Coal-Fired Power Generation Handbook. James G. Speight

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СКАЧАТЬ large amounts of coal – the coal is baked in hot furnaces to make coke, which is used to smelt iron ore into the iron needed for making steel. The high temperatures created from the use of coke gives steel the strength and flexibility needed for making bridges, buildings, and automobiles. The heat and the by-products produced from coal are also used to produce a variety of products such as methanol (methyl alcohol, CH₃OH) and ethylene (CH₂=CH₂) which can then be used to produce plastics, synthetic fibers, fertilizers, and medicines.

      Certain characteristics of coal ensure its place as an efficient and competitive energy source and contribute to stabilizing energy prices. Key factors include (i) the large reserves without associated geopolitical or safety issues, (ii) the availability of coal from a wide variety of sources, (iii) the facility with which coal can be stored in normal conditions, and (iv) the non-special and relatively inexpensive protection required for the main coal supply routes. Furthermore, retirements of older units, retrofits of existing units with pollution controls, and the construction of some new coal-fueled units are expected to significantly change the coal-fueled electricity generating fleet, making it capable of emitting lower levels of pollutants than the current fleet but reducing its future electricity generating capacity (GAO, 2012).

Deposits of coal, sandstone, shale, and limestone are often found together in sequences hundreds of feet thick. This period is recognized in the United States as the Mississippian and Pennsylvanian time periods due to the significant sequences of these rocks found in those states (i.e., Mississippi and Pennsylvania) (Table 1.4). Other notable coal-bearing ages are the Cretaceous, Triassic and Jurassic Periods. The more recently aged rocks are not as productive for some reason, but lignite and peat are common in younger deposits but generally, the older the deposit, the better the grade (higher rank) of coal (Ward, 2008).

      As with many industrial minerals, the physical and chemical properties of coal beds are as important in marketing a deposit as the grade. The grade of a coal establishes its economic value for a specific end use. Grade of coal refers to the amount of mineral matter that is present in the coal and is a measure of coal quality. Sulfur content; ash fusion temperatures, i.e., measurement of the behavior of ash at high temperatures; and quantity of trace elements in coal are also used to grade coal. Although formal classification systems have not been developed around grade of coal, coal grade is important to the coal user.

Table 1.4 The Geologic timescale.

Era	Period	Epoch	Duration (x 106)	Years ago (x 106)
Cenozoic	Quaternary	Holocene	10,000**
		Pleistocene	2	.01
	Tertiary	Pliocene	11	2
		Miocene	12	13
		Oligocene	11	25
		Eocene	22	36
		Paleocene	10	58
Mesozoic	Cretaceous		71	65
	Jurassic		57	136
	Triassic		35	190
Paleozoic	Permian		50	225
	Carboniferous		65	280
	Devonian		60	345
	Silurian		25	405
	Ordovician		65	425
	Cambrian		70	500
Precambrian			3,400	600

      Approximate

**To the present

      In terms of coal grade, the grade of a coal establishes its economic value for a specific end use (Ward, 2008). Grade of coal refers to the amount of mineral matter that is present in the coal and is a measure of coal quality. Sulfur content, ash fusion temperature (i.e., the temperature at which measurement the ash melts and fuses), and quantity of trace elements in coal are also used to grade coal. Although formal classification systems have not been developed around grade of coal, grade is important to the coal user.

      Another means by which coal is evaluated is through the rank of the coal, which is the most fundamental characteristic relating both coalification history and utilization potential of a coal. Volatile matter and maximum vitrinite reflectance are important values used to determine the worth of coking coals. However, because volatile matter is dependent on both rank and composition, coals of different composition may be assigned to the same rank value even though their levels of maturity may differ.

      Volatile matter is not considered to be a component of coal as mined but a product of the thermal decomposition of coal. Volatile matter is produced when coal is heated to 950°C (1740°F) (ASTM D3175) in the absence of air under specified conditions and contains, in addition to moisture, typically a mixture of low-to-medium molecular weight aliphatic hydrocarbon derivatives, aromatic hydrocarbon derivatives, with higher boiling oil and low-volatile tar. Volatile matter decreases as rank increases and when determined by the standard test method (ASTM D3175) can be used to establish the rank of coals, to indicate coke yield on carbonization process, to provide the basis СКАЧАТЬ