Geochemistry of coals, coal ashes and combustion wastes from coal-fired power stations

Abstract

Contents, concentration trends, and modes of occurrence of 67 elements in coals, coal ashes, and combustion wastes at eleven Bulgarian thermoelectric power stations (TPS) were studied. A number of trace elements in coal and coal ash have concentrations greater than their respective worldwide average contents (Clarke values). The highest values in coal ash are displayed by elements such as Rb, Cs, Ba, Cu, Sb, Bi, U, and Ag. Trace elements are concentrated mainly in the heavy accessory minerals and organic matter in coal. In decreasing order of significance, the trace elements in coal may occur as: element-organic compounds; impurities in the mineral matter; major components in the mineral matter; major and impurity components in the inorganic amorphous matter; and elements in the fluid constituent. A number of trace elements in the waste products, similar to coal ashes, exceed known Clarke contents. Trace elements are mainly enriched in non-magnetic, heavy and fine-grained fractions of fly ash. They are commonly present as impurities in the glass phases, and are included in the crystalline components. Their accessory crystalline phases, element-organic compounds, liquid and gas forms, are of subordinate importance. Some elements from the chalcophile (Cu, Zn, Ga, Ge, Pb, As, Sb), lithophile (Be, Ba, Ce, Hf, Sr, La, Zr, Mo, U) and siderophile (Sc, Cr, V) groups may release into the atmosphere during coal burning. For others, the combustion process appears to be a powerful factor causing their relative enrichment in the fly ash and rarely in the bottom ash and slag. Considerable amounts of Hf, some chalcophile elements (Zn, As, Tl, Pb) and Ag from stack emissions have probably entered the soil near TPS. Trace elements can also occur in water soluble forms in the fly ash (Li, Sr, Mo, Cs, V, Cr, Mn, As, Bi, B, F, Cl, Br, I) and probably contaminate the surface and subsoil waters. Some trace elements (Sr, Ba, Yb, Sc, Cd, Tl, Pb, Bi) may accumulate in the vegetation near TPS.