Study On Thermal Stability Of Organic Working Fluids And Its Influence On Cycle Optimization

Power recovery of medium-to high-temperature flue gas waste heat is beneficial to save fossil energy,reduce the emission of greenhouse gases and air pollutants,and reduce the heat pollution of waste heat to the environment.The organic working fluid power cycle,organic Rankine cycle(ORC)and organic transcritical cycle(OTC),which are applicable under the condition of medium-to low-temperature and mediumto small-capacity heat source,also belongs to the cycle selection range of the mediumto high-temperature flue gas waste heat power recovery system.However,under the condition of medium-to high-temperature flue gas waste heat source,the thermal stability of organic working fluids is poor,and most organic working fluids have no exact upper limit temperature data at present,which makes the determination of the maximum allowable upper limit value of power cycle temperature lack of basis,also affects the reliability of system optimization results and the objectivity of system performance evaluation.In this paper,under the condition that the mass flow of flue gas is 60kg/s and the initial temperature of flue gas is in the range of 300℃～600℃,for the regenerative ORCand the regenerative OTC system,the optimization of cycle parameters and the best combination of cycle and working fluids with the maximum net output power as the optimization objective,and the experiment of the thermal stability of organic working fluids are studied.The influence of the upper limit temperature of power cycle on the system optimization results when there is no thermal stability temperature data of the working fluid is discussed.The results show that when the thermal stability temperature of the working fluid is in the range of the net output power of the system increasing rapidly with upper limit temperature of power cycle,the upper limit temperature of power cycle higher than the thermal stability temperature of the working fluid will obviously overestimate the performance of the system,and vice versa.Toluene,ethanol and acetone,which have good performance but lack reliable thermal stability data,and cyclopentane with good performance as well as known thermal stability temperature,four kinds of organic working fluids were selected.The relationship between decomposition rate and temperature of the working fluids was obtained by long-time constant temperature heating at multiple temperatures and detection of the working fluids composition after heating by GC-MS.The upper limit of the allowable annual decomposition rate of working fluids in the system is determined to be 255℃,256℃,172℃ and 159℃ for cyclopentane,toluene,ethanol and acetone,respectively.When the initial temperature of flue gas is 300℃,350℃ and 400℃～600℃,the optimal combination of cycle and working fluid is pentane regenerative OTC,cyclopentane regenerative ORC and benzene regenerative ORC respectively,on condition that the upper limit temperature of power cycle is set as the thermal stability temperature of each working fluids.The maximum net output power of organic power cycle system under the condition of flue gas waste heat source of this paper is higher than that of traditional steam Rankine cycle system.Based on the experimental results of the main products and concentrations of cyclopentane thermal decomposition,the influence of the thermal decomposition of the working fluid on the cycle parameters and the net output power of the system is analyzed when the upper limit temperature of the cycle exceeds the thermal stability temperature of the working fluid.The results show that the maximum temperature of the cycle increases with the thermal decomposition of cyclopentane;when the rated upper limit temperature is higher than the thermal stability temperature of cyclopentane,the net output power of the system will increase first,and then decrease as the decomposition amount of working fluid is too large with the operation of the system.It shows that the power cycle system has a certain tolerance for the thermal decomposition of the working fluid.When the rated upper limit temperature is 280℃,the total net output work of cyclopentane regenerative ORC system is the largest in 20 years.