The exhaust heated unfired CCGT

Exhaust gases from the gas turbine are used to raise steam in the lower cycle without the burning of additional fuel (Fig. 7.3) the temperatures of the gas and water/steam flows are as indicated. A limitation on this application lies in the heat recovery system steam generator choice of the evaporation pres.sure (p ) is related to the temperature difference (Tft — T ) at the pinch point as shown in the figure, and a compromise has to be reached between that pressure and the stack temperature of the gases leaving the exchanger, (and the consequent heat loss ).  

We first consider how the simple analysis of Section 7.3, for the combined doubly cyclic series plant, is modified for the open circuit/closed cycle plant. The work output from the gas-turbine plant of Fig. 7.3 is 

the (arbitrary) overall efficiency of the whole plant is 

But if combustion is adiabatic, then the steady flow energy equation for the open-circuit gas turbine (with exhaust of enthalpy (//p)s leaving the HRSG and entering the exhaust stack with a temperature Ts greater than that of the atmosphere. To) is 

The arbitrary overall efficiency of the combined plant (Eq. (7.12)) may then be written 

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