[Gasification] Ash to land? Each case different

[Pels, J.R. (Jan)]

There is indeed a lot of information out there. Here is another long
contribution with comments on several issues raised today.

Indeed, each case is different. There is as least as much variation in
ashes as there is in fuels. And ashes from the same fuel may be
different in different installation types. 

On cadmium:
There are two main sources of Cd en Europe. Firstly in rural areas, Cd
is present because it is a contaminant in mineral fertilizers. When used
for over 100 years on fields, it accumulates even when annual dosage are
below the limits. The US may not be suffering from Cd because the time
that fields are used is shorter. Perhaps fertilizers with low Cd content
are use. The main source in industrialized areas, is pollution: metal
works and coal plants, including 'city gas' plants. Both from waste
water and from air emissions. For most of the times water and smoke
stacks were not filtered at all.
Cd when deposited on the land is in the top layer and is transported
with dust to places downwind. In the Netherlands we have Cd emitted long
ago by our English friends, the first to industrialize. On top of that,
Cd is mobile. It gets into the water and deposits. Rivers are dredged
and the sludge is used as fertilizer. 
Even Austria, which is not so industrialized has high Cd levels as a
result of industry in Germany, France, etc.
I grew up in region that was industrialized by wind mills in the 1700s.
The locations of the mills where paint pigments were produces are still
known for their contaminated soil. 

On contaminants:
Contaminants (Cd, Pb, PAHs) may be easy degradable, normally occurring,
immobilised or whatever. Fact is that legislation exists that puts
maximum levels. Most ashes I have analysed and assessed for
fertilizer/recycling so far exceed at least one and typically more than
two limits. Denmark has regulations for straw ash. They found that 90%
of ashes exceed the Cd levels and cannot be used as fertilizer.
Typically Cd was 10x the limit. Other notorious elements that spoil
utilisation are Pd, Zn, Cu, Mo and Se. When the wood has been machined
with steel tool, the levels of Ni, Cr and V are elevated.
Furthermore, the speciation is important. Cr(III) is immobile, but
Cr(VI) is mobile and extremely toxic. Not all PAKs are equally toxic.

On the question of PAH:
The question is right. Fact is that plant owners have to comply with
PAKs regulations that are supposed to guarantee safe levels for the
environment and human exposure. I am not going to question the
correctness of the legislation in this forum. Another fact is that all
gasification ashes I have analysed so far smelled of naphthalene. I know
there are PAHs in there, but my nose cannot detect levels or determine
whether they are at too high levels. I just know that they are in there
and I also know that part of them has moved through the air into my
body.

On tests for land fill or fertilizer use:
Most ashes we have tested so far have have been approved for land fill.
Some have not, but these are exceptions and they are from combustion of
not-so-clean biomass. Only one fly ash that did not pass the test for
land fill came from combustion of clean wood (park waste, thinnings,
etc.). However, when testing for utilisation as fertilizer, most ashes
do NOT pass these more stingent criteria. Bottom ashes may pass the
test, but bottom ashes are less relevant because a) they have less
available nutrients and b) they can be used in road contruction at lower
cost. Fly ashes on the other hand did not pass, except maybe one or two.


On use of ash in forests:
Decompostion of dead trees is different in different places. BC may have
a conservating climate or persistent trees. In other forest it takes
10-20 years. In the Tropics probably even faster.
In Scandinavia, the boreal forests are limited in Nitrogen. Ashes
contain no nitrogen, so the growth is not significantly influenced.
Ashes contain immobilized P that is released in 20 years or so. Ashes
contain Ca and Mg that are benificial against acidification (from air
pollution). Only K is leached fast, in about 3 years. The effect is that
there is a relatively fast (3 years) growth of organisms in the soil
that store this K. 
Furthermore, forestry is not nature. Forestry or silviculture is a form
of agriculture with trees and long cycles. The Swedish have a set of
well worked out recommendations to handle ash recycling. It is only
mandatory in case of whole tree harvesting. When tops, leaves and
needles are removed from the site and cannot return their nutrients. In
case of stem extraction, this forest residue PLUS natural weathering of
rock AND atmospheric deposition are enough to return the nutrients to
the original level. (This level may not be natural, but it is the level
the foresters want to have their 'crops' grow.) The ash is used 6 years
after clearcut and only immobilized ash is used with a slow release of
nutrient. Experiments in Sweden run for 25 years and this approach
appears to be safe and not negatively affect the forest stands. I am not
sure about biodiversity, etc. The conclusion is drawn from the
perspective of silviculture.

On lead in road sides:
Yes, I remember when the government issued warnings against picking
berries, etc. along the roads because of lead. I know alsow that
nowadays these warnings are slowly lifted.

On installation scale:
The ashes from Scandinavian CHP plants are produced in installations of
50-200 MW. When they use uncontaminated waste wood (bark and cut off
pieces) they tend to be okay for recycling. They are regularly analysed.
No problem with larger scale. When they shift fuel, the ashes are
analysed more intensely to establish that they are safe (or not) for
recycling.
Small scale units tend to fire a larger variety of fuels and tend to
have less analyses of ash, also by the one who is utilising the ashes
for whatever purpose. Therefore, my conclusion is that those small
volumes of ash with unknown or varying composition should be land filled
and for the larger ash volumes with controlled quality utilisation
options should be developed.

Jan Pels





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