[Stoves] Seeking feedback on a mixed-materials TLUD

Ronal W. Larson rongretlarson at comcast.net
Mon Jan 12 13:43:55 CST 2015


Bodie, cc List:

	Congratulations on getting as far as you have gotten.  I went to the two web sites (I recommend to all) you have below and can see you have been doing all the right things.

	Few thoughts inserted below, and in your list of questions


THE AMRITA STOVE:
A Front-Loaded Natural-Draft Top-Lit Updraft Gasifier

Objective

The guiding principle behind the Amrita TLUD is to design an improved cookstove that can be fabricated by the user at no cost. The design is similar both in materials and construction to the traditional “chulha” widely used in India, which augments its social viability. Additionally, the stove is designed to produce biochar, a valuable co-product of pyrolytic combustion.

	RWL1:   All good, valid objectives.  I am most concerned about #2 - looking like a traditional chulha.  Most TLUDs have top loading of the fuel or fuel can.  But maybe you can make the front load work.

Materials and Construction

The stove body is fabricated by hand from a mixture of clay, ash, and fiber. The clay provides cohesive strength, the ash serves as insulation, and the fiber provides tensile strength. The combination of these three with water produces a workable mix which becomes very durable and moderately insulative when dry. A hollow chamber around the combustion chamber/riser is packed with ash for insulation.

	[RWL2:  All valid.  Some work has been done on loading the clay with sawdust and/or charcoal to get a lower density.  I can’t see the configuration of the last sentence.  That riser looks rather short.

The primary air enters through 25 holes about 0.6 cm in diameter, evenly spaced across the bottom of the fuel canister. Secondary air enters near the bottom of the fuel canister under the door (area is about 50 cm2) and through a hole in the back (about 30 cm2). This air rises through a 1 - 2 cm wide space between the can and the stove body and then is forced through a 1 cm wide gap at the top of the canister where it mixes with the wood gases. The ratio of primary to secondary air supply is approximately 1 to 6 by surface area.

	RWL3:  I think you can cut the ratio of air supplies down closer to unity.  There is a lot of flow resistance in the fuel can.  The 0.6 cm holes can be smaller.  Repeat on the riser normally taller than you show.  One way to use the shorter riser is to strive for swirl;  see work of Kirk Harris in last few months - a fan blade shape (not a fan).
The fuel canister is a large food can (20.3 cm by 15.1 cm), and the combustion chamber/riser is a smaller food can (12.7 cm by 9.4 cm). Primary air enters through a similar food can in the front, modified into an oblong oval shape.

	[RWL4:  I’d try a food can taller than 12.7 cm.

	I suggest a good bit of primary air is also getting in via your large secondary air inlets - unless you have a really smooth surface for the fuel can to sit on.  Maybe that “sit-on” surface could be metallic or ceramic.

	The oblong food can could be replaced with an all clay aperture that is plugged with a wedge.  It is not clear how you are plugging that oblong - but you need a ver good plug to control primary air.

	(more below)
	
Designed by Bodie Cabiyo for Amrita SeRVe, www.amritaserve.org. For more information, contact bodie.cabiyo at gmail.com.

  

Operation

The fuel canister is first packed with small pieces of fuel (about 2 to 4 cm), and a layer of flammable material can be placed on top to catch the fuel. Otherwise, a few pieces of fuel soaked in kerosene can be placed on top of the fuel bed. The stove is started by top-lighting the packed fuel canister and placing the canister inside the stove body. Whether or not the door is immediately put in place doesn’t seem to make a difference. Once the stove starts to burn strongly, the temperature can be slightly moderated by blocking the primary air inlet.

	[RWL5:  The fact that the secondary air door and the “slight moderation of the primary air inlet makes no difference is of concern.  

Performance

Starting: Depending on the flammability of the fuel, starting can be difficult. With coconut shells as fuel, it typically takes 2 to 4 minutes of weak, smoky combustion before the flame reaches a point where it is sufficient to burn the smoke.

	[RWL6:  Can you expand in first clause?  What fuels give no problem?

Mid burn: The stove produces a very strong flame after the fuel has equally lit and the can is starting to heat up (about 4 or 5 minutes). With high-calorie fuel (i.e. coconutshell), the flame can reach up to 8 inches above the combustion chamber/riser in a vertical column. With low calorie fuel (i.e. grass and cow dung), the flame is much smaller but still vigorous. Without a pot, the flame produces no visible smoke and does not cause stinging to the eyes or produce smell. When a pot is placed on top of the flamespire, however, puffs of smoke and a very thin stream of smoke are produced periodically.

	[RWL7:  All these caused I think by too much primary air.  The last sentence problems presumably caused by wind,
Transition: If excess primary air is supplied near the end of the burn, the transition is almost entirely smokeless. The charcoal will continue to burn slowly and cleanly if left in the stove (though this is presumably damaging to the fuel canister).

	[RWL8:  Again evidence of too much primary.  Now your secondary is not being used at all.  The char is combusting from the bottom.  For those of us believing in char, the cook should be making money with this valuable commodity.

Efficiency: The stove will bring 2 L of water to a rolling boil in 12 minutes when placed on the burner after the fire has caught evenly. The turndown ratio, however, is very poor. At present, the stove essentially has two heat settings: high and very high. Even with tiny amounts of primary air, the flame is still very strong. Total burn time with coconut shell is about 30 minutes and 45 minutes with top-loading (at high heat). The stove yields about 20 to 25% biochar by weight if the char is removed. At any point during the combustion cycle, the stove body surrounding the combustion chamber/riser is cool to the touch.

	[RWL9:  You may think you have “tiny amounts of primary air”, but try a design where the 0.6 cm holes are 0.6 mm (maybe too small, but anything to further cut down on primary air).  There is excellent recent work on the list from Julien on the impact of these primary air holes.

	Really glad you wrote in.  Best of luck.
Designed by Bodie Cabiyo for Amrita SeRVe, www.amritaserve.org. For more information, contact bodie.cabiyo at gmail.com.

  





On Jan 12, 2015, at 10:06 AM, Bodie Cabiyo <bodie.cabiyo at gmail.com> wrote:

> Hi Fellow Stove Enthusiasts!
> 
> For the past few months I've been working on refining a TLUD design (attached) which is fabricated from a mud mixture and recycled cans. My intention is to design a efficient, biochar-producing stove that can be fabricated by the user. 
> 
> I've gotten the stove to a point where it burns well under certain conditions, but I'm facing some serious roadblocks that I can't seem to surmount on my own. I would love your feedback.
> 
> Here's the key problems I'm trying to solve:
> Weak starting. With my primary fuel (coconut shells), it takes about 2 - 4 smoky minutes for the stove to really get going and start burning the smoke. 
	[RWL10:  Some ideas above.  Anything like pine needles available for starting?
> Poor mixing. A couple of weeks ago, Crispin described a "textbook example" of poor mixing characterized by a column of flame that produces smoke when the pot is placed over it. I presume that with better mixing, the majority of the smoke would be combusted by the time it gets to the pot surface. 
	[RWL11:  You all need vertical distance to get mixing.  Try Kirk Harris’ small fan blade to achieve swirl.  Can sit right on top of the fuel.  You might also consider filling the fuel can only 70-80% with a set of secondary air holes or slits at that height.  Most of use doing TLUD designs have such holes, I believe
> Poor turndown ratio. As mentioned in the document, the stove has two settings: high heat and very high heat. Even with a tiny amount of primary air space, the heat stays very high. 
	[RWL12:  Repeat that I believe your primary air supply is not tiny.

> I'm eager to hear your comments! What do you think? Can I perfect this stove, or have I created a beast that can’t be tamed? :)
	[RWL13:  Many things done well here.  Congratulations on doing a lot of work before writing in. I strongly endorse trying to make a do-it-yourself clay stove - to get the cost to near zero.  You might try a parallel approach with a top-loading design - but I recognize that you are trying to make it look similar to the traditional - and that may have considerable value in India.

	I hope others will chime in.

	Ron
> 
> 
> Thank you in advance!
> 
> Bodie
> 
> PS. You might notice the absence of a proper WBT. I haven't bothered yet because I feel that I should address the above issues first. If it would be helpful for assessing the design, I certainly can give the results of a WBT.
> 
> -- 
> 
> Bodie Cabiyo
> 
> Professional: carbonalternatives.org
> Personal: concentricchange.wordpress.com
> 
> Food for Thought:
> "A mountain is composed of tiny grains of earth. The ocean is made up of tiny drops of water. Even so, life is but an endless series of little details, actions, speeches, and thoughts. And the consequences whether good or bad of even the least of them are far-reaching."
> - Sri Swami Sivananda
> <Amrita TLUD Overview.pdf>_______________________________________________
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