System and method for treating boil-off gas in ship
In a BOG treatment system, boil-off gas (BOG) discharged from a storage tank is compressed, most of the BOG is used as the fuel of vessel engines, and a remaining part of the BOG is liquefied by cold energy of BOG newly discharged from the storage tank and is returned to the storage tank, thereby efficiently utilizing the BOG. The BOG treatment system for a vessel includes a compressor compressing the BOG discharged from the storage tank; a medium pressure gas engine receiving at least a part of the BOG compressed by the compressor, as fuel; a heat exchanger exchanging heat between the remaining part of the BOG, which is not supplied to the medium pressure gas engine as fuel, and the BOG, which is discharged from the storage tank and is not compressed; and an expander decompressing the remaining part of the BOG cooled by the heat exchanger.
1. A method of processing liquefied natural gas (LNG) in a vessel, the method comprising:
providing a vessel comprising:
an LNG tank containing LNG and boil-off gas (BOG) of the LNG,
a gas fuel engine configured to consume a pressurized gas composition,
a first gas-to-gas pathway configured to process a gas composition comprising BOG to generate the pressurized gas composition for sending to the gas fuel engine, wherein the first gas-to-gas pathway comprises a heat exchanger and a bypass line bypassing the heat exchanger,
a gas-to-liquid pathway configured to liquefy a portion of the pressurized gas composition for returning to the LNG tank,
a second gas-to-gas pathway configured to send a gas phase generated in the gas-to-liquid pathway to the first gas-to-gas pathway, and
a liquid-to-gas pathway configured to process LNG from the LNG tank to supply a vaporized composition to the first gas-to-gas pathway; and
running one or more of the first gas-to-gas pathway, the gas-to-liquid pathway, the second gas-to-gas pathway, and the liquid-to-gas pathway,
wherein, when BOG discharged from the LNG tank exceeds an amount of the pressurized gas composition required at the gas fuel engine, the method runs the first gas-to-gas pathway with the heat exchanger, the gas-to-liquid pathway, and the second gas-to-gas pathway,
wherein running the first gas-to-gas pathway comprises:
discharging BOG from the LNG tank,
heating the gas composition comprising the discharged BOG at the heat exchanger,
pressurizing the heated gas composition at a first multi-stage compressor to generate the pressurized gas composition having a pressure of 5-20 bara, and
supplying part of the pressurized gas composition to the gas fuel engine;
wherein running the gas-to-liquid pathway comprises:
branching off a stream of the pressurized gas composition between the first multi-stage compressor and the gas fuel engine,
further pressurizing the pressurized gas composition at a second multi-stage compressor to generate a supercritical state LNG composition having a pressure of 80-400 bara,
cooling the supercritical state composition at the heat exchanger to generate a cooled composition by heat-exchanging with the gas composition of the first gas-to-gas pathway,
depressurizing the cooled composition at a depressurizer to generate depressurized composition that comprises a liquid-gas mixture,
generating a liquid phase and the gas phase from the liquid-gas mixture at a liquid-gas separator, and
returning the liquid phase to the LNG tank;
wherein running the second gas-to-gas pathway comprises sending the gas phase from the liquid-gas separator to the first gas-to-gas pathway for adding to the discharged BOG to provide the gas composition in the first gas-to-gas pathway between the LNG tank and the heat exchanger such that the gas composition is heated by heat-exchanging with the supercritical state composition at the heat exchanger;
wherein, when the discharged BOG from the LNG tank is smaller than an amount of the pressurized gas composition required at the gas fuel engine, the method does not run the gas-to-liquid pathway, runs the liquid-to-gas pathway, and runs the first gas-to-gas pathway with the bypass line in which the heat exchanger is bypassed via the bypass line such that the gas composition comprising the discharged BOG from the LNG tank bypasses the heat exchanger and is pressurized at the first multi-stage compressor to generate the pressurized gas composition having a pressure of 5-20 bara,
wherein running the liquid-to-gas pathway comprises:
discharging a portion of the LNG from the LNG tank,
vaporizing the discharged portion of the LNG at a vaporizer to generate the vaporized composition, and
sending the vaporized composition to the first gas-to-gas pathway, in which the vaporized composition is added to the discharged BOG to provide the gas composition such that the gas composition comprising the vaporized composition from the liquid-to-gas pathway bypasses the heat exchanger before the first multi-stage compressor in the first gas-to-gas pathway.
2. The method of claim 1 , wherein the first multi-stage compressor comprises a plurality of compressing units that are serially connected, wherein the second multi-stage compressor comprises a plurality of compressing units that are serially connected.
3. The method of claim 1 , wherein the vessel further comprises a third multi-stage compressor, wherein the first and third multi-stage compressors are arranged in parallel in the first gas-to-gas pathway for redundancy.
4. The method of claim 1 , wherein the vessel does not comprise a refrigeration cycle of a nitrogen coolant for cooling the supercritical state LNG in the gas-to-liquid pathway.