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Lecture 1: Basics of Combustion EngineeringCombustion Stoichiometry

1.1 Introduction

1.2 Definitions

1.3 Combustion stoichiometry for gaseous fuels

1.4 Combustion stoichiometry for liquid and solid fuels

1.5 Combustibles burnout for solid fuels


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Examples of industrial combustion systems

Boiler fired with pulverised coal

Pulverised coal flame


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Modern reheating furnace


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Flameless oxidation


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1.2 Definitions

Chemical Reactions, Atoms and Molecules in Combustion

H

1

2 O H O2 2 2+

The atoms are conserved (neither created nor destroyed)

Molecules are not conserved

Water is the productHydrogen and oxygen are

reactants

Atoms relevant in combustion are: C,H,O,N,S,Cl


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1.1 Definitions

Compounds of carbon and hydrogen are called hydrocarbons.

HYDROCARBONS

Aliphatic

Alkanes CnH2n+2Alkenes CnH2nAlkynes C

n

H2n-2

Alicyclic

CH2

CH2 CH2

Aromatic

Benzene

Benzene derivatives

Naphtalene


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The first ten members of the unbranched-chain alkanes series are:

CH4 methane C6H14 hexane

C2H6 ethane C7H16 heptane

C3H8 propane C8H18 octaneC4H10 butane C9H20 nonane

C5H12 pentane C10H22 decane


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Table 1.1 Names of aliphatic hydrocarbons

CnH2n+1-CnH2n-2CnH2nCnH2n+2n

C5H11- PentylC5H8 PentyneC5H10 PenteneC5H12 Pentane5

C4H9-ButylC4H6 ButyneC4H8 ButeneC4H10 Butane4

C3H7-PropylC3H6 ButyneC3H10PropeneC3H8 Propane3

C2H5- EthylC2H2 EthyneC2H4 EtheneC2H6 Ethane2

CH3- MethylCH4 Methane1

Alkyl

group

AlkyneAlkeneAlkaneNo.of C

atoms


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Other molecules relevant in combustion are:

Haloalkanes R-X CH3Cl (chloromethane)

Alcohols R-OH C2H5OH (ethanol)

Amines R-NH2 CH3NH2 (methylamine)

Aldehyde R-COH CH3COH (ethanal)

Ketons R-CO-R CH3COCH3 (propanone)

Carboxylic Acid R-COOH CH3COOH (ethanoic acid)


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Amount of substances, mole and mass fractions

1 mole contains 6.023 x 1023 particles (atoms, molecules)

For a mixture of species:

n (total number of moles)=ni

xn

nii

= M x Mmean i i=


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Mole (volume) fractions and mass fractions:

w i = =num ber of kg of species " i"

total num ber of kg in the system

= =

n M

n M

x M

x M

i i

k kk

i i

k kk

xi = =number of moles of species "i" in 1kg of mixture

total number of moles in 1kg of mixture

= =

w M

M

w M

w M

i i

m e a n

i i

k kk

/

/

/

/1


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Equation of state for gases and gas mixtures

F p T c( , , ) = 0or

F p T( , , ) =

0The perfect gas equation:

p V n R T= or cp

R T=

= =

p M

R T

p

R Tw

M

m e a n

i

i


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The perfect gas law:

p V n R T=

Under constant pressure and temperature one mole (kmol)

of any ideal gas occupies the same volume.

At normal conditions:

p = 760 Tr (1 Tr=133,322 N/m2)

T = 273.15 K (0 C)

1 kmol of gas = 22.418 mn3

1 mol of gas = 22.418 dmn3

R = 8,314 J/kmol/K


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1.2 Combustion Stoichiometry for Gaseous Fuels

Stoichiometric Combustion

Combustion is said to be stoichiometric if fuel and oxidizer

consume each other completely forming only CO2

and H2O

CH 2O 2H O CO4 2 2 2

+ + stoichiometric

CH 3O 2H O CO O4 2 2 2 2+ + + lean

CH O H O 0.5CO 0.5CH4 2 2 2 4+ + + rich


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1.3 Combustion Stoichiometry for Gaseous Fuels

Mole fraction of fuel in a stoichiometric mixture

1 kmol fuel O products (CO H O)2 2+ + 2

xfuel stoich in oxygen, _ _ =+

=number of moles of fuel

total number of moles (fuel oxygen)

=+1

1

xfuel stoich in air, _ _ / . .=

+ =

+1

1 0 211

1 4 762


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For example:

CO 0.5O CO2 2+ xfuel,stoich_in_oxygen = 1/1.5=2/3

xfuel,stoich_in_air= 1/(1+0.5/0.21)=0.2958

C H 5O 3CO 4H O3 8 2 2 2+ +

Xfuel,stoich_in_oxygen = 1/(1+5)=1/6

xfuel,stoich_in_air= 1/(1+5/0.21)=0.0403


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Excess air ratio (air equivalence ratio)

= =( / )

( / )

( / )

( / )

x x

x x

w w

w w

air fuel

air fuel stoich

air fuel

air fuel stoich

Fuel equivalence ratio =1

Rich combustion > 1< 1

Stoichiometric combustion = 1

Lean combustion > 1

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