AMBO UNIVERSITY HACHALU HUNDESSA CAMPUS SCHOOL

AMBO UNIVERSITY HACHALU HUNDESSA
CAMPUS
SCHOOL OF MECHANICALAND INDUSTRIAL
ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEERING
er Plant Engineering
pter 3
s and Combustion
2022/2023
By : Abubeker N.

INTRODUCTION
 Any material that can be burned to release thermal energy
called a fuel.
 Most familiar fuels consist primarily of hydrogen and carbon.
 They are called hydrocarbon fuels and are denoted by th
general formula CnHm.
 Hydrocarbon fuels exist in all phases, some examples bein
coal, gasoline, and natural gas.

A chemical reaction during which a fuel is oxidized and a large quanti
f energy is released is called combustion.
The oxidizer most often used in combustion processes is air, for obvio
easons—it is free and readily available.
ure oxygen O2 is used as an oxidizer only in some specializ
pplications, such as cutting and welding, where air cannot be used.
During a combustion process, the components that exist before t
eaction are called reactants and the components that exist after th
eaction are called products
troduction Cont…

The increasing worldwide demand for energy has focuse
attention on fuels, their availability and environmental effects
The fuels available to utility industry are largely nuclear an
ossil, both essentially nonrenewable.
Fossil fuels originate from the earth as a result of the slo
decomposition and chemical conversion of organic material.
Coal represents the largest fossil-fuel energy resource in th
world.
troduction Cont…

Classification of fuels
‘Fuel’ refers to a combustible substance capable of releasing h
during its combustion.
In general fuels have carbon, hydrogen and sulphur as the ma
combustible chemical elements.
Sulphur is found to be relatively less contributor to the total h
released during combustion.
Fuels may be classified as
 solid,
 liquid and
 gaseous fuel depending upon their state.

oal is the most common solid fuel.
oal is a dark brown/black sedimentary rock derived prima
om the unoxidized remains of carbon-bearing plant tissues.
Coal is a general term that encompasses a large number of so
rganic minerals with widely differing compositions
roperties, although all are essentially rich in amorous (with
egular structure) elemental carbon.
assification of fuels Solid fuel

t can be further classified into different types based upon t
omposition.
Composition can be estimated using either “proximate analysis”
y “ultimate analysis”.
According to geological order formation, coal may be of t
ollowing type
lant debris peat lignite Brown coal sub-bitumino
oal Bituminous coal semi-bituminous coal sem
nthracite coal Anthracite coal graphite.

With increase percentage of carbon.
Decrease volatile matter.
Decrease moisture content.
Increasing heating value
Lignite:- lowest grade coal.
- Containing moisture as high as 30% and high volatile matter.
Bituminous :- the largest group.
-Contains 46-86% of fixed carbon and 20-40% of volatile
matter.
-The lower volatile, the higher the heating value.
Anthracite:- contains more than 86% fixed carbon and
less volatile matter.

ccording to ASTM ( American Society of Testing and Materials)
Peat is not regarded as a rank of a coal.
eat contains up to 90% moisture and is not attractive as utility fu
ank carries the meaning of degree maturation ( carbonization) a
a measure of carbon content in coal.
ignite is considered to be low rank and anthracite to be high rank

There are two types of coal analysis.
1. Proximate analysis.
2. Ultimate analysis.
Both done on a mass percent basis.
Both of these methods may be based on:
o an as received basis,
o useful for combustion calculations; and
o a moisture free basis

ximate analysis
his is the easier of two types of coal analysis and the one which supp
eadily meaningful information for coal’s use in steam generators.
determines the mass percentages of fixed carbon, volatile ma
moisture, and ash. Sulfur is obtained a separate determination.
ed carbon + volatile matter + moisture + ash = 100% by mass.
FC = 100-( VM + M + A)
ixed carbon is the elemental carbon that exists in coal.
n proximate analysis, its determination is approximated by assuming i
e the difference between the original sample and the sum of vola
matter, moisture, and ash.

The volatile matter is that portion of coal, other than water vap
which is driven off when the sample is heated in the absence
xygen in a standard test (up to 1750°F or 7 min).
t consists of hydrocarbon and other gases that result fro
istillation and decomposition.
Moisture is determined by a standard procedure of drying in
ven.
his does not account for all the water present, which includ
ombined water and water of hydration.
There are several other terms for moisture in coal.
One, inherent moisture, that existing in the natural state of coal a
onsidered to be part of the deposit, excluding surface water.

Ash is the inorganic salts contained in coal.
t is determined in practice as the noncombustible residue af
he combustion of dried coal in a standard test (at 1380°F).
Sulfur is determined separately in a standard test, given
ANSUASTM Standards D 2492.
Being combustible, it contributes to the heating value of t
coal. It forms oxides which combine with water to form acids

The proximate analysis indicates the behavior of coal when it is heated.
When 1gram sample of coal is subjected to a temperature of about 10
for a period of 1 hour, the loss in weight of the sample gives the mois
content of the coal.
When 1 gram sample of coal is placed in covered platinum crucible
heated to 9500C and maintained at that temperature for about 7 minute.
There is a loss in weight due to the eliminating of moisture and vola
matter.
The latter may now be determine since moisture has been calculated fr
previous test.
Volatile matter consists of hydrogen and certain hydrogen-car
compound.

When 1 gram sample of coal is placed in uncovered platinum cruc
and heated to 7200C until the coal is completely burned, a constant we
reached, which indicates that there is only ash remaining in the crucible
Complete combustion of coal is determined by repeated weighing of
sample.
This difference does not represent all the carbon that was in the coal.
ome of the carbon may have been in the form of hydrocarbons which
ave been distilled off while determining the volatile matter.
The amount of volatile matter indicates whether the coal will burn w
hort or long flame and it tends to produce smoke.
he more volatile the coal, the more it will smoke.

imate Analysis
The ultimate analysis gives the chemical elements t
omprise the coal substance, together with ash and moisture.
The coal substance consists of organic compounds of carb
ydrogen and oxygen derived from the original vegeta
matter.
The analysis shows the following components on mass basis:-
Carbon(C), oxygen(O), hydrogen(H), nitrogen(N), sulphur(
moisture(M) and ash(A)
C + H + O + N + S + M + A= 100% by mass

Fuels in liquid form are called liquid fuels.
Liquid fuels are generally obtained from petroleum and its b
products.
These liquid fuels are complex mixture of differe
hydrocarbons, and obtained by refining the crude petroleum oil.
Commonly used liquid fuels are petrol, kerosene diesel, aviatio
fuel, light fuel oil, heavy fuel oil etc.
assification of fuels Liquid fuel

Percentage by volume composition of some of liquid fuels is given belo
 Table 3.1 Composition of liquid fuels
Liquid fuels offer following advantages over solid fuel.
 Better mixing of fuel and air is possible with liquid fuel.
 Liquid fuels have no problem of ash formation.
 Storage and handling of liquid fuels is easy compared to solid fuels.
 Processing such as refining of liquid fuels is more convenient.
assification of fuels
assification of fuels Liquid fuel Cont…

These are the fuels in gaseous phase.
Gaseous fuels are also generally hydrocarbon fuels derived fro
petroleum reserves available in nature.
Most common gaseous fuel is natural gas.
Gaseous fuels may also be produced artificially from burnin
solid fuel (coal) and water.
Some of gaseous fuels produced artificially are coal gas, produc
gas etc.
assification of fuels Gaseous fuels

Volumetric analysis of gaseous fuels is presented in Table 3.2
 Table 3.2 Composition of gaseous fuels
Gaseous fuels offer all advantages as there in liquid fuels
except ease of storage.
assification of fuels Gaseous fuels Cont…

Combustion Stoichiometry
A chemical reaction during which a fuel is oxidized and
large quantity of energy is released is called combustion.
On a mole or a volume basis, dry air is composed of 2
percent oxygen, 78.1 percent nitrogen, 0.9 percent argon, a
small amounts of carbon dioxide, helium, neon, and hydroge
In the analysis of combustion processes, the argon in the
is treated as nitrogen, and the gases that exist in trace amou
are ignored.

Then dry air can be approximated as 21 percent
xygen and 79 percent nitrogen by mole numbers.
Therefore, each mole of oxygen entering a
ombustion chamber is accompanied by 0.79/0.21
.76 mol of nitrogen (Fig. 3–1).
That is,
FIGURE 3–1 Each kmol of O
is accompanied by 3.76 kmol o
ombustion Stoichiometry Cont…

A frequently used quantity in the analysis of combustio
processes to quantify the amounts of fuel and air is the air
uel ratio AF.
It is usually expressed on a mass basis and is defined as th
ratio of the mass of air to the mass of fuel for a combustio
process (Fig. 3–2).
That is,
FIGURE 3–2 The air–fuel ra
represents the amount of air u
unit mass of fuel during a com
process.
mass m of a substance is related to the
ber of moles N through the relation m= NM,
e M is the molar mass.

t is often instructive to study the combustion of a fuel
ssuming that the combustion is complete.
A combustion process is complete if all the carbon in the f
urns to CO2, all the hydrogen burns to H2O, and all the su
if any) burns to SO2.
That is, all the combustible components of a fuel are burned
ompletion during a complete combustion process (Fig. 3–3).
FIGURE 3–3 A combustion process is complete if all the
combustible components of the fuel are burned to completion.

Conversely, the combustion process is incomplete if t
combustion products contain any unburned fuel or compone
such as C, H2, CO, or OH.
Insufficient oxygen is an obvious reason for incompl
combustion, but it is not the only one.
Incomplete combustion occurs even when more oxygen is pres
n the combustion chamber than is needed for compl
combustion.
This may be attributed to insufficient mixing in the combusti
chamber during the limited time that the fuel and the oxygen are
contact.
Another cause of incomplete combustion is dissociation, wh
becomes important at high temperatures.

The minimum amount of air needed for the complete combusti
of a fuel is called the stoichiometric or theoretical air.
Thus, when a fuel is completely burned with theoretical air,
uncombined oxygen is present in the product gases.
The theoretical air is also referred to as the chemically corre
amount of air, or 100 percent theoretical air.
A combustion process with less than the theoretical air is bound
be incomplete.

The ideal combustion process during which a fuel is burn
completely with theoretical air is called the stoichiometric
heoretical combustion of that fuel.
 CH4 +2(O2+3.76N2) CO2 + 2H2O + 7.52N2
 No unburned fuel
 No free oxygen on products
Notice that the products of the theoretical combustion contain
unburned methane and no C, H2, CO, OH, or free O2.

The amount of air in excess of the stoichiometric amount
called excess air.
The amount of excess air is usually expressed in terms of t
stoichiometric air as percent excess air or percent theoretic
air.
For example, 50 percent excess air is equivalent to 1
percent theoretical air, and 200 percent excess air is equivale
o 300 percent theoretical air.
Of course, the stoichiometric air can be expressed as 0 perce
excess air or 100 percent theoretical air.

Amounts of air less than the stoichiometric amount are cal
eficiency of air and are often expressed as percent deficiency
ir.
For example, 90 percent theoretical air is equivalent to 10 perc
eficiency of air.
The amount of air used in combustion processes is also expressed
erms of the equivalence ratio, which is the ratio of the act
uel–air ratio to the stoichiometric fuel–air ratio.

Combustion
stoichiometry
Combustion
process
Complete Incomplete
Amount of
air
Theoretical air
Excess air
Deficiency
of air

. Ethane (C2H6) is burned with 20 percent excess air during
combustion process, as shown in Fig.. Assuming comple
combustion and a total pressure of 100 kPa, determine (a
the air–fuel ratio and (b) the dew-point temperature of th
products.
ombustion Stoichiometry Examples

Enthalpy Of Formation (ΔHf)
A combustion reaction is a particular kind of chemic
reaction in which products are formed from reactants with th
release or absorption of energy as heat is transferred to o
from the surroundings.
In some substances like hydrocarbon fuels which are man
in number and complex in structure the heat of reaction o
combustion may be calculated on the basis of known value
of the enthalpy of formation, ΔHf of the constituent of th
reactants and products at the temperature T0 (referenc
temperature).

The enthalpy of formation (ΔHf) is the increase in entha
when a compound is formed from its constituent elements
their natural form and in a standard state.
The standard state is 25°C, and 1 atm. pressure, but it m
be borne in mind that not all substances can exist in natu
form, e.g. H2O cannot be a vapour at 1 atm. and 25°C.
Property values at the standard reference state are indicated b
a superscript (°) (such as h° and u°)
The expression of a particular reaction, for calculat
purposes, may be given as :
nthalpy Of Formation (ΔHf) Cont…

Consider the formation of CO2 from its elements, carbon an
oxygen, during a steady-flow combustion process (Fig. 3–4).
Both the carbon and the oxygen enter the combustion chamb
at 25°C and 1 atm.
The CO2 formed during this process also leaves t
combustion chamber at 25°C and 1 atm.
FIGURE 3-4 The formation of CO2 du
steady flow combustion process at 25C
atm.

Therefore, some heat is transferred from the combustio
chamber to the surroundings during this process, which i
393,520 kJ/kmol CO2 formed.
The process described above involves no work interactions.
Therefore, from the steady-flow energy balance relation, th
heat transfer during this process must be equal to th
difference between the enthalpy of the products and th
enthalpy of the reactants.
That is,
 Q = Hprod - Hreact =-393,520 Kj/kmol

The enthalpy of reaction hR is defined as the difference betwee
he enthalpy of the products at a specified state and the enthalp
f the reactants at the same state for a complete reaction.
or combustion processes, the enthalpy of reaction is usuall
eferred to as the enthalpy of combustion hC, which represen
he amount of heat released during a steady-flow combustio
rocess when 1 kmol (or 1 kg) of fuel is burned completely at
pecified temperature and pressure (Fig. 3–5).
FIGURE 3–5
The enthalpy of combustion repres
amount of energy released as a fuel is
during a steady-flow process at a s
state.

It is expressed as
 hR = hC = Hprod – Hreact
The enthalpy of formation , which can be viewed as
enthalpy of a substance at a specified state due to its chemi
composition.
To establish a starting point, we assign the enthalpy
formation of all stable elements (such as O2, N2, H2, and C
value of zero at the standard reference state of 25°C and 1 at
That is, = 0 for all stable elements.

Now reconsider the formation of CO2 (a compound) from
elements C and O2 at 25°C and 1 atm during a steady-flo
process.
The enthalpy change during this process was determined to
-393,520 kJ/kmol.
However, Hreact = 0 since both reactants are elements at t
standard reference state, and the products consist of 1 kmol
CO2 at the same state.
Therefore, the enthalpy of formation of CO2 at the standa
reference state is -393,520 kJ/kmol

That is,
The negative sign is due to the fact that the enthalpy of 1 kmo
CO2 at 25°C and 1 atm is 393,520 kJ less than the enthalpy o
mol of C and 1 kmol of O2 at the same state.
n other words, 393,520 kJ of chemical energy is relea
leaving the system as heat) when C and O2 combine to form
mol of CO2.

 Therefore, a negative enthalpy of formation for
compound indicates that heat is released during th
formation of that compound from its stable elements.
 A positive value indicates heat is absorbed.

Examples
1. Determine the enthalpy of combustion of methane (CH
at 25°C and 1 atm, using the enthalpy of formation da
from Table A–26. Assume that the water in the products
in the liquid form.

Adiabatic Flame Temperature
In the absence of any work interactions and any changes
kinetic or potential energies, the chemical energy release
during a combustion process either is lost as heat to th
surroundings or is used internally to raise the temperature o
the combustion products.
The smaller the heat loss, the larger the temperature rise
In the limiting case of no heat loss to the surroundings (
=0), the temperature of the products reaches a maximum
which is called the adiabatic flame or adiabat
combustion temperature of the reaction

FIGURE 3-4 The temperature of a combustion chamber become
maximum when combustion is complete and no heat is lost to th
urroundings (Q = 0).
The adiabatic flame temperature of a steady-flow combustio
rocess is determined from Eq. below by setting Q = 0 and W = 0
t yields
diabatic Flame Temperature Cont…

Heating values of fuels
The “calorific value or heating value” of the fuel is defined a
the energy liberated by the complete oxidation of a unit mass o
volume of a fuel.
the heating value of the fuel is defined as the amount of he
released when a fuel is burned completely in a steady-flo
process and the products are returned to the state of the reactants
In other words, the heating value of a fuel is equal to the absolu
value of the enthalpy of combustion of the fuel.
It is expressed in kJ/kg for solid and liquid fuels and kJ/m3 f
gases.
If a fuel contains hydrogen water will be formed as one of th
products of combustion.

If this water is condensed, a large amount of heat will b
released than if the water exists in the vapour phase.
For this reason two heating values are defined ; the higher o
gross heating value and the lower or net heating value.
The heating value depends on the phase of the H2O in th
products.
The heating value is called the higher heating value (HHV
when the H2O in the products is in the liquid form, and it
called the lower heating value (LHV) when the H2O in th
products is in the vapor form
eating values of fuels Cont…

The higher heating value, HHV, is obtained when the wat
ormed by combustion is completely condensed.
The lower heating value, LHV, is obtained when the wat
ormed by combustion exists completely in the vapour phase.
Thus
where
 m = Mass of water formed by combustion,
 hfg = Enthalpy of vaporisation of water, kJ/kg,
 ug = Specific internal energy of vapour, kJ/kg, and
 uf = Specific internal energy of liquid, kJ/kg.

Determination Of Heating Values
The calorific value of fuels can be determined either from chemi
analysis or in the laboratory.
Solid and Liquid Fuels
Dulong’s formula: Dulong suggested a formula for the calculation of t
calorific value of the solid or liquid fuels from their chemi
composition which is as given below.
Gross calorific value
where C, H, O and S are carbon, hydrogen, oxygen and sulphur
percentages respectively in 100 kg of fuel.
In the above formula the oxygen is assumed to be in combination w
hydrogen and only extra surplus hydrogen supplies the necessary heat.

e calorific value of solid and liquid
ls is determined in the laboratory by
mb calorimeter’.
is so named because its shape
embles that of a bomb. Fig. shows
schematic sketch of a bomb
orimeter.
etermination of Heating values Cont…
aboratory method (Bomb calorimeter)

The calorimeter is made of austenitic steel which provide
considerable resistance to corrosion and enables it to withstand
high pressure.
n the calorimeter is a strong cylindrical bomb in which
combustion occurs.
The bomb has two valves at the top. One supplies oxygen to th
bomb and other releases the exhaust gases.
A crucible in which a weighted quantity of fuel sample is burnt i
arranged between the two electrodes as shown in Fig.

The calorimeter is fitted with water jacket which surround
the bomb.
To reduce the losses due to radiation, calorimeter is furth
provided with a jacket of water and air.
A stirrer for keeping the temperature of water uniform and
thermometer to measure the temperature up to an accurac
of 0.001°C are fitted through the lid of the calorimeter.

Procedure.
To start with, about 1 gm of fuel sample is accurately weig
nto the crucible and a fuse wire (whose weight is known
stretched between the electrodes.
t should be ensured that wire is in close contact with the fuel
To absorb the combustion products of sulphur and nitrogen 2
of water is poured in the bomb.
Bomb is then supplied with pure oxygen through the valve to
amount of 25 atmosphere.
The bomb is then placed in the weighed quantity of water
he calorimeter.

The stirring is started after making necessary electri
connections, and when the thermometer indicates a stea
temperature fuel is fired and temperature readings are record
after 1/2 minute intervals until maximum temperature
attained.
The bomb is then removed ; the pressure slowly relea
through the exhaust valve and the contents of the bomb
carefully weighed for further analysis.

he heat released by the fuel on combustion is absorbed by the surroun
water and the calorimeter.
rom the above data the calorific value of the fuel can be found in
ollowing way :
et
wf = Weight of fuel sample (kg),
w = Weight of water (kg),
C = Calorific value (higher) of the fuel (kJ/kg),
we = Water equivalent of calorimeter (kg),
t1 = Initial temperature of water and calorimeter,
t2 = Final temperature of water and calorimeter,
tc = Radiation corrections, and
c = Specific heat of water.

Heat released by the fuel sample = wf × C
Heat received by water and calorimeter
Heat lost = Heat gained
Value of c is 4.18 in SI units

Examples
1. The following particulars refer to an experiment
determination of the calorific value of a sample of co
containing 88% C and 4.2% H2. Weight of coal = 0.848 gm
weight of fuse wire 0.027 gm, of calorific value 6700 J/gm
weight of water in the calorimeter = 1950 gm, wat
equivalent of calorimeter = 380 gm, observed temperatu
rise = 3.06°C, cooling correction = + 0.017°C.
 Find the higher and lower calorific values of the coal.

seous Fuels
e calorific value of gaseous
ls can be determined by
nker’s gas calorimeter.
. illustrates Junker’s gas
orimeter.
principle is some what similar
Bomb calorimeter ; in respect
heat evolved by burning the
is taken away by the water.
Fig. Junker’s gas calorimeter

In its simplest construction it consists of a combustio
chamber in which the gas is burnt (in a gas burner).
A water jacket through which a set of tubes called flues pa
surrounds this chamber.
Thermometers are incorporated at different places to measu
the temperatures.

Procedure.
A metered quantity of gas whose calorific value is to b
determined is supplied to the gas burner via a gas meter whic
ecords its volume and a gas pressure regulator which measure
he pressure of the gas by means of a manometer.
When the gas burns the hot products of combustion trav
upwards in the chamber and then downwards through the flue
and finally escape to the atmosphere through the outlet.

The temperature of the escaping gas is recorded by the thermomet
fitted at the exit and this temperature should be as close to roo
temperature as possible so that entire heat of combustion is absorbed b
water.
The cold water enters the calorimeter near the bottom and leaves near th
top.
Water which is formed by condensation of steam is collected in a pot.
The quantity of gas used during the experiment is accurately measure
by the meter and temperature of ingoing and outgoing water a
indicated by the thermometers.
From the above data the calorific value of the gas can be calculated.

Examples
Following results were obtained when a sample of gas wa
ested by Junker’s gas calorimeter :
Gas burnt in the calorimeter = 0.08 m3, Pressure of gas supp
= 5.2 cm of water, Barometer = 75.5 cm of Hg. Temperature o
as = 13°C, Weight of water heated by gas = 28 k
Temperature of water at inlet = 10°C, Temperature of water
utlet = 23.5°C, Steam condensed = 0.06 kg.
Determine the higher and lower calorific values per m3 of th
as at a temperature of 15°C and barometric pressure of 76 c
f Hg.

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