Heat optimisation pradeep kumar

Optimisation of heatconsumption

Heat loss from shell radiation

Heat loss inproduct

Heat loss inPreheaterExit gas

Cooler exit air

Where does the heat go ?

For perpetual pyro process in a kiln the heat required is only heat of clinker mineral formation,ie., 380 - 400 Kcal/kg clinker. 280 – 350 Kcal/kg clinker is wasted which is about 40 - 45 % .The dream of design engineer is to make heat losses to minimum and how to optimize the heat consumption.

The preheater heat losses

Pre heater gas temperatureDesign of cyclonesNo of stagesDust lossInlet / out let velocity ratioLocation of meal distribution boxes

Preheater( Surface) heat losses through radiation

Calciner gas retention timeCombustion effciency

Coal residue & raw meal residueLocation of firing nozzles

Different flames

Normal flame

Flame with lowSecondary air tempDistorted nozzle

Flame –poorhood geometryOr distorted nozzle

Flame at the center

Flame downward

Flame upward

Flame length

Long flame, unstable coating,High back end temp Low shell temperature

Short intense divergent flameGood for burningLow back end temperaturePoor refractory life, highShell temperature

Convergent flameGood for burningGood for refractoryStable coatingLow shell temperature

The Ideal Flame

hot !short !stable !

T"long" flame

"short" flame

Complete combustion:- CO = 0- SO2, NOX ↓

Homogeneous:- no temperature peaks- no local CO on the clinker bed

Longer flame increase the back end temperature resulting inHeat loss at kiln exit and hot meal clogging

Burning zone, Flame-profile• Low momentum burner

• High momentum burner

rings12m (~3xD) burning zone

Rotaflam ~16 m Flame !☺!

rings

~23 m Flame

17m (~4xD) burning zone

! !

Burner Operation

Duo Flex burner

M.A.S burner, unitherm

Pyrojet

Greco

Lafarge Multi Channel burner

Pillard Roto Flame

Cement kiln flame types

Straight flame – essentially external recirculation

Type-1 flameWeak internal recirculation & external recirculation

Type-2 flameStrong internal recirculation & external recirculation

Objectives

“It is desirable to operate the kiln at the lowest fuel consumption. This must be consistent with the highest practical output at an acceptable market quality.”

WHY TO DO IT

• To get a detailed view of the kiln line performance• Evaluate exact data for heat consumption,

production,...• Basis for comparison

– impact of investment or modifications carried out

– comparison to other plants• Detect weak points - Action Plan• Detect optimization potential• Check of sensors, weigh feeder,...

Σ massin = Σ massout

MASS BALANCE

Σ heatin = Σ heatout

HEAT BALANCE

“Energy cannot be created or destroyed but may be converted from one form to another”

Energy in = Energy out

Boundary selection

• Any boundary shape can be chosen.• Every stream that crosses the envelope must be taken into

account.• The boundary line is chosen so that the boundary points

are:– useful for the balance goals– easily accessible for reliable measurements

KilnKiln

P/HP/H

CoolerCooler

Boundary selection

Dust Exists System Dust Does Not Exit Boundary

KilnSystem

Dust

Kiln Feed

Measuring point (t/h)

KilnSystem

Dust

Kiln Feed

Clinker Clinker

Boundary selection and streams

Kiln System

Primary air

Clinker

Fuel

cooling air

False airKiln feed

Kiln exitgases

Water spray

Exit dust

Returndust

S heatin = S heatout

Wall losses

Heat balanceCooler exhaust gas

Bypass gas and dust

Heat Transfer MechanismsHeat Transfer Mechanisms

• Conduction

• Convection

• Radiation

200°C 50°Cheat

ConductionConduction

• Transfer of heat from the hotter to colder part of a body

• By direct molecular contactFurnace wall

Hot gas

1200°C

Cold air

25°C

L

Q kAT TL

h c=−Q

ConvectionConvection

• Natural convection: – fluid moving from difference of density due to different

temperatures

• Forced convection:– fluid is moved by the action of an external device

hot airhot aircold air cold air

natural convection forced convection

( )Q hA T Tw f= −

RadiationRadiation

• Energy transferred by electro-magnetic radiation

( )Q A T T= −σ 14

24

2000°C 50°CQ

HEAT TRANSFER

• Radiant heat transfer

• Free convection(occurs by natural thermal draft, at low wind velocities)

• Forced convection (occurs at high wind velocities)

Convection

Radiation

Air

Chemical Reaction

• Endothermic reaction - heat is consumed– Calcium Carbonate breaks down to CaO (lime)

and CO2 when heated – it takes heat in Þ the reaction is endothermic.

• Exothermic reaction - heat is released– CaO (lime) reacts with Silica and the cement

minerals are formed– the process gives out heat Þ the chemical

reactions is exothermic.

Two types of heat

Latent HeatLinked to modification by chemical reaction,

change in state, change in structure

Sensible HeatAbsorbed or released by a

substance

The heat to remove from a material to cool it down to the reference temperature (usually 0ºC).

Q = M × Cp (T) × (T - T0)

M = specific mass Cp (T) = specific heat of a material at temperature TT = temperature of M

Sensible heat

Qf = mf × ( LHVf + Cpmean f (Tf) × Tf )Qf : heat from fuel (kcal/h)mf : fuel flow rate (kg/h)LHVf : fuel low heat value (kcal/kg)Cpmean f : mean specific heat of fuel (kcal/kg.ºC)Tf : fuel temperature (ºC)

ORh = m • CV

h : heat from fuel (kcal/kg clk)m : specific fuel consumption (kg/kg clk) CV : calorific value of fuel (kcal/kg fuel)

Heat from fuel

Incomplete combustion

• The kiln exit gases might contain some un burnt gases (CO, H2, CH4)

• The combustion heat from those fuels must be included as a out stream

Qic = mCO ×LHVCO + mH2 × LHVH2 + mCH4 ×LHVCH4

The heat loss through the gas can be calculated to:

h = m•(CO%•12640+H2%•10800+ CH4% • 35 840)m = specific gas quantity (Nm3/kg clk)

Heat of Reaction

• Heat of reaction is the difference between the heat absorbed in decarbonating the limestone and the heat released in forming the clinker minerals

• It should be noted that raw meal chemistry affects the reaction heat, the heat absorbed by the process gets bigger as the LSF of the materials rises

• 420 kcals/kg clinker is used if little else is known

Clinker theoretical heat of formation

• The heat required to form clinker from dry raw mix• ZKG formula (German formula):

Qt = 4.11 Al2O3 + 6.47 MgO + 7.64 CaO - 5.11 SiO2 - 0.60 Fe2O3

• If no clinker analysis: assume Qt = 420 kcal/kg ck• Must be added to the clinker heat content as latent heat or as a separate

output heat stream.

CaF2 addition reduce the heat of reaction considerably butIt has the other implications.

Heat of formation

• Heat of dehydration of clay (endothermic)

• Heat of decarbonation of CaCO3 + MgCO3 (endothermic)

• Heat of formation of clinker (exothermic!)

• General assumption for the three: 1750 kJ / kg clk 0r• 400 Kcal/kg cl

Qw : heat loss through wall (W)

atot : total heat transfer coefficient (W/m².C)

A : shell area (m²)

T : shell temperature (ºC)

Ta : ambient temperature (ºC)

( )Q A T Tw tot a= −α

Heat loss through kiln shell

0

5

10

15

20

25

30

35

40

45

50

55

60

65

100 200 300 400 500 600

T - T° (C)

W/M2C

v = 14 m/s wind

1312

11

10

9

8765

43

21

v = 0 m/s (free convection)

SS = 0.9 Ambient T° - 20°C

Global heat transfer coefficient

Radiation and convectionheat transfer coeffcient( Total)

Radiation and Convection

Shell Losses vs Shell Temperatures

Wind Velocity 0 m/s

Wind Velocity 1.5 m/S

SHELL TEMPERATURE ºC

Kca

l/(m

2.m

in)

25

0

22

5

20

0

17

5

15

0

12

5

10

50 100 150 200 250 300 350 400

Radiation losses = 4*10 -8 * ( T4- Ta4) Kcal/h m2

Convection losses = 80.33*((T+Ta)/2) -0.724*(T-Ta)1.333

Convection losses = 28.03*((T+Ta)/2) -0.351*V 0.805(T-Ta) *D -0.195*(T –Ta)

If wind velocity is > 3 m/s

Surface heat losses

0 2 4 6 81680

1690

1700

1710

1720

1730

700

750

800

850

900

950

1000

Exit Oxygen %

kcal

/kg

wet

kcal

s/kg

(dr

y)

WET

DRY

Kiln Heat ConsumptionEffect of Kiln Exit Oxygen

How is cooling accomplishedHow is cooling accomplished

Heat transferby radiation

and convection Heat movesto clinker edgeby conduction

Air flows overclinker cooling

surface

Heat Transfer in ClinkerHeat Transfer in Clinker

• Convection - Surface to Air• Conduction - Inside to Surface• Heat transfer is driven by temperature difference

• Takes place at the clinker surface

• To maximize it:– Increase the air/material contact time with:

• Deeper bed ( ⇒ more power)• Slower air flow (⇒ larger cooler)

Heat Exchanger TypesHeat Exchanger Types

CounterflowParallel flow

Co-current

Air

Material

Air

Material

Cross-flow

Material

Air

material

air

T

material

air

T

material

T

Old conventional grate platescreate sand blasting effect or fluidizationThis creates poor heat exchange

Modern cooler plates flow resistancebranch off the air , createsless fluidization , better heat exchange

Cross flow Counter current

Mechanical flow regulator

Heat Exchange Between Clinker and AirHeat Exchange Between Clinker and Air

Temperature

Bed

thic

knes

s

clinker

air

Fixed bed

Fluidized bed

Air in

Air out

Clinker

Air in

Air out

Clinker

Temperature

Bed

thic

knes

sclinker

air

More efficient recovery with fixed bed

General Truths (All Coolers)General Truths (All Coolers)

1. The hotter the inlet temperature the hotter the clinker outlet temperature.

2. The hotter the cooling air temperature the hotter the clinker outlet temperature.

3. The longer the air/material contact time the cooler the clinker outlet temperature.

SYSTEM DATA COLLECTION

• Process• Type of kiln• Nominal capacity• Supplier• Fuel and firing system• Type of burner nozzle• Dust reintroduction system• Dimensions of main equipment• Data on fans, drives, etc.

OPERATING DATA

• Various operating data (rpm, kW, temperature and pressure profiles along kiln system, grate speed, undergrate pressures, etc.)

• Electric power readings (before / after test)

• Chemical analysis of raw meal, dust(s) and clinker, LSF, SR, AR, etc.

Kiln

Gas outlet

Wall lossesmisc

Clinker formation

Clinker heat Cooler vent

Sec.airTert.air

fuel

Sec.air

misc

Tertiary air

Kiln Heat balance

Cooler heat losses

Optimised air flow &air distribution & sealing

Clinker nodulesFiner fraction &big balls causesbad heat exchange

Radiation losses from walls

Exit air temperature

Measurement Plan

• Duration of an audit ?• What to measure? How to measure?

– Material balance– Gas flows – Heat Balance

• Frequency of sampling and measurements? • Which analyses have to be carried out ?• Which further data are to be collected ?

• All referred to 1 kg clinker Production = .... t/h

• Reference temperature = 0°C Specific

• Ambient temperature = ...°C Heat cons. = .... kJ/kg clk

Specification Heat(kg/kg clk), Temp.(Nm3/kg clk)(kW etc.) °C (kJ/kg clk) (%)

Fuel combustion - primary firing -- secondary firing -

Burnable matter in kiln feed -Raw meal: sensible heatFuel: sensible heatPrimary air: sensible heatCooler air: sensible heat -

Total of inputs -100%

INPUT DATA SUMMARY

Specification Temp. Heat(kg/kg clk),(Nm3/kg clk) °C (kJ/kg clk) (%)(kW etc.)

Heat of formationWater evaporation: - kiln feed

- water spray (s)Exhaust gas: - sensible heat

- dust sensible heatdust CaO-loss- unburnt gases (CO, etc)

Cooler: - waste air sensible heat- middle air sensible heat- clinker exit sensible heat

Bypass losses: - sensible heatdust sensible heatdust CaO-loss- unburnt gases (CO, etc)

Radiation and Convection: - Preheater- Rotary kiln- Cooler- Tert air duct

Rest (difference)Total of outputs100%

OUTPUT DATA SUMMARY

1. INPUT from sensible heatFUEL from combustion

RAW MEAL from sensible heatfrom sensible heat of water

COMBUSTION AIR from sensible heat of all theair supplied (prim. sec.)

Total input

2. OUTPUTHeat of formationEvaporation of water from raw mealExhaust gas sensible heatDust sensible heatIncomplete combustion (CO)Clinker exit temperatureCooler exhaust gasesLosses due to radiation and convectionWater cooling (Recupol inlet chute)Difference

Total output

kJ/kg clk25

5560

2571

67

5750

1750237075425—59

100540

—152

5750

%0.4

96.7

0.40.2

1.2

100

30.441.213.10.4—

1.01.79.4—

2.6

100

Wet ProcesskJ/kg clk

153343

3017

20

3425

175050631421—50

2764524214

3425

%0.4

97.6

0.90.5

0.6

100

51.114.89.20.6—

1.58.1

13.21.20.4

100

Semi-Dry (Lepol)

kJ/kg clk13

3150

54—

6

3223

175013

63618—63

423297

—23

3223

%0.4

97.6

1.7—

0.2

100

54.30.4

19.70.6—

2.013.19.2—

0.7

100

Dry Preheater (4-Stage)

HEAT BALANCE EXAMPLES

Cooler Balance

Tertiary air Vent air0,65 Nm³/kgck 1,14 Nm³/kgck

Secondary air 719°C 293°C0,27 Nm³/kgck Coal mill Raw mill

1029°C 0,00 Nm³/kgck 0,00 Nm³/kgck

Clinker95.6001465°C

Grate surface 52,20 m²Standard load 44,0 t/d/m²

Cooling air Clinker2,07 Nm³/kgck 95.600

4°C 107°C

Elements of Mass BalanceElements of Mass BalanceTertiary air

Ventair

Comp 1 Comp 2 Comp 3 Comp 4 Comp 5 Comp 6 Comp 7 Comp 8

Secondaryair

clinker

clinker

mCK1

mCK2

mSA mTA mCM

mVA

mF1 mF2 mF3 mF4 mF5 mF6 mF7 mF8

Coalmill air

Rawmill air

mRM

Elements of Heat BalanceElements of Heat BalanceTertiary air

Ventair

Comp 1 Comp 2 Comp 3 Comp 4 Comp 5 Comp 6 Comp 7 Comp 8

Secondary

air

clinker

clinker

HCK1

HCK2

HSA HTA HCM

HVA

HF1 HF2 HF3 HF4 HF5 HF6 HF7 HF8

Walllosses

HWL

Coalmill air

Rawmill air

HRM

Mass BalanceMass BalanceIn Out

clinker (mCK1) clinker (mCK2)cooling air (SmFi) secondary air (mSA)

tertiary air (mTA)coal mill air (mCM)raw mill air (mRM)vent air (mVA)

In = Out

secondary air flow: calculated by differenceshould be validated against a kiln balance

Heat BalanceHeat BalanceIn Out

clinker (HCK1) clinker (HCK2)cooling air (SHFi) secondary air (HSA)

tertiary air (HTA)coal mill air (HCM)raw mill air (HRM)vent air (HVA)wall losses (HWL)

In = Out

secondary air heat : calculated by differencegood to validate it against kiln heat balance

secondary air temperature: calculated from secondary air heat

Temperature Stratification of air above Clinker BedTemperature Stratification of air above Clinker Bed

Secondary &Tertiary air

Air tocoal mill

Ventair

1400°C

300°C 250°C200°C

175°C 125°C100°C

1000°C700° C

500°C

400°C 150°C

Measuring Actual Bed DepthMeasuring Actual Bed Depth

floor

Cooling EfficiencyCooling Efficiency

clinkerin input heat clinkerby lost heat

=η

inck

outck

inck

outckinck

hh

1hhh

−=−

=η

Qualifies the cooling of the clinker but not the clinker cooler.More cooling is possible with more air but that does not improve the cooler efficiency.An efficient cooler would give same cooling with less air.

Cooler lossCooler loss

cooler loss = all heat not recovered by combustion air (secondary or tertiary)

cooler loss = heat content of clinker leaving cooler (hck out)

+ heat content of vent air

+ heat content of coal mill air

+ heat content of raw mill air

+ wall heat losses

Often a specification in supplier process performance guarantee

Operating results cooler

C

oole

r effi

cien

cy [%

]

Combustion air Nm³/kgKl.0,75 0,8 0,85 0,9 0,95

60

65

70

75

80

85

Standard - cooler New - competition REPOL RS

New-type coolers

Old-type coolers

Heat saving 1.Run the plant stable , continously with consistent production2.Minimise the false air ingress by giving efficient seals3.Optimise the flame as per our requirement4.Minimise the variation in airflow, meal flow and fuel flow rate5.Reduce the radiation losses by giving proper insulation6.Optimise the cooler operation and cooler efficiency7.Optimise the cyclone efficiency in the preheater8.Minimise the variation in chemistry of raw meal and ash in fuel

by efficient blending.9. Addition of mineralisers reduce the heat of reaction by 20 – 30 Kcal/ kg.cl after

thorough study on the rheological properties of cement. CaF2, Dolomite and slag are good mineralisers.

Set point

Naturaland acceptable variation

high variationNot acceptable Variation is a devil in any process

Satellite cooler

rotary cooler

grate cooler

Recuperation z one C ooling zone

static gratedirect aeration chamber aerationchamber aeration

Grate coolerWith stationaryinlet

Walking floorpyrofloor

Cross bar cooler

Improvement in

technology

Rotary disc cooler

MMD cross bar

IKN

Poly trackPyro floor

?

Pyro step

1.31Kg/kg cl0.1Dust in tertiary duct7

0.180.16%1Excess air in kiln12

-0.1-0.17Deg C10Temperature of primary air

11

0.220.22%1Excess air in calciner13

-0.25-0.17Kcal/kgcoal100Heat content in coal9

1615.7Kg/kgcl0.1False air through hood14

1918.7Kg / kg cl0.1False air through inlet seal

15

0.680.66%1Amount of primary air10

0.470.45%1Moisture in coal8

1.30.12Kg/kg cl0.1Dust from kiln6

1.11.11Kcal/kgcl1Heat of reaction5

-73-74.37%1Carbon in rawmeal4

5.65.6%1Hydrate water3

1.81.8%1Feed moisture2

-0.5- 0.57Deg c10Feed temperature1

Heat, kcal/kgclILC

Heat, kcal/kgclSLC

unitbyA change ins.noHeat calculation

1.11.1Kg / kg cl0.1Raw meal27

1.2Kg/ kg cl0.1False air cyclone C122


1.21.1Kcal/kg cl1standard Cooler loss28


Cyclone efficiency29

- 0.1%1Cyclone- K129



1918.8Kg/ kg cl0.1False air cyclone K521

1210.9Kg/ kg cl0.1False air cyclone K420

6.95.8Kg/ kg cl0.1False air cyclone K319



1615.7Kg / kg cl0.1False air calciner16

Heat, kcal/kgclILC

Heat, kcal/kg clSLC

unitbyA change ins.no

-0.26%1Cyclone- K533

-0.38-0.28%1Cyclone- C436

-23-21.8Change from 4 to 5 stage41

1.61.6%1By pass of kiln gases40

-11-10.0Change from 5 to 6 stages

42

0.13%1Recarbonation, KS38

43

44

0.510.37%1Recarbonation, CS39

0.82-0.74%1Cyclone- C537

-0.34- 0.24%1Cyclone- C335

-0.27-0.18%1Cyclone- C234

-0.21-0.14%1Cyclone- C134




Heat, kcal/kgclILC

Heat, kcal/kg clSLC

unitbyA change ins.no

Heat loss from shell radiationInsulation effect of refractoriesoptimised coating ,300mm thk

Flame Shape & flame length

Ring formation shootsthe gas velocity

takes the heat fartherinto the kiln, increasesthe back end temperature

Exit gas velocity at kiln inlet= 10 m/s

v=15 -16 m/s

SteadyFeed rateWith lessFluctuationOf calcination

Heat losses from kiln

Parasite air ( ingress of false air entry) at inlet , outlet hood&preheater.Primary air & coal transportair are all false entry.

Fluctuations In process

Hood take offV=5 m/s

Well controlled air flowFuel flow

4 –stage preheater

5 –stage preheater 6 –stage preheater

Conversion of 4 stage preheaterTo 5 stage preheater saves 28 Kcal/kg cl

Conversion of 5 stage preheaterTo 6 stage preheater saves 14 Kcal/kg cl

calciner

769.34.40Total Output

6.0Radiation Loss from Cooler

20.2Radiation Loss from Kiln

37.1Radiation Loss from Preheater

4.8Heat of Evaporation of Moisture

20.91110.1881.0Heat Through Clinker

91.62930.2521.2Heat Through Cooler Vent

410.0Heat of Clinkerisation

7.93360.2360.1Heat of PH Exit Dust

170.83360.2472.1Heat of PH Exit Gases

Kcal/kg clinkerdeg CKcal/kg degCkg/kg clinker

HeatTemperature Sp.heat capacity Mass flow

Heat Output relative to 0 deg C

769.34.40Total Input

412.3Heat of Coal Combustion in Calciner

295.7Heat of Coal Combustion in Kiln

1.1560.2380.1Sensible heat of Coal and Conveying Air

2.8600.2870.2Sensible heat of Coal

1.3460.2380.1Sensible Heat of PH Leak Air

27.1460.2472.4Sensible heat of Cooling air

8.1Heat through combustibles in raw meal

21.0600.2121.6Sensible heat of Kiln Feed

kcal/kg Clinkerdeg CKcal/kg degCkg/kg clinker

HeatTemperature Sp.heat capacity Mass flow

Heat Input relative to 0 deg C

Specific heatConsumption=Total heat output –Total sensible heat769.3-61.3 = 709

Kcal/kg cl

433.63.43Total Heat output

6.0Radiation

20.90.1881111.00Clinker

91.60.2522931.24Excess air

6.70.2369490.03Tertiary air dust

176.30.2689490.69Tertiary air

5.00.2410490.02Secondary air dust

127.10.27110490.45Secondary air

kcal/kg clinkerkcal/kg oCdeg Ckg/kg cl

HeatSpecific heatTemperatureMass flow

reference: 0 deg CHEAT OUTPUT

433.63.43Total Heat Input

4.6Fan energy

27.10.247462.38Cooling air

19.10.26414500.05Dust

382.80.26414501.00Clinker

kcal/kg clinkerkcal/kg oCdeg Ckg/kg cl

HeatSpecific heatTemperatureMass flow

reference: 0 deg CHEAT INPUT

NORMAL OPERATING CONDITION

Automation further helps to run the plant more stable by reducing the meal, fuel flow and air flow.

Running the kiln continuously with consistent production is the best way to reduce the fuel and power bills.

For consistent production we must have short , Convergent and intense flame, less chemistry variationof raw meal , less variation in ash content of fuel and stable cooler operation. Automation further helps to run

Thanks for your attention

Technology

Heat optimisation pradeep kumar