Hi John And Colleagues,
As I am back to access my computer today after a week away, I see a question
that needs an answer relating to a DIY gasifier using the Fluidyne gas making
principles.
I have a gasifier (Fluidyne design) that works excellently with two inch chunks
of wood, but when we switched to burning briquettes (compressed shavings and
sawdust) everything changed so much that we no longer can sustain gas
production, plus have extreme amounts of slipped char and cyclone particulates.
In a downdraft gasifier that has a sealed fuel hopper, the stack gases that
form around the fuel is mainly wet steam. Any waste fine materials that are
just compressed, swell and fall apart usually before the reach carbonization.
This then fills the interstitial space with fines, and increases the pressure
drop across the bed. It shuts the gasifier down, and yes you get large amounts
of fines in the outlet gas, usually due to channelling through the reduction
zone. Combustible gas can still be made in such circumstances, but will contain
tar and a lot of CO2.
Has anyone experience in gasifying such fuel?
We have tested many of these briquetted combinations over the years, and learnt
long ago, that the only briquetting process suitable for sealed hopper
downdraft gasifiers, are made in a screw press type, with heated dies to obtain
a thermally bonded briquette. We tested these in Germany in our Pacific Class,
and behaved very similar to wood blocks. You can see one on the Fluidyne
Archive under Agricultural Fuels. www.fluidynenz.250x.com Compressed
briquettes are best suited as combustion fuel in a boiler or stoves, not a
gasifier.
The swelling from steam in the hopper causes bridging that did not occur with
the chunks, but that should be easily overcome, perhaps with a monorator hopper?
Thanks,
John Blount
The swelling is the first stage of disintegration, by which time the fuel can
be quite soggy and not want to carbonize until it begins to enter the oxidation
zone at the nozzles. The original fuel size block, either wood or briquette,
does not change shape until it enters the oxidation phase, then, a segmentation
process begins of the char. Because of the amount of fines released by
compressed briquettes, the surface areas increase rapidly changing all the gas
making parameters, which end up blocking the reduction zone.
A monorator hopper would not change the way the fuel segments at oxidation, and
possibly the only way that I can see you using these compressed briquettes, is
to see if they can be pre-carbonized in a charcoal making process. I doubt if
this would provide the thermal bonding of the fines, which releases the lignin
bonding (I think), and they might still fall apart.
Hope that explains your situation.
Doug Williams,
Fluidyne Gasification.
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