RST-112

RECENT LOW-NOx GAS AND OILBURNER APPLICATIONS

by

Robert A. Lisauskas

Richard W. Green

and

Perumal Thamaraichelvan

Riley Stoker CorporationWorcester, Massachusetts

Presented at the

EPRIIEPA 1993 Joint Symposium onStationary Combustion NOx Control

Miami Beach, FloridaMay 24-27, 1993

Riley Stoker CorporationPO.

ox 15040: Worcester MA 01615-0040A Me0beq of the Deutsahe gabcock Group

ABSTRACT

INTRODUCTION

Over the past several years Riley hasintroduced several new low NOx gas and oilcombustion systems to the U.S. PowerIndustry. These systems are now beinginstalled on commercial operating systems.

Riley Axial Staged Return (ASR) flow burnershave been installed on several large gas-firedpackage boilers. The ASR burner is based onpatented Deutsche Babcock technology, andhas been used successfully on new and existingpower plants in Europe since 1984. Low NOxASR burners are installed on more than 70 gasand liquid fired boilers in various firingarrangements. Following extensive large scaleprototype testing at the Riley ResearchCenter, Riley also now offers its Swirl TertiaryStaged (STS) burner for boiler retrofitapplications. The STS burner can beretrofitted within existing burner openings onmost multiple burner wall-fired boiler

RECENT LOW NOx GAS ANDOIL BURNER APPLICATIONS

Robert A. LisauskasRichard W. Green

Perumal Thamaraichelvan

Riley Stoker Corporation5 Neponset Street

Worcester, Massachusetts 01606

An overview of Riley Stoker low NOx gas and oil burner technology is presented. Riley has introducedthe Axial Staged Return (ASR) flow burner for industrial packaged boiler applications. NOxperformance data are provided for several recent package boiler installations with ratings o f over 230million Btulhr. Field data are used to examine the influence of flue gas recirculation and external airstaging on NOx reduction in commercial operating systems. A new burner concept, the Axial FlameStaged (AFS) burner, is also described for small industrial boiler applications.

The AFS burnercombines the NOx reduction benefits of both air and fuel staged combustion. In addition, an updateis given on the development and application of the Riley STS gas and oil burner for larger wall-firedboiler systems. Both pilot and full-scale STS burner emission data are presented.

installations. Six STS burners were recentlyretrofitted on an industrial process steam wall-fired boiler. Currently 30 STS burners arebeing retrofitted on a 638 MW oil. and gas-fired utility boiler.

This paper describes the design and operatingprinciples of both the ASR and STS burners.Recent field emission test results from severalU.S. boiler installations are presented. Inaddition, a new burner concept, the AxialFlame Staged (AFS) burner is described. TheAFS burner employs both air and fuel stagedcombustion to control NOx emissions. AFSburners are currently being installed in Europein burner sizes up to 40 million Btu/hr.

ASR Burner Description

The basic features and operating principles ofthe Axial Staged return flow (ASR) burner are

illustrated in Figure 1. Development of theASR burner began in 1983. 12 The ASRburner is a parallel flow burner design with thecombustion air divided into primary andsecondary air streams. An air register is usedto impart swirl to the primary air entering theinner burner throat. Secondary air isintroduced axially through secondary air tubeslocated concentrically in the annular spacebetween the inner primary air throat and theburner periphery. The primary and secondaryair stream velocities are designed to aspiratehot furnace flue gas into the annular spacebetween the primary and secondary airstreams. Depending on the specific burnerdesign, approximately 20 to 30% of the fluegas produced is recirculated in this manner.This internal flue gas recirculation is animportant feature of the ASR burner. Initiallythe aspirated flue gas acts as a dividing layerbetween the primary and secondary airstreams. Later, as fuel and air mix this fluegas serves to lower both temperature and theoxygen concentration in the primary flamezone.

In summary, NOx control in the ASR burneris achieved through:

Axial staging of the primaryand secondary air streamsRecirculation of self-aspiratedhot furnace gases

The ASR burner can also be integrated withother combustion control measures foradditional NOx reduction. For example, ASRburners can be combined with furnace airstaging ports such as tertiary or overfire airports. ASR burners can also be combinedwith external or separate recirculated flue gasextracted from the furnace stack and mixedwith the incoming combustion air.

ASR burners are designed to fire a wide rangeof gaseous and liquid fuels. As shown inFigure 1, gas is fired through a series of gaslances that can be axially positioned and

individually rotated in the burner throat.These can be made while the burner is inoperation. The gas lances in combination withthe primary air swirler are designed to producestable flames across a wide operating range.Burner turndown ratios of 8 to 1 or betterhave been achieved even with gas supplypressures as low as 3 psig. ASR burners canalso be designed for turndown ratios of 20 to1 or more if required.

Both light and heavy liquid fuels are firedthrough a centrally located steam assistedatomizer. This atomizer is designed tominimize steam flow. Steam atomization flowrequirements are on the order of 4 to 5% ofthe oil flow.

ASR BURNER PERFORMANCE

Package Boiler Applications

Riley has supplied ASR burners on fourboilers at three locations. Each is a Riley MHwatertube package boiler equipped with asingle burner. One of the installations is aburner retrofit application while the other tworepresent new unit construction. Burner firingcapabilities range from 230 to 275 millionBtu/hr on these units.

Furnace plan views for two new boilerinstallations are shown in Figure 2. Unit Ahas a capacity of 180,000 lb/hr steam flow. Itis designed to fire both natural gas and No. 2fuel oil. In addition to a single ASR burnerrated at 230 million Btu/hr, it is equipped withan external flue gas recirculation (FGR)system. Flue gas is extracted from theeconomizer outlet and mixed with thecombustion air entering the burner. Thisexternal FGR system provides 13 to 20% fluegas to the burner in addition to the flue gasaspirated by the burner itself. Air foil mixingdevices are installed in the supply ductwork toinsure good mixing of flue gas with thecombustion air.

A second new ASR burner furnace installationis also illustrated in Figure 2. Unit B is a200,000 lb/hr Riley MHW package boiler firedwith natural gas. It is equipped with a singleASR burner with a maximum firing capacity of275 million Btu/hr. The Unit B furnace designis notably different than the more conventionalUnit A design. Unit B has a wider furnace inthe flame zone region. This wider furnacedesign serves to: (1) decrease burner area heatrelease, thereby reducing flame zonetemperature, and (2) improve the flow of fluegas internally recirculated through the burnernozzle. This unique furnace design is intendedfor applications with more severe NOx controlrequirements.

Like Unit A, Unit B is also equipped with-anexternal FGR system designed to mix up to20% additional flue gas with the burner airsupply. Unlike Unit A, Unit B is alsoequipped with air staging ports on each side ofthe furnace chamber. These air ports servethe same function as overfire air on large wall-fired boilers. The air staging ports areinstalled at two different furnace depths topromote uniform mixing of air and furnaceflue gases. As in the Riley overfire air portdesign, each air staging port is separated intoone third/two third flow areas, each with itsown control damper. A front view of theboiler with the associated external flue gas andair staging ductwork is shown in Figure 3.

Field Test

Short term field tests were recently performedon package boilers A and B to characterizethe emission performance of each boiler onnatural gas over its operating range. Theresults of these tests are summarized inFigures 4 and 5.

The effects of external flue gas recirculationand amount of external air staging on full loadNOx emissions are shown in Figure 4. Testsperformed on Unit B demonstrated theperformance benefits of air staging on NOx

reduction. Increased amounts of air stagingresulted in reduced NOx emissions both withand without the combined effect of externalFGR. NOx versus load for various amounts offlue gas recirculation is summarized in Figure5. Burners on both units were designed tooperate with 10% excess air. Measured excessoxygen levels at full load varied from 1.4 to2.7% during the test program. As is commonon many package boiler firing systems, thecombustion air on both units is not preheated.

Both units A and B achieved their specifiedrequired emission performance limits. Unit Awith external FGR, but without air stagingports, achieved NOx levels of 40 ppm (dry 3%02) or less with carbon monoxide (CO)emission of less than 10 ppm. Unit B, whichis also equipped with air staging ports and hasthe larger furnace design, is able to operate atNOx levels of less than 20 ppm. CO underthese operating conditions was 65 ppm. Testson Unit B also demonstrated the tradeoffbetween NOx and CO emissions during naturalgas firing. By reducing the amount of FGRand air staging, CO can be reduced to lessthan 20 ppm but with a rise in NOx to 29

PPM .

STS Burner Description

Development of the Riley STS burners begani n 1991. The goal of this development effortwas to adapt Deutsche Babcock low NOx gasand oil burner technology for retrofit on U.S.wall-fired boilers with commonwindbox/multiple burner firing arrangements.3

An illustration of a commercial STS burner isshown in Figure 6. Because the STS burner isintended for retrofit applications, it is designedto fit within existing burner openings. In orderto reduce the size of the required burneropening, the STS burner does not incorporatethe ASR burner flue gas self-aspiration system.However, the STS burner does not includeboth primary and secondary air zones.

Primary air is delivered to the center of theburner which contains gas lances, central oilgun and a primary air fixed vane swirler. Thegas lance and oil gun configurations are similarin design to the fuel injection system employedon the ASR burner. A secondary air registerwith curved overlapping blades providesvariable swirl control for the outer secondaryair. A separation gap is employed between theinner primary air core and outer secondary airannulus to help maintain distinct primary andsecondary combustion zones.

The STS burner is also equipped with movableflow control shrouds which slide over theprimary and secondary air inlet passages.These flow control shrouds serve twopurposes. First they can be used to balancewindbox air flow to each burner. A flowmeasuring pressure impact probe is positionedin the primary air passage as a relativeindicator for balancing air flow in each burner.Secondly, the shrouds can also be adjusted tocontrol the primary/secondary air splitindependently of swirl vane position. Partiallyclosing the shroud at low loads enhances theturndown capability of the burner bymaintaining sufficient swirl for flamestabilization.

STS BURNER PERFORMANCE

Large Pilot-Scale Testing

A prototype 85 million Btu/hr STS burner wastested at the Riley Research Center burnertest facility located in Worcester,Massachusetts. The objectives of the testprogram were the following: (1) tocharacterize the burner on natural gas and No.6 fuel oil under U.S. boiler operatingconditions, and (2) to evaluate the sensitivityand trade-off of various burner designcomponents and adjustments on NOxemissions and other combustion performanceparameters.

Pilot-scale test results for the Riley commercial

STS burner configuration are shown in Figures7 and 8 for natural gas and No. 6 oil. Thefuel oil used for these tests was a 2% sulfur oilwith a fuel nitrogen content of 0.5% and anasphaltene content of approximately 12%.Combustion air for these tests was preheatedto approximately 5000F.

Unstaged NOx and CO emissions as a functionof excess oxygen concentration are summarizedin Figure 7. The combined effects of externalflue gas recirculation (FGR) and air staging onSTS burner performance are described inFigure 8. Flue gas for these tests wasextracted from the furnace exit and introducedinto the windbox with the primary andsecondary air. Air staging was achievedthrough the use of overfire air ports (OFA)located downstream of the firing wall. NOxemissions of less than 90 ppm were achievedon No. 6 oil using OFA and less than 20 ppmon natural gas using both OFA and FGR.Due to the cooler thermal environment of thepilot single burner test facility, these emissionlevels are somewhat lower than those reportedfor European field tests. 3

A number of burner design configurationswere evaluated in the Riley pilot-scale testprogram including the use of a separateconcentrated flue gas stream to divide theprimary and secondary air streams. However,the final STS burner design configuration doesnot include this feature. Our testing showedthat the incremental improvement in NOxreduction provided by this separation layer didnot justify the required additional design oroperational complexity.

STS Burner Field Application

The first U.S. commercial application of theRiley STS burner is on an existing 175,000lb/hr process steam boiler located at a papermill. This Babcock and Wilcox designed boilerwas originally equipped with a wood-firedgrate. As shown in Figure 9, the unit is nowexclusively gas-fired. In 1992 this unit was

retrofitted with six STS gas-fired burners withindividual firing capacities of 38 million Btu/hr.Combustion air is preheated to above 4000F.The installation does not include a flue gasrecirculation system.

Fuel load unstaged NOx emissions for this unitare compared with pilot-scale STS burneremission results in Figure 10. The measuredfield emissions are more than 50% belowreported uncontrolled NOx levels. Asexpected, the field emission data areapproximately 60% higher than pilot test NOxvalue due to the more than fourfold increasein burner area heat release. One of thebiggest combustion problems to overcome onthis older industrial boiler was the largeamount of air infiltrating through casing leaksand various furnace penetrations. This isreflected in the higher measured 0 2 levels inthe boiler outlet duct.

Utility Boiler Retrofit

Thirty STS burners are currently beinginstalled on a 638 MW gas and oil fired utilityboiler. This retrofit is taking place atConsumers Power Company's Karn Unit No.4, located near Essexville, Michigan. KarnUnit No. 4, shown in Figure 11, is a Rileywall-fired boiler installed in 1974. It isequipped with thirty burners arranged in threerows on a single firing wall. Each burner hasa firing capacity of approximately 240 millionBtu/hr. This unit was originally designed tofire crude oil but later switched to No. 6 oilfiring.

The purpose of the retrofit is to convert KarnUnit No. 4 to natural gas firing. Because ofgas supply limitations at the Karn Station, onlythe lower two rows of STS burners will becapable of gas firing. The top STS burnerrow, or one third of the burners, will continueto fire No. 6 oil only. The retrofit does notinclude the installation of OFA or FGR. Unitrestart with the STS burner firing system isscheduled for May of 1993.

AFS Burner

Riley's parent, Deutsche Babcock, hasintroduced a newly patented low-NOx multi-fuel burner design for small industrial andcommercial firing applications. This is theAxial Flame Staged (AFS) burner. The AFSburner incorporates two low-NOx combustioncontrol features:

Axial staging of primary andsecondary air.Staged fuel addition.

As shown in Figure 12, the AFS burneremploys both internal and external gasinjection nozzles. Fuel staging is achieved bysplitting the gas input between the internal gasnozzles located in the primary air passage andthe outboard external gas nozzles locatedaround the burner periphery.

Deutsche Babcock is currently installingmodular AFS burners with firing capacities of20 to 40 million Btu/hr on firetube boilers andsmall industrial watertube boilers in Germany.AFS burners are designed for combustion airtemperatures of up to 200°F, but can bealtered for air preheat applications up to500°F.

Gas staged burner results for several recentcommercial installations are compared inFigure 13 for various operating loads. Testresults from Deutsche Babcock's 35 millionBtu/hr pilot test facility are also shown. Eachof these results represent differences infurnace heat release rates and operatingconditions. The results also show the effect ofcombustion fuel staging with flue gasrecirculation. NOx emissions of less than 25ppm can be achieved with increasing amountsof flue gas recirculation. CO emissions of 5 to30 ppm have been achieved on gas-fired fieldoperating units. AFS burner performance onlight fuel oil is summarized in Figure 14.

Summary

Riley offers several low-NOx gas and liquid burner systems designed to meet a wide range ofindustrial and power industry combustion requirements. ASR burners have proven themselves onseveral recent package boiler applications. ASR natural gas fired burners are able to achieve NOxemission levels of <0.1 lb/MM Btu. Considerably lower emission levels can be achieved when theASR burner is combined with other combustion modifications such as external flue gas recirculationand air staging.

Riley STS burners are now being retrofitted on multiple burner wall-fired boilers. The firstcommercial STS burner retrofit has achieved NOx reductions greater than 50% on natural gas.

Compact AFS burners, employing fuel staging techniques, are now being installed on small industrialboilers in Europe. These burners have been shown to be capable of meeting German gas and oilemission standards.

References

1.

R. Oppenberg, "Primary Measures Reducing NOx Levels in Oil-and Gas-Firing WatertubeBoilers", presented at Conference of the Association of German Engineers, Duisberg,Germany (September 1986)

2.

H.P. Niepenberg, "NOx Emissions and Reduction Measures in Industrial and Power StationSteam Generators," Gas Warme International, V38 (5), pp. 311-321, (June/July 1989)

3.

R. Lisauskas and C. Penterson, "An Advanced Low NOx Combustion System for Gas and OilFiring," presented at the 1991 Symposium on Stationary Combustion NOx Control -EPA/EPRI, Washington, D.C. (March 1991)

4.

R. Lisauskas et al, "Experiment Investigation of Retrofit Low NOx Combustion Systems",Proceedings: 1985 Symposium on Stationary NOx Control. Vol 1, EPRI CS-4360 (January1986)

Riley Stoker Corporation 1993

The Company reserves the right to make technical and mechanical changes or revisions resulting fromimprovements developed by its research and development work, or availability of new materials inconnection with the design of its equipment, or improvements in manufacturing and constructionprocedures and engineering standards.

Figure 1.

Low-NO X ASR Burner.

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Figure 2.

ASR Burner Package Boiler Applications.

Figure 3.

Riley MHW Boiler With ASR Burner Low NO X Combustion SystemWith External Flue Gas Recirculation And Air Staging.

NOx

3%02

PPM

N Ox

3%02

PPM

100

80

60

40

20

I

0 5 10 15 20 25

0 20 40 60 80 100

FOR, % OF TOTAL FLUE GAS

PERCENT MCR LOAD

Figure, 4.

The Effect Of External Flue Gas Recirculation

Figure S.

NOx Versus Load Field Data For NaturalAnd Air Staging On ASR Burners NOx

Gas-Fired ASR Burners.Emissions - Natural Gas Firing.

SYMBOL STAGED AIR I Ut4IT

0-*

A

Q 0 B

12 B

0 15 B

18 B

SYMBOL UNIT STAGEDAIR ,% FGR %

A 0 18-23

B 1 5 10-13

B 18 22

Figure G.

Riley STS Burner.

Oil Gun With Diffuser

Gas Cones

3 %

02

PPM

Figure 7.

160

140

120

100

N OX80

60

40

20

0

FLUE GAS 02, %

The Tvt Of Excess Air On Prototype(85 x 10 Btu/hr) STS Burner NOx And COEmissions - Riley Pilot Facility, No FGR.

NOx

Y

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02

PPM

100

80~

60 I-

0

Figure 8.

19 - NO.8 OIL (OFA-0%)

0

NAT. DAS

NO. 8 OIL ( OFA -16-18%)

FOR, %

The Effect Of Flue Gas RecirculationAnd Overfire Air On STS Burner NOxEmissions.

Figure 9.

Industrial 175,000 lb/hr Boiler Retrofitted With STS Burner.

NOx

axoz

PPM

BURNING AREA HEAT RELEASE, x10 Btu/hr/ft

Figure 10.

Comparison Of Pilot And Field STS Burner NO..

Figure 11.

STS Burner Retrofit Under Construction - 638 NIW Gas AndHeavy Oil Fired Consumers Power Company Karn Unit No. 4.

AIR

BARREL NATURALADJUSTER GAS--r

OPPOSEDJ

LOUVERDAMPER

Figure 12.

AFS Air And Fuel Staged Burner.