System for supplying fuel to high-pressure natural gas injection engine having excess evaporation gas consumption means

Abstract

Provided is a fuel supply system for a high-pressure natural gas injection engine. The fuel supply system includes: a BOG compression unit configured to receive BOG, which is generated in a storage tank, from the storage tank and compress the received BOG to a pressure of 12 to 45 bara; a reliquefaction apparatus configured to receive and liquefy the BOG compressed by the BOG compression unit; a high-pressure pump configured to compress the BOG liquefied by the reliquefaction apparatus; a high-pressure gasifier configured to gasify the BOG compressed by the high-pressure pump and supply the gasified BOG to the high-pressure natural gas injection engine; and an excess BOG consumption unit configured to consume excess BOG corresponding to a difference between an amount of BOG generated in the storage tank and an amount of BOG required as fuel for the high-pressure natural gas injection engine.

Description

TECHNICAL FIELD[0001]The present invention relates to a fuel supply system for a high-pressure natural gas injection engine, and more particularly, to a fuel supply system for a high-pressure natural gas injection engine having an excess boil-off gas (BOG) consumption unit, which can consume excess BOG corresponding to a difference between an amount of BOG generated in the storage tank and an amount of BOG required as fuel for the high-pressure natural gas injection engine, when a larger amount of BOG than required as fuel for the high-pressure natural gas injection engine is generated.BACKGROUND ART[0002]Recently, the consumption of natural gas, such as liquefied natural gas (LNG) or liquefied petroleum gas (LPG), has been rapidly increasing throughout the world. Liquefied gas is transported in a gaseous state through onshore or offshore gas pipelines, or transported to a remote consumption place while being stored in a liquefied state inside a liquefied gas carrier. Liquefied gas,...

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Benefits of technology

[0036]The present invention may provide a fuel supply system for a high-pressure natural gas injection engine having an excess boil-off gas (BOG) consumption unit, which can consume excess BOG corresponding to a difference between an amount of BOG generated in the storage tank and an amount of BOG required as fuel for the high-pressure natural gas injection engine, when a larger amount of BOG than required as fuel for the high-pressure natural gas injection engine is generated.

[0037]As opposed to the related art in which the BOG is compressed to a low pressure of about 4 to 8 bara, the fuel supply method for the high-pressure natural gas injection engine according to the present invention compresses the BOG to a medium pressure of about 12 to 45 bara and then reliquefied. As the pressure of the BOG increases, the liquefaction energy decreases. Therefore, the liquefaction energy consumed in reliquefaction may be reduced.

[0038]Also, in the fuel supply system for the high-pressure natural gas injection engine according to the present invention, since the pressure of the BOG in the BOG reliquefaction is a medium pressure higher than that of the related art, the liquefying point of the BOG increases. Therefore, thermal stress applied to a heat exchanger for reliquefaction is reduced, and a heat duty of a high-pressure gasifier is reduced, leading to a reduction in the size of the apparatus.

[0039]Also, since the liquefied BOG compressed to a medium pressure is compressed to a high pressure, power of a high-pressure pump is reduced.

[0040]Also, in the fuel supply system for the high-pressure natural gas injection engine according to the present invention, a nonflammable mixed refrigerant is used as a refrigerant of a reliquefaction apparatus for the BOG reliquefaction. Therefore, the fuel supply method according to the present invention is more efficient than a conventional nitrogen refrigeration cycle, and can reliquefy the BOG more safely than a conventional mixed refrigerant cycle.

[0041]The fuel supply method of the fuel supply system may supply all of the liquefied BOG to the high-pressure natural gas injection engine during the operation of the high-pressure natural gas injection engine. That is, the high-pressure natural gas injection engine may require a larger amount of fuel than an amount of the BOG generated in the LNG storage tank for a considerable period of time during the voyage of the marine structure. In this case, all of the liquefied BOG is supplied to the high-pressure natural gas injection engine, thereby preventing the generation of flash gas when the liquefied BOG is returned to the LNG storage tank. Also, it is possible to considerably reduce energy that is consumed by subcooling for reducing the generation of flash gas when the liquefied BOG is returned to the LNG storage tank. A conventional Mark III reliquefaction apparatus of Hamworthy Gas Systems (the technology disclosed in International Patent Publication No. WO 2007 / 117148) compresses the BOG to a pressure of 8 bara and liquefies the BOG at a temperature of −159° C. In this case, since the saturation temperature of the BOG is about −149.5° C., the BOG is subcooled by about 9 to 10° C. The BOG needs to be subcooled by such a degree in order to prevent the generation of flash gas when the liquefied BOG is returned to the LNG storage tank. However, since the liquefied BOG is compressed by the high-pressure pump while the liquefied BOG is supplied as fuel for the high-pressure natural gas injection engine, the LBOG saturated by the increased pressure can stably maintain the overcooled state later. Therefore, according to the present invention, the liquefied BOG may be liquefied by overcooling as much as 0.5 to 3° C., preferably about 1° C., as compared to the saturation temperature at the corresponding pressure, and then supplied as fuel.

Problems solved by technology

The generated natural gas may increase the internal pressure of the storage tank and accelerate the flow of the natural gas due to the rocking of the vessel, causing structural problems.

However, the LNG stored in the storage tank is maintained at an ambient pressure state, and therefore, if a pressure of the liquefied BOG is excessively high, flash gas may be generated when the BOG is returned to the storage tank.

Consequently, the BOG needs to be compressed to the above-mentioned low pressure of about 4 to 8 bara, in spite of low reliquefaction efficiency.

Also, it is well known in the art that it is technically inappropriate to compress BOG to a pressure higher than the above-mentioned pressure.

Meanwhile, since the nitrogen refrigeration cycle uses nitrogen gas (N2) as a refrigerant, the liquefaction efficiency is low.

Also, the mixed refrigerant cycle uses a refrigerant mixed with nitrogen and hydrocarbon gases as a refrigerant, the stability is low.

The nitrogen reverse Brayton cycle used for the offshore LNG liquefaction apparatus is relatively simple in the configuration of the apparatus and thus is advantageous to a limited vessel or offshore plant, but has low efficiency.

The mixed-refrigerant Joule-Thomson refrigeration cycle used for the onshore LNG liquefaction plant has relatively high efficiency but is complicated in the configuration of the apparatus because a separator needs to be used for separating a mixed refrigerant when a gaseous state and a liquid state coexist due to the feature of the mixed refrigerant.

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