Emission characteristic and transformation mechanism of hazardous trace elements in a coal-fired power plant

Coal-fired power plant is considered to be main anthropogenic emitter for emission of hazardous trace elements (HTEs), which can cause great damage on environment and human health. Emission characteristic of nine HTEs, namely Hg, Zn, Sb, Pb, Cd, As, Cr, Mn and Ba was carried out on a 320 MW coal-fired power plant equipped with SCR, ESP and WFGD. US EPA method 29 was used for flue gas HTEs sampling at inlet or outlet of each APCD simultaneously. Solid or liquid samples like feed coal, ESP ash, bottom ash, fresh slurry, waste water, gypsum were collected at the same time. Scanning electron microscope, energy dispersive spectrometer and X-ray power diffraction spectrometry were adopted to characterize the physical and chemical structure characteristic of some solid samples. Results show that mass balance rate of the HTEs for each device and the system is in acceptable range. Mass amount of HTEs distributed in bottom ash and ESP ash accounts for large proportion, while little of them escapes into atmosphere (<0.05%). The mass amount of Mn and Ba in bottom ash is higher than that of other HTEs. Particulate HTEs are the main form in flue gas before ESP. ESP + WFGD has high removal efficiency for the HTEs in flue gas (99.96-99.99%), in which ESP makes a great contribution. During coal combustion, Mn and Ba enrich in bottom and fly ash equally while As, Cd, Sb, Zn, Pb and Cr will be prone to enrich in fly ash. Concentration of the HTEs in flue gas emitted to atmosphere is relatively low (3.5 × 10−3-4.30 μg/m3) with emission factor of 0.001-1.473 g/1012 J. Finally, transformation mechanism of the studied HTEs during coal combustion is discussed.