Study On Evolutionary Mechanism Of Polycyclic Aromatic Hydrocarbons In Denitration Unit Of Coal-fired Power Plant

Organic compounds in the environment are harmful to human health.Coal combustion is one of the main sources of organic compounds.Although air pollution control devices?APCDs?are equipped in coal-fired power plants to reduce pollutant emissions,organic pollutant cannot be effectively removed in these devices.The organic pollutant in flue gas can be partially degraded in selevtivity catalytic reduction device?SCR?,but without complete degradation,the concentration of polycyclic aromatic hydrocarbons?PAHs?even increase in the flue gas.In order to reduce the pollution of organic compounds from coal-fired power plants,it is meaningful to study the reaction mechanism of organic pollutants to PAHs in SCR.In this study,the conversion of benzene was studied in simulated SCR unit.It was found that benzene could convert to PAHs in SCR reactor.This phenomenon was more obvious with the increase of temperature.Increasing the oxygen content in flue gas weakened the conversion of benzene to PAHs,while NO and NH₃ promoting the formation of PAHs.Combined with the result of benzene adsorption and conversion on the surface of the conventional SCR catalysts,the polymerization mechanism of benzene to PAHs in SCR was proposed:benzene could react with dicarbon hydrocarbons to form intermediate A.Then,there were two further reaction paths based on intermediate A.Path one was that the second dicarbon hydrocarbons added on the phenyl radical site to form intermediate B,and path two was that the addition of dicarbon hydrocarbons to the radical site on the branch of intermediate A to form intermediate C.Finally,both intermediates B and C could form naphthalene.The branched chain of intermediate C could be captured by the surface of SCR catalyst to form maleate,weakening the polymerization of benzene to PAHs.The increasing temperature inhibited the adsorption of benzene and the formation of maleate,resulting in more naphthalene formed from intermediate C,which aggravated the polymerization of benzene to PAHs in SCR unit.In the flue gas components?O₂,NO,NH₃?,the increase of O₂content promoted the formation of maleate on the catalyst surface,preventing the Path two and reducing the polymerization of benzene to PAHs;NO and NH₃ occupied oxidation site on the surface of catalyst and inhibited the formation of maleate,leading to the further reaction of intermediate C to naphthalene,which aggravated the polymerization of benzene to PAHs in SCR unit.According to the characterization of the conventional SCR catalysts,catalysts were prepared with several active components,and their influence on benzene conversion were studied.Although the conversion of benzene was low on the TiO₂,there were still PAHs found in the tail gas.Few intermediates were observed on the catalyst surface.On V/TiO₂,W/TiO₂ and V-W/TiO₂,the conversion of benzene was enhanced,with lower concentration of PAHs in the tail gas.The formation of maleate influenced the concentration of PAHs in the tail gas.With the highest amount of maleate on W/TiO₂,which could consume the intermediates of PAHs formation,the concentration of PAHs in the tail gas decreased significantly.Therefore,W showed a significant weakening effect on the formation PAHs from benzene.