My volcano

While I waited for my kontiki kilns to be fabricated, this summer I wanted to char a lot of biomass that I've accumulated. We had a little mini pond excavated last winter and I didn't get all the dirt spread out, so there is this mound in the paddock. One day it occurred to me that I could scoop out an inverted cone in the top, like a caldera. So when a friend came over looking for some biochar, we decided to get busy and make some:







In the initial iteration, it's just a pit. The photos are from the third or fourth burn and you can see how the edges of the rim have been a bit chewed up. This happened when I poured water on to quench the embers...I poured it along the edge and it shattered the weak terra cotta that had formed during firing. Since then, I have coated the interior with mud and put a piece of pipe into the mound to intercept the bottom of the cone and serve as a drain for the quench water. I now plug the pipe and pour the water right into the centre, then uncork and drain after I'm sure everything is out. Getting the leachate is a nice bonus.

A full caldera yields about two feed sacks of char. I've had good results with feedstock up to 6 cm diameter...beyond that the middle of the pieces don't always pyrolise completely.

Very cool idea Phil, and good results.  I really like this.

Now that is NICE! good job Phil and thank you for posting great photos for all of us to ooh and ahhh over (I know I am).

Redhawk

This system seems easy to operate. I really liked the spontaneous design. Can you give more information about how it works?

Hi Güneş -

The basic concept behind a pit or cone kiln is flame cap pyrolysis. This type of burning consumes all the volatile fractions as the gases pass through the layer of active flames on top. In doing so, they use up the oxygen at the surface of the fire and keep the charred material from burning all the way to ash. The most refined version of this type of kiln is the kon-tiki developed by Ithaka Institute and there is lots of great info on their site, including a how-to for a simple pit kiln.

My version is only novel because I dug it out of the top of a pile of dirt.

Phil Stevens wrote:Hi Güneş -

The basic concept behind a pit or cone kiln is flame cap pyrolysis. This type of burning consumes all the volatile fractions as the gases pass through the layer of active flames on top. In doing so, they use up the oxygen at the surface of the fire and keep the charred material from burning all the way to ash. The most refined version of this type of kiln is the kon-tiki developed by Ithaka Institute and there is lots of great info on their site, including a how-to for a simple pit kiln.

My version is only novel because I dug it out of the top of a pile of dirt.

So there is no CO2 emission? I am really new to pyrolysis obviously...

Phil Stevens wrote:

My version is only novel because I dug it out of the top of a pile of dirt.

Maybe so, but as one of those with really old knees, I *really* like the idea. The last time I made biochar in a pit, I gave up, dumped more fresh branches and dirt on top of the char and ashes until it was a mound, and planted a black current on top. That was a decade ago, and the mound is now flat, but the current is still happy! I will keep this idea in mind as we clean up branches from storm damage.
I've never tried to turn cedar into biochar - any reason I shouldn't? It's too dangerous from the wildfire perspective to leave it all on the ground.

The key in making charcoal in this manner is in continually feeding the fire in a way that newly added fuel (branches and other wood scraps) are constantly layered over the top of the burning fuel below.  As soon as the wood has a good char on it and the wood is clearly burning well, another layer of wood is thrown over the top of it.  With each successive layer of fuel, the coals below are starved of air and are preserved.  Its that suffocating effect of multiple layers that quickly builds a lot of coals at the base of the fire.

If you feed the fire too quickly, you don't get sufficient charing on the wood.  If you feed it too slowly, your wood will burn up and you'll not be left with any charcoal.  You've got to find that sweet spot where you keep the fire hot but you don't burn up the very thing you are trying to create.

I find it works best when the fuel sock is all of similar thickness.  That way you aren't burning up the small stuff while waiting for the thicker stuff to sufficiently char.  I'll start with the thicker stock first, and then bury it with the smaller stuff as the burn goes along.  When I do a burn like this now, I'll stockpile a lot of wood in advance, and then I'll keep the fire going for several hours before I finally turn on the hose and quench the fire.  In that time, I can burn upwards of a third of a cord of wood, and I can get a foot or more of charcoal at the bottom of my 5' diameter fire pit this way --- enough to fill two big wheel barrows with charcoal.

With that much charcoal, I'll need at least a cubic yard of finished compost to turn it into biochar, or about 3 yards of uncomposted biomass in a hot pile.  I like to add the un-charged charcoal to the new compost pile and then turn it (Berkley method) every 3 days for 21 days.  That seems more than sufficient to turn that charcoal into biochar.

Jay - Cedar will char just fine. I just did a couple of burns with lots of pine and macrocarpa (Monterey cypress) chunks and it came out great. Mac wood reminds me of cedar, with the same sort of aroma and rot resistance. I love getting loads of cypress wood for home heating because it smells so nice when it burns.

Having the pit up at my level is nice for the back as well as the knees, since there's less bending going on.