Crazy idea for a charcoal kiln

(I thought this might go in this forum, too...no hard feelings if an admin deletes this thread, or the comment it duplicates from the thread I'm about to link to.)

Discussing the pollution from making biochar on another thread, the praises of a double-walled retort for efficient and clean operation were sung. And I obnoxiously shot my mouth off about what I see as flaws in the design. I like it well enough, but I think too much of the heat goes up the stack, especially right at the beginning, meaning more fuel must be packed around the inner retort (see bottom diagram). Thoughts of how to improve on that design, to make more charcoal from less wood by directing more heat where it can be used, have pestered me on and off ever since.

After much incubation and percolation, here's my notion of an improved homebrew kiln: the flame from pyrolysis gas heats the wood to be pyrolyzed from the center out, and some heat from the chimney is re-cycled to warm the intake air. It only takes one barrel, and a few other pieces. I think two diameters of stove pipe might be good enough for this job. The best diameters, and especially the best lengths above the barrel lid, are very much open questions, of course.

In the illustration, black represents steel, brown is wood (stuff in the center is starter fuel: more can be added once the draft gets going, if the operator sees the need), and grey arrows show how I expect air & gas to flow.

The section of larger-diameter pipe that touches the main load of wood should have a reasonably good seal to the bottom of the barrel, and a gap between it and the lid of the barrel large enough that pyrolysis gasses can meet incoming air and be burned, but not so large that the charcoal continues to burn after pyrolysis finishes. I didn't illustrate this, but it might make sense to put crennelations in the top of the pipe, both to shape the flow of pyrolysis gasses for a better flame, and to gain better control of the total size of the gap.

The chimney that runs up the center can be attached to the bottom of the barrel in a way that allows good airflow, and attached to the pipe around it with a grating or with stiff wires, wich would serve to hold the starter fuel and also to hold the flame from the pyrolysis gas.

An air intake cowl is attached to the barrel lid, and can be the same diameter as the pipe below it. Aside from pre-heating intake air, it holds the initial charge of starter fuel. I could imagine fitting a choke onto the air intake, but that seems like an unnecessary complication.

Flammable insulation like straw or chaff might be added among the main charge of wood if it didn't interfere with dense packing, to improve the efficiency of the early stages of the process. If high-temperature insulation were available, I think it would help the most where the air intake cowl meets the top of the lid. The outside of the barrel could be insulated, but by the time it gets very hot, all of the wood would have begun to pyrolyze. A small depression, maybe full of ashes, dug under the center of the barrel might be worth the resulting efficiency gains.

Holes in the chimney to allow a secondary burn might be a good idea, but I don't know enough to say if the intake air would be sufficient.

Did you build one of these?

Hello, first post in the forum. I am a small and medium scale biochar enthusiast (and permaculture of course) and have some self taught permaculture and agroecology training and have made a smal biointensive garden using also biochar and mulches.

I find your design to be a very good idea, but I have some observations.

The two barrel retort ´´wastes energy´´ by radiating outwards and upwards but your design uses the energy that leaves from the sides, this could lead to a very hi temperature that could deform the barrels and could also redduce the amount of biochar you make (hi temp gives you less biochar) and make very alcaline biochar. Also, hi temp could lead to faster pyrolisis making maybe to much smoke to burn cleanly.

A way to avoid this theoretical hi temp problem would be to open some holes in the upper part of the iner tube, how many and how big would be open questions too, maybe a sliding mechanism would work to tune this.

On the other hand a very hi smoke generation would help to tap wood vinager and syngas, an afterburner could help with the untaped material.

Hope you build it, I may do so too, but am currently working in another kiln. Good luck

Good point. So I guess no insulation should be installed around the outside of it. My intent was to start with as little fuel as possible outside the retort, and for that, insulating around that barrel wouldn't help much anyway.

I think it might also be valuable to build the prototype with some adjustable apertures in strategic locations.

triato wrote:The two barrel retort ´´wastes energy´´ by radiating outwards and upwards but your design uses the energy that leaves from the sides, this could lead to a very hi temperature

If the outside isn't insulated, this system should waste about the same amount of energy as the old design once it really gets going. If the system is sized appropriately, cold air flowing over the outer surface of the retort should provide some self-regulation. Now that you mention it, though, I should probably be ready for that to work differently than I expect. And it would lead to a gradient in quality of the product, with well-cooked char toward the center, and relatively "rare" char toward the edges of the retort.

A friend of mine is currently learning to weld: he and I are working on a soap nut project at the moment, but afterward I might raise this topic with him. I'm curious to see if the idea is really practical.

what about this design?  seems like a relatively simple construction and operation.  I'm sure there are drawbacks that I'm not familiar with.  how would this compare to other methods?

about the diferent grades at diferent distances from the heating source, that can also be a good thing, you would get very recalcitrant char at the center and charcoal with oil condensates at the sides (be sure to mix well and test the PH before using). However if temp is to hi at the center I understand dioxines (toxic) may be created.

Good luck again, let me know how it goes when you do it.

happy hollidays

Hey guys, look at what i've bumped onto. May be old news for you, but i thought it was way cool.

http://www.youtube.com/watch?v=hZD6hrVhZGc&feature=related

Here is another design which I believe is better in that you get two chars in one burn. No wasted wood for the retort. This is using the TLUD method.

http://biochar.bioenergylists.org/content/JRo-video

Ken

How does what you are making differ from coke? There is such a thing as: Wood Coke.