[Stoves] combustion of char -- spontaneous and dangerous

Paul Anderson psanders at ilstu.edu
Sat Feb 23 12:42:13 CST 2013 

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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>
>
> -- 
> 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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