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1
CHAPTER I
INTRODUCTION
================================================================
1.1 Introduction:
Coal is a kind of fossil fuel. It is a combustible, sedimentary organic rock and is a
complex mixture of organic chemical substances containing carbon, hydrogen and oxygen
in chemical combination, together with smaller amount of nitrogen and sulfur. It is
physically and chemically a complex heterogeneous substance composed of organic and
inorganic material. The organic materials are derived mainly from partially decomposed
plant debris, which have undergone various degrees of decomposition in the peat swamps
under the influence of high pressure and temperature caused by overlying sediments over a
long period of several hundred million years 1-3
. It occurs in stratified deposits both near
the earth‟s surface and at various depths.
1.2 Origin and formation of coal:
Coal was formed from partially decomposed (and subsequently metamorphosed)
plant debris which had collected in regions where waterlogged or swampy conditions
prevailed. Generally coal occurs as an intimate mixture of complex organic mass and
inorganic matter 4. It is formed from vegetation which has been consolidated between other
rock strata and altered by the combined effects of pressure and heat over millions of years
to form coal seams. The buildup of slit and other sediments, together with movements in
the earth‟s crust buried these swamps and peat bogs, often to great depths. Coal was found
to consist mainly of detritus from higher terrestrial plants 5. Thus once plant debris has
2
accumulated under the correct conditions, the formation of peat gradually occurs. As the
peat is formed then after coalification process, in essence, the progressive change in the
plant debris as it becomes transformed from peat to lignite and then through the higher
ranks of coal to anthracite. The metamorphism of the plant debris not only relies on
geological time but also on temperature and pressure.
When the organic debris or peat is buried beneath overburden, various physico-
chemical processes occur as part of the metamorphosis. The major influences are believed
to be resulting heat and pressure developed because of the overlying sedimentary cover
(overburden). This leads to changes in the constituents of the debris such as an increase in
the carbon content, alteration of the functional groups, alteration of the various molecular
structures ultimately resulting in the loss of water, oxygen and hydrogen with the increased
resistance to solvents, heat and oxidation 6-8
. A schematic representation of the
coalification process is shown in Figure 1F-1. The theory and formation of coal require
that the original plant debris eliminate oxygen and hydrogen continuously under the
prevailing conditions, ultimately leading to a product containing approximately 90 % w/w
carbon, i.e., anthracite. In order for this formation to proceed, chemical principles require
that oxidation reactions be completely inhibited. However, in the early stages of
coalification, micro-organism may play an important role and somewhat paradoxically;
they may interact with the plant material under aerobic conditions as well as under
anerobic conditions. The formation of coal under the slow conditions generally referred to
a geological time may, nevertheless be regarded as occurring in the absence of oxygen and
hydrogen from the original organic molecules 6.
3
Coal is generated from various types of organic precursors under a broad range of
chemical reactions. It is highly heterogeneous in nature and consists of compounds with
distinctive appearances and chemical composition. These are referred to as macerals,
which are classified into three categories; vitrinite, liptinite and inertinite. The shiny, glass
like vitrinite, a major component of coal, is oxygen-rich with moderate hydrogen and
aromatic content. On the other hand, the waxy liptimite is hydrogen-rich and highly
aliphatic. Finally charcoal like inertinite is carbon rich and contains a high aromatic
content.
Living plants→ Dead Organic Matter
↓
Biological and Chemical Degradation of organic Material to peat
↓
COAL
Figure 1F-1: Schematic representation coalification Process
1.3 Classification of coal:
Coal consists of various organic compounds that were generally derived from
ancient plants and have subsequently undergone changes in molecular and physical
structures during the transition to coal. Thus, there is a need to accurately describe the
various coals in order to identify the end use of the coal and, also, to provide data which
can be used as a means of comparison of the various worldwide data coals 9. Prior to 19
th
4
century, coal was classified according to appearance e.g. bright coal, black coal or brown
coal. A number of classification systems have been developed 10
.
1.3.1 The Seyler classification:
The Seyler classification is based on the carbon and hydrogen contents of coals
determined on a dry mineral basis 11-12
. This method gives the range and interrelationship
of the properties of coal including parameters such as moisture content and swelling
indexes.
1.3.2 The ASTM classification:
The ASTM classification system adopted in 1938 as a standard means of
specification used in USA and in many other parts of the world and is designated as D388
in ASTM standard 13
. In this classification the higher rank coals are specified by fixing
carbon > 69 wt%, or from volatile matter < 31 wt%, on a dry mineral free basis. Lower
rank coals are classified by calorific value (on moisture and mineral matter free basis).
1.3.3 National coal Board classification:
This classification of coal proposed in 1946 by the UK department of scientific
and industrial research. There are two parameters: the quantity of volatile matter
determined on a dry mineral matter free basis, and the Gray-king coke type assay, a
measure of coking as designed in the British Standards.
1.3.4 Classification by type:
This type of classification depends upon the nature and biochemical alteration of
the original plant ingredients. They are humic, cannel and bog-head coal.
5
1.3.5 Classification by rank:
The classification by rank is made on the basis of the coalification 14
, which gives
the carbon content in lignite, sub-bituminous, bituminous and anthracite shown in Figure
1F-2. Coal rank increases with the amount of fixed carbon but decreases with the amount
of moisture and volatile matter. This is the most commonly used method for coal
classification. The different ranks of coal have different structural parameters.
Figure 1F-2: Classification of coal by rank under the burial pressure, heat and
time
1.3.6 Classification by grade:
Depending upon the amount and nature of the mineral impurities associated coals
are classified by grade. The extraneous matter forms most of the non combustible
impurities of coal. The ultimate constituents of coal are carbon, hydrogen, oxygen and
nitrogen which bear a direct relation to the rank. Anthracites are differentiated from
bituminous coals by agglomeration characteristics and fix carbon. Bituminous coals are
Peat Sub-bituminous Lignite
Bituminous
Anthracite
6
distinguished from sub-bituminous coals by agglomeration properties and partially by
heating value while sub-bituminous coals are distinguished from lignite by heating value.
The classification of commercial grades of Indian coals was evolved based on the ash
contents, moisture and calorific value. Coal grading board was established in 1925.
1.4 Structure and Composition of coal:
Although, coal has been utilized for several years the exact chemical nature of its
structures are still not fully known. Coals of different age have different chemical
composition and therefore, different structures. Even within a certain age group (or rank)
of coal, such as lignite or bituminous coals, the structure may vary depending on the
environment in which a particular coal was formed. The principal elements from which
coals are composed are the same ones which are made up of wood and other vegetal matter
are C, H and O together with lesser amounts of S, N and other elements characteristics of
the inorganic matter. The composition of a coal involves both the elemental composition
and the functional groups that are derived there from. The structure of coal molecules are
highly complex and are difficult to define, as the macromolecules of coal are not composed
of repeating mono-organic units 15
. The structure of coal were characterized by values of
some parameters (such as chemical composition, types of cross-links, carbon and hydrogen
aromaticities, size distribution of the macromolecules, size of aromatic cluster etc.) as well
as the total organic matter of the coal or its major petrographic components i.e. vitrinites.
1.4.1 Physical Structure:
The chemical makeup of coal is highly complex and not amenable to
straightforward analysis. In the chemical origin of coal, the maturation process may cause
7
chemical changes to the individual chemical constituents of the tissue, this same
maturation process may also cause physical changes of the tissue that may cause it to
appear as a recognizable entity in the coal. The tissues of the original plants can themselves
contribute to the physical structure of coal that has led to the development of coal
petrography. Coal is heterogeneous both macroscopically and microscopically. The
microscopically distinguishable components are classified as macerals, which usually
originates from plant tissues and is divided into three major groups: vitrinite, liptinite
(exinite) and inertinite. These are optically homogeneous discrete organic materials in coal.
They have considerably different chemical composition with H/C. The properties of
macerals vary from coal to coal.
Coal has macromolecular character and there is non-polymeric component of small
organic molecules embedded in the polymeric matrix. Also, the structure of coal is varying
in different geographical age and areas. By its large variety, coal offers an interesting
subject for research to a far larger number of scientific disciplines 16
. Although research on
physical structure of coal has been carried out over half a century, several issues remain
unresolved and incomplete. X-ray diffraction has been applied to the characterization of
physical structure of coal for better understanding of ordered packing of macromolecules
17-22.
1.4.2 Chemical Structure:
Coal is physically and chemically complex having a physical microstructure that
is discernibly derived from plants and a chemical structure containing a wide variety of
polymeric organic compounds and crystalline minerals. The chemical structure of coal is
8
described by the structural parameters such as molecular weight, carbon aromaticity,
aromatic and aliphatic structures and functional groups 23
.
The structure of coal solid depends to a significant extent on the arrangement of the
functional groups within the material. The functional groups within the coal contain the
elements C, H, O, N and S. The functional groups are the most reactive components of
coal. The relative and absolute amounts of various groups vary with coal rank and maceral
type. The significant carbon containing groups found in coals are-carboxyl, hydroxyl,
carboxylic acid and methoxy. The nitrogen containing groups include aromatic nitriles,
pyridine, carbozoles, quinolines and pyroles. Sulfur is primarily found in thiols, dialkyl
and aryl-alkyl thioethers, thiophese groups and disulfides. The relative and absolute
amounts of the various groups vary with coal rank and maceral type. Aromaticity of coal
molecules increases with coal rank.
The problem of ascertaining the molecular structure of the organic part of coal is
that coal is not structurally dependent on a simple molecule but on a complex mixture of
molecules which varies according to the type of coal. Many workers now agree that coal is
macromolecular in nature 24
. This macromolecule network consists of clusters of aromatic
carbon that are linked to other aromatic structures by bridges. Bridges between aromatic
clusters are formed from a wide variety of structures. Most bridges are thought to be
aliphatic in nature but may also include other atoms such as oxygen and sulfur. Those
bridges that contain oxygen as ethers are thought to have relatively weak bond strengths.
Other bridges are made up of aliphatic functional groups only. Some bridges consist of a
single bond between aromatic clusters; this is known as bi-aryl linkage. Due to the large
9
variety of functional groups that make up the bridge structure of coal, bridges have a large
distribution of bond strengths. This distribution of bond strengths becomes important
during the pyrolysis process as the weakest bonds are broken first. There are other
attachments referred to as side chains and are thought to be consists mainly of aliphatic and
carbonyl functional groups. The existence of largely aromatic and hydro-aromatic „cluster‟
as suggested by many X-ray and chemical degradation studies are accepted 25
. These
„cluster‟ are linked by covalent bond to form network. Several different models for the
structure of coal have been suggested by many workers 25
. The macromolecular skeletal
structure of bituminous coal is shown in Figure 1F-3.
This investigation of the chemical structures of coal had led to comprehensive and
well-defined results on the basis of development of spectroscopic methods. These are
FTIR, UV-visible spectroscopy, X-ray structural analysis, solid state NMR etc. X-ray
diffraction studies provide useful information about the internal arrangement of atoms in
coal. This arrangement in coal is a vital factor, which affects many physical properties in
relation with utilization of coal.
1.4.3 Structural variation with coal rank:
The structure of coal, also, changes with its coal rank. The Table 1T-1 shows the
relationship of rank with coal characteristics.
1.4.4 Trace metals and mineral matter in coal:
In coal technology the term „mineral matter‟ and „ash‟ are often used
interchangeably. Ash is actually the residue remaining after complete combustion of the
10
organic portion of the coal matrix. Thus, the constituents of ash are formed as a result of
chemical changes which take place in the mineral matter during the combustion (ashing)
process. These changes usually involve the breakdown of complex chemical structures
with the formation of the metal oxides.
The elements present in coal are C, H, N, O and S of which four elements occur in
inorganic locations is called mineral matter. The mineral content of coals varies
considerably and may even be as high as 35% by weight of the coal. The use of coal as a
fuel has directed interest toward the mineral constituents of coals but with the tendency to
an increased use of coal for power generation as well as for proposed gasification and
liquification plants that will enable coal to act as a source of liquid and gaseous fuels.
O
H
S
H
H
O NH2
C
H
H
H2
H2H
O
O
C
H
O
SO
C
CH
H
HHH
H
H
H
CH2
CC
H2
H2
H2
H2
H
HHC
H
H
H
HH
H2HH
C HH
H
S
C
H2
H
H
H
H
H
O
C
O
HH H
OO
H
C
C
C
NO
CH2H
HH
HH
O
H
C
C
C
HH
HH
HHH
O
H HH
HH H2
H2
H2
H2HHO
H2
H
H
CH
HH
H
H
H H
O
HH
H HH
H O
H
C N
H
H
O H H
C
H
O
H H
H
H HH
O
H
H
O
H
O
H
H
HH
HH
H
H
H
H
H
H2H2
H2 H
H
H
H
H
H2
H
Figure 1F-3: A model structure of bituminous coal
11
Coal mineral matter originated from the inorganic constituents of the
vegetation which acted as the precursor to coal and from the mineral matter that was
transported to the coal bed. It is common to all and has been recognized from the time
when coal was first mined for general use that coal contained some material other than the
main coal substance. The amount and type of minerals found in coal varies widely and
depends in coal history. Silicon (as silicates) is often the major elemental component of
the mineral matter of coal. Aluminum and iron are generally next to abundance in coal
followed by calcium. The most abundant minerals are the clay minerals of which illite,
kaolinite and montmorillonite. Pyrite is the common sulfide mineral while sulfates are
relatively rare but increase with weathering. Carbonates form readily in non-acid areas and
quartz may occur in concentrations as high as 20 % of the total mineral matter. Sulfide
minerals often constitute as much as 25 % of the coal mineral matter.
In coal the study of metal content is difficult because some metals emitted during
coal combustion have great impacts on the environment and on our health. Trace metal are
important because they are linked with environmental issues and the health of plants,
animals and human beings. The metals come in contact in coal mining, coal crushing, coal
storage, coal beneficiation and coal combustion which may lead to different kind of
deseases e.g. quartz may lead to silicosis to the mine workers. Leaching of As, B, Pb etc.
from coal or fly ash during rain contaminates the surface and ground water. Water is a
good solvent which can leach some of the toxic metal from coal 26
.
12
Table 1T-1: General variations in structural characteristics with ranks.
Coal rank Carbon
(wt %)
Nature of monomers Nature of cross-links
Lignite 30-50 Small, largely single-ring
systems extensively
substituted with O-
functional groups (-
COOH,-OH, -OCH3), 1
oxygen per 3 to 4
carbons.
Many hydrogen bonds,
salt bonds, few aliphatic
cross-links, water is an
important structural
components.
Sub-bituminous 50-60 Ring systems with some
larger rings, O-groups in
ring less than lignites, 1
oxygen per 5 to 6
carbons.
Mixture of hydrogen
bonds and probably
ethers, some aliphatic
links.
Bituminous
60-70 Mixture of ring systems,
1 oxygen per 9 carbons
mainly-OH functional
groups.
Mixture of aliphatic and
cross-links
70-75 Significant increases in
amounts of larger rings, 1
oxygen per 12 carbons,
almost entirely ring ether
and –OH.
Mostly aliphatic types,
some link biphenyl
types.
75-80 Degree of condensation
of aromatic still grates,
very few O-groups, down
to 1 oxygen per 20
carbons.
Non-reactive aliphatic
bridges and bi-phenyl-
type links
Anthracite 95 Highly condensed,
aromatics, graphitic,
commonly multiple
rings, functional groups
rare, only 1 oxygen to
about 100 carbons.
Almost entirely direct
aromatic-aromatic
links.
13
1.4.5 Effect of structure on reactivity of coal:
The structural properties of a coal from certain region have a direct relationship
with its reactivity. The small aromatic cluster, the high concentration of organic oxygen
functional groups and the presence of inorganic species as ion exchangeable cations are
unique features of low rank coals. Each of these features should influence the reactivity of
low rank coals, thereby, the low rank coals will show distinctly different reactivity
compared to bituminous coals.
The carboxylic acid group is one of the most important of the organic functional
groups. The carboxylic acid group under relatively mild conditions decomposes to carbon
dioxide below 450˚C 27
. Oxygen functional groups can, also, promote β-bond scission 28
,
which may be an important process in the degradation of the coal structure. The role of
ether, carboxyl and other groups in wood pyrolysis and combustion has been discussed 29
;
it is reasonable to assume that analogous reactions would occur in low rank coals. Other
possible roles for oxygen functional groups include ether cleavage and the cleavage of
aliphatic bridges linked to aromatic rings bearing a phenolic groups.
The inorganic constituents of low rank coals have significant impacts on utilization
and conversion processes so that a consideration of their properties and behavior is at least
as important as for the carbonaceous portion of the coal.
1.5 Uses of coal:
There are so many important uses of coal. In power stations steam coal which is,
also, known as thermal coal is used to generate electricity. Currently 39 % of the world‟s
electricity is generated by coal based thermal power station. Coal is essential for iron and
14
steel production, 64 % of steel production worldwide comes from iron made in blast
furnaces which uses coal. In many countries coal is converted into a liquid fuel by
liquefaction process which can be refined to produce transport fuels and other oil products.
In cement factory coal is used as an energy source.
Other important uses of coal include alumina refineries, paper manufacturers and
the chemical and pharmaceutical industries. Refined coal tar is used in the manufacture of
chemicals such as creosote oil, naphthalene, phenol and benzene. Coal, is also, an essential
ingredient in production of activated carbon, carbon fibre and silicon.
1.6 Coal: Geology of Indian Measures
Coals of India belong to two principal geological periods: lower Gondwana coal of
Permian age and Tertiary coal of Ecocene to Miocene age. The Assam coal belongs to
Gondwana and Teritary ages. But the Gondwana coal present in disjointed and lenses
along the Himalayan foothills from Bhutan Duars to Sadiya. This coalfield has not been so
far exploited because of the thickness of the seams and transportation difficulties. It is the
Tertiary coal that is found in thick workable seams and, therefore, mining has developed
for its extraction in several places but the quality of coal is not very good as its organic
sulphur content is high and carbon content is relatively low.
1.6.1 Coal from Assam (North East India)
In North East India the major coal bearing formations are found in the states of
Assam, Meghalaya, adjoining territories of Nagaland and Arunachal Pradesh. The coals of
Assam under Northeastern Coalfield Limited (NECL), Margherita in Northeastern region
of India show some abnormalities in their physico-chemical properties which are attributed
15
to the marine environment and peculiar depositional conditions. The marine depositional
conditions were responsible for the perhydrous nature of the coal and decrement of oxygen
content by organic sulphur replacement. They contain high amount of sulphur (2-7 wt %),
about 70- 80 wt% (in general) of which is organic in nature, which is unique in the
country.
The tertiary coals of Assam are generally very low ash and perhydrous in nature.
Besides their normal use as fuels, they are ideally suited for direct hydrogenation for
production of synthetic oil. Assam coal is mostly of sub-bituminous variety. Assam is said
to contain approximately 260 million tons of coal reserve which continues to be valuable
energy source especially for the various industries like brick, paper and ceramic industries
utilize coal from Assam and, also, for the liquefactions of coal.
Northeastern coalfield, Margherita of Coal India Limited (CIL) is at present
performing coal mining activities at Makum coalfield in Tinsukia district of Assam. It lies
between latitudes 27˚ 13΄-27˚23΄ N and longtitutes 95˚35΄-96˚00΄ E in the Tinsukia district
of Assam, India. It covers an area of about 130 sq. km. Out of 259.37 Million tones of
proven coal reserve in Assam; Makum coalfield has 249.65 Million tones. At present, there
are five working collieries in this field which are located at Ledo, Baragolai, Tipong, Tirap
and Tikak. The map of Assam showing Makum Coalfied, India and their geological
positions are shown in Figure 1F-4 and Figure 1F-5 and Table 1T-2.
16
Table 1T-2: Geological positions of Makum collieries, Assam
Collieries Lattitudes Longitudes
Ledo
Boragulai
Tikak
Tipong
Tirap
27˚18˝N
27˚16˝N
27˚17˝N
27˚18˝N
27˚18˝N
95˚51˝E
95˚55˝E
95˚55˝E
95˚51˝E
95˚51˝E
Figure 1F-4: Map of Assam showing the location of Makum coalfield (not in scale)
17
Figure 1F-5: Geological map of Makum coalfield
1.7 Status of research and development in the subject:
The structural properties of coal have been much attention among the coal
chemists due to their importance in chemical reactivity as well as in various utilizations.
Detailed structural characterization has found to be extremely difficult and, therefore,
research on coal structure is still a challenging task and continues to be pursued
intensively. The basic chemical structure that has been widely accepted today was built up
from X-ray diffraction 30
, Infrared spectroscopy 31
, Nuclear magnetic resonance 32
,
Florescence spectroscopy studies 33
and other physico-chemical properties. On the basis of
these studies it is observed that-
The structure of coal is homogeneous in nature.
18
Coal consists of aromatic layers about 20-30 Å in diameter aligned parallel to near
neighbours at distance about 3.5 Å and aromaticity of coal increases with
increasing rank.
The aromatic carbon in coals occur in layers composed of 4 to 5 condensed rings is
about 30 for coal with 87 to 94 % of total carbon.
Coal (upto 90 % C) contains an appreciable proportion of small layers containing 1
to 3 rings. These small condensed aromatic regions form part of large units which
may themselves be linked to other such units by aliphatic or alicyclic materials or
by 5 membered rings to form large bucket sheet.
Coal is composed of essentially small but heterogeneous condensed aromatic ring
system. The sum of aromaticity and alicyclicity from lignite to higher rank
bituminous coal being constant.
Coal is composed of a 3D cross-linked macromolecular structure.
All these observations are debatable and vary for coals from different regions. The
concept of macromolecular and molecular phase has led to much radical thinking and
research activity in coal research 34-37
.
Since 1960 coal chemists attempted to create models coal structure which represent a
characteristics of coal organic matter. One of the main aims was to construct an “average
structural unit” of coal organic matter. Structural unit were described in classically using
atoms, chemical bonds and some functional groups. The works on this subject were started
as early as 1942 38
. Later various works have been done on structural studies on coal 39-42
.
19
Hirsch first reported that coal consisted of 50-80 % graphitic, with primarily 89 %
ordered structure 43
. Ergun later, using x-ray scattering, concluded that coal is less aromatic
and contains large quantities of amorphous region 44
. X-ray diffraction of coal is a subject
of intense study of several present and past workers 45-52
. Blayden et al. 53
who postulated
that coal contains some aromatic layers aligned parallel to near neighbors at distance about
3.5 Å. The first basic X-ray diffraction studies on coal structure were carried out by simple
data processing 54-57
. Hirsch 54
indicated that coal (upto about 90 % C) contained an
appreciable proportion of small layers consisting of one to three rings. In addition Cartz
and Hirsch 56
stated that these small condensed aromatic regions form part of layered units
which may themselves be linked to other such units by aliphatic or alicyclic materials five-
memberd rings to form large bucket sheets. A number of workers have been attempting to
derive a representative structure of coal among which first proposed by Given 58
and then
Wiser 59
. Some other workers, also, studied on different aspects of coal structure 60-62
.
In Wertz‟s work 63-64
a radial distribution function (RDF) obtained from Fourier
transform of molecular scattering was used to examine the molecular-level structuring of
coals. According to some other workers 65-69
coal consists of primary macromolecules of
poly-aromatic poly-nuclear structure with some hetero-atom groups and their secondary
networks, later of which are derived from aromatic ring stacking, aliphatic side-chain
entanglement and hydrogen bond cation bridges, charge-transfer interaction through
oxygen functional groups.
The study on structure of coal in India was reported By Mahadavan et al. 47
as early
as 1929, which concluded the aromatic layered structure in coals. Mazumder et al. 70
and
20
Mazumder and Lahiri 71
considered coal to be consist of essentially small but
heterogeneous condensed ring systems.
The detailed spectroscopic studies carried out on Indian coals have not been
reviewed as yet as per our literature survey. Not much X-ray diffraction and spectroscopic
work on Assam coals have been reported although extensive research works were done on
other aspects 72-82
particularly sulphur problem of the coal.
1.8 Motivation and outline of the present work
There has been an enormous increase in the global demand for energy in recent
years as a result of industrial development and population growth. The role of coal for
development is remarkable. Coal has, also, potentiality for inter-fuel substitution in
replacing oil, second most popular sources of energy. The existing accessible stock of oil
and natural gas will last for approximately another 70 years while the world‟s most
abundantly available fossil fuel, having a potential reserve of about 1940 billion tones, will
be able to supply at least for three centuries. Coal reserves are more than double the
world‟s petroleum reserves. It has the capability to meet future needs with high reliability
83. The use of coal as an energy source and as a source of organic chemicals feedstock may
become more important in the future 84
. Coal alone can contribute 39 % of the world‟s
electricity 85
. Almost 70 % of electricity is generated from coal in India 85
. India ranks 4th
position in the world in coal reserves with 10 % of the world‟s reserves 85
. Estimated coal
reserve of India is over 92 billion tones 86
.
Coal is made up of heterogeneous materials and its composition varies with places
of occurrences. It is chemically and physically complex, having a physical microstructure
21
that is discernibly derived from plants, and chemical structure containing a wide variety of
polymeric organic compounds and crystalline minerals. The structure of coal also changes
with its matrix 38-41
. Visibly although coal is heterogeneous in composition, there are many
regular and repeating features which have definite physical or chemical structure. Although
it has been studied for more than 50 years, the structure of coal has not yet been
satisfactorily established. The concept of macromolecular and molecular phases in coal had
led to radical changes in thinking and in research activities in respect of structure of coals.
Thus, the structure of coals and the behavior of associated mineral matter; sulfur and trace
element have been serious obstacles on the way of utilization of coal. Detailed structural
characterization has been found to be extremely difficult and, therefore, research on coal
structure is still a challenging task and continues to be pursued intensively. Because of the
relationship of coal structure to its reactivity in various coal processing and utilization
techniques, the structural properties of coal have been receiving much attention among
coal chemists and, thus, a deeper understanding of its chemical composition and structural
characteristics could result in substantial improvement in coal processing and utilization.
The basic chemical structure of coal that has been widely accepted today was built up from
X-ray diffraction, Infrared Spectroscopy, Nuclear magnetic resonance, Fluorescence
spectroscopic studies and other supplemented physicochemical properties.
The diffuseness of the x-ray pattern of coal has been attributed to particles in which
the arrangement of carbon atoms is that of a graphite crystal, but with extremely small size
of the crystallite 1, 40
. These sheets like crystals of negligible thickness tend to accumulate
in parallel groups in which the adjacent sheets have no fixed orientation with respect to
22
each other except that they are mutually parallel. Thus coal possess a turbostatic structure 1,
40, 41, 87, 88 which means that coal contains stacked aromatic layers which are roughly
parallel and equidistant, but with each layer having a completely random orientation in the
plane and about the layer normal. The basic diffraction studies on coal structure indicates
that the typical carbon content in coal are arranged in a macromolecular structure of
condensed aromatic rings that form layer units, with bridges or cross-links formed by
aliphatic and/ or other ethers conferring them a certain structural order 89-91
.
North East India has a substantial deposit of high sulphur coals of approximately
930 million tones 92
which are important sources of energy for the country. The nature of
the organic constituents of Assam coal has been of interest for many years and currently of
special importance owing to its structural aspects. Thus basic knowledge of coal structure
is crucial importance for an understanding of the physical properties of coal and chemistry
of conversion processes such as gasification, liquification, combustion and carbonization
93, 94. Due to the occurrence of low carbon and oxygen but with high sulphur content North
East coals are referred to as abnormal coal 72
. These are of different characteristics in their
chemical composition and physical properties. This variation takes place even in depth
wise also. Thus the variation of coal matrix from place to place will effect on coal structure
and, hence, study of structural parameters of these coals is, also, important for coal
characterization. Apart from the routine analysis of minerals, low and high temperature ash
in coal, the X-ray powder diffraction can be used to determine the short-range structural
features and to describe the details of the average polycyclic aromatic unit in coal. It
includes short-range structural features, structural models, the relationships between aryl/
23
alkyl carbon ratio and the determination of the size of the average polycyclic aromatic unit
in coal. Although some works on Assam coal 95-100
were carried out using radial
distribution function study, but the works on structural model from it, alkyl/ aryl carbon
ratio, aromaticity etc. of Assam coal have not been carried out 101
. This type of works,
however, were carried out on some coals from other places such as APC 401 20
from
Pittsburg seam No.3 from Buchanan country, sapropelitic coals 102
from Russia etc. Fourier
Transform Infrared (FT-IR) spectroscopy is currently one of the most powerful and
versatile technique for the characterization of coal structure. It has several advantages
including relatively low instrumental cost as well as fast and easy operation. FT-IR
technique can be used to supplement the results obtained from XRD study. Thus, structural
properties of coal from North-East India have been receiving much attention among coal
researcher for better understanding of its chemical composition and structural
characteristics in substantial improvement in coal processing and utilization.
24
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