[Stoves] Changing rate of pyrolysis front in TLUDs

[Paul S. Anderson]

.Quoting Dean Still <dstill at epud.net>:

> Dear Paul,
>
> I have observed in T-LUD stoves that it is the reduction of air feeding the
> fire, both primary and secondary, that controls the gasification process.

Dean and all, the gasification is the making of the gases (smokey gases) and
depends on the primary air ONLY.  The burning of the gases is with the
secondary air, so secondary air does not control the gasification (gas making)
process.

> So
> if very little air is allowed into the fire then the height of the chimney
> above the fire will have less effect if air drawn into the fire continues to
> be limited. Is this how you envision the process?

I think that is not correct.  But when you say "fire" I have some 
doubts because
you are not specifying the gasification process or the gas-burning process, or
both.  Also, just saying "air" does not clearly say if you mean the primary or
the secondary air, and that could make a difference between what you mean and
what I might be thinking you mean to say.

However, chimney height is a crucial element that is "fixed" for a specific
stove and cannot be adjusted for low heat and high heat.  The chimney needs to
be sufficient for the high heat, and then it is also sufficient (with surplus)
for the low heat.
>
> If too much air is drawn into the batch of wood, gasification stops and
> direct combustion begins which creates smoke, etc...

Almost correct.  See the difference here:   [In updraft gasifiers like the
TLUDs], if too much air is drawn into the batch of wood [via the primary-air
inlets], gasification [is excellerated, making massive amounts of smokey
gases].  [But if some of that excessive primary air includes oxygen 
that passes
all the way through the pile of fuel, then] direct combustion begins [and that
is the burning of the smokey gases, not creation of the smokey gases.]

NOTE:  In the gasifiers, SOME small amount of the initially created gases is
actually burned right in the area of the pyrolysis front.  This must occur in
order to have some released exothermic heat to sustain the pyrolysis which is
an endothermic (heat needing) function.  That small amount of air to sustain
that combustion in order to sustain pyrolysis is not considered to be
"secondary" air.

Of course, the "gurus" of gasification and combustion (Andrew, Tom, Tom and
more) can discuss it all in even better details, including the exceptions to
the rules.

Paul


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