The present thesis revolves around the challenges involved in removal of nitrogen oxides in biomass fired power plants. Nitrogen oxides are unwanted byproducts formed to some extent during almost any combustion. In coal fired plants these byproducts are removed by selective catalytic reduction, however the alkali in biomass complicate matters. Alkali in biomass severely deactivates the catalyst used for the selective catalytic reduction in matter of weeks, hence a more alkali resistant catalyst is needed. In the thesis a solution to the problem is presented, the nano particle deNOx catalyst. Through the thesis the one-pot sol-gel synthesis of this nano particle catalyst, have been optimised by evaluation of each synthesis step. Resulting in a highly active catalyst comprising amorphous vanadia on a high surface area crystalline anatase carrier. Due to the high surface area, loadings of 20 wt.% vanadia could be obtained without exceeding the V2O5 monolayer coverage. Explaining the very high activity corresponding to a factor of 2, compared to an industrial reference. Even at high vanadia loadings the catalyst did not show any sign of increased SO2 oxidation, compared with a low vanadia industrial reference catalyst. Furthermore long-term activity measurements at normal operating temperature revealed that the catalyst did not display any sign of deactivation. The catalyst showed very high resistance towards potassium poisoning maintaining a 16 times higher activity than the equally poisoned industrial reference catalyst, after impregnation of 225 mole potassium/g of catalyst. A catalyst plate was synthesised using 20 wt.% sepiolite mixed with nano catalyst, supported by a SiO2-fibre mesh. Realistic potassium poisoning was performed on the catalyst plate, by exposure in a potassium aerosol for 632 hours at 350 C. Owing to physical blocking of potassium by sepiolite fibres the composite catalyst showed a further increase in potassium resistance compared with the unsupported catalyst. Finally a refined mechanism was proposed for the nano particle SCR catalyst explaining insitu FTIR observation done on the system. Most importantly it indicated that the V=O bond did not break during the SCR reaction, suggesting that another oxygen is responsible for the activity of the active vanadia site.
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Nørklit Jensen, Jørgen, Fehrmann, Rasmus, Riisager, Anders
Technical University of Denmark, Department of Chemical Engineering, 2013