It is a black mineral of plant origin which is chemically, a complex mixture of elemental carbon, compounds of carbon containing hydrogen, oxygen, nitrogen and sulphur.
Formation of coal
Coal is believed to have been formed about 300 million years ago under the Earth by a process called carbonization.
Carbonization is the process of slow conversion of vegetable matter to coal under the Earth due to the action of high pressure, high temperature, anaerobic bacteria and absence of oxygen.Types of coal
Depending upon the extent of carbonization, coal can be classified into four types as follows:
Classification of coal
| Type of Coal | Carbon content | Commonly known as |
|---|---|---|
| Peat | 11% | - |
| Lignite | 38% | Soft coal / brown coal |
| Bituminous | 65% | Household coal |
| Anthracite | 96% | Hard coal |
Peat is the first stage in the conversion of vegetable matter to coal while anthracite is the last.
Destructive distillation of coal
(Coking of coal)
The process of heating coal in the absence of oxygen to obtain useful products is called destructive distillation of coal.
Laboratory method of destructive distillation of coal
Aim
Destructive distillation of coal
Materials required
Two hard glass test tubes marked A and B, delivery tubes, clamp stand, burner, rubber stoppers, pieces of coal and water.
Principle
The volatile matter present in coal escapes on heating coal to a high temperature in the absence of oxygen.
Procedure
- Small pieces of coal are taken in test tube A.
- Test tube A is fitted with a rubber stopper carrying a delivery tube and is clamped to the clamp stand.
- Test tube B containing water is clamped vertically to the clamp stand.
- The apparatus is assembled as shown in the figure.
- The burner is lighted and the test tube A is heated first gently and then intensely.
Products formed and their uses
| Product | Formed/collected in | Uses |
|---|---|---|
| Coal Tar (complex mixture of carbon compounds) | Bottom of the test tube B. Liquid residue insoluble in water | Can be distilled to obtain: Benzene — solvent Toluene — manufacture of explosive TNT Naphthalene — insect repellent |
| Coal gas (CH4+CO+H2) | Combustible gas insoluble in water. Escapes through the side tube | Industrial fuel |
| Liquor ammonia (NH4OH) | Soluble in water present in test tube | Manufacture of nitrogenous fertilizers |
| Coke (98%C) | Solid residue left behind in test tube A | i) Reducing agent in metallurgy ii) Manufacture of water gas and producer gas — Industrial fuel |
Flow chart of the process taking place in a coking plant (same as destructive distillation)
Uses of coal
- On destructive distillation, coal yields useful products like coal gas, coal tar, ammonia and coke.
- Generation of electricity
- Domestic fuel for cooking and heating
- Manufacture of synthetic petroleum and natural gas
- To heat water in steam engines and boilers
