Flame for cement kilns kppradeep kumar

Combustion techniques
and
Coal flame for cement
kiln

If coal is mixed it is burnt

PDF created with pdfFactory Pro trial version www.pdffactory.com

Always to be remembered

If coal is mixed it is burnt

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

Kinetics of coal combustion in kilns
The coal combustion phenomenon takes place in a cement
Rotary kiln takes place in four stages.( for normal coal)
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 C
lignite = 250 O C
anthracite = 400 O C
pet coke = 800 O C

2. Devolatilisation
Devolatisation process starts after the coal particles attain a
temperature of 350 to 400 O C . At this temperature the coal
bond structure breaks up to yield carbon monoxide , hydrogen
and hydrocarbons.

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 the
volatile mix with air after being emitted from the coal particles
and the rate of chemical reaction.

4. Residual char burning

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


heat Volatile matter evolution
And burning

oxygen Char gasification
CO2 and
H2O combustion
NOX
SOX
etc char


Coal combustion process


Main Processes in Coal Combustion

homogeneous
coal particle combustion CO2, H2O, …
volatiles
p-coal, d=30-
70µm
heterogeneous
combustion CO2, H2O, …
char

devolatilization

tdevolatile=1-5ms tvolatiles=50-100ms tchar=1-2sec

t


Stages of coal combustion

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-condensable
Volatiles ( gases) , condensable volatiles ( tars) , and
carbonaceous char

Oxidation of the combustible volatiles ; and finally
Char oxidation.


Reburning

Excess air
homogeneous
combustion
volatiles CO2, H2O, NO…

heterogeneous
combustion
char CO2, H2O, NO… CHi∙

devolatilization
CO2, H2O, N2…

CHi∙ + NO ↔ HCN
HCN + NO ↔ N2 + …


Staged Combustion

Fuel Rich
homogeneous
combustion
volatiles CO, CO2, H2O, N2…

heterogeneous
combustion
char CO, CO2, H2O, N2… O2

Devolatilizatio
n CO2, H2O, N2…


Combustion time as a function of particle
diameter
The combustion time , T = K ( D ) 1.5
D = diameter of coal particle
K = constant characteristic of coal quality
low for bituminous coal
high for pet coke

That is why we maintain high residue for bitumen coal and low
residue for pet coke (pet coke has low volatile)
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 between
Combustion air and coal particles is required. This needs a high
flame momentum with high primary air velocity and low % of
Primary air.


For bituminous coal
For pet coke
And anthracite

Relationship between coal types,compostion
and grinding fineness

Petcoke < 10 < 1.0
4%< + 0.09 mm
0 %< + 0.2 mm


0.5 sec 0.1 Combustion time
for a particular coal
particle
Total combustion time

Ignition time

Burning time of gases

pause

Burning time of carbon(char)


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 CHARS
When 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 Classification
Tenuisphere 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

Smaller, this balloon- like spheres and thinner its walls ,faster the
combustion. It is very difficult to form such spheres(ceno spheres) from
Pet coke because of low volatile
presence.It needs very high
energy and longer
retention time in ignition zone.

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.

Cenosphere


Combustion of char
Once the ignition has occurred the critical reactions as far as
a 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 coal
promotes 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 thermal
Inertia of reacting species , shift the flame and reduces the flame
temperature.
Volatile matter
The volatile rich coal has a high porosity offering a larger space
area for combustion hence requiring a lower ignition temperature
than volatile less coal ( eg anthracite , pet coke etc).Thus coal rich
volatile matter > 30% decomposes with higher rate and
promotes faster combustion . Volatile rich coal form small
cenospheres with thin walls and decompose faster.


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


Effect of coal moisture content on degree of combustion
Vs distance from the burner.
0 100

1 90

2 80

Degree of combustion
3
70
Moisture content

4
60
High moisture
5
50
6
40
7
30
Low moisture
8
20

9 10

10
1 2 3 4 5 6 7 8 9 10 0
Distance from burner (m)

Effect of coal moisture on flame temperature vs
distance from burner
1700

1600
Flame temperature , deg . C

1500
Low moisture
1400 High moisture
1300

1200

1100

1000

900

800

1 2 3 4 5 6 7 8 9 10

Effect of volatile content on degree of combustion vs
distance from burner
100

90

80

Degree of combustion
70

60
low volatilite , 9.8 %
50

40

30
High
Volatilite , 38 % 20

10

1 2 3 4 5 6 7 8 9 10 0

Effect of secondary air velocity on flame temperature Vs
distance from the burner
1800 10

1700 9

Secondary air velocity, m/ s
1600
8
Flame temperature, deg C

1500
7
1400
6
1300
5
1200

1100 4

3
1000
2

900 1
800
1 2 3 4 5 6 7 8 9 10 0

Effect of oxygen level on exit gas heat loss
Heat loss Kcal

In complete
Optimum operating range
combustion

-0.5 0 0.5 1.0 1.5 2.0 2.5 3.0
oxygen level in kiln exit , %

Simulated oxygen content for an ideal flame

Oxygen concentration in kiln


Flame


Cement kiln flame types

Straight flame –essentially external recirculation

Type-1 flame
Weak internal recirculation external recirculation

Type-2 flame
Strong internal recirculation external recirculation

Straight flame of single channel burner

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

Multi channel burner
For multi – annual burners , enhancing the internal recirculation flow
patterns can increase the flame stability. This can be accomplished by
reducing the momentum of the inner zones while increasing the
momentum of the outer air zones , or by mounting a bluff body flame
stabilizer in front of the primary stream.


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 by
conduction 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 a
relatively a small portion of the total heat volume. Many tried
to keep the flame close to charge but it has negative influence
as 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 which
is 13 – 18 % , 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.


We do positioning of the
burner for centering the
flame.The positions
1,2,3, 4 and 7are close
to the refractory and
they are away from the
charge.
1 2 3 Positions9 and 8
are close to charge .
4 5 6 Only 5 is close to charge
and refractory and this is
7 8 9 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
negative
impact.Position 1,4 & 7
is very close to refractory
and it can burn the
Burner positioning refractory.

Flame positioning towards the charge
There is an illusion if the burner is kept just above the charge or
Impinges the charge burning is better but it is on the other way. In heat
exchange 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 highly
possible 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-red
entirely transparent and their radiation equals zero.Therefore , the presence
of these gases is only ballast. On the other hand gases with a higher no.of
atoms such as H2), CO2 and SO2 develop a considerable thermal radiation
due to their absorption bands in the IR range.CO2 radiates more than the
others.


The radiation active constituents of the pulverized coal flame are
a. The CO2 content of the flame gases
b. The H2O content of the flame gases
c. 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 temperature
2. An increase in the concentration of CO2
3. An increase in kiln diameter ( to have 13 % degree of filling)

Thick coating increases the degree of filling , reduces the effective diameter
300 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 and
with a minimum generation of carbon monoxide , nitrogen
Oxides 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 transfer
from 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 of
the kiln as well as a nodular clinker with a low dust content and
correctly developed clinker minerals.

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


Flame momentum

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

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


Secondary air
Velocity= 5 – 6 m/s

Ignition This depends on the If the jet has good
This depends upon pressure difference momentum
Rate of mixing of between secondary it will pull back
sec air air region and the flue gases ,causing
and coal particles, primary air region. external recirculation.
size of, Higher the pressure This is an indication of
the fuel particle difference higher Sec,air entrainment into
and volatile inner Circulation. the primary air jet.Multi
Content and the Channel burners do this
injection Job efficiently. This reduces
Velocity. the NOX formation.


Secondary
air

Ejector effect
Inside Outside
circulation circulation

Ejector effect
Secondary air

Secondary air Recirculated combustion
Ignition area taking area gas area
Axial outer stream
Swirl coal +transport air
Swirl inner stream


Different flames Flame at the center

Normal flame

Flame with low Flame downward
Secondary air temp
Distorted nozzle

Flame upward
Flame –poor
hood geometry
Or distorted nozzle


Different flames
Normal snappy flame
forms dense and
stable coating

Indication of first dam
Long , lazy flame
With unstable coating

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


Secondary air velocity Vs flame length
Secondary air
Velocity influences flame
length and shape

Higher the secondary
air velocity longer is
the flame.Here we have
to increase the flame
momentum by increasing
the primary air velocity
at the tip
Higher the sec.air velocity
Lower the hot air pressure region.
Hence we have to increase
the pressure drop at the
tip to pull back more
secondary air towards
the flame. Coating
at the tip , called shark teeth,
increases the secondary
air velocity and so increases the flame length.


Flame trouble shooting
Pulsating flame with CO Peaks at the kiln inlet
1. Fluctuations in coal flow
Check the flow promoters.clean the bin as there may be coating
formation.check the liners in the cone. Bin dusting pressure
should be maintained. Coal flow discharge chute can have coating
formation. 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 and
fluidise the coal , change its bulk density and hence the flow.
Pump is a volumetric transport device.
2.insufficient secondary air temperature and flow variation
Optimise the clinker bed in cooler and cooling air to recuperate more
heat. Reduce the variations in the under grate pressure as well as
hood pressure pulsation. If shock blasters are there adjust the
time interval to avoid pressurization of hood.


3. Insufficient transport air velocity or coal injection
velocity

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 250 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 the
pipe line.


Flame characterised by a long blackcore
( long plume) , increased CO-value at the
kiln inlet
1.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/s
tip 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 coal
Check 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 blowe has increased or blower filter
got choked. If coal pipe got punctured inside the burner coal will mix
with 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
channel
Remove the foreign matter. When we do the casting ensure
that 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.


As per Pillard, burner needs changes when
coal quality changes.
Coal property Effect Burner
adjustment
Volatile Flame shortens and Reduce swirl air,
increases Burning zone Increase axial air,
Temperature rises move Burner into kiln

Grindability Fineness decreases,
Increase swirl air,
decreases flame lengthens and
retract burner
temperature drops

Heat input drops, Increase swirl air,
Heating value
Flame lengthens and reduce axial air
decreases
Sintering Temperature and increase coal
drops feed


Characteristic of Flames with and without recirculation
Flame with recirculation Flame without recirculation
Fuel / air mixing Good Poor
Reducing/ oxidizing Oxidizing conditions exist Reducing condition occur in
conditions throughout the flame fuel rich part of the flame and
in the area of flame
impingement
Flame impingement None- recirculating gases Flame impingement occurs
protect refractory and product on refractory where jet
from direct contact expands to hit the wall( 11-
14 %)
Carbon monoxide level CO only produced in High levels of CO produced
significant quantities below at oxygen levels as high as 2
0.5 % –4%

Heat release pattern Rapid mixing gives high Poor fuel/ air mixing gives
flame temperature and good gradual heat release with
heat transfer long flame

Kiln stability Good flame shape with stable Heat release pattern
hat release pattern considerably affected by
Gives stable operation changes in secondary air
temperature, excess air , fuel
quality etc.


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 temperature
A = combustion air required kg / kg coal
Hv = heating value of fuel
S = specific heat of combustion gas ( 0.29)

Heat flux and combustion intensity
Hr = mf C v Where Hr = combustion intensity, kw/ m2
Lf π D mf = fuel flow rate, Kg/ s
Cv = net calorific value , kj/ kg
Lf = flame length , m
D = kiln internal diameter , m


Swirl coefficient( Swirl number)
As per M.A.S/ Burner,
I tan .R e.tan
Sn =
I ax .R e.ax
Where

Sn = swirl coefficient ( swirl number)
I tan = momentum of swirl air in tangential momentum
Re.tan = momentum of swirl air in axial direction
I ax = equivalent radius of swirl air duct
R e.ax
= equivalent radius of the axial air duct

Axial index

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


Tangential index:

This index refers to re-circulations internally to the flame,
Which has influence in the ignition of the particles and
flame spread. The tangential index has close relationship
With the position and intensity of the first temperature peak
In the kiln. Usually , during burner design the dimensions
Of the nozzles at the tip are calculated in order to allow
The 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 burner
designer should calculate the tip dimensions to get lower values
of tangential index in the burner operational range. On the other
hand , if the basis of project indicates that the process would
require wide and short flames , then the designer should
calculate The burner operational range to present higher
tangential indexes.

Turbulence index:
This index refers to the position of both temperature
peaks in the kiln.During the calculation of the burner tip
dimensions 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 of
fuel type , fuel preparation ( moisture and fineness ),
secondary air temperature and kiln dimensions.With relationship
to that minimum value of the turbulence index it should be
pointed out that:
• Bituminous coal finely ground ( 90 < 170 ) would require
lower turbulence indexes than petroleum coke ground to
the same fineness.

One system operating with 100 % petroleum coke ground to
90 < 170 would require higher turbulence index than another
system 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 role
in the study of the thermal NOX generation.
Some additional factors , not directly related to characteristic
dimensionless indexes must be considered during burner
design. The first one refers to the secondary air conditions
( temperature , velocity distribution , dust content, etc). The
second factor is the burner pipe penetration into the kiln
cylinder in view that the length of this penetration has proved
To interfere in both kiln performance and clinker quality.
Finally , the firing hood geometry has some influence in the flame
characteristics as it interferes with secondary air flow pattern


Conclusion

After taking account of all considerations above , it is possible
to conclude that the combustion plays a major role in the
rotary kiln operation , but any improvement in this area should
be faced , first of all , as a cooking problem and merely as a
firing 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 have
Proper judgment about the flame being formed by the burner.


Thank you for your kind
attention


Flame for cement kilns kppradeep kumar

Recomendados

Recomendados

Más contenido relacionado

Destacado

Destacado (19)

Más de pradeepdeepi

Más de pradeepdeepi (16)

Flame for cement kilns kppradeep kumar