ENV 4101/5105 Elements of Air Pollution
Combustion
Basics
Prepared by: Ying Li
4/9/2015
Aerosol & Particulate Research Laboratory
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Combustion Basics
• Fuel
• Combustion Stoichiometry
• Air/Fuel Ratio
• Equivalence Ratio
• Air Pollutants from Combustion
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Fuel
 Gaseous Fuels
• Natural gas
• Refinery gas
 Liquid Fuels
•
•
•
•
Kerosene
Gasoline, diesel
Alcohol (Ethanol)
Oil
 Solid Fuels
• Coal (Anthracite, bituminous, subbituminous, lignite)
• Wood
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Fuel
 Properties of Selected Fuels
CH4
C2H6
C3H8
Other HCs
H2S
Heating Value
(106 J/m3)
(wt%)
Natural gas (No.1)
87.7
5.6
2.4
1.8
2.7
43.2
Natural gas (No.2)
88.8
6.4
2.7
2.0
0.0004
41.9
H
N
O
(Ultimate analysis)
C
S
Heating value
(106 J kg-1)
(wt%)
Gasoline (No.2)
86.4
(Approximate analysis)
Anthracite
(PA)
Bituminous
(PA)
Subbituminous
Lignite (ND)
Carbon
(CO)
12.7
0.1
Volatile matter
0.1
Moisture
0.4-0.7
Ash
Heating value
(106 J kg-1)
(%)
(%)
(%)
(%)
77.1
3.8
5.4
13.7
27.8
70.0
20.5
3.3
6.2
33.3
45.9
30.5
19.6
4.0
23.6
30.8
28.2
34.8
6.2
16.8
Which one has a higher energy density per mass?
Do they burn in the same way?
Data from Flagan and Seinfeld, Fundamentals of Air Pollution Engineering, 1988, Prentice-Hall.
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Combustion Stoichiometry
 Combustion in Oxygen
Cn H m  O2  CO2  H 2O
1.
2.
Can you balance the above equation?
Write the reactions for combustion of methane and
benzene in oxygen, respectively.
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
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Combustion Stoichiometry
 Combustion in Air (O2 = 21%, N2 = 79%)
Cn H m  (O2  3.78N 2 )  CO2  H 2O  N 2
1.
2.
Can you balance the above equation?
Write the reactions for combustion of methane and benzene
in air, respectively.
Answer
m
m
m


Cn H m   n  (O2  3.78N 2 )  nCO2  H 2O  3.78 n   N 2
4
2
4


CH 4  2(O2  3.78N 2 )  CO2  2H 2O  7.56N 2
C6 H 6  7.5(O2  3.78N 2 )  6CO2  3H 2O  28.35N 2
1. What if the fuel contains O, S, Cl or other elements?
2 Is it better to use O2 or air?
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Air-Fuel Ratio
 Air-Fuel (AF) ratio
AF = m Air / m Fuel
Where:
m air = mass of air in the feed mixture
m fuel = mass of fuel in the feed mixture
Fuel-Air ratio: FA = m Fuel /m Air = 1/AF
 Air-Fuel molal ratio
AFmole = nAir / nFuel
Where:
nair = moles of air in the feed mixture
nfuel = moles of fuel in the feed mixture
What is the Air-Fuel ratio for stoichiometric combustion of
methane and benzene, respectively?
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Air-Fuel Ratio
 Rich mixture
- more fuel than necessary
(AF) mixture < (AF)stoich
 Lean mixture
- more air than necessary
(AF) mixture > (AF)stoich
Most combustion systems operate under lean conditions.
Why is this advantageous?
Consider the combustion of methanol in an engine. If the Air-Fuel
ratio of the actual mixture is 20, is the engine operating under rich
or lean conditions?
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Equivalence Ratio
Equivalence ratio: shows the deviation of an actual
mixture from stoichiometric conditions.
( FA) actual ( AF ) stoich


( FA) stoich ( AF ) actual
The combustion of methane has an equivalence ratio Φ=0.8
in a certain condition. What is the percent of excess air (EA)
used in the combustion?
How does temperature change as Φ increases?
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Formation of NOx and CO in Combustion
 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
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Air Pollutants from Combustion
Source: Seinfeld, J. Atmospheric Chemistry and Physics of Air Pollution.
How do you explain the trends of the exhaust HCs, CO,
and NOx as a function of air-fuel ratio?
How do you minimize NOx and CO emission?
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Quick Reflections
• Fuel
• Combustion Stoichiometry
• Air/Fuel Ratio
• Equivalence Ratio
• Air Pollutants from Combustion
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