Process Burner Spacing

Burner-to-burner interaction is aphenomenon that is characterisedby the creation of a dense firecloud, an increase in NOx emissions andan altered heat flux distribution in thefurnace. It occurs frequently in multiple-burner heaters, where vertical cylinderheaters have been observed to have theseverest interactions. Although thisproblem has been around for a while, ithas not been well understood. Only ad-hoc solutions have been introduced tomitigate it. However, a comprehensivestudy has now been completed, whichreveals a dimensionless interactionparameter that could identify conditionsthat are likely to produce burner-to-burner interaction. Observations ofinteractions were made on lab-scale,small-scale and full-scale burners tovalidate the parameter.

Two case studies are presented usingCFD modelling. The first study showsthat interaction was unlikely in avertical cylindrical furnace where thespacing between burners was decreased.The other study confirms interaction

observed in the field and suggestsmodifications that were successfullyimplemented and led to the eliminationof burner-to-burner interaction. Specificrules of thumb, which are based onexperience, are given to predict thelikelihood of interactions in ethylene-cracking furnaces.

The problemIn response to the increased demand toreduce NOx emissions from industrialheaters, new burner technologies havebeen developed to meet the morestringent emissions regulations.1 Most ofthese technologies use some type offurnace gas recirculation (Figure 1) toreduce the flame temperature, so thatthermal NOx emissions can be reducedsubstantially.2 However, diluting the fuel-air mixture with furnace gases results inlower burning rates, which often makesthe flames significantly longer. Bothlonger flames and increased volumetricheat release density (Btu/hr/ft3 orMW/m3) in the newer multi-burnerindustrial furnaces have led to frequent

occurrences ofa phenomenonoften referred to asburner-to-burner(BtoB) interaction.

The occurrence of this phenomenon isindicated by the formation of a large,bright yellow fire cloud and by anincrease in NOx emissions compared tosingle-burner performance. Althoughthis problem has been observed in alltypes of multi-burner heaters, it is muchmore severe in vertical cylindrical heaterscompared to other types such as cabinheaters.

It would be desirable to obtain asimple rule of thumb to characterise andpredict the onset of BtoB interaction.However, the complexity of the problemlimits the application of each rule to thespecific case from which it was deduced.Some of the BtoB interaction rules thathave been proposed for verticalcylindrical heaters are based on theheat-release rate and the geometricaldimensions of the furnace. The ruleusually takes the form of a criterionvalue that demarcates the absence ofinteraction from the onset ofinteraction. Figure 2 shows thedimensions discussed in the followingrules. One such rule uses the heat releaseof a single burner per unit distancebetween two adjacent burners (Btu/hr/ftor MW/m). A second rule uses the totalheat release of the heater per unit area(Btu/hr/ft2 or MW/m2) of the outermost

Process burner spacingBurner-to-burner interaction can increase NOx emissions and cause flame

impingement on process tubes. Predictive tools and rules of thumb have beendeveloped to help identify when there may be interaction problems

Mahmoud Fleifil, Michael Lorra, Charles E Baukal, Jeff A Lewallen and Daniel E WrightJohn Zink Company LLC

TTHHEERRMMAALL TTEECCHHNNOOLLOOGGYY

PPTTQQ QQ22 22000066w w w. e p t q . c o m

1

Figure 1 Schematic of furnace gas recirculationFigure 2 Plan view of a vertical cylindrical furnace withdimensions used in rules of thumb (B = burner and T = tube)

jzink.qxd 6/6/06 10:20 AM Page 1

burner circle, which is the circle thatpasses through the centres of theoutermost burners. A third rule uses thetotal heat release of the heater per unitarea (Btu/hr/ft2 or MW/m2) of the tubecircle, which is the circle that passesthrough the centres of the heaterprocess tubes. A fourth rule uses thetotal heat release per unit volume of theheater (Btu/hr/ft3 or MW/m3).

In extreme cases, when the criterionvalue is close to zero for no interactionand relatively large for full interaction,these rules can correctly predict theperformance of a multi-burner heater.However, predicting the accurateconditions for the onset of interaction isa challenge for any of these rules. Inother words, it is easy to predict thesufficient conditions (extreme cases) forinteraction, but it is difficult to predictthe necessary conditions (onsetconditions) for the interaction. Theserules fail for two reasons:oversimplification of the problem andfailure to account for all significantparameters. For instance, during aresearch study of this phenomenon, itwas observed that BtoB interactiondepends on both fuel pressure and fuelcomposition, while none of the aboverules account for such a dependence.

BtoB interaction, sometimes referredto as flame-to-flame interaction (FtoF),is a complex phenomenon that makesthe behaviour of the multiple-flameconfiguration very different from that ofan individual flame. Increased flamelength, increased NOx emissions, poormixing, modified heat flux distributionand decreased surrounding gasentrainment are some of the results ofFtoF interaction. While aerodynamicinteraction of the jets in a multiple-jetsystem is considered to be the mostimportant factor that governs FtoFinteraction, the presence of combustionis also a major factor. In other words, theshape of the flame not only responds tothe turbulence in the surrounding

medium, but also to the combustionitself, which creates eddies that canaffect neighbouring flames.

In vertical cylindrical heaters, theBtoB spacing is more critical than in box-type heaters, because the burner spacinghas an impact on the flue gas flow intothe centre of the furnace. If the spacingbetween burners is too small, theinteraction of the flames can actuallyblock the flow of flue gases from thecircumference of the furnace into thecentre. This generates a low-pressurezone in the centre of the furnace thatcan effectively pull the flames towardthe centre of the furnace. This increasesthe tendency for the flames to mergetogether and create a large fire cloud thatcan impinge on process tubes. Anothereffect is the creation of a reducing(oxygen-deficient) atmosphere in thecentre of the furnace. Since the flue gasesare blocked, the O2 concentrationdecreases at the centre, leading toincreased CO formation on that side ofthe flame and creating the potential forincreased flame lengths. Longer flamescan also alter the design heat fluxdistribution, as more heat is transferredhigher in the furnace. NOx emissionsmay also increase because there areinsufficient flue gases on the inside ofthe burner circle that can be pulled intothe flames. Many low-NOx burners usefurnace gas entrainment to reduce theflame temperature, which reduces NOxemissions.

An extensive theoretical andexperimental investigation wascompleted to develop a suitable

criterion for identifying interaction invertical cylindrical heaters. Details ofthat investigation are given elsewhere.3

The developed criterion depends onboth physical and chemical parameters;namely, the geometry of the burnersand the fuel composition. Interaction isdefined here as the influence of flameson each other that leads to the creationof a yellow flame cloud (due to theformation of soot particles) betweenburners. A dimensionless number wasdeveloped that can be used as a measurefor the interaction strength. Both lab-scale and small-scale burners were usedin the experimental investigation.

Consider two diffusion flames, asshown in Figure 3, that are adjacent toeach other. There is a critical height atwhich jets interact (Zc). The flameheight is designated here as Lf. If theflame height is less than the criticalheight (Lf < Zc), no FtoF interaction willoccur. If the flame height is greater thanthe critical height (Lf > Zc), the flameswill interact. FtoF interaction of a groupof diffusion flames arranged in a circle,as in a vertical cylindrical furnace, canbe characterised in a similar manner.

Experimental studyTwelve lab-scale burners were built. Eachwas a single-jet burner with its diffusionflame stabilised on a conical bluff bodydownstream of the fuel jet. The objectiveof testing with a simple lab-scale burnerwas to study the FtoF interaction on afundamental basis and to verify theinteraction criterion for configurationswith a large number of burners. Theseburners were tested in a small labfurnace equipped with sensors formeasuring fuel pressure, temperature,flow rate, flue gas temperature, furnacedraft, wet and dry gas analysis of excessoxygen, carbon monoxide and nitrogenoxides.

The parameters were varied to studytheir effects on FtoF interactionindependently. Interaction among the

PPTTQQ QQ22 22000066

2


Figure 3 Condition of interaction between two adjacent diffusion flames. Lf < Zc (no flame interaction); Lf > Zc (flame interaction)

Computational fluiddynamics (CFD) is apowerful tool for analysingcomplicated fluid flowproblems in combustion


flames was characterised by the existenceof a yellow fire cloud between adjacentflames. The strength of interaction wasevaluated by direct visual observation ofthe yellow cloud. A somewhat subjectiveinteraction index was used to scale thestrength of the interaction. FtoFinteraction was observed to increase asthe fuel pressure decreased, for the sameheat release rate. FtoF interactionincreased as the burner circle diameterdecreased. The dependence of thedimensionless interaction parameter onthe number of burners was governed by aspacing ratio, defined as thedimensionless ratio of the burnerdiameter to the distance between twoadjacent burners. Two configurationswere tested to investigate the impact ofthe number of burners on FtoFinteraction. Four and six burners wereused where the distance betweenadjacent burners was kept equal in bothconfigurations. The interaction in thecase of six burners was weaker than thatwith four burners. Excess air had aninsignificant effect on the strength of theFtoF interaction.

Small-scale burnersThe objective of testing with small-scalediffusion burners was to study the FtoFinteraction on a true process burnerdesign and to verify the interactioncriterion for all the relevant parameters.The following control parameters werevaried: heat release, fuel pressure, fuelcomposition, excess air, burner circlediameter and number of burners. Six fuelcompositions of various combinations ofnatural gas (>90% CH4), hydrogen andpropane were tested.

In these experiments, FtoF interactionwas evaluated by direct visualobservation of the yellow fire cloudbetween the flames. Changing both theheat-release rate and fuel pressure, with afixed fuel nozzle diameter, did not affectFtoF interaction significantly. Keepingthe heat-release rate per burner constant,a parametric study was then carried outin which only one control parameterwas varied at a time. Interactionincreased when: fuel pressure decreased,burner circle diameter decreased, thenumber of burners increased and theexcess O2 increased. Fuels with morepropane tended to have higherinteraction than fuels with morehydrogen, which tended to have lowerinteraction. Fuels with more natural gashad interactions in-between. Thiscorresponds with flame length, aspropane tends to make flames longer,while hydrogen tends to make flamesshorter, for a given burner configuration.

Three distinct interaction regimeswere found: non-interaction, transitionand strong interaction. The interactionparameter could be used to demarcatethe transition from no interaction tostrong interaction.

Full-scale field burnersData were collected from 12 fieldinstallations, which had full-sizedprocess burners of the same design asthe small-scale burners. There were bothinteracting and non-interacting cases.Using the field data, the dimensionlessinteraction parameter was computed foreach case. The three interaction regimeswere evident, although the transitionregime was narrower. The narrowtransition regime was due to thesubjectivity involved in visuallyindexing the fire cloud as well as thesmall number of cases that fell into thistransition regime.

Numerical studiesComputational fluid dynamics (CFD) isa powerful tool for analysingcomplicated fluid flow problems incombustion.4 CFD has been usedextensively to study burners and heatersin the process industries. Oneparticular topic of interest hasbeen FtoF interaction. Thefollowing two examples illustratesome of the issues and results. Thefirst example shows a case wherereducing the spacing betweenburners did not cause FtoFinteraction. The second exampleshows how CFD was used tosimulate an existing FtoFinteraction problem in anoperating heater and the solutionto that problem by modificationof the burners.

CFD was used to study theeffects of reducing the burner

circle diameter in avertical cylindrical furnacefrom 8.667.43ft(2.642.26m) to determineif FtoF interaction wouldbe a problem. The burnercircle is centred at themiddle of the furnace andgoes through the centre ofeach burner. The furnacehad six burners mountedin the floor in a circularpattern, surrounded bytubes next to the wall(Figure 4). The yellow ringbetween the burners andthe tubes is known as aReed wall. Its purpose is tocontrol the gas flow in the

furnace and minimise the chance forflame impingement on the tubes.

Each burner was fired at 9.3MMBtu/hr(2.7MW). Two different fuel composi-tions were fired simultaneously througheach burner. One fuel had approxi-mately 40% hydrogen and 44%methane, with some other minorcomponents. The other fuel was about23% methane and 61% nitrogen, withother minor constituents. Each burnerwas operated with 15% excess air. Theexhaust gas temperature was about1600F (870C).

Turbulence, equilibrium combustionchemistry and thermal radiation wereincluded in the CFD model. The flameboundary for the calculated flamepattern (not show here) is defined inthis case as the location where thecomputed carbon monoxide concen-tration is 1500ppm by volume on a wetbasis, indicating that no FtoFinteraction was predicted after reducingthe burner circle diameter. Other resultsalso predicted no flame impingementon the process tubes.

The second study involved anoperating furnace in a refinery.5 Theproblem observed in a xylene reboiler


3


Figure 4 Cutaway view of six low-NOx burners mountedin the floor of a VC furnace

Figure 5 Cutaway view of a xylene reboiler withsix floor-mounted low-NOx burners


was that the flames from the ultra-lowNOx burners were very long and had thepotential to damage the process tubes inthe convection section at the top of thefurnace. This phenomenon hadpreviously been observed in vertical/cylindrical furnaces with ultra-low NOxburners. The problem was related to theflow pattern within the furnace, as it didnot allow complete mixing of thecombustion air with the fuel, but ratherdistorted the flame prior to burnout.

The geometry of the verticalcylindrical furnace is shown in Figure 5.The small wall around the burners is aReed wall and is used to heat the coldflue gases coming from the tubes. Thishelps enhance flame stability.Equilibrium combustion chemistry,turbulence and thermal radiation wereused in these computations.

Figure 6a shows a CFD simulation ofthe burners as they were originallyinstalled. The figure shows an iso-surface of OH, which is a good indicatorof flame shape. The results reveal thatthe flames from adjacent burnersmerged together to produce a singlelong flame, which was confirmed byobservations of the operating furnace.This burner had two primary fuel tipsthat fired fuel inside the burner tile, andfour secondary fuel tips firing outsidethe tile. The solution to this flameinteraction problem was to change theburner so that only three of thesecondary tips actually fired. The CFDresults for that configuration are shownin Figure 6b. This solution wasimplemented and tested in theoperating furnace and found to yieldqualitatively the same result theflames became distinct and burned outat the appropriate height. FtoF

interaction was negated, which reducedthe flame length and eliminated flameimpingement on the process tubes.

Ethylene furnacesOne example of an application whereFtoF interaction is an issue is inethylene-cracking furnaces. Onecommon type of low-NOx burner usedin the floor of these furnacesincorporates fuel staging and furnacegas recirculation (Figure 7a). While thiscombination of techniques reduces NOx,it also tends to increase flame length.

Longer flames move the location of thepeak heat flux higher in the furnace anddecrease the heat flux at the floor of thefurnace. These burners are so effective atpulling in surrounding furnace gasesthat, if spaced too closely together, theycan compete for the same gases. Thiscan cause NOx emissions to increase andflame quality to decrease. On extremelytight spacing, the burners can negativelyimpact the natural furnace fluid flowcurrents, leading to FtoF interaction andflame impingement on process tubes.

Another burner was developed to


4


Figure 6 Elevation view of CFD predictions of flame shape: (left) unmodified burners and(right) modified burners

Figure 7 Burner used in ethylene-cracking furnaces with fuel staging and (left) uncontrolled and (right) controlled furnace gas recirculation


help control the interaction of low-NOxburners that incorporate furnace gasrecirculation, where the spacingbetween burners is tight. These burnerscontrol the recirculation of furnacegases by using a venturi and internalducting, and introduce the fuel andfurnace gas mixture vertically (Figure7b). These burners have low NOxemissions, with better radiant heat fluxprofiles and flame quality than theprevious design. They also allow closerburner-to-burner spacing.

Burner technologies must be correctlyapplied to get predictable and reliableperformance from the furnace. Thisincludes low NOx, no flame impinge-ment and the desired heat fluxdistribution in the furnace. There aretwo parameters that can be used topredict burner performance in thisapplication: heat release per burner as afunction of burner spacing and totalheat release for the furnace geometry.

The first of the two parameters iscalled linear heat intensity (LHI). Itquantifies flame intensity along afurnace wall or along the length of thefurnace. LHI is calculated by taking theheat release of all the floor burners in arow and dividing by the length of thefurnace. Typical units for this would beMMBtu/hr/ft or MW/m. For example, ifthere are ten burners in a row, eachfiring 10MMBtu/hr (2.91MW), and thetotal length of the row is 30ft (9.1m),the LHI would be (10 burners) x(10MMBtu/hr/burner)/30ft = 3.33MMBtu/hr/ft (3.2MW/m). The LHI can beused to determine the extent of possibleburner-to-burner interactions and howmuch NOx emissions may increasecompared to NOx measurements from asingle-burner test where there is no FtoFinteraction.

Different burner designs are more orless sensitive to LHI. Generally speaking,all burner technologies will workpredictably when the LHI values are lessthan 2MMBtu/hr/ft (1.9MW/m).Between 2 and 3.5MMBtu/hr/ft (1.9 and3.4MW/m), the burner technology playsan important role and the burnervendor should be consulted andprevious experience of the type ofburner at similar LHI levels considered.The previous example suggests thatBtoB interaction would be a concernfor the conditions specified. Above4MMBtu/hr/ft (3.8MW/m), BtoBinteraction is likely and makes itdifficult to predict burner and heaterperformance.

The second parameter, called hearthheat intensity (HHI), is a measure of

flame intensity for a given floor area. HHIis calculated by taking the heat release ofall floor burners in a furnace anddividing by the floor area inside the box.Typical units for HHI would beMMBtu/hr/ft2 or MW/m2. For example, ifthere are ten burners firing 10MMBtu/hr(2.9MW), each in a total floor area of400ft2 (37m2), the HHI would be (10burners) x (10MMBtu/hr/burner)/400ft2

= 0.25MMBtu/hr/ft2 (0.79MW/m2). TheHHI measures the furnace-to-burnercoupling effect. If the HHI is too high,the potential for flame rollover increases.If the intensity of the heat on the floor ofthe furnace is higher than the burner cansustain, a short circuit in the naturallyoccurring furnace currents can occur,creating the potential for flame rollover.This rollover will reduce efficiencies andrun lengths, and in severe cases reducetube life.

Generally speaking, there will beminimal burner-to-furnace interactionwhen the HHI values are less than0.3MMBtu/hr/ft2 (0.95MW/m2). Between0.3 and 0.45MMBtu/hr/ft2 (0.95 and1.4MW/m2), the possibility for negativeinteraction exists and each burnertechnology should be compared toexisting furnaces to determine thepossibility and extent of the interaction.Above 0.5MMBtu/hr/ft2 (1.6MW/m2),burner-to-furnace interaction is likelyand heater performance parameters suchas efficiency and run length will bedifficult to predict reliably. For theprevious example with HHI =0.25MMBtu/hr/ft2, the rule of thumbsuggests minimal interaction for thoseconditions.

ConclusionsBurner-to-burner interaction can increaseNOx emissions, cause flame impinge-ment on process tubes and alter the heatflux distribution in the furnace. Thisproblem has become more prevalent aslow-NOx burner designs use more fuelstaging and furnace gas recirculation andfiring rate densities increase. Empiricalrules of thumb have been developed, butare often limited to fairly specific

geometric configurations and operatingconditions. As the result of an extensiveexperimental investigation, a generalinteraction parameter has beendeveloped to help predict when theremay be an interaction problem for a widerange of operating conditions. CFDmodelling can be used to both predictpotential problems and to recommendsolutions. Rules of thumb have beendeveloped for ethylene-cracking furnacesto predict when there is an increased riskof interaction problems.

References1 Baukal C (editor), Industrial Burners Hand-

book, CRC Press, Boca Raton, Florida, 2004.2 Baukal C, Industrial combustion pollution and

control, Marcel Dekker, New York, 2004.3 Fleifil M, Karan J, Colannino J, Demystify-

ing burner-burner interactions in industrialvertical cylindrical process heaters,presented at the AFRC-JFRC JointInternational Combustion Symposium,Maui, Hawaii, 1013 October 2004.

4 Baukal C E, Gershtein V Y, Li X (editors),Computational fluid dynamics in industrialcombustion, CRC Press, Boca Raton, Florida,2001.

5 Henneke M, Smith J D, Jayakaran J D, LorraM, Computational fluid dynamics (CFD)based combustion modeling, Chapter 9,The John Zink Combustion Handbook, BaukalC (editor), CRC Press, Boca Raton, Florida,2001.

Mahmoud Fleifil has been an acousticsengineer at John Zink Company LLC inTulsa, Oklahoma, USA, since 1999 andhas a PhD from MIT. Email: [email protected] Lorra is a senior CFD engineer atJohn Zink Company LLC in Tulsa,Oklahoma, USA, and has a PhD fromRuhr-Universitaet Bochum, Germany.Email: [email protected] E Baukal, PE is director of theJohn Zink Institute in Tulsa, Oklahoma,USA, and has a PhD from the University ofPennsylvania. Email: [email protected] A Lewallen, PE is a senior designengineer at John Zink Company LLC inTulsa, Oklahoma, USA, and has a BS inmechanical engineering from theUniversity of Tulsa. Email: [email protected] E Wright is the ethylene marketdirector at John Zink Company LLC inTulsa, Oklahoma, USA, and has a BS inchemical engineering from BYU. Email: [email protected]



5

Burner-to-burner interactioncan increase NOx emissions,cause flame impingement on process tubes and alterthe heat flux distribution inthe furnace


Process Burner Spacing

Documents

Transcript of Process Burner Spacing