Studies on Lignite Quality Depending on Storage Conditions

A small increase in the relative humidity in the air in a coal deposit can cause a 1% increase in the moisture content of the deposit resulting in the probability of spontaneous ignition. By depositing the freshly extracted coal over the coal already in the deposit, there is a direct contact between the two surfaces that have different characteristics in terms of physical and chemical properties, so that the latter acts as a primer.

The coal with a higher temperature gives up the excess temperature to the coal with a lower temperature, thus initiating the formation of self-heating nuclei followed by self-ignition ones. The phenomenon is easy to observe in the colder periods of the season and especially usually after rain, when the vapors resulting from the exchange of temperature between the two types of coal are released into the atmosphere.

The common cause is the movement of water vapor through the deposit correlated with the adsorption on the coal granules. The heat of condensation of vapor at storage temperature is about 580 cal./gram of water. Condensing the amount of water required to increase the content from 3% of the weight of the coal to 4% leads to an increase in the temperature of the coal by more than 170C. This increase in temperature is sufficient to increase the oxidation rate by 5 times.