Coal

Coal

Template:EnergyPortal Coal (IPA: /ˈkəʊl/) is a fossil fuel formed in swamp ecosystems where plant remains were saved by water and mud from oxidization and biodegradation. Coal is a readily combustible black or brownish-black rock. It is a sedimentary rock, but the harder forms, such as anthracite coal, can be regarded as metamorphic rocks because of later exposure to elevated temperature and pressure. It is composed primarily of carbon along with assorted other elements, including sulfur. It is the largest single source of fuel for the generation of electricity world-wide, as well as the largest source of carbon dioxide emissions, which is considered the primary cause of global warming. Coal is extracted from the ground by coal mining, either underground mining or open pit mining (surface mining).

Etymology

The word "coal" is of Aryan origin, and appears in many Germanic languages (German language Kohle, Swedish language kol), also giving the name for element carbon in those languages—charcoal is wood rendered to carbon and carbonic compounds by pyrolysis.

Types of coal

As geological processes apply pressure to peat over time, it is transformed successively into:

• Lignite - also referred to as brown coal, is the lowest rank of coal and used almost exclusively as fuel for steam-electric power generation. Jet is a compact form of lignite that is sometimes polished and has been used as an ornamental stone since the Iron Age.
• Sub-bituminous coal - whose properties range from those of lignite to those of bituminous coal and are used primarily as fuel for steam-electric power generation.

• Bituminous coal - a dense coal, usually black, sometimes dark brown, often with well-defined bands of bright and dull material, used primarily as fuel in steam-electric power generation, with substantial quantities also used for heat and power applications in manufacturing and to make coke.
• Anthracite - the highest rank; a harder, glossy, black coal used primarily for residential and commercial space heating.
• Graphite - technically the highest rank, but difficult to ignite and is not so commonly used for ignition.

Early use

Outcrop coal was used in Britain during the Bronze Age (2-3000 years BC), where it has been detected as forming part of the composition of funeral pyres. It was also commonly used in the early period of the Roman occupation. Evidence of trade in coal (dated to about AD200 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 via the Car Dyke for use in drying grain. Coal cinders have been found in the hearths of villas and military forts, particularly in Northumberland, dated to around AD400. In the west of England contemporary writers described the wonder of a permanent brazier of coal on the altar of Minerva at Aquae Sulis (modern day Bath) although in fact easily-accessible surface coal from what is now the Somerset coalfield was in common use in quite lowly dwellings locally.

However, there is no evidence that the product was of great importance in Britain before the High Middle Ages, after about AD1000. Mineral coal came to be referred to as "seacoal," probably because it came to many places in eastern England, including London, by sea. This is accepted as the more likely explanation for the name than that it was found on beaches, 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 13th century, when underground mining from shafts or adits was developed. In London there is still a Seacoal Lane (off the north side of Ludgate Hill) where the coal merchants used to conduct their business. An alternative name was "pitcoal," because it came from mines. It was, however, the development of the Industrial Revolution that led to the large-scale use of coal, as the steam engine took over from the water wheel.

Uses today

File:DSCN4524 ashtabulacoalcars e2.jpg

Coal rail cars in Ashtabula, Ohio.

Coal as fuel

See also Clean coal and Fossil fuel power plant

Coal is primarily used as a solid fuel to produce electricity and heat through combustion. World coal consumption is about 5.3 billion tonnes annually, of which about 75% is used for the production of electricity. The region including the People's Republic of China and India uses about 1.7 billion tonnes annually, forecast to exceed 2.7 billion tonnes in 2025. The USA consumes about 1.0 billion tons of coal each year, using 90% of it for generation of electricity. Coal is the fastest growing energy source in the world, with coal use increasing by 25% for the three-year period ending in December 2004 (BP Statistical Energy Review, June 2005).

When coal is used for electricity generation, it is usually pulverized and then burned in a furnace with a boiler. The furnace heat converts boiler water to steam, which is then used to spin turbines which turn generators and create electricity. The thermodynamic efficiency of this process has been improved over time. "Standard" steam turbines have topped out with some of the most advanced reaching about 35% thermodynamic efficiency for the entire process, which means 65% of the coal energy is rejected as waste heat into the surrounding environment. Old coal power plants, especially "grandfathered" plants, are significantly less efficient and reject higher levels of waste heat. The emergence of the supercritical turbine concept envisions running a boiler at extremely high temperatures and pressures with projected efficiencies of 46%, with further theorized increases in temperature and pressure perhaps resulting in even higher efficiencies Approximately 40% of the world electricity production uses coal. The total known deposits recoverable by current technologies, including highly polluting, low energy content types of coal (i.e., lignite, bituminous), might be sufficient for 300 years' use at current consumption levels, although maximal production could be reached within decades (see World Coal Reserves, below).

A more energy-efficient way of using coal for electricity production would be via solid-oxide fuel cells or molten-carbonate fuel cells (or any oxygen ion transport based fuel cells that do not discriminate between fuels, as long as they consume oxygen), which would be able to get 60%–85% combined efficiency (direct electricity + waste heat steam turbine). Currently these fuel cell technologies can only process gaseous fuels, and they are also sensitive to sulfur poisoning, issues which would first have to be worked out before large scale commercial success is possible with coal. As far as gaseous fuels go, one idea is pulverized coal in a gas carrier, such as nitrogen. Another option is coal gasification with water, which may lower fuel cell voltage by introducing oxygen to the fuel side of the electrolyte, but may also greatly simplify carbon sequestration.

Coking and use of coke

Main article: Coke (fuel)

Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents are driven off by baking in an oven without oxygen at temperatures as high as 1,000 °C (1,832 °F) so that the fixed carbon and residual ash are fused together. Metallurgic coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Coke from coal is grey, hard, and porous and has a heating value of 24.8 million Btu/ton (29.6 MJ/kg). Byproducts of this conversion of coal to coke include coal tar, ammonia, light oils, and "coal gas".

Petroleum coke is the solid residue obtained in oil refining, which resembles coke but contains too many impurities to be useful in metallurgical applications.

Gasification

High prices of oil and natural gas are leading to increased interest in "BTU Conversion" technologies such as gasification, methanation and liquefaction.

Coal gasification breaks down the coal into its components, usually by subjecting it to high temperature and pressure, using steam and measured amounts of oxygen. This leads to the production of syngas, a mixture mainly consisting of carbon monoxide (CO) and hydrogen (H₂).

In the past, coal was converted to make coal gas, which was piped to customers to burn for illumination, heating, and cooking. At present, the safer natural gas is used instead. South Africa still uses gasification of coal for much of its petrochemical needs.

The Synthetic Fuels Corporation was a U.S. government-funded corporation established in 1980 to create a market for alternatives to imported fossil fuels (such as coal gasification). The corporation was discontinued in 1985.

Gasification is also a possibility for future energy use, as the produced syngas can be cleaned-up relatively easily leading to cleaner burning than burning coal directly (the conventional way). The cleanliness of the cleaned-up syngas is comparable to natural gas enabling to burn it in a more efficient gas turbine rather than in a boiler used to drive a steam turbine. Syngas produced by gasification can be CO-shifted meaning that the combustible CO in the syngas is transferred into carbon dioxide (CO₂) using water as a reactant. The CO-shift reaction also produces an amount of combustible hydrogen (H₂) equal to the amount of CO converted into CO₂. The CO₂ concentrations (or rather CO₂ partial pressures) obtained by using coal gasification followed by a CO-shift reaction are much higher than in case of direct combustion of coal in air (which is mostly nitrogen). These higher concentrations of carbon dioxide make carbon capture and storage much more economical than it otherwise would be.

Liquefaction

Coal can also be converted into liquid fuels like gasoline or diesel by several different processes. The Fischer-Tropsch process of indirect synthesis of liquid hydrocarbons was used in Nazi Germany for many years and is today used by Sasol in South Africa. Coal would be gasified to make syngas (a balanced purified mixture of CO and H₂ gas) and the syngas condensed using Fischer-Tropsch catalysts to make light hydrocarbons which are further processed into gasoline and diesel. Syngas can also be converted to methanol, which can be used as a fuel, fuel additive, or further processed into gasoline via the Mobil M-gas process.

A direct liquefaction process Bergius process (liquefaction by hydrogenation) is also available but has not been used outside Germany, where such processes were operated both during World War I and World War II. SASOL in South Africa has experimented with direct hydrogenation. Several other direct liquefaction processes have been developed, among these being the SRC-I and SRC-II (Solvent Refined Coal) processes developed by Gulf Oil and implemented as pilot plants in the United States in the 1960s and 1970s.

Another direct hydrogenation process was explored by the NUS Corporation in 1976 and patented by Wilburn C. Schroeder. The process involved dried, pulverized coal mixed with roughly 1wt% molybdenum catalysis. Hydrogenation occurred by use of high temperature and pressure synthesis gas produced in a separate gasifier. The process ultimately yielded a synthetic crude product, Naptha, a limited amount of C₃/C₄ gas, light-medium weight liquids (C₅-C₁₀) suitable for use as fuels, small amounts of NH₃ and significant amounts of CO₂.

Yet another process to manufacture liquid hydrocarbons from coal is low temperature carbonization (LTC). Coal is coked at temperatures between 450 and 700°C compared to 800 to 1000°C for metallurgical coke. These temperatures optimize the production of coal tars richer in lighter hydrocarbons than normal coal tar. The coal tar is then further processed into fuels. The Karrick process was developed by Lewis C. Karrick, an oil shale technologist at the U.S. Bureau of Mines in the 1920s.

All of these liquid fuel production methods release carbon dioxide (CO₂) in the conversion process, far more than is released in the extraction and refinement of liquid fuel production from petroleum. If these methods were adopted to replace declining petroleum supplies, carbon dioxide emissions would be greatly increased on a global scale. For future liquefaction projects, Carbon dioxide sequestration is proposed to avoid releasing it into the atmosphere, though no pilot projects have confirmed the feasibility of this approach on a wide scale. As CO₂ is one of the process streams, sequestration is easier than from flue gases produced in combustion of coal with air, where CO₂ is diluted by nitrogen and other gases. Sequestration will, however, add to the cost.

Coal liquefaction is one of the backstop technologies that could potentially limit escalation of oil prices and mitigate the effects of transportation energy shortage that some authors have suggested could occur under peak oil. This is contingent on liquefaction production capacity becoming large enough to satiate the very large and growing demand for petroleum. Estimates of the cost of producing liquid fuels from coal suggest that domestic U.S. production of fuel from coal becomes cost-competitive with oil priced at around 35 USD per barrel, (break-even cost). This price, while above historical averages, is well below current oil prices. This makes coal a viable financial alternative to oil for the time being, although production is not great enough to make synfuels viable on a large scale.

Among commercially mature technologies, advantage for indirect coal liquefaction over direct coal liquefaction are reported by Williams and Larson (2003). Estimates are reported for sites in China where break-even cost for coal liquefaction may be in the range between 25 to 35 USD/barrel of oil.

Harmful effects

Coal mining

Coal mining causes a number of harmful effects. When coal surfaces are exposed, pyrite (iron sulfide), also known as "fool's gold", comes in contact with water and air and forms sulfuric acid. As water drains from the mine, the acid moves into the waterways, and as long as rain falls on the mine tailings the sulfuric acid production continues, whether the mine is still operating or not. If the coal is strip mined, the entire exposed seam leaches sulfuric acid, leaving the infertile subsoil on the surface and begins to pollute streams by acidifying and killing fish, plants, and aquatic animals who are sensitive to drastic pH shifts.

By the late 1930s, it was estimated that American coal mines produced about 2.3 million tonnes of sulfuric acid annually. In the Ohio River Basin, where twelve hundred operating coal mines drained an estimated annual 1.4 million tonnes of sulfuric acid into the waters in the 1960s and thousands of abandoned coal mines leached acid as well. In Pennsylvania alone, mine drainage had blighted 2,000 stream miles by 1967.

Coal burning

Combustion of coal, like any other fossil fuel, produces carbon dioxide (CO₂) and nitrogen oxides (NO_x) along with varying amounts of sulfur dioxide (SO₂) depending on where it was mined. Sulfur dioxide reacts with oxygen to form sulfur trioxide (SO₃), which then reacts with water to form sulfuric acid (see Acid anhydride for more information). The sulfuric acid is returned to the Earth as acid rain.

Emissions from coal-fired power plants represent the largest source of carbon dioxide emissions, which have been implicated as the primary cause of global warming. Coal mining and abandoned mines also emit methane, another cause of global warming. Since the carbon content of coal is much higher than oil, burning coal is a more serious threat to the stability of the global climate, as this carbon forms CO₂ when burned. Many other pollutants are present in coal power station emissions, as solid coal is more difficult to clean than oil, which is refined before use. A study commissioned by environmental groups claims that coal power plant emissions are responsible for tens of thousands of premature deaths annually in the United States alone. Modern power plants utilize a variety of techniques to limit the harmfulness of their waste products and improve the efficiency of burning, though these techniques are not subject to standard testing or regulation in the U.S. and are not widely implemented in some countries, as they add to the capital cost of the power plant. To eliminate CO₂ emissions from coal plants, carbon capture and storage has been proposed but has yet to be commercially used.

Coal and coal waste products including fly ash, bottom ash, boiler slag, and flue gas desulferization contain many heavy metals, including arsenic, lead, mercury, nickel, vanadium, beryllium, cadmium, barium, chromium, copper, molybdenum, zinc, selenium and radium, which are dangerous if released into the environment. Coal also contains low levels of uranium, thorium, and other naturally-occurring radioactive isotopes whose release into the environment may lead to radioactive contamination. While these substances are trace impurities, enough coal is burned that significant amounts of these substances are released, resulting in more radioactive waste than nuclear power plants. Mercury emissions from coal burning are concentrated as they work their way up the food chain and converted into dangerous biological compounds that have made it dangerous to eat fish from many waterways of the world. Due to its scientifically accepted connection with climate change, the world's reliance on coal as an energy source, and health concerns in areas with poor air pollution controls, The Economist recently labeled the burning of coal "Environmental Enemy No. 1."

Energy density

The energy density of coal is roughly 24 Megajoules per kilogram.

The energy density of coal can also be expressed in kilowatt-hours, the units that electricity is most commonly sold in, to estimate how much coal is required to power electrical appliances. The energy density of coal is 6.67 kW-h/kg and the typical Thermodynamic efficiency of coal power plants is about 30%. Of the 6.67 kW-h of energy per kilogram of coal, about 30% of that can successfully be turned into electricity - the rest is waste heat. (Coal power plants obtain approximately 2.0 kW-h per kg of burned coal)

As an example, running one 100 Watt computer for one year requires 876 kW-h (100 W × 24 h × 365 {days in a year} = 876000 W-h = 876 kW-h). Converting this power usage into physical coal consumption: : ${\frac {876\ \mathrm {kW\cdot hours} }{2.0\ \mathrm {kW\cdot hours/kg} }}=438\ \mathrm {kg\ of\ coal} =967\ \mathrm {pounds\ of\ coal}$.

It takes 438 kg (967 pounds) of coal to power a computer for one full year. One should also take into account transmission and distribution losses caused by resistance and heating in the power lines, which is in the order of 5 - 10%, depending on distance from the power station and other factors.

Relative carbon cost

Because coal is at least 50% carbon (by mass), then 1 kg of coal contains at least 0.5 kg of carbon, which is ${\frac {0.5\mathrm {kg} }{\mathrm {12} \cdot \mathrm {kg/kmol} }}={\frac {1}{24}}\mathrm {kmol}$ where 1 mol is equal to N_A (Avogadro Number) particles. This combines with oxygen in the atmosphere during combustion, producing carbon dioxide, with an atomic weight of (12 + 16 × 2 = mass(CO₂) = 44 kg/kmol). ${\frac {1}{24}}\mathrm {kmol}$ of CO₂ is produced from the ${\frac {1}{24}}\mathrm {kmol}$ present in every kilogram of coal, which once trapped in CO₂ weighs approximately ${\frac {1}{24}}\mathrm {kmol} \cdot {\frac {44\mathrm {kg} }{\mathrm {kmol} }}={\frac {11}{6}}\mathrm {kg} \approx 1.83\mathrm {kg}$.

This fact can be used to put a carbon-cost of energy on the use of coal power. Since the useful energy output of coal is about 30% of the 6.67 kW-h/kg(coal), we can say about 2 kW-h/kg(coal) of energy is produced. Since 1 kg coal roughly translates as 1.83 kg of CO₂, we can say that using electricity from coal produces CO₂ at a rate of about 0.915 kg(CO₂) / kW-h, or about 0.254 kg(CO₂) / MJ.

Coal fires

There are hundreds of coal fires burning around the world. Those burning underground can be difficult to locate and many cannot be extinguished. Fires can cause the ground above to subside, combustion gases are dangerous to life, and breaking out to the surface can initiate surface wildfires. Coal seams can be set on fire by spontaneous combustion or contact with a mine fire or surface fire. A grass fire in a coal area can set dozens of coal seams on fire. Coal fires in China burn 109 million tonnes of coal a year, emitting 200 million tonnes of carbon dioxide. This amounts to 2-3% of the annual worldwide production of CO₂ from fossil fuels, or as much as emitted from all of the cars and light trucks in the United States. In Centralia, Pennsylvania (a borough located in the Coal Region of the United States) an exposed vein of coal ignited in 1962 due to a trash fire in the borough landfill, located in an abandoned anthracite strip mine pit. Attempts to extinguish the fire were unsuccessful, and it continues to burn underground to this day. The Australian Burning Mountain was originally believed to be a volcano, but the smoke and ash comes from a coal fire which may have been burning for over 5,500 years.

The reddish siltstone rock that caps many ridges and buttes in the Powder River Basin (Wyoming), and in western North Dakota is called porcelanite, which also may resemble the coal burning waste "clinker" or volcanic "scoria". Clinker is rock that has been fused by the natural burning of coal. In the Powder River Basin approximately 27 to 54 billion tonnes of coal burned within the past three million years. Wild coal fires in the area were reported by the Lewis and Clark Expedition as well as explorers and settlers in the area.

Production trends

File:2005coal.PNG

Coal output in 2005

In 2005, China was the top producer of coal with almost one-third world share followed by the USA and India, reports the British Geological Survey.

World coal reserves

US coal regions

In 2003 it was estimated that there was around one exagram (1 × 10¹⁵ kg or 998 billion tons) of total coal reserves accessible using current mining technology, approximately half of it being hard coal. The energy value of all the world's recoverable coal is 27 zettajoules, which is expected to last 200 years. At the current global total energy consumption of 15 terawatt, there is enough coal to provide the entire planet with all of its energy for 57 years.

British Petroleum, in its annual report 2006, estimated at 2005 end, there were 909,064 million tons of proven coal reserves worldwide (9.236 × 10¹⁴ kg), or 155 years reserve to production ratio. This figure only includes reserves classified as "proven", exploration drilling programs by mining companies, particularly in under-explored areas, are continually providing new reserves. In many cases, companies are aware of coal deposits that have not been sufficiently drilled to qualify as "proven".

The United States Department of Energy uses estimates of coal reserves in the region of 1,081,279 million short tons (9.81 × 10¹⁴ kg), which is about 4,786 BBOE (billion barrels of oil equivalent). The amount of coal burned during 2001 was calculated as 2.337 GTOE (gigatonnes of oil equivalent), which is about 46 million barrels of oil equivalent per day. Were consumption to continue at that rate those reserves would last about 285 years. As a comparison, natural gas provided 51 million barrels (oil equivalent), and oil 76 million barrels, per day during 2001.

Of the three fossil fuels coal has the most widely distributed reserves; coal is mined in over 100 countries, and on all continents except Antarctica. The largest reserves are found in the USA, Russia, Australia, China, India and South Africa.

Proved recoverable coal reserves at end-1999 (million tonnes)

Country	Bituminous (including anthracite)	Sub- bituminous	Lignite	TOTAL
United States of America	115891	101021	33082	249994
Russian Federation	49088	97472	10450	157010
People's Republic of China	62200	33700	18600	114500
India	82396		2000	84396
Australia	42550	1840	37700	82090
Germany	23000		43000	66000
South Africa	49520			49520
Ukraine	16274	15946	1933	34153
Kazakhstan	31000		3000	34000
Poland	20300		1860	22160
Serbia	64	1460	14732	16256
Brazil		11929		11929
Colombia	6267	381		6648
Canada	3471	871	2236	6578
Czech Republic	2114	3414	150	5678
Indonesia	790	1430	3150	5370
Botswana	4300			4300
Uzbekistan	1000		3000	4000
Turkey	278	761	2650	3689
Greece			2874	2874
Bulgaria	13	233	2465	2711
Pakistan		2265		2265
Iran (Islamic Rep.)	1710			1710
United Kingdom	1000		500	1500
Romania	1	35	1421	1457
Thailand			1268	1268
Mexico	860	300	51	1211
Chile	31	1150		1181
Hungary		80	1017	1097
Peru	960		100	1060
Kyrgyzstan			812	812
Japan	773			773
Spain	200	400	60	660
Korea (Democratic People's Rep.)	300	300		600
New Zealand	33	206	333	572
Zimbabwe	502			502
Netherlands	497			497
Venezuela	479			479
Argentina		430		430
Philippines		232	100	332
Slovenia		40	235	275
Mozambique	212			212
Swaziland	208			208
Tanzania	200			200
Nigeria	21	169		190
Greenland		183		183
Slovakia			172	172
Vietnam	150			150
Congo (Democratic Rep.)	88			88
Korea (Republic)	78			78
Niger	70			70
Afghanistan	66			66
Algeria	40			40
Croatia	6		33	39
Portugal	3		33	36
France	22		14	36
Italy		27	7	34
Austria			25	25
Ecuador			24	24
Egypt (Arab Rep.)		22		22
Ireland	14			14
Zambia	10			10
Malaysia	4			4
Central African Republic			3	3
Myanmar (Burma)	2			2
Malawi		2		2
New Caledonia	2			2
Nepal	2			2
Bolivia	1			1
Norway		1		1
Republic of China	1			1
Sweden		1		1
TOTAL	519062	276301	189090	984453

Major coal exporters

Exports of Coal by Country and year (million tonnes)

Country	2003	2004
Australia	238.1	247.6
United States	43.0	48.0
South Africa	78.7	74.9
Former Soviet Union	41.0	55.7
Poland	16.4	16.3
Canada	27.7	28.8
People's Republic of China	103.4	95.5
South America	57.8	65.9
Indonesia	107.8	131.4
Total	713.9	764.0

See also

• Charcoal
• Clean coal
• Coal assay
• Coal dust
• Coal Measure (stratigraphic unit)
• Coal mining
• Coal mining debate
• Coal-tar
• World Coal Institute
• Energy value of coal
• Fluidized bed combustion
• Future energy development
• Gasification
• Granular material
• Australian Coal Alliance
• History of coal mining
• List of environment topics
• Major coal producing regions
• World energy resources and consumption

References

• The Face of Decline: The Pennsylvania Anthracite Region in the Twentieth Century. Cornell University Press. 2005. ISBN 0-8014-8473-1. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Rottenberg, Dan (2003). In the Kingdom of Coal; An American Family and the Rock That Changed the World. Routledge. ISBN 0-415-93522-9.
• Robert H. Williams and Eric D. Larson (December 2003). "A comparison of direct and indirect liquefaction technologies for making fluid fuels from coal" (PDF). Energy for Sustainable Development. VII: 103–129.
• Outwater, Alice (1996). Water: A Natural History. New York, NY: Basic Books. ISBN 0-465-03780-1.
• Smith, Duane A. (1993). Mining America: The Industry and the Environment, 1800-1980. Lawrence, KS: University Press of Kansas. p. 210. ISBN 0870813064. {{cite book}}: Unknown parameter |month= ignored (help)

External links

• http://www.coalonline.org/site/coalonline/content/home
• World Coal Institute
• Coal: Facts & Figures
• MSNBC report on coal pollution health effects in the United States
• Clean coal technologies
  • Advanced methods of using coal (Japanese Coal Energy Center)
• USDOE Hydrogen from Coal Research
• Coal Preparation, Journal of
• Wyoming Coal from the University of Wyoming
• Coal - origin, purification and consumption
• History of coal seams and the practice of coal mining in North Staffordshire, UK