[Stoves] Accidental TLUD technique discovery

Crispin Pemberton-Pigott crispinpigott at outlook.com
Mon Nov 14 13:07:00 CST 2016


Dear Paul and All

Two things spring out of your last contribution:
‎
"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.

The complete (ok, as good as complete) gasification of the char is possible with very little oxygen for a couple of reasons. O2 in the fuel is sufficient to burn the Hydrogen with little left over. So all it needs is one O for each C. In practice it means ‎a bit more than an equal mass of O for the mass of C. (16 for 12). The gases rising from below are likely to contain some O2 and some CO2. Also water vapour which will react on the surface of the charcoal. All this means that in a damp environment the splitting of water, the splitting of CO2 and the free O2 can result in the combustion of the char at least to CO and some H2. This will burn in the secondary fire above.

With damp fuel this is far more likely than with dry fuel because it is much harder to get the water gas shift reaction going with the Oxygen level is high, the temperature low and the H2O sparse and the char surface scattered.

The second thing is that there are commercial and very small pyrolysers  that are BLUD with fuel added on top and the fire 'in the middle'. Riaz at CAU is measuring the emissions and performance of one right now as his thesis work. The combustion can be quite good in terms of completeness. The difference is that the MPF is rising, not descending, and the gas is really wet. The gas is high energy compared with a TLUD, according to Hirendra Charkrabarti who builds large ones.

It can be refueled indefinitely from the top. The 300 kW(e) gasifier at YDD also works this way‎. It produces no char at all, reacting the last of it in a fluidised bed of ceramic sand as the air enters the bottom.

‎Regards
Crispin



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.)

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.

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.  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.

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.  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.

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.

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

On 11/14/2016 6:11 AM, neiltm at uwclub.net wrote:
> On 13 Nov 2016 at 14:45, Paul Anderson wrote:
>
>> Dear Neil,
>>
>> Very nice description of what you accomplished / learned.  Very useful
>> information.
>>
> Thank you.
>
>> Question:   "riddling" is a UK word for "shaking"?
>>
> Yes, if 'shaking' is American English for agitating the grate to release
> ash and fines to maintain air flow?
>
>> What you have described refers to when the fuel is sufficiently damp to
>> hinder proper / expected TLUD performance with a descending Migratory
>> Pyrolytic Front (MPF)that does not sustain the processes (plural).  But
>> not so wet that there can be no MPF pyrolysis.  There is some
>> mini-burning of some of the created gases so that the pyrolysis can
>> continue at the MPF.
>>
> Yes, that seems a good description.
>
>> But the pyrolytic gases that move above the MPF are either:
>> A.  too cold to be combustible with the secondary air later, or
>> B.  too insufficient (quantity) to sustain the secondary combustion, or
>> C.  have too much water vapor (therefore are "diluted" to sustain the
>> secondary combustion, or
>> D.  combination of the above, or something else.
>>
> All three I would guess.
>
>> Whatever the case, things are not working.
>>
>> So, you are showing that a small, frequently tended, additional zone of
>> combustion (another fire that I will call the "middle fire" because it
>> is between the MPF and the secondary combustion) can be made perched on
>> top of the charcoal that was created by the MPF that continues to move
>> downward.
>>
> Yes.  Although this all started from the outset and the way all TLUDs are
> started with some sort of starter fire on top of the main charge, if only
> the top layer of the main fuelling soaked in alcohol.
>
>> That small additional "middle fire" could be with several variations.
>> Please tell us about your experiences and observations regarding the
>> following (You might do some intentional further experiments to confirm
>> or deny the following):
>>
>> The middle fire must get SOME oxygen from somewhere to be sustained.
>> The O2 either has come up through the fuel-MPF-charcoal (which is quite
>> possible because it is blowing past the MPF where the pyrolysis is so
>> weak because of the damp fuel.)
>>
>> OR
>>
>> The O2 is coming downward from above, circulating in and keeping things
>> hot enough for pyrolysis to occur.
>>
> It will be both.  With the fuel charge built up to just below the level
> of the secondary air holes, when it is lit it is very obvious,
> particularly with the Reed fan stoves, especially at the point of
> starting the fan and observing the flame disturbance that the secondary
> air is probably enough directed air regardless of the primary air from
> below.  I guess that with keeping the pyrolising mass at that level
> through refuelling, this remain true.  With this morning's burn it seemed
> that the main pyrolysis front was contiguous with the pyrolysis of the
> added fuel, that they were not discernably separate layers, there being
> no body of non combusting char between them, and the level of fuelling
> gradually lowered with the pyrolysis of the main charge.  I hope that
> made sense?  It is perhaps worth remembering that the height of the
> secondary air holes is a mere 5 inches from the ground.  Only through a
> batch burning of very fine chip does the MPF noticeably lower beneath a
> combustion chamber largely remaining full of non combusting char.
>
>> OR
>>
>> Your stove might be one with some side holes in the wall of the fuel
>> chamber.
>>
> It isn't.
>
>> Whatever the case, there are a couple of options:   (What did you observe?)
>>
>> A.  The middle fire is actually more of pyrolysis (and a shortage of O2)
>> so there is more "smoke" (pyrolytic gases) created and they rise
>> together with whatever is from the MPF to come into contact with the
>> secondary air, giving some cooking fire as is normally associated with a
>> TLUD with dry fuel.
>>
>> OR
>>
>> B.  The middle fire is actually a "diffusion fire" with pyrolysis and
>> burning of gases AND even some char-gasification, resulting in a rather
>> traditional fire perched on top of the charcoal in the TLUD.
>>
> I think it is more B than A, except that as explained above I think it is
> probably misleading to think in terms of two separated combustion
> processes. That might be possible to achieve in this stove though.
> Firstly the main charge would have to be very fine such that it retained
> a significant body of non combusted char above the MPF, which it
> invariably does with fines.  Under these conditions as can be imagined,
> placing fresh wood on top of the char where the MPF is at the bottom of
> the stove with a mass of non combusting char above it, results only in
> the slow build up of uncombusted smoke, but I would expect it to be
> possible to separately ignite such a top layer of fuel, certainly if near
> the secondary air holes.  I've not conducted this specific experiment, so
> cannot say if fresh wood at a mid chamber level would find sufficient O2.
> I imagine separately lighting such a mid layer might be too problematic
> because of restricted access and hot gasses extinguishing the ignition
> flame as has always happened when I have tried to sustain a match below
> the level of the secondary holes. A small braiser's LPG torch however
> will ignite such a layer.
>
> Bearing in mind the squatness of the stove, the mid fire experiment would
> be more meaningful with a taller fuel chamber I think, otherwise the
> separation of layers is likely to be too uncertain in so few inches. I
> feel clear that when large lumps of wood are burned as the entire fuel
> charge, the vigorous unrestricted primary air flow ensures that even
> where there is char above the MPF when it has reached the bottom, the
> entire body of char is consuming very rapidly, and producing a nice blue
> flame from the top of the char.  The large amount of primary air present
> from the outset of the firing I feel ensures a continuous close
> connection between the MPF, and the fuel igniting immediately above it.
> With completely dry medium sized wood as a TLUD this stove is a 10 minute
> towering inferno.  By introducing larger fuel size, damper and denser
> material such as my hoard of mice opened plum stones, the burn time can
> be extended easily to 20 minutes or as much as 30, which is a lot easier
> to cook with of course.
>
>> To test the difference between A and B, what is needed is that the
>> middle fire is down lower into the metal fuel chamber (cylinder), not up
>> near the top.   So, does the same advantage occur if the cylinder is
>> sufficiently taller?
>>
> I would need to modify one of my taller tincan stoves to replace the
> bottom of the fuel chamber with a wire or expanded metal grate, so can't
> answer that Q very quickly,  but have been intending to revisit these.
>
>> RESULTS:
>>
>> 1.  If A above, that final combustion might have low emissions of CO and
>> PM such as with normal TLUD usage.
>>
> I think I see what you are getting at, but what I'm having difficulty
> with is envisaging achieving this without a separate ignition at a half
> way stage of a batch burn.  It might be a possible if tricky experiment,
> but rather impractical otherwise surely?  But perhaps I have
> misunderstood?  Achieving this 'dual front' stage naturally from start
> up, the mid fire would presumably either have to produce its own MPF
> (more statically than migrating), or not consume the body of char left
> unburnt by the initial MPF, at least at as fast a rate as the descent of
> the initial MPF?  Can a batch loaded TLUD be made to split into two
> 'fires' by top fuelling before the batch has consumed?  I don't know the
> answer to that, but I feel as sure as I can be that is not what is
> happening in my stove.
>
>> 2.  But if B above, the emissions would probably be more similar to a
>> regular "contained" fire (not an open fire like 3-stone type).
>>
> Yes, I think so.  This stove is sold with no instructions or mention of
> being used as a TLUD, so most will simply light a small fire in the
> bottom of it and continuously feed it.  It does work well that way, and
> placing a lit pine cone in the bottom and another 2 or 3 and/or some
> twigs on top is an almost instant fire needing no preparation.  But it
> does tend to burn with less smoke as a TLUD, and is my preferred way to
> start it. Sustaining it indefinitely beyond the batch loading is where it
> gets most interesting.
>
>> As I said, I like what has been discussed.  Pardon me if I have
>> misunderstood something.
> I don't think so, except perhaps where you conjecture a fire 'perched on
> top of the charcoal'.  Probably not achievable in these stoves at least.
>
>> Looking forward to your next message or
>> comments by others.
>>
> Thanks for your thought provoking response.  I hope this reply clarifies
> more than confuses!
>
> Here is a question though?  What happens if building a conventional open
> fire, not straight onto the ground, but on top of a base of wet or damp
> wood instead? The base wood would eventually dry out and be consumed, but
> surely to the advantage of the main fire above it which might be the more
> stable for such a base?  I think this is essentially what I achieved in
> miniature, and enclosed of course.
>
> Neil Taylor
>
>> 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
>>
>> On 11/13/2016 7:54 AM, neiltm at uwclub.net wrote:
>>> There have been days of rain here in the SE of England, and my outdoor
>>> wood, consisting at the moment mainly of Hazel coppiced two or three
>>> years ago for runner bean poles, but now too weak and snappy to be
>>> re-used, but ideal easy fuel for the (ebay/amazon) Chinese wood gas camp
>>> stoves, has been getting progressively wetter as the softening (rotten)
>>> fibres cease to be able to repell water as well as more freshly or
>>> quickly dried or seasoned wood.
>>>
>>> So this morning, despite having loaded the stove yesterday for this
>>> morning's breakfast, and kept the stove indoors, the wood simply hadn't
>>> dried out well enough to sustain the pyrolitic front once the top
>>> ignition layer of well dried fuel had been exhausted.  This is an old
>>> learned lesson, that there is no use, beyond a subtle difference at least
>>> in stratifying a TLUD in the hope that it will be going well enough by
>>> the time it reaches a lower layer to cope with meeting wet wood.  There
>>> is an extent to which you can get away with it, but it's too easy to
>>> miscalculate, although it does have the potential if you can manage it
>>> for a strong boiling flame to turn itself into a simmer flame at the
>>> point where you want that.
>>>
>>> Rather than tip the whole smoky mess out into a tin and put the lid on
>>> it, I hoped that it might not be too far off and might sustain if I just
>>> put more dry tinder and candle wax gratings on top, as is often the case
>>> with a failed start.  Three times I did this.  It did not want to burn
>>> that damp wood.  Still I didn't bail out, but instead contented myself
>>> with feeding the top with thin dry wood, and cooked on that, continuing
>>> refuelling as I went, just as you would if continuing the burn of a TLUD
>>> batch AFTER it had all pyrolised because you still want a fire.  In other
>>> words this morning I worked this in reverse, refuelling from the outset
>>> as slowly, the batch burned right down by the end of the cooking.
>>>
>>> It was immediately apparent that this way round had several advantages
>>> over the more expected batch burn, then refuel to extend.
>>>
>>> The fire was a much steadier middling heat level throughout the cook,
>>> which was better suited to what I was cooking.
>>>
>>> The fire was easier to sustain for the length of time and amount of the
>>> cooking, needing less rather than more tending as the batch burned down.
>>>
>>> The problem of sustaining a fire in these stoves after the batch has
>>> burned are twofold:
>>>
>>> Firstly ash builds up and chokes the primary air requiring riddling,
>>> (there are no lower on the side primary air holes).  Secondly, unless the
>>> lumps of wood are large, so much small char builds up with only the base
>>> ignited, that no amount of riddling will permit refuelling without smoke.
>>>    If the fuel is large, the char is burning vigorously with its own blue
>>> flame, but this results in the refuelled fire losing its base eventually.
>>>    Refuelling works great for a while, but then unless refuelling is
>>> sustained at quite a high rate, the base of the fire simply disappears,
>>> leaving insufficient embers to ignite new fuel.  However, even with these
>>> vigorous stoves maxing out on primary air, not even trying to design for
>>> a soot free fire, it is possible to add too much fuel and start to make
>>> smoke, something that the cleaner burning 1:6 air ratio stoves suffer
>>> from to a much greater extent.  In general though it is the greater
>>> forgiveness of lack of fine tuning and fuel fussiness that is the
>>> strength of these stoves, and why IMO they are so much better in camping
>>> situations, where adaptability to whatever fuel is available is key.
>>>
>>> But my persistence this morning resulted in discovering a different way
>>> of using these stoves that worked well, once I adapted to continuing the
>>> refuelling from the outset, and which I can easily adapt to doing by not
>>> initially filling the stove right to the top.
>>>
>>> There is a clear advantage, in camping type situations at least, but
>>> possibly elsewhere such as refugee camps maybe where there might be a
>>> problem of sufficient dry wood, to know that there is a good way of
>>> utilising a batch loading of wetter wood, by lighting and refuelling a
>>> conventional fire on top of it with dry wood until it will sustain the
>>> rest of the batch, or perhaps with decreasing feeding of dry wood
>>> depending on how wet the wood is, or strength of flame wanted.
>>>
>>> There is also an advantage in gaining greater heat control, and the
>>> primary air supply will remain more constant with reloading at the start
>>> of the burn rather than at the end.  The wet wood batch sitting
>>> underneath the dryer wood fire acts as a buffer, a stabiliser of the
>>> small fire on top in a way that simply lighting a small fire straight
>>> onto the grate does not.  This must be because there is a stable, but
>>> slow pyrolytic front constantly at the base of it.
>>>
>>> I'm not advocating this as a generalised preferred way of using TLUD's,
>>> merely sharing, for me at least, a newly discovered versatility that
>>> definitely has worthwhile application in some circumstances, but in
>>> general does perhaps show that some TLUD designs that have or can permit
>>> less restricted primary air can be highly versatile and adaptable stoves,
>>> capable of utilising in the same physical stove a very wide variety and
>>> condition of fuel, and being controllable through fuel
>>> modification/selection rather than, or in addition to stove damping.
>>>
>>> I can't help but conjecture that where an end user has spent their
>>> lifetime daily tending a three stone fire, they will have such a wealth
>>> of experience with fire and fuels that they can bring to maximise the
>>> potential in a TLUD, that if that TLUD permits greater versatility
>>> through permitting at least a maximum possible of primary air, then this
>>> surely might be one key to continued adoption of a stove, that it permits
>>> wide adaptation and uses the lifetime empirically gained knowledge of how
>>> to use fire that the user brings to it.  If on the other hand it's design
>>> is too specialised, and cannot adapt, its future will be the more
>>> dependent on the supply of a consistent and affordable fuel that it works
>>> well with.
>>>
>>> I'm guessing that when Paal Wendelbo used to say 'start with the fuel and
>>> build the stove around that', what he was mostly doing was adjusting the
>>> size/proportions of his Peko Pe stove and primary/secondary air ratios to
>>> most suit the fuel he found?  Good principle where the fuel and cooking
>>> needs are more of a constant, but the Chinese camp stoves, as with the
>>> similar Bush Buddy stoves and derivatives (like the Solo range) with wire
>>> grates, maximising the primary air, encourages an endless learning curve
>>> that teaches the user how to get just what they want from them.  It
>>> teaches you no less about biomass, different woods, their condition and
>>> preparation, and fire making, than an open fire does.
>>>
>>> There is real satisfaction in that, just as I've just been reading about
>>> traditional hay making with a hand scythe delivers, despite back breaking
>>> labour, a profound happiness and contentment.  Another unquantifiable
>>> dimension to contend with, LOL.
>>>
>>> Maybe that sort of unsuspected intangible is why Bangladesh once came top
>>> of an early 'world happiness survey' in the 90s, despite great poverty
>>> and loss of life in natural disasters.  I can easily afford to cook
>>> breakfast on natural gas in our centrally heated kitchen, but something
>>> that gives me a satisfaction I find hard to resist, drives me out even on
>>> frosty mornings, to cook it on a simple wood stove instead.
>>>
>>> But here is what we are intended to aspire to in the affluent west:
>>>
>>> http://www.telegraph.co.uk/technology/2016/10/12/man-spends-11-hours-tryin
>>> g-to-make-cup-of-tea-with-wi-fi-kettle/
>>>
>>> Tears rolling down cheeks!  A candidate for the Darwin awards perhaps?
>>>
>>> I don't think so.  I'll stick with my eccentricity!
>>>
>>> Neil Taylor
>>>
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