[Stoves] combustion of char

Mon Feb 25 14:46:11 CST 2013

I would guess that to get O2 uptake, you'd have to use a large amount of
char in order to be able to measure the resulting small weight percent
increase. That could be inconvenient, or downright dangerous if spontaneous
combustion is indeed a risk (large amount of char + atm of pure O2 ???).

Josh

On Mon, Feb 25, 2013 at 1:36 PM, Frank Shields <frank at compostlab.com> wrote:

> Stovers,****
>
> ** **
>
> I think wetting, keeping dry or covering new chars may just be putting of
> the potential problem. ****
>
> ** **
>
> A test might be:****
>
> 1) Weigh a container****
>
> 2) add biochar and weigh****
>
> 3) purge with nitrogen (200c)****
>
> 4) weigh tare and dry char****
>
> 5) purge with air (oxygen) (25c)****
>
> 6) weigh tare and biochar****
>
> 7) purge with butane (25c)****
>
> 8) weigh tare and biochar****
>
> ** **
>
> Step 4 you get the moisture****
>
> Step 6 you get oxygen up-take and potential for spontaneous combustion****
>
> Step 8 you get butane activity****
>
> ** **
>
> If this method looks good, when we get some time we will try it.****
>
> We need to get relative O2 up-take values compared to potential for
> Spontaneous Combustion to be able to report a SC potential Index.   ****
>
> Butane activity value just easily added at this time. ****
>
> ** **
>
> Regards****
>
> ** **
>
> Frank****
>
> ** **
>
> ** **
>
> Frank Shields****
>
> Control Laboratories, Inc.****
>
> 42 Hangar Way****
>
> Watsonville, CA  95076****
>
> (831) 724-5422 tel****
>
> (831) 724-3188 fax****
>
> www.controllabs.com****
>
> ** **
>
> ** **
>
> ** **
>
> ** **
>
> ** **
>
> *From:* Stoves [mailto:stoves-bounces at lists.bioenergylists.org] *On
> Behalf Of *Harold Annegarn
> *Sent:* Saturday, February 23, 2013 12:50 AM
>
> *To:* Discussion of biomass cooking stoves
> *Cc:* James Robinson; Glenn Stracher
> *Subject:* Re: [Stoves] combustion of char****
>
> ** **
>
> 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
>
>
>
>
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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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-- 
Josh Kearns
PhD Candidate, Environmental Engineering
University of Colorado-Boulder
Visiting Researcher, North Carolina State University

Director of Science
Aqueous Solutions
www.aqsolutions.org

Mobile: 720 989 3959
Skype: joshkearns
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