Boil-off gas reliquefaction system for ship

Abstract

The present invention relates to a boil-off gas reliquefaction system for a ship. The boil-off gas reliquefaction system for a ship according to the present invention, whereby boil-off gas generated from a liquefied natural gas (LNG) storage tank for a ship is reliquefied, comprises: a first heat exchanger for recovering cold heat from a first boil-off gas generated in the storage tank; a multi-stage compressor for compressing the boil-off gas that has passed through the heat exchanger; a high-pressure gas line for supplying high-pressure gas that is supplied from the multi-stage compressor; a low-pressure gas line for supplying the boil-off gas, which is supplied from the multi-stage compressor, as a low-pressure gas by using a decompression valve; a second heat exchanger in which heat exchange occurs between the high-pressure gas line and the low-pressure gas line downstream of the decompression valve; and a third heat exchanger by which the boil-off gas that has passed through the second heat exchanger is cooled by means of heat medium oil of a cryogenic chiller.

Description

Marine boil-off gas reliquefaction system

[0001] The present invention relates to a system for re-liquefying boil-off gas (BOG) generated by the natural vaporization of liquefied gas, and more specifically, to a marine boil-off gas re-liquefaction system capable of re-liquefying BOG generated in a ship's liquefied natural gas storage tank and transferring it back to the tank to maintain the internal pressure of the tank.

[0002] To reduce greenhouse gas emissions resulting from the use of fossil fuels, the use of clean fuels with low greenhouse gas emissions is being encouraged. Representative examples include liquefied natural gas (LNG), liquefied hydrogen, and ammonia; among these, liquefied natural gas (LNG) is stored and supplied for use in maritime and land transportation as well as power generation facilities.

[0003] Liquefied gas, produced by liquefying gas at low temperatures, has the advantage of increasing storage and transport efficiency because its volume is significantly smaller than that of the gas.

[0004] In addition, liquefied gases, including liquefied natural gas, can remove or reduce air pollutants during the liquefaction process, so they can be viewed as eco-friendly fuels that emit fewer air pollutants during combustion.

[0005] Since the liquefaction temperature of natural gas is an extremely low temperature of -163°C at atmospheric pressure, liquefied natural gas is sensitive to temperature changes and evaporates easily.

[0006] As a result, although storage tanks for liquefied natural gas are insulated, external heat is continuously transferred to the tanks; consequently, during the transportation process, the liquefied natural gas naturally vaporizes inside the tanks, generating Boil-Off Gas (BOG).

[0007] Conventionally, all vaporized gas was consumed in the low-pressure fuel consumption unit, or any remaining BOG after consumption in the low-pressure fuel unit was liquefied using the cold energy generated when supplying liquefied gas (LNG) fuel to the main engine (ME-GI) and consumed in the high-pressure fuel unit or transferred to a tank to regulate tank pressure. However, due to recent concerns that methane slip from low-pressure fuel engines used in generators causes continuous environmental pollution, the use of low-pressure fuel engines is being restricted.

[0008] Prior art related to such BOG reliquefaction includes Application No. 20-2016-0003957 (Reliquefaction system for boil-off gas for ships) and Application No. 10-2016-0120835 (Reliquefaction system and method for boil-off gas).

[0009] However, there is a problem in that the cold energy of the LNG used for the main engine cannot be utilized during cargo loading and unloading when the main engine is not operating, or during standby after the main engine has stopped, and the low-pressure power generation engine cannot be used either, causing BOG in the LNG tank to continuously accumulate and the tank pressure to increase rapidly.

[0010] The purpose of the ship boil-off gas reliquefaction system according to the present invention is to basically provide a ship boil-off gas reliquefaction system capable of reliquefying BOG generated in a ship's liquefied natural gas storage tank and transferring it back to the tank to maintain the internal pressure of the tank.

[0011] In addition, the marine evaporative gas reliquefaction system according to the present invention aims to utilize evaporative gas as high-pressure fuel gas and low-pressure fuel gas, and to recover the cold energy of the evaporative gas in this process.

[0012] A marine evaporated gas reliquefaction system for reliquefying evaporated gas generated from a marine liquefied natural gas (LNG) storage tank according to the present invention comprises: a first heat exchanger for recovering cold heat from a first evaporated gas generated in the storage tank; a multi-stage compressor for compressing the evaporated gas that has passed through the heat exchanger; a high-pressure gas line for supplying high-pressure gas supplied from the multi-stage compressor; a low-pressure gas line for supplying the evaporated gas supplied from the multi-stage compressor as low-pressure gas using a pressure reducing valve; a second heat exchanger in which heat exchange occurs between the high-pressure gas line and the low-pressure gas line that has passed through the pressure reducing valve; and a third heat exchanger for cooling the evaporated gas that has passed through the second heat exchanger through a heat transfer fluid of an ultra-low temperature chiller. The system is characterized by using the second evaporated gas that has passed through the third heat exchanger for the recovery of cold heat in the first heat exchanger, and then re-injecting it into the storage tank through an expansion means.

[0013] Here, the temperature of the first evaporated gas transferred from the storage tank to the multi-stage compressor and the second evaporated gas that has passed through the third heat exchanger are compared, and the second evaporated gas is supplied to the expansion means by bypassing the first heat exchanger.

[0014] The marine evaporated gas reliquefaction system according to the present invention has the advantage of being able to reliquefy the evaporated gas using the cold energy of the evaporated gas by utilizing a second heat exchanger in which heat exchange occurs between the high-pressure gas line and the low-pressure gas line passing through the pressure reducing valve.

[0015] In addition, there is an advantage in efficiently recovering the cold energy of the evaporated gas by comparing the temperature of the second evaporated gas that has passed through the third heat exchanger with that of the first evaporated gas and supplying it to the first heat exchanger or bypassing it.

[0016] FIG. 1 is a conceptual diagram illustrating a ship boil-off gas reliquefaction system according to a preferred embodiment of the present invention.

[0017] FIG. 2 is a conceptual diagram illustrating a ship boil-off gas reliquefaction system according to another preferred embodiment of the present invention.

[0018] <Explanation of Symbols>

[0019] 10: Storage tank

[0020] 20: First heat exchanger

[0021] 30: Multistage compressor

[0022] 40: Pressure reducing valve

[0023] 42: Heater

[0024] 50: Second heat exchanger

[0025] 60: Third heat exchanger

[0026] 70: Ultra-low temperature chiller

[0027] 80: Control unit

[0028] 85: Three-way valve

[0029] 90: Means of expansion

[0030] TT1, TT2: Temperature sensors

[0031] Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the attached drawings.

[0032] FIG. 1 is a conceptual diagram illustrating a ship evaporated gas reliquefaction system according to a preferred embodiment of the present invention.

[0033] As illustrated in FIG. 1, the ship's boil-off gas reliquefaction system according to the present invention is used to reliquefy boil-off gas generated from a ship's liquefied natural gas (LNG) storage tank, and the ship's boil-off gas reliquefaction system comprises a storage tank (10), a first heat exchanger (20), a multi-stage compressor (30), a high-pressure gas line (HL), a low-pressure gas line (LL), a second heat exchanger (50), a third heat exchanger (60), and an expansion means (90).

[0034] First, when the internal pressure of the storage tank (10) exceeds a certain pressure, it opens and the evaporated gas generated in the liquefied gas storage tank (10) is supplied to the first heat exchanger (20), and after the cold heat is recovered through the second evaporated gas described later, it is supplied to the multi-stage compressor (30).

[0035] Here, the multi-stage compressor (30) is provided in multiple stages to compress evaporated gas through multiple stages, and an intercooler may be provided between the compressors to cool the evaporated gas whose temperature has risen while passing through each stage of the compressor.

[0036] As the evaporated gas passes through this multi-stage compressor (30), it can be compressed to a high pressure, for example, about 300 bar and 40 degrees Celsius, while alternating between compression and cooling.

[0037] And the evaporated gas (hereinafter referred to as 'high-pressure gas') compressed to high pressure while passing through the multi-stage compressor (30) can be supplied along the high-pressure gas line (HL) to a high-pressure evaporated gas demand point provided on the ship, for example, an ME-GI engine (HP) that receives evaporated gas compressed to about 300 bar and 40 degrees as fuel.

[0038] Low-pressure boil-off gas demanders (LP: e.g., DFDE, boiler, etc. that receive boil-off gas compressed to about 6 bar as fuel) that receive boil-off gas compressed to a relatively low pressure receive boil-off gas through the low-pressure gas line (LL).

[0039] Accordingly, the evaporated gas passing through the multi-stage compressor (30) is depressurized using the pressure reducing valve (40), and the depressurized evaporated gas is supplied to the low-pressure gas line (LL).

[0040] Here, the evaporated gas remaining after supplying high-pressure fuel to the main engine ME-GI engine (HP), or in the case where the main engine is not operating while anchored, the high-pressure evaporated gas recovers the cold heat of the low-pressure gas line (LL) through the second heat exchanger (50).

[0041] Here, the low-pressure evaporated gas passing through the second heat exchanger (50) is heated to room temperature through the heater (42) and delivered to the low-pressure evaporated gas demand point (LP).

[0042] The evaporated gas passing through the second heat exchanger (50) is cooled through a third heat exchanger (60) using a separate heat transfer fluid, and the heat transfer fluid used for cooling is re-cooled through an ultra-low temperature chiller (70) to continuously cool the evaporated gas. For example, the heat transfer fluid cooled in the ultra-low temperature chiller (70) is a heat transfer fluid of -100°C or lower supplied to the third heat exchanger (60).

[0043] Here, the second evaporated gas that has passed through the third heat exchanger (60) is used for the recovery of cold heat in the first heat exchanger (20) and is reinjected into the storage tank (10) through the expansion means (90).

[0044] Here, the expansion means (90) is expanded by a Joule-Thomson valve (JTV) and re-liquefied, the cooling evaporated gas re-liquefied by the Joule-Thomson valve is separated into gas and liquid in a storage tank (10), and the re-liquefied evaporated gas in a liquid state is recovered to a liquefied gas storage tank (10).

[0045] Next, we will look at other embodiments.

[0046] FIG. 2 is a conceptual diagram illustrating a ship evaporated gas reliquefaction system according to another preferred embodiment of the present invention.

[0047] The embodiment illustrated in FIG. 2 compares the temperature of the first evaporated gas moving from the storage tank (10) to the multi-stage compressor (30) with the temperature of the second evaporated gas passing through the third heat exchanger (60), and bypasses the first heat exchanger (20) to supply the second evaporated gas to the expansion means (90).

[0048] In this embodiment, a temperature sensor (TT1) for measuring the temperature of the first evaporated gas and a temperature sensor (TT2) for measuring the temperature of the second evaporated gas are each installed, and the signal transmitted from the temperature sensors (TT1, TT2) is transmitted to a control unit (80), and according to the transmitted signal, the control unit (80) controls a three-way valve (85) to determine whether to flow into the expansion means (90) through the first heat exchanger (20) or to flow into the expansion means (90) without passing through the first heat exchanger (20).

[0049] That is, the second evaporated gas, which passes through the third heat exchanger (60) that exchanges heat with the heat medium of the ultra-low temperature chiller (70), is cooled to approximately -100 degrees Celsius, and the decision to recover cold heat is made based on the temperature of this second evaporated gas and the temperature of the evaporated gas generated in the storage tank (10).

[0050] As described above, the main technical concept of the present invention is to provide a evaporative gas reliquefaction system for ships. Since the embodiments described above with reference to the drawings are merely examples, the true scope of the present invention should be determined by the claims.

Claims

1. A marine boil-off gas reliquefaction system for reliquefying boil-off gas generated from a marine liquefied natural gas (LNG) storage tank, A first heat exchanger for recovering the cold heat of the first evaporated gas generated in the above storage tank; A multi-stage compressor that compresses the evaporated gas passing through the above heat exchanger; A high-pressure gas line supplying high-pressure gas supplied from the above-mentioned multi-stage compressor; A low-pressure gas line that supplies evaporated gas supplied from the above-mentioned multi-stage compressor as low-pressure gas using a pressure reducing valve; A second heat exchanger in which heat exchange occurs between the commercial high-pressure gas line and the low-pressure gas line passing through the pressure reducing valve; It is composed of a third heat exchanger that cools the evaporated gas passing through the second heat exchanger through the heat transfer fluid of an ultra-low temperature chiller, and A shipboard evaporated gas reliquefaction system characterized by using the second evaporated gas, which has passed through the third heat exchanger, for cold heat recovery in the first heat exchanger, and then re-injecting it into the storage tank through an expansion means.

2. In Paragraph 1, A marine evaporated gas reliquefaction system characterized by comparing the temperature of the first evaporated gas transferred from the storage tank to the multi-stage compressor with the temperature of the second evaporated gas that has passed through the third heat exchanger, and bypassing the first heat exchanger to supply the second evaporated gas to an expansion means.

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