Editing Combustion (section)

==Types==

===Complete and incomplete===
{{see also|pyrolysis}}

====Complete====
[[File:Methane-combustion.svg|thumb|The combustion of [[methane]], a [[hydrocarbon]]]]
In complete combustion, the reactant burns in oxygen and produces a limited number of products. When a [[hydrocarbon]] burns in oxygen, the reaction will primarily yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides. Carbon will yield [[carbon dioxide]], sulfur will yield [[sulfur dioxide]], and iron will yield [[iron(III) oxide]]. Nitrogen is not considered to be a combustible substance when oxygen is the [[Oxidizing agent|oxidant]]. Still, small amounts of various nitrogen oxides (commonly designated [[NOx|{{chem|NO|''x''}}]] species) form when the air is the oxidative.

Combustion is not necessarily favorable to the maximum degree of oxidation, and it can be temperature-dependent. For example, [[sulfur trioxide]] is not produced quantitatively by the combustion of sulfur. {{NOx}} species appear in significant amounts above about {{convert|2800|F|C}}, and more is produced at higher temperatures. The amount of {{NOx}} is also a function of oxygen excess.<ref name="NOx formation">[http://www.alentecinc.com/papers/NOx/The%20formation%20of%20NOx_files/The%20formation%20of%20NOx.htm The formation of NOx]. Alentecinc.com. Retrieved on 2010-09-28.</ref>

In most industrial applications and in [[fire]]s, [[air]] is the source of oxygen ({{chem|O|2}}). In the air, each mole of oxygen is mixed with approximately {{val|3.71|ul=mol}} of nitrogen. Nitrogen does not take part in combustion, but at high temperatures, some nitrogen will be converted to [[NOx#Thermal|{{chem|NO|''x''}}]] (mostly [[Nitric oxide|{{chem|NO}}]], with much smaller amounts of [[Nitrogen dioxide|{{chem|NO|2}}]]). On the other hand, when there is insufficient oxygen to combust the fuel completely, some fuel carbon is converted to [[carbon monoxide]], and some of the hydrogens remain unreacted. A complete set of equations for the combustion of a hydrocarbon in the air, therefore, requires an additional calculation for the distribution of oxygen between the carbon and hydrogen in the fuel.

The amount of air required for complete combustion is known as the "theoretical air" or "stoichiometric air".<ref>{{cite report |url=https://books.google.com/books?id=jqkXAAAAYAAJ&pg=RA1-PA26R |title=Central Boiler Plants |year=1989 |page=Glossary 26 |publisher=US Department of the Army |id=TM 5-650}}</ref> The amount of air above this value actually needed for optimal combustion is known as the "excess air", and can vary from 5% for a natural gas boiler, to 40% for [[anthracite]] coal, to 300% for a [[gas turbine]].<ref>{{cite web |title=Engineering Toolbox: Optimal Combustion Processes - Fuel vs. Excess Air |url=https://www.engineeringtoolbox.com/fuels-combustion-efficiency-d_167.html |access-date=15 May 2023 |date=2003 }}</ref>

====Incomplete====
{{see also|Charring}}
Incomplete combustion will occur when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. It also happens when the combustion is quenched by a heat sink, such as a solid surface or flame trap. As is the case with complete combustion, water is produced by incomplete combustion; however, [[carbon]] and [[carbon monoxide]] are produced instead of carbon dioxide.

For most fuels, such as diesel oil, coal, or wood, [[pyrolysis]] occurs before combustion. In incomplete combustion, products of pyrolysis remain unburnt and contaminate the smoke with noxious particulate matter and gases. Partially oxidized compounds are also a concern; partial oxidation of ethanol can produce harmful [[acetaldehyde]], and carbon can produce toxic carbon monoxide.

The designs of combustion devices can improve the quality of combustion, such as [[Oil burner|burners]] and [[internal combustion engines]]. Further improvements are achievable by [[catalytic]] after-burning devices (such as [[catalytic converter]]s) or by the simple partial return of the [[exhaust gas]]es into the combustion process. Such devices are required by [[environmental legislation]] for cars in most countries. They may be necessary to enable large combustion devices, such as [[thermal power station]]s, to reach legal [[emission standards]].

The degree of combustion can be measured and analyzed with test equipment. [[HVAC]] contractors, [[firefighters]] and [[engineers]] use combustion analyzers to test the [[Fuel efficiency|efficiency]] of a burner during the combustion process. Also, the efficiency of an internal combustion engine can be measured in this way, and some U.S. states and local municipalities use combustion analysis to define and rate the efficiency of vehicles on the road today.

Carbon monoxide is one of the products from [[incomplete combustion]].<ref>{{cite web|title=Incomplete combustion process|url=https://www.greenfacts.org/glossary/ghi/incomplete-combustion-processes.htm}}</ref> The formation of carbon monoxide produces less heat than formation of carbon dioxide so complete combustion is greatly preferred especially as carbon monoxide is a poisonous gas. When breathed, carbon monoxide takes the place of oxygen and combines with some of the hemoglobin in the blood, rendering it unable to transport oxygen.<ref>{{cite web|title=Burning showing incomplete combustion|url=https://www.sciencelearn.org.nz/resources/747-what-is-fire}}</ref>

====Problems associated with incomplete combustion====

=====Environmental problems=====
These oxides combine with [[water]] and [[oxygen]] in the atmosphere, creating [[nitric acid]] and [[sulfuric acids]], which return to Earth's surface as acid deposition, or "acid rain." Acid deposition harms aquatic organisms and kills trees. Due to its formation of certain nutrients that are less available to plants such as calcium and phosphorus, it reduces the productivity of the ecosystem and farms. An additional problem associated with [[nitrogen oxides]] is that they, along with [[hydrocarbon]] pollutants, contribute to the formation of [[ground level ozone]], a major component of smog.<ref name="education.seattlepi.com">{{cite web|title=Environmental Problems associated with incomplete combustion |work= Seattle PI - Education|date= 25 February 2014|url= http://education.seattlepi.com/environmental-problems-associated-combustion-hydrocarbons-5621.html|last1= Reeder|first1= Elizabeth}}</ref>

=====Human health problems=====
Breathing [[carbon monoxide]] causes headache, dizziness, vomiting, and nausea. If carbon monoxide levels are high enough, humans become unconscious or die. Exposure to moderate and high levels of carbon monoxide over long periods is positively correlated with the risk of heart disease. People who survive severe [[carbon monoxide poisoning]] may suffer long-term health problems.<ref>{{cite web|title=Carbon Monoxide Poisoning|date=8 December 2020|url=https://ephtracking.cdc.gov/showCoRisk.action}}</ref> Carbon monoxide from the air is absorbed in the lungs which then binds with [[hemoglobin]] in human's red blood cells. This reduces the capacity of red blood cells that carry oxygen throughout the body.

===Smoldering===
[[Smoldering]] is the slow, low-temperature, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel. It is a typically incomplete combustion reaction. Solid materials that can sustain a smoldering reaction include coal, [[cellulose]], [[wood]], [[cotton]], [[tobacco]], [[peat]], [[Plant litter|duff]], [[humus]], synthetic foams, charring [[polymers]] (including [[polyurethane foam]]) and [[dust]]. Common examples of smoldering phenomena are the initiation of residential fires on [[upholstered furniture]] by weak heat sources (e.g., a cigarette, a short-circuited wire) and the persistent [[combustion of biomass]] behind the flaming fronts of [[wildfire]]s.

===Spontaneous===
[[Spontaneous combustion]] is a type of combustion that occurs by self-heating (increase in temperature due to [[exothermic]] internal reactions), followed by thermal runaway (self-heating which rapidly accelerates to high temperatures) and finally, ignition.
For example, phosphorus self-ignites at room temperature without the application of heat. Organic materials undergoing bacterial [[composting]] can generate enough heat to reach the point of combustion.<ref>{{cite web |url=http://www.soilandmulchproducernews.com/index.php/frontpage-articles-hidden/160-a-perfect-storm-mulch-fire-dynamics-and-prevention |title=A Perfect Storm: Mulch Fire Dynamics and Prevention |publisher=Soilandmulchproducernews.com |access-date=2018-07-12 |archive-date=2018-07-01 |archive-url=https://web.archive.org/web/20180701011314/http://www.soilandmulchproducernews.com/index.php/frontpage-articles-hidden/160-a-perfect-storm-mulch-fire-dynamics-and-prevention |url-status=dead }}</ref>

===Turbulent===
Combustion resulting in a turbulent flame is the most used for industrial applications (e.g. [[gas turbine]]s, [[gasoline engine]]s, etc.) because the turbulence helps the mixing process between the fuel and [[oxidizer]].

===Micro-gravity===
[[Image:Microgravity Burning.jpg|thumb|Colourized gray-scale composite image of the individual frames from a video of a backlit fuel droplet burning in microgravity]]

The term 'micro' gravity refers to a gravitational state that is 'low' (i.e., 'micro' in the sense of 'small' and not necessarily a millionth of Earth's normal gravity) such that the influence of [[buoyancy]] on physical processes may be considered small relative to other flow processes that would be present at normal gravity. In such an environment, the thermal and [[flow transport dynamics]] can behave quite differently than in normal gravity conditions (e.g., a [[candle]]'s flame takes the shape of a sphere.<ref>[https://web.archive.org/web/20011118103426/http://spaceflight.nasa.gov/history/shuttle-mir/science/mg/nm21460011.htm Shuttle-Mir History/Science/Microgravity/Candle Flame in Microgravity (CFM) – MGBX].  Spaceflight.nasa.gov (1999-07-16). Retrieved on 2010-09-28.</ref>). Microgravity combustion research contributes to the understanding of a wide variety of aspects that are relevant to both the environment of a spacecraft (e.g., fire dynamics relevant to crew safety on the [[International Space Station]]) and terrestrial (Earth-based) conditions (e.g., droplet combustion dynamics to assist developing new fuel blends for improved combustion, [[materials fabrication processes]], [[Thermal management (electronics)|thermal management of electronic systems]], multiphase flow boiling dynamics, and many others).

===Micro-combustion===
Combustion processes that happen in very small volumes are considered [[micro-combustion]]. The high surface-to-volume ratio increases specific heat loss. [[Quenching]] distance plays a vital role in stabilizing the flame in such [[combustion chamber]]s.