Coal and Petroleum

The materials we obtain from nature are natural resources. They are of two kinds, depending on how much is available.

• Inexhaustible (renewable) resources are present in unlimited amounts and will not run out by human use — for example, sunlight and air.
• Exhaustible (non-renewable) resources are present in limited amounts and can be used up — for example, coal, petroleum, natural gas, minerals and forests.

Coal, petroleum and natural gas are called fossil fuels because they were formed from the dead remains of living organisms (plants and animals) that were buried deep under the Earth millions of years ago. Under great heat and pressure, and in the absence of air, these remains slowly changed into fuels.

Because fossil fuels took millions of years to form and are being used up rapidly, they are exhaustible and will not last forever. This is why we must conserve them — use them carefully and avoid wasting them — so that they are available for the future.

Coal is a hard, black solid fossil fuel. It was formed from the remains of large trees and forests that were buried under the soil millions of years ago. The slow process by which dead vegetation became coal under heat and pressure is called carbonisation. Coal is mainly made of carbon.

Coal is widely used as a fuel for cooking, in railways (in the past) and especially in thermal power stations to produce electricity, and in industries. But coal is much more than just a fuel — when it is heated strongly in the absence of air (a process called destructive distillation), it gives several useful products:

• Coke — a tough, porous, almost pure form of carbon. It is used in the extraction of metals (such as iron) and as a clean fuel.
• Coal tar — a thick, black, smelly liquid. It is a mixture of about 200 substances and is used to make dyes, paints, explosives, perfumes, plastics, medicines and roofing materials. (The black material used to make roads, called bitumen, is a petroleum product that has now largely replaced coal tar for road surfacing.)
• Coal gas — a gas obtained during the processing of coal. It was once used for street lighting and is now used as a fuel in industries.

Petroleum is a dark, oily liquid fossil fuel. It was formed from the dead remains of tiny sea plants and animals that settled at the bottom of the sea, were covered by sand and clay, and changed into petroleum over millions of years. Because petroleum is so valuable, it is often called black gold.

Petroleum taken from the ground (crude oil) is a mixture and is not useful as it is. It is separated into useful parts at an oil refinery by a process called refining (fractional distillation), which separates the mixture based on different boiling points. The separated parts, called fractions, include: petroleum gas (LPG, for cooking), petrol (fuel for cars), diesel (for trucks and trains), kerosene (for stoves and lamps, and as jet fuel), lubricating oil (to reduce friction), paraffin wax (candles, polishes) and bitumen (for surfacing roads).

Natural gas is another important fossil fuel, found above petroleum in the ground. It is a clean fuel and easy to transport through pipes. CNG (Compressed Natural Gas) is used to run vehicles and causes less pollution than petrol or diesel; natural gas is also used to make fertilisers and other chemicals.

Since these fuels are exhaustible and burning them pollutes the air, we must conserve them. We can save fuel by walking or cycling for short trips, using public transport, switching off engines at traffic lights, keeping vehicles serviced, and using clean fuels like CNG and renewable sources where possible. In India, the Petroleum Conservation Research Association (PCRA) advises people on saving fuel.