Page 127 - Advanced Gas Turbine Cycles

Chapter 6. ‘Wet’ gas turbine plants 99

Combined STIG
Steam

Water

Air
Fig. 6.12. Combined STIG plant (after Frutschi and Plancherel [I]).

6.4.1.2. The combined STIG cycle
The combined STIG cycle (Fig. 6.12) was described by Frutschi and Plancherel [I].
Steam is raised at two pressure levels in the waste heat boiler. Superheated steam at the
higher pressure level expands through a steam turbine before injection into the compressor
discharge air stream. Low pressure steam is injected (STIG fashion) into the combustion
chamber. Attainable efficiency for this plant may in theory reach about 50%. In a variation
of this combined cycle (the Foster-Pegg plant), the steam turbine drives a second high
pressure compressor.

6.4.1.3. The FAST cycle
Another modification of the combined STIG cycle is the so-called advanced steam
topping (FAST) cycle. Now the double steam injection process (before and after
combustion) of the combined STIG cycle of Fig. 6.12 is replaced by a single steam
injection into the combustion chamber, after expansion in the steam turbine and reheating
in the HRSG (Fig. 6.13). In one version the steam turbine and the gas turbine are on the
same shaft, jointly driving the electrical generator. To call this cycle a steam topping cycle
is somewhat misleading, since it is essentially a doubly open combined cycle in that heat
rejection from the (upper) gas turbine is rejected to a (lower) main steam turbine cycle.
This lower cycle now includes reheating, steam leaving the steam turbine being reheated
before a second expansion in the gas turbine. But, of course, the steam is exhausted with
the gas and is not finally condensed, and there is no recirculation of water.

6.4.2. Developments of the EGT cycle

There have been a larger number of proposals for recuperated cycles with water
injection and evaporation, but all these can be interpreted as modifications of the EGT
plant, which is essentially a ‘wet’ CBTX cycle, as explained above.