- Wood charcoal: It is obtained by destructive distillation of wood in a crude method. It is prepared by slow burning of logs in a domed type of earthen structure under controlled conditions of air supply. It takes many days to obtain charcoal. During the process, the volatile matter and water are expelled to the atmosphere. The temperature of heated air is not allowed to increase beyond about 300°C. It burns rapidly with a clear flame without smoke. It is soft and dark black in colour. It is mainly used for domestic purposes.
- Coke: It is prepared by removing the volatile matter from bituminous coal. It is hard, brittle, and porous. It is generally prepared by heating in an electric furnace, followed by water sprays. It contains carbon, 2% Sulphur, and small quantities of hydrogen, nitrogen, and phosphorus. It is mainly used in blast furnace to produce heat and reduce iron ore.
- Briquetted coal: It consists of finely ground coal or coke mixed with a suitable binder and pressed together to form blocks or briquettes of any shape. In the briquetted form, the heating value of low-grade coal is increased.
- Pulverised Coal: The crushed coal to powder is called pulverised coal. The fineness of powdered coal is so adjusted that it floats during the burning process. This gives better contact between air and fuel, which results in very high combustion efficiency. The pulverising method is used for steam raising in boilers by using low grade and rough fuels. The advantages of coal pulverising are as follows:
- Flexibility of control
- Complete combustion with lesser excess air
- High flame temperature
Secondary Fuels
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