Editing Combustion (section)

===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.