[Stoves] Limiting factor for secondary burn?

[Paul S. Anderson]

Frank and all,

Thank you Andrew and Crispin for helping with the explanations.  One main
comment below:

Quoting frank <frank at compostlab.com>:

> If we get the fuel to pyrolysis at a temperature of ~ 400 C (starved of
> O2) and char accumulates above as in the TLUD will the char ever start
> to gasifier with the heat increasing to ~800 C?

What you seek is a contradiction of the basis for the TLUD.  To gasify 
the char
you need O2 in that zone, but not arriving through the raw fuel zone and the
phyrolysis zone that are both below the char.

> To get the char to help out here in the TLUD (temp ~800 and changing all
> pyrolysis to CO) do we need to find a way to supply more O2 between the
> pyrolysis gases and the char?

Frank, it is the basis of the DOWNDRAFT gasifiers to force the pyrolysis gases
through a hot bed of char, thereby getting cleaner off-gas coming out.  
UPDRAFT
gasifiers do not work that way, whether top lit or bottom fired.  All UPDRAFT
units of which I am aware give tarry (dirty) gases that are great for burning
but terrible for going into internal combustion engines.

> If we wait until enough char has critical
> mass (or add outside char on top of the small fuel before igniting) will
> adding a small outside vent to allow the char gasification to continue
> so all the pyrolysis gases going through it will oxidize(?) to CO? Do we
> need to stretch the distance of the pyrolysis zone from the char that
> builds up - having three fun fires (pyrolysis(bottom),
> gasification(center)  and combustion(top) to make the PS (perfect stove)?

Instead of saying bottom, center, top, try saying first, second, third.  The
sequence IS in that order in a DOWNdraft unit (pyrolysis first on top,
char-gasification second in center, and finally the combustion of the gases
after leaving the gasifier), and the gases can be clean.  The sequence has not
been shown to be practical in an UPdraft situation (pyrolysis first on the
bottom, etc).

The simplicity and usefulness of the UPdraft are in making dirty gases 
that burn
well for thermal applications.  The difficulty and "much-sought-usefulness" of
the DOWNdraft gasifier are in making (or getting close to making) clean gases
for IC engines and other uses of high-quality clean combustible gases.

So far in the history of gasifers, you "can't have your cake and eat it too."
Or  you "can't have your simple updraft and clean gases too."

That must be taken into account when setting your research agenda.

Paul

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