Novel integrated system of LNG regasification / electricity generation based on a cascaded two-stage Rankine cycle, with ternary mixtures as working fluids and seawater as hot utility

Abstract

This paper presents a novel compact system of liquefied natural gas (LNG) regasification/electricity generation, with a two-stage cascade Rankine cycle, using the ternary mixtures methane-ethylene-propane (CH4–C2H4–C3H8) and ethane-propane-butane (C2H6–C3H8–C4H10) in the first and second stages, respectively, with seawater as the hot utility and LNG as the heat sink. The system was simulated using the Aspen Plus® simulator, considering not only power generation subsystem with the recovery of the LNG cold energy, but also the conventional LNG pumping and boil-off gas (BOG) recovery facilities, as commonly implemented in a European standard LNG terminal – with natural gas send out specifications compliant with a high pressure transmission network. After optimization, the system achieves a thermal efficiency of 23.7% and an exergy efficiency of 79.18%. Performance was found to increase when compared to a conventional system with a simple Rankine cycle, with propane as working fluid, or a two-stage cascade Rankine cycle with ethane/propane as working fluids. The economic assessment also favours the novel system over the conventional systems. The improvement obtained comes from an adequate working fluid selection, instead of a highly complex design or from the availability of high-grade hot utilities.