Study On Flue Gas Denitration: Catalytic Oxidation Of NO Over Mn-based Catalysts And Biological Integrated Method

Because emission of nitrogen oxides can result in a series of human health and environmental problems,it has attracted increasing attentions from countries around the world.However,China's energy structure determines that coal will still be the dominating energy in our country for a quite long time in the future.Hence,it becomes more important to control the nitrogen oxides emission from coal combustion flue gas.Through analyzing the current removal technologies for nitrogen oxides,the catalytic oxidation of NO plays a vital role in both wet processes for denitration and dry methods for denitration.High cost of gas oxidants or liquid oxidants limits their practical industry applications,but NO oxidation catalysts have the distinct advantage at the price.Therefore,the successful development of NO oxidation catalysts can not only improve the existing dry methods for denitration but also make large-scale industrialization of wet methods for denitration possible.Mn-based catalysts exhibits good catalytic activity among the numerous non-noble metal catalysts of NO oxidation,but the current reported Mn-based catalysts usually doped with different metals are all mixed phases.Although Mn⁴⁺performs better catalytic activity than Mn³⁺,the best catalytic activity in these catalysts has not been understood.Moreover,the optimal temperature range for NO oxidation catalysts is very narrow,and usually exceeds 250°C.Apart from these,the research on catalytic oxidation mechanisms of NO are mainly focused on noble metal catalysts,but the oxidation mechanisms of NO over non-noble metal catalysts are still few.Above the current conditions,the NO catalytic oxidation activity over?-,?-,?-and?-MnO₂ were systematically investigated.And on this basis,the?-MnO₂ performed the best catalytic activity was selected to further improve its low-temperature catalytic activity and extend its active temperature window through modifying the synthetic method and adding surfactants.Furthermore,both the catalytic oxidation mechanism and the SO₂ inhibitory mechanism of the best catalyst were also systematically investigated by in-situ diffuse reflectance infrared Fourier transform spectroscopy?DRIFTS?.Whether the following treatment method is liquid-phase absorption–biological integrated method,or direct biological method after NO oxidation,the core part of these methods is biodegradation of nitrite and nitrate as the catalytic oxidation of NO to NO₂ can well solve the low mass transfer efficiency of NO.Therefore,an aerobic denitrifying bacterium Y1obtained by our research group was chosen to optimize its nitrite and nitrate degradation performance,and then the influences of absorbents on its degradation were investigated.Finally,the nitrogen removal characteristics of the strain were analyzed in the simulated absorption solution.These results will provide important experimental references for building a new selective catalytic oxidation absorption–biological reduction integrated method to remove NO_X.The main research conclusions are as follows:?1?The?-,?-,?-and?-MnO₂ catalysts were synthesized by a hydrothermal method,and their conversion–temperature curves were typical volcanic type curves.In terms of both low temperatures and high temperatures,?-MnO₂exhibited better activity than the other three catalysts,and the maximum conversion of NO over?-MnO₂ achieved 91%at around 250°C with a mass space velocity of 48 000 mL g^-1 h^-1.The four catalysts showed stable catalytic activity as there were no noticeable decrease in NO conversion within 30 h at their optimal temperature of 275,275,250 and 325°C,respectively.Besides,?-Mn O₂ behaved efficiently in wider ranges of O₂ concentrations and flow rates.?2?The activity of the four catalysts was related to the specific surface area,the mobility of oxygen species,surface adsorbed oxygen and tunnel structures of MnO₂.Among these factors,the tunnel structure and surface chemisorbed oxygen might play a more important role in NO oxidation reaction.Because?-Mn O₂ possessed a randomly intergrown structure of ramsdellite and pyrolusite,this distinctive tunnel structures could lead to more vacancies and point defects,and higher amount of chemisorbed oxygen.?3?On the basis of determining the best crystal structure of?-MnO₂,the PEG-modified Mn-based catalysts were prepared by the precipitation method.The crystal forms of the three catalysts,which were calcined at 200,300 and400°C,respectively,were all?-MnO₂,and there was no significant difference in their catalytic activity.Moreover,the catalytic activities of the three catalysts were much better than that of the other catalysts.With a mass space velocity of48 000 mL g^-1 h^-1,the highest NO conversion reached 93%at the reaction temperature of 220°C,and the NO conversion was still above 50%as the reaction temperature is as low as 130°C.In addition,NO conversion of the three catalysts kept above 50%in a temperature span of about 250°C?from 130°C to380°C?.The catalyst exhibited the best activities among the non-noble metal based catalysts reported in the literatures as far as we know.Moreover,they demonstrated comparable activity to or better activity than some noble based catalysts.The catalyst calclined at 300°C displayed good stability,and exhibited a good adaptability in a wide range of O₂ concentrations and flow rates.?4?PEG could not change the crystal structure of the catalysts,but it could increase the specific surface area,average pore diameter,and pore volume.The SEM results further demonstrated that adding PEG could alter the surface morphology of the catalysts.Moreover,the addition of PEG could not only improve the redox properties of the catalysts,but also enhance the adsorption of O₂ and NO at the catalysts'surface.At the same time,the results also indicated that the catalytic activity of the catalysts had a positive correlation with the amount of surface chemically adsorbed oxygen,and adding PEG could increase the amount of chemically adsorbed oxygen on the catalysts'surface.?5?When NO was introduced into the reaction cell,three kinds of nitrate species,which were monodentate nitrate,bidentate nitrate and bridged nitrate,were formed on the surface of the catalyst.And these nitrate species could still be generated when there was no O₂ existence in the reaction gases.The above results indicated that the chemisorbed oxygen or lattice oxygen in the catalyst could react with NO,and the consumed chemisorbed oxygen or lattice oxygen was supplied by the gas O₂.Whether at low temperature or at high temperature,the nitrate species could be formed on the catalysts'surface.However,different nitrate species had different thermal stabilities.In the three nitrates species,the monodentate nitrate,which was directly related to the catalytic activity of the catalysts,had the worst thermal stability.The hydroxyl groups could be consumed in the NO catalytic oxidation process,but their consumptions were irreversible,implying that hydroxyl was not the main factor influencing the catalytic oxidation of NO.In summary,the possible mechanism for NO oxidation over the catalyst was as follows:First,NO adsorbed on the surface of the catalyst to form nitrosyls.Second,the adsorbed nitrosyls could be oxidized to nitrates species by the chemisorbed oxygen or lattice oxygen.Third,the nitrates generated at higher temperature became decomposed.Finally,NO₂,which was due to the decomposition of nitrates,was released from the catalyst.?6?Whether?-MnO₂ was synthesized by the hydrothermal method or by the PEG modified precipitation method,both H₂O and SO₂ could result in the deactivation of the catalyst.The catalyst deactivation caused by H₂O was reversible,while SO₂ resulted in an irreversible inactivation of the catalyst.Compared with the results of SO₂,the service life of the catalyst was prolonged when H₂O and SO₂ were co-existed.The inhibitory mechanism of SO₂ on the?-Mn O₂ catalyst was as follows:SO₂ was easily adsorbed on the surface of the catalyst to form sulfates.These stable sulfates could consume the catalyst's active sites to further hinder the formation of nitrates,leading to an irreversibly deactivation of?-MnO₂.?7?Strain Y1 had the same optimal experimental conditions for removing nitrite or nitrate:sodium citrate as the carbon source,C/N=16,30°C,pH=7,and 120 r/min.Therefore,the strain's optimal degradation conditions for nitrogen whether it was nitrite-N or nitrate-N in liquid phase,would not change.?8?Strain Y1 could grow and remove nitrite and nitrate in the culture solutions containing NaHCO₃ or Na₂SO₃,but the inhibitory effect of Na₂SO₃was much higher than that of NaHCO₃ on the growth and biodegradability of strain Y1.Although NaHCO₃ had no difference in degrading nitrite and nitrate,Na₂SO₃ had more significantly inhibitory influences on nitrite removal than on nitrate removal.