Sawdust stove, a.k.a. Finnish Candle throws lots of smoke when fire dies down

Hi,

I am trying to build small portable woodstoves that burn pellets and/or wood chips that I can deploy in my greenhouse for really cold nights without venting them. I am not too concerned about carbon monoxide as no one will be in the greenhouse until after they have burned out and I have a CO detector as well.

To that end, I have built a small model. It consists of an 8" black stove pipe about 12" tall with a hardware cloth screen-cylinder open below the cap at the bottom of the pipe. I fill the pipe with wood pellets, leaving the cylinder in the middle open to allow air from below. I then light it and add a 2' chimney consisting of 6" black pipe. A basic Finnish candle with an added chimney. Once lit, the surface burns quite well and there is no detectable smoke coming out. After a couple of hours though, after all the pellets have carbonized, the flame goes out and as the coals die down the stove starts throwing lots of smoke. This continues for another half hour (or more - not sure), then dies down some more and stops smoking, but continues to throw some heat for many hours afterward.

My guess is that once the coals start to die down, it needs more oxygen. I am not sure the best way to handle it. I have read with rocket stoves, it is important to gradually feed fresh wood to keep the flame going. I know this is not strictly a rocket stove since it is solely batch heating, but it seems to be the simplest way to heat a greenhouse without having to tend a fire.

Does anyone have some suggestions on either how to fix this problem or else another concept for a portable, small, batch heater that will burn for a long time?

Thanks,
Scott

I love this topic!
Its one of my own preoccupations .
I aim to use the charcoal created, but a clean complete burn all the way through to ashe is also desirable, and smoke never is.

Since you are burning pellets, you should be able to use a TLUD(Top Lit Up Draft)design instead of a sawdust stove design.
This should help minimize smoke.
You don't mention secondary air inlets at the base of the chimney.
That should also help minimize smoke.
Smokeless firepits utilize this.
They also use an outer cylinder that preheats the secondary combustion air.


You can buy a knock off solo stove/ firepit that should work for you,for not too much, but here's a decent diy video that shows how to make one from stock pots:
https://youtu.be/wG-JGsNAFk0?si=lMuUz_jCrvn6udop

Smaller holes in the inner pot would let you load it with pellets.
Light it on the top( as in Top Lit)usually using a little alcohol, and the combustion air will be drawn up through the bottom holes(as in Up Draft).



For your specific situation, I would build a tlud out of a 3 feet long inner cylinder of 8" stove pipe,with holes ringing it 5"up from the bottom and a mesh bottom inside, 1 inch up from that.
Put the cap on the bottom, affixed with tap screws , then screw the cap to a wooden base, for stability.
The 6 inch distance between the bottom mesh and the wood is there to keep the wood base from charring.

Because you are trying to heat this space, an outer cylinder for preheating secondary air might be counter productive.
Instead we can add an  8" to 6" reducer to your chimney and space it 1/4 to 1/2" from the top of your tlud, allowing for secondary air.

Ideally, you heat a mass using this.
Water or solid, maybe a metal basket full of rocks.
I use the tank of a junked gas water heater to capture the heat from my pellet burning, TLUD greenhouse heater.
Unfortunately I have yet to seal said greenhouse, so firing it is rather pointless.

You seem well on your way to better outcomes!