Loss Optimisation in Boilers

8/21/2019 Loss Optimisation in Boilers

1/50

The Heat Balance and

Efficienc of Steam Boilers

1

By : Debanjan Basak


2/50

The efficient utilization of fuel in steam boilersis primarily determined by the following three

factors :

Complete combustion of the fuel in thefurnace

2

Deep cooling of the combustion productsduring their passage through heatingsurfaces.

Minimization of heat losses to theenvironment.


3/50

Heat absorbed by workingfluid in boiler :

Heat content of superheated steamfrom boiler

Heat content of reheated steam from

3

boiler

Heat content of boiler blow- downwater


4/50

Heat in Fuel

Heat in atomi sing steam

Sensible he at in fuel

INPUT Pulver iser or crusher power Boiler Circulating pump power

Primary air fan power

Recirculating gas fan power

Heat supplied by moisture in entering air

Heat in Final superheater steamHeat in desuperheater water

Heat in feedwater

Boundary envelope Heat in blowdown water OUTPUT

HEAT BALANCE IN STEAM GENERATOR

4

Heat in reheat s team outHeat in desuperheater water

Heat in reheat steam i n

Unburnt carbon in a sh

Heat in dry gas

Moisture in fuel

Moisture from burni ng hydrogen

Moisture in ai r

LOSSES Heat in atomising steam

Unburnt gases

Radiation and convection

Sensible he at in slag

Heat in mill rejects Reference : PTC ; 4.1

Sootblowing


5/50

Efficiency

Input = Heat input

Output = Input – losses

5

Efficiency = Input

Output


6/50

The Heat balance Equation

The distribution of the heat supplied tothe boiler as useful heat and lost heat is

the basis for compiling the heat

6

balance of a steam boiler


7/50

The Heat Balance Equation

Q = Q1 + (Q2 +Q3 + Q4 +Q5 + Q6) where :

Q= available heat of burnt fuel

7

Q1= heat absorbed by working fluid

Q2 to Q6 = heat losses

Dividing both sides by Q and expressing as a %age we get :100 = q1 +(q2 +q3+q4+q5+q6)


8/50

Methods of determining BoilerGross efficiency

Direct method

Inverse balance method

8


9/50

The direct method

Uses the heat balance equation

Measures Q and Q1

Method is insufficientl inaccurate

9

Accurate measurements of certainparameters like mass flow rate of steamand fuel,heating value of fuel etc is not

possibleNot a preferred method


10/50

The inverse balance method

Uses the equation

Efficiency = 100 –(q2 +q3+q4+q5+q6)

Determines the sum of heat losses

10

s more accurate met o s nce t e sum othe heat losses roughly constitutes 10% ofQ1.

All the items can be reliably measured

Is the SOLE method used for determiningefficiency


11/50

Heat Losses in Steam Boilers

Item Symbol % as relative loss of Q

Waste Gases q1 4 - 7

Incomplete q2 0 – 0.5

11

Unburnt Carbon q3 0.5 - 5

Cooling throughlining

q4 0.2 - 1

Pysical heat of

removed slag

q5 0 - 3

Sum of heat losses 6 - 12


12/50

Chart depicting heat losses

Waste gases

Incomplete combstion

12

Unburnt carbon

Cooling through lining

Physical heat of

removed slag


13/50

13

na ys s o eat osses nSteam Boilers


14/50

Dry Flue gas loss

Is the largest component of heat loss

It depends on the absolute difference

between the enthal of waste ases

14

and the enthalpy of cold air supplied.

The enthalpy of waste gases dependson the exit temperature of waste gases

and its volume.


15/50

Dry Flue gas loss

It is the heat lost to the atmosphere bydry component of flue gas

Also referred as stack loss

15

Carbon and sulphur in coal burn to formthe dry component of flue gas

Carbon can burn to form either CO or

CO2


16/50

Components of Dry Flue gas

Carbon- Dioxide

Carbon Monoxide

16

Nitrogen

Sulphur Dioxide – can be neglected forIndian coals


17/50

Seigert Formula for estimation of Dry Fluegas loss

% loss = K ( T – t )

% CO2

17

K is a constant whose value are taken as :

For anthracite : 0.68

For Bituminous coal : 0.63

For coke : 0.70


18/50

Wet flue gas loss

Wet products of combustion are obtainedfrom the following components in fuel :

Moisture

18

–1 Kg of hydrogen burn to form 9 Kg of

moisture

The heat lost with the mass of the water

vapour along with flue gas to the atmosphereis known as wet flue gas loss


19/50

Sensible Heat of water vapour

This is the amount of sensible heatabsorbed by moisture in coal

Is calculated b :

19

( Wet FG loss – (GCV – NCV)) KJ/Kg fuel


20/50

Moisture in combustion air

This is the amount of heat absorbed bythe moisture present in cold air

However this loss is ver small and is

20

normally not calculated


21/50

Heat loss with waste gases

By reducing the temperature of waste gases by 22degree cent it is possible to increase boiler efficiencyby 1%.

he exit tem erature of waste ases de ends on

21

feed water temperature at economizer inlet and andcold air temperature at air heater inlet.

The exit temperature depends on the moisturecontent of fuel used.A higher moisture content

results in increase of heat loss for the same heatingsurface due to higher volume of combustion.


22/50

Heat loss with waste gases

A cost economic approach is taken fordetermining the design optimal flue gas exittemperature.

22

ne o t e ma n cons erat ons n m t ng t eFGET is the low temperature acid corrosion inair preheaters.The waste gas temperature isgenerally kept within 140- 160 deg cent to

prevent acid condensation.


23/50

Reasons for high FGET

Improper soot blowing

Deposition on furnace wall tubes

23

Low temperature at air heater inlet

High moisture content of fuel

High excess air ratioHigh air infiltration


24/50

Clean

Tube

1200ºC

Scale

Both side

depositsSoot

300ºC

m t ng etaTemp of 450ºC

200M

Kcal/hr/m2150M

Kcal/hr/m2

155M

Kcal/hr/m2

120MKcal/hr/m2

Typical Heat transfer and temperature drop across furnace tube


25/50

Heat loss with unburnt carbon

In the combustion of solid fuels, unburnt cokeparticles are carried off from the combustionchamber by flue gases.

Durin the short time the are resent in the hi h

25

temperature zone of the flame, these particles evolvevolatile mater but remain partially unburnt.

Under normal operating conditions, these unburntcarbon loss may range from 0.5% to 5%

More the VM of fuel less is the heat loss.


26/50


This loss can be divided into :

Carry over loss

Loss with slag or bottom ash

26

e carry over oss s muc pre om nant overthe loss with slag.

The carry over loss is determined by thecarbon content in Fly ash samples collected

from ESP hoppers.The slag loss is determined by the carboncontent in bottom ash samples.


27/50


The carry over loss depends on :

Excess air ratio

27

The slag loss depends on :

Improper fineness of pulverized fuelfrom mills


28/50

Heat loss by Incompletecombustion

The products of combustion containgaseous combustible substances suchas CO,H2 or CH4.

28

Their afterburning beyond the boilerfurnace is practically impossible sincethe temperature of gases and

concentrations of combustiblecomponents and oxygen are too low.


29/50


The heat that may be produced by afterburningthese components constitutes the heat loss dueto incomplete combustion.

29

the concentration of CO and to a lesser extentH2 in flue gas.

However analysis for incomplete combustionshould always be done for all components of

flue gas since even a slight quantity of CH4 mayhave a noticeable effect .


30/50


Heat loss due to incomplete combustionsubstantially depends on excess air ratio andboiler load conditions.

30

eoret ca y, t oroug nterm x ng o ueand oxygen ensures that this heat loss maytake place only for K


31/50


With reduced load conditions, the exitrate of fuel and air through the burnersdecreases causing improper mixing

31

which results in increase in heat loss.

Also the temperature of the combustionzone decreases with reduced load

causing an increase in incompletecombustion


32/50

Unaccounted losses

Radiation loss

Loss due to unburnt volatilehydrocarbons

32

Loss due to combination of carbon andwater-vapour

Mill rejection

Physical heat carried by bottom and fly-ash


33/50

Heat Loss by Radiation

Since the temperature of the boiler liningsand casings are higher than the surroundings,they give up heat to the environment.

33

g er s t e temperature o t e n ngs ancasings higher is this heat loss.

In general the boiler casings should beinsulated in such a way that the average

temperature of the casings is not more than55 degree cent.


34/50

Typical summary of Radiation and

Unaccounted losses

BoilerRating

Approximate Range in %

020 0.6 – 1.7

34

050 0.38 – 1.2

060 0.35 – 1.15

100 0.3 – 1.0

200 0.25 – 0.93

500 0.2 – 0.88


35/50

Heat Loss by Cooling

In rough calculations, the heat flux from theboiler surfaces to the surroundings is taken atan average level of 200 – 300 W/m2

35

t ncreas ng o er power, t e a so uteand relative heat loss q5% becomes lower asthe total heat release and the volume ofcombustion products increase more quickly

than the area of exposed boiler surfaces.


36/50

Heat loss with physical heat ofslag

The slag removed from the bottom of a boilerfurnace has rather high temperature andpossesses a significant amount of heat.

36

the slag bath and is lost irreversibly

In dry bottom furnaces the temperature ofslag is 600- 700 deg cent.

In slagging bottom furnaces the temperatureof flowing slag is 1400 – 1600 degree cent.


37/50

Heat loss with physical heat ofslag depends on

Total ash content of fuel

37

furnace

Enthalpy of slag

Method of removal of slag fromfurnace.


38/50

Theoretical Air

It is the quantity of air required by afuel which will provide just sufficientoxygen for complete oxidation of the

38

fuel

It is also known as the stoichometric air

requirement


39/50

Quantity of theoretical air

The theoretical air is calculated from the

Ultimate analysis of fuel by the formula :

39

4.31 x 83

C 8 ( H -O

8) S Kg/Kg of fuel

•The value within the bracket denotes the quantity of

oxygen reqd.

•The factor 4.31 gives the amount of air required to supply

this oxygen


40/50

Excess Air

The actual amount of air required for completeburning of a fuel is always more than the theoreticalair.

40

his additional quantity of air required over thetheoretical air is known as excess air

Present designs allow for 20% excess air for coalfired boilers

THE SUM OF THEORETICAL AIR AND EXCESS AIR ISTHE TOTAL AIR REQUIOREMENT FOR A BOILER


41/50

Reasons for excess air

To overcome the uncertainties in thefuel-air mixing process

o ensure intimate mixin of fuel and

41

oxygen at point of injection

To account for errors in ultimateanalysis of fuel


42/50

Quantity of excess air

Excess air =

(O2 % X100)/ (21 – O2%)

42

OR

(CO2 %(max) x 100)/CO2% - 100


43/50

Conclusion

The quantity of excess air adverselyaffects boiler efficiency

he uantit of excess air needs to be

43

optimized for achieving maximumefficiency of boiler


44/50

Monitoring of excess air

By residual carbon dioxide in flue gas

By residual oxygen in flue gas

44

By residual carbon- monoxide influe gas


45/50

12

11

Total Loss

10

9 Optimum Total air

8 Dry Gas loss

7

o s s %

Optimisation of To tal Air supply

45

6

5

4

3 Aux . Pow er

Unburnt Gas

2

1 Comb. In ash

14 15 16 17

Carbon- Dioxi de %

B o i l e r


46/50

Optimisation of Total Air

• Unburnt gas (primarily CO) is formed just a littlehigher than the optimum CO2

• Measurement of CO is a better method of

46

• Oxygen measurement in flue gas can be misleading

due to air ingress

• Modern combustion control utilizes CO measurement

in addition to Oxygen measurement


47/50

Saturation

700

600

Air Defici ent Air Rich

500

400

Graph Depicting variation of Oxygen with Carbon Monoxide in Flue gas

f a n d i s c h a r g e

47

Ideal Operating Point

300

200

100

5 6 7 8 At ID Discharge

1 2 3 4 5 At AH Inlet

p p m v

o l C O

a t I D

% Oxygen In Flue gas


48/50

6 Mill s equally loaded

6 Mills unequally loaded

500

5 Mills equally loaded

400

i s c h a r g e

Effect of mill operation on CO breakpoint for a 500 MW PF boiler

48

300

200

100

2 3 4 5

p p m v

o l C O

a t I D f a n

d

% Oxygen In Flue gas at Air Heater Inlet


49/50

CO2 %

CO2 %

I n c r e a s i n g

Trend o f Boiler Gases with variations in excess air

49

O2 %

SO2 ppm

-20 -10 0 10 20Excess Air %age

D e c r e a s i n g


50/50

50

Date post:	07-Aug-2018
Category:	Documents
Upload:	rashm006ranjan
View:	213 times
Download:	0 times

Loss Optimisation in Boilers

Documents