POWER February 2011 - 44

INSTRUMENTATION & CONTROL
Automated Exhaust Temperature
Control for Simple-Cycle Power
Plants
A common concern for gas turbine power plants is treating exhaust gases to
comply with laws restricting pollutants present in the gases that are emitted
into the ambient atmosphere. The challenge for designers is to control
the exhaust gas operating temperature within a range that maximizes performance
of the oxidation and reduction catalysts.
By Dr. Mark A. Buzanowski and Sean P. McMenamin, Peerless Mfg. Co.
I
n simple-cycle power plants that use
gas turbines, ambient air is compressed
and, at high pressure, mixed with fuel,
generating thermal energy in a turbine
combustion chamber. As a result of the
combustion process, exhaust gas contains,
among other components, nitrogen oxides
(NOx
) and carbon monoxide (CO).
NOx has several different forms, including
nitric oxide (NO) and nitrogen dioxide
(NO2
) and
). NOx emissions react with other
compounds in the atmosphere to form secondary
products such as ozone (O3
nitric acid (HNO3
). Because of the negative
environmental impacts associated
with NOx
Expansion joint
emissions, simple-cycle power
plants are subject to emissions regulations
restricting the amount of NOx
the atmosphere.
emitted into
The Selective Catalytic
Reduction Process
One process that has been developed to remove
NOx
from emission gases is selective
catalytic reduction (SCR). The SCR process
is capable of removing over 95% of
NOx
from the gas turbine exhaust gas. This
technology is generally considered a best
available control technology, one that represents
the most stringent NOx
emissions
control process that is technologically feasible
and cost effective (Figure 1).
An SCR placed in the exhaust duct of
a simple-cycle combustion turbine reduces
the concentration of NOx
gases by converting NOx
nitrogen (N2
) and water (H2
O) molecules,
which are not considered pollutants. This
reduction is performed with the aid of a
catalyst (referred to here as the reduction
catalyst) and a gaseous reactant, which is
typically ammonia (NH3
). Ammonia in the
44
in the exhaust
into diatomic
Tempering air system
1. An exhausting process. An exhaust duct of a simple-cycle turbine equipped with
a selective catalytic reduction system is shown in this heat-recovery steam generator view.
Courtesy: Peerless Mfg. Co.
Reduction
catalyst
Ammonia injection grid
Oxidation catalyst
Perforated plate
system is often derived from anhydrous
ammonia, aqueous ammonia, or urea. The
ammonia is added into the exhaust gas
containing NOx
the catalyst.
As this mixture of NH3
and NOx
prior to interaction with
passes
through the catalyst bed, a high percentage
of NOx
reacts on the catalyst surface,
decomposing into diatomic nitrogen and
water molecules, thereby reducing the NOx
level in the exhaust gas.
The operating temperature of the reduction
catalyst has a critical effect on the
NOx
conversion efficiency. Depending on
the catalyst active component chemical
formulation and its substrate, the reduction
catalyst is typically utilized at about
850F, and the operating temperature cannot
exceed 1,050F for prolonged periods
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of time. Operating the reduction catalyst at
lower temperatures allows for adding more
of the active component (typically vanadium
oxide) to the catalyst. As a result, the
catalyst volume can be reduced and the
catalyst life span increased.
Typically, the simple-cycle turbine exhaust
gas temperature exceeds the temperature
range required by the reduction
catalyst, and the exhaust gas must be cooled
down. Consequently, in a typical application,
simple-cycle power plants require air
blowers for injecting ambient air (so-called
tempering air) into the exhaust system to
bring the exhaust temperature within the
operating range of the reduction catalyst.
In addition to forming NOx
, the combustion
process generates numerous other
oxides, some of which are produced as a
POWER | February 2011

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POWER February 2011

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