Benefit of chimneys in burning biochar

In a recent discussion here, I was writing that I had heard that chimneys are useful in burning biochar.  Then I realized that I don't know why.  

I burned a batch of biochar today, and I was observing the chimney. At first, when I put the chimney on, it created a lot of smoke and the fire went down.  That doesn't usually happen to me.  I took the chimney off and looked at the fire.  I think the fire wasn't getting enough air, because when I opened the wood up so it wasn't closed off, the smoke went away.  The air and fire could travel through the barrel much more easily.   One thing that I did notice is that when I put the chimney on, the flames go way higher in the barrel.  They will go up the chimney.  That makes me wonder if it is reburning the smoke, like a catalytic converter.  

I thought that part of the justification for the chimney was to keep the heat flowing up smoothly.  Is that limiting the amount of the air?

Does anyone here know what the purpose of the chimney is in burning biochar?

Thanks,
John S
PDX OR

I think it really depends on the design of the kiln/burner.

If your TLUD barrel burns clean on its own, I'm not sure a chimney adds much value.

In a barrel burner like mine, the top two thirds is essentially a hot chimney. I lets me shove small trees vertically into the burn zone, pack the new coals through an open window in front, re-establishes the draft if I don't have the wet/dry fuel mix quite right, and (most importantly) keeps most of the smoke out of my face.

In more elaborate low oxygen kilns, the chimney draws off the various noxious gases produced from pyrolysis and allows them to be burned off elsewhere or even routed back around to heat the kiln.