Experimental And Mechanism Investigation On Ozonation Coupled Catalytic Oxidation For Removal Of No_x And Cl-VOCs In Flue Gas

With the ultra-low emission transformation of coal-fired boilers in large power stations approaching the end,the pollutant emissions of small and medium-sized industrial boilers,kilns and non-electric industries are becoming more and more obvious.Specifically,nitrogen oxides（NO_x）and volatile organic compounds（VOCs）will promote the generation of particulate matter,organic secondary aerosols and near-ground ozone.In particular,chlorine-containing volatile organic compounds（Cl-VOCs）have stable chemical properties and strong biological toxicity,possibly inducing the formation of high carcinogens such as dioxins.Collaborative control will be the future trend.Ozone synergistic-oxidation technology has the advantages of strong adaptability,good low-temperature performance,high removal efficiency.It is an ideal removal scheme and has been applied in many industrial projects.In order to improve the economy and adaptability of ozone oxidation technology,this paper mainly carried out the following work:1).A method for pre-oxidation of NO to NO₂ via residual oxygen in flue gas was proposed.The influence mechanism of alkali metal K,H₂O,and SO₂ on the oxidative intermediates was also revealed.In addition,the tolerance properties of alkali metal,sulfur,and water of Mn-Ce O_xcatalysts were enhanced by Ce doping.The characterization results showed that potassium converted the strong acid sites into medium/weak acid sites,which promoted surface oxygen vacancies,chemisorbed oxygen,surface adsorbed water/hydroxyl and easily reducible Mn-Ce O_xspecies.However,excess potassium would reduce the specific surface area and redox performance,resulting in deactivation.Therefore,the influence of K exhibited a loading-dependent duality.Further,the key intermediates of NO oxidation were further discovered as well as the duality mechanism of potassium,i.e.,the appropriate K could inhibit the formation of low-reactivity monodentate nitrates,while excessive K promoted the direct binding of NO to active sites and generated stable low-reactivity nitrates.Furthermore,competitive adsorption of H₂O caused slightly reversible deactivation.SO₂ induced stable sulfate/sulfite species on the catalyst,causing irreversible sulfur poisoning,and the effect of potassium on sulfur poisoning was also two-sided.2).Dichloromethane（DCM）was selected as the typical Cl-VOCs,and the catalytic oxidation of DCM by O₂ was carried out over Mn/H-ZSM-5 catalyst.The effects of reaction temperature,surface acidity,and H₂O/SO₂ on chlorine-containing by-products were also revealed.The HCl selectivity at high temperature was closely related to DCM degradation activity and catalyst acidity.Too high or too low acidity was not conducive to the generation of HCl.The chlorinated by-products in incomplete oxidation at high temperature were mainly CH₃Cl,CHCl₃,and CCl₄,while at low temperature,a small amount of more toxic by-products such as benzyl chloride and chlorobenzene were detected,indicating the poor degradation.In addition,H₂O molecules promoted the generation of hydroxyl species and improved the HCl selectivity.SO₂ led to a slight decrease in HCl/Cl₂ selectivity and promoted the generation of chlorinated organic by-products.3).The individual and simultaneous ozonation of NO,DCM were carried out.The effects of molar ratio,catalyst acidity and other factors on the key intermediates of NO/DCM ozonation were investigated,and the interaction mechanism between NO_x and DCM in the co-ozonation was also revealed.For individual ozonation of NO_x/DCM,the process of NO oxidation and NO₂ deep ozonation were mainly affected by the O₃/NO molar ratio.DCM degradation activity and HCl selectivity of the weakly acidic samples were higher.For co-ozonation of NO_x/DCM,DCM had no effect on the NO primary oxidation,but exhibited an inhibitory effect on the NO₂ deep oxidation.O₃ preferentially oxidized NO/NO₂ at O₃/DCM ratio<1.7,and the DCM degradation was“shielded”and hardly occurred.Further,NO₂ and N₂O₅ could promote the conversion of the key intermediates in DCM ozonation,i.e.,aldehydes and carboxylates species,corresponding to the“synergistic effect”.Compared with catalytic oxidation,there were fewer macromolecular chlorinated by-products in ozonation process.Moreover,H₂O molecules generated more surface-active hydroxyl groups under ozone interference,which improved the HCl selectivity and degradation activity.Besides,O₃ molecule protected the active component from being directly reacted by SO2.For ozone decomposition,cactus-like Mn O₂-IV exhibited the highest de O₃activity and the excellent tolerance to H₂O/SO₂,which maintained over 88%de O₃ efficiency in the high-humidity and sulfur-containing conditions.Further,the detailed interference mechanism of O₂/O₃/H₂O/SO₂ molecules on Mn O₂-IV were revealed,and a novel mechanism to decompose O₃ via surface-active hydroxyl/intermediates were established.4).The“SNCR+SCR+Ozonation”route was applied in the 2×200MW coal-fired boiler to achieve ultra-low emission.This technical route has strong load adaptability and high denitration efficiency,which saves～50%investment cost and gets good environmental protection benefits.Further,in the combined oxidation test of"oxygen primary oxidation+ozone deep oxidation+wet absorption",Mn Co Ce O_x catalyst was used to pre-oxidize NO and replaced the ozone dosage of 0.7～0.8 molar ratio.The nitrogen balance error was 6～8%,achieving a high removal efficiency.It indicated that using pre-oxidation could significantly save investment and operation costs.