The Behaviour of Potentially Hazardous Elements in Fluidized Bed Combustion Fly Ash

Petr Sulovsky Dept. of Mineralogy, Petrology & Geochemistry, Masaryk University Brno, Czech Republic

The influence of fluidized bed combustion (FBC) parameters on the mode of occurrence of potentially hazardous elements (PHE) in fly ash and building materials produced thereof are of great environmental importance, as they can substantially influence the amount of contaminants released from this very common industrial waste. The methods used to characterise the PHE speciation involved modified sequential extraction procedure (followed by ICP-ES determination of extracts chemistry), XRD analysis and electron microprobe (in polished resin mounts). The automated EDS analysis and classification of individual grains, combined with XRD determination of major crystalline phases, were used to characterise the bulk phase composition of fly ash. In general, the FBC fly ash composition distinctly differs from that of fly ash produced by conventional combustion, above all by the absence of glass, mullite and other high-temperature phases, showing on the other hand much higher calcium sulphate content and higher portion of unmetamorphosed or only slightly thermally influenced coal minerals (micas, feldspars, clay minerals). The differences are also reflected by the bulk chemistry of trace elements and their speciation. These were studied with electron microprobe - through element mapping and spot analyses of whole fly ash particles or individual "mineral" phases in these particles. The WDS analyses involved the determination of both major (Si, Al, Ca, S, Fe, K, Na, Mg, Mn, Ti) and trace elements (As, Ba, Co, Cu, Ni, Pb, V, Zn). The acquired sets of data were large enough to be processed by multivariate correlation analysis and factorial analysis. The latter disclosed several latent variables, influencing the occurrence of PHE's. Some of them are fixed prevalently to aluminosilicate phases, corresponding to thermally metamorphosed micas (Cr, V), feldspars (Ba), metakaolinite (Cr, Co, Ba).

Another very significant factor is oxidic Fe+Mg+Mn minerals. In fly ash from conventional boilers they host preferably Zn, Cr, Cu and some V, in FBC fly ash the most characteristic admixtures are Co, Zn and Pb. In the former case, the matrix phase has mostly the spinel structure (Zn and Cr being therefore firmly fixed), while in FBC they are closer to maghemite or goethite structure, able to adsorb more Co. Calcium sulphates are another common species in fly ash. Their proportion of is much higher in FBC ashes. The relatively very low levels of PHE in Ca sulphates cause that this factor explains the variations in PHE content from 8 to 20%. In fly ash from conventional boilers and fine ash from FBC (samples from textile filter), the greatest affinity to Ca sulphate factor show anions - As, V, sometimes Cr, while in FBC bed ash it is Ni and Ba. In the latter, some significance has a factor with dominant role of Ti. It explains the variations in PHE content usually from 20%, the highest factor load have vanadium (analogue to natural rutile) and lead.

The last important latent variable, which was revealed only in fly ash from conventional combustion, is an association of PHE fixed to the surface of fine particles - Zn, Ni, Cu, V, Pb. It occurs only in the samples coming from conventional boilers. It corresponds to the condensation model originally proposed by Linton and Natusch in 1975. The absence of such phenomenon in FBC can be explained by lower temperatures of combustion (around 825 °C), not enabling the PHE to volatilise and subsequently condense on the particle surface. Comparison of these two approaches to the study of PHE speciation showed that sequential extraction yields results less easy to interpret and less detailed, yet sufficient for the monitoring of optimisation of FBC process parameters, and in fly ash stabilisation.