Waste Energy Recovery – Including Pressure and Thermal Energy – From LNG Regasification

Abstract

The world has been concentrating on waste heat recovery for several decades, with the vast majority of achievements making a significant contribution to the development of industry and economy. The attention has recently been paid to waste energy from cold streams. This work focuses on the recovery of waste cold energy released from Liquefied Natural Gas (LNG) regasification process, including pressure and thermal energy. A direct expansion configuration involving different steps of expansion and mass flow rate extraction at intermediate pressure levels is adopted in the mathematical models for pressure and cold energy recovery. An equation of state for methane (the main component of LNG) is employed to calculate LNG thermodynamic properties in a long-range phase transition of the regasification process. A direct configuration of the organic Rankine cycle (ORC) is employed to recover waste cold energy. The modified Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) equations of state (EOS) play important roles in calculation of thermodynamic properties of the organic working fluids used in the ORC. Several fluids are examined to select the most suitable one as the ORC working fluid. All the models are developed and solved by using MATLAB. By adopting propane as the working fluid in the ORC, the multi-stage expansion along with thermal energy extraction is shown to be the most thermodynamically efficient configuration with the recovery of up to 170 kJ pressure energy for each kg of the flowing LNG, and up to 74 kJ from thermal energy recovery.