Coal-Fired Power Generation Handbook. James G. Speight

Types

      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