Gas Material, Processing and Power Technologies
Abstract
@Osaka Gas Co., Ltd. developed a highly efficient system for sending out boil-off gas (BOG) generated in LNG tanks. This system utilizes LNG cold energy to liquefy BOG, and can save the electric power required to send out BOG by 30-60% compared with conventional system. Cold energy storage (CES) technology is adopted in the system for stable BOG liquefaction despite fluctuating LNG cold energy.
Comparison of BOG send-out system
- Conventional send-out system
At LNG receiving terminals, BOG is compressed to send-out pressure (2 to 7.5 MPa) by a high-pressure compressor in order to maintain a tank pressure of several kPa, and is sent out along with gas vaporized by an LNG vaporizer. In this method, the compressor requires a large amount of electric power in order to raise BOG to send-out pressure. - BOG liquefaction system using cold energy storage
In the newly developed system, BOG compressed to about 0.9MPa by a low-pressure compressor is pre-cooled with S&T or plate-fin type heat exchangers, and is mixed with LNG. Then BOG and LNG mixture is passed through CES vessel and gas-liquid separator, and is sent to LNG vaporizer.
Since the pressure of the liquefied BOG is raised to send-out pressure by the secondary pump, this system can save the electric power of BOG compressor.
Furthermore, a constant volume of BOG can be liquefied under fluctuations of LNG flow rate caused by daily gas demand using CES vessel.
Cold energy storage vessel
@Since more than the amount of LNG required to liquefy BOG can be obtained
during the day when the demand is high, BOG is completely liquefied by
mixing with LNG and some cold energy is left over. In CES vessel, this
surplus cold energy is stored by freezing the phase change materials (PCM).
On the other hand, since there is insufficient cold energy for liquefying
BOG at night because of the low demand, BOG is not liquefied and inlet
fluid of CES vessel becomes two-phase flow. However, this fluid is completely
liquefied by melting PCM.
Benefits
- Comparison of electricity in BOG send-out
The table below shows the amount of electricity required to send out 1ton of BOG. With conventional high-pressure compression, the amount of electricity required for compressor increases as send-out pressure becomes higher. With BOG liquefaction, on the other hand, the amount of electricity required for send-out changes very little, because the pressure of the liquefied BOG is raised with LNG pump.BOG send-out method Send-out pressure (MPa) Electric power for BOG send-out (kWh/t-BOG) High-pressure compression 2.0 146 7.5 227 BOG liquefaction 2`7.5 98 - Effect of electricity savings by liquefaction
(BOG liquefying at 15t/h)
The table below shows the effect of electricity saving with this system. As send-out pressure from receiving terminals will continue to increase with growing demand for natural gas in the future, this system is suited for BOG send-out facilities at receiving terminals.Send-out pressure Reduction of electricity 2 MPa About 800 kW (About 7 million kWh/year) 7.5 MPa About 2,050 kW (About 18 million kWh/year)
Overview of plantiSenboku LNG terminalj
