Tar was the name given to the resinous liquid which exudes from pine wood during slow combustion, or by cooking oleoresin (obtained from a wounded conifer) in an open pot. Pitch is obtained by further heating the tar until a carbonised product is obtained. The condensed volatile component of the oleoresins was collected by stretching a sheepskin across the top of the pot to produce oil of turpentine. Pine tar has been used to treat chronic dermatitis, and has found use as an expectorant. Pyrolysis is the thermal decomposition of wood in the absence of oxygen, resulting in the production of combustible and non-combustible gases and vapours, plus a residue of charcoal. Wood is decomposed in a stepwise manner: the hemicelluloses are decomposed in the temperature range 200-260oC, cellulose at 240-350oC, and lignin at 280-500oC. Gas, tar, oil, wood vinegar and wood alcohol are produced in the wood carbonisation processes, with yields depending upon starting material and processing conditions. The crude tar and oil fractions are very complex mixtures, which are distilled into light and heavy oil fractions. The heavy oil fraction can be processed to creosote. The phenolic constituents of the tar can be used in adhesives for plywood. Wood alcohol contains about 60% methanol combined with various impurities. The wood vinegar fraction can be purified to give acetic acid. The non-condensable wood gas (a mixture of carbon dioxide, carbon monoxide, methane, hydrogen, hydrocarbons) is used in industrial charcoal production to pre-dry the wood and to flush out the retorts. Heating of wood at about 1000oC yields wood gas, the exact composition of which depends upon process conditions and starting material. The process can be carried out as a pyrolitic process, or in the presence of air, oxygen, or steam. With gasification in air (producer gas), a gas of lower calorific value. Gasification in oxygen and steam yields water gas. Cleaning then enriching the gas with hydrogen, produces synthesis gas, which can be used industrially for the production of methanol. Other possible products include ammonia, methane and other higher aliphatic hydrocarbons.
Contributor: Dr Callum Hill FIMMM