petrochemicals & gas processing applying ultra-low-no … · reason, technip applies flame...

In recent years, the US and theEuropean Union have imposedstringent emissions legislation on

ethylene plant operators. Othercountries have recently imposed similarlegislation. In the EU, two directives arein force: notably, the IPPC (integratedpollution prevention and control)directive from 1998 and the 2003/87/ECdirective from 2003, which mandatesthe implementation of a greenhouse gasemissions trading scheme as well asceilings on allowable emissions. TheIPPC directive is related to theapplication of best but proven availabletechniques to minimise greenhouse gasemissions. Nitrogen oxides (NOx),formed during combustion in thepresence of nitrogen, are regarded ascontributors to the greenhouse effect. Toreduce NOx emissions in ethylenefurnaces, the operator has the option toeither reduce already-formed NOx viaflue gas treatment or to minimise theformation of NOx at the source, which isthe combustion process in the radiantsection (the firebox) of the ethylenefurnace. In either case, a properunderstanding of the combustionkinetics and fluid dynamics in thefirebox chamber is required to arrive at afurnace design that combines stable andefficient combustion, as well as meetingenvironmental NOx and CO emissionsconstraints.

NOx and combustion kineticsA kinetics model describing theoxidation of hydrocarbons up tokerosene and JP8 is used as a basis,containing some 250 species and 5000reactions. The model also includes 30species and 260 reactions to describe theformation and disappearance of NOx.

In the combustion of methane andhydrogen-containing fuels, NOx can beformed by two reaction paths: the so-called prompt NOx and the thermal NOx

mechanisms. Thermal NOx is formed as

a result of the oxidation of nitrogen toNOx through a reaction path thatinvolves oxygen, hydrogen andhydroxyl radicals. Also, the directreaction between nitrogen and oxygenmolecules contributes to the formationof this pollutant species via the thermalmechanism. High temperatures, highresidence times and excess oxygenfavour thermal NOx. Avoiding local highflame temperatures, recirculationpatterns and excess air can reduce theformation of thermal NOx.

Prompt NOx is the conversion ofnitrogen through a radical reactionnetwork, which involves the formationof hydrocarbon radicals with HCN as anintermediate. Prompt NOx is favouredby excess hydrocarbons, is lesstemperature dependent than thermalNOx and the reactions are fast compared

to thermal NOx. Avoiding local excessunburned hydrocarbons and keepingthe flame lean of fuel can reduce theformation of prompt NOx.

Another mechanism important inNOx control is the reduction of already-formed NOx. The reduction of NOx isfavourable in areas of the flame withhigh concentrations of NO2, low NOand which are lean of oxygen atmoderate temperatures.

Burner and firebox designIn a modern ultra-low NOx fireboxdesign, the objective of the designer is tominimise NOx by influencing all threeoccurring mechanisms. The burner isthe prime source of NOx. Thecombustion characteristics of the burnervery much determine the final NOx

emission values of the ethylene furnace.

Applying ultra-low-NOx

burners Application of computational fluid dynamics in the design of a high-flux ethylene

furnace equipped with ultra-low NOx vortex burners. The fundamental kineticsmechanisms that are important in NOx formation under combustion are described

Simon Barendregt, Iek Risseeuw and Frank WaterreusTechnip Benelux

PPEETTRROOCCHHEEMMIICCAALLSS && GGAASS PPRROOCCEESSSSIINNGG

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Figure 1a Flame rollover due to localrecirculation patterns

Figure 1b Flame stabilisation by bottomand sidewall combination firing

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Nevertheless, if the combination ofburner characteristics and firebox fluiddynamics does not fit, even whenapplying ultra-low-NOx burners, finalNOx figures can be two- or three-fold thefigures obtained in a burner testperformed in a test furnace.

Modern ethylene furnaces withethylene capacities of over 200 000 tpain each firebox cell have been designed.The firing required in such a firebox cellis 130MW or more. To put this heat in,a substantial portion of the heat(typically 50–80%) needs to be suppliedby large-duty diffusion flame bottomburners. Diffusion flame burners in tallboxes may become unstable due tounpredicted recirculation patterns.When applying techniques to reduceNOx, such as multiple-level fuel and/orair staging, the tendency of the flamesto become unstable increases. For thisreason, Technip applies flame

stabilisation by adding low-NOx flat-flame radiant wall or balcony upshotburners (Figures 1 and 2).

When including ultra-low-NOx

burners in a modern high-capacityethylene furnace, Technip appliesrigorous CFD-NOx studies during thedesign stage of the project. For thispurpose, Technip, in combination withthe Politecnico di Milano, hasdeveloped a CFD-NOx simulator able topredict both burner flame characteristicsand emission values on NOx and CO.

CFD-NOx simulatorThe combustion kinetics model appliedin the CFD-NOx simulator contains some250 species involved in a reactionnetwork of 5000 reactions. The reactionnetwork involved in the prompt NOx,thermal NOx and NOx re-reburningmechanism accounts for about 260reactions involving 30 species. The mesh

of a CFD model for an industrial furnaceeasily contains more than a million gridcells. Including such a rigorous kineticsmodel in a CFD simulation leads toexcessive and unpractical calculationtimes. Even with modern high-speedcomputers equipped with a lot ofmemory, such an approach is unfeasible.

However, use can be made of thecharacteristics of the combustionprocess, which allows a different butfeasible approach. In the combustionprocess, the overall temperature andfluid dynamics patterns are determinedby the combustibles and air fed to theburner and the main flue gascomponents. The reactions involved inthe formation of pollutants, such asNOx, PAH and soot, do not influencesignificantly the overall temperature,velocity pattern and concentrationprofiles of the main species.

This phenomenon allows decouplingof the Nox kinetics from the rigorous CFDsimulations. In the CFD-NOx simulator, arigorous CFD model of the furnace is firstset up. This contains a detailed model ofone or more burners in a furnacesegment. However, this CFD model usesa simplified kinetics model of thecombustion process. This is sufficientlydetailed to allow a proper estimation ofthe temperature, velocity andconcentration profiles of the main fluegas components. The CFD-NOx simulatoralso contains the NOx kinetics post-processor that takes care of the detailedcombustion kinetics calculations using

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Figure 2 Balcony upshot (left) and bottom burners (right) in a Technip-designed GK6furnace showing stable combustion patterns

Figure 3 Composition maps of selected components, as predicted by the CFD-NOX model

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the results of the CFD calculations.

NOx postprocessorThe NOx postprocessor identifies thecritical zones for NOx and otherpollutants formation. These criticalzones are identified by large gradients oftemperature, velocity and compositionprofiles. Here, the original CFD griddetail is maintained. The vicinity of theburners and the places in the fireboxwhere local recirculation occurs aretypically considered critical for NOx

kinetics. However, other parts of thefirebox chamber are considered lesscritical. These places are characterisedby low temperature, low velocity andlow concentration gradients. In theseareas of the furnace, the coupling ofCFD grid cells is performed.

The NOx postprocessor typicallyreduces the number of grid cells by afactor of 10 to 20 and applies thereafterthe rigorous combustion kinetics thatinvolve all reactions responsible forpollutant formation.

Numerical approachAlthough the grouping methodologyreduces the number of cells drastically,the resulting grid and the complexity ofthe full kinetics combustion modelrequire a specially developed numericalalgorithm to keep computer calculationtimes within reasonable limits.

The large system of non-linearalgebraic equations is initially solvedwith successive substitutions using theCFD results as first estimate. Whenresiduals have reached a lower value, amodified global Newton method isapplied to reach the final solution. Thismethod solves the main diagonalstructures and approximates the extra-diagonal elements of the Jacobianmatrix. This methodology savesmemory allocation, which is the realbottleneck in large systems of equationsto be solved. To increase computationalefficiency, derivatives are evaluatedsymbolically rather than numerically.

Physical modelThe NOx postprocessor solves the massbalance in each grid cell reactor,accounting for convection, diffusionand chemical reactions. Local massfractions of each component arecalculated. Mass and temperaturefluctuations largely affect the reactionsinvolved in NOx formation, so themodel calculates these effects in detail.

The model has been validated bycomparing results with those obtainedunder controlled laboratory conditions.For this, the well-known and published

Sydney bluff body burner data are used.This burner produces a diffusion flamefrom a methane/hydrogen mixed fuel.

The model results compare very wellwith published laboratory data.

Figure 3 shows the maps of selected

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Figures 4a, b and c NO as a function of radial position at 13, 30 and 45mm from thebluff body burner calculated using the kinetics postprocessor — continuous line, fullkinetics mechanism; dashed line, prompt NOx suppressed

a

b

c


components as predicted by the NOx

postprocessor. NO is significantlyproduced through the prompt NOx

mechanism. This is confirmed by theconsiderable intermediate amounts ofHCN, which is an active component inthe prompt NOx mechanism. Asexpected, NO2 is formed close to thelean side of the flame. In the so-calledneck zone, the flame is close toextinction. In this zone, formation ofsome C2H6 can be observed. C2H6 isformed through recombination ofmethyl radicals. Other small amounts of products from recombinationreactions of hydrocarbon radicals, suchas formaldehyde, can be observed as well.

In Figure 4, the comparison betweenpredicted and experimental NO profilesis presented

LSV ultra-low-NOX burnerThe LSV burner is based on proprietaryknow-how from Air Products andChemicals Inc. The burner ismanufactured and commercialised byJohn Zink Company under a licencefrom Air Products and Chemicals Inc.Technip, under anagreement with thesecompanies, has exclusiverights for application ofthe LSV burner in ethylenefurnaces and for retrofitapplications of existingethylene furnaces ofdifferent make.

The LSV burner hasbeen successfully appliedin two Technip-designedsteam reformers currentlyoperated by Air Products,including a recent 110

MM SCFD hydrogen plant in Westlake,Louisiana, USA. It has also been selectedby a European refinery and Technip fora 80 MM SCFD hydrogen steamreformer project in North Europe. Asecond smaller hydrogen plant inEurope was successfully retrofitted withLSV burners in July 2005, and recently amajor ethylene producer selected theLSV burner for a large GK6 liquid-cracking furnace. This project is in theengineering phase.

LSV burner principlesTo reduce NOx emissions in the stackeffluent, the LSV burner and the furnacedesigners are creating special flow andtemperature patterns for fuel, air andcombustibles to create local conditionsthat are unfavourable to NOx formationor favour the reduction of already-formed NOx.

At the same time, the flame needs tobe stable not only at design conditionsbut also over a large turndown ratio.Flame rollover that may cause flameimpingement on the radiant coil shouldbe avoided at all times. The stability ofthe flame is determined by the burner

design and by the firebox geometry andheat flux patterns. Unfortunately, theconditions favouring low NOx aregenerally contradictory to theconditions for creating stable flames.This is a major reason why, in someinstances, larger-capacity ultra-low-NOx

bottom burners, which are nowgradually being applied in ethylenefurnaces, face severe problems of flameinstability, such as flame rollover.

In the LSV burner design, flamestability is reached by creating a large-scale vortex in the centre of the flame.This vortex is created by mixing part ofthe air with a small portion of the fuel atdissimilar velocities (Figure 5). The LSVburner, contrary to other ultra-low-NOx

burners, does not contain metallic orceramic flame stabilisers, which are aknown source of prompt NOx.

The LSV stabiliser has a largeturndown ratio, providing superiorflame stability performance with avariety of fuels and compositions.Typically, the LSV burner has aturndown ratio of 1 to 10 or more,keeping the flame stable.

The remainder of the combustion airis staged concentrically around thevortex. Specially designed zipper lancescreate an optimum mixing stage in thelargest portion of the fuel. Thetechnology applied in the proprietaryzipper lances (Figure 6) is inspired bystealth aircraft technology, wheresimilar mixing principles are applied to

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Figure 4d CO profiles vs radial position at two distances from the bluff body burner,calculated using the kinetics postprocessor

Figure 5 LSV burner design improvesflame stability

Figure 6 LSV burner in service

d


keep jet engine exhaust temperaturesdown to avoid upper atmosphere IRdetection.

When fuel lances are exposed to hotradiation, overheating causes cokingover time, and extensive foulingproblems may occur. Due to theirefficient mixing capability, the zipperlances do not require extension into thecombustion space, and overheating andfouling are avoided.

Simulation of the LSV burnerTechnip has applied its proprietary CFD-NOx simulator to investigate theperformance of the LSV burner in aradiant firebox environment, such as anethylene furnace or hydrogen steammethane reformer.

Besides the earlier-mentionedlaboratory burner simulation, furthervalidations of the CFD-NOx simulatorwere done using the LSV burnerperformance data in the Westlake steamreformer and the European steamreformer project in the test furnace. TheCFD-NOx simulator successfully predictsthe temperature distribution in thefirebox as well as the flame shape andthe profile of the heat flux to the radiantcoil. Furthermore, the NOx predictioncompares very well with the measuredplant data.

Concerning the application of theLSV burner for Technip-designedethylene furnaces, each projectencompasses a demonstration test of amultiple-burner arrangement, includingmultiple bottom and sidewall burners,in a John Zink test furnace to bewitnessed by the end user.

A CFD-NOx simulation of both thetest assembly and the GK6 or SMKfurnace to be equipped with the LSVburner is also part of the design anddemonstration activities. During thisphase, burner design parameters such asthose used to obtain desired flame-length and heat-release patterns arefinally chosen. In a CFD-NOx simulationof the LSV burner, three distinct zonescan be recognised (Figure 7).

In Zone 1, where the first smallportion of the total fuel comes intocontact with the central air stream, localunburned hydrocarbons cause theformation of prompt NO, and the NOconcentrations are relatively high.However, due to rapid mixing in the LSV,the absolute volume of such high NOconcentrations is small and,consequently, the absolute amount ofNO stays low. In Zone 2, whereremaining volumes of air and fuel are fedin a staged manner, the NO formed inZone 1 by means of the prompt NOx

mechanism, as well as molecular N2, areboth oxidised to NO2 via the thermalNOx route. When combustibles areentering Zone 3, combustion in the LSVburner is virtually complete.Concentrations of unburned hydro-carbons are extremely low, and any airremaining is only the stoichiometricexcess air fed to the burner. Conditionsin Zone 3 are favourable for thereduction of NO2 to NO and N2. The NOx

left in the flue gas leaving Zone 3determines the final achieved NOx levels.These are low and consist mainly of NOwith low amounts of NO2.

LSV burner application inethylene furnacesTechnip is currently performing adetailed CFD study as part of theengineering of a GK6 ethylene furnacein which a LSV burner will be applied.Intermediate results of this study arepresented in Figure 8, showing theperformance of a LSV burner in a GK6cracking furnace.

In this case, the LSV burner’sperformance in a bottom-firedconfiguration was studied to judge itsperformance without flame stabilisationfrom sidewall burners. The CFD analysisshows a stable combustion withoutflame impingement on the radianttubes. Due to the prevailing flowpatterns, the relative hot flue gases flowat the refractory side where theprevailing flow is upward. The relativecold flue gases are near the cold planeformed by the radiant tubes, where theprevailing flow is downward. Outsidetube-wall temperatures evolve smoothlyand local hot spots are avoided. Theaddition of some sidewall firingimproves this situation even more.Further CFD design study-work iscurrently ongoing. The results are notavailable for inclusion here, but are

expected to be available for presentationsoon.

Critical factorsDue to the current legislation in manycountries to reduce emission levels ofpollutant species in industrial furnaces,the design of the burner in combinationwith the design of the cracking furnacehas become a critical factor in theeventual successful performance of thefurnace after the first start-up.

In order to arrive at a sound design,detailed knowledge of the parametersinfluencing NOx formation as well asthose determining the behaviour of theflame and combustion in the totalassembly of burners, radiant coils andfirebox is required.

To obtain such detailed knowledge,Technip, in close co-operation withscientific researchers at the Politechnicodi Milano, has developed a CFD-NOx

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Figure 7 CFD-NOx simulation of the LSV burner in a firebox environment

Figure 8 CFD simulation of the LSV burnerperformance in a GK6 cracking furnaceshowing stable combustion pattern andthat no relatively hot gases are presentnear the tube lanes


simulator, which allows for the accurateprediction of flame behaviour and NOx

and other pollutant emission levels underindustrial conditions.

Technip and Air Products have appliedCFD-NOx simulations on Air Products’proprietary ultra-low-NOx burner (LSV)installed in an existing industrial steam-reforming furnace. The proprietary LSVburner has also been selected for aTechnip-designed GK6 ethylene furnace,which is currently at the engineeringstage. As part of the engineering effort, adetailed CFD-NOx study is currently ongoing.

LSV Burner is a mark of Air Products. Thisarticle is prepared from a paper presented atthe March 2006 ARTC in Kuala Lumpur.Special recognition is extended to thefollowing individuals for their help inpreparing the ARTC paper as well as thiscurrent article: Alessio Frassoldati, Guido BuzziFerraris, Tiziano Faravelli and Eliseo Ranzi,Politecnico di Milano, Milano, Italy. XianmingJimmy Li, Air Products and Chemicals Inc,Allentown, Pennsylvania, USA, and Ashok RPatel, John Zink Company LLC, Tulsa,Oklahoma, USA.

Simon Barendregt is vice president,business development and technology, withTechnip Benelux in Zoetermeer, TheNetherlands. Email: [email protected] Risseeuw is senior thermal ratingengineer with Technip Benelux inZoetermeer, The Netherlands, where he isresponsible for the design of heat-transferequipment and burners. Email: [email protected] Waterreus is senior process engineerwith Technip Benelux in Zoetermeer, TheNetherlands. Email: [email protected]

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