Simulation And Optimization Of BOG Re-liquefaction Process Of LNG Bunkering Vessel

Clean Energy,represented by the liquefied natural gas(LNG),is vital in curbing climate change and transforming the energy mix.China is implementing the Clean fuel project to replace the traditional oil fuel with LNG fuel,but it faces the dilemma of insufficient LNG fuel filling facilities,therefore,inland LNG bunkering vessel(LNGBV)was proposed to meet the demand of LNG-powered ship.Since the first launch of LNGBV in 2013,there has been little research on the management of the cargo tank,which is the core component of LNGBV.In this paper,an independent type C LNGBV cargo tank with the net capacity of 3000 m³ was taken as the research target,in which boil-off gas(BOG)generation model is established.Based on the results of simulation model,the design and optimization of BOG re-liquefaction process is proposed,which can be used for engineering design of BOG re-liquefaction.The generation characteristics of BOG in the process of static evaporation and dynamic bunkering of LNG cargo tank are studied.In the static evaporation simulation of LNG cargo tank based on Fluent,the insulation layer thicker,the evaporation rate(BOR)lesser.When the insulation layer thickness is 350mm,which is acceptable,the BOR is 0.306%;The effect of initial filling ratio and ambient temperature on the evaporation rate is approximately linear,the BOR increases from 0.115%to 0.306%when initial filling ratio increases from 10%to 90%,when the ambient temperature increases from 5°C to 45°C,the BOR increases from 0.159%to 0.422%.In HYSYS-based dynamic LNG bunkering simulation,the temperature and pressure of the receiving ship decrease gradually,but the temperature and pressure of the bunker ship does not change obviously.The influence of temperature on the generation of BOG is significant,and there is a positive correlation between the generation of BOG and the amount of bunkering.Moreover,lowering the temperature of receiving ship is beneficial to shortening the bunkering time.Based on the generation characteristics of BOG in LNG cargo tank,a re-liquefaction process of BOG based on reverse Brayton refrigeration cycle is proposed,and the key parameters of the process are analyzed.The increase of the outlet pressure of the nitrogen compressor is beneficial to the increase of the pressure difference before and after the expansion,so as to obtain more refrigerating capacity.The increase of precooling temperature is benefit to the increase of enthalpy difference,thus decrease the energy consumption.As the outlet pressure of nitrogen expander increases,the unit energy consumption decreases and then increases.The increase of the outlet pressure of BOG compressor helps to reduce the unit energy consumption.The expansion pressure of nitrogen has different effects on the unit refrigerating capacity and the energy consumption of BOG compressor.Optimization of BOG re-liquefaction process for LNGBV was carried out by exergy analysis,and the adaptability of the optimized process was studied.After optimization of the BOG re-liquefaction process by genetic algorithm,the nitrogen circulation rate is 3476.00 kg/h,the liquefaction rate is 91.95%,the energy consumption SEC is 1.157 kwh/kg _LNG,the refrigeration coefficient COP is 0.1929,increase by 9.60%,the exergy efficiency is 26.4%,increase by 10.5%.By adjusting nitrogen circulation flow rate,the change of refrigerating capacity in different amount of BOG can be realized,which has good adaptability.