Probably overcomplicated ideas for biochar kiln design... but could it work?

So... I've been really digging into the idea of biochar and a bunch of the different production methods and kiln design; and I was somewhat inspired by the YouTube videos from Living Web Farms where they show their 3 massive 1 ton (I think?) kilns that are all linked together.  And I really liked the idea of how they are able to use the excess gases from one burn to start up the next one rather than needing more waste fuel to start the reaction.

I also like how they are trying to capture some of the heat for other uses, but that is less directly relevant to my questions here.


It got me thinking about ways to potentially use even less waste fuel to start the process.

And my first idea came when I remembered how with dominos, each domino can knock over a domino that is a certain % bigger than it is. So in theory, you can start with a standard sized domino and gradually scale them up one at a time until the Nth one in line is big enough to crush a car (I want to say they actually tried this on mythbusters?).

Seeing as how each biochar burn has a certain amount of gas left over, and that residual gas doesn't need to be enough to actually burn the next pile but simply to heat up the retort and contents enough to start the reaction going so it can continue heating itself, each consecutively larger container of wood (or whatever) should have somewhat more gas left over to heat a somewhat larger retort on the next round (no idea how much it would scale up each time, but my intuition says it may actually be fairly quick?). Very similar to the dominos.

So, I guess the first question: Is there anything wrong with that line of thought?

Assuming this chain of retorts was set up similar to the one shown by those Living Web Farms videos, where the excess gas is used for starting the next reaction, is there any reason you couldn't theoretically start with a fairly small retort and "domino" it up to one of those big 1 ton retorts without ever having to add any more waste? And in THEORY, if the system was efficient enough at capturing excess gases, you might be able to do all that with something like the kindling used for one of those little paint can TLUD retorts? (Maybe more because you're probably using thicker metal walls that need to be heated, but still...)


And then I had another idea.  I think it would probably work best if used with the domino idea, since I don't think this would be as effective on one of those massive retorts...

If you took that smallest retort and didn't even use any waste fuel at all to heat it up, but instead took a fresnel lens and pointed it either at the little retort (maybe more than one lens to spread the heat around?) or at a chunk of steel or something in contact with the retort to heat it up to the necessary temperatures, would you be able to fully gasify the wood (or whatever your feedstock is) without needing any external "fuel" aside from the concentrated solar energy?

In theory, this could actually allow you to capture ALL of the gases from this initial charring process without needing to burn any of them, I think... because there wouldn't have been any combustion involved in the entire process of that first retort, until those gases were moved over to the second one to start up a burn to heat up the second retort.


I fully acknowledge this may be overcomplicated, but is there any reason it wouldn't theoretically work? I have to say, I have gotten myself pretty fascinated with the idea, but it's entirely possible I'm overlooking something obvious... I'm curious to see what others have to say about the ideas though.


If I could eventually work towards creating a system like this and also integrating ways to capture and make use of the heat, that would be really cool.

I have a fresnel lens I removed from an old projection TV and I've played around with it.  The problem with a fresnel lens is that it only creates heat in a tiny spot.  If you point it at a retort made of something like a barrel, it will burn a hole in that spot without really spreading any heat to any larger area.  I don't know of any way to "spread" the heat you need over a large enough area and still have enough heat to char wood.

Trace Oswald wrote:I have a fresnel lens I removed from an old projection TV and I've played around with it.  The problem with a fresnel lens is that it only creates heat in a tiny spot.  If you point it at a retort made of something like a barrel, it will burn a hole in that spot without really spreading any heat to any larger area.  I don't know of any way to "spread" the heat you need over a large enough area and still have enough heat to char wood.

Hmm....

I confess, I've never actually had the opportunity to mess around with a fresnel lens, but here are a couple of thoughts in response to that problem:

1) I've read that there is a difference in the features of a fresnel lens you would want to prioritize if you're using it to project an image vs to collect light. Is it possible there would be a better lens for this than one from a TV?

Even if that is true though, it doesn't mean that your overall point wouldn't still be an issue.  But it could make the overall system more efficient for each of the next points...

2) As I understand it, there are some fresnel lenses which focus the light to a single point and others which focus it into a line - which can be used to focus the energy on a pipe or something. That could be used to spread it out.

Also, if you were to put that barrel you mentioned closer to your lens than the actual focal point, that should spread out the heat somewhat, I believe. Technically, the same amount of energy is still hitting the barrel, but spread out over a larger area.


3) Another part of my thought was, if burning through the retort was a concern, I could either make it out of a different material that couldn't be melted through as easily by the lens (like stainless steel?) OR use a chunk of a material like that which could absorb a bunch of the heat and act like a heat sink essentially and transfer the energy to the retort (might have to be designed specifically to make solid contact with the heatsink).

The chunk being used as the heatsink should be able to absorb a lot more and be much harder to burn through.

Potentially combining that with the concept of moving inside the focal point to spread the energy out a bit on the metal and possibly (likely?) using more than one - at least a second on the opposing side for example - to even the heat distribution out some.


Again, I don't know how realistic these thoughts are, but at least in my head it feels like I have answers to the questions lol.

To use the fresnel lense I would make a rotating retort, possibly out of refractory.
Rotating it would spread the heat around.

You could do the domino effect, but it would be relatively complicated .

Re your "domino chain" idea...

Having made some batches of charcoal (larger diameter material, to make lumps for cooking) and biochar (fine twiggy feed material, to make fine char pieces), my feeling is you may be trying to solve a problems that in practice doesn't exist?

I think - If I understand the proposal correctly - that you are trying to avoid wasting feed stuff to heat the retorts?

When making biochar, I usually end up with some pieces that are too coarse to work well as biochar feed. And conversely, when making charcoal, I usually have fine pieces that burn too quickly to be included in the charcoal making process. These wastes are ideal to use as that initial fuel load to get the process started.

All that said; I have more recently moved away entirely from retort based approaches to an open fire in a trench, that is continually topped up with brash and then quenched. I can process far more finished biochar in the same time, compared to using retorts, at the "cost" of reduced efficiency. The idea is that the the continual flame front above the bed of embers protects the biochar, which builds up in layers. The resulting char is typically fine, friable, and plentiful.

William Bronson wrote:To use the fresnel lense I would make a rotating retort, possibly out of refractory.
Rotating it would spread the heat around.

You could do the domino effect, but it would be relatively complicated .

Interesting! I had ruled out refractory in my mind thinking it would be far too insulating. But after seeing your comment, I did a bit of Googling and found out that you can get it in varying degrees of porosity/density.

The denser stuff conducts more heat (which totally makes sense), and actually holds up better to even higher temps as well. And you can build up layers of multiple types within the same kiln if you want.

So I am still trying to think through how an effective rotary system would work, but at least it seems like the use of refractory is back on the table. (Which may not be news to you, but it is to me! Lol)

The domino effect idea sounds like something that might work at industrial scale but I'm dubious about doing it on the homestead. OTOH, I'm eager to see where your experimentation goes.

On the point of the fresnel lens, if it's concentrating the energy too tightly, just bring it out of focus until it isn't. If the focal length of the lens is e.g. two feet, point it at the retort from 2'2" away and you'll see it concentrating the light over a much broader area than the pinpoint.  

You're correct that you can move the lens back and open up the focal point, but it very quickly becomes too cool to do anything.  I picture it like this.  I don't know the exact measurement of my lens offhand, but for simplicity sake, lets say it is 3ft x 3ft.  It doesn't create any more light or heat energy, it simply focuses it, whether to a point, or as you said, less often, a line.  Either way, you have no more heat than you would coming through a 3x3 window.  If you put you hand in front of a 3x3 window, you will feel warmth, but not really heat.  If you focus it into one tiny point, you can reach between 3-5000 degrees F.  You would have to figure out a way to get several hundred degrees onto the wood itself while restricting oxygen.  Problem with that is that wood is a pretty good insulator, so if you could put a piece of wood directly against a piece of metal, you could get that piece of wood hot enough, but I don't know if you could get enough heat past that.  I'm just not able to imagine a way to do this.  If you can, I'm all in :)  I would love to be able to create charcoal without using any extra wood to do it, and without losses.