Influence of surface area properties on mercury capture behaviour of coal fly ashes from some Bulgarian power plants

In order to reveal the enrichment/depletion behaviour of Hg during coal combustion, a separation procedure was applied on three fly ash (FA) samples from two Bulgarian thermoelectric power plants, Maritza 3 TPP and Varna TPP, as follows: (1) FAs from the 1-st and 3-rd rows of the electrostatic precipitators (ESP) of Maritza 3 TPP; and (2) FA from the 5-th row of ESP of Varna TPP. Five fractions were obtained from each bulk FA sample as a result of the sequential separation scheme applied, namely: water leachate of FA; water washed FA (PAW); char concentrate (FAC); magnetic fraction (PAM); and FA residue (FAR). Mercury content of bulk samples and separated fractions, Brunauer-Emmett-Teller (BET) specific surface area, Barrett-Joyner-Halenda (BJH) mesopore volumes, micropore surface area, monolayer capacity, and micropore volumes for porous texture characterization of the FAs and their respective char fractions were also measured. The data obtained reveal that char, which contains mainly unburned organic matter, has an important role in the partial capture and retention of Hg in FAs. Fly ash char fraction (FAC) generated from lower rank lignites (Maritza 3 TPP) has greater enrichment in Hg compared to FAC originated from higher rank bituminous coals (Vama TPP). There is a strong positive correlation between Hg concentration and the BET/Langmuir specific surface area, BJH mesopore volume, micropore surface area and monolayer capacity of the char fraction derived from FA of the 3rd ESP row at Maritza 3 TPP, which suggests that Hg adsorbs on both the mesopores and micropores of the char particles and by volume filling of the pores for the lignite-derived fly ash char sample. In contrast, Hg concentration shows strong positive correlation with the micropore surface area and moderate positive correlation with micropore volume and monolayer capacity of the char fraction obtained from the FAs of the 5th ESP row in Varna TPP. Thus, the adsorption on the micropores of the char particles and subordinate volume filling of the char micropores can be expected as preferable mercury adsorption mechanisms for the bituminous-derived fly ash chars. (C) 2013 Elsevier B.V. All rights reserved.