Open Top Flame Cap Biochar Kiln – Second Design

A critique of my first attempt to make biochar with an open top kiln and how to solve some of the problems.

In my last post I laid out a design idea for an open top flame cap biochar kiln made from a 55 gallon steel drum cut in half. The results, I have to report, were somewhat disappointing. Fortunately, in analyzing the problems with the first design I was able to come up with an alternative that is just as easy to construct and way more effective.

The Good: As advertised, the open top flame cap kiln did produce a huge amount of charcoal in a relatively short period of time. Merely by building a fire and piling on wood when the coals begin to turn to form ash I was able to produce about eight times the amount of charcoal as I could have in my TLUD retort in the same amount of time.

The Bad: Unfortunately, the open top kiln was way smokier than it should have been. I couldn’t seem to achieve the vortex condition that is supposed to recirculate the smoke back into the flames to be completely consumed. Moreover, despite making some improvements to the design, the quality of the charcoal was not good with way more ash than it should have had and way more uncharred material.

Admittedly, the wood I used was not as dry as it could have been. And atmospheric conditions were cold and windy and wet (It is winter after all) But if the design was good that should not have been sufficient to cause the disappointing results I experienced.

Analysis: In looking over the designs for open top kilns I had seen I soon realized that almost all of the designs were quite a bit deeper in profile than mine. Many of them also had metal screens that fit around them. The screens performed three functions:

1 to keep the wind from disturbing the combustion

2 to provide a layer of insulation to the kiln and allow pyrolysis to occur and

3 to allow rising air to be warmed before reaching the edge of the kiln, thus allowing for more complete combustion.

Solution: The solution to these issues was ridiculously simple. First I got a ten gallon bucket. I place it on blocks to insulate it from the ground.

Next I took a 55 gallon drum – from a previous that I had cut the bottom out of – and put it around the bucket, raising it off the ground with blocks as well.

Then Then, I simply built a pile of pieces of VERY DRY WOOD, and lit it. (I removed the screen in the photo to make it easier to see.)

Lastly, I replaced the barrel.

Notes: I found I had to raise the barrel a bit higher off the ground to allow more air into the space between the containers but once I did the fire burned steadily and very hot with very little smoke. The vortex at the edge of the bucket was clearly visible. Within a period of an hour all the chunks of wood had been turned into very high grade charcoal with very little ash. When all the flames had died down, I poured a gallon of water over the fire to douse it and jammed the lid on the bucket to deprive the remaining hot coals of oxygen.

The whole process was quite painless. Instead of tending and worrying about the fire (and having to lug 15 gallons of water up the hill in order to quench it) I was able to let the kiln do it’s thing and work on several other projects while the burn was going on.


This method is BY FAR the simplest least worrisome method of making biochar in one’s backyard. While my interest in the project is for gardening, eventually turning the charcoal into biochar, I can readily see how this method could be used by those wishing to produce their own charcoal for fuel to power home blacksmith forges and smelters or simply to produce cooking fuel for barbecues.

The only caveat I would offer to this method is my observation that with open top kilns – even more than TLUD retorts – it is ESSENTIAL that you use dry feed stock. This is particularly true if you are trying it on a small scale as most back yard DIY folks will be doing it. The reason is simple: In order for complete pyrolysis to occur YOU NEED A REALLY HOT FIRE! In a ten gallon bucket, Green branches, and wet punky wood ( like old pallets that have been left out in the rain ) by themselves, simply can’t produce a hot enough fire to drive off the volatile gasses and moisture and produce usable charcoal for biochar.

I hope this blog post has been informative for readers who happen upon it. If so, I invite you to share blog with anyone who is interested in tinkering. Word of mouth keeps me going.

Happy Tinkering!

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