The Development Of A Full-scale Evaluation System For SCR And Preparation And Characterization Of SCR Catalysts With Keggin Structure

Energy and Environment are the basic condition of human survival and development.With the rapid development of economy,the increasing exploitation and utilization of fossil fuels have brought some serious problems of environmental pollution.The emission of nitrogen oxides can cause air pollution,acid rain and the greenhouse effect.Selective Catalytic Reduction?SCR?is the main method to control NO_x emission.Based on the application of SCR catalysts,a full-scale reaction system was developed in order to evaluate the activity of the SCR catalysts and low-temperature SCR catalysts with heteropoly acid were investigated.In this paper,a full-scale evaluation system used for honeycomb-type NH₃-SCR catalysts was developed.System composition,control method and operation of the full-scale evaluation system were introduced in detail.This system could provide many gas conditions in the temperature range of 100-450°C,air velocity with100-539 m³/h,NO concentration with 500-3200 mg/m³ and SO₂ concentration with50-3000 mg/m³.Furthermore,flue-gas temperature,gas flow,pollutant concentrations could be easily adjusted.The system running was stable for long time with less fluctuation which ensured the reliability of results.In addition,this system using NH₃oxidation to realize NO gas supply,which could reduce the cost and was suitable for batch activity evaluation on honeycomb-type SCR catalysts.Through running the system continuously for a long time,it was found that after reaching stability,the system was stable with less fluctuation and was sensitive to the changes of the flue gas conditions,which could be used for the effect of different flue gas condition for SCR catalysts and research on the laws of the running.From the detection of the sulfur content on the surface of forming catalysts and the running situation of the catalysts under the condition with high sulfur content,it was found that NH₄HSO₄ was enriched from the lower end of SCR reactor.Furthermore,NH₄HSO₄ was difficult to enrich at its dew point temperature of 230°C and its decomposition temperature of 270°C.In this paper,the SCR catalyst with Keggin structure was synthesized by simple impregnation method and the effect of?NH₄?₃PMo₁₂O₄₀ loading and calcination temperature on the catalyst was studied.The results showed that with loading increase,the surface area of catalysts decreased and the catalyst with 20%loading exhibited the best SCR activity.The structure and SCR performance of the catalyst was affected by calcination temperature.Under 450°C calcination temperature,the part of the Keggin structure was destructed.When calcination temperature reached 500°C,the Keggin strucuture was difficult to be observed.The SCR performance and surface acidity of catalysts decreased with an increase of calcination temperature.It suggested that the Keggin structure could increase the surface acid leading to improve SCR performance.To further investigate the effect of the Keggin structure on the low-temperature NH₃-SCR,the catalysts with and without the Keggin structure were synthesized.The physicochemical characteristics of the obtained catalysts were characterized by many techniques.The results indicated that the catalysts with the Keggin structure exhibited more surface Bronsted and Lewis acid sites,meanwhile the catalysts showed a significantly promoted performance in SCR reaction and SO₂ poisoning resistance.In order to understand the effect of Keggin structure on SCR reaction,a series of in situ DRIFT analyses were conducted to investigate the surface-adsorbed species on the catalysts during the SCR reaction.Furthermore,the effect of NH₄⁺in the Keggin structure was also investigated.Results indicate that both the L-H and E-R mechanisms occurred in the NH₃-SCR reaction over the catalysts.But NO_x species activity were improved and more NH₄⁺ions participated in reaction over the catalyst with the Keggin structure,thus promoting SCR activity.