Reasons for Melt Formation

Bestway Cement Limited Chakwal Production Department 27.06.16 Kiln Drive Amps It is the most important sensor used to evaluate the burning zone state. The running set point will change according to the raw material composition. Amperage value changes with ring formation. Amperage value changes with the size of the clinker. The absolute value is important only when amperage is high (fuse protection limitations). The trend is a very good indication of burning zone length and temperature; therefore the amps should be recorded in such a way to give a good indication in its variations. A drastic increase of the amps trend, followed by a fast decrease is an indication of loss of coating or a broken ring inside the kiln. A target amps range should be established (depending on raw mix and other factors) through which this variable can fluctuate safely. A drastic increase in amps could indicate a possible mechanical condition problem. Secondary Air Temperature It should be kept as stable as possible by the automatic cooler control system (+/85oF or 39oC). Absolute value is not important because most indications are incorrect due to instrument inaccuracy. Trend is very important as it shows variations of material from the kiln and the gas temperature variations to the kiln. The secondary air temperature should be as high as possible in respect of the various refractory temperatures in front of the kiln. Hood Draft Absolute value is important and is automatically controlled at a constant value (usually looped to cooler exhaust fan damper / fan). It should be low as possible (to reduce in-leakage). It is the separation between cooler and kiln and it should be always constant to avoid influences of changes in the cooler gas flow to the kiln operation. A poor regulation of this signal will induce variations in the kiln gas flow and could bring the kiln into cycling. Under Grate Pressure The absolute value should be maintained constant with automatic control. The value varies with the cooler bed depth and with 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 under grate pressure, bed depth and secondary air temperature to be valid. Kiln Speed It should be related to feed rate.

It should be set with a constant feed ratio. Small variations of the kiln speed (1 to 3 revs) could be used to control the burning zone state. However, kiln speed variations should be used only after the fuel flow rate has been used as the primary controller on long wet and dry kiln. Often if over used, speed variations in the material loading of the kiln and will lead the kiln into a cycle. On large temperature variations in burning zone like raw material pushes, the kiln speed must be used with wide variations (as example 30 revs or even a complete stop) to avoid raw materials to go in the cooler and to re-heat the burning zone. Fan Draft It should be related to the oxygen level, fuel flow rate and feed rate. It is the main controller to keep the temperature profile along the kiln. The rpm variations should be small during kiln normal operation +/- 15 rph. During kiln pushes and kiln slow speed, large speed variations will be required to maintain the chain gas temperature under safe limit of the chain system.

Variable that you cannot control Quality and characteristics of the raw materials Quality of the fuel used as a example: heat value, ash content, volatile matter and moisture level Dust quality and quantity returned to the kiln Accuracy of the feeders Chain system design Accuracy and good response of all control loops and sensors of the kiln system For these variables that he cannot control, the operator should be kept informed of any changes done and should make sure that those variables are kept inside an acceptable range to maintain a good kiln stabilization.

VARIABLES WHICH THE OPERATOR CAN CONTROL Material feed to the kiln Fuel feed to the kiln Speed rotation of the kiln Temperature profile along the kiln Draft at the feed end of the kiln Supply of combustion air Retention time of the material in the kiln Temperature of the combustion air However, some restrictions are sometimes given on the utilization of those variables and may vary from plant to plant due to local conditions and are usually the following:

Set point on the maximum speed of the kiln Set point on the maximum feed rate to the kiln Automatic loop set point for oxygen level and ID fan speed Settings on the burner pipe and its position Primary air settings and fuel tip velocity Set point on cooler fans flow Set point for the under grate pressure and the clinker bed depth in the cooler

RAW, UNBURNED FEED IN CLINKER COOLER Indicators: On rush of raw feed into and beyond burning zone “Black feed” position advanced more than ½ way under the flame “Black-out” in burning zone Red grates in cooler Rapid rise in cooler grate and clinker discharge temperatures Cooler drag-chain amperage increases rapidly Actions 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 on auxiliary drive) Reduce fuel and ID fan speed in accordance with standard slowdown procedures to protect the kiln back end temperature 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 to Avoid Re-occurrence Accelerate frequency of visual observations of burning zone for early detection of impending cooler upsets Evaluate kiln output rates vs. capabilities and kiln operating stability

KILN UPSET CONDITIONS – SLOW DOWN Kiln cannot operate in a stable condition at all times and it will eventually get colder. Sometimes heavy rush of feed might end up too far under the flame (halfway under the flame). Operator must then decide if he will be able to control the heavy feed load only by increasing the fuel flow, or if he needs to reduce the kiln speed. The determinant factors to be considered are the oxygen level, back-end temperature, movement of the feed rush, and conditions in the cooler. Assuming that it is impossible to maintain the same kiln speed, the operator must determine how much the kiln speed can be slowed down depending on the magnitude of the push. Only experience can tell the operator how much the kiln has to be slowed down. In those cases the following rules apply all the time; When in doubt, the kiln speed reduction should be greater than required. This will help to overcome the problem faster. Never allow the raw, unburned feed to enter into the cooler, even if it means that the kiln must be stopped and rotated on ¼ turn or the auxiliary drive. As soon as the kiln has been slowed down, the operator must; Try to keep the back-end temperature within acceptable limits variation (i.e. 75 to 100oF). Cut down the fuel flow to maintain the oxygen level above 0.5% and to keep backend temperature inside the limit range. Evaluate the cooler conditions, grate speed, grate temperature, inlet temperature of gravel bed collection system and secondary air temperature. Often, the cooler will have to be switched on manual mode to protect it to become overheated as often regulation of cooler are made for small variations only. As the kiln is on slow speed, it is necessary to determine when the speed can be increased back again. In that case, the oxygen level is the main factor to look at. Try to keep the same values on the fuel flow and ID fan speed in order to observe the O2 variations. When O2 level increases, the burning zone is warming up. As soon as the oxygen level start to increase, increase the kiln speed slowly i.e. +/3 rph if the burning zone is still dark and very dusty. Page 58 of 98

Observe the reaction of the O2 level if it keeps on raising. If so, increase kiln speed by 3 rph at every 10 minutes, and keep on increasing as long as the oxygen level does not drop. Keep fuel flow always one step ahead of normal operation level, (i.e. when rotation speed is at 24 revs, set fuel flow for 27 revs), and ID fan speed accordingly.

Stop increasing kiln speed approximately 1 ½ to 2 hours after initial slow speed in order to overcome any upset in the material load of the kiln created by the speed variation. Restart increase the kiln speed after reception of material from slowdown is over. Kiln speed increase should be reduced around 80 revs as we are getting near full production (i.e. 2 revs at every 20 minutes). When full production has been reached (i.e. 85 revs) the fuel flow must be kept one step higher than normal for at least 2 more hours, and then it must be cut down slowly following the normal operating procedures to get the load at the end of the flame again. A slowdown made in time and at proper speed will requires about 20 minutes before to restart to increase the kiln speed again (as O 2 start to go back up). Usually, the kiln should be back in full production about 4 hours after the slowdown was made. Slowdown t/h Fuel flow ID fan O2 Operation Example: speed revs 86 93 102 m3/h 530 rpm 1.5% Normal 66 72 85 m3/h acc. O2 0.7 Slowdown level 36 40 50 m3/h acc. O2 0.7 slowdown level

Root cause analysis of Cooler 1st Grate tripping and melt formation inside cooler Reasons for Melt formation inside cooler 1. 2. 3. 4.

Quality variation : Decrease in LSF Over burning Actual fuel TPH high against SP Low CF silo level

Important cooler control measures 5. Under grate pressure 6. Air flow 7. Hood draft How can you get to know that kiln is going towards melt formation?

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The best available parameter with kiln operator is Kiln Main drive Amp that can tell you the exact situation inside kiln. In case of melt formation it is observed that kiln main drive (amp) suddenly increases and then decrease. This is sign for melt formation. Never wait for quality to give you the results rather push them to take spot sample of LSF when you observe such an abnormality. Have eyes in field to check the physical condition of kiln. The cross LSF value will confirm that a decrease in quality have upset the condition inside the kiln. Carefully monitor and analyzer kiln main drive (amp) trends. The variation shows the story, of the variation increases then coating has fall in that turn. During coating fall kiln main drive amp trend is quite different.

Steps to be taken           

A sudden increase in Kiln main drive amp is direct sign of melt formation inside kiln. Immediately open kiln main drive trend on your other LCS and analyze the trend variation. Reduce rpm, fuel and feed. Increase ID fan Cool your kiln as much as you call to avoid this melt material to enter inside cooler. Open 2nd grate fans, reduce TAD dampers up to 65 % to maximum secondary air. Decrease secondary air temperature up to 900 C Have an eye in the field to check the physical condition of kiln. Person in the sight will confirm you melt formation inside kiln. If you observe under grate pressure increasing above 8 KPa , further reduce kiln rpm and increase ID fan. Reduce fuel at main, reduce feed and make kiln dead slow. If you do not take drastic steps then the melt can be controlled and it will trip your first grate.

Why melt is not transported by cooler? Melt material is liquid type not solid. Cooler can only transport solid material or dusty material. If you think that cooling melt inside cooler will help address the issue, never because due to extreme high temperature and liquid phase air cannot enter and bed resistant increases. In coming material from kiln further increase the load on cooler grate finally leading to its tripping. What went wrong? Facts and Figure: I.

Kiln main drive amp increase at hrs.

II. III. IV. V. VI. VII.

Maintenance at flap of C5 was done at 1800 Hrs. Cooler grate tripping occurred at 1900 Hrs. A probable reason is that grate over loading is maintenance at C5 flaps. Maintenance of flap should be done while kiln is shutdown. Low rpm of kiln some time add to melt formation because material residence time inside kiln increases. Kiln rpm are may be less as compared to feed. Since the kiln is being operated at less feed due to CF silo level, the fuel may be higher as compared to feed. Fuel at kiln main burner also have an error ± 5 %, may be this error have forced this high melt formation.

Root cause