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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Modeling of selective catalytic reduction (SCR) of nitric oxide with ammonia using four modern catalysts

Sharma, Giriraj 01 November 2005 (has links)
In this work, the steady-state performance of zeolite-based Cu-ZSM-5, vanadium based honeycomb monolith catalysts (V), vanadium-titanium based pillared inter layered clay catalyst (V-Ti PLIC) and vanadium-titanium-tungsten-based honeycomb monolith catalysts (V-Ti-W) was investigated in the selective catalytic reduction process (SCR) for NO removal using NH3 in presence of oxygen. The objective is to obtain the expression that would predict the conversion performance of the catalysts for different values of the SCR process parameters, namely temperature, inlet oxygen concentration and inlet ammonia concentration. The NOx emission, its formation and control methods are discussed briefly and then the fundamentals of the SCR process are described. Heat transfer based and chemical kinetics based SCR process models are discussed and widely used rate order based model are reviewed. Based on the experimental data, regression analysis was performed that gives an expression for predicting the SCR rate for the complete temperature range and the rate order with respect to inlet oxygen and ammonia concentration. The average activation energy for the SCR process was calculated and optimum operating conditions were determined for each of the catalyst. The applicable operating range for the catalyst depends on the NO conversion as well as on the ammonia slip and the N2O and NO2 emission. The regression analysis was repeated for the applicable range and an expression was obtained that can be used to estimate the catalyst performance. For the Cu-ZSM-5, the best performance was observed for 400oC, 660 ppm inlet ammonia concentration and 0.1% inlet oxygen concentration. For the V based honeycomb monolith catalyst, the best performance was observed for 300oC, 264 ppm inlet ammonia concentration and 3% inlet oxygen concentration. For the V-Ti based PLIC catalyst, the best performance was observed for 350oC, 330 ppm inlet ammonia concentration and 3% inlet oxygen concentration. For the V-Ti-W based honeycomb monolith catalyst, the best performance was observed for 300oC, 330 ppm inlet ammonia concentration and 3% inlet oxygen concentration. The conversion performance of all of these catalysts is satisfactory for the industrial application. At the operating conditions listed above, the N2O emission is less than 20 ppm and the NO2 emission is less than 10 ppm. The results were validated by comparing the findings with the similar work by other research groups. The mechanism of SCR process is discussed for each of the catalyst. The probable reactions are listed and adsorption and desorption process are studied. The various mechanisms proposed by the researchers are discussed briefly. It is concluded that V-Ti-W and Cu-ZSM-5 catalyst are very promising for SCR of NOx. The expressions can be used to estimate the conversion performance and can be utilized for optimal design and operation. The expressions relate the SCR rate to the input parameters such as temperature and inlet oxygen and ammonia concentration hence by controlling these parameters desired NOx reduction can be achieved with minimal cost and emission.
2

CARBON NANOTUBE SUPPORTED METAL CATALYSTS FOR NO<sub>x</sub> REDUCTION USING HYDROCARBON REDUCTANTS

Santillan-Jimenez, Eduardo 01 January 2008 (has links)
Nitrogen oxides (NOx) are atmospheric pollutants that pose a serious threat to both the environment and human health. Although catalytic deNOx technologies for engines working under stoichiometric air-to-fuel ratios (i.e., most gasoline engines) are already available, their performance is unsatisfactory under excess air conditions like those under which diesel engines operate. The selective catalytic reduction of NOx with hydrocarbon reductants (HC-SCR) is a potential deNOxsolution for diesel engines, whose operating temperatures are 150-500 ºC. Given that is unlikely for a single catalyst to show acceptable activity throughout this entire temperature span, the use of two catalysts is proposed in this dissertation. Whereas several catalysts active at high temperatures (>300 ºC) are already available, a catalyst showing an acceptable performance at low temperatures (<300 ºC) is yet to be found. Platinum group metals (PGMs) supported on activated carbon have been identified as promising low temperature HC-SCR catalysts. However, these materials show three main drawbacks: 1) the propensity of the carbon support to undergo combustion in an oxidizing environment, 2) a narrow temperature window of operation; and 3) a high selectivity towards N2O (as opposed to N2). To address the first limitation, the use of multi-walled carbon nanotubes (MWCNTs) as the support has been investigated and found to yield catalysts displaying a higher resistance to oxidation. Further, the acid activation of MWCNTs prior to their use as catalyst support has been explored, following reports than link carrier acidity with improved catalyst performance. In turn, the use of PGM alloys as the active phase has been examined as a means to improve catalyst activity and selectivity. Additionally, kinetic, spectroscopic and mechanistic studies have been performed in an attempt to probe structure-activity relationships in the MWCNTs-based formulations showing the best deNOx performance. The fundamental insights gained through these studies may inform further improvements to HC-SCR catalysts. Finally, the synthesis of the most promising formulations has been scaled-up using commercial metal monoliths as the catalyst substrate and the resulting monolithic catalysts have been tested in a diesel engine for activity in the HC-SCR reaction.
3

Development of Spatially-Resolved FTIR – Gas Concentration Measurements inside a Monolith-Supported Selective Catalytic Reduction Catalyst

Hou, Xuxian 04 June 2013 (has links)
The diesel engine is growing in popularity due to its energy efficiency and solving the emissions issues associated with diesel engine exhaust would clear the way for further growth. The key pollutants are NOx, particulate matter and unburned hydrocarbons. Selective catalytic reduction (SCR) catalysis is likely the best choice for NOx control. In SCR, NH3 selectively reacts with NOx to form N2 – the selectivity refers to NH3 reacting with NOx instead of the abundant O2. Urea is used as the NH3 source, being injected into the exhaust as an aqueous solution where the urea decomposes and NH3 is generated. Spatial resolution characterization techniques have been gaining attention in the catalysis field because of the higher level of information provided. In this thesis, a new spatial resolution technique, called SpaciFTIR (spatially-resolved, capillary-inlet Fourier transform infra-red spectroscopy), was developed, which overcomes the interference of water in the detection of NH3 in an earlier developed technique, SpaciMS (spatially-resolved, capillary-inlet mass spectrometry). With the new test method, three SCR topics were addressed. First, the three key SCR reactions were spatially resolved. These are the standard SCR reaction (2NO + 2NH3 + 1/2O2 = 2N2 + 3H2O), the fast SCR reaction (NO + NO2 + 2NH3 = 2N2 + 3H2O), and NO2-SCR, (6NO2 + 8NH3 = 7N2 + 12H2O). Results show that in the presence of NO2, but at a NO2/NOx ratio < 0.5, the fast SCR reaction proceeds followed by the standard SCR reaction, i.e. in series. If the NO2/NOx ratio exceeds 0.5, the NO2-SCR and fast SCR reactions occur in parallel. Compared to the standard integral test method, this spatial resolution technique clearly showed such trends. Secondly, the spatial resolution technique was used to characterize the effects of thermal aging on catalyst performance. It was found that for a highly aged catalyst, there was a radial activity profile due to an inhomogeneous temperature distribution in the process of aging. Aging effects on various key SCR reactions, i.e. NO oxidation, NH3 oxidation, and the reduction reactions, were studied. Last but not least, for the purpose of passive SCR system development, transient NH3 storage profiles along the monolith channel were measured with SpaciFTIR. Passive SCR is a system where the NH3 is generated on an upstream catalyst, such as a three-way catalyst or lean-NOx trap, instead of via urea injection. In such a system, NH3 is therefore not constantly being fed to the SCR catalyst, but “arrives” in pulses. Factors such temperature, NH3 concentration, pulsing time, flow rate and thermal aging were investigated. For the first time, NH3 migration was observed and its effect on SCR reactions along the length of catalyst was studied.
4

Selective Catalytic Reduction (SCR) of nitric oxide with ammonia using Cu-ZSM-5 and Va-based honeycomb monolith catalysts: effect of H2 pretreatment, NH3-to-NO ratio, O2, and space velocity

Gupta, Saurabh 30 September 2004 (has links)
In this work, the steady-state performance of zeolite-based (Cu-ZSM-5) and vanadium-based honeycomb monolith catalysts was investigated in the selective catalytic reduction process (SCR) for NO removal using NH3. The aim was to delineate the effect of various parameters including pretreatment of the catalyst sample with H2, NH3-to-NO ratio, inlet oxygen concentration, and space velocity. The concentrations of the species (e.g. NO, NH3, and others) were determined using a Fourier Transform Infrared (FTIR) spectrometer. The temperature was varied from ambient (25 C) to 500 C. The investigation showed that all of the above parameters (except pre-treatment with H2) significantly affected the peak NO reduction, the temperature at which peak NO reduction occurred, and residual ammonia left at higher temperatures (also known as 'NH3 slip'). Depending upon the particular values of the parameters, a peak NO reduction of around 90% was obtained for both the catalysts. However, an accompanied generation of N2O and NO2 species was observed as well, being much higher for the vanadium-based catalyst than for the Cu-ZSM-5 catalyst. For both catalysts, the peak NO reduction decreased with an increase in space velocity, and did not change significantly with an increase in oxygen concentration. The temperatures at which peak NO reduction and complete NH3 removal occurred increased with an increase in space velocity but decreased with an increase in oxygen concentration. The presence of more ammonia at the inlet (i.e. higher NH3-to-NO ratio) improved the peak NO reduction but simultaneously resulted in an increase in residual ammonia. Pretreatment of the catalyst sample with H2 (performed only for the Cu-ZSM-5 catalyst) did not produce any perceivable difference in any of the results for the conditions of these experiments.
5

An experimental investigation of the urea-water decomposition and selective catalytic reduction (SCR) of nitric oxides with urea using V2O5-WO3-TiO2 catalyst.

Johar, Jasmeet Singh 01 November 2005 (has links)
Two flow reactor studies, using an electrically heated laminar flow reactor over Vanadia based (V2O5-WO3/TiO2) honeycomb catalyst, were performed at 1 atm pressure and various temperatures. The experiments were conducted using simulated exhaust gas compositions for different exhaust gases. A quartz tube was used in order to establish inert conditions inside the reactor. The experiments utilized a Fourier transform infrared (FTIR) spectrometer in order to perform both qualitative and quantitative analysis of the reaction products. Urea-water solution decomposition was investigated over V2O5-WO3/TiO2 catalyst over the entire SCR temperature range using the temperature controlled flow reactor. The solution was preheated and then injected into pure nitrogen (N2) stream. The decomposition experiments were conducted with a number of oxygen (O2) compositions (0, 1, 10, and 15%) over the temperature range of 227oC to 477oC. The study showed ammonia (NH3), carbon-dioxide (CO2) and nitric oxide (NO) as the major products of decomposition along with other products such as nitrous oxide (N2O) and nitrogen dioxide (NO2). The selective catalytic reduction (SCR) of nitric oxide (NO) with urea-water solution over V2O5-WO3/TiO2 catalyst using a laboratory laminar-flow reactor was investigated. Urea-water solution was injected at a temperature higher than the vaporization temperature of water and the flow reactor temperature was varied from 127oC to 477oC. A FTIR spectrometer was used to determine the concentrations of the product species. The major products of SCR reduction were NH3, NO and CO2 along with the presence of other minor products NO2 and N2O. NO removal of up to 87% was observed. The aim of the urea-water decomposition experiments was to study the decomposition process as close to the SCR configuration as possible. The aim of the SCR experiments was to delineate the effect of various parameters including reaction temperature and O2 concentration on the reduction process. The SCR investigation showed that changing parameter values significantly affected the NO removal, the residual NH3 concentration, the temperature of the maximum NO reduction, and the temperature of complete NH3 conversion. In the presence of O2, the reaction temperature for maximum NO reduction was 377?C for ratio of 1.0.
6

Development of Spatially-Resolved FTIR – Gas Concentration Measurements inside a Monolith-Supported Selective Catalytic Reduction Catalyst

Hou, Xuxian 04 June 2013 (has links)
The diesel engine is growing in popularity due to its energy efficiency and solving the emissions issues associated with diesel engine exhaust would clear the way for further growth. The key pollutants are NOx, particulate matter and unburned hydrocarbons. Selective catalytic reduction (SCR) catalysis is likely the best choice for NOx control. In SCR, NH3 selectively reacts with NOx to form N2 – the selectivity refers to NH3 reacting with NOx instead of the abundant O2. Urea is used as the NH3 source, being injected into the exhaust as an aqueous solution where the urea decomposes and NH3 is generated. Spatial resolution characterization techniques have been gaining attention in the catalysis field because of the higher level of information provided. In this thesis, a new spatial resolution technique, called SpaciFTIR (spatially-resolved, capillary-inlet Fourier transform infra-red spectroscopy), was developed, which overcomes the interference of water in the detection of NH3 in an earlier developed technique, SpaciMS (spatially-resolved, capillary-inlet mass spectrometry). With the new test method, three SCR topics were addressed. First, the three key SCR reactions were spatially resolved. These are the standard SCR reaction (2NO + 2NH3 + 1/2O2 = 2N2 + 3H2O), the fast SCR reaction (NO + NO2 + 2NH3 = 2N2 + 3H2O), and NO2-SCR, (6NO2 + 8NH3 = 7N2 + 12H2O). Results show that in the presence of NO2, but at a NO2/NOx ratio < 0.5, the fast SCR reaction proceeds followed by the standard SCR reaction, i.e. in series. If the NO2/NOx ratio exceeds 0.5, the NO2-SCR and fast SCR reactions occur in parallel. Compared to the standard integral test method, this spatial resolution technique clearly showed such trends. Secondly, the spatial resolution technique was used to characterize the effects of thermal aging on catalyst performance. It was found that for a highly aged catalyst, there was a radial activity profile due to an inhomogeneous temperature distribution in the process of aging. Aging effects on various key SCR reactions, i.e. NO oxidation, NH3 oxidation, and the reduction reactions, were studied. Last but not least, for the purpose of passive SCR system development, transient NH3 storage profiles along the monolith channel were measured with SpaciFTIR. Passive SCR is a system where the NH3 is generated on an upstream catalyst, such as a three-way catalyst or lean-NOx trap, instead of via urea injection. In such a system, NH3 is therefore not constantly being fed to the SCR catalyst, but “arrives” in pulses. Factors such temperature, NH3 concentration, pulsing time, flow rate and thermal aging were investigated. For the first time, NH3 migration was observed and its effect on SCR reactions along the length of catalyst was studied.
7

Quantum Chemical Simulation Of No Reduction By Ammonia (scr Reaction) On V2o5 Catalyst Surface

Uzun, Alper 01 January 2003 (has links) (PDF)
The reaction mechanism for the Selective Catalytic Reduction (SCR) of NO by NH3 on V2O5 surface was simulated by means of density functional theory (DFT) calculations performed at B3LYP/6-31G** level. As the initiation reaction, ammonia activation on V2O5 was investigated. Coordinate driving calculations showed that ammonia is adsorbed on Br&oslash / nsted acidic V-OH site as NH4 + species by a nonactivated process with a relative energy of -23.6kcal/mol. Vibration frequencies were calculated as 1421, 1650, 2857 and 2900cm-1 for the optimized geometry, in agreement with the experimental literature. Transition state with a relative energy of -17.1kcal/mol was also obtained. At the end of the Lewis acidic ammonia interaction calculations, it was observed that ammonia is hardly adsorbed on the surface. Therefore, it is concluded that the SCR reaction is initiated more favorably by the Br&oslash / nsted acidic ammonia adsorption. As the second step of the SCR reaction, NO interaction with the preadsorbed NH4 + species was investigated. Accordingly, NO interaction results in the formation of gas phase NH2NO molecule with a relative energy difference of 6.4kcal/mol. For the rest of the reaction sequence, gas phase decomposition of NH2NO was considered. Firstly, one of the hydrogen atoms of NH2NO migrates to oxygen. It then isomerizes in the second step. After that, the reaction proceeds with the isomerization of the other hydrogen. Finally, a second hydrogen atom migration to the oxygen leads to the formation of N2 and H2O. Total relative energy for this reaction series was obtained as -60.12kcal/mol, in agreement with the literature.
8

Selective catalytic reduction of nitrogen oxides with ammonia over microporous zeolite catalysts

VENNESTROM, PETER NICOLAI RAVNBORG 14 October 2014 (has links)
With increasing legislative demands to remove nitrogen oxides (NOx) from automotive diesel exhaust, new catalyst systems are investigated and intensely studied in industry as well in academia. The most prevailing catalytic method of choice is the selective catalytic reduction (SCR) where non-toxic urea is used as a reductant for practical reasons. Usually urea is stored in a separate tank and once injected into the exhaust system it hydrolyses into the more aggressive reductant NH3 and CO2. 4 NH3 + 4 NO + O2 -> 4 N2 + 6 H2O (NH3-SCR reaction) In regions where vanadium is not banned cost effective V2O5/WO3/TiO2 NH3-SCR catalyst systems can be used. Vanadium based are well understood, but they do however not provide stability above ca. 550 °C for longer periods of time. In exhaust treatment systems where the temperature is either high or where high temperature excursions are experienced from e.g. regeneration of particulate filters, zeolite based catalysts are therefore today the most promising candidates as high-temperature stable and non-toxic catalysts for the NH3-SCR reaction. Among the most promising candidates are the Cu- and Fe-based zeolites. Usually Fe based zeolites show good performance in the temperature range 250-500 °C and reasonable stability, whereas Cu-based zeolites show good low-temperature activity in the 180-400 °C range. The presence of copper does however also lead to a lower stability of the catalyst material. Since the low-temperature activity is of paramount importance it is necessary to improve this behavior. Therefore the purpose of this project is to investigate: - The deactivation mechanism of copper based zeolites - The influence of the zeolite framework on stability and activity These investigations should mostly be carried out on model systems such as Cu-ZSM-5 and Cu-IM-5. Recently it was found that zeolite materials with the CHA-type structure show increased hydrothermal stability, most likely originating from the small 8-MR window openings in the structure. Part of the project should therefore also include investigations on this type and other similar structures, and therefore entail: - Synthesis, in-depth characterization and catalytic testing of Cu-SSZ-13 and Cu-SAPO-34 (both structures having the CHA-type framework) - Theoretical DFT calculations on relevant parameters found by the in-depth investigation of the afore-mentioned materials - Synthesis and testing of similar materials with 8-MR windows to elucidate the influence of the zeolite sub-structure i.e. if different ring sizes in the structure influences the catalytic performance Relevant characterization techniques include, besides conventional methods, in situ methods such as: high resolution (transmission) electron microscopy, infrared (and raman) spectroscopy together with X-ray absorption spectroscopy. These are all techniques that will complement each other to produce invaluable results. Zeolites are today applied in many and diverse applications both within automotive and environmental catalysis, but also within the petrochemical and renewable chemistry. The findings of this project are therefore also believed to contribute to a more comprehensive understanding of this class of materials, relevant to many areas of heterogeneous catalysis, and therefore have the potential, to create research and business with very high impact. / Vennestrom, PNR. (2014). Selective catalytic reduction of nitrogen oxides with ammonia over microporous zeolite catalysts [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/43217 / TESIS
9

Systém pro snížení NOx / NOx Reduction System

Karafa, Pavel January 2017 (has links)
This diploma thesis deals with the issue of nitrogen oxides emissions in exhaust gases and possibilities of their reduction. The task of the thesis was analysis of systems for NOX reduction by contemporary diesel engines, design and construction of NOX reduction device for given diesel engine, then verify functionality of this system compiled from commercially available components. In the last part of thesis available measurements will be made with an analysis of achieved results.
10

Snižování oxidů dusíku z proudu spalin na speciálních katalyzátorech / Reduction of nitrogen oxides in flue gas on special catalysts

Vávra, Jan January 2018 (has links)
The diploma thesis is focused on experimental reduction of nitrogen oxides on special catalysts. The latest and state-of-the-art flue gas cleaning technologies are used. Selective catalytic reduction results in the desired level of pollution. It is necessary to meet the prescribed emission limit. A ceramic honeycomb filter based on vanadium and titanium is used as the catalyst. The entire measurement is carried out on the experimental INTEQ II unit, which is installed in the flue gas cleaning laboratory at NETME Center. It is shown which operating parameters achieve better efficiency of flue gas cleaning. Comparison of the BASF and CERAM catalysts is also performed. Finally, a material balance of the system is performed and a new external electric heater is designed to accelerate the heating process.

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