EJ-SEVIER Applied Catalysis B: Environmental 10 ( 1996) 139- 156 8: ENVIRCNMENTAL

Simultaneous control of particulate and NO, emissions from diesel engines

Jerry C. Summers, Stkphane Van Houtte, Dimitrios Psaras *

Rhbne-Poulenc, Inc., Cranbury, NJ 08512, USA

Received 5 October 1995; revised 24 November 1995; accepted 12 December 1995

Abstract

In view of increased concerns regarding the effects of diesel engine particulate and NO, emissions on human health and the environment, legislators are currently reviewing and proposing legislation targeting the reduction of these pollutants. The reported serious health risks of particulate matter on the respiratory system and its carcinogenic effects, along with the known contributions of NO, in acid rain and ground ozone formation, demand that the enacted legislation reflect in severity the health and environmental threats. As a consequence, diesel engine manufacturers and users are under increasing pressure to greatly reduce the engine’s exhaust emissions.

A system which is currently being proposed for the simultaneous control of diesel particulate matter and NO, emissions involves the use of a cerium fuel-borne catalyst/filter/EGR system. This paper describes the principles of operation of Rhane-Poulenc’s cerium fuel-borne catalyst and the factors that govern its use.

Keywords; Emissions control; Diesel engine; NO,; Particulate matter; Particulate filter; Cerium; Fuel additives; Particulate filter regeneration

1. Introduction

Diesel engines are the primary power plant of vehicles used in heavy duty applications. This includes buses, large trucks, and off-highway construction and mining equipment. Furthermore, diesel engines are winning an increasing share of the light duty vehicle market worldwide [l]. In Europe, for example, 100% of

* Corresponding author. Tel.: (+ l-609) 860-4508

0926.3373/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PZZ SO926-3373(96)00028-8

140 J.C. Summers et al./Applied Catalysis B: Environmental 10 (1996) 139-156

heavy duty, ca. 60% of light duty commercial vehicles and ca. 20% of passenger cars are diesel powered.

The popularity of the diesel engine revolves around its fuel efficiency, reliability, and durability. The high compression ratios, along with relatively high oxygen concentrations in the diesel combustion chambers, are responsible for the fuel efficiency and low CO and hydrocarbon emissions when contrasted to a comparable gasoline engine [2]. However, these same factors result in higher NO, emissions [3]. In some urban environments diesel powered vehicles are the largest mobile source of NO, pollution. For example, in California it is reported that heavy duty diesel trucks and urban buses are the largest NO, emitters on a per vehicle basis when compared to any other vehicle category [4].

The contribution of NO, to ozone formation is well known [5] and its regulation has prompted considerable research. Table 1 traces NO, legislated standards in the United States for on-road diesel powered trucks and buses. As this table indicates considerable reductions in NO, have been legislated and further reductions are anticipated. The latest proposals in California advocate a 2.0 g NOJBHP-hr standard (2.5 g/BHP-hr HC + NO,, 0.5 g/BHP-hr HC maximum) in the year 2004 [6].

Along with higher NO, levels, another emission characteristic of the diesel engine is its emanation of particulate matter. Particulate matter, by EPA definition, is any mass that collects on an exhaust filter for a specified engine or vehicle after a designated engine operating cycle and location at an exhaust temperature maintained at a constant 52°C [7]. This material consists primarily of carbon, upon which heavy hydrocarbons, sulfates and water are adsorbed [8-l 11. Their small physical size is described by the mass-median aerodynamic diameter which is reported to be in the range of 0.01-0.25 p,m with more than

Table I California and U.S. federal heavy-duty engine emissions standards

Standards

1984-87 Federal

Trucks (g/BHP-hr)

NO,

10.7

PM

N/A 1988-89 Federal 10.7 0.6 10.7 1990 Federal 6.0 0.6 6.0 1991 Federal 5.0 0.25 5.0 1991 California 5.0 0.25 5.0 1993 Federal 5.0 0.1 5.0 1994 Federal 5.0 0.1 5.0 1994 California 5.0 0.1 5.0 1996 Federal 5.0 0.1 5.0 1998 Federal 4.0 0.1 4.0 2004 Proposal 2.0 0.1

(2.5 HC + NO,)

Buses (g/BHP-hr)

NO, PM

10.7 N/A 0.6 0.6 0.25 0.1 0.1 0.07 0.05 0.05 0.05

J.C. Summers et al/Applied Catalysis B: Environmental 10 (1996) 139-1.56 141

75% smaller than 1 pm [12-141. This small size of the particulate and the adsorbed hydrocarbons have attracted considerable attention by the health agencies [ 151. Health studies indicating mutagenic activity by the particulate matter have prompted National Institute for Occupational Safety and Health (NIOSH) and International Agency for Research on Cancer (IARC) to declare diesel exhaust a potential and probable human carcinogen [ 16,171.

Various technologies have been developed to control particulate matter (Table 2). Diesel oxidation catalysts have proved effective in not only reducing CO and gaseous HC but also the heavy hydrocarbon species that condense or adsorb at 52°C on the collected soot [18]. Since sulfate also condenses on the sampling filter under these conditions, diesel oxidation catalysts had to be formulated so that their conversion of exhaust SO, to sulfuric acid was minimized [19-211.

Combustion chamber, injection timing, injection pressure and air induction modifications were used to reach the 1991 U.S. HD standard of 0.25 g/BHP-hr particulate matter and 5.0 g/BHP-hr NO, [22]. Further refinements of the 1991 engines, the development of sophisticated electronic controls, fuel injection systems, and in some cases the application of flow-through oxidation catalysts along with mandated lower sulfur fuel have brought the 1994 engines in compliance with legislation (0.1 g/BHP-hr particulate matter, 5.0 g/BHP-hr NO,). In 1998, the HD diesel engine emissions challenge reflects a tightening of the NO, standard to 4.0 g/BHP-hr. Further engine modifications and control technologies will be applied to meet these standards. However, as particulate matter and NO, emission limits are concurrently lowered, the NO, vs. particu- late matter trade-off becomes more prevalent [23]. A preferred method for resolution of this dilemma is a high performance and highly durable lean NO,

Table 2 Diesel emissions and some of their control technologies (diesel fueled engines only)

Emission Engine system changes Exhaust aftertreatment Fuel modification

HC Fuel delivery Oxidation catalysts a Reduced aromatics, increased cetane number b

CO’ - _ _

NO, Exhaust gas recirculation, Lean NO, catalysts, Fuel additives injection timing, fuel delivery filter/fuel additives improvements, intercooling

PM Fuel delivery, timing, engine Oxidation catalysts, Reduced sulfur, reduced component/combustion chamber filter/fuel additives, aromatics, cetane number modification, air induction filter/burner systems

Oxidation catalysts are used in European applications to meet a NO, + HC standard. In the United States, no HC control is currently needed due to the inherently low HC emissions from diesel engines. b Cetane Number depends on both, fuel composition and the presence of cetane improvers (additives). ’ CO emissions from diesel engines is inherently very low and their mediation has generally not been an issue.

142 J.C. Summers et al./Applied Catalysis B: Enuironmental 10 (1996) 139-156

catalyst technology [24,25]. In the absence of such a catalyst, fuel injection strategies such as injection retardation, high pressure fuel injection and EGR (exhaust gas recirculation) are all potential candidate technologies for achieving lower NO, and particulate matter levels (assuming alternate or mixed fuels are not considered) [3,26].

Limited EGR operation in the light and medium load regime can limit increases in NO, with minimal particulate matter and performance penalty [27]. Under heavy load conditions, the high quantities of EGR required result in high levels of particulate. These high levels cannot be lowered sufficiently by flow-through oxidation catalysts [28]. Consequently, more drastic particulate matter control technologies need to be considered and applied. A technology which has been proven to greatly reduce particulate emissions is the particulate filter [29].

In order to increase reliability and durability of the particulate filter, several techniques of filter regeneration have been proposed. They have ranged from forced regeneration methods such as in-line burners [30], electrically assisted regeneration [3 11, microwave regeneration [32], washcoat-type catalysts applied to the filter [33,34] and fuel-borne catalysts [35].

A fuel-borne catalyst which continues to show considerable promise in the diesel emissions reduction area is Rhone-Poulenc’s cerium based catalyst. This paper will describe the properties and application of this catalyst on an engine system targeting the simultaneous reduction of NO, and particulate matter.

2. Experimental

2.1. Microreactor experiments

Soot for microreactor experiments was generated by introducing a mixture of hydrocarbons, oxygen and argon through a cooled injector by way of a piston pump into a oven maintained at 1200°C. The hydrocarbon mixture consisted of 1-methylnapthalene, 2,2,4,4,6,8,8 heptamethylnonane and hexadecane in con- centrations of 142, 226, and 226 g/mol, respectively. Immediately following the oven, the combustion products were collected on a filter. For the cerium-doped soot, cerium-based additive was added to the feed hydrocarbons prior to their introduction to the pyrolysis unit. The reactivity of the collected carbon was measured by using a microbalance reactor. The reactor atmosphere was con- trolled by using an argon/oxygen feed.

2.2. Engine experiments

Engine experiments were conducted using a rebuilt DDC 6V92TA DDECII engine operating on a Chicago Transit Authority Bus Cycle using a dynamome-

J.C. Summers et al./Applied Catalysis B: Environmental IO (1996) 139-156 143

ter. Fuel feed to the engine was pre-mixed with cerium-based additive to achieve the desired fuel concentrations. Emissions were measured using an engine dynamometer and a full dilution tunnel using analytical methods prescribed by the U.S. EPA for heavy-duty engines.

2.3. Fuel-borne catalyst description

The cerium active component of the fuel-borne catalyst used in these studies is present in an organic matrix at a concentration of 33% (w/w). The additive has a specific gravity of 1.39 g/cc (at lS’C>, its Brookfield viscosity at 18°C is 150 m Pa s, its low-shear viscosity is 10 m Pa s and 6 m Pa s at 25” and 50°C respectively. Tests have shown it to be diesel fuel compatible with no reaction by-product formation.

2.4. Diesel particulate filters

A variety of diesel particulate filters and filter configurations are available in the market today. For the studies described in this paper, cordierite monoliths were used. A pictorial representation of its filtering mechanism is given in Fig. 1. The cordierite monolith filter is extruded in a manner that results in longitudinal honeycomb pattern channels with porous walls. Alternate channel plugs in the entrance and exit of the monolith force the exhaust flow through the porous walls which act as the filtering surface.

Fig. 1. Schematic of a cordierite honeycomb particulate filter (channels greatly exaggerated).

144 J.C. Summers et al/Applied Catalysis B: Enuironmental 10 (1996) 139-156

3. Results and discussion

3.1. NO, reduction

A combination of engine-related technologies for controlling NO, and partic- ulate emissions from trucks and buses have been and are being developed. Lower sulfur fuels, modified injection timing strategies, engine hardware modi- fications and oxidation catalysts have been used to meet 1994 U.S. heavy duty truck engine standards [36,37]. In 1998 when the on-highway diesel standards require a decrease in NO, emissions from 5.0 g/BHP-hr to 4.0 g/BHP-hr further engine hardware and software refinements will be used [38]. A technol- ogy which has seen rather limited application in 1994 light-duty diesel engines, but is expected to be more extensively used in the on-highway diesel engines in the future for NO, reduction, is EGR.

Generally, the application of EGR to a diesel engine decreases NO, emissions while at the same time it increases particulate matter [39]. This trade-off in emissions is more significant when attempts are made to achieve very low NO, emissions by increasing the EGR rates (Fig. 2) [40]. The relatively high rates of EGR required to decrease NO, to levels proposed by the State Implementation Plan (SIP) of California (2.0 g NOJBHP-hr standard beginning 2004), result in unacceptably high levels of particulate matter. These particulate matter increases cannot be brought to within legislated limits (0.1 g/BHP-hr) by the current oxidation catalysts.

3.2. Post combustion chamber particulate reduction

Typical 1994 light and medium duty engine particulate emission levels are in the range of 0.12 g/BHP-hr. Approximately 60% of this particulate matter is

, , 1 1

Closed Loop EGR + Improved Combustion + Oxidation Catalyst

~_

Closed Loop Cooled EGR + Improved Combustion + Oxidation Catalyst

Fig. 2. Effect of EGR on engine NO, and particulate emissions. Figure adapted from Ref. [41].

J.C. Summers et al./Applied Catalysis B: Environmental IO (1996) 139-156 145

composed of the soluble organic fraction @OF), while the remaining 40% is the insoluble organic fraction (IOF). Oxidation catalysts, mainly developed for the 1994 engines, affect only the SOF portion of the particulate matter, leaving the IOF component mostly intact. With efficient oxidation catalysts (SOF conver- sions > 60%), the 0.12 g/BHP-hr engines can be brought to within the legislated 0.1 g/BHP-hr 1994 particulate level. However, the same catalyst cannot lower the particulate matter to within legislated limits if the engine’s raw exhaust IOF emissions exceed 0.1 g/BHP-hr (Fig. 3). IOF exceeding 0.1 g/BHP-hr is predicted for engines which emit < 3.0 g/BHP-hr NO, [41]. In addition, in cases where EGR is used, the IOF fraction of the particulate is increased, while the SOF fraction is decreased. This seriously diminishes the effectiveness of diesel oxidation catalysts. To date, the only technology capable of such reductions in particulate matter is the diesel particulate filter (Fig. 3).

3.3. Particulate reduction using particulate filters

There are two objectives when using a diesel particulate filter to lower diesel particulate emissions. The first is to efficiently achieve the required filtering efficiency by collecting emitted particulate matter. The second is to prevent excessive backpressures by the filter, which will result in excessive fuel consumption and engine performance penalties (Fig. 4). The filtering objective can be met rather easily using a variety of filtering media. The low diesel engine operating temperatures along with high diesel particulate oxidation temperatures

0.16

E 0.14

& 0.12

B

? 0.1

.P

.$ 0.08 E w a, 0.06

iij

2 0.04 m

a 0.02

Fig. 3. Effects of oxidation catalysts and filters on particulate emissions. 1994 raw engine and 1994 + oxidation catalyst results are experimental results. EGR data are estimates. Filter efficiencies are based on data reported in Ref. [51].

146 J.C. Summers et al./Applied Catalysis B: Enuironmental 10 (19961 139-156

Fig. 4. Filter operation schematic

( > 420°C) [42,43] make the second objective difficult. In order to meet this objective, either the filter needs to be frequently replaced, or it needs to be continuously or intermittently regenerated. In practice, the costs associated with filter replacement are relatively high rendering filter regeneration as the only feasible method. Furthermore, the low diesel engine exhaust temperatures require that external means of filter regeneration be used. A method which has been proven effective in filter regeneration involved the addition of a cerium catalyst in the diesel fuel [44].

3.4. Oxidation of particulates collected in diesel particulate filters

The oxidation reactions of ‘hydrocarbons’ taking place during the regenera- tion of the diesel particulate filter can be generally described by the following equation ’ :

zC(,) + C,H, +x0, -+ gC0 + C,H, + CO, + wH,O + AH

The heat produced by the reaction per unit time, is dependent on the rate of the reaction. Although an oversimplification, this rate can be approximated by the Arrhenius expression:

&n/at = D, AeeEaiRT

In a static or low gas flow environment, the rate expression appears to be first order in feed oxygen concentration [43]. At higher 0, concentrations, the diffusion of oxygen to the reaction site is the rate limiting step (0,). As a consequence, the reaction rate appears to be independent of oxygen concentra-

’ Equation ignores any nitrogen containing compounds, elemental nitrogen, and oxygenated hydrocarbons present. Additionally, it combines the multitude of hydrocarbon species present into a single entity on the reactant and product side.

J.C. Summers et al./Applied Catalysis B: Environmental 10 (1996) 139-156 147

tion. At high temperatures, the apparent energy of activation (E,) is overcome by the temperature term (T) and the reaction is limited by oxygen diffusion (Do). In model reactor experiments, the apparent activation energy of the reaction (E,) has been measured to be between 160 and 172 kJ/mol [43]. This E, barrier is overcome and particulate oxidation can commence by raising the reactant temperature to values between 420°C and 600°C [42,43]. The wide range observed is due to uncertainties introduced by the variable compositional nature of the actual diesel particulate matter. The instantaneous heat experienced by the filter (Q,) can be described as the difference between the net heat release of the reaction (Q,) and the convective and radiative heat losses to the environment ( Qh).

aQ,/& = fiQp/dt - aQ,/&

In typical diesel exhaust conditions, an excess of oxygen is always present ( > 5% 0,; lean environment). The heat transferred to the filter during engine operation is a function of the apparent reaction rate and heat losses to the environment. Once ignited, a large amount of collected particulate results in a condition where the reaction rate is auto-accelerated by the increased energy available. In the presence of porous particulate matter (low 0, diffusion resistance), peak adiabatic temperatures in excess of 2500°C [45,46] can over- come the heat loss term (Q,) resulting in excessive heat buildup on the filter material (Q,). Such exotherms result in particulate filter failure [47].

During transient engine operation, parameters such as the exhaust oxygen concentration, exhaust flow and temperature and particulate morphology are defined by the demands placed on the engine by the operator. Ideally, once particulate oxidation is initiated, the reaction exotherms should be controlled by the quantity of particulate matter available for oxidation on the filter. This would be the case if the exhaust gas temperature is sufficiently high at frequent intervals to overcome the required activation energy and system thermal losses, thereby limiting the quantity of material available for oxidation. Unfortunately, typical diesel engine operation does not reach such exhaust gas temperatures.

3.5. Cerium assisted filter regeneration

3.5.1. Microreactor experiments: carbon oxidation; E, decrease Catalysts have been investigated as a method for lowering the activation

energy of the carbon and hydrocarbon oxidation reaction [48]. In gasoline and diesel oxidation catalyst applications, the catalytic material is applied directly to a relatively high surface area support which is then applied to the substrate support (typically a cordierite monolith). Although this method is applicable for gas-solid interactions, in the case of diesel particulate matter, direct interaction between the catalytic site and the particulate itself is limited to the solid-solid contact surface. This contact frequency is sufficiently low, that it results in

148 J.C. Summers et al. /Applied Catalysis B: Environmental IO (1996) 139-156

essentially no IOF reduction ( < 5%) when using washcoat-type oxidation catalysts [49].

An effective method for increasing the surface contact between the catalyst and particulate matter is the introduction of the catalyst in the fuel prior to, or in the combustion chamber. In doing so, the combustion process integrates the catalyst in the particulate resulting in extensive surface contact. Enhancement of the particulate matter/catalytic surface contact does not alter the energy of activation of the reaction beyond that of the previously described low dispersion case. However, the higher frequency of contact between reactants and the catalyst sites increases the integral heat released per unit time. Under conditions where this heat cannot be quickly dissipated, the localized system is non-isother- mal resulting in an increased apparent reaction rate.

Microreactor experiments on model CeO, containing particulate matter show a decrease in the apparent ignition temperature (inflection point of conversion vs. temperature plot in Fig. 5) upon the introduction of the catalyst. As it can be seen in Fig. 5, the incorporation of 3% (w/w> Ce decreases the ignition temperature of the particulate matter by 1OO’C (from 550°C to 450°C). Using the above described Arrhenius equation, the apparent energy of activation of the CeO, containing particulate matter is calculated to be 118-120 kJ/mol. This is a decrease of approximately 50 kJ/mol when compared to virgin particulate matter.

3.5.2. Engine experiments: efSects of fuel cerium on raw engine emissions The catalytic activity of fuel born cerium is first observed in the combustion

chamber. By monitoring the raw emissions of an engine operating on cerium

(no Cerium)

AF 400 500

Temperature ( “C)

Fig. 5. Microreactor experiments. Effect of cerium on the oxidation of carbonaceous matter. Adapted from Ref. 1521.

J.C. Summers et &./Applied Catalysis B: Enaironmental 10 (19961 139-156 149

loppm 125ppm 147ppm

Ceriumconcentration in Fuel

Fig. 6. Effect of cerium fuel concentration on the sum of raw engine particulate emissions after operating the engine for 10 consecutive Chicago Transit Authority Driving Cycles.

containing diesel fuel, one is able to see a drastic linear decrease (> 40%) in total non-cerium IOF with increased cerium concentration in the fuel (Figs. 6-8). The SOF content of the particulate matter is also decreased by 11%. At the same time, the sulfate emissions are unchanged. It can be speculated, that the observed high IOF conversion with the relatively low SOF conversion are direct results of particulate formation and measurement mechanisms. As the particulate forms inside the combustion chamber, cerium catalyst is also incorporated in its

??I

SOF (Lube Oil Derived Fraction)

0 I

0 50 100 150

Cerium Concentration in Fuel (ppm)

Fig. 7. Effect of cerium fuel concentration on the sum of raw engine SOF emissions after operating the engine for 10 consecutive Chicago Transit Authority Driving Cycles.

150 J.C. Summers et al./Applied Catalysis B: Environmental 10 (1996) 139-156

E 2 12 0 z 3 11

E .p t

10

‘E al

B

9

0 20 40 60 60 100 120

Cerium Concentration in Fuel (ppm)

140 160

Fig. 8. Effect of cerium fuel concentration on the sum of raw engine IOF emissions after operating the engine for 10 consecutive Chicago Transit Authority Driving Cycles.

bulk. As the particulate travels down the exhaust pipe and into the filter, additional SOF condenses on the surface of the particulate. The collected particulate, prior to its oxidation, may be exposed to higher temperatures which may drive part of the SOF off its surface. This SOF travels down the dilution tunnel and condenses on the high surface area of the small particulate which escaped the filtering medium and collected on the emissions collection and measurement filter. As a consequence, when oxidation of the filter collected IOF occurs, there is a disproportionate amount of IOF oxidized.

3.5.3. Cerium effects on filter regeneration The effect of cerium on the oxidation of particulate matter collected on

particulate filters is demonstrated in Fig. 9. There are two general regimes of

$ SO-

B 60-

J 40-

20 -

0 / I

200 300 400 500 600

Time (seconds)

Fig. 9. Effect of cerium on the regeneration temperature of carbon collected in a particulate filter 1531

J.C. Summers et al/Applied Catalysis B: Enoironmental 10 (1996) 139-156 151

50ppmCe 750 - TExhsusl

--- Tenocouple 1

" = 2000 rpm; pm.3 = 2.6 bar; TExhau - 250 deg. C

Time [sl 0

Fig. 10. Regeneration behavior of cerium containing particulate matter at 250°C stochastic point [53].

filter regeneration; the first, at temperatures above about 400°C and the second below this temperature. As it was discussed above, at temperatures greater than 400°C the apparent energy of activation for the oxidation of CeO, containing particulate is overcome. As a result, a continuous burning of the particulate is observed. At temperatures below 400°C oxidation of the particulate follows a stochastic regeneration profile. That is, particulate is collected in the filter until a threshold value is reached. At this point, partial regenerations occur which maintain the filter to essentially an equilibrium loading condition (Fig. 10).

To date, there is not sufficient data to explain this phenomenon. One may speculate that the variability in diesel particulate matter composition, in combi- nation with the cerium catalyst results in hydrocarbon material light-off tempera- tures which are well below 400°C. Given sufficient concentrations of these hydrocarbons and a high reaction rate, sufficient exotherms are generated to continuously burn the collected particulate. This speculation is supported by a number of studies of diesel particulate matter which show a wide variability of SOF composition, including fuel and lube type hydrocarbons whose composition depends on the type of engine used and its operational regime.

3.6. Collection of residual CeO,

Following particulate matter oxidation in the filtering media, the ceria resid- ual ash is efficiently captured in the filters. The resultant backpressure increase due to the buildup of CeO, in the filter over time is largely a function of the filter volume, the cerium fuel content, and the collected CeO, apparent bulk density. During system design, the first two parameters (filter volume, cerium fuel concentration) are determined based on engine backpressure limits, and

152 J.C. Summers et al./Applied Catalysis B: Enuironmental 10 (1996) 139-156

catalytic activity requirements. The CeO, apparent bulk density, on the other hand, is determined by the conditions prevailing in the filter medium during regeneration. These can vary significantly from one driving situation to another. Table 3 compiles the apparent bulk densities and corresponding average CeO, crystallite sizes of powders generated in a series of engine, vehicle and laboratory studies. The first two entries of Table 3 were obtained from urban bus demonstration programs. The third entry was obtained from an engine dy- namometer study. The fourth entry in Table 3 (sample 4) generated under steady state operation was exposed to the highest temperature (650°C) prior to its collection on the particulate filter. Taking into consideration that this tempera- ture is well above the particulate oxidation light-off temperature, it is reasonable to assume that of the time the exhaust reached the particulate filter, it was mainly composed by gaseous emissions and CeO, with minimal, if any, non-metallic particulate matter. Under these conditions, there was limited oppor- tunity for exotherms of any consequence to occur, and the collected CeO, to sinter to a more condensed form. This hypothesis is supported by experiments conducted using oven heating as a source for sintering activation energy. As it can be seen in Table 3, significant sintering occurred when the 650°C CeO, generated under steady state engine operation was heated for four hour at 800°C (sample 4a) and 1000°C (sample 4b). The apparent bulk density increased dramatically (from 0.2 g/cm3 to 1.0 g/cm3). Examination of field data, shows that engines operated on different transient field schedules result in higher bulk densities (samples 1, 2, 3). This indicated that significant sintering occurs during field filter regeneration.

The impact of apparent bulk density on filter lifetime is illustrated in Table 4. Examination of the contents of this table shows the importance of apparent bulk density of CeO, on filter lifetime as a function of vehicle fuel economy and catalytic cerium fuel concentration. For example, a filter equipped medium heavy-duty truck with a fuel economy of 10 miles/gallon and with 25 ppm of Ce in the fuel will be 50% filled at 150000 miles if the collected CeO, has an apparent bulk density of 0.5 g/cm3 but only one fourth filled if the apparent bulk density is 1.0 g/cm3.

Table 3 Ceria apparent bulk density and crystallite size during variable operating conditions

Source of CeO, powders Apparent bulk density (g/cm3) CeO, crystallite size (A)

1. Athens bus field program 0.8 > 50 2. Lyon bus field program 1.2 44 3. Engine study - driving cycle 0.7 36 4. Engine study - steady state ( = 650°C) 0.2 15 4a. Steady state powder heated: 4 h, 800°C 0.5 44 4b. Steady state powder heated: 4 h, 1000°C 1.0

IO

25

25 0.5 50 0.75 37.5 1.0 25

25 0.5 20 0.75 15 1.0 10

J.C. Summers et al./Applied Catalysis B: Environmental 10 (1996) 139-156 153

Table 4 Filter lifetime as a function of CeO, apparent bulk density

Fuel economy Cerium content Apparent bulk density Percent filter filled after 150,000 miles (miles/gal) (ppm) (g/cm3)

4 25 0.5 _a

0.75 93.8 1.0 62.5

40 25 0.5 0.75 1.0

a The filter is completely filled at 120,000 miles.

12.5 9.4 6.3

3.7. Storage and delivery of the catalytic additive

Cerium catalyst fuel dosing can be achieved in three ways. The first method is for it to be introduced in all diesel fuel at the refinery stage. The second is for it to be mixed at the diesel fuel pump during vehicle fueling. The third is for it to be delivered to the fuel on-board the vehicle. Each method has its advantages and disadvantages. Addition at the refinery stage would be simplest, but would require creation of another diesel fuel grade. Mixing the catalyst at the vehicle re-fueling stage is convenient for fleet vehicle operation, but it is rather cumbersome for vehicles that are not centrally fueled. On-board cerium delivery is a very attractive method for all vehicles if cerium concentration is sufficiently high, so that the service requirements are minimized.

Traditionally, a cerium(II1) naphthenate compound was used as the additive. Although proven very effective on the centrally fueled Athens bus fleet [50], this form of cerium fuel-borne catalyst presents difficulties in its implementation. With approximately 6% cerium concentration in the naphthenate form, an on-board system would have to be serviced approximately every 31000 miles. * A more acceptable cerium concentration is that of the EOLYS’” system. The EOLYS’” concentration is 30% w/w cerium. As a able to achieve 156000 miles on a single cartridge.

2 Based on a typical medium heavy-duty vehicle: 25 ppm [Ce] consumption; 3 1 additive cartridge.

result, the same vehicle is

in the fuel; 10 miles/gallon fuel

154 J.C. Summers et al. /Applied Catalysis B: Enaironmental 10 (1996) 139-156

4. Conclusions

The combination of effective EGR strategies for NO, reduction with effective ways to counter the resultant increases in particulate matter present a viable method for achieving an environmentally friendly diesel engine. Vehicles equipped with a fuel-borne cerium catalyst and a particulate filter enable the use of EGR as a credible method of NO, reduction. Although a pure filter system is able to meet this objective in the short term, cerium enhances its long term viability.

Simple microreactor soot oxidation experiments show that ceria catalyzes soot combustion. The catalytic effects of fuel-borne cerium are first observed in raw engine particulate matter decreases during the combustion of the diesel fuel. Furthermore, the catalytic effects are observed on filter collected matter as a considerable decrease in the ‘soot’ light-off temperature. The combination of these two features enhances the durability of the particulate filter by (a) decreasing the frequency of filter regeneration (lower particulate matter emission rate) and (b) by decreasing the particulate matter filter loading needed for the onset of equilibrium regeneration.

Finally, it is important to recognize that the introduction of a fuel component must meet fuel, fuel component and engine system component compatibility. The delivery of the catalyst must be very cost effective and a ‘turn-key’ operation, and the emissions from the engine must not introduce any new health and environmental dangers.

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