Article pubs.acs.org/IECR
Heterogeneous Ammonia Storage Model for NH3−SCR Modeling Jian Gong,*,†,‡ Kushal Narayanaswamy,⊥ and Christopher J. Rutland† †
Engine Research Center, University of WisconsinMadison, 1500 Engineering Drive, Madison, Wisconsin 53706, United States General Motors, Global Research and Development, 30500 Mound Road, Warren, Michigan 48092, United States
⊥
S Supporting Information *
ABSTRACT: Single- and dual-site ammonia storage models are initially developed and calibrated from ammonia TPD experiments. The dual-site model gives better agreement with the experimental measurements, but neither of these models is able to adequately represent the observed ammonia storage trends over a wide range of temperatures. An alternative heterogeneous single-site model considering the heterogeneity of ammonia storage sites is proposed that more accurately predicts ammonia storage in the temperature window of 150 °C−400 °C. When the heterogeneous storage site model is combined with kinetics for NH3 and NO oxidation for simulating standard SCR, the NH3 and DeNOx trends as well as the NH3 inventories during reaction conditions reported for lab reactor studies are replicated.
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INTRODUCTION Lean-burn exhaust aftertreatment systems are typically required to provide high nitrogen oxides (NOx) conversion efficiency (>90%) over a wide temperature window (150−550 °C) to meet stringent NOx regulations. Urea/ammonia (NH3) selective catalyst reduction (SCR) systems have been successfully applied on lean-burn engines for NOx emission control.1,2 Non-noble metal catalysts like vanadium (V), iron (Fe), and copper (Cu) supported zeolites are among the most active catalysts for the urea/NH3 SCR process. In the past few years, significant efforts have been put into Fe and Cu exchanged zeolites, which are more active than V-based systems at low temperatures.3−5 It was also found that Cu-zeolites show higher low temperature activity due to superior ammonia storage at low temperature and lower sensitivity to the NO2/ NOx ratio as compared to Fe-zeolites.4,6−8 Recently, Cuzeolites with a chabazite (CHA) structure, like Cu-SAPO-349,10 and Cu-SSZ-13,11−14 have exhibited superior performance due to their high thermal stabilities.13,15,16 Also, there is growing interest in metal exchanged SCR catalysts for NOx control in lean-burn gasoline engines.17−21 Recently, a passive selective catalytic reduction (passive SCR) system, including closecoupled three-way catalysts (TWCs) and SCR catalysts, was proposed and demonstrated by Li et al.19,22 to control NOx emission on a light-duty gasoline engine. Effective NH3 generation23−25 in the TWCs and high NH3 storage capability in the SCR at medium temperatures are the keys to successfully apply the passive SCR system. For diesel and gasoline engines, understanding NH3 storage characteristics is critical to achieve high DeNOx performance by using SCR catalysts. Until recently, the understanding of the NH3 storage has been improved to a good extent by applying advanced © 2016 American Chemical Society
diagnostic tools and well-designed experimental protocols.11,26−29 In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature-programmed experimental protocols were utilized to probe the active sites and reaction pathway on Cu-zeolites with a chabazite structure.26,28,29 It was found that NH3 can adsorb in the form of NH4+ species on a high degree of heterogeneity Brønsted acid sites.9,15 Also, NH3 was observed to absorb in the form of molecular NH3 on Lewis acid sites, including exchanged Cu ion in the zeolite framework and some extraframework Al.15,26 A high variety of adsorption sites creates a great challenge to accurately model NH3 adsorption. Parallel to the improved experimental understanding of the nature of a catalyst, catalyst models have been extensively developed in the past decade.30 Mathematic models such as diesel oxidation catalyst31,32 and diesel particulate filter33−36 have been successfully applied on aftertreatment control strategies development37−40 and system integration41 for hydrocarbons and particulate emissions control. Similarly, robust and accurate SCR models are needed for SCR catalyst development to control NOx emissions. A variety of detailed models5,42−44 as well as global kinetic models45−49 for NH3− SCR are available in the literature. As discussed, one of the main challenges for SCR model development is accurate description of ammonia storage in a wide range of operating conditions. The accurate description of NH3 adsorption and desorption phenomenon is the basis for correct prediction of Received: Revised: Accepted: Published: 5874
March 19, 2016 May 5, 2016 May 6, 2016 May 6, 2016 DOI: 10.1021/acs.iecr.6b01097 Ind. Eng. Chem. Res. 2016, 55, 5874−5884
Article
Industrial & Engineering Chemistry Research the NH3−SCR catalytic performance. NH3 adsorption and desorption has previously been described by single-site models49,50 and detailed multiple-sites models.5,44 A Temkintype adsorption isotherm kinetic model was widely used over V-based and zeolite-based SCR catalysts.46,50 In this approach, it is common to assume that the adsorption is a nonactivated process, and the activation energy of the desorption process is a linear function of the adsorbed species surface coverage. Recently, a dual-site approach was developed by Colombo et al.51 on a Fe-zeolite. The dual-site storage model resulted in the accurate description of NH3 adsorption and desorption in studied temperatures. In the present work, a single-site NH3 storage model and a dual-site NH3 storage model are developed and compared based on the NH 3 TPD experiments. An improved heterogeneous single-site NH3 storage model with a temperature dependent heterogeneous desorption energy is presented. This heterogeneous single-site storage model is further validated at standard SCR conditions.
experimental data obtained from this protocol was described and used in this study. The isothermal SCR protocol was conducted over a range of temperatures (150 to 400 °C) at ORNL. Table 1 shows the Table 1. Isothermal SCR Protocol step
description
1.0 1.1 1.2 1.3
cool NH3 storage NH3 oxidation NO oxidation/NH3 inventory NO SCR NO SCR NH3 inventory
1.4 1.5
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NH3 (ppm)
NO (ppm)
NO2 (ppm)
O2 (%)
T (°C)
0 350 350 0
0 0 0 350
0 0 0 0
10 0 10 10
600→T T T T
350 0
350 350
0 0
10 10
T T
detailed gas composition for each step in the protocol. The NH3 storage started in step 1.1 with NH3 uptake measured in the absence of O2 and NOx. This step yielded a measurement of the total NH3 storage capacity while avoiding the complications introduced by NH3 oxidation. The rate of NH3 oxidation was measured in step 1.2 after O2 was turned on. After step 1.2 reached steady state, NO was turned on in step 1.3. Integrating the NOx reduced by the NH3 stored on the catalyst during this step yielded a measure of the NH3 inventory. Standard SCR was conducted in step 1.4. In step 1.5, NH3 inventory measurement was repeated after standard SCR reaction by feeding NO. The protocol was repeated at temperatures of 400 °C, 350 °C, 300 °C, 250 °C, 200 °C, and 150 °C.
EXPERIMENTAL SECTION Catalyst. The catalyst for this study is a commercial small pore Cu-chabazite SCR catalyst. The catalyst is a 100% ion exchanged zeolite containing 2.8% Cu.52 The exchanged zeolite was coated on a cordierite ceramic honeycomb substrate. The specification of the Cu SCR catalyst is shown in Table S1. The core sample was hydrothermally degreened at 700 °C for 4 h in humidified air (∼10% H2O, 20% O2, balance N2) in a laboratory furnace. Experimental Methodology. In order to study the SCR performance, a small core sample was extracted from SCR catalyst brick. Experiments were performed at Oak Ridge National Laboratory (ORNL) in a fixed flow bench reactor system. The catalyst was located in a position inside the heated zone of the furnace so as to achieve a near-isothermal condition. Two test protocols were utilized in this study. Temperatureprogrammed desorption (TPD) experiments were carried out to quantify NH3 adsorption and desorption. In ammonia TPD experiments, a SCR catalyst was exposed to 350 or 420 ppm of NH3 and 5% H2O for about 70 min at 150 °C and followed by a temperature ramp for about 80 min. The TPD experiments began with a NH3 adsorption step, which consists of a stepwise increase in NH3 concentration (from 0 to 350 or 420 ppm), while the catalyst temperature was held constant at 150 °C. After the catalyst was saturated with NH3 (as evidenced by a steady-state outlet concentration being the same as the inlet concentration), the flow of NH3 was shut off. At this point, the catalyst was held at the 150 °C while NH3 isothermally desorbed. When the outlet NH3 concentration dropped below 5 ppm (indicating the end of isothermal desorption), the catalyst temperature was increased to 550 °C at a rate of 5 °C min−1. An experimental protocol to characterize the key reactions that control SCR NOx conversion performance was used to study the kinetics. The isothermal protocol consists of a series of step changes in inlet gas composition designed to measure reaction rates and NH3 inventories under SCR operating conditions. Each step in the protocol was allowed to run until the outlet gas concentrations reached a steady state. Standard space velocity (at 1 atom and 273 K) is 60,000 h−1, and 5% H2O was maintained in all the tests. However, only part of the
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KINETIC MODELING Reactor Model. A single-channel, one-dimensional (1D) model used to describe the monolith converter has been developed with the following assumptions: 1) steady state; 2) axisymmetric geometry; 3) neglect axial diffusion of mass and heat (axial Peclet number, defined as the ratio of axial diffusion time to the axial convection time, is about 1000 for heat and mass transport, which indicates the dominance of the convective heat and mass transfer17,53). The mass conservation equation of bulk gas species is described by ∂cj , g ∂cj , g = −km , jGsa(cj , g − cj , s) +u (1) ∂x ∂t th where cj,g is the concentration of the j bulk gas species, while cj,s is the concentration of the jth surface species. In eq 1, Gsa is the geometric surface area to catalyst volume ratio. The widely used film model is applied to account for mass transfer between bulk gas and washcoat by assuming that the reaction rate is equal to the diffusion rate km , jGsa(cj , g − cj , s) = GcaR j
(2)
In eq 2, Gca is the specific catalyst surface area, and km,j is the mass transfer coefficient of jth species. Also, stored NH3 at the kth storage sites is described by Ωk 5875
∂θk = ∂t
Nj
∑ RNH
3, k
j=1
(3) DOI: 10.1021/acs.iecr.6b01097 Ind. Eng. Chem. Res. 2016, 55, 5874−5884
Article
Industrial & Engineering Chemistry Research where Ωk is the storage capacity of kth storage sites, and θk is the NH3 coverage at site k. The energy equation of the bulk gas is written as ⎛ ∂Tg ∂Tg ⎞ ρg Cpg ⎜ +u ⎟ = −hg Gsa(Tg − Ts) ∂x ⎠ ⎝ ∂t
necessary to describe multiple NH3 desorption peaks during NH3 desorption. Single-Site Storage Model. A single-site storage model has been found to be sufficient for kinetic models valid from 150 °C and higher temperatures in previous studies.50 Also, it was observed that NH3 desorbing from Brønsted acid sites and Lewis acid sites showed analogous profiles at temperature above 200 °C.26 This indicates that one adsorption site may be adequate to describe desorption dynamics since both types of sites have similar acid strengths. In order to make the model simple to be used, a single-site ammonia storage model is initially developed to model the NH3 adsorption and desorption characteristics. The catalyst active site density as well as the kinetic parameters is calibrated by minimizing the difference of outlet NH3 concentration between the model and TPD experiments using the least-squares regression algorithm. The active site density of this catalyst, which is calibrated from ammonia TPD experiments, turns out to be 250 mol/m3. Estimated kinetic parameters are given in Table 2.
(4)
Here, hg is the heat transfer coefficient between the catalyst surface and gas. To complete the 1D model, the monolith or surface energy equation is included which lumps the washcoat and substrate N
ρs cp , s
j ∂Ts ∂ 2T = λs 2s + hg Gsa(Tg − Ts) + Gca ∑ R j( −ΔHj) ∂t ∂x j=1
(5)
The mass transfer coefficient is determined by the correlations with the Sherwood number kj =
ShDj dh
. Similarly,
the heat transfer coefficient is calculated from the Nusselt number hg =
Nuλg dh
Table 2. Kinetic Parameters of a Single-Site Ammonia Storage Model
. Other symbols and notations used in the
above equations are given in the nomenclature. Also, the washcoat mass transfer is not considered in this study, which also has been done in several other NH3−SCR modeling studies.44,50,54 There are several reasons for neglecting the washcoat mass transfer in this work: 1) the washcoat mass transfer will not likely affect the NH3 storage characteristics and eventually the NH3 storage model development; 2) the NH3− SCR performance data were collected in the temperature from 150 to 400 °C, at which the washcoat mass transfer may be minor; 3) the catalyst we studied is small-pore zeolite, on which the washcoat mass transfer has less effects on the SCR performance compared to large-pore zeolite catalysts.
parameters
value
units
Aad Ade Eade
1.5 1.00 × 1011 162.18
1/s mol/m3/s kJ/mol
ϵcat Ωsite
0.73 250.00
mol/m3
The outlet NH3 concentrations from the NH3 TPD experiment (inlet NH3 = 350 ppm) and simulation are presented in Figure 1. There is a total uptake of ammonia for
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RESULTS AND DISCUSSION NH3 Storage Modeling. Temkin isotherm is widely used to describe the NH3 adsorption and desorption process. Assuming a nonactivated adsorption rate constant,50,55 an adsorption rate expression can be described by eq 6 RNH3 , ad = Aad ·cNH3 , g ·(1 − θ )
(6)
In the Temkin isotherm model, indirect adsorbate−adsorbate interactions on adsorption isotherms have been taken into account by observing that the heats of adsorption would decrease with increasing coverage during the desorption process.3 In other words, the heterogeneity of the catalyst surface is modeled by considering the desorption energy as a linear function of surface coverage, which has been used in several other models to simulate desorption.55−57 The NH3 desorption rate equation is shown in eq 7, and ϵcat is a model constant, which describes the dependence of desorption activation energy Eade on NH3 coverage θ ⎡ Ea (1 − ϵcat θ ) ⎤ ⎥·θ RNH3 , de = Ade ·exp⎢ − de ⎢⎣ ⎥⎦ RT
Figure 1. Single-site and dual-site NH3 storage models from the NH3 TPD experiment (NH3 = 350 ppm, T = 150 °C, SV = 60K h−1).
about 2 min, and thereafter the ammonia starts to break through. During isothermal desorption, the single-site model gives slightly lower ammonia desorption. When the temperature is increasing, the model seems to overpredict ammonia desorption with a slightly higher but flat desorption peak. It seems that there is a large overlap between the NH3 desorption from Brønsted acid sites and Lewis acid sites around 200 °C, which is consistent with other studies of NH3 adsorption on small pore Cu-CHA SCR catalysts.26 Desorption of ammonia is complete at 450 °C, which is correctly predicted by the model. Dual-Site Storage Model. In order to compare to the singlesite storage model, a dual-site ammonia storage model is developed and calibrated. The first site is used to model weakly
(7)
Single-site and multiple-sites NH3 storage models are both widely used to model the dynamic NH3 adsorption and desorption process. Generally, it is ideal to use less adsorption sites to simplify the model, and therefore less calibration effort is needed. However, adding additional adsorption sites may be 5876
DOI: 10.1021/acs.iecr.6b01097 Ind. Eng. Chem. Res. 2016, 55, 5874−5884
Article
Industrial & Engineering Chemistry Research
protocol experiment shown in Table 1 is simulated using the single-site and dual-site models at temperatures from 150 to 400 °C with an inlet ammonia concentration of 350 ppm. At a specific temperature, the amount of NH3 adsorption can be calculated from the model as below
adsorbed ammonia and physisorbed ammonia on acid sites for a correct description of NH3 desorption at low temperatures. Similar to the single-site model, the Temkin isotherm model is applied on the first site considering there is a high degree of heterogeneity of acid sites of weakly adsorbed NH3. The second site is used to model strongly bonded NH3 in order to describe NH3 desorption at relatively high temperatures. However, a simple Langmuir isotherm model is utilized on the second site by realizing that most of the NH3 adsorbed at high temperature is from the Brønsted acid site of bridging Al− OH−Si and Lewis acid sites of exchanged Cu ion, which may have similar acid strengths. The rate expressions for NH3 adsorption and desorption on each site are described by eq 8−eq 11. Estimated kinetic parameters are summarized in Table 3. The desorption energy
CNH3 , ab , cal = Ωcat ·θeq
where the equilibrium NH3 coverage θeq is obtained by setting dθeq
= RRad − RRde = 0 at equilibrium. Here Ωcat is the active site density of the catalyst. From the experiments, the amount of NH3 adsorption can be calculated from eq 13 P CNH3 , ab ,exp = (xNH3 , in − xNH3 , out ) ·SV · dt R uT (13) dt
∫
In Figure 2, ammonia storages at different temperatures are compared between the single-site and dual-site models under
Table 3. Kinetic Parameters of a Dual-Site Ammonia Storage Model site 1
(12)
site 2
parameters
value
units
parameters
value
units
Aad,1 Ade,1 Eade,1
0.75 13.25 43
1/s mol/m3/s kJ/mol
Aad,2 Ade,2 Eade,2
21.5 1.97E7 88
1/s mol/m3/s kJ/mol
ϵcat,1 Ωsite,1
0.89 207
mol/m3
ϵcat,2 Ωsite,2
28
mol/m3
at the first site (weak adsorption) is 43 kJ/mol at zero coverage, which is much lower compared to 88 kJ/mol of the second site (strong adsorption). The heterogeneity constant ϵcat,1 is calibrated to be 0.89 on the first site, which is consistent with our initial assumption of a high degree of heterogeneity of adsorption sites of weakly adsorbed NH3. Also, it is interesting to see that the total number of storage site density of this dualsite model is very comparable to that of the single-site model (235 vs 250 mol/m3) RNH3 , ad ,1 = Aad ,1·cNH3 , g ·(1 − θ1)
(8)
⎡ Ea (1 − ϵcat ,1θ1) ⎤ de ,1 ⎥ ·θ1 RNH3 , de ,1 = Ade ,1·exp⎢ − RT ⎢⎣ ⎥⎦
(9)
RNH3 , ad ,2 = Aad ,2 ·cNH3 , g ·(1 − θ2)
(10)
⎡ Eade ,2 ⎤ ⎥ ·θ2 RNH3 , de ,2 = Ade ,2 ·exp⎢ − ⎣ RT ⎦
(11)
Figure 2. Comparison of ammonia storage capacities at different adsorption temperatures between single-site and dual-site models.
steady-state conditions. Surprisingly, none of these two models give acceptable predictions in the amount of NH3 storage at different temperatures. The single-site model significantly underpredicts ammonia storage at temperatures greater than 150 °C, and the differences between model and experiments increase with temperature. The differences indicate that the single-site model gives a higher desorption rate (or a lower desorption activation energy) at a higher temperature, which leads to less amount of ammonia adsorbed. The dual-site model overpredicts with a constant bias at temperature higher than 150 °C. The single-site model shows a linear relationship between the adsorbed ammonia and temperature, which is consistent with the experimental observations.3 The dual-site model shows a slight deviation from the linearity when temperature is higher than 150 °C. Through these comparisons, it can be seen that ammonia storage models calibrated from TPD experiments are not adequate to accurately predict the ammonia storage over a wide range of temperatures. It is necessary to validate the storage model to different temperatures after TPD calibrations. Heterogeneous Single-Site Storage Model. Theoretically, the dynamic ammonia adsorption and desorption can be addressed and modeled by using a large number of sites. However, this approach will definitely result in an increase in complexity, a higher computation cost, and a significant calibration effort. To keep the storage mode as simple as possible, an alternative approach using a single storage site is presented.
Ammonia TPD comparison between the single-site and dualsite model is shown in Figure 1. The dual-site model accurately describes ammonia adsorption as well as the single-site model. During isothermal desorption, the dual-site model shows slightly higher desorption of ammonia, which is attributed to the lower desorption energy of the dual-site model compared to that of the single-site model. When temperature is ramping up, the dual-site model captures desorption much better than the single-site model. This is probably due to a low storage site density of the second site in the dual-site model. Ammonia Storage at Different Temperatures. The singlesite and dual-site ammonia storage models were calibrated and compared with the ammonia TPD experiments at 150 °C in the previous section. In order to further evaluate the storage models, the ammonia storage step (step 1.1) of the isothermal 5877
DOI: 10.1021/acs.iecr.6b01097 Ind. Eng. Chem. Res. 2016, 55, 5874−5884
Article
Industrial & Engineering Chemistry Research In the previous single-site model, classic Temkin isotherm was used by considering the adsorbent−adsorbate interaction to model the coverage dependence desorption energy. On the other hand, based on the recent experimental studies of ammonia storage,9,15,28 there are different groups of adsorption sites (weak acid sites like P−OH and Si−OH groups and extraframework Al, physisorbed NH3 molecules) with distinct adsorption strengths at low temperatures. At high temperatures, only NH3 adsorbed on Brønsted acid sites and Lewis acid sites with similar acid strengths stays. This indicates the heterogeneity of ammonia adsorption is temperature dependent. This temperature dependent heterogeneity of adsorption sites is missed in the classic Temkin isotherm. In order to capture the temperature dependent heterogeneity of ammonia adsorption sites in a single-site model, a temperature dependent ϵcat is introduced rather than using a constant value. A higher value of ϵcat indicates more heterogeneous NH3 adsorption sites. The ammonia desorption rate equation from the previous single-site model is then modified as ⎡ Ea (1 − ϵcat (T ) ·θ ) ⎤ ⎥·θ RNH3 , de = Ade ·exp⎢ − de ⎢⎣ ⎥⎦ RT
Figure 4. Cumulative adsorbed ammonia at temperatures from 150 to 400 °C (solid line: experiments; dash line: heterogeneous single-site model).
(14)
The temperature dependent term ϵcat(T) is calibrated based on the ammonia adsorption step in the isothermal test protocol at temperatures of 150−400 °C, while the kinetics of adsorption and desorption and the total storage site density are the same as the previous single-site model. The results of ammonia storage capacity at different temperatures are given in Figure 3. With the heterogeneous single-site storage model,
Figure 5. Calibrated values of temperature dependent term ϵcat(T).
groups of adsorption sites with different adsorption strengths at relatively low temperatures. Figure 6 shows the variation of the desorption energy with NH3 surface coverage for the three different approaches. The
Figure 3. Comparison of ammonia storage capacities at different adsorption temperatures for single-site, dual-site, and heterogeneous single-site with temperature dependent ϵcat(T) ammonia storage models. Figure 6. Comparison of desorption energy as a function of NH3 coverage.
predicted cumulative ammonia adsorption profiles are in good agreement with the experiments at different temperatures, which are described in Figure 4. The relationship between ϵcat and temperature can be readily described by a quadratic function as shown in Figure 5. In this quadratic function, the value of ϵcat decreases with temperature. The calibrated ϵcat results in a low value of 0.07 at 400 °C and a high value of 0.73 at 150 °C. This is consistent with the experimental observations15,26,28 that the adsorption sites (only strong Brønsted acid sites and Lewis acid sites with similar acid strengths) are less heterogeneous at high temperatures, while the adsorption sites are more heterogeneous with distinct
single-site model shows that the desorption energy is linear to the NH3 surface coverage, which results from the classic Temkin isotherm model (see eq 7). This linear dependence is necessary by ignoring the heterogeneity of various families of adsorption sites. The dual-site model shows significantly different desorption energy between the two sites. Site 1 representing the weakly adsorbed NH3 site shows a relatively lower desorption energy (
