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- Lindholm, Anna Maria, 1977, et al.
(författare)
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Reduction of NOx over a combined NSR and SCR system
- 2010
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 98:3-4, s. 112-121
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Tidskriftsartikel (refereegranskat)abstract
- Flow reactor experiments are performed over a model Pt/Ba/Al catalyst, an Fe-beta sample, and over a combined system where the Pt/Ba/Al catalyst is placed upstream the Fe-beta sample. The combined system show a superior NOx removal efficiency and a lower ammonia slip compared to the single Pt/Ba/Al catalyst at all temperatures examined. Ammonia, formed during the rich period over the Pt/Ba/Al, is stored in the SCR catalyst and consumed in the following lean period which results in a decrease of the NH3 slip and an increase of the NOx removal efficiency. The optimum temperature for NOx removal of the combined system is 300 degrees C; at this temperature a remarkably high NOx removal efficiency of 99.5% is achieved. The SCR catalyst exhibit a high ammonia adsorption capacity at 200 degrees C which results in a very low NH3 yield (3%) and a high N-2 yield (85%) for the combined NSR and SCR system. There is a benefit when NO2 is present in the feed at low temperatures. The NOx removal efficiency of the combined system increases due to the increase in the amount of NOx stored over the Pt/Ba/Al sample. Furthermore, the amount of NOx removed over the combined system is influenced by the amount of hydrogen in the rich periods. A higher hydrogen concentration enhances the NOx removal efficiency at lower temperatures. At higher temperatures an optimum hydrogen concentration exists due to ammonia inhibition of the SCR reaction when an excessively high H-2 concentration is used. (C) 2010 Elsevier B.V. All rights reserved.
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| 4. |
- Sjövall, Hanna Maria, 1977, et al.
(författare)
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A Kinetic Model for the Selective Catalytic Reduction of NOx with NH3 over an Fe-zeolite Catalyst
- 2010
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 1520-5045 .- 0888-5885. ; 49:1, s. 39-52
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Tidskriftsartikel (refereegranskat)abstract
- The selective catalytic reduction of NOx with ammonia over all Fe-zeolite catalyst was investigated experimentally and a transient kinetic model was developed. The model includes reactions that describe ammonia storage and oxidation, NO oxidation, selective catalytic reduction (SCR) of NO and NO2, formation of N2O, ammonia inhibition and ammonium nitrate formation. The model call account for a broad range of experimental conditions in the presence of H2O, CO2 and O-2 at temperatures from 150 to 650 degrees C. The catalyst stores ammonia at temperatures up to 400 degrees C and shows ammonia oxidation activity from 350 degrees C. The catalyst is also active for the oxidation of NO to NO2 and the oxidation reaches equilibrium at 500 degrees C. The SCR of NO is already active at 150 degrees C and the introduction of equal amounts of NO and NO2 greatly enhances the conversion of NOx at temperatures up to 300 degrees C. The formation of N2O is negligible if small fractions of NO2 are fed to the reactor, but a significant amount of N2O is formed at high NO2 to NO ratios. An ammonia inhibition oil the SCR of NO is observed at 200 degrees C. This kinetic model contains 12 reactions and is able to describe the experimental results Well. The model was validated using short transient experiments and experimental conditions not used in the parameter estimation and predicted these new conditions adequately.
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| 6. |
- Sjövall, Hanna Maria, 1977, et al.
(författare)
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Detailed kinetic modeling of NH3 SCR over Cu-ZSM-5
- 2009
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Ingår i: Applied Catalysis B: Environmental. - 0926-3373 .- 1873-3883. ; 92:1-2, s. 138-153
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Tidskriftsartikel (refereegranskat)abstract
- A combination of flow reactor experiments and detailed kinetic modeling was used to study the selective catalytic reduction (SCR) of NOx with NH3 over Cu-ZSM-5. The model was developed in several steps and the SCR mechanism presented here is based on our earlier work on two subsystems: ammonia storage and ammonia oxidation with and without water, and NOx adsorption and NO oxidation. In this work, the subsystems were combined with reaction steps for the reduction of nitrogen oxides with ammonia. The SCR mechanism involves a reaction between adsorbed NO2 and NH3, formation of HNO2 and HNO3 and the final reduction to N2 or N2O. Seven experiments that describe the catalytic activity using various feed combinations of NO, NO2 and NH3 in both dry and wet feeds were used in the model development. These experiments investigate the SCR of NO over a wide temperature range, the influence of using various NO to NO2 ratios at 175 °C, the influence of changing NO to NO2 ratio at 350 °C, the NO oxidation in wet feed, and the ammonia inhibition at low temperature. The model was also validated using six new experiments. The predicted NOx conversions and formations of N2O correlated well with the experimental results and show that the detailed kinetic model developed in this study could be used successfully to describe several experimental observations over a wide range of temperatures.
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| 9. |
- Sjövall, Hanna Maria, 1977
(författare)
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Selective Catalytic Reduction of NOx with NH3 - Kinetic Modeling and Experimental Studies using Zeolite Based Catalysts
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Emissions from the combustion of fossil fuel contain several pollutants which can be converted over a catalyst to less harmful products. An efficient way to reduce nitrogen oxides in a lean environment is to apply selective catalytic reduction of these oxides with ammonia (NH3 SCR). Metal ion exchanged zeolite catalysts have proven to be active and selective for NH3 SCR. In this thesis, iron and copper zeolite catalysts are evaluated, and the activity for NOx reduction is studied at various temperatures and feed concentrations. DRIFT spectroscopy is applied to investigate the surface species that are formed during NH3 SCR conditions, and kinetic models are developed that describe the catalytic activity of the catalysts. The Cu-ZSM-5 catalyst is active in a wide temperature range, but the NO conversion decreases at high temperature due to ammonia oxidation. During NO oxidation, species such as nitrite or nitrate can be identified on the surface and are believed to be involved in the SCR mechanism. High concentrations of ammonia inhibit the NO reduction at 175ºC, possibly due to competition for sites that are needed also in the formation of nitrite or nitrate species. The NOx conversion is enhanced by the introduction of about equal amounts of NO and NO2. The NOx reduction over the Fe-zeolite catalyst is similar to the reduction over the Cu-ZSM-5 catalyst. The iron catalyst is, however, less active for NO reduction at low temperature, but more active in the high temperature range. It is also more sensitive to ammonia inhibition, has a lower NH3 storage capacity, produces less N2O at low NO2 to NOx ratios, and is less active for NO and NH3 oxidation.Two models are developed in this work. A detailed model which describes the experimental observations made for the Cu-ZSM-5 catalyst, and a global model which describes the activity over a supplier Fe-zeolite catalyst. Both models account for a broad range of experimental conditions at a wide temperature range. The models are validated using experimental conditions not included in the parameter estimation and predict these new conditions adequately.The poisoning effect of hydrocarbons is investigated and the results show that the presence of hydrocarbons inhibits the NOx conversion over the catalyst (Fe-beta). The effect is greater in presence of n-octane than in presence of propylene, and the inhibition increases with increased concentration of hydrocarbons. Propylene and n-octane reduce the conversion of NO as well as the conversion of a mixture of NO and NO2.
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