7.kiln System Review

PYROPROCESSING

Friday, February 5, 2021

INTRODUCTION 

Basis: Introduction to Process:  kiln system description  kiln pyro-processing zones

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Objective - increase understanding of  kiln system benefits & constraints  interaction between equipment selection, clinker chemistry, & process control You can become a rotary cement kiln pyro-processing expert Friday, February 5, 2021

TYPES OF ROTARY KILNS 

Wet-Process Kilns  WSK, PLD, SEA, (RMD K1, K2)

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Semidry Kilns  Lafarge France

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Dry Kilns  STC, BFD, KAM, ALP, SCK, EXS K4,  JPA K1, BTH

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Preheater Kilns  WHL, (Demopolis), BTH, JPA K2

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Precalciner Kilns  EXS K5, RMD, SCK II, (Balcones) Friday, February 5, 2021

KILN HEAT CONSUMPTION

5.5

Million BTU/ton

4.5

3.5

2.5 1.5 Long Wet

Long Dry

Preheater

Precalciner

Friday, February 5, 2021

ROTARY KILN LENGTH Drying

Preheating

Calcination

Clinkerization Cooling

Kiln Types

Long Wet Long Dry 55 to 60%

Preheater

85 to 90%

Precalciner

Kiln Inlet

Kiln Processes

Kiln Discharge Friday, February 5, 2021

EVOLUTION OF ROTARY KILN 

Decreasing capital costs per tonne of clinker  Primary driver

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Reduction in specific heat consumption  Secondary benefit  Greatest benefit is wet to dry

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Environmental issues  Driver in new kiln design Operations: lever advantages while understanding process constraints Friday, February 5, 2021

WET PROCESS 

Feed enters the kiln in the form of slurry

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Moisture content up to 40%

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Additional dehydration zone requires the kiln be longer than dry kilns

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Requires more fuel

ADVANTAGES Uniformly blended feed Lower dust losses More suitable for moist climate regions Friday, February 5, 2021

WET PROCESS OPERATIONAL COMMENTS 

Small kilns  no economy of scale  small “divisor” for fixed costs

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High specific heat consumption  waste fuels

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Robust process  waste fuels  high moisture alternative raw materials

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Brick life limitations  high heat loading in BZ Friday, February 5, 2021

SEMIDRY PROCESS 

Grate Process or Lepol Kiln

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Kiln exit gases pass through the granular feed bed

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Partly calcined feed enters the kiln

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Kiln controls are much more difficult. from cyclones

Auxilary Stack

Material

350°C Cold chamber = Drying

Grate

900°C Hot chamber = Decarbonation

120°C

Material : Nodule feed= SEMI-WET PROCES Granular feed = SEMI-DRY PROCESS

400°C

to the Kiln

to cyclones

Friday, February 5, 2021

DRY PROCESS 

Operate with a high back-end temperature

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Typically have chain sections at the feed end

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Suitable for cogeneration of electrical power

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Issues  combustion emissions  operator control practices

Friday, February 5, 2021

CHAINS 

Chains provide large surface area in small length of the kiln Example:

12’ x 450’ kiln; 750 t/d 18,750 ft2 (1742 m2) chains equivalent kiln length = 500’ i.e. kiln length for the same heat exchange = 950’ 

Without chains, heat consumption is much higher but can be recuperated (Alpena)

Friday, February 5, 2021

CHAIN DESIGN Optimum: Decreasing SHC vs Dust Generation 

SHC improves with increased chains  Dust is important heat exchange mechanism 

Entrained dust S.A.  2x chain S.A.

 Dust generation is often limiting factor   

Wet kilns - possibility to trap dust 

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Chains increase gas velocity Dust pick-up = kV 2 slurry flow problems in plastic zone

Dry kilns - limit dust generation 

poor trapping  uniform chain density Friday, February 5, 2021

LONG DRY PROCESS OPERATIONAL COMMENTS 

Highly competitive  if entire process (plant) optimised  St. Constant - extremely low cost

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Reasonable specific heat consumption  not a precal but not wet process  not “forced” to use waste fuels

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Volatile cycles must be recognized & managed  BTH, JPA

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Tendency to overburn  long burning zone, lower quality Friday, February 5, 2021

PREHEATER KILNS 

Feed is preheated and partly calcined  by the kiln exit gases in cyclone tower

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Heat exchange mechanism  cold material in suspension with hot gases in cyclones

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Tower plug-ups  High concentration of alkalis, sulfur and chlorine in the kiln exit gases  often requires bypass

Friday, February 5, 2021

CYCLONES

Friday, February 5, 2021

PRECALCINING SYSTEMS either

through rotary kiln (AT)

Combustion Air or through separate duct (AS) either

in the preheater

Precalcining Performed or in a separate vessel Friday, February 5, 2021

44 STAGE STAGE PREHEATER PREHEATER WITH WITH PRECALCINER PRECALCINER Gas out 360°C Feed in 50°C 540°C

1st stage

350°C

2nd stage 690°C 530°C

880°C

3rd stage 630°C

4th stage

850°C

(Precalciner) 750°C 1200°C

Drying Calcination

Burning Zone

Cooling

Friday, February 5, 2021

ADVANTAGES OF PRECALCINING (In a Preheater System) 

Smaller dimensions of rotary kiln  Lower specific thermal load of kiln tube  Increased refractory life  Possibility of large capacities (8000 t/d per kiln)

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Independent calcining fuel control  More stable kiln operation  Fire low grade fuel in precal

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Lower downtime (stability, refractory life)

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Increase capacity of existing pre-heater kilns

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Volatile problems  more easily controlled Friday, February 5, 2021

PRECALCINER COMBUSTION PERFORMANCE CONCERNS 1) Fuel Distribution 2) Retention Time 3) Air-Gas supply favorable 4) Raw Mix / Fuel / Air Mix Uniform Friday, February 5, 2021

THERMAL PROFILES Dry Process

Wet Process

Clinkering

Cooling

4450

2200

3850

2000

3500

1800

3150

1600

2800

1400

2450

1200

2100

Gas

1000

1750

800

1400 Material

600

1050

400

700

200

350 20 10

Feed End

80 20

100

140

30

40

180 50

60

220 70

260

300

340

80

90 100 110

380

420

460

120 130 140

500

540

150 160

2400

Slurry preheat

Evaporation

Feed preheat

Calcination

Clinkering

Cooling

4450 4200

2200

3850

2000

3500

1800

3150

1600

2800

1400

2450

1200

2100

1000

Gas

1750

800

1400

600

1050

400

350 20

ft m

700

Material

200

10

80 20

100

140

30

40

180 50

60

220 70

260

300

340

380

80

90 100 110

420

460

500

540 ft

120 130 140 150 160

m

Feed End

Friday, February 5, 2021

deg. F

4200

deg. C

Calcining

deg. F

deg. C

Feed preheating 2400

THERMAL PROFILES Pre-heater

Calcining

Clinkering

Cooling

4450 4200 3850

2000

3500

1800

3150

1600

2800

1400

2450

1200

2100

Gas

1000

1750

800

1400 1050

Material

400

700

200

350

Feed preheating

Calcining

Clinkering

40 10

60 20

80

100 30

120 140 40

4450 4200

2200

3850

2000

3500

1800

3150

1600

2800

1400

2450

1200

Gas

2100

1000

1750

800

1400 Material

600

1050

400

700

200

350 20

20

Cooling

deg. F

2200

600

2400

deg. F

deg. C

2400

deg. C

Preheating

Pre-calciner

40

60

80

100

120

140

ft

ft 10

m 1

2

20

30

40

m

3

Flash Calciner

Friday, February 5, 2021

ZONES OF THE KILN

1) Evaporation of free water 2) Dehydration of water combined in clay 3) Calcining 4) Sintering 5) Cooling

Friday, February 5, 2021

EVAPORATION ZONE (100 - 400 °C) 100-400 °C: H2O (l) + heat  H2O (g) H = + 44.2 kJ/mol Wet Process 2400

Slurry preheat

Evaporation

Feed preheat

Calcination

Clinkering

Cooling

4450 4200

2200

3850

2000

3500

1800

3150

1600

2800

1400

2450

1200

deg. F

deg. C

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2100

1000

Gas

1750

800

1400

600

1050

400

700

Material

200

350 20 10

80 20

100

140

30

40

180 50

60

220 70

260

300

340

380

80

90 100 110

420

460

500

540 ft

120 130 140 150 160

m

Feed End

Friday, February 5, 2021

DEHYDRATION ZONE (350 - 650 °C) 

350-650 °C:  Clay loses water of crystallization 2SiO2•Al2O3•2H2O + heat  2SiO2•Al2O3 + 2H2O H = +274 kJ/mol

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400 °C:  Decarbonization of magnesium carbonate MgCO3 + heat  MgO + CO2 H = +120 kJ/mol  Vapourization, oxidation of organics & suphides FeS2 + O2  Fe2O3 + SO3

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550 °C:   

Decomposition of calcium carbonate 900 °C in pure state occurs sooner due to impurities and acidic environment Friday, February 5, 2021

CALCINING ZONE (600 - 1200 °C) 

600-900C  break down of clays minerals into oxides Al2O3•SiO2  Al2O3 + SiO2

 solid solution reactions with clay oxides CaO, SiO2, Al2O3  CA, C12A7, CS, C2S.CC, C2S 

850-900°C  rapid decomposition of calcium carbonate CaCO3  CaO + CO2 H = +474 kJ/mol

 free-lime reacts first with silica 2CaO + SiO2  2CaO.SiO2

H = -143 kJ/mol

 free-lime then reacts with alumina & iron 2CaO + Al2O3, Fe2O3  2CaO.Al2O3, 2CaO.Fe2O3 Friday, February 5, 2021

CALCINATION ZONE SPEED OF DECARBONATION 

Temperature of the material,  determines the partial pressure of decomposition of CaCO3 to CO2.

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Gas temperature  controls the transfer of heat from decarbonated material to the crystals of SiO2, Fe2O3, Al2O3

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The partial pressure of CO2,  sum of the decomposition pressure of CaCO3 and the pressure of the combustion gases

Friday, February 5, 2021

CALCINING ZONE (600 - 1200 °C) 

Also at 850-900°C  Free CaO combines with SO3 to give anhydrite CaO + SO3  CaSO4

 SO3 affects the compounds formed CaO + Na2O, K2O  Na2SO4, K2SO4, 3K2SO4.Na2SO4

 If alkalis in excess Na2O + C3A  NaC8A3 K2O + C2S  KC23S12

Friday, February 5, 2021

ALKALI SULPHATE REACTIONS (850-900 °C) 

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Changes in alkali balance may change quality  cycling which is often not noticed in clinker sampling  erratic setting times, strengths, flow problems Build-ups, rings 3(K2SO4.Na2SO4)  double salts: calcium langbenite, syngenite  flow problems NaC8A3  quicker setting time, higher water demand  lower strength development KC23S12  behaves like C2S; but stable compound, will not form C3S  lower C3S (strength), higher f-CaO (setting time)

Friday, February 5, 2021

BEGNINING OF BURNING ZONE KILN OPERATORS VIEW  

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Upper transition (5-15 min) Exothermic clinkerization reaction starts  analoguous to a combustion  beginning of reaction termed “Ignition Point” Ignition Point  temperature of the material rises very quickly  visually look up the BZ and under the flame  material changes from black to white within 5-6 ft As the clinkerization progresses  presence of the flux  the material starts to stick on walls and coating  then starts tumbling at an angle of ~45 degre

Friday, February 5, 2021

CLINKERING ZONE (1200-1450 °C) 

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1200°C  Belite (C2S) is completely formed 2CaO + SiO2  2CaO.SiO2 H = -125 kJ/mol  C12A7, CaO becomes C3A  Solid solution of C4AF 4CaO + Al2O3 + Fe2O3  C4AF H = -50.4 kJ/mol 1250- 1300°C  C3A and C4AF liquefy and constitute the flux.  Alkalis, SO3 volatilized R2SO4, CaSO4 + heat  R2O, CaO + SO2 + 1/2 O2  if reducing condition: Fe2O3  2FeO + 1/2O2 1310-1450 °C CaO + C2S  C3S H = -125 kJ/mol  facilitated by presence of liquid; also solid-solid reaction Friday, February 5, 2021

END OF BURNING ZONE KILN OPERATORS VIEW 

Lower limit :  Difficult to precisely define end of BZ  Could be at 2-3 or 15 feet from the nosering  Depends on the temperature of the material in the burning zone  Position of the burner pipe important

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One arbitrary rule is that the front end of the coating is where the burning zone ends

Friday, February 5, 2021

COOLING ZONE (1400-1250 °C) 

1400°C to 1250°C  -C2S crystallizes to the more hydrolizable ß-C2S  If slowly cooled C3S will decompose to C2S, CaO (Birefringence)  Alkali sulfates precipitate from the melt  C3A and C4AF crystallize  Molten sulfates crystallize

Friday, February 5, 2021

PHASE DIAGRAM

Friday, February 5, 2021

HEAT OF REACTION IN THE KILN HEAT ABSORBED (kJ/kg Clinker) 1) Heating of raw materials from 20 to 450°C 714 2) Dehydration of clay 168 3) Heating of material from 450° to 900°C 820 4) Decarbonation at 900°C 1995 5) Heating the decarbonated material from 900° to 1400°C 525 6) Heat for melting 105 4327 1) 2) 3) 4)

HEAT LIBERATED Crystallization of dehydrated clay Formation of clinker phases Cooling of clinker from 1400°C to 20°C Cooling of gases from 900°C to 20°C

42 420 1512 588 2562

Heat necessary for producing 1 kg clinker

1765 Friday, February 5, 2021

CONCLUSION 

Kiln system design  driven by capital cost reduction  each system has advantages & disadvantages  Thermal loading is important concept

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Heat input  calcination - quanitity  clinkerization - quality

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Complex, obscure thermo-reactions  explain operational reactions  important impact on quality Friday, February 5, 2021

GLOSSARY Terminologies Primary air: Secondary air: Tertiary air:

Combustion air introduced through a burner pipe. Combustion air from an adjacent part of the process. Typically, hot gasses from the clinker cooler. Combustion air introduced from a point in the process not adjacent to the burner. Typically, hot clinker cooler gases used in the precalciner. Kiln back end: Feed end of the kiln, kiln inlet. Kiln front end: Kiln clinker discharge. Burning zone: Hottest area of the kiln. C3S is formed here. Calcining zone: Area of kiln/preheat tower where the material is heated, driving off all gases short of fusing it. Clinker cooling zone: Front end at the kiln where clinker is somewhat cooled prior to discharge. Clinker quenching: Quick, air cooling of clinker in the clinker cooler. ID fan: Induced Draft fan used to draw air through the kiln Kiln coating: Natural build-up on the kiln refractory that protects the refractory and insulates the kiln. Volatilization: Evaporation of elements at high temperatures. Friday, February 5, 2021