Coal Geology

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<p>GEOLOGI BATUBARA 2005</p> <p>Geologi batubara</p> <p>Formation of coal deposits250 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.</p> <p>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.</p> <p>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).</p> <p>Characteristics of different types of coalsType of coal Peat Lignite Bituminous Coal Anthracite Carbon Water (%) (%) 5 30 65 90 90 40 3 3 Fuel value (MJ/kg) Very low Low High High</p> <p>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.</p> <p>Composition of coalComposed primarily hydrocarbons and small amounts of O-, N-, and S-containing compounds. Compared with petroleum, coal contains a higher percentage of aromatic hydrocarbons.</p> <p>Pros and cons of coalAdvantages:Very large resource base Relative cheap to mine and transport by rail</p> <p>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.</p> <p>Environmental effects of the use of coal as an energy source</p> <p>Convert coal to cleaner fuels Increase H/C ratio of the coal C + H2O CO + H2 131.4 kJx2 CO + H2O CO2 + H2 + 41.4 kJ CO + 3H2 CH4 + H2O +206.3 kJ 2C + 2H2O CH4 + CO2 15.1 kJSteam-reforming (900oC)</p> <p>Geologi batubaraPolished section of bituminous coal.</p> <p>vitrinite exinite</p> <p>inertinite</p> <p>Geologi batubaraTwo broad categories of coalCoal 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'.</p> <p>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.</p> <p>Geologi batubara</p> <p>Geologi batubara</p> <p>Geologi batubara</p> <p>Geologi batubaraCoalification The transformation of plant material into coal takes place in two stages, biochemical degradation and physico-chemical degradation.</p> <p>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.</p> <p>Geologi batubaraPhysico-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.</p> <p>Geologi batubaraConcept of Coal Rank The 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.</p> <p>Geologi batubaraTable 1. From Diessel (1992) indicates the difference in rank parameter with increase in rank.</p> <p>Rank Stages</p> <p>specific %volatile energy % in situ %carbon matter (gross in moisture (daf) (daf) MJ/kg)50 60 71 80 86 90 91 92 95 &gt;65 &gt;60 52 40 31 22 14 8 2 14.7 23 33.5 35.6 36 36.4 36 35.2 75 30 5 3 20m 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.</p> <p>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 teloinertinites 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.</p> <p>Geologi batubara</p> <p>Inertinite, macrinite horizons, semifusinite/fusinite fragments.</p> <p>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)</p> <p>Geologi batubaraThe 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.</p> <p>Geologi batubara</p> <p>In fluorescent mode, exinite ( liptinite) bodies fluoresce markedly. Inertinite horizons display minimal fluorescence. Fluorescent light. 40x dry objective.(12k jpeg).</p> <p>Geologi batubaraUsing 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.</p> <p>Geologi batubaraThe 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 &amp; Wolff-Fischer , 1987).</p> <p>Geologi batubaraMacerals 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 &gt;1.5.</p> <p>Geologi batubaraThe 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).</p> <p>Geologi batubaraMicrolithotypes Increasing in scale the macerals of coal form microlithotypes, these are bands &gt;50m wide. There are three main classes of microlithotypes those that contain one type of maceral (monomaceralic), two types of maceral both with a proportion &gt;5% (bimaceralic) and &gt;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.</p> <p>Geologi batubaraTable 3 : Classification and composition of microlithotypes, found in coal (adapted from Stach et al. 1982) Maceral Composition Microlithotype Group (mineral free) Vitrite &gt;95% Vitrinite Monomaceralic Liptite &gt;95% Liptinite Inertite &gt;95% Inertinite Clarite &gt;95% Vitrinite + Liptinite Bimaceralic Durite &gt;95% Inertinite + Liptinite Vitrinertite &gt;95% Vitrinite + Inertinite Vitrinite+Liptinite+Inertinite Duroclarite each &gt;5% Trimaceralic Vitrinertoliptite Vitrinite &gt; Liptinite, Inertinite Liptinite &gt; Vitrinite, Inertinite Clarodurite Inertinite &gt; Vitrinite, Liptinite</p> <p>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 .</p> <p>Geologi batubara</p> <p>Routine Analyses</p> <p>Geologi batubaraMaceral Analysis A 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...</p>