[Gasification] Ash to land? - separated ash fractions

[Pels, J.R. (Jan)]

Tom,

The tests we did were on fluidized bed gasification using shreddered
demolition wood. I'm talkig about the fly ashes. I screened after
cooling and collecting ashes in a drum. I used lab screens of 20, 38,
45, 63, 90, and 180 micrometer (sorry, can't find the mesh numbers). I
found that nearly anything above 180 micrometer is char. The fractions
between 90 and 180 micron is mostly sand (attrition from bed) and both
fractions below 38 micron have nearly the same composition. Those were
the true ash particles and they had lower unburned carbon than the
larger fraction (except for the sand fraction). The fractions between 38
and 63 are mix between ash and small char particles. When you remove the
char particles from the composition, by combustion or by calculation,
all four fractions become virtually identical. 

In other experments we found that the volatile elements Pb, K, Cd and Hg
do have a tendency to concentrate in the ash collected at cold
temperature. But other problem elements, Zn, Ni, Cr, Cu, Ba did not.
There is insufficient gain in removing ash a high temperature. But
relevant fractions: sand for road construction and char for reburning
can be removed by cold screening. 

For clean biomass the situation may be less hopeless, not because in
that fuel the distribution is better, but because it has much lower
contents of contaminants, so a difference in Pb or Cd, might just be
what you need to get a clean ash fraction.

By the way, the bottom ash of demolition wood is also quite interesting:
It contains sand, molten glass fragments with sand sticking to it,
nails, pieces of brick/concrete/tiles. Even coins and door ornaments.

On screening of fuel:
That is almost a necessity to avoid frequent shut downs. For demolition
wood it is absolutely necessarey, but in every truck load of even the
cleanest biomass you can find stuff like metal wire or big nails, even
lead strips, plastic bags, glass.

Jan

 

========================================
Dr. Jan R. Pels
ECN - Biomass, Coals and Environmental Research
P.O. Box 1, 1755 ZG Petten, The Netherlands
telephone: +31-224-564884; fax: +31-224-568487
mobile: +31-6-10923218
e-mail: pels at ecn.nl
========================================
 
 

> -----Original Message-----
> From: gasification-bounces at listserv.repp.org 
> [mailto:gasification-bounces at listserv.repp.org] On Behalf Of Tom Miles
> Sent: Friday, May 11, 2007 3:49 PM
> To: 'Discussion of biomass pyrolysis and gasification'
> Subject: Re: [Gasification] Ash to land? - separated ash fractions
> 
> >Quick tests showed that screening ash is equally effective as 
> >separation at
> certain
> >temperatures.
> >Jan
> 
> Jan,
> 
> Do you mean screening the ash once it has been cooled and 
> removed from the gasifier? I would think that would be 
> difficult due to the similarity in particle size. 
> 
> We have found that screening the fuel is one solution. Remove 
> everything below 1/4 in (6 mm). Unfortunately this can be 1/3 
> of the fuel. 
> 
> Tom
> 
> 
> 
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