Complete Combustion: Higher vs Lower Air Temp

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Discussion Overview

The discussion revolves around the effects of air temperature on the combustion of fuel in air, specifically addressing whether higher temperature air provides more moles of air for complete combustion compared to lower temperature air. The scope includes theoretical considerations of combustion and the physical properties of air at different temperatures.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that higher temperature air has more moles of air for combustion than lower temperature air.
  • Another participant questions the assumptions regarding the amount of air, asking whether it is based on constant mass or constant volume.
  • It is noted that cold air is denser, containing more O2 molecules per volume, which means less air by volume is needed for combustion, assuming all else is equal.
  • Some participants clarify that higher temperature air is less dense, leading to less air and O2 available for combustion, contradicting the original claim.
  • A participant emphasizes that the density of air changes with temperature, affecting the amount of air available for combustion in a constant volume scenario.
  • A resource is shared that provides tools for calculating combustion parameters, including flame temperature and combustion products composition.

Areas of Agreement / Disagreement

Participants express disagreement regarding the relationship between air temperature and the amount of air available for combustion. While some acknowledge that higher temperature air is less dense and contains less O2, others initially interpret the question differently, leading to confusion about the implications for combustion.

Contextual Notes

The discussion highlights the dependence on conditions such as pressure and humidity, which may influence the combustion process and the density of air at different temperatures.

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If i am trying to burn a fuel in air am i right in saying if i have a higher temerature compared to a lower temperature of air, the higher temperature air has more moles of air to burn than the lower temperature air for complete combustion to occur?
 
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How much air? A constant mass? Constant volume?

At a given pressure, air is less dense at higher temperature (less mass per unit volume).
 
In general, yes. Cold air is denser (all else being equal), and therefore has more O2 molecules per volume, and so less air (by volume) is required to accomplish the same combustion. This of course does not hold true if the "all else" is not equal. Ari pressure has a significant effect and humidity can make a huge difference.
 
LURCH said:
In general, yes.

The OP asked whether having a higher temperature provided more air for combustion. That's clearly not true.
 
Ah yes; I read it as saying higher temperature requires more air, but upon re-read, I see that was almost the oposite of the poster's intent.

Engineerign, higher temperature air is thinner and contains less O2 than colder air (again, all else being equal), and therefore provides a better comustion reaction.
 
It's not that a higher temperature air contains less 02, it is simply less dense. Since you are dealing with a constant volume device, then the lower density means less air (coincidentally less O2).
 
Hi there:

Here is a free MS Excel spreadsheet on combustion -- http://engware.i-dentity.com/pf/combustion.xls.

For any fuel and oxidant combustion scenario whwre stoichiometry is => 1, one can calculate the combustion flame temperature, combustion products composition on both weight and mole basis, stoichiometry and stoichiometric ratio.

Thanks,

Gordan
 

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