Kiln Control Variables

7/29/2019 Kiln Control Variables

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Kiln Control Variables


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Before explaining the control variables one has to understand first the

meaning of a variable and a controller.

So what is the meaning of a variable?A variable is a physical quantity that can be measured and /or

changed; like weight, pressure, temperature, speed, level, flow, etc.

What is a process control?

A process control is the regulation of the physical variables to obtain

a desired end result.

Then the following question should also be answered

What are the goals of process control? Process controls are for Optimizing operation conditions in the kiln

1- to maximize production.

2-to achieve maximum energy efficiency.


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1. BURNING ZONE CONTROL

Burning zone temperature is not only one of the most important

kiln control variables we have in any rotary kiln system, butbut it is also the hardest and difficult to control, understand and

monitor. Now, rotary kilns are equipped with state of the art

technology for measuring the different temperature and pressures

etc. needed to control burning zone temperature.

Even with the help of this technology, the operator should be

trained to be able to make use of these instruments to evaluate the

data provided to be able to control the situation in the burning

zone.


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Apart from this the kiln operator should be trained also to make visual inspection of

the burning zone

The interference of dust coming from the grate cooler with the secondary air, shifting

of the burning zone caused by change in burnability of the raw mix or change flame

characteristics, may affect the reading of the pyrometer without any real significant

change in the burning zone condition and temperature.

T.V cameras in the kiln hood are there to help the operator to see part of the burning

zone, However kiln interior after the flame area, color of the clinker and its

granulation size, intensity and momentum of the flame, the type of coat in the

burning zone etc, are not revealed by the cameras.


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Viewing kiln from the kiln-hood

It is quite dangerous to look at it without protective glass

Appearance of clinker

The visual inspection of the kiln hood can give an indication of

the quality of the clinker by the size and color of the hot clinker

If the clinker is overburned, it is burned in a hotter atmosphere,

the cascading movement of the clinker bed in the burning zone

becomes higher, and clinker grains become bigger. In such

condition the color


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The feed behind the flame

It is difficult to see behind the flame, but in spite of this the operator should spare no

effort in trying to see behind the flame because it is the region of the kiln where

early detection of possible kiln upsets can be seen

The dark feed under the flame

Looking into the burning zone, one will observe a color change of the lower part of the

feed bed under the flame from dark to bright. This point in the burning zone is of great

importance to the operator.

In normal operation this dark feed is stationed nearly at the first quarter of the flame

length starting from the flame end. When the burning zone is cooling down the dark

feed tend to move furthers towards the direction of the kiln outlet. When the burning

zone is warming up the dark feed will start to retreat in the direction of the kiln inlet.


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Whenever the flame is shortened the dark feed will move in the direction of the kiln

outlet.

When the kiln feed is harder to burn than usual and the flame is shortened and

when kiln material loading is higher, then the dark bed moves toward the kilns outlet

because the burning zone is cooling down

When the flame is made longer but still strong enough and there is enough heat to

raise the temperature of the feed so the dark feed can move toward the kiln inlet

area.

When the feed is easier to burn than usual this dark feed will move in the direction

of the inlet area because the feed will need less heat than usual to arrive to the

temperature that give it the bright red whitish color.


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Color of the coating

Color of coating tells a great deal about the condition in the burning zone. The color of

the coating in the intense heat area should be whitish yellow.

When the color changes to red or orange red the burning zone is cooling down.

If it starts to change into white then it is heating up and if the color becomes dazzling

the burning zone is over heated.

The coating can be considered as heat storage in the burning zone. But the coating with

its temperature stored in, will not be able to do much if material lumps rush into the

burning zone.

So we can say it is the obligation of the kiln operator look after the changes in the

following items since any change in any one of them will surely indicate a change in

burning zone temperature.


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1- Clinker color

2- Clinker size

3- Type of cascading movement of the clinker under the 4- flame itself.

4- Feed bed before the directly before the flame area

5- Dark feed position

6- Appearance of secondary air coming from the cooler and the quantity of dust in it

7- Coating condition

8- Flame shape and color


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2- KILN EXIT GAS ANALYSIS CONTROL

Oxygen, Carbon monoxide, Carbon di-oxide and NOx measurements are made round the

clock in any rotary kiln. The instruments used in determining these compounds must

work as long as the kiln is in operation i.e. fuel is fired in the kiln system.

Now the rotary dry kiln works with very high production that normally not less than

4200t/day and any disturbance can represent considerable loss.

Does he work with combusting gas at the inlet? Does he get the correct oxygen content in

the kiln-inlet area?

Oxygen should be in the range of not less than 0.7 and that represents the border of

insufficient oxygen level in the kiln and not more than 3.5% in the kiln inlet and that

represents the border of high excess air in the kiln. for optimum operation the advisable

oxygen level should be between 1and 1.5% Same values can be applied to precalciner.

Now if a kiln works steady at maximum production yet with more than 3% oxygen or the

kiln works with deficiency of oxygen of 0.7% then in such condition the kiln works in a

stable condition, therefore there is no point in introducing any major changes because

this may upset the kiln.


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The moves in such condition should be delicate and in a slow but steady way, so the

balance of the kiln would not be disturbed.The strategy in such condition is to think first what to do, then starts the process of fine-

tuning in a way not disturbing the stability of the kiln in any way. The time between each

small change and another should be not less than one hour.

The whole process of fine-tuning should take from 10 to -16 hrs.

With the other control variables in consideration the, following are the basic steps to

follow as regards exit gas control:

Secure sufficient oxygen and no carbon monoxide in the kiln atmosphere.

Stabilize the kiln oxygen and make it level off and stay within a definite narrow range.

Optimize by fine-tuning the oxygen in the kiln in small steps to bring the oxygen level

to the range of 1-1.5% for optimum kiln efficiency.


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There is a relation between the burning zone temperature and the

concentration of NOx in the kiln atmosphere.

When the temperature of the flame of the main burner changes in the burning

zone.

Also when the Oxygen in the main burner increases in the burning zone, the

NOx concentration increases.

In any rotary cement kiln it is nice to work in an efficient way by making the kiln work

with as short burning zone as possible i.e. short flame and high flame temperature

that in turn means high NOx content. Therefore as a general rule if the kiln work with

say 1500ppm of NOx then start to be 2000ppm then the kiln tends to:

have more draft i.e. excess Oxygen.

may starts to have higher burning zone temperature.


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If a kiln works normally with 1600ppm of NOx concentration in the kiln out

let, then the concentration goes down to 600ppm so that indicates the

following:

- there is loss of oxygen in the burning-zone.

- the burning-zone temperature is lowering and starts to cool down.

-the operator should not wait until reduction condition prevails in the burning zone

but when the concentration of the NOx starts to decrease he should stop that

trend of cooling down of the burning zone.

In any rotary kiln the register of oxygen recording of this variable is in the range of 0-

5%.It is advisable not work over long period of time at the upper limit of 5%. The

operator does not know if the oxygen is 5.4 or 6 or 8-or15%In short the operator does

not know what are the combustion conditions in his kiln.


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It is the duty of the kiln operator to stabilize the oxygen percent after lit- up

of the kiln at a level below 5% Oxygen at the kiln inlet gas analyzer.

O2 CO CO2 NOx

Fuel Increases down up down down

Decreases up down up up

I.D. fan Increases up down up up

Decreases down up down down

Flame Decreases - - - up

Temp. Increases - - - down

Feed rate Increases down - up -

Calcine Decreases up - down -


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3. Fuel rate Control

Before the kiln is fired for any reason i.e. long shutdown or short shutdown the

following requirements should strictly be met:

The gas analyzer is calibrated and working properly.

Sufficient air is present to achieve complete combustion of fuel

strong Source of heat enough to ignite the fuel is available.

The primary air fan is running and the draft is properly set.

The I.D. fan is running condition and the draft is adjusted in a way that prevent

any sudden, delayed, explosive ignition of the fuel.

After the lit-up the operator should:

1- Inspect the gas analyzer for CO

2- Never attempt to control the burning zone temperature by merely increasing or

decreasing the fuel in the main burner.

3- The kiln operator should anticipate the reactions caused by the change in I.D fan

speed or the fuel rate or both.


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He should remember that

An increase in fuel rate results in higher kiln inlet temperature, low oxygen

percent in exit gas and higher burning zone temperature.

An increase in I.D. fan speed results in higher kiln inlet temperature, and

lower kiln burning zone temperature and higher oxygen in the kiln inlet.


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Rules in fuel rate control

Always check the oxygen content and CO analyzer in the trends or recorders on

the monitor or the panel after any adjustment of fuel rate. It should be

mentioned here that the burning zone reacts slowly to any fuel rate adjustment.

There is a time delay until a noticeable change in temperature takes place.

Since these time delays can be as much as at least 10 to 15 minutes, especially

after an upset condition

This is what is known as a cyclic operation condition.

Due to this time delay the operator should be trained to anticipate these fluctuations

of extreme temperatures and make their fuel adjustment before the actual change in

temperature takes place.


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The early warning signals are:

Oxygen% without fuel increases B.Z. is heating up

Or I.D fan adjustment decreases B.Z. is cooling down

Kiln drive torque increased B.Z. is heating up

Decreases B.Z. is cooling down

NOx

increased B.Z is heating up

Decreased B.Z is cooling down

CO2 increased B.Z is cooling down

Decreased B.Z is heating up

Kiln inlet temp*. Higher than normal heating upLower than normal cooling down


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The kiln operator should control the fuel rate on the basis of the following

variables:

First: the burning zone temperatureSecond: the kiln inlet temperature

Third: the percent of oxygen in the kiln inlet.

Simple control logic of adjusting fuel rate strictly and solely on the basis of

condition on the burning zone alone seldom yields satisfactory stable and

efficient kiln operation.


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What the kiln operator should do during severe kiln upsets in

relation to fuel rate in the kiln?

During kiln severe upsets, the kiln burning zone will be blackened and the

unburned fine clinker will over flow the cooler and the visibility in the kiln hood

will be nil. The kiln operator natural and spontaneous reaction to the situation will

be increasing the fuel rate to the kiln regardless the real situation would be. This

is a natural tendency of all operators because he understands that a lot of heat isneeded to bring back the burning zone to its normal operation condition.

Since the kiln speed in this condition is greatly reduced and since the visibility is

greatly reduced, the operator will think that the burning zone is of very low in

temperature and he will leave the fuel rate very high for a long time in the kiln

without alteration, waiting for the burning zone to clear. In most cases the kiln will

be very hot in the burning zone and the burning zone will be in an overheated

condition. The following are to be considered during such condition


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4.kiln speed control

Any kiln cannot run over extended period of time in stable condition at which

nothing in operation parameters is changed.

There should be an appropriate action the operator must take in case of kiln upset ;

this upset may not be due to operator error but due to

Change in the physical and chemical characteristics of the kiln feed.

Change in the mineralogical characteristics of the raw materials.

Change in the homogeneity of the kiln feed and change in the standard deviation of

say L.S.F. or silica modulus.

Change in the fuel chemical composition or in its temperature in case of fuel oil.

Change in calcination degree coming from the precalciner.


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This action is called slow down procedure

Slowing kilns speed depends on the experience of the operator and no literature

can teach him how to do it, but actual training and accumulation of experience isthe key.

But one rule has to be followed at all times during kiln operation:

Never allow raw and unburned kiln feed to go to the grate cooler

Even if this means to stop the kiln and reheat it on quarter turnOr auxiliary drive.

When the operator slow down the kiln there are number of changes that can take

place. The operator should be alerted to be able to notice the indication of these

changes and take appropriate actions:-


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Kiln inlet temperature will start to increase, but this is undesirable, since

this temperature should be held within 20C.

Reducing the I.D. fan speed will help reducing this increase intemperature of the kiln inlet.

Reducing kiln draft resulting from I.D. fan speed decrease, will make the

oxygen content less in the kiln exit gas and there will not be enough

oxygen for fuel to complete combustion

What is the effect of slower kiln speed on the cooler and secondary and

tertiary air temperature?

when the operator decreases kiln speed, less material will enter the cooler. The

direct result will be lower secondary and tertiary air temperatures and the under

grates pressure will decrease


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When should the operator decide to increase the kiln speed again

in the time of upset periods in the kiln?

Never increase kiln speed before there is definite sign of burning zonewarming up, due to less feed coming to the kiln burning zone. The

behavior of the operator during this period should be as follows:

Never hold the kiln on very low speed until the kiln reaches normal

clinkering temperature. Start increasing the speed as soon as there is a sign of warming up of

the burning zone.

The more the kiln speed increases, the smaller should be the increments

and the longer the time intervals between each speed increase.

The operator should remember that:

The way the kiln speed is increased is the most important

factor in getting the kiln into stable conditions again


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1- EXAMPLE

An operator started to increase the kiln speed in large steps at a time of

15rph i.e 0.25rpm, until the kiln reached 150rph i.e.2.5rpm, then he extended

the time between each speed increment and also he carried out the

increase in smaller steps. The rate was 9rph i.e.0.15rpm from150 to186rph

i.e.3.1rpm, 6rph from 186 to 198rph i.e. 3.3rpm and 3i.e.0.055rph from 198 to

210rph.i.e 3.5rpm.

Time interval is irrelevant here because it depends on many factors we

cant put into consideration during explaining this example.


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EXAMPLE 2:

In the last example if the kiln operator started to increases the speed say at

0.5 rpm for a short period, then he went at 1rpm then quickly jumped to 2

rpm then to 3rpm and finally to 3.5 rpm then surly he will be obliged to

backup on the kiln speed since the burning zone was not ready for these

speeds. Since every time he increases the speed the kiln inlet temperature

will drop, he therefore has to start reheating the kiln-burning zone all over

again.


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EXAMPLE 3:

Here in this example the condition is opposite to what the second example

showed:

The operator waited too long to increase kiln speed. The result he suddenly

found that the burning zone gained heat very rapidly, forcing him to

increase kiln speed in large increments and in short time intervals. In such

case full kiln speed can be maintained but very soon the kiln would be

overloaded and the operator had to slow the kiln speed and eventually

decreased feed

The important rule in kiln speed increase is that:

Whenever the operator increases kiln speed, kiln inlet temperature will decreases,

requiring increasing fuel rate to the kiln. Any increase in the fuel rate will require an

adjustment in kiln fuel rate and an increase I.D. fan speed. Both should be increase in a

manner equal to that practiced in increasing kiln speed.


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5.Clinker cooler control

Cooler conditions influence the burning process in the kiln and

consequently the quality of the clinker. The cooler upset condition or the

bad operation practices of the grate cooler can lead stable condition to

pass through extended periods of upset condition with the eventual

results of losing production and higher fuel consumption and perhaps

damage of the equipment.


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5.1Variables in cooler control

Clinker temperature should be as low as possible to save valuable heat and

protect clinker-conveying system.

Secondary and tertiary air temperatures should be as stable and as high as

possible especially for secondary air temperature, because this is important for

kiln operation stability and good fuel efficiency.

Cooler exit gas or excess gas temperature should be as low as possible, and

volume as small as possible to decrease the amount of heat wasted to the

atmosphere.

Kiln hood pressure should be negative.

Depth of the clinker-bed is kept at its optimum height to the keep the freepassage of air through clinker bed.

To keep the cooler controlling parameters in such a way that protect the cooler

components from overheating.


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The basic control variables in the cooler

The cooler grate speed determines the clinker bed depth and its residence time

in the cooler.

The quantity of air delivered by cooler fans to the different grate compartments

and the way of distributing it on these compartments.


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5.2Under-grate pressure and air flow rate

Under-grate pressure is governed by:

Depth of the clinker bed over the grate

Average particle size of the clinker in the cooler

Temperature of the clinker in the cooler

Amount of air introduced into the cooler

The thicker the clinker bed over the grate plates in any cooler stage, the

higher the resistance and require more pressure and force from the fan to

push the air through this bed.


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Control of the clinker bed thickness:

The clinker bed thickness is controlled by the speed of cooler grates. A deeper

bed can be obtained by slowing of the grate speed. Therefore, it is logical to

obtain and maintain a constant clinker bed depth by keeping the under grate

pressure constant regulating coolers grate speed.


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Air low rate:

Sometimes the kiln suddenly starts to discharge high amount of clinker due to

the falling down of a coat in the kiln-burning zone. The reaction of the cooler

control is to speed up the grate to maintain the under-grate pressure constant.

This will result in reducing the residence time of the clinker in the cooler resulting

hotter clinker leaving the cooler.


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6. Secondary and tertiary air temperature control

The source of both of the secondary and tertiary air is the grate cooler first

stage.

Both of them are drawn form either the kiln hood or in some cases the

tertiary air is drawn form the middle of the roof of the first stage of the

grate cooler. This ensure that the temperature of the secondary air to be

higher than the tertiary air.

Secondary air temperature has a direct influence on the shape of the flame

and the point of ignition of the fuel; therefore irregular secondary air

temperature can cause irregular flame characteristics that can cause

shifting in the burning zone.


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Control of the secondary air temperature during normal kiln

operation:

Every kiln operator understands that he must maintain the secondary air

temperature as high as possible because it contains the maximum amount of

heat that can be recovered from the clinker.

The second advantage of higher secondary air temperature is that it gives the

operator the chance and therefore the advantage of burning the clinker nearer

to the nose ring of the kiln.

Tertiary air temperature, should not be more than 900C since the temperature

in the precalciner should not exceed this limit.


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Factors affecting secondary and tertiary air temperature:

The speed of the clinker bed on the grate cooler in relation to the volume

and temperature of air introduced into the cooler to cool that bed of

clinker.

The grain size distribution of the clinker and the temperature of that

clinker discharged from the kiln to the cooler.

The rule of thump in secondary air temperature is that

1.The higher the speed of the grate the less bed clinker thickness will

be and the lower the secondary and tertiary air temperatures.

2.The lower the speed of the grate the higher bed clinker thickness

will be and the higher the secondary and tertiary air temperature.


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This can simply be explained if all other factors are remained unchanged

as follows:

When the grate cooler speed is reduced the clinker speed is lowered, the

direct result will be higher clinker bed thickness and the air will find more

chance to absorb more heat from the clinker in the cooler and visa versa.

When the cooler speed is increased the clinker bed is lowered and the

chance of cooling air to absorb heat will decrease resulting in lower

secondary and tertiary air temperature.


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Clinker grain size and its effect on secondary and tertiary air

temperature:

For example consider a kiln in an upset condition and the clinker

produced was either small nodules, or in worst case condition is dust. The

operator in this condition would reduce kiln speed; so amount of clinker

discharged to the kiln is reduced. This would lead to lower secondary air

temperature.


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The operator should never attempt to hold thesecondary air temperature

At its normal operating level in an upset

condition in the kilnAnd the clinker is dusty or having lower liter

weight


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Temperature of clinker discharged from and its effect on secondary

and tertiary air temperatures

By changing the character of the main burner flame, the burning zone can be

shifted very near to kiln nose ring outlet, thus the clinker can be discharged to the

cooler very hot, and so the secondary air temperature will be higher.

Controls of secondary air and tertiary air tempera

To raise secondary air temp to reduce secondary air temp

Decrease cooler grate speed increase cooler grate speed

Increase clinker bed depth in cooler decrease clinker bed depth in cooler

Increase size of clinker particles decrease size of clinker particles

Increase amount of clinker decrease amount of clinker

Burn closer to the nose ring of the kiln burn further back in the kiln


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7. kiln hood pressure control

The kiln I.D. fan draws certain amount of air through the kiln and another amount

of air is forced into the cooler by the cooler fans. In any modern kiln the excess

air volume resulting from the fact that the amount of air from the cooler and used

as secondary and tertiary air is smaller than the amount of the air forced into the

cooler for cooling of clinker. In any modern kiln system will have excess air in the

cooler.

Factor affecting kiln hood drafts

The I.D. fan speed

The volume of air intruded by the cooler fans


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Never to operate with positive pressure in the kiln hood

This will result in very troublesome operation condition. Clinker fine particles

will be blown into the kiln hood area and causing the following problems:The kiln outlet seal will be damaged

Flying red-hot dust will cover the kiln platform

Dust emission will make viewing the burning zone very poor.

Instrumentation in the area of kiln hood will be greatly damaged.

8. Kiln exit - gas temperature control

The kiln inlet area conditions are:

Kiln inlet temperature.

The percent of oxygen in the gases in the kiln inlet.

The pressure and draft condition in the kiln inlet area


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8.1Kiln inlet temperature

The kiln inlet temperature means the temperature of the exit gas from the kiln

exit that leaves the kiln tube to go to :

By pass system

Kiln riser duct

The precalciner if the system has inline-precalciner

The lower most cyclones if the system has off line-precalciner

What are the ideal conditions in the kiln-inlet?


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Factors affecting kiln inlet temperature:

I.D. fan speed increase or decrease

Fuel rate increase or decrease Kiln feed rate

Kiln speed

1- If all other factors remain stable without change and the operator

increases the I.D. fan speed then the kiln inlet temperature increases.

2- If all other factors remain stable without change and the operator

increases the fuel rate to the kiln main burner then the kiln inlet

temperature increases.

3- If feed rate decreases then the kiln inlet temperature increases.

If the operator decreases I.D. fan speed, decreases fuel rate or increase feed

rate, the kiln inlet temperature increases.


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Kiln speed and its relation to kiln inlet temperature:

Since sometimes the kiln material charge is changed say due to ring in the kiln or

mechanical default or electrical failure in the feed dozing equipment, this will

require change in the kiln speed.

Whenever kiln speed is increased, the kiln inlet temperature decreases. In kiln

operation, any change in kiln speed, kiln fuel rate, and /or I.D. fan speed cause

changes at both the kiln inlet and the kiln out let i.e. burning zone.


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8.2Kiln inlet draft control

The draft in the kiln inlet area is created by the action of the kiln I.D. fan. This

local static pressure is negative and the suction is measured and indicated bythe instrument. This explains why the negative pressure is changed by either

decreasing or increasing under certain kiln condition.

indication of the negative pressure decreases.

The physical meaning of changing of draft in the kiln inlet area is, the speed of

the air in side the kiln either becomes higher or lower depend on the change in

the draft.

The main cause of changing the draft in kiln inlet is changing the speed of the

I.D. fan or the speed of the by-pass fan.


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9. Feed rate control

One of the most important factor in the control of the feed into the kiln is the

accuracy of the kiln raw meal feed weigh-feeder i.e the dosing system is reliableand working properly.

The factors which are determine the kiln feed rate are

Kiln speed

Kiln cross-sectional feed loading or volumetric load

Kiln cross sectional thermal loading

What is the relation between kiln speed and the kiln feed rate?

1- If the feed rate changed but kiln speed is kept constant, the burning zone

conditions is remains unchanged from 25 to30 minutes until the different feed rate

arrives to this zone. After a short time, the kiln inlet temperature will start to

change making it necessary to readjust the I.D. fan speed and fuel rate


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2- If the feed rate held constant and kiln speed is changed, there will be an

immediate reaction in the burning zone and the kiln inlet temperature and this

will demand an adjustment in I.D. fan speed and fuel rate in the same time

with the changing in the kiln speed


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But when the operator after achieving stable condition in the kiln try again to add

more feed and what are the criteria to be questioned before he takes such act?

The operator or kiln manager can try to add extra tons to the kiln production

when :

He has additional grinding capacity in the raw grinding department to cover the

additional kiln feed and/or enough raw meal in kiln feed storage silos.

The kiln operates in stable condition over reasonably extended period of time

with stable burning zone temperature and kiln inlet temperature prior to the

intend feed increase.

The oxygen reading in the kiln inlet is above 1.5 for at least one shift.

The black feed in the burning zone, during the last shift is consistently located

behind or directly under the end of the flame inside the kiln.

In the addition to the excess oxygen the kiln has, it is preferably to have some

reserve rpms in the I.D. fan speed to be utilized if needed.


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It is necessary that the kiln department manager evaluate what effect the

proposed change will have on the overall prospects of kiln condition, fuel

efficiency, clinker size, burnability and adjustments that may have to be madeto fuel rate, I.D. fan speed, kiln speed, and other variables.

Documents

Kiln Control Variables