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Coal is a combustible organic sedimentary rock that is formed from the accumulation and preservation of plant materials, usually in a swamp environment (Speight, 2013). Along with crude oil and natural gas, coal is one of the three most important fossil fuels, such as for the generation of electricity and provides approximately 40% of electricity production on a worldwide basis.

For the past two centuries, coal played this important role – providing coal gas for lighting and heating and then electricity generation with the accompanying importance of coal as an essential fuel for steel and cement production, as well as a variety of other industrial activities. In fact, subject to environmental concerns, coal remains an important source of energy in many countries, but this does not give the true picture of the use of coal for electricity production. During that time, the coal industry has been pressured into serious considerations related to the environmental aspects of coal use and has responded with a variety of on-stream coal-cleaning and gas-cleaning technologies (Speight 2013, 2020).

In fact, coal has a long-term history of use (Table 1.3) (Freese, 2003). For example, outcrop coal was used in Britain during the Bronze Age (3000 to 2000 BC), where it has been detected as forming part of the composition of funeral pyres. The earliest recognized use (approximately 4000 BC) is from the Shenyang area where Neolithic inhabitants had begun carving ornaments from black lignite, but it was not until the Han Dynasty (202 BC to AD 220) that coal was also used for fuel.

Using Britain we find examples of the longevity of coal use. In Roman Britain the Romans exploited coal from all the major coalfields in England and Wales by the end of the 2^nd century AD. Evidence of trade in coal (dated to approximately AD 200) has been found at the inland port of Heronbridge, near Chester, and in the Fenlands of East Anglia, where coal from the Midlands was transported for use in drying grain. Coal cinders have been found in the hearths of villas and military forts, particularly in Northumberland, dated to approximately AD 400. In the west of England contemporary writers described the wonder of a permanent brazier of coal on the altar of Minerva at Aquae Sulis (Waters of Sulis; modern-day Bath) although in fact easily accessible surface coal from what became the Somerset coalfield (southwest England) was in common use in quite lowly dwellings locally. There is also evidence of the use of coal for iron-working in the city during the Roman period.

Table 1.3 History of coal use.

Time frame	Use
Stone Age	Coal may have been used for heating and cooking.
AD 100-200	The Romans use coal for heating.
1300s	In the American southwest, Hopi Indians use coal for heating.
1673	Explorers to America discover coal.
1700s	The English find coal produces a fuel that burns cleaner and hotter than wood charcoal.
1740s	Commercial coal mines begin operation in Virginia.
1800s	James Watt invents the steam engine and uses coal to produce the steam to run the engine. The Industrial Revolution spreads to the United States as steamships and steam-powered railroads become the main forms of transportation, using coal to fuel their broilers. During the Civil War, weapons factories begin using coal. By 1875, coke replaces charcoal as the primary fuel for iron blast furnaces to make steel. 1880s: Coal is first used to generate electricity for homes and factories.
1900s	Coal accounts for more than three-quarters of the total energy used in the United States, but is later supplanted by oil and natural gas for transportation and residential applications. Coal reemerges later as an affordable, abundant domestic energy resource to support the growing demand for electricity. In the late 1900s, environmental issues force a reduction in the amount of coal used for power generation. Clean Coal technologies were developed in the United States to allow coal to be used in an environmentally friendly manner.

The Somerset coalfield included pits in the north Somerset, England, area where coal was mined from the 15^th century until 1973. It is part of a wider field which covered northern Somerset and southern Gloucestershire in England. There is documentary evidence of coal being dug from this coalfield in the 14^th century and continuing until the 16^th century. During the early part of the 17^th century coal was largely obtained by excavating the outcrops or driving an incline, which involved following the seam into the ground. Only a small amount of coal could be obtained by these methods and so bell pits took their place – a bell pit is so-named because in cross section the pit resembles an upturned bell. The bell pit is a primitive method of mining coal where the coal lies near the surface on flat land. A shaft is sunk to reach the coal which is then excavated and removed by means of a bucket (much like a well). No supports are used and mining continues outward until the mine becomes too dangerous (or collapses) at which point another mine is started.

Mineral coal came to be referred to as sea coal (seacoal), probably because it came to many places in eastern England, including the northeast coast 50 to 100 miles south of the Scottish border. This is accepted as the more likely explanation for the name of the coal, having fallen from the exposed coal seams above or washed out of underwater coal seam outcrops. These easily accessible sources had largely become exhausted (or could not meet the growing demand) by the 13^th century when underground mining from shafts or adits was developed. An alternative name was pit coal (pit coal), because it came from mines. It was, however, the development of the Industrial Revolution (18^th century to 19^th century) that led to the large-scale use of coal, as the steam engine took over from the water wheel. Looked at from another angle, the Industrial Revolution was impossible without coal.

Currently, in the United States, coal is used primarily to generate electricity. The coal is burned in power plants to produce almost 40% of the electricity that is used each year. Coal is also used in the industrial and manufacturing industries. For example, the steel industry uses 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 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) or during the second stage of coalification, after consolidation of the coal by movement of solutions in cracks, fissures, and cavities. Mineral components of plant origin are not easily recognized in coals because they tend to be disseminated on a submicron level. The primary mineral components incorporated during plant deposition tend to be layered with and intimately inter-grown with the organic fraction, whereas the secondary mineral matter tends to be coarsely inter-grown and associated with cleat, fractures, and cavities. Therefore, secondary minerals may be more readily separated (cleaned or washed) from the organic matrix to improve the value of the material.

More information related to coal character and properties is derived from geological studies of coal – which includes a wide variety of topics, including coal formation, occurrence, and properties but which is outside of the purview of this book but is described in detail elsewhere (Speight, 2013).