Carbon dioxide reliquefaction system and carbon dioxide reliquefaction method which use closed cycle

Abstract

The present invention relates to a carbon dioxide reliquefaction system and a carbon dioxide reliquefaction method which use a closed cycle. A carbon dioxide evaporation gas reliquefaction system according to the present invention comprises: an engine using a liquefied gas as fuel; a storage tank for storing liquefied carbon dioxide; an evaporation gas compressor, which compresses carbon dioxide evaporation gas evaporated in the storage tank, so as to generate a compressed evaporation gas; a first evaporation gas heat exchanger for cooling the compressed evaporation gas; and a refrigeration cycle in which a refrigerant circulates in order to provide cold energy to the first evaporation gas heat exchanger, wherein the refrigeration cycle includes a refrigerant compressor for compressing the refrigerant heated by heat exchange in the first evaporation gas heat exchanger, a refrigerant control valve for expanding the compressed refrigerant and supplying same to the first evaporation gas heat exchanger, and a second evaporation gas heat exchanger, which exchanges heat with the carbon dioxide evaporation gas evaporated in the storage tank, so as to cool the refrigerant that passed through the refrigerant control valve.

Description

[Technical Field]

[0001] The present invention relates to a carbon dioxide liquefaction system which is used in a liquefied carbon dioxide carrier fueled by liquefied gas and is configured to handle boil-off gas of liquefied carbon dioxide though a reliquefaction process employing a closed cycle, and a carbon dioxide liquefaction method using the same.[Background Art]

[0002] A combustion process for producing energy from fossil fuels involves generation of carbon dioxide. With increased use of fossil fuels, global carbon dioxide emissions are rising. Carbon dioxide has been identified as one of the greenhouse gases (GHGs) responsible for global warming.

[0003] Although the global warming potential of carbon dioxide is relatively low compared to other greenhouse gases, carbon dioxide is classified as a key greenhouse gas due to the fact that carbon dioxide accounts for about 80% of total greenhouse gas emissions and emissions of carbon dioxide can be regulated.

[0004] Based on various international agreements, each country has regulated carbon dioxide emissions. This has led to a demand for carbon dioxide handling technologies, such as carbon capture, utilization, and Storage (CCUS), which is a technology that captures carbon dioxide from various industrial sites and stores the captured carbon dioxide in a separate isolated place to reduce the amount of carbon dioxide released into the atmosphere.

[0005] CCUS technology involves liquefying and transporting captured carbon dioxide for separate handling. There have proposed various handling methods for the liquefied carbon dioxide, such as injecting the liquefied carbon dioxide into an empty space remaining after oil extraction for stable storage, or using the liquefied carbon dioxide, instead of water, as a high-pressure injection fluid during oil extraction.

[0006] To liquefy carbon dioxide, there are two main approaches: an open-cycle system in which carbon dioxide is compressed and then cooled through heat exchange with carbon dioxide decompressed through a Joule-Thomson valve; and a closed-cycle system which uses a separate refrigerant having a lower saturation temperature than CO 2 .

[0007] Despite being capable of achieving high efficiency with a small amount of refrigerant compared to the open-cycle system, the closed-cycle system requires a separate refrigeration system.

[0008] Therefore, there is a need for an optimized carbon dioxide liquefaction system that can meet both system efficiency and cost requirements.[Disclosure][Technical Problem]

[0009] It is an aspect of the present invention to provide a carbon dioxide liquefaction system employing a closed cycle and a carbon dioxide liquefaction method using the same.

[0010] More specifically, the present invention aims to provide a carbon dioxide liquefaction system that may be used in a liquefied carbon dioxide carrier fueled by liquefied natural gas (LNG) and can reduce the amount of a refrigerant circulating in a refrigeration cycle by cooling the refrigerant using cold heat of boil-off gas generated in a liquefied carbon dioxide storage tank, and a carbon dioxide liquefaction method using the same.

[0011] It is another aspect of the present invention to provide a carbon dioxide liquefaction system that can reduce the internal pressure of a carbon dioxide cargo tank (storage tank) by returning reliquefied carbon dioxide to the cargo tank to reduce the temperature of vapor inside the tank, and a carbon dioxide liquefaction method using the same.

[0012] The technical problems to be solved by the present invention are not limited to those mentioned above, and other technical problems not mentioned will be apparent to those skilled in the art from the detailed description of the invention.[Technical Solution]

[0013] In accordance with one aspect of the present invention, a carbon dioxide boil-off gas reliquefaction system includes: an engine fueled by liquefied gas; a storage tank storing liquefied carbon dioxide; a boil-off gas compressor compressing carbon dioxide boil-off gas from the storage tank to generate a compressed boil-off gas; a first boil-off gas heat exchanger cooling the compressed boil-off gas; and a refrigeration cycle in which a refrigerant is circulated to supply cold heat to the first boil-off gas heat exchanger, wherein the refrigeration cycle includes: a refrigerant compressor compressing the refrigerant heated through heat exchange in the first boil-off gas heat exchanger; a refrigerant control valve expanding the compressed refrigerant and supplying the expanded refrigerant to the first boil-off gas heat exchanger; and a second boil-off gas heat exchanger cooling the refrigerant having passed through the refrigerant control valve through heat exchange with the carbon dioxide boil-off gas from the storage tank.

[0014] The second boil-off gas heat exchanger may supply the carbon dioxide boil-off gas heated through heat exchange with the refrigerant to the boil-off gas compressor.

[0015] The refrigeration cycle may further include: a refrigerant heat exchanger, wherein the second boil-off gas heat exchanger may primarily cool the refrigerant having passed through the refrigerant control valve through heat exchange with the carbon dioxide boil-off gas from the storage tank, and the refrigerant heat exchanger may secondarily cool the primarily cooled refrigerant through heat exchange with liquefied gas supplied to the engine and may supply the carbon dioxide boil-off gas heated through heat exchange with the refrigerant to the boil-off gas compressor.

[0016] The carbon dioxide boil-off gas reliquefaction system may further include: a reliquefied boil-off gas recovery line connecting the first boil-off gas heat exchanger to the storage tank; and a first boil-off gas control valve disposed on the reliquefied boil-off gas recovery line.

[0017] The storage tank may include a spray nozzle line disposed at an upper portion inside the storage tank, wherein the spray nozzle line may be connected to the reliquefied boil-off gas recovery line to spray supercooled boil-off gas into the storage tank.

[0018] The carbon dioxide boil-off gas reliquefaction system may further include: a recirculation line connecting the first boil-off gas heat exchanger to the storage tank, wherein boil-off gas not cooled in the first boil-off gas heat exchanger may be recirculated to the boil-off gas compressor through the recirculation line.

[0019] The carbon dioxide boil-off gas reliquefaction system may further include: a second boil-off gas control valve disposed on the recirculation line and expanding the boil-off gas not cooled in the first boil-off gas heat exchanger to generate a gas-liquid mixed boil-off gas; and a boil-off gas separator disposed on the recirculation line and separating the gas-liquid mixed boil-off gas into a reliquefied boil-off gas in a liquid state and a boil-off gas in a gaseous state.

[0020] The boil-off gas separator may be connected at a lower side thereof to the reliquefied boil-off gas recovery line and may be connected at an upper side thereof to the recirculation line, the reliquefied boil-off gas in the liquid state may be supplied to the storage tank through the reliquefied boil-off gas recovery line, and the boil-off gas in the gaseous state may be supplied to the first boil-off gas heat exchanger through the recirculation line to be heated through heat exchange and may be recirculated to the boil-off gas compressor.

[0021] In accordance with another aspect of the present invention, a carbon dioxide boil-off gas reliquefaction method includes: a compressed boil-off gas generation step in which carbon dioxide boil-off gas generated in a storage tank is supplied to a compressor to generate a compressed boil-off gas; a cooling step in which the compressed boil-off gas is cooled through heat exchange with a refrigerant circulating in a refrigeration cycle; and a recovery step in which reliquefied carbon dioxide resulting from cooling in the cooling step is recovered, wherein the cooling step includes: a refrigerant circulation step in which the refrigerant is circulated in the refrigeration cycle, the refrigerant circulation step includes a refrigerant cooling step in which the refrigerant is cooled, and the refrigerant cooling step includes: cooling the refrigerant through heat exchange with the carbon dioxide boil-off gas; and supplying the carbon dioxide boil-off gas heated through heat exchange to the compressor.

[0022] The compressed boil-off gas generation step may further include: a first heat exchange step in which the carbon dioxide boil-off gas generated in the storage tank is subjected to primary heat exchange.

[0023] The refrigerant cooling step may further include: a first refrigerant cooling step in which the refrigerant is primarily cooled through heat exchange with the carbon dioxide boil-off gas; and a second refrigerant cooling step in which the primarily cooled refrigerant is secondarily cooled through heat exchange with liquefied gas supplied to an engine.

[0024] The cooling step may further include: a gas-liquid mixed boil-off gas generation step in which the compressed boil-off gas is expanded to generate a gas-liquid mixed boil-off gas; and a boil-off gas separation step in which the gas-liquid mixed boil-off gas is separated into a reliquefied boil-off gas in a liquid state and a boil-off gas in a gaseous state.

[0025] The carbon dioxide boil-off gas reliquefaction method may further include: a recirculation step in which the reliquefied boil-off gas in the liquid state is sprayed into the storage tank for recovery and the boil-off gas in the gaseous state is recirculated to the boil-off gas compressor for reliquefaction.

[0026] The recirculation step may include recirculating the boil-off gas in the gaseous state to the boil-off gas compressor through a first boil-off gas heat exchanger disposed on a recirculation line such that the boil-off gas in the gaseous state is heated while cooling the compressed boil-off gas in the first boil-off gas heat exchanger and is resupplied to the boil-off gas compressor through the recirculation line.[Advantageous Effects]

[0027] Embodiments of the present invention provide a carbon dioxide re-liquefaction system employing a closed cycle and a carbon dioxide re-liquefaction system using the same.

[0028] Specifically, the carbon dioxide re-liquefaction system may be used in a liquefied carbon dioxide carrier fueled by liquefied natural gas (LNG) and can reduce the amount of a refrigerant circulating in a refrigeration cycle by cooling the refrigerant using cold heat of carbon dioxide boil-off gas generated in a liquefied carbon dioxide storage tank.

[0029] In addition, the carbon dioxide re-liquefaction system can eliminate the need for a separate low-temperature compressor by allowing the carbon dioxide boil-off gas to be heated through heat exchange and to be mixed with a recirculating stream of gaseous carbon dioxide prior to introduction into a compressor.

[0030] In addition, the carbon dioxide re-liquefaction system can reduce the internal pressure of a carbon dioxide cargo tank (storage tank) by returning reliquefied carbon dioxide to the cargo tank to reduce the temperature of vapor inside the cargo tank

[0031] In addition, the carbon dioxide re-liquefaction system can be operated continuously since a liquefied carbon dioxide storage tank is always generating boil-off gas.

[0032] The advantageous effects of the present invention are not limited to those mentioned above, and other advantageous effects not mentioned will be apparent to those skilled in the art from the detailed description of the invention.[Description of Drawings]

[0033] FIG. 1 and FIG. 2 are simplified schematic diagrams of a carbon dioxide boil-off gas reliquefaction system according to one embodiment of the present invention.[Best Mode]

[0034] The above and other aspects, features, and advantages of the present invention will become apparent from the detailed description of the following embodiments in conjunction with the accompanying drawings.

[0035] The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting. For example, it will be understood that the terms "includes", "comprises", "including" and / or "comprising," when used in this specification, specify the presence of stated features, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups. In addition, when an element or layer is referred to as being "on," "connected to," or "coupled to" another element or layer, it may be directly on, connected to, or coupled to the other element or layer or intervening elements or layers may be present.

[0036] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be understood that the embodiments described below are provided for thorough understanding of the invention by those skilled in the art and should not be construed as limiting the scope of the invention, and that the present invention is not limited to the following embodiments and may be embodied in different ways by those skilled in the art.

[0037] In the following description of the present invention, the term "means of transport" may refer to a vehicle equipped with a liquefied carbon dioxide storage tank and capable of transport people or goods by land or sea. In description of embodiments of the present invention, a ship operating at sea will be used as an example of the means of transport.

[0038] In addition, in some embodiments of the present invention, the ship may be a liquefied carbon dioxide carrier (LCO 2 carrier). However, it should be understood that the present invention is not limited thereto and the following embodiments can be applied equally to any vessel equipped with a liquefied carbon dioxide storage tank.

[0039] In addition, as used herein, the term "ship" may include non-self-propelled floating offshore structures, as well as self-propelled vessels.

[0040] A liquefied carbon dioxide carrier according to one embodiment of the present invention described below may include at least one dual-fuel engine as a propulsion engine or a power generation engine, wherein the dual-fuel engine can be selectively fueled by a gaseous fuel, a fuel oil, or a mixture thereof.

[0041] Here, the gaseous fuel may be stored in a liquefied state, that is, in the form of a liquefied gas, in an onboard fuel tank, and may be supplied in a liquid state to an engine, or may be converted into a gaseous state by vaporization before being supplied to the engine. For example, the liquefied gas may be selected from among hydrocarbon-based liquefied gases, such as liquefied natural gas (LNG), liquefied ethane gas (LEG), liquefied petroleum gas (LPG), liquefied ethylene gas, and liquefied propylene gas, and non-hydrocarbon-based liquefied gases, such as liquefied ammonia (NH 3 ) and liquefied hydrogen.

[0042] In some embodiments of the present invention described below, natural gas will be used as an example of the gaseous fuel, such that liquefied natural gas may be stored in the fuel tank.

[0043] Here, the engine is a dual-fuel engine that can be fueled by natural gas, among various engines used in the ship. For example, the engine may include at least one of an MAN Electronic Gas-Injection (ME-GI) engine, an eXtra long stroke Dual Fuel (X-DF) engine, or a DF engine (Dual Fuel Diesel Electric (DFDE) engine), a Dual Fuel Diesel Generator (DFDG). However, it should be understood that the present invention is not limited thereto.

[0044] Hereinafter, a carbon dioxide boil-off gas reliquefaction system according to one embodiment of the present invention will be described with reference to FIG. 1 and FIG. 2.

[0045] A liquefied carbon dioxide carrier according to this embodiment includes a liquefied carbon dioxide system, wherein the liquefied carbon dioxide system may include a liquefied carbon dioxide storage system, a boil-off gas processing system, and a fuel supply system (not shown).

[0046] The liquefied carbon dioxide storage system includes at least one storage tank 100 for storing liquefied carbon dioxide. The storage tank 100 is a pressurized tank and may be pressurized to a certain pressure or higher when in operation to keep carbon dioxide in a liquid state.

[0047] The boil-off gas processing system includes a boil-off gas compressor compressing boil-off gas discharged from the storage tank 100 and a first boil-off gas heat exchanger 220 cooling the compressed boil-off gas from the boil-off gas compressor 210, wherein carbon dioxide boil-off gas generated in the carbon dioxide storage tank 100 may be introduced into the boil-off gas compressor 210 through a boil-off gas supply line BL.

[0048] Specifically, the boil-off gas processing system further includes a second boil-off gas heat exchanger 120 on the boil-off gas supply line BL, wherein the boil-off gas discharged from the storage tank 100 may be subjected to heat exchange while passing through the second boil-off gas heat exchanger 120 and may then be introduced into the boil-off gas compressor 210 for compression.

[0049] The carbon dioxide boil-off gas introduced through the boil-off gas supply line BL may be heated through heat exchange with a low-temperature refrigerant circulating in a refrigeration cycle 500 described below before the carbon dioxide boil-off gas is introduced into the boil-off gas compressor.

[0050] The boil-off gas introduced into the boil-off gas compressor may be compressed to a pressure allowing the boil-off gas to be at least partially liquefied during cooling in the first boil-off gas heat exchanger 220.

[0051] The boil-off gas processing system may further include: an intercooler 211 cooling the boil-off gas heated during compression in the boil-off gas compressor 210 prior to supply to the first boil-off gas heat exchanger 220; and a buffer tank 212 temporarily storing the cooled boil-off gas from the intercooler 211 prior to supply to the first boil-off gas heat exchanger 220. Accordingly, the compressed boil-off gas discharged from the boil-off gas compressor 210 may be cooled in the intercooler 211, may then stay in the buffer tank 210, and may then be introduced into a hot fluid channel of the first boil-off gas heat exchanger 220 while flowing along the boil-off gas supply line BL.

[0052] The first boil-off gas heat exchanger 220 cools the compressed boil-off gas introduced therein through the boil-off gas supply line BL through heat exchange to generate a cooled compressed boil-off gas.

[0053] For example, the compressed boil-off gas introduced into the first boil-off gas heat exchanger 220 through the boil-off gas supply line BL may be cooled through heat exchange with a low-temperature refrigerant circulating in a refrigeration cycle 500 described below.

[0054] Alternatively, the compressed boil-off gas introduced into the first boil-off gas heat exchanger 220 through the boil-off gas supply line BL may be cooled through heat exchange with both the low-temperature refrigerant circulating in the refrigeration cycle 500 described below and a gaseous boil-off gas resulting from gas-liquid separation in a boil-off gas separator 260 described below.

[0055] The boil-off gas processing system further includes: a reliquefied boil-off gas recovery line RL connecting the first boil-off gas heat exchanger 220 to the storage tank 100 to allow the cooled compressed boil-off gas from the first boil-off gas heat exchanger 210 to be returned to the storage tank 100 through the reliquefied boil-off gas recovery line.

[0056] Specifically, a first boil-off gas control valve 240 may be disposed on the reliquefied boil-off gas recovery line RL such that the cooled compressed boil-off gas is supplied to the storage tank 100 after passing through the first boil-off gas control valve 240.

[0057] The first boil-off gas control valve 240 expands the cooled compressed boil-off gas to generate reliquefied boil-off gas in a liquid phase, wherein the reliquefied boil-off gas in the liquid phase may be in a supercooled state. Accordingly, the first boil-off gas control valve 240 may be a Joule-Thomson valve configured to expand cooled boil-off gas through an isenthalpic process.

[0058] The reliquefied boil-off gas recovery line RL may be connected to a spray nozzle line 110 disposed at an upper portion inside the storage tank 100 such that the supercooled reliquefied boil-off gas from the first boil-off gas control valve 240 can be sprayed into the storage tank. As the supercooled reliquefied boil-off gas is sprayed into the storage tank, the internal pressure of the storage tank 100 can be effectively reduced.

[0059] The boil-off gas processing system further includes: a recirculation line GL connecting the first boil-off gas heat exchanger 220 to the storage tank 100 to allow the compressed boil-off gas subjected to heat exchange through the first boil-off gas heat exchanger 220 to be recirculated to the boil-off gas compressor 210.

[0060] Specifically, a second boil-off gas control valve 250 and a boil-off gas separator 260 are disposed on the recirculation line GL. Accordingly, as the compressed boil-off gas is subjected to heat exchange in the first boil-off gas heat exchanger 220, the compressed boil-off gas is transformed into a cooled compressed boil-off gas in a liquid state, which, is, in turn, transformed into a gas-liquid mixed boil-off gas in which a reliquefied boil-off gas in a liquid state is mixed with a boil-off gas in a gaseous state while passing through the second boil-off gas control valve 250 and then is supplied to the boil-off gas separator 260.

[0061] The boil-off gas separator 260 separates the gas-liquid mixed boil-off gas into the reliquefied boil-off gas in the liquid state and the boil-off gas in the gaseous state. The boil-off gas separator 260 and the storage tank 100 may be connected to each other through both the reliquefied boil-off gas recovery line RL and the recirculation line GL.

[0062] Specifically, the boil-off gas separator 260 may be connected at a lower side thereof to the storage tank 100 through the reliquefied boil-off gas recovery line RL to allow the reliquefied boil-off gas in the liquid state, obtained from gas-liquid separation in the boil-off gas separator 260, to be returned to the storage tank 100 through the reliquefied boil-off gas recovery line RL.

[0063] That is, the reliquefied boil-off gas in the liquid state may be returned to the storage tank 100 for storage through either the reliquefied boil-off gas recovery line RL or the recirculation line GL, depending on conditions of the system. For example, when ice (dry ice) is formed in one of the reliquefied boil-off gas recovery line RL and the recirculation line GL during operation of the system, the reliquefied boil-off gas may be returned to the storage tank 100 without passing through the other line in which ice is not formed.

[0064] The boil-off gas separator 260 is connected to the storage tank 100 through the recirculation line GL. Accordingly, the gaseous boil-off gas obtained from gas-liquid separation in the boil-off gas separator 260 may be recirculated through the recirculation line GL.

[0065] Specifically, the recirculation line GL may be connected to the boil-off gas compressor 210. Accordingly, the gaseous boil-off gas recirculated to the boil-off gas compressor 210 through the recirculation line GL may be heated through heat exchange in the first boil-off gas heat exchanger 220 before the gaseous boil-off gas is supplied to the boil-off gas compressor 210.

[0066] Here, the heated gaseous boil-off gas may join a stream of boil-off gas heading toward the boil-off gas compressor 210 after being discharged from the storage tank 100 and having been heated through the second boil-off gas heat exchanger 120 before the heated gaseous boil-off gas is supplied to the boil-off gas compressor 210. As the gaseous boil-off gas recirculating through the recirculation line GL is mixed with the stream of the carbon dioxide boil-off gas from the storage tank 100, boil-off gas having a temperature higher than ambient temperature (about 35°C) or higher can be supplied to the boil-off gas compressor 210.

[0067] That is, since the warmer carbon dioxide boil-off gas is resupplied to the boil-off gas compressor 210, it is possible to eliminate the need for a separate low-temperature compressor.

[0068] In addition, a three-stream heat exchanger may be used as the first boil-off gas heat exchanger 220 to allow the gaseous boil-off gas recirculated to the boil-off gas compressor 210 through the recirculation line GL and the low-temperature refrigerant circulating through the refrigerant circulation line ML to exchange heat with the compressed boil-off gas delivered from the boil-off gas compressor 210 through the boil-off gas supply line BL.

[0069] That is, the compressed boil-off gas is cooled by the gaseous boil-off gas supplied from the boil-off gas separator 260 and the refrigerant circulating in the refrigeration cycle 500, while the gaseous boil-off gas is heated prior to resupply to the boil-off gas compressor 210.

[0070] The refrigeration cycle 500 may include a refrigerant compressor 510 compressing the refrigerant and a refrigerant control valve 520 controlling the flow of the refrigerant supplied to the first boil-off gas heat exchanger 220.

[0071] In addition, the refrigeration cycle 500 further includes a refrigerant cooler 511 cooling the refrigerant heated during compression in the refrigerant compressor 510. Accordingly, the refrigerant introduced into the refrigerant compressor 510 through the refrigerant circulation line ML may be compressed in the refrigerant compressor 510, may then be cooled in the refrigerant cooler 511, and may then be supplied to the first boil-off gas heat exchanger 220 through the refrigerant circulation line ML.

[0072] Furthermore, the refrigeration cycle 500 may further include a second boil-off gas heat exchanger 120. Specifically, the second boil-off gas heat exchanger 120 may be disposed on the refrigerant circulation line ML, preferably downstream of the refrigerant cooler 511. That is, the refrigerant having passed through the refrigerant cooler 511 is cooled in the second boil-off gas heat exchanger 120 through heat exchange with the carbon dioxide boil-off gas from the storage tank 100.

[0073] Alternatively, as shown in FIG. 2, the refrigeration cycle 500 may further include a second boil-off gas heat exchanger 120 and a refrigerant heat exchanger 310. Specifically, the second boil-off gas heat exchanger 120 and the refrigerant heat exchanger 310 may be disposed on the refrigerant circulation line ML, preferably downstream of the refrigerant cooler 511.

[0074] That is, the refrigerant having passed through the refrigerant cooler 511 is primarily cooled in the second boil-off gas heat exchanger 120 through heat exchange with the carbon dioxide boil-off gas from the storage tank 100 and is secondarily cooled in the refrigerant heat exchanger 310 through heat exchange with liquefied gas LNG supplied as fuel from a fuel tank 300 to an engine 400.

[0075] Here, heat exchange in the refrigerant heat exchanger 310 allows the LNG to be heated prior to supply to the engine 400 while allowing the refrigerant to be secondarily cooled. Thus, the refrigerant heat exchanger 310 may act as vaporizer that vaporizes LNG supplied from the fuel tank 300 to the engine 400.

[0076] Alternatively, the refrigeration cycle 500 may further include a vaporizer (not shown) vaporizing LNG supplied as fuel to the engine 400 downstream side of the refrigerant heat exchanger 310. Here, the refrigerant heat exchanger 310 may be used as a preheater to preheat LNG from the fuel tank 300 prior to supply to the vaporizer.

[0077] The refrigerant control valve 520 may be disposed at a downstream side of the refrigerant heat exchanger 310.

[0078] The refrigerant control valve 520 may be a flow control valve that regulates the flow rate of the cooled compressed refrigerant (that is, the low-temperature refrigerant) supplied from the refrigerant heat exchanger 310 to the first boil-off gas heat exchanger 220 through the refrigerant circulation line ML, and may have a function of expanding the cooled compressed refrigerant from the refrigerant heat exchanger 310 through an isenthalpic process.

[0079] Accordingly, the low-temperature refrigerant having passed through the second boil-off gas heat exchanger 120 and the refrigerant control valve 520 may be introduced in a liquid state into the first boil-off gas heat exchanger 220, may be vaporized through heat exchange in the first boil-off gas heat exchanger 220, and may be introduced in a gaseous state into the refrigerant compressor 510.

[0080] That is, the low-temperature refrigerant circulating through the refrigerant circulation line ML is heated through heat exchange with the compressed boil-off gas from the boil-off gas compressor 210. Specifically, the low-temperature refrigerant may become a high-temperature refrigerant while cooling the carbon dioxide boil-off gas in the first boil-off gas heat exchanger 220, and the high-temperature refrigerant may be introduced into the refrigerant compressor 510 through the refrigerant circulation line ML to be circulated in the refrigeration cycle 500.

[0081] Here, the refrigerant circulating in the refrigeration cycle 500 may be a hydrocarbon refrigerant, such as propane or ammonia. However, it should be understood that the present invention is not limited thereto and the refrigerant may include any refrigerant suitable for reliquefying the carbon dioxide boil-off gas while circulating in the refrigeration cycle 500.

[0082] Next, a carbon dioxide boil-off gas reliquefaction method according to one embodiment of the present invention will be described with reference to the carbon dioxide boil-off gas reliquefaction system according to the present invention as described above.

[0083] The carbon dioxide boil-off gas reliquefaction method may be used in a carbon dioxide carrier provided with a dual-fuel engine and may include a compressed boil-off gas formation step, a cooling step, and a recovery step.

[0084] The compressed boil-off gas formation step is a step in which carbon dioxide boil-off gas generated in the storage tank 100 is supplied to the boil-off gas compressor 210 to be compressed while passing through the boil-off gas compressor 210.

[0085] Specifically, the carbon dioxide boil-off gas from the storage tank 100 may be subjected to heat exchange while passing through the second boil-off gas heat exchanger 120 and may then be introduced into the boil-off gas compressor 210 for compression.

[0086] Here, the carbon dioxide boil-off gas introduced into the second boil-off gas heat exchanger through the boil-off gas supply line BL may be heated through heat exchange with a low-temperature refrigerant circulating in the refrigeration cycle 500 and may then be introduced into the boil-off gas compressor.

[0087] The compressed boil-off gas from the boil-off gas compressor 210 may be cooled in the intercooler 211, may then stay in the buffer tank 210, and may then be introduced into a hot fluid channel of the first boil-off gas heat exchanger 220 while flowing along the boil-off gas supply line BL.

[0088] The cooling step is a step in which the compressed boil-off gas is supplied to the first boil-off gas heat exchanger 220 to be cooled through heat exchange with the refrigerant circulating in the refrigeration cycle, and may include a refrigerant circulation step in which the refrigerant is circulated in the refrigeration cycle.

[0089] In addition, the refrigerant circulation step may include a refrigerant cooling step in which the heated refrigerant is cooled. In the refrigerant cooling step, the refrigerant heated through heat exchange with the compressed boil-off gas is compressed while passing through the refrigerant compressor and is then cooled by expansion while passing through the refrigerant control valve. Then, the cooled refrigerant cools the compressed boil-off gas supplied from the storage tank 100 while passing through the first boil-off gas heat exchanger 220.

[0090] Specifically, in the refrigerant cooling step, the refrigerant having passed through the refrigerant compressor 510 and the refrigerant cooler 511 may exchange heat with the carbon dioxide boil-off gas from the storage tank 100 while passing through the second boil-off gas heat exchanger 120.

[0091] Subsequently, the cooled refrigerant passes through the refrigerant control valve 520 disposed downstream of the refrigerant heat exchanger 310. As the cooled compressed refrigerant is expanded while passing through the refrigerant control valve 520, the temperature of the refrigerant can be further reduced.

[0092] Alternatively, the refrigerant cooling step may include: a first refrigerant cooling step in which the refrigerant having passed through the refrigerant compressor 510 and the refrigerant cooler 511 is primarily cooled through heat exchange with the carbon dioxide boil-off gas from the storage tank 100 while passing through the second boil-off gas heat exchanger 120; and a second refrigerant cooling step in which the primarily cooled refrigerant is secondarily cooled through heat exchange with liquefied gas (LNG) supplied as fuel from the fuel tank 300 to the engine 400 while passing through the refrigerant heat exchanger 310.

[0093] Here, the refrigerant cooled through the second refrigerant cooling step passes through the refrigerant control valve 520 disposed downstream of the refrigerant heat exchanger 310. As the cooled compressed refrigerant is expanded while passing through the refrigerant control valve 520, the temperature of the refrigerant can be further reduced.

[0094] Subsequently, the cooled refrigerant exchanges heat with the compressed boil-off gas while passing through the first boil-off gas heat exchanger 220. Through heat exchange in the first boil-off gas heat exchanger 220, the compressed boil-off gas is cooled. The cooled compressed boil-off gas may be supplied to the first boil-off gas control valve 240.

[0095] Here, the cooled compressed boil-off gas is transformed into a reliquefied boil-off gas in a liquid state by expansion while passing through the first boil-off gas control valve 240. Then, the reliquefied boil-off gas in the liquid state may be returned to the storage tank 100.

[0096] Specifically, the reliquefied boil-off gas is in a supercooled state and may be sprayed into the storage tank upon recovery thereto. As the reliquefied boil-off gas in the supercooled state is sprayed into the storage tank 100, the internal pressure of the storage tank 100 can be effectively reduced.

[0097] The carbon dioxide boil-off gas reliquefaction method may further include: a gas-liquid mixed boil-off gas generation step in which the compressed boil-off gas from the cooling step is expanded to generate a gas-liquid mixed boil-off gas; and a boil-off gas separation step in which the gas-liquid mixed boil-off gas is separated into a reliquefied boil-off gas in a liquid state and a boil-off gas in a gaseous state.

[0098] In addition, the boil-off gas separation step may include: a recovery step in which the separated reliquefied boil-off gas is recovered by being spraying into the storage tank 100; and a recirculation step in which the separated gaseous boil-off gas is recirculated to the boil-off gas compressor 210 for reliquefaction.

[0099] Specifically, when the refrigerant is not sufficiently cooled, the compressed boil-off gas having passed through the first boil-off gas heat exchanger 220 is supplied to the second boil-off gas control valve 250. The compressed boil-off gas is transformed into a gas-liquid mixed boil-off gas by expansion while passing through the second boil-off gas control valve 250.

[0100] Subsequently, the gas-liquid mixed boil-off gas is supplied to the boil-off gas separator 260 to be separated into a gaseous boil-off gas and a liquid boil-off gas. Here, the separated liquid boil-off gas may be recovered by being sprayed into the storage tank 100, and the separated gaseous boil-off gas may be recirculated to the boil-off gas compressor 210 for reliquefaction.

[0101] That is, when there is no or insufficient cold heat for heat exchange in the first boil-off gas heat exchanger 220, the compressed boil-off gas may be supplied to the second boil-off gas control valve 250 for recovery.

[0102] In the recirculation step, the gaseous boil-off gas may be recirculated to the boil-off gas compressor 210 after passing through the first boil-off gas heat exchanger 220. Here, the gaseous boil-off gas, from which cold heat has been recovered while passing through the first boil-off gas heat exchanger 220, may join a stream of boil-off gas heading towards the boil-off gas compressor 210 after being discharged from the storage tank 100 and having been heated through the second boil-off gas heat exchanger 120 before the gaseous boil-off gas is resupplied to the boil-off gas compressor 210.

[0103] As described above, the present invention provides a carbon dioxide re-liquefaction system employing a closed cycle and a carbon dioxide re-liquefaction system using the same.

[0104] Specifically, the carbon dioxide re-liquefaction system may be used in a liquefied carbon dioxide carrier fueled by liquefied natural gas (LNG) and can reduce the amount of a refrigerant circulating in a refrigeration cycle by cooling the refrigerant using cold heat of carbon dioxide boil-off gas generated in a liquefied carbon dioxide storage tank.

[0105] In addition, the carbon dioxide re-liquefaction system can eliminate the need for a separate low-temperature compressor by allowing the carbon dioxide boil-off gas to be heated through heat exchange and to be mixed with a recirculating stream of gaseous carbon dioxide prior to introduction into a compressor.

[0106] In addition, the carbon dioxide re-liquefaction system can reduce the internal pressure of a carbon dioxide cargo tank (storage tank) by returning reliquefied carbon dioxide to the cargo tank to reduce the temperature of vapor inside the cargo tank

[0107] In addition, the carbon dioxide re-liquefaction system can be operated continuously since a liquefied carbon dioxide storage tank is always generating boil-off gas.

[0108] Although some embodiments have been described herein, it should be understood that these embodiments are presented by way of example only and that various modifications, changes, alterations, and equivalent embodiments can be made by those skilled in the art without departing from the spirit and scope of the present disclosure. Therefore, it should be understood that these embodiments are provided for illustration only and are not to be construed in any way as limiting the present disclosure. The scope of the present disclosure should be defined by the appended claims and the claims and their equivalents are intended to cover such modifications and the like as would fall within the scope and spirit of the invention.

Claims

1. A carbon dioxide boil-off gas reliquefaction system comprising: an engine fueled by liquefied gas; a storage tank storing liquefied carbon dioxide; a boil-off gas compressor compressing carbon dioxide boil-off gas from the storage tank to generate a compressed boil-off gas; a first boil-off gas heat exchanger cooling the compressed boil-off gas; and a refrigeration cycle in which a refrigerant is circulated to supply cold heat to the first boil-off gas heat exchanger, wherein the refrigeration cycle comprises: a refrigerant compressor compressing the refrigerant heated through heat exchange in the first boil-off gas heat exchanger; a refrigerant control valve expanding the compressed refrigerant and supplying the expanded refrigerant to the first boil-off gas heat exchanger; and a second boil-off gas heat exchanger cooling the refrigerant having passed through the refrigerant control valve through heat exchange with the carbon dioxide boil-off gas from the storage tank.

2. The carbon dioxide boil-off gas reliquefaction system according to claim 1, wherein the second boil-off gas heat exchanger supplies the carbon dioxide boil-off gas heated through heat exchange with the refrigerant to the boil-off gas compressor.

3. The carbon dioxide boil-off gas reliquefaction system according to claim 1, wherein the refrigeration cycle further comprises: a refrigerant heat exchanger, the second boil-off gas heat exchanger primarily cools the refrigerant having passed through the refrigerant control valve through heat exchange with the carbon dioxide boil-off gas from the storage tank, and the refrigerant heat exchanger secondarily cools the primarily cooled refrigerant through heat exchange with liquefied gas supplied to the engine and supplies the carbon dioxide boil-off gas heated through heat exchange with the refrigerant to the boil-off gas compressor.

4. The carbon dioxide boil-off gas reliquefaction system according to claim 1, further comprising: a reliquefied boil-off gas recovery line connecting the first boil-off gas heat exchanger to the storage tank; and a first boil-off gas control valve disposed on the reliquefied boil-off gas recovery line.

5. The carbon dioxide boil-off gas reliquefaction system according to claim 4, wherein the storage tank comprises a spray nozzle line disposed at an upper portion inside the storage tank, the spray nozzle line being connected to the reliquefied boil-off gas recovery line to spray supercooled boil-off gas into the storage tank.

6. The carbon dioxide boil-off gas reliquefaction system according to claim 1, further comprising: a recirculation line connecting the first boil-off gas heat exchanger to the storage tank, wherein boil-off gas not cooled in the first boil-off gas heat exchanger is recirculated to the boil-off gas compressor through the recirculation line.

7. The carbon dioxide boil-off gas reliquefaction system according to claim 6, further comprising: a second boil-off gas control valve disposed on the recirculation line and expanding the boil-off gas not cooled in the first boil-off gas heat exchanger to generate a gas-liquid mixed boil-off gas; and a boil-off gas separator disposed on the recirculation line and separating the gas-liquid mixed boil-off gas into a reliquefied boil-off gas in a liquid state and a boil-off gas in a gaseous state.

8. The carbon dioxide boil-off gas reliquefaction system according to claim 7, wherein the boil-off gas separator is connected at a lower side thereof to the reliquefied boil-off gas recovery line and is connected at an upper side thereof to the recirculation line, the reliquefied boil-off gas in the liquid state is supplied to the storage tank through the reliquefied boil-off gas recovery line, and the boil-off gas in the gaseous state is supplied to the first boil-off gas heat exchanger through the recirculation line to be heated through heat exchange and is recirculated to the boil-off gas compressor.

9. A carbon dioxide boil-off gas reliquefaction method comprising: a compressed boil-off gas generation step in which carbon dioxide boil-off gas generated in a storage tank is supplied to a compressor to generate a compressed boil-off gas; a cooling step in which the compressed boil-off gas is cooled through heat exchange with a refrigerant circulating in a refrigeration cycle; and a recovery step in which reliquefied carbon dioxide resulting from cooling in the cooling step is recovered, wherein the cooling step comprises: a refrigerant circulation step in which the refrigerant is circulated in the refrigeration cycle, and the refrigerant circulation step comprises a refrigerant cooling step in which the refrigerant is cooled, the refrigerant cooling step comprising: cooling the refrigerant through heat exchange with the carbon dioxide boil-off gas; and supplying the carbon dioxide boil-off gas heated through heat exchange to the compressor.

10. The carbon dioxide boil-off gas reliquefaction method according to claim 9, wherein the compressed boil-off gas generation step further comprises: a first heat exchange step in which the carbon dioxide boil-off gas generated in the storage tank is subjected to primary heat exchange.

11. The carbon dioxide boil-off gas reliquefaction method according to claim 9, wherein the compressed boil-off gas generation step further comprises: a first heat exchange step in which the carbon dioxide boil-off gas generated in the storage tank is subjected to primary heat exchange.

12. The carbon dioxide boil-off gas reliquefaction method according to claim 9, wherein the cooling step further comprises: a gas-liquid mixed boil-off gas generation step in which the compressed boil-off gas is expanded to generate a gas-liquid mixed boil-off gas; and a boil-off gas separation step in which the gas-liquid mixed boil-off gas is separated into a reliquefied boil-off gas in a liquid state and a boil-off gas in a gaseous state.

13. The carbon dioxide boil-off gas reliquefaction method according to claim 12, further comprising: a recirculation step in which the reliquefied boil-off gas in the liquid state is sprayed into the storage tank for recovery and the boil-off gas in the gaseous state is recirculated to the boil-off gas compressor for reliquefaction.

14. The carbon dioxide boil-off gas reliquefaction method according to claim 13, wherein the recirculation step comprises recirculating the boil-off gas in the gaseous state to the boil-off gas compressor through a first boil-off gas heat exchanger disposed on a recirculation line such that the boil-off gas in the gaseous state is heated while cooling the compressed boil-off gas in the first boil-off gas heat exchanger and is resupplied to the boil-off gas compressor through a recirculation line

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