During LNG storing and transporting, heat from surrounding will cause LNG temperature to rise. The rise of temperature will lead to boil-off gas (BOG) generation. Boil-off gas produced in storage tank should be remove to maintain tank pressure at safe level.
BOG can be handled by several methods.
BOG Utilization as Fuel
In previous project, we use this option to utilize boil-off gas. Boil-off gas from atmospheric storage tank is compressed by BOG compressor. Then its temperature is increased by using small natural draft ambient air vaporizer before being fed to gas engine.
Although it is quite promising method to handle BOG, not all BOG can be utilized as fuel. Therefore, BOG utilization as fuel is secondary method to handle BOG.
Compressing BOG to Gas Pipeline
This method is suitable when supply of gas to customer is continuous. Gas from LNG storage tank is compressed until it met pressure requirement in pipeline. Its temperature is increased in vaporizer.
Boil-off gas can be recondensed so that it is converted into LNG. BOG recondensing facility consists of LNG low pressure pump, BOG recondenser package complete with recondensed BOG buffer tank, LNG high pressure pump, and BOG compressor. Boil-off gas is compressed by using BOG compressor. It is then contacted with LNG coming from LNG LP pump. Contact between BOG and LNG occurs at BOG recondenser. Flow of LNG to recondenser is adjusted to ensure that all BOG is recondensed. Recondensed BOG will be stored in storing section at BOG recondenser. After that, LNG and recondensed BOG is pumped by LNG HP Pump.
Boil-off Gas Reliquefaction
The function of BOG reliquefaction is to cool and liquefy BOG through heat exchange. LNG then is supplied to vaporized or stored back to LNG storage tank. As I know, there are at least two methods of boil-off gas reliquefaction.
BOG Reliquefaction using LNG as Refrigerant
In this method, BOG pressure is boosted by BOG compressor and then contacted with LNG in heat exchanger. LNG is used as refrigerant.
BOG Reliquefaction using Nitrogen as Refrigerant
The facility consists of heat exchanger (cold box), expander-compressor, and BOG compressor. Figure below shows simplified process flow diagram applied for BOG reliquefaction for LNG carrier.