GEOLOGI BATUBARA2005

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Formation of coal deposits

• 250 millions years ago, the climate was mild and moist; huge ferns and other plants grew in great abundance in tropical freshwater swamps and bogs, which covered many regions of the earth.

Formation of coal deposits (Continued)

• When the luxuriant growth died, some of the plant material sank under water before it could be oxidized by O2.

• In the absence of air, little decomposition occurred; the material accumulated and became buried under sediments; in time, it was compressed and converted into a porous brown organic material that we know as peat.

Formation of coal deposits (continued)

• In some locations, the peat became more deeply buried, increasing pressure compressed it and changed it into a harder material called lignite.

• Over thousands of years, deeper burial and the resulting increase in temperature and pressure transformed the lignite into various grades of bituminous coal (also known as soft coal).

• In certain areas mountains formed, very high temperature and pressure were associated with mountain-making processes, under these conditions bituminous coal was converted into anthracite (or hard coal).

Characteristics of different types of coals

Type of coal Carbon (%)

Water (%)

Fuel value (MJ/kg)

Peat 5 90 Very low

Lignite 30 40 Low

Bituminous Coal 65 3 High

Anthracite 90 3 High

Note: values may vary considerably with the source of coal

With each step of transformation from peat to anthracite, chemical reactions occur. Volatile compounds are released, the water content of the material decreases; and the carbon content increases. At the same time, the materials becomes harder and brighter.

Composition of coal

• Composed primarily hydrocarbons and small amounts of O-, N-, and S-containing compounds.

• Compared with petroleum, coal contains a higher percentage of aromatic hydrocarbons.

Pros and cons of coal

• Advantages:– Very large resource base– Relative cheap to mine and transport by rail

• Disadvantages:– Emission of air pollutants SO2, NO2 as well as

greenhouse gas CO2.– Coal burning produces large quantities of ash.– Mining posts safety and health threat to

miners– Coal mine drainage is highly acidic,

contaminating local streams.– Strip-mining damages landscape.

Note: Ash is the mineral residue left after complete combustion.

Environmental effects of the use of coal as an energy source

Convert coal to cleaner fuels

• Increase H/C ratio of the coal

C + H2O CO + H2 –131.4 kJ x2

CO + H2O CO2 + H2 + 41.4 kJ

CO + 3H2 CH4 + H2O +206.3 kJ

2C + 2H2O CH4 + CO2 –15.1 kJ

Steam-reforming (900oC)

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exinitevitrinite

inertinite

Polished section of bituminous coal.

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Two broad categories of coal

Coal is an organic sediment consisting of a complex mixture of substances.humic More common and originates from peat deposits consisting mostly of organic debris deposited in situ (autochthonous). sapropelic Derived from redeposited (allochthonous) resistant plant fragments such as spores or aquatic plants. The sapropelic coals can be further subdivided into: cannel coal Cannel coal is made up principally of uniformly sized plant fragments eg spores boghead coal Consists mainly of alginite (a coal maceral derived from algae).

Peat is formed from the deposition of organic material with a restricted supply of oxygen. Peat forming environments are known generally as 'mires'.

Geologi batubaraMires may be classified as limnic or paralic

Paralic deposits imply that there was a hydrological connection with the sea at the time of peat deposition. Mires may be found along coastal lowlands; as back barrier lagoons, estuaries and deltas.

Peat forming environments isolated from the sea, for example slowly subsiding basins produce limnic coal deposits.

The type of original plant input, the availability of nutrients, climatic conditions, the level of the water table, the pH and Eh conditions all help to determine the type of peat that is formed. Every part of the ecosystem of the peatland or mire may be represented in the peat, including the large trees, herbaceous shrubs, grasses, aquatic plants and the micro-organisms that break down the organic material.

For a coal to be developed, the peat has to be buried and preserved. The process that converts peat to coal is called coalification. The degree of coalification which has taken place determines the rank of the coal.

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Coalification

The transformation of plant material into coal takes place in two stages, biochemical degradation and physico-chemical degradation.

Geologi batubaraBiochemical degradation involves chemical decomposition of botanical matter assisted

by organisms. In tropical environments, this process may be faster, since the warm moist conditions are ideal for the organisms that assist in this process such as bacteria and fungi. However plant growth is also more rapid and so the increased rate of decomposition may be balanced by plant growth. In tropical conditions high rates of evaporation need to be coupled with high precipitation to maintain plant growth and peat accumulation.

In cooler climates the growth rate of vegetation may be cyclical in nature and slower since the seasonal variation in conditions is greater. The conditions are less ideal for fungi and bacteria so the slower growth rate is matched by a slower rate of biochemical degradation.

Humification affects the soft contents of the plants cells before the cell walls, which consist of cellulose, hemicellulose and lignin which is the most resistant compound.

Humification begins with the oxidation of plant matter and attack by aerobic organisms such as fungi, insects and aerobic bacteria. Hydrocarbons are extracted from the tissue and the material left behind is relatively enriched in oxygen and carbon. Semifusinite, an inertinite maceral may be formed in this manner.

Various humic substances are formed at this time, these are acidic in nature. If this continues the plant material will be completely degraded into carbon dioxide and water. When the plant material or degraded plant material is buried below the ground water table aerobic organisms and oxidation can no longer attack the material. Anaerobic bacteria may still decompose the plant matter until it reaches a depth or conditions unsuitable for these organisms. Anaerobic bacteria utilise the oxygen in the plant matter, so all molecules may be attacked even the more resistant compounds. However the softer tissue may be more rapidly affected.

Biochemical coalification ends at the rank of sub-bituminous coal, when humic substances have polymerised.

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Physico-chemical coalification which follows is caused by conditions of burial. The overburden which is deposited, the heat flows in the earth's crust and tectonic heat and pressure change the chemistry and structure of the altered organic material. The same conditions are applied to all the macerals.

Water is squeezed out and pore size is reduced as pressure increases and oxygen and hydrogen are released during thermal cracking. Water and carbon dioxide are the first products released.

When rank reaches medium volatile bituminous coal demethanation begins.

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Concept of Coal RankThe rank of a coal refers to the degree of

coalification endured by the organic matter. It is estimated by measuring the moisture content, specific energy, reflectance of vitrinite or volatile matter (these are known as rank parameters). See Table 1 for details of the different rank stages.

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Rank Stages%carbon

(daf)

%volatile matter (daf)

specific energy

(gross in MJ/kg)

% in situ moisture

random maxwood 50 >65 - - - -

peat 60 >60 14.7 75 0.20 0.20

brown coal 71 52 23 30 0.40 0.42

sub-bituminous 80 40 33.5 5 0.60 0.63

high volatile bituminous coal 86 31 35.6 3 0.97 1.03

medium volatile bituminous coal 90 22 36 <1 1.47 1.58

low volatile bituminous coal 91 14 36.4 1 1.85 1.97

semi-anthracite 92 8 36 1 2.65 2.83

anthracite 95 2 35.2 2 6.55 7

Table 1. From Diessel (1992) indicates the difference in rank parameter with increase in rank.

% vitrinite reflectance

Geologi batubaraClassification of organo-petrographic constituents

Coal is used industrially for a number of purposes, since it is a highly variable product its use is dependent on the individual properties of a seam or part thereof. Coal is as variable as the conditions in the mire, during and after peat accumulation. To accommodate this a number of classification systems have been developed so that the coal is used appropriately. The four most commonly used systems are the

– International Organisation for Standardisation (ISO) – American Society for Testing Materials (ASTM) – Australian Standard (AS) – British Standards Institution (BSI) classifications.

Coal beneficiation engages methods to improve coal quality for a particular industrial application, if the coal needs to be altered before use . The most important of these include, washing to remove ash and desulfurisation.

Petrography is the microscopic study and description of coal and rocks. The petrography of coal is important since it affects the physical and chemical nature of the coal. Coal crushing, grinding, handling, washability, gasification, liquefaction combustion and carbonisation are affected by the petrography of the coal.

Coal 'type' refers to the petrographic constituents in coal.

Geologi batubaraMacerals

The smallest microscopically recognisable entities in coal are called macerals, they are analogous to minerals in rocks. However they differ since minerals have an homogeneous chemistry and an orderly internal structure, while coal macerals consist of a mixture of compounds.

The chemical and physical properties of macerals vary with coal rank. Coal macerals are distinguished by: – their optical characteristics of colour – relief of the polished surface – morphology – reflectance and fluorescence

Macerals differ because they represent different parts of the original plant material and micro-organisms that contributed to the peat. The mode of preservation, that is whether or not the organic fragments were oxidised before being preserved, is also considered in the classification of macerals.

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Inertodetrinite set in a vitrinite groundmass. Inertodetrinite set in vitrinite

groundmass. Reflected light, X40 oil objective.(2 20k jpegs)

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There are three maceral groups vitrinite, inertinite and liptinite (exinite). These are all subdivided into maceral subgroups and macerals. Several systems of nomenclature exist world wide. The Australian Standard (AS3856-1986) is modified from the system used by the International Committee for Coal and Organic Petrology, which is continually reviewing their terminology and classification. Table 2 gives a classification based on the Australian standard system.

Geologi batubaraThe liptinite group is relatively enriched in hydrogen

compared with the other two groups, inertinites have a greater carbon content and vitrinites have an intermediate chemistry. As rank increases the differences in the chemistry between the groups diminishes.

The vitrinite group generally represents the woody plant material eg. stems , trunks, roots and branches. Liptinite includes the more resistant parts of plants like spores, cuticles and resins and inertinite is material that has been oxidised prior to coalification.

Vitrinite and inertinite are subdivided into maceral subgroups depending on size and the degree of gelification. Three prefixes are used to divide the macerals into maceral subgroups. Telo- and detro- differentiate between the size of the individual particles and gelo- means the material has been gelified.

Geologi batubara based on the Australian Standard system of nomenclature (AS2856-1986) [* refers to brown coal macerals]Maceral Group Maceral Subgroup Maceral

Textinite*Texto-ulminite *

E-ulminite*Telocollinite

Attrinite*Densinite*

DesmocolliniteCorpogelinitePorigelinite*EugeliniteSporiniteCutiniteResinite

LiptodetriniteAlginite

SuberiniteFluorinite

ExsudatiniteBituminiteFusinite

SemifusiniteSclerotinite

InertodetriniteMicrinite

Gelo-inertinite Macrinite

Table 2: Classification of macerals into subgroups and groups,

Vitrinite

Telovitrinite

Detrovitrinite

Gelovitrinite

Liptinite

InertiniteTelo-inertinite

Detro-inertinite

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Vitrinite in bituminous coal is dark to light grey in colour, depending on the rank. Telovitrinite represents intact fragments of plant matter, the original plant cell structure may be visible in this maceral subgroup. Detrovitrinite results from smaller fragments (must be >20æm in greatest dimension (AS3856-1986)) that often form a groundmass for other macerals. Gelified material produced before or during coalification becomes gelovitrinite which is relatively uncommon. The vitrinite maceral group is desirable technologically.

Geologi batubaraThe inertinite maceral group originates from the same

material as the vitrinite group however the oxidation endured before coalification has changed its optical properties and chemistry. It is much lighter coloured (varying from light grey, to white, to yellow) in comparison with vitrinite. Cell structure is visible in telo-inertinites which are subdivided according to the degree of oxidation. These macerals may exhibit a high degree of relief. The oxidation may have occurred at any time before peat preservation. Forest fires which oxidise wood, can occur during peat accumulation, burnt leaves and wood (charcoal) result in the formation of the maceral fusinite. Since it has a high carbon content to start with the composition of this maceral does not vary with rank Plant material may have already started to gelify and break down before oxidation, this oxidised gel can form the maceral macrinite.

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Inertinite, macrinite horizons,semifusinite/fusinite fragments.

In fluorescent mode, inertinite bodies display varying degrees of fluorescence intensity, with macrinite horizons marginally more intense than semifusinite and fusinite fragments. It has been found that fluorescent inertinite fuses during carbonisation.(2x20k jpegs)

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The final maceral group is liptinite, these macerals include the parts of plants that because of their chemistry are more resistant to physical and chemical degradation. Quantitatively this maceral group is usually much less common, than the other two maceral groups. Above approximately 1.25% mean random vitrinite reflectance liptinite is indistinguishable from vitrinite. Spores, cuticles (found on the surface of leaves and stems), waxes and resin are included in this group, as is alginite which is the remains of algae and rarer substances that may only be detected using fluorescence microscopy. In general this group has a grey to brownish appearance with distinctive morphology and is highly fluorescent when irradiated with short wave (ultra violet or blue) light.

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In fluorescent mode, exinite ( liptinite) bodies fluoresce markedly. Inertinite horizons display minimal fluorescence. Fluorescent light. 40x dry objective.(12k jpeg).

                              

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Using a microscope individual species of plants and micro-organisms that contributed to the peat deposit have been identified, by studying the type of spores and pollen, plant tissue or fungal spores. This can then be used to correlate some seams or seam splits, gain a rough estimate of the age of the deposit or provide some insight into the environment of deposition. Knowing the environment of deposition may be useful to predict changes in coal quality. For example anticipating high sulfur and nitrogen values and changes in ash content.

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The quantity of each maceral group varies both between and within coal seams. Technologically vitrinite is usually the most desirable maceral group, since it contains more hydrogen and oxygen, but these elements decrease with increasing rank. The name inertinite is a misnomer as it is not all inert. The lower reflecting inertinite within a sample has been found to be reactive during carbonisation (Diessel & Wolff-Fischer , 1987).

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Macerals differ in their specific gravity, this can be used to separate the coal from the mineral matter in crushed samples and also to separate out some inertinite (eg. for coal liquefaction). Liptinite macerals are the lightest group followed by vitrinite then inertinite. Fusinite the most carbon rich inertinite has a specific gravity >1.5.

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The Hardgrove grindability of a sample is also a function of the petrographic composition, fusinite is the easiest to grind, vitrinite is the next softest while liptinite macerals are the hardest to grind because of their waxy nature (Tsai 1982).

Geologi batubaraMicrolithotypes

Increasing in scale the macerals of coal form microlithotypes, these are bands >50æm wide. There are three main classes of microlithotypes those that contain one type of maceral (monomaceralic), two types of maceral both with a proportion >5% (bimaceralic) and >5% of all three maceral groups (trimaceralic).

Microlithotype composition shown in Table 3 adapted from Stach et al. (1982). The arrangement of microlithotypes is important technologically because of localised reactions during combustion, carbonisation, liquefaction and gasification. For example vitrinertite since it contains some inertinite is more likely to produce a stronger coke.

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(adapted from Stach et al. 1982)Maceral Composition

(mineral free)Vitrite >95% VitriniteLiptite >95% LiptiniteInertite >95% InertiniteClarite >95% Vitrinite + LiptiniteDurite >95% Inertinite + Liptinite

Vitrinertite >95% Vitrinite + Inertinite

DuroclariteVitrinite+Liptinite+Inertinite each >5%

Vitrinertoliptite Vitrinite > Liptinite, InertiniteLiptinite > Vitrinite, InertiniteInertinite > Vitrinite, Liptinite

Bimaceralic

Trimaceralic

Clarodurite

Table 3 : Classification and composition of microlithotypes, found in coal

Microlithotype Group

Monomaceralic

Geologi batubaraLithotypes

In hand specimen coal is often banded, reflecting change in material and conditions in the mire. These bands are termed lithotypes and there are several systems of classification according to rank and preference.

Classification of brown coals may be based on colour, texture, desiccation pattern, strength and degree of gelification.

The classification of black coal lithotypes is given in Table 4 (from Diessel 1992). The properties of lustre, type of fracture, proportion of banding and mineral matter content are used to distinguish the coal lithotype. Each individual band must be greater than 5mm in width.

The terms vitrain, clarain, durain and fusain refer to the classification of Stopes (1919) which has been extended to apply to a wider range of coals. In practice during logging of core or a coal face, an estimate is made of the percentage of coal brightness , this is useful as a guide to the composition of the coal and technological application. The brightness log may also be used in determining roof and floor limits of mining .

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Routine Analyses

Maceral AnalysisA maceral analysis is carried out on prepared polished

grain mounts (or pellets), the coal is crushed and embedded in a mounting medium and the surface is polished for microscopy. The analysis usually involves counting a thousand points on a grain mount which is covered by evenly spaced traverses using a mechanical stage. Each time the centre of the image falls on a maceral, that maceral is entered into a point counter. The result is a volume percentage of each of the different macerals present in the sample.

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Vitrinite ReflectanceFor bituminous or black coals the most commonly used

rank parameter is vitrinite reflectance, which is measured routinely. Standards are followed for the preparation of samples. The sample is crushed to a particular size then embedded in a mounting medium, the surface is cut and polished and viewed under a microscope using reflected light (light is directed onto the surface of the polished surface and reflected back to the eye piece and measuring device). Reflectance is measured on the maceral telovitrinite and standards indicate the method to be used; International (ISO7404 1984 part 5), Australian Standard (AS 2486-1989) and ASTM (D2798-88).

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This graph illustrates the increasing, then decreasing relative fluorescence intensity with increasing coal rank (Ro). The maxiumum of fluorescence intensity is at 1.1% Ro max.

Geologi batubaraDescription

Bright coal

(vitrain)

Banded bright coal BbMainly bright coal containing thin (less than 5mm) dull coal bands ranging in proportion between 10 and 40%; even fracture.l

Banded coal

(duroclarain)

Banded dull coal

Clarodurain)

Dull coal

(durain)

Fibrous coal

(fusain)

Shaly coal CsContains between 30 and 60% of clay and silt either in intimate mixture with coal or in separate bands each less than 5mm thick.

Coaly shale,

mudstone,

sandstone etc.

Shale, mudstone, siltstone, sandstone,

etc.

Any sediment containing less than 10% carbonaceous matter.

Table 4: The classification of black coal lithotypes (from Diessel 1992).Lithotype

BVitreous to subvitreous lustre; even to conchoidal fracture; brittle; may contain up to 10% dull coal bands less than 5mm thick.

BDContains bright and dull coal bands (all less than 5mm) ranging in proportion between 40 and 60% each.

DbMainly dull coal containing thin (less than 5mm) bright bands in proportion between 10 and 40%; uneven fracture.

Any sediment containing 60 to 90% finely disseminated carbonaceous matter.

DMatt lustre and uneven fracture; may contain 10% of bright coal bands less than 5mm thick.

FDull with satin sheen; friable; may contain up to 10% of other coal lithotypes less than 5 mm thick.

Proximate AnalysisThe degree of physico-chemical coalification of a coal is indicated in a

proximate analysis. There are international standards for the procedure of the proximate analysis; British Standard (BS 1016, part 3, 1973), Australian Standard (AS 1038.3-1989) ASTM (D3173-89). A sample is heated to approximately 900oC so that all the oxygen, hydrogen, sulfur and nitrogen are released as volatile matter (VM). The amount of residual char remaining is inversely proportional to the volatile matter and is called the amount of fixed carbon (FC). The moisture of the sample is also measured in this analysis by measuring the difference in mass of the sample before and after heating. The final variable measured in this analysis the ash content. Ash is determined by heating coal in a furnace at a particular rate up to 815oC in the Australian and British Standards and 750oC in the ASTM procedure.

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Ultimate Analysis An ultimate analysis is performed on coal samples when further information is required.

The following standards outline the procedure to be adopted for the determination of an ultimate analysis; British Standard (BS 1016, part 6, 1977), Australian Standard (AS 1038.6) and ASTM (D3176-89). The quantity of the principal elements in coal; carbon, hydrogen, oxygen and sulfur, are determined. Usually the results of a proximate and ultimate analysis are enough to indicate how the coal may be utilised. Sulfur and nitrogen can be pollutants when the coal is combusted or carbonised. Alternatively some sulfur may be useful as a catalyst during coal liquefaction.

• Carbon and hydrogen are found in coal as hydrocarbons. The quantity of these is determined by heating an air dried sample in a stream of oxygen and collecting the CO2 and H2O produced.

• Nitrogen is determined using the Kjeldahl method. The nitrogen is converted to ammonium sulfate in sulfuric acid. Titration is then used to find the amount of ammonium sulfate.

• Four quantities are usually quoted for the sulfur content. – Total sulfur includes the three forms of sulfur measured. – Organic sulfur is that which is bound in organic molecules (hydrocarbons) and originates

from the sulfur in the plant matter that contributed to the peat deposit. – Sulfide sulfur is that usually found in the form of pyrite and marcasite and is most easily

removed by crushing, followed by washing or float sink methods. • Oxygen is usually calculated from the difference of the other elements from 100%

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Mineral MatterMineral matter is different to the ash quantity of a proximate analysis. Mineral

matter is the quantity of minerals observed in a grain mount and is usually included in a maceral analysis. Ash is a smaller quantity since it is found by heating the sample and the hydrous minerals are altered in the process.

Minerals may be washed into the mire during peat deposition, or may result from air falls due to volcanic activity. Other minerals precipitate out of ground water solution. Dissolved minerals may also occur in the surface and pore water of the sample. This type of mineral matter is referred to as 'adventitious'.

Plants themselves contain inorganic compounds and organo-metallic complexes, that can be added to the peat. This type of mineral matter forms 'inherent ash'.

Mineral matter may occur in thin bands, fill cracks or fissures or be intimately associated with the coal matrix. Cell lumens often contain minerals or mineral matter may replace the cell structure eg. siderite and pyrite. It may also occur finely dispersed within the coal matrix. Discrete bands of mineral matter and infillings are more easily removed during coal washing.

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Geologi batubaraMineral Group Mineral Composition Occurence

illite-sericitedominant-abundant

montmorillonite rare-common

Kaolinitecommob-very common

halloysite rare

pyrite FeS2 rare-common

marcasite FeS2 rare

melnikovite rare

sideritecommon-very common

ankeritecommon-very common

calcitecommon-very common

dolomitecommon-very common

hematite rarequartz rare-common

magnetite very rarerutile very rare

limonite rare-commongoethite rare

raresphalerite rare

galena rareapatite very rare

phosphorite very raresulfates baryte rare

zircon rarebiotite very rare

staurolite very raretourmaline very rare

garnet very rareepidote very rare

orthoclase rarechlorite raregypsum rare

bischofitevery rare-uncommon

sylvinvery rare-uncommon

halitevery rare-uncommon

kieserite very rare-rare

Table 5: From Stach et al. Minerals that are found in coal.

Clay

Iron disulfides

carbonates

oxides

hydroxides

sulfides

salts

phosphates

silicates

Mineral matter in coal is incombustible and so is left as a residue from technological applications. Mineral matter affects the coal processing and handling. Hard minerals increase the wear and tear on equipment during handling and crushing. The quantity of ash and its composition is important to determine the method of its removal, either as a dry ash or a slag. The composition of coal ash can affect the product formed. In the case of coking, the qualitity of the steel produced is affected by the elements in the ash (eg. phosphorous is undesirable). Other elements are pollutants eg. sulfur. Ten elements are routinely determined and are expressed as oxides, thes are SiO2, Al2O3, CaO, Na2O, K2O, Fe2O3, TiO2, MgO ,P2O3 and SO3.

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The most abundant minerals in coal are clays, these are variable in their chemistry. Dominant minerals include kaolinite, illite montmorillonite and illite-montmorillonite mixed layer clays. Bands of clay minerals are useful as marker beds to correlate seams across a coalfield. Clays which have swelling properties such as those in the montmorillonite group, expand in contact with water. This reduces strength and can be hazardous during mining. Clay minerals are converted to silica and alumina during ashing.

Carbonates are the next most common minerals. The main minerals are siderite, ankerite , calcite and dolomite. These decompose to give metal oxides and carbon dioxide.

Sulphides and oxides are commonly found in coal. Table 5. (from Stach et al., 1982) lists minerals found identified in coal and their approximate abundance.

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Ash FusionThe temperature at which the ash melts

determines whether ash will be removed dry or as a slag during combustion. If the ash fusion temperature is lower than furnace temperature , the ash melts and is removed as a slag. However the viscosity of the slag is also dependent on the ash composition.

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ReferencesBailey J.G.(1993) Training Course of Coal Production, Utilisation and

Environmental protection. Bailey J.G.(1993) Training Course of Coal Production, Utilisation and Environmental protection. UNDP/PACE-E

Diessel C.F.K. (1992) Coal- bearing depositional Systems. Springer-Verlag.

Diessel C.F.K. & Wolff-Fischer E. (1987) Coal and Coke petrographic investigations into the fusibility of Carboniferous and Permian Coking Coals. International Journal of Coal Geology 9: 87-108.

Stach E., Mackowsky M-Th., Teichmller M., Taylor G.H., Chandra D., Teichmller R. (1982) Stach's Textbook of Coal Petrology. 3rd edition. Gerbrder Bortraeger, Berlin, Stuttgart.

Tsai S.C. (1982) Coal Science and Technology 2- Fundamentals of coal Beneficiation and Utilisation. Elsevier Scientific Publishing Co.

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