Coal And Petroleum
Carbon And Its Compound of Class 10
COAL AND PETROLEUM
A fuel is a material that has energy stored inside it. When a fuel is burned, the energy is released mainly as heat (and some light). This heat energy can be used for various purposes like cooking food, heating water, and for running generators in thermal power stations, machines in factories and engines of motor cars. Most of the common fuels are either free carbon or carbon compounds. For example, the fuels such as coal, coke and charcoal contain free carbon whereas the fuels such as kerosene, petrol, LPG and natural gas, are all carbon compounds.
When carbon in any form (coal, coke, charcoal, etc.) is burned in the oxygen (of air), it forms carbon dioxide gas and releases a large amount of heat and some light :
Most of the fuels which we use today are obtained from coal, petroleum and natural gas. Actually, coal, petroleum and natural gas are known as fossil fuels. Fossils are the remains of the pre-historic animals or plants buried under the earth, millions of years ago. Coal, petroleum and natural gas are known as fossil fuels because they were formed by the decomposition of the remains of the prehistoric plants and animals (fossils) buried under the earth, long, long, ago.
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. It is found in deep coal mines under the surface of earth.
HOW COAL WAS FORMED ?
Coal was formed by the decomposition of large land plants and trees buried under the earth millions of years ago. It is believed that millions of years ago, due to earthquakes and volcanoes, etc., the forests were buried under the surface of the earth and got covered with sand, clay and water. Due to high temperature and high pressure inside the earth, and in the absence of air, wood was converted into coal.
HOW PETROLEUM WAS FORMED ?
Petroleum oil (and natural gas) were formed by the decomposition of the remains of extremely small plants and animals buried under the sea millions of years ago. It is believed that millions of years ago, the microscopic plants and animals which lived in seas, died. Their bodies sink to the bottom of the sea and were soon covered with mud and sand. The chemical effects of pressure, heat and bacteria, converted the remains of microscopic plants and animals into petroleum oil and natural gas just as they converted forest trees into coal. This conversion took place in the absence of oxygen or air. The petroleum thus formed got trapped between two layers of impervious rocks (non-porous rocks) forming an oil trap.
Petroleum is dark coloured, viscous, and foul smelling crude oil. 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 fuels such as petrol, kerosene, diesel and LPG are obtained from petroleum.
WHY DO SUBSTANCES BURN WITH A FLAME OR WITHOUT A FLAME ?
We are all familiar with a candle flame. A candle, cooking gas (LPG), and kerosene oil, all burn with a flame. A flame is the region where combustion (or burning) of gaseous substances takes place. So, a flame is produced only when gaseous substances burn.
Flames are of two types :
(a) Blue flame and
(b) Yellow flame.
(a) When the oxygen supply (or air supply) is sufficient, then the fuels burn completely producing a blue flame. This blue flame does not produce much light, so it said to be non luminous (or non light-giving) flame. In a gas stove, cooking gas (LPG) burns with a blue (non-luminous flame).
The gas stove has holes (or inlets) for air to mix properly with cooking gas. The cooking gas gets sufficient oxygen from this air and hence burns completely producing a blue flame. Thus, complete combustion of cooking gas takes place in a gas stove.
(b) When the oxygen supply (or air supply) is insufficient, then the fuels burn incompletely producing mainly a yellow flame.
The yellow colour of flame is caused by the glow of hot, unburnt carbon particles produced due to the incomplete combustion of fuel. This yellow flame produces light, so it is said to be a luminous (light-giving) flame.
When wax is burned in the form of a candle, it burns with a yellow, luminous flame. When a candle is lighted, the wax melts, rises up the wick and gets converted into vapours. In a candle, there is no provision for the proper mixing of oxygen (of air) for burning wax vapours. So, In a candle the wax vapours burn in an insufficient supply of oxygen (of air) which leads to incomplete combustion of wax. The incomplete combustion of wax in a candle produces small unburnt carbon particles. These solid carbon particles rise in the flame, get heated and glow to give out yellowish light. This makes the candle flame yellow and luminous. The unburnt carbon particles then leave the candle flame as soot and smoke. Thus, incomplete combustion of wax takes place in a candle.
Those solid and liquid fuels which do not vaporize on heating, burn without producing a flame. For example, coal and charcoal burn in an ‘angithi’ without producing flame. They just glow red and give out heat. Coal and charcoal contain some volatile substances. So, when coal or charcoal are ignited, the volatile substances present in them vaporize and they burn with a flame in the beginning. When all the volatile substances present in coal and charcoal get burnt, then the remaining coal or charcoal just glows red and gives heat without producing any flame.