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The optimal volume of a combined system of LNT and SCR catalysts
Journal of Industrial and Engineering Chemistry 17 (2011) 382–385
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Short communication
The optimal volume of a combined system of LNT and SCR catalysts Choong-Kil Seo, Hwanam Kim, Byungchul Choi *, Myung Taeck Lim Chonnam National University, Republic of Korea
A R T I C L E I N F O
Article history: Received 10 August 2010 Accepted 20 October 2010 Available online 13 May 2011 Keywords: Diesel engine Catalyst LNT (Lean NOx Trap) SCR (Selective Catalytic Reduction) SV (Space Velocity)
A B S T R A C T
This paper aims to find the optimal volumes of the LNT (Lean NOx Trap) and SCR (Selective Catalytic Reduction) catalysts for higher de-NOx performance and suppression of N2O and NH3 in a combined system of LNT + SCR. The basic characteristics of the LNT catalyst were identified first at various space velocities (SV), and then the effects of the variation in volume of the combined system were investigated. The NOx conversion, NH3 production, and N2O emissions were evaluated of four differently sized LNT– SCR systems. Considering all together the NOx conversion, cost of the precious metal, NH3 slippage, and N2O generation, the optimal volume ratio of LNT and SCR catalysts was found to be 1:1 for the LNT–SCR system. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction As diesel engines have high power and good fuel economy with low CO2 emissions, their market share is increasing in the sectors of commercial vehicles as well as passenger cars [1]. Because of the intrinsic nature of diesel combustion, however, nitrogen oxides (NOx) are generated in the localized high-temperature reaction zones, and particulate matters (PM) are formed in the diffusive combustion zones. The exhaust emission standards for vehicles have become extremely strict in most of the countries. European Union (EU) recently published the final Euro VI regulation, setting the limits of 10 ppm ammonia on the European Transient and European Steady-State Cycles [2]. Several technologies are being implemented in order to reduce these harmful emissions. Typical ways of reducing NOx emissions are urea-SCR, which dispenses urea as a reducing agent; lean NOx catalyst (LNC) [1], which uses hydrocarbons (HCs) as the reductant [3]; and LNT (Lean NOx Trap) [4,5]. Urea-SCR is the most effective among these techniques, and is employed mainly on heavy-duty diesel (HDD) engines [5,6]. The SCR technique, which uses ion-exchanged non-precious metals (Fe, Cu), has such problems as poor NOx conversion at low temperatures, ammonia slip, the need for urea refill, and the freezing of urea in cold weather. Based on three-way catalysts technology for gasoline engines, LNT has emerged as a solution to
* Corresponding author at: Chonnam National University (CNU), Automobile Research Center of CNU, Republic of Korea. Tel.: +82 62 530 1681; fax: +82 62 530 1689. E-mail address: [email protected] (B.C. Choi).
reduce NOx in lean-burn gasoline engine exhaust gas. Since diesel engines operate only under lean conditions, the NOx emitted under these conditions have to be adsorbed on the adsorbent (Ba, K) [2,7] before being reduced by the reductants (CO, HC, H2) that are generated during the short-rich operations of the engines [8–10]. Several groups of engineers have already evaluated the performance of a combined system of LNT + SCR with light-duty diesel engines [11–15]. The LNT catalyst was reported in those studies to remove NOx but also produce some NH3 during rich operation [12,15]. The NH3 formed by the LNT was then utilized by the downstream SCR catalyst to further reduce NOx in the exhaust gas [12,15]. The present authors also reported basic reaction characteristics and hydrothermal and sulfur aging of the combined system of LNT + SCR [16]. Although the system is somewhat complicated, it may provide a way of lowering NOx emissions with negligible NH3 slip. The system cost can potentially be lowered by reducing the amount of precious metals in the LNT, thereby eliminating clean-up catalyst for NH3 oxidation. In order to comply with the 10 ppm NH3 regulation, the combined system of LNT + SCR has to be more thoroughly investigated than is presently the case. However, it is important to find the optimal volumes of the combined system of LNT + SCR in terms of the system volume and de-NOx performance. The Ford Motor Company is recently in the process of exploring the catalyst volume of the combined system of LNT + SCR [17]. However, there are no results of in-depth research into the catalyst volume and its effects on N2O formation. Using a model gas catalytic reactor, this study aims to explore the optimal volumes of the LNT and SCR catalysts for higher de-NOx performance and suppression of N2O and NH3 in a combined system of LNT + SCR.
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2010.10.033
C.-K. Seo et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 382–385
383
Table 2 Model gas components for evaluating the combined system of LNT + SCR.
2. Experimental details 2.1. Catalysts and characterization The catalysts (LNT and SCR) used in this experiment are commercial catalysts for automobiles. The Brunauer–Emmett– Teller (BET) surface area and mean pore size of the catalysts were measured by N2 adsorption at 77 K through a BET analyzer (Micromeritics, ASAP-2020). The precious metal content and alumina molar ratios of the catalysts were determined by an Xray microanalyzer with a 15 kV electron beam (Shimadzu, EPMA1600). Table 1 shows the analytical results for the catalysts. The LNT catalyst (600 cpsi), with a precious-metal combination of Pt/ Pd/Rh (3.3/0.7/0.3 in wt.%), was supported by Ba/g-Al2O3. Fezeolite(TMI) was used as the SCR catalyst (400 cpsi); the Fe content was 1.8 wt.% and the Si/Al ratio was 11:1. 2.2. Experimental apparatus To identify the fundamental characteristics of NOx in the combined system of LNT and SCR, a model gas catalytic reactor system was used [19]. The total flow rate of the model gas was 2 L/ min. The water content was adjusted to 1.5% by the saturated vapor pressure of water. A quartz reaction tube with an inner diameter of 19 mm and length of 350 mm was used as the catalyst reactor. The catalyst temperature was measured by fixing a K-type thermocouple (0.5 mm in diameter) at the center of the catalyst system. The gas composition after the catalytic reaction was measured both quantitatively and qualitatively every 4.5 s using an FTIR (Fourier transform infrared) spectrometer (Midac, I2000) that was equipped with a gas cell of 4 m path length. 2.3. Evaluation of the de-NOx performance The NOx conversion of the combined system of LNT + SCR was investigated in a transient of varying catalyst temperature from 150 to 600 8C with a rising rate of 4 8C/min. The NOx conversion was calculated by Eq. (1). R C NOx outlet dt NOx conversion ð%Þ ¼ 1 R 100 (1) C NOx inlet dt In the equation, C NOx inlet and C NOx outlet respectively mean the concentrations of NOx at the inlet and outlet of the reactor. Table 2 shows the basic components of the model gas used in the evaluation. 3. Results and discussion It is important to investigate the basic characteristics of the reduction reactions of the LNT catalyst that is arranged upstream in order to understand the operating characteristics of the combined system of LNT + SCR. The NH3 formed in the LNT catalyst acts as a reductant in the SCR catalyst downstream in the combined system of LNT + SCR; therefore, the combined system exhibits better deNOx performance when compared with the LNT catalyst alone. In this context, the effect of SV (Space Velocity) in the LNT catalyst becomes an important factor in NOx conversion and the generation of NH3 as a reductant for the SCR catalyst downstream of LNT. According to previous studies, a maximum of 4% of CO and 1.3% of
Gas component
Lean
Rich
O2 (%) C3H8 (ppmC1) CO (ppm) NO (ppm) CO2 (ppm) H2 (ppm) H2O (%) N2
10 450 500 500 50,000 0 1.5 Balance 1.5 28,000 55
0 0 30,000 0 0 13,000 1.5 Balance 0.89 28,000 5
l SV (h1) Time (s)
H2 can be generated by an in-cylinder method of 1.7-l CRDI (Common Rail Direction Injection) engine [18]. Fig. 1 shows the result after supplying a lean exhaust gas for 55 s and then rich exhaust gas for 5 s, including reductants of 3% CO and 1.3% H2 (l = 0.89). A method of reducing the volumes of the catalysts with the flow rate fixed at 2000 mL/min was adopted in the present study in order to investigate the effect of SV. Fig. 1(a) shows performance differences of 10–40% in the NOx conversion at catalyst temperatures below 350 8C and no big difference in performance at catalyst temperatures above 350 8C in the case of SV = 14,000/h on the basis of SV = 28,000/h. A performance reduction of 5–25% is seen at temperatures below 350 8C when SV = 56,000/h, which is a reduction of 50% in the catalyst volume. The NOx conversion performance drops due to the shortened time for reaction according to the increase in the flow rate within catalysts during a series of processes such as storage, reaction of NOx, and desorption of products according to the increase in SV of the LNT catalysts. Fig. 1(b) shows the generation of NH3, which is an intermediate product generated in the rear portion of the LNT. At SV = 14,000/h, the peak NH3 generation appears at a catalyst temperature of 200 8C. At SV = 56,000/h, the greatest generation of NH3 is seen at a catalyst temperature of 250 8C. As SV increases, more NH3 is generated. The intermediate product of NH3 is formed due to the reduction of the adsorbed NOx species, according to the stoichiometry in the following global reaction equation (2). The NH3 formed this way reacts with the adsorbed NOx to yield N2, as shown in Eq. (3). The intermediate product NH3 increases due to the shortened reaction time as a result of the increase in SV on the LNT catalyst, which acts as a reductant for the SCR catalyst downstream of the combined system of LNT + SCR. BaðNO3 Þ2 þ 8H2 ! 2NH3 þ BaO þ 5H2 O
(2)
BaðNO3 Þ2 þ 10NH3 ! 8N2 þ 3BaO þ 15H2 O
(3)
Fig. 1(c) shows the concentration of generated N2O at various catalyst temperatures. N2O is a greenhouse gas which affects global warming 310 times more than CO2. The amount generated N2O can represent the NOx storage capacity of the LNT catalyst. At SV = 14,000/h, there is a trend of rapid reduction in generated N2O from a peak at 150 8C to 350 8C. Due to the large catalyst volume, the NOx storage capacity is large, and thus, large amount of N2O is generated. Under the smallest catalyst volume with SV = 56,000/h generated N2O reaches a peak of 7 ppm at 250 8C, but shows the lowest level of N2O among all of the cases tested. Under large SV,
Table 1 Specifications of the LNT and SCR catalysts. Catalyst
Components
Relative weights (wt.%)
BET (m2/g)
Mean pore size (nm)
LNT SCR
Pt/Pd/Rh/Ba/Ce/Zr Fe-(Si/Ai)zeolite
3.3/0.72/0.31/12.58/7.97/4.49 1.82–(11)
36.15 121.00
11.824 2.23
[(Fig._1)TD$IG]
[(Fig._2)TD$IG]
C.-K. Seo et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 382–385
384
80
80
NOx conversion(%)
100
NOx conversion(%)
100
60 40 SV 56, 000 1/ h SV 28, 000 1/ h
20
60 40 1. 5( L NT) +0. 5( SC R) 1( L NT) +1( SC R) 0. 75( L NT) +1. 25( SC R) 0. 5( L NT) +1. 5( SC R)
20
SV 18, 000 1/ h SV 14, 000 1/ h
0
0 150
250
350
450
150
550
250
Temperature( ºC)
80 SV 28, 000 1/ h SV 14, 000 1/ h
50 40 30
1. 5( L NT) +0. 5( SC R) 1( L NT) +1( SC R)
25
SV 18, 000 1/ h
NH3 slip(ppm)
NH3 concentration(ppm)
30
SV 56, 000 1/ h
60
0. 75( L NT) +1. 25( SC R) 0. 5( L NT) +1. 5( SC R)
20 15 10
20
5
10
0
0 150
250
350
450
150
550
250
Temperature( ºC)
25
SV 28, 000 1/ h
550
10 5
N2O slip(ppm)
15
1. 5( LNT) +0. 5( SCR) 1( LNT) +1( SCR) 0. 75( LNT) +1. 25( SCR)
25
SV 18, 000 1/ h SV 14, 000 1/ h
0. 5( LNT) +1. 5( SCR)
20 15 10 5
0 150
30
SV 56, 000 1/ h
20
350 450 Temperature(ºC)
(b) NH3 slip
(b) NH3 generation
N2O concentration(ppm)
550
(a) NOx conversion
(a) NOx conversion
70
350 450 Temperature(ºC)
250
350
450
550
Temperature( ºC)
(c) N2O generation
0 150
250
350
450
550
Temperature(ºC)
(c) N2O slip
Fig. 1. Impact of the Space Velocity of the LNT catalyst: (a) NOx conversion; (b) NH3 generation (ppm); (c) N2O generation (ppm).
Fig. 2. The selection of the optimal volume of the combined system of LNT + SCR: (a) NOx conversion; (b) NH3 slip (ppm); (c) N2O slip (ppm).
the NOx storage capacity is small, and so the generated N2O is small. For the optimal volume of the combined system of LNT + SCR, the NOx conversion rate and the cost of systematization should be considered together. Fig. 2 shows an evaluation of four LNT + SCR
catalysts with different combined volumes when supplied with lean exhaust gases for 55 s and then with rich exhaust gas for 5 s containing reductants of 3% CO and 1.3% H2 (l = 0.89) for 5 s. Fig. 2(a) shows the NOx conversion rate versus the volume of the combined catalysts. The 1.5(LNT) + 0.5(SCR) combination shows
C.-K. Seo et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 382–385
the best de-NOx performance, and the next best de-NOx performance is by the 1.0(LNT) + 1.0(SCR) combination, which is 5–10% worse than the best combination. A bigger volume of LNT is unfavorable in terms of the cost of the LNT catalyst that contains lots of precious metals, whereas the 0.5(LNT) + 1.5(SCR) combination denotes a combination of reduced volume LNT catalyst and increased SCR catalyst volume. This implies a de-NOx performance reduction of 5–20% in comparison with the 1:1 volume combination, and a maximum conversion rate of 60% at a catalyst temperature of 300 8C. Fig. 2(b) presents the results of the NH3 slip from the combined system of LNT and SCR. From the 1.5(LNT) + 0.5(SCR) combination slips about 10 ppm of NH3 at a catalyst inlet temperature of 350 8C. Less than 5 ppm of NH3 slips from the 0.5(LNT) + 1.5(SCR) combination. This is because, as the LNT catalyst volume increases, generated NH3 increases, and then some NH3 slips without reaction into the SCR catalyst. The 1.5(LNT) + 0.5(SCR) combination yields the maximum performance which is 10% better at 200 8C and even better at lower temperatures, as compared to the 1:1 volume combination. However, the combination is unfavorable in terms of the cost for LNT catalyst as compared to the 1:1 combination, and it also has the problem of high level of NH3 slip. Therefore, the 1.0(LNT) + 1.0(SCR) combination should be the best choice considering de-NOx performance, the system cost, and NH3 slip all together. Fig. 2(c) shows slipped N2O for the same catalysts. A large LNT volume is again associated with a higher level of N2O emission below 300 8C. The 1.5(LNT) + 0.5(SCR) combination with a large LNT catalyst volume shows a high level of N2O generation due to its large NOx storage capacity, and the 0.5(LNT) + 1.5(SCR) combination has a low level in generated N2O due to the small LNT catalyst volume. Considering together the NOx conversion efficiency, precious metal cost, NH3 slip, and N2O generation, the optimal volume ratio of the combined system of LNT + SCR is 1:1. 4. Conclusions This study aims to find the optimal volumes of the LNT and SCR catalysts for higher de-NOx performance and suppression of N2O as well as NH3 in a combined system of LNT + SCR. Regarding the basic characteristics of the reduction reactions of LNT catalysts, it was confirmed that as SV increases, the de-NOx performance decreases, and more NH3 is generated. At large SV, the NOx storage capacity decreases and thus, the amount of
385
generated N2O decreases. Regarding the optimal volumes of the LNT and SCR catalysts in a combined system of LNT + SCR, the 1.5(LNT) + 0.5(SCR) combination with a large LNT catalyst volume yielded the best de-NOx performance, and the next best was the 1.0(LNT) + 1.0(SCR) combination, which was 5–10% worse than the best combination. However, a bigger volume of LNT was taken unfavorable in terms of the cost as it contained more precious metals. As the LNT catalyst volume increased, NH3 generation increased, and then some of this NH3 slipped without reacting on the SCR catalyst. The 1.0(LNT) + 1.0(SCR) combination was take to yield the best de-NOx performance within the reasonable range of the system cost and NH3 slip. As the LNT volume increased, the N2O emissions also increased. Finally, considering the NOx conversion rate, precious metal cost, NH3 slip, N2O generation, the optimal volume ratio determined of the LNT and SCR catalysts in the combined system of LNT + SCR was 1:1.
Acknowledgements This study was carried out as part of the ‘‘Mid-term Technology Development Project’’ supported by the Ministry of Knowledge and Economy, Korea; we appreciate this support for the study. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
C.H. Lee, B.C. Choi, Int. J. Automot. Technol. 8 (2007) 667. T.V. Johnson, SAE 2010-01-0301 (2010). B. Westerberg, C. Kunkel, C.U.I. Odenbrand, Chem. Eng. J. 92 (2003) 27. A. Lundstro¨m, B. Andersson, L. Olsson, Chem. Eng. J. 150 (2009) 544. L. Xu, W. Watkins, R. Snow, G. Graham, R. McCabe, SAE 2007-01-1582 (2007). J.G. Kim, H.M. Lee, J. Ind. Eng. Chem. 14 (2008) 841. A. Schuler, M. Vostmeier, P. Kiwic, J. Gieshoff, W. Hautpmann, A. Drochner, H. Vogel, Chem. Eng. J. 154 (2009) 333. G.W. Lee, B.H. Shon, J.G. Yoo, J.H. Jung, K.J. Oh, J. Ind. Eng. Chem. 14 (2008) 457. D.J. Kim, J.W. Kim, S.J. Choung, M. Kang, J. Ind. Eng. Chem. 14 (2008) 308. W.P. Partridge, J.S. Choi, Appl. Catal. B: Environ. 91 (2009) 144. L. Castoldi, L. Lietti, I. Nova, R. Matarrese, P. Forzatti, F. Vindigni, S. Morandi, F. Prinetto, G. Ghiotti, Chem. Eng. J. 161 (2010) 416. N.W. Cant, M.J. Patterson, Catal. Today 73 (2002) 271. L. Lietti, P. Forzatti, I. Nova, E. Tronconi, J. Catal. 204 (2001) 175. R. Snow, D. Dobson, R. Hammerle, S. Katare, SAE 2007-01-0469 (2007). J. Parks, V. Prikhodko, SAE 2009-01-2739 (2009). L. Xu, R. McCabe, W. Ruona, G. Cavataio, SAE 2009-01-0285 (2009). R. Bonzi, L. Lietti, L. Castoldi, P. Forzatti, Catal. Today 151 (2010) 376. J.R. Theis, J.A. Ura, R.W. McCabe, SAE 2010-01-0300 (2010). C.K. Seo, H.N. Kim, B.C. Choi, 6th Tokyo Conf. on Advan. Catal. Sci. & Tech./5th Asia Paci. Cong. on Catal., 2010, p. 177.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Short communication
The optimal volume of a combined system of LNT and SCR catalysts Choong-Kil Seo, Hwanam Kim, Byungchul Choi *, Myung Taeck Lim Chonnam National University, Republic of Korea
A R T I C L E I N F O
Article history: Received 10 August 2010 Accepted 20 October 2010 Available online 13 May 2011 Keywords: Diesel engine Catalyst LNT (Lean NOx Trap) SCR (Selective Catalytic Reduction) SV (Space Velocity)
A B S T R A C T
This paper aims to find the optimal volumes of the LNT (Lean NOx Trap) and SCR (Selective Catalytic Reduction) catalysts for higher de-NOx performance and suppression of N2O and NH3 in a combined system of LNT + SCR. The basic characteristics of the LNT catalyst were identified first at various space velocities (SV), and then the effects of the variation in volume of the combined system were investigated. The NOx conversion, NH3 production, and N2O emissions were evaluated of four differently sized LNT– SCR systems. Considering all together the NOx conversion, cost of the precious metal, NH3 slippage, and N2O generation, the optimal volume ratio of LNT and SCR catalysts was found to be 1:1 for the LNT–SCR system. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction As diesel engines have high power and good fuel economy with low CO2 emissions, their market share is increasing in the sectors of commercial vehicles as well as passenger cars [1]. Because of the intrinsic nature of diesel combustion, however, nitrogen oxides (NOx) are generated in the localized high-temperature reaction zones, and particulate matters (PM) are formed in the diffusive combustion zones. The exhaust emission standards for vehicles have become extremely strict in most of the countries. European Union (EU) recently published the final Euro VI regulation, setting the limits of 10 ppm ammonia on the European Transient and European Steady-State Cycles [2]. Several technologies are being implemented in order to reduce these harmful emissions. Typical ways of reducing NOx emissions are urea-SCR, which dispenses urea as a reducing agent; lean NOx catalyst (LNC) [1], which uses hydrocarbons (HCs) as the reductant [3]; and LNT (Lean NOx Trap) [4,5]. Urea-SCR is the most effective among these techniques, and is employed mainly on heavy-duty diesel (HDD) engines [5,6]. The SCR technique, which uses ion-exchanged non-precious metals (Fe, Cu), has such problems as poor NOx conversion at low temperatures, ammonia slip, the need for urea refill, and the freezing of urea in cold weather. Based on three-way catalysts technology for gasoline engines, LNT has emerged as a solution to
* Corresponding author at: Chonnam National University (CNU), Automobile Research Center of CNU, Republic of Korea. Tel.: +82 62 530 1681; fax: +82 62 530 1689. E-mail address: [email protected] (B.C. Choi).
reduce NOx in lean-burn gasoline engine exhaust gas. Since diesel engines operate only under lean conditions, the NOx emitted under these conditions have to be adsorbed on the adsorbent (Ba, K) [2,7] before being reduced by the reductants (CO, HC, H2) that are generated during the short-rich operations of the engines [8–10]. Several groups of engineers have already evaluated the performance of a combined system of LNT + SCR with light-duty diesel engines [11–15]. The LNT catalyst was reported in those studies to remove NOx but also produce some NH3 during rich operation [12,15]. The NH3 formed by the LNT was then utilized by the downstream SCR catalyst to further reduce NOx in the exhaust gas [12,15]. The present authors also reported basic reaction characteristics and hydrothermal and sulfur aging of the combined system of LNT + SCR [16]. Although the system is somewhat complicated, it may provide a way of lowering NOx emissions with negligible NH3 slip. The system cost can potentially be lowered by reducing the amount of precious metals in the LNT, thereby eliminating clean-up catalyst for NH3 oxidation. In order to comply with the 10 ppm NH3 regulation, the combined system of LNT + SCR has to be more thoroughly investigated than is presently the case. However, it is important to find the optimal volumes of the combined system of LNT + SCR in terms of the system volume and de-NOx performance. The Ford Motor Company is recently in the process of exploring the catalyst volume of the combined system of LNT + SCR [17]. However, there are no results of in-depth research into the catalyst volume and its effects on N2O formation. Using a model gas catalytic reactor, this study aims to explore the optimal volumes of the LNT and SCR catalysts for higher de-NOx performance and suppression of N2O and NH3 in a combined system of LNT + SCR.
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2010.10.033
C.-K. Seo et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 382–385
383
Table 2 Model gas components for evaluating the combined system of LNT + SCR.
2. Experimental details 2.1. Catalysts and characterization The catalysts (LNT and SCR) used in this experiment are commercial catalysts for automobiles. The Brunauer–Emmett– Teller (BET) surface area and mean pore size of the catalysts were measured by N2 adsorption at 77 K through a BET analyzer (Micromeritics, ASAP-2020). The precious metal content and alumina molar ratios of the catalysts were determined by an Xray microanalyzer with a 15 kV electron beam (Shimadzu, EPMA1600). Table 1 shows the analytical results for the catalysts. The LNT catalyst (600 cpsi), with a precious-metal combination of Pt/ Pd/Rh (3.3/0.7/0.3 in wt.%), was supported by Ba/g-Al2O3. Fezeolite(TMI) was used as the SCR catalyst (400 cpsi); the Fe content was 1.8 wt.% and the Si/Al ratio was 11:1. 2.2. Experimental apparatus To identify the fundamental characteristics of NOx in the combined system of LNT and SCR, a model gas catalytic reactor system was used [19]. The total flow rate of the model gas was 2 L/ min. The water content was adjusted to 1.5% by the saturated vapor pressure of water. A quartz reaction tube with an inner diameter of 19 mm and length of 350 mm was used as the catalyst reactor. The catalyst temperature was measured by fixing a K-type thermocouple (0.5 mm in diameter) at the center of the catalyst system. The gas composition after the catalytic reaction was measured both quantitatively and qualitatively every 4.5 s using an FTIR (Fourier transform infrared) spectrometer (Midac, I2000) that was equipped with a gas cell of 4 m path length. 2.3. Evaluation of the de-NOx performance The NOx conversion of the combined system of LNT + SCR was investigated in a transient of varying catalyst temperature from 150 to 600 8C with a rising rate of 4 8C/min. The NOx conversion was calculated by Eq. (1). R C NOx outlet dt NOx conversion ð%Þ ¼ 1 R 100 (1) C NOx inlet dt In the equation, C NOx inlet and C NOx outlet respectively mean the concentrations of NOx at the inlet and outlet of the reactor. Table 2 shows the basic components of the model gas used in the evaluation. 3. Results and discussion It is important to investigate the basic characteristics of the reduction reactions of the LNT catalyst that is arranged upstream in order to understand the operating characteristics of the combined system of LNT + SCR. The NH3 formed in the LNT catalyst acts as a reductant in the SCR catalyst downstream in the combined system of LNT + SCR; therefore, the combined system exhibits better deNOx performance when compared with the LNT catalyst alone. In this context, the effect of SV (Space Velocity) in the LNT catalyst becomes an important factor in NOx conversion and the generation of NH3 as a reductant for the SCR catalyst downstream of LNT. According to previous studies, a maximum of 4% of CO and 1.3% of
Gas component
Lean
Rich
O2 (%) C3H8 (ppmC1) CO (ppm) NO (ppm) CO2 (ppm) H2 (ppm) H2O (%) N2
10 450 500 500 50,000 0 1.5 Balance 1.5 28,000 55
0 0 30,000 0 0 13,000 1.5 Balance 0.89 28,000 5
l SV (h1) Time (s)
H2 can be generated by an in-cylinder method of 1.7-l CRDI (Common Rail Direction Injection) engine [18]. Fig. 1 shows the result after supplying a lean exhaust gas for 55 s and then rich exhaust gas for 5 s, including reductants of 3% CO and 1.3% H2 (l = 0.89). A method of reducing the volumes of the catalysts with the flow rate fixed at 2000 mL/min was adopted in the present study in order to investigate the effect of SV. Fig. 1(a) shows performance differences of 10–40% in the NOx conversion at catalyst temperatures below 350 8C and no big difference in performance at catalyst temperatures above 350 8C in the case of SV = 14,000/h on the basis of SV = 28,000/h. A performance reduction of 5–25% is seen at temperatures below 350 8C when SV = 56,000/h, which is a reduction of 50% in the catalyst volume. The NOx conversion performance drops due to the shortened time for reaction according to the increase in the flow rate within catalysts during a series of processes such as storage, reaction of NOx, and desorption of products according to the increase in SV of the LNT catalysts. Fig. 1(b) shows the generation of NH3, which is an intermediate product generated in the rear portion of the LNT. At SV = 14,000/h, the peak NH3 generation appears at a catalyst temperature of 200 8C. At SV = 56,000/h, the greatest generation of NH3 is seen at a catalyst temperature of 250 8C. As SV increases, more NH3 is generated. The intermediate product of NH3 is formed due to the reduction of the adsorbed NOx species, according to the stoichiometry in the following global reaction equation (2). The NH3 formed this way reacts with the adsorbed NOx to yield N2, as shown in Eq. (3). The intermediate product NH3 increases due to the shortened reaction time as a result of the increase in SV on the LNT catalyst, which acts as a reductant for the SCR catalyst downstream of the combined system of LNT + SCR. BaðNO3 Þ2 þ 8H2 ! 2NH3 þ BaO þ 5H2 O
(2)
BaðNO3 Þ2 þ 10NH3 ! 8N2 þ 3BaO þ 15H2 O
(3)
Fig. 1(c) shows the concentration of generated N2O at various catalyst temperatures. N2O is a greenhouse gas which affects global warming 310 times more than CO2. The amount generated N2O can represent the NOx storage capacity of the LNT catalyst. At SV = 14,000/h, there is a trend of rapid reduction in generated N2O from a peak at 150 8C to 350 8C. Due to the large catalyst volume, the NOx storage capacity is large, and thus, large amount of N2O is generated. Under the smallest catalyst volume with SV = 56,000/h generated N2O reaches a peak of 7 ppm at 250 8C, but shows the lowest level of N2O among all of the cases tested. Under large SV,
Table 1 Specifications of the LNT and SCR catalysts. Catalyst
Components
Relative weights (wt.%)
BET (m2/g)
Mean pore size (nm)
LNT SCR
Pt/Pd/Rh/Ba/Ce/Zr Fe-(Si/Ai)zeolite
3.3/0.72/0.31/12.58/7.97/4.49 1.82–(11)
36.15 121.00
11.824 2.23
[(Fig._1)TD$IG]
[(Fig._2)TD$IG]
C.-K. Seo et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 382–385
384
80
80
NOx conversion(%)
100
NOx conversion(%)
100
60 40 SV 56, 000 1/ h SV 28, 000 1/ h
20
60 40 1. 5( L NT) +0. 5( SC R) 1( L NT) +1( SC R) 0. 75( L NT) +1. 25( SC R) 0. 5( L NT) +1. 5( SC R)
20
SV 18, 000 1/ h SV 14, 000 1/ h
0
0 150
250
350
450
150
550
250
Temperature( ºC)
80 SV 28, 000 1/ h SV 14, 000 1/ h
50 40 30
1. 5( L NT) +0. 5( SC R) 1( L NT) +1( SC R)
25
SV 18, 000 1/ h
NH3 slip(ppm)
NH3 concentration(ppm)
30
SV 56, 000 1/ h
60
0. 75( L NT) +1. 25( SC R) 0. 5( L NT) +1. 5( SC R)
20 15 10
20
5
10
0
0 150
250
350
450
150
550
250
Temperature( ºC)
25
SV 28, 000 1/ h
550
10 5
N2O slip(ppm)
15
1. 5( LNT) +0. 5( SCR) 1( LNT) +1( SCR) 0. 75( LNT) +1. 25( SCR)
25
SV 18, 000 1/ h SV 14, 000 1/ h
0. 5( LNT) +1. 5( SCR)
20 15 10 5
0 150
30
SV 56, 000 1/ h
20
350 450 Temperature(ºC)
(b) NH3 slip
(b) NH3 generation
N2O concentration(ppm)
550
(a) NOx conversion
(a) NOx conversion
70
350 450 Temperature(ºC)
250
350
450
550
Temperature( ºC)
(c) N2O generation
0 150
250
350
450
550
Temperature(ºC)
(c) N2O slip
Fig. 1. Impact of the Space Velocity of the LNT catalyst: (a) NOx conversion; (b) NH3 generation (ppm); (c) N2O generation (ppm).
Fig. 2. The selection of the optimal volume of the combined system of LNT + SCR: (a) NOx conversion; (b) NH3 slip (ppm); (c) N2O slip (ppm).
the NOx storage capacity is small, and so the generated N2O is small. For the optimal volume of the combined system of LNT + SCR, the NOx conversion rate and the cost of systematization should be considered together. Fig. 2 shows an evaluation of four LNT + SCR
catalysts with different combined volumes when supplied with lean exhaust gases for 55 s and then with rich exhaust gas for 5 s containing reductants of 3% CO and 1.3% H2 (l = 0.89) for 5 s. Fig. 2(a) shows the NOx conversion rate versus the volume of the combined catalysts. The 1.5(LNT) + 0.5(SCR) combination shows
C.-K. Seo et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 382–385
the best de-NOx performance, and the next best de-NOx performance is by the 1.0(LNT) + 1.0(SCR) combination, which is 5–10% worse than the best combination. A bigger volume of LNT is unfavorable in terms of the cost of the LNT catalyst that contains lots of precious metals, whereas the 0.5(LNT) + 1.5(SCR) combination denotes a combination of reduced volume LNT catalyst and increased SCR catalyst volume. This implies a de-NOx performance reduction of 5–20% in comparison with the 1:1 volume combination, and a maximum conversion rate of 60% at a catalyst temperature of 300 8C. Fig. 2(b) presents the results of the NH3 slip from the combined system of LNT and SCR. From the 1.5(LNT) + 0.5(SCR) combination slips about 10 ppm of NH3 at a catalyst inlet temperature of 350 8C. Less than 5 ppm of NH3 slips from the 0.5(LNT) + 1.5(SCR) combination. This is because, as the LNT catalyst volume increases, generated NH3 increases, and then some NH3 slips without reaction into the SCR catalyst. The 1.5(LNT) + 0.5(SCR) combination yields the maximum performance which is 10% better at 200 8C and even better at lower temperatures, as compared to the 1:1 volume combination. However, the combination is unfavorable in terms of the cost for LNT catalyst as compared to the 1:1 combination, and it also has the problem of high level of NH3 slip. Therefore, the 1.0(LNT) + 1.0(SCR) combination should be the best choice considering de-NOx performance, the system cost, and NH3 slip all together. Fig. 2(c) shows slipped N2O for the same catalysts. A large LNT volume is again associated with a higher level of N2O emission below 300 8C. The 1.5(LNT) + 0.5(SCR) combination with a large LNT catalyst volume shows a high level of N2O generation due to its large NOx storage capacity, and the 0.5(LNT) + 1.5(SCR) combination has a low level in generated N2O due to the small LNT catalyst volume. Considering together the NOx conversion efficiency, precious metal cost, NH3 slip, and N2O generation, the optimal volume ratio of the combined system of LNT + SCR is 1:1. 4. Conclusions This study aims to find the optimal volumes of the LNT and SCR catalysts for higher de-NOx performance and suppression of N2O as well as NH3 in a combined system of LNT + SCR. Regarding the basic characteristics of the reduction reactions of LNT catalysts, it was confirmed that as SV increases, the de-NOx performance decreases, and more NH3 is generated. At large SV, the NOx storage capacity decreases and thus, the amount of
385
generated N2O decreases. Regarding the optimal volumes of the LNT and SCR catalysts in a combined system of LNT + SCR, the 1.5(LNT) + 0.5(SCR) combination with a large LNT catalyst volume yielded the best de-NOx performance, and the next best was the 1.0(LNT) + 1.0(SCR) combination, which was 5–10% worse than the best combination. However, a bigger volume of LNT was taken unfavorable in terms of the cost as it contained more precious metals. As the LNT catalyst volume increased, NH3 generation increased, and then some of this NH3 slipped without reacting on the SCR catalyst. The 1.0(LNT) + 1.0(SCR) combination was take to yield the best de-NOx performance within the reasonable range of the system cost and NH3 slip. As the LNT volume increased, the N2O emissions also increased. Finally, considering the NOx conversion rate, precious metal cost, NH3 slip, N2O generation, the optimal volume ratio determined of the LNT and SCR catalysts in the combined system of LNT + SCR was 1:1.
Acknowledgements This study was carried out as part of the ‘‘Mid-term Technology Development Project’’ supported by the Ministry of Knowledge and Economy, Korea; we appreciate this support for the study. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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