Urea-SCR System Research Articles

A solving new method for the urea-selective catalytic reduction (SCR) system in a diesel engine using coupled hyperbolic-parabolic partial differential equations (PDEs)

To control the diesel engine urea SCR system with high accuracy, firstly, the partial differential equations of the SCR system are simplified through variable substitution and the method of characteristic lines to eliminate the partial derivative terms of the hyperbolic partial differential equations in the flow direction. The backward difference method is used to solve the problem, and the adaptive time step is adjusted to improve computational efficiency. Secondly, the Levenberg-Marquardt algorithm is applied to identify the model parameters per second based on the 1800-s test bench data. By combining the experimental data with the parameter identification results, this paper calculated the downstream NOx concentration with 99.5 % accuracy. Finally, the 1800s transient test data was applied to a commonly used single-state SCR control model, and cell numbers 1–4 of the cases were numerically simulated. It was found that the reduced-order model had a computation time of 1 s but was less accurate. When the test data was applied to the model presented in this study, the calculation time was 27s, and the model's calculation results show that the average error of the downstream NOx concentration is 16.95 ppm, which is 14.3 ppm lower than that of the two-cell one-state model.

Developments And Challenges In Urea-Scr Aftertreatment System: Review

Due to their efficiency and reliability, diesel engines are regarded as the most effective power generators in the manufacturing and haulage industries. However, they are plagued by the negative effects of exhaust emissions, particularly oxides of nitrogen (NOx), which are subject to increasingly strict emission rules, prompting the search for alternative technologies. Urea-SCR is a sophisticated energetic emissions reduction technique that uses a catalyst and urea-water solution (UWS) to convert nitrogen oxides (NOx) into harmless byproducts. Current study deals with the various factors effecting Urea-SCR system implementations, along with advantages, and future challenges.

Newly developed detailed urea decomposition mechanism by marine engine urea-SCR system crystallization test and DFT calculations

The problem of urea crystallization in selective catalytic reduction (SCR) systems has been a concern for a long time, especially in vehicle engine after-treatment. Compared with the vehicle engine SCR system, the layout space of the marine engine SCR system is more spacious, but the impingement of urea-water solution (UWS) on the wall as well as the formation of urea deposits still cannot be avoided in many cases. In this paper, the urea deposits in the SCR systems of 10 MW marine two-stroke and 140 kW marine four-stroke engines are taken as the research objects. The research results showed that the composition of urea deposits is affected by the composition of exhaust gas. Besides biuret, ammelide, cyanuric acid (cya) and its homologues, the urea deposits contain non-urea decomposition by-products such as particulate matter (PM) and sulfate. In addition, most of the existing urea decomposition mechanisms are obtained by inferring the intermediate reaction pathways through thermogravimetric analysis (TGA), which has the disadvantage of kinetic parameters being inaccurate calculation due to oversimplified reactions, and cannot explain the coexistence phenomenon of cya and its homologues in the urea deposits of marine SCR systems. Therefore, based on density functional theory (DFT), a new detailed urea decomposition mechanism is proposed in this paper, including 15 species and 23 reactions. This mechanism introduces homologues of urea, biuret, and triuret, complementing the reaction pathways for forming cya and its homologue, and is more reliable in theory. On this basis, the correctness of the proposed urea decomposition mechanism is validated by using the TGA experimental data of urea deposits and the urea decomposition kinetic model, and machine learning.

The importance of individual spray properties in performance improvement of a urea-SCR system employing flash-boiling injection

The appropriate mixing of a urea–water solution (UWS) with exhaust gases in a selective catalytic reduction system is crucial to efficiently reduce nitrogen oxides and diminish the deposition of liquid wall film. One of the methods to enhance the mixing of the UWS with the exhaust gases is a flash-boiling injection. Its positive effect has been linked with a reduced Sauter mean diameter (SMD) and improved evaporation, but the flash boiling influences all the spray parameters. For low-injection-pressure cases, the spray angle generally increases, and spray-tip penetration reduces. Moreover, the reduction of the droplet size is not manifested only by reduced average droplet diameter such as SMD, but also the shape of the droplet-size distribution is changed when the injected liquid is superheated. It has not been evaluated which of these factors is the most important. It is also unknown whether the synergy between these factors is an essential element. In this study, we aim to fill these gaps and provide the numerical analysis of the influence of individual parameters of urea–water solution sprays on the exhaust system’s performance, with particular reference to the changes caused by flash boiling.It was found that none of the spray parameters influenced by the flash boiling could alone explain the mixing enhancement. Nevertheless, the alteration of the droplet size distribution led to the highest simultaneous reduction of wall film and improvement of ammonia distribution uniformity. However, the most substantial drop in the wall film formation was achieved when the spray angle was increased.The results led to the conclusion that the most positive effect of the flash boiling results from the combination of all of the considered parameters, with the most significant contribution coming from the reduced droplet size.

Spray strategy and mixer optimization in Urea-SCR system based on frozen flow modeling

An integrated after-treatment device model was established by Converge and the experiments were conducted on rapid urea deposits platform under low and medium load conditions. The model was validated by back pressure of catalysts, mixer wall temperature and distribution of urea deposits. In order to reduce the computational costs, the urea injection strategy and mixer structure was optimized based on the frozen flow method. The effects of frozen flow method on the simulation errors and computational costs were evaluated by film mass, film distribution and cycle numbers. The results show that the simulation errors after frozen flow were larger under low load condition but lower under medium load condition. However, the frozen flow method could effectively reduce the cycle numbers under both low and medium load conditions, which was reduced by 9.05% and 17.77%. Furthermore, the cycle numbers were reduced by 31.23% in 5 times injection simulation. With the optimal scheme under low and medium load conditions, the urea deposition rate was reduced by 43.51% and 23.16%. The mass of NH3 was increased by 23.99% under medium load condition. The NH3 uniformity coefficient increased by 5.04% and 2.69%, and the mixer back pressure declined by 24.40% and 27.44%.

Measurements of nitrous oxide and ammonia emissions from diesel automobile under the real-world driving conditions using laser absorption spectrometer

Nitrous oxide (N2O) is a greenhouse gas and contributes to ozone depletion, and ammonia (NH3) contributes to the formation of secondary particulate matter. To reduce global warming and particulate matter, the real-world levels of N2O and NH3 emitted from automobiles is required to be examined. However, there are a few studies on the measurements of N2O and NH3 in automobile exhaust under the real-world conditions. In this study, we have measured the real-world emissions of N2O and NH3 from the latest diesel vehicle equipped with an urea-SCR system. N2O was measured by an on-board system using mid-IR laser spectroscopy and NH3 was measured using a sensor-based measurement technique. As a result, NH3 was not emitted, while N2O was emitted intermittently when vehicle was running.

Atomization Effects of Collision Droplets by Dimples for Application to Urea SCR System

Atomization Effects of Collision Droplets by Dimples for Application to Urea SCR System

Melting and Heat Transfer Characteristics of Urea Water Solution According to a Heating Module’s Operating Conditions in a Frozen Urea Tank

The urea-selective catalytic reduction (SCR) system, a nitrogen oxide reduction device for diesel vehicles, is a catalytic system that uses urea water solution (UWS) as a reducing agent. This system has a relatively wide range of operating temperatures. However, the freezing point of the reducing urea solution used in this system is −11 °C. When the ambient temperature dips below this freezing point in winter, the solution may freeze. Therefore, it is important to understand the melting characteristics of frozen UWS in relation to the operating conditions of the heating device to supply the minimum amount of aqueous solution required by the system in the initial stage of normal operation and startup of the urea–SCR system. In this study, we artificially froze a liquid solution by placing it along with a heating module in an acrylic chamber to simulate a urea solution tank. Two types of heating modules (P120 and P160) consisting of two heating elements and heat transfer bodies were used to melt the frozen solution. The melting characteristics of the frozen solution were observed, for example, changes in the temperature distribution around the heating module and the cross-sectional melting shape with the passage of time since the start of the power supply to the heating module. The shape of melting around the heating module differed depending on the level of UWS relative to the heater inside the urea tank. In case 1, it melted in a wide shape with an open top, and in case 2, it melted in a closed shape. This shape change was attributed to the formation of internal gaseous space due to volume reduction during melting and the heat transfer characteristics of the fluid and solid substances.

Spraying and Mixing Characteristics of Urea in a Static Mixer Applied Marine SCR System

The most effective de-NOx technology in marine diesel applications is the urea-based selective catalytic reduction (SCR) system. The urea-SCR system works by injecting a urea solution into exhaust gas and converting this to NH3 and CO2. The injection, mixing, and NH3 conversion reaction behavior of the urea-water solution all have a decisive effect on the performance of the system. To improve de-NOx efficiency, it is important to provide enough time and distance for NH3 conversion and uniform distribution prior to the solution entering the catalyst. In this study, therefore, the characteristics of gas flow, NH3 conversion, and its distribution are investigated with a static mixer by means of numerical methods, providing a special advantage to ship manufacturing companies through the optimization of the urea-SCR system. The results show that the inclusion of the mixer induces strong turbulence and promotes the NH3 conversion reaction across a wider region compared to the case without the mixer. The mean temperature is 10 °C lower due to the activated endothermic urea-NH3 conversion reaction and the NH3 concentration is 80 PPM higher at 1D than those without the mixer. Moreover, the uniformity of NH3 distribution improved by 25% with the mixer, meaning that the de-NOx reaction can take place across all aspects of the catalyst thus maximizing performance. In other words, ship manufacturing companies have degrees of freedom in designing post-processing solutions for emissions by minimizing the use of the reduction agent or the size of the SCR system.

A conceptual design and numerical analysis of the mixerless urea-SCR system

In the present study, an innovative design of the urea-selective catalytic reduction (SCR) system without conventional mixing elements was developed. The aim was to obtain a high degree of urea decomposition, and uniform ammonia distribution at the inlet to the catalyst, while minimising the liquid film deposition and keeping the compact design. The concept of the design was based on creating high turbulences and elongating the flow paths of the droplets. The design was verified through a series of numerical simulations based on the Reynolds-averaged Navier–Stokes (RANS) approach and a discrete droplet model (DDM) spray representation. The analysis included various operating conditions as well as subcooled and superheated sprays. A uniform ammonia distribution was achieved regardless of the operating points and spray properties. Additionally, in the case of the flash-boiling injection, a further reduction of the wall film was observed.

Simultaneous estimation of ammonia injection rate and state of diesel urea-SCR system based on high gain observer

Selective catalytic reduction (SCR) controller achieves high NOx conversion and low NH3 slip primarily by means of controlling the rate and timing of injection of urea solution. No any low-cost commercial sensor can directly measure the actual ammonia (NH3) injection rate currently; thus, it is necessary to propose a numerical method to estimate the actual NH3 injection rate, which helps stabilization of emission control performances of a diesel SCR system during its service life cycle. A three-state nonlinear model was established and a method for designing high-gain observer (HGO) was studied so that those state variables such as NH3 injection rate, NOx concentrations, NH3 concentrations and NH3 coverage ratio can be estimated simultaneously. Such design method only requires bounded unknown input dynamics rather than any assumption of the fact how the unknown input changes. The simulation verification was performed by the ETC test cycles and qualitative and quantitative analysis of estimation errors were carried out. Compared with the extended Kalman filter (EKF) observer, the proposed high-gain observer has a higher estimation accuracy and better target traceability.

Influence of Operating Conditions on Urea Deposit Formation in the Urea-SCR System on a Diesel Engine Exhaust-like Test Bench

Influence of Operating Conditions on Urea Deposit Formation in the Urea-SCR System on a Diesel Engine Exhaust-like Test Bench

Development of a Selective Mixed-Potential Ammonia Sensor for Automotive Exhausts

One of the most effective technologies in decreasing large-scale NOx emission produced by diesel engine vehicles is Urea-SCR (selective catalytic reduction) system. In order to prevent inducing excessive ammonia to the environment, an NH3 sensor is required at the exit of this system [1, 2]. In this study, highly selective ammonia sensors were developed to detect ammonia emissions from automotive exhaust.The sensors were fabricated with 8-YSZ electrolyte, a platinum reference electrode and a working electrode of Au-V2O5 (mass ratio 85/15), screen-printed on an alumina substrate. A platinum resistor was printed at the backside of the support to control the sensor temperature. The measured sensor’ response (ΔV) is the potential difference between reference and working electrodes. Figure 1 shows the responses of two identical sensors to 100 ppm CO, NO2, NO and 20 ppm of NH3 at four different temperatures. It can be seen that the sensors respond to all gases at lower temperatures while by increasing temperature to 600°C, the selectivity to NH3 is greatly improved. The selectivity of sensors was also confirmed by testing other possible interfering gases: no responses were observed for 20ppm of hydrogen and 100ppm of a hydrocarbon mixture. The stability of such sensors was studied at 550°C and 600°C. Since the sensors show no long-term stability at 600°C (electrode degradation), but remain stable at 550 °C, investigations were made to decrease the working temperature while maintaining selectivity. After testing different mass percentages of V2O5 in working electrode, we observed that by increasing this value to 50%, the working temperature of selective ammonia sensors could be decreased to 550°C with stable responses. Further investigations will be performed in order to gain deeper insight in sensing mechanism of V2O5 based working electrodes, which governs the sensor’s performance.

Effect of Atomization of Impact Droplets by Surface Texturing using High-Speed Camera

The promoting evaporation of urea-water solution by atomization is required to improve the purification efficiency of NOx for the urea SCR system. This study proposed an innovative method of atomization which processed a surface texture to the impingement walls of the spray droplet. The atomization mechanism in this method was verified by the high qualitative visualization of the impingement droplets.

Impact dynamics and morphology of urea-water-solution droplets impinging on a hot plate under urea-SCR relevant conditions: Influence of surface tension

NOx conversion efficiency of urea-selective-catalytic reduction (SCR) systems are governed by dispersion of urea-water-solution (UWS) injected in exhaust manifold. Impingement of larger size urea droplets on mixture distribution fans as well as to the internal surfaces of SCR systems will lead to formation of deposits, which has potential to deteriorate the effectiveness of urea-SCR system. In this work, detailed analyses on droplet-wall interactions of UWS droplets impinging on a hot plate under urea-SCR-relevant conditions have been presented. The effect of lowering surface tension of UWS on droplet morphology and impact dynamics were also explored. The surface tension of UWS was lowered from 73.7 to 30.2 mN/m by adding a surfactant (DDA75).Distinct modes of droplet impact viz., deposition, thermal atomization, rebound and breakup were identified. The DDA75 droplets showed a significant increase in maximum spreading factor (βmax) by 37% due to 59% reduction in surface tension. New empirical correlation was developed to predict βmax for UWS and DDA75 droplets, which considers the effect of wall temperature on spreading process; the predicted βmax values for different liquids including water, hydrocarbon and alternative fuels viz., n-heptane, n-decane, Jatropha biodiesel and camelina-based alternative jet-fuel had a mean error less than 8.2% across all wall temperature conditions. The drop-size distributions of secondary DDA75 droplets revealed considerably narrower drop-size distribution (up to 36%) compared to UWS droplets. The surfactant-added-UWS droplets have the potential to enhance NOx reduction in SCR systems through better evaporation and mixing and also through reduction in the formation of urea deposits.

Urea-SCR System Development in the Mitigation of NOx Emissions from Diesel Engines – A Review Study

Introduction: Due to energy demand concerns, diesel engines have gained much attraction recently compared to petrol engines because of their higher thermal efficiencies. However, they emit a larger amount of NOx emissions into the atmosphere. Objective: Nitrogen oxides are known as important ambient air pollutants that are responsible for health problems, forest damage, and buildings corrosion. Therefore, using emissions control strategies for diesel engines is required in order to have a cleaner environment. Urea-SCR (selective catalytic reduction of NOx by urea) after-treatment system is considered as one of the most efficient techniques available to reduce engine-out NOx emissions sufficiently. Conclusion: This review article discusses all the methods suggested to diminish nitrogen oxides emissions and then presents a comprehensive survey on developing urea-SCR unit -whether from catalyst development aspect or from injection system modification point of view- in diesel engines to meet strict emissions regulations.

Effect of UWS spray angle and positioning of injector on ammonia concentration in Urea-SCR system

Compounds like Urea and Ammonium formate can be used to generate the reducing agent, NH3 in De-NOx Selective Catalytic Reduction Systems (SCR). In Urea-SCR, optimized Urea-Water Solution (UWS) injection is a key factor for ammonia (NH3) distribution in SCR mixing chamber. The present simulation study emphasizes the effect of UWS injection strategies and positioning of the injector on NH3 conversion efficiency through parametric studies at temperature range of 300–400°C using CFD code AVL FIRE. Simulations were carried out for three spray angles of UWS injection at two different UWS injector positions. For lower spray angles, the NH3 concentration was low and it found to increase with an increase in spray angle. The optimized spray angle is necessary in balancing both NH3 genartion and wall interaction. The 3D-simulations considering residence time for various spray angles determine the time to achieve maximum NH3 concentration in SCR mixing domain. The shifting of injector position leads to variation in urea decomposition and distribution of NH3 in the SCR chamber.

분사 방향에 따른 Urea-SCR 후처리 시스템의 배출가스 저감 성능 예측

분사 방향에 따른 Urea-SCR 후처리 시스템의 배출가스 저감 성능 예측

Study of reducing deposits formation in the urea-SCR system: Mechanism of urea decomposition and assessment of influential parameters

In the Selective Catalytic Reduction (SCR) system of a diesel engine, the decomposition of urea will produce the reducing agent NH3, and by-products which will form deposits. To reduce deposits, this work studied the relationship between liquid film temperature and reactants. Further, the mixers with different structures and the injectors with different injection parameters were designed and compared with different designs about the location and mass of deposits. The evaporation and pyrolysis process of urea aqueous solution in an SCR system were simulated by using computational fluid dynamics (CFD) coupled with a detailed urea decomposition mechanism. The results indicated that the by-products (biuret, CYA, and ammelide) would be produced at 300, 380, and 437K, respectively. With increasing liquid film temperature, the mass fraction of NCO− and by-products also increased. The turbulence intensity varied when they were generated by the different mixer structures. In lower turbulence intensity of mixer1, with the decrease of injection angle and pressure, the mass and distribution regions of liquid film and deposits were obviously reduced. In higher turbulence intensity of mixer2, the urea droplets were entrained by strong turbulence. The decrease of the injection angle and pressure did not significantly reduce the liquid film and deposits.

Urea Injection and Uniformity of Ammonia Distribution in SCR System of Diesel Engine

Abstract The kinetic mechanism of chemical reaction was used to calculate the coupling of the fluid kinetics with the urea decomposition reaction and the SCR reaction kinetics. Combined with the engine test and simulation, the distribution uniformity of the urea injection and ammonia gas was studied. Through numerical simulation on urea spray and exhaust flow in Urea-SCR system, the flow field characteristics in whole after-treatment system are gotten. By using numerical calculation in different urea injection angle and orifice sizes, the urea-crystallization and ammonia distribution have been studied and the optimal urea spray angle and nozzle size are given.