urea-SCR Systems Research Articles

Numerical Study on Compact Design in Marine Urea-SCR Systems for Small Ship Applications

With increasingly stringent emissions legislation, such as that stipulated by the International Maritime Organization, for nitrogen oxide emission reduction in marine diesel engines, the imperative of curtailing nitrogen oxide emissions from marine diesel engines is intensifying. Consequently, the significance of aftertreatment technologies, including diesel particulate filters (DPFs) and selective catalytic reduction (SCR), is poised to grow substantially. In particular, a redesign is required to reduce the size of DPF and SCR systems for application in small ships. In this study, we varied the shape of the filters in DPF and SCR systems, aiming to achieve a distinct flow pattern and enable overall miniaturization. The performance metrics, including the nitric oxide (NO) reduction rate, NH3 slip rate, and pressure drop, of the redesigned models were compared with those of the conventional model. Computational fluid dynamics simulations were used to compare the performance of the redesigned model with that of the conventional model in terms of NO reduction and pressure drop. The redesigned system achieved a NO reduction rate of 6.9% below that of the conventional system, offering additional noteworthy benefits such as a 50% reduction in both pressure and overall length.

Solid–Gas Sorption System for Ammonia Storage and Delivery Driven by Engine Waste Heat for NOx Reduction of Diesel Engine

A new design of a sorption-selective catalytic reduction (SCR) system is proposed to improve ammonia storage density and meet the ammonia demand for high NOx conversion efficiency at a relatively lower temperature (<100 °C) compared to urea-SCR systems. The major components are a main unit and a start-up unit that each contain a metal halide ammine as the sorbent. The start-up unit can operate without any external heat source, but spontaneously releases ammonia at the ambient temperature and is only used when the main unit is being warmed up for action. The selection criteria for the metal halide ammine for each unit is discussed. The working pair of SrCl2 as the main ammine and NH4Cl as the start-up ammine is further analyzed as an example to be used in the sorption-SCR system for a diesel engine, the NOx emissions of which were experimentally measured in different operation modes. Based on the experimental data of engine emissions and kinetic models of the chemisorption between ammines and ammonia, the dynamic performance of the sorption system with a total capacity of 180 L sorbent composite in different layouts was investigated and compared. It was found that the achievable desorption conversion degree was lower in smaller reactors and was more sensitive to operating conditions in smaller reactors compared to larger reactors. This suggests that a system using a small reactor layout requires some extra volume to completely meet the required capacity compared to a larger reactor layout. However, because systems with large reactors tend to respond slowly, as they have more thermal mass and take a longer time for preparation, there is a design trade-off required to have optimal performance and balance between the main unit and the start-up unit. In the case studied in this work, a system using three rechargeable reactors with a volume of 60 L each was found to be the preferable layout; it could have about a 90% desorption conversion degree and required around 10 min of warm-up time. Meanwhile, the coupled start-up unit should have a capacity of around 165 mL at least.

Automated Model Calibration for Urea-SCR Systems Using Test-Rig Data

Automated Model Calibration for Urea-SCR Systems Using Test-Rig Data

Simplified Modeling and Analysis of Surface Temperature Distribution in Electrically Heated Catalyst for Diesel Urea-SCR Systems

Impending emission regulations of diesel engines for construction machineries would regulate nitrogen oxide emissions strictly in cold operating conditions. The urea-based selective catalytic reduction (urea-SCR) system coupled with the electrically heated catalyst (EHC) has been considered as a potential measure to meet the strict emission regulations by promoting evaporation and thermal decomposition of urea–water solution in cold operating conditions. Analyzing the thermal conditions in the EHC is crucial for the optimized operation and control of EHC-based urea-SCR systems under various engine operating conditions. In the current study, we introduce a simple one-dimensional analysis scheme to characterize the surface temperature distribution in the EHC based on energy conservation and the theories of forced internal convection. Since the EHC has a complicated internal structure with fine flow cells inside it, a flow cell in the EHC is extracted for the one-dimensional heat transfer analysis. EHC operation parameters such as exhaust gas flow rate and supplied electric power to the EHC are scaled to be applied for the flow cell analysis. The adequacy of the analysis scheme is then validated by surface temperature measurement results at the EHC outlet. The validation results showed over 95% prediction accuracy of the 1D analysis scheme in the operating conditions of a heavy-duty diesel engine. Based on proven reliability, the effects of geometric and operation parameters on the surface temperature distribution in the EHC were analyzed and discussed using the analysis results.

Effects of thermal treatments on characteristics and morphological variations in the deposits of urea-SCR systems

Selective catalytic reduction (SCR) systems are employed by automobile manufacturers for the abatement of environmental pollutants like oxides of nitrogen (NOx) emitted from exhaust gases of diesel engines. In SCR, the urea-water solution (UWS) is injected to exhaust gases in the form of a spray to generate the reducing agent NH3. Deposit formation at lower temperatures is a major concern with this technology. The deposits not only create backpressure but also leak NH3 to the environment as they deplete. It is very important to know the depletion characteristics of deposits formed at lower temperatures in order to assess the NH3 leakage to the environment when the engine exhaust gases attain higher temperatures. In the present work, deposits formed at a low-temperature range of 150–200°C for continuous run along with UWS injection were investigated. Additionally, they were aged at 300°C in the absence of UWS to check the variation in characteristics with the rise of temperature. By gravimetric analysis, it is inferred that the deposits formed at higher pre-age temperatures are less prone to depletion as the temperature increases. The elemental analysis using energy-dispersive X-ray spectroscopy (EDX) indicates slight variation in carbon, nitrogen and oxygen compositions for all the pre-age conditions. As an extended study, the byproducts at pre-age and post-age conditions were investigated through X-ray diffraction (XRD). The compounds like cyanuric acid (CYA) and biuret were not observed when pre-age samples were aged at 300°C. Instead, the compounds like ammelide, ammeline, triuret and melamine were observed. Scanning electron microscope (SEM) study revealed morphological changes in both pre-age and post-age samples. Further, the crystallinity variations were also observed for the changes in the heating cycles during deposit formation. The gravimetric analysis of deposits in pre-age and post-age conditions helps in predicting the amount of deposits for transient load cycles.

Effects of Temperature and Turbulence on Characteristics and Morphological Variations of Urea Deposits in Urea-SCR Systems. A Study Using SEM and XRD

The Urea-Water Solution (UWS) is sprayed into the diesel engine exhaust gases of automobiles in order to generate reducing agent NH3 in Urea-Selective Catalytic Reduction (SCR) systems adopted to diesel engines of modern automobiles to mitigate oxides of nitrogen (NOx). Although it is a very efficient technique, deposit formation at low temperatures is a major concern. The growth of deposits not only reduces the efficiency of NH3 generation but also generates non-depleting complex compounds. The temperature of exhaust gas and the Reynolds number of the flow have significant effects on this deposit formation. In this study, the compositional variation and the crystallinity of the deposits are investigated using Energy Dispersive X-ray Spectroscopy (EDX) and X-ray diffraction analysis (XRD) with respect to variation in temperature and flow rate at the preliminary stage. The elemental analysis is done for different samples using EDX that indicates urea as a major constituent in most of the deposits for a temperature range of 423-523K for overdosed conditions. The XRD results indicate the evolution of various traces of urea-related compounds at different temperatures and for turbulent flow of Reynolds Number 9000-32000. Using Scanning Electron Microscopy (SEM), the morphological variations of the deposits have been studied for various temperature and flow conditions. The comparative study of the morphology of deposits during the initial stages of deposit formation indicates an extent to which growth can occur and probable areas of deposits for further growth.

The Reactions and Role of Ammonia Slip Catalysts in Modern Urea-SCR Systems

The modern mobile aftertreatment systems (ATSs) include ammonia slip catalyst (ASC) located after the selective catalytic reduction (SCR) catalyst. Mainly copper, iron and vanadium SCR catalysts are applied commercially. SCR systems are mostly located after diesel particulate filter (DPF), which is thermally a demanding location for SCR and ASC. ASC properties are integrated to control strategy together with the SCR catalyst to reach NOx conversions above 95%. Reactions by Pt loading, Pt layer radial location (bottom, top, full layer) and feed conditions were simulated experimentally in synthetic gases on Cu, Fe and V based SCR catalysts in this study. NH3 slip after SCR unit was simulated in controlled (NH3/NO ≤ 1) or uncontrolled (NH3 only or NH3/NOx >> 1) conditions. The axial location of Pt relatively to the flow direction dominated to the radial location of Pt in reaction rates. The results showed significant differences in ammonia conversions and reaction selectivity (to N2, NO, N2O) by the Pt layer location (bottom or top) together with a thick SCR layer. The Pt loading was optimized to the range of 2–5 g/cft to keep a good selectivity to N2 as well as to avoid N2O/NOx formation and the Pt contamination effect on SCR. Pt-ASC was necessary to remove remaining NH3 and CO after SCR but the prevention of N2O formation is an essential part of design. Experimental simulations with 2-layer ASC with a Cu-chabazite (CHA) catalyst resulted in a complete mapping by the Pt loading as a function of space velocity and temperature as fresh and hydrothermally aged.

Improvement of NOX Reduction Rate of Urea SCR System Applied for an Non-Road Diesel Engine

Urea SCR technology has been widely used to reduce NOx emissions of diesel engines. Despite remarkable development for decades, more advanced control and optimization of urea SCR systems are still required as global NOx emissions standards are expected to become more stringent. This study investigated several influential parameters of urea SCR system to improve NOx reduction efficiency. This study uses a commercialized UWS (Urea Water Solution) supply system and a SCR catalyst which was installed in the exhaust line of a non-road CRDI (Common Rail Direct Injection) diesel engine. From this study, it was found that the low space velocity of SCR catalyst is essential for high NOx reduction efficiency, especially at low temperatures. Early injection of UWS enhances the overall NOx reduction efficiency if UWS injection was carefully controlled to avoid urea deposits. Rich injection of UWS with AOC is a good strategy for high NOx reduction efficiency. However, NOx reproduction in the AOC, which has an adverse effect on the overall NOx reduction rate, occurs at high exhaust gas temperatures. System insulation also can improve the NOx reduction efficiency by a few percentage points.

NOx abatement and N2O formation over urea-SCR systems with zeolite supported Fe and Cu catalysts in a nonroad diesel engine

NO and NO2 emissions over urea-based selective catalytic reduction (SCR) systems were investigated in a nonroad diesel engine. For catalysts of the SCR systems, two types of metal ion-exchanged zeolites were used; one is Fe, the other Cu. The Fe/zeolite was composed of Fe only, however, the Cu/zeolite consisted of Cu (80%) and Fe (20%). Nitrogen oxides (NOx) conversion efficiency of the Fe/ and Cu/zeolite catalysts remained greater than 80% in the temperature range of approximately 300 and 450 °C. The Cu/zeolite catalyst retained high efficiency above approximately 450 °C, however, the Fe/zeolite exhibited a drastic decrease resulting from the deterioration of NO conversion efficiency due to the ammonia storage capability of Fe/zeolite, which was inferior to that of Cu/zeolite. It was also found that NO was oxidized to NO2 in the diesel oxidation catalyst and diesel particulate filter, and NO2/NOx ratio at the SCR inlet consistently maintained thermodynamic equilibrium irrespective of the SCR catalyst. The ratio was higher than 50% between 200 and 410 °C. N2O emission that forms in SCR systems through side reactions during NOx reduction was also studied because N2O is a greenhouse gas with approximately 300 times greater global warming potential than carbon dioxide. The ratio of N2O at the SCR outlet to NOx at the SCR inlet for both catalysts was higher between 200 and 410 °C than at other temperatures, which implies that N2O formation is considerably involved in the NO2/NOx ratio. The Cu/zeolite catalyst in the urea-SCR system exhibited higher NO and NO2 conversion efficiencies while maintaining a lower amount of N2O formation compared to the Fe/zeolite catalyst.

Theoretical and Experimental Study on Flow Characteristics of a Diaphragm Pump for Urea-SCR Systems

Urea selective catalytic reduction (SCR) is the primary technology used to reduce the nitrogen oxides (NOx) of diesel engine exhaust. To meet the requirements of an SCR system, a novel type of miniature diaphragm pump was designed. Based on the theory of large deflection of annular plates, the equilibrium equations of a diaphragm with a rigid inclusion were established, and the equations were solved by the nondimensional method and the finite difference method. Theoretical and approximated flow model for this pump were proposed. A theoretical relationship between back pressure, rigid inclusion size and volumetric efficiencies were calculated. To verify the validity of theoretical model, a prototype pump was fabricated and tested. Experimental results demonstrated that the flow is proportional to the pump speed. The deviation between theoretical, approximated flow and experimental flow was less than 4 % and 9.4 %, respectively. The difference between theoretical and experimental volumetric efficiency was varied from 2.7 % to 6.1 % when back pressure changed from 0 to 0.9 MPa. The volumetric efficiency was growing with the increasing of the rigid inclusion size. The pressures in the working chamber showed almost the same overall trends between the theoretical results and experimental values. The experimental results show that the proposed theoretical model is effective.

A NOx Emission Model Incorporating Temperature for Heavy-Duty Diesel Vehicles with Urea-SCR Systems Based on Field Operating Modes

The selective catalytic reduction (SCR) is the most commonly used technique for decreasing the emissions of nitrogen oxides (NOx) from heavy-duty diesel vehicles (HDDVs). However, the same injection strategy in the SCR system shows significant variations in NOx emissions even at the same operating mode. This kind of heterogeneity poses challenges to the development of emission inventories and to the assessment of emission reductions. Existing studies indicate that these differences are related to the exhaust temperature. In this study, an emission model is established for different source types of HDDVs based on the real-time data of operating modes. Firstly, the initial NOx emission rates (ERs) model is established using the field vehicle emission data. Secondly, a temperature model of the vehicle exhaust based on the vehicle specific power (VSP) and the heat loss coefficient is established by analyzing the influencing factors of the NOx conversion efficiency. Thirdly, the models of NOx emissions and the urea consumption are developed based on the chemical reaction in the SCR system. Finally, the NOx emissions are compared with the real-world emissions and the estimations by the proposed model and the Motor Vehicle Emission Simulator (MOVES). This indicates that the relative error by the proposed method is 12.5% lower than those calculated by MOVES. The characteristics of NOx emissions under different operating modes are analyzed through the proposed model. The results indicate that the NOx conversion rate of heavy-duty diesel trucks (HDDTs) is 39.2% higher than that of urban diesel transit buses (UDTBs).

A Convergence Study on the Effects of NH 3 /NOx Ratio and Catalyst Type on the NOx Reduction by Urea-SCR System of Diesel Engine

Diesel engines have important advantages over its gasoline counterpart including high thermal efficiency, high fuel economy and low emissions of CO, HC and CO2. However, NOx reducing is more difficult on diesel engines because of the high O2 concentration in the exhaust, marking general three way catalytic converter ineffective. Two method available technologies for continuous NOx reduction onboard diesel engines are Urea-SCR and LNT. The implementation of the Urea-SCR systems in design engines have made it possible for 2.5l and over engines to meet the tightened NOx emission standard of Euro-6. In this study, we investigate the characteristics of NOx reduction with respect to engine speed, load, types of catalyst and the NH3/NOx ratio and present the conditions which maximize NOx reduction. Also we provide detailed experimental data on Urea-SCR which can be used for the preparation for standards beyond Euro-6.

An NOx Sensor-Based Direct Algebraic Approach-Newton Observer for Urea Selective Catalytic Reduction System State Estimations

NOx sensor-based state estimations for urea-based selective catalytic reduction (SCR) systems have attracted much attention in the past several years because of their significant importance in achieving high NOx conversion efficiency and low ammonia slip at low operation cost. Most of the existing SCR state estimation techniques require sophisticated design processes and significant tuning efforts, which may prevent them from widespread applications to production urea-SCR systems. In addition, the existing SCR state observers may not be able to achieve fast and accurate estimations due to the corresponding slow estimation error dynamics. The purpose of this study was to design a straightforward and effective NOx sensor-based SCR state estimation algorithm for decoupling post-SCR NOx sensor signals (NOx concentration, ammonia concentration), and for estimating ammonia coverage ratio of the urea-SCR systems. A singular-perturbation-based approach was applied to attain the reduced-order SCR model by decoupling the fast NO and NH3 concentration dynamic models from the slow ammonia coverage ratio dynamics model. Based on the reduced-order model, a direct algebraic approach (DAA)-Newton observer was proposed for estimating ammonia coverage ratio. The achieved ammonia coverage ratio estimation was applied to estimate the post-SCR NOx and NH3 concentrations. Simulation verification results under US06 cycle proved the effectiveness of the proposed method in accurately estimating the aforementioned key SCR states. The proposed observer can potentially be popularly applied to the production SCR systems for the advanced SCR control systems and on-board diagnostics.

Effect of UWS injection at low exhaust gas temperature on NOx removal efficiency of diesel engine

Urea-SCR systems have been widely used in diesel vehicles according to the strengthened NOx (Nitrogen Oxides) emission standard. The NOx removal efficiencies of the latest well optimized urea-SCR system are above 90 % at moderate exhaust gas temperature of 250 ~ 450 °C. However, a large amount of NOx is emitted from diesel vehicles at cold start or urban driving conditions, when the exhaust gas temperature is not high enough for SCR catalyst activation. Although many researchs have been stuied to improve NOx conversion efficiency at these low temperature conditions, it is still one of important technical issues. In this study, the effect of UWS injection at low exhaust gas temperature conditions is studied. This study uses a 3.4 L diesel engine equipped with a commertial urea SCR system. As a result, it is found that about 5 % of NOx removal efficiency is improved in the NRTC test when UWS injection starts at the SCR inlet temperature of 150 °C compared to 200 °C. It is also found that urea deposits can be formed on the wall of exhaust pipe, when the local wall temperature is lower than temperature of urea decomposition.

Modelling and control of urea-SCR systems through the triple-step non-linear method in consideration of time-varying parameters and reference dynamics

Owing to the dynamic operation mode of urea selective catalytic reduction (urea-SCR) systems, advanced control strategies are required to improve urea dosing control. A new control-oriented model presentation of urea-SCR systems is developed in this study. A novel controller based on the triple-step non-linear method is designed. The controller drives the non-linear system with time-varying parameters to track the variable ammonia coverage ratio. Unlike the existing triple-step non-linear method, the third design procedure in the proposed method is adjusted as an [Formula: see text] error feedback control. The proposed method provides a concise design process so that derivation of the control law can be simple and straightforward. The robustness of the controller against measurement noises and system uncertainties is analysed. A transient simulation is conducted to evaluate the effectiveness of the proposed control strategy.

Regulatory Update: CARB Passes USA’s First Regulations for Heavy-Duty Diesel Engine Replacement Filters

Emission regulations play a significant role in assuring that manufacturers of internal combustion engines utilize emission control devices. For on-road, heavy-duty Diesel vehicles, any replacement of such emission devices during the initial warranty period (5 years, 100,000 miles, or 3000 hours of operation in California, whichever comes first) is covered by the manufacturer, while any replacement thereafter is the vehicle owner’s responsibility. Vehicle owners who, past the initial warranty period, opt to replace such devices using the OEM-approved parts have come to the realization that this is a costly option, hence demanding lower-cost replacement devices commonly known as “aftermarket” parts. Not so fast, says the California Air Resources Board (CARB), as, historically speaking, aftermarket parts, especially when produced by non-OEM manufacturers, are not required to meet the same benchmark as the OEM-produced parts do. This is where the market and the regulations meet once again: Since automotive pollution is of major public interest and is clearly regulated, one can simply understand why regulatory agencies, such as the CARB, would be inclined to devise rules and guidelines for replacing components of emission control systems. On Friday 22 April 2016, the CARB officially adopted its drafted rules and guidelines for replacement parts for Diesel Particulate Filters (DPF), the first of its kind for Diesel exhaust emission control components in the USA. For now, the CARB rules are limited to 2007 through 2009 model year heavy-duty Diesel engines for on-road platforms; these were the vehicles having lower engine-out NOx and without urea SCR systems. With such CARB regulations for replacement DPF parts now becoming hard-wired into the California Code of Regulations, one wonders how such rules will gradually impact other landscapes of replacement DPF technologies and markets outside of California. One potential impact may be on the other 49 US states, a jurisdiction of interest to the US EPA. Another may be that its various versions could be adopted or worked into existing regulations in other countries or regions, such as within the EU member countriesor regions, such as within the EU member countries. California replacement DPF rule-making did not come as a surprise. The need for a structured approach for lower-cost replacement DPFs had been felt for some time. On the one hand, lower-cost replacement DPFs were needed to provide relative financial ease for vehicle owners; on the other, such DPFs had tomeet emission reduction performance benchmarks satisfactory to the public at large. Indeed, lack of such regulations had created a singularity between California and the rest of the country. In California, vehicle owners were so far left with only one choice: the higher-cost OEM-approved replacement DPFs; outside of California, the picture has not been as crisp: There existed (and continues to exist) replacement DPFs over a large spectrum of the good, the bad, and the ugly. While some non-OEM replacement DPF producers appear to yield replacement parts having suitable qualities, it is however not hard to notice other replacement DPFs available in the market barely meeting minimally acceptable performance benchmarks. It is hence fair to predict the recent rule-making in California, indeed the first of its kind, will come without neither its challenges nor its widespread impact; most notably, (a) there will be an initial period where these newly adopted technology, quality and rules will help greatly fine-tune the replacement DPF markets, while such markets will help fine* Mansour Masoudi mansour.masoudi@emissol.com

이유체 노즐 미립화 특성이 요소 열분해에 미치는 영향

Recently, there has been rising interest in applying urea-SCR systems to large marine diesel engines because the International Maritime Organization (IMO) has decided to enforce NOx reduction regulations. Generally, in the case of urea-SCR of the marine diesel engine, a type of twin fluid atomizer has been using for injection of the urea solution. This study conducted to investigate an effect of the atomization of external-mixing twin fluid nozzle on the conversion efficiency of reductant. The lab-scaled experiment device was installed to mimic the urea-SCR system of the marine diesel engine for this study. In a low temperature inflow gas condition which is similar with the exhaust temperature of large marine diesel engine, this study found that the conversion efficiency of reductant of when relative big size urea solution droplets are injected into exhaust gas stream can be larger than that of when small size urea solution droplets are injected. According to results of this study, the reason was associated with decrease of reaction rate constant caused from temperature drop of inflow gas by assist air of twin fluid atomizer.

Study on HNCO Hydrolysis and Reduction in Marine Urea-SCR Systems

Study on HNCO Hydrolysis and Reduction in Marine Urea-SCR Systems

우레아염 퇴적 방지를 위한 믹서 위치 및 하우징 형상에 따른 벽면 유동특성에 관한 연구

Abstract: The Urea-SCR system commercialized shows a remarkable performance to reduce NOx emission in heavy duty diesel engines. However, Urea-water solution injected upstream a mixer, which is set up inside a exhaust pipe to promote exhaust gas-atomized droplet mixing, bumps up against the wall of a exhaust pipe as the droplets flow downstream through the exhaust gas. The urea deposited on the wall of the exhaust pipe is changed into the Urea-salt, resulting in the decreased life-time of the SCR catalysts. Therefore, the develop-ment of the urea deposition avoidance technologies is being treated as an important issue of the Urea-SCR systems. An experimental study was carried out to investigate the effects of the wall flow characteristics around the mixer-housing assembly with the variation of the mixer housing surrounding and supporting the mixer, which is designed to increase the wall flow and then to reduce droplet deposition. The flow character-istics was investigated by using a hot-wire anemometry for 2-D simplified duct model, and the housing tilt angles and the position of the mixer were changed : angle of 0°, 1°, 2°, 3°, and mixer positions of 0L, 0.5L, 1L. The results showed that the wall flow onto the exhaust pipe was improved with changing the tilt angle of the mixer housing, and the wall flow improved more when the position of the mixer was on 1L.

Control of Urea SCR Systems for US Diesel Applications

Resume — Commande des systemes de RCS a l’uree destines aux applications Diesel US — Cet article a pour objet de qualifier les defis en matiere de commande des systemes de reduction catalytique selective (RCS) dans une solution a base d’uree, pour la conversion des NOx au sein de groupes motopropulseurs Diesel. Nous montrons l’importance de la maitrise du stockage d’ammoniac ainsi que les difficultes a l’estimer face aux incertitudes du systeme. Les dynamiques de stockage lentes du catalyseur de RCS s’averent constituer un facteur de limitation majeur quant aux performances des systemes de RCS au cours d’un fonctionnement transitoire. Enfin, nous montrons l’effet de l’incertitude des capteurs sur les performances du systeme et comment les capteurs de NOx peuvent etre utilises en association avec une correlation d’entree pour distinguer une deviation de NOx d’un glissement d’ammoniac.