Reaction Mechanism Study About Impact Of Bromine On Mercury’s Speciation Transformation In Flue Gas

Mercury is a global pollutant. With the improvement of Man’s awareness ofenvironmental protection, how to take effective mercury control measures in coal-firedpower plants has caused widespread attention. Mercury is very toxic even in very lowconcentration. Among the pollutants emitted by coal-fired power plants, we have only alittle understanding of mercury emission emission behavior. In this paper, anequilibrium analysis was carried out to determine the speciation and distribution ofmercury in flue gas during coal combustion in detail; simultaneously, it is studied thatthe homogeneous reaction mechanism between mercury and bromine-containingsubstances such as HBr and BrO with quantum chemistry methods.An equilibrium analysis was carried out to study the effect of bromine on thespeciation of mercury of high Cl coal and low Cl coal flue gas within the temperaturerange of325K-1125K. The results showed that, with the addition of bromine, theformation of HgCl2（g） was inhibited for high Cl coal, and the formation of both HgCl2（g）and Hg（g） is inhibited for low Cl coal. As the O₂（g） content increased, HgBr₂（g）observably decreased. SO₂（g） hardly changed the speciation of mercury. The chemicalthermal equilibrium analysis results showed that the formation of HgBr₂（g） was mainlydependent on the Br₂（g） content.Quantum chemistry methods were used to calculate the geometry parameters andfrequency of reactants and products. The calculated results were compared with theNIST database in order to determine the most accurate ground state geometries. Thethermodynamic parameters of HgO, HgBr and HgBr₂were obtained from the frequencyanalysis. The results showed that the maximum relative error of enthalpy difference andentropy were3.37%and1.79%, respectively. The results provide a base for studyingreaction mechanism of mercury with HBr, BrO by quantum chemistry.Three mechanism of homogeneous mercury oxidation in the coal flue gas werestudied which supplemented the original mercury oxidation kinetics of reactionmechanism. The method of B3PW91which come from density functional theory wasused to find the transiton state and calculate the energy and pre-exponential factor byfrequency analysis and thermal energy calibration. The transition state theory was usedto calculate the reaction rate constants in the temperature scale of298.15-2000K. Thecalculated kinetic parameters provided a theorecical basis for further study of mercury oxidation kinetic model in the flue gas.