Quesitons on Gas firing a lab furnace

[wiyosaya]

I am in the process of building a small furnace aimed at technical ceramics work, and I would like to use a carbon crucible. The carbon crucible's limit temperature is ~ 430 C in an oxidizing atmosphere and ~ 2800 C in a neutral atmosphere. My plans are to have the furnace vent to atmosphere so that it will not be under a vacuum. I intend to use bottled O₂ to supply oxygen to the burner. If I adjust the mixture of fuel gas and oxygen so that the flame itself burns neutral, will this be enough to reduce degradation of the carbon crucible to the point where the crucible will survive temperatures in excess of 430C or will O₂ from the atmosphere result in severe degradation of the crucible?

Also, I am in search of a fuel gas. My first thought is Acetylene; however, I understand that Praxair makes a gas called "http://www.praxair.com/praxair.nsf/0/3C9FBEECABD517C6852570610002D0C4/$file/P-10116.pdf" " which claims a slightly higher temperature than acetylene when combined with O₂. As well, Starflame C is supposed to produce considerably less soot and be more stable than acetylene. Does anyone have experience with this gas?

What other fuel gases might be suitable for the application?

Thanks in advance for your help.

 

[FireStorm000]

I have no personal experience in the area, but my best guess is that diffusion of gases is going to be a problem. Gases have a tendency to mix together, thus you're likely to have oxidation as long as the furnace is open to the atmosphere.

I think that if you have the furnace at a slightly higher pressure than the outside air it will be less of an issue. When doing air cooling for PC cases you can keep some of the dust out by having the air pressure inside the case higher than outside: Air flows out of the gaps thus preventing dust from entering, and where the air enters through fans, the dust can be filtered out. I think the same thing could apply to your furnace. If you have a pressure gradient then you could push the gas out.

Also it will more than likely be important to have a stoichiometric mixture of whatever fuel you chose to use. Too much fuel and you have an explosion risk, too much oxygen and your crucible disintegrates.

If you were to fill the crucible with a neutral gas like nitrogen (I'm not sure it's neutral at super high temperatures) or helium, and then burn your mixture in it, you shouldn't have too much problem with oxidation. You could maybe have a pressure release valve to keep the pressure from climbing to much as you added your fuel air mixture. You might have to make it double walled too if you want to keep the outside from oxidizing.

I'd have to do a lot more research before I could help you design this, but that's my initial thoughts on the matter.