Study On NO_x Removal From Flue Gas By Ozone Assisted Low Temperature SCR Technology

Nitrogen oxides（NO_x）,as the main flue gas pollutants,emitting into the atmosphere can seriously harm the environment and human health.The traditional selective catalytic reduction technology（SCR）operates stably in the middle and high temperature range and has high NO_x removal efficiency.However,the SCR technology has gradually exposed the problems of narrow temperature window（300⁴20℃）and environmental pollution caused by ammonia leakage.Moreover,part of the flue gas does not have the traditional SCR working conditions,which results in the NO_x removal effect not meeting the treatment requirements.Therefore,It is urgent to explore low temperature SCR technology to remove NO_x of flue gas.In this paper,the research of ozone assisted low temperature SCR technology was carried out,aiming to efficiently remove NO_x from flue gas.First of all,the study of NO oxidation by O₃ was carried out to make the molar ratio of NO/NO₂ to 1:1 to provide conditions for fast SCR.Secondly,the position of ozone injection was optimized to improve the NO_x removal efficiency of flue gas under low temperature conditions.Finally,the low emperature SCR catalysts were prepared to study the effect and mechanism of NO_x removal in ozone assisted SCR system.The research results are as follows:（1）The influence factors of NO oxidation by ozone,including ozone residence time,reaction temperature,ozone injection volume were studied and the oxidation products of NO were also analyzed by FTIR.The results showed that the oxidation efficiency of NO by ozone increased with the increase of treatment time and O₃/NO molar ratio,but it decreased with the increase of flue gas temperature.When the reaction time was 2 s and the O₃/NO molar ratio was 0.5,the NO oxidation efficiency reached 50%.While the NO/NO₂ molar ratio was 1,and no other high valent NO_x products were detected.The NO oxidation efficiency droped significantly when the temperature exceeded 200℃.（2）The CeO₂ catalyst was prepared by precipitation method and its activity was evaluated.Then,the effects of different O₃/NO molar ratio,ozone injection position,water vapor and SO₂on NO_x removal in the ozone assisted SCR system efficiency at low temperature were investigated.The experimental results showed that compared with NH₃-SCR,ozone assisted SCR exhibited better NO_x removal efficiency at low temperature.The NO_x removal efficiency in the ozone assisted SCR system increased with the increase of the O₃/NO molar ratio.When the O₃/NO molar ratio was 0.5,the molar ratio of NO to NO₂ reached 1,and the NO_x removal efficiency was the highest.In addition,the ozone was injected before ammonia（O₃-NH₃-SCR）had a higher NO_x removal efficiency than that after ammonia（NH₃-O₃-SCR）.This was because that part of O₃ could react with NH₃ to produce ammonium nitrate in NH₃-O₃-SCR system,resulting in the decrease of NH₃ concentration,finally,the NO_x removal efficiency was decreased.Compared with NH₃-SCR system,the conversion efficiency of NO_x in NH₃-O₃-SCR and O₃-NH₃-SCR system was increased by 25.16%and 31.41%at the same temperature of250℃,respectively.The presence of water vapor and SO₂ had an inhibitory effect on the removal of NO_x.However,compared with the NH₃-SCR,the inhibitory effect of water vapor and SO₂ on the removal of NO_x in the O₃-NH₃-SCR reaction was significantly reduced.（3）The effects of different molar ratio of Ce and W on the activity of CeO₂-WO₃ catalysts were investigated.The results showed that incorporation of W into CeO₂ catalyst not only increased the amount of active sites on the catalyst surface,but also improved the redox performance of the catalyst.When the Ce/W molar ratio was 3:1,at the reaction temperature range of 200⁴00℃and the GHSV of 25000 h^-1,the NO_x removal efficiency of Ce3W1 catalyst reached 90%¹00%.In addition,the effects of water vapor and SO₂ on the removal of NO_x by CeO₂-WO₃ catalysts in the ozone assisted system were also investigated.Compared with NH₃-SCR,water vapor and SO₂ had less inhibitory effect on Ce3W1 catalytic activity in ozone assisted SCR system.SO₂ had no effect on the NO oxidation process,and the inhibitory effect of SO₂ on the denitration could be attributed to the formation of sulfate and the sulfation of the metal active sites on the surface of the catalyst,which caused the clogging of the catalyst pores and the decrease of the active sites,resulting in a decrease of catalytic activity.However,it was noted that compared with the NO_x removal process in the traditional SCR system,the catalyst showed better resistance to SO₂ poisoning in the O₃-NH₃-SCR system.Therefore,the O₃-NH₃-SCR system had the advantage of prolonging the service life of the catalyst.