Coal

From Thermal-FluidsPedia

Coal has been used longer than any other form of fossil fuel. Even early cavemen used coal (known as black rock) for heating purposes. In the United States and many other industrial countries, fuel wood was the dominant energy source until the sixteenth century. In the early seventeenth century, aft er many forests had been cleared and wood became scarce, coal substituted wood as the main source of fuel. Th e coal extracted from shallow streams was high in sulfur and burned with an irritating smell but also had some advantages. It burned at higher temperatures than wood and was more desirable for the smelting of irons and other metals. Th e coal byproduct, coke, was widely used in glass blowing, brick and tile manufacturing, and production of high purity metals that transformed the field of metallurgy (a). Another byproduct of coal, the coal gas, revolutionized the field of lighting, first on the streets of London and soon thereafter in other large European cities (See box “Coal Gas”). As more and more coal was extracted, the need for better drilling equipment, safety devices, and hauling carts increased. Th is led to important innovations ranging from reciprocating pumps to atmospheric engines and wagon tramways.

In the early twentieth century, coal accounted for 90% of all energy supplies in the world, before it lost its dominance as an energy source to petroleum, a liquid fossil fuel that could easily be used in internal combustion engines. Today, coal accounts for a quarter of the world’s primary energy consumption, and generates 40% of all electricity. Similar figures for the US are 25% and 50% (1).

1 Types of Coal
2 Reserves and Resources
3 Environmental Concerns
4 References
5 Additional Comments
6 Further Reading
7 External Links

Types of Coal

The quality of coal is typically categorized by its rank and grade. Th e rank of the coal represents its morphological development from peat to lignite (or brown coal), sub-bituminous, bituminous (soft coal), and anthracite (hard coal). The higher the coal is ranked, the greater is its carbon content,resulting in more energy being liberated when it is burned (Table 1). The grade of the coal determines its purity. Coal is of a better grade if its sulfur content is less and burns with lower emissions; coal is classified into low, medium, and high grades.

Table 1. Coal Ranks and their Properties.
Rank	Age (million years)	Carbon Content	Heating Value (kJ/kg)*
Anthracite	350	85-95%	34,000 and up
Bituminous	300	45-85%	25,000-35,000
Sub-bituminous	100	35-45%	20,000-25,000
Lignite	60	25-35%	10,000-20,000
*Higher Heating Values.

Table 2 Major Recoverable Coal.

Whether coal ranks higher or lower depends largely on how it was formed. The first stage of coal formation involves the compression of vegetation under the heavy weights of water and ground materials. These materials gradually turn into a dark-brown, compact organic material known as peat. Over time, peat is compressed and heated to form lignite. Lignite is a soft , brownish-black coal, containing about 30% carbon. It is also the lowest quality and the most abundant type of coal in the world. Traces of the texture of the original wood may even be found in pieces of lignite. At greater depths, lignite is transformed into sub-bituminous, bituminous, and ultimately anthracite coal. Therefore, it is expected to find anthracite in the very old deposits that reside in the deep layers and to find lignite in the younger deposits and in those closer to the surface.

Reserves and Resources

As Table 2 indicates, just five countries, the US, Russia, China, India, and Australia, own about 70% of the total world coal reserves – estimated at 976 billion metric tons. Despite what one might expect, with the exception of Iran, there are no coal mines in the Middle East.

Figure 1 US actual and projected coal production.

Figure 2 World coal consumption outlook by region.

Large coal deposits are found in 38 US states, with more than half of these in only three states: Wyoming, West Virginia, and Kentucky. Anthracite is most commonly found in Pennsylvania and accounts for only 1% of the US’s coal reserves. About 70% of coal is bituminous; the rest is essentially lignite. Eastern and Mid-continent coalfields contain mostly bituminous coal, while sub-bituminous coal is predominantly found in the Western states. Most lignite is mined in Texas, Montana, and North Dakota.

Worldwide coal production, which reached 4.8 billion metric tons in 2004, accounted for more than a quarter of all energy sources and about 90% of all electricity generated. In the United States, coal production grew exponentially from 1860 to 1910. As we switched our source of primary consumption to petroleum and natural gas, coal production (and consumption) leveled off until 1972, whereupon the rate started to rise again (2). Th e latest figures indicate coal consumption at one billion metric tons in 2004. Th e US’s proven, identified, and ultimate (identified and undiscovered) coal reserves are estimated at 247 billion, 1557 billion, and 3968 billion metric tons, respectively (3). Th e actual and projected US coal productions are shown in Figure 1.

Although the US has the most coal reserves in the world, it is China that produces (and consumes) the most coal. The trend is expected to continue, and in the next two decades, coal use is projected to increase by another 36%, with the largest increases projected for China and India (Figure 2). In Europe, however, substantial declines in coal use are projected in favor of cleaner natural gas (4).

Environmental Concerns

Figure 3 Impact of various energy technologies on global warming and overall air quality.

Coal is probably the dirtiest of all fossil resources; it produces more carbon per unit energy content, has the highest percentage of sulfur, and produces more nitric oxides. Th e relative impact on global warming and overall air quality is considered to be the greatest, producing twice carbon dioxide as oil for the same amount of energy (See Figure 3). Depending on how deep the coal is buried, coal can be extracted by digging tunnels or strip-mining. Hazards associated with tunnel operation include cave-ins, explosions from dust buildup, carbon monoxide poisoning, and lack of sufficient ventilation. Although tunneling operations harm miners the most, strip-mining has the greatest environmental impacts. Th ese include the destruction of fertile surface-soil, permanent changes in the landscape, and the possibility of acidic or alkaline drainage to the surface (Figure 4). In many industrial countries, public and environmentalist pressure has forced local governments to require coal mining companies to reclaim the land and restore it to its original form aft er strip-mining.

Figure 4 Shale oil can potentially meet a great portion of the future energy demand.

Since coal is formed primarily underneath swamp beds, it contains a large amount of sulfur. To remove the sulfur, coal is crushed into small chunks and washed inside large water tanks. Since sulfur is heavier than coal, most sink to the bottom and is removed. Some of the sulfur is chemically bonded to carbon (organic sulfur) and cannot be washed off this way; it must be removed by adding chemicals that react with sulfur and break it loose.

Although most sulfur is removed by physical and chemical processes, the coal burned in power plants still contains considerable amounts of sulfur which, when burned, produce sulfur dioxide. In addition to sulfur dioxide, nitrogen oxides and particulates are produced. Modern power plants use wet or dry scrubbers to remove the remaining sulfur in their smokestacks before it is released into the atmosphere. In wet scrubbers, limestone slurry is sprayed onto the fl ue gases where it combines with sulfur to form a paste that is left behind. Dry scrubbers consist of a fixed sorbent bed of activated carbon, char, and alumina impregnated with copper. Large bag houses remove larger particles while electrostatic precipitators filter smaller particulates.

In addition to ecological concerns, coal extraction can be quite dangerous. It is estimated that several thousand miners are killed each year as a result of mine explosions, cave-ins, and carbon monoxide poisoning. As the energy crunch becomes tighter and the use of coal becomes more widespread, the number of mine accidents and fatalities are expected to increase.

Of course, coal has a number of advantages. It is relatively cheap; it can be transported by truck, ship, and rail; it is easy to store and burn, and can be liquefied to produce synthetic oil. Every ton of coal will yield about 5.5 barrels of liquid fuel (5). It should be noted, however, that although there exists a large quantity of coal reserves to be exploited, if coal is used to substitute the current demand for petroleum, the coal supply would serve as only a very temporary solution to the problem (6).

References

(1) 2004 estimates; a. World Coal Institute (http://www.worldcoal.org); b. US Geological Survey (http://energy.usgs.gov/coal.html).

(2) Barlett , A., “Fundamentals of the Energy Crisis,” Th e American Journal of Physics, 40, September 1978. pp. 876-888.

(3) EIA Website (htt p://www.eia.doe.gov/cneaf/coal/reserves/chapter1.html).

(4) Energy Information Agency Fact Sheet (http://www.eia.doe.gov).

(5) Lumpkin, R.E., “Recent Progress in the Direct Liquefaction of Coal,” Science, 239, p. 873, 1988.

(6) Hatfield, C. B., “How long oil supply grow?” M. King Hubbet Center for Petroleum Supply Studies, Newsletter #97/4-1-6, October 1997.

(7) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Additional Comments

(a) Coke is produced by carbonization of coal in an oxygen-deficient environment and high temperatures.

External Links

National Energy Technology Laboratory: Th e Strategic Center for Coal (http://www.netl.doe.gov/coal).

National Petroleum Technology Office (http://www.npto.doe.gov).

US Geological Survey (http://www.usgs.gov).

Organization of Petroleum Exporting Countries (OPEC) (http://www.opec.org).

Society of Petroleum Engineers (http://sae.org).

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Coal

From Thermal-FluidsPedia

Contents

Types of Coal

Reserves and Resources

Environmental Concerns

References

Additional Comments

Further Reading

External Links

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