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Presented at:AFRC 2014 INDUSTRIAL COMBUSTION SYMPOSIUM

September 7-10, Houston, TX, USA

Know the Secret of Fired Heater Design and Operationwith Ultra Low NOx Burner

Sultan Ahamad, Rimon VallavanattBechtel Corp, Houston, TX

Introduction

New burner technologies have been developed tomeet the more stringent emission requirements.Most of these new technologies use internal flue gasrecirculation and fuel staging to reduce NOx. Theseburners are often referred to as Ultra Low NOxBurners (ULNB). The techniques used in ULNB forreducing NOx emissions from process heaters affectkey aspects of burner and heater performance.

The existing guidelines for burner specifications aregenerally based on conventional and Low NOxburners. These guidelines may not be sufficient asUltra Low NOx burners behave differently.

NOx Basics

Nitrogen oxides formation is commonly groupedinto three mechanisms or types: (a) Fuel NOx (b)Thermal NOx and (c) Prompt NOx.

(a) Fuel NOx

Fuel NOx is the result of reactions between fuel-bound Nitrogen and Oxygen in the combustion air.The conversion rates of the fixed nitrogen in the fuelcan be as high as 50 to 60% percent of the nitrogenpresent. Most refinery fuel gases do not have fuelbound Nitrogen; therefore there is virtually no fuelNOx for fuel gas firing.

(b) Thermal NOx

Thermal NOx is refers to the oxidation of molecularnitrogen contained in the air or fuel by oxygen. Therate of Thermal NOx formation is very sensitive tolocal flame temperature. The formation of ThermalNOx is described by following reactions:

N2 + O => NO + NN + O2 => NO + O

The first reaction has high activation energy andtherefore requires high temperature for NOxformation. The formation of a NOx molecule fromthe first reaction results in a release of an N atomwhich rapidly forms another NOx molecule.

The formation rate of Thermal NOx increasesexponentially with increasing flame temperature andis also directly proportional to residence time ofreactant in the peak flame zone. The key parametersof Thermal NOx formation are temperature, Oxygenand Nitrogen concentrations, and residence time inthe flame zone.

(c) Prompt NOx

Prompt NOx is a newly recognized mechanism ofNOx formation. Although it only represents a smallportion of the NOx formed, it becomes a significantemissions source with Ultra-Low NOx burners.

Under the fuel-rich conditions of the flame zone,particularly areas where the stoichiometry is lessthan 0.6, both HCN and NH3 can be formed throughan extremely rapid reaction of Hydrocarbon radicalswith atmospheric Nitrogen. As these componentsenter areas of the combustion zone where additionaloxygen is available, HCN and NH3 are oxidized to

form CO2, H2O and NOx.

The major source of NOx in a fuel gas fired heater isThermal NOx emissions created through high-temperature reactions of Nitrogen and Oxygenpresent in the combustion air. Since the reaction ishighly temperature-dependent, the best directmethod of lowering NOx emissions is to lower thepeak flame temperature. Various burner suppliershave developed specialized Ultra Low NOx burnerto accomplish these goals.

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Ultra Low NOx Burners

Ultra Low NOx Burners have design features andoperating philosophy, which are different from aconventional burner. An Ultra Low NOx burner usesfuel staging and flue gas inspiration (internal fluegas recirculation).

In fuel staging, combustion is carried out in stages,one fuel-rich and the other fuel-lean. Fuel gas isinjected into the combustion zone in two stageswhich creates a fuel-lean zone and delayscompletion of the combustion process. This keepscombustion away from the stoichiometric mixture offuel and air where flame temperature peaks.

These burners are designed to re-circulate relativelycooler flue gas from the firebox back into thecombustion zone using the pressure energy of fuelgas. This re-circulation of largely inert flue gasesinto the combustion zone reduces the peak flametemperature and average O2 concentration.

The fuel-air mixture gets diluted by re-circulatedflue gas resulting in lower combustion rates. Lowerburning rates increase the flame length which canoften result in flame to flame interaction as well asflame impingement on tubes.

Flame to flame interaction can result in the mergingof flames from individual burners. Merged flame ismuch longer than stand-alone flame. The prolongedflames can tilt towards the tubes and flame tails canimpinge on tubes.

Key Considerations for Process Fired Heaterswith Ultra Low NOx Burners

Every design step in fired heater and burner designis critical for optimum performance. Oversight atany stage, from fuel gas supply to flue gas disposalsystem, will adversely affect burner performanceand hence fired heater capacity, overall thermalefficiency, and run length. Proper burner selection,number of burners, fired duty per burner, andlocation of burners etc. are very important foroptimum performance of a fired heater. Followingare the most critical parameters that are need to beevaluated when designing a fired heater with UltraLow NOx burners:

(1) Burner Design Parameters(2) Heater design parameters(3) Fuel Gas Delivery System(4) Operating Parameters

A fired heater with well-designed burners with goodengineering practice should be able to attainfollowings:

Minimize flame-to-flame interaction

Prevent leaning of flames

Prevent flame impingement

Prevent burner to re-circulate flue gas from itsown flame

Meet the emission requirements

Fuel StagingFlue Gas Recirculation

(Using Primary Fuel Gas)Flue Gas Recirculation

(Using Staged Fuel Gas)Enhanced Flue Gas

Recirculation

Figure-1 : Basic Concept of NOx Reduction Methods

Secondary

Zone

Primary

Zone

Combustion

Air

Primary

Fuel Gas

Staged

Fuel Gas

Flue Gas

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(1) Burner Design Parameters:

2.1 Selection of number of burners

Increasing the number of burners will reduce theheat liberation per burner. This will result in theshortest possible flame length and more uniformheat distribution.

Two separate CFD modeling were performed;one for a fired heater with thee burners and onewith four burners. A comparison of Peak HeatFlux Profile for the three and four burnerarrangements was plotted for the tubes. CFDmodeling (as shown in Figure-2) predicting asignificant reduction in peak heat flux for thefour burner arrangement.

The tube skin temperature was also reducedsignificantly with the four-burner arrangement.This resulted in significant improvements inoverall fired heater run length.

2.2 Burner Layout

2.2.1 Burner to Burner Spacing

Ultra-Low NOx burners require maximumclearance between the tiles of neighboringburners. This space is required for internal fluegas recirculation. For example, in a verticalcylindrical heater with burners too closetogether, the inner portion of burner tips may nothave internal flue gas recirculation. This canproduce longer flames and produce higher NOx.

2.2.2 Burner Circle Diameter

Traditionally, burner circle diameter in a verticalcylindrical heater is calculated using followingformula:

= �+

ૡ) )

Where:BCD = Burner Circle DiameterDb = Burner Controlling diameter (generally, frontplate Outside Diameter)Cb = Clearance between burner controlling diametern = Number of Burners

In a competitive market, just barely meetingAPI-560 minimum clearance requirement can beexpected to minimize overall cost. Burnerinstalled with minimum API clearance can causeflame impingement for slight change in fuelpressure or upset in fired heater operation.

This formula works well for conventional burnerand still need to be checked for ULNB. Thecalculation considers actual size of burners toprovide required burner circle diameter.A new formula is required in addition to theabove formula which considers spacing betweentiles of neighboring burners. The followingformula can be used to calculate the burnercircle diameter in a vertical cylindrical heaterwith Ultra Low NOx Burners:

=�+

ૡ)ܖܑܛ )

Where:BCD = Burner Circle DiameterDt = Burner tile Outside DiameterCt = Clearance between burner tilesn = Number of Burners

A general clearance guideline is to use 1 Inchper MMBtu/hr of heat release. For example, ifburner heat release is 10 MMBtu/hr, thenclearance between tiles should be 10 Inch.Another suggested guideline is keeping burnercenter to center spacing as 2 times the burner tileoutside diameter.

2.2.3 Burner in Two Concentric Circles

It has been a common practice to install burnersin two circles for conventional burners (seeFigure-3).The two circle arrangement providesbetter clearance between tubes and burners. Inmost of the cases, an inner circle is added duringfired heater revamp to provide additional firingrate.

This is not a preferred option with Ultra LowNOx Burners. The inner circle burners do not

Figure-2 : Peak Radiant Heat Flux Profile with 3 and 4 Burners

22,000

24,000

26,000

28,000

30,000

32,000

34,000

36,000

1 2 3 4 5 6 7 8 9 10

Pe

akFl

ux,

Btu

/hr/

ft2

Tube Number (Counted from Inlet)

3 Burners

4 Burners

Know the Secret of Fired Heater Design and Operation with Ultra Low NOx Burner

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get the cold flue gases for internal flue gasrecirculation. This lack of internal flue gasrecirculation increases the NOx emission fromthe inner circle burner. The overall NOx fromthe heater will go up and may not meet theemission requirement.

2.3 Heat Liberation and Overdesign

A good engineering practice for specifying theoverdesign for a burner as per API-560 is asfollows:

5 or fewer burners : 120%

6 or 7 burners : 115%

8 or more burners : 110%

It has been common practice in the industry tooverdesign the burners. Additional margin onburner design is sometimes applied on top ofmargin on fired heater design. Overdesign shiftsthe burner design point away from the actualoperating point.

In a case study, burners were designed with120% margin on required firing rate. The draftavailable at heater design condition is 0.6InchWC. However the draft utilized at heaterdesign condition will be 0.41 InchWC instead of0.6 InchWC actually available. This overdesignon the burner reduces the draft loss requirementsfor actual heater operating condition whichresults in a larger flame dimension. It is highlyrecommended to minimize the overdesign toavoid increasing the overall flame dimension.

An adequate air pressure drop should be used for

Ultra Low NOx burners for forced draftapplication. In this type of application, higher airpressure drop across the burner (say 2 InchWC)can significantly improve burner performance.The higher air pressure drop results in betterflame pattern, minimize tube impingement, andlower NOx. It becomes more critical inapplication with large number of burners in asingle chamber such as in a Coker heater. Thecoupling of flames results in much higher NOxvalues than expected based on shop test of asingle burner.

2.4 Burner Performance Shop Test

The burner test is critical for Ultra Low NOxBurners. With conventional and low NOxburners, the performance seen in the test issimilar to multi-burner heaters. However thismay not be always true with ultra-low NOxburners. If burners are not placed with sufficientspacing, the flames may merge, resulting inlonger flame length. It may also result in flameimpingement on furnace tubes. Ultra-low NOxburners are more prone to flame interaction inmulti-burner heaters. Therefore, it is alwaysrecommended to carry out a burner test. Also, ifa burner supplier has a sufficiently large heater,try to test two or more burners with the samespacing as the actual heater. The test furnaceconditions (e.g. Bridge wall temperature, draftetc.) should replicate that of the actual heater.

It is critical to measure flame dimensions duringtest. Visually establish the flame dimensionsmay not be correct. It is recommended to use

Desired layout: Burners in one circle Non-desired layout: Burners in two circles

Figure-3 : Burner Layout Options

Know the Secret of Fired Heater Design and Operation with Ultra Low NOx Burner

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CO probing to establish more reliable flamedimension. CO levels must be low in the fireboxoutside of the flame envelope. CO levels ofmore than 2000 ppm are generally considered asflame.

Many users do not specify a burner test if theyhave used a similar burner in the past. Eventhough operating conditions and heat liberationare similar performance test is stillrecommended. A small difference in tip drilling(which is very common) can have a big impacton NOx emissions and flame stability.

(2) Heater design parameters:

3.1 Burner to Tube Spacing and Flue GasRecirculation

It is suggested to maximize burner to tubeclearance not only to avoid flame impingementon tubes but also for better internal flue gasrecirculation. This spacing becomes even morecritical for a vertical cylindrical heater. In awell-designed furnace, the hot flue gases shouldflow upward through the center of the heater.The cold flue gas should flow downward behindand along the tubes. A smaller clearancebetween the burner and tubes may not allowcold flue gases to flow downward. The smallspace will be filled with hot gases. Refer toFigure-4 for a typical flue gas flow profile inheaters with a normal firebox and a tight firebox.

API recommends a clearance between tubes andburners. For natural draft, fuel gas fired burners;this can be translated into the followingequations:

ି =

+ �.

Where:CB-T = Burner to Tube Clearance, ftQB = Burner Liberation, MMBtu/hr

For Ultra Low NOx application, adding 6 inch toAPI recommended spacing seems to provideneeded clearance for proper flue gasrecirculation. The modified equation shall be:

ିି =

+ �

Table-1 provides suggested clearances for UltraLow NOx Burners.

Table-2 provides summary of results form a casestudy comparing the required minimum TubeCircle Diameter (TCD) for a fired heater withconventional burners and Ultra Low NOxBurners.

3.2 Bridge Wall (Firebox) Temperature

Ultra Low-NOx burner performance is muchmore dependent on firebox temperaturecompared to conventional burners. Burner NOxand other emissions are a function of fireboxtemperature. Higher firebox temperature leads tohigher NOx formation.

The firebox temperature calculation methods

Normal firebox : With flue gas re-circulation Tight firebox: With no or low flue gas re-circulation

Figure-4 : Firebox flue gas flow profile

Burner Burner

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generally used are empirical correlations basedon experimental data. The heat transfer in theradiant section is calculated with widely usedLobo-Evans method. This method assumescomplete flue gas mixing in the firebox such thatthere are no longitudinal or transversetemperature gradients. The equation for radiantheat transfer is as follows:

∝

= .ૠቈ൬

൰

−�൬

൰

+ �ૠ( (�−

Where:qr = Total heat absorbed in Radiant Section, Btu/hrTg = Average Radiating flue gas temperature inradiant section, °RTs = Temperature of cold surface (Tube MetalTemperature), °RαAcp = Equivalent Cold Plane area, ft2

F = Overall exchange factor

This correlation was developed based onoperating data from heaters using conventionalburners. Ultra Low NOx burner flames aresignificantly different than a conventional burnerflame. An Ultra Low NOx burner flame is coolerthan conventional burner flame. A lower flametemperature will result in lower heat transfer inthe radiant section. Therefore, the flue gastemperature leaving the radiant section will behigher than that of a heater with conventionalburners. One must account for this whendesigning fired heater. An Ultra Low NOxburner installed in an existing fired heater(burner retrofit) may result in loss of capacity oroverall thermal efficiency.

The overall heat transfer may not be affected inapplications where the heater has a convection-section preheat coil followed by a radiant-section in which further heating occurs. In thiscase, the total heat absorption for both theradiant and convection coils will be about thesame, regardless of the Bridge WallTemperature.

However, it is very important to correctlyestimate Bridge Wall Temperature where thereare two or more separate services, one in theradiant section and one or more in theconvection section. If the Bridge WallTemperature is higher than expected, theradiant-section absorption will be relatively lowand the convection-section absorption relativelyhigh. In this case, such as when the radiantsection consists of a process coil and theconvection section consist of a steam-generatingcoil. An error in the predicted bridgewalltemperature would result in an error in one orboth of the predicted duties as well as in theguaranteed heater efficiency.

An accurate prediction of bridgewalltemperature is required, not only for emissionprediction, but also to correctly predict heatabsorptions and other heater performanceparameters.

In a case study, an estimated performance of afired heater with conventional burner and UltraLow NOx burners were analyzed. The casestudy was for a vertical cylindrical heater with

Table-1 : Suggested clearance for Ultra Low NOx BurnerHeat release,MMBtu/hr

Vertical clearance tocenterline roof tubes /refractory (verticalfiring), ft-inch

Horizontal clearancebetween tubes andburner centerline, ft-inch

Horizontal clearancefrom centerline ofburner to unshieldedrefractory, ft-inch

Clearance betweenopposing burner(horizontal firing), ft-inch

2 10'-11" 2'-5" 1'-6" 14'-5"

4 15'-7" 3'-0" 2'-0" 21'-7"

6 20'-4" 3'-6" 2'-6" 28'-10"

8 24'-11" 4'-0" 3'-0" 36'-0"

10 29'-7" 4'-6" 3'-6" 43'-3"

12 34'-4" 5'-0" 4'-0" 46'-10"

14 39'-0" 5"-6" 4'-6" 50'-5"

16 43'-9" 6'-0" 5'-0" 54'-0"

18 48'-5" 6'-5" 5'-6" 60'-0"

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process coil in radiant section only. Convectionsection is used for waste heat recovery (steamcoil). The Bridge wall temperature withconventional burners is estimated as 1,560°F ascompared to 1,620°F with Ultra Low NOxburners. This increase in bridge wall temperaturedo not only affect the firing rate & NOxemission but also affect the mechanical integrityof heater components like tube supports,structural steel, refractory etc.

3.3 Heat Density per unit area

Heat density is not used for conventionalburners. However, it is a very significantparameter for Ultra Low NOx Burners. Thereason for this is the importance of flue gasinternal circulation in ULNB.

The following formula can be used to determinearea heat density:

� = ∗ �

Where:

Qb = Burner Heat Releasen = Number of BurnersA = Floor Area (inside the tube circle or between tuberows)A = πd2/4 (for Vertical Cylindrical Heaters)d = Tube Circle diameterA = LxW (for Cabin / Box Heaters)L = Heater LengthW = Heater width (tube center to center)

Flue gas flow patterns in the radiant section ofproperly designed process fired heaters areusually similar irrespective of heater geometry /type. In most fired heaters, tubes are placedalong the walls, and the burners reside in themiddle. As heat is transferred from the flue gas,it cools and flows downward close to tubes. Thisresults in a region of hot upward gas flow in themiddle of the heater and cool downward flowadjacent to the tubes. However, this recirculationdecreases as over-firing increases. An over-firedheater may have very little flue gas recirculation.The extreme case is plug flow case with no fluegas downward flow.

This over-firing characteristic can easily becorrelated with heat density. It is recommendedto limit area heat density to a maximum of250,000 Btu/hr.ft2.

Two heaters with different area heat density but

with similar firing rate, bridgewall temperatureand excess, will have different NOx. The onewith lower heat density will likely to have lowerNOx.

3.4 Common Air Plenum Design

Common combustion air plenums are sometimesprovided to better control the combustion air.These need to be designed to provide equal air toall the burners. It is possible to achieve relativelyuniform combustion air flow to each individualburner for a well-designed burner plenum. It isalways recommended to carry out aComputational Fluid Dynamic (CFD) analysis toensure proper air distribution to each burner.

The example in Table-3 demonstrates how airmal-distribution can starve some burners ofcombustion air.

It is very common to target 2% or even lowerexcess O2 (2% O2 is equivalent to 10% excessair). Most new fired heaters are able to operateat lower excess air because of lower tramp air(lower air leakage) due to better sealing ofcasing.

As noted in the table, a heater operating with10% excess air will have a few burners starvingfor air at 10% air mal-distribution. A lowercombustion air supply would result in sub-stoichiometric operation for that burner. Thiswill increase CO emissions.

(3) Fuel Gas Delivery System

The Ultra Low NOx Burner tip orifices are verysmall compared to conventional burners. Burnerignition ports in most of Ultra Low NOx burnersare 1/16 inch. These small ports can and will getplugged if the fuel gas delivery system is notdesign to reliably supply clean, dry fuel.

Table-2 : Case Study: Required Minimum Tube CircleDiameter for Conventional vs ULNB

ConventionalBurner

ULNB

Burner design liberation, MMBtu/hr 10.0 10.0

Number of burners 8 8

Burner spacing, Inch 30” 40”

Burner circle diameter, ft-inch 6’-6” 8’-9”

Burner to tube clearance, ft-inch 4’-0” 4’-6”

Min. tube circle diameter, ft-inch 14’-6” 17’-9”

Know the Secret of Fired Heater Design and Operation with Ultra Low NOx Burner

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The fuel delivery system for Ultra-Low NOxburner applications must have a filter /coalescerlocated downstream of the knockout drum. Useof stainless steel fuel piping downstream of thefilter/coalescer is also recommended. Fuelpiping downstream of the filter/coalescer mayrequire insulation and heat tracing depending onlocal site conditions.

(4) Operating Parameters:

4.1 Carbon Monoxide (CO) emission:

Most fired heater emission requirements alsolimit CO emission. The factors influencing NOxemission (firebox temperature, excess air &residence time) also influence CO emission,although unfortunately in opposite direction.

NOx emission is reduced as firebox temperaturedecreases but CO emission increase.

CO emission is generally not an issue at heaterdesign conditions where firebox temperature isrelatively high. However, at low heaterthroughput CO emission will increase.

Carbon Monoxide is produced by incompletecombustion of fuel gases. The basic combustionprocess includes formation of CO as anintermediate product before it is converted toCO2. CO is easy to convert into CO2 if sufficientOxygen, temperature and mixing is available.The minimum ignition temperature of CO in airis 1,128°F. A firebox temperature higher thanthe ignition temperature is required to limit COemission. Burner supplier must be consulted forminimum firebox temperature below which COemission is not guaranteed. Generally, COemission from a burner is guaranteed at afirebox temperature above 1300°F.

4.2 Draft

Draft is defined as negative pressure at any pointinside a fired heater. Draft loss across the burneris the pressure drop of combustion air in theburner. Higher available airside pressure drop tothe burner almost invariably improves flamepattern and the ability to effectively use internalflue gas recirculation in burners. It is veryimportant to provide the actual calculated draftavailable to the burner supplier.

Site elevation (altitude above sea level) is one ofthe most often overlooked parameters whilecalculating draft. Stack effect reduces withincrease in altitude. If draft is not calculatedbased on actual site elevation, the burner as wellas the heater will have capacity limited due toinsufficient draft.

Following correlation can be used to calculatethe draft (Inch WC) generated in a heater.

= . ∗ ∗ �ቊ

−

ቋ

Where:H =Height, ftPatm = Atmospheric Pressure, psiaTamb =Ambient Air Temperature, °RTfg =Flue Gas Temperature, °R

Table-3 : Common Air Plenum – Air Mal-distribution

DesignExcess Air

(%)

% Mal-distribution

MaximumExcess Air

(%)

MinimumExcess Air

(%)

15 5 20.75 9.25

10 26.5 3.5

15 32.25 (-)2.25

10 5 15.5 4.5

10 21.0 (-)1.0

15 26.5 (-)6.5

Table-4 :Operating at high draft-Effect of draft change for 40 ft tall firebox

High draftoperation

After draftchange

Draft at Arch, InchWC 0.20 0

Draft at Floor, InchWC 0.6 0.4

Excess Air, % 15 (-) 6

Figure-5 : Effect of Site Elevation on Draft Available at Heater Floor

0.2

0.3

0.4

0.5

0.6

0.7

20 30 40 50 60

Dra

ftat

Flo

or,

Inch

WC

Firebox Height, ft

Elevation = 0

Elevation = 5,000 ft

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Atmospheric pressure (Patm) reduces withincrease in site elevation resulting in reduction isavailable draft. Figure-5 provides a comparisonof draft at site elevation of 0 ft and 5,000 ft.

Fired heater operators tend to operate the heaterat higher draft to be on safer side. However,when there is a large change in draft, it canadversely affect the excess air. Example in table-4 indicates that air flow rate becomes sub-stoichiometric for a draft change from 0.2 InchWC to 0 InchWC. This change may not eventrigger the low draft alarm (depending on settingof instrument).

4.3 Air Leakage

Process fired heaters operate under negativepressure. Even small openings can allowsignificant quantities of air to leak into theheater. Air leaking into the furnace increases theO2 measured at arch or stack, independent of theair actually coming through the burners. Theactual air coming through burner shall be lowerthat measure by Oxygen analyzer. This mayincrease the NOx emissions. When furnace airleakage is high, the air coming through theburner may not be enough for completecombustion of the fuel, causing the flames togrow larger and possibly impinge on the furnacetubes. In such a situation, the heater may have tobe operated at a higher excess O2, sacrificingheater efficiency in order to prevent flameimpingement.

There are many sources of air leakage in aprocess heater. It can be tube penetrations, sightports, joints between heater walls, out of serviceburners etc. It is very important to seal any andall noticeable openings in order to prevent air

leakage for proper burner operation. Sight

Conclusion

With the introduction of next generation Ultra-lowNOx burners, there have been major reductions inNOx emissions. These burners are operating closerto their limits of flame stability and completecombustion. Ultra-low NOx burners are very proneto flame interaction in multi-burner heaters.

Ultra-Low NOx burners along with fired heater needto be specified correctly to get an optimal heater

operation. Proper selection of fuel gas supplysystem, number of burners, fired duty per burner,location of burners, fired heater design and operatingparameter are very important for optimumperformance of a fired heater. Every design step infired heater and burner design is critical. Anoversight at any stage, from fuel gas supply to fluegas disposal, will adversely affect burner as well asfired heater performance.

Many fired heater design and operating parametersmay not be of significance with conventional burnerbut become important for Ultra Low NOx burners.

References:

(1) API-560 : Fired Heaters for General Refinery Service(2) API-535: Burners for Fired Heaters in General

Refinery Services(3) Lobo, W. E., Evans, J. E.; Heat Transfer in Radiant

Section of Petroleum Heaters, Trans. Am. Inst.Chem. Engrs. 35, pp.748-778, 1939.

(4) Beer JM, Chigier NA. Combustion Aerodynamics,Applied Science Publishers Ltd.,:1972.

Sultan Ahamad is a Sr. FiredHeater Specialist at Bechtel Corp.in Houston, Texas. He has morethan 16 years of experience in thedesign, engineering andtroubleshooting of fired heaters andcombustion systems for therefining and petrochemicalindustries. He graduated from the

Indian Institute of Technology in Roorkee, India, with a degreein chemical engineering. He is a member of API Sub-Committeeon Heat Transfer Equipment. He can be reached by email:sahamad@bechtel.com

Rimon Vallavanatt is the Sr. Principal Engineer at BechtelCorp. in Houston, Texas. He hasmore than 40 years of experience inthe design, engineering andtroubleshooting of fired heaters,thermal oxidizers, boilers and flares.He graduated from the University ofKerala in India with a degree inmechanical engineering. He alsoreceived a degree in industrial

engineering from St. Mary’s University in San Antonio, Texas.Mr. Vallavanatt is a registered professional engineer in the stateof Texas, and he has served on the American PetroleumInstitute’s subcommittee on heat transfer equipment for the past30 years. He can be reached by email: rvallava@bechtel.com