Parametric optimization of Brayton /organic trans-critical combined cycle for flue gas waste heat recovery

A combined cycle composed of an air based open Brayton cycle (BC) and an organic trans-critical cycle (OTC) is presented for power recovery from flue gas of initial temperature around 600 degrees C. BC is employed as the top cycle to span the temperature gap between the heat source and the OTC bottom cycle, and offers a desirable temperature matching between the heat source's heat rejection process and the heat addition process of the cycle. Air is selected as working fluid for BC owing to its outstanding thermal stability. Parametric optimization on the combined cycle is carried out to maximize the net power output of the system at a given mass flow rate of the flue gas. The results show that the choice of OTC working fluid and pressure ratio value of the BC dominate the system performances, while the values of other parameters like the initial temperature of air in BC expansion process, the initial pressure and temperature of the organic fluid in OTC expansion process, and the condensing temperature of OTC also affect the system performances. Benzene, toluene, heptanes, acetone and R113 are screened as candidates for OTC working fluid, and maximum net power output of 175.87kJ/(kg-flue gas) is achieved with benzene. (C) 2015 Published by Elsevier Ltd.