Answer. The main constituents of fuels (coal, petroleum and natural gas) are hydrocarbons. When hydrocarbons-are burnt in air, CO2 and H2O along with heat are produced. This reaction is called combustion. Alkanes burn in the presence of excess of air or oxygen to produce a lot of heat, carbon dioxide and water. It is highly exothermic reaction, i.e. it produces a lot of heat. The basic combustion reaction is
CH4 + 2O2 ———> CO2 + 2H20 + heat
The heat energy produced is used for various processes, in homes, transportations, as well as in industries.