COMBUSTION OF FUEL

By: Prof K. M.Joshi,

Assi. Professor, MED,

SSAS Institute of Technology, Surat. 12:57:42

The burning of fuel in presence of air is

known as combustion. It is a chemical

reaction taking place between fuel and

oxygen at temperature above ignition

temperature.

Heat is released during combustion

process.

Introduction

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The substances taking part in combustion are

called reactants and the substances produced,

during combustion are called products of

combustion.

fuel + air = products of combustion + heat

If the heat is released during the chemical process, it

is called an exothermic reaction. When heal is

absorbed from the surroundings during the chemical

reaction, it is called an endothermic reaction.

By: P

rof. K

. M. J

oshi

When combustion of fuel takes place, the constituents of fuel react with oxygen. The molecular mass of various constituents of fuel are given below :

Constituent Molecular mass C 12

02 32

H2 2

S 32

N2 28

The relation between kg mass and kg mole of a substance is given by

mass

Number of moles = ------------------

molecular mass

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Combustion Equations

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Carbon :

C + O2 = CO2

1 mole of C + 1 mole of 02 = 1 mole of C02 ......... (1)

12 kg of C + 32 kg of 02 = 44 kg of C02

Hydrogen :

2 H2 + 02 = 2 H20

2 moles of H2 + 1 mole of 02 = 2 moles of H20 ......... (2)

4 kg H2 + 32 kg 02 = 36 kg H20

1 kg C + 8/3 kg 02 = 11/3 kg C02

By: P

rof. K

. M. J

oshi

12:57:43

Methane :

CH4 + 202 = C02 + 2H20

1 mole CH4 + 2 moles 02 = 1 mole C02 + 2 moles H20 ...... (4)

16 kg CH4 + 64 kg 02 = 44 kg C02 + 36 kg H20

1kg CH4 + 4 kg 02 = 11/4 kg C02 + 9/4 kg H20

Sulphur :

S + O2 = So2

1 mole of S + 1 mole of 02 = 1 mole of S02 ......... (3)

32 kg S + 32 kg 02 = 64 kg S02

1 kg S + 1 kg 02 = 2 kg S02

By: P

rof. K

. M. J

oshi

These equations are helpful for calculating the amount of

oxygen required to burn the fuel. Instead of supplying only

oxygen, the fuel is generally burnt in presence of air which

contains mainly O2 and N2 with small amount of CO2. argon

etc. N2 and other gases in air' do not take part in

chemical reaction.

The composition of air is approximately as under :

02 N2 Total air

by volume 21% 79% 100%

1 mole 3.76 mole 4.76 mole

by mass 23% 77% 100%

1 kg 3.35 kg 4.35 kg

12:57:43

By: P

rof. K

. M. J

oshi

12:57:43

The ratio of mass of a constituent of a mixture or a compound to the total

mass of a mixture or compound is called mass fraction.

e.g. consider a gas CH4,

Molecular mass of CH4 = 12 + 4 x 1 = 16 kg,

in which C is 12 kg and H2 is 4 kg.

Mass fraction B

y: P

rof. K

. M. J

oshi

Mole fraction

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The ratio of volume of gas in a mixture to the total volume of mixture

is called the mole fraction. It is also equal to the ratio of no. of moles

of gas in a mixture to the total no. of moles of mixture.

e.g. for octane gas C8H18, no. of moles of C is 8 and that of H2 is 9.

Total no. of moles = 8 + 9 = 17

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The no. of moles of 02 required for complete combustion of 1

mole of fuel is known as the theoretical amount of 02 (molar

basis). OR

The mass of 02 required for complete combustion of 1 kg of

fuel is known as the theoretical amount of 02 (mass basis).

The mass of 02 and N2 together (i.e. the air) required for

complete combustion of 1 kg of fuel, is known as minimum air

requirement. It is also called the theoretical or stochiometric

amount of air.

A mixture of theoretical air and fuel for complete

combustion of fuel is called stochiometric mixture or

chemically correct mixture.

Minimum Air Requirement

In practice, the combustion of fuel is never complete

with the theoretical amount of air because of imperfect

mixing of fuel and air. Mixture of fuel and air is never

homogeneous.

So, to ensure the complete combustion of fuel- usually

the actual air supplied is more than the theoretical air

required for complete combustion of fuel.

The difference of actual air supplied and the theoretical

or stochiometric air required for complete combustion of

fuel is called excess air.

Excess air B

y: P

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. M. J

oshi

12:57:43

Air Fuel Ratio

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If actual AF ratio is more than stochiomelric AF ratio, mixture of air

and fuel is said to be weak mixture or lean mixture. If actual AF ratio is

less than stochlometric AF ratio, mixture is said to be a rich mixture.

Mixture strength or equivalence ratio is defined as the ratio of

stochlometric AF ratio to actual AF ratio.

If mixture strength is more than 100%, it represents a rich mixture.

If mixture strength is less than 100%. it represents a weak mixture.

12:57:43

Consider 1 kg of fuel. It contains x1 kg of carbon. x2 kg of

hydrogen, x3 kg of sulphur and x kg of oxygen.

The amount of oxygen required for complete combustion can be

calculated using eq. (1), (2) and (3).

1 kg carbon requires 8/3 kg of O2

x1 kg carbon requires ( 8/3 X x1 ) kg of O2

Similarly

x2 kg of hydrogen requires 8 x2 kg of O2

x3 kg of sulphur requires x3 kg of O2

Total oxygen required = 8/3 x1 + 8 x2 + x3 kg

x kg oxygen is already presents in the fuel.

Calculation for minimum air

requirement per kg of fuel

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x1 x2 x3 x

x1 x2 x

x3

x1 x2 x

x3

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Multiply the percentage volume of each constituent by

their respective molecular masses. It gives the

proportionate mass of each constituent in the flue gas.

Add these proportionate masses of all constituents to

get total mass.

Divide the mass of each constituent by total mass and

express it as percentage. This gives the mass analysis of

flue gas.

Conversion of volumetric analysis into mass analysis or gravimetric analysis

12:57:43

The amount of heat energy produced by combustion of unit

mass or volume of fuel is called its calorific value (CV).

For the solid and liquid fuels, unit mass is considered and

the unit of CV will be kJ/kg. For gaseous fuel, unit volume is

considered under NTP and the unit of CV will be kJ/m3.

Most of the fuels contain hydrogen. During combustion

process. H2 combines with 02 and forms steam (water vapour).

If this water vapour is condensed at constant temperature,

large amount of heat is released.

Calorific Value Of Fuel

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On account of this, two types of calorific values are defined.

(1) Higher Calorific Value (HCV) :

The higher calorific value is defined as the total heat liberated by

combustion of unit mass of fuel when the water vapour formed by

combustion is completely condensed at constant temperature

releasing its latent heal.

(2) Lower Calorific Value (LCV) :

The lower calorific value of fuel is defined as the net heat liberated

by combustion of unit mass of fuel when the water vapour formed

by combustion exists completely in vapour phase.

12:57:43

Determination of calorific value of fuel by bomb calorimeter

Calorific value of solid and liquid fuels can be determined with the help of

bomb calorimeter.

The basic principle used for determination of calorific value of fuel is that

the known quantity of fuel is burnt and the heat energy liberated is

transferred to a medium of known mass and sp. heat and rise in

temperature of that medium is measured.

Construction :

The calorimeter C consists of a thick

walled bomb B made of stainless steel. It

has a capacity of about 650 c.c. and it is

designed to with stand high pressure upto

200 atmosphere.

The non return oxygen valve V and a

release valve U are connected to a bomb

at the top.

The crucible made of silicon or quartz is carried on support ring R

which can slide on insulated pillars P. The fuse wire W is passed though

the slots kept in pillars. The fuse wire is connected to electrical circuit.

The sensitive thermometer T is used to measure the temperature of

water filled in the calorimeter. Stirrer is provided in the water to stir the

water.

Working :

A pillet of solid fuel whose calorific

value is to be determined is prepared

and 1 gm of this pillet is kept into the

crucible.

A known quantity of water

(approximately 2500 c.c.) is filled

around the bomb inside the

calorimeter. The crucible and the

calorimeter are weighed before starting

the experiment.

The oxygen is admitted until a pressure of about 25 atmosphere.

The stirring of water is continued throughout the experiment and

the temperature readings are taken every minute.

After five minutes when the equilibrium sets, the fuel is ignited.

The temperature of water rises quickly due to heat energy released

by the combustion of fuel.

After the temperature has reached its maximum value, it again

starts falling due to heat transfer losses to the surroundings.

At the end of experiment, the release valve is opened so that the

pressure inside the bomb reduces to atmospheric pressure.

12:57:47

Let,

mf = mass of fuel

CV = calorific value of fuel

mc = mass of calorimeter

CC = sp. heat of calorimeter

mw = mass of water

Cw = sp. heat of water

δt = rise in temperature of water and calorimeter

Heat released from combustion of fuel =

heat gained by calorimeter and water

mf x CV = mc CC δt + mw Cw δt

When other quantities are known, CV of fuel can be calculated.

12:57:47

By: P

rof. K

. M. J

oshi

Junkers gas calorimeter

12:57:47

By: P

rof. K

. M. J

oshi

Construction :

Junkers gas calorimeter consists of a combustion chamber

surrounded by water jacket.

A gas pipe line is connected with a burner kept in combustion

chamber.

12:57:47

A gas flow meter and

pressure regulator are

provided in a gas pipe line.

Thermometers are used to

measure the temperature of

water at inlet and outlet.

Condensate from the gases

is collected in condensate pot.

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Working :

The gas whose calorific value is to be measured is supplied

through a pipeline to the gas burner where it is burnt.

The flow rate of gas is measured by a flow meter. The pressure

of gas is measured by a manometer attached to the pressure

regulator.

The heat produced by combustion of gas is absorbed by the

cold water flowing through water jacket. The gases are cooled

upto room temperature as far as possible, so that entire heat

released from the combustion may be absorbed by circulating

water.

The temperature of cooling water at inlet and outlet and exit

gas temperature are measured. Mass flow rate of cooling water

is also measured. Volume flow rate of gas is converted to STP

condition.

12:57:48

By: P

rof. K

. M. J

oshi

12:57:48

Enthalpy Of Reaction

During the constant pressure process, the heat energy released by

combustion of fuel at STP is called enthalpy of reaction.

It is also known as heating value or heat of reaction or calorific value of fuel

at constant STP.

It is denoted by Qp or H0.

Applying 1st law of thermodynamics to combustion process at constant

pressure,

12:57:48

By: P

rof. K

. M. J

oshi

Enthalpy Of Formation

It is assumed that the reactants C

and O2 and the product CO2 both

enter and leave the combustion

chamber at STP. 12:57:49

The chemical energy of fuel carbon is 393520 kJ/kg mole. Above reaction

is exothermic, so the amount of heat energy transferred from system to

surroundings is equal to 393520 kJ/kg mole which is equal to chemical

energy of carbon.

Enthalpy datum for all naturally occuring stable elements is assigned as

zero enthalpy, e.g. the stable form of gases oxygen, hydrogen and nitrogen

are respectively O2, H2 and N2 while the natural form of carbon is.

graphite. All these elements are assigned zero enthalpy of formation.

So. the change in enthalpy of above chemically reactive system at STP

is [0 - 393520 ] = - 393520 kJ/kg mole

Negative value indicates that heat energy is released in forming the

compound from its stable elements. A positive value will Indicate that the

heat energy is absorbed in formation of compound from its stable

elements.

12:57:49

By: P

rof. K

. M. J

oshi

Adiabatic flame temperature is defined as the theoretical

temperature attained by the products of combustion in an

adiabatic process assuming complete combustion.

If the combustion chamber is completely insulated, the

chemical energy released during combustion will be utilized to

raise the temperature of products of combustion alone and no

heat will be transferred to the surroundings.

In this situation, the combustion process is said to be

adiabatic.

12:57:49

Adiabatic Flame Temperature

In actual practice, the actual flame temperature is less than

adiabatic flame temperature due to following reasons :

(1) Heat transfer to the surroundings since the system can not

be made perfectly insulated.

(2) Combustion is never complete.

(3)At high temperatures, the gases may not be stable, they

may dissociate and absorb energy internally from the

system, thus reducing the flame temperature.

Factors affecting adiabatic flame temperature;

(1) Heat losses from combustion chamber

(2) Incomplete combustion of fuel

(3) Dissociation of gases

(4) Excess or deficient air

12:57:49

As adiabalic flame temperature is the maximum temperature in

combustion chamber, it helps in selecting the material for

combustion chamber and design of combustion chamber.

Adiabatic flame temperature can be reduced by increasing the

air fuel ratio.

It is lower if excess air is supplied, since the total heat of

reaction of fuel is utilized by more no. of moles of products of

combustion.

The adiabalic flame temperature is also lower with deficient air

as heat of reaction is not fully released since 02 available is not

sufficient to burn the entire fuel.

Adiabatic flame temperature is maximum with stochiometric air.

12:57:50

By: P

rof. K

. M. J

oshi

Consider a perfectly insulated combustion chamber. The reactants are

supplied at P0, T1 and the products leave at P0 , T2.

According to definition, T2 is adiabaltic flame temperature. From steady

energy equation, with negligible changes in KE and PE.

Where Hp = enthalpy of products and HR = enthalpy of reactants

As combustion chamber is completely insulated, Q = 0

No work is done. So W = 0

12:57:50

Equation for adiabatic flame temperature

As the products and reactants are respectively at temperatures T2 and T1,

their enthalpy is more than enthalpy of formation by the amount equal to

difference between enthalpy at that temperature and enthalpy at STP.

Using this equation, adiabatic flame temperature T2 can be calculated

with the help of chemical reaction equation, enthalpy of formation

table and enthalpy of gas table.

12:57:50

By: P

rof. K

. M. J

oshi