[Stoves] [Fwd: Superficial Velocities of Power gasifiers and WoodGas Stoves]

Mon Jul 30 12:33:43 EDT 2007

-------- Original Message --------
Subject: 	Superficial Velocities of Power gasifiers and WoodGas Stoves
Date: 	Mon, 30 Jul 2007 10:30:14 -0600
From: 	Thomas Reed <tombreed at comcast.net>
To: 	GASIFICATION <GASIFICATION at listserv.repp.org>, Gayathri 
<gayathri at cgpl.iisc.ernet.in>, Art Lilley <artsolar at aol.com>, Jim 
Fournier <jim at planetwork.net>, Agua Das <aguadas at onebox.com>, Paul 
Anderson <psanders at ILSTU.EDU>, Chuck Stevenson <chuckmunk at comcast.net>, 
"STOVES at LISTSERV.REPP.ORG" <STOVES at LISTSERV.REPP.ORG>, Pravina Parikh 
<parikh at somaiya.edu>, Shivayam Ellis <shivayam.ellis at comcast.net>, Phil 
Romig Jr <promigjr at mines.edu>, Dag Nummedal <Dag.Nummedal at is.mines.edu>, 
Van Blackwood <van.blackwood at gmail.com>, Anuradda Ganesh 
<aganesh at me.iitb.ac.in>

Dear Gasifiers (and Stovers):

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 and 
typically generate 20-25% charcoal from reasonably dry fuels.  Our paper

TESTING & MODELING THE WOOD-GAS
TURBO STOVE
T. B. Reeda,b, E. Anselmoa and K. Kircherc1
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 
1.5, so if you burn 1 kg of fuel you will generated ~ 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.  

-- 
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