Study On Emission Of Mercury And Its Mechanism In The Process Of Thermal Treatment

Coal fly ash, which is discharged from coal-fired power plants, is a fine ash collectedduring the process of flue gas dedusting. Generally speaking, coal fly ash possesses a strongerabsorption capacity for gaseous mercury, increasing the likelihood of mercury enrichmentbecause of its high surface area and containing many activity substances. Nowadays, the coalfly ash has been widely used in building material, agriculture, metallurgy, environmentalprotection industries, and most of them involve thermal treatment of the fly ash. The mercuryenriched in the fly ash is easily released with increasing temperatures, making it a potentialthreat to the environment. So it is meaningful to research the migration characteristics ofmercury from the coal-fired fly ash during thermal treatment in the coal-fired fly ash reuseindustry. In this work, the coal fly ash sample used was collected from the Zhujiang PowerPlant in Guangzhou, was screened into four size fractions (>150μm,150-75μm,75-48μmand <48μm). And the morphological features, BET surface area, porosity, chemicalcomposition and mineral composition were detected. On this basis, the fly ash samples wereheated in different temperatures by tubular furnace，from200℃t o1200℃degree with each100℃intervals, and with each50℃from500℃to800℃. Gaseous mercury species (Hg0andHg2+) emitted during thermal treatment were absorbed using the Ontario Hydro Method, andmercury species (extracted mercury, semi-mobile mercury and non-mobile mercury) wereextracted by sequential extraction procedure of EPA Method3200. The results demonstratedthat, during the thermal treatment, gaseous mercury emissions from coal fly ash began at200℃and the maximum mercury emission temperature was about400℃. When thetemperature was higher than400℃, the concentration of the total mercury did not increase.With the decreasing of the particle size, total and the semi-mobile mercury concentrationdecreased. When the temperature was higher than500℃, the remaining mercuryconcentration did not change any more. There were mainly three aspects of the oxidization ofthe Hg0. On one hand, the activity substances on the surface of the fly ash, such as the carbon,TiO2, and CaO, which had the effect on the Hg0oxidization from200℃to400℃. On theother hand, with the combustion of the carbon from500℃to800℃, the Hg0emitted had beenoxidized to Hg2+. And there still little Hg0, which could be oxidized by the oxygen when thetemperature higher than1000℃. The mercury adsorption and oxidizationwere highlydependent on the contents and characteristics of unburned carbon. In the fractions with thecoarser particle size, the carbon contents increased. The carbon content was well-correlated with the the total mercury emitted rate, the Hg0oxidization rate and the semi-mobile mercurycontent, and the correlation coefficient were r=0.9118，r=0.8610, and r=0.9866, respectively.