Advanced design and analysis of BOG treatment process in LNG fueled ship combined with cold energy utilization from LNG gasification

Abstract

The use of re-gasified liquefied natural gas (LNG) and boil-off gas (BOG) as a fuel in LNG fueled ships generally results in a waste of cold energy. Meanwhile, when the ship is docked, BOG should be re-liquefied to minimize the LNG loss using external energy. To efficiently utilize wasted cold energy, a novel BOG treatment concept combined with air liquefaction is proposed and a new process for recovering cold energy of BOG and fueled LNG is designed and analyzed in this paper. The system is divided into three sections: the BOG and LNG regasification into the fuel gas supply system (FGSS), liquid air energy storage and liquid air energy release. The proposed system is analyzed in three ways: energy analysis by the first law of thermodynamics, heat exchanging analysis by the heat flow, and exergy analysis by the second law of thermodynamics. The energy efficiency of the BOG and LNG gasification into the FGSS, air storage and air release sections are 34.2%, 93.4%, and 52.3%, respectively. The overall heat exchange efficiency is 42.28% and the exergy efficiency is 30.4%. The economic analysis of the system is investigated on the lifecycle of 20 years. This study not only combines the BOG treatment process and the FGSS of the LNG ship but also reduces the energy consumption of the system onboard.