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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