A Fuel is a material that can be made to react, usually with oxygen, to give off useful energy, usually in the form of heat energy. It contains stored energy in the form of chemical energy. That chemical energy has to have been made from another kind of energy, because energy cannot be destroyed or made, only changed from one form to another.
A fossil fuel is a fuel that has been made from the remains of living materials, millions of years ago.
300 million years ago nuclear reactions (nuclear energy) gave out light that reached the Earth (light energy). Plants absorbed the light energy through photosynthesis and changed the carbon dioxide from the air into sugars, which they then stored as carbohydrates (chemical energy) Animals ate some of these plants, storing some of the chemical energy and using the rest for respiration and other life processes (heat energy, chemical energy, kinetic energy). The remains of the plants and animals have now been changed into other forms, oil, coal and gas, which contain some of the chemical energy that was stored in these organisms.
The giant ferns of the Carboniferous Period, 360 million years ago fell into the swamps. There was not enough oxygen to allow bacteria to break them down fully and over millions of years their remains were changed into coal. Sometimes you will see the imprints of fossilized ferns in lumps of coal and this gives the lie to the creationist theories of oil and coal formation.
Crude oil and natural gas were both formed from the remains of tiny sea creatures around the same time as coal.
As the bodies of the tiny marine organisms died and fell to the sea floor they were covered with sediment. The weight of the layers above the marine organisms caused immense pressure and heat, turning them into crude oil and natural gas, because no oxygen was present.
Sometimes people imagine giant caves deep under the Earth's surface full of oil or gas. Not so, both crude oil and gas are found in the tiny pores in rocks like limestone. They are both less dense than water so tend to rise. If there is a dome structure in the rock layers, with the dome cap being made of impermeable rock, the oil and gas will collect under the dome.
Fossil fuels are not being formed any more. The correct conditions have not been maintained on Earth for 300 million years. The oceans no longer teem with the life that they did during the carboniferous period.
Fossil fuels reserves are being used up rapidly. No more fossil fuels are being formed, therefore at some point we will run out of fossil fuels.
We do discover new geological rock formations that have acted as oil and gas reservoirs from time to time. These will extend the time scale over which we can depend on fossil fuels, but finding new reserves will not extend this time scale indefinitely. One day there will be no more oil reserves to find. The other important point is that the easily extracted oil reserves have all been found, so oil extracted from any newly discovered reserves will be more expensive than our current supplies.
Coal is largely made of carbon. When carbon burns in air it forms carbon dioxide. Most scientists believe that increasing levels of carbon dioxide (CO2) in the atmosphere are changing the Earth's climate, causing global warming. China, USA, the former USSR, India and Africa have large coal reserves. One of the obstacles to obtaining an international agreement on cutting carbon dioxide emissions has been the unwillingness of certain countries to switch away from their own, low cost indigenous coal to expensive, imported oil and natural gas. (Oil and natural gas produce less carbon dioxide per unit of heat produced.). There are about 120 years worth of coal left in the ground at current extraction rates.
Crude oil is a complex mixture of hydrocarbons. Hydrocarbons are compounds that contain carbon and hydrogen only.
Before crude oil can be used it needs to be separated into 'fractions' at an oil refinery. The petrol, diesel, kerosene, gas oil and fuel oil that are separated from the crude oil are mixtures, too, but less complex mixtures than the original one. Each fraction is a mixture of hydrocarbons that have similar boiling points and burning properties. This means that they can be used as fuels more easily than the original crude oil.
When any fraction of crude oil burns carbon dioxide and water vapour are made. Less carbon dioxide is released per unit of heat produced than with coal. We have about 100 years of proven reserves of oil, though hopefully improved extraction technologies and new discoveries will extend this.
Natural gas is also a mixture. In its natural form it contains methane, varying proportions of ethane, propane and butane and the poisonous gas hydrogen sulfide.
Refining the gas removes the hydrogen sulfide and turns it into sulfur, which is then used to make sulphuric acid.
The known reserves of natural gas are very limited, only about 70 years worth of gas remain.
When natural gas burns it releases carbon dioxide and water vapour, but less carbon dioxide per unit of heat produced than either coal or crude oil.
Natural gas is being depleted at an ever increasing rate because countries can cut their carbon dioxide emissions by switching from coal fuelled power stations to gas fired ones.
To call coal, oil and gas fossil fuels gives the impression that they can only be used as fuels. This is not the case. In the 1960s most of the chemical industry was based on chemicals obtained by heating coal in a vacuum, called destructive distillation. Coal has been replaced by oil and natural gas as the main raw material for the chemicals that we all use and need every day.
We should be using the oil and gas to make medicines, polymers and secondary raw materials instead of burning them. Our chemical industry may have to switch back to coal as a raw material at some point in the future. The next question must be,
'What happens when the coal runs out?'