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.

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.

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%.

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.

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.

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