Combustion Problem

Mike T., Swampeast MO

http://www.process-heating.com/CDA/ArticleInformation/Energy_Notes_Item/0,3271,82221,00.html

Please tell me if I have this right...

If you use natural gas for welding (using pure oxygen instead of "air") the flame temperature is around 4,600°F.

But, this still isn't the true temperature of combustion as at that temperature there was a lot of disassociation (the water changing back to hydrogen & oxygen and the carbon dioxide changing back to carbon monoxide and oxygen).

The flame temp in this case is still quite high though because there was very little else to raise to this temperature (eg. the non-combustibles in the natural gas).

Now if we use room air the flame temperature in real-life is reduced to around 3,200°F. There is still some disassociation, but most of the reduction comes from having to raise the temperature of all the non-combustibles in the room air.

Now the questions...

Carbon dioxide and water are the products of the combustion. Are they produced at a still higher temperature that is "drug down" so rapidly that it can't be measured?

Is the yellow portion of a "normal" flame providing a cooler "insulating" layer (presumably with a much higher ratio of non-combustibles) to reduce disassociation?

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Is that perforated mesh in the burner of the Vitodens trying to "grab" as much of the true combustion temperature as possible, transferring this to the HX nearly instantly via radiation, and essentially acting as the "yellow portion" of the flame? And while this is occuring are the non-combustibles essentially "exploding" through the holes in the mesh without picking up as much heat as they "should" because the heat "wants" to be relieved in the most efficient means possible?

jerry scharf

ignition temperature

Mike,

I think this is the right chemical explanation, but there may be something that I'm missing.

If we say we're working with methane and air than you have an ignition temperature of around 1100F. If we say that most all of the combustion is CH4 + O2 -> H20 and C02, then you heat the combustion materials up to the ignition point. At that point, the chemical reaction occurs and the combustion products go up to some peak flame temperature (say 3200F) and then down to the final exhaust temperature based on the device extracting the heat.

As long as there is mixing of the combustion gases and there are not other things that are altering the combustion process (pressure, catalyst, etc.) any part of the combustible mix reaches 1100F combusts at that temperature.

So you can't have combustion mix at 3200F. It's all H20 and CO2. I hve no idea what the proper thing to call that H20 is. I think I learned it as water vapor, just to confuse your other post a bit more.

I'm guessing that the difference in peak flame temp between an air burn and a pure O2 burn is the fact that you have all that nitrogen that isn't involved in the reaction but gets heated. The combustion energy of the two should be the same.

enjoy,
jerry