Natural charcoal is a hard, porous, highly carbonaceous product formed during the heating of wood without access (or with limited access) of air in furnaces and retorts (sometimes in campfires as well). Depending on the type of wood, 1 m3 yields 140-180kg of coal, 280-400 kg of liquid products, and nearly 80 kg of fuel gas. The heat of combustion of wood charcoal is 30,000-35,000 kJ/kg (7,000-8,100 kcal/kg). The density of birch coal is 380 kg/m3. Pines (300 kg/m3) and spruces (260kg/m3) yield less dense coal.
The high porosity of wood charcoal is responsible for its significant adsorption properties. At normal temperature wood charcoal is capable of combining with the oxygen in air; this is the reason for cases of spontaneous combustion of wood charcoal. The moisture content of wood charcoal unloaded from furnaces and retorts is 2-4 percent. During storage, the moisture content rises to 7-15 percent. The ash content of charcoal should not exceed 3 percent and thecontent of volatiles, not more than 20 percent. A distinctive feature of wood charcoal is the low content of such admixtures as phosphorus and sulfur, which makes it indispensable in some metallurgical processes.
Wood charcoal is widely used in the production of crystalline silicon, carbon disulfide, activated carbon, and electrocarbon articles and as fuel for domestic use.