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Babcock & Wilcox 1

A.D. LaRue

M.A. Costanzo

J.A. FulmerK.J. Wohlwend

Babcock & WilcoxBarberton, Ohio, U.S.A.

Presented to:ASME International Joint Power Generation Conference

July 24-25, 2000Miami, Florida, U.S.A.

Update of B&Ws Low NOx Burner Experience

BR-1702

AbstractBabcock & Wilcox has installed low NO

xcombustion equip-

ment in more than 180 PC-fired steam generators representing

over 78,000 MW of electrical generation. These include new

boiler and retrofit applications covering a diverse range of coals

and furnace geometries. This paper presents a summary of theresults and experience gained from these installations. In addi-

tion, a few selected units are considered in more detail relative

to emission performance and case-specific technical issues.

These include situations with low NOx burners only, and also in

combination with air staging ports (OFA) to provide even greater

NOx

reduction. NOx

emissions at or below 0.20 lb/million Btu

are being maintained long term in some utility boilers firing

reactive coal, and may offer an option to SCRs in some situa-

tions. Significant advancements in the area of low NOx

PC-fired

burner design will also be presented. The most advanced plug-

in burners provide a step reduction in NOx

emissions, as may

be expected, but along with a simultaneous step reduction in

unburned carbon.

Progress in combustion emission control from gas/oil firedretrofits will also be presented. This includes a recent applica-

tion, plus latest developments concerning a new gas-fired de-

sign which sharply reduces NOx

for gas, or gas-capable PC-

fired burners.

Introduction and BackgroundThe marketplace for electrical power generation in the United

States is undergoing massive changes. Deregulation is leading

to major growth in gas-fired combined cycle systems to aug-

ment capacity and/or compete with conventional boiler/steam

turbine plants which are predominantly coal-fired. Increasingly

restrictive emission regulations are simultaneously requiring

additions of equipment to further reduce airborne emissions of

NOx, SO

2and particulate from the power industry. The billions

of dollars being expended for back end clean up systems add to

the complexity as well as expense of operation and maintenance.Meanwhile plant staffing remains on a downward trend driven

by cost control, and mega mergers are intended to further re-

duce overall staffing and costs.

Success in an economically deregulated (re-regulated is more

like it) market will increasingly depend on selecting the most

cost effective means of complying with emission regulations.

Low NOx

burners provide a means for coal-fired burners to sig-

nificantly reduce NOx emissions at the source, in the combus-

tion process. Low NOx

burners, sometimes with air staging ports

(NOx

ports), can reduce NOx

emissions to levels required by

the Title IV Phase I and II of the Clean Air Act Amendments.

Phase II limits most wall-fired (coal) units to 0.46 lb/million

Btu. Thats history. NOx

reductions to the 0.15 lb/million Btu

level, or even lower, are becoming a requirement in some statesand are expected to become widespread in the near future. The

0.15 lb/million limit appears to be at or beyond the capability

of most low NOx combustion systems, even given favorable coal

attributes. B&W has successfully developed an ultra low NOx

burner design which has reduced NOx

below these limits dur-

ing large scale prototype tests. The first retrofit of these DRB-

4ZTM burners is now in the early stages of commissioning, and

may demonstrate this capability for wall-fired boilers.

For situations where low NOx burners cannot fully satisfy

the NOx

emission limits, conventional wisdom has been that

they still offer the most cost effective first step in NOx

control.

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2 Babcock & Wilcox

While generally true, this will not always be the case, depend-

ing on the existing level and final NO x emission target. Selec-

tive Catalytic Reduction (SCR) technology enables NOx

reduc-

tion to 0.15 lb/million Btu or lower and is becoming a necessity

at many U.S. power plants. Given the use of SCR, the need for

highly effective low NOx burners is less certain. It depends on

the existing NOx

emission levels and the final target value. In

some cases, especially where the present NOx level is fair ly low,

it may prove more cost effective to supply a larger SCR to handle

the burden. Otherwise, low NOx

burners continue to carry the

first responsibility for significantly reducing the formation of

NOx.

Collective experience in the power industry has shown that

burners labeled low NOx actually exhibit considerable varia-

tion in NOx reduction capability among suppliers. NOx perfor-

mance depends on the quality of the technology embodied in

the design, as well as the durability of the equipment under the

severe conditions in which they must function. Decisions based

on low capital cost can backfire if the equipment fails to per-

form well, or cant take the heat. Air staging systems, to aug-

ment low NOx burners, face similar criticism. Basic systems

may or may not provide satisfactory short-term performance.

Best long-term results depend on a careful analysis of the

furnace geometry and physical constraints, coupled with numeri-

cal modeling to determine the most suitable size and arrange-

ment of NOx ports. NOx ports which provide adjustable air flow

mixing characteristics enable performance optimization during

commissioning. Neural networks are being employed to assist

in NOx reduction and to reduce day-to-day fluctuations.

B&Ws leadership in the low NOx burner business is evi-

denced by the breadth of experience, the depth of NOx

reduc-

tion achieved by these systems and continued investment in

R&D to advance the performance of equipment. This paper pro-

vides an overview of B&Ws experience, some examples of re-

cent applications, and descriptions of latest developments in low

NOx burner design.

NOx

Formation and ControlNO

xformation during the combustion process occurs mainly

through the oxidation of nitrogen in the combustion air (ther-

mal NOx) or from oxidation of nitrogen bound to the organic

matter in the coal (fuel NOx). Thermal NO

xis the dominant for-

mation mechanism for natural gas and for fuel oils with little or

no fuel nitrogen. Thermal NOx increases exponentially with tem-

perature and is suppressed by techniques which reduce flame

temperature and reduce concentration of oxygen. These include

low NOx burners, air staging systems, and flue gas recircula-

tion to the combustion equipment.

Fuel NOx formation, while complex, can be inhibited by re-

duction of oxygen concentration and temperature during theearly stages of combustion. Fuel NO

xis the primary formation

mechanism during coal combustion, and is highly dependent

on coal properties. Highly reactive coals, with low fixed carbon

to volatile matter ratio, and coals with low nitrogen content dis-

play lower NOx forming tendencies. Fuel NOx can be suppressed

by low NOx burners, air staging systems, and by fuel switching

to more reactive coal.

Low NOx burners regulate the rate of air introduct ion during

the early stages of combustion, usually by use of multiple air

zones and hardware to control mixing rates. This reduces oxy-

gen availability as the coal devolatizes and reduces peak flame

temperatures, limiting NOx

formation. Reducing air mixing rates

with the coal can cause undesirable consequences as well, such

as reduced flame stability or increased unburned carbon. Mul-

tiple air zones in low NOx

burners make it possible to swirl or

otherwise induce recirculation sufficiently for good flame sta-

bility with just a portion of the air flow. The remaining air flow

is more gradually mixed downstream to complete combustion.

Air staging systems divert a portion of the combustion air from

the burners, typically 10 to 30%, for introduction downstream

in the process.

Again, diverting air from the burners can impair flame sta-

bility and this must be considered when adding air staging to an

existing burner system. Diverting large quantities of air from

the burners more effectively reduces NOx. However, operating

the burners below theoretical air flow (substoichiometric) causes

corrosive gases and compounds to form during combustion of

coals with high sulfur content. These can damage unprotected

steel tubing in the burner zone. A well designed air staging sys-

tem produces rapid thorough mixing of the staged air with prod-

ucts of combustion from the lower furnace, and minimizes un-

burned combustibles (UBC).

B&W Low NOx Coal Burner ExperienceB&W low NO

xburners have been installed in over 180 PC-

fired units (Table 1), amounting to over 78,000 MWe of capac-

ity. The Dual Register Burner (DRB) was developed and then

deployed in 71 new boiler applications dating from 1971, to

satisfy new source performance standards (NSPS). NSPS limits

were initially 0.7 lb/million Btu and were reduced to 0.6 and

0.5 lb/million Btu over the next decade. DRBs were applied to

utility units ranging in size from under 100 to 1300 MW e ca-

pacity. The DRB proved uniquely capable of satisfying these

limits without the need for air staging systems. With reactive

coals, like subbituminous Powder River Basin coals, the DRB

frequently reduced NOx to 0.3 - 0.4 lb/million Btu. The DRB,

first of the first generation low NOx burners, showed itself tobe quite effective as a NOx

control device. However, the me-

chanical design was not well suited for retrofit application to

existing boilers, and it tended to suffer binding and warpage

after long term use.

In the mid 1980s, B&W embarked on development of a sec-

ond generation low NOx

burner. This burner was intended for

retrofit use in existing boilers, as well as for new boilers. The

DRB-XCL was developed for use in wall-fired boilers from

all manufacturers, with fuel inputs up to the ~300 million Btu/

hr range. The DRB was mechanically reconfigured to an axial

Table 1

B&W Low NOx

Combustion Systems

Wall Fired and Pulverized Coal

78,226 MW186 Boilers

5,558 Burners

Burner Type Units Burners Capacity, MWe

DRB 71 3,284 41,065DRB-XCL 102 1,854 29,216LNCB 8 196 4,939DRB-4Z 5 224 3,006

Totals 186 5,558 78,226

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Babcock & Wilcox 3

flow arrangement, to facilitate air flow control. The aXial Con-

trol Low (XCL) NOx burner (Figure 1) uses a simple, reliable

sliding air damper to control air flow to each burner. A linear

actuator is used to automate this damper. Each burner is equipped

with a pitot grid to measure air flow just past the entrance of the

burner. The air then travels through one of two parallel paths

the inner zone or outer zone. The inner zone is equipped with

adjustable vanes which swirl the air to stabilize the flame. Theouter zone is equipped with fixed vanes as well as adjustable

vanes. Together they improve peripheral air distribution in the

outer zone and swirl the majority of the air more moderately for

mixing later in the flame. The inner and outer adjustable vanes

enable emission optimization on-line during commissioning, and

are then fixed in place.

The new hardware design provided a highly reliable means

of adjusting the components. The burner nozzle hardware op-

tions include Conical Diffusers (lower NOx) and bladed impel-

lers (shorter flames, lower UBC). The mechanical structure of

the DRB-XCL has also proven to be highly rugged and reli-

able. The main outer barrel is constructed of heavy steel plate

with a system of stiffeners which accommodate thermal expan-

sion without distortion. All front end components are high qual-

ity stainless steel. A Flame Stabilizing Ring on the coal nozzle

proved to be a mechanical weakness of early versions of the

DRB-XCL. Further development work was performed to en-

able elimination of this hardware while maintaining emission

performance.

Referring again to Table 1, the DRB-XCL was the burner

of choice for over 100 PC-fired boilers over the past decade,

amounting to nearly 30,000 MWe

of capacity. These included

some applications for new boilers, but primarily were retrofits

to wall-fired boilers. These represent considerable diversity in

capacity (20 to 1100 MWe); in burner input (50 to 300 million

Btu/hr firing rate); in coal (lignite to low volatile bituminous);

and in burner arrangement (single wall, opposed wall, roof fired,

and plug-in for cell burners). The burners were used with air

staging ports in some cases, and increasingly so as time goes on

to meet lower emissions standards.

Figure 2 summarizes NOx emission performance for the DRB

and DRB-XCL PC-fired burner population. The graph dis-

plays EPA Continuous Emission Monitor (CEM) published NOx

emission data, or full load test data where it is available, for

110 units. This is not hand-picked data, but rather displays all

units for which data was readily available. Note that the popu-

lation includes boiler applications with B&W DRB combustion

equipment installed back as far as 1975 as original equipment.

The NOx

limit at that time was 0.7 lb/million Btu. All of the

units are at or lower than their original NOx guarantee level.

The graph also includes XCL emission points, which include

retrofit and some new boiler applications. The right hand por-

tion of Figure 2 characterizes the data by fuel and combustion

system. Clearly the coal rank, as it relates to reactivity, has a

major impact on emissions. The less reactive medium and high

volatile bituminous result in the higher emission levels, while

subbituminous and staged subbituminous produce the lowest

NOx. There is not much difference in actual unstaged emission

data between the DRB and DRB-XCL. The DRB-XCL is

known to produce lower NOx than the DRB under the same con-

ditions. However, much of the XCL data is from retrofits into

Outer ZoneAdjustableVane

Inner ZoneAdjustableVane

BladedImpeller

Stationary Vane

Air MeasuringPitot Grid

SlidingDisc

DistributionCone

Pulverized CoalandPrimary Air Inlet

Figure 1 Low NOx DRB-XCL burner.

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

0 200 400 600 800 1000 1200

5

5 5

5

5 55

55

55

55 5

5

555

5

555555

55555555

5

555 5

5555555

55

5

5

55

55555

5

55

5

5

5

555

55

5

5

55

5 55

5

55555

55555

5555555

5

5

55

555

555

Capacity (MWe)

NO

x(lb/millionBtu)

NOx Emissions Units Characteristics(lb/million Btu)

>0.50 10 Bituminous or medium volatilebituminous

0.40 - 0.499 39 Bituminous

0.30 - 0.399 36 Subbituminous/lignite or staged bituminous

0.20 - 0.299 24 Subbituminous/lignite or staged bituminous

0.10 - 0.199 1 Staged bituminous

B&W Low NOx

Combustion SystemsNOx

EmissionsWall Fired and Pulverized Coal

(EPA-CEM data or full load test data when available)Burners Installed 1975 to 1999

Figure 2 B&W low NOx

combustion systemsNOx

emissions.

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4 Babcock & Wilcox

hotter, more compact furnaces versus the DRB. This offsets

much of the lower NOx

characteristic of the XCL. The staged

cases are with DRB-XCL burners, and show NOx emission lev-

els around 0.25 lb/million Btu with subbituminous coal, with

two cases in the 0.15 to 0.20 range.

Recent RetrofitExperiences with DRB-XCL

A few examples of recent retrofit s are described here to pro-

vide an indication of performance on specific applications.

Karn 2The first case is for Consumers Energy Karn Station, Unit 2.

This is a moderately sized single-wall-fired unit with a capac-

ity of 265 MWe. The B&W boiler is equipped with six EL-76

pulverizers serving 24 circular (original) burners located on the

front wall (Figure 3). The vertical burner spacing is cell-likefor the upper and lower pairs, with only 6 ft (1.83 m) separating

burners. The burners are grouped horizontally in pairs also, to

fit between two (partial depth) division walls which partition

the furnace into thirds. The furnace is 60 ft wide x 23 ft deep x

94 ft [18.3 x 7 x 28.65 m] high (hopper to roof ). The unit fired

bituminous coal with FC/VM ~1.5, sometimes blended with

PRB. The new low NOx

system was designed to fire up to 100%

of either coal. The low NOx

combustion system consists of 24

DRB-XCL burners which fit into the original burner openings.

No pressure part modifications were necessary to accommodate

the burners. New lighters were also supplied with the burners.

An air staging system was included to further reduce NO xemissions for Karn 2. B&W numerically modeled several alter-

nate NOx port arrangements to evaluate upper furnace mixing

effectiveness. The best solution made use of six NOx ports po-

sitioned ~10 ft (3 m) above the top burner row on the opposite

firing wall. Dual Zone NOx

ports (Figure 4) provided control of

air mixing rates from the NOx

ports. The Dual Zone NOx

port

provides high jet penetration when required by using the core

zone, and by limiting swirl in the outer air zone. Near field mix-

ing can be increased by adding more swirl to the air passing

through the outer air zone. An automated sliding sleeve damper

regulates total air flow to the port. A pitot grid measures air

flow to the ports, providing information to assist in commis-

sioning adjustments. The scope included complementary equip-

ment for the NOx ports, including the windbox, pressure part

openings, and connecting ductwork.

Dynamically Staged Variable Speed (DSVSTM) rotating

classifiers were provided to control fineness with the EL-76

pulverizers. This enables adjusting fineness to higher levels to

reduce UBC with bituminous coal, or to lesser fineness and

higher throughput with PRB coal. The DSVS classifiers

proved capable of fineness in excess of 90% passing 200 mesh,

but were normally operated with moderate f ineness having 68%

minus 200 mesh and 99.7% minus 50 mesh and 94.7% minus

100 mesh. The customer purchased additional equipment to

improve coal flow control, including gravimetric feeders, new

PA fans, and valves for balancing coal flow among the coal pipes.

Pre-retrofit emissions were tested and NOx was found to be

0.84 lb/million Btu at full load, with unburned carbon at 8.9%

in the flyash. NOx emissions were reduced to 0.36 lb/million

Btu with the new B&W equipment, which amounts to 57% re-

duction, while UBC went to 8.3% (Figure 5). UBC could be

further reduced by increasing fineness.

Belews Creek 2A recent retrofit in the category of large, opposed-f ired unitswas at Duke Energys Belews Creek, Unit 2. This unit has a

gross capacity of 1168 MWe. The B&W supercritical steam gen-

erator is equipped with 10 vertical spindle mills which supply

40 2 nozzle cell burners (80 throats). The furnace is 89 ft

wide x 51 ft deep x 190 ft high ( 27 x 15.5 x 40 m). (See Figure

6) The original burner arrangement had vertical spacing of 4.5/

10.4/4.5 ft (~1.4/3.2/1.4 m), resulting in a very high burner zone

Figure 3 Consumers EnergyKarn 2.

NOx PortSupport System

LinearActuator

Total FlowAir Damper

BiasingAir Damper(Inner Zone)

AdjustableVane

Inner Air Zone

Pitot Grid

Outer Air ZonePitot Grid

SlidingLinkage

Figure 4 Dual air zone NOx port with total air flow control.

5 6 7 8 9 10

1

0.8

0.6

0.4

0.2

0

NO

xEmissions(lb/m

illionBtu)

Unburned Carbon (%)

Figure 5 Consumers EnergyKarn 2 emissions reductions.

Low NOx

Original

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6 Babcock & Wilcox

million Btu with 3.8% LOI. However, a simple adjustment per-

mitted operation in either of two modes. The lowest NOx

mode

averaged 0.26 lb/million Btu with 4.9% LOI. The lowest LOI

mode averaged 0.37 lb/million Btu NOx with only 1.3% UBC.

Figure 9 compares NOx

emissions and LOI for the standard

DRB-XCL and DRB-4ZTM burners, along with the plug-in

(HV-high velocity) versions of these burners. Clearly the thirdgeneration 4Z burners indicate significant improvements in NO

x,

with potential for simultaneous reductions in NOx and LOI emis-

sions with the plug-in design.

The first commercial application for the DRB-4ZTM is in the

installation stage at the time of this writing. Start-up is sched-

uled for May 2000 with tuning work to follow that summer.

Reliant Energy selected this technology, in combination with

Dual Zone NOx ports, to further reduce NOx emissions at their

W. A. Parish Unit 6 (Figure 10). This 690 MWe

unit fires a vari-

ety of subbituminous coals or natural gas. The combustion sys-

tem includes 56 DRB-4ZTM burners, arranged in seven rows of

8, which are installed in place of the original DRB burners. Nu-

merical modeling was performed by B&W to determine the best

arrangement of NOx ports for mixing effectiveness, within con-

straints from existing components. This resulted in a total of 12

Dual Zone NOx

ports (6 front, 6 rear). Preliminary results may

be available by time of presentation.

Low NOx Oil/Gas Combustion SystemsNOx reduction for oil- and gas-fired utility boilers is actively

continuing on new and existing units. Much of the U.S. domes-

Figure 10 Reliant EnergyW.A. Parish 6.

XCL HV-XCL 4Z HV-4ZBurner Type

0.5

0.4

0.3

0.2

0.1

0

10

8

6

4

2

0

NO

x(lb/MBtu)

LOI(%)

NOx

LOI

Figure 9 B&W low NOx

burner emissions; CEDFunstaged,bituminous coal.

Figure 11 XCL-S oil/gas low NOx

burner. Figure 12 Taiwan Power CompanyHsieh-Ho Units 1-4.

Air

Measurement

Grid

LinearActuat or

Optional

Isolation Valves

I-Jet

Oil Gun

Burner

Support System

Externally

Adjust able Spin

Vanes

Sliding

Air Damper

Core

Air DamperAdjust able

Hemispherical

Gas Spuds

tic action involves low NOx

combustion systems and SCR for

units which primarily or exclusively fire natural gas.

B&W recently conducted an in-depth development campaign

to reduce NOx emissions from our gas-capable burners. An ex-

tensive series of tests in the CEDF resulted in a new gas ele-

ment design for these burners which exhibited major NOx

re-

duction capability. Back-to-back tests in the XCL-STM burner(Figure 11) demonstrated just under 50% NO

xreduction with

the new fuel element design vs. the standard HEMI gas element

design. Actual emissions were less than 0.1 lb/million Btu with

the advanced design, and CO emissions were less than 50 ppm

in either case. These results were without use of flue gas recir-

culation (FGR) or air staging ports. Subsequent tests with FGR

reduced emissions NOx

to 30 ppm.

The new design is adaptable to other low NOx burners, in-

cluding the DRB-4ZTM. The first contract making use of the new

gas technology combined with PC-fired 4Z burners will go into

service in the summer of 2001.

Internationally, low NOx combustion systems often include

capability to fire fuel oil as the primary fuel. Recently B&W

performed retrofits to four oil-fired units in Taiwan to reduce

NO x emissions. These were Units 1-4 at Taiwan Power

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Babcock & Wilcox 7

Copyright 2000 by The Babcock & Wil cox Company,All rights reserved.

No part of th is work may be publi shed, trans lated or reproduced in any form or by any means, or incorporated into any information ret rieval system,without the written permission of the copyright holder. Permission requests should be addressed to: Market Communications, The Babcock &Wilcox Company, P.O. Box 351, Barberton, Ohio, U.S.A. 44203-0351.

Disc laimer

Although the informat ion presen ted in this work is bel ieved to be re liable, this work is publ ished with the understanding that The Babcock & Wi lcoxCompany and the authors are supplying general information and are not attempting to render or provide engineering or professional services.

Neither The Babcock & Wilcox Company nor any of i ts employees make any warranty , guarantee, or representation, whether expressed or impl ied,with respect to the accuracy, completeness or usefulness of any information, product, process or apparatus discussed in this work; and neither The

Babcock & Wilcox Company nor any of its employees shall be liable for any losses or damages wi th respect to or resul ting from the use of , or t heinability to use, any information, product, process or apparatus discussed in this work.

Companys Hsieh-Ho Power Station, which are rated at 512 MWe

each. The furnaces are 52 ft wide x 45 ft deep and 150 ft high

(15.85 x 13.7 x 45.7 m). The original parallel flow burners were

arranged four wide by three high, in an opposed fired pattern

(Figure 12). Taiwan Power contracted with B&W to supply a

new low NOx

combustion system which included 24 XCL-STM

oil burners and lighters; 8 Dual Zone NOx

ports; NOx

port

windboxes, dampers, and air flow measurement equipment; and

pressure part openings. The new equipment reduced full load

NOx to an average of 141 ppm with less than 50 ppm CO (ppm@ 6% O2), satisfying guarantees.

SummaryB&W is a leading supplier of low NOx combustion systems

to the utility boiler industry. The DRB-XCL has proven to be

highly effective, typically reducing NOx to the 0.2 to 0.5 lb/

million Btu range, depending on system design and coal prop-

erties. Continued investment in combustion technology has re-

sulted in third generation designs, capable of significant fur-

ther emission improvement. The latest advancements in low NOx

combustion equipment design include plug-in versions of the

DRB-XCL and DRB-4ZTM burners. These burners indicate step

reductions in unburned carbon emissions compared to the in-

dustry standard DRB-XCL, in combination with further NOxreduction. Reducing NOx at its source by advanced combustion

systems can reduce the size, or potentially the quantity of SCRsystems necessary to meet regulated limits.

Low NOx oil and gas combustion systems can provide full

compliance with emission requirements in many situations, do-

mestically or internationally. B&Ws latest generation of low

NOx gas burners indicate nearly 50% NOx reduction compared

to our previous design. The gas element is designed for use in

advanced coal-fired burners in addition to oil/gas burners.