Coal was formed from the remains of plants.  Most of the plants that formed our coal grew in the carboniferous period which was between 340 and 280 million years ago.

When I was a child living in England we used coal for heating our house and I used to look at the shapes of plants on the pieces of coal where they had been split.

Carbon

Coal is black.  Often the blacker it is the higher quality the coal.  The blackness is Carbon.  (Not all forms of Carbon are black.  Diamond is Carbon, but Diamonds are not usually included in the fossil fuels.)

Carbon burns in air, assuming there is no restriction in the air supply, with a simple equation:

C + O₂  → CO₂

That is, Carbon combines with Oxygen producing Carbon dioxide.

This is the main way that burning coal produced heat.  But Carbon is not the only thing in coal.  There is also a significant amount of Sulphur.  The Sulphur burning equation is similar:

S + O₂ → SO₂

That is, Sulphur dioxide is produced.  Sulphur dioxide dissolves in water producing the weak acid sulphurous acid.  It will also react with Oxygen again, making sulphuric acid which is a strong acid.

Burning coal certainly puts pollution into the atmosphere.  It is also a major source of the greenhouse gas, carbon dioxide.  In coal seams there is often large amounts of Methane.  This is sometimes used as Coal Bed Methane. If it is not used, it will get released to the atmosphere ewhen the coal is mined.  Methane is an important Greenhouse Gas.

Clean Coal

The idea of clean coal is to remove the carbon dioxide and do something with it.  This has already been done to some extent, but mostly the technology is still to be developed.

Some of the ways of removing and sequestering the carbon dioxide will be expensive, even assuming that they are practical.  Other ways have been suggested that might even be profitable.

Coal Reserves

There are many different estimates of how long our coal will last.  The World Coal Institute’s estimate is 130 years.  However, this is only counting proven reserves.  It is reasonable to expect that the total recoverable reserves will be higher.  Of course we should recognise that people will tend to take the deposits that are easier to recover first, so a slow increase in coal price is likely.

Sources