[Stoves] combustion of char

Sun Feb 24 15:01:59 CST 2013

Dear Harold

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, 
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.

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.

Best wishes,

Kevin
  ----- Original Message ----- 
  From: Harold Annegarn 
  To: Discussion of biomass cooking stoves 
  Cc: James Robinson ; Glenn Stracher 
  Sent: Saturday, February 23, 2013 4:49 AM
  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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