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CHEMICAL FUELS
Definition
Chemical fuel is a combustible carbonaceous material which
on proper burning in air gives large amount of heat that can
be used economically for domestic and industrial purposes.
Eg.- wood, charcoal, coal, kerosene, petrol, diesel,
producer gas, water gas, natural gas, etc.,
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During the process of combustion, C and H of the fuel
combine with oxygen of air to form C! and H!
respectively.
"ince the heat content of combustion products C!, H!,
etc. being lower than that of reactants #C, H, etc. of fuel$, the
chemical fuel release heat during their combustion process.
Classification
% &ased on their origin' 1) Primary (Natural) fuels
2) Secondary (Derived) fuels
% (hese are again subdivided into solid, li)uid and gaseous
according to the physical state.
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Table-1. Classification of Fuels
*hysical state *rimary fuel "econdary fuel
"olid +ood, *eat, Charcoal, Coke
Coal, ignite
i)uid Crude petroleum *etrol, erosene,Diesel, "ynthetic
petrol
as /atural gas *roducer gas,
+ater gas, Coal gas,
&iogas, *
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Characteristics of a Goo Fuel% High calorific value.
% 0oderate ignition temperature.% ow moisture content% ow content of non-combustible matter.% 1n case of solid fuel, the ash content should be
less and the si2e should be uniform.% 3eadily available in bulk at low cost.% *roducts of combustion should not be harmful.% Combustion should be easily controllable.% 1t should be safe, convenient and economical
for storage and transport.
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Calorific !alue
1t is defined as 4the amount of heat liberated when
unit mass #or unit volume in the case of a gaseous
fuel$ of fuel is completely burnt in air or oxygen5Units
%"olid or i)uid fuels - cal6g or kcal6kg or 76kg%aseous fuels 8 kcal6m9 or 76m9
%Gross or Hi"her calorific !alue #GC$%
1t is defined as 4the amount of heat liberated when
unit mass 6 volume of the fuel is burnt completely
in air and the products of combustion are cooled
to room temperature5
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% &et or Lo'er calorific !alue #&C$%
1t is defined as 4the amount of heat produced when
unit mass6volume of fuel is completely burnt in air and the products of combustion are allowed to escape into
the atmosphere5
% C: ; /C: < atent heat of condensation of steam
% /C: ; C: 8 = > 0ass of hydrogen > atent heat
of steam
%/C: ; C: 8 ?.?= > @ of hydrogen > atent heat
of steam
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(rinci)le
A known mass of the fuel sample is burnt completely inexcess of oxygen.
(he liberated heat is absorbed by water and calorimeter.
(he heat lost by burning fuel is the heat gained by water
and calorimeter.
(he calorific value of the fuel is calculated from the
measured data.
Determination of calorific value using
&omb Calorimeter
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*bser!ations an Calculations0ass of the fuel sample taken ; m g
0ass of water taken in the copper calorimeter ; + g
+ater e)uivalent of calorimeter ; w g1nitial temperature of water ; tB
? C
inal temperature of water ; t!? C
Heat liberated by burning of fuel ; Heat absorbed by waterand calorimeter
m x C: ; #+ < w$ #t! - tB$
C: ; #+ < w$ #t! - tB$ cal6g
m
C: ; #+ < w$ #t! - tB$ > .B > B?9 76kg
m
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Calculation of /C:
1f H ; *ercentage of hydrogen in fuel, then
% +ater formed by combustion of Bg of fuel ; B x H
! B??
; ?.?=H g
% atent heat of water formed ; ?.?=H > FG cal6g
% /C: ; C: 8 atent heat of water formed
; C: 8 ?.?=H > FG cal6g
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(he water e)uivalent of the calorimeter is determined by
burning a fuel of known calorific value and using the above
e)uation.(he fuels used for this purpose are ben2oic acidHC:; ,9!F kcal6kg and naphthalene HC:;=, kcal6kg
(roble+,% A solid fuel of ?.F g was burnt in a &omb calorimeter and
the temperature of B?? g of water raised from ! to
!.FoC. 1f the fuel contains G@H, calculate its /C: I C:.
% J+ater e)uivalent of calorimeter ; F?.?g, atentheat of steam ; FG cal 6 degK
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o/s Calori+eter
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(rinci)le
% A known volume of gaseous fuel sample is burnt in thecombustion chamber of a &oyLs calorimeter.
%(he released heat is )uantitatively absorbed by cooling
water, circulated through copper coils surrounding the
combustion chamber.%(he mass of cooling water and its rise in temperature
are noted.%(he mass of water produced by condensation of steam
is calculated.
%(he calorific value of the fuel sample is then calculatedfrom these data.
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*bser!ations an Calculations
:olume of fuel burnt at "(* in time, t ; : m9
0ass of cooling water circulated in time, t ; + kg
"teady temperature of incoming water ; tB? C
"teady temperature of outgoing water ; t!? C3ise in temperature ; #t! - tB$
? C
0ass of water produced from steam condensation ; m kg
Heat released by combustion of fuel ; Heat absorbed by water
: x C: ; + #t! - tB$
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% C: ; + #t! - tB$ kcal6m9
: ; + #t! - tB$ > .B k76m9
:
% atent heat of steam per m9
of fuel sample ; m > FG kcal :
% /C: ; + #t! - tB$ - m >FG kcal
: : ; J+ #t! - tB$ - m >FG K .B k76m9
: :
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Soli Fuels
+ood M*eat Mignite M&ituminous coal MAnthracite
Carbon content, calorific value and hardness
0oisture content, C,,/ and " content, volatile matter
Coal
Coal is a highly carbonaceous matter that has been formed as a result of alteration of vegetable matter #e.g., plants$
under certain favorable conditions.
%1t mainly composed of C, H, /, and , besides
non-combustible inorganic matter
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Analsis of coal an its si"nificance
The )ro0i+ate analsis involves the determination ofmoisture, volatile matter, ash, and fixed carbon.
%(his gives )uick and valuable information regarding
commercial classification and determination of
suitability for a particular industrial use.
The ulti+ate analsis involves the determination of
carbon, hydrogen, sulphur, nitrogen, oxygen and ash.
%(he ultimate analysis is essential for calculating heat
balances in any process for which coal is employed
as a fuel.
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*roximate analysis*roximate analysis
i% Moisture' An air-dried coal sample is weighed into a
dry silica crucible and heated for about one hour at
BB?? C in an electric hot air-oven. (he crucible is
cooled first in air then in a desiccator and then
weighed.
@ moisture ; oss in weight > B??
+t. of coal taken
% 0oisture in coal evaporates during the burning of coal and
it takes some of the liberated heat in the form of latent heatof evaporation.% 0oisture lowers the effective calorific value of coal.% esser the moisture content better is the )uality of coal as a
fuel.
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ii% $olatile +atter ' (he dried sample of coal left in the
crucible in step #i$ is then covered with a lid and placed
in a muffle furnace, maintained at =F?? C. (he crucible
is taken out after G minutes of heating. 1t is cooled firstin air then in a desiccator and finally weighed.
@ :olatile matter ; oss in weight > B??
+t. of coal taken
% A high volatile matter content means that a high proportion
of fuel will distill over as vapour and a large portion of which
escapes unburnt. "o, higher @ of volatile matter in coal is undesirable.% A high volatile matter containing coal burns with a long flame,
high smoke and has low calorific value.% esser the volatile matter, better is the rank of coal.
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iii% Ash, (he residual coal left in the crucible in step #ii$ is
then heated without lid in a muffle furnace at G??-GF?? C,
until a constant weight of residue is obtained.
@ Ash ; +t. of ash left > B??
+t. of coal taken
Ash-for+in" constituents in coal are unesirable
for the follo'in" reasons, % (he calorific value of the coal is decreased% (he removal and disposal of ash poses problems
% (he ash deposited in the fire bars interferes withcirculation of air
% 1f the ash fuses to form a clinker on the fire bars, it
hinders air circulation and also promotes corrosion
of the fire bars.
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i!% Fi0e carbon, 1t is reported as the difference
between B?? and the sum of the percentages ofmoisture, volatile matter and ash content of a coal
sample.
% Higher the percentage of fixed carbon, greater is its
calorific value and better is the )uality of coal. % reater the percentage of fixed carbon, smaller is the
percentage of volatile matter. 1t is the fixed carbon
which burns in the solid state.
% 1nformation regarding the percentage of fixed carbon
helps in designing of the furnace and the fire-box.
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Ulti+ate Analsis
i% Carbon an Hro"en, An accurately weighed coal
sample #B-! g$ is burnt in a current of oxygen in combustionapparatus. As a result C and H of the coal are converted into
C! and H! respectively. (hese are absorbed respectively
in H and CaCl! tubes of known weights. (he increase in
the weights of H and CaCl! tubes corresponds to theamount of C! and H! formed respectively.
C < ! M C!
B!
! H < C! M !C9 < H!
H! < B6!! M H!
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@ Carbon ; 1ncrease in wt .of H tube > B! > B??
+t. of coal taken >
@ Hydrogen ; 1ncrease in wt. of CaCl! tube > ! >B?? +t. of coal taken >B
% C and H in coal directly contribute towards the calorific value
of the coal.% Higher the percentage of C and H, better is the )uality of the
coal and higher is its calorific value.
ii% &itro"en, Determined by digesting a known )uantity #Bg$ of
powdered air-dried coal sample in a NeldhalLs flask with conc.H!" in the presence of !" as a catalyst. After the solution
becomes clear, it is treated with excess of /aH.
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(he liberated ammonia is distilled into a known volume of
standard acid solution. (he volume of unused acid is then
determined by back titration with standard /aH solution.rom the volume of acid used by ammonia liberated, the
percentage of nitrogen is calculated.
@ /itrogen ; +t. of nitrogen > B??
+t. of coal taken
+here,
+t. of nitrogen ; :ol. of acid used > /acid > B
B???
(hus, &itro"en 2 $ol. of aci use 3 &aci 31.4
5t. of coal ta6en
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% iii%Sul)hur, A known amount of coal
sample is burnt completely in a bomb
calorimeter. "ulphur present in coalis oxidi2ed to sulphates. (he ash left after
combustion from the bomb calorimeter is
extracted with dil. HCl. (he acid extract isthen treated with barium chloride solution
to precipitate sulphate as barium
sulphate. (he precipitate is filtered,washed, ignited and weighed.
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Sul)hur 2 5t. of aS*4 obtaine 3 78 3 199
5t. of coal ta6en 3 877
% "ulphur containing coal is not suitable forthe preparation of metallurgical coke as it
adversely affects the properties of the
metal.% xides of sulphur pollutes the
environment and leads to corrosion.
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#i!% Ash, (he ash content of coal sample is
determined as described under proximate
analysis.
#!% *0"en, *0"en 2 199 : #C ; H ; & ; S ; Ash%
(he lower the oxygen content, the more is thematurity of coal and greater is its calorific value
( t l fi i
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(etroleu+ refinin"
(he various fractions condensed and collected
at different heights of the column are '
Gases #oilin" ran"e< belo' 799C%<
(etroleu+ ether #79-=99 C%<
Gasoline or )etrol #49-1899 C%< &a)htha #189-1>99 C%<
?erosene #1>9-8@99 C%<
Diesel or fuel oil or li"ht "as oil #8@9-7899C%< Hea! oil #789-4999 C%<
(araffin 'a0< As)halt< etc.< #abo!e 4999 C%.
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Crac6in"
%Crac6in" is efine as the )rocess of eco+)osition of
hi"her +olecular 'ei"ht hrocarbons #hi"her boilin"%
into lo'er +olecular 'ei"ht hrocarbons #lo' boilin"%.
% Cracking process involves breaking of C-C and C-H bonds.% 1t produces low boiling alkanes and alkenes.% A small amount of carbon and hydrogen are also produced.
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Cracking
Cracking
CB?H!! CFHB! < CFHB?
Decane *entane *entene
(hermal Cracking
JCarried out at high temperature
and pressure in the absence of
catalystK
Catalytic CrackingJCarried out in the presence of a
catalyst #Al!
9
< "i!
$ at a much
lower temperature and pressureK
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A!anta"es of fluiie-be crac6in"
% A high degree of mixing is achieved and conse)uently
a good contact is established between the catalyst and
the feed stock vapours.
%(his results in a higher yield.
% (he regeneration of the inactive catalyst can be
carried out continuously without interrupting the
production of gasoline unlike in fixed-bed catalytic
cracking.
Befor+in" of )etrol
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Befor+in" of )etrol
Cataltic Befor+in", 1t is the process of upgrading gasoline
#increasing its octane number$ in presence of a catalyst
%(he increase in octane number of straight run gasoline occurs
through structural modifications such as conversions of straight
hydrocarbons into branched, cyclic and aromatic hydrocarbons.
% Befor+in" (rocess, (he feed stock #straight run gasoline$ is
preheated to remove " and / content to acceptable limits to
avoid platinum catalyst being poisoned. (he vapours of thefeed stock is mixed with hydrogen and preheated to F??? C.
(he mixture is compressed #BF-F? atmosphere$ and then fed
into a series of three cylindrical reactors containing the
platinum catalyst supported on alumina-silica base. (he
reformed products are fractionated to get stabili2ed gasoline.
Befor+in" reactions
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Befor+in" reactions% Iso+erisation, (he conversion of straight chain
hydrocarbons into branched chain hydrocarbons.
% Dehro"enation, Dehydrogenation of Cycloalkanes toproduce aromatic compounds.
CH9
#CH!$D CH9
CH9
CH
CH9
CH!
CH9
CH!
!-methyl petanen-hexane
CH9
CH9
< 9 H!
Cyclohexane &e2ene
< 9H!
0ethyl cycloheane (oluene
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% Cclisation an ehro"enation' Cyclisation of straight
chain hydrocarbons followed by dehydrogenation to
produce aromatic hydrocarbons.
CH9 CH9#CH!$D H!<
n-Hexane Cyclohexane
< 9H!
Cyclohexane &en2ene
% Hro crac6in", Hydro cracking of n-*araffins to produce
light gases that are removed from gasoline fraction.
CH9 #CH
!$E CH
9 CH9 #CH
!$9 CH
9< H!
Cat!
n-Decane n-*entane
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?noc6in"
(he explosive violent noise coming out from the 1C engines
due to instataneous burning of the last portion of the fuel rapily
is known as knocking
% (he efficiency of power production in spark ignited
internal combustion #1C$ engines is related to the
compression ratio #C3$.% (he C3 is the ratio of the cylinder volume #:B$ at the
end of the suction stroke to the volume #:!$ at the
end of the compression stroke of the piston.
% (his ratio is always greater than one, since :B being
greater than :!.
%
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(he tendency to knock decreases as
follows'
n-alkanes> mono substituted alkanes >
cycloalkanes > alkenes > poly substituted
alkanes > aromatics.
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A!erse effects of "asoline 6noc6
% 1t increases the fuel consumption.
% 1t results in decreased power output.% 1t causes mechanical damage by overheating of the
cylinder parts.% (he driving becomes rather unpleasant.
The 6noc6in" in IC en"ines can be +ini+ie throu"h
the follo'in" +easures,
% &y a suitable change in engine design.% &y using high rating gasoline.% &y using anti-knocking agents.
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*ctane &u+ber % Among alkanes, n-heptane knocks severely,
while under identical conditions, !,!,-trimethyl pentane
#iso-octane$has a high resistance to knocking.% or the scale proposed to indicate the anti-
knock properties of gasoline, n-heptane was
arbitrarily assigned an octane number of 2ero
and iso-octane was arbitrarily assigned a
value of B??.%
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*ctane nu+ber is efine as the )ercenta"e
b !olu+e of iso-octane in a +i0ture of iso-
octane an n-he)tane blen<'hich has the sa+e 6noc6in" characteristics
as the "asoline sa+)le< uner the sa+e set
of conitions.
% (hus a gasoline with an octane number of=?, has the same knocking characteristics as a
mixture of iso-octane and n-heptane containing
=?@ by volume of iso-octane. "ince iso-octanehas good anti-knock properties, it is clear that
greater the octane number, greater is the
resistance to knocking.
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Anti-6noc6in" A"ents
(he octane rating of gasoline samples
can be increased by the addition ofcertain organometallic compounds
called anti-knocking agents and the
process is called 4doping5. An extensively used anti-knocking
agent is tetraethyl lead #(E$,
*b#C!HF$.
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*roposed 0echanism'
*b#C!HF$ <! *bO! <ther products
3CH!O < ! 3CH < HO!
*b < H O *b#H$*bO! < 3CH! *b <3CH!O
HO! < *b HO < *b!
*b#H$ < HO *b! <H!
(etraethyl lead helps to cut self ignition broughtabout by compression by generating a smooth
supply of free radicals
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Unleae (etrol
*etrol wherein the enhancement of octane rating is
accomplished without the addition of lead compounds isreferred to as unleaded petrol.
% (o improve its octane number, Concentration of high octane
components #like isopentane isooctane et!ylben"ene
isopropyl ben"ene etc$ is increased by the process of
reforming.% Compounds like met!yl tertiary butylet!er (#$%&) can also
be added to improve octane number of unleaded petrol.
0(&E provides oxygen #of ether group$ for combustion of
petrol in 1C engines, thereby reducing considerably the
formation of peroxy compounds #which causes knocking$.
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A!anta"es of unleae )etrol
% (he harmful effects of discharge of poisonous lead and its
compounds through the exhaust of automobiles is avoided.% ne of the maNor advantages of using unleaded petrol
is that it permits the attachment of a catalytic converter to
the exhaust pipe in automobiles.
Cataltic con!erter contains rhoiu+ catalst.% 1t converts the toxic gases such as C and / to harmless
C! and /!.
% 1t also oxidi2es hydrocarbons into C! and H!.
eaded petrol cannot be used in automobiles e)uipped with
catalytic converter as the lead present poisons the catalyst
thus destroying the active sites.
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Gaseous Fuels
Liuefie )etroleu+ "as #L(G% or bottled gas or refinery gas
% 1t is obtained as a byproduct, during the cracking of heavy oils
or from natural gas.% * is dehydrated, desulphurised and traces of odorous
organic sulphides #mercaptans$ are added to give warning of
gas leak. 1t is supplied under pressure in containers under the
trade name like 1ndane, &harat gas, etc.% 1t has the calorific value of about !F??? kcal 6m9.% * consists of hydrocarbons of such volatility that they can
exist as gas under atmospheric pressure, but can be readily li)uefied under pressure.% (he main constituents of * are n-butane, isobutene,
butylenes and propene.
* i id l d d ti f l P d
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% * is widely used as a domestic fuel. Psed as an
alternative fuel for 1C engines, since it permits the
attainment of high compression ratios without producing
knocking
A!anta"es of L(G o!er "asoline as a +otor fuel% 1t is cheaper than gasoline.% 1t readily mix with air.
% 1t is highly knock resistant,% 3esidue and oil contamination is less, as it burns cleanly.
Disa!anta"es of L(G o!er "asoline as a +otor fuel
% Handling has to be done under pressure.% * is advantageous only in engines working under
high compression ratio.% 1ts octane number is )uite low.% 1ts response to blending is very poor.
5 t
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5ater "as
% 1t is essentially a mixture of combustible gases, C and H!.
% 1t is also known as blue gas because it burns with a blue flame
due to the combustion of carbon monoxide.% (he calorific value of water gas is about B???? -BB??? 76m9.% (he average composition of water gas is as follows'
C' ? 8 F@ Q H! ' F 8 F?@ Q C! ' @ Q /! ' @
Manufacture, 1t is produced by passing alternatively steam
and little air through a bed of red hot coke maintained atB???? C.
(rinci)le, +hen steam is blown through a bed of hot coke
#B???? C$ water gas is produced.
C < H! #steam$ M C < H! RH ; B9B. 7% (he reaction being endothermic in nature, the temperature
of the coke bed gradually decreases with continuous passage
of steam and the drop in temperature must be prevented. or
this the steam supply is temporarily cut off and air is blown in.
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(he over all reaction during air blow is the formation of C.
!C < ! M !C RH ; - !!B.= 7
(his reaction being exothermic increases the temperature of
the coke bed to about B???? C. (hus by blowing steam and
air alternatively, the temperature of the coke bed can be
maintained at B????
C.
Uses % 1t is used for the production of hydrogen.% 1t is extensively used for the manufacture of methyl alcohol
and synthetic petrol.% 1t is used as a fuel in glass and ceramic industries.% Enriched water gas #mixed with hydrocarbons$, which
burns with luminous flame is used as illuminating agent.
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(roucer "as
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(roucer "as% 1t is essentially a mixture of carbon monoxide and nitrogen.% 1t is prepared by passing air mixed with little steam over a
red hot coal or coke bed maintained at aboutBB??
?
C.% (he average composition of producer gas is as follows'
C' !F-9?@ Q /! ' F?-FF@ Q H! ' B?@ Q C! ' F@Q
Hydrocarbons ' !-9@.% (he calorific value of producer gas is ??? - F??? 76m9.
Manufacture, (he producer is charged with coke from the
top and the charge is heated to about BB??? C. A mixture of
air and steam is passed over red hot coke bed through the
inlet at the bottom. (he producer gas goes out through theoutlet at the top.
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Beactions that ta6es )lace in ifferent ones of the fuel be
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Beactions that ta6es )lace in ifferent ones of the fuel be
% *0iation one, (his is the lowest part of the coke bed.
Here, the carbon of the coke burns in presence of excess of
air to give carbon dioxide.
!C < ! M !C! RH ; - 9=9.F 7
% Beuction one, Carbon dioxide produced in the oxidation
2one then rises through the hot bed and is reduced by coke to C.
!C! < C M !C RH ; - BG9.F 7
(he over all reaction in the formation of carbon monoxide
being exothermic, the fuel bed gets heated up beyond BB??? C. At high temperature'
- the ash forms clinkers or slag which are rather difficult to
remove.
- the grate bars and refractory lining get distorted.
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1n order to avoid these problems in the producer, a
reduction in temperature is achieved by passing air saturated
with steam instead of air alone.
1n the reduction 2one, steam gets reduced to water gas.
C < H!
#steam$ M C < H!
RH ; B9B. 7
(his endothermic reaction brings down the temperature to
the
optimum level.
% Distillation one, (his is the upper most part of the fuel bed,
where the distillation of volatile matter of coke 6 coal occurs.
Uses,- 1t is used as a fuel in the manufacture of steel, glass,
A!anta"es of cataltic crac6in"
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A!anta"es of cataltic crac6in"
% (he octane number of gasoline produced is high.
% (he yield of gasoline is also high.% (he process can be better controlled.% (he product contains a very little amount of undesirable
sulphur.% (here is a saving in production costs since high temper-
atures and high pressures are not needed.% 1n catalytic cracking, external fuel is not re)uired.
(he necessary heat is obtained by burning off the coke
deposited on the catalyst itself, during the regeneration
process.% (he gasoline formed contains much less gum and gum
forming compounds.% Catalysts are selective in their action, and therefore, they
permit cracking of only high boiling hydrocarbons.
%Fluiie #+o!in"% be cataltic crac6in",
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Fluiie #+o!in"% be cataltic crac6in",
(he finely divided catalyst bed #Al!9 < "i!$ is fluidi2ed by the
upward passage of feed stock vapours #Heavy oil, gas oil, etc$
in a cracking chamber #called 3eactor$ maintained at FF?
?
C./ear the top of the reactor, there is a centrifugal separator
#called cyclone$, which allows only the cracked oil vapours to
pass onto the fractionating column but retains the catalyst
powder in the reactor itself. (he catalyst powder gradually
becomes heavier due to the deposition of carbon and settles tothe bottom, from where it is forced by an air blast to the
regenerator #maintained at ??? C$. After cracking, the products
are fractionated into gases, gasoline, gas oils and residual oils.
(he heavier oil fractions may be cracked in a second-stagecracking.
1n regenerator, the spent catalyst is stripped of the adsorbed
oil by passing steam and then decarboni2ed by a hot air blast,
under controlled conditions. (he heat liberated during this
regeneration is used to raise steam and to preheat the catalyst.
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% H. 3. 3icardo, with the help of a variablecompression engine
% showed that in actual practice, the power
increases to a% maximum and then falls rapidly with furtherincrease in the
% C3. (he C3, corresponding to the maximum
power output, is% known as highest useful compression ratio
#HPC3$.
Mechanis+ of ?noc6in"
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Mechanis+ of ?noc6in"% 1n 1C engines, the gasoline and air drawn into the cylinder is
compressed by the piston and ignited by an electric spark.
% As the flame front travels towards feed end of the combustion
chamber, rapidly expanding combustion gases, compress the
remaining un-burnt fuel ahead of flame front and raise its
temperature.% 1f the flame front travels rapidly at optimum speed, the
combustion of un-burnt fuel takes place but smoothly.% 1f the flame front travels too slowly, the entire last portion of
the fuel-air mixture may get heated up beyond its ignition
temp. and undergo instantaneous explosive combustion.
(his produces thermal shock wave which hits cylinder walls
and piston. (his result in emitting of characteristic rattling
sound called knocking or pinking.
Liui Fuels
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Liui Fuels
(etroleu+ #Crue oil%
% An important primary li)uid fuel.% 1t is a dark colored viscous oil found deep in the earthLs
crust.% 1t is believed to have been formed millions of years ago by
anaerobic decay of marine plant and animal life under the
influence of high temperature and pressure.% 1t is mainly a complex mixture of hydrocarbons #like straight-
chain paraffins, cycloparaffins, olefins and aromatics$ with
small amounts of other organic compounds containing /, and ", and traces of inorganic compounds.
% (he average composition of crude oil is'
C' 9 - G@Q H' BB - BF @Q ", / and ' ?.B- F@.
(h h i f th h i l ti th t l d t
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% (he mechanism of the chemical reactions that lead to
knocking is not clear.% 1t is believed that chemical reactions that are of importance
are cracking and the oxidation of the hydrocarbons.% *robably the reactions proceed by a chain reaction.% 1t was recogni2ed that the structures of the fuel
hydrocarbons determines largely their knocking tendency.
% (he tendency to knock decreases as follows'
n-alkanesS mono substituted alkanes S cycloalkanes S
alkenes S poly substituted alkanes S aromatics.
% (he tendency to knock depends not only on the fuel usedbut also on the engine design, shape of head, location of
plug, etc., and also upon the running conditions.
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% Automobile gasolineTs have octane number ranging from GF% to =F. Aviation gasolineTs have a greater knock resistance% and their octane numbers are greater than B??. 1n such cases% the octane numbers are computed using the relationship,
% ctane number ; J *ower number 8B?? K < B??% 9% where, power number is an arbitrary number proportional to% the power being extracted by the engine.
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% About ?.F ml of (E per liter is added for motor fuel and about
B ml of (E per liter is generally added for aviation petrol.% 1t is believed that during combustion of gasoline, (E forms
*b and *b.% (hese species act as free-radical chain inhibitors and thus
curtail the propagation of the explosive chain reaction and
thereby minimi2ing knocking.
% 1f (E alone is used, the species *b and *b may getdeposited on engine parts and cause mechanical damage.% (he vapours of *b and *b may pollute the air.% 1n order to minimi2e the air pollution and damage to engine
parts, (E is always used along with ethylene dibromide
or ethylene dichloride.% (he functions of these ethylene derivatives is to convert the
less volatile *b and *b into more volatile *b&r ! or *bCl!
which escapes into air along with exhaust gases.
Cataltic Crac6in" +ethos
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Cataltic Crac6in" +ethos
% Fi0e-be cataltic crac6in", (he catalyst #Al!9 < "i!$
in the form of powder or pellets is placed on the grid in thecatalytic chamber. (he vapours of the feed stocks #Heavy oil,
gas oil, etc.$ are passed through the bed of catalyst main-
tained at F?-F??? C. About F?@ of the feed stock is
converted into gasoline together with elemental carbon
which gets deposited on the surface of the catalyst. Cracked
vapours are next subNected to fractionation in a fractionating
column wherein gasoline is separated from un-cracked heavy
oil. (he catalyst loses its activity because of the deposition
of carbon and also due to the adsorption of oil vapours. Accordingly, the catalyst re)uires regeneration after -B?
hours. During regeneration time, the cracking process is
interrupted and the adsorbed oil is stripped off by passing