Study On Energy Matching Between BOG Treatment And LNG Regasification Process

Liquefied natural gas(LNG)will inevitably produce boil off gas(BOG)during transportation,storage etc.,because storage tanks and pipelines cannot reach absolute insulation.The existence of BOG will cause the tank pressure to rise,the tank structure will be damaged,there is a big safety hazard,it needs to be reasonable recovery and utilization.However,the traditional BOG treatment process has problems such as high energy consumption,energy waste and environmental pollution.At the same time,the BOG treatment process ignores the large amount of cold energy released by the LNG regasification process.In view of the above problems,this paper explores and compare the three types of BOG treatment processes from the perspective of integrating the global energy flow of LNG.First of all,in order to reduce BOG recondensation system energy consumption,determine the best BOG recondensation system.This paper proposes four improvement methods: precooling,aftercooling,increasing the number of recondensation stages and compressor stages from the perspective of thermodynamics,and combining them to obtain 12 BOG recondensation systems.By analyzing the parameters of the BOG recondensation system,it is found that the compressor outlet pressure,LNG split rate and recondenser inlet temperature have important effects on system performance.The net present value and energy consumption of the system increase gradually with the increase of BOG content.The optimal system configuration is different under different BOG content.Considering the economy and energy consumption of the system,the two-stage recondensation system with pre-cooling is recommended.Secondly,in order to further reduce system energy consumption and energy waste,the organic Rankine cycles were introduced on the basis of the best BOG recondensation system,and the idea of integrating BOG recondensing with LNG cold energy power generation was proposed.On the one hand,the organic Rankine cycle is used to recover the cold energy released by LNG during the regasification process.On the other hand,the electricity generated by the organic Rankine cycle can be used for the power consumption of compressors and pumps in the recondensation process,effectively reducing the overall energy consumption of the LNG receiving station.Compared with the BOG recondensing system,the integrated system has a maximum reduction of 30.6%.Through the optimization andcomparison of the cold energy power generation system in the LNG regasification process,it is determined that the economic efficiency of the one-stage integrated system is the best.Then,from the perspective of BOG combustion utilization,two combined BOG combustion-LNG cold energy generation systems were constructed in series and parallel.It is found that the existence of optimal evaporation and condensation temperatures resulted in the maximum net power output and net present value of the two different forms of BOG combustion and LNG cold energy generation systems.Under different BOG contents,the net present value of the BOG combustion-series LNG cold energy power generation system is greater than the BOG combustion-parallel LNG cold energy power generation system.Finally,the three types of BOG treatment processes are compared.The BOG combustion-LNG cold power generation system has the largest net power output,then the integrated system of BOG recondensation and LNG cold energy generation,BOG recondensation system is the smallest.BOG content,interest rate and electricity price have an important influence on the economics of BOG treatment process.As the BOG content increases,the system’s net present value gradually increases,the system’s net present value decreases as the interest rate increases.Under the same BOG and interest rate conditions,when the electricity price is high,the BOG combustion-LNG cold power generation system is more suitable,when the electricity price is low,the integrated system of BOG recondensation and LNG cold energy generation is more suitable.