Charcoal is a light, black residue, consisting of carbon and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen (see char and biochar). It is usually an impure form of carbon as it contains ash; however, sugar charcoal is among the purest forms of carbon readily available, particularly if it is not made by heating but by a dehydration reaction with sulfuric acid to minimise the introduction of new impurities, as impurities can be removed from the sugar in advance. The resulting soft, brittle, lightweight, black, porous material resembles coal
Charcoal has been made by various methods. The traditional method in Britain used a clamp. This is essentially a pile of wooden logs (e.g. seasoned oak) leaning against a chimney (logs are placed in a circle). The chimney consists of 4 wooden stakes held up by some rope. The logs are completely covered with soil and straw allowing no air to enter. It must be lit by introducing some burning fuel into the chimney; the logs burn very slowly and transform into charcoal in a period of 5 days’ burning. If the soil covering gets torn (cracked) by the fire, additional soil is placed on the cracks. Once the burn is complete, the chimney is plugged to prevent air from entering. The true art of this production method is in managing the sufficient generation of heat (by combusting part of the wood material), and its transfer to wood parts in the process of being carbonised. A strong disadvantage of this production method is the huge amount of emissions that are harmful to human health and the environment (emissions of unburnt methane). As a result of the partial combustion of wood material, the efficiency of the traditional method is low.
Charcoal Burner, Bouth Woods
Improved methods use a sealed metal container, as this does not require watching lest fire break through the covering. However, on-site attendance is required, and also this method sacrifices part of the material for generating process heat – with the associated low yield. At Bulworthy Project in the UK, charcoal production supports an experiment in low-impact living and nature conservation. Modern methods employ retorting technology, in which process heat is recovered from, and solely provided by, the combustion of gas released during carbonisation. (Illustration:). Yields of retorting are considerably higher than those of kilning, and may reach 35%-40%.
Examples of large industrial, but clean, industrial technologies are the Lambiotte shaft furnace, and the Reichert retort. A recently developed technology is the Condensing Retort developed by Clean Fuels. This latter technology is suitable for medium to large industries.
The last section of the film Le Quattro Volte (2010) gives a good and long, if poetic, documentation of the traditional method of making charcoal. The Arthur Ransome children’s series Swallows and Amazons (particularly the second book Swallowdale) features carefully drawn vignettes of the lives and the techniques of charcoal burners at the start of the 20th century, in the Lake District of the UK.
The properties of the charcoal produced depend on the material charred. The charring temperature is also important. Charcoal contains varying amounts of hydrogen and oxygen as well as ash and other impurities that, together with the structure, determine the properties. The approximate composition of charcoal for gunpowders is sometimes empirically described as C7H4O. To obtain a coal with high purity, source material should be free of non-volatile compounds (sugar and a high charring temperature can be used). After charring, partial oxidation with oxygen or chlorine can reduce hydrogen levels. For activation of charcoal see activated carbon.
Common charcoal is made from peat, coal, wood, coconut shell, or petroleum. “Activated charcoal” is similar to common charcoal, but is made especially for use as a medicine. To make activated charcoal, manufacturers heat common charcoal in the presence of a gas that causes the charcoal to develop lots of internal spaces or “pores.” These pores help activated charcoal “trap” chemicals.
Commercial charcoal is found in either lump, briquette, or extruded forms:
Lump charcoal is made directly from hardwood material and usually produces far less ash than briquettes.
Pillow shaped briquettes are made by compressing charcoal, typically made from sawdust and other wood by-products, with a binder and other additives. The binder is usually starch. Some briquettes may also include brown coal (heat source), mineral carbon (heat source), borax, sodium nitrate (ignition aid), limestone (ash-whitening agent), raw sawdust (ignition aid), and other additives.
Hexagonal sawdust briquette charcoal are made by compressing sawdust without binders or additives. Hexagonal Sawdust Briquette Charcoal is the preferred charcoal in countries like Taiwan, Korea, Middle East, Greece. It has a round hole through the center, with a hexagonal intersection. Mainly for barbeque uses as it does not emit odor, no smoke, little ash, high heat, and long burning hours (exceeding 4 hours).
Extruded charcoal is made by extruding either raw ground wood or carbonized wood into logs without the use of a binder. The heat and pressure of the extruding process hold the charcoal together. If the extrusion is made from raw wood material, the extruded logs are then subsequently carbonized.
Japanese charcoal removes pyroligneous acid during the charcoal making. Therefore when burning, there are almost no stimulating smells or smoke. The charcoal of Japan is classified into three kinds.
White charcoal (Binchōtan) is very hard and has a metallic sound.
Ogatan is made from hardened sawdust. It is most often used in Izakaya or Yakiniku restaurants.
The characteristics of charcoal products (lump, briquette, or extruded forms) vary widely from product to product. Thus it is a common misconception to stereotype any kind of charcoal, saying which burns hotter or longer etc.