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Experimental and numerical research of the selective catalytic reduction system for diesel engine cars
Abstract
The aim of the dissertation was to conduct experimental and numerical research for a
selective catalytic reduction (SCR) system of a passenger cars with a diesel engine. The research consists of comparing the results for two different SCR systems, existing and new developed. The developed SCR system is aimed at introducing it to the secondary market (Aftermarket), which is also associated with the development of its own mixer design.
Due to increasingly stringent emission standards in particular nitrogen oxides (NOx), the SCR systems have recently been invented and installed in diesel cars around the world. These systems must be validated during emission tests on the reduction of NOx to the appropriate limit, in order to authorise a car do drive. To achieve this goal a coupled approach needs to be applied incorporating both extensive experimental research and advanced numerical methods based on computational fluid dynamics (CFD).
Therefore, in the research work various design variants of the SCR system and mixers
at different operational parameters were studied. Several solutions were investigated under conditions that reflected the real operating conditions of the diesel engine operation. Among other things, pressure drops on monoliths, gas distribution and conversion of nitrogen oxides were tested and analyzed on prototypes in Tenneco laboratories. Furthermore, for the purpose of numerical model development, laser scanning was used to extract 3D models of the real geometries of the system elements by using a reverse engineering approach.
A commercial code ANSYS Fluent was used to perform the multiphase computational
fluid dynamics studies. A careful analysis has been done for the subsequent processes occurring in the system, i.e. the evaporation and mixing of the reactants prior to the catalyst, proper distribution of flow through the catalyst and selection of appropriate thermal conditions for the process. Attention was given to the implementation of the SCR reaction kinetics. The CFD model was then validated against the experimental data showing good agreement between the measured and simulated parameters.
The final design of the replacement SCR part was compared with an original system
delivered by the original equipment manufacturer. It was found that application of the new mixer in the replacement SCR system led to slightly lower NOx emission, which was confirmed in the certification unit through emission tests in a car on the chassis dynamometer.
https://bip.polsl.pl/nadania_dr/damian-kurzydym/
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In-service problems of selective catalytic reduction systems for reduction of nitrogen oxides This paper describes the development of selective catalytic reduction systems which are used for reduction of nitrogen oxides in diesel engines in passenger vehicles. The paper presents a methodology of application development and solutions for problems encountered in service. The main solution which allows the requirements of Euro 6 limit to be met (as applied by various car manufacturers), is the introduction of the SCR system as an upgrade of the existing aftertreatment systems that meet the Euro 5 emission standard consisting of a diesel oxidation catalyst (DOC) and a particulate matter filter (DPF).
It is difficult to simulate both the flow field and the chemical reaction using, respectively, the flow state and kinetics calculations and actually reflect the influence of the gas flow state on the chemical change in a selective catalytic reduction (SCR) system. In this study, the flow field and the chemical reaction were therefore coupled to simulate a full Cu-Zeolite SCR system and the boundary conditions of the simulation were set by a relevant diesel engine bench test which included the exhaust temperature, the mass flow, and the exhaust pressure. Then, the influence of the gas flow state on the NOx conversion efficiency was investigated. Specifically, an orthogonal experimental design was used to study the influence of the injection parameters (position, angle, and speed) on the NH3 distribution by establishing the NH3 uniformity coefficient γ at the SCR catalyst capture surface in the flow field simulation. Then, the velocity capture surface of the SCR catalyst front section was sliced into coupled data transfer interfaces to study the effects of exhaust temperature, ammonia to NOx ratio (ANR), and the NO2/NOx on the NOx conversion efficiency. This was used as guidelines to optimize the SCR system control strategy. The results showed that a 1150 mm injection position, a 45°injection angle, and a 23 m/s injection velocity provided the most uniform NH3 distribution on the SCR catalyst capture surface. For constant injection parameters, the NOx conversion efficiency was the highest when the exhaust temperature was 200°C-400°C, the ANR was 1.1, and NO2/NOx was 0.5.
The article presents the results of experimental research and their comparison with CFD simulations for the original selective catalytic reduction system and WALKER replacement. The research was performed to develop the WALKER universal mixer. The SCR prototype without mixer and with the proposed mixer were tested and compared with the original VW part. The next step was reverse engineering, which consisted in scanning the tested parts with a laser and processing their point cloud in Leios2 program. Reverse engineering has allowed the reconstruction of 3D geometry of the tested parts in the Catia v5 program and then preparation their models for computational fluid dynamics. Numerical simulations were carried out in the Ansys Fluent program, thanks to which several quantities were determined e.g. uniformity index of gas flow through the monolith and coefficient of variation as a measure of mixing degree, which have a significant impact on the design of the mixer and the SCR system.
Selective catalytic reduction (SCR) has been exhibited as a promising method of NOx abatement from diesel engine emissions. Long-term durability is one of the key requirements for the automotive SCR system. A high NOx conversion, droplet distribution and mixing, and fluid film and solid deposit formation are the major challenges to the successful implementation of the SCR system. The current study is therefore three-fold. Firstly, high-speed images disclose detailed information of the spray impingement on the heated impingement surface. The spray impingement investigation took place in a specially-designed optically-accessible visualization chamber where the Z-type shadowgraph technique was used to capture the high-speed images. Wall temperature has a great influence on the film formation and wall wetting. A higher wall temperature can significantly increase the droplet evaporation, and consequently, wall wetting decreases. The numerical analysis was performed based on the Eulerian-Lagrangian approach using STAR CCM+ CFD code. Secondly, the resultant phenomena due to spray-wall impingement such as fluid film generation and transport, solid deposit formation, and thermal decomposition were recorded using a high-speed camera operating at a low frame rate. Infrared thermal imaging was used to observe the spray cooling effect after impingement. Spray impingement caused local cooling, which led to wall film formation, which introduced urea crystallization. Finally, solid deposits were analyzed and characterized using Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). FTIR analysis revealed that urea decomposition products vary based on the temperature, and undecomposed urea, biuret, cyanuric acid, ammeline, and melamine can be formed at different temperatures. TGA analysis showed that accumulated deposits were hard to remove. Moreover, complete thermal decomposition of deposits is not possible at the regular exhaust temperature, as it requires a comparatively long time span.
To improve ambient air quality, several countries have adopted regulations setting stringent limits on vehicular tailpipe emissions of particulates. The issue of high particulate emissions has been mostly addressed for diesel vehicles with the widespread adoption of diesel particulate filters (DPFs). Attention is now turned to gasoline direct injection (GDI) technology, which provides improved fuel economy and performance, but also increased particulate emissions, as compared to the port fuel injection (PFI) engines. Europe has set a particle number (PN) limit on emissions from GDI vehicles, while China has expanded that to include all gasoline vehicles. In the USA, these are regulated through particle mass (PM) limits. To meet these regulations, it is anticipated that gasoline particulate filters (GPFs) will be widely applied to gasoline exhaust after-treatment. GPF technology has rapidly advanced, and already a wide range of pore size distribution and cell geometries are being offered to minimize back pressure and offer high ash storage capacity, high filtration efficiency, and, in the case of filters combined with three-way catalytic functionality, high conversion of gas-phase criteria pollutants. This review summarizes representative studies on particulate emissions from gasoline engines, the nature of the particulates, and the advances in GPF technology.
The article presents the issues concerning the specific distance emission test results comparison of the exhaust gases from vehicles with Diesel engines. The tests in real driving conditions included: (in the first stage) the test on the proposed test route, based on the type approval test parameters and (in the second stage) testing in accordance with the European Union guidelines, where the research route was divided into urban, rural and motorway sections. The obtained results – in various test conditions – were used to compare the measured values of specific distance emissions, carbon dioxide, carbon monoxide, hydrocarbons, nitrogen oxides and the particle number. Not performing the road conditions measurements of particulate mass is in line with the requirements of the RDE tests, in which the focus was placed mainly on the assessment of nitrogen oxides and the particle number. The performed tests allowed making a comparison between the specific distance emission test results of exhaust components and the values obtained in the type approval test. These tests were also used to verify the legitimacy of performing full RDE tests and validating the specific distance emission results obtained in the proposed tests.
The paper describes results of experiments and CFD simulation for an original catalytic converter part and WALKER's replacement. Pressure drop was measured for catalysts, which influences on exhaust backpressure, an important parameter affecting the engine power, fuel consumption, emission of harmful gases and wear components. First the pressure drops was examined in the original catalyst and in prototypes of WALKER's. The prototypes differed in that their monoliths had different lengths, density of channels and contents of precious metals (PGM). In the next stage reverse engineering was applied, which included laser scanning of parts and then processing the point clouds in Leios2 program. Recreated 3D geometry of catalysts in Catia v5 program was prepared for computational fluid dynamics. For all studied cases the measured data were used to calculate coefficients of porous media model i.e. viscous and inertial resistances, which were used in the Ansys Fluent program. Static pressure drops on the monoliths were then predicted by the CFD model for different gas mass flow rates through the catalysts. Exhaust velocities and flow uniformity were also analysed. Comparison of the results for all parts allowed for stating, which prototype is better and which has operating parameters comparable to the original part.
Selective catalytic reduction (SCR) using Urea-Water Solution (UWS) is a NOx reduction method that many diesel engine manufacturers adopt to meet new regulations for mobile applications (cars, trucks, etc.,) throughout the world. However, this technology still faces many scientific and technological challenges. Indeed, the UWS injected spray can impact on the walls/mixer or lead to a marked inhomogeneity of ammonia at the SCR catalyst inlet; worse, this spray can affect the monolith by forming solid deposits. This work aims to investigate numerically such conditions using recently developed multidimensional models for the UWS evaporation and urea thermal hydrolysis upstream of a typical SCR catalyst. This study highlights the different timings of water evaporation and urea thermal hydrolysis processes in cold and hot conditions. It is found that the UWS Leidenfrost temperature is the main parameter of UWS modelling and numerical simulation. In cold conditions, for which the temperature of the walls is smaller than the UWS Leidenfrost temperature, the deposits are formed mainly on the mixer and near the pipe bends. In this case, water vaporizes first and more than 99% of the injected urea sticks on the wall forming a liquid film of pure urea. In such conditions, the main role of the mixer is to significantly increase the wall surface area covered by the liquid film, which may enhance the thermal hydrolysis processes for cold conditions. In hot conditions, for which the temperature of the walls is greater than the UWS Leidenfrost temperature, the deposits are formed on the inlet section of the monolith also because of the short distance between the UWS injector and the SCR monolith. Besides, the mixer does not play its role in such hot conditions, since the UWS spray droplets mainly splash on the mixer hot walls, and quickly leave towards the monolith. Finally, this study showed that deposits formation is an inherent phenomenon of UWS injection (whether in hot or cold conditions). They may be very problematic for the exhaust system durability since it can lead to the formation on the wall of solid by-products (Cyanuric acid or ammelide …) due to urea polymerization. In addition, deposits generally lead to a maldistribution of ammonia upstream of the SCR catalyst brick, as shown in the particular configuration of the exhaust pipe studied here.
Europe's regulation of passenger car emissions has been proven to have failed when it comes to nitrogen oxide emissions (NOx) by diesel engines. Due to historical decisions favouring diesel technology, Europe has become a diesel island with no equal worldwide. As a result, virtually every European citizen breathes in air which is deemed harmful to human health. Real driving emissions (RDE) testing by means of portable emissions measurement systems (PEMS) can potentially eliminate the discrepancy between lab and road tests, and will complement the dynamometer type-approval procedure from September 2017 onwards. Despite the significant potential of PEMS testing, the emission assessment has been watered down through politics to provide the automotive manufacturers with additional lead-time. In this way, the lab to road gap is not eliminated but only decreased. This means that diesel cars will continue to over-emit NOx until the 2020s at earliest. This has consequences for effectively bringing down local air quality issues, especially in low emission zones (LEZ).
This paper presents a review of the European emission regulation history up to date and makes a comparison with the approach in the other important car markets globally. One can conclude that a substantial update of the European regulatory framework concerning automotive emissions is required, while ambitious post-2021 targets should be set if Europe does not want its automotive industry to lose its competitive position in the global market. In addition, an equilibrium should be sought for between sustainable personal transport in the form of zero-emission vehicles (ZEV), and a sustainable economic climate for the automotive industry. The former is needed if LEZs are to effectively bring down pollutant levels in cities.
Sustainable mobility is a legal and public demand. An important measure to clean the exhaust gas of diesel engines is the conversion of nitric oxides with selective catalytic reduction (SCR). The development of SCR systems can be strongly supported by numerical methods. This review article is to document the status of the CFD‐simulation method with respect to the preparation of the injected urea‐water solution, the turbulent transport of ammonia and the assessment of solid deposits coming from liquid film. Comparisons with measurements for realistic sprays and mixing sections are given.
NH3 is a precursor of PM2.5 which deteriorates urban air quality, affects human health and impacts the global radiation budget. Since vehicles are important sources of NH3 in urban areas, we have satisfactorily studied the possibility of measuring NH3 emissions from gasoline and SCR-equipped diesel light-duty vehicles during real driving on-road operation using a portable FTIR. The performance of the portable FTIR resulted to be comparable to that of a laboratory-based FTIR during a series of experiments performed in the Vehicle Emission Laboratory (VELA) using the World-harmonized Light-duty Test Cycle (WLTC). Higher on-road NH3 emission factors were obtained for the gasoline vehicle than for the diesel. High NOx emissions were measured from the diesel vehicle, indicating a low efficiency of the DeNOx system, SCR. On-road NH3 emission factors were ∼2 times lower than during the laboratory tests at 23 °C for both vehiclesNH3 emissions were not observed for the diesel vehicle during cold start operation. However, NH3 cold start emissions from the gasoline vehicle were up to 2 orders of magnitude higher than during the entire road trips, ranging from 45 to 134 mg km⁻¹. Cold start emissions are of paramount importance as they commonly take place in urban areas. Hence, future urban reductions in PM2.5 might need to take into consideration the introduction of NH3 emissions limits for passenger cars.
Mobile sources contribute about 44% of outdoor toxic emissions, approximately 50% of cancer risk and at around 74% of noncancer risk health problems. Catalytic converter is quite needed in removing the pollutant and in preventing a health problem. The main problem in the catalytic converter is low oxidation resistance when operated at high temperature. Therefore, this paper aimed to develop catalytic converter material in high-temperature operation at around 1100 °C using FeCrAl foils as a metallic catalytic converter which coated by γ-Al2O3. This research is conducted using 3 various techniques such as ultrasonic bath for 3, 4, and 5 hours, Nickel (Ni) electroplating for 30, 45 and 60 minutes and the combination of ultrasonic bath and electroplating technique. Oxidation resistance analysis was conducted using tube furnace under argon gas for 60 hours in 3 cycles. Mass changes analysis of treated samples is showed by degradation mass. Lowest mass change of by ultrasonic bath samples is 0.3 wt%, for a combination of ultrasonic and electroplating samples is 0.3 wt% shown by UT 3 hours as well as 0.6 shown by EP 30 min. Parabolic rate constant is obtained by the time calculation based on the mass change of treated and untreated samples. It shown that UB 3 h is lowest parabolic rate constant of 2.258 × 10-20 g2 cm-4s -1 and UB 5 h is 1.13 × 10-20 g2 cm-4s -1. Lowest mass gain and lowest parabolic rate constant are become an indicator that the samples and that technique are recommended to fabricate the catalytic converter.
In the presented paper a simple method was applied to calculate a viscous and inertial resistant coefficient of a porous material substituting a flared fin heat sink geometry. Based on the performed calculations plots showing the variation of the resistant coefficients with heat sink height and distance between fins were made. In addition, empirical relations that can be applied for a calculation of required resistant coefficients were proposed. Finally a comparison of a flow characteristic both for the real heat sink and its porous representation for selected cases was shown. In the analysis an air flow in room temperature was investigated. Analyses were performed for a range of Reynolds number: 8.3·103 – 1.7 104. All the necessary numerical calculations were made in ANSYS Fluent commercial code, Microsoft Excel and Matlab.
The purpose of this study is to identify parameters influence on the particle deposition within fin and tube heat exchanger of air-conditioning systems by CFD analysis. First the basic sketch of periodic geometry drawn and meshing operation including boundary conditions was performed. Then the gas side properties and flow parameters were solved by ANSYS Fluent 14.5 software. Lagrangian equations used for modeling dispersed phase and effect of injecting various particle sizes evaluated by Rosin-Rammler model. Moreover turbulence effect was investigated using discrete random walk model. Consequently it is observed that deposition rate is highly affected by particles diameter in which leads to 90% deposition for particles over 30 mu m in diameter.
V2O5/WO3-TiO2 selective catalytic reduction (SCR) catalysts with a V2O5 loading of 1.7, 2.0, 2.3, 2.6, 2.9, 3.2 and 3.5 wt. % were investigated in the fresh state and after hydrothermal aging at 600 °C for 16 h. The catalysts were characterized by means of nitrogen physisorption, X-ray diffraction and X-ray absorption spectroscopy. In the fresh state, the SCR activity increased with increasing V loading. Upon aging, the catalysts with up to 2.3 wt. % V2O5 exhibited higher NOx reduction activity than in the fresh state, while the catalysts with more than 2.6 wt. % V2O5 showed increasing deactivation tendencies. The observed activation and deactivation were correlated with the change of the VOx and WOx surface coverages. Only catalysts with a VOx coverage below 50% in the aged state did not show deactivation tendencies. With respect to tungsten, above one monolayer of WOx, WO3 particles were formed leading to loss of surface acidity, sintering, catalyst deactivation and early NH3 slip. An optimal compromise between activity and hydrothermal aging resistance could be obtained only with V2O5 between 2.0 and 2.6 wt. %.
The promotion effect of ceria modification on the low-temperature activity of V2O5-WO3/TiO2 catalyst was evaluated for the selective catalytic reduction of NO with NH3 (NH3-SCR). The catalytic activity of 1wt% V2O5-WO3/TiO2 was significantly enhanced by the addition of 8wt% ceria, which exhibited a NOx conversion above 80% in a broad temperature range 190-450°C. This performance was comparable with 3wt%V2O5-WO3/TiO2, indicating that the addition of ceria contributed to reducing the usage of toxic vanadia in developing low-temperature SCR catalysts. Moreover, V1CeWTi exhibited approximately 10% decrease in NOx conversion in the presence of 60ppm SO2. The characterization results indicated that active components of V, W and Ce were well dispersed on TiO2 support. The synergetic interaction between Ce and V species by forming V-O-Ce bridges enhanced the reducibility of VCeWTi catalyst and thus improved the low-temperature activity. The sulfur poisoning mechanism was also presented on a basis of the designed TPDC (temperature-programmed decomposition) and TPSR (temperature-programmed surface reaction) experiments. The deposition of (NH4)2SO4 on V1CeWTi catalyst was much smaller compared with that on V1Ti. On the other hand, the oxidation of SO2 to SO3 was significantly promoted on the CeO2-modified catalyst, accompanied by the formation of cerium sulfates. Therefore, the deactivation of this catalyst was mainly attributed to the vanishing of the V-Ce interaction and the sulfation of active ceria.
Ammonia (NH3) is one of the most promising fuels regaining interest in recent years, as an alternative to
fossil fuels. It is often suggested as a more favorable solution than supplying pure hydrogen due to its higher
volumetric energy density. In this study a dual fuel CI engine running on ammonia and diesel mixture was
tested, as a means to utilize ammonias potential in diesel engines. The injection strategy was port injection
for ammonia, where it was mixed with air, and direct injection of diesel oil. A set of different NH3-diesel
mixtures were tested. Due to partial substitution of diesel fuel by NH3 the elementary composition of fuel
has changed. It increased nitrogen’s and reduced carbon’s mass in the combustion chamber. Consequently,
the exhaust species have also changed. The Fourier Transform Infrared Spectroscopy was used to analyze the
composition of flue gas. On top of investigating changes in CO2, CO, HC, NOX, residual ammonia molar fraction
was observed. Possibility of using residual ammonia to activate selective catalytic reduction (SCR) system and
its effectiveness in different operating points of engine were investigated. As part of the research, a numerical
model of operation of the used SCR system was also developed. Results from the experiments have served to
validate numerical model. Created model was then used to analyze wider spectrum of operating conditions of
the engine and SCR.
The article presents an experimental and numerical study of nitrogen oxides reduction in diesel car selective catalytic reduction (SCR) system. Due to increasingly stringent emission standards, SCR systems have recently been installed in diesel cars around the world. The system under investigation is a recently developed replacement part dedicated for secondary market (aftermarket). The analysis includes the whole aftertreatment system, i.e. the urea injection, the mixer and two-stage SCR system, and is aimed at meeting the desired performance and requirements of the European emission standards. Within the experimental work various design variants of the SCR system and mixers at different operational parameters were studied. Several solutions were investigated under conditions that reflected the real operating conditions of the diesel engine operation. Among other things, pressure drops on monoliths, gas distribution and conversion of nitrogen oxides were tested and analyzed. Furthermore, for the purpose of numerical model development, laser scanning was used to extract 3D models of the real geometries of the system elements. A commercial code ANSYS Fluent was used to perform the multiphase computational fluid dynamics studies, in which the urea-water solution droplets were tracked in a Lagrangian frame of reference and the catalysts, were treated as porous media. Attention was given to the implementation of the SCR reaction kinetics. The developed model, with implemented kinetics, was verified by comparing the results of numerical calculations with the analytical solution. The CFD model was then validated against the experimental data showing good agreement between the measured and simulated parameters. The final design of the replacement part was compared with an original system delivered by original equipment manufacturer (OEM) for this engine. It was found that application of the new mixer and separated SCR converters of the replacement part led to slightly lower NOX emission, and increased NH3 slip and pressure drop, but kept on acceptable level.
It is shown how the deactivation of a diesel SCR (Selective Catalytic Reduction) catalyst by compounds in the exhaust gases influences the kinetics of the catalyst. Results are given for the fresh (4.56 % V2O5/TiO2) catalyst and the ones used in the rig for 890 h and 2299 h. The reactions of 700 ppm NH3 and 2 % O2 in Helium yielded N2, N2O, and NO at increasing temperatures. Simulations were performed with COMSOL Multiphysics using a 3-D model of the catalyst system. The experimental values of the products N2, N2O, and NO were very nicely fitted by the kinetic model used. All three ammonia oxidation reaction rates were of the first order in the concentration of NH3. A preliminary study using non-isothermal conditions showed the maximal temperature increase to be 0.15 K. Thus, further simulations were done with an isothermal model. The deactivation reduces the pre-exponential factors and the activation energies for the formation of N2, N2O, and NO. The formation of N2 is not substantially influenced by deactivation. The changes in the kinetics of the catalytic NH3 oxidation by deactivation is reported for the first time in the present study.
Testing of real driving emissions (RDE) with portable emission measuring system (PEMS) in an appropriate road circuit became an obligatory element of new type approval of passenger cars since September 2017. In several projects the Laboratory for Exhaust Emissions Control (AFHB) of the Berne University of Applied Sciences (BFH) performed comparisons on passenger cars with different PEMS’s on chassis dynamometer and on road, considering the quality and the correlations of results. Particle number measuring systems (PN PEMS) were also included in the tests. The present paper informs about influences of E85 on RDE on two flex-fuel-vehicles, discusses some aspects of different ways of evaluation with different programs, shows comparison of different types of PN PEMS and represents the effects of simulation of slope on the chassis dynamometer.
The article presents comparative study of nitrogen oxides removal systems based on selective catalytic reduction (SCR). The systems under investigation are an original part of a passenger car with a diesel engine and a recently developed WALKER’s replacement part. Development of a SCR system is performed by a thorough analysis of the processes and subsequent devices of the system in order to meet the desired performance and requirements of the European emission standards. For this purpose, experimental research was conducted for the SCR systems at different operational parameters. Proposed solutions were investigated under conditions that reflected the real operating conditions of the diesel engine. Among other things, gas distribution and conversion of nitrogen oxides were tested and analyzed. In the next step, laser scanning was used to extract 3D models of the real geometries. This allowed for preparing the SCR systems for comparative simulations by means of ANSYS Fluent computational fluid dynamics (CFD) software. The developed numerical model takes into account the flow of flue gases, as well as the injection, evaporation and decomposition of the urea-water solution. The geometry encompasses the inlet part, the mixer and catalyst, which was treated as a porous medium. Attention was given to the implementation of the catalytic reaction kinetics, which was represented by a global mechanism. The implementation was verified by comparison with analytical solution. The overall CFD model was then validated against the experimental data showing good agreement between the measured and simulated parameters.
Catalytic converters with non-linear channel structures were prepared using 3D printing and tested in the oxidation of methane in a simulated dual-fuel engine exhaust stream. The design used a simple repeating angular offset between adjacent layers, which was sufficient to introduce complexity with minimal software programming. All 3D printed substrates were mechanically stable and, following washcoating with a composite catalyst, demonstrated higher catalytic activity in methane oxidation than a commercial honeycomb substrate. The methane conversion at e.g. 510 °C was 12.6% on the commercial sample, 72.6% for 90°, 80.1% for both 30° and 45°, and 89.6% for the 60° oriented structures. This enhancement is attributed to the increased turbulence/mass transfer and surface area than are possible using conventional straight-channelled substrates. Computational fluid dynamics (CFD) analysis confirmed that the higher methane conversion over 3D printed substrates is due (at least partially) to its higher turbulence kinetic energy. Backpressures over the 3D printed structures were also experimentally measured and compared with the conventional honeycomb monolith.
Selective catalytic reduction with NH3 (NH3-SCR) is the most efficient technology to reduce the emission of nitrogen oxides (NO
x
) from coal-fired industries, diesel engines, etc. Although V2O5-WO3(MoO3)/TiO2 and CHA structured zeolite catalysts have been utilized in commercial applications, the increasing requirements for broad working temperature window, strong SO2/alkali/heavy metal-resistance, and high hydrothermal stability have stimulated the development of new-type NH3-SCR catalysts. This review summarizes the latest SCR reaction mechanisms and emerging poison-resistant mechanisms in the beginning and subsequently gives a comprehensive overview of newly developed SCR catalysts, including metal oxide catalysts ranging from VO
x
, MnO
x
, CeO2, and Fe2O3 to CuO based catalysts; acidic compound catalysts containing vanadate, phosphate and sulfate catalysts; ion exchanged zeolite catalysts such as Fe, Cu, Mn, etc. exchanged zeolite catalysts; monolith catalysts including extruded, washcoated, and metal-mesh/foam-based monolith catalysts. The challenges and opportunities for each type of catalysts are proposed while the effective strategies are summarized for enhancing the acidity/redox circle and poison-resistance through modification, creating novel nanostructures, exposing specific crystalline planes, constructing protective/sacrificial sites, etc. Some suggestions are given about future research directions that efforts should be made in. Hopefully, this review can bridge the gap between newly developed catalysts and practical requirements to realize their commercial applications in the near future.
To analyze the effect of a gasoline particulate filter (GPF) attachment on the emissions of gasoline direct injection (GDI) vehicles, this study compares the emission results of three types of vehicles: conventional GDI vehicles, vehicles with a GPF at the close-couple catalytic converter (CCC), and vehicles with a GPF at the under-floor catalytic converter (UCC). Regulated particulate matter (PM) and particle number (PN) emitted from test vehicles were measured using gravimetric methods and condensation particle counter (CPC) equipment. In addition, this study analyzed nanoparticle size distribution, organic carbon (OC), elemental carbon (EC), and ammonia (NH3) using EEPS, OC-EC analyzer, and HFIR equipment. In cases of regulated particle emissions, both PM and PN satisfy EURO 6c and are reduced when a GPF is attached. Particulate emissions are especially reduced when the GPF is attached at the UCC position. This is believed to be why a soot layer is formed in stable flow. Emissions of nanoparticles and OC/EC are high in US06 mode at high driving speed. This is considered to be the influence of the regeneration of the GPF as the temperature of the exhaust gas rises. The emission of NH3 is also highest in US06 mode, which is related to catalytic conversion efficiency.
Urea aqueous solution is injected into the hot exhaust flow which acts as the source of ammonia in SCR systems to reduce the NOx emissions of diesel vehicles. However, urea decomposition thus ammonia generation is not always perfect, resulting in solid urea deposit formation accumulated in exhaust pipe wall, on the catalyst surface and mixing element. Decomposition of urea and mitigation of solid deposit formation is a great challenge to implement automotive SCR systems. This study presents a combined experimental and numerical investigation of UWS spray, droplet breakup, urea decomposition and deposit formation of SCR systems. The investigation firstly focused on fundamental spray characteristics of UWS and its effects on urea decomposition. The experiment was conducted with a commercial pressure driven SCR injector into an optically accessible test chamber and z-type shadowgraph imaging was implemented to capture the spray images. Relevant dimensions of spray images were estimated by digital image processing. The injection quantity of the experimental injector was measured to get the injection rate. Temperature distribution in the exhaust pipe and ammonia mass fraction was measured for different exhaust gas temperatures to estimate the urea decomposition. Finally, FTIR was used to analyze the chemical composition of solid deposit formed due to incomplete urea decomposition. The numerical assessment was carried out by using STAR CCM+ CFD code. The Eulerian-Lagrangian approach was implemented for the modeling of multiphase flow and urea water spray droplets. The investigation reveals that higher injection pressure increases the risk of wall impingement and wall wetting that leads to deposit formation. Though injection pressure is dominant on droplet size, exhaust gas temperature has also significant effects on droplet size and evaporation. Spray injection causes local cooling that effects on urea decomposition and relatively higher exhaust gas temperature is required for the complete decomposition of urea. Major chemical composition identified in the deposit sample are unreacted urea, biuret, and cyanuric acid.
This paper contains the analysis of the mixture homogeneity in two types of static mixers, Koflo and Kenics, based on both experimental and CFD study. The research was also expanded to the recognition of the pipe that was used as a background. The scope of work includes the analysis of the CoV coefficients (as a measure of mixing degree) obtained for the mentioned devices and their comparison. The research showing the impact of such parameters i.e., pressure drop or L/d ratio on the mentioned mixture homogeneity was also presented. What is more, the axial dispersion model was introduced to describe the devices as non‐ideal reactors. According to this, new relations for the CoV predictions (taking into consideration the turbulence intensity, the deviations from well‐known ideal states, and the mixing elements' shape) were developed. As a result, it was proven that static mixers are highly efficient devices because the obtained CoVs were at around 2% in the laminar flow and even less than that in the turbulent regime. When the pipe was used, the satisfactory value of the CoV was obtained when the Reynolds number was increased to 2600. This article is protected by copyright. All rights reserved
This chapter provides an introduction to driving cycles. Driving cycles are defined and classified with respect to each application and each application’s special attributes. A detailed discussion on the characteristics of each category (chassis dynamometer and engine dynamometer, modal and transient) is performed, with emphasis placed on cycle representativeness issues. Furthermore, the various metrics used to assess the technical properties and shortcomings of test cycles are detailed. The last part of the chapter describes the procedure followed when developing a test cycle; both the collection of driving data as well as the cycle construction techniques are discussed.
A model based mathematical optimization methodology to optimize the precious metal loading profile (PGM loading) in zone-structured catalytic converters is developed. To carry out this task, a multi zone-structured optimization formulation, where the catalyst is divided into N zones axially to obtain a non uniform optimal PGM loading profile, which can be tested experimentally, is used. The effects of the PGM loading on washcoat diffusion limitations is also considered. The objective is to optimize the spatial distribution of loading for a fixed amount of precious metal to maximize the chemical conversion efficiency under transient operation. To achieve this, the transient 1D + 1D model is solved with the help of implicit solver DASPKADJOINT and translated into a non-linear optimization problem that can be solved with any derivative based nonlinear programming (NLP) solvers. The model is applied to two example cases: CO oxidation on a Pt/Al2O3 based Diesel Oxidation Catalyst catalyst (minimizing cold-start emissions) and CH4 oxidation on Pd/Al2O3 (minimizing deactivation effects). In both the cases it was observed that the optimal solution with maximum PGM loading in the channel entrance region improved the performance of the catalysts. The methodology presented is generic and can be transferred to different systems with different chemistries, which may result in significantly different optimization results and loading patterns.
Urea-SCR has received worldwide attention for reducing the harmful NOx emission from present diesel engines. But this faces lot of challenges such as complete decomposition of reducing agent urea and its deposition at the bottom of exhaust tail pipe, lack of uniform distribution of the urea-decomposed ammonia during the continuous running of the engine. This study is involved with CFD evaluation of urea decomposition rate by adopting different urea injection angles and nozzle positions. Also, urea atomization and evaporation/decomposition to ammonia and ammonia distribution on tail pipe cross-sectional area are investigated. Exhaust tail pipe is fitted with guided pipe at different angles. Also, urea and air are injected at different pressures respectively, in the twin-flow nozzle. The CFD analysis indicated that, the ammonia conversion rate is well improved using guided pipe fitted at 30° inclination with exhaust tail pipe. The CFD analysis is validated by engine experiments. It was proven that, the conversion increased for the 5bar urea and 1bar air.
Selective catalytic reduction based on urea water solution as ammonia precursor is a promising method for the NOx abatement form exhaust gasses of mobile diesel engine units. It consists of injecting the urea-water solution in the hot flue gas stream and reaction of its products with the NOx over the catalyst surface. During this process flue gas enthalpy is used for the urea-water droplet heating and for the evaporation of water content. After water evaporates, thermolysis of urea occurs, during which ammonia, a known NOx reductant, and isocyanic acid are generated. The uniformity of the ammonia before the catalyst as well as ammonia slip to the environment are important counteracting design requirements, optimization of which is crucial for development of efficient deNOx systems.
Urea Water Solution (UWS) is injected to generate NH3 in Selective Catalytic Reduction (SCR) system of modern automobiles. Thermal and fluid dynamic conditions such as temperature and Reynolds number of the flow favors ammonia generation in terms of heat transfer to UWS droplets by forced convection. During extremely cold weather conditions and low exhaust temperatures, the overdosing of UWS results in deposits of urea and its byproducts. As deposit depletion changes the stoichiometry of NOx/NH3, any predictive method becomes complementary to experimental studies on deposit formation. In the present work, we experimentally investigated deposit formation and its rate by a newer concept of usage of Stainless Steel (SS) foils considering temperature and flow rate as variables. According to numerical results, the droplet evaporation of UWS decreases as flow rate increases. For a fixed rate of UWS quantity of deposits decrease with increase in temperature and flow rate. Accordingly, structural changes are observed. Numerical values of time dependent deposit formation found slightly superior to the experimental values. The study revealed that deposit areas at low temperatures are comparable to numerical values. Phenomenological model is proposed to find deposit conversion factor for low temperatures (150-250°C), which helps in tuning of UWS dosage strategy to prevent NH3 slip.
This review paper summarizes major and representative developments in vehicular emissions regulations and technologies from 2015. The paper starts with the key regulatory advancements in the field, including newly proposed Euro 6 type regulations for Beijing, China, and India in the 2017-20 timeframe. Europe is continuing developments towards real driving emissions (RDE) standards with the conformity factors for light-duty diesel NOx ramping down to 1.5X by 2021. The California heavy duty (HD) low-NOx regulation is advancing and may be proposed in 2017/18 for implementation in 2023+. LD (light duty) and HD engine technology continues showing marked improvements in engine efficiency. Key developments are summarized for gasoline and diesel engines to meet both the emerging criteria and greenhouse gas regulations. LD gasoline concepts are achieving 45% BTE (brake thermal efficiency or net amount of fuel energy gong to the crankshaft) and closing the gap with diesel. Projections indicate tight CO2 regulations will require some degree of hybridization and/or high-performing diesel engines. HD engines are demonstrating more than 50% (BTE) using methods that can reasonably be commercialized; and proposals are developed for reaching 55% BTE. Lean NOx control technologies are summarized, including SCR (selective catalytic reduction), SCR filters, and combination systems. Emphasis is on durability, N2O, and greatly reduced emissions. Diesel PM (particulate matter) reductions are evolving around the nature of soot and the distribution in the filters. Gasoline direct injection (GDI) particulates carry PAHs (polycyclic aromatic hydrocarbons) through the three way catalyst, but filters can remove most of them. Gasoline particulate filter regeneration is now better understood. Improved understanding of oxidation catalyst formulations are reported with further quantification of the impact of precious metal formulations. Finally, the paper discusses some key developments in three-way catalysts, with improved understanding of the catalyst-support interactions, and the introduction of a low-mass cellular substrate that improves TWC cold start performance.
Background/Objectives: This research analyzes the influence of Urea SCR System's mixer and optimization of the Catalyst's position through post technologies in order to satisfy the Euro 6 regulations. Methods/Statistical Analysis: The simplified modeling was facilitated to interpret the current exhaust system. Considering the latter as the background, ANSYS was used for the Analysis. As for the method of Analysis, 5 flux variables were considered from exhaust flux of 1,000rpm~3,000 rpm in increments of 500 rpm. Findings: When the flux passes through the mixer, the speed dispersion increased and as the rpm increased, the results showed that 1,000 rpm compared to 3,000 rpm, the back pressure elevated over 500 Pa. By assessing the degree of uniformity, the uniformity expanded faster at high rpm and in low rpm, the flux itself is low so the uniformity gradually increased as it reached the latter part. Overall efficiency of over 95% was achieved at approximately 15~20 cm and in the points after that, the degree of uniformity gradually decreased in the graph. Therefore, it was determined that optimal position for the SCR catalyst should be 20~50 cm from the posterior of the mixer. Application/Improvements: This research was analyzed of the distribution uniformity effect in the exhaust system, the analysis result could be optimized of catalyst-position and improved uniformity index.
In terms of energy efficiency and exhaust emissions control, an appropriate design of cooling systems of climatic chambers destined to vehicle certification and/or perform scientific research is becoming increasingly important. European vehicle emissions certification (New European Driving Cycle, NEDC) stablishes the position of the wind-simulation blower at 200 mm above floor level. This height is fixed and kept constant independently of the vehicle tested. The position of the blower with respect to the vehicle can modify the external forced convection under the car, where after-treatment devices are located. Consequently, the performance of such devices could be modified and emission results during the certification cycle could be non-representative of real-world driving conditions. The aim of this work is to study the influence of different wind blower-vehicle relative heights on the air velocity and temperature profiles under the car by means of a simple computational fluid dynamics (CFD) approach.
Most modern diesel engine aftertreatment systems comprise functions to oxidize hydrocarbons and CO, reduce particulate emissions (mass and number), and reduce NOx enabling compliance with ever tightening regulations. A typical system layout based on SCR as a means to reduce NOx is shown in Fig. 20.1a.
This chapter delineates the mechanistic aspects of the NO–NH3–O2 reacting system, also known as the standard SCR reaction. The standard SCR technology was first developed in the 1970s, and thus has a long history in the research area of catalyst development as well as the associated reaction mechanisms.
This chapter will provide an overview of the major legislative and technology developments related to SCR for mobile applications. Regulatory initiatives are moving forward in all markets in the light-duty (LD), heavy-duty (HD), and non-road (NR) (agriculture and construction) sectors. NOx regulations are quite tight, forcing deNOx aftertreatment in all US LD diesel applications, and most HD and NR applications in the US, Europe, and Japan. CO2 regulations are also tightening worldwide, driving deNOx technologies on diesel engines. Knowledge of engine technologies will help define required deNOx aftertreatment needs. In the HD sector, because of the fuel consumption versus NOx trade-off, deNOx means deCO2. As such, 98 % selective catalytic reduction (SCR) deNOx to result in elimination of EGR (exhaust gas recirculation) and minimum operating cost and CO2 emissions is desired. The situation is similar on the LD side. Once SCR is added to the vehicle to meet the NOx regulations, it will likely be used to maximum efficiency in the context of consumer acceptance of ammonia consumption and replacement. On-board ammonia delivery systems for the SCR catalyst principally comprises liquid urea and gaseous ammonia types. Liquid ammonia systems are in their third generation of cost reduction and simplification. The gaseous ammonia system is attractive in LD applications due to the heavy weighting of cold start emissions in the regulations, and the ability to deliver ammonia at low exhaust temperatures. The field of catalysts, including oxidation catalysts, SCR catalysts, and ammonia slip catalysts is very dynamic. Oxidation catalysts are used to generate NO2 for better low-temperature performance of certain SCR catalysts (like Fe-zeolites). However, they can have undesirable N2O emissions, and NO2 production decreases with palladium content, when replacing platinum. Zeolite SCR catalysts are evolving beyond the copper and iron varieties into other formulations with better low-temperature and/or high-temperature performance. Vanadia catalysts have made recent impressive gains in high-temperature durability. High-efficient SCR systems generally are run with excess ammonia. Ammonia slip catalysts (ASC) are becoming more selective towards nitrogen, minimizing N2O, and NOx by-product formation. Finally, system design considerations are discussed, as well as emerging technologies. The author closes with comments on the future outlook and opportunities.
Titania supported vanadia catalysts have been widely used for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) in diesel exhaust aftertreatment systems. Vanadia SCR (V-SCR) catalysts are preferred for many applications because they have demonstrated advantages of catalytic activity for NOx removal and tolerance to sulfur poisoning. The primary shortcoming of V-SCR catalysts is their thermal durability. Degradation in NOx conversion is also related to aging conditions such as at high temperatures. In this study, the impact that short duration hydrothermal aging has on a state-of-the-art V-SCR catalyst was investigated by aging for 2 hr intervals with progressively increased temperatures from 525 to 700°C. The catalytic performance of this V-SCR catalyst upon aging was evaluated by three different reactions of NH3 SCR, NH3 oxidation, and NO oxidation under simulated diesel exhaust conditions from 170 to 500°C. The catalytic activity for SCR was stable when this V-SCR catalyst was aged up to 630°C. Significant SCR reactivity loss was observed when aging at temperatures above 680°C. SEM/EDS and N2 adsorption measurements were applied to elucidate the structure change during the aging process. Increasing the hydrothermal aging temperature results in not only the decrease of the BET surface area and BJH pore volume of the V-SCR catalyst as measured by N2 adsorption, but also the crack formation of the washcoat as illustrated by SEM.
This overview summarizes the development and application of vanadia-based urea/NH3-SCR for mobile applications. The focus is on heavy-duty diesel engines where this technology has been put to the market on a broad scale. A short history of vanadia SCR technology for diesel engines and how the technology emerged as the choice for mobile diesel engines is introduced together with related emission legislation. The layout of a typical mobile vanadia SCR system is described including different sensors and control strategies. Design considerations important for mobile vanadia SCR systems are discussed as well as washcoated and fully extruded catalysts. Some attention is put on enhancing NOx conversion by using vanadia-based SCR catalysts together with an oxidation catalyst. Durability and different deactivation mechanisms for vanadia-based SCR catalysts, relevant for mobile applications are discussed.
The presented work describes how numerical modeling techniques were extended to simulate a full Selective Catalytic Reduction (SCR) NOx aftertreatement system. Besides predicting ammonia-to-NOX ratio (ANR) and uniformity index (UI) at the SCR inlet, the developed numerical model was able to predict NOx reduction and ammonia slip. To reduce the calculation time due to the complexity of the chemical process and flow field within the SCR, a semi-1D approach was developed and applied to model the SCR catalyst, which was subsequently coupled with a 3D model of the rest of the exhaust system. Droplet depletion of urea water solution (UWS) was modeled by vaporization and thermolysis techniques while ammonia generation was modeled by the thermolysis and hydrolysis method. Test data of two different SCR systems were used to calibrate the simulation results. Results obtained using the thermolysis method showed better agreement with test data compared to the vaporization method. Decreasing the distance between the injector and SCR inlet decreased ANR and UI, while NOx concentration at the SCR exit increased but ammonia slip decreased. For a constant distance between the urea injector and the SCR inlet, putting a mixer closer to the injector changes ANR slightly while UI remains nearly the same.
The paper describes numerical simulations of flow with combustion in a coal-fired grate boiler. The work focuses on the NOx reduction process utilizing SNCR (selective non-catalytic reduction) technology. An integration of 12-step reduced mechanism with commercial CFD (computational fluid dynamics) code was presented. The SNCR reaction chemistry was fully coupled with the turbulent fluid mechanics and heat transfer via user-defined interface. The reduced mechanism was compared to 7-step global model which is commonly used in CFD codes. The comparison was done in the first step via plug-flow calculations. A better performance of 12-step reduced mechanism was demonstrated. Second stage of comparison was done with the CFD code. Final NO prediction at the boiler outlet for both mechanisms is similar and sufficiently accurate in relation to measurement. Nonetheless contours of NO concentrations suggest different NO rate in the initial stages of reduction. A similar trend was observed for secondary reactants NH3, HNCO. Large differences in calculated values at the outlet were observed. Disparity in secondary pollutants predictions is also observed. The general trend in NH3, N2O, HNCO and CO predictions was similar as in the plug flow calculations. These results suggest necessity of more detailed verification in a pilot or industrial scale to evaluate which mechanism is more accurate for coupled fluid dynamics computations.