Frank, Harold,
Interesting discussion, made more so by the fact that I making drums of
char yesterday.
Here is a paper that suggests that initial heating of coal has more to
do with H20 sorption than oxygen. Rank and particle size being other
factors.
Where upon oxygen moves to center stage.
http://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/14_1_TORONTO_05-70_0059.pdf
Glad to hear your lab lives on Frank.
Alex
On 23/02/2013 3:49 AM, Harold Annegarn wrote:
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 <[email protected]
<mailto:[email protected]>> 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
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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
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