[Stoves] is this new?

[Crispin Pemberton-Pigott]

Dear Ron

 

>[RWL2a:  I am not sure I can agree with this last thought.   Such a multi-purpose stove would certainly be very helpful in education and for research purposes.  

Indeed when I train people in stove design principles I start with an open fire, then add one Vesto part at a time demonstrating how it changes the fire and its controllability. When all the parts are in place the fire is controllable. Once the trainees understand how the various controls work they can design their own stoves using any group of features from open fire to Vesto (there are 5 stages).

>But I assume that if a buyer is most interested in making char, that features that do other tasks and that are even detrimental to making char are not necessarily desirable in the char-making stove market, especially if not coming for free.]

Char making requires the most features.

>RWL2b:  Not sure when you joined "stoves" - but the early char-making stove discussions (>1996) were mostly (all?) natural draft.   The intent then was to make char more sustainably than as mostly practiced in the bush - for later use in char-using stoves.  Biochar and blowers (as I recall) came much later ]

OK. Interesting. Has anything been demonstrated showing that char made in a stove is a reasonable fuel in a charcoal stove? I can add that in December I ran the BLDD6 downdraft stove at the SeTAR lab (very successfully) with fully coked left-overs from an mbaula fire as fuel. It was added to the new coal, meaning that the fire was started with coal that had some volatiles. It is nearly smokeless from start to finish. My point is that crummy over-coked fuel can be burned if the fire is already going. If the stove char turns out to be difficult to light, that would be a way of using it as fuel anyway.

>Your objective seems to have been well met - especially with the reduced amount of secondary air in the latest mods..

>Any way to guess what the change in measured efficiency would have been with the old and new hole arrangements?  I am assuming more than half the secondary air holes were deleted (?)

The number was reduced from 80-something to 73, as I recall. 

> Also, I am not understanding the 35 and >60% values.  

The thermal efficiency when cooking Is 35% when cooking with wood on high power. The thermal efficiency on low power when burning small amounts of charcoal is in the 60’s. This typically happens if you let the fire burn out to the very end. The efficiency of a lot of stoves rises a lot as the power drops on condition that the excess air is self-limiting. The easiest way to get that is the have counter-flow preheating of the primary air (which the Vesto has).

>[RWL2d:  Agreed - but I am asking a different question - trying to alert others to how little primary air is needed when making char.   

It is extremely small and even small leakage allows in too much to make really large % yields from the initial wood.

>Way down the road we might see char produced at different temperatures go for different prices - but there also are presently proponents for a wide range of temperatures.

Interesting. If they will take, and the raw fuel is available a market will develop.

>I cannot endorse the idea of retorts - which often have even less control over production temperature than do most char-making stoves.   

Then it is not a good retort! That is obvious. Make a good retort and you won’t have the problem. We can’t blame an entire class of product because someone did a poor job of building one.

>Runaway pyrolysis (uncontrolled final temperatres) is a common problem with any retort, given the exothermic character of all char-making.    

Well, build it so it does not do that. There are lots of commercial retort systems (typically with a linking gas pipe to get the initial reaction going).

>It is not necessary to lose much char in a char-making stove.  With the Lucia, there is not even any oxygen ever traveling through the fuel bed.]

Wood contains a lot of oxygen so there is no air needed to make char. There is still some loss of carbon as it leaves in gaseous form.

>[RWL2f:    Not yet completely understood.  I believe that virtually every char-making stove (Lucia excepted - but it is not a TLUD) meets the standard of your first sentence.  So I assume the first sentence means radially (usually inward ,  in a few cases also outward;  not talking here about vertical flows?)

To get to the bottom the secondary air preheating tube, the air must travel vertically downwards, being preheated as it goes. This is taking place continuously in the chamber below the primary air controller. This counterflow pulls ‘backwards’ on the gases rising in the chamber, self-limiting the flow rate. If you compare this with, for example a Rocket Stove, it passes perhaps ¼ of the total air into the chamber.

>And the self-regulation exists similarly in all TLUDs (always more secondary air if more primary air).  

No, that is a different case altogether. If the secondary air can enter at the bottom (normally it does) then it is a straight upward path to the top. The Davies packed bed gasifier is a good example of this. The outer metal sheath has an opening at the bottom and another at the top directing the hot air at the top of the pyrolysis bed. The air does not enter at the top and start heading downwards, heating as it goes.

>Marc's questions relate to the downward flow of secondary air when the primary air is all blower-supplied.  That topic still seems noteworthy - and not (?) in the Vesto?

The air entering a Vesto stove travels downwards for both primary and secondary air. The primary air is sometimes preheated to as much as 250 C before entering the bottom of the chamber. The secondary is re-heated after that point between the air tube and the combustion chamber (a conventional effect) to about 500 C (max).

>[RWL2h:   This is a very important design topic.   I gather that the Envirofit folk are stating they have a new (NASA developed) stainless with much longer life (and I think no means of keeping the flame from the wall).  

I believe it as an aluminum-oxide alloy invented at Oak Ridge in the ‘80s for which they are still finding applications.

>I support your thoughts on using ceramics - but (having tried) know these also have problems.  Any materials specialists reading this can provide a lot of help to the stove community.    

The paper I wrote for the POCA project covers the most important aspects, still not widely accepted in the stove community which seeks light, insulative ceramics for assist initial high temperature effects (reduced smoke early in the fire).

>The (interior cylinder) approach that Marc has described should have the potential to lower the chimney steel temperature a lot (?) as the secondary air pre-heating removes wall heat.  I presume that the metal in the fuel area is not as big a problem - and wonder if anyone reading this can also report, as Crispin has, on wall metal life.  I personally never ran a design long enough to learn anything on the life of tin cans.    Ron ]

If you use metal you have to cool it at the back. A Rocket stove could easily be made to do that by eliminating the air passing under the fuel and using the chamber (empty) at the back to create preheating for primary and secondary air. This was employed in the STOvs Cooker in Senegal and the Lion Stove (Libubhesi and Tau) Stoves. None of those are metal stoves, but they still work the same way. It renders a Rocket stove into something close to a Vesto with a reduction in excess air, preheating (which can burn damp fuel) and gives more controllability.

I showed a side-feeding Vesto for long sticks at ETHOS one year. It is in Tami’s lab in Illinois. Paul has seen it.

Gotta run

Regards

Crispin

 

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