Ejector design for compressorless BOG utilization system for a LNG fueled ship

In general LNG fueled ships, boil-off gas (BOG) produced by heat leakage into the storage tank is directly utilized as fuel for engines and generators by transport using compressors, which have many moving parts making the maintenance difficult, and require much power consumption for pressurizing the BOG. This paper proposes a new BOG utilization system with a compressorless configuration adopting an ejector and a static mixer. In this system, the BOG is transported and liquefied by the ejector and the static mixer, respectively, and then temporally stored before fueling. This paper also introduces an innovative optimization for ejector, which is a key component and should sufficiently entrain the BOG with less motive fuel gas. A preliminary thermodynamic analysis was performed to identify the effect of the ejector entrainment ratio on the effectiveness of BOG liquefaction. Considering system configurations, two ejectors depending on the motive pressure were initially designed by applying a theoretical method. A new optimization method with a modification procedure including maximizing the entrainment ratio in relation to key parameters and resizing to satisfy the requirements was applied to the initial ejector designs with validated CFD methodology. After the modification procedure, the entrainment ratios were enhanced up to 101% compared to before. The results showed that the designed ejectors achieved the liquefaction effectiveness of 10.3, which is close to that of a commercial one. Additionally, the performance characteristics for various off-design BOG conditions based on the designed ejector were also investigated.