Experimental And Mechanism Investigation On Catalytic Removal Of NO_x And VOCs Over Mn- Based Catalysts

Human activities and industrial process would emit NO_x,SO₂,VOCs and PM,which could lead to haze,photochemical smog and acid rain after a series of atmospheric chemical reactions.Due to the“Ultra-Low Emissions”retrofit on coal-fired power plant,the total emission pollutants are all reduced and the environmental issues are getting better.However,Ministry of Environmental Protection reported in“Bulletin on the state of China's ecological environment of2018”that ground-level ozone was the only one of the air pollutants that increased from 2017 to2018.Because NO₂ photolysis reaction was accelerated by VOCs in atmosphere,the ground-level ozone quickly generated.Ground-level ozone as the new air pollutant directly endangers human health.At present,the environmental pollution issues are still severe,so it is essential to carry out the work of NO_x and VOCs treatment.Simultaneous removal of multi-pollutants by ozone oxidation technology can treat complex and low temperature flue gas,thereby achieving great success in industrial application.But any technologies that standstill will be knocked out.Therefore,our group mumbers continuously try to improve this technology these years,and my work were:?a?Reduce ozone dosage,using O₂ in the flue gas to oxidize NO to NO₂;?b?Widen application fields,achieving VOCs degradation under industrial related conditions.Transition metal?Cu,Fe,Ce,Co?was used as loading metal to improve LaMnO3+?perovskites NO oxidation activity and SO₂ resistance.Results show that substitution of La was more favorable than Mn,and Co loading obtained the best NO oxidation efficiency.The best LaCoMnO catalyst was further treated by HNO₃,leading to a loose skeletal structure and better catalytic acitivity.SO₂ poisoning was the main obstacle for oxidizing catalyst application,and there are still no ideal methods to improve the catalyst tolerance for high level SO₂.This paper mainly focused on the flue gas that has low SO₂ initial concentration,for example,biomass boiler flue gas.Results show that the NO conversion efficiency of LaCoMnO-A maintained stable even when SO₂ concentration increased to 80 ppm.However,the SO₂ poisoning appeared as 100 ppm SO₂ was injected.Therefore,it is feasible to achieve low concentration resistance,which provides a new field for oxidizing catalyst application.The effect of MnO₂ catalyst morphology on NO oxidation activity was investigated.H-MnO₂has hollow sphere structure and MnO₂-R has rod-like structure,both achieving better NO conversion efficiency than the amorphous MnO₂ catalyst?MnO₂-C?.In addition,Fe loading on H-MnO₂ abtained further improvement on NO oxidation conversion.In the SO₂ tolerance experiment,Fe and Ce loading on MnO₂-R can protect the Mn⁴⁺from being reduced by SO₂,thereby enhancing the SO₂ resistance.The mechanisms of NO oxidation over H-MnO₂ and MnO₂-R were summarized by in-situ DRIFTS measurement,and the NO?^{M n9}+)NO₂^{-O 2}?NO₂ was considered as the main NO oxidation pathway for these two catalysts.Because toluene is one of the most common VOCs that has relatively higher concentration in waste gas,it was selected as the target VOCs in this paper.The toluene degradation efficiency was only⁷%for the homogeneous reaction with O₃.It is necessary to use catalyst to increase the toluene degradation efficiency.Moreover,because VOCs and normal air pollutants were co-existing in flue gas,it is essential to investigate their interaction effect in the simultaneous removal process.This paper mainly focused on two aspects for toluene degradation:?a?Conduct toluene catalytic oxidation by ozone under industrial related conditions;?b?Investigate the interaction effect of NO oxidation and toluene degradation process.Manganese was supported on?-Al₂O₃,SiO₂ and TiO₂ to test toluene oxidation using ozone at 120 ^oC.MnO_x/?-Al₂O₃ obtained nearly 100%toluene degradation efficiency without O₃residual.Then it was selected to study the effect of reaction temperature,O₃/C₇H₈ molar ratio and C₇H₈ initial concentration.Results show that the optimal reaction temperature was 100¹20 ^oC and the C₇H₈ initial concentration did little effect on toluene degradation efficiency.In the SO₂and water vapor tolerance experiment,SO₂ and water vapor could both lead to the catalyst poisoning,but the toluene degradation efficiency could recover as SO₂ and water vapor were stopped.MnO_x/?-Al₂O₃ mantained at⁸0%toluene degradation efficiency even with 200 ppm and 3.6 vol.%water vapor,which gives a guidance on the industrial application.?-,?-,?-,?-MnO₂ catalysts were used to investigate the interaction effect of NO oxidation and toluene degradation.NO addition just slightly raised up the toluene degradation temperature for 10²0 ^oC without decreasing its value.Toluene strongly affected NO conversion,which directly banned the NO oxidation at low temperature.However,this inhibition effect became much weaker at higher temperature,and NO conversion efficiency gradually recovered to the equivalent value as if without toluene addition.Results demonstrated that NO oxidation initialized temperature approached to the temperature of equilibrium in toluene degradation.Therefore,promoting toluene catalytic activity at low temperature could advance NO initialized temperature,thereby guaranteeing desirable NO conversion and toluene degradation efficiency.At last,this paper introduced an industrial application case using simultaneous removal of multi-pollutants by ozone oxidation technology on a biomass boiler.Because the alkaline metal in the flue gas would lead to SCR catalyst deactivation and there is no suitable temperature window for SCR catalyst,the simultaneous removal of multi-pollutants by ozone oxidation technology is the best option for this biomass flue gas treatment.The final NO_x emission can be controlled below 15 mg/Nm³,and there is no O₃ residual as the O₃/NO<2.1.