Trace element (Hg, As, Cr, Cd, Pb) distribution and speciation in coal-fired power plants

•Cd is evenly distributed in bottom ash, fly ash and gypsum samples from coal-fired power plants.•Pb, Cr and As are enriched in fly ash. Hg is enriched in gypsum.•Most of Hg, As and Pb are in the residual fraction of fly ash.•Most of Cd and Cr are in the effective fraction in desulfurization gypsum.•Stability of these trace elements in gypsum is lower than the compounding stability in fly ash.

This work reports the distribution of selected trace elements, including Hg, As, Cr, Cd and Pb, from different sampling locations in a 300 MW coal-fired power plant. The plant is equipped with low NOx burners, a selective catalytic reduction (SCR) denitrification system, an electrostatic precipitator (ESP) and a wet flue gas desulphurization system (WFGD). It was found that Cd is approximately evenly distributed between the bottom ash, fly ash and gypsum. Pb, Cr and As are mostly concentrated in the fly ash. Mercury, being the most volatile element, has the lower concentration in the solid form among the five target elements. The concentrations of As, Cr and Pb are higher in fly ash than in the gypsum, in contrast to Hg and Cd. FGD gypsum samples and fly ash samples from four coal-fired power plants were collected. Samples were leached and analyzed for the speciation of trace elements by a five-step extraction procedure modified from the Community Bureau of Reference (BCR) method. The speciation study indicates that more residual fraction for Hg, As and Pb than that of effective fraction in both fly ash and gypsum. However, Cr and Cd have more effective fraction than that of residual fraction in both fly ash and gypsum. In fly ash, 60 to 90% of the trace elements exist in the residual fraction. The stability of trace elements in the gypsum is weaker than that of the fly ash in the environmental resource utilization. This may mean that the trace elements should be retained in the fly ash as much as possible.