The main element in coal that is oxidised to form carbon dioxide is carbon (C). During combustion, coal reacts with oxygen (O2) present in the air, and the carbon within the coal combines with oxygen to form carbon dioxide (CO2) and release energy in the form of heat and light.

The basic chemical reaction for the oxidation of carbon in coal to form carbon dioxide can be represented as:

C (coal) + O2 (oxygen) → CO2 (carbon dioxide)

Carbon is the primary combustible component of coal, and as it reacts with oxygen during combustion, the stored chemical energy in the coal is released, leading to the formation of carbon dioxide as a byproduct.