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Chemistry CHE 12 - Visvesvaraya Technological University e

By Beatrice Mason,2014-04-16 13:24
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Specific heat of water: 4.187 KJKg-1C-1 and rise in temperature: 4.8oC Combustion of fossil fuels ? coal and crude oil contain up to 3% sulphur. (ii) a reducer (causes the reduction of metallic ions to metal),

    Notes by Pushpa Iyengar, East point college of Engineering, Bangalore

    FUELS

    The discussion relates to fossil or chemical fuels

    Definition

    Fuel is a carbonaceous combustible substance, which on combustion liberates a large

    amount of energy in the form of heat.

Classification

    On the basis of occurrence, fuels are classified as primary and secondary fuels

    ? Primary fuels occur in nature and are used without processing.

    ? Secondary fuels are obtained by chemical processing of primary fuels.

    On the basis of physical state, fuels are classified as solid, liquid and gaseous fuels

     Fuels

     Secondary Primary

     Solid Liquid Gaseous Solid Liquid Gaseous

    E.g. Coal Crude oil Natural gas Charcoal Petrol Coal gas

     Wood (Petroleum) Coke Diesel Water gas

Calorific Value

    The quality of a fuel is determined by the amount of energy released per unit mass or

    volume referred to as calorific value.

Definition

    ? Calorific value of a fuel is the amount of heat liberated when a unit mass or a unit

    volume of the fuel is burnt completely in air or oxygen.

    ? Fuels generally contain hydrogen in addition to carbon. During combustion, the

    hydrogen is converted to steam.

    ? In the determination of calorific value of the fuel if the products of combustion

    are cooled to ambient temperature (room temperature), the latent heat of steam is

    also included. This is referred to as gross calorific value (GCV) or higher calorific

    value.

    ? In practice, the products of combustion are allowed to escape and the amount of

    heat realized is lesser than the GCV (since the latent heat of vaporization is not

    released). This is net calorific value (NCV) or lower calorific value.

     GCV = NCV + latent heat of steam

    ? Gross Calorific value is the amount of heat liberated when a unit mass or a unit

    volume of the fuel is burnt completely in air or oxygen and the products of

    combustion are cooled to ambient temperature.

    ? Net Calorific value is the amount of heat liberated when a unit mass or a unit

    volume of the fuel is burnt completely in air or oxygen and the products of

    combustion are allowed to escape.

     Oxygen Determination of Calorific Value of a Solid Fuel - Bomb Calorimeter

     B Stirrer

    Wires for ignition Thermometer

     Lid

     A

     Sample

    ? Construction

    ? The bomb calorimeter (shown in the fig.) consists of an outer cylindrical steel

    vessel (bomb) with an airtight screw and an inlet for oxygen.

    ? The bomb has a platinum crucible with a loop of wire. The ends of the wire

    project out and can be connected to a source of electric current.

    ? The bomb is immersed in a rectangular vessel (calorimeter) containing water,

    which is continuously stirred.

    ? A Beckmann thermometer is introduced into the calorimeter.

Working

    ? A known mass of the fuel is made into a pellet and taken in the crucible.

    ? Oxygen is passed through the bomb.

    ? A known mass of water is taken in the calorimeter and is closed with the lid.

    ? The initial temperature of water is noted.

    ? The ends of the wire are connected to an electric source so as to ignite the fuel.

    ? The heat released is absorbed by water. The temperature of water rises.

    ? The final temperature is noted.

Calculation

    Let

    m = mass of fuel

    W = mass of water

    w = water equivalent of calorimeter = initial temperature of water 1

    tt = final temperature of water 2

    s = specific heat of water

    GCV ( solid fuel) = (W+w) ? (t-t) ? s 21

     m

If the fuel contains x% hydrogen, NCV of the fuel is calculated as follows

    2 atoms of hydrogen produce one molecule of water

    2g of hydrogen produce 18 g of water

    x g of hydrogen produce 9 g of water

    x % hydrogen ? 9 ? x g of water = 0.09 ? x g of water

     100

    NCV = GCV - latent heat of steam formed

     = GCV - 0.09 ? x ? latent heat of steam

     Latent heat of steam = 2454 kJ kg-1

    -1 1 calorie = 4.187 kJ kg The calorific value of a liquid fuel can be determined using bomb calorimeter.

Determination of Calorific Value of a Gaseous Fuel - Boy’s Calorimeter

     TT2 1 Gas Exit

     Water outlet Water inlet

     Tubes for circulating Condenser for water cooling water vapour formed Combustion during the chamber of the combustion calorimeter Gas meter Condenser outlet Gas inlet

     Burner

Construction

    ? Boy‘s calorimeter (shown in fig.) consists of a combustion chamber surrounded

    and T attached. by water tube with two thermometers T1 2

    ? There is a burner in the chamber, which is connected to a gas tube.

Working

    ? A known volume of water is passed through the tubes.

    ? The initial temperature is noted when the two thermometers show the same

    constant temperature.

    ? A known volume of the gas (measured using a meter) is passed through the tube

    and burnt in the combustion chamber.

    ? The heat liberated is absorbed by the water in the tubes.

    ? The final temperature of water is noted.

    ? The gaseous products are cooled and condensed into a measuring jar.

Calculation:

    Let

    V = volume of gas burnt

    W = mass of water

    t

     = initial temperature of water 1

    t = final temperature of water 2

s = specific heat of water

    v = volume of water collected in the measuring jar

    GCV( gaseous fuel) = W ? s ? (t- t) 21

     V

    NCV ( gaseous fuel) = GCV - latent heat of steam formed

     = GCV - latent heat of steam ? volume of water collected.

Formulae for Solving Numerical Problems:

    ? GCV (solid fuel) = (W+w) ? (t-t) ? s 21

     m

    ? NCV (solid fuel) = GCV - latent heat

     = G.C.V. - (0.09 ? % of H) ? latent heat

    ? GCV( gaseous fuel) = W ? s ? (t- t) 21

     V

    ? NCV ( gaseous fuel) = GCV latent heat

     = G.C.V. amount of water collected ? latent heat

     V

     = G.C.V. v ? latent heat

     V

     (1 cm3 of water ? 1 g of water)

Numerical Problems

Problem 1: Calculate the gross calorific value and net calorific value of a sample of coal

    0. 5g of which when burnt in a bomb calorimeter, raised the temperature of 1000g of

    water from 293K to 301.6K. The water equivalent of calorimeter is 350 g. The specific

    heat of water is 4.187 kJ kg-1-1, latent heat of steam is 2457.2kJkg. the coal sample

    contains 93% carbon, 5% hydrogen and 2% ash.

    m = mass of the fuel = 0.5 g

    W = mass of water taken = 1000 g

    w = water equivalent of calorimeter = 350 g

    t= initial temperature of water = 293 K 1

    t = final temperature of water = 296.4 K 2-1-1s = specific heat of water = 4.187 kJ kgK

     GCV (solid fuel) = (W+w) ? (t-t) ? s 21

     m

    -1-1 = (1000 +350) g ? (296.4 -293)K ? 4.187 kJ kgK

     0.5g

    -1-1K

    0. 5g -1 = 3 8437 kJ kg

     = 1350 g ? 3.4 K ? 4.187 kJ kgNCV (solid fuel) = GCV - latent heat

     = G.C.V. - (0.09 ? % of H) ? latent heat

    -1-1 = 38437 kJ kg - (0.09 ? 5) ? 1105.7 kJ kg -1-1 = 38437 kJ kg 1106 kJ kg -1 = 37331 kJ kg

Problem 2: Calculate the gross calorific value and net calorific value of a gaseous fuel,

    30.012m of which when burnt raised the temperature of 3.5kg of water by 8.2K. Specific -1-1. -1heat of water is 4.2 kJ kgKLatent heat of steam is 2.45 kJ kg. The volume of water 3-1collected is 6.5cm . Latent heat of steam is 2457.2kJ kg 3 V = volume of the gas burnt = 0.015 m

    W = mass of water = 3.5 kg

    t- t= rise in temperature = 15.6 K 21 -1-1 s = specific heat of water = 4.2kJ kgK3v = volume of water collected = 6.5 cm

     GCV( gaseous fuel) = W ? s ? (t- t) 21

     V

    -1-1 = 3.5 kg ? 4.2 kJkgK ? 15.6 K 3 0.012m

     -3 = 11073 kJm

     NCV( gaseous fuel) = GCV latent heat

     = G.C.V. - amount of water collected ? latent heat

     V

    -3-3-1 = 11073 kJm 6.5 ? 10 kg ? 2457.2kJkg

     0.012

    3 (1 cm of water ? 1 g of water)

    -3-3-1 = 11073 kJm 6.5 ? 10 kg ? 2457.2kJkg

     0.015 -3-3 = 11073 kJm 1065 kJm-3 = 10008 kJm

    Cracking of Petroleum

    ? Heavy oil is a major fraction of petroleum refining. It is converted to petrol by

    cracking.

     Definition:

    ? Cracking is the breaking down of high boiling high molecular mass petroleum

    fractions

    ? ( heavy oil) into smaller fragments.

Fluidized Catalytic Cracking

    ? Heavy oil is cracked using zeolite (Y type) catalyst with a rare earth oxide.

    ? Heavy oil is heated to about 580K in a preheater and passed through a riser To fractionating column(shown in fig.) into a reactor. column

     Flue Stripper gases

     Reactor Regenerator

     Steam

     Riser column

     Air

     Steam / oil

    ? The reactor contains finely powdered catalyst maintained at about 970K.

    0? The heavy oil undergoes cracking. K.

    ? The temperature falls to about 820? The cracked product is fractionated to give petrol.

Regeneration of Catalyst

    ? After some time, the catalyst gets deactivated due to the deposition of carbon and

    oil on its surface.

    ? Steam is passed through the riser column.

    ? The deactivated catalyst is led into a regenerator through which air is passed.

    ? Air oxidizes C to CO and steam removes the oil. 2

Reforming of Petrol

    ? Reforming is a process carried out to improve the octane number of petrol by

    bringing about changes in the structure of hydrocarbons.

    ? The changes in structure could be isomerization, cyclization or aromatization..

    ? Isomerization straight chain hydrocarbons are converted to branched

    hydrocarbons

     CH

     - CH - CH - CH - CH- CH - CH? CH - CH - CH - CH - CH- CH 32222 23 3222 3

     n- heptane

     CH 3

     methyl hexane

    ? Cyclization straight chain hydrocarbons are converted to cyclic compounds

     CH - CH - CH - CH - CH- CH - CH? - CH 32222 23 3

     n- heptane methyl cyclohexane

    ? Aromatization cyclic compounds are dehydrogenated.

     - CH - CH 3 3

     methyl cyclohexane toluene

    ? Reforming is carried out by passing the petrol through Pt supported on alumina at

    0-2about 500C and 50 kg cm pressure.

Knocking in IC Engines

    ? The power output and efficiency of an IC engine depends on the Compression

    ratio which is the ratio of the volume of the cylinder at the end of the suction

    stroke to the volume of the cylinder at the end of the compression stroke.

     Volume of cylinder at end of suction stroke

    Compression ratio =

     Volume of cylinder at end of compression stroke

    ? Under ideal conditions, in an IC engine the petrol-air mixture drawn into the

    cylinder of the engine undergoes compression and then ignited.

    ? The hydrocarbons in petrol undergo complete combustion and the flame

    propagates smoothly.

    ? Sometimes, due to deposits of carbon on the walls of the cylinder the

    hydrocarbons in petrol form peroxy compounds.

    ? The accumulated peroxides decompose suddenly and burst into flames producing

    shock waves.

    ? The shock wave hits the walls of the engine and the piston with a rattling sound.

    ? This is knocking.

    ? The reactions that take place in an IC engine are given below (taking ethane as an

    example for the hydrocarbon present in petrol):

     Under ideal conditions

    C

    H + 7/2 O ? 2 CO + 3HO 26222

H + O ? CH O-O- CH26233

     (Dimethyl peroxide) Under knocking conditions

     CH O-O- CH ? CHCHO + HO 3332C

    CHCHO + 3/2 O ? HCHO + CO + HO 3222

    HCHO + O ? HO + CO 222

? Note that the overall reaction is the same under both the conditions. One molecule

    of ethane reacts with 7/2 molecules of oxygen forming carbon dioxide and water

    with the release of energy.

    ? Under ideal conditions, the energy is released at a uniform rate.

    ? Under knocking conditions, the energy is released slowly at first followed by a lag

    (formation of peroxides) and finally the energy is released at a very fast rate

    (decomposition of peroxides).

     Ill effects of knocking

    1. Decreases life of engine

    2. Causes Piston wrap

    3. Consumption of fuel is more

     Octane Number

    ? The resistance to knocking offered by petrols is expressed in terms of an arbitrary

    scale called octane number

? Octane number is the percentage by volume of isooctane present in a mixture of

    isooctane and n heptane which has the same knocking characteristic as the

    petrol under test.

? The octane value of isooctane is arbitrarily taken as 100 and that of n heptane as

    zero.

    ? Different standard mixtures ( 90:10; 80:20, 75:25 etc) of isooctane and nheptane

    are prepared and the compression ratio of each of these is determined under

    standard conditions.

    ? The compression ratio of the fuel under test is determined under the same

    conditions.

    ? Suppose the compression ratio of the fuel is same as that of 80 :20 mixture, the

    octane number of the fuel is 80.

     Cetane Number:

    ? The resistance to knocking offered by diesels is expressed in terms of an arbitrary

    scale called cetane number

? It is the percentage by volume of cetane present in a mixture of cetane and ?-

    methyl naphthalene which has the same knocking characteristic as the diesel

    under test.

Prevention of Knocking

? Addition of lead tetraethyl (TEL) to Petrol:

    Lead tetraethyl decomposes the peroxides formed and prevents knocking.

    In the process, lead gets deposited on the inner walls of the engines and at spark plugs.

    Hence dichloroethane and dibromoethane are added along with tetraethyl lead.

    These convert the lead into lead halides, which are volatile and escape with exhaust

    gases.

    ? The release of lead compounds pollutes the atmosphere.

    ? Catalytic converters (rhodium catalyst) are used in IC engines to convert CO in

    the exhaust to CO. Lead tetraethyl used as anti knocking agent poisons the 2

    catalyst and hence leaded petrol is not advisable in such IC engines.

    ? Addition of MTBE: Methyl tertiary butyl ether (MTBE) is added to petrol (unleaded petrol) to boost its

    octane number. The oxygen of MTBE brings about complete combustion of petrol

    preventing peroxide formation and hence knocking is prevented.

     MTBE can be used as antiknocking agent in IC engines with catalytic converter.

     Power Alcohol:

    ? This is alcohol-blended petrol.

    ? Gasohol is a blend of 10 85% of absolute ethanol and 90 15% of petrol by

    volume and is used as a fuel in the United States. Absolute alcohol is used in the

    preparation of Power alcohol to prevent phase separation.

    ? Alcohol contains higher percentage of oxygen than MTBE and hence brings about

    complete oxidation of petrol more effectively.

    ? Therefore power alcohol has better antiknocking characteristics than unleaded

    petrol.

    Advantages of power alcohol

    ? power output is high

    ? does not release CO, causes less pollution.

    ? alcohol is obtained from molasses, a agricultural product and hence renewable.

    ? biodegradable.

    ------------------------------------

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