Charcoal making was not an important objective of the Eindhoven
Woodburning Stove Group; even clean burning was not initially considered
important by the Steering Committee. The objective was saving fuel in
the preparation of food.
The Downdraft stove was the first clean burning stove we developed. It
could be fed continuously but utilising the heat was not straightforward.
There was no deliberate partitioning of primary and secondary air; the
layer of burning char was not thick enough to deplete the air completely
of oxygen; whatever oxygen was left after passing through the char bed
was used to burn the volatiles. If the fuelbed was not too thick, enough
oxygen passed through to burn the volatiles downstream.
If the fuelbed thickness was increased the stove worked for a short
period as a gasifier but only while the chimney was very hot from the
proper burning period. After that there was smoke everywhere.
We did have a shot at producing charcoal in a stove that would burn only
the volatiles. Wood sat in a vertical cylinder, closed at the top with
an opening at the bottom. Air was drawn into that opening to burn the
wood in that vicinity. Around the wood filled cylinder an insulated
chimney surrounded it. The idea was that burning part of the wood would
produce hot gas which could be burned at the top under a pan. The hot
gas heated the cylinder, charring the wood inside.
It didn't work and we had no time to look into proper dimensioning which
might have resulted in something that did work.
Peter (smoke) Verhaart
On 13/01/2012 14:25, [email protected] wrote:
Tom (cc Andrew and list):
Thanks. Professor Prasad is certainly one of my heroes in
stove work. It was fun to read (I need to re-read) this report on a
helpful down draft design. But char production was not a part of his
analysis. I will look more closely to see if there is something
there to allow for a char-making design. I think the main issue is
predicting something on needed chimney heights - which seems to be in
there. But it appears to me that both primary and secondary air were
traveling through the fuel - whereas I presume a need to separate the
air supplies - as in the TLUD.
The message (below) by Andrew to the stoves list was at least in
part generated by some off-list conversation that we have been having
on BLDD and char-making. The issue is why are almost all gasifiers
(which can be operated to give sizeable char output) based on BLDD,
but (apparently) all (?) char-making stoves are TLUD?
I hope anyone knowing of a BLDD char-maker will let us know.
Few items below also on Andrew's e-mail..
------------------------------------------------------------------------
*From: *"Tom Miles" <[email protected]>
*To: *"Discussion of biomass cooking stoves"
<[email protected]>
*Sent: *Thursday, January 12, 2012 5:23:21 PM
*Subject: *Re: [Stoves] Inverted top lit updraught
Krishna Prasad described the downdraft stove in a presentation to ETHOS in
2004
http://www.vrac.iastate.edu/ethos/ethos05/proceedings2004/presentations/pras
adbiomasscookstoves.pdf
A picture and WBT for Peter Verhaart's down draft barbeque can be
found at:
http://www.stoves.bioenergylists.org/verhaartbarbeque
Tom Miles
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of
[email protected]
Sent: Thursday, January 12, 2012 2:48 PM
To: Discussion of biomass cooking stoves
Subject: [Stoves] Inverted top lit updraught
One for Peter Verhaart to comment on perhaps: we probably all know by now
the genesis of the inverted down draught stoves which Ronal and Tom Reed
expounded early on this list and concurrently Paal was developing
with his
early Peko Pe and we understand how burning the pyrolysis offgas can offer
very low particulates compared with burning whole wood in a conventional
updraught stove but is the same true of a down draught stove if the
primary
air is similarly controlled?
*[RWL: I think Piet Verhaart's thoughts would be excellent (for
others: Piet's doctoral work was on a BLDD stove). But for this
purpose, I am interested right now in whether any BLDD is providing
(lots of) char.*
The advantage of stratified down draught ( i.e. where the air moves down
through the charge of wood as that also descends through the grate) would
seem to be that the fire can be continually stoked. The disadvantage
is all
the extra pipe work and either needing a hot plate or sunken pots to
maintain the chimney depression required to suck the primary air down.
[RWL: I think Professor Prasad's paper shows a BLDD design that
doesn't suffer from these two drawbacks. But also it doesn't seem to
produce char.
Down draught devices are normally intended to gasify all the fuel, often
with extra air supplied in the "throat" but what if one was not
particularly
concerned if a high char ash were left?
[RWL: I have recently been talking with Agua Das [cc'd] about his
"Dasifier" - which is incredibly efficient (very high temperatures)
with an "ejector" supplying this extra air. He says he can produce
lots of char as well. This is not what I have had in mind - but
could be attractive if that intermediate secondary air can be
introduced and controlled economically.
For the sake of staying on topic can we avoid the "b" word and just
discuss the concept?
[RWL: Hmm. "b" word ? I trust it is OK to talk of saved char
(intended for a "b" purpose).
Andrew - thanks for these thoughts.
Ron
AJH
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