Exergy recovery during LNG gasification using ambient air as heat source
Résumé
The market of liquefied natural gas (LNG) is growing and presents the advantage of diversifying supplies. The LNG is fed at a temperature of -162°C and at atmospheric pressure. This study aims at the recovery of the LNG exergy during its gasification and heating before being delivered trough the network. An energy and exergy based analysis is performed for the definition of engine-cycle architectures using the ambient air as a heat source through an intermediate fluid, and LNG as a cold sink. The use of ambient air led to manage frosting problem. The intermediate two phase fluid with a controlled evaporating pressure, recovering heat from the air, is used to avoid the direct contact of humid air with very cold heat exchanger surface in order to limit the frost formation. In this paper, the challenge is to find the best engine-cycle architectures with minimized overall exergy destruction, moving up the efficiency of the entire system. Different systems including Organic Rankine cycles (ORCs) and Brayton cycles, operating as simple or cascade cycles are investigated. Pure working fluid, as well as mixtures of hydrocarbons are considered.