump 55. In the low-temperature power generation system, the temperature of the flue gas at the tail end of the gas boiler 40 is around 140°C. The heat from the flue gas is introduced into the evaporation chamber to lower the temperature of the flue gas and reduce thermal pollution to the environment. The raw materials in the evaporation chamber 45 come from two sources: seawater pool 46 and washing water flowing out of the washing chamber 33. The heat source for these materials comes from the compressed air heat exchanger 14 in the air separation subsystem and the flue gas at the tail end of the gas boiler 40 in the low-temperature power generation system. This heat is used to evaporate the seawater. The generated high-temperature steam flows through the hot side of the feedwater preheater 39 in the low-temperature power generation system, and its heat is used to preheat the feedwater of the gas boiler 40. The high-concentration seawater in the evaporation chamber 45 is discharged into the waste liquid pool 31 to maintain the raw material concentration in the evaporation chamber at a low level. In the seawater purification subsystem, the purified water produced is stored in the purified water pool 43 to supply the water needs of each subsystem.

[0038] In this embodiment, the water in the purified water tank 43 of the low-temperature power generation system is pumped into the feedwater processor 38 by the boiler feedwater pump 37 to remove dissolved oxygen and other gases from the feedwater, preventing corrosion of the boiler feedwater pipes and other auxiliary equipment. The water treated by the feedwater processor 38 flows through the cold side channel of the feedwater preheater 39, using the heat from the high-temperature distilled water drawn from the evaporation chamber 45 to preheat the boiler feedwater and increase its initial temperature before entering the boiler. The tail flue gas generated by the gas boiler 40 is introduced into the evaporation chamber 45, and the generated superheated steam is introduced into the turbine 41 to do work. The steam in the turbine 41 does work, driving the generator rotor to rotate. Part of the generated electricity is used for plant power consumption, and most of it is processed and input into the power grid. In the turbine, the exhaust steam after the superheated steam has done work passes through the hot side channel of the exhaust steam primary heat exchanger and the hot side channel of the LNG fourth-stage heat exchanger to reduce the exhaust steam temperature and improve the generator set operating efficiency. The exhaust steam flows back to the purified water tank after two stages of cooling and condensation.

[0039] In this embodiment, in the cold chain cooling subsystem, natural gas flows sequentially through the cold side passage of the LNG third-stage heat exchanger 8 and the cold side passage of the LNG fourth-stage heat exchanger 9; the refrigeration unit utilizes inexpensive off-peak electricity for cooling, and its refrigerant flows sequentially through the second refrigerant circulation pump 49, the cold side passage of the cold air first-stage heat exchanger 50, and the cold side passage of the exhaust steam first-stage heat exchanger 51; air flows in from the air inlet, sequentially through the cold storage air valve 53, the fan 54, and the hot side passage of the cold air first-stage heat exchanger 50, using the cooling capacity of the refrigeration unit to cool the air once; the cold air flow at the outlet of the cold air first-stage heat exchanger 50... The ice slurry solution generated by the seawater desalination subsystem is used to provide secondary cooling for the cold storage air in the hot side channel of the ice slurry heat exchanger 35. The cold air from the outlet of the hot side channel of the ice slurry heat exchanger 35 flows into the hot side channel of the LNG three-stage heat exchanger 8, using the remaining cooling capacity of natural gas to reduce the temperature of the cold storage air to -30℃ to -25℃. The ice bucket water pump 36 pumps some water from the purified water pool 43 into the ice storage bucket 26. The ice storage bucket 26 is cooled by liquid oxygen produced by the air separation subsystem mentioned above, producing ice at -30℃ to -25℃. This ice can be used for the freezing and preservation of seafood.

[0040] In summary, in this liquefied natural gas (LNG) cold energy cascade utilization system, the heat generated by air compression is used for the evaporation and purification of seawater in the seawater purification subsystem. The liquid oxygen product generated in the separation stage is also reused, and the cooling capacity of the liquid oxygen is used in the cold chain cooling subsystem. Finally, the gaseous oxygen enters the gas boiler in the cryogenic power generation system for combustion. In the seawater purification subsystem, a combined freezing and distillation method is proposed, which fully utilizes the heat generated by the air separation subsystem and the cryogenic power generation system. The generated high-temperature steam is used to heat the boiler feedwater in the cryogenic power generation system, and the cooling capacity generated during the freezing process is used in the cold chain cooling subsystem. Through the nested cooperation between various subsystems, this system improves energy utilization, generates high-quality industrial products, and saves operating costs.

Claims

1. A cascaded utilization system for cold energy of liquefied natural gas, characterized in that: This includes the LNG trunk line, air separation subsystem, seawater purification subsystem, cryogenic power generation system, and cold chain cooling subsystem; The LNG inlet (1) supplies fuel to the gas boiler (40) through the LNG trunk line. The LNG trunk line is equipped with an LNG primary heat exchanger (4), an LNG secondary heat exchanger (6), an LNG tertiary heat exchanger (8) and an LNG quaternary heat exchanger (9) in sequence along the fuel supply direction. The air separation subsystem is connected to the LNG primary heat exchanger (4), which is used to exchange heat between the cryogenic fuel in the LNG trunk and the compressed air in the air separation subsystem. The seawater purification subsystem is connected to the LNG secondary heat exchanger (6), which is used to exchange heat between the low-temperature fuel in the LNG trunk and the ice maker in the seawater purification subsystem. The seawater purification subsystem includes a seawater pool (46), an ice maker (30), and an evaporation chamber (45). The seawater in the seawater pool (46) is fed into the ice slurry formed by the ice maker (30) and then flows into the purification water pool (43) after washing. The seawater in the seawater pool (46) is fed into the high-temperature steam formed by the evaporation chamber (45) and then flows into the purification water pool (43) after cooling. The evaporation chamber (45) is heated by the combustion exhaust gas of the gas boiler (40) and the compressed air of the air separation subsystem. The high-temperature steam generated by the evaporation chamber (45) is used to heat the treated water delivered from the purification water pool (43) to the gas boiler (40). The cold chain cooling subsystem is connected to the LNG three-stage heat exchanger (8), which is used to exchange heat between the cryogenic fuel in the LNG trunk line and the cold storage air in the cold chain cooling subsystem. The cold chain cooling subsystem includes a refrigeration unit (48), which provides cooling to the cold side of the cold air first-stage heat exchanger (50) and the exhaust steam first-stage heat exchanger (51). The exhaust steam generated by the turbine (41) flows into the exhaust steam first-stage heat exchanger (51) for initial cooling and is then transported to the LNG four-stage heat exchanger (9). The exhaust steam undergoes a second cooling process, and the condensate formed after the second cooling flows into the purification water pool (43). The cold storage air supplied by the cold storage air inlet (52) flows into the cold air primary heat exchanger (50) for a first cooling process and then is supplied to the LNG tertiary heat exchanger (8) for a second cooling process. The cold storage air that has been cooled again is then supplied back to the cold storage. After the cold air primary heat exchanger (50) performs a first cooling process, it is fed into the ice slurry heat exchanger (35) for a second cooling process and then supplied to the LNG tertiary heat exchanger (8). The cold source of the ice slurry heat exchanger (35) is the washed ice slurry. The outlet of the liquid oxygen tank (19) of the air separation subsystem is connected to the inlet of the cold side channel of the liquid oxygen heat exchanger (23), the outlet of the cold side channel of the liquid oxygen heat exchanger (23) is connected to the inlet of the cold side channel of the ice storage tank (26), the outlet of the cold side channel of the ice storage tank (26) is connected to the gas boiler (40), and the purified water tank (43) is used to deliver purified water to the ice storage tank (26). The cryogenic power generation system is connected to the LNG fourth-stage heat exchanger (9), which is used to exchange heat between the cryogenic fuel in the LNG trunk line and the exhaust steam output from the turbine.

2. The liquefied natural gas cold energy cascade utilization system according to claim 1, characterized in that: The air separation subsystem includes a molecular sieve purifier (15), an expander (17), and a distillation column (18). Compressed air flows through the LNG primary heat exchanger (4) for cooling and then passes through the molecular sieve purifier (15) for purification and the expander (17) for cooling before the product is fed into the distillation column (18). The top outlet of the distillation column (18) is connected to the liquid nitrogen tank (20), and the bottom outlet is connected to the liquid oxygen tank (19).

3. The liquefied natural gas cold energy cascade utilization system according to claim 1, characterized in that: A gas-fired boiler (40) is used to supply superheated steam to a steam turbine (41) and drive the steam turbine (41) to rotate, wherein the rotating shaft of the steam turbine (41) is coaxial with the rotor of the generator (42).

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