DOT regulations have stipulated that when charcoal on transit catches fire, it should not be put out using water

DOT regulations

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DOT regulations have stipulated that when charcoal on transit catches fire, it should not be put out using water. The burning charcoal should be located and removed from the rest. This is because under certain conditions, charcoal can react with air causing it to spontaneously heat and ignite. Fresh charcoal is more likely to heat spontaneously as compared to dry charcoal. The conditions that lead to spontaneous heating include lack of enough airing and cooling before the charcoal is shipped. Another reason is charcoal becoming wet hence heating spontaneously and catching fire (Wolters, Pagni, Frost, & Cuzzillo, 2010). Putting out fire with water leads to wetting of the charcoal hence leading to unstable conditions and necessary heat that leads to the spontaneous ignition of charcoal. The process of drying charcoal is endothermic and hence it lowers the temperature of the charcoal. On the other hand, putting out fire with water that leads to wetting of the charcoal is an exothermic process. The heat that is liberated can lead to an acceleration of the heating of the charcoal and hence cause fire.

Charcoal does not comprise of pure carbon only. It is made up of aromatic compounds, hydrocarbon, sulphur and other compounds. Charcoal also has very low coefficient of heat dissipation and hence it is a good heat insulator. When stored, some components of charcoal go through oxidation and hence release heat and gases. Once water diffuses into the pores of the charcoal it ends up wetting the surface and hence catalyzing oxidation reactions. Heat is released but is not immediately dissipated because charcoal is an insulator (Wolters, Pagni, Frost, & Cuzzillo, 2010). Eventually there is a build up of heat that causes an ignition of the gases hence resulting into a fire. Therefore, charcoal is termed as having a slow oxidation potential.

Reference

Wolters, F., Pagni, P., Frost, T., & Cuzzillo, B. (2010). Size Constraints on Self Ignition Of Charcoal Briquets. Fire Safety Science, 593-604.