Methane, coal, and gasoline are fossil fuels. When these and other fossil fuels burn, they react with oxygen from air to produce heat.
Called combustion, burning is one of the most important chemical reactions known to man.
The combustion reaction always has these these components:
fuel molecules + O2 --> CO2 + H2O
When a fossil fuel reacts with oxygen a rearrangement of their constituent atoms takes place. When the reaction is completed, the original substance is gone. Its atoms have been rearranged to form carbon dioxide (CO2) and water (H2O). One result of this rearrangement of atoms is a release of heat. The quantity of heat released is called the heating value of the fuel. For example, a gallon of most petroleum products (gasoline, fuel oils, etc.) has a heating value of approximately 140,000 BTU*. A pound of coal has a heating value between 6,000 and 15,000 BTU. A cubic foot of natural gas has a heating value of about 1,000 BTU.
A useful rule of thumb is that 1 gallon of oil is equivalent to about 10 pounds of coal and to about 150 cubic feet of natural gas. When these quantities of fuel are burned, approximately the same quantity of energy is released in the form of heat.
Methanol is not a fossil fuel, but when it burns, CO2 and H2O are also produced. It has a heating value of 57,000 BTU / Gal per gallon.