Low-Temperature Carbonization

the processing of solid fuels, such as coal, shale, or peat, by heating them in the absence of air in furnaces at temperatures up to 500°–550°C. During the process, the fuel is broken down into a solid residue called semicoke (50–70 percent yield), a primary gas (80–100 m³/ton), tar (8–25 percent yield), and a water condensate.

Industrial low-temperature carbonization was first employed in the early 19th century for the extraction of illuminating oils from solid fuels; such fuels were later supplanted by petroleum products. During World War I and World War II, a process was developed in Germany for the manufacture of artificial liquid fuel. In later years, improved methods were again used in complex systems for the extraction of tar, semicoke, and gas. The best tar is obtained from shale and boghead coal; the best semicoke is extracted from calcined coal. The yield and quality of products obtained are determined by the nature of the raw material, as well as by the method used and by furnace design.

Furnaces differ in the method by which heat is supplied; they may feature external heating through the walls of the furnace chamber or internal heating by direct contact between the raw material and the heating medium, be it a gas or solid. Furnaces are usually of the continuously operating type. The latest methods of low-temperature carbonization involve the use of a fluidized bed, rapid heating, and other techniques.