Fuels and Their Classification

UNIT V

FUELS

•

Fuels are naturally occurring(fossil fuel) or

artificially manufactured combustible organic

substances which is a source of heat or raw

material for industry. Fuel is a substance that

undergoes combination with O

2

 in air to

liberate heat. They can be solids, liquids or

gases.

•

They have carbon and hydrogen. The chief

fuels are coal and mineral oil. Fuels can be

either of vegetable (organic- coal etc) or

mineral (inorganic – sulphur, iron pyrites etc)

origin, can be also nuclear, like enriched

uranium oxide.

Definition

1. Defined as any source of heat energy.

2. It is a combustible substance containing carbon

as main constituent which on proper

     burning gives large amount of heat that can be

used economically for domestic and

      industrial purposes.

3. It is any chemical or any reactant which produces

energy in a form that can be used for

    producing power.

Classification

Fuels are classified as follows

              I:      

Solid, liquid and gases

              II: i) 

Natural or raw fuels or primary

fuels.

                         Ex: Wood Peat, coal, petroleum

                  ii) 

Manufactured or processed or

artificial or secondary fuels

                  Ex: Coke, charcoal, kerosene, petrol

III : i) 

Primary fuels

   ii) 

Secondary fuels

                               Primary fuels are directly used

for the function of heat and its technical

utilization as such. They may be solids, liquids

or gases. Ex: Coal, wood, petroleum.

•

 Secondary fuels are one from which other

fuels are manufactured, which is then utilized

as fuel. These are obtained from primary

fuels. Ex Coal gas, water gas, producer gas etc

are obtained from primary fuel coal.

Natural fuels:

Solid Fuels

                Ex; Wood, peat, lignite, bituminous,

anthracite coal

Liquid

              Ex: petroleum

Gaseous

          Natural gas from petroleum wells.

Manufactured fuels

Solid:

 Charcoal, high and low temperature coke.

Liquid

 Petrol or gasoline, kerosene, diesel,

ethanol, methanol

Gaseous:

 Coal gas, coke oven gas, producer gas,

water gas.

Industrial fuels:

I : Power generating fuels :

            These are burned in power plants for

generating energy.

II : Process fuels:

            These are used in industrial furnaces and

driers to produce heat which is needed in

various industrial processes.

Calorific value :

•

The amount of heat given out by different

fuels will be different. This difference is

compared based on calorific value. Calorific

value is defined as the amount of heat

produced in calories during the combustion of

one gram of a substance in oxygen. Its unit is

kcalg

-1

.

•

Calorific value is measured by the number of

British thermal units (BTU) liberated by the

combustion of one lb of a solid fuel or one

cubic feet of a gaseous fuel. 1 BTU is the

amount o heat required to raise the

temperature of one lb of water through 1

o

F.

Calorific value can be measured using bomb

calorimeter.

Flash point:

•

For a liquid fuel it is the temperature at which

it emits vapour which is kindled by a flame.

The lowest temperature, at which oil

continues to burn after a flame has been

applied to it, is its ignition point. Flash point of

gasoline is 45

o

C, for kerosene it is 73

o

F.

Ignition temperature:

•

It is the temperature below which a solid fuel

will not catch fire and burn.

Essential requirements of fuels:

•

A good fuel should have high calorific value and

low moisture content.

•

It should be of low cost and readily available.

•

It should be easily transportable.

•

It should not give any undesirable by products

during burning.

•

It should leave only little ash or non- combustible

material. If they have large percentage of non-

combustible material left as ash then the calorific

value decreases.

•

The combustion product of the fuel must not

be harmful.

•

It should burn in air with efficiency without

producing much smoke.

•

Its combustion should take place

spontaneously and should be controllable.

•

Particle size of solid fuels should be uniform.

•

It should produce high temperature, called as

pyrometric burning effect. This highest

temperature obtained by combustion depends

upon   i) calorific value     ii) Velocity of

combustion   iii) space in which it is burnt iv)

physical state of fuel.

•

      If same quantity of solid, liquid and gaseous

fuels is burnt in same amount of air, gaseous fuel

gives the highest pyrometric effect, liquid less

effect and solid the least.

•

Fuels should have proper ignition point. If

have low ignition temperature then fire

hazards can happen. But high ignition

temperature makes fuel safe for

transportation and storage, but not favorable

for starting fire. So moderate ignition

temperature and moderate velocity of

combustion is favored for a good fuel.

Energy characteristics of a fuel:

Energy characteristics of a fuel depends on

•

Heat of combustion

: It is the amount of heat

obtained by burning a unit mass or volume of a

fuel.

•

Energy content

 : It is the amount of potential

heat energy contained in a unit volume of a fuel

and measured as the volume of fuel in m

3

corresponding to one ton of reference fuel,

whose heat of combustion is

292.4kj/kg(700kcal/kg)

•

Solid and liquid fuels contain organic

combustible part(C,H,O) and a mineral in

combustible part which is moisture and

inorganic compounds like silicates,

phosphates, sulphates, sulphites of metals like

Ca, Al, Fe, K and Na. These mineral substances

are converted into metal oxides on burning of

solid fuel and remaining ash.

•

Sulphur as sulphides is oxidized to SO

2

.

Moisture and minerals lower the energy

ratings of fuels, raise the cost of

transportation and complicates processing

technology. When fuel is heated without

access of air (pyrolysis) gaseous and liquid

products from that are contained as vapours

in the pyrolysis products. This vapour and gas

mixture also containing the fuel water is called

volatile matter.

Water Gas:

•

Water gas is often called as blue gas since the

flame formed will be blue in colour when

burnt. Water gas is a mixture of CO and H

2

. It

is produced by passing steam over red hot bed

of coke at 1400 to 1000 

0

C.

 H

2

O + C → CO + H

2 

     -29 kcal.

•

This is an endothermic reaction, so

temperature falls from 1400 to 1000

0

C. At this

temperature CO

2

 is also formed.

                2H

2

O  +  C  →  2H

2

  +  CO

2  

    -19 kcal

•

This CO

2

 has no calorific value. Formation of

water gas depends on the decomposition of

steam by coke at 758

o

C, at this temperature

25.3% steam is decomposed so the

composition will be 56.2% of H

2

, 7.8% CO and

27%CO

2

. At 1000

o

C 93% stem is decomposed

so the composition will be 50.7% H

2

, 48%CO

and 1.3% CO

2

.

•

And at 1125

o

C 99.4% of steam is decomposed

so the composition will be50.9%H

2

, 48.5% CO

and 0.6%CO

2

. To maintain the percentage of

decomposition ie at 1000

o

C or above, blast of

steam is stopped and air blast is passed to

raise the temperature to 1400

o

C.

•

Then pass stem blast and then pass air blast

and so on. Since air forms explosion with

water gas, water gas is removed by passing

steam for one minute, this steam is called

purge steam. The average composition of

water gas is 48% of H

2

, 1% of CH

4

, 42% of CO,

3% of CO

2

 and6% of N

2

. The calorific value of

water gas is about 300 Btu/cu ft.

Uses:

–

Used as a source of H

2

 and as a reducing agent in

metallurgy.

–

It is added to coal gas for street lighting.

–

Used to synthesis chemicals like alcohols and

oxygenated compounds.

Producer Gas:

•

Producer gas is a mixture of CO and N

2

. This a

cheap individual fuel obtained from low grade

coal. This is prepared by passing air over a bed

of white hot coke or solid carbonaceous fuel

at 1000 – 1400

o

C.

•

           C  + ½ O

2

  + 2N

2 

(air)  → CO  + 2N

2

                                                          ΔH  = -27.06kcal

•

 During preparation, actually CO

2

 is first

produced; it then reacts with coke to give CO.

Instead of coke and coal, peat, wood waste,

spent tar can be used. But largely used raw

material is coke and coal. Coke or anthrasite

or charcoal is used if producer gas is used in

gas engines for power production. It is

manufactured in a plant known as “ gas

producer”.

•

The fuel is stacked in a grate in the cylindrical

furnace, made of fire clay refractory bricks.

The blast of air is blown through the grate.

The fuel is fed from the top. Gas produced

leaves the furnace through the gas outlet.

2C  + O

2

 →2CO        + 58000 cal

C  + O

2

  →  CO

2

        +97000 cal

CO

2

 + C →  2CO       -39000 cal

•

The CO

2

 formed is reduced to CO by heated

coke and as the reaction is endothermic, the

temperature of the bed is kept high at above

1000

o

C. The proportion of CO increases with

increase in temperature.

•

Producer gas has lower calorific (1300kcal/m

3

)

value of any gaseous fuel and the temperature

of its flame is the lower of any fuel. Its calorific

value is 150Btu per cubic feet. Its average

composition is 10% H

2

, 8% CH

4

,20% CO,

4%CO

2

 and 64% N

2

.

Uses:

•

 It is used for heating coal gas retorts, in

furnace and gas engines.

Oil Gas:

•

This oil is used in labs. It is obtained by

cracking heavy oil like kerosene, during which

a mixture of gaseous hydrocarbons are

produced.

          C

16

H

34

  →6CH

4

  +  2C

2

H

4

  +  C

2

H

2

  +  4C

•

The average composition of oil gas is 25-30%

CH

4

,50-55%H

2

,10-12%CO and 3%CO

2 

. Its

calorific value is 4500-5400kcal/m

3

.

Natural gas:

•

This is a mixture of methane (80- 98 %),

ethane, propane, butanes, pentanes, CO

2

, N

2

etc.  Its calorific value is 1200-1400kcal/m3.

This is found in vicinity of coal mines or oil

fields.

•

It is used in domestic, industrial and as a

chemical raw material for various syntheses. It

accumulates in underground reservoirs either

with or with out petroleum oil. Natural oils

obtained from oil wells may be either wet or

dry.

i) Dry Variety

•

This type contains mainly of methane and

does not contain either gasoline vapour or

crude petroleum. Its composition will be

96%CH

4

, 0.8%C

2

H

4

 and 3.2%N

2

.

ii) Wet variety

•

This type consists of a mixture of methane

and higher HC, like n-propane, n-butane.

Isobutene, isopentane etc. It is obtained from

oil producing wells along with petroleum.

Calorific value of this variety is higher than dry

variety because of its high percentage of

heavier unsaturated molecules.

By products from natural gases:

•

The products formed like methane, ethane,

propane, butane, natural gasoline and LPG are

of industrial importance. These are separated

from crude gas as liquids by the absorption of

higher HC in petroleum or catalytically

polymerizing higher HC into liquid HC. Natural

gas is transported in the form of liquid.

•

The gas contains undesirable compounds like

H

2

O, H

2

S, which should be removed before

usage. The gas obtained by fermentation of

organic matter in sewage is also called sewage

natural gas. The sewage is collected and

subjected to anaerobic fermentation, were by

the nitrogenous matter in it is converted into

nitrates and a part of the carbonaceous

matter is converted into sewage natural gas

consisting mainly of methane and CO

2

.

•

This gas contains 70% methane and 30%CO

2

.

The calorific value of the gas is 62.5Btu/Cu ft.

•

Cow dung is the main source of natural gas.

Cow dung on fermentation gives gas with

composition as in sewage gas. The residue

formed in this process is used as manure.

Natural gas burns with smoky flame in

ordinary burners. So use special appliances.

•

Natural gas can be liquefied under pressure

and cooling to -121

o

C. This can be mixed with

other fuels to increase their calorific value.

This gas is used for the manufacture of carbon

black.CH

4

 natural gas used for the

manufacture of formaldehyde and methanol

by controlled oxidation. Helium is also

recovered from natural gas.

Coal gas:

•

Coal gas is obtained from the destruction of

coal ie when coal is heated in the absence of

air. One ton of coal gives12000cubic feet of

coal gas at 1000

o

C and the yield is 18%.The

yield can be increased to 22% by increasing

the temperature to 1400 -1500

o

C.

•

Coal gas produced by this process is a mixture

of gases and about 95% of it is combustible.

The compounds present in the mixture

depend on the temperature of carbonization.

The average composition of the gas is 49% of

H

2

, 32% of CH

4

, 8% of CO,4-5% acetylene and

ethylene, 4% of N

2

, 1% of O

2

and 1% of CO

2

.

•

H

2

, CH

4

 and CO are non-illuminating but heat

producing, N

2

, O

2

 and CO

2

 are inert or

diluents. H

2

S, CS

2

, NH

3

, cyanogens etc are

present as impurities. Its calorific value is

about 4900kcal/m

3

.

•

This gas is used to provide inert atmosphere in

number of preparations, used to provide

reducing atmosphere in metallurgical

operations, in smelting of metals and alloys,

used as fuel and illuminanent, used in

producing fuel gases and in making graphite,

and does not condense on cooling.

Gobar or bio gas:

•

This is a mixture of methane, CO

2

, H

2

 and H

2

S.

The average composition is 55% CH

4

, 7.4% H

2

,

35%CO

2

, 2.6%N

2

 and traces of water. This is

produced by the anaerobic fermentation of

animal waste in the absence of O

2

 and in the

presence of H

2

O.

•

The carbohydrates, proteins and fats in animal

waste are broken down by bacteria to form

CH

4

. The residue formed during the process

can be used as manure since the essential

nutrients like N

2

 and phosphorous remains

intact.

•

The gas is highly flammable and is used as

fuel. Natural gas is also biogas, which results

from long time decay of animal and vegetable

matters brought about by bacteria’s at high

temperature, high pressure etc. The average

calorific value is 5300kcal/m

3

.

•

The limitation of this gas is that its formation

depends on cattle dung and the burner or

lamp should be with in ten meters of the

plant.

LPG (Liquid petroleum gas):

•

This is a petroleum gas obtained by the

fractional distillation of petroleum, which

catches fire readily. It is a mixture of butane as

major part along with propane, ethane and

alkenes.

•

The high alkene variety of LPG contains 27% of

n-butane, 25%2-butane, 435 of butane, 2.5%

of propene and2.5% of propane. The low

alkene variety contains 70% of butane, 18.3%

of 2-butane, 1.2% of butane.11.3% of propane

and 0.2% of ethane.

•

This is prepared from natural gas or gas from

the cracking units of petroleum refineries.

This can be liquefied under pressure. For

house hold use 14kg of it is liquefied and

bottled.

•

The pressure of the gas at room temperature

within the cooking cylinder is about 3kg/cm

2

in closed condition. The pressure decreases

to0.034kg/cm

2

 by the operation of the

cylinder regulator.

•

The mixture of gas has no smell so to detect

leakage a strong smelling substance

ethylmercaptan is added. Its calorific value is

29780kcal/m

3

.

Coal products:

•

Besides being used as a fuel coal is converted

into     a) coal gas and coke by destructive

distillation   b) gaseous fuels like water,

producer gases etc and c) liquid fuels by

hydrogenation. The destructive distillation of

coal is known ad carbonization.

•

Coal is carbonized at high temperature ie

1200

o

C-1400

o

C or at low temperature ie

600

o

C-650

o

C. In either case the main products

are coal gas, coal tar and coke. The products

formed vary in quality.

Low temperature carbonization

•

At low temperature process, less gas is

obtained but with high calorific value. This

process gives high yield of fluid tar of low

viscosity with high percentage of phenol and

Nitrogen compounds, high percentage of tar

acids, motor spirit and fuel oil. High

temperature tar is the principal source of

aromatic compounds like benzene, toluene,

naphthalene, anthrascene and phenol.

•

Low temperature tar is converted into motor

spirit and diesel oil by hydrogenation. Low

temperature coke is soft and fireable, burns

without smoke and contains 5 to 10 % of

volatile matter. High temperature coke is hard,

suitable in metallurgy and burns with smoke.

•

Low temperature  carbonization  is done when

i) low temperature semi coke as a domestic

fuel is needed  ii) complete gasification of coal

is required  iii) smoke less, free burning coke is

required   iv) to obtain coal tar suitable for

conversion in to liquid fuels for use as motor

spirit.

•

In this process the yield of gas is low, quantity

of tar is more and has high percentage of

paraffin substance, yield of ammonia is low,

aromatic content of tar is low and tar acid

content is high.

High temperature carbonization

•

The need of this process is to manufacture

coal gas and coke and several valuable by-

products like coal tar, ammoniacal liquor, gas

carbon and coke. Coal gas is used for heating

and lighting. Coal tar on fractional distillation

gives benzene, toluene, naphthalene, carbolic

acid, creosote oil( used for preparing dyes,

drugs, perfumes and explosives).

•

Coal tar pitch is used for road making, directly

used for preserving timber and for water

proofing gunny bags. Ammoniacal liquor on

boiling with milk of lime yields ammonia,

which is fixed by H

2

SO

4

 as ammonium

sulphate,  which is used as fertilizer. Gas

carbon is hard and is a good conductor of heat

and electricity. It is used in making electrodes.

Liquid fuel from coal:

•

 Coal is pyrolysed in the presence of   sulphur

resisting catalyst like molybdenum, in the

atmosphere of H

2

 at 300-500

o

C. At 200-250

o

C

H

2

 combines with N

2

 of coal to produce

ammonia and with O

2

 of coal to produce

water.

•

During this process three fractions are

obtained  i) crude spirit, the top fraction   ii)

middle oil, the middle fraction and   iii) heavy

oil , which is the residue. Middle oil can be

converted to spirit and can be used in motors.

•

Using different catalyst different hydrocarbons

can be separated. The liquid fuel obtained is

fractionally separated into gasoline and the

higher boiling fraction is an excellent diesel

fuel. Using cobalt as catalyst olefin’s and

paraffin’s are produced.

Petroleum products:

•

Petroleum, a liquid fuel, is also called as rock

oil which is obtained from the ground either

by natural seepage or by drilling wells. It

consists of liquid oils, hydrocarbons, natural

gas, waxes, sulphur, nitrogen, and oxygen.

 Formation of petroleum:

•

Inorganic theory - F

ormed by the action of

steam or water on heavy metal carbides

existing in the interior of the earth.

•

Organic theory

 - Formed by the

decomposition of marine organic matter

deposit by bacterial action in the absence of

oxygen.

•

      It can be formed by the decomposition of

organic matter deposit by the catalytic action

of inorganic matter.

Composition of petroleum:

•

 It is a mixture of major part of hydrocarbons,

minor amount of naphthenic acids, Sulphur,

nitrogen and oxygen derivatives of

hydrocarbons.  It contains 83-87% of carbon,

11-14 % of hydrogen and 0-4% of S,N and O

2

.

The composition will be constant since only

eight or nine homologous series of

compounds are present.

•

The hydrocarbons present are n- paraffin’s,

iso-paraffin’s, olefins, napthenes, and

aromatics. Next to paraffin’s napthenes  are

abundant and then aromatics and olefin’s.

Sulphur present is a pollutant so it should be

removed.

Classification of Petroleum:

•

The classification is based on composition of

i) Paraffin base which contains hydrocarbons

of paraffin series.

 ii) Asphaltic base which contains hydrocarbons

of naphthalene ring or naphthalene product.

iii) Mixed base which contains both paraffin

and asphaltic base.

Products obtained from petroleum

distillation:

•

Petroleum ether  -    20-60

o

C   -     solvent

•

Benzene  -               70-90

o

C   -      dry cleaning

•

Ligroin   -                80-120

o

C   -    solvent

•

Rubber solvent   -   100-160

o

C   -    rubber

solvent

•

White spirit   -        150-200

o

C    -     solvent

•

Solvent naphtha -  >200but < 300

o

C    -

solvent

•

Kerosene    -        >200 -  <300

o

C    -   fuel

•

Lubricating oil  -   >300

o

C

•

Vaseline/grease    -                     lubricant,

medicine, cosmetics

•

Diesel oil (gas oil)        -            used in diesel

engines

•

Heavy fuel oil              -             fuel for generating heat.

•

Paraffin wax                -              candles, waxed paper,

polishes.

•

petroleum wax             -             water proofing coating

on metals

•

Petroleum jelly            -              cosmetics, coating

metals.

•

Bitumen                      -               road making, roofing,

paints, coating pipes

•

Liquid paraffin or white oil     - medicine.

Octane number of a fuel:

•

It is the percentage of iso-octane which must

be added to n- heptanes in order to obtain a

mixture which matches the knocking

characteristics of the fuel under examination,

in a one cylinder motor engine under standard

conditions. The octane number of isooctane is

100 and for n-heptanes it is zero.

•

Isooctane and other branched paraffin’s under

high compression have less knocking

tendency, so better fuel for modern vehicles

and aero plane. Aviation gasoline has an

octane number 100 and ordinary gasoline,

which is a mixture of large amount of straight

chain paraffin’s and small amounts of olefins,

naphthenes and aromatics, has an octane

number ranging from 73 to20.

Cetane number:

•

The suitability of a fuel for diesel engine is

expressed as cetane number. It is defined as

the percentage of cetane which is to be mixed

with methyl naphthalene in order to obtain a

fuel which matches the performance of the

fuel under examination. For diesel engines

cetane is normal hexadecane.

Knocking:

•

Knocking is a sharp metallic sound produced

in the internal combustion engine. This results

in huge loss of energy in the engine.

Antiknock compounds:

•

These are compounds used to prevent

knocking in engines. Compounds used

tetraethyl lead (largely used in gasoline and

petrol) and iron penta carbonyl.

Industrial gases:

•

Following are the gases commonly used for

industrial purposes.

•

   1) H

2

   2) O

2 

  3) N

2

   4) CO

2

   5) acetylene   6)

ethylene   7) helium.

1) Hydrogen:

•

This is prepared by the electrolysis of water.

Uses:

•

In the synthesis of ammonia and then

converting ammonia into fertilizers.

•

For hydrogenating oils to make fats or in the

hardening of fatty oils.

•

As a fuel for rockets.

•

For the hydrogenation of coal.

•

For hydrogenating water gas to produce

methanol.

•

For producing HCl.

•

In filling meteorological balloons.

•

As a cooling gas  for alternators in place of air

•

In producing an inert atmosphere in making

tungsten filaments for electric lambs.

Carbon dioxide:

•

This is obtained as a by product in the

production of hydrogen by steam-water gas or

during fermentation of glucose.

 Uses:

•

Solid CO2 is for refrigerating ice creams, meat

and other foods.

•

Liquid CO2 is used for blasting coal.

•

The gas is used for preparing aerated water.

Acetylene:

•

This is prepared by the reaction of calcium

carbide with water.

Uses:

•

As an illuminant for marine and air ships, light

house etc

•

For welding, cutting of steel, mixed with oxygen it

produces heat of 3500

o

C.

•

For preparing neoprene rubber and vinyl resins.

•

As a fuel gas.

Ethylene:

•

This is obtained from petroleum refinery

gases.

•

It is used for producing polythene and styrene,

for preparing large number of organic

compounds.