A Transient Model For The Performance Of Dual Fuel (Natural Gas/Diesel) Catalytic Converter Sallamie, N.1, Kazemeini, M.2, Badakhshan, A.3, Soltanieh, M.2 and Estiri, M.4 1. Dept. Of Chemical Eng., Iran University Of Science and Technology, Farjam, Narmak, Tehran, I.R.Iran, Tel: ++9821-7896621 Fax: ++9821-7896620 E-mail:
[email protected] 2. Dept. Of Chemical Eng., Sharif University Of Technology, P.O.Box 11365-8639, Tehran, I.R.Iran, EMail:
[email protected] 3. Dept. Of Chemical and Petroleum Eng., University Of Calgary, Calgary, Alberta, Canada T6G 2G8 4. Air Quality Control Co., Tehran, I.R.Iran
Abstract Exhaust gas emissions from mobile sources and its important role in urban pollution has currently pushed much research towards alternative fuel systems for use in road vehicles. One of the best replacements is the natural gas/diesel dual fuel which offers an alternative to standard compression ignition diesel engines. In the dual fuel system, the primary fuel is natural gas which may replace as much as 90% of the diesel fuel, with a small amount of diesel fuel required to ensure effective ignition. Methane, the main constituent of natural gas, has a high combustion efficiency. Dual fuel engines produce lower NOx emissions and fewer particulates. However, at moderate engine loads, lower fuel combustion efficiency leads to significant hydrocarbons and carbon monoxide in the exhaust. Therefore, an oxidizing catalytic converter is necessary in the exhaust system to reduce these emissions to an acceptable level. Based upon experimental data gained with an Isuzu 4BE1 dual fuel engine, a transient two dimensional model is introduced in this work for the monolithic oxidizing catalytic converter, while at the same time considering chemical reaction and transport phenomena. It is shown that first order kinetics may well fit methane and carbon monoxide catalytic conversion. Furthermore, effectiveness factor is used to describe simultaneous mass transfer and reaction in the catalyst phase. Temperature and composition dependent physical properties are also employed. Predicted steady-state conversions are then compared with experimental data to verify the validity of the model. Finally, some extensions of the model, including the effectiveness factor changes and temperature profiles in the transient state are presented. Ultimately, it is concluded that the model is properly capable to describe the performance of the converter.
Introduction Based on its high H/C ratio, clean combustion nature and abundant sources all over the world, natural gas can be easily and widely used for vehicular applications. The natural gas/diesel dual fuel system is a common alternative to the diesel fuel , keeping the benefits of the efficient diesel cycle [1]. The main constituent of the fuel mixture is natural gas which can replace up to 90% of the required diesel fuel, due to its high knock resistive characteristics. A small amount of diesel fuel is simultaneously injected into the cylinder to ensure effective and well distributed compression ignition. The diesel dual fuel engine produces lower NOx emission and less particulates compared with the standard diesel engine. However, at low to intermediate engine loads, poor combustion efficiency, leads to high levels of unburned hydrocarbons (HC), mainly composed of methane, and also carbon monoxide (CO) in the exhaust [2]. Application of oxidizing catalytic converter is suggested as a practical solution to
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minimize HC and CO emissions of the dual fuel system and practically examined, while at the same time, low exhaust temperature of the dual fuel engine and high resistance of methane to oxidization are the severe problems [3]. Catalytic converters has been widely used with Otto (gasoline) engines since 1976. Three way catalysts (TWC) are the most common type reducing unburned hydrocarbons, carbon monoxide and NOx at the same time. Many experimental studies and different theoretical models have been published in this domain, however, performance and operating conditions of the dual fuel (natural gas/diesel) catalytic converters are rarely noticed. Compared with an Otto engine operating near stoichiomertic condition, the dual fuel runs at lean fuel mixture with much more excess oxygen in the exhaust (usually more than 6%) and lower exhaust temperature resulted from high compression ratio. In this paper, a two dimensional transient model is introduced for the dual fuel catalytic converter, including reaction and transfer phenomena. In this model the laminar flow in the ceramic monolith channels is described with the plug flow model. Model outputs are then verified using steadystate experimental data and proper adaptation is observed. Finally, some model extensions are presented.
Experimental Results The experimental data used in this work has been gained with a 4-cylinder Isuzu 4BE1 naturally aspirated 3.6 lit diesel engine, equipped with dual fuel system and installed on an eddy current dynamometer. The engine exhaust was connected to the catalytic converter and engine speed and load were controlled and set by an electronic control system. An automatic sampling system was arranged and connected to the analyzers to measure the upstream and downstream concentrations of the ideal species. Table 1 shows test equipments used for measurement of important variables, together with their operating range and repeatability. The specifications of the monolithic catalytic converter, consisted of two cylindrical cordierite ceramic pieces in series, are given in Table 2. Main experimental results gained are conversion-temperature curves of HC and CO in the steady-state condition. The operational procedure in a constant engine speed and variable load have been previously described by Sallamie et. al. and different experimental curves have been published [4,5].
Transient Theoretical Model Chemical reaction and transfer phenomena are the basic approaches included in the general transient model. The gas velocity is assumed to be uniform at the cross section of the converter, as verified and suggested in different reports [6]. As Sallamie et. al. investigated different forms of chemical reaction rates, first order reaction rates are selected for catalytic conversion of methane and carbon monoxide in the dual fuel exhaust environment with excess oxygen available [7]. Effectiveness factor is used to model the simultaneous diffusion and reaction in the catalyst solid phase. To account for heat and mass transfer between gas and solid phases, film theory is used and it is assumed that dimensionless Nusselt and Sherwood numbers are constant in the channels after the short entrance region and fully-development of the flow [8]: Shi = Nu =
K gi . Dh Dgi
= 3.0
(1)
h. Dh = 3.0 kg
(2)
Composition and temperature dependent physical properties are used from Reid et. al. [9], considering proper mixing rules for gases. Evaluation of methane and carbon monoxide 2
conversions in the catalytic converter outlet has been the basic goal for the development of the model. Assuming a cylindrical element, energy and mass balances for gas and solid phases are noticed as follows: −U
C gi z
− K gi . av (C gi − C si ) = 0
− g . UC pg
(i = 1 to 2)
Tg
− h. a v (Tg − Ts ) = 0 z K gi . av (C gi − C si ) − a( x ). i . Ri (C s , Ts ) = 0
(1 − ) k
2Ts z 2
+ (1 − )
= (1 − ) s C ps
(3) (4)
(i = 1 to 2)
(5)
T k ( r s ) + ha v (Tg − Ts ) + a ( x ) i . R i (C s , Ts )( − H i ) r r r
Ts t i =1 : CO
i =2 : CH4
The continuum model is selected, in other words, the overall heat transfer in the channels are considered, instead of thermal interaction among adjacent channels. Adiabatic catalytic converter with equal gas flow in each channel are also considered [10]. Axial heat and mass transfer terms are ignored in comparison with corresponding convective terms. Radiation is not included in the model and due to low pressure of the engine exhaust (less than 2 atm), gas phase is supposed to be ideal. It is proved that the heat transfer between the fluid and catalyst has the slowest response, so that the only equation including the transient term, is the energy balance in the gas phase, while the time-dependent terms in other equations are ignored [11]. The equations are modified to the dimensionless form and solved with numerical methods. Initial and boundary conditions are known and may be summarized as follows: Ts=Ts0 at t = 0 (7) in Tg=Tg at z = 0 (8) Cg=Cgin at z = 0 (9) Ts =0 z Ts =0 z Ts =0 r
2Ts r 2
=0
at z = 0
(10)
at z = L
(11)
at r = 0
(12)
at r = R
(13)
Equation (13) shows that the linear temperature profile in the solid catalyst wall is assumed [10,12]. It is noted that the first order reaction rates used for catalytic conversion of methane and carbon monoxide are those published by Sallamie et. al. for the dual fuel (natural gas/diesel) catalytic converter [7]. Steady-state and transient outputs of the model may be studied. It is also possible to investigate the effects of many different variables in the system, however, only a few samples will be presented. In Figure 1, the model outputs of methane and carbon monoxide vs. temperature at constant 1500 rpm are compared with corresponding experimental curves for the fresh catalyst, while, aging phenomena may be separately investigated. Figure 2 shows the comparison between predicted trends of conversion vs. temperature of methane and carbon monoxide with experimental data at constant 2600 rpm and steady-state condition. Figure 3 gives the predicted methane and carbon monoxide effectiveness factor changes vs. temperature at the converter outlet in the steady-state condition. Solid phase temperature
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profile across the converter in transient condition is illustrated in Figure 4, at three different times. These are the times passed from the entrance of the engine exhaust into the cold catalytic converter. Finally, Figure 5 shows methane and carbon monoxide conversion changes at the converter outlet for three different converter lengths.
Discussions It is necessary to note that each constant speed for diesel dual fuel engine translates into approximately constant mass flow rate in the exhaust. Furthermore, it is usually possible to exert higher levels of loads at high engine speeds. Consequently, the exhaust mass flow rates and temperatures will be higher at high speeds. As it is evident in Figure 1, active catalytic areas is the separating parameter among three sets of curves. Due to confidential nature of the converter, measurement of the active catalytic surface was not possible, so, this parameter is used to properly adapt model outputs with experimental data when other governing parameters are well established in the model. It is clear that this adaptation will not be possible with changing the parameters if other important relations are not correctly included in the theoretical model [11]. In this manner, an approximation for the active catalytic area is resulted. The adaptation between experimental results and model outputs at constant speed of 2600 rpm observed in Figure 2 seems acceptable, however, it is not as exact as the case in Figure 1. Figure 3 shows that the methane effectiveness factor is always very close to unit, while, for carbon monoxide, the amount decreases severely at temperatures higher than 200 o C. It is logical that because of the low rates of methane conversion, diffusion in the catalyst pores is not the controlling step, while, due to high conversion rates of carbon monoxide which increases with increasing temperature, diffusion in the catalyst solid phase is the limiting step. As Figure 4 illustrates, at 60 seconds passed from the entrance of the flow into the cold converter, steady-state performance for the catalytic converter is reached. Decreasing trend of temperature across the converter is sign of the converter heating process. Figure 5 is dedicated to the effect of the converter length on the methane conversion. It is proved that the methane conversion is much more sensitive to the length of the catalytic converter [11]. These data are aquisited at 414 oC where the methane conversion is very low. The increase in the converter length is not desirable, in addition to higher pressure drop across the converter and space limitations.
Conclusion The natural gas/diesel dual fuel is a satisfactory alternative to the conventional diesel fuel from economical, technical and emission reduction aspects. High CO and HC emissions at low loads may be highly reduced with the aid of an oxidizing catalytic converter. A two dimensional transient model coupled with first order reaction rates for methane and carbon monoxide conversion created a proper adaptation with experimental results, while, sets of model predictions are presented for the improvement of the converter performance.
Acknowledgment The authors are grateful to Sharif University of Technology and Alternative Fuel Systems Inc. for their financial support and Professor M.D.Checkel for his technical guidance.
Nomenclature 4
av : gas-solid contact surface area per unit volume of the reactor (m2/m3) a(x) : active catalytic surface area per unit volume of the reactor (m2/m3) Ci : concentration of the i specie (mol/m3) Cpi : specific heat of the i specie (J/kg.K) Di : diffusion coefficient of the i specie (m2/s) Dh : channels hydraulic diameter (m) h : heat transfer coefficient (J/W.K) − H : heat of the reaction (J/mole) ki : thermal conductivity of the i specie (J/W.K) Kji : mass transfer coefficient of the i specie (m/s) L : monolith length (m) r : radial axis in cylindrical system (m) R : monolith radius (m) Ri : reaction rate of the i specie (mol/m2.s) Sh : Sherwood No. (dimensionless) t : time (s) T : temperature (K) U : superficial gas velocity (m/s) z : longitudinal axis (m) : void volume of the bed per unit volume of the reactor (dimensionless) : effectiveness factor (dimensionless) : density (kg/m3)
Subscripts and Superscripts g : gas in : inlet s : solid
References [1] Badakhshan, A. and Mirosh, E.A.; “The Multiple Diesel Dual Fuel Transport Engine; Its Merits in Energy Conservation and Environmental Protection”; The Second North American Conference and Exhibition “Clean Air 96”, Orlando, Florida, USA, 1996. [2] Karim, G.A.; “Examination of Some Measures for Improving the Performance of Gas Fueled Diesel Engines at Light Load”; SAE Paper No.912366, 1991. [3] Sallamie, N., Kazemeini, M., Soltanieh, M., Checkel, M.D., Badakhshan, M. , Zheng, M. and Mirosh, E.A.; “Catalytic Converter Performance in Dual Fuel (Natural Gas/Diesel) Engines”; CHISA Conference, Republic of Czech, 1998. [4] Sallamie, N., Kazemeini, M., Soltanieh, M., Checkel, M.D., Badakhshan, A. , Zheng, M. and Mirosh, E.A.; “Catalytic Converter Reducing Dual Fuel (Natural Gas/Diesel) Emissions”; Scientica Iranica (International Journal), Accepted for Publication, 1999. [5] Sallamie, N., Kazemeini, M., Soltanieh, M., Badakhshan, A., Checkel, M.D. and Zheng, M.; “The Performance of Catalytic Converters in Dual Fuel (Natural Gas/Diesel) Engines”; 3rd National Iranian Conference Of Chemical Eng., The University Of Oil Industry, Ahwaz, 1998. [6] Heck, R.H., Wei, J. and Katzer, J.R.; “Mathematical Modeling of Monolithic Catalysts”; AIChE, 22, 1976. [7] Sallamie, N., Kazemeini, M., Badakhshan, A. and Soltanieh, M.; “ An Investigation On The Kinetics Of Methane and Carbon Monoxide Conversions In The Dual Fuel (Natural Gas/Diesel) Engine Catalytic Converters”; 4th National Iranian Conference Of Chemical Eng., Sharif University Of Technology, Tehran, 1999. [8] Villermaux J. and Schweich, D.; “Is The Catalytic Monolith Reactor Well Suited to Environmentally Benign Processing?”; Ind. Eng. Chem. Res., 33, 1994. [9] Reid, R.C., Prausnitz, J.M. and Polling, B.E.; “The Properties of Gases and Liquids”; 4th Ed., McGraw-Hill, Singapore, 1988. [10] Zygourakis, K.; “Transient Operation of Monolith Catalytic Converters, A Two Dimensional Reactor Model and the Effect of Radially Nonuniform Flow Distribution”; Chem. Eng. Sci., 44, 1989. [11] Sallamie, N.; “Optimized Reduction Of The Dual Fuel (Natural Gas/Diesel) Engine Emissions Using Catalytic Converters”; Ph.D. Thesis, Sharif University Of Technology, Tehran, Iran, 1999.
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[12] Cundari, D. and Nuti, M.; “A One-Dimensional Model for Monolithic Converter: Numerical Simulation and Experimental Verification of Conversion and Thermal Responses for Two-Stroke Engine”; SAE Paper No.901668, 1990.
Table 1: Instruments Used for the Measurement of Important Parameters Measured Quantity Exhaust O2 Exhaust CO2 Exhaust CO Exhaust THC Exhaust NOx Dynamometer
Fuel Consumption Air Flow
Instrument Model and Serial Taylor Servomex OA.137 Beckman 864 NDIR Beckman 864 NDIR Beckman GC 72.5 FID Beckman 955 Chemiluminescent Mid-West MD1014W Eddy Current GSE Scale GSE 550 ASME Nozzle per SAE J244
Range
Resolution / Repeatability
0-25%
~0.1%
0-20%
~0.05%
0-1%
~0.01%
0-1.2%
5 ppm
0-0.2%
1 ppm
0-6000 rpm 0-200 KW 0-60 Kg
1 rpm 0.1 KW 5g
0-200 L/s
1 L/s
Table 2: Specifications Of The Monolithic Catalytic Converter Shape Cell Density Diameter Length Surface Area No. Of Cells/Cross Section Volume Hydraulic Diameter No. Of Pieces
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Cylinder 31 Cell/Cm2 15.24 Cm 15.24 Cm/Piece 182.41 Cm2 5655 1502.69 Cm3/Piece 0.149 Cm 2
a(x)=14000 m2/m3 a(x)=17000 m2/m3 a(x)=21000 m2/m3 100.00
CO
Conversion (%)
80.00
60.00
40.00
Methane
20.00
0.00 100.00
200.00
300.00
400.00
500.00
600.00
Temperature (Deg. C)
Figure 1: A comparison between model outputs and experimental data for CO and CH4 conversions at constant 1500 rpm
100.00
Experimental Data a(x)=14000 m2/m3 80.00
Methane Conversion (%)
a(x)=17000 m2/m3
60.00
40.00
20.00
0.00 300.00
400.00
500.00
600.00
700.00
Temperature (Deg. C)
Figure 2: A comparison between model outputs and experimental data for CO and CH4 conversions at constant 2600 rpm
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1.00
0.80
Effectiveness Factor
Methane CO 0.60
0.40
0.20
0.00 100.00
200.00
300.00
400.00
500.00
600.00
Temperature (Deg. C)
Figure 3: CO and CH4 effectiveness factor changes at the converter outlet
280.00
Temperature (Deg. C)
240.00
200.00
60 Sec. 160.00
30 Sec. 15 Sec.
120.00
80.00 0.00
0.20
0.40
0.60
0.80
1.00
Dimensionless Length
Figure 4: Model prediction for temperature profile across the converter
8
Inlet Temp.= 414 Deg. C 25.00
L= 0.6096 m L= 0.3048 m L= 0.1524 m
Methane Conversion (%)
20.00
15.00
10.00
5.00
0.00 0.00
20.00
40.00
60.00
80.00
Time (s)
Figure 5: Model prediction for methane conversion changes vs. time at different converter lengths
A Transient Model For The Performance Of Dual Fuel (Natural Gas/Diesel) Catalytic Converter 9
Sallamie, N.1, Kazemeini, M.2, Badakhshan, A.3, Soltanieh, M.2 and Estiri, M.4 1. Dept. Of Chemical Eng., Iran University Of Science and Technology, Farjam, Narmak, Tehran, I.R.Iran, Tel: ++9821-7896621 Fax: ++9821-7896620 E-mail:
[email protected] 2. Dept. Of Chemical Eng., Sharif University Of Technology, P.O.Box 11365-8639, Tehran, I.R.Iran, EMail:
[email protected] 3. Dept. Of Chemical and Petroleum Eng., University Of Calgary, Calgary, Alberta, Canada T6G 2G8 4. Air Quality Control Co., Tehran, I.R.Iran
Abstract Exhaust gas emissions from mobile sources and its important role in urban pollution has currently pushed much research towards alternative fuel systems for use in road vehicles. One of the best replacements is the natural gas/diesel dual fuel which offers an alternative to standard compression ignition diesel engines. In the dual fuel system, the primary fuel is natural gas which may replace as much as 90% of the diesel fuel, with a small amount of diesel fuel required to ensure effective ignition. Methane, the main constituent of natural gas, has a high combustion efficiency. Dual fuel engines produce lower NOx emissions and fewer particulates. However, at moderate engine loads, lower fuel combustion efficiency leads to significant hydrocarbons and carbon monoxide in the exhaust. Therefore, an oxidizing catalytic converter is necessary in the exhaust system to reduce these emissions to an acceptable level. Based upon experimental data gained with an Isuzu 4BE1 dual fuel engine, a transient two dimensional model is introduced in this work for the monolithic oxidizing catalytic converter, while at the same time considering chemical reaction and transport phenomena. It is shown that first order kinetics may well fit methane and carbon monoxide catalytic conversion. Furthermore, effectiveness factor is used to describe simultaneous mass transfer and reaction in the catalyst phase. Temperature and composition dependent physical properties are also employed. Predicted steady-state conversions are then compared with experimental data to verify the validity of the model. Finally, some extensions of the model, including the effectiveness factor changes and temperature profiles in the transient state are presented. Ultimately, it is concluded that the model is properly capable to describe the performance of the converter.
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