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<div class="moz-cite-prefix">Dear Stovers AND to all who make
biochar or TLUDs with char-saving.<br>
<br>
The two messages below are very important. And they lead to
asking this question:<br>
<br>
When the quantities of char production are small, such as from a
domestic-size TLUD stove, or from a biochar barrel (as with
RE:Char), does this danger of spontaneous combustion pose a threat
to households, etc?<br>
<br>
Because Frank's experience was with a small sample of char, I
suspect that the danger to households could be very real!!! <br>
<br>
Therefore, what are the correct and incorrect ways of dealing with
TLUD chars that are being saved and stored?<br>
<br>
It is interesting that no stories of spontaneous combustion in
stored cookstove TLUD chars have reached us. How big is this
danger?<br>
<br>
Paul<br>
<pre class="moz-signature" cols="72">Paul S. Anderson, PhD aka "Dr TLUD"
Email: <a class="moz-txt-link-abbreviated" href="mailto:psanders@ilstu.edu">psanders@ilstu.edu</a> Skype: paultlud Phone: +1-309-452-7072
Website: <a class="moz-txt-link-abbreviated" href="http://www.drtlud.com">www.drtlud.com</a></pre>
On 2/23/2013 2:49 AM, Harold Annegarn wrote:<br>
</div>
<blockquote
cite="mid:CAK2BUKTs-=r=5OAkvDck+NS5myFjr7FrYmKxRtc9Eedqn6CNsA@mail.gmail.com"
type="cite">Response from Harold Annegarn
<div><br>
</div>
<div>I make reference not to char, but to coal. Perhaps the same
will apply to freshly prepared char surfaces.</div>
<div><br>
</div>
<div>The following information arises from the phenomenon of
spontaneous combustion in underground coal and in coal slag
heaps. </div>
<div><br>
</div>
<div>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. </div>
<div><br>
</div>
<div>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. </div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
<div>.</div>
<div>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.</div>
<div><br>
</div>
<div>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!</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>Best regards</div>
<div>Harold</div>
<div><br>
<br>
<div class="gmail_quote">On 23 February 2013 03:43, Frank
Shields <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:frank@compostlab.com" target="_blank">frank@compostlab.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Stovers,<br>
<br>
FYI<br>
After having about 30 compost samples in a drying oven go up
in flame it<br>
looked like it started with a single char sample placed in
the corner. I<br>
decided to do a little research by asking Hugh McLaughlin.
Another question<br>
I had for him was why does some biochar samples gain weight
in a drying oven<br>
set at 105c.<br>
<br>
So it seems soon after a char is produced it wants to take
up oxygen. If<br>
bagged in a sealed container and sent to the lab soon after
being made the<br>
dry sample will take up oxygen in the oven therefore gaining
weight. If a<br>
sample is wetted soon after being made the water prevents
oxygen from being<br>
taken up. Then if it becomes dry it wants oxygen, increases
temperature and<br>
can catch fire under the right (wrong) conditions. Char is a
great insulator<br>
so the heat can be trapped and build up. If there is organic
materials<br>
around it can flame. So before storage better give it time
to take up<br>
oxygen. Or if wetted do not let it dry out and keep bags of
char in single<br>
layers.<br>
<br>
The forced air drying oven was left on overnight and it was
in the morning<br>
we found all the burned up samples. It was placed against
the wall so we are<br>
lucky we still have a lab!<br>
<br>
Regards<br>
<br>
Frank<br>
<br>
<br>
<br>
<br>
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</blockquote>
</div>
<br>
<br clear="all">
<div><br>
</div>
-- <br>
Harold Annegarn<br>
Department of Geography Environmental Management and Energy
Studies<br>
University of Johannesburg<br>
Mobile +27 (0)83 628 4210 <br>
Fax +27 11 559 2430<br>
Office +27 11 559 3927<br>
</div>
<br>
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</pre>
</blockquote>
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