Perfecting The Biochar Kiln

A report on some upgrades to the “Tin Man” biochar retort.

In my first post on biochar I outlined A design for a biochar kiln. In the post I expressed delight at how well it had performed. However, as I have continued to experiment I have noticed some very definite flaws in the original design that I have attempted to address with my latest iteration of the nested barrel retort.


A. One major problem that has come to the fore as the weather has turned colder was the rate at which the kiln/retort loses heat to the atmosphere. During the summer when the ambient temperature was mild, this was not so evident. But when the ambient temperature is at or near freezing, the thin walls of the steel barrels release heat too quickly. This results in an inefficient, smoky burn indicating major pollutants are being released into the atmosphere.

B. In addition, because of the heat loss, it becomes harder to maintain the necessary temperatures to achieve pyrolysis. In cold weather I have had several runs where I experienced incomplete charring of the material in the inner retort.

C. I have also had problems with maintaing a good burn due to insufficiently dry fuel. Here in the Pacific Northwest from November to March it rains….

… a lot.

Thus all of the material I had been using as fuel was now soaked. It is possible to burn even wet wood if you can get your coals hot enough. But as I noted above, due to the heat loss, it was difficult to achieve that critical mass of heat, thus I had several frustrating runs where my burn material never fully combusted but simply smoked and smoldered.

D. lastly, I had some questions about whether the two new stoves I had built were even drawing properly or not.


The last issue was the easiest to address. To ensure that oxygen was not an issue I enlarged the vent holes top and bottom. In addition I added a screen at the bottom of the barrel to keep my fuel from plugging the primary air intake.

Trying to solve the heat loss problem was somewhat tricky. At first I thought that simply insulating the barrel with some rock wool batts would suffice. However, this proved problematic because after a test run I noticed the steel in the barrel was RED HOT.  In addition I learned that although rockwool doesn’t combust, in the presence of high heat it does melt.

In thinking about the problem, it occurred to me, As pyrolysis starts around 400 degrees Fahrenheit, I didn’t really need to achieve kiln like temperatures. Thus super rocket mass heater temperatures were over kill. All I really needed was to come up with enough insulation to keep the temperature consistently hot so that pyrolysis could occur.

The solution which I arrived at, which seems to be working better, was to use a third barrel to construct an outer jacket. This jacket slides down around the burn barrel with a 3/4 inch air space which gives a small amount of insulation. In addition, the air space is open at the bottom allowing air to flow upwards pre-heating it before it enters the secondary air holes.

As those with stove experience know pre-heating air prior to combustion results in a cleaner more efficient burn. I will be continuing to test this out. But So far, it appears that this solution is successful at addressing the heat loss problem.

In Diagram form the new design looks like this:

Packing The Retort and Fuel Chamber:

The last issue I have found with the double barrel retort design is the importance of packing the retort and the fuel chambers carefully. Few of the YouTube videos out there stress this. However, I have found it critical to take some care when loading the stove. The retort can’t be packed too tightly or the pyrolysis gasses can’t escape leading to an incomplete charring.

Likewise, with the burn chamber, it needs to be as full as you can get it while still allowing air vertical air channels. Care needs to be taken not to block the primary or secondary air holes. No oxygen, no fire. No fire, no pyrolisis.

Lastly, I’m here to tell you that you can include some wet wood in the burn chamber… BUT NOT TOO MUCH. A vast majority of your fuel should be dry and solid. The logic here that is simple. To achieve pyrolysis you need a consistently hot fire. Adding too much wet fuel will make it difficult to achieve the consistently hot temperatures necessary for pyrolysis.

One final note: After getting myself coated in soot several times trying to remove the retort after firing, I finally got smart and added a handle to make it easier to remove.

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