Coal-Fired Power Generation Handbook. James G. Speight
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 for purchasing and selling, or to establish burning characteristics.
All types of coal contain fixed carbon, which provides stored energy, plus varying amounts of moisture, ash, volatile matter, mercury, and sulfur. However, the physical properties of coal vary widely, and coal-fired power plants must be engineered to accommodate the specific properties of available feedstock and to reduce emissions of pollutants such as sulfur, mercury, and dioxins which reduce power plant efficiency. The efficiency of a coalfired power plant is typically represented defined as the amount of heat content in (Btu) per the amount of electric energy out (kWh), commonly called a heat rate (Btu/kWh). Expected improvements in power plant efficiency mainly arise from the substitution of older power plants with new plants that have higher efficiency.
Calorific value is one of the principal measures of the value of a coal as a fuel and is directly influenced by mineral impurities. Coal mineralogy is not only important to combustion characteristics, but also as materials that can be passed on to secondary products such as metallurgical coke. Alkali-containing compounds derived from coal minerals can contribute to excessive gasification of coke in the blast furnace and attack of blast furnace refractories, whereas phosphorus and sulfur from coal minerals can be passed on to the hot metal, thus reducing its quality for steelmaking.
Mineral matter may occur finely dispersed or in discrete partings in coal and is generally grouped according to origin. A certain amount of mineral matter and trace elements are derived from the original plants. However, the majority enters to coal precursor either during the initial stage of coalification (being introduced by wind or water to the peat swamp)