Combustion Basics

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.

In practical engines and power plants the source of

heat is the chemical energy of substances (fuels). This
energy is released during the chemical reaction of the
fuel with oxygen. Heat is released during the reaction.
 Fuel
The most important fuel elements are carbon and
hydrogen, and most fuels consists of these and
sometimes a small amount of sulphur.

The fuel may contain some oxygen and a small quantity

of water vapour, nitrogen or ash (incombustibles).

Where fuels contain some oxygen this oxygen is

available for the combustion process, and so the
fuel requires a smaller supply of air.
Fuel
 Oxygen in Air

For a given amount of fuel there is a definite amount of

oxygen required for its complete combustion (therefore
a definite amount of air).

To ensure complete combustion of fuel, it is usual to

supply air in excess of the amount required for
chemically correct combustion.
 Source of Oxygen
The most convenient source of oxygen supply is that of
the atmosphere.

Air also contains Nitrogen and traces of other gases.

It is usual in combustion calculations to take air as:

23.3% O2 and 76.7% N2 by mass;
21% O2 and 79% N2 by volume.

The small traces of other gases in dry air are included in the nitrogen,
which is sometimes called atmospheric nitrogen.
 Fuel

Gaseous Fuels (chemically simplest)

Natural gas
Refinery gas
Liquid Fuels (mostly hydrocarbons)
Kerosene
Gasoline, diesel
Alcohol (Ethanol)
Oil
Solid Fuels
Coal (Anthracite, bituminous, subbituminous, lignite)
Wood
C O2 CO2
1
H2 O2 H2 O
2
 C8 H18 A O2 B CO2 D H2O

C8 H18 A O2 8 CO2 9 H2O

C8 H18 12.5 O2 8 CO2 9 H2O

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 A O2 8 CO2 9 H2O

C8 H18 12.5 O2 8 CO2 9 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

Proximate analysis gives the percentages of inherent

moisture, volatile matter, and combustible solid called
fixed carbon.
The fixed carbon is found as a remainder by deducting
the other quantities.
Analysis of Liquid Fuels
Analysis by volume of a Typical Natural Gas
 Fuel
Properties of Selected Fuels
CH4 C2H6 C3H8 Other HCs H2S Heating Value
(wt%) (106 J/m3)
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

(Ultimate analysis) C H N O S Heating value

(wt%) (106 J kg-1)
Gasoline (No.2) 86.4 12.7 0.1 0.1 0.4-0.7

(Proximate analysis) Carbon Volatile matter Moisture Ash Heating value

(%) (%) (%) (%) (106 J kg-1)
Anthracite (PA) 77.1 3.8 5.4 13.7 27.8
Bituminous (PA) 70.0 20.5 3.3 6.2 33.3
Subbituminous (CO) 45.9 30.5 19.6 4.0 23.6
Lignite (ND) 30.8 28.2 34.8 6.2 16.8

Data from Flagan and Seinfeld, Fundamentals of Air Pollution Engineering, 1988, Prentice-Hall.
 Stoichiometric air-fuel ratio

A stoichiometric mixture of air and fuel is one that

contains just sufficient oxygen for the complete
combustion of the fuel.
 Combustion Stoichiometry

Combustion in Oxygen

Cn H m O2 CO2 H 2O

1. Can you balance the above equation?

2. 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
 Combustion Stoichiometry

Combustion in Air (O2 = 21%, N2 = 79%)

Cn H m (O2 3.78N 2 ) CO2 H 2O N 2

1. Can you balance the above equation?

2. 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

 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 molar ratio

AFmole = nAir / nFuel
Where: nair = moles of air in the feed mixture
nfuel = moles of fuel in the feed mixture
 Air-Fuel Ratio

Rich mixture
- more fuel than necessary
(AF) mixture < (AF)stoich

Weak 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?
 Percentage Excess Air

actual A/F ratio - stoichiometric A/F ratio

stoichiometric A/F ratio

A- Air

F- Fuel
 Mixture Strength

Stoichiometric A/F ratio

Mixture Strength
Actual A/F ratio
 Exhaust and Flue Gas Analysis

The products of combustion are mainly gaseous and

include vapour.

For analysis, the samples are usually cooled to a

temperature below saturation temperature of the
steam.

When steam is NOT included, the analysis is said to

be dry.

An analysis that includes the steam (from the

exhaust) is called a wet analysis.
Problem 1

A sample of dry anthracite has the following composition

by mass:

C 90%; H 3%; O 2.5%; N 1%; S 0.5%; ash 3%.

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

A sample of dry anthracite has the following composition

by mass:

C 90%; H 3%; O 2.5%; N 1%; S 0.5%; ash 3%.

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.

C8H18 + 20(O2 + 3.76N2) xCO2 + yH2O + zO2 + wN2

C8H18 + 20(O2 + 3.76N2) 8CO2 + yH2O + 7.5O2 + 75.2N2

Problem 1

A sample of bituminous coal gave the following ultimate

analysis by mass:

C 81.9%; H 4.9%; O 6%; N 2.3%; ash 4.9%.

Calculate:
(i) the stoichimetric A/F ratio;

(ii) the analysis by volume of the wet and dry products of

combustion when 25% excess air is supplied.

(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

Calculate the stoichiometric A/F ratio for benzene

(C6H6), and wet and dry analysis of the combustion
products.
Problem 2

Calculate the stoichiometric A/F ratio for benzene

(C6H6), and wet and dry analysis of the combustion
products.

13.2/1; [wet:]CO2 16.13%; H2O 8.06%; N2 75.81%;

[dry:] CO2 17.54%; N2 82.46%
Example 7.3 & 7.4 from Eastop & McConkey

Ethyl alcohol is burned in a petrol engine. Calculate:

(i) The stoichiometric A/F ratio;
(ii) The A/F ratio and the wet and dry analyses by volume of
the exhaust gas for a mixture strength of 90%;
(iii) The A/F ratio and the wet and dry analyses by volume of
the exhaust gas for a mixture strength of 120%.
Example 7.3 & 7.4 from Eastop & McConkey

Ethyl alcohol is burned in a petrol engine. Calculate:

(i) The stoichiometric A/F ratio;
(ii) The A/F ratio and the wet and dry analyses by volume of
the exhaust gas for a mixture strength of 90%;
(iii) The A/F ratio and the wet and dry analyses by volume of
the exhaust gas for a mixture strength of 120%.

(i) 8.957/1; (ii) 9.952/1, [dry:]13.45% of CO2, 84.31% of N2;

(iii) 7.47/1, [dry:] 8.77% of CO2, 82.46% of N2.
Example 7.3 & 7.4 from Eastop & McConkey

For the stoichiometric mixture (just calculated), calculate:

(i) the volume of the mixture per kilogram of fuel at a
temperature of 650C and a pressure of 1.013 bar;
(ii) The volume of the products of combustion per kilogram
of fuel after cooling to a temperature of 1200C at a
pressure of 1 bar.
Example 7.3 & 7.4 from Eastop & McConkey

For the stoichiometric mixture (just calculated), calculate:

(i) the volume of the mixture per kilogram of fuel at a
temperature of 650C and a pressure of 1.013 bar;
(ii) The volume of the products of combustion per kilogram
of fuel after cooling to a temperature of 1200C at a
pressure of 1 bar.

(i) 9.219 m3 (ii) 9.219 m3

Problem 3

In the actual combustion of benzene (C6H6) in an

engine the A/F ratio was 12/1. Calculate the wet and
dry analysis of the combustion products.

CO2 13.38%; CO 3.94%; H2O 8.66%; N2 74.03%

Problem 4

The analysis of a supply gas is as follows: H2 49.4%;

CO 18%; CH4 20%; C4H8 2%; O2 0.4%; N2 6.2%; CO2 4%.
Calculate:
(i) The stoichiometric A/F ratio;
(ii) the wet and dry analysis of the products of
combustion if the actual mixture is 20% weak
(i) 4.062 (by volume)
(ii) wet: CO2 9.0%, H2 O 17.5%, O2 3.08%, N2 70.4%
Problem 5

Propal alcohol (C3H7OH) is burned with 50 percent

excess air. Write the balanced reaction equation for
complete combustion and determine the air-to-fuel
ratio.

15.5 kg air/kg fuel

Problem 6

A fuel mixture of 20 percent by mass methane (CH4)

and 80 percent by mass ethanol (C2H6O), is burned
completely with theoretical air. If the total flow rate of
the fuel is 31 kg/s, determine the required flow rate of
air.

330 kg/s
Problem 6

A certain natural gas has the following volumetric

analysis: 65 percent CH4, 8 percent H2, 18 percent N2,
3 percent O2, and 6 percent CO2. This gas is now
burned completely with the stoichiometric amount of
dry air. What is the airfuel ratio for this combustion
process?

9.42 kg air/kg fuel

Problem 7

Methane (CH4) is burned with dry air. The volumetric

analysis of the products on a dry basis is 5.20 percent
CO2, 0.33 percent CO, 11.24 percent O2, and 83.23
percent N2.
Determine (a) the airfuel ratio and (b) the percentage
of theoretical air used.

(a) 34.5 kg air/kg fuel, (b) 200 percent

The combustion reaction is a particular kind of
reaction in which products are formed from
reactants with the release or absorption of energy as
heat is transferred to or from the surroundings.
Theoretical And Actual Combustion Processes
Complete combustion: If all the carbon in the fuel burns to
CO2, all the hydrogen burns to H2O, and all the sulfur (if any)
burns to SO2.

A combustion process is complete if all the combustible components

of the fuel are burned to completion.

44
Theoretical And Actual Combustion Processes

Incomplete combustion: If the combustion products contain

any unburned fuel or components such as C, H2, CO, or OH.

Reasons for incomplete combustion: 1 Insufficient oxygen,

2 insufficient mixing in the combustion chamber during the
limited time that the fuel and the oxygen are in contact, and 3
dissociation (at high temperatures).

45
THEORETICAL AND ACTUAL
COMBUSTION PROCESSES
Reasons for incomplete combustion:

1. Insufficient oxygen;

2. insufficient mixing in the combustion chamber during the

limited time that the fuel and the oxygen are in contact, and

3. dissociation (at high temperatures).

Oxygen has a much greater tendency to combine with

hydrogen than it does with carbon. Therefore, the hydrogen in
the fuel normally burns to completion, forming H2O.

Some of the carbon, however, ends up as CO or just as plain

C particles (soot) in the products.
46
 Stoichiometric or theoretical air: The minimum amount of air
needed for the complete combustion of a fuel. Also referred to
as the chemically correct amount of air, or 100% theoretical air.

Stoichiometric or theoretical combustion: The ideal

combustion process during which a fuel is burned completely
with theoretical air.

The complete combustion process with no free oxygen in

47
the products is called theoretical combustion.
Excess air: The amount of air in excess of the stoichiometric
amount. Usually expressed in terms of the stoichiometric air as
percent excess air or percent theoretical air.

50% excess air = 150% theoretical air

200% excess air = 300% theoretical air.

90% theoretical air = 10% deficiency of air

48
Deficiency of air: Amounts of air less than the stoichiometric
amount. Often expressed as percent deficiency of air.

Equivalence ratio: The ratio of the actual fuelair ratio to the

stoichiometric fuelair ratio.

49
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.
50
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:

When the existing chemical bonds

are destroyed and new ones are
formed during a combustion process,
The microscopic form of energy of a
usually a large amount of sensible
substance consists of sensible, latent,
energy is absorbed or released.
51
chemical, and nuclear energies.
 ~
ENTHALPY OF FORMATION h f

The standard enthalpy of formation of a compound is the

change of enthalpy that accompanies the formation of 1
mole of a substance in its standard state from its
constituent elements in their standard states.

(The most stable form of the element at 1 bar of pressure

and the specified temperature, usually 298.15 K or 25
degrees Celsius).
 ~
ENTHALPY OF FORMATION h f

The standard enthalpy change of formation is measured in

units of energy per amount of substance. Most are defined
in kilojoules per mole, or kJ mol1.

All elements in their standard states (oxygen gas, solid

carbon in the form of graphite, etc.) have a standard
enthalpy of formation of zero, as there is no change
involved in their formation.

To establish a starting point, we assign the enthalpy of formation of all

stable elements (such as O2, N2, H2, and C) a value of zero at the
standard reference state of 25C and 1 atm.
 ~
ENTHALPY OF FORMATION h f

The standard enthalpy change of formation is used in

thermodynamics to find the standard enthalpy change of
reaction.

This is done by subtracting the sum of the standard

enthalpies of formation of the reactants from the sum of
the standard enthalpies of formation of the products, as
shown in the equation below.
The molar enthalpy of reaction may be calculated using:
The enthalpy of formation hf : The amount of energy
absorbed or released as the component is formed from its
stable elements during a steady-flow process at a specified
state.

57
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.

For the fuels with variable

composition (i.e., coal, natural gas,
fuel oil), the heating value may be
determined by burning them
directly in a bomb calorimeter.
58
Problem 5

Calculate the molar enthalpy of reaction at 250C of

ethyl alcohol, C2H5OH.
-1 231 428 kJ/kmol
Problem 5
The enthalpy of combustion of propane gas, C3H8, at
250C with the H2O in the products in the liquid phase is
-50 360 kJ/kg. Calculate the enthalpy of combustion
with the H2O in the vapour per unit mass of fuel and
per unit amount of substance of fuel.

-46 364 kJ/kg; -2 040 030 kJ/kmol

Partial Combustion
 BOMB CALORIMETER
A calorimeter is a device used for calorimetry, the science of
measuring the heat of chemical reactions or physical changes as
well as heat capacity.
Specific heat is the amount of heat per unit mass required to
raise the
temperature by one Kelvin:
Substance c in J/gm K c in cal/gm K
Aluminum 0.9 0.215
Bismuth 0.123 0.0294
Copper 0.386 0.0923
Brass 0.38 0.092
Gold 0.126 0.0301
Q = heat added (energy) Lead 0.128 0.0305

c = specific heat Silver 0.233 0.0558

Tungsten 0.134 0.0321
m = mass Zinc 0.387 0.0925
T = change in temperature Mercury 0.14 0.033
Alcohol(ethyl) 2.4 0.58
Water 4.186 1
Ice (-10 C) 2.05 0.49
Granite 0.79 0.19
Glass 0.84 0.2
 Question Time

How does the presence of N2 in air affect the outcome of

a combustion process?

Nitrogen, in general, does not react with other chemical

species during a combustion process but its presence
affects the outcome of the process because nitrogen
absorbs a large proportion of the heat released during the
chemical process.

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 Question Time

How does the presence of moisture in air affect the

outcome of a combustion process?

Moisture, in general, does not react chemically with any of

the species present in the combustion chamber, but it
absorbs some of the energy released during combustion,
and it raises the dew point temperature of the combustion
gases.

1/16/2017 68
 Question Time

What does the dew-point temperature of the product

gases represent? How is it determined?

The dew-point temperature of the product gases is the

temperature at which the water vapor in the product gases
starts to condense as the gases are cooled at constant
pressure. It is the saturation temperature corresponding to
the vapor pressure of the product gases.

1/16/2017 69
 Question Time

Is the number of atoms of each element conserved

during a chemical reaction? How about the total number
of moles?

The number of atoms are preserved during a chemical

reaction, but the total mole numbers are not.

C6 H 6 7.5(O2 3.78N 2 ) 6CO2 3H 2O 28.35N 2

1 + 7.5 + (7.5 * 3.78) 6 + 3 + 28.35

36.85 37.35
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 Question Time

Which is more likely to be found in the products of an

incomplete combustion of a hydrocarbon fuel, CO or
OH? Why?

CO.

Because oxygen is more strongly attracted to hydrogen

than it is to carbon, and hydrogen is usually burned to
completion even when there is a deficiency of oxygen.

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 Question Time

What is enthalpy of combustion? How does it differ from

the enthalpy of formation?

Enthalpy of combustion, which represents the amount of

heat released during a steady-flow combustion process.
[Whereas]
Enthalpy of formation is the enthalpy of a substance due
to its chemical composition. The enthalpy of formation is
related to elements or compounds whereas the enthalpy of
combustion is related to a particular fuel.

1/16/2017 72
 Question Time

The enthalpy of formation of N2 is listed as zero. Does

this mean that N2 contains no chemical energy at the
standard reference state?

No. The enthalpy of formation of N2 is simply assigned a

value of zero at the standard reference state for
convenience.

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 Question Time

Which contains more chemical energy, 1 kmol of

H2 or 1 kmol of H2O?

1 kmol of H2. This is evident from the observation that

when chemical bonds of H2 are destroyed to form H2O a
large amount of energy is released.

1/16/2017 74
Air and fuel-vapour mixtures

If the NCV of hydrogen is 10 000 kJ/kg, calculate the

calorific value of the combustion mixture per cubic
metre at 1.013 bar and 150C.

Molar mass of air = 28.96 kg/kmol

 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
 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?
Please attempt problems 7.1 to 7.12

Acknowledgement:

Prepared by: Ying Li