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Technology

BOG Re-liquefaction System

Because LNG is handled at the ultra-low-temperature of −160°C, partial gastification due to natural heat input from the outside cannot be avoided in LNG facilities. This gas is called boil-off gas (BOG). The pressure of BOG is boosted by compressors in receiving terminals, and sent along with LNG gasified by LNG vaporizers to city gas companies and the fuel systems of power plants.
As one means of processing the gas, a BOG re-liquefaction system was established with the use of cold LNG.

The following advantages can be obtained through application of the BOG re-liquefaction system to receiving terminals:

  1. Controlling the concentration of stored LNG by returning reliquefacted BOG to LNG tanks
  2. Energy saving through the reduction of the input of compressors

The re-liquefaction system is intended to cool and liquefy BOG through heat exchange between BOG pressurized to 0.5 to 1 MPa, and LNG supplied to a vaporizer. The LNG that was used as a refrigerant causing its temperature to rise is sent to a vaporizer. Liquefied BOG is sent back to an LNG tank, or to a vaporizer along with the LNG used as a refrigerant after being pressurized with a pump according to the above-said objectives.

General BOG re-liquefaction system

BOG re-liquefaction system (with hot gas bypass control)

  1. An LNG receiving terminal with a higher gas outlet pressure has greater energy savings.
  2. The system is designed to re-liquefy BOG without interfering with the mission of the LNG receiving terminal, the reliability of a stable gas supply, because there is no limitation on the amount of LNG sent to an LNG vaporizer at the time of operating or stopping BOG re-liquefaction facilities, or at the time of load changes.
  3. BOG can be re-liquefied at a stable pressure by constantly controlling the balance between the flow of BOG and the flow of refrigerant LNG.
  4. A system for re-liquefying BOG at a stable pressure can be provided by integrating a function for controlling hot gas bypasses with the BOG re-liquefaction system even when the balance between the flow of BOG and the flow of refrigerant LNG is not controlled.