Utilization of engine exhaust gas in boil-off gas reliquefaction system of a LNG Carrier ship.

Boil-off gas (BOG) generated due to heat in-leak into the storage tanks of LNG carrier ships is recondensed using refrigeration devices. Reverse Brayton cycle (RBC) with nitrogen as refrigerant or cascade system with propylene and ethylene as refrigerants are used for BOG reliquefaction. In order to reduce power consumption of reliquefaction process, vapour absorption refrigeration system (VARS) that utilizes engine exhaust gas of LNG carrier ship may be a suitable choice as a precooler for BOG reliquefaction process. In this paper, onboard reliquefaction systems are coupled with VARS and their performances are evaluated using exergy. Steady state simulations with the aid of Aspen Hysys 8.6® show that when RBC based reliquefaction system is precooled with VARS, exergy efficiency improves by 5.7%. However, precooling by VARS in cascade system improves efficiency by 22.8% as cascade system has a higher ability to absorb refrigeration supplied by VARS resulting in higher reliquefaction.