Study On Emissions Of Hg/SO₂/NO_x And Photocatalytic Removal Mechanisms

Combustion has been targeted as one of the most significant anthropogenic sources of pollutants emissions to the atmosphere.Some power plants implemented ultralow emission?ULE?technology to reduce pollutant emissions.Correspondingly,the techniques required to achieve these engineering specifications should be further developed to measure pollutants with high precision and selectivity.Besides,in-depth understanding of the emissions is essential for efficient combustion and the control of gaseous pollutants.SO₂,NO_x and mercury have been identified as hazardous compounds to public health and the environment because of the volatility,persistence,and bioaccumulation in the biosphere.Fortunately,photocatalysis has been proven to be a practical advanced oxidation technique for disposal of mercury.More encouringly,the TiO₂-based photocatalytic technique has great potential to remove SO₂,NO simultaneously.Thus,the emphases of this study were placed on emissions of gaseous pollutants from combustion and the involved mechanisms through photocatalysis technology.The results will provide some theoretical guidance for follow-up studies and applications of this technology.First,this study investigated the reliability of FTIR on the simultaneous measurement of various gases through comparing with the standard method.Detailed and quantitative evolution profiles of the gases were determined through FTIR evaluation.And the focus was on the effects of various parameters on emissions and the relationship between different components.According to the results,N₂O released from DS and La Loma combustion was0.15³.9 g/kg_fuel and 0.19⁴.43 g/kg_fuel,respectively.The N₂O resulted from DS combustion was almost twice more than La Loma,while NO_x was much lower.In addition,both N₂O and NO_x increased with?,while SO₂ increased with reaction temperature.Different from La Loma,the content of mercury was in a range of 30.9 and 35.2?g/m³ under stable combustion of sewage sludge.With the increase of temperature from 640°C to 880°C,the percentage of Hg⁰ decreased by 14.78%.This might be connected with the influences of other flue gas components.Second,simulated flue gas system was set up based on the combustion campaign.With the combination of photocatalysis,the mechanisms involved in Hg⁰ oxidation was clarified.TiO₂ samples exhibit a mixture-type crystalline phase at 400°C.After doping Ce into TiO₂,the rutile diffraction peaks decreased,indicating that Ce doping inhibited the formation of rutile.The lattice spaces of 0.31 nm,0.27 nm and the EDS mapping further demonstrated that CeO₂ nanoparticles were highly dispersed throughout the nanofibers.Hg⁰ removal efficiency increased with the increasing of Ce content in the TiO₂ up to 0.3%,and catalyst with 0.3%Ce showed the highest efficiency under UV irradiation.The superior activity of 0.3%Ce should be correlated with the high surface area and better performance of electron-hole separation.In the atmosphere containing SO₂,both UV and catalyst had adverse effects on Hg⁰ oxidation,that was assigned to more SO₂ reacted with catalyst in the presence of UV.In contrast,UV and catalyst were beneficial for Hg⁰ oxidation with the existence of HCl.Moreover,the average Hg⁰ removal efficiency in the coexistence of SO₂ and NO was better than that in the presence of SO₂ alone.Further results indicated that the competition reaction between SO₂/NO and Hg⁰ for the oxidants was not serious.Subsequently,various TiO₂-based nanofibers were fabricated to elucidate the photocatalytic properties of SO₂ and NO removal.According to the results,SO₂ could not be oxidized without light,and no adsorption or photolysis existed during NO removal.Under optimal condition,the oxidation efficiency of SO₂,NO was 100%and 54.05%,respectively.Some Ti⁴⁺was reduced to Ti³⁺during the process.SO₃^2-was formed because of the fast reaction between SO₂ and H₂O,while a great deal of SO₄^2-was generated relating to the oxidation of SO₃^2-or HSO₃^-by O₂ or NO₂.Moreover,a small amount of NO₂^-and NO₃^-were also formed,which is likely due to the reaction between NO₂ and H₂O.During the simultaneous removal of SO₂,NO and Hg⁰,the performance of TiO₂-based catalysts was better than pure TiO₂.In contrast with SO₂,NO promoted SO₂ as well as Hg⁰ removal.Based on the experimental data,the modified L-H model was employed to simulate the photocatalytic performance.The model fits good with tests and reaction kinetic equations for various catalysts were illustrated accordingly.According to orthogonal analysis,the factors affecting SO₂/NO removal are represented in descending order of influence:initial NO concentration>initial SO₂ concentration>oxygen content>flue gas humidity.