Combustion of Fuels We know that fuels produce energy only when they are burnt in the presence of oxygen. This process of burning of fuels in the presence of oxygen to produce energy is called combustion. The combustion of fuels can take place if following two essential conditions are satisfied:

1. Heating the fuel to its ignition temperature
No substance can catch fire unless it is heated up to a certain minimum temperature. This minimum temperature at which a substance begins to burn is called its ignition temperature. The ignition temperature of every substance is definite. For example, the ignition temperature of white phosphorous is 35°C. It means, white phosphorous will start burning only if it is heated up to 35°C.

To understand ignition temperature in detail watch this video

2. Presence of supporter of combustion
Those substances which help in combustion of fuels are called supporter of combustion. For example, oxygen is a supporter of combustion because in the absence of oxygen fuels cannot burn.

Characteristics of a Good Fuel
A fuel can be considered good if it possesses following characteristics:

1. The cost of fuel should be low and it should be easily available.
2. There shouldn’t be any problem in its storage and transportation.
3. Its ignition temperature should be proper.
4. Its calorific value must be high.
5. It should not produce poisonous gases.
6. No residue should be left behind after its combustion.

Test Your Understanding and Answer These Questions:

1. What is combustion?
2. What are the essential conditions for burning of fuels?
3. What is ignition temperature?
4. What is supporter of combustion?
5. What are characteristics of a good fuel?

When fuels are burnt, heat is produced. The amount of heat produced by different types of fuels on burning is expressed in terms of calorific value. Calorific value of a fuel may be defined as the amount of heat produced on complete burning of 1 gm of fuel. S.I. unit of calorific value of fuels is kilojoule per gram (KJ/g). For example, when one gram of wood is burnt completely it produces 17 kilojoules heat. Therefore, the calorific value of wood is 17 KJ/g. in similar manner, when 1 gm of kerosene oil is burnt completely it produces 48 kilojoules heat. So, the calorific value of kerosene oil is 48 KJ/g. the calorific value of different types of fuels is given in following table.

Table of calorific value of fuels

Significance of Calorific Value
It becomes clear from table given above that different fuels have different calorific values, i.e. different fuels produce different amounts of heat on burning. The calorific value of fuels helps us to decide that which fuel is good for us. This is done by comparing the calorific values of fuels with each other. Usually, a fuel having higher calorific value is considered to be a good fuel.

1)   Case of Hydrogen Gas
Hydrogen gas has the highest calorific value of 150 KJ/g among all the fuels. So, hydrogen gas is considered to be an extremely good fuel. However, hydrogen gas is not used as a fuel in homes and industries. The main reasons for this are:

1. Hydrogen gas is highly combustible and it burns with explosion when lighted.
2. The storage and transportation of hydrogen gas from one place to another is very difficult.
3. The cost of production of hydrogen gas is very high. So, it is an expensive fuel.

2)   Case of Methane and Butane
Almost all the fuels consist of atoms of hydrogen and carbon. Since, the calorific value of fuels depends on the percentage of hydrogen present in them so the fuels which have higher percentage of hydrogen will have higher calorific value as compared to those fuels which have lower percentage of hydrogen. The percentage of hydrogen present in methane is 25% whereas the percentage of hydrogen present in butane is 17%. Due to this reason, methane has higher calorific value as compared to butane.

Test Your Understanding and Answer These Questions:

1. What is calorific value?
2. Give calorific value of different fuels?
3. What is the significance of calorific value?
4. Which fuel has higher calorific value out of methane and butane?

 Natural gas is a very good fuel which is formed by slow anaerobic decomposition of organic matter like plant and animal wastes inside the earth. Usually, natural gas is found over petroleum in oil wells; however there are also some oil wells which give only natural gas. Natural gas is a mixture of three gases namely methane, ethane and propane. But, the chief constituent of natural gas is methane.

Natural gas can be easily compressed by applying pressure. In the compressed state it is called compressed natural gas or CNG. Compressed natural gas is used as a fuel in vehicles because it burns easily and does not cause air pollution. Besides it, natural gas is also used as a fuel in homes, industries and thermal power plants.

The top natural gas producing countries in the world are: Russia, U.S.A., Canada, U.K. and Algeria. In India reserves of natural gas are found in Krishna-Godavary delta, off-shore areas of Mumbai, Tripura and Jaisalmer.

Test Your Understanding and Answer These Questions:

1. What is natural gas?
2. What is compressed natural gas?
3. What is composition of natural gas?
4. Name the chief constituent of natural gas.

Petroleum is a dark coloured viscous fossil fuel which was formed millions of years ago by anaerobic digestion of remains of sea animals that got buried in the earth. It is also called crude oil. Petroleum is present in the earth trapped between non-porous rocks in the form of large reservoirs. Petroleum is extracted from such reservoirs by drilling holes in the earth’s crust and sinking pipes in them.

The top petroleum producing countries in the world are: Saudi Arabia, U.S.A., Russia, Iran and Mexico. Unfortunately, Indian does not have rich resources of petroleum. In India petroleum is extracted in: Bombay High, Ankleshwar in Gujarat and Rudra Sagar and Lakwa in Assam.

Fractional Distillation of Petroleum
Petroleum is a mixture of large number of hydrocarbons. So, before using it, it becomes essential to separate it into its various fractions. The process which is used to separate various fractions of petroleum is known as fractional distillation. The fractional distillation of petroleum is done in a tall distillation tower.

To separate various fractions of petroleum, first of all petroleum is heated to a temperature of 400⁰C. At this temperature all the fractions of petroleum except asphalt begins to vaporize. Then these vapours are fed into the bottom of the distillation tower. In the distillation tower, the vapours of petroleum rises up, get cooled and condense into different liquids at different heights. The remaining gases which do not liquefy are taken out from the top of the distillation tower. Finally, the various fractions of petroleum that condensed at different heights of distillation tower are collected and stored. The fractions of petroleum obtained in the distillation tower from bottom to the top are: asphalt, lubricating oil, fuel oil, diesel oil, kerosene oil, petrol and petroleum gas.

All the fractions of petroleum except asphalt and lubricating oil are used as fuel because they can easily burn and produce a large amount of heat. The fuel oil is heavy grade oil which is used as fuel in ships. The diesel oil is used as fuel in heavy vehicles like tractors, trucks, buses, railway engines etc. kerosene oil is usually used in homes for cooking food. Petrol is used to power cars, scooters, motor cycles and piston engined aircrafts.

The gas collected from the top of the distillation tower is called petroleum gas. It is a mixture of many gases like butane, propane and ethane. However, the main constituent of petroleum gas is butane. The number of carbon atoms present in petroleum gas ranges between C₁ to C₄. Petroleum can be easily liquefied by applying pressure. In the liquid state it is called liquefied petroleum gas or LPG. Cooking gas cylinders used in homes contain liquefied petroleum gas. LPG is used in homes to cook food because it burns easily with a blue flame and produce a lot of heat. Also it does not produce smoke and other poisonous gases during combustion.

Precautions during leakage of LPG
 Liquid petroleum gas is an odourless gas. So, to detect its leakage one more gas called ethyl mercaptan is added in it. In case of detection of leakage of LPG following precautions should be used:

    1. Do not light the match stick.
    2. Do not switch on or off electrical appliances.
    3. Open all the windows and doors to allow the gas to escape.
    4. Immediately close the valve of cylinder and knob of burner.
    5. Inform the gas agency.

Test Your Understanding and Answer These Questions:

1. What is petroleum or crude oil?
2. How is petroleum formed?
3. What is fractional distillation?
4. Explain fractional distillation of petroleum.
5. What is lpg?
6. Which gas is mixed in lpg to detect its leakage?
7. Which gas is mixed in lpg to detect its leakage?

Coal has been used as a main source of energy by human beings for centuries. It is formed millions of years ago by anaerobic digestion of remains of large plants that got buried in the earth due to earthquakes.

Occurance of Coal
Almost in every country big reserves of coal are present. In our country coal is found mainly in coal mines of Jharia and Bokaro in Bihar and Raniganj in West Bengal. In addition of it coal is also found in Orisa and Madhya Pradesh.

Composition of Coal
Coal consists of various types of substances such as carbon, hydrogen, oxygen, sulphur, nitrogen etc. however, the chief component of coal is carbon. The quality of coal depends on the percentage of carbon present in it. On the basis of difference of percentage of carbon, coal can be classified into four types. The variety of coal having lowest percentage of carbon is called peat. Peat consists of about 27% carbon. The variety of coal having highest percentage of carbon (95%) is anthracite. And the rest of two varieties of coal called lignite and bitumen contains about 35% and 80% of carbon respectively. Out of these four varieties of coal, anthracite is considered to be the most superior quality of coal.

Uses of Coal
When coal is burnt, it produces a large amount of energy in the form of heat and light. Due to these reasons coal is used;

    1. in homes to cook food.
    2. in industries for heating purpose.
    3. in thermal power plants to generate electricity.
    4. in the manufacture of coke, coal gas and coal tar.

Test Your Understanding and Answer These Questions:

1. What is peat?
2. What is anthracite?
3. How coal is formed?
4. Name the most superior quality of coal?
5. Name the variety of coal having highest percentage of carbon?

Those fuels which are formed millions of years ago from the anaerobic decomposition of remains of living organisms, buried under the earth are called fossil fuels. Fossil fuels are mainly of 3 types. The names of 3 fossil fuels are Coal, petroleum and natural gas. It is believed that millions of years ago the remains of plants and animals got buried under the earth because of earthquakes. Due to presence of high temperature, pressure and absence of oxygen in the earth, these buried remains of plants and animals were gradually converted into fossil fuels like coal, petroleum and natural gas. Combustion of fossil fuel is the main source of energy for modern world. Thus, fossil fuels are the cheapest and most effective source of energy.

Formation of Fossil Fuels
This process of formation of fossil fuels took place millions of years ago because of prevalence of special conditions on the crust of earth. But these conditions do not exist anymore. Thus, no new reserves of fossil fuels are being formed now. It means, we have finite reserves of fossil fuels. And within few years these reserves of fossil fuels will get exhausted if we used the fossil fuels at the present rate. Due to this reason, fossil fuels are classified as nonrenewable source of energy.

Test Your Understanding and Answer These Questions:

1. What are fossil fuels?
2. Name 3 fossil fuels?
3. Explain formation of fossil fuels.

 The fixed dome type bio gas plant consists of a closed underground digester tank made up of bricks which has a dome shaped roof also made up of bricks. This dome shape roof of the digester tank functions as gas holder and has an outlet pipe at the top to supply gas to homes.

The slurry is prepared by mixing water in cattle dung in equal proportion in mixing tank. The slurry is then sent into the digester tank with the help of inlet chamber. It should be noted that slurry is fed into the digester tank up to the point where the dome of the roof starts. Inside the digester tank, the complex carbon compounds present in the cattle dung breaks into simpler substances by the action of anaerobic microorganisms in the presence of water. This anaerobic decomposition of complex carbon compounds present in cattle dung produces bio gas and gets completed in about 60 days. The bio gas so produced starts to collect in dome shaped roof of bio gas plant and is supplied to homes through pipes. The spent slurry is replaced from time to time with fresh slurry to continue the production of bio gas.

Test Your Understanding and Answer These Questions:

1. How does fixed dome type biogas plant work?

 The floating gas holder type bio gas plant consists of a dome shaped gas holder made of steel for collecting bio gas. The dome shaped gas holder is not fixed but is moveable and floats over the slurry present in the digester tank. Due to this reason, this biogas plant is called floating gas holder type biogas plant.

Slurry is prepared by mixing water in cattle dung in equal proportion in mixing tank. The slurry is then injected into a digester tank with the help of inlet pipe. The digester tank is a closed underground tank made up of bricks. Inside the digester tank, the complex carbon compounds present in the cattle dung breaks into simpler substances by the action of anaerobic microorganisms in the presence of water. This anaerobic decomposition of complex carbon compounds present in cattle dung produces bio gas and gets completed in about 60 days. The bio gas so produced starts to collect in floating gas holder and is supplied to homes through pipes. And the spent slurry is replaced from time to time with fresh slurry to continue the production of bio gas.

Test Your Understanding and Answer These Questions:

1. How floating gas holder type bio gas plant works?

Biogas or (Gobar Gas) is a mixture of gases which is produced by anaerobic digestion of bio-mass in the presence of water. Generally, biogas is prepared by using animal dung, sewage, fallen leaves, rotten vegetables, straw and human excreta as bio-mass. The gases present in biogas are methane, hydrogen, hydrogen sulphide and carbon dioxide. But, the main constituent of biogas is methane which contributes about 65% part of the biogas.

Biogas a Boon to Farmers!
Biogas is an ideal fuel which burns without producing smoke. The heat produced by biogas is very high and it does not leave any residue behind after burning. Due to these reasons biogas is used in homes as a fuel for cooking food, and in agriculture to run engines. Biogas is also used for street lighting by producing electricity. In addition to these uses the spent slurry left behind in the overflow tank is a rich source of nutrients like nitrogen and phosphorous. So, it can be used as manure in fields. Thus, we see that using biomass to produce bio gas serves many purposes especially for farmers. That is why biogas is considered to be a boon to the farmers.

Biogas Plant Designs
In India mainly two designs of biogas plants are used to produce bio gas. These are:

    1. Floating gas holder type biogas plant and
    2. Fixed dome type biogas plant

In both bio gas plants a mixture of cattle dung and water also called slurry is used to produce bio gas.

Test Your Understanding and Answer These Questions:

1. What is biogas?
2. What is slurry?
3. What is composition of biogas?
4. Biogas is boon to a farmer, comment.
5. Name the main constitute of biogas plant.
6. Name two designs of biogas plant.