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Chapter 2 | Microcomponents in Coal
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Peat, the material from which coal is formed, consists of loosely consolidated layers of various combinations of plant and mineral matter. Peat accumulates in “peat swamps,” “bog lands,” and “mires.” Over millions of years, burial, compression by overlying sediments, and the eflects of heat (from proximity to volcanic sources or depth in the earth) cause peat to very gradually change to coal. Coal is an extremely complex and predominantly organic rock. To be classified as coal, the rock must contain less than 50 % ash-forming mineral matter. In the United States, individual coal beds may be as thin as a few inches or as thick as 200 ft, which is very rare. The bed may cover areas as small as a few square yards or as large as several counties .From the time the peat is buried, it goes through a series of chemical and physical changes called “coalification,” which leads to coals of various ranks. Coalification is a continuous process involving increases in temperature and pressure resulting from burial under different layers of earth. Higher depths of burial and higher temperatures increase the rate of the coalification process through the elimination of moisture and other volatile elements. In eflect, “Coalification is a baking process in the earth, under pressure. As it proceeds, coalification produces coals of increasing hardness and calorific value and results in a reduction of tar, oil and gas” .Coal is considered to be composed of two principal parts—an organic part, which is inherited from the remains of plant parts, and an inorganic part. The micro-components and microstructures that make up the organic part are called “macerals,” which are considered to be the building blocks of coal in the same way minerals are the building blocks of rocks. There are three principal types of macerals, which are optically discrete particles of organic material in coal.1. Inertinite is maceral material derived from the partial carbonization of the coal-forming materials by fire or intensive degradation by microorganisms.2. Vitrinite is derived from woody tissues and is the most abundant maceral in coal.3. Liptinite is derived from spores, needles and leaf cuticles, plant resins, and similar materials.