Study On BOG Re-liquefaction And Calorific Value Adjustment Of LNG

As a kind of high-quality and clean energy,Liquefied Natural Gas(LNG)has the advantages of convenient storage and transportation,good security performance and little environmental pollution after combustion,and it has been widely applied in urban gas,ship and vehicle fuels.Due to the cryogenic property of LNG,the heat leakage from environment to storage tanks will make LNG evaporate as Boil-Off Gas(BOG)even if storage tanks have excellent thermal insulation performance.The direct emission of BOG will cause energy losses.BOG can be effectively recycled through traditional compression re-liquefaction or condensation re-liquefaction,which needs equipping with low-temperature compressors and condensers,resulting in high investment cost and complex process.Besides,in the practical applications of LNG,it is necessary to adjust the calorific value of LNG to meet the different requirements of users.Through researches of BOG recovery and LNG calorific value adjustment technologies at home and abroad,a process scheme based on cryogenic liquid was proposed to achieve BOG re-liquefaction and LNG calorific value adjustment at the same time,and the investigation on the process scheme was conducted.The main work is as follows:(1)The process of BOG re-liquefaction and LNG calorific value adjustment was designed.BOG in LNG storage tanks was mixed up in a jet-mixer with some cryogenic liquid,then mixed up with the raw material of LNG for the second time,and the finished product of LNG was obtainined with the realization of BOG re-liquefaction and LNG calorific value adjustment.(2)The simulation of BOG re-liquefaction and LNG calorific value adjustment was performed.The process simulation model was established by using Aspen Plus software,and the characteristics of the finished LNG were simulated by the method of single factor anlysis based on the SRK equation.The influence rule of the relevant process parameters on the characteristics of the finished LNG was anslyzed.(3)The process parameters of BOG re-liquefaction and LNG calorific value adjustment were optimized.With the re-liquefaction rate of BOG as the objective function,the process parameters were optimized by determining the decision variables and constraints,and the re-liquefaction rate of BOG was increased by 17.3%.At the same time,the components and calorific value of the finished LNG conform with relevant national standards and meet the different requirements of users.(4)Economic analysis of the process scheme was completed.For the process of BOG re-liquefaction and LNG calorific value adjustment of a certain NG liquefaction station,the economy and sensitivity analysis of the process scheme were performed.The results show that the process scheme has good investment returns and a certain anti-risk capability.