[Stoves] FD-TLUD Research at Colorado State University -- ETHOS Presentations

Julien Winter winter.julien at gmail.com
Wed Mar 9 20:23:38 CST 2016


Hello All;


At the 2015 and 2016 meetings of ETHOS, we got a tantalizing preview of the
research being done at Colorado State University on TLUD functioning.  I am
bringing these conference papers to the attention of the “Stoves”
discussion list, because they give us an idea of the current state of the
art for forced draft TLUDs.

Jessica Tryner (1, 2) and James Tillotson (3) presented papers on their
thesis research.  Their work was part of a $10M fund (Elliott Levine, 2016)
distributed over five years among various institutions by the US Department
of Energy for research on various improved cookstoves and their testing.
These TLUD presentations show us what we might expect in forthcoming
papers.  For the moment, we have to wait patiently for the full story,
because there is a limit to what someone can say in a 12-minute talk, and
because graduate students should present their work at a thesis defense
before they publish.

(1) Tryner, J. 2015.  Achieving Tier 4 Emissions and Efficiency in Biomass
Cookstoves.  ETHOS Conference Proceedings, January 25, 2015, Kirkland,
Washington
http://www.ethoscon.com/wp-content/uploads/2015/04/DOE-Jessica-Tryner-Achieving-Tier-4-Emissions-and-Efficiency-in-Biomass-Cookstoves.pdf

(2) Tryner, J; Marchese, AJ.  2016.  Achieving Tier 4 Emissions and
Efficiency in Biomass Cookstoves.   ETHOS Conference Proceedings, January
29 -31, 2016, Kirkland, Washington.  The talk was called "When is a TLUD
not a TLUD"
http://ethoscon.com/pdf/ETHOS/ETHOS2016/Tryner.pdf

(3) Tillotson, J; Tryner, J; Marchese, AJ.  2016.  Effects of stove design
and fuel bed properties on TLUD operation and performance.   ETHOS
Conference Proceedings, January 29 -31, 2016, Kirkland, Washington
http://ethoscon.com/pdf/ETHOS/ETHOS2016/Tillotson.pdf

I was in the audience for (1), and heard (2) via a webinar.  Below I have
added a brief description of some slides and made a few comments.

MATERIALS AND METHODS

We see a photo of the experimental TLUD plus some fuels and gas burners in
Tryner 2015: slide_6, and Tillotson2016: slide_5.  Treatments:
a) They are using a forced draft TLUD that allows for independent
adjustment of measured primary and secondary air flow rates.
b) There is an array of syngas burners with secondary air holes of
different sizes, that apparently can create a swirl angle and downward
angle.  There is also a concentrator ring that can be placed on the top of
the fuel chamber (reactor) or the top of the burner, and a gap for
secondary air.
c) There are four different fuels: chips of corn cobs, eucalyptus, and
Douglas fir; and lodgepole pine pellets.
d) A 3-phase sequential  operating regime of Phase_1 normal TLUD mode, then
Phase_2 refueling onto the residual char, then Phase_3 char burnout
(Tryner2016: slide_6, Tillotson2016: slide_7)
They measured:
a) CO, H2 and the hydrocarbon composition of non-condensable gases rising
out of the fuel bed.  (From Ms. Tryner’s webinar, I believe that tars were
condensed out of the syngas, but not measured.)
b) CO and PM2.5 emissions in the exhaust.
c) heat accumulation in water contained in a cooking pot
d) mass loss from the fuel
e) fuel bed temperature.

RESULTS

A temperature regime for the fuel bed (Tillotson2016: slide_9) shows the
flaming pyrolytic front (FPF) moving down through a fuel bed.  The FPF
reaches the top thermocouple, then the middle thermocouple, then the bottom
thermocouple during the first 20 minutes of Phase_1 (TLUD mode).  At the
end of Phase_1, the ignition front (FPF) has hit the grate.  Phase_2 starts
when 200 grams of fuel are added on top of the char, and causes a switch
from TLUD mode to updraft gasifier mode, that is, with the hot-zone of
combustion at the bottom with the heat rising upward through the newly
added biomass.  Because there is no flame at the top to the new fuel, a new
Top-Lighting is done but there is no downward Migratory Pyrolytic Front
because there is no oxygen remaining in the gases that are being created.
After 40 minutes, Phase_3 char burnout starts.  From my own experience with
TLUD temperatures, I think that they are presenting data for a TLUD
operating at high power in Tillosen2016: slide_9 (higher rates of primary
air are possible, but the stove starts to emit fly ash).   I think that we
can assume that all the subsequent figures in Tillosen2016 are at high
power.

Tillosen2016: slide_11 (Tryner2016: slide_7) show the concentrations of
non-condensable gases in the syngas for the TLUD at high power.  It is
interesting to see the importance of CO, and the increase in H2 with
pellets for Phase_1 TLUD-mode.  These are not soot forming gases.  The
potential soot-making hydrocarbons are not in high concentrations.  There
was not a great increase in hydrocarbons for Phase_2 Updraft-mode, yet
other slides in these presentations show an increase in PM2.5 soot for
Phase_2 compared to Phase_1.  The answer could be in the tars which were
not measured (?), because we know that updraft gasifiers are tar-makers.
Is the ‘Devil’ in the minor syngas components?  Initially in Phase_2, water
will be evaporated adding difficulty to maintaining the new flame at the
top of the new fuel.

People building TLUDs are of necessity interested in the design of syngas
burners.  Jessica Tryner mentioned in her webinar that for the various
forced draft burners, some designs were better than others for Phase_1 TULD
mode, however, burner design could not correct problems in PM2.5 emissions
in Phase_2 updraft mode (see Tillotson: slide_15).  Tillotson_2016:
slides_12-14 show treatments on burner design and secondary air flow, and I
think they are self-explanatory.   Note that these are forced-draft burners
that rely on syngas entrainment into fast-moving, fine jets of secondary
air.  Thus, the conclusions in these slide may not extrapolate to low
pressure, nor to natural draft burners that use concentrators and downward
air jets to get mixing of syngas and secondary air.

We can’t talk about the CFD modeling yet, because that is still a work in
progress.

The optical work conducted at the University of Toronto is quite
interesting.  Tryner 2016: slide_14 shows OH radical chemiluminescence,
which shows us the location of rapid chemical reactions and high heat.
According to her webinar, simulated syngas is introduced through glass
beads in the bottom of the box, and secondary air is introduced through
narrow slots in the side walls.  We can see traces of the OH in secondary
air streams, but it is interesting how the hot gases collide in the middle
of the burner, and move both upwards and downward.  This downward movement
could direct syngas to the sides of the burner.   The bottom video of
Tryner 2016: slide_15 shows a planar laser induced fluorescence (PLIF)
pattern produced by seeding the syngas with 5% acetone.  The acetone is a
tracer for unburned syngas, which we see moving toward the sides of the
burner, and into the underside of the secondary air jets.   This movement
of syngas could be driven both by entrainment into the high momentum
secondary air, as well as being pushed aside by the hot gases as seen in
the OH image above.  This is interesting work, because it corroborates what
some of us working TLUDs have seen; a ring-shaped cloud of ‘smoke’ rising
from the fuel bed, and a clear view to the fuel bed down the middle of the
burner.  This is why it is possible to build a gas burner slightly wider
than the TLUD, to gain more horizontal space for flame.  (To see the videos
in the slides, I had to install the freeware “Foxit Reader” (
www.foxitsoftware.com), which is a good program for pdf viewing and
annotations.)

These presentations show that a lot of interesting work has been done by
the Colorado State U. team.  These presentations are the tip of the iceberg
of what they must have found, so we must wait patiently for the papers
(which we are told will be open access).

What we need for the future is research on burners for natural draft TLUDs,
the role of tars in producing soot in syngas flames, and the inclusion of
minimally processed fuels (that can be tar-makers).   How does tar
composition and combustion change with gasification temperature?  ND-TLUDs
turn down to lower gasification temperatures than FD-TLUDs.  Pellets and
‘cubic’ chips are great fuels for creating paradigmatic TLUD gasification.
Thicker fuels (e.g. >2 cm) and planar wood chips can produce gasification
conditions that are more complex than 6 mm pellets.


(Thanks to Paul Anderson for editing my text, and adding a few
clarifications)

Cheers,
Julien
-- 
Julien Winter
Cobourg, ON, CANADA
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