Ammonia boil-off gas reliquefaction system and method for ship

Abstract

An ammonia boil-off gas reliquefaction system and method for a vessel are disclosed. The ammonia boil-off gas reliquefaction system includes: a compression unit for multi-stage compression of boil-off gas, the compression unit including a first compressor receiving and compressing boil-off gas generated from ammonia supplied to a storage tank of a ship, and a second compressor further compressing the boil-off gas compressed in the first compressor; a condenser that cools the boil-off gas compressed by the compression unit; and a reliquefied gas recovery line along which the ammonia cooled and condensed in the condenser is conveyed to the storage tank, in which the temperature of the boil-off gas in the compression unit is adjusted by diverting the ammonia from the reliquefied gas recovery line portion to be supplied to the compression unit.

Description

Technical Field

[0001] The present invention relates to a system and method for reliquefying ammonia boil-off gas from a ship, and more particularly, to a system and method for reliquefying ammonia boil-off gas from a ship, wherein the system compresses and cools boil-off gas generated from an ammonia storage tank in a ship using ammonia as fuel for an onboard engine, thereby reliquefying the ammonia boil-off gas. Background Art

[0002] As global warming intensifies, efforts are underway around the world to reduce greenhouse gas emissions. Following the expiration of the 1997 Kyoto Protocol, which committed developed countries to reducing greenhouse gas emissions, in 2020, the 195 Parties to the Paris Agreement on Climate Change, adopted at the 21st Conference of the Parties to the United Nations Framework Convention on Climate Change in Paris, France, in December 2015 and entering into force in November 2016, agreed to undertake various efforts to reduce greenhouse gas emissions.

[0003] Along with this global trend, there is growing interest in renewable energy (or renewable energy), such as wind power generation, photovoltaic power generation, solar thermal power generation, bioenergy, tidal energy, geothermal energy, etc., as pollution-free energy that can replace fossil fuels and nuclear energy, and various technologies are being developed in this field.

[0004] Liquefied gases, including liquefied natural gas (LNG), can eliminate or reduce air pollutants during the liquefaction process and are therefore considered environmentally friendly fuels that provide low air pollutant emissions during combustion. Consequently, the consumption of liquefied gases, such as LNG (Liquified Natural Gas) and LPG (Liquified Petroleum Gas), has rapidly increased globally in recent years. Liquefied gases, obtained at ultra-low temperatures, have a smaller volume than natural gases, offering the advantage of improved storage and transportation efficiency.

[0005] Liquefied natural gas (LNG) is a colorless, transparent liquid obtained by cooling natural gas, primarily composed of methane, to approximately -162°C. Its volume is approximately 1 / 600 of that of natural gas. Therefore, natural gas is liquefied for efficient transportation.

[0006] Although liquefied petroleum gas (LPG) has different liquefaction temperatures depending on its composition, LPG mainly composed of propane can be liquefied at a low temperature of about -42°C at normal pressure and can be stored in a liquid state at a temperature of up to about 45°C at a pressure of 18 bar and up to 20°C at a pressure of 7 bar.

[0007] On the other hand, conventional LPG carriers employ a fuel supply system that uses relatively inexpensive heavy fuel oil, such as marine fuel oil C, as ship propulsion fuel. However, due to increasingly stringent international emission regulations on the use of heavy fuel oil, the fuel supply system needs to adopt separate fuel tanks (LSHFO tanks) to store low-sulfur heavy fuel oil, and the demand for environmentally friendly fuel supply systems that meet international environmental regulations is increasing.

[0008] In recent years, the use of fuel supply systems that use LPG or LNG and the boil-off gas generated therefrom as propulsion fuel for LPG or LNG carriers has increased. Furthermore, with the trend towards increasingly stringent international emission regulations, the number of ships using LNG and other fuels as propulsion fuel is increasing, not only for LPG or LNG carriers but also for ships in general.

[0009] Examples of marine engines capable of using natural gas as fuel include gas engines such as DFDE, X-DF engines, and ME-GI engines.

[0010] The DFDE is a four-stroke engine and uses the Otto cycle, in which relatively low-pressure natural gas of approximately 5.5 barg is injected into the combustion air inlet and compressed by an upwardly moving piston.

[0011] The X-DF engine is a two-stroke engine that uses natural gas at a pressure of approximately 15 barg as fuel and employs the Otto cycle.

[0012] The ME-GI engine is a two-stroke engine and uses a diesel cycle, in which high-pressure natural gas at a gauge pressure of approximately 300 bar is injected directly into the combustion chamber when the piston approaches top dead center.

[0013] However, while LNG is considered a cleaner fuel than other fossil fuels used in typical ships, it still produces carbon dioxide when burned, and LNG-fueled ships emit carbon dioxide during operation. Summary of the Invention

[0014] Problems to be solved by the invention

[0015] The International Maritime Organization (IMO), a specialized agency established by the United Nations to standardize international shipping routes, traffic regulations, port facilities and other related matters, has set a goal of reducing greenhouse gases by 50% by 2050 and 100% by 2100 (zero GHG emissions) compared to 2008 levels. It is expected that regulations in various countries and regions will be strengthened.

[0016] The Energy Efficiency Design Index (EEDI), the IMO's mandatory CO2 emissions reduction regulation for newly built ships, initially mandated EEDI Phase 1, based on CO2 emissions from 2013 to 2014, with a 10% reduction starting in 2015. EEDI Phase 3 was planned for implementation in 2025, with incremental increases every five years. However, for LPG carriers, EEDI Phase 3 will be implemented early, starting in 2022, two years after EEDI Phase 2. With rapidly tightening regulations on CO2 emissions from ships, it may be difficult to achieve CO2 emissions compliance in the future using only LNG or LPG as fuel.

[0017] Therefore, various studies have been conducted on environmentally friendly ship fuels capable of reducing carbon dioxide emissions, and recently, ship engine technology capable of using ammonia as a fuel together with other types of fuels such as LNG or LPG has been studied and developed.

[0018] Ammonia (NH3) is a substance with three hydrogen atoms bonded to one nitrogen atom, forming strong hydrogen bonds between molecules, making it easy to liquefy. Ammonia has a boiling point of -33.34°C and a melting point of -77.73°C at standard temperature and pressure.

[0019] Because ammonia is easier to store than LNG and, although slightly lower in specific energy and energy density than HFO, does not emit carbon dioxide, it has attracted attention as an environmentally friendly marine fuel to meet increasingly stringent international standards for greenhouse gas emissions.

[0020] Since ammonia has a boiling point of -33.34°C, which is higher than the boiling point of LNG and still above room temperature, tanks configured to store ammonia for use as a marine fuel produce boil-off gas.

[0021] One aspect of the present invention is to provide a technology capable of efficiently liquefying boil-off gas generated from ammonia to be supplied as fuel, thereby preventing fuel waste while safely maintaining the pressure of a storage tank.

[0022] Technical means to solve the problem

[0023] According to one aspect of the present invention, there is provided an ammonia boil-off gas reliquefaction system for a ship, comprising: a compression unit for multi-stage compression of boil-off gas, the compression unit comprising a first compressor for receiving and compressing boil-off gas generated from an ammonia storage tank provided to the ship, and a second compressor for further compressing the boil-off gas compressed in the first compressor;

[0024] a condenser that cools boil-off gas compressed by the compression unit; and

[0025] a reliquefied gas recovery line along which the ammonia cooled and condensed in the condenser is conveyed to a storage tank,

[0026] The temperature of the boil-off gas in the compression unit is adjusted by partially diverting ammonia from the reliquefied gas recovery line to supply it to the compression unit.

[0027] Preferably, the ammonia boil-off gas reliquefaction system further comprises: a temperature regulating line along which ammonia is partially diverted from the reliquefied gas recovery line and transported to the second compressor as boil-off gas after being compressed in the first compressor; a heat exchanger provided to the reliquefied gas recovery line and further cooling the ammonia condensed in the condenser; and a first pressure relief valve provided to the temperature regulating line and depressurizing the ammonia diverted from the reliquefied gas recovery line, wherein the ammonia depressurized in the first pressure relief valve is mixed with the boil-off gas downstream of the first compressor after heat exchange with ammonia in the liquefied gas recovery line through the heat exchanger.

[0028] Preferably, the first pressure relief valve is controlled in a cascade control manner by comparing the ammonia discharge temperature downstream of the first pressure relief valve on the temperature regulating line with the ammonia discharge temperature downstream of the heat exchanger on the liquefied gas recovery line.

[0029] Preferably, the compression unit is a three-stage compressor, further including a third compressor that receives and compresses the boil-off gas compressed by the first compressor and the second compressor, and the system further includes an intercooler that cools the compressed boil-off gas received from the second compressor and supplies the cooled boil-off gas to the third compressor.

[0030] Preferably, the ammonia boil-off gas reliquefaction system further comprises: a side flow line along which ammonia is partially diverted from the reliquefied gas recovery line and supplied to the first compressor of the compression unit as boil-off gas; a heat exchanger provided to the reliquefied gas recovery line and further cooling the ammonia condensed in the condenser; and a first pressure relief valve provided to the side flow line and depressurizing the ammonia diverted from the reliquefied gas recovery line, wherein the ammonia depressurized in the first pressure relief valve is mixed with the boil-off gas upstream of the first compressor after heat exchange with the ammonia in the liquefied gas recovery line through the heat exchanger and then supplied to the first compressor.

[0031] Preferably, the first pressure relief valve is controlled in a cascade control manner by comparing the ammonia discharge temperature downstream of the first pressure relief valve on the side stream line with the ammonia discharge temperature downstream of the heat exchanger on the liquefied gas recovery line.

[0032] Preferably, the ammonia boil-off gas reliquefaction system further includes an intercooler cooling the compressed boil-off gas received from the second compressor and supplying the cooled boil-off gas to the second compressor.

[0033] Preferably, the ammonia boil-off gas reliquefaction system further comprises: a receiver disposed between the condenser and the heat exchanger on the reliquefied gas recovery line and receiving ammonia cooled in the condenser; a vent line along which the gas separated from the receiver is discharged; and a control valve provided to the vent line, wherein the control valve is controlled according to an ammonia liquid level detected in the receiver to adjust the ammonia liquid level in the receiver.

[0034] Preferably, the ammonia boil-off gas reliquefaction system further comprises: a second pressure relief valve provided to the reliquefied gas recovery line and for reducing the pressure of the ammonia further cooled by the heat exchanger, wherein the second pressure relief valve is controlled according to a value selected from an ammonia discharge pressure downstream of the compression unit and an ammonia pressure upstream of the second pressure relief valve.

[0035] Preferably, the ammonia boil-off gas reliquefaction system further comprises: a knock-out drum for receiving boil-off gas discharged from the storage tank and supplying a gas component of the boil-off gas to a first compressor of the compression unit, wherein liquid contained in the boil-off gas is separated by the knock-out drum and discharged from the knock-out drum.

[0036] According to another aspect of the present invention, there is provided an ammonia boil-off gas reliquefaction method for a ship, comprising: compressing boil-off gas generated from ammonia in a storage tank of the ship by multi-stage compression of a compression unit including a first compressor and a second compressor;

[0037] condensing the boil-off gas compressed by the compression unit through a condenser, and then transferring the condensed boil-off gas to a storage tank along a reliquefied gas recovery line; and

[0038] A portion of the reliquefied gas recovery line is diverted to ammonia to supply the ammonia to the compression unit to adjust the temperature of the boil-off gas in the compression unit.

[0039] Preferably, the ammonia in the reliquefied gas recovery line is partially diverted along the temperature regulating line and, after being compressed in the first compressor, is conveyed to the second compressor as boil-off gas to regulate the temperature of the boil-off gas introduced into the second compressor; the reliquefied gas recovery line is provided with a heat exchanger for further cooling the ammonia condensed in the condenser; the temperature regulating line branches off from the reliquefied gas recovery line upstream of the heat exchanger; the ammonia diverted along the temperature regulating line is decompressed in the first pressure relief valve and, after heat exchange with the ammonia in the reliquefied gas recovery line through the heat exchanger, is mixed with the boil-off gas downstream of the first compressor.

[0040] Preferably, the ammonia in the liquefied gas recovery line is partially diverted along the side flow line and transported as boil-off gas to the first compressor of the compression unit to adjust the temperature of the boil-off gas supplied to the compression unit; the liquefied gas recovery line is provided with a heat exchanger for further cooling the ammonia condensed in the condenser; the side flow line branches off from the reliquefied gas recovery line upstream of the heat exchanger, and the ammonia diverted along the side flow line is decompressed in the first pressure relief valve and, after heat exchange with the ammonia in the reliquefied gas recovery line through the heat exchanger, is mixed with the boil-off gas upstream of the compression unit and supplied to the first compressor.

[0041] Preferably, the first pressure relief valve is controlled in a cascade control manner by comparing the ammonia discharge temperature downstream of the first pressure relief valve with the ammonia discharge temperature downstream of the heat exchanger on the liquefied gas recovery line.

[0042] Effects of the Invention

[0043] Embodiments of the present invention provide an ammonia boil-off gas reliquefaction system and method, which can reduce greenhouse gas emissions during ship operation while meeting regulatory standards established by international agreements by supplying ammonia, an environmentally friendly fuel, as fuel for ship engines.

[0044] In particular, the ammonia boil-off gas reliquefaction system according to the present invention compresses and cools boil-off gas generated from an ammonia storage tank to reliquefy the boil-off gas, which is then recovered or supplied as fuel, thereby stably maintaining the pressure in the storage tank while preventing waste of ammonia fuel.

[0045] In addition, the ammonia boil-off gas reliquefaction system according to the present invention can maximize the heat recovery efficiency of the side stream and reduce the cost of system installation and operation by partially diverting the condensed ammonia to supply it to a heat exchanger and exchange heat with the main ammonia stream to be reliquefied, and then transporting the heat-exchanged ammonia between the first compressor and the second compressor to adjust the temperature of the compressed ammonia boil-off gas or transporting the heat-exchanged ammonia to the upstream side of the compression unit to adjust the temperature of the ammonia boil-off gas to be supplied to the first compressor. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 FIG. 1 is a schematic diagram of a ship ammonia boil-off gas reliquefaction system according to a first embodiment of the present invention.

[0047] Figure 2 FIG. 4 is a schematic diagram of a ship ammonia boil-off gas reliquefaction system according to a second embodiment of the present invention. DETAILED DESCRIPTION

[0048] For a fuller understanding of the operating advantages and objectives attained by its practice, reference should be made to the accompanying drawings in which preferred embodiments of the invention are illustrated, and the description thereof.

[0049] Hereinafter, the features and effects of the exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that the same components will be denoted by the same reference numerals throughout the specification and drawings.

[0050] As used herein, the term "vessel" may refer to any type of vessel provided with an engine capable of utilizing ammonia as onboard engine fuel, and may include self-propelled vessels, such as liquefied petroleum gas carriers (LNG carriers), liquefied natural gas carriers (LNG carriers), liquid hydrogen carriers, ammonia carriers, container carriers, crude oil carriers, bulk carriers including mineral carriers or grain carriers, specialized vessels including offshore wind turbine installation vessels (Wind Turbine Installation Vessels), roll-on / roll-off ships, and non-self-propelled offshore floating structures.

[0051] As used herein, "ammonia-fueled engines" include both engines fueled solely by ammonia and engines fueled by ammonia in combination with other marine fuels (e.g., liquefied natural gas, liquefied petroleum gas, heavy fuel oil, marine gas oil, diesel, etc.), and encompass both propulsion engines and power generation engines of a ship. Multiple propulsion engines and power generation engines may be provided as needed.

[0052] In addition, embodiments of the present invention can be applied to reliquefaction systems for any type of liquefied gas that can be transported in a liquid state by liquefying at ultra-low temperatures and that can generate boil-off gas during storage. For example, such liquefied gas can include liquefied petrochemical gases such as liquefied natural gas (LNG), liquefied ethane gas (LEG), liquefied petroleum gas (LPG), liquefied ethylene gas (Ethylene Gas), and liquefied propylene gas (Propylene Gas), as well as ammonia. In the following embodiments, ammonia is used as an example as a typical liquefied gas.

[0053] Figure 1 FIG. 1 is a schematic diagram of an ammonia boil-off gas reliquefaction system for a ship according to a first embodiment of the present invention.

[0054] like Figure 1As shown, the ammonia boil-off gas reliquefaction system according to the first embodiment is configured to reliquefy ammonia boil-off gas generated from a storage tank (T) provided on a ship and storing ammonia supplied as fuel for an onboard engine, so that the reliquefied boil-off gas can be returned to the storage tank or supplied as fuel. Because ammonia gas is toxic, classification regulations require that the design pressure of the storage tank be increased to allow operation without exhaust even at room temperature, or that a system capable of treating the ammonia boil-off gas generated in the storage tank be installed. The system according to this embodiment is a system capable of treating ammonia boil-off gas by reliquefaction, wherein the ammonia boil-off gas generated in the storage tank is transported to a compression unit (100) through a knock-out drum (400) to be compressed in the compression unit, cooled and condensed in a condenser (200), and recovered through a receiver (250), a heat exchanger (300), etc.

[0055] refer to Figure 1 According to the present embodiment, the ammonia boil-off gas reliquefaction system includes: a compression unit (100) for multi-stage compression of boil-off gas, which includes a first compressor (100A) for receiving and compressing boil-off gas generated from ammonia in a storage tank (T) provided in a ship, and a second compressor (100B) for further compressing the boil-off gas compressed in the first compressor; a condenser (200) for cooling the boil-off gas compressed by the compression unit; a reliquefied gas recovery line (RL), along which the ammonia cooled and condensed in the condenser is transported to the storage tank; and a temperature regulating line (TL), along which ammonia is diverted from the reliquefied gas recovery line and is transported to the second compressor as boil-off gas after being compressed in the first compressor.

[0056] Ammonia boil-off gas discharged from the storage tank (T) passes through a knock-out drum (400), so that only the gaseous component of the boil-off gas is supplied to the first compressor (100A) of the compression unit. Liquid (condensate) contained in the boil-off gas is separated by the knock-out drum and discharged to the lower side of the knock-out drum.

[0057] The compression unit (100) may be a three-stage compressor, including a first compressor (100A) that receives and compresses boil-off gas, a second compressor (100B) that further compresses the boil-off gas compressed in the first compressor, and a third compressor (100C) that further compresses the boil-off gas compressed in the second compressor and delivers the further compressed boil-off gas to a condenser. The compression unit may be, for example, a three-stage centrifugal compressor including the first to third compressors or a three-stage reciprocating compressor with reciprocating pistons. The number of stages of the compression unit may be increased as needed.

[0058] The boil-off gas compressed in the compression unit (100) is supplied to the condenser (200) to be cooled therein. In the condenser (200), the boil-off gas compressed in the compression unit (100) is cooled and reliquefied, for example, by cooling the ammonia boil-off gas through heat exchange with a cold heat source.

[0059] The ammonia cooled and condensed in the condenser (200) is transported along the reliquefied gas recovery line (RL) to the receiver (250) to be received therein in a sub-cooled state. Since the ammonia transported to the receiver is in a sub-cooled state, although some vapor is generated from nitrogen, air, etc., no boil-off gas is generated. In the receiver, gas is separated from the condensed ammonia and vented along the vent line (CL), and the liquid is transported along the reliquefied gas recovery line (RL) to a storage tank.

[0060] The ventilation line (CL) is provided with a control valve (CV) which is controlled according to the ammonia liquid level in the receiver. By controlling the control valve, the gas discharged along the ventilation line can be adjusted together with the ammonia liquid level inside the receiver.

[0061] The reliquefied gas recovery line (RL) is provided with a heat exchanger (300) downstream of the receiver to further cool the ammonia condensed in the condenser. In the system according to this embodiment, the heat exchanger can be, for example, a plate heat exchanger or a PCHE.

[0062] To ensure a cold and hot source for the heat exchanger (300), a temperature regulating line (TL) branches off from the reliquefied gas recovery line (RL) on the upstream side of the heat exchanger. The temperature regulating line (TL) is provided with a first pressure relief valve (JTV1) to reduce the pressure of ammonia diverted from the reliquefied gas recovery line. An ammonia side stream that branches off from the temperature regulating line (TL) and is cooled by the pressure reduction in the first pressure relief valve (JTV1) is supplied to the heat exchanger (300), where the ammonia side stream exchanges heat with the main ammonia stream flowing along the reliquefied gas recovery line (RL). That is, in the heat exchanger (300), the ammonia in the reliquefied gas recovery line (RL) exchanges heat with the ammonia that is diverted along the temperature regulating line (TL) and cooled by the first pressure relief valve (JTV1). Ammonia passing through the heat exchanger (300) along the temperature regulating line (TL) is delivered to the downstream side of the first compressor (100A) of the compression unit and mixed with the boil-off gas to be compressed in the first compressor and then supplied to the second compressor (100B). In the liquefaction system according to this embodiment, the cycle efficiency can be maximized by applying a plate heat exchanger or PCHE to cool the ammonia by heat exchange between the side stream and the main stream without a quenching operation.

[0063] The first pressure relief valve (JTV1) can be controlled in a cascade control manner by comparing the ammonia discharge temperature (T1) downstream of the first pressure relief valve in the temperature regulation line with the ammonia discharge temperature (T2) downstream of the heat exchanger in the liquefied gas recovery line. Therefore, the temperature of the boil-off gas compressed in the first compressor and then introduced into the second compressor can be adjusted without providing a separate intercooler between the first and second compressors.

[0064] Therefore, the boil-off gas compressed in the first compressor (100A) of the compression unit is mixed with decompressed ammonia to allow temperature adjustment after being diverted along the temperature adjustment line (TL), and then supplied to the second compressor (100B) for compression therein. The intercooler (150) is provided between the second and third compressors of the compression unit, so that the boil-off gas compressed in the second compressor can be cooled by the intercooler (150) and supplied to the third compressor. In this way, the reliquefaction system according to this embodiment adopts a three-stage compressor, in which a side stream is provided between the first and second compressors, and the intercooler is provided between the second and third compressors, thereby being able to adjust the temperature of the boil-off gas while preventing overheating.

[0065] The ammonia boil-off gas compressed by the compression unit including the third compressor (100C) is supplied to the condenser (200) to be cooled and condensed therein, and passes through the receiver (250) and the heat exchanger (300).

[0066] The reliquefied gas recovery line (RL) is provided with a second pressure relief valve (JTV2) which reduces the pressure of ammonia that has been additionally cooled by passing through the heat exchanger (300). The second pressure relief valve (JTV2) is controlled by a pressure value (pressure control), specifically a pressure value selected from the ammonia discharge pressure (PT1) downstream of the compression unit (i.e., at the third compressor) and the ammonia pressure (PT2) upstream of the second pressure relief valve (selection function).

[0067] The ammonia cooled in the heat exchanger (300) and decompressed through the second pressure relief valve (JTV2) can be returned to the storage tank (T) or can be delivered to the fuel supply line to be directly supplied as engine fuel.

[0068] As described above, according to the first embodiment, by reliquefying the boil-off gas generated from ammonia supplied as fuel, ammonia waste can be prevented while maintaining storage tank pressure. Specifically, a portion of the condensed ammonia is diverted and sent to a heat exchanger for heat exchange with the main flow of ammonia to be reliquefied. The stream is then sent between the first and second compressors to adjust the temperature of the compressed ammonia boil-off gas. This maximizes the efficiency of heat recovery from the side stream, reduces the number of system components, lowers installation costs, and helps secure onboard space on the vessel.

[0069] Next, Figure 2 FIG. 4 is a schematic diagram of an ammonia boil-off gas reliquefaction system for a ship according to a second embodiment of the present invention.

[0070] like Figure 2 As shown, the ammonia boil-off gas reliquefaction system according to the second embodiment is configured to reliquefy ammonia boil-off gas generated from a storage tank (T) provided on a ship and storing ammonia supplied as fuel for an onboard engine, so that the reliquefied boil-off gas can be returned to the storage tank or supplied as fuel. Because ammonia gas is toxic, classification regulations require that the design pressure of the storage tank be increased to allow operation without venting even at room temperature, or that a system capable of treating the ammonia boil-off gas generated in the storage tank be installed. The system according to this embodiment is a system capable of treating ammonia boil-off gas by reliquefaction, wherein the ammonia boil-off gas generated in the storage tank is transported to a compression unit (100) through a knock-out drum (400) to be compressed in the compression unit, cooled and condensed in a condenser (200), and recovered through a receiver (250), a heat exchanger (300), etc.

[0071] Now refer to Figure 2 According to the present embodiment, the ammonia boil-off gas reliquefaction system includes: a compression unit (100) for multi-stage compression of boil-off gas, which includes a first compressor (100A) for receiving and compressing boil-off gas generated from ammonia in a storage tank (T) provided in a ship, and a second compressor (100B) for further compressing the boil-off gas compressed in the first compressor; a condenser (200) for cooling the boil-off gas compressed by the compression unit; a reliquefied gas recovery line (RL), along which the ammonia cooled and condensed in the condenser is transported to the storage tank; and a side flow line BL, along which ammonia is diverted from the reliquefied gas recovery line portion and supplied as boil-off gas to the upstream of the compression unit to be supplied to the first compressor.

[0072] Ammonia boil-off gas discharged from the storage tank (T) passes through a separator drum (400) along a gas delivery line (GL), so that only the gaseous component of the boil-off gas is supplied to the first compressor (100A). Liquid (condensate) contained in the boil-off gas is separated by the separator drum and discharged to the bottom of the separator drum.

[0073] The compression unit (100) may be a two-stage compressor including a first compressor (100A) that receives and compresses the boil-off gas and a second compressor (100B) that further compresses the boil-off gas compressed in the first compressor. The compression unit may be, for example, a two-stage centrifugal compressor including the first compressor and the second compressor, or a two-stage reciprocating compressor with reciprocating pistons. The number of stages of the compression unit may be increased as needed.

[0074] The boil-off gas compressed in the compression unit (100) is supplied to a condenser (200) to be cooled therein. In the condenser (200), the boil-off gas compressed in the compression unit (100) is cooled and reliquefied, for example, by heat exchange with a cold heat source to cool the ammonia boil-off gas.

[0075] The ammonia cooled and condensed in the condenser (200) is transported along the reliquefied gas recovery line (RL) to the receiver (250) to be received therein in a sub-cooled state. Since the ammonia transported to the receiver is in a sub-cooled state, although some vapor is generated from nitrogen, air, etc., no boil-off gas is generated. In the receiver, gas is separated from the condensed ammonia and vented along the vent line (CL), and the liquid is transported along the reliquefied gas recovery line (RL) to a storage tank.

[0076] The ventilation line (CL) is equipped with a control valve (CV) which is controlled according to the ammonia level in the receiver. By controlling the control valve, the ammonia level in the receiver and the gas discharged along the ventilation line can be regulated together.

[0077] The reliquefied gas recovery line (RL) is provided with a heat exchanger (300) downstream of the receiver to further cool the ammonia condensed in the condenser. In the system according to this embodiment, the heat exchanger can be, for example, a plate heat exchanger or a PCHE.

[0078] To ensure a cold and hot source for the heat exchanger (300), a side stream BL branches from the reliquefied gas recovery line (RL) on the upstream side of the heat exchanger. The side stream BL is provided with a first pressure relief valve (JTV1) to reduce the pressure of ammonia diverted from the reliquefied gas recovery line. The ammonia side stream that branches along the side stream BL and is cooled by the first pressure relief valve (JTV1) is supplied to the heat exchanger (300), where the ammonia side stream is heat-exchanged with the main ammonia stream flowing along the reliquefied gas recovery line (RL). That is, in the heat exchanger (300), the ammonia in the reliquefied gas recovery line (RL) is heat-exchanged with the ammonia that is diverted along the side stream BL and cooled by the first pressure relief valve (JTV1). The ammonia that passes through the heat exchanger (300) along the side stream BL is transported to the upstream side of the compression unit, mixed with the boil-off gas stream discharged from the storage tank, and supplied to the first compressor (100A) of the compression unit through a knock-out drum. In the liquefaction system according to the present embodiment, the cycle efficiency can be maximized by applying a plate heat exchanger or PCHE to perform heat exchange between the side stream and the main stream without the need for a quenching operation to cool the ammonia, and the side stream is sent back to the upstream side of the compression unit for temperature adjustment of the boil-off gas supplied to the compression unit.

[0079] The first pressure relief valve (JTV1) can be controlled in a cascade control manner by comparing an ammonia discharge temperature (T1) downstream of the first pressure relief valve with an ammonia discharge temperature (T2) downstream of a heat exchanger on a liquefied gas recovery line.

[0080] Therefore, the boil-off gas passing through the knock-out drum is mixed with reduced-pressure ammonia to allow temperature adjustment after being diverted along the side flow line BL and compressed in the first compressor (100A). The intercooler (150) is provided between the first and second compressors in the compression unit, so that the boil-off gas compressed in the first compressor can be cooled by the intercooler (150) and supplied to the second compressor. In this way, the reliquefaction system according to this embodiment adopts a two-stage compressor, in which the side flow is supplied along the side flow line upstream of the compressor, and the intercooler is provided between the first and second compressors, thereby being able to adjust the temperature of the boil-off gas while preventing overheating.

[0081] The ammonia boil-off gas compressed by the compression unit is supplied to the condenser (200) to be cooled and condensed therein, and passes through the receiver (250) and the heat exchanger (300).

[0082] The reliquefied gas recovery line (RL) is provided with a second pressure relief valve (JTV2) which reduces the pressure of ammonia further cooled by the heat exchanger (300). The second pressure relief valve (JTV2) is controlled by a pressure value (pressure control), specifically a pressure value selected from the ammonia discharge pressure (PT1) downstream of the compression unit (i.e., at the second compressor) and the ammonia pressure (PT2) upstream of the second pressure relief valve (select function).

[0083] The ammonia decompressed through the second pressure relief valve (JTV2) after being cooled in the heat exchanger (300) may be returned to the storage tank (T) or may be delivered to the fuel supply line as engine fuel.

[0084] As described above, according to the second embodiment, by reliquefying the boil-off gas generated from ammonia supplied as fuel, it is possible to prevent ammonia waste while maintaining storage tank pressure. Specifically, a portion of the condensed ammonia is diverted and sent to a heat exchanger for heat exchange with the main flow of ammonia to be reliquefied. The flow is then sent upstream of the compression unit to adjust the temperature of the ammonia boil-off gas introduced into the compression unit. This maximizes the cold and heat recovery efficiency of the side stream, reduces the number of system components, and helps secure space onboard the vessel.

[0085] It is obvious to those skilled in the art that the present invention is not limited to the above embodiments, but can be modified or improved in various ways within the scope of the technical gist of the present invention.

Claims

1. An ammonia boil-off gas reliquefaction system for ships, characterized in that: include: a compression unit for multi-stage compression of boil-off gas, the compression unit comprising a first compressor for receiving and compressing boil-off gas generated from ammonia in a storage tank provided to the vessel, and a second compressor for further compressing the boil-off gas compressed in the first compressor; a condenser to cool the boil-off gas compressed by the compression unit; and a reliquefied gas recovery line along which the ammonia cooled and condensed in the condenser is transported to the storage tank, wherein the temperature of the boil-off gas in the compression unit is adjusted by diverting the ammonia from the reliquefied gas recovery line portion to supply it to the compression unit.

2. The ammonia boil-off gas reliquefaction system according to claim 1, characterized in that: Further including: a temperature regulating line along which the ammonia is diverted from the reliquefied gas recovery line portion and, after being compressed in the first compressor, is conveyed to the second compressor as boil-off gas; a heat exchanger provided to the reliquefied gas recovery line and further cooling the ammonia condensed in the condenser; as well as a first pressure relief valve, provided to the temperature regulating line and decompressing the ammonia diverted from the reliquefied gas recovery line; The ammonia decompressed in the first pressure relief valve is mixed with the boil-off gas downstream of the first compressor after undergoing heat exchange with the ammonia in the liquefied gas recovery pipeline through the heat exchanger.

3. The ammonia boil-off gas reliquefaction system according to claim 2, wherein: The first pressure relief valve is controlled in a cascade control manner by comparing the ammonia discharge temperature downstream of the first pressure relief valve on the temperature regulating line with the ammonia discharge temperature downstream of the heat exchanger on the liquefied gas recovery line.

4. The ammonia boil-off gas reliquefaction system according to claim 3, characterized in that: Further including: an intercooler that cools the compressed boil-off gas received from the second compressor and supplies the cooled boil-off gas to a third compressor, The compression unit is a three-stage compressor, further comprising a third compressor configured to receive and compress the boil-off gas compressed by the first compressor and the second compressor.

5. The ammonia boil-off gas reliquefaction system according to claim 1, wherein: Further including: a side flow line along which the ammonia is diverted from the reliquefied gas recovery line section and conveyed as boil-off gas to the first compressor of the compression unit; a heat exchanger provided to the reliquefied gas recovery line and further cooling the ammonia condensed in the condenser; as well as a first pressure relief valve, provided to the side flow line and decompressing the ammonia diverted from the reliquefied gas recovery line; The ammonia decompressed in the first pressure relief valve is heat-exchanged with the ammonia in the liquefied gas recovery line through the heat exchanger, mixed with the boil-off gas upstream of the first compressor, and then supplied to the first compressor.

6. The ammonia boil-off gas reliquefaction system according to claim 5, characterized in that: The first pressure relief valve is controlled in a cascade control manner by comparing an ammonia discharge temperature downstream of the first pressure relief valve on the side flow line with an ammonia discharge temperature downstream of the heat exchanger on the liquefied gas recovery line.

7. The ammonia boil-off gas reliquefaction system according to claim 6, wherein: Further including: An intercooler cools the compressed boil-off gas received from the second compressor and supplies the cooled boil-off gas to the second compressor.

8. The ammonia boil-off gas reliquefaction system according to claim 2 or 5, characterized in that: Further including: a receiver disposed between the condenser and the heat exchanger on the reliquefied gas recovery line and receiving the ammonia cooled in the condenser; a vent line along which the gas separated from the receiver is discharged; as well as A control valve is provided to the ventilation line, The control valve is controlled according to the ammonia liquid level detected in the receiver to adjust the ammonia liquid level in the receiver.

9. The ammonia boil-off gas reliquefaction system according to claim 8, wherein: Further including: a second pressure relief valve, provided to the reliquefied gas recovery line and decompressing the ammonia further cooled by the heat exchanger; The second pressure relief valve is controlled according to a value selected from an ammonia discharge pressure downstream of the compression unit and an ammonia pressure upstream of the second pressure relief valve.

10. The ammonia boil-off gas reliquefaction system according to claim 9, wherein: Further including: a knock-out drum receiving the boil-off gas discharged from the storage tank and supplying the gas component of the boil-off gas to the first compressor of the compression unit, The liquid component contained in the boil-off gas is separated by the separation drum and discharged from the separation drum.

11. A method for reliquefying ammonia boil-off gas for ships, characterized in that: include: compressing boil-off gas generated from ammonia in a storage tank of the vessel through multi-stage compression of a compression unit including a first compressor and a second compressor; condensing the boil-off gas compressed by the compression unit through a condenser, and then transferring the condensed boil-off gas to the storage tank along a reliquefied gas recovery line; and The ammonia is partially diverted from the reliquefied gas recovery line to supply the ammonia to the compression unit for adjusting the temperature of the boil-off gas in the compression unit.

12. The method for reliquefying ammonia boil-off gas according to claim 11, wherein: The ammonia in the reliquefied gas recovery line is partially diverted along the temperature adjustment line and, after being compressed in the first compressor, is transported to the second compressor as boil-off gas to adjust the temperature of the boil-off gas introduced into the second compressor; The reliquefied gas recovery line is provided with a heat exchanger to further cool the ammonia condensed in the condenser; The temperature regulating line branches off from the reliquefied gas recovery line upstream of the heat exchanger; as well as The ammonia diverted along the temperature regulating line is decompressed in a first pressure relief valve and is mixed with the boil-off gas downstream of the first compressor after heat-exchanging with the ammonia in the reliquefied gas recovery line through the heat exchanger.

13. The method for reliquefying ammonia boil-off gas according to claim 11, wherein: The ammonia in the liquefied gas recovery line is diverted along a side flow line portion and fed as boil-off gas to the first compressor of the compression unit to adjust the temperature of the boil-off gas supplied to the compression unit; The liquefied gas recovery pipeline is provided with a heat exchanger to further cool the ammonia condensed in the condenser; The side flow line branches off from the reliquefied gas recovery line upstream of the heat exchanger; as well as The ammonia diverted along the side flow line is decompressed in the first pressure relief valve and, after heat exchange with the ammonia in the reliquefied gas recovery line through the heat exchanger, is mixed with the boil-off gas upstream of the compression unit and supplied to the first compressor.

14. The method for reliquefying ammonia boil-off gas according to claim 12 or 13, wherein: The first pressure relief valve is controlled in a cascade control manner by comparing the ammonia discharge temperature downstream of the first pressure relief valve with the ammonia discharge temperature downstream of the heat exchanger in the liquefied gas recovery line.

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