Kiln and Cooler –Control and Optimisation

By

Dr Y C Naidu

Cement Plant Typical View

Basics of Kiln and Cooler

• Kiln for pyroprocessing of raw material to make clinker

• Technology development from wet – semi-wet/semi dry- preheater kilns- precalciner kilns

• Cooler to cool clinker for further material conveying and also recuperation of heat

Precalcination

DefinitionPrecalcination is the addition of a second firing point and combustion chamber at the base of preheater with separate ducting of hot air from clinker cooler through Tertiary air duct.

REACTIONS OF CLINKER BURNING

• BELOW AND - EVAPORATION OF FREE WATER AT 100 0C

• 500 – 600 0C - RELEASE OF COMBINED WATER FROM CLAY

• 600 – 700 0C - MgCO3 DECOMPOSITION • 600 – 900 0C - CaCO3 DECOMPOSITION

• 700 – 900 0C - FORMATION OF CaO.Al2O3, 2CaO.FeO, 2 CaO.SiO2 AND 12 CaO.7Al2O3 BEGINS

REACTIONS OF CLINKER BURNING

• 900 – 1200 0C - FORMATION OF C3A, C4AF AND C2S

• 1250 – 1350 0C - FORMATION OF LIQUID (FROM MAINLY C3A AND C4AF) AND

COMMENCEMENT OF C3S FORMATION

• 1350 – 1450 0C - FORMATION OF C3S

• 1350 – 1250 0C - CRYSTALLISATION OF LIQUID INTO C3A AND C4AF

SP: Suspension Preheater Kiln

ILC-E: In-Line Calciner using Excess air

ILC: In-Line Calciner

SLC: Separate Line Calciner

SLC-I: Separate Line Calciner with In-line Calciner

TEMPERATURE PROFILE OF FLS-SLC-I KILN SYSTEM (TYPICAL)

CONFIGURATION OF FLS INLINE CALCINER (LOW NOX)

CONFIGURATION OF FLS SEPARATE LINE CALCINER

Configuration of FLS SLC-I Calciner

Configuration of FLS Downdraft Calciner

POLYSIUS LOW PRESSURE CYCLONES

POLYSIUS LOW PRESSURE CYCLONES

Precalcination

DefinitionPrecalcination is the addition of a second firing point and combustion chamber at the base of preheater with separate ducting of hot air from clinker cooler through Tertiary air duct.

Advantages of the Precalcinator Technology

1. Reduced the thermal loading the BZ of the kiln.2. Increased refractory life.3. Increased kiln availability.4. Increased specific kiln capacity up to 2.50times of SP kiln.5. Reduced kiln size of a given capacity.6. Smoother operation of kiln.7. Slight decrease in heat consumption under favorable

conditions.8. Reduce energy losses in bypass with alkalis / chloride /

sulphate bearing raw meal.9. Increase in out put of existing SPkilns up to 50%.10. Economical conversion of wet process kiln to dry process.

Factors Influencing the Decarbonation Rate

TemperatureHeat Transfer AreaMineral Composition

Factors Influencing the Combustion of Coal

Surface area of the coal particleVolatile matter of the coalPartial pressure of oxygen

Combustion and Decarbonation

In a typical precalciner both combustion and decarbonation are occurring simultaneously.

Rate of combustion and rate of decarbonation influence each other.

Raw meal is suspended in a rising stream of hot gases resulting from combustion.

Residence Time in Precalciner

The entire surface area of the raw meal particle is able to absorb the heat from the gases and release CO2.

The effective time of heat transfer in the effective temperature is very short, 2-3 seconds.

According to particle size the retention changes. Large particles rise slowly taking more time than smaller particles which rise up fast.

Precalcining - Particle Size - Combustion

In each particle of raw meal, calcining reaction proceeds at a decreasing rate from the surface towards the centre.

At a particular time a particle will consist of an uncalcined core surrounded by calcined shell. The shell is porous and filled with CO2. The shell develops insulating effect against heat transfer and expansion of CO2.

Temperature and particle diameter influence the rate of calcination.

Calcination at Constant Temperature

It is critical to regulate the temperature. Cement plants employ a PID loop between precalciner temperature and coal feed rate.

Precalciner temperature is controlled at 5°C.

Tertiary Air Duct - Advantages

Heat from the fuel firing in the kiln is used for i) Calcining the balance 10% ii) For heating the material from around 860-900°C 1400-

1450°C.. iii)Heat quantity is generally balanced by the heat of formation of clinker

mineral, an endothermic reaction.

The rest of the heat in the rotary kiln is used to meetI) The radiation lossesii) Exhaust gas heat loss andiii)Heat lost with the clinker.

The quantity of fuel fired in the kiln is therefore is independent of variation in the kiln output.

The heat requirement for entire production are met by increasing fuel to the precalciner.

Maintain constant precalciner temperature through PID controller at varying output

Kiln Control Parameters

• Material Control– Kiln Feed– Fine Coal

• Air Control– PH air by rpm– Cooler vent air by rpm

• Mechanical control– Kiln rpm

Kiln control parameters

• Kiln inlet CO/O2

• Kiln exit temp

• Cooler vent temperature

• Degree of calcination

• CO/O2 at Precalciner outlet or Bottom cyclone outlet

TEMPERATURE PROFILE OF HUMBOLDT KILN SYSTEM WITH PYROCLON LOW NOX CALCINER

Cooler Control

• Cooling fan air volumes

• Cooler strokes

• Cooler bed through speed of grates

• Air Blasters

Fixed inlet air beam system

Fig.2.2.25

Air blasters above static grate lineFig 3.15

Air blasters between steps of static grate

Fig 3.16

Efficiency of satellite cooler

Efficiency (%) = Heat in secondary air x 100 Heat in hot clinker entering cooler = 65 - 70 % for dry process kilns

Factors influencing the efficiency of heat transfer in satellite cooler

* size of the clinker particles, surface area of the clinker* cooling air rate, cooling air temperature, cooling air velocity* frequency of contact between the clinker and air

Effect of size of clinker particles

Coarse particle tend to cool slowly because the surface area for given mass

of clinker, in other words the contact area of heat transfer between clinker and air, reduces. This can adversely affect the

heat transfer efficiency.

Effect of air velocity

Higher the cooling air rate and lower the cooling air temperature, better is the heat transfer. Higher cooling air velocity improves the heattransfer efficiency upto a point, beyond which, the fine clinker entrainment and resultant dust cycles between kiln and cooler reduce the heat transfer efficiency.

Disadvantages of 1st GenerationGrate Coolers

* Low clinker bed depth due to grate inclination upto 10 degrees* Poor air distribution due to excessively large air chambers* High requirement of cooling air for overcoming poor distribution of air and achieving satisfactory cooling of all clinker.* Unsealed air chambers due to internal drag chain* Snowman formation due to platform or water-cooled chute in the inlet shaft* High grate slope and high air flow causing risk of red river due to air slide effect on fine clinker.

Improvements in 2nd Generation Grate Coolers

- Improvement in Air Distribution By * Optimisation of chamber sub division * Location of drag chain outside cooler- Increase Heat Recovery , Grate Plate Life and reduced cooling air due to higher bed thickness at inlet achieved by reduction in grate width and slope.- Control of Red Rivers by * Limiting grate inclination to maximum 3 degrees * Better clinker distribution by reduced grate width at inlet by incorporating ‘Horse- Shoe’ arrangement * Reducing specific air loading * Vertical back wall to eliminate risk of snowman formation.

Modern grate coolers

- Air beam coolers * Direct aeration of grate plates mounted on aerated hollow beams.

- Fully static grate coolers with separate reciprocating mechanical elements for conveying and distribution of clinker over the grate.

Fig.2.2.4

Air beam technology

Air beam technologyFig.2.2.5

Air beam technology

Fig.2.2.6

Advantages of Direct Aeration 1. Reduced heat consumption of kiln 2. Low preheater fan and cooler fans power consumption 3. Reduced cooler vent air for dedusting 4. Higher specific grate loading and retrofit possibility - potential for production increase 5. Extended grate plate life, reduced grate riddlings - Reduced maintenance and high operational reliability 6. Stable flame and positive effect on kiln operation

Thermal Efficiency of Grate Cooler

• Recuperation of clinker heat through secondary and tertiary air denotes thermal efficiency.• Lowest clinker temperature after recuperation zone implies highest thermal efficiency.• Heat recuperation depends on clinker bed depth on grate and distribution of air over the grate.• Due to unsatisfactory air distribution traditional coolers need to use more air to prevent over heating of grate.• Modern coolers use less air for same heat recuperation due to improved air distribution.

Process design parameters of grate cooler

* Clinker loading , tpd/m2

* Air loading, Nm3/m2/min

* Bed thickness, mm

* Grate width loading, tpd/m

* Compartment division for compartment aeration

CONSIDERATIONS FOR PROCESS DESIGN OF GRATE COOLER

•For given clinker to recuperation air (secondary air + tertiary air) ratio in terms of Nm3/Kgcl heat recuperation depends on * bed depth * average cooling air velocity through grate (m3/m2/min) * air distribution over the grate.•Higher the clinker temperature, maintaining higher bed depth and higher cooling air velocity will increase heat recuperation.•For given average air velocity and distribution over the grate limiting clinker bed height exists beyond which further heating of air does not take place.

Contd..

Contd..•Compartmentalization to prevent excessive shot circuiting through cooled clinker on downstream side.• Minimum cooling air velocity required to prevent thermal over - loading and over heating of grate.•Cooling air should not fluidize clinker or form blow chimneys. •High bed thickness, high clinker temperature and high air velocity require high air pressure.•Very high air pressure can lead to leakages through gaps between grates and side walls etc.•Temperature of clinker leaving the cooler depends on downstream equipment safety.

Clinker Bed Thickness

-Inclined grate cooler with 10 deg. : 400- 500mm grate inclination

- Combi-Cooler with 5/10 deg. : 500 mm inclination

- Combi - Cooler with 1st Grate at : 700 mm 3 deg. inclination:

- Modern air - beam cooler : 800 mm

Bed thickness, air loading and pressure drop

* Pressure drop across the clinker bed increases with bed thickness, air velocity and clinker temperature and decreases with clinker size.

* The bed thickness, clinker temperature, air velocity, pressure drop across clinker and under grate pressure to be developed by cooling air fans decrease from inlet end to outlet end of clinker.

Fan pressures for conventional coolers:

First fan : 750 mmWGLast fan : 300 mmWG

Fan Pressures for modern coolers:

First fan : 1000 mmWG

FAN PRESSURES

Red River

- Narrow stream of uncooled red hot fine clinker flowing down the cooler.

- Red river occurs due to uneven depth and size segregation of clinker across the width of cooler resulting in :

* Diversion of cooling air through bed of coarse clinker and less bed thickness.

* Fluidisation of fine clinker and air slide effect.

Fig.3.11

Typical course of a red river

Remedial measures for Red River

• Reduction in cooler width at inlet to give deeper bed and greater uniformity across width of cooler.

• Use of wedge plates with higher pusher face along path of red river, to divert fine material and mix it

with other clinker.

• Use of air beam technology.

Snowman Formation

•Discharge of sticky clinker containing residual liquid from kiln causing buildup at cooler inlet.

•Clinker with relatively high content of alkali or sulphate.

Remedial Measure for Snowman Formation

• Installation of air blasters in back wall of the cooler and on the sides as well as between the steps of

static inlet grates.

• Top surface refractory covering on either side of the cooler should slope towards the centre of the cooler.

• Extra air input to the clinker drop zone.

• Pushing the burner pipe slightly into the kiln to create a longer cooling zone in the kiln itself.

• Improved quality and process controls to avoid irregular kiln feed chemistry and unstable operation of kiln.