Role of CFD in Evaluating SCR NOx Reduction Performance

Manoj Sampath, Figen LacinTenneco

AbstractIn recent years, significant effort has been put into the reduction of diesel exhaust emissions, primarily

on Particulate Matter (PM), i.e. soot, HC, CO and NOx to meet stringent emission standards. For reducing NOxemissions, Selective Catalytic Reduction (SCR) deNOx technology has been widely used by the diesel industry.Typically SCR system consists of decomposition tube, urea dosing module, static mixer and a SCR catalyst. Inthe dosing process, liquid urea (Diesel Exhaust Fluid, DEF) is injected into the domain by injector and in mostcases, with the help of mixer, the urea decomposes into ammonia (NH3) and isocyanic acid (HNCO) throughthermolysis and hydrolysis process. At the SCR catalyst, NH3 undergoes surface reaction with the coatedmaterials and reduction process takes place, where the NOx is converted into N2 and H2O. Computational FluidDynamics (CFD) is one of the common tool which is used in design stages to evaluate SCR design performanceby predicting the ammonia distribution in the gas phase reactions. This paper is focused on developing CFDmethodology to conduct SCR spray modeling analysis using commercial software and discuss the outputparameters to evaluate the design performance. The challenges of validating the predicted results with theexperimental data is also discussed. The work is concluded by proposing ways to utilize the CFD results to bestaddress the SCR performance in NOx reduction process.

IntroductionExhaust emissions from stationary and mobile sources pollutes the air and pose a serious threat to the

environment and health safety. Most harmful emissions such as PM, HC, CO, NOx and SOx are typicallycaused by the diesel combustion in the internal combustion engine. In order to control these emissions, EPA wascreated to enforce regulation on amount of emissions that can be allowed from the sources. These legislationspaved the way to introduce exhaust after-treatment systems to control the emissions. Emissions of HC and COare reduced by utilizing the Diesel Oxidation Catalyst (DOC) through oxidation and PM is reduced by utilizingDiesel Particulate Filter (DPF), where the soot is burned to ashes through regeneration process. For NOxabatement, technologies like lean NOx catalyst, adsorbers and SCR were developed and among which SCRDeNOx has been widely implemented.

In SCR systems, liquid urea, a reacting agent, is typically introduced in the exhaust flow to convert theNOx. This liquid urea called as AdBlue or DEF which contains 32.5 wt % of urea in water. In the NOxreduction process, DEF is injected in the decomposition tube where ammonia (NH3) is evaporated from ureaand reacts with NOx with the presence of Oxygen (O2) on the SCR catalyst bed forming nitrogen (N2) and water(H2O). The entire NO reduction process can be subdivided into two main reaction stages, gas phase reactionsand surface reactions.

The region of gas phase reactions start from the inlet of the decomposition tube till the inlet of SCR catalyst.For DEF dosing, an urea injector is used to atomize the spray for better evaporation. In some cases, a staticmixer might be used to atomized the spray and help to improve the evaporation of urea droplets. The spraycharacteristics of the injector is also an important factor in urea evaporation. In general, the generation ofammonia happens in three main reactions [1,2], 1. Formation of solid or liquid urea after evaporation of water

2. Thermolysis of urea into ammonia and iso-cyanic acid, 3. Hydrolysis of iso-cyanic acid,

For an efficient system, the decomposition tube needs to be optimized so that it promotes complete evaporationof urea in NH3 without any deposit on the wall[3,4,5].

The NH3 generated from the gas phase reactions undergoes series of chemical reactions [6,7] as it passesthrough the catalyst pores that are coated with materials for NOx reduction. In addition to these reactions, the

overall NOx reduction process is affected by so many factors such as the catalyst types, gas temperatures,amount of dosing, NH3 mal-distribution, etc.

ObjectiveThe decomposition tube design is one of the important parameter for SCR performance. The exhaust

flow interaction with the urea spray and evaporation of urea are key factors in the design. From design phase toproduction phase, conducting experiment test to determine each design performance may not be viable in termsof cost and time. Hence for the development phase, numerical predictions such as CFD are widely used toconduct various design iterations to achieve an optimized one to meet the requirements. The scope of this paperis mainly focused on developing a CFD methodology on how to optimize the design and how the results can beinterpreted to address the product performance. The study is conducted on a typical SCR system consisting of adecomposition tube with a static mixer along with SCR catalyst. AVL Fire, a commercial available CFD code, isused for conducting urea spray modeling analysis. The numerical analysis mainly deals with the gas phasereactions happening in the decomposition tube.

Urea spray modelingUrea spray modeling analysis predicts the formation of NH3 and HNCO from the liquid urea evaporation

and decomposition for the given exhaust flow and urea dosing conditions. The entire modeling is done byapplying best practices for SCR applications. More detailed theory on this section can be found in the solverguide of AVL Fire manual [8]. Since there have been many literatures [9, 10, 11, 12] available on governingequations, this paper focuses on CFD results and interpretation.

CFD analysisThe CFD analysis is carried out using the commercial software, AVL Fire. This section explains the

CFD domain used, model setup and boundary conditions.5.1 CFD Model and Mesh

Two CFD models were created for the study, one with mixer and one without mixer. Both design areidentical in terms of geometry and meshing except for the presence of mixer. The CFD model for mixer designis shown in Fig. 1. The model is a sub-system of an exhaust after-treatment system, which contains thedecomposition tube, a static mixer and SCR. The decomposition tube is 3” in diameter with a wall thickness of1.8mm. A static mixer is placed in optimized location with respect to injector to improve urea evaporation. Theinlet is approximately 11” upstream of mixer to have the exhaust flow fully developed before the sprayinteraction happens. SCR inlet is located 15” downstream from the injector tip, having enough mixing length inorder to promote higher evaporation. The SCR is based on Fe Zeolite coating with 8”diameter and 10” length.Since the primary focus is on urea evaporation and decomposition, the CFD model is captured till the SCRoutlet. The outlet of the domain is an extrusion of the SCR outlet, without any outlet cone and open to ambientconditions. For simplification, the thickness of the decomposition. tube, substrate MAT are not captured, butappropriate boundary conditions are applied to account for the heat transfer.

Fig. 1: CFD Model used for spray analysisThe computational mesh is generated using Fire FAME Meshing and shown in Fig 2, 3 and 4. To capture

the thermal boundary layer, 2 prism layers were used in all the fluid domains. The final mesh composed of 2.4million elements with a maximum cell size of 4mm and consists of tetrahedral and hexahedral elements. Fig. 3

also shows the mesh extrusion from the SCR outlet to form the domain outlet. Local refinement of the mesh isdone near the vicinity of the injector and mixer to capture the spray interaction with the mixer as shown in Fig.4. Fluid, SCR and outlet domains are connected through conformal meshing without any arbitrary interface.

Fig. 2: CFD Mesh: Without mixer

Fig. 3: CFD Mesh: With mixer

Fig. 4: CFD Mesh: Closer view of the injector and mixer

The transient simulation is setup with delta t of 0.001 sec with a maximum of 30 sub iterations. Themodules activated for this study are porosities, general gas phase reactions, spray, wall film and thin walls. Thedomain is discretized using ‘Least Sq. Fit’ derivations with the ‘Simple’ coupled solver. For turbulence, k-epsilon with standard wall function is selected. The gravity is turned on to include the gravity effects on thedroplets. For better convergence, the delta t and under-relaxation factors are the main tuning parameters. ForSCR pressure loss, the resistance values are calculated based on the CPSI, wall thickness and coatinginformation from catalyst. The inlet gas is composed of different species typically found in exhaust gas. TheAdblue property is defined as multi-component liquid with 67.5% of water and 32.5 % of urea. The urea isinjected after the exhaust flow is fully developed inside the domain. The typical input for the urea dosing is thetotal mass for that cycle and duration for which the injection is on. For this analysis, a single hole urea injectorwith a spray angle of 48 degree and injection velocity of 20 m/sec is applied. For the particle sizes, the diametervs probability curve is fed into the solver. These data are calculated from the injector PDPA data conducted onthe specific injector. The breakup parameter and spray parameters are tuned to reproduce the spraycharacteristics of that injector measured from PDPA before applied to the spray analysis. The droplet trajectoriesare calculated up to the SCR inlet and after that the droplets are assumed to be fully evaporated.

Two flow conditions are selected to predict the SCR performance for both cases, with and withoutmixer. The exhaust gas mass flow rate, inlet gas temperature, dosing rate and outlet pressure conditions aregiven in Table 1. The injector properties defined in section 5.2 are applied. The viscous and inertial resistance

values used to model the porosities are 3.34E+07 1/sq.m and 8.08 1/m respectively. For the pipe wall, adiabaticconditions are applied to simulate the insulations around the decomposition tube. For the mixer wall, inlet gastemperatures are applied as an initial conditions.

CasesMassflowrate Temperature

Dosingrate

Dosingduration

Outlet

Pressure

Kg/hr C g/sec ms PaLow Flow 250 330 0.09 191.49 0High Flow 760 410 0.15 319.15 0

Table 1. Boundary conditionsPressure loss is one of the base variable to define a system performance and most industries widely

accept the CFD predicted results for quantification. For this case, static pressure and total pressure drop from the inlet to domain outlet are considered as the pressure loss of the system.

In the analysis, two uniformity indices are calculated at the SCR inlet face; one for the axial velocity andone for the ammonia distribution. The velocity uniformity index, is defined by, (4)Where V is average axial velocity and A is area of the cross section. The urea uniformity index, , is defined by, (5)Where is the mole fraction of NH3.

CFD ResultsThe system static and total pressure loss from inlet to outlet for the cases with and without mixer is

shown in Fig. 5. From the results it was observed that at high flow rate, the mixer contributed twice the pressureloss of the no mixer system. Based on the back pressure requirement, the mixer can be tuned to meet the targets.With the current design, at the high flow conditions the total pressure loss for the mixer case predicted to be 9.3kPa.

Fig. 5: System pressure [Pa] lossThe cross sectional velocity contours at high flow conditions for both cases are shown in Fig. 6 and 7. With nomixer design, the flow separates right at the SCR inlet cone, resulting in the core impingement as shown in Fig.8 a. For the mixer design, the flow gets accelerated at the mixer region and have more uniform distribution atthe SCR inlet as shown in Fig.8 b.

Fig. 6: No mixer: Velocity magnitude [m/sec] contours

Fig. 7: With mixer: Velocity magnitude [m/sec] contours

(a) no mixer (b) with mixer

Fig. 8: SCR inlet velocity [m/sec] contoursTo quantify this distribution, as defined in Eqn.4, velocity uniformity index is calculated for both designs andshown in Fig. 10. The uniformity index of 1 shows that the flow is evenly distributed across the face and typicalindustry target of having flow or velocity uniformity is around 0.95. From the results shown in Fig. 9, it isobserved that the mixer helps to improve the uniformity by ~2% at low flow and ~8% at high flow and alsohelps to meet the flow uniformity target. The NH3 uniformity for both cases are shown in Fig. 10.

Fig. 9: Velocity uniformity at the SCR Inlet Fig. 10: NH3 uniformity at the SCR Inlet

The uniformity is calculated at the time when ammonia generation is at maximum in this paper. If the NH3

generation is fluctuating, then the NH3 uniformity is calculated in a different way of weighing and averageapproach. At low flow, the mixer case NH3 uniformity is 0.92, a ~55% improvement from the no mixer case andat the high flow, the mixer case NH3 uniformity is 0.74, a 48% improvement from the no mixer case. Theseresults shows that the mixer needs to be optimized to improve the NH3 uniformity. For the current scope of thispaper, the mixer optimization results are not included. Urea uniformity index is important to identify the hot spots and cold spots of the ammonia distribution at theSCR inlet for better NOx reduction process. The mass fraction of NH3 distribution at the SCR inlet for thesecases are shown in Fig. 11 to Fig. 14. At low flow, the mixer helps to distribute the NH 3 more uniform across

the SCR face, but at high flow the gradients are high at the core and its needs more diffusion. By having a moresmooth transition of the cone design will also help to improve the uniformity for this case.

Fig. 11: Low Flow: No mixer NH3 contours Fig. 12: Low Flow: With mixer NH3 contours

Fig. 13: High Flow: No mixer NH3 contours Fig. 14: High Flow: With mixer NH3 contours

Validation challengesTo gain confidence in the numerical predictions, experiments are conducted in flow lab or engine

dynamometer and then compared with the CFD results. For backpressure and velocity distribution, CFD resultsare often accepted. But for the problems involves advance physics or sophisticated modeling, correlationbecomes quite challenging and often misinterpreted. In the SCR spray modeling, the main quantificationparameter is NH3 uniformity across the SCR face. This parameter is often compared to the NOx reductionefficiency from the tests.

Fig. 15: Test vs CFD: Correlation process flow chartThe flow chart shown in Fig. 15 shows the gap between these correlation parameters. From the chart, it isexplained that the CFD predictions are mainly based on gas phase reactions, whereas experiments are based onboth gas phase and surface reactions. Even though the assumption on the flow chart shown in Fig. 15 shows thathigher SCR inlet NH3 uniformity results in better NOx reduction, there are many factors influences this NOxreduction process. Main such factors include, inlet NO to NO2 ratio, type of SCR reactions at different operatingtemperature, type of SCR catalyst and oxidation. In addition to that, the resolution of measurement or numberof measurement points is also a crucial factor. It would be ideal to measure the actual amount of NH3 formedwithout any utilization towards NOx reduction and its uniformity, from the flow lab measurements to compareit with CFD. One way to achieve this is by implementing a hydrolysis catalyst to deactivate the SCR reactionsor by having the inlet gas composition without any NOx. This approach help to have, only NH3 at the SCRoutlet, but still oxidation factor is purely dependent on the gas temperatures. Nevertheless, it is a quitechallenging task to measure NH3 directly. Like NOx, NH3 is also a harmful emission which cannot be releasedto ambient, hence the X-Y probe at the SCR outlet may not feasible for large amount of the species. Even if theexhaust samples are taken out through a FTIR, there is a limitation in amount of NH3 in ppm that can bemeasured.

Results interpretationGiven the challenges in validating the CFD predictions, the question becomes how to interpret the

predicted results to quantify the system performance. Since the spray modeling solves for the flow and reactionshappening in the decomposition tube, the results of NH3 generated and its distribution can be used to predictthe design performance qualitatively. Instead of solving for absolute numbers, a % difference or a trend, wouldbe the best way to quantify the results. For example, this study conducted here is used to compare the designswith and without mixer.

Similarly for a design change in the mixer, the initial design result serves as baseline for anyimprovement. CFD mainly addresses the decomposition tube design performance in terms of backpressure,flow distribution and a trend wise ammonia formation. For an overall system performance based on tail pipeNOx or NOx reduction efficiency, still flow lab or engine dynamometers or the actual test on a prototype ismore appropriate way to quantify.

Conclusion

The CFD methodology of conducting urea spray modeling for SCR applications is presented using acommercial CFD software. The physics needed for such spray analysis is also discussed and the models that areactivated for this study are also presented. CFD spray analysis using a simple decomposition tube integratedwith a static mixer and SCR is conducted and the results were discussed. Main performance quantification forthe decomposition design is identified as the flow distribution at the SCR inlet and specifically NH3 uniformity.From the studies conducted, it was shown that a static mixer helps to improve the ammonia distribution at theSCR inlet significantly and also helps to generate more ammonia. The main challenge in validating the CFDresults with experimental data is that the comparison variable between the CFD and test are not the same.Proposals of comparing only NH3 from test are provided and limitations of such proposals are also discussed. Insummary, CFD is a key tool in design and development of decomposition tube in SCR applications, and byconducting urea spray modeling analysis, CFD can determine the design performance qualitatively byaddressing the NH3 flow distribution in the domain.

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