1. Solid fuel.
2. Liquid fuel.
3. Gaseous fuels.
Each of these fuels may be further subdivided into two types :
(a) Natural fuels, and (b) Prepared fuels.
What are Solid Fuels?
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 followed :
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Wood |
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 values of wood is about 19700 k J/kg.
2. Peat :
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Peat |
3. Lignite or brown coal :
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Lignite or brown coal |
4. Bituminous coal :

5. Anthracite coal :

It is made by heating wood with a limited supply of air to a temperature not less than 2800 c. It is a good prepared solid fuel, and is used for various metallurgical processes.
7. Coke :

8. Briquette coal :
It is produced from the finely ground coal by moulding under pressure with or without a binding material. Briquetted coal has the advantages of having, practically, no loss of fuel through grate openings and thus it increases the heating value of the fuel.
9. Pulverised coal :

What are Liquid fuel?
Almost all the commercial fuels are derived from natural petroleum(or crude oil). The liquid fuels consist of hydrocarbons. The natural petroleum may be separated into petrol or gasoline, paraffin oil or kerosene, fuel oils and lubricating oils by boiling the crude oil at different temperatures and subsequent fraction distillation or by a process such as cracking. The solid products like Vaseline and paraffin wax are recovered from the residue in the still. The following are some important liquid fuels:
1. Petrol or gasoline :
It is the lightest and most volatile liquid fuel, mainly used for light petrol engines. It is distilled at a temperature from 650 to 2200C.
2. Paraffin oil or kerosene :
It is heavier and less volatile fuel than liquid fuel, and is used as heating and lighting fuel. It is distilled at a temperature from 2200 to 3450.
3. Heavy fuel oils :
The liquid fuels distilled after petrol and kerosene are known as heavy fuel oils. These oils are used in diesel engines and in oil-fired boilers. This is distilled at a temperature from 3450 to 4700C.
What are Gaseous fuels ?
The natural gas is, usually, found in or near the petroleum fields, under the earth's surface. It, essentially consist of marsh gas or methane(CH4)together with small amount of other gases such as ethane(C2H6), carbon dioxide(CO2) and carbon monoxide(CO). The following prepared gases, which are used as fuels, are important :
1. Coal gas :
It is also known as a town gas. It is obtained by the carbonisation of coal and consists mainly of hydrogen, carbon monoxide and various hydrocarbons. It is very rich among combustible gases, and is largely used in towns for street and domestic lighting and heating. It is also used in furnaces and for running gas engines. Its calorific value is about 21000 to 22500 kj/m3.
2. Producer gas :
It is obtained by the partial combustion of coal, coke, anthracite coal or charcoal in a mixed air-steam blast. It is, mostly, used for furnaces particularly for glass melting and also for power generation. Its manufacturing cost is low, and has a calorific value of about 5000 to 6700 kj/m3.
3. Water gas :

4. Mond gas :
It is produced by passing air and a large amount of steam over waste coal at about 6500C. It is used for power generation and heating. It is also suitable for use in gas engines. Its calorific value is about 5850 kj/m3.
5. Blast furnace gas :
5. Blast furnace gas :


6. Coke oven gas :
It is by-product from coke oven, and is obtained by the carbonisation of bituminous coal. Its calorific value varies from 14500 to 18500 kj/m3. It is used for industrial heating and power generation.
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