| MadSci Network: Chemistry |
Dear User, Charcoal, and coal in general, has what is called inindustry "slow oxidation potential". What does this mean ? Charcoal is not pure carbon. It contains varied amounts of hydrocarbons, aromatic compounds, sulphur and sulphur derivatives and so forth. In addition, charcoal has a very low coefficient of heat dissipation, which means that it is a good heat insulator. Upon storage, assorted components of charcoal undergo slow oxidation, releasing heat and various gases (H2 and CO in particular). Inner surface of charcoal is enormous, due to its highly porous structure which in turn results from the fact that charcoal has once been a living plant or suchlike (hence, had cellular structure). Water in small quantities diffuses into the pores and wets the surface, thus catalyzing all sorts of oxidation reactions on the interface. Heat is released, and it does not dissipate readily because charcoal is an insulator. Eventually, build up of heat ignites the gases and the hydrocarbonds, resulting in fire. Exact mechanisms are not available, but the above short description is a satisfactory model which explains spontaneous combustion of charcoal. Similar events lead to combustion of other porous products such as wood shavings, loose flour and straw. Especially hazardous are oil-soaked rags (as is well known to artists) because oil undergoes slow air oxidation into peroxides which are prone to self-ignition. In addition to purely chemical processes, ignition of biodegradeable materials is often triggered by bacterial growth - bacteria rapidly grow in wet shavings or paper, the temperature builds up and at a certain point chemical oxidation takes over, resulting in fire. There is not much information available on the net, however for a quick start take a look at: http://www.saftek.com/worksafe/bull94.txt Hope it helps. A.G.E.
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