Coal

Coal

Coal is a hard rock which can be burned as a solid fossil fuel. It is mostly carbon but also contains hydrogen, sulphur, oxygen and nitrogen. It is a sedimentary rock formed from peat, by the pressure of
rocks laid down later on top.

Coal is a fossil fuel and is the altered remains of prehistoric vegetation that originally accumulated in swamps and peat bogs. The energy we get from coal today comes from the energy that plants absorbed from the sun

Coal, one of the most important primary fossil fuels, a solid carbon-rich material that is usually brown or black and most often occurs in stratified sedimentary deposits.

Coal is defined as having more than 50 percent by weight (or 70 percent by volume) carbonaceous matter produced by the compaction and hardening of altered plant remains—namely, peat deposits. Different varieties of coal arise because of differences in the kinds of plant material (coal type), degree of coalification (coal rank), and range of impurities (coal grade). Although most coals occur in stratified sedimentary deposits, the deposits may later be subjected to elevated temperatures and pressures caused by igneous intrusions or deformation during orogenesis (i.e., processes of mountain building), resulting in the development of anthracite and even graphite. Although the concentration of carbon in Earth’s crust does not exceed 0.1 percent by weight, it is indispensable to life and constitutes humankind’s main source of energy.

Facts about coal

·         Coal is a rock that can burn.

·         Coal is an energy source.

·         Coal is a sedimentary rock.

·         Coal is an energy mineral (legally a mineral, scientifically a rock).

·         Coal is fossil fuel (because it is derived from fossil plant remains).

·         Coal is a solid hydrocarbon (because it consists mostly of carbon, hydrogen, and oxygen; in contrast, oil is a liquid hydrocarbon, and natural gas is a gaseous hydrocarbon).

·         Coal and the peat it comes from are part of the carbon cycle.

How is Coal Formed?

Coal is formed from the physical and chemical alteration of peat. Peat is composed of plant materials that accumulate in wetlands (bogs and fens). which break down through the process of peatification. lf peats are buried, then the peats can be altered into different ranks of coal through the process of coalification.

Peat and Peatification

Peat is soil-like, partially decayed plant material that accumulates in wetlands. Most people learn that coal is formed in swamps, but this is not completely accurate. The term “swamps” can be applied to many different types of wetlands, but coal only forms from peat-accumulating wetlands. Hence, peat deposits form in wetlands, but not all wetlands form peat. Peat-accumulating wetlands, also called peatlands or mires, include peat bogs, peat fens, and peat forests.

For peat to accumulate, the accumulation of plant debris in a mire must exceed the rate of bacterial decay of the plant debris. The process of partial decomposition of plant material in swampy, waterlogged environments is called peatification.

Peatification involves bacterial decay. The surface layer of most peats is dominated by aerobic bacterial decay (with oxygen) and detritus-eating organisms, so has high decay rates. If conditions in a wetland don’t allow for peat accumulation to exceed the rate of aerobic bacterial decay, or local rates of erosion, a peat will not accumulate or be preserved. In stagnant standing water, or in pore ater within the peat. itself, oxygen can become depleted. This happens naturally in organic-rich waters in which oxygen is used up by the aerobic decay process. Under anaerobic (without oxygen) conditions, the bacterial decay rate is greatly reduced, and peat accumulates. Many peatlands at high latitudes (e.g., northern Canada and Siberia) have extensive peats greater than 15 feet thick. Some modern tropical peats (e.g. Sumatra) may exceed 50 feet in thickness. If these modern peats were buried to great depths, under the right geologic conditions, they could become coal.

Peat deposits are quite varied and contain pristine plant parts (roots, bark, spores, etc.), decayed plant parts, decay products, sediment, and charcoal from fires in the peat swamp. They also contain minerals (mineral matter) from sediment carried into the swamp by wind and water. The variable constituents in peat lead to a wide variety of organic and inorganic constituents in the resulting coals. The constituents within coal can vary both laterally and vertically in a coal seam, influencing the coal’s physical appearance and quality.

coalification

The stages of physical and chemical changes that proceed from peat through coal are called coalification, and are classified and described by rank. In general, rank categories are defined based on recognizable changes in coal parameters. Slightly different categories and parameters are used in different countries, but in general, proceed in increasing rank from lignite, to sub-bituminous, to bituminous, to anthracite coal. Different ranks of coal cannot always be differentiated based only on their physical appearance. Testing of different parameters is often needed to differentiate coal ranks.

1. Peat

A mass of recently accumulated to partially carbonized plant debris. Peat is an organic sediment. Burial, compaction, and coalification will transform it into coal, a rock. It has a carbon content of less than 60% on a dry ash-free basis. a brown deposit resembling soil, formed by the partial decomposition of vegetable matter in the wet acidic conditions of bogs and fens, and often cut out and dried for use as fuel and in gardening. Peat, also known as turf, is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs.

2. Lignite

Lignite is the lowest rank of coal. It is a peat that has been transformed into a rock, and that rock is a brown-black coal. Lignite sometimes contains recognizable plant structures. It has a carbon content of between 60 and 70% on a dry. A soft brownish coal showing traces of plant structure, intermediate between bituminous coal and peat. Lignite, often referred to as brown coal, is a soft, brown, combustible, sedimentary rock formed from naturally compressed peat. It is considered the lowest rank of coal due to its relatively low heat content. It has a carbon content around 60–70 percent

3. Sub bituminous

Sub bituminous coal is a lignite that has been subjected to an increased level of organic metamorphism. This metamorphism has driven off some of the oxygen and hydrogen in the coal. That loss produces coal with a higher carbon content (71 to 77% on a dry ash-free basis). Sub-bituminous coal is a type of coal whose properties range from those of lignite to those of bituminous coal and are used primarily as fuel for steam-electric power generation. Sub-bituminous coals may be dull, dark brown to black, soft and crumbly at the lower end of the range, to bright jet-black, hard, and relatively strong at the upper end. They contain 15-30% inherent moisture by weight and are non-coking (undergo little swelling upon heating)

4. Bituminous

Bituminous is the most abundant rank of coal. Bituminous coal is formed when a sub bituminous coal is subjected to increased levels of organic metamorphism. It has a carbon content of between 77 and 87% on a dry ash-free basis and a heating value that is much higher than lignite or sub bituminous coal. Bituminous coal or black coal is a relatively soft coal containing a tarlike substance called bitumen or asphalt. It is of higher quality than lignite coal but of poorer quality than anthracite. Formation is usually the result of high pressure being exerted on lignite

5. Anthracite

Anthracite is the highest rank of coal. It has a carbon content of over 87% on a dry ash-free basis. Anthracite coal generally has the highest heating value per ton on a mineral-matter-free basis. It is often subdivided into semi-anthracite, anthracite, and meta-anthracite on the basis of carbon content. Anthracite is often referred to as "hard coal". coal of a hard variety that contains relatively pure carbon and burns with little flame and smoke. Anthracite, often referred to as hard coal, is a hard, compact variety of coal that has a submetallic luster. It has the highest carbon content, the fewest impurities, and the highest energy density of all types of coal and is the highest ranking of coals

Question b. Is coal a sedimentary or metamorphic rock?

Coal is an organic sedimentary rock that forms from the accumulation and preservation of plant materials, usually in a swamp environment. Coal is a combustible rock and, along with oil and natural gas, it is one of the three most important fossil fuels. Coal has a wide range of uses; the most important use is for the generation of electricity.

Coal is a readily combustible rock containing more than 50 percent organic matter (carbon) by weight, and 70 percent carbonaceous material by volume including inherent moisture, which was formed from the compaction and alteration of plant remains.

Is coal a sedimentary or metamorphic rock?

Because coal undergoes physical and chemical changes as a result of increased heat, there is sometimes a misconception that coal is a metamorphic rock. Coal is a sedimentary rock. Coal is altered through biological and burial-thermal processes into different ranks. Many sedimentary rocks are also altered through burial-thermal processes (increasing cementation, etc.). 

Of the three categories of rocks, sedimentary, igneous and metamorphic, sedimentary is the closest type that coal could fit into. Coal was not derived from molten material, so is not igneous by definition. It was not transformed under temperature and pressure from a pre-existing rock into a metamorphic rock. Coal was lithified by compressive forces from loose material at relatively low temperature. This is most similar to how sedimentary rock is formed, which is generally the compaction and cementation of inorganic material. The primary difference between coal and other sedimentary rocks is that by definition "rock" is an aggregate of minerals (minerals being inorganic solid material of specific chemical formula and crystaline structure), whereas coal is made of plant material primarily. Plants are not minerals, but geologists have made an exception for coal, since it is a natural earth solid material that is best characterized as a rock versus anything else we could classify it as, such as life, water, or air.

Question c. Using A Van Krevelen  diagram classifying kerogens

Van Krevelen diagrams are graphical plots developed by Dirk Willem van Krevelen (chemist and professor of fuel technology at the TU Delft) and used to assess the origin and maturity of kerogen and petroleum. The diagram cross-plots the hydrogen: carbon atomic ratio as a function of the oxygen: carbon atomic ratio.

A plot of atomic oxygen/carbon (O/C) versus atomic hydrogen/carbon (H/C) originally used to classify coals and predict compositional evolution during thermal maturation, but adapted to classify kerogen types I, II, III, and IV. More common is the pseudo-Van Krevelen diagram, where Rock-Eval pyrolysis oxygen index ([OI] mg HC/g TOC) is plotted versus hydrogen index ([HI] mg HC/g TOC).

A Van Krevelen  diagram is one example of classifying kerogens, where they tend to form groups when the ratios of hydrogen to carbon and oxygen to carbon are compared.

Type I: Sapropelic

Type 1 oil shales yield larger amount of volatile or extractable compounds than other types upon pyrolysis. Type1 kerogen oil shales provide the highest yield of oil and are the most promising deposits in terms of conventional oil retorting. Containing alginite, amorphous organic matter, cyanobacteria, freshwater algae, and land plant resins

·         Hydrogen: carbon ratio > 1.25

·         Oxygen: carbon ratio < 0.15

·         Shows great tendency to readily produce liquid hydrocarbons.

·         It derives principally from lacustrine algae and forms only in anoxic lakes and several other unusual marine environments

·         Has few cyclic or aromatic structures

·         Formed mainly from proteins and lipids

Type II: Planktonic

Type II kerogen is common in many oil shale deposits.

It is based on marine organic materials, which are formed in reducing environments.

Although pyrolysis of Type II kerogen yields less oil than Type I, the amount acquired is still sufficient to consider Type II bearing rocks as potential oil sources

·         Plankton (marine)

·         Hydrogen: carbon ratio < 1.25

·         Oxygen: carbon ratio 0.03 to 0.18

·         Tend to produce a mix of gas and oil.

Several types:

–Sporinite: formed from the casings of pollen and spores

–Cutinite: formed from terrestrial plant cuticle

–Resinite: formed from terrestrial plant resins and animal decomposition resins

–Liptinite: formed from terrestrial plant lipids (hydrophobic molecules that are soluble in organic solvents) and marine algae

Type III: Humic

Kerogen Type III is formed from terrestrial plant matter that is lacking in lipids or waxy matter. It forms from cellulose, the carbohydrate polymer that forms the rigid structure of terrestrial plants,TypeIII kerogen involving rocks are found to be the least productive upon pyrolysis and probably the least favorable deposits for oil generation

·         Land plants (coastal)

·         Hydrogen: carbon ratio < 1

·         Oxygen: carbon ratio 0.03 to 0.3

·         Material is thick, resembling wood or coal.

·         Tends to produce coal and gas.

·         Has very low hydrogen because of the extensive ring and aromatic systems

Type IV: Residue

·         Hydrogen: carbon ratio < 0.5

·         Type IV kerogen contains mostly decomposed organic matter in the form of polycyclic aromatic hydrocarbons.

·         They have no potential to produce hydrocarbons.

Question d. Vitrinite reflectance

Vitrinite reflectance is a measurement of the maturity of organic matter with respect to whether it has generated hydrocarbons or could be an effective source rock.

Vitrinite reflectance is a measure of the percentage of incident light reflected from the surface of vitrinite particles in a sedimentary rock. It is referred to as %Ro. Results are often presented as a mean Ro value based on all vitrinite particles measured in an individual sample.

Connection between vitrinite and kerogen

The maturation of vitrinite is a kinetic process. The relationship between %Ro and hydrocarbon generation is dependent on the chemistry of the vitrinite as well as the chemistry of the kerogen.

Vitrinite: Is one of the primary components of coals and most sedimentary kerogens. Vitrinite is a type of maceral, where "macerals" are organic components of coal analogous to the "minerals" of rocks. Vitrinite has a shiny appearance resembling glass (vitreous).

Vitrinite Reflectance Measurements

All organic maceral with lower reflectance than inertinite,and higher reflectance than liptinite are usually given the generic name vitrinite and thier reflectance is measured.

For Ro measurements from coals, the situation is a little better becouse hydrogen rich vitrinite such as desmocollinite and hydrogen poor vitrinite such as pseudovitrinite can be distinguished by comparing their reflectance and relief with those of telecollinite and semi-fusinite which can be found easly in most humic coals.

The measurement of vitrinite reflectance is defined by both ISO and ASTM standards methods (as well as other national standards).  ISO 7404-5 and ASTM D2798 both state that measuring the vitrinite reflectance of coal is to be done with a specially configured microscope using a calibrated photometer.  The microscope is configured for incident illumination with green light where the illuminating light may be either plane-polarized or not.  The reflected light intensity is measured with either a photometer, a spectrophotometer or with a digital camera fitted to the microscope.

The system is calibrated with special Vitrinite Reflectance Standards, supplied by CRAIC.  The sample is then placed on the sample stage and brought into focus.  Using the standard methodology, at least 100 measurements are made of the sample.  This allows the user to test blends that contain coals of different ranks.  From the data, the mean and standard deviation of all the readings are calculated as percent reflectance.  The spread of the individual reflectance values is also plotted as a histogram.  This allows the user to determine the different macerals and types of macerals in a sample.  This data will give an indication of the rank of the coal sample.

The study of vitrinite reflectance (or VR) is a key method for identifying the maximum temperature history of sediments in sedimentary basins. The reflectance of vitrinite was first studied by coal explorationists attempting to diagnose the thermal maturity, or rank, of coal beds. More recently, its utility as a tool for the study of sedimentary organic matter metamorphism from kerogens to hydrocarbons has been increasingly exploited. The key attraction of vitrinite reflectance in this context is its sensitivity to temperature ranges that largely correspond to those of hydrocarbon generation (i.e. 60 to 120 °C).

This means that, with a suitable calibration, vitrinite reflectance can be used as an indicator of maturity in hydrocarbon source rocks. Generally, the onset of oil generation is correlated with a reflectance of 0.5-0.6% and the termination of oil generation with reflectance of 0.85-1.1%. The onset of gas generation ('gas window') is typically associated with values of 1.0-1.3% and terminates around 3.0%. However these generation windows vary between source rocks with different kerogen types (vitrinite is typically abundant in 'Type III' kerogen-rich source rocks), so a conversion to 'Transformation Ratio' (TR) can be applied to create a kerogen-specific maturity parameter.

The vitrinite reflectance value represents the highest temperature that the vitrinite maceral (and source rock) has experienced, and is routinely used in 1D burial modelling to identify geological unconformities in sedimentary sections.

Typically, vitrinite reflectance data is presented in units of %Ro, the measured percentage of reflected light from a sample which is immersed in oil (%Ro = % reflectance in oil).

The lack of vitrinite macerals in marine shales with little terrestrial input often requires alternative maturity parameters instead of vitrinite reflectance such as heating the sample to determine the hydrocarbons present (Rock-Eval Tmax in industry jargon), biomarker equivalences and other maceral reflectance parameters (e.g. liptinite reflectance).

However the problem still exists to some extent , becouse even the telocollinite in coals with high contents of desmocollinite or pseudovitrinite has relatively high or lower initial hydrogen content than that in ‘’normal’’coals.

Question e. Stable Isotopes in relation to geochemical method.

Stable Isotopes

an aspect of geology based upon the study of natural variations in the relative abundances of isotopes of various elements. Variations in isotopic abundance are measured by isotope ratio mass spectrometry, and can reveal information about the ages and origins of rock, air or water bodies, or processes of mixing between them.

An isotope is an atom whose nuclei contain the same number of protons but a different number of neutrons. Isotopes are broken into two specific types: stable and unstable. These unstable isotopes are commonly referred to as radioactive isotopes. There are approximately 300 known naturally occurring stable isotopes. Stable isotope is largely concerned with isotopic variations arising from mass-dependent isotope fractionation, whereas radiogenic isotope geochemistry is concerned with the products of natural radioactivity.

For most stable isotopes, the magnitude of fractionation from kinetic and equilibrium fractionation is very small; for this reason, enrichments are typically reported in "per mil" (‰, parts per thousand).[1] These enrichments (δ) represent the ratio of heavy isotope to light isotope in the sample over the ratio of a standard.

The stable isotope ratios of various elements (e.g., H, C, O, S) have numerous uses to improve the understanding of the genesis and formation of hydrothermal and magmatic ore deposits, as well as having various applications to mineral exploration. However, stable isotope data has not been routinely collected during mineral exploration for various reasons related to cost per sample, the speed at which analytical data can be collected, and uncertainty regarding the benefits of stable isotope measurements to mineral exploration

The power of the stable isotope technique resides in the fact that the most commonly studied elements (H, C, O, and S) also constitute the major components of Earth reservoirs (water, air, lithosphere, and organic matter). For example, hydrogen and oxygen isotopic studies of natural waters have a distinct advantage over studies using other chemical indicators because hydrogen and oxygen are the principal constituents of aqueous solutions

Carbon Isotopes

There are three naturally occurring isotopes of carbon: 12, 13, and 14,12C and 13C are stable, occurring in a natural proportion of approximately 99:1.14C is produced by thermal neutrons from cosmic radiation in the upper atmosphere, and is transported down to earth to be absorbed by living biological material.

 Isotopic ally, 14C constitutes a negligible part but, since it is radioactive with a half-life of 5.700 years, it is radio-metrically detectable.

Since dead tissue doesn't absorb 14C, the amount of 14C is one of the methods used within the field of archeology for radiometric dating of biological material.

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