Editing Bituminous coal (section)

==Origin==
[[File:Canal Run shadows (5179305812).jpg|thumb|[[Okefenokee Swamp]], a modern peat-forming swamp]]
Like other ranks of coal, bituminous coal forms from thick accumulations of dead plant material that are buried faster than they can decay. This usually takes place in [[peat bogs]], where falling plant debris is submerged in standing water. The stagnant water excludes oxygen, creates an acidic environment, and slows decay. The dead plant material is converted to [[peat]].<ref>{{cite web|url=http://www.fe.doe.gov/education/energylessons/coal/gen_howformed.html|title=How Coal Is Formed|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|df=dmy-all}}</ref>

Peat is mostly a mixture of [[cellulose]], [[hemicellulose]], and [[lignin]] that originally made up the woody tissue of the plants.<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/> This implies that chemical processes during coalification remove most of the oxygen and much of the hydrogen, leaving carbon, 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>

During coalification, the maturing coal increases in carbon content, decreases in hydrogen and volatiles, increases in its heating value, and becomes darker and more lustrous.{{sfn|Blatt|Middleton|Murray|1980|page=335}} Chemical changes include [[Dehydration reaction|dehydration]] (which removes oxygen and hydrogen as water), [[decarboxylation]] (which removes oxygen as [[carbon dioxide]]), and demethanation (which removes hydrogen as [[methane]]). By the time the coal reaches bituminous rank, most dehydration and decarboxylation has already taken place, and maturation of bituminous coal is characterized by demethanation.<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> During coalification at bituminous rank, coal approaches its maximum heating value and begins to lose most of its volatile content.<ref>{{cite web |title=Coal rank |url=http://www.uky.edu/KGS/coal/coal-rank.php |website=Kentucky Geological Survey: Earth Resources -- Our Common Wealth |publisher=University of Kentucky |access-date=28 November 2020}}</ref>

As [[carbonization]] proceeds, [[aliphatic compound]]s (carbon compounds characterized by chains of carbon atoms) are replaced by [[aromatic compound]]s (carbon compounds characterized by rings of carbon atoms) and aromatic rings begin to 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]]. This is accompanied by an increase in vitrinite reflectance, used to assess coal rank.<ref name=UKVitriniteReflectance/>

During coalification, the pressure of burial reduces the volume of the original peat by a factor of 30 as it is converted to coal.{{sfn|Boggs|2006|p=234}} However, the increase in rank of maturing coal mostly reflects the maximum temperature the coal reaches. Neither the maximum pressure, nor the nature of the original plant material, nor the length of burial is nearly as important.{{sfn|Blatt|Middleton|Murray|1980|page=335}} The temperature range for maturation of bituminous coal is from {{convert|85 to 235|C||sp=us}}.<ref>{{cite journal |last1=Barker |first1=Charles E. |last2=Goldstein |first2=Robert H. |title=Fluid-inclusion technique for determining maximum temperature in calcite and its comparison to the vitrinite reflectance geothermometer |journal=Geology |date=1 October 1990 |volume=18 |issue=10 |pages=1003–1006 |doi=10.1130/0091-7613(1990)018<1003:FITFDM>2.3.CO;2|bibcode=1990Geo....18.1003B }}</ref><ref>{{cite web |title=Burial temperatures from coal |url=http://www.uky.edu/KGS/coal/coal-burial-temperature.php |website=Kentucky Geological Survey: Earth Resources -- Our Common Wealth |publisher=University of Kentucky |access-date=28 November 2020}}</ref> The bitumen that characterizes bituminous coal forms under approximately the same conditions at which [[petroleum]] is formed in petroleum source rocks. Bituminization is accompanied by peak methane generation in medium to low volatile bituminous coal. This makes these bituminous coals "gassy" and precautions must be taken against methane explosions. If the coal reaches temperatures above about {{convert|235|C||sp=us}}, bitumen breaks down (''debituminization'') and the coal matures to anthracite.<ref name="UKBituminousCoal"/>