Urea Selective Catalytic Reduction Research Articles

Understanding the Electrochemically Induced Conversion of Urea to Ammonia for Direct Ammonia SCR Processes

Background Nitrogen oxides (NOx) emissions are much concerned these days. NO2is not only one of the air pollutants, but it will also react in the atmosphere to form ozone and acid rain. An effective method to remove NOx is the selective catalytic reduction (SCR) process and ammonia is a known effective reductant for this process. The urea SCR, where urea solution is injected into an SCR chamber to produce ammonia and then remove NOx on the SCR catalyst, faces challenges with lower exhaust temperature and harmful deposits on SCR catalyst. In contrast, direct NH3 SCR expands the SCR temperature “window” while eliminating deposits [1]. However the storage of pressurized ammonia becomes an issue. To confront these issues, Botte has proposed a new method, the electrochemically induced conversion of urea to ammonia (eU2A) process [2, 3]. The eU2A reactor provides the flexibility to decouple the ammonia generation from the SCR process at low temperature as indicated in Fig. 1. The eU2A process not only uses urea storage instead of ammonia but also provides direct NH3dosing with co-generation of hydrogen, which could potentially enable better efficiencies of the SCR catalyst. Experimental results have shown that generation of ammonia from urea could be promoted by an applied voltage on Ni based catalysts [2], resulting in higher yield of ammonia when compared to thermal hydrolysis of urea to ammonia (THU) as shown in Fig. 2 [3]. However, the mechanism of the urea conversion to ammonia via the electrochemical process is still unknown. In this work, we implement in-situspectroscopy techniques to provide a better understanding of the eU2A process in alkaline media. Methodology A three-electrode reactor was designed and constructed at the Center for Electrochemical Engineering Research (CEER) for the production of ammonia from urea using chemical and electrochemical methods. Nickel based catalysts were used as working and counter electrodes for the cell. Results The eU2A experiments were conducted and compared with the THU process for different parameters including applied voltage and concentration of supporting electrolyte (KOH). In-situ Raman spectroscopy was employed to identify and monitor the intermediates and products formed during the reaction on the catalyst surface. Ex-situFTIR spectroscopy was utilized to identify the intermediates and products in the bulk solution during the reaction. By comparing time-dependent Raman and FTIR spectra for the eU2A experiments with the THU experiments, a possible reaction pathway for the conversion of urea to ammonia in the eU2A process is proposed. The findings and the proposed mechanism will be presented at the meeting. References Johannessen, T., “3rd Generation SCR System Using Solid Ammonia Storage and Direct Gas Dosing: Expanding the SCR window for RDE.” in Directions in Engine-efficiency and Emissions Research (DEER). 2012. Dearborn.G. G. Botte, “Electrolytic cells and methods for the production of ammonia and hydrogen.” US Patent 0095636A1, 2009.Lu, F. and G. G. Botte, “Electrochemically Induced Conversion of Urea to Ammonia.” ECS Electrochemistry Letters, 2015. 4(10): p. E5-E7. Figure 1

Installation and characteristics of urea-selective catalytic reduction systems for nitrogen oxide reduction in marine diesel engine

Since 1 January 2016, the International Maritime Organization Tier III regulations concerning nitrogen oxides took effect. International Maritime Organization Tier III targets new ships weighing over 400 ton engaged in international sailing, especially those entering and leaving emission control areas. The number of emission control areas, including sulfur emission control areas, is forecasted to increase in Japanese seas and elsewhere; thus, all new ships intended for international sailing purposes must consider appropriate responses to these regulations. Currently, urea-selective catalytic reduction systems are frequently used to satisfy International Maritime Organization Tier III regulations in marine diesel engines, but these decrease ship engine performance due to increased engine exhaust gas pressure. Thus, following a study by Ryu et al., this research compares engine exhaust gas pressures at different engine loads (25%, 30%, 35%, 40%, 45%, and 50%) with the application of a thinner metal catalyst to a selective catalytic reduction reactor; metal catalyst thickness was reduced by 30% or more from the original sheet thickness of 0.1 t. Furthermore, 40% urea was injected into the exhaust gas line to conduct a comparative analysis of nitrogen oxide reduction. By changing the metal catalyst’s sheet thickness, nitrogen oxide was reduced by more than 80%, while the engine exhaust gas pressure was lowered by approximately 13%–28%.

Mechanisms of Increased Particle and VOC Emissions during DPF Active Regeneration and Practical Emissions Considering Regeneration.

Mechanisms involved in increased particle and volatile organic compound (VOC) emissions during active and parked active regenerations of a diesel particulate filter (DPF) were investigated using heavy-duty trucks equipped with both a urea selective catalytic reduction system and a DPF (SCR + DPF) and a DPF-only. Particle emissions increased in the later part of the regeneration period but the mechanisms were different above and below 23 nm. Particles above 23 nm were emitted due to the lower filtering efficiency of the DPF because of the decreasing amount of soot trapped during regeneration. Small particles below 23 nm were thought to be mainly sulfuric acid particles produced from SO2 trapped by the catalyst, being released and oxidized during regeneration. Contrary to the particle emissions, VOCs increased in the earlier part of the regeneration period. The mean molecular weights of the VOCs increased gradually as the regeneration proceeded. To evaluate "practical emissions" in which increased emissions during the regeneration were considered, a Regeneration Correction Factor (RCF), which is the average emission during one cycle of regeneration/emission in normal operation, was adopted. The RCFs of PM and VOCs were 1.1-1.5, and those of PNs were as high as 3-140, although they were estimated from a limited number of observations.

An experimental and numerical study on effects of exhaust gas temperature and flow rate on deposit formation in Urea-Selective Catalytic Reduction (SCR) system of modern automobiles

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.

Performance evaluation of a urea-selective catalytic reduction system in a marine diesel engine

A numerical simulation of the droplet distribution at the exhaust duct for a static mixer and injection location was performed for marine selective catalyst reduction. Through experiments with this auxiliary marine urea-selective catalyst reduction system, the ammonia slip, selective catalyst reduction system pressure drop, and NOx reduction rate in accordance with the space velocity (i.e. engine load) were investigated. Under incremental and random experimental conditions, the ammonia slip of this urea-selective catalyst reduction system is below 3 ppm, which satisfies International Maritime Organization Tier III regulations.

Simulation study on improving the selective catalytic reduction efficiency by using the temperature rise in a non-road transient cycle

In this study, the transient nitrogen oxide reduction performance of a urea selective catalytic reduction system installed on a non-road diesel engine was tested on an engine dynamometer bench over a scheduled non-road transient cycle mode. Based on the measurement results, the characteristics of the transient selective catalytic reduction behaviours of nitrogen oxide reduction were evaluated. Also, in this study, the effects of several thermal management strategies for improving the selective catalytic reduction efficiency was investigated by transient selective catalytic reduction simulations. The kinetic parameters of the current simulation code for selective catalytic reduction were calibrated and validated by comparison with the measurement data. As a result of this study, it was found that a thermal management strategy utilizing a partial temperature rise in the transient time domain can be an efficient approach for improving the transient selective catalytic reduction efficiency, in comparison with the temperature rise over the entire cycle period. Furthermore, this study can provide some guideline data for the magnitude and the duration of the temperature rise required to obtain the target selective catalytic reduction efficiency over the non-road transient cycle mode. In the last part of this study, the impact of the variation in the space velocity on the transient selective catalytic reduction efficiency was assessed using transient selective catalytic reduction simulations.

Quasi 1D modeling of two-phase flow and deposit formation for urea-selective catalytic reduction systems

A quasi 1D model of two-phase flow for a urea-selective catalytic reduction (SCR) system is developed which can calculate not only the generation of reducing agent but also the formation of deposits in the exhaust pipe. The gas phase flow is solved through Euler method, variables are stored on staggered grids, and the semi-implicit method for pressure-linked equation (SIMPLE) algorithm is applied to decouple the pressure and velocity. The liquid phase is treated in a Lagrangian way, which solves the equations of droplet motion, evaporation, thermolysis, and spray wall interaction. A combination of a direct decomposition model and a kinetic model is implemented to describe the different decomposition behaviors of urea in the droplet phase and wall film, respectively. A new 1D wall film model is proposed, and the equations of wall film motion, evaporation, thermolysis, and species transport are solved. The position, weight, and components of deposits can be simulated following implementation of the semi-detailed kinetic model. The simulation results show that a decrease in the exhaust temperature will increase the wall film region and the weight of deposits. Deposit components are highly dependent on temperature. The urea-water-solution (UWS) injection rate can affect the total mass of wall film and expand the film region, but it has little influence on deposit components. An increase in exhaust mass flow can decrease the total weight of deposits on the pipe wall because of the promotion of the mass and heat transfer process both in the droplets and wall film.

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.

운전 조건을 고려한 승용차용 요소첨가 선택적 촉매환원장치의 내부 유동 해석에 관한 연구

Abstract : Diesel vehicles should be equipped with urea-selective catalytic reduction(SCR) system as a high- performance catalyst, in order to reduce harmful nitrogen oxide emissions. In this study, a three-dimensional Eulerian-Lagrangian CFD analysis was used to numerically predict the multiphase flow characteristics of the urea-SCR system, coupled with the chemical reactions of the system’s transport phenomena. Then, the numerical spray structure was modified by comparing the results with the measured values from spray visualization, such as the injection velocity, penentration length, spray radius, and sauter mean diameter. In addition, the analysis results were verified by comparison with the removal efficiency of the nitrogen oxide emissions during engine and chassis tests, resulting in accuracy of the relative error of less than 5%. Finally, a verified CFD analysis was used to calculate the interanl flow of the urea-SCR system, thereby analyzing the characteristics of pressure drop and velocity increase, and predicting the uniformity index and overdistribution positions of ammonia.

Numerical analysis of NOx reduction for compact design in marine urea-SCR system

ABSTRACT In order to design a compact urea selective catalytic reduction system, numerical simulation was conducted by computational fluid dynamics tool. A swirl type static mixer and a mixing chamber were considered as mixing units in the system. It had great influence on flow characteristics and urea decomposition into ammonia. The mixer caused flow recirculation and high level of turbulence intensity, and the chamber increased residence time of urea- water-solution injected. Because of those effects, reaction rates of urea decomposition were enhanced in the region. When those mixing units were combined, it showed the maximum because the recirculation zone was significantly developed. NH3 conversion was maximized in the zone due to widely distributed turbulence intensity and high value of uniformity index. It caused improvement of NOx reduction efficiency of the system. It was possible to reduce 55% length of the chamber and connecting pipe without decrease of NOx reduction efficiency.

An Experimental Investigation into DEF Dosing Strategies for Heavy Duty Vehicle Applications

Copyright © 2015 SAE International.In recent years urea selective catalytic reduction (SCR) has become the principal method of NOx abatement within heavy duty (HD) diesel exhaust systems; however, with upcoming applications demanding NOx reduction efficiencies of above 96 % on engines producing upwards of 10 g·kWh−1 NOx, future diesel exhaust fluid (DEF) dosing systems will be required to operate stably at significantly increased dosing rates. Developing a dosing system capable of meeting the increased performance requirements demands an improved understanding of how DEF sprays interact with changing exhaust flows. This study has investigated four production systems representing a diverse range of dosing strategies in order to determine how performance is influenced by spray structure and identify promising strategies for further development. The construction of an optically accessible hot-air flow rig has enabled visualisation of DEF injection into flows representative of HD diesel exhaust conditions. High-speed and laser sheet imaging have been applied to capture the injection event and analyse spray development within the flows. Results from ambient shadowgraphy show the extent of variation in spray structure that exists between the systems; further quantified with droplet size distribution data collected using phase Doppler interferometry (PDI). Imaging within the exhaust section indicates that the structure of a spray has a significant impact on droplet entrainment within the flow, in turn affecting the level of spray-wall impingement seen. This suggests knowledge of dosing strategy will be critical for optimal system design and enabling near future dosing rate demands to be met.

A least squares estimation-based parameterisation method for a control-oriented model of selective catalytic reduction systems

Nowadays, heavy duty diesel engines are usually equipped with urea selective catalytic reduction (SCR) systems to remove the NOx emissions. To design a high performance urea dosage controller for the SCR systems, a control-oriented SCR model is essential. In the last decade, a few control-oriented SCR models have been proposed. However, a systematic calibration method is missing in the literature. To solve the calibration issue, we propose a novel parameterisation method. The main idea is to re-parameterise the model such that it becomes linear in the new parameters. This allows the model parameters to be determined analytically with the least squares estimation (LSE) technique. The proposed method is tested on our test bench with a few modifications to achieve improved robustness. The results confirm that the calibrated model is able to predict both the NOx and NH3 emissions reasonably well. The key system dynamics are always successfully captured.

Urea-SCR Process Control for Diesel Engine Using Feedforward-Feedback Nonlinear Method

Urea selective catalytic reduction (urea-SCR) process control is very important for exhaust gas aftertreatment in diesel engines. A control system tracking ammonia coverage ratio is designed to achieve high NOx conversion efficiency and low ammonia slip for urea-SCR process in this paper. The control system consists of a nonlinear feedforward controller based on flatness and a PI feedback controller to deal with the uncertainties and disturbance. Furthermore, a tuning method of controller parameters is presented based on randomized algorithms. Finally, the simulation results are given to demonstrate the effectiveness of the proposed control scheme.

Thermo fluid effect of the urea thermal decomposition in a lab-scaled reactor

Urea decomposition in the urea selective catalytic reduction (urea-SCR) system is achieved under a multiphase flow field coexisting with the injected urea solution droplets in the liquid phase, the inflow gas in the gas phase, and solid urea particles. For this reason, urea decomposition is closely connected with thermo-fluid dynamics as well as various chemical reactions. An experimental study was performed to investigate urea decomposition under various thermo-fluid conditions, with different temperatures and velocities of inflow gas, and urea solution quantities. The inflow gas conditions were similar to the exhaust conditions of a large marine engine. A urea solution injector of twin fluid type was designed, as well as a lab-scale exhaust system to control the temperature and velocity of the inflow gas. The spray performance of the designed urea solution injector was almost identical under all the experimental conditions used, so the spray’s effect on the urea decomposition was minimized. The total conversion efficiency considered with both NH3 and HNCO was closely related to the urea injection quantity and convective heat transfer of the inflow gas as well as the temperature of the inflow gas. The conversion efficiency of NH3 was larger than that of HNCO under all experimental conditions, unlike for the theoretical thermolysis reaction. The difference between the conversion efficiency of NH3 and HNCO was affected by convective heat transfer and urea injection quantity as well as the temperature of the inflow gas.

Sliding-mode observers for urea selective catalytic reduction system state estimations based on nitrogen oxide sensor measurements

The state estimations for urea-based selective catalytic reduction systems have received growing interest in the past few years because of their importance to selective catalytic reduction controls and onboard diagnostics, as well as the high cost and difficulties of direct sensor measurements. Most of the existing estimation techniques suffer because of the ammonia cross-sensitivity from the nitrogen oxide sensor and require an ammonia sensor to be available downstream of the selective catalytic reduction, which may increase the costs of production urea-based selective catalytic reduction systems. The purpose of this study is to design observers for estimating the key states (the nitrogen oxide concentration, the ammonia concentration, and the ammonia coverage ratio) of the urea-based selective catalytic reduction system at the selective catalytic reduction outlet in various situations, using only a nitrogen oxide sensor. The sliding-mode observers were designed to ensure high robustness against the ammonia cross-sensitivity from the nitrogen oxide sensor and uncertainties from the selective catalytic reduction inputs. Simulation verification results under the Federal Test Procedure 75 test cycle showed that the proposed observers were able to estimate the states accurately, in hot and cold exhaust conditions, for both single-cell selective catalytic reduction systems and two-cell selective catalytic reduction systems. The proposed observer-based estimation technique has the potential for reducing the cost of advanced selective catalytic reduction control systems and for applying it to onboard diagnostics.

Integrated Selective Catalytic Reduction–Diesel Particulate Filter Aftertreatment: Insights into Pressure Drop, NOx Conversion, and Passive Soot Oxidation Behavior

Integrating urea-selective catalytic reduction (SCR) and diesel particulate filter (DPF) technologies into a single device has the potential to reduce the complexity of current diesel aftertreatmen...

Effects of Various Factors on the Conversion Efficiency of Urea Solution in a Urea Selective Catalytic Reduction System

In a urea selective catalytic reduction (SCR) system, the urea solution is injected into hot exhaust gas, after which the urea solution becomes ammonia that acts as a reducing agent for de-NOx through evaporation, thermolysis, and hydrolysis. The formation of the reducing agent from urea decomposition is closely connected with thermofluid dynamics as well as various chemical reactions. An experimental study was performed to investigate urea decomposition in a low-temperature environment that is similar to the emission gas temperature of a large marine diesel engine. Also, this study investigated urea decomposition in conjunction with thermofluid dynamics related to the urea SCR system driving conditions. The modeled exhaust pipe was designed to control the inflow gas temperature and velocity. The urea solution injector was chosen to obtain almost identical spray performance, regardless of the urea solution flow rate, to exclude the effect of the spray on urea decomposition. A multicomponent Fourier transf...

On-road measurement of regulated pollutants from diesel and CNG buses with urea selective catalytic reduction systems

In this study, emissions from 13 buses operated in Beijing, including two Euro-III diesel buses, four Euro-IV diesel buses, three Euro-V diesel buses and four Euro-V CNG buses, were characterized in real world conditions. All of the buses tested were fitted with selective catalytic reduction (SCR) systems except for the Euro-III diesel buses. A SEMTECH-DS was used for testing the gaseous pollutants, and an electric low pressure impactor (ELPI) was used for measuring of particle numbers and size distributions. A comparison was made based on emission performance of these buses by employing the VSP approach and fuel- based emissions factors. Diesel buses emitted less CO and THC but more NOx and PM pollutants than CNG buses. The NOx reduction efficiencies of the SCR systems for CNG buses were higher because of the high exhaust temperature and high NO2/NOx ratio, whereas the efficiencies for diesel buses were lower. This resulted in extremely low NOx emissions from CNG buses, but the high NO2/NOx ratio needs further study. Failures of urea injection in the SCR systems were detected in this research, which resulted in very high NOx emissions. The CNG buses also emitted smaller numbers of particles and less particle mass with the presence of oxidation catalysts. Diesel buses satisfying the Euro-V standard performed better than Euro-IV and Euro-III diesel buses in terms of emission performance, except for more nuclei mode particles. Most of time, the Euro-IV diesel buses show no advantages in CO and NOx emissions compared with the Euro-III diesel buses.

Improved low-temperature activity for marine selective catalytic reduction systems

Through the international work carried out recently by International Maritime Organization Marine Environment Protection Committee, the need for improved selective catalytic reduction systems for marine applications at low and transient temperatures has been acknowledged. It is of importance to improve the low-temperature activity to achieve compliance with the upcoming stricter International Maritime Organization NOX regulation (Tier III, 80% reduction) for ships. This study therefore investigates the possibilities to improve the low-temperature activity for a commercial vanadium-based urea-selective catalytic reduction catalyst for marine applications. By altering the NO2/NOX ratio, the reaction proceeds in the fast selective catalytic reduction reaction regime, which is especially beneficial at low and transient temperatures and in combination with SO2 and H2O. For both steady-state and transient modes, it is suggested that an improved low-temperature activity for the marine selective catalytic reduction system can be achieved by increasing the NO2/NOX ratio to 50%, whereby utilizing the fast selective catalytic reduction reaction scheme. The fast selective catalytic reduction can significantly improve the NOX reduction already at 250 °C in the presence of water or sulfur. The fast selective catalytic reduction will, however, require an increased NO2 fraction in the exhaust gases, preferably 50%, which may be achieved by placing an oxidation catalyst upstream of the selective catalytic reduction. The use of an oxidation catalyst upstream of the selective catalytic reduction can offer a possibility to achieve International Maritime Organization Tier III compliance at low and transient temperatures, but to avoid formation of ammonium nitrates and possible ammonium sulfates, temperatures should be kept above 250 °C in combination with marine fuels with a maximum of 0.10 wt% S.

A study of the effects of NH3 maldistribution on a urea-selective catalytic reduction system

The maldistribution of urea/NH3 at the selective catalytic reduction inlet and the resulting NH3/NOx ratio variation have been reported to have negative effects on the NOx conversion efficiency and NH3 slip in heavy-duty diesel applications. Maldistribution is caused by incomplete mixing and decomposition of the injected urea water solution within the exhaust flow in the aftertreatment system. How NH3 and the NH3/NOx ratio are distributed at the selective catalytic reduction inlet in an engine aftertreatment system and how the resultant maldistribution affects the system performance and the kinetics are presented in this combined experimental and modeling study. The distribution profiles of the selective catalytic reduction inlet NH3 and NH3/NOx ratio were determined through engine experiments. The NH3 maldistribution effects on the selective catalytic reduction performance and kinetics were quantified by using a multi-channel one-dimensional model that takes into consideration the NH3 maldistribution. It was determined that the NH3 maldistribution is a major factor causing the difference in catalyst performance in the engine exhaust as compared to the laboratory reactor environment, and these results explain specific differences in the model calibrations determined from the laboratory reactor and engine data. Improving the selective catalytic reduction inlet NH3 uniformity can improve the NOx conversion efficiency by up to 7% and reduce NH3 slip by 10–20 ppm. A single-channel one-dimensional selective catalytic reduction model calibrated to the engine data that accounts for the NH3 maldistribution phenomena can be applied for the development of on-board diagnostic and control strategies.