Remember, when you burn coal you are really promoting a chemical reaction – a chain
reaction. When coal is heated to a high enough temperature in the presence of air, the
carbon in the coal combines with oxygen of the air to form either carbon dioxide (CO2) or
carbon monoxide (CO). These, of course, are both gases. Which gas is formed depends
upon the quantity of oxygen present. The CO means that the coal is only partially burned;
the CO can be combined with more oxygen to form CO2.