CLASSIFICATION OF FUELs

they are classified as shown below.

1. According to the nature of their exist (state or phase)

(a) Solid fuels (b) Liquid fuels (c) Gaseous fuels

2. According to the nature of their origin

 (a) Natural fuels (Primary fuels or fossile fuels)

(b) Artificial fuels (Secondary fuels or prepared fuels.

SOLID FUELS:

Most solid fuels are obtained by mining. They burn in solid phase in the presence of oxygen. The solid fuels consist mainly carbon together with hydrogen, sulphur and some incombustible ash. Wood, peat, lignite, bituminous coal, anthrocite coal. Etc. are examples of solid fuels.

Wood: Wood is used to a considerable extent as a domestic fuel. It contains about 50% carbon. 6% hydrogen, 43% oxygen and 1% ash. The calorific value of air dried wood ranges from 10500 kJ/kg to 14650 kJ/kg. The wood is cheaply available, but the disadvantages of wood as a industrial fuel is its large moisture content and low calorific value.

Coal: Coal is formed from vegetation in different stages ranging from peat to anthracite. Anthracite is most valuable, and peat the least. The stages of coal formation is summarised below

Plant debris → peat → lignite → brown coal → bituminous coal → anthracite Coal

This series is sometimes referred as peat to anthracite.

Peat: Peat is the first stage in the formation of coal from wood. It is associated with large amount of water, and it may be as much as 90%, and only 10% solid. It is dried for about 1 to 2 months before using as a fuel. Its calorific value (1500 kJ/kg) is slightly greater than wood. During burning it emits large smoke. It is not considered as an important fuel.

Lignites :- Lignites contain high percentage of moisture (20 to 60%) and small percentage of carbon. They possess low calorific value and the lignite is the next stage to peat in coal formation. Lignites with 20% moisture are brown in colour, and are referred as brown coals. Lignite is used for generation of thermal power, fertiliser manufacture and for making briquettes to be used as domestic fuel. Its calorific value is about 21000 kJ/kg.

Bituminous coal: Bituminous coal contains 78 to 90% carbon, 20 to 45% volatile matter and small percentage (4-6%) moisture. It represents third stage of coal formation in peat anthracite series i.e., it forms after lignite. The calorific value is about 31600 kJ/kg. These coals are used for making metallurgical coke, coal gas and for domestic heating. It burns with long yellow and smoky flames.

Anthracite coal: Anthracite coal contains 90 to 98% carbon and little volatile matter which is less than 8%. It represent the last stage of coal formation. It burns without smoke, with a short, non-luminous flame giving intense local heating. The calorific value of fuel is about 35000 kJ/kg. and is suitable to use in steam generating units.

Charcoal :- Charcoal or wood charcoal contains about 80% carbon and is obtained by heating (burning) wood out of contact with air. Burning of wood in the absence of air is referred as destructive distillation. Water and various kinds of volatile matters are expelled. Because of its purity and low ash (3%) content charcoal is most widely used as metallurgical coal.

Coke :- Coke is obtained by heating bituminous coal in the absence of air. The time of heating is about 45 hours at a temperature of 100° C. The process of heating coal in the absence of air is called carbonisation of coal. Coke contains about 90% carbon, upto 2% sulphur and small quantities of hydrogen, nitrogen and phosphorus. It is mainly used in blast furnace as a fuel as well as a reducing agent. Its calorific value is about 29000 kJ/kg.

In certain applications coal is used in the form of briquettes. Briquettes are prepared from the fine power of coal or coke by compressing the material under pressure. Coal and coke may also be used in the from of fine powder form. Coal in a fine powder form can be obtained by crushing. And it is referred as pulverized coal.

Advantages, Limitations and Applications of Solid Fuels :

Solid fuels have the following advantages:-

• Cheaper than other fuels.

• Coal is raw material for producing other fuels such as coke, producer gas and water gas.

• Risk of fire is minimum.

Limitations :

• Contains incombustible matter.

• Need large amount of storage space.

• Properties are not uniform.

• Slow deterioration may take place with time. Handling is not easy, and therefore may be costly.

Applications :

Solid fuels are used in furnaces, boiler units for steam generation. They are also used for domestic applications.

LIQUID FUELS:

The liquid fuels are obtained from petroleum by distillation. Petroleum (crued oil) is found under the earth’s crust. Liquid fuels are also obtained from coal (coal tar, benzol etc.)

Petrol or gasoline :Petrol is the lightest and most volatile liquid fuel. It is mostly used as a fuel for petrol engines, air craft engines (aviation petrol). Its calorific value is about 46800 kJ/kg.

Kerosene :It is also known as paraffin oil. It is obtained by fraction distillation of crued oil between 140° C and 280° C. It is heavier and volatile than petrol and is used for illumination as well as heating purposes. It is also used as a fuel in farm tractors and jet engines. Its calorific value is about 46470 kJ/kg.

Diesel oil:Diesel oil may be obtained by straight distillation of several crued oils. This is used in Diesel engines. Its calorific value is about 46000 kJ/kg.

Heavy fuel oil :It is obtained by distillation of crued oil between 200°C to 360° C. Fuel oils are mostly used in steam power plant, heating furnaces and in Diesel engines. Its calorific value is 43900 kJ/kg.

Advantages, Limitations and Applications of Liquid Fuels :

Advantages:-

• Relatively higher calorific value

• Burns without leaving a solid residue and with less smoke

• Can be stored for long periods without deterioration.

• Requires less storage space

• Easy to handle and control

• Requires a small volume of excess air

• Relative cleanliness 

Limitations:-

• May have higher sulphur content

• Highest cost

• Risk of fire, and

• Require costly containers to store.

Applications :-

Liquid fuels are commonly used in internal combustion engines (i.e., petrol and diesel engines). They are also used in oil-fired furnaces.

GASEOUS FUELS:

Gaseous fuels may be classified as natural gas and manufactured gas. The natural gas is obtained from the earth’s crust. Gaseous fuels may be obtained by thermal treatment (distillation) of coal or oilThe common examples of manufactured gases are coal gas, producer gas, water gas, mond gas, coke oven gas and blast furnace gas.

Natural gas: Natural gas is found in the petroleum under the earth’s surface. It consists of methane (C*H_{4}) and ethane (C_{2}*H_{1}) together with small amounts of hydrogen sulphide, carbon dioxide and nitrogenIts calorific value is about 21000kJ / (m ^ 3)

Coal gas: It is obtained by the destructive distillation of bituminous coal in a fireclay retort. It consists of carbon monoxide, carbon dioxide, hydrogen and methane. It is used in boilers, gas engines, and also for lighting and heating applicationsIts calorific value is about 20110 kJ / (m ^ 3)

Producer gas: Producer gas is obtained by the partial burning of coal or coke in a mixed air-steam blast. It consists of carbon monoxide, nitrogen and small amount of hydrogen. It is mainly used in power generation and in glass melting furnaces. It has low calorific value (5024kJ / (m ^ 3)) because of higher content of nitrogen.

Water gas (Blue gas): Water gas is produced by blowing the steam through red hot coal or cokeIt consists of carbon monoxide and hydrogen along with small amount of carbon dioxide and nitrogen. Its calorific value is about 11700 kJ/m³. It is used in heating furnaces.

Mond gas: It is produced by passing air and large amount of steam over waste coal at 650° C. It is used in furnaces, gas engines, power generation and for heating purposes. Its calorific value is about 6490kJ / (m³)

Coke oven gas: Coke oven gas is obtained by the destructive distillation Of coal in air-tight coke ovens. It is made as by-product in the production of metallurgical coke from coke ovens. It is used in industrial heating and power generation. Its calorific value is about 19250kJ / (m³)

Blast furnace gas: It is obtained as by-product in the production of Pig iron from blast furnace. It is used in power plants, gas engines and for preheating the air blast for furnaces. Its calorific value is about 3440kJ / (m³)

Advantages, Limitations and Applications of Gaseous Fuels :

Advantages:-

• Easy combustion is possible with better control.

• Minimum air is needed for complete combustion.

• Can be easily conveyed through pipe lines, and supply to furnaces can be quickly adjusted.

• Can be produced with low grade coals.

• Clean to use

• They can be used directly in I.C., engines (gas engines).

• Burn without smoke and produce no ash.

• Free from impurities like sulphur and solid residence.

Disadvantages :-

• Require large storage tanks.

• Risk of fire and explosion are greater

• Tendency to leak.

Applications :

Gaseous fuels are used in gas engines, in boilers for steam raising. It is also used in metallurgical furnaces. The other applications of gaseous fuels include domestic and industrial heating.