boiler effy

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Boiler Efficiency

Air Heater Performance

Presentation Outline

1 March 2011

T K Ray NTPC 2


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Loss due to moisture in air.

Loss due to moisture in fuel.

Loss due to comb. gen. moisture.

Dry Exhaust Gas Losses~ 4.6%Fuel Energy100%

Heat gained by boiling water 38%

Hot gas

Flue gas

Heat loss from furnace surface.

Unburned carbon losses.Incomplete combustion losses.

Loss due to hot ash .

Heat gained by

SH & RH 38%

Heat gained by economizer & air pre-heater 11%

1 March 2011 T K Ray NTPC 3


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Boiler EfficiencyFor utility boilers efficiency is generally calculated by heat

loss method wherein the component losses are calculated

and subtracted from 100.

(Boiler Efficiency = 100 - Losses in %)Commonly used standards are

-

ASME PTC 4.2: Coal Pulverizers

ASME PTC 4.3: Air Heaters

BS 2885 (1974) IS: 8753: 1977

DIN standards1 March 2011 T K Ray NTPC 4


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100*)(

B H

hmhm

f

iiee

+

=

&&

Boiler Efficiency determination

The % of heat input to the boiler absorbed by the working fluid

i. Input /output method

100*100 B H

L

f +

=

ii. Heat Loss method



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Parameters required for computing Boiler Efficiency

AH inlet and exit FG O 2 / CO 2 /CO

AH inlet and exit FG temp

Primary / Secondary air temp at AH inlet Dry/Wet bulb temperatures

Ambient ressure b r bs

Proximate Analysis & GCV of Coal

Combustibles in Bottom Ash and Fly ash



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Boiler Losses Typical valuesDry Gas Loss 4.56Unburnt C Loss 1.50

Hydrogen Loss 3.29Moisture in Fuel Loss 2.53Moisture in Air Loss 0.12

oss .Radiation/Unaccounted Loss 0.89Total Heat Loss 12.93

Boiler Efficiency 87.51Heat credit 0.44



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DFG Loss (kJ/kg of fuel) =

DFG Loss (%) =Seigert formula:2%

)(

CO

T T K air fg

)(6.30*)]267100

()(12

100[ _2

air fg Ain T T C S C

COCO+

+

fgt pg

air fgt pa L fg T C

T T C AT +

=

100*

)(**

90

*90 _2_2

+=

L

inout A

COCO

O2 in / CO 2 in measured, A L known

K ~0.63 for bituminous coal

100*9.0*21

100*9.0*

_2

_2_2

_2

_2_2

out

inout

out

out in L

OOO

CO

COCO A

=

=



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Typically 20 0C increase in exit FG temp ~ 1%

reduction in boiler efficiency.

temp in AH outlet common ducts leading to ESP.



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Wet Flue Gas Loss (kJ/kg of fuel) =

)]25(2.42442)25(88.1[100

9air fg T T

H M ++

+



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Moisture in combustion air loss (kJ/kg fuel)=

)(*88.1** air fga T T h M

a M =Dry air for combustion kg/kg of fuel

h =kg moisture per kg dry air

1 March 2011

)267100[.

_2

2 Aina C COCO M

++

=

N2, CO 2, CO=% volume in dry gasC, S=% in fuel

T K Ray NTPC 11


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Incomplete gas (CO) loss (kJ/kg fuel)=

23717 kJ/kg = CV of burning 1 kg of carbon in CO to CO 2

23717*)]267100

(*)(3

7[*

28

12_

2 AinC

S C COCO

CO+

+

1 March 2011

CO 2, CO=% volume in dry gas

C, S=% in fuel

T K Ray NTPC 12


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Measure of effectiveness of Combustion processand Mill performance

Loss in kJ/kg of fuel: c= % of carbon in ash

Combustible in Ash Loss

33820*100cA

= ass o as g g o ue Carbon burnt to CO 2 =33820 kJ/kg (8077 kcal/kg)

Compute Boiler efficiency loss % due to c in Ash


23717 kJ/kg = CV of burning 1 kg of carbon in CO to CO 2


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HEAT CREDIT

Heat Credit due to Coal Mill Power

= [MP * 859.86 * 100] / [Coal FLOW * GCV * 1000]

Coal Flow Rate Coal FLOW Tons/Hr

Total Coal Mill Power MP kWh

GCV of Coal Kcal/Kg


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Probable measurement errors and resultingerrors in efficiency calculations

Parameter

Measurement

error, %

Error in calculated SG

Efficiency, %

Heat value (coal) 0.50 0.03Orsat analysis 3.00 0.30

Exit FG temp 0.50 0.02

1 March 2011

Inlet air temp 0.50 0.00Ult. anal. of coal (C) 1.00 0.10

Ult. anal. of coal (H ) 1.00 0.10

Fuel moisture 1.00 0.00

T K Ray NTPC 15


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Unburnt Carbon Loss (Controllable)

Cunburnt is a measure of effectiveness of comb. process

Cunburnt includes the unburned constituents in FA and BA

Focus to be on FA due to uncertainty in repeatability and

+50 PF fineness fractions to be < 1%



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Influencing Factors - Unburnt Carbon Loss

Type of mills and firing system Furnace size

PF fineness (Pulveriser problems) Coal FC/VM ratio, coal reactivity

Air damper / register settings

Burners design / condition

Burner balance / worn orifices

Primary Air Flow / Pressure



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Dry Gas loss reduction requires

Boiler operation at optimum excess air

Cleanliness of boiler surfaces Reduction of tempering air to mill

e uct on n a r ngress Cleaning of air heater surfaces and proper

heating elements / surface area



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Air Optimization


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Air Heater in Fossil Fired Plant

The rotating cylinder packed withthousands of sq m of specially formed

sheets of heat transfer surfaces.

As it revolves, heat of FG is absorbedthrough one half.

1 March 2011 T K Ray NTPC

The accumulated heat is released to the incomingair as the same surfaces pass through other half.

The heat transfer cycle is continuous as the

surfaces are alternately exposed to the outgoinggas and incoming air.

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Air-in-Leakage (~13%) Gas Side Efficiency (~ 68 %)

X ratio (~ 0.76)

Flue gas temperature drop (~220 0 C)

Air side temperature rise (~260 0 C)

The indices are affected by changes in entering

air or gas temperatures, their flow quantities and

coal moisture.



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AH Leakage The leakage of the high pressure air to

the low pressure flue gas due to

Differential Pressure

increased seal clearances in hot condition seal erosion

improper seal settings.

Direct flow of air through gaps betweenrotating and fixed structure

Leakage gap area x (density x P)1/2

Entrained air in elements carried via

rotation from air side to gas side

Rotor Turndown HE grows

radially more than the CE,rotor goes outward and

downward

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Leakage paths

Increased AH leakage leads to

Reduced AH efficiency

Increased fan power consumption

Higher gas velocities that affect ESP performance

Loss of fan margins leading to inefficient operation and at times

restricting unit loading1 March 2011 T K Ray NTPC 23


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AH Leakage

Typically air heater starts with a baselineleakage of 6 to 10% after an overhaul

What we measure is mainly leakagethrough radial seals at hot & cold end

substantial and has a major effect on heat

transfer but nominal effect on APC

Leakage is expressed as a % of inlet gas

flow and not a % of fan input flow



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This leakage is assumed to occur entirely between air inlet and gas outlet

Empirical relationship using the change in concentration of O2 or CO2 in

the flue gas

100*9.0*21

100*9.0*

_2_2

_2

_2_2

=

=

inout

out

out in L

O

OOCO

COCO A

%1.17

100*9.0*7.5218.27.5

_

=

=

Method of determination of O2 or CO2 should be the same at inlet

and outlet wet or dry (Orsat)

O2 dry = O 2 wet / (1- FG Moisture)1 March 2011 25


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Data Collected / MeasuredO2 or CO 2 in FG at AH Inlet

O2 or CO 2 in FG at AH OutletTemperature of gas entering/leaving air heater

Temperature of air entering/leaving air heater

Diff. Pressure across APH on air & gas side

1 March 2011 T K Ray NTPC

CO 2 measurement is preferred due to high absolute values; In case of any

measurement errors, the resultant influence on lkg. calculation is small.

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Factors affecting APH performance

Operating excess air levels PA/SA ratio

Inlet air / gas temperature

Coal moisture

Air ingress levels

Upstream ash evacuation Soot blowing

No. of mills in service

Maintenance practices



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Boiler Efficiency

Air Heater Performance

Presentation Outline

1 March 2011

Use of Performance Evaluation Software

T K Ray NTPC 28


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&

Accurate determination of Boiler Efficiency & Air Heater

performance poses many challenges.

Acceptance Test accuracy is not cost justified, but theres a

need to obtain representative & accurate performance

data & trend the same. Diagnostic tests are required for the following

Root cause analysis of different problems

Identifying reasons for boiler inefficiency

To verify the feedback from online instruments



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Thanks

1 March 2011 32

[email protected]

T K Ray NTPC

boiler effy

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