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

[Julien Winter]

Dear Paal and Crispin; and Stovers;

Thanks for your suggestions on how to use bundles grasses as cookstove fuel.

I gave it a go in my natural draft, 3.87-L paint can TLUD with a 15 cm
tall, 8 cm Ø chimney fitted to the paint can lid.  Secondary air
entered through a 4 mm gap under the lid.  Primary air entered through
twenty-five 4 mm Ø holes in the bottom of the can.  I used well-dried
wheat straw, and started the fire with a dab of barbeque lighter.
Ambient air was 4°C, and a dog was used as a windbreak.

Bundling the straw was a definite improvement over filling the can
with loose straw, but not an unqualified success.

Lightly-packed, loose straw produced a smoky shouldering fire.  There
were preferential flow paths of the primary air through the fuel, so
soon after the TLUD was lit, the ignition fire followed the best path
to the bottom of the can.  The bottom fire then consumed all the
oxygen, and resulted in shouldering in the unconsumed fuel above which
produced unburned smoke.

Loose bundles of straw that occupied about 75% of the cross-sectional
area of the TLUD produced a hot, steady gas fire, but with some
unburned smoke (see below). The ignition fire also bypassed fuel, but
there remained a clear path for primary air to reach flaming pyrolysis
higher up in the fuel.  The gas fire stayed alight, but there was
periodic unburned smoke, symptomatic of patchy shouldering.   The
straw was burnt out after about 5 minutes.

Tightly bundled straw, and bundles that fully occupied the TLUD, shouldered.

As noted above, I didn't get a completely smokeless fire with a loose
bundle of straw.  I think this relates to my wheat straw coming
higgledy piggledy from a bale.  With grasses it could be important to
primary air flow that the bundles have stems at the bottom and leaves
at the top.  Also, grass species vary considerably in size and
leaf/stem ratio.  Tropical grasses with coarse stems likely present a
denser fuel that is better aerated, hotter burning and longer-lasting
than low bulk density, cool-season grasses.

Now with some experience under my belt, I will wait for a good rain to
wash the sand and salt off the roads, a warm spring day, and get on my
motorcycle to see if can find Alex English.

Cheers,
Julien


-- 
Julien Winter
Cobourg, ON, CANADA