[Stoves] combustion of char
Art Donnelly
art.donnelly at seachar.org
Sat Feb 23 20:28:31 CST 2013
Frank,
Glad to hear it was a near miss and not a disaster, but thanks for using
this as an opportunity to flag this important safety issue. I want to make
sure we take away a clear message in terms of best practices for handling
our inventories of biochar.
- Is auto-ignition only likely to occur in char that has been wetted?
- Is there a low risk of auto-ignition in char that has been dry
quenched?
- Is particle size a risk factor: IE is powdered char more likely to
ignite?
- When you say: "So before storage better give it time to take up
oxygen." What is the safest way to do this?
We (the Estufa Finca project) have picked over 3-tons of cook-stove biochar
from just 32 households over the past 7 months. I currently have an
inventory of approx. a ton. It's piled in woven-plastic feed sacks, under a
shed roof. It was all quenched dry in the steel pails we provide all
participants and we only buy dry biochar. However this is a tropical humid
environment. So we assume the char is taking on moisture. As we are now
starting to develop demand, I expect to see the volume of material on hand
go up substantially in the coming year. We are trying to figure out how to
avoid the problem you just had. As someone pointed out it's not a matter of
whether it is a matter of when.
chao
Art
On Sat, Feb 23, 2013 at 2:00 PM, <stoves-request at lists.bioenergylists.org>wrote:
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> Today's Topics:
>
> 1. combustion of char (Frank Shields)
> 2. Re: combustion of char (Harold Annegarn)
> 3. Re: combustion of char (Alex English)
> 4. Re: combustion of char (Crispin Pemberton-Pigott)
> 5. Re: combustion of char -- spontaneous and dangerous
> (Paul Anderson)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 22 Feb 2013 17:43:06 -0800
> From: "Frank Shields" <frank at compostlab.com>
> To: "'Discussion of biomass cooking stoves'"
> <stoves at lists.bioenergylists.org>
> Subject: [Stoves] combustion of char
> Message-ID: <009601ce1167$20a1ee70$61e5cb50$@compostlab.com>
> Content-Type: text/plain; charset="us-ascii"
>
> 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
>
>
>
>
>
>
> ------------------------------
>
> Message: 2
> Date: Sat, 23 Feb 2013 10:49:54 +0200
> From: Harold Annegarn <hannegarn at gmail.com>
> To: Discussion of biomass cooking stoves
> <stoves at lists.bioenergylists.org>
> Cc: James Robinson <jamesrobinson77 at gmail.com>, Glenn Stracher
> <stracher at ega.edu>
> Subject: Re: [Stoves] combustion of char
> Message-ID:
> <CAK2BUKTs-=r=
> 5OAkvDck+NS5myFjr7FrYmKxRtc9Eedqn6CNsA at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> 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 --------------
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> >
>
> ------------------------------
>
> Message: 3
> Date: Sat, 23 Feb 2013 10:37:42 -0500
> From: Alex English <english at kingston.net>
> To: Discussion of biomass cooking stoves
> <stoves at lists.bioenergylists.org>
> Subject: Re: [Stoves] combustion of char
> Message-ID: <5128E246.8050503 at kingston.net>
> Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"
>
> 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 <frank at compostlab.com
> > <mailto: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
> > <mailto: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
> >
> >
> > _______________________________________________
> > 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/
> >
>
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> http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20130223/3a06a693/attachment-0001.html
> >
>
> ------------------------------
>
> Message: 4
> Date: Sat, 23 Feb 2013 11:14:19 -0500
> From: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com>
> To: "'Discussion of biomass cooking stoves'"
> <stoves at lists.bioenergylists.org>
> Cc: 'Glenn Stracher' <stracher at ega.edu>, "Prof Lodoysamba, Sereeter"
> <lodoysamba at gmail.com>
> Subject: Re: [Stoves] combustion of char
> Message-ID: <0ac201ce11e0$d9ca1890$8d5e49b0$@gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Dear Harold and Other Oxygen Breathers
>
>
>
> The uptake of oxygen by coke (which is after all, a form of biomass) is
> surely taking place in exactly the same manner as described below.
>
>
>
> While testing the performance of lignite burning stoves at the SEET
> Laboratory in Mongolia about 2 years ago, Prof Lodoysamba and I did some
> very long tests that included a complete burnout to see what would happen.
> The experimental method was this: run the stove through the usual ignition
> and refuelling cycle as prescribed by the social science team. Then leave
> the recording instruments on long after the standard ?90% of fuel burned?
> time window. The stove was left burning overnight and the recording
> instruments tracked the gases, six temperatures and the mass.
>
>
>
> When the fire became very small and extinguished there was eventually no
> detectable change in mass. As the instruments were left on for many hours
> after that a very surprising thing happened: the stove gained weight and
> continued to do so for some time.
>
>
>
> It will take some time out dig out the tests in question but the effect
> was noticed immediately we processed the data. What, after all, was the
> final burned-out mass of the fuel?
>
>
>
> For some stoves, that answer is the initial fuel minus everything that
> burns, leaving the ash. For other stoves, there is some char left (in this
> case coke, which is charred coal, which is compressed biomass). The coke
> might be in the ash in small pieces, or it is on the edges of the fuel bed
> where it got cold and never burned completely. We tested one stove that
> left considerable unburned material because it was so poorly designed that
> the fire simply went out after a while leaving more than 20% of the
> burnable fuel mass unburned.
>
>
>
> At the time we considered that the ceramic components of the stove
> (usually bricks and clay) were absorbing moisture from the air; that in the
> heating cycle the stove components first lost moisture and then regained it
> when they could, tapping any humidity in the air passing through the stove.
> This is a reproducible effect and is a consideration for testers using a
> mass-based and chemically balanced emissions measurement method. All
> ceramic (clay) stoves have to be watched careful for the loss or gain of
> moisture during the test.
>
>
>
> However this observation that the char may be collecting oxygen following
> the termination of the ?fire? as we know it is most interesting and may
> explain, in certain cases and partially, the increase of mass by the stove.
>
>
>
> I have no doubt that there is a concurrent ceramic effect, that the stove
> gains weight, but it appears the fuel is also going to gain mass. The
> implications for determining the moisture content of fuel is obvious.
> Perhaps the moisture measurements of char remaining, charcoal and coke
> should be conducted in a nitrogen atmosphere, or in a low pressure
> environment, or??
>
>
>
> We should take some char, especially high temperature char, and place it
> on a microbalance for a few days to observe the uptake of O2. If this can
> be done in a sealed, water vapour-free, high O2 box the effect could be
> separated from the absorption of moisture (which also occurs).
>
>
>
> Regards
>
> Crispin
>
>
>
> +++++++++
>
>
>
> 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
>
> -------------- next part --------------
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> URL: <
> http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20130223/e79472d7/attachment-0001.html
> >
>
> ------------------------------
>
> Message: 5
> Date: Sat, 23 Feb 2013 12:42:13 -0600
> From: Paul Anderson <psanders at ilstu.edu>
> To: Discussion of biomass cooking stoves
> <stoves at lists.bioenergylists.org>
> Cc: James Robinson <jamesrobinson77 at gmail.com>,
> "biochar at yahoogroups.com" <biochar at yahoogroups.com>, Hugh
> McLaughlin
> <wastemin1 at verizon.net>, Bob Fairchild <
> solarbobky at yahoo.com>, Luke
> Iseman <luke at re-char.com>, Glenn Stracher <stracher at ega.edu>
> Subject: Re: [Stoves] combustion of char -- spontaneous and dangerous
> Message-ID: <51290D85.6080503 at ilstu.edu>
> Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"
>
> Dear Stovers AND to all who make biochar or TLUDs with char-saving.
>
> The two messages below are very important. And they lead to asking this
> question:
>
> 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?
>
> Because Frank's experience was with a small sample of char, I suspect
> that the danger to households could be very real!!!
>
> Therefore, what are the correct and incorrect ways of dealing with TLUD
> chars that are being saved and stored?
>
> It is interesting that no stories of spontaneous combustion in stored
> cookstove TLUD chars have reached us. How big is this danger?
>
> Paul
>
> Paul S. Anderson, PhD aka "Dr TLUD"
> Email: psanders at ilstu.edu Skype: paultlud Phone: +1-309-452-7072
> Website: www.drtlud.com
>
> On 2/23/2013 2:49 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 <frank at compostlab.com
> > <mailto: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
> >
> >
> >
> >
> > _______________________________________________
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> > 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
> >
> >
> > _______________________________________________
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> >
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> End of Stoves Digest, Vol 30, Issue 24
> **************************************
>
--
Art Donnelly
President SeaChar.Org
US Director, The Farm Stove Project
Proyecto Estufa Finca
<http://email2.globalgiving.org/wf/click?c=1Oy%2FmZbgIyjS5WI580KXwShvfKBcF2eaJvtN7Pi6p7Jl%2FiR4938EMMCBwY%2FuYALeA%2BQYUWN4RpvnxBsBC7e2%2BGIHcONTozBmvsUU5LTL%2FTNk4Q3vxE%2BKdXTV2cxIsFplSPh%2F9nMG3bQMQf4bz9ZK9SHMy46Z8OPLAtMAnPG9SKkPuLCWvofBTLC%2BImqax%2BZTkkF2RvDri5UdgH19NHjHOBj5WMUrS4L62Z2xxUJbBsJdDUOfeifheNFXH546Xm0yul4P2stm%2FTUOJxYnI0nFjXEaYfzxDSc%2FwgqVkR1t0USDHk30%2Fgt9UpDpyzLj37HWtnNQ0q8Jh1gZCkB4Y1Fgbg394gYFkyNqFN4MchxO2Js%3D&rp=wrhiOr2wAxUyDMDlMSqbOkKa0FpPoiCSHffb%2ByfHGClRxIFjEIrUDwAF%2BFD%2BpAPuvam9BDwvSMcadhFv7aFwKoyAXYrFk00%2B92xPIeMHXaTDJ3x0VIj6ZYwjm1win65o&up=YDTqBOjidbCUo%2Far1oAtZjp5ji73zPEvmoO14mevuXzIDUdb6Ac9W13SPOXmzL5NflZkH0HxLp0v4dT9UwEHDV0wSZ1qusv09bIKkUliWs4%3D&u=LHuflw_1TAib_lgCu2JvQw%2Fh0>
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