Excess Air Calculation

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Dr. Albrecht Kaupp Page 1

Excess air in combustion

Issue Incorrect amount of air in fuel combustionaccounts for the largest losses in combustionsystems.

LearningObjectives

Understanding the technical jargonbehind combustion technologies

Knowing the very basics of combustionfundamentals

Appreciating the various methods tocalculate excess air levels

Being able to relate O and !Omeasurements to fuel properties

Assessing fuel cost reduction potentials

Applying "uic# estimates to calculateenergy losses


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Excess air in combustion Page 2

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1. A crash course in combustion principles

As shown in lecture $% all fuels consist mostly of atomic !arbon&!'% (ydrogen &('% Oxygen &O'% )itrogen &)'% *ulfur &*'% minerals&ash' and water &(O'.

+uel combustion means to let the molecular Oxygen &O' in air reactwith the combustible components of a fuel. As an example the fuel!arbon &!' reacts with Oof the air to generate !arbon ,ioxide&!O'. If the reaction is incomplete !arbon -onoxide &!O'% adeadly gas% is generated.

It is worthwhile to point out that all combustion products such as!O% !O% )Ox% !n(m% *O% *O% except for the water generated bycombustion of ( to (O% are harmful. /here is essentially nothing

benign in stac# gas% except the water vapor. And even the watervapor% because it reacts with *O to sulfuric acid &(*O$' is notreally harmless.

/his environmental and health threat of stac# gas is one morereason to reduce energy consumption per unit of product output.

2. The excess air parameter

One #g of fuel re"uires a certain minimum of ambient air to be fullycombusted. 0e call this minimum amount of air the 1stochiometricair2 or sometimes also 1the theoretical air2 to combust the fuel. /hestochiometric air would completely combust the fuel to !arbon,ioxide &!O'% water &(O'% and *ulfur ,ioxide &*O' if *ulfur is

present. If the fuel does not get enough air for combustion it willgenerate smo#e and a potential unhealthy mixture of stac# gas

products. In addition energy is wasted. /he same applies if too

much excess air is used for combustion. A less trivial issue incombustion technology is therefore to ensure the proper amount ofair that minimi3es environmental impact and fuel consumption. +orconvenience we define the 1stochiometric air2 as the air to fuelratio% A+ g air4#g fuel'% and the excess air factor as

5A 6-ass of air g' to combust one #g of fuel

*tochiometric air &A+'

/he A+ is a property of a fuel that can be calculated from theultimate chemical composition of the fuel.

able 1!Air-to-fuel ratio of various fuels


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#uel Phase A# $O2 max%et

$O2 max&r'

7ery light fuel oil li"uid 8$.9 8.:;

8$.==

(eavy fuel oil li"uid 8.$; 8$.8$

Bun#er ! li"uid 8.; 8;.

?eneric Biomass &maf' solid :.@@ 89.>8

!oal A solid ;.>9 8;.=>

= 8= B' gas 8:.:: 88.;:

!arbon solid 88.$$ 8.==

3. Terminology and equations

5xcess air and the excess air factor were defined in the previousparagraph. )ote% that both parameters describe the samephenomena.

+or instance saying a burner re"uires = C excess air to correctly

combust fuel oil% is the same as saying the burner operates at anexcess air factor of 8.. A ideal combustion process would re"uire =C excess air or has an excess air factor of 8.

A combustion process re"uiring 8== C excess air uses twice asmuch air as necessary% or in other words has an excess air factor oftwo.

/he technical literature and car industry reserves the ?ree# symbol


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excess air' will re"uire a 1oxygen trim2 that closely monitors excessair and adjusts it.

Operating very close to the minimal amount of air &6 stochiometricair 6 theoretical air' has the inherent danger of smo#e and !Ogeneration.

Once is #nown it is fairly easy to calculate the mass of stac# gasgenerated from the combustion process by

m*? 6 mf&8 D A+' E mash

It is worthwhile to examine the last e"uation. In case the boiler doesnot have any lea#s% where stac# gas escapes we can be assured thatthe mass entering the boiler must also leave the boiler through eitherthe chimney or the ash bin.

In the case of oil we #now mash6 =.

/herefore m*? 6 mf&8 D A+'

6 mfD mfA+

6 fuel D combustion air

)ote% that the term combustionairrefers to dry air% excluding thehumidity in air that could be anything from 8 to = grams ofmoisture per #g of air.

. !eri"ation o# excess air #actor$

/he amount of excess air can not be measured directly% but is ratherderived from a measurement of either the Oor !Ocontent of the

stac# gas. 0hether one measures O or !O is irrelevant for thecalculation of the excess air% or % as long as one has obtained anaccurate measurement of either Oor !O. As previously shown inlecture ;% various sensors and methods exist to measure Oor !O.

/here is no simply and also accurate e"uation to calculate if Oor!Ois #nown. /he correct e"uation based on a !Omeasurement is

6 8 8+

!O

!O

7

7

max

*?

A+

where

!O max 6 the maximum !Ocontent of the dry stac# gas at stochiometric combustion. ?iven in volume C


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7*? 6 dry stac# gas in mn4#g at stochiometric condition7A+ 6 airEtoEfuel ratio expressed as mn

4#gmn

6 normal cubic meter at = o! and 8.=8: bar.

/he factor f 6V

V

SG

AF

is between =.> to =.>9 for fuel oils.

It is between =.>@ and 8 for solid fuels and between =.> and 8.> forgases.

It is best to calculate and generate appropriate charts expressing

as a function of either O or !O in the stac# gas by computersoftware.

One should appreciate the complexity involved% that has resulted in"uite a number of simplistic e"uations. -ost commonly usede"uations are

6 8 8

+

CO

CO

max

68

8 O

All e"uations apply only if no !O and (is found in the stac# gas.In case of incomplete combustion% !O is found in the stac# gas. In

this case is given as

6 8+

CO g

g

V

V

SG

AF

max

where g 6

& '

. .

CO CO

CO H

8==

8== =: 8:

+

)ote that !O is commonly measured in ppm and 8=%=== ppm 6 8C.!O contents of 8%=== ppm 6 =.8 C are considered high in thecombustion of li"uid and gaseous fuels.

%. Excess air #actors #ound in practice

As mentioned% the excess air factor of a burner furnace or boiler is ayardstic# about its efficiency as well as the s#ill of the operator.


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*tandard average figures are

?as burners% forced draft 8.8 E 8.

Atmospheric gas burners 8.: E 8.:

Oil burners 8.8: E 8.

!oal dust burners 8. E 8.

!oal firing &mechanical' 8. E 8.:

!oal firing &hand' 8.: E .:

/hese are best values that can be achieved with careful monitoring

and constant adjustment of the combustion air at varying loads. Inreality energy auditors may see much higher numbers.

&. 'et stac( gas "ersus dry stac( gas "alues

/here can be a considerable amount of confusion and misjudgmentof the situation if one does not clearly indicate whether Oor !Omeasurements were conducted on either a wet or dry stac# gas

basis. As discussed in lecture ;% chemical cell sensors for Oand !Omeasure on a dry stac# gas basis while Firconium Oxide sensorsmeasure the Oon a wet and hot stac# gas basis.

Of the four options OC &dry'% OC &wet'% !OC &dry' !OC&wet' only one value needs to be measured. /he others arecalculated based on the ultimate chemical composition of the fuel.


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E,E-$IE

Tas( 1

In actual field wor# one measures or calculates !O on a 1dry2basis% meaning the gas sample is cooled to almost ambienttemperature and the stac# gas is in a saturated state% with littlemoisture left.

!alculate !O maxin a dry stac# gas and complete table 8 on page $.

Tas( 2

Assume the !Ocontent of stac# gas from a C &dry basis'. (owever we donGt #now thecomposition of the C !O .

5xcess air for Butane at > C !Oe"uals C.

5xcess air for ropane at > C !Oe"uals C.

Tas( 3

Assume the !Ocontent of stac# gas from a coal fired boiler wasmeasured at 8$ C &dry basis'. (owever we donGt #now the ultimatechemical analysis of the coal. (ow large could the error beH

/int! Use the 15xcess Air2 graphics for Carbon% Coal Aand Coal B% aswell asBunker Coil and complete the following table.

#uel 0 3 $O24&r' 565actor Excess air

!arbon &89' 8$ 8 :=

!oal A &8' 8$

!oal B &8' 8$

Bun#er ! &9' 8$


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NOTES

Tas(

!omplete the following sentences

5xcess air of C is e"uivalent to 6

5xcess air of = C is e"uivalent to 6

5xcess air of 8== C is e"uivalent to 6

,ecreasing excess air close to = C may have the followingundesirable side effects

a'

b'

c'

Increasing excess air beyond 8== C may have the followingundesirable side effects

a'

b'

Tas( %

It is fairly easy to ma#e mista#es in the assessment of the excess air%if it is not "uite clear what basis% either 1dry2 or wet stac# gas isused.

)ote% very rarely sensors that measure !Oin the wet stac# gas areused. Assume the wrong !O max&wet' is used. !alculate the errorfor excess air at 9 C !Oand 8 C !OforBunker Coil% by usingthe e"uation

6 8 8

+

CO

COfmax

/he fEfactor calculated by the software is always based on a dry gascomposition. /he values for !O max and !O refer to a dry gasmeasurement.

Tas( &


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,ecide whether the following statements are true or false.

tatement rue #alse

8. A !Ocontent of : C was measured in the stac# gas of a coal fired boiler% but the stac# loo#ed clean. . /he excess air of a coal fired boiler is 8= C. . A very low excess air means a high energy efficiency. $. /he !O content decreases whenever excess air increases.

:. /he !O content increases whenever excess air increases. ;. /he !O content increases whenever excess air decreases to = C. 9. *mo#e generation is a sign of too little excess air. @. *mo#e generation is a sign of too much excess air. >. /he excess air increases at low boiler load. 8=. Increased !arbon -onoxide generation is always a sign of too little excess air. 88. *olid fuels have always a higher !O maxthan


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tatement rue #alse

=. A wea# !Oadsorbent solution will underestimate the boiler efficiency. 8. A wea# Oadsorbent solution will overestimate the boiler efficiency. . /oo much excess air increases corrosion. . /oo much excess air increases the flame temperature. $. /oo much excess air decreases the pressure in a boiler.

Tas( )

5xplore the shape of the 15xcess Air2 curve. 0hat happens% if !Ois very close to =H

/int! /a#e any 15xcess Air2 printout and extend the !Oaxis to the left.!alculate the excess air for 8 C% =.: C% and =.8 C !O..

/a#e the e"uation for and set f68

6 8 8

+

CO

CO

max

,ecrease !O in steps from 8 to =.==8C for !Omax 68C

!O &Cdry basis' 8 =.: =.8

5xcess Air&C'

Tas( *

Often the power consumption of the forced draft fan% andoccasionally an additional induced draft fan of the boiler is not#nown or data is not available. On the other hand the annual fuelconsumption and annual operating hours are fairly well #nown.

Use the e"uation

P =

+

p v

M F

in 0att

where p 6 pressure drop across a fan in ascal


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v 6 volume flow in m

4s of air or stac# gas

-D+ 6 system efficiency of fan and electric motor &about =.$ to =.@'

to roughly estimate the hourly electricity consumption of a boilerthat combusts := #g4h ofBunker Cat := C excess air.

teps -esults 0P in /2O

" cm ) cm + cm 19 cm

ressure drop in ascal

A+ ratio of Bun#er !% #g4#g fuel

Jeal air #g4#g fuel

Jeal air m4#g of fuel

#0h4hour at -D+6=.;

)ote% the pressure drop across the system depends very much onthe height of the chimney% the stac# gas temperature and the settingof the fan louvers.

Tas( +

In the field one is never sure whether the measuring e"uipmentwor#s properly. 0ea# adsorbent solutions% or chemical cells% shortcircuits in the Firconia Oxide sensors or incorrect calibration willyield false results. A clever way to safeguard against undetectedmeasurement errors is to measure both the Oand !Ocontent ofthe gas. In this case one can ta#e advantage of the fact that O and!Oare not independent from each other. Once one value is #nown

the other can be calculated. Use the software to explore thefunctional relation of O&!O' and !O&O'.

Excess Air Calculation

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