K-Precalciners1.pdf

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Learning Objectives Precalciners

 Understand the principle of precalcination.

 Know the basic precalciner types

 Know the main features of the various precalciner types

 Know the advantages of precalcination

 Understand the principle of low NOx precalciners

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Precalciners: Content

Content

 Principle and theoretical aspects

 Precalciner design

 ILC precalciners

 Pre-combustion chambers

 Low NOx precalciners

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Energy Balance of Process Steps for Clinker Burning

Endothermic Processes: kJ/kg cli kcal/kgcli

Dehydration of clays 165 40

Decarbonisation of calcite 1990 475

Heat of melting 105 25

Heating of raw materials 2050 490

(0 to 1450 °C)

Total endothermic 4310 1030

Exothermic Processes: kJ/kg cli kcal/kgcli Recrystallistion of dehydrated clay 40 10 Heat of formation of clinker minerals 420 100

Crystallisation of melt 105 25

Cooling of clinker 1400 335

Cooling of CO2 (ex calcite) 500 120 Cooling and condensation of H2O 85 20

Total exothermic 2550 610

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AT (Air Through) and AS (Air Separate)

PA TA PA SA Definitions: PA = Primary Air SA = Secondary Air TA = Tertiary Air TA

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Precalciner – Preheater Arrangements

Pre-Combustion

In-Line Off-Line Hybrid Separate Line

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Requirements I

 Process:

High calcination degree

Good control of calcination degree

No material drop-out (meal, ashes etc) No build-ups on walls

Simple and rapid start-up procedure Forgiving operating behaviour

Flexibility regarding fuel ratio BZ / PC Safe regarding equipment overheating Minimum primary air requirement

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Requirements II

 Combustion:

High burnout degree of fuels Best mixing of air (O₂) with fuel Instant ignition of all fuels

Direct return of fuel residues to kiln

Optimum combustion monitoring / control

 Fuels:

Suitable for all fuel types (high flexibility) Insensitive to changes of mix of fuels

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Requirements III

 Emissions:

Reduction of pollutants from BZ

No (low) generation of pollutants in PC firing Possibility for SNCR

 Tertiary Air System:

No dust deposits and dust cycles No hot dust handling

Reliable O₂-control for BZ and PC firing

 Design:

Easy integration in preheater Stepwise upgrading possible

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Precalciner Control

 Via Fuel Rate

Normally 45% to 60% of total fuel

 According to Calcination Degree

of bottom cyclone hot meal (via LOI);

Normally 85% to 95% apparent calcination degree

 According to Gas Temperature at exit of bottom cyclone;

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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

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O

₂

Control for PC/BZ: Tertiary Air Damper

p_{ID fan}

p_{C5 exit}

Var. Dp₁

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O

₂

Control for PC/BZ: Kiln Riser Orifice

p_{ID fan}

p_{C5 exit}

~p_amb Dp₁

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Precalciner Elements

 Long tubes “gooseneck type”  Large volume “vessel type”  Tertiary air frontal impact

 Tertiary air tangential inlet

 Orifice

 Bends, curves and vessels

 Multiple burners

 Hot spot with and without control

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Dimensioning Criteria for Precalciners

1. Gas Retention Time (for combustion in pure air) decisive for complete combustion

Fuel Reactivity Gas Retention Time low > 3.5 sec medium > 2.5 sec high > 2.0 sec

2. Meal Retention Time

decisive for complete calcination

Actual meal retention times are 6 to 12 seconds, at the above gas retention times.

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

decisive design criteria.

Fuel Reactivity Examples:

Low: Petrol coke

Medium: Bituminous coal, natural gas High: Lignite, fuel oil

Inline Calciners:

Due to less favourable conditions for combustion (presence of kiln gas, imperfect mixing of

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Standard Precalciners

In-Line and Off-Line AT

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Pre-Combustion Chambers

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Low NOx Precalciners

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Precalciners for Lump Fuels

(Tires etc): Future Development

FLS, Polysius, KHD,

Blue Circle, Ash Grove

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PC for Solid AFR: BC-Lafarge AFR Ram

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Pre-Calciner: Typical Problems

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

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Main Benefits of Precalciner Technology

1. More stable kiln operation due to better kiln control via 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