Page 145 - Handbook of Energy Engineering Calculations

Injecting steam or water into a GT’s combustor can significantly increase
power output, but either approach also degrades overall CC efficiency. With
steam injection, steam extracted from the bottoming cycle is typically
injected directly into the GT’s combustor, Fig.10. For advanced GTs, the
steam source may be extracted from either the high-pressure (h-p) turbine

exhaust, an h-p extraction, or the heat-recovery steam generator’s (HRSG) h-
p section.

FIGURE 10 Water or steam injection can be used for both power
augmentation and NO , control. (Power.)
x

Cycle economics and plant-specific considerations determine the steam
extraction point. For example, advanced, large-frame GTs require steam
2
pressures of 410 to 435 lb/in (gage) (2825 to 2997 kPa). This is typically
higher than the economically optimal range of h-p steam turbine exhaust
2
pressures of 285 to 395 lb/in (gage) (1964 to 2722 kPa). Thus, steam must be
supplied from either the HRSG or an h-p turbine extraction ahead of the
reheat section.
Based on installed-cost considerations alone, extracting steam from the
HRSG is favored for peaking service and may be accomplished without

altering the reheat steam turbine. But if a plant operates in the steam-injection
mode for extended periods, extracting steam from the turbine or increasing
the h-p turbine exhaust pressure becomes more cost-effective.
Injecting steam from the HRSG superheat section into the GT increases