2 Heat recovery steam cycle Report

Page 1

Politecnico di Milano MSc. Energy Engineering – Power Production Energy Conversion A Prof. Gianluca Valenti PROJECT 2 ANALYSIS OF A ONE- PRESSURE LEVEL HEAT RECOVERY STEAM CYCLE a.y. 2015/16 Giulia Boschi Omar Brembilla Alessandro Mosca 2 I) TOT CORRECTION The presence of a HRSG introduces pressure drops in our gas turbine. Hence, the gas cycle will change according to the following diagram. As the diagram shows, t he coupling of the gas turbine with the HRSG leads to a change in the TOT. Our p urpose for the moment is to find out the new TOT, which will be a fundamental importance parameter in the analysis of the Recovery Steam Cycle that will follow. Focusing on the last part of the expansion, the only one in which we can define an adiabatic efficiency for the absence of air coolings, the situat ion is depicted in the T-s diagram on the right , and the problem is solved performing the calculations that follow. 𝜂𝑎𝑑 = ℎ𝑐𝑐(𝑇𝑐𝑐 = 𝑇𝑂𝑇′)− ℎ5(𝑇5 = 𝑇𝑂𝑇) ℎ𝑐𝑐(𝑇𝑐𝑐 = 𝑇𝑂𝑇′)− ℎ𝑐𝑐′(𝑇𝑐𝑐′) 𝑠𝑐𝑐(𝑇𝑐𝑐 = 𝑇𝑂𝑇′; 𝑝𝑐𝑐 = 𝑝𝑎𝑡𝑚 + Δ𝑝) = 𝑠𝑐𝑐′(𝑇𝑐𝑐′; 𝑝𝑎𝑡𝑚) 3 Here we have set a system of two equations in two unknowns, which are: - 𝑇𝑐𝑐 = 𝑇𝑂𝑇′ [𝐾] - 𝑠𝑐𝑐 [ 𝑘𝐽 𝑘𝑔𝐾] Note that we are supposing that the enthalpy jump between the two isobars is constant. This is possible only because the last expansion portion is small. Running the Excel so lver, we obtain the following result to the above system. In particular, the value i n which we are interested is the new TOT, which is the new turbine outlet temperature of the exhaust gases, the same temperature at which the gas will enter the HRSG. 𝑇𝑐𝑐 = 𝑇𝑂𝑇′ = 823.3 𝐾 = 550.2 °𝐶 < 544.6 °𝐶 = 𝑇𝑂𝑇 = 𝑇5 The result is consistent since a smaller expansion has to lead to a higher temperature. This has two main consequences for the moment; the first is that with a smaller…