Modeling Studies on Lean NOx Reduction by a Sequence of LNT

Several experimental studies have been conducted to determine the NOx reduction by a series of LNT (lean NOx trap) and SCR (selective catalytic reduct...
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Modeling Studies on Lean NOx Reduction by a Sequence of LNT−SCR Bricks Arun S. Kota, Dan Luss,* and Vemuri Balakotaiah* Department of Chemical & Biomolecular Engineering, University of Houston, Houston, Texas 77204, United States S Supporting Information *

ABSTRACT: Several experimental studies have been conducted to determine the NOx reduction by a series of LNT (lean NOx trap) and SCR (selective catalytic reduction) catalytic bricks. An important goal is to minimize the required precious metal loading in the LNT while keeping the NOx emission below a specified level. We present a mathematical model of this system using hydrogen as the reductant. Simulations are used to determine the influence of the architecture of the LNT−SCR bricks, nonuniform precious metal loading in the LNT bricks, and the cycle time at temperatures in the range of 200−350 °C. The simulations lead to the following observations: (a) Low temperature reduction is the limiting step in the optimization of precious group metal (PGM) loading in LNT. (b) The NOx conversion increases as the number of the sequential bricks (with total length fixed) increase and reaches an asymptotic limit. From a practical point of view, there is little incentive in using more than two sequential pairs. (c) Nonuniform precious metal loading of the LNT bricks results in only a minor improvement in the deNOx performance. (d) The cycle time has a significant impact on the NOx conversion. In the simulated example, the NOx conversion at low temperatures is increased by about 15−20% by reducing the cycle time by a factor of 2. (e) Even at low temperature operation, diffusional limitations in the washcoat are most likely to be important in the LNT but not in the SCR operation. The NOx conversion and ammonia selectivity are reduced when washcoat diffusion is dominant in the LNT.

1. INTRODUCTION Lean burn gasoline and diesel engines are becoming increasingly popular because of their high fuel efficiency and low CO2 emissions. However the oxygen rich environment (with A/F ratios around 16:1−17:1) increases NOx emissions, and its removal is a major technological challenge. Leading technologies for NOx emissions reduction include: (a) NOx storage and reduction (NSR) and (b) selective catalytic reduction (SCR). The NSR process is carried out over a Lean NOx Trap (LNT) which is a monolith, washcoated with a bifunctional catalyst. The conventional LNT catalyst comprises of precious metals (Pt, Rh, etc.) and oxides of alkaline earth metal (Ba, K) supported on a high surface area metal oxide carrier like Al2O3, MgO, etc. The most common LNT catalyst is Pt/BaO/ Al2O3.1−3 Alternate lean and rich fuels are periodically fed to the LNT. NOx is stored on the trapping material (BaO) during the lean feed (consisting of NO and O2) period. It is subsequently purged by a rich feed (containing reducing agents like H2, hydrocarbons, etc.) to form N2 and H2O. Formation and consumption of the NH3 occurs during the reduction of the stored NOx by a complex network of reaction steps.1,4 Experiments revealed that NH3 generated in the upstream of the LNT reacts with stored NOx at the downstream until it breaks through the LNT. Major concerns in the operation of the LNT are the following: reduction of the NOx, slippage of NH3, and the cost of the precious metal catalyst. Selective catalytic reduction (SCR) which uses NH3 as a reductant is a widely used commercial technology for NOx abatement. In mobile applications urea can be used to generate NH3. Urea-SCR recently has found widespread application in the exhaust aftertreatment of automobiles and heavy duty © 2012 American Chemical Society

vehicles. One of the main disadvantages of Urea-SCR is the cost and space required for the urea injection system. Various catalysts like V2O5/TiO2 and metal exchanged zeolites like Cu/ Fe-ZSM5 are used for selective catalytic reduction of NOx. The use of V2O5/TiO2 for the abatement of NOx in the exhaust gases is regulated in some countries because of the toxicity and poor stability at high temperatures (T > 400 °C). Metal zeolites like Cu/Fe ZSM5 perform better over a wide range of temperatures,5,9 are environmentally friendly, and have good durability with respect to hydrothermal aging. Olsson et al.6−8 developed a global and a microkinetic model of NH3 SCR reaction on Cu-ZSM5 catalysts. Recently Metkar et al.9 have reported detailed transient and steady state kinetic studies of SCR using Fe-ZSM5 catalyst. Hybrid technologies which combine the functionalities of LNT and SCR offer an opportunity to overcome the disadvantages of either the LNT or SCR. An economic incentive in using the combined LNT−SCR system is the potential lowering of the expensive precious metal loading. Theis et al.10 studied the advantages of using a sequence of LNT−SCR bricks at high temperatures. Lindholm et al.11 evaluated the performance of Pt/Ba/Al2O3 along with Fe-Beta catalyst on NOx conversion. They reported a significant improvement in deNOx efficiency of the sequential catalytic system. Lietti et al.12 and Seo et al.29 studied the NOx reduction by a sequential pair of LNT and SCR catalysts. They reported a pronounced improvement in NOx removal efficiency especially Received: Revised: Accepted: Published: 6686

January 20, 2012 March 15, 2012 March 27, 2012 March 27, 2012 dx.doi.org/10.1021/ie300190c | Ind. Eng. Chem. Res. 2012, 51, 6686−6696

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at low temperatures ( 100 (which falls in the Knudsen regime), diffusion limitations in the LNT catalyst are pronounced at all the temperatures for NO oxidation, storage, and regeneration reactions. The SCR kinetic parameters indicate that diffusion limitations are not significant at high temperatures for which λD ≈ 100. However, Metkar et al.28 have shown that diffusion limitations are important even at 200 °C for fast and NO2 SCR reactions on Fe-zeolite catalyst. Figure 11 shows the influence of washcoat diffusion on NOx conversion at 237 °C. If the bulk diffusion is dominating in the

Figure 11. Influence of washcoat diffusion on NOx conversion for different values of diffusivity in the washcoat (the catalyst configuration corresponds to that used for Figures 8 and 9).

washcoat (λD ≈ 10) only a small change in the conversion is observed. When the diffusion in the washcoat is in the Knudsen regime, a pronounced effect on the NOx conversion occurs. However, the dependence of the NOx conversion on the length of the LNT brick is qualitatively the same as that observed 6694

dx.doi.org/10.1021/ie300190c | Ind. Eng. Chem. Res. 2012, 51, 6686−6696

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RΩ = effective transverse length scale (m) t = time (s) Ts = monolith temperature (K) u̅ = average fluid velocity in the fluid phase (m/s) Xjm = cup mixing mole fraction of species j Xj,wc = mole fraction of species j in washcoat z = axial coordinate (m)

results to laboratory experiments, the simulations reported here were restricted to LNT and SCR bricks of length 2 cm. However, nearly identical results are obtained in the isothermal case studied for longer bricks provided the space velocity (u/L) ̅ remains constant. This is no longer true for the nonisothermal case.

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ASSOCIATED CONTENT

Greek Letters

S Supporting Information *

Kinetic constants and the rate expressions for the LNT and SCR reactions along with the information on the values of Weisz modulus. This material is available free of charge via the Internet at http://pubs.acs.org/ .

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 713-743-4305. E-mail: [email protected] (D.L.). Tel.: 713743-4318. E-mail: [email protected] (V.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work reported was supported by the U.S. DOE National Energy Technology Laboratory (DE-FC26-05NT42630). This report was prepared as an account of work sponsored by an agency of United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, or process, disclosed.

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αν = platinum surface area per unit washcoat volume (m2/ m3) δc = washcoat thickness (m) εwc = porosity of washcoat λ = length ratio of LNT to SCR λD = ratio of gas phase to washcoat diffusion θBa(NO3)2(f,s) = fractional surface coverage on fast or slow BaO sites occupied by NOx θj,S = fractional coverage of NH3 adsorption sites on SCR catalyst θj,X = fractional coverage of NH3 and H2O adsorption sites on Al2O3 support of LNT catalyst θv(f, s) = fractional surface coverage of vacant fast or slow sites νm,j = stoichiometric coefficient of species j in reaction m νl,BaO(f, s) = stoichiometric coefficient in reaction l on fast or slow storage sites τC = convection time (s) τD = diffusion time (s) τRj = reaction time for the reaction j (s)

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NOMENCLATURE A = pre-exponential factor (for units see Appendices provided in the Supporting Information) CBaO(f,s) = total concentration of fast or slow BaO sites (mol of BaO/m3 washcoat) CiPt = total platinum concentration of the catalyst (mol of Pt/ m3 washcoat) Cref = reference concentration (mol/m3) CS = total concentration of NH3 adsorption sites (mol/m3 washcoat) CTm = total concentration in fluid phase (mol/m3) CX = concentration of adsorption sites for H2O and NH3 on Al2O3 (mol/m3 washcoat) Dm,j = diffusivity of species j in gas phase (m2/s) De,j = diffusivity of species j in washcoat (m2/s) kc(j, z) = mass transfer coefficient of species j at axial position z (m/s) ko(j, z) = overall mass transfer coefficient of species j at axial position z (m/s) ke(j, z) = external mass transfer coefficient of species j at axial position z (m/s); same as kc(j, z) ki(j, z) = internal mass transfer coefficient of species j at axial position z (m/s) k = reaction rate constant (for units see Appendices in the Supporting Information) L = length of the monolith (m) m = sensitivity of the rate constant to Pt concentration n = no. of LNT−SCR zones Rads,j = rate of adsorption of species j (mol/m3washcoat·s) Rdes,j = rate of desorption of species j (mol/m3washcoat·s) RV,m = rate of reaction m (mol/m3washcoat·s) 6695

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dx.doi.org/10.1021/ie300190c | Ind. Eng. Chem. Res. 2012, 51, 6686−6696