[Stoves] The Art of Using Grass Bundles in TLUD Stoves

[Julien Winter]

Hello stovers;

Crispin offered me the following advice for burning wheat straw in a
gasifier cookstove:

> "It sounds like you could afford to reduce the primary air supply. If you have things right you should be able to turn it off completely. If you are able to generate a condition that is primary-air-starved, you can then regulate it."

> "It is not exactly true that a TLUD (or a gasifier or semi-gasifier) has to burn from the top down only. As long as the control over the primary air is complete you can create a good burn. In order to create a decent gas you have to have 'smoke production' so some air has to get into the fuel unless you have a self-heating retort like a WorldStove product. They run on oxygen in the fuel only, basically. "

> "Pack in the fuel if you can."

Following that advice, I packed the paint can with as much straw as I
could.  I put the can on top a paint-can lid which blocked off all
primary air.  Then I put a on a handful of pellets soaked I barbecue
lighter,  put on a chimney with a 4 mm gap for "secondary" air, and
threw on a match (see attached photo).

After the wood pellets burned, the can of straw slowly burned away for
a couple of hours.  In the last half hour, I allowed full access of
primary air through the twenty-five 4 mm diam. holes in the bottom.
There was a slow, but steady production of smoke throughout the burn.
At the end, when all smoke production stopped, I smothered the fire.
The can was full to the top of charred straw (which will, of course,
compress down to a smaller volume) and some ash in the bottom.

There was a gentle production of heat throughout the burn, but I don't
think it would boil water.  To get more heat, I need more primary air.
 I can adjust that easily using putting an adjustable aperture on a
paint can lid upon which the can sits.

Swanky, lab bench TLUDs burning straw with forced air at 0.18 kg/(m(2)
s) reach temperatures up to 1200 C (van der Lans et al., 2000;  Zhou
et al., 2005).  I don't suppose anyone knows the maximum temperatures
for straw in a choked paint can, do they?  Any guesses?

Periodically during the burn, I tried to ignite the smoke with a
propane torch, but failed.  Are there any opinions on why it didn't
burn, and any solutions?

I am working on wheat straw as a surrogate for rice straw in
Bangladesh.  It would be nice to get unpelleted rise straw to work in
their cookstoves because it is a free source of fuel.  Whatever fuel
is used, my colleagues and I will work with stove owners on uses for
their biochar.

Cheers, Julien



van der Lans, RP; Pedersen, LT; Jensen, A; Glarborg, P; Dam-Johansen,
K.  2000.  Modelling and experiments of straw combustion in a grate
furnace.  Biomass & Bioenergy  19: 199-208

Zhou, H; Jensen, AD; Glarborg, P; Jensen, PA; Kavaliauskas, A.  2005.
Numerical modeling of straw combustion in a fixed bed.  Fuel  84:
389-403
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