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Re-evaluation and Modeling of a Commercial Diesel Oxidation Catalyst Young-Deuk Kim Department of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
Woo-Seung Kim* Department of Mechanical Engineering, Hanyang UniVersity, 1271 Sa3-dong, Sangnok-gu, Ansan, Gyeonggi-do 426-791, Republic of Korea
A modeling approach to predict the performance of a commercial diesel oxidation catalyst (DOC) under actual vehicle operating conditions is presented in this study. Prior to completing this prediction, the performance characteristics of DOCs, as previously published, are examined to validate the currently developed in-house computational code. Steady-state experiments with DOCs mounted on a light-duty four-cylinder 2.0-L turbocharged diesel engine then are performed, using an engine-dynamometer system to calibrate the kinetic parameters such as activation energies and pre-exponential factors of heterogeneous reactions. The reaction rates for CO, HC, and NO oxidations over a fresh Pt/Al2O3 catalyst are determined in conjunction with a transient one-dimensional (1D) heterogeneous plug-flow reactor (PFR) model with diesel exhaust gas temperatures in the range of 150-450 °C and space velocities in the range of (1-5) × 105 h-1. To determine the kinetic parameters that best fit the experimental data, a two-step optimization procedure is introduced. First, the results from the conjugated gradient method (CGM) with individual temperatures for each species are plotted in an Arrhenius plot to generate proper intermediate guesses from initial guesses for all preexponential factors and activation energies. Here, the kinetic parameters for CO oxidation are calibrated to provide the best fits to the lowest temperature data with fixed initial activation energy without implementing the first-step tuning procedure, because of complete conversion of CO over the temperature range of 150-450 °C. Kinetic parameters for all species then are obtained simultaneously by searching the best fits to experimental data using the CGM from the intermediate guesses for all species. The prediction accuracy of the model through first step optimization procedure against experimental results is slightly improved by performing a second-step optimization procedure, and the simulation results of optimized kinetic parameters are compared to the experimental data obtained at both 1500 and 2000 rpm. Introduction Diesel engines with direct fuel injection are becoming increasingly important in automotive applications, for both performance and fuel economy. The low fuel consumption of diesel vehicles leads to reduced emissions of the greenhouse gas carbon dioxide (CO2). However, diesel engines exhaust environmentally harmful carbon monoxide (CO), unburned hydrocarbons (HCs) such as paraffins, olefins, aldehydes, aromatic compounds, as well as nitric oxides (NOX), sulfur dioxide (SO2) and sooty particles that contain carbon in both solid form and the form of the so-called volatile organic fraction (VOF). A fuel-efficient mode typically produces high NOX raw emissions and low particulate emissions from NOX-particulate tradeoff. Therefore, advanced combustion technologies such as lean premixed compression ignition (PCI) low-temperature combustion have been investigated to simultaneously reduce NOX and particulate emissions.1 Furthermore, the use of emission control devices including diesel particulate filters (DPFs), diesel oxidation catalysts (DOCs), lean NOX traps (LNTs), selective catalytic reduction (SCR), and sulfur traps is imperative to comply with future stringent emission regulations. A DOC in the diesel exhaust after-treatment system can be used as an integral component to effectively reduce engine raw emissions, in combination with the aforementioned aftertreatment components. The main function of a commercial DOC * To whom correspondence should be addressed. Tel.: +82-31-4005248. Fax: +82-31-418-0153. E-mail:
[email protected].
is to reduce the soluble organic fraction (SOF) of the diesel particulate matter, as well as the gaseous CO and HC emissions. In addition, to improve the NOX conversion over a SCR catalyst, the DOC is usually placed upstream of the SCR catalyst to enhance the fast SCR reaction (4NH3 + 2NO + 2NO2 f 4N2 + 6H2O) using equimolar amounts of NO and NO2. Here, a ratio of NO2/NOX above 50% should be avoided, because the reaction with NO2 only (4NH3 + 3NO2 f 3.5N2 + 6H2O) is slower than the standard SCR reaction (4NH3 + 4NO + O2 f 4N2 + 6H2O).2,3 The DOC is also used in conjunction with a DPF, which is called a continuously regenerating trap (CRT), to oxidize NO for passive particulate combustion by NO2 and oxidize fuel for heat management with active particulate regeneration.4-6 Thus, as the diesel after-treatment architecture becomes more complicated with increasingly tight emission standards, more-accurate mathematical modeling of aftertreatment components is required to reduce the number of required experiments and tests. Many numerical and experimental attempts3-10 have been conducted to analyze the chemicophysical characteristics and conversion performances of DOCs and to examine the effect of DOCs in the diesel exhaust after-treatment architecture. In the spatially one-dimensional (1D) heterogeneous plug-flow reactor (PFR) model, which is the most widely used model in the automotive industry, the concentration and temperature gradients in axial direction must be considered in a full-scale monolith reactor.11 Axial dispersion can be neglected due to laminar flow in the channels, very short residence times, and
10.1021/ie801509j CCC: $40.75 2009 American Chemical Society Published on Web 06/19/2009
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high Peclet number (Pe) values. The accumulation of mass and heat in the gas phase and mass in the solid phase can also be neglected for a quasi-steady PFR model, because their time constants are typically much smaller than that of the solid thermal response.12 On the other hand, heat accumulation and conduction in the solid phase must be considered, along with the heat and mass transfer between a flowing gas phase and a solid phase. Gieshoff et al.3 experimentally reported that, using a platinum catalyst, it is possible to increase the NO2 fraction in the NOX up to the maximum given by the thermodynamic equilibrium between NO and NO2 in O2 using a synthetic gas mixture without CO and HC contents. They also showed a significant effect of platinum catalyst on NOX conversion from engine bench experiments for HSO (hydrolysis, SCR, and oxidation catalysts) and VHSO (oxidation catalyst and HSO) systems at various engine operation points. Kandylas et al.,4 Triana et al.,5 and York et al.6 presented modeling approaches aimed at predicting CRT system behavior with a 1D quasi-steady PFR model. Kandylas et al. used the experimental results of the platinum catalyst study conducted by Gieshoff et al.3 to assess the kinetic constants of the rate expressions. Triana et al.5 performed steady-state experiments covering a wide range of operating conditions with a heavy-duty diesel engine mounted on a dynamometer test bench and developed a 1D DOC model to predict the oxidation of CO, HC, and NO with the apparent kinetic parameters (activation energies and pre-exponential factors) obtained from the model calibration. York et al.6 developed a 1D DOC model that was based on laboratory microreactor data to describe the kinetics of the CO, HC, and NO oxidation reactions. They validated the model using engine bench data measured over both low- and high-temperature driving cycles. Voltz et al.7 derived Langmuir-Hinshelwood (LH)-type rate expressions for the oxidation reactions of CO and C3H6 on a pelleted Pt/Al2O3 catalyst. Most of the previous and current studies have adopted oxidation kinetic rates of the LH type proposed by Voltz et al.7 and calibrated the activation energy and frequency factor dependent on the catalyst under consideration by manual or computer-aided methodology. Tanaka et al.8 used the gas diffusion into the catalyst layer, adsorption and desorption from catalyst sites, and oxidation reaction kinetics models to simulate the precise transient performance of C3H6 over a model catalyst consisting of a mixture of Pt/γ-Al2O3 and zeolite. Sampara et al.9 reported the global oxidation reaction rates for CO, C3H6, H2, and NO in the presence of excess O2 over a laboratory-scale reactor with simulated diesel exhaust between 200 °C and 415 °C over wide concentration ranges. They solved a 1D isothermal PFR model by the steady-state assumption to predict exit concentrations and validated the kinetic model against the engine test results with a full-scale reactor mounted on a light-duty diesel engine. Wang et al.10 developed a 1D isothermal PFR model to examine the performance characteristics of a commercial Pt/Al2O3 catalyst. To calibrate the kinetic parameters of the model reactions, they performed steady-state experiments with a lightduty exhaust gas recirculation (EGR)-mounted diesel engine on a dynamometer test bench. As shown in the aforementioned literature, some of the previous studies regarding DOC modeling developed a simplified 1D reactor model, such as a 1D isothermal or quasi-steady PFR model, with a kinetic model based on laboratory-scale experiments. A kinetic model for a laboratory-scale reactor was scaled-up and transferred to the full-scale monolith model with validation of the model using engine bench data. Some previous studies of DOC modeling based on engine-dynamometer test
results were also conducted using a simplified 1D reactor model or incomplete kinetic parameters to describe the performance of commercial DOCs. However, note that the experimental data measured using a real diesel engine with EGR exhibit morecomplex characteristics, in comparison with those obtained from laboratory-scale experiments. In this work, a transient 1D heterogeneous PFR model has been developed to simulate the performance of the commercial DOCs based on enginedynamometer test results. A two-step optimization procedure has been developed for calibrating the kinetic parameters. We first re-evaluate the conversion behavior of DOC used in Triana et al.5 to validate the currently developed in-house computational code. Here, the kinetic parameters are calibrated instead of using the apparent kinetic constants suggested by Triana et al.5 Based on the spatially 1D heterogeneous PFR model developed in this study, we then performed steady-state experiments covering a wide range of operating conditions with the DOC mounted on a light-duty four-cylinder 2.0-L turbocharged diesel engine to calibrate the kinetic parameters of heterogeneous reactions and to evaluate the performance characteristics of the DOC. The kinetic parameters for CO, HC, and NO to best-fit the experimental data are determined using a two-step optimization procedure introduced in this study. Based on the obtained kinetic parameters, the predicted exit concentrations of all species are compared to the experimental data. Experimental Section Engine and Test Cell Instrumentation. The engine used for this research is a four-cylinder, in-line, 2.0-L engine with a rated power of 85 kW at 4000 rpm and a peak torque of 260 N m at 2000 rpm. This engine is operated with an EGR, highpressure common rail direct injection, and a turbocharged/ aftercooled system. The diesel engine is connected to a 160kW EC dynamometer controlled with a PUMA 5.3 control system. The test cell computer controls the engine speed, records all the test cell data, and controls the emissions bench. This test cell has a full emissions bench with a Horiba Model MEXA-9100DEGR analyzer. The CO and CO2 instruments use a nondispersive infrared detector, the HC analyzer uses the heating flame ionization detection (FID) method, the NOX analyzer uses the chemiluminescence principle, and the O2 instrument is a paramagnetic detector. Gas and substrate temperatures are measured using K-type thermocouples, and pressures are also measured. Lower sulfur content in diesel fuel inhibits poisoning of the oxidation catalyst to extend its life, limits the formation of sulfates, and reduces particulate matter (PM) emissions. Therefore, the diesel fuel used in the experiments is an ultralow sulfur diesel with a cetane number of 57.1, and the density is 824 kg/m3 at 15 °C. The commercial DOC is mounted in the exhaust stream ∼2 m from the engine. Figure 1 shows a schematic representation of the test cell setup, where a valve configuration is used to select the sampling between the upstream and downstream ports. Testing and Sampling Procedures. To achieve the range of temperatures and space velocities representative of engine operation, 18 steady-state conditions are considered for the experimental test matrix. Experiments are performed by varying the temperatures (150-450 °C) and space velocities of the engine speed and load operating range ((1-5) × 105 h-1) to evaluate the CO, HC, and NO oxidations over representative engine operating conditions. Figure 2 shows the variations in space velocity against exhaust gas temperatures measured under two different engine speed conditions. The exhaust gas temperatures in the inlet and outlet of the DOC and substrate
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mil cordierite substrate. The catalyst evaluated in the experiment is 110 mm in diameter and 95 mm long. An active platinum surface area (aPt) and the dispersion of platinum (DPt) are deduced from the CO adsorption isotherm with the use of eqs 1 and 2.10,13,14 The CO:Pt chemisorption stoichiometry is assumed to be 0.7,10,14,15 whereas the surface area per Pt atom (am) is assumed to be 8.07 Å2.10,13,14 sPt (%) )
VadsNAnam × 100 Vmmw
aPt ) sPtmPt DPt (%) ) Figure 1. Schematic of the test cell setup.
Figure 2. Correlation between space velocity and exhaust gas temperature at (0) 1500 and (O) 2000 rpm.
temperatures along the centerline of the monolith cross section are measured to use as initial and boundary conditions for a 1D PFR model coupled with chemical reaction kinetics. A switch valve is used to sample emissions upstream and downstream of the DOC. The engine load varies from 10% to 100% (80% for 1500 rpm) at two different engine speeds of 1500 and 2000 rpm with 10% (∼20 N m) increments starting from the minimum load to the maximum load for each engine load. Approximately 12 min are required to obtain each steady-state condition, and prior to each engine speed test, the engine is run sufficiently for warm-up and stabilization. Because only one emissions bench is used for all tests, it is necessary to sample upstream and downstream of the DOC at different times in arranged order for every steady-state condition. The NO2 emissions are determined by sampling the NO and NOX emissions and then taking the difference between them. The first emissions sample at each mode is taken after ∼4 min for engine stabilization and is upstream of the DOC at the NO mode of the NOX analyzer. The second sample is upstream of the DOC at the NOX mode of the NOX analyzer, and NO and NOX emissions downstream of the DOC are taken using the same sampling sequence with a sampling time of 2 min. Diesel Oxidation Catalyst. A fresh commercial DOC with platinum supported on a Al2O3 washcoat is used for all of the experiments. A Pt/Al2O3 catalyst is coated on a 400 cpsi/6
sPt M × 100 amNA
(1a) (1b) (2)
Equations 1a and 1b lead to a specific platinum surface area of 7.1 m2/gPt, and the dispersion of platinum (DPt) is calculated to be 2.9%. Model Description. Chemical Reaction Kinetics Model. To describe the reactions in the oxidation catalyst, three basic chemical reactions for CO, HC, and NO are considered, as shown in Table 1. The hydrocarbon constituents of diesel exhaust generally span a range from C1 and C40 and are centered in the C9-C12 range.5 Because of low HC emissions (
