Fuel supply method for high-pressure natural gas injection engine

Abstract

Provided is a fuel supply method for a marine structure using a high-pressure natural gas injection engine. BOG stored in a stored in the storage tank is compressed to a pressure of 12 to 45 bara (absolute pressure) and then reliquefied. A reliquefaction apparatus includes a cold box configured to exchange heat between a refrigerant and the BOG, a compression unit configured to compress the refrigerant heated by the cold box, an expansion unit configured to expand the compressed refrigerant to drop the temperature thereof, and a plurality of gas-liquid refrigerant separators configured to separate the refrigerant into a gaseous refrigerant and a liquid refrigerant. A gaseous refrigerant and a liquid refrigerant separated by the gas-liquid refrigerant separator disposed at an upstream side are again mixed and supplied to the gas-liquid refrigerant separator disposed at the most downstream among the plurality of gas-liquid refrigerant separators.

Description

TECHNICAL FIELD[0001]The present invention relates to a fuel supply method for a high-pressure natural gas injection engine, and more particularly, to a fuel supply method for a marine structure using a high-pressure natural gas injection engine, for example, a MAN Electronic-Gas Injection (ME-GI) engine, in which boil-off gas (BOG) generated in a liquefied natural gas (LNG) storage tank is compressed to a medium pressure, reliquefied, compressed to a high pressure, gasified, and then supplied to the high-pressure natural gas injection engine.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, such as LNG or LPG, is obtained by cooling ...

AI Technical Summary

Benefits of technology

The present invention provides a fuel supply method for a marine structure using liquefied gas generated from a liquefied gas storage tank. This method compresses and reliquefies the BOG to a high pressure, reducing the energy consumption during liquefaction and reducing the size of the apparatus. The method also uses a nonflammable mixed refrigerant for safer reliquefaction. Additionally, the method ensures that all the liquefied BOG is used as fuel for the high-pressure natural gas injection engine, reducing energy waste and flash gas generation during the process. The liquefaged BOG is compressed and supplied as fuel without significant subcooling, as the increased pressure maintains the overcooled state.

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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