[Stoves] Correcting a misconception that approaches myth status

[kgharris]

Crispin,

I have been following your insights as you have shared them with us.  I value what you are saying.  I like your earlier statement that the pipe is a solution for a problem that should not exist in the first place.  If my stove was providing adiquate air and mixing then my Wonderwerk Strata combustor would have no effect.  It would not be necessary.

Reading through this responce to Dr. Anderson I can follow your logic until I get to the last paragraph.  I do not disagree with you, I just wonder how it works.  The heat of pyrolysis rises through the fuel in a BLDD, heating the fuel and driving out moisture.  I wonder what the mechanism is for this raising heat in a down draft stove.  Is the gas not moving down, carrying the heat downward?  Heat is a property of material, not material itself, and so it is independent of gravity and buoyancy.  The only mechanism I can see for heat to move upward is radiant heat, just like a TLUD MPF radiates heat downward.  I would like to know more about how this works.  I mean no contention here, I just want to know.

Thank you,

Kirk
  ----- Original Message ----- 
  From: Crispin Pemberton-Pigott 
  To: 'Discussion of biomass cooking stoves' 
  Sent: Monday, December 15, 2014 12:39 PM
  Subject: Re: [Stoves] Correcting a misconception that approaches myth status


  Dear Paul

   

  You can't just jump in with such classifications without asking first. 

   

  >.In 2004 - 05 I re-named it Top Lit UpDraft (TLUD) which is also not totally accurate.   None of the names acknowledge the Migratory Pyrolytic Front (MPF) that is the dominant feature of what is commonly called TLUD micro-gasification.



  I think you should have a look at a BLDD working and watch the migratory pyrolytic front rising through the fuel instead of descending into it. You might change your opinion.


  Note:
  A DD gasifier is ignited at the BOTTOM, and the fire (hot zone of gasification) REMAINS at the bottom, and the unit can be operated with continuous fuel entry into the top.



  No that is not necessarily true at all!  One could make the same claim for all top lit stoves with a high superficial air velocity.  In a BLDD there is a gas production process started by a fire at the bottom. Same as TLUD.  There is a migratory pyrolytic front that works its way upwards. Same as a TLUD. When it reaches the top, there is a fuel chamber full of char, same as a TLUD. The gas produced can be burned immediately under the grate or taken elsewhere to be burned in a gas burner. Same as a TLUD.


  A IDD or TLUD or MPF gasifier is ignited at the TOP, and the fire (hot zone of gasification) slowly MOVES to the bottom, after which time the fire zone stays at the bottom and the unit operates as if it were a regular UpDraft (UD) gasifier if anyone puts more fuel into the top.  

  Fundamentally different.

   

  Again, no. It is the same!  When the migratory pyrolytic front reaches the top of a BLDD fuel chamber the combustion continues at the top drafting air in (if allowed) and does exactly what you describe but upside down. The big difference is that the BLDD can be refuelled by placing more fuel on top and the MPF continues to work its way into the new fuel, upwards, until it again reaches the fully charred condition.


  Because of Tom Reed and also Paal Wendelbo who worked totally independently, we have something new for cookstoves.   They did not invent pyrolysis.   They were not the first people to have a fire in a barrel making gases and have the flames only up at the top where secondary air was available.   (I saw that in the 1950s in a burn barrel at my home.) But Reed and Wendelbo were the ones who CONTROLLED it and understood it and made it all small enough to be useful in a cookstove.   And the rest is history.   And TLUD history is still being made.

  That may all be true, but the BLDD stoves with MPF have been in use literally for centuries. BLDD combustors with large fuel chambers were used for making coal gas and the only way a useful gas can be created is by having a MPF in the fuel bed. Obviously it was controlled or they would not have been able to get a reliable product.

   

  A TLUD is a bottom-lit downdraft stove turned upside down so that can't be refuelled. Deal with it.

   

  The only practical difference is that the heat from pyrolysis in a DD stove rises within the fuel supply, heating it and driving out moisture sooner than it would in a TLUD where the heat all rises, save by radiation. There is an advantage to this with coal or wet charcoal or wet wood. Otherwise, not really any practical difference. I find that with a BLDD and wood pellets, for example, the airflow velocity through the interstitial spaces becomes more important in a DD stove than an UD one in order that the rising heat does not create run-away gas generation inside the fuel bed. If too much gas is produced it is hard to burn it all. Apart from this small consideration, and having built lots of each, I observe no difference at all between these architectures in terms of how the fuel is pyrolysed and the options for burning the gas.

   

  Regards

  Crispin

   



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