Diesel Particulate Emission Control

PROCESSING TECHNOLOGY ELSEVIER

Fuel Processing Technology 47 ( 1996) 1-69

Review article

Diesel particulate emission control John P.A. Neeft, Michiel Makkee *, Jacob A. Moulijn Department of Chemical Process Technology, Delft University of Technology, P.O. Box 5045,26W The Netherlands

GA De@.

Received 30 June 1995; revised 6 October 1995; accepted 6 October 1995

Abstract This paper reviews the emission control of particulates from diesel exhaust gases. The efficiency and exhaust emissions of diesel engines will be compared with those of otto engines (petrol engines). The formation of particulates (or “soot”), one of the main nuisances of diesel exhaust gases, will be briefly outlined. The effects of various emission components on human health and the environment will be described, and subsequently the emission standards for particulates and for NO,, which have been introduced worldwide, will be summarized. Possible measures for reducing exhaust emissions of particulates and NO, will be discussed, such as the use of alternative fuels, modifications to the engine and the use of aftertreatment devices. It will be made clear that aftertreatment devices may become necessary as diesel emission standards become more stringent, in spite of important progress in the other fields of reducing exhaust emissions. Selective catalytic reduction via hydrocarbons, ammonia or urea, a possible aftertreatment method for NO, emission control, will be discussed briefly. Filters for collecting particulates from diesel exhaust gases will be examined in more detail and aftertreatment control systems for particulate removal will be reviewed. These can be divided into (i) non-catalytic filter based systems which use burners and electric heaters to bum the soot once it has been collected on the filter; (ii) catalytic filter-based systems which consist of filters with a catalyst coating, or filters used in combination with catalytically active precursor compounds added to the diesel fuel; and (iii) catalytic non-filter-based systems in which gaseous hydrocarbons, carbon monoxide and part of the hydrocarbon fraction of the particulates are oxidized in the exhaust gases. Finally, recent trends in diesel particulate emission control will be discussed, indicating the growing importance of the catalytic solutions: the fast introduction of non-filter-based catalysts for diesel engines and the possible application of filters in combination with catalytically active precursor compounds added to diesel fuel. Keywords: Diesel; Particulate

* Corresponding

emission

author.

0378-3820/96/$15.00 Copyright PII 0378-3820(96)01002-8

0 1996 Elsevier Science B.V. All rights reserved.

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Technology 47 (1996) 1-69

1. Introduction

The most common internal combustion engines used in western society are Otto and diesel engines. The combustion processes occurring in these engines are often incomplete, and undesirable by-products are formed. This can be attributed to the combined effect of high temperatures and, in the case of otto engines, low air-to-fuel ratios, or, in the case of diesel engines, combustion under substoichiometric oxygen conditions around evaporating fuel droplets. In Table 1 the abundant undesirable compounds found in the exhaust gases of these engines are compared with those found in flue gases of other common combustion processes such as gas, residual oil, and coal combustion for power generation, municipal waste incineration, and the burning of wood or coal in open fires or stoves. These emissions are typical for the technology available in the 1980s and early 1990s. The emissions of carbon monoxide (CO), volatile hydrocarbons (HC) and dust or particulates from internal combustion engines and small-scale burning of coal and wood are quite high compared with the emissions from the large-scale combustion processes used in municipal waste incineration and power generation. The emissions of nitrogen oxides (NO,) are of the same order of magnitude for all the combustion processes mentioned in Table 1. Apparently, the local temperatures reached in the hotter parts of the reactors are to a certain degree independent of the size and type of reactor. Recently, catalytic

Table 1 Typical emissions of combustion processes strongly upon process and fuel specifications co

(g per kg of fuel). The data are averages;

dust or particulates

HC SO,,

Power generation Gas ’ Heavy residual oil Coal

emissions

References

0.1-0.3 0.5-2 0.1-2

0.05-0.08 0.2-0.7 0.03-O. 1

2-4 5-10 l-10

0 15-30 S-20

0 1 0.05-2

t11

Municipal waste combustion

0.2-2

0.02-O. 1

1-3

0.5-1.5

0.05-0.5

L3.41

Wood or coal, open fires/stoves

20-120

2-50

l-5

2-10

l-20

[51

Diesel engine b Otto engine ’ Otto engine plus 3-way catalyst a

3-30 20-200 2-30

0.5-10 IO-50 0.5-5

5-20 lo-60 0.2-4

0.5-5 0.1-l 0.1-l

l-10 0.1-0.4 0.05-0.3

t6.71’ [6,7] ’

a Natural gas (with a low sulphur content). b Light-duty engines. ’ Fuel consumption in ref. [7] in litres per 100 km [S]; in this calculation be 0.75 kg I- ’.

the gasoline

depend

t11 lL21

l61

density was assumed to

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3

reduction of NO,r has been applied in power generation combustion plants, which decreases NO,Y emission typically by one order of magnitude. The emissions of sulphur oxides (SO,,) are proportional to the sulphur content of the fuel, and are therefore high for the combustion of residual oil, coal, wood and diesel fuel and low for the combustion of gas. The current values for oil and coal combustion in power generation and for municipal waste incineration are much lower in many cases because of the large-scale introduction of SO,V removal from flue gases. The small-scale processes, the burning of wood and coal in open fires and stoves and the use of engines for transport purposes, are most common in densely populated areas. It is therefore not surprising that these are held mainly responsible for the high concentrations of pollutants found in urban environments [5]. In Fig. 1 the contribution of traffic to overall emissions of CO, HC (including polynuclear aromatic hydrocarbons (PAHs)), NO,V, SO, and particulates is depicted. It is clear that traffic contributes to a large extent to CO, HC, and NO, emissions, and to a lesser extent to SO, and particulate emissions. These data apply to the Netherlands [9], however; for the USA similar data have been reported [lo], with typical traffic contributions to total pollutant emissions of 70% for CO, 50% for HC, 50% for NO,, 5% for SO,, and 20% for particulates. With growing urbanization and increasing numbers of cars, exhaust gas emissions started to pose a serious threat to air quality, particularly in urban areas. For this reason, in the USA, the 1970 Clean Air Act Amendments were issued, and standards were set for light-duty vehicles (passenger cars>. At the same time in Europe similar legislation was introduced. When the standards were tightened in subsequent years, catalytic convertors for otto engines became necessary. This led to the successful development and introduction of the three-way catalyst. Nowadays, in western society, practically all new petrol cars are equipped with a catalytic convertor, reducing CO, HC, and NO, emissions typically by 80-95%. These catalytic convertors consist of monoliths treated with an alumina washcoat and containing noble metal catalysts. Although high conversions have been accomplished, optimization of these catalytic convertors still continues: their performance during cold start behaviour is being optimized and the search for catalysts which do not depend on scarce and expensive noble metals has not yet ended. The successful introduction of three-way catalysts also influenced the attitude towards diesel engine emissions. As diesel exhaust gases had always been considered clean in comparison with otto exhaust gases (particulate concentrations were not legislated at the time), not much attention had been paid to catalysts for decreasing diesel engine emissions. After the introduction of the three-way catalysts and the subsequent tightening of emission standards to what was technologically possible for otto engines, the view on diesel exhaust gases had to be updated. Nowadays, CO and HC emissions from diesel engines are about the same as those of modem, catalytically equipped otto engines. However, the NO, and certainly the particulate emissions from diesel engines are much higher (see Table 1). For this reason, the search for particulate and NO, reduction techniques for diesel exhaust gases was initiated, and in 1982 particulate standards for diesel engines were introduced [ll]. As these standards have tightened over the years, and will tighten further in years to come, measures for reducing diesel engine particulate and NO,, emissions have become necessary.

-

!%

59%

15%

3%

Other*

*: combustion processes only

emissions

Power generation*

Industry*

Road traffic

BBlP recess

0

m

49%

NO (as NO&

Fig. 1. Relative contribution of emission sources to air pollutants. Source: CBS [9].

14%

-

aerosols (particulates) -

11%

2%

48%

Hydrocarbons

P

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5

Diesel emission control has been considered regularly over the last two decades. In 1991, Lox et al. [12] published an overview on the basics of the control of diesel engine emissions. In a more recent paper, Johnson et al. [13] gave a similar overview, followed by a review on the sampling of particulates and their soluble organic fraction (SOF), characterization of individual PAHs, and the effect that specific aftertreatment techniques could have on diesel emissions. Two review articles by Scheepers and Bos [14,15] focused on incomplete combustion products in general and PAHs in particular; the first deals with their origin, the second treats their toxicology. Diesel particulate emission reduction is a field of research in which disciplines such as automotive and chemical engineering, fuel processing and catalysis come together. The problem is complex, as these various disciplines contribute many parameters which all play a role, and it is therefore unlikely that a monodisciplinary approach could provide a satisfactory solution. The present review aims to be comprehensive for engineers and scientists from these disciplines, and to provide sufficient background information to bridge the gap between them. In this way, we hope to make a contribution to a multidisciplinary approach to the reduction of diesel engine emissions. The aim of this paper is two-fold. The first objective is to give an overview of relevant background information on particulate emissions from diesel engines, and the second goal is to provide a review on the reduction of these emissions. In Section 2, the two most common internal combustion engines, otto and diesel engines, will be briefly described and their exhaust emissions will be compared. In Section 3, the formation of diesel particulates will be briefly outlined. In Section 4, the effects of the emissions subjected to regulation, and of particulates in particular, on human health will be described. In Section 5, emission legislation on diesel particulate and NO, emissions will be discussed. The second objective of this paper, to provide a review of the reduction of particulate emissions from diesel engines, will be dealt with in Section 6. As the emissions of NO, and particulates are often found to correlate strongly, NO, emission reduction strategies will also be briefly discussed. A short summary will be given in Section 7. Finally, in Section 8 the current situation and future outlook will be outlined for emission reduction techniques with respect to diesel engines.

2. Diesel versus otto engines 2.1. Applications

Otto and diesel engines were first constructed at the end of the nineteenth century, and owe their names to their inventors, Nicolaus August Otto and Rudolf Diesel. Otto engines were successfully installed in cars in the beginning of the twentieth century. Diesel engines were more difficult to put to practical use owing to their more robust construction, which made them very heavy. At the time only trucks could be powered by diesel engines. Fuel injection posed another problem: the technology for injecting fuel at high pressures, at precise times and for precise durations, necessary for constructing a smoothly running, high speed diesel engine, was not available in those days. These early problems are still encountered: nowadays diesel engines are often used for large-scale,

J.P.A. Neefr ef d/Fuel

1980

Processing Technology 47 (1996) 1-69

1984

1988

1992

Fig. 2. Relative contribution of diesel, LPG and Otto fuelled engines in passenger cars in The Netherlands. Source: CBS [16].

stationary applications where high numbers of revolutions per minute are not needed. These engines are all of the direct injection (DI) type, and both four-stroke and two-stroke engines are used. In the automotive industry, diesel engines are mainly used in heavy-duty vehicles. These are also of the DI type, and have better fuel efficiencies than otto engines. Because of their more robust construction, the lifetimes of these engines are generally much longer than those of otto engines; lifetimes of 1000000 km for buses and trucks are not uncommon. Although Otto engines are more commonly used in light-duty vehicles (passenger cars), diesel engines can also be applied, which are in that case normally of the indirect-injection (IDI) type. In ID1 engines the fuel is mixed with air after injection and partially combusted in a prechamber before entering the main chamber for complete combustion. ID1 diesel engines run more smoothly than DI diesel engines, and emissions are generally lower. The main drawback of ID1 engines is an increased fuel consumption, although it is still considerably lower than the fuel consumption of otto engines. Because of lower fuel prices (partly because of favourable tax levels) and gradual public acceptance of diesel powered passenger cars, the percentage of diesel engines in passenger cars in Europe has increased steadily over the last decade. This is illustrated in Fig. 2 for the Netherlands [16]. It should be kept in mind that the total number of passenger cars increased during this period by 25%, resulting in an absolute increase of all car types. In addition, in the Netherlands the use of liquefied petroleum gas (LPG) is widespread compared with other countries. The percentage of diesel fuelled passenger cars also fluctuates strongly among European countries owing to different tax policies (from less than 1% in Sweden to over 30% in Belgium [ 121). 2.2. Fuel e$kiency One of the fundamental differences between otto and diesel engines is the way in which the reactants, air and fuel, are introduced into the cylinder. As a result, two important quantities in automotive engineering, the air-to-fuel ratio and the compression ratio, differ profoundly for the two types of engine.

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I

Table 2 Typical values for compression ratio, h and efficiency for diesel and Otto engines Engine type

Diesel

otto

Compression ratio A (air to fuel ratio) Effective efficiency a

16-24 1.1-6 0.30-0.45

7-10 0.9-1.1 0.25-0.30

a kWeffuent

mechanical

energy

p

r JCKnergy

of fuel burnt.

In otto engines, air and fuel are mixed before introduction into the cylinder. The air-to-fuel ratio is constant and is often chosen to be stoichiometric, i.e. about 14.6 on an air-to-fuel weight basis. The parameter A, which is defined as fhe ratio of air available to air required for complete combustion, is a commonly used measure for the

air-to-fuel ratio. Under stoichiometric conditions, h equals unity. The mixture is compressed in the cylinder and ignited by a spark plug. Combustion is fast, causing a high pressure peak. As the fuel must not ignite too soon, high-quality petrol has poor self-ignition properties. The pressure in the cylinder before spark-assisted ignition should not be too high in order to avoid self-ignition of the petrol-air mixture. As a result, the compression ratio cannot be designed higher than 10. The compression ratio is the ratio of the maximum (piston in lowest position) to the minimum (piston in highest position) combustion chamber volume.

In diesel engines only the air is compressed. Just before the piston reaches its highest position, diesel fuel is sprayed into the compressed air. The amount of fuel injected depends upon the load of the engine and, as the amount of air compressed in the engine is constant, the air-to-fuel ratio h varies between 1.1 and 6. As diesel fuel has good self-ignition properties, the fuel droplets ignite spontaneously. Fuel is injected over a certain time span, to avoid a too high a pressure. A result of the self-ignition of the fuel in diesel engines is that the maximum pressure in the cylinder is not limited by the fuel properties, as it is in Otto engines. Material properties determine the maximum pressures that can be employed. From thermodynamic considerations it can be deduced that the higher the compression ratio, the more efficiently the thermal combustion energy can be converted into mechanical energy. In practice, in addition to material properties, factors as friction losses and ignition behaviour play an important role in the choice of compression ratios for diesel engines. Typical values for the compression ratio, air-to-fuel ratio and efficiency are shown in Table 2. The efficiency is expressed as the fraction of the heat of combustion that is converted into useful mechanical energy (kW,,,,,,/kW,,,). From Table 2 it is clear that diesel engines are more efficient than Otto engines. This difference is caused by (in order of importance): (i) energy losses at the throttle valve, which controls the flow of the mixture of air and fuel into the cylinder in Otto engines (“pumping losses”); (ii> differences in compression ratio; and (iii) differences in air-to-fuel ratio. 2.3. Characterization of exhaust emissions The compositions of exhaust gases emitted from diesel and Otto engines differ considerably owing to the differences in combustion, as described above. Typical

8

J.PA. Neefr et al./ Fuel Processing Technology 47 (1996) l-69

Table 3 Typical diesel and Otto exhaust gas compositions: harmless compounds Compound

Unit

02

Vol%

CO, J-W

Vol% Vol% Vol%

N2

Diesel

otto

5-15 2-12 2-10 70-75

0.2-2 10-13.5 10-12 IO-75

exhaust gas compositions are listed in Tables 3-5, in which the combustion products are subdivided in three groups: harmless compounds, regulated harmful compounds (harmful compounds subjected to regulation) and unregulated harmful compounds. The first group is harmless in the sense that the compounds have no direct adverse effect on health. Of course, carbon dioxide does contribute to the greenhouse effect. 2.3.1. Harmless combustion products Typical concentrations of exhaust gas compounds of diesel and otto engines that have no adverse effect on health are listed in Table 3 [17]. Diesel engines run using a surplus of air, whereas Otto engines operate at near stoichiometric conditions, these being a prerequisite for three-way catalysts. It should be mentioned that research and development is being performed to develop so-called lean-burn engines: Otto engines that operate at higher values of A, with the potential of much lower NO, emissions. The surplus of air necessary for diesel engines is reflected in the 0, values given in Table 3 [17]. The lower limits given for CO, and H,O for diesel engines result from operation at idle or low load conditions, when only a small part of the incoming air is used for combustion. 2.3.2. Regulated emissions Emissions of compounds subject to regulation are listed in Table 4. Emissions of sulphur di- and trioxide depend upon the fuel sulphur content, which is legally restricted within maximum values. The maximum permissible sulphur content is currently 0.3 wt% in EC countries, but will be reduced by 1996 to 0.05 wt%, as it is in the USA [l 11. A concentration of 0.05 wt% sulphur in diesel fuel yields exhaust gas concentrations of 5-30 ppm SO,, depending on the air-to-fuel ratio.

Table 4 Typical diesel and otto exhaust gas compositions: regulated harmful compounds Compound

Unit

Diesel

co

Vol% Vol% “C ,” Vol% mgm-’

0.01-0.1 0.1-6 0.005-0.05 0.5-l 0.003-0.06 0.04-0.4 20-200 I-10 Proportional to fuel S content

HC NO, Particulates SO,

Otto

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Table 5 Typical diesel and Otto exhaust gas compositions: unregulated harmful compounds [6]. In mg per mile a Compound

Diesel

Otto

Aldehydes Ammonia Cyanide Benzene Toluene PAHs

0 2 1 6 2 0.3

60 4 10 80 240 0.2

a For 4-5 cylinder Volkswagen passenger cars.

Hydrocarbons comprise a large number of compounds, of which the low molecular weight hydrocarbons (methane, ethene, ethyne) occur in the highest concentrations [6]. PAHs are also present, though in much smaller concentrations. Diesel particulates are defined by the US Environmental Protection Agency (EPA) as “all compounds collected on a pre-conditioned filter in diluted diesel exhaust gases at a maximum temperature of 325 K (125°F)“. These particulates consist of soot nuclei (carbon) including inorganic material, adsorbed hydrocarbons (often referred to as SOF: soluble organic fraction), SO, (or sulphuric acid) and some water. A schematic representation of the composition is given in Fig. 3. The size of the individual soot spheres is 5: 25 nm and the size of the total particulates is = 200 mn, as will be discussed in Section 3. 2.3.3. Unregulated emissions Besides the harmful compounds limited to within maximum concetrations by emission standards, many other compounds which are also harmful are present in diesel and

Soot sphere (about 25 nm in diameter), including some anorganic material

0

Hydrocarbons adsorbed on microporous surface of soot sphere

??

Droplet of condensed hydrocarbons. On soot spheres as well as in gas phase

0

Droplet of sulfate and adhered water. On soot spheres as well as in gas phase

Fig. 3. Composition of diesel particulates. After Johnson et al. [ 131.

10

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47 (1996) 1-69

Otto exhaust gases. The most important ones are listed in Table 5. Aldehydes are held responsible for the typical diesel exhaust odour. PAHs, which are present partly in the gas phase and partly adsorbed onto the soot spheres, consist of a large number of compounds which originate from lube oil, diesel fuel, and the radical hydrocarbon degradation and formation processes occurring during combustion [18-211. These PAHs are thought to have adverse health effects, as will be discussed in Section 4. Many studies have been undertaken to identify individual PAHs in diesel particulates and in diesel exhaust gases (e.g. [6,18,22-29]), as some of these compounds have been found to be carcinogenic [15]. PAH emissions are reported to increase with increasing PAH content of the fuel 130-321, but also with increasing total aromatic fuel content [33].

3. Dynamics of soot formation Soot formation occurs in the high temperature, fuel-rich reaction zone around individual fuel droplets, where fuel hydrocarbons are oxidized under substoichiometric oxygen conditions. In this reaction zone the oxidation reaction is limited by the oxygen concentration. Oxygen transport occurs by diffusion through the flame front, and this type of reaction zone is therefore called a “diffusion flame”. Another flame type which occurs in combustion processes is the “premixed flame”; the combustion of a premixed amount of fuel and air. Temperatures in premixed flames are higher than in diffusion flames. As NO, formation depends strongly upon temperature and oxygen concentration (see, e.g., [34,35]), premixed flames give rise to much larger NO, emissions than diffusion flames. As studies on soot formation in diesel engine cylinders pose practical problems, many studies have been performed under model conditions. Diffusion flames in burners are often studied. Soot formation processes in burner diffusion flames do not fundamentaIly differ from those in diesel engines. Therefore, the most relevant results of studies on burner diffusion flames will be discussed in this section. The formation of soot is thought to take place via a number of elemental steps: pyrolysis, nucleation, sur$ace growth and coagulation, aggregation and oxidation. These processes take place on different time scales, ranging from a few microseconds (initial nucleation processes) to some milliseconds (completion of soot formation, oxidation, and cooling by cylinder expansion). This is shown schematically in Fig. 4 as a function of time; n is the total number of nuclei per cylinder volume [36]. Pyrolysis is the process in which gas-phase molecules form soot precursor molecules by free radical mechanisms. Two different types of pyrolysis can be distinguished: pyrolysis in oxygen-free reaction zones (often called “pure pyrolysis”) and pyrolysis in oxygen-containing reaction zones. Diffusion flame studies in burners at atmospheric pressure and relatively low temperatures indicate that pyrolysis is an order of magnitude faster in the presence of small amounts of oxygen (as O,, 0 - or - OH) than in oxygen-free diffusion flames (see [37,38], and references therein). This accelerating effect decreases with increasing temperature and with decreasing oxygen(air)-to-fuel ratio. In both types of pyrolysis it is generally accepted that both aliphatic and aromatic

J.P.A. Neeji et al./Fuel Approximate

Processing Technology 47 (1996) 1-69

number

of soot nuclei

11

present (-)

and coagulation

10 ’

10-b

10 5

10-e

10-3

Time after local nucleation Fig. 4. Particulate

10 2 (s)

number density as a function of time. After Smith 1361.

fuel molecules will first break down into olefins and then form acetylene [37-411. This molecule is generally thought to be a major soot precursor. Nucleation is the process in which soot precursor molecules grow into small soot nuclei. Nucleation is, in fact, a misleading term, as soot nuclei can be regarded as large PAH molecules [42]. Inception would therefore be a more appropriate name for this process [43]; however, “nucleation” remains the commonly used term. The oxidation of pyrolysed diesel fuel molecules takes place at high temperatures and at high concentrations of reactive compounds such as ions and radicals of hydrocarbons, 0 - and - OH. Under these conditions, the decomposition rate of soot nuclei is lower than their rate of reaction with other unsaturated, charged or radical hydrocarbons, resulting in a net growth of the soot nuclei. The reactions involved in this nucleation process have been studied extensively, and both radical-type and ion-type reaction paths have been proposed [37,41,44]. Smith [36] compared a number of studies and concluded that the initial number of soot nuclei in a diesel cylinder (at a pressure of 6.5 MPa) amounts to = 2.5 X lo*’ nuclei rnm3. Physically this translates to a density of 250 nuclei per cubic micrometre! Sur$ace growth is the process in which the precursor molecules grow from some l-2 nm to lo-30 nm. The most important reaction is thought to be the fast addition of acetylene or polyacetylene molecules by mechanisms similar to nucleation. The H/C ratio of the soot decreases during this process. This is the result of the addition of polyacetylene, which has a much lower H/C ratio than the original nuclei, and of dehydrogenation reactions [37]. The rate of soot formation during surface growth was reported to depend only upon the number of nuclei present [39]. Surface growth accounts for most of the soot formed (mass base) [43]. It occurs a few ps to 0.05 ms after the formation of nuclei. Simultaneously, another process takes place: coagulation. Small soot particulates collide and coalesce, forming larger, still more or less spherical particles. Although this process contributes to the growth of the particle, surface growth is still assumed to be the major growth process during this stage of soot formation [37]. Aggregation or chain-firming coagulation, which starts at 0.02-0.07 ms after nucleation [36], accounts for the formation of the well-known “fractal” structure of soot. This process occurs outside the cylinder; no chain-like structures are found by

12

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cylinder sampling [37]. The soot spheres, now of the order of 20 nm, collide at a rate proportional to the square number of spheres, and form chain-like structures with final dimensions of some hundreds of nanometres [37]. Oxidation of soot also takes place, lowering the soot tail-pipe emission. During surface growth and coagulation, oxidation reactions do not seem to play an important role. The main oxidizing species are reported to be * OH, 0 - , and 0, [37,44], although CO, and H,O may also oxidize soot to some extent [44]. As the amount of soot oxidized appears to be a function of temperature, time, and the concentration of oxidizing species [44,45], soot oxidation is thought to take place both inside and outside the cylinder. Khan estimates from a semi-empirical model that about 60% of the soot formed is oxidized ([36]). Lepperhof and Houben [45] report that up to 95% of the soot formed is subsequently oxidized. In the exhaust tail-pipe, exhaust gases cool down. Hydrocarbons of relatively low vapour pressure, sulphates and sulphuric acid plus bound water will condense on the soot. The resulting conglomerates are called particufutes; a schematic representation has already been shown in Fig. 3 in Section 2.3.2.

4. Adverse health effects 4.1. Regulated emissions

The adverse health effects of diesel exhaust compounds are difficult to assess; a large number of studies have been published, but still little is known about the effects that the compounds present in diesel exhaust gases have on human health. One of the reasons is that most studies concern effects on animals or cells. The fact that toxic effects of single constituents may be enhanced or inhibited by other components in the mixture further complicates the interpretation of the outcome of these studies. Ambient air concentrations of pollutants as CO, HC, NO, and particulates are higher in urban than in rural areas. As was depicted in Fig. 1, traffic contributes significantly to the emissions of these compounds. The higher concentrations may cause such adverse health effects as decreased lung function or lung cancer. A possible effect of air pollution on cancer mortality was examined in a number of studies comparing cancer incidenccein rural and urban areas, as reviewed in [46]. Cancer mortality in cities appears to be significantly higher than in rural areas. Effects on respiration were often studied during periods of extreme air pollution, when smog was formed. A distinction has been made between smog periods occurring in summer and in winter. Summer smog is formed during periods of high HC and NO, concentrations combined with intense sunlight. Photochemical reactions can cause the formation of very reactive compounds, of which ozone (0,) is considered to be biologically the most active. Health effects involve a decrease of lung function and irritation of the upper respiratory tract. The effects increase with exposure time and physical exercise, and affect not only people with decreased resistance but also healthy people. However, no

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13

increased mortality could be demonstrated [47]. Damage to vegetation also occurs at increased ozone concentrations [48]. Winter smog is formed during episodes of quiet and cold weather, because of the increased burning of fossil fuels for heating or power supply purposes combined with slow discharge of pollutants in the stagnant atmosphere. Large concentrations of SO, and particulates occur, which cause all kinds of respiratory problems, notably to people with decreased resistance [47]. In a limited number of extreme winter smog episodes, mortality increased statistically; e.g. during the smog period in December 1952 in London the excess death number due to smog was estimated to be 4000 [34,47]. Although they are now less severe, extreme smog periods in the 1990s are also suspected of causing increased death rates 1491. 4.2. Possible adverse health effect of particulates Diesel particulates are so small that they can penetrate the respiratory tract of the human or animal lung and are deposited in the pulmonary region of the lung, where they may cause adverse health effects. In outdoor air, diesel particulates contribute to the total concentration of so-called total suspended particulates (TSP). A fraction of this TSP, denoted PM-10 (roughly, the particulates smaller than 10 pm), enters the human lung. All diesel particulates can be classified as PM-IO. 4.2.1. Epidemiological studies A limited number of studies has been carried out assessing the effects of PM-10 or diesel exhaust gases on human health. It has been found difficult to correlate causes of death to exposure to diesel engine gases or PM-IO during a person’s life. It has been made plausible that a statistical relationship exists between exposure to diesel exhaust gases and particulates and a tendency towards lung disorders and a higher daily mortality [50-521. No evidence has been found, however, for an increased lung cancer mortality or a greater number of cases of lung disease in studies up to the early 1980s [50]. In a recent review by Mauderly [53] a large number of epidemiological studies has been reevaluated. These data “suggest that long-term employment in jobs with substantial exposures to diesel exhaust is associated with a 20% to 50% increase in risk for lung cancer” [53]. The risks of lower, more common exposures (e.g. in urban environments) are more difficult to assess. It is often assumed that the risk is proportional to the concentration of pollutant. These lower risks apply, however, to a large public; a significant part of the world’s human population is exposed to diesel exhaust and particulates. In addition to a possible effect on lung cancer, diesel particulates are also suspected of increasing bladder cancer. Mauderly reviews the epidemiological studies performed on bladder cancer increase, and comes to a conclusion similar to that found for lung cancer: “there appears to be a small positive risk for bladder cancer among long-term workers in occupations with high presumed exposures to diesel exhaust” [53]. 4.2.2. Laboratory studies When investigating the genotoxic effect of diesel exhaust gases, or of specific compounds occurring therein, a distinction can be made between in vitro and in vivo

14

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Fuel Proces.\rrr<~Tec~lrtrolo,~y47 f 1996) l-69

types. In vitro studies subject bacteria (the Ames test) or mammalian cells to diesel exhaust gases or extracts of diesel particulates (e.g. [31,33,54,55]), and in vivo studies expose live animals (rats, mice, and hamsters) to diesel exhaust gases (e.g. [53,56,57]). Such studies lead to the conclusion that diesel exhaust gases contain mutagenic substances, viz. PAHs and nitro-PAHs, and that diesel exhaust is definitively a carcinogen for rats. The results for mice were less consistent, and for hamsters no adverse effect was found at all [53]. Filtered diesel exhaust gases did not give rise to tumour development in any of the laboratory animals. The in vivo studies indicate that tumours develop only at very high particulate concentrations, typically in the order of lo2 to lo4 pg rnd3. These results cannot reasonably be extrapolated to the low concentrations of particulates of IO-’ to IO2 pg m -3 found in ambient air [48,53], as a threshold value may exist below which no mutagenic effects occur [53,58,59]. From bacterial and animal testing it can only be concluded that diesel exhaust gases are a probable human carcinogen [6Ol. More recently, it has been suggested that it is the carbon nucleus of the soot particulates which makes a significant contribution to tumour induction [61,62]. The relevance of this statement should not be underestimated. It would affect the strategies for reducing the emission of hazardous compounds by using aftertreatment devices which have recently been developed. High concentrations of other poorly soluble respirable particles (quartz, coal dust, titanium oxide, etc.) also cause increased incidence of lung tumour, which has been attributed to “stress” of the lungs [53,61]. At this time it is not yet clear by which mechanism lung tumours develop in exposed animals. The above mentioned stress of the lungs due to the very high particle concentrations may be a cause, and another possibility could be the chemical genotoxity of soot particulate-related compounds. Suggested mechanisms of carcinogenicity have been reviewed by Scheepers and Bos [15]. Mauderly argues that, as long as the mechanisms by which tumours develop in animals are not clear, it is very difficult to extrapolate the data from animal studies to potential human risks [53]. Specific mutagenic compounds found in diesel particulates, such as 1-nitropyrene [63-651 and benzo[a]pyrene [66,67], have also been studied. Indications exist that the polar fraction of PAHs, which mainly consists of oxygenated PAHs, accounts significantly for the mutagenicity of the diesel particulate extracts. Nitro-PAHs seem to contribute to a lesser extent to this activity [ 151. Benzo[ alpyrene is often used as an indicator for PAHs, of which several have been shown to be mutagenic or carcinogenic (e.g. [15,46,67]). 4.2.3. Risk estimates Finally, risk estimates for diesel exhaust exposure have also been made. Such studies, as reviewed recently [ 15,531, define so-called unit risks, which denote the probability of the development of cancer on a lifetime basis per particulate concentration in pg mm3 and which roughly range from 10e5 to lO-3 pg-’ m3, under the assumption that no threshold limit exists. Extrapolation of data from in vivo inhalation studies results in lower estimated unit risks than extrapolation of epidemiological data. This is surprising, as in the in vivo studies exposure concentration is much higher, as was described above, and which might cause additional effects. The estimated particulate concentration in the

J.P.A. Neej et al./ Fuel Processing Technology 47 (1996) 1-69

15

Netherlands is = 5 pg m-3 [151 (against 2-3 pg mm3 in the USA [53]), and the average life expectancy is 74 years [68l, resulting in an estimated cancer risk due to diesel exhaust in the order of 10m6 to 10e4 per year. In the Netherlands, a cancer risk higher than 10m6 per year is regarded as being unacceptable [67], therefore, particulate concentrations should be decreased. The concentration of benzo[a]pyrene is also estimated to be unacceptably high (leading to risks higher than 10m6 per year) [67]. 4.3. Other negative aspects Besides having adverse effects on human and animal health, diesel exhaust gases have other undesired properties. Diesel particulates contribute significantly to the soiling of buildings, as the “blackness” (light absorption properties) of diesel particulates, which in environmental studies are often denoted as “black smoke”, is much higher than the blackness of other fine particulates. Soiling is mainly an aesthetic problem, which cannot easily be quantified. This effect, however, is largely recognized by the general public [69]. In addition, this soiling can cause substantial damage to the materials exposed, which is due to a combination of chemical and polluting effects [48,67]. It is well-known that SO, and NO, contribute to acidification. Although diesel exhaust gases hardly contribute to total SO, emissions, their contribution to total NO, emission is substantial, as was described in Section 1. Acidification presents a major environmental problem on a continental scale, whereas the effects of diesel exhaust and particulates on health and on soiling are mainly local problems. Among other things, acidification contributes to the decline of forests, the damage to ecosystems (“death lakes”), and the reduction of ground water quality [48]. Finally, diesel particulates reduce visibility and diesel exhaust gases have an unpleasant smell, presumably caused by aldehydes present in the gases. 5. Legislation 5.1. General

In the early 1970s automotive exhaust emission regulations were introduced in both Europe and the United States of America. These first standards applied to light-duty vehicles, a general term for passenger cars. These standards, expressed in grams of regulated compound per mile, were so lenient that passenger cars could easily fulfil them. In the following years the regulations were tightened, leading to the development of the three-way catalyst. The development of light-duty vehicle emission standards from 1970 onwards is illustrated in Fig. 5 for NO, emissions in the USA and the state of California. The latter has the most stringent emission regulations worldwide. In the same figure NO,V standards for heavy-duty diesel engines are also shown, expressed in grams per brake-horse-power hour (1 bhp-h = 0.746 kWh). Standards for CO and HC show the same trend when a distinction is made between light-duty vehicle standards and heavy-duty diesel standards. Particulate standards were introduced in 1982 only for light-duty vehicles and not until 1990 for heavy-duty engines [ 111. Fig. 5 indicates the

16

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-

Processing Technology 47 (1996) I-69

Federal emission regulations Emission regulations in the state of California

3

cl

c^

1970

1975

1980

1985

1990

1995

2000

2005

3:

Year

5. Development of US-Federal and Californian NO, standards over three decennia. Standards apply for diesel and for otto engines. 1 bhp-h = 0.746 kWh.

Fig.

gradual tightening of emission standards over the years but also the difference in attitude towards Otto and diesel engines. Passenger car (Otto engines) NO, standards were tightened quickly up until 1982, whereas heavy-duty diesel standards were not introduced until 1984. However, by 1998 they will have tightened to a much greater extent than light-duty vehicle NO, standards, which will tighten further over the same period. The development in the tightening of diesel standards shadows that of Otto standards some 15 years later. From these numbers one can deduce that the development of diesel emission reduction measures follows the development of otto emission reduction measures at a delay of some 15 years. Besides setting standards, a reproducible way for measuring exhaust gas emissions as a function of engine load and speed had to be developed. Test cycles were introduced, in the form of speed and load patterns simulating engine behaviour on the road in different situations such as urban and motorway driving. Test cycles will be described in Section 5.5. Exhaust emission standards and test cycles differ from country to country and often per year. A short summary follows for current and future diesel particulate emission standards for the major regions of car usage and manufacturing (i.e. Europe and the US). A note has to be added concerning the need for emission standards for diesel particulates. Some governments appear to have issued more stringent emission standards because of scientific reports on the mutagenicity or carcinogenicity of diesel particulates. According to Stiiber [59], careless or even biased interpretations of toxicological or epidemiological studies seem to have led to an overreaction, at least as far as the relationship between lung cancer and diesel particulates is concerned. The concern shown, for instance, in a number of papers by Walsh (who wrote numerous such papers in the last decade, e.g. [70,71D can be ascribed to an incomplete interpretation of literature data. This does not mean that diesel particulate standards are superfluous. However, as St&er reasons, they should be based on a careful cost/benefit analysis that takes into account known or suspected health effects, overall nuisance (odour, visibility, material damage) and the increase of diesel engine usage.

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17

Table 6 Diesel particulate standards of the European Community [ 1I] Engine type Test cycle Unit of standard

Light-duty diesel vehicle ECE- 15 + EUDC gkn-’

Heavy-duty diesel vehicle ECE R-49 13-mode gperkWh

Implementation date a

Particulate standard

Particulate standard

NO, standard

0.36 b

9.0

0.15 0.12

7.0 5.0

1993 (Euro-I) 1994 (Euro-I) 1996 (Euro-II) 1999 (Euro-III) ’

DI 0.19 0.14 0.10 0.04

(HC+NO,) standard IDI 0.14 0.14 0.08 0.04

DI 1.36 0.97 0.9 0.5

ID1 0.97 0.97 0.7 0.5

a Implementation dates are summarized per year. For more detailed data see [ 111. Implementation dates are given for al1 cars (entry into service). New car types have to fulfil new limits earlier (type of approval date). b Higher if engine smaller than 85 kW, see [I 11. ’ Euro-III legislation has not been accepted yet. Implementation date and standards are anticipated values.

5.2. Europe Current and prospective diesel particulate standards for the member states of the European Community (EC) are listed in Table 6, together with the test cycles used. The emission standards operative in 1993 and 1994 are denoted as “Euro-I” standards, and the limits to be implemented in 1996 are referred to as “Euro-II” standards. “Euro-III” standards are still under discussion and will probably become effective in 1999. 5.3. United States of America In the United States of America, the 49 States have implemented the standards listed in Table 7. Because of greater air pollution problems, California has its own, more stringent standards.

Table 7 Diesel particulate standards in the USA [l

11

Engine type Test cycle Unit of standard

Light-duty diesel vehicle FI-P 75 g per mile b

Heavy-duty diesel vehicle EPA cycle g per bhp-h ’

Implementation date a 1987 1991 1994 1998

Particulate standard 0.2

NO, standard 1.0

Particulate standard

NO, standard

0.08

0.4

0.25 0.10 0.10

5.0 5.0 3.15 d

a Implementation dates are summarized per year. For more detail see [I 11.Implementation dates am given for all cars (entry into service). New car types have to fulfil new limits earlier (type of approval date). bl.6gpermiIe=lgkm-‘. ’ 1 gperbhp-h=l.34gperkWh. d HC+NO, standard.

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Table 8 Speed, load and relative weight factor of the 13 points in the European Mode

4 5 6 7 8 9 10 11 12 13

13-mode test

Speed

Load/%

Weighting

Idle Intermediate Intermediate

0 10 25 50 75 100 0 100 75 50 25 10 0

0.083 0.080 0.080

Intermediate Idle Rated Rated Rated Rated Rated Idle

factor

0.080 0.250 0.083 0.100 0.020 0.020 0.020 0.083

5.4. Other countries Many other countries have diesel emission standards, which are often derived from the emission standards for the EC, USA or Japan. In general, they are less stringent than the USA Federal emission standards. California has the most stringent standards in the world [ 111. 5.5. Test cycles

Vehicle emissions are determined in emission test cycles. A distinction is made between test procedures for light-duty vehicles and those for medium plus heavy-duty engines. In addition, different countries have their own emission test procedures. In Table 8, the test points of the European 13-mode test for heavy-duty engines are given; the emissions are measured at these 13 speed and load points, and then an overall emission is calculated using the weighting factors. In the USA, the heavy-duty emission test consists of a transient test cycle. For light-duty vehicles transient test cycles are used in both Europe and the USA. A transient test cycle prescribes the speed (and load) of a vehicle as a function of time. The vehicle is driven according to this speed pattern, and emissions are simultaneously measured. Test cycles vary as to whether or not a cold start forms part of the cycle and which typical driving conditions are emphasized. The US transient test cycle for heavy-duty engines focuses on metropolitan driving conditions, the underlying thought being that air quality in the USA is a human health problem mainly in large cities. These metropolitan driving conditions result in relatively low average exhaust gas temperatures and, therefore, in low NO, emissions. As can be seen in Table 8, in the European heavy-duty engine 13-mode test the two points at full load are heavily biased by high weighting factors, resulting in relatively high NO, data. The European heavy-duty test cycle is currently under review.

J.P.A. Neefr et al./ Fuel Processing Technology 47 (1996) l-69

6. Reduction

19

of emissions

Reduction of NO, and particulate emissions has become necessary from an environmental point of view, as was discussed in Section 4. In Section 5 it was outlined that the USA and European governments have set standards in order to force the relevant technology to be developed. Several techniques are promising for reducing emissions from diesel engines. Modified or alternative &els have been studied, trying to correlate emission levels with certain fuel specifications in order to optimize fuel composition towards low emissions. Some engine modifications have proved very effective in reducing diesel engine emission levels and have been implemented in recent years. Finally, after-treatment techniques have been studied extensively for NO,Vand also for particulate removal from diesel exhaust gases. 6.1. Changes in fuel

The use of alternatives for gasoline and diesel fuels has been studied extensively. Motives are multiple, and include the surplus agricultural area of Europe, a recycle of carbon in an attempt to diminish the emission of the greenhouse gas CO,, and a decrease of emissions of legislated compounds. A discussion of these motives would be beyond the scope of this review. The effects of alternative fuels on the performance and the emissions of diesel engines will be summarized; for costs, energy savings and CO, emissions of alternative fuels we refer to refs. [72] and [73]. Diesel engines can run on a large variety of fuels. Attention has been paid in particular to “normal” diesel fuels with a modified composition, to “biofuels” (vegetable oils and ethanol) and to compressed natural gas (CNG) and methanol. Alternative fuels can be used pure or mixed with normal diesel fuel. Modified vegetable oils such as rapeseed or sunflower oil can be used in diesel engines quite easily; the use of ethanol or methanol demands more adaptation, as the self-ignition properties of these alcohols are poor. Their cetane number is low, and consequently their octane number high, which means that application in Otto engines would be much more straightforward. 6.1 .I. Modi$ed diesel fuels The properties of diesel fuels can be varied in many ways. A large number of studies has been published on the effect of modified fuels on exhaust emissions. The most important properties, which can affect particulate emission, are thought to be the fuel sulphur content, the density, the volatility and possibly the cetane number [29,74-801. The effects are greater for DI engines than for ID1 engines. A higher fuel cetane number decreases the emissions of CO and HC in both types of engine, and also decreases NO, emissions slightly, although this effect is larger on DI engines than for ID1 engines [79]. However, increasing the cetane number by using cetane improvers does not seem to influence particulate emissions [78]. In a number of studies (e.g. Wall and Hoekman [78]), the aromatic content was also found to influence particulate emissions. However, in these studies the aromatic content was closely correlated to the cetane number and fuel density. Breaking this correlation, the fuel density was found to influence emissions

20

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Processing

Technology 47 (1996) 1-69

Particulates reduction, % of Euro-l limit

Sulphur

Density

Cetane

Aromatics Total Pdy

~!-IFJj~~

Property changes Fig. 6. Influence of fuel parameters on particulate emissions. Typical range of emission decrease (% EURO-I standard; 0.36 g per kWh) changing variable according to added values [SO].

to a much larger extent than the fuel aromatic content [79,80]. Other studies report a correlation between the di-, tri- or polyaromatic hydrocarbon fuel content and particulate emissions, while no correlation exists with the overall fuel aromatic content or with the monoaromatic content of the fuel [76,81]. Fuel properties can also be improved by hydrodesulphurization, to remove sulphur from the fuel, and subsequent further hydrotreating, to remove aromatic compounds. At the same time these two treatments increase the cetane number and decrease the density of the fuel. Fuels modified in this manner give lower particulates emissions [81], but reductions generally do not exceed 1.5%, as is depicted in Fig. 6 [80]. This figure shows the assessed particulate reduction, expressed as a percentage of the Euro-I limit (0.36 g per kWh for heavy-duty engines), for various changes made in fuel properties. It demonstrates that an important part of the particulate emission reduction must be ascribed to a reduction in sulphate when fuel sulphur content is decreased. It has been found that, independent of engine type and fuel sulphur content, between 1 and 3% of the fuel sulphur is converted to sulphate which is emitted as particulates [6,78-841. The rest of the fuel sulphur is mainly emitted as sulphur dioxide. Fig. 6 shows that a reduction of the fuel sulphur content from 0.20 to 0.05 wt% resulted in a 7-12% particulates reduction of the Euro-I limit. A decrease in fuel density from 840 to 800 kg mT3 similarly resulted in a decrease of up to 13%. A large variation in data was observed for different engines, and in some cases even a slight increase of particulate

J.P.A. Neeji et al./ Fuel Processin,q Teclvroio~y 47 (1996)

l-69

21

emission was observed. Increasing the cetane number by 5 units was found to result in a O-5% particulates reduction of Euro-I. Changing the total aromatic content had no influence, and for polyaromatics no definite conclusions could be drawn in this study, as the effects were found to be small and to differ for different engines [80]. The relatively large effect on particulate emissions by decreasing fuel sulphur levels was also reported in other studies. Van Beckhoven [77] found a lo-30% decrease in particulate emissions for DI engines after decreasing the fuel sulphur content from 0.30 to 0.05 wt%. The decrease was again reported to be much smaller for ID1 engines. Bartlett [75] reports an average 7% reduction in particulates for light-duty vehicles of both DI and ID1 type after a similar decrease in fuel sulphur content (0.30 to 0.05 wt%). The significance of the fuel sulphur level increases with decreasing emission standards. Assuming a SO,-to-SO, conversion of 2%, the effect of a decrease in fuel sulphur content from 0.2 to 0.05 wt% would result in a decrease in particulate emission by about 0.05 g per kWh [85], independent of the total particulate emission. This 0.05 g per kWh is about 13% of the Euro-I particulate standard and 33% of the Euro-II standard. Therefore, at low particulate emissions (that means, at low emissions of particulate constituents other than sulphate), the fuel sulphur level is the most important fuel parameter determining particulate emissions. It has been shown that the SO,-to-SO, conversion factor is independent of fuel sulphur level [84]. Fuel properties also have an effect on NO, emissions; higher cetane numbers were reported to have the largest effect in decreasing NO, emissions ([86] and references cited therein). The effect of fuel properties on NO, emissions is, however, lower than the effect on particulate and hydrocarbon emissions [77]. 6.1.2. Alcohols Owing to their poor self-ignition properties, methanol and ethanol cannot be applied as such in standard diesel engines. They can be used in four different ways, as follows. 1. Methanol or ethanol can be blended with diesel fuel. A distinction is made between unstabilized blends, which have to be made on board the vehicle, and stabilized blends, to which “stabilizers” are added to avoid separation of the two components. Examples of such stabilizers are “high order” Cs-C,, alcohol compounds [87] or naphtha [88]. The amount of methanol or ethanol in blends is limited owing to engine malfunction at high alcohol contents: 30 ~01% of alcohol in diesel fuel is reported to be the upper limit [89-911. The engine malfunctioning is caused by a decrease in cetane number when the alcohol content is increased. The diesel fuel provides the necessary self-ignition properties of the mixture, but at high alcohol contents a cetane improver has to be added to the blend. 2. Dual injection of diesel fuel and alcohol can be applied. The diesel fuel is injected first and then the alcohol. This method of injection is called pilot injection. The alcohol is ignited by the burning diesel fuel. In this case, higher alcohol-to-diesel fuel ratios are possible, up to 90% [89,92]. 3. Alcohol can be carburetted into the intake air. Again, at high alcohol-to-diesel ratios, problems are encountered for diesel engines. At higher loads the alcohol, which is compressed together with the intake air, ignites too early, despite its relatively poor ignition properties. This is because of the high compression ratios employed. The

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Alcohols Relative emissions,

diesel fuel = 100%

I 0 Experimental

n

300 -

data

Range of experimental

data

200 -

100 --

O-

I

co

n

HC

NOx

Particulates

Fig. 7. Emissions of CO, HC, NO, and particulates using alcohol fuels, relative to emissions using diesel fuel. Data from [87,89,95-981.

premature ignition is observed as engine knock. At lower loads, high alcohol contents result in misfiring and flame quenching owing to lower intake air temperatures as a result of the high heat of vaporization of the alcohols. The maximum possible alcohol content is reported to be 20% [89], although at medium engine loads higher alcohol contents of up to 50% (on a fuel energy basis) are possible [92]. 4. Pure alcohols have also been used, either in combination with an ignition improver, which is usually a nitrogen-based compound [92,93], or in combination with glow or spark plugs to ensure ;:nition. An advantage of the use of ignition improvers is that no extensive engine modifications are necessary. The only changes required ‘are a consequence of the lower volumetric heat of combustion of methanol or ethanol, due to which the volumetric fuel consumption increases considerably (by roughly 200% and 150% for methanol and ethanol, respectively) [92,94,95]. A larger fuel tank, fuel pump, and injection nozzle are needed. A drawback is that considerable amounts (typically 5-20%) of ignition improver are needed, which is expensive [92]. The exhaust gas emissions of methanol or ethanol fuelled engines are related to those of diesel fuelled engines in Fig. 7. Emissions fluctuate between different studies, and optimization of the engine with respect to general performance and emissions could still often be improved. Sometimes very high emissions of hydrocarbons, aldehydes, and CO have been reported. The aldehyde emissions, notably formaldehyde and acetaldehyde [95,97], are a matter of concern, as these compounds cause the typical smell of alcohol fuelled engines. In the Clean Air Act of the USA, a performance criterium for “Clean alternative fuels” has been adopted in which a formaldehyde standard of 0.15 g per mile was specified [l 11. Therefore, oxidation catalysts are a prerequisite for alcohol fuelled engines.

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23

As can be read from Fig. 7, the emission of CO tends to increase slightly using alcohol or alcohol/diesel fuels compared with standard diesel fuels; the emission of HC increases, the emission of NO, decreases and particulate emissions decrease significantly. 6.1.3. Compressed natural gas Diesel engines also have to be modified to use compressed natural gas (CNG). Spark plugs are necessary if the engine is to run on gas alone, but only slight modifications of the diesel engine suffice if dual fuel operation is to be used. Injected diesel fuel ignites the natural gas which is carburetted into the intake air and therefore premixed. Although the use of CNG in combination with diesel fuel slightly reduces particulate and NO, emissions, CO and HC emissions are greatly increased [99-1021. From an emissions point of view, the use of pure CNG seems more beneficial. Preliminary results from a CNG city bus project in the Netherlands indicate that modified diesel engines with spark ignition and fuelled by pure CNG have very low emission levels for all components [102]. A recent paper [lo31 corroborates these findings; stoichiometric combustion of CNG in combination with a three-way catalyst in trucks and buses resulted in emissions which were much lower than the Euro-II limits. Fuel consumption of these spark ignited engines run on pure CNG is lower than that in comparable Otto engines, but higher than in diesel engines [loll. 61.4. Vegetable oils The use of vegetable oil requires fewer modifications to the engine or fuel than does

the use of alcohols. Diesel engines run smoothly on vegetable oils, as these fuels have similar properties to diesel fuels. However, the higher viscosity of vegetable oils causes deposits and malfunctioning of the engine in the longer term. For this reason vegetable oils are modified via transesterification (reaction with methanol or ethanol at = 325 K over an alkali hydroxide catalyst), breaking down the triglyceride bonds and yielding methyl esters of the oils and glycerol, which is subsequently removed from the fuel [73,104,105]. Rapeseed oil is converted in this way to rapeseed oil methyl (ethyl) ester (RSME). Sunflower oil, soya bean oil and many other oils can also be transesterified and used in this manner in diesel engines. Emissions of regulated compounds for RSME are shown in Fig. 8. The emissions of CO do not change remarkably when using RSME instead of diesel fuel. A slight overall decrease can be seen if the results of a number of studies are considered together, as is done in Fig. 8. Hydrocarbon and particulate emissions decrease, whereas NO, emissions slightly increase. Data in Fig. 8 refer to hot-start driving cycles; particulate emissions using RSME were found to increase much more in cold-start driving cycles, resulting in an overall cold-start increase of particulate emissions of 105165% compared with diesel fuel [72]. 6.1 S. Practical applications of alternative fuels

Although a large number of studies have revealed the potential use of alternative fuels, practical applications are still scarce. On a small scale, rapeseed methyl ester is being used in some European countries (in Austria it has replaced about 5% of the total

24

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Processing Technology 47 (1996) 1-69

Vegetable Relative emissions,

oils

diesel fuel = 100%

2ooj,,::,,....1...

co

HC

NO,

Particulates

Fig. 8. Emissions of CO, HC, NO, and particulates using vegetable oil fuels, relative to emissions using diesel fuel. Data from [72,104,106,107].

diesel fuel consumption [73]). In Brazil an extensive project called Pruulcool has been undertaken, in which ethanol replaces gasoline. Large government involvement underlies these programs; low oil prices preclude projects being initiated without government involvement. At current prices both ethanol and vegetable oils are more expensive than diesel fuel [72]. Methanol is the cheapest alternative fuel, its price per amount of heat of combustion being close to the prices of oil-based fuels [89,94]. However, as methanol is not a renewable fuel (being made from coal or natural gas), the interest in ethanol is greater. In general, therefore, vegetable oils, alcohol or dual fuelling (diesel plus alcohol, vegetable oil or CNG) cannOt be expected to be used on a large scale in the near future. Specific, subsidized applications in areas with high population densities must however not be ruled out; the use of CNG or alcohols in buses certainly has potential. A small but significant reduction of diesel regulated emissions can be achieved by modifications in diesel fuels. In particular, lower diesel fuel sulphur contents would help to reduce particulate emissions. Maximum sulphur levels, as permitted by legislation, are decreasing (0.05 wt% in the USA since October 1993, and in Europe the current 0.2 wt% will be lowered to 0.05 wt% from October 1996 [ll]). Even lower levels, down to 0.005 wt% or less, are feasible, as has been demonstrated in Sweden [108]. This was, however at the expense of an increase in fuel price (considerable subsidies were also necessary in Sweden [l 1I) and inferior lubricating properties of the fuel. 4.2. Engine modijcations In 1981, when diesel particulate standards were first proposed, it was generally believed that filter-based particulate removal systems would be developed in time to

J.PA. Neef et al./Fuel

Carbon particulate

Processing Technology 47 (19%) 1-69

emission

25

(g/kWh)

with swirl 0.20 i

i

/

without

swirl

0.10

0.00 600

1000 Peak injection

1400 pressure

1800

at rated power (bar)

Fig. 9. Particulate emissions at different peak injection pressures. Adapted from Zelenka et al. [ 1091.

fulfil the future standards [109]. As will be discussed in subsequent sections, these filter-based systems stumbled on many practical problems. As a consequence, the redesign of engines is currently the main strategy to meet current emission standards. A number of modifications can be applied to decrease NO, and particulate emissions: Optimization of the combustion chamber geometry can be used to improve the formation of fuel-air mixtures. High air swirl rates are often employed [86,109,1 lo]. - Injection timing. NO, emissions can be reduced by retarding injection of the fuel. However, fuel consumption is increased in this way. The effect on particulate emission is subject to debate: some authors report an increase in particulate emissions [86,111,112], others a decrease [113-1151. Because of incomplete combustion as a result of the lower temperatures, hydrocarbon and CO emissions increase when injection is retarded 11131. In modem diesel engines, electronic injection control is often applied. Electronic control allows flexibility in the choice of settings for fuel injection. In this way, both particulate and NO, emissions can be reduced [ 109,116]. Higher injection pressures [109,1 lo]. The efficiency of combustion depends upon the degree to which the fuel droplets can be nebulized. As injection pressures become higher, finer fuel droplets will cause more efficient combustion, resulting in a decrease in particulate emissions and a slight increase in NO, emission because of the increased combustion temperatures [ 1171. The reduction in particulate emission is due mainly to a reduction in the solid (non-SOF) carbon fraction of the particulates [117], as can be seen in Fig. 9 (data are taken from Zelenka et al. [109]>. In this figure, the effect of swirl in the combustion chamber is also shown. Note that the particulate emission is expressed as carbon particulate emission, which is lower than the emission of total particulates (carbon + SOF + sulphates and bound water). Turbocharging. Owing to higher oxygen pressures, particulate emissions decrease because combustion is more efficient [ 1171. If intercooling is used, inlet air charge temperatures decrease, which decreases combustion temperatures and thus favours low NO, emissions [l 101. ??

??

??

26

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0.6 0.4

0.5 nf 0 .a =. * RI=

P

0.4 0.3

0.2 0.1

0.1

140 120

I

Intake manifold temperature

100 60 60 40 20 EGR

- %

Fig. 10. Effect of EGR and “cooled EGR” on NO, and particulate emissions and on intake manifold temperature 1861.Solid line, uncooled EGR; broken line, cooled EGR.

Oil consumption minimization. As the particulates partly consist of unburnt fuel and lube oil, important reductions in particulate emissions could be established if lube oil consumption could be decreased [ 109,117]. Exhaust gas recirculation (EGR). Partly recirculating the exhaust gases to the air intake reduces NO, emissions owing to the increased heat capacity of the combustion gases, causing a lower combustion temperature and a decrease in oxygen content of the combustion gases [112]. Particulate emissions, however, tend to increase, as do fuel consumption and engine wear. Application of EGR has, to date, been limited mainly to light-duty, ID1 vehicles [109,110,118]. Herzog et al. [86] report a much more moderate increase of particulate emissions if the intake manifold is kept at constant temperature, which they refer to as “cooled EGR”. An example is given in Fig. 10, showing NO, and particulate emissions and also intake manifold temperature as a function of the amount of EGR. Similar results are reported by Needham et al. [115]. Havenith et al. [114] showed that the effect of “cooled” EGR could be

J.P.A. Neeji et al./ Fuel Processing Technology 47 (1996) 1-69 Particulate

emission

(g/km

27

or g/kWh)

(0)

0

Starting-point (1)

(no measures)

-

Trade-off

curve by:

- increased EGR rates retarded injection timings (2) ----_. Shifted higher

wade-off

curve by:

injection

pressures

coaled EGR oxidation catalyst (ncs ISEfiO” 6.X.4)

t NO,-emission

Fig. 11. Schematic example of particulate-N4 measures.

(g/km

or g/kWh)

trade-off curves and the effect of several emission reduction

attributed to an effect on the air-to-fuel ratio; if the air-to-fuel ratio at higher EGR rates is kept constant by increasing intake air pressure, the favourite trade-off curves were also measured at higher intake manifold temperatures. The extent of these engine modifications is, however, bound by restrictions, as can be illustrated by the so-called trade-off curves existing for certain parameters. Measures for decreasing NO, emissions will unfortunately increase fuel consumption and particulate emission. An example of a trade-off curve for NO, versus particulate emission is given in Fig. 11. Starting with an engine without EGR at standard injection timing, NO, emissions can be decreased by increasing the EGR rate or retarding the injection timing. However, particulate emissions are increased by these measures. This results in the solid-line trade-off curve. Other measures, such as reduced oil consumption, higher injection pressures, cooled EGR or the use of an oxidation catalyst, shift the whole curve to the lower, broken-line trade-off curve. Similar trade-off curves exist for fuel consumption versus NO, emission. Currently, engine modifications such as optimization of the combustion chamber geometry, higher injection pressures, EGR, electronic injection timing, and intercooled turbocharging are used in combination with oxidation catalysts (which reduce the SOF content of the particulates) [119]. With one or with combinations of these measures both Euro-I and Euro-II standards can be complied with. Euro-II standards can be met for heavy-duty engines equipped with modem injection pumps and electronic injection control. Medium and light-duty diesel engines would become too expensive if similar measures had to be taken and, therefore, require further modifications to fulfil Euro-II standards [llOl. It is believed that Euro-III standards also can be met by a careful optimization of the above mentioned techniques, focusing on EGR [ 110,114]. Finally, it should be noted that the three most important automobile markets each have their own emission control. Engine types vary between the European, American, and Japanese markets, as do the test cycles in which the emissions are measured. For example [ 1101: on the Japanese market heavy-duty engines are usually naturally aspired engines, and the Japanese 13-mode test emphasizes emissions at light load operation of the engine. This would favour the use of so-called pilot injection, in which electronically controlled injection of a small amount of fuel takes place before the main fuel injection,

28

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causing shorter injection delays, less premixed combustion and, therefore, lower NO, emissions. In contrast, turbo-charged engines are mainly found on the European market, and in the European 13-mode test high-load operation is heavily weighted (as was discussed in Section 5.5. Ignition delays are therefore already short, causing pilot injection to be much less effective in meeting European NO, emission standards. 6.3. After-treatment The third option for reducing diesel exhaust gas emissions is to eliminate them after they have been formed. Catalysis plays an important role in this field, although non-catalytic methods can also be applied to reduce particulate emissions. As the CO and HC emissions from diesel engines are low and already fulfil standards, attention has been focused on the reduction of particulate and NO, emissions. As the reduction of these emissions is correlated by trade-off curves (Section 6.2, it is useful to discuss reducing both particulates and NO,. In the case of it proving possible to reduce only one emission by aftertreatment, both emission standards could still be met by optimizing the engine to decrease emissions of one compound and using aftertreatment to reduce the emissions of the other. This may, however, have implications for fuel consumption or engine performance, as was discussed in the previous section. 6.3.1. Removal of NO, The removal of NO, from diesel exhaust gases is a field of active research. Reduction of NO, in the presence of oxygen and water is an attractive reaction for aftertreatment purposes. Applications would be possible not only to remove NO, from the exhaust gases of diesel and lean-bum engines, but also to remove NO, from the flue gases of large combustion plants used for heating or power generation. Considering the options for removing NO, from exhaust gases, it is useful to discriminate between stationary and non-stationary NO, sources. Gas temperatures vary greatly between, for instance, different applications of diesel engines or other sources of NO, as power generation plants. On the one hand, the more or less steady exhaust gas temperatures of some large diesel engines make NO, reduction possible but, on the other hand, the temperature fluctuations in smaller and non-stationary operated diesel engines cause NO, reduction to be much more difficult. A number of catalytic reactions to reduce NO, in gases which contain oxygen have been studied. Catalytic decomposition of nitrogen oxide into molecular nitrogen and oxygen would be the most favourable reaction; however, the catalysts developed so far are deactivated by oxygen (reversibly) and by SO, (irreversibly) [120,121]. As a result, nitrogen oxides have to be reduced by chemical reactions with other compounds. Several reduction reactions are possible, although the oxygen content and the relatively low temperature of diesel exhaust gases make the use of non-catalytic reduction impossible [35,122]. Non-selective catalytic reduction (NSCR), as used in three-way catalysis in Otto engines, where oxygen is first removed by reaction with CO or HC and then NO, with the reductants, is not feasible owing to the excessive oxygen concentration of diesel exhaust gases. The option remaining is selective catalytic reduction (SCR). SCR has been studied using a number of reductants, of which carbon monoxide (CO), hydrocar-

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bons (HC), and ammonia (NH,) or urea (NH,-CO-NH,) are most often reported. SCR using CO and HC as reductants is sometimes referred to as NSCR. SCR using CO is difficult, as CO preferentially reacts with oxygen. NO, conversions of some tens of per cent have been reported, but only at low oxygen concentrations of typically less than 1 ~01% [123,124]. SCR using HC has received considerable attention. Although HC also reacts more easily with oxygen than with NO, under oxidizing conditions, this preference is less pronounced than in the case of CO. A three to four-fold surplus of hydrocarbons (expressed as ppm C,>, relative to the NO, concentration (ppm NO,), is necessary to obtain reasonable NO, conversions of the order of 50% [ 122-1271. As such a surplus of hydrocarbons is not usually present in diesel exhaust gases (see Tables 1 and 4 for typical diesel engine HC and NO, emissions), hydrocarbons, or diesel fuel, would have to be added to the diesel exhaust gases [126]. Engine modifications could also be undertaken, aiming at higher hydrocarbon emissions [86,128]. The catalysts studied are mostly of zeolite type, for instance copper exchanged ZSM-5 [124-127,129], cobalt exchanged ZSM-5 [130], and copper-mordenite [123]. C,, C, and C, hydrocarbons have been studied as reductants, and when ZSM-5 catalysts are used they perform much better than methane [124,126]. NO, conversions of 50% and higher are reported at oxygen concentrations in the range of O-10 ~01% [124]. The optimum temperature for the Cu-ZSM-5 catalysts is ~625-775 K [124,126,127]. An important drawback of these zeolite-based catalysts is that the NO, conversion decreases in the presence of water, and partial conversion can be maintained only at high hydrocarbon-to-NO, ratios [ 122,123,126]. Further deactivation in the presence of water occurs at elevated temperatures (Cu-Mor: above 875 K [ 1231; Cu-ZSM-5: 975 K [127]) owing to instability of the zeolites. Another drawback is the formation of N,O and HCN over Cu-ZSM-5 catalysts [127,131]. Recently, platina catalysts have also been reported to catalyse the reaction of hydrocarbons with NO,, reaching conversions of > 50% [ 127,132,133]. However, NO, conversions over alumina supported platina drop at elevated temperatures [ 1321, and also Pt/ZSM-5 catalysts deactivate quickly [133]. Furthermore, these catalysts are also active for SO, oxidation, resulting in increased particulate emissions [127]. SCR using ammonia or urea is a proven technology for large stationary combustion plants (power plants, heaters and boilers in the process industry). Although it is in principle not a very elegant technology, it is the only one which performs reliably in practice. In Japan, the USA, and Europe, large-scale application of SCR has been introduced in the last two decades [35,134], and Japan has taken a leading role in SCR technology since the 1980s [35]. Aftertreatment of NO, emissions in exhaust gases of Table 9 Important reactions in the SCR process with ammonia or urea as reagent 4NH, +4NO+O, 4NH, + 6N0 4NH, + 30, 4NH, + 50, H,N=CO=NH,

+H,O

+ 4N, + 6H,O + 5N, + 6H,O + 2N, + 6H,O --) 4N0+6H,O + 2NH, +CO,

(1) (2) (3) (4) (5)

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mobile diesel engines is more difficult because of the non-stationary operation. Furthermore it is unattractive, as a separate reductant would have to be carried on board. In large-scale commercial SCR, ammonia is used as a reagent. A number of reactions between NO and NH, in the presence of oxygen can take place; the main reactions are listed in Table 9 [122]. Reaction (1) is the most desirable reaction of stoichiometric amounts of NH, and NO,. At higher temperatures, reactions (2)--(4) also play a role, causing a deviation from the NO/NH, stoichiometry. Reactions producing N,O (nitrous oxide) are also possible [122]. Although V,O, catalysts, supported on TiO,, are widely used for this reaction (e.g. [135-137]), zeolite type catalysts have also been studied and applied. An extensive review on SCR catalysts has been published by Bosch and Janssen [351. SCR using ammonia has been studied for non-stationary diesel engines, but as ammonia poses health and practical problems (ammonia is a toxic gas that has to be stored under pressure), an alternative in the form of urea is generally proposed [ 122,137-1411. Urea, a solid which is highly soluble in water, can be injected as an aqueous solution into diesel exhaust gases, where it decomposes according to reaction (5) in Table 9. High NO, conversions have been reported using ammonia or urea in diesel exhaust gases over copper exchanged ZSM5 [122,135,136], cerium exchanged mordenite [ 142,143] and vanadium type catalysts [ 135,137]. Catalysts were found to store considerable amounts of NH, under the reaction conditions, which results in rather slow responses to temperature changes or changes in NO, concentration [ 1351. In addition, the NH, storage behaviour of catalysts changes with changing temperature, causing unwanted ammonia slip at fast temperature increases [135]. If an oxidation catalyst is used downstream to the SCR catalyst, NH, slip can be efficiently eliminated without decreasing the NO, conversion [ 1361. Work in our own laboratory showed that this NH, is converted very selectively into N,; no increases in NO, NO,, or N,O concentrations were measured [144]. Sharp changes in NO, concentration can be avoided without the occurrence of NH, slip or N,O formation by continuous overdoses of urea [145]. SO, oxidation is, however, enhanced if an oxidation catalyst is used. A Ce-mordenite catalyst has been developed which exhibits very low SO, oxidation properties even at higher temperatures (up to 820 K), while NO, conversions are still reported to be high and up to 20% excess NH, could be used without NH, slip occurring [ 143,146]. The stability of this catalyst is the subject of further research. A number of other problems have also been reported for SCR in diesel exhaust gases. The use of V,O,/TiO, catalysts is, for instance, limited due to SO, oxidation (and subsequent sulphate emissions) at temperatures above 675 K [ 1351. At low temperatures, on the other hand, side-products are formed which can deactivate the catalyst. These side-products originate from urea (HCNO, biuret, triuret, and polymerization products) or from ammonia and sulphate (NI-I,HSO, and (NH,),SO,) [135]. The temperature window within which these catalysts operate is therefore rather narrow, from about 525 to 675 K. SCR is nowadays a suitable aftertreatment technique for use in stationary diesel applications; a number of papers report applications for large marine diesel engines [137-1391. The narrow temperature window of the catalyst is not a large drawback in

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these applications, as the system can be installed closer to or further from the engine to find a suitable temperature. The temperature can be kept sufficiently constant because of the stationary operation of the engine. Summarizing, SCR with hydrocarbons is still far from being used in large-scale applications for non-stationary diesel engines; limitations due to catalyst activity and stability still need further study. SCR with NH, or urea is more promising, and is a feasible option in certain applications where exhaust gas temperatures fall within a narrow temperature window (475-675 K), so that commercially available V,O,/TiO, catalysts in combination with an oxidation catalyst can be used. At higher exhaust gas temperatures SO, oxidation becomes a problem and other catalyst formulations will have to be found. The reductant dosage problem also needs further study. It is hoped that in future sufficiently active NO,Xdecomposition catalysts will be developed. 6.3.2. Removal of particulates Many options have been considered for removing particulates from diesel exhaust gases. As most are based on the use of a filter, the different filter types will first be outlined. Different techniques for removing accumulated soot from the filters (“regeneration of the filter”) will then be discussed. A distinction can be made between techniques in which soot combustion is achieved via a temperature increase, and techniques in which regeneration takes place at relatively low temperatures by the use of catalysts. Finally, a non-filter-based option in the form of open monolith (“flow through”) oxidation catalysts will be described. traps. The use of a filter is the most straightforward method for reducing particulate emissions. In diesel engine applications, filters have to satisfy specific requirements. In spite of high exhaust gas flow rates, the pressure drop over the filter must be low to avoid decreasing engine performance. The filter must be able to withstand the high temperatures of diesel exhaust gases (up to 875 or 975 K); but also briefly to withstand temperatures of over 1250 K which are due to the exothermic reactions occurring during the batch-wise oxidation of accumulated soot. Finally, of course, the particulate collection efficiency must be high. Several types of filter have been described in the literature, and the most important ones will be discussed below. The general term for filters which remove particulates from diesel exhaust gases is “particulate trap”. Unmistakably the most studied and applied particulate trap is the wall flow monolith. It consists of a ceramic structure with parallel channels, of which half are closed at the upstream end in an alternate, checkerboard manner, and the other half are closed at the downstream end. Thus, exhaust gases are forced to flow through the porous walls, which then act as filters. A schematic representation of the wall flow monolith is given in Fig. 12. The two factors determining the collection efficiency are the wall thickness and the mean pore diameter, which are in the order of 0.3-0.6 mm and lo-40 pm, respectively. Coming Glass Works [ 1471 and NGK Insulators [148] both produce wall flow monoliths made of cordierite (2MgO-2Al,O,-5Si0,). Recently, wall flow monoliths made of silicon carbide have also been developed, which are reported to have similar collection efficiencies at lower pressure drops [ 149-1511.

6.3.2.1. Particulate

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-.-...porous

c’ollected

wall soot

cleaned exhaust gases

exhaust

gases

Fig. 12. Schematic representation of a wall flow monolith. By closing alternate. channels, the gas through the porous walls, which act as a filter.

flow is forced

Ceramic foams consist of blocks of porous ceramic material. The pores are interconnected, allowing gases to flow through the material. The number of pores per unit of length determines the collection efficiency and the pressure drop. This number has to be at least 30 mesh (1 pore per 0.085 mm) to obtain a reasonable collection efficiency (40-60%) [152]. Pieces of ceramic foam with different numbers of pores per unit length can be used in series to improve particulate collection efficiency [152,153]. Candle filters consist of crosswise woven ceramic fibres surrounding punched metal support tubes. These filters were developed and are applied by Daimler-Benz. Collection efficiencies of these particulate traps increase rapidly with increasing amounts of collected soot. However, the pressure drop, initially quite low, rises quickly at the same time [154]. Metal wool filters consist of a housing filled with compressed metal wool. A disadvantage is that the metal wool is able to oxidize sulphur dioxide to sulphur trioxide at high exhaust gas temperatures, resulting in sulphate mist formation and consequent increased particulate emissions [ 1553. Wire mesh jlters are made of a mesh of metal wires. Johnson Matthey has made an extensive study of these filters, using radial flow metal wire meshes which were installed at the exhaust gas manifold in order to make use of the high temperatures of the exhaust gases close to the engine [156,157]. Filters as described above can be divided into surface filtration or deep filtration filters. In surface filtration a layer of soot is deposited on top of the filter, which results in an increase in collection efficiency but also in a rather fast increase in filter pressure

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Table 10 Type of filtration aad typical particulate collection efficiencies of a number of particulate traps Type of particulate trap

Type of filtration

Typical particulate collection efficiency

References

Wall flow monolith Ceramic foam Candle filter Wire mesh

surface depth surface

60-95% 40-70% 70-99%

[94,158-1601 [152,153] [1541

depth depth

20-50% 50-80%

[I561 [1611

Metalwool

drop when this soot layer becomes thicker. Deep filtration type filters have a more constant collection efficiency and pressure drop. However, the collection efficiency is often rather low, and thick filters with high pressure drops are needed. Typical collection efficiencies for the different particulate traps are given in Table 10. These data can be seen as only an indication of collection efficiencies, as they are influenced by such factors as the size of the trap, the exhaust gas flow rate, the exhaust gas temperature, the soot loading, the trap length to diameter ratio and probably other parameters. Collection efficiencies for candle filters and wall flow monoliths are the highest. The preference for wall flow monoliths is probably because, besides their high collection efficiency, they are easy to handle in comparison with the other types of particulate traps. The life of a trap under practical conditions is limited by the accumulation of ash, which in the long term will clog the filter. Wall flow monoliths clogged by ash have been reported in a number of studies (e.g. [162-1651, but candle filters [154], ceramic foams [ 1661 and wire mesh filters [ 1671 have also been reported to suffer from ash clogging. Barris et al. [164] studied ash accumulation in a wall flow monolith for a normal sulphur level fuel (0.21 wt%) with a high ash level oil (1.6 wt%> and for a low-sulphur fuel (0.03 wt%) with an ashless oil (< 0.01 wt%). Oil ash level and fuel sulphur level proved to be important parameters in reducing ash accumulation, as hardly any ash was deposited by the low-sulphur fuel and the clean oil. A number of authors used oil ash levels to calculate ash accumulation rates, which are in the order of 1 kg of ash per 100000 km of engine life [ 166-1681. It seems that not all of the ash is recovered in the trap, according to Barris et al. [ 1641, possibly accumulation of metals or ash in the lubricating oil decreases the ash emission to some extent. MacDonald and Simon [168] argue that data on concentrations of inorganics in oil would support this view. The ash accumulated in a wall flow monolith is preferably collected at the down-flow ends of the channels [162,165], and is reported to have a higher density (140 kg me31 than diesel particulates (40 kg me31 [164]. The actual installation of the particulate trap in a vehicle is also a point of consideration. Particulate traps have been installed instead of the silencer and are reported to reduce noise to the same degree or only to a slightly smaller degree than a standard silencer [168-1711. The size and length-to-diameter ratio of a trap will depend upon the specific application, as a compromise has to be found between pressure drop, which would favour large traps, and space restrictions imposed by car manufacturers, favouring smaller compact traps. Konstandopoulos and Johnson [172], on reviewing the

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literature, come to the conclusion that such parameters are usually determined empirically. They found two convenient “rules of thumb”: the trap length is more or less equal to the trap diameter, and the trap volume is about as large as the engine displacement. Finally, electrostatic precipitation or agglomeration should also be mentioned as a way to collect particulates. Diesel particulates have aerodynamic diameters of typically 0.1 ,um, which makes them too small to be collected by mechanical means based on sedimentation forces, such as cyclones. Agglomeration by means of an electric field has been studied, which is possible as the particulates are already charged as a result of the combustion process during which they are formed (as was described in Section 3) [ 1731. A corona discharge can be used to increase the charge of the particulates and thus the agglomeration rate [174,175]. The agglomerated particulates with diameters greater than 1 pm can then be collected by mechanical means such as a cyclone. However, even only an increase in particulate diameter would be beneficial with respect to health effects, as was proposed by Weaver et al. [94]. The increase in fuel consumption due to a corona discharge (about 15 kV), electrostatic precipitator and cyclone was reported to be 3% [175]. The different means of removing particulates from diesel exhaust gases all have their pros and cons. In practice, a filter type is chosen on the basis of its availability, its pressure drop versus collection efficiency characteristics, and its reliability under practical conditions (resistance against vibrations and high temperatures). Electrostatic precipitation or corona discharge has hardly been applied, which is probably because of the high voltages that would be required on board. Deep filtration type filters such as ceramic foam, wire mesh and metal wool have been used frequently. Their advantage over surface type filters is a larger capacity for accumulating ash. Ash accumulation clogs surface type filters in the long term. A disadvantage of deep filtration type filters, compared with wall flow monoliths, is that these filters are not yet produced on a large scale. The user has to choose the material, the size and geometry of the filter and how it should be packed in a can which can be mounted in the exhaust pipe. Wall flow monoliths are produced commercially, their size varies with the engine size and the collection efficiency can be adjusted by the wall characteristics (wall thickness and pore diameter). Procedures for mounting wall flow monoliths in a can have been developed. In addition, canned wall flow monoliths can be supplied. This makes wall flow monoliths very easy to use, which probably is a major reason for their high popularity. 6.3.2.2. Non-catalytic regeneration of particulate traps. When using a particulate trap, a regeneration technique is necessary to prevent the trap becoming clogged with collected soot which, in the longer term, would result in engine malfunction due to increased back-pressure. Several techniques for removing soot from the particulate trap have been proposed. Most of them are based on soot combustion, initiated by an increase in temperature. However, so much energy is required to reach combustion temperatures that only periodic regeneration of the particulate trap is feasible. Consequently, the amount of soot trapped in the particulate filter has to be monitored. In most studies, the pressure drop over the filter is used as an indicator for the accumulated amount of soot. At a certain predefined value, regeneration of the particulate trap is started.

J.P.A. Neef et d/Fuel Oxygen

concentration

Processing Technology (~01%)

,120o __,I

47 (1996) 1-69

“C

35

__,_1100 “C

15 T 10 “C 5

O0.0

0.5

1.0

1.5

2.0

Gas flow rate (N-m3 min.‘) Fig. 13. Influence of gas flow rate and oxygen concentration on maximum temperature in a 1.7 1 wall flow monolith trap during regeneration of 15 mg particulates at 875 K. After Higuchi and coworkers. 0, trap resisted; + , trap failed.

A number of regeneration techniques will be summarized briefly. First, however, the general features of batch-wise soot combustion will be discussed. Supplying heat for soot combustion costs fuel, so it should be kept to a minimum. For several techniques ignition alone suffices; if soot combustion starts at the inlet face of the particulate trap, adjacent soot will ignite spontaneously. A combustion front propagates through the trap, and temperatures are reported to be highest in the centre of the particulate trap at the outlet end near the end of the regeneration [148]. Too high a temperature should be avoided, as the trap may be damaged, as will be discussed later in this section. Self-propagating combustion may take several minutes to burn the trap clean. The amount of soot that has been collected should neither be too low nor too high before regeneration commences. If it is too low, the “combustion front” will extinguish, leaving part of the particulate trap uncleaned. In a subsequent regeneration the amount of soot may then locally be too high. Large amounts of soot will lead to very high temperature gradients, which may result in the trap cracking or in extreme cases even melting. Threshold temperatures for cordierite wall flow monoliths are reported to be 1275 K for cracking and 1475-1700 K for melting [148,176,177]. The problem of particulate traps cracking or melting is often reported at low exhaust gas flow rates (low speeds) [ 1781. Higuchi et al. [ 1481 reported similar findings: the maximum temperature in the particulate trap increases with decreasing exhaust gas flows and with increasing oxygen concentrations, as is shown in Fig. 13. Optimization of the total amount of particulates collected, before a regeneration can start, is therefore a tedious task, as minimum and maximum soot limits exist. The amount of heat needed to burn off the accumulated soot can be decreased by bypassing the filter during regeneration [ 168,176,178- 18 11, avoiding superfluous heating of the bulk of the exhaust gases. As emissions would temporarily increase during the bypass operation [ 178,180], systems containing dual particulate traps have been developed [ 181-1841. The bypass technique also enables regeneration in a different regime to full exhaust gas flow: the oxygen supply can be limited to control the oxidation rate of

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the soot. A number of papers report successful tests, using a separate air blower to supply a controlled amount of air [171,183-1851. Burners. One of the first ways proposed to increase the exhaust gas temperature was the use of a burner installed upstream of the particulate trap. The exhaust gas temperature is periodically raised to ignite the accumulated soot. Propane or diesel fuel is used to feed the burner. Typical regeneration times are reported to vary from a few to ten minutes [148,161,168,171,177-181,185-1881. The use of a burner incurs a decrease in fuel economy, ranging from 0.5% (at a 350 mile regeneration interval [1681) to 5% (at smaller regeneration intervals) [168,177-1811. At the Ford Motor Company, the regeneration interval was optimized for fuel economy. An increase in regeneration interval leads to a rise in back-pressure and consequently a decrease in fuel economy owing to impaired engine performance. A decrease in regeneration interval leads to higher burner losses. A 64 miles regeneration interval was found to be optimal, which resulted in an average decrease in fuel economy of 3.3% (back-pressure plus burner fuel consumption) [178]. In a later study [179], the same research group used a bypass during regeneration which resulted in a 1.8% decrease in fuel economy. Burner-assisted regeneration does, however, pose several problems. The ignition of the burner has to be very reliable; if the burner misfires, fuel will collect on the soot in the particulate trap, and the following regeneration could be so fierce that the trap could be damaged [ 1861. Finally, burner-assisted regeneration equipment is costly and complex, which may become an important limitation to successful commercialization if the problems conceming the reliability of regeneration and the long-term stability of the ceramic trap can be overcome. Application in light-duty vehicles in particular is unlikely because of the high costs [171]. Although a number of burner-assisted regeneration systems have been patented and many road tests have been performed, we know of no commercial applications of these systems to date. Elecrricul hearers. Regeneration can also be induced by using electric heaters, electric glow plugs or electric wires installed before or in the upstream boundary of the particulate trap. When regeneration is started, the heated plugs or wires cause the soot to ignite. A combustion front moves through the particulate trap in a downstream direction; the plugs or wires can often be switched off before regeneration has ended, as it propagates spontaneously. The decrease in fuel economy of electrically assisted regeneration is reported to be higher than for burner-assisted regeneration, as the electricity has to be generated on board, causing heat losses elsewhere in the engine [179]. Fuel penalties reported range from 1 to 5% [179,183,189]. A much discussed problem posed by electrical regeneration is that the capacity of common batteries is insufficient; power required for regeneration is in the order of kilowatts. In most studies, therefore, a larger battery was used. However, Pischinger et al. [190] solved the problem by dividing the trap into a number of segments which were regenerated alternately. Hayashi et al. [ 1761reported a better performance (although more energy was needed to initiate regeneration) if the exhaust gas flow was reversed during regeneration. This can be explained as follows. During normal regenerations, the heat is carried off by gas

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flowing into the downstream channel where no soot is accumulated. By reversing the gas flow during regeneration, the extra heat generated by soot combustion is transported to where the soot has not yet been burnt. Using a model trap in which the regeneration could be followed visually, Hiithwohl et al. [191] studied the propagation rate of the combustion front after ignition by an electrical resistance heater. They report an initial propagation rate of 0.5 mm s- ’, which decreased to 0.2 mm s- ’ at the end of flame propagation. This propagation rate was found to increase significantly to 1.5 mm s- ’ if a catalytic coating or catalytic (Mn or Fe) fuel additives were used [191]. Simon and Stark [ 1891 also studied electrical regeneration in combination with fuel additives. A number of electrical heaters or glow plugs were tested. They conclude that high soot loadings are indispensable to regenerate a loaded particulate trap, but that melting or cracking of the trap material often occurs at these high loads. Although fuel additives enhance the regeneration performance, problems with soot ignition (minimal contact between electrical igniter and soot) and flame propagation cause electrical regeneration to be too unreliable. Researchers from Donaldson Co. report fleet tests on a large number (over 700) of buses equipped with two parallel wall flow monolith traps with upstream installed electrical heaters. The main problem to be solved remains the lack of reliability of the system [ 1831. Intake or exhaust gas throttling. The engine itself can also be used to produce exhaust gases with elevated temperatures which are high enough for particulate trap regeneration. Reduction of the combustion air mass flow rate, thus reducing the air-to-fuel ratio, is the most straightforward method. Throttling of the intake air or the exhaust gases has been studied and applied. Both result in a decrease in engine power output and in an increase in fuel consumption [177,181,192]. Exhaust gas temperatures can be raised by typically 200 K at medium and high loads by both methods. Under normal driving conditions, regeneration of particulate traps has been described as very efficient; a trap can be burnt clean in a couple of minutes. However, regeneration does not occur at low engine speeds and loads, a problem that was discerned in the early 198Os, shortly after commercialization of the wall flow monolith [147,177,193]. Ludecke and Dimick [ 1811 studied low speed regeneration with intake air throttling, using a catalyst in combination with a variety of filter types. Trap regeneration never succeeded at speeds below 40 km h- ’. Pattas et al., who have explored exhaust gas throttling in detail [ 192,194], applied this regeneration method in combination with cerium fuel additives (to be discussed) to a bus fleet in Athens [165,195] and also on passenger cars 11961. Although quite a number of field tests have been performed, hardly any data have been published with respect to fuel consumption. Only one study on two light-duty diesel cars mentions that fuel consumption does not change notably [196]. Miscellaneous techniques. A number of other, non-catalytic methods have been used to regenerate particulate traps. We will confine ourselves to a brief overview. In the last four years a number of papers have been published, dealing with “reverse air flow regeneration” [ 197-2011. These systems make use of pressurized air (from the brake system) to blow soot countercurrently from the trap into a collection or combus-

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tion system [ 1991. For this operation, the exhaust gases must be vented elsewhere during regeneration, and two parallel traps have to be used, which are operated and regenerated alternately [ 198,199]. A rotating monolith has also been applied, a part of which is regenerated continuously [ 197,201]. Quite high air flow rates or pulsations are needed to remove the soot efficiently from the particulate trap. Although short road tests have been performed, a number of problems have not yet been solved: soot removal from the particulate trap is reported to be incomplete, resulting in a gradual increase in back-pressure [197,198], soot combustion (heating it electrically or with a small burner) was reported to be difficult, and, although not mentioned in these papers, space requirements probably limit practical applications. Microwave irradiation has also been proposed as an effective means of particulate trap regeneration [202,203]. The soot itself can be heated using microwave irradiation, whereas ceramic materials such as cordierite are transparent to microwave radiation and are therefore not heated [202,203]. An elegant option is to use microwave irradiation to ignite the soot at the beginning of the filter, using plugs made of special microwave-susceptible materials. Flame propagation would ensure proper cleaning of the entire trap. This procedure might be more efficient with respect to complete regeneration in comparison to heating the soot itself [202]. 6.3.2.3. Catalytic regeneration of particulate traps. The use of catalysts in soot removal from particulate traps is a logical choice: catalysts for the oxidation of carbonaceous materials, e.g. graphite, activated carbon, and coal, have been studied for a long time. Catalytic oxidation of soot is in many ways analogous to these oxidation reactions, in which many catalysts have appeared to be active. There are several ways in which the catalyst can be applied for oxidation of soot or hydrocarbons in diesel exhaust gases. Catalytic coatings, catalytic fuel additives and injection of catalysts will be reviewed in this section. Flow-through oxidation catalysts will be dealt with in Section 6.3.2.4. Catalytic coatings. The most obvious and straightforward way to make use of a soot oxidation catalyst is to coat the particulate trap with a layer of catalyst. A washcoat is probably not very important in this case, as the contact between soot and catalyst is thought to depend mainly upon the outer surface of the catalyst layer. Of course, an increase in surface area of the catalyst becomes important if gas-phase reactants are to be oxidized. Several catalyst formulations have been proposed; experiments performed with these catalysts range from small-scale laboratory experiments to full-scale diesel engine tests. The conditions which occur in particulate traps are difficult to imitate in model studies. Specific conditions such as gas flow rate, soot density and the exact manner in which the particulates are deposited on the catalyst layer are not precisely known, and the influence of these parameters on the soot oxidation rate has not been assessed in any detail. Numerous tests, therefore, involve full-size, catalyst-coated particulate traps installed downstream of real diesel engines, and give a good indication of the practical use of these traps, which are often referred to as catalytic trap oxidizers (CTOs). In these tests, particulates from the diesel exhaust gases are deposited on the CTOs. Meanwhile, the pressure drop over the filter is monitored, which is an indicator for the amount of

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particulates collected on the trap. The activity of the catalysts can be expressed as a temperature: qgnr Tes or T,,. Gil” is the temperature at which the soot ignites when a soot-loaded filter is heated, Teq -is the temperature at which the soot combustion rate equals the soot deposition rate, and T,, is the temperature at which 50% of the soot is burnt in a linear temperature programme. These temperatures are often compared with the corresponding temperatures found for a particulate filter without a catalytic coating. Care must be taken when comparing the “ ignition temperatures’ ’from different studies, as many varying definitions exist. In a number of studies noble and base metals have been compared with respect to their performance in CTOs. Degussa [204] found that precious metal catalysts barely decreased 7& or had no effect at all, whereas base metal catalysts decreased & by up to 100 K. Mazda’s findings were similar [186]: noble metal CTOs only gave a O-70 K reduction in Tign, whereas base metal CTOs gave a 140 K reduction in Tig,,. Aachen Technical University in cooperation with FEV Motorentechnik and Heraeus [205] also report low activities for noble metal catalysts, but found that vanadium or molybdenum coatings were able to decrease Teq by up to 100 K. The addition of alkali metal promoters further decreased the ignition temperature by about 50 K. Other studies contradict the conclusion that base metal catalysts have a higher activity than noble metal catalysts: Mitsubishi 12061, General Motors [207] and Nippon [166] report noble metal catalysts to be more active. Catalyst formulations are not usually specified in large-scale engine tests, but a number of observations can be made concerning the activity of catalysts under practical conditions. Johnson Matthey [ 156,157,208] reported that their JM4 and JM13 catalysts, working under a large range of driving conditions without a regeneration system, reduce particulate emissions by about 50%. Mitsubishi [ 1521 optimized a catalyst to have a low sulphur dioxide oxidation activity. Temperatures to regenerate a CT0 were found to surpass 675 K. Finally, Degussa [209] measured reductions of qs,, by about 80 K using base metal catalysts. The most important information that these data provide is that no catalyst, whether containing noble or base metals, is able to reduce Tq or Tign to temperatures at which continuous removal of soot from diesel exhaust gases becomes feasible under a wide range of operating conditions. Although it has been claimed that CTOs operate continuously, exhaust gas temperatures during these tests were so high that applicability under a wide range of practical conditions is questionable, to say the least (Johnson Matthey: temperatures of 625-675 K are required from time to time; Nippon: temperatures surpass 675 K for 5 min every 45 min) [156,157,166,208]. Most studies on soot oxidation catalysts were performed on a laboratory scale. As the experimental procedures used in the different studies varied strongly, it was difficult directly to compare the activities of catalysts between one study and another. We will confine ourselves to an overview of the studies reported in the literature. Ahlstriim and Odenbrand [210] found high activities for CuO, MnO,, Cr,O,, Ag, and Pt at relatively low temperatures. V,O, was found to be more active at higher temperatures. The oxides of Co, Fe, MO, and Pb showed hardly any activity. In this study, the catalytic soot oxidation rates, expressed relative to soot oxidation over Al,O, as a blank, were low, however, and did not exceed a factor of 5.

40

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Marinangeli et al. [211] compared Pt(/Pd) with Cu/Cr catalysts, and found that the noble metal catalysts were more active. They used NO and SO, in their gas feed, but did not report NO, NO,, SO, or SO, concentrations. Otto et al. [212] reported barium catalysts which reduce the temperatures at which soot bums by about 100 to 150 K. Hoffmann and Rieckmann 12131studied the oxidation of diesel particulates catalysed by several base metal oxides on a number of filters. They found rather high activities for Ca, Cu, Ce, Pd/Pt, V and Zn catalysts. The filter material and type of filter were, however, reported to have a larger influence on the soot oxidation activity than the catalytic material. Inui and coworkers [214,215] examined multi-compound catalysts and reported Fe/La,O,/Pt, Co/La,O,/Pt and Ni/La,O,/Pt catalysts to be very active in comparison with other catalysts. However, the temperatures at which these catalysts showed high reaction rates were less than 100 K lower than temperatures required for the uncatalysed reaction. In another study, Fe/La,O,/Cu and Fe/Mn,O,/Cu catalysts were found to be similarly active. It was suggested that the different metals in the catalysts played different roles: Pt (or Cu> was thought to promote transfer of oxygen from the gas phase to the separate Fe/La,O, or Fe/Mn,O, particles which was then transferred to the carbon by the partially reduced iron oxide [216]. Mendoza-Frohn, LGwe and Schijnwiese performed a model study on a similar Co/La/Pt catalyst. They found decreases in soot combustion temperatures of up to 200 K, depending on the preparation of the catalyst/soot mixture [217-2201. In another study, they reported alumina supported cobalt to be as active in soot oxidation as cobalt aluminates which were formed at high temperatures. They conclude that these results contradict the commonly accepted redox mechanism of the catalysis of carbon oxidation [219]. The Institut Fran+ du P&role and Rhane Poulenc developed a series of catalysts, which were active under practical conditions and very active in laboratory tests: one of their catalysts burnt soot at 635 K at a rate similar to that for soot without a catalyst at 875 K [221]. Watabe et al. [222] found many base and alkali metals to be active soot oxidation catalysts. Using alumina, silica or silica-alumina supported catalysts, they found several alkali salts (LiCl, LiF, KCl, KF, CsCl) and base metal compounds (NI-I,VO,, Cu(NO,),, CuCl,, CrCl,, (NH,),Mo,O,,, MnCl, and RuCl,) to decrease Z’s, by much more than 100 K. They also studied catalysts composed of Cu(NO,), plus a second and sometimes a third compound, and found composite catalysts of Cu(NO,), with NiCl,, Pl$NO,),, NbCl,, Na,MoO,, GeCl,, FeCl, or Na,WO, to lower T,, by some 300 K. CuClJKCl catalysts were also found to be very active, but unfortunately they vaporize at elevated temperatures (875- 1175 K). Addition of molybdenum or vanadium resulted in stable but still very active catalysts. Other studies have been performed on the activity of copper chloride catalysts, either on their own or in combination with an alkali metal or with vanadate or molybdate. The activity of single metal chlorides has been reported by a number of authors, who all found copper chloride to be the most active catalyst. Murphy et al. [223] impregnated low concentrations of chlorides in soot, and found a decrease in catalytic activity in the

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range CuCI, > H,PtCl, > CoCl, > MnCI, (CoCl, is the assumed cobalt compound; “CoCl” was reported). Later, Hillenbrand and Trayser [224] also reported CuCl, to be an active catalyst. They found that the addition of NaCl increased the activity of CuCl,, CoCl, and Co(NO,), catalysts. Impregnating soot with these catalysts resulted in high activities (Tis,, < 675 K). Combinations of cobalt, copper or silver with vanadium or molybdenum were also examined in a number of studies. Setzer et al. 12251reported that both copper and cobalt vanadates and copper molybdates catalysed soot oxidation (- 100 K decrease in Ts,) [225]. Ahlstrom and Odenbrand found an optimum in the V,O,/CuO molar ratio of = 9: 1. The addition of small amounts of platinum yielded a further increase in catalytic activity. It should be noted that only relative activities were given [226]. Silver vanadates have been reported as soot oxidation catalysts in the patent literature by Degussa [227] and Engelhard 12281, again in combination with small amounts of platinum. In examples given in these patents, these catalysts were found to be moderately active (50 K decrease in Tes and about 115 K decrease in temperature at 50% bum-off, respectively). The combination of copper, vanadium and potassium was studied extensively by Ciambelli and coworkers [229-2321. They found this catalyst, which was prepared from potassium and copper chlorine precursors, to decrease Sign by 250-300 K [229,230]. Recently, it was reported that potassium also increases the reaction rate of soot oxidation over titania supported copper catalysts. Potassium was found to interact with the support, causing a decrease in the rate of sintering of the small TiO, support particles [233]. The support material might also influence the activity of the catalyst. Davies et al. found that the activities of supported Pt and Pd catalysts depend more on the (unspecified) washcoats than on the choice of catalytic material [234]. In another study, certain washcoat or support materials were themselves reported to be quite active [235]. Oxidation and storage of sulphur oxide compounds are important criteria for the choice of a support material, as will be discussed later in Section 6.3.2.4. Comparing a number of support materials, van Doom et al. [235] found lanthanum oxide to be a fairly active soot oxidation catalyst. The activity of this material did not decrease after SO, poisoning. Whether it oxidizes SO, or stores SO, or SO,, was not reported. It is difficult to compare catalytic activities found in different studies, as experimental conditions often differ profoundly. The intrinsic properties of the mixture of soot and catalyst, such as the carbon material used, the catalyst-to-soot ratio and the contact between soot and catalyst, and also the temperature history of the sample and the oxygen concentration, are major parameters determining catalytic activity [236]. A more general but very interesting observation on catalytic activity can be made when comparing the model studies with engine tests. In model experiments, the measured catalytic activities, expressed as a decrease in the temperature of soot ignition or of reaching a certain soot conversion, range from some tens up to more than 300 K. The use of CTOs behind real diesel engines resulted in activities which barely surpassed 100 K. There can be various causes for this difference in catalytic soot oxidation reactivity in model and engine tests. One of the most important causes appears to be the way in which the soot is brought into contact with the catalyst. A number of authors mention

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this as being an important parameter in catalysed soot oxidation [214,217,235-2391. In a recently published study, our research group [236] tried to quantify the contact between soot and catalyst. Soot, collected on catalyst material or trapped in segments of a wall flow monolith coated with a catalyst, burnt at temperatures similar to those found in model experiments with poor contact between soot and catalyst. Enhancing the contact in model experiments significantly increased soot oxidation rates. It was concluded that contact is a key parameter in catalysed soot combustion, and that contact between soot and catalyst in particulate filters under practical conditions is too poor to achieve high soot combustion rates. The importance of this contact also explains the high activities found for chloride based catalysts such as CuCl, and M&l2 [222,223]. These chlorides have low melting points and high partial pressures [240] and might thus be redistributed over the soot, thereby enhancing contact between soot and catalyst. An interesting observation was reported by Cooper and Thoss [241], which might explain the high activity of platina found in some studies. They measured high soot oxidation activities over a platinum catalyst, and showed that this was caused by oxidation of NO to NO,, followed by a subsequent reaction of soot with NO, NO + l/20,

+ NO,

NO;!+C-+CO+NO By placing a platinum catalyst upstream of the particulate trap, Cooper and Thoss showed that contact between soot and catalyst is not a prerequisite in the case of platinum catalysed soot oxidation. An important drawback of platina catalysts is their high activity for SO, oxidation. Cooper and Thoss found the SO, conversion to SO, to be of the same order of magnitude as the conversion of NO to NO,. This high SO, oxidation in conjunction with NO oxidation is not surprising, as it is known that in one of the oldest chemical processes (the so-called “lead chamber process”) SO, is oxidized catalytically by NO, (formed continuously by oxidation of NO). Later, platinum was used in sulphuric acid production. Xue and coworkers also studied NO and SO, oxidation over Pt on SiO,. They concluded that the oxidation of SO, to SO, is more highly favoured than the oxidation of NO to NO, [242,243]. This NO/NO, mechanism might also explain the sometimes rather high activity of noble metal catalysts at low temperatures, as reported for instance in the studies by Vergeer et al. [187] and by Degussa [204]. These studies report very slow pressure drop increases over particulate traps at low temperatures. Similar observations were described by Marinangeli et al. [211], who used platina or palladium catalysts in combination with 100 ppm NO and 50 ppm SO, in their gas feed. Neither of these studies report SO, or SO, concentrations; given the above considerations, they would be expected to show a rather large SO, concentration. Catalyst mobility has been studied to a certain extent in catalysed carbon and graphite oxidation studies. Baker [244,245] observed directly the mobility of small catalyst particles in the oxidation of graphite by controlled atmosphere electron microscopy. Several kinds of mobility were observed, resulting in pits, channels, or recession of

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MobIlitytemperature 15Oo_

1000_

500_ T,=

0 500

I 1000

, 1500

0.51 lmelting

I 2000

polntj

I 2500 Melting

Fig. 14. Relationship between mobility temperature and bulk melting temperature particles supported on graphite). From Baker [244].

K

3001 pmtlKI

(10 nm metal or metal oxide

edges in the graphite. Baker observed the temperature at which small (10 nm) catalyst particles became mobile, and found a clear correlation with the melting point of the metal or metal oxide, as shown in Fig. 14 [244]. The temperatures at which the catalyst particles became mobile coincided with the Tammann temperature of the materials studied (which is the temperature at which surface atoms of the material become mobile, approximately half the bulk melting temperature, in K). A similar relationship between the onset temperature of catalytic activity and bulk melting temperature was found [244]. As the catalyst particles were very small (nm scale), these values cannot be directly extrapolated to catalytic coatings where particles may be much larger. Moreover, the particles are likely to interact with the support material. In heterogeneous catalysis literature, this mobility of small catalyst particles on a support is well documented, as it constitutes one of the major causes of catalyst deactivation by sintering. Results from McKee and coworkers [246,247] indicate that physical mixtures of eutectic salt particles with carbon materials give rise to low oxidation temperatures because of the low melting points of the catalysts. Support for high activities for catalysts with a low melting point does exist in soot oxidation literature. Metal chloride catalysts, mentioned earlier to be very active soot oxidation catalysts, have very low melting temperatures. The addition of certain alkali salts is known to decrease the melting temperature of, for example, CuCl, even further owing to the formation of eutectic salts [248]. This, in combination with the observations of Hillenbrand and Trayser [224], indicates an influence of catalyst melting temperature on the activity of soot oxidation catalysts. AhlstrGm and Odenbrand [226] argued that the increase in soot oxidation rate which they found by varying the copper content in a copper/vanadium catalyst towards a CuO/V,O, ratio of = 9 can be explained by the formation of a eutectic salt. Lin and Friedlander [249,250] demonstrated the mobility of a sodium catalyst oxidizing soot on a fibrous filter. They explained this mobility by “surface migration” of sodium, as sodium (in the form of sodium hydroxide) has a very low melting point and is mobile at even lower temperatures. For the CuCl system, mobility by the gas phase can also explain the high catalyst activity.

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Summarizing, we may conclude that many precious metal, base metal oxide and multiple compound catalysts are active soot oxidation catalysts. Used as catalytic coatings, however, no large activities (expressed as Sign or Te,) are found: the studies by Degussa, Mazda, Aachen Technical University, Mitsubishi, General Motors, Nippon and Johnson Matthey, using precious metals coatings and base metal coatings, showed a relatively low activity. A possible explanation for these observations is that the contact between soot and catalyst is poor under practical conditions. Some indications exist that a limited number of catalysts (certain metal chlorides, alkali metal catalysts, eutectic salts) give rise to high soot or carbon oxidation rates because of contact established by the mobility of the catalytic material. Cutulyric fuel additives. Metallic fuel additives were studied extensively as cetane improvers or smoke suppressors at the end of the 1970% as reviewed by Howard and Kausch [251]. The amount of soot formed during combustion processes in both diesel engines and other combustion engines decreases when these additives are used (e.g. fuel additives based on Ba, Ca, Fe, and Mn [94,251-2571). Three mechanisms are thought to play a role. When alkali or alkaline earth additives are used ions or radicals are formed, which can remove soot precursors (mechanism 1) or inhibit the nucleation of soot precursors (mechanism 2). A number of transition metals catalyse the oxidation of soot at a later stage of the soot formation process (mechanism 3). The most effective additives, Ba and Ca, are thought to decrease soot emissions mainly by mechanisms 1 and 2 [251]. More recently, much attention has been paid to fuel additives acting as soot oxidation catalysts. In order to reach high efficiencies of soot combustion, soot is collected on a filter, as the residence time in the diesel exhaust is much too short to oxidize the activated soot. A decrease in raw engine soot emissions might be helpful here, but the main objective of the additive is to increase the soot oxidation rate. It is, therefore, not surprising that some additives described under mechanism 3 have also been studied extensively in recent diesel engine applications. Fuel additives are organometallic compounds which can be dissolved in diesel fuel. The exact nature of the organic part of the fuel additive is of limited importance, as after combustion metal oxide or metal sulphate particles remain, which are well distributed within the diesel particulates. Of course, physical properties do matter, such as stability and miscibility parameters. Among others, carbonate, carboxylate, dicyclopentadiene and naphthenate organometallic compounds are often used as additives. The different metals will be dealt with in more detail below, as the choice of the metal is thought primarily to determine the activity of the fuel additive. The organic part of the additive determines its fuel solubility, and for some metals this solubility is found to be a problem. Blending the additive with the fuel can result in a decreased stability: fuel additives tend to form deposits and sometimes an increase of sedimentation from the fuel itself is found [258]. Water might also pose a problem: deposits are observed when water is added to diesel fuels containing cerium or manganese additives, whereas fuel without additives is stable [189]. These problems might be overcome by on-board mixing of fuel and additive, using a second small fuel tank [180,259,260]. Although numerous papers have been published dealing with fuel additives, little attention has been paid to the exact nature and distribution of the resulting species in the

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particulates. Some metals are present in the particulates as oxide (cerium [ 16.5,189], copper [169,178,261,262], and manganese [189]), others as sulphate (lead [178,261-2631, sodium [264], possibly manganese [265]). Other important parameters such as particle size, homogeneity of the additive distribution in the soot layer and interaction between soot and additive have hardly ever been determined: the size and distribution of the resulting particles were measured in only two studies. In the case of lead, the particles were found to be somewhat smaller in size than the elementary soot particles (5-25 versus lo-40 nm). A uniform distribution of lead throughout the soot particulates was observed [263]. A similar homogeneous distribution was found for platinum and iron, together with even smaller particle sizes of 3-6 nm [254]. Several metals are applied as fuel additives, as can be seen in Table 11. A large range of concentrations is used; this can influence both the activity and the life of the filter, which becomes clogged by ash as will be discussed shortly. Literature data are ambiguous on the relative activity of the additives: some authors find copper to be the most active fuel additive [163,262,266], others find manganese [36,259,260] or manganese plus copper [258] to be more active than copper alone. The most common fuel additives will be discussed briefly below. Cerium. Pattas and coworkers have reported extensively on the use of cerium additives [165,195,196,272,273]. According to a recent paper [165], cerium reduces the amount of soot emitted (by 20-40%) and decreases the soot ignition temperature. Furthermore, their choice of fuel additive was influenced by the reported extremely low toxicity of cerium oxide [195]. Extensive city bus field tests have been performed in Athens (up to 100000 km), in which exhaust gas throttling was used to ensure particulate trap regeneration [165,195]. No data on fuel efficiencies, which would be expected to decrease due to exhaust gas throttling, were given, however. Copper. Lubrizol researchers [169,258,274-2761 performed a large number of field tests using low concentrations (15-40 mg 1-l ) of copper additives. No regeneration means were applied; 40 mg 1-l of Cu fuel additive resulted in a low, constant pressure drop over a wall flow monolith. Health risks due to the emission of copper were not judged to result in hazards to human health or to the environment. The amount of copper emitted was found to be only 2% of the copper collected in the trap. In a study on possible adverse health effects of the Lubrizol copper additive [275], results from modelling indicated that ambient concentrations are estimated to increase only slightly. Although copper is known to foul engine parts such as pistons and injectors, no deposits were found in these studies [169,276]. Another known problem of copper as fuel additive, its oxidizing behaviour towards the fuel, was not mentioned in these articles. Field tests were performed, demonstrating the reliability of these additives [ 169,258]. In a recent study by Ford [170], copper octoate (132 mg 1-l Cu) was also found to be a very active fuel additive. Exhaust gas temperatures of 590 K or higher, during a few seconds or more, sufficed to regenerate a soot loaded trap. A palladium coating was used to oxidize CO and HC. Researchers from General Motors [ 1801 reported that their copper naphtbenate additive (83 mg l- i Cu) was not active enough to regenerate a filter mounted downstream of a 6.2 1 indirect injected light-duty truck.

carbonate, sulphonate carboxylate, naphthenate, sulphonate carboxylate, carbonate ? naphthenate, carboxylate, octoate, pivaloylpinacolonate acetate, naphthenate, octoate + (Ce, Mn, Ni, Pb) organometallic compounds

acetylacetonate, carboxylate, ferrocene, naphthenate t-butoxy carboxylate, methylcyclopentadienyl-Mn-tricarbonyl, oxide, sulphonate

t-butoxy naphthenate naphthenate, tetra-ethyl carboxylate

Ba Ca Ce Ce+Mn cu Cu +others

Fe Li Mn

Na Ni Pb zn

a Diesel fuel density is assumed to be 0.84 kg I- ’.

Organometalhc compounds

Metal

Table 11 Organometallic compounds used as diesel additives

6-80 350- 1600 110-220 ?

13-2200 2-25 7-1700

500-3700 100-I 100 20-300 35-65 S-200 9-120

Concentration (mg metal per 1fuel) a

]264] ]257] [ 178,179,189,261-263.2661 [2591

[252,253,257] [163,178,257-2591 [165,189,258,259] 11891 [ 169,178- 180,259,261,262,264,266] Cu/Ce: [179] Cu/Mn: [178,179,187,266-2681 Cu/Ni: [ 1791 Cu/Pb: [178,261] [257-259.264.2691 [2641 [178,187,189,258,259,262,268,270,271]

References

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Iron. Volkswagen performed engine tests using iron additives in quite high concentrations (170-2200 mg additive per litre of fuel). No regeneration was necessary, but high back-pressures due to the large pressure drop over the filter resulted in a decrease in fuel economy of about 7%. Iron emissions were found to be low; ranging from 2% to an undetectable amount of the iron used as additive [269]. The “Gesellschaft fur Abgasentgiftungsanlagen” also used an iron additive (ferrocene or dicyclopentadienyl iron). Their system, which is commercially available for fork-lift trucks, is claimed to operate without a regeneration device. We could not find any publications on this system in the open literature. Manganese. A large number of tests have been performed using manganese fuel additives (see Table 11); however, no extensive long-term testing has been reported. Emissions of manganese after the filter were found to vary: 0.2, 5, or 15% of the manganese used ([265], [270] and [277], respectively). The higher Mn emissions in the last study might be due to the use of a Coming EX-54 particulate trap, which is known to have a worse particulate collection efficiency than the commonly used Coming EX-47 [160]. Mn street level concentrations were calculated and were found to be far below the threshold limit value of Mn,O, (the manganese oxide with the lowest threshold limit value, viz. 1 mg me3>. Different authors mention manganese street level concentrations (by the emission of diesel fuel additives not collected in the filter) of the order of 1 pg me3 [259,265,270,277]. As described above, regeneration was found to be necessary in most tests, apart from those in which copper or iron was used as a fuel additive. Means of regeneration such as throttling the exhaust gases, increasing the engine speed or load and using fuel burners have all been tested [165,178-180,261,266,267,278]. Regeneration is one of the prerequisites for the successful use of fuel additives, another is the long-term stability of the filter. Ash was found to be collected very efficiently by wall flow monoliths (many authors find collection efficiencies higher than 95% for the metal oxide or sulphate), which causes long-term clogging of the filter. Ash is collected in the inlet channels, throughout the whole filter, but concentrations are reported to be higher on the downflow side of the filter [165]. As was discussed in Section 6.3.2.1, ash also results from diesel fuel, lubricating oil and engine wear, which also contributes to clogging of the filter [ 164,167]. In fact, it is surprising that high trap lives (> 100000 km) have been reported in the literature [165,169]. This might well be due to the use of very clean oils with low ash contents; details were not reported in either study. Even so, diesel fuel additives still give rise to a substantial ash accumulation, which can easily be calculated to be in the order of 1 kg per 100000 km. The consequences for the practical application of fuel additives are not clear at this moment, as only a few long-term field tests have been performed. A particulate trap life of 100000 km is, however, short in comparison with the life of a heavy-duty diesel engine (of the order of 1000000 km), and trap cleaning or replacement has to be foreseen for practical applications of traps in combination with fuel additives. The ash collected on the filter consists of a loose, fluffy material. The literature is not consistent concerning the ease of removal of collected ash from a particulate trap, from being described as easy (just shaking [268]), to simple (using such means as counterflow and an ultrasonic bath [165] or dissolving the ash in acid [259]) to difficult [189]. The

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ash does not react with the cordierite of wall flow monoliths, nor does it influence the filtration properties [165,268]. The ash from a sodium based fuel additive, which was in the form of sodium sulphate, could easily be washed off a filter owing to its good solubility [264]. Many problems still remain before fuel additives can be introduced onto the market for general application. The reliability of the filters has to be increased (many authors witnessed melted or broken wall flow monoliths due to temperatures or thermal stresses being too high during regeneration [ 189,271,277]); engine deposits have to be avoided (combustion chamber deposits or fuel injector nozzle deposits [163,178,180,187,189, 2621); and a proper way of cleaning the accumulated ash from the filter at certain service intervals has to be developed. However, as the extensive field tests described above showed, fuel additives form an interesting and promising route for the catalytic removal of soot from diesel exhaust gases. Injection of reagenfs before the filter. A third option for applying a catalyst in combination with a particulate trap is the use of specific reagents injected into the diesel exhaust gas upstream of the trap. These reagents consist of either (i) organic compounds which can be oxidized over a catalytic coating applied to the trap or (ii) catalytic materials which can increase the reactivity of the soot. Both types of reagent will be discussed briefly. The use of organic compounds has been proposed by a number of authors. McMahon et al. [161] performed some preliminary work, in which propane was oxidized at a minimum temperature of 535 K over a catalyst mounted upstream of the particulate trap. The heat released from this reaction was then used to ignite the soot collected on the particulate trap. A similar laboratory study was performed by Noirot et al. [279], using hexane or diesel fuel as the organic compounds that were oxidized. Bandy and Graboski [280] also used diesel fuel, injected upstream of a platina coated particulate trap. At temperatures of 715 K and above, the trap could be successfully regenerated. The reported decrease in fuel economy was 1.6%. Ma and coworkers [281,282] studied the effectiveness of several common organic solvents, with or without dissolved Fe or Mn compounds. No catalytic coating was applied in this study. Several common chemicals such as acetone, methanol, ethanol, diesel fuel itself, and certain alkanes (ranging from pentane to heptane) were tested. Acetylacetone was found to be the most effective for decreasing the soot ignition temperature. This could be decreased further by addition of iron or manganese compounds to the injected chemicals. Daimler-Benz [154,283-2851 performed an extensive study in which reagents based on either copper chloride (CuCl) or copper per-chlorate (Cu(ClO,),> were injected into diesel exhaust gases upstream of a loaded trap to increase the reactivity of the soot. Very low diesel exhaust gas temperatures, even as low as 500 K, were sufficient to regenerate the traps, which were candle filters (see Section 6.3.2.1). Attempts to reduce the emission of copper and chlorine were reported; however, copper emissions remained in the order of 1 g of metal per regeneration. Although field tests have been performed with this system [154,285], the main objective was to test candle filters, not reagent-induced regeneration [ 1541. The drawbacks of reagent-induced regeneration are numerous. High temperatures are needed in the case of hydrocarbon addition and large amounts of metals are used to

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increase soot reactivity. Furthermore, exhaust emissions increase when reagents are used. When injecting hydrocarbons or diesel fuel before the particulate trap, hydrocarbon emissions, although not reported in these studies, probably increase. When nitrogen-containing chemicals are used, NO, emissions increase [283], and the use of perchlorate or chloride augments hydrogen chloride emissions 12831.In our opinion, the formation of hazardous chlorinated PAHs can also not be ruled out. Recent research has revealed that polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans can easily be formed in the presence of copper chloride, hydrogen chloride, oxygen and carbon at temperatures of 475-675 K. This reaction also proceeds in the absence of hydrogen chloride, though at a lower reaction rate [286,287]. These drawbacks will probably prevent reagent-induced regeneration from being put into practice on a large scale. 6.3.2.4. Flow-fhrough oxidation catalysts. When the deadlines for tighter diesel particulate standards came nearer and catalyst and automobile manufacturers recognized that reliable solutions for particulate removal would probably not be available in time, a simpler solution was proposed to reduce part of the diesel particulates. In the engine tail-pipe temperatures are still high (325-775 K) and a fraction of the hydrocarbons, which would otherwise be collected as part of the particulate matter at lower temperatures (< 325 K; the fraction is then called SOF), is still in the gas phase. This fraction can easily be burnt without first collecting it together with the soot on a particulate trap. A so-called flow-through oxidation catalyst, an open monolith with a washcoat and an oxidation catalyst, can be used. Particulate reduction will depend strongly upon the load of the engine. At low loads the particulates contain large amounts of SOF, making relatively large particulate reductions possible. At higher loads, the amount of SOF decreases, resulting in lower particulate reductions. Typical particulate reductions of 20-50% are possible for common driving cycles. These high conversions can be attributed partly to adsorption of hydrocarbons onto the washcoat of the catalyst, which was found to occur at exhaust gas temperatures below 475 K. At higher temperatures the type of catalyst will dictate to what degree hydrocarbons will desorb before oxidation takes place [288,289]. Reduction of SOF of over 50% was reported using a standard catalyst with light-off temperatures well above 475 K in relatively cool exhaust gases (on average < 475 K) [290]. So far, mainly noble metal catalysts have been reported as effective flow-through oxidation catalysts. The first catalysts described in the literature were platinum catalysts on alumina washcoats. At higher exhaust gas temperatures, however, platina tends to convert SO, to SO,. Alumina is known for its SO, storage behaviour, and, at occasionally high temperatures, this results in high SO, releases forming sulphate mists (e.g. [291-2941). Horiuchi et al. [294] revealed that two types of sulphur storage can occur on alumina washcoats: the formation of aluminium sulphate at high temperatures and the adsorption of sulphate or sulphate droplets at low temperatures. The adsorbed sulphate can easily desorb during temperature excursions, giving rise to the above mentioned sulphate mist. The chemically bonded sulphate is more difficult to release; high temperatures of at least 775 K are needed before aluminium sulphate decomposes. This sulphate formation and storage is a key point in the development of flow-through

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oxidation catalysts. Much depends on the fuel sulphur content; the lower the fuel sulphur content, the higher the activity of catalysts can be without excessive sulphate emissions. This problem was already mentioned in an early study on oxidation catalysts, which recommended a limit of 0.05 wt% sulphur in fuel 12951.A reduction towards this 0.05 wt% is currently being implemented. Lower fuel sulphur levels are unlikely to be implemented on a large scale in years to come because of the resistance of oil companies to make the large investments involved and of the public to pay higher fuel prices. However, in Scandinavian countries the sacrifice has been made and fuel sulphur levels are already much lower. From an aftertreatment point of view, lower diesel sulphur levels would be extremely beneficial. Both washcoat improvement and catalyst optimization have received considerable attention in recent years, with the aim of reducing sulphate formation over the catalyst and also as sulphate adsorption on the washcoat. As far as washcoat improvements are concerned, only a few authors have specified optimized washcoat formulations. Silica is reported to be superior to alumina because it exhibits low sulphur storage [291,296-3001: alumina was found to store about 0.85 wt% sulphur, as against silica, which stores = 0.3 wt% [291,296]. Other papers report improved performance using titania or zirconia washcoats [301,302]. Ogura et al. [303] reported that adsorption of SOF is necessary to obtain catalytic activity at low temperatures, but that sulphate adsorption should be avoided. Although silica was found to perform better than alumina, a combined silica-alumina washcoat performed even better. Titania hardly adsorbs any SOF at all. In a joint paper by Degussa and Volkswagen [304], a new washcoat was reported with low sulphur adsorption properties in combination with a low sintering rate. A reference catalyst was found to decrease in dispersion from 21 to 4% after 32 h of catalyst ageing, whereas the new generation of catalysts essentially retained its original dispersion (15 down to 13% dispersion). The composition of the new washcoat was not disclosed. Substrates often used are ceramic (cordierite) monoliths. Some authors mention the use of metal substrates. Brear et al. [305] report an initial worse performance of a catalyst based on a metal substrate compared with a ceramic substrate equivalent. However, after short-term ageing of this catalyst, performance was similar. Concerning catalyst improvements, palladium has been suggested to (partially) replace platinum, to suppress sulphate formation at higher temperatures [292,296,297,306]. Carbon monoxide and hydrocarbon conversion curves are shifted slightly towards higher temperatures for palladium catalysts compared with platina catalysts. In addition, the sulphur oxidation behaviour is shifted to much higher temperatures. Besides, palladium or combined platina/palladium catalysts are cheaper than platina catalysts [306]. Combinations of noble metal (Pt or Pd) and base metal catalysts have been found to result in good SOF catalysts without significant SO, formation [303]. Base metal oxides were found to be responsible for the SOF oxidation, and CO and HC oxidation was established with small amounts (0.018 g 1-l) of platinum [307,308] or larger amounts (1.8 g 1-l) of palladium [309]. Similar results were reported by Wyatt et al. [292]: suppression of SO, oxidation over platina was found to be possible by the addition of several metal oxides. Vanadium pentoxide performed best. This has already been patented by Degussa [3 lo,31 11. Furthermore, partial replacement

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Conversion(%)

100 '50 ZOO 250

XC

350

400

45C

500

Temperaiure(C)

Fig. 15. Effect on CO, HC (propene) and SO, conversion curve of vanadium addition to a platina based catalyst. Adapted from [292].

of Al,O, by TiO,, in addition to the V,O, already present, appeared very efficient to decrease SO, oxidation [310,312]. An example of the suppression of SO, oxidation by V,O, is shown in Fig. 15 (from Wyatt et al. [292]); CO and HC conversions were slightly inhibited by the addition of vanadium, but SO, oxidation was retarded significantly, which is apparent from the shift to much higher temperatures. Sulphur storage was found to be lower for the Pt/V/Al catalyst compared with a Pt/Al catalyst [292]. In many papers it is argued that the most active catalysts for CO, hydrocarbon and SOF oxidation do not perform as well for particulate reduction. Catalyst performance should be optimized towards low SO, oxidation. Moderately active catalysts with low SO, oxidation rates perform best as far as particulate removal is concerned. Only when very low fuel sulphur levels are used (e.g. 0.01 wt%) do the most active catalyst formulations perform the best for particulate reduction [305]. Sulphur compounds are reported to influence CO and HC conversions. At low temperatures, oxidation of CO and HC was found to be hindered by higher SO, concentrations [305,313]. Also, Engler et al. 13061 report a decrease in hydrocarbon oxidation activity, but CO oxidation rates were found to be similar for 0.05 and 0.26 wt% sulphur in the fuel. The poisoning of flow-through oxidation catalysts has been studied to some extent. Sulphate storage was found to deactivate catalysts, shifting CO and hydrocarbon combustion curves to higher temperatures. Using a platina catalyst (1.8 g l-r), this difference was reported to amount to some 50 K [306]. This was thought to be caused by adsorption of SO, on platina at low temperatures, which would inhibit hydrocarbon adsorption [314]. Harayama et al. [315] gave data on the initial uptake of sulphate, and showed that sulphate emissions became constant only after several US transient test cycles. Apart from sulphur oxides, phosphorus and zinc species were found to poison precious metal catalysts. Beckmann et al. [304], comparing the influence of sulphur, phosphorus and zinc, concluded that sulphur oxides were the main poisoning species. Adsorption behaviour differed remarkably: phosphorus and zinc (originating principally from lubricating oil) were found mainly in the entrance part of the open monolith, whereas sulphur was found throughout the entire monolith [304,306,308]. In agreement with this observation, sulphur was found throughout the whole washcoat, but zinc,

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phosphorus and calcium were detected only on the surface of the washcoat [304,308]. Voss et al. [308] found a linear increase in the amounts of phosphorus and zinc on the catalyst with catalyst ageing time. The amount of sulphur on the catalyst, on the other hand, became stable after a relatively short time. Finally, SOF and sulphate formation could also result in deactivation of the catalyst over the longer term, as was shown by Ogura et al. [303], using a Pt/Al,O, catalyst at temperatures of 545 K maximum. After a durability test of 15 000 km they found the catalyst surface to be completely blocked by a layer of adsorbed sulphate and SOF. This observation also explains the results of Ball and Stack [296], who report deactivation of palladium and platina catalysts after ageing in diesel exhaust gases (675 K maximum). When using silica washcoats, however, they did not find any deactivation for a palladium catalyst. Also, Khair [299] and Khair and Bykowski [300] found a difference in deactivation behaviour between alumina and silica washcoat based catalysts. Catalysts with an alumina washcoat showed higher deactivation and a larger sulphate desorption than similar catalysts with silica washcoats after ageing at high fuel sulphur levels and relatively low temperatures, not exceeding 575 K. Flow-through oxidation catalysts have been introduced in the diesel engine market quite quickly, and currently many catalyst manufacturers develop or sell these catalysts for diesel engine application (Degussa, Engelhard, Johnson Matthey, Kemira and Nippon among many others; Khair and Bykowski [300] mention 17 companies supplying diesel catalytic convertors). As early as 1989 Volkswagen introduced a passenger car onto the market equipped with a diesel oxidation catalyst [316]. The current technology still has major drawbacks; at very low temperatures the activity of current catalysts is not high enough, and at high temperatures sulphate formation poses problems. This is summarized in Fig. 16 [306]. Catalyst and washcoat formulation and amount can therefore vary between different diesel applications; more active catalysts should be used for engines with low average exhaust gas temperatures than for those with high exhaust gas temperatures. Within this context, the difference between light and heavy-duty engines must be

100 50

Change

of particulate

Organic

mass concentration

(8)

solubles

0 -50 -100

250

350 Exhaust

Fig. 16. The temperature et al. [306].

window of current generation

gas temperature

flow-through

oxidation

450 (“C) catalysts.

Adapted from Engler

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mentioned. The SOF content of the particulate matter is relatively high in light-duty engines, and in the US and European driving cycles the emphasis is on light load operation of these engines. It is because of this that flow-through oxidation catalysts are currently being retrofitted onto these engines; certainly in Europe the introduction of these catalysts is happening quickly. In heavy-duty engines, the merits of these catalysts are much lesss as the SOF content of heavy-duty particulates is lower and SO, oxidation becomes more important. In the European 13-mode test, the emphasis is on high loads and speeds at which high exhaust gas temperatures occur, causing SO, oxidation to become critical. In the US heavy-duty transient cycle, testing conditions are milder, and thus the introduction of flow-through oxidation catalysts for heavy-duty engines stands a better chance on the US market. Although the oxidation of CO and gaseous hydrocarbons is not necessary in order to meet emission standards in the near future, SOF oxidation can certainly help to eliminate health hazards caused by potential mutagens among the PAHs. A number of studies have been performed regarding the mutagenicity of diesel emissions with or without a catalyst (making use of Ames tests). A general tendency towards decreased mutagenicity attributable to the use of a flow-through oxidation catalyst was observed [317-3191. However, in some studies an increase of mutagenicity per mass unit of emitted SOF was found [319], or the mutagenicity of exhaust gases was even reported to increase with the use of a flow-through catalyst 13201. Using particulate traps, similar observations have been reported [270,321]. In some studies not only particulate-bound hydrocarbons @OF) were found to cause the mutagenicity of diesel exhaust gases; gas-phase hydrocarbons were also identified as mutagenic [13,31X]. The use of flow-through oxidation catalysts or particulate traps could well shift the relative contribution to mutagenicity of particulate-bound and gaseous hydrocarbons. This would mean that the mutagenicity of both groups of hydrocarbons should be assessed in order to evaluate the total effect of these aftertreatment devices [ 131.

7. Summary In this paper, the background of the emission of particulate matter from diesel exhaust gases has been reviewed. The diesel engine in modem society was outlined as a fuel efficient and durable internal combustion engine. With respect to its emissions, however, many other combustion processes, including Otto engines equipped with three-way catalysts, perform better. Emission standards have therefore been proposed to reduce significantly the harmful emissions of diesel engines, notably particulates and NO,. Several measures for reducing particulate and NO, emissions were described. In the first place, emissions could be reduced by optimizing the fuel composition, or by using an alternative fuel. Although significant, emission reductions obtained in this way were not sufficient to attain the emission targets desired. Secondly, emissions were also diminished by modifying the engine. Technological advances could result in reduced overall emissions. If both targets, i.e. reduction of particulates and of NO,, were not achievable, an interesting policy would be to reduce one emission at the cost of an

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increased emission of the other (which would then be removed by aftertreatment). These measures were cost effective and were put into practice, but probably would not be sufficient in the long run. Thirdly, and finally, emissions could also be decreased by a number of aftertreatment techniques. Some of these aftertreatment techniques (i.e. selective catalytic reduction of NO, with hydrocarbons, particulate removal using traps combined with catalytic coatings or with oxidizing agents) were clearly still too experimental to be applied within the next few years. Some of the other techniques (traps plus a burner or electrically assisted regeneration, a trap combined with fuel additives, flow-through oxidation catalysts, selective catalytic reduction of NO, with ammonia or urea) could possibly play a role in emission reduction of both particulates and NO, in the near future. Future application of any of these techniques will depend upon many stimuli; technological improvements of the various emission reduction techniques, legislative steps accelerating emission reductions, automotive interest to implement the various techniques, cost, and, last but not least, public acceptance of the different measures.

8. Current position and future outlook Papers dating from the early 1980s imply that the general view of those days was that particulate trap-based systems would become available within a few years. The development of catalyst, burner, electrically or throttle assisted regeneration would be a matter of a few years. By the mid and late 1980s this view had changed, as trap regeneration techniques in general proved to be unreliable. Engine manufacturers started developing low emission engines, and catalyst manufacturers put their efforts into non-filter-based oxidation catalysts. The particulate standards of 1991 and 1994 (USA) and 1993 (Europe) were met in this way, and the limits of the near future were expected to be achieved in a similar way. At the beginning of the 1990s it became clear, however, that other measures would have to be taken. In this respect, diesel engines can no longer be viewed as one large family of essentially identical engines. Strategies to fulfil standards will depend upon the specific application of the diesel engine. The most urgent are measures for buses, as, by 1996, the US limit for particulates will decrease from 0.1 to 0.05 g per bhp-h (0.067 g per kWh). Buses have heavy-duty engines (1996 standard: 0.1 g per bhp-h), but because of their use in urban environments bus limits will be tightened. These limits cannot be met by engine modifications alone, but at least three other possibilities exist to meet these standards. In the first place, fuels leading to low particulate emissions could be used, such as CNG, LPG or methanol. An oxidation catalyst would suffice to decrease the increased hydrocarbon, aldehyde and CO emissions. A second option consists of a particulate trap in combination with a burner or electrically assisted regeneration. Extensive field tests have been initiated, but successful application in the short term seems doubtful because of problems regarding the reliability of this regeneration method [183]. The third option consists of a trap in combination with a fuel additive (cerium [165] or copper [169]). All three options have drawbacks, namely: (1) high costs for reconstruction of engines and additional fuel

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Emissions

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in g/kwh European

I

Current

engine

Euro-II

emission

standards

Euro-III

/

Injection Reduced

timing control oil consumption

Higher injection pressures Reduced oil consumption EGR

Fig. 17. Strategy towards low particulate emissions for medium and heavy-duty engines. Medium-duty Ricardo engine. Data from Needham et al. [ 1101,after Zelenka et al. [ 1091.

distribution systems, (2) high costs for the complex regeneration system, and (3) logistic problems with additive distribution. In addition, the reliability of the two latter options has still to be proven on a large scale. The outlook is completely different with regard to other heavy-duty engines. Engine modifications such as injection at higher pressures, enhanced electronic injection, EGR, turbocharging, and decreasing the consumption of lubricating oil, in combination with an improved fuel, are thought to prove sufficient to fulfil US 1998 and Euro-II (1996) particulate and NO, standards, and also the anticipated Euro-III standards (1999) [ 1IO,1 141. In Fig. 17 the measures for particulate reduction are shown schematically for European standards. In a similar approach towards future US limits, Zelenka et al. [109] foresee a role for oxidation catalysts in heavy-duty applications. Concerning the reduction of NO, emissions, many engine developers see EGR, in combination with a fully optimized engine, as the technique for meeting future NO, standards. Researchers at Ricardo, however, point out that SCR catalysts (“DENOX catalysts”) could also stand a chance, if not before Euro-III, then certainly in the beginning of the next century [llOl. Concerning medium-duty diesel engines, the situation is somewhat different. Medium-duty engines are less durable and less expensive than heavy-duty engines. Measures like modem high pressure injection pumps and electronic injection timing are so expensive that application on medium-duty diesel engines is less probable. As a result, not all engine modifications which can reduce emissions of heavy-duty engines towards future standards will be applied to medium-duty engines. Oxidation catalysts will therefore probably be needed to reach standards in the second half of the 1990s [86]. Light-duty diesel engines (passenger cars and light-duty trucks) are expected to be able to meet the standards of 1994 and 1996 with oxidation catalysts and EGR. No expensive engine modifications will be implemented. In general terms, it can therefore be concluded that to date, engine modifications are still the prime measures for reducing particulate and NO, emissions from diesel engines.

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Flow-through oxidation catalysts have been introduced in light-duty diesel engines, but other aftertreatment techniques have not yet proven reliable on a large scale. Reduced fuel sulphur levels in the USA, shortly also to be implemented in Europe, have contributed only to a small extent towards the achievement of current emission standards. In the future, the importance of fuels for reducing engine emissions will remain at the same level: the results are significant but small. The further decrease of sulphur levels and increase of cetane numbers through (mild) hydrotreating will have a positive effect on engine emissions. Engine modifications such as EGR, electronic injection control, turbocharging, and high injection pressures will remain important for emission reduction, but the general expectation is that future emission standards can no longer be met by engine modifications and improved fuel specifications alone. The role of the flow-through oxidation catalyst will gain importance (and will no longer be used primarily for marketing reasons, as is the view of some car manufacturers at this moment [322]). It is likely that these catalysts will be implemented in some specific applications such as buses from 1996 onwards and in medium and light-duty engines in a later stage. Finally, electrically regenerated traps, or traps in combination with fuel additives, are other options that will possibly be implemented in years to come.

Acknowledgements This review article was written within the project “Removal of soot and NO, from diesel exhaust gases” (Project No. 628311-Ol), sponsored by the Dutch Ministry of Housing and the Environment. Financial, technical and scientific support is gratefully acknowledged. The authors further acknowledge the critical commentary on this manuscript by L.C. van Beckhoven, D.M. Heaton, R.C. Rijkeboer, and P.T.J. Scheepers, and the assistance in preparing the final manuscript by L. Taylor.

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Verbmnnung

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und Miiglichkeiten

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