Thermal decomposition of coals

Often the terms carbonization and pyrolysis are used almost interchangeably. Pyrolysis has the broader meaning: breaking apart of molecules by application of heat or thermal energy. As discussed in Chapters 19 and 22, pyrolysis processes could be run to make gases or liquids rather than solids as the primary product. Carbonization, more narrowly defined, refers to conversion of a starting material into carbon, or a carbon-rich solid. It is entirely possible, and indeed often done, to pyrolyze a hydrocarbon feedstock for the purpose of carbonization, but carbonization is not simply pyrolysis by another name. Carbonization can be effected without using heat as the primary driving force, a good example being carbonization of sucrose (ordinary table sugar) by pouring concentrated sulfuric acid on it; carbonization occurs very promptly and very effectively. Carbonization driven by thermal energy usually requires temperatures >500 °C.

A carbonaceous solid that appears to have passed through an intermediate fluid state when being produced is called a coke. Carbonaceous solids that do not pass through such a fluid state during formation are chars. These definitions apply to carbonization processes using any feedstock, including biomass, petroleum, and polymers. All coals, regardless of whether they are caking or coking coals, leave a solid carbonaceous residue at the end of the carbonization process. Chars, if heat-treated to extreme temperatures, e.g. ≥2500 °C, do not form graphite, while cokes do. That is, chars are non-graphitizable, while cokes are graphitizable [A].