Editing Combustion (section)

==Chemical equations==

===Stoichiometric combustion of a hydrocarbon in oxygen===

Generally, the [[chemical equation]] for [[Stoichiometry|stoichiometric]] combustion of a [[hydrocarbon]] in oxygen is:
:<math chem="">\ce{C}_x \ce{H}_y + \left(x+{y\over 4}\right)\ce{O2->} x\ce{CO2} + {y\over 2} \ce{H2O}</math>

For example, the stoichiometric combustion of [[methane]] in oxygen is:
:<chem>\underset{methane}{CH4} + 2O2 -> CO2 + 2H2O</chem>

===Stoichiometric combustion of a hydrocarbon in air===

If the stoichiometric combustion takes place using air as the oxygen source, the [[nitrogen]] present in the air ([[Atmosphere of Earth]]) can be added to the equation (although it does not react) to show the stoichiometric composition of the fuel in air and the composition of the resultant flue gas. Treating all non-oxygen components in air as nitrogen gives a 'nitrogen' to oxygen ratio of 3.77, i.e. (100% &minus; {{chem|O|2}}%) / {{chem|O|2}}% where {{chem|O|2}}% is 20.95% vol:
:<math chem="">\ce{C}_x \ce{H}_y + z\ce{O2} + 3.77z\ce{N2 ->} x\ce{CO2} + {y\over 2} \ce{H2O} + 3.77z\ce{N2}</math>
where <math>z = x + {y\over 4}</math>.

For example, the stoichiometric combustion of methane in air is:
:<math chem="">\ce{\underset{methane}{CH4} + 2O2} + 7.54\ce{N2-> CO2 + 2H2O} + 7.54\ce{N2}</math>

The stoichiometric composition of methane in air is 1 / (1 + 2 + 7.54) = 9.49% vol.

The stoichiometric combustion reaction for C{{sub|α}}H{{sub|β}}O{{sub|γ}} in air:
:<math chem>\ce{C_\mathit{\alpha}H_\mathit{\beta}O_\mathit{\gamma}} + \left ( \alpha + \frac{\beta}{4} -\frac{\gamma}{2} \right ) \left ( \ce{O_2} + 3.77 \ce{N_2} \right ) \longrightarrow \alpha \ce{CO_2} + \frac{\beta}{2} \ce{H_2O} + 3.77 \left ( \alpha + \frac{\beta}{4} -\frac{\gamma}{2} \right ) \ce{N_2}</math>

The stoichiometric combustion reaction for C{{sub|α}}H{{sub|β}}O{{sub|γ}}S{{sub|δ}}:

:<math chem>\ce{C_\mathit{\alpha}H_\mathit{\beta}O_\mathit{\gamma}S_\mathit{\delta}} + \left ( \alpha + \frac{\beta}{4} -\frac{\gamma}{2} + \delta \right ) \left ( \ce{O_2} + 3.77 \ce{N_2} \right ) \longrightarrow \alpha \ce{CO_2} + \frac{\beta}{2} \ce{H_2O} + \delta \ce{SO_2} + 3.77 \left ( \alpha + \frac{\beta}{4} -\frac{\gamma}{2} + \delta \right ) \ce{N_2}</math>

The stoichiometric combustion reaction for C{{sub|α}}H{{sub|β}}O{{sub|γ}}N{{sub|δ}}S{{sub|ε}}:

:<math chem>\ce{C_\mathit{\alpha}H_\mathit{\beta}O_\mathit{\gamma}N_\mathit{\delta}S_\mathit{\epsilon}} + \left ( \alpha + \frac{\beta}{4} -\frac{\gamma}{2} + \epsilon \right ) \left ( \ce{O_2} + 3.77 \ce{N_2} \right ) \longrightarrow \alpha \ce{CO_2} + \frac{\beta}{2} \ce{H_2O} + \epsilon \ce{SO_2} + \left ( 3.77 \left ( \alpha + \frac{\beta}{4} -\frac{\gamma}{2} + \epsilon \right ) + \frac{\delta}{2} \right ) \ce{N_2}</math>

The stoichiometric combustion reaction for C{{sub|α}}H{{sub|β}}O{{sub|γ}}F{{sub|δ}}:

:<math chem>\ce{C_\mathit{\alpha}H_\mathit{\beta}O_\mathit{\gamma}F_\mathit{\delta}} + \left ( \alpha + \frac{\beta-\delta}{4} -\frac{\gamma}{2} \right ) \left ( \ce{O_2} + 3.77 \ce{N_2} \right ) \longrightarrow \alpha \ce{CO_2} + \frac{\beta-\delta}{2} \ce{H_2O} + \delta \ce{HF} + 3.77 \left ( \alpha + \frac{\beta-\delta}{4} -\frac{\gamma}{2} \right ) \ce{N_2}</math>

===Trace combustion products===

Various other substances begin to appear in significant amounts in combustion products when the [[Adiabatic flame temperature|flame temperature]] is above about {{val|1600|ul=K}}. When excess air is used, nitrogen may oxidize to {{chem|link=nitric oxide|NO}} and, to a much lesser extent, to {{chem|link=nitrogen dioxide|NO|2}}. {{chem|link=carbon monoxide|CO}} forms by [[disproportionation]] of {{CO2}}, and {{chem|link=hydrogen|H|2}} and {{chem|link=hydroxyl radical|OH}} form by disproportionation of {{H2O}}.

For example, when {{val|1|ul=mol}} of [[propane]] is burned with {{val|28.6|ul=mol}} of air (120% of the stoichiometric amount), the combustion products contain 3.3% {{chem|O|2}}. At {{val|1400|ul=K}}, the [[Chemical equilibrium|equilibrium]] combustion products contain 0.03% {{chem|NO}} and 0.002% {{chem|OH}}. At {{val|1800|ul=K}}, the combustion products contain 0.17% {{chem|NO}}, 0.05% {{chem|OH}}, 0.01% {{chem|CO}}, and 0.004% {{chem|H|2}}.<ref name="EquiWeb">{{cite web |url=http://www.crct.polymtl.ca/equiweb.php |title=Equilib-Web |date=8 March 2022 |first1=Christopher W. |last1=Bale |first2=Eve |last2=Bélisle |access-date=15 May 2023 |publisher=Centre for Research in Computational Thermochemistry, Polytechnique Montreal}}</ref>

[[Diesel engines]] are run with an excess of oxygen to combust small [[Particle|particles]] that tend to form with only a stoichiometric amount of oxygen, necessarily producing [[NOx|nitrogen oxide]] emissions. Both the United States and European Union [[Emission standard|enforce limits]] to vehicle nitrogen oxide emissions, which necessitate the use of special [[catalytic converter]]s or treatment of the exhaust with [[urea]] (see [[Diesel exhaust fluid]]).

===Incomplete combustion of a hydrocarbon in oxygen===

The incomplete (partial) combustion of a [[hydrocarbon]] with oxygen produces a gas mixture containing mainly {{chem|CO|2}}, {{chem|CO}}, {{H2O}}, and {{chem|H|2}}. Such gas mixtures are commonly prepared for use as protective atmospheres for the [[Heat treating|heat-treatment]] of metals and for [[Carburizing|gas carburizing]].<ref>ASM Committee on Furnace Atmospheres, ''Furnace atmospheres and carbon control'', Metals Park, OH [1964].</ref> The general reaction equation for incomplete combustion of one [[Mole (unit)|mole]] of a hydrocarbon in oxygen is:

: <chem>\underset{fuel}{C_\mathit{x} H_\mathit{y}} + \underset{oxygen}{\mathit{z} O2} -> \underset{carbon \ dioxide}{\mathit{a}CO2} + \underset{carbon\ monoxide}{\mathit{b}CO} + \underset{water}{\mathit{c}H2O} + \underset{hydrogen}{\mathit{d}H2}</chem>

When ''z'' falls below roughly 50% of the stoichiometric value, [[Methane|{{chem|CH|4}}]] can become an important combustion product; when ''z'' falls below roughly 35% of the stoichiometric value, elemental [[carbon]] may become stable.

The products of incomplete combustion can be calculated with the aid of a [[material balance]], together with the assumption that the combustion products reach [[Chemical equilibrium|equilibrium]].<ref>{{cite journal | title = Exothermic atmospheres | journal = Industrial Heating | page = 22 | date = June 2013 | url = http://www.industrialheating.com/articles/91142-exothermic-atmospheres | access-date = 5 July 2013 | archive-date = 9 November 2023 | archive-url = https://web.archive.org/web/20231109214347/https://www.industrialheating.com/articles/91142-exothermic-atmospheres | url-status = dead }}</ref><ref name="ExoCalc">[http://www.industrialheating.com/ExoCalc] ExoCalc</ref> For example, in the combustion of one [[Mole (unit)|mole]] of propane ({{chem|C|3|H|8}}) with four moles of {{chem|O|2}}, seven moles of combustion gas are formed, and ''z'' is 80% of the stoichiometric value. The three elemental balance equations are:
* Carbon: <math>a + b = 3</math>
* Hydrogen: <math>2c + 2d = 8</math>
* Oxygen: <math>2a + b + c = 8</math>

These three equations are insufficient in themselves to calculate the combustion gas composition.
However, at the equilibrium position, the [[water-gas shift reaction]] gives another equation:

: <chem>CO + H2O -> CO2 + H2</chem>; <math>K_{eq} = \frac{a \times d}{b \times c}</math>

For example, at {{val|1200|ul=K}} the value of ''K{{sub|eq}}'' is 0.728.<ref name="ReacWeb">{{cite web|url=http://www.crct.polymtl.ca/reacweb.htm |title=Reaction-Web |publisher=Crct.polymtl.ca |access-date=2018-07-12}}</ref> Solving, the combustion gas consists of 42.4% {{H2O}}, 29.0% {{CO2}}, 14.7% {{chem|H|2}}, and 13.9% {{chem|CO}}. Carbon becomes a stable phase at {{val|1200|ul=K}} and {{val|1|ul=atm}} pressure when z is less than 30% of the stoichiometric value, at which point the combustion products contain more than 98% {{chem|H|2}} and {{chem|CO}} and about 0.5% {{chem|CH|4}}.

Substances or materials which undergo combustion are called [[fuel]]s. The most common examples are natural gas, propane, [[kerosene]], [[Diesel fuel|diesel]], petrol, charcoal, coal, wood, etc.

===Liquid fuels===

Combustion of a [[liquid fuel]] in an oxidizing atmosphere actually happens in the gas phase. It is the vapor that burns, not the liquid. Therefore, a liquid will normally catch fire only above a certain temperature: its [[flash point]]. The flash point of liquid fuel is the lowest temperature at which it can form an ignitable mix with air. It is the minimum temperature at which there is enough evaporated fuel in the air to start combustion.

=== Gaseous fuels ===
Combustion of gaseous fuels may occur through one of four distinctive types of burning: [[diffusion flame]], [[premixed flame]], [[autoignitive reaction front]], or as a [[detonation]].<ref name=":0">{{Cite journal|last=Bradley|first=D|date=2009-06-25|title=Combustion and the design of future engine fuels|journal=Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science|language=en|volume=223|issue=12|pages=2751–2765|doi=10.1243/09544062jmes1519|s2cid=97218733}}</ref> The type of burning that actually occurs depends on the degree to which the [[fuel]] and [[oxidizer]] are mixed prior to heating: for example, a diffusion flame is formed if the fuel and oxidizer are separated initially, whereas a premixed flame is formed otherwise. Similarly, the type of burning also depends on the pressure: a detonation, for example, is an autoignitive reaction front coupled to a strong shock wave giving it its characteristic high-pressure peak and high [[detonation velocity]].<ref name=":0" />

===Solid fuels===
[[File:disfig1.svg|thumb|upright=1.5|A general scheme of [[polymer]] combustion]]

The act of combustion consists of three relatively distinct but overlapping phases:
* '''Preheating phase''', when the unburned [[fuel]] is heated up to its flash point and then [[fire point]]. Flammable gases start being evolved in a process similar to [[dry distillation]].
* '''Distillation phase''' or '''gaseous phase''', when the mix of evolved flammable gases with oxygen is ignited. Energy is produced in the form of heat and light. [[Flame]]s are often visible. Heat transfer from the combustion to the solid maintains the evolution of flammable vapours.
* '''Charcoal phase''' or '''solid phase''', when the output of flammable gases from the material is too low for the persistent presence of flame and the [[charring|charred]] fuel does not burn rapidly and just glows and later only [[Smouldering|smoulders]].