Simulation and Analysis of the Combined Power Process of LNG Cold Energy Recovery for Electricity Production

Abstract

Abstract:The model of LNG cold energy recovery consisting of a Brayton cycle and a Rankine cycle is constructed in Aspen Plus. The effects of the inlet and outlet pressure of the turbines, ammonia-water flow rate, condensing temperature and the compressor outlet pressure to the thermal efficiency of the combined power cycle have all been performed sensitivity analysis. The result shows that the thermal efficiency increases with the turbines inlet pressure and the compressor outlet pressure. Meanwhile, decreasing the ammonia-water flow rate and condensing temperature can also achieve such effects. InstructionWith economy rapid development, natural gas [1] will be mainstream energy all over the world. In China, the storage of natural gas (NG) is abundant, but the origin is remote. The NG being liquefied is a convenient way to transport to other places. On the terminal to receive liquefied natural gas (LNG), LNG is pressed by pump, vaporized to gas in heaters, and delivered to users in the end. LNG contains so much cold energy to be recycled during vaporization, and the energy from LNG releases can reach to 830~890kJ/kg. However, when it is regarded as fuel, the LNG cold energy recovery can be used to generate electricity [2] , air separation [3] , cryogenic grinding [4] and light hydrocarbon separation [5] etc. Making most use of LNG cold energy can't only save energy but also bring considerable economic benefits.Many scholars have already researched on the combined power cycle. T. Miyazaki and Y.T. Kang [6] et al. have proposed a power cycle by combining waste heat with LNG cold energy and analyzed how ammonia water concentration, the inlet and outlet pressure of turbines and coefficient of heat transfer of condenser influence the combined power cycle. Wang and Li [7] et al. have a new study of the power cycle to recovery LNG cold energy with a low heat source. The result shows that the thermal and exergy efficiency increase as the heat source temperature and the turbine inlet pressure increases and condensing temperature decreases. Manuel Romero Gómez et al. [8] have presented a novel power cycle consisting of a combination of a closed Brayton cycle with a stream Rankine cycle, and analyzed the effects of some key parameters including the temperature at the compressor inlet, the compression ratio, the temperature at the Brayton cycle turbine and the LNG pressure to the cycle performance.In this paper, the model of the combined power cycle of LNG cold energy recovery is constructed in Aspen Plus. How the turbine inlet and outlet pressure, the compressor outlet pressure, ammonia water flow rate and condensing temperature influence the cycle energy efficiency are made sensibility analysis so that the evidence of improving the cycle efficiency can be presented. The Model of the Combined Power Cycle