If coal is mixed it is burnt

Combustion techniquesechniquesand

Coal flame for cement kiln

Always to be remembered

If coal is mixed it is burnt

If flame is wrong everything goes wrongwhatever you may do with chemistry or higher heat input through calciner or kiln.The burning zone needs heat and it can beonly obtained from well shaped radiantflame.i.e., short, snappy and convergent flame .

Kinetics of coal combustion in kilns

1.Heating Heating of coal particles takes place by conduction and convection till ignition takes place is reached.Ignition temperature of

bituminous coal = 300 O Clignite = 250 O Canthracite = 400 O C pet coke = 800 O C

2. DevolatilisationDevolatisation process starts after the coal particles attain atemperature of 350 to 400 O C . At this temperature the coalbond structure breaks up to yield carbon monoxide , hydrogenand hydrocarbons.

The coal combustion phenomenon takes place in a cement Rotary kiln takes place in four stages.( for normal coal)

3.Volatile burning

The volatiles that are formed burn in gas phase and the rate of burning depend upon two factors , the rate at which thevolatile mix with air after being emitted from the coal particlesand the rate of chemical reaction.

4. Residual char burning

The residual char is the solid carbon left after completedevolatilisation. As the reaction progresses the residual char starts to take up 70 to 80 % 0f the total burning time.

heat

oxygen

Volatile matter evolutionAnd burning

Char gasificationand combustion

CO2H2ONOXSOXetc char

Coal combustion process

Main Processes in Coal Combustion

coal particlep-coal, d=30-

70µm

devolatilization

volatiles

char

homogeneouscombustion

heterogeneouscombustion

CO2, H2O, …

CO2, H2O, …

tchar=1-2sectvolatiles=50-100mstdevolatile=1-5ms

t

Coal particle drying , and then heating-up to the Pyrolysis reaction temperature

Heating up to the pyrolysis reaction temperature

Pyrolysis of the coal particle to produce non-condensableVolatiles ( gases) , condensable volatiles ( tars) , and carbonaceous char

Oxidation of the combustible volatiles ; and finallyChar oxidation.

Stages of coal combustion

Reburning

devolatilization

volatiles

char

homogeneouscombustion

heterogeneouscombustion

CO2, H2O, NO…

Excess air

CO2, H2O, NO…

CO2, H2O, N2…

CHi·

CHi· + NO ↔ HCN

HCN + NO ↔ N2 + …

Staged Combustion

Devolatilization

volatiles

char

homogeneouscombustion

heterogeneouscombustion

CO, CO2, H2O, N2…

Fuel Rich

CO, CO2, H2O, N2…

CO2, H2O, N2…

O2

The combustion time , T = K ( D ) 1.5

D = diameter of coal particleK = constant characteristic of coal quality

low for bituminous coalhigh for pet coke

The faster the combustion are gases are removed and replaced by fresh air( hot secondary air), the faster the coal particles burn.To fulfill this precondition , a high relative velocity betweenCombustion air and coal particles is required. This needs a highflame momentum with high primary air velocity and low % ofPrimary air.

That is why we maintain high residue for bitumen coal and lowresidue for pet coke (pet coke has low volatile)

Combustion time as a function of particlediameter

For pet cokeAnd anthracite

For bituminous coal

The residue on 90 mic is 5-7 % of the volatiles

Relationship between coal types,compostionand grinding fineness

Petcoke < 10 < 1.04%< + 0.09 mm0 %< + 0.2 mm

Normally the residue on 90 mic is50 % of the volatiles %.

Ignition time

Burning time of gases

pause

Burning time of carbon(char)

0.5 sec 0.1 Combustion timefor a particular coalparticle

Total combustion time

The physical processes influencing pulverized coal combustion

• Turbulent/swirling flow of air and coal.• Turbulent/convective/molecular diffusion of

gaseous reactants and products.• Convective heat transfer through the gas and

between the gas and coal particles.• Radiative heat transfer between the gas and

coal particles and between the coal/air mixture and the furnace walls.

COAL COMBUSTION CHARSWhen coal is combusted in air it burns in a two step process. In the first step

gases are driven out of the coal structure leaving behind a carbon char that burns

in the second step. These chars play a critical role in combustion in that they must

burn up in the reaction zone of the furnace or be carried out of the furnace as

unburnt carbon in fly ash. This unburnt carbon represents an inefficiency as well

as an economic loss because the energy in the unburnt carbon is not being used.

Excess unburnt carbon also destroys the ability of the fly ash to be use as a cement

in a variety of applications.

In modern combustion systems coal is usually ground into a fine powder

(-200 mesh or - 74 micrometers) that features many single maceral particles

. This is significant in that the various macerals tend to have different reactivities

and therefore burn at different rates. Because the different maceral groups form

chars with different morphologies, it is possible to analyze coal combustion chars

to gain information about the nature and reactivity of coals being combusted.

The vitrinite macrerals form chars that take the form of hollow spheres, centispheres

. Semifusinite macerals form centispheres with a lacey or honeycomb structure, and fusinite

maceral char come through the combustion process unfused.

Coal Combustion Char ClassificationTenuisphere Fused or partially fused hollow spherical or angular char with walls

less than 10 micrometers and porosity greater than 85 %

Crassisphere Fused to partially fused hollow spherical or angular char with walls

thicker than 10 micrometers and porosity greater than 75 %

Tenuinetwork Partly fused, thin-wall char with internal network structure and porosity

greater than 75%

Mesophere Partly fused, thin-wall char with internal network structure and

porosity 40-60%

Inertoid Unfused particle with a rectangular to irregular shape and low porosity of

5-40%

Solid Unfused particle with a rectangular to irregular shape and no porosity

Fusinoid Unfused particle resembling fusinite with original plant cell structure

Mixed Porous Mixed particle showing both fused and unfused sections with fused

porous section dominant

Mixed Dense Mixed particle showing both fused and unfused sections with unfused

porous section dominant

Skeletal Unfused, angular but highly burnt out char, still resembling fusinite

Mineraloid Char with over 50% mineral matter

Description: The object in the center of the field is a typical tenuisphere.

It is characterized by its spheroidal shape, open center, and thin walls.

The char forms such hollow spheres , also called Cenospheres before

Mixing with with Oxygen to form gases of various oxides.It easily bursts

Into micro particles of carbon.

Smaller, this balloon- like spheres and thinner its walls ,faster thecombustion. It is very difficult to form such spheres(ceno spheres) fromPet coke because of low volatilepresence.It needs very high

energy and longerretention time in ignition zone.

Cenosphere

Combustion of charOnce the ignition has occurred the critical reactions as far asa good combustion in kiln is concerned are:

H + O2 = OH + O

C n H m+ O = C n-1 Hm+C O

CO + OH = CO2 + H

2CO +O2 +M = 2CO2 + M

H2O + O = 2 OH

2C + O2 = 2 CO

Effect of coal properties on combustion Moisture content

A moisture content of 1 to 1.5 % in the pulverized coalpromotes combustion.In the presence of hydroxyl ions(OH)-, the formation CO and CO2 takes place by chain reaction.on the other hand a higher moisture content increases the thermalInertia of reacting species , shift the flame and reduces the flame temperature.Volatile matterThe volatile rich coal has a high porosity offering a larger spacearea for combustion hence requiring a lower ignition temperaturethan volatile less coal ( eg anthracite , pet coke etc).Thus coal richvolatile matter > 30% decomposes with higher rate andpromotes faster combustion . Volatile rich coal form smallcenospheres with thin walls and decompose faster.

AshAsh is an inert component of coal and an increase in quantityleads to increase in heating time due to added thermal inertia.Most of the combustible particles of coal will be covered by ashand hence less surface is available for oxygen diffusion. Thisincreases the burning time and the residual char causing anincrease in flame length. Overall there is delay in combustion,elongates the flame . If there is a cloud of clinker dust , what willhappen? This dust will absorb the radiated heat from flame ,reduce the heat flow to the refractory( and coating) and getreheated with more stickiness. Hence optimized cooler airflow with good clinker bed , overallcooler efficient operation will enhance the combustion efficiency.

Effect of coal moisture content on degree of combustion Vs distance from the burner.

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90

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0

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Degree of com

bustion

Distance from burner (m)

Moistu

re co nten

t

High moisture

Low moisture

1 2 3 4 5 6 7 8 9 10Distance from burner (m)

High moistureLow moisture

Effect of coal moisture on flame temperature vsdistance from burner

1700

1600

1500

1400

1300

1200

1100

1000

900

800

Flame tem

p erature , deg . C

1 2 3 4 5 6 7 8 9 10

Degree of com

bustion

Distance from burner (m)

low volatilite , 9.8 %

High Volatilite , 38 %

100

90

80

70

60

50

40

30

20

10

0

Effect of volatile content on degree of combustion vsdistance from burner

1 2 3 4 5 6 7 8 9 10Distance from burner (m)

Effect of secondary air velocity on flame temperature Vsdistance from the burner

1800

1700

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1100

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0

Seco

nda

ry a

ir v

eloc

ity,

m/

s

Flam

e te

mpe

rat u

re, d

e g C

Effect of oxygen level on exit gas heat loss

-0.5 0 0.5 1.0 1.5 2.0 2.5 3.0

In completecombustion Optimum operating range

oxygen level in kiln exit , %

Hea

t lo

ss K

cal

Oxygen concentration in kiln

Simulated oxygen content for an ideal flame

Flame

Cement kiln flame types

Straight flame –essentially external recirculation

Type-1 flameWeak internal recirculation external recirculation

Type-2 flameStrong internal recirculation external recirculation

Straight flame is stabilized ( single pipe burner) by the strength of the external recirculation flow establishedby the shear forces between the primary and secondary air streams

For multi – annual burners , enhancing the internal recirculation flowpatterns can increase the flame stability. This can be accomplished byreducing the momentum of the inner zones while increasing the momentum of the outer air zones , or by mounting a bluff body flamestabilizer in front of the primary stream.

Straight flame of single channel burner

Multi channel burner

Heat transfer from coal dust flames Flue gases or flame gases respectively pass their heat to the environment mainly by radiation and only to a small degree byconduction and convection. Normally 13 % ( ideal) of the kiln volume is usually filled , therefore major portion of the heat is transferred to kiln refractory lining with kiln feed receiving arelatively a small portion of the total heat volume. Many triedto keep the flame close to charge but it has negative influenceas coal may get trapped and cause reducing conditions in the charge which causes reduction of Fe2 O3 and also volatile recycling of alkalis and sulfur.If it is close to charge whichis 13 – 18 %(degree of filling) , the heat radiated to refractory weakened and causes poor heat exchange.The heat is carried always by the flue gases only to result in high backend temperature.If kiln has stable and optimum coating then then we get the best heat exchange as it acts as the best heat reservoir.

7 8 9

Burner positioning

We do positioning of the burner for centering the flame.The positions1,2,3, 4 and 7are close to the refractory and they are away from the charge.Positions9 and 8 are close to charge .

Only 5 is close to charge and refractory and this is best as the flame in this gives the best thermal distribution to do effective burning.Position 8 & 9 is very close to charge if coal is trapped it has serious negativeimpact.Position 1,4 & 7 is close to refractory and it gives more thermal stresses on refractory.

4 5 6

1 2 3

Flame positioning towards the charge

There is an illusion if the burner is kept just above the charge orImpinges the charge burning is better but it is on the other way. In heatexchange process 85 % of the heat is radiated to refractory and 15 % to the charge. If flame is kept above the flame.( Beyond the plume it is invisible)If we are not careful the char takes more time to burn out and hence it is highlypossible the char gets trapped , form local reducing condition , reduce the haematite ( vicious redox cycle), spoils the liquid and increase the recycle of sulfurous cycles.

The rules of radiation of solids cannot be applied by the radiation of flames.Monatomic and diatomic gases like N2 and O2 are in the range of infra-redentirely transparent and their radiation equals zero.Therefore , the presenceof these gases is only ballast. On the other hand gases with a higher no.ofatoms such as H2), CO2 and SO2 develop a considerable thermal radiationdue to their absorption bands in the IR range.CO2 radiates more than theothers.

The radiation active constituents of the pulverized coal flame area. The CO2 content of the flame gasesb. The H2O content of the flame gasesc. C the content of suspended dust in the flame gases

The following requirements result in promoting the heat transfer By the gases in the clinkering zone

1. An increase in the flame temperature2. An increase in the concentration of CO23. An increase in kiln diameter ( to have 13 % degree of filling)

Thick coating increases the degree of filling , reduces the effective diameter300 mm thickness is considered as ideal to improve the refractory life as Well as the heat exchange process.

Multi channel burner

Traditional burner

Function of burner or requisites of a good flame.

1.The burner must be able to burn fuel with a low excess air andwith a minimum generation of carbon monoxide , nitrogenOxides and volatile recycling like SO2 etc.

2 The burner must be able to produce a short, narrow , and strongly radiant flame which is a requisite for good heat transferfrom flame to material in the sintering zone of the kiln.

3. The flame formation must be conducive to the formation of a dense . stable coating on the refractory in the burning zone ofthe kiln as well as a nodular clinker with a low dust content andcorrectly developed clinker minerals.

4.The burner must use as little primary air possible since primaryair is basically false air.

Flame momentum

The burner in kiln functions as an injector, the purpose ofwhich is to draw the secondary air coming from the coolerinto the flame in order to burn the fuel as near the center ofKiln as possible.The explains why momentum of the burneris deciding factor for the flame formation. Multi channel burner makes a faster entrainment of secondaryair than single channel burner.Higher the momentum better theentrainment of secondary air and faster the combustion of fuel.

momentum or impulse = % primary air * velocity of primary airfor normal coal = 1200 – 1500 % m/sfor petcoke > 2500 % m/s Momentum obtained by low primary air % and higher velocityis better than higher primary air % and lower velocity

Secondary airVelocity= 5 – 6 m/s

IgnitionThis depends uponRate of mixing of sec airand coal particles, size of,the fuel particle and volatileContent and the injectionVelocity.

This depends on the pressure differencebetween secondaryair region and primary air region. Higher the pressure difference higher inner Circulation.

If the jet has goodmomentum it will pull backthe flue gases ,causingexternal recirculation.This is an indication ofSec,air entrainment intothe primary air jet.MultiChannel burners do thisJob efficiently. This reduces the NOX formation.

Outsidecirculation

Insidecirculation

Ejector effect

Ejector effect

Ignition areaSecondary air taking area

Recirculated combustiongas area

Secondary air

Secondary air

Axial outer streamSwirl coal +transport airSwirl inner stream

Different flames

Normal flame

Flame with lowSecondary air tempDistorted nozzle

Flame –poorhood geometryOr distorted nozzle

Flame at the center

Flame downward

Flame upward

Different flamesNormal snappy flameforms dense andstable coating

Indication of first dam

Long , lazy flameWith unstable coating

To be remembered: if burner pipe is at the center that does not mean flameis in center. Visualizing is the best thing to do and it should be done fromright and left peeping holes . If there is a peeping hole just above the burner inthe center help us further to center the flame. A good uniform coating is a fairlygood criterion for a good flame. Uniform shell temperature around the shell isgood indication.

Secondary air Velocity influences flamelength and shape

Higher the secondaryair velocity longer isthe flame.Here we haveto increase the flamemomentum by increasingthe primary air velocityat the tipHigher the sec.air velocityLower the hot air pressure region.Hence we have to increasethe pressure drop at thetip to pull back more

secondary air towards the flame. Coatingat the tip , called shark teeth,increases the secondaryair velocity and so increases the flame length.

Secondary air velocity Vs flame length

Flame trouble shootingPulsating flame with CO Peaks at the kiln inlet

Check the flow promoters.clean the bin as there may be coating formation.check the liners in the cone. Bin dusting pressureshould be maintained. Coal flow discharge chute can have coatingformation. Un uniform gap between screw flight and casing.Firing pump discharge flaps -counter weight needs adjustment .dedusting for coal feeders device is to be optimum.If FK pump seal is leaking transport air can go inside the pump screw andfluidise the coal , change its bulk density and hence the flow.Pump is a volumetric transport device.

1. Fluctuations in coal flow

2.insufficient secondary air temperature and flow variationOptimise the clinker bed in cooler and cooling air to recuperate moreheat. Reduce the variations in the under grate pressure as well ashood pressure pulsation. If shock blasters are there adjust thetime interval to avoid pressurization of hood.

3. Insufficient transport air velocity or coal injectionvelocity

Check the material /air ratio. It is 4 kg coal/ cu .m air to 6 kgs, cu m.The velocity is 25 to 30 m/s . If it is not so, modify the transport pipe inner dia or increase the transport air volume.

Too long transport air pipe. The maximum length is 100 meters.Avoid sharp bends as these bends will cause pressure variations.during lay out itself it should be considered.

Check the coarseness of coal.Too coarse coal can settle in thepipe line.

Flame characterised by a long blackcore( long plume) , increased CO-value at thekiln inlet1.Too high coal injection velocity. It is normally 25 - 30 m /s

Increase the coal pipe annular space at tip to have 25 - 30 m/stip velocity.Some plants plants run with < 25 m/s also.

2.Insufficient mixing of coal and secondary air( delay in combustion)Low secondary air temperature. Arrest false air ingress through nose ring by cooling fan or false outlet sealing.

3.Coarse coalCheck the separator . Increase the fineness.

Flame burning at the burner tip or sometimes coal drops

1.Too low coal injection velocity .

This may be due to rotor gap fo blower has increased or blower filtergot choked. If coal pipe got punctured inside the burner coal will mixwith primary air flow and damage further.

check for wear of the coal injection pipe tip.Monitor filter DP and rotor gap ( blower pressure )

2. Excessive swirl air

Optimize the swirl air

Flame burning unilaterally

1 .Partial pipe clogging due to foreign matter existing in coal channelRemove the foreign matter. When we do the casting ensurethat wet castable mix should not drop into the burner.close the burner by a plate while casting

2. Worn out centering element for coal channel or air channel.Unequal spacing of air flow annular space.

Aligning of burner channels so that annular spacing is equal for coal flow pipe as well for primary airflow

3. Coal pipe got punctured inside the burner and the coal flows into primary air flow channels.

Change the burner along with coal injection pipe.

Burner adjustment

EffectCoal property

As per Pillard, burner needs changes whencoal quality changes.

Volatileincreases

Flame shortens andBurning zone Temperature rises

Reduce swirl air,Increase axial air,move Burner into kiln

Grindabilitydecreases

Fineness decreases,flame lengthens andtemperature drops

Increase swirl air,retract burner

Heating value decreases

Heat input drops,Flame lengthens and Sintering Temperaturedrops

Increase swirl air,reduce axial air and increase coal feed

Heat release pattern considerably affected by changes in secondary air temperature, excess air , fuel quality etc.

Good flame shape with stable hat release patternGives stable operation

Kiln stability

Poor fuel/ air mixing gives gradual heat release with long flame

Rapid mixing gives high flame temperature and good heat transfer

Heat release pattern

High levels of CO produced at oxygen levels as high as 2 – 4 %

CO only produced in significant quantities below 0.5 %

Carbon monoxide level

Flame impingement occurs on refractory where jet expands to hit the wall( 11-14 %)

None- recirculating gases protect refractory and product from direct contact

Flame impingement

Reducing condition occur in fuel rich part of the flame and in the area of flame impingement

Oxidizing conditions exist throughout the flame

Reducing/ oxidizing conditions

PoorGoodFuel / air mixingFlame without recirculationFlame with recirculation

Characteristic of Flames with and without recirculation

Burner swirl number = tangential momentum( N) * characteristic swirl radius(m)

Axial momentum(N)*charateristical channel radius(m)

Flame temperature (T) = Hv / 1.11A s

T = theoretical flame temperatureA = combustion air required kg / kg coalHv = heating value of fuelS = specific heat of combustion gas ( 0.29)

Hrmf C vLf Dπ

= Where HrmfC vLfD

= combustion intensity, kw/ m2

= fuel flow rate, Kg/ s= net calorific value , kj/ kg= flame length , m= kiln internal diameter , m

Heat flux and combustion intensity

Swirl coefficient( Swirl number)

As per M.A.S/ Burner,

Sn = I tan .R e.tan

I ax . R e.ax

Where

Sn = swirl coefficient ( swirl number)

I tan = momentum of swirl air in tangential momentumRe.tan = momentum of swirl air in axial direction

I axR e.ax

= equivalent radius of swirl air duct

= equivalent radius of the axial air duct

This index refers to the generation of gaseous re-circulationsexternally to the flame/ It is directly to the aspiration and mixing of secondary air by both primary air and fuel / conveyingair streams. The axial index also has some relation toreicirculation at the kiln area and the formation of build-ups at the nose-ring called the so-called “ shark teeth”

Axial index

Tangential index:

This index refers to re-circulations internally to the flame,Which has influence in the ignition of the particles andflame spread. The tangential index has close relationshipWith the position and intensity of the first temperature peakIn the kiln. Usually , during burner design the dimensionsOf the nozzles at the tip are calculated in order to allowThe variation of this index inside a predetermined range,depending on the adjustment of the primary air components.So , if the basis of design indicates narrower flames , the burnerdesigner should calculate the tip dimensions to get lower valuesof tangential index in the burner operational range. On the otherhand , if the basis of project indicates that the process wouldrequire wide and short flames , then the designer should calculate The burner operational range to present highertangential indexes.

Turbulence index:This index refers to the position of both temperaturepeaks in the kiln.During the calculation of the burner tipdimensions the turbulence index is checked to be above a minimum value all over the range of adjustment of the burner.Usually this minimum value is calculated as a function offuel type , fuel preparation ( moisture and fineness ),secondary air temperature and kiln dimensions.With relationshipto that minimum value of the turbulence index it should be pointed out that:• Bituminous coal finely ground ( 90 < 170 ) would requirelower turbulence indexes than petroleum coke ground to the same fineness.

One system operating with 100 % petroleum coke ground to90 < 170 would require higher turbulence index than anothersystem operating with the same coke ground to 99 < 170

Dispersion index:

This index refers to the conditions of dispersion of the pulverized fuel cloud in the primary and secondary air streams. the dispersion index is related to the intensity of both Temperature peaks and as consequence , plays a major rolein the study of the thermal NOX generation.

Some additional factors , not directly related to characteristicdimensionless indexes must be considered during burnerdesign. The first one refers to the secondary air conditions( temperature , velocity distribution , dust content, etc). Thesecond factor is the burner pipe penetration into the kilncylinder in view that the length of this penetration has provedTo interfere in both kiln performance and clinker quality.Finally , the firing hood geometry has some influence in the flamecharacteristics as it interferes with secondary air flow pattern

Conclusion

After taking account of all considerations above , it is possibleto conclude that the combustion plays a major role in the rotary kiln operation , but any improvement in this area shouldbe faced , first of all , as a cooking problem and merely as afiring problem. It must be considered all the predominant Variables of the process and not only those related to the Oxidation of a fuel. Statement by - Peter J Mullinger

Adelaide combustion institute

Though the burner is very efficient we should know how use it.An experienced man must know how to look into the kiln to haveProper judgment about the flame being formed by the burner.

Thank you for your kind Attention

K.P.Pradeep kumar