[Stoves] [Fwd: Superficial Velocities of Power gasifiers and WoodGas Stoves]
frank
frank at compostlab.com
Mon Jul 30 13:50:33 EDT 2007
Tom and stovers,
I am still trying to get hold of all the calculations and how this is
measured. But looking at the calculations it seems the diameter (cross
sectional area) of the fuel chamber is used. I would think if we wanted
to know the SV in the fuel bed we would need to calculate the air space
within the fuel (subtracting the volume of the fuel narrowing the
channel) that would result in a lot faster SV than in the open
container. I am thinking this is important because of the physical
properties of the rush of air going past the combustion producing the
heat needed for pyrolysis. This change in speed removes needed heat from
the fuel depending on the percent open area. Is this not important?
To measure this in different fuel packing we would need to inject
something like CO2 in the air flow at one end of the fuel chamber and
measure it at the other at a time when there is no fire. The time it
takes could be compared to a section with no fuel to determine the
increased SV(?) Something like that.
It seems there is a lot more to do with different 'small' fuels
regarding packing and air flow that make the conditions right for
combustion. Then we add carbon density of the fuel particle and
surface area and it would seem we could come up with some basic
requirements for small fuel.
We receive a lot of potential fuels from across the country for testing
for potential compost feedstocks and fuel waste products and are in the
process of getting equipment that can be used in the testing. Problem is
time and money if I am to do this on company time.
Frank
Thomas Reed wrote:
>-------- 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:
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>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
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>(Note: Gas velocity at 3.7 kWe should be 0.63, not 0.063.)
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>Question: Is diameter calculated on cylinder or at end of taper?
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> *MAXIMUM SV SUMMARY*
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>Power level Reactor Dia Vs (300K)
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>kWe m m/s
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>*3.7* 0.15 *0.21*
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>*20* 0.25 *0.25*
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>*100* 0.4 *0.36*
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>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,
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>* SUPERFICIAL VELOCITY - THE KEY TO DOWNDRAFT GASIFICATION*
>T. B. Reed, R. Walt, S. Ellis, A. Das, S. Deutch
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>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
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>(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.
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>** 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.)
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>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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--
Frank Shields
Soil Control Lab
42 Hangar way
Watsonville, CA 95076
(831) 724-5422 tel
(831) 724-3188 fax
frank at compostlab.com
www.compostlab.com
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