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Fuels and Combustion
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Recommended Books
1. Fuels and Combustion; M.L. Smith & K.W. Stinson
2. Fuels and Fuel Technology ; W. Francis & M.C. Peters
3. Fuel – Solid , Liquid and Gaseous; J.S.S. Brame &
J.G. King
4. Hydrocarbon Fuels; E.M. Goodger
5. Coal Conversion Processes; Stanley & Lee
6. Fuel Testing: Laboratory Methods in Fuel
Technology; G.W. Himus
7. Methods of Analysis of Fuels and Oils; J.R. Campbell
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Fuel
A substance which produce heat
either by combustion or by nuclear
fission / fusion
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Classification of Fuels
Fuels can be classified as solid, liquid and gaseous fuels.
Solid fuels : wood, coal, charcoal and coke Liquid fuels : petrol, kerosene, diesel, alcohol etc Gaseous fuels : methane, propane, butane, hydrogen, coal gas, gobar gas etc
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Classification of Fuels
Primary Fuels: Naturally occuring e.g.
coal, wood, natural gas
Secondary Fuels: Which are derived from
primary fuels e.g. kerosene, coke etc
Naturally occurring
Artificially prepared
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Combustion
Combustion is the conversion of a
substance called a fuel into chemical
compounds known as products of
combustion by combination with an
oxidizer.
The combustion process is an exothermic
chemical reaction, i.e., a reaction that
releases energy ???
Combustion or Burning
Combustion or burning is the sequence
of exothermic chemical reactions between a fuel and
an oxidant accompanied by the production of heat and
conversion of chemical species. The release of heat can
result in the production of light in the form of either glowing or
a flame. Fuels of interest often include organic compounds
(especially hydrocarbons) in the gas, liquid or solid phase.
A flame is the visible, gaseous part of a fire. It is caused by a highly exothermic reaction taking place.
Deflagration
Deflagration "to burn down" is a term describing
subsonic combustion that usually propagates through thermal
conductivity; hot burning material heats the next layer of cold
material and ignites it. Most "fire" found in daily life,
from flames to explosions, is deflagration. Deflagration is different
from detonation, which is supersonic and propagates
through shock.
Simply it is a rapid high energy release combustion event that
propagates at subsonic speeds, driven by the transfer of heat.
Smouldering (or smoldering
Smouldering (or smoldering) is the slow, low-
temperature, flameless form of combustion, sustained by
the heat evolved when oxygen directly attacks the
surface of a condensed-phase fuel. Many solid materials
can sustain a smouldering reaction,
including coal, cellulose, wood,cotton, synthetic foams,
charring polymers including polyurethane foam etc
Common examples of smouldering phenomena are the
combustion of biomass behind the flaming front
of wildfires
Incineration
Incineration is a waste treatment process that involves
the combustion of organic substances contained in waste materials.
Incineration and other high temperature waste treatment systems
are described as "thermal treatment". Incineration of waste materials
converts the waste into ash, flue gas, and heat. The ash is mostly
formed by the inorganic constituents of the waste, and may take the
form of solid lumps or particulates carried by the flue gas. The flue
gases must be cleaned of gaseous and particulate pollutants before
they are dispersed into the atmosphere. In some cases, the heat
generated by incineration can be used to generate electric power.
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Combustion
Fuel + Oxidizer => Products of combustion + Energy
Fuel ?
Oxidizer ?
Products of Combustion ?
Incomplete Combustion ?
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Basic Flame types
Premixed: Fuel and oxidizer are mixed first
and burned later
Nonpremixed: Combustion and mixing
occur simultaneously
Fundamental Definitions
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Combustion
Air-Fuel Ratio; A/F
Oxygen-Fuel Ratio; O/F
Stoichiometric or Theoretical A/F
Excess Air % excess air = 100[(A/F)actual - (A/F)theo ]/(A/F)theo
120% of theoretical air ? Fuel- Rich flame: If there is an excess of fuel
Fuel - lean flame : if there is an excess of oxygen
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Combustion
1 kg of C needs ? kg of O2
1 kg of H2 needs ? Kg of O2
1 kg of Sulphur needs ? kg of O2
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Combustion
Problem:
A fuel contains by mass 88 % carbon, 8 %
H2, 1% S and 3% ash. Calculate the
stoichiometric air/fuel ratio.
Ans: ?
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Fundamental Definitions
Calorific value
Amount of heat librated by the combustion of unit quantity of fuel. kcal/ kg , kcal / m3
Gross Calorific Value (G.C.V) or HCV
heating value measurement in which the product water vapour is allowed to condense
Net Calorific Value (N.C.V) or LCV
heating value in which the water remains a vapor and does not yield its heat of vaporization
HHV = LHV + (mwater /mfuel)ʎwater
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Flash Point
The lowest temperature at which a liquid fuel
gives enough vapours in air which produce a
momentary flash when exposed to a flame
Fire point
The lowest temperature at which a liquid fuel
vapours in air produces a continuous flame
when exposed to a flame
Fundamental Definitions
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Density
Specific gravity
Viscosity
Viscosity Index
Aniline Point
Pour Point
Carbon Residue
Fundamental Definitions
Viscosity Index
The viscosity index is an arbitrary number
indicating the effect of change of
temperature on the kinematic viscosity of
an oil. A high viscosity index signifies a
relatively small change of kinematic
viscosity with temperature.
The viscosity index of an oil is calculated
from its viscosities at 40 and 100°C.
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viscosity Index
Naphthenic base oils change more than
paraffinic base oils. Certain synthetic
lubricants change much less than paraffinic
oils. Hence, the VI of an oil is important in
applications where an appreciable change
in temperature of the lubricating oil could
affect the start up or operating
characteristics of the equipment
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Dynamic Viscosity Vs Kinematic Viscosity
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Viscosity Index
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Theoretical Flame Temperature:
It is the temperature attained by the
products of combustion of fuel when
there is no loss of heat to the
surroundings
Flue Gas: It is the gaseous product of
combustion of fuel
Fundamental Definitions
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Coal
Origin of Coal
Coal has been formed by the partial decay of plant materials accumulated million of years ago and further altered by the action of heat and pressure
In situ Theory: coal occupies the same site where the orignal palnts grew
Drift Theory: plants were uprooted and drifted by rivers to get deposited
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Coal classification
• Peat :
• Lignite: soft coal and the youngest
• sub-bituminous
• Bituminous:
• semi-bituminous:
• Anthracite: hard and geologically the
oldest composed mainly of carbon
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Coal Analysis
Proximate analysis of coal
• Determines only fixed carbon, volatile matter,
moisture and ash
• Useful to find out heating value (GCV)
• Simple analysis equipment
Ultimate analysis of coal
• Determines all coal component elements: carbon,
hydrogen, oxygen, sulphur, etc
• Useful for furnace design (e.g flame temperature,
flue duct design)
• Laboratory analysis
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Proximate analysis
Moisture Content :
Moisture in coal must be transported,
handled and stored
Since it replaces combustible matter, it
decreases the heat content per kg of coal
Aids radiation heat transfer
1-2 gm 72 mesh coal at 105-110 C till
constant weight
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Proximate analysis
Volatile Matter:
Consist of CH4, hydrocarbons, H2 and CO,
and incombustible gases like CO2 and N2
Proportionately increases flame length, and
helps in easier ignition of coal
Sets minimum limit on the furnace height
and volume
72 mesh coal 900-950 C for 7 minutes
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Proximate analysis
Ash Content :
• Ash is an impurity that will not burn
• Reduces handling and burning capacity.
• Increases handling costs.
• Affects combustion efficiency and boiler
efficiency
• Causes clinkering
• 1-2 gm 72 mesh 800 C (burned)
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Proximate analysis
Fixed carbon:
Solid fuel left in the furnace after volatile
matter is removed
consists mostly of carbon
may contains some H2, O2, S and N2
gives a rough estimate of heating value
of coal
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