Improvement Of Rankine Circulation System Using LNG Cold Energy

Natural Gas occupies a decisive position in the world’s primary energy structure at present, and has an increasing proportion year by year in the world’s primary energy structure. China’s natural gas market grows with high investment. Pipeline transporting accounts for the majority in natural gas land transportation, and LNG shipping is the only way of natural gas marine transportation. LNG which is transported by ship is received and stored in LNG terminal. LNG is gasificated then transported to natural gas users. Because of the low temperature storage processes, LNG can release much cold energy in gasification process. The cold energy can be recovered by Rankine Cycle.In this paper, working fluids of Rankine Cycle are further studied. Principle of working fluids selection in different Rankine Cycle is determined. Model of utilization of LNG cold energy is built by means of study of LNG cold energy, LNG exergy and LNG gasification characteristics. The Cycle optimizes the process of simple Rankine Cycle according to the Model. Advantages of the new process compared to the simple process are analyzed by means of exergy recovery rate, cold energy utilization and power generation. Three parts are studied in this paper.(1) Scope of application of common working fluids are studied, taking simple Rankine Cycle Process as example. The effect of working fluids condensing pressure, condensing temperature and critical temperature on cycle process are studied to determine the principle of working fluids selection. Working fluids condensing pressure should be micro-positive pressure. Working fluids condensing temperature should be Δt℃higher than LNG gasification temperature. Temperature cross must be avoided. Working fluids dryness In the steam turbine export should be greater than0.88. Working fluids critical temperature should be higher than the heat source temperature in the subcritical circulating. And working fluids critical temperature should be lower than the heat source temperature in the supercritical circulating.(2) The direction of optimization process is determined by study of LNG cold energy, LNG exergy and LNG gasification characteristics. LNG gasification curve is divided into a few areas by LNG bubble point curves, transition line in two phase flow region and LNG Dew point curve. LNG cold energy in every area is recovered respectively. The new Rankine Cycle Process Model of subcritical pressure is built.(3)The new Rankine Cycle Process is built according to the model, including the subcritical pressure and supercritical pressure. The effect of the heat source temperature variation on exergy recovery rate, cold energy utilization and power generation is analyzed. The heat source temperature rise can improve exergy recovery rate, cold energy utilization and power generation. Exergy recovery rate, cold energy utilization and power generation are studied taking gasification pressure of2MPa as an example. The new process can improve exergy recovery rate, cold energy utilization and power generation. The new process is much better than simple process.