Everything you need to know about KILN CONTROL THEORY
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Kiln Control Theory
5.1 Principle of kiln operation
Kiln is operated according to general principles but there are no absolute rules.
Operating the kiln means attaining 3 goals by setting the controls and checking the variables.
- A) Quality
Produce good clinker with good free lime
- B) Stability
Kiln system must be kept stable. Stay aware of the variables that will affect the system’s stability.
- C) Optimization
Strive for optimum production level at the lowest possible cost:
– Lowest meat consumption
– Highest production rate
5.2 Operation target
- Highest production level under stable conditions.
- Oxygen level as low as possible (depending on the variations in fuel mixture).
- Kiln exit temperature as low as possible
– Flame as short as possible
- Secondary air temperature as high as possible but stable.
– Temperature not above liquid phase temperature in front of kiln (damage scale refractory).
- Primary air as low as possible.
- Clinker temperature.
– under 110°C as it could promote quality problems (false set) the during grinding process.
5.3 Kiln system stability – pre-requisite conditions
5.3.1 Stable feed
- Feed quality range should be:
+/- 0.4 in CaO
+/- 0.4 in SiO2
+/2 0.2 in Al2O3
- C3S variations in clinker of +/- 3%
- Approximately 25% relative potential liquid phase in the clinker is good for kiln burning and fuel saving.
- Good and accurate feed rate indication.
- Linear at low, medium and high feed rate.
- Calibration by physical weight of the material to be done at every major kiln shut-down.
- Good feed and speed ratio.
5.3.2 Stable dust reintroduction to kiln
- Dust collector cleaning cycle sequencing.
- Dust circuit configuration should provide stable reintroduction.
- Monitoring of the dust return flow is an added advantage.
- If dust wasting required, wasting should not upset dust return to kiln.
5.3.3 Stable water spray injection
- Good spray injects into gas stream, not into the material load.
- Good regulation of water injection with respect to ESP inlet temperature control (will cause changes in oxygen level).
5.3.4 Good chain system design (wet & long dry)
- Acts as a good dust curtain (dust trap).
- Wet kiln design favours a good “plastic” zone.
- Chain tonnage between 12 and 14% of clinker production for large kiln and 10 to 12% for small kiln.
- Good chains should stand high temperatures that in turn enable high production rates.
5.3.5 Good control of hood pressure
- Hood pressure control is critical because it prevents cooler variations from upsetting the burning zone conditions.
- Should be set as close as possible to zero, while remaining negative.
5.3.6 Stable secondary air temperature
- Variations during normal operation should not exceed =/- 30°C (as stable as possible).
- As hot as possible, without damaging the nosering or refractory.
5.3.7 Good production level
- Lower limit (under 60% capacity tends to become unstable).
- Upper limit.
– kiln size (maximum flame size).
– I.D. fan capacity (oxygen level).
– Precipitator capacity (emission problems).
– Chain system (maximum temperature).
– Cooler system capacity (high clinker temperature).
- Good feed rates and kiln speed ratios should be developed experimentally to find the best material load depth in kiln for a given set of reference conditions.
- The optimum operation level experimentally developed for each kiln system should be used as a permanent setpoint for the kiln operator.
5.3.8 Constant fuel quality to allow constant heat input to kiln
When the burning zone temperature is acceptable and the back end temperature is on target, the kiln has a perfect temperature profile. The next goal is to keep it that way and optimize the oxygen to a minimum level.
5.4 Main parameters – trend and absolute value
5.4.1 Back end state
Back end temperature:
- Setpoint varies with kiln production level.
- Absolute value allows one to draw conclusions about efficiency of kiln operation.
- Bet trend important for operation.
- With the use of a water spray in the back end, the water volume will slow reactions of the back end temperature.
Chain inlet gas temperature:
- This temperature is generally a limiting parameter for the kiln operation.
- Chain gas temperature setpoint is fixed by the metallurgy of chain system and its design.
- Absolute value – very important – must be held below the allowed temperature for the chains.
5.4.2 Burning zone state
Burning zone temperature indicator:
- Importance of this instrument is often overestimated.
- Precise only when kiln conditions are clear.
- The secondary air dust influences in the indication of the instrument and readings are in error when kiln gets hot and dusty.
- Must be correlated with kiln amps indication and secondary air temperature to ensure the validity of the reading.
Shell temperature (scanner):
- Importance of this instrument is underestimated.
- Is often used only to detect hot spots on the kiln shell, estimation on brick thickness, and coating in the kiln.
- On bigger charts, it gives a good indication of the burning zone temperature profile.
- Fastest indication of a slowly-moving ignition point.
- Very good indication of flame variations due to the burner design, pipe position, flame shape, and direction.
- Absolute value is important to detect hot spots and rings.
- Trends indicate changes in burner system, kiln speed, and operation conditions which are affecting the temperatures profile of the burning zone.
- NOxlevel has been measured in the range between 500 and 2000 PPM in the Lafarge group kilns.
- NOxincreases with excess air and is strongly correlated to low excess air levels. At higher levels, i.e. 3% and greater, NOx is weakly dependent on excess air.
- Correlation between burning zone temperature and NOxlevel is good.
- NOxis generally, but not systematically, correlated with the kiln drive. NOx gives a truer picture of burning conditions. Kiln drive amps will change due to ring formation and degradation where as the NOx signal will not.
- NOxcorrelates better with clinker liter weight than free lime.
- NOxsignal is affected by fuel changes. Natural gas flames yield the highest NOx level.
5.4.3 Other parameters
Secondary air temperature:
- Should be kept as stable as possible (automatic).
- Absolute value is not important because most indications are incorrect due to instrument inaccuracy.
- Trend is very important; it shows variations in material and gas flow rate.
- Absolute value is important and is automatically controlled at a constant value (usually looped to cooler exhaust fan damper/fan).
- Should be as low as possible (reduce inleakage).
- It is the separation between cooler and kiln and it should be always constant to avoid influence of changes in the cooler gas flow to the kiln operation.
Feed end pressure:
- Trend and absolute value help to detect build-up in the kiln.
- It is a very important instrument if many ring problems occur in a plant.
Cooler exhaust gas temperature:
- Absolute value is an important limitation for protection of the dust collection system.
- Trend is a good indication of clinker, gas flow variations, and clinker temperature on the horizontal grate.
- More sensitive and reliable as most clinker temperature indicator.
Clinker discharge temperature:
- Trend has no importance for cooler operation.
- Absolute value is an important limitation for safety of the clinker transport system.
- Clinker temperature measurements are generally not very accurate. The cooler exhaust temperature is a more reliable clinker discharge temperature indication.
Kiln drive amps:
- Running setpoint will change according to the raw material composition (free lime maintained constant).
- Amperage value changes with ring formation.
- Amperage value changes with size of clinker.
- The absolute value is important only when amperage is high (fuse protection limitations).
- Trend is a very good indication of burning zone length and temperature.
- See description attached.
- The absolute value should be maintained constant with automatic control.
- The value varies with the cooler bed depth and with the clinker size. It must be held constant in order to achieve a constant secondary air temperature. Also, only a constant air flow can allow the relationship between undergrate pressure, bed depth, and secondary air temperature to be valid.
- Variations of the undergrate pressure trend are reflected by variations in the cooler grate speed. Undergrate pressure together with cooler drive amps should give an indication of the clinker size.
- Undergrate pressure setpoint should be at least 15 inches (H2O).
- Trend and setpoint are very important.
- Should be maintained as constant as possible.
- Can supply indications about general kiln conditions, pushes, burning zone temperature, and ring formation.
- Should be kept as low as possible, without going into CO range.
Main parameters used to evaluate the kiln:
- Kiln amps: Accurate 80% of the time.
- NOXanalyzer: Nitrogen oxide level can be related to burning zone temperature.
- Back end temperature: Should be kept at constant level, and is related to I.D. fan, oxygen level, and fuel flow rate.
- Gas temperature: Should be kept at constant level according to feed rate.
- Oxygen level: Related to I.D. fan speed.
- Hood pressure: Control of excess air by cooler exhaust fan.
- Secondary air temperature: Temperature of combustion air carried back to kiln from the cooler. Definate reactions and temperature variations during a kiln push.
Main parameters used to operate the kiln:
- Fuel flow: Should be related to feed rate.
- Kiln speed: Should be related to a constant feed rate.
- Draft fan: Should be related to oxygen level, fuel flow rate, and feed rate.
- Undergrate pressure: Related to cooler grate speed during pushes on kiln.
5.5 Kiln control basics
5.5.1 Control of the three basic variables
Conditions in the kiln are indicated by:
- burning zone temperature OK too low too high
- back end temperature OK too low too high
- oxygen level OK too low too high
Because any of these conditions can be within the allowable range, below the minimum allowable value or above the maximum value.
In most conditions, except for emergencies or upset conditions, the operator will find that these 3 variables can be maintained within reasonable limits by means of adjustments of one or more of 3 basic controls, which are:
- fuel flow to the burner;
- change in the kiln speed and/or feed rate;
- change in the speed on the I.D. fan.
5.5.2 Control philosophy
Regarding the results of the control two criteria appear over whelming:
- burning zone rate (clinkerization)
- back end state (decarbonation)
- Our chief concern is to ensure a proper burning zone temperature, the final criteria that we intend to set, being the free line at the kiln discharge.
- However we have to prepare the future, in other word to maintain the back end temperature in a suitable range to control decarbonation.
What we have now at the chain discharge is going to be in the burning zone in about 2 hrs. Control such temperatures is thus a prerequisite to proper burning zone control.
Burning zone control:
The purpose is to ensure quick and generally minor changes of the burning zone.
2 controllers may be used:
- The fuel rate (with or without O2 adjustment)
- The kiln speed (slowing down or speeding up the kiln can move the burning zone front)
These controllers have an immediate effect on the burning zone temperature.
The short term effect on back end temperature is quite low.
In emergency state ??? or very hot burning zone, both controllers can be used.
Back end zone control:
On a short term basis, the back end temperature of the kiln depends chiefly on the variation of the ratio
X = lbs of gas / lbs of clinker
So to have a quick reaction on changes of the back end temperature; 3 controllers can be used:
- the feed rate
- the draft (IDFAN)
- the fuel with constant O2
Playing with draft and/or fuel will affect the burning zone state.
The cooler is an amplifier of the kiln instability.
- The secondary air must be kept as constant as possible.
- Very often a good way to stabilize a cycling kiln is to work on the cooler and stabilize it.
- Grate speed or under grid pressure setting point.
- First 3 compartments air flow.
5.5.3 Kiln operation fundamental rules
- A) Keep a constant feed rate to the kiln.
- B) Keep a constant kiln speed during normal running. Do not change the speed to control the kiln during normal running.
- C) Find the optimum production level of the kiln and keep this level of production as a permanent operation setpoint.
- D) Keep corrections to the fuel supply as small as possible, but react promptly.
- E) The temperature profile along the kiln should be as smooth as possible.
- F) Corrections for the draft through the kiln should be small and should principally be made to keep the temperature constant.
- G) The secondary air temperature must be as uniform as possible.
- H) Watch the kiln burning zone of possible and the kiln panel instruments constantly, and react immediately.
5.6 Optimization of kiln system
- Operate kiln with higher free lime.
- Reduce air leakages:
– near nosering area and seal;
– air inleakage of kiln hood;
– around blast pipe;
– hood pressure should be set as close as possible to zero to prevent inleakage;
– primary air should be kept as low as possible.
- Reduce length of precooling zone by adjusting:
– burner pipe position;
– kiln speed (feed/speed ration);
– oxygen should be maintained at minimum and should be looped in with I.D. fan speed;
– good flame tip velocity.
- Good chemical composition
– good burnability factors;
– constant mix.
- All fans should be maintained well within their rated capacity, and attention should be given to the operating position on the fan curves.
5.6.1 Kiln limitation factors
- Gas chain temperature (near 940°C depends on chain quality)
- Diameter of kiln
– gas velocity in kiln 20-25M/sec in free kiln section and 9M/sec. in chain section maximum);
– flame erosion on kiln wall.
- Dust emission at precipitator stack.
- I.D. fan capacity.
- Cooler capacity
– no red clinker should be present after third compartment;
– clinker discharge temperature should be about 75°C;
– cooler exit gases should be about 175°C;
– Bed depth near 15 inches will allow a very good undergrate pressure with sufficient fan capacity (20 inches in hot zone).
5.6.2 Emergency condition 1: Raw, unburned feed in clinker cooler
- Onrush of raw feed into and beyond burning zone.
- “Black feed” position advances more than halfway under the flame in burning zone.
- “Black-out” in burning zone.
- Red grates in cooler.
- Rapid rise in cooler grate and clinker discharge temperatures.
- Cooler drag-chain amperage increases rapidly.
Possible effects and dangers:
- Thermal damage to cooler grates and grate drive mechanism.
- Flame extinguishment in burning zone.
- Fire on clinker conveyor belts.
- Excessively high temperatures in coal mill air circuit.
Warning: Watch for incomplete combustion when visibility in burning zone is severely restricted.
Action to take:
First and foremost – do not wait until raw feed is in the cooler, act when the first signs of impending problems are visible in the burning zone.
- Immediately reduce kiln speed to minimum (or turn kiln on auxiliary drive).
- Reduce fuel and I.D. fan speed i accordance with standard slowdown procedures to protect kiln back end.
- Reduce cooler grate drive speed (switch to manual control) to allow material in cooler more time for cooling.
- Adjust cooler air flow rates to obtain maximum cooling without the hood pressure going positive.
- Advise all unauthorized personnel to stay clear of the firing floor, cooler, and coal mill area.
Preventive Measures for Reoccurrence:
- Accelerate frequency of visual observation of burning zone for early detection of impending cooler upsets.
- Evaluate kiln output rates vs. cooler capabilities and kiln operating stability.
5.6.3 Emergency condition 2: Large ring broken loose in kiln
- Visual observation of large junks in burning zone.
- Sudden drop in kiln back end draft.
- Large drop in oxygen content of kiln exit gases.
- Hood pressure tending toward positive side.
- Sudden change in kiln drive amperage.
Possible effects and dangers:
- Overloading cooler with unburned feed.
- Onrush of excessive amounts of feed into the burning zone.
- Damage to cooler drive and grates.
- Large pieces jamming cooler hammer mill.
- Red hot clinker leaving cooler.
Action to take:
When amount of feed and ring fragments in burning zone is extremely large:
- Immediately reduce kiln speed to minimum.
- Reduce fuel and I.D. fan speed to keep back-end temperature under control.
- Switch cooler grate control to manual and reduce grate speed.
- Adjust cooler air flows to maximum flow possible; without that the hood pressure goes positive.
- Have personnel on stand by to watch the cooler and the hammermill for possible overloading, overheating, and jamming.
Possible preventive measures for reoccurrence:
- Laboratory to reevaluate chemistry of kiln feed (including dust return rates) for possible elimination of ring formation. If no solution in this area is possible, then:
- Initiate regular schedule to remove rings and heavy build-up by means of special devices designed for this purpose.
- Initiate regular procedures to displace the burning zone location on a daily basis (i.e. reposition burner every morning).
5.6.4 Emergency condition 3: burning zone dangerously hot
- Clinker balling (“sausage”) in burning zone.
- Coating dripping off the wall.
- Sliding molten clinker bed in burning zone.
- Burning zone temperature recording too high.
- Cooler undergrate pressures too high.
- Yellow-white burning zone.
Possible effects and dangers:
- Loss of coating and thermal damage to refractory.
- Red spots on the kiln shell.
- Thermal damage to cooler and kiln hood components.
- Reduce fuel flow rate to minimum until sausaging stops.
- Increase kiln speed approximately 5-10 RPH until sausage is broken.
- Provide maximum possible air in cooler (without hood pressure going positive).
- Reduce primary air flow.
Then, as soon as the primary objective of breaking the agglomeration is accomplished.
- Reduce the kiln and I.D. speed and increase fuel flow rate to restore normal operating conditions.
- If “sausaging” is frequent and the result of easy-burning mix, have laboratory evaluate possibility of providing a mix with less percentage of liquid content.
- Make more frequent, vigilant observation of burning zone conditions.
- Evaluate flame position and shape to determine if thinner, longer flame is feasible.
5.6.5 Emergency condition 4: Sudden, sharp rise in back-end temperature
- Feed shortage.
- Combustibles in exit gas.
- I.D. fan speed too high.
- Kiln speed too low.
- Chain “fire”
Possible effects and dangers:
- Chain fire on wet and dry kilns.
- Thermal damage to back end, dust collector, and preheater tower equipment.
- Delayed ignition of fuel in back end of the kiln.
- Immediately de-energize electrostatic precipitator.
- Immediately reduce fuel flow rate and I.D. fan speed to obtain less than 0.3% oxygen in exit gas.
Warning: Do not cut off fuel flow rate completely as this could trigger an explosion.
- Increase kiln speed and feed rate.
- Warn personnel to stay clear of kiln back end.
- Do not open any doors in kiln back end.
Then, as soon as the primary objective of bringing the back-end temperature under control is accomplished:
- Return kiln control variables to normal to restore normal operating conditions.
- Check out back end to determine if thermal damage has occurred.
- Do not operate kiln feed for more than 10 min.,
- Provide alarms and properly maintain kiln instrumentation to obtain warnings before the back-end temperature gets out of hand.
- Maintain close vigilance over combustion, back end, and feed flow conditions during kiln starts, shutdowns, and upsets.
5.6.6 Emergency condition 5: Red clinker at cooler discharge
- High drag-chain amps.
- Sudden drop in undergrate pressure (grate out).
- Excessively high undergrate pressure (cooler overloaded)
- Cooler loaded with coating and ring fragments.
- Stalagmite formation at cooler inlet.
- Uneven cross-sectional loading of cooler.
- Insufficient air flow into cooler.
Possible effects and dangers:
- Thermal damage to cooler components.
- Thermal damage to clinker transport equipment.
- Immediately make a visual check of the cooler to determine reason for red-clinker discharge:
– If cooler grate out, shut kiln down.
– If cooler overloaded, reduce kiln speed to minimum and reduce cooler-grate drive speed to allow more time for cooling.
- Increase air flow into cooler.
- Activate water spray at cooler discharge and reroute clinker to prevent damage to conveyor belts.
- A) On frequent grate failures:
- Investigate for possible faulty grate-installation methods by maintenance department.
- Investigate quality of grates and bolts used.
- B) On frequent one-sided loading of cooler bed:
- Investigate possible cooler-design changes.
- Investigate possibilities for elimination of stalagmite (“snowmen”) formation at cooler inlet.
- C) On frequent overloading of cooler due to upsets:
- Slow kiln speed down before raw feed enters cooler or cooler can become overloaded (make your corrective moves before things get out of control).
5.6.7 Emergency condition 6: “A chain fire”
- Rapid, sudden rise in intermediate and exit-gas temperatures
- By visual observation
Possible effects and dangers:
- Melt-down and loss of chains.
- Damage to kiln shell in chain-system area.
- On wet-process kilns: Steam explosion.
- Thermal damage to kiln back-end equipment.
Warning: Under no circumstances should there be water added at the feed end.
- Immediately, reduce fuel rate to minimum (but don’t shut fuel off completely!!!). At he same time, reduce I.D. fan speed to obtain zero combustibles and less then 0.3% oxygen.
- Increase kiln speed and feed rate to maximum until the back-end temperature is under control.
- On wet-process kilns: Clear all personnel from firing floor.
- Avoid operating the kiln for more then 10 min. when there is a feed shortage.
- Establish and enforce maximum permissible operating limits for intermediate and/or exit gas temperatures.
5.6.8 Emergency condition 7: Sudden, high-positive hood pressure
- I.D. fan failure.
- Large ring or buildup broken loose inside kiln.
- Instrumentation failure of cooler air flow, cooler stack damper, or I.D. fan control.
- Steam explosion on wet-process kilns.
Possible effects and dangers:
- All personnel on firing floor is in peril.
- Thermal damage to equipment on firing floor and hood.
- Danger of back-fire in coal system.
- Immediately, clear all personnel from firing floor.
- Immediately, reduce fuel rate to minimum and increase I.D. fan speed.
- Reduce cooler air-flow rates into undergrate compartments.
- Open cooler excess air damper manually.