Carbon dioxide reliquefaction system and carbon dioxide reliquefaction method using closed cycle

Abstract

The invention relates to a carbon dioxide boil-off gas reliquefaction method. The carbon dioxide boil-off gas reliquefaction method according to the present invention is characterized by comprising: a step of forming compressed boil-off gas by supplying carbon dioxide boil-off gas evaporated in a storage tank to a compressor in a carbon dioxide transport using a dual-fuel engine; a cooling step of supplying the compressed boil-off gas to the first boil-off gas heat exchanger and cooling the compressed boil-off gas by heat exchange with a refrigerant circulating in the refrigeration cycle; and a recovery step of spraying the reliquefied carbon dioxide cooled by the cooling step into the storage tank and recovering the reliquefied carbon dioxide, in which a second cooling step or a third cooling step is additionally performed during the cooling step according to an operation mode of the carbon dioxide transportation tool.

Description

TECHNICAL FIELD

[0001] The present application relates to a carbon dioxide liquefaction system and method using a closed cycle in a liquefied carbon dioxide transport vehicle using a liquefied gas as a fuel, in which boil-off gas of liquefied carbon dioxide is treated by re-liquefaction. BACKGROUND

[0002] Fossil fuels generate carbon dioxide when combusted to produce energy. As the use of fossil fuels increases, carbon dioxide is emitted in large amounts and has been designated as one of the greenhouse gases (GHG) causing global warming.

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

[0004] Based on various international agreements for reducing carbon dioxide emissions, many countries regulate the emission of carbon dioxide, and as one of the technologies derived from these agreements, there is a need to develop carbon dioxide treatment technologies (e.g., Carbon Capture Utilization & Storage (CCUS) and the like) that 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.

[0005] The CCUS technology is a technology that liquefies and transports disposed carbon dioxide for separate treatment, in which liquefied carbon dioxide is injected into a space left after oil extraction to be stored in a stable state or is injected as a high-pressure spray instead of water at the time of oil extraction.

[0006] Carbon dioxide is liquefied by an open cycle method in which heat exchange is performed by pressure reduction through a Joule-Thomson valve after compression or a closed cycle method in which a separate refrigerant having a lower saturation temperature than CO2 is used.

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

[0008] Accordingly, there is a need to develop an optimal reliquefaction system that satisfies not only system efficiency but also cost effectiveness. SUMMARY

[0009] TECHNICAL PROBLEM

[0010] An object of the present application is to provide a carbon dioxide liquefaction system using a closed cycle and a carbon dioxide liquefaction method.

[0011] Specifically, the present application aims to provide a carbon dioxide liquefaction system using a closed cycle and a carbon dioxide liquefaction method in which, in a liquefied carbon dioxide carrier using liquefied natural gas (LNG) as fuel, boil-off gas of liquefied carbon dioxide is recovered by reliquefying the liquefied carbon dioxide using cold and heat of the liquefied natural gas.

[0012] The present application also aims to reduce the internal pressure of a carbon dioxide cargo tank (storage tank) by reducing the vapor temperature in the cargo tank by returning the reliquefied carbon dioxide to the cargo tank.

[0013] The present application is not limited to the above object, and other objects of the present application will become apparent to those skilled in the art from the following description.

[0014] TECHNICAL SOLUTION

[0015] According to one aspect of the present application, 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 of supplying boil-off carbon dioxide gas evaporated from a storage tank to a compressor to generate compressed boil-off gas; a cooling step of supplying the compressed boil-off gas to a first boil-off gas heat exchanger and cooling by heat exchange with a refrigerant circulating in a refrigeration cycle; and a recovery step of injecting the reliquefied carbon dioxide cooled by the cooling step into the storage tank to recover the reliquefied carbon dioxide, wherein the cooling step further includes a second cooling step or a third cooling step depending on an operation mode of the carbon dioxide carrier.

[0016] Preferably, the cooling step further includes a refrigeration cycle circulation step of circulating a refrigerant to be cooled, and in the refrigeration cycle circulation step, the refrigerant heated by heat exchange with the compressed boil-off gas is compressed when passing through a refrigerant compressor and then expanded and cooled when passing through a refrigerant control valve.

[0017] Preferably, when the carbon dioxide transport means is operated in the gas mode, a second cooling step is further implemented, the second cooling step comprising supplying the compressed boil-off gas, which is subjected to primary cooling in the cooling step, to a second boil-off gas heat exchanger to implement secondary cooling of the compressed boil-off gas by heat exchange with the liquefied gas supplied to the engine.

[0018] Preferably, the compressed boil-off gas subjected to secondary cooling is expanded to generate re-liquefied boil-off gas in a subcooled liquid state.

[0019] Preferably, the compressed boil-off gas subjected to secondary cooling is supplied to a first boil-off gas control valve configured to expand the compressed boil-off gas subjected to secondary cooling to generate re-liquefied boil-off gas in a subcooled liquid state.

[0020] Preferably, the re-liquefied boil-off gas in a subcooled liquid state is returned to the storage tank through one of the re-liquefaction recovery lines selected from the first re-liquefaction recovery line and the second re-liquefaction recovery line, and when said one of the re-liquefaction recovery lines is blocked by dry ice, the re-liquefied boil-off gas in a subcooled liquid state is diverted to the re-liquefaction recovery line that is not blocked to recover the re-liquefied boil-off gas.

[0021] Preferably, when the carbon dioxide transport means is operated in the oil mode, a third cooling step is further implemented, 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 generate 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 re-liquefied boil-off gas and gaseous boil-off gas.

[0022] Preferably, the re-liquefied boil-off gas is injected into the storage tank to recover the re-liquefied boil-off gas, and the carbon dioxide re-liquefaction method further comprises a recirculation step in which the separated gaseous boil-off gas is recirculated to the boil-off gas compressor to re-liquefy the separated gaseous boil-off gas, the recirculation step comprising recirculating the gaseous boil-off gas to the boil-off gas compressor through the first boil-off gas heat exchanger such that the gaseous boil-off gas is heated while the compressed boil-off gas is cooled, and the heated gaseous boil-off gas is supplied again to the compressor through a recirculation line.

[0023] According to another aspect of the present application, there is provided a carbon dioxide boil-off gas reliquefaction system including: an engine fueled by a liquefied gas; a storage tank storing liquefied carbon dioxide; a boil-off gas compressor compressing a boil-off gas generated from the liquefied carbon dioxide in the storage tank to generate a 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 once-cooled boil-off gas supplied from the first boil-off gas heat exchanger by 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.

[0024] 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 provided on the reliquefaction recovery line to expand the boil-off gas subjected to the secondary cooling by the second boil-off gas heat exchanger into a supercooled state.

[0025] Preferably, an injection nozzle line is provided above the storage tank, the injection nozzle line being connected to the reliquefaction recovery line to inject the supercooled boil-off gas into the storage tank.

[0026] 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 provided on the reliquefaction recovery line to expand the boil-off gas subjected to the secondary cooling by the second boil-off gas heat exchanger into a supercooled state.

[0027] Preferably, the carbon dioxide boil-off gas reliquefaction system further includes a second boil-off gas control valve provided on the reliquefaction recovery line and expanding the boil-off gas not subjected to the secondary cooling in the second boil-off gas heat exchanger to generate a gas-liquid mixture of the boil-off gas, and a boil-off gas separator separating the gas-liquid mixture of the boil-off gas into a reliquefaction boil-off gas and a gaseous boil-off gas.

[0028] Preferably, the boil-off gas separator is connected to a liquefaction recovery line at a lower portion thereof and to a reliquefaction recovery line at an upper portion thereof, the reliquefaction 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 reliquefaction recovery line, and the gaseous boil-off gas supplied to the first boil-off gas heat exchanger is heated by heat exchange and is recycled to the boil-off gas compressor.

[0029] 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 provided on the reliquefaction recovery line to expand the boil-off gas subjected to the secondary cooling by the second boil-off gas heat exchanger into a supercooled state.

[0030] Preferably, the second boil-off gas heat exchanger and the storage tank are connected to the second reliquefaction recovery line, and a second boil-off gas control valve is provided on the second reliquefaction recovery line.

[0031] Preferably, the carbon dioxide boil-off gas reliquefaction system further includes a boil-off gas separator provided on the second reliquefaction recovery line and separating the gas-liquid mixture of the boil-off gas into reliquefied boil-off gas and gaseous boil-off gas, and a valve configured to divert the flow of the boil-off gas between the second boil-off gas control valve and the boil-off gas separator.

[0032] Preferably, the boil-off gas separator is connected to a third liquefaction recovery line at a lower portion thereof and to a recycle line at an upper portion thereof, 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 recycle line, and the gaseous boil-off gas supplied to the first boil-off gas heat exchanger is heated by heat exchange and recycled to the boil-off gas compressor.

[0033] Preferably, the refrigeration cycle further includes a refrigerant compressor compressing the refrigerant heated during the heat exchange in the first boil-off gas heat exchanger, and a refrigerant control valve configured to expand the compressed refrigerant and supply the expanded refrigerant to the first boil-off gas heat exchanger.

[0034] Advantageous effects

[0035] The present application provides a carbon dioxide reliquefaction system using a closed cycle and a carbon dioxide reliquefaction method.

[0036] Specifically, the present application provides a carbon dioxide liquefaction system using a closed cycle and a carbon dioxide liquefaction method in which, in a liquefied carbon dioxide transport vehicle using liquefied natural gas (LNG) as fuel, boil-off gas of the liquefied carbon dioxide is recovered by reliquefying the liquefied carbon dioxide using cold heat of the liquefied natural gas.

[0037] In addition, the reliquefied carbon dioxide is returned to a carbon dioxide cargo tank to reduce the internal pressure of the storage tank by reducing the vapor temperature of the cargo tank.

[0038] In addition, since no vapor 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.

[0039] In addition, if there is no cold heat or insufficient cold heat in the liquefied gas, an existing Joule-Thomson valve and a separator (boil-off gas separator) are used to selectively operate as the case may be.

[0040] The present application is not limited to the above-mentioned objects, and other objects of the present application will become more apparent to one of ordinary skill in the art from the following description. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 and Figure 2 is a schematic diagram of a carbon dioxide boil-off gas reliquefaction system according to one embodiment of the present application. DETAILED DESCRIPTION

[0042] The above and other aspects, features, and advantages of the present application will become more apparent to one of ordinary skill in the art by reading the following detailed description of embodiments that follow in conjunction with the accompanying drawings.

[0043] The terms used herein are for the purpose of describing particular embodiments only and are not intended to be limiting. As used herein, the use of the terms "comprises", "comprising", "includes", "including" and the like are specifically intended to be open-ended and to mean that other features can be added thereto without departing from the scope of the present application. Also, it should be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements can be present.

[0044] Hereinafter, exemplary embodiments of the present application will be described in detail with reference to the accompanying drawings. It should be understood that the exemplary embodiments are provided for more complete and thorough disclosure of the present application and to fully convey the scope of the present application to those skilled in the art, and the present application is not limited to the following embodiments only, and the present application can be embodied in various ways by those skilled in the art.

[0045] In the present application, the transport vehicle can be a transport vehicle provided with a liquefied carbon dioxide storage tank and transportable via land or sea. In the following description, the transport vehicle is a maritime vessel.

[0046] Further, in some embodiments of the present application, the vessel can be a liquefied carbon dioxide carrier (LCO2 carrier). However, it should be understood that the following embodiments can be equally applicable to any type of vessel provided with a liquefied carbon dioxide storage tank.

[0047] The vessel used herein can include a self-propelled vessel and a non-self-propelled floating marine structure.

[0048] Further, the liquefied carbon dioxide carrier according to one embodiment of the present application set forth below can be provided with at least one dual-fuel engine as a propulsion engine or as a power generation engine, which can use a gaseous fuel and a fuel oil as a fuel, or alternatively a mixture thereof.

[0049] The gaseous fuel can be stored in the fuel tank on the ship in the form of a liquid gas, which can be vaporized and supplied to the engine as a gas or a liquid. For example, the liquid gas can be selected from hydrocarbons (e.g., liquefied natural gas (LNG), liquefied ethane gas (LEG), liquefied petroleum gas (LPG), liquefied ethylene gas (LEG), liquefied propylene gas (LPG)) and non-hydrocarbons (e.g., liquefied ammonia (NH3), liquefied hydrogen, and the like).

[0050] In one embodiment set forth below, natural gas will be set forth as an example of a gaseous fuel, and thus the fuel tank can store liquefied natural gas.

[0051] Herein, the engine refers to a dual-fuel engine, and can be fueled with natural gas used in a ship. For example, the engine can 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 application is not limited thereto.

[0052] Hereinafter, referring to Figure 1 and Figure 2 a carbon dioxide boil-off gas reliquefaction system according to one embodiment of the present application will be set forth.

[0053] A liquefied carbon dioxide transport according to the present embodiment includes a liquefied carbon dioxide system, which can be provided with a liquefied carbon dioxide storage, a boil-off gas treatment, and a fuel supply (not shown).

[0054] The liquefied carbon dioxide storage includes one or more storage tanks (100) that store liquefied carbon dioxide. The storage tank (100) is a pressurized tank, and can be operated in a pressurized state higher than a certain pressure to maintain carbon dioxide in a liquid state.

[0055] The boil-off gas treatment section can include a boil-off gas compressor (210) that compresses the 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).

[0056] Specifically, the carbon dioxide boil-off gas generated in the carbon dioxide storage tank (100) can be introduced into the boil-off gas compressor (210) through the boil-off gas supply line (BL), and can be compressed in the boil-off gas compressor (210) to generate compressed boil-off gas. Preferably, the compressed boil-off gas is compressed to a pressure such that at least a portion of the compressed boil-off gas can be liquefied during cooling in the first boil-off gas heat exchanger (220).

[0057] In addition, the boil-off gas treatment section can further include an intercooler (211) configured to cool the boil-off gas whose temperature has risen 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 intercooler (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) can be cooled in the intercooler (211) through the boil-off gas supply line (BL), and can reside in the buffer tank (212) before being introduced into the hot fluid flow path of the first boil-off gas heat exchanger (220).

[0058] The first boil-off gas heat exchanger (220) serves to cool the compressed boil-off gas, and can generate compressed and cooled boil-off gas by exchanging heat with the compressed boil-off gas introduced therein through the boil-off gas supply line (BL).

[0059] 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) can exchange heat with the low-temperature refrigerant circulated in the refrigeration cycle (500) set forth below, and can be cooled by the heat exchange to generate compressed and cooled boil-off gas.

[0060] Alternatively, the compressed boil-off gas introduced into the first boil-off gas heat exchanger (220) through the boil-off gas supply line (BL) can exchange heat with the low-temperature refrigerant circulated in the refrigeration cycle (500) set forth below and with the gaseous boil-off gas separated by gas-liquid separation in the boil-off gas separator (260), and can be cooled by the heat exchange to generate compressed and cooled boil-off gas.

[0061] The boil-off gas treatment section can further include a second boil-off gas heat exchanger (230) located on the boil-off gas supply line (BL). The second boil-off gas heat exchanger (230) can be disposed downstream of the first boil-off gas heat exchanger (220).

[0062] Specifically, the second boil-off gas heat exchanger (230) can perform secondary cooling on the compressed and once-cooled boil-off gas supplied from the first boil-off gas heat exchanger by exchanging heat with liquefied gas supplied to the engine (400).

[0063] For example, when the carbon dioxide transport means is operated in a gas mode, the second boil-off gas heat exchanger (230) can receive the compressed and once-cooled boil-off gas from the first boil-off gas heat exchanger (220) and perform heat exchange on the received boil-off gas with liquefied natural gas (LNG) 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 such that the compressed and cooled boil-off gas is supplied after undergoing secondary cooling.

[0064] Accordingly, the second boil-off gas heat exchanger (230) can be a vaporizer that vaporizes the LNG supplied from the fuel tank (300) to the engine (400).

[0065] Alternatively, the second boil-off gas heat exchanger (230) can further provide a vaporizer (not shown) disposed downstream of the second boil-off gas heat exchanger (230) that vaporizes the LNG supplied as fuel for the engine (400) such that the second boil-off gas heat exchanger (250) can be used as a preheater for preheating the LNG before being supplied from the fuel tank (300) to the vaporizer.

[0066] In addition, the second boil-off gas heat exchanger (230) is connected to the storage tank (100) via a reliquefaction recovery line (RL) whereby the compressed and cooled boil-off gas that undergoes secondary cooling as it passes through the second boil-off gas heat exchanger (230) can be returned to the storage tank (100).

[0067] Specifically, a first boil-off gas control valve (240) is provided on the reliquefaction recovery line (RL) such that the compressed and cooled boil-off gas that undergoes secondary cooling can be supplied to the storage tank (100) through the first boil-off gas control valve (240).

[0068] Alternatively, the second boil-off gas heat exchanger (230) can be connected to the storage tank (100) and the first reliquefaction recovery line (RL1) and a first boil-off gas control valve (240) can be provided 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).

[0069] The first boil-off gas control valve (240) is used to expand the compressed and cooled boil-off gas subjected to secondary cooling to generate the reliquefied boil-off gas in a supercooled 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.

[0070] Further, the reliquefaction recovery line (RL) is connected to the upper injection nozzle line (110) in the storage tank (100) such that the reliquefied boil-off gas in a liquid state that has been supercooled upon passing through the first boil-off gas control valve (240) can be injected into the storage tank (100) through the upper injection nozzle line (110). Here, the injection of the reliquefied boil-off gas in a supercooled liquid state effectively reduces the internal pressure of the storage tank (100).

[0071] 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 that has not been subjected to secondary cooling by the second boil-off gas heat exchanger (230) is recirculated to the boil-off gas compressor (210).

[0072] Alternatively, the compressed and secondarily cooled boil-off gas generated by heat exchange in the second boil-off gas heat exchanger (230) can generate dry ice that can clog the first reliquefaction recovery line (RL1) upon passing through the first boil-off gas control valve (240).

[0073] Accordingly, the reliquefaction system according to the present application can further include a second reliquefaction recovery line (RL2) connecting the second boil-off gas heat exchanger (230) and the storage tank (100) and a second boil-off gas control valve (250) provided on the second reliquefaction recovery line (RL2).

[0074] The second boil-off gas control valve (250) expands the compressed and cooled boil-off gas supplied from the second boil-off gas heat exchanger (230) and subjected to secondary cooling to generate the reliquefied boil-off gas in a supercooled 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, similar to the first boil-off gas control valve (240).

[0075] Furthermore, 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 by the second boil-off gas heat exchanger (230) is recirculated to the boil-off gas compressor (210).

[0076] In particular, when the compressed and cooled boil-off gas does not undergo sufficient secondary cooling due to a lack or deficiency of cold heat in the second boil-off gas heat exchanger during operation of the carbon dioxide transport means in the oil mode, the second boil-off gas control valve (250) and the boil-off gas separator (260) are preferably provided on the recirculation line (GL) for recirculating the compressed and cooled boil-off gas through the recirculation line (GL).

[0077] Here, the compressed and once-cooled boil-off gas not subjected to secondary cooling in the second boil-off gas heat exchanger (230) generates a gas-liquid mixture of boil-off gas upon passing through the second boil-off gas control valve (250), which is a mixture of the reliquefied boil-off gas in a liquid state and gaseous boil-off gas, and the generated gas-liquid mixture of boil-off gas is preferably supplied to the boil-off gas separator (260).

[0078] Alternatively, the boil-off gas separator (260) can be provided on the second reliquefaction recovery line (RL2) and can be connected to the recirculation line (GL) at an upper portion thereof and to the third reliquefaction recovery line (RL3) at a lower portion thereof.

[0079] The second reliquefaction recovery line (RL2) can also be provided with a valve (251) to supply the boil-off gas stream to the boil-off gas separator (260), which is preferably provided between the second boil-off gas control valve (250) and the boil-off gas separator (260) in the second reliquefaction recovery line (RL2). Thus, the valve (251) is preferably a control valve or a 3-way on-off valve.

[0080] That is, the compressed and once-cooled boil-off gas that has not been subjected to secondary cooling in the second boil-off gas heat exchanger (230) generates a gas-liquid mixture of boil-off gas that is a mixture of liquefied boil-off gas in a liquid state and gaseous boil-off gas when passing through the second boil-off gas control valve (250), and the generated gas-liquid mixture of boil-off gas is supplied to the boil-off gas separator (260).

[0081] The boil-off gas separator (260) preferably separates the gas-liquid mixture of boil-off gas into liquefied boil-off gas in a liquid state and gaseous boil-off gas. Specifically, the boil-off gas separator (260) is connected to the storage tank (100) at a lower portion thereof by a liquefied recovery line (RL) or a third liquefied recovery line (RL3), and the liquefied boil-off gas in a liquid state obtained by performing 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) by the liquefied recovery line (RL) or the third liquefied recovery line (RL3).

[0082] Further, the boil-off gas separator (260) is connected to the storage tank (100) by a recycle line (GL) so that the gaseous boil-off gas obtained by performing gas-liquid separation of the gas-liquid mixture of boil-off gas in the boil-off gas separator (260) can be recycled by the recycle line (GL).

[0083] Specifically, the recycle line (GL) is connected to the boil-off gas compressor (210), and the gaseous boil-off gas is preferably recycled by the recycle line (GL) after being merged with the boil-off gas flowing from the storage tank (100) to the boil-off gas compressor (210).

[0084] When recycled to the boil-off gas compressor (210) by the recycle line (GL), the gaseous boil-off gas is preferably heated and then supplied to the boil-off gas compressor (210) while recovering cold heat by heat exchange in the first boil-off gas heat exchanger (220).

[0085] In addition, a three-stream heat exchanger can be applied to the first boil-off gas heat exchanger (220) so that the gaseous boil-off gas recycled to the boil-off gas compressor (210) by the recycle line (GL) and the low-temperature refrigerant circulating in the refrigerant circulation flow line (ML) can be subjected to heat exchange with the compressed boil-off gas supplied from the boil-off gas compressor (210) by the boil-off gas supply line (BL).

[0086] That is, the gaseous boil-off gas supplied from the boil-off gas separator (260) and the refrigerant circulated in the refrigeration cycle (500) can cool the compressed boil-off gas, and the gaseous boil-off gas can be heated and recirculated to the boil-off gas compressor (210).

[0087] The refrigeration cycle (500) can include a refrigerant compressor 510 configured to compress a refrigerant, and a refrigerant control valve (520) configured to control the refrigerant flowing to the first boil-off gas heat exchanger (220).

[0088] The refrigeration cycle (500) further includes a refrigerant cooler (511) that cools the refrigerant whose temperature has been raised during compression in the refrigerant compressor (510), wherein the refrigerant introduced into the refrigerant compressor (510) through the refrigerant circulation circulation line (ML) is compressed in the refrigerant compressor (510) to generate compressed refrigerant, which in turn can be cooled in the refrigerant cooler (511) and then supplied to the first boil-off gas heat exchanger (220) through the refrigerant circulation circulation line (ML).

[0089] The refrigerant control valve (520) can be a flow regulation valve that regulates the flow rate of the cooled and compressed refrigerant (i.e., low-temperature refrigerant) supplied from the refrigerant cooler (511) to the first boil-off gas heat exchanger (220) through the refrigerant circulation circulation line (ML), and can have a function of expanding the cooled and compressed refrigerant in the refrigerant cooler (511) through an isenthalpic process.

[0090] Therefore, after passing through the refrigerant cooler (511) and the refrigerant control valve (520), the low-temperature refrigerant can flow into the first boil-off gas heat exchanger (220) in a liquid state, and can be vaporized by heat exchange, thereby flowing into the refrigerant compressor (510) in a gaseous state.

[0091] That is, the low-temperature refrigerant circulating through the refrigerant circulation flow line (ML) is heated by heat exchange with the compressed boil-off gas supplied through the boil-off gas compressor (210) while cooling the carbon dioxide boil-off gas in the first boil-off gas heat exchanger (220), thereby becoming a high-temperature refrigerant, and the high-temperature refrigerant is introduced into the refrigerant compressor (510) through the refrigerant circulation flow line (ML) in the first boil-off gas heat exchanger (220) to circulate in the refrigeration cycle (500). The refrigerant circulating in the refrigeration cycle (500) can be a hydrocarbon-based refrigerant (e.g., ammonia or propane). However, it should be understood that the present application is not limited thereto, and any refrigerant suitable for re-liquefying the carbon dioxide boil-off gas while circulating in the refrigeration cycle (500) can be selected and applied.

[0092] Hereinafter, a method of re-liquefying carbon dioxide boil-off gas according to one embodiment of the present application will be described with reference to a carbon dioxide boil-off gas re-liquefying system according to an embodiment of the present application.

[0093] In a carbon dioxide transport vehicle using a dual-fuel engine, the method of re-liquefying carbon dioxide boil-off gas can include a compressed boil-off gas generating step, a cooling step, and a recovering step.

[0094] In the compressed boil-off gas generating step, the carbon dioxide boil-off gas evaporated from the storage tank (100) is supplied to the boil-off gas compressor (210) and compressed while passing through the boil-off gas compressor (210). Here, the compressed boil-off gas can be cooled in the intercooler (211) through the boil-off gas supply line (BL) and can stay in the buffer tank (210) before being introduced into the hot fluid flow path of the first boil-off gas heat exchanger (220).

[0095] 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 by heat exchange with the refrigerant circulating in the refrigeration cycle.

[0096] In the refrigeration cycle circulation step, the refrigerant heated by heat exchange with the compressed boil-off gas is compressed while passing through the refrigerant compressor and then expanded and cooled while passing through the refrigerant control valve, and 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).

[0097] On the other hand, the cooling step can further include a second cooling step depending on the operation mode of the carbon dioxide transport means.

[0098] Specifically, when the ship is operated in the gas mode, the second cooling step can be performed by supplying the compressed boil-off gas, which is subjected to the first cooling in the cooling step, to a second boil-off gas heat exchanger.

[0099] In the second cooling step, the compressed and first-cooled boil-off gas is supplied to the second boil-off gas heat exchanger (230) to be subjected to second cooling by heat exchange with the liquefied gas supplied to the engine (400).

[0100] For example, the second boil-off gas heat exchanger (230) can receive the compressed and first-cooled boil-off gas from the first boil-off gas heat exchanger (220), and can perform heat exchange of the compressed and first-cooled boil-off gas with liquefied natural gas (LNG) 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 such that the compressed and first-cooled boil-off gas is second-cooled and supplied to the engine (400).

[0101] Specifically, as shown in FIG. 2, Figure 1 the compressed boil-off gas subjected to the second cooling by the second cooling step can be supplied to the first boil-off gas control valve (240). The reliquefied boil-off gas in a supercooled liquid state, which has been supercooled as it passes through the second boil-off gas control valve (240), is preferably injected into the storage tank for recovery. Here, since the reliquefied boil-off gas in the supercooled liquid state is injected into the storage tank, the pressure in the storage tank (100) is effectively reduced.

[0102] Alternatively, as shown in FIG. 2, Figure 2 the compressed boil-off gas subjected to the second cooling by the second cooling step is preferably returned to the storage tank through one of the recovery lines selected from among the first reliquefied recovery line (RL1) and the second reliquefied recovery line (RL2).

[0103] For example, when the compressed boil-off gas subjected to the second cooling is returned through the first reliquefied recovery line (RL1), the compressed boil-off gas subjected to the second cooling generates the reliquefied boil-off gas in a supercooled liquid state as it passes through the first boil-off gas control valve (240), and the reliquefied boil-off gas in the supercooled liquid state can be injected into the storage tank (100) for recovery. Since the reliquefied boil-off gas in the supercooled liquid state is injected into the storage tank (100), the pressure in the storage tank (100) is effectively reduced.

[0104] On the other hand, when the cold heat of the LNG is excessively used during the second cooling step, the temperature of the carbon dioxide drops below the triple point and dry ice is generated, and the first reliquefaction recovery line (RL1) or the second reliquefaction recovery line (RL2) can be clogged by the generated dry ice, thereby preventing the carbon dioxide from returning to the storage tank (100). Therefore, it is desirable to control the amount of the LNG to prevent the generation of the dry ice.

[0105] However, when the first reliquefaction recovery line (RL1) or the second reliquefaction recovery line (RL2) is clogged by the 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 transfer the reliquefied boil-off gas to the reliquefaction recovery line (RL) that is not clogged to recover the reliquefied boil-off gas.

[0106] For example, when the first reliquefaction recovery line (RL1) is clogged by the 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 can be transferred to the second reliquefaction recovery line (RL2) without performing heat exchange with the cold heat of the LNG in the second boil-off gas heat exchanger (230).

[0107] Here, the second boil-off gas control valve (250) is provided on the second reliquefaction recovery line (RL2) such that the compressed and once-cooled boil-off gas expands to generate the reliquefied boil-off gas when passing through the second boil-off gas control valve (250), and the reliquefied boil-off gas returns to the storage tank (100) along the second reliquefaction recovery line (RL2).

[0108] Alternatively, when the first reliquefaction recovery line (RL1) is clogged due to the generation of the dry ice, the amount of the cold heat of the LNG supplied to the second boil-off gas heat exchanger (230) can be controlled to perform heat exchange, and the compressed boil-off gas subjected to the secondary cooling by the heat exchange can be transferred to the second reliquefaction recovery line (RL2) for recovery.

[0109] Here, the compressed boil-off gas subjected to the secondary cooling passes through the second boil-off gas control valve (250) on the second reliquefaction recovery line (RL2), and can be formed into the reliquefied boil-off gas in a supercooled liquid state when passing through the second boil-off gas control valve (250). The reliquefied boil-off gas in the supercooled liquid state can be injected into the storage tank (100) along the second reliquefaction recovery line (RL2) for recovery.

[0110] Preferably, a valve (251) is provided on the second reliquefaction recovery line (RL2) and is controlled such that the reliquefied boil-off gas in the supercooled liquid state can be supplied to the storage tank (100).

[0111] That is, when the ship is operated in the 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).

[0112] When the ship is operated in the oil mode, the cooling step can further include a third cooling step.

[0113] Specifically, the third cooling step preferably further includes a boil-off gas-liquid mixture generation step in which the compressed and once-cooled boil-off gas is expanded to generate a gas-liquid mixture of the boil-off gas, and a boil-off gas separation step in which the gas-liquid mixture of the boil-off gas is separated into reliquefied boil-off gas and gaseous boil-off gas.

[0114] In the boil-off gas separation step, the gas-liquid mixture of the boil-off gas is separated into reliquefied boil-off gas and gaseous boil-off gas, and the boil-off gas separation step includes a recovery step in which the separated reliquefied boil-off gas is injected into the storage tank (100) to recover 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) to reliquefy the gaseous boil-off gas.

[0115] That is, the third cooling step is preferably performed when the compressed and once-cooled boil-off gas does not undergo secondary cooling due to the absence or insufficiency of cold heat in the second boil-off gas heat exchanger.

[0116] 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 the compressed boil-off gas is cooled, and the heated gaseous boil-off gas is again supplied to the boil-off gas compressor (210) through a recirculation line (GL) to be merged with the boil-off gas flowing from the storage tank (100) toward the boil-off gas compressor (210) to be recirculated.

[0117] As described above, the present application provides a carbon dioxide reliquefaction system using a closed cycle and a carbon dioxide reliquefaction method.

[0118] Specifically, the present application provides a carbon dioxide liquefaction system using a closed cycle and a carbon dioxide liquefaction method, in which in a liquefied carbon dioxide transport means using liquefied natural gas (LNG) as fuel, boil-off gas of liquefied carbon dioxide is recovered by re-liquefying the liquefied carbon dioxide using cold and heat of the liquefied natural gas.

[0119] In addition, the re-liquefied carbon dioxide is returned to a carbon dioxide cargo tank to reduce the internal pressure of the storage tank by lowering the vapor temperature of the cargo tank.

[0120] In addition, since no vapor 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.

[0121] In addition, if there is no cold and heat or insufficient cold and heat in the liquefied gas, an existing Joule-Thomson valve and a separator (boil-off gas separator) are used to selectively operate depending on the situation.

[0122] While some embodiments have been set forth in the foregoing description, it is understood that these embodiments are presented by way of example only, and that various substitutions, changes, alternatives, and equivalents can be used without departing from the spirit and scope of the present application. Therefore, it is understood that these embodiments are presented for illustrative purposes only, and are not to be construed as limiting the present application in any way. The scope of the present application is to be defined by the appended claims, and their equivalents, and it is intended that all such modifications and alterations be included within the scope of the present application.

Claims

1. A carbon dioxide reliquefaction method for a carbon dioxide transport vehicle using a dual-fuel engine, the method comprising: a compressed boil-off gas generation step in which boil-off carbon dioxide gas is supplied from a storage tank to a compressor to generate 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 by 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 injected into the storage tank to recover the reliquefied carbon dioxide, wherein the cooling step further includes a second cooling step or a third cooling step depending on an operation mode of the carbon dioxide transport vehicle.

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

3. The carbon dioxide reliquefaction method according to claim 1, wherein the second cooling step is further implemented when the carbon dioxide transport vehicle is operated in a gas mode, the second cooling step includes supplying the compressed boil-off gas subjected to primary cooling in the cooling step to a second boil-off gas heat exchanger to implement secondary cooling of the compressed boil-off gas by 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 generate reliquefied boil-off gas in a supercooled 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 is configured to expand the compressed boil-off gas subjected to secondary cooling to generate reliquefied boil-off gas in a supercooled liquid state.

6. The carbon dioxide reliquefaction method according to claim 5, wherein the reliquefied boil-off gas in the supercooled liquid state is returned to the storage tank through one of a first reliquefaction recovery line and a second reliquefaction recovery line, and when the one of the reliquefaction recovery lines is blocked by dry ice, the reliquefied boil-off gas in the supercooled liquid state is diverted to a reliquefaction recovery line that is not blocked to recover the reliquefied boil-off gas.

7. The carbon dioxide reliquefaction method according to claim 1, wherein the third cooling step is further implemented when the carbon dioxide transport vehicle is operated in an oil mode, the third cooling step including: a vapor gas-liquid mixture generation step in which the compressed vapor gas subjected to primary cooling is expanded to generate a vapor gas-liquid mixture of vapor gas; and a vapor gas separation step in which the vapor gas-liquid mixture of vapor gas is separated into reliquefied vapor gas and gaseous vapor gas.

8. The carbon dioxide reliquefaction method according to claim 7, wherein the reliquefied vapor gas is injected into the storage tank to recover the reliquefied vapor gas, the carbon dioxide reliquefaction method further includes a recirculation step in which the separated gaseous vapor gas is recirculated to a vapor gas compressor to reliquefy the separated gaseous vapor gas, the recirculation step includes recirculating the gaseous vapor gas to the vapor gas compressor through the first vapor gas heat exchanger so that the gaseous vapor gas is heated while the compressed vapor gas is cooled, and the heated gaseous vapor gas is again supplied to the compressor through a recirculation line.

9. A carbon dioxide vapor gas reliquefaction system, comprising: an engine fueled with a liquefied gas; a storage tank storing liquefied carbon dioxide; a vapor gas compressor compressing a vapor gas of the liquefied carbon dioxide generated in the storage tank to generate a compressed vapor gas; a first vapor gas heat exchanger cooling the compressed vapor gas; a second vapor gas heat exchanger secondary cooling the vapor gas once cooled supplied from the first vapor gas heat exchanger by 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 vapor gas heat exchanger.

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

11. The carbon dioxide reliquefaction system according to claim 10, wherein an injection nozzle line is provided above the storage tank, the injection nozzle line is connected to the reliquefaction recovery line to inject the supercooled vapor gas into the storage tank.

12. The carbon dioxide reliquefaction system according to claim 9, wherein the second vapor gas heat exchanger and the storage tank are connected to a recirculation line through which the vapor gas not subjected to secondary cooling in the second vapor gas heat exchanger is recirculated to the vapor 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 that has not been subjected to secondary cooling in the second boil-off gas heat exchanger to generate a gas-liquid mixture of boil-off gas, and a boil-off gas separator separating the gas-liquid mixture of boil-off gas into re-liquefied boil-off gas and gaseous boil-off gas.

14. The carbon dioxide re-liquefaction system of claim 13, wherein the boil-off gas separator is connected to a liquefaction recovery line at a lower portion thereof and to the recirculation line at an upper portion thereof, the re-liquefied 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 by heat exchange and recirculated to the boil-off gas compressor.

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

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

17. The carbon dioxide re-liquefaction system of claim 16, further comprising: a boil-off gas separator disposed on the second re-liquefaction recovery line and separating the gas-liquid mixture of boil-off gas into re-liquefied 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 re-liquefaction system of claim 17, wherein the boil-off gas separator is connected to a third liquefaction recovery line at a lower portion thereof and to a recirculation line at an upper portion thereof, the re-liquefied 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 by heat exchange and recirculated to the boil-off gas compressor.

19. The carbon dioxide re-liquefaction system of claim 9, wherein the refrigeration cycle further comprises: a refrigerant compressor compressing the 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 supply the expanded refrigerant to the first boil-off gas heat exchanger.

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