Combustion
Combustion
Combustion
Combustion
Any material that can be burned to release thermal
energy is called a fuel.
Most familiar fuels consist primarily of hydrogen and
carbon. They are called hydrocarbon fuels and are
denoted by the general formula CnHm Hydrocarbon fuels
exist in all phases, some examples being coal, gasoline,
and natural gas.
The small traces of other gases in dry air are included in the nitrogen,
which is sometimes called atmospheric nitrogen.
Fuel
C8 H18 12.5 ( O2 3. 76 N 2 )
8 CO2 9 H2 O 47 N 2
Composition of air:
23.3% O2 and 76.7% N2 by mass;
21% O2 and 79% N2 by volume.
C8 H18 A O2 B CO2 D H2O
C8 H18 12.5 ( O2 3. 76 N 2 )
8 CO2 9 H2 O 47 N 2
Ultimate Analysis
An accurate chemical analysis by mass of the
important elements in the fuel is called the Ultimate
analysis.
The elements usually included are carbon, hydrogen,
oxygen, nitrogen and sulphur.
Proximate Analysis
Data from Flagan and Seinfeld, Fundamentals of Air Pollution Engineering, 1988, Prentice-Hall.
Stoichiometric air-fuel ratio
Combustion in Oxygen
Cn H m O2 CO2 H 2O
Answer
m m
Cn H m n O2 nCO2 H 2O
4 2
CH 4 2O2 CO2 2H 2O
C6 H 6 7.5O2 6CO2 3H 2O
Combustion Stoichiometry
Answer
m m m
Cn H m n (O2 3.78N 2 ) nCO2 H 2O 3.78 n N 2
4 2 4
Rich mixture
- more fuel than necessary
(AF) mixture < (AF)stoich
Weak mixture
- more air than necessary
(AF) mixture > (AF)stoich
F- Fuel
Mixture Strength
Calculate:
(i) the stoichimetric A/F ratio;
(ii)the actual A/F ratio and the dry and wet analysis of the
products of combustion by mass and by volume, when
20% excess air is supplied.
Example 7.1 from Eastop & McConkey
Calculate:
(i) the stoichimetric A/F ratio;
(ii)the actual A/F ratio and the dry and wet analysis of the
products of combustion by mass and by volume, when
20% excess air is supplied.
(i) 11.245
(ii) 13.494 / 1
One kmol of octane (C8H18) is burned with air that contains
20 kmol of O2. Assuming the products contain only CO2,
H2O, O2, and N2, determine the mole number of each gas in
the products and the airfuel ratio for this combustion
process.
One kmol of octane (C8H18) is burned with air that contains
20 kmol of O2. Assuming the products contain only CO2,
H2O, O2, and N2, determine the mole number of each gas in
the products and the airfuel ratio for this combustion
process.
Calculate:
(i) the stoichimetric A/F ratio;
(i) 10.8/1
(ii) CO2 14.14%; H2O 5.07%; O2 4.08%; N2 76.71%; CO2
14.89%; O2 4.30 %; N2 80.81%
Problem 2
330 kg/s
Problem 6
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Theoretical And Actual Combustion Processes
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THEORETICAL AND ACTUAL
COMBUSTION PROCESSES
Reasons for incomplete combustion:
1. Insufficient oxygen;
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Deficiency of air: Amounts of air less than the stoichiometric
amount. Often expressed as percent deficiency of air.
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Predicting the composition of the
products is relatively easy when
the combustion process is
assumed to be complete.
With actual combustion
processes, it is impossible to
predict the composition of the
products on the basis of the
mass balance alone.
Then the only alternative we
have is to measure the amount
of each component in the
products directly.
A commonly used device to
analyze the composition of Determining the mole fraction of the
combustion gases is the Orsat CO2 in combustion gases by using
gas analyzer. the Orsat gas analyzer.
The results are reported on a
dry basis.
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ENTHALPY OF FORMATION AND
ENTHALPY OF COMBUSTION
Disregarding any changes in kinetic and potential energies, the energy change
of a system during a chemical reaction is due to a change in state and a change
in chemical composition:
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Heating value: The amount of heat
released when a fuel is burned
completely in a steady-flow process
and the products are returned to the
state of the reactants. The heating
value of a fuel is equal to the
absolute value of the enthalpy of
combustion of the fuel.
Higher heating value (HHV): When
the H2O in the products is in the
liquid form.
Lower heating value (LHV): When
the H2O in the products is in the
vapor form.
The higher heating value of a fuel is equal to the sum of the lower heating
value of the fuel and the latent heat of vaporization of the H2O in the products.
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36.85 37.35
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CO.
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Air and fuel-vapour mixtures
Thermal NOx
- Oxidation of atmospheric N2 at high temperatures
N 2 O2 2 NO
NO 12 O2 NO2
- Formation of thermal NOx is favorable at higher temperature
Fuel NOx
- Oxidation of nitrogen compounds contained in the fuel
Formation of CO
- Incomplete Combustion
- Dissociation of CO2 at high temperature
CO2 CO 12 O2
Air Pollutants from Combustion
Acknowledgement: