When coal is heated without air, it does not burn but produces many by-products. This process of heating coal in the absence of air is called destructive distillation of coal.
The main by products are:
The destructive distillation of coal can be carried out in the laboratory. The apparatus is set up as shown in the below figure.
Destructive Distillation of Coal
Finely powdered coal taken in a test tube is heated. As the coal breaks down coke, coal tar, ammonia and coal gas are produced. Coal tar collects at the bottom of the second test tube and coal gas escapes out through the side tube. The ammonia produced is absorbed in the water forming ammoniacal liquor (ammonium hydroxide). The black residue left in the 1st tube is called coke.
Coke is a fuel in its own right which can be used in homes and factories. But it is largely used as a reducing agent in the extraction of metals from their ores. It is also used in making fuel gases like water gas which is a mixture of carbon monoxide and hydrogen.
Another by product namely coal tar which is a mixture of different carbon compounds can be used to make soap, fats, dyes plastics perfumes, drugs, pesticides, explosives etc.
The other by product, ammonia is used for making fertilizers such as ammonium sulphate, ammonium superphosphate etc.
The coal gas, also called as a town gas is a mixture of hydrogen, methane and carbon monoxide which are combustible making the coal gas an excellent fuel having high calorific values.
It is wrong to suppose that the coal industry is declining. World coal production is doubled in the last thirty years and it is the most widespread of the fossil fuels. The coal mining industry is becoming efficient and safer.