[Stoves] Smokeless transition

[Julien Winter]

Hello Marc;

I have not devoted any effort to studying the transition from the first
phase of fuel pyrolysis to char gasification in natural draft TLUDs.
However, I have observed good transitions when there is a good primary air
supply, and when there is good gas conductivity in the char bed.  For me
this happens with wood pieces and wood chips.  The char bed from pellets
seems to impose too much resistance to gas conductivity.   Char bed
conductivity is important because the draft will be largely developed in
the reactor and to a much lesser extent in the gas burner, once the pyrogas
flame has gone out.

For charcoal to burn, it needs a good steam of primary air.  Once it goes
out, it is had to relight.

I have found that some of the older literature is better at describing the
basics of combustion.  Evans and Emmons (1977) describe the problem with
charcoal this way:

"If an isolated piece of wood charcoal is ignited, it will not continue to
burn unless one blows an oxidizer, i.e. air, on to it." ... "Unfortunately,
in order to maintain combustion, *air must be blown *at the charcoal
burning surface at high mainstream velocities; velocities that are high
enough to make the flow turbulent." ... "*The lowest recorded mainstream
air velocity at which the charcoal would self-sustain its own combustion
was 7.7 m/sec" with a surface temperature of 775 to 800 °C"*

 (Evans, DD; Emmons, HW. 1977. Combustion of wood charcoal. Fire Research
1:57-66)
So, to conclude, effective charcoal combustion need a faster rate of
primary air than was being supplied in the first phase of fuel pyrolysis.

I hope that helps,

Julien.
-- 
Julien Winter
Cobourg, ON, CANADA
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