Coal Type

The ultimate microscopic constituents of coal are called macerals. The three main groups are characterized by their appearance, chemical composition, and optical properties. In most cases, the constituents can be traced back to specific components of the plant debris from which the coal formed [10]. The three maceral groups are vitrinite, exinite (sometimes also referred to as liptinite), and inertinite, which in turn can be subdivided into finer classifications. Only the three maceral groups will be introduced here, as extensive discussions of petrography and its relevance to industrial processes can be found elsewhere [1,8,10].

Vitrinite group macerals are derived from the humification of woody tissues and can either possess remnant cell structures or be structureless [8]. Vitrinite contains more oxygen than the other macerals at any given rank level. Exinite group macerals were derived from plant resins, spores, cuticles, and algal remains, which are fairly resistant to bacterial and fungal decay. Exinite group macerals exhibit higher hydrogen content than the other mac-erals, especially at lower rank. The inertinite group macerals were derived mostly from woody tissues, plant degradation products, or fungal remains. These are characterized by a high inherent carbon content that resulted from thermal or biological oxidation [8]. Petrographic analysis has many uses. Initially it was primarily used to characterize and correlate seams and resolve questions about coal diagenesis and metamorphism, but later it influenced developments in coal preparation (i.e., crushing, grinding, and removal of mineral constituents) and conversion technologies [10]. Industrially, petrographic analysis can provide insight into the hardness of a coal (i.e., its mechanical strength) as well as the thermoplastic properties of a particular coal, which are of significant importance in the coking industry.

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