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Title:
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Characterisation of major component leaching and buffering capacity of RDF incineration and gasification bottom ash in relation to reuse or disposal scenarios
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Author(s):
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Published by:
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Publication date:
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ECN
Environment & Energy Engineering
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2-4-2012
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ECN report number:
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Document type:
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ECN-W--12-031
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Article (scientific)
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Number of pages:
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10
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Published in: Waste Management (Elsevier), , 2012, Vol.32, p.759-768.
Abstract:
Thermal treatment of refuse derived fuel (RDF) in waste-to-energy (WtE) plants is considered a promising
solution to reduce waste volumes for disposal, while improving material and energy recovery from
waste. Incineration is commonly applied for the energetic valorisation of RDF, although RDF gasification
has also gained acceptance in recent years. In this study we focused on the environmental properties of
bottom ash (BA) from an RDF incineration (RDF-I, operating temperature 850–1000 C) and a RDF gasification
plant (RDF-G, operating temperature 1200–1400 C), by evaluating the total composition, mineralogy,
buffering capacity, leaching behaviour (both at the material’s own pH and as a function of pH) of
both types of slag. In addition, buffering capacity results and pH-dependence leaching concentrations of
major components obtained for both types of BA were analysed by geochemical modelling. Experimental
results showed that the total content of major components for the two types of BA was fairly similar and
possibly related to the characteristics of the RDF feedstock. However, significant differences in the contents
of trace metals and salts were observed for the two BA samples as a result of the different operating
conditions (i.e. temperature) adopted by the two RDF thermal treatment plants. Mineralogy analysis
showed in fact that the RDF-I slag consisted of an assemblage of several crystalline phases while the
RDF-G slag was mainly made up by amorphous glassy phases. The leached concentrations of major components
(e.g. Ca, Si) at the natural pH of each type of slag did not reflect their total contents as a result of
the partial solubility of the minerals in which these components were chemically bound. In addition,
comparison of total contents with leached concentrations of minor elements (e.g. Pb, Cu) showed no
obvious relationship for the two types of BA. According to the compliance leaching test results, the
RDF-G BA would meet the limits of the Italian legislation for reuse and the European acceptance criteria
for inert waste landfilling. RDF-I BA instead would meet the European acceptance criteria for non hazardous
waste landfilling. A new geochemical modelling approach was followed in order to predict the leaching
behaviour of major components and the pH buffering capacity of the two types of slags on the basis of
independent mineralogical information obtained by XRD analysis and the bulk composition of the slag. It
was found that the combined use of data regarding the mineralogical characterization and the buffering
capacity of the slag material can provide an independent estimate of both the identity and the amount of
minerals that contribute to the leaching process. This new modelling approach suggests that only a limited
amount of the mineral phases that control the pH, buffering capacity and major component leaching
from the solid samples is available for leaching, at least on the time scale of the applied standard leaching
tests. As such, the presented approach can contribute to gain insights for the identification of the types
and amounts of minerals that control the leaching properties and pH buffering capacity of solid residues
such as RDF incineration and gasification bottom ash.
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