(70) Therefore, the risk of sulphuric acid dew point attack ... - DTI Home

4.3.3 Gas Turbine Steam Injection for Power Augmentation
The injection of steam into a gas turbine for NOx control is well established.
Steam injection into the combustor reduces combustion temperatures by
diluting both the level of oxygen in the combustion air and the heat generated
from combustion of the fuel. However the use of steam injection for power
augmentation is considered to be somewhat under developed for larger heavy
duty gas turbines.
Injecting large amounts of steam for power augmentation creates a fairly
efficient power only plant that effectively makes the steam turbine, condenser
and cooling tower in a combined cycle redundant. Aero-derivative engines
such as LM 5000 and the Alison 501 were proven in the mid 80s to be suitable
for steam injection power augmentation [63] . The LM 5000 can absorb all the
steam generated from the heat recovered from its own exhaust, and in doing so
increase its power output by up to 15%.
The reason for the aero-derivative engines success at power augmentation was
because aero-engines were initially designed to pick up load faster than heavy
industrial gas turbines and consequently have a greater surge margin when
operating in an industrial application. Some of this surge margin can therefore
be exploited to accept steam injection.
However the limited electrical generation capacity of these gas turbines (<
50MWe) means that the benefits of this application have been limited to
mostly small scale industrial power supply uses. Although in some cases
several of these aero-derivative gas turbines have been successfully combined
to form a reasonably sized utility plant (i.e. 7 x 50MWe). Currently no gas
turbine greater than 50 MWe has been designed which allows for such heavy
steam injection that the need for a separate steam turbine is removed (as is the
case for the LM5000).
However, elaborate arrangements where steam/water is injected into large gas
turbines are in the development stage, albeit that relatively few have actually
been constructed. A current list and description of these proposals has been
generated by Foster-Pegg [64] . Those which have reached demonstration status
include: -
• Simple Steam Injected Gas Turbine “SIGT” Cycle: this system involves
moderate steam injection into the combustor of a standard large gas
turbine and has been used to augment power under hot ambient conditions.
• Humid Air Turbine or “HAT” Cycle: this system involves evaporating
moisture into the air flowing into a gas turbine and requires a special gas
turbine in order to operate effectively. It has been highlighted as
particularly appropriate for gasification combined cycles.
The retrofit of gas turbine steam injection for power augmentation may be
particularly attractive in areas where there is existing industrial scale open
cycle GT plant and a demand for greater generating capacity. A variety of
HRSG designs could be used in this application, but it is a developing
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