Charcoal is made by heating wood or other organic materials above 400° C (750° F) in an oxygen-starved environment. The process, called pyrolysis, is exothermic – meaning it gives off heat once started. The more volatile elements in the wood, such as hydrogen, oxygen, and some carbon, combine to form gasses that escape from the wood. Some methods burn the volatile gasses to prevent them from escaping as pollution, at the same time producing surplus heat. Others collect them for later use as biofuels. While charcoal-making is generally quite easy to do, control of the various factors that affect the process can make a big difference in the yield and properties of the final product. The wood used, heating temperature, air availability, and time of processing all play a role. So keep these things in mind as you set out to make your own biochar or wood charcoal.
- Simmons, F.C., 1957. Guides to Manufacturing and Marketing Charcoal in the northeastern states. USDA Forest Service Station Paper No. 95, Upper Darby, PA.
- FAO. 1985 Industrial charcoal making.
- FAO Forestry Paper 63. Food and Agriculture Organization of the United Nations. Rome.
- FAO. 1987 Simple technologies for charcoal making.
- FAO Forestry Paper 41. Food and Agriculture Organization of the United Nations. Rome.
Charcoal kilns are containers typically made of brick, earth, or metal that are filled with wood which is set aflame inside the kiln. Once the burning wood generates enough heat, airflow to the kiln is reduced so the process of carbonization proceeds without burning up all the wood. Earthen kilns can be dangerous, requiring vigilance around-the clock to be sure the heat from the fire within doesn’t escape. Brick kilns, while widely used in some developing countries, may not provide adequate control over heat and airflow to efficiently convert wood to charcoal. Metal kilns afford greater control and may be portable, and generally cost more to build and maintain.
- Newton, J.J., ca. 1982. Charcoal making: Newton’s method. Undated photocopy from author’s archive.
- Newton, J.J., 1982. Small batch charcoal making: Newton’s method. Photocopy from author’s archive.
- Baldwin, H.I., 1958. The New Hampshire charcoal kiln. Fox Forest Bulletin 11, New Hampshire Forestry and Recreation Commission. 84p.
- Design and instructions for making a New Hampshire-style metal charcoal kiln from readily-available materials (with appendix).
A retort is a sealed metal container, filled with wood, similar to a kiln except that heat is applied from outside the container. Factors like temperature, airflow inside the container, and exhaust can be strictly controlled with a retort. Retorts typically require an external source of fuel to heat the container to the point at which pyrolysis will start. Once the reaction starts to produce volatile gasses, they can then be redirected outside the chamber and burned to produce heat needed to keep the reaction going. Retort designs must take into account the relatively high temperatures needed to transfer adequate heat through the wall of the container. They are often constructed of heavier gauge steel than kilns to endure the high temperatures.
- Producing and marketing natural wood "lump" charcoal home
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Dr. Phil Radtke
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