<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META content="text/html; charset=utf-8" http-equiv=Content-Type>
<META name=GENERATOR content="MSHTML 8.00.6001.19400">
<STYLE></STYLE>
</HEAD>
<BODY bgColor=#ffffff>
<DIV><FONT face=Arial>Dear Harold</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>I think there may be an additional "fire mechanism"
associated with coal, that is not associated with charcoal. Almost invariably,
coal has sulphur, in the form of FeS2 (pyrite) or pyrrhotite, </FONT></DIV>
<DIV><FONT face=Arial>FeS(1-x). When it is exposed to air and water, plus
microbial action, it oxidizes readily. In addition, of course, there are many
organic compounds in the coal, and some of them may also oxidise on their own
accord, perhaps being hastened or accelerated by the heat from the biological
reactions.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Charcoal does not have sulphur in the sulphide form, so
one of the possible "ignition mechanisms" in coal is absent in charcoal.
However, I recall reading somewhere, and unfortunately don't have the reference
now, that under certain conditions, the production of charcoal can yield
"products of pyrolysis" that are pyrophoric, ie, they ignite when exposed to
air. As I recall, these pyrophoric compounds were produced relatively easy, when
charcoal was produced under vacuum conditions. I don't recall if such pyrophoric
products can be made in atmospheric pressure retorts, or in "air blown" charcoal
making devices.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Best wishes,</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Kevin</FONT></DIV>
<BLOCKQUOTE
style="BORDER-LEFT: #000000 2px solid; PADDING-LEFT: 5px; PADDING-RIGHT: 0px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: black"><B>From:</B>
<A title=hannegarn@gmail.com href="mailto:hannegarn@gmail.com">Harold
Annegarn</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A
title=stoves@lists.bioenergylists.org
href="mailto:stoves@lists.bioenergylists.org">Discussion of biomass cooking
stoves</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Cc:</B> <A title=jamesrobinson77@gmail.com
href="mailto:jamesrobinson77@gmail.com">James Robinson</A> ; <A
title=stracher@ega.edu href="mailto:stracher@ega.edu">Glenn Stracher</A>
</DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Saturday, February 23, 2013 4:49
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [Stoves] combustion of
char</DIV>
<DIV><BR></DIV>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 href="mailto:frank@compostlab.com"
target=_blank>frank@compostlab.com</A>></SPAN> wrote:<BR>
<BLOCKQUOTE
style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8ex; PADDING-LEFT: 1ex"
class=gmail_quote>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>_______________________________________________<BR>Stoves
mailing list<BR><BR>to Send a Message to the list, use the email
address<BR><A
href="mailto:stoves@lists.bioenergylists.org">stoves@lists.bioenergylists.org</A><BR><BR>to
UNSUBSCRIBE or Change your List Settings use the web page<BR><A
href="http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org"
target=_blank>http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org</A><BR><BR>for
more Biomass Cooking Stoves, News and Information see our web
site:<BR><A href="http://www.bioenergylists.org/"
target=_blank>http://www.bioenergylists.org/</A><BR><BR></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>
<P>
<HR>
<P></P>_______________________________________________<BR>Stoves mailing
list<BR><BR>to Send a Message to the list, use the email
address<BR>stoves@lists.bioenergylists.org<BR><BR>to UNSUBSCRIBE or Change
your List Settings use the web
page<BR>http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org<BR><BR>for
more Biomass Cooking Stoves, News and Information see our web
site:<BR>http://www.bioenergylists.org/<BR><BR></BLOCKQUOTE></BODY></HTML>