[Stoves] T-LUD with central core: derived from Dung Berry Fireballs

[Paul S. Anderson]

Stovers,

Lanny's comment makes sense:
>> The central combustion [hollow vertical core surrounded by fuel in a 
>> canister] seems to keep the top gasses lit. Could this be
>> the gasifier "spark plug" that is needed to keep top lit application
>> flaming when burning dificult to burn fuel?

Richard commented about
> attempting to top lite the hollow core briquette in our briquette
> gassifying experiements in south africa. Problem was that the hole was
> too inviting and it would [end] up being a toplit, bottom-burned briquette..
>
> You are right about the need for a central air feed though. Something
> is needed to supply air up through the center at least when you are
> trying to gassify biomass to our experience.

When a T-LUD is of small diameter (such as 4 inches -- 10 cm -- as in the
Woodgas Campstove) and there is a reasonable draft of primary and secondary
air, the sustaining of the combustion is not much of a problem.  But when the
diameters are 6 inches or 8 or larger, the gases (especially if at a low
turn-down setting) can be rather wispy and/or favoring one side more than the
others.  Therefore, especially in those cases, a "flame holder" or 
"spark plug"
could be of great assistance.

Based on the comments from Lanny and Richard, I offer an idea that I have not
yet had a chance to try, but hope to do soon.  It involves a central core, but
a specific type:

1.  The core must not allow fire (via falling embers, etc) to fall to 
the bottom
and to cause a bottom-lit situation.  Therefore, the core is mostly closed and
rather small in diameter.  I envision a hollow metal tube perhaps 1 cm in
diameter with perhaps two of 1/8th inch holes (about 3 mm) placed at 1 cm
vertical spacing.  More holes can be added if needed.

2.  There is a clear passage for primary air to get to the bottom of the
vertical hollow core.  If there is too much passage of air, the core might be
partially closed at the top.  The tube is to end about 2 - 3 cm below 
the level
of the ring of secondary air holes.

3.  Initially, most of the air coming through the pipe will be primary air to
support the top-lit pyrolysis, even if that air enters the fuel area laterally
via the lower holes in the tube.  But any excess air at the top will
essentially be secondary air providing oxygen to the combustion of those
pyrolysis gases, sustaining a flame in the center of the fuel pile.  This is
the weakest spot for T-LUD combustion, even in the 4-inch fuel containers.  So
this flame will be a nice addition.

4.  As the pyrolysis proceeds downward, the top of the tube will be 
exposed, so
all air coming out of that end will be secondary air.  This must NOT be an
excessive amount if we want to avoid a "torch effect" of a tall vertical flame
rising substantially higher than the flames supported by the lateral secondary
air holes in the container sides.  Perhaps a "diffuser cap" to spread that
stream of air out to the sides will be necessary.

5.  Let's consider the case of when the pyrolysis is about half completed.
   a.  Any air movement in the lowest holes on the tube is of little 
consequence
because other primary air is entering at the bottom.
   b.  Holes that are in the zone of flaming pyrolysis are either sending extra
air (with O2) into contact with the hot fuel and hot char (good for 
keeping the
center area hot) or there could be some pyrolysis gases coming into the 
tube via
those holes, resulting in a flaming combustion of those gases with the rising
air.  This also is good because it is acting like secondary air that is
introduced into the heart of the mass of fuel to burn some gases, but
constrained inside the hollow tube.  This indeed could be the
flame-holder/spark-plug.
   c.  Whether the air is going outward through the holes in the tube or if
gases are going inward into the tube via those holes will be influenced by the
amount of pressure (forced-air T-LUD) of the primary air AND by the
constriction or openness of the top of the tube.  Too open at the top means
fast moving air with gases being sucked into the tube.  Too closed at the top
means only providing air with O2 outward in to the four main levels (bottom
with raw fuel, lower middle with flaming pyrolysis, upper middle with 
char, and
top with combustoin of pyrolysis gases in the open space because the 
fuel level
has shrunk downwards.

This should work.  Needs lots of experimentation.  But success could mean the
ability to have much larger diameters of T-LUD gasifiers because there 
could be
several of these vertical hollow cores in one fuel container (perhaps 60
centimeters diameter??).

Note:  We already know that large diameter T-LUD will function and create
substantial quantities of pyrolysis gases.  But the combustors (the burner
heads) to handle such quantities are not well developed.

Although I like the above concept, I conclude by saying that there might be
better and easier ways of sustaining the fire and/or other ways to get
secondary air into the center of the gases (or getting the gases from the
center out to the mixing areas with the secondary gases)

Paul
-- 
Paul S. Anderson, Ph.D., Geography professor - Emeritus
Telephone:  USA-309-452-7072 (residence and office)
Internet site:  www.ilstu.edu/~psanders
For my gasifier stoves info, go to:
http://bioenergylists.org/contributors#Paul_Anderson



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