[Stoves] combustion of char -- spontaneous and dangerous

Kevin kchisholm at ca.inter.net
Tue Feb 26 07:32:18 CST 2013


Dear Crispin


  ----- Original Message ----- 
  From: Crispin Pemberton-Pigott 
  To: 'Discussion of biomass cooking stoves' 
  Sent: Tuesday, February 26, 2013 5:57 AM
  Subject: Re: [Stoves] combustion of char -- spontaneous and dangerous


  Dear Kevin

   

  Wow. Well I had not considered either of those two. What is perhaps the logical approach is to look at the processes that could account for 100 g of gain.



  # Firstly, I can't see the weight gain, in that the weight at 2815 is virtually identical to the weight at 6567, as shown on your previous chart. However, the range of your scale seems to be from 0 to at least 100 kilograms.  100 grams in 100 kilograms = 1 in a thousand = 0.1%. It may be a question of electronic precision being significantly more than the mechanical accuracy of the scale.

   

  You can take it that the initial fire was with 5 kg of coal and then a reload with 4.6 = 9.6 kg of coal, 25% moisture, roughly 10% ash. From that we could calculate the missing fraction of fuel mass which would be most of the non-ash portion. I can't see there being enough carbon left to pick up that much O2.



  # With 10% Ash, the ash would be about 1 kg. A weight change of 100 grams would thus be about 10% CaCO3 is 44% CaO, and 56% CO2. The addition of the CO2 to CaO roughly doubles its weight. To get a 100 gram weight gain, one would need roughly 100 grams of "free" CaO in the ash. This would be roughly 10% CaO, or roughly 20% limestone in the final ash.  Free lime in that quantity is unlikely, in that it would most likely be tied up with other reactive ash components, such as Feo, Sio2, and Al2O3.  Equally, it is unlikely that the3re would be free FeO, in that it is very reactive to silica. 

   

  There is no doubt that the reversal idea is not correct because there is a temperature record and the interior air temperature remained well above the outdoor temperature (the building is heated).



  # If these reactions jhad occurred, then it is likely that there would be a bump in ash temperature, in that these slagging reactions are exothermic.  Note that these reactions, if they were present, are independent of room temoperature.... Lime calcines at about 897 C, and Fe3O4 is formed when FeO drops below about 680C in the presence of oxygen. I can't see any "roughness" in any of the other lines, at the time of the "weight gains", so this seems to confirm it is not an oxidation effect.



  # Yet another possibility is that your "Heavy stove + sensitive scale" was actually functioning as a seismograph.... is it possible that there might be a quarry nearby doing blasting, or heavy trucks or trains going down their respective roads?



  Best wishes,



  Kevin

   

  Here is the set of temperatures with the Mass, test date 12 Jan 2010:

   



  You can save and stretch that. The 1 deg rise in temperature to -12.5° as the sun comes up is visible at the end. The temperature in the stove is slightly higher than the air in the room. The mass curve is generally the same shape as the charts by Battacharyya for coal (with a different time scale - but remember this is coke).

   

  It was hardly an ideal set of measurements.  We were just looking to see the cooling. The thermal efficiency of the stoves drops to -250% at times because it pulls in more cold air than it generates as heat - the fire just acts as a fan to throw heat from the room up the chimney.

   

  I am still prepared to write off the mass change as an instrument artifact - wind or something but it is a curious phenomenon to consider.

   

  Regards

  Crispin

   

   

  Dear Crispin

   

  I can think of two possibilities for the possible weight gain:

   

  1: If there was backdraft, as a result of the windy conditions, it might be possible that particles of char or creosote fell from the chimney, or were blown back, to the stove system, creating an increase in weight of the stove system. This material then burned off. Note that the weight at about 2815 is the same as at the very end of the chart, where the red line continues past 6567

   

  2: The oxygen that was picked up would be not only as oxygen, but as CO2. What might have happened is that is that the ash, containing FeO could easily pick up oxygen and become oxidized to Fe3O4, and limestone components in the ash could have been calcined to CaO, which could easily react with CO2 from the atmosphere, to "airslake" back to CaCO3. The "jiggle in the weight" may be a result of "local fire" re-calcining" the lime fraction, or "re-reducing the iron components.

   

  Any likelihood that either of these guesses might be accurate?

   

  Best wishes,

   

  Kevin



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