Exergy Analysis of Heat Recovery Steam Generator: Effects of Supplementary Firing and Desuperheater

Comprehensive exergy analysis of a heat recovery steam generator (HRSG) with two levels of delivered pressure is presented. The effects of supplementary firing as well as desuperheater set-point are considered to evaluate the exergy destruction of HRSG components. Burner firing rate is limited to a value that corresponds to the maximum allowable temperature of tube metal of high-pressure (HP) superheater. According to the exergy analysis performed in the current study, the exergy efficiency of HRSG is about 80% which means 20% of flue gas exergy (entering HRSG) is dissipated by HRSG destruction (∼14%) and stack exergy loss (∼6%). The stack exergy loss drops continuously as supplementary firing raises. It has also been determined that increasing the rate of supplementary firing boosts the exergy efficiency in the absence of water spray and reduces it when desuperheater is working. In addition, the exergy delivered to steam turbine shows a linear growth with burner heat while it is hardly affected by the set-point of desuperheater. Also, it is found that exergy loss through the stack is not sensitive to desuperheater set-point while it is on the decrease as burner duty raises. HP steam flow will raise with increasing the firing and/or decreasing the desuperheater set-point. HP evaporator has the most contribution in exergy destruction among HRSG components (∼40%), whereas HP superheater and desuperheater are components with a maximum sensitivity of exergy destruction to the amount of water spray.