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

Explanation

`E_(a)` for the combustion reaction is very high for burning of coal but low for burning of petrol. Yet so much energy is not available at room temperature. On applying flame, a part of coal or petrol and air in contact with the flame absorb heat which provides the necessary `E_(a)` and combustion starts. The heat liberated further provides `E_(a)` for the combustion to continue.