Research On LNG Regasification Cold Energy Comprehensive Utilization System On FSRU Platform

Floating storage and regasification unit(FSRU)is a new receiving terminal of liquefied natural gas(LNG)on the sea.In recent years,FSRU has developed rapidly in the marine transportation industry.LNG will produce huge cold energy during gasification process.If this part cold energy is not recycled,it will not only cause waste of cold energy,but also cause cold pollution to the ocean.After consulting a large number of domestic and foreign documents,it is found that the high recovery efficiency can be achieved by cascade utilization of the cold energy released from the LNG gasification process.In view of FSRU's operating environment,this paper studies and analyzes the low-temperature Rankine cycle power generation technology using LNG cold energy,and further use of low-grade cold energy after LNG power generation for desalination.In the process of storage and transportation,FSRU will produce more boil off gas(BOG),which needs to be treated and recovered.Therefore,a LNG cold energy comprehensive utilization system is constructed,which combines low temperature Rankine cycle power generation,desalination and BOG treatment.The specific research and analysis results are as follows:Aiming at the incompleteness of LNG cold energy recovery in simple power generation system,a set of cold energy utilization system combining power generation and seawater desalination is constructed.Six schemes are selected from common refrigerants which conform to the LNG cold energy cascade utilization curve.The influences of different schemes of refrigerants combination on the system net output power and turbine dryness at all levels are analyzed.It is found that R1150,R23,R290 and R600 a working medium combination scheme can make the net output power of the system reach the maximum value.For the BOG problem generated in the process of LNG storage and transportation,the BOG direct compression LNG cold energy utilization system and BOG recondensation LNG cold energy utilization system are proposed respectively.The simulation results show that the total exergy loss of BOG recondensation system is 12704 kw,which is 6.08% less than that of BOG direct compression system,the net output power of the system is 4961 kw,the system exergy efficiency is 28.08%,which is 8.43% and 11.1% higher than that of BOG direct compression system.It seems that the advantages of BOG recondensation LNG cold energy utilization system are more obvious.This paper analyzes and discusses the problem of excessive exergy loss of LNG heat exchanger 1,LNG heat exchanger 3 and working medium heat exchanger 3 in the LNG cold energy comprehensive utilization system under pure working medium.By analyzing the heat transfer curves of the three heat exchangers,it is found that the heat transfer curves of the cold and heat flow do not match.It is proposed to use mixed working medium to optimize the analysis,after analyzing common working fluids,the mixed working fluid of the first level working fluid is R14,R170 and R600 a are the mixed working medium of the third level working medium.On the premise of ensuring the normal operation of the system,eight mixed working medium schemes are obtained.Through the comparative analysis of the influence of the eight mixed working medium schemes on the system parameters,such as the saturation pressure of working medium,the gas phase fraction,the net output work of the system,the desalination amount,the exergy efficiency of the system and the economic benefits of the system,the best combination of working fluids in the three-stage circulation system is selected as follows: first-stage RA1(R1150: R14 = 9: 1),second-stage R23,and third-stage RC1(R290: R170 = 9: 1).After the optimization of working medium,the system parameters are further analyzed,It is found that the parameters that have a great influence on the system are respectively the condensation pressure of the three-stage Rankine cycle,the LNG temperature at the outlet of LNG heat exchanger 4 and the working medium temperature at the outlet of working medium heat exchanger 4.Aiming at these parameters,the effects on the net output power,exergy efficiency,desalination capacity and economic benefits of the system are simulated and analyzed respectively,and the parameters are optimized by genetic algorithm to maximize the system exergy efficiency.Taking the maximum system exergy efficiency as the objective function,the system net output power after parameter optimization is 5376 kw,which is 6.1% higher than before optimization,the system efficiency is 31.37%,6.12% higher than before optimization,the system fresh water output is 177 t / h,28.2% higher than before optimization,the system economic benefit is 4138 yuan / h,which is 4.02% higher than before optimization.In addition,according to the inquiry of China's domestic market,the estimated total investment price of the initial input equipment of the system is 37 million yuan,the annual economic benefit of the system is 30.31 million yuan,and the cost recovery cycle is 15 months.