Siemens LMV 5 Series, Технические инструкции, Страница: 201 / 374

LMV Series Technical Instructions
Document No. LV5 ‐ 1000
SCC Inc. Page 29 Section 6
Considerations when Using O
2
Trim with FGR
Some burners use a high percentage of FGR (FGR flow compared to air flow) to lower NOx emissions.
Since flue gases change in %O
2
and are being drawn back into the blower in significant quantities, these
types of burners are inherently more difficult to trim the %O
2
in the stack.
The reason behind this increased difficulty is the oxygen content of the air / FGR mixture is dependent
on the %O
2
in the stack, which adds another dynamic variable into the system. On a burner without FGR,
the oxygen content of the air at the blower intake is always a constant 20.9% O
2
. Naturally, a larger
percentage of FGR (20%) has the potential to vary the oxygen content of the air / FGR mixture at the
blower intake more than a small percentage of FGR (5%) would.
Moreover, a large percentage of FGR can change the oxygen content of the air / FGR mixture at the
blower intake in a way that can set up a "cycle of intensification". An example of this cycle is below:
1. The %O
2
in the stack increases for some reason.
2. The %O
2
in the FGR also increases.
3. The oxygen content of the air / FGR mixture at the blower intake increases, which increases the %O
2
in the stack even more.
Naturally, the example above would also hold if the %O
2
in the stack decreased, except the cycle of
intensification would serve to push the burner rich instead of lean.
In addition to the cycle of intensification that is inherent to many FGR burners, the mechanical design of
the burner / boiler influences the repeatability of the FGR flow. The repeatability of the FGR flow has a
large influence on how well the O
2
trim system can work. Obviously, non ‐ repeatability in the FGR flow
will cause non ‐ repeatability in the %O
2
read in the stack and therefore the operation of the O
2
trim.
Two different methods of inducing FGR into a burner are illustrated and explained below, and their
behaviors from an FGR flow repeatability standpoint are discussed.
Method shown in Figure 6 ‐ 12:
Pressure P1 relative to pressure P2 (differential pressure across the FGR damper) changes drastically
with firing rate. Very little differential pressure will be generated across the FGR damper at low fire,
requiring the FGR damper to be mostly open to achieve even minimal FGR flow at low fire. As firing rate
increases, P1 will decrease and P2 will increase, yielding much more differential pressure across the FGR
damper. As a result, the FGR damper will need to be ramped closed as the burner is ramped up to high
fire. Due to the much higher differential pressure at high fire the FGR damper might be oversized for
effective control at high fire. Other points to consider:
1. If using O
2
trim, trimming with the air actuator (set to "air influenced") closed should serve to
decrease P1 and decrease the FGR flow slightly (depending on the position of the stack damper).
2. At low fire when the FGR valve is mostly open, even small changes in the differential pressure across
the FGR damper will cause large changes in FGR flow.
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