Study On Heavy Metals Online Monitoring, Release And Adsorption Mechanism During Combustion

With the rapid development of economy and accelerated process of urbanization, air pollutant emissions from coal and municipal solid waste combustion increase substantially. The pollution of heavy metals has attracted a significant attention in the world because of its extreme toxicity, persistence and high degree hazardous. And, there are several important issues to be resolved such as heavy metals online monitoring method of combustion flue gas and the adsorption mechanism of merury on the carbon-based sorbents. So it is necessary to focus on understanding the evaporation and reaction mechanism. By seeking economical and efficient method to control heavy metal pollution, it can provide an effective basis on the development of the emission standards and environmental regulations.Normal methods for measuring heavy metals in flue gas can only get an average value, which takes a long time for sampling and analysising. For modern industrial process whose scales increased continuously, it is important to monitor the concentration of heavy metal to guarantee the safety of combustion process. An on-line analysising method has been developed to continuously measure the heavy metal concentrations. Calibration is achieved by introducing a standard aerosol prepared in-situ using an ultrasonic nebuliser coupled with solvent system. The calibration can improve atomization efficieny, eliminate inspired interference of H2O and CO2in aerosol, and ensure the moisture content of the matrix composition consistency. The on-line systems can be used for direct analysising of heavy metal pollutants in gaseous effluents of industrial processes domestically. The best of detection limit is gained as0.5-10-3mg/m3. The system has several advantages such as short response time, high sensitivity and wide dynamic range etc, which provides an effective means for monitoring and controlling heavy metal pollutants emissions from combustion flue gas.The on-line analysis system has been used with a laboratory fluidized-bed reactor to measure continuously measure the heavy metal concentration in exhaust gases to determine the roles of temperature during mineral matrix (porous Al2O3and sepiolite), coal, municipal solid waste and sludge combustion process. The results indicate that the heavy metal vaporization process is strongly influenced by temperature. The vaporization of three heavy metals (Cd. Pb and Zn) increased with the temeperature arising from500℃to900℃. During coal combustion at900℃, the release rate of Cd is34.0%while the release rate of Zn is only14.3%. The release mechanism of heavy metals during combustion is complicated and including internal mass transfer and gasification of heavy metals. The behavior of heavy metals depends strongly on the proportion of ash mineral (Al2O3and SiO2). Thus, this experimental setup is a very useful tool for studying the vaporization of heavy metals during the combustion process.The finite clusters of four-six fused benzene rings are structured for the basis models of carbonaceous surface. The cluster models are structured to represent the carbonaceous surface. The B3PW91/RCEP28DVZ combination was used for Hg and I, and the basis of6-31G (d) is used for nonmetal atoms (C, H, O, N, S etc.). The average bond length of C-C and C-H is calculated as1.41A and1.39A, respecitvely. Both bond lengths and angles of the optimized structure are in good agreement with the experimental data. Defective carbonaceous surfaces are applied to understand the influence of atomic structure and properties. The results indicate that there are more electronic remove to surface from the inside of carbonaceous materials, which increase the active sites and promote the adsorption reaction.The adsorption mechanism of different forms of mercury on carbonaceous sorbents is not clear. Density functional theory is applied to understand the binding mechanism of mercury halides (HgCl, HgCl2b, HgBr, HgBr2, HgI and HgI2) on carbonaceous. The electronics are found translate from mercury atom to carbon atom by analyzing Mulliken total atomic charges. The adsorption of mercury on carbonaceous surface is proved stable by atomic bond population and frontier orbital analysis. The adsorption energies are calculated between-29.6and-66.4kJ/mol when Hg is adsorbed on carbonaceous surface. The adsorption energies are calculated between-123.6and-239.1kJ/mol Hg adsorbed on defective carbonaceous surface. So Hg0is much easier to adsorb on defective carbonaceous surface. The adsorption of mercury-halogen species on carbonaceous surface belong to chemisorption and include several adsorbing and desorbing processes. HgBr2strongly adsorbs on carbonaceous surface and not likely to desorb, while the adsorption capacity of HgCl2is relatively weak. Thus, Br-modified carbon-based sorbents are verified as effective sorbents for mercury capture. It would provide a foundation for finding economic and effective method of mercury control by understanding the mechanism of mercury species adsorption on carbonaceous surface.The effect of flue gas on Hg0adsorption on carbonaceous surface is still not clear by experimental results. A theoretical investigation of the effects of flue gas (HCl、NOx、SO2、 CO�'�CO2) on the mercury adsorption mechanism on carbonaceous surfaces is presented to study the interaction of mercury and different flue gas with carbonaceous surface. All of the possible approaches of flue gas adsorption on the edge sites of the carbonaceous surface are considered first, and the results indicated that the adsorption of flue gas belong to chemisorption. There is a possibility that NO2and HCl dissociates from carbonaceous surface. Compared with Hg0adsorption on carbonaceous surface, it is clearly that flue gas is easier adsorbed. The stablility of adsorption followed by SO2> CO2> HCl> CO> NO> SO2> Hg0The effects of flue gas on Hg adsorption are complicated and depend on the concentration. The adsorption of flue gas molecular will increase the adsorption capacity of the carbonaceous surface for Hg0by increasing the activity of its neighboring sites. However higher concentrations of flue gas decrease the Hg adsorption capacity because they competes for the activate sites on the carbonaceous surface. This result may explain the seemingly conflicting experimental results reported in the literature concerning the influence of flue gas on mercury adsorption on carbonaceous surface.