(MEBG) - Coal is a readily combustible rock with more than 50 percent of its weight comprising carbonaceous material, that was created from altered plant remains, similar to those in peat . In fact, most coal is fossilized peat.
Peat, incidentally, is an unconsolidated deposit of plant remains from a water-saturated environment, such as a bog or mire. Structures of the vegetal matter can be seen, and, when dried, peat burns freely.
Coal is formed by the physical and chemical alteration of peat (coalification) by processes involving bacterial decay, compaction, heat, and time. Coal is an agglomeration of many different complex hydrocarbon compounds, some of which owe their origin to the original constituents in the peat.
Peat deposits are actually quite varied and contain everything from pristine plant parts (roots, bark, spores, etc.) to decayed plants, decay products, and even to charcoal if the peat caught fire. Peat deposits typically form in a waterlogged environment where plant debris is accumulated.
In order for the peat to become coal, it must be buried by sediment. Burial causes compaction of the peat and, consequently, water is squeezed out during the first stages of burial. Continued burial and the addition of heat and time cause the complex hydrocarbon compounds in the deposit to start to break down and alter in a variety of ways.
The gaseous alteration products (methane is one) are typically expelled from the deposit and the deposit becomes more and more carbon-rich as other elements drop out. The material proceeds from plant debris, peat, lignite, sub-bituminous coal, bituminous coal, anthracite coal, to graphite (a pure carbon mineral).
Rocks are made up of grains of one or more minerals. Similarly, coal, an organic deposit, is made up of organic grains called macerals. Coal petrographers, people who study coal under the microscope, separate the macerals into three maceral groups, each of which are composed of several maceral types.
The kinds of coal, in increasing order of rank), are lignite (brown coal), sub-bituminous, bituminous, and anthracite. These classes are further divided into subclasses based on their degree of alteration (measured by volatile-matter content, BTU's, or by petrographic means.
The chief uses of coal are electricity generation, heat, and cocking coal for iron and steel making. Each of these uses has specific requirements, but generally a high Btu (British Thermal Unit ) value, and a low sulfur, ash, and moisture content are desirable. The important properties of coal are dependent upon the specific industrial use of the coal. Undesirable chemical constituents in coal such as sulfur, chlorine, sodium, and various hazardous air pollutants may be important for some uses of coal.
The coal is a property that determines how easily these chemical constituents and the ash content of the coal can be reduced through preparation before the coal is used. Important handling properties include grindability, content of scaling agents (chlorine and sodium cause scaling in boilers), and ash fusion.
Ash fusion is a property that indicates whether the ash totally melts (low ash fusion) and must be removed from the boiler as a liquid, or forms "clinkers" or cinders (high ash fusion) that must be removed as a solid. Boilers are designed to burn coals with specific ash fusions for this reason.
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