Gas Turbine Handbook : Principles and Practices

Gas Turbine Exhaust Treatment 139
same amount of NO x
reduction. Steam flow requirements are nominally
1.5-2.0 times those required for water injection. The steam
injection rate for a typical aeroderivative gas turbine operating at
25 megawatts is 3.3% of the compressor inlet air flow. As a function
of mass flow (steam to fuel), ratios of up to 2.4 by weight have been
used 10 .
However, there is also a limit to the amount of steam that may
be injected into the gas turbine in general, and into the primary combustor
zone in particular. The maximum total steam injection into the
gas turbine is between 5% and 20% of existing air flow. The allowable
amount of steam to be injected into the combustor primary zone is
limited by the flameout characteristics of the combustor. Also the allowable
amount of steam injected for power augmentation is set by
mechanical considerations and compressor pressure ratio limitations.
On a per-pound basis, steam contains more expansion energy
than air; the Cp of steam is approximately twice the Cp of air. The
power output gain from steam injection is about 4% for each 1% of
steam injected (where % of steam injected refers to main turbine
flow). The following graph (Figure 9-5) shows the effect of a constant
5% steam injection on the load carrying ability of a typical gas turbine.
For comparative purposes, the output variation with ambient
temperature is shown for the same turbine without steam injection.
SELECTIVE CATALYTIC REDUCTION
Selective catalytic reduction (SCR) is a process in which NO x
is removed from the exhaust gas stream by the injection of ammonia
(NH 3
) into the stream and the subsequent chemical reaction in
the presence of a catalyst. For a given gas condition (temperature,
gas composition, etc.) the performance of the SCR is a function of
the catalyst type and geometry, the residence time of the gas in the
reactor, and the amount of ammonia injected upstream of the reactor.
Selection of the catalyst is specific to the temperature in which
it is expected to operate. The ammonia utilized in the process may
be either anhydrous or aqueous. The injection systems differ slightly
depending on the type of ammonia injected. The basic chemical reactions
11 are: