[Stoves] Smokeless transition

[Paul Anderson]

Marc,

All  of the other comments are useful.   I only add a couple of thoughts:

1.  If you have some (even one) irregularly larger piece of fuel at the 
bottom, it will continue to give gases (potential smoke) for a while 
after all of the surrounding fuel pieces have become char.   That very 
small amount of pyrolytic gase sometimes is not sufficient to sustain 
the upper flame, and the result will be smoke.

2.  When the MPF (migratory pyrolytic front) approaches the bottom, the 
radiant heat that goes downward has no "next layer" of biomass to dry, 
torrify and pyrolyze.   Instead, it radiates to the grate and back to 
the already hot fuel, making it hotter and pyrolyzing faster.   That is 
why there is a slight upward spike in the pyrolytic gases and the 
resultant flame at the top.   Seeing that, the cook has the tendency to 
try to turn down the flame, which is exactly the wrong thing to do.   
The pyrolysis needs to finish strongly at the very end, to avoid that 
small amount of final gases from being TOO small to sustain  the upper 
flame (similar to the comment in #1 above).

We all hope that this helps.   Please (everyone) submit comments on 
experiences that support or refute  the comments made to the readers.

Paul

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
Email:  psanders at ilstu.edu
Skype: paultlud      Phone: +1-309-452-7072
Website:  www.drtlud.com

On 12/12/2014 9:33 AM, Dean Still wrote:
> Hi All,
>
> We added more bottom air until all the wood had turned to charcoal so 
> there was no smoke at the transition.
>
> Best,
>
> Dean
>
> On Fri, Dec 12, 2014 at 4:32 AM, Ray Menke <ray.menke at gmail.com 
> <mailto:ray.menke at gmail.com>> wrote:
>
>     If the fuel is not dry, or doesn't let enough air through, the stove
>     will need more primary air.  A small computer fan mounted in the
>     bottom of a tin can or 2 liter juice bottle will provide that excess
>     air.  (I use an old motorcycle battery with clip-clip leads.)  Most of
>     the time, it is not needed, but it does solves the problem in a hurry.
>     It also melts grates and the walls of the burn chamber.  A sacrificial
>     burn chamber liner from scrap metal or old stovepipe works well, and
>     grates can be made from 1/8" thick steel.
>     Another solution might be to mix some pencil size torrefied wood in
>     with the almond husks.  (one or two inches in length.)  The torrefied
>     wood is guaranteed dry, and will loosen the packing of the husks so
>     more air can enter.
>     My stoves are based on Paul Anderson's Champion TLUD design, and have
>     a 2" tube at the bottom for primary air.  A funnel to direct air into
>     the tube can be made by taping tin or cardboard around the open end of
>     the olive can.
>     Ray
>
>     On Fri, Dec 12, 2014 at 4:57 AM, kgharris <kgharris at sonic.net
>     <mailto:kgharris at sonic.net>> wrote:
>     > 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
>     <mailto: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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