Research On LNG Cold Energy Two-stage Recovery System

With the rapid development of society,the world’s increasing energy consumption has brought a series of environmental problems.The development of new energy sources and the improvement of energy efficiency have become the main research directions in today’s society.LNG(liquefied natural gas)is an efficient and clean energy source in recent years.It has become a new force in the world’s energy structure change.LNG will release 830～860kJ/kg of cold energy in the process of vaporization.Due to cost,site and technology,most LNG cold energy utilization projects are currently inefficient.This article addresses the existing problems in the existing LNG cold energy utilization project,according to the release characteristics of cold energy and physical exergy at different evaporation pressures of LNG,the evaporation process of LNG is divided into two different temperature sections(S1 temperature section and S2 temperature section,in which S1 is the storage temperature in the LNG tank rising to-90℃.S2 is raised from-90℃ to ambient state).Based on engineering practice and theory,a set of LNG cold energy two-stage utilization system is designed,A single-stage organic Rankine cycle(ORC)system in which a low-temperature waste heat is used as a heat source and cold energy released during vaporization of LNG is used as a cold source is used as a first-stage cold energy recovery system absorbs cold energy and physical exergy in the S1 temperature section;an ice storage system as a second-stage cold energy recovery system,it absorbs the cold energy and physical exergy of the S2 temperature section.The system achieves the combined use of power generation and cooling.The effect has solved the problems such as unstable vaporization,high cost,and lack of space in LNG refueling stations.Then based on Aspen Plus software established LNG cold energy two-stage recovery system model.Four working fluids(R170,R290,R290/R600 and R1270)have been selected as the first-stage system working fluid;50%ethylene glycol aqueous solution is selected as the second-stage system working fluid.This article simulates the system by changing the evaporation pressure,the condensing pressure,and the inlet temperature of the expander in the first-stage system,and select the most suitable working fluid for this system according to the evaluation parameters of the system(heat exchanger UA value,system net output power(Wnet),thermal efficiency(ηth),system cooling capacity(Qw),cold energy utilization(ηcold),and enthalpy efficiency(ηex).The results show that R170 is the best working fluid for the first-stage recovery system,the evaluation parameters of the system(heat exchanger UA value,system net output power(Wnet),thermal efficiency(ηth),system cooling capacity(Qw),and cold energy The utilization rate(ηcold)and the enthalpy efficiency(ηex))are 540.5192 cal/(sec·k),7.7742kw,25.03%,19.48kw,45.57%,and 27.73%,respectively.Finally,the simulation data of cold energy recovery system and the daily operation data of Xinyang gas station in Nanning are used to carry out the preliminary experimental bench construction and engineering economic analysis.The results show that the LNG cold energy two-stage recovery system is expected to save electricity daily 380.85 kw,the annual electricity savings of 68,500 yuan,the total investment in system equipment is 257,500 yuan,and the annual operating cost is 20,000 yuan,and the calculated investment recovery period of the system is estimated to be 5.7 years.