Multi-objective Analysis And Optimization Of Thermal Performance Of LNG Regasification System

Liquefied Natural Gas（LNG）is a clean and efficient primary energy,which has a very broad application prospect in present energy industry and daily life in a society where low-carbon environmental protection is advocated.LNG is usually stored and transported under normal pressure and-163°C.Although the entire process will be fully insulated,the inevitable heat leakage will cause part of the LNG to evaporate inside the low-temperature storage tank and become low-temperature Boil-off Gas（BOG）.This part of BOG must be processed,otherwise it will cause the pressure inside the LNG storage tank to continue to rise and cause danger.LNG must be regasified before entering the end of use,which process is accompanied by the release of a large amount of cold energy.LNG cold energy combined with ship waste heat can achieve higher efficiency and more forms of energy recovery and utilization when the object of use is an LNG-powered ship.BOG will be directly discharged or burned and the cold energy released during LNG regasification will also be directly dispersed into the environment in most practical operations.Therefore,to change these treatment methods that are neither energy-saving nor environmentally friendly,the energy-saving and high-efficiency LNG phase change systems,namely BOG reliquefaction system and LNG regasification system,are designed to process BOG and LNG cold energy,which has significant energy saving and Environmental protection significance.In this thesis,a combination of theoretical analysis and process simulation is used to conduct in-depth analysis and research on the LNG phase change system.Firstly,an improved new BOG reliquefaction system,consisting of two evaporative refrigeration cycle subsystems and one BOG subsystem,for processing BOG continuously generated in storage tanks is proposed in this thesis.In order to study the cooling mode with higher efficiency,the working conditions of three different refrigerant combinations were simulated and analyzed.Multi-objective optimization of the three working conditions system based on genetic algorithm,with Specific Energy Consumption（SEC）as the objective function,comparative analysis of the core parameters affecting the BOG reliquefaction system,and comparative analysis with the reference system.According to the optimization results,the flow of LNG produced by BOG reliquefaction has increased to a certain extent under the condition that the input power remains basically unchanged.Compared with the basic conditions,the performance of optimized condition 2 has a greater improvement,and its Coefficient of Performance（COP）,exergy efficiencyηand reliquefaction rate increased by 10.00%,10.65%and7.43%,respectively,while SEC A decrease of 9.82%.In particular,the optimized Case 3,which used mixed refrigerants in both the high-temperature refrigeration subsystem（HTRS）and the low-temperature refrigeration subsystem（LTRS）,showed the best performance,with a COP of 0.322,an exergy efficiency,,of 0.519,and an SEC of 0.506 k W·h/kg_LNG.Secondly,two LNG energy comprehensive utilization systems suitable for LNG-powered ships are proposed in this paper,which comprehensively considers the regasification of LNG,the utilization of cold energy generated in the regasification process,and the utilization of waste heat when LNG is used as fuel for ships.The methods of cold energy recovery and utilization include organic Rankine cycle power generation,cooling of air conditioning systems,cold storage,and fresh-keeping rooms,while the ship waste heat recovery includes the use of liner cooling water as the heat source of the Rankine cycle and the use of ship exhaust gas to drive the supercritical carbon dioxide cycle for power generation.Taking a typical LNG power ship as the research object,the LNG energy comprehensive utilization system is modeled and analyzed based on the theory and method of process simulation.Then the system is subjected to thermodynamic analysis and economic analysis,and is evaluated by five indicators such as net output power,and the influence of LNG evaporation pressure on system performance is studied.The results show that the net output power of system 1 is 465.14 k W,which is 6.6%higher than system2.The total exergy losses of system 1 and 2 are 1430.69 k W and 1775.51 k W respectively,and system 2is 24.24%more than system 1.From the perspective of economic indicators,the total investment of system 1 is about 4.32%higher than that of system 2,but from the perspectives of the Levelized Energy Cost（LEC）and the Pay-Back Period（PBP）,the LEC of system 1 is 0.1013$/k W·h,and the investment cost can be recovered in about 6.5 years,which has a lower net output power cost than system 2,and can recover the investment cost in a shorter period of time.Therefore,the overall performance of system 1 is better than system 2 through comprehensive analysis.In summary,the LNG regasification related system is taken as the research object to design and simulate the BOG reliquefaction system and the LNG energy comprehensive utilization system in this thesis.Then the analysis of thermodynamics and economics is used to evaluate the system performance,and the multi-objective optimization method is used to improve the system performance.It has far-reaching significance and practical engineering value in saving energy,saving money and protecting the environment.