Experimental And Mechanism Study On The Mercury, Arsenic And Selenium Transformation And Emission Control During Coal Combustion

Trace elements emitted from coal combustion have become an increasingly important environmental concern due to its potential threats to human health, global agricultural and social sustainability. Among the toxic metals of interest are most volatile trace elements, Which could not be removed in polluted control equipments and produce direct atmosphere emission. In this work, we focused on describing the evaporation, transformation and partition behavior of mercury, arsenic and selenium during coal combustion with experimental and computational methods. Because of their low concentration and high volatility, it is very difficult to determinate mercury, arsenic, selenium in coal quickly. A microwave technique for digesting coal and coal ash samples was developed and evaluated for quantifying low levels of Hg, As, Se by cold vapor atomic fluorescence spectroscopy and HG-ICP-AES. Results showed that the method meets three criteria: firstly, to digest all sample material completely and consistently, secondly, to reduce considerably the digestion time, thirdly, to maintain a low analytical blank. The modes of occurrence of an element are important factors used in anticipating the behaviour of the element during coal cleaning and combustion, as well as during weathering and leaching of the coal. The mode of occurrence of trace mercury, arsenic and selenium were investigated using sequential chemical extraction procedure. It was found that mercury and arsenic in coal mainly occurs in bound-sulfide and residue. There are a variety of occurrence modes of selenium in different coals. Three form of sulphur in the coals were determined. The distribution of mercury and arsenic in six parts of different densities of the coal were also investigated. The result showed that the concentration of mercury and arsenic are tended to enrich in high densities fractions. But the mass distributions of elements in the coals are dispersive. The behaviour of mercury during coal combustion was investigated in different temperatures in bench scale. The attention was also focused on the characteristic of mercury transformation and the mercury speciation in flue gas during combustion in a large-scale utility boiler. Coal, slag, and fly ashes were sampled from a 300MW utility boiler. Ontario-Hydro method was applied to determine the mercury speciation in flue gas. The experimental data indicate that the majority of mercury goes into flue gas. The content of Hg2+ in flue gas is about 55%-69% and the content of Hg0 is about 31-45%. Potential chemical reaction mechanisms involved were proposed. In addition, the absorption mechanism of mercury in fly ash was studied. It was found that the mercury concentration in fly ash is independent of the particle size and has positive correlation with the LOI of fly ash. The behaviour of arsenic and selenium during coal combustion was investigated in different temperatures in bench scale. The result of chemical thermodynamic equilibrium calculation and that of experiment data were compared. An emissions study for arsenic and selenium was conducted at a 300-MW coal-fired plant equipped with an electrostatic precipitator. Gaseous arsenic and selenium were sampled using EPA method 29. The modes of occurrence of arsenic in ash samples were studied using a modified sequential chemical extraction method. There are no appreciable differences in the arsenic solubility fractions between the different fly ash particle sizes. Four probable reaction mechanisms were discussed: arsenic is dissolved by silicate melting mass, reacts with some chemical components in fly ash and produces stable substance, adsorbed by fly ash and condensed on the surface of fly ash. The adsorption of mercury and mercury chloride on a CaO(001) surface were investigated by the density functional theory(DFT) by using Ca9O9 cluster embedded in an electrostatic field represented by 178 point charges at the crystal CaO lattice positions. The present calculations show that CaO injection could substantially reducing gaseous mercury chloride, but have no apparent effect on the mercury, which is compatible with the available experimental results. The research method will provide the valuable information for the optimizing and selecting sorbent of the trace element in flue gas.