Training on
Technologies for Converting Waste Agricultural Biomass into Energy
Organized by
United Nations Environment Programme (UNEP DTIE IETC)
23-25 September, 2013
San Jose, Costa Rica
Basics of Combustion
Surya Prakash Chandak
Senior Programme Officer
International environmental Technology Centre
Division of Technology, Industry and Economics
Osaka, Japan
BASICS OF COMBUSTION
• Combustion
 Generation of heat through rapid chemical reactions of
fuels is known as combustion
• Products of Combustion
-
CO2
H2O
NO2
SO2
CO,
HCs,
NOX, SOX, ….
Complete Combustion
Incomplete
Combustion
BASICS OF COMBUSTION
 Main parameters for proper combustion
- Temperature: To initiate and sustain combustion
- Turbulence: For proper mixing of fuel and air
- Time: Sufficient for complete combustion
3T’s : Time, Temperature,
Turbulence
BASICS OF COMBUSTION
• Combustion
 Flame of different fuels
BASICS OF COMBUSTION
• Combustion Reactions
 During combustion, molecules undergo chemical
reactions.
 The reactant atoms are rearranged to form new
combinations (oxidized).
 The chemical reaction can be presented by reaction
equations.
 However, reaction equations represent initial and final
results and do not indicate the actual path of the reaction,
which may involve many intermediate steps and
intermediate products.
 This approach is similar to thermodynamics system
analysis, where only end states and not path mechanism
are used.
BASICS OF COMBUSTION
• Combustion Reactions
 Types of combustion reactions:
- Exothermic: Heat is released
- Endothermic: Heat is absorbed
BASICS OF COMBUSTION
• Combustion Reactions
Exothermic
Endothermic
+3000
C + 4H + 4O
+2000
Break two
“O=O” bonds
Form two “C=O”
bonds
+ 988 kJ/mol
-1598 kJ/mol
C + 4H + 2O2
CO2 + 4H + 2O
+1000
Break four “CH” bonds
Form four “O-H”
bonds
+ 1644 kJ/mol
0
CH4 + 2O2
(Reactants)
-1836 kJ/mol
Net energy change
-802 kJ/mol
Exothermic – gives off heat
energy
-1000
CO2 + 2H2O (Products)
BASICS OF COMBUSTION
• Combustion Reactions




 Some fundamental reactions of combustion:
C + O2  CO2 + 33.8 MJ/kg-C
2H2 + O2  2H2O + 121.0 MJ/kg-H
S + O2  SO2 + 9.3 MJ/kg-S
2C + O2  2CO + 10.2 MJ/kg-C
 Note: Above equations are in accordance with
conservation of mass. For example consider the first
reaction:
- 1 kmol C + 1 kmol O2  1 kmol CO2, or
- 12 kg C + 32 kg O2  44 kg CO2, or
- 0 vol. C + 1 vol. O2  1 vol. CO2.
BASICS OF COMBUSTION
• Combustion Reactions



 In fuels, the combustion reactions are more complex than
above:
In general, air is used in combustion than pure oxygen
Fuels consists of many elements such as C, H, N, S, O
In addition to complete combustions, fuels undergo incomplete
combustions too.
 Heat generation during combustion:
- Combustion reactions together with enthalpies of
components could be used to predict the net heat
generation.
- This needs identification of all the combustion products.
BASICS OF COMBUSTION
• Composition of Air
 On a molar (or volume) basis, dry air is composed of:
–
20.9% oxygen O2
–
78.1% nitrogen N2
–
0.9% CO2, Ar, He, Ne, H2, and others
 A good approximation of this by molar or volume is: 21%
oxygen, 79% nitrogen
 Thus, each mole of oxygen is accompanied 0.79/0.21 =
3.76 moles of nitrogen
BASICS OF COMBUSTION
• Composition of Air
 At ordinary combustion temperatures, N2 is inert, but
nonetheless greatly affects the combustion process
because its abundance, and hence its enthalpy change,
plays a large part in determining the reaction
temperatures.
-
This, in turn, affects the combustion chemistry.
Also, at higher temperatures, N2 does react, forming
species such as oxides of nitrogen (NOx), which are a
significant pollutant.
BASICS OF COMBUSTION
• Stoichiometry and Air/Fuel Ratios
 Oxidation all the elements or components in a fuel is known
as complete combustion or “Stoichiometric Combustion”.
 The amounts of fuel and air taking part in a combustion
process are often expressed as the ‘air to fuel’ ratio:
mair
AFR 
.
m fuel
 Minimum amount of air (or oxygen) required to have a
complete combustion is represented by Stoichiometric Ratio
AFRstoich.
 For a fuel CxHyOz
AFRStoich

34.32  4 x  y  2 z 
.
12 x  y  16 z 
BASICS OF COMBUSTION
• Stoichiometry and Air/Fuel Ratios
 Eg: Combustion of Methane
CH4 + 2(O2 + 79/21N2 )  CO2 + 2H2O + 158/21N2
Therefore, AFRStoich = (232 + 22879/21)/(12 + 41) = 17.16
Fuel
Very light fuel oil
Light fuel oil
Medium heavy fuel oil
Heavy fuel oil
Generic Biomass
Coal A
LPG (90 P : 10 B)
Carbon
Phase
liquid
liquid
liquid
liquid
solid
solid
gas
solid
AFRStoich
14.27
14.06
13.79
13.46
5.88
6.97
15.55
11.44
BASICS OF COMBUSTION
• Stoichiometry and Air/Fuel Ratios
 In order to obtain complete combustion, supply of excess
amount of air (or oxygen) is required in practice.
 The amount of excess air required depends on the
properties of the fuel and the technology of the
combustion device.
 Amount of excess air is usually represented by the
equivalence ratio, φ, or the ‘lambda’ ratio λ:
BASICS OF COMBUSTION
• Stoichiometry and Air/Fuel Ratios
 Eg:
Fuel
Type of Furnace or Burners

Pulverized Coal
Crushed coal
Coal
Fuel oil
Acid sludge
Natural coke ovens and
refinery gas
Blast furnace gas
Wood
Bagasse
Black liquor

Completely water-cooled furnace for slag-tap or dry-ashremoval
Partially water cooled furnace for dry-ash-removal
Excess air %
by weight
15 – 20
15 - 40
Cyclone furnace – pressure or suction
 Spreader stroker
 Water-cooled vibrating grate stroker
 Chain-grate and traveling grate strokers
 Underfeed stroker
10 - 15
30 – 60
30 – 60
15 – 50
20 - 50
 Oil burners, register type
 Multi-fuel burners and flat-flame
Cone and flat-plate-type burners, steam-atomized
 Register-type burners
 Multi-fuel burners
Intertube nozzle-type burners
Dutch oven and Hofft-type
All furnaces
Recovery furnace for kraft and soda-pulping processes
5 – 10
10 - 20
10 - 15
5 – 10
7 - 12
15 - 18
35 – 50
25 - 35
5-7
BASICS OF COMBUSTION
• Combustion Reactions of Fuels
 Complete combustion of hydrocarbons:
y  2x 
y
y  2x 




CH y O x  1 
O

3
.
76
N

CO

H
O

3
.
76
1

 2

 N 2  Heat.
2
2
2
4 
2
4 


 Incomplete combustion of hydrocarbons :
C x H y O z  pO 2  3.76 N 2    CO   H 2   CH 4  r NOX  s O 2 
  CO2   H 2O  3.76 p N 2  Heat.
BASICS OF COMBUSTION
• Estimation of Heating Values
 Eg: Methane:
CH4 + 2(O2 + 79/21N2 )  CO2 + 2H2O + 158/21N2
Enthalpies
CH4 : -4.667 MJ/kg; O2 : 0.0; N2 : 0.0
CO2 : -8.942 MJ/kg; H2O : -13.423 MJ/kg (Gas) / -15.866 MJ/kg (Liquid)
(i) Net Calorafic Value
NCV = - (Hproducts – Hreactants)/mass of CH4
= - [{-8.94244 + -13.423218} – {-4.66716}]/16 = 50.125 MJ/kg
(ii) Gross Calorafic Value
GCV = - (Hproducts – Hreactants)/mass of CH4
= - [{-8.94244 + -15.866218} – {-4.66716}]/16 = 55.622 MJ/kg
Note: NCV = GCV – (Mwater/Mmethane)hfg
= 55.622 – (36/16)2.443 = 50.125 MJ/kg.