Therefore, those materials used for production of heat energy are termed as fuels.
Nature has furnished or given us unlimited source of energy of different forms.
What is a fuel?
A fuel can be defined in different ways as follows----
(1) Any source of heat energy is called as fuel.
(2) Any substance which by oxidation, is made a source of heat for practical
purposes is called as fuel.
(3) Any combustible substance which may be burnt to supply heat that can
be used economically for domestic and industrial purposes.
Raw coal is directly used for domestic heating and as source of
other products obtained by processes of dry distillation which may be
conveniently called carbonization. During carbonization process, there is
always decrease of the amount of gaseous and volatile matter when the fuel is being heated.
Modern concept of fuels : It is now well established fact that ,it is not necessary for a fuel
to undergo combustion or oxidation .
For example, electrical energy when used as a source of heat is also called fuel.
Nuclear energy may also be used as a source of heat.
According to the modern definition, a fuel is any fissionable material which produces energy in a form that can be used for producing power.
Classification of fuels:
On the basis of physical states, fuels can be classified into three groups:
a) Solid state
b) Liquid fuels and
c) Gaseous fuels.These are further sub-classified according to their origin as natural or artificial.
The clarification is summarized in below table
<table border="2">
<tr>
<td>State</td>
<td> Natural Fuels</td>
<td>Artificial Fuels</td>
</tr>
<tr>
<td>1. Solid Fuels</td>
<td>Coal,wood,shale,peat,liginite</td>
<td>Charcoal, wood, coke, petroleum coke,<br> Briquettes.</td>
</tr>
<tr>
<td>2. Liquid Fuels</td>
<td>Natural gasoline,crude petroleum</td>
<td>Petrol, Kerosene, diesel oil, ligroline oil, <br>coal tar, alcohol, gas-oil.</td>
</tr>
<tr>
<td>3. Gaseous Fuels</td>
<td>Natural gas (Marsh gas),</td>
<td>Producer gas, water gas, coke over gas ,<br> coal gas, oil gas ,Blast furnace gas,<br> acetylene, hydrogen,<br>Liquid Petroleum Gas (LPG)</td>
</tr>
It seem from the table that , whatever may be the origin of the fuel ( natural or synthetic) - both contain all the three phase states viz, solid
liquid and gaseous.
Artificial or synthetic fuel is prepared by processing natural fuel by
different ways. for example , natural fuels like coal , wood , shale , peat ,
petroleum natural gas etc . are known as primary fuels , where as coke ,
charcoal , petrol , kerosene , coal tar , producer gas , coke - oven gas , compressed
butane etc . are called secondary fuels . Secondary fuels are therefore , produced by
processing primary fuels .
Properties of a Good Fuel
A good fuel should possess following properties.
1. High Calorific Value:
The calorific value of a fuel is the amount of heat produced by the
complete combustion of a unit weight or volume of the fuel . The fuel with
high calorific value is more useful.
a.In case of solid and liquid fuels that the calorific value is usually expressed in
BTU per pound or calories per gram.
1 calorie per gram = 1.8 BTU per pound.
b. In case of gaseous fuels the calorific value is expressed in BTU per cubic
feet at 60 % F and 30 inch pressure of Hg.
1 kcal / m = 0.1077 * BTU / Ft.
1BTU / Ft = 9.3 kcals / m.
2. Moderate Ignition Temperature :
The temperature at which a solid, liquid or gaseous fuel catches fire and continues to burn without further heat is called ignition temperature.
It is also called as ignition point .The value of ignition temperature decreases due to presence of moisture and foreign material.
i. If the ignition temperature of the fuel is low it is easy to start fire but it is dangerous for storage and transport.
ii. If the ignition temperature of the fuel is high it is difficult to start fire but it is easy for storage and transport.
Therefore, an ideal fuel should have moderate ignition temperature.
3. Moderate velocity of combustion:
The temperature attained by combustion in case of a particular fuel
depends upon its velocity of combustion , its calorific value and the space
used for combustion . It the velocity of combustion is very low , then the
liberated may get radiated instead of raising the temperature . Secondary the
velocity of combustion should not be very high .
4.Low % of Non - combustible matter :
After combustion of fuel , the non - combustion matter like ash or
clinker remains behind which also reduces the heating value of the fuel . A
good fuel can have high calorific value , only if it has low % of non -
combustion matter (ash).
5.Products of composition :
The gaseous products of combustion of fuel should not pollute the
atmosphere and should not be harmful or poisons for human lives. Gases like
carbon monoxide , sulphur-dioxide , hydrogen sulphide , phosphine etc . are
some of the harmful gases.
6.Ease of transport :
The good fuel is easy to handle , store and transport. Solid and liquid
fuel can be transported very easily from one place to another .On the other
hand transportation of gaseous fuels is costly and can fire hazards.
7.Low cost :
A fuel should be readily available in bulk at low cost . The cost of
production , storage and transport should be low.
Criterion of Selection of Fuel
Following factors are of primate importance while selecting a fuel for
a particular purpose.
i. The fuel must have reasonably high velocity of combustion.
ii. It should have high calorific value.
iii. It must have proper ignition temperature.
iv. It must have low % of non combustion matter .
v. It must have low cost.
vi. It must be available in sufficient quantity.
vii. It must be easy for transport.
Calorific Value of Fuels.
Different fuels produce different amount of heat on burning. The
usefulness of a fuel is measured in terms of its calorific value . The calorific
value of fuel may be defined as :
i. The amount of heat liberated in calories by the complete combustion of
a combustion substance with oxygen and the condensation of the
products to the desired temperature , OR
ii. The amount of heat liberated by burning unit weight or unit volume of
the fuel.
Let us consider the combustion of carbon and hydrogen .
C + O2 ------- CO2 + 97 .644
12g 32g 44g.
H2 + O ------- H2O
2g 16g 18g.
It is evident from the above equations that 12 g of carbon give
97. 644 calories of heat , therefore 1 g of carbon would give 97644 / 12 =
8137 cals of heat , similarly 2g of hydrogen give 69 .000 calories of heat ,
when steam is condered to water .
Units for Calorific Value :
i.BTU .
For solid fuels the calorific value is expressed in BTU per pound or
in KCU per kilogram . One BTU in the hear required to raise the temperature of
one pound of water from 60 % F to 61 % F .
ii. Calorie :
A calorific which is the unit of heat , may be defined as , the heat
required to raise the temperature of one kilogram of water from 15% C to
16% C.
1 BTU = 252 cals = 0.252 kcals .
1 kcal = 3.968 BTU .
1 kcal/kg =1.8 BTU/1b .and = 0.1077 * BTU/ft3.
iii. KCU:
One KCU is the amount of heat required to heat one kilogram of
water from 15 % C to 16 % C.
The mean calorific value of any fuel . containing carbon and
hydrogen can be calculated as follows :
Total calories = (% carbon * 8137 ) + ( % Hydrogen * 34500 )
--------------------------------------------------------
100
iv.KJ/g :
The calorific value of a fuel is usually expressed as kilojoules per
gram . For example the calorific value of charcoal is 33 kilojoules per
gram. It is a common unit of measuring calorific values in kilojoules per gram.
The calorific values of some common fuels are given in table 6 . 2.
Problem:
i. Calculate the calorific value of carbon from given reaction
C + O2 ---- CO2 + 387 kJ
: 12 g of carbon produces heat = 387 kJ
: 1 g of carbon produces heat = 387 / 12 = 32 . 25 kJ
In case of gaseous fuels calorific values are calculated from the sum
of the calorific values of the constituent gases. In case of fuel containing
heat is also deducted for moisture present in the fuel.
Determination of Calorific Values of a Fuel
Calorific value of a solid or liquid fuel can be determined by
burning a definite amount of fuel in a Bomb calorimeter . In such a
under constant pressure .
@
Thus , C9H6O + 10 O2 -------------- 9 CO2 + 3 H2O
Anthracitic coal
Procedure:
a. Take a known mass of the fuel whose calorific value is to be determined.
b.Take a known mass of water in a beaker and then burn the fuel.
c.The fuel in burning will produce heat , that will be absorbed by water
taken in beaker.
d.The temperature of water starts rising.
e.The rise in temperature is noted when all the fuel is burnt. It is
found out from temperature of water..
f.Thus , knowing the mass of water taken in a beaker , specific
formula ,
Heat produced Q = m * S * t Joules .
-------------------------------------------
where , m = mass of water
S = specific heat of water
t = rise in temperature of water .
This quantity heat has been produced by burning ' X '
grams of fuel.
We can find out the calorific value of the fuel in joules per gram by
dividing heat Q by the mass water ' X ' grams of the fuel burnt.
: Calorific Value = Amount of heat produced
--------------------------------------------
Mass of fuel burnt.
= Q /X Joules per gram.
= Q kJ/ gram.
-----------
X * 1000.
Example :A beaker containing 400 grams of water at 25 % C heated by
burning 2 . 5 grams of charcoal. The final temperature of water become 55 % C .
Solution :
i. Mass of water = 400 g .
ii.Specific heat of water =4 .2 J/g% C.
iii.Intial temperature of water = 25 % C.
iv. Final temperature of water = 55 % C.
v. Rise in temperature of water (t) = 55 - 25 = 30 % C.
Formula :
Heat absorbed by water ( Q ) = m * S * t Joules
= 400 * 4. 2 * 30
= 50400 Joules
= 50 kJoules.
This amount of heat ( Q ) absorbed by water has been produced by
burning 2 . 5 grams of charcoal fuel.
: Calorific value of the fuel = Amount of heat produced
--------------------------------
Mass of fuel burnt
= Q / X
= 50 / 2 .5
= 20 . 16 kJoules gram.
Hence calorific value of the fuel is 20 . 16 .
The calorific value of gaseous fuels can be determined by burning a
definite volume of the fuel completely at atmospheric pressure in a chamber
surrounded by coils through which water is allowed to flow at constant pressure .
Other properties of fuel .
(1) Ignition Temperature : It is defined as , the temperature at which a fuel
catches fire and continues to burn without further heat . Each fuel has definite
ignition temperature value . The ignition temperature is influence by a
number of factors such as -- i. Nature of fuel ii .combustion environment
iii.presence of moisture iv. Size and shape of the experimental value vi.
particle size and presence of foreign material etc .
The value of ignition temperature decrease due to the presence of
moisture and foreign material .
( 2 ) Flash point : It is the property of a liquid fuel like gasoline , kerosene ,
petrol , alcohol etc . It is defined as , the minimum temperature at which a
sample of a fuel oil gives off enough vapour that catches fire but does not
continue to burn in presence of air by naked flame or electric discharge .
( 3 ) Fire point :It is defined as , 'the temperature at which the oil vapours will
catch fire and continue to burn ' . Both flash point and fire point are used to
detect dissolved impurities much more volatile than the bulk of the oil .
( 4 )Coke number : It is defined as , ' the amount of carbon deposited by a
weighed sample of oil in a standard apparatus under a set of
experimental conditions . ' Better grade lubricating oils have lower coke
number .
( 5 ) Viscosity :This is the properly of lubricating oils . It is expressed in ,
centipose . In USA . Saybolt Universal Viscosity ( SUV ) is generally
employed in petroleum industry . It is expressed as ' the number of seconds
required for 60 c.c. of the oil to run through a standard orifice from a cylinder
filled to a defined level at a standard temperature . '
Solid Fuels .
Solid fuels contain combustible organic materials and an
incombustible or minerals part known as ballast. There are two types of solid fuels .
Among the solid fuels wood , coal and charcoal are commonly used .
The heat of combustion of a fuel depends upon the composition of the fuel
and on its moisture content .
Coal :
Coal is the most important natural solid fuel . It is combustible solid
fuel , usually formed by the burial of partically decomposed vegetation in post
geological ages . After that pressure and temperature converted them to coal .
The calorific values and uses of some of the solid fuels are
summarized in table 6 . 3 .
Chemically , coal is highly complex organic matter with varying
amount of water , trapped as well as combined , together with nitrogen
and sulphur in the form of organic and inorganic matter . Sulphur is
an important impurity which affects the quality of a fuel . The contents of
impurities in coal make different grades of coal e .g .
a.Anthracite
b.Bituminonus coals -low volatile type fixed carbon , medium
volatile type and high volatile type.
c.Sub -bituminous coal or black lignite.
d.Pulverized coal .
Destructive Distillation of coal .
During destructive distillation, coal is thermally pyrolyed or
distilled by heating in the absence of air . During this process coal is
converted into variety of solid , liquid and gaseous products . The quality as
well as the amount of each product depends upon the temperature used in the
pyrolysis and the variety of coal .
When soft coal and other kinds of solid fuels are heated under
different in various conditions , they undergo complicated changes and lead
to the formation if new solid , liquid or gaseous products .
The method of high temperature processing of solid fuels can be
divided in to three main types -- a. Pyrolysis b. Gasification
c.Hydrogenation .
a.Pyrolysis : In this process the heating of the fuel in a closed reactors is
carried out in the absence of air , the low temperature pyrolysis , called semi-
coking is carried out at 500 - 580 % C , The temperature pyrolysis , called
coking is carried out at 900 -1050 % C .
b.Gasification :In the process the transformation of the organic part of
solid fuel into combustible gases is carried out by incomplete oxidation with
air ,steam or their mixture in gasifies at a temperature of about 1000 % C .
c.Hydrogenation :In this process the treatment of the fuel with hydrogen
is carried out at high temperature and pressures of catalyst.
The process of carbonisation .
The bituminous coal on low heating losses moisture and gases that
are driven out . At about 270 % C , some H2S and olefine gases are evolved. At
about 350 % C active decomposition of the coal substance takes place with
increases and below 800 % C , gas is the main product.
The quality of the coke produced mainly depend on -- i. the type
of coal used ii. the temperature of carbonisation and iii. the rate of cardonisation.
These factors control the porosity , reactivity end the amount of volatile matter
retained in the coke .
(A) Low temperature carbonisation .
The low temperature carbonisation is carried out mainly for
the manufacture of domestic fuel. The coke produced is called low
temperature coke and contains about 5 to 15 % volatile matter.
This coke is easily ignited and it is a valuable , smokeless , domestic
coal and process used for carconisation.
Disadvantages :
i.It produces heavy air pollution.
ii.By this process , poor quality of the soft coke is produced.
iii.There is loss of liquid and gaseous by products .
(B) High temperature carbonization
The high temperature carbonization is used for the
production of pure , strong , hard and porous metallurgical coke. The yield of
the coke is 65 to 75 % and contain 1 to 3 % volatile matter. Its calorific value
is about 5000 - 6000 k. cals'/m3 which is lower than that produced in LTC.
Chemicals from coal
Chemicals from coal were initially and mostly obtained by
destructive distillation of coal , furnishing mainly the aromatic compounds
such as -- benzene , toluene , xylene , napthalence and methyl - naphthalenes.
When coal is thermally pyrolysed or distilled by heating in the absence of air ,
and amount of each product depends on the temperature used in pyrolysis and
CO2 , CO , H2S , NH3 , N2 . The products other than coke are known as coal
Coking of Coal
Coking is used mainly for processing of fuels such as coals . In
coking , coals are heated in the absence of air to a temperature to form
volatile products and solid reminder known as coke .
Chamber ovens.
i.It consists of narrow rectangular chambers made of silica bricks . These
chambers are tightly closed so that no air is admitted.
ii. The heating is carried out by burning gases like producer gas , blast
furnace gas or coke oven gas .
iii.The chambers are also fitted with charging doors having 3 and 4 openings
at the top and discharging doors at the bases .
iv.Each oven is separated from the neighboring one by a vertical flue in
which the fuel gas burns and the ovens get heated from both sides.
v.The hot gases leaving the fuels are allowed to pass through the chequer
brick - work in the next oven . The air and fuel gas pass through other
chamber which been already heated .
vi.When coal is heated in the coke oven , moisture is first expelled and at
about 300 - 450 % C the coal takes place .
vii.As about 500 % - 500 % C the coal passes through a plastic state and it is
and solidification to form semi coke.
viii..As about 500 % - 500 % C the coal passes through a plastic state and it is
and solidification to form semi coke.
ix.The process of carbonisation takes place in the coal charge starting from
the two side walls of the oven .
x.Each oven holds about 20 tonnes of coal charge and the time required
for carbonisation is about 12 to 20 hours .
xi.The final temperature goes around 1100 % C and the yield of coke is
about 70 % .
xii.After carbonisation the red hot coke is pushed into a coke car where the
coke comes in contact with spray of cooling water .
xiii.The excess water is evaporated and the coke is screened to different
sizes.
xiv.The gases from the coke ovens are converted into different hyproducts .
Byproducts of coal .
The gaseous mixture that leaves the coke ovens contains ammonia ,
sulphur , volatile hydrocarbons , tar etc . The important byproducts
like light oil , tar ammonium sulphate , finely divided sulphur , ammonium
