Chemical Engineering September 2022 - 37

fuel-to-air ratio. There are many benefits of utilizing
a parallel positioning system instead of a standard,
single-point position.
Parallel positioning provides precise fuel-and-air ratio
control because there is an actuator directly coupled
to the control valve, whether it be the air damper, gas
metering valve or FGR valve to ensure repeatability. At
all points in the combustion process, it is always going
to repeat.
Parallel positioning utilizes dedicated servos that are
FIGURE 6. This fully metered control system includes cross-limiting capabilities
and individual device actuators
During the first step, the combustion reduced to flame
temperature between the heat exchanger is insufficient.
FGR can be added to the first-step combustion to reduce
the flame temperature itself. The fuel injected in
the second step may be staged to blend it with furnace
gases and fuel station of the second fuel injection step,
which is common for this system.
The typical packaged boiler using a two-step fuelinjection
system achieves around 30 ppm NOx without
FGR. If FGR is added to the two-stage injection process,
NOx can be reduced to 9-10 ppm.
Applying the two-step air injection method to very
large furnaces, such as field-erected boilers, requires
two combustion systems because of the sheer
volume required.
In the air-injection method (Figure 5), the first step is a
very rich, or what is called sub-stoichiometric, combustion.
In this type of combustion, there will be only partial
combustion in the first step.
In the second step, the rest of the combustion air is introduced,
and usually the heat transfer between the two
injection steps is not sufficient to reduce the temperature.
Therefore, FGR has to be introduced in the second
injection step along with air.
The second combustion step is responsible for most of
the thermal NOx production, so it is important to control
the temperature of the flame. In the field, this is often referred
to as over-fire air on field-erected boilers. By utilizing
the two-step air-injection method, NOx can be reduced to
30 ppm. With Number 6 oil (a high-Btu-content residual
oil), NOx can be reduced to 140 ppm, whereas it normally
would be around 375 ppm.
Advanced controls
To utilize these combustion technologies or processes,
advanced controls, such as parallel positioning systems
or fully metered systems, are required. Additional
options include variable speed drives (VSD) and O2 trim
airflow-trimming systems (described in the upcoming
section). These systems provide precise control, which
is needed to control the fuel-to-air ratio and achieve a
good mixture with the firing technologies.
Parallel positioning. Basically, parallel positioning
is a control system that optimizes the burner's
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high-resolution actuators, and each one of the fuels or
air has its own dedicated servo. The air damper has
its own dedicated servo. The gas control valve has its
own dedicated servo, and if FGR is present, there is
a servo on the FGR valve that will control the amount
of recirculation. Every actuator is also equipped with
a feedback signal to improve safety. If an actuator
does not reach its position, it will signal an alarm and
shut down.
Each of these can have its own separate curve that
will be applied. The air-fuel curve for parallel positioning
is a software function. Because of that, it is possible
to modify a curve or even have multiple curves for the
fuels to meet the requirements for the various burner
technologies. Also, because it is a software function,
there is the capability of adding a variable-frequency
drive (VFD), VFD bypass and O2 trim systems.
Since high-resolution actuators are used, there is
Take the stress out of
managing your
compressed air system.
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Chemical Engineering September 2022

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