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

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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

.

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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.

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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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