COOLflow™ Case Study 106 Encina Station Unit No. 4

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, the Encina Station Unit No. 4, 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 air ports on each wall. Table 1 shows the boiler data.

Table 1

Unit No. 4

No. Burners 24
No. Over Fire Ports 8
Max. Combustion Air/FGR flow, kpph 3,450
Boiler/Turbine Output, MW (gross) 325
Steam Flow @ MCR, kpph 2,278
Nominal Combustion Air Temp., °F 467
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 20%. COOLflow modeling technology helped design modifications to stream-line the windbox and air supply ducting including a SOFA port system, increase the FGR flow rate, and maximize the efficiency of the FGR mixing device and system. Fig. 1 shows the physical windbox model.

The most significant modification made to Unit 4 was the installation of new SOFA ducts that route pure combustion air through the OFA ports, rather than 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 air from the combustion air supply ducting upstream of the FGR airfoils.

The TODD Result

A comparison of the model and field data for the mass flow distribution of Unit 4 is seen in fig. 3. The data suggests that before modeling, the variations between the individual burners were up to ±10%, and that the rear windbox was receiving significantly more combustion air/FGR than the front windbox. After modeling, the mass flow deviations between burners are within approximately ±1%. The actual field data shows the mass flow deviations between burners are approxi-mately ±5%. The field data also indicates that after retrofit, the front to rear mass-flow bias is within ±2.3%.

The FGR model and field data taken at startup for Unit 4 is seen in fig. 4. Before retrofit, the model exhibited large FGR imbalances between the front and rear. After retrofit, the model and field data are in agreement with one another. Both the field and model data indicate that the FGR distribution between the 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 29.4 ppm is close to the lowest ever achieved for a 327 MWe utility boiler without post-combustion NOx controls. The CO level for this condition was approximately 300 ppm while operating at approximately 1.5% excess O2 . Table 2 compares the before and after retrofit results for Unit 4.

Table 2

Encina Unit 4 Low NOx Results 327 MW

Baseline Test Post Retrofit Test
Excess %O2 (wet) 0.86 1.50
OFA/SOFA (%) 12 21
%FGR 7.13 ~20
NOx (ppm @ 3% O2 ) 93 29.4
CO (ppm @ 3% O2) 48 307