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Name: Jay
Status: student
Grade: 9-12
Location: FL
Country: USA
Date: Fall 2009


Question:
What level of oxygen needs to be present in the air in order for combustion to occur?



Replies:
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.

Vince Calder


Jay,

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.

Best wishes,

Tom Collins



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