MadSci Network: Chemistry

Re: why does wet charcoal easily spontaneously combust?

Date: Fri Jan 28 11:09:25 2000
Posted By: Artem Evdokimov, Postdoc
Area of science: Chemistry
ID: 942863407.Ch

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

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:

Hope it helps.


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