[Stoves] Distributing Secondary Air from the Outside or Center of TLUD gas burner

Julien Winter winter.julien at gmail.com
Wed Dec 17 10:05:42 CST 2014


Hello All;

I have been doing a bit of testing on TLUD pyrogas burners over the past 9
months.

I have been working on two approaches.  (1) bringing secondary in from the
circumference, and (2) bringing air in from the middle (with Kirk Harris)

1) AIR FROM THE CIRCUMFEREANCE

A robust design that I have come up with was illustrated in the recent
"experimental vignette" I posted on the effect of changing riser height,
Increasing ND-TLUD riser height accelerates gasification and increases bed
temperature"
http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-December/009596.html
In this example, preheating secondary air was not done, because it was not
necessary for the experiment.

Prototypes of this burner were reported in a posting to this list "Trials
on TLUD Gas Burners - Burner Diameter"  with and without preheating
secondary air.
http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-July/009043.html

The burner is robust in the sense that it works over a very wide range of
gasification rates without producing visible smoke.  The burner has not yet
been tested for CO and particulate emission over a power curve or turndown
curve.  That needs to be done.

The features of this burner are:

a)  The diameter of the burner's outer cylinder is 1.3 x the diameter of
the TLUD reactor.  This was demonstrated in the experimental prototypes to
increase the horizontal space for gas flame expansion.  The result was the
bulk gas flame doesn't form a central conical peak, by has lower multiple
peaks instead.

b) The holes for secondary air are not near the top of the reactor, but
elevated.  This allows space for rising pyrogas to become entrained in the
jets of secondary air as they enter through the air holes.  In earlier
experiments, I observed that pyrogas does not rise straight up for the fuel
bed, but moves toward the side walls of the reactor.  This means that
nothing is needed to force pyrogas to the sides.

c) There is a "deflector ring" above the secondary air holes.  I found that
this ring was necessary to prevent vertical flamelets arising from the
secondary air holes.  These vertical flamelets produce smoke, because there
is not enough turbulence, and the flame is cooled as it passes up the riser
walls.  The "deflector ring" is not quite the same as a "concentrator
ring".  The deflector is only wide enough to prevent vertical
flamelets.  The aperture of the deflector ring is kept wide so that
it doesn't restrict the gas flame.  What we what are horizontal flamelets,
and it is interesting to see that you can get both a horizontal and
vertical flamelets arising from the same air hole.

d) There are no obstructions over the mouth of the TLUD reactor.   It is
important that secondary air can settle toward the fuel bed when
gasification rate is low.  When the gasification rate is high, the bottom
of the gas flame is around the level of the secondary air holes.  When the
stove is turned down, the secondary air settles down to the middle of the
fuel bed, and the bottom of the gas flame is in contact with the fuel bed.

e) Secondary air holes are relatively large.  The size of secondary air
holes was tested in an experiment that has not yet been reported.  If the
secondary air holes were small, the gas flame was smaller and the
gasification rate was lower than with larger air holes.  If the secondary
air holes are two large, then become ineffective at controlling and shaping
the jets of secondary air.  When the secondary air holes are the right
size, they impart sufficient structure to the jets of secondary air,
projecting them inwards, whilst not imposing a high frictional/turbulent
resistance to secondary air entry.   The size of secondary air holes
is important.  The best size needs to be determined for each stove design.

Round secondary air holes are better than a continuous secondary air gap,
or rectangular holes.   Buoyance within the gas burner is not uniformly
distributed, but varies according to flame activity.  I find that round
holes are better able to self-regulate air flow according to the localized
differences in draft.  That is because they impose a discrete and
symmetrical resistance to air entry.  If you have a continuous gap, then
the secondary air flow can't compartmentalize, so at low gasification rates
you can get too much secondary air, the gas/air mixture is too lean, and
you get smoke and flameouts.

One of the key assumptions guiding the design of this burner is to allow
space for the gas flame to go where it wants to, and provide no
interference.   That is an assumption that is open to be challenged by
other designs.


2) AIR FROM THE CENTER

A while back, Kirk Harris posted the design for his new Wonderwerk TLUD
stove  ... and it got no comments!!!  I suspect that many people didn't
realize the significance of what he has done.  He introduced secondary air
to the center of the burner through slits in a horizontal pipe.  Take a
look at his stove, because he has achieve a large turn down capacity and
very low CO emissions.
http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-October/009408.html

Kirk was kind enough to send me a description of his design before it was
publically released.   Kirk's burner is linear.  Following his lead, I have
done a very similar think, only circular.  We have been bantering back and
forth by email since.

Some pictures of the central burner are attached in a pdf.

The first picture is of the burner running by itself.  The second picture
is of the central burner mounted in a peripheral burner similar to the one
described above.

This is a prototype under development, and we are not yet sure if it will
work.  The geometries have to be adjusted to that there is enough space for
the flame to expand.

One of the problems with the central burner on its own is that flame is
consistently symmetrical as with Kirk's linear burner nor as with burner
described above.  That means that you can get a taller flame in one
quadrant than the others.

The central distributer is being combined with peripheral secondary air to
see if the flame can be made consistently symmetrical, and to see if there
is better mixing of pyrogas with secondary air, if the pyrogas passes
through two opposing streams of air.  The soot around the underside of the
burner in the second picture shows that this is a work in progress.  If
secondary air is also added from the outside, then less air is drawn from
the middle, because the draft in the burner has to be shared between the
two modes of feeding secondary air.

This burner has to be tested on a full range of gasification rates.

The final test will be to see if very low CO and particulate emissions can
be achieved.  The burner in Kirk's Wonderwerk TLUD stove is a very
practical design that is easy to build.  This new burner is trying to take
a good idea and make it complicated.  If you can take something simple and
make it complicated, then that must be an improvement :-}

Food for thought,
Cheers,
Julien


-- 
Julien Winter
Cobourg, ON, CANADA
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