[Stoves] Lots of smoke after a few minutes of clean burning

[Julien Winter]

Dear Mango;

One cause of a clean burn, followed by smoke, is if the flaming pyrolysis
reaches the grate before it has passed through all the fuel.  When this
happens, you end up with a bottom-lit updraft gasifier, with a charcoal
fire at the bottom.  The syngas exiting the fuel bed is rich in tars,
relatively low in reactive H2 and CO, so is very hard to ignite, even with
a pilot flame.

This can happen for three reasons:
1) You poured an accelerant on the fuel bed prior to igniting the stove,
and the accelerant ran down into the fuel bed.  Wet some fuel particles
with accelerant, then add them to the top of the stove.
2) Burning embers from the top of the fuel bed fell down through pore space
and transferred the fire to the bottom.  Embers of small kindling on top of
coarse fuel particles can do this.
3) The fuel bed (e.g. wood chips) has a high resistance to primary air flow
-- a higher resistance that down the side of the fuel bed by the reactor
walls.  When this is the case, the primary air will travel mostly up the
side walls, and the flaming pyrolysis will case the source of the primary
air down to the grate.  The 'chasing the air' is called 'channelling'.
Channelling problems are more frequent when the primary air flow is low.

Perhaps you can find an answer to you problem in the above.

You can try Paul Anderson's suggestion of cutting back primary air.

However, I find that if the fire has hit the grate, a high rate of primary
air prevents a lot of smoke, because it provides suffient air to support
both char combustion on the grate and flaming above, and raises the
temperature in the fuel bed to crack the tars.  Much of the syngas may burn
in the fuel bed.

Cheers,
Julien

-- 
Julien Winter
Cobourg, ON, CANADA
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