Mineralogy, geochemistry and mercury content characterization of fly ashes from the Maritza 3 and Varna thermoelectric power plants, Bulgaria

The goal of the present study is to characterize the mineralogy and geochemistry of the bulk fly ashes (FA) and some separated fly ash fractions sampled from each row of electrostatic precipitators (ESP) of two Bulgarian thermoelectric power plants (TPP), namely Maritza 3 and Varna, and to reveal the influence of the mineral matter and chemical composition on the FAs mercury capture behaviour. A separation procedure was conducted on three selected fly ash samples as follows: (1) FAs from the Maritza 3 1st-row ESP; (2) FAs from the Maritza 3 3rd-row ESP; and (3) FA from the Varna 5th-row ESP. Following the sequential separation procedure, five fractions were obtained from each bulk FA sample including water leachate of FA, water-washed FA (FAW), char concentrate (FAC), magnetic fraction (FAM), and FA residue (FAR). Phase-mineralogical composition, the concentration of 30 trace elements, and mercury content of bulk FA samples and separated FA fractions were determinded.The data obtained reveal that the phase-mineral composition of the char-rich fly ash fractions isolated from the FAs of the Maritza 3 1st-row ESP consists mainly of char and quartz with lesser contents of anhydrite and hematite; while the FAC fraction derived from the FAs of the 3rd-row ESP in Maritza 3 is highly enriched in char and anhydrite with insignificant amounts of magnetite-hematite and quartz. The results indicate that there is a synergetic effect between char and Ca sulphate, and anhydrite could play a significant role in the Hg capture in the fly ashes from Maritza 3. In contrast to the FAC fraction from Maritza 3, the FAC fraction isolated from the FAs of the Varna 5th-row ESP contains almost pure char with insignificant amount of magnetite and quartz. Mercury capture in the FA from the Varna 5th-row ESP is strongly dependent on the bituminous-derived FA char and the mineral matter does not have significant influence on the Hg capture behaviour. It was established that very high enrichment of trace elements in the studied FAs does not occur, excluding the concentrations of Cd, Mo, As, Mn, and Ag in Maritza 3 FAs, and the contents of Be, Cd, Cs, Rb, Se, and Ag in Varna FAs. The high capture behaviour of Hg by char and sulphates in the Maritza 3 3rd-row ESP is accompanied by simultaneous capture behaviour of hazardous and volatile elements such as Cd, Mo, and As, and probably others when compared with FAs from the 1st-row ESP in the same TPP. This observation indicates that the capture mechanism of such elements tend to be similar to that of mercury.