[Stoves] Natural draft TLUD comments

[psanders at ilstu.edu]

Dear Stovers,

Dr. Priya Karve asked me a question and my answer includes things that I have
not written down before.  So I am sending it to all Stovers.  The topic is the
Agni cookstove that is the ARTI version of the natural draft TLUD that I
designed in India in late 2005, based on the principles of the Champion Stove
that won the "Cat Pee" clean combustion award at Stove Camp 2005.  Sorry the
measurements are in inches.  6 inches = 15 cm.  Just multiply the 
inches by 2.5
to get approximate centimeters.

Dear Priya,

Thanks for writing.  All is well here, but progress is never as fast as one
might desire.  My replies are below:

Quoting Priyadarshini Karve <priyadarshini.karve at gmail.com>:

> Dear Paul,
>
> How are you?
>
> As you are aware we have been promoting the TLUD stove that you designed for
> ARTI in the name of 'Agni stove' for the last couple of years. It is an
> excellent high power stove, particularly suited for commercial applications
> like roadside eateries. However, a big limitation of the stove is the need
> to frequently refill the fuel holder at the bottom. This is particularly
> cumbersome for the roadside eateries, which require the high power over long
> durations without interruption. One suggestion has been to make the fuel
> holder in the form of a large box, which will hold larger quantities of
> fuel. the top of the fuel holder will still be the same (otherwise it will
> not fit into the stove). I wanted to know your views on this before trying
> it out.

Please do conduct the experiments and let us know the results.  I have not
experimented recently with significantly larger fuel containers.  My concerns
are about the size of the fire.  If the SAME amount of primary air enters, the
larger surface area will result in spreading the limited air over a large area
of thin pyrolysis, with lower temperatures and possibly uneven pyrolysis.  And
if larger amounts of primary air are allowed in, the amount of combustible
gases will be very large, with either too much flame or some gases not being
combusted.  With those cautionary remarks, I now give the positive comments.

I am quite sure that the successful Chinese TLUD by Daxu (over US$100 
price when
without subsidy) has a larger box-type fuel container, but I have never 
seen the
amount of flame.  Because you are targeting street vendors, they might 
like the
larger amount of heat.  I would not recommend a square-ish fuel container.  I
believe that the corners would have cool spots and delays of pyrolysis,
resulting in possible non-combusting smoke when the fuel batch is almost
finished.  I would suggest that you consider an 8.5 inch diameter fuel
container (anywhere between 8 and 9 inches).  Radius of 4.25 when squared is
18, which is double that of radius of 3 when squared = 9.  That means double
the surface area, and double the volume for each vertical inch of fuel height.

Your work with the street vendors is highly commendable, and is ample 
reason for
your intended experimentation and possible product adjustments.  But for
residential cooking, I doubt that the fuel container should be much 
larger than
the 6 inch diameter.  (Please show that I am incorrect.)

I believe that the Agni design has an internal chimney ("riser" before the pot
position) that is taller than is necessary.  I have one here.  It is 15 inches
tall, as I recommend.  But you also have an additional 3 to 4 inches of
vertical movement of the coupler, giving over 18 inches rising if the fuel
canister is short, or if the canister support is higher than need be.  You can
observe the flame height when in operation.  If the riser is tall (with 
greater
draft), the flame height is shorter (more intense mixing and burning).  If the
riser is shorter with less draft, the flame height is taller.  And there needs
to be a balance between riser height and draft to have the flame at the right
height.  This is all complicated further by the need for high heat and low
heat, which is mainly controlled by the amount of primary air allowed 
to enter.

Therefore, if you can get the combustion you need by having a slightly shorter
internal chimney, you can increase the height of the fuel canister.

Also, the grate inside the Agni fuel canister is perhaps an inch too 
tall.  The
grate can be resting on the top of the air inlet duct.

Also, the Agni has a platform to hold the fuel canister in place.  You could
lower the platform maybe 3 inches.

Putting all of these height changes together, you could gain perhaps 6 
inches of
additional height in the fuel canister.  The current available height for fuel
is 5.5 inches, so you could double your fuel capacity and therefore 
double your
"burn time" for one batch of fuel.  I have made numerous prototypes with tall
fuel canisters even 12 or more inches high.  In general, height is NOT a
serious limiting factor to proper pyrolysis.  The reason I have not continued
with tall fuel canisters is that they can become awkward when emptying out the
char.  In Cambodia in 2006 we made a great forced-air TLUD with two heights of
fuel (about 9 inches and about 16 inches in containers slightly taller) 
and two
diameters (4 and 5 inches).  The tall units were difficult to empty, but that
was partly because the canister was inside another cylinder and had to be
lifted out.  You might have much better results with the single-walled fuel
canisters that you are using because you can add another handle for better
lifting and dumping.

So, double the available height and double the surface area (8.5 inch 
diameter)
and you will have FOUR times the fuel capacity.  Awesome!!!!!  We await 
results
of your trials, whether successful of not.

An additional advantage of the larger fuel containers is the ability to 
use more
"stick-type" fuel.  You (the Karve's) are well informed about placing small
diamter sticks vertically into the fuel canisters of TLUD cookstoves.  In the
larger canisters, those sticks can be longer and more of them (but do not
increase the diameter of the sticks that are down low because the time needed
to pyrolyze the centers increases rapidly).  All readers:  It is quite
important to not allow for large air spaces between the fuel pieces.  I 
like to
place wood chips or wood shavings onto the top of the loaded sticks, 
then shake
it to have it fall down into the spaces, add some more, shake again, until it
seems "full".  If you do it incorrectly, the resultant fire will not be 
optimal
and will tell you to try again until you can get it right.
>
> Another question is regarding the fire control arrangement. At present we
> have a small plate which can be moved to control the air inlet at the bottom
> of the fuel holder. But that too is a bit cumbersome if one is standing up
> to cook. Is there any way we can control the fire nearer to the top of the
> stove?

I do not know any easy solution for this difficulty.  I have made fuel
containers with an upward-curved neck for the pipe with the primary air, and
have even used that neck as part of the handle.  Problems are cost increases
and making sure there are no air leaks.  If the air channel is in contact with
the fuel canister, there can even be some preheating of the primary air, but I
do not have any evidence of significant improvement by doing that.  
Perhaps you
can find a manufactured solution that would be a nice addition to this stove.
>
> Would like to know if you have done any experiments regarding these two
> issues, before we start our own experiments.

Those are all the experimental results that I can think of for the two 
issues of
fuel container size and position of the primary air entry.  I hope it helps.

Please tell us more about the numbers of Agni TLUDs sold and how they 
are being
used, plus any stories about fuels, etc.

Paul
>
> Thanks and regards,
>
> Priya
> --
> Dr. Priyadarshini Karve
> # Project Co-ordinator, Appropriate Rural Technology Institute, India
> (Please visit: www.arti-india.org)
> # Director, Samuchit Enviro Tech Pvt Ltd, India
> # Co-Editor, 'Shaikshanik Sandarbh', a Marathi bimonthly on science and
> education




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