The supplementary firing is one of the techniques which are used to increase the output power of the combined cycle power plants (CCPP). The low construction cost per generated power encourages designers to consider it in the new CCPP. In this paper the thermal and exergy analyses of HRSG for various operating conditions in variation of loads and variation of ambient temperature carried out. They are based on the performance test data at different operating conditions. The objective of these analyses is to present the effects of supplementary firing on gross power output, combined cycle efficiency and the exergy loss in Heat Recovery Steam Generator (HRSG) devices at different ambient temperatures and different gas turbine loads. This study is based on 420 MW Neka combined cycle power plant operation and performance data sheet. The results show that the most effective components for the exergy losses are stack, HP Evaporator and HP Superheater at the rated load. The results reveal that although the supplementary firing increases the gross output power of the combined cycle power plant, however it increases the total exergy loss of HRSG and consequently decreases the total exergy efficiency. Moreover, it shows that the most effective components for the exergy losses in this case are HP Evaporator and HP Economizer at partial loads. Also, LP Evaporator and LP Superheater have the same effect at the different ambient temperature.

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