[Terrapreta] On the spontaneous combustion of char

[Bob Hawkins]

    It was mentioned that char can spontaneously combust, and since then,
several questions
 about this have been asked.  Below, is an explanation for why this occurs
and how to
 prevent it.  I hope this answers everyones concerns.


    We collect the char in an air-tight barrel as it falls out of our
pyrolysis unit.
Char fresh from the machine is still hot and it only takes a little bit of
oxygen to get it
glowing hot and burn up all its surroundings, reducing the char to ashes. If
no oxygen is
present, it won't continue to burn once it is out of the reactor. This
reactivity is caused
by several factors and is easily preventable, however.  The freshly produced
char has never
been exposed to oxygen, and during the pyrolysis process, reactive organic
species are
created on the surface of the char (this includes the surface of the pore
structure).
These organic compounds react exothermically with oxygen, and when char is
fresh and in
a big pile, there is enough heat in the char pile to start these reactions.
Once these
reactions begin, and there is air getting to the char, the coals are now
lit, so to speak.
Also, when the char is collected in an air-tight container, there is still
enough heat in
the char as it exits the pyrolyzer for small amounts of volatiles to enter
the gas phase.
When the char totally cools, these volatiles will re-condense in and on the
char.  If there
still exists gas-phase volatile organic compounds in the char pile, and they
are exposed
to oxygen, they react exothermically, providing enough heat to get the char
going.


    This can be prevented by a few methods.  The char has to be exposed to
air at somepoint,
 so never allowing it to come in contact with oxygen is not really
practical.  The trick is
to expose the char to air slowly, and while not in a big pile.  Usually,
just transferring
the char from its collection container into a separate storage container
will prevent the
char from igniting.  The reason for this is that the volatile organic gasses
trapped in
the char pile are allowed to escape, and the highly reactive surfaces on the
char are
allowed to react with the air, but in the process of moving from one
container to another,
the heat from the reaction can escape and does not cause the char to heat
up.  After the
char has been transfered, grinding the char into smaller pieces repeats the
process just
mentioned, and further prevents the char from igniting.

    One thing to remember, char contains energy that is released as heat
when it reacts
with oxygen.  Char, or charcoal, can catch fire at any time if it is hot
enough and there
is enough air.  So, if you are storing the char in a huge container
(industrial size
storage), and it is 100 degrees F outside, and there is air drafting through
the container
(maybe from a hole somewhere), it is possible that the char may catch fire,
much like a
pile of compost/wood waste gets hot enough to burn..  Therefore, careful
attention must
be paid to storage of char.
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