Dry distillation is the
heating of
solid materials to produce
gaseous products (which may condense into liquids or solids). The method may not involve
Pyrolysis or
Thermolysis. The products are
condensed and collected. This method usually requires higher temperatures than classical
distillation. The method has been used to obtain liquid
fuels from
coal and
wood. It can also be used to break down mineral
salts such as
sulfates through thermolysis, in this case producing
sulfur dioxide/
sulfur trioxide gas which can be dissolved in water to obtain
sulfuric acid. By this method sulfuric acid was first identified and artificially produced. When substances of vegetable origin, e.g. coal, oil shale, peat or wood, are heated in the absence of air (dry distillation), they decompose into gas, liquid products and coke/charcoal. The yield and chemical nature of the decomposition products depend on the nature of the raw material and the conditions under which the dry distillation is done. Decomposition within a temperature range of 450 to about 600°C is called
Carbonization or low-temperature degassing. At temperatures above 900°C, the process is called
Coking or high-temperature degassing. If coal is
gasified to make
coal gas or carbonized to make coke then
Coal tar is among the by-products.