Experiment And Mechanism Analysis On Selective Catalytic Reduction DeNO_x Catalyst

Selective catalytic reduction (SCR) catalyst for NOx reduction becomes the research focus of domestic and foreign research institutions with the emission standards developing stricter in the worldwide.In this study, the powder SCR catalyst prepared with analytical pure chemicals was researched by experiment and theoretical analysis. The denitration reaction mechanism of V2O5-WO3-MoO3/TiO2catalyst was studied and its denitration performance was compared with V2O5-WO3/TiO2and ViO5-MoO3/TiO2catalysts. The physical and chemical properties of V2O5-WO3-MoO3/TiO2catalyst was analyzed based on its microscopic characterizations, such as specific surface area, pore volume, mean pore diameter, phase composition and grain size. Also, the mass percentage of each chemical in the catalyst was optimized.The honeycomb catalyst of V2O5-WO3-MoO3/TiO2was prepared with industrial pure chemicals to investigate the impurities effects on the denitration performance, which were contained in the chemicals. Meanwhile, the forming process was optimized during the honeycomb catalyst preparation. The denitration performance of honeycomb V2O5-WO3-MoO3/TiO2catalyst under actual flue gas was tested on a SCR DeNOx test bench which was built at a300MW unit of a coal-fired power plant in Shandong Province. In this test, the effects of space velocity, catalyst volume, reaction temperature, NH3to NOx ratio, NOx initial concentration, soot depositing time, and catalyst life on the catalyst denitration performance were revealed.The denitration performance of powder V2O5-WO3-MoO3/TiO2catalyst was researched under the exhaust of a Lister Petter TR1heavy direct injection single-cylinder diesel engine, and the catalyst deactivation mechanism during diesel engine operation was analyzed.1. The powder V205-W03-Mo03/Ti02catalyst was prepared with analytical pure chemicals of ammonium vanadate, ammonium tungstate, ammonium molybdate and anatase titanium dioxide. The denitration performance of this catalyst was researched under simulated flue gas composed of NO, SO2, O2and N2. The research results show that, the Denitration efficiency of the catalyst loading W and Mo is higher than the catalyst only loading W or Mo, when the same total amount of active substance is loaded. In high temperature range, W can effectively enhance the activity of the catalyst and inhibit the side reactions caused by Mo. In low temperature range, Mo can effectively improve the activity of the catalyst to compensate for the insufficient activity of W. Thereby the temperature window of the catalyst loading both W and Mo is broadened. The catalyst has best denitration performance with uniform and dense particles and complete microporous structure under V load at1.0%w/w, W load at4.5%w/w, Mo load at4.5%w/w. The sintering temperature has close relationship with the characterizations of the catalyst, such as specific surface area, pore volume, average pore diameter, degree of crystallization, phase transform and catalytic activity. The catalyst sintered at350℃had the highest Denitration efficiency of99.1%, but its chemical structure was not stable. The catalyst sintered at450℃had a stable chemical structure and complete crystallization with the active material dissolved into the anatase TiO2lattice, but its Denitration efficiency reduced to89.1%. The catalyst, sintered by350℃+450℃two-stage sintering method, had a stable chemical structure, and its specific surface area increased by5.0%, pore volume enlarged by1.9%, average pore diameter reduced by1.5%, Denitration efficiency peak increased by3.0%, and the temperature window approximately broadened by40℃compared with the catalyst sintered at450℃.2. The Denitration efficiency of the catalyst prepared with industrial pure chemicals and analytical pure chemicals were tested and compared. The research results show that, the industrial pure H2C2O4causes the catalyst serious deactivation, and the catalyst prepared with industrial purity of V, W, Mo and analytical purity of H2C2O4has low cost, high Denitration efficiency and wide temperature window. Forming properties have close relationship with the mold structure, the forming agent type and the precursor preparation process. The density and uniformity of the precursor can be improved by optimizing the forming process. After powder screening, rolling, beating and sealed storing, the precursor has preferable honeycomb-forming properties, strong hardness and abrasive resistance.3. The operating parameters effects on the catalyst denitration performance were researched on the SCR DeNOx test bench which was built at a300MW unit of a coal-fired power plant in Shandong Provence. The research results show that the overall trend of the catalyst denitration efficiency is reduced with the space velocity increasing, but within a certain range, the denitration efficiency is relative stable. When the denitration efficiency reaches80.0%, increasing the amount of catalyst used in the SCR system to improve the denitration efficiency is not effective, but it will result in SO2oxidation increase, meanwhile, it will result in higher cost of the SCR system. The volume of catalyst used in the SCR system should be arranged according to the design standards and emission requirements for each coal-fired power plant. The ratio of the catalyst denitration efficiency to catalyst volume should be optimized to avoid other resources wasting by singlely pursuiting the high denitration efficiency. Compared with the results in the simulated flue gas, the denitration efficiency in the actual flue gas is more sensitive to temperature variation, and the denitration efficiency change is larger with temperature varying at the high-temperature range. The denitration efficiency, however, is less sensitive to NH3concentration variation, which reach70.0%at NH3:NOx of0.9. The denitration efficiency can be improved by enlarging NH3:NOx and the SO2oxidation will decrease at the same time. The V2O5-WO3-MoO3/TiO2catalyst has strong adaptability to the change of the NOx initial concentration, and its denitration efficiency is larger than70.0%with the NOx initial concentration in the range of300-1700ppm. The denitration efficiency of the catalyst reduces with the soot depositing time extension, and it declines sharply at the beginning of the catalyst continuous operation. In the first month of the catalyst continuous operation, the denitration efficiency decreases by1.0%per78h, in the range of2to12months, the denitration efficiency decreases by1.0%per740h.4. The effect of the diesel engine operating parameters on the catalyst denitration performance was researched with the exhaust of a Lister Petter TR1heavy direct injection single-cylinder diesel engine. The results show that, the denitration efficiency and the by-product N2O concentration decrease with the space velocity increase, while the NH3slip increases. With the diesel engine load increasing, the denitration efficiency declines and the decline rate keeps on rising, at the same time, the by-product N2O concentration decreases and the NH3slip increases continuously. Under different loads, the catalyst denitration efficiencies are all greatly influenced by the reaction temperature and the trends of denitration efficiency varying with reaction temperature rising are similar, but the varying values are quite different. Diesel engine load change can lead the catalyst temperature window (denitration efficiency>70%) varying largely, at the same time, the starting temperature of NH3/N2O side reaction and the N2O concentration change together. With the increase of NH3volume, the denitration efficiency increases first and then remains unchanged, while the concentration of by-product N2O increases and increase rate keeps stable, at the same time, the NH3slip increases and the increase rate rises gradually. With the diesel engine operation time extension, the carbon content on the catalyst surface area increases and the catalyst denitration efficiency decreases slowly, but the effect of carbon deposition on denitration efficiency is weak. Diesel engine start-up, shut-down times increase accelerates the reduction of catalyst denitration efficiency, meanwhile, the side reaction of NH3/N2O becomes weak, and the NH3slip increases.This study is funded by shandong province science and technology development plan:high efficiency selective catalytic reduction catalyst deactivation mechanism research and industrial application (NO.2011GSF11716) and Shandong Power Ltd technology projects:NOx emissions control technology research on large capacity coal fired boilers (NO.2008ZB-19). The author’s experiment and life at the university of Birmingham, UK is supported by China Scholarship Council (NO.2011622102).