Fossil fuels are formed over millions of years by the burial of plant and animal remains. They accumulate slowly under the cover of sediments and are thus locked away from the reach of oxygen. The cover of sediments prevents their oxidation and decay while their weight squeezes out of the water and other volatile materials. Fossil fuels are thus energy-rich molecules of carbon originally made by plants with the help of solar energy. You can not appreciate that fossil fuels are produced by an unusual sequence of earth processes operating over millions of years. If we continue to use them at a rate which is faster than the earth can recreate them, they will eventually be exhausted. Examples of fossil fuels are coal, petroleum and natural gas.
Coal deposits are the distilled remains of large land plants which flourished in Vast shallow swamps about 300 million years ago. These remains buried in slowly sinking basins and preserved as a store-house of energy. Today, we mine coal from these ancient deposits to meet our energy requirements.
Coal consists largely of the atoms of carbon, hydrogen, and oxygen and the small amount of sulphur it comes in three main varieties- LIGNITE, BITUMINOUS coal, and ANTHRACITE. They vary in their carbon content, volatile matter, and moisture. For example, lignite contains 38% carbon, 19% volatile matter and about 43% moisture. Anthracite, at the other extreme, contains 96% carbon, 1% volatile matter and only 3% moisture. Bituminous coal which is 65% carbon, is intermediate between these two, and by far the most important as fuel.
Coal, when heated in absence of air, gives coke. You can obtain coal-tar and coal-gas from coal by a simple process known DESTRUCTIVE DISTILLATION of coal.
Coke, like charcoal, is a good fuel and burns without smoke. However, it is seldom used as fuel. For, it is more valuable when used as a reducing agent in the extraction of metals from their ores.
Coal-tar is a mixture of many carbon compounds. For about 100 years coal-tar was the chief source of important carbon compounds used in the dyer, explosives paints, synthetic fibers, drugs, and pesticides. Today, these are largely derived from petroleum products.
Coal gas is a mixture of hydrogen, Methane and carbon monoxide. This mixture is an excellent fuel. It was used for lighting homes, factories and streets, and for the domestic cooking until recently.
It is believed that petroleum is formed from micro-organisms living in the sea. When these organisms die, they sink to the bottom and gradually covered by sand and clay. Over a period of millions of years, these remains are converted into hydrocarbons by heat, pressure and catalytic action. These hydrocarbons rise through porous rocks until they are intercepted by some impermeable rocks, forming an oil trap. If the reason in subsequently uplifted the oil trap will be found to lie under land.
Petroleum is obtained by drilling a hole through the impervious cap-rock. Early drilling for oil was mainly carried out on land. The first oil well was drilled in Pennsylvania, U.S.A., in 1859.
Petroleum is a mixture of several hydrocarbons. Crude oil is a brown-black liquid. When refined it separates into various fractions of carbon hydrocarbons. The refining process exploits the fact that its different compounds have distinctly different boiling points. They can thus be separated in a large fractionating column. Crude oil fed at the base is heated to about 400 degree Celsius. At this temperature, all the compounds except asphalt vaporize. As the mixture of hot vapors rises in the column, it cools. The compounds with the highest boiling point liquefied first and are collected. Then, a little higher in the column. The compounds next in line liquefies and so on while the residual gases escape from the upper part of the tower. The components thus separated at various heights in the column are easily trapped, collected and stored.
The useful components obtained from the fractional distillation of petroleum, from the base upward are asphalt, lubricating oil, paraffin wax, fuel oil, diesel kerosene, petrol and petroleum gas. Except for asphalt, lubricating oil and paraffin wax, all other fractions readily burn, producing heat, and are used in fuels.
Fuel oil is mostly used in industries to heat boilers and furnaces. This oil is a better fuel than coal. For, unlike coal with leaves enormous amounts of ash which must be removed regularly, fuel oil burns completely.
Petrol and diesel are mainly used in automobiles and locomotives. Two-wheelers, three-wheelers, and most motor cars run on petrol. Heavy vehicles such as buses, trucks and railway engines run on diesel. Diesel is also used to run pumps in the fields and to generate electricity on a small scale.
Kerosene is used for domestic purposes. It is used in a pressure or wick stove to obtain heat. It is used for the lightning hurricane or petromax lamps. A special grade of kerosene is also used as an aviation fuel in airplane jet engines.
Petroleum gas is a mixture of ethane, propane, and butane. Its main constitution is butane which burns readily, giving a lot of heat. Butane is easily liquefied under pressure. It is supplied in the liquid form in cylinders and is commonly known as LIQUID PETROLEUM GAS ( LPG ).
When we open the valve of the cylinder thereby reducing the pressure, the liquid vaporizer and flows to the burner. On being lighted it burns with a blue flame. One gram of this gas gives about 50 kilojoules of energy.
Liquefied Petroleum gas should be used with care as accidental leakages can cause an explosion and also death if the gas is inhaled in the large quantity. To help detect such leakages, the LPG is mixed with a small amount of another strongly smelling gas. Should a leakage be smelt on entering a kitchen, all doors and windows should be flung open to allow the gas to escape and the tube and joints systematically checked for defects. Care should be taken to refrain from using an open flame or any electric appliance until the defect has been rectified.
Natural gas readily burns to produce heat. It is also a rich source of hydrogen gas which is needed in the manufacture of fertilizers. It occurs along with oil. Many oil wells produce natural gas as a by-product. There are, however, some wells that produce only natural gas.
Natural gas mainly contains methane (CH4). Its great advantage is that it can be used directly for burning in home and factories where it can be supplied through pipes. Although transporting gas through the long-distance pipeline is initially expensive, it is economical in the long run, as it eliminates the need for additional storage and transport. Recently we have discovered new fields of natural Gas in several parts of different countries. New pipelines are therefore being planned to be laid and several parts of country to facilitate the use of these energy resources
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