[Stoves] Strata combustor general principles

Mon Sep 14 17:47:45 CDT 2015

Roberto,

Thank you for your excellent response.

Yes, it now has several tubes.  This is to increase the mixing capacity for a higher power flame.  I forgot to mention in the previous email that the Venturi works in addition to the draft to bring in secondary air.  The change in pressure is the main contribution of the Venturi.  

I am a fan of Alexis Belonio.  We had a good talk at Aprovecho after ETHOS 14.  The pipe to conduct the wood gas to an lpg like device is a exciting idea.  I don't like the idea of cooling the gas unless it is to be stored, because so much heat is lost.  For a TLUD, burning the gas while it is hot makes sense.

A narrow pipe as you describe would work also to create a Venturi effect.  I can see how this would work.  You write that the holes are at the top of the central ring hole.  I put the tubes at the bottom of the combustor for two reasons:  better draft and more time for the gasses to finish burning.  This works for my needs.  When the flame hits the pot, it cools and some of the fuel is lost as PM, so I like to finish the burning before the flame hits the pot.  The pot seeing the entire flame will increase radiant heat to the pot surface but also can loose some of the conductive heat transfer as I have heard described at Aprovecho.  I am not familiar with your arrangment, whether this will happen or not.  Which is best, radiant or conductive I don't know, but your experience seems to add a vote for radiant.  I am happy that you are getting the good results that you want.  This is good.  There are many ways to burn wood gas.

Some of us attending Stove Camp at Aprovecho this summer, Steve, Cody, Ricardo, and myself tried using a second layer of tubes.  The test looked good at first until we realized that something was wrong with the sensor/computer read out.  When the filter was examined, it was very dirty.  This being said, some other thoughts need saying.  This combustor, as a 4 tube combustor had earlier tested at 1 mg/min, but clean at only 3 kw.  We wanted 5 kw, so we added the second row to this combustor to try to increase its capacity to the higher power level.  Thus we ran the stove at a higher power level.  All we proved was that this particular combination did not work at 5 kw.  I have been trying variations of this combination since, and the results are similar.  It really doesn't seem to work.  The single layer is better.  More tubes, as suggested by Cody Anderson at stove camp, is the direction I am trying.  It seems to work, but only if the entire array can be used.

The pipes themselves reduce the open area of the combustor, so reducing the overall combustor opening higher up is not needed.  The combustor is left wide above the stationary fan to allow room for the flame to finish burning.  The rate of combustion does not need to be increased.  Improving on 1 mg/min would not be that meaningful.  1/2 mg/min is not too different from 1 mg/min.  What I need to do is increase the usable power level.

Pre-mixing with a TLUD is not possible since as soon a it mixes, it burns.  It is a mixed-while-burning device.  Venturi gas mixers used for fossil gasses can pre-mix because the gas is cool and compressed.  TLUD wood gas is hot and at near atmospheric pressure.  The TLUD Venturi gas mixer will look quit different than a fossil gas Venturi mixer because of this.  The TLUD Venturi mix is on a par with a pre-mixed system, quite good.  Actually, it is quite excellent.

To cool the gas, a complex cooling and filtering system is needed, which would be expensive.  All TLUDs mix the hot gasses with various amounts of completeness.  The TLUD Venturi mixer is capable of mixing hot gasses, and doing it with a high amount of completeness.  Using a retort might be more appropriate for wood gas that is to be cooled.

Burning some of the gas below the tubes is a good topic.  I have found that with no flame below the tubes, soot is deposited on the bottoms of the tubes and the exhaust can be dirty.  Too much air below the tubes negates the value of the tubes and produces considerable smoke. Probably because of too much air cooling and diluting the flame.   Allowing a moderate amount of flame below the tubes keeps the tubes and exhaust clean, and does seem to help increase the possible power level.  The pre-tube secondary air can perform another task, if it is done right.  It can be used to blow the wood gas evenly over the tube mixer array.  This is very helpful because it avoids over or under loading any part of the array, making better use of the entire array.  The simple Strata designs of a year ago using the 5 point star or single tube arrangments seem to work better with various stoves.  The current design with more parallel tubes is more tempermental, and will need the stove to be designed for the combustor.

Thank you, Roberto for your thoughtful response and good ideas,

Kirk
  ----- Original Message ----- 
  From: Roberto Poehlmann 
  To: stoves at lists.bioenergylists.org 
  Sent: Monday, September 14, 2015 12:25 PM
  Subject: [Stoves] Strata combustor general principles

  Hi Kirk,

  Thanks for sharing this knowledge.
  I see that the Strata combustor has evolved to a design with several secondary tubes. This remind me the secondary tubes of some double combustion woodstoves, but without the use of the venturi effect.

  I always see the TLUD combustion chamber as a smoke or woodgas generator. On top of the combustion chamber, exists multiple alternatives to put different kind of combustors. One alternative is the Strata combustor, as you propose. Another alternative is a pipe to conduct the woogas to a lpg like burning device, as Alexis Belonio did in Phillipines.

  With that in mind, i have constructed a combustor wich also utilizes the venturi effect. Is a vertical tube with two rows of secondary holes on top of the central ring hole. The diameter of the central ring hole (and the tube) is smaller than a standar TLUD ring hole. Inside the tube, because of the drop in pressure, it sucks secondary air to allow a good mixing of the gases.
  Because of the small diameter of the tube (2 inches aprox.), i can have a low consumption rate of pellets, without the risk of smoke traspassing through the flame. Another advantage of this design, is that the pot "see" the entire flame.
  With this design, i see that 2 or 3 rows of secondary holes, generate a very good looking flame, without any perceptible contamination.

  Have you tried to put 2 layers or rows of secondary tubes in the Strata combustor? Maybe you can also reduce the top hole of the combustion chamber, and with the additional tubes, you can increase the rate of combustion.

  Another question or idea: have you tried to have a premixing flame?
  I tried to do that, but when the air enter below the secondary flame, the air reacts with the smoke and generate a pilot flame below the principal flame.
  It is possible to cool down the smoke before mixing it with air, to have a premixing flame? I see that in some woodgas generator to feed internal combustion engines, so maybe is possible.

  But, after the intend to have a premixing flame, i have had a new idea. It is the increase of the pilot air below the principal flame or terciary flame, to reduce it at very high combustion rates. Part of the smoke will be burn inside the combustion chamber, before reaching the principal flame.

  Greetings
  Roberto Poehlmann

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