Study On Occurrence Of Mercury In Coal Fly Ash And Its Release Characteristics In The Heat Treatment Process

Mercury is a natural element throughout the world, which is widely used in all areas oflife. Because of its volatile, the role of persistent, long-range transport of pollution andbiological characteristics of enrichment, mercury has become a toxic metal element of themost atmospheric pollutants in the world widely. Research findings on mercury sourcesshowed that coal burning is an important source of mercury in the environment. As China isundergoing rapid economic development stage, the demand for coal increases year by year,which makes our emissions of mercury, have increased year by year correspondingly, ourcountry has become the largest country mercury emissions in the world recognized. In thecoal byproducts, the mercury is concentrated in coal fly ash. When the fly ash undergoesrecycling process, especially in the process of heating treatment, mercury in fly ash willreleased into the environment again. Therefore, it is necessary to clarify the status of theoccurrence and the release characteristics of mercury in coal fly ash during heating treatmentprocess so to control or reduce mercury emissions in coal fly ash during the heat treatment,which make the theoretical basis for the next step to control mercury emission.The coal fly ash used in his study was collected in Guangzhou Zhujiang Power Plant,which was divided in non-magnetic ash sample and four segment densities ash samples of<2.05g/cm3,2.05-2.10g/cm3,2.10-2.15g/cm3, and>2.15g/cm3by magnetic and gravitationalseparation. Experiments were carried out by using a variety of analytical methods todeterminate physical and chemical properties of fly ash samples, and applying EPA Method3200extracts to determined different forms of mercury in fly ash during heating process. Andthe results showed that the total mercury content in coal fly ash of Guangzhou ZhujiangPower Plant was about0.304μg/g,66.8-77.1%of which was semi-mobile mercury (mainlyHg0), followed by extractable inorganic mercury (mainly Hg2+), accounting for18.5-28.1%,and the non-mobile mercury(mainly Hg2Cl2) is the minimum content of3.2-5.5%. Mercuryof the fly ash was not assigned in the magnetic component. After gravitational separation, thedensity of segments>2.10g/cm3had the highest total mercury content of an average of0.330μg/g, which is mainly attributed to its larger average specific surface than other densitysegments. And statistical correlation analysis also showed a positive correlation between the presence of semi-mobile mercury and surface area of fly ash (correlation coefficientsR=0.915). In addition, there were positive correlations between semi-mobile mercury andCaO content and MnO content in fly ash (RCaO=0.929，RMnO=0.880), while negativecorrelation was present in the average particle size of coal fly ash and Al2O3content (Rparticlesize=-0.911，RAl2O3=-0.818). On the other hand, after magnetic separation, non-magnetic ashsample had a total mercury content of about0.331μg/g. Although fly ash samples of totalmercury changed after gravitational separation, the proportion of extractable inorganicmercury, semi-mobile mercury and non-mobile mercury was still the same. Results of heattreatment show that, in the gravitational separated fly ash samples, three forms of mercuryreleased massively during heating of temperature range300-400oC. When the heatingtemperature rose to500oC, more than92%of total mercury had emitted in the air. Thechanges in the composition of fly ash after separation caused some impact in the mercuryemission, but generally speaking, the release curve of four ash samples are very similar. Andthree forms of mercury maintain a similar releasing ratio of air in the low-temperature heatingprocess. The total mercury of non-magnetic ash released into the air more thoroughly,98%ofwhich had released at heating temperature of400oC. The presence of magnetic componentincreased the conversion temperature of solid to gas phase of mercury and improved thecapture ability of mercury by fly ash. In addition, release and transformation mechanism ofmercury during the heating process can be explained based on the adsorption mechanism ofthe active site and Mars-Maessen reaction mechanism: when the heating treatmenttemperature is low, the mercury relying on physical adsorption force began to escape from flyash surface, which was accounting for a large proportion of the releasing mercury; when thetemperature is high, the chemical adsorption force was destroyed, and thus the mercuryadsorbed on the active site started to escape from fly ash as well, which was accounting for avery small proportion.