TOKYO GAS TOP > Technical development > For the stable production of city gas > LNG-BOG Reliquefaction Technology

LNG-BOG Reliquefaction Technology

Objective


In order to use the boil-off gas (BOG) resulting from the entry of heat into LNG as feedstock for making city gas, its pressure must be raised to at least the sendout pressure of city gas. As the sendout pressure rises, so too does the required power and cost. Tokyo Gas has therefore developed a low-cost BOG pressurization process and is operating this process.

 

Present Status


A study of processes for producing high-pressure city gas by pressurizing and then adjusting the calorific value of BOG showed the greatest cost reduction to be achievable by reliquefying BOG using the cold energy of LNG, pressurizing it in a liquid state, and then adjusting its calorific value and vaporizing it. This process was adopted at the Ohgishima LNG Terminal and Hitachi LNG Terminal, where it has contributed to further reduction of the cost of producing city gas. This technology won the Japan Institute of Energy Prize in 1999.

 

How it works


The newly developed process significantly reduces the cost of pressurization by performing the bulk of pressurization in a liquid state and using the thermal energy of seawater transferred to the liquid in the vaporizer for pressurization. The pressurization cost reduction generated by this process increases as reliquefaction pressure falls.
 
  • BOG generated pressurized to reliquefaction pressure (around 0.5 MPa) using a compressor
  • Cooling and reliquefaction of pressurized BOG by LNG using a pre-chilling heat exchanger and plate-fin type heat exchanger
  • Pressurization of reliquefied BOG to LNG sendout pressure by pressurization pump
  • Reliquefied BOG with LNG for cooling are fed to open rack vaporizer (ORV) after adjustment of calorific value
  • Mix of LNG and reliquefied BOG sent to the vaporizer is warmed and vaporized by seawater to produce high-pressure city gas
 

BOG reliquefaction facilities and process

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