(522d) Design of Cascade Organic Rankine Cycle Utilizing LNG Regasification Process and CO2 Liquefaction Process

After the problems of global warming have been issued, development of environmental-friendly power generation attracted the attention of many researchers. One available technology for imtigating the global warming problem is CCS, which is composed of carbon capture, liquefaction of CO2, transportation and storage. This technology can reduce significant amount of CO2 emission but it has been reported that the CCS technology costs lots of energy so reducing the total energy assumption is neccessary for applying CCS to real plants.

Meanwhile, liquefied natural gas (LNG) is one of the most widely used form of fossil fuel with high levels of energy production and low greenhouse gas emissions. Also, due to the extremely low temperature of LNG, it can produce some additional electricty if there is a organic rankine cycle (ORC) utilizing the physical exergy of LNG. After the LNG imported is transported to regasificatioin termainal, it should be evaporated to be delivered to using site. Here, the regasification process can be utilized as heat sink of organic rankine cycle in order to produce some additional electiricy. Also, if the site of CO2 liquefaction process is placed near by the LNG regasification terminal, the CO2 can be adopted as heat source of the ORC, which uses the LNG cold exergy.   

In this study, a cascade organic Rankine cycle which uses LNG cryogenic exergy was proposed. The proposed system utilizes LNG process as a heat sink so that it can produce additional power as well as obtain natural gas (NG), that is, LNG regasification process is adopted to produce additional power. For producing maximum amount of power, it is designed to be three stage cascade cycle. The first two stages are adopted to recover the LNG cold exergy directly and the third stage is used to recover additional exergy from the first two stages. Each stage uses binary working fluid to fit the temperature glide of LNG evaporation process. The best combination of working fluid is selected through irreversibility minimization by varying flow rate, composition and pressure of working fluids. After selecting the working fluids, process optimization is performed via a parametric study. The heat source is obtained from CO2 from the carbon capture process. Therefore the CO2 liquefaction process which is essential for CO2 capture process is substituted by the proposed process so it can reduce around 50% of CO2 liquefaction energy. An additional heat necessary to vaporize each working fluids is brought from free heat sources such as sea water. As a result, the proposed process produces 151.78 kJ/h kgLNG under a 25℃ heat sources and showed 21.85% second law efficiency without any additional energy injected except for LNG cold exergy.