Development Of V₂O₅-WO₃/TiO₂ Catalyst And Its Application In NH₃-SCR Of NOx At Low Temperatures

Nitrogen oxides (NOx) remain as one of the major sources of air pollution, which greatly contributes to the greenhouse effect, ozone depletion and formations of photochemical smog and acid rain. Many countries have made stringent emission limits for flue gases from combustion facilities and vehicles. The selective catalytic reduction (SCR) of NOx with NH3 is worldwide used as the most effective technology for removal of NOx from stationary sources such as power boilers and combustion furnaces. The V2O5-WO3/TiO2 catalysts have been widely used for decades, with a relatively narrow temperature window of 300-400 ℃. In China, unlike in developed countries, a considerably huge amount of NOx is from a large number of coal-fueled industrial furnaces, such as heating boilers, cement klins, sintering machines and so on. The flue gas temperatures are mostly in a range of 150℃ to 300℃ and out of the working temperature window of commercial SCR catalysts for denitration (de-NOx). Moreover, Euro Ⅵ diesel buses equipped with SCR systems failed to mitigate on-road NOx emissions to meet the expected regulation. The NOx emission factor of Euro V diesel buses was 37% lower than that of Euro Ⅵ diesel buses, but it still exceeded the Euro Ⅴ standard by 180%. The real-word NOx emission factors were sensitive to changes of average speed. In other words, NOx emissions are higher than the Euro Ⅵ/Ⅴ limits because of the low exhausting temperatures.Considering the drawbacks of current SCR technology for denitration at low tempeartures, it is urgent to explore how to reduce the NOx emission at low flue gas temperatures at 200 to 400℃, which will have considerable applications not only to industrial combustion facilities but also to diesel engines. Systematic investigation has been performed to clarify the influences of supports, preparation method and vanadium precursors on catalytic activity and its resistance to SO2 and H2O poisoning. In this work, fundamental studies on catalytic mechanism and industrial application are carried out for V2O5-WO3/TiO2 catalyst, and the main research achievements are listed as follows:1. Optimization and modification of the TiO2 and WO3/TiO2 supports. The TiO2 support was prepared through atmospheric hydrolysis method. The active carbon (AC) was added as template which would facilitate the formation of mesoporous. Based on the preparation of TiO2 support, the catalyst using F-doped supports exhibited higher catalytic activity, especially for low-temperature denitration reaction. And for the F-doped catalyst there were higher surface atomic concentrations of V and W. But the excess F would was washed into the solution, causing issues of treatment of waste liquids2. Improved low-temperature activity of V2O5-WO3/TiO2 for denitration using different vanadium precursors. A catalyst made according to the solvothermal method using vanadyl acetylacetonate (VO(acac)2) as the vanadium precursor was obatained to have high catalytic activity and good resistance to poisoning of SO2 and H2O at 220-450 ℃. Via FTIR and TGA analyses it clarified that over the catalyst a dynamic balance between the formation and decomposition of ammonium sulfite or sulfate is possibly built at low temperatures, for example,220℃. To eliminate the influence of organic solvent, the study tests grinding as a safety method for V2O5WO3-TiO2 catalyst preparation. Catalysts were prepared by grinding method using different vanadium precursors, and were further evaluated and characterized to clarify the relationship between surface morphology of vanadium oxides and performance. Catalyst characterization revealed that the VO(acac)2 precursor promoted the formation of polymeric vanadia species and low valence of vanadium oxides to decrease the difficulty of NO reduction reactions over the catalyst. The study would provide a new and low-cost technology to prepare the V2Os/WO3-TiO2 catalyst with good de-NOx activity.3. Integration and optimization of Diesel Oxidation Catalyst (DOC) and SCR two-stage process. The performances to removal of NO, CO and HC by NH3-SCR and DOC catalysts were studied. The conversions of CO and HC were too low over SCR catalyst, which cannot be improved by doping Pd. Thus, a two-stage process combining DOC and SCR was tested. The results showed that all species were simultaneously removed, and NO was oxidized to NO2 via DOC to benefite denitration at low temperatures. Moreover, CO or HC and its intermediate products can participate in SCR reaction as reductant to obtaine higher NO conversion than theoretical value based on NH3/NO ratio. Comparing with a Euro V monolith catalyst, a monolith with a catalyst coating layer of 23 wt.% enabled the similar NOx reduction. Thus, with suffuciatly increased coating catalyst, the coated monolith can meet the Euro V emission standard.4. Abrasion resistance and prediction of durable hours of washcoated monolith catalyst. Monolith SCR catalysts coated with V2O5-WO3/TiO2 powder were prepared by varying binder and coating thickness. Comparing with a monolith extruded with 100% V2O5-WO3/TiO2 powder, a coated monolith with a catalyst-coating layer of 260 μm in thickness exhibited the similar initial NOx reduction activity at 250℃. After 4-h abrasion (attrition) in an air stream containing 300 g/m3 fine sands (50-100μm) at a superficial gas velocity of 10 m/s, the catalyst still has the activity as a 100% molded monolith does in a 24-h activity test and it retains about 92% of its initial activity at 250℃. Estimation of the equivalent durable hours at a fly ash concentration of 1.0 g/m3 in flue gas and a gas velocity of 5 m/s demonstrated that this coated monolith catalyst is capable of resisting abrasion for 13 months without losing more than 8% of its initial activity. The result suggests the great potential of the coated monolith for application to de-NOx of flue gases with low fly ash concentrations from, such as glass and ceramics manufacturing processes.5. Apparent chemical kinetics of washcoated honeycomb catalyst. The effects of gas linear velocity (LV), gas hourly space velocity (GHSV) and channel size on NO conversion were investigated to seek an apparent chemical kenetic model for prediction of denitration rate and also for reactor design of industrial application. The overall rate constant Ko, mass transfer coefficient Kg, reaction rate constant Ks and kinetic parameters (Ea and A) were calculated. The results showed that the active energy for washcoated honeycomb catalyst was about 30 kJ/mol, lower than the value of commercial SCR catalysts, indicating that the denitration reaction was easier to happen over the self-made monolith catalyst. Finally, the simulation calculation was performed under arbitrary conditions to provide valuable information for design of industrial SCR system.