Solid fuels can be classified into two types, they are primary fuels and secondary fuels.
- Wood: It is the most commonly used and easily available natural fuel. It has been used for cooking and other purposes for centuries. The important constituents of wood are cellulose fiber and water. On burning, it gives ash, which is used for cleaning utensils in rural and even urban areas. The calorific value of wood varies with its type and moisture content. It ignites easily at about 250°C and is used for igniting other fuels.
- Peat: It is a mixture of water and decayed vegetable matter. It contains a large amount of moisture and requires drying in sun for about one to two months before use. It is the first stage in the formation of coal from wood.
- Lignite: It is transformed from peat. It is brown in color and is also known as brown coal. Airdried lignite contains 10%–20 % moisture. It is used as a low-grade fuel. It is non–caking and burns with a large smoky flame.
- Bituminous coal: It is transformed from lignite. It has a shining, black appearance and comes in caking and non–caking varieties. It ignites easily and burns with a long yellow and smoky flame.
- Anthracite coal: It is transformed from bituminous coal. It is hard, brittle, and lustrous in appearance. It is non–caking and burns without smell, smoke, or flame. It is difficult to ignite. This coal has minimum ash, volatile matter, and moisture. Its calorific value is the highest and is suitable for steam generation in thermal power plants.
- 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 into 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 color. 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 furnaces 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 briquette form, the heating value of low-grade coal is increased.
- Pulverized Coal: The crushed coal to powder is called pulverized 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 pulverizing method is used for steam raising in boilers by using low-grade and rough fuels. The advantages of coal pulverizing are as follows:
(a) Flexibility of control
(b) Complete combustion with lesser excess air
(c) High flame temperature