[Stoves] Comments about T-LUDs

[Paul S. Anderson]

Dear Stovers,

This is a reply to 10 or more messages about ¡§Comments about T-LUDs¡¨.  Much
has been snipped.    T-LUDs are ¡§Top-Lit UpDraft¡¨ pyrolytic 
gasifiers.   This
identifier can be spelled as:

T-LUD      To emphasize the desired pronunciation of    ¡§Tea-Lud¡¨
TLUD       Nice to eliminate the hyphen, but hard to pronounce if seen for the
first time.
tlud       Lower case seems to get lost in the rest of the text

But all of them do specifically identify a distinctive process of gasification
with close-coupled combustion.

*************************
Drew wrote on 2006-10-02

I have not heard of anyone duplicating my sphere insulated tlud 
experiments, but
would like hear about others experiances in trying them if anyone has.

Paul replies:
I have not used the ¡§sphere-insulated¡¨ method partly because 2 or 3 
inches of
spheres would occupy much of my small fuel-canister space.  But as I go to
larger T-LUDs, your method might have some real advantages.   I hope you and
others will gather more experiences with these spheres (or other possible
objects) that can cover, insulate, and protect from wind currents the top of
the fuel in the T-LUDs.  This could be quite important with the use of
small-particle fuels like rice husks or coarse sawdust that can be (but should
not be) blown out of the stove as raw fuel or embers or cool char.
****************

Several people made comments about the flow of the air.

 From Jeff Davis:
Or is it that it is much easier to move the air through the IDD than the DD?

Paul says:  Not really.  Of course natural draft is possible and is always
¡§upward¡¨, but really the amounts of air and the air pressure (positive or
negative) to get the air to move is rather low (high pressure air flows are
generally not needed).  I am speaking of the needed primary air used in the
making of the gases, which is in relatively small amounts.

Jeff  asks:  What if one blows hard through a cigarette?

Paul says:  While the principle of making the gases (smoke) can be illustrated
in the cigarette example, I like to think of gasifiers as being useful devices
that operate with controls.

**********************
Crispin wrote:
It was a very good explanation of air supply.  I agree that DD mode is 
difficult
to do on a small scale.
In my experience close coupling the gas burning to the gas generating is more
reliable if the stove is small.

Paul says:  Almost right.  Close-coupled burning can be done with the 
large and
the small gasifiers of all types.  But, for the small ones, remote burning is
generally problematic and would add costs that are not justified with the
overall small energy produced.

Also, because the small gasifiers are generally not focused on producing clean
gases (which in large gasifiers are cooled and used in IC (internal 
combustion)
engines), the small gasifiers make tarry gases and we are very happy with
burning those tars to get the heat.  So, keeping the gases hot and 
burning them
right away makes much more sense than allowing for the distance of remote
burning and having to face the difficulties of the cooling of the gases (heat
loss) and the undesirable accumulation of the tars as liquids or solids.
************************
On Mon, 2 Oct 2006 01:33:38 -0400, either Andrew or Jeff wrote:
> A down draft gasifier will gasify charcoal.
              [and]
Similarly a tlud will gasify char.

Paul replies:  Yes, the T-LUD __can__  gasify the char, but it occurs at a
different time (after pyrolysis) and can result in heat damage to the lower
part of the fuel canister.

But DD gasifiers __do__ gasify the char AND must do it during the time when
pyrolysis is taking place in a different part of the gasifier.  If it did not
consume the char, the DD gasifier must have a char removal system (which would
add to the device cost and result in less energy production in relation to the
amount of char removed).

*******************
On Mon, 02 Oct 2006 11:37:53 -0500, Andrew wrote:

in a kiln or stove I think you are bound always to burn some char
before you create pyrolysis gases, I think there is a slight
difference in the way the tlud works compared with the down draught
and that is to do with how the heat is transferred  because the
convection currents work differently. So I say that char will always
burn preferentially to off gas in the pyrolysis zone and that its the
combustion of a very small amount of char that drives the pyrolysis in
the tlud.

Paul replies:  I do not know.  This deserves some study by the chemists in
laboratories, and becomes IMHO a minor issue in the designing of our stoves.

Crispin commented also on Andrew¡¦s words above:
This sounds right to me - I have been wondering all along why the TLUD is
considered to be an IDD.  It is not really the same after all.  Micro
currents of air/gas will definitely affect the way heat moves around the
pellets (or elements) of the pyrolysis front.  It can't work exactly the
same when inverted.  Your analysis looks spot on to me!

Paul says:    ļ)     Finally we have a technical reason to not use the IDD
(Inverted DownDraft) designation that Tom Reed created.

Seriously, I think Tom was not implying that everything is exactly the 
same when
a DD is turned upside down to become an IDD, but rather to convey the 
important
issue (in terms of the direction of the air flow) that the IDD with its
pyrolysis that is BEFORE the char-gasification zone is like the DD gasifiers
(BEFORE) and is NOT like the standard Up-Draft (UD) gasifiers where the
pyrolysis is AFTER the primary air has passed through the layer of hot 
char.   And ever this comment by me is a generalization because of the 
many possible
positions of air entries into the various devices.

***********
Tom Reed wrote:

Dear AJH, Paul and Aul:

Good question.

I differentiate gasifiers many ways, but for this the best is

TAR BURNING, CHAR MAKING = Conventional downdraft gasifiers, inverted (more
recently called also TLUD) and blowing air into a lit cigar (not 
inhaled). These are most appropriate for biomass which is 80% volatile, 
only 20% fixed
carbon.

CHAR BURNING, TAR MAKING = Conventional updraft gasifiers (Lurgi for 
coal, Purox
for MSW), and an inhaled cigar.  Most appropriate for coal  which is 
80% fixed C
and only 20%  valuable volatiles.

An important distinction is what I began to call "FLAMING PYROLYSIS" (c 
1982). This is similar to what you observe in the burning match 
(FLAMING COMBUSTION,
except that it occurs in the presence of a large excess of biomass, so is only
a partial combustion.

Paul replies:
I had never heard before the origin of the term ¡§flaming pyrolysis¡¨.  
I would
have thought that the term was much older than the early 1980s.   And I like
the emphasis on ¡§the presence of a large excess of biomass, so is only a
partial combustion.¡¨  And the ¡§partial¡¨ refers to very little of the 
biomass
being combusted to support the pyrolysis process to liberate so much gases.

***************
Tom Reed also wrote:  (I leave it here for all to re-read so that I can 
make my
reply in the correct context):
The Downdraft and Inverted downdraft (or TLUD, thanks, Paul Anderson)
are in many ways similar, but in one way quite different.

The superficial velocity (gas production rate/area of pyrolysis zone) of
the the Downdraft gasifier must be greater than ~0.05 m/s or natural
convection begins to take over and smoke comes out the top.

The SV for the IDD (TLUD) is typically less than 0.1 or you will blow
the charcoal bed out the top and sparks and soot all over.  The heat
rate of downdraft gasifiers is typically 1 M Btu/ft2-hr, while for our
IDD/TLUD camp stove it is ~100,000 Btu/hr-ft2.

Paul comments:
A.  Typing error???   Is it  ..../ft2-hr    or is it   ..../hr-ft2  ?    (and
what does that mean?)

B.  While what Tom says is probably true,  (I do not work with such 
numbers and
therefore rely on Tom¡¦s science and data), the issue is that the DownDraft
gasifiers operate at substantially more than the MINIMUM of 0.05 m/s of 
primary
air and do (probably MUST) provide oxygen into the char-gasification 
zone.  Tom
and others, please tell us what would be a typical superficial velocity in an
operating DD gasifier.

But on the other hand, the 0.1 m/s in the T-LUDs is stated as a MAXIMUM, so in
reality the actual value might be only 0.05 or even less when in turn-down
mode.  My point is to emphasize what Tom has pointed out as one way 
that the DD
and IDD (T-LUD) are quite different.

***************************
Next topic is the ¡§J¡¨ stove, starting with comments by Tom Reed and by AJH:

Tom Reed wrote:
The "J" stove, developed about 1984 by Mike Antal at U. Hawaii and Peter
Verhaart (?) in the Netherlands is a VERY simple true downdraft
gasifier.  I hope this gets comments from both on the origins and maybe
some pictures of the J stove.  The J stove probably formed my
development of the TLUD in 1985.

AJH wrote in reference to the above note:
Peter has posted to us about his downdraught barbecue stove so I think
you are right. Also when Elsen was making his chardust he used just
such an approach as a pyrolyser and flare.

The thing that strikes me about both down draught and top lit
updraught (alias inverted down draught o.k. Martin ;-)) is that the
incoming air keeps the mass of fuel outside the active zone cool, so
this limits how much fuel is undergoing reaction at any one moment
without the need to add fuel in Paul's trickle feed fashion.

Paul replies:
A.  For clarification, I only described and named the ¡§trickle fuel¡¨ or
¡§trickle feed¡¨ way of adding fuel.  I was not the first to use that method,
and I certainly am not advocating it and do not want it attributed to me.

B.  I want to question whether or not a ¡§J¡¨ stove is a gasifier.  My 
¡§test¡¨
of what is a gasifier revolves around the creation of the combustible gases
SEPARATELY IN TIME AND PLACE from the combustion of those gases.  The
separation can be as little as fractions of seconds and only millimeters of
space, but those separations are identifiable.  And in practice, it should be
possible for the gasification (gas-making) stage to be sustainable WITHOUT
combustion of the created gased (resulting in a smoky mess), but the whole
process is controlled and the combustion can be re-ignited or 
extinguished with
only a modest effort.  Simply to have smoldering tips of sticks in a J-stove
would not be sufficient to show the sustained gas-making function necessary to
qualify as a gasifier.  But use of a fan to assure continuing flow of primary
would be allowed and, in fact, be necessary for the gasifier to function when
the combustion was extinguished and the draft from the chimney is weak or
absent.

The key issue, as I see it, is in the physical nature of the fuel mass and the
ability of the primary air to pass through the fuel with appropriate 
resistance
to maintain a zone of pyrolysis or char gasification.

If the fuel is a collection of loose sticks, IMHO the result will be a rather
traditional fire that burns the sticks without meeting the above stated
requirement of separation of gas creation from gas combustion.

If, on the other hand, the fuel is like a pile of woodchips as used in the
WoodGas CampStove and other T-LUD gasifiers, I believe that the J-stove could
be operational as a gasifier of some sort (DD or IDD or ¡§J-DD¡¨ as in
¡§J-stove-DownDraft¡¨), especially if some provision for entry of 
secondary air
is made for combustion of the created gases.

In the case of Peter V.¡¦s barbecue stove, it might operate either way.  I do
not know about Elsen¡¦s device.  And maybe Tom can tell us more about what he
saw in 1984 with Mike Antal in Hawaii.  I do know that the gigantic 
¡§J¡¨ style
burner in the building where we had Stove Camp 2005 on the Aprovecho 
Campus was
operated with loose sticks and did sustain a nice traditional fire that 
was not
being operated like a gasifier.

************************
The final two messages are from Andrew and Crispin.  The topic is mainly about
combustion that
¡§needs to be under the depression of the chimney and hence the heat has to be
extracted via a hob plate.¡¨

I request some clarification of what is the ¡§depression of the 
chimney.¡¨  (Is
that the same as the ¡§negative pressure created by a chimney¡¨?  If 
so, then I
understand and agree.)

Also, what is a ¡§hob plate¡¨?   (If that could be a ¡§plancha¡¨ or a ¡§stove
top¡¨ or the placement of the sunken pot that Crispin mentions, (all of them
prevent the outside air from leaking into the chimney and reducing the 
draft or
negative pressure), then I understand and agree.)

So, I leave their messages below (snipped), but I end my comments here.

Thanks to everyone for their efforts to explain or understand what is going on
in T-LUD gasifiers.

Paul

*******************************
Paul had written that: ... T-LUD technology lends itself to making very SMALL
gasifiers.

And Jeff had asked:
> Or is it that it is much easier to move the air through the IDD than the DD?

And AJH commented:
Also much easier to make use of the heat as you can just stick a pot
on it, a natural draught down draught stove will need a long chimney
(the long upright of the J) to draw air down through the fuel, this
means the secondary combustion area needs to be under the depression
of the chimney and hence the heat has to be extracted via a hob plate.

**********************************
And Crispin wrote:
Dear Andrew

There is a version of a coal stove which was early days before the downdraft
I made for JHB shacks.  It was a single pot stove that sat fully into the
top where you mention the hob would be.  It clamped on top of a Vesto body.

> ...the secondary combustion area needs to be under the depression
> of the chimney and hence the heat has to be extracted via a hob plate.

Sink the pot.  It is only necessary to make the hole the right size and
place the chimney draft on one side to get a huge increase in heat transfer
plus lower losses from the pot.

The lid of the pot sits flush with the deck.  Lots of stove do this and so I
made a box with the pot sunk (at pot 2 position) with a removable (loose
insert) downdraft (cross draft??) burner sitting at Pot 1 position.  It
works really well with chacoal, coal or wood.  I think it costs $50.

The first pot is placed in a grate over the 'back' of the downdraft fire and
boils very well I must say.

The whole box of a stove stands on 4 round tube legs.

******************
-- 
Paul S. Anderson, Ph.D., Geography professor - Emeritus
Telephone:  USA-309-452-7072 (residence and office)
Internet site:  www.ilstu.edu/~psanders
For my gasifier stoves info, go to:
http://bioenergylists.org/contributors#Paul_Anderson



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