[Stoves] Smokeless transition

kgharris kgharris at sonic.net
Fri Dec 12 04:57:33 CST 2014


Marc,

I have also been struggling with this transition problem.  It has become more pronounced since the rains started, so humidity may be a contributing factor.  Also, the ND-TLUD I am using has turn-down capability and the transition problem seems to share some similarities with turn-down problems.  This makes sense because in both cases the wood gas production is being reduced and the temperature of the flame is lower.  This made me think that perhaps the problem could be addressed in the same way that turn-down is addressed, with either hot char or pilot flame support.  This has so far not worked with pilot flame support.  What has helped is giving the stove lots of primary air to get the char hot before the flame reduces.  Entering this phase with the stove turned to a low setting always results in smoke.  This is in keeping with something that Crispin wrote about, using paper to cover excess primary holes.  When the pyrolysis front reaches the bottom, it burns the paper and lets extra primary air in to heat the char, which helps get through this phase without smoke.  It might be a pain to add this paper to each burn.  

Fuel depth may have some input here.  With a large load the char will start burning at the bottom and will not be near the decreasing wood gas to heat and ignite it.  Directing air down onto the top of the char and heating some of it to red hot may help.  This is one of the methods of supporting turn-down by using red hot char.  Julian Winter once suggested a purched char method for turn-down which did work.  Placing a small amount of char or wood which will become char in a wire basket inside the reactor just below the secondary air inlet provides red hot char to support the secondary flame during turn-down.  This would have the deminishing transition wood-gas passing through red hot char to be heated and ignited.  This might work for the transition and also provide turn-down. 

The primary air in this phase will be used mainly for the last bit of pyrolysis and the char will gradually increase burning as the pyrolysis slows down.  This produces a temperature low point before the char heats up.  Excess primary air just before this phase can heat the char to keep the temperature up.  Keeping the temperature up is important.

Please post anything you might learn.

Kirk
Santa Rosa, CA. USA
  ----- Original Message ----- 
  From: Marquitusus 
  To: stoves at lists.bioenergylists.org 
  Sent: Friday, December 12, 2014 1:01 AM
  Subject: [Stoves] Smokeless transition


  Hi Stovers,


  There is a point in the TLUD process that I find specially difficult to understand and control: the transition from wood gasification to char gasification. Maybe you can help me to do it.


  At this point,  when the hydrocarbon fuel is near to finish, my experiments with almond husks as fuel for ND-TLUD shows 2 possible endings:
             1- The flames in the burner turns to blue color (due to high CO presence in gases) and the char pyrolisis continues until ashes. No smoke present.
             2- The flames in the burner extinguishes and the smoke appears.


  I made a lot of tests, with variations in ND-TLUD design (riser height, quantity of primary and secondary air, type of concentrator, etc.)  and still I'm not able to say which factor combination is the answer for a smokeless transition. Sometimes I have smoke, and sometimes I don't. 


  I suspect the answer has to do with chemical fuel composition, as when I use wood pellets, almost never have smoke.


  So this is the point where I am today. When char gasification begins, I have to lower the primary air? Lower the secondary air? How I can ensure the flame won't extinguish?


  It would be very useful for me to read your comments about this.


  Thanks very much,


  Marc






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