Koukouzas, and 2 co-authors reported in 2011 that the present study investigates the heavy metal content of coal fly ash (FA) samples coming from three different sampling points (secondary cyclone, cooler and filter) of a pilot plant combustion facility. The combustion experiments were carried out in a 0.1 MW Circulated Fluidized Bed (CFB) boiler using South African coal, with the addition of limestone for sulfur capture. FA was tested for the presence of selected heavy metals using inductively coupled plasma optical emission spectrometry (ICP-OES); batch leaching tests were conducted as well. The samples were also characterized in terms of their microstructure, chemical and mineralogical composition, total surface area and particle size distribution. Most of the studied metals (Cd, Cr, Cu, Ni, Mn, Zn) showed enrichment in the fine, filter FA particles, while Pb was mostly concentrated in the cooler sample. Regarding leaching characteristics of the examined samples, Cr was found to occur in
considerable amounts. Although the use of CFB technology for the combustion of solid fuels steadily gets bigger worldwide, only a very limited number of studies have environmentally assessed CFB-coal FAs to date. Thus, the current study aims to contribute toward building a more integrated knowledge on the
environmental impact of this abundant power production by-product.
The authors concluded that the current research study can be summarized as follows:
• The examined fly ash samples had two particle modes: (a) coarse in the secondary cyclone and (b) fine in the filter fly ash. Those two particle modes were accompanied by inversely proportional values of specific surface area: low in the case of secondary cyclone ash and high in this of filter ash.
• The heavy metal content evaluation of the fly ash samples indicated their enrichment in the fine particles of the filter ash, with the exception of Pb which was mostly concentrated in the cooler ash (due to the lower temperature after the cooler with a concomitant condensation of flue gas in this region). In this context, it is of great
importance for the environmental consequences of the heavy metals that conventional particle-removal devices effectively capture the fine fly ash particles, which means that they will also capture the incorporated heavy metals.
• Leaching evaluation of the filter ash samples showed that Cr concentration was in considerable amounts, with respect to the EN conditions. Under the TCLP conditions, Cr exceeded the EPA Land Disposal Restriction Limit while Cu and Mn were found to be on the high level regarding the US EPA National Secondary Drinking Water Standard. In general, trace elements were found to be more mobile under TCLP conditions, the results of which indicated the worst case scenario, due to strongly acidic conditions in comparison to EN 12457-2.
• Chromium should be carefully monitored. In general, leaching experiments showed that the elemental composition of the leachate may not proportionally reflect the elemental composition of the whole ash sample (the rate at which the elements will leach from the ash sample is dependent on the form in which the element is present and the location of the element within the ash matrix or absorbed onto the ash particle surface).
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