[Stoves] combustion of char

Sat Feb 23 02:49:54 CST 2013

Response from Harold Annegarn

I make reference not to char, but to coal. Perhaps the same will apply to
freshly prepared char surfaces.

The following information arises from the phenomenon of spontaneous
combustion in underground coal and in coal slag heaps.

Exposure of freshly broken coal surfaces, or in situ coal that has been in
water filled voids that is subsequently exposed to air (oxygen) (as occurs
when previously mined areas by bord & pillar extraction is exposed by strip
mining) results in a chemical reaction known as oxysorption ==> oxygen
reacts with carbon/carbonaceous matter at the surface of the coal in an
exothermic reaction that results in the oxygen being absorbed into the
chemical structure of the coal without necessarily being emitted as CO..
This oxysorption can occur at room temperatures and does not require
elevated temperatures or a spark to start.

Thereafter, basic physics applies: if thermal energy is released from a
chemical reaction, that heat is either conducted, convected or radiated
away. If the rate of production is faster than the rate of dissipation,
then the temperature will rise in the zone where the oxysorption is taking
place. The balance between conduction, convection and radiation may change
as the local temperature increases; likewise the reaction rate will
increase. There must be some convection possible, otherwise the available
oxygen will be consumed and the process will stop, If the local convection
is large, then the surface remains cool and the reaction proceeds without
large temperature rise. If the convection is limited, local temperature
will increase, and eventually may reach the point where volatile matter
that is driven off reaches ignition point, i.e. spontaneous combustion has
commenced.

It is not a matter of whether spon com occurs, it is just a matter of when.
For this reason, modern practise of stockpiling coal discards and fines
requires continual compaction of the surface to limit air ingress, and
final sealing of surfaces with a meter or more of soil.

Although I I have not read anything about oxysorption on char, it is
logical that similar processes are occurring, The char is a freshly
prepared surface with high specific surface area due to the voids created
by driving off volatile material, and with many dangling bonds at these
surfaces. Not for nothing is this material also known as "activated
charcoal" with good properties for absorbing ions e.g. from water for water
purification.
.
To test this it would be possible to char wood in a dry nitrogen or helium
stream, although this would result in a different surface chemistry than if
the same material is passed through the same heating cycle in the presence
of oxygen. If the material is then cooled, and placed on a balance with
automatic recording, the the change of weight with time can be recorded. If
this is done in a closed system, with dry air passed over, then one could
isolate the effect of oxygen absorption from the absorption of H2O from the
atmosphere if this was done in open laboratory air with variable relative
humidity.

In the case of the spontaneous combustion accident described by Frank
Shields, it appears that the raising of the temperature to just over 100
degrees C assisted in creating a heat generation rate and oxygen supply to
the char samples that allowed the oxysorption to proceed to the point at
which full ignition commenced - yes, he is lucky to have a lab still!

I have copied this message and thread to Glenn Stracher, an expert on
spontaneous combustion on coal, who has a much deeper understanding of
these matters than the qualitative handwaving that I have given above.

Best regards
Harold

On 23 February 2013 03:43, Frank Shields <frank at compostlab.com> wrote:

> Stovers,
>
> FYI
> After having about 30 compost samples in a drying oven go up in flame it
> looked like it started with a single char sample placed in the corner. I
> decided to do a little research by asking Hugh McLaughlin. Another question
> I had for him was why does some biochar samples gain weight in a drying
> oven
> set at 105c.
>
> So it seems soon after a char is produced it wants to take up oxygen. If
> bagged in a sealed container and sent to the lab soon after being made the
> dry sample will take up oxygen in the oven therefore gaining weight. If a
> sample is wetted soon after being made the water prevents oxygen from being
> taken up. Then if it becomes dry it wants oxygen, increases temperature and
> can catch fire under the right (wrong) conditions. Char is a great
> insulator
> so the heat can be trapped and build up. If there is organic materials
> around it can flame. So before storage better give it time to take up
> oxygen. Or if wetted do not let it dry out and keep bags of char in single
> layers.
>
> The forced air drying oven was left on overnight and it was in the morning
> we found all the burned up samples. It was placed against the wall so we
> are
> lucky we still have a lab!
>
> Regards
>
> Frank
>
>
>
>
> _______________________________________________
> Stoves mailing list
>
> to Send a Message to the list, use the email address
> stoves at lists.bioenergylists.org
>
> to UNSUBSCRIBE or Change your List Settings use the web page
>
> http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
>
> for more Biomass Cooking Stoves,  News and Information see our web site:
> http://www.bioenergylists.org/
>
>

-- 
Harold Annegarn
Department of Geography Environmental Management and Energy Studies
University of Johannesburg
Mobile +27 (0)83 628 4210
Fax +27 11 559 2430
Office +27 11 559 3927
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20130223/31d64410/attachment.html>