Transcript Fuels and Combustion

Fuels and Combustion
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Recommended Books
1.
2.
3.
Fuels and Combustion; M.L. Smith & K.W. Stinson
Fuels and Fuel Technology ; W. Francis & M.C. Peters
Fuel – Solid , Liquid and Gaseous; J.S.S. Brame &
J.G. King
4.
5.
6.
7.
Hydrocarbon Fuels; E.M. Goodger
Coal Conversion Processes; Stanley & Lee
Fuel Testing: Laboratory Methods in Fuel
Technology; G.W. Himus
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 occuing 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 ???

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Combustion

Combustion or burning is a complex
sequence of exothermic chemical
reactions between a fuel (usually a
hydrocarbon) and an oxidant
accompanied by the production of heat
or both heat and light
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Combustion

Fuel + Oxidizer => Products of combustion + Energy
 Fuel
?
 Oxidizer ?
 Products of Combustion ?
 Incomplete Combustion ?
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Fundamental Definitions

Basic Flame types
Premixed: Fuel and oxidizer are mixed first
and burned later
Non-premixed: Combustion and mixing
occur simultaneously
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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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Fundamental Definitions

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
 Firepoint
The lowest temperature at which a liquid fuel
vapours in air produces a continuous flame
when exposed to a flame
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Fundamental Definitions
Density
 Specific gravity
 Viscosity
 Pour Point
 Carbon Residue

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Fundamental Definitions
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
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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
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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
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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 :
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Ash is an impurity that will not burn
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Reduces handling and burning capacity.
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Increases handling costs.
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Affects combustion efficiency and boiler
efficiency
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Causes clinkering
•
1-2 gm 72 mesh 800 C (burned)
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Proximate analysis
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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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