[Stoves] Riser Height and a 'Counter-Current' Woodgas Burner - YouTube Vid

[Julien Winter]

Hello Stoves;

Thanks to everyone who offered helpful comments.  I will try to address
them here
 ============
Jock:

I think that your work with flame retention disks was very interesting.  If
I have the right place, you discuss the flame retention disks in the pdf
attached to this post:
http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-August/009159.html
The flame retention disk sits over the centre of the TLUD reactor so that
rising woodgas moves to the sides the reactor to meet the incoming
secondary air.

A flame retention disk is worth at try with this burner, because is would
make it into a try counter current burner.  I have two reservations,
however: (1) a flame retention disk would prevent glowing char which could
aid in cracking tars, and piloting ignition; (2) at very low turndown, I
find it best to let the flame flicker over the char surface.  Wherever
there is a flame, buoyancy will draw other woodgas to it.  I have found
that with larger diameter retention disks, forcing a low levels of woodgas
to the sides dilutes them below their flammable concentration, or inhibits
piloting a locally unignited gas.    All the same, there may be benefits to
the retention disk that are more important ... it is a hypothesis to be
tested.

I share your concern about burners that pass flames through a small hole
such as a concentrator.  In general the concentrator method has been a good
stepping stone in the path of TLUD development, but there have been some
problems with soot production.   Aside from the possibility that a
sharp-edged concentrator creates soot, there may also be a problem with
tall flames if they radiate heat too much heat, cooling of the edges of the
flame and causing unburnt soot.   Like you, one of my objectives is to get
hot flat flames.   The counter-current burner in my video uses the space
below the central riser and above the char as the initial space for the
flame; that is the full width of the TLUD reactor.   All the same, at
medium gasification rates, a lot of the flame will be in the central
riser.  Provided that the secondary air and wood gas were well mixed at the
bottom of the flame, that may not be a big problem.   However, emissions of
soot and CO may occur, especially high rates of woodgas production.   These
are all hypotheses to be tested with gas analyzers.

People may want to cook with blue flames, but that is hard to do with fuel
that contains lignin, which is an excellent source for the precursors of
soot.  Using a discontinuous annular burner, specialized for pellet fuels,
and working over only half the potential range of turndown,  I have been
able to achieve flames with a lot of blue.   I haven’t succeeded over a
wide range of woodgas production rates.  I am designing “generalist”
burners that will work with pellet fuels, nut shells, and chips and chunks
of wood.   Those fuels will produce a five-fold range in gasification
rates, and wide range in woodgas supply and composition.  For that reason,
I have to keep the geometry of my burners fairly open to accommodate large
flames.   I don’t expect to get blue flames with a ‘one size fits all’
burner.  Yellow flames are OK, provided we burn off all the soot.  As
pointer out earlier by Paul Medwell, incandescent soot is a source of
radiant heat.
 [
http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2016-January/011193.html
l]

================

Marquitusus:

I get better mixing with the counter-current burner than I do with a
concentrator hole or with a ring of air holes in the side of the TLUD
reactor wall.  The latter, especially, tends to create a central conical
flame.   The purpose of the counter-current burner is to get vigorous
mixing over the top of the fuel bed, or at the base of the gas flame.   I
have not tried to measure flame temperatures.   Testing for polluting
emission and for heat transfer efficiency have yet to be measured.   In a
sense, my video is a research proposal; it stimulates many questions.


===============

Crispin;

Interesting ideas.  I will deal with primary and secondary air control in a
separate thread.

===============

Ray;

That is a nice TLUD cooker that you have built, and an impressive pile of
char.  Now that the Canadian Government has declared it legal to add
biochar to soil, I am thinking of building a TLUD cooker in my local
community garden, so people can roast flesh and make a cup of tea.

That is a well-made butterfly valve for controlling primary air, but how
good is it for fine control of low flow rates?

I don’t have primary air regulators on my prototypes for a couple of
reasons:
1) They are prototypes, so I don’t spend time on some aspects that are
necessary for working stoves.
2) I run each trial at a ‘set rate’ of primary air by using grates with
different numbers of air holes.  Obviously, this is not a precise control
of primary air; for that I  would need a regulator and an  anemometer (and
I really need to get one).  However, when I have a series of grates of,
e.g. 1, 1.5, 2, 3, 5, 6, 10% grate area, my trials are very repeatable.
Results based on grate aperture are of more practical significance to stove
makers, because they can see how they need to design their regulators.
(But note that size of the air holes has an effect on the resistanct to air
flow.)  From a scientific standpoint, it is important to measure the
superficial velocity of primary air.

If you are able to run your stove with your primary air regulator closed,
then you must be leaking primary air in somewhere else.

===============

Paul and Crispin;

I have used vertical pieces of spruce lumber in an 18 cm diameter, 30 cm
tall ND-TLUD.  Because the air channels are vertical it is possible to
generate a strong chimney effect inside the reactor.  The resulting gas
flame was 50 cm tall.   The temperature in the TLUD rose to 1200 °C. That
is what I call “flash-fried goat” mode.

==============

Thanks for all your feedback.
I must now address Crispin suggestion of regulating primary and secondary
air in a new thread.

Cheers,
Julien.


-- 
Julien Winter
Cobourg, ON, CANADA
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