[Gasification] Ash to land? - separated ash fractions

[Pels, J.R. (Jan)]

The principle of 'staged cooling and particle collection' was
investigated by Obernberger and co-workers for COMBUSTION systems in the
last decade. The idea is to collect the bulk of the ash at high
temperatures and rest at low temperatures, where volatile problem
elements are concentrated. He was able to concentrate the majority of
problem elements in a small ash fraction. They got some patents awarded
for the idea and for a plant using the principle. The bulk of the ash
can be utilised. The smaller fraction could be landfilled or used to
recover metals. It was his solution to lower the Cd content in ashes
produced from clean Austrian wood and make them suitable for fertilizer
use. Actually, I don't know whether this is already implemented. It was
through his work that I first came in touch with this Cd issue. 

We (at ECN) tried to do the same for GASIFICATION in CFB. We quickly
found that metals behave somewhat different in gasification and that we
could not make a clear separation of problem elements. Moreover, it
turned out that the larger ash particles are much more useful in the gas
cooler. They sandblast the surface removing secondary char (resulting
from tar). In the end, the 'value' of different ash qualities was lower
than the 'value' of sand blasting the cooler. Quick tests showed that
screening ash is equally effective as separation at certain
temperatures.

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
========================================

> The principal mechanism for thermal fractionation or 
> separation of the inorganics in gasification and combustion 
> is the volatilization at relatively low temperatures and 
> condensation to submicron particles (or onto particles).