Simulation of boil-off gas recovery and fuel gas optimization for increasing liquefied natural gas production

Abstract

In a natural gas liquefaction plant, boil-off gas (BOG) is generated in both liquefaction and loading processes. During the holding mode (no loading operation), the fuel gas supply, including the BOG generated in the liquefaction process, is less than the fuel gas demand. Therefore, make-up gas is added to the fuel gas system. BOG recovery systems are designed to absorb the maximum amount of BOG during liquefied natural gas (LNG) loading to ships (loading mode). Additional BOG is generated during the loading process (loading mode). At this stage, the fuel gas supply is higher than the demand. To avoid fuel gas flaring, BOG from the liquefaction process is reduced by cooling the LNG from the main heat exchanger (MHE) to a lower temperature, which consequently reduces LNG production by more than 1%. This study aimed to optimize liquefaction, storage, loading, fuel gas and BOG recovery processes to increase LNG production. In this study, a natural gas liquefaction plant producing 8 mtpa (million tonnes/ year), where LNG production was limited by the capacity of the BOG recovery system and fuel gas demand, was modelled using UniSim software. In the model, the optimization of BOG recovery during the holding mode increased LNG production by 46,850 tonnes/ year. The volumetric flow rate of BOG generated during LNG loading was reduced by decreasing the temperature of the ship’s cargo tanks, decreasing the loading pumps’ discharge pressure, reducing the discharge pressure of the ship’s compressors, and reducing the loading rate. This study shows that BOG generated during loading can be significantly reduced by around 50% from the initial rate, which increases LNG production by around 36,576 tonnes/ year. In total, the potential production increase from BOG recovery and fuel gas optimization was around 90,260 tonnes/ year or equivalent to 1.4 cargo of LNG per year.