Hi,
My understanding of the "Imbert design" is that the superficial velocity is
not a consideration, per se, in the sizing of the combustion zone.
The capacity most fixed bed gasifiers in decided by the rate of pyrolysis,
which as we all know is the slowest step in the gasification process.
The sizing thus has to do with the completion of pyrolysis before the fuel
reaches the combustion zone.
The combustion zone delivers heat to the pyrolysis reaction by radiation
from the hot lobes and by convection from the eddies created.
The zone, in my understanding, is sized so that it provides sufficient heat
transfer area and residence time for completion of pyrolysis in the zone
above it.
Obviously, the characteristics of the fuel, esp. size, devolatization
temperature and rate, are important factors in sizing this zone.
As for the reduction zone, there are reasons for a higher SV.
01. The reduction reaction is much faster and the residence time required by
the gas is much lower.
02. The original Imbert design did not insulate the reduction zone, so there
was a need to keep this zone as small as possible.
03. The slighty higher velocities, as mentioned in one of the earlier posts,
helps in carrying over the fine dust which may lead to tunneled flows, if
allowed to remain.
04. As Arnt has pointed out in numerous posts earlier, the quantum of
reverse reactions would increase, generating a lot more soot if the gas
stayed in the zone longer than required.
Regards
Shivanand Ramachandran
Infinite Energy (P) Ltd
First Floor, Baba House,
149-A, Kilokri
New Delhi - 110 014
Ph : 91-11-65191937, 65273819
Fax : 91-11-26903696
----- Original Message -----
From: "Brian D Paasch" <[email protected]>
To: "Discussion of biomass pyrolysis and gasification"
<[email protected]>
Sent: Tuesday, August 10, 2010 12:24 AM
Subject: [Gasification] Imbert chemistry question
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
_______________________________________________
Gasification mailing list
[email protected]
http://listserv.repp.org/mailman/listinfo/gasification_listserv.repp.org
http://gasifiers.bioenergylists.org
http://info.bioenergylists.org
_______________________________________________
Gasification mailing list
[email protected]
http://listserv.repp.org/mailman/listinfo/gasification_listserv.repp.org
http://gasifiers.bioenergylists.org
http://info.bioenergylists.org
