Minimum Oxygen for Combustion
Date: Fall 2009
What level of oxygen needs to be present in the air in
order for combustion to occur?
This is a very complicated question. Three factors are required for
combustion: fuel, oxidant, ignition source. In some cases auto-ignition
occurs and no ignition source is required. First, it depends upon what the
fuel is. The configuration and ignition temperature are important. The flow
of oxidant is important and non-linear. Too fast and the fuel cannot get hot
enough, too slow and not enough oxidant can reach the fuel. The definitive
text on ignition is "Ignition Handbook" by Vytenis Babrauskas. Starting a
fire is a very complicated subject.
The answer to this question relies upon which fuel is being
combusted. Some fuels, especially those with high carbon contents,
require much more oxygen for combustion. Also, the amount of oxygen
will affect what the products of combustion are, hydrocarbons give
CO2 and water under complete combustion conditions and CO or soot
(Carbon) under conditions with less oxygen. Some fuels contain
oxygen atoms in the their makeup, eg methanol CH3OH, and these
require less oxygen from the air to completely combust, hence their
use in applications with a high fuel/air ratio eg top fuel drag racing.
Finally, the percentage of oxygen will affect the conditions that
are required to get full combustion, lower proportions of O2 will
require more harsh conditions. It is worth bearing in mind, though,
that there is a minimum amount of O2 that is required for complete
combustion under any conditions: the stoichiometric mixture where
there is exactly the right ratio of fuel to air so that all the
oxygen and fuel is used in the reaction. It is possible to determine
this from the balanced equation of combustion for the fuel, but it
still depends on what the fuel is.
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Update: June 2012