unit iv[dr tanushree]

7/30/2019 Unit IV[Dr Tanushree]

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Fuels and Combustion


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Definition

Fuel - combustible substance having carbon as major

constituent.

Reacts with oxygen leading to the formation of the

pro uct w t t e evo ut on o eat at a rap rate.

The energy content of the products is less than the

reactants.

The excess energy is liberated as heat energy.


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Fuels and Combustion

Fuel is a combustible substance, which on combustionproduces a large amount of heat, which can be used forvarious domestic and industrial purposes.

The process of combustion involves oxidation of carbon,hydrogen etc. of the fuels to CO2, H2O, and the difference inthe energy of reactants and the products are liberated as large

amoun o ea energy w c s u ze .

Fuel + O2 Products + Heat

The primary or main source of fuels are coal and petroleumoils, the amounts of which are dwindling day by day. Theseare stored fuels available in earth's crust and are generallycalled "fossil fuels".


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Classification Primary fuels: Natural fuels

Solid

Liquid

Secondary fuels: Derived

Solid Liquid

Gas


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Classification of fuels


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Calorific value of fuels

The most important property of fuel to be taken into

account is its calorific value or the capacity to supplyheat.

The calorific value of a fuel can be defined as "the totalquantity of heat liberated when a unit mass or volume

of the fuel is burnt completely".


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Units of heat

1. Calorie - Calorie is the amount of heat required to

raise the temperature of one gram of water through

one degree centigrade.

2. Kilocalorie (or) kilogram centigrade unit - This is

calories. This may be defined as "the quantity of

heat required to raise the temperature of one

kilogram of water through one degree centigrade".

Thus 1 kcal = 1000 cal.


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Units of heat

3. British thermal unit (B. Th. U.) - This is defined as"the quantity of heat required to raise the

temperature of one pound of water through onedegree Fahrenheit". This is English system unit.

1 B. Th. U. = 252 cal = 0.252 k cal.

1 k cal = 3.9 8 B. Th. U.

4. Centigrade Heat Unit (C. H. U.) - This is the

"quantity of heat required to raise the temperatureof one pound of water through one degreecentigrade".

Thus, 1 k cal = 3.968 B. Th. U. = 2.2 C. H. U.


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Units

Amount of heat required to raise the temp. of

Calorie: 1g of water through 1oC

Kilocalories : 1K of water throu h 1oC

British Thermal

Unit (B. Th. U) : 1pound of water through 1oF

Centigrade

Thermal Unit (C.T.U) : 1pound of water through 1oC


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GCV/HCV Gross / Higher calorific value

Amount of heat produced, when unit

mass/volume of the fuel is burnt completelyand the products of combustion have beencooled to room temperature.

If the products of combustion are condensed toroom temp., the latent heat of condensation of

steam also gets included in the measured heat.Hence higher or gross calorific value


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NCV/LCV- Net / Lower calorific value

In practice, water vapor are not condensed but

allowed to escape. Hence lesser amount ofheat is available.

s t e net eat pro uce , w en un t massof the fuel is burnt completely and the productsare permitted to escape.

LCV = HCV Latent heat of water vapor produced

= HCV Mass of hydrogen x 9 x Latent heat of steam


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Calculation

m = mass of fuel pellet (g)

W = mass of water in the calorimeter (g)w = water equivalent of calorimeter (g)

=

t2 = final temperature of calorimeter.

HCV = gross calorific value of fuel.


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Corrections Fuse wire correction. Heat liberated during sparking should be

subtracted from heat liberated.

Acid correction. Fuels containing Sulphur and Nitrogen if oxidized,the heats of formation of H2SO4 and HNO3 should be subtracted (as

the acid formations are exothermic reactions).

Cooling correction. The rate of cooling of the calorimeter frommaximum temperature to room temperature is noted. From this rate of

cooling (i.e., dt/min) and the actual time taken for cooling (t min) then

correction (dt t) is called cooling correction and is added to the (t2 .

t1) term.


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Corrections


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Combustion Combustion is an exothermic chemical

reaction accompanied by the increase ofheat.

Flue gases: CO,CO2, N2, O2, SO2, H2O

2


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Combustion of carbon

C (s) O2 (g) CO2 (g)

12 32 44

(1)

12Kg carbon requires 32 kg oxygen for combustion

C kg carbon in the fuel requires oxygen

for complete combustion = 32/12 * C


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Combustion of Hydrogen

H2 (s) O2 (g) H2O (g)

4 32 36

(2)

4 Kg hydrogen requires 32 kg oxygen for combustion

for complete combustion = 32/4 * H

From the total amount of hydrogen In the fuel part of hydrogenare non combustible in the form of water

8 parts of oxygen combines with 1 part of hydrogen

The amount of hydrogen available for combustion = (H-O/8)

Hence oxygen required for combustion of hydrogen = 32/4 (H-O/8)


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Combustion of Sulphur

S (s) O2 (g) SO2 (g)

32 32 64

(3)

32Kg Sulphur requires 32 kg oxygen for combustion

S kg sulphur in the fuel requires oxygen

for complete combustion = 32/32 *S


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On combining the three eqns. , the theoretical oxygenrequirement for the combustion of C, H, and S

= 32/12 * C + 8 ( H-O/8) + S

Air contains 23 % by weight of oxygen and 21 % byvolume of oxygen

=

100/23 * [32/12* C+ 8 (H-O/8) + S

Minimum volume of air required for combustion =100/21 * [32/12* C+ 8 (H-O/8) + S


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Dulongs Formula

The approximate calorific value of a fuel can be determined by knowing theamount of constituents present:

Gross or higher calorific value (HCV) from elemental constituents of a fuel.

H = 34500 kcal/kg; C = 8080 kcal/kg; S = 2240 kcal/kg

Ox en resent in the fuel is assumed to be resent as water fixed

hydrogen).Available Hydrogen = Total hydrogen - Fixed hydrogen

= Total hydrogen - 1/8 mass of oxygen in fuel.

Dulongs formula for calorific value from the chemical composition of fuelis,


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Knocking

It is a kind of explosion due to rapid rise of pressure in an internal

combustion (IC) engine.

Gasoline and air(1:17) mixture is compressed and ignited by aspark where oxidation of HC takes place.

Products of oxidation drives the piston down in cylinder, if

Sudden oxidation takes place and mixture detonates resulting anexplosive sound called knocking which leads to loss of power.

Compression ratio= Gaseous vol. in the cylinder at the

end of suction stroke/ volume at the end ofcompression stroke.

Compression ratio depends on the nature of constituents of the fuel.

Efficiency of IC engine increases with compression ratio


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Knocking tendency depends on the chemical structure

of fuel, which decreases in the following order.

Straight chain paraffins > Branched chain paraffins >

Cycloparaffins > Olefins > Aromatics

Knocking results in loss of efficiency of the engine

Knocking characteristics of a combustion engine fuel

Antiknock quality of a fuel is expressed as its octane

number

It is defined as the % of isooctane in a mixture of n-heptane and isooctane which has the same knocking

characteristics of the petrol under the same set of

conditions.


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Octane numberIsooctane best antiknocking properties and

assigned O.N of 100

n-heptane poor antiknocking property andassigned O.N. of 0

Most effective antiknock agent is tetraethyl lead(TEL) along with ethylene dibromide which preventsdeposition of lead by forming volatile lead halides

Others are tetramethyl lead, tertiary butyl acetate,diethyl telluride

Greater the octane number, greater is the resistance ofknocking


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Diesel knock

Long ignition delays lead to accumulation of more vapours in

the engine and when ignited an explosion results as thecombined effect of temperature and pressure. This is

responsible for diesel knock.

Diesel Index: Specific gravity (API) X Aniline point (oF)/100

API American Petroleum Institution


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Cetane number

Long ignition delays lead to accumulation of more vapours in

the engine and when ignited an explosion results as the

combined effect of temperature and pressure. This is

responsible for diesel knock.

There is a delay period between injection of diesel fuel


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Cetane number

There is a delay period between the injection of diesel fuel and its ignition.If this delay period is large, too much fuel accumulates in the cylinder andburn very rapidly and causes diesel knock.

Increasing delay period occurs in the series : n-paraffin < Olefins


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Structures

CH3-CH2-CH2-CH2-CH2-CH2-CH3 n-heptane (O.N = 0)

CH3-C(CH3)2-CH2-CH(CH3)-CH3 2,2,4 trimethylpentane (O.N. =100)

CH3

CH3-(CH2)14-CH3 n-hexadecane (C.N = 100)

2-methyl naphthalene (C.N. = 0)


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Gaseous Fuels

Natural gas:

Found in oil wells.

Can be liquified by compression and cooling

Methane : 70-90%; Ethane :5-10% and other gases like CO2 -N2 etc are found in traces

Calorific value : 12,000-14,000Kcal/m3

Dry gas Natural gas + crude oil

Wet gas Natural gas + petroleum


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Applications

Domestic fuel

Raw material for the manufacture of carbon black (filler for

rubber) and hydrogen (used in ammonia synthesis)

Microbiological fermentation of methane yields synthetic

protein used as animal feed.


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Compressed Natural Gas (CNG)

Natural gas compressed to high pressure of 1000 atm. substitute for petroland diesel

Less polluting as during combustion as it doesnt evolve nitrogen, carbonand sulphur

Initial cost of engine designing - high

Advantages over LPG:Ignites at higher temperature

Cheaper

Lesser CO emission than gasoline

Mixes well with air than liquid fuels

No emission of smoke, SO2, SO3, etc.,


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LPG Obtained as a byproduct during cracking process or from

natural gas

Consists of hydrocarbons of such volatility that they can existas gas at atmospheric pressure but can be liquified underpressure.

Deh drated, desul hurised and traces of or anic sul hides

mercaptans are added warning of leak and compressed underpressure

Calorific value 27,800Kcal/m3

Constituents are n-butane, iso-butane, butylene and

propane with trace amounts of propylene and ethane Domestic, industrial and automobile fuel.


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Advantages/Disadvantages

Advantages:

Cheaper than gasoline High efficiency and heating rate

Complete combustion with no smoke

High knock-resistant

Disadvantages

Difficult to handle as fuel Limited to only to certain classes of vehicles

Poor blending characteristics


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Bio-gas

Raw materials : Animal dung, poultry wastes, vegetable wastes,

waste paper and cotton clothes, plant wastes,

human excreta, birds excreta etc.,


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Forms of biogas

Natural gas biogas obtained long period decay of animal and vegetable

matters inside the earth

Gobar gas anaerobic fermentation of cattle dung

Can also be produced by sewage wastes and organic wastes

Constituents of biogas:

Methane = 50-60% - makes it an excellent fuel

CO2 = 30-40%

H2S = traces

H2 = 5-10%N2 = 2-6%

Calorific value : 1200Kcal/m3


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Gobar gas

Dung + water slurry fed in to digester tank undergoes

fermentation anaerobic bacteria evolution of bio gas

fresh feed to let inside the well (Refer book for figure)

Uses / Advantages:

, ,

Burns without smoke and harmful gases

Waste from biogas plant manure

Economical

Limitation:

Need to have gas lamp or burner within 10metres of the plant.


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Biodiesel Diesel equivalent biofuel from renewable biological

materials such as vegetable oils or animal fats usually a

transesterified oil

Used alone or can be blended with petrodiesel

B factor to state the amount of biodiesel in any fuel mix.Pure biodiesel B100; Fuel containing 30% biodiesel islabelled B30

Also derived from triglycerides of plant (BOTADIESEL)or animals (ZOODIESEL)


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Biodiesel from plant and animal

sources

Plant oils are from sunflower, soyabeans, jatropha, corn,canola, safflower and rapeseed oil

From animal source, it is less prevalent though the process iscomparatively cheaper than extraction from plant source

Made by transesterification where organically derived oils(vegetable oils, animal fats) are combined with alcohol(methanol) and chemically altered to form fatty esters such asmethyl ester. The process results in two products

- methylesters chemical name for biodiesel- Glycerine valuable by-product sold for use in production of

soap

unit iv[dr tanushree]

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