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{{Image|Anthracite coal.jpg|right|275px|Anthracite coal (American 25 cent coin shown for scale).}}


{{Image|Anthracite coal.jpg|right|275px|Anthracite coal (coin shown for scale).}}  
'''Coal''' is a [[Carbon|carbon-containing]] rock formed by the debris from the decay of ferns, vines, trees and other plants which flourished in swamps millions of years ago. Over time, the debris became buried and the actions of [[bacteria]], [[heat]] and [[pressure]] transformed the debris first into [[peat]] (a precursor of coal) and then into the various types of coal as we know them today.<ref name=Perry>{{cite book|author=Green, Don W. and Perry, Robert H. (Editors)|title=Perry's Chemical Engineers' Handbook|edition=6th Edition|publisher=McGraw-Hill|year=1997|id=ISBN 0-07-049479-7}}</ref><ref name=Marks>{{cite book|author=Eugene A. Avallone, Theodore Baumeister and Ali Sadegh (Editors)|title=Marks' Standard Handbook for Mechanical Engineers|edition=11th Edition|publisher=McGraw-Hill Professional|year=2006|id=ISBN 0-07-142867-4}}</ref><ref name=Kreith>{{cite book|author=Frank Kreith (Editor)|title=The CRC Handbook of Mechanical Engineering|edition=1st Edition|publisher=CRC Press|year=1998|id=ISBN 0-8493-9418-X}}</ref> In more technical terminology, that process of transformation is referred to as ''metamorphosis'', ''coalification'' or ''lithification''.


'''Coal''' is a carbonaceous rock which occurs as a result of anaerobic decomposition of plant matter and [[lithification]] of the resulting material. Coal occurs in three main forms: [[anthracite]], [[bituminous coal]], and [[lignite]], depending on the degree of lithification. 
[[Coal mining]] occurs for coal deposits that exist deep underground, as well as one that are at or near the surface of the ground. Because of the various degrees of transformation that occurred during the forming of coal deposits in different locations, the composition of coal varies from one deposit to another. No two coals are the same in every respect. In general, coal consists of [[carbon]], [[hydrogen]], [[oxygen]], [[nitrogen]], [[sulphur]] and [[mineral]] matter (including compounds of [[silicon]], [[aluminium]], [[iron]], [[calcium]], [[magnesium]] and others).
Coal is primarily [[carbon]], with traces of [[sulfur]], [[oxygen]], [[hydrogen]], [[nitrogen]], and other elements.<ref>needs further discussion - quantities, ash content, etc.</ref> The processes of decomposition and lithification drive off most of the oxygen, hydrogen and nitrogen from the plant material, leaving primarily carbon.  


Due to its high carbon content and solid, easily-handled form, coal is used for fuel, and has been for hundreds of years (see [[Coal mining, history of|history of coal mining]]). Coal can be burned directly, or it can be converted to [[coke]] by anaerobic heating (called '''coking'''), which alters the physical properties to provide a more uniform and more combustible product.<ref>need more discussion of coking</ref> Coal is used in [[steel]]making both to provide heat and to add controlled amounts of carbon to the iron.<ref>needs expansion</ref>
== Coal classification ==


Coal deposits are found in many places around the world, though the deposits which have had the largest economic impacts are those in the Rhine/Ruhr area of [[Germany]], [[Wales]] and the Midlands of [[England]], and numerous parts of the U.S., especially Pennsylvania (in the 19th century) and Wyoming (in ther 21st century). Currently, coal production in [[China]] is rapidly increasing to meet soaring demand from China's industrialization.<ref>need statistics and more history</ref>
There are many compositional differences between the coals mined from the different coal deposits worldwide. The different types of coal are most usually classified by '''rank''' which depends upon the degree of transformation from the original source (i.e., decayed plants) and is therefore a measure of a coal's age.<ref name=Perry/><ref name=Speight>{{cite book|author=J.G. Speight|title=The Chemistry and Technology of Coal|edition=2nd Edition, Revised and Expanded|publisher=CRC Press|year=1994|id=ISBN 0-8247-9200-9}}</ref> As the process of progressive transformation took place, the [[heating value]] and the fixed carbon content of the coal increased and the amount of volatile matter in the coal decreased. The method of ranking coals used in the [[United States of America]] and [[Canada]] was developed by the [[ASTM|American Society for Testing and Materials]] (ASTM) and is based on a number of parameters obtained by various prescribed tests:


==Economic use==
* ''Heating value'': The [[Energy (science)|energy]] released as heat when coal (or any other substance) undergoes complete [[combustion]] with oxygen.
===Industrial Revolution===
* ''Volatile matter'': The portion of a coal sample which, when heated in the absence of air at prescribed conditions, is released as [[gas|gases]]. It includes [[carbon dioxide]], volatile [[organic chemistry|organic]] and [[inorganic chemistry|inorganic]] gases containing sulfur and nitrogen.  
Large-scale exploitation of coal began concurrently with the [[Industrial Revolution]], and coal was one of its primary ingredients. Coal was used to make iron, and later steel, and was used to power steam engines which were used to transport very large quantities of goods (relative to previous animal-driven transport) and to pump out deep mines of coal and iron.<ref>Needs much more discussion</ref>
* ''Moisture'': The water inherently contained within the coal and existing in the coal in its natural state of deposition. It as measured as the amount of water released when a coal sample is heated at prescribed conditions. It does not include any free water on the surface of the coal. Such free water is removed by air-drying the coal sample being tested.  
===United States===
* ''Ash'': The inorganic residue remaining after a coal sample is completely burned and is largely composed of compounds of silica, aluminium, iron, calcium, magnesium and others. The ash may vary considerably from the mineral matter present in the coal (such as [[clay]], [[quartz]], [[pyrite]]s and [[gypsum]]) before being burned.
Currently, burning of coal provides about one-quarter of the electric power of the United States, and accounts for about one-quarter of energy use worldwide. About 90% of coal mined in the United States is burned to produce electricity.<ref>needs to be fleshed out with more accurate statistics, and references</ref>
* ''Fixed carbon'': The remaining organic matter after the volatile matter and moisture have been released. It is typically calculated by subtracting from 100 the percentages of volatile matter, moisture and ash. It is composed primarily of carbon with lesser amounts of hydrogen, nitrogen and sulfur.
===China===
Coal produces over 80% of [[China|China's]] energy; 2.3 billion metric tons of coal were mined in 2007.  Despite the health risks posed by severe air pollution in cities (see [[Beijing]]) and international pressure to reduce greenhouse emissions, China’s coal consumption is projected to increase in line with its rapid economic growth. Most of the coal is mined in the western provinces of Shaanxi and Shanxi and the northwestern region of Inner Mongolia. However most coal customers are located in the industrialized southeastern and central coastal provinces, so coal must be hauled long distances on China’s vast but overextended rail network. More than 40% of rail capacity is devoted to moving coal, and the country has been investing heavily in new lines and cargo-handling facilities in an attempt to keep up with demand. Despite these efforts, China has suffered persistent power shortages in industrial centers for more than five years as electricity output failed to meet demand from a booming economy. Demand for electricity increased 14% in 2007. Severe snowstorms in late January 2008 seriously disrupted the rail and electrical systems, at a time when some 200 million city workers were attempting to visit their home villages during the Lunar New Year holiday.<ref>David Lague, "Chinese Blizzards Reveal Rail Limits," [http://www.nytimes.com/2008/02/01/world/asia/01china.html?_r=1&hp&oref=slogin ''New York Times'' Feb. 1, 2008]  </ref>


==Coal mining==
The ASTM ranking system is presented in the table below:
 
{| class = "wikitable" align="center"
|+ Classification of Coals by Rank<ref name=Perry/><ref name=Marks/><ref name=Kreith/><sup> (a)</sup>
!rowspan="2"|<br><br><br><br><br>Class or<br>Rank||rowspan=2|<br><br><br><br><br><br>Group||colspan="2"|<br>Fixed Carbon<sup> (b)</sup><br>(wt % dry mmf) ||colspan="2"|<br>Volatile Matter<sup> (b)</sup><br>(wt % dry mmf)||colspan="2"|Gross<br>Heating Value<sup> (c)</sup><br>(M[[Joule|J]]/[[Kilogram|kg]] moist mmf)
|- align="center"
!Equal or<br>greater<br>than ||<br><br>Less than||<br>Greater<br>than ||<br>Equal or<br>less than ||Equal or<br>greater<br>than ||<br><br>Less than
|-
|Anthracitic<br><br><br>||Metaanthracite<sup> (d)</sup><br>Anthracite<sup> (d)</sup><br>Semianthracite<sup> (d)</sup>|| align="center"|98<br>92<br>86||align="center"|<br>98<br>92||align="center"|<br>2<br>8||align="center"|2<br>8<br>14||&nbsp;||&nbsp;
|-
|Bituminous<br><br><br><br><br><br>||Low-volatile bituminous<sup> (d)</sup><br>Medium-volatile bituminous<sup> (d)</sup><br>High-volatile A bituminous<br>High-volatile B bituminous<br>High-volatile C bituminous<sup> (e)</sup><br>High-volatile C bituminous<sup> (f)</sup>||align="center" |78<br>69<br><br><br><br><br> ||align="center"|86<br>78<br>69<br><br><br><br>||align="center"|14<br>22<br>31<br><br><br><br>||align="center"|22<br>31<br><br><br><br><br>||align="center"|<br><br>32.55<br>30.23<br>26.74<br>24.41||align="center"|
<br><br><br>32.55<br>30.23<br>26.74
|-
|Subbituminous<br><br><br>||Subbituminous A<br>Subbituminous B<br>Subbituminous C|| &nbsp;||&nbsp;||&nbsp;||&nbsp;||align="center"|24.41<br>22.09<br>19.30||align="center" |26.74<br>24.41<br>22.09
|-
|Lignite<br><br>||Lignite A<br>Lignite B||&nbsp;||&nbsp;||&nbsp;||&nbsp;||align="center"|14.65<br><br>||align="center"|19.30<br>14.65
|-
|colspan="8"|(a) This classification does not include a few coals (referred to as unbanded coals) having unusual physical and chemical<br>properties falling within the fixed carbon and heating value ranges of the high-volatile bituminous and subbituminous ranks.<br>
(b) Percentage by weight on a dry and mineral matter free basis (mmf).<br>
(c) [[Gross Heating Value]] on a moist and mineral matter free basis. Moist refers to the natural inherent water contained in<br>a coal but does not include visible water (if any) on the surface of the coal. Multiply MJ/kg by 430.11 to convert to [[U.S. customary units|Btu]]/[[U.S. customary units|lb]].<br>
(d) Coals containing 69 wt % or more fixed carbon on a dry mmf basis are ranked according to their fixed carbon content<br>regardless of their Gross Heating Value.<br>
(e) A high-volatile C bituminous coal that may be agglomerating or non-agglomerating.<ref name=Perry/><ref name=Glossary>{{cite book|author=Klaus K.E. Neuendorf, James P. Mehl and Julia A. Jackson|title=Glossary of Geology|edition=5th Edition|publisher=American Geological Institute|year=2005|id=ISBN 0-922152-76-4}}</ref><br>
(f) A high-volatile C bituminous coal that is an agglomerating coal, which means that it tends to become sticky and to ''cake''<br> when heated. The agglomerating character of a coal is determined by heating a sample to 950 °C under certain conditions.<br>If the residue is coherent and supports a weight of 500g without pulverizing, the coal is classified as being agglomerating.
|}
 
The anthracitic coals, with the highest contents of fixed carbon and lowest contents of volatile material,
have the highest rank. The lignite coals, with the lowest contents of fixed carbon and highest contents of volatile matter, have the lowest rank. The bituminous and subbituminous coals (in that order) are ranked between the anthracitic and lignite coal. The diagram below provides the estimated percentage of the world's coal reserves for each coal rank. It also provides the typical uses of each coal rank. 
 
As a broad generality, the anthracitic coals have the highest heating value and the lignite coals have the lowest heating values.
{{Image|Coal Rank & Uses.png|center|455px|Diagram of the typical uses and the estimated percentage of the world's coal reserves for each coal rank.<ref>[http://www.worldcoal.org/coal_info.asp Coal Information] World Coal Institute</ref>}}
 
There are other coal classification systems developed by the [[International Organization for Standardization]] (ISO), the [[United Kingdom]] and perhaps others.<ref name=Speight/><ref>{{cite book|author=Sunggyu Lee|title=Alternative Fuels|edition=First Edition|publisher=CRC Press|year=1996|id=ISBN 1-56032-361-2}}</ref>
 
==Coal analysis==
 
The composition of a coal is usually reported in terms of its '''proximate''' analysis and its '''ultimate analysis''':
 
*The proximate analysis consists of four items: fixed carbon, volatile matter, moisture and ash, all on a weight percent basis.
 
*The ultimate analysis provides an element-by-element composition of the coal's organic fraction, namely:  carbon, hydrogen, oxygen and sulfur, all on a weight percent basis.


{{main|Coal mining history}}
Both the proximate and the ultimate analysis may be reported on an ''as received'' (ar) basis, a ''dry'' (d) or ''moist'' basis, an ''ash-free'' (af) basis, a ''mineral matter-free'' (mmf) basis and various combinations of those bases. For example, an analysis may report the basis to be: as received (ar), dry and ash-free (daf), moist and ash-free (maf), dry and mineral matter-free (dmmf) or moist mineral-matter free (moist mmf).
{{Image|Coal strip mining.jpg|right|375px|Strip mining of coal.}}


Coal tends to exist in ''seams'', which are lateral layers under the earth which may vary in height from 1 or 2 feet to dozens of feet. [[mine (resource exploitation)|Mining]] of coal seams is achieved in several different ways. "Strip" mines  scrape coal from the earth's surface; they may be large open pits, or if on a mountain, result in ribbons chewed away from around the perimeter of a mountain at each level where a seam of coal exists. So-called "drift" mines angle horizontally into a mountainside and may be very shallow (i.e., not tall enough for a person to stand up in). "Shaft" mines, also called deep mines, reach down vertically to open into person-sized horizontal tunnels which may be miles from the surface.
Ash and mineral matter are two distinctly different entities. Mineral matter consists of the various minerals contained in the coal. Ash is the inorganic solids remaining after the coal is completely combusted. The ash is usually less than the mineral matter because of the weight changes that take place during coal combustion such as the loss of gaseous carbon dioxide from mineral carbonates, loss of water from silica minerals and loss of sulfur (as gaseous sulfur dioxide) from iron [[pyrite]]s.


Strip mines remove any top soil with bulldozers to get at coal near the earth's surface.  Coal is excavated from the ground in what becomes large pits, or else ribbons of stripped land stretch around a mountain.  After strip mining has exhausted the available surface coal, the mining company often abandons the site with no restoration, leading to severe erosion problems (with resultant flooding or pollution) and to an unsightly landscape that cannot support plant growth due to the lack of topsoil. Drift mines may be used after strip mining has used up surface coal.  Because drift mines tend to be shallow, special equipment may be required to mine them, on which, for example, workers can lie flat on short vehicles moving inside the tunnel.  Drift mining is common in the extreme southwest corner of Virginia.  Deep mines are similar to those for any other mineral deposit found deep enough in the earth that the cost of removing the overburden is prohibitive. Shafts are dug and veins of coal are excavated and transported to the surface.<ref>needs more discussion</ref>
Some examples of proximate and ultimate analyses are given in the table below:


{|border="0" align="center"
{| class = "wikitable" align="center"
|+ Examples of Proximate and Ultimate Analyses <ref>{{cite book|author=Chris Higman and Maarten van der Burgt|title=Coal Gasification|edition=2nd Edition|publisher=Gulf Professional Publishers|year=2008|id=ISBN 0-7506-8528-X}}</ref>
!rowspan="2"|<br><br><br><br>Coal Rank||colspan="4"|Proximate Analysis<br>(wt % ar) ||colspan="5"|Ultimate Analysis<br>(wt % maf)||rowspan=2|Net<br>Heating<br>Value<br>(maf)<br>(MJ/kg)
|- align="center"
!Fixed<br>carbon||Volatile<br>matter||<br>Moisture ||<br>Ash ||<br>C||<br>H||<br>O||<br>N||<br>S
|-
|Anthracite||align="center"|81.8||align="center"|7.7||align="center"|4.5||align="center"|6.0||align="center"|91.8||align="center"|3.6||align="center"|2.5||align="center"|1.4||align="center"|0.7||align="center"|36.2
|-
|Bituminous||align="center"|54.9||align="center"|35.6||align="center"|5.3||align="center"|4.2||align="center"|82.8||align="center"|5.1||align="center"|10.1||align="center"|1.4||align="center"|0.6||align="center"|36.1
|-
|Subbituminous||align="center"|43.6||align="center"|34.7||align="center"|10.5||align="center"|11.2||align="center"|76.4||align="center"|5.6||align="center"|14.9||align="center"|1.7||align="center"|1.4||align="center"|31.8
|-
||Lignite||align="center"|27.8||align="center"|24.9||align="center"|36.9||align="center"|10.4||align="center"|71.0||align="center"|4.3||align="center"|23.2||align="center"|1.1||align="center"|0.4||align="center"|26.7
|-
|-
|valign="center" |{{Image|Mining.gif|right|250px|Three common mining methods.}}
|colspan="12"|'''Notes''':<br>
|valign="center" |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
• wt % = percent by weight &nbsp; &nbsp; ar = as received &nbsp; &nbsp; maf = moisture and ash free<br>
|valign="center" |{{Image|Mining2.gif|right|250px|Imagined view "from above" of a mined seam of coal.}}
• C = Carbon &nbsp; &nbsp; H = Hydrogen &nbsp; &nbsp; O = Oxygen &nbsp; &nbsp; N = Nitrogen &nbsp; &nbsp; S = Sulfur <br>
• Multiply [[Net Heating Value]]s in M[[Joule|J]]/[[Kilogram|kg]] by 430.11 to convert to [[U.S. customary units|Btu]]/[[U.S. customary units|lb]].
|}
|}


===Deep and drift mining safety===
==Coal reserves and production statistics==
 
Economically recoverable coal deposits exist in more than 70 nations and in every major region of the world ([[Africa]], [[Asia]], [[Australia]], [[Europe]], [[North America]] and [[South America]]).  It has been estimated that the worldwide proven reserves of coal amounted to about 848 gigatonnes (Gt) as of 2007.<ref name=WorldEnergy>[http://www.worldenergy.org/documents/ser2007_final_online_version_1.pdf 2007 Survey of Energy Resources] World Energy Council 2007</ref> Proven coal reserves are those coal deposits that have been confirmed by exploration, drilling and other means, and which are economically and technically extractable.
 
It has also been estimated that the worldwide production (i.e., [[Mine (resource extraction)|mining]]) of coal amounted to about 5.54 gigatonnes (Mt) as of 2007.<ref name=WorldCoal>[http://www.worldcoal.org/assets_cm/files/PDF/coalfacts08.pdf Coal Facts] 2008 Edition (with 2007 data)</ref> If that rate of production remains constant, the proven reserves will last about 150 years.<ref name=WorldEnergy/>
 
The tables below list the distribution of coal reserves and coal production nation-by-nation as of 2007:
{| border="0" cellpadding="25" align="center"
|
{|class = "wikitable" align="center"
|+ Worldwide<br>Proven Coal Reserves<br>(in 2007) <ref name=WorldEnergy/>
! Nation!!Reserves<br>(Gigatonnes)
|- align="center"
|United States||245
|- align="center"
|[[Russia]]||151
|- align="center"
|China||125
|- align="center"
|Australia||75
|- align="center"
|[[India]]||51
|- align="center"
|[[South Africa]]||50
|- align="center"
|[[Ukraine]]||35
|- align="center"
|Others||116
|- align="center"
|'''Total'''||'''848'''
|}
|
{| class = "wikitable" align="center"
|+ Worldwide<br>Coal Production<br>(in 2007) <ref name= WorldCoal/>
! Nation!!Production<br>(Megatonnes)
|- align="center"
|China||2549
|- align="center"
|United States||981
|- align="center"
|India||452
|- align="center"
|Australia||323
|- align="center"
|South Africa||244
|- align="center"
|Russia||241
|- align="center"
|[[Indonesia]]||231
|- align="center"
|Others||522
|- align="center"
|'''Total'''||'''5543'''
|}
|}
 
==Coal as a fuel==
 
{{main|Conventional coal-fired power plant}}
Due to its relatively high carbon content and solid, easily-handled form, coal is used for fuel, and has been for hundreds of years (see [[Coal mining, history of|history of coal mining]]). As a fuel, coal is the largest source of energy for the generation of electricity worldwide. In 2005, coal fuelled 40% of the world's electricity generating power plants.<ref name=IEA>[http://www.iea.org/textbase/nppdf/free/2006/key2006.pdf International Energy Association, 2006, Key Energy Statistics] ([[International Energy Agency]])</ref><ref name=EIACh.5>[http://www.eia.doe.gov/oiaf/ieo/electricity.html International Energy Outlook 2008: Chapter 5] (Energy Information Administration, U.S. DOE)</ref>
 
A major component of the [[combustion]] [[flue gas]]es produced by burning coal as a fuel is [[carbon dioxide]] (CO<sub>2</sub>), which is not a [[pollutant]] in the traditional sense since it is essential to support [[photosynthesis]] for all plant life on [[Earth]]. However, carbon dioxide is a ''[[greenhouse gas]]'' considered to be a contributor to ''[[global warming]]''. It is the most abundant anthropogenic (human caused) greenhouse gas in the [[Earth's atmosphere]]. As shown above, coal may contain from about 70 to more than 90 weight percent carbon, which burns almost completely to carbon dioxide. Hence, coal is the [[fossil fuel]] with the largest "carbon footprint".


Early mining methods led to very unsafe mines which often were not even represented on maps at all, or were represented inaccurately.<ref>Children in coal-mining areas sometimes can't resist entering old mines, which can be especially dangerous.</ref> Even using the best known methods, underground coal mining is hazardous workIn addition to the hazard of simple cave-ins, miners have to worry about their tunnels flooding, accumulation of "bad air" (gases lacking enough oxygen), accumulation of explosive gases resulting in fires and/or cave-ins, and many other unexpected problems. Bad air and water can suddenly flood a tunnel if a pocket of the non-oxygenated gas or water is reached without warning when removing coal from a seam.
===Currently in the United States===
 
Coal-fired power plants provided about 50 percent of the electric power generated in the United States during 2007.<ref name=EIA-1>[http://www.eia.doe.gov/cneaf/electricity/epa/epates.html Summary Statistics for the United States]] ''Electric Power Annual'' (2007) published by the [[Energy Information Administration]] of the [[U.S. Department of Energy]]</ref> About 92% of the coal [[Mine (resource extraction)|mined]] in the United States is burned to produce electricity.<ref name=EIA-1/><ref name=EIA-2>[http://www.eia.doe.gov/cneaf/coal/page/acr/table1.html Coal Production and Number of Mines by State and Mine Type] ''Annual Coal Report'' (2007) published by the Energy Information Administration of the U.S. Department of Energy</ref>
 
The consumption of coal in the United States by sector (as a percentage of the total coal mined in 2007) was 92.7 % for electric power generation, 2.0 % for production of coke, 5.0 % for use in other industries and 0.3 % for residential and commercial heating.<ref name=EIA-3>[http://www.eia.doe.gov/cneaf/coal/page/acr/acr.pdf Annual Coal Report (2007), Executive Summary] Published by the Energy Information Administration of the U.S. Department of Energy</ref>
 
===Currently in China===
Coal produces over 80% of China's energy; 2.3 billion metric tons of coal were mined in 2007.  Despite the health risks posed by severe air pollution in cities (see [[Beijing]]) and international pressure to reduce greenhouse emissions, China’s coal consumption is projected to increase in line with its rapid economic growth. Most of the coal is mined in the western provinces of Shaanxi and Shanxi and the northwestern region of Inner Mongolia.  However most coal customers are located in the industrialized southeastern and central coastal provinces, so coal must be hauled long distances on China’s vast but overextended rail networkMore than 40% of rail capacity is devoted to moving coal, and the country has been investing heavily in new lines and cargo-handling facilities in an attempt to keep up with demand. Despite these efforts, China has suffered persistent power shortages in industrial centers for more than five years as electricity output failed to meet demand from a booming economy. Demand for electricity increased 14% in 2007.
 
==Other uses of coal==
 
{{main|Destructive distillation|Coal gasification|Synthesis gas|Fischer-Tropsch}}
Coal can be converted to [[coke (fuel)|coke]] by the process of [[destructive distillation]] which removes the volatile matter and alters the physical properties to provide a more uniform and more [[Combustion|combustible]] product with a higher carbon content.<ref>Many [[Petroleum refining processes|petroleum refineries]] produce a similar product, called ''petroleum coke'', by a process known as [[delayed coking]].</ref> The process is often referred to as the ''coking'' or ''carbonization'' of coal. One of the major uses of coke is in the making of [[steel]] where coke is utilized in [[blast furnace]]s to reduce [[iron ore]] ([[iron oxide]]) to molten [[pig iron]], a form of iron with high carbon content. Removal of most of the carbon from the pig iron yields steel that is used for construction of bridges, high-rise buildings, manufacturing of automobiles and major household appliances (refrigerators, cooking stoves and washing machines), and a host of other products. Coke is also used in the production of [[phosphorus]] and of [[calcium carbide]].
 
Coal can be converted, by a process known as [[coal gasification]], into a gas with the same [[heat of combustion]] as many [[natural gas]]es and referred to as ''synthetic natural gas'' (SNG).<ref>[http://nicholas.duke.edu/ccpp/ccpp_pdfs/synthetic.gas.pdf Synthetic Natural Gas (SNG): Technology, Environmental Implications, and Economics] Munish Chandel and Eric Williams, Climate Change Policy Partnership, [[Duke University]], January 2009</ref><ref>Beychok, M.R., ''Process and environmental technology for producing SNG and liquid fuels'', U.S. EPA report EPA-660/2-75-011, May 1975</ref> Coal gasification can also be used to produce a mixture of [[carbon monoxide]] and hydrogen gases referred to as [[synthesis gas]] (or syngas) which has a heat of combustion that is much less than that of natural gas. Syngas can be burned as a fuel or it can be converted into automotive fuels like [[gasoline]] and [[diesel oil]] through the [[Fischer-Tropsch process]].<ref>[http://www.purdue.edu/dp/energy/pdf/Basics1-CoalGasification-July06.pdf Coal Gasification & Fischer-Tropsch] Brian H. Bowen and Marty W. Irwin, The Indiana Center for Coal Technology Research, [[Purdue University]], July 2006</ref>
 
==Coal mining==


Following known best practices can reduce the likelihood of extensive loss of life during catastrophic mining accidents.  Poor safety records of some mine owners led to the formation of labor unions around the world, and today there remains a high degree of solidarity among mine workers.  Mining deaths still occur periodically that arguably could have been prevented with appropriate safety equipment, training and safety procedures.
''See [[Coal mining]] for details.


==Notes==
== References ==
{{reflist}}
{{reflist}}[[Category:Suggestion Bot Tag]]

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(PD) Photo: U.S. Geological Survey / Andrew Silver
Anthracite coal (American 25 cent coin shown for scale).

Coal is a carbon-containing rock formed by the debris from the decay of ferns, vines, trees and other plants which flourished in swamps millions of years ago. Over time, the debris became buried and the actions of bacteria, heat and pressure transformed the debris first into peat (a precursor of coal) and then into the various types of coal as we know them today.[1][2][3] In more technical terminology, that process of transformation is referred to as metamorphosis, coalification or lithification.

Coal mining occurs for coal deposits that exist deep underground, as well as one that are at or near the surface of the ground. Because of the various degrees of transformation that occurred during the forming of coal deposits in different locations, the composition of coal varies from one deposit to another. No two coals are the same in every respect. In general, coal consists of carbon, hydrogen, oxygen, nitrogen, sulphur and mineral matter (including compounds of silicon, aluminium, iron, calcium, magnesium and others).

Coal classification

There are many compositional differences between the coals mined from the different coal deposits worldwide. The different types of coal are most usually classified by rank which depends upon the degree of transformation from the original source (i.e., decayed plants) and is therefore a measure of a coal's age.[1][4] As the process of progressive transformation took place, the heating value and the fixed carbon content of the coal increased and the amount of volatile matter in the coal decreased. The method of ranking coals used in the United States of America and Canada was developed by the American Society for Testing and Materials (ASTM) and is based on a number of parameters obtained by various prescribed tests:

  • Heating value: The energy released as heat when coal (or any other substance) undergoes complete combustion with oxygen.
  • Volatile matter: The portion of a coal sample which, when heated in the absence of air at prescribed conditions, is released as gases. It includes carbon dioxide, volatile organic and inorganic gases containing sulfur and nitrogen.
  • Moisture: The water inherently contained within the coal and existing in the coal in its natural state of deposition. It as measured as the amount of water released when a coal sample is heated at prescribed conditions. It does not include any free water on the surface of the coal. Such free water is removed by air-drying the coal sample being tested.
  • Ash: The inorganic residue remaining after a coal sample is completely burned and is largely composed of compounds of silica, aluminium, iron, calcium, magnesium and others. The ash may vary considerably from the mineral matter present in the coal (such as clay, quartz, pyrites and gypsum) before being burned.
  • Fixed carbon: The remaining organic matter after the volatile matter and moisture have been released. It is typically calculated by subtracting from 100 the percentages of volatile matter, moisture and ash. It is composed primarily of carbon with lesser amounts of hydrogen, nitrogen and sulfur.

The ASTM ranking system is presented in the table below:

Classification of Coals by Rank[1][2][3] (a)





Class or
Rank






Group

Fixed Carbon (b)
(wt % dry mmf)

Volatile Matter (b)
(wt % dry mmf)
Gross
Heating Value (c)
(MJ/kg moist mmf)
Equal or
greater
than


Less than

Greater
than

Equal or
less than
Equal or
greater
than


Less than
Anthracitic


Metaanthracite (d)
Anthracite (d)
Semianthracite (d)
98
92
86

98
92

2
8
2
8
14
   
Bituminous





Low-volatile bituminous (d)
Medium-volatile bituminous (d)
High-volatile A bituminous
High-volatile B bituminous
High-volatile C bituminous (e)
High-volatile C bituminous (f)
78
69




86
78
69



14
22
31



22
31






32.55
30.23
26.74
24.41




32.55
30.23
26.74

Subbituminous


Subbituminous A
Subbituminous B
Subbituminous C
        24.41
22.09
19.30
26.74
24.41
22.09
Lignite

Lignite A
Lignite B
        14.65

19.30
14.65
(a) This classification does not include a few coals (referred to as unbanded coals) having unusual physical and chemical
properties falling within the fixed carbon and heating value ranges of the high-volatile bituminous and subbituminous ranks.

(b) Percentage by weight on a dry and mineral matter free basis (mmf).
(c) Gross Heating Value on a moist and mineral matter free basis. Moist refers to the natural inherent water contained in
a coal but does not include visible water (if any) on the surface of the coal. Multiply MJ/kg by 430.11 to convert to Btu/lb.
(d) Coals containing 69 wt % or more fixed carbon on a dry mmf basis are ranked according to their fixed carbon content
regardless of their Gross Heating Value.
(e) A high-volatile C bituminous coal that may be agglomerating or non-agglomerating.[1][5]
(f) A high-volatile C bituminous coal that is an agglomerating coal, which means that it tends to become sticky and to cake
when heated. The agglomerating character of a coal is determined by heating a sample to 950 °C under certain conditions.
If the residue is coherent and supports a weight of 500g without pulverizing, the coal is classified as being agglomerating.

The anthracitic coals, with the highest contents of fixed carbon and lowest contents of volatile material, have the highest rank. The lignite coals, with the lowest contents of fixed carbon and highest contents of volatile matter, have the lowest rank. The bituminous and subbituminous coals (in that order) are ranked between the anthracitic and lignite coal. The diagram below provides the estimated percentage of the world's coal reserves for each coal rank. It also provides the typical uses of each coal rank.

As a broad generality, the anthracitic coals have the highest heating value and the lignite coals have the lowest heating values.

(CC) Image: World Coal Institute
Diagram of the typical uses and the estimated percentage of the world's coal reserves for each coal rank.[6]

There are other coal classification systems developed by the International Organization for Standardization (ISO), the United Kingdom and perhaps others.[4][7]

Coal analysis

The composition of a coal is usually reported in terms of its proximate analysis and its ultimate analysis:

  • The proximate analysis consists of four items: fixed carbon, volatile matter, moisture and ash, all on a weight percent basis.
  • The ultimate analysis provides an element-by-element composition of the coal's organic fraction, namely: carbon, hydrogen, oxygen and sulfur, all on a weight percent basis.

Both the proximate and the ultimate analysis may be reported on an as received (ar) basis, a dry (d) or moist basis, an ash-free (af) basis, a mineral matter-free (mmf) basis and various combinations of those bases. For example, an analysis may report the basis to be: as received (ar), dry and ash-free (daf), moist and ash-free (maf), dry and mineral matter-free (dmmf) or moist mineral-matter free (moist mmf).

Ash and mineral matter are two distinctly different entities. Mineral matter consists of the various minerals contained in the coal. Ash is the inorganic solids remaining after the coal is completely combusted. The ash is usually less than the mineral matter because of the weight changes that take place during coal combustion such as the loss of gaseous carbon dioxide from mineral carbonates, loss of water from silica minerals and loss of sulfur (as gaseous sulfur dioxide) from iron pyrites.

Some examples of proximate and ultimate analyses are given in the table below:

Examples of Proximate and Ultimate Analyses [8]




Coal Rank
Proximate Analysis
(wt % ar)
Ultimate Analysis
(wt % maf)
Net
Heating
Value
(maf)
(MJ/kg)
Fixed
carbon
Volatile
matter

Moisture

Ash

C

H

O

N

S
Anthracite 81.8 7.7 4.5 6.0 91.8 3.6 2.5 1.4 0.7 36.2
Bituminous 54.9 35.6 5.3 4.2 82.8 5.1 10.1 1.4 0.6 36.1
Subbituminous 43.6 34.7 10.5 11.2 76.4 5.6 14.9 1.7 1.4 31.8
Lignite 27.8 24.9 36.9 10.4 71.0 4.3 23.2 1.1 0.4 26.7
Notes:

• wt % = percent by weight     ar = as received     maf = moisture and ash free
• C = Carbon     H = Hydrogen     O = Oxygen     N = Nitrogen     S = Sulfur
• Multiply Net Heating Values in MJ/kg by 430.11 to convert to Btu/lb.

Coal reserves and production statistics

Economically recoverable coal deposits exist in more than 70 nations and in every major region of the world (Africa, Asia, Australia, Europe, North America and South America). It has been estimated that the worldwide proven reserves of coal amounted to about 848 gigatonnes (Gt) as of 2007.[9] Proven coal reserves are those coal deposits that have been confirmed by exploration, drilling and other means, and which are economically and technically extractable.

It has also been estimated that the worldwide production (i.e., mining) of coal amounted to about 5.54 gigatonnes (Mt) as of 2007.[10] If that rate of production remains constant, the proven reserves will last about 150 years.[9]

The tables below list the distribution of coal reserves and coal production nation-by-nation as of 2007:

Worldwide
Proven Coal Reserves
(in 2007) [9]
Nation Reserves
(Gigatonnes)
United States 245
Russia 151
China 125
Australia 75
India 51
South Africa 50
Ukraine 35
Others 116
Total 848
Worldwide
Coal Production
(in 2007) [10]
Nation Production
(Megatonnes)
China 2549
United States 981
India 452
Australia 323
South Africa 244
Russia 241
Indonesia 231
Others 522
Total 5543

Coal as a fuel

For more information, see: Conventional coal-fired power plant.

Due to its relatively high carbon content and solid, easily-handled form, coal is used for fuel, and has been for hundreds of years (see history of coal mining). As a fuel, coal is the largest source of energy for the generation of electricity worldwide. In 2005, coal fuelled 40% of the world's electricity generating power plants.[11][12]

A major component of the combustion flue gases produced by burning coal as a fuel is carbon dioxide (CO2), which is not a pollutant in the traditional sense since it is essential to support photosynthesis for all plant life on Earth. However, carbon dioxide is a greenhouse gas considered to be a contributor to global warming. It is the most abundant anthropogenic (human caused) greenhouse gas in the Earth's atmosphere. As shown above, coal may contain from about 70 to more than 90 weight percent carbon, which burns almost completely to carbon dioxide. Hence, coal is the fossil fuel with the largest "carbon footprint".

Currently in the United States

Coal-fired power plants provided about 50 percent of the electric power generated in the United States during 2007.[13] About 92% of the coal mined in the United States is burned to produce electricity.[13][14]

The consumption of coal in the United States by sector (as a percentage of the total coal mined in 2007) was 92.7 % for electric power generation, 2.0 % for production of coke, 5.0 % for use in other industries and 0.3 % for residential and commercial heating.[15]

Currently in China

Coal produces over 80% of China's energy; 2.3 billion metric tons of coal were mined in 2007. Despite the health risks posed by severe air pollution in cities (see Beijing) and international pressure to reduce greenhouse emissions, China’s coal consumption is projected to increase in line with its rapid economic growth. Most of the coal is mined in the western provinces of Shaanxi and Shanxi and the northwestern region of Inner Mongolia. However most coal customers are located in the industrialized southeastern and central coastal provinces, so coal must be hauled long distances on China’s vast but overextended rail network. More than 40% of rail capacity is devoted to moving coal, and the country has been investing heavily in new lines and cargo-handling facilities in an attempt to keep up with demand. Despite these efforts, China has suffered persistent power shortages in industrial centers for more than five years as electricity output failed to meet demand from a booming economy. Demand for electricity increased 14% in 2007.

Other uses of coal

For more information, see: Destructive distillation, Coal gasification, Synthesis gas, and Fischer-Tropsch.

Coal can be converted to coke by the process of destructive distillation which removes the volatile matter and alters the physical properties to provide a more uniform and more combustible product with a higher carbon content.[16] The process is often referred to as the coking or carbonization of coal. One of the major uses of coke is in the making of steel where coke is utilized in blast furnaces to reduce iron ore (iron oxide) to molten pig iron, a form of iron with high carbon content. Removal of most of the carbon from the pig iron yields steel that is used for construction of bridges, high-rise buildings, manufacturing of automobiles and major household appliances (refrigerators, cooking stoves and washing machines), and a host of other products. Coke is also used in the production of phosphorus and of calcium carbide.

Coal can be converted, by a process known as coal gasification, into a gas with the same heat of combustion as many natural gases and referred to as synthetic natural gas (SNG).[17][18] Coal gasification can also be used to produce a mixture of carbon monoxide and hydrogen gases referred to as synthesis gas (or syngas) which has a heat of combustion that is much less than that of natural gas. Syngas can be burned as a fuel or it can be converted into automotive fuels like gasoline and diesel oil through the Fischer-Tropsch process.[19]

Coal mining

See Coal mining for details.

References

  1. 1.0 1.1 1.2 1.3 Green, Don W. and Perry, Robert H. (Editors) (1997). Perry's Chemical Engineers' Handbook, 6th Edition. McGraw-Hill. ISBN 0-07-049479-7. 
  2. 2.0 2.1 Eugene A. Avallone, Theodore Baumeister and Ali Sadegh (Editors) (2006). Marks' Standard Handbook for Mechanical Engineers, 11th Edition. McGraw-Hill Professional. ISBN 0-07-142867-4. 
  3. 3.0 3.1 Frank Kreith (Editor) (1998). The CRC Handbook of Mechanical Engineering, 1st Edition. CRC Press. ISBN 0-8493-9418-X. 
  4. 4.0 4.1 J.G. Speight (1994). The Chemistry and Technology of Coal, 2nd Edition, Revised and Expanded. CRC Press. ISBN 0-8247-9200-9. 
  5. Klaus K.E. Neuendorf, James P. Mehl and Julia A. Jackson (2005). Glossary of Geology, 5th Edition. American Geological Institute. ISBN 0-922152-76-4. 
  6. Coal Information World Coal Institute
  7. Sunggyu Lee (1996). Alternative Fuels, First Edition. CRC Press. ISBN 1-56032-361-2. 
  8. Chris Higman and Maarten van der Burgt (2008). Coal Gasification, 2nd Edition. Gulf Professional Publishers. ISBN 0-7506-8528-X. 
  9. 9.0 9.1 9.2 2007 Survey of Energy Resources World Energy Council 2007
  10. 10.0 10.1 Coal Facts 2008 Edition (with 2007 data)
  11. International Energy Association, 2006, Key Energy Statistics (International Energy Agency)
  12. International Energy Outlook 2008: Chapter 5 (Energy Information Administration, U.S. DOE)
  13. 13.0 13.1 Summary Statistics for the United States] Electric Power Annual (2007) published by the Energy Information Administration of the U.S. Department of Energy
  14. Coal Production and Number of Mines by State and Mine Type Annual Coal Report (2007) published by the Energy Information Administration of the U.S. Department of Energy
  15. Annual Coal Report (2007), Executive Summary Published by the Energy Information Administration of the U.S. Department of Energy
  16. Many petroleum refineries produce a similar product, called petroleum coke, by a process known as delayed coking.
  17. Synthetic Natural Gas (SNG): Technology, Environmental Implications, and Economics Munish Chandel and Eric Williams, Climate Change Policy Partnership, Duke University, January 2009
  18. Beychok, M.R., Process and environmental technology for producing SNG and liquid fuels, U.S. EPA report EPA-660/2-75-011, May 1975
  19. Coal Gasification & Fischer-Tropsch Brian H. Bowen and Marty W. Irwin, The Indiana Center for Coal Technology Research, Purdue University, July 2006