The Lucia Stove’s Coaxial Gasification

Since my post about pyrolyzing biomass stoves, I’ve been trying to understand how the Lucia Stove works.  A lot of the pyrolyzing stoves are “TLUDs” (Top Lit Updraft).  My understanding was that TLUDs really only work well in batch mode, while the Lucia Stove promises that more fuel can be added during a burn run.  Unfortunately the WorldStove site and YouTube Videos contain just enough information to confuse me, so I went digging around the Stoves Mailing list and finally found what I was looking for, in an explanation by Nat Mulcahy to some questions posed by Clement Davis.

I’m reproducing it here to make it easier to find in the future.  I’ve also cleaned up some of the formatting.

in answer to Clement’s questions about the LuciaStove

nataniele at aim.com
Wed Feb 11 09:42:31 CST 2009

Hello Clement,

Thanks for the great questions. I’ll do my best to answer them all; if you are like me the more I learn about anything the more I want to ask, so please do not hesitate to ask if more question arise.
I’ll try to answer in order:

1. “Please explain ‘coaxial gasification'”

Coaxial Gasification differs from stratified gasification normally found in Imbert, TLUD (Tom and Paul), or Downdraft type gasifiers. In traditional gasifiers, either from top to bottom or bottom to top you find a series of layers (the order of which vary based on design) but usually include Fuel, Flaming Combustion (Reed) or Combustion, Drying, Pyrolisis, Reduction, Oxidation, and Ash. These are the “solid components”, and then there are the gaseous zones which also vary from gasifier to gasifier. In the case of Kelly or Cross Draft, the zones at times could be defined as partial spheres but still have the various components.

The layering in a LuciaStove differs in that the same elements are present but that they are arranged in a coaxial fashion. Combustion occurs in a much reduced form, and only on the outer most perimeter of the top of the combustion chamber. The Pyrolysis zone occurs in a central vertical column (which because of a series of controlled vortexes, also acts as the filter and cyclone) and all the rest happens in a very thin outer cylindrical shell just below the combustion ring.

The primary advantages of the LuciaStove in Coaxial Gasification mode are: improved emissions, more surplus gas can be produced per unit of feedstock, fuel can be added at any time, does not have to be a closed chamber, adjustable flame, more complete combustion of fuel, and can deal with changing fuel size and type during operation with no adjustments.
In the Pyrolytic Mode of operation there is only combustion of the feedstock during the first 40 seconds or so of operation, after that any feedstock in the chamber or added to the chamber is simply pyrolysed by the heated inert gases, and the char produced never gets a chance to combust.

The Primary Advantages of the LuciaStove in Pyrolytic mode are: Controlled char production with out the need for quenching, controllable char qualities (Ph, pore size, etc), even lower emissions because the char is used as a filter.

2. “Is the stove drawing gas from the combustion chamber into the gap to meet the incoming 2nd air?”

Yes, the lower part of the chamber is designed in such a way that gases are extracted and no air can enter the combustion chamber if not from the top.

3. “Can you confirm the type of blower used to provide the forced air (cfm+watts)?”

The LuciaStove can work with almost any blower or fan and in some applications it works without a fan. The cfm is not as critical so long as pressure differences are maintained. All fans have both flow vs pressure curve and the pressure point is a critical parameter for the function of the LuciaStove.

4. “Is the blower the muffin fan from an old computer?”

While the LuciaStove if often built using muffin fans, the blower used in the demonstration was a cross flow or tangential fan used in our pellet stove, wood stove, and fireplace insert applications.

5. “During the demonstration was the Luciastove running in gasification, pyrolysis or coaxial mode?”

During the demonstration at ETHOS, the LuciaStove was running in Coaxial Gasification mode.

6. Is there a need to “change parts” to make this stove run in different modes?

No, there is not a need to change parts. All LuciaStoves can function in either mode; it takes some practice which is why we offer training classes for groups that do humanitarian work.

7. “Was there any fuel added during the run?”

Boy howdy was there, a veritable cornucopia of feedstock options to the delight or any pyromaniac or stover! Thanks to Paul and Charlie we had all sorts of nifty different things to toss in, including, bark, wood chunks, pellets, small branches, even snow-soaked pellet mash as we ran out of fuel the top notch stuff.

8. “Was there any smoke during startup or refuelling?”

Yes, during start up, it takes about 40 seconds for the LuciaStove to become “smokeless” after that, an experienced operator can maintain indefinite smokeless operation.

Thanks for the super questions and a special thanks to Mark, Angie and Andy for putting together such a wonderful conference.

Cheers,
Nat Mulcahy
www.WorldStove.com
www.youtube.com/user/worldstove

Thus endeth the lesson.

17 thoughts on “The Lucia Stove’s Coaxial Gasification

  1. Matt

    Hi there,

    so… is there a way to describe in three sentences how this stove works? I know quite a bit about the batch driven types, just can’t picture the inner setup – as in what goes and / or flows from where to where.

    THANKS!

    Matt

  2. Matt

    Also, what really puzzles me is:

    If the air draws pyrolysis gas out of the inner chamber via the venturi effect in the slots below the inner cylindrical wall:

    What stops the gas to burn off with the available air while still half way up between the two walls? Why does it only burn once it gets ejected at the top? Why does the flame / reaction not move back downwards into the space in the double wall?

    Is the air content too low for that there, and it needs additional air after being ejected into the top centre where there is atmospheric air? Or is it not mixed well enough there?

    Has this anything to do with the third and / or fourth (!!) wall that is mentioned somewhere as one of the possible configurations?

    I really would like to understand how this thing works. Is there a real comprehensive manual anywhere?

    Cheers,

    Matt

  3. eas Post author

    Matt, I’ve never operated a Lucia stove, and in attempting to duplicate the basic principle using a forced air stove, I’ve been reminded how little intuition I have for fluid dynamics.

    However, I think I understand enough to answer some of your questions. I think you have a basic understanding of how it works.

    One think that might help is to realize that the downward flow of gasses in the inner chamber is pretty slow. Most of the pyrolysis gases rise up and are burned in the flame cap. There is, however, enough downward flow of hot gases to heat and decompose the solid fuel. I think any pyrolysis gas that get sucked into the main airflow is fairly dilute, and gets cooled off a lot, so there isn’t much chance of it igniting between the walls.

    Hope that helps.

  4. Ogeya Mbeo

    Its puzzling to learn so much about Lucia stove from this page. I have been searching through the internet for more information besides the youtube illustrations that doesn’t explain so much.

    I am an interested student in biomass energy conversion technologies from Kenya.

    You have explained quite abit but I still don’t have answers to some questions.

    1. How does stove height affect the pyrolitic process owing that the only source of energy for pyrolitic process is the inert gases blocked by the coaxial flame

    2. Is there need for primary air through the biomass combustion bed?

    3. What are the typical pyrolisis temperatures while using Lucia stove?

    4. How is the pyrolisis affected by non-uniform biomass sizes and distribution? for instance while using irregular agricultural residue as the feedstock.

    Otherwise it has been a great pleasure visiting this website because it has given me an indepth understanding of how the Lucia stove works

  5. Ogeya Mbeo

    One more question.

    You answered Matt, and said that the gases that are pushed out to the external chamber mixes with air and is diluted. Is it that they are diluted that they cannot combust? and have you studied the combustible gas concentration gradient within the pyrolitic chamber. And can you obtain the temperature gradients within the chamber too?

  6. eas Post author

    Ogeya, thanks for your questions. I’m afraid I can’t provide you with good answers for most. I’ve tinkered with the Everything Nice stove design a little, but I haven’t done careful measurements and experiments. I will offer what I can:

    1. I’ve made short versions of the everything nice stove, and tall versions, both worked roughly equally well. It did seem that it might make a difference that there isn’t too big a gap between the top of the fuel bed and the top of the inner cylinder at the start of a run.

    2. No, no primary air is needed, it would actually work against the design and you’d end up with a TLUD

    3. No idea

    4. I’ve used wood pellets for most of my runs, because I wanted to eliminate a possible source of variability, and because I got tired of chopping up twigs and small branches.

    I’d encourage you to try and contact Nate Mulchay, the creator of the Lucia stove. His stove may not be well documented, but he seems to be interested in sharing knowledge.

  7. Brill

    I am a combustion engineer and stovemaker and I have a hard time believing many of these claims. I’ve studied the biochar stove space pretty intensely, and I think this is just a modified TLUD. There is no way any significant portion of gas is sucked out of the inner chamber via the venturi effect. The flow rate of gas is simply too low in natural draft mode. It’s much more likely that the partially occluded holes in the bottom chamber act like the holes in a TLUD. Gas evolves upwards through the inner chamber and mixes with incoming secondary air creating the flame cap. What this stove does well is preheat the incoming secondary air. By having that second annular ring, the incoming secondary air is in close contact with heat. This is likely to improve combustion. However, this is not a new kind of design. The champion TLUD designed by Paul Anderson achieves the same ends.

    Also, in none of these videos is the user ever shown quenching the char. This is because, like all TLUDs, this device is not self-quenching. There is simply no way to prevent an open-topped stove from taking in oxygen.

  8. Matt

    Hi Brill and everyone else,

    been a while, now I’m back by coincidence.

    Note: All of the following refers to the Lucia stove and my replication that I made from cans, and only in pyrolysis mode. I am NOT referring to the everything nice stove, and I am not referring to the Lucia stove in gasification mode or TLUD stoves!

    Brill, I have built a simplified version of the Lucia stove out of cans, just to play and learn. I probably spent about a hundred hours or so and went through about two dozen versions, but in the end it worked. I can assure you that if the geometry is right, especially that of the venturi jets at the bottom which are necessary to “suck” gases out of the inner chamber, there is significant downwards flow of gases in the inner chamber, and it works in pyrolysis almost exclusively. I agree that this is no easy feat to achieve and requires very careful tuning of geometries, but it is absolutely doable.

    Despite the geometry limitations of what I can achieve with just cans (there are much more skilled people out there who can do sheer magic with simple tools and cans, but unfortunately I’m not one of them), the venturi suction and downwards draft in the inner chamber that I could achieve were strong enough to have a fully working pyrolysis mode. A minute or so after lighting the stove (I am usually using wood pellets), there is almost zero glow in the fuel left, with the exception of a very thin cylindrical space that is “lining” the inner wall where there is a bit of glowing in very few places, usually only near the bottom. From looking at it I can say with certainty that at least 95% of the fuel is pitch black, and zero combustion takes place there. Also, after the burn, there is almost zero ash, but almost all the pellets are still there, in the form of char.

    That pyrolysis is actually happening can be nicely observed when throwing a few new – or, even better, slightly wet – pellets into the stove. You can see the moisture coming out of the new pellets in the form of tiny white steam clouds; but these little clouds do NOT rise up from the fuel as one would expect, instead, they “fall” down and get sucked into the gaps between the pellets relatively quickly, and disappear into the fuel – clear proof of down draft. No, they do not simply get thinned with other gases and become invisible and rise; they clearly visible just drop into the fuel.

    Bottom line: It is really the case and actually achievable that the rising air in the double jacket, in passing the venturi jets near the bottom, “sucks” gases out of the inner chamber, which then “sucks” hotter gases from the flames at the very top down into and through the fuel, which heats the fuel up and pyrolyses it. It really works that way; the tin stove prototype sitting on my table here does exactly that, and I am sure with a better geometry – which the Lucia stove surely has – it will work a whole lot better still.

    Limitations of this stove concept that I have observed (which may not be an issue in indoor cooking, but as I am looking for backpacking stoves, this is different for me):

    – If blown out by wind, it is extremely hard to relight it, simply because there are absolutely no ambers to begin with. If not relit within seconds, it usually is almost impossible. Interestingly, this could also be seen as an advantage, because it means that if the stove should be tipped over, there will be a lot of smoke from the still hot pyrolysis fuel, but very most likely there will be no fire caused by it because there simply are almost no embers!

    – If blown out by wind, it instantly smokes A LOT.

    – In pyrolysis mode, the stove can only be operated for a limited time and not uninterruptedly, since there is no really pracical way to remove the char while it is running, and eventually the char will fill the stove so no new fuel can be added.

    – My prototype does not work well enough to be very tolerant to changing fuel quality and sizes. With wood pellets, it works fine; if I add other types of fuel, it often starts to smoke, or even goes out. That in my opinion is not a problem of the general concept, but only due to the imperfection of my specific geometries. (I stopped further work on this stove after I realised that the above issues make it undesirable for backpacking, at least for me. With more work, maybe the fuel tolerance could be improved.)

    – My prototype which is made from a 1 litre paint can does not have a hell of a lot of power output. I can cook on it all right (1 litre of water boils in about 12 minutes, which isn’t too bad), but compared with other (simpler, but gasifying) wood stoves also built from cans it seems that for the amount of wood I put in, I should get more heat output.

    – One thing that I see as a disadvantage with this concept (in pyrolysis mode) is that how you put the fuel into the stove MASSIVELY influences how the stove works – at least with my prototype version, and I think it is unavoidable with this concept. The reason is that the size, the variation of size and the shape of the fuel pieces have a huge influence on how easily gases can flow downwards through the fuel; also, the higher the fuel inside the stove, the slower the gases will flow, too. This means that the fuel and how it is put into the stove changes the balance between the “flow resistance” of gases INSIDE the inner chamber, and OUTSIDE in the double wall. Now, if the flow resistance inside is too HIGH (read: small fuel bits stacked very tightly to a tall height), the venturi jets will not be able to suck a lot of gas out of the inner cylinder, resulting in too little hotter gases following downwards from the upper flames, resulting in lack of pyrolysis rate, resulting in the stove going out. If the flow resistance inside is too LOW (read: unevenly sized fuel bits with large gaps between them, stacked less high), the stove can burn the fuel very quickly, even too quickly; this could be eliminated with a regulating mechanism that would allow the operator to artificially slow down gas flow.

    By the way, I have built versions both with a spiraling “flame cap”, just like the Lucia stove, and with a “straight” flame cap, as in, the upper gas jets just aiming straight at the centre of the stove. I could not identify any significant difference in performance, stability or anything else – except the spiraling flame cap looks a lot cooler. I don’t believe it is significant for the stove’s function, and think it is an over-hyped feature of the Lucia.

    The real beauty of the Lucia lies in its perfect balance of all other geometries, leading to perfect balance of flows and pressures inside and in the double wall.

    I love the idea of cooking carbon negative by using pyrolysis, but at least for outdoor adventures, for now I have to give up on that idea because I could not make a pyrolysis stove that is practical and reliable enough for the circumstances in such an application. That’s not to say it can’t be done; I just ran out of patience to work my way through dozens and dozens of cans with knife, pliers, drill and other tools. Maybe I’ll try it again one day.

    One thing seems clear: Any pyrolysis stove will always be much more complex, bulky and heavier than a gasifying stove of comparable power output. Unfortunately, that is a big drawback for backpacking trips.

    That’s all I can think of for now; hope it helps someone!

    Keep experimenting!

    Cheers,

    Matt

  9. Matt

    Clarification: I have never had the chance to lay my hands on the actual Lucia stove. My “replication” is based on the information about the Lucia stove that I could get from the internet.

    Cheers,

    Matt

  10. Matt

    Brill,

    just read your post again and feel the need to say this:

    From my own experience with my can replica of the Lucia stove, if designed right, the “flame cap” does indeed close off the top of the stove well enough to prevent air or oxygen from entering the pyrolysis chamber.

    Perfect size and spacing of the jets and ideal gas velocity provided (!), the separate flames coming out of the jets at the top merge together instantly after they get ejected from the jets, and create a barrier for air entering from the top.

    I know it sounds hard to achieve – and believe me, it is, as it takes a hell of a lot of tuning and working out the right geometries! – but it works.

    But…

    No but! I really does.

    How? I’ll try and explain.

    A flame produces gases.

    A part of the gases that are the product of the flame cap get sucked into and through the fuel.

    Now here is the trick: The suction at the bottom of the inner chamber, created by the venturi jets, must be JUST right.

    If it is too little, not enough hot gases from the upper flame get sucked into the fuel, and the fuel doesn’t get pyrolysed, the stove goes out.

    If suction inside the inner cylinder gets too much, the flame “lid” won’t “hold”, and air will get sucked through it in some places and into the inner chamber, turning the pyrolysis zone into at least a partial combustion zone.

    This is the difficult thing to achieve, and it is even more difficult with varying fuel quality, quantity, size, and size variation. You can throw in whatever you want and it will get pyrolised (at least as long as the process is going), but your choice alters the flow resistance inside, and therefore alters the whole process. If you do it wrong, it goes out. (This may be less of an issue with larger versions, I’m not sure; I only went up to the size of a 1 litre paint can.)

    Note that the Lucia stove is built and offered in an almost infinite number of geometry variations. In my opinion, this is to optimise the geometry and the balance as described above for the type of fuel that the user is likely going to use most.

    It also means that, as far as I can see, the Lucia stove is probably not too good at dealing with a huge variety of fuels – unless you keep changing its geometry depending on what you burn.

    Matt

  11. Matt

    One more kind of important thing I forgot to mention:

    All my prototypes were completely without any fan or other forced movement of gases.

    I assume that using a fan would make the whole stove a lot easier to design, and possibly also much more tolerant to varying conditions such as fuel size and quality. But I never went that way because in my backpacking philosophy, the need of a fan is undesirable.

    So I have become a stover through backpacking, and because of that have probably chosen the most difficult way to build and operate pyrolysis stoves.

    It was hard, but I have learned a lot, which also helped me to improve my much simpler gasification and other type wood stove prototypes, which are a lot less interesting and only pyrolyse a small part of the fuel, but are also much easier to handle on the trail.

    Matt

  12. Bernard

    I think this is all PR BS. The LuciaStove does not have any advantages over other coaxial designs. Quite the contrary. It does not provide insulation in the layer where the fresh air flows upwards.

    “the lower part of the chamber is designed in such a way that gases are extracted and no air can enter the combustion chamber if not from the top.”

    Not true.

    It in fact combustion gases would be extracted (flow downwards in the center and then into the outer layer) then they would create a major heat loss – in addition they would also condense at the cold outer case and therefore a major source of energy would be lost. Even if they would not condense – they would be too cold and diluted to burn at the top.

    The major problem with almost all designs is that with daily use the combustion chamber even if made from stainless steel burns out after six months because the steel is not heat resistant. Only super alloys e.g. FeCrAl/Kanthal are useful at the heat source. Isn’t it funny that you cannot just buy the thing in small quantities < 500 and try it out?

  13. Matt

    Bernard, you are talking from a theoretical point of view.

    Maybe read my previous posts, because I have the impression that you haven’t.

    I have never had an actual Lucia stove in my hands, so I can not speak for that. I have however after making some dozen prototypes and a lot of hard work successfully recreated its functionality, most importantly, the actual downdraft inside, and pyrolysis, both WITHOUT the use of any fan.

    With that background, I believe I have the right to say something about what is and what is not true about the Lucia stoves:

    From my observation, you are right that there is quite a significant heat loss through the bottom of the inner chamber, where the hot gases are drawn outwards. The heat loss is not too much of an issue though because most of the heat gets fed back into the system when the gases are burned off at the top, some cooling of the gases on their way up inside the outer jacket aside.

    Condensation is only experienced when there is not sufficient heat in the outer jacket, which at least in my experimental stoves seems to be no problem. I only have condensation when I use unsufficiently dry fuels. Otherwise, the inside of the double jacket remains mostly clean. The same goes for the gases not burning (or not burning completely) at the top: From my observation, that can happen, but only when the stove is not hot enough yet in the first maybe two minutes after lighting it, or when the gas mix is too diluted – which means that your geometries are not right, and you have to do more work to achieve a better gas mix.

    The talk about the spirally shape of the gas jets at the top of the stove is either hype or lack of understanding of their own design. The direction the gas jets are pointing at their opening is the only factor that determines where the gases go, and how the flames are shaped. Whatever shape the channel has that leads the gases to the outlet of the jet has no influence on that whatsoever.

    I agree that not being able to buy the stove in small numbers and test it makes it hard for many people to believe that the concept actually works. I think that is marketing strategy – not to hide any shortcomings of the stove, but to prevent unexperienced and untrained “test operators” to ruin the reputation of a product that does work well if used in the correct way. It is also likely to be a means of protecting themselves from quick and dirty rip-offs, which could happen all too easily – once you have one in your hand that works, it’s easy to replicate, no matter how much work it took the developer to get it to this point. So while the strategy of only selling large numbers may suck in some regards, I fully understand it. Whether or not making money with wood stoves for third world applications is a valid business model remains to be seen.

    About your mentioned “burning out” problem I can only say: That applies to all designs, including the many designs that are being used by many millions of people every day around the globe. So what? If it burns out after half a year or so, make or buy a new one. If the design affords you dramatic savings in fuel expenses, it will still be saving you money even if you have to replace parts or even the whole stove regularly. Nothing lasts forever, and once the skills and know-how are there, it is no problem to replace what needs replacing. If that means people buy more stoves and parts from the manufacturer, good for the business model! As long as everyone including the customer wins, what’s wrong with that? No one will ever make the perfect stove that costs nothing, takes any fuel, burns clean, is highly efficient, and lasts forever. To criticise a stove not to be perfect is meaningless.

    Most importantly, I want to say this: While it is understandable that the feature of actual inner down draft is hard to believe and also hard to recreate for most people, I feel the strong need to repeat once again that from my observations this is very much possible EVEN WITHOUT A FAN – I know that because I did exactly that myself successfully with cans.

    It is quite tiring to read comments of some people in different places on the web who are claiming that “it’s not possible”. If you say that, what you really mean is that you can’t understand it, can’t believe it, or can’t do it yourself. Neither of those mean that it is impossible; saying that only means it is impossible for you, at least until you learn more and get better at making stoves.

    Instead of rubbishing a concept that you have not even tried to replicate yourself, why not take a few weekends, a pile of cans, tin snips, a drill, and do some experimenting? You may be surprised at what you find – although if you don’t put in the hours to build your experience and be able to actually make it work, you’ll probably come back here after two or three failed prototypes and claim that you have “proved that it doesn’t work”.

    Once and for all:

    THE CONCEPT OF INNER DOWN DRAFT AND PYROLYSIS IN A DESIGN SUCH AS THE LUCIA STOVE DOES WORK.

    But… NOT BUT.

    IT WORKS.

    Cheers,

    Matt

  14. Goran Steen

    Hallo all Lucia stove speculators!
    I have red this conversation intressted and will try to give my explanation of the process in a lucia stove or in an own garage experiment stove, that prove function in practice. As Matt said or proposed we can speculate about wy and how it works, not deny replicated practical function, because we can´t explain it. I propose following explaination:

    The downwarth drift of air into the chamber filled with fuel burns like an open fire during the first lightning up period of function..
    Heat is warming up fuel chamber wall during that startperiod. Convection starts between inner and outer wall and sucks secondary air from outside under the inner fuelchamber. The faster rising convection airstream drag air even from the small holes in the bottom of the fuelchamber and create an low preassure localy in the small holes, while it rises faster upwards.
    At some point enough heated gases are draged upp in the topp and entered over the fuel chamber and reaches an open air oxygen enviroment..Enlighting gases and the flames covers the fuelchamber opening and consumes the air that is draging slowly down in the fuelchamber. The airs oxygen content is radicaly reduced when it is entering fuel and the Lucia stove is working in a parlytic mode creating charcoal.

    There is no dought that air is draged down in fuelchamber, and during that process the oxygen is not present in the spaces between charcoal and fuelpices. This hot gas has not enough oxygen to burn, even when it´s mixed with secondary air. When these hot gases reaches the top nostril opening and come outside there is oxygen enough to provide full combustion in a thin layer covering the fuelchambers opening, like a “burning cap”. I therefore will point at “combustionchamber” as a missleading word. Heated fuelchamber under non oxygen circumstances.
    Thoughts about the morpholgy of the cap with spiral formed cannals for heated gases+ sec air create a more “spiral closing” movement compered with a linear directed stream of gases burning. I imagine it is blending better and “closes” the opening to fuelchamber in a better way as far as my experience seem to prove.. Linear burningcap with holes directed radialy to the center seem to be not efficiantly closing oxygen to enter down in fuel.

    Keep experimenting and speculating but not neglecting what is experimental replicated in lac of good explanation.
    Goran

  15. Cliff

    I have succeeded in building down draft pyrolysis stoves similar in principle to the LuciaStove using fans and without fans. I spent very little time at it, perhaps 4-5 days in all, so I probably lucked out in terms of the geometries. I do have a small pile of cut-up coffee cans that I went through to get to the ones I now use.
    I built my stoves out of two sizes of coffee cans and a piece of 5″ stove pipe I had laying around. For the fan version I used a 12v. PC fan running on about 9v. I found they work best with pellets, loaded to within 1-2″ of the top holes, and then don’t mess with it until it’s down to just the char, at which point the flame turns very blue and gets smaller. At that point I found it’s best to just cover it to put the flame out, then dump the charred pellets into a metal can and close it up until cool. I am consistently getting 100% biochar at the end when I do this – no unburned pellets. The char is about half the original volume and 12% of the original weight of the pellets.
    I’ve tried other biomass types and found I had to change the size of the lower holes since I was usually getting too much gas due to larger gaps between pieces. I can confirm much of what Matt has said above in his experiments about these stoves being finicky.
    I have been able to bring 9 liters of water to a rolling boil from 56 deg. F in 37 minutes using my fan-driven stove with no skirt on the pot but sheltered from any wind. I get a 75 min. burn time with 1150 gms. of pellets, which is about as much as I can fit into the two small coffee cans.
    I can also run my Stirling engine with 20″ fan blade for about 2 1/2 to 3 hours on just over a kilo of pellets using natural draft, no fan. It runs consistently at just over 500 rpm.
    You can view my videos of these here:
    http://www.youtube.com/user/approtechie

    Hope this is helpful.
    Cliff/aka Approtechie

  16. Sophia

    I see many cool sufficiency items such as water purifiers, solar cookers and the Lucia stove which are supposedly used in 3rd world and impoverished countries. Why can’t we buy them here in the USA? I don’t have the capability to build things and would prefer to buy them outright. Are they not sold?

  17. Charlie

    Regarding burning up metal stoves to extract energy, a tremendous amount of energy is expended in making steel, from ore extraction to final sheets. If calculations are made, the avoided usage of fossil energy by using wood stoves may be counterbalanced by the fossil energy expended in making the steel, so the whole exercise then becomes counterproductive in terms of avoided fossil energy usage and carbon emissions. I think it would be environmentally friendly to have a less efficient wood stove not reliant on materials that require a lot of fossil energy to construct the stoves e.g. clay, masonry, pumice stone, etc.

    I am toying with the idea of building a cement-sand stove based on the principles used in the Lucia stove design but the snag is how to bring about heat transfer from the inner chamber to the outer annular chamber without using metal, to induce the necessary extraction of pyrolysis gases.

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