[Stoves] TLUD pellets stove with a venturi burner.

Julien Winter winter.julien at gmail.com
Sun Oct 11 14:33:45 MDT 2015


Hi Roberto;

As you will be aware, trying to get natural draft burners to work over a
range of gas supply rates is the biggest challenge.  It is difficult to get
a simple geometry that gets best mixing and air/gas ratios over a broad
range of gasification rates.  At very high rates of gasification, we can
flash-fry a goat.  When we turn down the primary air in a TLUD, we don't
get enough gas to support a flame across the whole cross-sectional area of
the TLUD.

I have a burner that is 1.2 x the diameter of the TLUD, with a ring of
secondary air holes, and a narrow, horizontal deflector ring above the air
holes (to stop flamelets from going up the sidewalls of the burner).  It
works well for high gasification rates.  At low gasification rates, the
secondary air (not preheated, therefore relatively dense) falls to the
surface of the char bed, and the gas flame dances across the top of the
char.  One might call that an 'unstable flame,'  however what I see are
localized patches of pyrogas and oxygen reaching a flammable concentrations
and igniting.  If the rate of pyrogas supply is not too slow, the flame
flashes from burning areas to pilot the ignition of non burning areas.
Also, pyrogas is drawn toward the flame, increasing its localized
concentration.  Hence the flame appears to be dancing across the surface of
the char.  If the gasification rate is too slow, the piloting will fail and
the fame goes out.  (This is where a 3-4 small holes in the side wall of
the TLUD, and below the surface of the char can help to pilot re-ignition.)

In my case, I have found it best not to interfere with the 'dance', by
swirling the secondary air.   I just let things be.

I have tried some methods to concentrate or focus the pyrogas to see if
that helps with turned down gasification, but with no success.  I did this
by creating an annular gap using a disk over the mouth of the TLUD reactor,
and similarly, using a cylindrical gas wick.  I have tried having the
pyrogas rise though holes in a horizontal disk with secondary air blowing
across the holes.  The problem I found with these methods was that at low
rates of pyrogas, if the flame went out in one sector of the annular ring,
or in one of the holes in the plate,  the flame elsewhere didn't
efficiently pilot re-ignition.  These methods also interfered with pyrogas
being drawn into, and concentrated in the flame.  As a result I got an
increase in smoke emissions.

Again,  I decided not to interfere with the dancing flame, and just let
secondary air flow down to the surface of the char bed.

I am still trying, but I haven't succeed yet to get a simple burner
geometry that is optimal for both high and low gasification rates.  For the
same reason (I suspect) Kirk Harris has designed his burner with secondary
air introduced in stages to support progressively larger flames.

I think there are a some thing to watch out for when using staged secondary
air.  (1) the stages should occur is quick succession, because the most
reactive gases will burn off first, and you what to use that energy to
activate combustion of the tars;  (2) be careful not to stop combustion
with too much cool air and cool surfaces; and, (3) the staged air has to be
mixed into the flame, and not just added to the edge.

Personally, I am still seeing if I can add secondary air at only one level,
and mix it well with the pyrogas so I can get as close as possible to
pre-mixed flame conditions.  I may cheat with some pilot holes in the side
walls of the TLUD reactor.

I hope this is helpful.

Cheers,
Julien.

-- 
Julien Winter
Cobourg, ON, CANADA
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