[Gasification] Sodium in fuels

[doug.williams]

Hi Paul,

You ask:
> What is it about sodium that makes biomass difficult to burn or gasify?

Where sodium is present in biomass, it combines with potassium usually to 
form a eutectic flux, that assists ash fusion, and clinker formation.

> What biomass has high sodium and should be avoided?  Specifically, does
> coconut
> husk (coir) have high sodium?

The worst case fuel I have encountered was coppice willow, which has a lot 
of tip wood and leaves, and I advise to avoid trying to gasify the fines 
from these types of stick fuel crops. We found that you could drop the fines 
out of the throw from the harvester, by adjusting the distance to the 
collection trailer.

 Coconut  is not a problem in this regard, but does have a high siliceous 
ash content, which can cause problems if allowed to stay in the packed char 
bed. You should also know that husks from coconut have a very large air to 
surface ratio, and difficult to create a high temperature oxidation zone. 
The shell however, is a better fuel, but you still need to tune the gasifier 
for these fuels. When we tested coconut shell and husk, the best result was 
to smash up the shell into about 20mm bits with husk attached. It's not my 
favourite fuel!

> Is there a way to gasify high sodium fuels, such as by reaching a 
> sufficiently
> high temperature?

If you had only these fuels to work with, possibly the best way is a low 
temperature pyrolysis gas application for process heat, or you could go 
updraft, but still only for heat generation.  High temperature would not get 
you into a tar free gas making system.

> The late, great Gus Johanssen of South Africa told me once of husk
> drying sheds
> at some project on islands in the Indian Ocean, and implied that the
> husks were
> quite acceptable as fuels in his downdraft gasifiers.

This would have been in the Seychelles back in the 1980's, and they were 
gasifying the husk and shell together. I'm sure I have a report from that 
time, but it was at a time when tar was plaguing all who tried to make gas. 
If I get a chance, I will dig it out some info for posting tomorrow.
Hope this helps,
Doug Williams,
Fluidyne.