LNG-FSRU cold energy recovery regasification using a zeotropic mixture of ethane and propane

Abstract

This study developed a cold energy recovery regasification system for recovering and utilizing waste cold energy from liquefied natural gas floating storage regasification units, and analyzed its thermal, exergy, and economic efficiencies using a zeotropic mixture of ethane and propane. The single-stage version exhibited the highest net output, thermal efficiency, and exergy efficiency for a 6:4 ethane/propane mixture. The existing method (using only propane as working fluid) exhibited a thermal efficiency of 3.5%, exergy efficiency of 5.9% at 25 °C, and thermal efficiency of 3.8%. The exergy efficiency was 6.2% because the exergy loss was reduced by 300 MJ/h compared to that for the conventional method. The highest thermal and exergy efficiencies (6.1% and 10.9%) in the two-stage version were obtained for an 8:2 ethane/propane mixture. The thermal efficiency was 6.6% and exergy loss was 16,300 MJ/h compared to the existing method, showing a 10.9% improvement. Thermal and exergy efficiencies of the two-stage version were higher than those for the one-stage system by 2.7% and 4.7%, respectively, providing an annual net income of USD 3.60 million and reduced electricity production costs by 0.0021 USD/kWh. The system could reduce exergy loss and electricity production costs while increasing the annual net income.