World Scale Boil-off Gas Reliquefaction

Boil-off gas reliquefaction has become an industry standard for many LNG carriers, which no longer use gas as fuel for steam boilers, but tap others energy sources to generate power for propulsion. Well known examples are the ships of the Q-Flex and Q-Max class. Most of these vessels use simple nitrogen expander cycles with relatively high energy consumption (typically 800 kWh/t) and a moderate capacity of about 200 t/d LNG liquefaction.

Increasing LNG demand in Asia, especially in Japan after the nuclear catastrophe triggered innovative ideas to increase the LNG output of existing liquefaction plants. Boil-off gases and ship return gases from LNG tank farms have been identified quickly as promising source for additional LNG, as these gases are perfectly suitable for cryogenic processing. Obviously, dehydration and sweetening is not required. However, low pressure, low temperature and elevated nitrogen content pose challenges to an efficient design.

Linde Engineering has developed an innovative concept for a world scale boil-off gas reliquefaction unit with 0.5 to 1.0 mtpa capacity, which includes the following features:

• Boil-off gas compression starting from cryogenic conditions up to a transcritical pressure level
• Mixed refrigerant cycle with two heat exchanger bundles
• Load balancing between feed gas and refrigerant cycle compressors
• Nitrogen rejection into the gas turbine fuel gas with a double flash process
• Use of identical gas turbines as mechanical drive

Details of this concept and rejected alternatives will be discussed in depth in the full paper.