Fuels
Cost: Coke should be cheap and easily available.Manufacture of
Metallurgical Coke: Beehive OvenOtto Hoffmann oven(i) Beehive oven:
A beehive oven is a fire-brick chamber having a dome-shaped
structure. The dimensions of a typical oven are 4m and 2.5m high.
The roof is provided with a hole for charging the coal from the
top. Another hole, the discharging hole is provided in the
circumference of the lower part of the wall. A number of ovens are
built in a row with common walls between neighbouring ovens.
Fig. 5: Beehive coke ovenDemerits of Beehive ovens: The demerits
areNo recovery of byproducts, which are useful chemicals and are
allowed to escape.Lower coke yield due to partial combustionLack of
flexibility of operation(ii) Otto-Hoffmanns oven or By-product
Oven: The beehive ovens have been replaced by chamber ovens which
works on regenerative principle of heat economy. All the valuable
products are recovered from the outgoing flue gases.Construction:
It consists of no. of narrow rectangular chambers made of silica
bricks.
Fig. 6: A single chamber of Otto Hoffmanns ovenWorking: Coal is
charged into the chamber.The coke ovens are heated to 1200oC by
burning gaseous fuels.The process of carbonization takes place
layer by layer in the coal charge.As the coal adjacent to the oven
walls gets heated, a plastic zone is formed which moves away from
the walls towards the central zone.As the coal is converted into
coke, there is decrease in volume. This is because of the removal
of volatile matter in the form of tar and gas at about 500oC. At
further high temperature, the plastic mass solidifies into hard and
porous mass called coke.Regenerative principle is employed to
achieve as economical heating as possible.Regenerators are built
underneath the ovens.The flue gases pass their heat to the checker
brick work of regenerators until the temperature rises to
1000oC.Regenerators work on the principle of alternate heating and
cooling cycles. This is achieved by periodically changing the
direction of flow of gases through the vertical flues every 30 min
or so.Carbonization of a charge of coal takes about 11-18 hours.
After the process is complete, red hot coke is pushed outside by
means of a ram which is electrically driven. The coke falls into a
quenching car. The yield is 75 % of coal.
Fig. 7: Otto Hoffmanns Byproduct coke ovenRecovery of
byproducts: The gases and vapours evolved on carbonization in coke
ovens are not allowed to mix with the combustion and are collected
separately.The coke oven gas is treated separately for the recovery
of the valuable byproducts.
Fig. 8: Coke-Oven gas treatment plantRecovery of Tar: The gas
from the coke ovens is passed through a tower in which liquor
ammonia is sprayed.Tar and dust get collected in a tank. The tank
is provided with a heating coils to recover back ammonia.Recovery
of ammonia: The gases are then passed through a tower where water
is sprayed to recover ammonia. The ammonia can also be recovered by
dissolving it in H2SO4 to form (NH4)2SO4, which is then used as a
fertilizer. Recovery of Naphthalene: The gases are passed through a
cooling tower, where water at a low temperature is sprayed.The gas
is scrubbed with water until its temp. reduces.(iv) Recovery of
Benzol: The gases are then introduced into a light oil or benzol
scrubber, where benzene along with its homologue is removed and is
collected at the bottom.(v) Recovery of H2S and other S compounds:
are removed from the coke oven gas after the light oil has been
separated out.
The SO2 obtained can be used for the manufacturing of sulphuric
acid, which can be used to absorb NH3 from the coal gas.
Liquid Fuels: The importance of liquid fuels is the fact that
almost all combustion engines run on them.The largest source of
liquid fuels is petroleum. The calorific value of petroleum is
about 40000 kJ/kg.There are other supplements of liquid fuels such
as coal tar, crude benzol, syntheic liquid fuel made from coal
etc.Petroleum: The term petroleum means rock oil. It is also called
mineral oil. Petroleum is a complex mixture of paraffinic, olefinic
and aromatic hydrocarbons with small quantities of organic
compounds containing oxygen, nitrogen and sulphur. Composition:
ElementCarbonHydrogenSulphurOxygenNitrogenPercentage80-8711.1-150.1-3.50.1-0.90.4-0.9The
ash of the crude oil is 0.1%.Metals e.g., Silicon, iron, aluminium,
calcium, magnesium, nickel and sodium.Crude oil is a mixture of
straight chain paraffins and aromatic hydrocarbons e.g., benzene,
toluene, naphthalenes etc.Sulphur is present in the form of
derivatives of hydrocarbons such as alkylsulphides, aromatic
sulphides etc. Nitrogen is present in the form of pyridine,
quinoline derivatives, pyrrole etc. Comined oxygen is present as
carboxylic acids, ketones and phenols.The objectionable odour of
crude petroleum is due to the presence of sulphur compounds in
it.Classification of Crude PetroleumResidue obtained after
distillationNameContents
Paraffin wax
Asphalt
Paraffin wax and asphaltParaffin base
Asphaltic baseMixed baseStraight chain hydrocarbons and small
amounts naphthenes and aromatic hydrocarbonsAromatic and naphthenic
hydrocarbonsParaffins, naphthenes and aromatic
hydrocarbonsProcessing of Crude Petroleum: Petroleum is found deep
below the earth crust. The oil is found floating over salt water or
brine. Generally, accumulation of natural gas occurs above the
oil.
Fig. 9: Pumping of oilRefining of PetroleumCrude oil reaching
the surface, generally consists of a mixture of solid, liquid and
gaseous hydrocarbons containing sand and water.After the removal of
dirt, water and much of the associated natural gas, the crude oil
is separated into a no of useful fractions by fractional
distillation.The resultant fractions are then subjected to
purification known as refining of petroleum.Steps involved in
refining of petroleum:(i) Demulsification: The crude oil coming out
from the well, is in the form of stable emulsion of oil and salt
water, which is yellow to dark brown in colour.The demulsification
is achieved by Cottrells process, in which the water is removed
from the oil by electrical process. The crude oil is subjected to
an electrical field, when droplets of colloidal water coalesce to
form large drops which separate out from the oil.(ii) Removal of
harmful impurities: Excessive salt content such as NaCl and MgCl2
can corrode the refining equipment. These are removed by washing
with water.The objectionable sulphur compound are removed by
treating the oil with copper oxide. The copper sulphide so formed
is separated by filtration. (iii) Fractional Distillation: It is
done in tall fractionating tower or column made up of steel.In
continuous process, the crude oil is preheated to 350-380 oC in
specially designed tubular furnace known as pipe still.
Fig. 10: Fractional distillation of crude petroleumThe hot
vapours from the crude are passed through a tall fractionating
column, called bubble tower.Bubble tower consists of horizontal
trays provided with a no of small chimneys, through which vapours
rise.These chimneys are covered with loose caps, known as bubble
caps. These bubble caps help to provide an intimate contact between
the escaping vapours and down coming liquid.The temperature in the
fractionating tower decreases gradually on moving upwards.As the
vapours of the crude oil go up, they become gradually cooler and
fractional condensation takes place at different heights of
column.The residue from the bottom of the fractionating tower is
vacuum distilled to recover various fractions
Fig. 11: Vacuum distillation of residual oilThere is yet another
type of fractional distillation called Top-flashing.
In top flashing, there is better control of product composition,
but requires more pumps and instruments and hence is an expensive
process.
Fig. 11: Top FlashingCracking: Gasoline is the most imp fraction
of crude petroleum. The yield of this fraction is only 20% of the
crude oil. The yield of heavier petroleum fraction is quite high.
Therefore, heavier fractions are converted into more useful
fraction, gasoline.This is achieved by a technique called
cracking.Cracking is the process by which heavier fractions are
converted into lighter fractions by the application of heat, with
or without catalyst. Cracking involves the rupture of C-C and C-H
bonds in the chains of high molecular weight hydrocarbons.e.g:
Nearly 50% of todays gasoline is obtained by cracking. The
gasoline obtained by cracking is far more superior than straight
run gasoline.The process of cracking involves the following
chemical changes:Higher hydrocarbons are converted to lower
hydrocarbons by C-C cleavage. The product obtained on cracking have
low boiling points than initial reactant.Formation of branched
chain hydrocarbons takes place from straight chain
alkanes.Unsaturated hydrocarbons are obtained from saturated
hydrocarbons.Cyclization may takes place.Cracking can also be used
for the production of olefins from naphthas, oil gas from kerosene.
Cracking can be carried out by two methodsThermal Cracking: When it
takes place simply by the application of heat and pressure, the
process is called thermal cracking. The heavy oils are subjected to
high temperature and pressure, when the bigger hydrocarbons break
down to give smaller molecules of paraffins, olefins etc. The
thermal stability among the constitutents of petroleum fractions
increases asParaffins < naphthenes < aromatics(a) Liquid
Phase thermal cracking: The charge is kept in the liquid form by
applying high pressures of the range 30-100 kg/cm2 at a suitable
temperature of 476-530 oC. The cracked products are separated in a
fractionating column.The important fractions are: Cracked gasoline
(30-35%), Cracking gases (10-45%); Cracked fuel oil (50-55%).(b)
Vapour phase thermal cracking: By this method, only those oils
which vapourize at low temperatures can be cracked. The petroleum
fractions of low boiling range like kerosene oil, are heated at a
temp of 670-720 oC under low pressure.Mechanism of thermal
cracking: It follows free radical mechanism.Initiation
PropagationThe free radical formed are thermally unstable and
undergo fission at the b-position to yield a new radical and an
olefin.
Catalytic cracking: Cracking is brought about in the presence of
a catalyst at much lower temperatures and pressures. The catalyst
used is mainly a mixture of silica and alumina. Most recent
catalyst used is zeolite. The quality and yield of gasoline is
greatly improved by this method.
Advantages of catalytic cracking over thermal cracking:High temp
and pressure are not required in the presence of a catalyst.The use
of catalyst not only accelerates the cracking reactions but also
introduces new reactions which considerably modify the yield and
the nature of the products.The yield of the gasoline is higher.No
external fuel is required for cracking.The process can be better
controlled so desired products can be obtained.The product contains
a very little amount of undesirable sulphur because a major portion
of it escapes out as H2S gas, during cracking.It yields less coke,
less gas and more liquid products.The evolution of by-product gas
can be further minimized, thereby increasing t he yield of desired
product.Catalysts are selective in action and hence cracking of
only high boiling fractions takes place.Coke forming materials are
absorbed by the catalysts as soon as they are formed.Knocking and
antiknockingIn an internal combustion engine a mixture of gasoline
vapour and air is used as a fuel. After the reaction is initiated
by a spark, a flame should spread rapidly and smoothly through the
gas mixture and the expanding gas drives the piston down the
cylinder. In certain circumstances the rate of oxidation is so high
that the last portion of the mixture detonates, producing an
explosive sound called knockingThe tendency of fuel constituent to
knock is in the following orderStraight
chain>branched>olefins>cycloparaffins>aromaticsAn
antiknocking agent is used to increase the octane number of a
fuel.TEL ( Tetra ethyl lead)Methyl cyclopentadienyl Manganese
tricarbonylFerroceneIron pentacarbonylTolueneIso octane
Anti Knocking agentsOctane number: is defined as the percentage
of iso octane present in a mixture of iso-octane and n-heptane,
which has the same knocking characteristics as that of fuel under
examination, under same set of conditions.Thus a gasoline with an
octane no of 80, would give the same knocking as a mixture of iso
octane and n-heptane containing 80% of iso octane by volume.
Greater the octane number, greater is the antiknock property of the
fuel.Cetane Rating: Fuels required for diesel engine are in
contrast to petrol engine fuels, hence a separate scale is used to
grade the diesel oils as they cannot be graded on octane number
scale.The cetane number of a diesel oil is defined as the
percentage of cetane in a mixture of cetane and a-methyl
naphthalene which will have the same ignition characteristics as
the fuel under test, under same set of conditions.Cetane is
n-hexadecaneThe cetane rating of a fuel depend upon the nature and
composition of hydrocarbon. The straight chain hydrocarbons ignite
quite readily while aromatics do not ignite easily. Ignition
quality order among the constituents of diesel engine fuels in
order of decreasing cetane no, is as follows:n-alkanes>
naphthenes > alkenes > aromaticsAniline PointThis is an
approximate measure of the aromatic content of a hydrocarbon fuel.
It is defined as the lowest temperature at which a fuel oil is
completely miscible with an equal volume of aniline. Aniline is an
aromatic compound and aromatics are more miscible in aniline than
are paraffins.Hence, the lower the aniline point, the higher the
aromatics content in the fuel oil. The higher the aromatics
content, the lower the cetane number of the fuel.The aniline point
can thus be used to indicate the probable ignition behavior of a
diesel fuel.Diesel IndexThe Diesel Index indicates the ignition
quality of the fuel. It is found to correlate, approximately, to
the cetane number of commercial fuels. It is obtained by the
following equation
Diesel Index and cetane number are usually about 50. Lower
values will result in smoky exhaust
Gaseous FuelsAdvantages of gaseous fuelsLeast amount of
handlingSimplest burners systemsBurner systems require least
maintenanceEnvironmental benefits: lowest GHG and other
emissions36Gas fuels are the most convenient because they require
the least amount of handling and are used in the simplest and most
maintenance-free burner systems. Gas is delivered "on tap" via a
distribution network and so is suited for areas with a high
population or industrial density. However, large individual
consumers do have gasholders and some produce their own gas.Gaseous
FuelsClassification of gaseous fuels (A) Fuels naturally found in
natureNatural gasMethane from coal mines(B) Fuel gases made from
solid fuelGases derived from coalGases derived from waste and
biomassFrom other industrial processes (C) Gases made from
petroleumLiquefied Petroleum gas (LPG)Refinery gasesGases from oil
gasification(D) Gases from some fermentation37The following types
of gaseous fuels exist:Fuels naturally found in nature:- Natural
gas- Methane from coal minesFuel gases made from solid fuel- Gases
derived from coal- Gases derived from waste and biomass- From other
industrial processes (blast furnace gas) Gases made from petroleum-
Liquefied Petroleum gas (LPG)- Refinery gases- Gases from oil
gasificationGases from some fermentation processGaseous
FuelsCalorific value Fuel should be compared based on the net
calorific value (NCV), especially natural gasTypical physical and
chemical properties of various gaseous fuelsFuel Gas Relative
Density Higher Heating Value kCal/Nm3Air/Fuel ratio m3/m3 Flame
Temp oCFlame speed m/s Natural Gas 0.6 9350 10 1954 0.290 Propane
1.52 22200 25 1967 0.460 Butane 1.96 28500 32 1973 0.870 38Since
most gas combustion appliances cannot utlilize the heat content of
the water vapour, gross calorific value is of little interest. Fuel
should rather be compared based on the net calorific value. This is
especially true for natural gas, since increased hydrogen content
results in high water formation during combustion. Typical physical
and chemical properties of various gaseous fuels are given in this
table.Type of FuelsGaseous FuelsNatural gasMethane: 95%Remaing 5%:
ethane, propane, butane, pentane, nitrogen, hydrogen, CO, carbon
dioxide, other gasesHigh calorific value fuelDoes not require
storage facilitiesNo sulphurMixes readily with air without
producing smoke or soot 39Methane is the main constituent of
natural gas and accounts for about 95% of the total volume. Other
components are: ethane, propane, butane, pentane, nitrogen, carbon
dioxide, and traces of other gases. As methane is the largest
component of natural gas, generally properties of methane are used
when comparing the properties of natural gas to other fuels.
Natural gas is a high calorific value fuel that doesnt require any
storage facilities. It mixes with air readily and does not produce
smoke or soot. It has no sulphur content. It is lighter than air
and disperses into air easily in case of leak. Application of
Natural GasNatural gas is finding increasing use as a domestic,
transportation and industrial fuel.It is used as a raw material for
the synthesis of methanol, formaldehyde and other chemical
compounds.It is used as a source of H2Ammonia used for the process
for extracting nickel from its ore is made by reacting N2 with
natural gas.Varieties of LPG bought and sold include mixes that are
primarily propane (C3H8), primarily butane (C4H10) and, most
commonly, mixes including both propane and butane and isobutane
depending on the season in winter more propane, in summer more
butane[. Propylene and butylenes are usually also present in small
concentration. A powerful odorant, ethanethiol, is added so that
leaks can be detected easilyLPGLiquefied petroleum gas (also called
LPG, GPL, LP Gas, or liquid propane gas) is a flammable mixture of
hydrocarbon gases used as a fuel in cooking, heating appliances and
vehicles.It is used as a feedstock for the manufacture of various
chemicals and olefins by pyrolysis.Other industrial application
includes its use in portable blow lamps, soldering, welding,
annealing hardening, brazing, steel cutting etc. It is increasingly
used as an aerosol propellant and a refrigerant, replacing
chlorofluorocarbons in an effort to reduce damage to the ozone
layer. When specifically used as a vehicle fuel it is often
referred to as autogas.Application of LPGCNG is made by compressing
natural gas (which is mainly composed of methane [CH4]), to less
than 1% of the volume it occupies at standard atmospheric pressure.
It is stored and distributed in hard containers at a pressure of
200248 bar (29003600 psi), usually in cylindrical or spherical
shapes.Compressed Natural Gas(CNG)Compressed natural gas (CNG) is a
fossil fuel substitute for gasoline (petrol), diesel, or
propane/LPG. Although its combustion does produce greenhouse gases,
it is a more environmentally clean alternative to those fuels, and
it is much safer than other fuels in the event of a spill (natural
gas is lighter than air, and disperses quickly when released). CNG
may also be mixed with biogas, produced from landfills or
wastewater, which doesn't increase the concentration of carbon in
the atmosphere.Cars and locomotives.
Type of FuelsComparing FuelsFuel OilCoalNatural GasCarbon
8441.1174Hydrogen 122.7625Sulphur 30.41-Oxygen 19.89TraceNitrogen
Trace1.220.75Ash Trace38.63-Water Trace5.98-45A typical comparison
of carbon contents in oil, coal and gas is given in this table.
(Reflection time before continuing) Numericals on Combustion of
fuel1. Calculate the weight and volume of air required for
combustion of 3kg of carbon.2. Calculate the minimum weight of air
required for complete combustion of 1kg of fuel containing C=90%,
H=3.5%, O= 3.0%, S= 0.5%, water=1%, N=0.5% and rest is ash.3. A
gaseous fuel has the following composition by volume: H=20%, CH4=
5%, CO= 20%, CO2= 5% and N= 45%If 50% excess of air is used find
the weight of air acutally supplied per m3 of this gas Power
Alcohol: Ethyl alcohol is an important liquid fuel and when it is
used in an internal combustion engines, it is called power
alcohol.Generally gasoline and alcohol are present in the ratio of
4:1. Its octane no is 90.Manufacture of Power Alcohol: Ethyl
alcohol can be manufactured from hydrocarbons, carbohydrates,
starches and molasses, waste sulphite liquor from paper and pulp
mills.Ethyl alcohol is mainly manufactured from molasses. The
formation of alcohol involves the following steps:(i) Dilution:
High concentration of sugars in molasses (ii) Addition of dilute
sulphuric acid: For fermentation the pH of the solution should be
kept between 4 to 5.(iii) Addition of ammonium salts: When molasses
contain insufficient yeast food, some neutrient substanceslke
ammonium sulphate and ammonium phosphate are added to overcome the
deficiency of phosphorous and nitrogen.(iv) Addition of yeast: This
is the main step in the formation of alcohol from mollases. To the
solution a selected strain of yeast is added. The mixture s kept at
a temperature of 30 oC for 2-3 days.
(v) Distillation: It is carried out in a specially designed
plant called coffey still.
Distillation of wash/wort(vi) Azeotropic distillation: The
alcohol obtained after distillation is mixed with benzene or
carbontetrachloride and subjected to distillation. The benzene or
carbontetrachloride distills over taking away a portion of alcohol
and water leaving behind absolute alcohol.Advantages of Alcohol
blended gasoline:The octane no of alcohol is quite high. Hence on
blending with gasoline, it increases the octane no of
gasoline.There is no decrease in power output or increase in
specific fuel consumption on blending alcohol with gasoline.3. The
starting difficulties because of high boiling point of alcohol is
overcome, when it is used in blended form with gasoline.4.
Alcohol-petrol blend has a tendency to absorb any traces of
moisture present.Disadvantages of Alcohol blended Gasoline:The
calorific value of alcohol is low and hence the alcohol lowers the
calorific value of petrol.Since the air required for complete
combustion of blend is less than the air required for petrol.The
ratio of flow of blend is to be increased to 1.56 times to that of
pure petrol by altering the size of carburetor jet.