[Stoves] Accidental TLUD technique discovery

[Crispin Pemberton-Pigott]

Dear Neil

"why doesn't everyone do it that way?!"


Then I was a Boy Scout we used to have large meetings in the evening ‎around a large campfire. It was a stack as you describe with 6-8" logs at the bottom and kindling at the top. It looked like an Egyptian pyramid. It was always top lit so it didn't make smoke and bother whoever was downwind.

The wood was gathered from what was available which meant the large pieces in particular would be damp. They were dried from above. As time passed the fire (char) fell to the centre. This caused the in-drafting air to pass through the dampish wood and run into the char fire, whatever the height at the time. It would remain quite clean burning to the last straw, so to speak.

In that condition it could be viewed as a crossdraft fire with the pyrolysis gases being consumed in the char/coke fire in the centre that also provided the draft.

The technique also contained the fire as the largest logs would take ages to burn through. As I recall this type of bonfire was standard practice in the Scouting world for generations.

Regards
Crispin
‎

On 14 Nov 2016 at 12:12, Paul Anderson wrote:

> Neil,
>
> This is a very informative conversation.  I hope a number of Stovers are
> reading the messages.
>
> 1.  Your last comment is answered first:  An open fire on top of some
> damp fuel.  (or maybe even a contained fire (in a cylinder but ignited
> at the bottom, NOT a TLUD).   Much would depend on whether air can
> actually pass through the lower pile.   I suspect that very little air
> could be coming up through the damp fuel even if with rather loose or
> largeish pieces.   (Also seen in open bonfires that air cannot get into
> the central area, but that is partly because of the conventive flame on
> the outer edges and comsumption of the O2.)
>

When I built what I imagined might have been the structure of Pal
Wendelbo's ww2 Norwegian resistance stealth TL fire, I crossed sticks in
a right angle layered grid, thus maximising air flow potential through
the bottom of the stack. This worked impressively well, and I was even
able to observe the way in which the PF also migrated sideways to
completely consume the stack against the mild side breeze.  Not an
unburnt fragment of wood remained without touching the fire once lit,
quite unlike any conventional fire. A wonderful and so simple experiment
anyone can easily conduct, and there is no better 'teacher'.  I stood
there feeling I had just discovered the 'right' place to light a fire,
and why doesn't everyone do it that way?! With my little trekker stove
simply charging the pot with large pieces ensures a robust, barely
impeded primary air flow through from the grate at the bottom.  If I want
a slower burn, more moderate heat, then using very fine woodchip de facto
restricts the primary air flow, thus avoiding the towering inferno in
what is in all other respects the identical chip, merely size graded.  I
use a 'riddle' (seive?) with 3 different mesh sizes when I want to
experiment with uniform sized fuel, or simply use up a fine residue.

> 2.  Very interesting that your work is with a short small "camping
> stove" type that is not even marketed as a TLUD.   Yes, shortness does
> limit the ability to have observations related to height issues.  My
> responses are in regard to taller units.
>

Yes.  Rechecking the measurements now that the breakfast stove has cooled
down, distance from ground to bottom of secondary air holes is 4 1/4".
>From bottom of fuel chamber to bottom of secondary air holes is 3 1/8" !
If this seems ridiculously short, Tom Reeds first woodgas campstove, the
SL, measures 2 7/8" on fuel depth!  This ND stove is slighter larger in
the fuel chamber.  The internal diameter is 4 1/4" so the chamber is
actually wider than it is deep.

> 3.  It is easy for having two fire zones or pyrolysis zones with solid
> fuels in a normal TLUD operation with appropriate dry fuel (not
> discussing damp fuel at this moment).  (Secondary burning is with
> gaseous fuel.)
> Let the MPF decend a distance (3 to 5 inches) so that
> there is a layer of charcoal.  Then put additional fuel onto the top of
> the char layer (and sufficiently below the entry of the secondary air).
> The rising hot gases (maybe 400 to 500 deg C) will dry and then start to
> pyrolyse the new upper fuel layer.  That creates additional gases that
> all rise to the level of the secondary combustion.

Ah, thank you for that.  This is outside my experience, so I didn't know
this.  So are you saying that the smoke as it builds up from the
eventually pyrolising "additional fuel onto the top of the char layer" is
simply burned cleanly at all stages at the secondary air flame front?
This must be a nice versatility if so, since doing so would presumably
give you a hotter fire? This is perhaps then a potential in a tall enough
stove, presumably such as your Champion?  The closest I've come to this
has been with Tom's XL woodgas campstove, the big one, where I found that
when I wanted an extra hot fire for stir frying in a wok, I could get a
useful extra boost by adding more fuel during normal full fan operation
of the stove.

But....
>
> 4.  But if there is any surplus O2 getting past the MPF (as could be the
> case with the damp fuel) or entering downwards or through a side
> hole/leak, and if hot enough or with a spark, there could be some small
> combustion of some of the gases (similar to the MPF) at the level of the
> middle fire.
>

Yes, that makes sense.  Interesting possibilities.

> 5.  Concerning the possibility of the "middle fire" and the MPF being
> together all the way to the bottom, I doubt it.  IF the MPF is
> functioning at all, it moves downward and SOME char is created.

Yes, but so little in terms of thickness of layer in the case of the
trekker stove surely as to be insufficient to separate out separate
combustion layers? (see above dimensions).  Yet just as with the even
shallower Reed SL stove it clearly does make sense to say that there must
be a MPF since these stoves can be batch loaded, top lit, and burn
relatively steadily and smokelessly for up to half an hour, and much
longer still with pellets.

The trekker stove, as I seem to be calling this nameless Chinese offering
(do they understand branding in China?), also makes an excellent charcoal
burning stove, and sometimes the only way to tell when pyrolysis has
finished is by the colour of the still substantial flame when it turns
entirely blue.  There can be at the end of a burn, a quarter full of
char, all aflame. And I can't see why this wouldn't be occuring all the
way down? The larger, deeper stack of char resulting from burning fines
remains cool until the char has burned down, and only then can you
successfully add a little thin dry fuel to revive a fire.

> Tom
> Reed showed many years ago the reduced amount of char that results from
> using moist fuels (such as char production being only 10% or 8 % by
> weight of the initial fuel instead of about 20%.)  But I recall no
> mention of zero char being left behind.   And the "middle fire" has no
> conditions for somehow combusting (char-gasifying) charcoal that is
> below that middle fire.
>

Are you saying that a mid fire's MPF will not consume char below it, left
from the original MPF?

> 6.  Significance:  When a person is desparate and needs a fire with
> dampish wood, the person will meticulously tend to the fire, nursing it
> with small pieces.  But in common everyday usage, the cook commonly does
> NOT want to be tending the fire with great frequency (hence the appeal
> of the batch operations of TLUD stoves with rather consistent fire
> without much tending).  In that case, the "middle fire" over damp fuel
> in a TLUD is not likely to be of much interest.
>

One has to hope not, I agree.  The goal must be as you rightly state.
Yet it might be worth considering those situations of emergency,
displacement, migration, refugee camps, and of course the entire
backpacking/camping uses where sufficient dry wood will not always be
available.  Also as Tony Vovers wrote: " a lot of the local found Biomass
is not dry and storage is not easy in the tropics."  I expect we may
experience this for ourselves when we visit Dominica this coming
April/May.

Also, if as I hope to discover, there can be only advantages in
stratifying a bottom layer of damper wood in conjunction with top feeding
for the added heat level control and/or longevity and stability of the
burn then this could simply become another way of getting finer control
over the stove.  Perhaps it is not so with the larger taller NDTLUDs?,
but my experience of the smaller ones is that invariably the fire is
hottest towards the end of pyrolysis, which conversely may coincide with
wanting a more simmer heat.  Is it the case that this would be no
different in a larger taller stove, but for a primary air control as part
of the stove, or is the "consistent fire without much tending?" you
describe a characteristic of the stove design independent of any primary
air control?

This is outside my personal experience, but would it not be useful to
know how to burn wet wood successfully without undue struggle and even
perhaps to the advantage of a steadier burn in disaster/displacement
situations of any kind where the weather is wet?  If SOME wood can be
dried, or prepared from dry heartwood, but not enough so easily for the
entire fuel needs, then certainly this would be of interest to
backpackers with their Bush Buddy 5oz weight stoves I'm sure.

This morning's breakfast was cooked in similar fashion to yesterdays, but
this time intentionally!  I three quarters filled the pot with chunked
hazel which has remained outside uncovered and was damp/wet, and I
deliberately left the bark on and didn't split it!  I then filled up the
rest with 'stove top' dried and split wood.  So unused to doing things
this way, and with hindsight not really leaving enough working room for
the top fire, the thing reached the damp wood and went out very suddenly
while I was paying more attention to the cooking.  So some smoky mess
reviving it again, but not on the scale of yesterday!  I am confident
that I will soon have this just right.  A bit more smoke as things
settled down with the burn improving and needing less tending as it went
down. Leaving a kettle on for coffee untended was an added bonus, just
needing a little reviving after I'd eaten, but with sufficient char still
to bridge that time gap, something I really appreciate.

For doing things this way a deeper burn chamber would definitely be an
advantage, and I hope to try this soon, but by settling on maybe a half
fill of the damp/wet wood I think things should turn out well next time.

You have definitely inspired me to fresh experiments with and
modifications of my tallest, largest tincanium TLUDs, and I feel humbled
in such exalted company that you have found my experiences of some
interest.  Thank you,

Best wishes,    Neil Taylor

> On the other hand, it is well known that people do throw (place) new
> fuel into an operating TLUD, either while the MPF is still advancing
> downward in TLUD style, or after the MPF has reached the bottom and has
> become a charcoal burner with the primary O2 consumed in the char
> layer.)  In either case, the new fuel will first dry, then torrify, and
> then pyrolyze, producing gases that can be burned in the stove.  In some
> ways, this is like the "Flame Cap" method of producing charcoal.  Flame
> Cap is discussed on the Biochar Listserv because it is not with a
> cookstove emphasis  (unless we choose to have that discussion here also,
> with cookstove focus).  My website    drtlud.com   has some documents.
> Or search the Internet for   Flame cap .
>
> Personal note:  Even just a few years back I was fiercely defensive of
> the purity of designating TLUD stoves WITH MPF as the key distinguishing
> feature.   I still am in many ways.   But TLUDs have recently received
> such widespread recognition and "acceptance" in some circles that I am
> much more open to discussions about "fringe" topics and things like
> adding additional fuel during the MPF operations.
>
> Neil's contributions are of interest and considerable potential.
>
> Paul
>
> Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
> Email:  psanders at ilstu.edu
> Skype:   paultlud    Phone: +1-309-452-7072
> Website:  www.drtlud.com
>

_______________________________________________
Stoves mailing list

to Send a Message to the list, use the email address
stoves at lists.bioenergylists.org

to UNSUBSCRIBE or Change your List Settings use the web page
http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org

for more Biomass Cooking Stoves,  News and Information see our web site:
http://stoves.bioenergylists.org/