Editing Pyrolysis (section)

== General processes and mechanisms ==
[[File:Processes in the thermal degredation of organic matter.svg|thumb|upright=1.75|Processes in the thermal degradation of organic matter at atmospheric pressure.]]

Pyrolysis generally consists in heating the material above its [[decomposition temperature]], breaking chemical bonds in its molecules.  The fragments usually become smaller molecules, but may combine to produce residues with larger molecular mass, even [[Network covalent bonding|amorphous covalent solid]]s.{{citation needed|date=November 2022}}

In many settings, some amounts of oxygen, water, or other substances may be present, so that combustion, hydrolysis, or other chemical processes may occur besides pyrolysis proper.  Sometimes those chemicals are added intentionally, as in the burning of [[firewood]], in the traditional manufacture of [[charcoal]], and in the [[cracking (chemistry)|steam cracking]] of crude oil.{{citation needed|date=November 2022}}

Conversely, the starting material may be heated in a [[vacuum]] or in an [[inert atmosphere]] to avoid chemical side reactions (such as combustion or hydrolysis).  Pyrolysis in a vacuum also lowers the [[boiling point]] of the byproducts, improving their recovery.

When organic matter is heated at increasing temperatures in open containers, the following processes generally occur, in successive or overlapping stages:{{citation needed|date=November 2022}}

* Below about 100&nbsp;°C, volatiles, including some water, [[evaporate]]. Heat-sensitive substances, such as [[vitamin C]] and [[denaturation (biochemistry)|proteins]], may partially change or decompose already at this stage.
* At about 100&nbsp;°C or slightly higher, any remaining water that is merely absorbed in the material is driven off. This process consumes a lot of [[latent heat|energy]], so the temperature may stop rising until all water has evaporated. Water trapped in crystal structure of [[hydrate]]s may come off at somewhat higher temperatures.
* Some solid substances, like [[fat]]s, [[wax]]es, and [[sugar]]s, may melt and separate.
* Between 100 and 500&nbsp;°C, many common organic molecules break down. Most [[sugar]]s start decomposing at 160–180&nbsp;°C. [[Cellulose]], a major component of wood, [[paper]], and [[cotton]] fabrics, decomposes at about 350&nbsp;°C.<ref name="Zhou-2013" /> [[Lignin]], another major wood component, starts decomposing at about 350&nbsp;°C, but continues releasing volatile products up to 500&nbsp;°C.<ref name="Zhou-2013" /> The decomposition products usually include water, [[carbon monoxide]] {{chem2|CO}} and/or [[carbon dioxide]] {{chem2|CO2}}, as well as a large number of organic compounds.<ref name="Zhou-2017" /><ref name="Zhou-2015">{{Cite journal|last1=Zhou|first1=Hui|last2=Long|first2=YanQiu|last3=Meng|first3=AiHong|last4=Li|first4=QingHai|last5=Zhang|first5=YanGuo|date=April 2015|title=Thermogravimetric characteristics of typical municipal solid waste fractions during co-pyrolysis|journal=Waste Management|language=en|volume=38|pages=194–200|doi=10.1016/j.wasman.2014.09.027|pmid=25680236|bibcode=2015WaMan..38..194Z }}</ref>  Gases and volatile products leave the sample, and some of them may condense again as smoke. Generally, this process also absorbs energy. Some volatiles may ignite and burn, creating a visible [[flame]]. The non-volatile residues typically become richer in carbon and form large disordered molecules, with colors ranging between brown and black. At this point the matter is said to have been "[[Char (chemistry)|char]]red" or "carbonized".
* At 200–300&nbsp;°C, if oxygen has not been excluded, the carbonaceous residue may start to burn, in a highly [[exothermic reaction]], often with no or little visible flame.  Once carbon combustion starts, the temperature rises spontaneously, turning the residue into a glowing [[ember]] and releasing carbon dioxide and/or monoxide. At this stage, some of the [[nitrogen]] still remaining in the residue may be oxidized into [[nitrogen oxide]]s like {{chem2|link=nitrogen dioxide|NO2}} and {{chem2|link=dinitrogen trioxide|N2O3}}. [[Sulfur]] and other elements like [[chlorine]] and [[arsenic]] may be oxidized and volatilized at this stage.
* Once combustion of the carbonaceous residue is complete, a powdery or solid mineral residue ([[ash]]) is often left behind, consisting of inorganic oxidized materials of high melting point. Some of the ash may have left during combustion, entrained by the gases as [[fly ash]] or [[particulates|particulate emissions]].  Metals present in the original matter usually remain in the ash as [[oxide]]s or [[carbonate]]s, such as [[potash]]. [[Phosphorus]], from materials such as [[bone]], [[phospholipid]]s, and [[nucleic acid]]s, usually remains as [[phosphate]]s.