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.

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 starts emitting 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.

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 has turned 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.