Oops!! I forgot something in my original posting. Please see correction
below shown in capital letters, in the 3rd paragraph.
----- Original Message -----
From: "Kevin Chisholm" <[email protected]>
To: "Discussion of biomass pyrolysis and gasification"
<[email protected]>
Sent: Tuesday, August 10, 2010 8:05 PM
Subject: Re: [Gasification] Imbert chemistry question
Dear Brian
It is really quite simple...
You must have a "high intensity combustion, or reaction zone", to ensure
that all the oxygen present is reacted to CO2, to get very high
temperatures, so that the CO2 can be "back reacted" in the reduction zone to
give CO.
If you have a "low intensity burn" in the reaction zone, much of the oxygen
ends up burning the fuel to CO, and the reaction zone is cool. There is
insufficient residual heat to crack/BURN the tars and reduce the PRODUCTS OF
COMBUSTION TO MAINLY CO AND H2.
(Because of the presence of H2 and H2O, the detailed reactions are more
complex, but the principle holds.)
You get the "high intensity combustion" with high velocity air jets, and a
"right sized relationship" between fuel particle size and distance from the
nozzle tip and fuel bed center.
The "Reduction Zone" simply reduces CO2 and H2O back to CO and H2 + more CO.
If you have tar in your off-gas, it is because of insufficiently intense
combustion in the Reaction zone. That being the case, you need higher nozzle
velocity, and/or a larger fuel size, and/or a smaller nozzle circle
diameter, and/or a combination of all three.
Best wishes,
Kevin
----- Original Message -----
From: "Brian D Paasch" <[email protected]>
To: "Discussion of biomass pyrolysis and gasification"
<[email protected]>
Sent: Tuesday, August 10, 2010 7:19 PM
Subject: Re: [Gasification] Imbert chemistry question
Big thanks to all of you who offered feedback on my question! Your answers
were much more diverse than I expected. I had assumed that the matter was
ONLY related to chemistry but from the discussion that appears to be only
part of the issue.
One topic did surprise me... comments were made that the
combustion/oxidation zone is hotter than the char/reduction zone.
Apparently this newbee to the art has it backwards. I thought I'd read
somewhere in my documents that the char zone is hotter than the combustion
zone (but as of this moment, I can't find that alleged info). And didn't
the ironsmiths of days past use charcoal as fuel for their forge since it
makes for higher heat than raw wood? Furthermore, our thermocouples must
not be properly placed in our firetube. Our TC in the (putative) combustion
zone reads about 1500F (815C) and our TC in the (putative) char zone reads
about 1750F (950C) when we have the best looking flare we can make. So
apparently we have something amiss in our assumptions there too. (Although
we have no way to validate the accuracy of the signal that we're getting
from those TCs either.)
-brian
On Aug 9, 2010, at 2:54 PM, Brian D Paasch wrote:
Hi all,
Got a question about Imbert style downdrafts…. One of the obvious
characteristics of an Imbert style gasifier is the hearth restriction. The
combustion/oxidation zone is physically larger than the subsequent
charcoal/reduction zone. As best I can find in the literature, the size
change is worked out so that there is an approximate four-fold increase in
superficial gas velocity through the reduction zone versus the oxidation
zone. The actual velocity increase is even higher due to the higher temp
of the reduction zone over the oxidation zone and also to an increase of
total mass as the gasification of the solid fuel adds its molecular load
to the gas stream.
So my question is, why? Why did the engineers of the Imbert decide that
they needed a higher gas velocity through the reduction zone versus the
oxidation zone?
Thanks!
-brian
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