Kinetics Study Of Homogeneous And Heterogeneous Mercury Oxidation On The Surface Of Unburned Carbon By Halogen In Coal-fired Flue Gas

Mercury has caused great harm to human health because of its unique physical andchemical properties and strong toxicity. The emission of mercury by coal-fired powerplants has been considered as the main part from anthropogenic source of mercuryemissions. Gaseous elemental (Hg0) from coal-fired flue gas is the most difficult to controldue to its highly volatile and water insoluble. Converting Hg0to its oxidized form (Hg2+)can effectively control mercury emissions from coal-fired power plants. Halogens caneffectively oxidize Hg0. Currently the study of reaction mechanism mainly focus onhomogeneous oxidation and heterogeneous oxidation mechanism is unclear. Therefore, acomprehensive study of kinetic mechanism of both homogeneous and heterogeneousmercury oxidation is very important for controlling mercury emissions from coal-firedpower plant.With the theory of chemical kinetics, we develop a multiphase oxidation model ofhomogeneous mercury oxidation by halogens which relates to eight element C、H、O、N、S、Cl、Br、Hg and a model of heterogeneous mercury oxidation by halogens which relatesto unburned carbon (UBC) particles in flue gas. Using the multiphase oxidation model topredict transformation behavior of trace element Hg in coal combustion flue gas usingCHENKIN-PRO software in contrast to the available experimental data, to describingtransformation behavior more properly and comprehensively.We studied the homogeneous and heterogeneous mercury oxidation kinetics on thesurface of UBC particles by chloride in coal-fired flue gas. It can be found throughcomparison that the homogeneous oxidation model results have a large deviation withexperimental data. However, multiphase oxidation model results have a good agreementwith experimental data under the same conditions. With sensibility analysis ofheterogeneous oxidation model, the key reaction channel for heterogeneous mercuryoxidation by chloride was found. The effects of flue gas components on mercury oxidation by chloride are as follows: The increasing of O2concentration promote the oxidation of mercury.The increasing of H2O、NO and SO2concentration inhibit mercury oxidation. The diameter ofUBC has a great effect on mercury oxidation rate and the mercury oxidation rate issignificantly decreased with the diameter increasing.The homogeneous and heterogeneous mercury oxidation kinetics by bromide wasstudied. We developed a multiphase mercury oxidation model by bromide. It is also foundthat multiphase oxidation model results have a good agreement with experimental data.The key reaction channel for heterogeneous mercury oxidation by bromide was found withsensibility analysis of heterogeneous oxidation model. The increasing of O2concentrationpromote the oxidation of mercury; NO and SO2inhibits the oxidation of mercury. Themercury oxidation rate is significantly decreased with the UBC diameter increasing.We developed a multiphase mercury oxidation model by chloride and bromide. It canbe concluded that bromide shows stronger mercury oxidation ability than chloride bycomparison with chlorine and bromine oxidation model separately. Under homogeneousoxidation model, with a fixed concentration of bromide, the mercury oxidation rate curve isdecreased with the increasing of chloride concentration. Under multiphase oxidation model,the mercury oxidation rate curve is increased with the increasing of chloride concentration;The percentage of HgBr2decrease, however, the percentage of HgCl2increase. Withsensibility analysis of heterogeneous oxidation model by chloride and bromide, it is foundthat the key reaction channel for heterogeneous mercury oxidation by chloride and bromidein flue gas.The effects of flue gas components on mercury oxidation are: SO2、H2O inhibit theoxidation of mercury; O2promote the oxidation of mercury; The mercury oxidation rate isdecreased with the UBC diameter increasing, but, the decline is relatively smaller. Duringoxy-fuel combustion, the combustion atmosphere have little effect on mercury oxidation;During oxy-fuel combustion, with the increasing of O2, there is a slight increase in the rate ofmultiphase oxidation of mercury. H2O has a relatively large impact on mercury oxidation and inhibits the oxidation of mercury.