Coal and Petroleum — REET Science Notes

What are fossil fuels

Coal, petroleum and natural gas are together called fossil fuels, because they formed over millions of years from the remains of dead plants and animals buried deep inside the earth, through the action of extreme heat and pressure. They are non-renewable natural resources, meaning that once they run out, they cannot be replenished quickly, so it's necessary to use them economically.

Fossil fuels mainly contain carbon and hydrogen, and they release energy (heat) on burning, which is why they are used as fuels. Coal is mostly found in a solid state, petroleum in a liquid state, and natural gas in a gaseous state, but the original source of all three is the same — the remains of organisms that were alive millions of years ago.

Formation and grades of coal

Formation process

Coal forms when dead vegetation gets buried under layers of soil and slowly undergoes carbonisation

Peat

The first stage of coal formation; the lowest carbon content

Lignite

Brown coal; a low-grade coal

Bituminous

The most widely used coal, of medium-to-high quality; common for both domestic and industrial use

Anthracite

The best grade of coal; the highest carbon content and heating capacity, and it produces the least smoke

Carbonisation

The slow process by which the carbon content of buried vegetation keeps increasing over time while moisture and other elements decrease — the greater the carbonisation, the better the grade of coal produced

Products obtained from coal

Coke

A hard, porous form of carbon; used in the steel industry for the reduction of metal — obtained from the destructive distillation of coal in the absence of air

Coal tar

A black, thick liquid with a strong smell; hundreds of useful substances (naphthalene, dyes, medicines, explosives, etc.) are obtained from it, which is why it is also called a storehouse of chemicals

Coal gas

A gas used as a source of fuel and light, obtained from destructive distillation; it was once used to light streets

Petroleum and its refining

Petroleum formed from the remains of micro-organisms living in the sea, which stayed buried under layers of sand and mud for millions of years and were converted into petroleum by the action of heat and pressure. Because of its diverse and valuable uses, it is also called 'black gold'. In its natural state, petroleum is a mixture of various gaseous, liquid, and solid components, which are separated at a petroleum refinery using the method of fractional distillation. To reach underground petroleum reserves, deep wells are drilled, and the oil is brought to the surface and transported to the refinery through pipelines or tankers.

Crude oil is first heated at the refinery and turned into vapour, and this vapour is then sent into a fractionating column, where the temperature keeps decreasing progressively towards the top of the column. Components with a lower boiling point reach the upper part of the column as vapour and condense there, while heavier components with a higher boiling point condense lower down. It is on this principle that petroleum gas, petrol, kerosene, diesel and other products are obtained separately.

Main products from fractional distillation

Natural gas and fuel conservation

The main component of natural gas is methane. It is compressed at high pressure and used as vehicle fuel in the form of Compressed Natural Gas (CNG); it is also used in power generation and as a source of hydrogen in the fertiliser industry. In India, the main coal reserves are in Jharkhand, West Bengal, Odisha, Chhattisgarh and Madhya Pradesh, while petroleum is mainly obtained from Assam, Gujarat and the Mumbai High region (in the Arabian Sea). Since fossil fuels take millions of years to form and reserves are limited, using them economically and adopting alternative sources of energy is essential.

Burning fossil fuels releases gases such as carbon dioxide, sulphur dioxide and nitrogen oxides into the atmosphere, which increase air pollution and the greenhouse effect. Since these fuels take millions of years to form while the rate of consumption is very fast, reserves will keep shrinking in the coming decades. This is why, along with using fuel economically, adopting renewable energy sources such as solar and wind energy has also become necessary. At the school level, making students aware of habits like avoiding unnecessary use of electric fans and vehicles, and promoting public transport, is a practical step towards conserving fossil fuels.

LPG, used as cooking gas, is mainly a mixture of butane and propane gases, which is compressed into a liquid form and filled into cylinders. Both LPG and CNG are actually colourless and odourless, so strong-smelling substances such as ethyl mercaptan are deliberately added to them, to allow leaks to be detected quickly.

How to teach this to students

Practice questions

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