EVERYTHING YOU NEED TO KNOW ABOUT KILN CONTROL VARIABLES

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EVERYTHING YOU NEED TO KNOW ABOUT KILN CONTROL VARIABLES

 

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BURNING  ZONE

THE burning-zone temperature is  not only one of the  most important kiln control  variables  but  also  the  most  difficult one to  monitor.  Despite the fact that burning-zone conditions in modem kilns are measured and monitored by means of sophisticated instruments, kiln operators should always be thoroughly trained to visually observe and evaluate the burning zone. this capability for visual inspection is an absolute requirement because there are times when these instruments are out of service. Visual verification is also needed when instrument readings are questionable due to dust interference in the burning-zone environment.

This control function is not merely a check to see Whether good clinker is being burned, but is also an evaluation of the clinker, the coating,  the flame, and the air stream. Color is important in evaluating the clinker, the coating, and the flame. General behavior of the clinker, air stream, and flame can be studied, as well as shape of flame and coating, and size of clinker being discharged.

the pyrometer records the temperature of a comparatively small area. A change in temperature registered by this instrument docs not always mean a true overall temperature change in the burning zone, and therefore docs not always indicate a need for adjustment in the kiln controls. As a matter of fact a change in pyrometer reading could be caused by a shifting of the burning zone due to a change in the flame characteristics, or by dust interference entering the burning zone with the combustion air from the cooler.

With reference to the television monitor, it must be kept in mind that the camera does not “see” the burning zone as a whole. what can be seen on the TV screen is only a fraction of the zone. Here again, dust interference could make visibility  in the burning zone poor or even impossible at times. Finally, either one or both instruments could fail. All these factors emphasize the need for training a new operator thoroughly in the art of “eye-balling” the burning zone.

Viewing the Kiln Interior. At this point it is important to learn how to look properly into the burning zone. Although this seems elementary, it should not be forgotten that the burning zone  is  an  extremely  luminous light source. Although filtering glasses are used,  the  light  source  is  so strong that focusing the eyes into it for too long a time could cause partial blindness. One should look no longer than one minute at a time into the fire. If longer viewing is required, look aside for a  few  seconds  occa­sionally to rest the eyes. Looking steadily too long at the flame results in the eye losing its ability to see details.

The question of what type of colored filter glass to use must be left to each individual operator, as one person can see better with one particular glass than with another, but the same glass may not suit someone else. Burning with a natural gas flame usually requires a darker colored glass than an oil or coal fire would, because of the greater luminosity of a gas flame. As a rule, one should always use a glass that enables him to see under and behind the flame. Once a certain glass has been chosen, the operator should stay  with this glass at all times in order that proper judgment of the conditions in the burning zone can be made. Sometimes an operator has the habit of overburning the clinker, and another may have the reputation of consistently underburning the clinker. A very effective measure to counteract these habits is to equip the “hot’ operator with a brighter glass and the “cold” operator with a darker glass to compensate for their misjudgment of the burning-zone temperature.

Many control rooms, in modern cement plant  design,  are  located  far away from the ftring floor and thus demand more reliable and accurate instrumentation to monitor the burning-zone temperature. Some of  these plants are highly successful in this endeavor but there are also plants where this is a source of concern. Generally, it is easier to satisfy  the require­ ment of recording accurate burning-zone temperature trends  on kilns  with little dust interference. On the  other hand, dry-process kilns, with their commonly poor visibility, are much more difficult to monitor for burning­ zone temperature. Although desirable, absolute accuracy as to specifying prevailing temperatures is not so important  as  the the temperature  trend that takes place in the kiln. In other words, as  long  as  the  instrument registers a corresponding temperature drop when  the kiln is actually cool­ing down and an increasing  temperature when the kiln heats up, the requirement of temperature monitoring of the burning zone is fulfilled.

How frequently  should one look into the burning zone?  There is nos answer to this question.   Naturally,  on remote-controlled  kilns there is no need for the operator to pace the firing floor as frequently as on kilns th are  almost  exclusively   manually   controlled. It is  on  these  manual’ controlled kilns where experienced operators sometimes become overconfident and think that it would be perfectly safe to leave the kiln alone for periods in excess of 40 min. This action, however, is against good buning practice.  The secret of every good operator is his ability to recognize change in kiln condition  at the time a change takes place  and not late For this reason  a good operator will never leave a kiln unchecked  for tr long a time.  When things are going smoothly, the kiln should be checked every half hour, with more frequent checks if adjustment are being made.

Appearance of Clinker.  The quickest, although not the most accurate means to check the clinker for quality is by observation of the color at the size of  the clinker.   Some operators  have become  so proficient  theory can tell fairly well how good the clinker has been burned by mere looking  at a handful  of clinker.   It should  be pointed  out, however,  the clinker  size  alone  does  not  give  a  true  indication  of  clinker  quality clinker size is influenced by such factors as feed composition  and kiln speed.  Generally speaking, well-burned clinker is dark, almost black color, and the “hotter” the clinker is burned, the larger the clinker nodule become.

If the clinker is overburned (that is, burned hotter), the free-lime conditions drops, the liter weight gets heavier, the clinker gets larger in size, me dense (less porous), and darker in color, compared with a clinker that is n overburned.  Conversely, underburning the clinker (burning colder) caus the free-lime content to increase, and the liter weight to lighten.  Also  clinker  gets  smaller  in  size  (dustier),  more  porous,  and  the  color lighter-more nearly  brown.

Until now the appearance of the clinker after it has been burned a cooled has been discussed. Now consider the clinker in the burning zone For this a reliance on eyesight is necessary.

The kiln feed, when it approaches clinkering temperatures, undergrdrastic changes in its physical and chemical characteristics. As soon liquid formation of the constituent begins, the feed becomes slugg because the chemical reactions tend to make the feed soft and visco Formation of balls of feed (clinkers) now commences, and the mater starts to ride up higher on the rising wall of the kiln. Instead of sliding down the wall, the feed bed starts to cascade over itself.  By noting the cascading action in the vicinity of the flame the observer will notice an increase in sluggishness of the feed at higher clinkering temperatures, the feed will climb higher on the kiln wall,  with more turbulent cascading. Assuming that other variables remain constant, larger clinker balls are formed at higher temperatures.

Any deviation of color from the orange-yellow should be investigated to determine the cause  and what adjustments need to be made to the kiln operation.

The Feed Behind the Flame. The manner in which the operator controls the position of the raw feed has a significant influence on operation of the kiln, and is one of the important key indicators of operating stability. Changes in this raw-feed posiiton are an early warning signal that burning­ zone conditions are about to change.

Special attention  has to be given  to the physical characteristics  of  the feed behind the flame. Although in most cases it will be difficult for the operator to see behind the flame because of the flame shape and dust interference, nevertheless, no effort should be spared in trying to see as far back as possible, because it is in this region of the kiln where an early detection of possible kiln upsets can be made.

The Dark  Feed.   When one looks into the burning  zone, one will ob­

serve a sharp color change of the lower part of the feed bed under the flame from dark to bright as  shown  in  Fig.  16.1.  This  point  in  the  burning zone is of great importance to the·operator. The importance of this lies in the fact that the position of this dark feed gives the kiln operator one of the earliest indications of when the burning zone tends to warm  up  or cool down. Under normal and stable conditions the position of the dark feed remains stationary approximately one-quarter of the distance into the flame (as shown in the figure). If it moves fartl1er under the flame (towards the front of the kiln), the burning zone  is cooling down; if it shifts in the direction of tl1e kiln rear, the burning zone is warming up.

Any change in flame  characteristics will have a direct effect  on the position of the dark feed. For example, whenever the flame is shortened the dark feed will move closer to the discharge end of the kiln. ·A change in position of the dark feed therefore does not always mean that  the burning zone is either heating up or cooling  down.  The position  of the dark feed can move because of a change in the shape of the flame, a change in burnability of the feed, or a change in the feed loading of the kiln.· The dark feed will move closer under the flame) when the flame is shortened, the feed is harder to burn, the feed loading of the burning zone increases, or the burning zone cools down. It will move away from the flame (toward the feed end) when the flame is made longer (and enough heat is available to drive the dark feed back), the feed is easier to burn, the feed loading in the burning zone decreases, or the burning zone heats up.

Any change in the position of the dark feed must be viewed in the light of all of these influences. The operator must be able to see the dark feed whenever he looks into the kiln, and he must regulate operation of the kiln so as to achieve this end.   A slow shifting of the dark feed in either direction can usually be counteracted by a small change in the fuel input rate in order to keep the feed in its proper place.

The dark feed should never be allowed to move further under the flame than one-half of toe flame length. When it becomes necessary to counter­ act the condition in which the feed has “slipped” too far under the flame, the operator can, if the means to do so are available, change the flame length to restore the dark feed position to the proper relation with the flame again.  Fig. 16.1 shows how this is accomplished.  Figure A depicts a condition in which the dark feed has advanced too far under the flame. By increasing the primary air pressure and temperature, and if possible the secondary air temperature, the flame will be shortened as shown in Figure Note that the position of the dark feed has not changed, but the relation between feed and flame is in its proper perspective again because the flame is shorter. This shortening of the flame results in more heat being released in the critical area where it is most needed.

Some operators take advantage of this procedure quite freely whenever they encounter the described condition. However, one has to consider ­ other aspect caused by such a change in the flame structure. The shorter fire results in a bushier flame that is more in contact with the coating, the coating being thus exposed to a greater heat. For this reason a shortening of the flame should never be carried out before the operator has assured himself that the coating will  be able  to withstand the extra  heat. For example, if a hot spot on the kiln shell is  already in existence in the critical area, it would be a serious mistake to shorten the flame and release more heat over the already weakened area.

Combustion  Air  From  the  Cooler. Considering  only  the  general appearance of the air coming from the cooler under normal operating conditions, not much attention need be given to this factor because the appearance remains nearly constant. Under upset conditions, however, this factor becomes very significant.

When insufficiently burned clinker is allowed to enter the cooler, the air stream carries a large amount of fine dust particles· back into  the kiln which can obstruct visibility in the burning zone. Furthermore,  the  sec­ondary air temperature is  most likely low in such a case, causing a change in the flame structure, with the result that the  ignition  point  of  the  fuel moves further into the  kiln. Combustion conditions are poor (cold flame) because of the dust-entrained atmosphere in the burning zone. All these conditons may make the burning zone appear to the eye to be “cold,” al­though  this  is  not  necessarily  the  case  at  all  times.   The area of  highest intensity  could  already  have  approached  normal  clinkering   temperature again although the  front  is  still  black  and  cold.  Whenever  dust obstructs the view in the burning zone, the operator must make a special effort to see under and behind the flame. Experience has established the rule that, whenever dusty conditions prevail in the burning zone, the clinkering con­ditions under and behind the flame, rather than the color of the front of the burning zone, govern subsequent corrective measures after an upset.

Color of the Coating. The color of the coating tells a great deal about temperature conditions in the burning zone. Under normal operating con­ditions, the color of the coating in the hottest area ranges between yellow and white. When the color changes to orange or red, the zone is cooling down; if it changes to white, the burning zone is heating up.

A large portion of heat is transferred to the feed by radiation and conduction from the kiln wall (see Chapter 2). Thus it becomes understandable that the temperature of the coating is very important for the burning pro­cess. In addition, the coating acts as a heat storage in the burning zone. Having a hot coating will in many instances  enable the operator to fight out a heavy onrush of feed. On the other hand if the coating loses tem­perature rapidly at the time a heavy load of feed enters the burning zone,there is usually no alternative but to slow down the kiln to avoid bad clinker.

Besides the color of the coating, the general appearance of the coating is also important The operator should try to detect weak spots, loss of coat­ing, or formation of rings in the burning  zone.  Early  detection  is  of utmost importance whenever a change in the coating structure takes place. It is the operator’s duty to maintain or rebuild the coating to protect the refractory and the kiln shell from damage by overheating.

A condition that can cause a great deal of damage to the kiln if it is not corrected immediately is when the  burning  zone  becomes  so overheated  that the clinker starts to ball up  or even  worse,  begins  to  liquefy.  This  con­dition is extremely hard on the coating as the coating, becoming soft (liquefying),  starts  to  come  oft.  Whenever  this  happens,  the operator  has to  disregard  the  production  of  good  clinker,  and  concentrate  on  protection of the refractory and the kiln shell. Failure to make  immediate corrections can result in red spots appearing on the kiln exterior, a sure indication that something  is  wrong.

In summary, it is important to visually inspect the burning zone for changes in the following items (any of which could signal the possibility of change in burning-zone temperature):

  1. Clinker color
  2. Clinker size
  3. Cascading action of the clinker bed in vicinity of the flame
  4. Feed-bed appearance behind the flame
  5. Dark feed position
  6. Appearance of secondary air coming from the cooler
  7. Coating conditions
  8. Flame shape and color

Knowing what to look for in the burning zone and being able to recognize changes in the items listed above doesn’t necessarily indicate that one is now ready to control the burning-zone temperature. Simply stated and as mentioned earlier, burning a kiln is not merely a function of adding fuel when the burning zone gets cool or conversely, reducing fuel when the burning zone gets hot Burning-zone temperature control must be con­sidered with other kiln control variables before corrective action can be taken.

Since it is difficult to measure the actual burning-zone temperature with pyrometers, several (old and new) methods have been tried to relate other kiln variables with possible changes  in  this  burning-zone  temperature. The most recent and noteworthy method in this respect is the monitoring of the NOx content in the kiln exit gases and relating this to the burning­ zone temperature changes (See Fig. I 6.2).

Kiln burning is a matter of detecting any pertinent changes as early as  possible and making countering adjustments in small steps. It can best be described as a control by anticipation, i.e., trying to establish what the kiln conditions (burning-zone temperature) will be a few minutes later. Waiting to make an adjustment until the full effect of a large deviation is registered on the instrument can lead to unstable operation and is not consistent with good burning practice. This applies not only to burning­ zone temperature but to all other main control functions.

How can an operator predict what the burning-zone temperature will be in, e.g., 10, 20, or 30 min hence?   The proven  indicators of approaching changes in burning-zone temperature are:

 

a) The kiln drive torque or amperage.

b)The oxygen content in the exit gases (provided the fuel rate and I.D. fan speed remained unchanged).

C) The back-end temperature (chain outlet, kiln inlet calcining zone) at that  time  that  equals  the  travel  time  of  the  feed  from  the respective thermocouple to the inlet of the burning zone.

d)The aforementioned NOx content in the exit gases.

e)The C02 content in the exit gases.

f)The kiln-feed end draft.

These  constitute  the  primary  early-warning  instrument  readings  that alert an operator to upcoming changes in burning-zone temperature.

 

KILN EXIT-GAS  ANALYSIS

Oxygen, carbon monoxide  (CO), .and carbon dioxide (C02) have been extensively discussed in Chapter 5.  From these discussions it has become apparent  that  the exit-gas  analyzer  combined  with  its  02  and  CO recorder in the control room is one instrument no kiln can do without.  As  a measure of safety precautions this is also one of the instruments that irrevocably MUST be in operation at all times while the kiln is operating, i.e., when fuel is fired into the kiln.

This gas analyzer, because it samples “dirty” kiln gases and takes the sample at a location where high temperatures prevail, has a tendency to malfunction frequently unless almost daily preventive maintenance is carried out on this unit. The location, where the sample probe is installed, is also a key point to consider as false air inleakage could distort the true contents of 02 and C02 in the exit gases. Kiln operators must be on the look-out for such malfunctions and notify the foreman or instrument shop whenever a malfunction occurs or is suspected.  Operating a kiln without the gas analyzer functioning properly is a dangerous thing to do and can lead to catastrophic accidents. This is one instrument that doesn’t allow room for compromises; it simply has to work and must be repaired as quickly as possible when it doesn’t. These are strong words but considered worth mentioning for the simple reason that many accidents have happened in the past in some cement plants because the gas analyzer wasn’t working properly.

For example, a few years ago a kiln  operator noticed  an  irregularity  in the gas-analyzer recording wherein more  than  2%  carbon  monoxide  (CO) was indicated but, at t11e same time the oxygen content showed peak levels of 5%. In this example, the operator made  the  wrong  assumption, specifically he believed  tl1e oxygen  (02) to be correct and the CO recording to be in error.  Result: A fire in the kiln back end.  Lesson: Anytime that the presence  of carbon monoxide (CO) is indicated, reduce  the fuel rate immediately and ask questions later. In such situations it helps to re­ member the triangular relationship for combustion discussed in Chapter 5. In the above example fuel was present (i.e., CO which is unburned carbon), air for combustion became available from possible inleakage of ambient air into the system at the kiln back end, and the only missing link for a flre or explosion at the feed end was heat for ignition. The third component for this accident to occur was most likely provided by the high temperature of the exit gas or the electrostatic precipitator which by itself is an igniter in the true sense. To safeguard against such accidents, most electrostatic precipitators are interlocked with the fuel rate. In other words, the precipitator is automatically deenergized whenever CO is present for more than 1 min in the exit gas.

Earlier, it was mentioned that both too much (more than 3.5%) and too little (less  than  0.7%)  oxygen  in  the  kiln  exit  gases  represents  inefficient operation  and  that  1.0-1.5% 02  is  a  level  that  results  in  optimum operat­ing   conditions.      This  applies   to   all  kilns  with   the  exception   of   a precalciner  kiln  that  has  no  tertiary  air supply  to  the  flash  calciner.   In tl1ese types of precalciner kilns, all the combustion air requirements for the flash calciner are supplied  from the kiln, hence higher oxygen  levels are required at the kiln exit to secure complete combustion of the fuel given to the  flash   furnace.          However,   the  same  principle  would  apply  at  the preheater cyclone outlet, i.e., for the gases  after t11e flash furnace.   Here too,  tl1e  oxygen  content  should  not  be  less  than  0.7%,  not  more  than 3.5%,  and ideally  between  1.0-1.5% during  normal  kiln  operations.    In plants where low-grade fuels with large variations in heat value are used for firing in the flash furnace, it is advisable to hold the percent oxygen  at a higher  level than  the  indicated  range  to secure sufficient  air availability when  surges  of  higher  heat-value  fuels  occur.    Since precalciner  kilns operate  with  two  combustion-process  locations,  it  is  advisable  to  equip tl1ese kilns with two gas analyzers, one at the kiln exit and the other at the flash calciner outlet. In this manner, both combustion processes can be independently controlled from each other.   However, on precalciners  that are equipped with only one analyzer at the preheater tower exit, an operator might  have  considerable  difficulty  in  relating  changes  in  02  or  CO  to either one of the two burning locations.

The question arises: “What is to be done when the kiln operates steadily but continuously with oxygen contents higher than say 3% or lower than 0.7%?” In such instances, because of prevailing stable conditions, no drastic changes should be made as such moves could upset the delicate balance of the kiln. Rather, it is advisable to fine-tune the controls in small steps, allowing ample time of about 1 h between each to make sure this balance and stability is not disturbed. Fine tuning in this manner may take up to 8 or 16 hand is referred to as optimizing the kiln operation. As with any other kiln control variable an operator follows the following basic steps in matters of exit-gas control:

  1. Secure tl1e safety of tl1e equipment and personnel (sufficient 02 available and no CO showing)
  2. Stabilize tl1e kiln (02 to level off and stay within a certain narrow range)
  3. Optimize (fine-tune) the 02 in small steps to bring the oxygen content into the range of 1.00-1.5% to obtain optimum kiln efficiency.

The majority of kilns are equipped with analyzers that test only for contents of oxygen and carbon monoxide which are considered the two key variables in combustion control. Some kilns also have  separate  carbon dioxide analyzers and monitors (see Chapter 5) but these are not considered as absolute requirement, rather they are excellent tools to forewarn the operator of upcoming changes and are helpful in optimizing the kiln operation.

In recent years, a great deal of work has been done in some plants in the utilization of recordings of NOx in the kiln exit gases for determining changes in burning-zone temperature. Originally these NOx analyzers were installed and used for the purpcse of emission control to satisfy environmental  specifications.    Engineers,  being  familiar  with  the process of NOx formation in a rotary-kiln flame, have recognized the relationship between   the  NOx  in  the  exit  gas   and  the  burning-zone   temperature.

Different  NOx  concentrations  are found  in  the  exit  gas  when  the  com­bustion  temperature  (flame  temperature)  in the burning  zone changes. Higher flame temperatures result in higher NOx.  Furthermore, when the kiln excess  air  (02)  increases,  the  NOx  also  increases.  It  must  be mentioned that NOx cannot be related to the actual prevailing temperatures in the burning zone but it is an excellent and rapid indicator of changes that have taken place there. In many respects the above reactions to flame temperature and excess air in the kiln lead to contradictory objectives in kiln operation and have been the subject of much controversy.  Naturally, in matters of environmental control, one strives to obtain a NOx content as low as possible.  For an efficient kiln operation, however, one tries to operate the kiln with as short a burning  zone as possible  (short flames, high flame temperatures) which in tum means high NOx contents.  This problem is especially acute on natural gas-fired kilns because they usually operate with higher NOx contents than coal-  or oil-fired kilns.

As a general rule, kiln exit-gas components are affected as follows, assuming all other variables remain constant.

Strip chart recordings of oxygen often display large variations in range even when  the kiln operation is stable. This is referred too as process “noise” and is a normal occurrence on many kilns. However, when this so­ called “noise” exceeds a range of± 0.5% in short-time intervals, this could be an indication of irregular fuel or air-flow rates taking place within the system and should be investigated. Likewise, since process “noise” is common to the oxygen analyzer, the instrument should be checked when­ ever the oxygen chart traces a straight line for a long period of time.

Most oxygen-recording charts register this variable within the range of 1-5 %. It is important to remember the following: When the recording is pegged out at the maximum for a long period of time (i.e., draws a straight line at the top), one doesn’t know if the oxygen is at 1 or at 15%. In short, an operator under these circumstances wouldn’t know what the com­bustion conditions are. it is therefore good burning practice to always set the fuel rate and/or J.D. fan speed so that the oxygen recording is always at least below the 5% mark whenever fuel is being fired in to the kiln. This recommendation also applies and is of equal importance during times of kiln starts and warm-ups. As a matter of fact, as soon as a main fire is lit in the kiln, it is the operator’s first duty thereafter to stabilize the oxygen content at a level that is at least below the 5% oxygen mark.

FUEL-RATE  CONTROL

Before any main fire ( except the kiln warm-up  torch) can be lit in the kiln, one has to ascertain  the following requirements are met:

  1. Sufficient air is present to achieve complete combustion.
  2. The gas analyzer is operational and functioning properly
  3. Sufficient heat is present to readily ignite the fuel
  4. The I.D. fan is running and the kiln draft is properly regulated to prevent an abrupt, delayed, explosive ignition of the flame.
  5. The firing floor is cleared of any unauthorized personnel.
  6. The primary-air fan is running its flow rate properly set.

Once the fuel has ignited and a proper flame obtained, the operator must immediately check the gas-analyzer recording to ascertain that no combustibles (CO) are showing after a time delay of 30 s. After this, necessary adjustments (fuel or ID. fan) to bring the oxygen (02) content below the 5% mark must be made. These are standard  and elementary steps that apply to any kiln when lighting the main fire.  Another rule that is practiced in most plants for safety reasons is to cut off the fuel com­pletely whenever the flame is not lit after 30 s.  The kiln is then purged for at least 5 min to rid the kiln of any combustible gases before another attempt is made to reignite the fire.

The operator should never attempt to control burning-zone  temperature by merely increasing or decreasing the fuel rate, nor should he, for example, increase the !D. fan speed to permit raising the fuel rate because the exit gas is deficient in oxygen. To operate in this manner is sure to lead to trouble, because no consideration is being given to the temperature at the back end of the kiln, where the feed is being prepared for calcination. Increasing the !.D. fan speed in order to be able to add more fuel (in case of low oxygen) will cause the back-end temperature to rise. This however is the wrong thing to do, because the back-end temperature plays a vital role in the operation of the kiln, and cannot be allowed to freely float up and down.

Whenever the operator makes a change in either the !.D. fan speed, the fuel rate, or both, he has to anticipate the possible reactions caused by the change, remembering that an increase in fuel rate results in higher back-end temperature, low percentage of oxygen in exit gas, and higher burning­ zone temperature, and an increase in I.D. fan speed will result in higher back-end temperature, more oxygen in the exit gas, and lower burning-zone temperature. From this it becomes obvious that the fuel rate alone does not govern the burning-zone temperature.  Without changing the fuel rate, a change in the burning-zone temperature can be caused merely by altering the kiln draft with I.D. fan speed. Nevertheless it should be emphasized that a fuel-rate adjustment will give the fastest reaction whenever a change in burning-zone temperature is required.

The first rule in fuel-rate control is: Always check the oxygen and CO­ analyzer recording before and after any fuel-rate adjustment is made. It will not take long for a new kiln operator to learn that the burning zone will react slowly to any fuel-rate adjustment. There is an inherent time delay until a noticeable change in temperature takes place.  Since  these  time delays can be as much as 10 min, particularly after an upset operating con­dition,  it is  quite  common  among  new  operators  to  either  over-fuel  or under-fuel the kiln for a prolonged length of time. The end result of this action is a burning zone that alternates from “cold” to excessively hot, a condition generally referred to as a cycling kiln.  Because  of  this  time delay, operators must become skillful in anticipating these swings of ex­treme temperatures and make their fuel adjustment before the actual change in temperature takes place. For  proper timing of these adjustments the operator checks other instruments which could give him a positive sign that a turnaround in temperature conditions in the burning zone is imminent.

The time lag between burning-zone temperature  and a fuel-rate adjustment has been previously mentioned. With direct-fired coal systems this lag time is even more pronounced since it takes several minutes between the time the coal rate to the mill is adjusted and the change to be noticed at the burner tip.

Much  work  has been done with  automatic  fuel-rate  control.   On pre­calciner kilns, the fuel rate to the flash calciner is usually automatically controlled in a closed loop that uses the fourth-stage, preheater cyclone exit­ gas  temperature   as  the  input   variable. Simply  stated,  when  this temperature is below the setpoint, more fuel is given to the flash calciner and, conversely  less fuel when  the temperature  is above this setpoint.

Constraints in this loop are  the same  as with  the fuel-rate  adjustments  at the rotary-kiln firing end, namely the oxygen must conform to certain predetermined  levels  and no CO is  allowed  to show.  Since a flash  furnace is much smaller than a rotary kiln, the  controller  must  be  tuned  so  that there will be a fast response by the coal feeder whenever a change in this fourth-stage,  cyclone outlet  temperature  takes  place.

Conventional fuel-rate control on wet, dry, and preheater kilns can be manually or automatically controlled.  When  automatic  control prevails, the control concept should preferably  consider as input variables at least the burning-zone temperature, the back-end temperature, and the percent oxygen in the kiln exit gases. A simple control logic of adjusting the fuel rate strictly and solely based on the burning-zone temperature alone seldom yields satisfactorily stable and efficient kiln operations.

During times of severe kiln upsets, i.e., when underburned fine clinker has entered the cooler and “blackened” out the burning zone, an operator often tends to make the mistake of overfueling the kiln. This is a natural tendency of all operators because a lot of heat is needed to bring the burn­ ing-zone temperature back to normal. Since tl1e kiln speed is drastically reduced in  such instances and visibility is extremely poor, they tend to leave the fuel rate at high levels for too long a period of time waiting for the burning zone to clear. However the kiln could already be in an over­ heated condition.  Things to consider during these conditions are:

a)Operating the kiln with no or very little oxygen  contents  (even when no combustibles are showing) often does not produce the desired heating of the burning  Reason: The dust in the kiln and low oxygen combined together produce lower flame temperatures.    Solution: Try a little less fuel to  operate  at  a slightly higher oxygen content of, e.g., 0.5-0.9%.

b)Once a heavy onrush of material has passed through  the burning zone, there invariably is a lighter load behind  it that causes a rapid increase in temperature when it arrives in the burning This rapid temperature increase can usually not be detected early enough because of the .dusty material in  the  cooler.  Solution: Watch for a definite  sign  that  tl1e  load  is  becoming  lighter (oxygen increases, NOx increases, kiln amp increases, cooler undergrate  pressure  decreases)  and  start  to  increa’e  the kiln  speed at that time.  Depending  on  the  severity  of  this  light  load  it might become necessary to  start  reducing  fuel  rate  at  this  time also  (the effect  on  back-end   temperature  should  not  be  forgotten) to prevent the burning zone from becoming too hot.  In short, this is  a time where  anticipation  of upcoming  changes  becomes of primary importance.

KILN-SPEED  CONTROL

A kiln can never be expected to operate in a stable condition for an indefinite length of time because changes do take place in the kiln regard­ less of how stabilized the operation may appear. Sooner or later an adjust­ ment in kiln speed has to be made if the kiln is to continue to produce a good clinker. The commonest action the operator must take in case of an apparent upset is tl1e so-called “slow-down” procedure, as described in the following paragraphs, and shown graphically in Fig. 16.2.

After the kiln has been operating for a long period of time under balanced conditions the operator detects a heavy onrush of feed directly behind the flame. Whenever this happens, the operator must determine whether he will be able to hold the heavy feed load with an increase in the fuel rate alone, or are conditions such that the dark feed could pass too far under the flame even with the fuel rate increase? It is important that this decision be made at the earliest time possible. Oxygen percentage in the exit gas, back-end temperature, and conditions in the cooler are the deciding factor:; to be considered.

Assume that all indications are that it will not be possible to maintain the same kiln speed, and therefore speed will have to be reduced. It then becomes necessary to determine how much the kiln can be slowed. Depending on the magnitude of the “push,” 5-10 rpm less will  be sufficient in some instances. In other cases the onrush is so heavy  that  the kiln has to be slowed down to minimum speed. Only experience will tell the operator how much the kiln has to be slowed. One rule, however, applies at all times: Never allow raw, unburned feed to enter the cooler, even if this means that the kiln has to be stopped and reheated on quarter turns or on auxiliary drive.

After the kiln has been slowed, there are, of course, a number  of changes that take place, and the operator must be alert to watch for these changes and take appropriate corrective action. First, the back-end temperature will start to increase.  This is undesirable because this temperature should be held within ± 20 degrees.  Reducing the I.D. fan speed will aid in  holding  this  temperature. Because of the decrease  in the kiln  draft resulting from the I.D. fan-speed reduction, there will be insufficient oxygen available for complete combustion. This is corrected by reducing the fuel rate until the exit-gas analyzer again indicates from 0.4-0.8% oxygen in the gas.

Because of the slower kiln speed, less material will enter the cooler, thus both secondary air temperature and undergrate pressure will decrease. Therefore, reduce the bed grate speed in the cooler to maintain more or Jess the same undergrate pressure. This will be difficult to do. On very slow kiln speed it is almost impossible to obtain the same undergrate pressure as on full speed. Under slow-speed operation, do not attempt to hold the secondary air temperature on the same level as on full kiln speed, espe­cially when the clinker is slightly underburned, because of the danger of the clinker not being sufficiently cooled when it leaves the cooler.

While the kiln is on slow speed it is necessary to decide how soon the speed can be raised again. First of all, never increase the kiln speed before there is a definite sign of the burning zone warming up, or there is a defi­ nite indication that the burning zone will warm up within a few minutes because of a lightening of  the load. Do not hold the kiln on very low speed until the burning zone has reached normal clinkering temperature; start increasing the speed as soon as there is a sign  of  warming  up. Signals and instruments to watch for these early warnings are discussed in Fig. 16.3. The more the kiln speed is increased, the smaller the speed increments should be and tlie longer tl1e time interval between each speed increase. The way in which the kiln speed is increased is probably the most important factor in getting the kiln into stable conditions again. Consider the three examples shown in Fig. 16.3.

Example  1: This shows  an  ideal  execution  of  a speed-up procedure.The operator started to increase the kiln speed in large steps of 5 rph at a time until he reached kiln speed 50. From then on he extended the time between each speed increase and carried out the increases in smaller steps as the speed was increased.   These were at the rate of 3 rph from 50 to 62, 2 rph from 62 to 66, and 1 rph from 66 to full speed of 70 rph.

Example  2:   In this  example  the operator  used  the wrong judgment.He started to raise the kiln speed too fast before the burning zone was ready for it. Very soon he had to backup on the kiln speed and had to start re­ heating the burning zone all over again.

Example   3: Here  the  opposite  condition  occurred. The  operator waited too long on slow kiln speed. Suddenly the burning zone started to gain heat very rapidly, forcing him to raise the kiln speed in large increments and in short time intervals. For a short time full kiln speed was able to be maintained,  but very  soon the kiln became overloaded and the operator had to slow the kiln down once again.

Remember  that every  time tl1e kiln  speed  is increased, tl1e back-end

temperature drops, requiring more fuel  to  burn  with  each  speed  increase. This requires an adjustment in the !.D. fan speed as well as in the fuel rate. Both are increased in a manner similar to the kiln-speed increase in Example  I  (Fig.  16.3).

The ideal example given in Example I by no means suggests that the kiln will operate in a stable manner again for a long time on full speed. In time intervals of 2-3 h another push, each time less severe, will be en­countered. Each time the slow-down period will be shorter and each time the chances are better that the kiln can be held at the normal operating speed. Sometimes one slow-down will be sufficient, at other times two or threee such slow-down sequences have to be undertaken until the kiln again is stable. The chances of avoiding a cycle in the kiln are minimized when tاe operator executes a slow-down sequence according to the ideal example given. The aforementioned discussion  applies primarily  to long-wet and dry kilns.  Preheater and precalciner kilns react differently.

Kiln Rollback. The feed bed in a rotary kiln occupies up to 10% of the cross-sectional area of tl1e kiln. While the kiln is rotating, the center of gravity of this material is displaced to one side (on the rising side) of the 6 o’clock position (bottom center of the cross section), the torque resulting from this displacement being opposed by the driving torque applied to the kiln. When power to the drive motors is cut off for any reason, the off­ center position of the load causes the kiln to rotate in the reverse direction until the feed bed comes to rest at the 6 o’clock position. The reverse rotation of rollback can reach a maximum speed that exceeds the normal running speed of the kiln, thereby damaging the drive gear.

Another problem associated with rollback is that of restarting a kiln that has stopped while loaded. If left to rotate freely after a stop, the kiln will come to rest witl1 the load of the feed bed at or near the 6 o’clock position. In the past the relatively small rotary kilns could be started from this position without much difficulty, as the kiln drives then in use were capable of overcoming the initial torque to set the kiln in motion. With the advent of the 500-foot or longer giants, kiln manufacturers had to find a way to overcome static fricton and inertia in order to accomplish initial acceleration with a minimum of strain on the drive gear and motor. This resulted in the concept of kiln rollback control. In this method of control, instead of perntitting the kiln to reverse rotation, or roll back, when the drive motor is stopped, a brake is applied to the drive unit to stop the kiln at the instant the motor circuit is broken with the feed-bed position in an inclined angle to the rising side. Thus the gravitational force of the kiln feed bed acting downard is frozen by the brake so that it can be used advan­tageously when the kiln is started again.

When the operator presses the kiln-drive start button, the following se­quence of actions takes place automatically; first the brakes are released permitting the kiln to roll backwards (Fig. 16.4) with the feed bed past the low point of the kiln circle. Next the feed bed, now being on the down side of the kiln, causes the kiln to start to roll, or rotate, forward in the normal direction of rotation, at which time the kiln drive motor is acti­vated to keep the kiln revolving in the forward direction.

This system has proved its usefulness and has contributed significantly to the present long life of drive gears and motors of large rotary kilns.

The most  commonly asked question is: “What is the optimum kiln speed for efficient and stable kiln operation?” There is no clear-cut answer because each kiln is different in its requirement. The kiln slope and the resultant feed velocity in the kiln is one factor that must be considered. Another important factor is the feed loading of the kiln, i.e., the output rate that can be achieved at a given kiln speed without sacrificing operating stability.  Typically, specific volume loading of kilns varies from a low of approximately 5% to a high of 10%.

Dry and wet kilns with steep slopes should be operated at slower speeds to prevent large onrushes of feed waves into the burning zone (and into the cooler) during upset conditions. Such kilns usually  operate normally  in the range of 50-60 rph. Other less-sloped dry- and wet-process kilns generally run at speeds of 60-72 rph whereas preheater  and precalciner kilns operate at much higher speeds. Kilns that consistently and periodi­cally exhibit difficult burning conditions in the burning zone as a result of incomplete feed calcination sometimes exhibit better operating  stability when the kiln-speed target is lowered and the kiln is concurrently operated at a higher percent specific loading. This, however, is in direct conflict with  the  “old”  heat-transfer  law  for  rotary  kilns  which  states   that more efficient and favorable heat-transfer conditions exist when the kiln is operated at high kiln speed and low percent feed loading. Hence, instead of lowering the kiln-speed target and operating at a deeper bed depth to over­come frequent operating instability, it might make more sense to  equip such kilns with more heat exchangers in the calcining zone to improve the overall heat transfer behind the burning zone. Regardless of the !din-speed target (top speed under normal operation) it is good practice to set this target at a speed that is approximately 5-15 rph lower than the maximum permissible. The kiln must be allowed some “elbow room” at the top to make it possible to further increase the kiln speed in an emergency when the burning zone has become dangerously overheated.

 

 

 

 

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