Modification Mechanism And Application Study Of Scr Catalyst Based On Regulating Structures And Performances Of Multiple Active Sites

With the rapid development of social economy,the energy consumption increased year by year.However,China's coal-based energy structure will not change fundamentally for a long time.Nitrogen oxides and mercury produced by coal combustion are the main air pollutants,which thereby resulted in a serious harm to human health and ecological environment.The selective catalytic reduction?SCR?is regarded as the most effective technology for NO_X removal at present,which has become the first choice for flue gas denitrification in China's coal-fired power plants.However,there have no mature application of mercury removal technology in China's coal-fired power plants up to now.Considering that the majority of power plants have installed the denitration equipment,dust removal equipment and desulfurization equipment,we can take use of SCR catalyst to oxide Hg⁰ to Hg²⁺,and then effectively remove Hg²⁺in the subsequent pollution control equipments.Consequently,this method can significantly reduce the cost of pollution control and has a much greater prospect.In China,the load of the boiler always changed.In coal-fired power plant,the SCR catalysts exhibit a low activity at low temperature leading to the poisoning of ammonium bisulfate.And that ultimately leads to the excess NO_X emissions by the denitration equipment during the period of low-load operation.In this paper,combined with the practical problems of coal-fired power plant operation,the commercial SCR denitration catalyst is modified to realize the multi-functionalization,and the catalyst formula for synergic control of NO_x and mercury and the full-load SCR catalyst formula suitable for the actual conditions of coal-fired flue gas in China are developed.The main results are as follows:?1?The study on the catalyst formula for synergic control of NO_x and mercury.The properties of SCR catalysts doped with different metal oxide were evaluated.The results show that the dopping of Cr₂O₃ will increase the redox properties and chemisorbed oxygen of the catalysts,and Cr₂O₃ also combines with V to form the Cr-O-V structure.The synergistic effect of the double active centers will improve the mercury oxidation ability,denitration performance and SO₂ oxidation ability of the catalyst,and the doping of Cr₂O₃ also promotes the SCR side reaction to generate N₂O.Based on the Cr-SCR catalyst,doping 1%MO_X can effectively reduce the generation of N₂O in the side reaction of Cr-SCR catalyst,but has little effect on the mercury oxidation ability and SO₂ oxidation ability of the catalyst.From the analysis of mercury oxidation mechanism,the doping of Cr₂O₃ can increase the generation of active chlorine in the catalyst,so that some of the mercury is oxidized by the Langmuir-Hinshelwood mechanism,and the others?Hg⁰?are reacted by the Mars-Maessen mechanism,thereby improving the mercury oxidation ability of the catalyst.For the bimetallic doped SCR catalysts,O₂,HCl and NO will promote the mercury oxidation ability of the catalyst,while the SO₂,H₂O and NH₃ will inhibit that.When the Cr₂O₃loading is 0.5%and the MOx loading is 1%,the mercury oxidation efficiency of the modified SCR catalyst in the simulated flue gas of 10 ppm HCl and 30 ppm NH₃ can also reach 85.2%,showing a good performance.It indicates that the investigated SCR catalyst about synergic control of NO_x and mercury can adapt to the flue gas conditions of low chlorine coal in China,and the catalyst can be installed in the last layer of the SCR denitration reactor to achieve the purpose of high-efficiency denitrification and mercury oxidation.?2?Research on full load SCR catalysts.The low temperature activity and stability of the catalyst is improved by doping BOx oxide,the results show that the doping of BOx oxide can significantly improve the denitrification ability of the catalyst in the temperature range of 250-350?,but when the BOx content is greater than 2%,it promotes the formation of N₂O and the oxidation of SO₂ in the side reaction,and reduces the denitration ability of the catalyst at 350-400?.It was studied that the denitration stability under simulated flue gas conditions at 275?where 500 ppm of SO₂ and 20 ppm of SO₃ were added,and the gas hourly space velocity is 10000 h^-1which is much larger than the actual condition.After 30 hours of operation,the denitration efficiency of the modified SCR catalyst was stabilized at 64.5%,while the denitration efficiency of the commercial SCR catalyst continued to decrease.The characterization results show that the ammonium sulfate salt deposition occurs in both catalysts,which leads to a decrease in the specific surface area,pore volume and the ratio of V⁴⁺/(V⁴⁺+V⁵⁺)of the catalyst.These are the reasons for the decrease in catalyst activity at low temperature.However,the amount of ammonium sulfate salt deposited on the modified catalyst is relatively lower.In addition,the deposited ammonium sulfate salt is chemically adsorbed on the surface of the catalyst to form NH₄⁺and SO₄^2-.However,the modified catalyst reduces the decomposition temperature of the ammonium sulfate salt in the catalyst,and the NH₄⁺decomposed at 275°C will react with the NO_x in the flue gas.The resulting SO₄^2-will interact with TiO₂ which is reversible and easy to regenerate.So sulfate will be formed partially on the surface of TiO₂.Therefore,after a certain period of time,the dynamic equilibrium is reached on the surface of the modified catalyst,so that the modified catalyst can be stably operated at this temperature,which indicates that the catalyst can be continuously and stably operated at low temperature after being modified,thereby realizing the full load operation on the coal-fired power plant.?3?Reloading studies of the active components of SCR catalysts.The effective loading of the active component on the surface of the catalyst is achieved by organic modification of the SCR catalyst.The results show that the SCR catalyst increases the functional group on the surface after organic modification,and promotes the adsorption of vanadium ions by the catalyst.When the concentration of the organic solution was6%and the concentration of vanadium ions was 3 g/L,the adsorption amount of the catalyst was 6.57 mg/g,which was 147%higher than that at the time of unmodified,and the conversion of SO₂/SO₃ was reduced by 28.9%compared to the unmodified catalyst.This indicates that the organic modification greatly increases the adsorption amount of vanadium on the surface of the catalyst,which is more beneficial to increase the activity of the catalyst and reduce the conversion rate of SO₂/SO₃ in the side reaction.The data of the kinetic experiments and the isotherm experiments show that the adsorption of vanadium ions by the organically modified catalysts is consistent with the quasi-second-order kinetic model and the Langmuir isotherm adsorption model.The use of the organic modification method to support the active component can reduce the initial concentration and the loading of the active component,which is highly advantageous for the regeneration of the SCR catalyst.?4?Study on complete process of SCR catalyst regeneration and modification.The complete process of the SCR catalyst regeneration and modification is designed and developed.The deactivated SCR catalyst can be regenerated or multi-functioned through the processes of deactivation reason diagnosis,purging,physical cleaning,chemical cleaning,activation,drying and calcination.Meanwhile,the flow field of the drying and calcination equipment is optimized.After optimization of the deflector,the velocity distribution uniformity deviation Cv value of the catalyst inlet in the tunnel furnace is 13.4%,which makes the velocity of the air entering the catalyst cross section more uniform,ensuring rapid and uniform drying and calcination of the catalyst.The production line for the catalyst regeneration and modification with an annual output of10,000 m³ has been established.The catalyst for synergic control of NO_x and mercury produced by the production line was applied to the 1000MW coal-fired unit.The experimental results showed that the mercury oxidation efficiency of the modified catalyst increased from 25%to 71.4%,and the conversion rate of SO₂/SO₃ was from0.447%up to 0.671%,while the activity remained basically unchanged.Field test results showed that the denitrification efficiency of SCR catalyst was more than 90%,the NOx concentration,NH₃ escape,mercury and its compounds at the outlet were less than 30 mg/m³,3 ppm,and 2?g/m³ respectively,all of which meet the ultra-low emission requirements.The regeneration and full-load modified SCR catalyst was applied to the 1000MW coal-fired unit.The results showed that the toxic elements in the catalyst were effectively removed after regeneration,and specific surface area was effectively recovered.In addition,the denitration efficiency of the catalyst was over 80%and the NH₃ escape was less than 3 ppm at 382?and 275?,so the full-load denitration of the SCR system is realized.