Exergy Recovery During Liquefied Natural Gas Regasification Using Methane as Working Fluid

The increased concerns about the effect of human activities on the climate have pushed natural gas among the most obvious solutions for the transition to a low-carbon economy. The growing importance and volumes of liquefied natural gas for transportation over long distances come as a consequence of this tendency. The liquefaction of natural gas requires a high amount of energy that can be recovered during the re-gasification phase. In this paper, a novel approach for this purpose is presented, where the main feature is the use of a combination of Rankine and Brayton cycles while retaining natural gas as the only working fluid of the system. The proposed system is optimized for cost and exergy efficiency using a bi-level multi-objective optimization procedure, where the master level is setup as a nonlinear optimization problem and solved using an evolutionary algorithm, while the slave level as a mixed integer-linear programming problem. The results of the optimization show that such system can potentially achieve high efficiencies (up to 60 % exergy efficiency for the power cycle and above 65 % plant thermal efficiency), at the cost of a significant capital investment for the heat exchanger network. By allowing a lower level of integration in the system a profitability of up to 98 kUSD/y can be achieved, while retaining significantly high performance.