CH05 Combustion Process Aman

7/23/2019 CH05 Combustion Process Aman

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EC551

EC551

THER AL ENGINEERING

HER AL ENGINEERING

EC551

EC551

THER AL ENGINEERINGHER AL ENGINEERING

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Combustion

ombustion

Processesrocesses

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Combustion

ombustion

Processesrocesses


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CHAPTER OUTLINECHAPTER OUTLINE

1.1. Fuels, mass balance, combustionFuels, mass balance, combustionequations, stoiciomet!ic an" non#equations, stoiciomet!ic an" non#

stoiciomet!ic anal$sis.stoiciomet!ic anal$sis.2.2. A%%lication on te 1A%%lication on te 1ststla& to combustionla& to combustion

%!ocess an" ental%$ o' combustion.%!ocess an" ental%$ o' combustion.


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P!o%e!ties o' mi)tu!eP!o%e!ties o' mi)tu!e

Combustion process involves mixture ofCombustion process involves mixture ofcomponents.components.

Mixture substances that has 2 or moreMixture substances that has 2 or more

components in forming (air).components in forming (air). Each component have their own properties Each component have their own properties

mole, mole fractions, molecular weight, gasmole, mole fractions, molecular weight, gasconstant etc.constant etc.


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+ole an" +ass F!action+ole an" +ass F!action

Mole fraction, y is escribe as!Mole fraction, y is escribe as!

Mass fraction, mf is efine as!Mass fraction, mf is efine as!

"otal number of molecules # mass $ summation of each"otal number of molecules # mass $ summation of eachiniviual component.iniviual component.

m

ii

N

Ny =

mixturem

componentindividuali

molesofnumberNi

=

=

=

m

ii

m

mmf =

mixturem

componentindividuali

massm

=

=

=


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+ole an" +ass F!action+ole an" +ass F!action

Molecular weight of mixture!Molecular weight of mixture!

"he mixture gas constant!"he mixture gas constant!

ii

iiMy

N

MN

N

mM

===

M

RR= KkmolkJ ./314.8


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I"eal as La& 'o! mi)tu!esI"eal as La& 'o! mi)tu!es

%alton moel $ total pressure & partial pressure%alton moel $ total pressure & partial pressure

'magats aw $ aitive volume mixture'magats aw $ aitive volume mixture

*or a mixture as a whole!*or a mixture as a whole!

N

N

VRTN

VRTN

P

P iii ==/

/

m

i

m

i

m

ii

N

N

V

V

P

Py ===


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CombustionCombustion

"heory"heory

%efinition $ ' chemical reaction uring which a fuel is%efinition $ ' chemical reaction uring which a fuel is

oxii+e an a large of energy is release. ' microscopicoxii+e an a large of energy is release. ' microscopic

process to convert store chemical energy into usefulprocess to convert store chemical energy into useful

heat energy.heat energy.

rimary element! *uel, -xygen (air) an eat.rimary element! *uel, -xygen (air) an eat.

*uel*uel 'ny material that releases energy when oxii+e, consist'ny material that releases energy when oxii+e, consist

hyrocarbon, exists in all phases.hyrocarbon, exists in all phases.

Common hyrocarbon fuels! gasoline (CCommon hyrocarbon fuels! gasoline (C//0/0/ octane),octane),

iesel (Ciesel (C02022121 oecane), methane (Coecane), methane (C), 3, etc), 3, etc


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-xygen (air)-xygen (air)

%ry air consists of 24.56 -%ry air consists of 24.56 -22, 7/.06 8, 7/.06 822, 4.56 argon an other gases., 4.56 argon an other gases.

Each mole -Each mole -22, it is accompanie by 4.75&4.20 $ 9.71 mol of 8, it is accompanie by 4.75&4.20 $ 9.71 mol of 8 22

't orinary combustion temp (:044't orinary combustion temp (:04444C) nitrogen oes not reactC) nitrogen oes not react

(inert gas), ;044(inert gas), ;04444C $ form 8-C $ form 8-xx..

Complete combustion prouct $ C-Complete combustion prouct $ C-22, , 22-, an 8-, an 822

into 22, -, -22, an -., an -.

yrogen will complete burn proucing yrogen will complete burn proucing 22-.-.


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eateat

lgnition temperatures of various fuels at atmospheric airlgnition temperatures of various fuels at atmospheric air


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Combustion E?uationCombustion E?uation

For a complete combustion process, the products are already well known

ie. CO2, H2O and N2(if applicable)


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nown mole numbers ie. fuel, air, certain proucts.

@et the un>nown mole numbers with letters, ie. a, b, c, x, y etc.@et the un>nown mole numbers with letters, ie. a, b, c, x, y etc.

Balance C-Balance C-22, C-, an unburne C with the number of C from the, C-, an unburne C with the number of C from the

fuel.fuel.

Balance Balance 22-, - an -, - an 22with from the fuel.with from the fuel.

Balance 4Balance 422from the other component Each mole -from the other component Each mole -22, it is, it isaccompanie by 4.75&4.20 $ 9.71 mol of 8accompanie by 4.75&4.20 $ 9.71 mol of 8 22


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'ir*uel Datio!'ir*uel Datio!

@et the number of moles of fuel e?ual to 0.@et the number of moles of fuel e?ual to 0.

'* ratio is a ratio to ?uantify the amount of air an fuel that exists'* ratio is a ratio to ?uantify the amount of air an fuel that exists

in the combustion process.


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CombustionCombustion3E)am%le 1*#143E)am%le 1*#14

%etermine the '* ratio for 0 >mol of octane burne%etermine the '* ratio for 0 >mol of octane burne

with air containing 24 >mol of 4with air containing 24 >mol of 422, assuming complete, assuming complete

combustion an excess oxygen exists in the proucts.combustion an excess oxygen exists in the proucts.

'ns 2.2 >g air&>g fuel'ns 2.2 >g air&>g fuel


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@toichiometric ("heoretical) air!@toichiometric ("heoretical) air!

"he minimum re?uire air for complete combustion to occur."he minimum re?uire air for complete combustion to occur.

Ahen a fuel is burne with stoichiometric air, no unburne fuel anAhen a fuel is burne with stoichiometric air, no unburne fuel an

free oxygen exists, ie.free oxygen exists, ie.

CCF 2(-F 2(-22F 9.718F 9.71822) ; C-) ; C-22F 2F 222- F 7.G28- F 7.G2822


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lf combustion air is supplie less than stoichiometric, it is terme aslf combustion air is supplie less than stoichiometric, it is terme as

air eficiencyair eficiency oror starve airstarve air..

046 starve air is e?ual to 546 stoichiometric air.046 starve air is e?ual to 546 stoichiometric air.

'ir eficiency is mostly practice in inustries where a soli fuel is'ir eficiency is mostly practice in inustries where a soli fuel is

converte into gaseous fuels, with minimum heat generation fromconverte into gaseous fuels, with minimum heat generation from

the chemical process. *or example, coal can be utili+e as a solithe chemical process. *or example, coal can be utili+e as a soli

fuel in steam engines. "he process will re?uire supply of excessfuel in steam engines. "he process will re?uire supply of excess

combustion air to gain maximum heat by complete combustion.combustion air to gain maximum heat by complete combustion.owever, coal can also be gasifie to form methane.


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'naly+ing Combustion roucts!'naly+ing Combustion roucts!

"heoretically, to achieve complete combustion re?uires the supply of"heoretically, to achieve complete combustion re?uires the supply of

excess air.excess air.

'ctual cases even the supply of excess air fails to ensure complete'ctual cases even the supply of excess air fails to ensure complete

combustion occurs. lt is almost impossible to preict the exactcombustion occurs. lt is almost impossible to preict the exactcomposition of composition proucts on the basis of masscomposition of composition proucts on the basis of mass

conservation alone.conservation alone.


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Effect of Moisture in Combustion 'ir!Effect of Moisture in Combustion 'ir!

lflf ry airry airis use as combustion air, than its chemical compositionis use as combustion air, than its chemical composition

can be written as acan be written as athth(-(-22F 9.718F 9.71822).).

lf the moisture content is significant (high humiity), the it shoullf the moisture content is significant (high humiity), the it shoul

be consiere in the combustion process abe consiere in the combustion process a thth(-(-22F 9.718F 9.71822) F 8) F 8vv,air,air((22-).-).

where the number of moles of moisture can be etermine fromwhere the number of moles of moisture can be etermine from


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CombustionCombustion3E)am%le 1*#243E)am%le 1*#24

Ethane (CEthane (C2211) is burne with 24percent excess air uring a) is burne with 24percent excess air uring a

combustion process. 'ssuming complete combustion an acombustion process. 'ssuming complete combustion an a

total pressure of 044>a, etermine!total pressure of 044>a, etermine!

i)i) "he air fuel ratio"he air fuel ratio

ii)ii) "he ew point temperature of the proucts."he ew point temperature of the proucts.


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CombustionCombustion3E)am%le 1*#(43E)am%le 1*#(4

' certain natural gas has the following volumetric' certain natural gas has the following volumetric

analysis! 726 Canalysis! 726 C,, 56 56 2,2, 06 806 82,2, 26 -26 -22 an 9percent C-an 9percent C-2.2.

"his gas is now burne with stoichiometric amount of air that"his gas is now burne with stoichiometric amount of air that

enters the combustion chamber at 24enters the combustion chamber at 2444

C, 0atm, an /46C, 0atm, an /46relative humiity. 'ssuming complete combustion an arelative humiity. 'ssuming complete combustion an a

total pressure of 0atm, etermine!total pressure of 0atm, etermine!

i)i) "he ew point temperature of the proucts."he ew point temperature of the proucts.


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CombustionCombustion3E)am%le 1*#*43E)am%le 1*#*4

-ctane (C-ctane (C//0/0/) is burne with ry air. "he volumetric) is burne with ry air. "he volumetric

'nalysis of the proucts on a ry basis is'nalysis of the proucts on a ry basis is

04.426 C-04.426 C-2,2, G.126 -G.126 -2,2, 4.//6 C-, /9./6 84.//6 C-, /9./6 822

%etermine!%etermine!

i)i) "he airfuel ratio"he airfuel ratio

ii)ii) "he percentage of theoretical air use"he percentage of theoretical air use

iii)iii) "he amount of "he amount of 22- that conenses as the proucts are- that conenses as the proucts are

coole to 2Gcoole to 2G44C at 044>a.C at 044>a.


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Energy 'nalysis of Combustion rocesses!! "hermoynamics point of view, the amount of heat energy

release uring a combustion process is the importantaspect to stuy.

Basically the heat energy release from a combustionprocess comes from the chemical energy contain in thefuel.


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Enthalpy of combustion, hc $the amount of heat releaseuring a steay flow of combustion process when 0>molfuel burn completely at specifie pressure # temperature.

c $ rouct reactant


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CombustionCombustion3E)am%le 1*# 5 1*#-43E)am%le 1*# 5 1*#-4

0)0) %etermine the enthalpy of combustion of li?ui octane%etermine the enthalpy of combustion of li?ui octane(C(C//0/0/) at 2G) at 2G44C an 0 atm. 'ssuming the water in theC an 0 atm. 'ssuming the water in theproucts is in the li?ui form.proucts is in the li?ui form.

2)2) i?ui propane (Ci?ui propane (C99//) enters a combustion chamber at) enters a combustion chamber at2G2G44C at a rate of 4.4G>g&min where it is mixe anC at a rate of 4.4G>g&min where it is mixe anburne with G46 excess air that enters the combustionburne with G46 excess air that enters the combustionchamber at 7chamber at 744C. 'n analysis of the combustion gasesC. 'n analysis of the combustion gasesreveals that all the hyrogen in the fuel burns to reveals that all the hyrogen in the fuel burns to 22--

(gas) but only 546 of the carbon burns to C-(gas) but only 546 of the carbon burns to C-22

, with, with

remaining 046 forming C-. mol)$ 00/504>K&>mol)

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CH05 Combustion Process Aman