Process of spontaneous combustion

Before coal openly burns, an unnoticed process of oxidation takes place. In this process oxygen from the air reacts with the carbon of the coal and carbondioxide is generated. This is an exothermic reaction, where heat will be released.

Normally, the heat is transported away by circulation of air, having a cooling effect.

For conditions favouring spontaneous combustion the air supply needs to be high enough to support the oxidation, but too small for a sufficient cooling. As a result the coal will heat up. The temperature rises and at about 80°C gases like carbon monoxide, carbon dioxide and water vapour are released. Above a certain critical temperature the combustion begins. The grain size and the surface structure of coal may also influence the suceptibility for spontaneous combustion

In the centre of large fires temperatures of 1000°C and higher can be reached, which can result in the pyro-metamorphoses or complete melting of the adjacent rocks.

Some Factors influencing spontaneous coal combustion are:

• air-ventilation
• climate: semi-arid to arid
• coal quality: low carbon content and large amounts of volatile components support combustion
• particle size: the smaller the particles, the larger the surface, the higher the risk
• geomorphologic setting and geological conditions
• human interactions such as mining activities
• changes of moisture and hydrological conditions