[Gasification] Superficial Velocities of Power gasifiers andWoodGas Stoves

Tue Dec 11 23:32:30 EST 2007

 Greetings Tom, and list.

 Tom, I wholeheartadly love your paper on SV, I've read and read and re-read
it many, many times, but have noticed a difference within our methods of
gasification.

In your post you speak of charcoal <quote> "because as the charcoal is
gasified it reaches a stage where it
collapses into dust which passes on to the cyclone or baghouse, and this
can be ~5% of the incoming biomass" <end quote>.

I see a difference in our way of doing things, I "slip" my char, intact when
it's about 1/6'th it's original size, I realize that at face value this
would appear to increase my percentage of incoming biomass that does not get
gasified to above 15-20% but, upon closer evaluation, the char fines or "
activated charcoal" as I affectionately call it, is re-introduced to another
gasifier (the wood chipper assy. unit)

 This method of slipped char negates the need for a cyclone, and only a
simple paper air filter (the type used on construction equipment, quite
large) is the only filter media after condensate removal, in my system
configuration.

 Condensate still needs to be dealt with, BUT, I'll hint at this, the amount
of condensate is so LOW.... and there I'll say good night..

Greg Manning,

Brandon, Manitoba, Canada

-----Original Message-----
From: gasification-bounces at listserv.repp.org
[mailto:gasification-bounces at listserv.repp.org]On Behalf Of Thomas Reed
Sent: Sunday, December 09, 2007 7:26 PM
To: GASIFICATION; Gayathri; Art Lilley; Jim Fournier; Agua Das; Paul
Anderson; Chuck Stevenson; STOVES at LISTSERV.REPP.ORG; Pravina Parikh;
Shivayam Ellis; Phil Romig Jr; Dag Nummedal; Van Blackwood; Anuradda Ganesh;
Robb Walt
Subject: [Gasification] Superficial Velocities of Power gasifiers andWoodGas
Stoves

Dear Gasifiers (and Stovers):

I came across the following in my "drafts" from a while ago.

I recently came across the following table sent by Prof. Mukunda* and would
like to draw our attention to the most important variable in
gasification/stove, the "Superficial Velocity".

Professor Mukunda has sent the following data:

Power level 	Reactor Dia 	Gas flow 	Gas density 	Gas velocity 	Reactor
Area 	Gas flow 	Vs (900K) 	Vs (300K)
kWe 	m 	kg/s 	kg/m3 	m/s 	m2 	m3/s 	m/s
3.7 	0.15 	0.0045 	0.4 	0.063 	0.018 	0.011 	0.637 	0.212
20 	0.25 	0.015 	0.4 	0.76 	0.049 	0.038 	0.764 	0.255
100 	0.4 	0.054 	0.4 	1.07 	0.126 	0.135 	1.074 	0.358

(Note:  Gas velocity at 3.7 kWe should be 0.63, not 0.063.)

Question:  Is diameter calculated on cylinder or at end of taper?

	*MAXIMUM SV SUMMARY*

Power level 	Reactor Dia 	Vs (300K)

kWe 	m 	m/s

*3.7* 	0.15 	*0.21*

*20* 	0.25 	*0.25*

*100* 	0.4 	*0.36*

The bold figures at the bottom say a million words about the most
important parameter in comparing various gasification systems, the
SUPERFICIAL VELOCITY.
The SV is defined as (m3 of gas/sec/(m2 of gasifier cross section) and
so has the units of m/s which is why it is called the "superficial"
velocity, since it isn't a real velocity.  However, it enables one to
compare gasifiers for a range wider that the 3.7 to 100 kW (above) on
the same basis.  Note that it usually refers to the diameter of the
gasifier at the point of pyrolysis. (Many gasifiers use a choke plate in
the charcoal zone and the SV is typically 4 times as large there, due to
the 2/1 change of diameter.)

I have been keeping track of the SVs of various gasifiers since I
learned the term in the 1970s. We wrote a paper on SV: The Key to
Biomass Gasification, in 2000,

* SUPERFICIAL VELOCITY - THE KEY TO DOWNDRAFT GASIFICATION*
T. B. Reed, R. Walt, S. Ellis, A. Das, S. Deutch

which you can find at http://www.woodgas.com/Superficial%20Velocity.pdf

(In the above list of authors Agua Das and I are still developing
improved small gasifiers at the Biomass Energy and Carbon Corporation,
Robb Walt is the president of the Community Power Corporation and Steve
Deutch is still in biomass at NREL (I think)).

In that paper there is some literature data on measured SVs.  The above
three figures from Prof. Mukunda represent some of the most solid data I
have now seen.   I hope that Robb Walt of CPC might like to weigh in
with their experience on CPC gasifiers.  Last time I heard, their 15 kW
had a maximum SV of ~0.3 m/s which fits nicely in the range of Mukunda's
gasifiers.  I hope that others will get out their slide rules and send
in some numbers and we can get more perspective on the ranges to be
expected.
------------------------------------------------------------------------
I'd like to stress that these numbers are the MAXIMUM SV reported for
steady state operation of Downdraft Biomass Gasifiers in which a pile of
biomass sits on top of a pile of charcoal with air passing into or down
through the fuel and gasifying MOST of the fuel with air.  I say MOST,
because as the charcoal is gasified it reaches a stage where it
collapses into dust which passes on to the cyclone or baghouse, and this
can be ~5% of the incoming biomass.

Pyrolytic gasifiers, like our WoodGas stove have much lower SV's
(.01-.05) m/sand typically generate 20-25% charcoal from reasonably dry
fuels.  Our paper

*TESTING & MODELING THE WOOD-GAS TURBO STOVE
T. B. Reed(a,b), E. Anselmo(a) and K. Kircher(c)1
*
a The Community Power Corporation, 8420 S. Continental Divide Rd., Su 100,
Littleton, CO 80127; b The Biomass Energy Foundation, 1810 Smith Rd.,
Golden, CO
80401; c The Colorado School of Mines, Golden, CO 80401on WoodGas stoves

(at http://www.woodgas.com/Turbo%20Stove%20MS-PITBC%20FINAL.pdf
<http://www.woodgas.com/Turbo%20Stove%20MS-PITBC%20FINAL.pdf>) lists SV
of pyrolytic gasification to generate clean gas for cooking as varying
from 0.02-0.06 m/s for low to high cooking.  Higher SVs resulted in
"fireflies", particles carried up in the air stream with wood chips, but
could have probably been higher for denser fuels such as pellets.
------------------------------------------------------------------------
It is sometimes difficult to measure SV accurately, but it can usually
be calculated approximately from easily available information.  The
cross sectional area (in m2) is easy to come by.  In open top downdraft
gasifiers and WoodGas stoves one generally does not know how much air is
being consumed.  However, gasification usually has an Air/Fuel ratio of
1.0-1.5, so if you burn 1 kg of fuel you will generated ~ 2.0-2.5 (A +
F) kg of gas.  The weight of air at sea level is 1.3 kg/m3, and in
Denver it is 1.0 kg/m3.
------------------------------------------------------------------------
I hope that this will get others to think in terms that can be compared
across the board, and I hope that some will send in their experience.

Onward with Power and Stoves,

TOM REED                    BEF/BEC

*  The CGP Lab in Bangalore is possibly the most advanced laboratory in the
world for widerange gasification research.  However, the IIT Bombay Lab may
contest this and I'd like other nominations.  I visited both labs several
times and was green with envy at the well funded scientific equipment they
had compared to what I had to fight for in my 8 years at the Solar Energy
Research Institute (SERI), now the  National (not very**) Renewable Energy
Lab, NREL here in Golden.

**  I say "(not very)" because, having worked there from 1977-1985, I
realized how political the mangement was.  While there are many excellent
scientists there they seldom have the freedom to do productive science
because their masters at DOE keep changing the focus to red herrings like
"hydrogen economy" or "cellulosics".  (In the 1980s they started great work
in algae production of oils;  gave it up in the 1990s.  In 1991 they started
a program in biodiesel that went nowhere.)

Recently NREL has formed a consortium with the Colorado School of Mines,
Colorado University, and Colorado State University (CSM, CU, CSU).  I hope
that this group can stay focussed better than NREL has.

_______________________________________________
Gasification mailing list
Gasification at listserv.repp.org
http://listserv.repp.org/mailman/listinfo/gasification_listserv.repp.org
http://gasifiers.bioenergylists.org
http://info.bioenergylists.org

No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.5.503 / Virus Database: 269.16.17/1179 - Release Date: 12/9/2007
11:06 AM

No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.5.503 / Virus Database: 269.17.0/1180 - Release Date: 12/10/2007
2:51 PM