Study on the mercury emission and transformation in an ultra-low emission coal-fired power plant

- The distribution, removal and emission characteristic of mercury in the ULE power plant were reported.
- Mercury transformation process was discussed systematically for each APCD by new method.
- Mercury in bottom and ESP ash had no effects on the soil.
- Attention should be given to the gypsum disposal, especially when using thermal treatment.

Mercury emissions from a Chinese ultra-low emission (ULE) coal-fired power plant burning a low chlorine and sulfur bituminous coal was investigated by using the Ontario Hydro method (OHM). A temperature-programmed decomposition desorption (TPDD) instrument, scanning electron microscope (SEM), and X-ray power diffraction spectrometry (XRD) were used to identify the mercury species, apparent morphology, and chemical composition of the coal and combustion by-products. As measured by the OHM, the elemental mercury emitted from the stack is the largest proportion of the total mercury (about 38.95%), almost 13 times more than the oxidized mercury. Mercury in bottom ash was found to be only 0.07% of the incoming total mercury. The elemental mercury was found to oxidize across the SCR, with 45.47% converted to oxidized mercury. This increase in mercury oxidation is significant, as it was shown to be removed by the WFGD system. The total mercury removed across all APCDs in order of most to least was WFGD > ESP > WESP. The WESP was shown to further remove mercury, whose removal rate is about 13.63%, resulting in lower mercury stack emissions. For this plant arrangement, the mercury emission factor was calculated to be 1.56 g/1012 J, less than the mean value of Chinese plants. The mercury content in the gypsum is higher than the limit (0.50 mg/kg) while bottom and ESP ash were shown to have no effect on the soil. Further investigation of thermal treatment on mercury in gypsum is needed before definitive conclusions on disposal can be drawn. Concentration of mercury measured in the WFGD and WESP waste water is higher than the limit (0.001 mg/L), and more importance should be given on the waste water from WFGD.