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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
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5
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5 55
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55555
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5
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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.