Chap 35 ppt

POWER PLANT ENGINEERING
(lecture notes)
© Mulugeta T.(MSc., Asst Professor)
@WSU, Mechanical Engineering Department

2
Most liquid hydrocarbon fuels are
obtained from crude oil by
fractional distillation.
Fuel: Any material that can be burned to
release thermal energy.
There are many fuels currently used in
combustion processes throughout the world,
and the most common are: Coal, Oils, Diesel
Oil, Gasoline, Natural Gas, Propane, Coke
Oven Gas, and Wood.
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 (usually treated
as octane C8H18), and natural gas.
CHAPTER-3:
3. Fuels and Combustion
3.1 Introduction to fuels of power plant

a). Solid Fuels
• Among solid fuels coal is commonly used source of energy
for Thermal power plant
• Coal fired power plant may need additional parts
– Coal handling, crushing, and delivery system
– Disposing of the solid residue or ash
– Firing and combustion bed
• Other solid fuels of power plant are wood, charcoal,
compressed coffee pulp, or sugar cane bagase, etc

b). Gaseous Fuels
• Gaseous Fuels are ideal and pose very few
problems in using them in power plant
• Common gaseous fuels are:
 Natural gas – from nature
 Liquefied Petroleum Gas - from refineries
 Producer gas - from coal or biomass
 Biogas - from biomass
 Hydrogen – from many sources

Cont…
• Advantages of Gaseous Fuels
– Mix more homogeneously with air for easy combustion
– Eliminate the distribution and starting problems
• Disadvantage
– Storage and handling Problem
• Therefore gaseous fuels are commonly used for stationary
power plants located near the source of which the fuel
available.
• Some of the gaseous fuel can be liquefied under pressure for reducing
the storage volume but this arrangement is very expensive and risky.

Natural Gas
 Found compressed in porous rock
and shale formations sealed in rock
strata of underground.
 Frequently exists near or above oil
deposits.
 Is a mixture of hydrocarbons and
non hydrocarbons in gaseous phase
or in solution with crude oil.
 Raw gas contains mainly methane
(60-90 %) plus lesser amounts of
ethane, propane, butane and
pentane, negligible sulfur, organic
nitrogen, carbon dioxide and helium
are present.
Underground layer oil deposit
Natural Gas may be used as
 Liquefied Natural Gas (LNG).
 Compressed Natural Gas (CNG).
Natural Gas can be made artificially it is
called substitute, or synthetic or
Supplemental Natural Gas (SNG).

Cont…
Preparation of Natural Gas:
1. Separation of liquid and gas: Liquid may be a hydrocarbon present in
the gas well along with the gas.
2. Dehydration: Water is corrosive and hydrates may form which will
plug the flow. Water will also reduce the calorific value of the gas.
3. Desulfurization: Presence of hydrogen sulfide is undesirable in power
production system due to its pollution. The gas is called sour. When
the sulfur is removed the gas is sweetened.
Advantages of Natural Gas
– Methane is a greenhouse gas with a global warming potential approximately 4 times that
of carbon dioxide.
– Its C/H ratio is lower than that of gasoline so its CO2 emissions are 22-25% lower (54.9
compared to 71.9 g CO2/MJ fuel).
– Has higher calorific values

c).Liquid Fuels
• Some of the liquid fuels being used in power production systems
are usually a derivatives of liquid petroleum or crude oil and are
commercial type of fuel.
• Are liquid hydrocarbons with different carbon hydrogen number
and structure.

Liquid Hydrocarbon
i. Paraffins(alkane)(CnH2n+2)
• straight chain compounds like methane, ethane,
propane, etc. or
• branched chain compounds (isomers) like iso-butane,
iso-heptane (like 2,2,3 tri-methyl butane or “triptane”)
and iso-octane (like 2,2,4 tri-methyl pentane).

Cont…
ii. Olefins (Alkene) and Diolefins: (CnH2n)
• Open chain unsaturated hydrocarbons with one or more double
bond like ethene or propylene which also have straight and
branched chain compounds.
 Both types of olefins produce gum when reacted with oxygen
which can block fuel filters.

Cont…
iii. Diolefins (Alkadiene): CnH2n-2
• These are olefins with 2 double bonds or triple
bond.
• Both types of olefins produce gum when reacted
with oxygen which can block fuel filters.

Cont…
iv. Napthenes or Cycloparaffins: (CnH2n)
– Have same general formula as mono olefins but are saturated
compounds with a ring structure.
• Examples are Cyclobutane, Butycylcolhexane, Decalin etc.

Cont…
v. Aromatics: (CnH2n-6)
• Ring structured unsaturated hydrocarbons with double bonds
but more stable than the paraffinic double bond
hydrocarbons.
• Examples are benzene, toluene, naphthalenes, and
anthracenes.

1. Fuel Properties
• In order to generate Heat Chemical Combustion of Fuel and oxygen
are required
• Therefore the fundamental knowledge of different types of Fuel and
their characteristics is essential in order to understand the combustion
process and combustion result.
• The various types of fuels like liquid, solid and gaseous fuels are
available for firing in boilers, furnaces and other combustion
equipments.
• The selection of right type of fuel depends on various factors such
as availability, storage, handling, pollution and landed cost of fuel.
Fuel+ Air Flue Gas+ Heat
Combustion

Influence of fuel property
others

Density and specific gravity
i. Density
• This is defined as the ratio of the mass of the fuel to the volume of
the fuel at a reference temperature, 15°C.
• Density is measured by an instrument called hydrometer. The
knowledge of density is useful for quantity calculations and
assessing ignition quality.
• The unit of density is kg/m3.
ii. Specific gravity
• This is defined as the ratio of the weight of a given volume of oil to
the weight of the same volume of water at a given temperature.
• The density of fuel, relative to water, is called specific gravity.
• The specific gravity of water is defined as 1.
• Since specific gravity is a ratio, it has no units. The measurement
of specific gravity is generally made by a hydrometer.

Viscosity
• The viscosity of a fluid is a measure of its internal resistance
to flow.
• Viscosity depends on temperature and decreases as the
temperature increases. Any numerical value for viscosity has
no meaning unless the temperature is also specified.
• Each type of oil has its own temperature - viscosity
relationship. The measurement of viscosity is made with an
instrument called Viscometer.
• Viscosity is the most important characteristic in the storage
and use of fuel oil.
• It influences the degree of pre-heat required for handling,
storage and satisfactory atomization.
• If the oil is too viscous, it may become difficult to pump,
hard to light the burner, and tough to operate. Poor
atomization may result in the formation of carbon deposits
on the burner tips or on the walls.
• Therefore pre-heating is necessary for proper atomization.

Flash Point
• The flash point of a fuel is the lowest temperature
at which the fuel can be heated so that the vapor
gives off flashes momentarily when an open flame
is passed over it.
OR
• is the lowest temperature at which it can form an
ignitable mixture in air
• Flash point of diesel is 52 oC (125 oF) or higher,
therefore, at ordinary ambient temperatures, it does not
form enough vapor for combustible mixture

Flammability limits
• Flammability limits are the minimum (lower)
and the maximum (upper) contents of fuel in
the fuel/air mixture, between which flame
propagation in this mixture is possible (the
mixture is flammable).
• Flammability limits mean the same as ignition
limits or explosion limits.

Volatility
• Fuels won’t burn till they vaporize
• Volatility is the temperature at which a given air-
vapor mixture is formed when under equilibrium
conditions at a pressure of one atmosphere, when
a given percentage of the fuel is evaporated.
• Volatility is a fuel’s ability to vaporize or change
from liquid to vapor
• According to this definition, one gasoline is more
volatile than another for any given percentage
evaporated, if it forms the given air-vapor
mixture at a lower temperature.

Pour Point
• The pour point of a fuel is the lowest temperature
at which it will pour or flow when cooled under
prescribed conditions.
• It is a very rough indication of the lowest
temperature at which fuel oil is readily pump able

Specific Heat
• Specific heat is the amount of heat needed to
raise the temperature of 1 kg of fuel by 1oC.
• The unit of specific heat is kcal/kgoC.
• The specific heat determines how much steam
or electrical energy it takes to heat fuel to a
desired temperature.
• Light oils have a low specific heat, whereas
heavier oils have a higher specific heat.

Heating Value of Fuels
– To compute the fuel conversion efficiency
– It can be measured using bomb calorimeter
– Measuring procedure
• Let water come to equilibrium while stirring to keep temperature uniform
• Ignite and burn fuel in bomb
• Measure water temperature rise
• Heating value measured by bomb called LHV
• HHV=LHV+Heat of water vaporization
– In combustion analysis, generally use LHV

Ash Content
• The ash value is related to the inorganic material in the
fuel oil.
• The ash levels of distillate fuels are negligible. Residual
fuels have more of the ash-forming constituents.
• These salts may be compounds of sodium, vanadium,
calcium, magnesium, silicon, iron, aluminum, nickel,
etc.
• Typically, the ash value is in the range 0.03-0.07 %.
Excessive ash in liquid fuels can cause fouling deposits
in the combustion equipment.
• Ash has erosive effect on the burner tips, causes
damage to the refractories at high temperatures and
gives rise to high temperature corrosion and fouling of
equipments.

2. Properties of Coal
• Coal is classified into three major types namely anthracite,
bituminous, and lignite.
• Anthracite is the oldest coal from geological perspective. It
is a hard coal composed mainly of carbon with little volatile
content and practically no moisture.
• Lignite is the youngest coal from geological perspective. It
is a soft coal composed mainly of volatile matter and
moisture content with low fixed carbon.
• Fixed carbon refers to carbon in its free state, not
combined with other elements.
• Volatile matter refers to those combustible constituents of
coal that vaporize when coal is heated.

Cont…
• There are certain
properties of coal
which are important in
power plant
application.
• They are swelling
index, grindablity,
weatherablity, sulphur
content, heating value,
and ash content and
ash softening
temperature.

Analysis of Coal
• There are two methods of coal analysis:
Ultimate analysis and
 Proximate analysis.
• The ultimate analysis determines all coal
component elements, solid or gaseous and the
proximate analysis determines only the fixed
carbon, volatile matter, moisture and ash
percentages.
• The ultimate analysis is determined in a properly
equipped laboratory by a skilled chemist, while
proximate analysis can be determined with a
simple apparatus.

(A)Proximate analysis:
• In the proximate analysis, moisture (M), Ash (A)
and volatile matter (VM) are determined. Fixed
carbon (FC) is obtained from the following
equation:
%FC=100% sample-(%A+%M+VM)
• Moisture: drying 1kg sample at 105 oC for 1hr
• Ash: complete combustion of 1kg sample at 700-
750 oC
• Volatile matter: weight loss after heating 1kg
sample at 950 oC for 7 minutes
• %FC=100% sample-(%A+%M+VM)

(B) Ultimate analysis:
• The main chemical elements in coal (apart from
associated mineral matter) are C, O, H, N and S.
• The chemical analysis is very important to
calculate material balance accurately and
calorific value of coal.
• For the ultimate analysis C, H, S and N are
determined by chemical analysis and expressed
on the moisture free basis.
• Ash is determined as in proximate analysis and
is calculated on moisture free basis.

3.2 Combustion
1. Principle of Combustion
• Combustion refers to the rapid oxidation of fuel accompanied by the
production of heat, or heat and light. Complete combustion of a fuel
is possible only in the presence of an adequate supply of oxygen.
• Oxygen is one of the most common elements on earth making up 20.9%
of our air. Rapid fuel oxidation results in large amounts of heat.
• Solid or liquid fuels must be changed to a gas before they will burn.
• Usually heat is required to change liquids or solids into gases. Fuel gases
will burn in their normal state if enough air is present.
• Most of the 79% of air (that is not oxygen) is nitrogen, with traces of
other elements. Nitrogen is considered to be a temperature reducing
dilutant that must be present to obtain the oxygen required for
combustion.
• Nitrogen reduces combustion efficiency by absorbing heat from the
combustion of fuels and diluting the flue gases

31
Combustion is a chemical reaction during
which a fuel is oxidized and a large quantity of
energy is released.
Each kmol of O2 in air is
accompanied by 3.76
kmol of N2.
The oxidizer most often used in combustion processes is air. Why?
On a mole or a volume basis, dry air is composed of 20.9% O2, 78.1% N2,
0.9% Ar, and small amounts of CO2, He, Ne, H2.
In the analysis of combustion processes, dry air is approximated as 21% O2
and 79% N2 by mole numbers.

32
In a steady-flow combustion process, the components that enter
the reaction chamber are called reactants and the
components that exit are called products.
 The fuel must be brought above its ignition
temperature to start the combustion. The
minimum ignition temperatures in atmospheric air
are approximately 260°C for gasoline, 400°C for
carbon, 580°C for hydrogen, 610°C for carbon
monoxide, and 630°C for methane.
 Proportions of the fuel and air must be in the
proper range for combustion to begin.
For example, natural gas does not burn in air in
concentrations less than 5% or greater than about
15%.
The mass (and number of atoms) of
each element is conserved during a
chemical reaction.
The total number of
moles is not
conserved during a
chemical reaction.

33
The air–fuel ratio (AF) represents the
amount of air used per unit mass of fuel
during a combustion process.
Air-fuel ratio (AF) is usually
expressed on a mass basis and is
defined as the ratio of the mass of
air to the mass of fuel for a
combustion process
m mass
N number of moles
M molar mass
Fuel–air ratio (FA): The reciprocal of air–fuel ratio.
Equivalence Ratio (ɸ): Normalizing
the actual fuel-air ratio by the
stoichiometric fuel-air ratio gives the
equivalence ratio,
ɸ= AFs/AFa
The subscript s indicates a value at the
stoichiometric condition.
ɸ <1 is a lean mixture,
ɸ =1 is a stoichiometric mixture,
and
ɸ >1 is a rich mixture.

34
2. Theoretical And Actual Combustion Processes
A combustion process is complete if all the
combustible components of the fuel are
burned to completion.
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.
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).
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.

35
The complete combustion
process with no free oxygen in
the products is called
theoretical combustion.
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.
Excess air: The amount of air in excess of the stoichiometry amount. Usually expressed in
terms of the stoichiometric air as percent excess air or percent theoretical air.
Deficiency of air: Amounts of air less than the stoichiometric amount. Often expressed as
percent deficiency of air.
50% excess air = 150% theoretical air
200% excess air = 300% theoretical air.
90% theoretical air = 10% deficiency of air

36
3. Enthalpy of Formation and Enthalpy of
Combustion
The microscopic form of energy of a
substance consists of sensible, latent,
chemical, and nuclear energies.
When the existing chemical bonds are
destroyed and new ones are formed
during a combustion process, usually a
large amount of sensible energy is
absorbed or released.
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:

37
Enthalpy of reaction hR : The difference between the enthalpy of the products at a
specified state and the enthalpy of the reactants at the same state for a complete
reaction.
Enthalpy of combustion hC : It is the enthalpy of reaction for combustion
processes. It represents the amount of heat released during a steady-flow
combustion process when 1 kmol (or 1 kg) of fuel is burned completely at a
specified temperature and pressure.
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.

38
h H H when T T C K
N h N h
C P R P R
o
e f
o
e i f
o
i
    
  
25 298
Products Reactants
Where:
•N-mole number
•hf-enthalpy of formation

39
Enthalpy of formation:
The enthalpy of a
substance at a specified
state due to its chemical
composition.

40
Enthalpy of Formation
When a compound is formed from its elements (e.g., methane, CH4, from C and H2), heat
transfer occurs. When heat is given off, the reaction is called exothermic. When heat is
required, the reaction is called endothermic. Consider the following.
The reaction equation is
C H CH 2 2 4
The conservation of energy for a steady-flow combustion process is
E E
Q H H
Q H H
in out
net
net

 
 
Reactants Products
Products Reactants

41
Q N h N h
Q h h h
net e e i i
net CH C H
 
  
 Products Reactants
1 1 24 2
( )
A common reference state for the enthalpies of all reacting components is established as
The enthalpy of the elements or their stable compounds is defined to be ZERO at
25o
C (298 K) and 1 atm (or 0.1 MPa).
Q h
h
net CH
CH
  

1 1 0 2 04
4
( ( ) ( ))
hf
o
hf
o
This heat transfer is called the enthalpy of formation for methane, . . The superscript (o)
implies the 1 atm pressure value and the subscript (f) implies 25oC data, is given in
Table A-26.
During the formation of methane from the elements at 298 K, 0.1 MPa, heat is given off (an
exothermic reaction) such that
Q h
kJ
kmol
net f
o
CH
CH
  
4
4
74 850,

42
The enthalpy of formation is tabulated for typical compounds. The enthalpy of formation of
the elements in their stable form is taken as zero. The enthalpy of formation of the
elements found naturally as diatomic elements, such as nitrogen, oxygen, and hydrogen, is
defined to be zero.
hf
o
hf
o
Substance Formula M kJ/kmol
Air 28.97 0
Oxygen O2 32 0
Nitrogen N2 28 0
Carbon dioxide CO2 44 -393,520
Carbon monoxide CO 28 -110,530
Water (vapor) H2Ovap 18 -241,820
Water (liquid) H2Oliq 18 -285,830
Methane CH4 16 -74,850
Acetylene C2H2 26 +226,730
Ethane C2H6 30 -84,680
Propane C3H8 44 -103,850
Butane C4H10 58 -126,150
Octane (vapor) C8H18 114 -208,450
Dodecane C12H26 170 -291,010

43
The enthalpies are calculated relative to a common base or reference called the enthalpy of
formation. The enthalpy of formation is the heat transfer required to form the compound
from its elements at 25o
C (77 o
F) or 298 K (537 R), 1 atm. The enthalpy at any other
temperature is given as
h h h hf
o
T
o
  ( )
Here the term is the enthalpy of any component at 298 K. The enthalpies at the
temperatures T and 298 K can be found in Tables A-18 through A-25. If tables are not
available, the enthalpy difference due to the temperature difference can be calculated from
h o
Based on the classical sign convention, the net heat transfer to the reacting system is
Q H H
N h h h N h h h
net P R
e f
o
T
o
e i f
o
T
o
i
 
      [ ( )] [ ( )]
Products Reactants
In an actual combustion process, is the value of Qnet positive or negative?
Exothermic
Endothermic

44
Example:
Butane gas C4H10 is burned in theoretical air as shown below. Find the net heat transfer per
kmol of fuel.
Balanced combustion equation:
C H O N
CO H O N
4 10 2 2
2 2 2
65 376
4 5 24 44
  
 
. ( . )
.
The steady-flow heat transfer is
Q H H
N h h h N h h h
net P R
e f
o
T
o
e i f
o
T
o
i
 
      [ ( )] [ ( )]
Products Reactants

45
Reactants: TR = 298 K
hf
o
hT h o
N h h hi f
o
T
o
i[ ( )] Comp Ni
kmol/kmol
fuel
kJ/kmol kJ/kmol kJ/kmol kJ/kmol fuel
C4H10 1 -126,150 -- -- -126,150
O2 6.5 0 8,682 8,682 0
N2 24.44 0 8,669 8,669 0
H N h h h
kJ
kmol C H
R i f
o
T
o
i  
 
 [ ( )]
,
Reactants
126 150
4 10Products: TP = 1000 K
hf
o
hT h o
N h h he f
o
T
o
e[ ( )] Comp Ne
kmol/kmol
fuel
kJ/kmol kJ/kmol kJ/kmol kJ/kmol fuel
CO2 4 -393,520 42,769 9,364 -1,440,460
H2O 5 -241,820 35,882 9,904 -1,079,210
N2 24.44 0 30,129 8,669 +524,482

46
H N h h h
kJ
kmol C H
P e f
o
T
o
e  
 
 [ ( )]
, ,
Products
1995188
4 10
Q H H
kJ
kmol C H
net P R 
 1869 038
4 10
, ,

47
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.
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.
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.
4. Heating values of fuels

48
The lower heating value is often used as the amount of energy per kmol of fuel supplied to
the gas turbine engine.
The higher heating value, HHV, is the heating value when water appears as a liquid in the
products.
HHV h h with H O in productsC C liquid   2
The higher heating value is often used as the amount of energy per kmol of fuel supplied to
the steam power cycle.
See Table A-27 for the heating values of fuels at 25oC. Note that the heating values are listed
with units of kJ/kg of fuel. We multiply the listed heating value by the molar mass of the fuel
to determine the heating value in units of kJ/kmol of fuel.
The higher and lower heating values are related by the amount of water formed during the
combustion process and the enthalpy of vaporization of water at the temperature.
HHV LHV N hH O fg H O  2 2

49
Example:
The enthalpy of combustion of gaseous octane C8H18 at 25oC with liquid water in the
products is -5,500,842 kJ/kmol. Find the lower heating value of liquid octane.
8 18 8 18 2 2
2
8 18 8 18 2
8 18
5,500,842 9 (44,010)
5,104,752
gas gasC H C H H O fg H OLHV HHV N h
kJ kmol H O kJ
kmol C H kmol C H kmol H O
kJ
kmol C H
 
 

8 18 8 18 8 18
8 18
8 18
(5,104752 41,382)
5,063,370
liq gasC H C H fgC H
liq
LHV LHV h
kJ
kmol C H
kJ
kmol C H
 
 

Can you explain why LHVliq< LHVgas?

50
5. Adiabatic Flame Temperature
The temperature of a
combustion chamber becomes
maximum when combustion is
complete and no heat is lost
to the surroundings (Q = 0).
In the limiting case of no heat loss to the surroundings (Q = 0), the temperature of the
products reaches a maximum, which is called the adiabatic flame or adiabatic
combustion temperature.
The determination of the adiabatic flame temperature
by hand requires the use of an iterative technique.
since
The temperature of the
products have when a
combustion process takes
place
adiabatically is called the
adiabatic flame temperature.

51
The adiabatic flame temperature of a fuel depends on
(1)the state of the reactants (dissociation)
(2)the degree of completion of the reaction
(3)the amount of air used
For a specified fuel at a specified state burned with air at a
specified state, the adiabatic flame temperature attains its
maximum value when complete combustion occurs with the
theoretical amount of air.
The maximum temperature
encountered in a
combustion chamber is
lower than the theoretical
or adiabatic flame
temperature.

52
The combustion equation is
C H O N
CO O H O N
8 18 2 2
2 2 2 2
4 12 5 3 76
8 37 5 9 188
  
  
( . ) ( . )
.
The steady-flow heat transfer is
Q H H
N h h h N h h h
Adiabatic Combustion
net P R
e f
o
T
o
e i f
o
T
o
i
 
     

 [ ( )] [ ( )]
( )
Products Reactants
0
Thus, HP = HR for adiabatic combustion. We
need to solve this equation for TP.
Example:
Liquid octane C8H18(liq) is burned with 400 percent theoretical air. Find the adiabatic flame
temperature when the reactants enter at 298 K, 0.1 MPa, and the products leave at
0.1MPa.

53
Since the temperature of the reactants is 298 K, ( )i = 0,h hT
o

H N h
kJ
kmol C H
R i f
o
i

   
 
Reactants
1 249 950 4 12 5 0 4 12 5 376 0
249 950
4 10
( , ) ( . )( ) ( . )( . )( )
,
Since the products are at the adiabatic flame temperature, TP > 298 K
2
2
2
2
2 2
2 2
Products
, ,
, ,
4 10
[ ( )]
8( 393,520 9364)
9( 241,820 9904)
37.5(0 8682)
188(0 8669)
( 7,443,845 8 9
37.5 188 )
P
P
P
P
P
P P
P P
o o
P e f T e
T CO
T H O
T O
T N
T CO T H O
T O T N
H N h h h
h
h
h
h
h h
kJ
h h
kmol C H
  
   
   
  
  
   
 


54
Thus, setting HP = HR yields
N h h h h he T e
oducts
T CO T H O T O T NP P P P P,
Pr
, , , ,.
, ,
    

8 9 375 188
7 193895
2 2 2 2
To estimate TP, assume all products behave like N2 and estimate the adiabatic flame
temperature from the nitrogen data, Table A-18.
242 5 7 193 895
29 6655
985
2
2
2
. , ,
, .
,
,
h
h
kJ
kmol N
T K
T N
T N
p
P
P



Because CO2 and H2O are triatomic gases and have specific heats greater than diatomic
gases, the actual temperature will be somewhat less than 985 K. Try TP = 960 K and 970K.
h K960
h K970
N he T eP ,
Produts

Ne
CO2 8 40,607 41,145
H2O 9 34,274 34,653
O2 37.5 29,991 30,345
N2 188 28,826 29,151
7,177,572 7,259,362
Interpolation gives:
TP = 962 K.

55
6. First-law Analysis of Reacting
Systems
Steady-Flow Systems
The energy balance (the first-law) relations developed are
applicable to both reacting and non-reacting systems. We
rewrite the energy balance relations including the changes in
chemical energies.
When the changes in kinetic and potential energies are
negligible, the steady-flow energy balance for a chemically
reacting steady-flow system: The enthalpy of a chemical
component at a specified state

56
Taking heat transfer to the system and work done by
the system to be positive quantities, the energy balance relation is
If the enthalpy of combustion for a particular reaction is available:
Most steady-flow combustion processes do not involve any work interactions. Also,
combustion chamber normally involves heat output but no heat input:

57
Closed Systems
Taking heat transfer to the system and work done by the
system to be positive quantities, the general closed-
system energy balance relation can be expressed for a
stationary chemically reacting closed system as An expression for the
internal energy of a
chemical component in
terms of the enthalpy.
Utilizing the definition of enthalpy:
The Pv terms are negligible for solids and liquids, and can be
replaced by RuT for gases that behave as an ideal gas.

58
Example:
A mixture of 1 kmol C8H18 gas and 200 percent excess air at 25o
C, 1 atm, is burned
completely in a closed system (a bomb) and is cooled to 1200 K. Find the heat transfer from
the system and the system final pressure.
Apply the first law closed system:

59
Assume that the reactants and products are ideal gases; then
PV NR Tu
The balanced combustion equation for 200 percent excess (300 percent theoretical) air is
C H O N
CO O H O N
8 18 2 2
2 2 2 2
3 12 5 376
8 25 9 141
  
  
( )( . ) ( . )

60
Q
h h
kJ
kmol C H
net CO
H O
O
N
K
o
C H
O
N
    
    
   
   
    
   
   
  
8 393 520 53 848 9364 8 314 1200
9 241820 44 380 9904 8 314 1200
25 0 38 447 8682 8 314 1200
141 0 36 777 8669 8 314 1200
1 208 450 8 314 298
37 5 0 8682 8682 8 314 298
141 0 8669 8669 8 314 298
112 10
2
2
2
2
8 18
2
2
298
6
8 18
( , , . ( ))
( , , . ( ))
( , . ( ))
( , . ( ))
( , . ( ))
. ( . ( ))
( . ( ))
.
To find the final pressure, we assume that the reactants and the products are ideal-gas
mixtures.
PV N R T
PV N R T
u
u
1 1 1 1
2 2 2 2



61
where state 1 is the state of the mixture of the reactants before the combustion process and
state 2 is the state of the mixture of the products after the combustion process takes place.
Note that the total moles of reactants are not equal to the total moles of products.
PV
PV
N R T
N R T
u
u
2 2
1 1
2 2
1 1

but V2 = V1.

62
Summary
• Fuels and combustion
• Theoretical and actual combustion processes
• Enthalpy of formation and enthalpy of combustion
• First-law analysis of reacting systems
– Steady-flow systems
– Closed systems
• Adiabatic flame temperature
• Second-law analysis of reacting systems

CHAPTER-5:
5. Combustion Mechanism, Combustion
Equipment and Firing Methods

5.1.Mechanisms of coal Combustion
• In the process of generating steam, the furnace or
burner system provided controlled, efficient
conversion, of chemical energy of fuel to heat
energy which is in turn is transferred to the heat
absorbing surface of the steam generator.
• To do this, the firing system introduced the fuel
and air for combustion, mix these reactants, ignite
the combustible mixture, and distribute the flame
envelop and the products of combustion.
• Furnaces can be broadly divided in to two types
–Grate-fired furnace
–Chamber type or flame furnace

Chamber type or flame
furnaceGrate-fired furnace

Kinetics of Combustion
• Up on heating, solid fuel particles first undergoes a stage of thermal
preparation, which consists in the evaporation of residual moisture and
distillation of volatiles.
• Fuel particles are heated to a temperature at which volatiles are evolved
rapidly (400-600 oC) in few tenths of a seconds.
• The volatiles are then ignited, so that the temperature around coke
particles increases sharply and its heating is accelerated.
• A high yield of volatiles produces enough heat to ignite coke particles
unless heated additionally from the external sources like radiant furnace
wall.
• The final stage is the combustion of coke particles at a temperature
above 800-1000 oC.
• In carbon-oxygen reaction, oxygen first adsorbed from gas volume on
the surface of particles and react chemically with carbon to form
complex carbon- oxygen compounds of type CXOY. which is then
dissociate to form CO2 and CO and resulting reaction at about 1200 oC
can be;
4C+3O2 = 2CO+2CO2

Cont…
• The ratio of primary products, CO/CO2, increases sharply with
the increasing temperature of burning particles, resulting
reaction at 1700 oC become;
3C+2O2 = 2CO+CO2 where the CO/CO2 ratio is 2
• The primary reaction products are continuously removed from
the surface of particles to the environment.
• In this process CO diffusing out encounters the oxygen diffusing
in to the reacting surface and reacting surface and react with it
with in the boundary layer of gas to form CO2.
• Consequently, the concentration of oxygen decreases sharply as
it approaches the reacting surface, while the concentration of
CO2 increases.
• At high temperature, CO can consume all the oxygen supplied,
which consequently will not reach the reacting solid surface and
endothermic reduction of CO2 to CO will occur with the high
combustion temperature maintained due to high heat release.

Hierarchical structure of chemical
kinetic schemes

Cont…
The degree of combustion, it may be forward,
equilibrium or backward combustion expression
depends on:
• Temperature and pressure of the state of
reaction
• Gibbs free energy

5.2 Combustion Equipment for
Burning Coal
Different sized coal fed in to the furnace of the
boiler combustion may occur in:
–Fuel bed furnace (course particle)
–Pulverized coal furnace (fine particles)
–Cyclone furnace (crushed particles)
–Fluidized bed furnace (crushed small
particles)

Fuel bed furnace (course particle)
• A grate is used at furnace bottom to hold bed of
fuel. There are two ways of feeding coal on to
the grate
–Over feeding
–Under feeding

CONT…
• In small boilers, the grate is stationary and coal is
fed manually by shovels but for more uniform
operating condition, higher burning rate and
greater efficiency, moving grates or stokers are
employed. Stokers may be of the following types:

Pulverized coal firing system
• There three stages in pulverizing process of coal: feeding,
drying, and grinding.
• Coal first ground to dust like sized and powdered coal is
then carried in the stream of air to be feed through
burners in to the furnace.
• As the entering coal particles get heated in high
temperature flames in the furnace, the volatile material
is distilled off and this reduces the coal particles to
minute sponge like masses of fixed carbon and ash.
• The volatile gases mix with the oxygen of the air, and get
ignited and burn quickly.
• Oxygen of the hot air reacts with the carbon surface to
release energy.

Interactive Combustion of Pulverized Coal Particles
For an isolated particle, a spherical envelope
flame is formed around the particle.
The flame acts as a sink for fuel and oxygen and
as a source for thermal energy.
If another burning particle is brought near the
particle, the CO released by the particles
competes for oxygen molecules.
Further the interstitial temperature and species
profiles are also affected.
At some point the flames surrounding each
particle will merge and a common flame is
formed around the particles.
If more particles are brought near, then a
common flame may be formed around an array
or cloud of particles.

The cyclone furnace
• The cyclone is essentially a water cooled horizontal cylinder
located outside the main boiler furnace, in which crushed
coal (60 mm size or less) is fed and fired with very high rates
of heat release.
• The cyclone is made to a diameter of 1.8-4m and its length is
1.2-1.3 times its diameter.
• The crushed coal is fed in to the cyclone from the left along
with primary air, which is about 20% of combustion. The coal
air mixture is entered tangentially, thus imparting a
centrifugal motion to the coal particles.
• The secondary air is also admitted tangentially at the top of
the cyclone at high speed (80-120m/s) imparting further
centrifugal motion.
• A small quantity of tertiary air is admitted at the center of
the cyclone downstream to complete combustion of the
remaining fuel, and greatly reducing NOx formation..

Fluidized bed furnace
• A fluidized bed is composed of fuel (coal, coke,
biomass, etc.,) and bed material (ash, sand, and/or
sorbent) contained within an atmospheric or
pressurized vessel.
• The bed becomes fluidized when air or other gas
flows upward at a velocity sufficient to expand the
bed.
• At low fluidizing velocities (0.9 to 3 m/s) relatively
high solids densities are maintained in the bed and
only a small fraction of the solids are entrained from
the bed.
• A fluidized bed that is operated in this velocity
range is refered to as a bubbling fluidized bed (BFB).
A schematic of a typical BFB combustor is illustrated
in figure.

FBC reduces the amount
of sulfur emitted in the form
of SOx emissions. Limestone is
used to precipitate out sulfate
during combustion, which also
allows more efficient heat transfer
from the boiler to the apparatus
used to capture the heat energy
(usually water tubes).

5.3 Fuel Handling System
• Coal delivery equipment is one of the major
components of plant cost and systems involve
are:
i) Coal delivery
• The coal from supply points is delivered by ships
or boats to power stations situated near to sea or
river whereas coal is supplied by rail or trucks to
the power stations which are situated away from
sea or river.
• The transportation of coal by trucks is used if the
railway facilities are not available.

ii) Unloading
• The type of equipment to be used for unloading the coal
received at the power station depends on how coal is received
at the power station.
• If coal delivered by trucks, there is no need of unloading device
as the trucks may dump the coal to the outdoor storage. Coal is
easily handled if the lift trucks with scoop are used.
• In case the coal is brought by railways wagons, ships or boats,
the unloading may be done by car shakes, rotary car dumpers,
cranes, grab buckets and coal accelerators. Rotary car dumpers
although costly are quite efficient for unloading closed wagons.
(iii) Preparation
• When the coal delivered is in the form of big lumps and it is
not of proper size, the preparation (sizing) of coal can be
achieved by crushers, breakers, sizers, driers and magnetic
separators.

iv)Transfer
• After preparation coal is transferred to the dead
storage by means of the following systems. 1. Belt
conveyors 2. Screw conveyors 3. Bucket elevators 4.
Grab bucket elevators 5. Skip hoists 6. Flight conveyor

Layout of Ash handling system
• Ash Handling System: Boilers burning pulverized coal have
bottom furnaces.
• The large ash particles are collected under the furnace in a
water-filled ash hopper, Fly ash is collected in dust collectors
with either an electrostatic precipitator or a bag house.
• A pulverized coal boiler generates approximately 80% fly ash
and 20% bottom ash.
• Ash must be collected and transported from various points of
the plants.
• Pyrites, which are the rejects from the pulverizes, are
disposed of with the bottom ash system.
• Three major factors should be considered for ash disposal
systems.
1. Plant site 2. Fuel source 3. Environmental regulation

5.4 Combustion chambers:
Types, geometries, various configurations
• In the combustion chambers of the furnaces, where
the aim is to exchange heat by radiation, the
arrangement of the set of burners may take various
configurations
•

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