Contents
Introduction
2
Colloidal Fuel
3
Calorific Value
3
Calorie
4
Kilocalorie
4
British thermal unit (BTU)
4
Centigrade Heat unit (CHU)
4
High Gross Calorific Value (HCV)
5
Lower or Net Calorific Value (LCV)
5
Bomb Calorimeter
5
Analysis of Coal
8
Proximate Analysis
9
Ultimate Analysis
11
Octane Number
14
Centane Number
15
Introduction
Fuel is a combustible substance, containing carbon as main
constituent, which on proper burning gives large amount of
heat, which can be used economically for domestic and
industrial purpose. For example wood, charcoal, kerosene,
petrol, natural gas etc.
During the process of combustion of a fuel (like coal), the
atoms of carbon, hydrogen etc. combine with oxygen with
simultaneous liberation of heat at a rapid rate. This energy is
liberated due to the "rearrangement of valence electrons" in
these atoms resulting in the formation of new compounds (like
CH2, H2O etc.)
Fuel
+
O2
more heat energy content
Products
+
lesser heat energy content
The products formed have less energy in them.
Classification of Fuels
Fossil fuels are classified according to their:
i) Occurrence and preparation
ii) State of aggregation
Heat
Chemical Fuels
Primary or
Natural
Secondary or
Derived
Solid
Liquid
Gaseous
Solid
Liquid
Gaseous
wood, peat,
lignite, coal
crude oil
natural gas
coke, charcoal,
petroleum, coal
tar, kerosene,
diesel, petrol
coal gas, oil gas,
bio gas
Colloidal Fuel
It is a suspension of finely powdered coal in fuel oil in the ratio
of 3:2 by weight. Usually some stabilizing agent is added during
the preparation of colloidal fuel in order to avoid the separation
of coal from the liquid fuel. Colloidal fuel possesses high
calorific value and is easy to handle than compared to powered
coal. Colloidal fuel is used in cement manufacture, brick firing,
ceramic firing, metal processing etc.
Calorific Value
Calorific value of a fuel is defined as the total quantity of heat
liberated when a unit mass of fuel is burnt completely.
Units
i) Calorie is the amount of heat required to raise the
temperature of one gram of water through one degree
centigrade.
ii) Kilocalorie is equal to 1000 calories. It is the quantity of
heat required to raise the heat of one kilogram of water through
one degree centigrade.
1Kcal = 1000 cal
iii) British thermal unit (BTU) is defined as the quantity of
heat required to raise the temperature of one pound of water
through one degree Fahrenheit. This is the English System
Unit.
1 B.T.U = 252 cal = 0.252 kcal
1 Kcal = 3.968 B.T.U
iv) Centigrade Heat unit (CHU) is defined as the quantity of
heat required to raise the temperature of 1 pound of water
through 1 degree centigrade.
1 Kcal = 3.968 B.T.U = 2.2 C.H.U
v) High Gross Calorific Value (HCV) is the total amount of
heat produced when unit mass of the fuel has been burnt
completely and the products of combustion have been cooled
to room temperature. (i.e. 15oC or 60oF)
vi) Lower or Net Calorific Value (LCV) is the net heat
produced when the unit mass of the fuel is burnt completely
and the products are permitted to escape.
∴ LCV = HCV - latent heat of water vapour formed
= HCV - mass of H x 9 x latent heat of steam
because, 1 part by mass of hydrogen produce 9 parts by mass
of water. The latent heat of steam is 587 Kcal /Kg.
Note: The calorific value is expressed in calorie/ gram or
Kcal/Kg or B.T.U in case of solid or liquid fuels. In case of
gaseous fuels, the units are Kcal/m3 or B.T.U/ft3.
Bomb Calorimeter
It is used to find the calorific value of solid and liquid fuels.
Working:
A known mass of the fuel is taken in clean crucible. The
crucible is then supported over the ring. A fine magnesium wire,
touching the fuel sample is then stretched across the
electrodes. The bomb lid is tightly screwed and the bomb filled
with oxygen to 25 atm. The bomb is lowered into a copper
calorimeter containing a known mass of water. The stirrer is
worked and the initial temperature of water is noted. The
electrodes are then connected to a 6 volt battery and the circuit
completes. The sample burns and heat is liberated. Uniform
stirring of water is continued and the maximum temperature
attained is recorded.
Note: The water equivalent of the calorimeter is determined by
burning a fuel of known calorific value and using the equation:
HCV of fuel (L) = (W+w) (t2-t1)
cal/g
x
where,
x = mass of fuel sample in g
W = mass of water in calorimeter
w = water equivalent in g of calorimeter, stirrer, thermometer
and bomb
t1 = initial temperature of water in calorie meter
t2 = final temperature of water in calorie meter
L = higher calorific value in fuel in cal/g
The fuels used for this purpose are benzoic acid (HCV = 6325
Kcal/Kg) and naphthalene (HCV = 9688 Kcal/Kg)
To calculate theoretical value of fuel, Dulong's formula is
applied.
HCV =
1
100
8080 C + 34500 H – O + 2240 S
8
HCV of Fuel Constituents:
Hydrogen - 34500
Carbon - 8080
Sulphur - 2240
Total mass of H2 in fuel - fixed H2
Total mass of H2 in fuel -1/8 mass of O2 in fuel
(∴ 8 parts of O2 combine with 1 part of H2 to form H2O.)
Analysis of Coal
In order to assess the quality of coal, 2 types of analysis are
made:
i) Proximate Analysis
ii) Ultimate Analysis
Proximate Analysis
Proximate Analysis of coal involves the following
determination:
(a) Moisture: For this about one gram of finely powdered air
dried coal sample is weighed in a crucible. The crucible is
placed inside an electric hot air oven, maintained at 105-110oC.
The crucible is allowed to remain in the oven for 1 hour and
then taken out and cooled in desiccators and weighed. Loss in
weight is reported as moisture in terms of moisture.
% of moisture = Loss of weight x 100
Wt. of coal taken
(b) Volatile Matter: It is also calculated in terms of percentage
% of volatile = Wt. loss due to removal of volatile matter x 100
matter
Wt. of coal taken
(c) Ash:
% of Ash = Wt. of ash left x 100
Wt. of coal taken
(d) Fixed Carbon:
% of fixed carbon= 100 - % of (moisture + volatile matter + ash)
Significance of Proximate Analysis
i) Moisture in coal evaporates during the burning of coal and it
takes some of the liberated heat in the form of latent heat
evaporation. Therefore, moisture lowers the effective calorific
value of coal. It quenches the fire in the furnace. Thus, lesser
the moisture content, better the quality of coal as a fuel.
ii) Volatile matter: A high volatile matter content means that a
high proportion of fuel will distil over as gas or vapour, so large
proportion of coal which escape remain unburnt.
A high volatile matter containing coal burns with a long flame
and has a lower calorific value. Hence, lesser the volatile
matter, the better the rank of coal.
iii) Ash: It is a useless, non-combustible matter which reduces
the calorific value of coal. Ash also causes trouble during firing
by forming lumps which block the interspaces of the gate on
which the coal is being burnt. Hence, lower the ash content,
better the quality of coal.
Presence of ash also causes early wear of furnace walls,
burning of apparatus and feeding mechanism.
iv) Fixed carbon: Higher the percentage of fixed carbon,
greater is its calorific value and better is the quality of coal.
Ultimate Analysis
Ultimate Analysis of coal involves the following
determination:
i) Carbon and Hydrogen:
Combustion of carbon
(12) C + O2
CO2 (44g)
Combustion of hydrogen
(2) H2 + ½ O2
H2O (18g)
The gaseous products of combustion are absorbed in KOH and
CaCl2 of known weights. The increase in weight of these are
then determined.
KOH + CO2
K2CO3 + H2O
CaCl2 + 7H2O
CaCl2.7H2O
% of C = Increase in wt. of KOH tube x 12
x 100
Wt. of coal sample taken x 44
% of H = Increase in wt. of CaCl2 tube x 2
x 100
Wt. of coal sample taken x 18
ii) Nitrogen:
To determine the % of N2, coal is heated with conc. H2SO4
along with K2SO4 (catalyst). From the volume of acid used by
ammonia liberated, the % of N2 in coal can be calculated as,
% of N2 = Volume of acid used x Normality x 14
Wt. of coal sample taken
x 100
iii) Sulphur:
It is determined from the washing obtained from the known
mass of coal, used in a bomb calorimeter for determination of
calorific value. During this, sulphur is converted into sulphate.
The washings are treated with BaCl2, when BaSO4 is
precipitated.
% of S = Wt. of BaSO4 obtained x 32 x 100
Wt. of coal
iv) Ash:
Ash determination is carried out as in proximate analysis.
v) Oxygen:
It is obtained by difference:
% of O = 100 - % of (C + H + S + N + ash)
Significance of Ultimate Analysis
i) Carbon and Hydrogen: Greater the percentage of carbon
and hydrogen, better is the coal in quality and calorific value.
Higher percentage of carbon in coal reduces the size of
combustion chamber required. The amount of carbon, the
major combustible constituent of coal depends on the type of
coal.
ii) Nitrogen: It has no calorific value and hence its presence in
coal is undesirable. Thus, a good quality coal should have little
N2 content.
Octane Number
Octane number is a standard measure of the performance of a
motor or aviation fuel.
The higher the octane number, the more compression the fuel
can withstand before detonating. In broad terms, fuels with a
higher octane rating are used in high-compression engines that
generally have higher performance. In contrast, fuels with low
octane numbers (but high cetane numbers) are ideal for diesel
engines. Use of gasolines with low octane numbers may lead to
the problem of engine knocking.
In Idia, two variants, "Speed 93" and "Speed 97", were
launched, with RON(Road Octane No.) values of 93 and 97.
India's economy-class vehicles usually have compression ratios
under 10:1, thus enabling them to use lower-octane petrol
without engine knocking.
Centane Number
Cetane number or CN is a measurement of the combustion
quality of diesel fuel during compression ignition. It is a
significant expression of the quality of a diesel fuel. A number
of other measurements determine overall diesel fuel quality these other measures of diesel fuel quality include density,
lubricity, cold-flow properties, and sulfur content.
It is a measure of a fuel's ignition delay; the time period
between the start of injection and the first identifiable pressure
increase during combustion of the fuel. In a particular diesel
engine, higher cetane fuels will have shorter ignition delay
periods than lower cetane fuels.
Generally, diesel engines operate well with a CN from 40 to 55.