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

[Ron]

Julien

   I am quite certain the problem is in your statement:

    "I put the can on top a paint-can lid which blocked off all
> 
> primary air."

    It was not Crispin's intent that you operate that way, only that you have full control over the quantity of primary air..  In your run, "primary" air was reaching eventually all the way to the bottom by coming through the secondary air circular gap.  You were always producing CO2, not CO and H2.

   Next try the "adjustable aperture" and start with it wide open.  The straw, if it will work, will have to be neither too loose nor too tight.  The problem with too loose is a falling ember getting to the bottom - when you will again generate CO2.  Trying for a "helical" fuel bundle, again of the right density, might help on that.  

   I once operated successfully with raspberry stalks (mostly with small vertical wood "twigs") - and eventually learned the right packing density.  If anyone has pyrolyzed with wheat straw successfully, this is a good time to speak up.  Maybe you'll just have to turn them into "pellets" which inherently have a better shape.

   Nat Mulcahy with his Lucia might pull this wheat straw geometry off, as he does not have a traveling pyrolysis front (or anything like an interior flame).  I disagree with how Crispin described its operation - which is not a retort. It works through downward flowing interior hot N2 and CO2 (not TLUD, but TLOD).  Using Venturi principles, those gases, now also containing pyrolysis gases, flow upward on the outside with secondary air ( not the oxygen in the fuel), getting premixed before combustion only at the top.  He needs a perfect flame "cap" at the top to pull this miracle off.

Ron

On Apr 4, 2013, at 7:50 PM, Julien Winter <winter.julien at gmail.com> wrote:

> 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
> <PaintCan_PackedWithStraw_04b.jpg>
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