FUELS

Introduction

Generally a fuel may be defined as a substance which on burning with oxygen in the atmospheric air, produces a large amount of heat. A fuel mostly contains carbon and hydrogen. The amount of heat generated is known as calorific value of the fuel. Fuel is the most important thing in the earth in terms of producing heat for different purposes. As 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.

Classification of fuels

The fuels may be classified into the following three general forms-

1.) Solid fuels

2.) Liquid fuels

3.) Gaseous fuels

Each of the fuels may be further subdivided into the following two types-

A.) Natural fuels

B.) Prepared fuels

Solid fuels

Solid fuels are those fuels which appears solid in physical state. 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.

The following solid fuels are mostly used in terms of utility-

1.) Wood: In previous days,it was extensively used as a fuel. It mainly consists of carbon and hydrogen. The wood is converted into coal when burnt in the absence of air. It is not considered as a commercial fuel, except in industries where a large amount of waste wood is available. The calorific value of wood Varies with its kind and moisture content. The average calorific value of wood is 19700 kj/kg.

2.) Peat: It is a spongy humid substance found in boggy land. It may be regarded as the first stage in the formation of coal. It has a large amount of water contents (upto 30%) and therefore has to be dried before use. It contains only 10% carbon.It has a characteristic odour at the time of burning and has a smoky flame. 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 bituminous coal and peat. It contains nearly 30 to 50% carbon. Rest it contains volatile matter and moisture.When dried,it crumbles and hence doesn’t store well. Due to its brittleness it is converted into briquettes,which can be handled easily. Its average calorific value is 25000 kj/kg.

4.) Bituminous coal: It represents the next stage of lignite in the coal formation. It contains 60 to 80% carbon and 20 to 40% volatile matter and moisture. It is weather resistant and burns with a yellow flame. The average calorific value of bituminous coal is 34000 kj/kg. The bituminous coal is of the following two types-

A.} CAKING BITUMINOUS COAL: It softens and swells on heating. Its pieces adhere together forming a pasty mass which makes firing difficult. It burns with a fairly long flame. Its specific gravity is 1.26 to 1.36. The caking variety is very useful for Manufacturing gas. It is also known as soft coal. Its average calorific value is 36000 kj/kg.

B.} NON- CAKING BITUMINOUS COAL: It burns with a shorter flame than the caking coal and gives off little or no smoke. Its specific gravity is 1.22 to 1.42. The non caking variety is mostly used as fuel for steam boilers, hence it is known as steam coal. Its average calorific value is 34000 kj/kg.

NOTE; If the carbon content is 78 to 81% then the bituminous coal is non- caking. If the percentage of carbon is 81 to 83% then it is slightly caking. In medium caking bituminous coal the carbon content is 83 to 85%. But if the carbon content is 85 to 90% then it makes coal strongly caking.

5.) Anthracite coal: It represents the final stage in the coal formation. It contains more than 90% carbon and very less volatile matter and moisture. It is best form of coal available. It is thus obvious that the anthracite coal is comparatively smokeless and has very little flame. It possesses a high calorific value of 38000 kj/kg and therefore very useful 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 id is used for various metallurgical processes.

7.) Coke: It is produced when coal is strongly heated continuosly for 45 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 higher calorific value than coal.

NOTE; If the carbonisation of coal is carried out at 500 to 700°C then the resulting coke is called lower temperature coke or soft coke. It is generally used as a domestic fuel. That coke produced at a temperature of 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. Sometimes it also used 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 binding materials used are pitch,coal tar, crude oil and clay etc. The briquetted coal has a 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 fine powder by pulverising machines. The pulverised coal is widely used in cement industry and also in metallurgical processes.

NOTE: Out of the all above solid fuels, anthracite coal is commonly used in all types of heat engines.

Liquid fuels

Almost all the commercial liquid fuels are derived from natural petroleum or crude oil. The crude oil is obtained from bore holes in the earth’s crust in certain part of the world. The liquid fuels consist of hydrocarbons. The natural petroleum may be separated into petrol or gasoline, paraffin oil or kerosene,fuels oil and lubricating oils by boiling the crude oil at different temperatures and subsequent fractional distillation or by process such as cracking. The solid products like vaseline and paraffin wax are recovered from the residue in the still.

The following liquid fuels are generally used-

1.) Petrol or gasoline: It is the highest and most volatile liquid fuel which is mainly used for light petrol engines. It is distilled at a temperature of 65 to 220°C.

2.) Kerosine or paraffin oil: It is heavier and less volatile fuel than the petrol and is used as heating and lighting fuel. It is distilled at a temperature of 220 to 350°C.

3.) Heavy fuel oils: The liquid fuels distilled after petrol and kerosene are generally known as heavy fuel oils. These oils are used in diesel engines and in oiled fired boilers. They are distilled at a temperature of 350 to 470°C.

Merits and demerits of liquid fuels over solid fuels:

Merits-

1.) Higher calorific value.

2.) Best control of consumption by use of valves.

3.) Better economy in handling.

4.) Low storage capacity is required.

5.) Practically no ashes.

6.) Higher efficiency.

7.) Non corrosive to boiler plates.

8.) Better cleanliness and freedom from dust.

9.) Non- deterioration in storage.

Demerits-

1.) Higher cost.

2.) Costly containers are required for storage and transport.

3.) Greater risk of fire.

Gaseous fuels:

The natural gas is usually find in or near the petroleum fields, under the earth’s surface. It essentially consists of methane together with small amount of other gases such as ethane, carbon dioxide and carbon monoxide.

Following gaseous fuels are generally used-

1.) Coal gas: It is also known as town gas. It is obtained by the carbonisation of coal and consist mainly hydrogen, carbon monoxide and various hydrocarbons. The quality of the coal gas depends upon the quality of the coal used, temperature of the carbonisation and the type of plant. 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 25000 kj/m^3.

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 5000 to 6700 kj/m^3.

3.) Water gas: It is mixture of hydrogen and carbon monoxide and is made by passing steam over incandescent coke. As it burns with a blue flame,it is also known as blue water gas. The water gas is usually converted into carburetted i.e. enriched water gas by passing it through a carburettor into which a gas oil is sprayed. It is usually mixed with coal gas to form town gas. The water gas is used in furnaces and for welding.

4.) Mond gas: It is produced by passing air and a large amount of steam over waste coal at about 650°C. It is used for power generation and heating. It is also suitable for use in gas engines. Its calorific value is about 5800 kj/m^3.

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 in gas engines,for steam raising in boilers and for preheating the blast furnace. It is extensively used as fuel for metallurgical furnaces. The gas leaving the blast furnace has a high dust content the proportion of which varies with the operation of the furnace. It has a low heating value of about 3700 kj/m^3.

6.) Coke oven gas: It is a by-product from coke oven and is obtained by the carbonisation of bituminous coal. Its calorific value varies from 15000 to 18000 kj/m^3. It is used for industrial heating and power generation.

Merits and demerits of gaseous fuels:

1.) The supply of fuel gas and hence the temperature of furnace is easily and accurately controlled.

2.) They are free from solid and liquid impurities.

3.) They don’t produce ash and smoke.

4.) They are directly used in internal combustion engines.

5.) The high temperature is obtained at a moderate cost by preheating gas and air with heat of waste gases of combustion.

6.) They undergo complete combustion with minimum air supply.

Demerits-

1.) They are readily inflammable.

2.) They required large storage capacity.

That was all important information about solid, liquid and gaseous fuels. Hope you all like this article.

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