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 different coals. To date, the principal practical application of coal petrology has been in the specification and selection of coals for carbonization.
According to its mode of origin, mineral matter within a coal seam may be classified into the following two categories:
MACROSCOPIC COMPONENTS OF COAL (visible to the necked eye):
Megascopically distinguishable ingredients of humic coals are recognized, Vitrain, Clarain, Durain and Fusain. These varieties of coal have been invested with the status of separate “rock types”, and are therefore termed as LITHOTYPES.
The micro-components (macerals) found in high and medium rank coals are:
· VITRAIN: Essentially bright glossy, brilliant in luster and homogeneous component of coal, having a massive texture and showing characteristics vitreous conchoidal fracture.
· CLARAIN: Bright component of coal in overall appearance, but less brighter than vitrain. It is hetrogeneous material with a banded structure and has a definite and smooth surface when fractured at right angles to bedding plane.
· DURAIN: Essentially dull component of coal, often with a suggestion of a slightly greasy black in overall appearance, & usually harder than bright coal.
it is hetrogeneous and has a firm granular texture.
· FUSAIN: It occurs in pockets or as patches rather than uniform brand, of soft, somewhat fibrous material resembling charcoal. It is highly friable and can be readily powdered by fingers.
Principle technique of applied coal petrology:
· Maceral analysis: It is a technique widely used for providing valuable information on the behavior of coals during carbonization. The analysis summarized in terms of total REACTIVES ( Vitrinite + exinite + 1/3 semi fusinite) and total INERTS (2/3 semi fusinite + other inertinite macerals + mineral matter).
· MICROLITHOTYPES Analysis: Macerals rarely occur at random within any lithyotype bands, under the microscope such a band can usually be seem to comprise thinner bands distinguished from each other consisting of different maceral associations, and which are known as microlithotypes. This analysis is sometimes used as an additional aid in the study of coals for carbonization.
Principal Maceral group (s)
Vitrinite
Vitrinite, exinite
Vitrinite, inertinite
Vitrinite, eximite, inertinite
Inertinite, exnite
Inertinite
Principal Microlithotypes group(s)
Vitrinite
Clarite
Vitrinertite
trimacerite
durite
inertite
Lithotype
Vitrain
Clarain
Durain
Fusain
Prinicipal microlithotype
vitrite
vitrite
Clarite
Vitrinertite
Trimacerite
Durite
Durite
Trimacerite
Inertite
In a given coal, exinite has a higher volatile matter than vitrinite, whereas inertinite has a lower value. The amount of mineral matter in most durains and fusains is considerably greater than in the corresponding vitrains, while it is intermediate in clarains. Qualitatively, the mineral matter content in vitrain generally tends to be lower in silica and alumina, and appreciably higher in alkalis, than in durain, while clarain is again intermediate. The mineral matter in fusain varies widely in composition.