Editing Coal (section)

==Formation of coal==
[[File:Struktura chemiczna węgla kamiennego.svg| thumb|Example chemical structure of coal]]

The conversion of dead vegetation into coal is called [[wikt:coalification|coalification]]. At various times in the geologic past, the Earth had dense forests<ref>{{cite web|title=How Coal Is Formed |url=http://www.fe.doe.gov/education/energylessons/coal/gen_howformed.html|url-status=live|archive-url=https://web.archive.org/web/20170118113211/http://www.fe.doe.gov/education/energylessons/coal/gen_howformed.html|archive-date=18 January 2017}}</ref> in low-lying areas. In these wetlands, the process of coalification began when dead plant matter was protected from [[oxidation]], usually by mud or acidic water, and was converted into [[peat]]. The resulting [[peat bog]]s, which trapped immense amounts of carbon, were eventually deeply buried by sediments. Then, over millions of years, the heat and pressure of deep burial caused the loss of water, methane and carbon dioxide and increased the proportion of carbon.<ref name="BGS">{{cite web | url=https://www.bgs.ac.uk/downloads/start.cfm?id=1404 | title=Coal  | publisher=[[British Geological Survey]] | date=March 2010}}</ref> The grade of coal produced depended on the maximum pressure and temperature reached, with [[lignite]] (also called "brown coal") produced under relatively mild conditions, and [[sub-bituminous coal]], [[bituminous coal]], or [[anthracite coal]] (also called "hard coal" or "black coal") produced in turn with increasing temperature and pressure.<ref name="EIA Coal Explained" /><ref>{{Cite book| url = https://books.google.com/books?id=_29tNNeQKeMC&pg=PA18| title = Paleobotany: The Biology and Evolution of Fossil Plants| isbn = 978-0-12-373972-8| author1 = Taylor, Thomas N| author2 = Taylor, Edith L| author3 = Krings, Michael| year = 2009| publisher = Academic Press| url-status = live| archive-url = https://web.archive.org/web/20160516231216/https://books.google.com/books?id=_29tNNeQKeMC&pg=PA18| archive-date = 16 May 2016}}</ref>

Of the factors involved in coalification, temperature is much more important than either pressure or time of burial.<ref>{{cite web |title=Heat, time, pressure, and coalification |url=http://www.uky.edu/KGS/coal/coal-heat-time-pressure.php |website=Kentucky Geological Survey |publisher=University of Kentucky |access-date=28 November 2020}}</ref> Subbituminous coal can form at temperatures as low as {{ convert|35 to 80|C||sp=us}} while anthracite requires a temperature of at least {{convert|180 to 245|C||sp=us}}.<ref>{{cite web |title=Burial temperatures from coal |url=http://www.uky.edu/KGS/coal/coal-burial-temperature.php |website=Kentucky Geological Survey |publisher=University of Kentucky |access-date=28 November 2020}}</ref>

Although coal is known from most geologic [[Period (geology)|periods]], 90% of all coal beds were deposited in the [[Carboniferous]] and [[Permian]] periods.<ref>{{cite book |last1=McGhee |first1=George R. |title=Carboniferous Giants and Mass Extinction: The Late Paleozoic Ice Age World |date=2018 |publisher=Columbia University Press |location=New York |isbn=9780231180979 |pages=98}}</ref> Paradoxically, this was during the [[Late Paleozoic icehouse]], a time of global [[Ice age|glaciation]]. However, the drop in global sea level accompanying the glaciation exposed [[continental shelves]] that had previously been submerged, and to these were added wide [[river delta]]s produced by increased [[erosion]] due to the drop in [[base level]]. These widespread areas of wetlands provided ideal conditions for coal formation.{{sfn|McGhee|2018|pp=88-92}} The rapid formation of coal ended with the [[coal gap]] in the [[Permian–Triassic extinction event]], where coal is rare.<ref name="Retallack1996">{{cite journal| last1=Retallack |first1=G. J.|last2= Veevers|first2=J. J.|last3= Morante|first3= R.|title=Global coal gap between Permian–Triassic extinctions and middle Triassic recovery of peat forming plants|journal=GSA Bulletin|volume=108|issue=2|pages=195–207|year=1996|doi = 10.1130/0016-7606(1996)108<0195:GCGBPT>2.3.CO;2|bibcode=1996GSAB..108..195R}}</ref>

Favorable geography alone does not explain the extensive Carboniferous coal beds.{{sfn|McGhee|2018|p=99}} Other factors contributing to rapid coal deposition were high [[oxygen]] levels, above 30%, that promoted intense [[wildfire]]s and formation of [[charcoal]] that was all but indigestible by decomposing organisms; high [[carbon dioxide]] levels that promoted plant growth; and the nature of Carboniferous forests, which included [[lycophyte]] trees whose [[determinate growth]] meant that carbon was not tied up in [[heartwood]] of living trees for long periods.{{sfn|McGhee|2018|pp=98-102}}

One theory suggested that about 360 million years ago, some plants evolved the ability to produce [[lignin]], a complex polymer that made their [[cellulose]] stems much harder and more woody. The ability to produce lignin led to the evolution of the first [[trees]]. But bacteria and fungi did not immediately evolve the ability to decompose lignin, so the wood did not fully decay but became buried under sediment, eventually turning into coal. About 300 million years ago, mushrooms and other fungi developed this ability, ending the main coal-formation period of earth's history.<ref>{{cite book |title=Unsettled: What Climate Science Tells Us, What It Doesn't, and Why It Matters |first=Steven E. |last=Koonin |location=Dallas |publisher=BenBella Books |date=2021 |isbn=9781953295248 |page=44}}</ref><ref>{{cite journal |last1=Floudas |first1=Dimitrios |last2=Binder |first2=Manfred |last3=Riley |first3=Robert |last4=Barry |first4=Kerrie |last5=Blanchette |first5=Robert A. |last6=Henrissat |first6=Bernard |last7=Martínez |first7=Angel T. |last8=Otillar |first8=Robert |last9=Spatafora |first9=Joseph W. |last10=Yadav |first10=Jagjit S. |last11=Aerts |first11=Andrea |last12=Benoit |first12=Isabelle |last13=Boyd |first13=Alex |last14=Carlson |first14=Alexis |last15=Copeland |first15=Alex |last16=Coutinho |first16=Pedro M. |last17=de Vries |first17=Ronald P. |last18=Ferreira |first18=Patricia |last19=Findley |first19=Keisha |last20=Foster |first20=Brian |last21=Gaskell |first21=Jill |last22=Glotzer |first22=Dylan |last23=Górecki |first23=Paweł |last24=Heitman |first24=Joseph |last25=Hesse |first25=Cedar |last26=Hori |first26=Chiaki |last27=Igarashi |first27=Kiyohiko |last28=Jurgens |first28=Joel A. |last29=Kallen |first29=Nathan |last30=Kersten |first30=Phil |last31=Kohler |first31=Annegret |last32=Kües |first32=Ursula |last33=Kumar |first33=T. K. Arun |last34=Kuo |first34=Alan |last35=LaButti |first35=Kurt |last36=Larrondo |first36=Luis F. |last37=Lindquist |first37=Erika |last38=Ling |first38=Albee |last39=Lombard |first39=Vincent |last40=Lucas |first40=Susan |last41=Lundell |first41=Taina |last42=Martin |first42=Rachael |last43=McLaughlin |first43=David J. |last44=Morgenstern |first44=Ingo |last45=Morin |first45=Emanuelle |last46=Murat |first46=Claude |last47=Nagy |first47=Laszlo G. |last48=Nolan |first48=Matt |last49=Ohm |first49=Robin A. |last50=Patyshakuliyeva |first50=Aleksandrina |last51=Rokas |first51=Antonis |last52=Ruiz-Dueñas |first52=Francisco J. |last53=Sabat |first53=Grzegorz |last54=Salamov |first54=Asaf |last55=Samejima |first55=Masahiro |last56=Schmutz |first56=Jeremy |last57=Slot |first57=Jason C. |last58=St. John |first58=Franz |last59=Stenlid |first59=Jan |last60=Sun |first60=Hui |last61=Sun |first61=Sheng |last62=Syed |first62=Khajamohiddin |last63=Tsang |first63=Adrian |last64=Wiebenga |first64=Ad |last65=Young |first65=Darcy |last66=Pisabarro |first66=Antonio |last67=Eastwood |first67=Daniel C. |last68=Martin |first68=Francis |last69=Cullen |first69=Dan |last70=Grigoriev |first70=Igor V. |last71=Hibbett |first71=David S. |title=The Paleozoic Origin of Enzymatic Lignin Decomposition Reconstructed from 31 Fungal Genomes |journal=Science |date=29 June 2012 |volume=336 |issue=6089 |pages=1715–1719 |doi=10.1126/science.1221748|pmid=22745431 |bibcode=2012Sci...336.1715F |hdl=10261/60626 |osti=1165864 |s2cid=37121590 |hdl-access=free }}</ref><ref>{{Cite web | url=https://www.scientificamerican.com/article/mushroom-evolution-breaks-down-lignin-slows-coal-formation |title = White Rot Fungi Slowed Coal Formation|website = [[Scientific American]]}}</ref> Although some authors pointed at some evidence of lignin degradation during the Carboniferous, and suggested that climatic and tectonic factors were a more plausible explanation,<ref>{{Cite journal|last1=Nelsen|first1=Matthew P.|last2=DiMichele|first2=William A.|last3=Peters|first3=Shanan E.|last4=Boyce|first4=C. Kevin|date=2016-01-19|title=Delayed fungal evolution did not cause the Paleozoic peak in coal production|journal=Proceedings of the National Academy of Sciences|language=en|volume=113|issue=9|pages=2442–2447|doi=10.1073/pnas.1517943113|pmid=26787881|pmc=4780611|bibcode=2016PNAS..113.2442N|issn=0027-8424|doi-access=free}}</ref> reconstruction of ancestral enzymes by phylogenetic analysis corroborated a hypothesis that lignin degrading enzymes appeared in fungi approximately 200 MYa.<ref>Ayuso-Fernandez I, Ruiz-Duenas FJ, Martinez AT: Evolutionary convergence in lignin-degrading enzymes. Proc Natl Acad Sci USA 2018, 115:6428-6433.</ref>

One likely tectonic factor was the [[Central Pangean Mountains]], an enormous range running along the equator that reached its greatest elevation near this time. Climate modeling suggests that the Central Pangean Mountains contributed to the deposition of vast quantities of coal in the late Carboniferous. The mountains created an area of year-round heavy precipitation, with no dry season typical of a [[monsoon]] climate. This is necessary for the preservation of peat in coal swamps.<ref>{{cite journal |last1=Otto-Bliesner |first1=Bette L. |author-link1=Bette Otto-Bliesner|title=Tropical mountains and coal formation: A climate model study of the Westphalian (306 MA) |journal=Geophysical Research Letters |date=15 September 1993 |volume=20 |issue=18 |pages=1947–1950 |doi=10.1029/93GL02235|bibcode=1993GeoRL..20.1947O }}</ref>

Coal is known from [[Precambrian]] strata, which predate land plants. This coal is presumed to have originated from residues of algae.<ref name="Tyler1957">{{Cite journal | doi = 10.1130/0016-7606(1957)68[1293:ACFPUH]2.0.CO;2 | bibcode = 1957GSAB...68.1293T | last1 = Tyler | first1 = S.A. | last2 = Barghoorn | first2 = E.S. | last3 = Barrett | first3 = L.P. | title = Anthracitic Coal from Precambrian Upper Huronian Black Shale of the Iron River District, Northern Michigan | journal = Geological Society of America Bulletin | volume = 68 | issue = 10 | page = 1293 | year = 1957 | issn = 0016-7606 }}</ref><ref name="Mancuso1981">{{Cite journal | doi = 10.2113/gsecongeo.76.4.951 | last1 = Mancuso | first1 = J.J. | last2 = Seavoy | first2 = R.E. | title = Precambrian coal or anthraxolite; a source for graphite in high-grade schists and gneisses | journal = Economic Geology | volume = 76 | issue = 4 | pages = 951–54 | year = 1981 | bibcode = 1981EcGeo..76..951M }}</ref>

Sometimes coal seams (also known as coal beds) are interbedded with other sediments in a [[cyclothem]]. Cyclothems are thought to have their origin in [[glacial cycle]]s that produced fluctuations in [[sea level]], which alternately exposed and then flooded large areas of continental shelf.<ref>Stanley, Steven M. ''Earth System History''. New York: W.H. Freeman and Company, 1999. {{ISBN|0-7167-2882-6}}  (p. 426)</ref>

===Chemistry of coalification===
The woody tissue of plants is composed mainly of cellulose, hemicellulose, and lignin. Modern peat is mostly lignin, with a content of cellulose and hemicellulose ranging from 5% to 40%. Various other organic compounds, such as waxes and nitrogen- and sulfur-containing compounds, are also present.<ref>{{cite book |last1=Andriesse |first1=J. P. |title=Nature and Management of Tropical Peat Soils |date=1988 |publisher=Food and Agriculture Organization of the United Nations |location=Rome |isbn=92-5-102657-2 |chapter=The Main Characteristics of Tropical Peats}}</ref> Lignin has a weight composition of about 54% carbon, 6% hydrogen, and 30% oxygen, while cellulose has a weight composition of about 44% carbon, 6% hydrogen, and 49% oxygen. Bituminous coal has a composition of about 84.4% carbon, 5.4% hydrogen, 6.7% oxygen, 1.7% nitrogen, and 1.8% sulfur, on a weight basis.<ref name=Perry>{{cite book |editor1-last=Robert Perry |editor2-last=Cecil Chilton |chapter=Chapter 9: Heat Generation, Transport, and Storage|first=William|last=Reid|title=Chemical Engineers' Handbook |date=1973 |edition=5}}</ref> The low oxygen content of coal shows that coalification removed most of the oxygen and much of the hydrogen a process called ''carbonization''.<ref>{{cite journal |last1=Ulbrich |first1=Markus |last2=Preßl |first2=Dieter |last3=Fendt |first3=Sebastian |last4=Gaderer |first4=Matthias |last5=Spliethoff |first5=Hartmut |title=Impact of HTC reaction conditions on the hydrochar properties and {{CO2}} gasification properties of spent grains |journal=Fuel Processing Technology |date=December 2017 |volume=167 |pages=663–669 |doi=10.1016/j.fuproc.2017.08.010}}</ref>

Carbonization proceeds primarily by [[Dehydration reaction|dehydration]], [[decarboxylation]], and demethanation. Dehydration removes water molecules from the maturing coal via reactions such as<ref name="hatcher-etal-1992">{{cite journal |last1=Hatcher |first1=Patrick G. |last2=Faulon |first2=Jean Loup |last3=Wenzel |first3=Kurt A. |last4=Cody |first4=George D. |title=A structural model for lignin-derived vitrinite from high-volatile bituminous coal (coalified wood) |journal=Energy & Fuels |date=November 1992 |volume=6 |issue=6 |pages=813–820 |doi=10.1021/ef00036a018}}</ref>

:2 R–OH → R–O–R + H<sub>2</sub>O

[[Decarboxylation]] removes carbon dioxide from the maturing coal:<ref name="hatcher-etal-1992"/>
:RCOOH → RH + CO<sub>2</sub>
while demethanation proceeds by reaction such as
:2 R-CH<sub>3</sub> → R-CH<sub>2</sub>-R + CH<sub>4</sub>
:R-CH<sub>2</sub>-CH<sub>2</sub>-CH<sub>2</sub>-R → R-CH=CH-R + CH<sub>4</sub>

In these formulas, R represents the remainder of a cellulose or lignin molecule to which the reacting groups are attached.

Dehydration and decarboxylation take place early in coalification, while demethanation begins only after the coal has already reached bituminous rank.<ref>{{cite web |title=Coal Types, Formation and Methods of Mining |url=http://epcamr.org/home/content/reference-materials/coal-types-formation-and-methods-of-mining/ |publisher=Eastern Pennsylvania Coalition for Abandoned Mine Reclamation |access-date=29 November 2020}}</ref> The effect of decarboxylation is to reduce the percentage of oxygen, while demethanation reduces the percentage of hydrogen. Dehydration does both, and (together with demethanation) reduces the saturation of the carbon backbone (increasing the number of double bonds between carbon).

As carbonization proceeds, [[aliphatic compound]]s convert to [[aromatic compound]]s. Similarly, aromatic rings fuse into [[polyaromatic]] compounds (linked rings of carbon atoms).<ref>{{cite journal |last1=Ibarra |first1=JoséV. |last2=Muñoz |first2=Edgar |last3=Moliner |first3=Rafael |title=FTIR study of the evolution of coal structure during the coalification process |journal=Organic Geochemistry |date=June 1996 |volume=24 |issue=6–7 |pages=725–735 |doi=10.1016/0146-6380(96)00063-0|bibcode=1996OrGeo..24..725I }}</ref> The structure increasingly resembles [[graphene]], the structural element of graphite.

Chemical changes are accompanied by physical changes, such as decrease in average pore size.<ref>{{cite journal |last1=Li |first1=Yong |last2=Zhang |first2=Cheng |last3=Tang |first3=Dazhen |last4=Gan |first4=Quan |last5=Niu |first5=Xinlei |last6=Wang |first6=Kai |last7=Shen |first7=Ruiyang |title=Coal pore size distributions controlled by the coalification process: An experimental study of coals from the Junggar, Ordos and Qinshui basins in China |journal=Fuel |date=October 2017 |volume=206 |pages=352–363 |doi=10.1016/j.fuel.2017.06.028|bibcode=2017Fuel..206..352L }}</ref>

===Macerals===
Macerals are coalified plant parts that retain the morphology and some properties of the original plant. In many coals, individual macerals can be identified visually. Some macerals include:<ref name= KO/>
* vitrinite, derived from woody parts
* lipinite, derived from spores and algae
* inertite, derived from woody parts that had been burnt in prehistoric times
* huminite, a precursor to vitrinite.
In coalification huminite is replaced by vitreous (shiny) ''vitrinite''.<ref>{{cite web |title=Sub-Bituminous Coal |url=http://www.uky.edu/KGS/coal/coal-sub.php |website=Kentucky Geological Survey |publisher=University of Kentucky |access-date=29 November 2020}}</ref> Maturation of bituminous coal is characterized by ''bitumenization'', in which part of the coal is converted to [[bitumen]], a hydrocarbon-rich gel.<ref>{{cite web |title=Bituminous Coal |url=http://www.uky.edu/KGS/coal/coal-bituminous.php |website=Kentucky Geological Survey |publisher=University of Kentucky |access-date=29 November 2020}}</ref> Maturation to anthracite is characterized by ''debitumenization'' (from demethanation) and the increasing tendency of the anthracite to break with a [[conchoidal fracture]], similar to the way thick glass breaks.<ref>{{cite web |title=Anthracitic Coal |url=http://www.uky.edu/KGS/coal/coal-anthracite.php |website=Kentucky Geological Survey |publisher=University of Kentucky |access-date=29 November 2020}}</ref>

{{anchor|Ranks}}

===Types===
[[File:Sydney Mines Point Aconi Seam 038.JPG|thumb|Coastal exposure of the Point Aconi Seam in [[Nova Scotia]]]]
[[File:Coal Rank USGS.png|thumb|upright=1.35|Coal ranking system used by the [[United States Geological Survey]]]]

As geological processes apply [[pressure]] to dead [[biotic material]] over time, under suitable conditions, its [[metamorphic grade]] or rank increases successively into:
* [[Peat]], a precursor of coal
* [[Lignite]], or brown coal, the lowest rank of coal, most harmful to health when burned,<ref name=Heal/> used almost exclusively as fuel for electric power generation
* [[Sub-bituminous coal]], whose properties range between those of lignite and those of bituminous coal, is used primarily as fuel for steam-electric power generation.
* [[Bituminous coal]], a dense sedimentary rock, usually black, but sometimes dark brown, often with well-defined bands of bright and dull material. It is used primarily as fuel in steam-electric power generation and to make [[coke (fuel)|coke]]. Known as steam coal in the UK, and historically used to raise steam in steam locomotives and ships
* [[Anthracite coal]], the highest rank of coal, is a harder, glossy black coal used primarily for residential and commercial [[space heating]].
* [[Graphite]], a difficult to ignite coal which is mostly used in pencils, or powdered for [[lubrication]].
* [[Cannel coal]] (sometimes called "candle coal"), a variety of fine-grained, high-rank coal with significant hydrogen content, which consists primarily of [[liptinite]]. It is related to boghead coal.

There are several international standards for coal.<ref>{{cite web |title=Standards catalogue 73.040 – Coals |url=https://www.iso.org/ics/73.040/x/ |publisher=[[ISO]]}}</ref> The classification of coal is generally based on the content of [[coal volatiles|volatiles]]. However the most important distinction is between thermal coal (also known as steam coal), which is burnt to generate electricity via steam; and [[metallurgical coal]] (also known as coking coal), which is burnt at high temperature to make [[steel]].

[[Hilt's law]] is a geological observation that (within a small area) the deeper the coal is found, the higher its rank (or grade). It applies if the thermal gradient is entirely vertical; however, [[metamorphism]] may cause lateral changes of rank, irrespective of depth. For example, some of the coal seams of the [[Madrid, New Mexico]] coal field were partially converted to anthracite by [[contact metamorphism]] from an igneous [[Sill (geology)|sill]] while the remainder of the seams remained as bituminous coal.<ref>{{cite journal |last1=Darton |first1=Horatio Nelson |title=Guidebook of the Western United States: Part C - The Santa Fe Route, with a side trip to Grand Canyon of the Colorado |journal=U.S. Geological Survey Bulletin |date=1916 |volume=613 |page=81 |doi=10.3133/b613|bibcode=1916usgs.rept....2D |hdl=2027/hvd.32044055492656 |hdl-access=free }}</ref>