Design Of Gas Supply System For LNG-fired Ship And Research Of Heat Exchanger

With the global environmental pollution becoming more and more serious and environmental problems becoming increasingly prominent,countries have begun to recognize that the development of clean,sustainable energy is an important trend in energy use today and in the future.The waste gas produced by oil-fueled ships is one of the main sources of air pollution today.At present,the air pollution caused by ship emissions is already very serious.Therefore,the International Maritime Organization(IMO)has issued stricter control to emissions from commercial ships.As a clean energy source,liquefied natural gas(LNG)is gaining more and more attention from various countries.The dual-fuel marine engines using LNG as fuel have been paid more and more attention by major shipping companies because of their low emission pollution.In order to comply with the requirements of national energy conservation and emission reduction,and accelerate the development of LNG-related industries,the following research work is carried out based on the gas requirements of Wartsila low and medium speed engines:(1)Design a gas supply system that meets the requirements of the engines.And the storage module,delivery module,and gasification heating module of the gas supply system are theoretically calculated,(2)According to relevant empirical formulas,under the same working conditions,the heat exchange area of common heat exchangers is calculated,and the heat exchange areas of different heat exchangers are compared.Reasonable selection of LNG heat exchanger according to heat exchange area is carried out,and the final choice is the printed plate heat exchanger(PCHE)as the core equipment of the gas supply system because of its small size and good heat exchange performance.(3)According to the shape of the inner flow channel of PCHE,the flow of supercritical LNG in the heat exchanger is analyzed from different angles through numerical simulation.The results show that both the mass flow of the fluid and the inlet pressure change have an effect on the thermal efficiency of the heat exchanger.The greater the mass flow rate and inlet pressure,the better the heat transfer effect.The convection heat transfer coefficient of supercritical LNG in the dual flow channel is different from that of the single flow channel(constant heat flow or fixed wall temperature).It reflects more complex trends.(4)Through the calculation of pump power,the selection of the appropriate cryopump model,and the in-depth study of the cryopump control scheme,it was found that when the air supply flow rate is greater than 0.6m~3/h,the inverter can be adjusted for the rotation speed of the pump to meet the requirements of the stage pressure gas supply.When the air supply flow rate is less than 0.6m~3/h,we need to use the fixed speed and built bypass adjustment method to continue the control under the condition of small flow rate.(5)For the boil-off gas(BOG)generated in the storage and transportation equipment,a new solution of re-liquefaction is proposed,and the designed nitrogen expansion re-liquefaction process is simulated by the process simulation software Aspen HYSYS to verify the feasibility.The results show that compared with other nitrogen expansion re-liquefaction process,the process in this paper is not only simple in equipment,but also able to utilize the cold energy of LNG exported to the engine,and the unit energy consumption of the process is low.At the same time,pre-cooling the BOG before entering the heat exchanger can effectively reduce the specific power consumption,increase the BOG liquefaction rate,and achieve the effect of energy saving.