The Theoretical Study Of Working Fluids For Rankine Cycle Utilizing Cold Energy Of LNG

Natural gas is a clean and efficient energy resource and will take an important role in the 21th century energy structure. LNG (Liquefied natural gas) is occupying important position in the energy supply and consume field for it has high-quality energy and good properties of transportation and storage. LNG releases great amount of cryogenic exergy during its evaporation. The rational use of this cold energy will bring about great economic and environmental benefits. This paper The paper focus on how to use the Rankine cycle to recycle the cold energy of LNG effectively and the main contents of this paper include:(1) The main public documentations and papers about the LNG utilization for power generation were consulted and studied, making a comparison about different utilization schemes of LNG cold energy.(2) The LNG physical exergy calculation and related factors were theorized. The LNG evaporation process and its heat exchange properties as well as the exergy loss were studied. The results showed that the evaporation pressure and methane mole fraction played a main role in the process of LNG physical exergy release. Depending on the different LNG evaporation pressure, several forms of Rankine cycle were established to recycle the LNG physical exergy, seeking proper temperature matching in heat exchanger and decrease of the exergy loss.(3) Screening principles of applicable working fluids were analyzed and some working fluids were selected. Cycles using different working fluids were simulated with the process simulator HYSYS, and the result showed that the performance of propane was better than the others. (4) Calculation showed that the system exergy loss mainly occurred in the heat exchanger and it was the mismatch of heat-transfer temperature between hot and cold fluids that lead to the result. It was suggested in this paper to use mixed to improve the performance of the system cycle. Theoretical calculations show that the cycling performance did not improve although the use of mixture can make a better match of the temperature within the heat transfer process in the heat exchanger.