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.
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 it could 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.
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.
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