TECHNICAL PUBLICATION

Riley Power Inc.

5 Neponset Street

Worcester, Massachusetts 01606

www.babcockpower.com

T-195

Low NOx Combustion System Solutions for Wall Fired,

T-Fired, and Turbo Fired Boilers

by

Craig A. Penterson

Manager, Fuel Equipment Design

Riley Power Inc.

Darrell E. Dorman

Sr. Staff Engineer, Fuel Equipment Design

Riley Power Inc.

Presented at

Clearwater Coal Conference

April 17-19, 2006

Clearwater, Florida

MEGA Symposium

August 28-31, 2006

Baltimore, Maryland
© Riley Power Inc. 2006

LOW NOx COMBUSTION SYSTEM SOLUTIONS FOR WALL

FIRED, T-FIRED, AND TURBO FIRED BOILERS

by:

C.A. Penterson, Manager, Fuel Equipment Design

Riley Power Inc., 5 Neponset Street, Worcester, MA 01606

Phone-508 854 4086

Fax-508 854 4670

cpenterson@babcockpower.com

D. E. Dorman, Sr. Staff Engineer, Fuel Equipment Design

Riley Power Inc., 5 Neponset Street, Worcester, MA 01606

Phone-508 854 3899

Fax-508 854 4670

ddorman@babcockpower.com

ABSTRACT

The need to develop cost effective combustion controlled solutions for reducing NOx

emissions in coal fired utility boilers has been a high priority for many years at Riley

Power Inc., a Babcock Power Company. Solutions have varied anywhere from burner

component modifications only to complete burner replacement combined with an

advanced air staging system.

This paper discusses the application and results of various combustion related

technologies for reducing NOx emissions on a wide variety of boiler designs including

traditional wall fired and T-fired furnace designs as well as a more unique furnace

design, the Riley Turbo Furnace. NOx reductions of 40-70% have been demonstrated

using advanced combustion controlled equipment with improved overall boiler

operation.

In these applications, computational fluid dynamic (CFD) modeling was used to refine

the design of low NOx combustion systems and to identify the initial startup settings to

minimize commissioning time. Results of the modeling will be discussed making

comparisons to actual field measurements.
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INTRODUCTION

Reducing NOx emissions from utility coal fired boilers continues to be a primary goal of

environmental authorities. Since the early 1990’s nearly all-large utility boilers have installed some

form of low NOx burner technology and/or overfire air (OFA) as a primary means or first step to

controlling NOx emissions. The cost is typically much less than implementing SCR systems and the

level of NOx reduction can range from 40-70% from uncontrolled levels. When combined with an SCR

system, LNB & OFA will lower the SCR inlet NOx and save on ammonia consumption cost. So, the

cost of LNB & OFA can often be justified by this savings as well.

With NOx emission requirements becoming more and more stringent, recent years have focused on

either replacing first or second generation low NOx burners or upgrading existing burners with newer

more advanced low NOx combustion technology. Not only will the newer burner technology decrease

NOx emissions an additional 10-15% or more, the equipment will provide greater operating longevity.

This paper discusses several recent experiences at Riley Power Inc., (RPI) where earlier vintage low

NOx burners were replaced or upgraded with the newer burner technology. These experiences

include the application of low NOx technology to a wide range of boiler types such as traditional or

more common wall fired and tangentially fired (T-fired) furnaces as well less common or unique Turbo

fired boilers, a proprietary boiler design of RPI.

In all cases, computational fluid dynamic, CFD modeling was performed to refine the final burner

design and further analyze the impact of the modification on furnace and boiler performance.

APPLICATION TO WALL FIRED BOILERS — LNB AND OFA

RPI supplies low NOx Controlled Combustion Venturi, CCV Burners and overfire air systems as a

mean for controlling NOx emissions from wall fired boilers firing pulverized coal. To date, RPI has

supplied over 2000 low NOx CCV Burners on 150+ utility boilers. Figure 1 shows a schematic of

RPI’s dual air zone low NOx CCV Burner.

Figure 1. RPI CCV DAZ Low NOx Coal Burner
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Unique key features of this design for controlling NOx include:

* Independent control of secondary and tertiary air streams to control near field stoichiometry

* Patented low NOx CCV type coal nozzle for fuel rich combustion with excellent flame 

attachment and flame length control

* 50-60% NOx reduction for burners only

Overfire air, (OFA) systems provide additional staging of the combustion air to further reduce the

NOx beyond levels achievable by only LNB’s. In most cases, 20-25% of the total combustion air is

introduced at a designated distance above the top elevation of burners to “stage” the lower furnace.

When supplied, OFA ports are installed above each burner column utilizing a 1/3-2/3-nozzle concept,

as shown in Figure 2. The design is based on extensive modeling and testing performed by RPI for

EPRI in the mid 1980’s (Reference 1). Separate on/off dampers are used to control the penetration

and mixing of overfire air over a wide range of operating loads. The result is low NOx while

maintaining low CO and flyash unburned carbon.

Figure 2. Typical OFA Port Design

A few years ago RPI was approached by a major Southeast utility to improve emissions performance

in two (2) wall fired boilers originally supplied by Foster Wheeler and later upgraded with the OEM’s

low NOx burners. Both boilers fire various eastern bituminous coals having a wide range of FC/VM

ratios (1.3-2.0). Both units also experienced relatively high NOx emissions (>0.6 lb/mmBtu), unstable

CO levels and relatively high-unburned carbon in the flyash.
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Figure 3. 500 MWg Boiler w/LNB’s

The first unit, a 500 MW opposed fired design, equipped with 24 burners and 6 HP 863 mills, is shown

in Figure 3. The OEM’s first generation LNB’s were replaced with RPI's CCV DAZ type low NOx

burners without any overfire air to meet NOx requirements. This project was a mechanical design

and installation challenge to avoid interference with multiple downcomers inside the windbox.

Because of the downcomer issues, the burner was supplied in multiple parts and assemblies and

subsequently connected inside the windbox. CFD modeling of the burner design was used to optimize

the design and geometry of the flame stabilizer ring. This was done to establish the best possible

internal recirculation zones necessary to produce low NOx with acceptable CO and UBC per furnace.

It was also used to establish the initial burner settings to reduce the time for commissioning to only

a 2-week period (Reference 4). A photograph of the installed burner is shown in Figure 4.
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Figure 4. RPI CCV DAZ Burner Installed in 500 MW Utility Boiler

Final results showed a significant reduction in NOx emissions throughout the load range, 

(see Figure 5). At full load, NOx was reduced over 30% while CO emissions were stabilized at < 50

ppm and flyash UBC averaged 5%.

Figure 5. Pre & Post Retrofit NOx Performance — 500 MWg Boiler
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Figure 6. 320 MWg Boiler w/LNB’s & OFA

The second unit identified by the same Southeast utility for additional emission reduction and

improved performance required a similar installation of new low NOx CCV DAZ burners but with

OFA as well. The challenge for this application was to effectively reduce NOx while maintaining low

CO emissions and low unburned carbon in a unit with abnormally short retention time in the upper

furnace for proper carbon burnout. This not only required CFD modeling of the burners but careful

CFD analysis of the entire furnace to ensure the OFA would properly mix with the flue gas products.

Figure 6 shows a cross section of the 320 MWg unit equipped with 16 front wall mounted burners

connected to 4 HP 863 mills. A photograph of the final installed low NOx burner geometry is shown

in Figure 7.
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Figure 7. RPI CCV DAZ Burner Installed in 320 MWg Utility Boiler

Start up and commissioning of this 320 MWg unit, with LNB’s and OFA, was completed in 2 weeks

again because of the careful design of the CCV DAZ burner and the use of CFD analysis in identifying

the initial “optimum” burner and OFA settings. Final performance demonstrated a NOx reduction of

40% from 0.58 to 0.34 lb/mmBtu, which subsequently was reduced further to <0.30 lb/mmBtu at 40%

boiler load. This is graphically depicted in Figure 8.

Figure 8. Pre & Post Retrofit NOx Performance — 320 MWg Boiler
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APPLICATION TO WALL FIRED BOILER — LNB COMPONENTS AND OFA

Another Southeastern utility was experiencing combustion related problems following the

installation of another OEM’s first generation low NOx burners in the late 1990’s. Two (2) years ago

RPI was approached by this utility with the intent on reducing NOx, CO, unburned carbon and to

improve flame attachment and eliminate any flame carryover into the backpass. The boiler, originally

designed and built by RPI in the late 1960’s, shown in Figure 9, is a 270 MWg boiler with 18 opposed

fired burners and EL Mills. Several options, including the installation of completely new burners,

overfire air and various combinations of both were considered. However, since the existing low NOx

burners were only five (5) years old and in reasonably good working condition, a more cost effective

approach was to simply modify or upgrade the existing burners with critical CCV burner components

and to add an overfire air system. The material cost would be about 50% of that estimated for

completely new burners and OFA plus shorter outage duration (Reference 2). The burners were

modified as shown in Figure 10 with CCV coal nozzles, burner heads, and air diverters. 

Figure 9. 270 MWg Boiler w/LNB’s & OFA
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Figure 10. Existing OEM XCL Burner Retrofitted with RPI CCV Components

The entire secondary side of the burner was reused, except an automatic actuator was installed to

regulate the positioning of the burner flow disc as a function of the boiler load.

CFD analysis was performed on the burners and furnace systems to ensure proper near and far field

mixing patterns were maintained.

The results of this retrofit, as shown in Figure 11, reduced NOx by over 30% while reducing CO

emissions and maintaining the same unburned carbon. The excessive flame carryover was greatly

diminished as demonstrated by a 90% reduction in SH attemperator spray flow (Reference 2).

Figure 11. Pre & post Retrofit NOx Performance — 270 MWg Boiler
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APPLICATION TO T-FIRED BOILER — OFA & BURNER RETIPPING

RPI provides overfire air and burner retipping as a means for controlling NOx emissions on T-fired

boilers with tilting coal burners. Overfire air can be provided as close coupled overfire air (CCOFA)

— at the top of the main burner windbox, or as separated overfire air (SOFA) — above the main

burner windbox. Burner coal and air tips are custom designed for each application to meet the sitespecific requirements. NOx reduction in excess of 50% from uncontrolled levels is possible for most

applications when SOFA is combined with burner retipping. RPI experience with two T-fired, low

NOx retrofit applications is presented in this section.

In the first application, RPI was contracted by a Midwestern utility to reduce NOx on their 125 MWg

CE boiler firing a Midwestern bituminous coal. The unit is shown in Figure12, and features four (4)

elevations of four (4) burners each, fed by four (4) CE 673 Raymond Bowl mills. RPI selected SOFA

and burner retipping as the means to lower NOx. CFD modeling was used to design one (1) elevation

of SOFA ports installed on the front and rear walls with non-tilting, fixed yaw nozzles. The original

OEM burner tips were replaced with reduced free area air nozzles and RPI flame attached coal nozzle

tips, which are shown in Figure 13. 

Figure 12. 125 MWg T-fired Furnace w/LNB’s & SOFA
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Figure 13. 125 MWg T-fired Furnace Flame Attached Coal Nozzle Tips

Figure 14 indicates NOx was reduced from a baseline value of 0.68 lb/MMBtu to 0.22 lb/MMBtu at

MCR. NOx reduction in excess of 65% from baseline levels was demonstrated across the boiler load

range during acceptance testing, while maintaining flyash UBC and CO emissions below 1.1% and 10

ppm respectively. Superheat and reheat steam temperatures were not adversely impacted, and RPI

met the contract boiler efficiency guarantee (Reference 3).

Figure 14. 125 MWg T-fired Furnace Pre & Post Retrofit NOx Performance

The second T-fired application is an ongoing Babcock Power Environmental, Inc (BPEI) multipollutant control contract to control mercury, SO3 and fine particulate, SO2, and NOx on a

Midatlantic utility boiler. The NOx reduction portion of the contract consists of combustion system

modifications coupled with a cascading SNCR / in-duct SCR system.

The unit has a peak rating of 114 MWg and is fired with Eastern bituminous coal alone or a

combination of coal and up to 10% clean wood waste on a heat input basis. It features four (4)

elevations of coal nozzles fed by four (4) 553A Raymond Bowl mills and is shown in Figure 15. 
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A single elevation of SOFA ports with limited directional control was installed without burner

retipping in 1996 as part of a gas reburn system. Natural gas reburning is no longer utilized on this

unit, however the SOFA system is used to reduce NOx emissions. Baseline NOx emissions exhibit a

high degree of scatter due to variations in operator control.

Figure 15. 114 MWg T-fired Furnace w/LNB’s & SOFA

RPI’s combustion modifications include new SOFA nozzles featuring automatic tilt and manual yaw

capability, burner retipping, and compartment damper modifications for improved air flow control.

CFD furnace modeling is being used to evaluate the SOFA nozzle design and determine the most

beneficial equipment settings for commissioning. 

The project features a bimodal NOx emission guarantee structure which varies from 0.28 lb/MMBtu

with the SNCR in non-cascading mode and SCR out of service to 0.10 lb/MMBtu with the SNCR in

cascading mode and SCR in service. This is shown in Figure 16 in comparison to the 2005 baseline

NOx emissions and the target NOx emissions from the burners and SOFA alone. The combustion

equipment portion of the project is currently in the design and fabrication phase. The unit outage is

scheduled for October 2006 with unit restart in November 2006.

Figure 16. Comparison of Baseline, Target, and Guaranteed NOx Emissions 114 MWg T-fired Furnace
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APPLICATION TO TURBO® FIRED BOILERS — LNB & OFA

The Riley Turbo fired boiler or “Turbo® Furnace” is a unique design developed by RPI in the 1960’s

for burning low volatile fuels such as petroleum coke and bituminous coal with low volatile content.

The boiler design is shaped like an hourglass with a row of burners mounted opposed at only one

elevation and aimed to fire downward on a 25 slope. The axial flow burners are called “Directional

Flame Burners” with two (2) rectangular coal nozzles/burner with air introduced above, below, and in

between the two nozzles. Some Turbo® furnaces have OFA introduced in the narrowest section or

throat of the furnace immediately above the burners.

In the 1970’s, when NOx emissions were becoming a concern, it was discovered these furnaces were

inheritantly low in NOx emissions because of the diffusion type of mixing and combustion resulting

from the axial flow burners (Reference 5 and 6). Approximately two (2) dozen Turbo® Furnaces are

in operation today ranging from 85-600 MWg in capacity firing various fuels from low volatile

petroleum coke and bituminous coal to higher volatile sub-bituminous and PRB coal. In the past five

(5) years, utilities have expressed interest in reducing NOx on these Turbo® Furnaces, to meet today's

more stringent NOx emissions requirements at the state and federal level.

Through CFD modeling and physical flow modeling, RPI has developed upgrades to the DF burners

to produce lower NOx emissions. Burner modifications have included new tilting or fixed low NOx

coal nozzles, windbox partitioning and other internal burner modifications. The key factors enabling

the lower NOx focused on better flame attachment, better air flow recirculation patterns and earlier

ignition and pyrolysis of the coal in the primary combustion zone. The modifications also provided

more independent control of the combustion air within the burner windbox and to the OFA for more

precise control of the burner zone stoichiometry.

A recent example of utilizing the upgraded equipment involved two (2) 600 MWg Turbo® Furnaces

firing PRB coal. The utility needed to reduce NOx emissions on these boilers from 0.35 to <0.21

lb/mmBtu to meet state mandated emission requirements. Each of the 600 MWg boilers, shown in

Figure 17, has 32 DF burners (16 per side) mounted opposed on a sloped surface and connected to four

(4) ball tube mill systems.

Physical flow modeling and CFD analysis were extensively used to analyze and develop the

modification details for the DF burners. After a review of numerous designs, the final configuration,

shown in Figure 18, included the following key features:

* New burner head inlet vanes

* New patented tilting coal nozzles, FSR’s and air diverters

* Windbox partitioning

* Modified center opening plating
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Figure 17. 600 MWg Riley Turbo® Furnace Firing PRB Coal

Figure 18. Low NOx Directional Flame Burner for RPI Turbo® Furnace 
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The CFD modeling carried further into the furnace to evaluate the use of OFA for additional NOx

control and to ensure proper CO and carbon burnout was maintained. The challenge of this furnace

design was short retention time for final carbon burnout and the lack of an arch in the upper portion

of the furnace, which aggravated the situation. Results of the modeling defined a need to install

additional air injection openings in the four (4) corners of the furnace above the OFA and in the lower

furnace below the main burner level to control CO.

With the modified combustion system installed, NOx emissions were effectively reduced to below 

0.2 lb/106Btu (see Figure 19) as required by the state. This represented a 43% reduction from preretrofit levels. Flyash unburned carbon was <0.5% and CO emissions averaged <300 ppm which was

considered good for this difficult furnace design. Without the additional air injection locations, CO

emissions would easily exceed 2,000 ppm.

Figure 19. Pre & Post NOx Performance — 600 MWg Turbo® Furnace Firing PRB Coal

Other significant benefits of the modified combustion system included:

* Improved SH & RH temperature distribution profiles

* Reduced attemperator spray flow

* Improved lower & upper furnace combustion conditions

* Reduced slagging and fouling
SUMMARY

Reducing NOx emissions from coal fired utility boilers using combustion control only continues to be

a cost effective approach for meeting environmental regulations. Recent experiences at Riley Power

have primarily involved upgrades to or replacement of existing first and second-generation low NOx

burners with or without overfire air. Actual field performance demonstrated NOx reductions of 

15-65% from pre-retrofit NOx levels. Overall boiler operation also improved in some cases with

reduced CO emissions and lower unburned carbon. In one installation, the superheat attemperator

spray flow quantity was reduced 90% because of less flame carryover into the backpass. RPI’s

experience presented in the paper, covers a wide range of boiler designs and types including wall fired,

T-fired, and Turbo fired furnace designs. The low NOx burner technology included patented designs

proprietary to Riley Power. CFD modeling was extensively used in each of the projects discussed to

refine the design details and to establish initial equipment settings to shorten commissioning time.

ACKNOWLEDGEMENT

The authors wish to acknowledge the significant contributions by Bonnie Courtemanche, Staff Engineer, Babcock

Power in the preparation of this paper. Bonnie was instrumental to the success of implementing low NOx burner

technology on the wall fired and turbo fired boilers discussed in this paper. We are grateful for her dedicated

support. 

REFERENCES

(1). Lisauskas, R.A, McHale, C.E, Alfonso, R. “Development of Overfire Air Design Guidelines 

for Front-Fired Boiler.” Presented at Joint Symposium on Stationary Combustion NOx

Control, 1987.

(2). Courtemanche, B, Esclavon, L., Curow, C. “CCV Components and Overfire Air Improve 

Existing First Generation Low NOx Burner Emissions,” 30th International Technical 

Conference on Coal Utilization and Fuel Systems, Clearwater, Florida, 2005. 

(3). Johnson, R., Borriello, S., Dodson, K., “Alternate Solutions for Reducing NOx Emissions from

Tangentially Fired Boilers,” Power Gen 94 Americas, Orlando, Florida, 1994.

(4). Courtemanche, B., Dorman, D., Yilmaz, A., Esclavon, L., “Reducing NOx Emissions and 

Commissioning Time on Southern Company Coal Fired Boilers with Low NOx Burners and 

CFD Analysis,” Combined Power Plant Air Pollution Control MEGA Symposium, Washington

DC, 2003

(5). Lisauskas, R.A., Marshall, J., “An Evaluation of NOx Emissions from Coal-Fired Steam 

Generators,” EPA/EPRI Joint Symposium on Stationary Combustion NOx Control, Denver, 

Colorado, 1980.

(6). Lisauskas, R.A., “Design and Operation of Coal-Fired Turbo Furnaces for NOx Control,” 

Committee on Power Generation Association of Edison Illuminating Companies, 1979.

© Riley Power Inc. 2006