| China’s coal-based primary energy structure will last for a long time,and the nitrogen oxide（NOx）emissions generated by coal combustion will also become a long-term environmental protection issue.NH3 selective catalytic reduction（NH3-SCR）is the most widely used denitration technology of middle and high temperature flue gas,and the performance of catalyst is the key to this technology.At present,the thermal power industry has achieved low emissions,effective control of NOx in flue gas,and NOx from the sintering flue gas at low temperature for iron and steel industry has become a most urgent need of emission reduction for atmospheric pollutants.Therefore,the development of denitration catalysts suitable for low-temperature sintering flue gas is of great significance to the green and sustainable development of China’s iron and steel industry.In this work,the influence of valence state and crystal type of Mn Ox on NH3-SCR denitration reaction was systematically studied at low temperature,as well as the influence of Ce O2 doping on the denitration performance of Mn O2 with different crystal types.On this basis,Mn-Ce/AC catalyst was prepared with activated coke（AC）as the support to study the denitration performance of NH3-SCR.Mn-Fe/AC and Mn-Ce-Fe/AC catalysts were prepared by doping iron oxide,and the influence of iron modification on the low-temperature activity,N2 selectivity and the resistance to sulfur and water poisoning of the catalysts were studied.On this basis,the SCR denitration performance of Mn-Ce/AC catalyst under the condition of actual sintering flue gas was studied.Firstly,different valence state of Mn Ox and different crystal of Mn O2 were prepared by hydrothermal method,and the SCR reaction gas in different form Mn Oxcatalyst surface adsorption and reaction characteristics were analyzed at low temperature.The results showed that with the increase of reaction temperature,N2O production amount from different valence of Mn Ox catalyst increased,and N2O generated directly by the NH3 oxidation also increased.NH3 species adsorbed on Mn O2were more easily oxidized by O2 directly.O2 promoted the conversion of ammonia to ammonium ions adsorbed at Lewis acid sites on Mn2O3.For different crystal types of Mn O2,NH3 species adsorbed onα-Mn O2 had high reactivity.The reaction activity of NH3 adsorbed onβ-Mn O2 with O2 was poor.NH3 species adsorbed onγ-Mn O2 were easily oxidized by O2.The NH3 adsorption capacity ofδ-Mn O2 was poor,and NH3species adsorbed on the surface ofδ-Mn O2 was difficult to react with O2.All the Manganese oxides showed high reactivity,but they all showed excessive oxidation.The oxidation capacity of Mn O2 decreased effectively with doping with cerium,especially the direct oxidation of Mn O2 to NH3 was inhibited,and the formation of N2O decreased.Secondly,on the basis of Mn-Ce oxides catalysts,Mn-Ce/AC catalyst was prepared with AC support by impregnation method.The effects of different Mn/Ce ratios and active component loading on the denitration activity and physical and chemical properties were studied,and the enhancement mechanism of Ce doping on the denitration activity of the catalyst was analyzed.The results showed that the denitration rate of Mn-Ce/AC catalysts with different Mn/Ce molar ratios was in the order of 0.3>0.1>0.5>0.7.After Ce doping,the reducibility of the catalysts was improved.More acidic sites were formed on the catalyst surface.In the NH3-SCR reaction of Mn-Ce/AC catalyst,Mn Ox played a major catalytic role,and the content of Mn4+in the catalyst significantly affected the SCR reactive activity.As a promoter,Ce O2 mainly played a synergistic catalytic role,promoting the transformation from Mn3+to Mn4+,accelerating the oxygen transformation from gaseous adsorption O2 to high activity Oα,improving the oxidation activity of NO,thus promoting“Fast SCR”and improving the denitration activity of the catalyst.The addition of promoter could significantly improve the performance of catalysts.In order to distinguish the influence mechanism of promoter on different components of catalysts,the effects of Fe doping on the denitration activity,sulfur resistance and physical and chemical properties of Mn/AC catalysts were studied firstly.The results showed that Mn-Fe/AC catalyst effectively weakened the oxidation of SO2 and inhibited the formation of（NH4）2SO4 and NH4HSO4 on the catalyst surface after Fe doping,showing a certain resistance to sulfur and water poisoning.In addition,the composite oxide formed by Fe doping and Mn generated new active sites,which promoted the formation of intermediate NH2NO and effectively improved the N2selectivity of the catalyst.On this basis,the influence of Fe doping modification on the denitration performance of Mn-Ce/AC catalyst was analyzed.The results showed that Fe doping could significantly reduce the surface area loss during the calcination process of Mn-Ce/AC catalyst.In addition,metal ions could enter the graphite structure of actived coke carrier and split it into smaller,randomly oriented graphite microcrystalline structure.Fe doping introduced more acidic sites,promoting the catalyst denitration activity.The real sintering flue gas composition was complex,and the flue gas flow fluctuation was large.Therefore,the application of denitration catalyst in sintering flue gas would also face problems such as the poisoning effect of sulfur,water,alkali metals and heavy metals,and ammonia escape under high space speed conditions,and other problems.In this work,the effects of three possible potassium species（KCl,K2SO4,KNO3）in sintering flue gas on catalyst inactivation were compared.The results showed that,on the one hand,potassium occupied oxygen vacancy around manganese ion and inhibited the adsorption of NH3 on manganese acid site;on the other hand,NO3–and SO42–acted as new acid sites,which compensated the negative effect of the decrease of manganese acid sites caused by potassium poisoning.The deactivation degree of Mn-Ce/AC catalyst caused by the interaction of potassium ion and their acid ions was KCl>K2SO4>KNO3.In addition,the results of ammonia escape experiment showed that ammonia escape began when the ammonia nitrogen ratio exceeded 0.4 at 100 oC.The ammonia escape was less than 30 ppm when the ammonia nitrogen ratio is 1 and above 150 oC.Besides,in the presence of sulfur and water,the results showed that the denitrification rate of Mn-Ce/AC catalyst remained above 70%at 175 oC,which showed a certain resistance ability to sulfur-water poisoning. |
