Coal Definition, Petrology and Analysis

7/21/2019 Coal Definition, Petrology and Analysis

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A. COAL DEFINITIONCoal is a fossil fuel. It is a combustible, sedimentary, organic

rock, which is composed mainly of carbon, hydrogen and oxygen. It

is formed from vegetation, which has been consolidated between

other rock strata and altered by the combined e ects of pressure

and heat over millions of years to form coal seams.Coal is the altered remains of prehistoric vegetation that

originally accumulated in swamps and peat bogs. The build-up of silt

and other sediments, together with movements in the earth s crust

!known as tectonic movements" buried these swamps and peat

bogs, often to great depths. #ith burial, the plant material was

sub$ected to high temperatures and pressures. This caused physicaland chemical changes in the vegetation, transforming it into peat

and then into coal.%ased on data provided by the International &nergy 'gency

and the %( )tatistical *eview of #orld &nergy+ Coal provides . /

of global primary energy needs and generates over 0 / of the

world1s electricity. It is also used in the production of over 2 / of

the world s steel. Total world coal production reached a record level

of 2344.3 5t in 4 , or .0/ more than in 4 4. The I&' reports that according to the 6erman 7ederal Institute

for 6eosciences and 8atural *esources there were 94 billion

tonnes of coal reserves remaining as of 4 4, or 0.: billion tonnes more than in 4 . These proved

reserves represent 0.9 years of production at current levels, up

from . years calculated last year and .: years in the

preceding year.;owever, other publications such as the %( )tatistical *eview

of #orld &nergy often refer to the #orld &nergy Council estimates of

global coal reserves. 'ccording to this source there are 3<4 billion

tonnes of coal reserves left, or years of coal output. These are the top ten coal producers in 4 +

Top Ten Coal Producers (2013e)PR China 3561Mt Russia 347Mt USA 904Mt South Africa 256Mt

n!ia 613Mt "er#an$ 191Mt n!onesia 4%9Mt Po&an! 143Mt



Austra&ia 459Mt 'a a hstan 120Mt

B. COAL PETROLOGY Coal petrology and the techni=ues used in coal petrology,

particularly optical microscopy, have important applications in a

number of areas related to coal and its derivative products as well

as in other areas not directly related to coal. The application of

organic petrology methods in archaeology in relation to the organic

gems and artifacts, environmental studies, spontaneous

combustion, forensic geology, and auto brakes is discussed. >ther

applications of coal petrography include those of a predictive

character, which are used to predict the hardgrove grindability of

coal.Coal is used in processes such as combustion, gasi?cation,

and li=uefaction and in carboni@ation for the manufacture of

metallurgical coke. Coal and its derivative products are also used as

precursors of other materials and in the production of chemicals.

There are two characteristics that inAuence the use of coal+ its

composition and its rank. Coal composition is, in turn, representedby two essentially independent factors+ type and grade.

Coal is a heterogeneous material, and evaluation of coal type

may be approached on two di erent levels+ the macroscopical and

microscopical, both of which form a part of coal petrology. The coal

metamorphism involves the physical and chemical transformation

from peat through bituminous coal through anthracite and meta-

anthracite to graphite. It is a function of heat and pressure acting

over a period of time. It is also denoted as the coal rank, which is

marked by a progressive decrease in moisture and volatile

functional groups with a conse=uent increase in the carbon content

of the coal.



*i+ure 2,1 CoalCoal formation began during the Carboniferous (eriod B known

as the ?rst coal age B which spanned : million to 4< million

years ago. The =uality of each coal deposit is determined by

temperature and pressure and by the length of time in formation,

which is referred to as its organic maturity .Initially the peat is converted into lignite or brown coal B

these are coal types with low organic maturity. In comparison to

other coals, lignite is =uite soft and its colour can range from dark

black to various shades of brown. >ver many more millions of years,

the continuing e ects of temperature and pressure produces furtherchange in the lignite, progressively increasing its organic maturity

and transforming it into the range known as sub-bituminous coals.

7urther chemical and physical changes occur until these coals

became harder and blacker, forming the bituminous or hard coals .

Dnder the right conditions, the progressive increase in the organic

maturity can continue, ?nally forming anthracite.



*i+ure 2,2 Coal 7ormation

Coal type is related to the type of plant material in the peat

and the extent of its biochemical and chemical alteration. Type can

be assessed in terms of variety of petrographic analysis. Coal

petrology is concerned with the origin, composition and properties

of the distinct organic and inorganic components of di erent coals.

To date, the principal practical application of coal petrology have

been in the speci?cation and selection of coals for carboni@ation.



*i+ure 2,3 Type of Coal

The degree of change undergone by a coal as it matures from

peat to anthracite B known as coali?cation B has an importantbearing on its physical and chemical properties and is referred to as

the rank of the coal. Eow rank coals, such as lignite and

subbituminous coals are typically softer, friable materials with a dull,

earthy appearance. They are characterised by high moisture levels

and low carbon content, and therefore a low energy content. ;igher

rank coals are generally harder and stronger and often have a black,

vitreous lustre. They contain more carbon, have lower moisturecontent, and produce more energy.

Coals are divided into lignitic, subbituminous, bituminous, and

anthracitic classes, and further subdivided into groups. Coals of

the bituminous class are most sought after in the C%5 process

because most properties are optimum at this rank. )peci?cally,

coals of hv'b through lvb are best. 5ore gas has been generated by

this point in the maturation process and retention capabilities have



been improved. 'lso, physical properties and mechanical properties

of the coal as a reservoir rock are optimum.

-a.&e 2,1 ')T5 Coal *ank

'nthracite is at the top of the rank scale and has a

correspondingly higher carbon and energy content and a lower level

of moisture. 6as content depends on the coal s rank, a measure of

the =uality and thermal maturity of the organic matter. 5echanicalproperties of the coal also depend on rank.

There are various means to establish rank. The ')T5

establishes percentage of ?xed carbon content and percentage of

volatile matter on a dry, ash-free basis as the standard for

designating ranks of coals at hv'b or higher in 'merica. In &urope,

4 the designation of rank may be based on percentage of carbon in

the elemental analysis on a dry, ash-free basis, rather than on a

percentage of ?xed carbon. Dniversally, an important criterion that

is highly accurate for the high-ranking coals most encountered in

C%5 pro$ects, and which is also independent of maceral content

variations, is the maximum vitrinite reAectance.

Comparisons of the methods for designating rank arepresented in table below



-a.&e 2,2 (arameters Fetermining Coal *ank

MACROSCOPIC COMPONENTS OF COAL ( visible to the nec e!

e"e#5egascopically distinguishable ingredients of humic coals are

recogni@ed+ Gitrain, Clarain, Furain and 7usain. These varieties of

coal have been invested with the status of separate Hrock types ,and are therefore termed as EIT;>TJ(&).

The four macroscopic components in coal are+• GIT*'I8+ &ssentially bright glossy, brilliant in luster and

homogeneous component of coal, having a massive texture

and showing characteristics vitreous conchoidal fracture.• CE'*'I8+ %right component of coal in overall appearance, but

less brighter than vitrain. It is hetrogeneous material with a

banded structure and has a de?nite and smooth surface when

fractured at right angles to bedding plane.• FD*'I8+ &ssentially dull component of coal, often with a

suggestion of a slightly greasy black in overall appearance, K

usually harder than bright coal. it is hetrogeneous and has a

?rm granular texture.• 7D)'I8+ It occurs in pockets or as patches rather than uniform

brand, of soft, somewhat ?brous material resembling charcoal.It is highly friable and can be readily powdered by ?ngers.



MICROSCOPIC COMPONENTS OF COALS (invisible to then$ e! e"e#

Lust as a rock is composed of several minerals so is the coal

composed of several organic constituents termed as macerals, the

organic e=uivalent of minerals !which are di erent types of

inorganic particles found in coals and other rocks".

The micro-components !macerals" found in high and medium rank

coals are+

• GIT*I8IT& !termed as ;D5I8IT& for peat and Eignite or low

rank coals, essentially woody materials"+ derived from plant

cell substances varing in appearance from being completely

structureless to exhibiting well discernible tissues. 5a$or

component of Gitrain and one of the two principal components

of Clarain.

• &xinite !EI(TI8IT& in low rank coals"+ derived from secretions

and waxy coatings of plants, and lower in reAectance than

vitrinite. The other principal component of clarain and durain.• I8*&TI8IT& !derived mainly from oxidised plant material"+ with

or without recogni@able plant structures, and higher in

reAectance than vitrinite. 5a$or component of 7usain. >ne of

the two principal components of Furain. In maceral analysis, it

is commonly subdivided into macerals MACRINITE%

MICRINITE% SEMIF&SINITE ' F&SINITE.

Investigating a coal for the purpose of utili@ation onvolves

knowing something about all these characteristic, none of which

should be separated from others. Coal =uality is a function of these

factors and their interactions, and coal petrology is the

fundamental discipline that contributes to the knowledge of coal

=uality. The petrology of coal may be expressed by a number of

fundamental parameters, including+



. The n$t )e o* the o)+$nic constit ent in terms of

macerals or maceral groups !an indicator of coal type"

4. The ,ine)$l ,$tte) , including the ma$or elements in the

coal or oxides in the ash, the minerals in the coal, theforms of sulfur, and the trace elements that may also be

present !indicator of coal grade"

. The vit)inite )e-ect$nce !which is usually taken as an

indicator of coal rank"

These parameters reAect the composition and rank of the coal

and are the primary factors that contribute the coal s speci?c

physical and chemical properties. The physical and chemical

properties in turn determine the overall =uality of the coal and its

suitability for speci?c purposes.

These are several methods which are used to determine the

coal properties+

(roperties 5ethodsChemical

(roperties

proximate analysis, ultimate analysis, and ash

analysis(hysical(roperties

density, speci?c gravity, pore structure, surfacearea, reAectivity

5echanical

(roperties

hardnessMabrasiveness friability, grindability,

dustiness index

Thermal

(roperties

calori?c value, heat capacity, thermal

conductivity, plastic, agglomerating index,

free-swelling index&lectrical

(ropertieselectrical resistivity, dielectric constant,

C. PRO IMATE ANALYSIS%y de?nition, coal must contain at least 9 / of its weight, or

2 / of its volume as organic, carbonaceous matter. ' proximate

analysis is a common laboratory procedure to provide fundamental

composition of the coal.

(roximate analyses of coal provide the percentagecomposition in coal of the following+



• 'sh.• 7ixed Carbon.• Golatile 5atter.• 5oisture Content.

-he tests are s/eci e! .$ /roce!ure 3172 AS-M Stan!ar!s, ach

of the four #easure! /ara#eters has si+ni cance to the C M

/rocess,

• 'sh The ash measured in the proximate analysis represents

that part of the mineral matter left after thermal degradation

of the sample by combustion !')T5 F- 20". ' small ! B4

gram" sample of the coal is completely burned in air at 249 N

49OC. The residue is the ash content. It has a value near that

of the percentage of mineral matter. 'n increasing ash

content, from a proximate analysis indicating mineral matter,

proportionately lowers the amount of methane that can be

adsorbed. 5ineral matter also has a deleterious e ect on

fracturing in the coal. %eing a determinant in limiting cleat

formation and gas content, mineral matter thus impacts two

of the most important parameters in the commercial C%5P

permeability and adsorbed methane capacity. The inorganic

particles that comprise the ash of the analysis are distributed

throughout the coal as clay minerals, carbonate minerals,

sul?de minerals !pyrite", and silica minerals !=uart@".

• 7ixed CarbonCarbon content increases with maturation until graphite

of / carbon would be reached ultimately. 7ixed carbon

from the preceding three tests is calculated using &=uation

bellow+%FC = 100 −( %Ash + H 2 O +%VM )

#here+7C Q calculated ?xed carbon of the coal

%Ash Q measured by ')T5 F- 20 H

2

Q measured by ')T5 F- 2



%VM Q measured by ')T5 F- 29

• Golatile 5atterGolatile matter is determined from the thermal

decomposition, without oxidation, of a -gm crushed sample!R: mesh" at <9 N4 OC for 2 minutes in a muSe furnace

!')T5 F- 29". Golatile matter and ?xed carbon of the

proximate analysis are used to specify higher coal ranks

above hv'b in the Dnited )tates.

• 5oisture Content5oisture content a ects methane adsorption capacity.

5oisture contents are determined !')T5 F- 2 " by heating asmall coal sample for hour in a vacuum or in a nitrogen

atmosphere to 2 N0OC. The weight loss as a percentage of

the original sample is reported as moisture content. %efore

beginning the analysis, the sample is crushed to R: mesh.

The percentages of ash, ?xed carbon, volatile matter, and

moisture of the proximate analysis may be presented on the

following bases+• 's receivedP(ercentages based on all four measured

components, which represent the coal as received in the

laboratory, approximating the conditions in the seam.• 'sh-free !'7"P(ercentages based on three measured

components without inclusion of ash.• FryP(ercentages based on the three components of volatile

matter, ?xed carbon, and ash.• Fry, ash-free !F'7"P(ercentages based on the two

components of volatile matter and ?xed carbon.

-a.&e 3,1 &xample of (roximate 'nalysis



*i+ 3,1, (roximate analysis of 7ruitland Coals. Copyright <<4 )(&

D. &LTIMATE ANALYSISDltimate analysis provides the elemental composition of

oxygen, carbon, hydrogen, sulfur, and nitrogen. The annual book of ')T5 standards presents the standard

method for ultimate analysis as procedure F- 2:. It speci?es that

carbon and hydrogen of the coal will be determined from the

gaseous products of the material s complete combustion !F- 23".

The total sulfur !F- 22", nitrogen !F- 2<", and ash !F- 20" are

to be determined from the entire material in separate calculations.7or lack of a suitable test for oxygen, its percentage content in

the coal is determined by subtracting from the sum of the

percentages of the other components. ' small error is taken for

granted but cannot be compensated for in the procedure because

some hydrogen and oxygen will be derived from the bound water of

clay, shale, or carbonate impurities in the coal.



The elemental analysis of coal obtained by this procedure,

when converted from a weight basis to a mole basis, provides the

ratios of >MC and ;MC used in the van revelen diagram2 to de?ne

the maturation state of coal. The following ultimate analysis applies to an mvb coal of the

%lue Creek seam of the #arrior basin that contains 0.34/ ash+ ! "

carbon, 3 .0:/U !4" hydrogen, 0. </U ! " nitrogen, .3 /U !0"

sulfur, .02/U and !9" oxygen, 9. 9/.

*i+ure 3,2 Gan revelen Fiagram

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Coal Definition, Petrology and Analysis