COOLflow™ Case Study 108
Pittsburg Station Unit No. 6
The Situation
The Environmental Protection Agency continues to
lower the allowable NOx emission rates through stringent
regulations. Some utilities, such as Mirant (formerly Southern Energy
California) in Pittsburg, CA, are researching and implementing
innovative concepts on the cutting edge of technology to
comply with these clean air regulations. They are proving
that advanced low NOx burner technology and windbox
modeling are cost competitive solutions in today's very tough
NOx reduction marketplace.
The Boiler
One unit in particular, Pittsburg Station Unit No. 6, is a
330 MWge power boiler, capable of delivering 2,150
kpph steam at 2,475 psig and 1,050°F, and of reheating
approximately 1,955 kpph steam from 558°F to 1,000°F. The
unit has two firing walls with two rows of six burners on
each wall. There is a row of six Over Fire Air (OFA) ports, one
above each burner column on each firing wall. The burners
and OFA ports are all in a common windbox. Flue Gas
Recirculation (FGR) is supplied to the air stream in four airfoil
sparger sections. Table 1 shows the boiler data.
Table 1
 |
Unit No. 6 |
|
|
|
| No. Burners |
24 |
|
|
| No. Over Fire Ports |
12 |
|
|
| Max. Combustion Air/FGR flow, kpph |
3,227 |
|
|
| Boiler/Turbine Output, MWge |
330 |
|
|
| Steam Flow @ MCR, kpph |
2,150 |
|
|
| Nominal Combustion Air Temp., °F |
560 |
|
|
| Model Scale |
1/12 |
|
|
The TODD® Solution
In early 2000, we used COOLflow technology to help
perform the turnkey retrofit of 24 low NOx, gas burners that
utilized advanced fuel gas injection techniques.
While performing an internal windbox inspection, we
noticed a large pressure drop in the FGR delivery system,
which was installed to balance the FGR in the system. This
large pressure drop limited the unit to 25% FGR. By
implementing COOLflow technology, we were able to
achieve the same FGR balance while removing the large pres-sure
drop, thereby returning the unit to its full FGR capacity.
 |
Fig. 1
|
COOLflow helped design modifications to streamline
the windbox and air supply ducting, including increasing the
amount of OFA from 8% to 12%, reducing the large
pressure drop within the FGR supply system, and increasing
the FGR from 25% to 30% - all while maximizing the
efficiency of the FGR mixing device. Fig. 1 shows the
physical windbox model.
The TODD Result
A comparison of the model and field data for the mass
flow distribution of Unit 6 is seen in fig. 2. The data suggests
that before modeling, the variations between the individual
burners were up to ±16%, and that the rear windbox was
receiving significantly more combustion air/FGR than the
front windbox, up to 5%. After modeling, the mass flow
deviations between burners were within approximately ±4%.
The actual field data shows the mass flow deviations
between burners were reduced to approximately ±7.5%. The
field data also indicates that after retrofit, the front to rear
mass-flow bias is much lower, within ±2.4%.
 |
Fig. 2: Mass Flow Deviation
|
The FGR field data taken both before and after the
retrofit startup of Unit 6 are seen in fig. 3. Before retrofit, the
unit exhibited FGR imbalances of up to ±4.5% between
burners. The front to rear FGR distribution was good,
however, this is due to the fact that the unit has separate
dampers with the front FGR damper set at 54% open and the
rear FGR damper set at 80% open. After retrofit, the field data
indicates that the FGR distribution between the individual
burners is within ±1.5% and is satisfactorily balanced
between the front and rear, with FGR dampers both at 80% open.
 |
Fig. 3: FGR Deviation
|
The improvement in the mass flow and FGR deviation
from the baseline conditions to the retrofit conditions was
significant. The resultant NOx emission rate of 36 ppm is
exceptionally low for a 330 MWge utility boiler without post-combustion
NOx controls. The CO level for this condition
was 133 ppm while operating at 0.88% excess O2. Table 2
compares the before and after retrofit results for Unit 6.
Table 2
|
Pittsburg Unit 6 Low NOx Results 330 MWge
|
|
| |
Baseline Test |
|
Post Retrofit Test |
|
| Excess %O2 (wet) |
1.38 |
|
0.88 |
|
| OFA/SOFA (%) |
8 |
|
12 |
|
| %FGR |
25 |
|
30 |
|
| NOx (ppm @ 3% O2) |
73 |
|
36 |
|
| CO (ppm @ 3% O2) |
403 |
|
133 |
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