117Bosch Rexroth AG Exhaust Gas Treatment

A Clean Solution – Exhaust Gas Treatment

AbstractTo comply with upcoming legislation for off-highway applications, especially the TIER 4 final emissions standard, significant reductions in particulate matter and nitrogen oxide emissions are necessary. A high-performance exhaust gas after-treatment system can help to meet these requirements. The following paper focuses on possible ways to reduce nitrogen oxide emissions in exhaust gas after-treatment technology using Selective Catalytic Reduction (SCR) and Diesel Exhaust Fluid (DEF). By using this technology and aiming to reduce nitrogen oxide emissions by 90 %, engine-out nitrogen oxide emissions of about 3 to 4 g/kWh can be achieved. With an increase in reduction efficiency of as little as 5 % to 95 %, even double the level of engine-out emissions would still be acceptable in terms of meeting the TIER 4 final emission standard. This would enable engine manufacturers to avoid internal engine measures, such as exhaust gas recirculation, and therefore provide an opportunity to save on space, cooling, fuel and costs.

Andreas KeuperDevelopment EngineerBosch Emission Systems GmbH

Jia HuangDevelopment EngineerBosch Emission Systems GmbH

Harald BresslerManager Engineering Bosch Emission Systems GmbH

Wolfgang AlbrechtGeneral Manager Bosch Emission Systems GmbH

118 Exhaust Gas Treatment Bosch Rexroth AG

To achieve optimal reduction efficiency of nitrogen oxides, it is important to understand the key influential parameters and their interaction within the complex selective catalytic reduction system. A theoretical analysis was developed to determine the influence of each parameter and its interaction on the overall efficiency within the framework of TIER 4 final emission tests for off-highway applications (the C1 steady-state test and the Non-Road Transient Cycle – NRTC). To determine the path to high reduction rates, the results of the theoretical analysis were then simulated using a Computational Fluid Dynamics (CFD) analysis before being tested on an engine test bench.

This paper describes the program, analyses the most in-fluential parameters, discusses how to achieve maximum nitrogen oxide conversion rates, and defines the limits of this technology within the scope of the investigation.

Introduction

The upcoming TIER 4 final emissions standard sets strin-gent emission limits, especially for Nitrogen Oxide (NOx) and Particulate Matter (PM). Engine manufacturers must invest heavily to comply with the new limits. While previ-ous emissions legislation could usually be met by using

a particulate filter (DPF) and various measures for NOx reduction inside the engine, the new emissions standard will require additional activities to reduce NOx. Figure 1 shows that there are different approaches to meeting this challenge. As a general rule, nitrogen oxide inside the engine can be reduced by optimizing combustion pro-cesses. The other option is exhaust gas after-treatment, which entails the catalytic breakdown of nitrogen oxides generated during combustion. Selective Catalytic Reduction (SCR) has proven to be the leading solution here in recent years and has thus established itself in the automotive sector. While intra-engine processes such as Exhaust Gas Recirculation (EGR) are limited in principle, SCR technology theoretically offers the potential to convert significantly more nitrogen oxide gases. Figure 1 also shows that higher SCR performance allows a greater degree of freedom for the layout of the entire system. If the TIER 4 final regulations can be met despite high NOx exhaust gas emissions before the catalytic converter, the engine-out NOx constraint is removed from the total cost optimization process. Although high engine-out NOx emissions lead to a higher consumption of Diesel Exhaust Fluid (DEF), the increased freedom offers scope to lower particle emissions, simplify the engine and decrease fuel

consumption.

TIER 4f

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Highly sophisticated engine (CRS, EGR, advanced cooling , 2-stage Turbo Charger )

Common Engine today (Wastegate TC, CRS)

Established engine designed for TIER1, TIER2 requests.

▶ TIER4f sets very stringent limits concerning NOx.

▶ With a high performance SCR even existing

engines can reach legal limits without major

changes on the ardware or engine calibration.

▶ With a matching engine calibration it is even

thinkable to avoid a DPF.

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Fig. 1: Possible engine and exhaust gas after-treatment strategies for TIER 4 final -compliant systems

Exhaust Gas Treatment 119Bosch Rexroth AG

As these considerations show, there are sufficient rea-sons to warrant a closer examination of the possibilities and limits of an SCR system – an effort which Bosch Emission Systems aims to support with this paper. A combination of laboratory tests, and simulation analyses, and engine test bench experiments form the core of this research. In addition to the essential question of feasibi-lity, practical implementation is therefore a main focus. A brief consideration of the factors leading to series imple-mentation is presented at the end of the paper.

Theory

Basics of selective catalytic reduction

Every combustion engine produces nitrogen oxides as a matter of principle. Due to the lean combustion of diesel engines, pollutants can easily be oxidized with a platinum-based oxidation catalyst. In contrast, nitrogen oxides cannot be reduced where there is a surplus of oxygen. To solve this problem, the SCR system injects DEF into the exhaust gas. In an SCR catalyst, the DEF is converted into gaseous ammonia, a strong reducing agent that enables the final reduction of nitrogen oxides via the SCR catalyst. As ammonia is a toxic gas,

it should be broken down completely to avoid ammonia slip23,19. The accurate dosing of DEF is thus essential for environmentally friendly operation.

The setup principle used for this investigation is illustra-ted in figure 2. It consists of a diesel oxidation catalyst, followed by the DEF dosing system, a mixer, and the SCR catalyst. At a later stage, the SCR catalyst can be designed with an integrated ammonia slip catalytic zone at the end of the monolith. Several sensors are used to regulate and control the system. Figure 2 shows a NOx sensor before the SCR catalyst to detect the NOx level in the exhaust gas – the most important value when calcu-lating the required amount of DEF. The NOx sensor at the end of the system is needed to monitor the remaining NOx content. Two temperature sensors are used to detect the temperature within the SCR catalyst as precisely as possible.

The most important factor within such a system is the appropriate selection of SCR technology. Different tech-nologies can be used according to the required overall performance, temperature conditions, and other influen-tial parameters (Fig. 6).

HC, CO PM NOx

SCR

NOx sensor

DPF

AdBlue DM

DOC Temp. sensor

∆p

NOx sensor

Mixer

Fig. 2: Setup of the selective catalytic reduction system under analysis

120 Exhaust Gas Treatment Bosch Rexroth AG

Catalytic characteristics

There is a range of SCR technologies on the market today, each with particular strengths, designed for different applications and system configurations.1 Along with vanadium-based catalysts, copper- and iron-zeolite technologies are available. All three types of catalyst were screened on a synthesis gas test bench in order to gain an understanding of how they work and the interrelationship between different key parameters.

Figure 3 shows the conversion performance of the three common representatives at different temperatures. The behavior illustrated here is the most important decision criterion for a catalyst technology, where conversion per-formance at a high temperature will prove to be crucial. As shown in figure 3, the iron-zeolite SCR is best suited for this requirement and is thus used for this investiga-tion10,13. This technology performs well at temperatures up to 500 °C and the coating can endure even higher temperatures, enabling the use of an actively regenerated particulate filter (DPF) in front. This type of catalyst has also already proven its reliability in serial application.14

Several additional aspects have to be considered when using an iron-zeolite catalyst, for instance the space velocity and NO2 / NOx ratio of the incoming exhaust gas. The following sections of the paper will present and explain these aspects.

The results illustrated in figure 4 show a high dependency on the catalyst’s temperature and the space velocity.

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Fig. 4: Iron-zeolite SCR catalyst technology – dependency of the NOx

conversion rate on space velocity

▶ In order to get the best system performance the catalyst

technology has to match the specific engine characteristic.

▶ For high-temperature applications the ferric-zeolite tech-

nology is favorable while the best conversion performance

at low temperatures can be reached with copper-zeolite

catalysts.

▶ Vanadium catalysts can hardly compete with the perfor-

mance of zeolite based catalysts but work also under

sulfureous conditions.

SV=30.000 1/h; NOx=200ppm; alpha=1.2; NO2/NOx=0.07; 5vol% O2; 5vol% H2O0

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Fig. 3: Comparison of different SCR catalyst technologies – NOx conversion rates at different temperatures

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This is a common characteristic for almost all kinds of catalysts.12,22,24 A minimum temperature is needed to start the reaction, and higher temperatures accelerate the chemical reaction. However, competing reactions can trig-ger and compromise NOx conversion performance. Each catalyst technology has its own optimal temperature. In terms of catalyst performance, a lower space velocity has a similar effect because it gives the reaction more time and hence allows higher NOx conversion rates.27,21

Specific to iron-zeolite catalysts is a strong dependency on the NO2/NOx ratio, as shown in figure 5. The reason for this lies in the reduction of nitrogen oxides in the SCR catalyst, which is roughly based on the three basic reactions shown below. Depending on the proportion of NO2 in the exhaust gas, different reaction paths are pos-sible.13,20 If there is no or very little NO2 present, reaction (1) will occur most, whereas reaction (3) takes place if there the main gas present is NO2. With a balanced NO2 / NOx ratio of around 50 %, reaction (2) is the prefer-red reaction path. In addition to a different stoichiometric

factor, the main difference between these reactions is their speed: reaction (2) is much faster than the others and hence shows the best performance – especially at low temperatures.16 A NO2 / NOx ratio of 50 % is thus a vital basic requirement if good NOx conversion performance is to be achieved.

Due to the fact that the exhaust emissions of a diesel engine do not usually contain a high proportion of NO2, this gas must be generated afterwards using a diesel oxidation catalyst (DOC). In order to obtain a NO2 / NOx ratio of 50 % in the most relevant areas of the engine map, the selection of suitable DOC technology is fundamental for high NOx conversion rates. This is the reason why both the iron-zeolite SCR and DOC catalysts are assessed together.

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Fig. 5: Iron-zeolite SCR catalyst technology – dependency of the NOx

conversion rate on the NO2/NOx

(1) 4NH3 + 4NO + O2 → 4N2 + 6H2O

(2) 2NH3 + NO + NO2 → 2N2 + 3H2O

(3) 8NH3 + 6NO2 → 7N2 + 12H2O

122 Exhaust Gas Treatment Bosch Rexroth AG

Testing conditions

Based on the laboratory results, literature and in-house experience, figure 6 shows the most influential para-meters for the conversion capability of an entire SCR system. In addition to catalyst technology, factors such as temperature conditions, uniformity, space velocity and dosing strategy influence system performance. It follows that the system design is a key consideration. An impor-tant step in obtaining representative results is to test theoretical assumptions under realistic conditions using a genuine exhaust system and real exhaust gas. Therefore a seven-liter engine designed for off-highway applications and a complete exhaust gas system was set up to serve as the subject for the following tests. The purpose of the investigation was to examine the major parameters and optimize them to achieve maximum NOx conversion rates and identify remaining limitations.

All these parameters must be considered with respect to emissions legislation. The TIER 4 final emissions standard, which represents a turning point for all off-highway applications in the near future, also provides the framework for this investigation. It is based on two different test procedures carried out on an engine test bench: the stationary “C1 test” and the “Non-Road Transi-ent Cycle”(NRTC). For an engine to be certified, including its exhaust gas system, it must pass both tests. Since this paper concerns the SCR system, the main focus lies on nitrogen oxide emissions and how to comply with legisla-tion limits.

high NOx conversion

Uniformity

Low space velocity

Optimized dosing strategy

Appropriate temperature

Suitable cat. technology

Controller concept Dynamic calibration

Optimized NO / NO2 – ratioVWT, Fe - Zeolithe, Ammonia Slip Cat .

Engine „closeness “ Thermal Management, Insulation Large cat . volumina

Suitable spray Mixer, Mixing length Mixture formation

▶ ASCR system should achieve NOx conversion rates up to 95 % in order to meet TIER4f

limits despite high NOx raw emissions.

▶ To achieve this, every parameter influencing the SCR performance needs to be

optimized individually.

Fig. 6: SCR systems – influential parameters on the NOx conversion rate

Exhaust Gas Treatment 123Bosch Rexroth AG

C1 test

For engines with a rated power between 56 kW and 560 kW, the C1 test consists of eight stationary operating points.5 As shown in figure 7, these points cover different loads at nominal, intermediate and idle speeds. The challenge for the SCR system in this test is that it covers different extremes: especially challenging are the high exhaust gas mass flow at nominal speed in combination with very high temperatures at operating point 1, and the very low temperatures at operating points 4 and 8. Using the official weighting factors of the emissions standard, operating point 1 has the highest influence on the overall test result (Fig. 7). As discussed with respect to SCR catalytic technology, iron-zeolite technology performs well at high temperatures. Taking both aspects of the C1 test and the high influence of operating point 1 into consideration, it is clear that SCR catalytic technology needs to perform well at high temperatures.

NRTC

The NRTC (Fig. 8) is a transient cycle derived from typical non-road applications that covers almost the entire engine map5. In contrast to the C1 test, the exhaust gas system does not have to endure extremely high temperatures. The challenge with this test is its highly transient behavior with strong peaks of NOx and exhaust mass, as well as a low starting temperature that makes good conversion rates at the beginning of the test virtually impossible.

The NRTC is carried out twice during the engine certifica-tion procedure. The first cycle commences at a standar-dized system temperature of 25 °C (cold NRTC), while the second follows after a delay of 20 minutes of soaking time after the first one. Due to the stored heat energy from the first NRTC, the start temperature of the second cycle is much higher (hot NRTC), enabling acceptable conversion rates even at the beginning of the test.

The relevant emissions are accumulated and divided by the mean power during the test.

Contribution to the C1 - Test result

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▶ For the NRMM C1 test it is obvious that the rated power

operating point contributes most NOx to the overall test-result.

▶ Hence the performance at this OP needs to be ensured.

▶ Because of the high temperature and exhaust mass flow at

rated power the catalyst size and technology is very important

Exhaust Mass Flow

Contribution to the C1 test result

Fig. 7: Operating points of the C1 test in a typical off-highway engine application showing the weighting of each point and its contribution to the overall NO, test result

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Fig. 8: Engine speed and torque during the Non-Road Transient Cycle (NRTC) test

124 Exhaust Gas Treatment Bosch Rexroth AG

Results and discussion

Exhaust gas and ammonia distribution

In most investigations that validate the performance of a catalyst, a uniform distribution of exhaust gas and ammonia is assumed. The result is optimal performance that can only be achieved in theory and does not necessarily represent conditions in real exhaust gas systems.7,8 It is necessary to consider the influence of this assumption for real-life applications.

The uniform usage of a catalyst can be understood in two ways: in addition to the distribution of exhaust gas itself – which could lead to different gas velocities inside the catalyst – it is even more important to ensure that ammo-nia and nitrogen oxides are present in the same place.17 If this is not the case, it is not possible to reduce all nitrogen oxides without a certain amount of ammonia slip.6,26

To meet these requirements and improve exhaust gas systems, Bosch Emission Systems developed advanced CFD models to simulate simultaneous spray and exhaust gas distribution. As a result of the simulation, the local stoichiometric ratio (alpha) could be established, provi-ding a very clear method of detecting the uneven distri-bution of ammonia and describing the consequences. If the local alpha value is higher than 1.0, there is too much ammonia present, leading to ammonia slip. In contrast, if the local value is lower than 1.0, there is insufficient ammonia present to convert all nitrogen oxides in the area. Clearly, it follows that only an even distribution of both reactants enables the highest NOx conversion rate in combination with only a minimum ammonia slip.

If the aim is to achieve high NOx conversion rates, a very even distribution is therefore required under all condi-tions.3,15 This represents a challenge because the DEF spray is redirected by the exhaust gas, depending on its mass flow.

a b

α [-] NOX conversion [%] Simulation Experiment

α deviation = +/ - 45% γ = 0.92

Conversion deviation = +/- 10%; γ = 96

Lean area low NOx conversion

rich area leads to ammonia slip

Redirection of DEF by Exhaust flow

▶ A homogenous DEF distribution is crucial for high NOx conver-

sion rates without the penalty of ammonia slip.

▶ In the shown example the DEF is redirected by the exhaust

flow wich leads to a bad distribution at high engine speeds.

α [-] NOX conversion [%] Simulation Experiment

α deviation = +/ - 2% γ = 0.99

Conversion deviation = +/- 2%; γ = 99

Homogenous distri- bution of ammonia

Protection of spray with BESG flow baffle.

▶ With the optimized geometry of the mixing pipe the injected

DEF is protected by the BESG flow baffle.

▶ This approach allows a nearly perfect ammonia

distribution – independent from the engine operating point.

Fig. 9: Exhaust gas and ammonia distribution (simulation vs. test result)

Exhaust Gas Treatment 125Bosch Rexroth AG

To avoid this effect, Bosch Emission Systems simulated various models and developed a flow baffle to be installed in front of the DEF injection to shield the spray from the exhaust gas. The mode of action and achieved advance-ment is shown in figures 9a and 9b. With the presence of a flow baffle, the behavior of the spray can be predicted for all engine speeds, allowing it to be directed to the designated spot of the mixing unit, guaranteeing good spray preparation. This is also beneficial for the system layout, since ammonia distribution is almost independent of the exhaust gas flow and geometry.

A positive side effect of the baffle is that wall contact by the urea spray can be avoided, meaning the risk of am-monia crystallization is also minimized with this method, and the minimum temperature for dosing release can be decreased.2,9 The latter has a significant effect during the first seconds of the NRTC because urea dosing and the associated NOx conversion then start earlier during the test procedure.

NO2 / NOX ratio

With reference to the laboratory test results shown in figure 5, it is essential to have a balanced NO2 / NOx ratio.18 For the test setup used in this investigation, this was the case for wide areas of the engine map as shown in figure 10. Only at operating points with a very low or very high load request does the NO2/NOx ratio not reach the target of 50 %. In both cases, this behavior can be explained by the exhaust gas temperature: to generate NO2, the DOC needs at least light-off temperature. This is not the case for low engine loads, which explains the poor proportion of NO2.16 On the other hand, the low NO2 level at high loads is linked to the instability of the gas at tem-peratures above 450 °C. While NO2 is less important at high temperatures because other chemical reaction paths are also fast under these conditions, at low temperatures a lack of NO2 has an adverse effect. Although an increase in platinum in the DOC could help to generate NO2 at an earlier stage, this would lead to an excessively high NO2 ratio at intermediate temperatures.

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Fig. 10: Engine test bench results for NO2 / NOx ratio upstream from the SCR catalyst

126 Exhaust Gas Treatment Bosch Rexroth AG

Space velocity

As the residence time has a large influence on chemical processes, the size of a catalyst plays an important role.4,6,11,21,27 The main criterion when determining the size of an SCR catalyst for TIER 4 final -compliant applications is the space velocity at full power. Not only is this the most important operating point during the C1 test procedure, it is also the point with the highest space velocity. Figure 11 shows the impact of catalyst size on conversion rates. As is clear to see, operating point 1 benefits most from a larger size. While operating points with lower space velocities can already achieve maximum performance with a medium-sized catalyst, the investiga-tion shows that space velocities above 40 000 1/h are not acceptable for the operating point 1 when aiming for high NOx conversion rates.

Because operating point 1 represents the worst-case scenario in terms of space velocity, the catalyst size is also suitable for the NRTC test procedure. Additionally, the increased size of the catalyst helps to compensate NOx peaks resulting from load alternation more effec-tively, so enhancing the overall test result.

Temperature

Temperature is one of the main parameters for all che-mical processes and thus also plays an important role in catalyst technology. Many different reactions take place inside a catalyst, so it is virtually impossible to identify a model that is best in class at all operating points considering that the temperature has a range of almost 500 K in the two test procedures. Today, catalysts are available that perform well either at low temperatures or at high temperatures. This study is based on iron-zeolite technology, which is stable at high temperatures. For low temperature activity, a copper-zeolite catalyst may be a better choice.

Despite this constraint, 95 % NOx conversion can be achieved in the C1 test with an optimally sized catalyst. This is possible because the relevant operating points with high engine-out NOx emissions feature temperatures within the working range of the catalyst and can reduce incoming nitrogen oxides down to the detection limit. Despite the low raw emissions at operating point 4 and operating point 8 (idle speed), these points are responsib-le for a significant share of the remaining tailpipe emissions

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Fig. 11: Improvement in NOx conversion for each operating point of the C1 test procedure when the maximum space velocity is decreased from 60 000 1/h to 40 000 1/h

before SCR after SCR

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Fig. 12: NOx emissions in C1 test with the weighted influence of each operating point

Exhaust Gas Treatment 127Bosch Rexroth AG

because the low exhaust gas temperature impedes an effective NOx conversion. While the temperature at idle speed is far too low for SCR operation, an increased temperature at operating point 4 could have a positive effect on overall conversion performance. To achieve this, extended insulation of the exhaust system could work, but might also cause trouble at high temperatures. Another solution would be to use engine measures such as post-injection to increase exhaust gas temperature.

In a brief test, the temperature was increased using a late post-injection to generate exothermic heat over the DOC. Although the DOC was able to oxidize all of the additional hydrocarbons and increase the temperature by 40 K, the recorded effect on the SCR system was smaller than had been predicted. The main reason for this is inhibited NO2 generation, since NO2 formation competes with the total combustion of hydrocarbons.

The NRTC test procedure is also a challenge for low tem-peratures at the beginning of the test. Two-thirds of the test-relevant nitrogen oxide emissions originate during the first 300 seconds of the cycle. A common method of improving the catalyst performance at this stage is to load it with ammonia and use the storage capability of

the zeolite structure. This allows an overall conversion rate of 93 % to be achieved over the NRTC despite the low catalyst temperature at the beginning of the test. Unfortunately, the ability to store ammonia depends on the catalyst temperature. At temperatures of more than 250 to 300 °C, the catalyst loses most of this capacity and desorbs surplus NH3. Due to the load profile of the NRTC, there are two phases during the test in which the temperature rises very quickly. In order to avoid ammonia slip at these points, there is the need for an intelligent dosing strategy that decreases the actual dosing quantity to consume the stored ammonia before it is released. This is even more critical for large catalysts in which the absolute mass of stored ammonia can also be very high. Fortunately, a large catalyst monolith also reacts with gre-ater inertia to temperature changes, so there is more time to adjust the load level of the catalyst. All measurements show that increased catalyst size has no further impact on the complexity and robustness of the load controller. Ammonia slip can still be held at a low level despite the catalyst volume (Fig. 13).

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Fig. 13: Temperature and emission change during an NRTC test procedure

128 Exhaust Gas Treatment Bosch Rexroth AG

Outlook for series application

Now that the feasibility of high NOx conversion rates has been proven both in theory and under practical condi-tions, the system must be evaluated from the perspective of series production. The following questions are of particular interest: How do the measures taken impact engine calibration? How robust is the operation of such a system? Do the advantages outweigh the cost and labor effort?

In this investigation, different catalyst sizes were realized by changing the catalyst length, not the diameter. This approach was chosen in order not to affect the distributi-on of ammonia and exhaust gas. Together with complex mixing techniques, this led to higher back pressure com-pared to common SCR solutions that aim for moderate conversion rates. For the sake of completeness, it was also important to know whether this had a significant impact on engine characteristics. Concerning the test setup, increased back pressure could be monitored, but the impact was only marginal. Particle emissions and fuel consumption showed variance only close to the detection limit.

Furthermore, system robustness is a key feature when it comes to the series application of this technology. Apart from mechanical robustness, this also means chemical robustness and a certain fault tolerance over the entire service life. As a matter of principle, a high-performance SCR system has to endure the same conditions as any other SCR system. At the same time, the requirements are in fact much higher, leaving less room for inaccuracies.

Although the results presented in this paper are based on pre-aged catalysts, further investigations should pay special attention to system stability over the entire life-time. The DOC should always be included in such an investigation because it tends to lose its capability to generate NO2, which is a necessary factor for high NOx conversion.

The final question as to whether a high-performance SCR pays off or not centers on possible savings on the engine side. While it is certainly possible for engines that emit high levels of NOx to meet TIER 4 final standards, it is necessary to consider the increased DEF consumption and catalyst size.

Exhaust Gas Treatment 129Bosch Rexroth AG

SummaryThe investigation shows that an optimized SCR system can achieve a significant reduction of NOx emissions. The design of an engine / exhaust gas system thus gains an additional degree of freedom, in the case of our test carrier allowing engine-out emissions of up to 6 or 7 g/kWh and remaining in line with the TIER 4 final emissions standard. In our test case, NOx conversion of 95 % is required to comply with requirements. To achieve results of this magnitude, the system setup needs to be optimized to create conditions for high-performance NOx reduction, especially as regards catalyst characteristics. The main milestones here are the optimization of exhaust gas and ammonia distribution, and the reduction of space velocity. Although the former is an advantage for any SCR system, it is compulsory for high-performance SCR.

The major factor that still limited overall conversion rates during the test procedure is temperature: operating points with a low engine load and at the beginning of the NRTC have low temperatures, meaning that the catalyst is unable to convert all incoming nitrogen oxides. To enhan-ce conversion performance further on, it is necessary to heat the catalysts rapidly or develop new catalyst techno-logy to increase performance at low temperatures with no compromises in the high-temperatures range.

At temperatures of more than 300 °C, the test carrier was able to reduce almost all incoming nitrogen oxides down to the detection limit with a certain amount of ammonia slip.

The additional ammonia helps to compensate remaining dosing and distribution inaccuracies and raises the partial pressure. Although the ammonia limit of the TIER 4 final emission standard was not exceeded in this investigation, an ammonia slip catalyst could help to increase system robustness for serial applications.

In general, a system designed to perform near its limits is more sensitive to changes and needs to be customized for specific applications. It follows that a simple transfer from one application to another cannot guarantee the same optimal performance. Another key issue in this context is ageing, and this must be taken into account. Future research into this area is recommended.

A high-performance SCR system is possible, however, and this solution can be considered an additional option in NOx reducing technologies..

130 Exhaust Gas Treatment Bosch Rexroth AG

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3 D. Chatterjee; Detaillierte Modellierung von Abgaskatalysatoren Dissertation, Naturwissenschaftlich - Mathematischen Gesamtfakul-tät der Ruprecht - Karls - Universität Heidelberg,

4 E. Ruiz-Martínez; J.M. Sánchez-Hervás, J.; Otero-Ruiz; Effect of operating conditions on the reduction of nitrous oxide by propane over a Fe-zeolite monolith Applied Catalysis B: Environmental 61 (2005) 306–315

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Exhaust Gas Treatment 131Bosch Rexroth AG

Acknowledgments

Last but not least, I would like to say a big thank you to all those colleagues who supported this investigation with their great experience, dedication and technical knowledge. An investigation of this scope requires the help of many different specialists, and naming them all is unfortunately not possible here. Nevertheless, there are some who made a large and decisive contribution to the research in addition to the co-authors:

Mr. Götz Flender, Robert Bosch GmbH (an expert in SCR technology and everything around the engine test bench)

Dr. Hartmut Lüders, Robert Bosch GmbH (division manager, responsible for research and development activities at Robert Bosch GmbH concerning exhaust gas after-treatment systems)

Mr. Chaiwat Jaruvatee, Bosch Emission Systems GmbH & Co. KG (responsible for CFD simulations and several solutions concerning the optimization of spray behavior)

Definitions/Abbreviations

DEF Diesel Exhaused Fluid Aqueous Urea Solution 32.5 %

DOC Diesel Oxidation Catalyst

DPF Diesel Particulate Filter

K Kelvin

NH3 Ammonia

NO2 Nitrogen dioxide

NOx Nitrogen oxides

PM Particulate matter

SCR Selective catalytic reduction

SV Space velocity

η Degree of efficiency

Appendix

Calculation of distribution to the overall test result for operating point i in the C1 test procedure:

Calculation of weighted NOx emissions for operating point i in the C1 test procedure:

[ ]

n

i WFikWP

WFihgNOxi

kWhgNOxi

⋅

⋅=

∑

[ ] 100% ⋅⋅

⋅=

∑ i

n

i=1i

ii

WFhgNOx

WFhgNOx

Distributioni

i=1