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Title:
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Biobrandstof van verontreinigde grond: kwaliteit vliegas bij vergassing en meestoken in poedercentrales
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Author(s):
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Published by:
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Publication date:
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ECN
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1997
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ECN report number:
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Document type:
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ECN-C--97-099
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ECN publication
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Number of pages:
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Full text:
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78
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Download PDF
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Abstract:
The cultivation of energy crops, e.g. fast growing willow on cheapcontaminated sludge soil, could reduce the cost of energy crops as biomass
fuel. The uptake of heavy metals from the soil could, however, endanger
potential of the crops as a fuel in co-firing, because of the expected impact
on the leaching behaviour of the fly ash. Increased leaching of heavy metals
from the fly ash could limit its use in the cement and concrete industry. To
avoid the inorganics (including heavy metals) of biomass from entering the
boiler, an option would be to gasify it first and to co-fire the fuel gas in
the boiler after a gas cleaning step. This, however, creates a separate
biomass ash stream with high landfilling cost, when it is not used as
building material, neutraliser or fertiliser. The literature on the quality
and leaching behaviour of fly ash from gasification and co-firing biomass is
very limited. For that reason the leaching behaviour was measured for a
mixture of fly ash and willow ash and for willow ash alone. The willow was
from clean soil and the leaching data are extrapolated to higher
concentrations of heavy metals, based on the extensive knowledge and
experience at ECN on leaching behaviour of solid waste streams. Willow from
contaminated soil likely contains higher concentrations of cadmium and zinc
compared to willow from clean soil. The co-firing ratio is probably limited
by the high expected concentration of calcium in the fly ash, which limits
its use in concrete and cement. The leaching behaviour and level of heavy
metals from fly ash is not greatly affected when co-firing clean willow.
Co-firing willow with higher concentrations of cadmium and zinc will give
increased leaching only when the fly ash is used under extreme pH conditions
or when it contains a significant amount of chlorine. Both are not very
likely. Furthermore, not the leaching behaviour of the pure ash, but of the
end product is important. The largest ash stream from gasification is
separated in the cyclone after the gasifier and contains high concentrations
of calcium, potassium and phosphor. It could therefore be used as fertiliser
or neutraliser. There is almost no regulation for heavy metals in
fertilisers, except that it can be used when concentrations do not exceed the
limits for the soil. The cyclone ash from gasification of clean willow
already shows high concentrations of heavy metals, exceeding the limits for
the soil. This could make it difficult to obtain permission to use it as
fertiliser or neutraliser, especially when the cadmium and zinc
concentrations are even higher as expected in the case of willow from
contaminated soil. The leaching of heavy metals from the fertiliser likely
determines its impact on the environment. The leaching of molybdenum from ash
from clean willow at pH>8 exceeds the limits stated for building materials
(very strict limits). At very low pH also elements like zinc, cadmium and
copper exceed the limits. With increased concentrations of cadmium and zinc,
extreme pH conditions and high amounts of chlorine in the ash could give very
high leaching levels. From this respect, it seems that the ash is not
suitable for use as fertilizer on soil with pH 6-8. Because the ash is very
fine it could also be very suitable for use as a filler in concrete, when the
high carbon content is decreased by better conversion in the gasifier. 62
refs.
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