Two-barrel charcoal retort – a safe way to produce charcoal

Two-barrel charcoal retort - a safe way to produce charcoal
Two-barrel charcoal retort - a safe way to produce charcoal

Two-barrel charcoal retort – a safe way to produce charcoal using two small barrels

Folke Gunther demonstrates how to construct and use a simple biochar kiln made from two metal barrels with sample images and explanations

In order to get a charcoal production method that

  • avoids emissions of methane during pyrolysis
  • is simple to build, even with small means
  • produces charcoal from the garden waste that is not easily added to the soil
  • makes a bit of atmospheric carbon reduction
  • supports the local ecosystem

— this simple backyard/allotment/home garden sized system can be used.

The unit consists of two steel barrels. Only.

The smaller with no alterations a all. It is filled with dry biomass and put upside down in the larger one.

In the larger one, holes are cut on the side, some centimetres (an inch) above the bottom, large enough to let in an ample amount of air.

The larger barrel shold reach at least ten centimeters (4 inch) above the smaller vessel, to get a good “chimney”-effect, i.e. a strong draught from the air inlets, and a hot fire throughout the process.

The equipment: Two metal barrels, the larger about 20 cm (8 in) wider and 10 cm (4in) higher than the smaller vessel. In the larger one you make air intakes some centimetres (about1 in) from the bottom that allow an ample amount of air intake. The smaller vessel, you do nothing with.

Fill the smaller vessel with biomass, preferrably dry to make the charring procedure more efficient. Use chips, dry grass, bamboo or Miscanthus stalks, twigs, sawdust or what you have. Even some hard bones are good to put in.

Put the material in as tight as possible.

Fill the vessel level with the top. It will be placed upside down.

Put the filled vessel on a pair of bricks to make the next step easier.

Put the larger drum upside down on top of the smaller vessel, so the bottom of the drum fits close to the top of the biomass filled vessel.

Hold the inner vessel tight to the bottom of the larger when turning it back so the content doesn’t spill out.

When you have done that, the smaller vessel stands upside down in the larger. No lids, no fastening. (Although itcould be fine with some cover, just to keep the material inside the vessel when turning. )

Put dry firewood in the space between the two vessels (and some on top). Light it.

Replenish the firewood, so it burns at least 30 – 40 minutes.

Two-barrel charcoal retort - a safe way to produce charcoal
Two-barrel charcoal retort – a safe way to produce charcoal

After 30-40 minutes, the pyrolysis of the biomass starts. The gases will emerge from the vessel through the chink between the vessel and the barrel and immediately take fire and heat the vessel more, so more gases emerge, and so on. It is easy to see when this happens.

To be sure that all the pyrolysis gases take fire, be sure that the air inlets really gives an excess of air, and the passage between the vessel and the jar is long enough (if it takes 1 second, you are on the safe side). Otherwise, the fire will smoke and powerful greenhouse gases will emerge into the atmosphere.

When the pyrolysis of the biomass have started, it is a good time to pepare the dinner. A wok is a delicious way to use the garden producs.
You have about 30-40 minutes to use the heat, which is just enough to prepare a meal.
Carbon dioxide negative cooking!

After about half an hour, the fire will stop burning rather abruptly. At that time, there is (hopefully) only charcoal left in the vessel. Let it cool at least an hour. If you take hot charcoal into the air, it may catch fire.

When the vessel has cooled down, turn everything back again (preferrrably over the soil in a garden plot, to take care of the ashes). The char is ready to dig down in the soil, to put into the compost or let it absorb urine for increased soil fertility. Also, put on the soil, it will deter snails.

Detailed info how the charcoal kiln works :

  • Fill the smaller vessel with dry biomass. The dryer it is, the easier it is to reach carbonisation temperaure.
    (To get the best char for soil amendment, this tmperature should be 350-450 C.)
  • Put the smaller vessel upside down in the larger.(Since there is no lid on it, you will need to put it on the ground, turn over the larger vessel upside down, and then turn everything back again.). When the biomass starts pyrolysing, the gases will emit through the chink between the vessels.
  • Fill the space between the vessels with dry firewood and light it. Since there is a good draught, it will start firing briskly, thus heating the biomass in the inner vessel. Add more firewood even during the next phase to be sure of complete charring.
  • After about half an hour, the biomass startsemitting pyolysis gasses. They will catch fire when they enter the burning firewood and the air on the outside, and further increase the heat of the biomass.The house-wall-well cycle
    It is important that the pyrolysis gasses really catch fire, otherwise methane (a 20 times worse greenhouse gas than carbon dioxide) will be emitted.
  • When the pyrolysis is over, i.e. the biomass hasturned into charcoal, the fire will go out and the char will start cooling. Wait for an hour, and then you have the char ready to be buried in the soil, in the compost, or used to adsorb urine.


This system is aimed to be a ‘backyard charring method’ . It is cheap and easy to construct and handle, and meant to replace the backyard burning of twigs, taking place everywhere, with a method of increasing soil fertility at the same time as reducing CO2in the air.

It is not meant as a large scale method, since the surplus gases, that could be of industrial use, are burnt and the useful heat is dissipated. (Although you could easily cook food on the fire if you put a grid on the top)

But it should be seen as an introduction to charring, and as an eye-opener for the usefulness of charcoal in the soil.

That the threat of climate change, in an infinitesimal way, simultaneously is reduced should be seen as a bonus.

Thank Folke Gunther for his great demonstrates

1 Comment

  1. Can you use sweet gum for this? I wanna make some willow charcoal and i have a field overrun with sweetgum

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