Page 159 - Advanced Gas Turbine Cycles

128 Advanced gas turbine cycles

a simple gas turbine plant. But there are additional complications, of higher irreversibility
in the HRSG (because of higher temperature differences), the possibility of regenerative
feed heating and the limitation on the temperature of the water at entry to the HRSG
economiser.
Rice found high CCGT efficiencies with gas turbine reheat at optimum pressure ratios
even higher than those discussed above.
The latest ABB GT24/36 CCGT plant ([16], see also Ref. [l] for a brief description)
employs reheating between the HP and LP turbines and a relatively high pressure ratio of
30. There are two thermodynamic features of this type of design. Firstly, the expansion
through the larger pressure ratio, but taken in separate HP and LP turbines with reheating
between them, means that the temperature leaving the LP turbine is not increased
substantially in comparison with non-reheated plants (about W"C, cf. 530-500°C); and
secondly that the pressure ratio for maximum (71)~~ becomes closer to that for the
maximum efficiency in the higher plant alone.
An extension of the approximate analysis of Section 7.6 suggests that the pressure ratio
for both the combined and higher level plants, for the example given there, should be about
48 which is higher than that used in the ABB plant (about 30).
Most modem CCGT plants use open air cooling in the front part of the gas turbine. An
exception is the GE MS9001H plant which utilises the existence of the lower steam plant
to introduce steam cooling of the gas turbine. This reduces the difference between the
combustion temperature T,,, and the rotor inlet temperature Tht The effect of this on the
overall combined plant efficiency is discussed in Ref. [l] where it is suggested that any
advantage is small.

7.8. Discussion and conclusions

It has been shown that the CCGT plant achieves a much higher overall efficiency
than the simple CBT plant, but the maximum efficiency is achieved at a substantially
lower pressure ratio than that giving optimum conditions in the latter plant.
With modem gas turbine inlet temperatures there is no advantage in supplementary
heating. However, reheating in the gas turbine may give high efficiency, but at a
higher optimum pressure ratio.
Irreversibility in the HRSG may be reduced by introducing dual pressure level steam
raising. This may increase the overall efficiency by about 2-3%, but going to triple
pressure levels adds relatively little further gain.
The introduction of feed heating into the steam cycle of a CCGT plant is a complex
matter and the following points are relevant.
(i) The simplest recuperative plant, with no regenerative feed heating and all the
feed water heated directly in the HRSG may not be feasible because of the limits
that have to be placed on the temperature Tb of the feed water entering the HRSG
(in order to avoid corrosion of the metal surfaces). However, a thermodynamic
performance the same as this simplest plant (no regenerative feed heating) can be
achieved by extracting from the exhaust gases the heat required to raise the feed