STEAM AND WATER INJECTION COMPARISON BASIC AND TUTORIALS

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. 3. 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 3 Water or steam injection can be used for both power augmentation and NOx control  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. 3. 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.

Cycle economics and plant-specific considerations determine the steam extraction point. For example, advanced, large-frame GTs require steam pressures of 410 to 435 lb / in2 (gage) (2825 to 2997 kPa).

This is typically higher than the economically optimal range of h-p steam turbine exhaust pressures of 285 to 395 lb / in2 (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 unit output by 21.8 MS., Case

Because the steam-injection system requires makeup water as pure as boiler feedwater, some means to treat up to 350 gal /min (22.1 L/ s) of additional water is necessary. A dual-train demineralizer this size could cost up to $1.5-million.

However, treated water could also be bought from a third party and stored. Or portable treatment equipment could be rented during peak periods to reduce capital costs. For the latter case, the average expected cost for raw and treated water is about $130/h of operation.

This analysis assumes that steam- or water-injection equipment is already in place for NOx control during distillate-fuel firing. Thus, no additional capital cost is incurred.

When water injection is used for power augmentation or NOx control, the recommended water quality may be no more than filtered raw water in some cases, provided the source meets pH, turbidity, and hardness requirements. Thus, watertreatment costs may be negligible.  

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