Research On Waste Heat Recovery And Utilization System Based On LNG Cold Energy

With the economic growth of countries around the world,the contradiction between energy supply and demand has become more and more serious.For the oil and gas field production process,the consumption of industrial waste heat is the most prominent,and efficient recycling of waste heat has become an important measure for energy conservation and emission reduction in China.On the other hand,China's natural gas utilization technology is becoming more mature,and the liquefied natural gas?LNG?industry has also made breakthrough progress,which broke the deadlock of the waste of the LNG cold energy resources,and developed many cold energy recovery projects and natural gas downstream users.This promotes the healthy development of the LNG industry.This paper will focus on the research of thermal power cycle integrated system based on LNG cold energy utilization,explore new methods of LNG cold energy utilization.It has carried out the research from the aspects of cycle comparison,effect of key thermal parameters and cycle optimization design.In the comparison of the thermal performance of the different power cycles,the organic Rankine cycle?ORC?and the Kalina cycle?KC?are used to recover the waste heat from the heavy oil exploitation process,the LNG cold energy is employed as the cold source.Thus,a combined cooling heating and power system is proposed.The thermodynamic model of the system was constructed and the influence of key thermodynamic parameters on the thermodynamic performance of the system was analyzed.Through analysis,it is found that increasing the turbine inlet temperature,the combined ORC system is more appropriate for waste heat recovery than the combined KC system,but combined KC system provides better energy use and exergy efficiency,while the reverse situation occurs when the evaporation pressure is increased.In addition,when the turbine inlet temperature is lower than 180?and the evaporation pressure is greater than 6MPa,the combined KC system has a higher energy return on investment.However,from the economic point of view,the combined ORC system has created more depreciation payback period and more benefits.In terms of the impact of key thermodynamic parameters on system performance,a combined KC/ORC power generation system using LNG cold energy as cold source is introduced.In order to evaluate the thermodynamic performance of the system,a thermodynamic model was established and the system was simulated using Aspen HYSYS software.The effects of turbine inlet temperature,NH₃ mass fraction,evaporation pressure and CO₂ capture pressure on the system were analyzed.The results showed that the optimal values of net output power,thermal efficiency and CO₂ capture quantity are394.685 kW,53.25%and 0.3 t/t_CO2(0.6 t/t_LNG),respectively.The optimum values of power recovery efficiency and exergy efficiency are 36%and41.42%,respectively.From the perspective of optimization,the nonlinear constrained optimization calculation of natural gas liquefaction cycle is carried out by C++,and the total power consumption of nitrogen expansion refrigeration cycle compressor is optimized as the objective function,and the optimal total power consumption of the compressor is 101.54 kW.The multi-objective optimization method is performed for the flash-binary cycle subsystem to optimize the exergy economy.The optimal results of the exergy efficiency,product unit cost,and minimum CO₂ emissions obtained by Pareto chart were75.84%,3.248$/GJ and 13.14 kg/MWhr,respectively.The decision makers can choose the most suitable scheduling plan according to the actual situation.