Combustion engineering. msc in thermal engineering

1. MSc in Thermal Engineering
Course : Combustion Engineering
Course code : MEngg 6132

2. What is a combustion?
1. It is a process of setting fire to a fuel in a controlled
manner where as fire refers to uncontrolled combustion
2. It’s a chemical process in which fuel is burnt in the
presence of oxidizer producing heat and light.
3. Combustion is a chemical process in which heat is
liberated due to exothermic chemical reaction.
There are 2 process
Exothermic process- Releases or liberation of heat causing
the temperature of the immediate surroundings to rise
Endothermic process- Absorbs heat and cools the
surroundings

4. • The global demand of energy is increasing every day. US
Energy Information Administration (EIA) projects a 28%
increase in energy use by 2040 . Figure 1.1 shows the
increase, by energy source, since 1990. The major
shareholders of energy resources are petro-product,
coal and natural gas at present. Even after 20 years, this
trend will not alter. Use of natural gas may cross the
demand of coal at that time. Renewable energy
generation took a higher gradient before a decade. It is
still far behind the fossil fuels. Nuclear energy is not at
all taking off because of the safety aspect mainly. In
electricity generation also, fossil fuel contributes around
70% share.

5. • Applications of combustion can be classified broadly into
two parts – domestic and industrial.
• The domestic applications cover cigarette lighters, kitchen
burners, room-heating fireplaces and so on.
• The industrial sector can be divided into three parts:
power generation, transport and others.
• Power generation sector includes mainly coal and gas
firing as fuel. It also requires some oil firing. Coal
combustion technologies has advanced through
pulverized fuel firing, fluidized bed firing to coal
gasification
• Transport is mainly dependent on internal combustion
(IC) engines

6. • IC engines are also used in land-based applications like
diesel generator sets.The aviation sector was once
dependent on IC engines. Now it depends on Brayton
cycle based propulsion.( Gas turbines)
• others include Manufacturing units, Production of
Cements, Fertilizers …..
TYPES OF FLAMES
• Combustion can be divided into two groups. In some
applications, fuel and oxidizers are mixed before going to
a combustion chamber. When they burn, a flame is
visible. The flame is acting as an interface. On one side of
the flame, burnt gas is present. On the other side, there
remains the unburned mixture.
• This type of combustion is called premixed flames.

7. In the other group, fuel and oxidizers enter into the
combustion chamber as separate streams. They are mixed
with each other and burn inside the combustion chamber.
A flame is visible here also, and the flame is demarcating
( line of division)fuel at one side and oxidizer on the other.
This type of flame is called a non-premixed flame.
• Another classification can be based on the consideration
of phases of the reactants. If two reactants ( fuel and air)
are in the same phase, it is called homogeneous
reaction. Gaseous hydrocarbon fuels burn in this mode
with air or oxygen. When coal and air react, as in
thermal power stations, one is in solid and the other in
gas phase. This is an example of heterogeneous reaction.

8. PROPERTY RELATIONS
• The relation between different equations of state is
called equation of state. Most commonly, equation of
state implies a relation between pressure, temperature
and volume.
• The simplest form of equation of state is of the following
form:
• PV = NR*T
• Where P represents Pressure, V is the volume, N is the
number of moles, is the Universal gas constant and is
equal to 8.314 kJ/kmolK for all gases
• Any gas which obeys the above equation of state is
known as ideal gas.

9. • The above equation of state can also be expressed on a
mass basis as follows:
PV=R* T = mT = mRT
The quantity R= is called the Characteristic gas constant
Unlike the universal gas constant, the value of the
Characteristic gas constant changes with gases.
The ideal gas equation of state is expressed more
conveniently in terms of specific properties on mass and
molar basis as follows:
Pv = RT
P = ρRT

10. Pv* = R*T
P = ρ*R*T
In the above equations, ρ and ρ* denote mass density and
molar density
The relation between internal energy or enthalpy with
temperature and specific volume (or pressure) is known as
calorific equation of state.
Mathematically this can be expressed as:
u = u(T,v) and h = h(T,P)
Thus, differential changes in these properties can be
written as:
du dT + dv





11. dh dT + dP
The specific heats at constant pressure and constant
volume are defined as
Cv = and Cp
• The ideal gas equation of state is a necessary and
sufficient condition for u and h to be functions of T only
Thus, for ideal gases:
Cv= and Cp =
The specific heats for ideal gases are therefore functions of
temperature only.


12. First Law of Thermodynamics
• The First Law of Thermodynamics is a statement of
conservation of energy. For a closed system (i.e., a
system with only energy transfer but no mass transfer
with the surroundings across the system boundary), the
First Law of Thermodynamics can be expressed as:
δQ = dE + δW
• Here δQ and δW denote the small heat and work
transfers across the system boundary during an
infinitesimal change of state of the system identified by
an infinitesimal change in thermodynamic property dE,
• E denotes the total energy of the sum and is expressed
as the sum of internal energy U, kinetic energy, KE and
potential energy PE.

13. Thus, one can write:
E =U + KE + PE
Changes in kinetic and potential energies are negligible
compared to the change in internal energy.
δQ = dU + δW
THERMO CHEMISTRY
It is the study of the heat energy which is associated with
chemical reactions and physical transformation.A reaction
may release or absorb energy. ( Exothermic and Endo
thermic reactions). Thermo chemistry focuses on these
energy changes particularly on the system energy
exchange with its surroundings. Thermo chemistry is useful
in predicting reactant and product quantities.

14. It is also used to predict whether a reaction is spontaneous
or non-spontaneous, favourable or unfavourable.
Reactant and Product
A reactant is a substance that starts a chemical reaction
and product is a substance that ends the chemical reaction
Eg: CH4 +2O2 → CO2 + 2H2O
Chemical equilibrium
Chemical equilibrium is the state in which both the
reactants and products are present in concentration which
have no further tendency to change in the properties of the
system.
The rate of forward reaction equals the rate of backward
reaction.

15. There are 2 types of chemical equilibrium
(i) Homogeneous equilibrium
(ii) Heterogeneous equilibrium
Homogeneous equilibrium
The equilibrium reactions in which all the reactants and the products
are in the same phase are known as Homogeneous equilibrium
N2(g) + O2(g) → 2 NO (g)
←
Heterogeneous equilibrium
The equilibrium reactions in which the reactants and the products are
present in different phases are known as Heterogeneous equilibrium.
The dissociation of solid calcium carbonate to give calcium oxide and
gaseous carbon dioxide
CaCo3(s) → CaO(s) + CO2 (g)
←

16. Enthalpy of formation
The enthalpy of formation is the increase in enthalpy when a
compound is formed from its constituent elements in their
natural form and in a standard state
The standard state is 25°C and 1 atm. Pressure
6C ( solid graphite) +6 H2(g) + 3O2 → C6H12O6 ( Glucose)
Adiabatic Flame Temperature
In a combustion process that takes place adiabatically( no heat
is added or removed) and with no work or changes in KE and PE
involved, the temperature of the products is referred to as
Adiabatic Flame Temperature. AFT is the maximum
temperature that can be achieved for the given reactants
because any heat transfer from the reacting substance and any
incomplete combustion would tend to lower the temperature
of the products.

17. The following points are worth noting
(i) The maximum temperature achieved through adiabatic
complete combustion varies with the type of reaction
and percentage of theoretical air is supplied. An
increase in A/F ratio will decrease the maximum
temperature
(ii) For a given fuel and given temperature and pressure
of the reactants the maximum AFT that can be
achieved is with stoichiometric mixture
(iii) AFT can be controlled by the amount of excess air that
is used.

18. Theoretical air and Excess air
For complete combustion we need air. The minimum
amount of air that supplies sufficient oxygen for the
complete combustion of fuel is called Theoretical air.
In practice it is found that complete combustion is not
achieved unless the amount of air supplied is somewhat
greater than theoretical amount, that is called as Excess air.
The amount of excess air supplied varies with the type of
fuel and the firing condition.

19. Stoichiometric A/F ratio
Stoichiometric ( Chemically correct) mixture of air and fuel
is one that contains just sufficient oxygen for complete
combustion of fuel.
A weak mixture has excess of air
A strong mixture has deficiency of air
% of Excess air =

20. Chapter 2
Combustion Mechanism
In the process of generating steam, the furnaces or burner
converts the chemical energy of the fuel to heat energy which
in turn transfers the heat to the steam generators to produce
steam.
Fuel bed combustion
Stoker( A machine used for feeding the coal into the furnace)
A grate is used at furnace bottom to hold a bed of fuel
There are 2 ways of feeding the coal on the grate
(i) Over feed
(ii) under feeding
An overfeed fuel bed combustion receives fresh coal on its top
surface

22. The air from below will tend to carry away the heat upward by
convection
Primary air ( Nitrogen and oxygen)along with water vapour
gets warmed up as it flows through the ash layer
When it passes to the Incandescent coke layer( 1200°C) the
reaction takes place initially C+ O2 =CO2. This is an exothermic
reaction where heat is released for combustion process
If the Incandescent coke layer is thick , CO2 may be partly or
fully reduced to CO
CO2 + C = 2CO
A slight water gas reaction may also take place with the
moisture from air
H2O + C = H2 + CO
This is an endothermic reaction ( heat is required)and bring

23. down the temperature of the bed and gas considerably.
The stream then passes through the distillation zone
where the Volatile matter is added
Then it is passed to the drying zone where moisture is
picked up and finally emerges above the fuel bed
Its contents are Nitrogen, carbon dioxide, carbon
monoxide, hydrogen, volatile matter and water vapour
After the secondary air is supplied, the fixed carbon in coal
leaves in the form of CO
Carbon combined with hydrogen in the volatile matter
leaves the bed as hydrocarbon gas

25. In small boilers, the grate is stationary and coal is fed manually
by shovels
For more uniform operating conditions, higher burning and
greater efficiency we go for moving grates or mechanical
stokers.
Travelling Grate Stoker
The grate surface is made up of series of cast iron bars joined
together by links to form an endless belt running over 2 sets of
sprocket wheels
An adjustable coal grate at the back of the hopper regulates the
depth of the fuel bed.
The grate can be raised or lowered.
Adjustment of grate speed , fuel bed thickness and air flow
controls the burning rate
Nothing but the ash remains on the grate by the time it

26. reaches the furnace rear.
The ash falls into the ash pit as the grate turns on the rear
sprocket to make the return trip

27. As the raw or green coal on the grate enters the furnace,
the surface coal gets ignited from the heat of the furnace
flame and from radiant heat rays reflected by the ignition
arch.
The fuel bed becomes thinner toward the furnace rear as
combustible matter burns off
Secondary air helps in mixing the gases and supplies
oxygen to complete combustion.

28. Pulverised Coal Firing System
Coal is first ground to dust like sizes and powdered coal is
then carried in a stream of air to be fed through burners
into the furnace
The volatile gases mix with the oxygen of the air, get ignited
and burn quickly
Oxygen of the hot air mix with carbon to release energy
Proper burning of fuel needs the supply of correct
proportions of air, mixing of fuel and air, high temperature
and adequate time to complete combustion reaction
The ash resulting from combustion
(i) Partly falls to the furnace bottom
(ii) Rest is carried in gas stream as fly ash

29. (iii) Deposited on the boiler heating surface
To burn the pulverised coal successfully , 2 conditions must
be satisfied
(i) Large quantities of very fine particles of coal, usually
would pass a 200 mesh sieve must ensure ready ignition
because of their large surface to volume ratio
(ii) minimum quantity of coarser particles should be present
since these coarser particles cause slagging and reduce
combustion efficiency
Greater surface area per unit mass of coal allows faster
combustion reaction because more carbon becomes exposed
to heat and oxygen
This also reduces the exhaust loss through chimney and
raises the steam generator efficiency

30. Advantages
(i) Low excess air requirement
(ii) less fan power
(iii) ability to use highly preheated air reducing exhaust
losses
(iv) higher boiler efficiency
(v) ability to burn a wide range of coals
(vi) fast response to load changes
(vii) ability to release large amount of heat in a boiler
(viii) ease of burning with gas and oil
(ix) ability to use flyash for making bricks
(x) less pressure losses

31. Disadvantages
(i) Added investment in coal preparation unit
(ii) added power needed for pulverising coal
(iii) investment needed to remove fly ash before ID fan
However the advantages far outweigh the disadvantages in
large utility and the net gain has led to the wide use of
pulverised coal firing system

32. Fuel –oil firing
Steps
( i) atomization (ii) Evaporation (iii) Mixing of evaporated
spray with air (iv) Combustion of spray
Atomization is the process of breaking the fuel oil particles
of hydrogen and carbon to extremely small droplets which
are easier to burn inside the combustion space
 immediately on being atomised, the oil vaporizes and a
thin film of vapour surrounds the oil droplet
 around the vapour film, there is a gas layer through
which oxygen diffuses

33.  reaction occurs at the flame front where the fuel reacts
with oxygen
 The combustion products diffuse out from the flame
front to the bulk fluid
 the flame front is an area of very rapid chemical reaction
and is a boundary between burnt and unburned gas
 the more rapid the combustion, the thinner the flame
front.

34. Combustion of gas
 burning of gas is very easy and clean
 no atomization is required
 combustion of 1 of natural gas requires roughly
20 of hot air
 proper mixing of gas and air can be ensured by
introducing the gas into the airflow in the form of thin jet
of high penetrability
 Because of good mixing , excess air required for
combustion is less

35. Mass transfer
In a mixture of fluid contains two or more components whose
concentration vary from point to point,
The constituents are transported from high concentration area
to low concentration area
This process of transfer of constituents of mixture from high
concentration area to low concentration area is known as
Mass transfer.
There are two types of mass transfer process namely
(i) Diffusion mass transfer
(ii) convective mass transfer
The diffusion mass transfer occur in stagnant liquid mixture
due to the concentration gradient of the constituent of the
mixture

36. The molecules of the constituent diffuse from high
concentration area to low concentration area in the mixture.
The convective mass transfer occur due to the bulk motion of
the fluid from one place to another place
Definitions
(i) Mass concentration:
The mass concentration of the component A is defined as the
mass of A per unit volume of the mixture
=
(ii) Molar concentration:
The molar concentration of the constituent A is defined as
the number of moles of A per unit volume of the mixture.
The molar concentration is expressed as

37. = The unit of molar concentration is kg-mol/
(iii) Mass fraction
The mass fraction is defined as the ratio of density mass
concentration of component A to the total mass density of the
mixture
=
(iv) Mole fraction
The mole fraction is defined as the ratio of number of moles of
the component A to the total number of moles of the mixture
= For the mixture of 2 components A and B
C= +

38. The perfect gas equation for the component A in the
gaseous mixture may be given as
V = T where is the partial pressure and is the number of
moles of component A . Total volume V, temperature T of
the mixture and is the universal gas constant ( = 8314 J/kg
molK)
The molar concentration is given by
=
(v) Molar average velocity
The molar average velocity of the mixture in the x –direction
is defined as
U = ( + ) where - statistical mean velocity of component A
and - statistical mean velocity of component B

39. (vi) Diffusion velocity
The diffusion velocities of component A and B with respect
to molar average velocity are defined as
Diffusion velocity of component A = - U
Diffusion velocity of component B = -U
The diffusion velocity indicate the motion of the component
relative to local average motion of the mixture
Ficks law of diffusion
The Ficks law of diffusion states that the mass flux of a
constituent A per unit area is proportional to the
concentration gradient
= -D where D is the diffusion coefficient in /s

40. Diffusion in gases
Consider a mixture of gas with 2 constituents A and B. The
diffusion mass transfer process occurs in 2 ways, the gas A is
diffusing into gas B at the same time the gas B is diffusing
into gas A. The Ficks law of diffusion for component A into
component B may be written for isothermal condition as
= -
Similarly for isothermal diffusion of component B into
component A maybe written as
= -

41. Convective mass transfer
In the diffusion mass transfer the bulk velocity is
insignificant where as in the convective mass transfer the
bulk velocity is significant. Both the components in a binary
mixture move with an appreciable velocity when the
convective mass transfer takes place in a binary mixture.
The convective mass transfer is classified into a natural
convective mass transfer and forced convewctive mass
transfer.
Analogy between momentum, heat and mass transfer
The momentum heat and mass transferequations for the
boundary layer flow maybe written as
Continuity equation
+ =0

42. Simultaneous heat and mass transfer
In many engineering process heat and mass transfer takes
place simultaneously. Few examples where the heat and
mass transfer takes place simultaneously are humidifiers,
dehumidifiers, cooling tower and evaporative condenser
When the simultaneous heat and mass transfer occurs , the
heat and mass coefficients are related by
= =
This equation is known as Lewis equation and when Sc= Pr,
the equation becomes =
Schmidt number = =

43. Prandtl number = =
Lewis number = =