Carbon dioxide re-liquefaction system and carbon dioxide re-liquefaction method utilizing closed cycle

Abstract

Disclosed is a carbon dioxide boil-off gas reliquefaction method. The carbon dioxide boil-off gas reliquefaction method includes: a compressed boil-off gas generation step in which evaporated carbon dioxide boil-off gas is supplied from a storage tank to a compressor to produce compressed boil-off gas; a cooling step in which the compressed boil-off gas is supplied to a first boil-off gas heat exchanger and cooled through heat exchange with a refrigerant circulating in a refrigeration cycle, and a recovery step in which reliquefied carbon dioxide cooled by the cooling step is sprayed into the storage tank for recovery of the reliquefied carbon dioxide, wherein the cooling step further include a second cooling step or a third cooling step depending on an operating mode of the carbon dioxide carrier.

Description

[Technical Field]

[0001] The present invention relates to a carbon dioxide liquefaction system and method utilizing a closed cycle, in which boil-off gas of liquefied carbon dioxide is processed through reliquefaction, in a liquefied carbon dioxide carrier using liquefied gas as fuel.[Background Art

[0002] Carbon dioxide is produced from fossil fuels upon combustion thereof for energy production. As the use of fossil fuels has increased, carbon dioxide is being emitted in large quantities and has been designated as one of greenhouse gases (GHGs) that contribute to global warming.

[0003] Although carbon dioxide has a lower global warming potential than other greenhouse gases, carbon dioxide is considered a very important greenhouse gas since carbon dioxide accounts for about 80% of all greenhouse gas emissions and emission of carbon dioxide can be regulated.

[0004] Based on various international agreements for reduction of emission of carbon dioxide, many countries regulate the emission of carbon dioxide and there is a need for development of carbon dioxide processing technologies, such as CCUS (Carbon Capture Utilization & Storage) and the like, which reduce the amount of carbon dioxide emitted into the atmosphere by recovering carbon dioxide generated from various industrial sites and storing the recovered carbon dioxide in a separate place, as one of technologies derived from these agreements.

[0005] The CCUS technology is a technique that liquefies and transports post-treated carbon dioxide for separate processing, in which the liquefied carbon dioxide is injected into a space left after oil extraction to be stored in a stable state or is sprayed at high pressure instead of water upon oil extraction.

[0006] Carbon dioxide is liquefied by an open cycle method in which heat exchange is performed through decompression of a Joule-Thomson valve after compression, or a closed cycle method in which a separate refrigerant having a lower saturation temperature than CO 2 is used.

[0007] Although the closed cycle method has higher efficiency with a smaller amount of refrigerant than the open cycle method, the closed cycle method requires a separate refrigeration system.

[0008] Therefore, there is a need for development of an optimal reliquefaction system that satisfies not only system efficiency but also costs.[Disclosure][Technical Problem]

[0009] It is an object of the present invention to provide a carbon dioxide liquefaction system and a carbon dioxide liquefaction method utilizing a closed cycle.

[0010] Specifically, the present invention is aimed at providing a carbon dioxide liquefaction system and a carbon dioxide liquefaction method utilizing a closed cycle, wherein boil-off gas of liquefied carbon dioxide is recovered through reliquefaction of the liquefied carbon dioxide using cold heat of liquefied natural gas, in a liquefied carbon dioxide carrier using liquefied natural gas (LNG) as fuel.

[0011] The present invention is also aimed at reducing an internal pressure of a carbon dioxide cargo tank (storage tank) through reduction in vapor temperature in the cargo tank by returning reliquefied carbon dioxide to the cargo tank.

[0012] The present invention is not limited to the aforementioned objects and other objects of the present invention will become apparent to those skilled in the art from the following description.[Technical Solution]

[0013] In accordance with one aspect of the present invention, there is provided a carbon dioxide reliquefaction method of a carbon dioxide carrier using a dual fuel engine, the method including: a compressed boil-off gas generation step in which evaporated carbon dioxide boil-off gas is supplied from a storage tank to a compressor to produce compressed boil-off gas; a cooling step in which the compressed boil-off gas is supplied to a first boil-off gas heat exchanger and cooled through heat exchange with a refrigerant circulating in a refrigeration cycle, and a recovery step in which reliquefied carbon dioxide cooled by the cooling step is sprayed into the storage tank for recovery of the reliquefied carbon dioxide, wherein the cooling step further includes a second cooling step or a third cooling step depending on an operating mode of the carbon dioxide carrier.

[0014] Preferably, the cooling step further includes a refrigeration cycle circulation step in which the refrigerant is circulated to be cooled, and, in the refrigeration cycle circulation step, the refrigerant heated by heat exchange with the compressed boil-off gas is compressed while passing through a refrigerant compressor and is then expanded and cooled while passing through a refrigerant control valve.

[0015] Preferably, when the carbon dioxide carrier is operated in gas mode, the second cooling step is further performed, the second cooling step including supplying the compressed boil-off gas subjected to primary cooling in the cooling step to a second boil-off gas heat exchanger to perform secondary cooling of the compressed boil-off gas through heat exchange with liquefied gas supplied to an engine.

[0016] Preferably, the compressed boil-off gas subjected to secondary cooling is expanded to produce reliquefied boil-off gas in a subcooled liquid state.

[0017] Preferably, the compressed boil-off gas subjected to secondary cooling is supplied to a first boil-off gas control valve, the first boil-off gas control valve being configured to expand the compressed boil-off gas subjected to secondary cooling to produce reliquefied boil-off gas in a subcooled liquid state.

[0018] Preferably, the reliquefied boil-off gas in the subcooled liquid state is returned to the storage tank through one reliquefaction recovery line selected from among a first reliquefaction recovery line and a second reliquefaction recovery line, and when the reliquefaction recovery line is blocked by dry ice, the reliquefied boil-off gas in the subcooled liquid state is diverted to an unblocked reliquefaction recovery line for recovery thereof.

[0019] Preferably, when the carbon dioxide carrier is operated in oil mode, the third cooling step is further performed, the third cooling step including: a boil-off gas-liquid mixture generation step in which the compressed boil-off gas subjected to primary cooling is expanded to produce a gas-liquid mixture of boil-off gas; and a boil-off gas separation step in which the gas-liquid mixture of boil-off gas is separated into reliquefied boil-off gas and gaseous boil-off gas.

[0020] Preferably, the reliquefied boil-off gas is sprayed into the storage tank for recovery thereof and the carbon dioxide reliquefaction method further includes a recirculation step in which the separated gaseous boil-off gas is recirculated to a boil-off gas compressor to reliquefy the separated gaseous boil-off gas, the recirculation step including recirculating the gaseous boil-off gas to the boil-off gas compressor through a first boil-off gas heat exchanger such that the gaseous boil-off gas is heated while cooling the compressed boil-off gas and the heated gaseous boil-off gas is resupplied to the compressor through the recirculation line.

[0021] In accordance with another aspect of the present invention, there is provided a carbon dioxide boil-off gas reliquefaction system including: an engine fueled by liquefied gas; a storage tank storing liquefied carbon dioxide; a boil-off gas compressor compressing boil-off gas of the liquefied carbon dioxide produced in the storage tank to produce compressed boil-off gas; a first boil-off gas heat exchanger cooling the compressed boil-off gas; a second boil-off gas heat exchanger secondarily cooling the primarily cooled boil-off gas supplied from the first boil-off gas heat exchanger through heat exchange with the liquefied gas supplied to the engine; and a refrigeration cycle in which a refrigerant is circulated to provide cold heat to the first boil-off gas heat exchanger.

[0022] Preferably, the second boil-off gas heat exchanger and the storage tank are connected to a reliquefaction recovery line, and a first boil-off gas control valve is disposed on the reliquefaction recovery line to expand the boil-off gas subjected to secondary cooling into a subcooled state through the second boil-off gas heat exchanger.

[0023] Preferably, a spray nozzle line is disposed above the storage tank, the spray nozzle line being connected to the reliquefaction recovery line to spray the subcooled boil-off gas into the storage tank.

[0024] Preferably, the second boil-off gas heat exchanger and the storage tank are connected to a recirculation line, through which the boil-off gas not subjected to secondary cooling in the second boil-off gas heat exchanger is recirculated to the boil-off gas compressor.

[0025] Preferably, the carbon dioxide boil-off gas reliquefaction system further includes: a second boil-off gas control valve disposed on the recirculation line and expanding the boil-off gas not subjected to secondary cooling in the second boil-off gas heat exchanger to produce a gas-liquid mixture of boil-off gas; and a boil-off gas separator separating the gas-liquid mixture of boil-off gas into reliquefied boil-off gas and gaseous boil-off gas.

[0026] Preferably, the boil-off gas separator is connected at a lower portion thereof to the liquefaction recovery line and is connected at an upper portion thereof to the recirculation line, the reliquefied boil-off gas is supplied to the storage tank through the liquefaction recovery line, the gaseous boil-off gas is supplied to the first boil-off gas heat exchanger through the recirculation line, and the gaseous boil-off gas supplied to the first boil-off gas heat exchanger is heated through heat exchange and recirculated to the boil-off gas compressor.

[0027] Preferably, the second boil-off gas heat exchanger and the storage tank are connected to a first reliquefaction recovery line, and a first boil-off gas control valve is disposed on the first reliquefaction recovery line to expand the boil-off gas subjected to secondary cooling into a subcooled state in the second boil-off gas heat exchanger.

[0028] Preferably, the second boil-off gas heat exchanger and the storage tank are connected to a second reliquefaction recovery line and a second boil-off gas control valve is disposed on the second reliquefaction recovery line.

[0029] Preferably, the carbon dioxide reliquefaction system further comprises: a boil-off gas separator disposed on the second reliquefaction recovery line and separating the gas-liquid mixture of boil-off gas into reliquefied boil-off gas and gaseous boil-off gas; and a valve configured to divert a flow of the boil-off gas between the second boil-off gas control valve and the boil-off gas separator.

[0030] Preferably, the boil-off gas separator is connected at a lower portion thereof to a third liquefaction recovery line and connected at an upper portion to a recirculation line, the reliquefied boil-off gas is supplied to the storage tank through the liquefaction recovery line, the gaseous boil-off gas is supplied to the first boil-off gas heat exchanger through the recirculation line, and the gaseous boil-off gas supplied to the first boil-off gas heat exchanger is heated through heat exchange and recirculated to the boil-off gas compressor.

[0031] Preferably, the refrigeration cycle further includes a refrigerant compressor compressing a refrigerant heated during heat exchange in the first boil-off gas heat exchanger and a refrigerant control valve configured to expand the compressed refrigerant and to supply the expanded refrigerant to the first boil-off gas heat exchanger.[Advantageous Effects]

[0032] The present invention provides a carbon dioxide reliquefaction system and a carbon dioxide reliquefaction method utilizing a closed cycle.

[0033] Specifically, the present invention provides a carbon dioxide liquefaction system and a carbon dioxide liquefaction method utilizing a closed cycle, wherein boil-off gas of liquefied carbon dioxide is recovered through reliquefaction of the liquefied carbon dioxide using cold heat of liquefied natural gas, in a liquefied carbon dioxide carrier using liquefied natural gas (LNG) as fuel.

[0034] In addition, reliquefied carbon dioxide is returned to the carbon dioxide cargo tank to reduce an internal pressure of the storage tank through reduction in vapor temperature of the cargo tank.

[0035] In addition, since no steam is returned through a separator (boil-off gas separator), the flow rate of carbon dioxide boil-off gas to a boil-off gas compressor (carbon dioxide compressor) is reduced, thereby reducing power consumption.

[0036] In addition, if there is no or insufficient cold heat in liquefied gas, an existing Joule-Thomson valve and a separator (boil-off gas separator) are used to selectively operate depending on the situation.

[0037] The present invention is not limited to the aforementioned objects and other objects of the present invention will become apparent to those skilled in the art from the following description.[Description of Drawings]

[0038] FIG. 1 and FIG. 2 are schematic diagrams of a carbon dioxide boil-off gas reliquefaction system according to one embodiment of the present invention.[Best Mode]

[0039] 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.

[0040] The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting. As used herein, the terms "comprises," "comprising," "includes," and / or "including," when used in this specification, specify the presence of stated features, but do not preclude the presence or addition of one or more other features. Further, it will be understood that, when an element is referred to as being "connected to" or "coupled to" another element, it may be directly connected or coupled to the other element or intervening elements may be present.

[0041] 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 are provided for complete disclosure and thorough understanding of the present invention by those skilled in the art 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.

[0042] In the present invention, a transportation vehicle may be one that is provided with a liquefied carbon dioxide storage tank and is transportable via land or sea routes. In the following description, the transportation vehicle is a maritime vessel.

[0043] Furthermore, in some embodiments of the present invention, the vessel may be a liquefied carbon dioxide carrier (LCO 2 carrier). However, it should be understood that the following embodiments may be equally applicable to any type of vessel provided with a liquefied carbon dioxide storage tank.

[0044] As used herein, the vessel may include self-propelled vessels and non-self-propelled floating offshore structures.

[0045] Furthermore, a liquefied carbon dioxide carrier according to one embodiment of the invention described hereinafter may be provided with at least one dual-fuel engine either as a propulsion engine or as a power generation engine, which may use gaseous fuel and fuel oil as fuel, optionally or as a mixture thereof.

[0046] The gaseous fuel may be stored in an on-board fuel tank in the form of liquefied gas, which can be vaporized and supplied either as a gas or as a liquid to the engine. For example, the liquefied gas may be selected from the hydrocarbon family, such as liquefied natural gas (LNG), liquefied ethane gas (LEG), liquefied petroleum gas (LPG), liquefied ethylene gas (LEG), liquefied propylene gas (LPG), as well as from the non-hydrocarbon family, such as liquefied ammonia (NH 3 ), liquefied hydrogen, and the like.

[0047] In one embodiment described below, by way of example, natural gas will be described as the gaseous fuel and thus the fuel tank may store liquefied natural gas.

[0048] Herein, the engine refers to a dual fuel engine and may be fueled by natural gas used in the vessel. For example, the engine may include at least one selected from among a MAN Electronic Gas Injection (ME-GI) engine, an extra-long stroke Dual Fuel (X-DF) engine, and a DF engine (Dual Fuel Diesel Electric (DFDE), Dual Fuel Diesel Generator (DFDG)). However, it should be understood that the present invention is not limited thereto.

[0049] Hereinafter, with reference to FIG. 1 and FIG. 2, a carbon dioxide boil-off gas reliquefaction system according to one embodiment of the present invention will be described.

[0050] The liquefied carbon dioxide carrier according to the present embodiment includes a liquefied carbon dioxide system, which may be provided with a liquefied carbon dioxide storage part, a boil-off gas processing part, and a fuel supply part (not shown).

[0051] The liquefied carbon dioxide storage part includes one or more storage tanks 100 that store liquefied carbon dioxide. The storage tanks 100 are pressurized tanks and may be operated in a pressurized state above a certain pressure to keep carbon dioxide in a liquid state.

[0052] The boil-off gas processing part may include a boil-off gas compressor 210 that compresses boil-off gas discharged from the storage tank 100, and a first boil-off gas heat exchanger 220 that cools the boil-off gas compressed by the boil-off gas compressor 210.

[0053] Specifically, 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 and may be compressed in the boil-off gas compressor 210 to produce compressed boil-off gas. Preferably, the compressed boil-off gas is compressed to a pressure that allows at least a portion of the compressed boil-off gas to be liquefied during cooling in the first boil-off gas heat exchanger 220.

[0054] In addition, the boil-off gas processing part may further include an intermediate cooler 211 configured to cool the boil-off gas, which has increased in temperature during compression in the boil-off gas compressor 210, before the boil-off gas is supplied to the first boil-off gas heat exchanger 220, and a buffer tank 212 configured to buffer the boil-off gas cooled in the intermediate cooler 211 before the boil-off gas is supplied to the first boil-off gas heat exchanger 220, wherein the compressed boil-off gas discharged from the boil-off gas compressor 210 may be cooled in the intermediate cooler 211 through the boil-off gas supply line BL and may reside in the buffer tank 212 before being introduced into a hot fluid flow path in the first boil-off gas heat exchanger 220.

[0055] The first boil-off gas heat exchanger 220 serves to cool the compressed boil-off gas and may produce compressed and cooled boil-off gas through heat exchange with the compressed boil-off gas introduced thereinto through the boil-off gas supply line BL.

[0056] 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 subjected to heat exchange with a low temperature refrigerant circulating in the refrigeration cycle 500 described below and may be cooled by heat exchange to produce the compressed and cooled boil-off gas.

[0057] Alternatively, the compressed boil-off gas introduced into first boil-off gas heat exchanger 220 through the boil-off gas supply line BL may be subjected to heat exchange with the low temperature refrigerant circulating in the refrigeration cycle 500 described below and with gaseous boil-off gas separated through gas-liquid separation in a boil-off gas separator 260 and may be cooled by heat exchange to produce the compressed and cooled boil-off gas.

[0058] The boil-off gas processing part may further include a second boil-off gas heat exchanger 230 on the boil-off gas supply line BL. The second boil-off gas heat exchanger 230 may be disposed downstream of the first boil-off gas heat exchanger 220.

[0059] Specifically, the second boil-off gas heat exchanger 230 may perform secondary cooling with respect to the compressed and cooled boil-off gas, which is supplied from the first boil-off gas heat exchanger and subjected to primary cooling, through heat exchange with liquefied gas supplied to the engine 400.

[0060] For example, when the carbon dioxide carrier is operated in gas mode, the second boil-off gas heat exchanger 230 may receive the compressed and primarily cooled boil-off gas from the first boil-off gas heat exchanger 220 and perform heat exchange of the received boil-off gas with liquefied natural gas (LNG), which is supplied from the fuel tank 300 to the engine 400 along a fuel supply line FL such that LNG is heated and supplied to the engine 400 and the compressed and cooled boil-off gas is supplied after undergoing secondary cooling.

[0061] Accordingly, the second boil-off gas heat exchanger 230 may be a vaporizer that vaporizes LNG supplied from the fuel tank 300 to the engine 400.

[0062] Alternatively, the second boil-off gas heat exchanger 230 may be further provided with a vaporizer (not shown) downstream of the second boil-off gas heat exchanger 250 that vaporizes the LNG supplied as fuel for the engine 400 such that the second boil-off gas heat exchanger 250 can be utilized as a preheater for preheating LNG before the LNG is supplied from the fuel tank 300 to the vaporizer.

[0063] In addition, the second boil-off gas heat exchanger 230 is connected to the storage tank 100 through a reliquefaction recovery line RL, whereby the compressed and cooled boil-off gas subjected to secondary cooling while passing through the second boil-off gas heat exchanger 230 can be returned to the storage tank 100.

[0064] Specifically, a first boil-off gas control valve 240 is disposed on the reliquefaction recovery line RL such that the compressed and cooled boil-off gas subjected to secondary cooling can be supplied to the storage tank 100 through the first boil-off gas control valve 240.

[0065] Alternatively, the second boil-off gas heat exchanger 230 may be connected to the storage tank 100 and a first reliquefaction recovery line RL1, and a first boil-off gas control valve 240 may be disposed on the first reliquefaction recovery line RL1 such that the compressed and cooled boil-off gas subjected to secondary cooling can be supplied to the storage tank 100 through the first boil-off gas control valve 240.

[0066] The first boil-off gas control valve 240 serves to expand the compressed and cooled boil-off gas subjected to secondary cooling to produce reliquefied boil-off gas in a subcooled liquid state. Preferably, the first boil-off gas control valve 240 is a Joule-Thomson valve that expands the cooled boil-off gas through an isenthalpic process.

[0067] Furthermore, the reliquefaction recovery line RL is connected to an upper spray nozzle line 110 in the storage tank 100 such that the reliquefied boil-off gas in a liquid state, which has been subcooled while passing through the first boil-off gas control valve 240, can be sprayed into the storage tank 100 through the upper spray nozzle line 110. Here, spraying the reliquefied boil-off gas in a subcooled liquid state effectively reduces the internal pressure of the storage tank 100.

[0068] The second boil-off gas heat exchanger 230 and the storage tank 100 are connected to a recirculation line GL, through which the boil-off gas not subjected to secondary cooling through the second boil-off gas heat exchanger 230 is recirculated to the boil-off gas compressor 210.

[0069] Alternatively, the compressed and secondarily cooled boil-off gas produced by heat exchange in the second boil-off gas heat exchanger 230 may produce dry ice, which blocks the first reliquefaction recovery line RL1, while passing through the first boil-off gas control valve 240.

[0070] Accordingly, the reliquefaction system according to the invention may further include a second reliquefaction recovery line RL2, which connects the second boil-off gas heat exchanger 230 and the storage tank 100, and a second boil-off gas control valve 250 disposed on the second reliquefaction recovery line RL2.

[0071] The second boil-off gas control valve 250 expands the compressed and cooled boil-off gas, which is supplied from the second boil-off gas heat exchanger 230 and subjected to secondary cooling, to produce reliquefied boil-off gas in a subcooled liquid state. Preferably, the second boil-off gas control valve 250 is a Joule-Thomson valve that expands the cooled boil-off gas through an isenthalpic process, like the first boil-off gas control valve 240.

[0072] Further, the second boil-off gas heat exchanger 230 and the storage tank 100 are connected to the recirculation line GL, through which the boil-off gas not subjected to secondary cooling is recirculated to the boil-off gas compressor 210 through the second boil-off gas heat exchanger 230.

[0073] Specifically, when the compressed and cooled boil-off gas has not undergone sufficient secondary cooling due to absence or insufficiency of cold heat in the second boil-off gas heat exchanger in operation of the carbon dioxide carrier in oil mode, the second boil-off gas control valve 250 and the boil-off gas separator 260 are preferably disposed on the recirculation line GL for recirculation of the compressed and cooled boil-off gas through the recirculation line GL.

[0074] Here, the compressed and primarily cooled boil-off gas, which has not undergone secondary cooling in the second boil-off gas heat exchanger 230, produces a gas-liquid mixture of boil-off gas that is a mixture of reliquefied boil-off gas in a liquid state and gaseous boil-off gas, while passing through the second boil-off gas control valve 250, and the produced gas-liquid mixture of boil-off gas is preferably supplied to a boil-off gas separator 260.

[0075] Alternatively, the boil-off gas separator 260 may be disposed on the second liquefaction recovery line RL2 and may be connected at an upper portion thereof to the recirculation line GL and at a lower portion thereof to a third reliquefaction recovery line RL3.

[0076] The second reliquefaction recovery line RL2 may be further provided with a valve 251 to supply a flow of boil-off gas to the boil-off gas separator 260, in which the valve 251 is preferably disposed between the second boil-off gas control valve 250 and the boil-off gas separator 260. Accordingly, the valve 251 is preferably a control valve or a three-way on-off valve.

[0077] That is, the compressed and primarily cooled boil-off gas, which is not subjected to secondary cooling in the second boil-off gas heat exchanger 230, produces a gas-liquid mixture of boil-off gas that is a mixture of reliquefied boil-off gas in a liquid state and gaseous boil-off gas, while passing through the second boil-off gas control valve 250, and the produced gas-liquid mixture of boil-off gas is supplied to a boil-off gas separator 260.

[0078] The boil-off gas separator 260 preferably separates the gas-liquid mixture of boil-off gas into reliquefied boil-off gas in a liquid state and gaseous boil-off gas. Specifically, the boil-off gas separator 260 is connected at a lower portion thereof to the storage tank 100 through the reliquefaction recovery line RL or the third reliquefaction recovery line RL3, and the reliquefied boil-off gas in a liquid state obtained through gas / liquid separation of the gas-liquid mixture of boil-off gas in the boil-off gas separator 260 is preferably returned to the storage tank 100 through the reliquefaction recovery line RL or the third reliquefaction recovery line RL3.

[0079] Further, the boil-off gas separator 260 is connected to the storage tank 100 through the recirculation line GL such that the gaseous boil-off gas obtained through gas-liquid separation of the gas-liquid mixture of boil-off gas in the boil-off gas separator 260 can be recirculated through the recirculation line GL.

[0080] Specifically, the recirculation line GL is connected to the boil-off gas compressor 210 and the gaseous boil-off gas is preferably recirculated therethrough after joining the boil-off gas flow from the storage tank 100 to the boil-off gas compressor 210.

[0081] When recirculated to the boil-off gas compressor 210 through the recirculation line GL, the gaseous boil-off gas is preferably heated and then supplied to the boil-off gas compressor 210 while cold heat is recovered through heat exchange in the first boil-off gas heat exchanger 220.

[0082] In addition, a three-stream heat exchanger may be applied to the first boil-off gas heat exchanger 220 such that the gaseous boil-off gas recirculated to the boil-off gas compressor 210 through the recirculation line GL and the low-temperature refrigerant circulating in the refrigerant circulation line ML can be subjected to heat exchange with the compressed boil-off gas supplied from the boil-off gas compressor 210 through the boil-off gas supply line BL.

[0083] That is, the gaseous boil-off gas supplied from the boil-off gas separator 260 and the refrigerant circulating in the refrigeration cycle 500 may cool the compressed boil-off gas and the gaseous boil-off gas may be heated and recirculated to the boil-off gas compressor 210.

[0084] The refrigeration cycle 500 may include a refrigerant compressor 510 configured to compress a refrigerant and a refrigerant control valve 520 configured to control a refrigerant flow to the first boil-off gas heat exchanger 220.

[0085] The refrigeration cycle 500 further includes a refrigerant cooler 511 that cools the refrigerant that has increased in temperature during compression in the refrigerant compressor 510, wherein the refrigerant introduced into the refrigerant compressor 510 through the refrigerant circulation line ML is compressed in the refrigerant compressor 510 to produce a compressed refrigerant, which in turn may be cooled in the refrigerant cooler 511 and then supplied to the first boil-off gas heat exchanger 220 through the refrigerant circulation line ML.

[0086] The refrigerant control valve 520 may be a flow regulation valve that regulates the flow rate of the cooled and compressed refrigerant, that is, the low temperature refrigerant, supplied from the refrigerant cooler 511 to the first boil-off gas heat exchanger 220 through the refrigerant circulation line ML, and may have a function of expanding the cooled and compressed refrigerant in the refrigerant cooler 511 through an isenthalpic process.

[0087] Accordingly, after passing through the refrigerant cooler 511 and the refrigerant control valve 520, the low temperature refrigerant may flow into the first boil-off gas heat exchanger 220 in a liquid state and may be vaporized by heat exchange to flow into the refrigerant compressor 510 in a gaseous state.

[0088] That is, the low temperature refrigerant circulating in the refrigerant circulation line ML is heated through heat exchange with the compressed boil-off gas supplied through the boil-off gas compressor 210 to becomes a high temperature refrigerant while cooling the carbon dioxide boil-off gas in the first boil-off gas heat exchanger 220, in which the high temperature refrigerant is preferably introduced into the refrigerant compressor 510 through the refrigerant circulation line ML to circulate in the refrigeration cycle 500. The refrigerant circulating in the refrigeration cycle 500 may be a hydrocarbon-based refrigerant, such as ammonia or propane. However, it should be understood that the present invention is not limited thereto and any refrigerant suitable for reliquefying the carbon dioxide boil-off gas while circulating in the refrigeration cycle 500 may be selected and applied.

[0089] Hereinafter, with reference to the carbon dioxide boil-off gas reliquefaction system according to the embodiment of the invention described above, a method of reliquefying carbon dioxide boil-off gas according to one embodiment of the present invention will be described.

[0090] In a carbon dioxide carrier using a dual-fuel engine, the method of reliquefying carbon dioxide boil-off gas may include a compressed boil-off gas generation step, a cooling step, and a recovery step.

[0091] In the compressed boil-off gas generation step, carbon dioxide boil-off gas evaporated from the storage tank 100 is supplied to the boil-off gas compressor 210 and is compressed while passing through the boil-off gas compressor 210. Here, the compressed boil-off gas may be cooled in the intermediate cooler 211 through the boil-off gas supply line BL and may reside in the buffer tank 210 before being introduced into a hot fluid flow path of the first boil-off gas heat exchanger 220.

[0092] The cooling step further includes a refrigeration cycle circulation 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 refrigerant circulating in the refrigeration cycle.

[0093] In the refrigeration cycle circulation step, the refrigerant heated through heat exchange with the compressed boil-off gas is compressed while passing through the refrigerant compressor and is then expanded and cooled while passing through the refrigerant control valve, in which 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.

[0094] On the other hand, the cooling step may further include a second cooling step depending on an operation mode of the carbon dioxide carrier.

[0095] Specifically, when the vessel is operated in gas mode, the second cooling step may be performed by supplying the compressed boil-off gas subjected to primary cooling in the cooling step to the second boil-off gas heat exchanger.

[0096] In the second cooling step, the compressed and primarily cooled boil-off gas is supplied to the second boil-off gas heat exchanger 230 to be subjected to secondary cooling through heat exchange with liquefied gas supplied to the engine 400.

[0097] For example, the second boil-off gas heat exchanger 230 may receive the compressed and primarily cooled boil-off gas from the first boil-off gas heat exchanger 220 and may perform heat exchange of the compressed and primarily cooled boil-off gas with liquefied natural gas (LNG), which is supplied from the fuel tank 300 to the engine 400 along the fuel supply line FL, such that the LNG is heated and supplied to the engine 400 and the compressed and primarily cooled boil-off gas is secondarily cooled and supplied to the engine 400.

[0098] Specifically, as shown in FIG. 1, the compressed boil-off gas subjected to secondary cooling through the second cooling step may be supplied to the first boil-off gas control valve 240. The reliquefied boil-off gas in a liquid state, which has been subcooled while passing through the second boil-off gas control valve 240, is preferably sprayed into the storage tank for recovery. Here, as the reliquefied boil-off gas in the subcooled liquid state is sprayed into the storage tank, the pressure in the storage tank 100 is effectively reduced.

[0099] Alternatively, as shown in FIG. 2, the compressed boil-off gas subjected to secondary cooling through the second cooling step is preferably returned to the storage tank through one recovery line selected from among the first reliquefaction recovery line RL1 and the second reliquefaction recovery line RL2.

[0100] For example, when the compressed boil-off gas subjected to secondary cooling is returned through the first reliquefaction recovery line RL1, the compressed boil-off gas subjected to secondary cooling produces reliquefied boil-off gas in a subcooled liquid state while passing through the first boil-off gas control valve 240, and the reliquefied boil-off gas in the subcooled liquid state may be sprayed into the storage tank 100 for recovery. As the reliquefied boil-off gas in the subcooled liquid state is sprayed into the storage tank 100, the pressure in the storage tank 100 is effectively reduced.

[0101] On the other hand, when cold heat of LNG is excessively used during the second cooling step, the temperature of carbon dioxide drops below the triple point to generate dry ice and the first reliquefaction recovery line RL1 or the second reliquefaction recovery line RL2 can be blocked by the generated dry ice to prevent the carbon dioxide from being returned to the storage tank 100. Therefore, it is desirable to control the amount of LNG so as to prevent generation of dry ice.

[0102] However, when the first reliquefaction recovery line RL1 or the second reliquefaction recovery line RL2 is blocked by dry ice generated due to the temperature of the carbon dioxide dropping below the triple point during the second cooling step, it is preferable to divert the reliquefied boil-off gas to the unblocked reliquefaction recovery line RL for recovery thereof.

[0103] For example, when the first reliquefaction recovery line RL1 is blocked by dry ice generated due to the temperature of the carbon dioxide dropping below the triple point during the second cooling step, the reliquefied boil-off gas may be diverted to the second reliquefaction recovery line RL2 without performing heat exchange with cold heat of LNG in the second boil-off gas heat exchanger 230.

[0104] Here, the second boil-off gas control valve 250 is disposed on the second reliquefaction recovery line RL2 such that the compressed and primarily cooled boil-off gas is expanded to produce reliquefied boil-off gas while passing through the second boil-off gas control valve 250 and the reliquefied boil-off gas is returned to the storage tank 100 along the second reliquefaction recovery line RL2.

[0105] Alternatively, when the first reliquefaction recovery line RL1 is blocked due to generation of dry ice, the amount of cold heat of LNG supplied to the second boil-off gas heat exchanger 230 may be controlled to perform heat exchange, and the compressed boil-off gas subjected to secondary cooling by heat exchange may be diverted to the second reliquefaction recovery line RL2 for recovery.

[0106] Here, the compressed boil-off gas subjected to secondary cooling passes through the second boil-off gas control valve 250 on the second reliquefaction recovery line RL2 and may be formed into the reliquefied boil-off gas in a subcooled liquid state while passing through the second boil-off gas control valve 250. The reliquefied boil-off gas in the subcooled liquid state may be sprayed into the storage tank 100 for recovery along the second reliquefaction recovery line RL2.

[0107] Preferably, the valve 251 is disposed on the second reliquefaction recovery line RL2 and is controlled such that reliquefaction boil-off gas in a subcooled liquid state can be supplied to the storage tank 100.

[0108] That is, when the vessel is operated in gas mode, the carbon dioxide boil-off gas is recovered through the first reliquefaction recovery line RL1 or the second reliquefaction recovery line RL2. When the carbon dioxide boil-off gas cannot be recovered through the first reliquefaction recovery line RL1, the carbon dioxide boil-off gas is preferably recovered through the second reliquefaction recovery line RL2, and when the carbon dioxide boil-off gas cannot be recovered through the second reliquefaction recovery line RL2, the carbon dioxide boil-off gas is preferably recovered through the first reliquefaction recovery line RL1.

[0109] The cooling step may further include a third cooling step when the vessel is operated in oil mode.

[0110] Specifically, the third cooling step preferably further includes a boil-off gas-liquid mixture generation step, in which the compressed and primarily cooled boil-off gas is expanded to produce a gas-liquid mixture of boil-off gas, and a boil-off gas separation step, in which the gas-liquid mixture of boil-off gas is separated into reliquefied boil-off gas and gaseous boil-off gas.

[0111] In the boil-off gas separation step, the gas-liquid mixture of boil-off gas is separated into the reliquefied boil-off gas and the gaseous boil-off gas, and the boil-off gas separation step includes a recovery step in which the separated reliquefied boil-off gas is sprayed into the storage tank 100 for recovery of the separated reliquefied boil-off gas, and a recirculation step in which the separated gaseous boil-off gas is recirculated to the boil-off gas compressor 210 for reliquefaction of the gaseous boil-off gas.

[0112] That is, the third cooling step is preferably performed when the compressed and primarily cooled boil-off gas has not undergone secondary cooling due to absence or insufficiency of cold heat in the second boil-off gas heat exchanger.

[0113] Preferably, in the recirculation step, the separated gaseous boil-off gas is recirculated to the boil-off gas compressor 210 through the first boil-off gas heat exchanger 220, in which the gaseous boil-off gas is heated while cooling the compressed boil-off gas and the heated gaseous boil-off gas is resupplied to the boil-off gas compressor 210 through the recirculation line GL to join the boil-off gas flow from the storage tank 100 toward the boil-off gas compressor 210 for recirculation.

[0114] As described above, the present invention provides a carbon dioxide reliquefaction system and a carbon dioxide reliquefaction method utilizing a closed cycle.

[0115] Specifically, the present invention provides a carbon dioxide liquefaction system and a carbon dioxide liquefaction method utilizing a closed cycle, wherein boil-off gas of liquefied carbon dioxide is recovered through reliquefaction of the liquefied carbon dioxide using cold heat of liquefied natural gas, in a liquefied carbon dioxide carrier using liquefied natural gas (LNG) as fuel.

[0116] In addition, reliquefied carbon dioxide is returned to the carbon dioxide cargo tank to reduce an internal pressure of the storage tank through reduction in vapor temperature of the cargo tank.

[0117] In addition, since no steam is returned through a separator (boil-off gas separator), the flow rate of carbon dioxide boil-off gas to a boil-off gas compressor (carbon dioxide compressor) is reduced, thereby reducing power consumption.

[0118] In addition, if there is no or insufficient cold heat in liquefied gas, an existing Joule-Thomson valve and a separator (boil-off gas separator) are used to selectively operate depending on the situation.

[0119] 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 invention. 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 invention. The scope of the present invention 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 reliquefaction method of a carbon dioxide carrier using a dual fuel engine, the method comprising: a compressed boil-off gas generation step in which evaporated carbon dioxide boil-off gas is supplied from a storage tank to a compressor to produce compressed boil-off gas; a cooling step in which the compressed boil-off gas is supplied to a first boil-off gas heat exchanger and cooled through heat exchange with a refrigerant circulating in a refrigeration cycle, and a recovery step in which reliquefied carbon dioxide cooled by the cooling step is sprayed into the storage tank for recovery of the reliquefied carbon dioxide, wherein the cooling step further comprises a second cooling step or a third cooling step depending on an operating mode of the carbon dioxide carrier.

2. The carbon dioxide reliquefaction method according to claim 1, wherein the cooling step further comprises a refrigeration cycle circulation step in which the refrigerant is circulated to be cooled, and, in the refrigeration cycle circulation step, the refrigerant heated by heat exchange with the compressed boil-off gas is compressed while passing through a refrigerant compressor, and is then expanded and cooled while passing through a refrigerant control valve.

3. The carbon dioxide reliquefaction method according to claim 1, wherein when the carbon dioxide carrier is operated in gas mode, the second cooling step is further performed, the second cooling step comprising supplying the compressed boil-off gas subjected to primary cooling in the cooling step to a second boil-off gas heat exchanger to perform secondary cooling of the compressed boil-off gas through heat exchange with liquefied gas supplied to an engine.

4. The carbon dioxide reliquefaction method according to claim 3, wherein the compressed boil-off gas subjected to secondary cooling is expanded to produce reliquefied boil-off gas in a subcooled liquid state.

5. The carbon dioxide reliquefaction method according to claim 3, wherein the compressed boil-off gas subjected to secondary cooling is supplied to a first boil-off gas control valve, the first boil-off gas control valve being configured to expand the compressed boil-off gas subjected to secondary cooling to produce reliquefied boil-off gas in a subcooled liquid state.

6. The carbon dioxide reliquefaction method according to claim 5, wherein the reliquefied boil-off gas in the subcooled liquid state is returned to the storage tank through one reliquefaction recovery line selected from among a first reliquefaction recovery line and a second reliquefaction recovery line, and when the reliquefaction recovery line is blocked by dry ice, the reliquefied boil-off gas in a subcooled liquid state is diverted to an unblocked reliquefaction recovery line for recovery thereof.

7. The carbon dioxide reliquefaction method according to claim 1, wherein when the carbon dioxide carrier is operated in oil mode, the third cooling step is further performed, the third cooling step comprising: a boil-off gas-liquid mixture generation step in which the compressed boil-off gas subjected to primary cooling is expanded to produce a gas-liquid mixture of boil-off gas; and a boil-off gas separation step in which the gas-liquid mixture of boil-off gas is separated into reliquefied boil-off gas and gaseous boil-off gas.

8. The carbon dioxide reliquefaction method according to claim 7, wherein the reliquefied boil-off gas is sprayed into the storage tank for recovery thereof, the carbon dioxide reliquefaction method further comprising a recirculation step in which the separated gaseous boil-off gas is recirculated to a boil-off gas compressor to reliquefy the separated gaseous boil-off gas, the recirculation step comprising recirculating the gaseous boil-off gas to the boil-off gas compressor through a first boil-off gas heat exchanger such that the gaseous boil-off gas is heated while cooling the compressed boil-off gas and the heated gaseous boil-off gas is resupplied to the compressor through the recirculation line.

9. 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 boil-off gas of the liquefied carbon dioxide produced in the storage tank to produce compressed boil-off gas; a first boil-off gas heat exchanger cooling the compressed boil-off gas; a second boil-off gas heat exchanger secondarily cooling the primarily cooled boil-off gas supplied from the first boil-off gas heat exchanger through heat exchange with the liquefied gas supplied to the engine; and a refrigeration cycle in which a refrigerant is circulated to provide cold heat to the first boil-off gas heat exchanger.

10. The carbon dioxide reliquefaction system according to claim 9, wherein the second boil-off gas heat exchanger and the storage tank are connected to a reliquefaction recovery line, and a first boil-off gas control valve is disposed on the reliquefaction recovery line to expand the boil-off gas subjected to secondary cooling into a subcooled state through the second boil-off gas heat exchanger.

11. The carbon dioxide reliquefaction system according to claim 10, wherein a spray nozzle line is disposed above the storage tank, the spray nozzle line being connected to the reliquefaction recovery line to spray the subcooled boil-off gas into the storage tank.

12. The carbon dioxide reliquefaction system according to claim 9, wherein the second boil-off gas heat exchanger and the storage tank are connected to a recirculation line, through which the boil-off gas not subjected to secondary cooling in the second boil-off gas heat exchanger is recirculated to the boil-off gas compressor.

13. The carbon dioxide reliquefaction system according to claim 12, further comprising: a second boil-off gas control valve disposed on the recirculation line and expanding the boil-off gas not subjected to secondary cooling in the second boil-off gas heat exchanger to produce a gas-liquid mixture of boil-off gas, and a boil-off gas separator separating the gas-liquid mixture of boil-off gas into reliquefied boil-off gas and gaseous boil-off gas.

14. The carbon dioxide reliquefaction system according to claim 13, wherein the boil-off gas separator is connected at a lower portion thereof to the liquefaction recovery line and is connected at an upper portion thereof to the recirculation line, the reliquefied boil-off gas is supplied to the storage tank through the liquefaction recovery line, the gaseous boil-off gas is supplied to the first boil-off gas heat exchanger through the recirculation line, and the gaseous boil-off gas supplied to the first boil-off gas heat exchanger is heated through heat exchange and recirculated to the boil-off gas compressor.

15. The carbon dioxide reliquefaction system according to claim 9, wherein the second boil-off gas heat exchanger and the storage tank are connected to the first reliquefaction recovery line, a first boil-off gas control valve is disposed on the first reliquefaction recovery line to expand the boil-off gas subjected to secondary cooling into a subcooled state through the second boil-off gas heat exchanger.

16. The carbon dioxide reliquefaction system according to claim 15, wherein the second boil-off gas heat exchanger and the storage tank are connected to a second reliquefaction recovery line, and a second boil-off gas control valve is disposed on the second reliquefaction recovery line.

17. The carbon dioxide reliquefaction system according to claim 16, further comprising: a boil-off gas separator disposed on the second reliquefaction recovery line and separating the gas-liquid mixture of boil-off gas into reliquefied boil-off gas and gaseous boil-off gas; and a valve configured to divert a flow of the boil-off gas between the second boil-off gas control valve and the boil-off gas separator.

18. The carbon dioxide reliquefaction system according to claim 17, wherein the boil-off gas separator is connected at a lower portion thereof to a third liquefaction recovery line and connected at an upper portion to a recirculation line, the reliquefied boil-off gas is supplied to the storage tank through the liquefaction recovery line, the gaseous boil-off gas is supplied to the first boil-off gas heat exchanger through the recirculation line, and the gaseous boil-off gas supplied to the first boil-off gas heat exchanger is heated through heat exchange and recirculated to the boil-off gas compressor.

19. The carbon dioxide reliquefaction system according to claim 9, wherein the refrigeration cycle further comprises: a refrigerant compressor compressing a refrigerant heated during heat exchange in the first boil-off gas heat exchanger; and a refrigerant control valve configured to expand the compressed refrigerant and to supply the expanded refrigerant to the first boil-off gas heat exchanger.

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