Chemical Energy Sources
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KOMUNITAS BLOGGER UNIVERSITAS SRIWIJAYA
Chemical Energy Sources Introduction to energy: Energy is an important aspect of human
activity. A large number of energy sources such as animal waste, firewood, coal, oil etc. are
now available. As these sources are depleting in nature. After some years the availability of
fossil fuels may come to an end. Hence scientists and engineers have been trained to
develop new sources of energy alternative to fossil fuels. Solar energy and nuclear energy
are possible alternatives which can replace fossil fuels. Fuels: Definition: A substance which
is used to produce heat, light and electricity by combustion is called a fuel. Classification of
fuels: Chemical fuels are classified into two types whether they are in solid, liquid or gaseous
state. a) Primary fuels: The naturally occurring fuels are called primary fuels. b) Secondary
fuels: Artificially prepared fuels are called secondary fuels. State Primary Secondary Solid
Coal, wood, peat lignite, bituminous coal Charcoal, Coke etc., Liquid Petroleum Petrol,
diesel, kerosene, alcohol, LPG Gas Natural gas Bio gas, water gas (CO + H2), Producer gas
(CO+ N2), Coal gas Importance of Hydrocarbons as fuels: The petroleum is a complex
mixture of hydrocarbons. The hydrocarbons may vary from low molecular weight into high
molecular weight. Crude petroleum is a mixture of alkanes, alkenes, alkynes, cycloalkanes,
aromatic hydrocarbons along with a small percentage of heterocyclic compounds. Some of
hydrocarbons present in the liquid fuels are as follows: Hydrocarbons Examples Alkanes
Methane, ethane, propane, butane etc., Alkenes Ethane, propene, butane, etc., Alkynes
Acetylene, methyl acetylene Cycloalkanes Cyclo propane, cyclo butane etc., Aromatic
hydrocarbons Benzene, toluene etc. Heterocyclic compounds Pyridiene, thiopene, pyrol etc.
Calorific value of a fuel: Definition: The quality of a fuel is expressed in terms of calorific
value. Calorific value is defined as the total quantity of heat produced when unit mass or unit
volume of the fuel is burnt completely in the presence of excess of air or oxygen. Units of
calorific value: (a)Calorie: It is the amount of heat required to rise the temperature of 1 gm of
water through 1° C. In SI units, calorific value is expressed in Joule 1 calorie = 4.183 J
(b) British thermal unit: It is the amount of the heat required to rise the temperature of one
pound of water by 1° F. It is called British system unit. 1 BTU = 252 cal. (c) Centigrade
Heat Unit (CHU): It is the quantity of heat required to rise the temperature of 1 pound of
water 1° C. 1 K cal = 2.2. CHU Types of calorific value: Calorific value have been
classified into 2 types 1) Gross calorific value 2) Net calorific value 1. Gross calorific value
[higher calorific value, HCV or GCV]: Chemical fuels usually contain H2. During combustion
H2 present in the fuel is converted into steam during the determination of calorific value in
the bomb calorimeter. Steam is condensed to H2O in the bomb calorimeter and hence the
latent heat of steam gets included in the measured quantity of heat. Therefore Calorific value
will be a little higher than the normal value. Hence it is called Gross Calorific value. Gross
calorific value is defined as the quantity of heat produced when 1 gm of fuel is burnt
completely and the products of the combustion are cooled to room temperature. 2. Net
calorific value (Lower calorific value, NCV or LCV): During the actual use of fuel combustion
products are not condensed to room temperature. Water vapour etc. are allowed to escape
into the atmosphere along with other hot combustible gases. Therefore calorific value will be
a little less than the gross calorific value. Hence it is called net calorific value. Net calorific
value is defined as the quantity of heat produced when one gram of fuel is burnt completely
and the products are permitted to escape. Net calorific value = (Gross calorific value)
– (Latent heat of steam) Comparative survey (Advantages and disadvantages) of
solid, liquid and gaseous fuels: Solid Liquid Gas 1. Not very clean, because both smoke and
air are produces Only smoke is produced and no ash is left behind. Hence liquid fuels are
clean Neither smoke not ash is produced. Hence gaseous fuels are very clean.
2.Transportation is difficult and much labour is involved Transportation is easier and can be
easily transfer through pipes Can be distributed through pipelines from storage tanks. Hence
transportation is easier. 3. Large excess of air is required for combustion. Less air is
sufficient Less air is sufficient 4. Rate of combustion cannot be controlled and a lot of heat is
wasted during the process Rate of combustion can be controlled easily which gives economy
of the fuel. Rate of combustion can be controlled and combustion takes place more
efficiently. 5. They cannot be used in IC Engines They can be used in IC engines They can
be used in IC engines 6. Calorific value is least. Calorific value is higher Calorific value is
higher 7.Thermal efficiency is least and burn with clinker formation Thermal efficiency is
higher than solid fuels. Thermal efficiency is highest 8. Storage is easy and there will be no
risk of fire hazards Highly inflammable and volatile and there will be a risk of fire hazards.
Highly inflammable and chances of fire hazards is highest. 9. Comparatively cheaper Costlier
than solid and gaseous fuels Except natural gas and Bio gas, other gaseous fuels are costly
Characteristics of a good chemical fuel: A good chemical fuel must have the following
characteristics: a) It should have high calorific value. b) It should be easy to transport and
store c) It should have low moisture content. d) It should contain lesser amount of noncombustible gases like CO2 , N2 etc., e) It should not produce harmful gases like SO2, SO3,
H2S, PH3, oxides of nitrogen etc., f) A good chemical fuel should be readily available at
cheaper rate. Experimental determination of calorific value of the given solid fuel or liquid fuel
using Bomb calorimeter Principle: The Calorific Value of solid and liquid fuels can be
determined by burning a known weight of fuel in oxygen under pressure in Bomb calorimeter.
The heat produced and absorbed by a known weight of H2O. By measuring the rise in
temperature of H2O the calorific value of a fuel can be determined. Procedure: The bomb
calorimeter consists of a strong steel vessel fitted with a valve for pumping O2. Known
weight of the fuel is taken in the platinum crucible and it is placed in the steel vessel. Two
copper wires are the introduced into the steel vessel for ignition of the fuel. O2 is filled into
the steel vessel at a pressure of 25 – 30 atmospheres. The steel vessel is placed in
the copper calorimeter enclosed in insulating material to prevent the loss of heat due to
radiation. A known weight of water is placed in the calorimeter. Calorimeter is fitted with a
mechanical stirrer and a Beckmann thermometer. The initial temperature of H2O in the
calorimeter is noted. The fuel is ignited by passing electric current. Rapid combustion takes
place. H2O in the calorimeter is stirred continuously using mechanical stirrer. The maximum
temperature attained by the water is noted. Calculation: Weight of the fuel taken = x g Weight
of water taken in the calorimeter = W g Water equivalent of calorimeter = w g Initial
temperature of water = t1 °C Final temperature of water = t2 °C Rise in
temperature = (t2 – t1)°C Let the gross calorific value = L Heat produced by the
fuel = xL Heat gained by water and apparatus = ( W + w) (t2 – t1) Heat lost by the
fuel= heat gained by the H2O xL = (W + w) (t2 – t1) L = (W + w) (t2 – t1)
cal/gms------------------------------------(1) x L = (W + w) (t2 – t1) 4.2 J/kg ---------------------------------(2) x x10 -3 Net Calorific Value = L – latent heat of steam Net Calorific Value
= L – 0.09 H x 587 ------------------------------- (3) Where H = % of hydrogen within the
fuel. Determination of water equivalent (w) of calorimeter: A known weight of the fuel of
known calorific value is burnt in the Bomb calorimeter. The rise in temperature of water is
determined. The water equivalent of calorimeter can be calculated from the equation (1).
Experimental determination of calorific value of a gaseous fuel using Boy’s
calorimeter Principle: A known volume of the fuel is burnt in Boy’s calorimeter at STP
in the presence of air. The heat produced is absorbed by circulating water. By measuring the
rise in temperature of the circulating water, the calorific value of the given gaseous fuel can
be calculated. Procedure: A known volume of the gaseous fuel whose calorific value is to be
determined is introduced into the burner under pressure at uniform rate in a known interval of
time. The combustion products are passed through spiral tube over which H2O flows at a
constant rate. During the process flowing H2O absorbs all the heat produced in the burner.
The steam formed during the combustion is condensed to liquid. It is collected and weighed.
The weight of H2O used for cooling during the same interval of time is also noted.
Calculations: The following readings are noted when steady conditions are established.
Volume of gas burnt @ STP in a particular interval of time = V cm3 Mass of water used for
cooling in the same interval of time = W kg Temperature of incoming water = t1°C
Temperature of outgoing water = t2°C Mass of water condensed in the same interval of
time = m Let gross cal value = L Heat produced by the fuel = heat grained by the water VL =
W(t2-t1) L= W(t2 – t1) cal/m3 V Mass of water condensed/m3 of the fuel = m/v kg
Latent heat of steam/m3 of the fuel = m/v x 587 Net Calorific Value = L – LHS Net Cv
= L – m/v x 587 cal /cm3 Fractionisation of crude petroleum: Refining of petroleum:
The crude petroleum consists of a mixture of hydrocarbons boiling between a wide ranges of
temperature. Hence crude petroleum is not suitable for technical purposes. The process of
fractional distillation of petroleum into different useful fraction and the removal of undesirable
impurities is called Refining of petroleum. The crude petroleum is brought to the surface of
the earth by bore wells and by means of pumping. The crude petroleum is washed with the
dilute H2SO4 to remove basic impurities and then with dil NaOH to remove acidic impurities.
It is then heated with steam coils. The emulsion breaks into two types. The upper layer is
taken out and subjected to fractional distillation with tall fractionating column. The following
fractions are collected. Fractions Length of carbon BP range Uses Gas C1 – C4 <
40°C Domestic gas fuel Gasoline (petrol) C5 –C10 40 – 120 °C Fuel
for IC engine Kerosene C10 – C15 160 -250 °C Domestic fuel Diesel C16
– C20 250 – 300 °C Fuel for diesel engines Heavy oils C20- C25 300 350 °C To produce petrol by cracking Pitch ( asphalt) > C25 Residue For surfacing
roads Several useful products are obtained by processing crude petroleum. They include
petroleum ether Benzene, lubricating oil, paraffin wax, petroleum cakes etc., Cracking of
petroleum: The process of cracking up of higher hydrocarbons into more volatile lower
hydrocarbons is known as cracking of petroleum. There are two types of cracking a) Thermal
cracking b) Catalytic cracking a) Thermal cracking: The heavy oil is subjected to high
temperature and pressure. The bigger hydrocarbons are broken down to given smaller
molecules of alkanes, alkenes, alkynes, hydrogen etc. The thermal cracking may be carried
out either in liquid phase or in vapour phase. In liquid phase thermal cracking, the heavy oil is
heated to 500 – 600°C under a pressure of about 100 atmospheres. After
cracking, the products are separated by fractional distillation. Ex: C8H18 C5 H12 + C3H5
Ocatane pentane propene C10 H22 C5 H12 + C5 H10 Decane pentane pentne BP =
174° C BP= 34° C In vapour phase thermal cracking, the heavy oil is vapourised
and then heated to about 600 to 650°C under pressure of 10 to 50 atmospheres.
Vapour phase cracking method require less time than liquid phase cracking. Petrol obtained
by vapour phase cracking has better antiknock properties. Catalytic cracking: In this process
the vapours of high boiling fraction are heated in the presence of catalysts such as Silica,
Alumina, Thoria (ThO2), Zirconium Oxide (ZrO), MnSO4 etc. Types of catalytic cracking a)
Fixed bed catalytic cracking b) Fluidized (moving) bed catalytic cracking Fluidized (moving)
bed catalytic cracking: In this process, the solid catalyst is finely powdered so that it behaves
as a fluid which can be circulated along with the vapors of heavy oil. The vapors of heavy oil
are mixed with finely divided catalyst and this mixture is passed into the reactor maintained
at 500°C. Cracking of heavy oil occurs. A centrifugal separator called cyclone is
provided in the reactor which allows only cracked vapours to pass into the fractionating
column. The catalyst gets separated in the reactor, is collected, regenerated and reused. The
different fractions like gasoline kerosene, diesel etc. are collected in the fractionating column.
Advantages of catalytic cracking: a) Catalytic cracking takes place at lower temperature and
pressure. a) The yield of petrol is high b) The process can be controlled easily and the
desired products can be obtained. c) The products contain higher amount of aromatic
hydrocarbons. Hence possess better antiknock characteristics. d) Catalysts are specific in
their action and therefore they permit cracking of high boiling hydrocarbon. Reformation of
petroleum: Gasoline used in automobiles mainly consists of a mixture of hydrocarbons;
namely alkanes, cycloalkanes and aromatics. Branched chain hydrocarbons and aromatic
hydrocarbons possess good combustion characteristics. Whereas straight chain
hydrocarbons have got poor combustion characteristics because former have higher octane
number and later compounds have lower octane number. Octane number of gasoline is the
percentage of Isooctane (Octane Number = 100) (v/v) in a mixture of isooctane and nheptane (ON= 0). Higher the octane number least is its tendency for knocking (rattling sound
in IC engines). IC engines require gasoline of octane number above 90 (in USA) and 75 -85
(in India). But gasoline obtained from primary distillation of crude oil contains mainly straight
chain hydrocarbons having octane number lessthan 60. Octane number of this gasoline can
be improved by structural modification i.e., by converting straight chain hydrocarbons into
cyclic, branched chain and aromatic hydrocarbons. This is done by catalytic reforming.
Reforming is a process of bringing about structural modifications, such as conversion of
straight chain hydrocarbons into branched, cyclic and aromatic in order to increase the
octane number. Reforming process is done by thermal reforming or by catalytic reforming.
Catalytic reforming: Catalytic reforming is a process of upgrading gasoline (increasing an
octane number) in presence of a catalyst Reforming conditions: Feed: straight run gasoline
Catalyst: pt supported on Al2O3-SiO2 base Temperature: 470 – 525 °C
Pressure: 15 – 50 atmospheres The main reforming reactions are a) Dehydrogenation
b) Dehydro cyclisation c) Isomerization d) Hydro cracking a) Dehydrogenation: Ex:
Cycloalkanes undergo dehydrogenation reactions + 3H2 Cycloalkanes C6H12 C6H6 (ON
> 100) b)Dehydrocyclisation: Ex: a straight chain hydrocarbon undergoes cyclisation by
dehydrogenation to produce aromatic hydrocarbons. H3C –CH2-CH2-CH2-CH2-CH3
+ H2 Hexane cyclo hexane + 3H2 Isomerization: (same mol. formula but different structural
formula) The straight chain hydrocarbons are converted into branched hydrocarbons H3CCH2-CH2-CH2-CH2-CH2-CH2-CH3 H3C-CH-CH2-CH2-CH2-CH3 N –heptane | CH3
2 –methyl hexane n-hexane 2, 2, dimethyl butane Hydro cracking :( cracking in
presence of hydrogen as a catalyst) n-heptane + H2 propane + n-butane Knocking of IC
engines:- In IC engine, mixture of gasoline and air is used as a fuel. This mixture of petrol
vapours and air is compressed and ignited by a spark in the cylinder which causes the
oxidation of hydrocarbon molecules. The ratio of original volume of the fuel air mixture (V1)
to that volume at the end of compression (V2) is called compression ration. Compression
ratio = V1/ V2 The efficiency of IC engine depends upon the compression ratio. Higher the
compression ratio higher is the efficiency. The high compression ratio depends on the quality
of the fuel. Beyond certain compression ratio fuel is converted into unstable hydrocarbon
peroxide and decomposes rapidly to give a number of gaseous products. This gives rise to
high pressure waves which knock engine walls producing sound. This is called knocking of
IC engines. The following are the disadvantages of knocking: a) It produces undesirable
sound b) It increases the fuel consumption c) It causes mechanical damage to engine by
overheating the engine parts. d) It results in decreases power output and driving becomes
unpleasant. Octane number: The quality of the petrol is determined by an arbitrary scale
called octane number. This was proposed by the Edger in 1972. Octane number of gasoline
is defined with reference to n- heptane and isooctane. The straight chain hydrocarbon n-
heptane which has poor burning characteristics and knocks the engine badly is arbitrarily
given an octane number zero. But the branch chain hydrocarbon iso-octane which has
excellent burning characteristics and very little tendency to knock the engine is given an
octane number 100. CH3 CH3 – (CH2)5 –CH3 CH3 – C –CH2
–CH2 – CH3 n -heptane CH3 (iso octane) octane no. = zero octane no. 100 [
2,2,4, - tri methyl petane) The octane number of petrol is the percentage by volume of iso
octane in the mixture of isooctane and n-heptane blend which has the same knocking
characteristics of the gasoline sample under investigation. Ex: 80 octane number fuel is one
which has the same combustion characteristics as 80: 20 (Mixture of isooctane and nheptane) The different gasoline samples rated by this octane number. Higher the octane
number least is the tendency of knocking and better is the quality of fuel. Automobiles petrol
has octane number ranging from 75 to 95. It has been found that knocking tendency is
largely related to chemical structure of the fuels. The decreasing tendency of fuels to
knocking is as follows: Alkane > [Branched chain] > cycloalkanes Alkenes>
Aromatics Aviation fuels (Aircraft fuels) Aviation petrols have greater knock resistance than
isooctane. Generally aviation fuel has an octane number greater than 100. in such cases,
octane number and computed by using the following relation. Octane Number=(power
number-100)/3 + 100 Where power number=power extracted by the engine. Cetane number:
The knocking characteristics of diesel oil are expressed in terms of centane number. The
suitability of diesel fuel is determined by its cetane number. Hexadecane is a saturated
hydrocarbon which has the least tendency of knocking the diesel engine as given arbitrarily
cetane number 100. α-methyl naphthalene which knocks the diesel engine badly has
given cetane number zero α-methyl naphthalene n-hexadecane cetane No.=zero
cetane No.=100 The cetane number of diesel oil is determined by comparing the burning
characteristics of the blend of specific composition of hexadecane and α-methyl
naphthalene. The ignition quality among hydrocarbons is as follows:
Alkanes>Napthenes>Alkenes>Branched chain alkanes>Aromatics. Prevention of
Knocking: Knocking can be prevented or reduced by using Ant knocking agents and
Unleaded petrol. Anti knocking Agents: Knocking can be reduced by adding some specific
compounds to the fuels which are called ant knocking agents. Tetraethyl lead is commonly
used as ant knocking agent. During the combustion TEL is converted into cloud of finely
divided lead dioxide particles in the cylinder. These particles react with hydrocarbon peroxide
molecules thereby controls the chain reaction. Thus knocking is prevented. The deposit of
lead dioxide is harmful to the engine. In order to eliminate this, a small amount of ethylene
dibromide is added to the petrol. In presence of this lead is removed from the cylinder as
volatile lead-bromide which escapes out through the exhaust pipe. The petrol containing lead
in the from TEL is called leaded petrol Unleaded petrol: Knocking can also be prevented by
mixing strain chain hydrocarbons such as isopentane, ethyl benzene. Tertiary butyl methyl
ether etc. These compounds reduce the formation of peroxy compounds and hence knocking
will be reduced. Petrol where knocking tendency can be reduced without the addition of TEL
is called unleaded petrol. One of the major advantages of using unleaded petrol is that it
allows the use of catalytic converted attached to the exhaust pipe in automobiles. The
catalytic converted is attached to the exhaust containing Rhodium catalyst which converts
the toxic gases like carbon monoxide into carbon dioxide and oxides of nitrogen into nitrogen
which are harmless. It also oxidizes unburnt hydrocarbon into carbon dioxide and water.
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