Open Top Flame Cap Biochar Kiln.

Tom builds an open top biochar kiln.

This past weekend I attended a fascinating biochar seminar put on by the Tilth alliance and Washington State University. The presenter, Kai Hoffman-Krull did an admirable job outlining the advantages of adding biochar to garden soil. But the highlight of the seminar for me was the demonstration of an open flame top kiln for making biochar. I have now had several posts on TLUD biochar retorts but this was my first experience with the open top variety. Of course I immediately wanted to come home and build my own.


The design of an open top kiln is incredibly easy. Quite simply, an open top biochar kiln is an open vessel in which you burn high carbon materials in such a way as to create charcoal instead of ash.

In the demo, they used a large propane tank cut in half. However they said the design could be scaled up or down to accommodate different needs. As I only have a small garden I decided to try and make one out of a 55 gallon drum cut in half and bolted together. I used an angle grinder to cut the barrel and a drill to make holes for the bolts. Given the thin gauge steel of the barrel, I added angle irons on the edges for support. The resulting container was sturdy, very simple to construct, and light enough to be lifted with one hand.


Like the retort, the open top kiln operates by heating wood to high temperatures in the absence of oxygen, driving off volatile gasses and moisture and leaving almost pure carbon (charcoal) behind. The chief difference between the retort and the open top is, instead of having two enclosures, one for feedstock and one for fuel, the open top combines them both into one container. Thus, the feedstock and fuel are one. Whatever you burn will be what you will turn into biochar.

How it works:

The operation is incredibly simple. Just collect your feedstock, set the kiln on a bare patch of ground and procure a source of water. Make a small fire in the kiln and each time coals are formed, you immmediately add more wood. Adding the wood is crucial as it deprives the coals of oxygen. Thus, the heat from the fire continues to drive off gasses and moisture from the coals without allowing them to burn to ash.

This cycle: watch for coals to form, add wood to starve coals of oxygen is repeated until either you run out of fuel or you fill the container with coals. Either way, at the end of the run, water is sprayed on the coals to extinguish the fire, leaving only charcoal and a small amount of ash behind.


The chief advantages of an open top kiln is 1) its simplicity to make and operate and 2) its capacity for making larger amounts of biochar than can be made in a retort in the same amount of time with the same amount of material.


The major drawbacks of the open top kiln are 1) the fact that it is not as clean burning as a retort and 2) the fact that it takes more maintenance and 3) that it requires a large volume of water at the end to quench the coals and stop them from burning.

Critical points:

A) Even more so than in a retort, it is critical that the feedstock you use in an open top kiln initially have a low moisture level. If you use wood that is too wet, the fire will be smoky and not nearly hot enough. I found this out the hard way as my initial use of fallen alder wood was not dry enough to create a hot enough fire to produce satisfactory coals. It wasn’t until I added a whole bunch of kiln dried dimensional lumber to the kiln that I was able to reduce smoke and get the fire hot enough to make proper coals.

B) If a flame top kiln is operating correctly fresh air should be sucked into the fire along the edges creating a vortex that circulates the smoke back into the fire for a secondary burn. Unfortunately, I did not see this. I assume this was because of the wet wood I was using which produced an undue amount of smoke, and the fact that it was a windy day. The next time I try this I will endeavor to use drier feedstock and possibly add a wind break of some sort to keep the wind from the fire.

C) The flame top kiln is safer than, say, a backyard bonfire. However, Because it is an open fire, caution must be taken to ensure safety. It is extremely important to monitor the fire at all times, to have a source of water on hand sufficient to extinguish the fire, and to obey local fire code and burn ban ordinances if they are in place.

Despite the problems I encountered with my test, In the end I was able to make a substantial amount of decent biochar in two hours. Much more than I would have been able to make with a standard two barrel TLUD retort. I did, however, notice that I had quite a lot of incompletely charred alder after the burn. I will have to add this to the next batch.

There are several other benefits to this design over a TLUD retort. Because of the simple trough shape it is quite easy, after the coals are quenched, to add the necessary compost and nutrient inoculants to the trough and simply mix them together with a hoe. Removing the finished biochar is then a simple matter of using a flat bladed shovel or simply tipping the trough over in the ground.


The open top flame cap biochar kiln design is quite good and fulfills my “three win” criteria: Cheap, Easy to build, Simple to operate. With a few modifications: Drier feedstock, and a windbreak I expect to be able to produce substantial amounts of biochar for my garden this year.

4 thoughts on “Open Top Flame Cap Biochar Kiln.

  1. I’m a mechanical engineering student developing a charcoal retort kiln product and would love to hear feedback from everyone! Responses are anonymous and are super helpful in development. Thanks! Follow this link for the short survey…
    Really in your experience, what make a fantastic charcoal kiln?
    ive never really seen an open concept like the one presented here. How do you regulate the temperature?
    How many lbs of lump coal does one burn produce?


    1. Regulation of temperature in an open top Charcoal Kiln is limited. So far I have been monitoring it with a infrared heat sensor. Since it is a small amount of fuel involved the temps are usually not excessive. (Less than 1000 degrees F. ) The real trick is to get the Kiln up to temperature as quickly as possible. Really dry small kindling and a propane torch usually gets the thing going fast. Also you really must have a wind screen surrounding the Kiln. Keeps the heat from escaping and allows the vortex effect at the edges to recirculate smoke back into 5he fire.


    2. As to how much charcoal is produced, you are only limited by the size of your container and the amount of feed stock you have. If you keep putting on wood every time the stuff on top develops ash, you can pretty much Mack as much charcoal as your Kiln will hold.


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