AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL RESEARCH © 2010, Science Huβ, http://www.scihub.org/AJSIR ISSN: 2153-649X doi:10.5251/ajsir.2010.1.2.203.212
Performance evaluation of silver-based catalytic filter system in a lightduty diesel engine Obodeh, O. * and Osuide, M. O. ** *Mechanical Engineering Department, **Chemistry Department, Ambrose Alli University, Ekpoma, Edo State, Nigeria. Email:
[email protected] ABSTRACT Experimental studies were carried out to investigate the performance of silver-based catalytic filter system (CFS) with regard to diesel exhaust emissions. The results were compared with those achieved with particulate filter (PF). The catalytic filter media consist of honeycomb substrate wash-coated with silver/alumina catalyst prepared by impregnation of commercial alumina beads with silver nitrate solution. Alumina rigidizer (Zircar ceramic, 99% alumina) was the binder used. The stable back pressure and high exhaust temperature observed on the engine when equipped with CFS indicate successful regeneration and satisfactory operation of the concept on the engine. Comparisons between the two emission reduction systems confirm that the CFS is effective at reducing diesel exhaust emissions except carbondioxide (CO2). As much as 51% reductions in particulate matter (PM), 42% in oxides of nitrogen (NOx), 63% in unburned hydrocarbon (HC) and carbonmonoxide (CO) were achieved with the CFS relative to the PF. The NOx reduction was observed in the same temperature range of the CO2 formation, implying the occurrence of the simultaneous removal of NOx and PM in an oxidizing atmosphere. It was shown that soluble organic fraction (SOF) and soot in PM are attributed to the reduction of NOx at lower and high temperatures, respectively. The oxidation of PM was enhanced by the coexistence of NO and O2. This method can be regarded as a combined process of PM trapping and catalytic reactions of PM oxidation as well as NOx reduction by SOF and soot. Therefore, it promises to be a desirable means of aftertreatment of diesel exhaust emissions. Keywords: Simultaneous Reduction, NOx and PM, Diesel Engine INTRODUCTION Early diesel catalysts used throughout the 1970s and 1980s, were simple oxidation formulations, virtually the same as those developed for gasoline cars. Certain disadvantages of these catalysts, such as increase sulphate and particulate matter (PM) emissions were gradually discovered by researchers (Sluder and West, 2000; Pfeifer et al., 2005; Qi and Yang, 2005; Arnby et al., 2005; Northrop et al., 2007). As a result, the overall air quality benefit from using diesel oxidation catalysts in enclosed space applications has been questioned (Northrop et al., 2007). The introduction of emission standards for highway diesel engines in the 1990s stimulated catalyst research, which led to the development of next generation diesel oxidation catalysts (Jobson, 2004; Johnson, 2006; Muller-Hass and Rice, 2006; Maus and Brück, 2007; Knecht, 2008; Johnson, 2008; Willems and Foster, 2009). These new catalysts, when used with low sulphur fuels ( ∠ 500ppm S), were able to control diesel particulate emissions in many diesel engine applications (Teraoka et al., 1995; Shangguan, 1996; Yang et al.,
1998; Miro et al., 1999; Noto et al., 2001; Ingramogunwumi et al., 2007). The first automotive application of diesel oxidation catalysts was the diesel-fuelled Volkswagen Golf “Umelt” in 1989 (Johnson, 2006). The catalyst was designed to reduce carbonmonoxide (CO) and hydrocarbons (HC) emissions. The use of a catalyst was not necessary to meet German or European emission regulations at that time. Rather, it was introduced as a sign of an environmentally responsible attitude of the car maker (Knecht, 2008). Since the introduction of the Euro II emission standards in the mid-1990s, the diesel oxidation catalyst has become a standard component in diesel fuelled cars in Europe (Johnson, 2006; Knecht, 2008). The task of the catalyst is to reduce the emissions of particulates, as well as to provide some reduction in CO and HC emissions (Knecht, 2008). Since the 1990s, there has been ongoing interest in the application of selective catalytic reduction (SCR) for mobile applications (trucks and diesel cars) (Delahay et al., 2005, Qi and Yang, 2005; Klingstedt et al., 2006; Kröcher et al., 2006; Rice et al., 2008;
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process for both NOx and PM emissions simultaneously (Kimura et al., 2002). Moreover NOx optimized engines offer less opportunity to optimize fuel efficiency and carbondioxide (CO2) emissions (Willems and Foster, 2009). A highly fuel efficient engine optimized for low PM will have significantly higher engine-out NOx levels, but in combination with an SCR system has the potential to achieve very low NOx emissions without degrading diesel fuel efficiency (Rice et al., 2008).
Willems and Foster, 2009). Since mobile applications of SCR would require a sophisticated active catalyst system and the availability of a reductant (urea) fuelling infrastructure, that technology was not the most attractive option for diesel trucks and cars (Schär et al., 2006). However, in the absence of more elegant oxides of nitrogen (NOx) reduction catalyst systems, SCR is an option for meeting future emission standards (Willems and Foster, 2009). Demonstration programmes showed that NOx emissions from existing heavy-duty Euro II engines (7g/kWh) can be reduced by SCR to levels below 2g/kWh i.e. the Euro V standard (Willems and Foster, 2009).
Willems and Foster, (2009) reported that SCR technology will enable European automobile manufacturers to comply with the Euro IV and Euro V emission standards (see Table 1) and at the same time, achieve fuel consumption levels 5 to 6% lower than those of equivalent Euro III engines. They have also demonstrated the potential for even lower emissions to meet future Euro VI limits. In addition, manufacturers are now starting to launch light-duty vehicles using this technology (Kim et al., 2009).
Whereas previous emission limits could be met by primary engine control measures, the limits for PM and NOx from diesel passenger cars in Euro VI represent a challenge for the developers of diesel engines and vehicles (Willems and Foster, 2009). Due to the thermodynamics of the diesel combustion process, it is difficult to optimize the combustion
Table 1 Present and proposed emission levels legislated in the EU for DI diesel engine passenger cars (for Euro I and Euro II emission levels, NOx values corresponds to the sum of NOx and HC emissions) Euro I
Euro II
Euro III
Euro IV
Euro V
Euro VI
(1992)
(1996)
(2000)
(2005)
(2009)
(2014)
NOx (g/km)
0.95
0.90
0.50
0.25
0.18
0.08
PM (g/km)
0.14
0.10
0.05
0.025
0.005
0.005
Source: (Willems and Foster, 2009)
2006). Catalytic filters combine surface filtration with catalysis (Sluder and West, 2000). They are capable of simultaneously removing sub-micron sized particulates from combustion gases and chemically reducing pollutants by catalytic reaction. The potential advantages of catalytic filters are those typical of multifunctional reactors (fewer process units, space and energy savings, cost reduction). Catalytic filters possess thermo-chemical and mechanical stability, high dust separation efficiency. In fact, the use of catalytic filter simplifies both maintenance and machine lay-out since no additional fluid is required (Min-Sun et al., 1998).
Nevertheless, urea- or ammonia SCR catalysts require an additional fluid to be tanked into vehicles and off-road machines. Particularly the latter ones work outside the ordinary infrastructure for long periods, which makes the maintenance demanding compared to on-road vehicles. For mobile sources such as vehicles, the toxicity and manipulation of NH3 together with problem of NH3 corrosion remain the major obstacles preventing the use of this technology (Liu et al., 2004). Again, Johnson (2008) points out that urea comes from natural gas feedstock and the processing involves an innate CO2 penalty. With a 5% urea consumption rate relative to fuel, the urea total contribution is said to be approximately 10 to 15%.
In this work the catalytic filter media consist of ceramic honeycomb substrate wash coated with silver/alumina catalyst. Ceramic honeycomb substrate provides a basis for enhanced catalyst support structures. As a catalyst support structure, it provides a way of constructing catalyst systems that are light weight, efficient and are thermally and chemically stable. The development of efficient, cost
During the last decade, the research on catalytic filter has been intensive and several catalytic formulations have been suggested. Among these, silver/alumina has shown high activity both in laboratory studies and in full-scale tests when applied to a diesel-driven passenger car (Arve et al., 2007; Klingstedt et al.,
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increasing the surface area of the substrate and improves catalyst binding (Pfeifer et al., 2005). The strength of the substrate is also enhanced by the washcoat.
effective diesel engine emissions control device is in great need in Nigeria to meet the stringent emission standards. The objective of this work was to develop a CFS for simultaneous chemical reduction of NOx and the removal of PM emissions from light-duty diesel vehicle and evaluate the performance of the system. Also to develop a system that is efficient, cost effective and would be able to replace in part emission control devices currently in use.
The CFS is made up of two chambers where the oxidation step in the inlet is followed by the soot collection/combustion process. The first chamber contains an oxidation catalyst consisting of ceramic honeycomb substrate coated with silver/alumina. Aside from oxidizing a portion of the NOx for soot combustion, the catalyst also oxidizes CO, HC and SOF portion of the PM. In the second chamber, the exhaust flows through a bare ceramic wall-flow filter. The filter is also a honeycomb design with alternate channels blocked at each end with ceramic material forcing exhaust to flow through the walls of the filter where gaseous components pass through and the soot is trapped (see Fig. 1).
Design and Operation of the Catalytic Filter A silver/alumina catalyst was prepared by impregnation of commercial alumina beads (A-201, LaRoche Industries Inc.) with a 0.022M silver nitrate solution of high purity (J. T. Baker) for 24 hours resulting in approximately 2 mass-% of silver in the catalyst. The catalyst was dried first at room temperature and therefore at 100oC followed by calcinations at 550oC for 3hours. The catalyst was wash-coated on ceramic honeycomb substrate. The primary reason for applying the catalyst to the ceramic substrate is to lower the regeneration temperature. Regeneration temperature is defined as the temperature at which the amount of PM being collected equals the amount being regenerated as measured by the change in pressure across the catalytic filter.
The trapped soot is then combusted by the NO2 generated by the catalyst. As PM accumulates within and on the porous walls of the ceramic, the backpressure on the engine increases. When the correct conditions are achieved, the PM burns and the backpressure decreases. This self-cleaning process is called regeneration. Typically, complete regeneration occurs when the temperature in the CFS exceeds 400oC (Liu et al., 2003).
Wash coating was applied to the catalyst support from water based slurry and the wet wash coated parts were then dried and calcined at 600oC. The wash-coat is a porous, high surface area layer bonded to the surface of the support. The binder used was alumina rigidizer (Zircar ceramic, 99% alumina). The washcoat improves performance by
The disassembled CFS is shown in Fig. 2. Its unit designed is 250mm in diameter and 400mm in length. It contained an oxidation catalyst and a wallflow filter in a ratio of 1:3 by volume
Fig. 1 Representative view of the magnified honeycomb structure with opposing channels blocked at alternate ends
.
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Fig. 2 Disassembled catalytic filter system
The size requirement is determined based on the engine power and emissions. All the modules are built with stainless steel and connected by V-clamps. The modular design provides easy access for filter maintenance at prescribed intervals.
was measured with EGA4 flue gas analyzer. The investigation consisted of two measurement series. In the first, particulate filter (PF) was fitted to the engine and in the second, the CFS was used. The engine speed was 3000rpm in all the experiment. All performance evaluations were carried out on stationary dynamometer test stand and at steady state conditions. The test procedures that were followed have been outlined by Kim et al. (2009).
MATERIALS AND METHOD: The engine used for this study was a modified Mercedes 1.7 litre directinjection, common rail diesel engine. The engine produces maximum torque of 180Nm from 1600 to 3200rpm. The maximum fuel rail pressure for the fuel system was 130MPa. Other specifications of the engine are as shown in Table 2. The specifications of the fuel used are also shown in Table 3.
Table 2 Test engine specifications
The CFS was prepared for laboratory evaluation by placing one 1.6mm diameter k-type thermocouple 25mm into the centre of the substrate face on both the inlet and outlet ends to measure temperatures. The thermocouples were installed through thermocouple fittings welded to the canister. A thinfilm strain-gauge based pressure sensors were installed at the inlet and outlet of the canister to monitor the pressure drop across the CFS. At the beginning of each test day, the instrumentation and engine were first warmed up. The engine was then set at the selected test speed and load and allowed to stabilize before taking readings. Exhaust emissions were measured with the aid of pocket gasTM-portable gas analyzer while PM
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S/No.
Specifications
Value
1.
Displacement
1.7litre
2.
Bore X Stroke
80 x 84mm
3.
Number of cylinder
4
4.
Piston bowl
Re-entrant
5.
Valves per cylinder
4
6.
Compression ratio
19.5:1
7.
Rated power
66kW @ 4200rpm
8.
Fuel system
Bosch common rail
9.
Injector orifice diameter
0.169mm
10.
Number of injector orifice
6
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Table 3 Specifications of the fuel used S/No.
Characteristics
Unit
Limit
1.
Specific gravity at 15/4
-
0.820 (max.)
2.
Distillation a) At 350oC
%wt
90 (min.)
b) Final boiling point (FBP)
o
385 (max.)
Colour
-
4.
Flash point
o
5.
Total sulphur
3.
C
3 (max.) C
66 (min.)
%wt
0.5 (max.)
-
No. 1 strip (max.)
Kinematics viscosity at 38 C
mm/s
1.6-5.5
8.
Cloud point
o
5 (max.)
9.
Carbon residue (conradson) on 10% residue
%wt
0.15 (max.)
10.
Strong acid number
mg of KOH/g
Nil
11.
Total acid number
mg of KOH/g
0.05 (max.)
12.
Ash content
%wt
0.01 (max.)
13.
Water by distillation
%vol.
0.05 (max.)
14.
Water by extraction
%wt
0.01 (max.)
15.
Diesel index
-
47 (max.)
6. 7.
o
Copper corrosion for 3hrs at 100 C o
C
Source: (NNPC, 2008)
The coexistence of NO and O2 in the exhaust gas also improve the catalytic activity for PM oxidation. This is probably due to the fact that NO2 is a better oxidant than O2. The fundamental role of the catalyst is to increase the oxidation rate of NO to NO2 and improve the formation of surface carbon species activated by adsorbed O atom denoted as C* [O] (Teraoka et al., 2000).
RESULTS AND DISCUSSION Fig. 3 depicts how PF and the CFS concepts reduced particulate matter (PM) emissions of the engine at 3000rpm. Relative to the PF, a reduction ranging from 35.7 to 51% was achieved with the CFS. By including catalytic agents, the temperature required to promote oxidation of PM is lowered to a more realistic level (Teraoka et al., 2000). This reduction in temperature requirement allows exhaust heat alone to promote regeneration of the filter. The lower temperature also reduces substrate failure (due to thermal stress) that is common with the supplementary heating techniques (Miro et al., 1999).
Fig. 4 illustrates the NOx emission rates at all studied loading points for both concepts. NOx reduction varied from 18.2 to 41.8%. The best performance of the CFS relative to the PF of 41.8% was achieved at idling (no load) condition.
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with hydrocarbon producing two N (ads) to form N2O. The other is the oxidation-reduction mechanism (Teraoka et al., 2000). NO reacts with oxygen to produce some reactive intermediates such as NO2, NO3 (ads), R-O-N=O, NCO and CN. These intermediates react further with hydrocarbons or partially oxidized hydrocarbons to form N2. Teraoka et al. (2000) and Liu et al. (2003) reported that most of the reactions occurring on metal oxide catalysts occur by this mechanism.
NOx is thermodynamically unstable and it can decompose to N2 and O2 which is the simplest approach to NOx removal. However, the decomposition reaction is inhibited by the high activation energy (Teraoka et al., 2000). Therefore, a catalyst is necessary to lower the activation energy in order to facilitate the reaction. Silver-based catalyst was used in this study because it is active for the direct decomposition of NOx at relatively low temperature. The main reactions occur as follows:
NO +
1 O 2 → NO 2 2
NO2 + 2C →
1 N 2 + 2CO 2
2CO + O2 → 2CO2
The soluble organic fraction (SOF) combustion also significantly contributes to the reduction of NOx. NOx is reduced by SOF (CxHyOz) at lower temperatures and by soot (C) at higher temperatures as follows:
(1)
(2)
C x H y O z + NO + O2 → CO 2 + N 2 + H 2 O
(4)
(3)
1 C + NO + O2 → CO2 + N 2
(5)
The mechanisms for NOx reduction can be classified into two categories. One is the adsorption/dissociation mechanism (Teraoka et al., 2000). This mechanism involves the adsorption of NO on the active metal oxide sites followed by dissociation into N (ads) and O (ads). O (ads) reacts
The results depicted in Fig. 5 showed that with the CFS, the HC reduction varied from 40.6 to 63% while those in Fig. 6 indicated that CO reduction varied from 41.2 to 62.1% relative to those obtained with PF.
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The O2- ion reacting as an oxidizer with the reducer (HC, CO) as follows:
( HC , CO) + O2 → CO2 + H 2 O
(6)
NO + ( HC, CO) → N 2 + CO2 + H 2 O
(7)
As illustrated in Fig. 7, at all loads, the use of the CFS increases CO2 emissions.
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Delahay, G.; Valade, D.; Guzman-Vargas, A. and Coq, B. (2005); Selective Catalytic Reduction of Nitric Oxide with Ammonia on Fe-ZSM-5 Catalysts Prepared by Different Methods, Applied Catalysis, Vol. 55, No. 2, pp. 149-155.
The CO2 emission was higher by 5.8 to 7.1% compared with the PF concept. CO function as the NO reductant over the catalyst surface according to the following reaction
2 NO + 2CO → N 2 + 2CO2
(8)
Ingram-Ogunwumi, R. S.; Dong, Q.; Murrin, T. A.; Bhargava, R. Y.; Warkins, J.L. and Heinel, A. K. (2007); Performance Evaluation of Aluminum Titanate Diesel Particulate Filters, SAE Technical Paper No. 2007-01-0656.
Equations (3) to (8) are responsible for the higher CO2 emissions level with the CFS. The exhaust temperature was above 275oC for about 50% of the operating time. This temperature profile is very favourable for successful regeneration of the catalytic filter and the results indicated that the CFS would satisfactory operate on a light-duty diesel engine.
Jobson, E, (2004); Future Challenges in Automotive Emission Control, Topics in Catalysis, Vol. 28, No. 1-4, pp. 191-199. Johnson, T. (2006); Diesel Emission Control in Review, Proceedings of the 2006 SAE World Congress, Detroit, Michigan, April 3-6, 2006.
CONCLUSION
Johnson, T. (2008); Diesel Engine Emissions and their Control: An Overview, Platinum Metals Review, Vol. 52, No. 1, pp. 23-37.
The stable back pressure and high exhaust temperature observed on the engine when equipped with CFS indicate successful regeneration and satisfactory operation of the concept on the engine. Comparisons between the two studied concepts also confirm that the CFS is effective at reducing diesel exhaust emissions except for CO2. As much as 51% reductions in PM, 42% in NOx, 63% in HC and CO were achieved with the CFS relative to the PF.
Kim, J.; Park, S. W.; Reitz, R. D. and Sung, K. (2009); Reduction of NOx and CO Emissions in Stoichiometric Diesel Combustion using a 3-Way Catalyst, Proceedings of the ASME Internal Combustion Engine Division 2009 Spring Technical Conference, ICES 2009, May 3-6, 2009, Milwaukee, WI, USA. Kimura, S.; Ogawa, H.; Matsui, Y. and Enomoto, Y. (2002); An Experimental Analysis of Low-temperature and Premixed Combustion for Simultaneous Reduction of NOx and Particulate Emissions in Direct Injection Diesel Engines, International Journal of Engine Research, Vol. 3, No. 4. pp. 231-237.
In conclusion, the effect of simultaneous catalytic removal of NOx and PM on diesel particulate filter coated by catalyst has been confirmed. This method can be regarded as a combined process of PM trapping and catalytic reactions PM oxidation as well as NOx reduction by SOF and soot. Therefore, it promises to be a desirable means of aftertreatment of diesel exhaust emissions.
Klingstedt, F.; Arve, K.; Eränen, K.; Murzind, Y. (2006); Toward Improved Catalytic Low-Temperature NOx Removal in Diesel-Powered Vehicles, Acc. Chem. Res., Vol. 39, pp. 273-282.
However, it is important to underscore the limitations of the approach taken in this study. Since these emissions indices were based upon fully-warmed, steady-state data, they do not include the important effects associated with cold-start and transient load/speed operation.
Knecht, W. (2008); Diesel Engine Development in View of Reduced Emission Standards, Energy, Vol. 33, pp. 264-271. Kröcher, O.; Devadas, M.; Elsener, M.; Wokaun, A.; Söger, N.; Pfeifer, M.; Demel, Y.; Mussmann, L. (2006); Investigation of the Selective Catalytic Reduction of NO by NH3 on Fe-ZSM-5 Monolith Catalysts, Applied Catalysis, Vol. 66, pp. 207-215.
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