Fuel Nptel
Fuel Nptel
Fuel Nptel
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Keyword: coal, rank of coal, moisture content, use of fuel
Introduction
Any naturally occurring carbon-containing material, when burned with air (or oxygen) produces
heat or energy (directly or indirectly). Fossil fuels can be classified according to their respective
forms at ambient conditions. Thus, there are solid fuels (coals), liquid fuels (petroleum) and
Coal is a brown to black naturally occurring combustible organic rock that originated by
accumulation and subsequent physical and chemical alteration of plant material over long period
of time. The plant debris accumulated in various wet environments, commonly called peat
swamps, in which trees, ferns and the like are deposited, and buried by sand, silt and mud. As a
result of temperature and pressure effects, metamorphosis of the woody material occurs to
produce the various types of coal. The initial transformation of vegetable materials probably
includes various types of degradation and decay due to some fungal and bacterial action. Slow
atmospheric oxidation may also take place. The course and rapidity of the vegetal decay are the
called the precursor of coal. This is primary transformation. The secondary transformation which
is rather a slower process of aging under substantially anaerobic condition, higher pressures and
elevated temperature.
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This progressive transformation of peat to higher coals is called coalification process.
Increasingly deeper burial under hundreds to thousands of feet of younger sediment is required to
advance coalification to the bituminous and anthracite stages. The pressure exerted by the weight
of the overlying sediments and the heat that increases with depth, as well as the length of
exposure to them, determines the degree of coalification, as well as the rank of coal.
There are two theories proposed for the mode of accumulation of the plant materials to transform
into coal.
1) In-situ theory-According to this theory, the coal seams are observed where once forest
grew. As the land was sinking slowly, the accumulated vegetation matters went under
water slowly and did not decompose and destroyed. In the course of time, the rate of
sinking of land was increased and coal forest was submerged under water. Again, land
along with the coal forest emerged out of water after sufficient time and this cycle went
on again and again, which is responsible of formation of coal strata and seams. The
evidence of this phenomenon is observed in the coal seam that the stem of fossil trees is
found standing erect with their roots protruding into the underclays. The uniformity in
thickness and composition of coal seams over wide areas suggests that the deposition of
2) Drift theory- This theory tells that, the plant material was transported with the stream of
water from one place to another, and finally get deposited in a place of swamp having
suitable condition like sediments. The coal seams of India are of drift origin. The
evidence of drift theory that the rocks associated with the coal seams are of distinctly
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sedimentary. The coal seam itself behaves like a sedimentary bed and they are observed
to branch out.
It is assumed that, at least some of the natural graphites are produced from anthracite by the
Although samples of each rank of coal have distinct physical and chemical characteristics, the
border line between two consecutive ranks of coal is difficult to determine. The first four
samples in the series have a nearly continuous gradation of a given physical and chemical
Peat is the result of insufficient transformation. The composition and properties of peat vary
greatly from one place to another, depending on the nature and type of the original plant material
and the extent of decay. It contains very large amount of water. Peat is not regarded as coal, but
it is an important fuel in those countries which have large deposits of peat. It is mainly used as a
domestic fuel. Moreover peat briquettes are largely used in steam boilers, power stations and gas
Lignite, may be termed as Brown coal, is lowest in rank and readily identified by its colour and
texture. It is soft, has a woody structure and disintegrates on drying. Dark brown or black colour
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is observed in lignite, having woody and amorphous nature. Lignite breaks in slabs after long
time exposure to weather. It is characterized by a high percentage of moisture, ranging from 30-
Black lignite grades are the preliminary stage of sub-bituminous coal, having of lower oxygen
content and higher heating value than lignite. Sub-bituminous coals are not distinguishable as a
class by appearance or physical properties. They are intermediate between black lignite and
Different bituminous coals vary in their appearance and properties. In general, they are harder
than sub-bituminous samples and exhibit cubical fracture in most of the time.
Next rank is anthracite coal which is again harder than bituminous and having an amorphous
texture. Anthracite is relatively dustless solid coal and burns with a smokeless flame. High
carbon content and low volatile matter and oxygen are the characteristics of anthracite coal.
As the rank of coal increases from peat to anthracite, the carbon content increases but, moisture,
volatile matter and oxygen contents decrease. The carbon content increases from 70% to 95%
The chemical composition of coal is not very clearly known. It is generally said that, coal
components are macromolecular with complex structures. The reason for appearance of different
components in different rank of coal is the variation of the structures of these macromolecules.
Waxes, resins, pectin, hemicelluloses, lignite etc. are found in low rank coal as peat and lignite.
The major coal fields in India are Gondwana coal field and Tertiary coal field. Gondwana coal
field covers the areas of West Bengal, Bihar, Orissa, Madhya Pradesh, Maharastra, Andra
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Pradesh etc. Tertiary coal field covers North East India, Tamilnadu, Rajasthan, J&K, Gujarat etc.
More than 98% coal is obtained from Gondwana coal field. Vast reserves of bituminous coal are
coal measures are high in moisture (3-10%) and high in volatile matter (30-36%). The coals of
Gondwana basins are mainly of sub-bituminous type. Coalfields of the Damodar valley of
Jharkhand and Bihar are the chief source of metallurgical coal in the country and most of the iron
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References:
1. Fuels and combustion, S. Sarkar, 2nd edition, Orient Longman Ltd., 1990.
2. Fuels combustion and furnaces, John Griswold, Chemical engineering series, McGraw
3. Chemistry of coal utilization, Ed. by Martin A. Elliott, John Wiley & Sons Inc., 1981.
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Key words: Petroleum; deposition of organisms; reservoir rock; natural gas, LPG
Liquid fuel
Petroleum is the naturally occurring liquid fuel and it accounts for the bulk of the liquid fuels.
Petroleum can be defined as a mixture of gaseous, liquid and solid hydrocarbons or hydrocarbon
derivatives that occur naturally within the geological traps. It is generally agreed that petroleum
was formed by processes similar to those which yielded coal, but was derived from small
animals, mainly marine animals, rather than plants. Dead organisms have been buried in mud
over millions of years. Further layers deposited over these mud layers, may reach a thickness of
thousands of feet. When a layer was particularly rich in broken sea shells, it was compacted into
limestone. Sandy layers become sandstone. The bodies of the organisms in the mud layer were
decomposed by high temperature and pressure of the earth layer and converted to fatty liquids
and solids. Heating these fatty materials over a very long time caused their molecules to break
into smaller fragments and recombine into larger ones.Hence a wide range of molecular size
found in crude petroleum was obtained by this process. Bacteria were usually present and helped
to remove oxygen from the molecule, which were mostly carbohydrates, comprising of carbon,
hydrogen and oxygen. These carbohydrate molecules were turned into hydrocarbons by the
bacterial action. High pressure of the overlying rock layers forced the oil to migrate from
compacted mud layer (shale) to less compacted limestone, dolomite, sandstone layers. The first
type of rocks (sedimentary rocks) has lesser permeability than the second type (reservoir rocks).
During this migration, the composition of oil may be changed due to filtration, adsorption etc.
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Accumulation of petroleum in sediments
Reservoir rock must possess fluid holding capacity and also fluid transmitting capacity. Most
reservoir rocks are coarser grained sedimentary rocks. Cap rock act as a seal to prevent the
escape of oil and gas from the reservoir rock. Typical cap rocks are clays and shells, rocks in
which the pores are very much finer than those of reservoir rock. It also has far lower
permeability than reservoir rock. Salts, anhydrites, gypsum which are called evaporates also act
as cap rock. In the following the diagram (Fig 1.) a probable accumulation pattern of gas, oil and
Once formed, the sedimentary rocks are subjected to various kinds of deformation, such as
folding and faulting. Anticlines are upfolds in layered rocks, which are important type of
structural traps in petroleum geology. Circular upfolds in the rocks are called “domes”. Synclines
are the opposite of anticlines, which are downfolds, usually occur between two anticlines. More
or less circular depressions in the layered rocks are called “basins”. Both these folds may extend
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from feets to miles long. Faults are breaks or fractures in rocks along which one side is moved
relative to other side. Fauting is important for entrapment and migration of oil at some places.
The distribution of fluids in a reservoir rock is dependent on densities of the fluids and
the detailed capillary properties of the rock. Taking the simplest case of a rock in which pores are
uniform size and evenly distributed, the fluids are distributed in the order, gas, oil, brine water in
the ascending order of density. The upper zone of the rock pores are filled mainly by gas (gas
cap), a middle zone, which is occupied mainly by oil with gas in solution and a lower zone, filled
by water. Usually, the gas-oil and oil-water contact is horizontal, but there are cases where these
are inclined.
Composition of petroleum
Petroleum is not a uniform material. Its composition can vary with the location, age and also
organic compounds of sulfur, oxygen and nitrogen, as well as compounds containing metallic
constituents, particularly vanadium, nickel, iron and copper. The hydrocarbon content may be as
high as 97%, for example, in the light paraffinic petroleum or as low as 50% or less as illustrated
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Gaseous fuels
Gaseous fuels are the most convenient fuel, which needs simplest and maintenance free burner
systems due to absence of mineral as impurities. Gaseous fuels may be divided into four types:
Natural gas
Producer gas
Water gas
Beside these, hydrogen and acetylene are two important gases those are widely used in the
industries.
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a) Gases obtained naturally
Natural gas
Natural Gas is a naturally occurring mixture of hydrocarbon and non-hydrocarbon gases found in
porous geological formations (reservoirs) beneath the earth's surface. It may be obtained, often as
an associated product of petroleum and also in gas reserve. The chemical composition and
heating value of natural gas varies with the reservoir source and the processing conditions.
Natural gas is primarily a mixture of methane with very little amount of C2 to C4 hydrocarbons.
In addition to fuel use, natural gas is a source of hydrogen for ammonia synthesis and a source of
The main constituent of coal mine gas is methane. When coal beds are formed through the
compression and heating of organic materials, methane is a gas formed as a part of the process of
coal formation, known as coalification. The methane content in coal seams generally increases
with the deepness of seam, and also with age. As the coal beds are mined, the entrapped or
adsorbed methane is released from coal seams. Methane can also be released as a result of
Producer gas is obtained by blowing air or air-steam mixture on burning bed of solid fuel, such
as coal. It is a fuel of low calorific value with principal components carbon monoxide and
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nitrogen. Hydrogen is also present in producer gas when air-steam mixture is used for blowing,
Water gas is formed by the reaction of steam and solid fuel as coal or coke at high temperature.
This gaseous fuel is a mixture of carbon monoxide and hydrogen in equal proportion. It’s
Coal gas is a moderate calorific value gas, obtained by the process of high temperature
carbonization of coal. Its main use is as domestic fuel. It is a rich source of hydrogen.
Wood gas is obtained by gasification of wood logs, chips etc by air. It is a mixture of CO2, CO,
CH4, some olefins, H2 and N2. Its calorific value is around 1660 kCal/m3.
The combustion gases obtained from blast furnace during iron ore extraction by coke is the blast
furnace gas. It is a low calorific value fuel with main constituent being hydrogen and carbon
dioxide.
Out of the gaseous hydrocarbons, the C3 and C4 compounds can be liquefied at room temperature
by the application of moderate pressure. This liquefied gas can be conveniently stored and
transported in light pressure vessels and known as Liquefied Petroleum Gas or LPG. The main
column of the crude distillation unit produces these hydrocarbon mixtures as a top product. The
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combustion characteristics of LPG differ greatly from other gaseous fuels. LPG has high
Liquid fuels are gasified to produce either gaseous fuel or synthesis gas. The raw materials used
for gasification are light distillates, middle distillates and heavy oils. Gasification is done by air
and steam, leading to the production of carbon monoxide and hydrogen along with smaller
molecular weight hydrocarbons. It is a high calorific value fuel because of the presence of
hydrocarbons in it.
Two important gaseous fuels, methane and hydrogen can be produced by the anaerobic
fermentation of organic waste in presence of microbes. The organic fractions of a waste are
degraded by several groups of anaerobic bacteria to produce volatile fatty acids (VFAs) and
hydrogen. VFAs and hydrogen are further converted by methanogenic bacteria to methane.
Hydrogen and acetylene both gases are produced from different chemical reactions in industries
either as byproduct or as per requirement. Hydrogen may be produced from electrolysis of water,
partial oxidation of liquid fuel, dissociation of ammonia, synthesis gas production etc. Acetylene
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Reference:
1. Modern Petroleum Technology, Vol 1, Upstream, Ed. by Richard A. Dawe, IP, 6th
2. Fuels and combustion, S. Sarkar, 2nd edition, Orient Longman Ltd., 1990.
3. Fuels combustion and furnaces, John Griswold, Chemical engineering series, McGraw
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Keywords: mining of coal; exploration of petroleum, drilling of petroleum, consumption
Coal
Coal mining
Coal mining is the process which involves activities such as removing the coal from earth and
preparing it for the market. The basic system of production techniques mainly consists of,
2) Transport of coal and waste products away from the active production location.
3) Ventilation with proper air distribution to provide sufficient air circulation to meet the
statutory regulations.
4) The control of the behavior of underground and surface opening required for coal
extraction.
b) Underground mining
GSI, CMPDI, SCCL, MECL etc. made a survey up to the maximum depth of 1200 metre to
estimate the reserve of coal in India and the report says that there is a cumulative total of
2,93,497 million tonnes of geological resources of coal in the country as on 1.4.2012. Two major
coal fields in India are Gondwana formation and Tertiary formation, where the first one is the
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About 75% of the coal in India is consumed in the power sector. In addition, other industries like
steel, cement, fertilizers, chemicals, paper and thousands of medium and small-scale industries
are also dependent on coal for their process and energy requirements.
Exploration
2) Reservoir rock, such as, porous and permeable limestone, dolomite or sandstone
3) A trapping mechanism, such as, anticline, faulted strata or any of the different kinds of
traps.
Petroleum geologists must do everything possible to search for areas, where all of these
conditions are met. One must gather as many clues as possible, the clues must be studied and
interpreted individually and then with a great deal of data compilation and imagination, a
Drilling oil and gas well is a complicated and expensive operation which needs versatile
knowledge of engineering and geosciences. Selection of drill site is an important criterion which
Surface condition must be suitable for drilling purpose. After completion of survey and selection,
the site will be cleared and leveled. All drilling equipments and necessary components are
moved to the location. The derrick is raised over the substructure where main bore hole will be
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made, and other equipments such as, engines, pumps, rotating and hoisting equipments are
aligned and connected. Drilling fluid is stored in the location. After all these arrangements, the
For oil and gas to flow into the drill casing or well bore from the porous rock, the required force
can be supplied in different forms, which are, 1) Dissolve gas drive, which is the result of
expansion of gas dissolved in oil, 2) Gas cap drive, which is done by the expansion of gas
contained in the reservoir above the oil, and 3) Water drive, where the force is exerted by the
upward pressure of water as it expands and moves into the regions of lower pressure as oil is
Fig 1. Reservoir drive mechanisms (a) Dissolved gas drive (b) Gas cap drive (c) Water drive
The availability of crude oil in the country increased from 18.51 million tonnes during 1970-71
to 201.31 million tonnes during 2010-11. During this period crude oil production increased from
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6.82 to 37.71 million tonnes. The availability of natural gas has steadily increased from a mere
0.65 BCMs during 1970-71 to 51.25 BCMs during 2010-11. (BCM=Billion cubic metre)
High speed diesel oil accounted for 38% of total consumption of all types of petroleum products
in 2010-11. This was followed by LPG (9.1%), petrol (9%), fuel oil (7%) and refinery fuel
(10.1%). ‘Low sulphur oil’ consumption was highest (52%) in industrial sector.
Natural gas has been used both for energy (69 %) and non-energy (31%) purposes and the
maximum use of natural gas is in power generation (46%) followed by fertilizers industry (28%)
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Reference:
1. Modern Petroleum Technology, Vol 1, Upstream, Ed. by Richard A. Dawe, IP, 6th
2. McGraw Hill Encylopedia of Science & Technology, no. 4, 9th edition, McGraw Hill,
2002.
3. Fuels and combustion, S. Sarkar, 2nd edition, Orient Longman Ltd., 1990.
4. Energy Statistics 2012, Central Statistics Office, Ministry of Statistics and Programme
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Keywords: property, analytical methods, coal carbonization, caking, swelling
Coal varies widely in composition and properties and they are used in different conditions. Coal
characteristics are determined by various kinds of analytical and testing data, which are of
scientific, technical and commercial interest. Following are the definitions of some coal
Ultimate analysis is the elemental analysis which determines the percentage composition of
carbon, hydrogen, oxygen, nitrogen and sulfur by weight. These elemental compositions are of
Proximate analysis reports moisture, volatile matter, ash and fixed carbon content of a fuel by
percentage weight, as defined by ASTM D 121. Moisture is the amount of water obtained from
the fuel by heating at a specific condition according to the standard method, without making any
chemical change to the fuel. Volatile matter consists of gases and vapors driven off during
pyrolysis under specified condition minus moisture, fixed carbon is the nonvolatile fraction of
coal, and ash is the inorganic residue remaining after combustion. Proximate analysis is the most
When coal is heated at high temperature in presence of air, heat liberated per unit weight of fuel
is called heating value or calorific value of that fuel. Calorific value can be determined either at
constant volume or constant pressure. Gross calorific value at constant volume is the amount of
heat liberated by combusting unit weight of coal at constant volume in an atmosphere of water
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vapour saturated oxygen, the original fuel and final products of combustion should be at 250C
Gross calorific value at constant pressure is similar to that of constant volume, only difference is
Net calorific value at constant volume is the amount of heat liberated by combusting unit weight
of coal at constant volume in an atmosphere of water vapour saturated oxygen, the original fuel
and final products of combustion should be at 250C and the water obtained by this process should
be in vapour state. Net calorific value at constant pressure is that, the combustion occurs at
Gross calorific value is also called higher heating value as it is higher than net calorific value.
The reason for this is, while determining net calorific value, the water remains in vapour state,
hence the heat of condensation is not taken in consideration, which is rather a part of gross
Ash content
Ash in coal, which is the remains when coal is burnt, is one of the materials of interest. Ash is
derived from the mineral matter content of coal. The inorganic materials which were actually the
part of the plant structures, constitute the ‘inherent’ mineral matter of coal, whereas, the
‘extraneous’ mineral matter is that which was introduced probably as air-borne dusts or water-
borne silts at the later stage of coalification. Mineral matter of coal predominantly consists of
kaolinite, pyrite and calcite and upon combustion; results in the oxides of silicons and metals,
such as, aluminium, iron and calcium. These oxides are the essential part of ash. When coal
burns, shales and other hydrated materials, which are also the constituents of mineral matter of
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coal, decompose and lose their water of hydration and also emit carbon dioxide, sulfur dioxide
gases. As there is a loss in weight, so, amount of ash of coal is always less than its mineral matter
content.
Moisture content
Moisture of coal may also become inherent or extraneous. Inherent moisture is the moisture
associated with coal inherently, which cannot be removed by only air drying, it can be removed
when coal is heated above 1000C. Extraneous moisture can be removed by air drying of coal.
Volatile matter
Volatile matter is the volatile part of coal when coal undergoes thermal decomposition. The
volatile part of organic mass of coal is the main constituent of it. The moisture content of coal is
not included in it. But volatile matter of may contain water, when hydrogen and oxygen of coal
produce water at high temperature of decomposition. The water of hydration, which is liberated
during decomposition, is also a part of volatile matter. It is observed that, as the maturity of coal
increases, volatile matter decreases. Fixed carbon is the non volatile part of organic mass after
decomposition. Ash is not included in it. Fixed carbon should not be confused with the total
carbon of coal. In anthracite, the values of fixed carbon and total carbon are almost equal,
whereas, for other coals, fixed carbon is less than total carbon.
The major elements present in coal are carbon, hydrogen, oxygen, sulfur and phosphorous. There
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Coal carbonization, caking index and swelling index
Coal carbonization is the process where coal is heated at high temperature without contact of air.
The decomposition product is of higher carbon content than the original coal. When coal is
converted to gaseous fuel by heating at sufficiently high temperature, the process is called
gasification of coal. During carbonization, some gaseous products are also produced.
During carbonization, some types of coal can produce lumps from its pulverized form. These
lumps are called cake and the process of cake formation is called caking. When these lumps can
meet the specification of some standard tests in terms of its hardness, brittleness etc. and also its
suitability to use in steel industry as a source of heat as well as a reducing agent, then they are
called coke. Coke is prepared in commercial coke oven by the process of carbonization at more
All types of coal do not show caking property. Some types of bituminous coals are caking in
nature. Other coals are non-caking. Hence, caking property of a coal is important for selection of
a particular type of coal in industrial purpose. Caking index is determined for coal to obtain its
binding or agglutinating property. This is defined as the maximum whole number ratio of sand to
coal in a 25 g mixture of those two, lumps produced by heating that mixture at a specified
condition which can withstand a 500 g weight, and the loose particles obtained by this process
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Swelling index is another important property of coal, where it is heated under specified
condition. The residue obtained is a swollen mass due to volatiles in coal are driven off. This
mass is compared with a standard chart where pictures of different swollen mass are given with a
definite number or unit, starting from 0 to 9, with an increment of ½. The number of the picture
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References:
1. Chemistry of coal utilization, 2nd supplementary volume, Elliott M.A.(ed.) Wiley, 1981
2. Fuels and combustion, S. Sarkar, 2nd edition, Orient Longman Ltd., 1990.
3. Fuels combustion and furnaces, John Griswold, Chemical engineering series, McGraw Hill
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The composition of petroleum varies with the location, age and also individual well. The high
proportion of carbon and hydrogen indicate that hydrocarbons are the major constituents of
petroleum. The principal types of hydrocarbon present in crude oil are normal, branched or
The properties of crude petroleum and its fractions can be determined by various ways.
Crude oils are roughly classified into different bases according to the nature of principal type of
hydrocarbons present in it. The bases are paraffin base, naphthene based, mixed base or
intermediate base and aromatic base. Paraffin based crude oils composed of mainly paraffins.
Mixed base or intermediate base crudes are lower in n-paraffins and higher in naphthenes
compared to that of the paraffin base oils. Naphthene base crudes are characterized by a high
percentage of naphthenes and almost no presence of any wax. Aromatic base crudes contain a
relatively high percentage of the lower aromatic hydrocarbons. This classification is arbitrary,
but long use of this concept makes this classification valuable to the technical persons.
0
API or API gravity is an empirical correlation which is actually a representation of specific
(API), as,
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.
.
.
. .
.
API gravity is used in petroleum chemistry instead of specific gravity. This is because; the
petroleum cuts are having specific gravities which are very near to each other. But use of API
gravity makes the difference of gravity wider between the two consecutive cuts, as specific
the fraction in a specified apparatus. As petroleum cuts are the mixture of different
hydrocarbons, they do not have a definite boiling point, but have a boiling range. Initial boiling
point (IBP) of a petroleum cut is the temperature when first drop of distillate comes out of the
condenser of the distillation apparatus. Final boiling point (FBP) is the maximum temperature
recorded at the end of the distillation. Distillation cannot be carried on beyond 3500C
Viscosity index is a property of petroleum fractions, which is defined as the rate of change of
viscosity with temperature. This is indicated by a number in an arbitrary scale ranging from 0 to
100, higher the number more is the viscosity index. A viscosity index of 100 means, oil which
ideally does not become thin at elevated temperatures or become viscous at lower temperature.
Mainly paraffinic base lubricating oils exhibit a viscosity index near 100.
Flash point and fire point are the two important properties of petroleum fractions. Flash point is
defined as the minimum temperature at which the fuel upon heating evolves vapour which after
mixing with the air give a sudden flash when a source of fire is brought in contact with it. Fire
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point is the minimum temperature at which the fuel vapour in admixture with air will produce a
continuous fire when a fire source is brought in contact with the vapour. So, the fire point is
more than the flash point for a particular petroleum fraction. Hence, flash point is more
During the transportation of heavy oil fractions in pipelines, there is a possibility of freezing the
oil within the pipe when it is transported at cold climate. Here pour point and cloud point are
the two properties of these oils which play important role. Pour point is the maximum
temperature, at which oil ceases to flow when it is cooled at specified condition. Pour point is
reported by adding 2.80C or 50F to this temperature, which is a caution to technical people.
Cloud point is the temperature at which oil becomes cloudy, when it is cooled at a specified
condition.
Burning quality of kerosene can be determined by its illuminating capacity and it is expressed
by two properties, smoke point and char value. Smoke point is defined as the maximum flame
height in millimeters when kerosene burns in a standard apparatus without producing any smoke
or shoot. Char value is the amount of char produced in milligrams on the wick of a standard
Carbon residue is the important characteristic of the oils which are used in engines, burners and
furnaces. The carbonaceous residue obtained after heating the oil at a specified rate due to
cracking and decomposition is called carbon residue. Two different types, Conradson and
Octane number determines the quality of gasoline or petrol. When gasoline is burned in a spark
ignition engine (petrol engine), it produces power. A good quality gasoline burns smoothly
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without making any noise in the engine. Whereas, burning of a bad quality gasoline is not
smooth and produces a sudden high pressure by burning all fuel at a time, which forms pressure
wave or detonation or knock to the engine. The octane number of a gasoline is a measure of
knocking tendency. Octane number is determined by comparing the performance of a model fuel
and the gasoline under test in a standard engine in laboratory. The model fuel is prepared by
mixing iso-octane (2,2,4 trimethyl pentane) whose octane number is assumed to be 100 and n-
heptane, whose octane number is assumed to be zero. So, octane number of a gasoline sample is
defined as the percent by volume of iso-octane in a model fuel (mixture of iso-octane and n-
heptane) whose knocking performance matches with the test gasoline. Hence, a gasoline with
octane number 90 means, its knocking tendency matches with a model fuel having 90 vol% iso-
octane. The descending order of octane number of the hydrocarbons is: aromatics> iso-
paraffins> naphthene> olefins> n-paraffins. There are many octane number improver. Tetraethyl
lead (TEL) is the most common improver. Now-a-days, octane number is improved by adding
alkylates to gasoline.
Ignition quality of diesel is expressed by cetane number. Diesel is injected in the hot
compressed air in the cylinder of a diesel engine, which then burns to produce power. If there is a
large time gap between the injection and ignition, there is an unwanted accumulation of fuel in
the cylinder, which suddenly burns at a time with a pressure wave, producing diesel knock. Like
octane number, here also a model fuel is prepared by mixing n-hexadecane or cetane (whose
cetane number is assigned to 100) and ∞-methyl naphthalene (whose cetane number is assumed
to be zero), at different volume proportions. The performance of the diesel under test is
compared with the model fuel. The percent by volume of n-hexadecane in a model fuel is the
cetane number of the test diesel whose diesel knocking performance matches with the model fuel
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when tested in a specified engine. Hence, if the performance of test diesel matches with the
performance of the model fuel having 45/55 blend of cetane and ∞-methyl naphthalene, then the
diesel is assigned to cetane number of 45. The descending order of cetane number of
Aniline point is also a property mainly of diesel. This is defined as the temperature at which the
oil under test is completely miscible with equal volume of aniline, when tested in a standard
apparatus. Aromatics dissolve aromatics easily. Hence, if the oil contains more aromatics, its
aniline point will be low. But if the oil contains more paraffin, its aniline point will be more.
,
Usually, it is observed that diesel index is three units higher than the cetane number, although it
Petroleum cuts contain ash but to a very negligible amount. Moisture content is also very less
which can be determined by standard test. Sulfur in petroleum oils remain in the form of sulfides,
disulfides, marcaptans, thiophenes and higher thiophenes. Heavier the cut more is the sulfur
content.
Calorific value of both petroleum oil and gaseous fuel are determined by bomb calorimeter and
Junker’s calorimeter. The gross calorific value of petroleum cuts vary from 10000 to11500
kcal/kg, the lower boiling cut, such as gasoline shows higher value and heavier oils show lower
value. Among gaseous fuels, natural gas and LPG has highest calorific value, which is in the
range of 43000 to 46000 kJ/m3. Other gaseous fuels are of moderate or low calorific value, such
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as the calorific value of producer gas, carbureted water gas and coal gas are 5000, 19000 and
References:
1. Fuels and combustion, S. Sarkar, 2nd edition, Orient Longman Ltd., 1990.
Company, 1987.
3. Fuels combustion and furnaces, John Griswold, Chemical engineering series, McGraw
http://www.kayelaby.npl.co.uk/chemistry/3_11/3_11_4.html
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Keywords: Characterisation, analytical methods, standards
Characterisation of coal and petroleum fractions, as well as gaseous fuels is done by some
specific test methods and apparatus. Some major characterization techniques are mentioned in
this section. Some standards, such as ASTM (American Society for Testing and Materials), BS
(British Standard), IS (Indian standard) etc. are followed to determine the properties.
matter, ash and fixed carbon content in terms of weight percentage. In this method, a specific
amount of finely powdered coal (1 or 2 g) is heated in a petri-dish for one hour at 105 to 1100C,
sample is cooled and weighed. The weight loss is the external moisture content. The coal sample
is then taken in a silica crucible and heated at 9250C for 7 min, the sample is quickly cooled to
avoid the oxidation and then weighed. The weight loss is expressed as volatile matter content.
About 1 g of powdered coal is taken in a silica dish and heated in a muffle furnace at 400 to
4500C for 30 mins, and then one hr at 725±250C. The weight of the incinerated residue is the ash
content. The fixed carbon is obtained by subtracting the weights of moisture, volatile matter and
Caking index for coal is determined by taking sample of the uniform mixtures of sand and coal
of total 25 g in different weight ratios. The samples taken in crucibles are kept inside a muffle
furnace at approximately 9250C for 7 min. Then the crucibles are removed and allowed to cool.
The lump or cake is observed to form from the mixtures. The cakes are taken out carefully and
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500 g weight is placed on the cakes. The percentage of loose particles comes out from the cakes
are determined separately for each cake which gives a measure of the binding property of the
coal sample. The maximum whole number ratio of sand to coal in the 25 g mixture which
produces a cake after heating that can withstand a weight of 500 g without producing 5% loose
Swelling index is one of the important properties of coal. Coal is heated under specified
condition on flame in a crucible. The swollen mass is cooled and compared with a standard chart
where profiles with numbers (which indicates swelling index) are given. The numbers are in
increasing order from 0 to 9 with a 0.5 increment. The higher the number better is the caking and
swelling characteristics.
Calorific value of coal is determined by Bomb calorimeter. It is a thick walled steel cylindrical
vessel with lid which is called Bomb. Two electrodes are inserted through the lid which are in
contact with fuse and fuel sample of known weight. An oxygen inlet valve is provided with the
lid through which high pressure oxygen gas (at about 25 to 30 atm) is supplied. Entire
arrangement is held in a calorimeter containing known weight of water and a mechanical stirrer
is provided to stir the water for uniform heating. A Beckmann thermometer is also provided to
measure the change in temperature of water due to combustion of fuel. Fig 1. shows the
schematic of the apparatus. The gross calorific value is calculated by the following formula
C. V m m T T T /m
Where, m1 and m2 are mass of water in calorimeter and water equivalent of bomb calorimeter
T1 and T2 are final and initial temperature of water sample. Tc is temperature correction for
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The major property of coke is its strength and hardness. Among many tests to determine
strength and hardness of coke, Micum test is important. In mecum test, 50 kg of coke samples
are taken in a metallic cylindrical drum (Micum trommel) of 1 m length and 1 m diameter, and
the drum is rotated about its horizontal axis at 25 rpm speed for 4 min. Iron angles are provided
inside the drum wall. This rotation gives a combined effect of abrasion and shatter to coke
samples. The particle size distribution after the test is determined by sieve analysis and this gives
the account of brittleness of coke during its handling and use. Micum index or M80 denotes the
method (ASTM D-86). This is the method of non-fractionating atmospheric distillation of light
cuts such as, gasoline, kerosene etc. A specific amount of liquid fuel is taken in a standard round
bottom flask with the arrangement of condenser and thermometer device. Fig 1. shows the
standard set-up , consisting of a flask, condenser, collector and thermometer. The fuel is heated
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by an electric heater at a specified rate and some part of vapour flows to the condenser through
the side limb of the flask. Other part of the vapour gets condensed and flows down as liquid to
the flask which acts as reflux. The condensed vapour from condenser is collected in the collector.
The volume percent distilled is plotted against corresponding temperature which provides the
Flash point and fire point of a definite petroleum cut are measured in a standard apparatus.
Different fractions may be classified based on their flash point, such as, fractions below 230C
flash point are highly inflammable, dangerous, between 23 to 660C are moderately inflammable
and above 660C are termed as safe. Gasoline lies in the first category, kerosene in the second and
higher oils, such as fuel oil and gas oils fall in the third. Abel apparatus is used for the oils
having flash point below 500C and Pensky-Martens apparatus is used for the oils having flash
point above that temperature. Both these apparata are of closed cup type. A specified amount of
oil is heated in a definite rate in a closed cup. A provision is made to remove a shutter to escape
the fuel vapor and air mixture and an external flame is introduced to test the flash. The
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temperature of the oil is determined by a thermometer at the time of flash. That is the flash point
of the oil. Flash point of heavy oils such as lubricating oil, crude oil or residue can be obtained
Pour point and cloud point of oil is determined by putting the oil in a specified test tube, where
thermometer is inserted. The oil is cooled at specified rate and at short interval of time the oil is
tested by tilting the test tube to see whether the oil is totally freezed. The maximum temperature
where oil does not show any flow, that temperature is recorded and the reporting of pour point is
done by adding 2.80C or 50F to that. Before that freezing temperature comes, haziness on the top
of the surface of the oil appears and that temperature is the cloud point of the oil.
Smoke point of kerosene is determined in a standard apparatus (Fig. 3) where kerosene is poured
in a specified holder (B) with wick on its top. The wick is lit and the height of its flame is so
adjusted that it gives a flame with round top, without any shoot tail. The maximum flame height
is measured by a mirror scale (C) at the back of the flame in the instrument. An exhaust (D) is
provided at the top of the apparatus. A lid with glass cover (A) is used to cover the flame from
wind.
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Burning quality of kerosene is determined by its char value. A standard lamp with glass chimney
is used. Definite amount of kerosene is poured in the lamp and wick is lit. The lamp is allowed to
keep lit for continuously 24 hours. The flame heights, initially and finally, are measured by the
graduation on the chimney. The char deposit on the wick is scraped and weighed. The mg of char
per kilogram of kerosene burned is expressed as char value of that kerosene. The decrease in
standard apparatus in which normally carbon dioxide, carbon monoxide, oxygen and nitrogen
gas composition in volume percentages are reported. Sometimes, hydrogen, methane and
unsaturated hydrocarbons can also be quantified in this apparatus. The apparatus consists of a
graduated burette upto 100 divisions from bottom to top, an aspirator bottle, a series of pipettes
having necessary fittings and a three way stop-cock connecting the top of the burette to the gas
stream. Each pipette contains an adsorbent for removing one gas. The adsorbents used are caustic
potash solution, freshly prepared alkaline pyrogallol solution and cuprous chloride in ammonia
solution for removal of CO2, O2 and CO respectively. Unsaturated hydrocarbons can be removed
by using bromine solution. The measured gas sample is passed to the pipettes one by one, by
opening the stopcock and raising and lowering the aspirator bottle several times for ensure
proper adsorption of gases in the respective adsorbents. The decrease in volume from the initial
volume after passage through every pipette gives the account of volume percent of the respective
gas. Fig.4 shows the typical figure of Orsat apparatus, where A is the graduated glass tube and E,
D, C and B are the pipettes containing KOH, alkaline pyrogallol, bromine solution and
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ammoniacal cuprous chloride solutions respectively, F is the stopcock, G is aspirator bottle, H is
rubber tube.
The gross calorific value at constant pressure of a gaseous fuel is determined by Junkers
calorimeter. The method is based on heating a particular amount of water by the heat evolved by
burning of the gaseous fuel. The calorific value is calculated by the following formula
∗ ∗ ,
.
.
Where, w= weight of hot water collected in kg, V= observed gas volume in m3, T= temperature
rise of water in 0C, Cp, water= Specific heat of water in kCal/kg.K, and K=[(Pa+Pg-Pw) in mm
Hg/760 mm Hg]×273/(273+t), where, Pa and Pg are the atmospheric and gauge pressure of gas,
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Reference
1. McGraw Hill Encylopedia of Science & Technology, no. 4, 9th edition, McGraw Hill,
2002.
2. Fuels and combustion, S. Sarkar, 2nd edition, Orient Longman Ltd., 1990.
Company, 1987.
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Keywords: Coal formation, rank, fuel ratio, basis
Coal is called a fossil fuel because it was formed from the remains of vegetation that grew as
long as 400 million years ago. Most of our coal was formed about 300 million years ago, when
much of the earth was covered by steamy swamps. As plants and trees died, their remains sank to
the bottom of the swampy areas, accumulating layer upon layer and eventually forming a soggy,
dense material called peat. Over long periods of time, the earth's surface changed, and seas and
great rivers caused deposits of sand, clay and other mineral matter to accumulate, burying the
peat. Sandstone and other sedimentary rocks were formed above peat layer, and the pressure
caused by their weight squeezed out the water from the peat. Increasingly deeper burial and the
heat associated with it gradually changed the material to coal. Scientists estimate that 3 to 7 feet
of compacted plant matter was required to form 1 foot of bituminous coal. There are many
compositional differences between the coals mined from the different coal deposits worldwide.
Coal formation is a continuing process and some of our newest coal is a mere 1 million years
old. Today, in areas such as the Great Dismal Swamp of North Carolina and Virginia, the
Okefenokee Swamp of Georgia, and the Everglades in Florida, plant life decays and subsides,
eventually to be covered by silts, sands and other materials. Perhaps millions of years from now,
For the use of coal in various purposes, there is a need of widely acceptable classification of
coal. The very old and earliest classification was based on the visual observation and the burning
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Lignite or brown coal: Brown or Black color woody substances with high moisture contents
Bituminous coal: Black in color, easily ignites, and burns with flame and smoke.
Anthracite: Black color and lustrous, difficult to ignite and burns without flame.
In 1837, Regnault first classified the coal based on chemical composition in five categories on
the basis of total oxygen and nitrogen percentage using the ultimate analysis of coal.
In 1844, Walter R. Johnson, divided them according to the ratio of fixed carbon to the volatile
matter which is defined as Fuel ratio. P Frazer (1887) used the fuel ratio to classify the various
Sub-bituminous 8-5
Bituminous 5-0
The different types of coal are usually classified by rank which depends upon the degree of
transformation from the original source (i.e., decayed plants) and is therefore a measure of a
coal’s age. As the process of progressive transformation took place, the heating value and the
fixed carbon content of the coal increased and the amount of volatile matter in the coal
decreased. The method of ranking coals used in the United States and Canada was developed by
the American Society for Testing and Materials (ASTM) and is based on a number of parameters
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The following table (Table 1) discusses about different grades of Indian coal, their characteristics
and uses.
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Analysis of coal is also reported in terms of some arbitrary basis. These are,
1) Run-of-mine (ROM)
When the coal directly obtained from a mine is analysed by elemental or proximate
2) As-received
After extraction from mine, coal is transported to the receiver. The analysis data obtained
3) Air-dried
When analytical data are collected after air drying the coal at a standard condition of
400C and 60% relative humidity, the data are called at air-dried basis.
4) Dry
When the effect of moisture content is removed from the analytical data, then that is said
as dry basis.
When the data are reported excluding the effect of ash content, then it is said d.a.f basis.
When effects of both moisture and mineral matter are removed from the analytical data,
then it is reported as d.m.m.f basis. This is the data of pure coal only.
7) Moist-mineral-matter-free
This is the basis where the effect of mineral matter is excluded, only pure coal and
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References:
1. McGraw Hill Encylopedia of Science & Technology, no. 4, 9th edition, McGraw Hill,
2002.
2. Fuels and combustion, S. Sarkar, 2nd edition, Orient Longman Ltd., 1990.
3. Fuels combustion and furnaces, John Griswold, Chemical engineering series, McGraw
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Introduction:
The technology required for recovering coal from the earth crust and transporting it into coal
processing unit, is the vital step of the coal mining. This technology is based on the following
operations:
1. Extraction of coal: the method used to break out the coal deposit into smaller lumps
3. Ventilation: the development of proper air circulation system within the mine
4. Ground control: to control and prevent the sagging of the underground or surface opening
a) Surface mining
b) Underground mining
Surface mining
Surface mining is generally used when the coal seams are found within 200 ft below the earth
surface. Very big utilities and machineries are used for removing soil and rocks to expose the top
layer of coal. After extraction of coal, the rocks and soils are returned to fill up the holes of the
mine and the whole surface on the ground is properly revived to its original state and can be used
for cultivation, industrialization etc. This process is comparatively less expensive compared to
underground mining.
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The prime considerations for this technique are geographic location, thickness and removal of
inter-seam waste and overburden and quality of the coal to be produced. Much of the overburden
contains layers of shale, limestone or sandstone and must be blasted before it can be removed.
After the overburden is removed coal is usually drilled and blasted, then loaded into coal haulers
with a shovel. Removal of overburden is called stripping and hence it is called strip-mining. The
b) contour mining
d) auger mining
Area mining is applied in relatively flat coal seams, where they are expanded in large area at
different depths. In this type of mining, overburden is removed by scrapers and placed outside
the mining area and loaded into trucks. Mining begins with drilling and blasting waste rocks to
expose the coal seams. Coal is removed and transported. The size and depth of the pit are
increased as mining progresses. This mining is adapted for the case when several seams lie in
parallel.
Contour mining and mountain top removal are used in hilly areas. Contour mining creates a shelf
or bench on the side of the hill. The mountain top mining process involves the removal of upto
1000 vertical feet of overburden to expose underlying coal seams. The overburden is often
scraped into the adjacent valley which is called a valley fill. Overburden is the soil and rocks to
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Augur mining is another type of surface method where overburden removal is uneconomic,
where terrain is too steep for overburden removal and where the underground method is
impractical or unsafe. It involves boring of large diameter holes into more or less horizontal coal
beds. In this method, exposed surface of the coal is drilled and removed by means of an auger.
Single, dual or triple auger heads can remove upto 90 inches of coal for a distance of about 200
ft.
Underground mining
When a coal seam does not appear near to the surface, it must be extracted by underground
process. Different underground methods may be classified as 1) room and pillar, 2) longwall.
Room and pillar is a mining technique in which the coal is extracted across a horizontal plane
making horizontal arrays of rooms and pillars to support the roof created due to extraction. In
this type of mining, the methods may be either continuous or conventional. The main differences
between the two methods are in the nature of equipments and the face of operation. In
continuous mining, the cycle begins with the continuous sumping cut into the coal face by a
continuous cutter or miner. A shuttle car is positioned behind the miner to receive and transport
the cut coal to the belt feeder. When the shuttle car is filled up, it moves away and the next
shuttle car is fed by the new cut and the cycle is continued.
In conventional mining, the breaking out of the coal from the face is done by cutting, drilling and
blasting operation. At the beginning, the cutting machine cuts a 3-4 meter slice horizontally
across the room width and then moves out to the next place to be cut. Then the drilling machine
moves in and drills holes into the cut face. This is followed by the blasting operation. Now the
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area is examined for safety for entering the loading machine and shuttle car. The cars come in for
The system of rooms and crosscuts driven in the production panels divides the panels into a
series of coal pillars. These coal pillars are extracted by the methods that allow the mining
operation to retreat toward the panel entries. Since methane gas may accumulate into the caves, a
Longwall mining is a typical form of underground coal mining where a long wall of coal is
mined in a single piece of around 3–4 km long and 250–400 m wide. Longwall mining has a
greater production activity than room and pillar arrangement and is safer, as mining is done
beneath a complete overhead steel canopy that moves forward as the face of coal deposit is
mined.
Three pieces of equipment are fundamental for modern longwall mining: armored face
conveyors, powered supports and the coal cutting machine. The cycle of face operations is based
on the movement of this equipment. The armored face conveyors are erected along the coal face
and are connected to a power support with the help of hydraulic jacks. The cutter loaded usually
slides along the top of the conveyor and breaks out a 20-30 in coal strip. The broken coal chunks
Other methods
Shortwall mining is an alternative method which uses the equipments of both room and pillar
and longwall mining. The shortwall layout is similar to the longwall panels except that the panel
width is 150-200 ft wide. A continuous miner loading shuttle cars substitute for the cutter-loader
face conveyor system. Shortwall mining is applied to relatively shallow coal seam.
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Hydraulic mining uses large amount of water at high pressure to break and convey the coal
from the working area. It is usually applied for a relatively thick coal seam. Jets of water are
Modification of the equipments and methods for underground mining system to overcome the
difficulties of mining is a great challenge for the future. Remotely controlled longwall and
continuous miners may be adopted for higher productivity and improve safety.
References:
1. Fuels and combustion, S. Sarkar, 2nd edition, Orient Longman Ltd., 1990.
2. McGraw Hill Encylopedia of Science & Technology, no. 4, 9th edition, McGraw Hill,
2002.
3. Elements of practical coal mining, 2nd edition, by D. F. Crickmer (Editor), David A.
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Keywords: run-of-mine coal, coal preparation, washing, Float and sink test
The raw coal extracted out from the coal mine is processed through different techniques to
achieve the desired qualities. These result in higher economic value of run of-mine (ROM)
coal. The environmental impacts of burning of coal should also to be kept in mind. The
mineral matters of the coal should be reduced during its processing such that, the
emissions of sulphur dioxide (SO2), carbon dioxide (CO2) and particulate matters are
minimized during burning. The technology of coal washing or coal preparation is applied to
produce specific desirable coal products from the run-of-mine coal without the change of the
physical identity.
In the early days, the coal in the form of lumps were supplied for domestic use and the
intermediate sizes were kept for the industrial use, whereas, the fines were rejected. The
sizing facilities were gradually developed. The sophisticated handling and screening facilities
were introduced into the market as per customer requirements. Recently, the demand of
smaller sized coal has increased. The larger sized coals are kept for their shipment. The
washing technique was first introduced in Europe in 1918 and later “Chance” washer was
used. The washer utilized sand and water as medium. In course of time, many other types of
washing technology have been introduced and then they were modified or rejected according
to the need.
Coal preparation
Coal preparation includes blending and homogenization, size reduction, grinding, screening
and handling. The most important step is coal beneficiation or cleaning. The cost of coal
preparation depends on the methods used and also on the degree of beneficiation required,
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which is greatly determined by the market demand of the product. Almost all coal used for
electric power generation and industrial boilers is either pulverized or crushed and sized
increases in net heat content but it reduces the dust, ash content, transportation and shipping
costs. Ash content also increases wear in coal grinding equipment and boilers.
Therefore, coal can be subjected to different levels of cleaning, depending upon its type, its
utilisation with consideration of the cost of cleaning. Very dirty coals containing large
amount of extraneous mineral material, could only match the market specification after
substantial cleaning. The final selling price of these coal is determined by the cost of the
cleaning steps. The equipments used for washing of coal include centrifuges, froth flotation
crushing
dewatering
thermal drying
blending
agglomeration or briquetting
Coal preparation process starts with crushing and screening of freshly mined coal, which
removes some of the non-coal material. Mechanical cleaning or “washing” is actually the
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liquid medium. The liquid medium may be an aqueous solution or organic liquid.
Sometimes a heavy, finely ground mineral, such as magnetite is added in the liquid medium
to ensure the removal of unwanted rock and mineral matter from coal particles. Wet or
“hydraulic” cleaning technique is a process which includes agitation of the coal-liquid feed
flotation to recover fine coal particles. To meet environmental regulations, modern wash
plants are able remove around 40 percent of the inorganic sulfur in coal. A rarely used
technique is dry technique in which coal and non-coal materials are segregated by vigorous
shaking and pneumatic air-flow separation for crushed feed coal. Dry technique is used
Prepared coal is usually dewatered to some extent as excess moisture lowers the deliverable
heat content in the coal and increases the weight of coal. Dewatering equipments includes
less costly vibrating screens, filters, or centrifuges to the more costly heated rotary kilns or
dryer units.
Washing of coal represents the most important step of coal preparation. The raw run-of-mine
coal must require some selective qualitative and quantitative analysis for finding out the most
suitable operating conditions for cleaning of coal to obtain the desired quality. Among these
Washability test
The washability test method can be used to investigate the cleaning characteristics of coarse-
and fine-coal fractions. However, especially with the fine-coal fractions, this test method may
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(1) To find the relative ease for separation of coal from the refuse based on the
The washability test is done by float and sink method. The float and sink test is an important
In the float and sink method, the freshly mined coal lumps are first crushed into different size
fractions such as, 50-25, 25-13, 13-06, 06-03, 03-0.5 mm through screen analysis. The
different fractions of the coal are separated by washing with different specific gravity organic
chloride or other inorganic salt. Each of the individual size fractions are subjected to
sequential float and sink tests with different density liquid. The liquid solutions of varying
density with a very small difference in specific gravity such as 0.01 are prepared within the
range of 1.25 and 1.9. Sometimes the density range may be broadened upto 2.25 depending
on the type of coal. The different size coal samples are immersed into organic solution of
known specific gravity, then the float and sink fractions of coal obtained in the washabilty
test are separated out. The ash content of each fraction is determined. The float-sink test can
be performed on samples ranging in size from bulk samples to bench-scale of coal samples.
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By using liquid of different specific gravity the coal samples are divided into number of
fractions with increasing order of specific gravity and hence, of ash value. From the results of
By the analysis of plots i) and ii), as shown in Figure 1, the ash content of the clean product
(float) and waste material (sink) are obtained by washing with a particular specific gravity of
liquid.
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Among the widely used washers, jig washer is one of the important one. In a jig washer,
coal is supported on a perforated tray and a continuous periodic flow of water is applied in
both upward and downward direction. While washing by this way, clean coal is accumulated
in the upper layer of the bed while unwanted heavy non-coal part settles at the bottom. The
A typical jig washer is shown in Figure 2, which is called baum jig. It consists of a U-shaped
chamber, divided vertically by a partition in two parts. One section is washing chamber and
another one is air chamber. Feed coal is fed in the washing compartment and compressed air
is passed in air compartment for generation of pulse in water. Cleaned coal carried out by the
water flow over a weir and the refuse sinks at the bottom. Refuse is removed time to time
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References
1. Blast furnace theory and practice, edited by J.H. Strassburger, Vol.1, Gordon and Breach,
3. Fuels: Solid , liquid and gaseous fuels , J. Brame and King, Kessinger Publishing, LLC,
2007.
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Keywords: Combustion, devolatilisation, mechanism, heating value
Coal can be used in a wide variety of ways, such as, direct burning to produce heat, to generate
steam, gasification and liquefaction. In combustion processes, pulverized coal is preferably used,
as finely ground coal has more surface area per unit weight than larger particles. Coal is finely
ground so that 70 to 80 percent by weight passes through a 200-mesh screen. Coal powder is
burned in a combustion chamber in a flow of air. The resulting energy is used to generate steam
On heating of pulverized coal sample in presence of air, the following changes occur:
1. Loss of entrapped gases inside the coal, such as, methane, ethane, carbon dioxide,
The decomposition of organic matter of pit and lignite starts at or below 1000C and of
bituminous coal at 200 to 3750C. Release of volatile matter due to thermal decoposition is termed
as devolatilisation. As the coal is heated, the weaker bonds of the coal compounds rupture at
lower temperatures and stronger bonds break at higher temperatures. The resulting products of
devolatilisation are oxides of carbon, pyrolysis water, hydrocarbons and hydrogen, which are
collectively referred to as volatiles. These escape through the solid carbon matrix to the
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surrounding environment. Except these products, some heavy and highly reactive species, such
as tar, may also undergo secondary reactions such as cracking and polymerization. The end
products of devolatilisation are carbon-rich residue and hydrogen-rich volatile part than the
parent coal.
The chemical changes of coal during heating depend on the type and rank of the coal, rate of
heating, maximum or peak temperature of heating, retention time at the highest temperature,
pressure, particle size etc. Among the occluded gases in the coal, carbon dioxide and methane are
driven off first at 2000C. Above this temperature, a certain amount of condensation occurs within
the coal mass, with the evolution of carbon dioxide and water. The extent of these reactions is
more for lower rank of coal. In the temperature range 200-5000C the organic sulfur compounds
of coal decomposes with the evolution of hydrogen sulfide and other organic sulfur compounds.
Along with these, decomposition of the nitrogenous compounds begins to release nitrogen and
ammonia. Except these gases, methane and its higher homologous compounds and olefins are
formed in this temperature range due to the decomposition of the higher organic coal
compounds. Oxygen content of coal is reduced to some extent and appears in the gases evolved
mainly as water and oxides of carbon. The evolution of hydrogen usually begins at around
400-5000C. At about 7000C, a sharp and rapid evolution of hydrogen and carbon monoxide
occur. In general, as the temperature increases, the release of hydrogen, carbon monoxide,
methane and nitrogen increase while higher hydrocarbons decrease. The expulsion of oil vapours
from coal starts at about 300-4000C, the yield of tar usually increases to a maximum at 500-
6000C. As the temperature increases, the aromatic nature of light oil and tar increases. Thus,
the liquids produced from coal at lower temperatures consist mainly of hydroaromatic
compounds, small quantities of higher olefins and paraffin hydrocarbons with very little aromatic
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compounds of the benzene series. Another product, higher temperature tar contains quite high
When devolatilization is carried out under inert conditions, it is termed as pyrolysis. Pyrolysis is
of commercial importance in coal liquefaction and in the production of chemicals from coal tar.
High temperature devolatilization carried out at above 1200 0C, is known as carbonization.
reactions in coal gasification processes. In addition to the above, coal can devolatilize in an
oxidizing or combusting environment, such as the that present within a combustor as discussed
above. Some coal samples on heating exhibit thermoplastic behavior with the formation a highly
viscous non-Newtonion liquid after melting. They are referred to as plastic coals or softening
coals. After solidification, these coals form 'cakes' and are therefore also called caking coals. The
nature of the decomposition products of coal depend on the rank of coal. The degree of
aromatization in coal structure increases with the increase in the rank of the coal. Lower and
higher rank coals, such as lignite and anthracite, decompose before melting and these are the
noncoking and weakly coking coals. Coals of intermediate rank, such as, some verities of
bituminous coals are coking. Thus, the primary physical changes which occur on heating a coal
sample depend on the differences between the tendencies of the coal to melt and decompose.
The process of devolatilization in small particles is found to be isothermal and usually kinetically
controlled, if the heating rates are not very high, however, as the particle size increases, the
surface to volume ratio decreases and the combustion may become primarily diffusion
controlled.
The devolatilization of large particles is significantly different from that of small particles, due to
the presence of heat and mass transfer resistances which act in these particles. These resistances
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not only affect the volatile release rate but also the product yield and distribution. Recently, there
has been a considerable interest in understanding the devolatilization behavior of large particles,
typically greater than 1 mm as, commercial fluidized-bed coal combustors and gasifiers make
The size of the coal particle is one of the important factor for devolatilization time. Usually,
longer time is needed for bigger particles to devolatilize than the smaller particles and this is
because the heating up of the former particles takes more time than the latter particles. It was
found that particles of sizes up to 2.60mm devolatilize in less than 10 sec after they are added in
Mechanism of devolatilisation
Pyrolysis reactions starts with the rupture of bonds. As lesser energy is required to break the C-C
bond(83-85 kcal/mole) than C-H bonds (98 kcal/mole), pyrolysis reactions commence when the
temperature is close to 4000C. The pyrolysis begins with the cracking of C-C bridges between
the ring systems resulting in the formation of free radical groups, such as -CH2, -O-, and other
bigger radicals. These free radicals are highly reactive and after combining to each other in the
gas phase produce smaller chain aliphatic, mainly methane) and water which diffuse out of the
coal particle. The polynuclear aromatic compounds start to condense with the elimination of
hydrogen. The final product due to condensation reactions is coke. Along with this, CO is also
produced by the cracking of heterocyclic oxygen groups at high temperatures. The following
typical reactions usually take place at different stages as the temperature is increased.
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Cracking:
Saturation:
CH3 + H* CH4
·
OH + H* H2O
·
Tar production:
R-CH2 + H· R-CH3
·
Condensation reaction:
The R radical is obtained from benzene, naphthalene, phenanthrene etc. In addition, oxides of
The heating (calorific) value of coal and char is a significant factor in the conversion of coal to
other useful forms of fuel, as well as in its direct use. The heating value of a fuel may be
calorimeter.
In the former method the calculation should be based on the ultimate analysis, which gives the
account of elementary constituents of carbon, hydrogen, oxygen, nitrogen, sulphur, ash and
moisture. A proximate analysis, which determines only the percentage of moisture, fixed carbon,
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volatile matter and ash, without determining the ultimate composition of the volatile matter,
cannot be used for computing the heat of combustion with the same degree of accuracy as an
ultimate analysis. The following table shows heating values of some of the coals
Anthracite 30080
Bituminous 33412-25047
Sub-bituminous 21319-20830
Lignite 16077
Anthracite has lower volatile matter than bituminous coal, hence the high-volatile variety of
Coal is one of the principal potential sources of fuel for energy generation and a valuable raw
material for industrial chemicals. But unfortunately, coal contains many impurities like sulfur,
nitrogen, sodium, potassium and other toxic impurities. To avoid environmental pollution, the
emission levels of these contaminants must be kept as low as possible during combustion.
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References:
nd
1. Fuels and combustion, S. Sarkar, 2 edition, Orient Longman Ltd., 1990.
2. Chemistry of coal utilization, 2nd supplementary volume, Elliott M.A.(ed.) Wiley, 1981
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Keywords: Combustion, ignition point, fixed bed, fluidized bed, overfeed, underfeed
Combustion of Coal
Combustion may be defined as the rapid high temperature complicated chemical reaction of
oxygen with carbon, hydrogen and sulphur of coal. These reactions follow mainly four steps.
2. Decomposition of these complexes with the generation of CO2 and H2O molecules
and formation of carboxyl (COOH), carbonyl (C=O) and phenolic -OH groups along
3. Decomposition of these groups to produce CO, CO2, H2, H2O and hydrocarbons such
4. Decomposition of aliphatic structure with the formation of CO, CO2 and H2O.
In low temperatures, the first step is developed faster than others. Oxygen molecules are
diffused through the pores into the internal surface and are attached to the coal surface by
physical adsorption. In this stage, the oxide layer formed due to the exposure of coal
surface to the air, prevents the diffusion of oxygen partially and oxidation rate is
decreased with time. The reactions between oxygen and coal are exothermic. The reaction
rate increases as the temperature increases and as a result coal reaches to ignition
temperature at about 175°C with the firing of a flame. The time required from the
Ignition point is the temperature at which the temperature of the combustible material
should be reached before it is combined with oxygen and combustion takes place. For
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complete combustion to take place, sufficient time must be allowed before the
form CO2. Incomplete combustion occurs when coal does not unite according to the
reaction stoichiometry. In this type of combustion carbon monoxide, CO, may be formed
which may be burned to carbon dioxide by the reaction with more oxygen. The hydrogen
and oxygen combine to produce H2O vapour. Sulphur is converted to SO2, which in
dissolution with water forms sulphuric acid. It is clear that the amount of oxygen required
for the combustion of a definite species or compound is fixed. Calorific value of each
air. In most of the cases, excess, amounting to double or more than the theoretical supply
of oxygen is required, depending upon the nature of the fuel to be burned and the method
of burning it.
The reason for this is that it is impossible to bring each molecule of oxygen in the air into
intimate contact with the particles in the fuel that are to be oxidized. It has been shown
experimentally that coal usually requires 50 per cent more than the theoretical net
calculated amount of air, or about 18 kg per kg of fuel either under natural or forced draft.
If less than this amount of air is supplied, the carbon burns to carbon monoxide instead of
Coal combustion is extensively used for both industrial and domestic purposes. Boilers of
power plants, industrial boilers and heat kilns consume most of the world’s reserve of coal.
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Space heating and domestic use is responsible for consumption of a large amount of coal in
each year.
The technology of the combustion of coal mainly depends on the type of coal firing
equipment, coal-feeding method and the type of combustion devices, such as fixed-bed
Fixed Bed Combustion: In fixed bed, coal particles are supported on a grate in the
combustion chamber. The combustion air is allowed to flow through the coal bed in upward
direction either by a chimney draft or by a fan. Coal may be fed to the bed in different ways
The grate may be fixed or movable. In case of overfeed technique, the fresh coal particles are
spread over the bed. The different combustion reactions may take place in different
combustion zone starting from grate to bed surface as shown in Fig.1. The combustion air
passes through the grate and moves upward. This becomes heated in the hot ash layer and the
combustion takes place in the coal-char with the formation of carbon dioxide. This reaction is
highly exothermic and generates heat. The bed-temperature rises rapidly in this zone and the
Depending on the thickness of the bed, two different types of combustion chamber may be
constructed, either shallow-bed or thick-bed. The air is supplied from the bottom as well at
top of the bed in both upward and downward respectively. Air supplied from the bottom is
called the primary air and the air supplied at the top is called the secondary air, which is
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Coal may be fed under the bed. This is known as underfeed technique. The burning coal
moves in cross current with respect to the flow of combustion air. The volatile matters,
moisture and combustion air pass through the bed with the formation of less smoke. Air is
supplied by a fan.
The use of fluidized bed combustion is advantageous over the traditional firing system as it
has simpler boiler design, higher combustion efficiency and reduced pollution problems.
If air is passed upward through a bed of solid particles, the bed of particles remains stationary
at low velocity. As the air velocity is increases, the particles become suspended in the air
stream. At a particular air velocity, the particles behave just like a boiling fluid. Under this
condition, the bed is called fluidized bed. If the air velocity is further increased, there will be
bubbling fluidized bed with the flow of air bubbles. At a certain higher velocity, the particles
are blown out of the bed. The particles can be recycled by mixing with the air feed. Then the
bed is called circulating fluidized bed. In a fluidized bed combustion, sand particles may be
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used as heating medium. The sand particles are first fluidized state and heated to reach the
ignition temperatures of coal at about 9000C . Then coal is injected into the bed and burnt.
Three types of fluidized bed combustion are used depending on the state of fluidization and
coal, type of fuel feed and fed into the combustion chamber. The atmospheric air, which acts
as both the fluidization air and combustion air, is delivered at a pressure and flows through
the bed after being preheated by the exhaust flue gases. The velocity of fluidising air is in the
range of 1.2 to 3.7 m/sec. The rate at which air is blown through the bed determines the
Pulverised coal combustion technology is mainly used for steam generation in the power
In combustion oxygen may be supplied by using fresh air or a stream rich in oxygen to ensure
complete combustion. Oxy fuel combustion is an example of a process where the second type
Oxy-Fuel combustion
plant. In a traditional power plant, the pulverized coal used for fuel is fired and the oxygen
comes from injected air. In oxy-fuel combustion, oxygen-enriched gas mix is used instead of
air (about 95% oxygen). The use of higher concentration of oxygen results in a very high
flame temperature in the boiler. Sometimes, the enriched oxygen stream with recycled flue
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gas (RFG) is used. This also may decrease the flame temperature to a level similar to that of a
conventional air-fired boiler. But the justification for using oxy-fuel is that it produces a CO2
flue gas. It has an advantage. The gas injected into the boiler with pulverized coal must be
preheated to meet operating conditions in the boiler. As the RFG is already a hot gas stream,
preheating is not required. After the combustion process, sulfur, water, particulate, and other
gases can be removed from the flue gas. The remaining gas consists mostly of carbon dioxide
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Reference:
1. Coal Combustion and Combustion Products, Xianglin Shen, Coal, Oil Shale, Natural
Bitumen, Heavy Oil and Peat, Vol-1, Ed. G. Jinshen, East China University of Science and
Technology, Encyclopaedia of Life Supports System.
2. http://www.netl.doe.gov/technologies/coalpower/cfpp/technologies/oxy_combustion.html
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Keywords: Coal tar, distillation, caustic washing, deoiling
Coal tar is obtained as a by-product while processing coking coal to form metallurgical coke in a
recovery-type coke oven plant. Coal tar accounts for around 3.5-4% of coke produced. Coal tar
pitch is a complex chemical mixture of phenols, cresols and xylenols (which together termed as
tar acids), polycyclic aromatic hydrocarbons (PAH), and heterocyclic compounds, which can be
distillation is coal tar pitch, which is further processed into coal tar pitch of desired chemical and
physical properties.
The primary objective of coal tar distillation process is to produce a number of tar acid products
from the crude tar. Tar distillation plant (TDP) consists of the following sections,
v) Recasting section
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Tar distillation section
The purpose of tar distillation is to 1) dehydrate the tar in the dehydration column, 2) remove the
pitch from dehydrated tar in pitch column and 3) separate tar oils in fractionating column.
The crude tar stored at elevated temperature in the storage tank is drawn through crude tar filter
and mixed with caustic soda pumped from caustic tank by dosing pump. The mixture is pumped
through tar vapour exchanger and steam-heated preheater to the bottom of the dehydration
column. In the column the crude tar is contacted with a relatively large stream of hot dehydrated
tar. The azeotropic water and oil mixture is vaporized and goes up to the top of the column and
condensed in a light oil condenser. A portion of the azeotropic light oil is sent back to the
column as reflux and the remaining portion is sent to an azeotropic distillation column. The
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bottom fraction of the dehydrator column is pumped at a high rate through pipe-still economizer
and heated. This bottom fraction is dehydrated tar, some part of which is sent back to the lower
In pitch column the dehydrated tar is mixed with a relatively large stream of hot circulating
pitch. The more volatile oils in the tar are vaporized and rose up through the column. Stripping
stream is injected in the column to run the operation. Crude pitch is drawn from the bottom of
the column by pitch circulating pump and heated by a pipe-still heater. Some part of this pitch is
put into the top of the column for contacting with the dehydrated tar.
Volatile portion along with the stripping steam are recovered from the pitch column and
separated into the light oil and water fraction, a middle oil fraction and a heavy oil fraction. The
light oil and water fraction combines with the same stream from the overhead of dehydration
column and are sent to light oil condenser and then to a decanter. Middle oil flows by gravity
through middle oil cooler either to middle oil buffer tank or directly to the mixing vessel in the
caustic washing section. Middle oil can be transferred from buffer tank to the caustic section as
per requirement.
Middle oil from the tar distillation section is counter currently contacted with a flow of 10%
caustic soda solution. The system consists of three mixing vessels and three separators, situated
alternatively. Middle oil, stripped of its tar acids, flows by gravity from top of the separators to
the middle tank. The caustic solution, which is sodium phenolate solution mainly after contacting
with oil, flows by gravity from the bottom of the separator to phenolate tank.
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De-oiling section
The sodium phenolate solution contains small amount of middle oil, which must be removed to
get good quality of tar acids. Sodium phenolate solution in buffer tank is pumped via overhead
exchanger into the top of the sodium phenolate stripping column. Stripping steam is introduced
at the bottom of the column which strips out the middle oil from the sodium phenolate solution.
The overhead vapour heats the incoming sodium phenolate solution and cools down. Clean
sodium phenolate solution is recovered from the bottom of the stripping column and sent to the
Springing section
The objective of this section is to recover tar acids from sodium phenolate solution by springing
with a carbon dioxide rich gas in a series of two packed column in counter flow. Gas is passed in
upward motion through the descending sodium phenolate solution in the first column, where
sodium carbonate is formed. The bottom of the first column is introduced at the top of the second
column where the stream is again contacted with carbon dioxide counter currently. The sodium
carbonate solution is sent to a separator from the bottom of the column. Crude tar acid collected
and stored in the tar acid buffer tank. Carbon dioxide rich gas is continuously bubbled through
the crude tar acid buffer tank to reduce the alkali and water content of tar acids.
Recasting section
In this section, the sodium carbonate solution from the springing section is concentrated with
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Recovery of tar acids
Crude wet tar acids recovered from springing plant contains little amount of water and pitch. It is
pumped to the top of the dehydration column which operates under vacuum, maintained by
ejector system. Azeotropic mixture of water and phenol is stripped out from tar acids and
removed as an overhead vapour. The dry tar acids obtained as bottom product is sent to a
depitching still which is nothing but a kettle reboiler operates under high vacuum. Crude tar
acids are vaporized and condensed in a condenser. The tar acids are flown to a buffer tank which
is fitted with a steam coil to prevent the solidification of tar acids. The phenolic pitch is collected
at the bottom of the kettle, mixed with the heavy oil and sent to a storage tank, jacketed with
steam to maintain the pitch in a free flowing state. The crude tar acids from the tank are pumped
to the primary distillation unit operated under high vacuum. During distillation, the crude tar
acids are separated into three fractions: crude phenol as overhead product, crude cresol as side
stream and crude xylenols/High boiling tar acids (HBTA) as the bottom product.
The crude phenol collected in a tank from this column is pumped to a vacuum column after
heating in a kettle. Pure phenol is collected at the top condenser. A portion of it is sent to the
column as reflux. The other portion is pumped to a storage tank. The residue of this column is
Crude cresol from the storage tank is pumped from the storage tank into a kettle to preheat and
then vacuum distilled in a column. The top product from this column is phenol, which is sent to
the crude phenol storage tank. The first side fraction is o-cresol, next one is a mixture of m- and
p-cresol and the bottom product is crude xylenol/HBTA mixture which is sent to xylenol/HBTA
storage tank.
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Another vacuum batch distillation is carried out to recover xylenol product and HBTA. Crude
xylenols is pumped from the storage tank to a preheater kettle and sent to high vacuum
distillation columns. Four cuts are distilled which require three different column arrangements.
The first cut is a mixture of m- and p-cresol; second cut is of mixed xylenols. Next cut is a
mixture of xylenols and HBTA mixture and the last fraction or residue is HBTA.
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Reference
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Keywords: Direct coal liquefaction, indirect coal liquefaction, EDS process, H-coal process
Coal liquefaction is a process where coal is converted into liquid fuels, mainly to provide
substitutes for petroleum products, which may be either used directly as fuel or converted into
chemicals or other liquid fuels. Coal liquefaction processes were first developed in the early 20th
century. The crude petroleum rich countries may not need this process for fulfilling their
requirement of liquid fuel but the countries which are lacking of crude but have a good reserve of
solid fuel like coal, a large scale applications of coal liquefaction process is entertained. The few
countries, where this process is mainly running are Germany (during World War II) and South
Africa since the year 1960s. USA is also running coal to liquid fuel plant successfully. A plant
using more than six million tons of coal annually could produce more than 3.6 million barrels of
diesel and napththa. China has expended fifteen billion dollar for coal to diesel fuel conversion
plant with the aim of replacing 10% of its oil import with coal converted liquid oils by the year
2013. The threat of depletion of conventional oil sources is another major reason for renewed
interest in the production of oil substitutes from coal since last three decades.
Hydrocarbon type liquid fuels are obtained from solid fuel like coal by the following routes
mainly:
1) Hydrogenation of coal
3) Refining of tar and oil obtained from carbonization of various soild fuels and oil shales
4) Gasification of solid fuels into synthesis gas and conversion of the gas into liquid fuels
and chemicals.
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The following block diagram (Figure 1) shows the options for coal to liquid conversion plants.
Coal liquefaction can be achieved either by direct or indirect processes. The difference between
these two processes lies in that, the indirect liquefaction process needs to go through gasification
first, while, direct liquefaction process involves producing partially refined synthetic oil from
coal. It is said by the technologists that, indirect liquefaction is more efficient than direct coal
Direct coal liquefaction was developed by Friedrich Bergius and it was started commercially in
Germany near World War II, to meet the huge demand of liquid fuel at that time.
Bergius process was modified and extended to fulfill today’s demand of liquid fuel. The coal is
ground so that it can be mixed into coal derived heavy oil recycled from the process to form a
coal-oil slurry feed. The slurry containing 30-50% coal is then heated in a reactor to about 450°C
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in a hydrogen atmosphere between 14-22 MPa pressures for about one hour. Different catalyst
used are tungsten or molybdenum sulfides, tin or nickel oleate. The reaction produces heavy oil,
One tonne of coal yields about one-half tonne of liquids. Processes have been developed to use
coals from low rank lignites to high volatile bituminous coals. Higher-rank coals are less reactive
The liquids produced have molecular structures similar to those found in aromatic compounds
and need further upgrading to produce specification fuels such as gasoline and fuel oil.
Direct liquefaction is of two types, single stage and two stage processes.
In single-stage direct liquefaction process, one primary reactor is used to get distillates. Here a
hydrotreating reactor is also joined along with the primary reactor to improve the quality of the
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o Kohleoel (Ruhrkohle, Germany)
Two reactors in series are used in two-stage direct liquefaction process. In the first stage, coal
dissolution is done where the process is operated either without a catalyst or with a low-activity
disposable catalyst. The heavy coal liquids produced in the first reactor are hydrotreated in the
second stage in the presence of a high-activity catalyst to produce desired distillate. The process
Among the different commercial processes, Exxon donor solvent process (EDS) and H-coal
In this process coal slurry is prepared using a recycled solvent and the slurry is mixed with H2,
preheated and fed to a simple up-flow tubular reactor. The reactor operates at 425-450 ºC and
2575 psig pressure. It is a non-catalytic process. The ligher product naphtha, a middle distillate
and a heavy distillate product are obtained. Heavy distillate mixed with some middle distillate
forms the recycle solvent. The recycle solvent is hydrogenated in a fixed-bed catalytic reactor
operated at 3700C and 1600 psig H2 pressure depending on the extent of hydrogenation, where
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either nickel-molybdenum or cobalt-molybdenum on alumina support is used as a catalyst. A
H-coal process
In this process, coal slurry is prepared with a recycle solvent that consists of a mixture of heavy
and middle distillates obtained by product fractionation and solids containing hydrocracker
product. H2 is added to the slurry, the mixture is preheated and fed to an ebullated bed
hydrocracker, which is the distinguishing feature of the process. The reaction conditions are :
temperature 425-455°C and H2 pressure 2900 psig. The catalyst used is either Ni-Mo or Co-Mo
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supported on alumina. The catalyst is fluidized by H2 and a pumped internal recycle stream. This
H-coal process is described in Fig 3 in the form of a flow diagram. The advantages of ebullated-
bed reactor over fixed-bed reactors are that, the reactor contents are well mixed and temperature
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References:
nd
1. Fuels and combustion, S. Sarkar, 2 edition, Orient Longman Ltd., 1990.
2. Direct Coal Liquefaction Overview Presented to NETL, John Winslow and Ed Schmetz,
Leonardo Technologies Inc., US Department of Energy, March, 2000.
3. http://www.thecanadianencyclopedia.com/articles/coal-liquefaction
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Keywords: Indirect liquefaction, Syngas, Fischer-Tropsh process, MTG process
Indirect liquefaction (ICL) of coal is the process for production of fuels with an intermediate step
syngas are carbon monoxide and hydrogen, which can be processed chemically in further steps
to produce a variety of different chemicals and fuels. Chemicals and fuels that can be made by
ICL include methanol (CH3OH), dimethyl ether (CH3OCH3) and Fischer-Tropsch diesel- or
gasoline-like fuels, and hydrogen (H2). ICL is practiced commercially in South Africa (for
The production of methanol by ICL processes, primarily for the use as a feedstock for different
ether by ICL is also drawing considerable interest as it is a potential feedstock of chemicals, can
be used in compression ignition engine along with diesel for its high cetane rating and non-
sooting nature, can be used as an alternative fuel for LPG because of its high calorific value and
non-toxic character.
The process of ICL involves two steps. The first step is the complete breakdown of coal structure
by gasification process to produce mainly syngas. Sulfur-bearing compounds are removed at this
step. Gasification products are reacted in the presence of a catalyst at definite temperature and
pressure. The synthetic liquid products include paraffins, olefins and alcohol, mainly methanol,
Fischer-Tropsch (FT) synthesis, named after the German inventors, Franz Fischer and Hans
Tropsch in the 1920s, is a well-known catalytic process to produce liquid transportation fuels
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from syngas. During World War II, FT synthesis process provided the liquid hydrocarbon fuels
required for the German in the war. After that, great efforts were made to refine and develop the
new technology for FT process, including catalyst development and reactor design. Depending
on the source of the syngas, the technology is often termed to as coal-to-liquids (CTL) and/or
gas-to-liquids(GTL).
In the FT process, carbon monoxide (CO) and hydrogen (H2) in syngas react catalytically to
convert into hydrocarbons of a series of molecular weights according to the following equation:
Where n is an integer. Thus, when n=1, the reaction represents the formation of methane, which
maximize the formation of hydrocarbons in the range of liquid fuels, which are higher value
products, by using suitable reaction conditions and catalysts. Some other side reactions occur in
CO + H2O → H2 + CO2
FT process produces hydrocarbons ranging from methane to higher molecular paraffins and
olefins, along with small amounts of organic alcohols and acids, depending on the reaction
conditions and catalyst used. The FT reaction is actually a condensation polymerization reaction
of CO. The most common catalysts used are iron, cobalt, nickel or ruthenium. The main focus of
FT process is the production of high molecular weight linear alkanes and diesel fuel. In addition
to the active metal part of the catalyst, they typically contain a number of promoters, including
potassium and copper, as well as high surface area binders/supports such as silica and/or
alumina. As the FT catalysts may be poisoned by the sulfur compounds present in the syngas,
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hence the gas should be made free of sulfur compounds before it is introduced to the
reactor. Originally cobult was used as the catalyst for FT process, but it is highly sensitive to the
sulfur compounds compared to Fe catalysts. Hence, syngas obtained from high sulfur coal is
treated with iron catalysts as those are not much sensitive towards sulfur.
FT reaction is highly exothermic and so, the control of the reaction temperature is a critical
factor. Three types of reactors, fixed bed, fluidized bed and slurry bed may be used commercially
in the process. Fixed bed reactor is known as Arge reactor, which was developed by Lurgi and
Ruhrchemie. Now most of the fixed bed reactors are replaced by slurry bed reactors, developed
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Fig. 1 shows a process flow diagram of FT process. The temperature range used in the FT
whereas low temperature FT process (>3000C) uses cobalt-based catalyst. Although higher
temperature shows faster reaction rate, yet, it favours methane production. For this reason, a
moderate temperature range is most desirable for the process. High temperature FT process is
extensively used in Sasol plant, South Africa, in their coal to liquid (CTL) plants. Low
temperature gas to liquid (GTL) plant is commercialized in Malaysia, built by Shell. Typical
Methanol to Gasoline process (MTG) was discovered by Exxon-Mobil scientists in the 1970’s.
Both the Fisher-Tropsch and MTG processes convert coal into synthesis gas before converting it
to the final liquid products. However, there are some basic differences between them in terms of
paraffinic hydrocarbons that require upgrading to produce commercial quality gasoline, jet fuel
and diesel. In contrary, MTG selectively converts methanol to one liquid product, a very low
After coal is gasified to produce synthesis gas. Methanol is produced from syngas in a methanol
convertor reactor operated at 220-275°C and 50-100 bar pressure on Cu/ZnO/Al2O3 catalyst.
After the production of methanol, it is fed to a fixed bed reactor system where in the first part,
In the second step, the equilibrium mixture is mixed with recycle gas and passed over specially
designed ZSM-5 catalyst to produce hydrocarbons and water. Most of the hydrocarbon products
are in the gasoline range. Most of the gas is recycled to the ZSM-5 reactor.
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MTG reactor effluent is separated into gas, raw gasoline and water. Raw gasoline is separated
into LPG, light gasoline and heavy gasoline. Heavy gasoline is hydro-treated to reduce durene
content. Heavy and light gasolines are re-combined into finished MTG gasoline. MTG process is
exothermic in nature. The Fig 2.describes the reactions involved in production gasoline via MTG
process.
Indian Scenario
Oil India Limited (OIL) has carried out an elaborate study regarding the conversion of various
shales and coals from north-east India into liquid fuels. It has been found that high sulfur, low
ash containing bituminous coal of NE India is quite suitable for liquefaction. OIL embarked a
technology and set up a 25 kg/day pilot plant in Duliajan, assam. The characteristic properties of
coal used are, ash: 2-10 wt%, volatile matter: 40-45% and sulfur: 1.5 to 6 wt%. The assay found
that the coal reserve of around 467 MMT may produce 200 MMT of liquid fuel.
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Reference
1. A Novel Synthesis Route for Coal Derived Syngas, M. Gogate, C. J. Kulik and S. Lee,
ACSFuel, 1100-1106, 1993.
2.http://www.exxonmobil.com/Apps/RefiningTechnologies/files/sellsheet_09_mtg_brochure.pdf
3. Synthetic fuel production by indirect coal liquefaction, Eric D. Larson and Ren Tingjin,
Energy for Sustainable Development, Volume VII, No. 4 l, December 2003.
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Coal gasification technology is efficiently used for converting coal to power, chemicals,
fertilizers, and fuels. This gasification process is a clean technology to decompose coal into
its different components in presence of steam and oxygen at high pressure and high
temperature. This leads to the production of synthesis gas, which is mainly a mixture of
carbon monoxide and hydrogen. Coal gasification can be utilized to produce methanol as
Power generation
Fertilizer
Methanol synthesis
Hydrogen
Hydrocarbons
It provides the only route to convert coal to hydrogen directly. In this process, coal is
combined with oxygen and steam to produce a combustible gas, waste gases, char, and ash.
Where, ‘m’ and ‘n’ depends on the composition of coal. The reactions in different stages of
i) CO + H2O CO2 + H2
ii) C + CO2 2 CO
iii) C + H2O CO + H2
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Gasification
Gas beneficiation.
The selection of an appropriate coal is the important step at the initial stage of coal
gasification. The various sources of coal samples are analyzed and compared in terms of their
costs and compositions. The comparison is generally made by the percentages of sulfur
content, fixed carbon, oxygen, ash and other volatile content. The sub-bituminous coal is
found to have the lowest percentage of sulfur content in comparison with lignite and
bituminous coal.
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One important step in the preparation of the syngas is the removal of acid gas, which can be
composed of H2S and CO2. In high temperature processes all sulfur components in the feed
are converted to undesired products, such as, H2S or COS, which are needed to be removed
by acid gas removal process. Therefore, a low sulfur content coal is a desirable feed.
Then ideally, the syngas mixture would have a 1:2 CO to H2 ratio. Steam can also be used to
produce more hydrogen by reacting it with carbon monoxide via the water-gas shift reaction
CO + H2O CO2 + H2
Coal preparation
In the pretreatment step, coal is first dried and crushed or pulverized for the fluid or entrained
bed gasifiers. The moisture of coals should be removed by drying. Some caking coals may
require partial oxidation to simplify gasifier operation. For feed to fixed bed gasifiers,
Gasification
The pre treated coal is charged into the gasification reactor where it reacts with oxygen (air)
and steam. The gasification reaction usually takes place at high temperatures from 800 to
1900°C and high pressure up to 10 MPa. When coal is burned with less than a stoichiometric
quantity of air, with or without steam, the product is a low-heat-content gas, which after
purification can be used as fuel gas. Using oxygen in place of air produces medium-heat-
content gas. Some of CO must be reacted with steam by shift conversion to get additional
hydrogen. The ashes from gasifier are removed as molten slag or dry condition.
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Shell Coal Gasification Process is the oldest technology and it was the first commercial plant
used in 1956.
The gas products from the gasifier contain a mixture of different gases such as, carbon
monoxide, carbon dioxide, hydrogen, methane, other organic vapours and hydrogen sulfide in
different concentrations. Nitrogen is also present in gas products if air is used. Other
impurities are particulates, and water vapour. The product gas is then purified prior to their
combustion through the removal of particulate matters, such as, coal dust, ash and tar
aerosols. The tar and oil are removed by gas quenching method, and then the acid gas is
removed by solvent treatment technology. The tar and oils are condensed in the gas
quenching and cooling section of the plant. Ammonia is removed from the gas in a scrubber
by using either aqueous or organic liquid. Acid gases such as H2S, COS, CS2, mercaptans,
and CO2 can be removed from gas by an acid gas removal step, where it is treated with a
solvent to absorb the acid gases. Methanol may be used as a solvent to separate acid gases
such as hydrogen sulfide and carbon dioxide from feed gas streams.
In the shift conversion process, H2O and a portion of the CO catalytically react to form CO2
and H2. After passing through an absorber for CO2 removal, CO and H2 remain in the product
gas. They are reacted in a methanation reactor to yield CH4 and H2O. Many processes are
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directly related with a coal gasification process, such as, oxygen plant, power and steam
Entrained-flow gasifiers.
In the moving bed gasifier the coal bed slowly moves downwards counter currently with
respect to air and is gasified. It has the lowest oxygen consumption. Moving bed gasifier can
operate at the lowest temperature, which inhibits the reaction rate and increase the
maintenance cost.
The fluid bed gasifier facilitates good mixing and it has low overall cost. However, the
conversion rate of carbon is lower in comparison with the other two types due to some carbon
lost with the ash. Also the fluid bed gasifier is appropriate for low rank coals like lignite, as
opposed to sub-bituminous. In the entrained flow gasifier, the fine coal particles react with
cocurrently flowing steam and oxygen. Since the gasifier operates at a high temperature, a
good conversion of about 99% is obtained and the destruction of tar and oil yields a very pure
syngas. However, the entrained flow gasifier has a high oxygen demand and also the high ash
Overall, the entrained flow gasifier is chosen as the gasifier technology for its high carbon
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Both the dry and wet coal feed can be used for the entrained gasifier. For dry coal feed, it
requires about 25% less oxygen consumption and which can optimize the gas production rate.
Whereas, a wet coal feed needs more oxygen consumption and it decreases the efficiency due
to the evaporation of water. So, a dry coal feed is much better than a wet feed for the
Two different types of gasifiers are used in dry-coal feed gasifiers, single stage and two stage
gasifiers. In single stage entrained-flow gasifier, gas with high purity is obtained. It ensures
low CO2 and high carbon conversion, in a two-stage gasifier, there is an increase in the
efficiency from single stage gasifier. The oxygen consumption is also less.
Out of the types of gasifiers mentioned above, the Noell gasifier and the CCP gasifier can be
considered. The Noell gasifier is a single stage gasifier. The CCP gasifier is relatively new
and is advantageous because it uses air as the oxidant, which is readily available. The carbon
conversion rate is 99.8%, with a variety of coals. Other advantages include lower NOx and
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Reference:
1. Kinetics of Coal Gasification, James L. Johnson, John Wiley & Sons; 1st edition. 1979.
2. Coal Combustion and Combustion Products, Xianglin Shen, Coal, Oil Shale, Natural
Bitumen, Heavy Oil and Peat, Vol-1, Ed. G. Jinshen, East China University of Science and
3. David A. Bell, Brian F. Towler, Maohong Fan, Coal Gasification and Its Applications ,
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Keywords: Crude, exploration, drilling, oil recovery
It has already been discussed in Module-1, lecture-2, how liquid petroleum is originated and
formed within the sedimentary rocks under the earth. These rocks make a source of reserve of
crude petroleum. The sedimentary rocks which have commercial value in terms of exploration,
are important. These are also called reservoir rocks. The structures of reservoir rocks are of
different types, such as, anticline, syncline, folds, faults, fractures, unconformities etc.
The two essential properties of reservoir rock are porosity and permeability. Porosity is the
storage capacity of the rock. This is sometimes expressed as porosity (%-void ratio) as:
Porosity (%) = × 100
Permeability is the rate of flow of fluid through the rock. It is expressed by Darcy’s law which
states that the rate of flow of a homogeneous fluid in a porous medium is proportional to the
pressure gradient and inversely proportional to the fluid viscosity. Permeability is expressed in
Darcies (D). As most of the petroleum reservoirs have permeability less than 1D, hence
For petroleum oil exploration, the knowledge of underground structures is necessary. In this
respect, the data about the properties of subsurface rocks in those structures should be acquired.
The properties which are encountered in gaining the data are density, elasticity, magnetic and
electrical properties of rocks. All these data collectively gives the idea of occurrence and
commercial exploration of crude petroleum at a definite reserve. The principle of obtaining those
data mainly depends on the use of magnetism, gravity and sound waves and the respective
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Magnetometer is a specially designed instrument which can detect minute differences in the
magnetic properties of various rock structures, which helps to find out the formations that might
contain oil. Except this, magnetometer provides the clue about the depth of basement rock, the
probability of finding the locations which may have anticlines or other oil-favourable structures.
Gravimeter detects differences in the gravity pull between the normal gravity and the gravity of a
subsurface structural formation. This gives the indication of the location and density of
Seismograph works on measuring the shock waves obtained from explosions initiated by
triggering small controlled charges of explosions in the bottom of the shallow holes in the
ground. Usually a series of detectors is placed in a predetermined fashion around the location of
the explosion. The acoustic waves travel outwardly in all directions and some are reflected back
to the ground surface by denser rock formations below. The formation depth is determined by
measuring the time elapsed between the explosion and detection of the reflected wave at the
surface.
Seismic geophysical work is also done on the water. A marine seismic project moves
continuously with detectors at a definite speed and at a definite depth. The detectors are towed
behind the boat. Explosives are charged at a position and time, determined by the speed of the
boat and the reflections are continuously detected at every position of the water level.
Another important exploration process is borehole logging. In this method a well is drilled and
various instruments are employed to log or acquire data at different positions of the well in terms
of the properties of the rocks, such as, electrical resistivity, radioactivity, acoustic or density.
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Drilling operation
Drilling oil and gas well is a complicated and very expensive operation which needs the
knowledge of many aspects of engineering and geosciences. The exploration and drilling
processes are planned in such a way so that the whole process becomes safe and cost-effective.
Oil and gas reservoirs are found in a variety of geographical areas. The wells drilled in these
reservoirs may be onshore or offshore. The depth of the well may vary from 1000 to 30,000 ft
and wells may be drilled either totally vertically or in many cases a larger part of the well may be
drilled in inclined or horizontal manner. Most of the drilling operation is done by rotary method
in which an abrasive bit is revolved at the end of a drilling stem or drill string. Generally, almost
all drilling rigs (the assembly which is used for drilling) are composed of the components: a
power plant, hoisting and rotary machinery, the drill column, a mud circulation system and
auxiliaries. The power used for drilling may vary from 250 to 2000 bhp, a heavy duty mud
pump needs around 700 bhp. Hoisting system is a large pulley system which is used to run and
pull the equipments (drill string and casing) into and out of the well. Rotary system is used to
rotate the drill string and therefore the drill bit, on the bottom of the borehole. Completing a well
and preparing for production of oil involves insertion of a casing which comprises of one or
more strings of tubing. The casing provides a permanent wall to the borehole, prevents cave-in,
blocks off unwanted water, oil or gas from another formation, provides a return passage for the
mud stream and provides control of the well during production. The mud circulating system is
used to circulate the drilling fluid or mud down the drill string, up the drill string to the borehole
annulus and for carrying the drill cuttings from the mouth of the bit to the surface. Drilling fluid
is usually a mixture of water, clay (bentonite), weighing material (barite) and chemicals.
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The mud is mixed and conditioned in the mud pits and then circulated downhole by large pumps.
1. To cool and lubricate the drilling bit and the drill string.
3. To form a gel to suspend drilled cuttings and any weighing materials, when the fluid
column is static.
5. To prevent squeezing or caving of formations and to plaster the sides of the borehole.
Mud is pumped through the mud pumps to the top of the drill string and mud then goes through
the string to the bottom. At the bottom, mud passes through the bit and then up the annulus,
carrying the drill cuttings to the surface. Before mud enters to the mud pit, the solids are
Recovery of oil when a well is first opened is generally by natural flow, forced by the pressure of
the gas or fluids that are contained within the reservoir. At the beginning, there may be a chance
of flush, hence at this stage, well should be carefully controlled. There are several ways which
serve to drive out the petroleum fluids from the formation to the surface, through the well. These
drives are classified as either natural flow (Primary recovery) or applied flow (Improved oil
recovery).
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Natural drive or Primary recovery
Petroleum is propelled out of the reservoir through the well by one of the three methods, or
In dissolved gas drive, the gas dissolved in petroleum oil exerts force to propel the oil as it tends
to come out of the solution because of the release of pressure at the point of drilling. Dissolved
gas drive is the least efficient method of natural drive as it is difficult to control the gas-oil ratio,
rapid drop in the bottom hole pressure and the recovery may be less than 20%.
If there is a gas cap above the oil reserve, then this compressed gas is utilized to drive the oil into
the well. This is gas-cap drive. Normally the gas-cap contains methane and other hydrocarbon
gases, which may be separated from the oil after the recovery, by compressing the gas. Natural
gasoline is a well known example of the recovered compressed hydrocarbon, which was
formerly known as casing-head gasoline or natural gas-gasoline. The normal percentage recovery
Water drive is the most efficient method of natural drive where the propulsive force comes from
the water accumulated under the oil. Water forces the lighter recoverable oil out of the reservoir
into the well. In water drive, rate of removal should be adjusted properly so that water moves up
evenly through the hole. The recovery in a properly operated water drive may be as high as 80%.
IOR is any activity which increases the recovery above that of the primary recovery. It may
include drilling extra wells or drilling horizontal wells, which intersect the reservoir areas which
may otherwise be missed. IOR may also be done by supplying energy to the reservoir. IOR can
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be done by the two broad ways, secondary recovery and enhanced oil recovery. Secondary
recovery involves adding external energy without making any fundamental changes to the
physical properties of the fluids. This energy is added either by water or gas injection. The
secondary gas or water injection mimic the naturally occurring processes of solution gas drive
Enhanced oil recovery which is sometimes known as tertiary recovery involves adding external
energy and creating fundamental changes to the physicochemical properties of the system. The
addition of external energy is in the form of using chemicals or heat to the reservoir to effect
changes in fluid density, viscosity, interfacial forces or to change the wettability which affects
the distribution of the oil, gas and water within the pores.
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References
1. Modern Petroleum Technology, Vol 1, Upstream, IP, edited by R.A. Dawe, 6th Edition,
3. Fuels and combustion, S. Sarkar, 2nd edition, Orient Longman Ltd., 1990.
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Keywords: Evaluation, characterization factor, TBP, ASTM, EFV
The assessment of a crude oil mainly involves the chemical evaluation of crude oil feedstocks by
petroleum testing laboratories. Each crude oil type has unique characteristics and no crude oil is
identical to the other. The results of crude oil assay testing provide extensive and detailed
analytical data for a particular crude oil which are necessary for refinery. In practice it is difficult
and very expensive to carry out full laboratory analysis of every type of crude oil at the refinery.
This has resulted in development of a number of computing methods that can predict the
methods, information about distillation characteristics, density, sulfur content, viscosity etc. of
According to U.S Bureau of Mines, eight bases of crude oil are designated depending on the
fraction no. 1, which boils at 482 to 5270F at atmospheric pressure and key fraction no. 2, which
condition. Table 1. shows different crude oil bases with their characterization factors and API
gravity. The bases of the crude oil are designated as paraffin, paraffin-intermediate, paraffin-
naphthene etc, depending on the nature of the low boiling and high boiling fractions of crude.
For example, in paraffin-naphthene base, the first word of the base name, such as ‘paraffin’
denotes the nature of the low boiling fraction and the second word, ‘naphthene’ indicates the
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Characterisation factor of a crude oil, defined by Universal Oil Product (U.O.P) is expressed as
Where, K is the characterization factor, TB is the molal average boiling point in oR and S is the
specific gravity at 600F. Bases of crude oil can be indicated primarily by this factor. Paraffin
base crude oils show the range of characterization factor as 12.9 to 12.15, for intermediate base
crude oils the range is 12.1 to 11.5 and for naphthene base crude oil it is 11.45 to 10.5. These
ranges are determined based on the properties of crude oil, such as, viscosity, aniline point,
Low boiling High boiling Key fraction 1 Key fraction 2 Low High
part part boiling boiling
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A crude oil is termed as ‘sour’ crude when it contains dissolved hydrogen sulfide in it, 0.05 cu ft
of hydrogen sulfide per 100 gallon of crude oil. At this level, the crude oil becomes dangerously
toxic. The crude oils which contain disulfides, mercaptans, thiophenes in a sufficient amount, are
said to be ‘high sulfur’ crude. Sometimes ‘high sulfur’ crude oils are erroneously termed as sour
crude. Such as, high sulfur crude oils of California, Venezuela and Mexico do not contain any
dissolved hydrogen sulfide in them; hence they should not be termed as sour crude.
curve where percentage distilled is plotted against respective temperature at which it is distilled.
Theoretically, a true boiling point (TBP) distillation is that where a very close separation is made
so that each component in the mixture is separated at its own boiling point and the quantity
present in the original mixture. Fig. 1 represents the TBP curve of a mixture containing two
components A and B which are present in the volume percent 30 and 70 respectively and their
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The stepwise plot shows the theoretical TBP plot with perfect fractionation while smooth curve
represents the actual curve with imperfect fractionation i.e incomplete separation of the two
components.
distillation with a little fractionation. This distillation is done by following the methods and
apparatus designated by ASTM (American Society for Testing and Materials). This type of
distillation is called ASTM distillation and the method of this distillation is termed as ‘ASTM
D158’ in ASTM standard. In this method 100 or 200 ml sample is distilled in a batch mode in
specified condition and apparatus. Fig. 2a and 2b show typical ASTM distillation curve for three
components compared to the typical TBP curve for the same three components and the same
(a) (b)
Fig. 2. (a) Comparison of TBP and ASTM distillation curves for a three component
mixture (b) for a complex system
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Equilibrium flash vaporization (EFV) is a type of separation of components of a petroleum
mixture where, the mixture under pressure is suddenly vaporized or flashed in a still and the
mixture is separated into a vapour and a liquid portion which remain at equilibrium. In fact, this
type of flash is observed in the refinery when crude oil containing appreciable amount of light
components under pressure is piped to separators at lower pressure and allowed to vaporize
suddenly through a pressure reducing valve. If a number of samples of the same composition are
flashed at the same pressure but at different temperatures between the bubble point and dew
Fig. 3 shows the comparison of the slopes of TBP, ASTM and EFV. It has been shown that 10-
Fig. 3. Comparison of the slopes of TBP, ASTM and EFV distillation curves
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The physical properties of petroleum oil vary gradually throughout the range of compounds by
which the oil is constituted. Distillation is a means of arranging these compounds in accordance
to their boiling points. The properties such as, viscosity, specific gravity, colour etc are found to
vary at each drop or fraction of the mixture distilled. The rate at which these properties change
can be shown by mid-percent curve. Actually, the viscosity or specific gravity of a petroleum
fraction is an average of that property of many drops which constitute the fraction. The concept
of mid-percent curve is that, if each drop is equally different from the last drop and it’s
succeeding one, then the property of the whole fraction is determined by the property of the drop
which comes at exactly 50% distilled. This would be the condition when a mid-percent curve is a
straight line. Although mid-percent curves are never exactly straight line, but they may be
straight for a short range of percent distilled and this short range, the average property of the
fraction is exactly the same as that of mid-percent fraction. There are some properties which are
not additive, such as, viscosity, API gravity, colour, flash point etc., for which this mid-percent
curve is not suitable. But the properties which are additive, such as, specific gravity, aniline
point, percent sulfur etc. can be utilized nicely in mid-percent curve to determine the average
property of the whole fraction. Although viscosity is not an additive property, but it has been
seen that, for a wide fraction of oil, the average viscosity is almost exactly the same as that of the
mid-point fraction.
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Reference:
1987.
3. Evaluation of crude oil quality, D. Stratiev, R.Dinkov, K. Petkov, K. Stanulov, Petroleum &
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3.3 Distillation
processes, collectively known as refining of petroleum. Petroleum refining starts with crude
oil distillation which is a physical separation process, followed by some chemical treatment
steps, such as cracking, reforming, hydrotreating etc to produce a wide range of petroleum
products of specific standard. Crude oil distillation is done at atmospheric pressure as well as
atmospheric distillation unit. A crude oil desalter is considered to be one of the facilities of
atmospheric distillation unit. Crude oil is first processed by desalter to remove salts, solids
and water before introduction to the atmospheric distillation unit. Desalted crude is then
fractionated into intermediate petroleum products or cuts, such as, light naphtha, heavy
naphtha, kerosene, gasoil and atmospheric residue (AR). Atmospheric distillation produces
cuts which boils upto 3500C. The whole or part of AR is treated in vacuum distillation unit to
produce either vacuum gas oil (VGO) or lubricating oil depending on the type of the crude oil
and vacuum residue (VR). The vacuum distillation unit used for production of VGO is called
a fuel-type unit and the unit used for producing lubricating oil is said to be a lube-type unit.
The atmospheric distillation unit consists of a desalter, an atmospheric tower, three side
strippers and a debutanizer/splitter. Crude oil is preheated and then sent to desalter. Crude oil
contains contaminants such as, salts, solids and water that may cause corrosion, fouling,
plugging and catalyst degradation in the refinery units. The salts contained in crude are
mainly NaCl, CaCl2, MgCl2 and they are soluble in the water associated with crude oil. This
solution forms water-in-oil emulsion which is broken by applying high voltage electrostatic
force. In the desalter, the salt containing water forms large drops, which coalesces and then
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settles by gravity. The desalted crude oil is then removed from the top of the vessel while
From the desalter, the crude oil is preheated by exchanging heat with products and pump-
around reflux streams and then heated by a pipe-still heater to a desired temperature of
around 3250C. At this temperature, the required degree of vaporization of crude occurs and
then it is introduced into the flash zone of atmospheric tower. The liquid portion of the crude
flows down to the bottom stripping section of the tower where the vapour portion or distillate
portion are vaporized with steam stripping. Steam is injected at the bottom of the tower which
strips out the distillate fractions from the crude and ascends through the column along with
the stripped vapour and the flashed vapour from the crude at the time of injection. This mixed
vapour steam comes in contact with the down-flowing internal reflux liquid on the trays
where mass transfer of the components occur by condensation and vaporization and this way
fractionation takes place. The internal reflux is created by condensing a portion of the
ascending vapour by exchanging heat with the pump-around reflux liquid. Pump-around
refluxes at different point of the tower at different temperatures are utilized for effective
products, such as, kerosene, light gas-oil and heavy gas-oil and the bottom product is
overhead condenser(s). This condensed liquid is called full boiling range naphtha which is
sent to a debutanizer to remove butane and lighter gas. The gases which are not condensed in
condenser and the debutanizer are taken out and sent to a gas separation unit to collect gases
like, methane, ethane, ethylene, propane, propylene, butane and butylenes. These gases have
definite use, such as, propane and butane constitute Liquefied Petroleum Gas (LPG),
methane, ethane are used as fuel and preparation of valuable chemicals, ethylene is the
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feedstock for first generation petrochemicals. Fig. 1 shows a process flow diagram of an
Before being introduced into the atmospheric tower, crude oil is heated in a crude furnace,
named pipe-still heater. Most crude oil starts cracking or decomposition in the temperature
range 340-3700C. Hence, the coil outlet temperature of the furnace should be selected in
such a way so as to avoid excessive thermal decomposition, which results in coking in the
furnace tube and poor quality and quantity of fractionated products. The typical furnace coil
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outlet temperature is maintained in the range 310-3700C depending on the nature of the crude
oil.
The full range naphtha withdrawn from overhead of the tower is stabilized in debutanizer
(stabilization means removal of light gas components from naphtha to maintain a definite
vapour pressure of it) and then sent to a splitter. In splitter, naphtha is separated into light
naphtha and heavy naphtha. As an alternative to use splitter, light naphtha and heavy naphtha
can be withdrawn as an overhead product and a side cut respectively from top of the
atmospheric tower. Kerosene, gas oils and atmospheric residue (AR) are obtained as the side
cut from the tower. Kerosene, light gas oil and heavy gas oil are steam stripped to remove the
light ends and stabilize the product. All these cuts and AR are collected after exchanging heat
Steam is introduced at the bottom of the distillation tower and to the side strippers to recover
the lighter fractions from the liquid stream or product. The rate of steam supplied is usually in
the range of 10-50 kg/m3. If heavy naphtha is withdrawn as side stream product, then a
reboiler is equipped with the stripper instead of introducing steam into it.
There are generally two types of reflux systems used in the atmospheric distillation. One is a
pump-around reflux system and another is overhead reflux system. In the case of pump
around reflux system, some of the ascending vapour is condensed at the top part of the tower
by exchanging heat with pump around reflux liquid and the condensed liquid flows down the
column as internal reflux. In the overhead reflux system, the overhead vapours are
condensed and this condensed liquid is sent back to the top section of the column as reflux.
Overhead reflux system may be again divided into two kinds, cold reflux and hot reflux. In
cold reflux system, vapour from the overhead of the tower is condensed by a condenser and
then enters to an overhead reflux drum, where oil, water and gases are separated. A part of
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the condensed oil is sent back to the top of the column as reflux and the remaining part is
collected as naphtha product. For hot reflux system, two sets of overhead condenser and
reflux drums are used in series. In the first condenser, heavier fraction of the overhead
vapours are condensed and sent to the first reflux drum. The lighter part goes to the second
condenser, condensed and collected in the second drum. The condensed liquid which is
accumulated in the first drum is sent back to the top of the tower as reflux. As the
temperature of this reflux liquid is more than that of the cold reflux liquid, hence, the system
is called hot reflux. Cold reflux is comparatively simple in operation, but hot reflux reduces
the corrosion in the tower top. For both the cases, the diameter of the tower top is smaller
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Reference:
1. Modern petroleum technology, Downstream, ed. by Alan G, Lucas, Vol 2, 6th edition, IP,
2. Modern petroleum refining processes, B.K.B.Rao, 4th edition, Oxford & IBH Publishing
3. Fuels and combustion, S. Sarkar, 2nd edition, Orient Longman Ltd., 1990.
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Keywords: Atmospheric residue, fuel-type distillation, lube-type distillation, vacuum gas oil
Atmospheric residue (AR) from atmospheric distillation tower contains several valuable cuts
this cut needs excessive temperature where, cracking or decomposition of crude starts resulting
in severe coke deposition. Hence, AR is recovered as a bottom product from the tower and
Crude oil can be categorized as lube-bearing crude and non-lube bearing crude. Non-lube
bearing crude cannot produce lubricating oil cut in vacuum distillation as this range of
hydrocarbons are not present in non-lube bearing crude. The unit for processing of non-lube
bearing crude and lube-bearing crude are known as fuel-type and lube-type vacuum distillation
column respectively. The former produces vacuum gas oil (VGO) and later produces lubricating
oil as the main distillate product. AR is introduced into the vacuum distillation column after heat
exchanging with distillation products, vacuum residue and pump-around reflux streams and
finally heated in a furnace at required temperature. Vacuum distillation furnace may be classified
into two types, wet and dry. In wet type, steam is injected into the furnace coils and that helps to
lower the partial pressure of feed as well as steam carries the feed vapours through the furnace
tube more rapidly. In dry type, steam injection is not done in the furnace. Steam injection lowers
the steam consumption in the vacuum ejector systems. The choice of the type depends on the
In a fuel type distillation, shown in Fig 1, AR is flashed at the required temperature in the
vacuum tower feed plate. The liquid portion of the flashed feed flows downward in the stripping
section and the vaporized part along with stripping steam goes up through the column. Light
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vacuum gas oil (LVGO) and heavy vacuum gas oil (HVGO) is withdrawn from the side trays at
their respective boiling ranges. These two cuts may be collected together as per requirement.
Vacuum residue (VR) is withdrawn from the bottom of the tower, after the VGO portions are
steam stripped. VR should have required penetration index (This is a property of bitumen/VR,
penetration of a bituminous material is the distance in tenths of mm, that a standard needle would
penetrate vertically, into a sample of the material under standard conditions of temperature, load
and time). A small part of the cold VR is recycled at the bottom of the tower to prevent coking.
The vacuum tower can be divided into stripping section, wash oil section, HVGO section and
LVGO section, from bottom to top, according to its product draw and working manner. Stripping
section is the bottom part of the distillation tower below feed plate, where stripping steam is
introduced. A mixture of the flashed vapour, stripped vapour from feed and steam flow up the
column and enter into the wash oil section. At this section, this mixture comes in contact with
internal reflux stream when any heavy oil fraction entrained in that vapour mixture is taken away
by the reflux and lighter fraction from reflux comes in the up-flow stream. This internal reflux is
called wash oil. The washed vapour stream goes up to the HVGO section and then to LVGO
section. HVGO and LVGO cuts are obtained from side draw trays by contacting with down-flow
reflux liquid. The internal reflux liquid is achieved by condensing the ascending vapour by cold
pump-around reflux stream. HVGO and LVGO obtained after condensation from their respective
trays and withdrawn as VGO product, either separately or together. A part of the wash oil from
wash oil section is withdrawn as side stream. This oil containing some fraction of heavy oil is
recycled to vacuum tower by mixing with the feed stream before heading to the furnace. The
overhead vapour of the vacuum tower, which is the mixture of steam and oil vapour, is
precondensed to remove most of the steam and oil. The uncondensed part is sent to the ejector
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system. Vacuum is created by using a series of ejectors and surface condensers. The condensed
overhead vapour and steam from pre-condenser and surface condensers are sent to an overhead
drum, where gas, slop oil and sour water were separated. In fuels-type vacuum distillation
column, no side strippers are employed, as VGO is the only side product obtained, whose
properties are mainly controlled by its metal content and carbon residue.
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The lube-type vacuum tower is shown in Fig 2. This type of distillation column produces lube-oil
base stock as the side stream, LVGO as the top product and VR as the bottom product. Three
types of lube-oil base stocks are withdrawn from three side draw trays, as light stock, medium
stock and heavy stock, which are all steam stripped in a side stripper to meet the viscosity and
carbon residue requirements of the stocks. Pump-around reflux is used to provide internal
refluxes. Steam is introduced at the bottom for stripping the feed liquid. Bubble-cap trays are
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The maximum allowable temperature of feed (AR) is determined by the type of feed to prevent
the cracking and coke formation in the furnace coils. Usually, the maximum temperature is kept
in the range of 400-4300C. The pressure at the tower top is maintained in the range 1.3 to 20 kPa.
The overall pressure varies depending on the type of furnace operation (wet or dry), feed
temperature and cut point temperature difference between VGO and VR.
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References:
1987.
3. Modern petroleum technology, Downstream, ed. by Alan G, Lucas, Vol 2, 6th edition, IP, John
4. Modern petroleum refining processes, B.K.B.Rao, 4th edition, Oxford & IBH Publishing CO.
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Keywords: Thermal cracking, visbreaking, hydrocracking, naphtha
3.4.1 Cracking
Cracking is a secondary process in the refinery where change in composition of the petroleum
fractions is done by the action of heat alone (thermal cracking) or heat in presence of a catalyst
(catalytic cracking). Cracking is the decomposition of C-C bond of hydrocarbon molecules. The
primary or first stage products of cracking are lower molecular weight compounds than the feed
and are mainly olefinic in nature. The second stage products come from the reactions like
products may be of the same molecular weight or higher than the feedstock. The fundamental
difference between thermal cracking and catalytic cracking is that, the former proceeds via free-
radical mechanism while the latter occurs mainly by a carbocation mechanism. Hydrocracking is
the cracking operation where hydrogen is introduced during cracking; hence it is a combination
Thermal cracking
Thermal cracking was first commercialized in 1912 to increase the yield of middle distillate
fractions (which boil in the range of 150-3500C) from crude oil. Visbreaking is a mild thermal
cracking operation which improves the viscosity of a heavy fraction (boiling range >2500C) by a
non-severe route. Coking is a severe thermal cracking operation whose target is to maximize
coke production from a heavy stock along with gas, gasoline and middle distillate. Although
thermal cracking is not practiced in most modern refinery and is replaced by catalytic cracking
yet there is some importance of thermal cracking depending on the products required or aimed.
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Table 1 shows the thermal cracking temperature range and the corresponding products obtained
thereof.
___________________________________________________________________________
Cracking temperature (0C) Name of the process Products
___________________________________________________________________________
425-460 Visbreaking Fuel oil
___________________________________________________________________________
i.e ethylene, which is produced by thermal cracking of straight run naphtha and gas-oils and this
Visbreaking
Visbreaking is a mild thermal cracking process utilized in the refinery to reduce the viscosity
and/or improve the pour point of a heavy oil, mainly residues, AR and VR both. A typical
visbreaker unit is shown in Fig. 1. In this process, the residual oil is heated at a desired
temperature in a furnace and then rapidly transferred to a soaking drum, where, cracking occurs
for a desired residence time up to the desired degree of cracking. The cracked products are
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quenched immediately by exchanging heat with gasoil or cold visbroken tar to stop the cracking
reaction then and there and prevent coking thereby. This type of visbreaking is called a soaker
type visbreaking. In this type, the cracking reactions are initiated in the furnace but the progress
Another type of visbreaking operation is coil type, where, the cracking reactions occur in the
furnace coil. Here the residence time of the reactions are kept as short as possible to avoid much
coking. A long coil is normally used to complete the desired degree of reaction. The cracking
temperature used in soaker type is normally lesser than that of coil type and hence, the coke
The products obtained from visbreaking are gas, middle distillates and stable fuel oil.
Visbreaking is often used to increase the middle distillate yield in the refinery by reducing the
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amount of heavy oil or bottom of the barrel (residual oil). This distillate fraction is used as a
blending agent of fuel oil to reduce its viscosity. This distillate replaces diesel in refinery which
is actually used as the blending agent. Hence, economics is strongly driven by the benefit of
Hydrocracking
Hydrocracking is an operation in which low-value gas oil with a high percentage of polynuclear
distillates which are of high value. The primary advantage of hydrocracking is that, it selectively
produces gasoline, diesel fuel or jet fuel by cracking and hydrogenation of polyaromatics,
without producing any unwanted low molecular weight gases. The first modern hydrocracking
unit was developed by Chevron in 1959 which is known as Chevron Isocracking Process.
Hydrocracking is widely accepted by the refiners because of its ability to produce high quality
products. Except gasoline and middle distillate this process gives lube oil base stocks and heavy
oil suitable as the feedstock for catalytic cracking. A flow diagram of hydrocracking unit is
shown in Fig. 2.
Light coker gas oils and light cycle oils were used as feedstocks for hydrocracking earlier, but as
the technology is improved, industry can take up heavier straight run vacuum gas oil, coker gas
oil and solvent deasphalted oils as the feed for the process. Hydrocarcking feedstocks are
complex mixtures of mainly paraffins, naphthenes and aromatics. The boiling range is 3450C+.
As the feedstock is passed through the reactor, aromatics are the first components to react;
mainly forming naphthenes, which boil in the same range of 3450C+. Hence the amount of
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naphthenes of the same range increases. After most of the aromatics are converted to naphthenes,
the naphthenes start to crack and the boiling point drops below 3450C.
Paraffins donot show significant cracking reaction in this single pass operation. Hydrocracking
reactions are inhibited by the presence of nitrogen and sulphur compounds. Those compounds
are mostly removed by this process when nitrogen compounds are converted to ammonia and
easier than the nitrogen ones, as the former requires lesser hydrogen partial pressure. A
precaution should be made in recycled hydrogen gas which should be free from any
contamination of ammonia.
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Hydrocracking can be done either thermally or catalytically, but the most common
hydrocracking is catalytic. Hydrocracking catalysts mainly consist of active metals on solid acid
supports where the acidic part of the catalyst serves the cracking function and the hydrogenation
function is provided by the metals. The acidic supports are mainly amorphous silica-alumina,
crystalline zeolite or a mixture of two. The metals that are used for performing hydrogenation
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Reference:
1. Modern Petroleum Refining Processes, B.K.B.Rao, 4th Ed.,Oxford & IBH Publishing Co. Ltd.
2002.
2. Modern petroleum technology, Downstream, ed. by Alan G, Lucas, Vol 2, 6th edition, IP, John
3. Petroleum Refining Processes, James G. Speight and Baki Ozum, Marcel Dekker Inc, ISBN:
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Keywords: Petroleum coke, fluid coking, delayed coking
3.4.1.2 Coking
Coking is a refinery operation that upgrades low-valued bottoms like atmospheric or vacuum
residue into higher-value petroleum coke. Petroleum coke is a high carbon coal like material.
The process is actually a severe thermal cracking which completely converts the petroleum
residues into coke and lighter products. The heavy distillate fraction which is produced in the
process is recycled. The temperature used in this process ranges from 500 to 6500C.
There are two basic forms of coking operation, Fluid coking and Delayed coking. The first one is
a continuous process and utilizes a fluid bed. The second process is a semi-continuous and uses
coke drums for accumulation of coke. Delayed coking is more commonly used process in
refinery. The name of the process ‘Delayed’ is due to the reason that, coking reaction occurs in
the coke drum rather than in the heater. Coke drums are used to hold or delay the heated
feedstock while the cracking takes place. The cracking/carbonization reactions involve
Fluid coking
Fluid coking is a continuous process in which heated feedstocks are sprayed into a fluidized bed
of hot coke particles. The reactor is maintained at 20-40 psi and more than 500°C. The feed
vapors are cracked on the fine coke particles while forming a liquid film on the coke particles.
The particles grow by layers until they are removed and new seed coke particles are added. The
main unit of fluid coking process is the combined scrubber-reactor assembly, where the scrubber
is fixed above the coker reactor as shown in the Fig 1. The residue feed is introduced into the
scrubber at about 5700C. It is heated with the effluent stream of the reactor.
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In this scrubber, the high boiling hydrocarbons are condensed and scrubbed out with the reactor
effluent vapors at about 5250C. This stream is recycled to the reactor mixing with the fresh feed.
The lighter hydrocarbons from the overhead of scrubber are sent to the fractionators for recovery
of lighter products. In the reactor, the cracking reactions take place to finally produce the coke
and lighter products. Coke is sent to the burner and about 20% coke is burnt here with air to
satisfy the heat requirements of the reactor in the cracking reactions. A part of the coke from the
burner is recycled to the reactor and the rest is taken as a marketable product coke. The flue gas
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Delayed coking
Delayed coking is the preferred choice of many refiners as it imparts the advantage of handling
very heavy residue. This process produces a significant amount of naphtha and diesel products.
Moreover, the yields of product can be tailored by adjusting the recycle and operating conditions.
Delayed coking is a semi batch process which uses alternative coke drums one of which is
switched off-line after filling. Except coke drum, a furnace, closed blow down, coke cutting and
handling and a water recovery system are included in the unit. Hot residual oil is mixed to the
bottom of the fractionators (heavy distillate from fractionators) and the combined stream is
heated in the furnace to initiate coke formation in the coke drums. Steam is injected in the
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furnace coil to avoid coking in the coil. Coke drum overhead vapour flows to the fractionator
where it is separated into wet gas, unstabilised naphtha, gas oil, heavy gas oil and recycle. The
Delayed coking unit is shown in Fig. 2. In the operating coke drum, the hot feed from the coker
furnace is injected into the bottom of the drum at high temperature and low pressure and is
cracked into gaseous products and coke. The gaseous products are sent to the fractionator for
product recovery and the solid coke is solidified in the coke drum. The other drum which is
offline and full of coke, is steamed, vented, and cooled prior to the drum being opened to
atmosphere. During the coke drum steam out and cooling period, all steam and hydrocarbon
vapours are directed to the blow down system where they are recovered. After this drum is
opened, the petroleum coke is cut from the drum using high pressure water jet. The coke is
dropped into a pit from the drum where water is separated from coke and recycled. Petroleum
coke or simply “coke” is similar to coal and is typically used for fuel in power plants.
There are two distinctive types of raw petroleum coke, one is calcined or green coke and another
Fuel-grade coke is spongy in texture and contains high amounts of sulfur. It can withstand high
heat and contains little ash. This type of coke is primarily used as a fuel in power generators. As
it contains high sulfur, hence, burning of it produces sulphur dioxide gas. So, a sulfur capture
system should be there to reduce the amount of sulfur released into the air and meet clean-air
standards.
Calcined petroleum coke is made by calcining or roasting petroleum coke just below the melting
point. This coke is commonly used as electrodes in the smelting industry for the production of
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Although, coke is a low value by-product compared to transportation fuel, there is a great
demand worldwide for even high sulphur petroleum coke as it is a very economical fuel.
Calcination grade raw petroleum coke (RPC) is produced at Barauni, BRPL, Digboi and
Guwahati refineries of Indian Oil Corporation (IOC). Fuel Grade Petcoke is product at Panipat
refinery of IOC.
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Reference:
1. The Chemistry and Technology of Petroleum, James G. Speight, 4th edition, CRC Press, 2010.
2. Modern petroleum technology, Downstream, ed. by Alan G, Lucas, Vol 2, 6th edition, IP, John
3. Modern Petroleum Refining Processes, B.K.B.Rao, 4th Ed.,Oxford & IBH Publishing Co. Ltd.
2002.
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