POWER February 2011 - 46

INSTRUMENTATION & CONTROL
in a higher temperature zone, the operating
temperature for the reduction catalyst
is limited to about 1,050F. This catalyst
maintains its high efficiency at 600F. The
operating temperature window is defined
by the oxidation catalyst's minimum effective
temperature and by the reduction catalyst's
maximum effective temperature.
Exhaust System Temperature
Control Arrangements
Developing control strategies for exhaust
gas temperature is challenging. They must
consider not only control schemes but also a
control methodology that takes into account
mechanical and control designs and their effects
on the overall efficiency of the simplecycle
system. The control system design
should be developed based on site-specific
requirements such as the required amount of
tempering air and a proper location for the
tempering air injection point. To select the
proper exhaust temperature control scheme,
the simple-cycle plant operating conditions
must be carefully evaluated.
Three strategies for injecting tempering
air are considered in this article. These
strategies differ in where they locate the
injection of tempering air. The injection
location will have a direct influence on the
process control design. For some strategies,
the control system can be designed
with a single-control loop, whereas for
multipoint tempering air injection points
multiple control loop designs and their interference
should be considered.
Inject at the Turbine Discharge. One
option is to inject tempering air at the discharge
of the turbine (Figure 2). Because
tempering air is injected at one location,
upstream of the emission control catalysts,
the exhaust flue gas is cooled down and
both catalysts (oxidation and reduction)
are maintained at the same operating temperature.
However, these catalysts have
been developed to control different pollutants
and operate efficiently at different
operating temperatures. By exposing
both catalysts to the same temperature, the
catalytic processes are not optimized. Ad3.
Improved injection of tempering air. By injecting tempering air downstream of
the oxidation catalyst, the operating temperature of the oxidation and reduction catalysts is
decoupled, and efficiencies of both oxidation and reduction catalysts are maximized. Courtesy:
Peerless Mfg. Co.
TE
100
TE
200
TE
300
TE
400
TE
500
TE
600
ditionally, the overall exhaust system pressure
drop may increase when tempering air
is injected at the turbine discharge.
Inject Downstream of the Oxidation
Catalyst and Upstream of the Reduction
Catalyst. In another approach, tempering
air is injected downstream of the
oxidation catalyst and upstream of the
reduction catalyst (Figure 3). The obvious
advantage of injecting tempering air
downstream of the oxidation catalyst is
that problems with overcooling of the oxidation
catalyst are alleviated. This is especially
important at partial loads, when the
exhaust temperature is low and excessive
flow of tempering air causes too low an
operating temperature for efficient oxidation
of pollutants. By injecting tempering
air downstream of the oxidation catalyst,
the operating temperature of the oxidation
and reduction catalysts is decoupled, and
efficiencies of both oxidation and reduction
catalysts are maximized.
Note that when tempering air is injected
downstream of the oxidation catalyst, the
ammonia required for the SCR process can
be mixed with tempering air and injected
into the exhaust system. In this arrangement
the assembly of the ammonia delivery
system is greatly simplified.
The control objective for strategies with
a single point of injection is to satisfy a
given setpoint for the reduction catalyst
operating temperature. In this case, the
oxidation catalyst temperature is not controlled
and either is the same as the reduction
catalyst temperature or equals the
exhaust gas temperature, depending on
whether tempering air is injected at the
turbine discharge or downstream of the
oxidation catalyst.
When using single-point injection, the
Flue gas
Perforated
plate
Oxidation
catalyst
Reduction
catalyst
Tempering air
46
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exhaust gas temperature is measured at one
location at the reduction catalyst. Standard
temperature measurement techniques are
employed, such as thermocouples with
temperature transmitters. To obtain a representative
sample, the exhaust temperature
is measured at multiple locations. The
average value of the operating temperature
is calculated and the signal is sent to a
controller as a process value. The location
of temperature sensors will have a critical
effect on determining the actual value of
the exhaust gas temperature. Typically, the
exhaust gas temperature fluctuates about
25F from the mean temperature.
Given this window of acceptable gas
temperatures for effective catalyst operation,
the dilution air damper will require a
control system that includes feedforward,
feedback, or a combination of feedforward/feedback
loops.
POWER | February 2011

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POWER February 2011

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