burner low nox

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FOSTER WHEELER ENERGY INTERNATIONAL

Services Division

TECHNICAL PAPER

WALL FIRED LOW NOx BURNER EVOLUTION FOR

GLOBAL NOx COMPLIANCE

FW AUTHORS:

Tom Steitz

Manager, Firing Systems

John Grusha

Director, Firing Systems andProduct Development

Ross Cole

Combustion Engineer

The 23rd International Technical Conference on

Coal Utilization & Fuel Systems

March 9-13, 1998

Clearwater, Florida USA



ABSTRACTLow NOx Burners for wall-fired boilers have substantially evolved

since their inception in the early 1970's. Foster Wheeler (FW) has

been a leader in the development and advancement of this technology

over these years. The intent of this paper is to present the

development and commercialization path of a new Foster Wheeler

Low NOx Burner (LNB) designed specifically for a deregulated

Utility Market and Industrial applications. In this market, Utilitiesand Industrial users demand high value solutions and equipment to

generate clean, environmentally friendly electricity. This Vortex

Series/Split Flame (VS/SF) LNB satisfies these demands by providing

a less complex, rugged and reliable solution to achieve low NOx

levels at optimum unit efficiency.

This technology is applicable for the EPA’s Title IV, Phase II Acid

Rain retrofits in addition to imminent Title I Ozone retrofit

regulations. It is also ideally suited for new/retrofit industrial and

utility plants worldwide. Significant capital and maintenance cost

advantages are available with superior performance.

COAL BURNER DESIGN EVOLUTION

A. Foster Wheeler Turbulent Burners (pre 1971) - IV Burner

Prior to the advent of low NOx requirements, conventional wall fired

coal burner designs consisted of a highly turbulent mixing burner,

designed to mix the fuel and air as rapidly as possible to maximize

combustion efficiency. FW’s turbulent Intervane (IV) burner was one

of the most efficient burners available in the power industry. The IV

burner is depicted in Figure 1. The burner was installed in two

separate modules, consisting of:

1. Secondary air register, including;

a) Single Air Zone

b) Multiple Radial Blade register vanes, linked

together

c) Large overall diameter

d) High swirl efficiency

e) Low Secondary Air pressure differentials 2. Fuel Injector

a) A tangential scroll inlet for optimum coal

distribution and swirl

b) An open ended nozzle for optimum mixing with

the secondary air

c) An inner barrel to house ignitor/main fuel

THROAT

WINDBOX AREA

INJECTOR

REGISTER

Intervane Burner

B. Foster Wheeler First Generation LNB (1976) - CF Burner

FW’s 1st generation low NOx Burner, the Controlled Flow (CF) LNB

depicted in Figure 2, was developed in the early 1970's and first

installed in 1976. The low NOx concept was to divide the secondary

air flow stream into two distinct paths to produce air separation from

the coal, or air staging, within the near burner throat area. This air

staging controls fuel bound nitrogen from mixing early with secondary

air, thus inhibiting fuel Nitrogen conversion to NOx. In addition, thiscontrolled mixing reduces peak flame temperatures, limiting the

formation of “thermal NOx” within the flame.

A sleeve damper shroud and was placed over the entire air register

assembly in order to balance and modulate air flow on a per burner

basis. This is a necessity when striving for optimum mixing for low

NOx in an open windbox arrangement. When burners are placed in

and out of service, this sleeve damper automatically adjusts for the

proper air flow per burner.

Primary Air/Coal Jet Control was an addition to the Controlled Flow

burner as well. This is a unique feature found only on Foster

Wheeler’s low NOx burners. It allows for the ability to control the exi

characteristics of the primary air and coal jet without changing primary air flow. This manual adjustment gives the boiler operations

personnel the on line flexibility to stabilize and optimize firing for a

wide range of coals with a single burner design.

Many of the proven mechanical features and sizing criteria from the

turbulent IV burner were carried through into the CF LNB. A

comparison of these two burners, in addition to later generations of

FW LNB’s is depicted in Table 1.

Figure 1

Figure 2



TABLE 1 Foster Wheeler Burner Designs

A B C D E F

Burner Type IV CF CF/SF IFS PF/SF VS/SF

Year <1971 1976 1979 1991 1994 1997

Number of Modules 2 2 2 2 1 or 2 1 or 2

Register

# Air Zones 1 2 2 2 2 2

Blade type Radial Radial Radial Radial Radial Axial

Flow Path Series Series Series Series Parallel Parallel

Relative Register Diameter 100% 120% 85% or 120% 85% or 120% 75% 65%

Swirl Efficiciency high high high high high high

dP low low low low low low

Fuel Injector

Inlet scroll scroll scroll scroll scroll or elbow scroll or elbow

Exit tip open open split split split split

Relative IB Diameter 100% 65% 65% 65% 35% 20%

Ignitor/Main Fuel IB IB IB IB or Ann IB or Ann IB or Ann

NOx

Reduction Potential Uncontrolled up to 30% 50-65% 50-65% 50-65% 50-65%

IB - Inner Barrel Ann -Secondary Air Annulus A - Axial Blades

C. FOSTER WHEELER SECOND GENERATION LNB (1979) - CF/SF

In 1979, FW introduced its 2nd Generation of Low NOx Burners, the

Controlled Flow/Split Flame (CF/SF) Burner. This burner is depicted

in Figure 3. The Split Flame fuel injector was a major advancement

in controlling NOx from the burner alone, more than doubling the NOx reduction potential of the CF burner. In essence, a multi-port

Split Flame nozzle (Figure 4) was added to the end of the fuel injector

of the CF burner. The burner air flow paths for this design are

depicted in Figure 5.

The Split Flame nozzle accomplishes additional NOx reduction by

internally fuel staging the coal. It is the key component of Foster

Wheeler’s low NOx burner technology. It is installed on over 100

units worldwide firing all types of fuels. The patented design

segregates the coal into multiple concentrated streams producing a

fuel staging effect which inhibits NOx formation. The coal separation

forces fuel devolatilization in very low O2 concentrations, promoting

the reaction of fuel bound nitrogen to form harmless gaseous N2. The

material utilized for the split flame tip is a high nickel chromium alloycasting to protect it from the high radiant heat fluxes in the burner

zone.

As retrofit opportunities increased in the late 1980’s, it became a

challenge to fit the full size Controlled Flow register into some older

boilers where the burners were more closely spaced. Spreading out

the burner pattern to accommodate the register, either horizontally or

vertically, became a costly undertaking and not always possible within

the given constraints of retrofit applications. Thus, FW began down-Figure 3



sizing the register assembly to fit into existing burner spacing. When

downsizing, the widths of the inner and outer register blades were

elongated to maintain a similar free area and pressure drop compared

to the full size registers. See Figure 6. The register diameter itself

was reduced by 30% with this design (Refer to Table 1).

Coal Nozzle Comparison

Split Flame (SF) Internal Fuel Staged

Nozzle (IFS)

Figure 4

Figure 5



Figure 6



TABLE 2 - Full Size vs. Down Sized Performance Comparison

For Identical Heat Liberation Burners

Unit Size

Boiler

OEM

Heat Lib,

MMBtu/hr Dt, in

Register

Diam, in

NOx

lb/MMBtu UBC, % # of LNB’s

180 FW 150 42 93 0.43 12 12

120 RS 150 41 64 0.44 12 8

Dozens of units have been retrofit with down-sized Controlled Flow

registers with equal or better performance. Refer to Table 2 for a

summary of two near duplicate FW LNB applications, the only

difference is that the first unit was designed for full-size registers and

the second for down-sized registers.

D. FOSTER WHEELER THIRD GENERATION LNB

(1991-1995) - IFS BURNER

In 1991, FW introduced the Internal Fuel Staged (IFS) Low NOx

Burner. This burner utilized the proven CF register in combination

with a revised fuel injector. The configuration of the split flame tip

was adjusted to increase the number of coal ports from four to six and

to add an internal swirl zone. These differences are depicted in

Figure 4.

The IFS burner was installed in 40 Utility units during Phase I of the

Clean Air Act for Acid Rain. Both full size and down-sized CF

registers were used with the IFS burner, depending on space

requirements. Ignitors could be located down the center inner barrel

or could be sighted through the secondary air annulus to save space in

the burner.

E. FOSTER WHEELER FOURTH GENERATION LNB

(1994-CURRENT) - PF/SF

As competition increased at the end of Phase I of the Clean Air Act,

customers continued to demand high value NOx Reduction Systems.

FW was prepared for this market with the introduction of the less

complex and simpler to operate Parallel Flow/Split Flame (PF/SF)

LNB (Figure 7). The inner air flow path is adjusted from a series to a

parallel flow path. The inner register assembly is moved away from

the fuel injector allowing even smaller diameter registers without a

sacrifice in performance. Based on laboratory and field results, the

swirl is removed from the inner air zone, replaced with a simple

conical air flow damper. For certain fuels, fixed swirlers can be addedto the inner zone as needed. Figure 6 depicts a comparison of the

Controlled Flow (Series Path) and Parallel Flow Registers.

Parallel Flow Split Flame (PF/SF) Low NOx Burner

The fuel injector for the PF/SF burner is also reduced in size by

limiting the size of the inner and outer barrel diameters. The scroll

inlet was maintained for early versions of this burner. However, the

scroll-less inlet, developed in 1996-1997 for the FW’s next generation

of LNB’s, has been adapted to subsequent PF/SF and other contracts(Table 3):

Figure 7



TABLE 3 - Scroll-less Burner Applications

Unit Size OEM

Current

Burner FW Burner # of LNB’s

1300 B&W Cell PF/SF 88

120 FW New Blr VS/SF 8

120 FW New Blr VS/SF 8175 RS KVB CF/SF 18

175 RS KVB CF/SF 18

To reduce installation costs and down-time, the PF/SF burner

possesses the attractive option of being fabricated and installed as a

single piece module. The reduced diameter and weight of the register

and fuel injector allows for easy rigging of a single piece module - this

allows for true “plug-in” retrofits from the burner front. This single

piece module is illustrated in the PF/SF burner of Figure 6 and is

depicted in the photograph of Figure 8.

F. FOSTER WHEELER FIFTH GENERATION LNB

(1997-CURRENT) - VS/SF

As deregulation rapidly descended on the Utility Market in the USA,

the demand continued for high value, cost effective NOx solutions.

By 1996, it was apparent that a new LNB design was required to meet

this market demand. FW responded by developing the New Vortex

Series/Split Flame (VS/SF) burner.

The performance of the previous generations of FW LNB’s had been

at or above industry levels. It was desired to maintain equal or

improved performance of these burners compared to previous LNB

designs. This includes the same throat aerodynamics, velocities, and

mixing patterns as the previous LNB design(s), but delivering the air

and fuel in a more simple, reliable manner. Thus, the “business end”

of the burner, where the actual fuel/air mixing and combustion is

initiated, remains the same as the established previous designs. Only

the air/fuel delivery system has been optimized.

Figure 6 illustrates the evolution of FW LNB technology over the past

22 years. The more compact and simple current designs offer many

fabrication, installation and maintenance advantages over it’s

predecessors with reliable and competitive emissions and combustion

performance.

VORTEX SERIES/SPLIT FLAME LOW NOx BURNER This Vortex Series/Split Flame (VS/SF) Low NOx burner is depicted

in Figure 9. The “Vortex” implies the type of air flow pattern the

register produces. The register is mated to the established FW Split

Flame Coal Nozzle which is adaptable to most OEM axial or

tangential coal nozzle inlets.

The configuration is even smaller and less complex than previous

designs. It is designed to reduce manufacturing costs and simplify

installation. It can be installed as a one-piece module saving

installation costs, and is more operational and maintenance friendly.

It was developed and designed employing the latest Computer Aided

Design (CAD) and Computational Fluid Dynamics (CFD) software.

Most importantly, it’s design continues to provide users with the

rugged and reliable features known of Foster Wheeler Low NOx

Burners.

Figure 8



DEVELOPMENT OF VORTEX SERIES LNBThe most noticeable difference between the Vortex Series and the

previous FW Low NOx burners is the method used to induce swirl to

the outer zone air flow. The split stream coal nozzle remains as an

integral component of the Vortex Series Burner. While the FW CF

(Series flow) and Parallel style register burners have numerous

adjustable vanes radially positioned to produce the swirl, the Vortex

Series design uses an adjustable “one piece” axial swirler to induce

swirl and provide flame shape control (See Figure 10). This less

complex arrangement reduces the number of “moving” components

from approximately twenty to only one.

As in all FW LNB designs, only one automatic adjustment is

necessary “on line”. It is the burner sleeve damper for individual

burner air flow control. The outer air register, inner air register, and

inner sliding tip are one-time adjustments during commissioningwhich operate as fixed dampers over the life of the boiler, unless a

major fuel or operational change warrants re-adjustment. The VS/SF

has four linear adjustments - the automatic sleeve damper, the sliding

outer air swirler, the sliding inner air cone, and the sliding fuel

injector tip. No complex links, turnbuckles, or control rings,

commonly used on other burner designs, exist with this burner.

Figure 9

Figure 10



RADIAL BLADES VERSUS AXIAL BLADESBased on an industry experience with low NOx burners, both radial

and axial swirl registers can produce the air flow patterns necessary

for achieving stable flame combustion, turndown, and emissions.

NOx emission reductions greater than 50% have been demonstrated

worldwide on both types firing a wide range of coals.

A key requirement of any low NOx burner register is to produce astrong swirling air flow pattern exiting the burner throats, coupled

with a well defined internal recirculation zone. The recirculation

pattern within this swirling vortex reduces the velocity and

penetration of the coal jet. This aids in establishing a flame front near

the coal nozzle tip. The coal, which ignites and devolatizes under fuel

rich conditions in this inert recirculation zone, prevents the complete

conversion of fuel bound nitrogen to NOx. The surrounding swirling

air shears the coal jet around its circumference, while radially

stratifying the flow with its angular momentum. Later, in the

combustion process, the swirl allows a gradual entrainment of fuel and

air to complete carbon burnout. The capability to adjust air flow

provides flame shaping as well as fuel and air mixing control,

reducing emissions and maximizing combustion efficiency.

COMPUTER FLUID AND COMBUSTION DYNAMICS

MODELINGAs part of the Vortex Series development program, CFD modeling of

the axial swirl register was conducted. Based on these results the

axial swirl design was shown to have significant swirl control.

Computational combustion dynamics modeling showed that an axial

swirler can provide significant flame shape control. The flame shape

of Run 1 of Figure 11 depicts the predicted temperature profile with

the swirlers fully inserted. The expanding temperature profile predicts a “flared out” flame pattern. This type of profile results with

high swirl. Run 2 depicts a flame profile for a retracted swirler and

less swirl. The results of the CFD modeling phase showed good flow

pattern correlation to other successful low NOx registers. This

supported proceeding to single phase flow modeling.

SINGLE PHASE FLOW MODELINGTo quantify the capability of the axial swirler register, a plastic burner

was constructed and extensively modeled in single phase air flow at

FW’s research labs in Livingston, NJ. The plastic model was a full

size replica of the burners used at FW’s Combustion and

Environmental Test Facility (CETF) in Dansville, NY. This testing

entailed comparing aerodynamics of a radial vane parallel register and

a radial vane register modified with an axial swirler. Several differentechniques were applied to quantify the swirl patterns, swirl numbers,

recirculation patterns and velocities. The single phase flow pattern

results validated the computer generated results.

Inserted - Full Swirl

Retracted - Moderate Swirl

Figure 11



PROOF OF CONCEPTAs the next step in the validation process, axial swirlers were

installed in the outer zone of two FWEC, radial vane parallel register

low NOx burners at FW’s CETF. The result was improved flame

stability throughout the load range with no degradation in NOx or

combustion efficiency.

Next, the concept was applied to an actual field installation on a 25MWg unit as a possible solution to flame instability when firing a

high moisture lignitic type coal. Previously, the unit could only be

operated with all ignitors in service. Installing fixed axial swirlers

allowed operation at MCR, without ignitors in service. Again

validation that an axial swirler produced positive performance.

FULL SCALE COMBUSTION TESTING AT CETFAs a final step in the development process, full scale testing of two 35

MMBtu Vortex Series/Split Flame Low NOx Burners was conducted

at Foster Wheeler’s CETF. To provide a direct performance

comparison, this program followed an extensive test program on the

Parallel register. Over 60 parametric test conditions were run

evaluating numerous performance aspects on several fuels rangingfrom low volatile to medium and high sulfur eastern bituminous.

Operational facets such as ignitor performance, turndown, gas co-

firing and effects of overfire air were tested and found to be adequate.

Results showed that the less complex Vortex Series/Split Flame Low

NOx Burner design produces slightly better emissions performance to

the previous LNB designs. Figure 12 depicts the comparative NOx

and UBC trend for two burners, validating results from previous

studies of the Vortex Series burner.

CONCLUSIONSIn summary, Foster Wheeler’s high performance Low NOx Burner

designs have evolved since NOx limits have been enacted world-wide

to control Acid Rain and Ozone formation. Market forces have

demanded that FW’s first generations of LNB designs be re-

engineered to be less complex, yet reliable and rugged while and

achieving superior NOx and unburned carbon performance. FW has

responded and systematically developed it’s current ParallelFlow/Split Flame (PF/SF) and Vortex Series/Split Flame (VS/SF)

burners. NOx reductions of 50-65% can be achieved with burners

only, with minimal impact on UBC and unit operation

Foster Wheeler low NOx burners are readily adaptable to any OEM

boiler. They possess simple adjustment devices. They can be

installed one-piece modules to save valuable outage time and costs.

They can mate to almost any coal conduit arrangement and burner

inlet, and are designed with the latest 3D CAD/CAM modeling

techniques. Seven boilers totaling 2200 MW and over 160 burners

have been, or are undergoing retrofit, with the PF/SF burners. The

VS/SF burner is in fabrication for two Utility contracts.

To meet the changing low NOx burner market requirements, Foster

Wheeler has continued to make a commitment to be a leader in this

technology. Foster Wheeler is already looking ahead to the next

generation of Low NOx burners. These burner will provide even

greater performance advantages through improved fuel/air mixing and

balances as well as swirl control.

Figure 12

burner low nox

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