[Stoves] Insulation and stove life

Tue Jun 18 10:34:17 CDT 2013

Dear Paul
  ----- Original Message ----- 
  From: Paul Olivier 
  To: Discussion of biomass cooking stoves 
  Sent: Friday, June 14, 2013 9:09 PM
  Subject: Re: [Stoves] Insulation and stove life

  Kevin,

  You write: Rice Hulls are very interesting. While they have a high percentage of highly siliceous ash, complete combustion could potentially make the ash available in a hazardous manner.  In Paul O's case, where he is interested in char making, I would guess that most of the siliceous components would remain embedded within the char particles, in a safe and non-hazardous manner. However, some ash particles would be liberated in that some of the char would be burned in the pyrolysis process.

  If, in a TLUD, air does not flow up the reactor in a uniform manner, channeling occurs, and zones that are incredibly hot are created. If the biomass contains amorphous silica, some of this silica will surely be converted into cristobalite at such high temperatures.

  # Certainly, channelling is a problem. It is caused by "non-uniform fuel" (both size and density), that has too much "primary air pressure." Unfortunately, once it starts, it tends to get worse. Additionally, as the channel diameter increases, even more air tends to flow up through it, creating a locally intense combustion condition. 

   When this mixture of biochar and ash containing cristobalite is removed from the reactor and handled, it would be so easy for the operator to breathe this deadly mixture.

  # Could to problem be reduced to an acceptable level, by lightly spraying teh Ash plus biochar with a light spray of water from a hand-operated sprayer bottle?

  The possibility of channeling is minimized if the biomass is thoroughly uniform. Also in the design of the stove, one has to be careful that the fan is not too powerful. Turning up a powerful fan a bit too high will cause channeling.
  # Yes, the fan pressure and flow must be controlled or limited, in a way to prevent channelling, or "spouting" of the fuel bed.

  Also rice hulls from most mills that I have seen in Vietnam are not properly dedusted. Consequently there is always some particulate matter in the syngas. This particulate matter might also contain cristobalite.

  # If the presence of dust is a problem, could you not configure a very simple "Rice Hull De-duster"? The equivalent to a "Seed Cleaner" that blew off teh dust and chaff, while allowing the hulls to be recovered should work well. BTW.... "gasifier gas" is not "syngas". "syngas is a gas of definite composition, intended for use as a 'feedgas to a synthesis process." It would more accurately be called "Producer Gas", "Rice Hull Gas", etc. 

  One simple way to solve these two problems is to use a pelleted fuel. Here the flow of air will be perfectly uniform. The pellet is relatively heavy and is not easily displaced within the reactor by a powerful fan. Also the pellet does not have much dust associated with it.
  # Certainly, more dense pellets would permit higher "under-bed pressure" without channelling. From a "big picture standpoint", one can certainly reduce the tendency for channelling by "de-dusting" the fuel, and then simply reducing the "under-bed pressure."

  So as you see, I have been working for several years on a unit to gasify loose rice hulls. But in the last few weeks I have begun to have serious doubts about the safety of such an apparatus. I feel strongly that the rice hull should be pelleted if it is to be used safely within a stove.
  # Simple pelletizing the rice hulls may not reduce the tendency for dust and particulate emissions, in that with the greater density, and reduced channelling tendency, one would be tempted to increase the "under-bed pressure", to increase air flow, and heat output rate. The 'local air velocity" may actually be high enough to scour off ash and carry it out of the stove.   

  Also please do not think about directly combusting any form of biomass that contains appreciable amounts of amorphous silica. For example, burning rice hull pellets is not a very good idea. And yet sadly, this is what is happening to most rice hull pellets in Vietnam. Imagine the poor workers who handle this ash.

  # I would suggest that in a TLUD producing char, there is still some "localized complete combustion", where it is possible for some of the silica ash to be freed up from the char, so that it can be blown away. Most would, of course, be trapped within the char. The higher the localized velocity through the bed, then teh greater the likelihood of ash being carried out of the stove with teh products of combustion.

  # What do you think of my previous suggestion of adding Ca(OH)2 to the RH fuel, before combustion? Basically, the Rice Hulls could be sprayed with "white-wash", and then be allowed to "sun dry." The idea is that the reactive silica would react with the Ca(OH)2, to form a "Calcium Silicate" of some sort, that would stay with the char, rather than venting into the Living space.
  Best wishes,

  Kevin

  Thanks.

  Paul Olivier

  On Sat, Jun 15, 2013 at 1:01 AM, Kevin <kchisholm at ca.inter.net> wrote:

    Dear Crispin

    Thanks very much for your helpful comments. 

    Clearly, a chimney that vents products of combustion outside the Living Space is advantageous, compared to a stove system that vents into the Living Space. 

    I was aware of the great importance of the size of the Particulate Matter. Basically, "big dirt particles" are not nearly as bad as are fine particles that can lodge deep in the lungs. Clearly also, "more bad-sized particles", with "bad type composition", are worse than fewer "safer sized particles", of "relatively neutral composition.

    The only way to "digitize the generalities" is with a scientifically sound test program that identifies and then measures the relevant parameters. Once numbers have been attached to the relevant parameters, then it is relatively easy to determine whether or not a given stove system is "safe" or "hazardous."

    Rice Hulls are very interesting. While they have a high percentage of highly siliceous ash, complete combustion could potentially make the ash available in a hazardous manner.  In Paul O's case, where he is interested in char making, I would guess that most of the siliceous components would remain embedded within the char particles, in a safe and non-hazardous manner. However, some ash particles would be liberated in that some of the char would be burned in the pyrolysis process. 

    Best wishes,

    Kevin

    ----- Original Message ----- 
      From: Crispin Pemberton-Pigott 
      To: 'Discussion of biomass cooking stoves' 
      Sent: Friday, June 14, 2013 10:51 AM
      Subject: Re: [Stoves] Insulation and stove life

      Dear Kevin

      I would like to add that using a fan will in all likelihood create much more flyash than without one. The advantage of a chimney is that the tends to suck the ash up the chimney, whereas a fan tends to blow it up from below into whatever outlet it can find. The difference in the room will be large, obviously.

      Secondly the size of the PM matters a lot. If the pyrolysis process releases very fine siliceous matter with a gentle push from a fan, that material can easily rise through the fuel bed.

      So we should be investigating size, total mass, chemical composition and whether the stove draws it from the stove and room or pushes it up from below.

      Maybe it is of no consequence at all, maybe it is a hazard.

      Regards
      Crispin

      Dear Rebecca

      Firstly, I am concerned about the "Cristobalite Link" below, in that it seems to overlap "silica", "cristobalite", and other forms of quartz. See: http://www.quartzpage.de/gen_mod.html showing how various forms of quartz exist at various temperatures.

      Note that "cristobalite" is one specific crystalline phase of quartz.

      Secondly, what is important is the "respirable" dust that is actually respired. 

      It is well known that respiring "silica dust" can cause "Silicosis". It is well known that "Small Particulate Matter emissions" from virtually any poor stove can cause serious health problems. Hence, the effort to design "stove systems" that minimize "Small Particulate Matter Emissions" into the living space, where they can potentially be respired.

      Rice Hull Ash, even at 90% silica in the ash pit, is not a problem, in that it is in the ash pit, where it is not respired. On the other hand, an "apparently superior fuel" that only had say 25% silica content in the ash pit would be vastly more hazardous, if it vented 10 or 100 times as much ash into the living space, in respirable form.

      This is where competent and meaningful stove design and testing comes into play. 

      As has been said many times, "It is not so much the fuel, but the stove system design, that is good or bad."

      Best wishes,

      Kevin

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  -- 
  Paul A. Olivier PhD
  26/5 Phu Dong Thien Vuong
  Dalat
  Vietnam

  Louisiana telephone: 1-337-447-4124 (rings Vietnam)
  Mobile: 090-694-1573 (in Vietnam)
  Skype address: Xpolivier
  http://www.esrla.com/ 

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