The Optimum Kiln System

9 )Pneumatic seals at both ends

4 )Hot spot (e.g. chamber) staged meal feed

2 )Large in-line calciner with AFR feeder

1 )5 stage preheater with low pressure drop cyclones

3 )Low NOx with staged TA swirl chamber

6 )High momentum multi channel burner

5 )Oversized kiln for AFR

7 )High efficiency cooler

10 )Splined tyre fixation

8 )Roller crusher

Pre-Calciner: Typical Problems

8 )Tertiary air duct blockage (elbow type only)4 )Too high / low

calcination degree

2 )Locally too high temp.

1 )CO at outlet of PC or cyclone

3 )Unburnt fuel particles in hot meal

6 )Refractory damage

5 )Material build-ups

7 )Tertiary air damper failure

Main Benefits of Precalciner Technology1 .More stable kiln operation due to better kiln controlvia two separate

fuel feed/control points

2 .More stable kiln operation due to controlled meal conditions at kiln inlet

3 .Reduced thermal load of burning zone

4 .Lower refractory consumption as a result of 1. to 3.

5 .More than double capacities possible with given kiln (10'000 t/d: 6 x 95m)

6 .Possibility of increasing capacity of existing kilns

7 .Reduced volatilisation of circulating elements

8 .Reduction of cycles (S, Cl, Na2O, K2O) with lower bypass rate / losses

9 .Makes short kilns possible with 2 stations, L/D < 12

10 .Possibilities of NOx reduction

11 .Lump fuel (AFR) utilization (in-line only)

True and Apparent Calcination Degree• True calcination degree:

Degree to which the calcination is completed, i.e. extent to which the CO2 is dissociated from the CaCO3.Extremes: Raw meal 0% (LOI=35%)

Clinker 100% (LOI= 0%)• Apparent calcination degree:

The calcination degree determined from a hot meal sample taken from the meal duct of the bottom cyclone

Dimensioning Criteria for Precalciners1. Gas Retention Time (for combustion in pure air)

decisive for complete combustion

Fuel Reactivity Gas Retention Timelow >3.5 sec

medium >2.5 sechigh >2.0 sec

2 .Meal Retention Timedecisive for complete calcinationActual meal retention times are 6 to 12 seconds,at the above gas retention times.

Calcination takes much less than that whichmeans that meal retention time is not a

decisive design criteria.

Fuel Reactivity Examples:Low: Petrol cokeMedium: Bituminous coal, natural gasHigh: Lignite, fuel oil

Inline Calciners:Due to less favourable conditions for combustion

(presence of kiln gas, imperfect mixing of tertiary air with kiln gas), the following rule of

thumb can be used for sizing this PC type:

Recommended gas retention time + 0.5 to 1 sec

Cyclone Preheater: Typical Problems

8 )Splash plate worn; splash box not clean

4 )Cyclone blockage

2 )False air

1 )Dip tube missing or damaged

3 )Meal deposit

6 )Meal flaps not working

5 )Too high velocity

7 )Thermocouple bad location

Counter-Current Heat Exchange

Co-Current Heat Exchange

Energy Turnover in a Grate Cooler

Cooler Efficiency h cooler

Q combustion air S Q loss

h cooler = = 1-

Q clinker from kiln S Q clinker from kiln

Tube Cooler

Problems of Conventional Grate Coolers

• Uneven clinker distribution• “Red River”• Air breaking through

(“Geyser”)• Overheated / burnt plates• Thin clinker bed• Poor UG-compartment sealing-> Poor recuperation (low hth)

Benefits of Modern Cooler Technology

• Smoother cooler operation• More stable kiln operation• Good recuperation (high hth)• Control of “Red River”• Less or no clinker fall

through• Smaller waste air system• Less space required• (Lower power consumption)

Fixed Grate CoolersSF Crossbar Cooler (FLS Fuller)Main Features• First radically different grate cooler!• Fixed line; no moving rows• Mechanical flow regulators; 1 per plate• Crossbars for clinker conveying• Standard modules; pre-installed• Different speeds across width possible• Size < 45 t/d m2• Air installed: ~2.0 Nm3/kg

Influence of Ratio on burning Process and Chemical CompositionInfluence on clinker

compositionInfluence on

burning process Up and down

Ratio

C3A C4AF

C3A C4AF

C3A C4AF

C3A C4AF

C3S C2SFree lime will increase

C3S C2S

Difficult to burnاLower ClinkeringTemperatureLower clinkerTemperature

Clinker reaction acceleratedDifficult to burn

Easy to burn

high

Low

High

Low

High

Low

1-Silica ratioSiO2

=Al2O3+ Fe2O3

2-Alumina ratio= Al2O3Fe2O3

3-Lime saturation factor= CaO

2.8SiO2 +1.18Al2O3+0.65Fe2O3

OrCaO

2.8 SiO2+0.65Al2O3+.35Fe2O

5 Stages of Hydration

Reaction Stage Kinetics of Reaction Chemical Process

1 .Initial hydrolysis Chemical Control : Rapid Dissolution of Ions

2 .Induction Period Nucleation control : low Continued dissolution of ions

3 .Acceleration Chemical Control : Rapid Initial formation of hydration products

4 .Deceleration Chemical and diffusion control : Slow

Continued formation of hydration products

5 .Steady State Diffusion control : Slow Slow formation of hydration products

Stage 5: Densification/ Steady Stage (can continue for years) Start of belite reactions and they can continue for years.Belite reactions also produce C-S-H and CH, forming a solid mass.

Longer length of this stage gives:•Greater concrete’s strength•Lower permeability•Greater durability

To promote continued hydration, moisture must be retained in the slab as long as possible

Factors Affecting Hydration

Major factors :-• Chemical Composition of Cement• Cement Type• Sulfate Content• Fineness• Water/Cement Ratio• Curing Temperature• Effects of SCMs and Admixtures

Mechanism of Heat generation

Reaction of calcium Silicates :

2 C3S + 7H C3S2H8 + 3CH H= -500 J/g

2 C2S + 7H C3S2H8 + CH H= -250 J/g

Reaction of tri calcium aluminateC3A + 3 CSH2 + 26 H C6AS3H32 H = -1350 J/g