04.04 Kiln Operations Guide Lines_ENG

KILN OPERATIONS GUIDE LINES

June 2006 / Version 1

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Our objectives • To produce more • To produce with regularity • To produce cheaper

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Stable kiln operation is key to achieving our objectives Rules R1 - Constant kiln volume load R2 - Constant cooler clinker bed depth R3 - Predefined tables for cooler fans R4 - Constant slightly negative kiln hood pressure R5 - Constant Oxygen and in excess at kiln back end R6 - Oxygen in excess at tower exit R7 - Constant ID fan outlet pressure R8 - Fuel amount proportional of to kiln feed rate – Production ramp up R9 - Constant calcination R10 - Maximize production to minimize heat consumption Prerequisites P1 - Raw mix preparation P2 - Fuel preparation P3 - Burner adjustment P4 - Reliable sensors P5 - Free lime follow-up 2

Rules R1 - Constant kiln volume load R2 - Constant cooler clinker bed depth R3 - Predefined tables for cooler fans R4 - Constant slightly negative kiln hood pressure R5 - Constant Oxygen and in excess at kiln back end R6 - Oxygen in excess at tower exit R7 - Constant ID fan outlet pressure R8 - Fuel amount proportional of to kiln feed rate – Production ramp up R9 - Constant calcination R10 - Maximize production to minimize heat consumption

3

R1 – Constant kiln load volume Why? Because Operating the kiln with an irregular bed depth • • • •

Makes the clinker pour irregularly into the cooler Influences heat exchanges between gases and material Impacts on the state of combustion in the burning zone Makes the kiln torque signal to be less representative of the kiln state

Indicators The kiln speed is proportional to the kiln feed

kiln speed = k · kiln feed The k factor characterizes each kiln In general, maximum kiln speed is reached at maximum kiln feed

Recommended value Kiln speed/kiln feed ratio has •to be calculated by the process engineer
Takes into account kiln dimensions, process, clinker reactivity.

Wet kiln

Grate preheater

Preheater

Precalciner

1.2 rpm

1.2 rpm

2 rpm

3.5 rpm

4

For process with grate coolers

R2 – Constant cooler clinker bed depth Why? To maximise & regularise heat recuperation

Indicators Constant bed depth

= constant under grate pressure measured in the first chambers

= Adjust cooler grate speed and never change the fan volume that impacts this under grate pressure…

Recommended values Cooler bed depth must be maximized. On new cooler generations, the clinker bed depth measured on the first grate can range from 500 to 800 mm.

5

For process with grate coolers

R3 – Predefined tables for cooler fans Why? • • •

To maximise heat recovery To supply secondary and tertiary air To cool enough the clinker and avoid clinker transport problems and maintain cement mills operation efficiency

Indicators Air flows Outlet cooler clinker temperature

Recommended values •In the first chambers •fluidized clinker bed •fixed fan volume, air flow density must be constant Chamber# Air flow density (Nm3/m2/.s)

1

2

3

1,5

1,3

1

•In the last chambers •non-fluidized bed •cooler fan volume proportional to clinker production rate. Make specific air flow (Nm3/kg clinker) adjustments in the last chambers to keep a Airflow/Clinker ratio constant Average air flow : 1,8 – 2,5 Nm3/kg clinker •Clinker temperature at the cooler outlet • Good performance = 100°C • Unacceptable > 150°C (cement false set…) • It is best to avoid having too cold clinker; the temperature inside the cement mill may not be sufficient to form enough semi-hydrates in the cement. 6

R4 – Constant slightly negative kiln hood pressure Why? • Positive pressure

gas/material puffing, spillages (safety issues)

If hood pressure is too negative

Impact

Risk of

Hood air inleak (false air)



Discharge end ring

Secondary air




CO

Levers Controlled by cooler exhaust fan which extracts excess air from the cooler

Recommended values Kiln hood pressure setting point • –2 to –7 mmWG. • depends on its stability: the more stable the pressure, the closer to zero it can be.

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R5 – Constant oxygen & in excess at kiln back end Why? • To avoid formation of CO which gives less energy where we want it C + O2 CO2 + 97.6 kcal C + ½ O2 CO + 29.4 kcal • To minimize volatilization & cyclic operations

• To guarantee uniform clinker quality especially sulfate fluctuations

Levers • Kiln back end O2 Increase Kiln back end O2 set point

If

CO level at kiln back end

> 100 ppm

Hot meal analysis on stage 4 •SO3

> 2,5 %

• Kiln back end O2 Standard deviation

Recommended values • For Precalciner kiln, the O2 set point ranges from 3 to 5% • Standard deviation measured by 1-minute average values over 24 hours 4,5 4

• Very good if

σ O < 0.2% 2

3,5 3 2,5 2 1

3

5

7

9

11

13

15

17

mi nutes

• Very bad if

σ O > 0.4% 2

19

21

23

25

27

29

31

4,5 4 3,5 3 2,5 2 1

3

5

7

9

11

13

15

17

19

21

2

2

2

2

31

minut es

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R6 – Oxygen in excess at tower exit Why? • Too low level of O2 will result in the formation of CO in preheater tower & possible explosion in the electrostatic precipitator: When the CO produced finds some oxygen, it burns as follows CO + ½ O2

•

CO2 + 68.2 kcal Explosive reaction if there is a spark!

Too high level of O2 will result in possible loss of production

Indicators • Tower exit O2 value • Also Take into account
Carbon included in the raw material
False air

Recommended values

• Generally 3% < O2 < 5%

9

For process with EP fan

R7 – Constant ID fan outlet pressure Why? • Stable pressure will prevent perturbations from raw mill, GCT to the kiln If ID fan outlet pressure positive Circuit head loss Fan curve ID fan volumetric flow Qv

O2


Indicators • ID fan outlet pressure must be controlled by the EP fan damper or variable speed drive.

Recommended values Depend on stability
ID fan outlet pressure must be slightly negative but as close to zero as possible (-10 mmWG).


If ID fan at the maximum limit with margin on EP fan, ID fan outlet pressure set point can be more negative.

10

R8 – fuel amount proportional to kiln feed rate – Production ramp up Why? Kiln feed increase

Fuel t/h 8,00

7,00

more material to be burned

6,00

B

5,00

4,00

proportional fuel increase Fuel = A · kiln feed + B B = constant function of heat wall losses

3,00

2,00

A

1,00

0,00 0

10

20

30

40

50

60

70

80

90

100

Kiln feed t/h

Ratio

Kiln Speed

Feed rate Ratio

Burner Fuel

Draft

Ratio

Indicators • Kiln specific heat consumption (SHC)

SHC Specific Heat Consumption kcal/k ck

1200 1175 1150 1125 1100 1075 1050

A

1025 1000 975 950 925 900

B

875 850 825 800 18

Recommended values •Create operation table for kiln feed to fuel rate taking in consideration • Kiln specific heat consumption • Heat wall losses • Fuel calorific value

23

28

33

38

43

48

53

58

Clinker t/h

•Questions to be raised in case of drift on SHC (A or B) • Calibration of the feeders • Raw mix chemistry • Raw mix uniformity • Fuel Calorific Value 11

For process with secondary burner

R9 – Constant Calcination Why? To avoid a shift of the burning zone.

Ignition point = Start of liquid phase

Calcination
Calcination  Indicators Bottom cyclones and riser duct temperatures Calcination level of bottom cyclones hot meal

Recommended values •Calcination level is controlled by temperature probe considered as representative (bottom cyclones, riser duct) with a control loop acting on secondary burner fuel rate. Set point around 850°C •The main burner / secondary burner ratio must be mastered on continuous basis to detect any drift (probe build-ups…). The main burner / secondary burner ratio does not vary too much (example: Precalciner kiln main burner = 40%, secondary = 60%) Type of kiln % calcination Preheater AT

55 - 70%

Preheater AS

90 – 92% 12

R10 – Maximize production to minimize Heat consumption Why? The maximum production minimizes the heat consumption:

SHC Specific Heat Consumption kcal/k ck

1200 1175 1150 1125 1100 1075 1050 1025 1000 975 950 925 900 875 850 825 800 18

23

28

33

38

43

48

53

58

Indicators • If the ID fan is at nominal ventilation : the only actuator to maintain kiln condition is the feed rate • If the ID fan is below nominal ventilation, give priority to feed rate and use fuel and ID fan to control combustion

Recommended values

ID fan draught margin

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Prerequisites P1 - Raw mix preparation P2 - Fuel preparation P3 - Burner adjustment P4 - Reliable sensors P5 - Free lime follow-up

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P1 - Raw mix preparation
Raw mix residues targets

100µm

10%

200µm

1%


Lime saturation

100C LSF = 2.8S + 1.18 A + 0.65 F

Range between 90 and 98 depending on fuel ashes and quality target

100(2.8S + 1.65 A + 0.35F − C ) ∆bc = S + A+ F +C

Range between –4 and +4 depending on fuel ashes and quality target

C3S = 4.07C − (7.6Ssol + 6.72A+ 1.43F) C3S is the potential C3S content of clinker when free lime is zero and calculation LOI=0. (Potential C3S target also depends on the chemical composition of the ashes generated by the fuel.) Potential C3S contained in raw mix as target is more sensitive than bc, LSF but bc, LSF calculations are more robust since these 2 lime saturation factors are less influenced by FX drifts.

15

P1 - Raw mix preparation
Effects of fuel ashes • Fuel ashes exhibit a very significant deficit in C in relation to S content (very high lime deficiency) • It must be compensated by using the lime from the raw mix to combine the excess S. • The C3S of the raw mix that have to be designed will be higher than the targeted C3S in clinker.

C3Sclinker = a.C3Sraw mix + b.C3Sashes
Example

with (a + b) = 1

%Ash fuel SHC . 100 b= Ratio (feed/clinker) . PCI fuel Fuel

when the free lime is zero

Kiln

Calorific Value (PCI)

ash

Heat Cons. (SHC)

J/g

%

J/g kk

24000

20,0

3500

Feed / kk

1,54

b = 0.019 ⇒ a = 0.981

C3Sclinker− b.C3Sashes C3Sraw mix = a 65− 0.019(-258) C3Sraw mix = = 71.2 0.981 16

P1 - Raw mix preparation


Silica ratio

S A+ F

SR =

SR

SR: 2.3 to 3.0 constant at ± 0.05

Burning

Liquid phase

Coating

Clinker

Cement strength

Low

Easy

Excessive, attack to bricks

Too thick, unstable

Balling, hard

Low

High

Hard

Low, high thermal load

No

Dusty, high free lime

High


Alumina ratio

AR = AR

A F

AR: 1.3 to 2.0 constant at ± 0.05

Liquid phase

Clinker

Cement strength

Low

Fluid

If AR 30, an emergency action plan must be implemented High KFUI: Possible reasons

Prehomogenization Mining

Variable materials quality

KILN FEED UNIFORMITY

Blockages

Raw mill feeders accuracy Homogenization

Analyser

Kiln dust management 18

P2 - Fuel preparation
Solid fuel residues Target

S fuel < 4%

200 µm

0% < (0.5×VM*) %

S fuel > 4%

90 µm

63 µm

*VM: volatile matter
Liquid fuel • Keep viscosity 2.0% under burning • 0.5% < Free CaO < 2.0% well burnt • Free CaO < 0.5% over burning

•Free lime stability

FLUI =

σ free lime 0.2 × free lime + 0.1