| Mercury is an extremely toxic trace element, can occumulated in organisms and enter hunman body through food chain. Mercruy poisoning can cause neurologic damage and infantile deformity. As a global pollutant, mercury has gained more and more attention. Coal-fired power plant is the main anthropogenic emission source of mercury. However, detail mechanisms about mercury emission and transportation during coal combustion remained unknown, especially in China.Mercury emission, transformation and control characteristics during coal combustion were investigated by tube furnace and drop tube furnace, utilizing mercury continuous emission monitor (Hg CEM). Studies of mercury about the emission characteristics and influences during heating, the speciation and control performance under oxy-fuel combustion, the oxidation by other flue gases, as well as the emission from a power plant were conducted in order to provide theoretical and practical information on mercury control during coal combustion.Mercury emission characteristics, including temperature and tensity, as well as speciation in flue gas during coal heating were investigated on a tube furnace. The mercury emission characteristics were analysed and used to identify the mercury form and distribution in coal. The relationship between mercury and coal sulfur was also studied. Results showed that Hg emission concentration displayed a multi-peak distribution. The first peak (P1) had the highest emission tensity at the temperature of 360℃-460℃, the second peak (P2), appeared at 445℃-850℃while the third peak (P3) corresponding to temperature higher than 930℃. The concentration of mercury released in P2 could be higher or lower than P3, depending on the coal type. The Hg0(g) percentage in separate Hg peaks was in the range of 38%-100%, in the order of P1>P2>P3. With the mercury emission characteristics and sink-float seperation process, the main forms of mercury and its content in coal were predicted. Mercury in coal is associated with sulfur, for some coals mercury and SO2 released at the same temperature.Mercury emission and speciation during coal combustion under O2/CO2 atmosphere were conducted on a tube furnace and a drop tube furnace. Influences of O2 concentration and CaO/Fe2O3 were also investigated. Results showed that the initial mercury emission was inhibited with both the tensity and temperature of P1 weakened. Mercury concentration and speciation in flue gas differed for different coals. When O2 increased to 30%, the HgT(g) concentration of bituminous coal notablely increased, while Hg (g) concretion was nearly not affected by O2 variation. Mercury control by CaO and the catalytic oxidation of Hg (g) by Fe2O3 can be promoted by O2/CO2 atmosphere. However, the increase of O2 concentration would prompt HgT(g) emission under CaO addition and prompted Hg0(g) oxidation and adsorption by Fe2O3.Homogeneous Hg0(g) oxidation by a single as well as two flue gas components of HCl/Cl2/SO2/NO/NO2 from 200℃-1000℃were investigated on a bench-scale quartz reactor. Results showed that HCl and Cl2 were the main Hg0(g) oxidants:Hg0(g) oxidation by HCl only occurred above 600℃, while Cl2 can significantly oxidize Hg0(g) even at room temperautre. Cl2 sarted to decompose at temperture higher than 400℃. Hg0(g) oxidation by NO was affected by NO concentration and decreased at higher temperature. O2, NO2 and SO2 alone exhibited limited Hg0(g) oxidation, less than 10%. The flue gas interactions effected Hg0(g) oxidation. Hg0(g) oxidation by HCl/NOx or Cl2/NOx was enhanced at 200℃-600℃and inhabited at 800℃～1000℃, wihle the addition of O2 further promoted Hg0(g) oxidation. SO2 strongly inhibited Hg0(g) oxidation by HCl. The coexist of SO2 and NO2 at 400℃can oxidize 48%～63%of the total Hg0(g) and the percent of Hg0(g) oxidized decrease sharply at 1000℃.A field measurement was conducted on a 200 MW pulverized coal fired boiler, using Ontario Hydro method (OHM) and EPA Method 26A to determine the speciation of Hg and halides in postcombustion flue gases, respectively. A model based on BP neural network was established and trained by ICR data in order to predict the mercury emission from real power plants and was compareded with this field data. Results indicated that, the total gas phase mercury (HgT(G)) in flue gas was 6-28μg/Nm3. As the flue gas cooling down, the percentage of oxidized mercury in total gas phase mercury (Hg2+(g)/HgT(G)) increased from 41% to about 74% across the electrostatic precipitator (ESP) outlet. The main halides measured in flue gas were HF and HCl, while the concentration of Cl2 and HBr were extremely low and no Br2 was detected in flue gas. Acid flue gas components, such as HCl, HF, SO2 and NO, showed a certain extent of promotion on Hg oxidation. The measured mercury emission factor (EMF) in this test was 5.63 g/1012J (13.1 1b/1012Btu). The BP neural network model was used to predict the mercury emission from this 200MW pulverized coal fired boiler, and results showed certain practical merits.
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