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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 cokeLiquid fuels        : petrol, kerosene, diesel, alcohol etcGaseous 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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Basic Flame types

Premixed: Fuel and oxidizer are mixed first and burned later

Non-premixed: Combustion and mixing occur simultaneously

Fundamental Definitions

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Combustion

Air-Fuel Ratio; A/FOxygen-Fuel Ratio; O/FStoichiometric 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

Firepoint

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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DensitySpecific gravityViscosityPour PointCarbon Residue

Fundamental Definitions

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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