Experimental Study On The Coordinated Removal Of Multiple Pollutants With Catalytic Materials

Nitrogen oxides（NO_x）and mercury（Hg）,as one of the major atmospheric pollutants from coal combustion,,have significant hazards to the ecology and human.Coal-fired power plants have been the largest anthropogenic source.Therefore,it is significant to limit the emissions from the coal-fired power plants.The SCR catalysts can maintain high denitrification efficiency and oxidize Hg⁰ to Hg²⁺which is easy to be removed by WFGD.However,The existing SCR unit is in the environment of high temperature,high ash and high sulfur,which easily leads to catalyst poisoning,failure and loss.At the same time,the oxidation process of Hg⁰ is also inhibited Therefore,it is of great market value and academic significance to realize the coordinated removal of multiple pollutants with catalytic filter materials,which take dust filter bag as carrier,loading low temperature catalyst.Firstly,a certain properties of polyphenylene sulfide（PPS）and polyimide（P84）were tested,and based on the single cycle loading and catalytic activity,catalytic filter carrier was pselected for subsequent exeriments.Then,the catalytic activity of three preparation methods was investigated.At the same time,the optimal values of such parameters as carrier pretreatment,titration procedure,bathing times and active loading were also clarified.The results show that compared with PPS,the NO_x and Hg⁰conversions of catalytic P84 were significantly higher,the increase rate reached about15.4%and 13.1%respectively.Besides,the saturation loading of P84 was 0.65,which means that P84 only needs 1.12 preparation cycles on average.Mn-Ce-Fe-Co-O_x/P84prepared by the in-situ method exhibited higher conversion of NO_x and Hg⁰ than the impregnation calcination or coating methods.it has significantly acceleration for the adsorption of Mn²⁺and Ce²⁺of the pretreatment of P84,and particularly,the activation modification performed best.Meanwhile,the bathing times and titration procedure also showed an great influence on catalytic activity,which may related to the residual concentration of KMnO₄ on P84.Secondly,MnO_x,Ce O_x,CoO_x and Fe₂O₃ were loaded on P84 carrier to support the transient experiments of the reaction conditions based on the best preparation parameters.The characteristics of Mn-Ce-Fe-Co-O_x/P84-ISM such as optimum catalytic temperature,minimum oxygen demand,applicable NO_x concentration,optimal ammonia-nitrogen ratio,resistance to SO₂ and H₂O were investigated.The results find that the optimum temperature of catalytic P84 prepared by ISM was about170°C,where the chemical oxidation rate of element mercury was maximized compared with other temperatures.And the minimum oxygen demand for catalytic reaction was about 5%,the concentration of O_αand O_βwere correlated with NO_xreduction and Hg⁰ oxidation respectively.NO_x could reacte with Hg⁰ to form Hg O and Hg_x（NO₃）₂（x equals 1 or 2）in a heterogeneous way.and NH₃ also perform compete adsorption with Hg⁰ on Lewis and Bronsted acid sites.Low-concentration SO₂ would be oxidized with Hg⁰ to generate Hg O and Hg SO₄ to enhance the mercury removal at low temperature,but the inhibitory effect on the denitration was still obvious.H₂O molecules would block reaction pores and release hydroxyl（OH-）,which facilitated the reduction of Hg²⁺to Hg⁰ effectively.Furtherly,SO₂ and H₂O all have restraint on co-removal of NO and Hg individually,especially,it was worse when they exists together,where it only reduced 44.2%NO_x and oxidized 54.8%Hg⁰ in the conditions of 75 ppm SO₂ and 8%H₂O.Thirdly,the optimal Mn-Ce-Fe-Co-O_x/P84-ISM catalyst was screened,then the fracture strength and physicochemical properties were evaluated by tensile-micro universal testing and BET,SEM,EDX and so on.The results reveal that the breaking strength and elongation of P84 modified by ISM were significantly reduced,and the hydrophobic property was also lower,which was beneficial for the subsequent loading of such catalyst as poorly adsorbed and less added.In addition,the oxides were uniformly distributed on the catalyst in a filamentous structure.It appeared four groups of H₂ reduction peaks in TPR experiment of the catalyst at 160～490℃,which indicated obvious low-reduction characteristics.The results of TPD desorption indicated that the adsorption active sites of NO_x and Hg⁰ on Mn-Ce-Fe-Co-O_x catalyst mainly acted in the form of weak acid and base sites,thereinto,NO_x was mainly in the form of-NO_y（y=2 or 3）in the medium-basic sites.And the decomposition weight loss ratio was also lower than 6.56%at 300～600℃,which exhibited a great temperature resistance.Mn-Ce O_x on catalytic P84 material was mainly copper with+4 oxidation state,which ensures the high low-temperature conversion of NO_x and Hg⁰.Besides,Co³⁺and Fe³⁺could also promote the low-temperature regeneration from Mn³⁺to Mn⁴⁺effectively.Finally,based on the internal catalytic composition and environmental conditions,the sulfur-resistant and water-resistant performance of the selected Mn-Ce-Fe-Co-O_x/P84-ISM catalyst was optimized.The influence of Fe and Co molar ratio,loading sequence,precursor and reaction temperature,NO₂ and HCl concentration were investigated.The results indicate that Fe O_x and CoO_x were benefit for the improvement of the resistance of SO₂+H₂O,but overload was just the opposite.Among them,the active Fe O_x especially perform better.The mixed loading of Fe promoted the enrichment of Mn-Ce O.the catalytic activity of Fe（NO₃）₃ as precursor was significantly higher than Fe₂O₃,which may benefit from the concerted reaction between Fe³⁺and Mn²⁺,Ce²⁺.As the ammonium sulfate decompose,NO_x and Hg⁰ conversions increased with the reaction temperature from 170°C to 260°C.Meanwhile,NO₂ increased the proportion of Fast SCR and eliminate deposited ammonium sulfate,while HCl promoted the oxidation of Hg⁰ to Hg Cl₂,therefore,low-concentration HCl and NO₂ all showed a promoting effect.