[Stoves] Bangladesh TLUD (was Re: No subsidies in TLUD char peoduction

[Julien Winter]

Hi All;

Mahbubul Islam and I have tried TLUDs cast as a single concrete cylinder.
You can see photo of a prototype here: http://www.biochar-bangladesh.org/
on the home page.  The concrete is made using Portland cement, sawdust,
sand, and ground biochar.  It is not refractory quality, so the interior of
the reactor is lined with clay, which can be replaced.  In another design,
there is a concrete ring above the concrete reactor with a space between
the two created with 12-16 spacers.

I have also done some experiments with burners, and you can see a video of
that here:  https://youtu.be/QmnUgj75TF0

We haven't done much work on this type of TLUD in over a year, because we
have concentrated efforts on the metal reactor cylinder in the Akha Stove.

What we really needed to develop concrete or ceramic reactors is research
money to pay for gas analysis and materials.  There are many design
parameters to adjust, and without gas measurements, we can't justify and
explain our decisions.  In the mean time, we have other things to work on.

There are a couple of drawbacks in the designs illustrated above.  They are
vulnerable to ambient wind, we don't preheat secondary air, and there are
no pilot holes in the reactor walls.  Pilot holes can be made, and
protection from wind could be designed.

The reactor bodies will be a heat sink.  I am not sure how big an issue
that would be, becaue gas flame is a the top of the stove, and heat in the
center of the char above the 'flaming pyrolysis' moves predominantly
upwards and not sideways, because air movement is upwards, and char is a
poor conductor of heat sideways.  (I have measured the horizontal profile
of temperature in metal TLUD reactors, and the temperature only drops in
the 1-2 cm near the reactor sidewalls.).

For burning pieces of wood more than 1 cm thick, some holes in the
sidewalls of the reactor would provide pilot lights to prevent accidental
shut-down of the reaction.  I belive this is important for thicker pieces
of fuel than 5 mm pellets, because it takes longer to reach pyrolysis
temperatures at the center of thick pieces of fuel.  That means that when
the reaction is turned down, char combustion on the surface of particles
becomes important to keep the primary pyrolysis reaction going.  If the
char combustion goes out because of insufficient oxygen, then it is tough
to get the reaction started again if a lot of the partially pyrolyzed wood
has charred surfaces.  Some holes in the side walls of the reactors keeps
some embers glowing, so the gas flame stays lit, and maintains draft.  The
primary air can be turned-up if need be.


We have to design these stoves with pilot lights so that it is not that
easy for the gas flame to go out accidentally.  If that happened in
semi-enclosed cooking spaces, the air would fill with smoke, and the cook
would get angry.  We are going to sell stoves, we need happy cooks.

I think it is important to work on ceramic and concrete reactors.  I needs
a research grant to do it properly.

(Just a side comment.  I have observed that the thicker the fuel the more
primary air is required for the primary pyrolysis reaction.  I believe that
this is because of the time it takes for the center of thick pieces of wood
to reach pyrolysis temperatures.  Because the time it takes to completely
char the wood, the heat from flaming pyrosysis is not sufficient, and heat
from char combustion becomes important.  Thus a higher flow rate of primary
air is required for a TLUD loaded with chunks of wood than pellet fuel.  As
fuel thickness increases, the yield of biochar decreases.)

Cheers,
Julien.

-- 
Julien Winter
Cobourg, ON, CANADA
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