Mercury Speciation And Transformation In Industrial Flue Gas

To implement the Minamata Convention on Mercury and protect the environment,it is essential to reduce the atmospheric mercury emissions from industrial process,such as coal-fired power plants and cement plants in China.Mercury speciation is not only the crucial parameter affecting atmospheric mercury emission control technologies,but also the key factor affecting the mercury transport and transformation in atmosphere and its environmental effects.Therefore,it is of great significance to understand the mercury speciation and transformation in industrial process and provide scientific support for atmospheric mercury emission control in China.In this study,we used fixed-bed instrument and one-dimension drop-tube furnace to study the mercury oxidation by chlorine.Based on E-R mechanism and L-H mechanism,the heterogeneous mercury oxidation model was built and verified using the experimental results.We figured out the mercury oxidation mechanism in coal combustion flue gas.We also developed different methods to identify the mercury chemical forms in coal combustion flue gas during the cooling process.Finally,we selected typical coal-fired power plants and cement plants to investigate the mercury behavior and effects of air pollution control devices(APCDs)on mercury transformation.With the increase of chlorine concentration in coal combustion flue gas,the reaction rate between the mercury and chorine first increased rapidly and then became steady.The mercury oxidation by chlorine follows the E-R mechanism.The mercury oxidation rate of chlorine impregnation in coal was higher than that of chlorine addition in flue gas through HCl gas.We used quartz membrane,Polytetrafluoroethylene(PTFE)membrane and nylon membrane to capture mercury,and applied Sequential Extraction Procedure(SSP)and High Performance Liquid Chromatography-Atomic Fluorescence Spectrometer(HPLC-AFS)to identify the mercury speciation.It was found that the mercury chemical speciation in coal combustion flue gas was predominantly in the form of complex compounds rather than simple compounds such as HgCl2 or Hg2Cl2.The change of mercury chemical forms was not detected in the cooling process of this study(310-170?).The results of SSP and Temperature Programmed Decomposition Technique(TPDT)showed that the mercury in fly ash of coal-fired power plant was mainly mobile mercury,accounting for 50-60%,and the peak temperature was 216-231?.The percentage of mobile mercury in gypsum was affected by the catalytic oxidation of SCR catalyst,and increased from 20% to 70%,while the percentage of semi-mobile mercury decreased from 80% to 20% with the increase of NOx reduction efficiency.The proportion of the peak with a temperature of 193-204? decreased from 40% to 10%,and the proportion of the peak with a temperature of 222-236? increased from 30% to 60%.There were three “mercury cycling” between rotary kiln system and raw mill system,coal mill system and kiln head system in cement production process,which caused mercury enrichment and over 90% of mercury input emitted into the atmosphere.The following factors contributed to the high proportion of oxidized mercury in flue gas of cement production: the gradually heating process caused by “mercury cycling”,the high concentration of particulate matters,the high Fe2O3 content in fly ash and the long residence time.The proportion of oxidized mercury in flue gas emitted into the atmosphere was 61% and 91%...