| Mercury is one of the most toxic heavy metal elements and can severely threat human health and environment. It is one of the priority pollutants which have caused great concern in the world. Mercury emitted from coal-fired power plants has been identified as the largest source of atmospheric mercury emission. Chinese energy is mainly from coal and Hg pollution from its combustion is a severe environmental issue so that the control on Hg emission from the combustion has been concerned widely.Distribution characteristics of mercury speciation in flue gas and the mercury removal through the existing purification equipments such as electrostatic precipitator (ESP) and wet flue gas desulfurization devices (WFGD) are extremely important for the forward technology of effective mercury control. Ontario Hydro Method (OHM) is considered as one of the most accurate methods for mercury speciation analysis in coal-fired flue gas. However, OHM with many bottles and big equipment could not be used in some of power plants with a narrow sample space. In this paper, a mercury sampling method with small portable equipment from coal-fired flue gas was studied based on the principle of OHM and then the Hg species in flue gas was sampled at the input and output of the individual purification. Distribution characteristics of mercury speciation in flue gas and the mercury coinstantaneous removal effect through individual purification equipment was investigated to provide a theoretical support for suggestion on the cost-effective control on mercury.Firstly, OHM and the simplified portable mercury sampling method (SPM) were established in laboratory and got close recoveries in mercury speciation and total mercury from both methods. The portable equipment with six and five small bottles was suitable for the coal with high-mercury and with low-mercury respectively. The profiles of the mercury speciation distribution in flue gas from two coals were similar with a small part of Hg in particles and a big part of Hg in gas, mainly in Hg0.Secondly, Hg content in flue gas was detected with SPM in three power plants and a mercury disposal and recycling plant. The results showed that the mercury speciation distribution was different due to different coals. Hg0 was mainly emitted in power plants 1 and 2 but Hg0 and Hg2+ were mainly emitted in power plant 3. The particulate mercury removal efficiency was closed to 100% by ESP and WFGD significantly removed Hg2+ in flue gas. This method could be applied in the smelting industry based on the testing results from the mercury disposal and recycling plant.Finally, the mercury content in coals, mercury speciation distribution characteristics and influencing factors were discussed based on the data in both laboratory and plants. Mercury content in burning coals of the power plants was changed greatly and the ash content in coal could affect on the content of particulate mercury in flue gas with a positive correlativity. The content in the oxidation mercury in flue gas was impacted by the chlorine content in coal, which presents a positive correlativity also. Mercury in three power plants was concentrated on the gypsum in WFGD but, in contrary, found in a little amount in fly and bottom ash. The total mercury removal rates of three plants were 73.51%,84.87% and 54.37%. The key to control mercury emissions should be to reduce the concentration of Hg0 in flue gas. Therefore it could be considered that the activated carbon with halogen is injected into the flue gas before the particulate control devices or some oxidation materials is added in the WFGD in order to improve mercury removal. |
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