Chemical Energy Sources

ii.

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.,

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.

d) It should contain lesser amount of non-combustible 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 = xg Weight of water taken in the calorimeter = W g Water equivalent of calorimeter = wg

Initial temperature of water Final temperature of water Rise in temperature Let the gross calorific value Heat produced by the fuel Heat gained by water and apparatus

= t1 °C = t2 °C = (t2 – t1)°C =L = xL = ( 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 n- heptane (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 Hexane

+ H2 cyclo hexane + 3H2

Isomerization: (same mol. formula but different structural formula) The straight chain hydrocarbons are converted into branched hydrocarbons H3C- CH2-CH2-CH2-CH2-CH2-CH2-CH3 CH3 N –heptane

n-hexane

2, 2, dimethyl butane

H3C-CH-CH2-CH2-CH2| CH3 2 –methyl hexane

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 – (CH2)5 –CH3 n -heptane octane no. = zero

CH3 CH3 – C –CH2 –CH2 – CH3 CH3 (iso octane) 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 n- heptane) 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 cetane No.=zero

n-hexadecane 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 αmethyl 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 leadbromide 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.