Biochar Kiln

The quest for the perfect backyard biochar kiln.


Chapter One: What is Biochar?

My working definition for biochar is simply charcoal that has been infused with a combination of nutrients, microbes, and moisture. When added as an amendment to garden soil, biochar’s porosity helps the soil hold on to nutrients, and moisture so that they are available to plants and microbes. Thus it improves the soil’s Tilth, or texture, and generally renders the soil healthier and more alive.

In addition, unlike organic compost which breaks down fairly rapidly, releasing gasses such as methane and carbon dioxide, biochar’s stable chemistry allows it to remain intact for years, in essence, sequestering carbon in the soil that would otherwise be lost to the atmosphere.

Finally, biochar can be made from any number of feedstocks. By using carbon rich organic waste material as your feed stock, you can effectively reduce your waste stream.

Trendy Topic:

Biochar has become a trendy topic among organic farmers, gardeners, and permaculturalists. Proponents of biochar talk about it like it’s some kind of holy grail, capable of solving a laundry list of environmental problems from soil depletion to greenhouse gas emissions. Others feel it is nothing more than the latest form of environmental snake oil, a feel good measure that diverts attention from the real environmental ills that plague us.

Experimenting With Biochar:
While I am skeptical of biochar’s ability to singlehandedly save the earth, the practical benefits of biochar listed above were compelling enough for me to want to try it in my own garden. Unfortunately, with our local sandy soil I would need quite a bit of biochar in order for it to have any effect. As commercially available biochar can be pricey, the whole experiment was looking prohibitively expensive and impractical. Thus, the only way I was going to be able to really try it out was to see if I could make my own biochar.

Chapter Two: The Project.

When I started looking, I found a number of clever designs that people had posted on YouTube for homemade backyard biochar kilns. After considering several of these I settled on building a “Tin Man” or nested barrel design as shown here. The reason I like this design is that a.) It is simple to construct, using no complicated tools or welding equipment. b.) It is simple to operate, requiring little in the way of monitoring. and c.) Because of it’s design It is comparatively efficient, providing a relatively clean burn with little smoke.

The design calls for two nested barrels, one large and one small, and length of stove pipe.

Two Barrels

A saw with a metal cutting blade is used to remove the lids of both barrels such that they can be reattached. A hole is cut in the lid of the larger barrel and the stovepipe attached. Lastly – and perhaps most other importantly, holes are drilled in the bottom of the smaller inner barrel and around the bottom and top of the outer barrel.

When finished, the whole contraption looks like this:

Chapter Three: Directions:

Step One: fill smaller barrel with feed stock (What you wish to turn into biochar.)

Step Two: Place small barrel inside larger barrel with the holes on the bottom.

Step Three: Fill spaces around smaller barrel with dry fuel. (Wood Scraps, paper, anything that will burn)

Step Four: Light the top of the fuel stack. Wait till the fire is really going.

Step Five: Place lid with vent stack on top and stand back.


Chapter Four: How Does it Work.

Perhaps the easiest way to begin is with a diagram:

As the fire in the outer barrel burns it heats the inner barrel. Because there is no oxygen in the inner barrel, the material in it doesn’t burn. Instead, when the inner barrel gets hot enough, through a process known as pyrolysis, the heat drives the volatile gases and moisture out of the biochar feed stock. These gases escape through the holes in the bottom of the barrel (A.) As they rise they ignite with the fire that is burning in the outer barrel. The gasses reignite at the top of the barrel in a secondary burn (B.) before being drawn up the chimney. (C.)


Amazingly, The first time I loaded and lit the kiln it performed exactly as it was supposed to. The kiln burned for about an hour with very little smoke creating a sizeable amount of charcoal. The only mishap I encountered was that I put my emergency plastic water jug too close to the kiln causing it to melt.


After several trials with this first prototype and a second, I have become convinced it is possible to produce charcoal in your backyard in a reasonably environmentally friendly manner. The only caveats I would offer is that 1.) the retort gets very very hot so make sure you don’t leave anything too close to it. and 2.) Be very careful about what you put into your kiln. Yesterday I tried out using an old salvaged coffee urn as a retort. When I drilled the vent holes in it, it seemed like it was made of aluminum, which melts at about 1400 degrees. After the burn I opened the outer barrel to find the urn had melted into a very artistic puddle:

Needless to say I didn’t get any biochar from that run.

I am now experimenting on some improvements to the original design which I will post soon.


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