Design and analysis of boil-off gas reliquefaction processes for the LNG-fueled ships

Abstract

• Three boil-off gas reliquefaction processes for LNG-fueled ships were proposed. • The boil-off gas self-expansion refrigeration cycle is used for the precooling. • Performance of each case was simulated and analyzed. • The process with all boil-off gas expanded for precooling is the best choice. An onboard reliquefaction process is necessary and the most common way to deal with boil-off gas (BOG) in LNG-fueled ships. Due to the additional restrictions, this kind of process must consider both the energy efficiency and the compactness. Based on the reverse Brayton process with N 2 as refrigerant, three new BOG reliquefaction processes are proposed in this paper by replacing the external precooling cycle with BOG self-expansion precooling cycle. The performance in terms of specific energy consumption (SEC), exergy efficiency and the size of the main heat exchanger (MCHE) of each process are simulated and optimized with the software Aspen HYSYS. According to the results, the process of case 1 is the best choice for the LNG-fueled ship, in which all BOG enters the expander for self-expansion precooling. The SEC, exergy efficiency and the size of MCHE of case 1 are 1.11 kWh/kg LNG , 24.88% and 191.9 kW, respectively, when the composition of feed BOG involves 95.5% C1, 1% C2 and 3.5 % N 2 at a temperature −80 °C and a pressure 113 kPa. Compared to the processes with external CO 2 or NH 3 precooling cycle, the SEC has been reduced by 15.2% and 14.6%, respectively, and the size of MCHE has been significantly reduced. A sensitivity analysis of case 1 process was also carried out and verified that it can be flexibly adapted to the change of BOG composition and temperature.