JSAE 20119072 SAE 2011-01-2092
HC-SCR on Silver-Based Catalyst: From Synthetic Gas Bench to Real Use Arnaud FROBERT , Stéphane RAUX IFP Energies nouvelles
Arnaud LAHOUGUE, Christian HAMON IRMA Technologies
Karine PAJOT, Gilbert BLANCHARD PSA Peugeot-Citroën
Copyright © 2011 Society of Automotive Engineers of Japan, Inc.
ABSTRACT The challenge for decreasing the emissions of compression ignition engines now remains mainly on NOx control. If the Lean NOx Trap (LNT) and Selective Catalytic Reduction by Urea (Urea-SCR) are very efficient, their extra-cost and management are a major issue for the OEMs. In that context, the selective catalytic reduction by hydrocarbons (HC-SCR) appears to be an interesting alternative solution, with a more limited NOx conversion efficiency but an easier packaging (diesel fuel as a reductant) and a limited price (reasonable coating cost / no PGM).
The gas velocity: unusual small GHSV values for automotive applications are needed to ensure a good NOx conversion rate.
•
The nature of the used reductant. Diesel fuel (ULSD) compared with decane lowers the efficiency, but synthetic Diesel fuel (SD) and, moreover, ethanol, largely improve it, especially for high GHSV.
INTRODUCTION Compression Ignition Direct Injection (CIDI) and Spark Ignition Direct Injection (SIDI) engines are well-known and recognized for their high CO2-saving potential. However, their lean-burn combustion yields to large amounts of NOx emissions [1]. If Exhaust Gas Recirculation (EGR) and optimized combustion have been developed and used since the end of the 90's, these two technologies are now reaching their limits.
In the framework of the RedNOx project, a prototype catalyst made of 2% silver on Alumina coated on cordierite was manufactured and tested on a synthetic gas bench. In parallel, an exhaust implementation study has been led to ensure the most suited conditions for injection. Thanks to SGB and simulation results, adapted engine tests have been designed and performed.
OEMs and suppliers actually work on DeNOx aftertreatment technologies. Urea-SCR is definitely the most efficient one (>95%), on a large range of temperature (200-500°C), without any major issue on aging and poisoning [2][3][4]. The challenges remain (i) on an accurate control with a good understanding of the catalyst behavior versus all influent parameters [5], (ii) on the reductant cost and delivery method and (iii) on the global cost and packaging of the solution: efforts have to be made on sensors and actuators accuracy and reliability [6][7][8]. That high potential allows moving the NOx / fuel economy trade-off more on the consumption side and having some significant CO2 savings, from -2 to -5% [9].
If the results on SGB are consistent with the literature with a maximum NOx conversion reaching about 70% and an operating range between 300 and 500°C, first engine results were very disappointing showing efficiencies lower than 15% for all the operating points. Three guidelines for improving these results were identified and studied: •
•
The way the reductant is introduced in the exhaust, especially the discontinued HC delivery due to the injection pattern
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cpsi. Two parts have been prepared, one for 3 laboratory testing (25 cm ) and the second one for the engine test-bench (2.5L).
Very impressive progresses have been made recently on Lean NOx Traps (LNT) [10][11]. The efficiency can reach 90% between 250 and 450°C and the packaging is easier than for a urea-SCR system. The drawbacks are (i) the versatile price of the solution due to the PGM market, (ii) the management of NOx / SOx purges and (iii) the aging and the irreversible sulfur poisoning. Moreover, the lean/rich switches induce an extra fuel consumption (+1 to +2%) and the solution is not applicable on all the engine operating range, what could be an issue for some driving cycles.
The main characteristics of the designed catalysts are summarized in the Table 1 hereunder. Support
Slurry
In that context, HC-SCR appears as a credible alternative. The major issue concerning the formulation of the active phase relates to the selectivity and especially the compromise between HC oxidation and NOx removal. For instance, the ion-exchanged base metal zeolite catalysts (e.g., Cu-ZSM-5) have a very low NOx activity on the considered temperature range. PGM-containing catalysts have a good NOx activity but in a very narrow temperature range [12]. The use of Silver on Alumina (Ag-Al2O3) is now recognized as the best solution for the HC-SCR thanks to its good selectivity on a large temperature range [13][14]. The most recent studies show encouraging results on a Synthetic Gas Bench (SGB), even if the efficiency strongly depends on the nature of HC being used [15]: paraffins, especially long-chain paraffins, yield to high efficiency, unlike aromatics. For low-temperatures, a continuous deactivation can be seen for most of the reductant used. Finally, alcohols like ethanol are very effective for NOx conversion without any deactivation issue, despite a rather high selectivity to ammonia. However, a NH3-SCR catalyst may be usefully fitted downstream of the HC-SCR one [16].
Diameter : 1" Length : 2" Volume : 0.025 L
Cordierite 400 cpsi 2% Ag on Alumina 150 g/L
Diameter : 5.66" Length : 6" Volume : 2.5 L
Table 1: Characteristics of the catalyst
REDUCTANTS
The RedNOx project focuses on the development of innovative formulations to improve both the low-temperature activity and the selectivity of the catalyst. The target application is a diesel powered passenger car. The engine-out NOx are supposed to be low with a low exhaust flow (high EGR-rate). Therefore, the reductant delivery has to be very accurate for very small injected quantities.
Different reductants have been used for the tests :
For those reasons, the transition from lab to engine has to be closely studied. Particularly, what are the prerequisites for a successful adaptation ? The paper focuses on this specific work, with the help of CFD-modeling and advanced measurement facilities. The involved partners concerning that part of the project are PSA Peugeot-Citroën as end-user and exhaust-line designer, IRMA Technologies for catalyst prototyping and laboratory testing and IFP Energies nouvelles for the engine bench tests.
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Decane, chosen for the in laboratory tests, since paraffins compose more than the half of european Diesel fuels
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Ultra-Low Sulfur Diesel fuel (ULSD). It is mainly composed of n-paraffins, naphtens and of some aromatics.
•
Synthetic Diesel fuel (SD), only used for some engine tests. This fuel, produced by the Fischer-Tropsch process from gas (GtL) has interesting properties: it is mainly paraffinic and the mean molecule is lighter than ULSD's one.
•
Ethanol, possible applications
reductant
for
some
The main properties of these four reductants are summarized in the Table 2.
CATALYST A state-of-the-art catalyst has been prototyped by IRMA, thanks to an extensive literature study [17]. A slurry made of 2% Ag on alumina has been prepared and coated at 150 g/L on a cordierite monolith of 400
2
Mean Molecule C H O NCV Density Comments Distillation Curve (°C)
%wt kJ/kg 3 kg/m 5% 50% 100%
ULSD CH1.886 85.9 13.5 0.6 42920 837 B5 167 277 354
SD CH2.15 84.8 15.2 20), but that phenomenon can also be seen for some more reasonable dosing values.
25
9.9
180
The DeNOx efficiency increases sharply to a peak value of 52%, which can be compared to the laboratory results for that temperature. After that peak, the conversion rate decreases quickly to reach a stabilized value of 12% after 10 minutes of injection.
30
9.6
120
Figure 12: 12: Deactivation phenomenon (Operating point n°2, decane)
35
9.3
6 3
40
9.0
0
0
The HC on NOx ratio for the same test than previously is plotted in Figure 11. The mean HC on NOx ratio is close to 6, convenient for the HC-SCR use.
5
10
12% eff. after 10min of injection !
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However, despite the good vaporization, the NOx conversion efficiency remains dramatically low, especially by using diesel fuel as reductant.
10
20
40
The integration of the raw HC signal of the Fast-FID analyzer, considering our diesel fuel as a CH1.85 molecule, gives a unit mass of 5.26g. Compared to the injection test, we have a recovery of 95%. Therefore, the vaporization estimated in the center of the catalyst inlet face is considered as quite good. Moreover, the results are perfectly consistent with the CFD calculation shown earlier.
NOx Efficiency [%]
12
Injection trigger [V]
14
8000 7000 6000 5000
ton = 2ms
4000
ton = 3 ms
6 Trigger [V]
We observe that, for a mean value corresponding to the desired setpoint of 6, the instantaneous HC on NOx ratio moves from 2 to 32, what represents an amplitude of nearly 600 % ! It was previously demonstrated with lab tests that the HC on NOx ratio has a dramatic impact on the NOx conversion efficiency. We thus assume that this pulsated over-injection cause a partial deactivation of the catalyst, possibly due to active site blocking by long chain hydrocarbons.
4 2 0 -2 1.0
1.1
1.2
1.3
1.4
1.5 time [s]
1.6
1.7
1.8
1.9
2.0
Figure 13: 13: HC level upstream of the catalyst for 2 exhaust injection patterns patterns (Operating point n°3, diesel fuel, HC/NOx = 6.5)
This deactivation phenomenon is illustrated by the example presented in Figure 12. Before the test, the HC-SCR catalyst was reset on a full load operating
The duration decreasing of the opening time leads to a 30% reduction of the unit injection. To deliver the
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The space velocity decreasing is a good way to improve the performances of the catalyst on a test bench application. It compensates the troubles linked to the pulsed injection.
same mass, the injections are thus more frequent, resulting in smoother variation of the HC recording. Unfortunately, that "smoothing" effect has only a low impact on the efficiency, at least for the tested reductant and operating point. It shows how challenging it is to find an accurate dosing system.
IMPACT OF THE REDUCTANTS The previous results were mainly presented for the decane used in the labs. As discussed in the introduction, the paraffinic structure of decane is favorable to the HC-SCR chemistry. Compared with a classical diesel fuel (ULSD), the results are supposed to be relatively optimistic. To better understand the role of the molecules kind, a synthetic diesel fuel, mainly paraffinic, was also used for some tests on the engine test bench. Ethanol, known as an effective reductant [15][16] was finally tested. It must be highlighted that ethanol is locally used as fuel in engines, and that HC-SCR by ethanol might be closely considered in lean-burn ethanol SIDI engines.
Impact of the engine pollutants and the space velocity Concerning the engine pollutants, a specific test was performed by including in the exhaust line a DOC and a DPF, just upstream of the reductant injection. Considering the exhaust temperature of the highlighted operating point, both DOC and DPF can be considered as perfectly efficient. The decreasing of the space velocity could provide a good help by keeping together the active species during a longer time. This speeding down effect was tested on the engine test bench thanks to an exhaust bypass, which allows a three time decreasing of the space velocity in the catalyst (see Figure 7).
The best results obtained on the synthetic gas bench are presented in Table 8. The HC on NOx ratio is adjusted for each reductant to obtain the best efficiency.
Both tests are presented on Figure 14, on the operating point n°2.
Facility Reductant Injection -1 GHSV (h ) NOx (ppm) DeNOx (%)
100 SCR alone (SV = 61000 1/h)
DeNOx efficiency (%)
90
DOC+DPF+SCR (SV = 61000 1/h)
80
Bypass+SCR (SV = 18000 1/h)
70 60
SGB ULSD Pump 30000 400 43
SGB Ethanol Pump 30000 400 85
SGB Decane Pump 30000 400 67
Table 8 : Impact of the reductant on DeNOx efficiency @ 350°C on SGB, HC/NOx adjusted
50 40 30
The expected hierarchy is respected. Ethanol is clearly the best candidate, even if the undesirable emissions of ammonia should be closely considered (not developed here).
20 10 0 0
2
4
6
8
10
12
14
16
HC on NOx ratio
The different reductants were also tested on the same operating point on the engine test bench. The space velocity is twice that of lab tests, to be more representative of real-life vehicle application.
Figure 14: 14: HC on NOx ratio ratio swept for Point 2 (decane) The influence of the DOC and the DPF seems slightly negative. We thus draw two conclusions: •
The engine-out HC are helpful for the HC-SCR,
•
PM and CO have no negative influence on the activity of our catalyst.
Facility Reductant Injection -1 GHSV (h ) NOx (ppm) DeNOx (%)
SGB Pump 30000 400 67
Engine Decane Injector 61000 150 16
Engine SD Injector 61000 150 20
Engine Ethanol Injector 61000 150 21
Table 9 : Impact of the reductant on DeNOx efficiency @ 350°C on engine, HC/NOx adjusted
Nevertheless, the space velocity has a dramatic impact on the NOx conversion: dividing the SV by a factor 3 rises the efficiencies close to those obtained in the lab (see Table 7). Facility Injection -1 GHSV (h ) NOx (ppm) DeNOx (%)
Engine ULSD Injector 61000 150 9
Diesel fuel leads to dramatically low results: the unfavorable molecule [15] magnifies the troubles linked to the pulsated injection and the relatively high space velocity.
Engine Injector 18000 150 52
The decane and synthetic diesel fuel present very similar results: about twice the diesel efficiency, what remains unacceptably low. The correlation between
Table 7: Impact of the space velocity on the DeNOx efficiency, HC/NOx adjusted (decane)
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the both reductants seems logical, due to the similitude of their properties.
on NOx ratio is never reached on the catalyst and some deactivation phenomena can be seen.
The ethanol results are quite disappointing compared to the laboratory ones and the litterature references [15][16].
Lowering the space velocity is a great help in that context. Thanks to longer residence times improving the contact between the reactants and the active sites, part of the deNOx efficiency is recovered. It thus appears that the correct operation of the HC-SCR requires a catalyst volume around twice that of the engine displacement, to obtain a SV lower than -1 30000 h [18].
From the findings developed in this paper , the experimental mean has been modified, especially concerning the reductant dosing. The pulsated injection system has been replaced by a micro volumetric pump that delivers at a low pressure the chosen reductant in the exhaust line through a capillary (Internal diameter = 0.5mm). The major drawback of this configuration, the poor vaporization quality, is compensated by the ethanol properties (complete distillation at 79°C, see Table 2). Thank s to the continuous introduction of the ethanol in the exhaust gases, the results are really much better than those presented in Table 9 : the peak efficiency is about 89%, close to the SGB results (Figure 15).
Lastly, the impact of the nature of the reductant is estimated. The diesel fuel appears to be the worst candidate for HC-SCR . Decane and synthetic diesel fuel show more interesting properties. Ethanol remains the most effective reductant, even if it stays very sensitive to the injection mean. Nevertheless, that reductant might be easily exploited for an ethanol fueled lean-burn application. The association of the HC-SCR catalyst with a NH3-SCR catalyst would allow very high performances and no undesirable ammonia emissions [16].
100 90
The ongoing work now focuses on the development of innovative formulation improving the selectivity of the catalyst and the low-temperature activity.
80 70
[%]
60 50 40 30
CONTACT
DeNOx efficiency Ethanol conversion efficiency
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Arnaud Frobert IFP Energies nouvelles - Lyon Energy Applications Techniques Division 69360 SOLAIZE, FRANCE
[email protected]
10 0 0
2
4
6
8 10 12 14 HC on NOx ratio
16
18
20
Figure 15: 15: HC on NOx ratio swept for Point 2 (ethanol) ethanol) and the improved injection system
Visit our website: http://www.ifpen.fr/
CONCLUSION
ACKNOWLEDGMENTS
The presented work has been initiated by the lack of efficiency of the HC-SCR seen on an engine exhaust line. These results were considered as particularly surprising in light of the actual performance of the catalyst in the synthetic gas bench.
The authors wish to thank Jean-Christophe Maronneaud for conducting the engine test bench experimental study.
Our investigations focused on the diverse elements that might cause this difference between the experimental means.
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It was first shown that the species issued from diesel combustion process – HC, CO and PM – have no negative impact on the HC-SCR behaviour.
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One of the main issues concerning the HC-SCR process on an engine lies on the reductant dosing in the exhaust gas. The use of an injector – even a low flow injector – proved to be a major drawback. The pulsated delivery of the reductant leads to over-injections and under-injections. The required HC
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14. Steven J. Schmieg, Thompson M. Sloane and Richard J. Blint (General Motors) – 2009. "Catalysts for Lean-Burn Engine Exhaust Aftertreatment Using Hydrocarbon Selective Catalytic Reduction". SAE paper 2009-01-2819.
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APPENDIX
7. Yong-Wha Kim and Michiel Van Nieuwstadt (Ford Motor Company) – 2006. "Threshold Monitoring of Urea SCR Systems". SAE paper 2006-01-3548.
CIDI: Compression Ignition Direct Injection
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DOC: Diesel Oxidation Catalyst
CFD: Computational Fluid Dynamics
DPF: Diesel Particulate Filter EGR: Exhaust Gas Recirculation
9. Pierre Macaudière (PSA Peugeot-Citroën) – 2010. "SCR as a technical response to Automotive environnemental challenges". MinNOx Conference 2010 (Berlin).
EUDC: Extra-Urban Driving Cycle GHSV: Gas Hourly Space Velocity PGM: Platinum Group Metals
10. F. Rohr, I. Grißtede and S. Bremm (Umicore AG) – 2009. "Concept Study for NOx Aftertreatment Systems for Europe". SAE paper 2009-01-0632.
PM: Particulates Matter NEDC: New European Driving Cycle
11. Ulrich Göbel (Umicore AG) – 2008. "Diesel NOx-aftertreatment systems for the Tier2/Bin5 legislation in North America". H030-06-272-8_17 MinNOx Conference 2008 (Berlin).
SGB: Synthetic Gas Bench SD: Synthetic Diesel
12. R. Burch, J.P. Breen, and F.C. Meunier, “A Review of the Selective Reduction of NOx with Hydrocarbons under Lean-Burn Conditions with Non-Zeolitic Oxide and Platinum Group Metal Catalysts”, Appl. Cat. B: Environ. 39, 283-303 (2002).
SIDI: Spark Ignition Direct Injection SV: Gas Hourly Space Velocity ULSD: Ultra-Low Sulfur Diesel
13. K.-i Shimizu and A. Satsuma, “Selective Catalytic Reduction of NO over Supported Silver Catalysts – Practical and Mechanistic Aspects”, Chem. Phys. 8, 2677-2695 (2006).
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