Burning of coal in absence of air to obtain some useful constituents is called destructive Distillation. Following constituents are obtained from this process :
(i) Coke : It is a hard porous substance which is almost the purest form of carbon. It is used in manufacture of steel and in extraction of many metals.
(ii) Coal-tar : It is a black liquid with unpleasant smell. It is a mixture of about two hundred substanes which are very useful in our day to day life. They are used in drugs, plastics, paints, photographic materials, synthetic dyes, roofing materials, perfumes, explosives etc. Insect repellants like naphthalene is also obtained from it. Earlier, it was also used for surfacing of roads.
(iii) Coal gas : This gas is also obtained during destructive distillation of coal. It is used as fuel in coal processing plants and in various other industrial units situated near coal processing plants. Coal gas was used by the Americans and the Britishers for street lighting.