[Gasification] Gasification of briquettes

[doug.williams]

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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