A compression-free re-liquefication process of LNG boil-off gas using LNG cold energy

Re-liquefying boil-off gas (BOG) of LNG is of great significance for energy conservation and environmental protection. BOG is usually compressed before being liquefied with nitrogen reverse Brayton cycle, which makes the system's capital investment and energy consumption high. A novel compression-free re-liquefication process of BOG using liquid nitrogen (LN2) is proposed. BOG is liquefied with latent heat of LN2, reducing the heat-transfer temperature difference in BOG condenser and the required amount of refrigerant. LN2 is produced using LNG pre-cooled Joule-Thomson cycle. Overall optimization is performed using genetic algorithm. Thermodynamic, economic, and environmental assessments are conducted based on the optimized results. Sensitivity analysis of the principal design parameters is performed. It is found that the proposed process is significantly profitable because it fully utilizes the cold energy of LNG and LN2 and eliminates the required cost for BOG compressor, turbine expander, and LNG regasification process. The minimum specific energy consumption of 0.74 kWh·kg−1 is achieved, and the figure of merit is up to 34.95%. The payback period of total capital investment is only one month. Carbon dioxide emission is reduced by 36242.66 kg·h−1 compared to the direct emission of BOG. Pinch temperature difference of 3∼5 °C is recommended for cryogenic heat exchangers.