Catalytic combustor

[justincorhad]

I’ve often wondered, does a catalytic combustor require oxygen to burn the smoke? Or is it a chemical reaction that doesn’t need any? I’ve seen diagrams of stoves without oxygen supply to the catalyst and some diagrams with.

 

[peakbagger]

It needs oxygen, all it does is lower the temperature of the oxidization.

 

[justincorhad]

It needs oxygen, all it does is lower the temperature of the oxidization.

I figured so, so I’m guessing most catalytic stoves do not have a separate air manifold just for the catalyst as pictured in the one diagram above? Would most just use excess air from the primary intake?

 

[peakbagger]

Once a stove is at steady state the big nasty is carbon monoxide (CO). CO is partially burnt fuel so its potential energy going up the stack. There are other volatiles along for the ride but if you get the CO they normally are long gone. In order for the CO to burn it has to have oxygen and enough turbulence to get the oxygen in contact with CO. Thus the honeycomb of the combustor which leads to turbulent flow and lots of surface area for the catalyst surface to be exposed to the gases, it also works to distribute the gas flow over the entire honeycomb. The reaction also needs a minimum temp around 1000 deg F for a non catalytic stove. Combustion temp is not a yes/no thing. As the temps goes up its more efficient, so to get 100% combustion without a lot of excess air needs higher temps. So the easy approach is put the combustor in the flame path and possibly insulate around it and run the stove with excess air. Excess air in the primary combstion zone also takes longer to heat the combustor up to temp so the alternative is to heat up air in the stove for secondary air and inject it right at the catalyst. That should get the combustor up to temperature quicker. The next step is install a oxygen sensor on the outlet of the stove and control the secondary air so that only the amount of air that is needed is injected. This speeds up light off of the catalyst and cuts down on air flow up the stack.

Ultimately its a trade off on complexity and cost to manufacture versus selling price. High temps means higher quality more expensive materials and better design to deal with expansion and contraction. Catalysts contain expensive metals and the bigger the catalyst the more material it uses.

 

[gthomas785]

My catalytic stoves (both VC) have separate primary and secondary air intakes. The secondary air mixes with the exhaust right after the exit from the firebox. Hope that helps.

 

[Highbeam]

Like when you breath air, the fire doesn’t consume all of the oxygen in the firebox. There is still plenty left after the primary combustion to support secondary combustion with a catalyst provided the stove design and user settings allow enough residence time of the smoke mixture in the catalyst. At high burn rates that many cat stoves are designed for they sometimes need extra air since the exhaust is whipping right through the catalyst.

It’s a fine line between a hybrid stove and a cat stove with secondary air feed.

 

[mellow]

Hybrids have been around for a long time, don't believe the hype that it is "new" technology, they have improved on existing technology.

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