Charcoal, petroleum cokes, coal, chars, and metallurgical coke are produced to be used as fuels, or in metallurgical applications as reducing agents. This chapter discusses production of other carbonaceous solids that find use because of their special properties as solids, not as fuels or as reducing agents. Premium carbon materials can be sold at prices well above the cost of the feedstocks, sometimes several US dollars per kilogram. Production and sale of such materials, even as by-product associated with a larger fuel conversion or refining operation, represents a significant revenue source.

Activated carbons

Activated carbons derive from a variety of feedstocks, including wood, coal, agricultural products, and peat. The usual method of production involves carbonizing the feedstock and reacting the char (either subsequently or simultaneously) with a reactive gas, to develop porosity and a high degree of internal surface area in the solid. Activated carbons have a wide range of uses in the adsorption of impurities from liquid or gas streams, including many applications in environmental protection or remediation.

Two strategies, physical or chemical activation, are used for production of activated carbons. These terms are commonly accepted and used in the activated carbon field, but unfortunately are slightly misleading, in that both rely on chemical reactions, albeit of different sorts. Physical activation begins with carbonization. Then, in a second, separate processing step, the char undergoes partial gasification, usually by reaction with steam or carbon dioxide. This gasification step actually does the activation, opening or creating porosity in the carbon, and increasing its internal surface area. In chemical activation, the feedstock is impregnated with a dehydrating agent that will degrade cellulose, such as phosphoric or sulfuric acids, and zinc chloride. The impregnated feedstock is then carbonized; during carbonization the added reagent decomposes part of the feedstock structure, thereby creating or opening internal porosity. Washing the reagent out of the now-activated char at the end of the carbonization step leaves the activated carbon.