Page 140 - Advanced Gas Turbine Cycles

112 Advanced gas turbine cycles
In the first type, heating of the steam turbine cycle is by the gas turbine exhaust with or
without additional firing (there is normally sufficient excess air in the turbine exhaust for
additional fuel to be burnt, without an additional air supply). In the second, the main
combustion chamber is pressurised and joint ‘heating’ of gas turbine and steam turbine
plants is involved.
Most major developments have been of the first (exhaust heated) system, with and
without additional firing of the exhaust. The firing is usually ‘supplementary’-burning
additional fuel in the heat recovery steam generator (HRSG) up to a maximum
temperature of about 750°C. However, full firing of exhaust boilers is used in the
repowering of existing steam plants.

7.4.1. The exhaust heated (unjred) 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 waterkteam flows
are as indicated. A limitation on this application lies in the heat recovery system steam
generator; choice of the evaporation pressure ( p,) is related to the temperature difference
(T6 - 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, TS
(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 circuitlclosed cycle plant. The work output
from the gas-turbine plant of Fig. 7.3 is
WH = (7)O)HFt (7.10)
(70)H is the (arbitrary) overall efficiency and F is the energy supplied in the fuel,
F = M,[CV],, where [CV], is the enthalpy of combustion of the fuel of mass flow Mf. The
work output from the steam cycle is
WL = NQL. (7.1 1)
in which is the thermal efficiency of the lower (steam) cycle and QL is the heat
transferred from the gas turbine exhaust.
Thus, the (arbitrary) overall efficiency of the whole plant is

(7.12)

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

HRO = HPS + WH + QL? (7.13)

’ Note that in Fig. 7.3, the steam entropy is scaled by a factor p = Ms/Mg, obtained from the heat balance,
M,(h4 - h6) = Mg I: Tdr, = M,(h, - h,) = M, Tds,. Point c is then vertically under point 6 (but point 6 may
not be precisely vertically below point S).