SAE TECHNICAL
`PAPER SERIES
`
`2001-01-0907
`
`Regeneration of Catalytic Diesel
`Particulate Filters
`
`J. Gieshoff, M. Pfeifer, A. Schäfer-Sindlinger,
`U. Hackbarth and O. Teysset
`Automotive Catalysts Division, dmc2 AG
`
`C. Colignon, C. Rigaudeau and O. Salvat
`PSA Peugeot Citroën
`
`H. Krieg and B.W. Wenclawiak
`Department of Chemistry, University of Siegen
`
`Reprinted From: Diesel Exhaust Emission Control: Diesel Particulate Filters
`(SP–1582)
`
`400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A.
`
`Tel: (724) 776-4841 Fax: (724) 776-5760
`
`SAE 2001 World Congress
`Detroit, Michigan
`March 5-8, 2001
`
`BASF-2001.001
`
`

`
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`ISSN 0148-7191
`Copyright 2001 Society of Automotive Engineers, Inc.
`
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`Printed in USA
`
`BASF-2001.002
`
`

`
`Regeneration of Catalytic Diesel Particulate Filters
`
`J. Gieshoff, M. Pfeifer, A. Schäfer-Sindlinger, U. Hackbarth and O. Teysset
`Automotive Catalysts Division, dmc2 AG
`
`2001-01-0907
`
`C. Colignon, C. Rigaudeau and O. Salvat
`PSA Peugeot Citroën
`
`H. Krieg and B.W. Wenclawiak
`Department of Chemistry, University of Siegen
`
`Copyright © 2001 Society of Automotive Engineers, Inc.
`
`ABSTRACT
`
`This paper will discuss a number of different matters
`relating to the regeneration of catalyst coated diesel
`particulate filters such as: impact of the catalyst on the
`soot ignition temperature, soot combustion rate and NO2
`generation.
`If catalytic coatings prove to be sufficient compared to
`certain fuel additives they could be used in second
`generation diesel particulate aftertreatment systems.
`Examples will be shown on how catalytic diesel
`particulate filters (“DPF”) can operate on a common rail
`passenger car diesel engine. Furthermore, an outlook is
`given on the future combination of particulate - and NOx -
`emission control for diesel passenger cars.
`
`INTRODUCTION
`
`The mandated EURO IV and LEV II legislation in
`2005/2007 for diesel passenger cars and the increasing
`concern about health effects of diesel exhaust emissions
`leads to numerous efforts to solve the issue of NOx and
`particulates of diesel vehicles. As shown in a previous
`SAE paper, a first car manufacturer has started to tackle
`the particulates issue for passenger cars by using a
`diesel particulate filter system requiring a fuel additive[1].
`This development is only the first step into the direction
`of ultra clean diesel vehicles. Since the mid 1980´s, a lot
`of efforts were made to solve the particulate emission
`problem on diesel engines [2].
`
`However, none of these measures were sufficient to
`solve
`the problem completely. Figure 1 gives an
`overview of the possibilities to reduce particulates by
`means of engine measures or by emission control
`technologies.
`
`Particulate Matter
`
`Engine Measures
`
`Aftertreatment Measures
`
`VGT-Charger
`
`Combustion Chamber
`
`Injection Timing
`
`Particulate Trap
`
`Oxidation Catalyst
`(SOF only)
`
`Figure 1:
`
`for
`Possibilities
`emission control
`
`diesel
`
`particulate
`
`Diesel particulates can be filtered with a very high rate of
`efficiency. The major obstacle is the regeneration of the
`particulate filter [4]. Numerous papers were published
`that presented different soot ignition coatings [5]. Only a
`few technologies are in the market today whose main
`use is for heavy-duty applications. One of them applies a
`pre-catalyst to oxidize NO to NO2 and to combust the
`collected soot with NO2 [6]. It is well known that NO2
`oxidizes soot at temperatures in the range between
`300°C and 400°C [7]. Notably, the broad application in
`passenger cars
`is
`limited due
`to
`its unfavorable
`temperature window for the particulate oxidation. In
`practice there is only one light duty vehicle currently
`available with a factory equipped DPF system using an
`additive to decrease the soot ignition temperature and to
`support the soot combustion process [1].
`
`This paper demonstrates the capabilities of catalytic
`diesel particulate filter coatings for oxidative removal of
`diesel particulates. Catalytic coatings were categorized
`according to their potential to reduce soot ignition
`temperatures. Furthermore, the paper describes the
`impact of O2 and NO2 on passive regeneration of
`passenger car diesel particulate filters. Finally, the NO2
`impact on active regeneration is discussed and an
`alternative for a filter regeneration strategy is given.
`
`BASF-2001.003
`
`

`
`between soot and active catalyst material. Figure 3
`shows an example of TGA with K4V2O7.
`
`100,00
`
`90,00
`
`80,00
`
`70,00
`
`60,00
`
`50,00
`
`40,00
`
`30,00
`
`20,00
`
`10,00
`
`0,00
`
`relative mass [%]
`
`0
`
`50
`
`100
`
`150
`
`200
`
`250
`
`300
`
`450
`400
`350
`temperature [°C]
`
`500
`
`550
`
`600
`
`650
`
`700
`
`750
`
`800
`
`Figure 3:
`
`Impact of K4V2O7 as catalytic material on
`soot ignition temperatures of Printex U
`
` K4V2O7 belongs to the class of molten salts which begin
`to liquefy before soot combustion starts. There are other
`materials with similar properties but they do not exhibit
`the same behavior. Table 1 lists these materials.
`
`Table 1: Decrease soot ignition temperature (50%
`conversion) for different catalyst materials.
`
`Material
`
`K4V2O7
`V2O5
`Ce-Mn-Ag mixed
`oxide/Pt
`Ag2V2O6
`CeO2/Pt
`MnO2/Pt
`
`Printex U
`[°C]
`- 190
`- 100
`- 60
`
`- 10 ~ 20
`- 10
`none
`
`Diesel Soot
`[°C]
`- 100
`- 10 ~ 20
`- 20
`
`none
`none
`none
`
`As the list in Table 1 exemplifies, the most active
`compounds contain vanadium. V2O5 formation of vanadia
`salts at elevated temperatures up to 600°C during soot
`combustion was found. Oxygen storage material like
`ceria just increases soot combustion rate, but reduction
`of the soot ignition temperature is not observable.
`
`A major disadvantage of TGA and DSC experiments
`was the test set-up does not represent the situation of
`wall flow particulate filters. The experiments are usually
`performed in small crucibles. In reality, the catalyst
`material is coated as a layer on the inner surface of the
`wall flow filter and a soot layer builds up on the top of
`catalyst. The exhaust gas passed these layers while
`flowing through the surface filter. In TGA and DSC
`experiments the gas does not flow through the sample
`mixture. Powder reactor experiments were designed to
`better represent actual conditions in wall flow filters but
`these tests also do not correlate completely to engine
`bench measurements with wall flow filters.
`
`SOOT IGNITION COATINGS
`
`Beginning in the mid 1980´s many attempts have been
`made to develop materials which are able to lower the
`soot ignition temperature. None of the materials proved
`to be sufficient to lower the soot combustion temperature
`significantly except the so-called molten salts, such as
`the ones based on vanadia-containing compounds [8].
`This class of compounds seemed to show promise, but
`recent
`investigations have shown some problems
`associated with these materials [9].
`
`TGA AND DSC EXPERIMENTS – Most investigations
`into
`soot
`ignition
`temperatures
`start
`with
`thermogravimetric analysis
`(TGA) or differential
`scanning calorimetric (DSC) experiments. Usually a well-
`defined artificial soot like Printex U (Degussa-Hüls AG)
`is used. The advantage in using artificial soot relates to
`the fact that this type of material does not contain any
`soluble organic fraction thus providing a well-defined
`starting point for all the tests. Soot collected from
`engines does not always have a defined SOF content.
`The concentration depends greatly on the sampling
`procedures. Figure 2 gives an example for TGA tests
`with Printex U. Tests were performed in a mixture of
`10% oxygen diluted in nitrogen in a temperature range
`between 25°C and 1000°C . The gas flow rate was
`adjusted to 50 ml/min with the temperature ramp
`20°C/min.
`
` Diesel Soot
` Printex U
`
`110,00
`
`100,00
`
`90,00
`
`80,00
`
`70,00
`
`60,00
`
`50,00
`
`40,00
`
`30,00
`
`20,00
`
`10,00
`
`0,00
`
`relative mass [%]
`
`0
`
`50
`
`100
`
`150
`
`200
`
`250
`
`300
`
`450
`400
`350
`temperature [°C]
`
`500
`
`550
`
`600
`
`650
`
`700
`
`750
`
`800
`
`Figure 2.
`
`Soot combustion of Printex U and diesel
`soot in TGA experiments.
`
`The soot combustion with Printex U starts at about
`520°C and finishes at around 720°C. The curves are
`very reproducible. The soot ignition temperature is
`defined as the temperature at 50% soot combustion. The
`ignition temperature of diesel soot without any catalyst is
`usually 100°C lower than Printex U.
`
`A concept has been proposed that by mixing certain
`materials with soot lead to either a change in the onset
`of the combustion or in an increase of the soot
`combustion rate [10]. A significant decrease of soot
`ignition temperature is only observable with a certain
`class of compounds, which
`improves
`the contact
`
`BASF-2001.004
`
`

`
`temperatures determined by
`ignition
`Table 2: Soot
`Powder Reactor tests
`
`Material
`
`w/o catalyst
`K4V2O7
`V2O5
`Ag2V2O6
`CeO2
`CeO2/Pt
`
`Printex U
`[°C]
`536
`398
`476
`512
`505
`525
`
`Diesel Soot
`[°C]
`478
`372
`433
`458
`453
`450
`
`As shown in Table 2, only the molten salt decreases the
`soot ignition temperature significantly. This confirms the
`TGA and DSC results. The observed soot ignition
`temperatures come closer to values observed for diesel
`soot in engine experiments. One important observation
`presents a drawback for materials based on molten salts
`such as K4V2O7. As mentioned above, under reducing
`conditions such as the one occuring locally during soot
`combustion, vanadia formation was observed in the
`crucibles of the TGA and DSC experiments. Notably,
`Ag2V2O6 decomposes into vanadia and metallic silver at
`temperatures below 600°C as shown in Figure 5.
`Decomposition of vanadia salts may pose limitations to
`their use
`in soot
`ignition catalysts. During
`filter
`regeneration temperatures up to 1000°C inside the trap
`were observed in engine test experiments.
`
`metallic
`silver
`
`vanadia
`
`metallic
`silver
`
`silver
`vanadat
`
`Figure 5:
`
`Vanadia and metallic silver formation during
`TGA and DSC tests.
`
`POWDER REACTOR TESTS – The powder reactor
`tests were performed with a mixture of Printex U and the
`catalyst material at a mass-ratio of 2:3. The model gas
`flows over the mixture deposited on a quartz filter. The
`tests reported here were also performed in a mixture of
`10 vol.-% oxygen in nitrogen. The gas flow rate was set
`to 50 ml/min at standard condititions. The temperature
`ramp was 10°C/min,
`the
`temperature range was
`between 25°C and 800°C.
`
`During these experiments, the soot combustion progress
`was monitored by mass spectrometry. The start of the
`soot combustion was determined when the oxygen
`signal (mass 32) started to drop. The soot ignition
`temperature was defined as the maximum of the CO2
`formation curve (mass 44). Figure 4 shows the soot
`combustion curves for different materials evaluated
`during the powder reactor experiments.
`
`Masse 32 [A]
`Masse 44 [A]
`
`1,2E-05
`
`1,0E-05
`
`8,0E-06
`
`6,0E-06
`
`4,0E-06
`
`MS-Signal [A]
`
`2,0E-06
`
`0,0E+00
`
`0
`
`50
`
`100
`
`150
`
`200
`
`250
`
`300
`
`450
`400
`350
`temperature [°C]
`
`500
`
`550
`
`600
`
`650
`
`700
`
`750
`
`800
`
`Figure 4.
`
`temperature when using
`ignition
`Soot
`catalyst components with Printex U
`in
`powder reactor tests
`
`The soot combustion temperature for both types of soot
`decreases significantly and approaches the level of
`diesel soot in engine tests. The lowest soot combustion
`temperature was again observed with potassium
`vanadate while ceria and mixtures of Pt and ceria had
`almost no impact on soot combustion temperatures [9].
`
`these powder
`results of
`The
`summarized in Table 2.
`
`reactor
`
`tests are
`
`BASF-2001.005
`
`

`
`Figure 7:
`
`Engine bench setup for filter regeneration
`tests.
`
`PASSIVE FILTER REGENERATION – As mentioned
`above, spontaneous regeneration can occur during
`deceleration phases after
`longer high
`temperature
`operation of diesel particulate filters. In this case NO2
`might initiate the regeneration at lower temperatures. A
`test with a step wise load increase was developed to
`investigate passive filter regeneration on the engine
`bench.
`
`The catalyst composition consisted of platinum and an
`oxygen storage compound as given in Tables 1 and 2.
`The precious metal loading ranged between 50 and 150
`g/ft3. The total washcoat loading was 50 g/l.
`
`The accumulated soot amount was roughly 20 g/filter
`after a soot loading phase of 2000 rpm at 50 Nm. The
`load, i.e. temperature was increased in 25 or 50°C steps
`from 200 to 550°C. During this test temperatures in the
`filter, back pressure and NO2 formation were monitored.
`Figure 8 shows an example for these experiments.
`
`200
`
`175
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`150
`
`125
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`100
`
`75
`
`50
`
`25
`
`p_5 [mbar]
`
`T_1_DPF [°C]
`
`T_4_DPF [°C]
`
`p_5 [mbar]
`
`0
`
`1000
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`2000
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`3000
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`4000
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`5000
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`6000
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`7000
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`8000
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`9000
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`0
`10000
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`time / s
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`550
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`500
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`450
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`400
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`350
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`300
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`250
`
`200
`
`T_1 T_4 DPF [°C]
`
`Figure 8:
`
`Passive regeneration via stepwise
`increase on a 2.0 l CR engine.
`
`load
`
`MODEL GAS TESTS WITH NO2 – It is known that NO2
`has an impact on soot oxidation/combustion since the
`late 1980´s [7]. In the case of diesel passenger cars the
`amount of NO or NO2 available from the engine and the
`temperature level are not sufficient to create continuous
`soot combustion. The impact of NO2 on soot ignition
`temperature was studied. During deceleration phases,
`after
`longer high
`temperature operation of diesel
`particulate filters, spontaneous regeneration can occur
`the soot oxidation at
`lower
`and NO2 may start
`temperatures. To elucidate this phenomenon model gas
`tests were performed as described above with 1000 vol.-
`ppm NO2 in addition in the feed gas. Figure 6 shows an
`example.
`
`Masse 32 [A]
`Masse 44 [A]
`
`1,2E-05
`
`1,0E-05
`
`8,0E-06
`
`6,0E-06
`
`4,0E-06
`
`2,0E-06
`
`0,0E+00
`
`MS-Signal [A]
`
`0
`
`50
`
`100
`
`150
`
`200
`
`250
`
`300
`
`450
`400
`350
`temperature [°C]
`
`500
`
`550
`
`600
`
`650
`
`700
`
`750
`
`800
`
`Figure 6:
`
`NO2 impact on soot ignition.
`
`NO2 decreases the soot ignition temperature but high
`NO2/soot ratios are necessary for this to occur. Similar
`effects are observable with NO.
`In
`this case Pt
`containing washcoats lead to in situ formation of NO2.
`The amount of NO2 formed is too small to combust all
`the soot and it is assumed that the complete combustion
`can only be an oxygen supported process. Ceria in the
`catalytic coating helps
`therefor
`to suppress CO
`formation.
`
`PASSENGER CAR ENGINE BENCH TESTS
`
`SET UP AND EXPERIMENTAL CONDITIONS – All
`engine bench experiments were performed either on a
`2.0l or on a 2.5l turbo charged common rail diesel
`engine. Figure 7 shows the setup for all experiments.
`Standard diesel fuel was used with a sulfur content of
`335 ppm and a cetane number of 53.6. The exhaust gas
`temperature was monitored upstream and downstream
`of a pre-catalyst and the particulate filter. Additionally
`four thermocouples were placed in the particulate filter
`and back pressure and lambda sensors were placed in
`the system. The CO, hydrocarbon and NOx emission
`measurement was done with a standard Pierburg
`analyzer system.
`
`BASF-2001.006
`
`

`
`Impact of the catalytic coating on the observed soot
`ignition
`temperatures was
`low.
`Interestingly, all
`regeneration processes occurred
`in
`the
`same
`temperature range between 400°C and 500°C. At this
`point significant NO2 formation on the coated particulate
`filters was observed (Figure 11). This is due to the fact
`that, according to thermodynamics and kinetics, a
`certain fraction of the NO is oxidized on Pt at this
`temperature (400°C).
`
`the regeneration
`Figure 11: NO2 concentrations at
`point after different catalytic diesel par-
`ticulate filter.
`
`The passive regeneration effects can be divided into
`three parts:
`
`1. Oxidation of SOF at lower gas temperatures
`between 250 to 350°C;
`
`2. Small NO2 impact on the soot between 350°C and
`400°C depending on NO/NO2 availability i.e. EGR
`rate;
`
`3. Carbon oxidation at high temperatures (> 450°C).
`
`Since passive regeneration does not occur at sufficiently
`low temperatures, modern diesel engines need an
`emission control aftertreatment, that is able to cover the
`low temperature operating conditions. Post injection of
`diesel fuel in the combustion chamber provides a
`method to induce filter regeneration.
`
`VIA
`REGENERATION
`FILTER
`ACTIVE
`POSTINJECTION – Regeneration
`tests with post
`injection were performed on the DW10, 2 liter Common
`Rail (“CR”) diesel engine from PSA. During post injection
`conditions a certain amount of hydrocarbons was
`injected into the combustion chamber after the main
`combustion at – 40 ° crank angle.
`
`Figure 12 shows the hydrocarbon concentration during
`post injection versus engine speed and load.
`
`then combusted on an
`These hydrocarbons are
`oxidation catalyst upstream of the catalyst coated diesel
`particulate filter. For all tests silicon carbide filters were
`
`Between 250°C and 350°C a strong back-pressure
`decrease was observed in the engine experiment.
`Analysis of the filter weight showed however that a
`significant amount of soot was left in the filter. Further
`tests showed that this effect was solely due to the
`oxidation of the SOF on the soot. Other experiments
`showed that real soot combustion is only observed at
`temperatures above 400°C (Figure 9).
`
`1000
`
`900
`
`800
`
`700
`
`600
`
`500
`
`400
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`300
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`200
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`01
`
`00
`
`P5[mbar]
`
`T_1_DPF [°C]
`
`p_5 [mbar]
`
`1000
`
`900
`
`800
`
`700
`
`600
`
`500
`
`400
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`300
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`200
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`100
`
`0
`
`T_1_DPF
`
`0
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`100
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`200
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`300
`
`400
`
`500
`time / s
`
`600
`
`700
`
`800
`
`900
`
`1000
`
`Figure 9:
`
`a
`of
`behavior
`Regeneration
`particulate filter on a 2.0 l CR engine.
`
`diesel
`
`The oxygen concentration in the raw gas upstream of
`the filter during this process was in the range of 6 to 14
`vol.-%. (Figure 10). Analysis of the oxygen mass flow,
`even with EGR, indicated that there was sufficient
`oxygen to oxidize all the soot. Based on stoichiometry
`(20 g soot/filter) leads to an significant decrease during
`the soot combustion process.
`
`18
`
`15
`
`12
`
`9
`
`6
`
`3
`
` O2_vK [Vol%]
`
`p_5 [mbar]
`
`O2_vK [Vol%]
`
`0
`
`1000
`
`2000
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`3000
`
`4000
`
`5000
`time / s
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`6000
`
`7000
`
`8000
`
`0
`9000 10000
`
`150
`
`125
`
`100
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`75
`
`50
`
`25
`
`0
`
` p5 [mbar]
`
`Figure 10: O2 concentrations and pressure drop at the
`regeneration point in front of a catalytic
`diesel particulate filter.
`
`BASF-2001.007
`
`

`
`pres s u re drop c oated DPF
`
`pres s u re drop dummy
`
`10 0
`
`200
`
`3 00
`
`40 0
`
`5 00
`
`60 0
`
`70 0
`
`8 00
`
`9 00
`
`100 0
`
`time / s
`
`1 00
`
`80
`
`60
`
`40
`
`20
`
`pressure drop [mbar]
`
`0
`
`0
`
`Figure 13:
`
`for a catalyst
`Back pressure curve
`coated diesel particulate filter at 550°C
`inlet temperature.
`
`filter regeneration proceeds rapidly within 60
`The
`seconds after heating up of the filter. Compared to the
`uncoated dummy,
`the
`regeneration speed
`is not
`significantly increased when a catalytic coating is applied
`on the filter (Figure 13). In most cases, about 550°C inlet
`temperature is necessary to provide a high regeneration
`speed. If the gas temperature at filter inlet is decreased
`the regeneration proceeded much slower.
`
`The impact of the catalyst on regeneration speed and
`soot ignition temperature is low due to the poor contact
`between catalyst and soot [11]. Figure 14 compares soot
`ignition temperatures with different catalyst coatings.
`The catalysts based on platinum combined with different
`oxygen storage
`compounds. The
`soot
`ignition
`temperature decreases about 60°C when vanadia-free
`catalyst coating B is applied. Certain vanadia-containing
`coatings led to better results, but stability is a major
`problem as previously mentioned.
`
`600
`
`580
`
`560
`
`540
`
`520
`
`500
`
`480
`
`Tign [°C]
`
`T-ign. DPF Dummy
`T-ign. DPF50b
`T-ign DPF37
`Figure 14: Comparison of the soot ignition temperature
`with different coating materials.
`
`used. An observed exotherm after the filter was also
`indicated in Figure 12.
`
`20
`
`40
`
`60
`
`100
`80
`Load [Nm]
`
`120
`
`140
`
`160
`
` 4813 -- 5500
` 4125 -- 4813
` 3438 -- 4125
` 2750 -- 3438
` 2063 -- 2750
` 1375 -- 2063
` 687.5 -- 1375
` 0 -- 687.5
`
`HC emission [ppm]
`
` 695.0 -- 720.0
` 670.0 -- 695.0
` 645.0 -- 670.0
` 620.0 -- 645.0
` 595.0 -- 620.0
` 570.0 -- 595.0
` 545.0 -- 570.0
` 520.0 -- 545.0
`
`temperature [°C]
`
`4000
`
`3500
`
`3000
`
`2500
`
`2000
`
`1500
`
`1000
`
`Engine Speed [rpm]
`
`4000
`
`3500
`
`3000
`
`2500
`
`2000
`
`1500
`
`1000
`
`Engine Speed [rpm]
`
`20
`
`40
`
`60
`
`80
`100
`Load [Nm]
`
`120
`
`140
`
`160
`
`Figure 12: Hydrocarbon emission map under post
`injection conditions versus temperature in
`front of the diesel particulate trap
`
`load conditions,
`the speed and
`Depending on
`temperatures up to 300°C are created upstream of the
`filter. A gas temperature level of 550°C to regenerate the
`filter was reached after 60 seconds. This time is also
`necessary to heat up the filter. The post injection was
`maintained for additional nine minutes to complete the
`regeneration. Figure 13 shows the back pressure curve
`of a 200 cpsi, 14 mil catalyst coated SiC diesel
`particulate filter compared with an uncoated SiC filter
`during regeneration via post injection at 1500 rpm,
`30 Nm.
`
`BASF-2001.008
`
`

`
`120
`
`110
`
`100
`
`90
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`0
`
`01
`
`P_5, p_6 [mbar]
`
`NO_ECO_vK [ppm]
`
`NO_ECO_nK [ppm]
`
`NO2_ECO_nK
`
`p_5 [mbar]
`
`NO before DPF
`
`NO after DPF
`
`p_5
`
`NO2 after DPF
`
`0
`
`100
`
`200
`
`300
`
`400
`
`500
`
`600
`
`tim e / s
`
`300
`
`275
`
`250
`
`225
`
`200
`
`175
`
`150
`
`125
`
`100
`
`75
`
`50
`
`25
`
`0
`
`NOx_ECO [ppm]
`
`A benefit of the catalyst coating is the suppression of CO
`and hydrocarbon emissions during filter regeneration.
`When soot oxidation on an uncoated filter has started
`high CO formation is usually observed. Figure 15
`illustrates this phenomenone.
`
`CO_vK, CO_nK [Vol%]
`
`0,5
`
`0,45
`
`0,4
`
`0,35
`
`0,3
`
`0,25
`
`0,2
`
`0,15
`
`0,1
`
`HC_v K [ppm]
`
`HC_nK [ppm]
`
`CO_ vK [V ol% ]
`
`CO_ nK [V ol% ]
`
`HC before DPF
`
`CO before DPF
`
`CO after DPF
`
`HC after DPF
`
`1 0000
`
`900 0
`
`800 0
`
`700 0
`
`600 0
`
`500 0
`
`400 0
`
`300 0
`
`200 0
`
`NOx v.K. [ppm]
`
`HC_vK, HC_nK [ppm]
`
`Figure 17: NO/NO2 trace during regeneration via post
`injection.
`
`The effects of vanadia-free catalyst coatings on passive
`and active particulate
`filter
`regeneration can be
`summarized as follows:
`
`1. Small decrease of the soot ignition temperature
`(20 –60°C);
`
`2.
`
`In situ formation or recycling of NO2 in the filter
`depending on NO concentrations and engine
`operation conditions;
`
`3. Oxidation of CO, hydrocarbons and SOF at normal
`operation conditions and during filter regeneration.
`
`Vanadia-containing materials, i.e. molten salts based on
`alkali metal salts and vanadia or vanadium oxide, lead to
`better results in terms of soot ignition temperature
`decrease because of a better contact between soot and
`catalyst. However, the thermal decomposition of many
`vanadia
`containing
`soot
`ignition
`catalysts
`at
`temperatures above 650°C and under regeneration
`conditions poses a challenge to their durability as diesel
`particulate soot ignition catalysts. Therefore vanadia-free
`materials are
`the more
`likely alternative
`for
`the
`particulate filter application.
`
`Since the effect of vanadia-free catalyst regeneration via
`post injection is mainly limited to suppression of CO and
`hydrocarbon emissions, another function of the catalyst
`coating might be interesting. If the pre-oxidation catalyst
`is replaced by a precious metal coated diesel particulate
`filter catalytic combustion of injected hydrocarbons can
`be used to generate the necessary heat for soot
`combustion directly on the filter. Figure 18 shows an
`example.
`
`,05
`
`00
`
`100
`
`200
`
`3 00
`
`4 00
`
`500
`
`600
`
`70 0
`
`80 0
`
`900
`
`100 0
`
`time / s
`
`100 0
`
`0
`
`0
`
`Figure 15: CO and HC emissions during regeneration
`of dummy.
`
`The combination of precious metal and oxygen storage
`component in catalyst coatings suppresses CO peaks
`during regeneration (Figure 16).
`
`CO_vK, CO_nK [Vol%]
`
`0,5
`
`0,45
`
`0,4
`
`0,35
`
`0,3
`
`0,25
`
`0,2
`
`0,15
`
`0,1
`
`,05
`
`00
`
`HC_v K [ppm]
`
`HC_nK [ppm]
`
`CO_vK [V ol% ]
`
`CO_nK [V ol% ]
`
`HC before DPF
`
`CO before DPF
`
`HC after DPF
`
`CO after DPF
`
`100
`
`200
`
`300
`
`400
`
`500
`
`600
`
`70 0
`
`80 0
`
`900
`
`1000
`
`time / s
`
`10000
`
`9000
`
`8000
`
`7000
`
`6000
`
`5000
`
`4000
`
`3000
`
`2000
`
`1000
`
`0
`
`0
`
`NOx v.K. [ppm]
`
`HC_vK, HC_nK [ppm]
`
`Figure 16: CO and HC emissions during regeneration
`of catalyst coated DPF.
`
`The impact of NO2 on the regeneration via post injection
`can be ignored. The amount of NO available is typically
`in the range of 100 to 200 vol.-ppm at low speed and low
`load conditions.
`
`Since the maximum amount of NO2 is formed between
`250 and 400°C, according to thermodynamic equilibrium,
`only small amounts were observed. This is due to the
`filter temperature moving in and out of this range during
`the regeneration process (Figure 17).
`
`BASF-2001.009
`
`

`
`MgO
`10,5%
`
`ZnO
`5,3%
`
`SO3
`35,9%
`
`P2O5
`13,5%
`
`CaO
`34,8%
`
`Figure 19:
`
`composition
`ash
`Oil
`determined by XRF
`
`in mass-%
`
`The composition corresponds well with results found in
`the literature [12]. Main components are: sulfur, calcium,
`magnesium and phosphorus.
`
`In a second step oil ash was deposited on a catalyst
`coated filter with the described procedure. 36.4 g of oil
`ash were found.
`
`To investigate the catalysts’ performance the filter was
`loaded with soot and regenerated via post-injection in
`the described engine bench test. For comparison,
`another filter was coated with the same catalyst and
`aged in the same way but without lubricating oil in the
`fuel. The results show surprisingly a small decrease in
`soot combustion temperature for the oil-aged filter.
`Figure 20 illustrates the back-pressure curves and the
`filter inlet temperatures for both traps.
`
`The results were confirmed when aging and
`procedures were repeated.
`
`test
`
`400
`
`350
`
`300
`
`250
`
`200
`
`150
`
`100
`
`05
`
`0
`
`p5 [mbar]
`
`T1 DPF50b with ash
`T1 DPF50b w/o ash
`p5 DPF50b with ash
`p5 DPF50b w/o ash
`
`p5 DPF 50b w/o ash
`
`p5 DPF 50b with ash
`
`200
`
`300
`
`500
`400
`time /s
`impact
`ash
`Oil
`temperatures.
`
`600
`
`700
`
`800
`
`900
`
`on
`
`regeneration
`
`800
`
`700
`
`600
`
`500
`
`400
`
`300
`
`200
`
`100
`
`T1 DPF [°C]
`
`0
`
`0
`
`100
`
`Figure 20:
`
`P_5, p_6 [mbar]
`
`100
`
`90
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`01
`
`0
`
`T_1_DPF [°C]
`
`T_4 [°C]
`
`p_5 [mbar]
`
`1000
`
`900
`
`800
`
`700
`
`600
`
`500
`
`400
`
`300
`
`200
`
`100
`
`0
`
`T_1_DPF - T_4_DPF [°C]
`
`0
`
`100
`
`200
`
`300
`
`400
`
`500
`time / s
`
`600
`
`700
`
`800
`
`900 1000
`
`Figure 18: Regeneration of a diesel particulate filter via
`catalytic
`combustion
`heating
`(fresh).
`Pressure-drop curve and
`temperatures
`inside the filter are shown
`
`For this test, the turbine outlet temperature is about
`250°C, well above the hydrocarbon light- off for the
`catalyzed filter. The regeneration proceeds fast and
`leads
`to a complete combustion of
`the soot.
`Investigations are currently ongoing to explore the need
`of extra oxidation capacity in the system to meet
`emission limits in the NEDC test cycle. First results were
`promising and demonstrated that current EU III emission
`levels were reached. For particulate emissions even the
`EU IV level was achieved.
`
`OIL ASH IMPACT ON CATALYST COATED DIESEL
`PARTICULATE FILTERS
` - An
`important aspect
`concerning catalyst coated diesel particulate traps is the
`impact of oil ash, i. e. particles from the combustion of
`lubricating oil. Does the oil ash residue in the filter
`influence the catalysts’ performance by lowering the
`contact between soot and catalyst?
`
`To find an answer the following experiments were
`carried out:
`To simulate filter regeneration a dedicated, in house
`developed diesel burner was employed which oscillated
`the inlet temperature of a silicon carbide diesel filter
`between 250°C and 900°C.
`
`Four liters of lubricating oil (Castrol GTX 15W40) were
`dissolved in 36 liters of diesel fuel with 300 ppm sulfur
`content. Oscillating the temperature in the described
`mode for 100 cycles burned this solution. An uncoated
`diesel trap caught the unburned particles. The filter
`retained 31.0 g of oil ash. Figure 19 shows the result of a
`chemical analysis by XRF for the ash composition.
`
`BASF-2001.010
`
`

`
`Some TGA and DSC experiments showed a positive
`impact of calcium – a main component of oil ash – on
`soot ignition temperatures, which might explain the
`unexpected observation.
`
`In conclusion, the oil ash resulting from the combustion
`of four liters presenting an oil consumption of 40.000 km
`in normal use of lubricating oil does not influence the
`performance of a catalyst coated particulate
`trap
`essentially but strongly increases the pressure drop of
`the filter.
`
`OUTLOOK ON THE FUTURE COMBINATION OF
`PARTICULATE - AND NOX - AFTERTREATMENT - The
`insufficient results for catalytic coatings are due to the
`limited contact between soot and active sites. Vanadia-
`containing materials lead to better results in terms of
`soot ignition temperature decrease; however, volatility of
`vanadia is an obstacle for application. A complete
`redesign of the system is necessary for proper function
`of catalytic materials on filters. Using the catalytic
`combustion of hydrocarbons to heat the system might be
`an acceptable method. Figure 21 ilustrates possible
`
`1. Oxidation of SOF occurs at low temperatures
`between 250 to 350°C;
`
`2. NO2 impact on the soot combustion depends on
`NO/NO2 availability i.e. EGR rate;
`
`3. Soot oxidation between 350°C and 400°C is low
`because of unfavorable NO2/C ratio;
`
`4. Oxygen concentration is more important;
`
`5. Carbon oxidation occurs at higher temperatures
`(> 450°C).
`
`Since the exhaust gas temperatures and NO emissions
`of modern common rail and unit injector diesel engines
`are low it is not possible to rely only on passive
`regeneration. Therefore, active measures
`like post
`injection are necessary
`to provide
`the proper
`temperature condition for filter regeneration.
`
`The effects of vanadia-free catalyst coatings on passive
`and active particulate
`filter
`regeneration can be
`summarized as follows:
`
`CO, HC, PM,
`NO, NO2
`
`CO, NO, NO2
`
`CO2, N2, H2O
`
`1. Small decrease of the soot ignition temperature
`(20 – 60°C);
`
`System 1:
`
`Exhaust
`gas
`
`System 2:
`
`Exhaust
`gas
`
`designs.
`
`DOC
`
`DPF
`
`NOx Adsorber
`
`HC, PM, NO,
`NO2
`
`NO, NO2
`
`CO2, N2, H2O
`
`DOC
`
`DPF
`
`SCR Catalyst
`System
`
`Urea
`
`2.
`
`In situ formation or recycling of NO2 in the filter
`depending on NO concentrations and engine
`operation conditions;
`
`3. Oxidation of CO, hydrocarbons and SOF at normal
`operation conditions and during filter regeneration.
`
`ACKNOWLEDGMENTS
`
`The authors like to thank Mr. R. Staab for preparation of
`samples. We also thank Mr. T. Prieger for the engine
`bench tests.
`
`REFERENCES
`
`1. O. Salvat, P. Marez and G. Belot, SAE Technical
`Paper No. 2000-01-0473
`2. P. Degobert “Automobiles and Pollution”, Society of
`Automotive Engineers Inc., Warrendale, PA, USA,
`(1995)401-425;
`3. C.S. Weaver, SAE Technical Paper No. 831713,
`1983; K. Ohno, K. Shimato, N. Taoka, H. Santae, T.
`Ninomiya, T. Komori and O. Salvat, SAE Technical
`Paper No. 2000-01-0185;
`4. E.S. Lox, B.H. Engler, E. Koberstein, Catalysis and
`Automobile Pollution Control II (Ed.: A. Cruq),
`Elsevier, Amsterdam, (1991)291;
`5. E. Koberstein, H.D. Pletka, H. Völker, SAE Technical
`Paper No. 830081, (1983); B. Engler, E. Koberstein,
`
`Figure 21:
`
`Possible setup for diesel particulate filter
`applications.
`
`The filter could replace the precatalyst in SCR systems
`because of its ability to form NO2. At this time it is not
`clear if sufficient oxidation capacity can be provided in
`the system to meet the CO and hydrocarbon emission
`targets. Further experiments are necessary to prove the
`feasibility of such a solution for the upcoming emission
`legislation.
`
`CONCLUSION
`
`The investigations of the soot combustion with catalyzed
`diesel particulate filters covered passi