GEK Wiki / Using-charcoal-for-media-in-the-packed-bed-filter
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Page history last edited by jim mason 11 years, 10 months ago


Prior to 2/1/09, the GEK came default with fine steel wool to use as the filter media.  Post 2/1/09 we're suggesting charcoal grains as the filter media.


We previously delivered steel wool as it is easy to install, cheap to replace, and very forgiving of newbies.  Early GEK runs showed much better filter performance with ground walnut shells, but if the filter got too hot they would start to pyrolysize and foul the gas.  Thus we changed to the easy solution of steel wool for starters.


Steel wool, however, is not the best performing filter media.  It only provides turbulence for rough separation, but has zero absorbency.  It will not cover any sins of tar production on early runs or mistakes given its lack of absorbency.


The best current suggestion to regain absorbency, while not having a pyrolysis risk, is charcoal.  Charcoal is a well known absorbent of hydrocarbons, with the dirty results easy to dispose of by putting it back into the reactor as fuel.


We suggest using charcoal grains between 1/8" and 1/4".  Use fully pyrolysized real wood charcoal.  Do not use pressed briquette charcoal, as it's usually high in voltatiles.  The goal would be to get the grains as small as possible while not going above about 5" h2o pressure drop across the filter at your max gas flow rate.  You can measure this drop with the GEK manometer and adjust grain size accordingly.


See the related article Filter Pressure Drop vs. Char Diameter for the math to predict pressure drop as filter media size and flow rate varies, as well as our experimential results to determine the same.


You will still need to capture the top of the charcoal to prevent it puffing up into the outtake on gas flow surges.  Ken Boak in UK is packing steel wool on top of the charcoal, with the default perf disk holding the whole thing in.


Others have suggested using various tar cracking catalytic materials as the filter media.  I doubt it is hot enough in the filter for any catalytic reactivity, but I need to go review the catalyst specs to be sure.  The various mineral catalysts would at least still be absorbent, even if they were not performing chemically.




Comments (2)

David Green said

at 9:07 am on Feb 8, 2009

You should remember that charcoal both absorbs (surface effect) and adsorbs (trapping of molecules in interior micro voids). The char produced by gasifiers is considered "activated charcoal" since it is produced at temperatures in excess of 900 degrees Celcius. Activated charcoal has a high percent of micro voids where the hydrocarbons have been driven out of the char matirix by heat and the reduction process. This makes recovery of the slip char very important to re-use as filter media.

jim mason said

at 5:44 am on Feb 9, 2009

copied over comment from deleted page
SteveU. said

at 9:44 am on Feb 3, 2009

Hello Jim Mason - The source for information and prefabricated wood gas tar catalyst would be in the solid and pellet wood burning stove industry. A mature technology. Experience says does require heating to 600-800F (316-550C) to be effective. Worse as an oxidizing process requires available oxygen and is exothermic. Not something energy efficient to use as a post reduction garbage truck. Now in between the pyrolosis and the reduction zones: thats food for thought. How to transport the char/charcoal around the catalytic core? Easy in a batch burn system, much harder in a continuos feed. Much too expensive, complex and High Tech for the direction I wish to go. But for those interested this where you can find the materials to explore with. SteveU.

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