COOLflow™ Case Study 107 Encina Station Unit No.5

The Situation

The Environmental Protection Agency continues to lower the allowable NOx emission rates through stringent regulations. These regulations are being met by several advanced methods in California. Some utilities, such as Cabrillo Power I LLC in Carlsbad, CA, are implementing innovative concepts on the cutting edge of technology. They are proving that advanced low NOx burner technology, along with windbox modeling and advanced Separated Over Fire Air (SOFA) port systems are cost competitive in today’s very tough NOx reduction marketplace.

The Boiler

Cabrillo Power I LLC researched the markplace for the ultimate NOx reduction solution to comply with clean air regulations. One unit in particular, Encina Station Unit No.5, is a Babcock and Wilcox “El Paso” type, opposed wall fired, natural circulation, forced draft design boiler. The combustion equipment consists of 24 burners and eight Over Fire Air (OFA) ports in the following configuration – three elevations of four burners, below one elevation of four OFA ports on each wall. Table 1 shows the boiler technical data.

Unit No. 5

No. Burners 24
No. Over Fire Ports 8
Max. Combustion Air/FGR flow, kpph 3,489
Boiler/Turbine Output, MW (gross) 346
Steam Flow @ MCR, kpph 2,405
Nominal Combustion Air Temp., °F 482
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/oil burners utilizing advanced fuel gas injection techniques and an advanced SOFA port system, while increasing the amount of Flue Gas Recirculation (FGR) to approximately 25%. TODD used the proprietary COOLflow physical modeling technology to design modifications to streamline the windbox and air supply ducting including a SOFA port system, to increase the FGR flow rate, and to maximize the FGR mixing device performance. Fig. 1 shows the physical windbox model.

The most significant modification made to Unit 5 was the installation of new SOFA ducts that route pure combustion air through the OFA ports, in lieu of air mixed with flue gas. The COOLflow modeling objective was to achieve a lower windbox O2 for a given amount of FGR, thus making the existing FGR system more effective. Fig. 2 shows a schematic of the SOFA duct modification, which takes the SOFA from the combustion air supply ducting upstream of the FGR airfoils.

The TODD Result

A comparison between the model and field data for the mass flow distribution of Unit 5 is seen in fig. 3. The data suggests that before modeling, the variations between the individual burners were up to ±8.3%, and that the rear wind-box was receiving significantly more combustion air/FGR than the front windbox. After modeling, the mass flow deviations between burners in the model are within approximately ±1.1%; the field data indicates the mass flow deviations between burners are approximately ±4.8%. The field data also indicates that after retrofit, the front to rear mass flow bias is within ±1.7%.

The FGR data from the model and field data taken at startup for Unit 5 are seen in fig. 4. Before the retrofit, the model exhibited FGR imbalances between the front and rear. After the retrofit, the model and field data are in agreement. Both the field and model data indicate that the FGR distribution between individual burners is acceptable, within ±3%, and is satisfactorily balanced between the front and rear.

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 19.8 ppm is the lowest ever achieved for a 346 MW utility boiler without post-combustion NOx controls. The CO level for this condition was approximately 300 ppm while operating at approximately 1.0% excess O2. Table 2 compares the before and after retrofit results for Unit 5.

Encina Unit 5 Low NOx Results 327 MW

Baseline Test
Post Retrofit Test
Excess %O2 (wet) 0.90 1.0
OFA/SOFA (%) 12 23
%FGR 9.57 25.3
NOx (ppm @ 3% O2) 59 19.8
CO (ppm @ 3% O2) 100 300