Concept and evaluation of energy-efficient boil-off gas reliquefiers in LNG carrier ships propelled by dual-fuel engines

Abstract

Ships carrying liquefied natural gas (LNG) are generally propelled by dual-fuel enginesthat use boil-off gas (BOG) or diesel as fuel. Fuelling the engine with pumped and vaporized LNG is operationally more convenient than compressing BOG. BOG is usually reliquefied using reverse Brayton cycle (RBC) or Claude cycles. In this paper, both these cycles are reconfigured to extract the cold of pumped LNG to reduce power consumption. Steady-state simulations are done on process simulator Aspen HYSYS V8.6®. Capturing the cold from the LNG fuel improves the exergy efficiency of RBC and Claude reliquefiers from 37.8% to 57.5% and 48.5% to 75.6% respectively. The surface area of heat exchangers in the Claude system is one-third that of RBC. Mass flow through compressor and turbine in Claude system is one-fifth and one-ninth of RBC, while volume flow at turbine inlet is only one-fourteenth in the Claude system.The evaluation shows that all equipment needs to be designed with sufficientoperational flexibility to enable the system toextract cold effectively from the LNG fuel supply because it varies widely during the voyage. Claude system with cold extraction from LNG fuel is highly compact and the most efficient reliquefier designed so far.