Trace elements in solid waste products from coal burning at some Bulgarian thermoelectric power stations

Abstract

The content, concentration trend and mode of occurrence of 40 trace elements in coal mixtures (coals and host rocks), fly ash, bottom ash and lagooned ash at four Bulgarian thermoelectric power stations (TPS) have been characterized. A complex of methods: separation, atomic emission and ICP spectroscopy, neutron activation, XRF, SEM, TEM, XRD, etc., was used. Trace elements in coal mixtures are concentrated mainly in the heavy fractions (above 2.9 g cm₋₃), authigenic minerals and organic matter; and to a lesser extent are present in the major detrital minerals and host rocks. A number of elements in the waste products, like coal mixtures (ash), exceed known Clarke contents. These are mostly the siderophilic, some lithophilic (Rb, Nb, Mo, Ba, REE, Hf) and chalcophilic (Cu, Zn, As) elements and U. The trace elements show different concentration trends in fly ash, but are more typical for non-magnetic, heavy and fine-grained fractions. They are commonly present as impurities in the glass phases, and are included in the structure of mineral phases. The accessory crystalline phases, element-organic compounds, liquid and gas forms, are of subordinate importance. Some elements, mostly from the chalcophilic (Cu, Zn, Ga, As, Sn, Sb) and lithophilic (Be, Y, Zr, Nb, Mo) groups, plus Co and U, show scattering trends into the atmosphere. For others, the combustion process appears to be a powerful factor causing relative increase in the fly ash. Considerable amounts of trace elements from stack emissions (Hf, As, Tl, Pb, etc.) have probably entered the soil near TPS. Trace elements can also occur in watersoluble forms (Li, Mo, Cs, halogen elements, V, As, Bi, etc.) in waste products and accumulate (Sr, Ba, Pb, etc.) in dump vegetation.

Keywords: fly ash; coal mixtures; trace elements

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