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What is Coal Petrography
The discipline wherein coal is studied as a rock consisting principally of macerals is called coal petrology (Petro: rocks; logy; the science of) and the description or classification of coal as a rock is referred to as coal petrography. Coal is not a uniform mixture of carbon, hydrogen, oxygen, sulphur, and other elements; nor is it, as is often implied, simply a uniform polyaromatic substance. Rather it is an aggregate of microscopically distinguishable, physically distinctive, and chemically different substances called “macerals" and “minerals." The study of coal maceral constituents based on the structure is important to know the coal type.
Two basic systems of petrography have been used over the years, the Stopes-Heerlen (SH) and the Thiessen Bureau of Mines systems. The former, almost universally now accepted, was developed by Marie C Stopes, a British Coal Scientist. The nomenclature system of Stopes began by her description of the different components of coal visible to the unaided eye, called litho types or rock types. Later, she revised her system of nomenclature to include components visible by microscopic examination of polished section of coal in reflected light. The term macerals was suggested for these microscopic components of coal, in analogy to minerals in rocks.
Microscopic Identification of Macerals
Macerals are microscopically identifiable components. The coal under study is powdered, mixed with binder, formed into a block and polished. These blocks are looked at through a microscope to study the structure and reflectance. Depending upon the structure and reflectance of the macerals, it is classified into different categories. The total macerals in coal can be broadly grouped into two major categories, namely the reactive and inert. The reactive group consists of the Vitrinites and Exinites and the inert group has Inertinites. With increasing percentage of inertinites, the coal properties like combustion or gasification reactives deteriorate very much. This is true both in the case of coking and non-coking coals. Mineral matter in coal is generally classified as inherent or extraneous. These are present in coal in addition to purely organic substance. Both macerals and the microlitho types contain small or larger amount of inorganic components.
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Petrography and Coal Combustion
The petrographic composition affects the ignition process, combustion, and the efficiency of combustion. Maceral types and reactivity need not have simple relations, all inertinites are not inert and not all vitrinites are reactive. The reflectance values of the maceral groups and the association of the different macerals, determined as micro lithotypes also need to be taken into account. Mineral matter also affect the type of char formed, and the situation gets further complicated by different char characterization.
Pulverised fuel combustion is used widely in power plants for the generation of electricity. Selection and testing of coals is an important factor for the efficient operation of boilers, and many different tests are carried out for this purpose. Coal petrography has gained importance in order to understand how combustion behaves from the fuel side. Changes occurring during pyrolysis determine the morphology of the char and the char types present affect the overall combustion efficiency. The type of char formed can depend on the macerals present, the rank of the coal, the particle size, and the temperature of char formation.
The main influence on char formation is associated with the organic part of the coal. The presence of certain types of minerals can affect the type of char formed. Reactivity can also be influenced by mineral matter. Vital properties such as flame stability and burnout efficiency can be affected by the presence or absence of certain minerals.
Even though it is evident that mineral matter adversely affects complete combustion, the total removal of mineral matter is impractical and is also undesirable as regards the requirements of modern burners. These inorganic compounds can be classified into four groups according to their origin:
- Inorganic matter from original plants
- Inorganic - organic complex minerals formed due to inorganic and organic interaction during the first stage of coalification process.
- Minerals introduced by water or wind into the coal deposits as they were forming.
- Minerals deposited, during the second phase of coalification process after consolidation of the coal, by ground water solutions in cracks, fissures, or cavities by alteration of primary minerals.
Many variables affect complete combustion including temperature, oxygen level, residence time, and char morphology (mainly structure, porosity, density, and optical texture). Some workers clearly state that vitrinite chars are more reactive than inertinite chars. Vitrinite char has been estimated to burn two to four times as fast as inertinite char. Some semifusinite has been found to ignite before vitrinite and burn faster.
The rank of a coal can be expressed by the reflectance of the vitrinite, which is related to C H and C/O ratios and volatile matter. The use of rank appears to be the most accurate means of predicting combustion behaviour.