(398g) Static Boil-Off Rate (BOR) Implication on Boil-Off Gas (BOG) in an LNG Regasification Terminal

Liquefied natural gas (LNG) is stored in the tank at its bubble point and any heat leak boils the natural gas (NG) to form boil off gas (BOG). BOG generation undone the compression work required for NG liquefaction. Thus, keeping the BOG to minimum increases the LNG value chain economics. Though BOG depends on the amount of heat leak, assessing it from the LNG storage tank is often based on the static value of BOR (Boil off rate), suggested by the tank vendors. Static BOR value depends on the worst case heat leak in the tank and always over predict the BOG generation because it assigns all the heat leak into the tank for BOG generation disregarding the heat leak to vapor that causes vapor expansion. This study investigates the impact of static BOR on BOG over prediction at different liquid levels in the tank. The analysis revealed that BOR varies from 0.012wt% per day at 80% tank fill to 0.12wt% per day at 10% tank fill. This means that for a 90m high tank, every 1meter decrease in LNG level over predicted BOR by 0.004%. Thus, instead of using static value of BOR for BOR predictions a variable value of level dependent BOR give more accurate predictions for BOG estimation and must be adopted.