Fossil fuels like coal, petroleum and natural gas are the remains of the prehistoric animals and plant life that was buried under the earth millions of years ago and decayed slowly. It was probably forced by the pressure of water into the spaces above the clays where it collected in large quantities with natural gas in cavities surrounded by the impervious rock. These anti-cyclines, as they are called, are shaped like inverted troughs and oil tends to collect under them.
Coal and petroleum have already been discussed under compounds of carbon and hydrocarbons. We will discuss the petroleum gases here.
Petroleum Gas.
The gas that comes packed in a cylinder to your house is a petroleum gas obtained by the fractional distillation of petroleum. It is a mixture of three hydro-carbons butane, propane and ethane with the major share going to butane. At normal pressure butane, propane and ethane are gases. They liquify under pressure, Hence the name Liquified Petroleum Gas (LPG), LPG has a high calorific value, 50 Kilo Joules per I gram.
It is liquified under pressure to facilitate a large quantity of gas into a small cylinder, A normal domestic cylinder contains about 14 kilograms of the gas. As a help in detection of any leakage, a strong smelling su’ stance ethyl mercaptan is added to the gas. It is a highly inflammable gas i.e., it catches fire easily.
Natural Gas.
Natural gas occurs deep in the earth crust either alone or along with the petroleum deposits. It is formed by the decomposition of vegetable matter lying under water acted upon by the anaerobic bacteria in the absence of oxygen.
Natural gas contains mostly methane with minor amounts of ethane and propane, its heating energy value ranges between 35–50 kJ/g, combusts with a clean flame and causes minimal pollution.
The natural gas is used as a source of hydrogen gas in the fertilizer industry. The methane in natural gas on heating decomposes to carbon and hydrogen, This hydrogen is combined with nitrogen gas to prepare ammonia. Ammonia is treated with acids to have ammonium salts which are used as fertilizers. Similarly the carbon formed on the decomposition of methane called the carbon black is used as a filler in the manufacture of tyres.
Synthetic Petroleum/Gas.
During the World War II petrol was not easily available and was severely rationed. Man’s quest for an alternative source enabled Germany to obtain synthetic petroleum from coal, It was in 1913 that Bergius produced the world’s first synthetic petrol. In this process the coal was ground to a paste with oil, treated with hydrogen under very high pressure in steel cylinders. The coal was turned into oil which on further treatment with hydrogen formed motor spirit. Over the time, with the petroleum products becoming available due to identification of new reserves the costly synthetic fuel could not hold ground.
The catalytic hydrogenation of coal gives us a gas as good as methane. Again the finely ground coal is heated with hydrogen gas under pressure. The complex hydrocarbon molecules present in coal react with hydrogen gas to form methane gas as one of the products
Fuels and Octane number.
The tendency of a fuel to auto ignite and ‘knock’ is measured by its octane number. A fuel with a low octane number has a significant tendency to auto ignite while a fuel with an octane number over 100 has a much lower tendency to auto ignite. The scale is based on the tendency to auto ignite of two hydrocarbons:
- 2,2,4-trimethylpentane, which used to be called iso-octane (hence the name of the scale), is given an octane number of 100.
- heptane is given an octane number of zero. The octane number of any fuel is the percentage of 2,2,4-trimethylpentane in a mixture with heptane that has the same tendency to auto ignite. So petrol with an octane number of 97 has the same tendency to auto ignite as a mixture of 97% 2,2,4-trimethylpentane and 3% heptane.
POLLUTION CAUSED BY THE FOSSIL FUELS.
Pollution by the burning of Coal
Burning of coal yields the following air pollutants:
- Carbon Dioxide
- Carbon Monoxide
- sulphur Dioxide
- Smoke
Carbon dioxide is formed when coal burns in a sufficient supply of oxygen. Although the carbon dioxide gas in not a polluting gas but its high concentration leads to changes in the climatic conditions. We know that the temperature of the earth is maintained by the energy balance of solar radiations that strike the earth surface and the heat that is reflected back to the space. This energy balance is maintained chiefly by carbon dioxide CO2. This thick layer of gas acts like a glass panel of a green house enabling the sunlight to pass through it but not allowing the heat to be radiated out into the space.
The increasing CO2 concentration by not allowing the radiation into the space is therefore warming the earth and a phenomena of global warming is being witnessed. Experts feel that while 2-3 degree increase in temperature may not seem much, it is thought to be sufficient to cause such things a partial melting of polar ice caps, rise in the level of oceans leading to submerging parts of the planet and dramatic changes in the climatic conditions.
Explain why fossil fuels are exhaustible natural resources
Fossil fuels are called exhaustible natural resources because they are limited in amount and can run out one day.
They were formed millions of years ago from the remains of dead plants and animals buried under the earth. Since this process takes such a long time, we cannot make new coal, oil, or natural gas quickly.
But humans use fossil fuels every day for electricity, transport, cooking, and industries. We are burning them much faster than nature can replace them. That’s why they are exhaustible—once they are gone, we cannot get them back in our lifetime.
✅ Advantages of Fossil Fuels
- High energy output – A small amount can produce a large amount of energy.
- Easily available – Found in many parts of the world and already in use.
- Reliable source – Can produce electricity or fuel 24/7 without depending on weather.
- Well-developed technology – Machines, vehicles, and power plants are already designed to run on them.
- Supports economy – Provides jobs and boosts industries like transport, electricity, and manufacturing.
❌ Disadvantages of Fossil Fuels
- Non-renewable – Once used up, they cannot be replaced quickly.
- Pollution – Burning coal, oil, and gas releases harmful gases (like CO₂, SO₂) that cause air pollution.
- Global warming – Carbon dioxide from fossil fuels is the main cause of climate change.
- Acid rain – Sulfur and nitrogen gases from burning coal and oil mix with rain, damaging crops, soil, and buildings.
- Health problems – Air pollution can cause asthma, lung diseases, and heart issues.
- Environmental damage – Mining coal and drilling oil can destroy land, forests, and water sources.