IntroductionFire and the heat from it drives the cement makingprocess, initiating the clinker producing reactions, and alarge proportion of cement making costs (fuel cost,refractories, maintenance) are linked to combustion.These are the obvious connections. Indirectly though,the combustion system also largely determines clinkerquality and the grindability of clinker thus affecting thecement mill power consumption and output as well. Insome cases, emissions restrict production, and combus-tion is a major factor in this too.

Delivering benefits through optimising combustion isthe business of FCT-Combustion Pty Ltd.. Scientificallybased techniques developed by FCT and validated overthe past 20 years have been proven to deliver large ben-efits to customers.

The effect of different burner designAll burner systems are not created equal - the nature offlames and heat transfer to the process certainly dependson the burner design, but just as much on the aerody-namics of the combustion environment. It is the combi-nation, therefore, of a burner design and airflowpatterns around it that determine the nature of theflame produced and the way the heat is transferred tothe material. A burner design developed in isolationwithout reference to aerodynamics will behave differ-ently in different kilns, and is notan engineering solution.

In cement kiln projects, the dif-ference between burners can besurprising. If benefits such as a 7%reduction in specific fuel consump-tion, 8% increase in production,7% increase in cement strength,and significant increases in refrac-tory life could be achieved, along-side higher percentages of muchcheaper fuels such as petcokebeing used, great results could beattained. In the case of a 1 milliontpa cement plant, such benefitscould run into several million dol-lars extra profit annually. Yet suchbenefits have been documentedwhere kiln and calciner burnershave been replaced by optimisedequipment.

Combustion equipment typical-ly represents a small percentage oftotal plant capital, but its perfor-mance strongly influences thereturn on investment from that

plant. Itis, therefore, false econ-omy to buy burners on price alone, as anon-optimised burner could cost a plant each year muchmore than the purchase cost of an optimised burner.

Combustion zone aero-dynamicsThe mass of combustion air, be it primary, secondary ortertiary in a cement kiln or calciner, is approximately anorder of magnitude more than the fuel. The velocity anddirection of this air is a major influence on the nature ofthe flame (size, shape, length, stability), as it is the inter-action between this combustion air and the fuel comingfrom the burner that defines the flame.

The secondary air from the cooler undergoes severalchanges in direction before it enters the kiln, leavingpotential for poor combustion aerodynamics in theregion of the kiln burner. Likewise the calciner tertiaryair inlets often produce stratification and recirculation inthe calciner vessel, leading to poor fuel/air mixing.

Small differences in cooler throat, cooler bull nose,kiln hood, tertiary air off-takes, secondary air tempera-ture/velocity and air leakage points to name a few, canhave an enormous effect on the air flow patterns in thecombustion environment of the rotary kiln.

Tertiary air inlet design and location as well as tertiaryair temperature and velocity will likewise determine the

airflow patterns in a flash calciner.

Total system design

considerationsWith the complexities of the cementprocess, changes and solutions needto be implemented with a broadperspective in mind. In combustionsystems especially, it is important toknow and understand the implica-tions of a new burner or fuel changeon fuel efficiency, production rate,product quality, refractory life, emis-sions and operating stability forexample. Without such proper inves-tigation prior to a change, optimaloutcomes are not possible, and dis-appointments are common.

In order to ensure that all rele-vant factors are taken into accountin combustion system design,process parameters and kiln systemdesign must be considered whendevising a burner for a given appli-cation. FCT employs experiencedcement and lime process experts and

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Kiln Burner DesignIlkfer Akman, Ufuk Durgut, Akcansa Cement,

and Con G Manias, Steven J Hill, FCT, investigate

the impact on plant economics of optimised

burner design.

Figure 1. Examples of water bead modelling ofkiln hoods.

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skilled combustion engineers to evaluate the whole system.

Part of the FCT design process includes the use of pro-prietary modelling techniques developed by the comp-any and used since 1984. These techniques for analysingthe fuel/air interaction in combustion environments andfor predicting heat release and heat flux profiles providevital information and design parameters used in FCTburner designs. Both physical and mathematical model-ling techniques are used in combination to arrive at thebest solution or burner design.

Water bead modellingWater bead modelling is used to examine the flow pat-terns inside a vessel such as a kiln or calciner. A model ofthe internal dimensions of a kiln, cooler, calciner or duct,for example, is drawn up from plant drawings and builtusing clear plastic material. Water, containing poly-styrene beads with the same density as water, is passedthrough it under fluid dynamic conditions representativeof the plant, and the resulting flow patterns observedwith the aid of photography. Water flow patterns will beidentical to those which air will follow in the plant, butmore easily observeable in the model. The process

indicates potential problems such as unstable or reverseflow, high and low velocity areas and the cause of anyundesirable flow patterns.

Acid alkali modellingAll combustion takes place in the following sequence:

Generally, ignition and chemical reaction (combus-tion) occur virtually instantaneously compared to mixing,so that the combustion process is controlled by the rateof fuel/air mixing. Hence the often heard phrase: “If it’smixed, it’s burnt”.

Acid alkali modelling is used to examine the fuel/airinteraction in a given combustion environment. A physi-cal model is designed and built in the same way as forwater bead modelling, but burners are included. Themodel is designed, built and operated observing fluiddynamic similarity with the real plant in preference togeometric similarity.

Air flow (represented by an acid solution) and fuel(represented by a phenolphthalein doped alkali solu-tion), are introduced into the model kiln at rates, con-centrations, velocities and positions representative of theactual plant conditions. The resulting reaction betweenthe acid (air) and alkali (fuel) is representative of thecombustion reaction. The pink colour of the alkali solu-tion fades on reaction with the acid, leaving a trail ofpink where there is un-reacted alkali, representing thekiln flame in the plant.

The model allows solutions to be developed andtested to ensure optimised combustion prior to installingand commissioning a new burner on a specific kiln.Variables such as primary air quantity and momentum,burner position and insertion distance, excess air require-ments and fuel injection velocities, can all be determinedfrom the model so that trial and error after installation iseliminated. In cases where the kiln aero-dynamics arenot good for combustion, physical changes to hood,cooler throat, off-take ducts, and the like, can be mod-elled and the most cost effective design solution deter-mined. In existing plants improvements have often beenmade with simple changes to refractory profiles.

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Figure 2. Modelling of a kiln burner.

Figure 3. Heat flux profiles determined from mathematical modelling.

Figure 4. Flame length determinations from physical modelling.Figure 5. FCT burners are designed from a total system perspective.

Mixing →→→ Ignition →→→ Chemical →→→ Dispersion of Reaction Combustion

Products

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rsMathematical modellingResults from physical modelling are corrected with math-ematical models that take into account the kiln or cal-ciner temperature (physical models are runiso-thermally) and the fuel properties. With some fuels,such as petcoke, the slow chemical reaction (combustion)may be significant and burn rates of these fuels must beconsidered in flame length determinations.

Modelling validationInitial validation of FCT modelling techniques was carriedout on a full size coal fired cement kiln in the late 1970s.Since that time, the company has conducted over twohundred model studies on operating plants, with theimplementation of model results proving the methodsused. The physical and mathematical modelling tech-niques used by FCT have been validated many times over,allowing their use with confidence in design or diagnos-tic applications.

Burner designDesigning kiln burners, therefore, is more than ensuringthey will be able to deliver the correct fuel rate. Processoperation, fuel properties, heat flux requirements, andkiln system aerodynamics evaluations should form partof the design process.

The most important factor is to match the momen-tum of the fuel and primary air coming from the burnerwith the momentum of the secondary air or tertiary airto achieve complete combustion within the limits of thekiln or calciner. At the same time, flame impingement onrefractory or kiln materials is to be avoided, while heatflux must be tolerable for the refractory materials.

Cement kiln burners (except for Gyro-Therm burners)are turbulent jet diffusion burners that rely on frictionbetween the burner jets (fuel and primary air) and thesurrounding air to entrain secondary air into the fuel jetcausing it to expand.

The relative momentum between burner and sur-rounding air is one of the key factors in designing aburner with the correct flame length and shape. TheCraya-Curtet parameter indicates burner design parame-ters required for a given kiln system and operating con-ditions for adequate secondary air entrainment andhence fuel/air mixing. Inadequate secondary air entrain-ment gives long lazy flames, high CO levels and can leadto kiln instability and build up problems. Excessiveburner momentum on the other hand can give a veryaggressive flame with refractory problems and poorthermal efficiency due to high primary air levels.

The physical modelling may also indicate some aero-dynamic issues that require correction through modifica-tions to vessels/ducts, or a burner design/ location tocounteract them.

Most burners have some degree of adjustability, butthis is of limited help for optimisation. Given the hugenumber of variables that exist and the relatively slowresponse time of the kiln to changes, optimisation ‘onthe run’ can be a time consuming andexpensive business. In fact, the majority of burnersadapted whilst in operation will never be truly opti-mised, as the inherent burner design will limit the truepotential.

Changes in product quality, fuel efficiency, refractorylife, etc, from a change in the burner setup can take

weeks, months or even years to evaluate. There arealways complicating factors in cement plants making itdifficult to draw conclusions quickly. The value of model-ling is in establishing optimum conditions on the model,and then applying this to the plant with minimal risk anddisruption.

Kiln burner case study

GeneralAkcansa Cimento has a plant in Buyukcekmece withthree kiln lines producing 1.85 million tpa of clinker.

Kiln 3 is a preheater kiln producing 1650 tpd. The kilnuses solid fuel, with oil and gas as back up and warm up

Figure 7. Kiln 3 original burner operating at 5% excess air.

Figure 8. Kiln 3 original burner operating at 30% excess air.

Figure 6. Model showing original flame in Akcansa kiln 3. Notepoor fuel/air mixing, long flame and impingement on clinkercharge in kiln.

Figure 9. New kiln 3 burner showing flame length at 5, 10 and30% excess air levels. Recommended condition to operate kilnwith 10 - 15% excess air.

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fuel. Akcansa Cimento was using a combination ofimported and local coal with up to 30% petcoke, but waskeen to use more petcoke since it is a cheaper fuel.

The main issues with the kiln system were that highlevels of CO were observed at the kiln inlet that becameworse with petcoke firing. Large build ups in the riserduct and kiln rings were also causing operational insta-bility and became unacceptable if the percentage of pet-coke increased above 30%.

Technical evaluations and modelling phaseFCT began the project with a site visit by two engineersto take plant measurements and gather operating dataand plant drawings. This information was evaluated anda model of the kiln was designed and built to simulatekiln 3 operations.

Operating the model in the laboratory under simu-lated plant conditions revealed the cause of the prob-lem. The kiln burner was not correctly designed for thekiln geometry and its operating conditions. The burnerfuel was not mixing properly with the primary and sec-ondary air, and a long lazy flame was produced in thekiln.

The flame was impinging on the clinker in the kiln,and when this occurs the sulphur in the clinker is readilyconverted to SO2 and carries through to the back of thekiln where it combines with lime and other minerals toproduce build up that affects kiln operation. As the alkalicontent of the feed material was sufficient to combinewith the sulphur introduced into the kiln system, it wasconcluded that the main reason for the sulphur volatili-sation was the flame impingement.

The original burner was also tested at varying excessair levels, as sometimes a flame can be significantlyimproved if a little more excess air is present. In this case,however, the flame was very poor even at high excess airup to 30%.

The model clearly showed that the existingburner design was producing a flame that was too longand lazy for good kiln operation, and that impingementof the flame on the clinker was undoubtedly driving sul-phur out of the clinker even at high excess air levels.

Following the technical investigations, the conclu-sions reached were that:

� The extant burner provides insufficient momentumto entrain the combustion air.

� The burner position and angle in the kiln causesflame impingement.

� The kiln is operated with too little excess air.

� Flame length is excessive at all reasonable excess airlevels.

New burner designFollowing the revelations of the modelling work, a newFCT burner was designed according to the FCT designparameters for momentum of the various burner jets incomparison to the secondary air momentum, and thenew design was again tested in the model. The resultswere obvious, as shown in Figure 9.

The new burner design was further tested to find theoptimal burner insertion distance/angle and swirl to axialmomentum ratio.

The results from modelling the new burner were:

� The new burner provides substantially shorter flamelength at all excess air levels.

� Satisfactory flame length and shape can be achievedat excess air levels of 10%.

� The burner performed best when aligned with thekiln axis.

� Correct burner momentum gives a shorter flame thanthe original burner.

� The burner performance was not particularly sensi-

Figure 12. Modelling new burner at different burner angles - per-formance is best with burner aligned with kiln axis.

Figure 10. Flame length against excess air levels for original and newkiln 3 burner.

Figure 11. Modelling new burner with different swirl to axialmomentum ratios.

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tive to insertion distance.

� Swirl must be below a critical level determined by themodelling or flame impingement would again beprevalent.

Project implementationFollowing the modelling and technical evaluations, theresults were translated into burner design parameters,and a new burner for kiln 3 was designed, manufacturedand delivered to Akcansa. The burner was installed andcommissioned in December 2002, and has operatedextremely well since then, truly reflecting the expecta-tions from the modelling work.

The highlights from the installation have been:

� Petcoke usage has now increased to 100%; previously30%.

� Clinker quality has improved dramatically, with 28 Dstrengths increasing by 10%.

� Almost no build up has been experienced in the kilnand riser, where previously build up was restrictingkiln performance even with 30% petcoke.

� A fuel saving of about 45 kcal/kg was seen by theplant, partly attributable to a change in the coal dos-ing system and partly to the new burner.

The kiln is now operating in a very stablecondition, producing excellent clinker more efficient-ly than before, giving an excellent payback on theburner investment.

Following the success of this burner installation,Akcansa have also replaced the burners on the remainingtwo kilns from the Buyukcekmece site with FCT burners.

ConclusionThe use of scientific techniques for optimisation of kiln combustion systems and for getting the best

performance from a cement kiln has been well proven inmany projects, and this case study again confirms the FCTapproach.

Kiln burners are relatively cheap items, but thewrong burner can have a very high cost in lost opportu-nity, if output, fuel consumption, clinker quality andrefractory life are not optimised. A kiln burner is oneitem of a plant that should not be evaluated by raw cap-ital cost alone, but according to its total impact on planteconomics.

Scientific techniques such as combustion and heattransfer modelling provide the basis for more confidencein designs, and reduce the risk of negative impact onplant economic performance._______________________�

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Figure 13. Akcansa kiln 3 burner being delivered to site.

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