Editing Coal (section)

===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>