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Different Types Of Fuels?


A fuel, may be defined as a substance (containing mostly carbon and hydrogen)which on burning with oxygen in the atmospheric air, produces a large amount of heat. The amount of heat generated is known as calorific values of the fuel. Since the principal constituents of a fuel are carbon and hydrogen, therefore, it is also known as hydrocarbon fuel. Sometimes, a few traces of sulphur are also present in it. The fuel may be classified into the following three general forms :

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 : 

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 : 

 Peat

It may be regarded as the first stage in the formation of coal. Its average calorific values is 23000 k J/kg.



3. Lignite or brown coal : 

    Lignite or brown coal

It represents the next stage of peat in coal formation, and is an intermediate variety b/w 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 values of bituminous coal is 33500 kj/kg. 



5. Anthracite coal :

It represents the final stage in the coal formation, and contains 90% of more carbon with a very little volatile matter. It possesses a high calorific value of about 36000 kj/kg and is, 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 2800 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 7000c,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 10000C,is known as “hard coke”. The hard coke is mostly used as a blast furnace for extracting pig iron from iron ores, and to some extent as a fuel in cupola furnace for producing cast iron. 






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 : 

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.


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 : 

It is mixture of hydrogen and carbon monoxide and is made passing steam over incandescent coke. As it burns with blue flame, it is also known as blue water gas. The water gas is usually converted into carburetted(enriched) water gas by passing with it through a carburetter into which a gas oil is sprayed. It is, usually, mixture with coal gas to from town gas. The water gas is used in furnaces and from welding.

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 : 
It is a by-product in the production of pig iron in the blast furnace. This gas serves as a fuel in steel works, for power generation and in gas engines, for steam raising in boilers and for preheating the blast for furnace. It is extensive used as fuel for metallurgical furnace. The gas, leaving the blast furnace, has a high dust content, the proportion of which varies with operation of the furnace. It has a low heating value of about 3750 kj/m3.

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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