The natural solid fuels are wood, peat, lignite or brown coal, bituminous coal and anthracite coal. The prepared solid fuels are wood charcoal, coke, briquetted coal and pulverised coal. These fuels are discussed, as follows:
1. Wood. It consists of mainly carbon and hydrogen. The wood is converted into coal when burnt in the absence of air. The average calorific value of wood is about 19700 kJ / kg.
2 Peat. It may be regarded as the first stage in the formation of coal. Its average calorific value is 23000 kJ / kg.
3. Lignite or brown coal. It represents the next stage of peat in the coal formation, and is an intermediate variety between bituminous coal and peat. Its average calorific value is 25000 kJ / kg.
4. Bituminous coal. It represents the next stage of lignite in the coal formation and contains very little moisture (4 to 6 %) and 75 to 90 % of carbon. The average calorific value of bituminous coal is 33500 kJ / kg.
5. Anthracite coal. It represents the final stage in the coal formation, and contains 90% or more carbon with a very little volatile matter. It possesses a high calorific value of about 36000 kJ / kg and is, therefore, very valuable for steam raising and general power purposes.
6. Wood charcoal. It is made by heating wood with a limited supply of air to a temperature not less than 280° C. It is a good prepared solid fuel, and is used for various metallurgical processes.
7. Coke. It is produced when coal is strongly heated continuously for 42 to 48 hours in the absence of air in a closed vessel. This process is known as carbonisation of coal. Coke is dull black in colour, porous and smokeless. It has a high carbon content (85 to 90%) and has a higher calorific value than coal:
If the carbonisation of coal is carried out at 500 to 700°C, the resulting coke is called lower temperature coke or soft coke. It is used as a domestic fuel. The coke produced by carbonisation of coal at 900 to 1100°C, is known as hard coke. The hard coke is mostly used as a blast furnace fuel for extracting pig iron from iron ores, and to some extent as a fuel in cupola furnace for producing cast iron.
8. Briquetted coal. It is produced from the finely ground coal by moulding under pressure with or without a binding material. The briquetted coal has the advantage of having, practically, no loss of fuel through grate openings and thus it increases the heating value of the fuel.
9. Pulverised coal. The low grade coal with a high ash content, is powdered to produce pulverised coal. The coal is first dried and then crushed into a fine powder by pulverising machines. The pulverised coal is Widely used in the cement industry and also in metallurgical processes.
