Lecture-9.-Fuels anCombustion for summary

DEFINITION:
FUELS are substances which when burned with oxygen or by
the process of burning produces heat and light. These are
materials burned to produce heat and power.
3 General Classification or Types of Fuels:
1. Solid Fuels – principal component is carbon.
2. Liquid Fuels – principal component is hydrocarbon.
3. Gaseous Fuels – principal component is hydrocarbon.

1. SOLID FUELS:
- includes coal, coke, peat, briquets, wood, charcoal
and waste products.

COAL:
- a black or dark brown, brittle,
compact, amorphous substance
or varied physical and chemical
composition produced by the
carbonization of pre-historic
vegetation.

Classification of Coal:
a) By Rank – degree of metamorphism or progressive
alteration in the natural series from lignite to anthracite.
b) By Grade –gradually determined by size designation,
calorific value, ash, ash-softening temperature and sulfur.
c) By Type or Variety – determined by nature of the original
plant material and subsequent alteration thereof.

Classification of Coal by Rank:
a) Anthracitic :
 meta-anthracite
 anthracite
 semi-anthracite
b) Bituminous:
 low volatile bituminous coal
 medium volatile bituminous coal
 high volatile A bituminous coal
 high volatile B bituminous coal
 high volatile C bituminous coal

Classification of Coal by Rank:
c) Sub-bituminous:
 sub-bituminous A coal
 sub-bituminous B coal
 sub-bituminous C coal
d) Lignitic:
 lignite
 brown coal

Coke:
– a solid fuel made by heating coal in the absence of air so
that the volatile components are driven off.
where:
Destructive Distillation – is
the process of heating coal in
the absence of air.

Peat:
– a highly organic material found by marshy or damp regions,
composed of partially decayed vegetable matters, cut and
dried for use as fuel.

Briquets:
– or briquettes, a compressed block of coal dust or other
combustible biomass materials such as charcoal, sawdust,
woodchips, peat or paper used for fuel.

Charcoal:
– a porous black solid, consisting of an amorphous form of
carbon, obtained as a residue when wood, bone, or other
organic matter is heated in the absence of air.

Bagasse:
– the dry pulpy residue left after the extraction of juice from
sugar cane.

2. LIQUID FUELS:
– includes any of the following: petroleum oils, gasoline,
kerosene, Diesel fuels, fuel oils, alcohol, coal tar and tar oil.

Petroleum Oils:
– are generally regarded to be formed from animal and
vegetable debris accumulating in sea basins or estuaries and
buried there by sand and silt. They are generally called
hydrocarbon (CnHm) compounds used as fuels which are
mainly composed of carbon and hydrogen.
Product composition of petroleum oils:
Carbon – 80 to 90%
Hydrogen – 12 to 14%
Nitrogen – 0.3 to 1%
Sulfur – 0.3 to 3%

Gasoline:
– a refined petroleum naphtha which by its composition is
suitable for use as carburetant in internal combustion
engine. It is the fraction of petroleum oil which is generally
regarded as light distillate. This is the most important
product of crude oil which is about 43% of the crude
processed.
where:
Naphtha  a generic term applied to readily vaporizable
hydrocarbon liquids such as liquids used as solvents for specific
purposes (cleaning, manufacture of rubber cement, paints and
varnishes.

Kerosene:
– a refined petroleum distillate having a flash point not below
22.8oC (as determined by the Abel tester) and suitable as an
illuminant when burn in a wick lamp. It has a boiling point
range of 150 oC to 300 oC.

Diesel Fuel:
– are petroleum distillates boiling in the range of 200 oC to
370 oC. These are used as fuels for heavy trucks, commercial
boats, stand-by power plants, etc.
Classification of Diesel Fuels:
a) Distillate Fuels – produced by distillation of crude oil
b) Residual Fuels – are those left after the distillation process
c) Blended Fuels – are mixture of straight distillate fuels with
crack fuel stocks.

Fuel Oils:
– these are liquid or liquefiable petroleum products burned
for the generation of heat in a furnace or firebox, or the
generation of power in an engine. They are specified loosely
as light, medium and heavy with maximum viscosities of 200
Redwood No.1 sec, 950 sec, and 350 sec at 35 oC respectively.
Bunker C is fuel oil No.6. This is further classified as low sulfur
and high sulfur depending on its sulfur content.

Alcohol:
– considered as fuel for internal combustion engines and
ethyl alcohol or sometimes called grain alcohol is the most
frequently used fuel. The modern name of ethyl alcohol is
ethanol. Two other alcohols that have been used as fuels are
methanol and isopropanol which are called methyl alcohol
and isoprophyl alcohol, respectively.

Coal Tar and Tar Oil :
– a product of destructive distillation of bituminous coal
carried out at high temperature. It is used principally in
reheating furnaces and open-hearth furnaces of steel works.
Typical Composition of Coal Tar:
Carbon = 86.7%
Hydrogen = 6%
Oxygen = 3.1%
Nitrogen = 0.1%
Sulfur = 0.8%
Ash = 0.1%
Moisture = 3.2%

3. GASEOUS FUELS:
– includes natural gas, coke-oven gas, blast-furnace gas, producer gas,
liquefied petroleum gas, acetylene and sewage-sludge gas.
Natural Gas:
– a combustible gas which is a mixture of hydrocarbon and non-
hydrocarbon gases. This is found in subsurface rock reservoirs with or
near accumulations of crude oil. Methane is the main component of this
gas.
Natural gas obtained from oil wells is called casing-head gas. Dry gas is
the term used when the gasoline from the natural gas is extracted or
recovered.

Coke-Oven Gas:
– is obtained as a by-product when making coke, and its
analysis depends upon the coal used and also upon the
method of operating the oven.
Blast-Furnace Gas:
– is a by-product of melting iron ore. Its analysis varies
considerably with the fuel used and the method of operating
the blast-furnace.

Producer Gas:
– a synthesis gas and is generated by blowing air or a mixture
of air and steam through a hot bed of coal or coke. This gas
consists of CO2, N2 and a small amount of H2 and CO.
Liquefied Petroleum Gas (LPG):
– is a mixture of specific hydrocarbons which can be liquefied
under moderate pressure at normal temperature but are
gaseous under normal atmospheric conditions. The chief
constituents of LPG are propane, propylene, butane, butylene
and isobutene mixed in any proportions or with air.

Acetylene:
– is primarily used in operations which require high flame
temperature, such as welding and metal cutting. This is
produced by the reaction of calcium carbide (CaCl) with water.
Sewage-Sludge Gas:
– a gas derived from sewage-disposal plants and used as fuel
for internal-combustion engines. These engines furnish
energy for driving the pumps in the sewage plants.

PROPERTIES OF FUELS AND LUBRICANTS:
1) Flash Point – the temperature to which oil must be
heated to give off sufficient vapor to form an
inflammable mixture with air.
2) Fire Point – the lowest temperature at which the vapor
of the fuel will continue to burn when ignited.
3) Pour Point – the lowest temperature at which oil will
flow under prescribed conditions.

4) Dropping Point – the temperature at which grease melts
(from semi-solid state to a liquid state).
5) Octane Number – the ignition quality rating of gasoline.
It is the percentage by volume of iso-octane in a mixture
of iso-octane and heptane that matches the gasoline in
anti-knock quality, as determined in the standard engine
under standard procedure.

6) Cetane Number - the ignition quality rating of Diesel fuel, which is the
percentage of cetane used in the mixture. This is determined by taking
the delay angle of the fuel in a standardized test engine. The delay angle
is the angle of crankshaft revolution between the beginning of the fuel
injection and the first appreciable rise in pressure due to combustion.
This is expressed by cetane number.
Diesel Index is also used to express ignition quality of fuels.
𝐷𝑖𝑒𝑠𝑒𝑙 𝐼𝑛𝑑𝑒𝑥 =
°𝐴𝑃𝐼 × 𝐴𝑛𝑖𝑙𝑖𝑛𝑒 𝐶𝑙𝑜𝑢𝑑 𝑃𝑜𝑖𝑛𝑡
100
where:
Aniline Cloud Point  is found by heating a mixture consisting of
equal volumes of the test sample and freshly distilled, water-free
aniline, C6H7N, until a clear solution is obtained.

7) Carbon Residue or Conradson Carbon – the carbonaceous
residue remaining after destructive distillation expressed in
percentage by weight of the original sample.
8) Viscosity Index – indicates the relative change in viscosity of
an oil for a given temperature change.

Hydrocarbons:
 Paraffins  𝐶𝑛𝐻2𝑛+2
 Olefins and Naphtenes  𝐶𝑛𝐻2𝑛
 Diolefins  𝐶𝑛𝐻2𝑛−2
 Aromatics  𝐶𝑛𝐻2𝑛−6
 Asphaltics  𝐶𝑛𝐻2𝑛−4
– is a substance which is a combination of carbon and hydrogen and
has a general formula of CnHm .
Main Types of Hydrocarbon:
Heptane, 𝐶7𝐻16, Octene, 𝐶8𝐻16, Hexadien, 𝐶6𝐻10, Benzene,
𝐶6𝐻6, Toluene, 𝐶7𝐻8
Examples:

LIST OF COMMON ALKANES (HYDROCARBON FUELS) :
Molecular Formula Name Molecular Formula Name
CH4 Methane C11H24 Undecane
C2H6 Ethane C12H26 Dodecane
C3H8 Propane C13H28 Tridecane
C4H10 Butane C14H30 Tetradecane
C5H12 Pentane C15H32 Pentadecane
C6H14 Hexane C16H34 Hexadecane
C7H16 Heptane C17H36 Heptadecane
C8H18 Octane C18H38 Octadecane
C9H20 Nonane C19H40 Nonadecane
C10H22 Decane C20H42 Icosane

ANALYSIS OF FUELS:
1) GRAVIMETRIC ANALYSIS OF FUELS (percent by mass or
weight analysis):
a) Ultimate Analysis of Fuel
Ultimate analysis of fuel shows the chemical elements on a weight-
percentage basis, together with the items ash and moisture. The
chemical composition of ultimate analysis of fuel are the following
(on weight percentages):
C – carbon
H2 – hydrogen
O2 – oxygen
N2 – nitrogen
S – sulfur
A – ash
M – moisture

ANALYSIS OF FUELS:
weight analysis):
Classifications of Ultimate Analysis:
I. As-received or as-fired basis:
(C + H2 + O2 + N2 + S + A + M = 100%)
II. Dry or moisture-free basis:
(C + H2 + O2 + N2 + S + A = 100%)
III. Combustible or ash-and-moisture free basis
(C + H2 + O2 + N2 + S = 100%)

ANALYSIS OF FUELS:
weight analysis):
b) Proximate Analysis of Fuel
Proximate analysis shows the following fuel compositions on a
weight-percentage basis such as:
FC - fixed carbon
VM - volatile matter
A - ash and
M - moisture.

ANALYSIS OF FUELS:
weight analysis):
Classification of Proximate Analysis:
I. As-received or as-fired basis:
(FC + VM + A + M = 100%)
II. Dry or moisture-free basis:
(FC + VM + A = 100%)
III. Combustible or ash-and-moisture free basis
(FC + VM = 100%)

ANALYSIS OF FUELS:
1) VOLUMETRIC ANALYSIS or MOLAL ANALYSIS OF FUELS
(percent by mols or percent by volume analysis):
Volumetric analysis is usually used for liquid and gaseous fuels.
Ex. C4H10 = 100%
CH4 = 70% and C2H6 = 30%
Note: Analyze how to convert a fuel formula into ultimate analysis
(percent by mass)

IMPORTANT PROPERTIES OF FUELS:
1) SPECIFIC GRAVITY OF FUELS
 Hydrometer
 Pycnometer
 Westphal Balance
Instruments used for measuring
specific gravity of fuel:
𝑺. 𝑮. =
𝝆𝒇𝒖𝒆𝒍
𝝆𝒘𝒂𝒕𝒆𝒓
– is the ratio between the density of any fuel to the density of a
standard substance (usually water).

a) Specific gravity of fuel at any temperature:
𝑆. 𝐺.𝑓 =
𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑓𝑢𝑒𝑙
𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟
=
𝜌𝑓
𝜌𝑤
=
𝛿𝑓
𝛿𝑤
𝑆. 𝐺.𝑓 =
𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑓𝑢𝑒𝑙
𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑎𝑖𝑟
=
𝜌𝑓
𝜌𝑎
=
𝛿𝑓
𝛿𝑎
3
3
3
4
.
62
1000
81
.
9
ft
lb
m
kg
m
kN f
f
w 



3
3
3
075
.
0
2
.
1
0118
.
0
ft
lb
m
kg
m
kN f
f
a 



where:
(𝑓𝑜𝑟 𝑙𝑖𝑞𝑢𝑖𝑑 𝑓𝑢𝑒𝑙𝑠)
(𝑓𝑜𝑟 𝑔𝑎𝑠𝑒𝑜𝑢𝑠 𝑓𝑢𝑒𝑙𝑠)

b) Standard Specific Gravity, S.G.s (liquid fuel existing at
temperature, t = 15.6oC or 60oF):
𝑆. 𝐺.𝑠 =
141.5
131.5 + 𝑜
𝐴𝑃𝐼
𝑆. 𝐺.𝑠 =
140
130 + 𝑜
𝐵𝑒
(𝑖𝑓 o𝐴𝑃𝐼 𝑟𝑎𝑡𝑖𝑛𝑔 𝑜𝑓 𝑓𝑢𝑒𝑙 𝑖𝑠 𝑘𝑛𝑜𝑤𝑛)
(𝑖𝑓 oBe 𝑟𝑎𝑡𝑖𝑛𝑔 𝑜𝑓 𝑓𝑢𝑒𝑙 𝑖𝑠 𝑘𝑛𝑜𝑤𝑛)
oAPI = degree API (American Petroleum Institute) rating of fuel
where:
oBe = degree Baume rating of fuel
oAPI and oBe are scales of density of fuel

c) Actual Specific Gravity, SGO:
𝑆. 𝐺.𝑜 = 𝑆. 𝐺.𝑠 × 𝐶𝑡
𝐶𝑡 = 1 − 0.0007 𝑡𝑓 − 15.6 (if the temperature tf is given in degree Celsius)
where:
If actual or operating temperature of the fuel is given (other than the
standard temperature), apply a thermal correction factor Ct to get
the actual density of the fuel at the given temperature.
𝐶𝑡 = 1 − 0.0004 𝑡𝑓 − 60 (if the temperature tf is given in degree Fahrenheit)
Note: Recall all relative properties of the substance such as mass, volume, weight and densities.
• At any temperature of the fuel, the mass is always constant.
• Volume increases as temperature increases while weight density decreases

2) HEATING VALUE OF FUELS
 Oxygen bomb calorimeter – used for solid and liquid fuels
Instruments used for measuring specific gravity of fuel:
– is the amount of energy (kilojoule or Btu) or heat obtained during
the combustion of the fuel at required temperature and pressure.
 Gas calorimeter – used for gaseous fuels

Lecture-9.-Fuels anCombustion for summary

a) Higher Heating Value, QH (ASME Value):
 the heating value obtained from fuel if it is assumed that the
water formed in the products of combustion is in the liquid
state.
 it is called the “Gross Calorific Value” by ASTM.
 it includes the latent heat of water.

Higher Heating Value Calculations:
i) ASME Formula – used for petroleum products with known oAPI rating.
𝑄𝐻 = 41,130 + 139.6 °𝐴𝑃𝐼 𝑘𝐽/𝑘𝑔
𝑄𝐻 = 17,680 + 60 °𝐴𝑃𝐼 𝐵𝑡𝑢/𝑙𝑏
ii) Bureau of Standards Formula – used for petroleum products if specific
gravity of the fuel is known.
𝑄𝐻 = 51,716 − 8,793.8 𝑆. 𝐺. 𝑓
2 𝑘𝐽/𝑘𝑔
𝑄𝐻 = 22, 230 − 3,780 𝑆. 𝐺. 𝑓
2 𝐵𝑡𝑢/𝑙𝑏

iii) Dulong’s Formula – used for fuels with known ultimate analysis.
𝑄𝐻 = 33,820 𝐶 + 144,212 𝐻2 −
𝑂2
8
+ 9,304 𝑆 𝑘𝐽/𝑘𝑔
𝑄𝐻 = 14,544 𝐶 + 62, 028 𝐻2 −
𝑂2
8
+ 4,050 𝑆 𝐵𝑡𝑢/𝑙𝑏
where:
C, H2, O2 and S  are the actual fractions by weight of carbon, hydrogen, oxygen
and sulfur in the fuel
Ultimate Analysis  the determination of the percentages by weight of the major
organic elements in the fuel such as carbon, hydrogen, oxygen,
nitrogen, sulfur, and ash.

iv) Shermann and Knoppf’s Formula – used for petroleum products with
known Baume rating.
𝑄𝐻 = 42,450 + 93 °𝐵𝑒 − 10 𝑘𝐽/𝑘𝑔
𝑄𝐻 = 18,250 + 40 °𝐵𝑒 − 10 𝐵𝑡𝑢/𝑙𝑏

b) Lower Heating Value, QL (ASME Value):
 is the heating value obtained from fuel it is assumed that the
water formed in the products of combustion is considered in
the vapor state or vaporized.
 it is called “Net Calorific Value by ASTM”.
 it excludes the latent heat of water.

Lower Heating Value Calculations:
𝑄𝐿 = 𝑄𝐻 − 𝑙𝑎𝑡𝑒𝑛𝑡 ℎ𝑒𝑎𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟
𝑄𝐿 = 𝑄𝐻 − ℎ𝑒𝑎𝑡 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑡𝑜 𝑣𝑎𝑝𝑜𝑟𝑖𝑧𝑒 𝑤𝑎𝑡𝑒𝑟 𝑖𝑛 𝑡ℎ𝑒 𝑝𝑟𝑜𝑑𝑢𝑐𝑡
𝑄𝐿 = 𝑄𝐻 − 𝑚𝑤 × ℎ𝑓𝑔
where:
𝑚𝑤 = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑖𝑛 𝑡ℎ𝑒 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑜𝑓 𝑐𝑜𝑚𝑏𝑢𝑠𝑡𝑖𝑜𝑛
𝑚𝑤 = 9 × % 𝐻2 𝑖𝑛 𝑡ℎ𝑒 𝑓𝑢𝑒𝑙
%𝐻2 = 26 − 15 𝑆. 𝐺.𝑠
ℎ𝑓𝑔 = 1,050
𝐵𝑡𝑢
𝑙𝑏
= 2,442
𝑘𝐽
𝑘𝑔
(latent heat of evaporation of 1 lb water at
77oF)
(S.G.s is the specific gravity at standard
condition, t = 60oF)

COMBUSTION OF FUELS:
Combustion - is a chemical reaction (chemical combination) at high
temperature of the combustible elements in the fuel with oxygen. Heat
energy and light being released in the process.
Essential elements for combustion to take place:
1. Fuel (main combustible elements are the carbon and hydrogen)
2. Air (only oxygen is necessary in the combustion while nitrogen acts
as a moderator)
3. Heat/Necessary turbulence
4. Sufficient space/volume

Theoretical Oxygen Required for Combustion (𝑾𝑶𝟐
):
- is the amount of oxygen needed to support the combustion process
measured in kg oxygen per kg of fuel.
Formulas:
a) If the ultimate analysis of the fuel is known, use Dulong’s formula:
𝑊𝑂2
= 2.67 𝐶 + 8 𝐻2 −
𝑂2
8
+ 𝑆
𝑘𝑔 𝑜𝑓 𝑂2
𝑘𝑔 𝑜𝑓 𝑓𝑢𝑒𝑙
𝑜𝑟
𝑙𝑏 𝑜𝑓 𝑂2
𝑙𝑏 𝑜𝑓 𝑓𝑢𝑒𝑙

):
b) If the fuel formula of hydrocarbon fuel is known (CnHm):
𝑊𝑂2
=
𝐴
𝐹
𝑘𝑔 𝑜𝑓 𝑎𝑖𝑟
× 0.231
,
where:
𝐴
𝐹
= 𝑖𝑑𝑒𝑎𝑙 𝑎𝑖𝑟 − 𝑓𝑢𝑒𝑙 𝑟𝑎𝑡𝑖𝑜 =
137.6 𝑛+0.25𝑚
12𝑛+𝑚
,
Formulas:

):
c) Using the balancing method of the combustion equation:
𝑊𝑂2
=
𝑛𝑜. 𝑜𝑓 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛 × 𝑀𝑊 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛
𝑛𝑜. 𝑜𝑓 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑓𝑢𝑒𝑙 × 𝑀𝑊 𝑜𝑓 𝑓𝑢𝑒𝑙
where:
MW = Molecular Weight (kg/kmol or lb/pmol)
Molecular Weight of Common Elements
 C = 12
 H2 = 1
 O2 = 16
 N = 14
 S = 32
Formulas:

The Air Required for Combustion:
In furnaces of boilers and internal combustion engines, the oxygen is
obtained from an air supply, air being composed of approximately 23%
oxygen and 77% nitrogen by mass.
The oxygen is the active element. Nitrogen, being an inert gas, takes no
active part, it acts as a moderator, dilutes the products of combustion and
as it absorbs some of the heat energy produced it reduces the
temperature of the combustion.

Properties of Air:
1) Volumetric Analysis (Molal Analysis)
𝑂2 = 21%, (0.21 𝑚𝑜𝑙 𝑜𝑓 𝑂2 / 𝑚𝑜𝑙 𝑜𝑓 𝑎𝑖𝑟)
𝑁2 = 79%, (0.79 𝑚𝑜𝑙 𝑜𝑓 𝑁2 / 𝑚𝑜𝑙 𝑜𝑓 𝑎𝑖𝑟)
2) Weight Analysis (Gravimetric Analysis)
𝑂2 = 23.1%, (0.231 𝑙𝑏 𝑜𝑓 𝑂2 / 𝑙𝑏 𝑜𝑓 𝑎𝑖𝑟)
𝑁2 = 76.9%, (0.769 𝑙𝑏 𝑜𝑓 𝑁2 / 𝑙𝑏 𝑜𝑓 𝑎𝑖𝑟)

Properties of Air:
3) Mol Relation
1 𝑚𝑜𝑙 𝑜𝑓 𝑎𝑖𝑟 = 0.21 𝑚𝑜𝑙 𝑜𝑓 𝑂2 + 0.79 𝑚𝑜𝑙 𝑜𝑓 𝑁2
4) Molecular Weight of Air
𝑀𝑊𝑎𝑖𝑟 = 28.97 𝑘𝑔/𝑘𝑚𝑜𝑙 𝑜𝑟 𝑙𝑏/𝑝𝑚𝑜𝑙
1 𝑚𝑜𝑙 𝑜𝑓 𝑁2 = 3.76 𝑚𝑜𝑙 𝑜𝑓 𝑂2

Diagrammatic Representation of Fuel Combustion with Air:
𝐹𝑢𝑒𝑙
𝑆𝑜𝑙𝑖𝑑
𝐿𝑖𝑞𝑢𝑖𝑑
𝐺𝑎𝑠
+ 𝐴𝑖𝑟 = 𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑠 𝑜𝑓 𝐶𝑜𝑚𝑏𝑢𝑠𝑡𝑖𝑜𝑛
𝐹𝑙𝑢𝑒 𝐺𝑎𝑠 𝐷𝑟𝑦 𝐺𝑎𝑠𝑒𝑠 + 𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒
+
𝑅𝑒𝑓𝑢𝑠𝑒 (𝐴𝑠ℎ + 𝑈𝑛𝑏𝑢𝑟𝑛𝑒𝑑 𝐶𝑎𝑟𝑏𝑜𝑛
+
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝐴𝑖𝑟 + 𝐸𝑥𝑐𝑒𝑠𝑠 𝐴𝑖𝑟
+ 𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒
𝑂𝑥𝑦𝑔𝑒𝑛 + 𝑁𝑖𝑡𝑟𝑜𝑔𝑒𝑛
=

Air-Fuel Ratio:
– is the ratio of the amount of air supplied to the amount of fuel burned
during the combustion process.
1) Types of Air-Fuel Ratio Values:
𝑎) 𝐴𝑖𝑟 − 𝐹𝑢𝑒𝑙 𝑅𝑎𝑡𝑖𝑜 𝑏𝑦 𝑀𝑎𝑠𝑠 =
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑎𝑖𝑟
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑓𝑢𝑒𝑙
𝑏) 𝐴𝑖𝑟 − 𝐹𝑢𝑒𝑙 𝑅𝑎𝑡𝑖𝑜 𝑏𝑦 𝑉𝑜𝑙𝑢𝑚𝑒 =
𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑎𝑖𝑟
𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑓𝑢𝑒𝑙
𝑐) 𝐴𝑖𝑟 − 𝐹𝑢𝑒𝑙 𝑅𝑎𝑡𝑖𝑜 𝑏𝑦 𝑀𝑜𝑙𝑠 =
𝑚𝑜𝑙𝑠 𝑜𝑓 𝑎𝑖𝑟
𝑚𝑜𝑙𝑠 𝑜𝑓 𝑓𝑢𝑒𝑙

Air-Fuel Ratio:
– is the ratio of the amount of air supplied to the amount of fuel burned
during the combustion process.
2) Classification of Air-Fuel Ratio:
𝑎) 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝐴𝑖𝑟 − 𝐹𝑢𝑒𝑙 𝑅𝑎𝑡𝑖𝑜, 𝐴/𝐹 𝑜𝑟 𝐴: 𝐹
𝑏) 𝐴𝑐𝑡𝑢𝑎𝑙 𝐴𝑖𝑟 − 𝐹𝑢𝑒𝑙 𝑅𝑎𝑡𝑖𝑜, 𝐴′/𝐹 𝑜𝑟 𝐴′: 𝐹

Theoretical Air-Fuel Ratio (A/F) Calculation:
– calculated when combustion requires only theoretical amount of air
(100%).
Theoretical Air-Fuel Ratio Formulas:
𝐴: 𝐹 = 11.5 𝐶 + 34.5 𝐻2 −
𝑂2
8
+ 4.3 𝑆,
𝑜𝑟
𝑙𝑏 𝑜𝑓 𝑎𝑖𝑟
1. If the ultimate analysis of the fuel is known, use the Dulong’s formula.

(100%).
𝐴: 𝐹 =
137.6 𝑛 + 0.25𝑚
12𝑛 + 𝑚
,
𝑜𝑟
𝑙𝑏 𝑜𝑓 𝑎𝑖𝑟
2. If the fuel formula (CnHm) of hydrocarbon fuel is known, use
Maleev’s equation:

(100%).
𝐴: 𝐹 =
𝑄𝐻
3,117
,
3. If the heating value of the fuel is known (usually for coals and oils),
use the Moore’s equation:
where: QH = heating value in kJ/kg

(100%).
𝐴: 𝐹 =
𝑊𝑂2
0.231
,
4. If the amount of oxygen required is known:
where: QH = heating value in kJ/kg

Actual Air-Fuel Ratio (A/F’) Calculation:
– calculated when an excess air is required or deficient air is supplied in
the combustion process.
𝐴′
𝐹
=
𝐴𝑐𝑡𝑢𝑎𝑙 𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝐴𝑖𝑟 𝑆𝑢𝑝𝑝𝑙𝑖𝑒𝑑
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝐹𝑢𝑒𝑙 𝐵𝑢𝑟𝑛𝑒𝑑
Cases of Actual Combustion:
1. If percent excess air (EA) is supplied:
𝐴′
𝐹
=
𝐴
𝐹
1 + %𝐸𝐴
2. If percent deficient air (DA) is known:
𝐴′
𝐹
=
𝐴
𝐹
1 − %𝐷𝐴
3. If actual combustion space data
known:
𝑚𝑎′ + 𝑚𝑓 = 𝑚𝑔
𝐴′
𝐹
=
𝑚𝑔
𝑚𝑓
− 1

Equivalence Ratio:
– is defined as the ratio between the theoretical air-fuel ratio to the
actual air-fuel ratio.
∅ =
𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑎𝑖𝑟 − 𝑓𝑢𝑒𝑙 𝑟𝑎𝑡𝑖𝑜
𝑎𝑐𝑡𝑢𝑎𝑙 𝑎𝑖𝑟 − 𝑓𝑢𝑒𝑙 𝑟𝑎𝑡𝑖𝑜
=
Τ
𝐴 𝐹
Τ
𝐴′ 𝐹
where: ∅ = 1, for stoichiometric mixture
∅ < 1, for fuel-lean mixture
∅ > 1, for fuel-rich mixture

BALANCING METHODS IN COMBUSTION REACTIONS (all balancing are done in mol values):
1. Combustion Equation with Theoretical Air (100% air):
Combustion
of
oducts
Air
Fuels Pr


  2
2
2
2
2 76
.
3
76
.
3 aN
O
cH
bCO
aN
aO
H
C m
n 




Steps to Balance the Ideal Combustion Equation (100% air):
a) Balance the carbon to find “b”. where: a = theoretical mols of oxygen
b) Balance the hydrogen to find “c”. b = no. of mols of CO2
c) Balance the oxygen to find “a”. c = no. of mols of water
3.76a = no. of mols of N2
Analysis:
fuel
lb
air
lb
or
fuel
kg
air
kg
fuel
of
MW
fuel
of
mols
air
of
MW
N
mol
O
mols
fuel
of
MW
fuel
of
mols
air
of
MW
air
of
mols
F
A
a ,
)
(
) 2
2







a
c
b
n
oduct
of
Mols
Total
b t 76
.
3
)
(
Pr
) 


TOTAL
t
O
H
TOTAL
O
H
O
H P
n
n
P
product
of
mols
of
no
total
water
of
mols
of
no
P
P
oduct
in
Water
of
essure
Partial
c 


 2
2
2
.
.
:
)
(
Pr
Pr
)
where: PTOTAL = PATM (if not given)

2. Combustion Equation with Excess Air:
- Supply only of the theoretical air amounts in furnaces sometimes would not burn the fuel completely. An amount
greater than the theoretical amount or an additional excess air must be supplied.
Combustion
of
oducts
Air
Fuels Pr


  2
2
2
2
2
2 %
)
76
.
3
)(
%
1
(
76
.
3
)
%
1
( aO
EA
N
a
EA
O
cH
bCO
aN
aO
EA
H
C m
n 







Steps to Balance the Actual Combustion Equation (with %EA):
a) Balance the carbon to find “b”.
b) Balance the hydrogen to find “c”.
c) Balance the oxygen to find “a”.
Analysis:
fuel
lb
air
lb
or
fuel
kg
air
kg
EA
fuel
of
MW
fuel
of
mols
air
of
MW
N
mol
O
mols
EA
fuel
of
MW
fuel
of
mols
air
of
MW
air
of
mols
F
A
a ),
%
1
(
)
(
)
%
1
(
'
) 2
2











a
EA
a
EA
c
b
n
oduct
of
Mols
Total
b t )
(%
76
.
3
)
%
1
(
)
(
Pr
) 




TOTAL
t
O
H
TOTAL
O
H
O
H P
n
n
P
product
of
mols
of
no
total
water
of
mols
of
no
P
P
oduct
in
Water
of
essure
Partial
c 


 2
2
2
.
.
:
)
(
Pr
Pr
)

3. Combustion With Deficient Air:
- Air supplied is below the theoretical amount required for combustion. Carbon monoxide is likely to be formed in
the products of combustion.
Combustion
of
oducts
Air
Fuels Pr


  dCO
N
a
DA
O
cH
bCO
aN
aO
DA
H
C m
n 








 2
2
2
2
2 )
76
.
3
(
)
%
1
(
76
.
3
)
%
1
(
Steps to Balance the Actual Combustion Equation (Deficient Air Supplied):
a) Balance first in theoretical air (100%) to find “a”.
  2
2
2
2
2 76
.
3
76
.
3 aN
O
cH
bCO
aN
aO
H
C m
n 




b) Balance in the actual deficient air supplied.
  dCO
N
a
DA
O
cH
bCO
aN
aO
DA
H
C m
n 








 2
2
2
2
2 )
76
.
3
(
)
%
1
(
76
.
3
)
%
1
(
Analysis:
fuel
lb
air
lb
or
fuel
kg
air
kg
DA
fuel
of
MW
fuel
of
mols
air
of
MW
N
mol
O
mols
DA
fuel
of
MW
fuel
of
mols
air
of
MW
air
of
mols
F
A
a ),
%
1
(
)
(
)
%
1
(
'
) 2
2











d
a
DA
c
b
n
oduct
of
Mols
Total
b t 



 76
.
3
)
%
1
(
)
(
Pr
)
TOTAL
t
O
H
TOTAL
O
H
O
H P
n
n
P
product
of
mols
of
no
total
water
of
mols
of
no
P
P
oduct
in
Water
of
essure
Partial
c 


 2
2
2
.
.
:
)
(
Pr
Pr
)

burned
fuel
kg
air
dry
kg
,
)
(
03
.
3
m
2
2
air
dry
CO
CO
N


PRODUCTS OF COMBUSTION ANALYSIS
















Carbon
Unburned
Ash
fuse
Vapor
Water
Gas
Dry
Gas
Flue
Combustion
of
oducts
Re
Pr
Orsat Analysis – is the volumetric analysis of the dry products of combustion or water-free analysis of the products of
combustion which include CO2, O2, N2, CO.
Flue Gas Analysis:
1) For the total mass of flue gas, mfg:
w
dg
fg
fg
m
m
m
products
in
water
of
mass
gas
dry
of
mass
m




r
f
refuse
fuel C
C
C
C
C
fuel
kg
carbon
unburned
kg




'
carbon,
unburned
of
amount
=
C'
:
where
fuel
kg
gas
flue
kg
C
CO
CO
N
CO
O
CO
m fg ,
'
)
(
3
)
(
7
8
11
2
2
2
2






2) For the mass of the dry gas, mdg:
fuel
kg
gas
dry
kg
C
CO
CO
O
CO
mdg ,
'
)
(
3
7
4
2
2
2





oducts
of
Analysis
Orsat Pr
CO
,
O
,
CO
:
where 2
2 
3) For the mass of water formed in the products of combustion, mw:
fuel
kg
water
kg
)
(%H
9
m 2
w 

4) For the mass of dry air supplied, mdry air:

fuel
kg
refuse
kg
,
mR R
R C
A 

Refuse Analysis:
1) Total mass of refuse, mR:
where: AR = ash in the refuse, kg refuse/kg fuel
CR = carbon in the refuse, kg carbon/kg fuel
2) Amount of carbon in the refuse, CR:
R
R
R
R
R
R
R
R
R
A
A
A
C
A
C
A
m
A













%
,
%A
:
given
is
refuse
kg
ash
kg
%A
If
a) R
R
From the principle of continuity of ash-fly: Afuel = Arefuse
fuel
kg
carbon
unburned
kg
A
A
A
C fuel
R
fuel
R ,
%


R
R
R
R
R
R
R
A
C
C
m
C
C
A













%
,
1
%
%C
:
given
is
refuse
kg
carbon
kg
%C
If
b) R
R
fuel
kg
carbon
unburned
kg
A
C
A
C
A
A
A
C fuel
R
fuel
R
R
R
R
R 




)
%
1
(
,
%

EXCESS AIR PERCENTAGE CALCULATIONS BASED ON PRODUCTS:
 
EA
F
A
F
A
%
1
'



a) For theoretical A/F:
S
O
H
C
F
A
3
.
4
8
5
.
34
5
.
11 2
2 









 
m
n
m
n
F
A



12
25
.
0
6
.
137
3117
H
Q
F
A

b) For actual A’/F:
769
.
0
'
03
.
3
' 2
2
2 fuel
N
CO
CO
N
C
F
A












Lecture-9.-Fuels anCombustion for summary

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