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Improvement of De-NOx Performance of a SCR System by Dual-Point Ammonia Injection in a Diesel Engine
Abstract
Selective catalytic reduction (SCR) catalysts is used widely to reduce NOx emissions from diesel engines. However, improvements in de-NOx performance are required to meet future standards of near-zero NOx emissions. This study examines the effectiveness of dual-point NH3 injection technology, in which gaseous NH3 is injected into both closed coupled and under floor SCR catalysts. The NOx removal efficiency of dual-point NH3 injection is compared with that of single-point injection in a World Harmonized Transient Cycle test. The results show that dual-point injection of NH3 gas can effectively reduce NOx emissions from diesel engines due to superior temporal and spatial distribution of NH3 gas in an SCR catalyst.
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The deNOx performance of a selective catalytic reduction (SCR) system using ammonium carbamate (AC) was investigated and compared with that of a urea water solution (UWS). The effects of the NH3/NOx (α) ratio, injection threshold temperature (Tinj), and a zero-dimensional ammonia adsorption-desorption model on NOx conversion efficiency were evaluated. World harmonized transient cycle (WHTC) and non-road transient cycle (NRTC) tests were conducted using a 3.9-L diesel engine over a Cu-zeolite catalyst. The NOx conversion efficiency of AC in the WHTC and NRTC was increased compared with that of UWS during the cold phase by 6.43% and 8.71%, respectively, and during the hot phase by 14.79% and 11.93%, respectively. Gaseous ammonia injection at a low Tinj can explain improved deNOx performance, as increasing Tinj leads to a decrease in NOx conversion efficiency. Increasing the α ratio effectively increases NOx conversion efficiency, but a high α ratio leads to ammonia slip. Ammonia injection using a model-based control increased cold-start deNOx performance. These promising results provide an alternative pathway to controlling NOx emissions from heavy-duty diesel engines.
This paper proposes a gain-scheduling (GS) control strategy for selective catalytic reduction (SCR) systems in diesel engines, to minimize both nitrogen oxides (NOX) emissions and ammonia (NH3) slip downstream of the SCR catalyst. The proposed GS controller switches among multiple linear time-invariant H∞ controllers based on the engine’s operating condition, and adopts three measurable GS parameters, that is, the amount of NOX entering the SCR catalyst, exhaust gas temperature in the catalyst, and ammonia slip measurement. While switching surfaces for the first two GS parameters are determined by simply dividing their operating ranges, to find optimal switching surfaces for the ammonia slip GS parameter, a novel method called Violation-Time-based Decision method is proposed. The efficiency of the proposed GS SCR controller is validated in simulations using real experimental data obtained from a diesel engine in the Non-Road-Transient Cycle (NRTC).
In this study, the N 2 O emission formation from heavy-duty diesel engine was studied by injecting ammonia gas and urea water solution (UWS) in Cu-zeolite SCR. Furthermore, the effect of exhaust gas temperature and NH 3 / NOx (α) ratio on the N 2 O formation were investigated. The ammonia was supplied using an ammonium carbamate (AC) based ammonia generating system. Experiments were performed under the European stationary cycle (ESC), world harmonized transient cycle (WHTC), and non-road transient cycle (NRTC). Ammonia gas injection produced a higher amount of tailpipe N 2 O than UWS injection for all test conditions. Compared with UWS an increment of 27% in N 2 O formation for ammonia gas was observed during the ESC test. The N 2 O emissions of ammonia gas injection in the WHTC and NRTC were increased compared with that of UWS during the cold phase by 63% and 55%, respectively, and during the hot phase by 57% and 74%, respectively. Meanwhile, NO and NO 2 emissions were decreased by injecting ammonia gas compared with UWS. The higher NH 3 /NOx (α) ratio led to higher tailpipe N 2 O and lower NO and NO 2 emissions. For each reductant test, the concentration of N 2 O formation was high at the exhaust gas temperature of 200-250 • C. These presented results provide useful information about the N 2 O formation from after-treatment systems of diesel engines when ammonia gas is used.
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.
The 2010 emissions standards for heavy-duty engines have established a limit of oxides of nitrogen (NOX) emissions of 0.20 g/bhp-hr. However, the California Air Resource Board (ARB) projects that even when the entire on-road fleet of heavy-duty vehicles operating in California is compliant with 2010 emission standards, the National Ambient Air Quality Standards (NAAQS) requirement for ambient particulate matter (PM) and Ozone will not be achieved without further reduction in NOX emissions. The California Air Resources Board (CARB) funded a research program to explore the feasibility of achieving 0.02 g/bhp-hr NOX emissions. This paper details the work performed on a heavy-duty diesel engine to explore the feasibility of various configurations of Traditional Technology (diesel oxidation catalyst-diesel particulate filter-selective catalytic reduction (SCR)) and Advanced Technology (passive NOX adsorber or diesel oxidation catalyst - SCR on Filter - SCR) to demonstrate ultra-low NOX emissions. Active and passive performance modifiers were also evaluated to demonstrate low NOX emissions, including heated dosing, gaseous dosing, and supplemental heat addition devices. The proposed Ultra Low NOX emission levels of 0.02 g/hp-hr require a significant shift in technology application to address cold start NOX emissions. Data are presented showing comparison in NOX reduction capability of the various configurations. All testing was conducted on the FOCAS-HGTR® system, which is a full flow, transient gas reactor bench for testing full sized catalyst systems.
Currently the advanced UREA-SCR system is one of the most popular technology to meet the strict emission standards as Euro IV or above. In order to reach high NOx conversion and low NH3 slip, the urea dosing rate needs to be calculated and controlled precisely. This paper describes an advanced UREA-SCR system based on model based control strategy and advanced diaphragm urea dosing pump. The strategy can be seen as a virtual closed-loop control method for the dosing rate calculation, and it was integrated with the advanced diaphragm urea dosing pump for the tailpipe urea dosing. A SCR chemical reaction model was developed to describe the concentration of different spices inside the SCR catalyst. Three different reactions were taken as the foundation of the model, including fast reaction, normal reaction and slow reaction. To simplify the process of calculation, the catalyst model was divided to three cells. In each cell, the NH3 storage ability was estimated and the results were taken as the feedback signals for the controller. A virtual closed-loop control method was developed to calculate the dosing rate. The model was tuned under different engine running conditions, the exhaust gas temperature was controlled between 250 and 400 C and the NH3/NOx molar ratio was controlled between 0.8 and 1.1. An advanced-diaphragm urea dosing pump was developed to finish the urea dosing activity. The results of transient engine running experiment show that the NOx conversion efficiency can reach 85 % while keeping the average NH3 slip under 15 ppm, the peak average NH3 slip under 30 ppm. And the dosing rate error can be controlled less than 3 %.
In this project, phenomena in a SCR catalyst, such as heat transfer and catalytic reactions, are modeled numerically. The model is simplified to be integrated on an electronic control unit. The calibration process for this model has been developed, which is performed on gas bench and validated on a vehicle equipped with a Urea-SCR system and a Rapid Prototype Control Unit. With this simplified SCR reaction model, it is possible to estimate NH3 consumption and properly control the urea injection quantity with less calibration efforts.
As the number of vehicles and environment pollution is increasing day by day, the emission regulation gets more stringent by the emission regulation authorities. Vehicle manufacturers' develop new ways and technologies to meet the norms levied for cleaner vehicles. Especially in diesel engines, NOx emissions are considered an important pollutant to be treated. In EURO 5 regulation NOx emission value is 0.18g/km for passenger cars which is further reduced to 0.08 g/km in EURO 6 regulation for diesel engines. In order to meet such stringent emission norms without compromising on engine performance, Selective Catalytic Reduction (SCR) is one of the solution to achieve EURO 6 NOx emission levels from diesel engines. In SCR technology the reduction of NOx is done through ammonia which is injected into exhaust stream in the form of Aqueous Urea solution known as DEF. To achieve better conversion efficiency, the injected DEF has to be uniformly distributed and mixed with the exhaust stream. In this system, a mixer is used for better mixing of ammonia with exhaust stream which in turn improve the NOx conversion efficiency. Mixer efficiency is measured through uniformity index of ammonia present in exhaust upstream SCR catalyst. The Uniformity Index of the ammonia up stream SCR catalyst can be calculated by the following equation [3]. Where 'c' is the value of ammonia upstream at the coordinate points of SCR catalyst, is the mean value of the ammonia values across the catalyst and 'n' is the total number of coordinate points. Uniformity Index depends on the mixer plate angles, exhaust flow profile, mixing length, impingement of injected DEF on mixer plates. Better mixer design results in better uniformity index that in turn results in better NOx conversion efficiency. This paper is an experimental comparison of the uniformity index for two dual stage plate mixers having different blade angles. At different engine operating points, the concentration of ammonia upstream SCR catalyst (at different co-ordinate points) is calculated which decides the uniformity index for the mixer and the effect of mixer design is analyzed The objective of this paper is to study the performance of two different dual stage mixers with different blade angles and bring out experimental results to compare and conceive the best mixer design for the current application. The mixers under study are Mixer A and Mixer B, with a difference of 10°-15° in their blade angles.
A fast preparation of the liquid urea water solution (UWS) is necessary to ensure high conversion rates in exhaust aftertreatment systems based on Selective Catalytic Reduction (SCR). Droplet wall interaction is of major importance during this process, in particular droplet breakup and the Leidenfrost effect. A deeper understanding of the underlying mechanisms is a basic requirement to calibrate CFD models in order to improve their prediction accuracy. This paper presents a detailed literature study and discussion about the major impact factors on droplet wall interaction. Measurements of the Leidenfrost temperature were conducted and the corresponding regimes classified based on optical observations. The pre- and post-impingement spray was analysed using the laser diffraction method. Further, the validity of spray initialisation based on measurements at room temperature was verified. The Leidenfrost effect, partial droplet evaporation, droplet breakup and evaporation at the catalyst are the crucial aspects of impingement modelling. Their implementation and calibration against measurement data is presented. Sensitivity studies of single impact factors were carried out based on two exhaust systems that are representative for the spectrum of typical automotive applications. A set of operating points was used that covers the whole range of typical operating conditions. Finally, a best practice setup was derived and validated with measured ammonia distributions. With the help of the presented numerical and experimental investigations the prediction accuracy of the simulated ammonia homogenisation could be significantly increased for inherently different exhaust pipe geometries.
Selective catalytic reduction (SCR) is a viable option for control of oxides of nitrogen (NOx) from diesel engines. Currently, copper zeolite (Cu-zeolite) SCR catalysts are favored for configurations where the exhaust gas temperature is below 450 oC for the majority of operating conditions, while iron zeolite (Fe-zeolite) SCR catalysts are preferred where NOx conversion is needed at temperatures above 450 oC. The selection of Cu-zeolite or Fe-zeolite SCR catalysts is based on the different performance characteristics of these two catalyst types. Cu-zeolite catalysts are generally known for having efficient NOx reduction at low temperatures with little or no NO 2, and they tend to selectively oxidize ammonia (NH 3) to N 2 at temperatures above 400 oC, leading to poor NOx conversion at elevated temperatures. Fe-zeolite catalysts are very efficient at NOx conversion at temperatures as high as 600 oC or higher, but they are not as efficient as Cu-zeolite catalysts at lower temperatures in the absence of NO 2. In this work, a combined SCR system consisting of an Fe-zeolite catalyst in front of a Cu-zeolite catalyst is tested and compared to a Cu-only system. The experiments are performed on a flow reactor with simulated diesel exhaust gas. It is seen that the operating temperature range of the SCR catalyst can be widened for a combined Fe-Cu system. At low temperatures, the Cu-zeolite improves NOx conversion efficiency vs. an Fe-only system. At elevated temperatures, the Fe-zeolite is more active. Also, one can overdose NH 3 at elevated temperatures with the combined Fe-Cu system without NH 3 slip, while the Fe-only system leads to substantial NH 3 slip when overdosing. A simple optimization of NH 3 to NOx ratio (alpha) is performed for the combined Fe-Cu system and compared to a Cu-only system. Also, the ratio of Fe-zeolite to Cu-zeolite is evaluated. Transient test results are also shown for these systems to determine the impact of a combined system on transient response at low temperatures.
A laboratory study was performed to optimize a zoned configuration of an iron (Fe) SCR catalyst and a copper (Cu) SCR catalyst in order to provide high NOx conversion at lean A/F ratios over a broad range of temperature for diesel and lean-burn gasoline applications. With an optimized space velocity of 8,300 hr-1, a 67% (by volume) Fe section followed by a 33% Cu section provided at least 80% NOx conversion from approximately 230oC to 640oC when evaluated with 500 ppm NO and NH3. To improve the lean lightoff performance of the SCR catalyst system during a cold start, a Cu SCR catalyst that was 1/4 as long as the rear Cu SCR catalyst was placed in front of the Fe SCR catalyst. When evaluated with an excess of NH3 (NH3/NO ratio of 2.2), the Cu+Fe+Cu SCR system had significantly improved lightoff performance relative to the Fe+Cu SCR system, although the front Cu SCR catalyst did decrease the NOx conversion at temperatures above 475oC by oxidizing some of the NH3 to N2 or NO.
Selective catalytic reduction (SCR) of NOx is coming into worldwide use for automotive diesel emissions control. To meet the most stringent standards, NOx conversion efficiency must exceed 80% while NH3 emissions or slip must be kept below 10-30 ppm. At such high levels of performance, non-uniformities in ammonia-to-NOx ratio (ANR) at the converter inlet can limit the achievable NOx reduction. Despite its significance, this effect is frequently ignored in 1D catalyst models. The corresponding model error is important to system integration engineers because it affects system sizing, and to control engineers because it affects both steady-state and dynamic SCR converter performance.A probability distribution function (PDF) based method is introduced to include mixture non-uniformity in a 1D, real-time catalyst model. The model is subsequently applied over a broad range of steady engine operating conditions to identify regimes where non-uniformity has greatest impact on NH3 slip limited conversion efficiency. Non-uniformity effects during transient emission cycles (US EPA HD-FTP and SET) are also studied for a 15 liter heavy-duty diesel which targets EPA-2010 standards and uses a copper-zeolite formulation. It is shown that the required ANR uniformity index is a function of not only the engine operating conditions and emissions test cycle, but also catalytic coating characteristics.
An ammonia supplying system using ammonium salt to reduce the NOx emissions of a MW marine engine
- H Woo
- H Raza
- W Kang
- S Choe
- M Im
- K Lim
- J Nam
- H Kim
Woo, H., Raza, H., Kang, W., Choe, S., Im, M., Lim, K., Nam, J., &
Kim, H. (2022). An ammonia supplying system using ammonium
salt to reduce the NOx emissions of a MW marine engine. Journal of Marine Engineering & Technology. https:// doi. org/ 10. 1080/
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