Modeling NOx Storage and Reduction for a Diesel Automotive

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Modeling NOx Storage and Reduction for a Diesel Automotive Catalyst Based on Synthetic Gas Bench Experiments Federico Millo,*,† Mahsa Rafigh,† Francesco Sapio,† Syed Wahiduzzaman,‡ Ryan Dudgeon,‡ Paolo Ferreri,§ and Eduardo Barrientos§ †

Department of Energetics, Politecnico di Torino, Corso Duca degli Abruzzi 24, Torino 10129, Italy Gamma Technologies, LLC, 601 Oakmont Lane, Westmont, Illinois 60559, United States § General Motors Global Propulsion Systems, S.r.l., Corso Castelfidardo 36, Torino 10138, Italy Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/31/18. For personal use only.

‡

ABSTRACT: To comply with stringent NOx emission regulations, automotive diesel engines require advanced aftertreatment catalytic systems, such as lean NOx traps (LNTs). Considering that test bench and chassis dyno experimental campaigns are costly and require a vast use of resources for the generation of data; therefore, reliable and computationally efficient simulation models are essential in order to identify the most promising technology mix to satisfy emission regulations. In the literature, a large number of simulation models for LNT kinetics can be found, realized for laboratory-scale samples and validated over synthetic gas bench (SGB) experimental tests, while full-size models validated over engine-dyno driving cycle data, crucial for industrial applications, are missing. In the current work, a simulation model of an LNT device is built to predict NOx storage and reduction, starting from SGB laboratory tests and finally validated over driving cycle data. The experiments including light-off, NOx storage and reduction (NSR), and oxygen storage capacity (OSC) characterization, were performed on a laboratory-scale sample extracted from a full-scale monolith. Light-off tests have been conducted under a temperature ramp cycle from 120 °C to 380 °C, while OSC and NSR tests were performed under isothermal conditions at five temperature levels, ranging from 150 °C to 400 °C. OSC tests were performed to characterize oxygen storage capacity of ceria sites and water gas shift (WGS) reaction over the precious metals by controlling inlet species concentrations with periodic lean/rich pulses. NSR experiments were then performed by alternating a lean inlet composition to reproduce adsorption/desorption of NOx with a rich inlet composition feed with three reductants (H2, CO, and C3H6) to replicate NOx reduction reactions. A global kinetic scheme was defined by means of a one-dimensional (1D) engine simulation fluid-dynamic code, GT-SUITE, to model oxidation reactions (CO, HC, NO), NOx adsorption/desorption, oxygen storage and NOx reduction reactions. The kinetic parameters were obtained using Arrhenius plots with the aim to minimize the error between simulated and experimental NOx, reductants, N2O and NH3 concentrations, reaching a satisfactory agreement with measurements.

1. INTRODUCTION Global upcoming stringent emission regulations (including real driving emissions, RDE) require reliable diesel vehicle aftertreatment systems capable of reducing tailpipe NOx to levels below the legislation limits.1,2 Different aftertreatment technologies for NOx reduction have been implemented during previous decades,3,4 and, in this context, lean NOx traps (LNTs), also known as NOx storage catalyst (NSC) and NOx storage reduction (NSR) catalysts, have been introduced as a promising technology for the abatement of NOx under lean-burn conditions.5 The operating principle of an LNT can be described based on lean/rich cyclic processes.6 The adsorption/desorption of NOx on the trapping component, for instance, BaO during the lean phase, results in the formation of nitrates and nitrites on the catalyst surface. When the active trapping sites are saturated, a short rich phase containing hydrogen (H2), carbon monoxide (CO), and hydrocarbons (HC), is actuated by means of fuel © XXXX American Chemical Society

post-injection. It is noteworthy that water gas shift (WGS) occurs at the precious metal sites, mainly Pt, as well as steam reforming and primary oxidation of NO, producing NO2, and the reverse reaction, producing NO from NO2, in addition to direct reduction of NO by means of HC, CO, or H2. Several studies have been dedicated to the analysis of the different stages of LNT system’s operation. The lean phase, which mainly contains the adsorption/desorption mechanisms, has been investigated in several works, such as refs 7−9. On the other hand, the rich period mechanisms and the effect of using different reductants have also been extensively explored in ref 10−13. Received: Revised: Accepted: Published: A

April 29, 2018 July 25, 2018 August 20, 2018 August 20, 2018 DOI: 10.1021/acs.iecr.8b01813 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Transferring the calibrated reactor-scale model to a full-scale component.

Figure 2. Extraction of LNT laboratory-scale samples from a full-scale monolith.

aftertreatment model based on experimental data for a full-size component using engine-out emissions could be challenging (see Figure 1). The present work extends what was presented in ref 18. Similar to that observed in ref 17, a Pt/BaO/Al2O3 LNT model for an 1.6 L automotive diesel engine has been built by means of a 1D multiphysics fluid-dynamic code, GT-SUITE. A dedicated experimental campaign was performed at the synthetic gas bench (SGB) on a core sample extracted from the full-size monolith. A global kinetic scheme has been defined and calibrated over SGB experimental data, evaluating the impact on the model fidelity of different reaction pathways. Furthermore, a transformation of the model to a full-size component has been performed, inheriting the chemical kinetics parameters from the laboratory-scale model without relevant modifications. The obtained model has been then validated over driving cycle data, including controlled catalyst regeneration events, thus considering the performance of the aftertreatment system with transient and real exhaust gas conditions as the input. This model can be crucial for industrial applications, especially in the early phase of powertrain development, since it can be used for multiple purposes such as optimization, development of control strategies, sensitivity analysis of different parameters (PGM loading, dimensions etc.), preliminary assessment of the capability of a powertrain architecture to fulfill legislation requirements, and virtual assessment of the aftertreatment system performance under real driving conditions. The activity

On the modeling side, different approaches have been investigated in the literature to model the LNT kinetics. A microkinetics approach accounts for all the elementary steps behind each reaction and could accurately describe LNT operations, as demonstrated in ref 14. However, such a detailed approach implies a significant calibration effort, because of its complexity, and requires a deep understanding of the kinetics, which could be obtained by means of an extensive laboratoryscale characterization. A global kinetic approach, instead, does not include all the reactions substeps, thus reducing the computational effort required, which makes the model suitable for industrial applications such as upscaling to the full-size component for driving cycles simulations. With a properly designed experimental protocol, global kinetics models could be calibrated to accurately represent LNT operations, as reported in refs 15−17. In ref 17, for instance, a single-channel three-way catalyst one-dimensional (1D) model has been extended to account for LNT operations, using a global kinetic scheme calibrated over experimental data obtained by means of synthetic gas on reactor samples, although the model has not been validated for the full-size component. Even if LNT kinetics has been widely investigated in the literature and several models have been developed, most of them refer to laboratory-scale samples and lack full-size validation over driving cycle data, which is crucial for industrial applications. In fact, due to overlapping of several phenomena, including transient conditions and complex gas mixture, building and calibrating an B

DOI: 10.1021/acs.iecr.8b01813 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

As depicted in Figure 3b, two thermocouples, with diameters of 0.5 mm each, were mounted in the gas flow upstream (TUS), and downstream, TDS, of the sample, while the other temperatures (T1 − T5) were not measured. Gas concentration measurements were performed with a multicomponent Fourier transform infrared (FTIR) analysis with a sampling frequency of 1 Hz. Moreover, lambda evaluation was performed by means of a wide band universal exhaust gas oxygen (UEGO) sensor and calculation from feed gas. The concentration of species at the outlet are measured, including CO, CO2, H2O, NO, NOx, NH3, N2O, C3H6, and C3H8. At the beginning and at the end of each test, a bypass signal is used to check the inlet gas composition alignment, relative to the nominal one. Since H2 measurements were not available downstream of the catalyst, the predicted catalyst outlet H2 concentration from the simulation model could be validated only indirectly by checking the other species concentrations. 2.2. Test Protocols. The tests can be categorized in three main groups: oxygen storage capacity (OSC), NOx storage and reduction (NSR), and light-off tests. 2.2.1. Oxygen Storage Capacity (OSC). The OSC experiments are performed at a space velocity (SV) of 30 000 1/h. Five inlet temperatures are tested, starting from 150 °C to 450 °C and divided into five intervals, equally spaced (in terms of inverse temperature, 1/T [1/K]), to improve resolution of the Arrhenius rate constants. The tests consist of a lean phase and a short rich phase by introduction of CO as the reducing agent. In this case, overlap between simultaneously feeding oxygen and CO is avoided by delaying the feed of CO until oxygen is completely shut off. The inlet gas composition is presented in Table 2. 2.2.2. NOx Storage and Reduction (NSR). The NSR tests are performed at a space velocity of 30 000 1/h, with sufficient

was performed on sulfur-free components, both in the laboratory-scale and in the engine-scale conditions. Since it is well-known that catalyst sulfation can appreciably affect the performances of LNTs,19 future activities will be performed to include this effect in the model.

■

2. EXPERIMENTAL ANALYSIS 2.1. Experimental Setup. The laboratory-scale samples were obtained from a full-scale monolith, as can be observed in Figure 2, with a diameter of 18.5 mm and a length of 96.6 mm. The core sample was aged in the oven under hydrothermal conditions prior to the experimental campaign. The main characteristics of the samples are reported in Table 1. Table 1. Main Characteristics of LNT under Investigation characteristic

value

substrate material cell density substrate wall thickness monolith dimensions (diameter × length) core sample dimensions (diameter × length) PGM loading PGM ratio (Pt:Pd:Rh)

cordierite 400 cpsi 0.1 mm 137.16 mm × 96.6 mm 18.5 mm × 96.6 mm 120 g/ft3 103:12:5

The experimental activity was performed at the ACA Center for Automotive Catalytic Systems of RWTH Aachen University through a laboratory gas bench, shown in Figure 3a. The sample was placed into an insulated cylindrical reactor, where the gases were mixed from compressed gas cylinders using mass flow controllers. Preliminary experiments confirmed that heat losses in the reactor chamber were negligible.

Figure 3. Experimental setup: (a) schematic view of laboratory gas bench and (b) thermocouple locations on the laboratory-scale sample. C


Article

Industrial & Engineering Chemistry Research Table 2. Inlet Gas Composition for OSC Experimentsa species

lean phase (60 s)

rich phase (30 s)

CO O2 CO2 H2O N2

0 ppm 0.5% 5% 10% balance

20000 ppm 0% 0% 10% balance

catalyst storage capacity. The inlet gas composition for lean/rich cycle is reported in Table 3. The reductant concentrations were selected such that NO reduction potential is held constant, i.e., ∼110 ppm of C3H6 can reduce the same quantity of NO as 1000 ppm of H2, based on reaction stoichiometry. NO is fed during the lean phase, together with O2 and after 900 s, NOx and O2 flows are shut off and reductants are injected in separate steps (H2, CO and C3H6) with the aim to clean off the NOx storage sites. Since engine-out NOx emissions mainly consist of NO, in the NSR test protocol, inlet NOx only includes NO, although NO2 adsorption/desorption kinetics can be indirectly characterized due to production of NO2 from oxidation of NO at mid- to highlevel temperatures. An example of measured NOx during NSR experiment in which H2 is used as the primary reductant is presented in Figure 4. 2.2.3. Light-Off Experiments. The oxidative behavior of the catalyst can be examined by means of light-off experiments through a temperature ramp cycle, which gives the possibility to characterize the CO and HC oxidation rates on the noble-metal component, as a function of the catalyst temperature.20 The temperature ramp cycle starts from 120 °C (393 K) to 380 °C (653 K) at a rate of 3 K/min. The experiments are performed at a space velocity of 30 000 1/h with the inlet feed composition shown in Table 4. Hydrocarbons are specified on a C3 basis and consist of a 2:1 molar ratio of propane (C3H8), representative of slow oxidizing HCs, to propylene (C3H6), representative of fast oxidizing HCs.

a

Concentrations given on a volume basis.

prereduction with H2 prior to each test run (∼900 s). Five inlet temperatures are tested, starting from 150 °C (423 K) to 400 °C (673 K), which are equally spaced in 1/T, where T is given in Kelvin. The experiments are ideally repeated at each temperature using H2, CO, and HC (represented by propylene, C3H6) as the reductant for each step. The test protocol consists of a first lean phase, inlet feed composition as in Table 3, which allows Table 3. Inlet Gas Composition for NSR Experimentsa species

lean (900 s)

rich, H2 (600 s)

rich, CO (600 s)

rich, HC (600 s)

reductant O2 CO2 H2O NO N2

0 10% 5% 5% 300 ppm balance

1000 ppm 0 0 5% 0 balance



a


adsorption of NOx, followed by a rich phase where a reductant is introduced to reduce the adsorbed species thus regenerating the

Figure 4. Measured NO and NO2 profiles from NOx storage and reduction experiments with 300 ppm of NO feed during lean phase and 1000 ppm of H2 during rich phase, space velocity (SV) of 30 000 1/h. D


Article

Industrial & Engineering Chemistry Research Table 4. Inlet Gas Composition for Light-Off Experimentsa species

composition

HC CO H2 O2 CO2 H2O N2

400 ppm C3 300 ppm 60 ppm 10% 5% 5% balance

at each temperature, an optimal rate is found and the relationship to temperature is determined by using the Arrhenius plot to yield an activation energy Ea and a preexponent multiplier A, as can be observed in Figure 5.

a


3. SIMULATION MODEL The LNT model was built for the reactor scale sample (using a cylinder 18.5 mm in diameter and 96.6 mm in length, corresponding to the full monolith length). The reaction mechanism is a global-type surface reaction mechanism using the turnover number reaction rate format. It incorporates NO, CO, and HC oxidation on PGM sites, NOx adsorption/desorption on barium sites, oxygen storage on ceria and NOx reduction reactions in addition to the WGS and steam reforming reaction occurring over PGM. Reaction rates are expressed by general form of Arrhenius term, as reported in eq 1: Ri =

(

E

Figure 5. Arrhenius plot used for calibration.

More details regarding the calibration approach and simulation model are found in ref 21. 3.1. OSC Model. The reactions in this step include the storage of oxygen on ceria sites during lean phase and the reduction of stored oxygen by means of CO during rich phase, more details regarding the kinetic scheme can be found in.21 Moreover, H2 and HCs are also capable to consume the stored oxygen. In addition, because of the presence of H2O in the inlet feed, during the rich phase, H2 is produced because of the WGS reaction over the precious metal Pt.22,23 The reaction mechanism can be described as reported in Table 5. Oxygen storage on ceria is modeled using an equilibrium function, which takes into account the increased storage capacity, as a function of temperature and limited by a maximum storage equilibrium curve.21 Calibration of WGS can be performed by taking into account the OSC experimental tests. At the beginning of the rich phase of the test, the produced H2 can be consumed reacting with oxygen molecules stored on ceria. Since, during the rich phase, O2 is not present in the inlet batch, after all the oxygen stored on ceria has been consumed the outlet H2 concentration is expected to reach a steady value, which could be directly related to the conversion of CO and H2O by means of the WGS reaction. Considering that (i) H2 measurements were not available and (ii) CO2 is not fed at the reactor inlet, the amount of H2 measured at the reactor outlet could be considered equal to that of CO2, using the stoichiometry of the reaction. Thus, WGS reaction can be calibrated considering CO2 emissions measured at the reactor outlet during rich phases, after steady-state conditions have been reached. Regarding the H2 and HC reactions in the OSC test, since the test protocol does not include HC and H2 at the inlet batch, a preliminary value from the literature was used, which was further tuned during NSR tests focusing on the reductant breakthrough. Moreover, G1 inhibition term is a Voltz-type inhibition function and the preliminary values are selected according to ref 21, which takes into account the competition of reactions occurring on PGM, including the concentrations of NO, CO, and HC. 3.2. NSR Model. During the NOx storage phase, the reaction mechanism includes NO oxidation on the PGM and NOx adsorption/desorption on the Ba sites, in the form of barium

)

Ai exp − RTai Ciθ Gi

(1)

in which R represents the rate of reaction i, Ai the pre-exponent multiplier, Eai the activation energy, Gi the inhibition terms (if required), Ci represents the reactant concentrations (in mol/ m3), and θ the the surface site coverage varying between 0 to 1. Calibration of the model is performed with the aim of minimizing an objective function: objective (error) function =

∫0

tend

|Cmeasured − Csimulated|species dt (2)

The calibration parameters, or independent variables, include site densities, pre-exponent multiplier, and activation energy for each reaction. Site density indicates the ratio between moles of active sites and the catalyst volume. In this work, reaction rate expressions have been implemented in the model as referred to the number of active sites; thus, the benefit of introducing active site density as a calibration parameter is to give the possibility to scale up or down a given chemical kinetics. In the case of a PGM site, when specific measurements are not available, a preliminary estimate of the active site density can be obtained multiplying the nominal PGM loading by the so-called “metal dispersion factor”, which is a factor accounting for the deviation between the ideal case (all the sites are active) and the real case (only some of the sites are active). Since the value set for the metal dispersion is generally coming from assumptions, further calibration of the active site density could be necessary to better capture the behavior of the component under investigation. In the case of storage sites (for instance, the barium site on the LNT, the zeolite site on a DOC and a SCR), the active site density physically represents the maximum storage capacity. Since the OSC and NSR tests are performed at constant temperature steps, an Arrhenius plot can be used to characterize the pre-exponent multiplier and activation energy for each reaction. Therefore, calibration is performed starting from the lowest temperature and moving to the next highest temperature; E


Article

Industrial & Engineering Chemistry Research Table 5. WGS, Oxygen Storage, and Reduction Reactions21 reaction

rate expression

K eq,WGS

activation temperature [K]

8.1 × 108

8832

K eq,WGS

H 2O + CO ←⎯⎯⎯⎯⎯→ CO2 + H 2

R1 =

Ce2O3 + 0.5O2 → 2CeO2

eq R 2 = k 2CO2(θCe − θCe2O3) 2O3

1.8

0

2CeO2 + CO → Ce2O3 + CO2

R3 = k 3CCOθCeO2

3.2

0

2CeO2 + H 2 → Ce2O3 + H 2O

R 4 = k4C H2θCeO2

12.6

0

1 1 1 2CeO2 + C3H6 → Ce2O3 + H 2O + CO2 9 3 3

R 5 = k5CC3H6θCeO2

ÄÅ G1 = ÅÅÅÅ1 + 9.467 exp ÅÇ

a

k1CCOC H2O −

pre-exponential factor [mol/s]

CCO2C H2

G1*

ÉÑ2

−215.34 C + 274.82 exp( T )CC H ÑÑÑÑ ( 63.451 T ) CO ÑÖ 3 6

ÄÅ Å × ÅÅÅ1 + 0.00061 exp ÅÇ

15

ÉÑ

ÄÅ

ÉÑ

0

2Ñ ÑÑ × ÅÅÅ1 + 5.554 × 106 exp( −6206.9 )C NOÑÑÑ. C 2C ( 9715.6 ÑÑÖ T ) CO C H Ñ T ÑÖ ÅÅÇ 3 6

Table 6. Kinetic Model for Light-Off Experiment on PGMa reaction

rate expression

CO + 0.5O2 → CO2

R6 =

H 2 + 0.5O2 → H 2O

R7 =

C3H8 + 5O2 → 3CO2 + 4H 2O

R8 =

C3H6 + 4.5O2 → 3CO2 + 3H 2O

R9 =

C3H6 + 3H 2O → 3CO + 6H 2

R10 =

C3H8 + 3H 2O → 3CO + 7H 2

R11 =

k6CCOCO2 G1 k 7C H2CO2 G1

k 8CC3H8CO2 G1 k 9CC3H6CO2 G1 k10CC3H6C H2O G1 k11CC3H8C H2O G1

pre-exponential factor [mol/s]

activation temperature [K]

8.5 × 103

3372

9.4 × 1014

13404

7.2 × 105

8546

2.4 × 1023

25506

1.2 × 109

10731

1.2 × 1013

20957

a

Data taken from ref 31. Note that G1 is the Voltz-type inhibition term present in the OSC kinetic scheme.

i −E y G = 1 + A expjjj a zzzC NO k RT {

nitrates and barium nitrites, using the reaction pathways proposed by ref 24. NO + 0.5O2 ↔ NO2

(3)

BaO + 2NO + 1.5O2 ↔ Ba(NO3)2

(4)

BaO + 2NO2 + 0.5O2 ↔ Ba(NO3)2

(5)

BaO + 2NO + 0.5O2 ↔ Ba(NO2 )2

(6)

BaO + 3NO2 → Ba(NO3)2 + NO

(7)

Ba(NO2 )2 + O2 ↔ Ba(NO3)2

(8)

(9)

Similar observations have been reported in ref 21. As far as reduction reactions of the stored NOx are considered, main reactions pathways involve H2, CO, and C3H6 as reductants, producing N2, CO2, and water as main products. Since H2 is also produced when CO and C3H6 are used as reductants (from WGS and steam reforming reactions, respectively), the calibration process is started from the cases where H2 is used as the primary reductant. In this case, the reaction mechanism is set according to26−28 Ba(NO3)2 + 8H 2 → BaO + 2NH3 + 5H 2O

In order to take into account the nonmonotonic storage capacity of Ba sites, with respect to temperature, a multiple-adsorption site model was considered,8,25 including three types of Ba sites, which differ in the active site density.21 Even if pure adsorption/ desorption test procedures were not available to detect if multiple temperature-dependent storage sites were present on the analyzed component, it was observed that multiple sites must be taken into account on the modeling side to properly model the LNT storage capacity. In particular, it was observed that, for temperatures of

Modeling NOx Storage and Reduction for a Diesel Automotive

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