利用LNG冷能与工业余热跨临界循环参数分析

Parametric Analysis for Trans-critical Cycle Utilizing LNG Cold Energy and Industrial Waste Heat

针对一种利用LNG冷能与工业烟气余热的单级跨临界循环展开研究,采用乙烷为循环工质,分析冷凝温度、蒸发压力、冷凝器夹点温差及烟气出口温度4个主要运行参数对循环性能(热效率、火用效率、净功率输出、乙烷及烟气质量流量)的影响。介绍LNG冷能与工业烟气余热单级跨临界循环模型,给出典型工况下各节点的参数。进行热力学分析,得到循环的热效率及火用效率表达式。采用Matlab软件中的遗传算法工具箱进行优化。热效率和火用效率随着冷凝温度的升高而降低,随着蒸发压力的增大而增大;随着冷凝温度的降低及蒸发压力的升高,循环系统净功率输出都增加;火用效率曲面和热效率曲面存在交叉线,热效率在蒸发压力较低而冷凝温度较高时低于火用效率;烟气的质量流量明显大于乙烷的质量流量,且都随着冷凝温度和蒸发压力的增大而增大。热效率和火用效率随着夹点温差的升高而降低,随着烟气出口温度的增大,热效率不变而火用效率降低,且火用效率曲面和热效率曲面存在交叉线;净功率输出随夹点温差的增大而降低,随烟气出口温度的改变而不变;烟气及乙烷的质量流量随夹点温差的增加都降低;随烟气出口温度的增加,乙烷质量流量不变而烟气质量流量增加。火用效率和净功率输出随着蒸发压力的升高,先增大再减小,存在最佳的蒸发压力(8 MPa左右),使得火用效率及净功率输出最大,而热效率随着蒸发压力的增大而增大;随着烟气出口温度的增加,热效率及净功率输出不发生变化,火用效率逐渐降低;蒸发压力升高,烟气及乙烷的质量流量都降低;烟气出口温度增大,乙烷质量流量不变而烟气质量流量增加。

The single-stage trans-critical cycle using LNG cold energy and industrial flue gas waste heat is studied.Using ethane as circulating working fluid,the influence of four main operating parameters including condensation temperature,evaporation pressure,condenser pinch point temperature difference and flue gas outlet temperature on cycle performance(thermal efficiency,exergy efficiency,net power output,ethane and flue gas mass flow))is analyzed.The single-stage trans-critical cycle model of LNG cold energy and industrial flue gas waste heat is introduced,and the parameters of each node under typical working conditions are given.The thermodynamic analysis is performed to obtain expressions of the thermal efficiency and the exergy efficiency of the cycle.The genetic algorithm toolbox in Matlab software is used for optimization.The thermal efficiency and the exergy efficiency decrease with the increase of the condensation temperature,and increase with the increase of the evaporation pressure.As the condensation temperature decreases and the evaporation pressure increases,the net power output of the cycle system increases.There is a cross line between the thermal efficiency surface and the exergy efficiency surface.The thermal efficiency is lower than the exergy efficiency when the evaporation pressure is lower and the condensation temperature is higher.The mass flow rate of flue gas is significantly larger than the ethane mass flow rate,and both increase with the increase of condensation temperature and evaporation pressure.The thermal efficiency and the exergy efficiency decrease with the increase of the temperature difference of the pinch point.As the flue gas outlet temperature increases,the thermal efficiency does not change and the exergy efficiency decreases,and there is a cross line between the exergy efficiency surface and the thermal efficiency surface.The net power output decreases with the increase of the temperature difference of the pinch point,and does not change with the change of the flue gas outlet temperature.The mass flow rates of flue gas and ethane decreases with the increase of the temperature difference of the pinch point.As the flue gas outlet temperature increases,the mass flow rate of ethane does not change and the flue gas mass flow increases.The exergy efficiency and the net power output increase first and then decrease with the increase of evaporation pressure.There is the best evaporation pressure(about 8 MPa),which maximizes the exergy efficiency and the net power output,while the thermal efficiency increases with the increase of the evaporation pressure.As the flue gas outlet temperature increases,the thermal efficiency and the net power output do not change,and the exergy efficiency gradually decreases.As the evaporation pressure increases,the mass flow rates of flue gas and ethane decrease.As the flue gas outlet temperature increases,the ethane mass flow rate remains unchanged,and the flue gas mass flow rate increases.