[Stoves] Strata combustor general principles

Thu Sep 17 21:20:34 CDT 2015

Frans,

It is exciting to have you bring up these concerns.  There is a question I have had for a long time, and your contribution reminds me about it.  It has to do with the pressures within the stove.  Perhaps you and the list can help answer this question.

First to address your concerns.  There is a sweet spot as to how much blockage the stove can manage.  It lies between to much, which blocks the flow and creates the attenuation you describe, and to little so that the combustor doesn't work.  I did not know that the venturi created a back pressure, I assumed that the back pressure was created by the obstruction of the tubes and fan.  I found the sweet spot by emperical experimentation.  I now know that this balance includes back pressure from the Venturi, if I understand you correctly.  So the equation now includes the resistance of three things: tubes, stationary fan blade, and Venturi back pressure.  I included your concern into the design without my even knowing it.  Its all a balance.  Thank you for your input, it is very good to help me understand more about the stove.  The combustor has been tested at Aprovecho and does work, so the idea is good.

The question I have had for so long is this:  When the wood gas emerges from the wood what is its pressure and how long does it take for the wood gas to drop to near atmospheric pressure?  When I measured the pressure in a stove with a manometer which measured down to 1/100th of an inch of water column, it measured at atmospheric pressure throughout the stove.  The water column did not move at all (I understand from Crispin that I must measure down to 1/1000th of an inch to find pressure differences).  Since the water column did not move when I measured the pressure at the pyrolytic front, that implies that the wood gas either reaches atmospheric pressure quickly or comes out of the wood at near atmospheric pressure.  Whatever the case, the wood gas comes out of the wood with enough force to push the atmosphere, which weighs twice as much as the wood gas, out of the stove.  That is a considerable amount of force.  I wonder how much pressure pyrolysis could build up in a confined container?  A lot I would guess, enough to cause an explosion.  Is the pressure of the emerging wood gas dependent on the pressure of the gas it emerges into?

Respectfully,

Kirk

  ----- Original Message ----- 
  From: Frans Peeters 
  To: 'Discussion of biomass cooking stoves' 
  Sent: Thursday, September 17, 2015 3:06 PM
  Subject: Re: [Stoves] Strata combustor general principles

  Dear,Kirk

         You don't see :it is not a closed compressed system !

  But any  reaction vessel (V1 ) with a  venturi  attenuation at the output gives  a  back reaction in all directions ,so flow  attenuation  under the grate air input .

  By over total power the air input BECOMES SMOKE OUTPUT TOO .! So invers to your idea ;

  A V1  rocket has an input COMPRESSOR powered by the traveling speed plus a constant fuel input so a nice VENTURY output .

  1and 2 you don't have it in a stove of half info .

         Yours

  Rocket scientist

  Van: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] Namens kgharris
  Verzonden: donderdag 17 september 2015 4:58
  Aan: Discussion of biomass cooking stoves
  Onderwerp: Re: [Stoves] Strata combustor general principles

  Dr. Boll and Crispin,

  Thank you for your well considered responses.  I especially like the idea of balancing the reactor fuel chamber volumn with the combustor size.  I will keep this in mind as I progress, and seek that balance.  

  I don't quite understand Crispins flame tube.  The stove appears to be loaded with charcoal, so there should not be any smoke.  If the clay is insulating clay than the tube would provide a hot environment for the CO to burn, if there is a source of secondary air.  If the stove burns wood, by what principles does the tube work?  Is air injected inside the tube?  This would make use of the Venturi effect like Roberto's stove.  The flame gasses accellerating into the tube would drop in pressure and the pressure difference would help the atmosphere push secondary air into the flame to equalize the pressure, providing good mixing.  This seems to me like it might work well if that is how it works.  Is there something else that I don't see?

  Kirk  

    ----- Original Message ----- 

    From: Boll, Martin Dr. 

    To: stoves at lists.bioenergylists.org 

    Sent: Wednesday, September 16, 2015 6:35 AM

    Subject: Re: [Stoves] Strata combustor general principles

    Kirk, 

    first of all, your stoves and thoughts about TLUD's are very interesting. And in my opinion

     the venturi-concept, you are talking about, -how ever managed- will be generally the next important step for better burning; - and for less blacked pots!!! 

    A presumption I made a year or two ago: 

    The (nominal-) power rate of a good working stove has a more or less narrow relation to the grate-surface, and/or the burning-chamber-volume.

     I calculated the given numbers of some different commercial stoves.  My calculations confirmed my guess.

    I dont remember the numbers, but it is easy to calculate the numbers out of available commercial data.

    Additional I remind to Crispin's postings, some time ago:

    He reported, that he made the burning-chamber of some coal stoves in Ulanbaator smaller, by just filling some space of the burning-chamber-space with bricks. And some surface of the grate. In combination with special incending-procedure, that gave better burning; even by transforming the may of stoves there, there was an enormous reduction (more than 90%) of air-pollution. 

    So I suspect, if you would use the right-balanced amount of burning-chamber-volume, you could get a step forward.  The volume within the venturi-riser-burner-tube should be included in the calculation; but in the tube there will be, -hopefully- a burning some similar to a LPG or NG flame, which will afford less volume. 

    You could take the power-rate which you achieve with low emission, for extrapolating the stove dimensions for your desired 5 kW output.

    Regards

    Martin

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