Propane burns invisibly

DaveC426913
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Main Question or Discussion Point

Here in Toronto we were rocked by a giant propane blast. It was so big that some thought it was nuclear dawn; it had both the characteristic mushroom cloud and a visible, expanding shockwave. Seeing the shockwave was awesome.

Despite being located in a residential area (:bugeye:), the only known casualties are a missing employee and a firefighter.

Anyway. That has nothing to do with my question.

The explosion was a giant yellow-orange fireball, just like gasoline or any other. Yellow-orange fire and black smoke is an indication of incomplete combustion. Natrually, this is because the propane was being ignited as it was mixing with air.


I was wondering what might happen in a hypothetical situation, if the air were first well-mixed with a large amount of propane. Would the fireball be blue? Invisible?
 

Answers and Replies

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I would say it should be blue because the air well mized with fuel would cause a complete combustion.
 
russ_watters
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I have propane for my house. The flame is blue.
 
DaveC426913
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I have propane for my house. The flame is blue.
Yes, I know it is on a small scale. On the other hand, isn't it set burning as it's mixed?

I guess I've gotten two things confused here. I was thinking that well-mixed propane burned invisibly. Now I'm not so sure.
 
turbo
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Well-mixed propane burns with a blue flame. If you have to tune up a grill or a gas stove (we have both) there is generally a damper on each burner's gas feed tube that is open to the air. This allows the gas to entrain air as it flows to the burner head Close the damper off and the flame will be yellow. Open the damper gradually until you get a clean-burning blue flame. That's the setting you want.
 
Astronuc
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We used to get nearly invisible flames with bunsen burners, but that's primarily NG (methane).

I think propane and gas flames have near stoichiometric mix and are blue, which indicates a hot flame. I used to adjust acetylene flames to get a bright blue 'feather' in the middle of a dark blue flame, which was almost invisible.

I think thermobaric bombs release a clear cloud fuel (butane), which when ignited, burns bright orange or yellow. LPG is usually a blend of propane and butane.
 
russ_watters
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Yes, I know it is on a small scale. On the other hand, isn't it set burning as it's mixed?
No, just like with a bunsen burner, a stove or grill mixes air with the gas via induction before it is burned.
 
chemisttree
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I don't think there would be a fireball... more like a firebang with a bight light (probably bluish white).
 
burning propane is not invisible to humans. The color you see is a black body radiation from the hot molecules. If it's blue, you're seeing the radiation from the CO2/H20 molecules that are very hot. If it's yellow or orange you're seeing glowing carbon atoms, which indicates a rich mixtures (incomplete combustion due to lack of oxygen).

A stoiciometric or leaner mixture will contain only blue flames.

----

Earl Stirling
Swiss Army Knife
http://exploratorium.edu
 
137
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does not the color of the flame directly relate to the air to fuel ratio? like when diesel trucks billow black smoke when they floor the accelerator?
 
The color only changes when there's not enough oxygen (burning rich). Those diesel particles are soot, which are clumps of carbon molecules... (and you're talking about smoke, not flame). The flames in those conditions would be orange.
 
137
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sorry, should have specified. but does that not also lend to the same idea?
 
nope
 

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