2 Analysis of a one pressure level combined cycle power plant

Page 1

Second project: analysis of a one-pressure level combined cycle power plant Simone Fedeli 892133 Luca Grippo 829683 Filippo Mecarelli 892120 Francesco Persico 893256 October 2017 1 The purpose of this analysis is to understand the main critical characteristics of a combined cycle power plant. We will investigate: • the power de cit of the gas turbine due to the presence of the HRSG at its outlet; • the optimal evaporation pressure for the steam cycle in the case of constant adiabatic isentropic e ciency of the steam turbine and with the presence of a correction for the e ciency at relatively low steam quality (below 0.98); • a complete and thorough II law analysis of the steam cycle in the second case (the one with the correction for the adiabatic isentropic e ciency of the turbine). 1 Gas turbine performance when coupled to the HRSG The presence of the HRSG means that the outlet pressure of the turbine is not atmospheric pressure anymore (101325 Pa , that is also the inlet pressure of the turbine, as we do not consider the presence of lters); the outlet pressure of the turbine is now increased of 3000 Pa , which is the value of the pressure drop of the exhaust gases inside the HRSG. The following diagram shows the new situation: As it is clear from the diagram, two key parameters change when the GT is 2 coupled to the HRSG: • the TOT becomes higher; • the power output of the gas turbine becomes lower. We assume the gas turbine to be a modern one, so to say, its blades are cooled by air bled from the compressor; this means that the actual path of the ex- pansion on the T −s diagram is not linear, and, more importantly, the usual isentropic relation T ·P 1−k k cannot be used, as the expansion process is clearly not adiabatic. We have to proceed in the opposite…