Why coal or petrol does not burn by itself in air but once initiated by flame, it continues to burn?

Explanation

The activation energy needed for the combustion of coal i.e. the oxidation of carbon is very high and is not available at room temperature. When brought in contact with the flame, a part of the coal in its contact absorbs heat as activationi energy. Since combusion is an exothermic reaction, the heat energy which is liberated further provides activation energy for the combusion to continue.