| As a new type of offshore LNG receiving terminal,LNG-FSRU has developed rapidly in the whole world due to its outstanding advantages over the traditional onshore LNG terminal.LNG will release a huge amount of cold energy when it is gasified under IFV gasification system in FSRU.If this part of cold energy can be used reasonably and efficiently,considered from the energy cascade angle,it is possible to make the FSRU a kind of offshore energy station that integrates natural outings and cold energy.It is found that to use the high grade cold which comes from the process of LNG gasification for power generation is currently one of the focus in the study of international scholars,during consulting and analyzing a large number of literature in domestic and overseas,and also combining with the characteristics of LNG gasification and FSRU characteristics.At the same time,the low grade cold energy in the process of gasification can be used in seawater desalination,which is one of the methods suitable for cold energy utilization for FSRU.With the object of IFV gasification system on the offshore LNG-FSRU,we researched and analyzed the LNG cold energy seawater desalination technology by the technical means of low temperature Rankie cycle and LNG cold energy seawater desalination technology by the use of direct contact with the refrigerant freezing desalination.Finally,according to the requirement of BOG gas treatment on FSRU,a comprehensive LNG cold energy utilization system is constructed which combined with power generation,seawater desalination and BOG treatment.The research and analysis results are as follows:This paper constructs a three-stage hybrid Rankine cycle power generation system using LNG cold energy.It was found that the optimal working quality of the three-stage hybrid system was R1150,R23 and R290 after performing the working medium matching and screening for the system.From the aspects of the system key equipment exergy efficiency,system exergy efficiency and system net output power,etc.,it is found that the main equipment in the hybrid three-stage system is used more reasonably in the temperature cascade configuration,and its energy efficiency is optimal,and the three-stage longitudinal circulation system is optimized in three different level 3 low temperature Rankine circulatory system projects.Among them,the net output work of the system of the mixed tertiary circulation system raises 1056.18 kW and 20.91 kW respectively.System exergy efficiency respectively increases to 21.41% from Level3 ’s 15.74% of the lateral and longitudinal 21.33%.Moreover,the working medium quality of the mixed tertiary circulation was significantly reduced compared with that of the three-stage longitudinal circulation.Furtherly based on the three-level Rankine mixed cycle power generation system,this paper puts forward LNG cold energy and comprehensive utilization composite system which is applicable to offshore LNG-FSRU power generation and cold-media direct contact with freezing seawater desalination compound.And it is compared and analyzed the power generation-seawater desalination complex system project and regular power generation and cold-media direct contact with freezing desalination direct series system solution proposed in this paper from the Angle of economics.In the result,the annual net economic benefit of the new project of power generation-seawater desalination complex system is 8.5 per cent higher than that of the direct series system based on the new three-stage hybrid Rankine cycle power generation system proposed in this paper.Finally,based on the analyzed insufficient mixed system,the original working medium was adjusted to the mixed working medium,and it is found that the net output of the system,the output of fresh water and the annual economic benefit are respectively raised from the original 4440.772 kW,98.6 t/h,and 19954,000 yuan to 4444.12 kW,114.18 t/h,and 20,334 million yuan by the system economy as the highest goal.Based on that,the two kinds of using LNG cold energy comprehensive utilization systems including power generation,desalination water and BOG gas treatment are established and compared,combining the FSRU’s necessary BOG gas treatment system with the LNG cold energy composite system.The results show that the output of desalination water and the system economy were 3068.46 KW,114.18t/h and 1483.15 million yuan/year respectively,combined with the net output work of LNG cold energy comprehensive utilization system1 of BOG direct compression process.The output of desalination water and system economy are 3672.95 kW,115.58 t/h,1728.07 million yuan/year respectively,combined with the LNG cold energy comprehensive utilization system 2 net output work of BOG re-condensation process.Therefore,it is determined that LNG cold energy comprehensive utilization system 2 is the final cold energy comprehensive utilization system. |
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