FAO - Charcoal, Carbonization Processes
The carbonization stage may be decisive in charcoal production even though it is not the most expensive one. Unless it is carried out as efficiently as possible, it puts the whole operation of charcoal production at risk since low yields in carbonisation reflect back through the whole chain of production as increased costs and waste of resources.
Wood consists of three main components: cellulose, lignin and water. The cellulose and lignin and some other materials are tightly bound together and make up the material we call wood. The water is adsorbed or held as molecules of water on the cellulose/lignin structure. Air dry or "seasoned" wood still contains 12-18% of adsorbed water. Growing, freshly cut or "unseasoned" wood contains, in addition, liquid water to give a total water content of about 40 to 100% expressed as a percentage of the oven dry weight of the wood.
The water in the wood has all to be driven off as vapour before carbonization can take place. To evaporate water requires a lot of energy so that using the sun to pre-dry the wood as much as possible before carbonization greatly improves efficiency. The water remaining in the wood to be carbonised, must be evaporated in the kiln or pit and this energy must be provided by burning some of the wood itself which otherwise would be converted into useful charcoal.