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Classification of Conventional Sources of Energy
The traditional sources of energy which are familiar to most people are called conventional sources of energy. The main conventional sources of energy are wood and fossil fuels (like coal, petroleum and natural gas).
The fuels derived from wood, coal and petroleum such as charcoal, coke, coal gas, petrol, diesel, kerosene, fuel oil and liquefied petroleum gas (LPG) are also known as conventional sources of energy (or conventional fuels). Even dung cakes (uple) is a conventional source of energy in our country.
In ancient times, the energy of flowing water and blowing wind were used through water-mills and wind-mills respectively, for very limited purposes like running flour mills (to grind grain to obtain flour) and water-lifting pumps. The energy of flowing water or wind is no longer used for these purposes.
The energy of flowing water and that of wind is now used to generate electricity. So, the energy of flowing water and of wind is now considered to be a non-conventional source of energy (or alternative source of energy). We will now discuss the most commonly used conventional sources of energy called fossil fuels which include coal, petroleum and natural gas. They are also called conventional fuels.
A natural fuel formed deep under the earth from the pre-historic remains of living organisms (like plants and animals) is called a fossil fuel. Coal, petroleum and natural gas are fossil fuels. Fossil fuels are dug out from the earth. Besides being used directly in homes, transport vehicles and industry, fossil fuels are the major source of energy for generating electricity in power plants. We will first understand how fossil fuels were formed.
How Fossil Fuels Were Formed
The plants and animals which died millions of years ago, were gradually buried deep in the earth and got covered with sediments like mud and sand, away from the reach of oxygen of air. In the absence of oxygen, the chemical effects of pressure, heat and bacteria, converted the buried remains of plants and animals into fossil fuels like coal, petroleum and natural gas. It should be noted that the buried remains of large plants were converted into coal whereas those of small plants and animals were converted into petroleum and natural gas.
Sun is the Ultimate Source of Fossil Fuels
We have just seen that fossil fuels have been produced from the remains of plants and animals that were buried in the earth long ago. Actually, it was the sunlight of long ago that made plants grow, which were then converted into fossil fuels. This can be explained as follows :
The green plants need sunlight energy to grow. They get this energy from sunlight, and store it in the form of carbon compounds. So, every leaf and every bit of wood in the plant was made with the energy of sunlight. The animals also eat plants, so the animal material is also a store-house of sun’s energy.
These plants and animals which were originally made by using sun’s energy have been converted into fossil fuels like coal, petroleum and natural gas. Thus, fossil fuels are energy-rich compounds of carbon which were originally made by the plants with the help of sun’s energy (solar energy).
Without sunlight, there could have been no coal, petroleum, natural gas, wood or any other fuel in this world. Today when we burn coal, petroleum or natural gas, we are actually making use of the sunlight energy that was stored by the plants millions of years ago. We will now discuss the three fossil fuels, coal, petroleum and natural gas, one by one. Let us start with coal.
Coal is a complex mixture of compounds of carbon, hydrogen and oxygen, and some free carbon. Small amounts of nitrogen and sulphur compounds are also present in coal (see Figure). It is found in deep coal mines under the surface of earth.
Coal is important because it can be used as a source of energy as such, or it can be converted into other forms of energy like coal gas, electricity and oil (synthetic petrol). The exploitation of coal as a source of energy made the industrial revolution possible.
When coal is burnt, the carbon present in coal reacts with the oxygen of air to form carbon dioxide. A lot of heat is produced during the burning of coal which makes it a good fuel.
The burning of coal, however, produces a lot of smoke which pollutes the air. When coal is subjected to destructive distillation (by heating strongly in the absence of air), then all the volatile material is removed from coal and coke is formed. Coke is 98% carbon.
Coke is a better fuel than coal because it produces more heat (than an equal mass of coal), and it does not produce smoke while burning. Thus, burning of coke does not cause air pollution. Coke is, however, more valuable when used as a reducing agent in the extraction of metals from their ores.
Uses of Coal
- Coal is used as a fuel for heating purposes in homes and in industry.
- Coal is used as a fuel in thermal power plants for generating electricity.
- Coal is used to make coke. And this coke is then used as a reducing agent in the extraction of metals.
- Coal is used in the manufacture of fuel gases like coal gas.
- Coal is used in the manufacture of petrol and synthetic natural gas.
Petroleum is a dark coloured, viscous, and foul smelling crude oil (see Figure). The name petroleum means rock oil (petra = rock; oleum = oil). It is called petroleum because it is found under the crust of earth trapped in rocks. The crude oil petroleum is a complex mixture of several solid, liquid and gaseous hydrocarbons mixed with water, salt and earth particles.
It also contains small amounts of other carbon compounds containing oxygen, nitrogen and sulphur. Thus, the crude petroleum oil is not a single chemical compound, it is a mixture of compounds. Petroleum is lighter than water and insoluble in it.
Petroleum occurs deep down under the earth between two layers of impervious rocks (non-porous rocks). Natural gas occurs above the Figure. This is petroleum (which is also petroleum oil trapped under the rocks. Petroleum is obtained by ca”ed crude oil).
It is used to obtain fuels drilling holes (called oil wells) into the earth’s crust where the presence petrol, diesel and kerosene, etc.of oil has been predicted by survey. When a well is drilled through the rocks, natural gas comes out first with a great pressure and for a time, the crude oil comes out by itself due to gas pressure.
After the pressure has subsided, the crude oil is pumped out of the oil well. It should be noted that many wells yield only natural gas but no oil. So, hundreds of wells may have to be drilled into the rocks and tested before getting an oil well from which oil can be produced profitably.
Petroleum is the crude oil which is a complex mixture of alkane hydrocarbons with water, salt and earth particles. Petroleum cannot be used as a fuel as such. So, before petroleum can be used as a fuel for specific purposes, it has to be purified or refined by the process of fractional distillation.
The fractional distillation of petroleum gives us the following fractions which can be used as fuels : Petroleum gas, Petrol (or Gasoline), Diesel, Kerosene and Fuel oil. The fuel oil obtained from petroleum is sometimes just called ‘oil’. The fraction of petroleum called lubricating oil is not used as a fuel.
Petroleum gas is used as a fuel for domestic heating purposes in the form of liquefied petroleum gas (LPG). Petrol is used as a fuel in motor cars, scooters, motor-cycles, and other light vehicles. Diesel is used as a fuel for heavy vehicles like buses, trucks, tractors, and railway engines.
Diesel is also used as a fuel to run water-pumps for irrigation purposes, and in diesel generators to produce electricity on a small scale. Kerosene oil is used as a household fuel. For example, kerosene is used in wick stoves or pressure stoves to cook food. Kerosene is also used as an illuminant (for lighting purposes) in hurricane lamps. A special grade of kerosene oil is used as ‘aviation fuel’ in jet aeroplanes.
Fuel oil is used in industries to heat boilers and in furnaces. Fuel oil is also used in thermal power plants for generating electricity. Fuel oil is a better fuel than coal because fuel oil bums completely and does not leave any residue. On the other hand, when coal is burned, it leaves behind a lot of ash which has to be removed regularly from the coal furnace.
The main constituent of petroleum gas is butane though it also contains smaller amounts of propane and ethane. Thus, we can say that petroleum gas is a mixture of three hydrocarbons : butane, propane and ethane. Butane, propane and ethane, all burn readily, producing a lot of heat. This makes petroleum gas a very good fuel.
Petroleum gas is obtained as a by-product in oil refineries from the fractional distillation of petroleum. It is also produced by the cracking of petrol.
Butane, propane and ethane are gases under ordinary pressure (atmospheric pressure) but they can be easily liquefied under pressure.
The petroleum gas which has been liquefied under pressure is called Liquefied Petroleum Gas (LPG) (see Figure). Thus, liquefied petroleum gas (or LPG) consists mainly of butane (alongwith smaller amounts of propane and ethane) which has been liquefied by applying pressure. In other words, the domestic gas cylinders like Indane contain mainly butane, with smaller amounts of propane and ethane, under pressure.
Petroleum gas is supplied in liquid form so that even a small cylinder may contain a good weight of the gas. A domestic gas cylinder contains about 14 kilograms of LPG (see Figure). A strong smelling substance called ethyl mercaptan (C2H5SH) is added to LPG cylinders to help in the detection of gas leakage.
Ethyl mercaptan has a foul smell resembling that of hydrogen sulphide gas which can be easily detected. The gas used for domestic cooking is called ‘Liquefied Petroleum Gas’ (LPG) because it is obtained from petroleum and it is liquefied by compression before filling into the cylinders.
Our domestic gas cylinders contain a mixture of liquefied butane, propane and ethane hydrocarbons under pressure. When we turn on the knob of the gas cylinder, the pressure is released and the highly volatile LPG gets converted into gas. This gas goes into the burner of our cooking stove. When a burning matchstick is applied to the burner, the gas burns with a blue flame producing a lot of heat. This heat is used for cooking food.
Advantages of LPG
- LPG has a high calorific value. So, it is a good fuel. The calorific value of LPG is about 50 kJ/g. That is, when 1 gram of LPG burns in a gas stove, it produces 50 kilojoules of heat energy.
- LPG burns with a smokeless flame and so does not cause air pollution (see Figure).
- LPG does not produce any poisonous gases on burning.
- LPG is easy to handle and convenient to store.
LPG is a very neat and clean domestic fuel.
Dangers of LPG
Though LPG is an extremely good domestic fuel but it can be dangerous too. LPG is a highly inflammable gas, that is, it catches fire easily. Any leakage of LPG from the gas cylinder, stove or the rubber pipe (connecting the cylinder and stove) will form an explosive mixture with air in the kitchen.
And on lighting the matchstick, an explosion will take place, the whole kitchen will be set on fire and the person working in the kitchen may get burnt. In fact, many such accidents occur quite often and should be avoided by observing a few precautions.
Precautions for Using LPG
1. Before lighting a matchstick we should make sure that there is no foul smell of the leaking gas in the kitchen, near the gas cylinder or gas stove. If we smell gas leakage on entering the kitchen, then the door and windows should be opened at once to allow the gas to escape.
This is because, though LPG is not a poisonous gas, but when inhaled in large quantities, it is quite harmful and can even cause death. The gas cylinder, rubber tubing and gas stove should then be checked to find out the source of gas leakage. The gas should be used only when the leakage has been set right by the gas mechanic. We should never light a matchstick or a candle during the leakage of LPG in the kitchen.
2. We should not use any hot flames like a kerosene lamp, kerosene stove or electric heater near the gas cylinder.
3. We should never use a leaking gas cylinder. The Gas Agency should be informed and the leaking gas cylinder replaced at the earliest.
4. We should handle the gas cylinder with care so that its valve does not get damaged.
5. The rubber pipe connecting the gas cylinder to gas stove should be checked periodically for any wear and tear.
6. In order to light the gas stove, we should first open the valve of the cylinder and then turn on the knob of gas stove.
7. In order to put off the gas stove, we should first close the valve of the cylinder and after that the knob of the gas stove should be turned off.
8. When the gas is not being used, the valve of the gas cylinder and the knobs of the gas stove must be kept closed.
Natural gas is another important fossil fuel. Natural gas burns easily and produces a lot of heat. Natural gas consists mainly of methane (CH4), with small quantities of ethane and propane. In fact, natural gas contains upto 95% methane, the remaining being ethane and propane. Natural gas occurs deep under the crust of the earth either alone or along with oil above the petroleum deposits.
Thus, some wells dug into the earth produce only natural gas, whereas others produce natural gas as well as petroleum oil. Natural gas is formed under the earth by the decomposition of vegetable matter lying under water. This decomposition is carried out by anaerobic bacteria in the absence of oxygen. Natural gas is used as a fuel for a number of purposes.
- Natural gas is used as a domestic and industrial fuel. Natural gas bums readily to produce a lot of heat. So, in many areas natural gas is used as the main fuel for domestic and industrial heating purposes.
- Natural gas is used as a fuel in thermal power plants for generating electricity.
- Compressed Natural Gas (CNG) is being used increasingly as a fuel in transport vehicles (like cars, buses and trucks) (see Figure ). It is a good alternative to petrol and diesel because it does not cause any air pollution.
Advantages of Natural Gas
1. Natural gas being a complete fuel in itself can be used directly for heating purposes in homes and industries.
There is no need to add anything else to it.
2. Natural gas is a good fuel because it has a high calorific value of up to 50 kJ/g. Moreover, natural gas burns with a smokeless flame and causes no air pollution. It also does not produce any poisonous gases on burning.
Natural gas is, therefore, an environment friendly fuel as compared to other fossil fuels.
3. A great advantage of natural gas is that it can be supplied directly from the gas wells to the homes and factories for burning through a net-work of underground pipelines, and this eliminates the need for additional storage and transport.