Fossil fuel: coal, oil and natural gas

The important elements in combustion of fuel are: Hydrogen and Carbon, with the final products: water and carbon dioxyde (CO2). Coal contains lots of Carbon and natural gas relatively more Hydrogen.

When carbohydrates are burned, in fact two elements H and C react with Oxygen. att.: Hydrogen is the most reactive one, so that one will be the first to consume the Oxygen.
If enough Oxygen is present, Carbons turn is there. Shortage of Oxygen creates carbon monoxyde (CO) or even you can see the black clouds of soot.
The formation of CO2 takes place all over the world and is considered a cause of the heating up of earth, including raise of see level. This could create big problems in the future. Last fifty years the amount of carbon dioxyde has an increase of about 15%.